T lymphocyte-dependent IL-10 down-regulates a cytokine storm driven by <i>Toxoplasma gondii</i> GRA24

As a model organism in the study of immunity to infection, <i>Toxoplasma gondii</i> has been instrumental in establishing key principles of host anti-microbial defense and its regulation. Here, we employed an attenuated uracil auxotroph strain of Type I <i>Toxoplasma</i> designated OMP to further untangle the early immune response to this parasitic pathogen. Experiments using αβ T cell-deficient <i>Tcrb</i>^<i>-/-</i> mice unexpectedly revealed that an intact αβ T lymphocyte compartment was essential to survive infection with OMP. Subsequent antibody depletion and knockout mouse experiments demonstrated contributions from CD4⁺ T cells and most predominantly CD8⁺ T cells in resistance. Using transgenic knockout mice, we found only a partial requirement for IFN-γ and a lack of requirement for Toll-like receptor (TLR) adaptor MyD88 in resistance. In contrast to other studies on <i>Toxoplasma</i>, the ability to survive OMP infection did not require IL-12p40. Surprisingly, T cell-dependent IL-10 was found to be critical for survival, and deficiency of this cytokine triggered an abnormally high systemic inflammatory response. We also found that parasite molecule GRA24, a dense granule protein that triggers TLR-independent IL-12 production, acts as a virulence factor contributing to death of OMP-infected <i>Tcrb</i>^<i>-/-</i> and <i>IL-10</i>^<i>-/-</i> mice. Furthermore, resistance against OMP was restored in <i>Tcrb</i>^<i>-/-</i> mice via monoclonal depletion of IL-12p40, suggesting that GRA24-induced IL-12 underlies the fatal immunopathology observed. Collectively, our studies provide insight into a novel and rapidly arising T lymphocyte-dependent anti-inflammatory response to <i>T. gondii</i> which operates independently of MyD88 and IL-12 and that depends on the function of parasite-dense granule protein GRA24.IMPORTANCEAs a model infectious microbe and an important human pathogen, the apicomplexan <i>Toxoplasma gondii</i> has provided many important insights into innate and adaptive immunity to infection. We show here that a low virulence uracil auxotrophic <i>Toxoplasma</i> strain emerges as a virulent parasite in the absence of an intact T cell compartment. Both CD4⁺ and CD8⁺ T lymphocytes are required for optimal protection, in line with previous findings in other models of <i>Toxoplasma</i> infection. Nevertheless, several novel aspects of the response were identified in our study. Protection occurs independently of IL-12 and MyD88 and only partially requires IFN-γ. This is noteworthy particularly because the cytokines IL-12 and IFN-γ have previously been regarded as essential for protective immunity to <i>T. gondii</i>. Instead, we identified the anti-inflammatory effects of T cell-dependent IL-10 as the critical factor enabling host survival. The parasite dense granule protein GRA24, a host-directed mitogen-activated protein kinase activator, was identified as a major virulence factor in T cell-deficient hosts. Collectively, our results provide new and unexpected insights into host resistance to <i>Toxoplasma</i>.