Platelets, anucleate cells derived from megakaryocytes (MKs) that are generated within the bone marrow, play an important role in the process of physiological hemostasis and in vascular repair. Low platelets in the blood stream result in bleeding risk in thrombocytopenic patients with liver failure, leukemia, or undergoing chemotherapy. Platelet transfusions remain the mainstay of treatment and require a constant supply of platelets. Because platelets from donor blood have a short life-span (only few days in storage), platelets are always in a short supply. In vitro generation of MKs and platelets from human induced pluripotent stem cells (hiPSCs) would provide a patient-specific renewable cell source of MKs and platelets to treat thrombocytopenic patients at risk of hemorrhage. We derived integration-free hiPSCs from peripheral blood cells of more than 20 individuals, and examined two methods of in vitro differentiation into MKs: i) co-culture on 10T1/2 cells or OP9 cells first developed by Takayama et al. (2010), and ii) a feeder-free and serum-free system by first forming embryoid bodies (EBs) in a chemically defined condition, similar to the recently published method of Pick et al. (2013). Although both methods gave rise with similar efficiency to CD41a+CD42a+ MKs with large cell size and high-ploidy DNA, we chose to focus on the feeder-free system that began with EB formation with centrifugal aggregation of hiPSCs (spin-EBs) because it is cheaper, faster, easier to scale up, and represents a chemically defined system. To investigate the effect of growth factors on hiPSC differentiation to MKs, we modified the spin-EB system to three steps: i) mesoderm induction and hematopoietic commitment in the presence of BMP4, VEGF, bFGF and SCF (day 0 to day 11), ii) hematopoietic progenitor and MK differentiation by adding TPO (day 11 to 14), and iii) MK maturation (day 14 to 19). To assess whether the FDA-approved pharmacological agent, Romiplostium (Nplate®, TPO analog), has a similar effect to TPO on MK differentiation from hiPSCs, we isolated hematopoietic progenitor cells at day 14, and differentiated them into MKs with Romiplostium or TPO. Our data demonstrated that Romiplostium (50 ng/ml) gave a 3-fold increase of CD41a+CD42a+ MKs, with similar dose-dependent kinetics as TPO. IL-11 has also been reported to enhance MK development. To test whether FDA-approved pharmacological IL-11, Oprelvekin (Neumega®), further stimulated MK differentiation from hiPSCs, we cultured hematopoietic progenitor cells from day 14 in the presence of Romiplostium and Oprelvekin for 5 days. Our data showed that Romiplostium and Oprelvekin synergistically promote megakaryocytic differentiation. In the presence of Romiplostium, 60 to 95 % of cells were CD41a+CD42a+ MKs. Addition of Oprelvekin significantly increased the number of CD41a+CD42a+ MKs, but not the percentage of CD41a+CD42a+ MKs, suggesting that Oprelvekin enhanced a proliferation of MK progenitors. So far, 10 hiPSC lines from several individuals have been tested using the combination of Romiplostium and Oprelvekin in the feeder-free and serum-free differentiation condition. We are currently investigating if the MKs and platelets generated by this defined and scalable system are as fully functional as those generated from bone marrow CD34+ cells from healthy donors.
* The first three authors contributed equally;
This study is supported in part by an NIH grant U01 HL-107446 and 2012-MSCRFII-0124 (to ZZ Wang).
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.