MODULATION OF PEPTIDE BINDING BY HLA-B27 POLYMORPHISM IN POCKETS A AND B, AND PEPTIDE SPECIFICITY OF B(ASTERISK)2703

The results in this study address three aspects of peptide binding to the disease-associated antigen HLA-B27 and its modulation by polymorph ism: the contribution of major anchor residues 2 and 9, the role of po cket B polymorphism in modulating peptide specificity and the binding properties of B2703, a subtype not found to be associated with spondy loarthropathy. Synthetic analogs of peptides naturally presented by B 2705 were used to demonstrate that residue 2 is essential, since Ala2 analogs bound marginally to B2705, 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 r esidues at this position. This was shown by quantitating the relative binding of Gln2 and Ala2 analogs, and by pool-sequencing of the peptid es bound in vivo to these mutants. Peptides naturally presented by B2 705 apparently bound with a lower affinity to pocket A variants with a ltered hydrogen bending to the peptide N terminus, including B2703. B inding of peptide analogs with changes at positions 2 or 9 suggested t hat in B2703 pocket A, interactions are weaker and pocket B interacti ons are stronger than in B2705. This can be explained by the effect o f the unique His59 change in B2703 in both pockets. Thus, B*2703 is p robably the HLA-B27 sub-type with the most stringent specificity for t he Arg2 peptide motif.