Coxiella burnetiiInhibits Apoptosis in Human THP-1 Cells and Monkey Primary Alveolar Macrophages

Abstract

Coxiella burnetii, the cause of human Q fever, is an aerosol-borne, obligate intracellular bacterium that targets host alveolar mononuclear phagocytic cells during infection. In all cell types examined, C. burnetii establishes a replicative niche in a lysosome-like parasitophorous vacuole where it carries out a lengthy infectious cycle with minimal cytopathic effects. The persistent and mild nature of C. burnetii infection in vitro suggests that the pathogen modulates apoptosis to sustain the host cell. In the current study, we examined the ability of C. burnetii to inhibit apoptotic cell death during infection of human THP-1 monocyte-derived macrophages and primary monkey alveolar macrophages. C. burnetii-infected cells demonstrated significant protection from death relative to uninfected cells following treatment with staurosporine, a potent inducer of intrinsic apoptosis. This protection correlated with reduced cleavage of caspase-9, caspase-3, and poly(ADPribose) polymerase (PARP), all proteolytic events that occur during apoptosis. Reduced PARP cleavage was also observed in cells treated with tumor necrosis factor alpha to induce extrinsic apoptosis. Apoptosis inhibition was a C. burnetii-driven process as infected cells treated with rifampin or chloramphenicol, inhibitors of bacterial RNA and protein synthesis, respectively, showed significantly reduced protection against staurosporine-induced apoptosis. C. burnetii infection affected the expression of multiple apoptosis-related genes and resulted in increased synthesis of the antiapoptotic proteins A1/Bfl-1 and c-IAP2. Collectively, these data suggest that C. burnetii modulates apoptotic pathways to inhibit host cell death, thus providing a stable, intracellular niche for the course of the pathogen's infectious cycle.

Coxiella burnetii modulates Beclin 1 and Bcl-2, preventing host cell apoptosis to generate a persistent bacterial infection

“…Thus, Cb is able to produce a persistent infection by means of inhibiting apoptosis. 34,35 Interestingly, we have observed that in Cb-infected cells numerous mitochondria are in close apposition to the CRV (Colombo MI et al unpublished observation). This intimate association suggests that the bacteria may actively manipulate critical mitochondria functions, such as the intrinsic apoptotic pathway.…”

“…37 Recent results have shown that Cb infection is able to inhibit the apoptosis induced by UV light and staurosporine in host cells. 34,35 As Cb infection is affected by Beclin 1 and Bcl-2, we wondered whether overexpression of these proteins would have an effect on the inhibition of apoptosis by Cb. Supplementary Figure 4a type of death was indeed apoptotic, we incubated the cells with Z-VAD-fmk (apoptosis inhibitor) for 2 h before UV irradiation.…”

“…42 Similarly, recent studies show that Cb inhibits apoptosis induction in host cells and this effect is mediated by the bacterial protein synthesis. 34,35 Cumulative evidence indicates that apoptosis and autophagy are interconnected pathways, implying that a common stimulus triggers the response of both pathways or that the activation of one of them excludes the other. 36 Consequently, we explored a possible role for Beclin 1 and Bcl-2 in the anti-apoptotic effect of Cb.…”

“…35 Besides, a recent study has shown that Cb infection modulates the expression of apoptosis-related genes. 34 In view of these results, an interesting point to analyze is how Cb modulates both autophagy and apoptosis, two pathways that are intimately associated, to develop a successful infection. In this study, we show that Cb recruits Beclin 1 to the CRV and that the development of this vacuole is favored by the overexpressed Beclin 1.…”

“…It has been shown that Cb infection inhibits apoptosis in host cells, allowing bacterial survival and multiplication. 34,35 One of the proposed mechanisms by which Cb exerts its anti-apoptotic effect is by preventing the release of cytocrome c, thus avoiding the activation of executioner caspases. 35 Besides, a recent study has shown that Cb infection modulates the expression of apoptosis-related genes.…”