XENOGENEIC ENDOTHELIAL-CELLS ACTIVATE HUMAN PROTHROMBIN

Background. Delayed xenograft rejection is characterized by platelet a ctivation and fibrin deposition and is thought to occur independently of complement activation. We have therefore investigated the potential for xenogeneic endothelial cells (EC) to regulate the conversion of p rothrombin to thrombin, a central component of the final common pathwa y of coagulation and all important platelet agonist. Methods and Resul ts. Quiescent porcine aortic EC (PAEC) were found to convert high leve ls of human prothrombin to thrombin (0.234+/-0.019 IU/ml) when compare d with human aortic EC (0.017+/-0 IU/ml, 30-min time point, chromogeni c assay; P<0.001). PAEC activation by human complement resulted in com parable levels of thrombin generation, Prothrombin conversion by PAEC as determined by generation of F1+2 (1.909+/-0.119 nmol/L) and formati on of thrombin-antithrombin III complexes (125.611+/-6.373 mu g/L) was significantly greater than the matched human aortic EC values (F1+2: 1.539+/-0.03 nmol/L, P<0.001; thrombin-antithrombin III: 1.833+/-0.104 mu g/L, P<0.001), Sequential analysis of prothrombin activation by PA EC indicated generation of the intermediate meizothrombin followed by autolytically accelerated thrombin formation. Subsequent experiments e stablished important cross-species' incompatibilities with respect to porcine thrombomodulin interaction with human thrombin and protein C i n that PAEC had, a reduced capacity to generate activated human protei n C in vitro. Conclusion. These observations indicate a potentially im portant molecular barrier involving blood coagulation that may impact on the planned clinical application of porcine transgenic organs.