Identification of a Neoplastic Stem Cell in Human Mast Cell Leukemia

Leukemic stem cells (LSCs) have recently been identified as an important target of therapy in various human leukemias and related blood cell disorders. Systemic mastocytosis (SM) is a rare hematologic neoplasm characterized by abnormal growth and accumulation of mast cells (MCs) in various organ systems, including the bone marrow (BM). Whereas patients with indolent SM (ISM) have a normal life-expectancy, patients with more advanced forms of SM have a poor prognosis. In these patients, neoplastic MCs are usually resistant against conventional drugs and various targeted drugs. MC leukemia (MCL) is the rare leukemic variant of advanced SM, defined by a rapidly devastating expansion of immature MCs in various hematopoietic organs and a poor prognosis with short survival times. Although MCL is considered a stem cell disease, little is known about the origin and phenotype of MCL-initiating LSCs. We examined the phenotypic and functional characteristics of putative LSCs in patients with aggressive SM (ASM, n=12) and MCL (n=6). Putative LSCs were identified and characterized phenotypically by flow cytometry. Highly enriched, sorted LSCs were injected into NOD-SCID-IL-2Rγ^-/- mice exhibiting a 220 amino acid isoform of human membrane-bound hSCF (NSG_SCF). We found that disease-initiating and propagating LSCs reside within a CD34⁺ fraction of the MCL clone. Whereas cell fractions containing CD34⁺ cells as well as highly enriched CD34⁺ cells produced engraftment in NSG_SCF mice with a MCL-like disease (43-77% human MCL cells in mouse BM after 10-22 weeks), no substantial engraftment was produced by MC-rich but stem cell-depleted, KIT⁺/CD34^─ cell fractions obtained from the same patients (<1% engraftment in mouse BM). In dilution experiments, engraftment of CD34⁺ cells was documented down to a minimum of 50 cells per mouse. The identity of engrafting MCL cells was confirmed by morphology, phenotyping and molecular studies demonstrating the presence of KIT mutations that were initially detected in the primary MCL samples used. Moreover, we were able to confirm long-term engraftment by successful serial transplantations into secondary recipient mice. In consecutive experiments, we were able to show that CD45⁺/CD34⁺/CD38^─ cells also produce leukemic engraftment in NSG_SCF mice. As assessed by flow cytometry, these CD34⁺/CD38^─ MCL LSCs were found to express several stem cells markers, including aminopeptidase-N (CD13), leukosialin (CD43), Pgp-1 (CD44), the IL-3R alpha-chain (CD123), AC133 (CD133) and CXCR4 (CD184). In addition, in most patients examined, MCL LSCs were found to display IL-1RAP, a surface antigen that is otherwise expressed in CML LSCs but is not expressed in normal stem cells. In addition, MCL LSCs were found to express various cell surface targets, including CD33 and CD52. By contrast, MCL LSCs did not express CD2, CD25, CD26 and CLL-1. The more mature progenitor cell fractions (CD34⁺/CD38⁺) were found to stain positive for CD13, CD33, CD43, CD44, CD90, CD117, CD123, CD133 and CD184. Mature clonal MCs expressed a similar phenotype, including molecular markers and targets, such as CD13, CD30 CD33, CD52 and CD184. In patients with ISM and aggressive SM (ASM), the CD34⁺/CD38^─ stem cells exhibited a similar surface marker profile compared to MCL, but expressed lower levels of CD133 and did not express IL-1RAP. In the validation phase of our study, we examined the effects of target-specific antibodies. As assessed by flow cytometry, the CD52-targeting antibody alemtuzumab was found to induce complement-dependent lysis of CD34⁺ and CD34⁺/CD38^─ cells in all MCL samples analysed. Furthermore, pre-incubation of MCL cells with alemtuzumab prior to injection into NSG_SCF mice resulted in a significantly reduced engraftment (2.7±4.1%) after 22 weeks. In conclusion, our data show that the MCL clone originates from a primitive hematopoietic stem cell that co-expresses CD34, CD123, CD133 and IL-1RAP but lacks CD25 and CD26. In addition, our data show that MCL LSC express a number of clinically relevant surface targets, including CD33, CD52 and CD117 (KIT). These observations may facilitate LSC detection and isolation in MCL and may lead to the development of novel LSC-eradicating treatment concepts in this highly aggressive and drug-resistant form of leukemia.