HLA-B27 and the pathogenesis of spondyloarthritis

The association of HLA-B27 with ankylosing spondylitis and other spondyloarthropathies ranks among the strongest between any HLA antigen and a human disease. Yet, in spite of intense research and advanced knowledge of the biochemistry and biology of major histocompatibility complex molecules, the mechanism of this association remains unknown. This review attempts a critical assessment of current pathogenetic hypotheses from evidence concerning the epidemiology of HLA-B27 association with disease, its peptide-binding specificity, and other aspects of the molecular biology and immunology of this molecule.     

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.     

Peptides: the cornerstone of HLA-B27 biology and pathogenetic role in spondyloarthritis

The association of human leukocyte antigen (HLA)-B27 to ankylosing spondylitis is one of the strongest between a major histocompatibility complex molecule and a disease. Yet, the basis for this association remains unknown. Several hypotheses, each based on a particular feature of HLA-B27, guide much of the current research on the pathogenesis of this disease, but none has yet satisfactorily explained its mechanism and the differential association of B27 subtypes to it. In this review, the pathogenetic role of HLA-B27 will be analyzed from a global perspective of its biology, emphasizing the interdependency of multiple molecular features and the likely influence of disease-modifying gene products. From this perspective, peptide binding emerges as the cornerstone of all other biological properties.     

Spontaneous inflammatory disease in HLA-B27 transgenic mice does not require transporter of antigenic peptides

HLA-B27 is strongly linked with a group of human diseases called spondyloarthropathies. Even though HLA-B27 as an MHC class I molecule would be expected to present endogenously processed peptides such as cytosolic or viral proteins, many of the B27-linked diseases begin after an infection with an enterobacteria, an exogenous antigen. In our previous studies, we have described development of spontaneous inflammatory disease in HLA-B27 transgenic mice expressing beta(2)m free heavy chains on the cell surface. In order to address the role of endogenous versus exogenous antigens and a role for Tap genes in the development of spontaneous diseases, mice lacking Tap-1 (knockout) were mated to HLA-B27/human beta(2)m transgenic mice. B27(+)/human beta(2)m(+) double-transgenic mice (without mouse beta(2)m) lacking the Tap-1 gene developed spontaneous inflammatory disease similar to wild-type Tap-1 gene-expressing counterparts. Our data demonstrate that peptide transporters (Tap) were not involved in the development of spontaneous inflammatory disease in B27(+)/human beta(2)m transgenic animals.     

HLA-B27 transgenic mice are susceptible to collagen-induced arthritis: type II collagen as a potential target in human disease

HLA-B27 is highly linked with a group of human diseases called spondyloarthropathies (SpA). Many of these disorders begin after an infection with an enterobacteria. The symptoms seen in patients with spondyloarthropathies are inflammatory pain in the spine and asymmetrical arthritis of lower limbs. Additional symptoms related to SpA include inflammation in the eyes, bowel, and skin. The autoantigen(s) in SpA are not known. Proteins such as collagen and proteoglycans have been thought to be potent autoantigens in arthritidis including B27-associated human diseases. Type II collagen is a common denominator among eyes and joints, affected tissues in B27-linked diseases. Moreover, a few reports indicated CII specific T cells and antibodies in patients with spondyloarthropathies. We and others have previously described development of spontaneous arthritis and nail disease in HLA-B27 transgenic animals. To determine whether CII may be a target antigen in the B27-linked diseases, B27 + m beta 2 m% (HLA-B27) transgenic mice lacking mouse beta 2m with and without human beta 2m) mice were immunized with type II collagen inside the barrier facility. Male HLA-B27 transgenic mice developed collagen-induced arthritis compared to transgene negative littermates or female counterparts. There was no difference in the incidence of arthritis in HLA-B27 transgenic mice with and without human beta 2m. Our data suggest that beta 2m free heavy chain of HLA-B27 may present soluble antigens such as type II collagen to trigger specific T cells contributing in the development of arthritis. Our data also suggest that CII may be a potential target antigen in the cartilage during the disease process.     

Peptide binding alpha1alpha2 domain of HLA-B27 contributes to the disease pathogenesis in transgenic mice

Human spondyloarthropathies are strongly associated with a major histocompatibility complex (MHC) class I allele, HLA-B27. HLA-B27 transgenic mice and rats demonstrate many features of these diseases further confirming the role of HLA-B27 in disease. Yet the exact role of this molecule in disease pathogenesis is not clearly understood. We have previously reported spontaneous arthritis and nail disease in HLA-B27 transgenic mice lacking beta2-microglobulin (B27+beta2m(o)). These observations along with binding studies of B27 derived peptides to HLA-B27 molecule itself led to two hypotheses: (i) HLA-B27 derived peptide as a source of autoantigen; and (ii) HLA-B27 functions as an antigen presenting molecule. In this report, we confirm spontaneous disease in transgenic mice expressing a hybrid B27 molecule with alpha1alpha2 domain of B27 and alpha3 domain of mouse H-2Kd. These mice developed spontaneous arthritis and nail disease when transferred from specific pathogen free barrier facility to the conventional area. Other control mice with MHC class I transgene (e.g., HLA-B7, HLA-Cw3, and H2-Dd) did not develop such disease. In a MHC reassembly assay, binding of similar peptides to both wild type and hybrid B27 molecules was observed. In addition, the hybrid B27 molecule lacks at least one of the 3 proposed peptides from the third hypervariable (HV3) region of HLA-B27. These data strongly suggest that HLA-B27 molecule is an antigen presenting molecule rather than a peptide donor in the disease pathogenesis.     

HLA-B27: a registry of constitutive peptide ligands

The very strong association of human leukocyte antigen (HLA)-B27 with spondyloarthritis might be related to its peptide-presenting properties. The natural polymorphism of this molecule influences both peptide specificity and disease susceptibility. In this study, we present a comprehensive compilation of known natural ligands of HLA-B27 arising from endogenous proteins of human cells, together with a statistical assessment of residue usage among constitutive peptide repertoires of multiple HLA-B27 subtypes. This analysis provides evidence that every peptide position, including "non-anchor" ones, may be subjected to selection on the basis of its contribution to HLA-B27 binding and also allows a quantization of residue preferences at known anchor positions. The present registry is intended as a basis on which to build up reliable criteria to assess the effect of HLA-B27 polymorphism on peptide presentation, for T-cell epitope predictions, and for molecular mimicry studies.     

Progression of HIV to AIDS: a protective role for HLA-B27?

HLA-B27 is known for its strong association with inflammatory spondyloarthropathies (SpA), a group of rheumatic diseases. Apart from playing its role in the onset of these inflammatory diseases, HLA-827 is so ubiquitous in the world that the carrying of this gene must have also have an advantage. There are some indications that a beneficial effect can be found as a less severe course of viral infections among B27-carriers. The literature on this subject was reviewed and revealed a favorable course of infection with influenza virus, herpes simplex type 2 virus, Epstein-Barr virus and, even more interesting, a protective effect of HLA-B27 in the progression of HIV infections. The course of HIV infection differs among individuals and is thought to be partly related to host-factor variability, reflecting broad genetic heterogeneity. The polymorphic human leukocyte antigens (HLA) are herein analyzed intensively with respect to this relationship. Cytotoxic T lymphocyte (CTL) responses, activated by HLA antigen presentation, are implicated in the control of HIV replication. An immunological explanation for the protective role for HLA B27 in HIV disease is that B27+ patients have a specific and strong CTL response against the p24 epitope, a conservative HIV protein that does not easily mutate. Some HLA genes seen in long-term non-progressors (LTNP) (>10 years disease free) are associated with a favorable prognosis. One of the alleles found predominantly in LTNPs is HLA-B27. More genetic factors seem to influence disease progression in HIV infections. Therefore, it would be interesting to further explore the influence of the genetic make up of these HIV-infected individuals. Knowledge of the immunogenetic profile might give clues for the individual course of the HIV infection, may influence the development of drug-resistant viruses and will possibly lead to a tailored therapeutic strategy in HIV-infected persons.     

HLA-B27-Restricted Cytotoxic T Lymphocyte Responses to Arthritogenic Enterobacteria or Self-Antigens are Dominated by Closely Related TCRBV Gene Segments. A Study in Patients with Reactive Arthritis

Identification of the T-cell receptors (TCR) used by synovial cytotoxic T lymphocytes (CTL) of patients with reactive arthritis (ReA) may be crucial to better understanding the pathogenetic mechanism underlying the HLA-B27 association ofspondylarthropathies. The authors, therefore, sequenced 25 TCRB chains from HLA-B27-restricted CD8⁺ CTL clones and two clonal lines specific for self- or Yersinia enterocolitica antigen isolated from synovial fluids of 3HLA-B27⁺ patients with ReA and PBL of one healthy HLA-B27⁺ individual. Fourteen non-HLA-B27-restricted CTL served as controls. Both autoreactive and Y. enterocolitica specific HLA-B27-restricted CTL used a highlylimited set of VB genes with preferential rearrangement of three closely related VB families (VB 13,14,17), suggesting that these families contain a preferred site for contact with the HLA-B27 molecule. In addition, the presence of limited TCRBJ usage,limited heterogeneity in CDR3 sequences and dominant clones from individual donors among these CTL indicate that TCRB chain usage is further restricted by a limited set of peptides bound to the HLA-B27 molecule. Limited TCR usage by SF CTL of ReA patients may lay a basis for therapeutical manipulation of the T-cell response in the spondylarthropathies.     

Old and new HLA associations with ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that primarily involves the axial skeleton and the sacroiliac joint, but may also affect peripheral joints and entheses. AS susceptibility is clearly attributable to genetic factors and the link between human leukocyte antigen (HLA)-B27 and AS is the strongest association between an HLA class I molecule and a disease. However, there is evidence for the involvement of other, non-B27 factors within the major histocompatibility complex (MHC) in AS susceptibility. MHC class I is clearly the most significant genetic region for the disease, although most of the genetic association of this region is driven by HLA-B27. Moreover, several studies have investigated the MHC class II region and its association with AS. This review summarizes the current findings concerning the MHC genetics of the disease, focusing in particular on the associations of HLA with AS found in different ethnic populations throughout the world, and the possible mechanisms underlying them.     

Molecular mimicry: Any role in the pathogenesis of spondyloarthropathies?

Ankylosing spondylitis and reactive arthritis are seronegative spondyloarthropathies, which are strongly associated with HLA-B27. Despite intensive investigation, the basis for this association is not clear. However, in recent years one favored hypothesis to explain this linkage has been that of molecular mimicry, i.e., sharing of linear or conformational epitopes common to microbial antigens and host structures. During the past few years several examples of molecular mimicry between HLA-B27 and microbial antigens have been described. Heat shock proteins, among others, have been considered as target candidates for autoimmune phenomena, because of the high degree of homology between bacterial and mammalian species. Reactive arthritis triggered byYersinia orSalmonella provides a unique model for studying the pathogenetic mechanisms underlying human inflammatory joint diseases in general, because the arthritogenic microbes are known and well-characterized. We have described two bacterial proteins that share amino acid homology with HLA-B27, namely YadA (Yersinia adhesin) and OmpH, outer surface proteins ofYersinia andSalmonella, respectively. Notably, the area of identity of these amino acid sequences is located in the same place on the HLA-B27 molecule as a hexapeptide identical betweenKlebsiella nitrogenase and HLA-B27, and a pentapeptide shared by aShigella flexneri protein and HLA-B27. We have investigated immune responses to a panel of synthetic peptides based on the HLA-B27-homologous portions of pathogen-specific antigens in patients with reactive arthritis and ankylosing spondylitis. One third of the patients have antibodies to the synthetic peptides. However, instead of recognizing the HLA-B27-homologous portion, the antibodies are directed against the flanking sequences of the synthetic peptides. The concept of the role of molecular mimicry between HLA-B27 and microbial antigens in the pathogenesis of spondyloarthropathies is discussed, with a conclusion that no convincing evidence for its significance exists at the present.     

HLA-B27 presents a peptide from a polymorphic region of its own molecule with homology to proteins from arthritogenic bacteria

A possible mechanism for the pathogenesis of HLA-B27-associated spondyloarthropathies is that peptides from arthritogenic bacteria with homology to endogenous self-peptides presented by HLA-B27, including those derived from HLA-B27 itself, could elicit an autoimmune T-cell response upon infection. We report here that an undecamer corresponding to the polymorphic region of HLA-B27 spanning residues 169–179 is presented in vivo by the B*2701, B*2704 and B*2706 subtypes, but was not detected in the B*2703-bound peptide pool. This peptide binds to B*2705 in vitro with sufficient affinity to allow its natural presentation by this subtype, but it binds with low affinity to B*2703. In spite of homology of this peptide to proteins from arthritogenic bacteria, its binding specificity does not correlate with current evidence concerning association of HLA-B27 subtypes to ankylosing spondylitis, suggesting that presentation of this peptide is not the critical feature that determines linkage of HLA-B27 to this disease.     

Demonstration of cross-reactivity between bacterial antigens and class I human leukocyte antigens by using monoclonal antibodies to Shigella flexneri

Bacterial envelope proteins which share immunodeterminants with the human leukocyte antigen (HLA) class I histocompatibility antigen HLA-B27 may invoke spondyloarthritic disease through the process of molecular mimicry in patients expressing this phenotype. Monoclonal antibodies generated by the immunization of BALB/c mice with envelope proteins of Shigella flexneri type 2a were tested for reactivity against cultured lymphoblastoid cell lines of defined HLA phenotype. As measured by flow microfluorometry, four immunoglobulin M monoclonal antibodies reacted preferentially with HLA-B27-positive lymphocytes (HOM-2, MM) as compared with a B27-loss mutant line (1065) or cells lacking major histocompatibility complex class I antigen (Daudi, K562). Monoclonal antibodies also reacted with mouse EL-4 cells transfected with and expressing the HLA-B7 gene. Western immunoblot analysis of isolated enterobacterial envelopes demonstrated that the reactive epitope was present on bacterial proteins with an apparent relative molecular mass of 36 and 19 kilodaltons. The structural basis for the cross-reactivity of bacterial antigen and HLA-B27 appeared to reside in the portion of the HLA molecule that is responsible for allotypic specificity (amino acids 63 through 83), since monoclonal antibodies were positive by enzyme-linked immunosorbent assay with synthetic polypeptides corresponding to this segment.     

Enhanced inflammatory reactivity in the pathogenesis of spondyloarthropathies

Pathogenesis of seronegative spondyloarthropathies such as ankylosing spondylitis and reactive arthritis is not known. Growing evidence indicates that microbial structures such as Chlamydia antigen and Yersinia antigen are present in the inflamed joints of patients with reactive arthritis. Microbial antigens can activate the host's inflammatory mechanisms. After the activation, the course of inflammation can be postulated to be affected by the host factors responsible for amplification of the inflammatory reaction and elimination of the foreign structures. Thus, the amplification, whether strong, moderate, or weak, may contribute to the degree of inflammatory tissue injury in patients with seronegative spondyloarthropathies. This review will discuss the role of increased inflammatory reactivity in the pathogenesis of HLA-B27 associated spondyloarthropathies, with special reference to reactive arthritis triggered by yersinia enteritis.     

What is the role of HLA-B27 in spondyloarthropathies?

HLA-B27 (Human Leukocyte Antigen-B27) accounts approximately for the one third of the overall genetic susceptibility to spondylorthropathies (SpAs). Up to 70 HLA-B27 subtypes have been reported all over the world with a decreasing north-south gradient of its frequency, which is reverse to that of endemic malaria. In an attempt to explain the possible role of HLA-B27 in SpAs pathogenesis, several theories have been suggested [1. Arthritogenic peptide, 2. Misfolding, 3. Cell surface HLA-B27 homodimers, 4. β2m (β2-microglobulin) deposition, 5. β2m-free/peptide free heavy chains of HLA-I, 6. Enhanced survival of some microbes in HLA-B27 cells, 7. ERAP1/ERAP2 (endoplasmic reticulum aminopeptidases 1 and 2) and Tapasin function, 8. β2m over-expression] each of which contributes up to a point to our understanding of HLA-B27 subtypes' role in SpAs manifestation. However, reviewing all the suggested hypotheses it seems logical to pass from a puzzle of distinct hypotheses to a global theory. This review summarizes the current knowledge of HLA-B27 aiming to understand its potential use in clinical practice of SpAs diagnosis and to direct its future studies.     

The naturally occurring polymorphism Asp116 → His116 , differentiating the ankylosing spondylitis-associated HLA-B*2705 from the non-associated HLA-B*2709 subtype,influences peptide-specific CD8 T cell recognition

HLA-B27 molecules are interesting because of their strong association with ankylosing spondylitis (AS) and reactive arthritis (ReA). A pathogenetic role for these molecules has been postulated in presenting a putative --bb--arthritogenic peptide to CD8 T cells. The HLA-B*2709 subtype, although differing by a single amino acid (His¹¹⁶ → Asp¹¹⁶ ) from the wide spread and strongly AS-associated subtype HLA-B*2705, is not found in patients. Since residue 116 interacts with the C terminus of the peptide, it is possible that the two subtypes differ in their antigen-presenting features. We show here that CD8 T cells can distinguish the two HLA-B27 subtypes when presenting a same epitope derived from Epstein-Barr virus-latent membrane protein 2. Moreover, alanine scanning mutagenesis analysis revealed that the peptide residues relevant for such recognition are different depending on whether HLA-B*2705 or -B*2709 molecules present the epitope. These results give support to the belief that functional differences determined by subtype-specific polymorphisms can have a pathogenetic relevance and open up a new scenario where subtle modifications within the peptide/HLA ligand might be responsible for the differential association between HLA-B27 subtypes and spondyloarthropathies.     

The pathogenesis of HLA-B27 associated arthritis: lessons from the B27 crystal

The most remarkable association between a major histocompatibility complex antigen and disease susceptibility - HLA-B27 and seronegative spondyloarthropathies, particularly ankylosing spondylitis - was discovered 20 years ago. During the two intervening decades advances in basic immunology and molecular biology have not only revealed the biosynthesis and structure of HLA-B27 but also given clues to the basic function of this molecule, the presentation of allele-specific peptides to CD8+ cytotoxic T cells. The recently reported three-dimensional structure of HLA-B27 and the identification of self-peptides bound to this major histocompatibility complex class I antigen can be viewed as a landmark in the understanding of the pathogenic role of HLA-B27. Based on crystallographic evidence, a peptide-binding motif can be postulated that should allow identification of HLA-B27 complexed peptides which may trigger an immune reaction causing arthritis.Abbreviations CTL CD8⁺ cytotoxic T-cells - TCR T-cell receptor - ₂m ₂-microglobulin - LMP low molecular mass polypeptide Correspondence to: H. Kellner     

HLA-B27: portraying immunodominant viral epitopes

Although the crystal structure of HLA-B27 has been known for a long time, only recently have X-ray diffraction studies of this molecule in complex with individual peptides become available. The report of three such structures involving viral epitopes that are immunodominant in HLA-B27-restricted T cell responses against influenza, Epstein-Barr and HIV viruses significantly improves our perception of critical aspects of the immunological and pathogenetic roles of HLA-B27, including (1) the molecular basis of its peptide-binding specificity and how this is modulated by subtype polymorphism, (2) the relationship between the structural and the antigenic features of immunodominant viral epitopes, (3) the basis for long term non-progression to AIDS of HIV-infected HLA-B27+ individuals, and (4) the structural features of microbial peptides influencing NK receptor engagement. Here, I discuss the implications of this and related studies for the relevance of HLA-B27 in host defense and as a pathogenetic molecule in spondyloarthritis.     

HLA-B27 and antigen presentation: At the crossroads between immune defense and autoimmunity

The HLA-B27 is historically studied as a susceptibility factor in spondyloarthropathies and, primarily, in ankylosing spondylitis (AS). Over the recent years however, it has been rediscovered as protective factor against some severe viral infections. This is due to the high capacity of virus-specific, HLA-B27-restricted CD8+ T cells for both intrinsic (i.e. polyfunctionality, high avidity, low sensitivity to Treg cell-mediated suppression) and extrinsic (i.e. rapid and efficient antigen processing and presentation) factors. It is tempting to speculate that these two aspects are not independent and that the association of B27 molecules to autoimmunity is the downside of this superior functional efficacy which, in given genetic backgrounds and environmental conditions, can support a chronic inflammation leading to spondyloarthropathies. Still, the pathogenic role of HLA-B27 molecules in AS is elusive. Here, we focus on the biology of HLA-B27 from the genetics to the biochemistry and to the structural/dynamical properties of B27:peptide complexes as obtained from atomistic molecular dynamics simulation. Overall, the results point at the antigen presentation as the key event in the disease pathogenesis. In particular, an extensive comparison of HLA-B*2705 and B*2709 molecules, that differ in a single amino acid (Asp116 to His116) and are differentially associated with AS, indicates that position 116 is crucial for shaping the entire peptide-presenting groove.