An HLA-B35-restricted epitope modified at an anchor residue results in an antagonist peptide

Peptides associated with HLA-B35 commonly have a proline or occasionally a serine residue in the P2 anchor position of the peptide, with a tyrosine at the C terminus. Based on this motif, we identified an octamer epitope from influenza A matrix protein which is presented by HLA-B35. The requirements for MHC binding and T cell receptor contact have been analyzed using analogs of this peptide with substitutions at positions 1, 2, 4, 7 and 8. The natural epitope contains a serine residue at P2 of the peptide. Substitution of this residue with proline (the favored amino acid in this position in B35-associated peptides) considerably enhances binding to HLA-B35 in the T2-B35 cell line, but the peptide is not recognized by the majority of CTL clones and can antagonize recognition of the index peptide. This suggests that a conservative substitution at the P2 anchor position results in a conformational change in the peptide-MHC surface exposed to the T cell receptor.