Studies of infused megakaryocytes into mice support a ''catch-and release'' model of pulmonary-centric thrombopoiesis

Many aspects of thrombopoiesis, the release of platelets from megakaryocytes (Mks), remain under debate, including where this process occurs. Murine lung in situ-microscopy studies suggested that a significant fraction of circulating platelets was released from lung-entrapped, marrow-derived Mks. We now confirm these that endogenous murine (m) Mks are immobilized in the lungs. Intravenously infused in vitro-differentiated, mMks and human (h) Mks are similarly entrapped, followed by shedding of their cytoplasm over [~]30 minutes with a peak number of released platelets occurring 1.5-4 hours later. However, while infused Mks from both species shed large intrapulmonary cytoplasmic fragments that underwent further processing into platelet-sized fragments, the two differed: many mMks escaped from and then recycled back to the lungs, while most hMks were enucleated upon first intrapulmonary passage. Infused immature hMks, inflammatory hMks, umbilical cord-blood-derived hMks and immortalized Mk progenitor cell (imMKCL)-derived hMks were also immobilized in the lung of recipient mice, and released their cytoplasm, but did so to different degrees. Pharmacologically-treated hMks support that membrane stiffness was key to pulmonary thrombopoiesis. Intraarterial infused hMks resulted in few Mks being immobilized in tissues other than the lungs and was accompanied by a blunted and delayed rise in circulating human platelets. These studies demonstrate that the lung entraps and processes both circulating Mks and released large cytoplasmic fragments consistent with a prior lung/heart murine study and support a pulmonary-centric "catch-and-release" model of thrombopoiesis. Thrombopoiesis is a drawn-out process with the majority of cytoplasmic processing of Mks occurring in the pulmonary bed. Key PointsO_LIInfused in vitro-differentiated megakaryocytes synchronously release cytoplasmic fragments selectively in the pulmonary bed. C_LIO_LILarge, released megakaryocyte fragments recycle to the lungs, undergo further fission, terminally forming platelets. C_LI