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-DR beta chains that are involved in the epitopes recognized by DRw52-like mAbs. Flow cytometric analysis of antibody binding to mouse fibroblast transfectants expressing wild-type or mutant DRw52-associated DR beta chains demonstrated that the amino acid at position 77, which is predicted to be on a solvent-accessible surface of the alpha-helix, contributes to the epitopes recognized by the 7.3.19.1 and 21r5 mAbs. In contrast, residues that are not predicted to occupy accessible positions 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, affect these epitopes. It is interesting that antibody binding is influenced by amino acid substitutions at DR beta positions 10 and 12 because the class II model predicts that residues at these positions are located in the middle of the floor of the peptide-binding site, with their side chains pointing down. These findings emphasize the functional importance of polymorphic residues whose side chains are not predicted to point into or up from the antigen-binding site and they raise the possibility that peptides contribute to the generation of these epitopes.