AHR and neoplasm: Additionally, metabolism of Trp leads to accumulation of its catabolites that induce T-cell apoptosis and promote Tregs through the activation of aryl hydrocarbon receptor (AhR).26,27 IDO1 is differentially expressed in human tumors, and can be expressed by several different cell types within the tumor tissue.28,29 Most murine tumors do not constitutively express IDO1, but it can be induced by IFNγ secretion in the tumor microenvironment.28,29