The in vivo function of a noncanonical TRAF2-binding domain in the C-terminus of CD40 in driving B-cell growth and differentiation.

The recruitment of tumor necrosis factor receptor-associated factors (TRAFs) 1, 2, 3, 5, and 6 to the CD40 cytoplasmic tail upon CD40 trimerization results in downstream signaling events that ultimately lead to CD40-dependent, thymus-dependent (TD) humoral immune responses. Previously, we have shown signaling through the C-terminal tail of CD40 in the absence of canonical TRAF-binding sites is capable of signaling through an alternative TRAF2-binding site. Here, we demonstrate that B cells from mice harboring CD40 with only the C-terminal tail can activate both canonical and noncanonical NFkappaB signaling pathways. Moreover, while lacking germinal center formation, several hallmarks of humoral immune responses including clonal B-cell activation/expansion, antibody isotype switching, and affinity maturation remain normal. This study demonstrates a new functional domain in CD40 that controls critical aspects of B-cell immunity in an in vivo setting.