Regulating NETosis: Increasing pH Promotes NADPH Oxidase-Dependent NETosis

pH-regulated single cell migration

Over the last two decades, extra- and intracellular pH have emerged as fundamental regulators of cell motility. Fundamental physiological and pathological processes relying on appropriate cell migration, such as embryonic development, wound healing, and a proper immune defense on the one hand, and autoimmune diseases, metastatic cancer, and the progression of certain parasitic diseases on the other, depend on surrounding pH. In addition, migrating single cells create their own localized pH nanodomains at their surface and in the cytosol. By this means, the migrating cells locally modulate their adhesion to, and the re-arrangement and digestion of, the extracellular matrix. At the same time, the cytosolic nanodomains tune cytoskeletal dynamics along the direction of movement resulting in concerted lamellipodia protrusion and rear end retraction. Extracellular pH gradients as found in wounds, inflamed tissues, or the periphery of tumors stimulate directed cell migration, and long-term exposure to acidic conditions can engender a more migratory and invasive phenotype persisting for hours up to several generations of cells after they have left the acidic milieu. In the present review, the different variants of pH-dependent single cell migration are described. The underlying pH-dependent molecular mechanisms such as conformational changes of adhesion molecules, matrix protease activity, actin (de-)polymerization, and signaling events are explained, and molecular pH sensors stimulated by H+ signaling are presented.

Hyperbaric oxygen therapy counteracts Pseudomonas aeruginosa biofilm micro-compartment phenomenon in murine thermal wounds

Background

Biofilm antibiotic tolerance is partly explained by the behavior of a biofilm as an independent pharmacokinetic micro-compartment. Hyperbaric oxygen therapy has been shown to potentiate antibiotic effects in biofilms. The present study investigates the effect of hyperbaric oxygen therapy (HBOT) on the biofilm micro-pharmacokinetic/pharmacodynamic behavior of tobramycin in an animal biofilm model.

Methods

Full-thickness necroses were created mid-scapular on mice by means of a thermal lesion. After four days, three 16 h seaweed alginate biofilm beads containing Pseudomonas aeruginosa PAO1 were inserted under the necrosis, and three beads were inserted under the adjacent non-affected skin. The mice were randomized to three groups I) HBOT for 1.5 h at 2.8 atm and 0.8 mg tobramycin/mouse subcutaneously; II) Tobramycin as monotherapy, same dose; III) Saline control group. Half the number of mice from group 1 and 2 were sacrificed, and beads were recovered in toto after 3 h and the other half and the placebo mice were sacrificed and beads collected after 4.5 h.

Results

Lower CFUs were seen in the burned group receiving HBOT at 3 and 4.5 h compared to beads in the atmospheric environment (p = 0.043 and p = 0.0089). At 3 h, no CFU difference was observed in the non-burned skin (HBOT vs atmospheric). At 4.5 h, CFU in the non-burned skin had lower CFUs in the group receiving HBOT compared to the corresponding atmospheric group (p = 0.02). CFU was higher in the burned skin than in the non-burned skin at 3 h when HBOT was applied (p = 0.04), effect faded out at 4.5 h.At both time points, the tobramycin content in the beads under burned skin were higher in the HBOT group than in the atmospheric groups (p = 0.031 and p = 0.0078). Only at 4.5 h a higher tobramycin content was seen in the beads under the HBOT-treated burned skin than the beads under the corresponding non-burned skin (p = 0.006).

Conclusion

HBOT, as an anti-biofilm adjuvant treatment of chronic wounds, counteracts biofilm pharmacokinetic micro-compartmentalization through increased available tobramycin and augmented bacterial killing.

Dysregulated neutrophil extracellular traps formation in sepsis

The migration and antimicrobial functions of neutrophils seem to be impaired during sepsis and contribute to the dysregulation of immune responses and disease pathogenesis. However, the role of neutrophil extracellular traps (NETs) remains to be clarified. The study aimed to analyse sequential phenotypic and functional changes of neutrophils during the time following the diagnosis of sepsis. We prospectively enrolled 49 septic and 18 non-septic patients from the intensive care unit (ICU) and emergency room (ER) and 20 healthy volunteers (HV). Baseline blood samples from septic and non-septic patients were collected within 12 h of admission to the hospital. Additional septic samples were drawn at 24, 48 and 72 h after baseline. Neutrophil phenotype and degranulation capacity were assessed by flow cytometry and NET formation was quantified by fluorescence. Neutrophils from septic patients exhibited increased CD66b, CD11b and CD177 expression but displayed reduced NET formation at baseline compared with non-septic patients and HV controls. Neutrophils expressing CD177 interacted less with platelets, were related to reduced NETosis and tended to indicate a worse sepsis outcome. In vitro experiments revealed that neutrophil function is compromised by the origin of sepsis, including the pathogen type and the affected organ. Assessing a decision tree model, our study showed that CD11b expression and NETosis values are useful variables to discriminate septic from non-septic patients. We conclude that sepsis induces changes in neutrophil phenotype and function that may compromise the effective capacity of the host to eliminate pathogens.

Role and Therapeutic Targeting Strategies of Neutrophil Extracellular Traps in Inflammation

Neutrophil extracellular traps (NETs) are large DNA reticular structures secreted by neutrophils and decorated with histones and antimicrobial proteins. As a key mechanism for neutrophils to resist microbial invasion, NETs play an important role in the killing of microorganisms (bacteria, fungi, and viruses). Although NETs are mostly known for mediating microbial killing, increasing evidence suggests that excessive NETs induced by stimulation of physical and chemical components, microorganisms, and pathological factors can exacerbate inflammation and organ damage. This review summarizes the induction and role of NETs in inflammation and focuses on the strategies of inhibiting NETosis and the mechanisms involved in pathogen evasion of NETs. Furthermore, herbal medicine inhibitors and nanodelivery strategies improve the efficiency of inhibition of excessive levels of NETs.

Neutrophil metabolomics in severe COVID-19 reveal GAPDH as a suppressor of neutrophil extracellular trap formation

Severe COVID-19 is characterized by an increase in the number and changes in the function of innate immune cells including neutrophils. However, it is not known how the metabolome of immune cells changes in patients with COVID-19. To address these questions, we analyzed the metabolome of neutrophils from patients with severe or mild COVID-19 and healthy controls. We identified widespread dysregulation of neutrophil metabolism with disease progression including in amino acid, redox, and central carbon metabolism. Metabolic changes in neutrophils from patients with severe COVID-19 were consistent with reduced activity of the glycolytic enzyme GAPDH. Inhibition of GAPDH blocked glycolysis and promoted pentose phosphate pathway activity but blunted the neutrophil respiratory burst. Inhibition of GAPDH was sufficient to cause neutrophil extracellular trap (NET) formation which required neutrophil elastase activity. GAPDH inhibition increased neutrophil pH, and blocking this increase prevented cell death and NET formation. These findings indicate that neutrophils in severe COVID-19 have an aberrant metabolism which can contribute to their dysfunction. Our work also shows that NET formation, a pathogenic feature of many inflammatory diseases, is actively suppressed in neutrophils by a cell-intrinsic mechanism controlled by GAPDH.

Inflammation as a Regulator of the Airway Surface Liquid pH in Cystic Fibrosis

The airway surface liquid (ASL) is a thin sheet of fluid that covers the luminal aspect of the airway epithelium. The ASL is a site of several first-line host defenses, and its composition is a key factor that determines respiratory fitness. Specifically, the acid-base balance of ASL has a major influence on the vital respiratory defense processes of mucociliary clearance and antimicrobial peptide activity against inhaled pathogens. In the inherited disorder cystic fibrosis (CF), loss of cystic fibrosis transmembrane conductance regulator (CFTR) anion channel function reduces HCO3- secretion, lowers the pH of ASL (pHASL), and impairs host defenses. These abnormalities initiate a pathologic process whose hallmarks are chronic infection, inflammation, mucus obstruction, and bronchiectasis. Inflammation is particularly relevant as it develops early in CF and persists despite highly effective CFTR modulator therapy. Recent studies show that inflammation may alter HCO3- and H+ secretion across the airway epithelia and thus regulate pHASL. Moreover, inflammation may enhance the restoration of CFTR channel function in CF epithelia exposed to clinically approved modulators. This review focuses on the complex relationships between acid-base secretion, airway inflammation, pHASL regulation, and therapeutic responses to CFTR modulators. These factors have important implications for defining optimal ways of tackling CF airway inflammation in the post-modulator era.

Immunomodulatory Effects of Bacterial Toll-like Receptor Ligands on the Phenotype and Function of Milk Immune Cells in Dromedary Camel

(1) Toll-like receptors (TLR) are a family of pattern recognition receptors that sense distinct molecular patterns of microbial origin. Although the immune cell composition of camel milk has been recently described, host-pathogen interaction studies in the camel mammary gland are still scarce. The present study aimed to use a whole milk stimulation assay for investigating the modulatory effect of selected Toll-like receptor (TLR) ligands on the phenotype and function of milk immune cells. (2) Methods-camel milk samples (n = 7) were stimulated in vitro with the TLR4 ligand LPS or the TLR2/1 ligand Pam3CSK4, and separated milk cells were evaluated for stimulation-induced shape change, the expression of cell surface markers, phagocytosis, apoptosis, ROS production, and NETosis. Stimulation with PMA was used as a control stimulation. (3) Results-all stimulants induced shape change in milk cells, change in the expression of several cell markers, and increased cell apoptosis and NETosis. In addition, stimulation with Pam3CSK4 and PMA was associated with enhanced ROS production, while only PMA stimulation resulted in enhanced bacterial phagocytosis by milk immune cells. (4) Conclusions-our data indicates selective modulating effects of the TLR ligands LPS and Pam3CSK4 on camel milk phagocytes. These results may have implications for the use of synthetic TLR agonists as immunomodulatory adjuvants of the immune response to intra-mammary vaccines against mastitis pathogens.

Direct observation of Mn distribution/speciation within and surrounding a basidiomycete fungus in the production of Mn-oxides important in toxic element containment

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn²⁺ absorption by a basidiomycete fungus, Coprinopsis urticicola, isolated from Ningyo-toge mine water samples, was investigated in the laboratory under controlled conditions utilizing electron microscopy, synchrotron-based X-ray analysis, and fluorescence microscopy with a molecular pH probe. The fungus' growth was first investigated in an agar-solidified medium supplemented with 1.0 mmol/L Mn²⁺, and Cu²⁺ (0-200 μM), Zn²⁺ (0-200 μM), or diphenyleneiodonium (DPI) chloride (0-100 μM) at 25 °C. The results revealed that Zn²⁺ has no significant effects on Mn oxide formation, whereas Cu²⁺ and DPI significantly inhibit both fungal growth and Mn oxidation, indicating superoxide-mediated Mn oxidation. Indeed, nitroblue tetrazolium and diaminobenzidine assays on the growing fungus revealed the production of superoxide and peroxide. During the interaction of Mn²⁺ with the fungus in solution medium at the initial pH of 5.67, a small fraction of Mn²⁺ infiltrated the fungal hyphae within 8 h, forming a few tens of nm-sized concentrates of soluble Mn²⁺ in the intracellular pH of ∼6.5. After 1 day of incubation, Mn oxides began to precipitate on the hyphae, which were characterized as fibrous nanocrystals with a hexagonal birnessite-structure, these forming spherical aggregates with a diameter of ∼1.5 μm. These nanoscale processes associated with the fungal species derived from the Ningyo-toge mine area provide additional insights into the existing mechanisms of Mn oxidation by filamentous fungi at other U mill tailings sites under circumneutral pH conditions. Such processes add to the class of reactions important to the sequestration of toxic elements.

Neutrophils' Extracellular Trap Mechanisms: From Physiology to Pathology

Neutrophils are an essential part of the innate immune system and the first line of defense against invading pathogens. They phagocytose, release granular contents, produce reactive oxygen species, and form neutrophil extracellular traps (NETs) to fight pathogens. With the characterization of NETs and their components, neutrophils were identified as players of the innate adaptive crosstalk. This has placed NETs at the center not only of physiological but also pathological processes. Aside from their role in pathogen uptake and clearance, NETs have been demonstrated to contribute to the resolution of inflammation by forming aggregated NETs able to degrade inflammatory mediators. On the other hand, NETs have the potential to foster severe pathological conditions. When homeostasis is disrupted, they occlude vessels and ducts, serve as sources of autoantigens and danger or damage associated molecular patterns, directly damage tissues, and exaggerate complement activity and inflammation. This review focusses on the understanding of NETs from their formation to their functions in both physiological and pathological processes.

Low NETosis Induced in Anaplasma phagocytophilum-Infected Cells

Anaplasma phagocytophilum are obligatory intracellular bacteria that preferentially replicate inside leukocytes by utilizing biological compounds and processes of these primary host defensive cells. In this study, bioinformatics analysis was conducted to further characterize A. phagocytophilum–host interactions using the neutrophil-like model of human Caucasian promyelocytic leukemia HL60 cells. We detected a hierarchy of molecules involved in A. phagocytophilum-HL60 interactions with overrepresentation in infected human cells of proteins involved in the reactive oxygen species (ROS) pathway and cell surface monocyte markers. As A. phagocytophilum phagocytosis by neutrophils is inhibited, the results suggested a possible explanation for our bioinformatics data: radical oxygen compounds could induce the killing of bacteria activating NETosis, a unique form of defense mechanism resulting in cell death that is characterized by the release of decondensed chromatin and granular contents to the extracellular space, forming neutrophil extracellular traps (NETs) to eliminate invading microorganisms. Thus, we confirmed the existence of a low NETosis induced in A. phagocytophilum-infected cells by immunofluorescence (IF) experiments. These results provide new insights into the complex mechanisms that govern immune response during A. phagocytophilum host interactions.

Zinc-doped ferric oxyhydroxide nano-layer enhances the bactericidal activity and osseointegration of a magnesium alloy through augmenting the formation of neutrophil extracellular traps

Implant-associated infections (IAI) and osseointegration disorders are the most common complications in orthopedics. Studies have shown that neutrophils surrounding implants play a vital role in regulating these complications. Although magnesium (Mg) and its alloys are considered promising biodegradable bone implants, their role in neutrophil-mediated antibacteria has not yet been examined. Considering the rapid corrosion of Mg, it is necessary to develop methods to inhibit its corrosion. To solve these issues, a zinc-doped ferric oxyhydroxide nano-layer modified plasma electrolytic oxidation (PEO)-coated Mg alloy (PEO-FeZn) was developed in this study, and its antibacterial, immune anti-infective, and osteogenic ability were systematically evaluated. The results showed that PEO-FeZn nano-layer enhanced the corrosion resistance, biocompatibility, bactericidal activity, and osteoblastic differentiation activity of the Mg alloy. Moreover, PEO-FeZn nano-layer inhibited immune evasion-related gene expression and contributed to the formation of neutrophil extracellular traps (NETs) by activating the extracellular release of DNA fibers and granule proteins, and thereby suppressing bacterial invasion and promoting osseointegration in vivo in Staphylococcus aureus (S. aureus)-infected rat femurs. Overall, the findings of this study could serve as a reference for the fabrication of highly biocompatible and corrosion resistant Mg alloys to address the challenges of IAI and osseointegration disorders. STATEMENT OF SIGNIFICANCE: The widely used metallic biomaterials usually come with the risk of IAI. As the first responder around the biomaterials, neutrophils could form NETs to defense against microorganism and promote tissue remodeling. Therefore, biomaterials addressing antibacterial and neutrophils-modulatory strategies are highly necessary in reducing IAI. To solve these issues, we grew PEO-FeZn nano-layers in situ on Mg alloy using a simple and green technique. The nano-layer not only enhanced the corrosion resistance and biocompatibility of Mg alloy, but also elevated the antibacterial and osteogenesis capability. Moreover, nano-layer contributed to NETs formation, thereby suppressing bacterial invasion and even promoting osseointegration in S.aureus-infected femurs. Accordingly, this functionalized multilayer coating with antibacterial immunity represents a novel therapeutic strategy for IAI and weak osseointegration.

Bicarbonate Effects on Antibacterial Immunity and Mucus Glycobiology in the Cystic Fibrosis Lung: A Review With Selected Experimental Observations

The primary defect in cystic fibrosis (CF) is abnormal chloride and bicarbonate transport in the cystic fibrosis transmembrane conductance regulator (CFTR) epithelial ion channel. The apical surface of the respiratory tract is lined by an airway surface liquid layer (ASL) composed of mucin comprising mainly MUC5A and MUC5B glycoproteins. ASL homeostasis depends on sodium bicarbonate secretion into the airways and secretion deficits alter mucus properties leading to airway obstruction, inflammation, and infections. Downstream effects of abnormal ion transport in the lungs include altered intrinsic immune defenses. We observed that neutrophils killed Pseudomonas aeruginosa more efficiently when it had been exposed to sodium bicarbonate, and formation of neutrophil extracellular traps (NETs) by neutrophils was augmented in the presence of increasing bicarbonate concentrations. Physiological levels of bicarbonate sensitized P. aeruginosa to the antimicrobial peptide cathelicidin LL-37, which is present in both lung ASL and in NETs. Sodium bicarbonate has various uses in clinical medicine and in the care of CF patients, and could be further explored as a therapeutic adjunct against Pseudomonas infections.

Roles, detection, and visualization of neutrophil extracellular traps in acute pancreatitis

Neutrophil extracellular traps (NETs) are produced in large quantities at the site of inflammation, and they locally capture and eliminate various pathogens. Thus, NETs quickly control the infection of pathogens in the body and play vital roles in immunity and antibacterial effects. However, evidence is accumulating that NET formation can exacerbate pancreatic tissue damage during acute pancreatitis (AP). In this review, we describe the research progress on NETs in AP and discuss the possibility of NETs as potential therapeutic targets. In addition, since the current detection and visualization methods of NET formation are not uniform and the selection of markers is still controversial, a synopsis of these issues is provided in this review.

Extracellular pH Controls Chemotaxis of Neutrophil Granulocytes by Regulating Leukotriene B4 Production and Cdc42 Signaling

Neutrophil granulocytes are the first and robust responders to the chemotactic molecules released from an inflamed acidic tissue. The aim of this study was to elucidate the role of microenvironmental pH in neutrophil chemotaxis. To this end, we used neutrophils from male C57BL/6J mice and combined live cell imaging chemotaxis assays with measurements of the intracellular pH (pH_i) in varied extracellular pH (pH_e). Observational studies were complemented by biochemical analyses of leukotriene B₄ (LTB₄) production and activation of the Cdc42 Rho GTPase. Our data show that pH_i of neutrophils dose-dependently adapts to a given pH of the extracellular milieu. Neutrophil chemotaxis toward C5a has an optimum at pH_i ∼7.1, and its pH_i dependency is almost parallel to that of LTB₄ production. Consequently, a shallow pH_e gradient, resembling that encountered by neutrophils during extravasation from a blood vessel (pH ∼7.4) into the interstitium (pH ∼7.2), favors chemotaxis of stimulated neutrophils. Lowering pH_e below pH 6.8, predominantly affects neutrophil chemotaxis, although the velocity is largely maintained. Inhibition of the Na⁺/H⁺ exchanger 1 (NHE1) with cariporide drastically attenuates neutrophil chemotaxis at the optimal pH_i irrespective of the high LTB₄ production. Neutrophil migration and chemotaxis are almost completely abrogated by inhibiting LTB₄ production or blocking its receptor (BLT1). The abundance of the active GTP-bound form of Cdc42 is strongly reduced by NHE1 inhibition or pH_e 6.5. In conclusion, we propose that the pH dependence of neutrophil chemotaxis toward C5a is caused by a pH_i-dependent production of LTB₄ and activation of Cdc42. Moreover, it requires the activity of NHE1.

Extracellular DNA Traps: Origin, Function and Implications for Anti-Cancer Therapies

Extracellular DNA may serve as marker in liquid biopsies to determine individual diagnosis and prognosis in cancer patients. Cell death or active release from various cell types, including immune cells can result in the release of DNA into the extracellular milieu. Neutrophils are important components of the innate immune system, controlling pathogens through phagocytosis and/or the release of neutrophil extracellular traps (NETs). NETs also promote tumor progression and metastasis, by modulating angiogenesis, anti-tumor immunity, blood clotting and inflammation and providing a supportive niche for metastasizing cancer cells. Besides neutrophils, other immune cells such as eosinophils, dendritic cells, monocytes/macrophages, mast cells, basophils and lymphocytes can also form extracellular traps (ETs) during cancer progression, indicating possible multiple origins of extracellular DNA in cancer. In this review, we summarize the pathomechanisms of ET formation generated by different cell types, and analyze these processes in the context of cancer. We also critically discuss potential ET-inhibiting agents, which may open new therapeutic strategies for cancer prevention and treatment.

Physiological and Pathophysiological Roles of Metabolic Pathways for NET Formation and Other Neutrophil Functions

Neutrophils are the most numerous cells in the leukocyte population and essential for innate immunity. To limit their effector functions, neutrophils are able to modulate glycolysis and other cellular metabolic pathways. These metabolic pathways are essential not only for energy usage, but also for specialized effector actions, such as the production of reactive oxygen species (ROS), chemotaxis, phagocytosis, degranulation, and the formation of neutrophil extracellular traps (NETs). It has been demonstrated that activated viable neutrophils can produce NETs, which consists of a DNA scaffold able to bind granule proteins and microorganisms. The formation of NETs requires the availability of increased amounts of adenosine triphosphate (ATP) as it is an active cellular and therefore energy-dependent process. In this article, we discuss the glycolytic and other metabolic routes in association with neutrophil functions focusing on their role for building up NETs in the extracellular space. A better understanding of the requirements of metabolic pathways for neutrophil functions may lead to the discovery of molecular targets suitable to develop novel anti-infectious and/or anti-inflammatory drugs.

Oxidative Injury in Ischemic Stroke: A Focus on NADPH Oxidase 4

Ischemic stroke is a leading cause of disability and mortality worldwide. Thus, it is urgent to explore its pathophysiological mechanisms and find new therapeutic strategies for its successful treatment. The relationship between oxidative stress and ischemic stroke is increasingly appreciated and attracting considerable attention. ROS serves as a source of oxidative stress. It is a byproduct of mitochondrial metabolism but primarily a functional product of NADPH oxidases (NOX) family members. Nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) is most closely related to the formation of ROS during ischemic stroke. Its expression is significantly upregulated after cerebral ischemia, making it a promising target for treating ischemic stroke. Several drugs targeting NOX4, such as SCM-198, Iso, G-Rb1, betulinic acid, and electroacupuncture, have shown efficacy as treatments of ischemic stroke. MTfp-NOX4 POC provides a novel insight for the treatment of stroke. Combinations of these therapies also provide new approaches for the therapy of ischemic stroke. In this review, we summarize the subcellular location, expression, and pathophysiological mechanisms of NOX4 in the occurrence and development of ischemic stroke. We also discuss the therapeutic strategies and related regulatory mechanisms for treating ischemic stroke. We further comment on the shortcomings of current NOX4-targeted therapy studies and the direction for improvement.

Methods for Quantifying Neutrophil Extracellular Traps on Biomaterials

Neutrophils rapidly accumulate at sites of inflammation, including biomaterial implantation sites, where they can modulate the microenvironment toward repair through a variety of functions, including superoxide generation, granule release, and extrusion of neutrophil extracellular traps (NETs). NETs are becoming increasing implicated as a central player in the host response to a biomaterial, and as such, there is a need for reliable in vitro methods to evaluate the relative degree of NETs and quantify NETs on the surface of biomaterials. Such methods should be relatively high throughput and minimize sampling bias. In this chapter, we describe two procedures, (1) fluorescent image analysis and (2) a NETs-based ELISA, both of which have been specifically optimized to quantify NETs generated from human neutrophils on electrospun polydioxanone templates. Both methods are valid and also compatible with tissue culture plastic, but have a variety of advantages and disadvantages. Therefore, both methods can be used to concomitantly study NETs on the surface of a biomaterial. Finally, while these methods were developed for electrospun templates in a 96-well cell culture plate, they may be easily adapted to a large scale and for other biomaterials, including but not limited to metallics, ceramics, and natural and synthetic polymers.

Research Note: Evaluation of the efficacy of engineered water nanostructures in inactivating airborne bacteria in poultry houses

Methods to control microbial contamination in confined livestock facilities are important to the health of both animals and workers. In addition, bacterial contamination is also a food safety issue. The most common disinfection technique employed in livestock facilities is the application of oxidizing agents (e.g., potassium peroxymonosulphate, chlorine, hydrogen peroxide, ozone). However, these techniques are associated with a number of limitations (e.g., toxicity, high cost, corrosiveness). Recently, engineered water nanostructures (EWNS) generated using an electrospray system was found effective in inactivating foodborne bacteria. Thus, this study investigated the efficacy of EWNS generated using a laboratory-scale electrospray system in inactivating bacteria found in poultry facilities. The effects of various operating conditions (distance between the injector and grounded electrode of the electrospray system, applied voltage, liquid pH and conductivity, liquid flow rate, and treatment time) on the efficacy were also assessed. In these various experiments, airborne bacterial samples were collected from a pullet room using tryptic soy agar plates and then exposed to EWNS under varying conditions. After treatment, the plates were incubated at 37°C prior to colony counting. Reductions in bacterial concentrations up to 1.26 logs were obtained. The results indicate that the EWNS generated by the electrospray system can be a potential chemical-free alternative to conventional disinfection methods. Future tests will focus on scaling up the system for larger scale trials.

Mechanism of neutrophil extracellular traps generation and their role in trophoblasts apoptosis in gestational diabetes mellitus

Gestational diabetes mellitus (GDM) is a metabolic syndrome occurring in pregnant women and increases the risk of placental dysplasia. Neutrophil extracellular traps (NETs) may play a critical role in placental dysplasia. NETosis (neutrophil cell death by NET release) depends on NADPH/ROS pathway. In view of the adiponectin which is widely believed to be reduced in GDM patients suppresses NADPH oxidase and ROS generation of neutrophil. We speculate that increased NET release is associated with hypoadiponectinemia. Trophoblast apoptosis is significantly increased in GDM patients, but it is not clear whether NETs promotes cell apoptosis. This study aims to reveal the mechanism of Neutrophil Extracellular Traps generation and their role in trophoblast apoptosis in Gestational Diabetes Mellitus. We investigated the generation of NETs by cell-free DNA (cf-DNA) quantification, live-cell imaging, and reactive oxygen species (ROS) measurement. ERK1/2 and p38 MAPK signalling pathway proteins were detected by western blotting. The Cell Counting Kit-8 (CCK-8) assay, flow cytometry, and western blotting were performed to explore the effects of NETs on trophoblast apoptosis. We found that adiponectin inhibited NET release by suppressing ROS production, and p38 MAPK and ERK1/2 proteins were involved in the process. Further, NETs promoted trophoblast apoptosis by activating the ROS-dependent mitochondrial pathway, which is mediated by ERK1/2 signalling. The current study demonstrated that hypoadiponectinemia is the cause of NETs formation and NETs promoting trophoblast apoptosis.

Neutrophil extracellular traps and organ dysfunction in sepsis

Sepsis is a clinical syndrome resulting from infection followed by inflammation and is one of the significant causes of mortality worldwide. The underlying reason is the host's uncontrolled inflammatory response due to an infection led to multiple organ dysfunction/failure. Neutrophils, an innate immune cell, are forerunners to reach the site of infection/inflammation for clearing the infection and resolute the inflammation during sepsis. A relatively new neutrophil effector function, neutrophil extracellular traps (NETs), have been demonstrated to kill the pathogens by releasing DNA decorated with histone and granular proteins. A growing number of pieces of shreds of evidence suggest that unregulated incidence of NETs have a significant influence on the pathogenesis of sepsis-induced multiple organ damage, including arterial hypotension, hypoxemia, coagulopathy, renal, neurological, and hepatic dysfunction. Thus, excessive production and improper resolution of NETs are of significant therapeutic value in combating sepsis-induced multiple organ failure. The purpose of this review is intended to highlight the role of NETs in sepsis-induced organ failure. Furthermore, the current status of therapeutic strategies to intersect the harmful effects of NETs to restore organ functions is discussed.

Neutrophil Extracellular Trapping Role in Cancer, Metastases, and Cancer-Related Thrombosis: a Narrative Review of the Current Evidence Base

Purpose of Review

Neutrophil extracellular trap (NET) formation is a newly discovered, reactive oxygen species-dependent regulated process, whereby neutrophils degranulate and extrude genetic material, after engulfing various infectious or neoplastic antigens, culminating in a measurable serologic footprint. Recent research has highlighted the involvement of NETs in cancer and cancer-related pathologies. We review the role of NET formation in cancer biology, prognosis and potential therapeutic modulators.

Recent Findings

Elevated NET levels are associated with cancer metastasis and may be modified by some anaesthetic-analgesic techniques during tumour resection surgery. It promotes tumour cell migration, angiogenesis and hypercoagulability. Although there are potential anti-NET formation therapeutics available, their role has not been formally assessed in cancer patients.

Summary

Limited available evidence suggests an association between elevated NET expression and cancer metastasis, but its validity as a prognostic indicator for cancer-related outcomes is inconclusive. Further observational and interventional studies are warranted to comprehend the potential prognostic and therapeutic role of NETs in cancer.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11912-021-01103-0.

Multi-facets of neutrophil extracellular trap in infectious diseases: Moving beyond immunity

Neutrophil extracellular traps (NETs) are networks of extracellular chromosomal DNA fibers, histones, and cytoplasmic granule proteins. The release of NET components from neutrophils is involved in the suppression of pathogen diffusion. Development of NETs around target microbes leads to disruption of the cell membrane, eventuating in kind of cell death that is called as NETosis. The very first step in the process of NETosis is activation of Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase upon signaling by innate immune receptors. Afterwards, produced Reactive oxygen species (ROS) trigger protein-arginine deiminase type 4, neutrophil elastase, and myeloperoxidase to generate decondensed chromatin and disrupted integrity of nuclear membrane. Subsequently, decondensed chromatin is mixed with several enzymes in the cytoplasm released from granules, leading to release of DNA and histones, and finally formation of NET. Several reports have indicated that NETosis might contribute to the immune responses through limiting the dissemination of microbial organisms. In this review, we discuss recent advances on the role of neutrophils, NETs, and their implications in the pathogenesis of microbial infections. Additionally, the prospective of the NET modulation as a therapeutic strategy to treat infectious diseases are clarified.

New Insights on NETosis Induced by Entamoeba histolytica: Dependence on ROS from Amoebas and Extracellular MPO Activity

NETosis is a neutrophil process involving sequential steps from pathogen detection to the release of DNA harboring antimicrobial proteins, including the central generation of NADPH oxidase dependent or independent ROS. Previously, we reported that NETosis triggered by Entamoeba histolytica trophozoites is independent of NADPH oxidase activity in neutrophils, but dependent on the viability of the parasites and no ROS source was identified. Here, we explored the possibility that E. histolytica trophozoites serve as the ROS source for NETosis. NET quantitation was performed using SYTOX^® Green assay in the presence of selective inhibitors and scavengers. We observed that respiratory burst in neutrophils was inhibited by trophozoites in a dose dependent manner. Mitochondrial ROS was not also necessary, as the mitochondrial scavenger mitoTEMPO did not affect the process. Surprisingly, ROS-deficient amoebas obtained by pre-treatment with pyrocatechol were less likely to induce NETs. Additionally, we detected the presence of MPO on the cell surface of trophozoites after the interaction with neutrophils and found that luminol and isoluminol, intracellular and extracellular scavengers for MPO derived ROS reduced the amount of NET triggered by amoebas. These data suggest that ROS generated by trophozoites and processed by the extracellular MPO during the contact with neutrophils are required for E. histolytica induced NETosis.

Phagocytosis, Degranulation and Extracellular Traps Release by Neutrophils-The Current Knowledge, Pharmacological Modulation and Future Prospects

Neutrophils are crucial elements of innate immune system, which assure host defense via a range of effector functions, such as phagocytosis, degranulation, and NET formation. The latest literature clearly indicates that modulation of effector functions of neutrophils may affect the treatment efficacy. Pharmacological modulation may affect molecular mechanisms activating or suppressing phagocytosis, degranulation or NET formation. In this review, we describe the role of neutrophils in physiology and in the course of bacterial and viral infections, illustrating the versatility and plasticity of those cells. This review also focus on the action of plant extracts, plant-derived compounds and synthetic drugs on effector functions of neutrophils. These recent advances in the knowledge can help to devise novel therapeutic approaches via pharmacological modulation of the described processes.

The Role of Neutrophil Extracellular Traps in Periodontitis

Periodontitis is a chronic, destructive disease of periodontal tissues caused by multifaceted, dynamic interactions. Periodontal bacteria and host immunity jointly contribute to the pathological processes of the disease. The dysbiotic microbial communities elicit an excessive immune response, mainly by polymorphonuclear neutrophils (PMNs). As one of the main mechanisms of PMN immune response in the oral cavity, neutrophil extracellular traps (NETs) play a crucial role in the initiation and progression of late-onset periodontitis. NETs are generated and released by neutrophils stimulated by various irritants, such as pathogens, host-derived mediators, and drugs. Chromatin and proteins are the main components of NETs. Depending on the characteristics of the processes, three main pathways of NET formation have been described. NETs can trap and kill pathogens by increased expression of antibacterial components and identifying and trapping bacteria to restrict their spread. Moreover, NETs can promote and reduce inflammation, inflicting injuries on the tissues during the pro-inflammation process. During their long-term encounter with NETs, periodontal bacteria have developed various mechanisms, including breaking down DNA of NETs, degrading antibacterial proteins, and impacting NET levels in the pocket environment to resist the antibacterial function of NETs. In addition, periodontal pathogens can secrete pro-inflammatory factors to perpetuate the inflammatory environment and a friendly growth environment, which are responsible for the progressive tissue damage. By learning the strategies of pathogens, regulating the periodontal concentration of NETs becomes possible. Some practical ways to treat late-onset periodontitis are reducing the concentration of NETs, administering anti-inflammatory therapy, and prescribing broad-spectrum and specific antibacterial agents. This review mainly focuses on the mechanism of NETs, pathogenesis of periodontitis, and potential therapeutic approaches based on interactions between NETs and periodontal pathogens.

Physiological concentrations of short-chain fatty acids induce the formation of neutrophil extracellular traps in vitro

Neutrophils represent the first line of host cellular defense against various pathogens. The most recently described microbicidal mechanism of these cells is the release of neutrophil extracellular traps (NET). Currently, a wide range of chemical and biological stimuli are known to induce this response; however, the effect of short-chain fatty acids (SCFAs) on the induction of NET is still unknown. SCFAs are produced mainly by bacterial fermentation of dietary fiber and are found in host tissues and blood. This study aimed to determine whether physiological levels of SCFAs can induce the formation of NET. Previously reported concentrations of SCFAs (as found in the colonic lumen and peripheral blood in postprandial and basal states) were used to stimulate the neutrophils. In order to determine the signaling pathway utilized by SCFAs, we tested the inhibition of the Free Fatty Acid 2 Receptor (FFA2R) expressed in neutrophils using CATPB, the inhibitor of FFA2R, genistein, an inhibitor of the downstream Gα/q11 proteins and DPI, an inhibitor of the NADPH oxidase complex. The SCFAs at colonic intestinal lumen concentrations were able to induce the formation of NET, and when tested at concentrations found in the peripheral blood, only acetic acid at 100 μM (fasting equivalent) and 700 μM (postprandial equivalent) was found to induce the formation of NET. The administration of the competitive inhibitor against the receptor or blockade of relevant G protein signaling and the inhibition of NADPH oxidase complex decreased NET release. SCFAs stimulate NET formation in vitro and this effect is mediated, in part, by the FFA2R.

β-Conglycinin induces the formation of neutrophil extracellular traps dependent on NADPH oxidase-derived ROS, PAD4, ERK1/2 and p38 signaling pathways in mice

β-Conglycinin is one of the key thermostable anti-nutritional factors in soybean, which has strong immunogenicity that usually leads to weaning in some young animals such as piglets and calves and allergic reaction in rats. Neutrophils are involved in the pathogenesis of an allergy. However, the contribution of functional neutrophils to allergy needs to be clarified. The formation of neutrophil extracellular traps is a novel effector mechanism of neutrophils and has been extensively investigated in recent years. To the best of our knowledge, there is no information available on β-conglycinin-induced NETs. In this study, β-conglycinin-induced NET formation in mice was examined via immunofluorescence analysis and fluorescence microplate reader. The mechanism of β-conglycinin-induced NETs was investigated using inhibitors and fluorescent microplate methods. The results showed that β-conglycinin induced the classical characteristics of NETs, which mainly consist of DNA as the backbone and decorated with histones, myeloperoxidase (MPO) and neutrophil elastase (NE). Moreover, β-conglycinin significantly induced the formation of NETs in a dose-dependent way. NET degrading enzyme DNase I markedly reduced β-conglycinin-induced NETs, which suggests that β-conglycinin indeed triggered the release of NETs. Further investigation showed that the quantitation of NETs was markedly decreased by the inhibitors of reactive oxygen species (ROS)-derived-NADPH oxidase, ERK1/2, p38, Rac and PAD4 signaling pathways, indicating the crucial role of these signaling pathways in β-conglycinin-induced NETs. Furthermore, our findings revealed that β-conglycinin induced the formation of NETs, which is dependent on NADPH oxidase-derived ROS, ERK1/2, p38, Rac and PAD4 signaling pathways. This study is the first to demonstrate the underlying mechanisms of β-conglycinin-induced NET formation, and it could be helpful to understand diarrhea caused by β-conglycinin overexposure in young animals and provides the corresponding theoretical basis for clinical applications.

Melatonin restores neutrophil functions and prevents apoptosis amid dysfunctional glutathione redox system

Melatonin is a chronobiotic hormone, which can regulate human diseases like cancer, atherosclerosis, respiratory disorders, and microbial infections by regulating redox system. Melatonin exhibits innate immunomodulation by communicating with immune system and influencing neutrophils to fight infections and inflammation. However, sustaining redox homeostasis and reactive oxygen species (ROS) generation in neutrophils are critical during chemotaxis, oxidative burst, phagocytosis, and neutrophil extracellular trap (NET) formation. Therefore, endogenous antioxidant glutathione (GSH) redox cycle is highly vital in regulating neutrophil functions. Reduced intracellular GSH levels and glutathione reductase (GR) activity in the neutrophils during clinical conditions like autoimmune disorders, neurological disorders, diabetes, and microbial infections lead to dysfunctional neutrophils. Therefore, we hypothesized that redox modulators like melatonin can protect neutrophil health and functions under GSH and GR activity-deficient conditions. We demonstrate the dual role of melatonin, wherein it protects neutrophils from oxidative stress-induced apoptosis by reducing ROS generation; in contrast, it restores neutrophil functions like phagocytosis, degranulation, and NETosis in GSH and GR activity-deficient neutrophils by regulating ROS levels both in vitro and in vivo. Melatonin mitigates LPS-induced neutrophil dysfunctions by rejuvenating GSH redox system, specifically GR activity by acting as a parallel redox system. Our results indicate that melatonin could be a potential auxiliary therapy to treat immune dysfunction and microbial infections, including virus, under chronic disease conditions by restoring neutrophil functions. Further, melatonin could be a promising immune system booster to fight unprecedented pandemics like the current COVID-19. However, further studies are indispensable to address the clinical usage of melatonin.

Pseudomonas Aeruginosa Induced Cell Death in Acute Lung Injury and Acute Respiratory Distress Syndrome

Pseudomonas aeruginosa is an important opportunistic pathogen responsible for the cause of acute lung injury and acute respiratory distress syndrome. P. aeruginosa isthe leading species isolated from patients with nosocomial infection and is detected in almost all the patients with long term ventilation in critical care units. P. aeruginosa infection is also the leading cause of deleterious chronic lung infections in patients suffering from cystic fibrosis as well as the major reason for morbidity in people with chronic obstructive pulmonary disease. P. aeruginosa infections are linked to diseases with high mortality rates and are challenging for treatment, for which no effective remedies have been developed. Massive lung epithelial cell death is a hallmark of severe acute lung injury and acute respiratory distress syndrome caused by P. aeruginosa infection. Lung epithelial cell death poses serious challenges to air barrier and structural integrity that may lead to edema, cytokine secretion, inflammatory infiltration, and hypoxia. Here we review different types of cell death caused by P. aeruginosa serving as a starting point for the diseases it is responsible for causing. We also review the different mechanisms of cell death and potential therapeutics in countering the serious challenges presented by this deadly bacterium.

Nuclear envelope rupture and NET formation is driven by PKCα-mediated lamin B disassembly

The nuclear lamina is essential for the structural integration of the nuclear envelope. Nuclear envelope rupture and chromatin externalization is a hallmark of the formation of neutrophil extracellular traps (NETs). NET release was described as a cellular lysis process; however, this notion has been questioned recently. Here, we report that during NET formation, nuclear lamin B is not fragmented by destructive proteolysis, but rather disassembled into intact full-length molecules. Furthermore, we demonstrate that nuclear translocation of PKCα, which serves as the kinase to induce lamin B phosphorylation and disassembly, results in nuclear envelope rupture. Decreasing lamin B phosphorylation by PKCα inhibition, genetic deletion, or by mutating the PKCα consensus sites on lamin B attenuates extracellular trap formation. In addition, strengthening the nuclear envelope by lamin B overexpression attenuates NET release in vivo and reduces levels of NET-associated inflammatory cytokines in UVB-irradiated skin of lamin B transgenic mice. Our findings advance the mechanistic understanding of NET formation by showing that PKCα-mediated lamin B phosphorylation drives nuclear envelope rupture for chromatin release in neutrophils.

Antiphospholipid antibody-activated NETs exacerbate trophoblast and endothelial cell injury in obstetric antiphospholipid syndrome

Despite the widespread use of antiplatelets and anticoagulants, women with antiphospholipid syndrome (APS) may face pregnancy complications associated with placental dysplasia. Neutrophil extracellular traps (NETs) are involved in the pathogenesis of many autoimmune diseases, including vascular APS; however, their role in obstetric APS is unclear. Herein, we investigated the role of NETs by quantifying cell‐free DNA and NET marker levels. Live‐cell imaging was used to visualize NET formation, and MAPK signalling pathway proteins were analysed. Cell migration, invasion and tube formation assays were performed to observe the effects of NETs on trophoblasts and human umbilical vein endothelial cells (HUVECs). The concentrations of cell‐free DNA and NETs in sera of pregnant patients with APS were elevated compared with that of healthy controls (HCs) matched to gestational week. APS neutrophils were predisposed to spontaneous NET release and IgG purified from the patients (APS‐IgG) induced neutrophils from HCs to release NETs. Additionally, APS‐IgG NET induction was abolished with inhibitors of reactive oxygen species, AKT, p38 MAPK and ERK1/2. Moreover, NETs were detrimental to trophoblasts and HUVECs. In summary, APS‐IgG‐induced NET formation deserves further investigation as a potential novel therapeutic target in obstetrical APS.

Neutrophil Metabolic Shift during their Lifecycle: Impact on their Survival and Activation

Polymorphonuclear neutrophils (PMNs) are innate immune cells, which represent 50% to 70% of the total circulating leukocytes. How PMNs adapt to various microenvironments encountered during their life cycle, from the bone marrow, to the blood plasma fraction, and to inflamed or infected tissues remains largely unexplored. Metabolic shifts have been reported in other immune cells such as macrophages or lymphocytes, in response to local changes in their microenvironment, and in association with a modulation of their pro-inflammatory or anti-inflammatory functions. The potential contribution of metabolic shifts in the modulation of neutrophil activation or survival is anticipated even though it is not yet fully described. If neutrophils are considered to be mainly glycolytic, the relative importance of alternative metabolic pathways, such as the pentose phosphate pathway, glutaminolysis, or the mitochondrial oxidative metabolism, has not been fully considered during activation. This statement may be explained by the lack of knowledge regarding the local availability of key metabolites such as glucose, glutamine, and substrates, such as oxygen from the bone marrow to inflamed tissues. As highlighted in this review, the link between specific metabolic pathways and neutrophil activation has been outlined in many reports. However, the impact of neutrophil activation on metabolic shifts' induction has not yet been explored. Beyond its importance in neutrophil survival capacity in response to available metabolites, metabolic shifts may also contribute to neutrophil population heterogeneity reported in cancer (tumor-associated neutrophil) or auto-immune diseases (Low/High Density Neutrophils). This represents an active field of research. In conclusion, the characterization of neutrophil metabolic shifts is an emerging field that may provide important knowledge on neutrophil physiology and activation modulation. The related question of microenvironmental changes occurring during inflammation, to which neutrophils will respond to, will have to be addressed to fully appreciate the importance of neutrophil metabolic shifts in inflammatory diseases.

SP-D attenuates LPS-induced formation of human neutrophil extracellular traps (NETs), protecting pulmonary surfactant inactivation by NETs

An exacerbated amount of neutrophil extracellular traps (NETs) can cause dysfunction of systems during inflammation. However, host proteins and factors that suppress NET formation (NETosis) are not clearly identified. Here we show that an innate immune collectin, pulmonary surfactant protein-D (SP-D), attenuates lipopolysaccharide (LPS)-mediated NETosis in human neutrophils by binding to LPS. SP-D deficiency in mice (Sftpd-/-) leads to excess NET formation in the lungs during LPS-mediated inflammation. In the absence of SP-D, NETs inhibit the surface-active properties of lung surfactant, essential to prevent the collapse of alveoli, the air breathing structures of the lungs. SP-D reverses NET-mediated inhibition of surfactant and restores the biophysical properties of surfactant. To the best of our knowledge, this study establishes for the first time that (i) SP-D suppresses LPS-mediated NETosis, (ii) NETs inhibit pulmonary surfactant function in the absence of SP-D, and (iii) SP-D can restore NET-mediated inhibition of the surfactant system.

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.

Neutrophil Extracellular Trap Formation: Physiology, Pathology, and Pharmacology

Neutrophil extracellular traps (NETs), a unique DNA framework decorated with antimicrobial peptides, have been in the scientific limelight for their role in a variety of pathologies ranging from cystic fibrosis to cancer. The formation of NETs, as well as relevant regulatory mechanisms, physiological factors, and pharmacological agents have not been systematically discussed in the context of their beneficial and pathological aspects. Novel forms of NET formation including vital NET formation continue to be uncovered, however, there remain fundamental questions around established mechanisms such as NADPH-oxidase (Nox)-dependent and Nox-independent NET formation. Whether NET formation takes place in the tissue versus the bloodstream, internal factors (e.g. reactive oxygen species (ROS) production and transcription factor activation), and external factors (e.g. alkaline pH and hypertonic conditions), have all been demonstrated to influence specific NET pathways. Elements of neutrophil biology such as transcription and mitochondria, which were previously of unknown significance, have been identified as critical mediators of NET formation through facilitating chromatin decondensation and generating ROS, respectively. While promising therapeutics inhibiting ROS, transcription, and gasdermin D are being investigated, neutrophil phagocytosis plays a critical role in host defense and any therapies targeting NET formation must avoid impairing the physiological functions of these cells. This review summarizes what is known in the many domains of NET research, highlights the most relevant challenges in the field, and inspires new questions that can bring us closer to a unified model of NET formation.

Post-Translational Modifications in NETosis and NETs-Mediated Diseases

Neutrophils undergo a unique form of cell death that generates neutrophil extracellular traps (NETs) that may help to neutralize invading pathogens and restore homeostasis. However, uncontrolled NET formation (NETosis) can result in numerous diseases that adversely affect health. Recent studies further elucidate the mechanistic details of the different forms of NETosis and their common end structure, as NETs were constantly found to contain DNA, modified histones and cytotoxic enzymes. In fact, emerging evidence reveal that the post translational modifications (PTMs) of histones in neutrophils have a critical role in regulating neutrophil death. Histone citrullination is shown to promote a rapid form of NET formation independent of NADPH oxidase (NOX), which relies on calcium influx. Interestingly, few studies suggest an association between histone citrullination and other types of PTMs to control cell survival and death, such as histone methylation. Even more exciting is the finding that histone acetylation has a biphasic effect upon NETosis, where histone deacetylase (HDAC) inhibitors promote baseline, NOX-dependent and -independent NETosis. However, increasing levels of histone acetylation suppresses NETosis, and to switch neutrophil death to apoptosis. Interestingly, in the presence of NETosis-promoting stimuli, high levels of HDACis limit both NETosis and apoptosis, and promote neutrophil survival. Recent studies also reveal the importance of the PTMs of neutrophils in influencing numerous pathologies. Histone modifications in NETs can act as a double-edged sword, as they are capable of altering multiple types of neutrophil death, and influencing numerous NET-mediated diseases, such as acute lung injury (ALI), thrombosis, sepsis, systemic lupus erythematosus, and cancer progression. A clear understanding of the role of different PTMs in neutrophils would be important for an understanding of the molecular mechanisms of NETosis, and to appropriately treat NETs-mediated diseases.

Frontline Science: Aggregated neutrophil extracellular traps prevent inflammation on the neutrophil-rich ocular surface

Eye rheum is a physiological discharge, which accumulates at the medial angle of the healthy eye soon after opening in the morning. Microscopic evaluation of eye rheum revealed the presence of viable neutrophils, bacteria, epithelial cells, and particles, aggregated by neutrophil extracellular traps. We observed that in the evening, during eye closure, high C5a recruited neutrophils to the tear film and activated them. In this hypoxic area rich in CO₂ , neutrophils fight microbial aggressors by degranulation. Immediately after eye opening, the microenvironment of the ocular surface changes, the milieu gets normoxic, and loss of CO₂ induces subtle alkalinization of tear film. These conditions favored the formation of neutrophil extracellular traps (NETs) that initially covers the ocular surface and tend to aggregate by eyelid blinking. These aggregated neutrophil extracellular traps (aggNETs) are known as eye rheum and contain several viable neutrophils, epithelial cells, dust particles, and crystals packed together by NETs. Similar to aggNETs induced by monosodium urate crystals, the eye rheum shows a robust proteolytic activity that degraded inflammatory mediators before clinically overt inflammation occur. Finally, the eye rheum passively floats with the tear flow to the medial angle of the eye for disposal. We conclude that the aggNETs-based eye rheum promotes cleaning of the ocular surface and ameliorates the inflammation on the neutrophil-rich ocular surfaces.

Progression of Cystic Fibrosis Lung Disease from Childhood to Adulthood: Neutrophils, Neutrophil Extracellular Trap (NET) Formation, and NET Degradation

Genetic defects in cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene cause CF. Infants with CFTR mutations show a peribronchial neutrophil infiltration prior to the establishment of infection in their lung. The inflammatory response progressively increases in children that include both upper and lower airways. Infectious and inflammatory response leads to an increase in mucus viscosity and mucus plugging of small and medium-size bronchioles. Eventually, neutrophils chronically infiltrate the airways with biofilm or chronic bacterial infection. Perpetual infection and airway inflammation destroy the lungs, which leads to increased morbidity and eventual mortality in most of the patients with CF. Studies have now established that neutrophil cytotoxins, extracellular DNA, and neutrophil extracellular traps (NETs) are associated with increased mucus clogging and lung injury in CF. In addition to opportunistic pathogens, various aspects of the CF airway milieux (e.g., airway pH, salt concentration, and neutrophil phenotypes) influence the NETotic capacity of neutrophils. CF airway milieu may promote the survival of neutrophils and eventual pro-inflammatory aberrant NETosis, rather than the anti-inflammatory apoptotic death in these cells. Degrading NETs helps to manage CF airway disease; since DNAse treatment release cytotoxins from the NETs, further improvements are needed to degrade NETs with maximal positive effects. Neutrophil-T cell interactions may be important in regulating viral infection-mediated pulmonary exacerbations in patients with bacterial infections. Therefore, clarifying the role of neutrophils and NETs in CF lung disease and identifying therapies that preserve the positive effects of neutrophils, while reducing the detrimental effects of NETs and cytotoxic components, are essential in achieving innovative therapeutic advances.

NETopathic Inflammation in Chronic Obstructive Pulmonary Disease and Severe Asthma

Neutrophils play a central role in innate immunity, inflammation, and resolution. Unresolving neutrophilia features as a disrupted inflammatory process in the airways of patients with chronic obstructive pulmonary disease (COPD) and severe asthma. The extent to which this may be linked to disease pathobiology remains obscure and could be further confounded by indication of glucocorticoids or concomitant respiratory infections. The formation of neutrophil extracellular traps (NETs) represents a specialized host defense mechanism that entrap and eliminate invading microbes. NETs are web-like scaffolds of extracellular DNA in complex with histones and neutrophil granular proteins, such as myeloperoxidase and neutrophil elastase. Distinct from apoptosis, NET formation is an active form of cell death that could be triggered by various microbial, inflammatory, and endogenous or exogenous stimuli. NETs are reportedly enriched in neutrophil-dominant refractory lung diseases, such as COPD and severe asthma. Evidence for a pathogenic role for respiratory viruses (e.g., Rhinovirus), bacteria (e.g., Staphylococcus aureus) and fungi (e.g., Aspergillus fumigatus) in NET induction is emerging. Dysregulation of this process may exert localized NET burden and contribute to NETopathic lung inflammation. Disentangling the role of NETs in human health and disease offer unique opportunities for therapeutic modulation. The chemokine CXCR2 receptor regulates neutrophil activation and migration, and small molecule CXCR2 antagonists (e.g., AZD5069, danirixin) have been developed to selectively block neutrophilic inflammatory pathways. NET-stabilizing agents using CXCR2 antagonists are being investigated in proof-of-concept studies in patients with COPD to provide mechanistic insights. Clinical validation of this type could lead to novel therapeutics for multiple CXCR2-related NETopathologies. In this Review, we discuss the emerging role of NETs in the clinicopathobiology of COPD and severe asthma and provide an outlook on how novel NET-stabilizing therapies via CXCR2 blockade could be leveraged to disrupt NETopathic inflammation in disease-specific phenotypes.

Anti-β2GPI/β2GPI induces human neutrophils to generate NETs by relying on ROS

Neutrophils participate in the regulation of pathogens by phagocytosis as well as by generating neutrophil extracellular traps (NETs). Antiphospholipid antibodies, particularly those targeting beta-2-glycoprotein I (β2GPI), stimulate monocytes, platelets, and endothelial cells with prothrombotic participation. This study aimed to explore NET generation in response to anti-β2GPI/β2GPI. A series of experiments involving the separation of primary human leukocytes, NETosis quantification using propidium iodide, exploration of NETosis by fluorescence microscopy, western blotting, examination of free Zn²⁺ using FluoZin-3, and reactive oxygen species (ROS) examination with dihydrorhodamine 123 were performed in this study. We found that anti-β2GPI/β2GPI triggered NETosis, resembling phorbol 12-myristate 13-acetate (PMA)-induced NETosis in magnitude and morphology. The anti-β₂ GPI/β₂ GPI complex in isolation stimulated NETs without relying on p38, protein kinase B (AKT), extracellular signal-related kinase (ERK) 1/2, and zinc signals. NET generation was unaffected by the NADPH oxidase suppressor DP1. The anti-β₂ GPI/β₂ GPI complex stimulated ROS generation without relying on NADPH oxidase, which may participate in NET generation triggered via the anti-β₂ GPI/β₂ GPI complex. In summary, our results indicate that the anti-β₂ GPI/β₂ GPI complex reinforced NET generation by relying on ROS. THE SIGNIFICANCE OF THE PAPER IN THE CONTEXT OF CURRENT KNOWLEDGE: Neutrophils as one of the first lines of defence and essential in the response to pathogen invasion. They eradicate bacteria via phagocytosis or by releasing antimicrobial proteins in degranulation. In this study, we explored the capability of anti-β₂ GPI/β₂ GPI to stimulate NETosis, demonstrating that anti-β₂ GPI/β₂ GPI is a promising method for triggering NET. Anti-β₂ GPI/β₂ GPI induced ROS generation without relying on NADPH oxidase, which contributes to NETosis independently of ERK1/2, Zn²⁺ , or AKT. Our results showed that anti-β2GPI/β2GPI triggered NETosis, resembling PMA-induced NETosis in magnitude as well as morphology. The anti-β₂ GPI/β₂ GPI complex in isolation stimulated NETs without relying on p38, AKT, ERK1/2, or zinc signals. The anti-β₂ GPI/β₂ GPI complex stimulated ROS generation without relying on NADPH oxidase, which may participate in NET generation triggered via the anti-β₂ GPI/β₂ GPI complex.

Furanoid F-Acid F6 Uniquely Induces NETosis Compared to C16 and C18 Fatty Acids in Human Neutrophils

Various biomolecules induce neutrophil extracellular trap (NET) formation or NETosis. However, the effect of fatty acids on NETosis has not been clearly established. In this study, we focused on the NETosis-inducing ability of several lipid molecules. We extracted the lipid molecules present in Arabian Gulf catfish (Arius bilineatus, Val) skin gel, which has multiple therapeutic activities. Gas chromatography⁻mass spectrometry (GC-MS) analysis of the lipid fraction-3 from the gel with NETosis-inducing activity contained fatty acids including a furanoid F-acid (F6; 12,15-epoxy-13,14-dimethyleicosa-12,14-dienoic acid) and common long-chain fatty acids such as palmitic acid (PA; C16:0), palmitoleic acid (PO; C16:1), stearic acid (SA; C18:0), and oleic acid (OA; C18:1). Using pure molecules, we show that all of these fatty acids induce NETosis to different degrees in a dose-dependent fashion. Notably, F6 induces a unique form of NETosis that is rapid and induces reactive oxygen species (ROS) production by both NADPH oxidase (NOX) and mitochondria. F6 also induces citrullination of histone. By contrast, the common fatty acids (PA, PO, SA, and OA) only induce NOX-dependent NETosis. The activation of the kinases such as ERK (extracellular signal-regulated kinase) and JNK (c-Jun N-terminal kinase) is important for long-chain fatty acid-induced NETosis, whereas, in F-acid-induced NETosis, Akt is additionally needed. Nevertheless, NETosis induced by all of these compounds requires the final chromatin decondensation step of transcriptional firing. These findings are useful for understanding F-acid- and other fatty acid-induced NETosis and to establish the active ingredients with therapeutic potential for regulating diseases involving NET formation.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.