Expansion of neutrophil precursors and progenitors in suspension cultures of CD34⁺ cells enriched from human bone marrow

The growth and differentiation of selected bone marrow CD34⁺ cells stimulated with hematopoietic growth factors in liquid cultures were evaluated to determine whether cell types that may be useful for reducing the neutropenia associated with high-dose chemotherapy (HDC) can be produced and quantitated in vitro. CD34⁺ cells enriched from bone marrow were cultured for up to 5 weeks in interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) with or without stem cell factor (SCF) (also termed c-kit ligand). The mixture of IL-3, GM-CSF and G-CSF resulted in an 18-fold increase in cells after 10 to 12 days of culture and a 94-fold increase after 21 days. A 3-fold increase in colony-forming unit granulocyte-macrophage (CFU-GM) was observed after 10 days of culture. The addition of SCF during the first 10 days of culture further augmented the proliferation of cell numbers to 24-fold and colony-forming cells (CFC) to 8-fold after 10 days while cell numbers increased 130-fold after 21 days. Two-color flow cytometry defined phenotypes expressing CD11b and CD15 that represented maturation stages of neutrophils. Maturation of neutrophils in these cultures could be followed by the initial appearance after 3 to 7 days of a CD15⁺CD11b^- phenotype representing promyelocytes, which gave rise after 2 to 3 weeks to a CD15⁺CD11b⁺ phenotype representing more mature neutrophil forms (metamyelocytes to segmented neutrophils). In contrast to normal neutrophil development, only a small fraction (10 to 15%) of the culture-derived neutrophils expressed CD16. These data define the kinetics and differentiation of neutrophils and neutrophil precursors from selected CD34⁺ cells in liquid cultures.