Blood Cells from Pluripotent Stem Cells: The Platelet Story

The combination of a constantly increasing demand for platelet transfusions, a static platelet donor base, the discovery of the megakaryocyte cytokine thrombopoietin, and the ability to differentiate embryonic stem cells (ESCs) and induced- pluripotent stem cells (iPSCs) into megakaryocytes and platelets has lead to a “Perfect Storm”, where a number of groups have declared that they have generated clinically relevant platelets in vitro. We believe that progress has been made towards this goal of generating platelets from in vitro-grown megakaryocytes but believe that these studies have yet to achieve a product approaching donor-derived platelets. We will discuss four challenges faced by the field, and the strategies and successes so far achieved to address them. Challenge #1 is that ESCs/iPSCs differentiate into primitive, yolk sac-like megakaryocytes. These cells yield a low number of platelets, and these primitive platelets have limited functionality. Strategies to achieve more definitive megakaryocytes and with better platelet yield and functionality will be presented. Challenge #2 is yield of platelets per beginning stem cell. At the moment, yields have been about 10^0-2 megakaryocytes/initial stem cell and the same number of platelets/final megakaryocyte. Given that a unit of platelets has >10¹¹ platelets, it is clear that the expense to generate this number of platelets is presently daunting. Strategies to increase the yield have been proposed including generating a self-replicating intermediate megakaryocyte progenitor that can be readily differentiated into functional megakaryocytes. Challenge #3 is that in vitro both megakaryocytes and platelets are injured by proteolysis of the glycoprotein Ibalpha ectodomain, desialylation, and apoptosis. The injured platelet subsequently have a short half-life in vivo. Some of the injuries may be by contaminating non-myeloid lineages and others to the growth of the cells under static condition in plastic dishes. Strategies that may limit these injuries will be discussed. Challenge #4 is the recognition that in vitro particles include actual platelets, but also cytoplasmic fragments from megakaryocytes and non-platelet cellular fragments of the same size range. A number of bioreactors have been proposed as well as direct infusion of megakaryocytes to release platelet in vivo as solutions and the success of these strategies will also be discussed. In summary, we believe that stem cell-derived platelets is not yet ready for primetime; however, we believe that by defining the challenges we will be on track to achieve this clinically valuable goal.