Tregs are enriched in the bone marrow niche, correlating with the secretion of immunoinhibitory factors, such as IL-10, IL-35, transforming growth factor-beta (TGF-β), and indoleamine 2,3-dioxygenase 1 (IDO1) from AML blasts [49,50]. Here, IDO1 is linked to acute myeloid leukemia.