Intramedullary megakaryocytes internalize released platelet factor 4 and store it in alpha granules

Summary Background Megakaryocytes express and store platelet factor 4 (PF4) in alpha granules. In vivo, PF4 is a clinically relevant, negative regulator of megakaryopoiesis and hematopoietic stem cell replication. These findings would suggest a regulated source of free intramedullary PF4. Objectives Define the source of free intramedullary PF4 and its intramedullary life cycle. Methods We interrogated both murine and human bone marrow–derived cells during megakaryopoiesis in vitro by using confocal microscopy and enzyme‐linked immunosorbent assay. With immunohistochemistry, we examined in vivo free PF4 in murine bone marrow before and after radiation injury and in the setting of megakaryocytopenia and thrombocytopenia. Results Exogenously added human PF4 is internalized by murine megakaryocytes. Human megakaryocytes similarly take up murine PF4 but not the related chemokine, platelet basic protein. Confocal microscopy shows that internalized PF4 colocalizes with endogenous PF4 in alpha granules and is available for release on thrombin stimulation. Immunohistochemistry shows free PF4 in the marrow, but not another alphagranule protein, von Willebrand factor. Free PF4 increases with radiation injury and decreases with megakaryocytopenia. Consistent with the known role of low‐density lipoprotein receptor–related protein 1 in the negative paracrine effect of PF4 on megakaryopoiesis, PF4 internalization is at least partially low‐density lipoprotein receptor–related protein 1 dependent. Conclusions PF4 has a complex intramedullary life cycle with important implications in megakaryopoiesis and hematopoietic stem cell replication not seen with other tested alpha granule proteins.