IDO1 and neoplasm: CAFs express the catabolic enzyme indoleamine 2,3-dioxygenase (IDO) and release a variety of factors such as transforming growth factor- β (TGF- β), vascular endothelial growth factor (VEGF), interleukins (IL)-6, IL-1 and IL-8, and prostaglandin E2 (PGE2) that directly or indirectly participate in the development of an immunosuppressive tumor milieu that promote tumor growth, angiogenesis and metastasis (10).