Chemical reactivity of an HLA-B27 thiol group

Cell Surface B2m-Free Human Leukocyte Antigen (HLA) Monomers and Dimers: Are They Neo-HLA Class and Proto-HLA?

Cell surface HLA-I molecules (Face-1) consist of a polypeptide heavy chain (HC) with two groove domains (G domain) and one constant domain (C-domain) as well as a light chain, B2-microglobulin (B2m). However, HCs can also independently emerge unfolded on the cell surface without peptides as B2m-free HC monomers (Face-2), B2m-free HC homodimers (Face 3), and B2m-free HC heterodimers (Face-4). The transport of these HLA variants from ER to the cell surface was confirmed by antiviral antibiotics that arrest the release of newly synthesized proteins from the ER. Face-2 occurs at low levels on the normal cell surface of the lung, bronchi, epidermis, esophagus, breast, stomach, ilium, colorectum, gall bladder, urinary bladder, seminal vesicles ovarian epithelia, endometrium, thymus, spleen, and lymphocytes. They are upregulated on immune cells upon activation by proinflammatory cytokines, anti-CD3 antibodies, antibiotics (e.g., ionomycin), phytohemagglutinin, retinoic acid, and phorbol myristate acetate. Their density on the cell surface remains high as long as the cells remain in an activated state. After activation-induced upregulation, the Face-2 molecules undergo homo- and hetero-dimerization (Face-3 and Face-4). Alterations in the redox environment promote dimerization. Heterodimerization can occur among and between the alleles of different haplotypes. The glycosylation of these variants differ from that of Face-1, and they may occur with bound exogenous peptides. Spontaneous arthritis occurs in HLA-B27+ mice lacking B2m (HLA-B27+ B2m-/-) but not in HLA-B27+ B2m+/- mice. The mice with HLA-B27 in Face-2 spontaneous configuration develop symptoms such as changes in nails and joints, hair loss, and swelling in paws, leading to ankyloses. Anti-HC-specific mAbs delay disease development. Some HLA-I polyreactive mAbs (MEM series) used for immunostaining confirm the existence of B2m-free variants in several cancer cells. The upregulation of Face-2 in human cancers occurs concomitantly with the downregulation of intact HLAs (Face-1). The HLA monomeric and dimeric variants interact with inhibitory and activating ligands (e.g., KIR), growth factors, cytokines, and neurotransmitters. Similarities in the amino acid sequences of the HLA-I variants and HLA-II β-chain suggest that Face-2 could be the progenitor of both HLA classes. These findings may support the recognition of these variants as a neo-HLA class and proto-HLA.

The oxidative folding and misfolding of human leukocyte antigen-b27

The major histocompatibility complex class I molecule human leukocyte antigen (HLA)-B27 is strongly associated with a group of inflammatory arthritic disorders known as the spondyloarthropathies. Many autoimmune diseases exhibit associations with major histocompatibility complex molecules encoded within the class II locus with defined immune responses either mediated by T or B-lymphocytes. Despite the association being known for over 30 years, no defined immune response and target autoantigens have been characterized for the spondyloarthropathies. Thus, the mechanism and role of HLA-B27 in disease pathogenesis remains undetermined. One hypothesis that has recently received much attention has focused around the enhanced propensity for HLA-B27 to misfold and the increased tendency of the heavy chain to dimerize. The misfolding of HLA-B27 has been associated with its redox status and this is postulated to be involved in disease development. Here we discuss the impact of the redox status on HLA-B27 biosynthesis and function.

Pathogenicity of Misfolded and Dimeric HLA-B27 Molecules

The association between HLA-B27 and the group of autoimmune inflammatory arthritic diseases, the spondyloarthropathies (SpAs) which include ankylosing spondylitis (AS) and Reactive Arthritis (ReA), has been well established and remains the strongest association between any HLA molecule and autoimmune disease. The mechanism behind this striking association remains elusive; however animal model and biochemical data suggest that HLA-B27 misfolding may be key to understanding its association with the SpAs. Recent investigations have focused on the unusual biochemical structures of HLA-B27 and their potential role in SpA pathogenesis. Here we discuss how these unusual biochemical structures may participate in cellular events leading to chronic inflammation and thus disease progression.

From HLA-B27 to spondyloarthritis: a journey through the ER

Almost four decades of research into the role of human leukocyte antigen-B27 (HLA-B27) in susceptibility to spondyloarthritis has yet to yield a convincing answer. New results from an HLA-B27 transgenic rat model now demonstrate quite convincingly that CD8(+) T cells are not required for the inflammatory phenotype. Discoveries that the HLA-B27 heavy chain has a tendency to misfold during the assembly of class I complexes in the endoplasmic reticulum (ER) and to form aberrant disulfide-linked dimers after transport to the cell surface have forced the generation of new ideas about its role in disease pathogenesis. In transgenic rats, HLA-B27 misfolding generates ER stress and leads to activation of the unfolded protein response, which dramatically enhances the production of interleukin-23 (IL-23) in response to pattern recognition receptor agonists. These findings have led to the discovery of striking T-helper 17 cell activation and expansion in this animal model, consistent with results emerging from humans with spondyloarthritis and the discovery of IL23R as an additional susceptibility gene for ankylosing spondylitis. Together, these results suggest a novel link between HLA-B27 and the T-helper 17 axis through the consequences of protein misfolding and open new avenues of investigation as well as identifying new targets for therapeutic intervention in this group of diseases.

HLA-B27 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta(2)m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to beta(2)m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation of misfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missing links between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

HLA-B27 misfolding and spondyloarthropathies

HLA-B27 plays a central role in the pathogenesis of many spondyloarthropathies and in particular ankylosing spondylitis. The observation that the HLA-B27 heavy chain has a tendency to misfold has raised the possibility that associated diseases may belong in a rapidly expanding category of protein misfolding disorders. The synthesis of the HLA-B27 heavy chain, assembly with beta2m and the loading of peptide cargo, occurs in the endoplasmic reticulum (ER) before transport to the cell surface. The evidence indicates that misfolding occurs in the ER prior to b2m association and peptide optimization and is manifested in the formation of aberrant inter- and intra-chain disulfide bonds and accumulation of heavy chain bound to the chaperone BiP. Enhanced accumulation ofmisfolded heavy chains during the induction of class I expression by cytokines, can cause ER stress resulting in activation of the unfolded protein response (UPR). Effects of UPR activation on cytokine production are beginning to emerge and may provide important missinglinks between HLA-B27 misfolding and spondyloarthritis. In this chapter we will review what has been learned about HLA-B27 misfolding in human cells and in the transgenic rat model of spondyloarthritis-like disease, considering it in the context of other protein misfolding disorders. These studies provide a framework to support much needed translational work assessing HLA-B27 misfolding and UPR activation in patient-derived material, its consequences for disease pathogenesis and ultimately how and where to focus intervention strategies.

The pathogenesis of ankylosing spondylitis

✓ Ankylosing spondylitis (AS) is a chronic inflammatory disease that can cause significant functional complications by affecting the sacroiliac joints and axial skeleton. Despite a longstanding knowledge about the familial associations of this disease, particularly among patients positive for human leukocyte antigen (HLA)–B27, the fundamental pathogenetic mechanism by which this disease arises in genetically susceptible individuals remains ill defined. Furthermore, the molecular predilection for characteristic articular site involvement remains under ongoing investigation. Current theories about the HLA-B27 association range from the presentation of novel arthritogenic peptides, to abnormal autoimmune stimulation, to anomalous microbial tolerance. The immune effectors of this damage include CD4+, CD8+, and natural killer cells, with marked heterogeneity at different sites. Biomechanical stresses may trigger this disease by exposing the body to previously immune-sequestered autoantigens or by providing a route for bacterial seeding. Environmental triggers such as infection have not been definitively established but may represent a primary pathogenic step in a molecular-mimicry process. In this article, the authors review the current literature on the origin and pathophysiology of AS, focusing on genetic and molecular associations, consequent pathomechanisms, and associated triggers. An improved understanding of the sequence of molecular events that predispose and initiate the onset of this disease will allow for more specific and targeted therapy and better avoidance of the significant side effects of systemic immunomodulation.

Pathogenesis of ankylosing spondylitis: current concepts

More than three decades after the discovery of HLA-B27 as a major genetic clue to the origins of ankylosing spondylitis, much has been learned about pathogenesis. However, the role of this major histocompatibility complex class I allele remains undefined. Studies from animal models have demonstrated that HLA-B27 overexpression can cause inflammatory disease with spondyloarthritis features, and together with investigations of patient-derived material, both innate adaptive and immune responses have been implicated. The gastrointestinal immune response to pathogens and even normal flora, with subclinical or overt inflammation, may play a role as an environmental component of these diseases. Although there has been a large conceptual emphasis on mechanisms involving autoreactive T-cell recognition of HLA-B27 complexes displaying arthritogenic peptides, and more recently non-canonical recognition of abnormal forms of HLA-B27 free of beta(2)m (heavy-chain dimers or monomers), it remains unclear whether immunological recognition plays a role in pathogenesis. The recognition that the HLA-B27 heavy chain misfolds during assembly, and causes endoplasmic reticulum 'stress', has led to the observation that this activates the unfolded protein response. This has opened additional areas of investigation into the response of immune system cells to protein misfolding, and suggested novel alternative concepts that may explain the role of HLA-B27 in pathogenesis. This chapter will discuss available data and current concepts regarding the pathogenesis of ankylosing spondylitis.

Aetiology and pathogenesis of reactive arthritis: role of non-antigen-presenting effects of HLA-B27

Spondyloarthropathies are inflammatory diseases closely associated with human leukocyte antigen (HLA)-B27 by unknown mechanisms. One of these diseases is reactive arthritis (ReA), which is typically triggered by Gram-negative bacteria, which have lipopolysaccharide as an integral component of their outer membrane. Several findings in vivo and in vitro obtained from patients with ReA and from different model systems suggest that HLA-B27 modulates the interaction between ReA-triggering bacteria and immune cells by a mechanism unrelated to the antigen presentation function of HLA-B27. In this review we piece together a jigsaw puzzle from the new information obtained from the non-antigen-presenting effects of HLA-B27.

Formation of HLA-B27 homodimers and their relationship to assembly kinetics

The human HLA-B27 class I molecule exhibits a strong association with the inflammatory arthritic disorder ankylosing spondylitis and other related arthropathies. Major histocompatibility complex class I heavy chains normally associate with beta(2)-microglobulin and peptide in the endoplasmic reticulum before transit to the cell surface. However, an unusual characteristic of HLA-B27 is its ability to form heavy chain homodimers through an unpaired cysteine at position 67 in the peptide groove. Homodimers have previously been detected within the ER and at the cell surface, but their mechanism of formation and role in disease remain undefined. Here we demonstrate, in the rat C58 thymoma cell line and in human HeLa cells transfected with HLA-B27, that homodimer formation involves not only cysteine at position 67 but also the conserved structural cysteine at position 164. We also show that homodimer formation can be induced in the non-disease-associated HLA class I allele HLA-A2 by slowing its assembly rate by incubation of cells at 26 degrees C, suggesting that homodimer formation in the endoplasmic reticulum may occur as a result of the slower folding kinetics of HLA-B27. Finally, we report an association between unfolded HLA-B27 molecules and immunoglobulin-binding protein at the cell surface.

HLA-B27 misfolding is associated with aberrant intermolecular disulfide bond formation (dimerization) in the endoplasmic reticulum

The class I protein HLA-B27 confers susceptibility to inflammatory arthritis in humans and when overexpressed in rodents for reasons that remain unclear. We demonstrated previously that HLA-B27 heavy chains (HC) undergo endoplasmic reticulum (ER)-associated degradation. We report here that HLA-B27 HC also forms two types of aberrant disulfide-linked complexes (dimers) during the folding and assembly process that can be distinguished by conformation-sensitive antibodies W6/32 and HC10. HC10-reactive dimers form immediately after HC synthesis in the ER and constitute at least 25% of the HC pool, whereas W6/32-reactive dimers appear several hours later and represent less than 10% of the folded HC. HC10-reactive dimers accumulate in the absence of tapasin or beta(2)-microglobulin, whereas W6/32-reactive dimers are not detected. Efficient formation of W6/32-reactive dimers appears to depend on the transporter associated with antigen processing, tapasin, and beta(2)-microglobulin. The unpaired Cys(67) and residues at the base of the B pocket that dramatically impair HLA-B27 HC folding are critical for the formation of HC10-reactive ER dimers. Although certain other alleles also form dimers late in the assembly pathway, ER dimerization of HLA-B27 may be unique. These results demonstrate that residues comprising the HLA-B27 B pocket result in aberrant HC folding and disulfide bond formation, and thus confer unusual properties on this molecule that are unrelated to peptide selection per se, yet may be important in disease pathogenesis.

The Cys-67 residue of HLA-B27 influences cell surface stability, peptide specificity, and T-cell antigen presentation

Cys-67 of HLA-B27 is located in the B pocket, which determines peptide-binding specificity. We analyzed effects of the Cys-67 --> Ser mutation on cell surface expression, peptide specificity, and T-cell recognition of HLA-B*2705. Surface expression was assessed with antibodies recognizing either native or unfolded HLA proteins. Whereas native B*2705 molecules predominated over unfolded ones, this ratio was reversed in the mutant, suggesting lower stability. Comparison of B*2705- and Cys-67 --> Ser-bound peptides revealed that the mutant failed to bind approximately 15% of the B*2705 ligands, while binding as many novel ones. Two peptides with Gln-2 found in both B*2705 and Cys-67 --> Ser are the first demonstration of natural B*2705 ligands lacking Arg-2. Other effects of the mutation on peptide specificity were: 1) average molecular mass of natural ligands higher than for B*2705, 2) bias against small residues at peptide position (P) 1, and 3) increased P2 permissiveness. The results suggest that the Cys-67 --> Ser mutation weakens B pocket interactions, leading to decreased stability of the mutant-peptide complexes. This may be partially compensated by interactions involving bulky P1 residues. The effect of the mutation on allorecognition was consistent with that on peptide specificity. Our results may aid understanding of the pathogenetic role of HLA-B27 in spondyloarthropathy.

Mutation of Cys-67 alters the thermodynamic stability of the human leukocyte antigen HLA0-B*2705

The B pocket of the class I major histocompatibility complex-encoded protein HLA-B*2705 has recently been suggested to be responsible for the misfolding of this HLA haplotype and thus to induce susceptibility to autoimmune inflammatory diseases. Four mutants of the B*2705 heavy chain were refolded in the presence of three control peptides. The monitoring of the thermal unfolding of the B*2705-peptide complexes by circular dichroism spectroscopy showed that all heterotrimeric mutants were markedly less stable than the corresponding complexes with the wild-type heavy chain. Among the four heavy chain mutations, the C67S change was investigated for unfolding and peptide binding properties because this position may mediate disulfide pair bridging and alter T-cell recognition of HLA-B*2705. Wild-type heterotrimers completely unfold in a single transition at mild acidic pH whereas increase of the pH to mild basic conditions induce only a partial biphasic unfolding. Cys-67 seems to play a crucial role in controlling the thermodynamic stability of the B*2705-peptide complexes as the C67S mutant unfolds faster and with a single transition, independent of pH. Fluorescence polarization and size exclusion chromatography of unfolding intermediates suggest that the peculiar unfolding of the B*2705 wild-type heavy chain cannot be explained by modified peptide binding properties but more likely by the formation of high molecular weight species.

Gorillas with spondyloarthropathies express an MHC class I molecule with only limited sequence similarity to HLA-B27 that binds peptides with arginine at P2

The human MHC class I gene, HLA-B27, is a strong risk factor for susceptibility to a group of disorders termed spondyloarthropathies (SpAs). HLA-B27-transgenic rodents develop SpAs, implicating HLA-B27 in the etiology of these disorders. Several nonhuman primates, including gorillas, develop signs of SpAs indistinguishable from clinical signs of humans with SpAs. To determine whether SpAs in gorillas have a similar HLA-B27-related etiology, we analyzed the MHC class I molecules expressed in four affected gorillas. Gogo-B01, isolated from three of the animals, has only limited similarity to HLA-B27 at the end of the alpha1 domain. It differs by several residues in the B pocket, including differences at positions 45 and 67. However, the molecular model of Gogo-B*0101 is consistent with a requirement for positively charged residues at the second amino acid of peptides bound by the MHC class I molecule. Indeed, the peptide binding motif and sequence of individual ligands eluted from Gogo-B*0101 demonstrate that, like HLA-B27, this gorilla MHC class I molecule binds peptides with arginine at the second amino acid position of peptides bound by the MHC class I molecule. Furthermore, live cell binding assays show that Gogo-B*0101 can bind HLA-B27 ligands. Therefore, although most gorillas that develop SpAs express an MHC class I molecule with striking differences to HLA-B27, this molecule binds peptides similar to those bound by HLA-B27.

HLA-B27 and immunogenetics of spondyloarthropathies

The arthritogenic peptide hypothesis has inspired research aimed at defining the peptide-presenting properties of HLA-B27 subtypes and their relation to ankylosing spondylitis. Various studies have shed new light on the influence of HLA-B27 polymorphism in modulating peptide binding and T-cell antigen presentation. Moreover, multiple factors along the antigen processing-loading pathway, including tapasin, contribute to shaping the HLA-B27 repertoire. Other studies have revealed significant peptide-binding similarities between HLA-B27 and subtypes of HLA-B39, supporting a role of this antigen in spondyloarthropathy. A putative pathogenetic role of the HLA-B27 heavy chain, initially suggested from studies in transgenic mice, is claimed on the basis of novel, yet circumstantial, evidence concerning an apparently unusual capacity of the heavy chain to form stable homodimers or misfold after biosynthesis. Finally, it appears that arthritogenic infections might downregulate HLA-B27 expression, favoring bacterial survival. The specificity and mechanism of this phenomenon are yet to be defined.

HLA-B27 misfolding: a solution to the spondyloarthropathy conundrum?

Compelling evidence indicates that HLA-B27 is directly involved in the etiopathogenesis of the spondyloarthropathies (SpAs). Several hypotheses based on its native antigenic structure, the peptides it presents and mimicry with bacterial epitopes, have been proposed. However, these potential mechanisms remain largely unsupported by human studies and transgenic animal models. Recent work demonstrating that HLA-B27 misfolds offers a novel alternative hypothesis. Here, we review this new information on the folding and assembly of HLA-B27, and discuss consequences of misfolding that could be relevant to the pathogenesis of SpAs.

Inflammatory disease in HLA-B27 transgenic rats

A spontaneous inflammatory disease in rats transgenic for HLA-B27 resembles the B27-associated human spondyloarthropathies. Colitis and arthritis, the two most important features, require T cells, gut bacteria, and high expression of B27 in bone marrow-derived cells. Control rats with HLA-B7 remain healthy. Most rats with HLA-Cw6 (associated with psoriasis vulgaris) remain healthy; a minority develop mild and transient disease. Rats with a mutant B27 with a Cys67-->Ser substitution resemble wild-type B27 transgenics, but with a lower prevalence of arthritis. A similar phenotype is seen in B27 rats co-expressing a viral peptide that binds B27. Disease-prone LEW but not F344 B27 rats develop high serum IgA levels concurrent with disease progression. Colitis is associated with high interferon-gamma, arthritis with high interleukin-6. Disease is similar in B27 LEW, F344, and PVG rats, but the DA background is protective.

CONCLUSIONS

The spondyloarthropathy-like disease in rats is specific for HLA-B27 but does not require Cys67. Arthritis but not colitis is particularly sensitive to B27 peptide-binding specificity. Genetic background exerts a strong influence, but some phenotypic differences exist between permissive strains that do not influence disease susceptibility. The data favor a role for B27 peptide presentation in arthritis, but other mechanisms to explain the role of B27 have not been excluded.

The role of HLA-B27 polymorphism and molecular mimicry in spondylarthropathy

Ankylosing spondylitis (AS), reactive arthritis (ReA) and other related spondyloarthropathies (SpAs) are characterized by a strong association with the major histocompatibility complex allele HLA-B27. Experimental evidence from humans and transgenic rodents suggests that HLA-B27 is itself involved in the pathogenesis of SpA. Population and peptide-specificity analysis of HLA-B27 suggest it has a pathogenic function related to antigen presentation. Putative roles for infectious agents have been proposed in ReA and suggested in AS. However, the mechanism by which HLA-B27 and bacteria interact to induce arthritis is not clear. Molecular mimicry between bacterial epitopes that cross-react with self-B27 peptides is the most persuasive explanation for the pathogenesis of SpA. The experimental studies reviewed here have greatly increased our knowledge of the structure, function and disease association of HLA-B27.

Alteration of HLA-B27 peptide presentation after infection of transfected murine L cells by Shigella flexneri

Shigella flexneri is a triggering agent for reactive arthritis in HLA-B27-susceptible individuals. Considering the intracellular multiplication of bacteria, it seems likely that bacterial peptides may be presented by the major histocompatibility complex (MHC) class I pathway. To examine this hypothesis, we infected HLA-B*2705- and/or human beta2-microglobulin-transfected murine L-cell lines with M90T, an invasive strain of S. flexneri. Bacterial infection induced no detectable modifications in the biosynthesis and expression level of HLA-B27, as assessed by immunoprecipitation, Northern blot analysis, and flow cytometry. Using confocal microscopy, we observed that bacterial infection induced a clustering of HLA-B27 molecules during macropinocytosis and before bacterial dissemination from cell to cell. Peptides naturally bound to HLA-B27 molecules were acid eluted from infected cells and separated by high-performance liquid chromatography. Major differences were observed in high-performance liquid chromatography profiles and in the nature of peptides presented following bacterial infection. Although most of the antigens presented were not accessed by Edman degradation, we obtained two sequences partially homologous to bacterial proteins. These peptides lacked the major HLA-B27 peptide anchor (Arg) at position 2, and one had an unusual length of 14 amino acids. These data suggest that alterations in the peptide presentation by HLA-B27 occur during infection, which could be relevant to the pathogenesis of HLA-B27-related arthritis.

Yersinia enterocolitica serotype O:3 alters the expression of serologic HLA-B27 epitopes on human monocytes

The expression of serologic HLA-B27 epitopes on leukocytes of patients with reactive arthritis or ankylosing spondylitis has been shown to be modified in the course of the disease. The purpose of this work was to study whether phagocytosis of arthritis-triggering microbes in vitro alters the expression of HLA-B27 molecules on human antigen-presenting cells and to characterize the underlying mechanisms. Human monocytes and HLA-B27- or HLA-A2-transfected human U-937 cells were exposed to Yersinia enterocolitica serotype O:3. The expression of different epitopes of HLA-B27 was monitored by using immunofluorescence, and their synthesis was determined by quantitative immunoprecipitation. Our results show that phagocytosis of Y. enterocolitica serotype O:3 changed the expression of serological HLA-B27 epitopes. This was due to the reduced synthesis of HLA-B27 molecules. The expression of especially the epitopes which depend on the presence of peptides in the antigen-binding groove was changed. The expression of the ME1 epitope, which has been shown to be important for T-cell recognition in patients with reactive arthritis, was decreased. Down-regulation of epitopes important for the T-cell recognition may impair the elimination of arthritis-triggering microbes and lead to persistent infection. In addition, Y. enterocolitica serotype O:3 seemed to alter the repertoire of peptides presented by the HLA-B27 molecules on human monocytes. This may have a role in the pathogenesis of reactive arthritis via an autoimmune mechanism.

Homocysteine modification of HLA antigens and its immunological consequences

Homocysteine-treated cells can be specifically lysed by cytotoxic T lymphocytes (CTL) identifiable in patients with ankylosing spondylitis and reactive arthritis. Sensitization of target cells involves disulfide bonding and the interaction between homocysteine and HLA antigens occurs in a pre-Golgi compartment in the cells. Salmonella-infected B cells are also lysed by homocysteine-specific CTL, suggesting that intracellular invading microorganisms may provide homocysteine which would gain access to the newly synthesized intracellular HLA molecules and modify them inside the cells. Two different mechanisms for homocysteine modification of HLA antigens are proposed: homocysteine could bind directly to the unpaired cysteine residues in HLA antigens, or it could bind indirectly to HLA antigens through cysteine-containing peptides bound to them. Thus, HLA antigens containing unpaired cysteine residues (e.g. HLA B27) could be modified by homocysteine directly or indirectly, while HLA antigens without unpaired cysteine residues (e.g. HLA A68) could only be modified indirectly. The results are discussed in relation to the potential involvement of homocysteine-specific CTL in ankylosing spondylitis and reactive arthritis, both of which are related to bacterial infections, associated with HLA B27, and considered to be autoimmune diseases.

A strategy for the synthesis and screening of thiol-modified peptide variants recognized by T cells

In this study we present a strategy for the identification of novel peptide conjugates which may be used to understand the molecular details of the recognition process or to potentially regulate T cell-mediated responses. The approach involves the incorporation of cysteine into a known peptide at a position of interest and subsequent chemical conjugation using thiol-specific agents. Conjugates derived from the nonapeptide QL9 that is recognized by CTL 2C had either enhanced or reduced activity compared to the original cys-peptides. Different classes of thiol-reactive agents (alkyl halides, alkylthiolsulfonates, and disulfides) were tested with increases in activity of over 100-fold. As with standard peptide analogs, the activity depended on the position of the cysteine within the peptide and the nature of the chemically linked functional group. Use of this approach in a cysteine 'scan' of all positions of the original peptide is cost effective and with the availability of many different thiol-specific functional groups will allow the screening of considerably larger libraries of chemically modified peptides than have been used to date. Additionally, these findings may provide insight into the pathogenesis of thiol agents involved in contact sensitivity reactions.