HLA-B27 and disease pathogenesis: new structural and functional insights

Refolding of HLA-B27 heavy chains in the absence of beta2m yields stable high molecular weight (HMW) protein forms displaying native-like as well as non-native-like conformational features: implications for autoimmune disease

Refolding of the heavy chain of the Class I HLA molecule, HLA-B27, in the absence of beta(2)m, yields soluble high molecular weight (HMW) oligomers reminiscent of the oligomeric forms of beta(2)m-free heavy chains (FHCs) of class I HLA antigens observed on cell surfaces. Here we examine the structural characteristics of HMW B27 in respect of features potentially relevant to autoimmunity, such as: (a) retention of native-like structure, since this could facilitate non-canonical interactions with T-cell receptors even in the absence of bound beta(2)m and peptide, or (b) presence of non-native structure, since this could yield novel (non-self) antigenic conformational epitopes that could elicit immune attack. We report that HMW B27 is characterized by high secondary structural content, structural stability, stability to proteolysis by trypsin, and structural features that are both partly native-like, and partly non-native-like, as assessed through the binding of conformationally-distinguishing and cross-reacting scFv antibodies specifically selected against HMW B27. We also present cell ELISA data with conformation-specific scFv antibodies that distinguish between lymphocytes from individuals who are healthy and B27 positive, and those who are B27 positive but suffering from ankylosing spondylitis.

Interaction of HLA-B27 homodimers with KIR3DL1 and KIR3DL2, unlike HLA-B27 heterotrimers, is independent of the sequence of bound peptide

HLA-B27 can form beta-2 microglobulin (beta2m)-associated heterotrimers (HLA-B27) and beta2m-free homodimers (B27(2)). Here, we study the role of complexed peptide in the interaction of these forms of B27 with the killer cell immunoglobulin (Ig)-like receptors KIR3DL1 and KIR3DL2 and with Ig-like transcripts LILRB1 and LILRB2. HLA-B27 tetramers complexed with three of five different naturally processed self peptides and three of seven pathogen-derived epitopes bound to KIR3DL1-expressing transfectants and NK cells. Heterotrimeric complexes containing peptides with charged amino acids at position 8 did not bind to KIR3DL1; however, studies with analogue peptides demonstrated that these are not the only peptide residues involved in binding. KIR3DL1 ligation by HLA-B27 inhibited NK cell IFN-gamma production in a peptide-dependent fashion. B27 but not HLA-A2, B7 or B57 heavy chains formed homodimers in the presence of peptide epitopes. B27(2) bound to KIR3DL1, KIR3DL2 and LILRB2 but not LILRB1. KIR3DL2 ligation by B27(2) inhibited NK and T cell IFN-gamma production. By contrast with HLA heterotrimers, B27(2) binding to KIR did not depend on the sequence of the bound peptide. Differences in KIR binding to classical HLA and B27(2) could be involved in the pathogenesis of spondyloarthritis.

The amino acid at position 97 is involved in folding and surface expression of HLA-B27

HLA-B27 confers susceptibility to ankylosing spondylitis (AS) but the mechanism linking this association remains unknown. Other properties unrelated to its natural role of antigen presenting function may be important in disease pathogenesis. We determined here the impact of N97D substitution on the folding and expression of HLA-B*2704 transfected in the 721.221 cell line. The mutation at position 97 abolishes the surface expression of non-conformational (HC10) and conformational (ME1) forms. The expression of ME1 forms was found to be absent in B*2704 N97D by immunoprecipitation and flow cytometry of fixed and permeabilized cell experiments with the conformation-sensitive ME1 antibody. However, immunoblotting cell lysates with HC10 revealed the presence of unfolded heavy chain (HC) and HC-dimer forms. The impact of the N97D mutation in the exit from the endoplasmic reticulum (ER) was analysed by western blot after endoglycosidase-H treatment, and it was found that B*2704 N97D was retained and accumulated as unfolded molecules. We tested for mutant association with transporter associated with antigen processing (TAP), calnexin (CNX), calreticulin (CLR) and beta2 microglobulin (beta2m). The wild-type B*2704 and N97D mutants were associated with TAP, CNX and CLR, although HC10 forms of mutant N97D interact more weakly with TAP. Only folded molecules of HLA-B*2704 were associated with beta2m. Surprisingly, the peptide-binding assay demonstrated the ability of unfolded N97D molecules to bind high-affinity peptides. It has been suggested that AS may arise because of aberrant folding of HLA-B27 molecules within the ER. Future work must therefore aim to clarify the functional connection between the unfolded protein response pathway in response to the accumulation of HLA-B27 in the ER. This mutant could be useful as a model for the misfolding of HLA-B27.

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.

HLA-B27 lacking associated β2-microglobulin rearranges to auto-display or cross-display residues 169-181: a novel molecular mechanism for spondyloarthropathies

Expression of the MHC class I allele, HLA-B27, is correlated with autoimmune disease. The misfolding and association of B27 heavy chains through non-native disulfide bonds has recently been implicated. Here, we propose that beta2m-free, peptide-free heavy chains support a helix-coil transition in the segment leading from the alpha2 domain to the alpha3 domain, facilitating rotation of backbone angles around residues 167/168, and allowing residues 169-181 (identical to a known B27 ligand) to loop around and occupy the molecule's own peptide-binding cleft. Such 'auto-display', occurring either within B27 molecules, or between B27 molecules, could provoke autoimmune attack.

HLA-B27 subtypes determination in patients with ankylosing spondylitis from Zulia, Venezuela

To perform an investigation regarding the distribution of the human leukocyte antigen (HLA)-B27 subtypes in the Zulian population with ankylosing spondylitis (AS), 48 unrelated Mestizos, HLA-B27 positive by serology, were studied using the polymerase chain reaction-specific sequence oligonucleotides probe (PCR-SSOP) and specific sequence primers (SSP) to analyze the polymorphism in exons 2 and 3 of the HLA-B27 gene. Only two of eight HLA-B27 subtypes studied (B*2701-B*2708) were found. The distribution of these alleles in the population of patients was: B*2705, 68.8%, and B*2702, 31.2%. B*2705 subtype showed significant association with patients being male. In the healthy controls, the most common subtype was B*2708. These results were compared with frequencies reported in other Mestizo and Spanish populations and showed significant differences, such as a high frequency of B*2702. Such results show that HLA*B2705 and HLA*B2702 are the subtypes most frequently associated with AS in our Mestizo population and suggest a possible protector role for HLA*B2708, which was found only in the healthy population.