Immune checkpoints regulate acute myeloid leukemia stem cells

Acute myeloid leukemia stem cells (LSCs) express major histocompatibility complex (MHC) class I and II and many different immune checkpoint ligands and receptors, in which respect they resemble professional antigen-presenting cells. In addition, LSCs reside in the bone marrow (BM), a primary and secondary lymphoid organ, surrounded by immune cells. The function of these immune checkpoints (ICs) in the regulation of an anti-tumor immune response is well studied and IC inhibitors (ICIs) became a standard of care in many solid tumors. However, ICIs have very limited efficacy in AML. Nevertheless, the expression especially of immune activating ligands and receptors on LSCs is somewhat unexpected, since these cells have to evade protective immunity. Many ICs have been shown to mediate direct signaling in AML blasts and LSCs and thereby regulate self-renewal, differentiation and expansion of leukemic cells. Thus, the expression of ICs on the cell surface or their soluble forms often correlate with worse survival. In this review we summarize recent data on selected ICs of the immunoglobulin superfamily (IgSF) and the tumor necrosis factor receptor superfamily (TNFRSF) that have a documented role in the regulation of LSCs, independent of their immune regulatory role, and might become novel therapeutic targets.