Ia molecules play a key role in antigen recognition by T lymphocytes. To analyze the structural features of the individual α and β chains relevant to the assembly of intact Ia molecules, mouse fibroblasts were contransfected with various combinations of haplotype- and isotype-mismatched Ia α/β gene pairs. Two important points emerged. First, the level of surface expression of a given haplotype-mismatched AαAβ pair appears to depend upon the α and β chain alleles involved. Second, transfection with some isotype-mismatched combinations such as EαdAβd results in a significant level of surface expression of a stable mixed-isotype dimer, which also appears to be normally expressed at a low level by an Iad-positive B lymphoma. Moreover, a T-cell hybridoma specific for human gamma globulin and restricted by the Ed molecule was found to be efficiently stimulated by the EαdAβd-positive transfectant in the presence of antigen. The stimulation was specifically inhibited by monoclonal antibodies directed to either the Ia or the L3T4 molecule. These findings suggest that the estimates of the potential number of Ia molecules available in an animal for restricting T-lymphocyte recognition of antigens must be revised
