In addition to aberrant expression of immune checkpoint markers, AML cells have been shown to secrete immune inhibitory soluble factors, such as IL-10, TGF-β and indoleamine 2,3-dioxygenase 1 (IDO1) [108,109,110], which push T cell polarisation towards induced Tregs, thus promoting T cell tolerance and leukaemia progression. Here, IDO1 is linked to leukemia.