Sites and kinetics of donor thrombopoiesis following transplantation of whole bone marrow and progenitor subsets

Introduction Little is known about the sites and kinetics of thrombopoiesis following bone marrow transplant. The spleen is a site of hematopoiesis in a healthy mouse, and hematopoietic activity increases in response to stress. We hypothesized that the spleen is a major site of early post-transplant thrombopoiesis. Methods We transplanted whole bone marrow (WBM) or lineage depleted progenitor subsets fractionated based on expression of c-kit and Sca-1 from transgenic mice expressing green fluorescent protein into lethally irradiated C57BL/6 recipients. We also transplanted whole bone marrow cells into healthy and splenectomized mice. Post-transplant megakaryopoiesis was assessed by measuring circulating platelet number, percent donor-derived platelets, bone marrow cellularity, splenic weight, megakaryocyte size, and megakaryocyte concentration from hour 3 to day 28 post transplant. Results Following transplant, circulating donor-derived platelets were derived only from c-kit expressing subsets. Donor-derived platelets first appeared on post-transplant day five. Splenectomy reduced the number of these earliest circulating platelets. Splenic megakaryopoiesis increased dramatically from day 7-14 post-transplant. However, splenectomy accelerated platelet engraftment during this time frame. Conclusion Overall, these results demonstrate that the first platelets are produced by c-kit expressing megakaryocyte progenitors in the bone marrow and spleen. After post-transplant day 5, the net effect of the spleen on thrombopoiesis is to slow engraftment due to immune effects or hypersplenism.