Four monoclonal antibodies reacting with distinct human Ia antigenic determinants have been used to demonstrate the coexpression of four distinct subsets, NG1, NG2, H40+-3/4+, and DC1 H40--3/4+, in the Ia pool of DR heterozygous or homozygous B cell lines. By two-dimensional peptide mapping the four subsets within the same Ia pool displayed structurally different beta as well as alpha subunits. The beta chain of the NG1 subset was shown to display considerable structural polymorphism when analyzed in two cell lines with distinct DR but similar DC phenotype, LG2 (DR1,1-DC1) and Raji (DR3, W6-DC1). In contrast, the beta chains of NG2, DC1 H40+-3/4+, and DC1 H40--3/4+ subsets of LG2 cells were shown to be very similar to their homologous Raji cell counterparts, thus indicating a relatively low structural polymorphism. Furthermore, the alpha chains of either one of the four subsets expressed in LG2 cells displayed very high structural similarities to the homologous counterparts in the Raji Ia pool, thus suggesting a relatively low polymorphism for the large Ia subunits described in this study. A striking feature deduced from this study was the selective subunit association of the distinct alpha-beta heterodimers.

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