Charles T. Esmon

Protein C prevents the coagulopathic and lethal effects of Escherichia coli infusion in the baboon.

Gram-negative septicemia elicits multiple abnormalities of the coagulation system. Although products of coagulation can lead to clot formation, thereby potentiating organ damage, recent work has shown that low concentrations of thrombin can protect animals from the shock state. Because these amounts of thrombin also lead to formation in vivo of the anticoagulant enzyme, activated protein C, we examined the role of protein C in modulation of Escherichia coli shock in baboons. First, we infused activated protein C and lethal concentrations of E. coli organisms, which prevented the coagulopathic, hepatotoxic, and lethal effects of E. coli. Second, using an antibody to protein C we blocked protein C activation in vivo to determine if this influenced the response to lethal and sublethal concentrations of E. coli organisms. Under these conditions the response to lethal concentrations of E. coli organisms was made more severe and the response to sublethal concentrations of E. coli was made lethal. The coagulopathic, hepatotoxic, and lethal responses in this latter case were prevented by infusion of exogenous protein C.

Extracellular histones are major mediators of death in sepsis.

Hyperinflammatory responses can lead to a variety of diseases, including sepsis. We now report that extracellular histones released in response to inflammatory challenge contribute to endothelial dysfunction, organ failure and death during sepsis. They can be targeted pharmacologically by antibody to histone or by activated protein C (APC). Antibody to histone reduced the mortality of mice in lipopolysaccharide (LPS), tumor necrosis factor (TNF) or cecal ligation and puncture models of sepsis. Extracellular histones are cytotoxic toward endothelium in vitro and are lethal in mice. In vivo, histone administration resulted in neutrophil margination, vacuolated endothelium, intra-alveolar hemorrhage and macro- and microvascular thrombosis. We detected histone in the circulation of baboons challenged with Escherichia coli, and the increase in histone levels was accompanied by the onset of renal dysfunction. APC cleaves histones and reduces their cytotoxicity. Co-infusion of APC with E. coli in baboons or histones in mice prevented lethality. Blockade of protein C activation exacerbated sublethal LPS challenge into lethality, which was reversed by treatment with antibody to histone. We conclude that extracellular histones are potential molecular targets for therapeutics for sepsis and other inflammatory diseases.

The regulation of natural anticoagulant pathways

Vascular endothelium plays an active role in preventing blood clot formation in vivo. One mechanism by which prevention is achieved involves a cell surface thrombin-binding protein, thrombomodulin, which converts thrombin into a protein C activator. Activated protein C then functions as an anticoagulant by inactivating two regulatory proteins of the coagulation system, factors Va and VIIIa. The physiological relevance of the protein C anticoagulant pathway is demonstrated by the identification of homozygous protein C--deficient infants with severe thrombotic complications. Recent studies suggest that this pathway provides a link between inflammation and coagulation.

Dysfunction of endothelial protein C activation in severe meningococcal sepsis

BACKGROUND Impairment of the protein C anticoagulation pathway is critical to the thrombosis associated with sepsis and to the development of purpura fulminans in meningococcemia. We studied the expression of thrombomodulin and the endothelial protein C receptor in the dermal microvasculature of children with severe meningococcemia and purpuric or petechial lesions. METHODS We assessed the integrity of the endothelium and the expression of thrombomodulin and the endothelial protein C receptor in biopsy specimens of purpuric lesions from 21 children with meningococcal sepsis (median age, 41 months), as compared with control skin-biopsy specimens. RESULTS The expression of endothelial thrombomodulin and of the endothelial protein C receptor was lower in the patients with meningococcal sepsis than in the controls, both in vessels with thrombosis and in vessels without thrombosis. On electron microscopical examination, the endothelial cells were generally intact in both thrombosed and nonthrombosed vessels. Plasma thrombomodulin levels in the children with meningococcal sepsis (median, 6.4 ng per liter) were higher than those in the controls (median, 3.6 ng per liter; P=0.002). Plasma levels, protein C antigen, protein S antigen, and antithrombin antigen were lower than those in the controls. In two patients treated with unactivated protein C concentrate, activated protein C was undetectable at the time of admission, and plasma levels remained low. CONCLUSIONS In severe meningococcal sepsis, protein C activation is impaired, a finding consistent with down-regulation of the endothelial thrombomodulin-endothelial protein C receptor pathway.

The interactions between inflammation and coagulation

Inflammation initiates clotting, decreases the activity of natural anticoagulant mechanisms and impairs the fibrinolytic system. Inflammatory cytokines are the major mediators involved in coagulation activation. The natural anticoagulants function to dampen elevation of cytokine levels. Furthermore, components of the natural anticoagulant cascades, like thrombomodulin, minimise endothelial cell dysfunction by rendering the cells less responsive to inflammatory mediators, facilitate the neutralisation of some inflammatory mediators and decrease loss of endothelial barrier function. Hence, downregulation of anticoagulant pathways not only promotes thrombosis but also amplifies the inflammatory process. When the inflammation–coagulation interactions overwhelm the natural defence systems, catastrophic events occur, such as manifested in severe sepsis or inflammatory bowel disease.

Familial protein S deficiency is associated with recurrent thrombosis.

Recent studies have demonstrated that protein C deficiency is associated with recurrent familial thrombosis. In plasma, activated protein C functions as an anticoagulant. This anticoagulant response requires a vitamin K-dependent plasma protein cofactor, referred to as protein S. Since the anticoagulant activity of activated protein C is dependent on protein S, we hypothesized that patients lacking functional protein S might have associated thrombotic disease. Two related individuals with otherwise normal coagulation tests are described whose plasma is not effectively anticoagulated with activated protein C. Addition of purified human protein S to their plasma restores a normal anticoagulant response to activated protein C. We have developed a rapid one-stage clotting assay for protein S to quantitate the level of protein S in their plasma. Plasma is depleted of protein S by immunoadsorption with immobilized antiprotein S antibodies. The resultant plasma responds poorly to activated protein C, but is effectively anticoagulated in a dose-dependent fashion upon addition of purified protein S or small quantities of plasma. The affected individuals possess less than 5% protein S activity. Using Laurell rockets, protein S antigen was detected in the plasma but was at reduced levels of 13 and 18% in the two individuals. When the barium eluate of the patient plasma was chromatographed on quaternary aminoethyl Sephadex, a single peak of protein S antigen devoid of protein S anticoagulant cofactor activity was detected early in the chromatogram. In contrast, the barium eluate from normal donors separated into two peaks, one emerging early and also devoid of anticoagulant cofactor, and the second peak with anticoagulant activity emerging later. The first peak of protein S antigen, from both the normal donor and the patient, chromatographed in the region of the complement component C4-binding protein-protein S complex. These studies suggest that protein S deficiency may result in recurrent thrombotic disease.

Recurrent venous thromboembolism in patients with a partial deficiency of protein S

Protein S is an antithrombotic plasma protein that serves as a cofactor for another plasma protein, activated protein C. Protein S is required for the expression of the anticoagulant effect of activated protein C, which inhibits blood clotting at the levels of factors V and VIII in the blood-clotting cascade. We postulated that patients deficient in protein S would have inadequate regulatory control of the clotting cascade and would be prone to thrombotic disease in a manner similar to that of patients congenitally deficient in protein C. To determine whether protein S deficiency is associated with recurrent thrombosis, we developed a functional assay for the plasma protein. With this assay, the protein S activity of normal persons ranges from 63 to 160 per cent. Using this test, we have now identified six unrelated persons with severe recurrent venous thrombosis who were deficient in protein S, with levels between 15 and 37 per cent while they were not receiving warfarin therapy. Our data suggest that the determination of protein S levels will be useful in the evaluation of patients with recurrent thrombosis.

Endotoxin enhances tissue factor and suppresses thrombomodulin expression of human vascular endothelium in vitro.

Endotoxemia is frequently associated clinically with disseminated intravascular coagulation (DIC); however, the mechanism of endotoxin action in vivo is unclear. Modulation of tissue factor (TF) and thrombomodulin (TM) expression on the endothelial surface may be relevant pathophysiologic mechanisms. Stimulation of human umbilical vein endothelial cells with endotoxin (1 microgram/ml) increased surface TF activity from 1.52 +/- 0.84 to 11.89 +/- 8.12 mU/ml-10(6) cells at 6 h (n = 11) which returned to baseline by 24 h. Repeated stimulation at 24 h resulted in renewed TF expression. Endotoxin (1 microgram/ml) also caused a decrease in TM expression to 55.0 +/- 6.4% of control levels at 24 h (n = 10) that remained depressed at 48 h. Both effects were dose and serum dependent. A temporary rise in TF expression accompanied by a sustained fall in TM expression comprise a shift in the hemostatic properties of the endothelium that would favor intravascular coagulation and may contribute to the pathogenesis of DIC in gram-negative septicemia.

Thrombomodulin Mutations in Atypical Hemolytic–Uremic Syndrome

BACKGROUND The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome. METHODS We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells. RESULTS Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement. CONCLUSIONS Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

The inhibition of blood coagulation by activated protein C through the selective inactivation of activated factor V

Abstract Activated Protein C was found to inhibit Factor Xa initiated clotting of plasma. Activated Protein C did not inhibit prothrombin activation by Factor Xa and Ca 2+ or Factor Xa, Ca 2+ and lipid. However, activated Protein C did inhibit prothrombin activation by Factor Xa, Ca 2+ and Factor Va or Factor Xa, Ca 2+ , lipid, and Factor Va. Excess Factor Va could reverse the inhibition of prothrombin activation. Incubation of Factor V with activated Protein C had little effect on Factor V activity either in the presence or absence of phospholipid. Preincubation of Factor V with activated Protein C had no effect upon the degree to which Factor V could be activated. When Factor V was activated with thrombin in the presence of activated Protein C, a rapid decline in Factor Va activity was observed. When activated Protein C was incubated with purified Factor Va in the absence of thrombin, a similar rapid decay in Factor Va activity was observed. Activated Protein C catalyzed decay of Factor Va activity was not obligately dependent on the presence of lipid. However, lipid enhanced the rate of inactivation. Analysis of sodium dodecyl sulfate gels of either Factor V or Va treated with activated Protein C indicated that activated Protein C degraded a slow migrating band of the Factor V doublet and that it also was able to degrade both the heavy and light chains of Factor Va. The results indicated that activated Protein C can inhibit Factor Xa initiated clotting by degrading Factor Va. Factor Va could be protected from activated Protein C inactivation by the presence of Factor Xa, suggesting that activated Protein C binds to Factor Va or near the Factor Xa binding site. The specificity of activated Protein C for Factor Va and the ability of Factor Xa to stabilize Factor Va may both play important functions in the regulation of blood coagulation.