Membrane-bound CD154, but not CD40-specific antibody, mediates NF-chi B-independent IL-6 production in B cells

CD40, a member of the tumor necrosis factor receptor (TNF-R) superfamily, i s expressed on the surface of B cells, where its engagement results in IL-6 secretion. in this study, we characterize the specific molecular requireme nts for CD40-mediated IL-6 production. Engagement of CD40 on either a B cel l line or normal mouse splenic B cells with a membrane-bound form of CD154 (also known as CD40L or gp39) induced IL-6 secretion as well as up-regulati on of IL-6 mRNA, but cross-linking CD40 with agonistic anti-CD40 mAb did no t, although these mAb induce many other CD40 activation events, including t he nuclear translocation of the transcription factor NF-kappa B. Using a mo use B cell line stably transfected with various human CD40 (hCD40) cytoplas mic truncation and point mutants, we show that the region from amino acids 202 to 225 in the cytoplasmic domain of CD40 is necessary for IL-6 secretio n. However, the carboxy-terminal 32 amino acids are not, although these res idues are required for CD40-mediated NF-kappa B activation. in addition, CD 40 mutants previously shown to lack binding to TRAF2 and -3 are fully capab le of inducing IL-6 production. Thus, CD40-mediated IL-6 induction is indep endent of NF-kappa B activation and the binding of TRAF2 and -3, but CD40 m ust be engaged by trimeric CD154 on cell membranes to activate production o f IL-6.