Exclusive expression of G-CSF receptor on myeloid progenitors in bone marrow CD34⁺ cells

Although granulocyte colony-stimulating factor (G-CSF) has been reported to act on cells of neutrophilic lineage, the administration of G-CSF to induce the mobilization of various haematopoietic progenitors into the circulation. We analysed the expression of receptors for G-CSF (G-CSFR) on human bone marrow and G-CSF-mobilized peripheral blood CD34⁺ cells, and examined the proliferation and differentiation capabilities of sorted CD34⁺G-CSFR⁺ and CD34⁺G-CSFR^- cells using methylcellulose clonal culture. Flow cytometric analysis showed that G-CSFR was expressed on 14.9 ± 4.9% of bone marrow CD34⁺ cells, most of which were included in CD34⁺CD33⁺ and CD34⁺CD38⁺ cell fractions. In clonal cultures, CD34⁺G-CSFR⁺ cells produced only myeloid colonies, whereas CD34⁺G-CSFR^- cells produced erythroid bursts, megakaryocyte and multilineage colonies. When incubated with the cytokine cocktail for 5 d, CD34⁺G-CSFR^- cells generated CD34⁺G- CSFR⁺ myeloid progenitors. In G-CSF-mobilized peripheral blood, CD34⁺ cells contained 10.8 ± 5.8% of G-CSFR⁺ cells, most of which were also myeloid progenitors, although CD34⁺G-CSFR^- cells contained a substantial number of myeloid progenitors. These results indicated that the expression of G-CSFR on CD34⁺ cells is restricted to myeloid progenitors, suggesting that the specific activity of G-CSF on myelopoiesis depends on the exclusive expression of its receptor on myeloid progenitors, and that the mobilization of various haematopoietic progenitors is not a direct effect of G-CSF in humans.