Absence of Correlation between Changes in the Number of Endothelial Progenitor Cell Subsets

Background and Objectives: Previously, various methodologies were used to enumerate the endothelial progenitor cells (EPCs). We now know that these methodologies enumerate at least three different EPC subsets: circulating angiogenic cells (CACs), colony-forming unit endothelial cells (CFU-ECs), and endothelial colony-forming cells (ECFCs). It is not clear whether there is a correlation between changes in the number of these subsets. The aim of the current study is to find an answer to this question. Materials and Methods: The number of all EPC subsets was quantified in the peripheral blood of nine pregnant women in their first and third trimesters of pregnancy. We enumerated 14 cell populations by quantitative flow-cytometry using various combinations of the markers, CD34, CD133, CD309, and CD45, to cover most of the reported phenotypes of CACs and ECFCs. Culturing technique was used to enumerate the CFU-ECs. Changes in the number of cells were calculated by subtracting the number of cells in the first trimester peripheral blood from the number of cells in the third trimester peripheral blood, and correlations between these changes were analyzed. Results: The number of CFU-ECs did not correlate with the number of ECFCs and CACs. Also, CACs and ECFCs showed independent behaviors. However, the number of CACs showed a strong correlation with the number of $CD133^{+}CD309^{+}$ cells (p=0.001) and a moderate correlation with the number of $CD34^{+}CD309^{+}$ cells (p=0.042). Also, the number of ECFCs was correlated with the number of $CD309^{+}CD45^{-}$ cells (p=0.029) and $CD34^{+}CD45^{-}$ cells (p=0.03). Conclusion: Our study showed that the three commonly used methods for quantifying EPC subsets represent different cells with independent behaviors. Also, any study that measured the number of EPCs using the flow cytometry method with a marker combination that lacks CD309 may be inaccurate.