Protein C pathway in sepsis.

The goals of this chapter are to provide a brief review of the biology of the protein C pathway and some of the features of the pathway that make it uniquely positioned to control microvascular coagulation and control the acute inflammatory response. Activated protein C works as an antithrombotic agent by inactivating factors Va and VIIIa. It is particularly effective at preventing microvascular thrombosis. Platelets may provide a margin of safety for activated protein C as an antithrombotic. Approximately 25% of the factor V/Va in plasma is contained within the platelet and hence resistant to time dependent inactivation by activated protein C. In addition, factor Va bound to the platelet surface is relatively resistant to inactivation by activated protein C. Activated protein C also facilitates clot lysis by inhibiting plasminogen activator inhibitor 1, a process that is accelerated markedly by vitronectin. Inflammatory cytokines like tumor necrosis factor alpha (TNFalpha) and interleukin-1beta (IL-1beta) downregulate two key components of the protein C activation complex, thrombomodulin and the endothelial cell protein C receptor resulting in decreased protein C activation. Activated protein C in turn has been shown in several animal models and in vitro to inhibit TNF elaboration in response to endotoxin. This inhibition appears to be due to diminished nuclear factor kappaB (NF kappaB) expression and nuclear translocation. Activated protein C has been shown to reduce the rate of death due to severe sepsis. This reduction may be due to both the anticoagulant effects as demonstrated by a reduction in D-dimer and inflammatory effects as demonstrated by a reduction in interleukin 6.