Background— Recently, it has been shown that stem/progenitor cells may repair damaged/lost endothelial cells in vein grafts and wire-injured arteries. In the present study, we investigated endothelial cell turnover and regeneration in apolipoprotein E (apoE)^−/−/transgenic mice carrying LacZ genes driven by an endothelial TIE2 promoter.

Methods and Results— To assess cell proliferation on the surface of aortas in apoE^−/− mice and wild-type controls, BrdU was injected into the tail vein and labeled on en face preparation. BrdU-positive cells on the aortas were observed occasionally in wild-type mice and frequently at sites prone to lesion development in apoE^−/− mice (0.18±0.1% versus 1.12±0.2%; P<0.001). Endothelial integrity tests demonstrated that the areas with high rate of cell turnover displayed Evans blue leakage, low levels of VE-cadherin expression, and increased cell attachment, as evidenced by Evans blue dye injection, immunostaining, and scanning electron microscopy, respectively. Furthermore, immunostaining for CD34, Sca-1, Flk-1, and CD133 indicated that ≈3% to 5% of total cells on the aorta were positive in apoE^−/− mice. En face double labeling using Ki-67 and progenitor markers revealed that 30% to 50% of progenitor⁺ cells expressed Ki-67, indicating a state of proliferation. To clarify the origin of endothelial progenitor cells participating in endothelial repair in apoE^−/− mice, a chimeric mouse model was created by bone marrow transplantation between apoE^−/− and LacZ^+/+/apoE^−/− mice. Ten months after bone marrow transplantation, ≈3% to 4% of total cells in the lesion-prone areas were β-gal positive in apoE^−/− with apoE^−/−/TIE2-LacZ bone marrow mice. When cells of aortas from chimeric mice were cultivated on Matrigel-coated plates, a capillary-like structure was found, which showed β-gal/CD31 or β-gal/von Willebrand factor double positivity. By a combined analysis of laser dissection microscopy and nest reverse transcription polymerase chain reaction, it was found that β-gal⁺ cells were mainly expressing CD31 and CD144.

Conclusions— Our findings provide the first quantitative data on endothelial turnover and repair by progenitor cells that are, at least in part, derived from bone marrow during development of atherosclerosis in apoE^−/− mice.