Endothelial progenitor cells are recruited into resolving venous thrombi

Background - The purpose of this study was to determine whether endothelial cells of bone marrow origin are involved in thrombus recanalization. Methods and Results - Irradiated mice were reconstituted with bone marrow from transgenic donors expressing green fluorescent protein (GFP) linked to the Tie2 promoter. Thrombi were formed in 2 groups of 6 mice. GFP-expressing cells were located and quantified in sections of the thrombi taken after 7 and 14 days. The cell markers Mac-3, F4/80, CD68 (macrophage), and vascular endothelial growth factor receptor 2 (VEGFR2; endothelial cells) were used to determine colocalization with GFP expression in tissue sections and peritoneal macrophages. The markers CD34 and VEGFR2 were used to quantify changes in circulating endothelial cells by flow cytometry of blood from 3 cohorts of wild-type animals that had either a thrombus induced (n = 18), a sham operation (n = 18), or no operation (n = 10). The number of GFP-expressing cells was found to increase by approximate to 3-fold in thrombi formed in transplanted animals between 7 and 14 days after induction (P = 0.0022). No GFP-expressing cells were found lining the new vascular channels that formed at either time interval, but many of the GFP-expressing cells also expressed Mac-3, CD68, and VEGFR2. Approximately twice as many circulating CD34(+)/VEGFR2(+) cells were found by day 3 in animals with thrombus compared with sham controls (CD45(-) , P=0.046 and CD45(-) , P = 0.016). Conclusions - Bone marrow - derived, Tie2-expressing cells were recruited into the thrombus during resolution but did not line the new vessels. Many of these cells expressed a macrophage phenotype and may represent a population of plastic stem cells that orchestrate thrombus recanalization.