Phenotyping of Neoplastic (CD34⁺/CD38⁻/CD123⁺) Stem Cells in Myeloid Malignancies Reveals Expression of Multiple Molecular Targets.

Recent data suggest that myeloid neoplasms are organized hierarchically in terms of self renewal and maturation of early progenitor cells, similar to normal myelopoiesis. In acute myeloid leukemia (AML), the NOD/SCID mouse-repopulating leukemic stem cells usually co-express CD123 with CD34, but lack CD38. So far, however, little is known about expression of other markers and targets on these progenitors. In the present study, expression of target antigens on CD34+/CD38− cells was analyzed by multicolor flow cytometry in patients with AML (n=18), myelodysplastic syndromes (MDS, n=6), chronic myeloid leukemia (CML, n=8), systemic mastocytosis (SM, n=9), and normal bone marrow (n=5). The IL-3Ra chain (CD123) was found to be expressed on CD34+/CD38− cells in a majority of all patients in all disease-categories. Independent of the type of disease, the vast majority of these stem cells also co-expressed aminopeptidase-N (CD13) and the target receptor CD44 in all patients. CD34+/CD38− progenitor cells expressed variable amounts of the Mylotarg^® receptor CD33, KIT (CD117), HLA-DR, and AC133 (CD133). With regard to AC133, two distinct subpopulations of progenitor cells were detected in many cases, namely a CD133+ and a clearly CD133- cell-fraction. In patients with AML, the levels of CD33 varied from patient to patient with a broad range of reactivity, whereas in most patients with MDS, CML, and SM, CD33 was found to be consistently expressed on most progenitors. In most patients, neoplastic stem cells did not express substantial amounts of the GM-CSF receptor alpha chain (CD116), Thy-1 (CD90), E-NPP3 (CD203c), MDR-1 (CD243), or PAR-2. In the normal bone marrow, CD34+/CD38− cells co-expressed CD13, CD44 and CD45, but did not express CD33, CD116, or CD123. In conclusion, neoplastic stem cells in various myeloid neoplasms appear to express a similar phenotype including target receptors such as CD13, CD33, and CD44. These antigens may thus be attractive targets of therapy in AML. However, since many of these targets are not expressed on all stem cells in all patients, the elimination of the entire clone may require combinations of targeted antibodies or use of additional drugs. In other cases (CD13, CD44, CD45), the target antigen is also expressed on normal stem cells, so that targeted therapy is likely to be an ablative maneuver and thus would require a combined stem cell transplantation approach.