Tumor IDO1 drives resistance to adoptive cell transfer by suppressing T cell recruitment and effector function

Adoptive cell transfer (ACT) has demonstrated potent anti-tumor efficacy in melanoma, yet therapeutic resistance frequently emerges within immunosuppressive tumor microenvironments. Indoleamine 2,3 dioxygenase 1 (IDO1) is a tryptophan catabolizing enzyme that generates kynurenine (Kyn), an immunomodulatory metabolite known to suppress effector T cell function. Here, we show that tumor overexpression of IDO1 undermines ACT efficacy through dual mechanisms of T cell exclusion and cytotoxic impairment. Using murine B16 melanoma cells engineered to overexpress IDO1 (B16IDO1), we observed elevated Kyn levels, reduced CXCL9/10 and CCL5 chemokine expression, and decreased intratumoral T cell infiltration. In vitro, IDO1 expressing tumor cells exhibited resistance to killing by activated CD8 PMEL T cells, a phenotype dependent on soluble factors in conditioned media and reversible with pharmacologic IDO1 inhibition. In vivo, adoptive transfer of either PMEL or TRP1 T cells, CD4 T cells specific for tyrosinase-related protein 1, failed to control B16IDO1 tumors, correlating with decreased infiltration, function, and survival. Pharmacologic blockade of IDO1 enhanced T cell infiltration and improved ACT mediated tumor control. Together, these findings identify IDO1 as a regulator of ACT resistance by suppressing T cell trafficking and effector activity. Complementary analyses of patient TIL, tumor, and serum samples are underway to assess correlations between IDO1/Kyn levels and immune cell burden, underscoring clinical relevance. Further studies extend this framework to human models, including IDO1⁺ melanoma xenografts and CAR T cells, to explore the translational potential of targeting the IDO1-Kyn-AHR axis to enhance cellular immunotherapy efficacy.