IDO2 and neoplasm: NK cell dysfunction in tumor microenvironments can generally be caused by tumor cells, myeloid-derived suppressor cells, macrophages, Tregs and platelets in a contact-dependent manner or via secretion of soluble factors such as transforming growth factor (TGF)-β, IL-10, indoleamine-2,3-dioxygenase, prostaglandin E2, and adenosine (145, 151).