CD40 Ligation Counteracts Fas-induced Apoptosis of Human Dendritic Cells

Dendritic cells (DC) are cells of the hematopoletic system specialized in capturing antigens and initiating T cell-mediated immune responses. We show here that human DC generated in vitro by culturing CD34+ cord blood progenitor cells in granulocyte macrophage colony stimulating factor plus tumor necrosis factor-alpha express the Fas antigen (APO-1, CD95) and undergo apoptosis upon triggering of Fas by mAb. However, only a proportion of the cells die in response to Fas ligation, an observation that may be related to the virtual absence of the bcl-2 protein in about half of the cells. Ligation of DC CD40 by culture on CD40L-transfected fibroblastic cells up-regulates the expression of bcl-2 and, concomitantly, renders DC virtually resistant to Fas-induced apoptosis. Parallel experiments with mature, interdigitating dendritic cells (IDC) isolated from tonsils revealed that IDC express Fas but do not enter into apoptosis following Fas ligation, a finding that may be explained by their high levels of bcl-2. Thus, upon encountering antigen-specific T cells, DC become resistant to Fas-induced apoptosis, as a consequence of CD40 ligation and possibly by mechanisms associated to the up-regulation of bcl-2 protein expression.