Retention of a differentiated endothelial phenotype by outgrowth endothelial cells isolated from human peripheral blood and expanded in long-term cultures

Rapid adequate vascularization by autologous human endothelial cells remains a limiting step in the treatment of ischemic tissues and the generation of new tissues. We have expanded outgrowth endothelial cells (OEC) from human peripheral blood and investigated their phenotypic stability in long-term cultures. Our goal has been to obtain suitable numbers of autologous endothelial cells for pro-angiogenic cell therapies. Mononuclear cells were isolated from human peripheral blood. During culture, cells were characterized for several endothelial and stem cell markers in mono- or in co-culture with mature endothelial cells. In cultures from peripheral blood, we observed cells with a variable ability to assume a differentiated endothelial phenotype. Most of the cells showed markers reported for endothelial progenitor cells or hemangioblasts (CD31, KDR, VE-cadherin, CD34, CD117, CD45) but failed to develop a differentiated phenotype. Caveolin-1 was not detectable in these cells by reverse transcription/polymerase chain reaction (RT-PCR) or immunofluorescence. Another cell type arising from the same cultures expressed a differentiated phenotype and was designated as an OEC. This subset as an OEC was expanded in long-term cultures and analyzed by immunofluorescence, flow-cytometry, and RT-PCR for a stable endothelial phenotype. OEC showed several markers of a differentiated endothelium, such as high levels of caveolin-1 throughout all tested passages, and the ability to form angiogenic sprouts in vitro. Thus, OEC in long-term expansion cultures from blood mononuclear cells are phenotypically highly stable, a feature that is an important prerequisite for using OEC from peripheral blood for autologous endothelial cell therapies.