CD40‐CD40 ligand interactions stimulate B cell antigen processing

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Abstract
The interactions between B cell CD40 and T cell CD40 ligand (CD40L) have been shown recently to play an important role in T cell‐dependent activation of B cells. Here, we show that the ligation of CD40 stimulates the processing of antigen by B cells. The activation of an antigen‐specific T cell hybrid by B cells co‐cultured with insect cells expressing recombinant CD40L or with a CD40‐specific monoclonal antibody requires less antigen and fewer B cells compared to control cells. The augmentation was observed both for processing initiated by antigen binding to and cross‐linking the surface immunoglobulin, and processing of antigen taken up by fluid‐phase pinocytosis. CD40 appears to affect a step in the intracellular processing of antigen, as CD40 has no effect on the presentation of an antigenic peptide which does not require processing. In addition, the CD40‐induced augmentation of processing is not attributable to the effect of CD40 ligation on the cell surface expression of B7, LFA‐1 or CD23. CD40 ligation does not affect the biosynthesis of the class II Ek molecules, and although ligation of CD40 induces B cell proliferation, the augmentation of processing does not require proliferation. The ability of CD40 to stimulate B cell antigen processing has the potential to influence significantly the outcome of antigen‐dependent T cell‐B cell interactions.