Topographic map of the HLA-A2 CREG epitopes using human alloantibody probes.

The topographic architecture of the epitopes expressed on the HLA-A2 glycoprotein using murine monoclonal antibody (mAb) probes indicates at least two sterically distinct domains. Previously, we have demonstrated using human HLA alloantibodies (aAb) that multiple determinants are expressed on each HLA antigen: the highly polymorphic private epitopes and the public determinants that are shared within a family of crossreactive groups (CREG). Our objectives now focus on probing the antigenic structure of the HLA-A2-28-9-B17 CREG using highly specific aAb in conjunction with mAb that have previously been used for structural studies. Both mAb-mediated blockage of complement-dependent cytotoxic aAb and reciprocal antibody (Ab) binding inhibition assays with quantitation by fluorescence flow cytometry have been utilized. We have found that xenogeneic mAb directed against A2-69, A2-B17, and A2-28 crossblock aAb of the same serologic specificity, and vice versa, indicating that the epitopes they respectively recognize are at least in close steric proximity. However, additional HLA-A2, A28, and B17 aAb of private specificity and A2-28-9 aAb of public specificity, for which there are no known mAb counterparts, paint an additional complexity not previously known. We conclude that at least four different alloepitopes can be expressed by each serologically defined HLA antigen. Based on the primary sequence data, we have assigned the location and the amino acid substitutions which most likely account for these discrete epitopes. The unique private determinants are located on the alpha 1 domain together with the interlocus A2-B17 epitope while the public epitopes A2-69, A2-28-9 and A2-28 are located on the alpha 2 domain.