Asthma and COPD: Basic Mechanisms and Clinical Management

Foreword

During the last century, transmissible and acute diseases dominated the interests of the research, clinical, and public health communities. Looking back, we can only marvel at the progress that has been made. Indeed, some contagious diseases have been eradicated, totally or virtually, in many parts of the world. Deaths from some acute events, such as myocardial infarction and stroke, have declined remarkably. As a result, quality of life, economic power, and life expectancy have all increased in a number of countries. Over the past 20 years in the United States, for example, life expectancy increased by about six years owing to a reduction in death rates of most major conditions. Chronic diseases of the airways, however, have worked against this favorable trend in life expectancy. Although death rates from asthma are relatively low, chronic obstructive pulmonary disease has a very significant impact on the total number of deaths worldwide. In the United States, it accounted for about 110,000 deaths in 1999, ranking as the fourth most common cause of mortality. Of even greater concern than the death toll from asthma and COPD is their considerable impact on those who live with these chronic diseases. Because of their lingering nature, they constitute an extraordinary burden that reduces the quality of life for the patients and many around them. Furthermore, these diseases have a negative impact on the economic potential of society, especially in developing countries. The burdens on the healthcare system are readily measurable – hospitalizations, emergency room visits, prescription drugs, respiratory therapy, long-term care, among others. But perhaps even more significant are the limitations that these diseases impose on the ability of their victims to fulfill their roles in school, in the workplace, and in the community, to care for their loved ones and, in many cases, even to care for themselves. The strength of a society resides in the independence and productivity of its people, and these qualities, in turn, hinge upon the people’s good health. Asthma and COPD are ominous threats to the strength of societies worldwide. At the end of the twentieth century, several events occurred that may lead to a transformation of this sad situation. First and foremost, the international scientific com-

munity began to arrive at the realization that the path to achievement of its ideal goal – elimination of the main cause of COPD, cigarette smoking – would be a rocky one and that its pursuit must be coupled with an intensive research effort to control and conquer this disease. With regard to asthma, an extraordinary research effort has yielded a greater understanding of the pathogenesis of this disease and a new armamentarium of therapeutic approaches that have proven to be remarkably effective. But there is no room for complacency! Another defining event has been a greater realization of the importance of chronic diseases, especially asthma and COPD, in the newly developed interests of the World Health Organization. This has been largely due to the work of Drs Murray and Lopez.1 They gave the research and public health communities great cause for alarm by demonstrating that the ranking of societal and individual burden from chronic respiratory diseases will rise from twelfth to fifth between the years 1990 and 2020. We, and the public at large, can only applaud the response of these communities. This book is further evidence of that response. First, it presents the best and newest of what is known about these two diseases. The roster of international contributors is stellar. In addition, the volume is comprehensive: all aspects of these two very prevalent diseases are addressed. The reader will soon recognize the complexity of the issues and appreciate the wonderful job that the text does of making them understandable. The most important and innovative feature of this volume, however, is its comparison, where appropriate, of the two diseases. Of course, asthma and COPD are different, but they also share a number of characteristics, and understanding one can greatly help us understand the other. In 1971, the CIBA Foundation sponsored a debate on “Identification of Asthma”.2 One participant led an extensive discussion on the definition of asthma and how it may be differentiated from chronic bronchitis. At its conclusion, another participant wisely observed: “The question that clinicians have to ask themselves before they can apply rational treatment is this: What is the mechanism?”

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Foreword

By comparing and contrasting asthma and COPD, this book helps answer that question. In the end, it is the patients and the societies in which they live who will benefit from this contribution Claude Lenfant, MD Bethesda, Maryland

1. Murray CJL, Lopez AD (eds). The Global Burden of Disease: A Comprehensive Assessment of Mortality and Disability from Diseases, Injuries and Risk Factors in 1990 and Projected to 2020. Cambridge, MA: Harvard University Press, 1996. 2. Porter R, Birch J (eds). Identification of Asthma, pp. 132–50. CIBA Foundation Study Group, no. 38. Edinburgh: Churchill Livingstone, 1971.

Contributors

Ian M. Adcock National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK [email protected] Carlo Agostini Department of Clinical and Experimental Medicine Clinical Immunology Branch Padua University School of Medicine Via Giustiniani 2 35128 Padova Italy Steven M. Albeda Department of Medicine Pulmonary Allergy and Critical Care Division University of Pennsylvania Medical Center 852 BRBII/III 421 Curie Blvd Philadelphia PA 19104-6160 USA Yassine Amrani Pulmonary Allergy and Critical Care Division University of Pennsylvania Medical Center 421 Curie Bvd 805 BRB II/III Philadelphia PA 19104-6160 USA

Morgan Andersson Department of Otorhinolaryngology Lund University Hospital S-221 85 Lund Sweden Nick Anthonisen University of Manitoba 753 McDermot Avenue Winnipeg R3E 0W3 MB Canada [email protected] Jon G. Ayres Department of Respiratory Medicine Birmingham Hartlands Hospital Bordesley Green East Birmingham B9 5SS UK [email protected] Peter J. Barnes Department of Thoracic Medicine National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK [email protected] John Britton Division of Respiratory Medicine City Hospital Hucknall Road Nottingham NG5 1PB UK [email protected]

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Contributors

A. Sonia Buist Oregon Health Sciences University Mail Code UHN 67 Sam Jackson Park Road 3181 SW Portland OR 97201 USA [email protected] Pierre-Regis Burgel Division of Medicine and Physiology University of California 505 Parnassus Avenue M-1325 Box 0130 San Francisco CA 94143-0130 USA [email protected] Edward J. Campbell Department of Medicine University of Utah School of Medicine 410 Chipeta Way Salt Lake City UT 84108 USA Gaetano Caramori National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK S. Cattapan Room A 342, Bldg 1 Hines Hospital 5th and Roosevelt Avenues Hines, IL 60141 USA [email protected]

George H.F Caughey Department of Medicine Cardiovascular Research Institute University of California Box 0911 San Francisco CA 94143-0911 USA [email protected] B.R. Celli Tufts Lung Station St Elizabeth’s Medical Center of Boston 736 Cambridge Street Boston MA 02135-2997 USA [email protected] Richard N. Channick Department of Medicine Division of Pulmonary and Critical Care Medicine University of California San Diego Med Center 9300 Campus Point Drive La Jolla CA 92037-1300 USA [email protected] Moira Chan-Yeung Respiratory Division Vancouver General Hospital 2775, Heather Street Vancouver B.C. V5Z 3J5 Canada [email protected] Kian Fan Chung National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK [email protected]

Contributors

D. W. Cockcroft Division of Respiratory Medicine Royal University Hospital Saskatoon SK S7N 0W8 Canada [email protected] Chris Corrigan Allergy and Respiratory Medicine 14th Floor, Hunts House Guys Hospital St Thomas’ Street London SE1 9RT UK [email protected] Adnan Custovic Department of Respiratory Physiology Wythenshaw Hospital Southmoor Road Manchester M23 9LT UK [email protected] Donna E. Davies Department of Medicine University of British Columbia Vancouver General Hospital Vancouver BC Canada [email protected] W. Bruce Davis Pulmonary Critical Care AF2024 Medical College of Georgia Augusta Georgia 30912-3135 USA [email protected] Dawn L. DeMeo Harvard Medical School Channing Laboratory 181 Longwood Avenue Boston MA 02115 USA

Sujal Desai King’s College Hospital London UK [email protected] Aaron Deykin Pulmonary and Critical Care Division Brigham and Women’s Hospital 75 Francis Street Boston MA 02115 USA [email protected] Jeffery M. Drazen Pulmonary and Critical Care Division Brigham and Women’s Hospital 75 Francis Street Boston MA 02115 USA [email protected] Ellen M. Drost Respiratory Medicine Unit Department of Medical and Radiological Science ELEGI/Colt Resarch Laboratories Medical School Teviot Place Edinburgh EH8 9AG UK Jonas S. Erjefalt Department of Physiological Sciences Lund University Hospital S-221 85 Lund Sweden Leonardo M. Fabbri Section of Respiratory Diseases Department of Medicine Oncology & Radiology University of Modena and Reggio Modena Largo del Pozzo 71.41100 Italy [email protected]

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Contributors

Marlies Feijen Department of Pediatrics University Hospital of Groningen PO Box 30.001 Groningen, 9700 RB The Netherlands [email protected] Jack Gauldie Department of Pathology and Molecular Medicine McMaster University 1200 Main Street West Hamilton Ontario L8N 3Z5 Canada [email protected] Jorrit Gerritsen Department of Pediatrics University Hospital of Groningen PO Box 30.001 Groningen, 9700 RB The Netherlands [email protected] John Gibson Department of Respiratory Medicine Freeman Hospital Newcastle upon Tyne NE7 7DN UK [email protected] Maurice Godfrey Center for Human Molecular Genetics University of Nebraska Medical Center Omaha NE 68198-5455 USA [email protected] Simon Godfrey Institute of Pulmonology Hadassah University Hospital Jerusalem Israel [email protected]

Lemark Grieff Department of Otorhinolaryngology Lund University Hospital S-221 85 Lund Sweden N.J. Gross Room A 342, Bldg 1 Hines Hospital 5th and Rossevelt Avenues Hines, IL 60141 USA [email protected] Ian Hall Division of Therapeutics University Hospital Nottingham NG7 2UH UK [email protected] David M. Hansell Royal Brompton and Harefield NHS Trust Sydney Street London SW3 6NP UK [email protected] James C. Hogg UBC McDonald Research Laboratories St Paul’s Hospital University of British Columbia 1081 Burrard Street Vancouver, BC V6Z 1Y6 Canada [email protected] Stephen T. Holgate RCMB Division School of Medicine University of Southampton Southampton General Hospital Southampton SO16 6YD UK [email protected]

Contributors

Gabor Horvath Division of Pulmonary and Critical Care Medicine University School of Medicine PO Box 016910 (R-47) Miami, FL 33101 USA [email protected] Sebastian L. Johnston Department of Respiratory Medicine National Heart and Lung Institute at St Mary’s Norfolk Place London W2 1PG UK [email protected] P. Jones Division of Physiological Medicine St George’s Hospital Medical School Cranmer Terrace London SW17 0RE UK [email protected] Susan Kennedy Occupational Hygiene Program University of British Columbia 2206 E Mall 3rd Floor Vancouver V6T 1Z3 Canada [email protected] Sergei A. Kharitonov Department of Thoracic Medicine National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK [email protected] Martin Kolb Centre for Gene Therapeutics Department of Pathology and Molecular Medicine McMaster University 1200 Main Street West Hamilton Ontario L8N 3Z5 Canada

Chakradhar Kotaru University Hospitals of Cleveland 11100 Euclid Avenue Cleveland Ohio 44106-5067 USA Sam Krachman Division of Pulmonary and Critical Care Temple University Philadelphia PA USA [email protected] Vera P. Krymskaya Pulmonary Allergy and Critical Care Division University of Pennsylvania Medical Center 421 Curie Blvd 805 BRB II/III Philadelphia PA 19104-6160 USA Geoffrey J. Laurent Centre for Respiratory Research Royal Free; and University College Medical School Rayne Institute 5 University Street London WC1E 6JJ UK [email protected] Aili L. Lazaar Department of Medicine Pulmonary Allergy and Critical Care Division University of Pennsylvania Medical Center 852 BRBII/III 421 Curie Blvd Philadelphia PA 19104-6160 USA [email protected]

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Contributors

William MacNee Respiratory Medicine Unit Department of Medical and Radiological Science ELEGI/Colt Resarch Laboratories Medical School Teviot Place Edinburgh EH8 9AG UK [email protected] Robin J. McAnulty Centre for Respiratory Research Royal Free; and University College Medical School Rayne Institute 5 University Street London WC1E 6JJ UK E. R. McFadden Jr Division of Pulmonary and Critical Care Medicine MetroHealth Medical Center – BG3 2500 MetroHealth Drive Cleveland, OH 44109-1998 USA [email protected] J. L. Malo Sacré Coeur Hospital 5400 W Gouin Boulevarde Montreal H4J 1C5 Canada [email protected] Fernando D. Martinez College of Medicine University of Arizona 1501 N Campbell Avenue PO Box 245018 Tucson AZ 85724-5018 USA [email protected] Simon Message Department of Respiratory Medicine National Heart and Lung Institute at St Mary’s Norfolk Place London W2 1PG UK [email protected]

Andrew W.P. Molyneux University of Nottingham Division of Respiratory Medicine City Hospital Nottingham NG5 1PB UK Jay A. Nadel Division of Medicine and Physiology University of California 505 Parnassus Avenue M-1325 Box 0130 San Francisco CA 94143-0130 USA [email protected] Paul M. O’Byrne St Joseph’s Hospital Firestone Chest and Allergy Unit 50 Charlton Avenue Hamilton Ontario L8N 4A6 Canada obyrnep@mcmaster Rory A. O’Donnell RCMB Division School of Medicine University of Southampton Southampton General Hospital Southampton S016 6YD UK Reynold A. Panettieri Pulmonary Allergy and Critical Care Division University of Pennsylvania Medical Center 421 Curie Blvd 805 BRB II/III Philadelphia PA 19104-6160 USA [email protected]

Contributors

Martyn Partridge The Faculty of Medicine Imperial College Charing Cross Campus 5th Floor Charing Cross Hospital Fulham Place Road London W6 8RF UK James E. Pease Leukocyte Biology Section Sir Alexander Fleming Building Imperial College School of Medicine London SW7 2AZ UK John Pepper Royal Brompton Hospital Sydney Street London SW3 6NP UK [email protected] Carl Persson Department of Clinical Pharmacology Institute of Laboratory Medicine Lund University Hospital S-221 85 Lund Sweden [email protected] Dirkje S. Postma Department of Pulmonology University Hospital Groningen 9713 GZ The Netherlands [email protected] Neil B. Pride Thoracic Medicine National Heart and Lung Institute Imperial College School of Medicine Dovehouse Street London SW3 6LY UK [email protected]

David Proud Johns Hopkins Asthma and Allergy Center 5501 Hopkins Bayview Circle Baltimore MD 21224 USA dproud@jhmi Irfan Rahman Respiratory Medicine ELEGI/Colt Research Laboratories University of Edinburgh Teviot Place Edinburgh EH8 9AG Scotland UK [email protected] Stephen Rennard Department of Internal Medicine University of Nebraska Medical Center 600 S 42nd Street Omaha NE 68198-5300 USA [email protected] L. Richeldi Section of Respiratory Diseases Department of Medicine Oncology & Radiology University of Modena and Reggio Modena Largo del Pozzo 71.41100 Italy M. Romagnoli Section of Respiratory Diseases Department of Medicine Oncology and Radiology University of Modena and Reggio Modena Largo del Pozzo 71.41100 Italy

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Contributors

Lewis J. Rubin Department of Medicine Division of Pulmonary and Critical Care Medicine University of California San Diego Med Center 9300 Campus Point Drive La Jolla CA 92037-1300 USA [email protected] Marina Saetta Department of Clinical and Experimental Medicine Clinical Immunology Branch Padua University School of Medicine Via Giustiniani 2 35128 Padova Italy Gianpietro Semenzato Department of Clinical and Experimental Medicine Clinical Immunology Branch Padua University School of Medicine Via Giustiniani 2 35128 Padova Italy [email protected] Steven D. Shapiro Department of Pediatrics Campus Box 8208 Washington University School of Medicine 660 South Euclid Avenue St Louis MO 63110 USA [email protected] Sat Sharma Section of Respirology University of Manitoba 753 McDermot Avenue Winnipeg R3E 0W3 MB Canada

Stephanie A. Shore Physiology Program Harvard School of Public Health 665 Huntingdon Avenue Boston MA 02115 USA [email protected] John R. Spurzem Department of Medicine Omaha VA Medical Center Omaha NE 68198-5300 USA [email protected] Scott A. Strassels Department of Pharmacy University of Washington Box 357630 Seattle WA 98195 USA [email protected] Sean Sullivan Department of Pharmacy University of Washington Box 357630 Seattle WA 98195 USA [email protected] Neil C. Thomson Department of Respiratory Medicine Western Infirmary Glagow G11 6NT UK [email protected] Martin J. Tobin Division of Pulmonary and Critical Care Medicine Loyola University Chicago Medical Center 2160 South First Avenue Maywood IL 60153 USA [email protected]

Contributors

Galen B. Toews Division of Pulmonary and Critical Care Medicine University of Michigan 1500 East Medical Center Drive 3916 Taubman Ann Arbor Michigan 48109-0642 USA [email protected] Rubin M. Tuder Division of Pulmonary Sciences and Critical Care University of Colorado Health Sciences Center Box C272 4200 East Ninth Avenue Denver CO 80262 USA Lena Uller Department of Physiological Sciences Lund University Hospital S-221 85 Lund Sweden Norbert F. Voelkel Division of Pulmonary Sciences and Critical Care University of Colorado Health Sciences Center Box C272 4200 East Ninth Avenue Denver CO 80262 USA [email protected] Adam Wanner Division of Pulmonary and Critical Care Medicine University of Miami School of Medicine PO Box 016960 (R-47) Miami Florida 33101 USA [email protected] J. Wedzicha Academic Respiratory Medicine Dominion House St Bartholomew’s Hospital London EC1A 7BE UK [email protected]

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Scott T. Weiss Channing Laboratory Brigham and Women’s Hospital 181 Longwood Avenue Boston MA 02115 USA [email protected] Timothy J. Williams Leukocyte Biology Section Sir Alexander Fleming Building Imperial College School of Medicine London SW7 2AZ UK [email protected] Ashley Woodcock Department of Respiratory Physiology Wythenshaw Hospital Southmoor Road Manchester M23 9LT UK [email protected] A.J. Woolcock formerly Institute of Respiratory Medicine Level 8 Building 82 Royal Prince Alfred Hospital Camperdown NSW 2050 Australia Miel Wouters Faculteit der Geneskude-Pulmonology Universiteit Maastricht PO Box 5800 6202 MZ Maastricht The Netherlands [email protected] Z. Xing Centre for Gene Therapeutics Department of Pathology and Molecular Medicine McMaster University 1200 Main Street West Hamilton Ontario L8N 3Z5 Canada

Preface

Asthma and chronic obstructive pulmonary disease are amongst the two commonest chronic conditions in the world today and both are predicted to increase. Because of their high prevalence and chronicity, these diseases impose an enormous and growing economic and social burden. Enormous strides have been made in our understanding of the basic mechanisms of asthma, with a much better appreciation of the inflammatory mechanisms involved and how this underlies the clinical features of the disease. This is one of the reasons why the management of asthma has improved enormously. Currently available medications are highly effective in most asthmatic patients, although there remain a small group of patients who are still not adequately controlled on existing treatments. But although asthma medications are very effective, many patients with asthma continue to have problems and asthma is still a common cause of hospital admission and time lost from work. There is therefore a need for further research in asthma and for the development of new and even more effective therapies. Although COPD is just as large a problem as asthma, there has been less attention given to this disease, and our understanding of the underlying basic mechanisms are far less advanced than for asthma. COPD has a very high morbidity and mortality and is a growing problem, particularly in developing countries. Treatment is less effective than in asthma, and none of the existing medications is able to reduce the progression of the disease. COPD is still commonly treated as poorly responsive asthma, yet the inflammatory process and effects are very different and there is little reason to think that the same treatments should be effective. There is a pressing need for much more research into underlying mechanisms of COPD, in order to identify novel therapies in the future. Management issues in COPD

are also different in many respects from those involved in asthma. Two of us (PJB and NCT) were involved in editing a book on Asthma: Basic Mechansism and Clinical Management. This was most successful and ran to three editions. In considering the next edition we thought that it would be very useful to include COPD as no other book had taken both these diseases together. In putting together this new volume on Asthma and COPD: Basic Mechanisms and Clinical Management we invited the two North American editors in order to make the book more international. We have retained the structure of the original Asthma book, but have added new chapters that are relevant to COPD. However, we have asked authors to consider both diseases in preparing their chapters. Of course, there is far more information about basic mechanisms pertinent to asthma than to COPD, but we hope that by contrasting this information and identifying areas of uncertainty, this may act as a stimulus to further research in COPD. We hope that this new book will be useful to researchers and to clinicians and will serve as a useful reference source. The format has been changed to make it more attractive and more easily read. Despite the advance of on-line publications on the Internet, we feel that there is still an important place for definitive reference books as a source of information. We would like to thank Margaret MacDonald and Simon Crump of Academic Press for all their help in putting together this book and we hope that you will enjoy the result. Peter J. Barnes London

Jeffery Drazen Boston

Stephen Rennard Omaha

Neil C.Thomson Glasgow

Chapter

Definitions

1

A. Sonia Buist Division of Pulmonary and Critical Care Medicine, Oregon Health and Science University, Portland, OR, USA

Until recently, the presence or absence of reversibility was considered to be the key distinction between asthma and chronic obstructive pulmonary disease (COPD) – with reversible airflow obstruction the hallmark of asthma, and irreversible airflow obstruction the hallmark of COPD. Better understanding of both diseases has brought new definitions that acknowledge the overlap and highlight the similarities and differences between them. The important change in our understanding is the recognition that chronic inflammation underlies both diseases. The nature of the inflammation differs, however, as does the response to antiinflammatory medications, as described in detail in later chapters.

DEFINITIONS Asthma In the most recent US asthma guideline, the Expert Panel 2 Report,1 asthma is defined as: A chronic inflammatory disorder of the airways in which many cells and cellular elements play a role, in particular, mast cells, eosinophils,T lymphocytes, neutrophils, and epithelial cells. In susceptible individuals, this inflammation causes recurrent episodes of wheezing, breathlessness, chest tightness, and cough, particularly at night and in the early morning. These episodes are usually associated with widespread but variable airflow obstruction that is often reversible either spontaneously or with treatment. The inflammation also causes an associated increase in the existing bronchial hyperresponsiveness to a variety of stimuli. COPD In the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines,2 COPD is defined as: A disease state characterized by progressive development of airflow limitation that is not fully reversible.The airflow limitation is usually progressive and usually results from an abnormal response of the lungs to noxious particles or gases.

SIMILARITIES AND DIFFERENCES Over the past 30 years, thinking about asthma and COPD has swung between the concept of asthma and COPD belonging to a spectrum of diseases that all cause airflow obstruction, to the concept of them as very different diseases, and most recently to them both being inflammatory diseases with important similarities and differences. The present thinking is illustrated in Fig. 1.1 from the GOLD guidelines, which shows both diseases causing airflow limitation, but through a gene–environment interaction involving different sensitizing agents, different cell populations in the inflammatory response, and a spectrum of reversibility. The airflow limitation resulting from the inflammatory process ranges from completely reversible (the asthma end of the spectrum) to completely irreversible (the COPD end of the spectrum). Table 1.1 highlights the most important similarities between asthma and COPD. Both are chronic inflammatory diseases that involve the small airways and cause airflow limitation; both result from gene–environment interactions;

ASTHMA Sensitizing agent

Asthmatic airway inflammation CD4 T lymphocytes Eosinophils

Completely reversible

COPD Noxious agent

COPD airway inflammation CD8 T lymphocytes Macrophages Neutrophils

Airflow limitation

Completely irreversible

Fig. 1.1. Schematic of the genesis of airflow obstruction in asthma and COPD.

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Asthma and Chronic Obstructive Pulmonary Disease

Table 1.1. Similarities between asthma and COPD

Both are chronic diseases Inflammation present in both Airflow obstruction Involvement of the small airways Mucus Bronchoconstriction Both are consequences of gene–environment interaction

and both are usually characterized by mucus and bronchoconstriction. Although the similarities are striking, it is the differences between the two diseases that define their natural histories and clinical presentations. The key differences are contrasted in Table 1.2. The first obvious difference is that the diseases involve different anatomic sites in the lungs. COPD affects both the airways and the parenchyma; asthma affects only the airways. The small airways are involved in both diseases, and the structural changes at this level are responsible for much of the lung function impairment associated with these diseases. Also, an important difference anatomically is that emphysema, an irreversible, destructive, parenchymal disease, is variably present in COPD, but is not present in asthma. Perhaps the single most important difference between the two diseases is the nature of the inflammation: it is primarily eosinophilic, CD4-driven in asthma and neutrophilic, CD8driven in COPD.1,2 The nature of the inflammation in turn affects the response to pharmacological agents. There is ample evidence now that inhaled corticosteroids are effective against the eosinophilic inflammation that is characteristic of asthma,1–5 but largely ineffective against the primarily neutrophilic inflammation seen in COPD – although this is not a completely consistent finding. The natural histories of asthma and COPD are very different. COPD is a chronic and progressive disease that is characterized by airflow limitation that is not fully reversible and by an accelerated decline in lung function. Asthma is a

chronic disease, but it is usually not considered a progressive disease, and it is not usually characterized by an accelerated decline in lung function, unless there are other risk factors such as smoking.9,10 The airflow limitation is fully reversible in the early stages of asthma but, at least in a subset of asthmatics, may become progressively less reversible as the disease becomes longstanding.11 The difference in the gene–environment interaction in the two diseases has already been alluded to: in asthma, the inflammation is a response to inhaled allergens. In COPD, the inflammation is a response to noxious particles and gases.

D I F F E R E N T I AT I N G B E T W E E N A S T H M A AND COPD It would be easy to differentiate between asthma and COPD if the latter occurred only in smokers and asthma in nonsmokers. In fact, there is a clear diagnostic bias on the part of physicians, with COPD more likely to be diagnosed in men and asthma in women.11 It is important to emphasize that both conditions may coexist in an individual, so many will have the clinical and pathophysiological features of both diseases. This makes differentiating the diseases sometimes challenging for the clinician, especially in older adults who are or have been smokers. The clinician can be guided by information in the clinical history, such as smoking history, age of onset of symptoms, history of atopic conditions, and description of acute episodes of shortness of breath (see Table 1.3). Asthma usually has its onset in early childhood. However, adult-onset asthma does exist, and many are unable to remember childhood events that would provide a clue to the early stages of asthma. Therefore, unless symptoms are continuous from childhood, the onset of asthma symptoms in adult life may be hard to interpret, especially in the presence of other risk factors such as smoking. COPD typically becomes clinically apparent in the sixth and seventh decades of life. If an individual is physically active, he or she may notice reduced exercise tolerance earlier.

Table 1.2. Differences between asthma and COPD

Characteristic

Asthma

COPD

Anatomic site of disease Nature of inflammation Reversibility of airway obstruction Response to inhaled corticosteroids Progression of disease

Airways involved Eosinophilic, CD4-driven Mostly reversible Inflammation reduced Chronic, but not characterized as progressive Normal or slightly accelerated Allergens are main drivers of inflammation

Airways and parenchyma involved Neutrophilic, CD8-driven Mostly irreversible Inflammation mostly nonresponsive Progressive airflow obstruction

Decline in lung function Gene–environment interaction

Accelerated Particles and gases are main drivers of inflammation

Definitions

5

Table 1.3. Clinical features of asthma and COPD

Clinical feature

Asthma

COPD

Age of onset

Usually early childhood, but may have onset at any age May be non-, ex-, or current smoker History of atopic disorder(s) common Common at all levels of severity except mild intermittent Of atopic disorders or asthma commonly present Normal in mild intermittent and mild persistent; airflow obstruction present at all other steps Characteristic of asthma Characteristic of asthma, usually  20% Usually normal

Mid–late adult life

Smoking history Atopy Exacerbations Family history Lung function

Reversibility of airflow obstruction Peak flow variability Diffusing capacity

COPD in developed countries is mostly a disease of smokers. This is not necessarily true in developing countries where other risk factors, such as heavy outdoor and indoor/occupational air pollution, may be important risk factors that are causally related to COPD.2 The relationship between asthma and smoking is complex. Individuals with asthma may be nonsmokers, smokers, or ex-smokers. Since asthma genes and genes leading to the susceptibility to develop airflow obstruction with smoking are common in the population, the likelihood that an individual may have both is high. One of the unresolved questions about asthma relates to the nature of the complex relationship between asthma and atopy. Most asthmatics are atopic, but not all atopic individuals have asthma. A history of atopic disorders, such as allergic rhinitis or eczema, is therefore common in asthma, but is not a characteristic of COPD. As noted above, because asthmatic/atopic genes are widespread in the population, it is not unusual for atopic disorders to coexist with COPD, but it is not a characteristic of the disease as it is for asthma. Pulmonary function tests can also provide guidance. Both diseases are characterized by airflow obstruction except in the early or mild stages. In asthma, lung function is still normal in patients with mild intermittent or mild persistent disease.1 COPD, in comparison with asthma, is defined by airflow limitation, and this becomes progressively greater as the disease advances. Fig. 1.1 shows the spectrum of reversibility ranging from completely reversible (asthmatic end) to completely irreversible (COPD end). Clinically, reversibility is defined as  12% increase in FEV1 (and at least 200 mL) over baseline.12 If clear-cut reversibility of airflow limitation is found, asthma is likely to be present. If the airflow limitation is irreversible, COPD is likely to be the diagnosis.

Usually smoker or ex-smoker Not a prominent feature Increase in frequency with increasing severity of disease Not usually a feature Airflow obstruction a hallmark of COPD Poorly reversible Often does not vary at all Abnormal when there is emphysema

OVERLAP BETWEEN ASTHMA AND COPD Not acknowledged in the definitions is the fact that longstanding asthma can lead to airway remodeling and partly irreversible airflow obstruction. So, in many (but not all) with longstanding asthma, there is an appreciable component of chronic irreversible airflow obstruction with reduced lung function and incomplete response (or at least, not complete reversibility) to a short-acting bronchodilator or to oral or inhaled corticosteroids.13,14 This complicates the diagnosis of asthma in older adults, and requires that the goals of treatment be modified since maintenance of normal lung function can no longer be a realistic goal. Not clear yet is whether early and aggressive treatment with anti-inflammatory drugs can prevent remodeling, or in what proportion of individuals with longstanding asthma remodeling occurs. Whether longstanding asthma with remodeling can be called COPD is intensely controversial. In so far as there is irreversible or poorly reversible airflow obstruction in the remodeled lungs, the term seems appropriate. Conceptually and practically, the recognition that remodeling is a feature of longstanding asthma in many (but not all) reinforces the notion that these diseases constitute a spectrum of disease, as illustrated in Figure 1.1, ranging from fully reversible to fully irreversible.

E X A C E R B AT I O N S The definition of asthma highlights the importance of exacerbations as a feature of asthma, and emphasizes the fluctuations of the disease.1 The definition of COPD does not include any mention of exacerbations.2 Nevertheless, they may be as important in the natural history of COPD as

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Asthma and Chronic Obstructive Pulmonary Disease

they are in asthma15–17 and account for approximately 70% of the COPD-related costs in the US.2 The commonest causes of exacerbations of COPD are infections of the tracheobronchial tree and air pollution,2,18–21 but the causes of about one-third of severe exacerbations cannot be identified. The commonest symptom of an exacerbation of COPD is increased breathlessness, often accompanied by wheezing, chest tightness, increased cough and sputum, change in color and/or tenacity of sputum, and fever. Enquiring about the nature, frequency, and length of exacerbations is an important part of the clinical history in COPD since exacerbations are an important contributor to the erosion of quality of life in severe disease, and should therefore be an important focus of management.

7.

8.

9. 10.

11.

12.

L I M I TAT I O N S O F T H E D E F I N I T I O N S Definitions for both asthma and COPD have limitations since they can reflect only our current understanding of the diseases, which is quite limited. Both diseases will continue to be redefined as our understanding of them deepens, and as new effective preventive strategies and treatments become available.

REFERENCES 1. National Asthma Education and Prevention Program Expert Panel Report 2: Guidelines for the Diagnosis and Management of Asthma, National Institute of Health, National Heart, Lung, and Blood Institute. NIH Publication 97-4051, 1997. 2. Pauwels RA, Buist AS, Calverley MA, Jenkins CR, Hurd SS. Global strategy for the diagnosis, management and prevention of chronic obstructive pulmonary disease. NHLBI/WHO Global Initiative for Chronic Obstructive Lung Disease (GOLD) Workshop Summary. Am. J. Respir. Crit. Care Med. 2001; 163:1256–76. 3. Haatela T, Järvinen M, Kava T et al. Comparison of a b2-agonist, terbutaline, with an inhaled corticosteroid, budesonide, in newly detected asthma. N. Engl. J. Med. 1991; 325:388–92. 4. Haatela T, Järvinen M, Kava T et al. Effects of reducing or discontinuing inhaled budesonide in patients with mild asthma. N. Engl. J. Med. 1994; 331:700–5. 5. Jeffery PK, Godfrey W, Ädelroth E et al. Effects of treatment on airway inflammation and thickening of basement membrane reticular collagen in asthma. Am. Rev. Respir. Dis. 1992; 145:890–9. 6. Keatings VM, Jatakanon A, Worsdell YM, Barnes PJ. Effects of inhaled and oral glutocorticoids on inflammatory indices in

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20.

21.

asthma and COPD. Am. J. Respir. Crit. Care Med. 1997; 155:542–8. Culpitt SV, Maziak W, Loukidis S et al. Effects of high dose inhaled steroid on cells, cytokines, and proteases in induced sputum in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1999; 160:1635–9. Confalonieri M, Mainardi E, Della Porta R et al. Inhaled corticosteroids reduce neutrophilic bronchial inflammation in patients with chronic obstructive pulmonary disease. Thorax 1998; 53:583–5. Peat JK, Woolcock AJ, Cullen K. Rate of decline of lung function in subjects with asthma. Eur. J. Respir. Dis. 1987; 70:171–9. Lange P, Groth S, Nyboe J et al. Decline of lung function related to the type of tobacco smoked and inhalation. Thorax 1990; 45:22–6. Dodge R, Cline MG, Burrows B. Comparisons of asthma, emphysema, and chronic bronchitis diagnoses in a general population sample. Am. Rev. Respir. Dis. 1986; 133:981–6. American Thoracic Society, Medical Section of the American Lung Association. Lung Function Testing: Selection of Reference Values and Interpretive Strategies. Official Statement of the American Thoracic Society, adopted by the ATS Board of Directors, March 1991. Busse W, Elias J, Sheppard D, Banks-Schlegel S. Airway remodeling and repair. Am. J. Respir. Crit. Care Med. 1999; 160:1035–42. Fish JE, Peters SP. Airway remodeling and persistent airway obstruction in asthma. J. Allergy Clin. Immunol. 1999; 104:509–16. Burge PS, Calverley PM, Jones PW et al. Randomised, double blind, placebo controlled study of fluticasone propionate in patients with moderate to severe chronic obstructive pulmonary disease: the ISOLDE trial. Br. Med. J. 2000; 320:1297–303. Gibson PG, Wlodarczyk JH, Wilson AJ, Sprogis A. Severe exacerbation of chronic obstructive airways disease: health resource use in general practice and hospital. J. Qual. Clin. Pract. 1998; 18:125–33. Reguerio CR, Hamel MB, Davis RB et al. A comparison of generalist and pulmonologist care for patients hospitalized with severe chronic obstructive pulmonary disease: resource intensity, hospital costs, and survival. SUPPORT Investigators. Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment. Am. J. Med. 1998; 105:366–72. Wilson R. The role of infection in COPD. Chest 1998; 113:242S–8S. Soler N, Torres A, Ewig S et al. Bronchial microbial patterns in severe exacerbations of chronic obstructive pulmonary disease (COPD) requiring mechanical ventilation. Am. J. Respir. Crit. Care Med. 1998; 157:1498–505. Anderson HR, Spix C, Medina S et al. Air pollution and daily admissions for chronic obstructive pulmonary disease in six European cities: results from the APHEA project. Eur. Respir. J. 1997; 10:1064–71. Chodosh S, McCarty J, Farkas S et al. Randomized double-blind study of ciproflaxacin and cefuroxime axetil for treatment of acute bacterial exacerbations of chronic bronchitis. The Bronchitis Study Group. Clin. Infect. Dis. 1998; 27:722–9.

Chapter

Epidemiology

2

Dawn L. DeMeo and Scott T. Weiss Channing Laboratory, Brigham and Women’s Hospital, Boston, MA, USA

This chapter discusses the epidemiology of both asthma and chronic obstructive pulmonary disease (COPD). After briefly contrasting the disease definitions, the chapter reviews incidence and prevalence data, risk factors, and natural history.

DEFINING THE DISEASES Asthma The study of asthma epidemiology has been plagued by lack of consensus regarding standards for diagnosis. Most definitions have included variable airflow obstruction; but asthma is a clinical syndrome, without a gold standard for its diagnosis. Epidemiology studies have used questionnaires to assess for the presence of disease, but are limited by recall and misclassification bias. Some have suggested that symptoms should be assessed in conjunction with airway hyperresponsiveness.1 Others argue that airway hyperresponsiveness and symptoms should be analyzed separately owing to the poor correlation between clinical asthma and hyperresponsiveness.2 Population-based epidemiology studies have demonstrated a low sensitivity of airway hyperresponsiveness for detecting asthmatic phenotypes, versus a sensitivity of greater than 90% in clinic studies.3 A standard definition of asthma is as a chronic inflammatory disease of the airways with variable reversible airflow obstruction. Beyond definitions, there are differences between languages for the words used to describe asthma symptoms. A novel solution to this problem has been used in the International Study of Asthma and Allergies in Children (ISAAC), which includes an asthma video questionnaire demonstrating clinical signs of asthma as an attempt to improve uniformity in surveying for asthma.4 COPD Before describing epidemiological trends for obstructive lung disease, agreement on definitions should be achieved such that trends in incidence, prevalence, morbidity, and mortality can be properly ascribed.

COPD includes chronic bronchitis and emphysema, and is characterized by airway obstruction that is fixed or only partially reversible. The degree of airflow obstruction assigned to a given patient depends upon the guidelines used, with some defining mild obstruction as a FEV1 greater than 65%, 70%, or 80% of predicted. As in the case of asthma, the lack of international standardization of criteria for diagnosis in COPD makes understanding relative incidence and prevalence more challenging. This is well illustrated by a study by Viegi et al.5 who compared the prevalence rates of COPD in a general population in the Po Delta Valley using European Respiratory (ERS) criteria, American Thoracic Society (ATS) criteria, and standard clinical criteria. In subjects 25–45 years of age: • ERS criteria revealed a 10.8% prevalence of COPD; • ATS criteria revealed a 27% prevalence; • clinical criteria showed a 9.9% prevalence. Similarly, in subjects aged 46 years or more: • ERS criteria revealed a 12.2% prevalence; • ATS criteria had a 57% prevalence; • clinical criteria showed a 28.8% prevalence. This example highlights the difficulty of comparison between international studies and the effort to understand COPD on a global scale. If such discrepant results are obtained within a single population, then the difficulty of comparison between populations is very clear. In summary, both asthma and COPD lack gold standards for diagnosis, which would facilitate epidemiological studies. As a result, comparison of studies of asthma and COPD between populations and between countries must be viewed in the light of differences in criteria used for disease diagnosis.

INCIDENCE Asthma is predominantly a disease of childhood, with more than 17.3 million persons having asthma in the United

8

Asthma and Chronic Obstructive Pulmonary Disease

COPD vary with the age of the population. Asthma is commonly diagnosed in early childhood; COPD is commonly diagnosed after age 60.

States, 12 million are children of less than age 16. In childhood, incidence rates for asthma are highest among the youngest age groups6,7 and among male children until puberty.8–11 In a recent study of an adult Swedish population, Toren and Hermansson12 found the incidence rate for adult-onset asthma to be highest among females of all ages greater than 20, with an incidence of 1.3 per 1000 personyears; among women 16–20 years of age the rate was 3 per 1000 person-years. Analysis of data from a prospective cohort study in Finland demonstrated no increase in incidence for asthma from 1982 to 1990 in adults aged 18–45 years.13 Early investigation into the increasing prevalence of asthma in the United States was noted in a review of medical records from Olmsted County, Minnesota, where the annual incidence of asthma was found to increase from 183 per 100,000 in 1964 to 284 per 100,000 in 1983. The most significant increase was in children aged 1–14 years, suggesting a potential cohort effect early in life. Despite this increased incidence in asthma among children from 1964 to 1983, constant rates were observed among adults.6 Although these data indicate that asthma incidence is increasing, minimal information is available for trends in COPD incidence. Incidence rates for asthma and

P R E VA L E N C E Recent trends in the prevalence of obstructive lung disease are suggested by an analysis of the National Health and Nutrition Examination Survey (NHANES III).14 This included subjects with asthma, chronic bronchitis, and emphysema (Fig. 2.1). In this cohort, outcome measures included a physician diagnosis of chronic bronchitis, asthma or emphysema, respiratory symptoms, and low lung function. Of note, for the purposes of evaluating this cohort the investigators defined low lung function as present when both the FEV1/FVC ratio was < 0.70 and the FEV1 was less than 80% of predicted. Of the investigated population of 20,050 adults, 6.8% had low lung function as thus defined; 7.2% of the population had an FEV1/FVC ratio less than 0.70 with an FEV1 greater than 80% predicted, and were not included as having low lung function. Of the entire population, 8.5% reported obstructive lung disease.

25 Current COPD Past Chronic Bronchitis or Asthma

Current COPD and Asthma Current Asthma

Percentage with condition

20

15

10

5

Black female

Black male

White female

≥85

75–84

65–74

45–64

25–44

17–24

≥85

75–84

65–74

45–64

25–44

17–24

≥85

75–84

65–74

45–64

25–44

17–24

≥85

75–84

65–74

45–64

25–44

17–24

0

White male

Age Fig. 2.1. Age-specific percentage of individuals, stratified by race and sex, with chronic obstructive pulmonary disease and asthma, current COPD, current asthma, and past chronic bronchitis or asthma. Reproduced from National Center for Health Statistics, Plan and Operation of the Third National Health and Nutrition Examination Survey, 1988–94, US Dept of Health and Human Services publication PHS 94-1308, 1994, with permission.

9

Epidemiology

Importantly, 63.3% of those with documented low lung function had no current or prior doctor diagnosis of obstructive lung disease. In addition to prevalence information regarding low lung function, data from NHANES suggests that there is still a significant proportion of disease that goes undiagnosed in the mild stages, thus leading to an underestimation of the true prevalence of obstructive lung disease. Asthma Data from the United States suggest an increase in prevalence of asthma in children as well as in older adults. During the last several decades studies have suggested an increase in prevalence worldwide of 5–6% per year. Data from the National Health Interview Survey reveal an increase of 75% in self-reported asthma rates from 1980 to 1994 (Fig. 2.2). This trend was demonstrated in all age and race strata as well as in both genders. The most significant increase was among:

COPD Susceptibility to cigarette smoke is not uniform. However, COPD is best understood by understanding first the trends for smoking in populations. Although projected smoking rates throughout the world have increased, smoking prevalence in the United States between 1983 and 1995 declined overall: • • • •

• children 0–4 years of age (increase of 160%); • persons 5–14 years of age (increase 74%).15 The prevalence among inner-city children is much higher.15–17 It has been suggested that a doctor’s diagnosis of asthma is made less frequently than asthma symptom reporting, raising concern that despite increasing prevalence there is still a tendency to underdiagnose asthma, and consequently underestimate true prevalence values.18 The increasing prevalence of asthma has been recapitulated in international data. The International Study of Asthma and Allergies in Children (ISAAC) has as its aim to describe, across 155 centers, the prevalence and severity of asthma in children in 56 countries.4 Phase 1 of this trial has demonstrated a large variation in the prevalence of asthma symptoms in children throughout the world, with the

from from from from

30% 32% 34% 41%

to to to to

24% 23% 26% 29%

in white women; in African American women; in white men; in African American men.

Stang et al.22 utilized smoking rates to create a mathematical model for estimating current COPD prevalence. Using their model, they estimated that 15.3 million people in the United States aged 40 years or more have COPD; this was a reasonable estimate compared to the spirometric prevalence of 17.1 million as estimated by the Third National Health and Nutrition Examination Survey. Using this model, they also predicted the prevalence of COPD in Germany (2.7 million), the United Kingdom (3.0 million), Spain (1.5 million), Italy (2.6 million), and France (2.6 million), and suggested smoking rates as a useful surrogate for estimating COPD prevalence.

Prevalence rates of asthma

16 14 Number (millions)

highest prevalence in centers from Australia, New Zealand, the United Kingdom, and Ireland18–21 (Fig. 2.3). While the prevalence of allergic rhinitis has been noted to be scattered in the groups with the highest prevalence of asthma, the lowest prevalence for rhinitis has been found in countries where the asthma prevalence was lowest, such as in Eastern Europe, Indonesia, Greece, and India. In addition to defining prevalence rates, the ISAAC study represents an effort to establish an international standard to facilitate comparability of data from epidemiological studies of asthma.

<18 years

18–44 years

45 years

12 10 8 6 4 2 0

* 1970

1975

*Younger than age 17 years.

1980

1985

1990

1995

Year

Fig. 2.2. Prevalence of asthma in individuals less than age 18, aged 18–44, and greater than age 45. Between 1979 and 1994 the prevalence of asthma increased in all three age groups. Reproduced from NHLBI Morbidity and Mortality Chartbook, 2000, p. 61, with permission. http://www.nhlbi.nih.gov/resources/docs/cht-book.htm

Percentage of sample 0

5

10

15

20

25

30

35

Australia, Irsland, New Zealand, UK Brazil, Canada, Peru, USA

1

France, Germany, Japan, Kenya, Sweden

1

Iran, Italy, Malaysia, Nigeria, Singapore, Spain Ethiopia, India, Mexico, Iterea, Pakistan

• 1

China, Indonesia, Greece, Russia

Fig. 2.3. Prevalence of wheeze measured in the last 1 2 months prior to survey in the International Study of Asthma and Allergy in Children. O n l y 28 of 56 of the participating countries are highlighted here. Reproduced from reference 102, with permission.

The World Health Organization prediction is that by 2020 C O P D will rise from being the twelfth to the fifth most prevalent disease worldwide, and from being the sixth most common cause of death to the third most common.^^ Recent prevalence estimates of C O P D in the United States suggest that approximately 15 million people have C O P D : 14.1 million with chronic bronchitis and 1.8 with emphysema in 1996 (Fig. 2.4). There was no change in the prevalence of emphysema from 1982 to 1996, although from 1983 to 1995 the prevalence of chronic bronchitis continued to increase. In a study of the Canadian population, prevalence rates of C O P D were 4.6% in the 55-64 age group, 5.0% in the 65-74 age group, and 6.8% in the greater than 75 age group.^ These data may be an underestimation, as there is a suggestion that C O P D prevalence rates are underestimated in the elderly, especially in those with lower incomes.^^ C O P D is thought to be underdiagnosed in both N o r t h American and European populations. The IBERPOC Project (Estudio Epidemiologico de la E P O C en Espana)

Prevalence rates of chronic bronchitis 16 r

<18 years

• 1 8 - 4 4 years

n 4 5 + years

14 12 10 -

1970

1975

1980

"Younger than age 17 years.

1985

1990

1995

Year

(b) 80 r

Prevalence rates of emphysema

ro 60 -

40 -

E

20

0 1980

1982

1984

1986

1990

1992

1994

1996

Year Fig. 2.4. Prevalence measures for chronic bronchitis and emphysema, (a) From 1983 to 1995 the prevalence of chronic bronchitis increased steadily, with most of the increase in those older than age 18 years, (b) Despite fluctuations from year to year there was no overall change in the prevalence rate of emphysema from 1982 to 1996; most of the burden of the disease has been in older individuals. Reproduced from NHLBI Morbidity and Mortality Chartbook, 2000, p. 56, with permission. http://www.nhlbi.nih.gov/resources/docs/cht-book.htm

11

Epidemiology

was a population-based study of prevalence of COPD in Spain.26 The prevalence of COPD in this population (26% current smokers, 24% ex-smokers, 76% men), defined according to European Respiratory Society criteria, was 9.1%. Only 22% of those diagnosed had a prior diagnosis, while 48% had prior respiratory symptoms. The WHO and the National Institute of Heart, Lung and Blood Diseases have collaborated in an effort to broach the increasing present and projected future burdens of COPD by implementing a Global Initiative for Obstructive Lung Disease (GOLD). GOLD aims to promote studies to understand the increasing prevalence of COPD worldwide, as well as to standardize the collection of data for international comparison27 (Fig. 2.5).

• If both asthma and COPD are underdiagnosed, the prevalence estimates underestimate the true burden of these diseases. • Variability in definitions of both asthma and COPD contribute to inexact prevalence estimates and problems with comparisons of prevalence data. • ISAAC (for asthma) and GOLD (for COPD) represent efforts underway to standardize the definitions used in studies to enhance international comparisons of incidence, prevalence, and burden of disease.

U T I L I Z AT I O N A N D H O S P I TA L I Z AT I O N TRENDS

Summary • The prevalences of both asthma and COPD are increasing in Western developed countries.

In the United States, the estimated cost for year 2000 for asthma was projected to be 12.7 billion dollars (8.1 for

Rate/100,000 population 40

30

20

10

Rate/100,000 population 0

0

10

20

30

40

50

60

70

80

Hungary (95) Ireland (95) Romania (96) Scotland (97) New Zealand (94) United Kingdom (97) England/Wales (97) USA (97) Northern Ireland (97) Australia (85)

Women

Men

Spain (95) Poland (96) Netherlands (95) Germany (97) Bulgaria (94) Portugal (96) Norway (95) Canada (95) Austria (97) Italy (93) France (95) Sweden (96) Israel (95) Japan (94) Greece (96)

Fig. 2.5. Age-adjusted death rates for chronic obstructive pulmonary disease by country and sex, for individuals aged 35 to 77. The year of data is shown in parentheses. Reproduced from reference 27, with permission.

12

Asthma and Chronic Obstructive Pulmonary Disease

direct cost, 2.6 related to morbidity, 2.0 related to mortality) and 30.4 billion dollars for chronic obstructive pulmonary disease (14.7 for direct cost, 6.5 related to morbidity, 9.2 related to mortality). Utilization of health services continues to increase for both diseases. An increase in health service use has been documented in many countries, including the United Kingdom, Canada, and the United States; the utilization increase has been concomitant with the documented increase in asthma prevalence.28–30 Increased hospital visits have been documented worldwide, including in England, New Zealand, the United States, Greece, Australia, and Canada.31–38 From 1971 to 1997, hospitalizations for asthma in the United States increased for children less than age 15, remained stable for people aged 15–44, and decreased for those greater than 45 years of age. Overall, hospitalizations with asthma as a primary diagnosis increased in the 1970s until the mid 1980s and then remained constant; this is in contrast to asthma as a secondary diagnosis which increased until 1997. From 1975 to 1995, office visits for asthma more than doubled, from 4.6 to 10.4 million.15 Based on the National Ambulatory Medical Care Survey, in 1995 more than 16 million visits were made to physicians for diagnoses related to COPD, increased from 9.3 million reported in 1985; 10 million were accounted for by chronic bronchitis and 4 million for chronic airways obstruction.This same survey noted nine million office visits coded for asthma in 1995. In 1995, there were 553,000 discharges coded as COPD or allied conditions. Again this may be a definitional problem; more than half of the discharge diagnoses were nonspecifically coded as COPD or allied conditions. In summary, increased health service utilization for asthma and COPD occurred in the last decade. Overall, hospital admissions and discharges increased for asthma and COPD.

M O R B I D I T Y A N D M O R TA L I T Y Asthma The New Zealand epidemic of asthma in the 1970s prompted a review of asthma deaths in Western countries; there was a notable increase of 1.5–2 fold in the asthma mortality rates between the mid-1970s and the mid-1980s.39 The highest mortality rates in the United States have been in the inner-city regions, with particularly high-risk populations studied in East Harlem, New York City, and Cook County, Chicago.17,40 One study found that socioeconomic and racial disparities were attributable to higher incidence of asthma exacerbations among inner-city children, with no excess utilization of medical resources.41 International comparisons of mortality rates have been limited by differences in recording statistics of cause of death.42 Comparison of mortality rates are difficult also because of the lack of standardized definitions for the disease, and because of environmental, genetic, socioeconomic, and occupational influences unique to a given population.

COPD Since 1960 there has been an increased mortality associated with COPD, and in 1998 COPD mortality in the United States increased with age for all racial and gender groups. COPD mortality rates in white men in the United States are the highest, but have remained stable since 1980. During this time period, rates have increased in African American men and have doubled in white and African American women. International mortality trends demonstrate high rates of deaths for COPD in many countries. These differences may be accounted for in part by different smoking behaviors including tobacco type, environment, infectious, and genetic factors. Differences among these death rates are striking, but again lack of standardization in coding practices and death certification as well as practice differences and quality of care are relevant when comparing estimates.27 Although overall asthma mortality remains low compared with COPD, mortality rates for both asthma and COPD have increased in the last decade. Differences in death rates for asthma and COPD between countries are multifactorial (genetic, environmental, occupational, socioeconomic), but differential coding of cause-of-death statistics hinders accuracy of estimates for both diseases.

SMOKING Burrows et al.43 have demonstrated that, for a given level of tobacco smoke exposure, FEV1 varies substantially (Fig. 2.6). In addition, the dose–effect relationship between cigarette smoking and FEV1 decline depends on when an individual is exposed. Dose and timing of tobacco smoke exposure have a differential effect on FEV1 depending on the stage of the life cycle (Table 2.1). Cunningham et al.44 observed that maternal smoking during pregnancy resulted in a 1.3% reduction in FEV1 when children were 8–12 years old. Tager et al.45 found that adolescents who smoke when aged 15–20 have an estimated 8% reduction in FEV1. The Vlagtwedde/Vlaardingen study46 demonstrated a large effect of cigarette smoking in decreasing maximal lung function in individuals less than age 20; this effect exceeded the effect of cigarette smoking on lung function decline seen in older subjects. Smoking is a notable risk factor for both asthma and COPD in children and adults. Overall, smoking is associated with an increase in asthma incidence.47,48 Passive exposure to cigarette smoke increases the risk for the development of asthma and allergic sensitization.49–51 There has also been a suggestion that nonspecific airways responsiveness is increased by environmental and personal smoke exposure.52 Maternal smoking is a risk factor for the development of asthma in children up to one year of age.53 In a case–control study of children whose mothers were heavy smokers, one group demonstrated an odds ratio of 2.15 among 3–4 year olds for the development of asthma; these data were

Epidemiology

30

who continued smoking for 5 years had further losses of several hundred milliliters of FEV1.56 However, chronic obstructive pulmonary disease has been identified in nonsmokers as well, with 4% of men and 5% of women reporting physician-diagnosed obstructive lung disease. Prevalence has been noted to increase with age, to be higher in women than in men, to be particularly high in Hispanic individuals, and to be higher in low-income versus affluent individuals.57

0 pack-years

20 10 0

30

0–20 pack-years

20 10

Summary • For asthma and COPD, smoking has a lifetime influence, starting in utero and continuing into older age. • Smoking is associated with increase airway responsiveness, both in asthma and COPD. • Smoking is sufficient but not necessary for the development of COPD. • Smoking is only one of several risk factors for asthma.

Percentage of population

0

30

13

21–40 pack-years

20 10 0

I N T E R M E D I AT E P H E N O T Y P E S 30

41–60 pack-years

20 10 0

30

61pack-years

20 10 0 40

60

80 140 100 120 Percentage predicted FEV1

160

Fig. 2.6. Distribution of percentage predicted forced expiratory volume in one second (FEV1) in adults with varying smoking histories as measured in pack-years. The proportion of smokers with normal flow decreased with increasing pack-year histories. Yet, many have nearnormal FEV1 with extensive smoking history. Subjects with “respiratory trouble” before age 16 were excluded. Medians and means  1 SD are shown for each group in the abscissae. Note that among the 425 persons with 20 pack-years, only 15% have an FEV1 of 60% of predicted or less. Reproduced from reference 43, with permission.

controlled for family history, past infections, gender, and other demographic variables.54 In a six-year follow-up, the odds ratio for asthma among those exposed to maternal smoking was 3.8.55 As noted above, the single most important risk for COPD is tobacco smoking, although only 10–15% of smokers actually go on to develop obstructive lung disease. Among those smokers already with a decreased FEV1, lung injury and subsequent decrements in lung function secondary to cigarette smoking are more dramatic. In the Lung Health Study, middle-aged smokers (with FEV1 between 55% and 90%)

Allergy Allergy represents immediate hypersensitivity to an antigen and is associated with an increased production of a specific immunoglobulin by sensitized lymphocytes. Elevations in specific IgE and/or total IgE, total eosinophil counts, and skin test reactivity to specific allergens have been used clinically to detect allergic individuals. As measured by skin test reactivity, allergy increases with age until about age 15, at which point it is maximal. The decline in skin test reactivity after age 35 confounds the measurement of this phenotype in older individuals susceptible to both asthma and COPD. This reported association between skin test reactivity and decline in FEV1 is not consistent in the literature. In retrospective studies, Taylor et al.58 and Frew et al.59 demonstrated no relation of skin test positivity to decline in FEV1. However, Gottlieb et al.60 investigated this prospectively in the Normative Aging Study and found that skin test positivity predicted increased annual rates of decline in both FEV1 and FEV1/FVC ratios. Allergic inflammation is characteristic of asthma; 80–90% of childhood asthmatics are atopic, and the degree of atopy appears to be associated with prognosis in childhood asthma.61 Studies have demonstrated that the asthmatic phenotype is associated with elevated serum IgE levels more so than skin test positivity,62 and that increased airways responsiveness is related to total serum IgE levels.63 Weiss has suggested that immediate type I hypersensitivity is a risk factor for the development of chronic obstructive lung disease, and suggests that atopy may influence childhood asthma and limit maximal lung function, accelerate FEV1 decline, and potentially enhance interaction with cigarette smoking to progress to the development of COPD.64 Hargreave and Leigh65 demonstrated, in a subset of COPD patients, that eosinophilic inflammation is important in COPD exacerbations, and potentially leads to a

14

Asthma and Chronic Obstructive Pulmonary Disease

Table 2.1. Effects of cigarette smoking at different stages of the life cycle

Life phase (gender)

Cigarette dose

Total FEV1 reduction

FEV1 reduction (mL/year per packs/day)

In utero (M & F) Adolescence (M) Adolescence (F) Adult (M) Adult (F)

? Intensity for 9 mo 15 cigs/day for 5 yrb 10 cigs/day for 5 yrb Variable Variable

27.3 mLa 390 mL 340 mL N/A N/A

36 104 136 13c 7c

M  male, F  female a Adjusted for gender and maternal smoking in the past year; based on 1.3% reduction and mean FEV1 = 2.1 liters, 1 pack/day in smoking mothers during pregnancy is assumed for relative FEV1 reduction (ref. 44). b Median values for cigarette smoking (ref. 45). c Estimated values (ref. 46). Adapted from Weiss ST, Silverman EK. Risk factors for the development of chronic obstructive pulmonary disease. In: Severe Asthma, New York: Marcel Dekker, 2000.

decline in lung function. These data indicate that, in both asthma and COPD, allergen sensitization may represent an intermediate phenotype which needs to be considered in understanding disease onset and progression. Airways responsiveness Airways responsiveness to methacholine and histamine has been used in population-based studies to help define individuals susceptible to the development of obstructive lung disease. This intermediate phenotype is a feature of both asthma and a subset of patients with COPD. Baseline levels of lung function, allergy, age, and cigarette smoking history all influence airways responsiveness. Airways hyperresponsiveness has been demonstrated to predict accelerated decline in lung function and the development of COPD.66 More recently, airways responsiveness has been demonstrated to predict COPD mortality.67 Airways responsiveness has been demonstrated to predict the development of asthma.68 The prevalence of airway hyperresponsiveness exceeds the prevalence of asthma; the former is about 20% in the general population. Data from the Childhood Respiratory Disease Study demonstrate that increased airway responsiveness predicts the development of asthma in children and young adults with a 2–3-fold risk.69 Some have found risk increased as much as 5-fold.70 Airways hyperresponsiveness in COPD patients may be demonstrated in 64–100% in situations where it is actually measured.71 Some individuals who develop COPD have an allergic asthma phenotype, as suggested by the Dutch hypothesis.72 Alternatively the hyperresponsiveness may be a consequence of COPD. Results from a 25-year longitudinal study in the Netherlands revealed that increased airways responsiveness is an independent risk factor for FEV1 decline.73 Among those with early-onset COPD, the degree of baseline airways responsiveness determines the response to cigarette smoking; those with early-onset COPD who

have increased airways responsiveness appear more sensitive to the effects of cigarette smoke and have an accelerated decline in FEV1.74 Gender-related influences The epidemiology of asthma is characterized by gender differences that vary with age. Asthma and wheezing have been demonstrated to be more prevalent in young boys than young girls.10 This trend disappears during puberty.75 A recent analysis of the European Respiratory Health Survey76 found that, during childhood, girls had a lower risk of developing asthma than did boys; about the time of puberty the risk was equal. After puberty the risk in women was higher than in men and was a consistent finding in the 16 countries included in this study. Women older than 20 years have higher prevalence and morbidity rates from asthma, and women are more like to present to the emergency department and be admitted with asthma.77 In the multicenter Asthma Collaboration Study,78 women were more likely to be admitted to the hospital and report ongoing symptoms at follow-up, although overall men had less outpatient care and lower pulmonary function. Men have been noted to have an increased risk for the development of chronic obstructive lung disease,79 and cigarette consumption clearly has a role in this gender difference. Yet, Prescott et al.80 have suggested that women are more susceptible to the development of COPD, and observed that smoking was associated with a greater decrement in FEV1 per pack-years of cigarette smoked when compared to male smokers. Mannino et al.81 analyzed data for deaths from obstructive lung disease from 1979 until 1993 and found that the mortality rates for men with COPD have started to stabilize but were continuing to increase among women, reflecting smoking trends. These gender differences most likely represent influences of both dose of tobacco exposure and underlying genetic and hormonal susceptibilities.

15

Epidemiology

Summary • In the adult years women have a higher prevalence of asthma. • The prevalence of COPD in women is increasing, with the prospect that it may equal that of men in the future, in keeping with the parallel trends of cigarette smoking and disease. • The gender differences between asthma and COPD raise speculation as to the nature of hormonal or genetic influences relevant to disease expression in each sex.

DEMOGRAPHICS In the United States, morbidity from asthma has been demonstrated in multiple studies to be greater in children of African American descent. In the United States, physician-diagnosed asthma has been reported in 13.4% of African American children and 9.7% of white children.82 African American children have also been reported to have greater limitation on activity due to asthma, with more hospital admissions and fewer doctors’ visits when compared with white children.83 Mortality from asthma has been higher for African American children when compared with children of other races since the mid-1980s.29,84–88 Studies in Chicago have demonstrated socioeconomic gradients and differing outcomes by race. In 1996, asthma hospitalization rates were more than twice as high as the United States’ rates overall. Age-adjusted mortality was 4.7 times higher in non-Hispanic blacks than in non-Hispanic whites.89 An association with poverty has been suggested,90 and it has also been suggested that severe asthma may occur more frequently in poorer communities.91,92 The association of lower socioeconomic status with increased asthma prevalence is most likely multifactorial: the effects of indoor air pollution, passive cigarette smoke exposure, allergen exposure, and reduced access to medical care may all be relevant. Using education as a surrogate for lower socioeconomic status, some have suggested an association with the development of obstructive lung disease. Bakke et al.93 demonstrated that completion of only primary schooling was associated with a 2.9 odds ratio for the development of obstructive lung disease when compared with those who achieved university level education. Exposure to smoking and occupational hazards decreased with increasing educational status. Overall in both asthma and COPD, there are substantial demographic differences between the prevalence, morbidity, and mortality outcomes.

AGE Infants born prematurely have a risk for asthma that is increased approximately 4-fold.94 There are data that breastfeeding is protective against asthma and, as noted, the risk for asthma increases in children exposed to cigarette smoke

in utero and in childhood. Asthma that begins after age 50 is thought to be more severe and less reversible than asthma that is incident in childhood.95 In childhood, the remission of asthma has been suggested to be about 50%.47,96,97 Less information is available on the epidemiology of asthma in the middle-aged or elderly, yet some suggest that older patients are more severely affected than younger patients.98 Some data support the proposition that adults may outgrow their asthma (with remission rates decreasing with increasing age).99 Other data suggest that remission of asthma and respiratory symptoms are uncommon.100 Aging has been associated with increased airway obstruction overall.101 The association of aging with the development of COPD most likely represents the cumulative insult of a lifetime of smoking and environmental exposure interacting with a susceptible host.

CONCLUSION Ninety percent of all childhood asthma is diagnosed before the age of 6 years. Since there is a crude inverse relationship between respiratory symptoms and level of lung function, it is not surprising that as lung function increases in childhood, respiratory symptoms decrease and often disappear. Thus, a large number of children are left with the intermediate phenotypes of increased airways responsiveness and/or allergy at the time that they reach their maximally attained level of lung function between the ages of 15 and 30. These intermediate phenotypes represent definable host characteristics that confer increased susceptibility to a variety of environmental exposures encountered in adult life, such as viral respiratory illness, occupation, allergen exposure, and perhaps most importantly, cigarette smoking (Table 2.2). Only 10–15% of cigarette smokers subsequently go on to develop fixed airflow obstruction. This is likely to be due to two factors: premature mortality as a result of a variety of fatal illnesses associated with cigarette smoking, and the fact that genetic susceptibility to cigarette smoking is only present in a minority of subjects. The most clearly defined susceptibility factors for premature or early-onset COPD are childhood asthma, increased airways responsiveness, and allergy. It is now absolutely clear that most airways hyperresponsiveness in Table 2.2. Risk factors

Smoking Gender Age Airway responsiveness Allergy

COPD

Asthma

 Male  female Old

 Female  male Young

 

 

16

Asthma and Chronic Obstructive Pulmonary Disease

adults antedates, precedes, and predicts the development of COPD. Obviously, this construct suggests that most genes that predict susceptibility to asthma may also be important genetic predictors of COPD susceptibility. Both asthma and COPD are defined as syndromes, and the definitions are loose; thus it is not surprising that there is substantial overlap between the conditions at any given age. Indeed, there is nothing to suggest that individuals cannot have both reversible and fixed airflow obstruction, and hence asthma and COPD at the same time. The importance of early life and in-utero events for the subsequent development of disease is a theme that is common in a variety of complex traits. These same issues also present themselves with disorders of the airways. The major barrier to applying this life-cycle approach to disease risk factors and natural history relates to the problem of recall bias and potentially missing or inadequate information about past events in both childhood and adult life, which may then distort the clinical picture. It is only through careful analysis of longitudinal cohort data that the true history of the relationship between the major environmental exposures and disease natural history can be deduced. We need to continue to gather such data, particularly data to link childhood asthma with adult COPD.

REFERENCES 1. Toelle BG et al. Toward a definition of asthma for epidemiology. Am. Rev. Respir. Dis. 1992; 146:633–7. 2. Pekkanen J, Pearce N. Defining asthma in epidemiological studies. Eur. Respir. J. 1999; 14:951–7. 3. Boushey HA et al. Bronchial hyperreactivity. Am. Rev. Respir. Dis. 1980; 121:389–413. 4. Asher MI et al. International Study of Asthma and Allergies in Childhood (ISAAC): rationale and methods. Eur. Respir. J. 1995; 8:483–91. 5. Viegi G et al. Prevalence of airways obstruction in a general population: European Respiratory Society vs American Thoracic Society definition. Chest 2000; 117(5 Suppl. 2):339S–45S. 6. Yunginger JW et al. A community-based study of the epidemiology of asthma: incidence rates, 1964–83. Am. Rev. Respir. Dis. 1992; 146:888–94. 7. McWhorter WP, Polis MA, Kaslow RA. Occurrence, predictors, and consequences of adult asthma in NHANESI and follow-up survey. Am. Rev. Respir. Dis. 1989; 139:721–4. 8. Broder I et al. Epidemiology of asthma and allergic rhinitis in a total community,Tecumseh, Michigan. III: Second survey of the community. J. Allergy Clin. Immunol. 1974; 53:127–38. 9. Broder I et al. Epidemiology of asthma and allergic rhinitis in a total community, Tecumseh, Michigan. IV: Natural history. J. Allergy Clin. Immunol. 1974; 54:100–10. 10. Dodge RR, Burrows B. The prevalence and incidence of asthma and asthma-like symptoms in a general population sample. Am. Rev. Respir. Dis. 1980; 122:567–75. 11. Martinez FD et al. Asthma and wheezing in the first six years of life. The Group Health Medical Associates. N. Engl. J. Med. 1995; 332:133–8. 12. Toren K, Hermansson BA. Incidence rate of adult-onset asthma in relation to age, sex, atopy and smoking: a Swedish populationbased study of 15,813 adults. Int. J. Tuberc. Lung Dis. 1999; 3(3):192–7.

13. Huovinen E et al. Incidence and prevalence of asthma among adult Finnish men and women of the Finnish Twin Cohort from 1975 to 1990, and their relation to hay fever and chronic bronchitis. Chest 1999; 115:928–36. 14. Mannino DM et al. Obstructive lung disease and low lung function in adults in the United States: data from the National Health and Nutrition Examination Survey, 1988–94. Arch. Intern. Med. 2000; 160:1683–9. 15. Mannino DM et al. Surveillance for asthma–United States, 1960–95. MM Weekly Report 1998; 47(1):1–27. 16. Crain EF et al. An estimate of the prevalence of asthma and wheezing among inner-city children. Pediatrics 1994; 94:356–62. 17. Persky VW et al. Relationships of race and socioeconomic status with prevalence, severity, and symptoms of asthma in Chicago school children. Ann. Allergy Asthma Immunol. 1998; 81: 266–71. 18. Sole D et al. Prevalence of asthma and related symptoms in schoolage children in Sao Paulo, Brazil. International Study of Asthma and Allergies in Children (ISAAC). J. Asthma 1999; 36:205–12. 19. Asher MI, Weiland SK. The International Study of Asthma and Allergies in Childhood (ISAAC). ISAAC Steering Committee. Clin. Exp. Allergy 1998; 28(Suppl. 5):52–66. Discussion 90–1. 20. Duhme H et al. Asthma and allergies among children in West and East Germany: a comparison between Munster and Greifswald using the ISAAC phase I protocol. Eur. Respir. J. 1998; 11:840–7. 21. Aguinaga Ontoso I et al. [The prevalence of asthma-related symptoms in 13–14-year-old children from 9 Spanish populations. The Spanish Group of the ISAAC Study]. Med. Clin. Barcelona 1999; 112(5):171–5. Erratum Med. Clin. Barc. 1999; 112(13):494. 22. Stang P et al. The prevalence of COPD: using smoking rates to estimate disease frequency in the general population. Chest 2000; 117(5 Suppl. 2):354S–9S. 23. Lopez AD, Murray CC. The global burden of disease, 1990–2020 [news]. Nat. Med. 1998; 4:1241–3. 24. Lacasse Y, Brooks D, Goldstein RS. Trends in the epidemiology of COPD in Canada, 1980–95. COPD and Rehabilitation Committee of the Canadian Thoracic Society. Chest 1999; 116:306–13. 25. Enright PL et al. Prevalence and correlates of respiratory symptoms and disease in the elderly. Cardiovascular Health Study. Chest 1994; 106:827–34. 26. Sobradillo V et al. [Epidemiological study of chronic obstructive pulmonary disease in Spain (IBERPOC): prevalence of chronic respiratory symptoms and airflow limitation]. Arch. Bronconeumol. 1999; 35:159–66. 27. Hurd SS. International efforts directed at attacking the problem of COPD. Chest 2000; 117(5 Suppl. 2):336S–8S. 28. Fleming DM, Crombie DL. Prevalence of asthma and hay fever in England and Wales. Br. Med. J. 1987; 294:279–83. 29. Gerstman BB et al. Prevalence and treatment of asthma in the Michigan Medicaid patient population younger than 45 years, 1980–86. J. Allergy Clin. Immunol. 1989; 83:1032–9. 30. Manfreda J et al. Trends in physician-diagnosed asthma prevalence in Manitoba between 1980 and 1990. Chest 1993; 103:151–7. 31. Mitchell EA. International trends in hospital admission rates for asthma. Arch. Dis. Child. 1985; 60:376–8. 32. Anderson HR, Bailey P,West S. Trends in the hospital care of acute childhood asthma 1970–78: a regional study. Br. Med. J. 1980; 281:1191–4. 33. Jackson RT, Mitchell EA. Trends in hospital admission rates and drug treatment of asthma in New Zealand. NZ Med. J. 1983; 96:728–30. 34. Halfon N, Newacheck PW. Trends in the hospitalization for acute childhood asthma, 1970–84. Am. J. Pub. Hlth 1986; 76:1308–11. 35. Priftis K et al.Time trends and seasonal variation in hospital admissions for childhood asthma in the Athens region of Greece: 1978–88. Thorax 1993; 48:1168–9.

Epidemiology

36. Carman PG, Landau LI. Increased paediatric admissions with asthma in Western Australia: a problem of diagnosis? Med. J. Aust. 1990; 152:23–6. 37. Kun HY, Oates RK, Mellis CM. Hospital admissions and attendances for asthma: a true increase? Med. J. Aust. 1993; 159:312–3. 38. Wilkins K, Mao Y. Trends in rates of admission to hospital and death from asthma among children and young adults in Canada during the 1980s. CMAJ 1993; 148:185–90. 39. Jackson R et al. International trends in asthma mortality: 1970–85. Chest 1988; 94:914–18. 40. Carr W, Zeitel L, Weiss K. Variations in asthma hospitalizations and deaths in New York City. Am. J. Pub. Hlth 1992; 82:59–65. 41. McConnochie KM et al. Socioeconomic variation in asthma hospitalization: excess utilization or greater need? Pediatrics 1999; 103(6):e75. 42. Thom TJ. International comparisons in COPD mortality. Am. Rev. Respir. Dis. 1989; 140(3 Pt 2):S27–34. 43. Burrows B et al. Quantitative relationships between cigarette smoking and ventilatory function. Am. Rev. Respir. Dis. 1977; 115:195–205. 44. Cunningham J, Dockery DW, Speizer FE. Maternal smoking during pregnancy as a predictor of lung function in children. Am. J. Epidemiol. 1994; 139:1139–52. 45. Tager IB et al. Effect of cigarette smoking on the pulmonary function of children and adolescents. Am. Rev. Respir. Dis. 1985; 131:752–9. 46. Xu X et al. Smoking, changes in smoking habits, and rate of decline in FEV1: new insight into gender differences. Eur. Respir. J. 1994; 7:1056–61. 47. Strachan DP, Butland BK, Anderson HR. Incidence and prognosis of asthma and wheezing illness from early childhood to age 33 in a national British cohort. Br. Med. J. 1996; 312: 1195–9. 48. Kaplan BA, Mascie-Taylor CG. Smoking and asthma among 23year-olds. J. Asthma 1997; 34:219–26. 49. Halken S et al. Passive smoking as a risk factor for development of obstructive respiratory disease and allergic sensitization. Allergy 1995; 50:97–105. 50. Wartenberg D, Ehrlich R, Lilienfeld D. Environmental tobacco smoke and childhood asthma: comparing exposure metrics using probability plots. Environ. Res. 1994; 64:122–35. 51. Cook DG, Strachan DP. Health effects of passive smoking. 3: Parental smoking and prevalence of respiratory symptoms and asthma in school age children. Thorax 1997; 52:1081–94. 52. Weiss ST, Utell MJ, Samet JM. Environmental tobacco smoke exposure and asthma in adults. Environ. Hlth Perspect. 1999; 107(Suppl. 6):891–5. 53. Weil CM et al. The relationship between psychosocial factors and asthma morbidity in inner-city children with asthma. Pediatrics 1999; 104:1274–80. 54. Infante-Rivard C. Childhood asthma and indoor environmental risk factors. Am. J. Epidemiol. 1993; 137:834–44. 55. Infante-Rivard C et al. Maternal smoking and childhood asthma. Am. J. Epidemiol. 1999; 150:528–31. 56. Anthonisen NR et al. Effects of smoking intervention and the use of an inhaled anticholinergic bronchodilator on the rate of decline of FEV1. The Lung Health Study. JAMA 1994; 272:1497–505. 57. Whittemore AS, Perlin SA, DiCiccio Y. Chronic obstructive pulmonary disease in lifelong nonsmokers: results from NHANES. Am. J. Pub. Hlth 1995; 85:702–6. 58. Taylor RG et al. Bronchial reactivity to inhaled histamine and annual rate of decline in FEV1 in male smokers and exsmokers. Thorax 1985; 40:9–16. 59. Frew AJ, Kennedy SM, Chan-Yeung M. Methacholine responsiveness, smoking, and atopy as risk factors for accelerated FEV1 decline in male working populations. Am. Rev. Respir. Dis. 1992; 146:878–83.

17

60. Gottlieb DJ et al. Skin test reactivity to common aeroallergens and decline of lung function. The Normative Aging Study. Am. J. Respir. Crit. Care Med. 1996; 153:561–6. 61. Nelson HS. The importance of allergens in the development of asthma and the persistence of symptoms. J. Allergy Clin. Immunol. 2000; 105(6 Part 2):S628–32. 62. Burrows B et al. Association of asthma with serum IgE levels and skin-test reactivity to allergens. N. Engl. J. Med. 1989; 320:271–7. 63. Sears MR et al. Relation between airway responsiveness and serum IgE in children with asthma and in apparently normal children. N. Engl. J. Med. 1991; 325:1067–71. 64. Weiss ST. Atopy as a risk factor for chronic obstructive pulmonary disease: epidemiological evidence. Am. J. Respir. Crit. Care Med. 2000; 162(3 Part 2):S134–6. 65. Hargreave FE, Leigh R. Induced sputum, eosinophilic bronchitis, and chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1999; 160(5 Part 2):S53–7. 66. Rijcken B et al. The association of airways responsiveness to respiratory symptom prevalence and to pulmonary function in a random population sample. Bull. Eur. Physiopathol. Respir. 1987; 23:391–4. 67. Hospers JJ et al. Histamine airway hyper-responsiveness and mortality from chronic obstructive pulmonary disease: a cohort study. Lancet 2000; 356:1313–17. 68. Xu X et al. Airways responsiveness and development and remission of chronic respiratory symptoms in adults. Lancet 1997; 350:1431–4. 69. Carey VJ et al. Airways responsiveness, wheeze onset, and recurrent asthma episodes in young adolescents. The East Boston Childhood Respiratory Disease Cohort. Am. J. Respir. Crit. Care Med. 1996; 153:356–61. 70. Weiss ST et al. Effects of asthma on pulmonary function in children: a longitudinal population-based study. Am. Rev. Respir. Dis. 1992; 145:58–64. 71. O’Connor GT, Sparrow D, Weiss ST. The role of allergy and nonspecific airway hyperresponsiveness in the pathogenesis of chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 1989; 140:225–52. 72. Sluiter HJ et al. The Dutch hypothesis (chronic non-specific lung disease) revisited. Eur. Respir. J. 1991; 4:479–89. 73. Rijcken B et al. Airway hyperresponsiveness to histamine associated with accelerated decline in FEV1. Am. J. Respir. Crit. Care Med. 1995; 151:1377–82. 74. Tashkin DP et al. Methacholine reactivity predicts changes in lung function over time in smokers with early chronic obstructive pulmonary disease. The Lung Health Study Research Group. Am. J. Respir. Crit. Care Med. 1996; 153(6 Part 1):1802–11. 75. Venn A et al. Questionnaire study of effect of sex and age on the prevalence of wheeze and asthma in adolescence. Br. Med. J. 1998; 316:1945–6. 76. de Marco R et al. Differences in incidence of reported asthma related to age in men and women: a retrospective analysis of the data of the European Respiratory Health Survey. Am. J. Respir. Crit. Care Med. 2000; 162:68–74. 77. Skobeloff EM et al. The influence of age and sex on asthma admissions. JAMA 1992; 268:3437–40. 78. Singh AK et al. Sex differences among adults presenting to the emergency department with acute asthma. Multicenter Asthma Research Collaboration Investigators. Arch. Intern. Med. 1999; 159:1237–43. 79. Menkes HA et al. Risk factors, pulmonary function, and mortality. Prog. Clin. Biol. Res. 1984; 147:501–21. 80. Prescott E et al. Gender difference in smoking effects on lung function and risk of hospitalization for COPD: results from a Danish longitudinal population study. Eur. Respir. J. 1997; 10:822–7. 81. Mannino DM, Brown C, Giovino GA. Obstructive lung disease deaths in the United States from 1979 through 1993: an analysis using multiple-cause mortality data. Am. J. Respir. Crit. Care Med. 1997; 156(3 Part 1):814–18.

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82. Taylor WR, Newacheck PW. Impact of childhood asthma on health. Pediatrics 1992; 90:657–62. 83. Coultas DB et al. Respiratory diseases in minorities of the United States. Am. J. Respir. Crit. Care Med. 1994; 149(3 Part 2):S93–131. Erratum AJRCCM 1994; 150:290. 84. Evans RD et al. National trends in the morbidity and mortality of asthma in the US: prevalence, hospitalization and death from asthma over two decades: 1965–84. Chest 1987; 91(6 Suppl.):65S–74S. 85. Clark NM et al. The impact of health education on frequency and cost of health care use by low income children with asthma. J. Allergy Clin. Immunol. 1986; 78(1 Part 1):108–15. 86. Nelson DA et al. Ethnic differences in the prevalence of asthma in middle class children. Ann. Allergy Asthma Immunol. 1997; 78(1):21–6. 87. Joseph CL et al. Prevalence of possible undiagnosed asthma and associated morbidity among urban schoolchildren. J. Pediatr. 1996; 129:735–42. 88. Weitzman M et al. Recent trends in the prevalence and severity of childhood asthma. JAMA 1992; 268:2673–7. 89. Thomas SD, Whitman S. Asthma hospitalizations and mortality in Chicago: an epidemiologic overview. Chest 1999; 116(4 Suppl. 1):135S–41S. 90. Duran-Tauleria E, Rona RJ. Geographical and socioeconomic variation in the prevalence of asthma symptoms in English and Scottish children. Thorax 1999; 54:476–81. 91. Mielck A, Reitmeir P, Wjst M. Severity of childhood asthma by socioeconomic status. Int. J. Epidemiol. 1996; 25:388–93.

92. Strachan DP et al. A national survey of asthma prevalence, severity, and treatment in Great Britain. Arch. Dis. Child. 1994; 70:174–8. 93. Bakke PS, Hanoa R, Gulsvik A. Educational level and obstructive lung disease given smoking habits and occupational airborne exposure: a Norwegian community study. Am. J. Epidemiol. 1995; 141:1080–8. 94. von Mutius E, Nicolai T, Martinez FD. Prematurity as a risk factor for asthma in preadolescent children. J.Pediatr. 1993; 123 :223–9. 95. Vergnenegre A et al. Comparison between late onset and childhood asthma. Allergol. Immunopathol. Madrid 1992; 20(5):190–6. 96. Jonsson JA, Boe J, Berlin E.The long-term prognosis of childhood asthma in a predominantly rural Swedish county. Acta Paediatr. Scand. 1987; 76:950–4. 97. Kelly WJ et al. Childhood asthma in adult life: a further study at 28 years of age. Br. Med. J. 1987; 294:1059–62. 98. Burrows B et al. Characteristics of asthma among elderly adults in a sample of the general population. Chest 1991; 100:935–42. 99. Panhuysen CI et al. Adult patients may outgrow their asthma: a 25-year follow-up study. Am. J. Respir. Crit. Care Med. 1997; 155:1267–72. Erratum AJRCCM 1997; 156(2 Part 1):674. 100. Ronmark E, Jonsson E, Lundback B. Remission of asthma in the middle aged and elderly: report from the Obstructive Lung Disease in Northern Sweden Study. Thorax 1999; 54:611–13. 101. Wise RA. Changing smoking patterns and mortality from chronic obstructive pulmonary disease. Prev. Med. 1997; 26:418–21. 102. Peat Jk, Li J. Reversing the trend: reducing the prevalence of asthma. J. Allergy Clin. Immunol. 1999; 103(1 Part 1):1–10.

Natural History

Chapter

3

Fernando D. Martinez Arizona Respiratory Center, University of Arizona, Tucson, AZ, USA

INTRODUCTION

METHODOLOGICAL APPROACH

There is increasing interest in developing a better understanding of the natural history of the two most frequent chronic lower respiratory diseases, asthma and chronic obstructive pulmonary disease (COPD). The main reason for this interest is the shift in the therapeutic approach to both conditions. In the case of asthma, a growing body of evidence suggests that, although several current treatments for the disease are extremely effective in controlling symptoms, none can change its natural course. It is thus evident that new approaches to the primary and secondary prevention of asthma are needed. These approaches will require a thorough understanding of the factors that determine the inception of asthma and its progression with age. In the case of COPD, it is apparent that, although smoking is the main demonstrated cause of this condition, a simple strategy of discouraging tobacco consumption has proven insufficient to prevent the enormous impact of nicotine addiction on public health. A new understanding of the factors that increase the risk of COPD in smokers seems a more realistic approach, and could perhaps contribute to palliate the increasing social toll of COPD. When discussing the natural history of asthma and COPD, an important issue is the overlap between these two conditions. After the early adult years, subjects with preexisting asthma who smoke are at increased risk of significant declines in lung function and may develop chronic airflow obstruction.1 It becomes thus a semantic issue if these subjects have asthma or COPD, because factors associated with both conditions have most likely contributed to their course. It is often difficult to determine whether smokers who develop asthma-like symptoms and who show significant declines in lung function have asthma or COPD. This issue cannot be resolved at this time, but recent studies indicating that inhaled corticosteroids cannot change the natural course of either asthma2 or COPD3 suggest that an absolute separation between these two conditions may be more semantic than practical with our current level of knowledge.

Any discussion about the natural history of a chronic condition ought to address the different hypothetical phases that such condition undergoes during its lifetime course (Fig. 3.1). There is first a pre-illness period during which subjects without overt disease have the susceptibility for the development of the condition, because of a genetic predisposition or because of injuries or developmental variations that have mutated one or more of the individual’s disease-associated phenotypes. During this pre-illness phase, the susceptible individual is not destined to develop the disease. More likely, he or she may be exposed to environmental factors that, in the presence of genetic variants that predispose to the chronic condition, can further modify their phenotype. We do not know whether genetic factors will be discovered that will be present in all affected individuals, a kind of major gene or universal “switch” that is necessary for the development of these conditions. More likely, both asthma and COPD are heterogeneous diseases, with multiple ways to final common pathways for an array of diverse pathogenetic mechanisms that give rise to more or less reversible airway obstruction. It would obviously be of great help for any prevention strategy if markers of susceptibility were identified that Inception ion

s es

r og

Chronic disease

Pr

No disease

Initial phase

Protection

Re

mi

ss

ion

Intermittent disease

No disease

Triggers Fig. 3.1. Hypothetical representation of the natural history of a chronic condition. For explanation please see text.

Asthma and Chronic Obstructive Pulmonary Disease

could be measured during this pre-illness phase. But the absence of a marker in a specific individual may not exclude susceptibility if that is not the pathway that may be active in that individual. Thus, because the marker phenotype may be influenced by determinants other than the factors that influence asthma or COPD risk, ascertainment of a marker may be fraught with difficulties. This “context-dependency”4 of the phenotypes of markers of disease creates a further layer of complexity for the identification of susceptible individuals. The above discussion points to the importance of determining the “true” time of incidence of the disease under study. Although the event that is easiest to ascertain is the age of initiation of symptoms, this event is modified by patients’ perceptions.What is important is that time of onset ought to be ascertained before the disease has become chronic; i.e. before the individual has evidence of physiological or anatomical changes caused by the disease itself and that predispose to persistent or recurrent symptoms. During this initial phase of the disease, the affected individual has some clinical manifestations, but the condition has not yet fully developed. Moreover, some individuals may not go beyond this initial phase, because remission or stabilization may be fostered by the presence of environmental or genetic protective factors. During this initial phase secondary prevention is possible, but again knowledge of the natural history becomes essential.

N AT U R A L H I S T O RY O F A S T H M A PHENOTYPES Asthma is a heterogeneous condition, and one of the main obstacles in understanding its natural history has been the lack of well-defined markers for the different disease phenotypes grouped under this common label. This hurdle has been addressed lately by a series of longitudinal studies that have assessed incidence and prevalence of asthma at different ages or defined the outcome of persons with asthma-like symptoms enrolled at different ages, and especially during childhood.5–10 Most cases of chronic, persistent asthma start in early life Several longitudinal studies have confirmed that, in most cases of chronic, persistent asthma, the initial clinical manifestations of asthma usually occur during the first five years of life.6,9,10 Studies have ascertained this age of onset of symptoms either by asking parents about the prevalence of wheezing or other asthma-like symptoms in their children at different ages, or by reviewing previous clinical charts in subjects who present to outpatient clinics with signs and symptoms of asthma at any age. Although these studies strongly suggest that asthma started at an early age in these individuals, a simple connection between these two phenomena is not warranted. Many children have asthma-like symptoms during viral infections in their preschool years, but most have transient conditions that subside with age, and only a minority will go on to have persistent asthma.11

Thus it is difficult to distinguish who will go on to develop chronic asthma from those who will not. Studies of this issue have suggested that there are at least three quite distinct groups of children with asthma-like symptoms coexisting up to the adolescent years (Fig. 3.2). The great majority of infants who wheeze during the first 1–2 years of life do so during viral infections, especially those caused by the respiratory syncytial virus (RSV). Most of these children will have one or only a few episodes of wheezing, with no further symptoms beyond the age of 2–3 years. This condition, which has been identified as transient wheezing of infancy,12 is the most frequent form of recurrent airway obstruction in this age group, affecting over twothirds of all infants with asthma-like symptoms. The main predisposing factors for transient wheezing of infancy are maternal smoking during pregnancy and lower levels of lung function, as assessed during the very first months of life and before any wheezing episode has occurred. Interestingly, infants (especially females) whose mothers smoke during pregnancy do have lower levels of lung function early in life that those whose mothers do not smoke.13 This has suggested the possibility that both inherited and acquired characteristics of the lung, the airways, or both may create the structural conditions for the development of airway obstruction during lower respiratory illnesses.14 The factors that determine the remission of early-life wheezing in these children are not well understood, but it is likely that growth of the airways may outpace that of the lung parenchyma after age 2–5 months, the period of the highest incidence of wheezing episodes.15 Thus, the critical size that the airways have to attain in order for airflow to be obstructed may not be as easily reached, thus making noisy breathing less likely.15 It is also possible that changes in the 40

Alternaria negative asthma Alternaria positive asthma Percentage of asthma group

20

30

20

10

0

0 0–1 1–2 2–3 3–4 4–5 5–6 6–7 Age at which asthma diagnosed (years)

Fig. 3.2. Age of diagnosis of asthma for children who are skin-test positive for Alternaria or skin-test negative for Alternaria at age 6 in the Tucson Children’s Respiratory Study. Adapted from reference 20.

21

Natural History

Transient early wheezers

IgE-associated wheeze/asthma

Nonatopic wheezers

Wheezing prevalence

regulation of airway tone may result in decreased bronchial hyperresponsiveness and decreased likelihood of airway obstruction with age.16,17 Young et al.18 analyzed data from a longitudinal study of over 250 children who were also enrolled at birth in Perth, Australia.They found that children who wheezed during the first two years of life had impaired lung function shortly after birth and before any episode of wheezing had occurred. However, they also subdivided their population into those children who wheezed only during the first year of life and those who were still wheezing during the second year or who started wheezing after age one. They found that abnormalities in lung function were present in both groups of children during the first month of life, but these abnormalities had resolved by age 12 months in those whose symptoms remitted by that age, whereas abnormalities persisted in those who were still wheezing during the second year of life.These data suggest that, among children who wheeze during the first year of life, unresolved airway abnormalities are associated with persistence of symptoms beyond that age. The factors that determine enduring or newly developed abnormalities in lung function during the first years of life are the subject of intense scrutiny, because they seem to be clearly associated with the risk of persistent asthma. Zeiger et al.19 showed an inverse relationship between duration of symptoms and level of lung function in school-age children with asthma, and this suggests that the earlier onset of asthma symptoms is associated with greater losses in lung function. Sensitization to local aeroallergens is strongly associated with increased risk of chronic asthma-like symptoms into adult life. Using this knowledge, Halonen et al.20 divided children who had a diagnosis of asthma by age 6 years into two main groups: those who at age six were skintest positive for Alternaria, the main aeroallergen associated with asthma in the study area,21 and those who were not. They observed that, in the majority of children in both groups, symptoms had started before age 3 years. However, among children with asthma who were skin-test negative to Alternaria, inception of the disease had occurred mainly during the first year of life; while in those who were skin-test positive to Alternaria, peak incidence occurred during the second and third years of life (Fig. 3.3). The fact that young children who will go on to develop chronic, atopy-related asthma by age six are diagnosed one or two years later than those with nonatopic asthma suggests that the mechanisms of disease are likely to be different in these two groups. Support for this contention comes from a slightly different analysis of the data from the same cohort on which Halonen and coworkers based their report quoted above. Stein et al.11 assessed the outcome of children with confirmed lower respiratory tract illnesses due to RSV (RSV-LRI), after adjusting for all other known risk factors for subsequent asthma, including skin test reactivity, maternal history of asthma and birthweight, among others. They found that, as previously reported, risk of wheezing during the school years was higher in children with a history of RSV-LRI than in those with no such history. However, the

0

3

6

11

Age (years) Fig. 3.3. Hypothetical yearly peak prevalence of wheezing for three different phenotypes in childhood. Prevalence for each age interval should be the sum of the areas under each curve. The dashed curves suggest wheezing can present different curve shapes due to many different factors, including overlap of groups. Adapted from reference 56.

risk decreased with age and was not statistically significant by early adolescence. There was also no association between RSV-LRI and subsequent risk of sensitization to local aeroallergens either at age 6 or at age 11 years.The only factor that was strongly associated with RSV-LRI in early life was diminished baseline levels of FEV1, as measured at age 11. Interestingly, these deficits were reversed by use of a bronchodilator, suggesting that they were likely to be due to increased bronchomotor tone. Longitudinal studies performed in the early 1980s had also suggested that the outcome of RSV-LRIs is usually benign and unrelated to increased allergic sensitization.22 It thus appears that at least three different forms of “asthma” coexist during infancy and early childhood (Fig. 3.2). Transient infant wheezing is quite frequent, is usually triggered by viruses (especially RSV), and is confined to the first three years of life. Other children who wheeze during viral infections in early life continue to have recurrent airway obstruction during the early school years. This form of nonatopic wheezing is associated with lower levels of lung function during the school years, but these lower levels seem to be reversible after use of a bronchodilator. Finally, children who will develop atopy-related asthma start having symptoms mainly during the second and third years of life. Chronic asthma is most often related to atopy There is now strong evidence indicating that, as a group, individuals with chronic asthma at any age between the school years and mid adult life are either sensitized to local aeroallergens, have elevated total levels of circulating IgE, or both.23 Although the association between asthma and total and specific IgE is well established, the nature of the association is not well understood. For years it was thought that sensitization to specific allergens, especially in early life, was a cause of asthma,24 and that development of specific IgE

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Asthma and Chronic Obstructive Pulmonary Disease

against these allergens was the first step in the natural history of the disease. The strong association between risk of having asthma and sensitization to the allergens of house dust mites in coastal regions seemed to argue in favor of this hypothesis, and strategies for the primary prevention of asthma based on avoidance of exposure to these allergens were proposed.25 However, several studies performed in desert areas with low exposure to house dust mites showed that the prevalence of asthma was either similar or even higher in these regions than that observed in zones where mite infestation rates were high.21,26 In desert regions, the allergens of the mold Alternaria appeared to be strongly associated with asthma. Moreover, recent studies in northern Sweden, where indoor exposure to either dust mites or molds is very low, showed that the prevalence of school-age asthma is very similar to that observed in southern Sweden, where exposure to house dust mites is high.27 Interestingly, only 50% of all schoolchildren with asthma are sensitive to known aeroallergens in northern Sweden (especially cat and dog), compared with over 90% of children in more temperate regions; but skin-test negative children with asthma have significantly higher IgE levels than nonasthmatic children.27 The association between specific sensitization to allergens and asthma also changes with time. Although most schoolchildren with asthma are sensitive to local aeroallergens, by the late adult years only a fraction of individuals with asthma are skin-test positive.28 This is in part due to the incidence of nonatopic asthma in late adult life, but is also due to the decreasing prevalence of skin-test reactivity to allergens in individuals who were sensitive to these allergens at a younger age.29 However, there seems to be little correlation between changes in skin test reactivity to aeroallergens and remission of asthma with age.30 All these studies suggest that the essential causal mechanism associated with asthma is not sensitization to any specific allergen, but more likely is an alteration in the regulation of responses to many different antigens with the potential of eliciting IgE responses, and especially to aeroallergens. Outcome of childhood asthma in adult life: remission of mild asthma and persistence of severe disease Only a few studies have addressed the outcome of childhood asthma during the adult years. These studies have used longitudinal assessment of asthma symptoms and lung function in children enrolled during the early school years or of birth cohorts for which assessment of asthma symptoms in early life was made through retrospective questionnaires. The longest ongoing study is that initiated by Williams and McNicol in Melbourne, Australia.31 Children with a history of recurrent episodes of wheezing were enrolled at the age of 7 years, together with a small group of controls without such a history. These individuals were then periodically reassessed, and the latest published data refer to information collected when their mean age was 35 years. Information about the age upon initiation of symptoms was obtained by use of questionnaires and, as a consequence, data on a crucial period of life for asthma the development

of asthma was almost certainly biased by preferential recall.32 The second study was the British 1958 birth cohort, in which over 18,000 subjects born between March 3 and March 9, 1958 were enrolled.5 Of these persons, 31% contributed information for ages 7, 11, 16, 23, and 33 years. As with the Melbourne study, information for the first seven years of life was obtained retrospectively at age 7. Kjellman and coworkers30 followed up, to age 30 years, 55 subjects with asthma aged 5–14 years and who were referred to a tertiary care unit in Sweden.These subjects had quite severe asthma: over 40% were hospitalized for the disease before age 16. Finally, the Tasmanian asthma survey enrolled over 8000 children born in 1961 who were also first contacted at age 7.8 Two thousand randomly chosen individuals were reexamined when they were 29–32 years of age. The most important findings of all four studies were that asthma remits in early adulthood in a large proportion of asthmatic children, and that the severity of asthma tracks significantly with age. This second point is important because it reflects a “stability” in the disease: most children with severe symptoms still have severe symptoms as adults, and asthmatic children with mild symptoms either have no asthma or have mild asthma as adults. • Children enrolled in the Melbourne study were divided at the time of enrollment into five groups according to their previous history of wheezing: a control group (no wheezing), a group with mild wheezy bronchitis (less than five lifetime episodes of wheezing associated with colds), a group with wheezy bronchitis (five or more such episodes), a group with asthma (wheezing apart from colds), and a group with severe asthma (selected at age 10 based on severe impairment of lung function). At age 35 years, only a quarter of children with both forms of wheezy bronchitis showed frequent asthma episodes (wheezing during the previous 3 months, but less than once a week) or persistent asthma (once a week or more). However, 50% of children with asthma and 75% of those with severe asthma at the time of enrollment had frequent or persistent wheeze at age 35.32 • Similar results have been reported based on the British 1958 study: 27% of all children whose parents reported they had wheezed before age 7 reported wheezing during the previous year at age 33.5 • In the Swedish study, severity of symptoms decreased significantly with age, especially for males, but only 16% were free of asthma symptoms at the time of the last visit (age 26–35 years). As stated earlier, children included in this follow-up had quite severe disease, but unfortunately no results of lung function testing were reported and severity cannot be compared with that of other studies. • For the Tasmanian study, 25.6% of subjects with “asthma or wheezy breathing” by age 7 reported current asthma at the age of 29–32 years, compared with only 10.8% of subjects without parental reports of childhood asthma. As with the Melbourne study, those with a history of more than ten attacks of asthma by age 7 were almost

23

Natural History

A consistent factor associated with persistent asthma in all four cohorts was evidence of an allergic predisposition. In the Melbourne study, severe asthma in early life was associated with significantly higher prevalence of allergic rhinitis at age 35. In the British cohort, allergic symptoms (e.g. allergic rhinitis or eczema) were significantly associated with persistence of symptoms into adult life. In the Swedish cohort, the proportion of subjects sensitized to three perennial allergens (cat, dog, horse) was much higher in subjects with persistent severe asthma than in those with mild disease. Finally, in the Tasmanian cohort, having a history of eczema in early life was also significantly associated with persistent wheezing at ages 29–32. Both the Tasmanian and the Melbourne studies assessed the association between persistence of asthma into adult life and lung function measured at enrollment (age 7 years). For both studies, low lung function at age 7 was a significant predictor of subsequent persistent wheezing. In the Melbourne study, lung function for each subgroup of subjects classified according to their wheezing history by age 7 was repeatedly assessed up to the age of 35 years. Children with severe asthma at age 10 had very low initial levels of lung function, and this was to be expected because deficits in lung function were used to classify them as having severe asthma in the first place. When compared with their peers, these children did not show further deficits in lung function growth with age, and by age 35 their position relative to subjects with milder symptoms or with no asthma was substantially unchanged. A similar pattern of relatively stable lung function growth was observed for patients with mild asthma who had shown loss of lung function at enrollment, albeit much less pronounced than persons with severe asthma. This finding is very relevant for our understanding of the natural history of chronic obstructive pulmonary disease, and will be discussed in more detail below. The potential role of bronchial hyperresponsiveness in early life has been explored only in a small study in the Netherlands.33 Children with asthma originally assessed at ages 6–14 years were subsequently restudied in early adult life. The proportion of young adults with a history of childhood asthma who were still wheezing was higher (43%) than that observed in the three cohorts described earlier. However, children included in this study were older than those enrolled in those studies, and the great majority were atopic. In this group of individuals, the two most important predictors of persistent wheezing into adult life were the initial level of lung function (as in both Australian cohorts) and the degree of bronchial hyperresponsiveness to histamine in early life. Relapse of asthma symptoms in patients whose asthma remitted in childhood Very little is known about the factors associated with relapses of asthma in adult life among subjects whose symp-

toms remitted during childhood. Only the British cohort study explored those factors. They identified a group of over 1300 persons with a history of wheezing illnesses from birth to age 16 years whose symptoms had remitted by age 23. This group was 50% more likely to report wheezing at age 33 than those with no history of asthma or wheezy bronchitis during childhood (P < 0.001). Interestingly, the association between early childhood asthma and relapses of asthma in adult life was independent of the two main determinants of the prognosis of childhood asthma, namely, allergies and smoking in adult life.5 The authors speculated that persistent, asymptomatic abnormalities in pulmonary function could explain their findings. The potential role of these abnormalities as determinants of COPD will be discussed below.

N AT U R A L H I S T O RY O F C O P D The natural history of COPD overlaps with that of the level of lung function (usually FEV1, expressed as percent of predicted value in relation to a certain height) during adulthood. Three are the main factors that can determine the level of lung function achieved at any age during adult life (Fig. 3.4). First, the individual may either start life with a low level of lung function or show a significant decline in lung function growth during the first years of life. The individual’s level of lung function will fall after a certain age, at the same rate as normal peers, but at a lower level overall. The level of lung function attained by late adolescence or early adult life will thus be lower. Through the normal process of aging, decline in lung function will naturally occur; and this decline, although parallel to that occurring in all the population, will nevertheless predispose to the attainment of a level of lung function (expressed as percentage of normal level in FEV1 (% normal level at age 20)

twice as likely to have persistent wheezing as adults than those who did not.

100 (c)

80

(a)

(d)

60 (b)

40 20 0 0

10

20

30 40 50 Age (years)

60

70

80

Fig. 3.4. Hypothetical mechanisms that may lead to a critically low level of lung function in adult life and to chronic airway obstruction (horizontal line): (a) normal growth and decline; (b) impaired lung growth with a lower plateau phase but a normal rate of decline compared to (a); (c) normal plateau with rapid initial decline in lung function and a subsequent normal rate of decline; (d) normal plateau with normal initial rate of decline but a subsequent accelerated loss in lung function. Adapted from reference 55.

24

Asthma and Chronic Obstructive Pulmonary Disease

early adulthood) that will be lower than the threshold for the expression of clinical symptoms of airway obstruction. A second potential mechanism is that of an early decline in lung function occurring either at the end of adolescence or in early adult life. Again, the main result of such a mechanism would be the attainment of lower level of lung function in these individuals as compared with the rest of the population. A third mechanism is that of a faster rate of decline of lung function during adult life. This may obviously occur both in individuals with a normal level of lung function at the beginning of adult life as in individuals who already have a diminished level of lung function to begin with. The essential determinants of the natural history of COPD are the rate of growth of lung function up to late childhood and the rate of decline of lung function during adult life. Growth of lung function during childhood The three most important determinants of growth of lung function during childhood are: • the level of lung function at birth; • the incidence of lower respiratory illnesses during the first years of life; and • the development of persistent asthma-like symptoms during childhood. Level of lung function at birth The availability of lung function tests that can be performed shortly after birth, and which appear to show good correlation with more invasive tests of airway function,34 has allowed us to study the potential role of the development of the lung in utero on subsequent levels of lung and airway function attained by the individual. Since these methods have been available only for the last 20 years, it is not possible to determine the role of airway function in the postnatal period on events occurring beyond the late teen years. Nevertheless, it is now apparent that there is quite significant tracking between the level of lung function measured shortly after birth and that measured during childhood.35 This may explain, at least in part, the strong association between lower respiratory illness in early life and level of lung function in late childhood and early adulthood (see the discussion below). The factors that determine growth of lung and airways in utero are only now beginning to be understood. The two main determinants of the level of lung function measured shortly after birth are gender and exposure to tobacco smoke products in utero.18 Boys have been found to have consistently lower maximal flows at functional residual capacity (VmaxFRC) throughout the first year of life in comparison with girls. Maternal smoking during pregnancy is associated with lower levels of VmaxFRC in both genders in comparison with unexposed infants.18 Postnatal exposure to tobacco smoke appears to be less important than prenatal exposure, at least in most developed countries.36 The effects of intrauterine exposure to tobacco smoke products persist

for the first year of life,18 and even beyond the first year up to the early adolescent years.37 The role of gender differences in airway function is not well understood. Females have larger airway size, relative to the size of lungs and to body size, as compared with males both at birth and during childhood, up to the plateau of lung growth.38 Although there are clear differences in the patterns of lung growth and development both in utero and during childhood between males and females, the role of these differences as determinants of risk for chronic obstructive pulmonary disease are not well understood. Role of lower respiratory illnesses in early life It has been known for decades that children who have lower respiratory illnesses during the first years of life have lower levels of lung function during childhood and into adult life.39 One possible explanation for this association is that viral infections, which are the main etiological factor for lower respiratory illnesses in early life, may damage lung and airways, and this may predispose for lower levels of lung function.40 This hypothesis seemed attractive, because it suggested a potential strategy for the prevention of early losses in lung function that could predispose to chronic obstructive pulmonary disease. However, several studies in which the techniques to assess lung function in infants, described earlier, were used showed that children who developed lower respiratory symptoms during viral infections in early life had diminished pre-illness levels of lung function.14,18,37 The hypothesis was thus suggested that lower levels of lung function observed after lower respiratory illnesses in early life could be explained by preexisting diminished lung function, the latter being therefore the link between early life episodes of airway obstruction and subsequent deficits in lung function. Unfortunately, the number of infants in whom lung function has been ascertained in early life and who have been followed for a number of years after birth is rather small. Therefore, the possibility that lower respiratory illnesses by themselves may alter lung and airway growth in groups of susceptible individuals cannot be excluded. It is unlikely that pre-illness lung function may explain all forms of infection-associated wheezing during early childhood, and it is legitimate to surmise that immune responses to the viruses themselves may also play a significant role. Bont and coworkers41 studied children who experienced recurrent wheezing episodes within one year of an original lower respiratory illness due to respiratory syncytial virus. In these children, IL-10 production by blood cells during the convalescent phase of the illness was significantly higher than IL-10 production by the same cells in children who had RSV but who did not have subsequent recurrent wheezing. Moreover, the IL-10 responses during the convalescent phase correlated significantly with the number of wheezing episodes occurring during the follow-up period. The mechanisms by which increased production of IL-10 by blood cells (most likely monocytes) leads to recurrent wheezing after RSV remain to be elucidated. Since IL-10 is

Natural History

a potent inhibitor of antigen presentation by macrophages,41 it is conceivable that IL-10 may suppress lower airway antiviral immune responses, thus enhancing the capacity of RSV and other viruses to invade local tissues and cause airway obstruction. On the other hand, IL-10 has been shown to increase airway responsiveness in animal models of allergen-induced airway obstruction.42 The mechanism by which IL-10 may exert this effect is not understood, nor is it known if similar mechanisms are present in humans. However, it is still possible that in human airways increased production of IL-10 in relation to a lower respiratory illness in early life may produce persistent alterations in the regulation of airway tone. Recent very long-term studies in England would support this contention.43 In these studies, men born between 1911 and 1930, whose birthweights, weights at one year, and childhood respiratory illnesses were recorded in early life, were studied at ages 59–70 years. Death from chronic obstructive disease, FEV1, and respiratory symptoms were the main outcome variables. The main early-life determinants of the level of FEV1 in old age were birthweight and history of bronchitis or pneumonia in infancy, and these effects were independent of smoking habit and social class. These data would thus suggest that both interuterine growth (and presumably lung development) and lower respiratory illnesses during the first year of life exert independent effects on the level of lung function attained late during adult life, and thus may be important determinants of the risk of COPD. Interestingly, a history of bronchitis or pneumonia in infancy was associated with increased risk of wheezing and persistent sputum production in adult life independently of birthweight, smoking habit, and social class. Persistent asthma-like symptoms during childhood It has been suggested that persistent asthma-like symptoms are a significant risk factor for the subsequent development of lower levels of lung function, and therefore, an important risk factor for the development of COPD. The hypothesis behind this contention has been that asthma is a progressive disease.44 It has thus been proposed that the presence of chronic airway inflammation is associated with significant lung remodeling and that the latter fosters a significant alteration in lung growth. Although some studies were able to show impairment in the development of lung function in children with asthma,44,45 the main outcome variable used in these studies was pre-bronchodilator lung function. More recently the Childhood Asthma Management Program (CAMP) study was specifically designed to test the hypothesis that mild to moderate childhood asthma is associated with significant deterioration in airway growth and that treatment with inhaled anti-inflammatory therapy could reverse this deterioration in lung function.2 Children aged 6–12 years were treated for 4–6 years with either an inhaled corticosteroid (budesonide), nedocromil, or a placebo. Postbronchodilator FEV1 was considered the outcome variable, because pre-bronchodilator FEV1 could be affected by the

25

degree of activity of the disease at the time of testing. The results of the study showed that, although children with asthma have levels of lung function that are significantly lower than those of children without the disease at any time between the ages of 6 and 15, these levels do not further deteriorate even among subjects who are not systematically treated with anti-inflammatory therapy. Moreover, systematic treatment with anti-inflammatory therapy improves bronchial hyperresponsiveness, but is not associated with a significant improvement in lung function with time. An interpretation of these results is that, in children with a diagnosis of asthma, most of the deterioration in lung function observed during the school years has already occurred by the age of 6–12 years. The results of studies by this author’s group suggest that children who go on to have persistent asthma-like systems during childhood start life with levels of lung function that are slightly but not significantly lower than those of children who will not have these persistent symptoms.46 It is thus likely that, although children with asthma may be predisposed to reaching early adult life with lower levels of lung function, these deficits in lung function growth occur mainly during the first years of life, and treatment with inhaled steroids after age 6 may not prevent the consequences of these deficits in lung function in adulthood. Determinants of early losses in lung function The factors that determine early losses in lung function at the age in which the plateau level of lung function is reached in early adult life have only recently been explored. Xuan et al.47 observed that, between the ages of 17 and 19 years, when growth in height had stopped, FEV1 continued to grow in both males and females. However, children who had recent episodes of wheeze and those with evidence of bronchial hyperresponsiveness showed a reduced rate of growth in airway caliber. Unfortunately, lung function was not assessed after use of a bronchodilator, and it is thus not possible from these data to assess whether the changes are due to increased airway tone or to irreversible alterations in airway structure. More recently, a preliminary report by Holberg et al.48 assessed the role of initiation of smoking during the adolescent years in determining deficits in lung function growth in late adolescence. These authors showed that, at age 16, the level of lung function achieved by males who had started smoking was significantly lower than that of those who had not. No such effect was observed in females. Determinants of increased slope of lung function decline Several longitudinal studies have addressed the role of different intrinsic and extrinsic factors on the rate of decline of lung function after the plateau phase.The role of cigarette smoking has been clearly and consistently established. Camilli et al.49 examined changes in FEV1 in over 1700 adults enrolled in a prospective study of a general population sample. Individuals who smoked more than ten cigarettes per day had excessive rates of decline in FEV1 as

26

Asthma and Chronic Obstructive Pulmonary Disease

compared with nonsmokers. The excess decline of smokers was age-dependent, particularly in men: much of the excess loss of lung function occurred between 50 and 70 years of age. In this study, effects of smoking on decline of FEV1 were greater in men than in women even when controlling for current cigarette dose. Interestingly, ex-smokers showed declines in FEV1 values that were similar to those of nonsmokers. The authors also examined the effect of quitting smoking on the decline of FEV1. In subjects younger than 35 years, quitting smoking during follow-up was associated with an actual increase in FEV1. In men above 50, smoking cessation early in the study led to a return to normal rate of functional decline during follow-up. Burrows and coworkers also reported that smokers who recalled a history of “respiratory trouble” before 16 years of age had significantly steeper rates of decline in lung function as compared with those with no such history.50 This effect was independent of a current or past diagnosis of asthma. It thus appears that intrinsic factors modify the effect of smoking on the rate of decline of lung function, and these factors may be related to events occurring during the first years of life. This conclusion is compatible with the observation by Barker and coworkers quoted earlier,43 that elderly individuals with a history of lower respiratory illness early in life were more likely to have lower levels of lung function than those with no such history. Although the factors that determine the link between “respiratory trouble” in early life and subsequent rate of decline in lung function during adult life are not well established, both genetic and environmental factors may play a role. In a preliminary analysis from the Tucson Longitudinal Study of Airway Obstructive Disease, the author’s group recently observed significant intra-family correlation in the rate of decline of lung function within smokers,51 suggesting that these rates of decline have an important genetic component. The observation by Stein et al.11 that subjects with a history of lower respiratory illnesses due to RSV have significant deficits in lung function growth by age 11 and that deficits are almost entirely reversible by use of a bronchodilator strongly suggest that alterations in the regulation of airway tone may be the link between respiratory illnesses in early life and subsequent risk of airway obstruction during adulthood. In support of this contention are the results of a longitudinal study by Postma and coworkers.52 These authors assessed the course of lung function after 2–21 years of follow-up in 81 nonallergic patients with considerable lung function impairment (<55% FEV1/FVC ratio) at the beginning of the study. They reported that a more favorable rate of change in FEV1 was not only associated with fewer pack-years of smoking but also with less nonspecific bronchial hyperreactivity and a higher degree of reversibility of airflow obstruction. These effects were independent of baseline FEV1 value, both in smokers and in ex-smokers. These results suggested the possibility that the rates of decline in lung function could be the consequence of an ongoing inflammatory process associated with bronchial

hyperresponsiveness in COPD, much like it was supposed to be present in asthma. It was thus suggested that treatment with inhaled corticosteroids could be associated with a lower rate of decline in lung function in subjects with COPD. The results of several long-term studies of the treatment of COPD with inhaled corticosteroids have been reported and recently reviewed.3 The results of these studies show that there seems to be an effect of inhaled steroids on lung function and symptoms during the first 3–6 months of use, but thereafter no further effects of inhaled corticosteroids on subsequent decline of lung function has been found. Although there could be subgroups of patients with COPD that may benefit from long-term use of inhaled corticosteroids, the data presently available do not support the contention that an ongoing inflammatory process is responsible for the decline in lung function observed in most patients with COPD. It thus appears that, although bronchial hyperresponsiveness and its associated alteration in the regulation of airway tone is an important risk factor for the development of COPD, the form of bronchial hyperresponsiveness associated with COPD may be different from that related to IgEmediated responses as observed in patients with asthma.53 A very recent observation by Palmer et al.54 may shed new light on this issue. These authors assessed bronchial responsiveness to histamine shortly after birth in a small group of normal newborns and then followed these subjects up to the age of 6 years. They found a significant correlation between bronchial responsiveness at age 1 month and FEV1 level at the age of 6 years, but no association between bronchial responsiveness at age 6 years and bronchial responsiveness at the age of 1 month. Both bronchial responsiveness at 1 month and at 6 years were independent determinants of the risk of wheezing at the age of 6 years. These studies thus suggest that a form of bronchial responsiveness present at birth may be an important risk factor for the development of respiratory symptoms in early life and an important determinant of the level of lung function at the beginning of the school years. Presumably, bronchial hyperresponsiveness at birth would also be a determinant of the level of lung function attained during the plateau phase, since measurements of lung function track markedly with age during childhood. This form of “intrinsic” bronchial responsiveness may differ in nature from that associated with atopic sensitization and may be less susceptible to the effects of anti-inflammatory therapy. The fact that it is present shortly after birth also suggests that genetic factors may explain a significant proportion of the link between lower respiratory illnesses in early life, level of lung function, and risk of COPD during adult life.

REFERENCES 1. Burrows B, Bloom JW, Traver GA et al. The course and prognosis of different forms of chronic airways obstruction in a sample from the general population. N. Engl. J. Med. 1987; 317:1309–14.

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2. The Childhood Asthma Management Program Research Group. Long-term effects of budesonide or nedocromil in children with asthma. N. Engl. J. Med. 2000; 343:1054–63. 3. Postma DS, Kerstjens HA. Are inhaled glucocorticosteroids effective in chronic obstructive pulmonary disease? Am. J. Respir. Crit. Care Med. 1999; 160(5 Pt 2):S66–71. 4. Martinez FD. Context dependency of markers of disease. Am. J. Respir. Crit. Care Med. 2000; 162(2 Pt 2):S56–7. 5. Strachan DP, Butland BK, Anderson HR. Incidence and prognosis of asthma and wheezing illness from early childhood to age 33 in a national British cohort. Br. Med. J. 1996; 312:1195–9. 6. Barbee RA, Dodge R, Lebowitz ML et al. The epidemiology of asthma. Chest 1985; 87(1 Suppl):21S–25S. 7. Oswald H, Phelan PD, Lanigan A et al. Childhood asthma and lung function in mid-adult life. Pediatr. Pulmonol. 1997; 23:14–20. 8. Jenkins MA, Hopper JL, Bowes G et al. Factors in childhood as predictors of asthma in adult life. Br. Med. J. 1994; 309:90–3. 9. Yunginger J, Reed CE, O’Connell EJ et al. A community-based study of the epidemiology of asthma. Incidence rates, 1964–1983. Am. Rev. Respir. Dis. 1992; 146:888–94. 10. Anderson HR, Pottier AC, Strachan DP. Asthma from birth to age 23: incidence and relation to prior and concurrent atopic disease. Thorax 1992; 47:537–42. 11. Stein RT, Sherrill D, Morgan WJ et al. Respiratory syncytial virus in early life and risk of wheeze and allergy by age 13 years. Lancet 1999; 353:541–5. 12. Martinez FD, Helms PJ.Types of asthma and wheezing. Eur. Respir. J. 1998; 27:3S–8S. 13. Tager IB, Ngo L, Hanrahan JP. Maternal smoking during pregnancy: effects on lung function during the first 18 months of life. Am. J. Respir. Crit. Care Med. 1995; 152:977–83. 14. Martinez FD, Morgan WJ, Wright AL et al. Diminished lung function as a predisposing factor for wheezing respiratory illness in infants. N. Engl. J. Med. 1988; 319:1112–17. 15. Martinez FD. Sudden infant death syndrome and small airway occlusion: facts and hypothesis. Pediatrics 1991; 87:190–8. 16. Montgomery GL, Tepper RS. Changes in airway reactivity with age in normal infants and young children. Am. Rev. Respir. Dis. 1990; 142(6 Pt 1):1372–6. 17. Burrows B, Sears MR, Flannery EM et al. Relation of the course of bronchial responsiveness from age 9 to age 15 to allergy. Am. J. Respir. Crit. Care Med. 1995; 152(4 Pt 1):1302–8. 18. Young S, Arnott J, O’Keeffe PT et al.The association between early life lung function and wheezing during the first 2 years of life. Eur. Respir. J. 2000; 15(1):151–7. 19. Zeiger RS, Dawson C, Weiss S. Relationships between duration of asthma and asthma severity among children in the Childhood Asthma Management Program (CAMP). J. Allergy Clin. Immunol. 1999; 103(3 Pt 1):376–87. 20. Halonen M, Stern DA, Lohman C et al. Two subphenotypes of childhood asthma that differ in maternal and paternal influences on asthma risk. Am. J. Respir. Crit. Care Med. 1999; 160:564–70. 21. Halonen M, Stern DA, Wright AL et al. Alternaria as a major allergen for asthma in children raised in a desert environment. Am. J. Respir. Crit. Care Med. 1997; 155(4):1356–61. 22. Pullen C, Hey E. Wheezing, asthma, and pulmonary dysfunction 10 years after infection with respiratory syncytial virus in infancy. Br. Med. J. 1982; 5:1665–9. 23. Burrows B, Martinez FD, Halonen M et al. Association of asthma with serum IgE levels and skin-test reactivity to allergens. N. Engl. J. Med. 1989; 320:271–7. 24. Peat JK, Tovey E, Toelle BG et al. House dust mite allergens: a major risk factor for childhood asthma in Australia. Am. J. Respir. Crit. Care Med. 1996; 153:141–6. 25. Platts-Mills TAE, Weck ALD. Dust mite allergens and asthma – a worldwide problem. J. Allergy Clin. Immunol. 1989; 83(2 Pt 1):416–27.

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26. Peat JK, Tovey E, Mellis CM et al. Importance of house dust mite and Alternaria allergens in childhood asthma: an epidemiological study in two climatic regions of Australia. Clin. Exp. Allergy 1993; 23:812–20. 27. Perzanowski MS, Ronmark E, Nold B et al. Relevance of allergens from cats and dogs to asthma in the northernmost province of Sweden: schools as a major site of exposure. J. Allergy Clin. Immunol. 1999; 103:1018–24. 28. Burrows B, Lebowitz MD, Barbee RA et al. Findings before diagnoses of asthma among the elderly in a longitudinal study of a general population sample. J. Allergy Clin. Immunol. 1991; 88:870–7. 29. Barbee RA, Kaltenborn W, Lebowitz MD et al. Longitudinal changes in allergen skin test reactivity in a community population sample. J. Allergy Clin. Immunol. 1987; 79:16–24. 30. Kjellman B, Gustafsson PM. Asthma from childhood to adulthood: asthma severity, allergies, sensitization, living conditions, gender influence and social consequences. Respir. Med. 2000; 94:454–65. 31. Williams H, McNicol KN. Prevalence, natural history, and relationship of wheezy bronchitis and asthma in children: an epidemiological study. Br. Med. J. 1969; 4:321–5. 32. Phelan PD. Asthma in Children and Adolescents: an Overview. London: Baillière Tindall, 1995. 33. Gerritsen J, Koeter GH, Postma DS et al. Prognosis of asthma from childhood to adulthood. Am. Rev. Respir. Dis. 1989; 140:1325–30. 34. Stocks J. Lung function testing in infants. Pediatr. Pulmonol. Suppl. 1999; 18:14–20. 35. Le Souef P,Turner S, Rye P et al. Pulmonary function at four weeks correlates with pulmonary function at 6 and 12 years. Am. J. Respir. Crit. Care Med. 2001; 163:A541. 36. Stein RT, Holberg CJ, Sherrill D et al. The influence of parental smoking on respiratory symptoms in the first decade of life: the Tucson Children’s Respiratory Study. Am. J. Epidemiol. 1999; 149:1030–7. 37. Tager IB, Hanrahan JP, Tosteson TD et al. Lung function, pre- and post-natal smoke exposure, and wheezing in the first year of life. Am. Rev. Respir. Dis. 1993; 147:811–17. 38. Pagtakhan RD, Bjelland JC, Landau LI et al. Sex differences in growth patterns of the airways and lung parenchyma in children. J. Appl. Physiol. 1984; 56:1204–10. 39. Burrows B, Knudson RJ, Lebowitz MD. The relationship of childhood respiratory illness to adult obstructive airway disease. Am. Rev. Respir. Dis. 1977; 115:751–60. 40. Samet JM, Tager IB, Speizer FE. The relationship between respiratory illness in childhood and chronic airflow obstruction in adulthood. Am. Rev. Respir. Dis. 1983; 127:508–23. 41. Bont L, Heijnen CJ, Kavelaars A et al. Monocyte IL-10 production during respiratory syncytial virus bronchiolitis is associated with recurrent wheezing in a one-year follow-up study. Am. J. Resp. Crit. Care Med. 2000; 161:1518–23. 42. Makela MJ, Kanehiro A, Borish L et al. IL-10 is necessary for the expression of airway hyperresponsiveness but not pulmonary inflammation after allergic sensitization. Proc. Natl. Acad. Sci. USA 2000; 97:6007–12. 43. Barker DJ, Godfrey KM, Fall C et al. Relation of birth weight and childhood respiratory infection to adult lung function and death from chronic obstructive airways disease. Br. Med. J. 1991; 303:671–5. 44. Peat JK, Woolcock AJ, Cullen K. Rate of decline of lung function in subjects with asthma. Eur. J. Respir. Dis. 1987; 70:171–9. 45. Agertoft L, Pedersen S. Effects of long-term treatment with an inhaled corticosteroid on growth and pulmonary function in asthmatic children. Respir. Med. 1994; 88:373–81. 46. Martinez FD, Wright AL, Taussig LM et al. Asthma and wheezing in the first six years of life. N. Engl. J. Med. 1995; 332:133–8.

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47. Xuan W, Peat JK, Toelle BG et al. Lung function growth and its relation to airway hyperresponsiveness and recent wheeze: results from a longitudinal population study. Am. J. Resp. Crit. Care Med. 2000; 161:1820–4. 48. Holberg CJ, Stern DA, Sherrill DL et al. Effect of smoking on the development of lung function in adolescence. Am. J. Respir. Crit. Care Med. 2001; 163:A260. 49. Camilli AE, Burrows B, Knudson RJ et al. Longitudinal changes in forced expiratory volume in one second in adults: effects of smoking and smoking cessation. Am. Rev. Respir. Dis. 1987; 135:794–9. 50. Burrows B, Knudson RJ, Cline MG et al. A reexamination of risk factors for ventilatory impairment. Am. Rev. Respir. Dis. 1988; 138:829–36. 51. Kurzius-Spencer M, Sherrill DL, Holberg CJ et al. Familial correlation of the slope of FEV1 in smoking and non-smoking pairs. Am. J. Respir. Crit. Care Med. 2000; 161:A580. 52. Postma DS, de Vries K, Koeter GH et al. Independent influence of reversibility of airflow obstruction and nonspecific

53.

54.

55.

56.

hyperreactivity on the long-term course of lung function in chronic airflow obstruction. Am. Rev. Respir. Dis. 1986; 134:276–80. Burrows B, Sears MR, Flannery EM et al. Relationships of bronchial responsiveness assessed by methacholine to serum IgE, lung function, symptoms, and diagnoses in 11-year-old New Zealand children. J. Allergy Clin. Immunol. 1992; 90(3 Pt 1):376–85. Palmer LJ, Rye PJ, Gibson NA et al. Airway responsiveness in early infancy predicts asthma, lung function, and respiratory symptoms by school age. Am. J. Respir. Crit. Care Med. 2001; 163:37–42. Weiss ST, Ware JH. Overview of issues in the longitudinal analysis of respiratory data. Am. J. Respir. Crit. Care Med. 1996; 154(6 Pt 2):S208–11. Stein RT, Holberg CJ, Morgan WJ et al. Peak flow variability, methacholine responsiveness and atopy as markers for detecting different wheezing phenotypes in childhood. Thorax 1997; 52:946–52.

Chapter

Genetics Jorrit Gerritsen, Marlies Feijen, and Dirkje S. Postma*

4

Departments of Pediatrics and Pulmonology,* University Hospital Groningen, The Netherlands

The inheritance of asthma and atopic diseases is evident from family histories. Notwithstanding the fact that twin studies have shown that heritability in asthma is as high as 60–70%,1,2 the mode of inheritance still is not clear.There is even more uncertainty about the mode of inheritance of chronic obstructive pulmonary disease (COPD). It is likely that continuous injury to the lungs by environmental factors, especially smoking, interacts with genetic factors in the development of COPD. This chapter discusses the current knowledge of the genetics of atopic diseases, asthma, and COPD.

H E R I TA B I L I T Y O F A S T H M A A N D C O P D Asthma Asthma tends to cluster in families (familial aggregation). Over 85 years ago Cooke and van der Veer3 concluded, on the basis of family studies of 504 patients, that inheritance is a definite factor in human allergy. Leigh and Marley4 showed that the prevalence of asthma was significantly higher (13%) in relatives of asthma patients than in relatives of a matched group of nonasthmatic subjects (6%). The question is whether this familial aggregation can be explained mainly by genetic factors, and what part the environment plays. A glossary of the common terms used in genetics is presented in Table 4.1. No single gene accounts for the familial segregation of asthma. Rather, data demonstrate that models with oligogenic loci (i.e. a handful of loci being responsible for most of the genetic control) provide the best fit to the data.5 However, there are strong environmental influences,6 which modify phenotypic expression (Fig. 4.1). In studies of the heritability of asthma and COPD, the absence of unambiguous criteria for the diagnosis of these diseases has been a major obstacle to gene finding. Different genes can lead to the same phenotype (genetic heterogeneity), or the same genotype may result in different phenotypes (pleiotropy). Genes with DNA sequence variants do not always express the phenotype (incomplete penetrance), and the specific phenotype under study can be

PATIENT CHARACTERIZATION

DNA analysis

Genetic analysis

Sub-phenotyping of the patient

Fig. 4.1. Schematic of the approach to finding susceptibility genes for diseases like asthma and COPD.

expressed without the genetic mutation (phenocopy). In asthma, polygenic inheritance (i.e. mutation(s) of different genes) also seems to be important. Thus asthma fits the definition of a complex genetic disease. COPD Both environmental and genetic factors contribute to the development of COPD. Active sustained cigarette smoking contributes to the origin of COPD in more than 90% of the affected population in subjects who are susceptible.7 However, in Caucasians only 10–20% of chronic heavy cigarette smokers develop symptomatic COPD, suggesting that genetic factors are likely to be important in determining which cigarette smokers are at risk to develop airflow limitation.8 Furthermore, some patients develop severe airflow limitation at an earlier age, again an indication for a genetic origin of COPD. Notwithstanding the influence of environmental factors, there is considerable evidence that COPD has a hereditary component as well. In a population of 5003 adults there were significant correlations in lung function in sibling pairs

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Asthma and Chronic Obstructive Pulmonary Disease

Table 4.1. Glossary of common genetic terms

Allele

An alternative form of the same gene present at a given genetic locus.

CentiMorgan (cM)

Relative genetic distance between two loci on a chromosome. The chance of recombining during a single meiosis is 1:100 if two loci are separated by 1 cM.

Complex disorder

A disorder coming from the inheritance of more than one genetic locus, influenced by environmental factors, and/or genotype-by-environment interaction.

Genetic map

A diagram of a particular chromosome in which the relationship of one genetic locus to another, both with respect to linear order and distance along the chromosome, is depicted.

Genome

All genetic material in a chromosome.

Genotype

The specific set of alleles inherited by a given individual.

Haplotype

The specific alleles on a chromosome inherited from one parent (haploid genotype). A haplotype typically refers to closely grouped loci.

Linkage

The inheritance of genetic material at two loci which, because of their physical proximity on the chromosome, do not assort independently.

Linkage disequilibrium

Association of a particular allele with a specific allele at a nearby locus more frequently than expected by chance.

Locus

The unique location of a gene or DNA sequence on a chromosome.

LOD-score

The log10 of the odds ratio, and reflects the (un)linkage of two loci.

Pharmacogenetics

The study of hereditary variations underlying differences among individuals in drug response.

Phenocopy

Individuals without the genotype show the study’s phenotype.

Phenotype

The physical characteristics of an individual.

Polymorphism

Frequency of >1% in the population of variant loci present at two or more alleles.

SNP

Single nucleotide polymorphism; i.e. those variants at a single nucleotide base that lead to inheritance of different alleles.

and in parents and their offspring, but only trivial correlations between spouses;9 and these findings have been replicated.10 Although accurate estimates of heritability are difficult, the heritability of FEV1, the major component of the expression of COPD available in clinical practice, is as high as 77%.11 Moreover, twin studies have shown that there is a closer similarity in lung function between monozygotic twins than between dizygotic twins. In a recent investigation in early-onset severe COPD, current and ex-smoking relatives of the probands had an increased risk to have reduced FEV1 and chronic bronchitis. This observation suggests a genetic risk factor for COPD that is expressed in response to cigarette smoking.12

population.13,14 The risk is thought to reflect sharing of both genes and environment. The genetic component of asthma has been studied in twin studies and segregation analyses (Fig. 4.2).

APPROACHES TO GENETIC STUDIES

MZ twins share virtually 100% of their genetic information and DZ twins 50%. A higher disease concordance in MZ twins compared with DZ twins can be ascribed to genetic factors, whereas the observations that in many genetically identical MZ twins one of the pair has asthma is a strong

A first-degree family member of a patient with asthma has a 2–6 fold higher risk to develop the disease than individuals unrelated to a patient with asthma in the general

Studies of twins The main assumptions in twin studies: • the environment for monozygotic (MZ) and dizygotic (DZ) twins is similar; • the twin pairs are otherwise representative of the general population; • in questionnaire-based studies, self-reported zygosity is correct.

Genetics

1

1

3

2

3 4

5

6 7

8

31

9

2

4

5

6

7

Confidence interval

8

9

Families

Genotyping using DNA markers

Localization of susceptibility genes

Fig. 4.2. Example of genetic analysis.

indication for environmental influences.2,15–20 A Swedish population-based study of 6996 twin pairs revealed that MZ concordance for self-reported asthma was 19.0% and DZ concordance 4.8%.21 In 8- to 18-year-old twins, genetic associations for asthma (as reported by questionnaire), atopy (revealed by skin-prick test) and bronchial hyperresponsiveness (assessed by hypertonic saline challenge) were ascertained, as odds ratios. The ratios were greater in MZ pairs compared with DZ pairs for asthma: 25.6 versus 1.9; for atopy 14.6 versus 2.5; and for airway hyperresponsiveness 14.1 versus 4.2. Since strong cross-sectional associations exist between these three traits, genetic factors are involved in the expression of each of these three traits.22 Only scanty information is available about the heredity of allergy as indicated by specific IgE levels and skin-prick test results, but the specific IgE response seems mainly to be determined by the environment.16,23,24 A twin study in smokers showed that there is a high risk to develop chronic airflow limitation in MZ twins, whereas this is not the case in DZ twins who smoke, even if they were raised apart.25,26 Segregation analysis The hypothesis that the aggregation of a trait in families comes from the action of a major gene has been tested by

segregation analysis, in which the number of individuals with a certain trait in a family is compared with the expected number using different genetic models of inheritance. The models used have been: • a Mendelian component (dominant or recessive gene model); • a non-Mendelian component (multiple genes with small effect); • a mixed model (a single gene on a polygenic background); or • a nongenetic environmental model. Segregation analysis identifies the genetic model which is the best fit to the data. From the optimal model the mode of inheritance, and parameters such as the penetrance, the heritability, and allele frequencies, can be estimated.27 A study in 13,963 patients with a self-reported family history of asthma, participating in the European Community Respiratory Health Survey, provided evidence for a two-allele gene with codominant inheritance.28 Familial aggregation of asthma and wheeze was also shown in four smaller studies, consistent with the action of multiple genes each with a small effect.5,29–31 Airway responsiveness to carbachol has been studied in nonasthmatic parents of patients with asthma and in healthy

32

Asthma and Chronic Obstructive Pulmonary Disease

controls.32 In the healthy controls, 10% of the parents had airway hyperresponsiveness, in contrast to 50% of the nonasthmatic parents of the patients with asthma. This familial clustering was also studied in complex segregation analyses of 83 and 131 families;31,33 the findings were similar but there was no evidence for a single major gene for airway responsiveness. A number of groups have studied the segregation of total serum IgE; the results have been quite variable. The best fitting models, using a single-locus approach, have pointed to a single major gene, either dominant,34 codominant,35,36 recessive,37 or mixed recessive inheritance.38 Polygenic models provide good fits in a multi-locus approach.31,39 A two-locus approach to total serum IgE levels in 92 Dutch families, ascertained through a proband with asthma, fitted the data significantly better than a one-locus model. It provided evidence for two unlinked loci; one single locus explained 56% of the variance of the level of total serum IgE, and the second 19%. Jointly the two loci accounted for 78.4% of the variability of total serum IgE levels.40

LINKAGE STUDIES Chromosome 11q The first reports of linkage of atopy, asthma, and rhinitis to the microsatellite marker D11S97 on chromosome 11q13 in seven extended UK pedigrees were presented by Cookson and coworkers in 1988 and 1989.41,42 They found a maximum LOD score of 5.58 for atopy, defined as an elevated IgE, raised allergen-specific IgE, or the presence of one or more positive skin-prick tests; the recombination fraction was 0.105. Linkage to the same marker was replicated by the same group in other population samples, among others in 64 nuclear families where a LOD score of 3.8 was found, at a recombination fraction of 0.067. It was suggested that atopy at this locus was inherited via the maternal line.43–45 Since those reports there have been a number of further independent publications claiming either positive46–48 or negative49–54 findings for linkage to this chromosomal region for asthma, airway hyperresponsiveness, and atopy. The negative finding of Zhu et al.54 can be ascribed to the age group (12 months) investigated. Almost all recent studies have failed to replicate the evidence for a candidate gene on chromosome 11q13 for asthma and atopy. Chromosome 5q Numerous candidate genes for asthma and atopy are located on chromosome 5q31 to q33. Linkage between total serum IgE levels and chromosome 5q was reported in a US Amish population and at the same time in a study of Dutch families who were ascertained through a proband with asthma.55,56 In the Dutch families it was shown that linkage exists with airway hyperresponsiveness to histamine in the same region of chromosome 5q, indicating that a gene governing airway hyperresponsiveness might be located near a gene regulating total serum IgE.57

In studies from Japan, the UK, and the US, chromosome 5q was also mentioned as a region containing one or more susceptibility genes for asthma.47,58–60 The finding of linkage to asthma or atopy on chromosome 5q has not been confirmed in many other studies.61–65 The importance of this region for the development of asthma and atopy remains unknown. Novel chromosomal regions of interest for asthma In an Australian and British sample, potential linkages (P < 0.001) were found between chromosome 4 and airway responsiveness, chromosome 6 and eosinophils, chromosome 7 and airway responsiveness, chromosome 11 and skin tests and total IgE, and chromosome 16 and total IgE.45 A US multicenter study was performed in 140 asthmatic families ascertained through two or more affected siblings with asthma in three racial groups – Hispanics, Caucasians, and Afro-Americans. The outcome was that different regions were linked in these different racial groups. In AfroAmericans, regions of interest for asthma-associated phenotypes were on chromosome 5p15 and 17p11.1 to q11.2, in Caucasians on chromosome 11p15 and 19q13, whereas in Hispanics on chromosome 2q33 and 21q21.59 In two European populations, consisting of 124 Finnish and Catalonian families, three phenotypic markers (total serum IgE, asthma, and specific IgE level for a mixture of the most common aeroallergens in Finland) were investigated and associated with the FceRII gene on chromosome 19p13. They found that chromosome 19p13 might harbor a genetic determinant of IgE-related traits, and concluded that other population samples are needed to verify this finding.66 The Hutteries are an American religious group that emigrated from the Tyrolean Alps in the 1500s. In the 1870s, about 900 of these members migrated to what is now South Dakota, and roughly half settled on three communal farms. The Hutteries have expanded dramatically and since they arose from less than 90 ancestors, they are a homogeneous population. If the subjects had either airway hyperresponsiveness to methacholine and reported asthma symptoms they were diagnosed as “strict” asthma. “Loose” asthma was defined if one of the two were positive. Linkages for “strict” asthma were found on chromosomes 19q13 and 21q21. For “loose” asthma these were 3p24.2–22, 5q23–31, and 12q15–24.1. This implies that also in a homogeneous population multiple susceptibility genes are involved in asthma.60 A more detailed study on chromosome 12q in Italian families revealed that asthma susceptibility factors are located on this chromosome.67 These findings have not been consistent; in a sibling-pair linkage analysis and transmission disequilibrium testing (Fig. 4.3) using four highly polymorphic microsatellite markers on 12q13–24, no linkage could be found with atopy.68 In 97 German families, including 415 single individuals and 156 sibling pairs, a genome-wide search was performed for asthma defined by clinical history and/or a history of at least 3 years of recurrent wheezing episodes in children over

Genetics

(a)

Allele frequency (log10 P value)

25

ApoE4

20 15 10 5 0 0

200 400 600 800 1000 1200 1400 1600 1800 2000

Distance (kb)

33

In 108 Dutch probands and their families, a genome-wide screen was performed using variance-component analysis. Significant linkage of IgE was found at chromosome 7q with a LOD score of 3.36, flanked by markers D7S820 and D7S821. Furthermore, linkage for previously reported regions on chromosome 5q and 12q was confirmed.70 The genome screen studies show that the findings in different populations are not consistent, although in most screens linkages have been found on chromosome 5q, 11q, and 12q. Thus far no linkage studies on COPD have been published. A summary of the results of genome screens in asthma is presented in Table 4.2.

(b)

C A N D I D AT E G E N E S

25

Allele frequency (log10 P value)

ApoE4 20 15 10 5 0 460 465 470 475 480 485 490 495 500 505 510 515 520 525

Distance (kb) Fig. 4.3. An example of transmission disequilibrium testing, showing the significance of SNP allele frequency differences in an affected Alzheimer’s disease population and age-matched controls. (a) Association data for dozens of ordered SNPs from a region of 2 million bases on either side of ApoE. When the allele frequencies of each SNP are compared in large series of Alzheimer’s disease patients and controls, a sharp peak of several SNPs can be readily observed in linkage disequilibrium, with no significant difference in the frequencies of background alleles. (b) If the peak is enlarged to illustrate a region of only 60,000 bases around ApoE, three SNPs from the map that are each highly significantly associated with Alzheimer’s disease can be identified. Only two genes, ApoC1 and ApoE, are coded in the physical DNA segment defined by the SNPs associated with Alzheimer’s disease. The association data from the SNP defining the ApoE4 polymorphism, known to be associated with earlier onset of disease, is also illustrated. Reproduced from reference 135, with permission.

age 3. Linkages were found at chromosome 2p (marker D2S2298) for asthma, airway responsiveness to methacholine and specific and total IgE; at chromosome 6p for asthma, eosinophils, total and specific IgE; at chromosome 9q for asthma, total and specific IgE; and at chromosome 12q for specific IgE.14 A genome-wide screen in France was conducted in 107 nuclear families with at least two siblings with asthma. The subjects were characterized by skin-prick tests, total serum IgE, lung function, and methacholine challenge. Linkages were found at the chromosomes 1p31 for asthma; at 11p13 for IgE and asthma; at 12q24.31 and 13q31 for eosinophil count; at 17q12 for asthma and skin-prick tests; and at 19q13 for airway responsiveness.69

After linkage of a disease to a specific chromosomal region, the region can be screened for candidate genes associated with the disease. A candidate gene has to meet the following criteria: • the product of the gene must be functionally relevant to the disease under study; • mutations within the gene have to alter the function; • the disease has to be linked to the chromosomal region harboring the candidate gene; • the disease has to show association with different alleles of the gene. Candidate genes in asthma Among the candidate genes studied in asthma are: • the b2-adrenergic receptor; • the b-chain of the high-affinity IgE receptor at chromosome 11q13; • the cytokine cluster at chromosome 5q31-q33; • the IL-4 receptor a-chain at chromosome 16p; • the major histocompatibility complex; • the tumor necrosis factor-a at chromosome 6. The b2-adrenergic receptor The gene encoding the b2-adrenergic receptor is located on chromosome 5q31. Nine polymorphisms have been identified in this gene; four code for a change in amino-acid sequence at positions 16, 27, 34, and 164 of the F13 residual protein (Fig. 4.4). Multiple studies show that these polymorphisms of the b2-adrenergic receptor do not contribute to the risk of asthma.71–74 However, they may play an important role in modifying the clinical severity; i.e. they are associated with nocturnal and/or severe asthma, as evidenced by using more corticosteroids and immunotherapy.71,75,76 The response to regular use of a b-agonist is impaired in patients homozygous for the genotype Arg16 of the b2-receptor. These patients may benefit by avoiding regularly scheduled b-agonists and might be candidates for earlier intervention with anti-inflammatory agents.77 In

34

Asthma and Chronic Obstructive Pulmonary Disease

Table 4.2. Results of genome screen in different populations

Chr 2p 2q 3p 3q 4q 5p 5q 6p 7p 9p 9q 10q 11p 11q 12q 13q 14q 16q 17p 17q 19q 21q

US asthma

Dutch

Australia

Hutteries

Germany Asthma

Hispanic

“Asthma” BHR/asthma symptoms BHR

BHR

BHR Afro-American Caucasian Caucasian

BHR

“Asthma” IgE/eos/atopy IgE/eos/BHR

Asthma BHR “Asthma”

Asthma

Asthma

Asthma

IgE Caucasian IgE/Skin-test Caucasian Hispanic Caucasian Caucasian

IgE Atopy

BHR

IgE/BHR Afro-American IgE Caucasian Hispanic

homozygous Arg-16 patients it has been shown that they are susceptible to clinically important increases in asthma exacerbations during chronic dosing with the short-acting b2agonist salbutamol.78 Children with the Gly16 polymorphism of the b2adrenergic receptor have been shown to have a decreased bronchodilation after the use of a short-acting b2-agonist compared with those with the Arg16 allele. It was suggested that the diminished response of the Gly16 variant to shortacting b2-agonists was related to the increased use of inhaled corticosteroids.73 A number of studies have focused on the b2-adrenoreceptor desensitization.79–81 The effect of two common polymorphic forms of the b2-adrenoreceptor on acute and long-term b2adrenergic receptor desensitization in human smooth muscle cells has been examined.81 Cells with any Arg16 allele showed significantly greater acute and long-term desensitization of isoprenaline-induced c-AMP formation than did cells without the Arg16 allele (54% versus 2%, P < 0.01, for short-term desensitization; and 73% versus 35%, P < 0.05, for long-term desensitization). Owing to the strong linkage disequilibrium it is difficult to assign causation to a single locus.

Asthma BHR/Asthma

The b-chain of the high-affinity IgE receptor The high-affinity IgE receptor (FceRI) consists of three chains, one a, one b, and two c chains. The abc1,2-complex is located on the surface of mast cells, basophils, eosinophils, and Langerhans cells. Binding of allergens to IgE on mast cells gives activation and excretion of cytokines such as IL-4, which in their turn increase IgE production of the B lymphocytes. The function of the a-subunit is ligand binding; the c-dimer mediates both the assembly of the receptor as well as signal transduction.The b-subunit amplifies the signal mediated by the c-dimer.82 Sequence variants in the a- and c-chains have not been associated to asthma or atopy.47 The b-chain is situated on chromosome 11q.83 A number of polymorphisms have been investigated in relation to atopy and asthma: the isoleucine (Ile)181/leucine (Leu)181 variant was associated with atopy, if the Leu-181 variant was inherited maternally.84,85 Also the Leu-181/Leu-183 variant, when inherited maternally, was associated with atopy.86 The results of the Leu-181/Leu-183 variants have not been replicated in Japanese, UK, Italian, and Dutch populations.50,87–90 One of the variants of a polymorphism resulting in two restriction sites for the restriction enzyme, RsaI, was

Genetics

Val34

Met (GTG

35

D P A H S R N P A L L F A S G N G P Q G M NH2 H D Arg16 Gly (AGA GGA) V Y A C T N N E Q I T Q Gln27 Glu (CAA GAA) A C R E C D Q D E 175 (CGG AGG) F T H W G N F V M T F F W W A K T V N L I R C Q D M N V R K G M G L I H E F W Y W H Q A Y I V H E V Y I V M S A A G F T S I D M Q I P A I A S V I N V I L L N L I V P V V L F S V L C S I V I F F W I G ATG) L A I V A L G M T L G S V T A S S F Y V P L W C Y V N S F G N V L D V I W I E T P L V L T F G F N V L V I A C A L V M L I L C V I I M V F T G M I P L I Y T A I S T I I V R A V D V Y S I G L C R S F T A R A R P K D F Y K Y K V L R I F N A F A N F F K H A Q T E R E K L K V C Q I L Q T E F T E T A K R Q L Q L L K K 84 (CTG CTA) S L L S S P F K I Y Q C S V H F R G E S K D Q R L N R R L S Q V E Q D G R T G H G L Thr164 Ile (ACC ATC) R S 366 (TAT TAC) S L Y G S Q E G T N G N S S Y G N G Y A K H V 351 (GGG GGC) E Q E K E N K L L C E D L P G T E D F V G H Q G T HOOC L L S D N T S C N R G Q S D I N D S P V

413 (CTG

CTA)

Fig. 4.4. Primary amino-acid sequence and proposed membrane topography of the human b2-adrenergic receptor. The darkened circles indicate codons where degenerate polymorphisms of the receptor were found. The four polymorphisms which result in changes in the amino-acid sequence are indicated. Reproduced from reference 136, with permission.

associated with atopy in a Japanese population, and with atopic dermatitis in a British population, which could however not be confirmed in another population.91–93 The Gly237→ Glu237 polymorphism, a mutation in about 5% of Australian and Japanese populations, was strongly associated with asthma and airway hyperresponsiveness.87,88 It can be concluded that the high-affinity receptor FceRIb is related to asthma and atopy, although it is plausible that other variants of the gene or other genes on chromosome 11 are more informative for the development of atopy and asthma. The cytokine cluster Chromosome 5q31–q33 region contains a cluster of genes coding for proinflammatory cytokines important in immune regulation (IL-4, IL-5, IL-9, IL-13, CD-14, and interferon-c). Two members of this cluster, IL-4 and IL-13, have been genetically and functionally implicated in the pathogenesis of asthma and atopy.94–97 These cytokines are

produced by Th2 cells and are capable of inducing isotype class switching of B cells to produce IgE. They also share a receptor component, IL4Ra, which is an important factor in the development or expression of atopy and asthma. The IL-13 receptor consists of one IL4Ra subunit and either a low-affinity IL13Ra1 or a high-affinity IL13Ra2 subunit. The complete receptor for IL-4 is composed of one IL4Ra subunit and an IL4Rc subunit. Van der Pouw Kraan was the first to establish that the IL13 1055TT genotype was associated with allergic asthma, and altered regulation of IL-13 production.95 In addition, an Arg130Gln polymorphism in exon 4 has been shown to be associated with high total serum IgE levels,96,98 atopic dermatitis,98 and asthma,97 – in German,96,98 American,99 British,68 and Japanese97 populations. These results provide evidence that variation in the IL-13 gene is involved in the pathogenesis of asthma, airway hyperresponsiveness, and skin-test reactivity. Further investigation in this region of the chromosome is required.

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Asthma and Chronic Obstructive Pulmonary Disease

The IL-4 receptor a-chain The production of IgE by the B cell and the switch of the T helper cell into the Th2 cell are mainly regulated by IL-4 and IL-13 and their receptors. IL-4 and IL-13 are located on chromosome 5q and share the IL-4 receptor a-chain which is located at chromosome 16p12. The polymorphisms of the IL-4 receptor (Ile50 allele, Arg551 allele) are predominantly linked to atopy, atopic asthma, and serum total IgE;99–103 but not in all populations.104 After a genome-wide screen in the Hutterites, the IL4Ra gene was examined as the 16p-linked susceptibility locus of asthma and atopy in both Hutterites and Hispanics.105 It appeared that there was an association to atopy or a lose definition of asthma. Since the alleles or haplotypes showing the strongest evidence differed between the Hutterite and Hispanic families, variation outside the coding region of the gene influences susceptibility.105 Others have found a lower IgE level in a population with a combination of the Pro478 and Arg551 allele.106 In one study it is shown that the IL4Ra*576R allele is associated with increased IgE and a low level of lung function in asthmatic subjects.90 From these studies it has become clear that the IL4Ra gene is of importance in the pathogenesis of atopy, IgE, and asthma. The precise place in larger populations has to be investigated. The human leukocyte antigen (HLA) region The human major histocompatibility complex (MHC) is a region of DNA on chromosome 6 (6p21.3) and contains over 200 genes, more than 40 of which encode leukocyte antigens.107 The HLA region and the tumor necrosis factor-a are located on chromosome 6p. The HLA molecules are membrane-bound glycoproteins involved in antigen presentation. The HLA class I is present on virtually every somatic cell, although the level of expression varies depending on the tissue. By contrast, class II genes are normally expressed by a subgroup of immune cells that includes B cells, activated T cells, macrophages, dendritic cells, and thymic epithelial cells. In the presence of interferon-c, however, other types of cells can express class II HLA molecules.107 Polymorphisms in the genes encoding the HLA class II molecules are associated with total serum IgE and specific IgE responses to several specific allergens, such as ragweed pollen, and cockroach.108–110 Also, associations with sensitization to cockroach allergens and the HLA-DRB1*01 alleles has been found.112 Others have failed to extend these findings to common major allergens, like house dust mite.110,111 In industrial isocyanate-induced asthma, certain HLA class II traits may be important.113 Although these are positive associations, the importance of this class II region for asthma is not universal.114,115 Tumor necrosis factor-a Inflammation of the airways is a key issue in asthma, as assessed by bronchial biopsies and bronchoalveolar lavage. One of the important cytokines in asthma is tumor necrosis factor-a; the gene encoding for TNF-a is situated on the

HLA class III locus of chromosome 6p near to the leukotriene A and B gene.107 The 308G→308A polymorphism (TNF1 allele) is associated with a 6- to 7-fold upregulation in transcription of TNF-a and results in a 5-fold increased risk of physician-diagnosed asthma requiring prophylactic medication. The patients studied had positive skin tests to one or more common aeroallergens and a family history of asthma and/or atopic disease in first-degree relatives.116 Positive associations between asthma (based on questionnaires) and TNF-a polymorphism (LTaNcol*1) have been found.117–119 Others have not replicated these findings,120 so no definitive statements can be made about TNF-a in asthma. Candidate genes in COPD There is one proven genetic risk factor for COPD: severe a1-antitrypsin deficiency. The human genes encoding a1-antitrypsin, a1-antichymotrypsin, and protein C inhibitor all map to human chromosome 14q32.1. The serine protease inhibitor (serpin) gene cluster also contains an antitrypsin-related sequence.121 Mutations in the 3 flanking region of the a1-antitrypsin gene have been reported to be associated with COPD, whereas in another study this could not be confirmed.122 A combination of a family history of COPD with homozygosity for His113/His139 mEH haplotype has been reported to be associated with rapid decline of lung function with an odds ratio of 4.9 and a P value of 0.04.123 Pulmonary surfactant, a lipoprotein complex essential for normal lung function, innate host defense of the lung, and the regulation of inflammatory processes known as the collectins, might play a role in the etiology of COPD. Transgenic mice that express the PDGF-B gene driven by the lung-specific surfactant protein C promoter have been shown to have distinct abnormalities in the developing and adult lung. This leads to a complex phenotype that encompasses aspects of both emphysema and fibrotic lung disease.124,125 Recently it has been confirmed, in humans, that polymorphisms of the surfactant protein gene are linked to COPD.126 In a Taiwanese population the TNF-308 polymorphism, which has previously been shown linked to asthmatic phenotypes, is disproportionately represented in patients with chronic bronchitis. Others found that TNFa*1/2 alleles are significantly associated with the presence of smoking-related COPD.127 Moreover, TNF-a may have an amplifying effect on the inflammatory process by activating NF-jB and other transcription factors to increase the expression of inflammatory genes, including IL-8. However, in a Caucasian population of smokers (86 COPD, and 62 healthy smokers) this was not confirmed. Moreover, there was no association between a polymorphism of TNF-a and level of FEV1 in COPD patients.128 Thus, more studies in larger groups are needed to assess the viability of TNF-a as a candidate gene in COPD. There are other, less well studied causative loci in COPD, including the IVS8 5T allele of the CFTR gene and the human mucin genes.129–131

Genetics

PHARMACOGENETICS

THE PERSPECTIVES OF GENETICS OF ASTHMA AND COPD

Individual variation in response to drugs is a major problem in clinical practice, but also in drug development. Such variation ranges from failure to respond to a drug to adverse drug reactions and drug interactions when several drugs are taken concomitantly.132,133 The research on single nucleotide polymorphisms (SNPs) of, for example, the b2-adrenergic, the glucocorticoid, and the leukotriene receptor will be important in identifying the response on the different drugs by the patients and will be a target to individualize therapy. An example is the effect of b2-adrenergic agents on desensitization in human airway smooth muscle. The Glu27 allele polymorphism of the b2adrenergic receptor on chromosome 5q was significantly associated with increased acute and long-term desensitization in human airway smooth muscle after pretreatment with isoprenalin.81 In a clinical trial of the efficacy of ABT-761, a compound derived from zileuton, the DNA-sequence variants of the 5lipoxygenase (ALOX5) were investigated. The 114 individuals receiving high-dose active treatment were compared with the placebo group. The average improvement of FEV1 was 18.8  3.6% in the wild-type patients and 23.3  6.0% in the heterozygous patients. This was in contrast with the ten patients with the mutant genotype who had no benefit from active treatment (change in FEV1 of 1.2  2.9%) (Fig. 4.5).134 These findings suggest that the use of genetic information to stratify treatment responses will become very important in the near future and will optimize therapy in asthma and COPD. Day 8 25

Day 84 P < 0.001

FEV1 (% change from baseline)

20 P  0.026

15

37

P  0.004

P  0.039

10 5 0 5

10 Fig. 4.5. Percentage change in FEV1 from pretreatment baseline for patients with the wild-type genotype at the ALOX5 core promotor locus treated for 8 and 84 days with either ABT-761g (dark bar) or placebo (light bar); and for patients with no wild-type alleles at this locus (open bar). Reproduced from reference 134, with permission.

In asthma and COPD the heterogeneity of the diseases with complex hereditary traits and many (sub)phenotypes are obstacles to the identification of genes. In the pathogenesis of asthma and COPD it is reasonable that the interactions between multiple genes and multiple environmental triggers are also important; again this will make it difficult to identify genes. When a linkage has been found in a certain population, repetition of the results in different populations is crucial. Collaborative efforts worldwide are needed if we are going to unravel the complex genetics of asthma and COPD. This will ultimately only be possible by close collaboration between, for example, clinicians, (molecular) geneticists, statisticians, and immunologists.

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102. Hershey GKK, Friedrich MF, Esswein LA, Thomas ML, Chatila TA. The association of atopy with a gain-of-function mutation in the a-subunit of the interleukin-4 receptor. N. Engl. J. Med. 1997; 337:1720–5. 103. Izuhara K, Yanaghihra Y, Hamasaki N, Shirkawa T, Hopkin JM. Atopy and the human IL-4 receptor a chain. J. Allergy Clin. Immunol. 2000; 106:S65–71. 104. Noguchi E, Shibasaka M, Arinami T et al. No association between atopy/asthma and the ile50val polymorphism of IL-4 receptor. Am. J. Respir. Crit. Care Med. 1999; 160:342–5. 105. Ober C, Leavitt SA, Tsalenko A et al. Variation in the interleukin 4 receptor gene confers susceptibility to asthma and atopy in ethnically diverse populations. Am. J. Hum. Genet. 2000; 66:517–26. 106. Mitsuyasu H, Yanagihara Y, Mao XQ et al. Cutting edge: dominant effect of Ile50Val variant of the human IL-4 receptor alpha-chain in IgE synthesis. J. Immunol. 1999; 162:1227–31. 107. Klein J, Sato A. The HLA system I. N. Engl. J. Med. 2000; 343:702–9. 108. Howell WM, Holgate ST. HLA genetics and allergic disease. Thorax 1995; 50:815–18. 109. Donfack J,Tsalenko A, Hoki DM et al. HLA-DRB1*01 alleles are associated with sensitisation to cockroach allergens. J. Allergy Clin. Immunol. 2000; 105:960–6. 110. Mansur AH, Williams GA, Bishop DT et al. Evidence for a role of HLA DRBI alleles in the control of IgE levels, strengthened by interacting TCR A/D marker alleles. Clin. Exp. Allergy 2000; 30:1371–8. 111. Young RP, Dekker JW, Wordsworth BP et al. HLA-DR and HLADP genotypes and immunoglobulin E-responses to common major allergens. Clin. Exp. Allergy 1994; 24:431–9. 112. Hollaway JW, Doull I, Begishvilli B et al. Lack of evidence of a significant association between HLA-DR, DQ and DP genotypes and atopy in families with HDM allergy. Clin. Exp. Allergy 1996; 26:1142–9. 113. Bignon JS, Aron Y, Ju LY et al. HLA class II alleles in isocyanateinduced asthma. Am. J. Respir. Crit. Care Med. 1994; 149:71–5. 114. Li PK, Lai CK, Poon AS et al. Lack of association between HLADQ and -DR genotypes and asthma in southern Chinese patients. Clin. Exp. Allergy 1995; 25:323–31. 115. Aron Y, Swierczewski E, Lockhart A. HLA class II haplotype in atopic asthmatic and non-atopic control subjects. Clin. Exp. Allergy 1995; 25:S65–S67. 116. Albuquerque RV, Hayden CM, Palmer LJ et al. Association of polymorphisms within the tumor necrosis factor (TNF) genes and childhood asthma. Clin. Exp. Allergy 1998; 28:578–84. 117. Moffatt MF, Cookson WO. Tumour necrosis factor haplotypes and asthma. Hum. Mol. Genet. 1997; 6:551–4. 118. Moffatt MF, James A, Ryan G et al. Extended tumour necrosis factor/HLA-DR haplotypes and asthma in an Australian population sample. Thorax 1999; 54:757–61. 119. Wa LK, Mansur AH, Britton J et al. Association between 308 tumour necrosis factor polymorphism and bronchial hyperreactivity in asthma. Clin. Exp. Allergy 1999; 29:1204–8.

120. Louis R, Leyder E, Malaise M, Bartsch P, Louis E. Lack of association between adult asthma and the tumour necrosis factor alpha-308 polymorphism gene. Eur. Respir. J. 2000; 16:604–8. 121. Rollini P, Fournier RE. A 370-kb cosmid contig of the serpin gene cluster on human chromosome 14q32.1: molecular linkage of the genes encoding alpha 1-antichymotrypsin, protein C inhibitor, kallistatin, alpha 1-antitrypsin, and corticosteroidbinding globulin. Genomics 1997; 46:409–15. 122. Sandford AJ, Spinelli JJ, Weir TD, Pare PD. Mutation in the 3 region of the alpha-1-antitrypsin gene and chronic obstructive pulmonary disease. J. Med. Genet. 1997; 34:874–5. 123. Sandford AJ, Chagani T, Weir TD et al. Susceptibility genes for rapid decline of lung function in the Lung Health Study. Am. J. Respir. Crit. Care Med. 2001; 163:469–73. 124. Hoyle GW, Li J, Finkelstein JB et al. Emphysematous lesions, inflammation, and fibrosis in the lungs of transgenic mice overexpressing platelet-derived growth factor. Am. J. Pathol. 1999; 154:1763–75. 125. Suga T, Kurabayashi M, Sando Y et al. Disruption of the klotho gene causes pulmonary emphysema in mice: defect in maintenance of pulmonary integrity during postnatal life. Am. J. Respir. Cell Mol. Biol. 2000; 22:26–33. 126. Guo X, Lin HM, Lin Z et al. Polymorphisms of surfactant protein gene A, B, D, and of SP-B-linked microsatellite markers in COPD of a Mexican population. Chest 2000; 117:249S–50S. 127. Sakao S, Tatsumi K, Igari H et al. Association of tumor necrosis factor a gene promotor polymorphism with the presence of chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 2001; 163:420–2. 128. Higham MA, Pride NB, Alikhan A, Morrell NW. Tumour necrosis factor-a gene promoter polymorphism in chronic obstructive pulmonary disease. Eur. Respir. J. 2000; 15:281–4. 129. Noone PG, Pue CA, Zhou Z et al. Lung disease associated with the IVS8 5T allele of the CFTR gene. Am. J. Respir. Crit. Care Med. 2000; 162:1919–24. 130. Keatings VM, Cave SJ, Henry MJ et al. A polymorphism in the tumor necrosis factor-a gene promotor region may predispose to a poor prognosis in COPD. Chest 2000; 118:971–5. 131. Vinall LE, Fowler JC, Jones AL et al. Polymorphism of human mucin genes in chest disease: possible significance of MUC2. Am. J. Respir. Cell Mol. Biol. 2000; 23:678–86. 132. Meyer UA. Pharmacogenetics and adverse drug reactions. Lancet 2000; 356:1667–71. 133. Wolf CR, Smith G, Smith RL. Pharmacogenetics. Br. Med. J. 2000; 320:987–90. 134. Drazen JM,Yandava CN, Dube L et al. Pharmacogenetic association between ALOX5 promoter genotype and the response to anti-asthma treatment. Nat. Genet. 1999; 22:168–70. 135. Roses AD. Pharmacogenetics and the practice of medicine. Nature 2000; 405:857–65. 136. Liggett SB. Genetics of b2-receptor variants in asthma. Clin. Exp. Allergy 1995; 25:89–94.

Pulmonary Physiology

Chapter

5

Neil B. Pride National Heart and Lung Institute, Imperial College of Science, Technology and Medicine, London, UK

Asthma is conventionally regarded as a disease confined to the airways, probably involving narrowing of all sizes of airways. The magnitude of narrowing usually varies considerably over time, either spontaneously or following treatment, but can become persistent. In contrast, airflow obstruction in chronic obstructive pulmonary disease (COPD) is due to a variable combination of predominantly fixed obstructive disease of the peripheral airways and loss of airway distending forces due to destructive changes in the lung parenchyma. Inflammatory changes in and around the airway wall are widespread in both asthma and COPD, and are also found in alveolar walls when emphysema is present. This chapter concentrates on the pathophysiology of the lungs in COPD and asthma.

A I R WAY F U N C T I O N Tests of overall airway function, whether measured during tidal breathing or forced expiration, reflect the summed effects of the total luminal cross-sectional areas of the airways which are available for gas flow. Before considering assessment by these tests, the factors which determine luminal caliber of an individual airway will be outlined. Factors determining airway caliber: area–transmural pressure curves At any lung volume, the luminal area of an airway depends on a combination of (1) luminal contents, such as mucoinflammatory plugs; (2) structural features of the airway wall; (3) functional features, notably the contractile state of airway smooth muscle; and (4) the effective transmural pressure of the airway (Ptm), which, except in central airways during forced expiration, tends to distend the airway. Therefore the cross-sectional intraluminal area (A) is determined by the elastic properties of the airway (i.e. its wall compliance which is the product of (2) and (3) above) and Ptm. Invivo secretions may occupy some of the lumen. A/Ptm curves are characteristically S-shaped with limits to their maximum size, perhaps set by the fully stretched dimensions of the basement membrane, and sometimes also limits to

minimum size preventing complete closure. Increases in airway caliber with lung inflation are determined by the accompanying increase in Ptm. Airway smooth muscle (ASM) The large differences in bronchoconstrictor responsiveness found in vivo cannot be explained by the differences in the contractility of human ASM in vitro as conventionally measured under static conditions; human ASM excised from normal lungs invariably contracts in response to bronchoconstrictor drugs such as histamine, and it has been difficult to demonstrate large interindividual differences in this response, even in the limited studies available using ASM from subjects with asthma.1 However, increased velocity of contraction has been shown in ASM derived from allergen-sensitized animals.2 Of equal potential importance, the behavior of ASM has been shown to be highly dependent on the volume and time history to which it is subjected.3 Thus it has been suggested that the normal tidal excursions of breathing4 (with occasional deep breaths) keep ASM in a relatively high-compliance state but that its state stiffens with lack of stretch. The direct relevance of these findings to asthma remain to be established (see Chapter 17). The most consistent abnormality of ASM is the increased mass found in asthma5 and, to a lesser extent, in COPD.6 These changes are responsible for some of the increase in wall thickening seen in both diseases. Thickening of airway wall Enhanced reductions in luminal caliber can occur with normal shortening of ASM if there is thickening of the airway wall internal to the contracting muscle. On average, the thickness of the airway wall was doubled in lungs from 18 subjects with fatal asthma.7 These changes involved all sizes of airways and all layers internal and external to the ASM. Calculations suggest that these changes would greatly enhance the effects of constrictor challenge but would hardly increase basal airway resistance.7 Less dramatic airway wall thickening was found in lungs from subjects with asthma who died of nonrespiratory causes8 and in patients with COPD9,10. It has also been hypothesized that rapid

44

Asthma and Chronic Obstructive Pulmonary Disease

expansion of the blood volume in and around the airway wall, acutely increasing its thickness, might be responsible for the airway narrowing that occurs after exercise or isocapnic hyperventilation in asthmatic subjects.11 Thickening of the wall external to ASM increasing the external perimeter may have effects of lung airway coupling. Airway wall compliance An increase in airway wall compliance would amplify the effects of ASM contraction, whereas a reduction might restrict the extent of narrowing with ASM contraction. In central intrathoracic but extrapulmonary airways, cartilage restricts the extent of luminal narrowing. In the trachea, the attachments of the muscle to the cartilage rings, at least in experimental animals, are such that smooth muscle contraction results in the formation of complete cartilage rings encircling the lumen.12,13 In the central conducting airways there are separate plates of cartilage, but these are also brought closer together to “fortify” the airway walls by ASM contraction. Apart from reducing wall compliance, this may limit narrowing and prevent luminal closure. As discussed above, contraction of ASM itself may directly decrease wall compliance.3,14 Conversely, occasional paradoxical decreases in maximum expiratory flow after bronchodilators in normal subjects have been attributed to enhanced collapsibility of central airways due to loss of the stabilization provided by the normal tonic contraction of ASM.15 A further factor preventing complete closure of the lumen is folding of the mucosa, preserving some lumen between the folds.16 There have been few direct measurements of compliance of individual airways in COPD or asthma. In COPD it would be expected that compliance of large conducting airways would be increased due to loss of cartilage and this would amplify the tendency for central airways to narrow when there is expiratory flow limitation. In the peripheral airways, fibrosis and other pathological changes would be expected to reduce their wall compliance. Surprisingly, tantalum bronchograms obtained at necropsy in lungs from patients dying with severe airflow obstruction showed no reduction in compliance of medium- and small-diameter airways at a standard static lung recoil pressure (PL); reduced compliance was only found in airways less than 1 mm diameter in emphysematous lungs.17 In contrast, in asthma direct intrabronchial measurements in humans have shown lower compliance of central airways in young adults than in healthy controls.18 Limited high-resolution computed tomography (HRCT) studies have shown normal expansion of large airways with lung inflation in mild asthma19 but reduced expansion with more severe asthma, presumably reflecting reduced wall compliance rather than a reduced increase in effective Ptm. Extra-airway distending forces: airway perimeter–lung coupling For the intrapulmonary airways, the most important factor stabilizing the airway wall against the effects of ASM contraction is the attachment of alveolar walls to the external perimeter of the airway wall.20 Theory, and most experimental work, suggest that these extra-airway forces would have their

greatest stabilizing effect at large lung volumes, so that a given amount of activation of ASM would lead to greater shortening at small rather than large lung volume. In humans in vivo, the magnitude of maximum bronchoconstriction to inhaled methacholine is greater at small than at large lung volume and is quite sensitive to small changes in volume above and below functional residual capacity (FRC).21 However, the magnitude of airway narrowing produced by submaximal doses of methacholine appears to be unchanged by moderate changes in lung volume, suggesting that ASM initially may contract freely and that the restraints applied by surrounding lung chiefly act to prevent extreme narrowing or closure.21,22 These restraining forces may be reduced by the modest loss of lung elastic recoil pressure (PL ) found in asthma; coupling between alveoli and airways may also be reduced by enlargement of the outer perimeter of the airway wall or by surrounding inflammatory changes. In COPD, loss of PL is much greater, while breaks in alveolar attachments to the airway perimeter23,24 and peri-airway inflammation would all be expected to reduce restraints to narrowing of smaller airways. There are few direct measurements demonstrating these mechanisms, but they provide a basis for interpreting the significance of changes in tests of “lumped” airway function and the causes of airway hyperresponsiveness. Airway responsiveness Bronchodilator response In clinical practice and in many trials of treatment, the size of the increase in airway function (usually FEV1) after inhaled bronchodilators (either b-agonists alone, or a combination of b-agonists and anticholinergic drugs) is used to make a practical distinction between asthma and COPD. Most patients with COPD show some improvement in airway function after treatment with bronchodilator drugs;25 in contrast to asthma, the response to anticholinergic drugs is similar to (some would say superior to) that to b-agonists.26 This suggests that most of the reversible component in COPD is due to vagal tone;25 and, because vagal motor innervation does not extend to the most peripheral airways, involves more central airways than those in which the major obstructive changes occur.This idea has been supported by a study using an intra-airway catheter.Total lung resistance was reduced equally (30%) by atropine and a b-agonist (fenoterol) aerosol; the b-agonist reduced resistance in all airways to a similar extent but atropine had a larger dilator effect in central than in peripheral (<3 mm diameter) airways.27 It is uncertain whether cholinergic “tone” is greater in COPD than in normal subjects; the absolute increment in FEV1 after antiCh cholinergic treatment is similar in COPD and normal subjects and a study of sensory stimulation of the nose failed to show an enhanced vagally induced bronchoconstriction in COPD compared with normal subjects.28 There is considerable day-to-day variation25 in the size of the bronchodilator response. Response to bronchodilators is usually assessed by changes in FEV1, maximum expiratory flow-volume

Pulmonary Physiology

(MEFV) curves, or resistance (conductance), and there appear to be no distinctive features between the response in COPD and in asthma, although in both the percentage change in resistance is usually greater than in FEV1. Improvement in airway function is usually accompanied by a decrease in FRC and an increase in inspiratory capacity and maximum inspiratory flow in both asthma and COPD; these changes may be more relevant to any improvement in exercise performance. Discrepancies between small changes in maximum flow and in volumes (FRC; residual volume, RV; and vital capacity, VC) probably account for some of the difficulty in predicting improvement in exercise tolerance after bronchodilators in COPD from resting measurements.29 Bronchoconstrictor response Airway hyperresponsiveness (AHR) to inhaled stimuli is a central feature of asthma, included in its definition, but is also found in COPD. Speculations on the origins of AHR in asthma have fluctuated over recent decades, although all give a central role to ASM contraction (Table 5.1). Whatever the initiating mechanism, the effect will be enhanced if baseline airway dimensions are reduced. Abnormalities due to inflammation of airway smooth muscle have predominated as explanations for AHR in asthma, while altered “geometric” or structural changes are regarded as important for AHR in COPD. AHR is characterized by an increase in threshold or sensitivity (lowest stimulus at which contraction is observed), an increase in reactivity (increase in airway narrowing for a given change in stimulus), and a loss of the plateau of maximum narrowing which is found in normal subjects.30,31 This plateau is lost in both asthma and COPD unless disease is very mild. Although reactivity may be lower in COPD than in asthma, in practice almost all information on AHR has been obtained in terms of the stimulus required to give a standard percentage change in airway function (most commonly 20% fall in FEV1) which is chiefly an indication of sensitivity. Patients with COPD show AHR to challenge with spasmogens such as inhaled histamine or methacholine and most of the other “indirect” stimuli used to provoke airway narrowing in asthma. The presence and intensity of AHR in

45

COPD is inversely related to baseline FEV1 (“geometric effect”).32,33 Bronchodilator responses are now usually reported as increase in absolute values (preferably as percentage predicted values) because percentage increase over baseline is enhanced as baseline declines. Bronchoconstrictor responses, however, are still reported as the dose required to produce a given change from baseline; it is widely assumed that when baseline airway function is reduced a smaller absolute reduction in airway cross-sectional area is required to produce a given percentage change from baseline in FEV1 or resistance. In addition more central deposition of inhaled drugs may accentuate the response. More specific structural changes, such as increased thickness of the airway wall (although this is less than in asthma), increase in ASM mass and isometric contractility,6 and loss of support to airways as a result of destructive changes in alveoli attached to the external airway perimeters34,35 may all contribute. The last mechanism has been most clearly shown in a study of methacholine response in patients with alpha 1-antitrypsin deficiency who retained a plateau of maximum narrowing; the plateau value of FEV1 decreased as PL decreased.36 Overall, however, no difference has been shown in AHR to histamine between patients with emphysema and those with predominant airway disease.37 The intensity of hyperresponsiveness in smokers predicts accelerated decline in FEV1, independent of the baseline spirometry.38 In contrast to asthma, almost all studies in COPD have failed to show attenuation of AHR by treatment for 2–3 months with inhaled corticosteroids;39 however, a small attenuation of AHR was shown after 3 years’ treatment in the Lung Health Study.40 In asthma, apart from inflammation and abnormal ASM, structural changes in the airway wall and reduced lung–airway coupling probably also contribute to AHR. Inflammation may be the initiating stimulus for increase in ASM mass and other structural wall changes, but it is unknown whether these changes regress if inflammation is suppressed for a long period. An inverse relation between baseline FEV1 and the intensity of AHR is found also in asthma and extends into the normal range of FEV1.41–43 As discussed below, peripheral lung resistance may be increased even when tests of overall airway function are normal. For a given level of FEV1, AHR is more intense in asthma than in

Table 5.1. Possible causes of airway hyperresponsiveness

Functional change

Implied mechanism

Enhanced mediator or neural responsiveness in airway mucosa

Mucosal inflammation

Increased contraction of ASM due to increased contractility and/or mass

Abnormal airway smooth muscle

Thickening of airway wall

Enhances effect of ASM contraction

Reduced lung–airway perimeter coupling

Loss of normal mechanism restricting airway narrowing

46

Asthma and Chronic Obstructive Pulmonary Disease

COPD (Fig. 5.1),42 presumably reflecting additional effects of airway inflammation and changes in ASM in asthma. This overlap in AHR between asthma and COPD can be interpreted as supporting the Dutch hypothesis.44 AHR related to atopy and predating the onset of smoking is common in the population and inevitably will occur in some patients with COPD. However, cross-sectional studies suggest that the prevalence of AHR increases in middle age in nonatopic smokers,45 so it remains possible that AHR is sometimes acquired and has a different pathogenesis in COPD than in asthma. Site of constrictor response In asthmatic subjects, induced airway narrowing is very heterogeneous among parallel airways, as shown by scans of regional ventilation which suggest that this heterogeneity occurs in conducting airways. High-resolution computerized tomography confirms airway narrowing is heterogeneous and involves all sizes of airways in both normal46,47 and asthmatic47 subjects. A potentially important area is the involvement of lung tissue in the constrictor response. In animals, increases in lung tissue resistance have accounted for a large part of the increase in pulmonary resistance after bronchoconstrictor aerosols, sometimes exceeding the contribution of narrowing of the conducting airways.48 The origins of the increased lung tissue resistance are uncertain; one possibility is contractile elements in the lung periphery, such as smooth muscle or myofibroblasts in alveolar ducts or small pulmonary vessels. But recent work suggests that heterogeneity of airway narrowing and constricted conducting airways may contribute to local distortion of alveolar expansion and geometry which may be responsible for peripheral lung effects.49–51 Studies are required to establish whether animal models of asthma involve very different sites of increase in resistance from those found in humans.

Log PC20 (mg/mL)

2

0

1

COPD

2

Asthma

0 0

20

40

60

80

100

FEV1 (% predicted) Fig. 5.1. Regression lines (95% CI) of log PC20 for histamine against baseline FEV1 (% predicted) in 81 subjects with asthma and 44 with COPD. Reproduced from reference 42, with permission.

Site of increased airflow resistance during tidal breathing COPD In mild disease, changes are found in the lung periphery. These changes are detected as nonuniform behavior of the lungs.52 As disease advances, an increase in peripheral lung resistance develops53 while total airway function remains within the normal range.When moderate airway obstruction (mean FEV1 50–60% of predicted) has developed, studies using an intrabronchial catheter have found that slightly more than half the total pulmonary resistance resides in airways of less than 3 mm internal diameter (Table 5.2).54 This proportion was similar for a group with predominant airway disease (“chronic bronchitis”) and for a group with physiological evidence of emphysema; despite a lower FEV1 in the emphysema group, total lung resistance was lower than in the chronic bronchitis group. For severe disease the best information comes from study of lungs at necropsy. Most of the increase in resistance in lungs of patients dying with COPD is in the peripheral airways of less than 2 mm diameter; on average these account for 75% of the total resistance at lung volumes comparable with those during tidal breathing in life.55–57 Despite pathological changes in their walls, such as mucus gland hypertrophy, the resistance of central airways is increased in only a minority of lungs,58 and even then it is associated with a proportionally greater increase in peripheral airway resistance. Although the increase in resistance is larger during expiration, inspiratory resistance is also increased. These post-mortem studies imply a reduction in peripheral airway dimensions even at a standard PL. This could result from intrinsic disease of the airways (obliteration and narrowing of airway lumina by scarring, wall thickening, inflammation, mucus plugs) or from loss of airway distension caused by a loss or breaks of the attached alveolar walls. In life there is an additional labile component to resistance, presumably resulting from bronchial muscle contraction (“vagal tone”) or mucosal edema, which probably involves more central airways. (See the section on airways responsiveness.) Nevertheless, the available evidence suggests that, when total airway resistance (measured at low flow, for example in a whole-body plethysmograph) is increased, in most patients with COPD it is mainly caused by a very large increase in peripheral airway resistance. A striking feature is the great parallel inhomogeneity of airway narrowing (and also of alveolar destruction) which results in great unevenness of ventilation – as can be visualized on a coarse basis by ventilation scans using radiolabeled gas or aerosols. In emphysema there is also a large increase in collateral ventilation,58,59 presumably reflecting a combination of hyperinflation and gas movement through breaks in alveolar walls; this should allow some gas exchange beyond occluded airways and prevent atelectasis. Asthma The airway narrowing in episodic asthma is due to varying combinations of smooth muscle contraction, mucosal

47

Pulmonary Physiology

Table 5.2. Peripheral lung resistance (Rp) as a percentage of total lung resistance (RL) in normal subjects, chronic persistent asthma, and COPD

Subjects (no.)

RL

Rp

Mean age (years)

FEV1 (% predicted)

Normal (5)

56

88

3.1

0.7

24

Chronic persistent asthma (10)

57

54

9.2

4.1

44

COPD Chronic bronchitis (7)

64

51

8.7

4.8

55

63

39

5.9

3.0

50

Emphysema (8)

(cmH2O per liter per second)

Rp/RL (%)

Reproduced from reference 54, with permission.

swelling, and luminal secretions. Pathological studies suggest that inflammation of the airway mucosa extends throughout the tracheobronchial tree, indicating the potential for narrowing to develop in all airway generations. Attempts to determine the serial site of airway narrowing in asthma have been relatively inconclusive. Narrowing of the large airways has been observed on occasions when bronchoscopy or bronchography have precipitated an asthmatic attack. Although endobronchial pressure measurements are the obvious approach to define the distribution of airflow resistance, intra-airway catheters may stimulate mucosal receptors and induce reflex bronchoconstriction and mucus production and can be used only in central airways without compromising airflow through the catheterized airway. While in a few studies intrabronchial catheters have been placed in central airways, more information has come from studies using a wedged-catheter. In one version, a 3-mm catheter is wedged in a right lower lobe bronchus and lateral pressure measured proximally. With this technique total lung resistance in normal subjects appears about 50% higher than without the presence of a catheter. In asymptomatic young subjects during remission of asthma, peripheral airways resistance remained slightly increased;27,54 in middle-aged patients with persistent airflow obstruction due to chronic asthma, there were increases in both central and peripheral resistance (Table 5.2).54 Allowing for the contribution of the extrathoracic airway to total pulmonary resistance, peripheral resistance accounts for about one-third of intrathoracic resistance in normal subjects and for about 50% in the subjects with chronic, persistent asthma. Another technique measures pressure–flow relations in the occluded lung beyond a bronchoscope wedged in a segmental bronchus. In asthmatic subjects in remission with normal total airways resistance and FEV1, a considerable increase in peripheral lung resistance has been found;60 this technique measures the combined resistance of peripheral airways and collateral channels. These results directly confirm earlier suggestions from pathological and physiological studies that there are residual

changes in the peripheral airways even in remission of asthma, when overall lung function is normal. As in COPD, a striking (and life-saving) feature of attacks of asthma is the enormous parallel inhomogeneity of ventilation resulting in some areas of the lung remaining available for gas exchange. This has been shown by ventilation scans, is implied by detailed analyses of pulmonary gas exchange and mechanics, and heterogeneity of individual airway constrictor responses after inhaling histamine has been directly shown using HRCT. Whereas in COPD these features may be attributed to inhomogeneity of “fixed” pathological changes, there is no such obvious explanation in asthma, nor is it known whether the pattern of inhomogeneity “repeats” or is highly variable between successive episodes. Site of flow limitation during forced expiration In both COPD and asthma, expiratory flow limitation (EFL) is found at all lung volumes during forced expiration; maximum expiratory flow is reduced at all lung volumes and is achieved with lower driving pressures than in normal subjects. Reduction in PL in emphysema enhances dynamic narrowing of the large intrathoracic airways on forced expiration so that there is much greater reduction in maximum expiratory flow than in maximum inspiratory flow.61 In the 1970s there was considerable interest in trying to localize the serial site of airflow limitation in airway disease by measuring the increase in maximum expiratory flow when the density of the expired gas was reduced by breathing a mixture of 80% helium and 20% oxygen.62 If maximum expiratory flow does not show the normal 60% increase when breathing helium–oxygen, this suggests that the major site of flow limitation is no longer in the central airways (as in normal subjects), but has moved to more peripheral airways where flow is presumed to be laminar and independent of density. This change is usually attributed to increased frictional pressure losses in narrowed peripheral airways.62 Most patients with COPD, even those with mild disease, showed a smaller (or absent) rise in maximum expiratory flow when breathing helium.63 Some asthmatic subjects consistently lose or consistently retain density

48

Asthma and Chronic Obstructive Pulmonary Disease

dependence of maximum flow with repeated attacks,64,65 but this is not invariably the case; in general, loss of density dependence becomes more common as expiratory airflow limitation increases in severity and is particularly observed in asthmatic subjects who smoke.66 Reduced density dependence of maximum expiratory flow should not be interpreted as indicating that only peripheral airways are involved, even if they are the site of flow limitation.52 Theoretically, the method analyzed the changes in the airways between the alveoli and the sites of expiratory flow limitation (“choke-points”), but it lost popularity when experimental studies showed that relatively small changes in geometry and position of choke-points could profoundly affect the helium response67 and that there was considerable variation in size of the baseline helium response in the normal population and in disease. Expiratory flow limitation during tidal breathing In normal subjects, levels of expiratory flow generated during tidal breathing at rest are much below maximum expiratory flow and only approach maximum levels during exercise at very high work loads. However, when maximum expiratory flow is severely reduced in COPD or in asthma, this limit may be reached during mild exercise and, when disease is severe, even at rest. Dynamic expiratory narrowing of central airways then develops and airflow resistance measured during expiration becomes unreliable as an indicator of “fixed” airway caliber. A simple test has been proposed to identify expiratory flow limitation (EFL) by applying a negative pressure of 5 cmH2O at the airway opening during tidal expiration and showing that this does not increase expiratory flow.68 Tidal EFL has been found in both asthma and in COPD when airflow obstruction is moderately severe.When EFL is present during tidal breathing at rest, the most effective way to increase total ventilation during exercise is to breathe at a larger lung volume, increasing end-expired lung volume (dynamic hyperinflation).

flow obstruction has developed, even if the predominant site of airway narrowing is in the peripheral airways, as in COPD, values of PEF and maximum flow throughout the vital capacity are reduced. Although changes in conductance and maximum expiratory flow are broadly related, their scaling need not be identical and some information can be obtained from empirical comparisons, particularly if patients with predominant emphysema are compared with asthma. Thus, for a given decrease in FEV1, resistance may be lower (see Table 5.2)54 and peak expiratory flow and maximum inspiratory flows higher in emphysema61 than in asthma. Of course in clinical practice the distinction between asthma and COPD is usually based on spontaneous or drug-induced variation (or lack of variation) in airway function. Summary See Table 5.3. Table 5.3. Site of airway narrowing during tidal breathing

• In COPD, the peripheral airways are the major site of fixed obstruction. When expiratory flow limitation develops, there may also be dynamic narrowing of central intrathoracic airways during expiration. • In acute asthma, distribution of airway narrowing is unknown but probably involves many sizes of airways. • In chronic asthma, there is narrowing of both central and peripheral airways. • Peripheral airway obstruction may persist during clinical remission of asthma. • In both asthma and COPD, extent of narrowing varies greatly between parallel airways.

A LV E O L A R F U N C T I O N Use of tests of airway function to distinguish asthma from COPD Many pathological changes in the airways distinguish asthma and COPD, but available simple measures of airway function have very limited ability to provide information on the site or nature of airway narrowing. Tests of overall airway function such as airway conductance or FEV1 breathing air reflect the dimensions of all the airways in series, but do not provide direct evidence on whether large or small airways are the site of airway narrowing. Because the resistance of peripheral airways at large lung volumes in normal lungs is low, for many years peak expiratory flow (PEF) has been regarded as reflecting mainly large airway function in normal subjects; but examining the relative changes in maximum flow on MEFV curves at large versus small volume only provides useful information about the site of airway disease when there are minor deviations from normal function. Once obvious air-

Perhaps the most striking distinction between asthma and COPD is that emphysema is not believed to develop in nonsmokers with asthma, even when asthma is longstanding and associated with persistent airway obstruction. This conclusion was originally based on post-mortem studies, but it is broadly supported by two tests of lung function – the carbon monoxide transfer coefficient, and the static expiratory pressure/volume curve of the lungs – both of which show less abnormality in asthma than in many patients with COPD. However, only in the last decade have studies of lung function been tested against a contemporary assessment of alveolar structure, as obtained by morphometry of lobectomy specimens or by HRCT; the latter shows permanent changes suggesting emphysema, not only in smokers, but sometimes also in never-smokers with the diagnosis of asthma.69,70 Abnormalities on HRCT scans have been reported also in many unusual diseases causing bronchioli-

Pulmonary Physiology

tis; these appear to be related to air trapping and locally reduced blood volume in the alveoli.71 At present it is not clear whether peripheral airway disease in COPD and asthma also produces distinctive changes in the HRCT scan independent of those due to emphysema. Pulmonary gas exchange Ventilation–blood flow imbalance: arterial blood gases The large alveolar–arterial PO2 difference found in both COPD and asthma is almost entirely due to ventilation– blood flow (V˙A/Q˙) imbalance. In both conditions, scans of pulmonary perfusion show gross regional abnormalities of blood flow in areas which are poorly ventilated giving broadly “matched” defects for the reduction in blood flow. In COPD various mechanisms have been proposed, whose relative importance is unknown: vascular remodeling or destruction in areas of emphysema, active constriction of the vessels in areas of severe alveolar hypoxia, or passive obstruction by the effects of increased alveolar pressure and distension. The latter two mechanisms might also be relevant in acute asthma. The altered distribution of flow is not confined to the gross abnormalities seen on scans, but is accompanied by much more subtle inequalities within a region. Changes in pulmonary blood flow do not, however, compensate fully for the unevenness of ventilation, so that gross inefficiency of pulmonary gas exchange develops. Some relatively subtle differences in the defects in gas exchange between COPD72 and asthma73 have been found using the multiple inert gas elimination technique (MIGET). In general, for a given severity of airways obstruction as assessed by FEV1, patients with asthma have less hypoxemia than patients with COPD. In hypoxemic patients with COPD, a large proportion of the blood flow perfuses lung with V˙A/Q˙ ratios under 0.1; such low ratios are not found in asthma, suggesting a greater compensatory reduction in blood flow to poorly ventilated areas in asthma (Fig. 5.2). A further factor is the increased total ventilation in severe asthma, although this has more effect on arterial PCO2. “Anatomical” shunt (V˙A/Q˙ ratio = 0) is unusual in COPD,72,74 except in the most severe exacerbations requiring assisted ventilation, despite probable complete occlusion of many peripheral airways. This suggests considerable collateral ventilation,58,59 which in part may be via destroyed alveolar walls. Even more surprisingly, anatomical shunt is not found in severe asthma, although in such attacks larger airways are often occluded by muco-inflammatory plugs and collateral ventilation is presumably confined to the pathways present in normal lungs. There is no evidence of a diffusion defect (difference between alveolar and end-pulmonary capillary PO2) at rest in either COPD74 or asthma.73 When 100% O2 is breathed there is an increased dispersion of pulmonary blood flow in both COPD75 and asthma,76 suggesting release of hypoxic pulmonary vasoconstriction. A striking difference between COPD and asthma is that chronic elevation of arterial PCO2 is rare in asthma, even when there is persistent airflow obstruction. The basic

49

mechanism raising arterial PCO2 when there is airflow obstruction is V˙A/Q˙ imbalance which reduces the efficiency of CO2 excretion in the same way that it impairs O2 uptake. However, arterial PCO2 can be kept normal by a small increase in total ventilation, which has a much smaller effect in preventing a fall in arterial PO2. The precise reasons for chronic elevation of PaCO2, occurring commonly in COPD but rarely in asthma, are unclear. Chronic hypercapnia occurs mainly in patients with COPD whose FEV1 is under 1.2 liters,77 a severity of obstruction which is associated with severe hyperinflation and mechanical disadvantage of the inspiratory muscles;78 changes of this magnitude are not often seen in chronic persistent asthma. In acute episodes of asthma, most patients increase total ventilation sufficiently to maintain arterial PCO2 in or below the normal range until a late stage, but a significant minority of patients develop hypercapnia.79 These patients tend to present with a similar PCO2 in each episode and may have a blunted ventilatory response. See Table 5.4 for a summary. Table 5.4. Pulmonary gas exchange

• Impaired O2 uptake and CO2 excretion are dominated by an imbalance in ventilation and blood flow in both COPD and asthma. • For a given severity of airflow obstruction, hypoxemia is less in asthma than in COPD, possibly reflecting greater pulmonary vasoconstriction in poorly ventilated areas in asthma. • Increased ventilation in asthma delays development of hypercapnia as airflow obstruction worsens.

Carbon monoxide transfer Tests of CO uptake were introduced in the 1950s in an attempt to mimic the lungs’ ability to take up O2 at rest and during exercise. Initially CO transfer was directly measured during tidal breathing by analyzing the difference between the concentration of an inspired CO mixture and mixed expired CO. However, this method was extremely sensitive to unevenness of ventilation, gave similar reduced CO uptake values in asthma and COPD, and did not reflect the true gas transfer characteristics of the alveoli. The single-breath CO transfer test (TLCO,sb) – also called CO diffusing capacity (DLCO) – measuring CO uptake during breath-holding at full inflation (total lung capacity, TLC) in contrast discriminates between asthma and emphysema. In this method TLCO is obtained as the product of the CO transfer coefficient (KCO) – also abbreviated as DLCO/VA or TLCO/VA – and the alveolar volume (VA). In airways obstruction, the appropriate alveolar volume is uncertain; the inspired CO mixture will tend to go to the best ventilated areas of the lung which will contribute disproportionately to the expired gas used for analysis so that

50

Asthma and Chronic Obstructive Pulmonary Disease

Normal

Episodic asthma

1.5 0.9

1.0

0.6

0.5

0.2

Shunt 0.0

0.0 0.0

0.1

1.0

100

10

0.0

Chronic severe asthma

0.01

1.0

100

Acute severe asthma

Ventilation and blood flow (L/min)

1.2 0.6

1.0 0.8

0.4 0.6

0.4

0.2

0.2 0.0

0.0 0.0

0.01

1.0

100

0.0

COPD type A

0.1

1.0

10

100

COPD type B 2.0

1.2

0.8 1.0 0.4

0.0 0.0

0.01

0.1

1.0

10

100

0.0

0.01

0.1

1.0

10

100

Ventilation/blood-flow ratio

Fig. 5.2. Frequency histograms of the ventilation/blood-flow ratios, calculated by the multiple inert gas technique, in a normal subject, in three patients with asthma, and in two patients with COPD. Note the different scaling of both axes in the various examples. The normal distribution of ventilation (open circles) and blood flow (solid circles) is over a narrow range of ventilation/blood-flow ratios and is unimodal. Patients with mild or chronic severe asthma have broader but still unimodal distribution of ventilation and blood flow, whereas those with acute severe asthma have a bimodal distribution of blood flow centered around V˙A/Q˙ ratios of 1.0 and 0.1. In COPD type A (emphysema), there is bimodal distribution of ventilation with areas of high V˙A/Q˙ ratio. In COPD type B (chronic bronchitis), blood flow distribution is bimodal with many alveolar units with V˙A/Q˙ ratio 0.1. Shunt (V˙A/Q˙ = 0) was absent in all patients. Reproduced from references 72 and 73, with permission.

Pulmonary Physiology

KCO is weighted towards the least diseased areas within an inhomogeneous lung. Smoking itself reduces T LCO by two distinct mechanisms: • Increases in blood CO, which reduces the pressure gradient across the alveolar–capillary membrane for CO uptake. • Independent of this effect, there is a roughly 10% reduction of T LCO in current smokers,80,81 which rapidly reverses after quitting and probably is due to a reduction in pulmonary capillary volume. In patients with airways obstruction and no additional lung pathology, KCO is a surrogate measurement of the perfused alveolar surface area per unit volume available for CO uptake. In COPD, reduction in KCO is related to the severity of “microscopic” emphysema (and hence reduced alveolar surface area) in the remaining functional lung,82,83 rather than to the extent of gross destructive changes.The fall does not indicate a true diffusion defect in the lung (failure for end-capillary PO2 to equilibrate with alveolar PO2).When airflow obstruction in COPD is predominantly due to airway disease, KCO is normal or only slightly reduced. In asthma, KCO is well preserved and even tends to increase above predicted values as FEV1 decreases.84,85 A possible explanation for this increase is that pulmonary vasoconstriction in poorly ventilated parts of the lung causes a redistribution of cardiac output to better ventilated areas. This leads to a local increase in pulmonary capillary volume and KCO, analogous to that which occurs throughout the lung when total cardiac output increases during exercise. Values of KCO in asthma and emphysema therefore tend to diverge as FEV1 falls. Because the measurements of KCO and T LCO,sb are made under the artificial circumstances of breath-holding at full inflation, they are predominantly indicators of lung structure rather than the efficiency of pulmonary gas exchange. See Table 5.5 for a summary. Table 5.5. Carbon monoxide transfer coefficient (KCO)

• In smokers there is a small reduction (about 10%) in KCO which reverses on quitting. • Reduction in KCO in COPD is related to severity of “microscopic” emphysema in surviving ventilated lung. • In asthma, KCO is normal and tends to increase slightly as airflow obstruction becomes more severe.

LUNG ELASTICITY AND LUNG VOLUMES Dynamic lung compliance Uneven ventilation of the lung leads to reduction in dynamic lung compliance. Indeed, a reduction in compliance as

51

breathing frequency is increased has been proposed as one of the most sensitive techniques for detecting mild airway disease, whether this is caused by asthma or by smoking.86 When significant airflow obstruction develops, dynamic compliance is considerably reduced during tidal breathing in both asthma and COPD. Static elastic properties of the lungs: expiratory pressure–volume curves COPD The similar reductions in dynamic compliance in asthma and all forms of COPD obscure considerable changes in the static elastic properties of the lungs in patients with severe emphysema. Two abnormalities of the static deflation pressure–volume (PV) curve of the lungs are thought to distinguish emphysema from airway obstruction solely caused by intrinsic airway disease: • an altered shape of the curve; • displacement of the whole PV curve to higher absolute volumes. Studies in the early 1980s showed that the plethysmographic technique may overestimate true lung volume in the presence of airway obstruction;87,88 therefore much of the earlier literature is difficult to interpret. Nevertheless, it seems likely that displacement of the PV curve to larger volumes is a much more prominent feature of emphysema than of intrinsic airway disease. Modeling the deflation PV curve as a single exponential allows a shape factor (k) to be obtained, which is virtually independent of the volume, relative expansion, and maximal distending pressure of the lungs, and so is not influenced by artefacts in the plethysmographic method.89 An increased value of k indicates an increase in static lung compliance, which is conventionally measured between FRC and (FRC + 0.5 L). But because static compliance varies with lung size and the volume range over which it is measured, comparisons of k are simpler. There seems little doubt that k is higher in emphysema than in asthma (Fig. 5.3),90,91 although not all studies have shown a clear relationship between k and the morphological changes of emphysema in lungs studied at post-mortem or after surgical removal.92–94 Because the worst areas of emphysema are poorly ventilated, changes in the PV curve reflect changes in the microscopic structure of the remaining functioning lung which accompany, and presumably precede, the gross morphological changes. Similar (but much smaller) changes occur with normal aging.89 Changes in the static PV curve of the lungs are partly responsible for increases in FRC and RV in COPD and are probably essential for any increase in TLC to occur. An increase inTLC acquired in adult life is suggested by loss of the zone of apposition of the diaphragm on images taken at full inflation. Loss of lung recoil pressure occurs at all volumes and leads to an increase in the neutral “relaxation” volume (Vr) of

52

Asthma and Chronic Obstructive Pulmonary Disease

the respiratory system (FRC); Vr may be further increased by reductions in chest wall recoil pressure.95 In patients with severe expiratory airflow obstruction, whether this is due to asthma or COPD, end-expiratory volume may be determined by dynamic factors rather than by the balance of static forces. Low values of expiratory flow then result in expiration being terminated by the initiation of the next inspiration before the respiratory system reaches Vr. In these circumstances, if the airway is occluded at end-expiration a positive alveolar pressure is present (“intrinsic” positive end-expiratory pressure, PEEPi) as a result of the effects of passive recoil of the lungs and chest wall. The increase in end-expired lung volume assists gas exchange by reducing the proportion of the respiratory cycle associated with very low mass movement of gas from the lungs, but the “dynamic hyperinflation” adds to the load placed on the inspiratory muscles. A combination of airway closure or near-closure and loss of lung recoil accounts for the increase in RV in severe COPD. The time to empty the lung is greatly prolonged with expiratory flow continuing at very low levels, presumably through dynamically narrowed airways, so that RV is limited finally by the breath-holding ability of the individual.

H

100

80

Mean k 0.414 cmH2O1 60

40 0 0

8

16

24

Chronic bronchitis CB

32

H

100

80

VL (% TLC)

Asthma Although asthma has been regarded as a disease confined to the airways without significant pathological changes in the lung tissue or air spaces, one transbronchial biopsy study has found alveolar tissue inflammation96 and, as discussed above, some physiological studies have suggested lung tissue may be involved in bronchoconstrictor responses.50,51 In addition most authors have found k is slightly increased and PL is slightly reduced (Fig. 5.3)90 in asthma; larger reductions in PL have been found by a few authors, including in a recent study97 in patients with chronic persistent asthma who had normal HRCT and CO transfer. Probably some change in the PV curve occurs in all forms of chronic airway obstruction with hyperinflation, perhaps due to stress relaxation.98 The consistent preservation of a normal or even increased value of carbon monoxide transfer coefficient (KCO) in nonsmokers with chronic asthma would not be expected in the presence of significant emphysema. Regardless of its cause, any loss of lung recoil pressure, together with enlargement of the airway outer diameter and periairway inflammation, would reduce the distension of the smaller airways by surrounding lung, even although the alveolar attachments themselves were normal. Adults who have had asthma in childhood99 may develop a slightly larger TLC than normal, but any increase in TLC is smaller than in emphysema. Studies before 1980 frequently showed large increases in TLC during acute exacerbations of asthma, but many of these increases were measurement artefacts;87,88 radiographic estimates suggest an increase in TLC of at most 0.3–0.5 liters.100,101 No change inTLC occurs after bronchodilator or bronchoconstrictor challenge.102 Increases in FRC and RV in asthma are mainly due to airway narrowing and closure. Studies with radioactive aerosols

Emphysema E

Mean k 0.206 cmH2O1 60

40 0 0

8

16

24

Asthma A

32

H

100

80

Mean k 0.155 cmH2O1 60

40 0 0

8

16

24

32

40

PL (cmH2O) Fig. 5.3. Range of expiratory pressure (PL)–volume (VL) curves in emphysema (n  24), COPD with primary bronchial disease (chronic bronchitis) (n  19), and asthma (n  24), compared with a range of values in age-matched healthy controls (H) (mean k  0.131 cmH2O1) (stippled areas). Lung volumes are expressed as percentage of TLC, which removes much of the effect of overestimation of lung volume by the plethysmographic technique. Reproduced from reference 90, with permission.

Pulmonary Physiology

show increased and patchy closure at RV in asthma,103 contrasting with the more uniform basal closure found in normal subjects which is believed to occur in the smallest airways. In asthma, closure may be facilitated by intraluminal secretions and/or ASM contraction and so may involve larger airways; in animal studies, closure of conducting airways with contraction of ASM is greater at small lung volumes, but can persist at large lung volumes.104 A large increase in RV with bronchoconstriction may indicate the patient has an increased risk of severe attacks.105 An increase in FRC occurs after bronchoconstrictor challenge even when this does not result in sufficient airway narrowing to cause EFL during tidal breathing;106 at this stage expiration is slowed by glottal narrowing107 and persistent activation of inspiratory muscles.108 The increase in FRC widens the airways and reduces the total work of the respiratory muscles during bronchoconstriction,109 but the precise neurological control mechanisms are unknown. When airflow obstruction increases in severe acute attacks, tidal EFL develops and RV rises and then ventilation can be sustained only by progressive increases in FRC with the development of PEEPi as in COPD. Assessment of contribution of alveolar disease to airway function Tests of maximum flow and resistance measure the combined effects of intrinsic airway disease and alveolar disease on airway function. In normal lungs, PL is a useful indicator of the forces distending the airways during breath-holding or breathing at low flows.110 In some patients with COPD and abnormal conductance–volume slopes, the relationship between conductance and PL remains normal.111–113 In general, the patients showing a normal conductance–lung

Table 5.6 Lung elasticity

• Loss of lung recoil, increased static compliance, and increase in TLC are characteristic of emphysema. • In mild COPD, loss of lung recoil may be the only mechanism reducing airway function. • Impairment of lung–airway coupling may not be fully reflected in disease by value of one lung recoil pressure. • In asthma there are small changes in static elastic properties, possibly related to stress relaxation of the lungs.

recoil relationship have radiologic and other functional evidence of emphysema and do not have the most severe reduction in airway function. In most patients with severe COPD, however, conductance is reduced at a standard PL, as is invariably the case in asthma.112 A normal relationship between conductance and PL does not necessarily imply the total absence of morphological changes in the airways because values of conductance are relatively insensitive to early changes in peripheral airways. Furthermore, the assumption that the pressure distending intrapulmonary airways can be approximated by PL is much more precarious in disease when the transduction of forces distending the alveoli to the external wall may be altered by breaks in alveolar walls or other causes of locally impaired lung–airway coupling and uneven distension of the lung. Maximum expiratory flow can also be related to PL.114 In the early stages of emphysema there may be loss of PL without reduction in maximum expiratory flow.91,115 But when

Table 5.7. Some extrapulmonary manifestations of COPD, which are not found in asthma

Manifestation

Pathogenesis

Increase in pulmonary artery pressure116 Right ventricular hypertrophy and dysfunction

Alveolar hypoxia 117

53

Secondary to increase in pulmonary artery pressure

Peripheral edema117

Secondary to hyperaldosteronism and/or “cor pulmonale”

Chronic hypercapnia78

˙ A/Q ˙ abnormality Severity of V and mechanical disadvantage of inspiratory muscles

Reduced inspiratory muscle strength118

Mechanical disadvantage due to hyperinflation and loss of muscle bulk, associated with cachexia

Reduction in skeletal muscle mass119

Deconditioning and/or cachexia

Weight loss120–122

Poor calorie intake and/or systemic “cachexins”

54

Asthma and Chronic Obstructive Pulmonary Disease

airflow obstruction develops, maximum expiratory flow is invariably reduced disproportionately to any reduction in PL in COPD (or asthma). See Table 5.6 for a summary

E X T R A P U L M O N A RY M A N I F E S TAT I O N S OF COPD Some well-known clinical manifestations outside the lungs are virtually confined to severe COPD (Table 5.7). It is uncertain whether this is because COPD is a more “systemic” disease with release of cytokines, mediators etc. into the circulation; or whether it simply reflects more severe changes in the lungs, the chronicity and unremitting nature of disease, and/or the greater age of subjects with COPD; or, as for example with pulmonary artery pressure, lack of measurements in asthma.

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Pulmonary Physiology

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57.

58.

59. 60.

61. 62. 63.

64.

65.

66.

67.

68.

69.

70.

71. 72.

73.

74.

75.

76.

77.

78.

55

functional–structural relationships. J. Appl. Physiol. 1983; 55:1733–42. Verbeken EK, Cauberghs M, Mertens I, Lauweryns JM, Van de Woestijne KP. Tissue and airway impedance of excised normal, senile and emphysematous lungs. J. Appl. Physiol. 1992; 72:2343–53. Hogg JC, Macklem PT, Thurlbeck WM. The resistance of collateral channels in excised human lungs. J. Clin. Invest. 1969; 48:421–31. Terry PB, Traystman RJ, Newball HH, Batra G, Menkes HA. Collateral ventilation in man. N. Engl. J. Med. 1978; 298:10–15. Wagner EM, Liu MC, Weinmann GG, Permutt S, Bleecker ER. Peripheral lung resistance in normal and asthmatic subjects. Am. Rev. Respir. Dis. 1990; 141:584–8. Stanescu D, Veriter C, Van de Woestijne KP. Maximal inspiratory flow rates in patients with COPD. Chest 2000; 118:976–80. Ingram RH, McFadden ER. Localization and mechanisms of airway responses. N. Engl. J. Med. 1977; 297:596–600. Fairshter RD, Wilson AF. Relationship between sites of airflow limitation and severity of chronic airflow obstruction. Am. Rev. Respir. Dis. 1981; 123:3–7. Fairshter RD, Wilson AF. Relationship between the site of airflow limitation and localization of the bronchodilator response in asthma. Am. Rev. Respir. Dis. 1980; 122:27–32. Partridge MR, Saunders KB. The site of airflow limitation in asthma: the effect of time, acute exacerbations of disease and clinical features. Br. J. Dis. Chest 1981; 75:263–72. Antic R, Macklem PT. The influence of clinical factors on site of airway obstruction in asthma. Am. Rev. Respir. Dis. 1976; 114:851–9. Jadue C, Greville H, Coalson JJ, Mink SN. Forced expiration and HeO2 response in canine peripheral airway obstruction. J. Appl. Physiol. 1985; 58:1788–801. Koulouris NG, Dimopoulou I,Valta P et al. Detection of expiratory flow limitation during exercise in COPD patients. J. Appl. Physiol. 1997; 82:723–31. Paganin F, Séneterre E, Chanez P et al. Computed tomography of the lungs in asthma: influence of disease severity and etiology. Am. J. Respir. Crit. Care Med. 1996; 153:110–14. Jaffuel D, Bousquet J, Paganin FJ. How can we assess remodeling using imaging? In: Howarth PH, Wilson JW, Bousquet J, Rak S, Pauwels RA (eds), Airway Remodeling, pp. 5–25. New York: Marcel Dekker, 2001. Hansell DM. Small airways diseases: detection and insights with computed tomography. Eur. Respir. J. 2001; 17:1294–313. Agusti AGN, Barberà JA. Contributions of multiple inert gas elimination technique to pulmonary medicine. 2: Chronic pulmonary diseases – chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Thorax 1994; 49:924–32. Rodriguez-Roisin R, Roca J. Contributions of multiple inert gas elimination technique to pulmonary medicine. 3: Bronchial asthma. Thorax 1994; 49:1027–33. Wagner PD, Dantzker DR, Dueck R, Clausen JL, West JB. Ventilation–perfusion inequality in chronic obstructive pulmonary disease. J. Clin. Invest. 1977; 59:203–16. Robinson TD, Freiberg DB, Regnis JA, Young IH. The role of hypoventilation and ventilation–perfusion redistribution in oxygen-induced hypercapnia during acute exacerbations of chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 2000; 161:1524–9. Corte P, Young, I. Ventilation–perfusion relationships in symptomatic asthma: response to oxygen and clemastine. Chest 1985; 88:167–75. Lane DJ, Howell JBL, Giblin B. Relation between airways obstruction and CO2 tension in chronic obstructive airway disease. Br. Med. J. 1968; 3:707–9. Bégin P, Grassino A. Inspiratory muscle dysfunction and chronic hypercapnia in chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 1991; 143:905–12.

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79. Mountain RD, Sahn SA. Clinical features and outcome in patients with acute asthma presenting with hypercapnia. Am. Rev. Respir. Dis. 1988; 138:535–9. 80. Sansores RH, Paré P, Abboud RT. Effect of smoking cessation on pulmonary carbon monoxide diffusing capacity and capillary blood volume. Am. Rev. Respir. Dis. 1992; 146:959–64. 81. Watson A, Joyce H, Hopper L, Pride NB. Influence of smoking habits on change in carbon monoxide transfer factor over 10 years in middle-aged men. Thorax 1993; 48:119–24. 82. McLean A, Warren PM, Gillooly M, MacNee W, Lamb D. Microscopic and macroscopic measurements of emphysema: relation to carbon monoxide gas transfer. Thorax 1992; 47:144–9. 83. Gevenois PA, De Vuyst P, de Maetelaer V et al. Comparison of computed density and microscopic morphometry in pulmonary emphysema. Am. J. Respir. Crit. Care Med. 1996; 154:187–92. 84. Keens TG, Mansell A, Krastins IRB et al. Evaluation of the single-breath diffusing capacity in asthma and cystic fibrosis. Chest 1979; 76:41–4. 85. Collard P, Njinou B, Nedjadnik B, Keyeux A, Frans A. Single breath diffusing capacity for carbon monoxide in stable asthma. Chest 1994; 105:1426–9. 86. Woolcock AJ,Vincent NJ, Macklem PT. Frequency dependence of compliance as a test for obstruction in the small airways. J. Clin. Invest. 1969; 48:1097–106. 87. Stanescu DC, Rodenstein D, Cauberghs M, Van de Woestijne KP. Failure of body plethysmography in bronchial asthma. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 1982; 52:939–48. 88. Rodenstein DO, Stanescu DC. Reassessment of lung volume measurement by helium dilution and by body plethysmography in chronic airflow obstruction. Am. Rev. Respir. Dis. 1982; 126:1040–4. 89. Gibson GJ, Pride NB, Davis J, Schroter RC. Exponential description of the static pressure–volume curve of normal and diseased lungs. Am. Rev. Respir. Dis. 1979; 120:799–811. 90. Colebatch HJH, Greaves IA, Ng CKY. Pulmonary mechanics in diagnosis. In: deKock MA, Nadel JA, Lewis CM (eds), Mechanism of Airways Obstruction in Human Respiratory Disease, pp. 25–47. Cape Town, SA: Balkema, 1979. 91. Petrik-Pereira R, Hunter D, Pride NB. Use of lung pressure– volume curves and helium–sulphur hexafluoride washout to detect emphysema in patients with mild airflow obstruction. Thorax 1981; 36:29–37. 92. Greaves IA, Colebatch HJH. Elastic behavior and structure of normal and emphysematous lungs post-mortem. Am. Rev. Respir. Dis. 1980; 121:127–36. 93. Paré PD, Brooks LA, Bates J et al. Exponential analysis of the lung pressure–volume curve as a predictor of pulmonary emphysema. Am. Rev. Respir. Dis. 1982; 126:54–61. 94. Nagai A, Yamawaki I, Thurlbeck WM, Takizawa T. Assessment of lung parenchymal destruction by using routine histological tissue sections. Am. Rev. Respir. Dis. 1989; 139:313–19. 95. Sharp JT, Van Lith P, Vej Nuchprayoon C, Briney R, Johnson FN. The thorax in chronic obstructive lung disease. Am. J. Med. 1968; 44:39–46. 96. Kraft M, Djukanovic R, Wilson S, Holgate ST, Martin RJ. Alveolar tissue inflammation in asthma. Am. J. Respir. Crit. Care Med. 1996; 154:1505–10. 97. Gelb AF, Zamel N. Unsuspected pseudophysiologic emphsema in chronic persistent asthma. Am. J. Respir. Crit. Care Med. 2000; 162:1778–82. 98. Rodarte JR, Noredin G, Miller C, Brusasco V, Pellegrino R. Lung elastic recoil during breathing at increased lung volume. J. Appl. Physiol. 1999; 87:1491–5. 99. Greaves IA, Colebatch HJH. Large lungs after childhood asthma: a consequence of enlarged airspaces. Aus. NZ J. Med. 1985; 15:427–34. 100. Blackie SP, Al-Majed S, Staples CA, Hilliam C, Paré PD. Changes in total lung capacity during acute spontaneous asthma. Am. Rev. Respir. Dis. 1990; 142:79–83.

101. Rothstein MS, Zelefsky MN, Eichaker PQ, Rudolph DJ, Williams MH. Radiographic measurement of total lung capacity in acute asthma. Thorax 1989; 44:510–12. 102. Kirby JG, Juniper EF, Hargreave FE, Zamel N. Total lung capacity does not change during methacholine-stimulated airway narrowing. J. Appl. Physiol. 1986; 61:2144–7. 103. King GG, Eberl S, Salome CM, Young IH, Woolcock AJ. Differences in airway closure between normal and asthmatic subjects measured with single-photon emission computed tomography and technegas. Am. J. Respir. Crit. Care Med. 1998; 158:1900–6. 104. Brown RH, Mitzner W. Airway closure with high PEEP in vivo. J. Appl. Physiol. 2000; 89:956–60. 105. Gibbons WJ, Sharma A, Lougheed D, Macklem PT. Detection of excessive bronchoconstriction in asthma. Am. J. Respir. Crit. Care Med. 1996; 153:582–9. 106. Tantucci C, Ellaffi M, Duguet A et al. Dynamic hyperinflation and flow limitation during methacholine-induced bronchoconstriction in asthma. Eur. Respir. J. 1999; 14:295–301. 107. Collett PW, Brancatisano AP, Engel LA. Upper airway dimensions and movements in bronchial asthma. Am. Rev. Respir. Dis. 1986; 133:1143–9. 108. Muller N, Bryan AC, Zamel N. Tonic inspiratory muscle activity as a cause of hyperinflation in asthma. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 1981; 50:279–82. 109. Wheatley JR, West S, Cala SJ, Engel LA. The effect of hyperinflation on respiratory muscle work in acute induced asthma. Eur. Respir. J. 1990; 3:625–32. 110. Butler J, Caro CG, Alcala R, DuBois AB. Physiological factors affecting airway resistance in normal subjects and in patients with obstructive respiratory disease. J. Clin. Invest. 1960; 39:584–91. 111. Jonson B. Pulmonary mechanics in patients with pulmonary disease, studied with the flow regulator method. Scand. J. Clin. Lab. Invest. 1970; 25:375–90. 112. Colebatch HJH, Finucane KE, Smith MM. Pulmonary conductance and elastic recoil relationship in asthma and emphysema. J. Appl. Physiol. 1973; 34:143–53. 113. Leaver DG, Tattersfield AE, Pride NB. Contributions of loss of lung recoil and of enhanced airways collapsibility to the airflow obstruction of chronic bronchitis and emphysema. J. Clin. Invest. 1973; 52:2117–28. 114. Stubbs SE, Hyatt RE. Effect of increased lung recoil pressure on maximal expiratory flow in normal subjects. J. Appl. Physiol. 1972; 32:325–31. 115. Demedts M, Aumann J. Early emphysema: ten years’ evolution. Chest 1988; 94:337–42. 116. Kessler R, Faller M,Weitzenblum E et al. “Natural history” of pulmonary hypertension in a series of 131 patients with chronic obstructive lung disease. Am. J. Respir. Crit. Care Med. 2001; 164:219–24. 117. MacNee W. Pathophysiology of cor pulmonale in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1994; 150:833–52 and 1158–68. 118. Arora NS, Rochester DF. COPD and human diaphragm muscle dimensions. Chest 1987; 91:717–24. 119. Bernard S, Leblanc P, Whittom F et al. Peripheral muscle weakness in patients with chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1998; 158:629–34. 120. Di Francia M, Barbier D, Mege JL, Orchek J. Tumor necrosis factor-a levels and weight loss in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1994; 150:1453–5. 121. Schols AM, Creutzberg EC, Buurman WA et al. Plasma leptin is related to proinflammatory status and dietary intake in patients with chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1999; 160:1220–6. 122. Landbo C, Prescott E, Lange P, Vestbo J, Almdal TP. Prognostic value of nutritional status in chronic obstructive pulmonary disease. Am. J. Respir. Crit. Care Med. 1999; 160:1856–61.

Airway Pathology

Chapter

6

James C. Hogg UBC McDonald Research Laboratories, St Paul’s Hospital, Vancouver, BC, Canada

N O R M A L A N AT O M Y The normal human bronchogram (Fig. 6.1) shows that the length of pathways from the trachea to the terminal airways differs depending on the pathway that is followed. Quantitative studies of the human airway branching system have established that it may take as as few as eight or as many as 24 divisions of airway branching to reach the gas-exchanging surface1 and that the small bronchi and bronchioles 2 mm in diameter are spread from the fourth to the fourteenth generation of airway branching.2 It has also been established that the total airway cross-sectional area expands rapidly beyond the 2 mm airways to provide for rapid diffusion of gas between the distal conducting airways and the gasexchanging surface. Therefore the central conducting airways larger than 2 mm internal diameter are the major site of resistance to airflow in the normal lung.3,4

The conducting airways are lined by epithelium and are surrounded by an adventitial layer.5 The submucosa between the epithelium and muscle layer is often referred to as the “lamina propria,” but this term is technically incorrect because many airways are not completely surrounded by muscle.6 Bronchi are defined by the presence of a layer of fibrocartilage external to the smooth muscle and tubuloalveolar glands, which communicate with the airway lumen via ducts.7 Bronchioles lack both cartilage and glands and become respiratory bronchioles when alveoli open directly into their lumen.7 The lining of the trachea and major bronchi consists of pseudostratified, ciliated columnar epithelium which gradually becomes more cuboidal with fewer ciliated cells as the alveolar surface is approached.6–8 Light microscopy reveals that the basal aspect of airway epithelial cells is attached to a thin basement membrane (80–90 nm width) that contains primarily type IV collagen and elastin.9 Transmission electron microscopy shows that a true basement membrane or basal lamina can be readily distinguished from the connective tissue observed with the light microscope.6,7 Quantitative studies10–12 have shown that the trachea and mainstem bronchi of normal subjects consist of: • • • •

Fig. 6.1. Postmortem bronchogram from an otherwise normal 19-yearold man who died suddenly for reasons unrelated to the lung. Note the difference in airway length depending upon the pathway that is followed.

30% cartilage; 15% mucous glands; 5% smooth muscle; a connective tissue matrix containing the bronchial arterial, venous, and lymphatic vessels.

With progression toward the periphery of the bronchial tree, the amount of cartilage and glands decrease, and the percentage of smooth muscle increases to account for approximately 20% of the total wall thickness in the bronchioles. The degree to which the smooth muscle surrounds the airway lumen varies according to site. In the trachea and mainstem bronchi, the airway smooth muscle is located within the posterior membranous sheath, whereas, in the bronchioles, it surrounds the entire lumen of the airway.7,8,13 Consequently, the same degree of muscle shortening has a smaller effect on the caliber of the trachea and central

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airways than on the distal bronchi and bronchioles.14 The adventitial layer consists of loose bundles of collagen admixed with blood vessels, lymphatics, and nerves. In the peripheral conducting airways, this layer interacts with the surrounding lung parenchyma through alveolar attachments that are distributed along the circumference of the adventitia. These alveolar attachments have the ability to limit the amount of airway narrowing produced by smooth muscle contraction, particularly at higher lung volumes.14 The systemic arterial supply to the bronchial tree originates from the ventral side of the upper thoracic aorta in the left hemithorax, while on the right the origin of the bronchial vessels is more variable. They may originate from the first to the third intercostal artery, from the right internal mammary artery, or from the right subclavian artery.8,15 Miller’s classic anatomical account8 showed that two to three arterial branches accompany each of the larger bronchi and that anastomoses between these branches form an arterial plexus in the outer wall of the airway. Small branches of this plexus penetrate the smooth muscle layer to form a capillary network below the epithelium. Short connecting branches extend from this plexus through the muscle layer to form a second plexus of venules along the outer surface of the airway smooth muscle (Fig. 6.2). In some species, this outer plexus contains large venous sinuses that can extend into the submucosal layer of the major bronchi, where there is no smooth muscle between the cartilage and epithelium.16 The venous blood from the first two or three subdivisions of the bronchial tree drains into the azygous and hemiazygous venous systems, which empty into the vena cava. The remainder of the bronchial venous flow drains directly into the pulmonary circulation, although there is debate as to how much enters at the precapillary, capillary

and postcapillary, levels.8,15 Airway disease increases the anastomotic flow between the pulmonary and the bronchial vascular systems; and in diseases such as bronchiectasis, injection of the bronchial arteries can result in rapid filling of the entire pulmonary vascular tree right back to the pulmonic valve.15 The bronchial circulation accounts for about 1% of cardiac output and delivers approximately 72 liters of blood to the conducting airways over 24 hours at a cardiac output of 5 L/min. Each liter of blood contains between 4 and 11  109 white blood cells, made up of approximately 60% neutrophils, 30% lymphocytes, 5% monocytes, 3% eosinophils, and 2% basophils. The neutrophils, remain within the vascular space unless an inflammatory response is present. In contrast, even in the absense of inflammation, lymphocytes, monocytes, basophils or eosinophils may migrate out of the vessels into the lung and airway tissue. The monocytes and lymphocytes are able to divide and multiply in the tissue and the basophils and eosinophils live surprisingly long lives.

T H E PAT H O L O G Y O F A S T H M A Postmortem studies At post-mortem, the lungs of patients who have died in status asthmaticus remain markedly hyperinflated after the thorax is opened. This hyperinflation is due to air trapping caused by widespread plugging of the segmental, subsegmental, and the smaller conducting airways by mucus and cellular debris.17 Although this lumenal content may extend to the respiratory bronchioles, it usually stops short of these structures and does not fill the alveolar airspaces. Examination of the cut surface of the lung reveals the plugged

Fig. 6.2. Photomicrograph of a bronchiole from a patient who died of asthma. This shows the location of submucosal capillaries (white arrow), connecting vessels passing between muscle bundles (dark arrow) to larger post-capillary venules located outside the muscle layer. On the right, the same features are shown at a higher magnification. Reproduced in colour between pages 56 and 57.

Airway Pathology

airways, but – in contrast to parenchymal destruction seen in hyperinflated emphysematous lungs – the parenchyma of the asthmatic lung remains intact. Huber and Koessler’s18 classic 1922 paper on the pathology of asthma reviewed 15 published cases and provided new data on six more. They noted that the pathology consisted of common features that allowed asthma to be distinguished from other conditions. They emphasized the presence of intraluminal mucus secretion, airway epithelial desquamation, and repair (e.g. goblet cell metaplasia), and airway inflammatory infiltrates consisting of an admixture of mononuclear cells and eosinophils and the presence of a thickened, “hyalinized” subepithelial basement membrane. Later studies based on electron microscopy and immunohistochemistry showed that this feature of the basement membrane was due to deposition of collagen fibrils and extracellular matrix below the true basement membrane, rather than thickening of the basal lamina. Huber and Koessler’s report noted that the tenacious plugs that fill the airway lumen consist of an exudate of plasma containing inflammatory cells, particularly eosinophils, mixed with epithelial cells that had sloughed from the airway surface. Using an eyepiece micrometer to measure the external airway diameters, they concluded that the walls of bronchi and bronchioli of more than 2 mm outside diameter were thickened compared with nonasthmatic persons, and that this difference was due to an increased thickness of all of the components of the airway wall. Over the next several decades, other reports confirmed and extended these findings.19–26 A comparison with Florey’s27 basic studies of the inflammatory process showed that the structural changes associated with asthma are consistent with an inflammatory process involving a mucus secreting surface. This knowledge had relatively little impact on the allergists, pulmonary physicians, and physiologists until the 1970s,28 because they were preoccupied with the concept that asthma was due to the sensitization of mast cells by IgE in a way that allowed specific antigen challenge to release mediators that caused an excessive contraction of airway smooth muscle. Bronchoscopic studies The nature of the asthmatic airways has been further revealed by studies of tissue obtained through the rigid and flexible bronchoscope. These techniques allowed investigators to obtain cells from living asthmatic patients by both bronchoalveolar lavage and bronchial biopsies.29–31 This brought physiologists and clinicians into closer agreement with the pathologist’s view that the inflammatory process was important in the pathogenesis of both bronchial hyperresponsiveness and reversible airflow obstruction. A very important conceptual development in understanding the nature of the inflammatory process is based on the discovery that murine CD4+ T cell clones could be divided on the basis of cytokine messenger RNA (mRNA) and proteins that they produced.32 Studies established that one type of T-cell clone (Th1) produced IL-2 and interferon-c but no

59

IL-4 or IL-5; whereas the second (Th2) produced IL-4 and IL-5, but no IL-2 or interferon-c. Both clones produced IL3 and GMCSF, and interactions between Th1 and Th2 subtypes allowed one type of clone to inhibit the other. As IL-4 is a mast cell growth factor that also stimulates IgE production, and IL-5 promotes the differentiation and survival of eosinophils, what has become known as the “Th2 response” became relevant to the inflammatory process in asthma. Robinson and associates33 were the first to offer the hypothesis that asthma represents a Th2 response using bronchoalveolar lavage and bronchial biopsy. Studies of surgically resected lung specimens from asthmatic patients subsequently established that a similar inflammatory process was also present in the smaller airways.34 It has now become clear that the structural features of the airways from asthmatic patients are the result of an inflammatory process involving tissue with a mucus-secreting surface. This response appears to be driven by a subset of CD4 T lymphocytes producing cytokines that result in an excess of eosinophils and an overproduction of IgE. The end-result is abnormal airway function, characterized by excessive airway narrowing in response to external stimuli, as well as reversible airways obstruction where smaller airways tend to close, leading to gas trapping and a requirement to breathe at a high lung volume. Although the majority of the symptoms produced by the process can be rapidly reversed with appropriate treatment, the process can be life-threatening under some circumstances. The relationship between airway structure and function The concept that the same degree of smooth muscle shortening will cause greater reduction in airway caliber when the wall is thickened by disease has been suggested by several authors.35,36 Moreno and associates36 calculated that the thickening of the airway wall observed in asthma would have only a minor effect on the caliber of the lumen of a fully dilated airway. However, when the smooth muscle in the airway shortens, the increased tissue between the muscle and lumen causes an excess reduction in airway caliber. Subsequent studies by James et al.37 showed that, in asthmatics, this increase in wall thickness was sufficient to close the lumen of the smaller airways when the smooth muscle shortening remained within the accepted normal range, suggesting that normal smooth muscle shortening acting in series with an abnormally thickened airway wall would narrow the airway lumen. It follows that the reduction in airway caliber produced by this mechanism would be reversed when the muscle lengthened by relaxation.This showed that an important feature of the pathology of asthma was the effect of the inflammatory process on the structure of the airway wall, and that these structural changes could result in excess airway narrowing whether or not smooth muscle contraction was abnormal. Wiggs et al.38,39 extended this concept using a computer model of the airways that could test the effect of structural changes on airway function. Their analysis (Fig. 6.3)

60 (a)

Asthma and Chronic Obstructive Pulmonary Disease

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Fig. 6.3. Data from Wiggs et al. (references 38 and 39) showing results from a computer model of the airways. The structural data from normal lungs were used to obtain the control measurements. These show that airway resistance increases from about 1 cmH2O per liter per second to reach a plateau of 12 cmH2O per liter per second, with maximum contraction of the airways smooth muscle. (a) When the structural features of asthmatic airways were used in the same computer model, the airway resistance continued to increase and did not reach a plateau at any physiologically meaningful value. (b) Data obtained from the model when the structural changes caused by asthma were limited to the central airway (i.e. those 2 mm diameter). Note that asthmatic changes in the central airways increased total resistance and resulted in a plateau slightly greater than the control lungs. (c) However, when the asthmatic changes were placed in the peripheral airway, the resistance increased without reaching a plateau. These data suggest that asthmatic changes in the peripheral airways have a much greater effect on overall airway function than changes that occur in the central airways. Reproduced from reference 38, with permission.

showed that maximum stimulation of the smooth muscle caused airway resistance to increase and reach a plateau in the normal lung. This finding was consistent with the previous observations by Woolcock et al.,40 who reported that the changes in the maximum volume of air that can be expired from the lung in one second (FEV1) reached a plateau in normal subjects when maximally stimulated by inhaled bronchoconstrictors. However, when the data on airway structure were changed from normal values to those found in asthmatic lungs, a similar degree of airway smooth shortening resulted in a sustained rapid increase in airway resistance without a plateau. When the effect of disease on the central and peripheral airway function was examined separately, they found that the increase in airway resistance was primarily due to the effect of disease on the peripheral airways, where smooth muscle shortening produced widespread airway closure. The concept that the peripheral airways are the major site of obstruction in asthma has now been confirmed by direct measurements in living asthmatic patients reported by Yanai et al.41 The changes that are produced in the airway surface and lumen also reduce the function of the conducting airways. Lambert42 was the first to systematically study the normal folding pattern of the bronchial mucosa, and suggested that asthma resulted in the replacement of the multiple mucosal folds that occur when normal airways narrow with fewer and larger folds that reduce airway caliber. Both Lambert and Wiggs et al.43 suggested that the mucosal folding pattern was controlled by the stiffness of the subepithelial layer relative to that of the surrounding airway tissue. This analysis suggested that changes in the subepithelial connective tissue that contribute to the appearance of the basement membrane could play a key role in determining the pattern of mucosal folding. Lambert and Wiggs et al. argued that the formation of a large number of folds in the normal airway placed a load on the airway smooth muscle that tended to prevent airway closure at low lung volumes. It followed that a change in this folding pattern in asthma may be one way in which the disease causes peripheral airway dysfunction. Many of the problems associated with mucosal function and its effect on airway caliber are under investigation, and these analyses should lead to a better understanding of small airway function in both health and disease. The caliber of the airway lumen is also influenced by airway surface tension.44 Normally, surface tension is low because these airways are lined with surfactant. Exudation of plasma on to the surface of the lumen will increase surface tension and cause the airways to narrow. This tendency towards an increased surface tension would be further increased by the secretion of mucus into the luminal exudate. The analysis of induced sputum has confirmed that plasma proteins, mucus, and inflammatory cells are present in the airway lumen even in mild asthma, suggesting that some of the abnormalities in airway function of asthmatics might be the result of the inflammatory exudate present in the lumen.

Airway Pathology

T H E PAT H O L O G Y O F C O P D The inflammatory process contributes to the pathogenesis of chronic cough and sputum production,45 peripheral airways obstruction,3,46 and emphysematous destruction of the lung surface47 that define chronic obstructive pulmonary disease (COPD). As tobacco smoking produces lung inflammation in everyone, and only 15–20% of heavy smokers develop COPD, cigarette smoke-induced airway inflammation must be amplified by other host or environmental risk factors to produce clinical disease.48 Chronic bronchitis Cough and sputum production are the features of airways disease that define chronic bronchitis49 and these symptoms can be present either with or without airways obstruction. Fig. 6.4 shows a normal bronchus with the connection between the epithelial lining of the bronchial lumen, the mucus gland, and duct. In early studies, Reid50 used the relative size of the mucus glands as a yardstick for measuring chronic bronchitis and

Fig. 6.4. Low-power photomicrograph of a normal bronchus showing the opening of a bronchial gland duct into the lumen and its connection with the submucosal gland. The bronchial muscle and cartilage are clearly labeled. The inflammatory process associated with chronic bronchitis involves the mucosa, gland ducts, and glands of the bronchi that are between 2 and 4 mm internal diameter. Original photograph taken by the late Dr William Thurlbeck.

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downplayed the influence of inflammation on increased mucus production. However, a reevaluation of this problem some years later showed that chronic bronchitis was associated with inflammation of the airway mucosal surface, the submucosal glands, and gland ducts, particularly in the smaller bronchi between 2 and 4 mm in diameter.45 The nature of the cells present in the inflammatory process of chronic bronchitis is currently under active investigation, and evidence from biopsy studies suggests that the CD8+ lymphocytes are present in excess numbers in smokers with chronic bronchitis.51,52 Sputum production represents the clearance of a mucoid inflammatory exudate from the lumen of the bronchi. This exudate is formed by an inflammatory process based on the microvessels of the bronchial circulation. It contains plasma proteins, inflammatory cells, and small amounts of mucus added from goblet cells on the surface epithelium and the epithelial glands. In established chronic bronchitis, the size of the bronchial mucous glands tends to increase,50 but Thurlbeck and Angus53 showed that there is no clear separation between patients with chronic bronchitis and controls. Chronic bronchitis also results in an increase in airway smooth muscle, a generalized increase in the connective tissue in the airway wall, degenerative changes in the airway cartilage, and a shift in epithelial cell type that increases the number of goblet and squamous cells.54-58 The site of airways obstruction The site of airway obstruction in COPD is in the smaller conducting airways that include bronchi and bronchioles of less than 2 mm diameter.3 Direct measurements of airways resistance in dogs4 and in similar measurements in postmortem human lungs3 established, in the normal lung, that the small peripheral airways offer very little resistance to air flow. Although van Brabant et al.59 subsequently argued that these peripheral airways account for a larger proportion of the total airways resistance in the normal lung, they agreed that the peripheral airways were the major site of obstruction in COPD. The anatomical features that have been proposed to contribute to the increase in peripheral airway resistance in COPD include destruction of alveolar support of the peripheral airways,60 loss of elastic recoil in the parenchyma supporting the airways,61 a decrease in the elastic force available to drive flow out of the lung,62 and structural narrowing of the airway lumen by the inflammatory process.3,63 The histological appearance of peripheral airways in cases of advanced COPD shows that a chronic inflammatory process is present in these airways and that this process changes the structural feature of the airway wall and narrows the lumen of these small airways. Although the destruction and loss of elastic recoil in the alveolar attachments supporting the airway could also cause these airways to narrow, the author’s group has argued that this is less important because lung inflation does not reverse the increase in peripheral airways resistance in COPD.3

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The lung inflammatory changes that are associated with cigarette smoking have been documented in autopsy studies,64-69 resected lung specimens,45,46,69 lung biopsies,70 and indirectly by examining bronchoalveolar lavage fluid.71–73 These studies support the concept that cigarette smokeinduced inflammation is present in the lungs of all smokers, including those with normal lung function. It follows that the inflammatory process must be amplified in the minority of cases that develop fixed airways obstruction and emphysematous lung destruction as a result of cigarette smoking, but the exact mechanism of this amplification is not understood.

Emphysema Emphysema has been defined as “abnormal permanent enlargement of airspaces distal to terminal bronchioles, accompanied by destruction of their walls without obvious fibrosis.”74 This definition emphasizes the destruction of the alveolar surface with a minimal reparative response in the lung matrix and the ability of this destructive process to reduce the gas-exchanging surface of the lung. The centrilobular form of emphysema (Fig. 6.5) results from dilatation and destruction of the respiratory bronchioles and is most closely associated with smoking.75,76 The panacinar

Fig. 6.5. (a) Normal lung photographed from the pleural surface after a postmortem bronchogram. The connective tissue outlining the secondary lobule is clearly seen (solid arrow) and a terminal bronchiole (TB) indicated by a clear arrow supplies a single acinus. (b) Photomicrograph of an acinus showing a terminal bronchiole, respiratory bronchiole (RB) and alveolar ducts (AD). Their structure is represented by the fine spray of contrast at the end of the terminal bronchiole shown in the bronchogram in (a). (c) Diagram of the lesion in centrilobular emphysema, showing the dilation and destruction of the respiratory bronchioles. (d) Postmortem bronchogram showing a centrilobular emphysematous lesion (CLE) outlined by bronchographic material. Parts (c) and (d) reproduced from references 76 and 79, with permission.

Airway Pathology

form of emphysema, on the other hand, results from a uniform destruction of the entire lobule and is characteristic of the lesions found in the lungs in a1-antitrypsin deficiency.77 The terms “distal acinar,” “mantle” or “periseptal” emphysema are used to describe lesions that occur in the periphery of the lobule and along the lobular septae particularly in the subpleural region. These lesions have been associated with spontaneous pneumothorax in young adults and bullous lung disease in older individuals, and may or may not be related to COPD.78 In far-advanced parenchymal destruction such as that frequently observed in end-stage COPD, these descriptive terms are not particularly helpful because the entire lung lobule is affected and many destroyed lobules coalesce to form single large lesions. Destruction of the lung by centrilobular emphysema results in hyperinflation of that lung region because the pressure–volume relationship of emphysematous lung keeps the tissue fully inflated at very low transpulmonary pressures.79 The decrease in lung elastic recoil diminishes the pressure available to drive air out of the lung during forced expiration,62 and the reduction in the lung surface area reduces the diffusing capacity and interferes with gas exchange.80 Acute exacerbations of COPD In a hospital-based study of 1205 admissions to five hospitals for acute exacerbations of COPD, infections accounted for 520, heart failure for 260, and 122 cases were attributed to a variety of etiologies that included arrhythmia, pulmonary embolism, pneumothorax, postoperative complications, and lung cancer.81 Interestingly, no cause was established for 303 of these 1205 admissions. Early studies by Stuart-Harris and associates82,83 showed that COPD patients with acute upper respiratory infections are more likely to have signs of infec(a)

63

tion in the lower airways than normal controls, and at least two studies have shown an association between acute illness and laboratory evidence of viral infection in patients with COPD.84,85 The normal lung is predisposed to bacterial infection by an acute viral illness because it increases the risk of aspiration of mucoid exudate containing large numbers of bacteria from the upper airways, and reduces both the mucociliary clearance and bacterial killing in the lower airways.86,87 Streptococcus pneumonia, Staphylococcus aureus and Hemophilus influenzae are the most common bacteria infecting the lower airways during an acute viral infection,88 and these infections are probably responsible for the positive effect of antibiotics in some cases of COPD.89 Unfortunately, very few pathological studies have attempted to define the anatomical features of an exacerbation because postmortem studies are confusing and very few biopsy studies have been attempted on these very sick patients. One such study by Saetta et al.90 implicated the eosinophil as the cause of acute exacerbations, but that study was based on patients where the FEV1 averaged 62  7% of predicted, and may not be relevant to the more usual case where FEV1 is much lower.83

S U M M A RY Asthma The pathology of asthma is dominated by widespread plugging of the segmental, subsegmental, and smaller conducting airways that leads to hyperinflation but not destruction of the parenchyma. These airway plugs are a manifestation of the fluid and cellular exudative phase of an inflammatory process based in the airway tissue (Fig. 6.6). (b)

Fig. 6.6. (a) Peripheral conducting airway from a patient who died of asthma. The airway lumen is filled with an inflammatory exudate that contains plasma proteins, inflammatory cells, sloughed epithelial cells, and mucus. This process that causes epithelial sloughing (not shown in figure) results in metaplastic changes in the epithelium, which increases goblet cells. There is also an increase in muscle and connective tissue, but the caliber of the lumen is not reduced. (b) Airway from a patient with COPD, where the caliber of the airways is reduced by a peribronchiolar inflammatory process with connective tissue deposition in the adventitia. Reproduced in colour between pages 56 and 57.

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This process is characteristic of inflammation involving any mucus-secreting surface, in that the exudate contains mucus as well as inflammatory cells, plasma protein, and sloughed epithelium. The cellular content of this process is consistent with a Th2 type of immune response. Quantitative postmortem studies have established that in addition to the exudate that plugs the airways, asthma is associated with sloughing and regeneration of the epithelium, an increase in the thickness of the epithelial basement membrane, an increase in the size of the bronchial glands, and hypertrophy and possibly hyperplasia of the airway smooth muscle. COPD The chronic bronchitis of COPD is defined by excess cough with sputum production and is associated with an inflammatory process located in the mucosa, gland ducts, and glands of intermediate-sized bronchi between 2 and 4 mm internal diameter.The airway obstruction in COPD is the result of an extension of this process to the smaller bronchi and bronchioles under 2 mm internal diameter, which gradually narrows the lumen to cause fixed airway obstruction (Fig. 6.6). Emphysematous destruction of the lung surface by an inflammatory process in the parenchyma contributes to the reduction in FEV1 by reducing lung elastic recoil and is responsible for reduced gas exchange. The acute exacerbations of COPD that occur with increasing frequency as the disease progresses have several causes that include infection, right heart failure, and pulmonary embolism in addition to other cases where no clear cause can be demonstrated.

REFERENCES 1. Horsfield K, Cumming G. Morphology of the bronchial tree in man. J. Appl. Physiol. 1968; 24:373–83. 2. Weibel ER. Morphometry of the Human Lung. New York: Academic Press, 1963. 3. Hogg JC, Macklem PT, Thurlbeck WM. Site and nature of airways obstruction in chronic obstructive lung disease. N. Engl. J. Med. 1968; 48:421–31. 4. Macklem PT, Mead J. Resistance of central and peripheral airways measured by a retrograde catheter. J. Appl. Physiol. 1967; 22:395–401. 5. Bai A, Eidelman DH, Hogg JC et al. Proposed nomenclature for quantifying subdivisions of the bronchial wall. J. Appl. Physiol. 1994; 77:1011–14. 6. Neutra M, Podykula H, Weiss L (eds). Histology, Cell and Tissue Biology, 5th edn, pp. 658–706. Amsterdam: Elsevier Biomedical, 1983. 7. Serokin S. In: Weiss L (ed.), Histology, Cell and Tissue Biology, 5th edn, pp. 788–868. Amsterdam: Elsevier Biomedical, 1983. 8. Miller WS.The blood vessels in the lung. In: The Lung, 3rd edn, pp. 73–84. Springfield, IL: Charles Thomas, 1943. 9. Roche WR, Beasley R, Williams JH, Holgate ST. Subepithelial fibrosis in the bronchi of asthmatics. Lancet 1989; i:520–4. 10. Dunnill MS. The pathology of asthma with special reference to changes in the bronchial mucosa. J. Clin. Path. 1960; 13:27–33. 11. Dunnill MS. Quantitative methods in the study of pulmonary pathology. Thorax 1962; 17:320–8. 12. Takizawa T, Thurlbeck WM. Muscle and mucous gland size in the major bronchi of patients with chronic bronchitis, asthma and asthmatic bronchitis. Am. Rev. Respir. Dis. 1971; 104:331–6.

13. Plopper CG, Ten Have-Opbroek AAW. Anatomical and histological classification of the bronchioles. In: Epler GR (ed.), Diseases of the Bronchioles, pp. 15–25. New York: Raven Press, 1994. 14. Macklem PT. Bronchial hyporesponsiveness. Chest 1985; 87:158S–159S. 15. Cudkowicz L. The Human Bronchial Circulation in Health and Disease. Baltimore: Williams & Wilkins, 1968. 16. Hill P, Goulding D, Webber SE, Widdicombe JG. Blood sinuses in the submucosa of the large airways of sheep. J. Anat. 1989; 162:235–47. 17. Rigler LG, Koucky R. Roentgen studies of pathological physiology of bronchial asthma. Am. J. Roeentgenol. 1938; 39:353–62. 18. Huber HL, Koessler KK.The pathology of bronchial asthma. Arch. Int. Med. 1922; 30:689–760. 19. Cardell BS, Pearson RSB. Death in asthmatics. Thorax 1959; 14:341–52. 20. Earle BV. Fatal bronchial asthma: a series of 15 cases with a review of the literature. Thorax 1953; 8:195–206. 21. Houston JC, de Navasquez S,Trounce JR.A clinical and pathological study of fatal cases of status asthmaticus. Thorax 1953; 8:207–13. 22. Dunnill MS, Massarella GR, Anderson JA. A comparison of the quantitative anatomy of the bronchi in normal subjects, in status asthmaticus, in chronic bronchitis and in emphysema. Thorax 1969; 24:176–9. 23. MacDonald IG. The local and constitutional pathology of bronchial asthma. Ann. Intern. Med. 1933; 6:253–77. 24. Messer J, Peters GA, Bennet WA. Cause of death and pathological findings in 304 cases of bronchial asthma. Dis. Chest. 1960; 38:616–24. 25. Richards W, Patrick JR. Death from asthma in children. Am. J. Dis. Child. 1965; 110:4–21. 26. Saetta M, di Stefano A, Rosina C, Thiene G, Fabbri LM. Quantitative structural analysis of peripheral airways and arteries in sudden fatal asthma. Am. Rev. Respir. Dis. 1991; 143:138–43. 27. Florey H. The secretion of mucus and inflammation in mucus membranes. In: Florey H (ed.), General Pathology, 3rd edn, pp. 167–96. London: Lloyd-Luke Medical Books, 1962. 28. Ciba Symposium on the Identification of Asthma. January 1971. 29. Djukanovic R, Wilson JW, Lai CKW, Holgate ST, Howarth PH. The safety aspects of fiberoptic bronchoscopy, bronchoalveolar lavage, and endobronchial biopsy in asthma. Am. Rev. Respir. Dis. 1991; 143:772–7. 30. Djukanovic R, Roche WR, Wilson JW et al. Mucosal inflammation in asthma. Am. Rev. Respir. Dis. 1990; 142:434–57. 31. Glynn AA, Michaels L. Bronchial biopsy in chronic bronchitis and asthma. Thorax 1960; 15:142–53. 32. Mosmann TR, Coffman RL. TH1 and TH2 cells: different patterns of lymphokine secretion lead to different functional properties. Annu. Rev. Immunol. 1989; 7:145–73. 33. Robinson DS, Hamid Q,Ying S et al. Predominant TH2-like bronchoalveolar T-lymphocyte population in atopic asthma. N. Engl. J. Med. 1992; 326:298–304. 34. Hamid Q, Song Y, Kotsimbos TC et al. Small airways inflammation in asthma. J. Allergy Clin. Immun. 1997; 100:44–51. 35. Friedman BJ. Functional anatomy of the bronchi. Bull. Pathophysiol. Respir. 1972; 8:545–51. 36. Moreno R, Hogg JC, Paré PD. Mechanics of airway narrowing. Am. Rev. Respir. Dis. 1986; 133:1171–80. 37. James AL, Paré PD, Hogg JC. The mechanics of airway narrowing in asthma. Am. Rev. Respir. Dis. 1989; 139:242–6. 38. Wiggs BR, Moreno R, Hogg JC, Hilliam C, Paré PD. A model of the mechanics of airway narrowing. J. Appl. Physiol. 1990; 69:849–60. 39. Wiggs BR, Bosken C, Paré PD, James A, Hogg JC. A model of airway narrowing in asthma and chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 1992; 145:1251–8. 40. Woolcock AJ, Salome CM, Yan K. The shape of the dose–response curve to histamine in asthmatic and normal subjects. Am. Rev. Respir. Dis. 1984; 130:71–5.

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41. Yanai M, Sekizawa K, Ohrui T, Sasaki H, Takishima T. Site of airway obstruction in pulmonary disease: direct measurements of intrabronchial pressure. J. Appl. Physiol. 1992; 72:1016–23. 42. Lambert R. The role of the bronchial basement membrane and airway collapse. J. Appl. Physiol. 1991; 71:666–73. 43. Wiggs BR, Hrousis CA, Drazen JM, Kamm RD. The implications of airway wall buckling in asthmatic airways. Am. J. Respir. Crit. Care Med. 1994; 149:A585. 44. Macklem PT, Proctor DF, Hogg JC. The stability of peripheral airways. Respir. Physiol. 1970; 8:191–203. 45. Mullen JBM,Wright JL,Wiggs B, Paré PD, Hogg JC. Reassessment of inflammation in the airways of chronic bronchitis. Br. Med. J. 1985; 291:1235–9. 46. Cosio M, Ghezzo M, Hogg JC et al.The relation between structural changes in small airways and pulmonary function tests. N. Engl. J. Med. 1978; 298:1277–81. 47. Janoff A. Biochemical links between cigarette smoking and pulmonary emphysema. J. Appl. Physiol. 1983; 55:285–93. 48. Speizer FE, Tager IB. Epidemiology of chronic mucus hypersecretion and obstructive airways disease. Epidemiol. Rev. 1979; 1:124–42. 49. Ciba Guest Symposium Report: Terminology, definitions and classifications of chronic pulmonary emphysema and related conditions. Thorax 1959; 14:286–99. 50. Reid L. Measurement of the bronchial mucous gland layer: a diagnostic yardstick in chronic bronchitis. Thorax 1960; 15:132–41. 51. O’Shaughnessy TC, Ansari TW, Barnes NC, Jeffery PK. Inflammation in bronchial biopsies of subjects with chronic bronchitis: inverse relationship of CD-8+T lymphocytes with FEV1. Am. J. Resp. Crit. Care Med. 1997; 155:382–7. 52. Saetta M, Di Stefano A, Turato G et al. T lymphocytes in the peripheral airways of smokers with chronic obstructive pulmonary disease. Am. J. Resp. Crit. Care Med. 1998;157:822–6. 53. Thurlbeck WM, Angus GE. The distribution curve for chronic bronchitis. Thorax 1964; 19:436–42. 54. Jamal K, Cooney TP, Fleetham JA, Thurlbeck WM. Chronic bronchitis: correlation of morphological findings and sputum production and flow rates. Am. J. Respir. Dis. 1984; 129:719–22. 55. Carlile A, Edwards C. Structural variations in the main bronchi of the left lung; a morphometric study. Br. J. Dis. Chest 1983; 77:344–8. 56. Mackenzie HI, Outhred KG. Chronic bronchitis in coal miners: antimortem/postmortem comparisons. Thorax 1969; 24:527–35. 57. Haraguchi M, Shemura S, Shirata K. Morphologic analysis of bronchial cartilage in chronic obstructive pulmonary disease and bronchial asthma. Am. J. Respir. Crit. Care Med. 1999; 159:1005–13. 58. Thurlbeck WM, Pun R, Toth J, Fraser RG. Bronchial cartilage in chronic obstructive lung disease. Am. Rev. Respir. Dis. 1974; 109:73–80. 59. van Braband T, Cauberghs M, Verbeken E et al. J. Appl. Physiol.: Respir. Environ. Exercise Physiol. 1983; 55:1733–42. 60. Dayman H. Mechanics of airflow in health and emphysema. J. Clin. Invest. 1951; 3031:1175–90. 61. Butler J, Caro C, Alkaler R, Dubois AB. Physiological factors affecting airway resistance in normal subjects and in patients with obstructive airways disease. J. Clin. Invest. 1960; 39:584–91. 62. Mead J, Turner JM, Macklem PT, Little J. Significance of the relationship between lung recoil and maximum expiratory flow. J. Appl. Physiol. 1967; 22:95–108. 63. Matsuba K, Thurlbeck WM. The number and dimensions of small airways in emphysematous lungs. Am. J. Pathol. 1972; 67:265–75. 64. McLean KA. Pathogenesis of pulmonary emphysema. Am. J. Med. 1958; 25:62–74. 65. Niewoehner DE, Kleinerman J, Reisst DB. Pathologic changes in the peripheral airways of young cigarette smokers. N. Engl. J. Med. 1974; 291:755–8.

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66. Auerbach O, Garfinkle L, Hammond EC. Relation of smoking and age to findings in lung parenchyma: a microscopic study. Chest 1974; 65:29–35. 67. Auerback O, Hammond EC, Garfinkle L, Benante C. Relation of smoking and age to emphysema: whole lung section study. N. Engl. J. Med. 1972; 286:853–7. 68. Petty TL, Silverds GW, Stanford RE, Baird ME, Mitchell MS. Small airway pathology is related to increased closing capacity and abnormal slope of phase III in excised human lungs. Am. Rev. Respir. Dis. 1980; 121:449–56. 69. Wright JL, Lawson LM, Paré et al. Morphology of peripheral airways in current smokers and ex-smokers. Am. Rev. Respir. Dis. 1983; 127:474–7. 70. Ollerenshaw SL, Woolcock AJ. Characteristics of the inflammation in biopsies from large airways in subjects with asthma and chronic airflow limitation. Am. Rev. Respir. Dis. 1992; 145:922–7. 71. Gadek JE, Fells JA, Crystal RG. Cigarette smoking induces a functional antiprotease deficiency in the lower respiratory tract of humans. Science 1979; 206:315–16. 72. Hunninghake GW, Crystal RG. Cigarette smoking and lung destruction: accumulation of neutrophils in the lungs of cigarette smokers. Am. Rev. Respir. Dis. 1983; 128:833–8. 73. Stone DJ, Galor JD, McGowan SE et al. Functional 1-protease inhibitor in lower respiratory tract of cigarette smokers is not decreased. Science 1983; 221:1187–9. 74. Snider GL, Kleinerman JL, Thurlbeck WM, Bengally ZH. Definition of emphysema. Report of a National Heart, Lung and Blood Institute, Division of Lung Disease Workshop. Am. Rev. Respir. Dis. 1985; 132:182–5. 75. Anderson AE, Hernandez JA, Holmes WL, Foraker AG. Pulmonary emphysema: prevalence, severity and anatomical patterns with respect to smoking habits. Arch. Environ. Hlth 1966; 12:569–77. 76. Leopold JG, Goeff J. Centrilobular form of hypertrophic emphysema and its relation to chronic bronchitis. Thorax 1957; 12:219–35. 77. Pratt PC, Kilborn KH. A modern concept of emphysemas based on correlations of structure and function. Hum. Pathol. 1970; 1:445–53. 78. Laurenzi GA, Toreno GM, Fishman AP. Bullous disease of the lung. Am. J. Med. 1962; 36:361–78. 79. Hogg JC, Macklem PT, Thurlbeck WM. The elastic properties of the centrilobular emphysematous space. J. Clin. Invest. 1969; 48:1306–12. 80. McLean A, Warren PM, Gilooly M, Lamb D. Microscopic and macroscopic measurements of emphysema: relation to carbon monoxide gas transfer. Thorax 1992; 47:144–9. 81. Connors AF, Dawson NV, Thomas C et al. Outcomes following acute exacerbation of severe chronic obstructive lung disease. Am. J. Respir. Crit. Care Med. 1996; 154:959–67. 82. Stuart-Harris CH. The role of bacterial and viral infection in chronic bronchitis. Arch. Environ. Hlth 1968; 16:586–95. 83. Stuart-Harris CH. Infection, the environment and chronic bronchitis. J. Roy. Coll. Phys. Lond. 1971; 5:351–61. 84. Monto AS, Higgins MW, Ross HW. The Tecumseh study of respiratory illness. VIII: Acute infection in chronic respiratory disease and comparison groups. Am. Rev. Respir. Dis. 1975; 111:27–36. 85. Smith CB, Golden CA, Canner RE, Renzetti AD. Association of viral and mycoplasmal pneumonia infections with acute respiratory illness in patients with COPD. Am. Rev. Respir. Dis. 1980; 121:225–32. 86. Knipe DN. Virus–host interactions. In: Fields BM, Knipe DN, Howley PM (eds), Field’s Virology, pp. 273–99. New York: Lippincott–Raven, 1996. 87. Hers JF, Masurel N, Mulder J. Bacteriology and histopathology of the respiratory tract in fatal Asian influenza. Lancet 1958; ii:1141–3.

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88. Lauria DB, Blumenfield HL, Ellis JT, Kilbourne ED, Rogers DE. Studies on influenza in the pandemic of 1957–58. II: Pulmonary complication of influenza. J. Clin. Invest. 1959; 38:213–65. 89. Anthonisen NR, Manfreda J, Warren CP et al. Antibiotic therapy in exacerbations of COPD. Ann. Int. Med. 1987; 106:196–204.

90. Saetta M, Di Stefano A, Maestrelli P et al. Airway eosinophilia in chronic bronchitis during exacerbations. Am. J. Respir. Crit. Care Med. 1994; 150:1646–52.

Fig. 6.2. Photomicrograph of a bronchiole from a patient who died of asthma. This shows the location of submucosal capillaries (white arrow), connecting vessels passing between muscle bundles (dark arrow) to larger post-capillary venules located outside the muscle layer. On the right, the same features are shown at a higher magnification.

(a)

(b)

Fig. 6.6. (a) Peripheral conducting airway from a patient who died of asthma. The airway lumen is filled with an inflammatory exudate that contains plasma proteins, inflammatory cells, sloughed epithelial cells, and mucus. This process that causes epithelial sloughing (not shown in figure) results in metaplastic changes in the epithelium, which increases goblet cells. There is also an increase in muscle and connective tissue, but the caliber of the lumen is not reduced. (b) Airway from a patient with COPD, where the caliber of the airways is reduced by a peribronchiolar inflammatory process with connective tissue deposition in the adventitia.

Airway Remodeling

Chapter

7

Rory A. O’Donnell, Donna E. Davies, and Stephen T. Holgate School of Medicine, University of Southampton, Southampton, UK

Airway remodeling may be defined as a process of sustained disruption and modification of structural cells and tissues leading to the development of a new airway wall morphology. Although this process was noted as long ago as 1922,1 interest in the mechanisms of airway remodeling in asthma has risen only in recent years. In chronic obstructive pulmonary disease (COPD), though airway structural changes are well recognized,2 the underlying pathogenesis has received less attention, possibly due to their more peripheral location and the overshadowing interest in the adjacent emphysematous tissue destruction. In both conditions a number of remodeling processes occur (Fig. 7.1 and Table 7.1). Principal among these changes are airway fibrosis, an elevation in smooth muscle mass, mucous metaplasia, and glandular hypertrophy, in addition to less well defined alterations of the bronchial vasculature and nerves. The outcome is a very abnormal airway wall. In asthma, segmental and subsegmental bronchial walls are thickened over their entire size range.3 In COPD, only the inner wall area of the large airways is convincingly thicker.4 The peripheral airways (2 mm diameter), normally devoid of either supporting cartilage or bronchial glands, are also conspicuously remodeled in COPD.5,6

N AT U R A L H I S T O RY A N D C L I N I C A L I M P O R TA N C E The natural history of remodeling is not well understood. Though it is surmised to be a consequence of long-term airways disease, studies have revealed early manifestations in asthmatics7 and young smokers,8 suggesting that remodeling is part of the primary pathology of the disease rather than the result of it. However, a considerable degree of variability in susceptibility to remodeling changes exists in both patient groups. In general, asthma is associated with a greater than normal decay in lung function over time.9 In a small proportion of asthma patients with severe chronic disease the decline in lung function is more progressive and associated with persistence of symptoms despite corticosteroid therapy, similar to a COPD-like clinical state.10 These

physiological changes may reflect not only the presence of airway structural modification but also the inadequacy of currently available treatments to modify the remodeling process. Airway remodeling may have a number of clinical consequences for both asthma and COPD. For example, bronchial hyperresponsiveness has been postulated to be a function of the physical effects of overall airway wall thickening, in addition to airway inflammation and smooth muscle reactivity.11 Individual remodeling changes, such as an increased smooth muscle bulk or an enhanced mucusproducing facility, have their own implications for both symptom severity and airway function.

MECHANISMS OF REMODELING Epithelial damage, airway inflammation, and epithelial repair Responses of the airway epithelial barrier to injury and manipulation and abnormalities in the ensuing processes of repair are the most likely causes of remodeling. In asthma, evidence suggests that epithelial repair processes have the crucial capacity, via intercellular signaling between epithelial and subepithelial (myo)fibroblasts, to influence matrix synthesis and the mass and composition of structures lying beneath it. It follows, that, in asthma, dysregulated injury and/or repair responses to innocuous environmental stimuli may result in more permanent changes to airway wall morphology. These epithelial–mesenchymal interactions have invited parallels with the intercellular communication found during branching morphogenesis, and remodeling has been suggested to reflect a reactivation of these early processes. In contrast with asthma, COPD is thought to be due almost exclusively to the toxic effects of cigarette smoke. It is possible that repetitive attempts by the epithelium to protect itself and repair the injury induced by this noxious agent can result in analogous mesenchymal responses. The importance of airway inflammation, integral and intimately linked to the process of epithelial injury, is

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Normal

Cigarette smoke

COPD Allergen

Asthma

Intact epithelium

Stress/damage

Altered epithelial phenotype

Proinflammatory cytokines Chemokines

Profibrogenic growth factors matrix proteins

Inflammatory cells

Fibroblasts

Smooth muscle cells

Blood vessels

Nerves

Fig. 7.1. Schematic of events leading to airway remodeling.

recognized in both diseases.12 In asthma, much interest has focused on the CD4 cell that is skewed toward a Th2 phenotype as the orchestrator of an immune response to allergens.13 The predominant pattern of Th2 cytokine expression results in an inflammatory phenotype where eosinophils and mast cells are prominent but macrophages, fibroblasts, and dendritic cells are also involved.14 The inflammatory milieu differs considerably in COPD. Eosinophils may play a role in a subset that has features in common with asthma,15 but their significance in the majority is outweighed by the contribution of macrophages and neutrophils with possibly a CD8 cell preponderance.16–18

Known perpetrators of epithelial damage include environmental agents such as cigarette smoke toxins, house dust mite allergen19 and pollen enzymes,20 with their effects being augmented by the airway inflammatory response to these agents. Thus generation of reactive oxygen species and the products of infiltrating inflammatory cells including arginine-rich eosinophilic proteins,21 mast cell tryptase,22 neutrophil elastase, and metalloproteases derived from eosinophils and mast cells,23,24 neutrophils and macrophages can all directly induce epithelial injury. The result in asthma is a fragile bronchial epithelium with weakening of junctional adhesion structures. Consequently, columnar

Airway Remodeling

69

Table 7.1. Table of remodeling processes in asthma and COPD

Remodeling process

Asthma

COPD

Cellular inflammation

Predominantly CD3, CD4 Eosinophils Mast cells

Predominantly CD3, CD8 Neutrophils Macrophages

Cytokine expression

Th2 profile – IL-4, IL-5, IL-13 RANTES

TNF-a, IL-8, LTB4

Airway wall thickening

Large and small airways

Large and small airways

Epithelium

Fragile, sloughing, denudation

Squamous metaplasia

Mucous metaplasia

Large and small airways

Large and small airways

Subepithelial basement membrane

Thickened

Normal

Diffuse excess matrix deposition

Large airways

Small airways

Submucosal glands

Hypertrophy

Hypertrophy Increased mucous to serous acini ratio

Smooth muscle

Increased mass in large and small airways

Increased mass in large and small airways

Large airway wall vascularity

Increased

Undetermined

epithelial cells become detached from the basal cells below them25 and are sloughed into the airway lumen.26–28 In COPD, although atrophy and shedding of the epithelium may occur,26 focal squamous metaplasia is much more common.12,29 Altered airway adhesion molecule expression, a hallmark of an active repair process, has been demonstrated in asthma and COPD. This includes enhanced expression of CD44,30 the integrins,31 and E cadherin32 in asthma, while in both asthma and COPD there is augmented expression of ICAM-1.33 Epithelial expression of the epidermal growth factor receptor (EGFR/c-erbB1) is also increased in asthma, and reflects the response seen in other injured epithelial tissues. The EGFR is the target of six distinct ligands: • • • • • •

epidermal growth factor (EGF); transforming growth factor a (TGF-a); amphiregulin (AR); heparin-binding EGF-like growth factor (HB-EGF); betacellulin (BTC); epiregulin.

Activation of the EGFR promotes both migration and proliferation of epithelial cells,34 and there is in-vitro and invivo evidence supporting a role for this receptor in bronchial epithelial repair.35,36 However, in asthma, increased EGFR expression does not appear to be coupled to an appropriate proliferative response by the repairing epithelium.37 This

impairment may explain why EGFR expression, as a marker of tissue injury, correlates positively with both asthma severity and the extent of subepithelial fibrosis.38 The impaired repair has been postulated to be due to the actions of the profibrogenic transforming growth factor beta (TGF-b) family of cytokines which are known inhibitors of epithelial proliferation39 and whose level is increased in asthmatic airways.40 One consequence of impaired EGFR-mediated repair is that the duration of activation of other non-EGFR dependent repair processes, involving release of inflammatory cytokines and profibrogenic growth factors, may be increased, thereby providing constant stimuli for chronic inflammation and remodeling.38 Although studies of the EGFR in COPD have been lacking, it has been reported that cigarette smoke results in enhanced EGFR expression in airways epithelium.41 Augmented expression of both epidermal growth factor (EGF) and TGF-b have been observed in subjects with chronic bronchitis,40 so a potential imbalance exists between proliferative and antiproliferative signaling in the airways in both asthma and COPD. However, as EGFR-mediated mitogen activated protein kinase (MAPK) activation can inhibit TGF-b signaling, the final outcome may differ in each disease depending on the absolute ratios of the EGF and TGF-b growth factor families and the relative levels of expression of their receptors. The relationship between epithelial injury and fibrosis has been demonstrated using co-cultures of bronchial epithelial

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cells and myofibroblasts. In these studies, damage inflicted upon the epithelial cells resulted in enhanced myofibroblast proliferation and collagen gene expression owing to the combined effects of a number of growth factors, including: • • • • • •

basic fibroblast growth factor (bFGF); FGF2; insulin-like growth factor (IGF); platelet-derived growth factor (PDGF); TGF-b; ET-1.42

In addition to these, a range of other potentially important epithelially derived products such as fibronectin, an extracellular matrix component with potent chemotactic properties for fibroblasts,43 and a range of proinflammatory cytokines44–47 are produced by epithelial cells during the repair process. Remodeling of the extracellular matrix The extracellular matrix is composed of a network of fibrous and structural proteins embedded in a hydrated polysaccharide gel to form the strong, resilient framework of the airway wall. Alteration in both the mass and composition of this structure is one of the primary aspects of remodeling in both asthma and COPD, with upregulated airway fibroblast activity being most directly responsible. The repair environment in the airways produces a number of changes in cytokine and growth factor expression to which the fibroblast is receptive. Studies in asthma have indicated that myofibroblasts, a cell population with morphological and biochemical features intermediate between those of fibroblasts and smooth muscle cells,48 are responsible for the particular pattern of matrix deposition in this disease. These cells, lying below and adjacent to the lamina reticularis in a layer referred to as the “attenuated fibroblast sheath” are major producers of collagenous and noncollagenous matrix proteins implicated in matrix accumulation, as well as wound contraction.49 In the absence of specific immunomarkers for this cell type, myofibroblasts are identified by specific ultrastructural criteria and the expression of a-smooth muscle actin (a-SMA) which is also found in smooth muscle cells and myoepithelial cells. Though myofibroblast hyperplasia has been demonstrated in the subepithelium in asthma,48,50 with their numbers correlating with the extent of subepithelial collagen thickness,50,51 their origin in asthma is uncertain. They appear early after allergen challenge, perhaps implying a quiescent precursor cell that differentiates to acquire myofibroblast features.48 Candidates for this include fibroblasts, smooth muscle cells or perhaps other primitive mesenchymal or structural cells. A number of factors have been identified which appear to alter fibroblast a-SMA expression: • most notably TGF-b;52 • heparin;53 • gamma-interferon;

• granulocyte–macrophage (GMCSF).

colony

stimulating

factor

Alternatively, proliferation or preferential chemoattraction of resident myofibroblasts already present beneath the bronchial epithelium may be occurring.50 For example, smooth muscle mitogens have been detected in BAL fluid after allergen challenge,54 and fibronectin has been observed to cause selective recruitment of myofibroblasts from a population of normal fetal lung fibroblasts.55 In COPD, studies investigating the presence or absence of myofibroblasts are lacking. The role of growth factors and inflammatory mediators The use of transgenic animal models of mediator overexpression has provided evidence to link both epithelial growth factor and inflammatory cytokine expression with matrix remodeling. TGF-b, overexpressed in the rat airway epithelium, has been shown to produce severe interstitial and pleural fibrosis,56 while overexpression of IL-6 or IL-11 induces subepithelial fibrosis in the lungs of experimental mice.57,58 Mouse models variously expressing the Th2 cytokines IL-5,59 IL-9,60 and IL-1361 have all, to a greater or lesser degree, developed like matrix changes. In contrast, mice overexpressing IL-4 have demonstrated inconsistent results in this regard.62,63 The TGF-b superfamily of cytokines are among the most studied with respect to fibrosis and remodeling. TGF-b is a multifunctional profibrotic growth factor and a key component in the regulation of tissue growth and differentiation, both during branching morphogenesis and the processes of wound repair. It stimulates fibroblast growth and is responsible for the differentiation of fibroblasts into myofibroblasts, causing myofibroblast hyperplasia.42,64,65 TGF-b promotes synthesis of extracellular matrix components by a number of cells including fibroblasts, bronchial epithelial cells, and macrophages,66 and it blocks matrix degradation by inhibiting proteolytic enzyme synthesis and augmenting the action of protease inhibitors. As stated earlier, studies have demonstrated augmented epithelial and submucosal TGF-b expression in both asthma and COPD.40,67,68 In addition, tissue macrophage TGF-b production is elevated in both diseases, while in asthma eosinophils and epithelial cells have also been identified as important sources.40,69,70 Moreover, in these conditions a significant correlation has been demonstrated between expression of TGF-b and the extent of subepithelial fibrosis and numbers of fibroblasts.40 TGF-b levels are also unaffected by corticosteroid treatment, and so this growth factor probably represents a key mediator of airway matrix remodeling. Another growth factor is platelet-derived growth factor (PDGF). This is produced mainly in the airways by macrophages but may also be derived from the epithelium and most airway inflammatory cell types. Eosinophils rather than macrophages are the more important source of PDGF in asthma.71,72 PDGF is known to promote fibroblast

Airway Remodeling

chemotaxis, fibrosis, and smooth muscle mitogenesis,73 and would seem to be a very plausible promoter of airway remodeling. Indeed, bronchial fibroblasts from asthmatic patients show enhanced responsiveness to this agent.50 Nevertheless, expression of PDGF has not been found to be elevated in the airways of asthmatics or patients with COPD,74,75 so its relevance in these diseases has yet to be demonstrated. Another family of peptides, the endothelins, are found in elevated quantities in both diseases.76–78 These agents, in addition to their vaso- and bronchoconstrictor properties, possess the ability to activate fibroblasts79 and so possibly also contribute to excess matrix deposition. Although epithelial EGFR expression has been shown in asthma to correlate with the extent of subepithelial fibrosis, the importance of the EGFR as an effector of matrix remodeling in cells of nonepithelial origin in asthma and COPD is uncertain and requires investigation. Stimulation of fibroblast-bound EGF and PDGF receptors is reported to work in concert with binding of b1 integrins on the surface of these cells by matrix components such as laminin or fibronectin to stimulate fibroblast chemotaxis and migration.80,81 Thus, alterations in growth factor and receptor expression, matrix composition, and adhesion molecule binding all have the potential to influence fibroblast activity in repair. However, although EGF immunoreactivity has been demonstrated in the submucosa of both asthmatic and COPD subjects, the number of cells found expressing EGF did not correlate with either basement membrane thickness or fibroblast number.40 Epithelial expression of TGF-a, another EGF ligand, results in lung fibrosis; but the effect appears to be mediated via the epithelium rather than by direct action on fibroblasts.34 The role of proteolytic enzymes Alteration in matrix turnover is but one of the ways in which proteolytic enzymes influence the remodeling process. Serine and matrix metalloproteases produced by a variety of inflammatory and stromal cells can digest all the major components of the extracellular matrix.82 Examples studied include MMP3 in asthma,83 MMP1 in COPD (with regard to its potential to induce emphysema),84 and MMP9 and neutrophil elastase in both diseases.24,85 Although in emphysema it is widely believed that a protease/antiprotease imbalance exists favoring the excessive proteolytic digestion of lung parenchyma, there is evidence to suggest that in the remodeled airway wall the reverse holds true. For example, although both MMP9 and neutrophil elastase are elevated in induced sputum of asthma and COPD patients, the augmented presence of these proteases is outweighed by a proportionally larger increase in their natural inhibitors TIMP1 and a1-antitrypsin,86,87 a situation which would favor fibrosis. The release of growth factors that are encrypted in the extracellular matrix or from their cell-membrane bound precursors is another way that proteases may contribute to remodeling. For example, both FGF and TGF-b are bound as inactive forms to heparan sulfate and decorin,

71

respectively,88,89 and can be released by the proteolytic activity of plasmin. In contrast, release of heparin-binding EGF, a potent smooth muscle mitogen, is dependent on MMP3-induced cleavage of its transmembrane precursor.34 In addition, mast cell tryptase acting via cellular PAR2 receptors acts directly on fibroblasts to promote both mitogenesis and collagen secretion,90 and is also mitogenic for epithelial91 and smooth muscle cells. Effects of the altered matrix on remodeling The capacity of the extracellular matrix to influence (directly and indirectly) cell development, migration, and proliferation is indicative of a more dynamic function than simply the provision of structural support.66,92 Matrix components such as decorin, versican, and fibronectin provide stimuli for inflammatory, epithelial and stromal cells by serving as ligands for adhesion molecules93 and by acting as a reservoir for release of cytokines, chemokines, and growth factors such as TGF-b.94 Glycosaminoglycans such as hyaluronic acid (HA), found to be increased in the BAL fluid of asthmatics,95 facilitate cell migration and proliferation during injury and repair,96 while survival of eosinophils is prolonged by interaction with the matrix proteins fibronectin and laminin, an effect due in part to eosinophil GMCSF release.97 Fibronectin is a matrix glycoprotein that has received much attention with regard to its role in repair.66,98 It is produced by many cell types, including bronchial epithelial cells,99 fibroblasts, and macrophages,67 and its production is upregulated by growth factors such as TGF-b100 and integrin ligation.101 Fibronectin is incorporated into the provisional matrix that forms after injury, where it acts as a cheomattractant for both epithelial cells and fibroblasts.43,99 Elevated quantities of fibronectin have been detected in bronchial lavage fluid in COPD, while in asthma it is found abnormally deposited in the sub-basement membrane.50,102 In both diseases the macrophage has been implicated as an important source of this protein.67

R E S U LT S O F M AT R I X R E M O D E L I N G I N ASTHMA AND COPD The pattern and distribution of matrix deposition in the remodeled airway differs between asthma and COPD. Interest in asthma has centered mainly on the larger airways, whereas in COPD what information is available applies generally to the small, peripheral branches of the bronchial tree. Matrix remodeling may be roughly subdivided into either thickening of the subepithelial basement membrane, or a pattern of deeper, more diffuse deposition. Subepithelial basement membrane matrix deposition in asthma and COPD Deposition of protein beneath the true epithelial basement membrane is characteristic of asthma. Electron microscopic and immunohistochemical analysis of bronchial

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biopsy specimens has shown that the true basement membrane, made up of the lamina rara and lamina densa, is normal in both size and composition and that the changes occur in the lamina reticularis. In this region, abnormal deposition of interstitial repair collagen subtypes I, III, and V takes place,102 in addition to noncollagenous matrix components that include fibronectin,102 laminin b2,103 and tenascin.104 Classical epithelially derived membrane subtypes, such as collagens IV and VII, are absent from this abnormal subepithelial matrix layer. Studies of COPD have not, to date, revealed an equivalent to the subepithelial fibrosis observed in asthma. Studies appear to show, rather, a basement membrane thickness within the normal range.16,26 Exceptions may exist, however, in a subset of patients with COPD who display some overlap with the asthmatic phenotype. These individuals who have a BAL eosinophilia and significant corticosteroid reversibility also show demonstrable basement membrane thickening, but the composition remains unknown.15 Further studies are needed to properly define the clinical and pathological characteristics of this subgroup. Diffuse matrix deposition in asthma and COPD Studies evaluating collagen deposition deep to the lamina reticularis in asthma have been few. An excess of collagen including types III and V have been found in large airway samples by some investigators,105,106 but not by others.102,107 Other matrix proteins and glycoproteins found in excess in this region in asthma include decorin, versican, and fibronectin.108 Excess matrix deposition in COPD, thus far, has consistently been identified only in the peripheral, noncartilaginous airways (2 mm diameter). However, though peribronchiolar fibrosis has been repeatedly demonstrated,29,109 the studies have usually been qualitative analyses of small airway morphology rather than specific evaluations of the extracellular matrix. However, one immunohistochemical investigation has demonstrated diminished quantities of the interstitial proteoglycans decorin and biglycan in the peripheral airways, with staining patterns for type IV collagen and laminin similar to those of control lungs.110

M U C O U S M E TA P L A S I A In both asthma and COPD, epithelial mucous metaplasia and hyperplasia occur with hypertrophy of the submucosal gland mass. Submucosal mucous glands are distributed throughout the cartrilagenous airways in normal individuals. An increase in both the number and size of mucus-secreting cells leads to enlargement of these tracheobronchial glands, this being a feature of both diseases.111,112 In COPD, unlike asthma, a variable degree of replacement of serous with mucous acini occurs.113 Epithelial mucous cell metaplasia is observed in both central and peripheral airways in asthma.114,115 However, postmortem studies suggest that a substantial degree of

individual variation exists with more pronounced peripheral airway changes found in those who died from asthma;114 this observation is compatible with the fact that most asthma deaths occur in association with excessive mucous production.116 In COPD, mucous metaplasia and hyperplasia are observed both centrally and peripherally,2,109,111 resulting in a more even distribution of surface secretory cells across the spectrum of airway size. Thus, the smaller (400 lm diameter) airways, which normally have only a sparse population of mucous cells, become important contributors to the excess mucous which characterizes this disease.8,117 The extent of mucous production varies widely across the COPD spectrum of diseases, so there is considerable variation in the degree of metaplastic change in this condition.112 The mucus produced in asthma and COPD is qualitatively and quantitatively abnormal with major alterations in both its cellular and molecular composition. For example, the elevated ratio of mucous/serous acini in COPD results in a more gel-like, thicker substance, lacking in the lysozymes and antiproteases which normally provide protection against both infection and proteolytic injury.118 In chronic and severe asthma, bronchial obstruction due to mucus plugging is found in both central and peripheral airways.114,119 In COPD, partial or complete occlusion of the small (2 mm diameter) airways with mucous plugs is common.5 In both diseases, replacement of surfactant lining the small airways with mucus results in increased surface tension at the air– liquid interface further predisposing to airway collapse.120 Expression of mucin genes, which encode the mucin glycoproteins, has been proposed as the principal factor governing the differentiation of epithelial cells into goblet cells.121 Both environmental and host factors, acting on the epithelium, have been shown to stimulate mucin gene upregulation and mucin secretion. Environmental factors include infectious agents122 and environmental pollutants,123 while host factors include inflammatory mediators124 and cell degranulation products.125,126 Acrolein, a low-molecularweight component of cigarette smoke, induces epithelial MUC5 AC gene expression, and mucous metaplasia in rats in vivo123 while the Th2 cytokines IL-4, IL-9, and IL-13 have been linked to both augmented mucin gene expression, particularly the MUC5AC and MUC2 genes, and goblet cell differentiation, using in-vivo animal models and in-vitro culture systems.60–62,127–130 These observations suggest that mucus overproduction represents an innate defense mechanism designed to limit the access of noxious stimuli to the epithelium. A central role has been postulated for the neutrophil in stimulation of airway mucin production. This is based on the observation of augmented MUC5AC expression by cultured epithelium when exposed to neutrophils,131 possibly through neutrophil elastase and oxidative stress, each of which has been shown to augment both epithelial MUC5AC mRNA and protein expression.126,131–133 It has been further proposed that the EGFR is involved in regula-

Airway Remodeling

tion of airway mucin synthesis,134,135 as mucin gene expression results from ligand-independent transactivation of the EGFR in response to oxidative stress.136 This effect can be blocked by selective inhibitors of the EGFR.131 Both EGF and EGFR expression are elevated in bronchial glandular tissue in asthmatic subjects compared with controls,137 suggesting a similar role for the EGFR in asthma. IL-13 may cause induction of mucin gene expression via an EGFRdependent pathway. This cytokine, like cigarette smoke components such as acrolein, is also believed to cause neutrophil infiltration through stimulation of expression of neutrophilic chemokines.

SMOOTH MUSCLE REMODELING The presence of an augmented airway smooth muscle bulk in both asthma and COPD is well established. In asthma, smooth muscle mass is increased in both large138 and peripheral139,140 airways, though whether hypertrophy or hyperplasia is the major contributor remains uncertain. Autopsy studies suggest the existence of two distinct patterns. The type 1 pattern has increased muscle mass due to hyperplasia restricted to large central airways, while in type 2 there is smooth muscle thickening throughout the bronchial tree caused predominantly by hypertrophy, particularly in the small airways, with a mild degree of hyperplasia in the larger airways.141 The situation is further complicated by the fact that a considerable degree of heterogeneity appears to be present. An increase in smooth muscle mass is not always demonstrated in mild asthma142–144 or in postmortem studies of asthmatics who died from other causes.145 Why different patterns of smooth muscle enlargement should exist in asthma or what factors predispose some but not other asthmatics to the development of either one is unknown. The peripheral airways are also the location of a large increase in smooth muscle mass in COPD,111,117,142,146 though it is suggested that the changes are less marked than those in asthma.142 Studies generally describe these changes under the heading of hypertrophy, but the extent to which hyperplasia plays a role is unclear. Estimates of smooth muscle enlargement in the larger airways in COPD have varied.147 Some investigators have observed no abnormality in smooth muscle area,138 while others have demonstrated up to a 2-fold increase in bronchial smooth muscle thickness, a finding attributable to both hyperplasia and hypertrophy.148 A clinical correlation with the presence of wheeze has been suggested, but there is a substantial degree of individual variation in both wheezing and this particular remodeling change.149 The mechanisms leading to these changes are not known. The exercise-like effects of smooth muscle spasm, the growth promoting potential of an enriched plasma environment due to vascular leakage,150 the mitogenic influences of mediators involved in the inflammatory response, or an intrinsic abnormality of smooth muscle itself have been

73

offered as explanations. However, most of our knowledge of factors promoting smooth muscle growth has come from analysis of cell cultures in vitro.151 An extensive range of potentially important smooth muscle mitogens has been identified, including: • • • •

inflammatory mediators; growth factors; enzymes; components of the extracellular matrix.151

It has been suggested that these can be subdivided into two broad categories depending on the receptor type through which they exert their major mitogenic influences. Group 1 includes mediators such as EGF and PDGF which activate receptors with intrinsic tyrosine kinase activity in human airway smooth muscle (ASM) cells. Group 2 includes those activating receptors coupled to GTP binding proteins such as thrombin.152 Although ET-1 and LTD4 are effective in animal models, they appear to have little effect on human ASM cells, though they are believed to potentiate the effects of thrombin and EGF.152 TGF-b has the ability to either promote or inhibit smooth muscle growth,153 while both EGF and EGFR immunoreactivity is augmented in asthmatic airway smooth muscle.137 Contractile agonists are found in both asthma and COPD, as are inflammatory mediators with known smooth muscle mitogenic potential such as TNF-a.17 A further association between inflammation and smooth muscle function has been suggested by the enhanced expression of adhesion molecules such as ICAM-1 and VCAM-1 on airway smooth muscle cells in response to TNF-a, IL-1, LPS, and IFN.154 Furthermore, the adherence of T cells to smooth muscle cells results in stimulation of ASM DNA synthesis.154 The capacity of these ASM to express adhesion molecules, synthesize ECM components and release inflammatory cytokines and chemokines including RANTES,155 eotaxin, and IL-8151 indicates that smooth muscle remodeling may itself be part of the inflammatory process.

VA S C U L A R A N D N E U R A L A LT E R AT I O N S Elevated airway-wall blood-vessel area has been demonstrated in asthma and is greater than in controls or subjects with COPD.139,142 However, it is unclear whether this vascular remodeling is primarily due to sprouting angiogenesis – the formation of new blood vessels156 – or to enlargement of the existing microvasculature. A study of the membranous bronchioles of subjects with asthma and COPD has suggested the latter,142 and indeed fatal asthma is known to be associated with dilatation of bronchial mucosal blood vessels, congestion, and wall edema.112 On the other hand, Li and colleagues157 have discovered changes suggestive of new vessel formation in mild asthma, and the microenvironment in asthma has been shown to possess the potential for

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angiogenesis. Mediators such as histamine, heparin, and tryptase all possess angiogenic properties, while expression of vascular endothelial growth factor (VEGF), a potent vascular growth factor, is upregulated by agents such as TGF-b, TNF-a and TGF-a that are involved in the inflammatory milieu of the asthmatic airway. There is also evidence in severe asthma of increased airway innervation. This could be due to the secretion of nerve growth factors from epithelial and inflammatory cells. In addition, in both asthma and COPD, the compromised epithelial barrier allows a greater exposure of nerve endings to environmental stimuli. Thus the potential for release of neurotransmitters such as the tachykinins substance P and neurokinin A is increased. These agents, in addition to their effects on vascular and smooth muscle homeostasis, can contribute to local inflammation with the attraction and activation of inflammatory cells.

EFFECTS OF THERAPY Whether any of the current anti-inflammatory strategies can significantly alter the course of remodeling is unknown. Long-term corticosteroid therapy has been shown to slow the annual rate of decline in lung function in asthmatics.158 However, other studies have suggested that a negative relationship exists between response to treatment and duration of disease.159,160 Added to this are reports showing persistent airflow obstruction in some patients, despite both oral and inhaled therapy.161 The implication is that early therapy may, to a limited extent, prevent remodeling, but established structural change is steroid-insensitive. In-vitro work suggests possible benefits from other current therapies such as b2 agonists, shown to inhibit smooth muscle cell proliferation,162 or theophylline, which has a similar effect on cultured fibroblasts.163 However the long-term in-vivo impact on remodeling remains unknown. It has been proposed that more effective strategies for the future would focus directly on remodeling pathways. Suggested approaches include blocking the actions of TGF-b in an attempt to limit fibrosis, or inhibition of EGFR signaling to prevent mucous metaplasia or smooth muscle hypertrophy. Careful assessment of the benefits would be required, however. Mucus overproduction, for example, may be fundamental in limiting the access of damaging environmental agents to the epithelium. A protective role for basement membrane thickening has been postulated also in restricting inflammatory cell passage to the epithelial layer above, while smooth muscle hypertrophy may possibly help to maintain bronchiolar caliber in severe emphysema.164 Thus if a new “anti-remodeling” agent were to emerge, the beneficial effects would have to be weighed against the consequences of interfering with what may, in some ways, be a valuable defense mechanism.

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Animal Models

Chapter

8

Stephanie A. Shore Harvard School of Public Health, Boston, MA, USA

Animal models have been used extensively in the process of discovery of the primary events that underlie asthma and chronic obstructive pulmonary disease (COPD). For example, the demonstration by Gross et al.1 that instillation of papain into the lungs of rats resulted in a syndrome similar to emphysema contributed to the prevailing elastase/ antielastase hypothesis regarding the pathogenesis of emphysema. Animal models have also been used to guide research into pharmacological interventions for the treatment of asthma and COPD. This topic is extensive, so this chapter focuses on only two models:

This chapter will focus on models developed using mice because of the advantages of using this species, including: • low cost; • a short breeding period; • the availability of inbred species with known characteristics; • good genetic markers; • a well-characterized immune system; • the ability to induce genetic modifications. Allergens used in these models have included:

• mouse models of asthma induced by ovalbumin sensitization and challenge and; • animal models of chronic bronchitis as created by exposure to high concentrations of SO2 gas. Each of these areas will be reviewed separately. In each case, the focus will be on changes in the mechanics of the airways, since it is airway obstruction associated with these conditions that ultimately causes lung dysfunction, disability, and death.

MOUSE MODELS OF ASTHMA INDUCED B Y A L L E R G E N S E N S I T I Z AT I O N A N D CHALLENGE Human asthma is characterized by intermittent reversible airway obstruction, airway hyperresponsiveness, and airway inflammation. Although there is no animal that spontaneously mimics all these traits, a syndrome with some features of asthma can be observed in certain horses and cats, and in the Basenji-greyhound cross of dogs. As a result, many investigators have chosen to develop animal models in which an asthma-like phenotype is induced by some intervention. The strategy most often employed is allergen sensitization and challenge. A variety of species have been utilized in such models, including dogs, sheep, guinea pigs, monkeys, rats, and mice.

• • • • •

ovalbumin (OVA); picryl chloride;2 sheep erythrocytes;3,4 short ragweed extracts;5 house dust mite allergen.6,7

Although the last named is likely to be particularly relevant given the importance of house dust mite allergies in asthma, most investigators have used OVA as the allergen, perhaps because of its relatively low cost and availability. Ovalbumin sensitization and challenge Many investigators have used models of asthma in which mice are first sensitized to OVA and then challenged with OVA via the lungs, but both the sensitization and challenge protocols vary widely among investigators. There are differences in the amount of OVA used to sensitize the mice, the route of sensitization (systemic versus airway), whether or not adjuvant is used, the number of sensitizations, and the number and method of pulmonary challenges (intranasal, intratracheal or aerosol). Both single and multiple challenges have been utilized. Mice are usually assessed 1–3 days after the last challenge. Consequently, the characteristics assessed are those related to the late response to allergen challenge, and airway obstruction is not usually evaluated. Consistent features of the model are increased ovalbuminspecific IgE and IgG1, eosinophilia (intraluminal, peribronchial, and perivascular), and recruitment of lymphocytes

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to the airways. Increases in Th2-type cytokines, either in BAL fluid, in lung tissue, or in lymph nodes draining the lungs also occur.3,8,9 Airway hyperresponsiveness (AHR) is often observed, though not always.3,10–12 There are few reports of comparisons of the various protocols in terms of their ability to induce AHR, but those available indicate that both systemic sensitization and pulmonary challenge are required to induce AHR.9 Hamelman et al.13 reported that multiple airway challenges without systemic sensitization can result in pulmonary eosinophilia and changes in the in-vitro responsiveness of the trachea to electrical field stimulation (EFS), but in-vivo AHR was not observed under these conditions. The challenge protocol used may influence AHR by altering the nature of the inflammatory response.9 The intensity of the challenge can also influence the development of AHR, although this varies across strains.11 Investigators using such models have consistently observed an important role for T cells in the development of an asthma-like phenotype.14,15 It is generally agreed that an important function of these cells is the generation of cytokines and their effects on IgE synthesis, eosinophil recruitment, and AHR; so a major focus of investigations using these models has been to understand the relationship among these three characteristics of the response to allergen. However, the results of these studies have not been consistent, perhaps because there are redundant pathways for the development of AHR that are influenced not only by the precise protocols used to sensitize or challenge the mice, but also by differences in the strain of mouse, and the method used to assess AHR. Role of IgE and mast cells in allergen-induced AHR Mast cells express receptors (FceRI) on their surface that bind the Fc portion of IgE with high affinity. Crosslinking FceRI receptors upon binding of allergen to IgE results in the secretion of a panel of mediators including histamine, eicosanoids, and cytokines, many of which have the capacity to elicit AHR. An important role for mast cells and IgE in the early response to allergen inhalation has been demonstrated in multiple species, including the mouse.16 However, their role in the late response to allergen and in allergeninduced AHR in particular is less clear. Animal models, particularly mast cell-deficient KitW/KitW-v mice which are genetically deficient in c-kit, the receptor for an important mast cell growth and survival factor, as well as mice genetically deficient in B cells and which lack IgE, have been utilized to help define this role. Activation of mast cells has the capacity to induce AHR. C57BL/6 mice treated with anti-IgE antibodies 20 minutes prior to measurements of airway responsiveness show increased responses to intravenous methacholine compared with animals treated with an irrelevant antibody.17 The increased responsiveness is not observed in mast celldeficient mice, but is restored when mast cells are reconstituted in these mice by infusion of bone marrow-derived cells. After passive sensitization of mice with OVA-specific

IgE, challenge with aerosolized OVA results in increased responsiveness to EFS of tracheae of these mice.18 Similar results are obtained after passive sensitization with OVAspecific IgG1, consistent with reports that this class of antibodies also participates in allergic responses in the mouse. Mice sensitized and challenged via the airways also develop increased tracheal responses to EFS, but these are not observed in B cell-deficient mice, suggesting that either IgE or IgG1 are necessary to induce this effect.13 Kung et al.19 reported that aerosol OVA challenge of systemically sensitized mast cell-deficient mice resulted in fewer eosinophils in BAL fluid and lung tissue and reduced eosinophil peroxidase levels compared with congenic controls, but these authors did not examine airway responsiveness. Kobayashi et al.20 performed similar experiments and did observe a reduction in AHR. Further, they observed that reconstitution of mast cell-deficient mice with bone marrow-derived cells restored the ability of these mice to develop AHR upon sensitization and challenge. Taken together, the results suggest that IgE acting on mast cells may contribute to allergen-induced AHR, but do not rule out a role for IgG1 or of effects of IgE on low-affinity receptors present on other cell types. In contrast, other investigators have not confirmed a role for either mast cells or IgE. MacLean et al.8 were unable to observe any difference between wild-type and B celldeficient mice in the lung eosinophilia, AHR, or increased serum cytokines induced by systemic sensitization and repeated aerosol challenge, despite unmeasurable levels of IgE and IgG1 in the B cell-deficient mice. Similar results were obtained by Hamelman et al.13 In addition, Takeda et al.21 reported that mast cell-deficient mice were identical to strain-matched controls in their ability to develop pulmonary eosinophilia and AHR after systemic sensitization and repeated airway challenge with ovalbumin. It is apparent that allergen-induced AHR can occur in the absence of an IgE response, since Brewer et al.11 reported that the AKR strain of mouse was among the strains with the most robust changes in airway responsiveness after ovalbumin sensitization and challenge, even though no increase in IgE was observed in this strain after sensitization. Hogan et al.22 also reported that IL-4 deficient mice, which failed to develop IgE after OVA sensitization, nevertheless had robust AHR upon airway challenge. At least part of these discrepancies are likely to be related to the precise protocol used to sensitize and challenge the mice. The studies cited above which support a role for IgE and/or mast cells in the response to allergen13,18–20 used protocols which resulted in the recruitment of only a few eosinophils, while those which suggested that IgE and/or mast cells were unimportant used protocols in which eosinophils constituted the majority of the cells in BAL fluid.8,11,21,22 Indeed, Kobayahsi and colleagues used two distinct protocols to sensitize and challenge mast cell-deficient mice and found reduced responses compared with strain matched controls when relatively mild protocols for airway challenge were employed, but no difference between mast

Animal Models

cell deficient and control mice when more frequent and robust airway challenges were used.20 They suggested, as have others,23,24 that AHR could be induced by at least two mechanisms, one involving mast cell activation in response to IgE crosslinking, and another involving eosinophils. The latter is discussed below. Role of eosinophils in allergen-induced AHR Eosinophils contain cationic proteins such as major basic protein (MBP) that have the capacity to induce AHR by damaging the airway epithelium, thus likely permitting greater access of inhaled bronchoconstrictors to the underlying airway smooth muscle.25 MBP also acts as an antagonist of M2 muscarinic inhibitory receptors on cholinergic nerves, resulting in greater release of aceytlcholine.26,27 Although the role of eosinophils in inducing AHR has been extensively investigated in murine models of asthma, it remains controversial. In support of a causal role for eosinophils in the etiology of allergen-induced AHR, Lee et al.28 generated transgenic mice that constitutively express IL-5 in the lung epithelium where IL-5: • promotes the differentiation of eosinophils from bone marrow precursors; • prolongs eosinophil survival by preventing apoptosis; • promotes eosinophil activation; • acts as an eosinophil chemoattractant. IL-5 trangenic mice have peribronchial and intraluminal accumulations of eosinophils even in the absence of allergen challenge. The mice also have hyperresponsive airways. While it is possible that effects of IL-5 on cells other than eosinophils, for example airway smooth muscle cells,29 might contribute to the AHR observed in these mice, the results suggest that expression of IL-5 in the airways is sufficient to induce airway eosinophilia, and that the presence of eosinophils may lead to AHR. Experiments performed in IL-5 knockout mice also support a role for eosinophils in allergen-induced AHR. Foster et al.30 used IP injection to sensitize C57BL6 mice that were genetically deficient in IL-5, and then repeatedly challenged the mice with aerosolized ovalbumin. Although levels of allergen-specific IgE were not altered, both accumulation of eosinophils in the airways and AHR were prevented in IL-5 deficient compared with wild-type mice. Importantly, the authors also reconstituted IL-5 in the knockout mice by nasal inoculation with a recombinant viral vector expressing IL-5. This treatment restored eosinophil levels in the peripheral blood, pulmonary eosinophilic inflammation, and AHR. Similar results were obtained by Hamelman et al.31 Hogan et al.22 also reported that, although AHR induced by allergen sensitization and challenge was maintained in IL-4 deficient C57BL/6  129Sv mice, treatment of these mice with a monoclonal antibody to IL-5 prior to aerosol challenge abolished both airway eosinophilia and AHR.

81

The observation of Brewer et al.,11 that there is a good correlation between airway eosinophils and AHR across multiple strains of mice all sensitized and challenged in the same way, also argues for a causal relationship between these factors, although the authors also suggested that this correlation could result from a genetic link between the genes which determine airway eosinophilia and those which determine AHR. In contrast to the results described above, data from other investigators do not support a causal relationship between eosinophils and AHR in murine models of allergen sensitization and airway challenge. Corry et al.32 treated BALBc mice with anti-IL-5 antibodies during a 4-week sensitization period prior to airway allergen challenge; they observed no change in allergen-induced AHR. Similar results were obtained by other investigators when they administered the anti-IL-5 antibody just prior to allergen challenge rather than during the sensitization period.20,33,34 Treatment with a soluble IL-5 receptor is also without effect on allergeninduced AHR.35 Despite the lack of effect of IL-5 antibodies on allergen-induced AHR, blood and/or lung eosinophils were markedly reduced in all these studies. The effect of the antibody on eosinophils indicates that an adequate amount of antibody was administered. Indeed Hessel et al.33 administered much higher amounts of antiIL-5 antibody than those required to inhibit eosinophilia and still found no effect on allergen-induced AHR. These results suggest that IL-5 is required for allergen-induced eosinophilia, but that AHR can occur even in the absence of eosinophils. The observation that AHR is observed in BALBc mice even when sensitization and challenge protocols which do not result in prominent airway eosinophilia are used6,10 also suggests that factors other than eosinophils can lead to AHR under these conditions. Genetic background may be an important determinant of the mechanism of allergen-induced AHR. Indeed, genetic background may in part explain the discrepancies in the results of studies examining the role of eosinophils in mediating AHR. The results discussed above supporting a role for IL-5 and eosinophils in allergen-induced AHR were obtained in C57BL/6 mice or offspring of C57BL/6 mice.22,28,30,31 In contrast, the studies which failed to demonstrate an important role for eosinophils in mediating the AHR observed following allergen sensitization and challenge were largely performed using BALBc mice.20,32–35 The BALBc mouse produces more allergen-specific IgE in response to allergen sensitization and challenge than does the C57BL/6 mouse,9,11 and it is possible that IgE and mast cell-mediated mechanisms are more important than eosinophils in this strain. Drazen et al.24 suggested that, because of genetic deficiencies in mast cell tryptase 7 and in secretory phospholipase A2 in the C57BL/6 mouse, these mice may lack the ability to develop IgE-dependent AHR. Although these specific genetic defects may not explain the observed effects on AHR, they are illustrative of the mechanisms that can lead to AHR, and how strain comparisons can help uncover them.

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Differences in the method used to assess AHR may also influence investigators’ ability to observe a role for IgE or eosinophils in its etiology. C57BL/6 are less prone to the development of AHR than are BALBc mice when identical protocols are used to sensitize and challenge the mice and pulmonary resistance (RL) is used as the outcome indicator.9,10 Indeed, AHR based on measurements of RL is sometimes not observed in C57BL/6 mice under conditions where BALBc mice develop robust responses. However, under these conditions AHR can be observed if dynamic compliance (Cdyn) rather than RL is used as the outcome indicator.9 These results indicate that the site of airway narrowing influenced by allergen sensitization and challenge may differ across mouse strains, and that methods insensitive to the lung periphery may fail to detect important changes. Summary The results suggest that in the mouse there are at least two pathways to the development of AHR following OVA sensitization and challenge. One involves IgE-dependent activation of mast cells and the consequent release of several classes of mediators. The other involves recruitment of eosinophils, perhaps through effects of IL-5 on the differentiation of bone marrow-derived precursors, and effects of eosinophilic proteins on target cells in the airways. As mouse models are used to further refine our understanding of the specifics of these pathways or to uncover other as yet uncharacterized mechanisms, it should be borne in mind that the protocol used to sensitize and challenge the mice, the choice of mouse strain, and the end-point used to assess AHR are likely to influence which of these mechanisms dominates. In this respect, some attempt by investigators in the field to standardize protocols appears warranted.

ANIMAL MODELS OF CHRONIC BRONCHITIS Chronic bronchitis is characterized by cough and mucus hypersecretion. Airway obstruction and airway hyperresponsiveness are also common features of this disease.36,37 Animal models have been developed which mimic many of these aspects of human chronic bronchitis. The methods used to generate these models include: • chronic exposure to high concentrations of SO2 gas (see below); • cigarette smoke;38–40 • endotoxin;41 • sodium metabisulfite;42,43 • air pollution.44 This review will focus on chronic SO2 exposure. In most of these studies, animals were exposed to SO2 at concentrations ranging from 200 to 500 ppm, 2–5 hours/day, for

periods of up to 6 months. Most of the published data concern effects in rats and dogs. However, because of the advantages of mouse models of disease as described above, some data generated in mice are included below. The effect of SO2 gas on ciliated epithelial cells In rats and dogs, there is sloughing of ciliated cells following acute exposure to SO2.45–47 After 2–4 days of exposure, ciliated cells are lost throughout the trachea and mainstem bronchi and replaced by one or two layers of small flat cells, whereas peripheral airways and alveoli appear normal.48 The central but not peripheral effects of SO2 are consistent with what could be expected from its highly water-soluble nature, and with studies that have shown that SO2 is removed from inhaled air primarily in the upper respiratory tract.49,50 Hence the lower airways and alveoli are likely to receive a much smaller dose of SO2 than the central airways. After 6 weeks of SO2 exposure, the epithelial layer in the trachea and bronchi begins to regenerate, it becomes thicker than normal, and some of the cilia begin to reappear. Lamb and Reid48 proposed that these changes in the epithelial lining might make it more resistant to injury. In ferrets, chronic exposure to SO2 leads to decreases in the number of cilia per cell, and causes widening of the intercellular spaces at the base of the epithelium.51 This widening may account for the increased epithelial permeability that has been reported in animals exposed to SO2.52 The effect of chronic SO2 exposure on mucus secretion In rats after chronic SO2 exposure, mucus is sometimes present in sufficient quantities as to be apparent either by direct visualization or in histological sections.53,54 There are also increases in the amounts of acidic and neutral mucins extracted from the lungs and tracheae,54 and elevations in the mucus content of bronchoalveolar lavage fluid.55 The amount of mucus recovered increases with increasing duration of exposure56 and may represent an adaptive response in that it protects the regenerating epithelial cells. Mucus hypersecretion is likely to result from both increased mucus synthesis and decreased mucus clearance. The size of the tracheal mucous glands, the abundance of mucus glycoprotein mRNA, and the number of epithelial secretory cells increases in rats exposed chronically to high concentrations of SO2.48,53,55–57 The secretory cells also extend into more peripheral airways. Mucus hypersecretion is also observed with chronic SO2 exposure in dogs.58-60 Tracheal mucus flow rates are reduced by approximately 50% in rats after 4 weeks of exposure to SO2,56 perhaps as a result of changes in the viscoelastic properties of the mucus as well as decreases in the number of ciliated epithelial cells. The nature of airway mucus glycoproteins changes over the course of chronic SO2 exposure in rats,48,54,57,61 consistent with observations in human bronchitis.62 Exposure to SO2 gas increases both acidic and neutral mucus glycoproteins in rat lung extracts, but the percentage increase is much greater for the neutral (PAS staining) mucins than for the acidic (Alcian Blue) staining mucins.54 Similar results

Animal Models

are obtained with chronic metabisulfite exposure.43 The number of PAS compared to Alcian Blue staining bronchiolar epithelial cells also increases with 4–6 weeks of SO2 exposure.48,56 Changes in the composition of mucus are observed with chronic SO2 exposure in dogs.59,60 These changes in mucus composition correlate with changes in its rheological properties.63 Similar results are obtained with chronic cigarette smoke exposure in dogs.64 The mechanisms accounting for mucus hypersecretion in SO2-exposed animals are not known. One possibility is that neutrophils are involved, since neutrophils are present in the airways of animals with SO2-induced bronchitis (see below), and neutrophil proteases are potent mucus secretagogues.65,66 It is also possible that SO2-induced airway injury renders the animals more susceptible to infection. Infections are an important component of human chronic bronchitis,67 and endotoxin exposure alone is known to induce mucus hypersecretion.41 Indeed in rats, mucus hypersecretion induced by chronic SO2 exposure is amplified by bacterial infection.55 Chronic SO2 exposure and airway inflammation Increases in polymorphonuclear (PMN) cells are observed in histological sections of large but not small airways or alveoli of rats with SO2-induced bronchitis.54,68 PMN are also observed in the airway lumen, as indicated by lavage,47 or by examination of mucus.69 Consistent with the histological results, PMN in the lavage fluid are observed earlier and to a greater extent in the central airways than in the peripheral lung. For example, in rats after 2 weeks of SO2 exposure, PMN comprise almost 30% of the cells in airway lavage fluid, but are not increased in lung lavage. After 4 weeks of exposure, the number of PMN in lung lavage does increase relative to air-exposed controls, but even then represents only 2% of total cells.47 Increased numbers of mononuclear cells as well as PMN are observed in histological sections of rats and dogs with SO2-induced bronchitis, but lavage lymphocytes and macrophages are not altered.47,54,70,71 Similarly, increased numbers of mononuclear cells are observed in the airway tissue,72,73 but not in the airway lavage fluid of patients with chronic bronchitis.74,75 It is not known why there are differences in the numbers of inflammatory cells in the airway lumen compared with the airway tissue. It is possible that there are important differences in the nature of the chemotactic factors and adhesion molecules expressed by epithelial versus subepithelial cells. Tracheae from chronic SO2-exposed rats show increased expression of the neutrophil chemokines KC and MIP-2,68 but the cell types responsible for this expression are not known. Airway obstruction and airway hyperresponsiveness Consistent with the airway obstruction observed in patients with chronic bronchitis, chronic exposure to SO2 gas for periods of 4 weeks causes a small (approximately 25%) but significant increase in pulmonary resistance (RL) in rats.54,76

83

In dogs, more substantive (2–3 fold) changes in RL are observed.70,77 The species difference in the magnitude of the response may relate to the mode of SO2 exposure, which was by nasal inhalation in rats, but through a surgically created tracheostomy in dogs. Because much of the SO2 that is inhaled is scrubbed in the nasal passages,49,50 it is likely that the dogs received a much greater dose than the rats of SO2 in their pulmonary airways. Chronic SO2 exposure also causes a substantial increase in airway responsiveness to inhaled aerosolized methacholine in rats, with more pronounced effects observed when RL rather than Cdyn was used as the outcome indicator.54,78,79 This finding has been interpreted to reflect the more pronounced effects on central than peripheral airways. These seem to be the first reports of increased airway responsiveness in an animal model of chronic bronchitis. Patients with chronic bronchitis also show increased airway responsiveness relative to normal subjects.36,37 One explanation for this phenomenon has been that patients with chronic bronchitis have preexisting hyperresponsiveness which predisposes them towards the development of bronchitis. The fact that airway hyperresponsiveness can develop with induction of a syndrome that strongly resembles many of the histopathological features of bronchitis suggests that this is not necessarily the case. We do not know the mechanistic basis for the airway hyperresponsiveness observed in rats with SO2-induced chronic bronchitis. Because airway responsiveness to intravenously administered methacholine is also observed,79 it is unlikely that changes in epithelial permeability leading to more rapid or increased uptake of inhaled methacholine across the epithelium accounts for the changes. Although inflammatory cytokines can induce alterations in the contractility of airway smooth muscle,80 it unlikely that the increased airway responsiveness observed in bronchitic rats is the result of alterations in the airway smooth muscle per se. Neither the sensitivity nor the maximal response to carbachol, eserine, acetylcholine, or serotonin is altered in tracheae of rats exposed chronically to SO2.54,78,81 The amount of airway smooth muscle is also unaffected.78 Instead, the observation that administration of carbocisteine, which reduces the amount of mucus in the airways, attenuates the increases in RL induced by SO2 exposure76 suggests that increased quantities of mucus present in the airways of rats with SO2-induced bronchitis may account for the increased resistance. Mucus hypersecretion may also contribute to the increased airway responsiveness observed with chronic SO2 exposure, since Moreno et al.82 have shown that changes in the diameter of the airway lumen due to the presence of mucus or to edema of the airway wall, even if small, can substantially increase changes in pulmonary mechanics induced by subsequent smooth muscle constriction. In dogs, chronic exposure to high concentrations of SO2 gas results in decreased, not increased, airway responsiveness to inhaled aerosolized methacholine, whereas responsiveness to intravenous methacholine is not altered.70 Similar results are obtained when cigarette smoke rather

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than SO2 gas is used to induce bronchitis.39 The effect is not specific to methacholine: airway responses to inhalation of two other chemically distinct mediators, histamine and PGF2a, are also reduced.77 We do not know why there are differences in the effects of chronic SO2 exposure on airway responsiveness in dogs and rats. Both species develop substantive changes in the epithelium, as well as chronic mucus hypersecretion and chronic airway inflammation.54,59,70 However, it is possible that species differences in the anatomy of the airways coupled with differences in the relative magnitude of the mucus hypersecretion may contribute. For its size, the dog has very large airways. It is possible that, in this species, the airways are sufficiently large that the increase in the thickness of the mucous layer overlying the epithelium resulting from mucus hypersecretion is without effect in augmenting the mechanical consequences of airway smooth muscle contraction. In contrast, the increased layer of mucus may be sufficient to reduce the penetration of methacholine to the underlying smooth muscle. Alternatively, there may be changes in the distribution of inhaled aerosols resulting from mucus hypersecretion83 that result in their deposition in larger airways that contribute less to the changes in pulmonary resistance that result from bronchoconstriction. Either explanation would account for the changes in the response to inhaled but not intravenous methacholine that are observed with induced bronchitis in dogs.39,70 The role of C-fibers in SO2-induced bronchitis in rats C-fibers, a class of unmyelinated sensory neurons which innervate the airways, are stimulated by inhaled irritants, including cigarette smoke and SO2 gas, substances known to induce chronic bronchitis in humans and animals. Stimulation results in central reflex changes in heart rate and breathing pattern, and also results in release of the tachykinins substance P and neurokinin A from the peripheral endings of these neurons. The actions of these neuropeptides include cough, mucus secretion, bronchoconstriction, and airway inflammation, events similar to those observed in chronic bronchitis.84 To test the hypothesis that C-fibers might contribute to the pathogenesis of chronic bronchitis, normal rats and rats treated neonatally with capsaicin to permanently destroy Cfibers85 were exposed to high concentrations of SO2 gas for periods of 1 day to 4 weeks.47,78 Surprisingly, RL increased to a greater extent after chronic SO2 exposure in the capsaicintreated rats. SO2 also resulted in more pronounced airway hyperresponsiveness in the capsaicin-treated rats.78 Airway inflammation was greater and progressed further down the respiratory tract.47 Taken together, these results support the hypothesis that, rather than contributing to the pathophysiological manifestations of bronchitis, C-fibers may protect the airways during induction of chronic bronchitis by SO2 exposure. While C-fibers can cause changes in the pattern of breathing, it is unlikely that such effects constitute the protection afforded against SO2 by C-fibers, since SO2-induced

changes in ventilation were not different in capsaicin- and vehicle-treated rats.47 The induction of chronic bronchitis in rats by SO2 exposure results in a 3-fold increase in the substance P content of the trachea.86 SP levels in induced sputum of patients with chronic bronchitis are also higher than in normal volunteers.87 Taken in light of the observations above, these changes would appear to represent an adaptive response to chronic irritation. Effects of chronic SO2 exposure in mice Adult male C57BL/6 mice were exposed to 250 ppm SO2 gas for 5 hours/day on 5 days/week for periods of 4 weeks. Pathological changes were primarily restricted to the epithelium and included loss of cilia, squamous metaplasia, and focal denudation. These changes were most pronounced in the central airways, with little change in the peripheral airways. The alveoli were normal. In contrast to the results obtained in the rats described above, no evidence was found of mucus plugging or increased amounts of mucus overlaying the epithelium in these mice. Anatomical differences between the two species may in part explain these differences in response to SO2: mouse airways do not contain mucous glands, and serous and mucous cells appear largely confined to the trachea. Johnson et al.88 also exposed mice to SO2, but used a continuous exposure albeit at a much lower concentration (40 ppm).They observed no changes in the lungs, despite changes in the nasal epithelium. To evaluate the extent of airway inflammation induced by SO2 exposure, mice were euthanized, tracheostomized, and a lavage procedure performed. Because of the more prominent central versus peripheral inflammation induced by SO2 in other species,47,54,71 bronchopulmonary inflammation was assessed by first performing an airway lavage (AL) and then performing a bronchoalveolar lavage. For the AL, 0.8 mL of sterile saline was inserted and quickly withdrawn. For the BAL this procedure was repeated 10 more times and the total return pooled. Mice were examined after either 1 week or 1 month of SO2 exposure. In each case, lavage was performed 24 hours after the last exposure. After 1 week of SO2 exposure, the total number of cells and the percentage of cells that were PMN were both greater in mice exposed to SO2 than in air-exposed mice in both the AL and the BAL (Fig. 8.1).There was no effect of SO2 exposure on any other cell type. As the author’s group had previously observed in rats,47 PMN increased to a greater extent in the AL than in the BAL (P  0.005). Similar results were obtained after 1 month of SO2 exposure. The results demonstrating greater percentage increases in PMN in the AL compared to the BAL of SO2 exposed mice also mimic the inflammation characteristic of human chronic bronchitis. Thompson et al.75 observed increased numbers of PMN in lavage fluid from patients with chronic bronchitis. These changes were more pronounced in bronchial than distal lavage samples.75 Effects of chronic SO2 exposure on airway obstruction were measured by whole-body plethysmography.89 The parameter assessed, Penh, is a dimensionless number that empirically correlates with bronchoconstriction in

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Animal Models

3

24 Air

SO2 – 1 week

SO2 – 1 month

Air SO2 – 250ppm

Penh (dimensionless)

PMN (% total cells)

20

16

12

8

*

*

2

1

4

0

0 AL

BAL

Fig. 8.1. PMN in airway lavage (AL) and bronchoalveolar lavage (BAL) from C57BL/6 mice exposed to filtered air or to SO2 gas for 1 week or 1 month. Results are means  SE of data from 35–55 animals in each group.

methacholine-challenged mice.89 Exposure to filtered air for up to 1 month had no significant effect on Penh (Fig. 8.2). In contrast, there was a significant increase in Penh both at 1 week and 1 month of SO2 exposure (P  0.001). Because whole-body plethsymography measures changes in the entire respiratory tract, including the upper airways (nose, pharnyx, larnyx), and because SO2 exposure has important effects on the nasal passages in mice,90–92 the author’s group sought to determine whether the effects observed on Penh were the result of nasal effects, or effects on the pulmonary airways. Penh was first measured with the mice anesthetized and breathing through their noses, and then after a tracheostomy had been created to bypass the upper airways. Penh was significantly greater (P  0.02) in anesthetized SO2-exposed than in anesthetized air-exposed mice. In air-exposed mice, insertion of a tracheostomy tube resulted in a significant increase in Penh (P  0.03). In contrast, in the SO2-exposed animals, there was no statistically significant increase in Penh upon insertion of the tracheostomy tube. Consequently, after tracheostomy, the difference in Penh between the air- and SO2-exposed animals was no longer significant. These results suggest that the increased Penh in the SO2exposed animals was primarily the result of increased airway obstruction imposed by the upper airways (nose, pharnyx, larynx). Once these airways were bypassed, the effect of SO2 was no longer observed. To confirm these results, the author’s group measured RL in air- and SO2-exposed animals that were anesthetized, tracheostomized, and ventilated and found no significant difference between the groups (1.60  0.019 and 1.66  0.04 cmH2O/mL per second, respectively, n  6–9).These results suggest that the changes in Penh induced by SO2 exposure are indicative of

0

1 Weeks

4

Fig. 8.2. Penh values for air- and SO2-exposed mice (10 in each group). Measurements were performed the day before initiating exposures and again 1 week and 4 weeks after SO2 exposure. For the 1-week and 4week elements, mice were studied 24 hours after the last SO2 exposure. Results are expressed as mean  SE. An asterisk indicates P  0.001 compared to pre-exposure values (0 weeks).

effects in the upper airways, with no significant effect of SO2 exposure on lower (pulmonary) airway caliber. Chronic exposure to SO2 gas has been shown to induce changes in the olfactory epithelium, including edema, loss of cilia, epithelial thinning, epithelial desquamation, and some serous exudation.90,91 SO2 exposure also increases the numbers of goblet cells in the nasal epithelium.92 Thus either nasal congestion or increased nasal secretions may have contributed to the increased nasal obstruction observed in these animals. The author’s group also measured airway responsiveness to inhaled aerosolized methacholine in air- and SO2exposed mice. In each animal, responsiveness to methacholine was measured prior to initiation of exposure, after 1 week of exposure, and again after 1 month of exposure. In mice exposed to filtered air, airway responsiveness did not change over time. However, chronic SO2 exposure was also without significant effects on airway responsiveness (Fig. 8.3). Summary of the SO2 data Table 8.1 summarizes the changes induced by chronic SO2 exposure in rats, dogs, and mice. In rats, chronic exposure to high concentrations of SO2 gas results in a syndrome that mimics many aspects of human chronic bronchitis. With repeated daily exposure over periods of 4–6 weeks, there is chronic injury and repair of epithelial cells, mucus hypersecretion, airway inflammation, increased airway resistance, and airway hyperresponsiveness. Similar results are obtained in dogs, although a critical aspect of the condition, airway

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Asthma and Chronic Obstructive Pulmonary Disease

Air-exposed mice

14 Pre

1wk

12

4wk

10

Change in Penh

Change in Penh

12

8 6 4

4wk

6 4

0

0 10

1000

1wk

8

2

30 100 300 Methacholine (mg/mL)

Pre

10

2

10

SO2-exposed mice

14

30 100 300 Methacholine (mg/mL)

1000

Fig. 8.3. Airway responsiveness to inhaled aerosolized methacholine in air- and SO2-exposed animals (10 in each group). Changes in Penh compared with saline aerosol were used as the outcome indicator. In each group, mice were studied prior to initiating exposures (Pre) and 1 week and 4 weeks after initiating exposures. Measurements were performed approximately 18 hours after the last SO2 exposure. There was no statistically significant effect of either air or SO2 exposures on airway responsiveness to methacholine. Results are expressed as mean  SE.

Table 8.1. Comparison of the characteristic traits observed in human chronic bronchitis and those observed in bronchitis induced by chronic SO2 exposure in rats, dogs, and mice

Trait

Humans

Rats

Dogs

Mice

Airway epithelial changes Mucous hypersecretion Airway inflammation Airway obstruction Airway hyperresponsiveness

Yes Yes Yes Yes Yes

Yes Yes Yes Minimal Yes

Yes Yes Yes Yes No

Yes No Yes Upper airways only No

hyperresponsiveness, is not observed. The cost of acquiring and boarding dogs may also make experiments in this species prohibitively expensive. As discussed above, there are numerous advantages to working with mice. However, while the syndrome induced by chronic SO2 exposure, at least in the C57BL/6 mouse, mimics the airway obstruction and airway inflammation of the human condition, the airway obstruction is primarily due to changes in the upper airways, and neither mucus hypersecretion nor airway hyperresponsiveness is observed – although it is possible that these features would develop in other mouse strains. For these reasons, the rat model may prove to be most suitable both for investigations of the mechanisms underlying the pathophysiological events that typify the condition in humans and for pharmaceutical studies.

REFERENCES 1. Gross P, Pfitzer E, Tolker M et al. Experimental emphysema: its production with papain in normal and silicotic rats. Arch. Environ. Hlth 1965; 11:50–8.

2. Garssen J, Nijkamp FP, Van Vugt E et al. T cell-derived antigen binding molecules play a role in the induction of airway hyperresponsiveness. Am. J. Respir. Crit. Care Med. 1994; 150(6 Pt 1):1528–38. 3. Wills-Karp M, Ewart SL. The genetics of allergen-induced airway hyperresponsiveness in mice. Am. J. Respir. Crit. Care Med. 1997; 156(4 Pt 2):S89–96. 4. Gavett SH, O’Hearn DJ, Li X et al. Interleukin 12 inhibits antigeninduced airway hyperresponsiveness, inflammation, and Th2 cytokine expression in mice. J. Exp. Med. 1995; 182:1527–36. 5. Chapoval SP, Nabozny GH, Marietta EV et al. Short ragweed allergen induces eosinophilic lung disease in HLA-DQ transgenic mice. J. Clin. Invest. 1999; 103:1707–17. 6. Tournoy KG, Kips JC, Schou C et al. Airway eosinophilia is not a requirement for allergen-induced airway hyperresponsiveness. Clin. Exp. Allergy 2000; 30:79–85. 7. Coyle AJ, Wagner K, Bertrand C et al. Central role of immunoglobulin (Ig) E in the induction of lung eosinophil infiltration and T helper 2 cell cytokine production: inhibition by a nonanaphylactogenic anti-IgE antibody. J. Exp. Med. 1996; 183:1303–10. 8. MacLean JA, Sauty A, Luster AD et al. Antigen-induced airway hyperresponsiveness, pulmonary eosinophilia, and chemokine expression in B cell-deficient mice. Am. J. Respir. Cell Mol. Biol. 1999; 20:379–87. 9. Zhang Y, Lamm WJ, Albert RK et al. Influence of the route of allergen administration and genetic background on the murine

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allergic pulmonary response. Am. J. Respir. Crit. Care Med. 1997; 155:661–9. Wilder JA, Collie DD, Wilson BS et al. Dissociation of airway hyperresponsiveness from immunoglobulin E and airway eosinophilia in a murine model of allergic asthma. Am. J. Respir. Cell Mol. Biol. 1999; 20:1326–34. Brewer JP, Kisselgof AB, Martin TR. Genetic variability in pulmonary physiological, cellular, and antibody responses to antigen in mice. Am. J. Respir. Crit. Care Med. 1999; 160:1150–6. De Sanctis GT, MacLean JA, Hamada K et al. Contribution of nitric oxide synthases 1, 2, and 3 to airway hyperresponsiveness and inflammation in a murine model of asthma. J. Exp. Med. 1999; 189:1621–30. Hamelmann E, Tadeda K, Oshiba A et al. Role of IgE in the development of allergic airway inflammation and airway hyperresponsiveness: a murine model. Allergy 1999; 54:297–305. Drazen JM, Finn PW, De Sanctis GT. Mouse models of airway responsiveness: physiological basis of observed outcomes and analysis of selected examples using these outcome indicators. Annu. Rev. Physiol. 1999; 61:593–625. Gelfand EW. Essential role of T lymphocytes in the development of allergen-driven airway hyperresponsiveness. Allergy Asthma Proc. 1998; 19:365–9. Martin TR, Galli SJ, Katona IM et al. Role of mast cells in anaphylaxis: evidence for the importance of mast cells in the cardiopulmonary alterations and death induced by anti-IgE in mice. J. Clin. Invest. 1989; 83:1375–83. Martin TR,Takeishi T, Katz HR et al. Mast cell activation enhances airway responsiveness to methacholine in the mouse. J. Clin. Invest. 1993; 91:1176–82. Oshiba A, Hamelmann E, Takeda K et al. Passive transfer of immediate hypersensitivity and airway hyperresponsiveness by allergen-specific immunoglobulin (Ig) E and IgG1 in mice. J. Clin. Invest. 1996; 97:1398–408. Kung TT, Stelts D, Zurcher JA et al. Mast cells modulate allergic pulmonary eosinophilia in mice. Am. J. Respir. Cell Mol. Biol. 1995; 12:404–9. Kobayashi T, Miura T, Haba T et al. An essential role of mast cells in the development of airway hyperresponsiveness in a murine asthma model. J. Immunol. 2000; 164:3855–61. Takeda K, Hamelmann E, Joetham A et al. Development of eosinophilic airway inflammation and airway hyperresponsiveness in mast cell-deficient mice. J. Exp. Med. 1997; 186:449–54. Hogan SP, Mould A, Kikutani H et al. Aeroallergen-induced eosinophilic inflammation, lung damage, and airways hyperreactivity in mice can occur independently of IL-4 and allergenspecific immunoglobulins. J. Clin. Invest. 1997; 99:1329–39. Galli SJ. Complexity and redundancy in the pathogenesis of asthma: reassessing the roles of mast cells and T cells. J. Exp. Med. 1997; 186:343–7. Drazen JM, Arm JP, Austen KF. Sorting out the cytokines of asthma. J. Exp. Med. 1996; 183:1–5. Gleich GJ. The eosinophil and bronchial asthma: current understanding. J. Allergy Clin. Immunol. 1990; 85:422–36. Elbon CL, Jacoby DB, Fryer AD. Pretreatment with an antibody to interleukin-5 prevents loss of pulmonary M2 muscarinic receptor function in antigen-challenged guinea pigs. Am. J. Respir. Cell Mol. Biol. 1995; 12:320–8. Jacoby DB, Gleich GJ, Fryer AD. Human eosinophil major basic protein is an endogenous allosteric antagonist at the inhibitory muscarinic M2 receptor. J. Clin. Invest. 1993; 91:1314–18. Lee JJ, McGarry MP, Farmer SC et al. Interleukin-5 expression in the lung epithelium of transgenic mice leads to pulmonary changes pathognomonic of asthma. J. Exp. Med. 1997; 185:2143–56. Hakonarson H, Maskeri N, Carter C et al. Autocrine interaction between IL-5 and IL-1b mediates altered responsiveness of atopic asthmatic sensitized airway smooth muscle. J. Clin. Invest. 1999; 104:657–67.

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30. Foster PS, Hogan SP, Ramsay AJ et al. Interleukin 5 deficiency abolishes eosinophilia, airways hyperreactivity, and lung damage in a mouse asthma model. J. Exp. Med. 1996; 183:195–201. 31. Hamelmann E, Takeda K, Haczku A et al. Interleukin (IL)-5 but not immunoglobulin E reconstitutes airway inflammation and airway hyperresponsiveness in IL-4-deficient mice. Am. J. Respir. Cell Mol. Biol. 2000; 23:327–34. 32. Corry DB, Folkesson HG,Warnock ML et al. Interleukin 4, but not interleukin 5 or eosinophils, is required in a murine model of acute airway hyperreactivity. J. Exp. Med. 1996; 183:109–17. Erratum J. Exp. Med. 1997; 185:1715. 33. Hessel EM,Van Oosterhout AJ,Van Ark I et al. Development of airway hyperresponsiveness is dependent on interferon-gamma and independent of eosinophil infiltration. Am. J. Respir. Cell Mol. Biol. 1997; 16:325–34. 34. Nagai H, Yamaguchi S, Inagaki N et al. Effect of anti-IL-5 monoclonal antibody on allergic bronchial eosinophilia and airway hyperresponsiveness in mice. Life Sci. 1993; 53:L243–7. 35. Yamaguchi S, Nagai H, Tanaka H et al. Time course study for antigen-induced airway hyperreactivity and the effect of soluble IL-5 receptor. Life Sci. 1994; 54:L471–5. 36. Mullen JB, Wiggs BR, Wright JL et al. Nonspecific airway reactivity in cigarette smokers: relationship to airway pathology and baseline lung function. Am. Rev. Respir. Dis. 1986; 133:120–5. 37. Parker CWBR, Reed CE. Methacholine aerosol as a test for broncial asthma. Arch. Intern. Med. 1965; 115:452–8. 38. Wanner A, Hirsch JA, Greeneltch DE et al. Tracheal mucous velocity in beagles after chronic exposure to cigarette smoke. Arch. Environ. Hlth 1973; 27:370–1. 39. De Sanctis GT, Kelly SM, Saetta MP et al. Hyporesponsiveness to aerosolized but not to infused methacholine in cigarettesmoking dogs. Am. Rev. Respir. Dis. 1987; 135:338–44. Erratum Am. J. Respir. Crit. Care Med. 1994; 149:following 1740. 40. King M, Wight A, De Sanctis GT et al. Mucus hypersecretion and viscoelasticity changes in cigarette-smoking dogs. Exp. Lung Res. 1989; 15:375–89. Erratum Exp. Lung Res. 1994; 20:471. 41. Harkema JR, Hotchkiss JA. In vivo effects of endotoxin on intraepithelial mucosubstances in rat pulmonary airways: quantitative histochemistry. Am. J. Pathol. 1992; 141:307–17. 42. Pon DJ, van Staden CJ, Boulet L et al. Hyperplastic effects of aerosolized sodium metabisulfite on rat airway mucus-secretory epithelial cells. Can. J. Physiol. Pharmacol. 1994; 72:1025–30. 43. Pon DJ, van Staden CJ, Rodger IW. Hypertrophic and hyperplastic changes of mucus-secreting epithelial cells in rat airways: assessment using a novel, rapid, and simple technique. Am. J. Respir. Cell Mol. Biol. 1994; 10:625–34. 44. Saldiva PH, King M, Delmonte VL et al. Respiratory alterations due to urban air pollution: an experimental study in rats. Environ. Res. 1992; 57:19–33. 45. Man SF, Hulbert WC, Man G et al. Effects of SO2 exposure on canine pulmonary epithelial functions. Exp. Lung Res. 1989; 15:181–98. 46. Hulbert WC, Man SF, Rosychuk MK et al. The response phase – the first six hours after acute airway injury by SO2 inhalation: an in-vivo and in-vitro study. Scanning Microsc. 1989; 3:369–78. 47. Long NC, Abraham J, Kobzik L et al. Respiratory tract inflammation during the induction of chronic bronchitis in rats: role of Cfibres. Eur. Respir. J. 1999; 14:46–56. 48. Lamb D, Reid L. Mitotic rates, goblet cell increase and histochemical changes in mucus in rat bronchial epithelium during exposure to sulphur dioxide. J. Pathol. Bacteriol. 1968; 96:97–111. 49. Dalhamm TSL. Acute effect of sulfur dioxide on the rate of ciliary beat in the trachea of rabbit, in vivo and in vitro, with studies on the absorptional capacity of the nasal cavity. Int. J. Air Poll.Water Poll 1961; 4:154–67. 50. Frank NR,Yoder RE, Brain JD et al. SO2 (35S-labeled) absorption by the nose and mouth under conditions of varying concentration and flow. Arch. Environ. Hlth 1969; 18:315–22.

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51. Miller ML, Andringa A, Rafales L et al. Effect of exposure to 500 ppm sulfur dioxide on the lungs of the ferret. Respiration 1985; 48:346–54. 52. Vai F, Fournier MF, Lafuma JC et al. SO2-induced bronchopathy in the rat: abnormal permeability of the bronchial epithelium in vivo and in vitro after anatomic recovery. Am. Rev. Respir. Dis. 1980; 121:851–8. 53. Reid L. An experimental study of hypersecretion of mucus in the bronchial tree. Br. J. Exp. Pathol. 1963; 44:437–45. 54. Shore S, Kobzik L, Long NC et al. Increased airway responsiveness to inhaled methacholine in a rat model of chronic bronchitis. Am. J. Respir. Crit. Care Med. 1995; 151:1931–8. 55. Jany B, Gallup M, Tsuda T et al. Mucin gene expression in rat airways following infection and irritation. Biochem. Biophys. Res. Commun. 1991; 181:1–8. 56. Lightowler NM, Williams JR. Tracheal mucus flow rates in experimental bronchitis in rats. Br. J. Exp. Pathol. 1969; 50:139–49. 57. Clark JN, Dalbey WE, Stephenson KB. Effect of sulfur dioxide on the morphology and mucin biosynthesis by the rat trachea. J. Environ. Pathol.Toxicol. 1980; 4:197–207. 58. Chakrin LW, Saunders LZ. Experimental chronic bronchitis: pathology in the dog. Lab. Invest. 1974; 30:145–54. 59. Bhaskar KR, Drazen JM, O’Sullivan DD et al. Transition from normal to hypersecretory bronchial mucus in a canine model of bronchitis: changes in yield and composition. Exp. Lung Res. 1988; 14:101–20. 60. Koshino T, Bhaskar KR, Reid LM et al. Recovery of an epitope recognized by a novel monoclonal antibody from airway lavage during experimental induction of chronic bronchitis. Am. J. Respir. Cell Mol. Biol. 1990; 2:453–62. 61. Broillet A, White R, Ventrone R et al. Efficacy of fenspiride in alleviating SO2 induced chronic bronchitis in rats and allergic rhinitis in guinea pigs. Rhinol. Suppl. 1988; 4:75–83. 62. Lopata M, Barton AD, Lourenco RV. Biochemical characteristics of bronchial secretions in chronic obstructive pulmonary disease. Am. Rev. Respir. Dis. 1974; 110:730–9. 63. Litt M, Khan MA, Chakrin LW et al. Effect of chronic sulfur dioxide inhalation on rheological properties of tracheal mucus. Biorheology 1976; 13:107–14. 64. King M. Experimental models for studying mucociliary clearance. Eur. Respir. J. 1998; 11:222–8. 65. Breuer R, Christensen TG, Lucey EC et al. Elastase causes secretory discharge in bronchi of hamsters with elastase-induced secretory cell metaplasia. Exp. Lung Res. 1993; 19:273–82. 66. Sommerhoff CP, Nadel JA, Basbaum CB et al. Neutrophil elastase and cathepsin G stimulate secretion from cultured bovine airway gland serous cells. J. Clin. Invest. 1990; 85:682–9. 67. Wilson R, Grossman R. Introduction: the role of bacterial infection in chronic bronchitis. Semin. Respir. Infect. 2000; 15:1–6. 68. Farone A, Huang S, Paulauskis J et al. Airway neutrophilia and chemokine mRNA expression in sulfur dioxide-induced bronchitis. Am. J. Respir. Cell Mol. Biol. 1995; 12:345–50. 69. Knauss HJ, Medici TC, Chodosh S et al. Cell vs noncell airway temporal response in rats exposed to sulfur dioxide. Arch. Environ. Hlth 1976; 31:241–7. 70. Shore SA, Kariya ST, Anderson K et al. Sulfur-dioxide-induced bronchitis in dogs: effects on airway responsiveness to inhaled and intravenously administered methacholine. Am. Rev. Respir. Dis. 1987; 135:840–7. 71. Seltzer J, Scanlon PD, Drazen JM et al. Morphologic correlation of physiologic changes caused by SO2-induced bronchitis in dogs: the role of inflammation. Am. Rev. Respir. Dis. 1984; 129:790–7. 72. Saetta M, Di Stefano A, Maestrelli P et al. Activated T-lymphocytes and macrophages in bronchial mucosa of subjects with chronic bronchitis. Am. Rev. Respir. Dis. 1993; 147:301–6.

73. Di Stefano A, Turato G, Maestrelli P et al. Airflow limitation in chronic bronchitis is associated with T-lymphocyte and macrophage infiltration of the bronchial mucosa. Am. J. Respir. Crit. Care Med. 1996; 153:629–32. 74. Martin TR, Raghu G, Maunder RJ et al. The effects of chronic bronchitis and chronic air-flow obstruction on lung cell populations recovered by bronchoalveolar lavage. Am. Rev. Respir. Dis. 1985; 132:254–60. 75. Thompson AB, Daughton D, Robbins RA et al. Intraluminal airway inflammation in chronic bronchitis: characterization and correlation with clinical parameters. Am. Rev. Respir. Dis. 1989; 140:1527–37. 76. Levrier J, Duval D, Lloyd KG. Study on the effect of oral administration of carbocysteine on ventilatory parameters in the SO2 inhalation model of bronchitis in the rat. Fundam. Clin. Pharmacol. 1992; 6:231–6. 77. Drazen JM, O’Cain CF, Ingram RH. Experimental induction of chronic bronchitis in dogs: effects on airway obstruction and responsiveness. Am. Rev. Respir. Dis. 1982; 126:75–9. 78. Long NC, Martin JG, Pantano R et al. Airway hyperresponsiveness in a rat model of chronic bronchitis: role of C fibers. Am. J. Respir. Crit. Care Med. 1997; 155:1222–9. 79. Drazen JM, Takebayashi T, Long NC et al. Animal models of asthma and chronic bronchitis. Clin. Exp. Allergy 1999; 29 (Suppl. 2):37–47. 80. Amrani Y, Panettieri RA, Frossard N et al. Activation of the TNFap55 receptor induces myocyte proliferation and modulates agonist-evoked calcium transients in cultured human tracheal smooth muscle cells. Am. J. Respir. Cell Mol. Biol. 1996; 15:55–63. 81. Touaty E, Gerber F, Fournier M et al. SO2-induced bronchopathy decreases airway sensitization with intratracheal ovalbumin in the rat. Bull. Eur. Physiopathol. Respir. 1986; 22:329–33. 82. Moreno RH, Hogg JC, Pare PD. Mechanics of airway narrowing. Am. Rev. Respir. Dis. 1986; 133:1171–80. 83. Sweeney TD, Skornik WA, Brain JD et al. Chronic bronchitis alters the pattern of aerosol deposition in the lung. Am. J. Respir. Crit. Care Med. 1995; 151(2 Pt 1):482–8. 84. Shore SALC, Gaston B, Drazen JM. Neural networks in the lung. In: Holgate ST (ed.), Immunopharmacology of the Respiratory System, pp. 123–45. San Diego: Academic Press, 1995. 85. Jancso G, Kiraly E, Jancso-Gabor A. Pharmacologically induced selective degeneration of chemosensitive primary sensory neurones. Nature 1977; 270:741–3. 86. Killingsworth CR, Paulauskis JD, Shore SA. Substance P content and preprotachykinin gene-I mRNA expression in a rat model of chronic bronchitis. Am. J. Respir. Cell Mol. Biol. 1996; 14:334–40. 87. Tomaki M, Ichinose M, Miura M et al. Elevated substance P content in induced sputum from patients with asthma and patients with chronic bronchitis. Am. J. Respir. Crit. Care Med. 1995; 151(3 Pt 1):613–7. 88. Johnson HD, Lincoln EM, Flatt RE. Sulfur dioxide (SO2) exposure and recovery effects on mice. Proc. Soc. Exp. Biol. Med. 1972; 139:861–4. 89. Hamelmann E, Schwarze J, Takeda K et al. Noninvasive measurement of airway responsiveness in allergic mice using barometric plethysmography. Am. J. Respir. Crit. Care Med. 1997; 156(3 Pt 1):766–75. 90. Giddons WEFG. Effects of sulfur dioxide on the nasal mucosa of mice. Arch. Environ. Hlth 1972; 25:166–73. 91. Min YG, Rhee CS, Choo MJ et al. Histopathologic changes in the olfactory epithelium in mice after exposure to sulfur dioxide. Acta Otolaryngol. 1994; 114:447–52. 92. Ukai K. Effect of SO2 on the pathogenesis of viral upper respiratory infection in mice. Proc. Soc. Exp. Biol. Med. 1977; 154:591–6.

Mast Cells and Basophils

Chapter

9

George H. Caughey Cardiovascular Research Institute and Department of Medicine, University of California at San Francisco, USA

Mast cells and basophils first came to attention over a century ago owing to their ample stocks of intracellular granules with unusual staining characteristics. For many years their origins, normal functions, and roles in disease were obscure, and in some respects remain so. Nonetheless, knowledge of their biology increased tremendously in the 1980s and 1990s.1–5 Until recently, researchers focused on their release of histamine and eicosanoids in acute allergic events, which were considered to be a corruption of hypothesized normal function of defending against invasion by parasites such as worms and ticks. However, studies in mice now suggest that mast cells contribute to innate immune defense and responses to immunologically nonspecific injury. These roles deviate from traditional concepts of mast cell and basophil participation in adaptive responses involving antigen recognition by IgE. In asthma investigations, attention is shifting from roles of these cells in acute responses to aeroallergens to roles in promoting persistent inflammation and remodeling in chronic disease. Their roles in other obstructive lung diseases have received less attention, but they may indeed contribute to conditions apart from asthma. This chapter summarizes current thinking about roles of mast cells and basophils in obstructive airway disease.

O R I G I N A N D FAT E Mast cells and basophils have shared origins but distinct distributions and fates. Mast cells, but not basophils, are normal residents of uninflamed airways. Mature mast cells rarely appear in blood, whereas mature basophils circulate and are recruited from blood to sites of allergic inflammation. Both cell types originate from shared progenitors in bone marrow; see Li and Krilis6 for a review. Immature mast cells released from marrow circulate briefly, exit the bloodstream to multiple tissue destinations, then differentiate, adopting a phenotype determined by their microenvironment. Basophils, on the other hand, mature in the marrow, circulate, then home to sites of inflammation if recruited to

do so. Tissue mast cells are not fixed in location, for they migrate towards airway epithelium after antigen challenge and traffic to lymph nodes from sites of antigen exposure. Mast cells probably also proliferate in tissues. Conditions such as intestinal parasitosis lead to large local increases in mast cells. Both types of cell survive degranulation and can restock their secretory granules with mediators. Lifespan in vivo has not been established, but in-vitro studies predict that basophils are short-lived compared with mast cells, which survive for weeks in culture. In humans, the aggregate mast cell mass is much larger than that of basophils, which usually comprise 1% of circulating leukocytes. Among mammals, basophil numbers vary widely, ranging from numerous in guinea pigs, few in humans and mice, to nearly nonexistent in dogs, in which they are arguably unimportant. There are genetic variants of mice with almost no mast cells but none with an inherited, selective deficit of basophils. Thus, it is easier to assess involvement of mast cells than of basophils in mouse disease models; see Galli5 for a review.

DEVELOPMENT AND HETEROGENEITY Paucigranular, mast cell-committed progenitors are released from marrow expressing surface receptor tyrosine kinase c-kit and low-affinity IgG receptor (FccRII), but not highaffinity IgE receptor, FceRI. In vitro, cells with mature characteristics, including FceRI and protease-rich granules, differentiate from progenitors under the influence of IL-6 and kit ligand. Presumably, similar events occur in vivo, with kit ligand produced by endothelial, stromal, and epithelial cells being critical for mast cell survival. Mice with defective c-kit or its ligand lack mast cells. Their importance to mast cell development is underscored by gain-of-function c-kit mutations in systemic mastocytosis and mast cell malignancy, and development of generalized mastocytosis in response to exogenous kit ligand. The importance of local production of kit ligand is suggested by the finding in mice that intratracheal kit ligand provokes mast cell-dependent hyperreactivity.7

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In vitro, a variety of cytokines (especially IL-3, -6, -9, and -10, and TGF-b1) determines the phenotype of maturing mouse mast cells. In the case of IL-9 (a candidate mouse “asthma gene”), airway overexpression in transgenic mice causes eosinophilic inflammation and hyperresponsiveness with mast cell hyperplasia.8 Mast cells vary in features, such as: • • • •

proteoglycan and protease content; metachromasia; granule ultrastructure; responses to degranulating stimuli, such as substance P.

These phenotypical variations appear reversible in a given cell and changeable in cell populations in response to infection and injury. Classically, rodent mast cells are divided into “mucosal” and “connective tissue” groups, although the phenotype distribution is not strictly bimodal. Human mast cells are sorted into subsets based on content of granule proteases.9 MCT cells express tryptases but not chymase, whereas MCTC express tryptases and chymase. Occasionally, chymase-only MCC mast cells are seen. Bronchi contain a mixture of types, whereas alveolar interstitium contains predominantly MCT. Because mast cells develop and are stimulated in tissues, some variation is due to differences in maturation, activation, or recovery from degranulation. Less is known of factors influencing basophil differentiation. c-Kit appears less important than for mast cells because mature basophils normally express little or no c-kit and because levels are largely unaffected by defects in c-kit causing profound mast cell deficits in mice. Nonetheless, c-kit human basophil-like cells circulate in asthma and atopy and manifest phenotypical changes in which c-kit could play a role.10

RECRUITMENT In asthmatics, increased numbers of basophils and activated mast cells appear in sputum after allergen challenge,11 which may reflect migration from submucosal sites and the bloodstream, respectively. As noted, mast cell precursors home to tissues even without inflammation. Constitutive homing and epithelial migration may involve responses to proteins such as: • • • • • • • •

C5a; RANTES; IL-8; MIP-1a; MCP-1; VEGF; fractalkine; kit ligand.

These may orchestrate movement singly or in combination. Cultured human mast cells express a broad repertoire of

chemokine receptors, which diminishes as cells mature, thus limiting mast cell movement after differentiation at a tissue site. Kit ligand is notable in that it is specific for mast cells in comparison to other leukocytes and is produced by airway cells. Mast cell migration into tissues depends on integrins and other adhesion molecules.12 Basophils express an array of chemokine receptors. Chemokines binding to CCR3 (such as RANTES and eotaxins) may be especially important.13,14 The multiplicity of chemoattractants predicts redundant recruiting pathways. Several chemoattractants also prime or activate one or both types of cell,14,15 enhancing their role in asthma pathogenesis beyond that of recruitment alone.

A C T I VAT I O N IgE-dependent activation Classic mast cell and basophil activation involves docking of allergens to IgE via FceRI expressed as an assemblage of subunits (abc2) in the plasma membrane. FceRIa expression is strongly influenced by the serum level of IgE itself.16 Aeroallergens with repeating epitopes attach to receptor achain-bound IgE, bridging receptors. Allergen-driven crosslinking initiates intracellular signaling characterized initially by phosphorylation of intracellular immunoreceptor tyrosine-based activation motif (ITAM) domains of receptor b- and c-chains. In turn, these recruit and activate nonreceptor tyrosine kinases, especially lyn, syk, and btk, which access pathways leading to exocytosis and synthesis of eicosanoids and cytokines. The b-chain is not essential in humans, but amplifies the signal. FceRI signals are damped by inhibitory receptors, such as FccRII and gp49, which possess intracellular immunoreceptor tyrosine-based inhibition motif (ITIM) domains.17 Phosphorylated ITIMs attract phosphatases, such as SHIP, which may inhibit FceRI signaling by dephosphorylating activated proteins in the signaling pathway. FccRII’s importance is particularly compelling.18 When IgG and IgE antibodies are raised against polyvalent antigen, “heterotypic” crosslinking of FccRII and FceRI by allergen bound to IgG and IgE inhibits signaling by FceRI. This may be a means by which “blocking antibodies” reduce atopic symptoms after allergen desensitization. IgE-independent activation Mast cells are activated by multiple nonimmunological inputs (Fig. 9.1). Physiological activators include: • • • • • • •

neuropeptides (e.g. substance P); purines (adenosine and ATP); byproducts of complement activation (e.g. C3a); eosinophil toxins; bacterial products (e.g. E. coli FimH); chemokines and lymphokines (e.g. IL-4); kit ligand.

Mast Cells and Basophils

INPUT

Antigen-specific/ “ADAPTIVE”: Ag  IgG Ag  IgE

93

Histamine OUTPUT Heparin MHC II

Antigen nonspecific/ “INNATE”: Substance P C3a C5a

Proteases: Tryptases; tPA Chymase; Cathepsin G Gelatinases A & B Carboxypeptidase A Dipeptidylpeptidase I

E.coli FimH

Cytokines: IL-4, IL-13 IL-5, IL-6 TNF-α

Kit ligand

Chemokines: IL-8, MIP-1α

NGF ATP Adenosine

CD40L

Growth factors: bFGF; VEGF NGF Eicosanoids: PGD2, LTC4

IL-4 IL-5

MAST CELL Fig. 9.1. Mast cell inputs and outputs. This contains a partial listing of factors influencing human mast cell production of mediators, with postulated roles in acute and chronic airway inflammation in asthma. Inputs are divided into those directly involving immunoglobulin-mediated “adaptive” responses and those involving immunologically nonspecific “innate/natural” immunity.

These inputs are immunologically nonspecific and provide the means by which products of activation participate in neurogenic inflammation and innate host defense.19 They also augment responses to allergen-specific mast cell activation (see below). Thus, innate and adaptive responses are not mutually exclusive. C3a, an agent of innate immunity, is an example, for C3a receptor-null mice are protected from sequelae of airway allergen challenge.20 It should be noted that human mast cell subpopulations do not respond uniformly to all stimuli. For example, lung mast cells tend to be less responsive than skin mast cells to substance P.21

allergic asthmatics are primed,24 presumably by exposure to cytokines in vivo. Priming may serve to activate cells when (and only when) necessary to protect the host; in asthmatics, dysregulated priming may contribute to the pathology of allergic inflammation. FceRI-mediated activation can also be opposed by physiological influences, including adrenergic agonists and inhibitory receptors with ITIMs, as discussed above. Inhibition of activation is the basis of emerging strategies to combat allergic disease, including use of adenosine receptor antagonists, cytokine and chemokine antagonists, and activators of ITIM-containing coreceptors.25

PRIMING AND INHIBITION

M E D I AT O R S

Priming describes the response to a substance that does not release mediators by itself, but enhances the effect of another stimulus, such as crosslinked FceRI. In cultured human mast cells, priming occurs with allergic cytokines, such as IL-4 and IL-5,22 and also with kit ligand and adenosine.23 Mechanisms of priming may be stimulus-specific and affect mediator synthesis and release in different ways. Interactions between primers are potentially complex, and, in the case of IL-4 and -5, may involve autocrine stimulation. In basophils, priming in vitro is especially impressive with IL-3, which augments release of histamine in response to MCP-4 and IL-4 in response to allergen, and is enhanced by eotaxin.15 Basophils harvested from

Eicosanoids, histamine, and proteases Stimulated mast cells and basophils release an astonishing variety of stored and newly synthesized “mediators”. These include prostanoids (Chapter 22), leukotrienes (Chapter 23), proteases (Chapter 28), chemokines (Chapter 26), lymphokines (Chapter 27), growth factors (Chapter 29), and nitric oxide (Chapter 31), the properties and pharmacology of which are considered in other cited chapters and are not reviewed extensively here. The principal eicosanoids synthesized after stimulation are PGD2 (whose importance in allergic airway inflammation is demonstrated in PGD receptor-null mice26) and LTC4, which is the major target of 5-lipoxygenase inhibitors

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and leukotriene receptor antagonists. The major granuleassociated constituents of human mast cells are histamine, serine proteases, and proteoglycans (heparin and chondroitin). Histamine’s importance is clearer in upper airway allergic disease, because antihistamines tend to be more effective in treating rhinitis than asthma. Proteases are the most abundant proteins in secretory granules,27 though this may not be true of normal basophils. As noted, human mast cells vary in expression of tryptases and chymase. In atopic and asthmatic individuals, circulating cells resembling hybrids of basophils and mast cells may express one or both types of protease.10 Human tryptases are a polymorphic family of trypsin-like proteases implicated in asthma based on in-vitro degradation of bronchodilating peptides, enhancement of bronchial contraction, promotion of airway smooth muscle and subendothelial fibroblast growth and collagen production, and proinflammatory properties.28 Studies in sheep and guinea pigs suggest that inhibitors of tryptases reduce allergeninduced airway inflammation and bronchoconstriction.29 Chymase is postulated to play roles in asthma by stimulating gland secretion and promoting airway remodeling via production of angiotensin II and activation of matrix metalloproteinases.27,30,31 Apart from helping to package and stabilize proteases,32 the role of heparin and other proteoglycans is less clear. However, because heparin delivered into airways in pharmacological doses is anti-asthmatic, heparin released from stimulated mast cells may attenuate inflammatory effects of mediators released from mast cells and other effector cells.33 There is little evidence that human basophils express mediators not also present in mast cells. However, proteins of unknown function are recognized by monoclonal antibodies and may be basophil-specific.10,34 Cytokines and chemokines Activated human mast cells express cytokines similar to those of Th2 cells, including IL-4, -5, -13, -16 and TNF-a. In airways in allergic rhinitis or asthma, mast cells are a substantial fraction of leukocytes expressing these “Th2” cytokines.35 The relative importance of mast cells and basophils versus lymphocytes as a source of these cytokines is unclear. However, mast cells differ from lymphocytes in storing cytokines in secretory granules. In the case of TNF-a, release from mast cell stores is a critical determinant of survival from peritonitis.36 Asthmatic basophils produce prodigious amounts of IL-4 and IL-13 after antigen-specific activation.37 The importance of these cytokines is strongly supported by studies in genetically modified mice, which suggest that airway overproduction causes epithelial hypertrophy, mucus metaplasia, eosinophilic inflammation, and hyperresponsiveness. Sustained overproduction of allergic cytokines by mast cells and basophils may heighten and perpetuate asthmatic inflammation. Chemokines expressed and secreted by mast cells include IL-8 and MIP-1a, which may recruit effector leukocytes to sites of inflammation.38

Growth factors Known cellular growth factors expressed by mast cells include bFGF,39 VEGF,40 and nerve growth factor.41 Lessknown growth factors are tryptases, which are mitogens for airway fibroblasts, smooth muscle, and epithelial cells,42–44 and may influence endothelial cells to form vessels, thereby promoting angiogenesis.45 Mast cell IL-4 is fibrogenic when presented in the context of cell-to-cell contact.46 These mediators suggest mechanisms by which mast cell and basophil mediators promote airway remodeling in the setting of persistent airway inflammation.

R O L E S I N H O M E O S TA S I S Basophils and mast cells are presumed to exist for reasons other than to promote sneezing, itching, and wheezing. Lacking informative animal models or natural deficiency states, investigators have few direct clues regarding normal basophil function. However, compelling evidence of roles for mast cells has emerged from studies using mast celldeficient mice, which suggest critical roles in innate as well as adaptive immune responses, including defense against bacterial peritonitis and pneumonia.36,47 In similar mouse models,5 mast cells promote a variety of immunologically nonspecific forms of inflammation, (e.g. ozone-inflicted lung injury48), providing further evidence that they are activated by IgE-independent pathways.

ROLES IN ASTHMA Animal models Several experiments in mice support a role for mast cells in asthma-like allergic inflammation, including eosinophilia, hyperresponsiveness, and epithelial remodeling;8,49–52 see Chapter 8. Other studies suggest that IgE and mast cells do not make major contributions; see Hamelmann et al.53 and Galli54 for reviews. Together, these studies reveal that IgE and mast cell dependence vary with strain of mouse, choice of antigen, mode of sensitization and challenge, and choice of physiological end-points. Mast cell dependence is easier to detect in mice sensitized and challenged locally than systemically, using lower (more physiological) amounts of antigen, without adjuvants. Overall, murine studies suggest that mast cells and IgE are not essential for development of eosinophilic inflammation but do influence kinetics and magnitude of its expression. Humans Notwithstanding some studies questioning its importance in mice, IgE’s long-suspected contributions to human asthma are supported by trials using anti-IgE antibodies, which reduce circulating IgE to nearly undetectable levels and decrease symptoms and corticosteroid use in moderate, steroid-dependent asthmatics55 and rhinitics.56 Anti-IgE joins a growing list of antiasthmatic drugs

Mast Cells and Basophils

influencing mast cells, basophils, and their products. These include: • • • • • • •

corticosteroids (which decrease mast cell numbers;57 cromones; b-adrenergic agonists; theophylline; heparin (which inhibit degranulation); IL-4 antagonists (which inhibit priming);58 leukotriene pathway inhibitors.

Others, effective in animal models, include tryptase inhibitors.29 Interestingly, mast cell desensitization to chronic bagonists used by asthmatics without corticosteroids may contribute to clinical deterioration.59 Furthermore, corticosteroids may protect mast cells from desensitization.60 Pharmacological evidence of roles for mast cells correlates with studies indicating activation in allergic airway disease.1,61 Mast cells release histamine and tryptase into bronchi following allergen challenge.62 They appear degranulated in an asthmatic airway, even in stable disease,63,64 and the percentage of mast cells expressing cytokines IL-4, IL-5, and TNF-a increases.35 Mast cell and basophil numbers rise in asthmatic airways and correlate with hyperresponsiveness to acetylcholine.65 However, fewer basophils are seen in asthmatic bronchial biopsies than mast cells or eosinophils, and they are more prominent in cutaneous than airway late-phase reactions.66 Nonetheless, their arrival coincides with development of late-phase bronchoconstriction.67 Airway basophils are thought to be the source of the late histamine release after allergen exposure because this occurs without a corresponding peak in tryptase, which is more abundant in mast cells. More basophils appear in asthmatic sputum with latephase responses to allergen than in those without such responses, and their numbers correlate with methacholine responsiveness.11 This supports the hypothesis that mast cells and basophils are important in early- and late-phase responses, respectively. The further hypothesis that these cells promote chronic, persistent asthma is speculative. However, this notion is supported by the studies summarized above suggesting that proteases, cytokines, and growth factors from chronically activated mast cells and basophils promote airway remodeling and sustain Th2-assisted IgE production and allergic inflammation.

ROLES IN CHRONIC OBSTRUCTIVE P U L M O N A RY D I S E A S E Several lines of evidence suggest connections between mast cells and COPD.68 Elevated levels of histamine and tryptase in smokers’ lavage fluid69 imply that mast cells are activated by smoke. Studies in mice suggest that mast cells promote airway injury and epithelial remodeling in response to ozone,48 which could explain some human responses to smoke. Most mast cells close to human bronchial glands

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express chymase,70 which stimulates gland cell secretion30 and may promote sputum production in bronchitis. Conceivably, mast cells contribute to emphysematous lung destruction by secreting matrix-altering proteases, such as chymase, tryptases, gelatinases, plasminogen activator, and dipeptidyl peptidase I.27 On the other hand, there is little evidence of increased mast cell numbers in COPD to match observed increases in neutrophils, macrophages, and lymphocytes. Overall, evidence of mast cell involvement in asthma is more compelling, although further investigation of connections between mast cells and COPD is warranted.

S U M M A RY Mast cells protect from certain types of infection and injury by contributing to innate and adaptive immune responses. The homeostatic roles of basophils are less clear. Both cell types participate in the pathology of atopy and asthma by deploying an arsenal of inflammatory mediators, including proteases, growth factors, chemokines, and “Th2” cytokines. Mast cells, being permanent airway residents, are more likely to encounter aeroallergens first and to participate in acute responses. Basophils, because of the time lag of recruitment, may be more important in late-phase responses. Support is mounting for the hypothesis that both types of cell magnify the pathology of persistent inflammation in chronic asthma, including stimulation of IgE production, recruitment of eosinophils, and remodeling of epithelium.

REFERENCES 1. Kaliner M. Asthma and mast cell activation. J. Allergy Clin. Immunol. 1989; 83:510–20. 2. Schulman ES. The role of mast cells in inflammatory responses in the lung. Crit. Rev. Immunol. 1993; 13:35–70. 3. Metcalfe DD, Baram D, Mekori YA. Mast cells. Physiol. Rev. 1997; 77:1033–79. 4. Dvorak AM. Cell biology of the basophil. Int. Rev. Cytol. 1998; 180:87–236. 5. Galli SJ. Mast cells and basophils. Curr. Opin. Hematol. 2000; 7:32–9. 6. Li L, Krilis SA. Mast-cell growth and differentiation. Allergy 1999; 54:306–12. 7. Campbell E, Hogaboam C, Lincoln P, Lukacs NW. Stem cell factor-induced airway hyperreactivity in allergic and normal mice. Am. J. Pathol.1999; 154:1259–65. 8. Temann UA, Geba GP, Rankin JA, Flavell RA. Expression of interleukin 9 in the lungs of transgenic mice causes airway inflammation, mast cell hyperplasia, and bronchial hyperresponsiveness. J. Exp. Med. 1998; 188:1307–20. 9. Irani AA, Schechter NM, Craig SS et al. Two types of human mast cells that have distinct neutr