BASIC SCIENCE FOR THE CLINICIAN

New Developments In the Immunobiology of HLA-B27:

Implications For Spondyloarthropathy Pathogenesis

 

 

Robert A. Colbert, MD, PhD

William S. Rowe Division of Rheumatology

Cincinnati Children’s Hospital Medical Center and the

University of Cincinnati College of Medicine

Cincinnati, OH

 

Running Title: HLA-B27 and Spondyloarthropathies

Key Words: HLA-B27, spondyloarthropathy, protein misfolding, unfolded protein response, arthritogenic peptides, dimers, leukocyte receptors

 

 

Address for correspondence:

Division of Rheumatology, ML4010

Cincinnati Children’s Hospital Research Foundation

3333 Burnet Avenue

Cincinnati, OH 45229-3039

Email: bob.colbert@chmc.org

Voice: 513.636.4934

Fax: 513.636.3328

 

Nonstandard abbreviations:

HLA, human leukocyte antigen; B27, HLA-B27; MHC, major histocompatibility complex; B2m, B2-microglobulin; ER, endoplasmic reticulum; ERAD, ER-associated degradation; UPR, unfolded protein response; TAP, transporter associated with antigen presentation; SpA, spondyloarthropathy; AS, ankylosing spondylitis; ReA, reactive arthritis; LILR, leukocyte immunoglobulin-like receptor.

 

Abstract

            Since the discovery of the striking relationship between HLA-B27 and susceptibility to ankylosing spondylitis numerous models have been proposed to explain the role of this molecule in disease pathogenesis. The initial focus was on conventional HLA-B27 complexes expressed on the cell surface as targets of a cross-reactive immune response by antibodies and/or CD8+ T cells. Experimental support for these hypotheses from translational studies has been limited, while data from animal models are largely inconsistent with these mechanisms. Recent observations indicate that folding of the HLA-B27 heavy chain and the formation of complexes containing B2-microglobulin and peptide display several exceptions to the rules established for most HLA class I molecules, which has led to new ideas about immunopathogenesis. The purpose of this review is to communicate these concepts. Ultimately, a better understanding of pathogenesis is likely to improve treatment strategies and provide greater and more sustained clinical responses.

 

Introduction

            This year marks the 30th anniversary of the remarkable discovery of the association between HLA-B27 and ankylosing spondylitis (AS) (1, 2). An illuminating personal account of this work written by Brewerton was recently published, and is well worth reading (3). This association has been extended to other forms of spondyloarthropathy (SpA) and replicated in numerous populations around the world. Genetic linkage between AS and the major histocompatibility complex (MHC) has been established (4), and spondyloarthropathy-like disease has been produced in HLA-B27 transgenic animals (5, 6). These and other observations support the idea that B27 plays a direct role in pathogenesis but only in the presence of other susceptibility alleles (7). Despite the detailed understanding of HLA-B27 structure and function that has emerged over the last 15 years, its role in pathogenesis remains a puzzle.

 

HLA Class I Structure and Function

            HLA class I complexes consist of a heavy chain (HC) non-covalently bound to B2-microglobulin (B2m) with a short peptide partially embedded within the folded HC. The peptides are derived from self-proteins or intracellular pathogens following intra-cytoplasmic protein degradation. These complexes are displayed on the cell surface where they can be recognized by CD8+ T cells and either tolerated, or when a pathogen-derived peptide is presented, target the cell for lysis. The HC is encoded within the MHC where there is extensive sequence variability, with over 700 HLA-A, B, and C alleles identified to date (8). Several human and mouse class I complexes have been crystallized revealing overall structural similarity (9, 10) despite considerable polymorphisms that result in peptide binding and T cell receptor recognition differences. This extensive molecular diversity is exploited by the immune system to enable recognition of pathogen-derived peptides presented on infected cells.

 

HLA Class I Assembly

            Many gene products require the assistance of other proteins and enzymes known as chaperones to fold and achieve their correct conformation, which is critical for proper function. For example, newly synthesized class I HCs initially fold and acquire a conformation that allows them to bind b2m and eventually their peptide cargo to be displayed on the cell surface. This assembly process occurs within the endoplasmic reticulum (ER) with subsequent transit through the Golgi, and is facilitated by the chaperones calnexin and calreticulin (11, 12). A class I-specific chaperone, tapasin, also participates in the assembly process by connecting HC·b2m heterodimers with the transporter associated with antigen processing (TAP) complex (13). This occurs during later stages of assembly where peptides are loaded into the peptide-binding groove and/or pre-existing peptides are optimized for high affinity binding (14). While sequence differences between HLA class I alleles have striking affects on immune recognition, a number of studies have also shown that they influence aspects of the assembly process (15, 16) including peptide loading efficiency, tapasin interaction, and interestingly, the early stages of HC folding.

 

HLA-B27 Misfolding

            We discovered an unusual characteristic of HLA-B27 where newly synthesized HCs folded slowly and were retained for prolonged periods in the ER in comparison to other alleles (17). A small proportion of HCs was destroyed via the ER-associated degradation pathway (ERAD), in contrast to internalization from the cell surface and degradation via endosomal/lysosomal pathways. ERAD is responsible for the disposal of misfolded and/or improperly assembled proteins, and is part of a quality control process that helps ensure that only properly folded and functional proteins exit the ER (18). Further investigation of the fate of newly synthesized B27 HCs revealed an even larger proportion of high molecular weight disulfide-linked complexes consisting primarily of HC-HC homodimers (19) and also HC associated with BiP (20), an ER chaperone that helps prevent misfolded proteins from aggregating (21). Importantly, BiP is also a key sensor that cells use to detect protein misfolding (see below) (22). Abnormal folding of B27 is related to some of the same amino acid residues that distinguish it from other alleles and influence peptide binding specificity, particularly Glu45 and Cys67 (17, 19). It appears that when the HC folds slowly the unpaired and reactive Cys67 is exposed to the oxidizing ER environment, and thus is susceptible to forming disulfide links with other proteins containing free cysteines. For reasons that are not entirely clear, the Glu45 is particularly important for slow folding (19). The presence of both Glu45 and a Cys67 is extremely uncommon among class I alleles (8), consistent with our observations so far suggesting that misfolding is uncommon, if not unique to the HLA-B27 family of alleles.

 

Expression of Aberrant Forms of HLA-B27 on the Cell Surface

            In addition to the misfolded complexes that form in the ER, aberrant forms of B27 are found on the cell surface, and include HC-HC homodimers (19, 23). These appear to form during endosomal recycling of class I (23), and similar forms can be produced in vitro by refolding HC in the absence of B2m (24). Cell surface dimers are much more prevalent when B27 is expressed in the absence of tapasin (19, 23), but they are also found in normal cells. In addition to dimers, there are other non-conventional forms of HLA-B27. Monomeric B27 HCs containing peptides but lacking B2m, have been found on the cell surface constituting ~10% of the entire B27 pool (25, 26). Initially these species were said to contain exceptionally long peptides (25), but this was not reported in a more recent study (26).

            Cell surface dimerization does not appear to be unique to HLA-B27, as it has been observed with HLA-B7 (19) and HLA-G (27), as well as certain mouse alleles (28). Although HC misfolding in the ER and cell surface dimerization both involve aberrant disulfide bond formation, the two processes appear to be temporally and spatially distinct (19). Furthermore, HLA-B7 and mouse class I HCs that form cell surface dimers, do not exhibit ER misfolding and dimerization unless they are expressed in the absence of B2m (19, 28), a condition that induces HC misfolding (29). Taken together these results suggest that ER and cell surface dimerization are distinct processes with different implications.

 

Molecular Mimicry and Arthritogenic Peptides

            To explain the link between gram-negative enteric pathogens and the development of B27-associated diseases such as reactive arthritis (ReA), hypotheses have traditionally focused on ‘molecular mimicry’. HLA-B27 or B27-bound peptides were thought to resemble bacterial peptides, and thus become the target of cross-reactive antibodies and/or CD8+ T cells (30-36). A modification of this idea invoked HLA class II molecules presenting peptides derived from the B27 HC and stimulating CD4+ T cells meant to be specific for bacterial peptides (37, 38). In the early 1990s Hermann et al. grew CD8+ T cells from the synovial fluid of patients with ReA and AS that recognized B27 in the absence of microbial peptides, providing evidence of autoreactivity (39). Despite these early encouraging results supporting the existence of ‘arthritogenic’ B27-bound peptides, only one subsequent study has identified a candidate, in this case derived from the vasoactive intestinal peptide receptor (VIP1R) (sequence RRKWRRWHL) (40). Indeed, the VIP1R peptide was chosen for study because of homology with an epitope from the Epstein Barr virus (sequence RRRWRRLTV) that is also presented by HLA-B27. Despite considerable homology between these epitopes the study showed limited evidence for CTL cross-reactivity. Ringrose et al. performed a comprehensive review of the literature, and concluded that there was insufficient evidence to suggest that SpA are autoimmune on the basis of cross-reactivity with bacterial sequences (41). Considering the dramatic expansion of sequence information for microbial and human genes including HLA alleles, it would be of interest to re-examine the similarity between HLA alleles and bacterial proteins to determine the relevance of molecular mimicry to potential B27-mediated disease mechanisms.

The development of animal models has provided an opportunity to address the question of whether autoreactive CD8+ T cells play a role in SpA-like disease. Rats, and under certain conditions mice that express HLA-B27 develop spontaneous arthritis and inflammatory disease (SID) that resembles B27-associated disease in humans (42-44). This is particularly true for HLA-B27/human B2m transgenic rats where the spontaneous inflammatory phenotype includes colitis, arthritis, alopecia and psoriatic skin/nail changes. In mice, inflammation is limited to joints and nails, and in one model disease is only seen when endogenous mouse B2m is absent (43). Interestingly, in the rat and B2m-deficient mouse models, development of the inflammatory phenotype does not appear to require CD8+ T cells (43, 45), and thus conventional recognition of HLA-B27·peptide·B2m complexes is unlikely to play a role in the pathogenesis of B27-associated disease. Although these rodent models mimic the human phenotype to varying degrees and incompletely, the striking overlap and specificity for B27, particularly with SID in rats (5), argues that the role of B27 is similar. Although the existence of arthritogenic peptides has not been ruled out, the paucity of supporting evidence combined with novel observations about the immunobiology of HLA-B27 has led to the formulation of new hypotheses about pathogenesis.

 

Novel Hypotheses

            The tendency for HLA-B27 HCs to misfold and form aberrant cell surface complexes has resulted in novel hypotheses to explain its role in the pathogenesis of SpA. These ideas differ fundamentally in terms of whether immune recognition of B27 is the triggering event. One idea is that cell surface B27 homodimers are specifically recognized (46) either by leukocyte receptors on NK or other immune cells (47), or by T cell receptors on CD4+ T cells (48). The other concept is that B27 HC misfolding and ER retention may generate ER stress, which affects the function of antigen presenting cells such as macrophages and/or dendritic cells, leading to a pro-inflammatory state (49). It should be noted that it is of course conceivable that more than one mechanism may actually be involved in pathogenesis.

 

Immunological Recognition of Aberrant Forms of HLA-B27

            The discovery of HLA-B27 HC homodimers (19, 23, 24) has led to the suggestion that such complexes might be recognized by leukocyte receptors, which are known to interact with conventional class I molecules (50, 51). These fall broadly into two families; killer immunoglobulin receptors (KIR) and leukocyte immunoglobulin-like receptors (LILR or LIR) (also known as immunoglobulin like transcripts (ILT)). These receptors are expressed to varying extents on natural killer (NK), myelomonocytic (monocytes/macrophages and/or dendritic cells), and certain T cells, and they can deliver activating or inhibitory signals, and thus have the capacity to influence the immune response and contribute to chronic inflammation. Two studies have reported recognition of B27 homodimers by leukocyte receptors (52, 53). Recognition patterns were very similar between classical and homodimer forms of B27, with only KIR3DL2 demonstrating specificity for homodimers (52, 53). It will be important to determine the extent to which other class I molecules express aberrant cell surface forms, and whether they are recognized in a similar fashion by leukocyte receptors.

            There is also evidence for non-conventional T cell recognition of aberrant forms of HLA-B27. CD4+ T cells have been grown from patients with AS which preferentially recognize B27 expressed on cells with antigen processing defects (TAP- or tapasin-deficient) (48, 54). The forms of B27 that are recognized have not been determined, although the possibility that homodimers might be involved has been raised. Since T cells can express LILR as well as T cell receptors, recognition might occur via these molecules. Non-conventional CD4+ T cell recognition of HLA-A2 has also been described (55), and thus it will be important to determine the specificity of B27 recognition in patients with AS.

 

Protein Misfolding, ER Stress, and Inflammatory Disease

            Biochemical evidence indicates that HLA-B27 misfolds results in HC retention (17, 19) and BiP binding in the ER (20). BiP is a key sensor of ER stress, and its sequestration by misfolded proteins appears to be involved in activating the ‘unfolded protein response’ (UPR) (56). The UPR is an evolutionarily conserved mechanism that allows the cell to increase its capacity to fold and secrete proteins when threatened by diverse stimuli such as protein misfolding, or alterations in redox status, glucose and amino acid availability, and calcium handling (22). The UPR also serves a physiological role in differentiating B cells by signaling ER expansion, which is critical during their development into the antibody-secreting factories known as plasma cells (57). When a UPR does not resolve ER stress, the end result can be cell death by apoptosis. ER stress can also generate an ‘ER overload’ response which has been reported to activate NF-KB (58). Although the mediators of this response have not been defined, recent evidence points toward an IRE1/TRAF-2-dependent pathway (59) that can also activate JNK (60). Transient expression of HLA-B27 but not HLA-B8 causes activation of UPR promoters (36 and unpublished results). In more preliminary studies we have evidence for UPR target gene induction in certain cells derived from HLA-B27/hB2m transgenic rats (manuscript in preparation). Interestingly, Gu et al. have found overexpression of a UPR target gene (BiP) in monocytic cells from inflammatory lesions of SpA patients relative to individuals with rheumatoid arthritis (61). Furthermore, several studies have demonstrated ‘non-antigen-presenting’ effects of B27 expression in cultured cells (62-65). While some of these effects could be related to protein misfolding, this remains to be determined. Thus, the relationship between B27 misfolding, a UPR and/or ER overload response, and an inflammatory phenotype, requires further investigation. Nevertheless, taken together these observations support the underlying hypothesis that HLA-B27 misfolding is sufficient to cause UPR activation. Relatively little is know about the consequences of ER stress in the immune system. NF-KB activation would be expected to have significant implications, since this transcription factor is a strong activator of many pro-inflammatory cytokine genes. Furthermore, there are several members of the NF-KB family whose expression may be cell specific, and thus sequelae of NF-KB activation may differ depending on cell type. While many of the toxins that induce a UPR also cause NF-KB activation, this is not true of all proteins that misfold, raising the possibility that ER overload requires a stronger or more prolonged stimulus. With regard to the immune consequences of UPR activation, two transcription factors that are involved (XBP1 and CHOP) can contribute to the induction of IL-6 synthesis (57, 66) independently of NF-KB activation. IL-6 is a pro-inflammatory cytokine with numerous immunological effects, including influences on monocyte differentiation into macrophages and dendritic cells (67). It will be important to confirm and extend these observations about HLA-B27 misfolding to understand the impact of this on the immune system in animal models and ultimately in humans.

 

Summary

            Despite compelling evidence that HLA-B27 is involved in the pathogenesis of spondyloarthropathies, understanding the nature of its role has so far eluded investigators. Although hypotheses have traditionally focused on the physiological function of this molecule, new discoveries raise the possibility that improper folding within the cell and/or aberrant forms expressed on the cell surface may create a stimulus that initiates or perpetuates a chronic inflammatory process. These disparate ideas underscore fundamental unanswered questions such as whether immunological recognition of HLA-B27 is necessary for the development of these disorders.

 

Acknowledgements

            A Pfizer Scholar Award, and grants from the Arthritis Foundation and the National Institutes of Health have supported this work. The author would like to thank members of his laboratory who have made important contributions to the work on HLA-B27 misfolding.

 

References

1.         Brewerton, D.A., F.D. Hart, A. Nicholls, M. Caffrey, D.C.O. James, and R.D. Sturrock. 1973. Ankylosing spondylitis and HL-A 27. Lancet 1:904-907.

2.         Schlosstein, L., P.I. Terasaki, R. Bluestone, and C.M. Pearson. 1973. High association of an HL-A antigen, W27, with ankylosing spondylitis. N. Engl. J. Med. 288:704-706.

3.         Brewerton, D.A. 2003. Discovery: HLA and disease. Curr. Opin. Rheum. 15:369-373.

4.         Rubin, L.A., C.I. Amos, J.A. Wade, J.R. Martin, S.J. Bale, A.H. Little, D.D. Gladman, G.E. Bonney, J.D. Rubenstein, and K.A. Siminovitch. 1994. Investigating the genetic basis for ankylosing spondylitis:  linkage studies with the major histocompatibility complex region. Arthritis Rheum. 37:1212-1220.

5.         Taurog, J.D., S.D. Maika, N. Satumtira, M.L. Dorris, I.L. McLean, H. Yanagisawa, A. Sayad, A.J. Stagg, G.M. Fox, A.L. O'Brien, M. Rehman, M. Zhou, A.L. Weiner, J.B. Splawski, J.A. Richardson, and R.E. Hammer. 1999. Inflammatory disease in HLA-B27 transgenic rats. Immunol. Rev. 169:209-223.

6.         Khare, S.D., H.S. Luthra, and C.S. David. 1998. Animal models of human leukocyte antigen B27-linked arthritides. Rheum. Dis. Clin. North America 24:883-894.

7.         Brown, M.A., B.P. Wordsworth, and J.D. Reveille. 2002. Genetics of ankylosing spondylitis. Clin. Exp. Rheumatol. 20:S43-49.

8.         Marsh, S.G.E., E.D. Albert, W.F. Bodmer, R.E. Bontrop, B. Dupont, H.A. Erlich, D.E. Geraghty, J.A. Hansen, B. Mach, W.R. Mayr, P. Parham, E.W. Petersdorf, T. Sasazuki, G.M.T. Schreuder, J.L. Strominger, A. Svejgaard, and P.I. Terasaki. 2002. Nomenclature for factors of the HLA system, 2002. Hum Immunol 63.

9.         Madden, D.R., J.C. Gorga, J.L. Strominger, and D.C. Wiley. 1992. The three-dimensional structure of HLA-B27 at 2.1 Å resolution suggests a general mechanism for tight peptide binding to MHC. Cell 70:1035-1048.

10.       Madden, D.R. 1995. The three-dimensional structure of peptide-MHC complexes. Ann. Rev. Immunol. 13:587-622.

11.       Pamer, E., and P. Cresswell. 1998. Mechanisms of MHC class I-restricted antigen processing. Ann. Rev. Immunol. 16:323-358.

12.       Antoniou, A.N., S.J. Powis, and T. Elliott. 2003. Assembly and export of MHC class I peptide ligands. Curr. Opin. Immunol. 15:75-81.

13.       Grandea III, A.G., T.N. Golovina, S.E. Hamiltn, V. Sriram, T. Spies, R.R. Brutkiewicz, J.T. Harty, L.C. Eisenlohr, and L. Van Kaer. 2000. Impaired assembly yet normal trafficking of MHC class I molecules in Tapasin mutant mice. Immunity 13:213-222.

14.       Cresswell, P., N. Bangia, T. Dick, and G. Diedrich. 1999. The nature of the MHC class I peptide poading complex. Imm. Rev. 172:21-28.

15.       Neefjes, J.J., and H.L. Ploegh. 1988. Allele and locus-specific differences in cell surface expression and the association of HLA class I heavy chains with b2-microglobulin: differential effects of inhibition of glycosylation on class I subunit association. Eur. J. Immunol. 18:801-810.

16.       Neisig, A., R. Wubbolts, X. Zang, C. Melief, and J. Neefjes. 1996. Allele-specific differences in the interaction of MHC class I molecules with transporters associated with antigen processing. J. Immunol. 156:3196-3206.

17.       Mear, J.P., K.L. Schreiber, C. Munz, X. Zhu, S. Stevanovic, H.-G. Rammensee, S.L. Rowland-Jones, and R.A. Colbert. 1999. Misfolding of HLA-B27 as a result of its B pocket suggests a novel mechanism for its role in susceptibility to spondyloarthropathies. J. Immunol. 163:6665-6670.

18.       Ellgaard, L., and A. Helenius. 2003. Quality control in the endoplasmic reticulum. Nature Rev Mol Cell Biol 4:181-191.

19.       Dangoria, N.S., M.L. DeLay, D.J. Kingsbury, J.P. Mear, B. Uchanska-Ziegler, A. Ziegler, and R.A. Colbert. 2002. HLA-B27 misfolding is associated with aberrant intermolecular disulfide bond formation (dimerization) in the endoplasmic reticulum. J Biol Chem 277:23459-23468.

20.       Colbert, R.A. 2002. HLA-B27 misfolding involves aberrant disulfide bond formation and BiP/GRP78 binding in the endoplasmic reticulum. Clin. Exp. Rheumatol. 20:585.

21.       Ma, Y., and L.M. Hendershot. 2001. The unfolding tale of the unfolded protein response. Cell 107:827-830.

22.       Kaufman, R.J. 2002. Orchestrating the unfolded protein response in health and disease. J. Clin. Invest. 110:1389-1398.

23.       Bird, L.A., C.A. Peh, S. Kollnberger, T. Elliott, A.J. McMichael, and P. Bowness. 2003. Lymphoblastoid cells express HLA-B27 homodimers both intracellularly and at the cell surface following endosomal recycling. Eur. J. Immunol. 33:748-759.

24.       Allen, R.L., C.A. O'Callaghan, A.J. McMichael, and P. Bowness. 1999. HLA-B27 can form a novel b2-microglobulin-free heavy chain homodimer structure. J. Immunol. 162:5045-5048.

25.       Urban, R.G., R.M. Chicz, W.S. Lane, J.L. Strominger, A. Rehm, M.J.H. Kenter, F.G.C.M. Uytde-Haag, H. Ploegh, B. Uchanska-Ziegler, and A. Ziegler. 1994. A subset of HLA-B27 molecules contains peptides much longer than nonamers. Proc. Natl. Acad. Sci. USA 91:1534-1538.

26.       Malik, P., P. Klimovitsky, L.W. Deng, J.E. Boyson, and J.L. Strominger. 2002. Uniquely conformed peptide-containing beta 2-microglobulin-free heavy chains of HLA-B2705 on the cell surface. J. Immunol. 169:4379-4387.

27.       Boyson, J.E., R. Erskine, M.C. Whitman, M. Chiu, J.M. Lau, L.A. Koopman, M.M. Valter, P. Angelisova, V. Horejsi, and J.L. Strominger. 2002. Disulfide bond-mediated dimerization of HLA-G on the cell surface. Proc. Natl. Acad. Sci. USA 99:16180-16185.

28.       Capps, G.G., B.E. Robinson, K.D. Lewis, and M.C. Zuniga. 1993. In vivo dimeric association of class I MHC heavy chains: possible relationship to class I MHC heavy chain-b2-microglobulin dissociation. J. Immunol. 151:159-169.

29.       Hughes, E.A., C. Hammond, and P. Cresswell. 1997. Misfolded major histocompatibility complex class I heavy chains are translocated into the cytoplasm and degraded by the proteasome. Proc. Natl. Acad. Sci. USA 94:1896-1901.

30.       Seager, K., H.V. Bashir, A.F. Geczy, J. Edmonds, and A. de Vere-Tyndal. 1979. Evidence for a specific B27-associated cell surface marker on lymphocytes of patients with ankylosing spondylitis. Nature 277:68-70.

31.       Geczy, A.F., K. Alexander, H.V. Bashir, and J. Edmonds. 1980. A factor(s) in Klebsiella culture filtrates specifically modifies and HLA-B27 associated cell-surface component. Nature 283:782-784.

32.       Schwimmbeck, P., D.T. Yu, and M. Oldstone. 1987. Autoantibodies to HLA-B27 in the sera of HLA-B27 positive patients with ankylosing spondylitis and Reiter's syndrome. J. Exp. Med. 166:173-181.

33.       Schwimmbeck, P.L., and M.B. Oldstone. 1988. Molecular mimicry between human leukocyte antigen B27 and Klebisella. Consequences for spondyloarthropathies. Am. J. Med. 85:51-53.

34.       Benjamin, R.J., and P. Parham. 1990. Guilt by association: HLA-B27 and ankylosing spondylitis. Immunol. Today 11:137-142.

35.       Ebringer, A. 1992. Ankylosing spondylitis is caused by Klebisella. Evidence from immunogenetic, microbiologic, and serologic studies. Rheum. Dis. Clin. North Amer. 18:105-121.

36.       Scofield, R.H., W.L. Warren, G. Koelsch, and J.B. Harley. 1993. A hypothesis for the immune dysregulation in spondyloarthropathies: contributions from enteric organisms, B27 structure, peptides bound by B27 and convergent evolution. Proc. Natl. Acad. Sci. USA 90:9330-9334.

37.       Davenport, M.P. 1995. The promiscuous B27 hypothesis. Lancet 346:500-501.

38.       Parham, P. 1996. Presentation of HLA class I-derived peptides: potential involvement in allorecognition and HLA-B27-associated arthritis. Immunol. Rev. 154:137-154.

39.       Hermann, E., D.T.Y. Yu, K.-H. Meyer zum Buschenfelde, and B. Fleischer. 1993. HLA-B27-restricted CD8 T cells from synovial fluids of patients with reactive arthritis and ankylosing spondylitis. Lancet 342:646-650.

40.       Fiorillo, M.T., M. Maragno, R. Butler, M.L. Dupuis, and R. Sorrentino. 2000. CD8+ T cell autoreactivity to an HLA-B27-restricted self-epitope correlates with ankylosing spondylitis. J. Clin. Invest. 106:47-53.

41.       Ringrose, J.H. 1999. HLA-B27 associated spondyloarthropathy, an autoimmune disease based on cross reactivity between bacteria and HLA-B27? Ann. Rheum. Dis. 58:598-610.

42.       Hammer, R.E., S.D. Maika, J.A. Richardson, J.-P. Tang, and J.D. Taurog. 1990. Spontaneous inflammatory disease in transgenic rats expressing HLA-B27 and human b2-m: an animal model of HLA-B27-associated human disorders. Cell 63:1099-1112.

43.       Khare, S.D., H.S. Luthra, and C.S. David. 1995. Spontaneous inflammatory arthritis in HLA-B27 transgenic mice lacking b2-microglobulin: a model of human spondyloarthropathies. J. Exp. Med. 182:1153-1158.

44.       Weinreich, S., F. Eulderink, J. Capkova, M. Pla, K. Gaede, J. Heesemann, L. van Alphen, C. Zurcher, B. Hoebe-Hewryk, F. Kievits, and P. Ivanyi. 1995. HLA-B27 as a relative risk factor in ankylosing enthesopathy in transgenic mice. Hum. Immunol. 42:103-115.

45.       May, E., M.L. Dorris, N. Satumtira, I. Iqbal, M.I. Rehman, E. Lightfoot, and J.D. Taurog. 2003. CD8ab T cells are not essential to the pathogenesis of arthritis or colitis in HLA-B27 transgenic rats. J. Immunol. 170:1099-1105.

46.       Edwards, J.C.W., P. Bowness, and J.R. Archer. 2000. Jekyll and Hyde: the transformation of HLA-B27. Immunol. Today 21:256-260.

47.       Allen, R.L., P. Bowness, and A. McMichael. 1999. The role of HLA-B27 in spondyloarthritis. Immunogen. 50:220-227.

48.       Boyle, L.H., J.C. Goodall, S.S. Opat, and J.S.H. Gaston. 2001. The recognition of HLA-B27 by human CD4+ T lymphocytes. J. Immunol. 167:2619-2624.

49.       Colbert, R.A. 2000. HLA-B27 misfolding: A solution to the spondyloarthropathy conundrum? Mol. Med. Today 6:224-230.

50.       Trowsdale, J. 2001. Genetic and functional relationships between MHC and NK receptor genes. Immunity 15:363-374.

51.       Vilches, C., and P. Parham. 2002. KIR: diverse, rapidly evolving receptors of innate and adaptive immunity. Ann. Rev. Immunol. 20:217-251.

52.       Allen, R.L., T. Raine, A. Haude, J. Trowsdale, and M.J. Wilson. 2001. Leukocyte receptor complex-encoded immunomodulatory receptors show differing specificity for alternative HLA-B27 structures. J. Immunol. 167:5543-5547.

53.       Kollnberger, S., L.A. Bird, S. M-Y, C. Retiere, V.M. Braud, A. McMichael, and P. Bowness. 2002. Cell surface expression and immune receptor recogntion of HLA-B27 homodimers. Arth. Rheum. 46:2972-2982.

54.       Boyle, L.H., and J.S.H. Gaston. 2003. Breaking the rules: the unconventional recognition of HLA-B27 by CD4+ T lymphocytes as an insight into the pathogenesis of the spondyloarthropathies. Rheumatology 42:404-412.

55.       Heemskerk, M.H., R.A. de Paus, E.G. Lurvink, F. Koning, A. Mulder, R. Willemze, J.J. van Rood, and J.H. Falkenburg. 2001. Dual HLA class I and II restricted recognition of alloreactive T lymphocytes mediated by a single T cell receptor complex. Proc. Natl. Acad. Sci. USA 98:6806-6811.

56.       Bertolotti, A., Y. Zhang, L. Hendershot, H. Harding, and D. Ron. 2000. Dynamic interaction of BiP and the ER stress transducers in the unfolded protein response. Nat. Cell Biol. 2:326-332.

57.       Iwakoshi, N.N., A.-H. Lee, P. Vallabhajosyula, K.L. Potipoby, K. Rajewsky, and L.H. Glimcher. 2003. Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1. Nat Immunol. 4:321-329.

58.       Pahl, H.L., and P.A. Baeuerle. 1997. The ER-overload response:  activation of NF-kappa B. Trends Biochem. Sci. 22:63-67.

59.       Kaneko, M., Y. Niinuma, and Y. Nomura. 2003. Activation signal of nuclear factor-kB in response to endoplasmic reticulum stress is transduced via IRE1 and tumor necrosis factor receptor-associated factor 2. Biol. Pharm. Bull. 26:931-935.

60.       Urano, F., X.-Z. Wang, A. Bertolotti, Y. Zhang, P. Chung, H. Harding, and D. Ron. 2000. Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1. Science 287:664-666.

61.       Gu, J., M. Rihl, E. Marker-Hermann, D. Baeten, J.G. Kuipers, Y.W. Song, W.P. Maksymowych, R. Burgos-Vargas, E.M. Veys, F. De Keyser, H. Deister, M. Xiong, F. Huang, W.C. Tsai, and D.T. Yu. 2002. Clues to the pathogenesis of spondyloarthropathy derived from synovial fluid mononuclear cell gene expression profiles. J. Rheumatol. 29:2159-2164.

62.       Ikawa, T., M. Ikeda, A. Yamaguchi, W.C. Tsai, N. Tamura, N. Seta, M. Trucksess, R.B. Raybourne, and D.T.Y. Yu. 1998. Expression of arthritis-causing HLA-B27 on Hela cells promotes induction of c-fos in response to in vitro invasion by Salmonella typhimurium. J. Clin. Invest. 101:263-272.

63.       Penttinen, M.A., C.I. Holmberg, L. Sistonen, and K. Granfors. 2002. HLA-B27 modulates nuclear factor kB activation in human monocytic cells exposed to lipopolysaccharide. Arthritis Rheum. 46:2172-2180.

64.       Virtala, M., J. Kirveskari, and K. Granfors. 1997. HLA-B27 modulates the survival of Salmonella enteritidis in transfected L cells, possibly by impaired nitric oxide production. Infect. Immun. 65:2436-4242.

65.       Ekman, P., M. Saarinen, Q. He, C. Gripenberg-Lerche, A. Grönberg, H. Arvilommi, and K. Granfors. 2002. HLA-B27 transfected (Salmonella permissive) and HLA-A2 transfected (Salmonella non-permissive) human monocytic U937 cells differ in their production of cytokines. Infect. Immun. 70:(in press).

66.       Hattori, T., N. Ohoka, H. Hayashi, and K. Onozaki. 2003. C/EBP homologous protein (CHOP) up-regulates IL-6 transcription by trapping negative-regulating NF-IL6 isoform. FEBS Lett. 541:33-39.

67.       Chomarat, P., J. Banchereau, J. Davoust, and A.K. Palucka. 2000. IL-6 switches the differentiation of monocytes from dendritic cells to macrophages. Nat. Immunol. 1:510-514.