In vivo genome-wide CRISPR screens identify SOCS1 as intrinsic checkpoint of CD4⁺ T_H1 cell response

Although CD8⁺ T cells undergo autonomous clonal proliferation after antigen stimulation in vivo, the expansion of activated CD4⁺ T cells is limited by intrinsic factors that are poorly characterized. Using genome-wide CRISPR-Cas9 screens and an in vivo system modeling of antigen-experienced CD4⁺ T cell recruitment and proliferation during a localized immune response, we identified suppressor of cytokine signaling 1 (SOCS1) as a major nonredundant checkpoint imposing a brake on CD4⁺ T cell proliferation. Using anti–interleukin-2 receptor (IL-2R) blocking antibodies, interferon-γ receptor (IFN-γR) knockout mice, and transcriptomic analysis, we show that SOCS1 is a critical node integrating both IL-2 and IFN-γ signals to block multiple downstream signaling pathways abrogating CD4⁺ T helper 1 (T_H1) cell response. Inactivation of SOCS1 in both murine and human CD4⁺ T cell antitumor adoptive therapies restored intratumor accumulation, proliferation/survival, persistence, and polyfunctionality and promoted rejection of established tumors. However, in CD8⁺ T cells, SOCS1 deletion did not affect the proliferation but rather improved survival and effector functions, which allowed for optimal therapeutic outcome when associated with SOCS1 inactivation in CD4⁺ T cells. Together, these findings identify SOCS1 as a major intracellular negative checkpoint of adoptive T cell response, opening new possibilities to optimize CAR-T cell therapy composition and efficacy.