Analysis of HLA-DR polymorphism by two-dimensional peptide mapping.

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Abstract
Two-dimensional peptide mapping was used to study the polymorphism of DR antigens, membrane glycoproteins composed of 2 chains, .alpha. and .beta., and encoded by the human major histocompatibility complex (MHC). Four DR antigens were purified by immunoabsorption from 4 human [Epstein-Barr virus-transformed] lymphoblastoid cell lines [WT-18, WT-48, WT-52, IBW-9] homozygous at the DR locus. After labeling with 125I, .alpha. and .beta. chains were separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and digested with pepsin. Comparison of the peptide maps showed a marked degree of polymorphism among .beta. chains; only 43% of peptides were common to all 4 chains; 15-21% of the spots were unique to a given chain. Only a limited variability was observed among .alpha. chains. Homology was 75% for the 4 chains; the percentage of unique peptides was very low. DR7 did not possess even a single unique peptide. The limited variability among .alpha. chains and the lack of private peptides in one of them suggest that the .beta. chain is the unique carrier of the alloantigenic specificities. Higher homology within the known crossreactive groups was not observed; the determinants responsible for crossreactivity are on different molecules. From a genetic point of view, because .beta. chains show allele-associated polymorphism, they are likely to be MHC encoded; the minor differences among .alpha. chains do not allow a similar conclusion. An analogy between these DR antigens and the mouse I-E/C antigens is indicated.