PDCD1 and cancer: The rationale for the combination therapy was due to secreted IFNγ from cytotoxic T cells driving expression of PD-L1/PD-1 and TDO2/IDO1 in cancer cells in an immunosuppressive feedback loop (53) because KYN binding to AhR inhibits the viability and function of cytotoxic T cells but expands regulatory T cells (54).