Amoebal Endosymbiont <em>Protochlamydia</em> Induces Apoptosis to Human Immortal HEp-2 Cells

<div><p><em>Protochlamydia</em>, an environmental chlamydia and obligate amoebal endosymbiotic bacterium, evolved to survive within protist hosts, such as <em>Acanthamobae</em>, 700 million years ago. However, these bacteria do not live in vertebrates, including humans. This raises the possibility that interactions between <em>Protochlamydia</em> and human cells could induce a novel cytopathic effect, leading to new insights into host-parasite relationships. Therefore, we studied the effect of <em>Protochlamydia</em> on the survival of human immortal cell line, HEp-2 cells and primary peripheral blood mononuclear cells (PBMC). Using mainly 4′,6-diamidino-2-phenylindole staining, fluorescent <em>in situ</em> hybridization, transmission electron microscopy, and also TUNEL and Transwell assays, we demonstrated that the <em>Protochlamydia</em> induced apoptosis in HEp-2 cells. The attachment of viable bacterial cells, but not an increase of bacterial infectious progenies within the cells, was required for the apoptosis. Other chlamydiae [<em>Parachlamydia acanthamoebae</em> and <em>Chlamydia trachomatis</em> (serovars D and L2)] did not induce the same phenomena, indicating that the observed apoptosis may be specific to the <em>Protochlamydia</em>. Furthermore, the bacteria had no effect on the survival of primary PBMCs collected from five volunteers, regardless of activation. We concluded that <em>Protochlamydia</em> induces apoptosis in human-immortal HEp-2 cells and that this endosymbiont could potentially be used as a biological tool for the elucidation of novel host-parasite relationships.</p>