Microglial STAT1-sufficiency is required for resistance to toxoplasmic encephalitis

Toxoplasma gondii is a ubiquitous intracellular protozoan parasite that establishes a life-long chronic infection largely restricted to the central nervous system (CNS). Constant immune pressure, notably IFN-gamma-STAT1 signaling, is required for preventing fatal pathology during T. gondii infection. Here, we report that abrogation of STAT1 signaling in microglia, the resident immune cells of the CNS, is sufficient to induce a loss of parasite control in the CNS and susceptibility to toxoplasmic encephalitis during the early stages of chronic infection. Using a microglia-specific genetic labeling and targeting system that discriminates microglia from blood-derived myeloid cells that infiltrate the brain during infection, we find that, contrary to previous in vitro reports, microglia do not express inducible nitric-oxide synthase (iNOS) during T. gondii infection in vivo. Instead, transcriptomic analyses of microglia reveal that STAT1 regulates both (i) a transcriptional shift from homeostatic to "disease-associated microglia" (DAM) phenotype conserved across several neuroinflammatory models, including T. gondii infection, and (ii) the expression of anti-parasitic cytosolic molecules that are required for eliminating T. gondii in a cell-intrinsic manner. Further, genetic deletion of Stat1 from microglia during T. gondii challenge leads to fatal pathology despite largely equivalent or enhanced immune effector functions displayed by brain-infiltrating immune populations. Finally, we show that microglial STAT1-deficiency results in the overrepresentation of the highly replicative, lytic tachyzoite form of T. gondii, relative to its quiescent, semi-dormant bradyzoite form typical of chronic CNS infection. Our data suggest an overall protective role of CNS-resident microglia against T. gondii infection, illuminating (i) general mechanisms of CNS-specific immunity to infection (ii) and a clear role for IFN-STAT1 signaling in regulating a microglial activation phenotype observed across diverse neuroinflammatory disease states. Author summaryThe brain, an immune-privileged organ, can be invaded and colonized by pathogens such as the opportunistic parasite, Toxoplasma gondii. How microglia, the resident immune cells of the brain, provide resistance to infection is an active area of investigation. In this study, we used a genetic approach to generate and study mice with microglia that lack STAT1, a critical transcription factor that confers protection against intracellular pathogens in both humans and mice. We find that despite robust activation and recruitment of immune cells from the blood to the brain during infection, STAT1 deficiency in microglia leads to increased brain parasite burden and uniform lethality in mice when challenged with T. gondii. Our bioinformatic analyses also indicate that STAT1 in microglia regulates (i) the expression of large families of genes associated with parasite killing (ii) a microglial activation state that has been classically seen in neurodegeneration. Our findings identify mechanisms by which microglia contribute to parasite control and contribute to a greater understanding of their cellular physiology during neuroinflammation.