CD40-mediated signals inhibit the binding of TNF receptor-associated factor 2 to the CD40 cytoplasmic domain

TNF receptor-associated factor 2 (TRAF2) is a signal-transducing protein associated with the CD40 cytoplasmic domain. It has been hypothesized that during signal transduction, TRAF2 must be released from CD40 in order for it to interact with downstream signaling molecules. We found that CD40 and TRAF2 were constitutively associated with each other in a human B cell line. Following stimulation with an anti-CD40 Ab, a decrease in the amount of CD40-associated TRAF2 was observed that could not be explained by a change in total level of either of the proteins. These results, as well as similar findings obtained with 293 cells overexpressing CD40 and TRAF2, suggested that CD40-mediated signals inhibited the CD40-TRAF2 interaction. We then conducted binding studies using CD40 cytoplasmic domain fusion proteins and TRAF2 derived from either control or CD40-stimulated cell lines. These in vitro studies also indicated that the binding of TRAF2 to the CD40 cytoplasmic domain was inhibited by CD40 stimulation. The results of these experiments, as well as differences between the in vitro and in vivo findings, indicated that multiple mechanisms were involved in the inhibition of the CD40-TRAF2 interaction by CD40 signals. Possible mechanisms of inhibition are discussed based on mapping of the TRAF2 binding site on the CD40 cytoplasmic domain.