THROMBOPOIETIN AUGMENTS EX-VIVO EXPANSION OF HUMAN CORD BLOOD-DERIVEDHEMATOPOIETIC PROGENITORS IN COMBINATION WITH STEM-CELL FACTOR AND FLT3 LIGAND

We studied the effects of stem cell factor (SCF) and flt3 ligand (FL) on the ex vivo expansion of human umbilical cord blood (CB)-derived CD 34(+) cells in combination with various cytokines, including interleuk in (IL)-3, IL-6, IL-11, and c-Mpl ligand (thrombopoietin, TPO), in a s hort-term serum-free liquid suspension culture system. Among the two-f actor combinations tested, SCF plus IL-3 most effectively expanded com mitted progenitor cells, including mixed colony-forming units (CFU-Mix ). The expansion efficiency (EE) of FL for each progenitor was inferio r to that of SCF in the presence of various cytokines, except TPO. IL- 6 significantly increased the EE for granulocyte/macrophage colony-for ming units (CFU-GM) obtained with SCF + IL-3 or FL + IL-3. Interesting ly, TPO markedly augmented the EE for committed progenitors, including CFU-GM, erythroid burst-forming units (BFU-E), and CFU-Mix, in the pr esence of SCF + IL-3 or FL + IL-3. The combinations of SCF + IL-3 + TP O + IL-6 or IL-ll maximally stimulated the expansion of committed prog enitors. The maximum EE for CFU-GM, BFU-E, and CFU-Mix was respectivel y 197-fold (day 14), 80-fold (day 7) and 51-fold (day 14). Other combi nations of cytokines without IL-3 failed to expand effectively these c ommitted progenitors. Our data demonstrate that it is possible to expa nd human CB-derived committed progenitors in vitro using SCF or FL wit h several other cytokines including TPO, and that IL-3 is the key cyto kine promoting the expansion of human hematopoietic progenitors in the presence of SCF or FL.