Travis J. Gould

Neutrophil Extracellular Traps Promote Thrombin Generation Through Platelet-Dependent and Platelet-Independent Mechanisms

Objective— Activation of neutrophils by microbial or inflammatory stimuli results in the release of neutrophil extracellular traps (NETs) that are composed of DNA, histones, and antimicrobial proteins. In purified systems, cell-free DNA (CFDNA) activates the intrinsic pathway of coagulation, whereas histones promote thrombin generation through platelet-dependent mechanisms. However, the overall procoagulant effects of CFDNA/histone complexes as part of intact NETs are unknown. In this study, we examined the procoagulant potential of intact NETs released from activated neutrophils. We also determined the relative contribution of CFDNA and histones to thrombin generation in plasmas from patients with sepsis. Approach and Results— NETs released from phorbyl myristate–activated neutrophils enhance thrombin generation in platelet-poor plasma. This effect was DNA dependent (confirmed by DNase treatment) and occurred via the intrinsic pathway of coagulation (confirmed with coagulation factor XII– and coagulation factor XI–depleted plasma). In platelet-rich plasma treated with corn trypsin inhibitor, addition of phorbyl myristate–activated neutrophils increased thrombin generation and shortened the lag time in a toll-like receptor-2– and toll-like receptor-4–dependent mechanism. Addition of DNase further augmented thrombin generation, suggesting that dismantling of the NET scaffold increases histone-mediated, platelet-dependent thrombin generation. In platelet-poor plasma samples from patients with sepsis, we found a positive correlation between endogenous CFDNA and thrombin generation, and addition of DNase attenuated thrombin generation. Conclusions— These studies examine the procoagulant activities of CFDNA and histones in the context of NETs. Our studies also implicate a role for the intrinsic pathway of coagulation in sepsis pathogenesis.

Extracellular DNA and histones: double-edged swords in immunothrombosis

The existence of extracellular DNA in human plasma, also known as cell‐free DNA (cfDNA), was first described in the 1940s. In recent years, there has been a resurgence of interest in the functional significance of cfDNA, particularly in the context of neutrophil extracellular traps (NETs). cfDNA and histones are key components of NETs that aid in the host response to infection and inflammation. However, cfDNA and histones may also exert harmful effects by triggering coagulation, inflammation, and cell death and by impairing fibrinolysis. In this article, we will review the pathologic nature of cfDNA and histones in macrovascular and microvascular thrombosis, including venous thromboembolism, cancer, sepsis, and trauma. We will also discuss the prognostic value of cfDNA and histones in these disease states. Understanding the molecular and cellular pathways regulated by cfDNA and histones may provide novel insights to prevent pathological thrombus formation and vascular occlusion.

Release of neutrophil extracellular traps by neutrophils stimulated with antiphospholipid antibodies: a newly identified mechanism of thrombosis in the antiphospholipid syndrome.

Antiphospholipid antibodies (aPL), especially those targeting β2‐glycoprotein I (β2GPI), are well known to activate endothelial cells, monocytes, and platelets, with prothrombotic implications. In contrast, the interaction of aPL with neutrophils has not been extensively studied. Neutrophil extracellular traps (NETs) have recently been recognized as an important activator of the coagulation cascade, as well as an integral component of arterial and venous thrombi. This study was undertaken to determine whether aPL activate neutrophils to release NETs, thereby predisposing to the arterial and venous thrombosis inherent in the antiphospholipid syndrome (APS).

Cell-Free DNA Modulates Clot Structure and Impairs Fibrinolysis in Sepsis

Objectives—Sepsis is characterized by systemic activation of inflammation and coagulation in response to infection. In sepsis, activated neutrophils extrude neutrophil extracellular traps composed of cell-free DNA (CFDNA) that not only trap pathogens but also provide a stimulus for clot formation. Although the effect of CFDNA on coagulation has been extensively studied, much less is known about the impact of CFDNA on fibrinolysis. To address this, we (1) investigated the relationship between CFDNA levels and fibrinolytic activity in sepsis and (2) determined the mechanisms by which CFDNA modulates fibrinolysis. Approach and Results—Plasma was collected from healthy and septic individuals, and CFDNA was quantified. Clot lysis assays were performed in plasma and purified systems, and lysis times were determined by monitoring absorbance. Clot morphology was assessed using scanning electron microscopy. Clots formed in plasma from septic patients containing >5 µg/mL CFDNA were dense in structure and resistant to fibrinolysis, a phenomenon overcome by deoxyribonuclease addition. These effects were recapitulated in control plasma supplemented with CFDNA. In a purified system, CFDNA delayed fibrinolysis but did not alter tissue-type plasminogen activator–induced plasmin generation. Using surface plasmon resonance, CFDNA bound plasmin with a Kd value of 4.2±0.3 µmol/L, and increasing concentrations of CFDNA impaired plasmin-mediated degradation of fibrin clots via the formation of a nonproductive ternary complex between plasmin, CFDNA, and fibrin. Conclusions—Our studies suggest that the increased levels of CFDNA in sepsis impair fibrinolysis by inhibiting plasmin-mediated fibrin degradation, thereby identifying CFDNA as a potential therapeutic target for sepsis treatment.

Delayed but not Early Treatment with DNase Reduces Organ Damage and Improves Outcome in a Murine Model of Sepsis.

ABSTRACT Sepsis is characterized by systemic activation of coagulation and inflammation in response to microbial infection. Although cell-free DNA (cfDNA) released from activated neutrophils has antimicrobial properties, it may also exert harmful effects by activating coagulation and inflammation. The authors aimed to determine whether deoxyribonuclease (DNase) administration reduces cfDNA levels, attenuates coagulation and inflammation, suppresses organ damage, and improves outcome in a cecal ligation and puncture (CLP) model of polymicrobial sepsis. Healthy C57Bl/6 mice were subjected to CLP, a surgical procedure involving two punctures of the ligated cecum, or sham surgery (no ligation/puncture). Mice were given DNase or saline by intraperitoneal injection 2, 4, or 6 h after surgery. Two hours after treatment, organs were harvested and plasma levels of cfDNA, interleukin-6 (IL-6), IL-10, thrombin-antithrombin complexes, lung myeloperoxidase, creatinine, alanine transaminase, and bacterial load were quantified. Survival studies were also performed. The CLP-operated mice had rapid time-dependent elevations in cfDNA that correlated with elevations in IL-6, IL-10, and thrombin-antithrombin complexes and had organ damage in the lungs and kidneys. Administration of DNase at 2 h after CLP resulted in increased IL-6 and IL-10 levels and organ damage in the lungs and kidneys. In contrast, DNase administration at 4 or 6 h after CLP resulted in reduced cfDNA and IL-6 levels, increased IL-10, and suppressed organ damage and bacterial dissemination. Deoxyribonuclease administration every 6 h after CLP also rescued mice from death. Our studies are the first to demonstrate that delayed but not early administration of DNase may be protective in experimental sepsis.

Histones link inflammation and thrombosis through the induction of Weibel-Palade body exocytosis.

Essentials Dysregulated DNA and histone release can promote pathological immunothrombosis. Weibel‐Palade bodies (WPBs) are sentinel‐like organelles that respond to proinflammatory stimuli. Histones induce WPB exocytosis in a caspase, calcium and charge‐dependent mechanism. A targetable axis may exist between DNA/histones and WPBs in inflammation and immunothrombosis.

Antiphospholipid Antibodies Promote the Release of Neutrophil Extracellular Traps: A New Mechanism of Thrombosis in the Antiphospholipid Syndrome

Antiphospholipid antibodies (aPL), especially those targeting β2‐glycoprotein I (β2GPI), are well known to activate endothelial cells, monocytes, and platelets, with prothrombotic implications. In contrast, the interaction of aPL with neutrophils has not been extensively studied. Neutrophil extracellular traps (NETs) have recently been recognized as an important activator of the coagulation cascade, as well as an integral component of arterial and venous thrombi. This study was undertaken to determine whether aPL activate neutrophils to release NETs, thereby predisposing to the arterial and venous thrombosis inherent in the antiphospholipid syndrome (APS).

Procoagulant effects of lung cancer chemotherapy: impact on microparticles and cell-free DNA.

Lung cancer is the second leading type of cancer, with venous thromboembolism being the second leading cause of death. Studies have shown increased levels of microparticles and cell-free DNA (CFDNA) in cancer patients, which can activate coagulation through extrinsic and intrinsic pathways, respectively. However, the impact of lung cancer chemotherapy on microparticle and/or CFDNA generation is not completely understood. The aim of the study was to study the effects of platinum-based chemotherapeutic agents on generation of procoagulant microparticles and CFDNA in vitro and in vivo. Microparticles were isolated from chemotherapy-treated monocytes, human umbilical vein endothelial cells, or cancer cells. Tissue factor (TF) and phosphatidylserine levels were characterized and thrombin/factor Xa generation assays were used to determine microparticle procoagulant activity. CFDNA levels were isolated from cell supernatants and plasma. A murine xenograft model of human lung carcinoma was used to study the procoagulant effects of TF+ microparticles and CFDNA in vivo. In vitro, platinum-based chemotherapy induced TF+/phosphatidylserine+ microparticle shedding from A549 and A427 lung cancers cells, which enhanced thrombin generation in plasma in a FVII-dependent manner. CFDNA levels were increased in supernatants of chemotherapy-treated neutrophils and plasma of chemotherapy-treated mice. TF+ microparticles were elevated in plasma of chemotherapy-treated tumour-bearing mice. Plasma CFDNA levels are increased in chemotherapy-treated tumour-free mice and correlate with increased thrombin generation. In tumour-bearing mice, chemotherapy increases plasma levels of CFDNA and TF+/phosphatidylserine+ microparticles. Platinum-based chemotherapy induces the shedding of TF+/phosphatidylserine+ microparticles from tumour cells and the release of CFDNA from host neutrophils.

Extracellular Histones Increase Tissue Factor Activity and Enhance Thrombin Generation by Human Blood Monocytes

Objectives: Sepsis is characterized by systemic activation of inflammatory and coagulation pathways in response to infection. Recently, it was demonstrated that histones released into the circulation by dying/activated cells may contribute to sepsis pathology. Although the ability of extracellular histones to modulate the procoagulant activities of several cell types has been investigated, the influence of histones on the hemostatic functions of circulating monocytes is unknown. To address this, we investigated the ability of histones to modulate the procoagulant potential of THP-1 cells and peripheral blood monocytes, and examined the effects of plasmas obtained from septic patients to induce a procoagulant phenotype on monocytic cells. Methods/Results: Tissue factor (TF) activity assays were performed on histone-treated THP-1 cells and blood monocytes. Exposure of monocytic cells to histones resulted in increases in TF activity, TF antigen, and phosphatidylserine exposure. Histones modulate the procoagulant activity via engagement of Toll-like receptors 2 and 4, and this effect was abrogated with inhibitory antibodies. Increased TF activity of histone-treated cells corresponded to enhanced thrombin generation in plasma determined by calibrated automated thrombography. Finally, TF activity was increased on monocytes exposed to plasma from septic patients, an effect that was attenuated in plasma from patients receiving unfractionated heparin (UFH). Conclusions: Our studies suggest that increased levels of extracellular histones found in sepsis contribute to dysregulated coagulation by increasing TF activity of monocytes. These procoagulant effects can be partially ameliorated in sepsis patients receiving UFH, thereby identifying extracellular histones as a potential therapeutic target for sepsis treatment.

Coagulation Abnormalities in Acute Pancreatitis

Acute pancreatitis (AP) is a potentially lethal disorder with no specific medical treatment. AP is characterized by a spectrum of symptoms, ranging from a local inflammatory process to the more severe form (acute necrotizing pancreatitis) which is associated with a systemic inflammatory response and a mortality rate of 27-45%. A number of risk factors have been identified for AP including alcohol abuse, gallstones, abdominal surgery/injury, cigarette smoking, cystic fibrosis, endoscopic retrograde cholangiopancreatography, hypercalcemia, hyperparathyroidism, hypertriglyceridemia, infection, pancreatic cancer, and injury to the abdomen (Pandol et al., 2007). Alcohol abuse and the development of gallstones account for the majority of AP cases. In AP, inappropriate intracellular activation of digestive enzymes within the pancreas (e.g. trypsin, chymotrypsin, elastase) is the main initiating event. The development of acute necrotizing pancreatitis is usually associated with pancreatic glandular necrosis. Acinar cell apoptosis, the release of cytokines, activation of coagulation, tissue ischemia, and tissue necrosis are key factors in the progression of the condition, as well as in the development of associated extrapancreatic complications (Steinberg & Tenner, 1994; McKay & Buter, 2003; Pandol et al., 2007).