HLA-DR RESIDUES ACCESSIBLE UNDER THE PEPTIDE-BINDING GROOVE CONTRIBUTE TO POLYMORPHIC ANTIBODY EPITOPES

Many residues involved in polymorphic antibody-binding epitopes on cla ss II molecules are located on the alpha-helix of DR beta chains. Alth ough they have received less attention, residues in the peptide-bindin g groove and second domain of the DR beta chain may also be critical f or polymorphic anti-DR antibody epitopes. In this study, we used trans fectants expressing site-directed mutations at positions in the HLA-DR beta(1) and beta(2) domains and flow cytometry to define the epitopes of several polymorphic anti-DR antibodies. Both DR(beta 10403) resid ues 14 and 25 were shown to be involved in the epitopes of mAbs DA6. 1 64, HU-20, Q5/6, and 50D6, and DR(beta 10701) residue 14 was shown to be critical for the epitopes of two DR7-specific mAbs, SFR16-DR7M and TAL13.1. Unlike most other residues shown to be important in antibody -binding epitopes, residue 14 is located in the floor of the peptide-b inding groove and residue 25 is in an outer loop, each with their side chains pointing down, such chat antibodies may directly contact these residues from below the binding groove. Two residues in the beta(2) d omain, beta 180 and beta 181, were also shown to be involved in the ep itopes of three polymorphic anti-DR mAbs, NFLD.D1, NFLD.M1, and LY9. A lthough these two residues are close to the transmembrane domain in th e linear sequence, their solvent accessibility in the DR1 structures i s quite impressive. Our data provide new evidence that residues access ible under the peptide-binding groove contribute to polymorphic antibo dy-binding epitopes.