Large Scale Generation of Functional Megakaryocytes From Human Embryonic Stem Cells (hESCs) Under Stromal-Free Conditions.

Abstract 2540

Poster Board II-517

Platelets collected from donors have very limited shelf life and are increasingly needed for transfusions. In contrast to donor dependent cord blood or bone marrow CD34+ stem cells, hESCs are a promising alternative source for continuous in vitro production of platelets under controlled conditions. Current procedures for in vitro generation of megakaryocytes/platelets from hESCs are not efficient and require undefined animal stromal cells. We have developed a novel system to generate megakaryocytes (MKs) from human ES cells under serum and stromal-free conditions. In the current system, hESCs are directed towards megakaryocytes through distinct steps including embryoid body formation and hemangioblast development (Lu et al, Nature Methods, 4:501–509, 2007). A transient bi-potential cell population expressing both CD41a and CD235a markers has been identified at the end of hemangioblast culture. These cells are capable of generating both MKs and erythroid cells as demonstrated by FACS sorting and CFU assays. TPO, SCF and IL-11 are used to further direct MK differentiation of hemangioblasts derived from human ES cells in suspension culture. Currently, up to 2.5×10⁷ MKs (CD41a+) can be generated from 1×10⁶ hESCs, which is approximately 10 times more efficient than recently reported methods (Takayama et al Blood, 111(11):5298–5306, 2008). Without further purification, >90% of live cells from the suspension cultures are CD41a+ and the majority of these cells are also CD42b+ (>70%). These in vitro derived MK cells have morphological characteristics of mature, polyploid MKs as shown by Giemsa staining and immunofluorescent staining of vWF in cytoplasmic granules. Importantly, proplatelet forming cells are constantly observed at the late stage of MK culture indicating that MKs generated in this system are able to undergo terminal differentiation under feeder-free conditions. Platelet-like particles are also detected in culture media by FACS. When plated on OP9 cells, these MKs generate functional platelets that are responsive to thrombin stimulation. In summary, we have established a novel system for the generation of platelet-producing MKs from human ES cells that is suitable for scale up and future preclinical and clinical studies.