Phenotyping of Disease-Initiating CD34⁺/CD38^─ Stem Cells in BCR-ABL1^─ MPN Reveals Expression of Multiple Cytokine Receptors and Resistance-Related Antigens

The classical BCR-ABL1-negative myeloproliferative neoplasms (MPN) are characterized by over-production of myeloid cells, disease-related mutations in certain driver-genes (JAK2, CALR, MPL) and an increased risk to transform to secondary acute myeloid leukemia (sAML). Although considered stem cell-derived neoplasms, little is known about the phenotype and functional properties of disease-initiating neoplastic stem cells (NSC) in MPN and sAML. Recent data suggest that MPN NSC reside in a CD34⁺ fraction of the malignant clone. Therefore, these cells are considered most critical target populations to be examined for expression of molecular and immunological targets with the aim to develop improved or even curative NSC-eliminating therapies, such as antibody-based or CAR-T cell approaches. Using a panel of monoclonal antibodies (n=40) and multicolor flow cytometry, we established the immunological phenotype and target expression profiles of putative CD34⁺/CD38^─ NSC and CD34^+/CD38⁺ progenitor cells in patients with polycythemia vera (PV, n=18), essential thrombocythemia (ET, n=29), primary myelofibrosis (PMF, n=38) and post-MPN sAML (n=11). In almost all patients, the putative MPN stem cells expressed the stem cell invasion receptors Hermes (CD44) and ADGRE5 (CD97), C1qR1 (CD93), the migration/adhesion receptor MIC2 (CD99), and the stem cell antigen AC133 (CD133). Contrasting normal stem cells, MPN NCS and sAML stem cells failed to express Thy-1 (CD90). Among the cytokine receptors tested, MPN NSC invariably displayed the TGFßR-related antigen endoglin (CD105), TPOR (CD110), SCFR KIT (CD117), IL-3RA (CD123), CXCR4 (CD184) and IGF-1R (CD221). NSC expressed particularly high levels of KIT and low levels of TPOR and IGF-1R. The IL-2RA (CD25) was identified on NSC in most patients with PMF and sAML, and in a few with ET, but not in patients with PV. Similarly, the GM-CSFR (CD116) was found to be expressed on NSC in most patients with PMF, a few with ET and no with PV. MPN NSC did not exhibit substantial amounts of M-CSFR (CD115), IL-3RB (CD131), FLT3 (CD135), NGFR (CD271) VEGFR-2 KDR (CD309), EPOR, MET or OSMRB. The CD34^+/CD38⁺ MPN progenitor cells displayed a similar profile of cytokine receptors. In addition, MPN and sAML progenitor cells expressed IL-1RAP and CLL-1 in most donors examined. We next examined the expression of various immunological targets and resistance-mediating immune checkpoint antigens on NSC and MPN progenitor cells. In all MPN patients and all sAML patients tested, NSC were found to express substantial amounts of Siglec-3 (CD33) and low levels of Campath-1 (CD52) and MDR-1 (CD243). In addition, MPN NSC and sAML stem cells invariably displayed the "don't eat" me checkpoint IAP (CD47) and the classical checkpoint PD-L1 (CD274). Exposure to interferon-gamma (200 U/ml, 24 hours) resulted in an upregulation of PD-L1 on NSC. In a subset of patients, MPN NSC expressed low levels of HB15 (CD83). In contrast, MPN NSC and sAML stem cells failed to express B7-1 (CD80), B7-2 (CD86), PD-L2 (CD273) and PD1 (CD279). MPN progenitor cells and sAML progenitors expressed an identical profile of cell surface targets and checkpoint antigens. Finally, we confirmed the disease-initiating capacity of MPN stem- and progenitor cells (CD34⁺ cells) using primary PMF cells in xenotransplantation experiments employing NSGS mice expressing human interleukin-3 (IL-3), granulocyte/macrophage colony-stimulating factor (GM-CSF) and stem cell factor (SCF). After 28 weeks post injection, engraftment of human CD45⁺ cells in the bone marrow of NSGS mice was found in 15/15 mice injected with bulk mononuclear cells (MNC) containing CD34⁺ cells and in 0/15 NSGS mice injected with MNC depleted of CD34⁺ cells. Together, MPN NSC reside in a CD34⁺ fraction of the malignant clone and display a unique phenotype, including cytokine receptors, immune checkpoint molecules and other target antigens. The phenotypic characterization of neoplastic stem cells should facilitate their enrichment and the development of NSC-eradicating treatment concepts in MPN.