Circulating Endothelial Cells and Circulating Endothelial Progenitor Cells in Polycythemia Vera.

Background: Angiogenesis and thrombosis, two conditions associated with perturbation of the vascular endothelium and increased CECs, are frequently observed in PV. We performed this study to quantify and characterize CECs in patients with PV and to determine whether CEC profile is associated with thrombotic complications. We also quantified CEPs in a subgroup of patients.

Methods: We used flow cytometry to prospectively analyze CECs and CEPs in the whole blood of healthy patients (n=20) and patients with PV (n=30; 13 treated with hydroxyurea, 12 undergoing phlebotomy alone, and 5 never treated). CECs (CD45−/CD31+/CD146+) were quantified and characterized to determine their apoptotic (annexin stain) or activation (CD106+) states. Cells with CD45−/CD31+/CD133+/VEGF-R2+ immunophenotype were considered CEPs.

Results: CEC levels in PV patients were higher compared to healthy controls (median 53 cells/mL [range, 11–392] vs 18.5 cells/mL [range, 4–66]; P< .0001). However, the proportions of apoptotic (17.3% [range, 0–87.5] vs 25.9% [range, 0–57.1]; P= .87) and activated (0.7% [range, 0–28.6] vs 0% [range, 0–57.1]; P= .14) CECs were similar between the two groups. In PV patients with high levels (≥2 SD above control mean or ≥53 cells/mL) of CECs (n=14), 5 (36%) developed thromboses. Four (25%) of 16 PV patients with CEC levels similar to healthy controls developed thromboses. The rates of thrombosis between the groups were not statistically different (P= .69). In addition, CEC count, activation, and apoptosis were similar between the hydroxyurea-treated group and the group not treated with hydroxyurea. We detected low numbers of CEPs in PV patients (n=25) that were similar to controls (4 cells/mL [range, 0–100] vs 8 cells/mL [range, 0–60]; P= .34).

Conclusions: CECs, but not CEPs, are significantly increased in PV patients. However, there appears to be no association between CEC number, activation or apoptotic state and the development of thromboses. In addition, hydroxyurea therapy does not appear to effect endothelial cells.