Haematopoietic action of flt3 ligand on cord blood-derived CD34-positive cells expressing different levels of flt3 or c-kit tyrosine kinase receptor: comparison with stem cell factor

We compared the effect of human flt3 ligand (FL) and stem cell factor (SCF) on cord blood (CB)-derived CD34(+) cells expressing different levels of flt3 or c-kit tyrosine kinase (TK) receptor in clonal cell culture. The c-kit receptor was expressed by 58.5+/-16.7% of CB CD34(+) cells (n=19), in which c-kit(high), c-kit(low), and c-kit(-) cell populations could be identified. In contrast, the flt3 receptor (FR) was weakly expressed on 58.6+/-8.3% (n=9) of CB CD34(+) cells. FL+erythropoietin (Epo) failed to support erythroid burst (BFU-E) formation by any subpopulation of CD34(+) cells. However, SCF+Epo supported BFU-E and erythrocyte-containing mixed (CFU-mix) colony formation from all subpopulations. Interestingly, FL markedly augmented CFU-mix colony formation supported by interleukin (IL)-3+Epo when CD34(+)c-kit(low) or CD34(+)FR(+) cells were used as the target. On the other hand, SCF significantly enhanced CFU-mix colony formation supported by IL-3+Epo when CD34(+)c-kit(high or low) and CD34(+)FR(+) cells were used. The replating potential of CFU-mix supported by IL-3+Epo+FL was greater when CD34(+)c-kit(low) or CD34(+)FR(+) cells were used. When the CD34(+)c-kit(low) cells were used, the number of lineages expressed in secondary cultures of CFU-mix colonies derived from primary cultures containing IL-3+Epo+FL or SCF was significantly larger than when the primary cultures contained IL3+Epo. Furthermore, the number of long-term culture-initiating cells found in CD34(+)FR(+) cells was larger than that in FR- cells. CB-derived CD34(+)c-kit(low) cells represent a less mature population than c-kit(high) cells, as reported previously. Therefore, these results indicate that both FL and SCF can act on primitive multipotential progenitors. However, it is still uncertain whether CB-derived CD34(+)FR(+) cells are less mature than CD34(+)FR(-) cells.