During immune challenge, T lymphocytes engage pathways of anabolic metabolism to support clonal expansion and the development of effector functions. Here we report a critical role for the non-essential amino acid serine in effector T cell responses. Upon activation, T cells upregulate enzymes of the serine, glycine, one-carbon (SGOC) metabolic network, and rapidly increase processing of serine into one-carbon metabolism. We show that extracellular serine is required for optimal T cell expansion even in glucose concentrations sufficient to support T cell activation, bioenergetics, and effector function. Restricting dietary serine impairs pathogen-driven expansion of T cells in vivo, without affecting overall immune cell homeostasis. Mechanistically, serine supplies glycine and one-carbon units for de novo nucleotide biosynthesis in proliferating T cells, and one-carbon units from formate can rescue T cells from serine deprivation. Our data implicate serine as a key immunometabolite that directly modulates adaptive immunity by controlling T cell proliferative capacity.

T cell subsets including effector (T), regulatory (T), and memory (T) cells are characterized by distinct metabolic profiles that influence their differentiation and function. Previous research suggests that engagement of long-chain fatty acid oxidation (LC-FAO) supports Foxp3 T cell and T cell survival. However, evidence for this is mostly based on inhibition of Cpt1a, the rate-limiting enzyme for LC-FAO, with the drug etomoxir. Using genetic models to target Cpt1a specifically in T cells, we dissected the role of LC-FAO in primary, memory, and regulatory T cell responses. Here we show that the ACC2/Cpt1a axis is largely dispensable for T, T, or T cell formation, and that the effects of etomoxir on T cell differentiation and function are independent of Cpt1a expression. Together our data argue that metabolic pathways other than LC-FAO fuel T or T differentiation and suggest alternative mechanisms for the effects of etomoxir that involve mitochondrial respiration.

Reactive oxygen species (ROS) are continuously produced as a by-product of mitochondrial metabolism and eliminated via antioxidant systems. Regulation of mitochondrially produced ROS is required for proper cellular function, adaptation to metabolic stress, and bypassing cellular senescence. Here, we report non-canonical regulation of the cellular energy sensor AMP-activated protein kinase (AMPK) by mitochondrial ROS (mROS) that functions to maintain cellular metabolic homeostasis. We demonstrate that mitochondrial ROS are a physiological activator of AMPK and that AMPK activation triggers a PGC-1α-dependent antioxidant response that limits mitochondrial ROS production. Cells lacking AMPK activity display increased mitochondrial ROS levels and undergo premature senescence. Finally, we show that AMPK-PGC-1α-dependent control of mitochondrial ROS regulates HIF-1α stabilization and that mitochondrial ROS promote the Warburg effect in cells lacking AMPK signaling. These data highlight a key function for AMPK in sensing and resolving mitochondrial ROS for stress resistance and maintaining cellular metabolic balance.

Highlights d Developed 13 C-infusion method for studying T cell metabolism in vivo d T cell glucose use and bioenergetics differ between cell culture and mouse models d Glucose metabolism in T cells changes dynamically over an immune response d Glucose-dependent serine biosynthesis supports T cell proliferation in vivo