Differences in the recognition by CTL of peptides presented by the HLA-B*4402 and the HLA-B*4403 molecules which differ by a single amino acid

The HLA-B*4402 and B*4403 molecules differ only at residue 156, which borde rs the peptide binding site. Strong in vivo allogeneic reactions mediated b y cytolytic T lymphocytes (CTLs) were reported in patients who received a b one marrow graft mismatched fur these B44 subtypes, indicating that HLA-B*4 402 and B*4403 molecules present distinct antigens. This could be due eithe r to the presentation of different sets oi antigenic peptides or to the rec ognition by CTLs of conformational epitopes formed by the MHC molecules alo ne or in association with antigenic peptides. To address this question, we compared the two B44 sublypes in their presentation to tumor-specific CTLs of three peptides, encoded by genes MAGE-3, MUM-1 and Tyrosinase. The pepti des bound with similar affinities to B*4402 or B*4403 molecules, as assesse d by lytic competition assays. One HLA-B*4402-restricted and one HLA-B*4403 -restricted CTL clone were derived against each peptide. When tested for ly sis of B*4402 and B*4403 cells incubated with the antigenic peptides, most CTLs showed a marked preference for one of the two B44 subtypes Using varia nt peptides incorporating single alanine substitutions, we compared a given CTLs' recognition of its antigenic peptide presented by both B44 subtypes. Some substitutions, which had no effect on the binding of the peptide, aff ected its recognition by the same CTL differently on B*4402 and B*4403 mole cules. These results imply that the conformations adopted by the same pepti de on the two HLA-B44 subtypes are different, me conclude that the B44 subt ype specificity of T cells results mostly from distinct conformations adopt ed by the same peptides in the two B44 molecules, This does trot exclude th e possibility that in some cases the B44 subtype specificity results from t he selective binding of a peptide to one subtype. We found several peptides , different from the three mentioned above, that contain the canonical HLA- B44 binding motif and bind to B*4403 but not to B*4402 molecules.