Neutrophils and the protein C pathway

Severe sepsis is a major health care problem, with mortality rates ranging from 28% to 50%. New insights into pathophysiology reveal that the hypotension and organ dysfunction that characterize this disorder reflect a complex interplay between the hemostatic and inflammatory systems (Esmon, Ann Med. 2002;34:598-605). This intimate linkage explains why strategies that block only one pathway have been ineffective. Thus, attempts to attenuate excessive activation of coagulation with heparin or antithrombin or the use of inhibitors of specific inflammatory cytokines have had no impact on survival. In contrast, recombinant activated protein C (APC), an agent that combines anticoagulant and anti-inflammatory properties, reduces mortality in severe sepsis (Bernard et al, N Engl J Med. 2001;344: 699-709).

Although the anticoagulant effects of APC are well understood, its anti-inflammatory properties are not. The work of Sturn and colleagues (page 1499) brings us one step closer. Examining the effects of protein C and APC on neutrophil function, these investigators demonstrated that both agents attenuate neutrophil chemotaxis in response to interleukin-8 (IL-8), formyl-Met-Leu-Phe, antithrombin, and C5a, effects that were inhibited by an antibody against endothelial protein C receptor (EPCR), a receptor that binds both protein C and APC. Using reverse transcriptase–polymerase chain reaction, Sturn and colleagues showed that neutrophils synthesize EPCR mRNA and, based on flow cytometric analysis, express EPCR on their surface. These findings raise the possibility that EPCR-mediated ligation of protein C or APC retards neutrophil migration in response to the types of inflammatory mediators that might be generated in sepsis. This inhibitory activity would be lost as protein C is consumed and would be restored by infusion of protein C or APC, a phenomenon that may contribute to the beneficial effects of APC in sepsis patients.

How do these findings build on previous investigations into the anti-inflammatory properties of protein C? APC infusion protects baboons from the lethal effects of Escherichia coli, and the levels of inflammatory cytokines rise when APC is inhibited (Taylor et al, J Clin Invest. 1987;79: 918-925). When administered to patients with severe sepsis, APC reduces IL-6 levels (Bernard et al, N Engl J Med. 2001;344: 699-709). Finally, in endotoxin-treated rats, APC decreases vascular permeability and leukocyte accumulation in the lungs. By contrast, neither heparin plus antithrombin nor active site-blocked factor Xa has activity in this model, suggesting that the beneficial effects of APC are independent of its anticoagulant properties (Murakami et al, Blood. 1996;87:642-647).

How does APC modulate the inflammatory response? In addition to augmenting protein C activation, EPCR shuttles APC from the endothelial cell surface into the nucleus where the APC/EPCR complex alters gene expression (Esmon, Ann Med. 2002;34:598-605). Gene expression profiling demonstrates that APC down-regulates functional NF kappa B expression in endothelial cells. This, in turn, suppresses expression of adhesion molecules in response to tumor necrosis factor, a phenomenon that could limit vascular injury by attenuating leukocyte attachment to the vessel wall (Joyce et al, J Biol Chem. 2001;276:11199-11203). APC also inhibits staurosporine-induced endothelial cell apoptosis, an effect dependent on EPCR and protease activated receptor-1 (Cheng et al, Nat Med. 2003; 9:338-342), and reduces the release of proinflammatory cytokines by monocytes, cells that also express EPCR (Esmon, Ann Med. 2002;34:598-605).

As a cost-effective and life-saving treatment for selected sepsis patients (Bernard et al, N Engl J Med. 2001;344:699-709), APC modulates coagulation and inflammation. The work of Sturn and colleagues adds to our understanding of the anti-inflammatory properties of protein C.