AHR and infection: Interestingly, whereas the level and activity of TDO in HCV or HCC has not been previously investigated, upregulation of hepatic IDO expression and an increased serum Kyn:Trp ratio was demonstrated in patients with chronic HCV infection and in infected chimpanzees.10, 11 Moreover, hepatic IDO expression decreased in animals that cleared the infection, but remained high in those that progressed to chronicity;10 (b) the immunosuppressive effects of TDO;12, 13 and (c) the immunosuppressive and tumorigenic effects of the AhR pathway.