NPPA and cancer: The immunosuppressive effects of Trp catabolism can be mediated by a two-pronged strategy, first by depletion of Trp, as Trp is essential for the proliferation and activity of liver-infiltrating T cells (72, 73) and secondly by the AHR-mediated immunosuppressive action of the KP metabolites, as has frequently been described in extra-hepatic systems such as the placenta (74) and different types of cancers (36, 75, 76).