Sibley, L. A. | Search bioRxiv

bioRxiv2022

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.

immunology↗

bioRxiv2022

Meningeal lymphatic drainage promotes T cell responses against Toxoplasma gondii but is dispensable for parasite control in the brain

The discovery of meningeal lymphatic vessels that drain the central nervous system (CNS) has prompted new insights into how neuroinflammation develops. In this study, we examined how T cell responses against CNS-derived antigen develop in the context of infection. We found that meningeal lymphatic drainage promotes CD4+ and CD8+ T cell responses against the neurotropic parasite Toxoplasma gondii, and we discovered changes in the antigen-presenting cell compartment of the dural meninges that potentially support this process. Indeed, compared to uninfected controls, mice chronically infected with T. gondii displayed a ten-fold increase in the total number of dendritic cells in the dural meninges. These cells upregulated MHC class II, CD80, and CD86 expression, sampled cerebrospinal fluid-derived protein, and were detected within meningeal lymphatic vessels in greater numbers during infection. Disrupting meningeal lymphatic drainage via ligation surgery resulted in reduced dendritic cell number and maturation in the deep cervical lymph nodes and impaired CD4+ and CD8+ T cell activation, proliferation, and IFN-{gamma} production at this site. Surprisingly, parasite-specific T cell responses in the brain remained intact following ligation, which may be due to activation of T cells at alternative sites during chronic infection, including lymph nodes that drain non-CNS tissue. Collectively, our work reveals that CNS lymphatic drainage supports the development of peripheral T cell responses against T. gondii but is nonetheless dispensable for host protection of the brain.

immunology↗