IDO1 and lung carcinoma: Indoleamine 2,3-dioxygenase 1 (IDO1), as a rate-limited enzyme necessary for catalytic tryptophan metabolism, is highly expressed in lung cancer cells, converting more than 95 % of tryptophan (Try) to kynurenine (Kyn), leading to tryptophan depletion and the accumulation of downstream products, then forming an immunosuppressive microenvironment [22,23].