A molecular insight on the association of HLA-B27 with spondyloarthropathies

Major histocompatibility complex class I peptide presentation after Salmonella enterica serovar typhimurium infection assessed via stable isotope tagging of the B27-presented peptide repertoire

Reactive arthritis (ReA) induced by infection with several gram-negative bacteria is strongly associated with expression of the major histocompatibility complex class I molecule HLA-B27. It is thought that due to the intracellular lifestyle of ReA-inducing bacteria, bacterial fragments can be presented by HLA-B27. Cytotoxic T cells recognizing such bacterial peptides or other induced host peptides could cross-react with self peptides presented in the joints, giving rise to disease. Studies to analyze the B27 peptide repertoire in relation to infection were severely hampered, as complex peptide profiles obtained from separate infected and noninfected cell preparations had to be compared. For this study, we applied a new approach to examine the effect of Salmonella enterica serovar Typhimurium infection on the B27 peptide repertoire presented by the HLA-B*2704 subtype associated with disease. Firstly, we showed that both host cell and S. enterica serovar Typhimurium proteins can be tagged metabolically with stable-isotope-labeled arginine. We then designed experiments so that either the tagged endogenous or tagged bacterial B*2704-presented peptide repertoires from infected cells could be analyzed by mass spectrometry from single peptide preparations that included uninfected controls. Using this new approach, we found no evidence for significant changes in endogenous B*2704 peptide presentation after infection or for any S. enterica serovar Typhimurium-derived B27-bound peptide. In conclusion, the hypothesis that S. enterica serovar Typhimurium induces changes in B27 peptide presentation could not be supported.

Novel HLA B*2714 and B*2708 allele associations in seronegative spondarthritis patients and haemophilia patients with chronic synovitis in India

HLA B27 association with seronegative spondarthritis (SSA) has been recognized for over a decade and its association with chronic synovitis in Haemophilia patients has been recently reported. We characterized the HLA B27 subtypes in a cohort of 98 B27 positive individuals with SSA (n = 45), haemophilia with chronic synovitis (n = 21) and normal controls (n = 32), using polymerase chain reaction-based sequence specific primers (PCR-SSP) and reverse line strip (RLS) techniques. The results revealed a significant association for a novel allele B*2714 in SSA along with the previously reported B*2705 allele. We also found a significant association between haemophilia patients with chronic synovitis from India and a hitherto unidentified allele, B*2708.

Molecular mimicry of an HLA-B27-derived ligand of arthritis-linked subtypes with chlamydial proteins

HLA-B27 is strongly associated with spondyloarthropathies, including ankylosing spondylitis and reactive arthritis. The latter disease is triggered by various Gram-negative bacteria. A dodecamer derived from the intracytoplasmic tail of HLA-B27 was a natural ligand of three disease-associated subtypes (B*2702, B*2704, and B*2705) but not of two (B*2706 and B*2709), weakly or not associated to spondyloarthropathy. This peptide was strikingly homologous to protein sequences from arthritogenic bacteria, particularly to a region of the DNA primase from Chlamydia trachomatis. A synthetic peptide with this bacterial sequence bound in vitro disease-associated subtypes equally as the natural B27-derived ligand. The chlamydial peptide was generated by the 20 S proteasome from a synthetic 28-mer with the sequence of the corresponding region of the bacterial DNA primase. Molecular modeling suggested that the B27-derived and chlamydial peptides adopt very similar conformations in complex with B*2705. The results demonstrate that an HLA-B27-derived peptide mimicking arthritogenic bacterial sequences is a natural ligand of disease-associated HLA-B27 subtypes and suggest that the homologous chlamydial peptide might be presented by HLA-B27 on Chlamydia-infected cells.

HLA B27 polymorphism in Western India

We have characterized HLA B27 alleles in a sample population of Maharastra, Western Indians (n = 51), with the aim to investigate the different subtypes present among this population. The study was carried out using polymerase chain reaction with sequence-specific primers (PCR-SSP) and reverse line strip (RLS) techniques. Significant new findings have arisen from this study: B*2704, B*2705, B*2707, B*2708 and B*2714 alleles were found to be present, and two novel B27 alleles, B*2708 and B*2714, were found in this Indian population. In addition, B*2714 was observed in a patient with ankylosing spondylitis. This association has not been previously reported in ethnic groups from India.

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.