A role for MHC class II antigens in B-cell activation.

Class II antigens of the major histocompatibility complex (MHC) have a well-defined role in restricting cellular interactions and presenting processed antigen to T cells. In addition, a fundamental role for Class II antigens in cellular activation has been suggested, following studies demonstrating that Class II antigen binding alters the proliferation of various cell types. This is further supported by biochemical evidence of signal transduction by second messengers after ligation of the Class II antigens. We have investigated the role of HLA Class II antigens in the activation of B cells. Both activated and resting B cells proliferate in the presence of Sepharose--conjugated anti-Class II antibodies. This proliferation was not epitope-restricted and was unaffected by low m.w. BCGF. Intracellular free calcium elevation was also examined as a marker of cellular activation. (Ca2+)i was increased after the binding and cross-linking of an anti-DR antibody. The above results further support the role of Class II antigens as signal-transducing molecules.