Differential expression and serologically distinct subpopulations of human Ia antigens detected with monoclonal antibodies to Ia alpha and beta chains

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Abstract
Studies with two monoclonal antibodies (DA6.147 and DA6.231) which react, respectively, with isolated human Ia α and β chains are reported. Both antibodies detect epitopes expressed on all DR‐heterozygous and DR‐homozygous cell lines tested (n = 17) and bind to the Epstein‐Barr virus‐negative cell line Ramos. Ia subunit specificity of the antibodies was determined by an adaptation of an electroblot technique which transfers separated la chains from polyacrylamide gels to nitrocellulose paper. In radioimmunobinding assays, peripheral blood B cell‐enriched fractions, phytohemagglutinin‐activated T cells and pokeweed mitogen‐activated cells gave strong reactions with DA6.231 (anti‐Ia β). In contrast, DA6.147 (anti‐Ia α) reacted only weakly, if at all, with peripheral B cells, pokeweed mitogen blasts and activated T cells. However, both antibodies bound to isolated Ia from activated T cells and peripheral B cells after Nonidet‐P40 solubilization of the cells and DA6.147+ antigens could be found in the cytoplasm of activated T cells by indirect immunofluorescence techniques. Results of serological inhibition procedures following fractionation of lymphoblastoid cell lysates on monoclonal antibody affinity columns showed that the DA6.147 α chain epitope is carried on only a minor subpopulation of human Ia.