Abstract 525: Endothelial Progenitor Cells Bind Platelets And Inhibit Aggregation And Adhesion Via Prostacyclin Secretion
Background - Interactions of Endothelial Progenitor Cells (EPCs) with blood cells play a central role in vascular biology and may influence platelet reactions and thrombosis. This study characterizes the in vitro differentiation of Peripheral Blood Mononuclear Cells (PBMCs) into EPCs and their impact on platelet function.
Methods and results - Cultured on fibronectin-coated dishes, PBMCs differentiate into early and late EPCs, which exhibit distinct morphological, phenotypical and functional characteristics. At day 0 of culture, cells have a rounded morphology and express leukocyte markers (87% CD14, 93% PSGL-1 and 46% L-selectin), whereas EPC specific labeling is not observed. At day 5, early EPCs form colonies and express EPC markers (55% CD34, 52% P-selectin and 16% VEGFR2) while leukocyte markers are down regulated (56% CD14, 52% PSGL-1 and 40% L-selectin). At day 10, late EPCs form a monolayer of spindle-shaped cells and highly express EPC markers (63% CD34, 56% P-selectin and 33% VEGFR2) while leukocyte markers are absent. EPC phenotype was confirmed by uptake of acetylated-LDL, and binding of ulexlectin under a confocal microscopy. Binding of early EPCs to platelets increases from 33% at baseline to 46% following activation, which is completely prevented by P-selectin, but not GPIIb/IIIa or L-selectin, blockade. However, the binding of late EPCs to platelets was not P-selectin, GPIIb/IIIa or L-selectin dependent. Early EPCs reduce platelet aggregation by 30%, whereas late EPCs inhibit completely platelet aggregation and significantly reduce platelet deposition (from 16.0% to 3.7%) on an ex-vivo capillary perfusion chambers coated with human type I collagen. This effect is likely prostacyclin-dependent, since it is inhibited by a cyclooxygenase inhibitor (indomethacin), but not by a NO chelator (PTIO).
Conclusions - EPCs differentiated from PBMCs interact with activated platelets by a mechanism that involves P-selectin only in the early stage of differentiation. EPCs inhibit platelet aggregation via prostacyclin secretion and reduce platelet deposition on a denuded capillary surface mimic. These findings add new insight into the biology of EPCs and define their potential role in the control of platelet function.