Regulatory T (Treg) cells, although vital for immune homeostasis, also represent a major barrier to anti-cancer immunity, as the tumor microenvironment (TME) promotes the recruitment, differentiation, and activity of these cells1,2. Tumor cells show deregulated metabolism, leading to a metabolite-depleted, hypoxic and acidic TME3, which places infiltrating effector T cells in competition with the tumor for metabolites and impairs their function4–6. At the same time, Treg cells maintain a strong suppression of effector T cells within the TME7,8. As previous studies suggested that Treg cells possess a distinct metabolic profile from effector T cells9–11, we hypothesized that the altered metabolic landscape of the TME and increased activity of intratumoral Treg cells are linked. Here we show that Treg cells display broad heterogeneity in their metabolism of glucose within normal and transformed tissues and can engage an alternative metabolic pathway to maintain suppressive function and proliferation. Glucose uptake correlates with poorer suppressive function and long-term instability, and high-glucose conditions impair the function and stability of Treg cells in vitro. Treg cells instead upregulate pathways involved in the metabolism of the glycolytic by-product lactic acid. Treg cells withstand high-lactate conditions, and treatment with lactate prevents the destabilizing effects of high-glucose conditions, generating intermediates necessary for proliferation. Lactic acid also contributes directly to epigenetic modifications through histone lactylation which may support the expression of Treg cell signature genes. Deletion of MCT1—a lactate transporter—in Treg cells reveals that lactate uptake is dispensable for the function of peripheral Treg cells but required intratumorally, resulting in slowed tumor growth and an increased response to immunotherapy. Thus, Treg cells are metabolically flexible: they can use ‘alternative’ metabolites in the TME to maintain their suppressive identity. Further, our results suggest that tumors avoid destruction by not only depriving effector T cells of nutrients, but also metabolically supporting regulatory populations
