Flt3(high) and flt3(low) CD34⁺ progenitor cells isolated from human bone marrow are functionally distinct

We generated monoclonal antibodies against the human Fit3 receptor end used them to study the characteristics of normal human bone marrow cells resolved based on Fit3 expression. Human CD34⁺ or CD34⁺Iin^- marrow cells were sorted into two populations: cells expressing high levels of Flt3 receptor (Flt3(high)) and cells with little or no expression of Flt3 receptor (Flt3(low)). Flt3 receptor was detected on a subset of CD34⁺CD38^- marrow cells, as well as on CD34⁺CD19⁺ B lymphoid progenitors and CD34⁺CD14⁺CD64⁺ monocytic precursors. FIt3 receptor was also present on more mature CD34^-CD14⁺ monocytes. In colony-forming assays, Flt3(high) cells gave rise mainly to colony-forming unit-granulocyte-macrophage (CFU- GM) colonies, whereas Flt3(low) cells produced mostly burst-forming unit- erythroid colonies. There was no difference in the number of multilineage CFU-Mix colonies between the two cell fractions. Cell cycle analysis showed that a large number of the Flt3(low) cells were in the G₀ phase of the cell cycle, whereas Flt3(high) cells were predominantly in G₁. Cell numbers in the suspension cultures initiated with Flt3(high) cells were maintained in the presence of Flt3 ligand (FL) alone, and increased in response to FL plus kit ligand (KL). In contrast, cell numbers in the suspension cultures started with Flt3(low) cells did not increase in the presence of FL, or FL plus KL. Upregulation of Flt3 receptor on Flt3(low) cells was not detected during suspension culture. CD14⁺ monocytes were the major cell type generated from CD34⁺Iin-Flt3(high) cells in liquid suspension culture, whereas cells generated from CD34⁺Iin-Flt3(low) cells were mainly CD71⁺GlycA⁺ erythroid cells. These results show clear functional differences between CD34⁺Flt3(high) and CD34⁺Flt3(low) cells and may have implications concerning the in vitro expansion of human hematopoietic progenitor cells.