Murine Immunity-Related GTPase M Proteins Regulate Immunity to Intracellular Infections

Abstract

Host cells are equipped with a robust defense program to defend against intracellular pathogens. The cytokine gamma-interferon (IFNg) activates expression of hundreds of genes that identify intracellular pathogens such as Chlamydia trachomatis and Toxoplasma gondii, target pathogen-containing vacuoles (PVs) with host effector proteins, and destroy the pathogen. In mice, the IFNg-induced defense program targeting Toxoplasma and C. trachomatis is controlled by three Immunity-related GTPase M (Irgm) proteins, which coordinate host effectors to efficiently target and destroy intracellular pathogens. The mechanisms by which Irgm proteins regulate cell-autonomous immunity are not well understood. Human cell-autonomous immunity differs substantially from the mouse system, and while the sole human IRGM ortholog is not involved in cell-autonomous immunity to Toxoplasma or Chlamydia, it has been linked with a variety of inflammatory diseases including Crohn’s disease and sepsis. The mechanism linking polymorphisms in human IRGM with autoimmunity is unclear, but murine Irgm knockout models recapitulate similar phenotypes of hyperinflammation associated with human IRGM. Additionally, mouse and human Irgm proteins have been implicated in similar fundamental cellular processes including autophagy and mitochondrial function. Collectively these findings support the central hypothesis that murine Irgm proteins regulate cellular membrane biology to both coordinate cell-autonomous immunity and regulate inflammation. In order to investigate this hypothesis, I leveraged existing Irgm-/- models as well as our novel pan-Irgm-/- mouse lacking all three Irgm proteins. I demonstrated that pan-Irgm-/- cells are completely defective for the targeting of Toxoplasma gondii with host effectors, while Chlamydia trachomatis remains robustly targeted by most but not all effectors. In spite of the differences in host-directed targeting of these two vacuolar pathogens, pan-Irgm-/- mice were defective for resistance to Toxoplasma and Chlamydia trachomatis both in vitro and in vivo. In pan-Irgm-/- mice infected with Chlamydia trachomatis, adaptive immunity compensated for the defect in cell-autonomous immunity to ultimately clear the infection. Consistent with the hypothesis that Irgm proteins limit inflammation, pan-Irgm-/- mice demonstrated increased inflammation in genital Chlamydia trachomatis infection, and increased inflammation and immunopathology in infection with Chlamydia muridarum. Collectively, my findings shed light on the elusive mechanisms of murine Irgm proteins, clarifying their roles in cell-autonomous immunity, defense against intracellular pathogens in vivo, and regulation of inflammation.