A dual role for both CD40-ligand and TNF-α in controlling human B cell death

Members of the TNF-R family are instrumental in controlling lymphoid cell death and survival. A major role in the regulation of murine and human B cell survival and differentiation has been attributed to CD40/CD40-ligand (CD40-L) interactions, but recent in vitro and in vivo data implicate that other receptor-ligand pairs might also be involved. We have used the human Burkitt lymphoma cell line Ramos as a model system to identify additional TNF-R/TNF family members that are implicated in the regulation of B cell apoptosis. Ligation of B cell receptor (BCR) with anti-IgM mAb for 48 h induced apoptosis in approximately 68% of the Ramos B cells. Interestingly, not only CD40 mAb but also rTNF-alpha could efficiently inhibit BCR-induced B cell death. In addition, activated T cells also prevented BCR-triggered apoptosis, and this effect was inhibited completely by a combination of blocking Abs against CD40-L and TNF-alpha. In contrast to the strong effect of BCR ligation, APO-1 mAb induced apoptosis in only +/- 18% of the Ramos cells after 48 h. Noticeably, addition of CD40 mAb or rTNF-alpha increased the percentage of cells (+/- 46%) undergoing apoptosis, which correlated with an increase of Fas/APO-1 membrane expression induced by CD40 or TNF-R ligation. Taken together, we show that CD40/CD40-L and TNF-R/TNF-alpha interactions not only postpone or prevent B cell death, but are also involved in sensitizing B cells for Fas-ligand (Fas-L)-dependent death.