Toxoplasma gondii infection has proven to be an ideal model to understand the delicate balance between protective immunity and immune-mediated pathology during infection. Lethal infection causes a collapse of T regulatory cells (Tregs) mediated by the loss of IL-2 and conversion of Tregs to IFN-γ-producing cells. Importantly, these Tregs highly express the Th1 transcription factor Tbet. To determine the role of Tbet in Tregs, we infected Tbx21^f/f -Foxp3^YFPCre and control Foxp3^YFPCre mice with the type II strain of T. gondii, ME49. The majority of Tbx21^f/f -Foxp3^YFPCre mice succumbed to a nonlethal dose. Notably, parasite burden was reduced in Tbx21^f/f -Foxp3^YFPCre compared with Foxp3^YFPCre control mice. We found that Tbx21^f/f -Foxp3^YFPCre mice have significantly higher serum levels of proinflammatory cytokines IFN-γ and TNF-α, suggestive of a heightened immune response. To test if CD4⁺ T cells were driving immunopathology, we treated Tbx21^f/f -Foxp3^YFPCre mice with anti-CD4-depleting Abs and partially rescued these mice. Broad-spectrum antibiotic treatment also improved survival, demonstrating a role for commensal flora in immunopathology in Tbx21^f/f -Foxp3^YFPCre mice. RNA sequencing analysis reinforced that Tbet regulates several key cellular pathways, including leukocyte activation, regulation of lymphocyte activation, and cell cycle progression, that help to maintain fitness in Tregs during Th1 responses. Taken together, our data show an important role for Tbet in Tregs in preventing lethal immunopathology during T. gondii infection, further highlighting the protective role of Treg plasticity in controlling immune responses to infection and the microbiota.

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