Fas-ligand (CD178) and TRAIL synergistically induce apoptosis of CD40-activated chronic lymphocytic leukemia B cells.

Chronic lymphocytic leukemia (CLL) B cells become sensitive to Fas (CD95)-mediated apoptosis 3 to 5 days after CD40 ligation. However, CD4+ cytotoxic T lymphocytes (CTLs) can kill CLL B cells via a Fas-ligand (CD178)-dependent process within 24 hours after CD40 cross-linking, when ligation of CD95 alone is insufficient to induce apoptosis. In addition to CD95, CD40-activated CLL cells also express DR5, a receptor for tumor-necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) that is expressed by CD4+ CTL. In addition, CD40 ligation in vitro and in vivo induces CLL cells to express the proapoptotic protein, BH3 interacting domain death agonist (Bid), which can facilitate crosstalk between mitochondrial-dependent, apoptosis-inducing pathways and death receptors, such as death receptor 5 (DR5). To evaluate whether ligation of CD95 and/or DR5 can induce apoptosis of CD40-activated CLL cells, we generated artificial cytotoxic effector cells that express both human TRAIL and CD178 (Chinese hamster ovary [CHO]-CD178/TRAIL) or only TRAIL (CHO-TRAIL) or CD178 (CHO-CD178). CHO-CD178/TRAIL cells were significantly more effective in killing CD40-activated CLL cells than either CHO-TRAIL or CHO-CD178 and, unlike the latter, could kill CLL cells 24 hours after CD40 ligation. We conclude that CD40 ligation induces CLL cells to express the proapoptotic molecule Bid and the death receptors CD95 and DR5, the latter of which can act synergistically to induce caspase-dependent apoptosis of CD40-activated CLL B cells.