IDO1 and gastric cancer: Once induced by TLR‐9, DC2s upregulate the expression of the B‐7 ligands, HLA‐DR antigen, and IDO, all of which promote Treg induction.37 It was demonstrated to be an immune‐evading mechanism, and under clinical conditions, it was related to certain nonimmune prognostic advantages in gastric cancer patients.37 IDO expression in this study was associated with tumor‐infiltrating Foxp3 + Treg cells and tumor‐infiltrating DC2 cells, but was correlated with a lower ratio of tumor‐infiltrating CD4/CD8 T cells.