IFN-γ-Induced Apoptosis and Microbicidal Activity in Monocytes Harboring the Intracellular Bacterium Coxiella burnetii Require Membrane TNF and Homotypic Cell Adherence

Abstract

IFN-gamma is critical for the protection against intracellular bacteria through activation of the antimicrobial machinery of phagocytes. Coxiella burnetii, the etiological agent of Q fever, is a strictly intracellular bacterium that inhabits monocytes/macrophages. We previously showed that IFN-gamma induced C. burnetii killing by promoting the apoptosis of infected monocytes. We show in this study that IFN-gamma-induced apoptosis of infected monocytes was characterized by a time- and dose-dependent activation of caspase-3. IFN-gamma-mediated caspase-3 activation and C. burnetii killing depend on the expression of membrane TNF. Indeed, TNF was transiently expressed on the cell surface of infected monocytes a few hours after IFN-gamma treatment. In addition, anti-TNF Abs inhibited IFN-gamma-mediated caspase-3 activation whereas soluble TNF had no effect on infected cells. Concomitantly, IFN-gamma induced homotypic adherence of C. burnetii-infected monocytes. The latter required the interaction of beta(2) integrins with CD54. When adherence was disrupted by pipetting, by a combination of Abs specific for CD11b, CD18, and CD54, or by an antisense oligonucleotide targeting CD18 mRNA, both cell apoptosis and bacterial killing induced by IFN-gamma were inhibited. Thus, adherence via CD54/beta(2) integrins together with membrane TNF are required to eliminate C. burnetii-infected cells through cell contact-dependent apoptosis. Our results reveal a new component of the antimicrobial arsenal mobilized by IFN-gamma against infection by intracellular bacteria.

AttenuatedCoxiella burnetiiPhase II Causes a Febrile Response in Gamma Interferon Knockout and Toll-Like Receptor 2 Knockout Mice and Protects against Reinfection

“…Previous reports have shown that activation by IFN-␥ of guinea pigs monocytes, murine fibroblasts, and primary mouse macrophages controls the replication of C. burnetii (11,15,42,53). IFN-␥ can also regulate nitric oxide production by macrophages infected with C. burnetii in the replication of phase I (6) and the replication of NMII in fibroblasts activated with IFN-␥ (19).…”

Differential Requirements by CD4+ and CD8+ T Cells for Soluble and Membrane TNF in Control of Francisella tularensis Live Vaccine Strain Intramacrophage Growth

“…Cell surface TNF has been shown to mediate cytotoxic activities by CD4 ϩ T cells, as measured via L929 fibroblast lysis assays (25) and thus can act via mechanisms other than up-regulation of NO. Furthermore, TNF in combination with IFN-␥ can synergistically activate a diverse number of other macrophage antimicrobial activities, such as expression of the tryptophan-limiting enzyme indolamine 2, 3-oxygenase (26,27), or induction of apoptosis (28). The combined action of these two cytokines may also redirect the intracellular trafficking of LVS to a compartment that does not support replication of the organism.…”

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

“…The nature of this growth restriction is ill defined, with production of toxic reactive oxygen and nitrogen species implicated (9). Dellacasagrande et al (16,17) have also invoked a novel mechanism of IFN-␥-mediated killing of C. burnetii whereby the cytokine induces apoptosis in infected monocytes by up-regulating synthesis of membranebound TNF. However, under this scenario, it is unclear how apoptosis per se would kill C. burnetii.…”

“…(This time point was considered 0 h postinfection.) The phase I MOI has been previously shown to result in initial infection of a high percentage of THP-1 cells with a few bacteria (16,17). A lower MOI was used for phase II organisms because they are roughly 10-fold more infectious for cultured cells than phase I bacteria (57).…”