Presence and activity of APC on MP-associated EPCR. HUVECs were incubated with APC (100 nM) and MPs isolated as described in “Materials and methods.” The activity of the APC on the MPs was estimated by a prior FVa inactivation step, followed by a prothrombinase reaction with the extent of thrombin generation reflecting the amount of residual FVa. The isolated MPs were also incubated with α₁-AT prior to the functional test to assess for APC specificity. Controls included APC in complex with sEPCR and the activity of 2 nM APC in the presence of 25 nM PC/PS vesicles was also assessed. Results are expressed as generated thrombin (nM) and represent mean (±SD) values of 3 experiments with samples in duplicates; *P < .005.