IDO2 and neoplasm: Apart from prolonged antigen stimulation, the dysfunction of CD8+ T cells induced by tumors is also influenced by the immunosuppressive factors in tumor microenvironments (TMEs), such as myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), regulatory T cells, IL-10, TGF-β and indoleamine 2,3-dioxygenase [7].