Toxoplasma gondii inhibits cytochrome c-induced caspase activation in its host cell by interference with holo-apoptosome assembly.

Inhibition of programmed cell death pathways of mammalian cells often facilitates the sustained survival of intracellular microorganisms. The apicomplexan parasite Toxoplasma gondii is a master regulator of host cell apoptotic pathways. Here, we have characterized a novel anti-apoptotic activity of T. gondii. Using a cell-free cytosolic extract model, we show that T. gondii interferes with the activities of caspase 9 and caspase 3/7 which have been induced by exogenous cytochrome c and dATP. Proteolytic cleavage of caspases 9 and 3 is also diminished suggesting inhibition of holo-apoptosome function. Parasite infection of Jurkat T cells and subsequent triggering of apoptosome formation by exogenous cytochrome cin vitro and in vivo indicated that T. gondii also interferes with caspase activation in infected cells. Importantly, parasite inhibition of cytochrome c-induced caspase activation considerably contributes to the overall anti-apoptotic activity of T. gondii as observed in staurosporine-treated cells. Co-immunoprecipitation showed that T. gondii abolishes binding of caspase 9 to Apaf-1 whereas the interaction of cytochrome c with Apaf-1 remains unchanged. Finally, T. gondii lysate mimics the effect of viable parasites and prevents holo-apoptosome functionality in a reconstituted in vitro system comprising recombinant Apaf-1 and caspase 9. Beside inhibition of cytochrome c release from host cell mitochondria, T. gondii thus also targets the holo-apoptosome assembly as a second mean to efficiently inhibit the caspase-dependent intrinsic cell death pathway.