IDO2 and neoplasm: Tumor cells can create an immunosuppressive microenvironment by secreting a large number of immunosuppressive cytokines (such as IL-10, TGFβ, and PGE2), recruiting immunosuppressive cells such as regulatory T cells (Tregs), activating negative regulatory pathways (CD80/CTLA4 and PD-1/PD-L1), and overexpressing immune regulatory enzymes such as indoleamine 2,3-dioxygenase (33), resulting in immune cell exhaustion or dysfunction, and affecting the normal anti-tumor function of the immune system.