The effect of platelet derived microvesicles on activated protein C (APC) resistance

Many patients with acute thrombosis have been shown to have acquired APC resistance. This also frequently occurs in normal pregnancy. This study examines the potential factors that are most likely to cause acquired APC resistance and the mechanism for the increased plasma levels of platelet derived microvesicles (PDM) that is associated with these conditions. In normal plasma we could cause a dose dependent reduction in the APC resistance ratio by the in vitro addition of collagen generated PDM (concentration to 30 x 10 / L), recombinant FVIII (concentration to 300 Ill/dl) but not monoclonal purified VWF (concentration to 300 lU/dl). These raised levels are the expected dotting factor and PDM concentrations that are found in pregnancy and as a response to an acute thrombotic event. The effect was synergistic between PDM and FVIII because the APC resistance ratio could be decreased to 62% of baseline with both compared to 76% and 78% with either agent alone. We found a strong association (r = 0.7) between immediately fixed whole blood EDTA PDM detected by flow cytometry and PDM plasma levels after centrifugation. By contrast, there was no difference in a sensitive platelet surface activation marker, P selectin between pregnant woman with elevated PDM and normal controls. These results suggest that the common phenotype of acquired APC resistance is caused by the interaction of increased levels of FVIII and PDM. It is likely that PDM circulate in vivo, providing a source of phospholipid and activated clotting factors and are not an in vitro artefact.