TDO2 and neoplasm: For instance, in a glioma mouse model, kynurenine produced by tumor cells was found to directly activate AHR in tumor-associated macrophages.32 This process was facilitated by the expression of tryptophan 2,3-dioxygenase (TDO-2) and indoleamine 2,3-dioxygenase-1 (IDO-1) enzymes, which are involved in kynurenine production.