Formation and functioning of the factor IXa-VIII complex on the surface of endothelial cells

The formation and functioning of the factor X activating complex on the surface of cultured human venous endothelial cells (HVEC) were investigated. To the HVEC monolayer human factors IXa, VIII, X, CaCl2, and S-2222 were added, and a gradually increasing activation of factor X was observed. The maximum activity of 88 nmol/L Xa/min was reached after a 12-minute lag phase. In the presence of thrombin-activated factor VIII (VIIIt) the same maximum activity developed in eight minutes, which suggests that VIIIt accelerates the formation of the IXa-VIII complex but does not influence its factor X-activating potential. Anti-VIII IgG did not affect the activity of the full-fledged complex. When anti-VIII IgG was added to the reaction mixture before factor VIII or during the lag phase of the reaction, it induced a concentration-dependent decrease of factor X activation. These results indicate that endothelial cells provide a binding surface for the IXa-VIII complex and that in the HVEC-bound complex factor VIII is protected from the effect of a specific antibody. However, the relatively slow development of the maximum activity indicates that HVEC only partially satisfy the surface criteria for the optimal assembly of the IXa-VIII complex.