Peptide-induced rescue of serologic epitopes on class I MHC molecules.

To monitor conformational changes in MHC class I structure induced by interaction with peptide or beta 2-microglobulin (beta 2-m), we have taken a serologic approach. Previous studies by us and others have defined circumstances wherein specific peptides can decrease serologic recognition of class I molecules. However, such blocking of serologic epitopes has often been interpreted as steric hindrance by peptide side chains. In this paper, we describe peptide-induced gains in recognition by mAbs 30-5-7, 34-1-2, and B22/249. In experiments with mAb 30-5-7, impaired reactivity, which resulted from an Ld loop mutation, was specifically rescued by the binding of a beta-galactosidase-derived peptide to the Ld mutant. In studies with mAb 34-1-2, poor Ld detection was enhanced by mutations in Ld at beta 2-m interaction sites or by changes within the peptide-binding groove. To evaluate whether known peptides in the Ld groove could influence 34-1-2 recognition, we tested six peptide ligands, four of which increased the reactivity of 34-1-2 with the Ld-expressing cell to various degrees (up to 14-fold). It is of interest that Ld mutations at position 9 and 95/97 made significant differences in the ranking of the peptides in regard to their ability to increase recognition by 34-1-2 and B22/249. This finding suggests that mutations in the binding groove can alter peptide conformation and result in secondary changes in class I structure. On the basis of the cumulative serologic data, we propose that the class I molecule displays considerable fluidity, and is structurally influenced by both beta 2-m and peptide.