A novel mechanism for TNFR-associated factor 6-dependent CD40 signaling

Members of the TNFR family play critical roles in the regulation of the immune system. One member of the family critical for efficient activation of T-dependent humoral immune responses is CD40, a cell surface protein expressed by B cells and other APC. The cytoplasmic domain of CD40 interacts with several members of the TNFR-associated factor (TRAF) family, which link CD40 to intracellular signaling pathways. TRAF2 and 6 appear to play particularly important roles in CD40 signaling. Previous studies suggest that the two molecules have certain overlapping roles in signaling, but that unique roles for each molecule also exist. To better define the roles of TRAF2 and TRAF6 in CD40 signaling, we used somatic cell gene targeting to generate TRAF-deficient mouse B cell lines. A20.2J cells deficient in TRAF6 exhibit marked defects in CD40-mediated JNK activation and the up-regulation of CD80. Our previous experiments with TRAF2-deficient B cell lines suggest that TRAF6 and TRAF2 may have redundant roles in CD40-mediated NF-kappaB activation. Consistent with this hypothesis, we found CD40-mediated activation of NF-kappaB intact in TRAF6-deficient cells and defective in cells lacking both TRAF2 and TRAF6. Interestingly, we found that TRAF6 mutants defective in CD40 binding were able to restore CD40-mediated JNK activation and CD80 up-regulation in TRAF6-deficient cells, indicating that TRAF6 may be able to contribute to certain CD40 signals without directly binding CD40.