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Chimeric antigen receptor (CAR) T cell therapies have revolutionized B cell malignancy treatment, but many patients with large B cell lymphoma (LBCL) experience primary resistance or relapse. To uncover resistance mechanisms, here we examine pre-infusion tumor biopsies and observe that increased immunoregulatory macrophages correlate with poor clinical responses. In murine models, CAR T cell-produced interferon-gamma (IFN-γ) upregulates inducible nitric oxide synthase (iNOS, NOS2) in immunoregulatory macrophages, impairing CAR T cell function. Proteomic profiling reveals that iNOS-expressing macrophages promote apoptosis and cell cycle arrest while downregulating protein synthesis machinery in CAR T cells. Metabolically, CAR T cells exhibit reduced glycolytic intermediates and altered tricarboxylic acid (TCA) cycle activity. Pharmacological inhibition of iNOS enhances CAR T cell treatment efficacy in vivo. Elevated levels of iNOS⁺CD14⁺ monocytes in leukaphereses are associated with non-durable responses to CAR T cells. Targeting iNOS in immunoregulatory macrophages, potentially by modulating CAR T-produced IFN-γ, could improve LBCL outcomes.