Expression of VEGFR-2 and AC133 by circulating human CD34(+) cells identifies a population of functional endothelial precursors

Emerging data suggest that a subset of circulating human CD34(+) cells have phenotypic features of endothelial cells. Whether these cells are sloughed mature endothelial cells or functional circulating endothelial precursors (CEPs) is not known. Using monoclonal antibodies (MoAbs) to the extracellul ar domain of the human Vascular endothelial receptor-2 (VEGFR-2), we have s hown that 1.2 +/- 0.3% of CD34(+) cells isolated from fetal liver (FL), 2 /- 0.5% from mobilized peripheral blood, and 1.4 +/- 0.5% from cord blood w ere VEGFR-2(+). In addition, most CD34(+)VEGFR-2(+) cells express hematopoi etic stem cell marker AC133. Because mature endothelial cells do not expres s AC133, coexpression of VEGFR-2 and AC133 on CD34(+) cells phenotypically Identifies a unique population of CEPs. CD34(+)VEGFR-2(+) cells express end othelial-specific markers, including VE-cadherin and E-selectin, Also, virt ually all CD34(+)VEGFR-2(+) cells express the chemokine receptor CXCR4 and migrate in response to stromal derived factor (SDF)-1 or VEGF, To quantitat e the plating efficiency of CD34(+) cells that give rise to endothelial col onies, CD34(+) cells derived from FL were Incubated with VEGF and fibroblas t growth factor (FGF)-2, Subsequent isolation and plating of nonadherent FL -derived VEGFR-2(+) cells with VEGF and FGF-2 resulted in differentiation o f AC133(+) VEGFR-2(+) cells Into adherent AC133(-)VEGFR-2(+)Ac-LDL+ (acetyl ated low-density lipoprotein) colonies (plating efficiency of 3%). In an In vivo human model, we have found that the neointima formed on the surface o f left ventricular assist devices is colonized with AC133(+)VEGFR-2(+) cell s. These data suggest that circulating CD34(+) cells expressing VEGFR-2 and AC133 constitute a phenotypically and functionally distinct population of circulating endothelial cells that may play a role In neo-angiogenesis. (C) 2000 by The American Society of Hematology.