Apaf-1 and caspase-9 are required for cytokine withdrawal-induced apoptosis of mast cells but dispensable for their functional and clonogenic death.

Cytokines promote survival of mast cells by inhibiting apoptotic pathways regulated by the Bcl-2 protein family. We previously showed that lymphocyte apoptosis can proceed via a Bcl-2-inhibitable pathway independent of the canonical initiator caspase, caspase-9, and its adaptor, Apaf-1. Here we report that mast cells lacking caspase-9 or Apaf-1 are refractory to apoptosis after cytotoxic insults but still lose effector function and ability to proliferate. In response to cytokine deprivation or DNA damage, fetal liver-derived mast cells lacking Apaf-1 or caspase-9 failed to undergo apoptosis. Nevertheless, the cytokine-starved cells were not functionally alive, because, unlike those overexpressing Bcl-2, they could not degranulate on Fcepsilon receptor stimulation or resume proliferation on re-addition of cytokine. Furthermore, mast cells lacking Apaf-1 or caspase-9 had no survival advantage over wild-type counterparts in vivo. These results indicate that the Apaf-1/caspase-9-independent apoptotic pathway observed in lymphocytes is ineffective in cytokine-deprived mast cells. However, although Apaf-1 and caspase-9 are essential for mast cell apoptosis, neither is required for the functional or clonogenic death of the cells, which may be due to mitochondrial dysfunction.