Protein C anticoagulant system--anti-inflammatory effects

Organ-specific protection against lipopolysaccharide-induced vascular leak is dependent on the endothelial protein C receptor

OBJECTIVE

To study the role of the endothelial protein C receptor (EPCR) in the modulation of susceptibility to inflammation-induced vascular leak in vivo.

APPROACH AND RESULTS

Genetically modified mice with low, <10% EPCR expression (EPCR(low)) and control mice were challenged with lipopolysaccharides in a mouse model of endotoxemia. Infrared fluorescence and quantification of albumin-bound Evans Blue in tissues and intravascular plasma volumes were used to assess plasma extravasation. Pair-wise analysis of EPCR(low) and control mice matched for sex, age, and weight allowed determination of EPCR-dependent vascular leak. Kidney, lung, and brain were the organs with highest discriminative increased Evans Blue accumulation in EPCR(low) versus control mice in response to lipopolysaccharides. Histology of kidney and lung confirmed the EPCR-specific pathology. In addition to severe kidney injury in response to lipopolysaccharides, EPCR(low) and anti-EPCR-treated wild-type mice suffered from enhanced albuminuria and profound renal hemorrhage versus controls. Intravascular volume loss at the same extent of weight loss in EPCR(low) mice compared with control mice provided proof that plasma leak was the predominant cause of Evans Blue tissue accumulation.

CONCLUSIONS

This study demonstrates an important protective role for EPCR in vivo against vascular leakage during inflammation and suggests that EPCR-dependent vascular protection is organ-specific.

Antibody SPC-54 provides acute in vivo blockage of the murine protein C system

Multiple protective effects of pharmacological activated protein C (APC) are reported in several organ pathologies. To help evaluate the endogenous murine PC system, we characterized a rat monoclonal anti-mouse PC antibody, SPC-54, which inhibited the amidolytic and anticoagulant activities of murine APC by>95%. SPC-54 blocked active site titration of purified APC using the active site titrant, biotinylated FPR-chloromethylketone, showing that SPC-54 blocks access to APC's active site to inhibit all enzymatic activity. A single injection of SPC-54 (10mg/kg) neutralized circulating PC in mice for at least 7days, and immunoblotting and immuno-precipitation with protein G-agarose confirmed that SPC-54 in vivo was bound to PC in plasma. Pre-infusion of SPC-54 in tissue factor-induced murine acute thromboembolism experiments caused a major decrease in mean survival time compared to controls (7min vs. 42.5min, P=0.0016). SPC-54 decreased lung perfusion in this model by 54% when monitored by vascular perfusion methodologies using infrared fluorescence of Evans blue dye. In LD50 endotoxemia murine models, SPC-54 infused at 7hr after endotoxin administration increased mortality from 42% to 100% (P<0.001). In summary, monoclonal antibody SPC-54 ablates in vitro and in vivo APC protective functions and enzymatic activity. The ability of SPC-54 to block the endogenous PC/APC system provides a powerful tool to understand better the role of the endogenous PC system in murine injury models and in cell bioassays and also to neutralize the enzymatic activities of murine APC in any assay system.

Down-regulation of the clotting cascade by the protein C pathway

The protein C pathway provides important biological activities to maintain the fluidity of the circulation, prevent thrombosis, and protect the integrity of the vasculature in response to injury. Activated protein C (APC), in concert with its cofactors and cell receptors, assembles in specific macromolecular complexes to provide efficient proteolysis of multiple substrates that result in anticoagulant and cytoprotective activities. Numerous studies on APC's structure-function relation with its cofactors, cell receptors, and substrates provide valuable insights into the molecular mechanisms and presumed assembly of the macromolecular complexes that are responsible for APC's activities. These insights allow for molecular engineering approaches specifically targeting the interaction of APC with one of its substrates or cofactors. Thus far, these approaches resulted in several anticoagulant-selective and cytoprotective-selective APC mutants, which provide unique insights into the relative contributions of APC's anticoagulant or cytoprotective activities to the beneficial effects of APC in various murine injury and disease models. Because of its multiple physiological and pharmacological activities, the anticoagulant and cytoprotective protein C pathway have important implications for the (patho)physiology of vascular disease and for translational research exploring novel therapeutic strategies to combat complex medical disorders such as thrombosis, inflammation, ischemic stroke and neurodegenerative disease.

Protein C activity and postoperative metabolic liver function after liver transplantation

BACKGROUND

Protein C is a natural thrombin antagonist produced by hepatocytes. Its levels are low in liver failure and predispose patients to increased risk for thrombosis. Little is known about the relationship between protein C activity and hepatic function after orthotopic liver transplantation (OLT).

METHODS

We measured protein C activity of 41 patients undergoing liver transplantation by the Staclot method (normal range, 70%-130%) preoperatively and then daily on postoperative days (POD) 0-5.

RESULTS

The mean protein C activity was low before OLT (34.3 ± 4.3%) and inversely correlated with the preoperative Model for End-Stage Liver Disease score (Spearman's r = -0.643; P < .0001). Mean activity increased significantly on POD 1 (58.9 ± 4.5%), and remained above preoperative levels through POD 5. Ten patients developed metabolic liver dysfunction defined by a serum total bilirubin >5 mg/dL on POD 7. These patients had significantly lower protein C activity from POD 3 (47.2 ± 9.6% vs 75.9 ± 5.8%; P = .01) to POD 5. Preoperative protein C activity correlated inversely with the severity of liver failure as indicated by preoperative MELD score.

CONCLUSION

Protein C activity recovered rapidly in patients with good allograft function but remained significantly lower in patients who had limited metabolic function as evidenced by increased total bilirubin levels.

Autoimmune diseases and venous thromboembolism: a review of the literature

Venous thromboembolism (VTE) is major health problem and is sometimes complicated by lethal pulmonary embolism (PE). Disturbances of the coagulation and anticoagulation systems are important risk factors for VTE. Comparative studies suggest that coagulation and innate immunity have a shared evolutionary origin. It is therefore unsurprising that the immune and coagulation systems are linked, with many molecular components being important for both systems. Systemic inflammation modulates thrombotic responses by suppressing fibrinolysis, upregulating procoagulant, and downregulating anticoagulants, and autoimmune disorders such as systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), and Behçet's syndrome have been linked to an increased risk of VTE. Recent reports have further shown that a majority of autoimmune and immune-mediated disorders are linked to an increased risk of venous thrombosis, PE, or VTE. For instance, a Swedish nationwide study found that the risk of PE was increased in the first year after hospitalization for 33 different autoimmune disorders. Especially high risks were noted for several autoimmune diseases such as immune thrombocytopenic purpura, polyarteritis nodosa, polymyositis/dermatomyositis, ulcerative colitis, and SLE. Another study from England, also based on hospitalization data, found that immune-mediated disorders were associated with an increased risk of VTE compared with other medical causes of hospitalization. Multiple mechanisms may operate and disease-specific factors, such as cardiolipin antibodies, have been identified. However, inflammation by itself appears to change the hemostatic balance in a thrombogenic direction. Recent epidemiological studies, together with previous experimental and clinical studies, indicate that autoimmune disorders should not only be viewed as inflammatory disorders, but also hypercoagulable disorders. Research to identify thrombotic risk factors, elucidate the mechanisms involved, and investigate prophylactic regiments is needed. The present review describes the epidemiological, clinical, and experimental evidence for the connection between VTE and autoimmune and immune-mediated disorders.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Thrombomodulin: a bifunctional modulator of inflammation and coagulation in sepsis

Deregulated interplay between inflammation and coagulation plays a pivotal role in the pathogenesis of sepsis. Therapeutic approaches that simultaneously target both inflammation and coagulation hold great promise for the treatment of sepsis. Thrombomodulin is an endogenous anticoagulant protein that, in cooperation with protein C and thrombin-activatable fibrinolysis inhibitor, serves to maintain the endothelial microenvironment in an anti-inflammatory and anticoagulant state. A recombinant soluble form of thrombomodulin has been approved to treat patients suffering from disseminated intravascular coagulation (DIC) and has thus far shown greater therapeutic potential than heparin. A phase II clinical trial is currently underway in the USA to study the efficacy of thrombomodulin for the treatment of sepsis with DIC complications. This paper focuses on the critical roles that thrombomodulin plays at the intersection of inflammation and coagulation and proposes the possible existence of interactions with integrins via protein C. Finally, we provide a rationale for the clinical application of thrombomodulin for alleviating sepsis.