Myeloid Lineage-Restricted Progenitors Contribute to Vascular Endothelium.

Endothelial progenitors of hematopoietic origin have a role in promoting recovery from vascular injury. Recently we have shown that following radiation damage, transplanted hematopoietic stem cells (HSCs) give rise to endothelial cell outcomes at the clonal level. This raises the question of whether prospectively identifiable HSC progeny have endothelial cell activity. Using β-galactosidase and EGFP expression as donor markers, we show that lineage-restricted myeloid progenitors can give rise to endothelial cells. Transplanted common myeloid progenitors (CMPs) and granulocyte/macrophage progenitors (GMPs) gave rise to similar numbers of endothelial cells in liver portal vein branches. Donor-derived endothelial cells expressing functional endothelial markers including CD31, von Willebrand factor (vWF) and Tie-2 were clearly distinguished from mature hematopoietic cells by the absence of CD45 and F4/80 expression. Confocal microscopy showed the co-localization of donor and endothelial cell marker expression in individual cells so that endothelial cells could be discriminated from perivascular cells. To evaluate cell fusion as a potential mechanism for the emergence of endothelial cells of hematopoietic origin, HSCs transgenic for EGFP and Tie-2-Cre recombinase were transplanted into ROSA26 reporter (R26R) mice. Activation of β-galactosidase reporter gene expression, indicative of cell fusion with a donor-derived hematopoietic cell, could be detected in hepatocytes but not in endothelial cells. However, due to the tissue-specificity of the Tie-2 promoter, endothelial fusion products in which the donor nucleus had failed to activate the Tie-2-driven Cre recombinase would have also lacked detectable reporter gene expression. To ensure that the absence of β-galactosidase expression in the donor-derived endothelial cells was not due to insufficient nuclear reprogramming, HSCs transgenic for EGFP and R26R were transplanted into Tie-2-Cre recipients. Donor-derived endothelial cells were again negative for both β-galactosidase expression and activity. To determine whether radiation injury is required for the incorporation of donor cells into the endothelium, a parabiosis model was utilized. Both long-term donor-derived endothelial cell outcomes and multilineage hematopoiesis were detected in parabiotic mice. These results demonstrate that CMPs and further lineage-restricted GMPs are capable of differentiating into endothelial cells in the absence of cell fusion. Importantly, radiation injury is not required for endothelial cell engraftment, indicating that circulating progenitors contribute to angiogenesis during steady-state conditions.