Identification of a leukemia-initiating stem cell in human mast cell leukemia

Mast cell leukemia (MCL) is a highly fatal malignancy characterized by devastating expansion of immature mast cells in various organs. Although considered a stem cell disease, little is known about MCL-propagating neoplastic stem cells. We here describe that leukemic stem cells (LSCs) in MCL reside within a CD34⁺/CD38^- fraction of the clone. Whereas highly purified CD34⁺/CD38^─ cells engrafted NSG_hSCF mice with fully manifesting MCL, no MCL was produced by CD34⁺/CD38⁺ progenitors or the bulk of KIT⁺/CD34^- mast cells. CD34⁺/CD38^- MCL cells invariably expressed CD13 and CD133, and often also IL-1RAP, but did not express CD25, CD26 or CLL-1. CD34⁺/CD38^- MCL cells also displayed several surface targets, including CD33, which was homogenously expressed on MCL LSCs in all cases, and the D816V mutant form of KIT. Although CD34⁺/CD38^- cells were resistant against single drugs, exposure to combinations of CD33-targeting and KIT-targeting drugs resulted in LSC-depletion and markedly reduced engraftment in NSG_hSCF mice. Together, MCL LSCs are CD34⁺/CD38^- cells that express distinct profiles of markers and target antigens. Characterization of MCL LSCs should facilitate their purification and should support the development of LSC-eradicating curative treatment approaches in this fatal type of leukemia.