IDO1 and infection: Infection with T. gondii activates inflammatory molecules—Interferon gamma (IFN-γ), Interleukin-2 (IL-2), and Tumor necrosis factor alpha (TNF-α)—which upregulate tryptophan-2,3-dioxygenase (TDO) and indoleamine-2,3-dioxygenase (IDO) while leading to degradation of tryptophan into kynurenine.