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Background: Cancer immunotherapy has generated significant excitement, mainly as a result of the development of immune checkpoint inhibitors. The blockade of PD-1 or its ligand with antibodies has resulted in impressive clinical efficacy. However, a subset of patients does not respond to biologic therapeutics, and another subset suffers from severe immune-related adverse events in certain cases. The modulation of the immune system with small molecules might yield surprising benefits. Methods: CD8+ cells were obtained through a magnetic cell sorting system (MACS), and their capabilities for IFN-γ release and PD-1 expression were analyzed. The in vitro effects of drugs were studied in a coculture system of tumor cells and activated CD8+ cells. We further isolated the primary tumor cells in tumor-bearing mice treated with CAI, DMF, 1-MT or a combination (CAI and DMF/CAI and 1-MT) and analyzed the percentages of CD8+ T cells and PD-1+CD8+ T cells among TILs. The selective anti-tumor immune reactions of the two drug combinations were confirmed in a coculture system consisting of B16-OVA cells and OVA-specific CTLs derived from OT-1 transgenic mice. The anti-tumor effects of the single drugs or combined therapies were assessed according to their capability to slow tumor growth and extend the life span of tumor-bearing mice, and they were compared with the effects of PD-1 antibody. Results: CAI increased IFN-γ release from activated T cells, which might strengthen the anti-proliferative and anti-metastatic effects on cancer cells. However, CAI also stimulated IDO1-Kyn metabolic circuitry in the tumor microenvironment and facilitated tumor cell immune evasion. Combining CAI with 1-MT or DMF disrupted PD-1 expression and promoted IFN-γ production in CD8+ T cells, and it also increased T lymphocyte infiltration in the tumor microenvironment, inhibited tumor growth and prolonged the life spans of tumor-bearing mice. Conclusion: Inhibitors of the IDO1-Kyn-AhR pathway could abolish the negative effects of CAI on CD8+ T cells and result in complementary and beneficial anti-tumor immune effects. The combination of CAI with 1-MT or DMF greatly augmented the ability of CD8+ T cells to kill malignant cells and showed a strong anti-cancer capability that was superior to that of either of the single agents was is comparable with that of anti-PD-1 antibody. The combinations of small molecules utilized in this study may serve as valuable new immunotherapy strategies for cancer treatment. [ABSTRACT FROM AUTHOR]