SUMMARY Inflammatory diseases are often driven by inappropriate responses of effector CD4 T cells (Teff). IL17, IFNγ, or dual-producing polyfunctional effector Th1, Th17, or Th17.1 T cells can become imbalanced with suppressive Treg CD4 T cells in a variety of disease settings, including inflammatory bowel diseases (IBD). A key therapeutic objective in efforts to shift the immunologic balance towards tolerance, therefore, is to selectively inhibit Teff and promote Treg. We have shown that Teff and Treg subsets utilize different metabolic programs that represent fundamental features of T cell biology. Here we explore one carbon (1C) metabolism and related microenvironmental factors to modulate CD4 T cells in inflammatory diseases. 1C metabolism integrates multiple nutrient inputs to provide intermediates for de novo methionine and purine synthesis and is commonly targeted with anti-folate drugs. An in vivo CRISPR screen of primary T cells in IBD with a 1C metabolism enzyme-focused gRNA library identified the mitochondrial enzyme Methylene-tetrahydrofolate Dehydrogenase 2 (MTHFD2) as conditionally essential for effector T cell proliferation and inflammation. MTHFD2 was upregulated in T cells a variety of inflammatory conditions and while MTHFD2-deficiency impaired Teff, MTHFD-deficient Treg had increased FoxP3 expression in both mouse and human T cells. Consistent with a role as a metabolic checkpoint on inflammation, MTHFD2-deficiency protected against IBD and other inflammatory diseases. Mechanistically, MTHFD2 inhibition suppressed mTORC1 activity, possibly through reduced methionine and/or interrupted purine synthesis. Importantly, local nutrients play key roles in 1C metabolism and T cell fate. To quantify T cell access to nutrients in vivo, we established Positron Emission Tomography (PET) tracer-based methods to directly image and measure nutrient uptake in vivo. These studies showed a sharp increase in T cell glucose uptake in inflammation. In addition, IBD is often associated with folate-deficiency. The effects of dietary folate on T cell 1C metabolism, mTORC1 signaling, and fate, however, are unclear. Because inflammation is associated with fevers and enzymes are temperature-dependent, we also tested fever conditions on T cell metabolism. We found fever led to increased cytokine production from Teff and mitochondrial Reactive Oxygen Species (ROS) specifically in Th1 cells that, surprisingly, led to Tp53-dependent apoptosis. These findings support the hypothesis that 1C metabolism is limiting and serves as a metabolic checkpoint to integrate local nutrients and physical conditions through MTHFD2 and mTORC1 signaling to provide new immunometabolic targets to modulate effector and regulatory T cells. To test this, we will: (1) Test the role and mechanism of MTHFD2 as a limiting enzyme in methionine and nucleotide synthesis essential for mTORC1 signaling and effector T cells; and (2) Determine how nutrient and microenvironmental factors such ...