IDO1 and neoplasm: The tumor environment contains an abundance of various types of immunosuppressive cells and inhibitory factors, such as tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), which secrete IL-10, transforming growth factor-β (TGFβ) and indoleamine-2,3-dioxygenase (IDO) to inhibit many important immunological processes [63, 64].