Microglial STAT1-sufficiency is required for resistance to toxoplasmic encephalitis

Abstract

Toxoplasma gondiiis 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-γ-STAT1 signaling, is required for preventing fatal pathology duringT.gondiiinfection. 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 previousin vitroreports, microglia do not express inducible nitric-oxide synthase (iNOS) duringT.gondiiinfectionin 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, includingT.gondiiinfection, and (ii) the expression of anti-parasitic cytosolic molecules that are required for eliminatingT.gondiiin a cell-intrinsic manner. Further, genetic deletion ofStat1from microglia duringT.gondiichallenge 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 ofT.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 againstT.gondiiinfection, 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.