PEPTIDES CONTROL THE GAIN AND LOSS OF ALLELE SPECIFICITY BY MUTATED MHC CLASS-I MOLECULES

To analyze the molecular basis of MHC class I allele-restricted peptid e recognition, a set of eight L(d)/L(q) mutants was constructed and te sted for peptide recognition by allele-restricted and peptide-specific CTL. The MHC molecules H-2L(d) and H-2L(q) differ at six amino acid p ositions (95, 97, 107, 116, 155, 157) located within the alpha(2) doma in of the molecule. Both molecules present the lymphocytic choriomenin gitis virus (LCMV) nucleoprotein-derived peptide RPQASCVYM and the mur ine cytomegalovirus (MCMV) pp89-derived peptide YPHFMPTNL to the respe ctive virus-specific CD8(+) CTL in a strictly allele-restricted fashio n. All mutated MHC class I molecules did still bind the LCMV peptide a nd seven of eight mutants retained MCMV peptide binding. The exchange Arg-->Trp at position 97 of L(q) in pocket C of the peptide binding gr oove prevented binding of the MCMV ligand and this loss was compensate d by the additional exchange of Ile-->Leu in position 95 (pocket F). W ithin the L(q) molecule, single mutations at either position 97 on the floor of the groove or position 155 of the wall sufficed for a gain o f LCMV peptide recognition by L(d)-restricted CTL. Altogether, six of eight mutants resulted in a gain of recognition by CTL specific for th e other allele. Thus, six of the eight mutants lost MHC-restricted rec ognition and were accepted by both L(d)- as well as L(q)-restricted CT L when presenting the LCMV peptide. Only one case of simultaneous reco gnition of the MCMV peptide by both L(d)- as well as L(q)-restricted C TL was noted. In other mutations, a gain of recognition by L(d)-restri cted CTL was associated with a loss of recognition of L(q)-restricted CTL. Analysis of extracted MCMV peptide from mutant molecules excluded quantitative differences in presented MCMV peptide as a reason for th e lack of CTL recognition. Altogether, the results show that, rather t han aminoacids at certain residue positions, individual peptides gover n MHC allele specificity of CTL recognition.