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Previous studies in our laboratory using primed lymphocyte typing (PLT) in an informative family have led to the identification of four traits and two regions, separable by recombination within the HLA-DR system. In the present study, we analysed the heterogeneity of HLA-DR antigens among members of this family as a first step towards defining the molecular entities responsible for the observed cellular reactivities. Cell lines of various genotypes derived from the members of this family were labelled biosynthetically with 35S-methionine. The total glycoproteins and immunoprecipitates prepared with two monoclonal anti-HLA-DR antibodies, two monoclonal anti-mouse Ia antibodies cross-reactive with HLA-DR, and a rabbit anti-HLA-DR (anti-p27,33) antiserum were fractionated by two-dimensional gel electrophoresis. In agreement with previous results, HLA-DR light chains were clearly polymorphic whereas the heavy chains looked rather monomorphic. The light chain subunits either segregated with one haplotype, or were shared by some but not all haplotypes or were common to all haplotypes. Whereas the haplotype-specific polypeptides correlated best with HLA-DR3 and HLA-DR5 specificities, the shared subunits may have corresponded to the cross-reactivities observed by PLT. Comparison of the patterns obtained with the various monoclonal antibodies and with the rabbit antiserum revealed that each monoclonal antibody bound a subset of HLA-DR light chains, all of which were present in the anti-p27,33 precipitates. This rabbit antiserum and one of the monoclonal antibodies immunoprecipitated a set of heavy chains not bound by the other 3 antibodies. Since overlapping pools of light chains were present in all five immunoprecipitates, these results suggest that different heavy chains may associate with the same light chains.

A cDNA clone encoding the human B cell alloantigen DC alpha chain (pDCH1) has been used to analyse the structure of the human and murine major histocompatibility complexes by the DNA filter hybridization technique. The pDCH1 probe hybridizes to a single DNA sequence present on chromosome 17 in the mouse genome. A restriction enzyme polymorphism enables us to map this sequence to the I-A subregion. Extensive restriction enzyme polymorphism detected in HLA-DR homozygous typing cells is reminiscent of the DR/MT linkage disequilibrium groups, suggesting that the pDCH1 probe could be useful for haplotype typing in the human population. The HLA-DR region appears more complex than the I region since a second DC-like hybridizing sequence is detected in the human genome in these experiments.