The peptide binding specificity of HLA-B27 subtypes

Detection of a novel HLA-B27 allele, B*2740, in Taiwanese volunteer bone marrow donors by sequence-based typing: curiosity rewarded

We report here a novel HLA-B allele, B*2740, discovered in Taiwanese volunteer marrow donors. The new sequence has nucleotide variation at position 527 (T-->A) as compared to B*2708. The nucleotide change caused an amino acid substitution from valine (V) to glutamic acid (E) at codon 152. Since B*2740 carries sequence confers to HLA-Bw6 public epitope we believe that this novel B*27 allele might have been generated from a gene conversion involving a Bw4-specific allele (probably B*2704) and a Bw6-specific allele.

HLA-B27 in the Greek Cypriot population: distribution of subtypes in patients with ankylosing spondylitis and other HLA-B27-related diseases. The possible protective role of B*2707

The major purpose of the present study was to investigate the frequency of human leukocyte antigen (HLA)-B27 alleles in healthy controls and in patients with ankylosing spondylitis (AS) and other HLA-B27-related diseases in the Greek Cypriot population. We selected 102 HLA-B27-positive individuals (60 controls and 42 patients). Typing of the HLA-B27 alleles was performed by polymerase chain reaction amplification with sequence-specific primers. Only two alleles were detected in the patient group: B*2702 (n = 31, 73.8%) and B*2705 (n = 11, 26.2%). The HLA-B*2707 allele was detected (n = 10, 16.7%) only in the healthy controls in addition to the B*2702 (n = 31, 51.7%) and B*2705 (n = 19, 31.7%) alleles. Our results show a restricted number of HLA-B27 subtypes associated with AS and other B27-related diseases and an elevated frequency of the B*2702 allele in the AS patients. The allele B*2707 seems to have a protective role in the population studied because it was found only in the healthy controls.

Peptides eluted from HLA-B27 of human splenocytes and blood cells reveal a similar but partially different profile compared to in vitro grown cell lines

The sequences and profiles of peptides which bind to HLA-B*2705 splenocytes and peripheral blood cells were compared with those previously published from in vitro long-term cell cultures. B*2705 peptide profile analysed by solid-phase Edman degradation and 15 individual peptide sequences determined by LC-MS/MS were partially similar to those defined from in vitro long-term cell cultures. Arg at P2 was found in 11 of 15 sequenced peptides (73.3%). This value is lower in comparison with other published data. Two sequences were matching to unknown proteins, which displayed similarity with myosin. These are first data on peptide sequences isolated directly from HLA-B27 molecules without prior in vitro propagation of the cells.

A naturally selected dimorphism within the HLA-B44 supertype alters class I structure, peptide repertoire, and T cell recognition

HLA-B*4402 and B*4403 are naturally occurring MHC class I alleles that are both found at a high frequency in all human populations, and yet they only differ by one residue on the alpha2 helix (B*4402 Asp156-->B*4403 Leu156). CTLs discriminate between HLA-B*4402 and B*4403, and these allotypes stimulate strong mutual allogeneic responses reflecting their known barrier to hemopoeitic stem cell transplantation. Although HLA-B*4402 and B*4403 share >95% of their peptide repertoire, B*4403 presents more unique peptides than B*4402, consistent with the stronger T cell alloreactivity observed toward B*4403 compared with B*4402. Crystal structures of B*4402 and B*4403 show how the polymorphism at position 156 is completely buried and yet alters both the peptide and the heavy chain conformation, relaxing ligand selection by B*4403 compared with B*4402. Thus, the polymorphism between HLA-B*4402 and B*4403 modifies both peptide repertoire and T cell recognition, and is reflected in the paradoxically powerful alloreactivity that occurs across this "minimal" mismatch. The findings suggest that these closely related class I genes are maintained in diverse human populations through their differential impact on the selection of peptide ligands and the T cell repertoire.

Functional conservation of HIV-1 Vpr and variability in a mother-child pair of long-term non-progressors

Increasing evidence suggests that HIV-1 Vpr is required in vivo for viral pathogenesis. Since Vpr displays multiple activities, little is known about which Vpr-specific activities are conserved in naturally occurring viruses or how natural mutations in Vpr might modulate viral pathogenesis in HIV-infected individuals. The goals of this study were to evaluate the functional variability of Vpr in naturally occurring viruses. The Vpr-specific activities of nuclear localization, induction of cell cycle G2 arrest and cell death were compared between viruses isolated from the fast progressing AIDS patients and a mother-child pair of long-term non-progressors (LTNPs). Wild-type Vpr activities were found in all of the viruses that were isolated from the fast progressing AIDS patients except for the truncated Vpr(IIIB) which lacked these activities. In contrast, defective Vpr were readily detected in viral populations isolated, over an 11-year period, from the mother-child pair. Sequence analyses indicated that these Vpr carried unique amino acid substitutions that frequently interrupted a highly conserved domain containing an N-terminal alpha-helix-turn-alpha-helix. Thus, Vpr activities are generally conserved in naturally occurring viruses. The functionally defective Vpr identified in the mother-child pair of LTNPs are likely to be unique and may possibly contribute to the slow disease progression.

HLA-B27: natural function and pathogenic role in spondyloarthritis

The human leukocyte antigen HLA-B27 is strongly associated with development of a group of inflammatory arthritides collectively known as the spondyloarthritides. We have set out to define the natural immunological function of HLA-B27, and then to apply this knowledge to understand its pathogenic role. Human leukocyte antigen class 1 molecules bind antigenic peptides for cell surface presentation to cytotoxic T lymphocytes. HLA-B27 binds and presents peptides from influenza, HIV, Epstein-Barr virus, and other viruses. This leads to vigorous and specific cytotoxic T lymphocyte responses, which play an important role in the body's immune response to these viruses. HLA-B27 thus carries out its natural function highly effectively. Although many theories have been proposed to explain the role of HLA-B27 in the pathogenesis of spondyloarthropathy, we favour those postulating that the pathogenic role of HLA-B27 stems from its natural function. For example, the 'arthritogenic' peptide hypothesis suggests that disease results from the ability of HLA-B27 to bind a unique peptide or a set of antigenic peptides. Additionally, a number of lines of evidence from our laboratory and other laboratories have suggested that HLA-B27 has unusual cell biology. We have recently demonstrated that HLA-B27 is capable of forming disulfide-bonded homodimers. These homodimers are expressed on the cell surface and are ligands for a number of natural killer and related immunoreceptors, expressed on a variety of cell types including natural killer cells, T lymphocytes and B lymphocytes, and members of the monocyte/macrophage lineage. We are currently investigating the possibility that such interactions could be involved in disease pathogenesis.

Identification of HLA-B27-restricted cytotoxic T lymphocyte epitope from carcinoembryonic antigen

Characterization of epitopes recognized by cytotoxic T lymphocytes (CTLs) in the sequence of tumor antigens is an important step in the development of tumor therapies. Because carcinoembryonic antigen (CEA) is a protein expressed in a high number of epithelial tumors, it is an interesting target to study. We screened for the presence of HLA-B27-restricted CTL epitopes from CEA by studying the binding to HLA-B27 of 31 synthetic peptides predicted to bind to this molecule. This afforded 16 peptides with moderate or high binding affinity. Immunization of HLA-B27 transgenic mice with the best binder peptides yielded 4 immunogenic peptides: CEA(9-17), CEA(9-18), CEA(138-146) and CEA(360-369). However, splenocytes from mice immunized with a vaccinia virus-expressing CEA recognized only CEA(9-18). These CTLs were of the CD8(+) phenotype, which upon stimulation with peptide specifically produced IFN-gamma. Moreover, they did not cross-react against peptides of region 9-18 from proteins of the CEA family. Our results show that CEA(9-18) may induce specific CTL responses against CEA.

Antigenic drift in the influenza A virus (H3N2) nucleoprotein and escape from recognition by cytotoxic T lymphocytes

Viruses exploit different strategies to escape immune surveillance, including the introduction of mutations in cytotoxic T-lymphocyte (CTL) epitopes. The sequence of these epitopes is critical for their binding to major histocompatibility complex (MHC) class I molecules and recognition by specific CTLs, both of which interactions may be lost by mutation. Sequence analysis of the nucleoprotein gene of influenza A viruses (H3N2) isolated in The Netherlands from 1989 to 1999 revealed two independent amino acid mutations at the anchor residue of the HLA-B27-specific CTL epitope SRYWAIRTR (383 to 391). A R384K mutation was found in influenza A viruses isolated during the influenza season 1989-1990 but not in subsequent seasons. In the influenza season 1993-1994, a novel mutation in the same CTL epitope at the same position was introduced. This R384G mutation proved to be conserved in all influenza A viruses isolated from 1993 onwards. Both mutations R384K and R384G abrogated MHC class I presentation and allowed escape from recognition by specific CTLs.

HLA-A*9, a probable secondary susceptibility marker to ankylosing spondylitis in Basque patients

HLA-B27 is strongly associated to ankylosing spondylitis (AS). The objective of our study was to analyze HLA-B27 association, B27 subtype distribution and frequency of other HLA class I and DR antigens in a group of Basque AS patients. HLA class I antigens were typed serologically and HLA-B27 and A9 subtypes were determined by DNA typing in samples from 46 patients with AS, 54 B27-positive spondyloarthropathies, 82 healthy subjects and 20 B27-positive controls. A class I HLA 9.2 kb PvuII restriction fragment length polymorphism (RFLP), previously associated with AS, was analyzed in a representative group of patients and controls. We found that HLA-B*2705 conferred a relative risk of 126 for AS in this group. HLA-A9 (A*2402) allele was significantly increased in AS patients compared with healthy controls and B27-positive control group (Pcorr<0.0001) and also increased in patients affected with peripheral arthritis. No association between class I HLA 9.2 Kb RFLP and AS was found. These results suggest that HLA-A*9 allele itself or another linked gene could act as a secondary and independent susceptibility allele to AS.

Overlapping peptide-binding specificities of HLA-B27 and B39: evidence for a role of peptide supermotif in the pathogenesis of spondylarthropathies

OBJECTIVE

Previous studies indicated the increase of HLA-B39 among HLA-B27 negative patients with spondylarthropathies (SpA). This study was performed to examine whether the natural ligands of HLA-B27 are capable of binding to HLA-B39.

METHODS

Peptides were synthesized according to the sequences of known natural ligands of HLA-B27 or B39 and were tested for their binding to HLA-B*3901 and B*2705 by quantitative peptide binding assay, using a TAP-deficient RMA-S cell line transfected with human beta2-microglobulin and HLA class I heavy chain genes.

RESULTS

Four of the 10 HLA-B27 binding peptides significantly bound to HLA-B*3901. All 4 peptides had hydrophobic/aromatic amino acids (Leu or Phe) at the C-terminus. In contrast, peptides with basic residues (Lys, Arg) or Tyr at the C-terminus did not bind to B*3901. In parallel experiments, 1 of the 2 natural ligands of HLA-B*3901 was found to bind to B*2705.

CONCLUSION

A subset of natural HLA-B27 ligands was capable of binding to B*3901. In addition to Arg at position 2 (Arg2), hydrophobic/aromatic C-terminal residues, such as Leu or Phe, seemed to be crucial for the cross-specificity. These results suggested that HLA-B27 and B39 recognize overlapping peptide repertoires, supporting the hypothesis that the peptides presented by both of these class I antigens play a role in the pathogenesis of SpA.

The signal sequence of lymphocytic choriomeningitis virus contains an immunodominant cytotoxic T cell epitope that is restricted by both H-2D(b) and H-2K(b) molecules

Infection of H-2b mice with lymphocytic choriomeningitis virus (LCMV) generates three well-characterized H-2D(b)-restricted immunodominant epitopes delineated in the NP, GP1, and GP2 proteins. Here we report that the H-2D(b)-restricted GP1 epitope GP33-41/43 (KAVYNFATC/GI) located in the signal sequence of LCMV is also the immunodominant epitope recognized by CTL at the surface of the same infected cells in the context of H-2K(b) restriction. The GP1 epitope bound to H-2D(b) and H-2K(b) molecules with comparable affinities. The respective binding processes involved different sets of peptide anchoring residues and required dramatically different conformations of the peptide backbone as well as rearrangement of residue side chains. The 10-mer peptide GP34-43 (AVYNFATCGI) was the optimal H-2K(b)-binding sequence and the 8-mer peptide GP34-41 (AVYNFATC) the minimal sequence for optimal H-2K(b)-restricted CTL recognition. Comparison of lytic activities of primary splenic anti-LCMV CTL from C57BL/6 (D(b+)/K(b+)), B10A.[5R] (D(b-)/K(b+)), and B10A.[2R] (D(b+)/K(b-)) mice against LCMV-infected or peptide-coated target cells expressing either one or the two MHC alleles revealed that the H-2K(b)-restricted component of the anti-GP1 CTL response was mounted independently of but as efficiently as its H-2D(b) counterpart. Analysis of the immune response against a GP1 variant that escapes CTL recognition showed that the GP1 epitope: (i) was likely the only immunodominant LCMV epitope in the context of H-2K(b), and (ii) could efficiently evade H-2D(b) and H-2K(b)-restricted CTL mediated lysis.

HLA-B27 (B*2701) specificity for peptides lacking Arg2 is determined by polymorphism outside the B pocket

B*2701 differs from B*2705-by three amino acid changes: D-->Y74, D-->N77, L-->A81, and from B*2702 only by two: D-->Y74 and T-->I80. Tyr74 is located in the C/F cavity of the peptide-binding site, and is unique to B*2701 among HLA-B27 subtypes. Binding of natural B*2705 and B*2702 ligands to B*2701, and to mutants mimicking subtype changes, was analyzed. In addition, sequencing of the peptides bound in vivo by B*2701 and the Y74 mutant was carried out. The main distinctive feature of B*2701 was its presentation of peptides with Gln2. Synthetic analogs bound in vitro similarly as the corresponding ligands with Arg2. Moreover, both Gln2 and Arg2 were dominant upon pool sequencing of B*2701-bound peptides, and 2 of 8 natural ligands contained Gln2. Suitability of Gln2 was largely determined by the Y74 change, as indicated by: 1) binding of Gln2 analogs to this mutant, and 2) detection of Gln2 by pool sequencing of Y74-bound peptides. B*2701 bound peptides with C-terminal aromatic or Leu residues, and interacted with these motifs more strongly than B*2702. The Y74 mutation alone was not responsible for poor binding of peptides with C-terminal basic residues to B*2701, since they bound efficiently and at least one was presented in vivo by this mutant. Most peptides bound to the A81 mutant worse than to B*2705, but frequently better than to B*2701 or B*2702, suggesting that other subtype changes were compensatory. The peptide specificity of B*2701 suggests that this subtype may determine susceptibility to spondyloarthropathy.

Frequency of HLA-B27 subtypes in a Danish population and in Danish patients with ankylosing spondylitis

Polymerase chain reaction in combination with sequence-specific oligonucleotide probes were used to analyze nine HLA-B27 subtypes among 51 healthy HLA-B27 positive Danish blood donors and 30 Danish HLA-B27 positive patients with ankylosing spondylitis (AS). In the group of healthy Danes we found two subtypes, B*2705 (90.2%) and B*2702 (9.8%), however, among the AS patients only the B*2705 subtype was detected. We did not find a significant evidence for associations between AS and a particular HLA-B27 subtype in a Danish population.

Recognition of chlamydial antigen by HLA-B27-restricted cytotoxic T cells in HLA-B*2705 transgenic CBA (H-2k) mice

OBJECTIVE

The association of reactive arthritis (ReA) with HLA-B27 and the presence of bacterial antigen in joints with ReA suggest that bacterial peptides might be presented by the HLA-B27 molecule and thus stimulate CD8 T cells. This study was performed to investigate the B27-restricted cytotoxic T lymphocyte (CTL) response to Chlamydia trachomatis, using the model of HLA-B27 transgenic mice.

METHODS

CBA (H-2k) mice homozygous for HLA-B*2705 and human beta2-microglobulin expression were immunized with C trachomatis or with the chlamydial 57-kd heat-shock protein (hsp57) coupled to latex beads. Cytotoxicity of lymphocytes from in vivo-primed transgenic mice was tested against C trachomatis-infected targets. Blocking experiments were performed with monoclonal antibodies (MAb) against class I major histocompatibility complex molecules.

RESULTS

A Chlamydia-specific lysis of both B27-transfected and nontransfected target cells was observed. This response could be inhibited by anti-B27 and anti-H2 MAb. CTL from mice immunized with hsp57 were not able to lyse Chlamydia-infected target cells, and Chlamydia-specific CTL could not destroy targets loaded with hsp57.

CONCLUSION

These results suggest the existence of at least 2 CTL populations in this mouse model: one recognizing peptide of bacteria-infected cells restricted by HLA-B*2705 and the other recognizing peptide of bacteria-infected cells restricted by the murine H-2Kk molecule. It does not appear that hsp57 is a major target for the CD8 T cell response directed against Chlamydia. This animal model opens the way for identifying bacterial epitopes presented by HLA-B27, and might thus help to clarify the pathogenesis of B27-associated diseases.

Binding of peptides naturally presented by HLA-B27 to the differentially disease-associated B*2704 and B*2706 subtypes, and to mutants mimicking their polymorphism

B*2704 and B*2706 are closely related HLA-B27 subtypes of which the former but not the latter is associated to ankylosing spondylitis. Their peptide specificity relative to other disease-associated subtypes was analyzed by testing binding of self-peptides naturally presented by B*2705 or B*2702, and synthetic analogs, to B*2704, B*2706, and site-specific mutants mimicking their changes. Peptides with basic, aliphatic or aromatic C-terminal residues bound to B*2705 with similar affinity. In B*2704 C-terminal aliphatic/ aromatic residues were preferred. B*2706 discriminated drastically between polar and nonpolar C-terminal residues, showing strong preference for Leu and Phe, and less than B*2704 for basic and Tyr residues. Loss of single acidic charges (D > S77, D > Y116) increased preference for C-terminal Leu and Phe, but allowed efficient binding of peptides with basic residues or Tyr. Their gain (V > E152, H > D114) maintained wide C-terminal specificity, but severely impaired binding, presumably by disrupting interactions with internal peptide residues. This was compensated by Y116 in the double D114Y116 mutant. The specificity of B*2704 and B*2706 was explained only partially by the separate effects of single mutations, indicating that novel properties arise from concomitant changes at various positions. For instance, specificity of B*2706 for nonpolar C-terminal residues required simultaneous removal of Asp77 and Asp116. B*2706 differed from B*2705, B*2702, and B*2704 in its lower suitability for C-terminal Tyr, suggesting that this feature might be relevant for HLA-B27 association to spondyloarthropathy.

Characterization of the peptide-binding specificity of HLA-B*7301

Previous studies showed the human MHC class I heavy chain HLA-B*7301 has a sequence very divergent from other class I alleles. Despite the unusual sequence, we predicted B*7301 would retain the peptide-binding function typical of other HLA-A, B and C glycoproteins, and sequence similarity to B*2705 in a region of the peptide-binding site known as the B pocket suggested B*7301 would bind peptides with Arg at position 2. To test this hypothesis, the peptide-binding specificity of B*7301 was investigated. Sequence analysis of peptides bound endogenously by B*7301 indeed found selectivity for nonamer peptides possessing Arg at position 2 and a preference for small nonpolar residues such as Pro or Ala at the C terminus was also revealed. B*7301 therefore possesses the potential to function as a conventional antigen presenting class I glycoprotein. Functional similarities between B*7301 and B*2705 are discussed in the context of the association of B*27 subtypes with susceptibility to ankylosing sponylitis and arthritic diseases.

T-cell receptor usage in alloreactivity against HLA-B*2703 reveals significant conservation of the antigenic structure of B*2705

B*2703 is an exceptional HLA-B27 molecule in that it differs from the most common B*2705 subtype by a unique amino acid change (His59) altering N-terminal peptide anchorage. To assess how this unusual feature affects the antigenic structure of HLA-B27, TCR usage by alloreactive CTL raised against B*2703 from two individuals was analyzed. Only few CTL recognized B*2703 from two individuals was analyzed. Only few CTL recognized B*2703 but nor ot at a lower level B*2705. Limited heterogeneity of these CTL was revealed by: 1) identity of TCR in two pairs of such CTL clones, 2) identity of beta chains, paired to distinct alpha chains, in two clonotypes, and 3) almost identical fine specificity of these two clonotypes with site-specific HLA-B27 mutants. These results indicate that B*2703 "private" epitopes are rare. TCR usage among anti-B*2703 CTL was analogous as in anti-B*2705 responses in the predominant and donor-independent usage of V beta segments from homology subgroup 4, more moderate and donor-dependent V alpha skewing, N+D beta diversity limited by motifs shared among clonotypes, and restricted J alpha heterogeneity. Homology of N+D beta motifs and J alpha segments of anti-B*2703 with anti-B*2705 TCR suggested significant sharing of peptide-associated epitopes between both subtypes. The results indicate that allospecific TCR are recruited by B*2703 following similar rules as in the anti-B*2705 response, and suggest that the B*2703 change keeps unaltered much of the antigenic structure of the molecule relative to B*2705. Therefore, most of the peptides bound to B*2703 should be the same and keep a similar conformation as in B*2705.

HLA-B27 and HLA-B73 polymorphism and its role on antigenicity, peptide presentation, and disease susceptibility

Current ideas on the structure, evolution, and disease association of HLA-B27 antigens are presented, with emphasis on the effect of HLA-B27 polymorphism on T-cell recognition and antigen-presenting properties of these molecules. The molecular relationship between HLA-B27 and HLA-B73, and its implications for a putative pathogenetic role of the later antigen are discussed.

The peptide-binding motif for the human transporter associated with antigen processing

Presentation of antigenic peptides by human leukocyte antigen class I molecules is dependent on peptide transport into the endoplasmic reticulum by the transporters associated with antigen processing (TAP) (Germain, R. N. 1994. Cell. 76:287-299). This translocation step is currently regarded as permissive for all peptides with COOH-terminal residues capable of binding to HLA class I molecules (Momburg, F., J. Roelse, J. C. Howard, G. W. Butcher, G.J. Hämmerling, and J.J. Neefjes. 1994. Nature (Lond.). 367:648-651). In this report, we show that the human transporter selects peptides according to a binding motif based on the strong effects on peptide affinity of the three NH2-terminal positions and the COOH-terminal residues. TAP favors strongly hydrophobic residues in position 3 (P3) and hydrophobic or charged residues in P2, whereas aromatic or acidic residues in P1, as well as Pro in P1 and P2, have strong deleterious effects. Selection of naturally presented peptides by the transporter is suggested by their higher average affinity for TAP, as compared to nonselected peptides. The TAP preferences in the three NH2-terminal positions correspond to those of the vast majority of human leukocyte antigen class I alleles, but they represent an obstacle for peptide supply to some alleles, e.g., the B7-like group. We propose that peptides binding to these alleles, and in general, peptides with TAP affinities below a certain threshold, may be transported as extended precursors.

Relevance of residue 116 of HLA-B27 in determining susceptibility to ankylosing spondylitis

Ankylosing spondylitis (AS) is an autoimmune disorder strongly associated with HLA-B27. A direct role of B27 molecules in the disease pathogenesis has been postulated, possibly by presenting to T cells an as-yet unidentified arthritogenic peptide that triggers the autoimmune response. There are nine HLA-B27 alleles differing from each other at one or more amino acid positions. It is important, for the identification of the arthritogenic peptide, to define which alleles, and therefore which polymorphic positions, predispose to the disease. Here, we report that HLA-B*2709 is not associated with AS, as it was not found in patients. HLA-B*2709 differs from the most frequent and disease-associated HLA-B*2705 allele for a single substitution (His vs. Asp) at position 116. Amino acid 116 is located at the bottom of the groove where the antigenic peptide sits, and it has been proven to influence the peptide-binding specificity of HLA class I molecules. The most likely interpretation of these data is that the differences in charge and size that accompany the His-to-Asp substitution exclude the acceptance of the arthritogenic peptide.

Modulation of peptide binding by HLA-B27 polymorphism in pockets A and B, and peptide specificity of B*2703

The results in this study address three aspects of peptide binding to the disease-associated antigen HLA-B27 and its modulation by polymorphism: the contribution of major anchor residues 2 and 9, the role of pocket B polymorphism in modulating peptide specificity, and the binding properties of B*2703, a subtype not found to be associated with spondyloarthropathy. Synthetic analogs of peptides naturally presented by B*2705 were used to demonstrate that residue 2 is essential, since Ala2 analogs bound marginally to B*2705, but the specificity of B*2705 for Arg2 is not absolute, and show that the contribution of basic residue 9 to binding was significant, but less than Arg2. The effect of single mutations in the B pocket was to decrease or--with the Glu > Met-45 mutation--totally shift pocket B specificity for Arg2 towards other residues at this position. This was shown by quantitating the relative binding of Gln2 and Ala2 analogs, and by pool-sequencing of the peptides bound in vivo to these mutants. Peptides naturally presented by B*2705 apparently bound with a lower affinity to pocket A variants with altered hydrogen bonding to the peptide N terminus, including B*2703. Binding of peptide analogs with changes at positions 2 or 9 suggested that in B*2703 pocket A, interactions are weaker and pocket B interactions are stronger than in B*2705. This can be explained by the effect of the unique His59 change in B*2703 in both pockets. Thus, B*2703 is probably the HLA-B27 sub-type with the most stringent specificity for the Arg2 peptide motif.

HLA-B27 subtypes in Asian patients with ankylosing spondylitis. Evidence for new associations

The aim of this study was to investigate the contribution of the different B27 subtypes to ankylosing spondylitis (AS) susceptibility. The polymerase chain reaction (PCR) in combination with the sequence-specific oligonucleotide probes (SSOs) was used to analyse the polymorphism in exon 2 and 3 of HLA-B27 in two Asian groups with different genetic HLA structures: Indian (I) and Thai (T) populations. The same number of AS patients (45) and healthy B27 positive donors (n = 17) from both populations were analysed in order to ascertain the B27 subtypes. Three different findings can be concluded from this study: 1) B*2707 has been found to be associated with AS in both populations. This association has not been previously reported in either ethnic group. 2) B*2704 is strongly associated with AS in the Thai patients (91% in AS vs. 47% in C; RR = 11.5; EF = 0.83). In contrast, B*2704 was found with similar frequency in Asian Indians AS patients and controls (41% in AS vs. 41% in C.). 3) B*2706 was found overrepresented in control populations and absent in AS patients (0% in AS vs. 47% in C.; pc < 10(-6)) showing the maximum value of protective fraction (PF = 1). The B*2706 negative association with AS has not been previously described in other ethnic groups and could indicate a protective effect of this subtype on AS susceptibility. The B*2706 allele has two changes relative to B*2704 at residue 114 (His to Asp) and 116 (Asp to Tyr) in the pockets D/E. The importance that these differences can play in the pathogenesis of AS are discussed.