Neutrophil extracellular traps (NETs) are part of the innate immune response to infections. NETs are a meshwork of DNA fibers comprising histones and antimicrobial proteins. Microbes are immobilized in NETs and encounter a locally high and lethal concentration of effector proteins. Recent studies show that NETs are formed inside the vasculature in infections and noninfectious diseases. Here we report that NETs provide a heretofore unrecognized scaffold and stimulus for thrombus formation. NETs perfused with blood caused platelet adhesion, activation, and aggregation. DNase or the anticoagulant heparin dismantled the NET scaffold and prevented thrombus formation. Stimulation of platelets with purified histones was sufficient for aggregation. NETs recruited red blood cells, promoted fibrin deposition, and induced a red thrombus, such as that found in veins. Markers of extracellular DNA traps were detected in a thrombus and plasma of baboons subjected to deep vein thrombosis, an example of inflammation-enhanced thrombosis. Our observations indicate that NETs are a previously unrecognized link between inflammation and thrombosis and may further explain the epidemiological association of infection with thrombosis.neutrophils | platelets | histones | red blood cells | chromatin

Cancer-associated thrombosis often lacks a clear etiology. However, it is linked to a poor prognosis and represents the second-leading cause of death in cancer patients. Recent studies have shown that chromatin released into blood, through the generation of neutrophil extracellular traps (NETs), is procoagulant and prothrombotic. Using a murine model of chronic myelogenous leukemia, we show that malignant and nonmalignant neutrophils are more prone to NET formation. This increased sensitivity toward NET generation is also observed in mammary and lung carcinoma models, suggesting that cancers, through a systemic effect on the host, can induce an increase in peripheral blood neutrophils, which are predisposed to NET formation. In addition, in the late stages of the breast carcinoma model, NETosis occurs concomitant with the appearance of venous thrombi in the lung. Moreover, simulation of a minor systemic infection in tumor-bearing, but not control, mice results in the release of large quantities of chromatin and a prothrombotic state. The increase in neutrophil count and their priming is mediated by granulocyte colony-stimulating factor (G-CSF), which accumulates in the blood of tumor-bearing mice. The prothrombotic state in cancer can be reproduced by treating mice with G-CSF combined with low-dose LPS and leads to thrombocytopenia and microthrombosis. Taken together, our results identify extracellular chromatin released through NET formation as a cause for cancer-associated thrombosis and unveil a target in the effort to decrease the incidence of thrombosis in cancer patients.

Thrombotic microangiopathies (TMAs) are a group of life-threatening disorders characterized by thrombocytopenia, fragmentation of erythrocytes, and ischemic organ damage. Genetic disorders, autoimmune disease, and cancer are risk factors for TMAs, but an additional, unknown trigger is needed to bring about acute disease. Recent studies suggest that DNA and histones are released during inflammation or infection and stimulate coagulation, thrombosis, thrombocytopenia, and organ damage in mice. We show that extracellular DNA and histones as well as markers of neutrophils are present in acute TMAs. Analysis of plasma from TMA patients of different clinical categories revealed elevated levels of DNAhistone complexes and myeloperoxidase (MPO) from neutrophil granules as well as S100A8/A9, a heterocomplex abundant in neutrophil cytosol. During therapy of thrombotic thrombocytopenic purpura, a subtype of TMAs often associated with severe ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13) deficiency, plasma DNA and MPO were inversely correlated with platelet counts, and their levels indicated amelioration or exacerbation of the disease. ADAMTS13 deficiency together with increased levels of plasma DNA and MPO were characteristic for acute thrombotic thrombocytopenic purpura. A minor infection often precedes acute TMA and extracellular DNA and histones released during the inflammatory response could provide the second hit, which precipitates acute TMA in patients with pre-existing risk factors. (Blood. 2012;120(6):1157-1164) IntroductionThrombotic microangiopathies (TMAs) are a heterogeneous group of life-threatening disorders characterized by thrombocytopenia, fragmentation of erythrocytes, and ischemic organ damage that includes thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). [1][2][3] TTP is often associated with a severe deficiency in the von Willebrand factor-cleaving protease known as ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs, member 13). The deficiency can be because of autoantibodies (acquired TTP) 4,5 or genetic mutations of ADAMTS13 (hereditary TTP). [6][7][8] Typical HUS is associated with and follows enterohemorrhagic Escherichia coli infection with bloody diarrhea (DϩHUS), occurs mainly in children, and is associated with acute renal failure. 9,10 Atypical, diarrhea-negative (DϪ) HUS is very difficult to distinguish from TTP and has clearly overlapping clinical findings. 1,3,11 In DϪHUS patients, several hereditary traits were recently identified that result in a hyperactivity of the alternative complement pathway, either by defective regulation of complement activation or by hyperfunctional mutations of complement factors. 3,11,12 Additional clinical conditions associated with TMAs are disseminated neoplasia; drugs including mitomycin C, quinine, and ticlopidine; hematopoietic stem cell transplantation; pregnancy; certain autoimmune diseases; and HIV infection. [1][2][3]13 Whereas almost all patients ...

Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related death. The biologic processes contributing to TRALI are poorly understood. All blood products can cause TRALI, and no specific treatment is available. A "2-event model" has been proposed as the trigger. The first event may include surgery, trauma, or infection; the second involves the transfusion of antileukocyte antibodies or bioactive lipids within the blood product. Together, these events induce neutrophil activation in the lungs, causing endothelial damage and capillary leakage. Neutrophils, in response to pathogens or under stress, can release their chromatin coated with granule contents, thus forming neutrophil extracellular traps (NETs). Although protective against infection, these NETs are injurious to tissue. Here we show that NET biomarkers are present in TRALI patients' blood and that NETs are produced in vitro by primed human neutrophils when challenged with anti-HNA-3a antibodies previously implicated in TRALI. NETs are found in alveoli of mice experiencing antibody-mediated TRALI. DNase 1 inhalation prevents their alveolar accumulation and improves arterial oxygen saturation even when administered 90 minutes after TRALI onset. We suggest that NETs form in the lungs during TRALI, contribute to the disease process, and thus could be targeted to prevent or treat TRALI. IntroductionTransfusion-related acute lung injury (TRALI) is a rare but serious complication of blood transfusion that occurs within 6 hours of transfusion and is characterized by hypoxemia, respiratory distress, and pulmonary infiltrates. 1 Over the years, prevention measures have resulted in a significant reduction in cases. However, TRALI is still the leading cause of transfusion-related mortality, and its prevalence is likely underestimated; one study suggested that more than 2% of cardiac surgery patients are affected. 2 Only supportive treatment is available to the patient, including mechanical ventilation and oxygen supplementation. Many of the severe cases have been linked to the presence of antineutrophil antibodies in the transfused product. 3,4 These antibodies bind to the recipients' neutrophils, activate them, and induce sequestration in the pulmonary capillaries, resulting in tissue injury. 5 Activated neutrophils can release neutrophil extracellular traps (NETs) 6 that are composed of DNA fibers decorated with histones and antimicrobial proteins 7 originally contained in the neutrophil granules. The structure and the composition of NETs allow them to trap and prevent the spread of pathogens and also to kill Gram-negative and Gram-positive bacteria, as well as yeast. 6 NET formation follows a specific pattern characterized by histone hypercitrullination, 8 chromatin decondensation, dissolution of the granular and nuclear membranes, and cytolysis. 9 Despite NETs' beneficial antimicrobial function, 6,10 their formation at the wrong time, in the wrong place, or in the wrong amount can have a negative effect on the host. NETs and their c...

Coronary heart disease is a major cause of death in the western world. Although essential for successful recovery, reperfusion of ischemic myocardium is inevitably associated with reperfusion injury. To investigate a potential protective role of ADAMTS13, a protease cleaving von Willebrand factor multimers, during myocardial ischemia/reperfusion, we used a mouse model of acute myocardial infarction. We found that Adamts13 ؊/؊ mice developed larger myocardial infarctions than wild-type control mice, whereas treatment of wild-type mice with recombinant human ADAMTS13 (rhADAMTS13) led to smaller infarctions. The protective effect of ADAMTS13 was further confirmed by a significant reduction of cardiac troponin-I release and less myocardial apoptosis in mice that received rhADAMTS13 compared with controls. Platelets adherent to the blood vessel wall were observed in few areas in the heart samples from mice treated with vehicle and were not detected in samples from mice treated with rhADAMTS13. However, we observed a 9-fold reduction in number of neutrophils infiltrating ischemic myocardium in mice that were treated with rhADAMTS13, suggesting a potent anti-inflammatory effect of ADAMTS13 during heart injury. Our data show that ADAMTS13 reduces myocardial ischemia/reperfusion injury in mice and indicate that rhADAMTS13 could be of therapeutic value to limit myocardial ischemia/reperfusion injury. (Blood. 2012;120(26):5217-5223) IntroductionCoronary heart disease is the leading cause of death in the western world with approximately 1 million myocardial infarctions (MIs) each year just in the United States. 1,2 Acute myocardial infarction (AMI) is caused by thrombotic occlusion of a coronary artery. Although rapid restoration of the coronary circulation is critical for successful treatment, reperfusion itself exacerbates injury of previously ischemic myocardium. 1 The exact mechanisms of myocardial ischemia/reperfusion (MI/R) injury are not fully understood. 1 Given that cardiac ischemia is either unpredictable (MI) or inevitable (in patients undergoing cardioplegic arrest), there is great interest in developing strategies to minimize injury. von Willebrand factor (VWF) and its cleaving protease ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type-1 motif, member 13) play a pivotal role in platelet adhesion and thrombus formation. By specifically cleaving the VWF A2 domain, ADAMTS13 digests the most thrombogenic ultra-large VWF multimers (UL-VWF) into smaller, less hemostatically active VWF molecules. In addition, ADAMTS13's action on VWF downregulates inflammatory responses. As a result, using experimental mouse models, ADAMTS13 was shown to reduce both thrombosis and inflammation, including atherosclerosis. [3][4][5] An increasing amount of clinical evidence points to the possibility that VWF and ADAMTS13 are involved in MI pathogenesis. 6 To test this experimentally, we used a mouse model of acute myocardial infarction. Using mice deficient in ADAMTS13 and treating wild-type mice with human rhADAM...

Platelets undergo several modifications during storage that reduce their posttransfusion survival and functionality. One important feature of these changes, which are known as platelet storage lesion, is the shedding of the surface glycoproteins GPIb-␣ and GPV. We recently demonstrated that tumor necrosis factor-␣ converting enzyme (TACE/ADAM17) mediates mitochondrial injury-induced shedding of adhesion receptors and that TACE activity correlates with reduced posttransfusion survival of these cells. We now confirm that TACE mediates receptor shedding and clearance of platelets stored for 16 hours at 37°C or 22°C. We further demonstrate that both storage and mitochondrial injury lead to the phosphorylation of p38 mitogen-activated kinase (MAPK) in platelets and that TACEmediated receptor shedding from mouse and human platelets requires p38 MAP kinase signaling. Protein kinase C, extracellular regulated-signal kinase MAPK, and caspases were not involved in TACE activation. Both inhibition of p38 MAPK and inactivation of TACE during platelet storage led to a markedly improved posttransfusion recovery and hemostatic function of platelets in mice. p38 MAPK inhibitors had only minor effects on the aggregation of fresh platelets under static or flow conditions in vitro. In summary, our data suggest that inhibition of p38 MAPK or TACE during storage may significantly improve the quality of stored platelets. (Blood.

There is an emerging interest in the diverse functions of neutrophil extracellular traps (NETs) in a variety of disease settings. However, data on circulating NETs rely largely upon surrogate NET markers such as cell-free DNA, nucleosomes, and NET-associated enzymes. Citrullination of histone H3 by peptidyl arginine deiminase 4 (PAD4) is central for NET formation, and citrullinated histone H3 (H3Cit) is considered a NET-specific biomarker. We therefore aimed to optimize and validate a new enzyme-linked immunosorbent assay (ELISA) to quantify the levels of H3Cit in human plasma. A standard curve made of in vitro PAD4-citrullinated histones H3 allows for the quantification of H3Cit in plasma using an anti-histone antibody as capture antibody and an anti-histone H3 citrulline antibody for detection. The assay was evaluated for linearity, stability, specificity, and precision on plasma samples obtained from a human model of inflammation before and after lipopolysaccharide injection. The results revealed linearity and high specificity demonstrated by the inability of detecting non-citrullinated histone H3. Coefficients of variation for intra- and inter-assay variability ranged from 2.1 to 5.1% and from 5.8 to 13.5%, respectively, allowing for a high precision. Furthermore, our results support an inflammatory induction of a systemic NET burden by showing, for the first time, clear intra-individual elevations of plasma H3Cit in a human model of lipopolysaccharide-induced inflammation. Taken together, our work demonstrates the development of a new method for the quantification of H3Cit by ELISA that can reliably be used for the detection of NETs in human plasma.

Platelets contribute to homeostasis of the tumor vasculature by helping prevent hemorrhage. Thus, we hypothesized that inducing thrombocytopenia would increase tumor vascular leakiness and facilitate the effective delivery of chemotherapeutic agents to tumors. In a mammary carcinoma murine model, platelet depletion induced bleeding specifically at the tumor site, favoring the accumulation of fluorescently labeled microspheres only in the tumor. Moreover, induction of thrombocytopenia in tumor-bearing mice before injection of paclitaxel increased its intratumoral accumulation and reduced growth of both slow-and fast-growing tumors, compared with mice with normal platelet counts that were treated only with paclitaxel. Histologic analysis confirmed the expectation of an increase in tumor apoptosis and a reduction in tumor proliferation in thrombocytopenic mice receiving chemotherapy. No increased toxicity was seen in other organs or blood cells. Taken together, our results indicate that low platelet count selectively induces leakiness of tumor vessels and favors the delivery of chemotherapy to tumor sites, enhancing its tumoricidal effects.