IDO  

Indoleamine-2,3-dioxygenase (IDO) is an intracellular enzyme which catalyzes the initial and rate limiting step in the degradation pathway of the essential amino acid tryptophan and is expressed in professional APCs, epithelial cells, vascular endothelium and tumor cells. IDO is an important enzyme in both cancer development and cancer progression. It supports inflammation in the tumor microenvironment, development of immune tolerance to tumor antigens in stromal and immune cells, suppression of T and natural killer (NK) cells, generation and activation of T regulatory cells (Tregs) and myeloid-derived suppressor cells, and promotion of tumor angiogenesis. It is reported that functionally active IDO protein is expressed in a wide variety of human hematologic malignancies, such as acute monocytic leukemia, acute lymphocytic leukemia, acute myeloid leukemia and T-cell leukemia/lymphoma, and solid tumors, such as breast cancer, colorectal cancer, endometrial cancer, gastric cancer, glioblastoma, gynecological cancers, head and neck cancers, non-small cell lung cancer (NSCLC), melanoma, and pancreatic cancer. As an immune checkpoint pathway, the IDO pathway is emerging as an important modality for the treatment of cancer.