Validation of a standardized method for enumerating circulating endothelial cells and progenitors: flow cytometry and molecular and ultrastructural analyses.

PURPOSE: Antigenic overlap among circulating endothelial cells (CEC) and progenitors (CEP), platelets, and other blood cells led to the need to develop a reliable standardized method for CEC and CEP quantification. These cells are emerging as promising preclinical/clinical tools to define optimal biological doses of antiangiogenic therapies and to help stratify patients in clinical trials. EXPERIMENTAL DESIGN: We report the experimental validation of a novel flow cytometry method that precisely dissects CEC/CEP from platelets and other cell populations and provides information about CEC/CEP viability. RESULTS: Sorted DNA/Syto16(+)CD45(-)CD31(+)CD146(+) CECs, investigated by electron microscopy, were found to be bona fide endothelial cells by the presence of Weibel-Palade bodies. More than 75% of the circulating mRNAs of the endothelial-specific gene, VE-cadherin, found in the blood were present in the sorted population. CECs were 140 +/- 171/mL in healthy subjects (n = 37) and 951 +/- 1,876/mL in cancer patients (n = 78; P < 0.0001). The fraction of apoptotic/necrotic CECs was 77 +/- 14% in healthy subjects and 43 +/- 23% in cancer patients (P < 0.0001). CEPs were 181 +/- 167/mL in healthy donors and 429 +/- 507/mL in patients (P = 0.00019). Coefficients of variation were 4 +/- 4% (intrareader), 17 +/- 4% (interreader), and 17 +/- 7% (variability over 0-72 h), respectively. Parallel samples were frozen by a standardized protocol. After thawing, coefficients of variation were 12 +/- 8% (intrareader), 16 +/- 10% (interreader), and 26 +/- 16% (variability over 0-14 days of frozen storage), respectively. CONCLUSIONS: This procedure enumerates a truly endothelial cell population with limited intrareader and interreader variability. It appears possible to freeze samples for large-scale CEC enumeration during clinical trials. This approach could be enlarged to investigate other angiogenic cell populations as well.

Circulating endothelial cell number and viability are reduced by exposure to high altitude.

High altitude and hypoxia are known to induce polycythemia, pulmonary hypertension, and vascular remodeling. The authors investigated a number of blood cell populations in 15 mountain trekkers before and after 12 days spent at >3000 m. Red blood cell and platelet count increased, whereas circulating hematopoietic stem cell (enumerated as CD34bright cells), circulating endothelial cell (CEC) and circulating endothelial progenitor (CEP) count significantly decreased. In particular, the authors observed a decrease in the count of viable CECs, and a decrease in the circulating levels of RNA of the endothelial-specific gene VE-cadherin, whereas the fraction of apoptotic/necrotic CECs was stable. These data suggest a unique pattern of modulation of surrogate markers of vascular remodeling induced by exposure to hypobaric hypoxia.