Activated platelets but not endothelial cells participate in the initiation of the consolidation phase of blood coagulation

To address the question of whether initiation of the consolidation phase of coagulation occurs on platelets or on endothelium, we have examined the interaction of coagulation factor XI with human umbilical vein endothelial cells (HLTVEC) and with platelets. In microtiter wells factor XI binds to more sites in the absence of HUVEC (1.8 x 10(10) sites/well, K-D = 2.6 nm) than in their presence (1.3 x 10(10) sites/well, K-D = 12 nm) when high molecular weight kininogen (HK) and zinc are present. Binding was volume-dependent and abrogated by HUVEC or Chinese hamster ovary cells and was a function of nonspecific binding of HK to the artificial plastic surface. Factor XI did not bind to HLTVEC or to HEK293 cell monolayers anchored to microcarrier beads. Activation of HLTVEC resulted in von Willebrand's factor secretion, but factor XI binding was not observed. Only activated platelets supported factor XI binding in the presence of HK and zinc (K-D = 8 nm, B-max = 1319 sites/ cell). Activation of factor XI was observed in plasma in the presence of platelets activated by the thrombin receptor activation peptide but not with activated HLTVEC. These results support the concept that activated platelets, but not endothelial cells, expose a procoagulant surface for binding and activating factor XI, thereby initiating the consolidation phase of coagulation.