Uncovering two neutrophil-committed progenitors that immediately precede the promyelocytes during human neutropoiesis

Technological advances have greatly improved our knowledge of myelopoiesis, for instance with the discovery of granulocyte-monocyte-DC progenitors (GMDPs), monocyte-DC progenitors (MDPs), common dendritic progenitors (CDPs) and common monocyte progenitors (cMoPs) based on sophisticated flow cytometry approaches. Concomitantly, little progress has been made to characterize the very early phases of human neutropoiesis, despite the recently reported eNePs, PM w/o eNePs, ProNeus, preNeus, all of them being, however, CD66b+neutrophil progenitors. More recently, we identified four SSCloLin-CD66b-CD45dimCD34+/CD34dim/-CD64dimCD115-cells as the earliest precursors specifically committed to the neutrophil lineage present in human bone marrow (BM), which we called neutrophil-committed progenitors (NCPs), namely from NCP1s to NCP4s. In this study, we report the isolation and characterization of two new SSChiCD66b-CD64dimCD115-NCPs that, by phenotypic, flow cytometric SSC, transcriptomic, maturation and immunohistochemistry properties/features, stand after NCP4s but precede the CD66b+Promyelocytes (PMs) during the neutropoiesis cascade. These cells, similarly to SSCloCD45RA+NCP2s/NCP3s and SSCloCD45RA-NCP1s/NCP4s, exhibit phenotypic differences in CD45RA expression levels and, therefore, were named as SSChiCD45RA+NCP5s and SSChiCD45RA-NCP6s. In addition, NCP5s result more immature than NCP6s, as determined by both their cell differentiation and proliferative potential, and by their transcriptomic and phenotypical features. Finally, by examining whether NCPs and all other CD66b+neutrophil precursors are altered in representative hematological malignancies, we found that, in patients with chronic-phase chronic myeloid leukemia (CP-CML), but not with systemic mastocytosis (SM), there is an increased frequency of NCP4s and all downstream neutrophil progenitors, particularly, the CD45RA-cells, like NCP6s, suggesting their involvement in CML pathogenesis. Altogether, our data advance our knowledge of human neutropoiesis.