We investigated the effects of recombinant human thrombopoietin (TPO) in co
mbination with various cytokines including erythropoietin (EPO), interleuki
n-3 (IL-3), interleukin-6 (IL-6), and stem cell factor (SCP) on megakaryopo
iesis, and the expansion of CD34(+)CD41a(+) cells from human cord blood CD3
4(+) cells with these cytokines under serum-free conditions. Human cord blo
od CD34+ cells were cultured in Megacult(TM) (Stem Cell Technologies Inc. V
ancouver, Canada) in the presence of recombinant growth factors. Colony-for
ming unit-megakaryocyte (CFU-M) colonies were counted on day 14. CD34(+)CD4
1a(+) and CD34-CD41a(+) cell expansion was analyzed using a serum-free liqu
id culture system for 7 days with recombinant growth factors. TPO alone had
a concentration-dependent effect on megakaryocyte colony growth. At concen
trations above 1 ng/ml, TPO supported significant CPU-Meg colony formation
in a concentration-dependent manner. The combination of TPO plus other cyto
kines, including EPO, IL-3, and SCP, resulted in a synergistic enhancement
of the number of CPU-Meg colonies, but IL-6 failed to enhance the effect of
TPO. The number of CD41a(+) cells increased after 7 days in liquid culture
of human cord blood CD34(+) cells with various cytokines (EPO, IL-3, IL-6,
SCP) combined with TPO, but SCP plus TPO only resulted in a significant sy
nergistic increment of CD34(+)CD41a(+) cells compared with TPO alone. The r
esults of the present study indicate that EPO, IL-3, and SCP can be synergi
stic with TPO to stimulate proliferation of CPU-Meg and suggest that SCP pl
us TPO can expand CD34(+)CD41a(+) cells to effect the rapid recovery of pla
telets in patients following stem cell transplantation.