HUMAN FLT3 LIGAND ACTS ON MYELOID AS WELL AS MULTIPOTENTIAL PROGENITORS DERIVED FROM PURIFIED CD34(-KIT PROTEIN() BLOOD PROGENITORS EXPRESSING DIFFERENT LEVELS OF C)

We studied the effect of human flt3/flk2 ligand (FL) on the proliferat ion and differentiation of purified CD34(+) blood progenitors which ex press different levels of c-kit protein in clonal cell culture in comp arison with that of stem cell factor (SCF). FL alone did not support s ignificant colony formation. However, FL significantly enhanced neutro phil colony (CFU-G) formation in the presence of granulocyte-colony st imulating factor (G-CSF) by peripheral blood (PB)-derived CD34(+)c-kit (-) cells which contained a large number of CFU-G. In addition, FL cou ld synergistically increase the number of CFU-G supported by a combina tion of interleukin (IL)-3 and G-CSF, as did SCF. As we reported previ ously, SCF showed a significant burst-promoting activity (BPA). In con trast, FL did not exhibit any BPA on PB-derived CD34(+)c-kit(high) cel ls in which erythroid-burst (BFU-E) was highly enriched. However, FL c ould synergize with IL-3 or GM-CSF in support of erythrocyte-containin g mixed (E-Mix) colony by PB-derived CD34(+)c-kit(high or low) or low cells in the presence of Epo. Replating of E-Mix colonies derived from CD34(+)c-kit(high) cells supported by IL3+Epo+SCF yielded more second ary colonies than those supported by IL-3+Epo or IL-3+Epo+FL. When PB- derived CD34(+)c-kit(low) cells which represent a more immature popula tion than CD34(+)c-kit(high) cells were used as the target, number of secondary colonies supported by IL-3+Epo, IL-3+Epo+SCF or IL-3+Epo+FL was comparable. However, the number of lineages expressed in the secon dary culture was significantly larger in the primary culture containin g IL-3+Epo+FL than in that containing IL-3+Epo. These results suggest that FL not only acts on neutrophilic progenitors, but also on more im mature multipotential progenitors.