Protection against CD95-induced apoptosis by chlamydial infection at a mitochondrial step.

Chlamydiae are obligate intracellular bacteria that infect human epithelial and myeloid cells. Previous work has established that chlamydiae are able to protect a cell against apoptosis induced by certain experimentally applied stimuli. Here we provide an analysis of this protective activity against the signal transduction during CD95-induced apoptosis. In HeLa cells overexpressing CD95, infection with Chlamydia trachomatis inhibited the appearance of apoptotic morphology, effector caspase activity, the activation of caspase-9 and -3, and the release of cytochrome c from mitochondria. However, caspase-8-processing and activity (measured as cleavage of Bid) were unaffected by the chlamydial infection. Similarly, infection with the species C. pneumoniae did not prevent the activation of caspase-8 but inhibited the appearance of effector caspase activity upon signaling through CD95. Furthermore, infection with C. trachomatis was able to inhibit CD95-induced apoptosis in Jurkat lymphoid cells, where a mitochondrial contribution is required, but not in SKW6.4 lymphoid cells, where caspase-8 directly activates caspase-3. Taken together, these data show that chlamydial infection can protect cells against CD95-induced apoptosis but only where a mitochondrial signaling step is necessary for apoptotic signal transduction.