title: IFN-gamma-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.
authors.affiliation: Unité des Rickettsies, Unité Mixte de Recherche 6020, Centre National de la Recherche Scientifique, Faculté de Médecine, Université de la Méditerranée, Marseille, France. jdellacas@yahoo.com
authors.forename: Jérôme
authors.initials: J
authors.lastname: Dellacasagrande
authors.forename: Eric
authors.initials: E
authors.lastname: Ghigo
authors.forename: Didier
authors.initials: D
authors.lastname: Raoult
authors.forename: Christian
authors.initials: C
authors.lastname: Capo
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Mege
mesh_terms: D017209:Apoptosis; D018821:CD18 Antigens / Q000378:metabolism; D053148:Caspase 3; D020169:Caspases / Q000378:metabolism; D002448:Cell Adhesion; D002460:Cell Line; D002462:Cell Membrane / Q000276:immunology; D016997:Coxiella burnetii / Q000276:immunology / Q000472:pathogenicity*; D003602:Cytotoxicity, Immunologic; D004789:Enzyme Activation; D006801:Humans; D007371:Interferon-gamma / Q000378:metabolism / Q000494:pharmacology*; D009000:Monocytes / Q000187:drug effects / Q000276:immunology* / Q000382:microbiology* / Q000473:pathology; D011994:Recombinant Proteins; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism*
chemical_list: D018821:CD18 Antigens; D011994:Recombinant Proteins; D014409:Tumor Necrosis Factor-alpha; D007371:Interferon-gamma; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases