Thrombopoietin enhances generation of CD34+ cells from human embryonic stem cells.

The role of thrombopoietin (TPO) in adult hematopoiesis is well-established. A recent report suggests that TPO and vascular endothelial growth factor (VEGF) play a role in promoting formation of early erythropoietic progenitors in a nonhuman primate embryonic stem cell (ES) model. No such report exists for human ES cells as yet. Because TPO may become an important factor promoting human ES cell-derived hematopoiesis, we sought to investigate whether TPO in combination with VEGF can enhance human ES-derived hematopoiesis in an EB-derived culture system. The emphasis of this work was to demonstrate the molecular mechanisms involved in this process, specifically the role of c-mpl and its ligand TPO. Human ES cells were cultured to the EB state, and EB-derived secondary cultures supporting hematopoietic differentiation were established: condition 1, control (stem cell factor [SCF] and Flt3 ligand [Flt3L]); condition 2, SCF, Flt3L, and TPO; and condition 3, SCF, Flt3L, TPO, and VEGF. Cells were harvested daily, starting at day 2 and continuing until day 8, for reverse transcription-polymerase chain reaction and Western blot. There was no evidence of expression of c-mpl and VEGF receptor on the gene or protein level until day 8, when the formation of well-established hematopoietic colonies began. This correlated with the formation of CD34+/CD31- negative progenitors, mostly found in blast-forming units-erythroid-like colonies. We concluded that TPO and VEGF play an important synergistic role in the formation of early ES-derived hematopoietic progenitors that occurs through the c-mpl and VEGF receptors. Disclosure of potential conflicts of interest is found at the end of this article.