Poxvirus IL-1 Receptor Homologs Grant McFadden1,* and Richard Moyer2 1
The John P. Robarts Research Institute and Department of Microbiology and Immunology, The University of Western Ontario, 1400 Western Road, London, Ontario, N6G 2V4, Canada 2 Department of Molecular Genetics and Microbiology, University of Florida College of Medicine, PO Box 100266, Gainesville, FL 32610-0266, USA * corresponding author tel: (519)663-3184, fax: (519)663-3847, e-mail:
[email protected] DOI: 10.1006/rwcy.2000.14013.
SUMMARY The IL-1 receptor homologs of poxviruses were the second examples discovered, following the poxvirus TNF receptor homologs, of `viroceptors' or virusencoded receptor mimics that function to bind and sequester cellular ligands away from their cognate cellular receptors. The prototypic member of this family, B15R of vaccinia virus, is a secreted member of the Ig superfamily, with highest sequence similarity to the ligand-binding domain of the cellular type II IL-1 receptor. The protein plays an important role in modulating the inflammatory cascade, particularly the fever response, following virus infection.
BACKGROUND
Discovery Routine sequencing of vaccinia virus Western Reserve (WR) revealed two open reading frames (B15R and B18R) with significant homology to type I IL-1 receptors (Smith et al., 1991). Subsequent publication of sequences from murine and human type II IL-1 receptors revealed that the B15R ORF has far more significant homology (McMahan et al., 1991) than that of B18R. The B15R gene encodes a 326 amino acid protein and is found in the vaccinia virus WR, Lister, and Ankara strains. A nearly identical gene is found in cowpox virus. A nonfunctional protein is
encoded by vaccinia Copenhagen, ectromelia virus, and variola virus (smallpox). The corresponding variola ORF is repeatedly disrupted by frameshifts and translation termination codons (Shchelkunov et al., 1993; Massung et al., 1994).
Alternative names IL-1 soluble receptor, ORF B15R (the vaccinia WR designation used throughout this discussion), ORF B16R (vaccinia Copenhagen), cowpox virus (B14R).
Structure The ORF is typical of poxvirus genes, contiguous and with no introns.
Main activities and pathophysiological roles The protein inhibits murine and human IL-1 , but not IL-1 or IL-1Ra proteins (AlcamõÂ and Smith, 1992, 1996). Affinities for human IL-1 are high (Kd 230 pM) which when coupled to very high levels of expression as compared with cellular receptors, lead to effective blocking of IL-1 binding to receptor-bearing cells and IL-1 -mediated activities in vitro (AlcamõÂ and Smith, 1992; Spriggs et al., 1992).
1602 Grant McFadden and Richard Moyer
GENE
Description of protein
Accession numbers
The protein consists of 326 amino acids (see Figure 1). There is an 21 amino acid N-terminal secretory signal sequence which is cleaved to generate the mature protein. The protein is characterized by the presence of three Ig domains which comprise most of the mature protein (Figure 1). All three Ig domains contain a pair of conserved cysteine residues which form intradomain disulfide bridges. These residues are present at positions 48 and 99, 143 and 194, and 242 and 309. There is no transmembrane sequence nor a cytoplasmic domain (Smith et al., 1991). While IL-1 levels are controlled extracellularly by this protein (ORF B15R), IL-1 levels are also controlled intracellularly in many orthopoxvirus strains by the SPI-2/crmA protein (a serpin) of vaccinia/cowpox virus. In vaccinia WR, the SPI-2 ORF is designated B13R. The B13R protein inhibits the proteinase interleukin-1 convertase (ICE) and prevents activation of precursor IL-1 (Ray et al., 1992).
Vaccinia virus WR: X56121, D01018 Vaccinia virus Ankara: U94848 Cowpox virus: M95202
Sequence In vaccinia WR, the gene is located 174 kb from the left end of the 190 kb genome in the HindIII B fragment, the rightmost terminal HindIII fragment of the viral genome. Transcription is in the rightward direction. The protein is synthesized and secreted `late' in infection, subsequent to DNA replication (AlcamõÂ and Smith, 1992).
PROTEIN
Accession numbers Vaccinia virus WR: 222697 Vaccinia virus Ankara: 2772816 Cowpox virus: 418180
Sequence See Figure 1.
Relevant homologies and species differences All active B15R proteins expressed by various orthopoxviruses are virtually identical. The most significant homology to a nonviral protein is to the rat type II IL-1 receptor (Bristulf et al., 1994) (Figure 2). Cleavage of an N-terminal signal precedes secretion into the infected cell media.
Figure 1 Sequence of the vaccinia WR IL-1 -secreted soluble receptor precursor. The signal sequence of approximately 20 amino acids is cleaved to generate the mature protein. The three immunoglobulin domains which dominate the protein are indicated in bold and are separated by small intervening peptides. The active proteins synthesized by vaccinia Ankara and cowpox virus are virtually identical.
Poxvirus IL-1 Receptor Homologs 1603 Figure 2 A `GAP' alignment of the B15R 326 amino acid ORF with a portion of the rat 416 amino acid type II IL-1 receptor precursor. The proteins are 39% similar and 29% identical through the regions compared. The conserved pairs of cysteine residues, conserved within the three immunoglobulin domains of both proteins are shown in bold.
BIOLOGICAL CONSEQUENCES OF ACTIVATING OR INHIBITING RECEPTOR AND PATHOPHYSIOLOGY The role of this protein was initially investigated by two groups using different model infections of mice. In one case, based on intracranial injection, it was reported that vaccinia virus WR deleted for the B15R ORF was attenuated (Spriggs et al., 1992). However, when mice were infected intranasally, deletion of the B15R ORF had the opposite effect, i.e. the infection was more virulent, the animals showing increased weight loss and a more generally severe illness (AlcamõÂ and Smith, 1992). It was proposed that the increased
illness was due to increased levels of IL-1 . Consistent with this prediction is the fact that IL-1 knockout mice are resistant to the induction of fever by chemical irritants (Kozak et al., 1995; Zheng et al., 1995). The specific link of IL-1 presence to fever suppression in vaccinia virus infections was proven when it was subsequently found that mice infected intranasally with wild-type vaccinia do not generate a fever despite signs of a severe infection and up to 30% loss of body weight. By contrast, animals infected with vaccinia deleted for the B15R ORF developed a significant fever which lasted up to 5 days (AlcamõÂ and Smith, 1996). Hence, it appears that intracellular IL-1 is mainly responsible for the control of fever in vaccinia virus-infected animals, despite the presence
1604 Grant McFadden and Richard Moyer of the viral encoded intracellular B13R ORF gene product (SPI-2/crmA) which prevents ICE (caspase 3) activation of proIL-1 to active IL-1 .
References AlcamõÂ , A., and Smith, G. L. (1992). A soluble receptor for interleukin-1 encoded by vaccinia virus: A novel mechanism of virus modulation of the host response to infection. Cell 71, 153±167. AlcamõÂ , A., and Smith, G. L. (1996). A mechanism for the inhibition of fever by a virus. Proc. Natl Acad. Sci. USA 93, 11029± 11034. Bristulf, J., Gatti, S., Malinowsky, D., Bjork, L., Sundgren, A. K., and Bartfai, T. (1994). Interleukin-1 stimulates the expression of type I and type II interleukin-1 receptors in the rat insulinoma cell line Rinm5F, sequencing a rat type II interleukin-1 receptor cDNA. Eur. Cytokine Netw. 5, 319±330. Kozak, W., Zheng, H., Conn, C. A., Soszynski, D., van der Ploeg, L. H., and Kluger, M. J. (1995). Thermal and behavioral effects of lipopolysaccharide and influenza in interleukin-1 betadeficient mice. Am. J. Physiol. 269, R969±R977. McMahan, C. J., Slack, J. L., Mosley, B., Cosman, D., Lupton, S. D., Bruton, L. L., Grubin, C. E., Wignall, J. M.,
Jenkins, N. A., and Brannan, C. I. (1991). A novel IL-1 receptor, cloned from B cells by mammalian expression, is expressed in many cell types. EMBO J. 10, 2821±2832. Massung, R. F., Liu, L. I., Qi, J., Knight, J. C., Yuran, T. E., Kerlavage, A. R., Parsons, J. M., Venter, J.C., and Esposito J. J. (1994). Analysis of the complete genome of smallpox variola major virus strain Bangladesh-1975. Virology 201, 215±240. Ray, C. A., Black, R. A., Kronheim, S. R., Greenstreet, T. A., Sleath, P. R., Salvesen, G. S., and Pickup, D. J. (1992). Viral inhibition of inflammation: cowpox virus encodes an inhibitor of the interleukin-1 converting enzyme. Cell 69, 597±604. Shchelkunov, S. N., Blinov, V. M., and Sandakhchiev, L. S. (1993). Genes of variola and vaccinia viruses necessary to overcome the host protective mechanism. FEBS Lett. 319, 80±83. Smith, G., Chan, Y. S., and Howard, S. T. (1991). Two vaccinia virus proteins structurally related to the interleukin-1 receptor and the immunoglobulin superfamily. J. Gen. Virol. 72, 511± 518. Spriggs, M. K., Hruby, D. E., Maliszewski, C. R., Pickup, D. J., Sims, J. E., Buller, R. M., and VanSlyke, J. (1992). Vaccinia and cowpox viruses encode a novel secreted interleukin-1 binding protein. Cell 71, 145±152. Zheng, H., Fltecher, D., Kozak, W., Jiang, M., Hofmann, K. J., Conn, C. A., Soszynski, D., Grabiec, C., Trumbauer, M. E., and Shaw, A. (1995). Resistance to fever induction and impaired acute-phase response in interleukin-1 beta-deficient mice. Immunity 3, 9±19.
