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Arginase 1 (Arg1), the enzyme catalyzing the conversion of arginine to ornithine, is a hallmark of IL-10-producing immunoregulatory M2 macrophages. However, its expression in T cells is disputed. Here, we demonstrate that induction of Arg1 expression is a key feature of lung CD4 + T cells during mouse in vivo influenza infection. Conditional ablation of Arg1 in CD4 + T cells accelerated both virus-specific T helper 1 (Th1) effector responses and its resolution, resulting in efficient viral clearance and reduced lung pathology. Using unbiased transcriptomics and metabolomics, we found that Arg1-deficiency was distinct from Arg2-deficiency and caused altered glutamine metabolism. Rebalancing this perturbed glutamine flux normalized the cellular Th1 response. CD4 + T cells from rare ARG1-deficient patients or CRISPR-Cas9-mediated ARG1-deletion in healthy donor cells phenocopied the murine cellular phenotype. Collectively, CD4 + T cell-intrinsic Arg1 functions as an unexpected rheostat regulating the kinetics of the mammalian Th1 lifecycle with implications for Th1-associated tissue pathologies., Competing Interests: Declaration of interests M.S. is an inventor on a patent describing the use of RBD ligands for cell-surface evaluation of CAT1/solute carrier family 7 member 1 (SLC7A1) and other solute carrier (SLC) expression (N.T. gave up her rights); he is a co-founder and head of the scientific board of METAFORA-Biosystems, a start-up company that focuses on metabolite transporters under physiological and pathological conditions., (Published by Elsevier Inc.)