IDO1 and cancer: Although both Ifnγ and IL-27 induce IDO1 and PD-L1 through STAT1 signaling [52, 53], under IL-1β stimulation IDO1 and PD-L1 appear to be regulated through different transcription machinery; however, these immune checkpoint proteins are co-expressed in cancer cells, indicating a cancer cell-intrinsic coupling of IDO1 and PD-L1 expression.