Studies on the Nature of the Relationship Between Ia Antigens and Fc Receptors on Murine B Lymphocytes

Abstract

Possible mechanisms for the specific inhibition by anti-Ia antibody of the binding of Ig complexes to B lymphocyte Fc receptors were evaluated. Capping of B cell Fc receptors did not lead to redistribution of antigens determined by genes in the I region or the I-A or I-C, I-E subregions as analyzed by indirect fluorescence and visual microscopy. In addition, the amounts of these antigens which could be quantitatively detected with indirect fluorescence and flow microfluorometry were unaltered by capping of Fc receptors. Thus, Ia antigens and B lymphocyte Fc receptors do not appear to be identical. Quantitation of Ia antigens after binding of Ig complexes to B cell Fc receptors under noncapping conditions revealed partial (6 to 10%) inhibition of detection of antigens determined by genes in the whole I region but no inhibition of I-A or I-C, I-E subregion antigens. Thus, the majority of Ia antigens do not, in the native state, appear to lie in close physical proximity to B cell Fc receptors. In view of the fact that the Fc portion of anti-Ia antibodies is not required to produce inhibition of binding to B cell Fc receptors, these observations suggest that this inhibition is due to a ligand-induced alteration of Ia antigens which leads to an interaction with the Fc receptors of B lymphocytes. Such Ia antigen-Fc receptor interactions may play a role in B cell activation.