Tbet expression by Tregs is needed to protect against Th1-mediated immunopathology during Toxoplasma infection in mice

T. 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 Tregs mediated by loss of IL-2, and conversion of Tregs to IFNγ 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 -Foxp3YFPCre and control Foxp3YFPCre mice with the type II strain of T. gondii , ME49. The majority of Tbx21 f/f -Foxp3YFPCre mice succumb to a non-lethal acute infection. Notably, parasite burden is comparable between Tbx21 f/f -Foxp3YFPCre and Foxp3YFPCre control mice. We found that Tbx21 f/f -Foxp3YFPCre mice have significantly higher serum levels of proinflammatory cytokines IFNγ and TNFα, suggestive of a heightened immune response. To test if CD4+ T cells were driving immunopathology, we treated Tbx21 f/f -Foxp3YFPCre mice with anti-CD4 depleting antibody and partially rescued these mice. Broad spectrum antibiotic treatment also improved survival, demonstrating a role for commensal flora in immunopathology in Tbx21 f/f -Foxp3YFPCre mice. RNA-seq analysis reinforced that Tbet regulates several key cellular pathways, including chromosome segregation, cytokine receptor activity and cell cycle progression, that help to maintain fitness in Tregs during Th1 responses. Taken together, our data shows an important role for Tbet in Tregs in preventing lethal immunopathology during Toxoplasma gondii infection, further highlighting the protective role of Treg plasticity to self and microbiota. ### Competing Interest Statement The authors have declared no competing interest.