Mutagenesis around residue 176 on HLA-B^*0702 characterizes multiple distinct epitopes for anti-HLA antibodies

Preformed antibodies against HLA-A,B,C molecules cause hyperacute rejection of transplanted allogeneic tissues. To understand better the molecular basis of hyperacute rejection, narrowly reactive anti-HLA-B^*0702 monoclonal antibodies have been studied. Previous epitope mapping studies of these monoclonal antibodies by mutating B^*0702 have conflicted with antibody-blocking studies. To resolve these discrepancies, we mutated B^*0702 residues around the antigenically important residue 176, and measured anti-B^*0702 antibody binding. Antibody MB40.2 binding is abrogated by mutations at residues 169, 180, and 182, close to residue 176 in the primary structure. However, MB40.2 binding is not affected by 12 other B^*0702 mutations close to residue 176 in the tertiary structure. This suggests that MB40.2 may recognize a sequential B^*0702 epitope including residues between positions 169-182. Antibody BB7.1 binding requires B^*0702 α₂-domain residues 166 and 169. Competition for B^*0702 residue 169 explains why MB40.2 and BB7.1 crossblock. Because BB7.1 binding also requires B^*0702 α₁-domain residues, BB7.1 may contact both α-helices, straddling the B^*0702 peptide-binding groove. Previous results showed that both MB40.2 and MB40.3 binding require B^*0702 residues 176/178. However, MB40.3 binding is not affected by any of 15 other mutations near residue 176. This suggests that MB40.3 does not contact residues 176/178; rather, residues 176/178 appear to affect MB40.3 binding by subtly influencing B^*0702 conformation. Thus, monoclonal antibodies influenced by a defined B^*0702 region around residue 176 appear to recognize three different types of epitopes. This suggests that human alloantibodies also recognize diverse types of HLA epitopes.