IDO2 and neoplasm: In addition, CD8+ CTL may upregulate the expression of PD-L1 (program death ligand-1) and indoleamine-2,3-dioxygenase in tumor cells, recruit Tregs in the tumor microenvironment, and then promote tumor immune escape by the production of CCL22 (C-C motif chemokine ligand 22) and IFN-γ (7).