Protective Effect of Activated Protein C in Murine Endotoxemia: Mechanism of Action.

Recombinant activated protein C (APC) reduces mortality of patients with severe inflammatory disease associated with multi-organ failure. APC exerts anticoagulant, anti-inflammatory, and cytoprotective effects. The contribution of these distinct APC activities to the overall therapeutic efficacy in septic patients is unknown. The aim of the study is to delineate the mechanism underlying the protective effect of APC in mouse endotoxemia. We first establish an experimental mouse model to demonstrate that recombinant murine APC reduces 6 day mortality of mice subjected to LPS-induced endotoxemia. APC treatment did not alter the extent of inflammatory cytokine release. Recombinant human APC did not exhibit therapeutic efficacy in this model. In contrast, recombinant human and mouse APC reduced to a similar extent experimentally induced arterial thrombus formation. The therapeutic efficacy of wild type recombinant murine APC was abolished in genetically engineered mice with reduced expression of the endothelial protein C receptor (EPCR). Recombinant mutant murine APC with greatly reduced anticoagulant potency was as effective as wild type murine APC in reducing mortality of mice subjected to LPS-induced septicemia. Mice homozygous for the Leiden polymorphism in the factor V (fV) gene, which renders coagulation factor V partially resistant to the anticoagulant effect of APC secondary to blocked fV proteolysis at R504 (R506 in humans), were refractory to the therapeutic benefit conveyed by administration of recombinant wild type mouse APC. In summary, these findings provide evidence that the therapeutic efficacy of recombinant APC is predominantly based on the ability of APC to interact with the endothelial protein C receptor, and that the anticoagulant activity of APC is not sufficient for achieving protection against mortality in a mouse model of endotoxemia. On the other hand, cleavage of fV at R506 appears necessary for retaining therapeutic efficacy in carriers of the fV Leiden allele.