Evasion of apoptosis and immune recognition during host adaptation of Chlamydia and Chlamydia-like bacteria
Final Report Abstract
Chlamydiae species are a group of very different, obligate intracellular bacteria. Some (such as Parachlamydia acanthamoebae (Pac) are symbionts of amoebae and cannot normally grow in human cells, others (for instance Chlamydia trachomatis) infect human epithelial cells and cause disease. This project tested the hypothesis that either differential recognition of the bacteria, and subsequent induction of apoptosis, or the differential expression of anti-apoptotic activities by the various bacterial species, contributes to this host tropism. We tested this hypothesis in various ways and pursued three lines of research. First we tested the transcriptional response to Pac in insect cells. Pac had been found to induce apoptosis in these cells but to be able to grow when apoptosis was inhibited. A number of potentially relevant transcriptional events were identified, the most promising is the down-regulation of the important insect regulator of caspase-activity, Diap1. In the second part of the project, we made the surprising observation that C. trachomatis actually has pro-apoptotic activity, which however does not induce apoptosis. This was surprising on the two levels, first that such activity can be detected although C. trachomatis has very well documented, strong anti-apoptotic activities; secondly, because a ‘sub-lethal activation’ of the apoptosis apparatus, although now established in the literature, at the time was very surprising. We investigated this pro-apoptotic activity more closely and arrived at the conclusion that C. trachomatis, as well as all other investigated intracellular pathogens, is recognized by the human epithelial cells in a process that involves such sub-lethal signalling of the mitochondrial apoptosis apparatus, involving Bax/Bak and activation the DNAse CAD. We obtained evidence that this signalling can be part of the immune response of these non-professional immune cells. Infected cells lacking the mitochondrial apoptosis apparatus secreted less cytokine and were reduced in their capacity to contain growth of C. trachomatis. The mitochondrial apoptosis apparatus therefore appears to act as part of the innate immune response to infection. In the third part of the project, we tested directly for apoptosis-induction by Pac and its relevance to infection of mammalian cells. Pac induced apoptosis in human epithelial (HeLa) cells. Surprisingly, when mitochondrial apoptosis was blocked by genetically interfering with the Bcl-2-signalling machinery of mitochondrial apoptosis, Pac was able to grow in these cells; similar findings were made in mouse macrophages. In macrophages, some role for the cytokine TNF was found in this induction of apoptosis and reduction of growth. We are currently conducting final confirmatory experiments to this finding. Taken together, this project identified mechanisms of recognition and apoptosis induction of evolutionarily very distinct species of Chlamydiae. It provides strong evidence that apoptosis can serve as a determining factor of host tropism of these bacteria, and the human-pathogenic Chlamydiaceae may indeed require their anti-apoptotic activity to be able to grow in human cells and to infect humans.
Publications
-
(2019) A non-death function of the mitochondrial apoptosis apparatus in immunity. EMBO J. 38(11)
Brokatzky, D., Dörflinger, B., Haimovici, A., Weber, A., Kirschnek, S., Vier, J., Metz, A., Henschel, J., Steinfeldt, T., Gentle, I.E. and Häcker, G.
