Immunity-Related GTPases in Cell-autonomous Resistance against Toxoplasma gondii

Immunity-related GTPases are interferon-inducible, indispensable resistance factors against a variety of intracellular bacterial and protozoal pathogens in mice. Irgm1 is the most powerful member of the IRG family. Previous studies showed that Irgm1 localizes mainly to the Golgi apparatus, and relocalizes to phagocytic cups during phagocytosis. Macrophages deficient in Irgm1 have impaired acidification of M. tuberculosis-containing phagosomes. In addition, a role of Irgm1 in promoting autophagy has been proposed based on EGFP-tagged Irgm1 overexpression experiments. During T. gondii infection, at least 6 IRG family members translocate from their resting localization onto the T. gondii parasitophorous vacuolar membrane and contribute to disruption of the T. gondii vacuoles. In the present study, Irgm1 has been shown to be associated with late endocytic/lysosomal compartments in addition to Golgi apparatus. N or C terminally EGFP-tagged Irgm1 is not localized to the Golgi and mislocalizes to the early/recycling endosomes as well as other unknown vesicular structures. Both Golgi and lysosomal localization of Irgm1 are mediated by a C-terminal amphipathic helix. Although Irgm1 is indispensable for mice to survive T. gondii infection, Irgm1 is not recruited to the T. gondii vacuole and is not absolutely necessary for the IFN-gamma-dependent cell-autonomous control of T. gondii. Using live cell imaging, the present experiments show the sequence of events by which IFN-gamma treated cells eliminate T. gondii. After the invasion of T. gondii, the vacuoles are coated with IRG proteins and subsequently disrupted. The stripped T. gondii are released to the cytosol and killed. The host cells finally undergo necrosis, and this presumably serves as a cell-autonomous resistance mechanism. The necrotic host cells release the proinflammatory mediator, HMGB1, to further enhance the host immune responses. Virulent T. gondii strains do not elicit IFN-gamma-dependent host cell death, but could not inhibit the death process once it is initiated by co-infection with an avirulent strain. The presented results show that in T. gondii infection, the IRG proteins contribute to the disruption of the T. gondii vacuole and this could be a trigger for the subsequent parasite killing as well as host cell death