The nonpolymorphic MHC class II isotype, HLA-DQA2, is expressed on the surface of B lymphoblastoid cells

The more centromeric of the two human MHC class II DQA genes, DQA2, has not previously been shown to express a protein product. However, its high degree of sequence similarity to DQA1, the fact that it is highly evolutionarily conserved, and the recent demonstration of its transcription in some cell lines suggest that a DQA2 alpha-chain may be expressed. Polyclonal anti-peptide antisera were generated and shown to be specific for DQA2 in Western blotting of Triton X-114 preparations of B lymphoblastoid cell lines. DQA2 expression is variable, although always at least threefold less than that of DQA1, and is found in all haplotypes examined. Immunoprecipitations of biotin-labeled cell surface proteins reveal that DQA2 appears at the cell surface. In keeping with others' findings, no evidence for an expressed DQB2 beta-chain was found, prompting investigation of the means by which DQA2 gains access to the cell surface. Antisera specific for allotypic and isotypic MHC class II beta-chains were used in sequential immunoprecipitation experiments to search for a beta-chain partner for DQA2, and a transfectant system was established in an attempt to promote pairing of DQA2 with a selected DQB1 (*0501) chain. In neither case was DQA2 found to dimerize with an MHC class II beta-chain. Despite this, DQA2 is capable of forming a complex with invariant chain.