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The balance between T helper type 1 (Th1) cells and other Th cells is critical for antiviral and anti-tumor responses, but how this fine balance is achieved remains poorly understood. Here, we dissected the dynamic regulation of Th1 cell differentiation during in vitro polarization, as well as in vivo differentiation upon acute viral infection, using scRNA-seq and multiple in vitro and in vivo CRISPR screens. We confirmed the role of known regulators and identified 5 novel regulators for Th1 differentiation. Among the novel regulators the neuropeptide receptor RAMP3, which is a component of the receptor for calcitonin gene-related peptide (CGRP), plays a cell-intrinsic role in Th1 cell-fate determination. Using a unique Th1/Th2 dichotomous culture system, we identified that RAMP3-CGRP interaction directly restricted the differentiation of Th2 cells but promoted Th1 differentiation through activation of downstream cyclic AMP (cAMP) signaling in T cells. Mechanistically, RAMP3 and cAMP signaling resulted in global changes in chromatin accessibility, blocking Th2 genes and specific induction of Th1 programs through activation of IFNγ-STAT1 pathway. Furthermore, both CGRP and RAMP3 were required for inducing effective anti-viral T cell responses to control acute viral infection. Our work reveals a novel neuro-immune circuit in which tissue itself participates in T cell fate determination by producing a neuropeptide during acute viral infection, which acts on RAMP3 expressing T cells to induce an effective anti-viral Th1 response.