Toxoplasma gondii infection confers resistance against Bim_S-induced apoptosis by preventing the activation and mitochondrial targeting of pro-apoptotic Bax

In order to accomplish their life style, intracellular pathogens, including the apicomplexan Toxoplasma gondii, subvert the innate apoptotic response of infected host cells. However, the precise mechanisms of parasite interference with the mitochondrial apoptotic pathway remain unknown. Here, we used the conditional expression of the BH3-only protein Bim_S to pinpoint the interaction of T. gondii with the intrinsic pathway of apoptosis. Infection of epithelial cells with T. gondii dose-dependently abrogated Bim_S-triggered release of cytochrome c from host-cell mitochondria into the cytosol, induction of activity of caspases 3, 7 and 9, and chromatin condensation. Furthermore, inhibition of apoptosis in parasite-infected lymphocytes counteracted death of Toxoplasma-infected host cells. Although total cellular levels and mitochondrial targeting of Bim_S was not altered by the infection, the activation of pro-apoptotic effector proteins Bax and Bak was strongly impaired. Inhibition of Bax and Bak activation by T. gondii was seen with regard to their conformational changes, the cytosol-to-mitochondria targeting and the oligomerization of Bax but not their cellular protein levels. Blockade of Bax and Bak activation was not mediated by the upregulation of anti-apoptotic Bcl-2-like proteins following infection. Further, the BH3-mimetic ABT-737 failed to overcome the Toxoplasma-imposed inhibition of Bim_S-triggered apoptosis. These results indicate that T. gondii targets activation of pro-apoptotic Bax and Bak to inhibit the apoptogenic function of mitochondria and to increase host-cell viability.