B cell receptor cross-linking triggers a caspase-8-dependent apoptotic pathway that is independent of the death effector domain of Fas-associated death domain protein

We have previously reported that B cell receptors, depending on the degree to which they are cross-linked, can promote apoptosis in various human B ce ll types. In this study, we show that B cell receptors can trigger two apop totic pathways according to cross-linking and that these pathways control m itochondrial activation in human Burkitt's lymphoma cells. Whereas soluble anti-tx Ab triggers caspase-independent mitochondrial activation, cross-lin ked anti-g Ab induces an apoptotic response associated with a caspase-depen dent loss of mitochondrial transmembrane potential. This B cell receptor-me diated caspase-dependent mitochondrial activation is associated with caspas e-8 activation. We show here that caspase-8 inhibitors strongly decrease cr osslinking-dependent B cell receptor-mediated apoptosis in Burkitt's lympho ma BL41 cells. These inhibitors act upstream from the mitochondria as they prevented the loss of mitochondrial membrane potential observed in B cell r eceptor-treated BL41 cells. Caspase-8 activation in these cells was also ev ident from the detection of cleaved fragments of caspase-8 and the cleavage of specific substrates, including Bid. Our data show that cross-linked B c ell receptors induced an apoptotic pathway involving sequential caspase-8 a ctivation, loss of mitochondrial membrane potential, and the activation of caspase-9 and caspase-3. Cells expressing a dominant negative mutant of Fas -associated death domain protein were sensitive to cross-linked B cell rece ptor-induced caspase-8 activation and apoptosis; therefore, this caspase-8 activation was independent of the death effector domain of Fas-associated d eath domain protein.