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Background— Receptor tyrosine kinases that include vascular endothelial growth factor (VEGFR)-1, VEGFR-2, and Tie-2 regulate cardiovascular development and physiological and pathological angiogenesis. We were interested in the phenotypic and functional characterization of peripheral blood cells expressing these receptors and their therapeutic potential in vascular injury. Methods and Results— VEGFR-1 + , VEGFR-2 + , and Tie-2 + cells constituted ≈3.0±0.2%, 0.8±0.5%, and 2.0±0.3%, respectively, of the total population of mononuclear cells in blood. Phenotypic analysis demonstrated that all 3 cell populations mainly expressed markers of monocytic/macrophage lineage. Only VEGFR-2 + and Tie-2 + cells phenotypically, morphologically, and functionally differentiated to endothelial cells after culture, whereas VEGFR-1 + cells did not. None of the cell types proliferated in vitro. Only freshly isolated VEGFR-2 + or Tie-2 + cells but not VEGFR-2 − or Tie-2 − cell populations significantly contributed to efficient endothelialization of balloon-injured femoral arteries of nude mice. Furthermore, these cells also differentiated into α-actin–positive smooth muscle cells. Administration of bromodeoxyuridine to animals transplanted with human endothelial progenitor cells showed that VEGFR-2 + and Tie-2 + cells proliferated in vivo. Conclusions— These data demonstrate that expression of VEGFR-2 and/or Tie-2 on peripheral blood cells defines functionally competent cell populations that proliferate in vivo and that contribute to reendothelialization. These findings may have implications for a cell-based approach in vascular diseases.

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