Epitope fine specificity of human anti-HLA-A2 antibodies identification of four epitopes including a haptenlike epitope on HLA-A2 at lysine 127

Anti-HLA-A2 CREG antibodies were purified from seven individuals by affinity chromatography. The binding of the purified antibodies to single or multiple amino acid variants of HLA-A2.1 was measured with an inhibition RIA. Substitutions at 10 amino acid residues in the polymorphic α₁ and α₂ domains were important for human antibody binding; eight of these have previously been shown to be important in the binding of murine anti-HLA-A2 CREG antibodies. Unlike any previously reported murine mAbs, the binding of antibodies from two individuals was eliminated by a substitution at the HLA-A2, -24, -28 shared loop amino acid residue lysine 127. Conversely, when the asparagine at residue 127 on the non-cross-reactive HLA-A3 was replaced with lysine, antibody binding was completely restored. The results further suggest that both λ- and κ-containing human antibodies that bind to this region may recognize lysine 127 as a haptenlike epitope. Anti-HLA-A2 antibodies that recognized a conformational epitope defined by changes at glycine 62 in the α₁ domain were predominanted by λ light chains whereas those that recognize an epitope defined by a loop residue at tryptophan 107 in the α₂ domain were predominated by κ light chains. The data are consistent with a model of restricted epitope recognition of HLA-A2 by human B cells that is similar to, but distinct from, epitope recognition by mouse B-cell hybridomas, and may help to explain the phenomenon of public or cross-reactive idiotypes in the HLA system.