HLA-DR-BETA-CHAIN RESIDUES THAT ARE PREDICTED TO BE LOCATED IN THE FLOOR OF THE PEPTIDE-BINDING GROOVE CONTRIBUTE TO ANTIBODY-BINDING EPITOPES

The purpose of this study was to identify polymorphic residues in HLA- DRbeta chains that are involved in the epitopes.recognized by DRw52-li ke mAbs. Flow cytometric analysis of antibody binding to mouse fibrobl ast transfectants expressing wild-type or mutant DRw52-associated DRbe ta chains demonstrated that the amino acid at position 77, which is pr edicted to be on a solvent-accessible surface of the alpha-helix, cont ributes to the epitopes recognized by the 7.3.19.1 and 21r5 mAbs. In c ontrast, residues that are not predicted to occupy accessible position s on the alpha-helix contribute to binding of the I-LR2 and UL-52 mAbs . Substitutions at positions 10, 12, and 51, but not 9, 11, and 13, af fect these epitopes. It is interesting that antibody binding is influe nced by amino acid substitutions at DRbeta positions 10 and 12 because the class II model predicts that residues at these positions are loca ted in the middle of the floor of the peptide-binding site, with their side chains pointing down. These findings emphasize the functional im portance of polymorphic residues whose side chains are not predicted t o point into or up from the antigen-binding site and they raise the po ssibility that peptides contribute to the generation of these epitopes .