Protein C concentrate controls leukocyte recruitment during inflammation and improves survival during endotoxemia after efficient in vivo activation

The prognostic utility of protein C as a biomarker for adult sepsis: a systematic review and meta-analysis

Background

Sepsis, the dysregulated host response to infection, triggers abnormal pro-coagulant and pro-inflammatory host responses. Limitations in early disease intervention highlight the need for effective diagnostic and prognostic biomarkers. Protein C’s role as an anticoagulant and anti-inflammatory molecule makes it an appealing target for sepsis biomarker studies. This meta-analysis aims to assess the diagnostic and prognostic value of protein C (PC) as a biomarker for adult sepsis.

Methods

We searched MEDLINE, PubMed, EMBASE, CINAHL and Cochrane Library from database inception to September 12, 2021. We included prospective observational studies of (1) adult patients (> 17) with sepsis or suspicion of sepsis that; (2) measured PC levels with 24 h of study admission with; and (3) the goal of examining PC as a diagnostic or prognostic biomarker. Two authors screened articles and conducted risk of bias (RoB) assessment, using the Quality in Prognosis Studies (QUIPS) and the Quality Assessment in Diagnostic Studies-2 (QUADAS-2) tools. If sufficient data were available, meta-analysis was conducted to estimate the standardized mean difference (SMD) between patient populations.

Results

Twelve studies were included, and 8 were synthesized for meta-analysis. Pooled analysis demonstrated moderate certainty of evidence that PC levels were less reduced in sepsis survivors compared to non-survivors (6 studies, 741 patients, SMD = 0.52, 95% CI 0.24–0.81, p = 0.0003, I² = 55%), and low certainty of evidence that PC levels were less reduced in septic patients without disseminated intravascular coagulation (DIC) compared to those with DIC (3 studies, 644 patients, SMD = 0.97, 95% CI 0.62–1.32, p < 0.00001, I² = 67%). PC could not be evaluated as a diagnostic tool due to heterogeneous control populations between studies.

Conclusion and relevance

Our review demonstrates that PC levels were significantly higher in sepsis survivors compared to non-survivors and patients with sepsis but not disseminated intravascular coagulation (DIC). Our evaluation is limited by high RoB in included studies and poor reporting of the sensitivity and specificity of PC as a sepsis biomarker. Future studies are needed to determine the sensitivity and specificity of PC to identify its clinical significance as a biomarker for early sepsis recognition.

Trial Registration PROSPERO registration number: CRD42021229786. The study protocol was published in BMJ Open.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-022-03889-2.

Relationships Among Circulating Levels of Hemostasis Inhibitors, Chemokines, Adhesion Molecules, and MRI Characteristics in Multiple Sclerosis

Background: Several studies suggested cross talk among components of hemostasis, inflammation, and immunity pathways in the pathogenesis, neurodegeneration, and occurrence of cerebral microbleeds (CMBs) in multiple sclerosis (MS).

Objectives: This study aimed to evaluate the combined contribution of the hemostasis inhibitor protein C (PC) and chemokine C-C motif ligand 18 (CCL18) levels to brain atrophy in MS and to identify disease-relevant correlations among circulating levels of hemostasis inhibitors, chemokines, and adhesion molecules, particularly in CMB occurrence in MS.

Methods: Plasma levels of hemostasis inhibitors (ADAMTS13, PC, and PAI1), CCL18, and soluble adhesion molecules (sNCAM, sICAM1, sVCAM1, and sVAP1) were evaluated by multiplex in 138 MS patients [85 relapsing-remitting (RR-MS) and 53 progressive (P-MS)] and 42 healthy individuals (HI) who underwent 3-T MRI exams. Association of protein levels with MRI outcomes was performed by regression analysis. Correlations among protein levels were assessed by partial correlation and Pearson's correlation.

Results: In all patients, regression analysis showed that higher PC levels were associated with lower brain volumes, including the brain parenchyma (p = 0.002), gray matter (p < 0.001), cortex (p = 0.001), deep gray matter (p = 0.001), and thalamus (p = 0.001). These associations were detectable in RR-MS but not in P-MS patients. Higher CCL18 levels were associated with higher T2-lesion volumes in all MS patients (p = 0.03) and in the P-MS (p = 0.003). In the P-MS, higher CCL18 levels were also associated with lower volumes of the gray matter (p = 0.024), cortex (p = 0.043), deep gray matter (p = 0.029), and thalamus (p = 0.022). PC-CCL18 and CCL18-PAI1 levels were positively correlated in both MS and HI, PC–sVAP1 and PAI1–sVCAM1 only in MS, and PC–sICAM1 and PC–sNCAM only in HI. In MS patients with CMBs (n = 12), CCL18–PAI1 and PAI1–sVCAM1 levels were better correlated than those in MS patients without CMBs, and a novel ADAMTS13–sVAP1 level correlation (r = 0.78, p = 0.003) was observed.

Conclusions: Differences between clinical phenotype groups in association of PC and CCL18 circulating levels with MRI outcomes might be related to different aspects of neurodegeneration. Disease-related pathway dysregulation is supported by several protein level correlation differences between MS patients and HI. The integrated analysis of plasma proteins and MRI measures provide evidence for new relationships among hemostasis, inflammation, and immunity pathways, relevant for MS and for the occurrence of CMBs.

Thrombomodulin induces anti-inflammatory effects by inhibiting the rolling adhesion of leukocytes in vivo

Thrombomodulin (TM) is an integral membrane protein expressed on the surface of vascular endothelial cells that suppresses blood coagulation. Recent studies have shown that TM exhibits anti-inflammatory effects by inhibiting leukocyte recruitment. However, the actual modes of action of TM in vivo remain unclear. Here, we describe the pharmacological effects of recombinant human soluble TM (TM alfa) on leukocyte dynamics in living mice using intravital imaging techniques. Under control conditions, neutrophils exhibited three distinct types of adhesion behavior in vessels: 1) "non-adhesion", in which cells flowed without vessel adhesion; 2) "rolling adhesion", in which cells transiently interacted with the endothelium; and 3) "tight binding", in which cells bound strongly to the endothelial cells. Compared to control conditions, local lipopolysaccharide stimulation resulted in an increased frequency of rolling adhesion that was not homogeneously distributed on vessel walls but occurred at specific endothelial sites. Under inflammatory conditions, TM alfa, particularly the D1 domain which is a lectin-like region of TM, significantly decreased the frequency of rolling adhesion, but did not influence the number of tight bindings. This was the first study to demonstrate that TM alfa exerts anti-inflammatory effects by inhibiting rolling adhesion of neutrophils to vascular endothelial cells in living mice.

Dystrophin deficiency promotes leukocyte recruitment in mdx mice

BACKGROUND

A growing body of evidence defines inflammation as a hallmark feature of disease pathogenesis of Duchenne muscular dystrophy. To tailor potential immune modulatory interventions, a better understanding of immune dysregulation in Duchenne muscular dystrophy is needed. We now asked whether dystrophin deficiency affects the cascade of leukocyte recruitment.

METHODS

We performed intravital microscopy on the cremaster muscle of wild-type and dystrophin-deficient mdx mice. Recruitment was triggered by preparation alone (traumatic inflammation) or in combination with scrotal TNFα injections. Neutrophilic infiltration of the cremaster muscle was assessed on tissue sections. Integrin expression on circulating neutrophils and serum levels of pro-inflammatory cytokines were measured by flow cytometry.

RESULTS

Mdx mice show increased rolling and adhesion at baseline (traumatic inflammation) and a more profound response upon TNFα injection compared with wild-type animals. In both models, neutrophilic infiltration of the cremaster muscle is increased. Upregulation of the integrins LFA-1 and Mac-1 on circulating leukocytes and pro-inflammatory cytokines IL-6 and CCL2 in the serum points toward systemically altered immune regulation in mdx mice.

CONCLUSION

We are the first to show exaggerated activation of the leukocyte recruitment cascade in a dystrophin-deficient organism in vivo.

Update on the role of endothelial cells in trauma

PURPOSE

This review gives an overview of physiological processes, mainly regarding vascular endothelial cells and their important role in hemostasis, information processing, and communication during trauma. An insight is given into molecules and cells involved in the first innate immune response through to the behavior of endothelial cells in developing trauma. The goal of this review is to show the overlap of crucial factors related to the endothelium and the development of trauma.

METHODS

A systemic literature search was performed using Google scholar and PubMed.

RESULTS

The results of the literature search showed that the endothelium, especially the vascular endothelium, is involved in various cellular and subcellular pathways of activation, suppression, and transfer of information. A variety of molecules and cells are orchestrated, subsequently the endothelium gets in contact with a traumatizing event.

CONCLUSION

The endothelium is one of the first barriers that comes into contact with exo- and endogenous trauma-related signals and is a pivotal point in activating subsequent pathways and cascades by transfer of information.

Activated protein C attenuates ischemia-reperfusion-induced acute lung injury

Ischemia-reperfusion (IR)-induced acute lung injury (ALI) is implicated in several clinical conditions, such as lung transplantation, acute pulmonary embolism after thrombolytic therapy, re-expansion of collapsed lung from pneumothorax, or pleural effusion, cardiopulmonary bypass, etc. Because mortality remains high despite advanced medical care, prevention and treatment are important clinical issues. Activated protein C (APC) manifests multiple activities with antithrombotic, profibrinolytic, and anti-inflammatory effects. We therefore conducted this study to determine the beneficial effects of APC in IR-induced ALI. IR-induced ALI was conducted in a rat model of isolated-perfused lung in situ. The animals were divided into the control group, IR group, and IR+APC group. There were six adult male Sprague-Dawley rats in each group. The IR caused significant pulmonary microvascular hyperpermeability, pulmonary edema and dysfuction, increased cytokines (tumor necrosis factor (TNF)-α, IL-17, CXCL-1), and neutrophils infiltration in lung tissues. Administration of APC significantly attenuated IR-induced ALI with improving microvascular permeability, pulmonary edema, pulmonary dysfunction, and suppression inflammatory response. The current study demonstrates the beneficial effects of APC in IR-induced ALI. This protective effect is possibly associated with the inhibition of TNF-α, IL-17A, CXCL1, and neutrophils infiltration in lung tissues. However, the current results were obtained in an animal model and it is still necessary to confirm these findings in human subjects. If we can demonstrate the benefits of APC to protect IR lung injury, we can postulate that APC is a potential therapeutic drug for lung preservation.

The olive oil-based lipid clinoleic blocks leukocyte recruitment and improves survival during systemic inflammation: a comparative in vivo study of different parenteral lipid emulsions

Although fish oil-based and olive oil-based lipid emulsions have been shown to exert anti-inflammatory functions, the immunomodulating properties of lipids are still controversial. Therefore, we investigated the anti-inflammatory effect of three different parenterally administered lipid emulsions in vivo: olive oil-based Clinoleic, fish oil-based Smoflipid, and soybean oil-based Lipofundin. We observed leukocyte recruitment in inflamed murine cremaster muscle using intravital microscopy and survival in a murine model of LPS-induced systemic inflammation and analyzed expression of leukocyte and endothelial adhesion molecules. Olive oil-based Clinoleic and fish oil-based Smoflipid profoundly inhibited leukocyte adhesion compared to Lipofundin during LPS-induced inflammation of the murine cremaster muscle. In the trauma model of cremaster muscle inflammation, Lipofundin was the only lipid emulsion that even augmented leukocyte adhesion. In contrast to Smoflipid and Lipofundin, Clinoleic effectively blocked leukocyte recruitment and increased survival during lethal endotoxemia. Flow chamber experiments and analysis of adhesion molecule expression suggest that both endothelial and leukocyte driven mechanisms might contribute to anti-inflammatory effects of Clinoleic. We conclude that the anti-inflammatory properties of Clinoleic are superior to those of Smoflipid and Lipofundin even during systemic inflammation. Thus, these results should stimulate further studies investigating parenteral lipids as an anti-inflammatory strategy in critically ill patients.

[Sepsis-associated Purpura Fulminans International Registry--Europe (SAPFIRE)]

BACKGROUND

Purpura fulminans is a rare life-threatening condition which is characterized by disseminated thrombosis in dermal and systemic microcirculation, cutaneous hemorrhages with progressing necrosis and multiple organ failure. The underlying pathogenesis is based on the disruption of the intrinsic anticoagulation cascade, with protein C deficiency being considered the leading factor in this process. In the majority of cases, the condition emerges as consumptive coagulopathy associated with severe sepsis.

OBJECTIVES

Epidemiological data on sepsis-associated purpura fulminans (SAPF) are scarce and evidence-based treatment guidelines have not been established yet. While restoration of the balance in the coagulation cascade is a declared therapeutic goal, evaluations of the efficacy of different therapeutic approaches in randomized clinical trials are still lacking. The causal role of individual microbial pathogens also requires comprehensive evaluation.

METHODS

A prospective multicenter Sepsis-Associated Purpura Fulminans International Registry-Europe (SAPFIRE) will be established in the first quarter of 2015. For the first time, participating centers will systematically collect information on etiology, clinical course, biomarkers, treatment, morbidity, and mortality of SAPF.

RESULTS

The SAPFIRE data will be periodically evaluated and disseminated. Retrospective analysis of each center's data and regular access to aggregated information collected by other centers will enable the participants to monitor and update care quality standards.

Anti-inflammatory functions of protein C require RAGE and ICAM-1 in a stimulus-dependent manner

By binding β₂-integrins both ICAM-1 and the receptor for advanced glycation end products (RAGE) mediate leukocyte recruitment in a stimulus-dependent manner. Using different inflammatory mouse models we investigated how RAGE and ICAM-1 are involved in anti-inflammatory functions of protein C (PC; Ceprotin, 100 U/kg). We found that, depending on the stimulus, RAGE and ICAM-1 are cooperatively involved in PC-induced inhibition of leukocyte recruitment in cremaster models of inflammation. During short-term proinflammatory stimulation (trauma, fMLP, and CXCL1), ICAM-1 is more important for mediation of anti-inflammatory effects of PC, whereas RAGE plays a major role after longer proinflammatory stimulation (TNFα). In contrast to WT and Icam-1^−/− mice, PC had no effect on bronchoalveolar neutrophil emigration in RAGE^−/− mice during LPS-induced acute lung injury, suggesting that RAGE critically mediates PC effects during acute lung inflammation. In parallel, PC treatment effectively blocked leukocyte recruitment and improved survival of WT mice and Icam-1-deficient mice in LPS-induced endotoxemia, but failed to do so in RAGE-deficient mice. Exploring underlying mechanisms, we found that PC is capable of downregulating intracellular RAGE and extracellular ICAM-1 in endothelial cells. Taken together, our data show that RAGE and ICAM-1 are required for the anti-inflammatory functions of PC.

RAGE controls activation and anti-inflammatory signalling of protein C

AIMS

The receptor for advanced glycation endproducts, RAGE, is a multiligand receptor and NF-κB activator leading to perpetuation of inflammation. We investigated whether and how RAGE is involved in mediation of anti-inflammatory properties of protein C.

METHODS AND RESULTS

We analyzed the effect of protein C on leukocyte adhesion and transmigration in WT- and RAGE-deficient mice using intravital microscopy of cremaster muscle venules during trauma- and TNFα-induced inflammation. Both, protein C (PC, Ceprotin, 100 U/kg) and activated protein C (aPC, 24 µg/kg/h) treatment significantly inhibited leukocyte adhesion in WT mice in these inflammation models. The impaired leukocyte adhesion after trauma-induced inflammation in RAGE knockout mice could not be further reduced by PC and aPC. After TNFα-stimulation, however, aPC but not PC treatment effectively blocked leukocyte adhesion in these mice. Consequently, we asked whether RAGE is involved in PC activation. Since RAGE-deficient mice and endothelial cells showed insufficient PC activation, and since thrombomodulin (TM) and endothelial protein C receptor (EPCR) are reduced on the mRNA and protein level in RAGE deficient endothelial cells, an involvement of RAGE in TM-EPCR-dependent PC activation is likely. Moreover, TNFα-induced activation of MAPK and upregulation of ICAM-1 and VCAM-1 are reduced both in response to aPC treatment and in the absence of RAGE. Thus, there seems to be interplay of the RAGE and the PC pathway in inflammation.

CONCLUSION

RAGE controls anti-inflammatory properties and activation of PC, which might involve EPCR and TM.

Activated protein C and its potential applications in prevention of islet β-cell damage and diabetes

Activated protein C (APC) is derived from its precursor, protein C (PC). Originally thought to be synthesized exclusively by the liver, recent reports have shown that PC is also produced by many other cells including pancreatic islet β cells. APC functions as a physiological anticoagulant with anti-inflammatory, anti-apoptotic, and barrier-stabilizing properties. APC exerts its protective effects via an intriguing mechanism requiring combinations of endothelial PC receptor, protease-activated receptors, epidermal growth factor receptor, Tie2 or CD11b, depending on cell types. Diabetes is a chronic condition resulted from the body's inability to produce and/or properly use insulin. The prevalence of diabetes has risen dramatically and has become one of the major causes of premature mortality and morbidity worldwide. Diabetes prevention is an ideal approach to reduce this burden. Type 1 and type 2 diabetes are the major forms of diabetes mellitus, and both are characterized by an autoimmune response, intraislet inflammation, β-cell apoptosis, and progressive β-cell loss. Protecting β-cell from damage is critical in both prevention and treatment of diabetes. Recent in vitro and animal studies show that APC's strong anti-inflammatory and anti-apoptotic properties are beneficial in preventing β-cell destruction and diabetes in the NOD mouse model of type 1 diabetes. Future preventive and therapeutic uses of APC in diabetes look very promising.

Anti-inflammatory effects of selected drugs on activated neonatal and adult neutrophils

AIM

In view of the central role of granulocytic neutrophils in the context of inflammatory reactions, the present study focuses on anti-inflammatory effects of drugs on activated neutrophils in neonates and adults.

METHODS

Sixteen blood samples of neonates and adults were investigated in a prospective study. Loss of deformability, morphological changes, and increases in neutrophil elastase were determined as measures of neutrophil activation due to incubation with the pro-inflammatory cytokine interleukin-8. For inhibition experiments, the blood samples were also incubated with the phosphodiesterase inhibitors milrinone and piclamilast, the protease inhibitor urinastatin, ketamine, protein C concentrate, and the nitric oxide donor FK 409. Changes in deformability were investigated with a cell transit analyzer, morphological changes by microscopic observation, and the extent of neutrophil elastase release with an enzyme immunoassay.

RESULTS

The drugs milrinone, piclamilast, urinastatin, ketamine, protein C concentrate and FK 409 showed deactivating effects on activated neutrophils in recommended clinical doses. They improved deformability as well as reduced pseudopod formation and the release of neutrophil elastase. The effects on neutrophils did not differ between neonates and adults despite their functional differences.

CONCLUSION

We conclude that these drugs may reduce the inflammatory response and improve microcirculation in neonates and adults during inflammation.

Diet-induced obesity reprograms the inflammatory response of the murine lung to inhaled endotoxin

The co-occurrence of environmental factors is common in complex human diseases and, as such, understanding the molecular responses involved is essential to determine risk and susceptibility to disease. We have investigated the key biological pathways that define susceptibility for pulmonary infection during obesity in diet-induced obese (DIO) and regular weight (RW) C57BL/6 mice exposed to inhaled lipopolysaccharide (LPS). LPS induced a strong inflammatory response in all mice as indicated by elevated cell counts of macrophages and neutrophils and levels of proinflammatory cytokines (MDC, MIP-1γ, IL-12, RANTES) in the bronchoalveolar lavage fluid. Additionally, DIO mice exhibited 50% greater macrophage cell counts, but decreased levels of the cytokines, IL-6, TARC, TNF-α, and VEGF relative to RW mice. Microarray analysis of lung tissue showed over half of the LPS-induced expression in DIO mice consisted of genes unique for obese mice, suggesting that obesity reprograms how the lung responds to subsequent insult. In particular, we found that obese animals exposed to LPS have gene signatures showing increased inflammatory and oxidative stress response and decreased antioxidant capacity compared with RW. Because signaling pathways for these responses can be common to various sources of environmentally induced lung damage, we further identified biomarkers that are indicative of specific toxicant exposure by comparing gene signatures after LPS exposure to those from a parallel study with cigarette smoke. These data show obesity may increase sensitivity to further insult and that co-occurrence of environmental stressors result in complex biosignatures that are not predicted from analysis of individual exposures.

The protein C pathway in cancer metastasis

Cancer is frequently associated with activation of blood coagulation, which in turn has been suggested to promote tumor growth and metastasis. Indeed, low molecular weight heparin treatment significantly prolongs the survival of a wide variety of patients with cancer. Based on this notion that anticoagulant treatment seems to benefit cancer patients, recent experiments aimed to elucidate the importance of the natural anticoagulant protein C pathways in cancer progression. Interestingly, these experiments showed that the repeated administration of exogenous activated protein C limits cancer cell extravasation in experimental animal models. In line, reducing endogenous activated protein C activity dramatically increased the number of experimental metastasis. These data thus strongly suggest that exogenous activated protein C administration may be a novel therapeutic avenue to limit cancer metastasis thereby prolonging overall survival of cancer patients. The current review provides an overview of recent data on the role of the protein C pathway in cancer metastasis. It discusses the potential of activated protein C as a novel target to reduce cancer progression, it points to several limitations of activated protein C administration in the setting of cancer cell metastasis and it suggest zymogen protein C as an attractive alternative.

CXCL1-triggered interaction of LFA1 and ICAM1 control glucose-induced leukocyte recruitment during inflammation in vivo

It is well acknowledged that proinflammatory stimulation during acute hyperglycemia is able to aggravate inflammatory diseases. However, the mechanisms of proinflammatory effects of glucose are controversially discussed. We investigated leukocyte recruitment after intravenous injection of glucose in different inflammatory models using intravital microscopy. Flow chamber experiments, expression analysis, functional depletion, and knockout of key adhesion molecules gave mechanistic insight in involved pathways. We demonstrated that a single injection of glucose rapidly increased blood glucose levels in a dose-dependent manner. Notably, during tumor necrosis factor (TNF) α-induced inflammation leukocyte recruitment was not further enhanced by glucose administration, whereas glucose injection profoundly augmented leukocyte adhesion and transmigration into inflamed tissue in the trauma model, indicating that proinflammatory properties of glucose are stimulus dependent. Experiments with functional or genetic inhibition of the chemokine receptor CXCR2, intercellular adhesion molecule 1 (ICAM1), and lymphocyte function antigen 1 (LFA1) suggest that keratino-derived-chemokine CXCL1-triggered interactions of ICAM1 and LFA1 are crucially involved in the trauma model of inflammation. The lacking effect of glucose on β(2) integrin expression and on leukocyte adhesion in dynamic flow chamber experiments argues against leukocyte-driven underlying mechanisms and favours an endothelial pathway since endothelial ICAM1 expression was significantly upregulated in response to glucose.

Is EPCR a multi-ligand receptor? Pros and cons

In the last decade, the endothelial cell protein C/activated protein C receptor (EPCR) has received considerable attention. The role initially attributed to EPCR, i.e. the enhancement of protein C (PC) activation by the thrombin-thrombomodulin complex on the surface of the large vessels, although important, did not go beyond the haemostasis scenario. However, the discovery of the cytoprotective, anti-inflammatory and anti-apoptotic features of the activated PC (APC) and the required involvement of EPCR for APC to exert such actions did place the receptor in a privileged position in the crosstalk between coagulation and inflammation. The last five years have shown that PC/APC are not the only molecules able to interact with EPCR. Factor VII/VIIa (FVII/VIIa) and factor Xa (FXa), two other serine proteases that play a central role in haemostasis and are also involved in signalling processes influencing wound healing, tissue remodelling, inflammation or metastasis, have been reported to bind to EPCR. These observations have paved the way for an exploration of unsuspected new roles for the receptor. This review aims to offer a new image of EPCR in the light of its extended panel of ligands. A brief update of what is known about the APC-evoked EPCR-dependent cell signalling mechanisms is provided, but special care has been taken to assemble all the information available about the interaction of EPCR with FVII/VIIa and FXa.

The protein C pathway and sepsis

After the discovery of the key components of the protein C (PC) pathway a beneficial effect on survival of the infusion of activated protein C (APC) in animal models of sepsis was demonstrated, leading to the development of recombinant human activated protein C (rh-APC) as a therapeutic agent. It soon became clear that rather than the anticoagulant and profibrinolytic activities of APC, its anti-inflammatory and cytoprotective properties played a major role in the treatment of patients with severe sepsis. Such properties affect the response to inflammation of endothelial cells and leukocytes and are exerted through binding of APC to at least five receptors with intracellular signaling. The main APC protective mechanism involves binding of the Gla-domain to the endothelial protein C receptor (EPCR) and cleavage of protease activated receptor 1 (PAR-1), eliciting suppression of proinflammatory cytokines synthesis and of intracellular proapoptotic pathways and activation of endothelial barrier properties. However, thrombin cleaves PAR-1 with much higher catalytic efficiency, followed by pro-inflammatory, pro-apoptotic and barrier disruptive intracellular signaling, and it is unclear how APC can exert a protective activity through the cleavage of PAR-1 when thrombin is also present in the same environment. Interestingly, in endothelial cell cultures, PAR-1 cleavage by thrombin results in anti-inflammatory and barrier protective signaling provided occupation of EPCR by the PC gla-domain, raising the possibility that the beneficial effects of rh-APC might be recapitulated in vivo by administration of h-PC zymogen to patients with severe sepsis. Recent reports of h-PC infusion in animal models of sepsis support this hypothesis.