CD40 ligation in B cell activation, isotype switching and memory development.

Interaction of CD40 on B cells with its ligand on activated T helper cells is crucial for the generation of mature T-dependent antibody responses. Experimental observations suggest that CD40 ligation acts at several points in B cell differentiation; however, it is not clear whether the consequences of ligation are direct or indirect. While its role in B cell activation is clearly direct we discuss the several ways in which it might drive isotype switching, the influence that it has on the development of memory B cells and the possibility that certain types of T-dependent antibody response are CD40L-independent. Experiments in which the ligation of CD40 is blocked in vivo reveal that early CD40 signals are important for the development of memory B cells but this may only reflect the ability of 'memory precursors' generated by CD40 ligation to accept subsequent programming or survival signals.