Protective immunity to the pathogen Chlamydia is dependent on a robust IFN-γ response generated by innate and adaptive lymphocytes. Here we assess the role of the macrophage in orchestrating a protective response in vivo to the murine pathogen, Chlamydia muridarum. During acute pulmonary and peritoneal infection, resident macrophages in both sites are infected with C. muridarum and adopt an inflammatory phenotype. In the lung, this activation is restricted to interstitial macrophages, which harbor higher levels of C. muridarum 16sRNA than alveolar macrophages. We examined innate and adaptive lymphocyte activation in the peritoneal cavity with macrophage depletion and with adoptive transfer of infected macrophages. These experiments demonstrate macrophage activation correlates with a protective IFN-γ response and effective control of C. muridarum. These studies suggest that a quantitative or qualitative alteration in macrophages may play a key role in the development of Chlamydiaassociated diseases.Keywords: Chlamydia r IFN-γ r Infection r Lung r Macrophage Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionChlamydia are obligate intracellular bacteria which have adapted to their respective hosts over millions of years [1]. In humans, Chlamydia trachomatis is a common pathogen, with a prevalence of 40 million active ocular infections and 90 million genital infections [2], while Chlamydia pneumoniae is a common cause of community-acquired pneumonia [3]. As the majority of Correspondence: Robert D. Inman e-mail: Robert.Inman@uhn.on.caChlamydia infections are asymptomatic [3][4][5], this bacteria is likely ubiquitous to a degree that is currently not recognized. Despite the widespread nature of Chlamydia, the biology of infection and host defense are not as well understood as other major human pathogens. This is due in part to limited availability of genetically modified Chlamydia [6] and because of the methodological challenges of Chlamydia culture and quantification.Chlamydia cycles through two unique life-cycle stages. The extracellular elementary bodies (EB) are infectious but nonreplicative, while the intracellular reticulate bodies (RB) are replicative but noninfectious [7]. The host response to Chlamydia engages all arms of the immune response. Innate immunity is essential for C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 3418 Eric Gracey et al. Eur. J. Immunol. 2015. 45: 3417-3430 limiting excessive Chlamydia growth during the early stages of infection and adaptive immunity acts to specifically terminate infection [8,9]. At a molecular level, many immune-related factors are important in the response to Chlamydia, however IFN-γ appears to have a central role in protection against Chlamydia, including C. muridarum [10]. Clinical studies have correlated IFN-γ levels with susceptibility to Chlamydia and postinfection complications [11][12][13]. IFN-γ is produced during both innate and adaptive phases of the immu...