This effect is tightly dependent on the tumor’s enzymatic landscape, particularly IDO1 expression levels, which explains ICA’s synergistic activity with anti-PD-1 therapy in both microsatellite instability-high (MSI-H) and microsatellite instability-low (MSI-L) CRC models (112); in these settings, ICA reduces intratumoral Treg infiltration and enhances CD8+ T cell cytotoxicity, even overcoming the inherent immunotherapy resistance of MSI-L tumors. Here, IDO1 is linked to colorectal carcinoma.