BINDING OF NONAMER PEPTIDES TO 3 HLA-B51 MOLECULES WHICH DIFFER BY A SINGLE AMINO-ACID SUBSTITUTION IN THE A-POCKET

The interaction between 9-mer peptides and HLA-BSI molecules was inves tigated by quantitative peptide binding assay using RMA-S cells expres sing human beta-microglobulin and HLA-B51 molecules. Of 147 chemically synthesized 9-mer peptides possessing two anchor residues correspondi ng to the motif of HLA-B5101 binding self-peptides, 27 peptides bound to HLA-B5101 molecules. Pro and Ala at position 2 as well as Ile at position 9 were confirmed to be main anchor residues, while Gly at pos ition 2 as well as Val, Leu, and Met at position 9 were weak anchor re sidues for HLA-B5101. The A-pocket is suspected to have a critical ro le in peptide binding to MHC class I molecules because this pocket cor responds to the N-terminus of peptides and has a strong hydrogen bond formed by conserved Tyr residues. Further analysis of peptide binding to HLA-B5102 and B*5103 molecules showed that a single amino acid sub stitution of Tyr for His at residue 171(B5102) and that of Gly for Tr p at residue 167 (B5103) has a minimum effect in HLA-B51-peptide bind ing. Since previous studies showed that some HLA-B51 alloreactive CTL clones failed to kill the cells expressing HLA-B5102 or HLA-B*5103, t hese results imply that the structural change of the A-pocket among HL A-B51 subtypes causes a critical conformational change of the epitope for TCR recognition rather than influences the interaction between pep tides and MHC class I molecules.