Mechanistic Studies of DNase I Activity: Impact of Heparin Variants and PAD4.

Background: Excessive production of neutrophil extracellular traps (NETs) in sepsis contributes to vascular occlusion by acting as a scaffold and stimulus for thrombus formation. Removal of extracellular DNA, the major structural component of NETs, by DNase I may reduce host injury. Objectives: (1) To determine how heparin variants (unfractionated heparin, enoxaparin, Vasoflux, and fondaparinux) affect DNase I activity, (2) to measure temporal changes in circulating DNA and DNase I in septic patients. Methods: DNA­histone complexes were treated with DNase I ± heparin variants and visualized via agarose gels. We compared the ability of DNase I ± heparin variants to digest NETs released by phorbol 12-myristate 13-acetate-stimulated neutrophils versus DNA­histone complexes released by necrotic HEK293 cells. Plasma DNA and DNase I levels were measured longitudinally in 76 septic patients. Results: Heparin enhances DNase I-mediated digestion of DNA­histone complexes in a size-dependent manner that does not require the antithrombin-binding region. In contrast, DNase I alone was able to degrade the DNA­histone component of NETs presumably due to peptidylarginine deiminase 4 (PAD4)-mediated histone citrullination that weakens DNA­histone interactions. In purified systems, PAD4 treatment of DNA­histone complexes enhanced the ability of DNase I to degrade histone-bound DNA. In septic patients, endogenous DNase I levels remained persistently low over 28 days, and there were no significant correlations between DNA and DNase I levels. Conclusion: Heparin enhances DNA-mediated digestion of DNA­histone complexes in a size-dependent manner that is independent of its anticoagulant properties. Citrullination of histones by PAD4 renders DNA­histone complexes susceptible to DNase I digestion. Endogenous DNase I levels are persistently decreased in septic patients, which supports the potential utility of DNase I as a therapy for sepsis.

Structural and Signaling Events Driving Aspergillus fumigatus-Induced Human Eosinophil Extracellular Trap Release.

Eosinophils are granulocytes classically involved in allergic diseases and in the host immune responses to helminths, fungi, bacteria and viruses. The release of extracellular DNA traps by leukocytes is an important mechanism of the innate immune response to pathogens in various infectious conditions, including fungal infections. Aspergillus fumigatus is an opportunistic fungus responsible for allergic bronchopulmonary aspergillosis (ABPA), a pulmonary disease marked by prominent eosinophilic inflammation. Previously, we demonstrated that isolated human eosinophils release extracellular DNA traps (eosinophil extracellular traps; EETs) when stimulated by A. fumigatus in vitro. This release occurs through a lytic non-oxidative mechanism that involves CD11b and Syk tyrosine kinase. In this work, we unraveled different intracellular mechanisms that drive the release of extracellular DNA traps by A. fumigatus-stimulated eosinophils. Ultrastructurally, we originally observed that A. fumigatus-stimulated eosinophils present typical signs of extracellular DNA trap cell death (ETosis) with the nuclei losing both their shape (delobulation) and the euchromatin/heterochromatin distinction, followed by rupture of the nuclear envelope and EETs release. We also found that by targeting class I PI3K, and more specifically PI3Kδ, the release of extracellular DNA traps induced by A. fumigatus is inhibited. We also demonstrated that A. fumigatus-induced EETs release depends on the Src family, Akt, calcium and p38 MAPK signaling pathways in a process in which fungal viability is dispensable. Interestingly, we showed that A. fumigatus-induced EETs release occurs in a mechanism independent of PAD4 histone citrullination. These findings may contribute to a better understanding of the mechanisms that underlie EETs release in response to A. fumigatus, which may lead to better knowledge of ABPA pathophysiology and treatment.

National Preclinical Sepsis Platform: developing a framework for accelerating innovation in Canadian sepsis research.

Despite decades of preclinical research, no experimentally derived therapies for sepsis have been successfully adopted into routine clinical practice. Factors that contribute to this crisis of translation include poor representation by preclinical models of the complex human condition of sepsis, bias in preclinical studies, as well as limitations of single-laboratory methodology. To overcome some of these shortcomings, multicentre preclinical studies-defined as a research experiment conducted in two or more research laboratories with a common protocol and analysis-are expected to maximize transparency, improve reproducibility, and enhance generalizability. The ultimate objective is to increase the efficiency and efficacy of bench-to-bedside translation for preclinical sepsis research and improve outcomes for patients with life-threatening infection. To this end, we organized the first meeting of the National Preclinical Sepsis Platform (NPSP). This multicentre preclinical  research collaboration of Canadian sepsis researchers and stakeholders was established to study the pathophysiology of sepsis and accelerate movement of promising therapeutics into early phase clinical trials. Integrated knowledge translation and shared decision-making were emphasized to ensure the goals of the platform align with clinical researchers and patient partners. 29 participants from 10 independent labs attended and discussed four main topics: (1) objectives of the platform; (2) animal models of sepsis; (3) multicentre methodology and (4) outcomes for evaluation. A PIRO model (predisposition, insult, response, organ dysfunction) for experimental design was proposed to strengthen linkages with interdisciplinary researchers and key stakeholders. This platform represents an important resource for maximizing translational impact of preclinical sepsis research.

Histoplasma capsulatum-induced extracellular DNA trap release in human neutrophils.

Neutrophils are leukocytes that are capable of eliminating both intra- and extracellular pathogens by mechanisms such as phagocytosis, degranulation, and release of neutrophil extracellular traps (NETs). Histoplasma capsulatum var. capsulatum (H. capsulatum) is a dimorphic fungus with a global distribution that causes histoplasmosis, a disease that is endemic in different geographic areas and is spreading worldwide. The release of NETs has been described as an important host defense mechanism against different fungi; however, there are no reports demonstrating that this process is implicated in neutrophil response to H. capsulatum infection. Therefore, the aim of this work is to investigate whether isolated human neutrophils release NETs in response to H. capsulatum and the potential mechanisms involved, as well as delineate the NETs antifungal activity. Using both confocal fluorescence and scanning electron microscopy techniques, we determined that NETs are released in vitro in response to H. capsulatum via an oxidative mechanism that is downstream of activation of the Syk and Src kinase pathways and is also dependent on CD18. NETs released in response to H. capsulatum yeasts involve the loss of neutrophil viability and are associated with elastase and citrullinated histones, however also can occur in a PAD4 histone citrullination independent pathway. This NETs also presented fungicidal activity against H. capsulatum yeasts. Our findings may contribute to the understanding of how neutrophils recognize and respond as immune effector cells to H. capsulatum, which may lead to better knowledge of histoplasmosis pathophysiology and treatment.

Eosinophils release extracellular DNA traps in response to Aspergillus fumigatus.

BACKGROUND: Eosinophils mediate the immune response in different infectious conditions. The release of extracellular DNA traps (ETs) by leukocytes has been described as an innate immune response mechanism that is relevant in many disorders including fungal diseases. Different stimuli induce the release of human eosinophil ETs (EETs). Aspergillus fumigatus is an opportunistic fungus that may cause eosinophilic allergic bronchopulmonary aspergillosis (ABPA). It has been reported that eosinophils are important to the clearance of A fumigatus in infected mice lungs. However, the immunological mechanisms that underlie the molecular interactions between A fumigatus and eosinophils are poorly understood. OBJECTIVE: Here, we investigated the presence of EETs in the bronchial mucus plugs of patients with ABPA. We also determined whether A fumigatus induced the release of human eosinophil EETs in vitro. METHODS: Mucus samples of patients with ABPA were analyzed by light and confocal fluorescence microscopy. The release of EETs by human blood eosinophils was evaluated using different pharmacological tools and neutralizing antibodies by fluorescence microscopy and a fluorimetric method. RESULTS: We identified abundant nuclear histone-bearing EETs in the bronchial secretions obtained from patients with ABPA. In vitro, we demonstrated that A fumigatus induces the release of EETs through a mechanism independent of reactive oxygen species but associated with eosinophil death, histone citrullination, CD11b, and the Syk tyrosine kinase pathway. EETs lack the killing or fungistatic activities against A fumigatus. CONCLUSIONS: Our findings may contribute to the understanding of how eosinophils recognize and act as immune cells in response to A fumigatus, which may lead to novel insights regarding the treatment of patients with ABPA.

Purinergic P2Y12 Receptor Activation in Eosinophils and the Schistosomal Host Response.

Identifying new target molecules through which eosinophils secrete their stored proteins may reveal new therapeutic approaches for the control of eosinophilic disorders such as host immune responses to parasites. We have recently reported the expression of the purinergic P2Y12 receptor (P2Y12R) in human eosinophils; however, its functional role in this cell type and its involvement in eosinophilic inflammation remain unknown. Here, we investigated functional roles of P2Y12R in isolated human eosinophils and in a murine model of eosinophilic inflammation induced by Schistosoma mansoni (S. mansoni) infection. We found that adenosine 5'-diphosphate (ADP) induced human eosinophils to secrete eosinophil peroxidase (EPO) in a P2Y12R dependent manner. However, ADP did not interfere with human eosinophil apoptosis or chemotaxis in vitro. In vivo, C57Bl/6 mice were infected with cercariae of the Belo Horizonte strain of S. mansoni. Analyses performed 55 days post infection revealed that P2Y12R blockade reduced the granulomatous hepatic area and the eosinophilic infiltrate, collagen deposition and IL-13/IL-4 production in the liver without affecting the parasite oviposition. As found for humans, murine eosinophils also express the P2Y12R. P2Y12R inhibition increased blood eosinophilia, whereas it decreased the bone marrow eosinophil count. Our results suggest that P2Y12R has an important role in eosinophil EPO secretion and in establishing the inflammatory response in the course of a S. mansoni infection.

Multifaceted roles of cysteinyl leukotrienes in eliciting eosinophil granule protein secretion.

Cysteinyl leukotrienes (cysLTs) are cell membrane-impermeant lipid mediators that play major roles in the pathogenesis of eosinophilic inflammation and are recognized to act via at least 2 receptors, namely, cysLT1 receptor (cysLT1R) and cysLT2 receptor (cysLT2R). Eosinophils, which are granulocytes classically associated with host defense against parasitic helminthes and allergic conditions, are distinguished from leukocytes by their dominant population of cytoplasmic crystalloid (also termed secretory, specific, or secondary) granules that contain robust stores of diverse preformed proteins. Human eosinophils are the main source of cysLTs and are recognized to express both cysLTs receptors (cysLTRs) on their surface, at the plasma membrane. More recently, we identified the expression of cysLTRs in eosinophil granule membranes and demonstrated that cysLTs, acting via their granule membrane-expressed receptors, elicit secretion from cell-free human eosinophil granules. Herein, we review the multifaceted roles of cysLTs in eliciting eosinophil granule protein secretion. We discuss the intracrine and autocrine/paracrine secretory responses evoked by cysLTs in eosinophils and in cell-free extracellular eosinophil crystalloid granules. We also discuss the importance of this finding in eosinophil immunobiology and speculate on its potential role(s) in eosinophilic diseases.

Isolation and functional assessment of eosinophil crystalloid granules.

Cell-free granules, upon extrusion from human eosinophils, remain fully competent to secrete granule-derived proteins in receptor-mediated processes in response to different stimuli. However, in order to avoid the shrinkage and damage of granules, as well as preserve their structure, properties, and functionality, the use of an optimized process of subcellular fractionation using an isoosmotic density gradient is needed. Here, we describe a detailed protocol of subcellular fractionation of nitrogen-cavitated eosinophils on an isoosmotic iodinated density gradient (iodixanol) and the isolation of well-preserved and functional membrane-bound specific granules.

Functional extracellular eosinophil granules: a bomb caught in a trap.

Eosinophils store a wide range of preformed proteins, including cationic proteins and cytokines, within their morphologically unique granules. Recently, we have demonstrated that cell-free eosinophil granules are functional, independent, secretory organelles and that clusters of cell-free granules are commonly found at tissue sites associated with various pathologic conditions. Cytolytic release of intact eosinophil granules produces extracellular organelles that are fully capable of ligand-elicited, active, secretory responses and are hence able to act as 'cluster bombs' that amplify the differential secretory properties of eosinophils. Herein, we review recent progress in elucidating the molecular mechanisms involved in the cytolytical release of intact cell-free functional eosinophil granules in a process associated with the liberation of eosinophil DNA traps (nets), a known aspect of the innate response recognized in various immune cells and pathological conditions. We also discuss the importance of clusters of cell-free eosinophil granules trapped in eosinophil DNA nets in disease and speculate on their potential role(s) in immunity as well as compare available data on DNA-releasing neutrophils.

Eosinophil crystalloid granules: structure, function, and beyond.

Eosinophils are granulocytes associated with host defense against parasitic helminths with allergic conditions and more recently, with immunoregulatory responses. Eosinophils are distinguished from leukocytes by their dominant population of cytoplasmic crystalloid (also termed secretory, specific, or secondary) granules that contain robust stores of diverse, preformed cationic proteins. Here, we provide an update on our knowledge about the unique and complex structure of human eosinophil crystalloid granules. We discuss their significance as rich sites of a variety of receptors and review our own recent research findings and those of others that highlight discoveries concerning the function of intracellular receptors and their potential implications in cell signaling. Special focus is provided on how eosinophils might use these intracellular receptors as mechanisms to secrete, selectively and rapidly, cytokines or chemokines and enable cell-free extracellular eosinophil granules to function as independent secretory structures. Potential roles of cell-free eosinophil granules as immune players in the absence of intact eosinophils will also be discussed.