PROLIFERATIVE PROPERTIES OF HUMAN UMBILICAL-CORD BLOOD MEGAKARYOCYTE PROGENITOR CELLS TO HUMAN THROMBOPOIETIN

We evaluated the effects of human thrombopoietin (TPO) alone, or in co mbination with other several hematopoietic factors, on megakaryocyte c olony growth from human umbilical cord blood CD34(+) cells in serum-de pleted agar cultures. The addition of TPO alone had a concentration-de pendent effect on the number of pure megakaryocyte colonies and of meg akaryocytes per colony (colony size). The maximally stimulating concen tration of TPO generated a greater number of megakaryocyte colonies an d larger megakaryocyte colony size compared with the stimulation obser ved with an optimal concentration of human interleukin-3 (IL-3) or ste m cell factor (SCF). At the high concentration of TPO that yielded the maximum colony numbers, a substantial proportion of megakaryocyte col onies contained 100 or more cells per colony. The combination of TPO p lus SCF resulted in a synergistic enhancement of both the number and s ize of megakaryocyte colonies. Among the combinations of TPO plus othe r cytokines tested, IL-3 plus TPO had a modest effect on mega karyocyt e colony numbers. The generation of megakaryocyte colonies from subpop ulations of CD34(+) cells was further examined. The addition of TPO al one induced a greater number of megakaryocyte colonies from CD34(+)CD4 1(+) cells compared with CD34(+)CD41(-) cells, and TPO plus IL-3 exert ed a synergistic effect on the number of megakaryocyte colonies only f rom CD34(+)CD41(-) cells. In contrast to the effects on colony numbers , TPO induced larger megakaryocyte colony sizes from CD34(+)CD41(-) ce lls, compared with CD34(+)CD41(+) cells. In the case of HLA-DR express ion, TPO and IL-3, administered singly or in combination, induced simi lar megakaryocyte colony numbers and sizes from CD34(+)DR(+) and CD34( +)DR(-) subpopulations. Ploidy analysis revealed that the majority of megakaryocytes generated from cord blood CD34(+) cells in serum-free l iquid cultures containing TPO displayed 2N and 4N values, suggesting t hat they were immature. These results indicate that, compared with IL- 3 and SCF, TPO has more potent proliferative effect on human cord bloo d megakaryocyte progenitors, leading to greater numbers of megakaryocy te progenitors triggered for both growth and cell division, and synerg izes with SCF to enhance megakaryocyte colony growth.