Endothelial Microparticles and Their Binding to Leukocytes in Patients with End-Stage Renal Disease.

Background: End-stage renal disease (ESRD) has been associated with endothelial dysfunction, a proinflammatory state and markedly increased cardiovascular risk. Endothelial microparticles (EMP) are released by activated or apoptotic endothelial cells. In vitro studies have shown that EMP can bind and activate neutrophils, suggesting that they may represent not only markers of endothelial status but also mediators of inflammation. The objectives of this study were to:

1. Assess the levels of free EMP and leukocyte-bound EMP in ESRD;

2. Determine whether EMP binding to neutrophils predicts the degree of neutrophil activation in patients with ESRD.

Methods: We measured the levels of endothelial microparticles (EMP) and EMP-leukocyte conjugates in 80 patients with ESRD and 35 normal controls. We performed flow cytometry in peripheral venous blood before hemodialysis to measure (1) EMP positive for CD31 (EMP₃₁), E-selectin (EMP_62E), CD54 (EMP₅₄); (2) EMP₅₄- and EMP_62E-leukocyte conjugates; (3) Neutrophil activation, assessed by expression of activation marker CD11b.

Results: Assessment of free EMP revealed that patients with ESRD had slightly elevated levels of free EMP₅₄ but not of free EMP_62E or EMP₃₁. However, levels of EMP₅₄-Lymphocyte, EMP₅₄-Monocyte, EMP₅₄-Neutrophil, EMP_62E-Lymphocyte and EMP_62E-Monocyte conjugates were clearly elevated in patients with ESRD. CD11b expression in leukocytes strongly correlated with levels of EMP₅₄-Neutrophil conjugates (R=0.67; p<0.0001) and EMP_62E-Neutrophil conjugates (R=0.66; p<0.0001). Furthermore, both types of conjugates independently predicted CD11b expression (adjusted p<0.05).

Conclusions: ESRD is associated with increased binding of EMP to leukocytes, resulting in normal levels of some species of free EMP. Therefore, measurement of both free EMP and EMP-leukocyte conjugates is necessary for adequately assessing EMP release in ESRD. Levels of EMP bound to neutrophils strongly predict neutrophil activation, suggesting that EMP induce neutrophil activation in vivo, consistent with in vitro studies. EMP may represent important mediators of the proinflammatory state in ESRD.