IDO1 and cancer: As for cytotoxic T cells, cancer cells can stimulate Tregs to inhibit CTL activity by releasing adenosine into TME and many other immunosuppressive mediators, including IDO, CXCL17, signal transducer, and transcriptional activator 3 (STAT3) to inhibit the activity of CTL and leading to poor production of IFN-γ (26–28).