BW4-REACTIVE AND BW6-REACTIVE ANTIBODIES RECOGNIZE MULTIPLE DISTINCT HLA STRUCTURES THAT PARTIALLY OVERLAP IN THE ALPHA-1 HELIX

Bw4 and Bw6 epitopes are expressed by mutually exclusive sets of HLA-B alleles and some HLA-A and HLA-C alleles. To test whether antithetica l structures are required to express Bw4 and Bw6 epitopes, we measured binding of Bw4-reactive and Bw6-reactive alloantibodies and mAbs to H LA-B7 variants. A triple substitution of HLA-B7 alpha-1 helix residues 80, 82, and 83 created Bw4 and destroyed Bw6 epitopes detected by all oantibodies and mAbs. Both Bw4-reactive and Bw6-reactive mAbs competed for binding to HLA-B7 variants with single substitutions at residues 82 and 83. Substitutions of residues H93 and D119 which form a salt br idge in HLA-A2 also permitted binding by both Bw4-reactive and Bw6-rea ctive mAbs, suggesting that Bw4 and Bw6 epitopes are conformationally dependent. Six Bw4-reactive mAbs showed four distinct patterns of bind ing to HLA-B7 variants. Detailed analysis of 74 HLA-B7 single-residue variants showed that Bw6-reactive SFR8-B6 binding was prohibited by mu tations altering the distal end of the alpha-1 helix and the nearby co nnecting loop. In contrast, Bw6-reactive BB7.6 binding required both a lpha-1 and alpha-2 helix residues. Thus, Bw4-reactive and Bw6-reactive Abs recognize multiple distinct HLA structures that partially overlap in the alpha-1 helix. As both Bw4 and Bw6 epitopes are expressed by s ome HLA-B7 variants, mutually exclusive expression of Bw4 and Bw6 epit opes in naturally occurring HLA class