Human megakaryocytic microparticles target murine hematopoietic stem cells to stimulate in vivo platelet biogenesis

One of the greatest challenges in the production of platelets ex vivo is implementing a system that does not require expensive media and time-consuming laboratory practices that rival the price of a platelet transfusion. Platelet transfusions are required for people who suffer from thrombocytopenia, a condition characterized by low platelet counts. Recently, new evidence has emerged suggesting that megakaryocyte derived microparticles (MkMPs) can be utilized to induce differentiation of the hematopoietic stem cell to megakaryocytes, the precursor of platelets through an ex vivo co-culture system without the cytokines traditionally used. Through endocytotic or membrane fusion events, MkMPs have been shown to deliver their miRNA rich cargo to the target cell and induce the target cell to commit to megakaryopoiesis (the differentiation of hematopoietic stem cell to megakaryocyte) and ultimately generate platelets. ☐ To determine the effectiveness of MkMPs in inducing platelet biogenesis in vivo, this project utilizes a thrombocytopenic mouse model, whereby thrombocytopenia is induced by an antibody targeting the important CD41 complex on the surface of platelets and megakaryocytes. Before the in vivo murine experiments, it was important to examine if human MkMPs can recognize and target murine hematopoietic stem and progenitor cells (HSPCs). By performing ex vivo co-cultures of the murine HSPCs with the human MkMPs, we confirmed that there was uptake of the human MkMP by the murine HSPCs, thus confirming that there is cross reactivity between the human MkMPs and the murine species and that the miRNA cargo of human MkMPs can likely be delivered to murine HSPCs. ☐ The next step was to determine if the human MkMPs can induce platelet biogenesis in non-thrombocytopenic mice. Mice were injected human MkMPs to examine if they could increase to platelet concentration in the mice. After examining three increasing dosages of human MkMPs, there was an observed dose dependent increase in platelet concentration in the mice. Further studies with thrombocytopenic mice showed human MkMPs can partially ameliorate the low platelet concentration resulting from the antibody-induced thrombocytopenia. Confirmation of this increase was performed by observing reticulated (newly synthesized) platelet levels. As demonstrated and expected, there was an increase in the percentage of circulating reticulated platelets confirming that human MkMPs are, in fact, causing de novo megakaryopoiesis and platelet biogenesis. ☐ Taken together, these data suggest that, in vivo, human MkMPs can target murine hematopoietic stem cells and induce them into de novo thrombopoiesis, thus increasing the platelet levels in non-thrombocytopenic mice and ameliorating the induced thrombocytopenia in thrombocytopenic mice.