Endothelial cells mediate the regeneration of hematopoietic stem cells

Recent studies suggest that endothelial cells are a critical component of the normal hematopoietic microenvironment. Therefore, we sought to determine whether primary endothelial cells have the capacity to repair damaged hematopoietic stem cells. Highly purified populations of primary CD31+ microvascular endothelial cells isolated from the brain or lung did not express the pan hematopoietic marker CD45, most hematopoietic lineage markers, or the progenitor marker c-kit and did not give rise to hematopoietic cells in vitro or in vivo. Remarkably, the transplantation of small numbers of these microvascular endothelial cells consistently restored hematopoiesis following bone marrow lethal doses of irradiation. Analysis of the peripheral blood of rescued recipients demonstrated that both short-term and long-term multilineage hematopoietic reconstitution was exclusively of host origin. Secondary transplantation studies revealed that microvascular endothelial cell-mediated hematopoietic regeneration also occurs at the level of the hematopoietic stem cell. These findings suggest a potential therapeutic role for microvascular endothelial cells in the self-renewal and repair of adult hematopoietic stem cells.