Recombinant Tpo Stimulates Maturation of Megakaryocytes Derived from Adult Peripheral Blood- but Not Cord Blood-CD34⁺ Cells.

Umbilical cord blood (CB) is a valuable source of stem cells for transplantation. However, platelet engraftment is slow, taking approximately 70 days for CB transplants versus 20 days for mobilized adult peripheral blood (PB) transplants. This time is not significantly shortened by the administration of recombinant thrombopoietin (rTpo). The cause for the delay in platelet engraftment following CB transplant is unknown. We hypothesized that developmental differences in size and ploidy of neonatal versus adult megakaryocytes (MKs) contribute to this delay. To mimic these two types of transplant in vitro, we compared CB to PB CD34⁺ cells cultured in adult bone marrow stromal-conditioned media (CM) or unconditioned media (UCM) for 14 days. Increasing doses of rTpo were added to the CM, and the resulting MK maturation was compared with that of UCM with maximal rTpo concentration. MK number and ploidy were determined by flow cytometry using CD41-FITC and propidium iodide, respectively. Increased ploidy levels were expressed as percentage of MKs with a ploidy ≥ 8N. Results represent an average of three independent experiments.

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As seen in the figure, PB-derived MKs reached highest ploidy levels in the presence of UCM + 100 ng/ml rTpo. When cultured in CM, they exhibited lower ploidy levels, regardless of Tpo concentration. In contrast, CB-derived MKs exhibited higher ploidy levels in response to CM with either 0 or 0.1 ng/ml (physiologic concentration) of rTpo, as compared to higher rTpo concentrations or UCM + 100 ng/ml rTpo. MK numbers increased in response to rTpo in a dose-response manner, regardless of the source of the MKs (data not shown). These results indicate that intrinsic differences between CB- and PB-derived megakaryocytes exist, and that maturation is regulated differently in neonatal versus adult MKs. While Tpo is a potent stimulator of MK maturation in PB-derived MKs, it appears to inhibit this process in CB-derived MKs. These differences may be relevant to understanding the delayed platelet engraftment following CB transplants.