CARM1-mediated hypoxanthine-enriched exosomes rewire inosine metabolism and impair CD8 T cell antitumor function.

Cancer cells utilize tumor-derived exosomes to suppress antitumor immunity. Herein, we identify co-activator-associated arginine methyltransferase 1 (CARM1) as a key regulator of exosome biogenesis and metabolite sorting that inhibiting CD8 T cell-mediated antitumor responses. Genetic ablation of CARM1 in breast cancer cells impairs immunosuppressive exosome secretion, enhancing CD8 T cell infiltration, proliferation, and effector function. Mechanistically, CARM1 dimethylates apoptosis-linked gene-2 interacting protein X (ALIX) at arginine 757, facilitating its interaction with endosomal sorting complex required transport (ESCRT) components, and promoting tetraspanin-enriched exosome biogenesis. CARM1-dependent ALIX methylation enables selective packaging hypoxanthine into exosomes through direct binding to the ALIX F676 pocket. Exosomal hypoxanthine disrupts inosine metabolism in activated CD8 T cells, inhibiting pentose phosphate pathway, glycolysis, nucleotide synthesis, and effector cytokine production. Co-administration of CARM1 inhibitor with inosine significantly enhances tumor-infiltrating CD8 T cell cytotoxicity, reduces PD-1TIM-3 exhausted CD8 T cells, and suppresses tumor growth. These findings establish the CARM1-ALIX-hypoxanthine axis as an immunosuppressive mechanism and suggest that combining CARM1 inhibition with inosine supplementation represent a promising therapeutic strategy for breast cancer.