Two-dimensional gel analysis of the polypeptides precipitated by a polymorphic HLA-DR1,2,w6 monoclonal antibody: evidence for a third locus.

The SDR1 monoclonal antibody reacts only with cells which express the HLA-DR1, 2,w6 or w8 allogeneic specificities. Two-dimensional nonequilibrium pH gradient/sodium dodecyl sulfate polyacrylamide gel electrophoretic analyses of SDR1 immunoprecipitates from [35S]methionine biosynthetically labeled cells revealed the typical pattern of Ia antigens, namely two polypeptides of about 34kDa and 29kDa (designated epsilon and beta-3) as well as the "basic invariant spot" of about 31 kDa. The epsilon and beta-3 polypeptides were only weakly represented in similar analyses of immunoprecipitates performed using a monomorphic HLA-DR monoclonal antibody, TDR31.1. The epsilon and beta-3 polypeptides of B lymphoblastoid cell lines homozygous for HLA-DR2 and w6 were structurally polymorphic as judged by two-dimensional gel analyses. This polymorphism was independent of the HLA-DR specificity. It is concluded that the SDR1 antibody recognizes a polymorphic set of Ia antigens that are coded by a locus other than HLA-DR. These antigens probably also express the MT1 (DC1, LB12) alloantigenic specificity and are most likely the human equivalent of the murine I-A subregion antigens.