Modulation at multiple anchor positions of the peptide specificity of HLA-B27 subtypes differentially associated with ankylosing spondylitis

Objective, To investigate the rules governing peptide binding to HLA-B*2705 , and to B*2704 and B*2706, which are 2 subtypes differentially associated with ankylosing spondylitis. Methods. Poly-Ala analogs carrying the HLA-B27 motif Arg-2, and substitutio ns at anchor positions P1, P3, or P Omega, were used to determine a binding score for each residue at each position. Binding was assessed in a quantit ative epitope stabilization assay, where the cell surface expression of HLA -B27 was measured by flow cytometry as a function of peptide concentration. Results. Peptide anchor residues contributed additively to B*2705 binding. About 15% of the natural B*2705 ligands used a deficient P3 or P Omega anch or, but never both, indicating that detrimental anchoring at one of these p ositions is always compensated by a good anchor at the other one. About 50% of the B*2705 ligands used suboptimal P1 residues. However, this was compe nsated with optimal P3 and/or P Omega anchoring. Peptides that were longer than decamers used good anchor residues at the 3 positions, suggesting more stringent binding requirements. B*2704 and B*2706 differed in their residu e specificity at P1, P3, and P Omega, The rules derived for B*2705 also app lied to the known ligands of these 2 subtypes, Conclusion. The B*2705, B*2704, and B*2706 peptide repertoires are limited by the allowed residue combinations described in this study. The differenti al association of B*2704 and B*2706 with spondylarthropathy correlates with differences in their peptide specificity at multiple anchor positions. How ever, it is now possible to predict the peptide features that determine thi s differential binding to both subtypes.