Flk-1 ؉ endothelial progenitors contribute critically to the definitive onset of hematopoiesis during embryogenesis. Recent studies have suggested that adult sources of endothelial cells also possess hematopoietic activity. In this study, we sought to determine whether transplantation of primary vascular endothelial cells (ECs) could enhance the hematopoietic recovery and survival of irradiated mice. C57Bl6 mice were exposed to sublethal and lethal doses of irradiation and were subsequently given transplants of either primary murine brain-derived ECs (MBECs) or fetal blood-derived ECs (FBECs). Mice that received a transplant with MBECs alone demonstrated accelerated BM cellular recovery, radioprotection of BM c-kit ؉ sca-1 ؊ lin ؊ progenitors and enhanced regeneration of c-kit ؉ sca-1 ؉ lin ؊ (KSL) stem/progenitor cells following irradiation compared with controls. MBEC transplantation also facilitated the recovery of circulating white blood cell and platelet counts following radiation exposure. Remarkably, 57% of mice that received a transplant with MBECs alone survived long term following 1050 cGy exposure, which was 100% lethal in control mice. FBEC transplantation was also associated with increased survival compared with controls, although these mice did not survive in the long term. These data suggest that reestablishment of endothelial cell activity can improve the hematopoietic recovery and survival of irradiated mice.
IntroductionStudies have shown that hematopoietic stem cells (HSCs) reside in close association with osteoblasts within the BM niche, and this association contributes to the maintenance of the HSC pool in vivo. 1,2 HSCs also reside in intimate anatomic proximity to vascular endothelial cells (ECs) from the earliest stages of embryonic development, 3 through their migration to the fetal liver, 4 and ultimately throughout their adult residence within the BM compartment. 5 During embryogenesis, mice lacking flk-1 ϩ endothelial precursor cells fail to initiate normal hematopoiesis, 3 and gene marking studies suggest that hematopoietic and vascular endothelial cells derive from a common precursor cell, the hemangioblast. 6 Similarly, bone marrow-, umbilical vein-, and yolk sac-derived endothelial cells elaborate growth factors that support the proliferation of myeloid, erythroid, and megakaryocytic progenitors in vitro. [7][8][9] Our laboratory has shown that primary human brain-derived endothelial cells produce a soluble hematopoietic activity that supports a 1 to 2 log expansion of the most primitive assayable human hematopoietic cell, which is capable of long-term repopulation in nonobese diabetic/severe combined immunodeficient mice. [10][11][12] Li et al 13 have also confirmed that murine brain-and heart-derived endothelial cells can support the maintenance of colony-forming unit-spleen day-8 colonies in vitro. Taken together, these data implicate vascular endothelial cells as a source of proliferative and regenerative signals for hematopoietic stem and progenitor cells.Although these stud...