Diverse stimuli engage different neutrophil extracellular trap pathways

Native/citrullinated LL37-specific T-cells help autoantibody production in Systemic Lupus Erythematosus

LL37 exerts a dual pathogenic role in psoriasis. Bound to self-DNA/RNA, LL37 licenses autoreactivity by stimulating plasmacytoid dendritic cells-(pDCs)-Type I interferon (IFN-I) and acts as autoantigen for pathogenic Th17-cells. In systemic lupus erythematosus (SLE), LL37 also triggers IFN-I in pDCs and is target of pathogenic autoantibodies. However, whether LL37 activates T-cells in SLE and how the latter differ from psoriasis LL37-specific T-cells is unknown. Here we found that 45% SLE patients had circulating T-cells strongly responding to LL37, which correlate with anti-LL37 antibodies/disease activity. In contrast to psoriatic Th17-cells, these LL37-specific SLE T-cells displayed a T-follicular helper-(T_FH)-like phenotype, with CXCR5/Bcl-6 and IL-21 expression, implicating a role in stimulation of pathogenic autoantibodies. Accordingly, SLE LL37-specific T-cells promoted B-cell secretion of pathogenic anti-LL37 antibodies in vitro. Importantly, we identified abundant citrullinated LL37 (cit-LL37) in SLE tissues (skin and kidney) and observed very pronounced reactivity of LL37-specific SLE T-cells to cit-LL37, compared to native-LL37, which was much more occasional in psoriasis. Thus, in SLE, we identified LL37-specific T-cells with a distinct functional specialization and antigenic specificity. This suggests that autoantigenic specificity is independent from the nature of the autoantigen, but rather relies on the disease-specific milieu driving T-cell subset polarization and autoantigen modifications.

Inhibition of neutrophil elastase prevents neutrophil extracellular trap formation and rescues mice from endotoxic shock

Neutrophil elastase (NE) is a serine protease stored in the azurophilic granules of neutrophils and released into the extracellular milieu during inflammatory response or formation of neutrophil extracellular traps (NETs). Neutrophils release NETs to entrap pathogens by externalizing their cellular contents in a DNA framework decorated with anti-microbials and proteases, including NE. Importantly, excess NETs in tissues are implicated in numerous pathologies, including sepsis, rheumatoid arthritis, vasculitis, and cancer. However, it remains unknown how to effectively prevent NET formation. Here, we show that NE plays a major role during NET formation and that inhibition of NE is a promising approach for decreasing NET-mediated tissue injury. NE promoted NET formation by human neutrophils. Whereas sivelestat, a small molecule inhibitor of NE, inhibited the formation of NETs in vitro , administration of free sivelestat did not have any efficacy in a murine model of lipopolysaccharide-induced endotoxic shock. To improve the efficacy of sivelestat in vivo, we have developed a nanoparticle system for delivering sivelestat. We demonstrate that nanoparticle-mediated delivery of sivelestat effectively inhibited NET formation, decreased the clinical signs of lung injury, reduced NE and other proinflammatory cytokines in serum, and rescued animals against endotoxic shock. Collectively, our data demonstrates that NE signaling can initiate NET formation and that nanoparticle-mediated inhibition of NE improves drug efficacy for preventing NET formation.

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents

'Reactive oxygen species' (ROS) is an umbrella term for an array of derivatives of molecular oxygen that occur as a normal attribute of aerobic life. Elevated formation of the different ROS leads to molecular damage, denoted as 'oxidative distress'. Here we focus on ROS at physiological levels and their central role in redox signalling via different post-translational modifications, denoted as 'oxidative eustress'. Two species, hydrogen peroxide (H₂O₂) and the superoxide anion radical (O₂^·-), are key redox signalling agents generated under the control of growth factors and cytokines by more than 40 enzymes, prominently including NADPH oxidases and the mitochondrial electron transport chain. At the low physiological levels in the nanomolar range, H₂O₂ is the major agent signalling through specific protein targets, which engage in metabolic regulation and stress responses to support cellular adaptation to a changing environment and stress. In addition, several other reactive species are involved in redox signalling, for instance nitric oxide, hydrogen sulfide and oxidized lipids. Recent methodological advances permit the assessment of molecular interactions of specific ROS molecules with specific targets in redox signalling pathways. Accordingly, major advances have occurred in understanding the role of these oxidants in physiology and disease, including the nervous, cardiovascular and immune systems, skeletal muscle and metabolic regulation as well as ageing and cancer. In the past, unspecific elimination of ROS by use of low molecular mass antioxidant compounds was not successful in counteracting disease initiation and progression in clinical trials. However, controlling specific ROS-mediated signalling pathways by selective targeting offers a perspective for a future of more refined redox medicine. This includes enzymatic defence systems such as those controlled by the stress-response transcription factors NRF2 and nuclear factor-κB, the role of trace elements such as selenium, the use of redox drugs and the modulation of environmental factors collectively known as the exposome (for example, nutrition, lifestyle and irradiation).

NETosis proceeds by cytoskeleton and endomembrane disassembly and PAD4-mediated chromatin decondensation and nuclear envelope rupture

Neutrophils are white blood cells specialized as the first line of host defense in the immune system. One way they protect organisms is through NETosis, in which they expel their DNA to form a web-like trap that ensnares pathogens and promotes clotting. However, NETs also mediate sterile inflammation, causing damage to the body. We used high-resolution live-cell microscopy to characterize the timing of dynamic cellular events leading to NETosis in human and mouse neutrophils and a neutrophil-like cell line. We discovered that NETosis proceeds by a stepwise sequence of cellular events that is conserved across species and requires the activity of the PAD4 enzyme for DNA to be released from the nucleus and cell membrane.

Neutrophil extracellular traps (NETs) are web-like DNA structures decorated with histones and cytotoxic proteins that are released by activated neutrophils to trap and neutralize pathogens during the innate immune response, but also form in and exacerbate sterile inflammation. Peptidylarginine deiminase 4 (PAD4) citrullinates histones and is required for NET formation (NETosis) in mouse neutrophils. While the in vivo impact of NETs is accumulating, the cellular events driving NETosis and the role of PAD4 in these events are unclear. We performed high-resolution time-lapse microscopy of mouse and human neutrophils and differentiated HL-60 neutrophil-like cells (dHL-60) labeled with fluorescent markers of organelles and stimulated with bacterial toxins or Candida albicans to induce NETosis. Upon stimulation, cells exhibited rapid disassembly of the actin cytoskeleton, followed by shedding of plasma membrane microvesicles, disassembly and remodeling of the microtubule and vimentin cytoskeletons, ER vesiculation, chromatin decondensation and nuclear rounding, progressive plasma membrane and nuclear envelope (NE) permeabilization, nuclear lamin meshwork and then NE rupture to release DNA into the cytoplasm, and finally plasma membrane rupture and discharge of extracellular DNA. Inhibition of actin disassembly blocked NET release. Mouse and dHL-60 cells bearing genetic alteration of PAD4 showed that chromatin decondensation, lamin meshwork and NE rupture and extracellular DNA release required the enzymatic and nuclear localization activities of PAD4. Thus, NETosis proceeds by a stepwise sequence of cellular events culminating in the PAD4-mediated expulsion of DNA.

Neutrophil Extracellular Trap-Associated CEACAM1 as a Putative Therapeutic Target to Prevent Metastatic Progression of Colon Carcinoma

Neutrophils promote tumor growth and metastasis at multiple stages of cancer progression. One mechanism through which this occurs is via release of neutrophil extracellular traps (NETs). We have previously shown that NETs trap tumor cells in both the liver and the lung, increasing their adhesion and metastasis following postoperative complications. Multiple studies have since shown that NETs play a role in tumor progression and metastasis. NETs are composed of nuclear DNA-derived web-like structures decorated with neutrophil-derived proteins. However, it is unknown which, if any, of these NET-affiliated proteins is responsible for inducing the metastatic phenotype. In this study, we identify the NET-associated carcinoembryonic Ag cell adhesion molecule 1 (CEACAM1) as an essential element for this interaction. Indeed, blocking CEACAM1 on NETs, or knocking it out in a murine model, leads to a significant decrease in colon carcinoma cell adhesion, migration and metastasis. Thus, this work identifies NET-associated CEACAM1 as a putative therapeutic target to prevent the metastatic progression of colon carcinoma.

Neutrophil extracellular traps activate IL-8 and IL-1 expression in human bronchial epithelia

Neutrophil extracellular traps (NETs) provide host defense but can contribute to the pathobiology of diverse human diseases. We sought to determine the extent and mechanism by which NETs contribute to human airway cell inflammation. Primary normal human bronchial epithelial cells (HBEs) grown at air-liquid interface and wild-type (wt)CFBE41o- cells (expressing wtCFTR) were exposed to cell-free NETs from unrelated healthy volunteers for 18 h in vitro. Cytokines were measured in the apical supernatant by Luminex, and the effect on the HBE transcriptome was assessed by RNA sequencing. NETs consistently stimulated IL-8, TNF-α, and IL-1α secretion by HBEs from multiple donors, with variable effects on other cytokines (IL-6, G-CSF, and GM-CSF). Expression of HBE RNAs encoding IL-1 family cytokines, particularly IL-36 subfamily members, was increased in response to NETs. NET exposure in the presence of anakinra [recombinant human IL-1 receptor antagonist (rhIL-1RA)] dampened NET-induced changes in IL-8 and TNF-α proteins as well as IL-36α RNA. rhIL-36RA limited the increase in expression of proinflammatory cytokine RNAs in HBEs exposed to NETs. NETs selectively upregulate an IL-1 family cytokine response in HBEs, which enhances IL-8 production and is limited by rhIL-1RA. The present findings describe a unique mechanism by which NETs may contribute to inflammation in human lung disease in vivo. NET-driven IL-1 signaling may represent a novel target for modulating inflammation in diseases characterized by a substantial NET burden.

The power from within - understanding the driving forces of neutrophil extracellular trap formation

Neutrophil extracellular traps (NETs) are one of the most intriguing discoveries in immunological research of the past few years. After their first description in 2004, the number of research articles on how NETs affect immunodefense, and also how they contribute to an ever-growing number of diseases, has skyrocketed. However, tempting as it may seem to plunge into pharmaceutical approaches to tamper with NET formation, our understanding of this complex process is still incomplete. Important concepts such as the context-dependent dual functions of NETs, in that they are both inflammatory and anti-inflammatory, or the major intra- and extracellular forces driving NET formation, are only emerging. In this Review, we summarize key aspects of our current understanding of NET formation (also termed NETosis), emphasize biophysical aspects and focus on three key principles - rearrangement and destabilization of the plasma membrane and the cytoskeleton, alterations and disassembly of the nuclear envelope, and chromatin decondensation as a driving force of intracellular reorganization.

The double-edged role of neutrophil extracellular traps in inflammation

While there are numerous studies showing that neutrophil extracellular traps (NETs) contribute to autoimmune inflammation and cause bystander tissue injury, human individuals with genetic impairments in NET formation curiously often suffer from exacerbated autoimmune diseases and/or chronic inflammatory conditions. These findings are confirmed in some mouse models of systemic lupus erythematosus (SLE) and gouty arthritis, where an absence of neutrophils or impairment of NET formation leads to exacerbation of autoimmunity and chronic inflammation. Thus, aside from their role as archetypical pro-inflammatory cells, neutrophils in general, and NETs in particular, can also interrupt the self-amplifying loop of cell activation and cell recruitment that characterizes neutrophilic inflammation. Here, we review the current state-of-the-science regarding anti-inflammatory and immune-regulatory action of NETs. We give an overview about the mechanistic involvement of NET-associated neutrophil serine proteases and suggest how tailored induction of NET formation could be exploited for the treatment of chronic autoinflammatory disorders.

Neutrophil Extracellular Trap Formation Correlates with Favorable Overall Survival in High Grade Ovarian Cancer

It is still a question of debate whether neutrophils, often found in the tumor microenvironment, mediate tumor-promoting or rather tumor-inhibiting activities. The present study focuses on the involvement of neutrophils in high grade serous ovarian cancer (HGSOC). Macroscopic features classify two types of peritoneal tumor spread in HGSOC. Widespread and millet sized lesions characterize the miliary type, while non-miliary metastases are larger and associated with better prognosis. Multi-omics and FACS data were generated from ascites samples. Integrated data analysis demonstrates a significant increase of neutrophil extracellular trap (NET)-associated molecules in non-miliary ascites samples. A co-association network analysis performed with the ascites data further revealed a striking correlation between NETosis-associated metabolites and several eicosanoids. The congruence of data generated from primary neutrophils with ascites analyses indicates the predominance of NADPH oxidase 2 (NOX)-independent NETosis. NETosis is associated with protein S100A8/A9 release. An increase of the S100A8/CRP abundance ratio was found to correlate with favorable survival of HGSOC patients. The analysis of additional five independent proteome studies with regard to S100A8/CRP ratios confirmed this observation. In conclusion, NET formation seems to relate with better cancer patient outcome.

Extracellular Traps: An Ancient Weapon of Multiple Kingdoms

The discovery, in 2004, of extracellular traps released by neutrophils has extended our understanding of the mode of action of various innate immune cells. This fascinating discovery demonstrated the extracellular trapping and killing of various pathogens by neutrophils. During the last decade, evidence has accumulated showing that extracellular traps play a crucial role in the defence mechanisms of various cell types present in vertebrates, invertebrates, and plants. The aim of this review is to summarise the relevant literature on the evolutionary history of extracellular traps used as a weapon in various kingdoms of life.

The action of Echis carinatus and Naja naja venoms on human neutrophils; an emphasis on NETosis

BACKGROUND

Neutrophils are the first line defense cells of the innate immunity. As a final defense, they discharge their de-condensed chromatin/DNA fibers, the NETs (Neutrophil Extracellular Traps), by a process called NETosis. Two types of NETosis have been currently described: the suicidal/delayed/classical-type, which is ROS dependent that results in the ejection of nuclear DNA, and the vital/rapid/early-type, which may or may not require ROS but, eject nuclear/mitochondrial DNA or both. Thus, Echis carinatus and Naja naja venoms are comparatively studied for their NET inducing property.

METHODS

Formation of NETs, cell viability, ROS, and Ca²⁺ levels are estimated. An in vivo toxicity study and possible cellular signaling have been addressed using immunoblots and pharmacological inhibitors.

RESULTS

E. carinatus and N. naja venoms respectively induce suicidal and vital NETosis. E. carinatus venom induces NETosis by activating NOX and PAD-4 enzymes in a ROS dependent manner via PKC/ERK/JNK signaling axis, while N. naja venom does it by activating PAD-4 enzyme, but independent of ROS requirement and as well as PKC/ERK/JNK activation.

CONCLUSION

For the first time our study demonstrates the distinct action of E. carinatus and N. naja venoms on the process of NETosis. NETosis being a newly explored area in snake venom pharmacodynamics, it is important to study its impact on the various pathophysiological properties induced by snake venoms.

SIGNIFICANCE

Understanding the varied actions of snake venoms on neutrophils/blood cells and the role of DNase are likely to provide insights for better management of snakebite pathophysiology.

NCF1-339 polymorphism is associated with altered formation of neutrophil extracellular traps, high serum interferon activity and antiphospholipid syndrome in systemic lupus erythematosus

​OBJECTIVES: A single nucleotide polymorphism in the NCF1 gene (NCF1-339, rs201802880), encoding NADPH oxidase type II subunit NCF1/p47phox, reducing production of reactive oxygen species (ROS) is strongly associated with the development of systemic lupus erythematosus (SLE). This study aimed at characterising NCF1-339 effects on neutrophil extracellular trap (NET) formation, type I interferon activity and antibody profile in patients with SLE. ​METHODS: Neutrophil NET-release pathways (n=31), serum interferon (n=141) and finally antibody profiles (n=305) were investigated in SLE subjects from Lund, genotyped for NCF1-339. Then, 1087 SLE subjects from the rheumatology departments of four Swedish SLE centres, genotyped for NCF1-339, were clinically characterised to validate these findings. ​RESULTS: Compared with patients with normal-ROS NCF1-339 genotypes, neutrophils from patients with SLE with low-ROS NCF1-339 genotypes displayed impaired NET formation (p<0.01) and increased dependence on mitochondrial ROS (p<0.05). Low-ROS patients also had increased frequency of high serum interferon activity (80% vs 21.4%, p<0.05) and positivity for anti-β2 glycoprotein I (p<0.01) and anticardiolipin antibodies (p<0.05) but were not associated with other antibodies. We confirmed an over-representation of having any antiphospholipid antibody, OR 1.40 (95% CI 1.01 to 1.95), anti-β2 glycoprotein I, OR 1.82 (95% CI 1.02 to 3.24) and the antiphospholipid syndrome (APS), OR 1.74 (95% CI 1.19 to 2.55) in all four cohorts (n=1087). ​CONCLUSIONS: The NCF1-339 SNP mediated decreased NADPH oxidase function, is associated with high interferon activity and impaired formation of NETs in SLE, allowing dependence on mitochondrial ROS. Unexpectedly, we revealed a striking connection between the ROS deficient NCF1-339 genotypes and the presence of phospholipid antibodies and APS.

Flavored e-liquids increase cytoplasmic Ca2+ levels in airway epithelia

E-cigarettes are noncombustible, electronic nicotine-delivery devices that aerosolize an e-liquid, i.e., nicotine, in a propylene glycol-vegetable glycerin vehicle that also contains flavors. While the effects of nicotine are relatively well understood, more information regarding the potential biological effects of the other e-liquid constituents is needed. This is a serious concern, because e-liquids are available in >7,000 distinct flavors. We previously demonstrated that many e-liquids affect cell growth/viability through an unknown mechanism. Since Ca2+ is a ubiquitous second messenger that regulates cell growth, we characterized the effects of e-liquids on cellular Ca2+ homeostasis. To better understand the extent of this effect, we screened e-liquids for their ability to alter cytosolic Ca2+ levels and found that 42 of 100 flavored e-liquids elicited a cellular Ca2+ response. Banana Pudding (BP) e-liquid, a representative e-liquid from this group, caused phospholipase C activation, endoplasmic reticulum (ER) Ca2+ release, store-operated Ca2+ entry (SOCE), and protein kinase C (PKCα) phosphorylation. However, longer exposures to BP e-liquid depleted ER Ca2+ stores and inhibited SOCE, suggesting that this e-liquid may alter Ca2+ homeostasis by short- and long-term mechanisms. Since dysregulation of Ca2+ signaling can cause chronic inflammation, ER stress, and abnormal cell growth, flavored e-cigarette products that can elicit cell Ca2+ responses should be further screened for potential toxicity.

Finegoldia magna, an Anaerobic Gram-Positive Bacterium of the Normal Human Microbiota, Induces Inflammation by Activating Neutrophils

The Gram-positive anaerobic commensal Finegoldia magna colonizes the skin and other non-sterile body surfaces, and is an important opportunistic pathogen. Here we analyzed the effect of F. magna on human primary neutrophils. F. magna strains ALB8 (expressing protein FAF), 312 (expressing protein L) and 505 (naturally lacking both protein FAF and L) as well as their associated proteins activate neutrophils to release reactive oxygen species, an indication for neutrophil oxidative burst. Co-incubation of neutrophils with the bacteria leads to a strong increase of CD66b surface expression, another indicator for neutrophil activation. Furthermore, all tested stimuli triggered the release of NETs from the activated neutrophils, pointing to a host defense mechanism in response to the tested stimuli. This phenotype is dependent on actin rearrangement, NADPH oxidases and the ERK1/2 pathway. Proteins FAF and L also induced the secretion of several pro-inflammatory neutrophil proteins; HBP, IL-8 and INFγ. This study shows for the first time a direct interaction of F. magna with human neutrophils and suggests that the activation of neutrophils plays a role in F. magna pathogenesis.

Cytonemes Versus Neutrophil Extracellular Traps in the Fight of Neutrophils with Microbes

Neutrophils can phagocytose microorganisms and destroy them intracellularly using special bactericides located in intracellular granules. Recent evidence suggests that neutrophils can catch and kill pathogens extracellularly using the same bactericidal agents. For this, live neutrophils create a cytoneme network, and dead neutrophils provide chromatin and proteins to form neutrophil extracellular traps (NETs). Cytonemes are filamentous tubulovesicular secretory protrusions of living neutrophils with intact nuclei. Granular bactericides are localized in membrane vesicles and tubules of which cytonemes are composed. NETs are strands of decondensed DNA associated with histones released by died neutrophils. In NETs, bactericidal neutrophilic agents are adsorbed onto DNA strands and are not covered with a membrane. Cytonemes and NETs occupy different places in protecting the body against infections. Cytonemes can develop within a few minutes at the site of infection through the action of nitric oxide or actin-depolymerizing alkaloids of invading microbes. The formation of NET in vitro occurs due to chromatin decondensation resulting from prolonged activation of neutrophils with PMA (phorbol 12-myristate 13-acetate) or other stimuli, or in vivo due to citrullination of histones with peptidylarginine deiminase 4. In addition to antibacterial activity, cytonemes are involved in cell adhesion and communications. NETs play a role in autoimmunity and thrombosis.

Neutrophil Subsets in Periodontal Health and Disease: A Mini Review

Neutrophils are amongst the most abundant immune cells within the periodontal tissues and oral cavity. As innate immune cells, they are first line defenders at the tooth-mucosa interface, and can perform an array of different functions. With regard to these, it has been observed over many years that neutrophils are highly heterogeneous in their behavior. Therefore, it has been speculated that neutrophils, similarly to other leukocytes, exist in distinct subsets. Several studies have investigated different markers of neutrophils in oral health and disease in recent years in order to define potential cell subsets and their specific tasks. This research was inspired by recent advancements in other fields of medicine in this field. The aim of this review is to give an overview of the current evidence regarding the existence and presence of neutrophil subsets and their possible functions, specifically in the context of periodontitis, gingivitis, and periodontal health.

Deadliest catch: neutrophil extracellular traps in autoimmunity

PURPOSE OF REVIEW

To summarize recent evidence on the pathogenic effects of neutrophils and neutrophil extracellular traps (NETs) in autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis.

RECENT FINDINGS

NETs can orchestrate innate and adaptive immune dysregulation through diverse mechanisms. NETs induce potent inflammatory responses and represent sources of many autoantigens, creating a feed-forward loop that may perpetuate disease and lead to organ damage. NETs are also increasingly relevant in atherosclerosis and could contribute to the increased risk of premature cardiovascular disease in patients with autoimmunity.

SUMMARY

NET formation is increased in a variety of autoimmune and autoinflammatory diseases and can have remarkable effects on cell and tissue-specific damage. Novel therapeutics that target NET formation or clearance is a promising strategy for clinical management of autoimmune diseases and may prevent chronic complications associated with these conditions.

Detection, Visualization, and Quantification of Neutrophil Extracellular Traps (NETs) and NET Markers

Neutrophil extracellular traps (NETs) have been identified as a key player in the pathogenesis of infection and inflammation in human and animals. On the one hand, NETs have been characterized as fundamental to the innate immune defense against different pathogens since they are able to entrap and immobilize invading pathogens. On the other hand, NETs have been shown to contribute to several diseases, based on their detrimental consequences. This chapter describes methods to detect NETs and NET markers in blood-derived isolated neutrophils of human, pigs, and horses in vitro, as well as NETs and NET marker detection in body fluids from in vivo studies. To avoid nonspecific background in NET-formation, a well-established isolation method for the neutrophils from fresh blood is needed. After stimulation of neutrophils to release NETs, NETs are stained with different antibodies to confirm the presence of extracellular DNA extrusion consisting of histone-DNA complexes, as well as granule components (e.g., myeloperoxidase or elastase). Furthermore, specific methods to quantify NETs and NET markers in the cerebrospinal fluid (CSF) and bronchoalveolar lavage fluid (BALF) are described in detail. In addition to immunofluorescence microscopy, quantification of NET markers from in vivo experiments in various body fluids is described (e.g., nuclease activity, free extracellular DNA, or cationic host defense peptides, such as the porcine PR-39 in BALF and CSF).

Neutrophils and neutrophil extracellular traps enhance venous thrombosis in mice bearing human pancreatic tumors

Pancreatic cancer is associated with a high incidence of venous thromboembolism. Neutrophils have been shown to contribute to thrombosis in part by releasing neutrophil extracellular traps (NET). A recent study showed that increased plasma levels of the NET biomarker, citrullinated histone H3 (H3Cit), are associated with venous thromboembolism in patients with pancreatic and lung cancer but not in those with other types of cancer, including breast cancer. In this study, we examined the contribution of neutrophils and NET to venous thrombosis in nude mice bearing human pancreatic tumors. We found that tumor-bearing mice had increased circulating neutrophil counts and levels of granulocyte-colony stimulating factor, neutrophil elastase, H3Cit and cell-free DNA compared with controls. In addition, thrombi from tumor-bearing mice contained increased levels of the neutrophil marker Ly6G, as well as higher levels of H3Cit and cell-free DNA. Thrombi from tumor-bearing mice also had denser fibrin with thinner fibers consistent with increased thrombin generation. Importantly, either neutrophil depletion or administration of DNase I reduced the thrombus size in tumor-bearing but not in control mice. Our results, together with clinical data, suggest that neutrophils and NET contribute to venous thrombosis in patients with pancreatic cancer.

Autophagy in neutrophils

Autophagy is a highly conserved intracellular degradation and energy-recycling mechanism that contributes to the maintenance of cellular homeostasis. Extensive researches over the past decades have defined the role of autophagy innate immune cells. In this review, we describe the current state of knowledge regarding the role of autophagy in neutrophil biology and a picture of molecular mechanism underlying autophagy in neutrophils. Neutrophils are professional phagocytes that comprise the first line of defense against pathogen. Autophagy machineries are highly conserved in neutrophils. Autophagy is not only involved in generalized function of neutrophils such as differentiation in bone marrow but also plays crucial role effector functions of neutrophils such as granule formation, degranulation, neutrophil extracellular traps release, cytokine production, bactericidal activity and controlling inflammation. This review outlines the current understanding of autophagy in neutrophils and provides insight towards identification of novel therapeutics targeting autophagy in neutrophils.

Insights into the study and origin of the citrullinome in rheumatoid arthritis

The presence of autoantibodies and autoreactive T cells to citrullinated proteins and citrullinating enzymes in patients with rheumatoid arthritis (RA), together with the accumulation of citrullinated proteins in rheumatoid joints, provides substantial evidence that dysregulated citrullination is a hallmark feature of RA. However, understanding mechanisms that dysregulate citrullination in RA has important challenges. Citrullination is a normal process in immune and non-immune cells, which is likely activated by different conditions (eg, inflammation) with no pathogenic consequences. In a complex inflammatory environment such as the RA joint, unique strategies are therefore required to dissect specific mechanisms involved in the abnormal production of citrullinated proteins. Here, we will review current models of citrullination in RA and discuss critical components that, in our view, are relevant to understanding the accumulation of citrullinated proteins in the RA joint, collectively referred to as the RA citrullinome. In particular, we will focus on potential caveats in the study of citrullination in RA and will highlight methods to precisely detect citrullinated proteins in complex biological samples, which is a confirmatory approach to mechanistically link the RA citrullinome with unique pathogenic pathways in RA.

Calcium signaling and regulation of neutrophil functions: Still a long way to go

Neutrophils are the most abundant leukocytes in blood and disruption in their functions often results in an increased risk of serious infections and inflammatory autoimmune diseases. Following recent discoveries in their influence over disease progression, a resurgence of interest for neutrophil biology has taken place. The multitude of signaling pathways activated by the engagement of numerous types of receptors, with which neutrophils are endowed, reflects the functional complexity of these cells. It is therefore not surprising that there remains a huge lack in the understanding of molecular mechanisms underlining neutrophil functions. Moreover, studies on neutrophils are undoubtedly limited by the difficulty to efficiently edit the cell's genome. Over the past 30 years, compelling evidence has clearly highlighted that Ca²⁺ -signaling is governing the key processes associated with neutrophil functions. The confirmation of the role of an elevation of intracellular Ca²⁺ concentration has come from studies on NADPH oxidase activation and phagocytosis. In this review, we give an overview and update of our current knowledge on the role of Ca²⁺ mobilization in the regulation of pro-inflammatory functions of neutrophils. In particular, we stress the importance of Ca²⁺ in the formation of NETs and cytokine secretion in the light of newest findings. This will allow us to embrace how much further we have to go to understand the complex dynamics of Ca²⁺ -dependent mechanisms in order to gain more insights into the role of neutrophils in the pathogenesis of inflammatory diseases. The potential for therapeutics to regulate the neutrophil functions, such as Ca²⁺ influx inhibitors to prevent autoimmune and chronic inflammatory diseases, has been discussed in the last part of the review.

Farnesol, a Quorum-Sensing Molecule of Candida Albicans Triggers the Release of Neutrophil Extracellular Traps

The efficient growth of pathogenic bacteria and fungi in the host organism is possible due to the formation of microbial biofilms that cover the host tissues. Biofilms provide optimal local environmental conditions for fungal cell growth and increased their protection against the immune system. A common biofilm-forming fungus—Candida albicans—uses the quorum sensing (QS) mechanism in the cell-to-cell communication, which determines the biofilm development and, in consequence, host colonization. In the presented work, we focused on the ability of neutrophils—the main cells of the host’s immune system to recognize quorum sensing molecules (QSMs) produced by C. albicans, especially farnesol (FOH), farnesoic acid (FA), and tyrosol (TR), with emphasis on the neutrophil extracellular traps (NETs) formation in a process called netosis. Our results showed for the first time that only farnesol but not farnesolic acid or tyrosol is capable of activating the NET production. By using selective inhibitors of the NET signaling pathway and analyzing the activity of selected enzymes such as Protein Kinase C (PKC), ERK1/2, and NADPH oxidase, we showed that the Mac−1 and TLR2 receptors are responsible for FOH recognizing and activating the reactive oxygen species (ROS)-dependent netosis pathway.

Neutrophil Extracellular Traps in Autoimmunity and Allergy: Immune Complexes at Work

Neutrophil extracellular traps (NETs) have been initially described as main actors in host defense owing to their ability to immobilize and sometimes kill microorganisms. Subsequent studies have demonstrated their implication in the pathophysiology of various diseases, due to the toxic effects of their main components on surrounding tissues. Several distinct NETosis pathways have been described in response to various triggers. Among these triggers, IgG immune complexes (IC) play an important role since they induce robust NET release upon binding to activating FcγRs on neutrophils. Few in vitro studies have documented the mechanisms of IC-induced NET release and evidence about the partners involved is controversial. In vivo, animal models and clinical studies have strongly suggested the importance of IgG IC-induced NET release for autoimmunity and anaphylaxis. In this review, we will focus on two autoimmune diseases in which NETs are undoubtedly major players, systemic lupus erythematosus (SLE), and rheumatoid arthritis (RA). We will also discuss anaphylaxis as another example of disease recently associated with IC-induced NET release. Understanding the role of IC-induced NETs in these settings will pave the way for new diagnostic tools and therapeutic strategies.

The Brain Entangled: The Contribution of Neutrophil Extracellular Traps to the Diseases of the Central Nervous System

Under normal conditions, neutrophils are restricted from trafficking into the brain parenchyma and cerebrospinal fluid by the presence of the brain–blood barrier (BBB). Yet, infiltration of the central nervous system (CNS) by neutrophils is a well-known phenomenon in the course of different pathological conditions, e.g., infection, trauma or neurodegeneration. Different studies have shown that neutrophil products, i.e., free oxygen radicals and proteolytic enzymes, play an important role in the pathogenesis of BBB damage. It was recently observed that accumulating granulocytes may release neutrophil extracellular traps (NETs), which damage the BBB and directly injure surrounding neurons. In this review, we discuss the emerging role of NETs in various pathological conditions affecting the CNS.

Machine Learning to Quantitate Neutrophil NETosis

We introduce machine learning (ML) to perform classification and quantitation of images of nuclei from human blood neutrophils. Here we assessed the use of convolutional neural networks (CNNs) using free, open source software to accurately quantitate neutrophil NETosis, a recently discovered process involved in multiple human diseases. CNNs achieved >94% in performance accuracy in differentiating NETotic from non-NETotic cells and vastly facilitated dose-response analysis and screening of the NETotic response in neutrophils from patients. Using only features learned from nuclear morphology, CNNs can distinguish between NETosis and necrosis and between distinct NETosis signaling pathways, making them a precise tool for NETosis detection. Furthermore, by using CNNs and tools to determine object dispersion, we uncovered differences in NETotic nuclei clustering between major NETosis pathways that is useful in understanding NETosis signaling events. Our study also shows that neutrophils from patients with sickle cell disease were unresponsive to one of two major NETosis pathways. Thus, we demonstrate the design, performance, and implementation of ML tools for rapid quantitative and qualitative cell analysis in basic science.

DGKα in Neutrophil Biology and Its Implications for Respiratory Diseases

Diacylglycerol kinases (DGKs) play a key role in phosphoinositide signaling by removing diacylglycerol and generating phosphatidic acid. Besides the well-documented role of DGKα and DGKζ as negative regulators of lymphocyte responses, a robust body of literature points to those enzymes, and specifically DGKα, as crucial regulators of leukocyte function. Upon neutrophil stimulation, DGKα activation is necessary for migration and a productive response. The role of DGKα in neutrophils is evidenced by its aberrant behavior in juvenile periodontitis patients, which express an inactive DGKα transcript. Together with in vitro experiments, this suggests that DGKs may represent potential therapeutic targets for disorders where inflammation, and neutrophils in particular, plays a major role. In this paper we focus on obstructive respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), but also rare genetic diseases such as alpha-1-antitrypsin deficiency. Indeed, the biological role of DGKα is understudied outside the T lymphocyte field. The recent wave of research aiming to develop novel and specific inhibitors as well as KO mice will allow a better understanding of DGK's role in neutrophilic inflammation. Better knowledge and pharmacologic tools may also allow DGK to move from the laboratory bench to clinical trials.

Neutrophil extracellular traps impair fungal clearance in a mouse model of invasive pulmonary aspergillosis

Neutrophil extracellular traps (NETs) are formed by polymorphonuclear neutrophils (PMN) and contribute to the innate host defense by binding and killing bacterial and fungal pathogens. Because NET formation depends on histone hypercitrullination by peptidylarginine deiminase 4 (PAD4), we used PAD4 gene deficient (Pad4^-/-) mice in a mouse model of invasive pulmonary aspergillosis (IPA) to address the contribution of NETs to the innate host defense in vivo. After the induction (24 h) of IPA by i.t. infection with Aspergillus fumigatus conidia, Pad4^-/- mice revealed lower fungal burden in the lungs, accompanied by less acute lung injury, TNFα and citH3 compared to wildtype controls. These findings suggest that release of NETs contributes to tissue damage and limits control of fungal outgrowth. Thus inhibition of NETosis might be a useful strategy to maintain neutrophil function and avoid lung damage in patients suffering from IPA, especially in those suffering from preexisting pulmonary disease.

ATP amplifies NADPH-dependent and -independent neutrophil extracellular trap formation

Neutrophils are the first immune cells to kill invading microbes at sites of infection using a variety of processes, including the release of proteases, phagocytosis and the production of neutrophil extracellular traps (NETs). NET formation, or NETosis, is a specific and highly efficient process, which is induced by a variety of stimuli leading to expulsion of DNA, proteases and antimicrobial peptides to the extracellular space. However, uncontrolled NETosis may lead to adverse effects and exert tissue damage in pathological conditions. Here, we show that the ATP channel pannexin1 (Panx1) is functionally expressed by bone marrow-derived neutrophils (BMDNs) of wild-type (WT) mice and that ATP contributes to NETosis induced in vitro by the calcium ionophore A23187 or phorbol 12-myristate 13-acetate (PMA). Interestingly, neutrophils isolated from Panx1^−/− mice showed reduced and/or delayed induction of NETosis. Brilliant blue FCF dye (BB-FCF), a Panx1 channel inhibitor, decreased NETosis in wild-type neutrophils to the extent observed in Panx1^−/− neutrophils. Thus, we demonstrate that ATP and Panx1 channels contribute to NETosis and may represent a therapeutic target.

Formation of neutrophil extracellular traps in mitochondrial DNA-deficient cells

Neutrophil extracellular trap (NET) formation plays an important role in inflammatory diseases. Although it is known that NET formation occurs via NADPH oxidase (NOX)-dependent and NOX-independent pathways, the detailed mechanism remains unknown. Therefore, in this study, we aimed to elucidate the role of mitochondria in NOX-dependent and NOX-independent NET formation. We generated mitochondrial DNA-deficient cells (ρ⁰ cells) by treating HL-60 cells with dideoxycytidine and differentiated them to neutrophil-like cells. These neutrophil-like ρ⁰ cells showed markedly reduced NOX-independent NET formation but not NOX-dependent NET formation. However, NET-associated intracellular histone citrullination was not inhibited in ρ⁰ cells. Furthermore, cells membrane disruption in NOX-dependent NET formation occurred in a Myeloperoxidase (MPO) and mixed lineage kinase domain like pseudokinase (MLKL)-dependent manner; however, cell membrane disruption in NOX-independent NET formation partially occurred in an MLKL-dependent manner. These results highlight the importance of mitochondria in NOX-independent NET formation.

Intrinsically Distinct Role of Neutrophil Extracellular Trap Formation in Antineutrophil Cytoplasmic Antibody-Associated Vasculitis Compared to Systemic Lupus Erythematosus

OBJECTIVE

Different studies have demonstrated that neutrophil extracellular traps (NETs) may be involved in the pathophysiology of both antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) and systemic lupus erythematosus (SLE). AAV and SLE are clinically and pathologically divergent autoimmune diseases with different autoantibodies. However, the respective autoantigens recognized in AAV and SLE have been shown to be an intricate part of NETs. This study aimed to examine whether the mechanisms of NET formation and the composition of NETs are distinct between AAV and SLE.

METHODS

To investigate this hypothesis, healthy neutrophils were stimulated with serum from patients with AAV (n = 80) and patients with SLE (n = 59), and the mechanisms of NET formation and NET composition were compared.

RESULTS

Both patients with AAV and patients with SLE had excessive NET formation, which correlated with the extent of disease activity (in AAV r = 0.5, P < 0.0001; in SLE r = 0.35, P < 0.01). Lytic NET formation over several hours was observed in patients with AAV, as compared to rapid (within minutes), non-lytic NET formation coinciding with clustering of neutrophils in patients with SLE. AAV-induced NET formation was triggered independent of IgG ANCAs, whereas SLE immune complexes (ICx) induced NET formation through Fcγ receptor signaling. AAV-induced NET formation was dependent on reactive oxygen species and peptidyl arginine deaminases, and AAV-induced NETs were enriched for citrullinated histones (mean ± SEM 23 ± 2%). In contrast, SLE-induced NETs had immunogenic properties, including binding with high mobility group box chromosomal protein 1 (mean ± SEM 30 ± 3%) and enrichment for oxidized mitochondrial DNA, and were involved in ICx formation.

CONCLUSION

The morphologic features, kinetics, induction pathways, and composition of excessive NET formation are all intrinsically distinct in AAV compared to SLE. Recognizing the diversity of NET formation between AAV and SLE provides a better understanding of the pathophysiologic role of NETs in these different autoimmune diseases.

Candida albicans triggers NADPH oxidase-independent neutrophil extracellular traps through dectin-2

Candida albicans is one of the top leading causes of healthcare-associated bloodstream infection. Neutrophil extracellular traps (NET) are known to capture and kill pathogens. It is reported that opsonized C. albicans-triggered NETosis is NADPH oxidase-dependent. We discovered a NADPH oxidase-independent NETosis pathway in neutrophil response to unopsonized C. albicans. While CR3 engagement with opsonized C. albicans triggered NET, dectin-2 recognized unopsonized C. albicans and mediated NET formation. Engagement of dectin-2 activated the downstream Syk-Ca²⁺-PKCδ-protein arginine deiminase 4 (PAD4) signaling pathway which modulated nuclear translocation of neutrophil elastase (NE), histone citrullination and NETosis. In a C. albicans peritonitis model we observed Ki67⁺Ly6G⁺ NETotic cells in the peritoneal exudate and mesenteric tissues within 3 h of infection. Treatment with PAD4 inhibitor GSK484 or dectin-2 deficiency reduced % Ki67⁺Ly6G⁺ cells and the intensity of Ki67 in peritoneal neutrophils. Employing DNA digestion enzyme micrococcal nuclease, GSK484 as well as dectin-2-deficient mice, we further showed that dectin-2-mediated PAD4-dependent NET formation in vivo restrained the spread of C. albicans from the peritoneal cavity to kidney. Taken together, this study reveals that unopsonized C. albicans evokes NADPH oxidase-independent NETosis through dectin-2 and its downstream signaling pathway and dectin-2-mediated NET helps restrain fungal dissemination.

Candida albicans as a dimorphic fungal pathogen is one of the top leading causes of overall healthcare-associated bloodstream infection worldwide. Invasive candidiasis affects more than 250,000 people each year and leads to more than 50,000 deaths. Upon stimulation, neutrophils release nuclear DNA that forms a web-like structure named neutrophil extracellular traps (NET). NET is known to capture pathogens and restrain the spread of infection in the host. It has been reported that opsonized C. albicans induces NET through NADPH oxidase. Here we show a NADPH oxidase-independent NETosis in response to unopsonized C. albicans. Signaling pathway leading to NETosis involves dectin-2 downstream Syk-Ca²⁺-PKCδ-PAD4/NE. In a C. albicans peritonitis model, NETotic cells are found in the peritoneal exudates and they adhere to mesenteric tissue. Treatment with PAD4 inhibitor or dectin-2 deficiency dampens the ability of neutrophil to undergo NETosis and facilitates the spread of fungus from the peritoneal cavity to kidney. Our work defines the molecular mechanism involved in NADPH oxidase-independent NET formation and sheds light on the role of dectin-2 in neutrophil anti-C. albicans function.

DNA-bound elastase of neutrophil extracellular traps degrades plasminogen, reduces plasmin formation, and decreases fibrinolysis: proof of concept in septic shock plasma

Activation of platelets and neutrophils in septic shock results in the formation of microvascular clots containing an intricate scaffold of fibrin with neutrophil extracellular traps (NETs) DNA. NETs contain multiple components that might impact endogenous fibrinolysis, resulting in failure to lyse clots in the microcirculation and residual systemic microthrombosis. We propose herein that the reservoir of human neutrophil elastase (HNE) on NETs may directly interfere with the fibrinolytic mechanism via a plasminogen proteolytic pathway. To investigate this mechanism, we constructed fibrin-NETs matrices by seeding and activating neutrophils onto a fibrin surface and monitored plasminogen activation or degradation. We demonstrate that the elastase activity of HNE-DNA complexes is protected from inhibition by plasma antiproteases and sustains its ability to degrade plasminogen. Using mass spectrometry proteomic analysis, we identified plasminogen fragments composed of kringle (K) domains (K₁₊₂₊₃, k_1+2+3+4) and the serine protease (SP) region (K₅-SP). We further demonstrate that patients with septic shock with disseminated intravascular coagulation have circulating HNE-DNA complexes, HNE-derived plasminogen fragments, a low plasminogen concentration, and a reduced capacity to generate plasmin onto fibrin. In conclusion, we show that NETs bearing active HNE-DNA complexes reduce plasminogen into fragments, thus impairing fibrinolysis by decreasing the local plasminogen concentration, plasminogen binding to fibrin, and localized plasmin formation.-Barbosa da Cruz, D., Helms, J., Aquino, L. R., Stiel, L., Cougourdan, L., Broussard, C., Chafey, P., Riès-Kautt, M., Meziani, F., Toti, F., Gaussem, P., Anglés-Cano, E. DNA-bound elastase of neutrophil extracellular traps degrades plasminogen, reduces plasmin formation, and decreases fibrinolysis: proof of concept in septic shock plasma.

Recombinant Thrombomodulin Suppresses Histone-Induced Neutrophil Extracellular Trap Formation

Histones, the major protein components of chromatin, are released into the extracellular space during sepsis, trauma, and ischemia-reperfusion injury, and subsequently mediate organ failure. Extracellular histones can promote endothelial damage and platelet aggregation, which can be suppressed by administration of recombinant thrombomodulin (rTM). The present study aimed to clarify whether histones can activate neutrophils to induce NET formation and whether rTM can prevent histone-induced NET formation. NET formation was analyzed in vitro by stimulating human neutrophils with histones in the absence or presence of rTM. NET formation was further analyzed in vivo by intravenous infusion of histones into rats with or without rTM. Histones induced NET release in a dose-dependent manner in vitro and NET release was induced as early as 1 h after stimulation. Histone-induced NET release was independent of NADPH oxidase. rTM suppressed histone-induced NET release in vitro as well as in vivo. The suppression might be mediated by rTM binding to histones, as suggested by analysis using a quartz crystal microbalance system. The present findings suggest that histones can activate neutrophils to form NETs and that rTM can inhibit histone-induced NET formation.

Blue and Long-Wave Ultraviolet Light Induce in vitro Neutrophil Extracellular Trap (NET) Formation

Neutrophil Extracellular Traps (NETs) are produced by neutrophilic granulocytes and consist of decondensed chromatin decorated with antimicrobial peptides. They defend the organism against intruders and are released upon various stimuli including pathogens, mediators of inflammation, or chemical triggers. NET formation is also involved in inflammatory, cardiovascular, malignant diseases, and autoimmune disorders like rheumatoid arthritis, psoriasis, or systemic lupus erythematosus (SLE). In many autoimmune diseases like SLE or dermatomyositis, light of the ultraviolet-visible (UV-VIS) spectrum is well-known to trigger and aggravate disease severity. However, the underlying connection between NET formation, light exposure, and disease exacerbation remains elusive. We studied the effect of UVA (375 nm), blue (470 nm) and green (565 nm) light on NETosis in human neutrophils ex vivo. Our results show a dose- and wavelength-dependent induction of NETosis. Light-induced NETosis depended on the generation of extracellular reactive oxygen species (ROS) induced by riboflavin excitation and its subsequent reaction with tryptophan. The light-induced NETosis required both neutrophil elastase (NE) as well as myeloperoxidase (MPO) activation and induced histone citrullination. These findings suggest that NET formation as a response to light could be the hitherto missing link between elevated susceptibility to NET formation in autoimmune patients and photosensitivity for example in SLE and dermatomyositis patients. This novel connection could provide a clue for a deeper understanding of light-sensitive diseases in general and for the development of new pharmacological strategies to avoid disease exacerbation upon light exposure.

Citrullination Licenses Calpain to Decondense Nuclei in Neutrophil Extracellular Trap Formation

Neutrophils respond to various stimuli by decondensing and releasing nuclear chromatin characterized by citrullinated histones as neutrophil extracellular traps (NETs). This achieves pathogen immobilization or initiation of thrombosis, yet the molecular mechanisms of NET formation remain elusive. Peptidyl arginine deiminase-4 (PAD4) achieves protein citrullination and has been intricately linked to NET formation. Here we show that citrullination represents a major regulator of proteolysis in the course of NET formation. Elevated cytosolic calcium levels trigger both peptidylarginine deiminase-4 (PAD4) and calpain activity in neutrophils resulting in nuclear decondensation typical of NETs. Interestingly, PAD4 relies on proteolysis by calpain to achieve efficient nuclear lamina breakdown and chromatin decondensation. Pharmacological or genetic inhibition of PAD4 and calpain strongly inhibit chromatin decondensation of human and murine neutrophils in response to calcium ionophores as well as the proteolysis of nuclear proteins like lamin B1 and high mobility group box protein 1 (HMGB1). Taken together, the concerted action of PAD4 and calpain induces nuclear decondensation in the course of calcium-mediated NET formation.

Neutrophil extracellular traps drive inflammatory pathogenesis in malaria

Neutrophils are essential innate immune cells that extrude chromatin in the form of neutrophil extracellular traps (NETs) when they die. This form of cell death has potent immunostimulatory activity. We show that heme-induced NETs are essential for malaria pathogenesis. Using patient samples and a mouse model, we define two mechanisms of NET-mediated inflammation of the vasculature: activation of emergency granulopoiesis via granulocyte colony-stimulating factor production and induction of the endothelial cytoadhesion receptor intercellular adhesion molecule-1. Soluble NET components facilitate parasite sequestration and mediate tissue destruction. We demonstrate that neutrophils have a key role in malaria immunopathology and propose inhibition of NETs as a treatment strategy in vascular infections.

Activation leads to a significant shift in the intracellular redox homeostasis of neutrophil-like cells

Neutrophils produce a cocktail of oxidative species during the so-called oxidative burst to attack phagocytized bacteria. However, little is known about the neutrophils' redox homeostasis during the oxidative burst and there is currently no consensus about the interplay between oxidative species and cellular signaling, e.g. during the initiation of the production of neutrophil extracellular traps (NETs). Using the genetically encoded redox sensor roGFP2, expressed in the cytoplasm of the neutrophil-like cell line PLB-985, we saw that stimulation by both PMA and E. coli resulted in oxidation of the thiol residues in this probe. In contrast to the redox state of phagocytized bacteria, which completely breaks down, the neutrophils' cytoplasmic redox state switched from its intital -318 ± 6 mV to a new, albeit higher oxidized, steady state of -264 ± 5 mV in the presence of bacteria. This highly significant oxidation of the cytosol (p value = 7 × 10^-5) is dependent on NOX2 activity, but independent of the most effective thiol oxidant produced in neutrophils, MPO-derived HOCl. While the shift in the intracellular redox potential is correlated with effective NETosis, it is, by itself not sufficient: Inhibition of MPO, while not affecting the cytosolic oxidation, significantly decreased NETosis. Furthermore, inhibition of PI3K, which abrogates cytosolic oxidation, did not fully prevent NETosis induced by phagocytosis of bacteria. Thus, we conclude that NET-formation is regulated in a multifactorial way, in part by changes of the cytosolic thiol redox homeostasis in neutrophils, depending on the circumstance under which the generation of NETs was initiated.

Histone H2A and Bovine Neutrophil Extracellular Traps Induce Damage of Besnoitia besnoiti-Infected Host Endothelial Cells but Fail to Affect Total Parasite Proliferation

Besnoitia besnoiti tachyzoites infect and develop in bovine endothelial cells in vivo and trigger the release of neutrophil extracellular traps (NETs) from bovine polymorphonuclear neutrophils (PMN). The purpose of this study was to analyze if pure B. besnoiti tachyzoite-triggered NETs would damage endothelial host cells and subsequently influence intracellular development and proliferation of B. besnoiti tachyzoites in primary bovine endothelial cells. For comparison purposes, isolated A23187-induced NETs were also used. Thus, we here evaluated endothelial host cell damage triggered by histone 2A (H2A) and B. besnoiti tachyzoite-induced NET preparations and furthermore estimated the effects of PMN floating over B. besnoiti-infected endothelium under physiological flow conditions on endothelial host cell viability. Overall, all treatments (H2A, B. besnoiti-triggered NETs and floating PMN) induced endothelial cell death of B. besnoiti-infected host cells. However, though host cell damage led to significantly altered intracellular parasite development with respect to parasitophorous vacuole diameter and numbers, the total proliferation of the parasite over time was not significantly affected by these treatments thereby denying any direct effect of NETs on intracellular B. besnoiti replication.

Neutrophils in Psoriasis

Neutrophils are the most abundant innate immune cells. The pathogenic roles of neutrophils are related to chronic inflammation and autoimmune diseases. Psoriasis is a chronic systemic inflammatory disease affecting ~2–3% of the world population. The abundant presence of neutrophils in the psoriatic skin lesions serves as a typical histopathologic hallmark of psoriasis. Recent reports indicated that oxidative stress, granular components, and neutrophil extracellular traps from psoriatic neutrophils are related to the initial and maintenance phases of psoriasis. This review provides an overview on the recent (up to 2019) advances in understanding the role of neutrophils in the pathophysiology of psoriasis, including the effects of respiratory burst, degranulation, and neutrophil extracellular trap formation on psoriatic immunity and the clinical relationships.

Effect of Adhesion and Substrate Elasticity on Neutrophil Extracellular Trap Formation

Neutrophils are the most abundant type of white blood cells. Upon stimulation, they are able to decondense and release their chromatin as neutrophil extracellular traps (NETs). This process (NETosis) is part of immune defense mechanisms but also plays an important role in many chronic and inflammatory diseases such as atherosclerosis, rheumatoid arthritis, diabetes, and cancer. For this reason, much effort has been invested into understanding biochemical signaling pathways in NETosis. However, the impact of the mechanical micro-environment and adhesion on NETosis is not well-understood. Here, we studied how adhesion and especially substrate elasticity affect NETosis. We employed polyacrylamide (PAA) gels with distinctly defined elasticities (Young's modulus E) within the physiologically relevant range from 1 to 128 kPa and coated the gels with integrin ligands (collagen I, fibrinogen). Neutrophils were cultured on these substrates and stimulated with potent inducers of NETosis: phorbol 12-myristate 13-acetate (PMA) and lipopolysaccharide (LPS). Interestingly, PMA-induced NETosis was neither affected by substrate elasticity nor by different integrin ligands. In contrast, for LPS stimulation, NETosis rates increased with increasing substrate elasticity (E > 20 kPa). LPS-induced NETosis increased with increasing cell contact area, while PMA-induced NETosis did not require adhesion at all. Furthermore, inhibition of phosphatidylinositide 3 kinase (PI3K), which is involved in adhesion signaling, completely abolished LPS-induced NETosis but only slightly decreased PMA-induced NETosis. In summary, we show that LPS-induced NETosis depends on adhesion and substrate elasticity while PMA-induced NETosis is completely independent of adhesion.

Intra- and Extracellular Degradation of Neutrophil Extracellular Traps by Macrophages and Dendritic Cells

Neutrophil extracellular traps (NETs) composed of nuclear DNA associated with histones and granule proteins are involved in the extracellular killing of pathogens. Excessive NET formation has been implicated in several noninfectious pathological conditions. The disposal of NETs is, therefore, important to prevent inadvertent effects resulting from the continued presence of NETs in the extracellular environment. In this study, we investigated the interaction of NETs released by freshly isolated, PMA-stimulated primary human neutrophils with primary human monocyte-derived macrophages or dendritic cells (DCs). NETs were internalized by macrophages, and removal of the protein component prevented engulfment of NETs, whereas complexation with LL-37 restored the uptake of "naked" (protein-free) NETs. NETs were also found to dampen the bacterial LPS-induced maturation of DCs. Cytokine profiling was conducted by using a multiplex array following the interaction of NETs with macrophages or DCs, and NETs alone were found to be noninflammatory, whereas immunomodulatory effects were noted in the presence of LPS with significant upregulation of IL-1β secretion, and a marked suppression of other LPS-induced factors including vascular endothelial growth factor (VEGF) in both cell types. Moreover, macrophage digestion of NETs was dependent on TREX1 (also known as DNaseIII), but not DNaseII, whereas extracellular DNase1L3-mediated degradation of NETs was observed for DCs. Collectively, these findings shed light on the interactions between NETs and phagocytic cells and provide new insights regarding the clearance of NETs, double-edged swords of innate immunity.

Anthracyclines Suppress Both NADPH Oxidase- Dependent and -Independent NETosis in Human Neutrophils

Neutrophil extracellular traps (NETs) are cytotoxic DNA-protein complexes that play positive and negative roles in combating infection, inflammation, organ damage, autoimmunity, sepsis and cancer. However, NETosis regulatory effects of most of the clinically used drugs are not clearly established. Several recent studies highlight the relevance of NETs in promoting both cancer cell death and metastasis. Here, we screened the NETosis regulatory ability of 126 compounds belonging to 39 classes of drugs commonly used for treating cancer, blood cell disorders and other diseases. Our studies show that anthracyclines (e.g., epirubicin, daunorubicin, doxorubicin, and idarubicin) consistently suppress both NADPH oxidase-dependent and -independent types of NETosis in human neutrophils, ex vivo. The intercalating property of anthracycline may be enough to alter the transcription initiation and lead NETosis inhibition. Notably, the inhibitory doses of anthracyclines neither suppress the production of reactive oxygen species that are necessary for antimicrobial functions nor induce apoptotic cell death in neutrophils. Therefore, anthracyclines are a major class of drug that suppresses NETosis. The dexrazoxane, a cardioprotective agent, used for limiting the side effects of anthracyclines, neither affect NETosis nor alter the ability of anthracyclines to suppress NETosis. Hence, at correct doses, anthracyclines together with dexrazoxane could be considered as a therapeutic candidate drug for suppressing unwanted NETosis in NET-related diseases.