CD40 induces resistance to TNF‐mediated apoptosis in a fibroblast cell line

CD40, a member of the TNF receptor family, has been characterized as an important T‐B cell interaction molecule. In B cells it co‐stimulates isotype switching, proliferation, adhesion and is involved in cell death regulation. In addition to B cells, CD40 expression was found on transformed cells and carcinomas. However, little is known about its functions in these cell types. Recent studies show that CD40 mediates the production of pro‐inflammatory cytokines in non‐hematopoietic cells, inhibits proliferation or induces cell death. In some cell types the apoptotic program triggered by CD40 is only executed when protein synthesis is blocked, suggesting the existence of constitutively expressed resistance proteins. Here we demonstrate that CD40, similar to the 55‐kDa TNF receptor (p55TNFR), has a dual role in the regulation of apoptosis in such cells. In the fibroblast cell line SV80 both CD40 and the p55TNFR trigger apoptosis when protein synthesis is blocked with cycloheximide (CHX). Simultaneous activation of both receptors results in markedly enhanced cell death. However, CD40 activation more than 4 h prior to a challenge with TNF/CHX paradoxically conferred resistance to TNF‐induced cell death. Protection correlated with NF‐κB induction and up‐regulation of the anti‐apoptotic zinc finger protein A20. Overexpression of A20 in turn rendered SV80 cells resistant to TNF cytotoxicity. In conclusion, our data provide evidence that CD40 may regulate cell death in non‐hematopoietic cells in a dual fashion: the decision upon apoptosis or survival of a CD40‐activated cell seems to depend on its ability to up‐regulate resistance factors.