TARC

TARC Osamu Yoshie* Department of Microbiology, Kinki University School of Medicine, 377-2 Ohno-Higashi, Osaka, 589-8511, Japan * corresponding author tel: +81-723-67-3606, fax: +81-723-67-3606, e-mail: [email protected] DOI: 10.1006/rwcy.2000.11025.

SUMMARY Thymus and activation-regulated chemokine (TARC) is a CC chemokine constitutively expressed most prominently in the thymus and also to a lesser extent in tissues such as lung, colon, and small intestine. TARC was the first CC chemokine shown to be chemotactic for lymphocytes but not for monocytes. It is produced by dendritic cells in various tissues and also by dendritic cells differentiated from blood monocytes in vitro. TARC and MDC, another CC chemokine that is very similar to TARC, act on CCR4. In the thymus, CCR4 is expressed mainly on CD4+ thymocytes in the cortex. Thus, TARC and MDC, which are expressed in the medullary dendritic cells and epithelial cells respectively, may be involved in guiding the migration of positively selected CD4+ thymocytes from cortex to medulla for negative selection. In peripheral blood, CCR4 is expressed by a subpopulation of CD4+CD45RO+ memory T cells, which are mostly if not exclusively polarized to TH2. Thus, it is likely that TARC and MDC play important pathophysiological roles in TH2-type immune responses.

BACKGROUND

Discovery TARC was cloned from freshly isolated normal peripheral blood mononuclear cells stimulated with phytohemagglutinin (PHA) for 72 hours. The 50 fragment of cDNA was originally identified by a method coined as signal sequence trap, which aims at the selective isolation of cDNAs encoding secretory and type I membrane proteins, i.e. those having

N-terminal signal peptides (Imai et al., 1996). The full-length cDNA was then isolated using the cDNA fragment as a probe. TARC was found to be constitutively and selectively expressed in the thymus and markedly, but rather slowly, induced in peripheral blood mononuclear cells upon PHA stimulation (Imai et al., 1996). These features, which were noted to be quite unique among the hitherto known CC chemokines, led to its naming.

Alternative names TARC was described as such in 1996 (Imai et al., 1996). Recently, murine TARC was reported by two groups (Lieberam and Foerster, 1999; Schaniel et al., 1999). One group termed it ABCD-2 (Schaniel et al., 1999). Its systemic name proposed at the Keystone Symposium in 1999 is CCL17 (Zlotnik and Yoshie, 2000).

Structure TARC is a typical CC chemokine with the four conserved cysteine residues. The mature protein has a molecular weight of 8083 and an isoelectric point of 9.7. TARC is distantly related to other CC chemokines. The amino acid identity of the mature TARC protein is 29% with RANTES, 28% with MIP-1 and MCP-3, 26% with MIP-1, and 24% with I-309, MCP-1, and MCP-2 (Imai et al., 1996). TARC is most similar in amino acid sequence to MDC (Godiska et al., 1997), with 37% identity. It is notable that the genes for TARC, MDC, and fractalkine are localized within 200 kb on chromosome 16q13 (Nomiyama et al., 1998). Furthermore, TARC and MDC share CCR4 (Imai et al., 1997,

1298 Osamu Yoshie 1998). It is thus likely that TARC and MDC were generated by gene duplication from a common ancestor. Human and murine TARC share a 65% amino acid identity in their mature forms (Lieberam and Foerster, 1999; Schaniel et al., 1999).

Main activities and pathophysiological roles So far, the most relevant if not exclusive receptor for TARC is CCR4 (Imai et al., 1997). Thus, much of the following discussion is also relevant for CCR4 functions. Another CC chemokine, MDC, shares CCR4 with TARC (Imai et al., 1998), but MDC may also act on some other unknown receptor(s) (Bochner et al., 1999; Schaniel et al., 1999). Thymocyte Migration in the Thymus One of the most notable features of TARC is its high constitutive expression in the thymus (Imai et al., 1996). In the murine thymus, TARC is expressed by medullary dendritic cells, while CCR4 is expressed by CD4+ cortical thymocytes. Accordingly, it is postulated that TARC is involved in the migration of positively selected CD4+ thymocytes from the cortex into the medulla. Murine MDC, which shares murine CCR4 with murine TARC, was also shown to attract CD4+CD8low cortical thymocytes (Chantry et al., 1999). Selective Attraction of Memory T Cells In human peripheral blood, CCR4-expressing cells were found to be a subset ( 20%) of CD4+ CD45RO+ memory T cells. Accordingly, TARC as well as MDC attracted CD4+CD45RO+ T cells in vitro (Imai et al., 1999). Furthermore, T cells expressing CCR4 were considered to be mostly polarized to TH2 since CCR4-positive T cells fractionated by anti-CCR4 and expanded with IL-2 preferentially gave rise to T cells that, when stimulated, produced IL-4 and IL-5 (Imai et al., 1999). In agreement with this, TH2 lines but not TH1 lines were found preferentially to express CCR4 (Bonecchi et al., 1998; Sallusto et al., 1998; Imai et al., 1999). In one study, however, CCR4-expressing T cells were shown to contain essentially all skin-homing memory T cells expressing the cutaneous lymphocyte antigen (CLA), which naturally contain both TH1 and TH2 cells, and a subset of other systemic memory T cells but not intestinal (4 7+) memory T cells (Campbell et al., 1999). The anti-TARC reactivity of venules and the infiltration of many CCR4(+) lymphocytes into

chronically inflamed skin but not into the gastrointestinal lamina propria was further demonstrated (Campbell et al., 1999). TARC has been shown to contribute to diverse disease states such as fulminant hepatic failure, atopic dermatitis, Hodgkin's disease, and asthma.

GENE AND GENE REGULATION

Accession numbers GenBank Human cDNA: D43767, NM_002987 Mouse cDNA: AJ242587 Human gene: AC004382

Chromosome location By somatic cell hybrid and radiation hybrid analyses, the human TARC gene (SCYA17 ) has been mapped to chromosome 16q13 (Nomiyama et al., 1997). Furthermore, the genes for TARC, MDC (SCYA22), and fractalkine (SCYD1) were found to be localized within 200 kb of each other (Nomiyama et al., 1998). The mouse genes for these three chemokines were also found to be clustered within 130 kb (Schaniel et al., 1999).

Cells and tissues that express the gene By northern blot analysis, TARC was shown to be expressed most prominently in the thymus and also in tissues such as lung, colon, and small intestine (Imai et al., 1996). In the murine thymus, TARC was found to be expressed in the medullary dendritic cells. In vitro, TARC was abundantly produced by human blood monocytes treated with GM-CSF or IL-3, especially in the presence of IL-4, as well as by dendritic cells derived from monocytes treated with GM-CSF+ IL-4 (Imai et al., 1999). TARC was also found to be one of the highly expressed genes in dendritic cells derived from human blood monocytes treated with GM-CSF+ IL-4+ TNF, as determined by the method of serial analysis of gene expression (Hashimoto et al., 1999). Expression of TARC was also detected in a fraction of dendritic cells derived from murine bone marrow cells treated with GMCSF (Lieberam and Foerster, 1999). In vivo, TARC expression was detected in dendritic cells derived from bone marrow and thymus at high

TARC 1299 levels, and in dendritic cells from lymph nodes and CD11c+ cells from the lung at lower levels (Lieberam and Foerster, 1999). The expression of TARC mRNA was also induced in murine splenic mature B cells at low levels by stimulation with anti-CD40+ IL-4 or anti-CD4 alone, and at higher levels by stimulation with anti-IgM (Schaniel et al., 1999). TARC expression was also seen in the basal epidermis in the atopic dermatitis-like skin lesions of NC/Nga mice and in a murine keratinocytic PAM212 cells that were treated with TNF, IFN , or IL-1 (Vestergaard et al., 1999).

PROTEIN

Accession numbers SwissProt: Human TARC: Q92583

Sequence See Figure 1.

RECEPTOR UTILIZATION TARC has been shown to be a highly specific functional ligand for CCR4 (Imai et al., 1997). Subsequently, MDC was also found to be a specific ligand for CCR4 (Imai et al., 1998). CCR4 was originally reported as a shared receptor for RANTES, MIP-1, and MCP-1. This conclusion was led by an assay measuring the stimulation of the Ca2+-activated chloride channel in Xenopus laevis oocytes that were injected with CCR4 cRNA (Power et al., 1995). However, in the studies using CCR4-transfected mammalian cells, TARC, but not any of these three CC chemokines, was shown to induce significant responses in both chemotactic and intracellular calcium mobilization assays (Imai et al., 1997). A claim that TARC was also a functional ligand for CCR8, the receptor for I-309 (Bernardini et al., 1998), has been refuted in a recent work (Garlisi et al., 1999).

IN VITRO ACTIVITIES

In vitro findings

Cellular sources that produce

Consistent with its highly selective expression in the thymus, 125I-labeled TARC binds to some human T cell lines but not to other types of cell line. Furthermore, 125I-labeled TARC binds to fresh and PHA/PMA-activated peripheral blood lymphocytes but not to monocytes or neutrophils. Consistently, TARC has been the first CC chemokine shown to be chemotactic for lymphoid cells but not for monocytes or neutrophils (Imai et al., 1996). TARC was originally shown to be chemotactic for the human T cell lines HUT102 and HUT78 (Imai et al., 1996). In later studies, TARC was shown to be selectively chemotactic for CD4+CD45RO+ memory T cells, which are polarized mostly if not exclusively to TH2 in both humans and mice (Imai et al., 1999; Lieberam and Foerster, 1999; Schaniel et al., 1999).

Dendritic cells are the major source of TARC. In the thymus, TARC is expressed constitutively by medullary dendritic cells. In vitro, TARC is produced constitutively by dendritic cells differentiated from peripheral blood monocytes (Hashimoto et al., 1999; Imai et al., 1999).

PATHOPHYSIOLOGICAL ROLES IN NORMAL HUMANS AND DISEASE STATES AND DIAGNOSTIC UTILITY

Figure 1 Amino acid sequence for human TARC. The signal peptide is underlined.

Role in experiments of nature and disease states

Important homologies TARC is most homologous to MDC. They share a 37% amino acid identity in their mature forms. Human and mouse TARC share a 65% amino acid identity (Lieberam and Foerster, 1999; Schaniel et al., 1999).

CELLULAR SOURCES AND TISSUE EXPRESSION

Hodgkin's lymphoma is characterized by the combination of Reed±Sternberg (RS) cells and a prominent

1300 Osamu Yoshie infiltration of lymphocytes, plasma cells, eosinophils, and histiocytes. The small lymphocytes immediately surrounding the RS cells are mostly CD4+ T cells. TARC was found to be expressed at a high level by RS cells of the classical nodular sclerosis and mixed cellularity subtypes of Hodgkin's lymphoma. Furthermore, lymphocytes surrounding RS cells were shown to be CCR4-positive and TH2-like (van den Berg et al., 1999). TARC production by RS cells may contribute to immune escape mechanisms in Hodgkin's disease.

IN THERAPY

Effects of therapy: Cytokine, antibody to cytokine inhibitors, etc. In a murine model of fulminant hepatitis, anti-TARC was shown to suppress T cell infiltration into the liver and protect mice from fatal liver damages (Yoneyama et al., 1998). In a murine model of asthma, antiTARC was shown to suppress eosinophilic infiltration and hyperreactivity of the airway (Kawasaki et al., unpublished).

Role of TARC in Fulminant Hepatic Failure The intravenous injection of heat-killed Propionibacterium acnes into mice induces granuloma formation in the liver. A subsequent challenge of these primed mice with a low dose of LPS induces acute and massive liver injury mimicking fulminant hepatic failure. Yoneyama et al. (1998) showed that the treatment of primed mice with anti-TARC just prior to the injection of LPS effectively suppressed T cell infiltration into the liver and fatal liver damage.

Role of TARC in Atopic Dermatitis NC/Nga mice are known spontaneously to develop atopic dermatitis-like lesions when kept in conventional conditions. Vestergaard et al. (1999) showed that TARC was expressed at a high level in the basal epidermis of atopic dermatitis-like lesions. On the other hand, MDC was expressed by dermal dendritic cells even in the normal skin but was also upregulated several-fold in the skin lesions (Vestergaard et al., 1999). These results support the role of these chemokines in the attraction of CCR4-expressing TH2-type T cells into the skin lesions.

Role of TARC in Asthma Mice primed with ovalbumin (OVA) and challenged with the inhalation of OVA develop pathologic conditions resembling asthma. Kawasaki et al. (unpublished) demonstrated that TARC was constitutively expressed in the lung and upregulated in allergic conditions induced by OVA inhalation. Furthermore, anti-TARC attenuated OVA-induced eosinophil infiltration and hyperreactivity of the airway. Similar results were obtained in mice treated with anti-MDC (Gonzalo et al., 1999).

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