Neutrophil Extracellular Traps and Microcrystals

Neutrophil extracellular traps in cancer

Neutrophil extracellular traps (NETs), which consist of chromatin DNA studded with granule proteins, are released by neutrophils in response to both infectious and sterile inflammation. Beyond the canonical role in defense against pathogens, the extrusion of NETs also contributes to the initiation, metastasis, and therapeutic response of malignant diseases. Recently, NETs have been implicated in the development and therapeutic responses of various types of tumors. Although extensive work regarding inflammation in tumors has been reported, a comprehensive summary of how these web-like extracellular structures initiate and propagate tumor progression under the specific microenvironment is lacking. In this review, we demonstrate the initiators and related signaling pathways that trigger NETs formation in cancers. Additionally, this review will outline the current molecular mechanisms and regulatory networks of NETs during dormant cancer cells awakening, circulating tumor cells (CTCs) extravasation, and metastatic recurrence of cancer. This is followed by a perspective on the current and potential clinical potential of NETs as therapeutic targets in the treatment of both local and metastatic disease, including the improvement of the efficacy of existing therapies.

Mild hyperthermia enhanced synergistic uric acid degradation and multiple ROS elimination for an effective acute gout therapy

BACKGROUND

Acute gouty is caused by the excessive accumulation of Monosodium Urate (MSU) crystals within various parts of the body, which leads to a deterioration of the local microenvironment. This degradation is marked by elevated levels of uric acid (UA), increased reactive oxygen species (ROS) production, hypoxic conditions, an upsurge in pro-inflammatory mediators, and mitochondrial dysfunction.

RESULTS

In this study, we developed a multifunctional nanoparticle of polydopamine-platinum (PDA@Pt) to combat acute gout by leveraging mild hyperthermia to synergistically enhance UA degradation and anti-inflammatory effect. Herein, PDA acts as a foundational template that facilitates the growth of a Pt shell on the surface of its nanospheres, leading to the formation of the PDA@Pt nanomedicine. Within this therapeutic agent, the Pt nanoparticle catalyzes the decomposition of UA and actively breaks down endogenous hydrogen peroxide (H2O2) to produce O2, which helps to alleviate hypoxic conditions. Concurrently, the PDA component possesses exceptional capacity for ROS scavenging. Most significantly, Both PDA and Pt shell exhibit absorption in the Near-Infrared-II (NIR-II) region, which not only endow PDA@Pt with superior photothermal conversion efficiency for effective photothermal therapy (PTT) but also substantially enhances the nanomedicine's capacity for UA degradation, O2 production and ROS scavenging enzymatic activities. This photothermally-enhanced approach effectively facilitates the repair of mitochondrial damage and downregulates the NF-κB signaling pathway to inhibit the expression of pro-inflammatory cytokines.

CONCLUSIONS

The multifunctional nanomedicine PDA@Pt exhibits exceptional efficacy in UA reduction and anti-inflammatory effects, presenting a promising potential therapeutic strategy for the management of acute gout.

Targeting Neutrophil Extracellular Traps in Gouty Arthritis: Insights into Pathogenesis and Therapeutic Potential

Gouty arthritis (GA) is an immune-mediated disorder characterized by severe inflammation due to the deposition of monosodium urate (MSU) crystals in the joints. The pathophysiological mechanisms of GA are not yet fully understood, and therefore, the identification of effective therapeutic targets is of paramount importance. Neutrophil extracellular traps (NETs), an intricate structure of DNA scaffold, encompassing myeloperoxidase, histones, and elastases - have gained significant attention as a prospective therapeutic target for gouty arthritis, due to their innate antimicrobial and immunomodulatory properties. Hence, exploring the therapeutic potential of NETs in gouty arthritis remains an enticing avenue for further investigation. During the process of gouty arthritis, the formation of NETs triggers the release of inflammatory cytokines, thereby contributing to the inflammatory response, while MSU crystals and cytokines are sequestered and degraded by the aggregation of NETs. Here, we provide a concise summary of the inflammatory processes underlying the initiation and resolution of gouty arthritis mediated by NETs. Furthermore, this review presents an overview of the current pharmacological approaches for treating gouty arthritis and summarizes the potential of natural and synthetic product-based inhibitors that target NET formation as novel therapeutic options, alongside elucidating the intrinsic challenges of these inhibitors in NETs research. Lastly, the limitations of HL-60 cell as a suitable substitute of neutrophils in NETs research are summarized and discussed. Series of recommendations are provided, strategically oriented towards guiding future investigations to effectively address these concerns. These findings will contribute to an enhanced comprehension of the interplay between NETs and GA, facilitating the proposition of innovative therapeutic strategies and novel approaches for the management of GA.

Proteomic analysis of sialoliths from calcified, lipid and mixed groups as a source of potential biomarkers of deposit formation in the salivary glands

Salivary stones, also known as sialoliths, are formed in a pathological situation in the salivary glands. So far, neither the mechanism of their formation nor the factors predisposing to their formation are known despite several hypotheses. While they do not directly threaten human life, they significantly deteriorate the patient's quality of life. Although this is not a typical research material, attempts are made to apply various analytical tools to characterise sialoliths and search for the biomarkers in their proteomes. In this work, we used mass spectrometry and SWATH-MS qualitative and quantitative analysis to investigate the composition and select proteins that may contribute to solid deposits in the salivary glands. Twenty sialoliths, previously characterized spectroscopically and divided into the following groups: calcified (CAL), lipid (LIP) and mixed (MIX), were used for the study. Proteins unique for each of the groups were found, including: for the CAL group among them, e.g. proteins from the S100 group (S100 A8/A12 and P), mucin 7 (MUC7), keratins (KRT1/2/4/5/13), elastase (ELANE) or stomatin (STOM); proteins for the LIP group-transthyretin (TTR), lactotransferrin (LTF), matrix Gla protein (MPG), submandibular gland androgen-regulated protein 3 (SMR3A); mixed stones had the fewest unique proteins. Bacterial proteins present in sialoliths have also been identified. The analysis of the results indicates the possible role of bacterial infections, disturbances in calcium metabolism and neutrophil extracellular traps (NETs) in the formation of sialoliths.

Novel perception of neutrophil extracellular traps in gouty inflammation

The self-limiting nature of the inflammatory flare is a feature of gout. The effects of neutrophil extracellular traps (NETs) on gout have remarkably attracted researchers' attention. Aggregated NETs promote the resolution of gouty inflammation by packing monosodium urate (MSU) crystals, degrading cytokines and chemokines, and blocking neutrophil recruitment and activation. Deficiency of NETs aggravates experimental gout. Thus, aggregated NETs are assumed to be a possible mechanism for the spontaneous resolution of gout. It is feasible to envisage therapeutic strategies for targeting NETosis (NET formation process) in gout. However, recent studies have demonstrated that levels of NETs are not associated with disease activity and inflammation in human gout. Moreover, the process of MSU crystal trapping is not affected in the absence of neutrophils. This review has concentrated on the mechanisms and associations between NETs and gout.

Protein kinase C isoforms mediate the formation of neutrophil extracellular traps

Neutrophils release extracellular traps (NETs) in response to numerous pathogenic microbes as the last suicidal resource (NETosis) in the fight against infection. Apart from the host defense function, NETs play an essential role in the pathogenesis of various autoimmune, inflammatory and malignant diseases. Therefore, understanding the molecular mechanisms of NETosis is important for regulating the aberrant or excessive NET release. Protein kinase C (PKC) is a serine/threonine kinase which is involved in various neutrophil functions, however, little is known about its implication in NETosis activated by various physiological and pharmacological stimuli. Since there are conventional, novel and atypical PKC isoforms (α, βI, βII, δ, and ζ) found in human neutrophils, we investigated their impact in NETosis, oxidative burst and spreading applying pharmacological approach. Using specific inhibitors of PKC isoforms, we showed that PKCβ, PKCδ, and PKCζ are involved in the oxidative burst, spreading and NETosis activated by calcium ionophore A23187, while only PKCβ is implicated in these functions activated by phorbol 12-myristate 13-acetate (PMA). The data obtained in our study might help in the development of new drugs useful for the treatment of autoimmune and inflammatory diseases associated with NETs.

Neutrophil extracellular traps induce the bone erosion of gout

OBJECTIVE

To investigate the relationships between monosodium urate (MSU) crystals -induced neutrophil extracellular traps (NETs) and bone erosion in gout.

METHODS

Animal models were used to study the relationship between NETs induced by MSU crystals and bone erosion. Neutrophils were treated with MSU crystals to induce NETs. The osteoblasts-like cells (OB) were then treated with NETs, and the supernatant was co-incubated with osteoclasts-like cells (OC). The NETs were digested with DNase, and the neutrophil elastase (NE) was inhibited with sivelestat sodium. Cell viability, mRNA, and protein expression were also assessed.

RESULTS

After treating OB with NETs, the cell viability decreased. Yet, after digesting the DNA and inhibiting NE, the viability was moderately improved. The expression level of osteoprotegerin (OPG) and alkaline phosphatase (ALP) was up-regulated, while the expression level of receptor activator of nuclear factor kappa-B ligand (RANKL) was down-regulated in the sivelestat sodium + MSU group compared with MSU group. The number of OC was significantly elevated. In contrast, the number of OB was not increased in the tibia after establishing the gout model. The supernatant obtained from OB was treated with NETs promoting OC differentiation. The expression level of receptor activator of nuclear factor kappa-B (RANK), tartrate-resistant acid phosphatase (TRAP), and cathepsin K (Cst K) was up-regulated in the MSU group compared with the normal control (NC) group.

CONCLUSION

NETs induced by MSU crystals could inhibit osteoblasts viability and enhance the activity of osteoclasts.

Neutrophil Extracellular Traps in Asthma: Friends or Foes?

Asthma is a chronic inflammatory disease characterized by variable airflow limitation and airway hyperresponsiveness. A plethora of immune and structural cells are involved in asthma pathogenesis. The roles of neutrophils and their mediators in different asthma phenotypes are largely unknown. Neutrophil extracellular traps (NETs) are net-like structures composed of DNA scaffolds, histones and granular proteins released by activated neutrophils. NETs were originally described as a process to entrap and kill a variety of microorganisms. NET formation can be achieved through a cell-death process, termed NETosis, or in association with the release of DNA from viable neutrophils. NETs can also promote the resolution of inflammation by degrading cytokines and chemokines. NETs have been implicated in the pathogenesis of various non-infectious conditions, including autoimmunity, cancer and even allergic disorders. Putative surrogate NET biomarkers (e.g., double-strand DNA (dsDNA), myeloperoxidase-DNA (MPO-DNA), and citrullinated histone H3 (CitH3)) have been found in different sites/fluids of patients with asthma. Targeting NETs has been proposed as a therapeutic strategy in several diseases. However, different NETs and NET components may have alternate, even opposite, consequences on inflammation. Here we review recent findings emphasizing the pathogenic and therapeutic potential of NETs in asthma.

Hyperuricemia and gout: effects on bone and articular cartilage (literature review)

Gout is a disease characterized by deposition of sodium monourate crystals in tissues which is the reason of inflammation among persons with hyperuricemia (HU). The prevalence of HU, which can be considered the first stage of gout formation, varies in different countries. Despite this, only a small number of persons with HU have been shown to develop symptoms of gout. Recent data suggest that HU is an independent risk factor for cartilage and bone damage. UA, both in the form of crystals and in a dissolved form, activates damage and potentiates cell death by releasing reactive oxygen species, activating the necroptosis pathway, neutrophil traps, synthesis of pro-inflammatory cytokines, and other pathogenetic mechanisms that cause the negative effects of HU and gout on articular cartilage and subchondral bone. The association of HU and osteoarthritis (OA) is well known and based on the common pathogenesis, but the direction of this relationship is still a debatable issue. The accumulated data suggest the need for a deeper study of the relationship of gout and asymptomatic HU with pathological processes leading to the development and progression of OA and disorders of bone metabolism.

Polystyrene nanoparticle exposure supports ROS-NLRP3 axis-dependent DNA-NET to promote liver inflammation

The widespread use of plastics and the rapid development of nanotechnology bring convenience to our lives while also increasing the environmental burden and increasing the risk of exposure of organisms to nanoparticles (NPs). While recent studies have revealed an association between nanoparticles and liver injury, the intrinsic mechanism of NP exposure-induced liver damage remains to be explored. Here, we found that polystyrene nanoparticle (PSNP) exposure resulted in a significant increase in local neutrophil infiltration and neutrophil extracellular trap (NET) formation in the liver. Analysis of a coculture system of PBNs and AML12 cells revealed that PSNP-induced NET formation positively correlates with the reactive oxygen species (ROS)-NLRP3 axis. Inhibition of ROS and genetic and pharmacological inhibition of NLRP3 in AML12 can both alleviate PSNP-induced NET formation. In turn, exposure of mice to deoxyribonuclease I (DNase Ⅰ)-coated PSNPs disassembled NET in vivo, neutrophil infiltration in the liver was reduced, the ROS-NLRP3 axis was inhibited, and the expression of cytokines was markedly decreased. Collectively, our work reveals a mechanism of NET formation in PSNP exposure-induced liver inflammation and highlights the possible role of DNase Ⅰ as a key enzyme in degrading NET and alleviating liver inflammation.

The Inhibition of Osteoblast Viability by Monosodium Urate Crystal-Stimulated Neutrophil-Derived Exosomes

Background and Objective

Bone erosion is common in patients with gout. The role of neutrophil-derived exosomes in gouty bone erosion remains elusive. This study aimed to investigate the functions of the neutrophil-derived exosomes in the development of bone erosion in gout.

Methods

Neutrophil-derived exosomes were collected and assessed by transmission electron microscopy and nanoparticle tracking analysis. Cell counting kit-8 assay was applied to evaluate cell viability, and cell apoptosis was assessed by flow cytometry. In addition, quantitative Real-time PCR and Western blotting were used to determine the expression levels of alkaline phosphatase (ALP), osteoprotegerin (OPG), and receptor activator of nuclear factor-κB ligand (RANKL). Neutrophil-derived exosomes were tagged with PKH67. The miRNA expression profiles of exosomes and human fetal osteoblasts (hFOB) were compared using high-throughput sequencing. Functional miRNAs transfected into hFOB after co-incubation with exosomes were selected and validated by preliminary qPCR.

Results

Neutrophil-derived exosomes were stimulated by monosodium urate (MSU). The exosomes could inhibit the viability of the hFOB, and the expression levels of ALP and OPG were down-regulated, while the expression level of RANKL was up-regulated. However, there was no significant difference in the viability of osteoclasts and the expression of nuclear factor of activated T cells 1. Exosomes were observed in the cytoplasm under a confocal microscopy, confirming that exosomes could be taken up by hFOB. In total, 2590 miRNAs were found, of which 47 miRNAs were differentially expressed. Among the delivered miRNAs, miR-1246 exhibited the highest level of differential expression. The viability of hFOB was reduced by miR-1246 mimics and increased by miR-1246 inhibitors. There was no significant difference in hFOB apoptosis rate between the miR-1246 mimic and miR-1246 inhibitor group. MiR-1246 overexpression decreased the expression levels of ALP and OPG, whereas increasing the expression level of RANKL. In contrast, miR-1246 inhibitor increased the expression levels of ALP and OPG, while decreasing the expression level of RANKL. Neutrophil-derived exosomes stimulated by MSU could increase the expression of miR-1246.

Conclusion

Neutrophil-derived exosomes stimulated by MSU could inhibit the viability of osteoblasts.

Centrality of Myeloid-Lineage Phagocytes in Particle-Triggered Inflammation and Autoimmunity

Exposure to exogenous particles found as airborne contaminants or endogenous particles that form by crystallization of certain nutrients can activate inflammatory pathways and potentially accelerate autoimmunity onset and progression in genetically predisposed individuals. The first line of innate immunological defense against particles are myeloid-lineage phagocytes, namely macrophages and neutrophils, which recognize/internalize the particles, release inflammatory mediators, undergo programmed/unprogrammed death, and recruit/activate other leukocytes to clear the particles and resolve inflammation. However, immunogenic cell death and release of damage-associated molecules, collectively referred to as "danger signals," coupled with failure to efficiently clear dead/dying cells, can elicit unresolved inflammation, accumulation of self-antigens, and adaptive leukocyte recruitment/activation. Collectively, these events can promote loss of immunological self-tolerance and onset/progression of autoimmunity. This review discusses critical molecular mechanisms by which exogenous particles (i.e., silica, asbestos, carbon nanotubes, titanium dioxide, aluminum-containing salts) and endogenous particles (i.e., monosodium urate, cholesterol crystals, calcium-containing salts) may promote unresolved inflammation and autoimmunity by inducing toxic responses in myeloid-lineage phagocytes with emphases on inflammasome activation and necrotic and programmed cell death pathways. A prototypical example is occupational exposure to respirable crystalline silica, which is etiologically linked to systemic lupus erythematosus (SLE) and other human autoimmune diseases. Importantly, airway instillation of SLE-prone mice with crystalline silica elicits severe pulmonary pathology involving accumulation of particle-laden alveolar macrophages, dying and dead cells, nuclear and cytoplasmic debris, and neutrophilic inflammation that drive cytokine, chemokine, and interferon-regulated gene expression. Silica-induced immunogenic cell death and danger signal release triggers accumulation of T and B cells, along with IgG-secreting plasma cells, indicative of ectopic lymphoid tissue neogenesis, and broad-spectrum autoantibody production in the lung. These events drive early autoimmunity onset and accelerate end-stage autoimmune glomerulonephritis. Intriguingly, dietary supplementation with ω-3 fatty acids have been demonstrated to be an intervention against silica-triggered murine autoimmunity. Taken together, further insight into how particles drive immunogenic cell death and danger signaling in myeloid-lineage phagocytes and how these responses are influenced by the genome will be essential for identification of novel interventions for preventing and treating inflammatory and autoimmune diseases associated with these agents.

Formation and Cellular Impact of Cholesterol Crystals in Health and Disease

Cholesterol crystals (CCs) were first discovered in atherosclerotic plaque tissue in the early 1900 and have since been observed and implicated in many diseases and conditions, including myocardial infarction, abdominal aortic aneurism, kidney disease, ocular diseases, and even central nervous system anomalies. Despite the widespread involvement of CCs in many pathologies, the mechanisms involved in their formation and their role in various diseases are still not fully understood. Current knowledge concerning the formation of CCs, as well as the molecular pathways activated upon cellular exposure to CCs, will be explored in this review. As CC formation is tightly associated with lipid metabolism, the role of cellular lipid homeostasis in the formation of CCs is highlighted, including the role of lysosomes. In addition, cellular pathways and processes known to be affected by CCs are described. In particular, CC-induced activation of the inflammasome and production of reactive oxygen species, along with the role of CCs in complement-mediated inflammation is discussed. Moreover, the clinical manifestation of embolized CCs is described with a focus on renal and skin diseases associated with CC embolism. Lastly, potential therapeutic measures that target either the formation of CCs or their impact on different cell types and tissues are highlighted.

The many roles of myeloperoxidase: From inflammation and immunity to biomarkers, drug metabolism and drug discovery

This review provides a practical guide to myeloperoxidase (MPO) and presents to the reader the diversity of its presence in biology. The review provides a historical background, from peroxidase activity to the discovery of MPO, to its role in disease and drug development. MPO is discussed in terms of its necessity, as specific individuals lack MPO expression. An underlying theme presented throughout brings up the question of the benefit and burden of MPO activity. Enzyme structure is discussed, including accurate masses and glycosylation sites. The catalytic cycle of MPO and its corresponding pathways are presented, with a discussion of the importance of the redox couples of the different states of MPO. Cell lines expressing MPO are discussed and practically summarized for the reader, and locations of MPO (primary and secondary) are provided. Useful methods of MPO detection are discussed, and how these can be used for studying disease processes are implied through the presentation of MPO as a biomarker. The presence of MPO in neutrophil extracellular traps is presented, and the activators of the former are provided. Lastly, the transition from drug metabolism to a target for drug development is where the review concludes.

The Immune System Throws Its Traps: Cells and Their Extracellular Traps in Disease and Protection

The first formal description of the microbicidal activity of extracellular traps (ETs) containing DNA occurred in neutrophils in 2004. Since then, ETs have been identified in different populations of cells involved in both innate and adaptive immune responses. Much of the knowledge has been obtained from in vitro or ex vivo studies; however, in vivo evaluations in experimental models and human biological materials have corroborated some of the results obtained. Two types of ETs have been described—suicidal and vital ETs, with or without the death of the producer cell. The studies showed that the same cell type may have more than one ETs formation mechanism and that different cells may have similar ETs formation mechanisms. ETs can act by controlling or promoting the mechanisms involved in the development and evolution of various infectious and non-infectious diseases, such as autoimmune, cardiovascular, thrombotic, and neoplastic diseases, among others. This review discusses the presence of ETs in neutrophils, macrophages, mast cells, eosinophils, basophils, plasmacytoid dendritic cells, and recent evidence of the presence of ETs in B lymphocytes, CD4+ T lymphocytes, and CD8+ T lymphocytes. Moreover, due to recently collected information, the effect of ETs on COVID-19 is also discussed.

A proposed role of neutrophil extracellular traps and their interplay with fibroblasts in ANCA-associated vasculitis lung fibrosis

ANCA-associated vasculitides (AAV) comprise three diseases: granulomatosis with polyangiitis, microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis. They are characterised by small vessel inflammation and have a broad range of clinical manifestations and multiorgan involvement which endanger the patient's life. An increasingly recognised complication of AAV, especially in MPA is lung fibrosis, for which no clearcut therapy in this context is available. The release of neutrophil extracellular traps (NETs) in these diseases has been related to the development of fibrosis, but the precise mechanisms are not fully unravelled. This review provides an overview of some of the important proteins known to compose NETs, and proposes some mechanisms by which these remarkable components may exert an impact on the different fibroblastic phenotypes leading to lung fibrosis.

NEUTROPHIL EXTRACELLULAR TRAPS RELEASE IN GOUT AND PSEUDOGOUT DEPENDS ON THE NUMBER OF CRYSTALS REGARDLESS OF LEUKOCYTE COUNT

OBJECTIVES

Microcrystal-induced arthritis is still an unresolved paradigm for medicine. Overt inflammation may be absent even when crystals occur in synovial fluid. Recently, the production of neutrophil extracellular traps (NETs) embedding monosodium urate crystals (MSU) has been proposed as a possible mechanism of the auto-resolution of the inflammatory phase during gout. We aimed to verify and quantify the release of NETs in synovial fluids during gout and pseudogout attacks and to compare any differences with respect to crystals and neutrophils number, and to analyze activation of necroptosis pathway in synovial fluid from crystal-induced arthritis.

METHODS

Synovial fluid samples were obtained by arthrocentesis from 22 patients presenting acute crystal-induced arthritis, gout or pseudogout (n = 11 each group), and from 10 patients with acute non-crystal arthritis as controls. NETosis was quantified in synovial fluid by nucleic acid stain and by quantification of human neutrophil elastase. Activation of p-MLKL was assessed by western blot.

RESULTS

We observed that synovial fluid neutrophils encountering MSU and CPPD crystals during episodes of gout and pseudogout release NETs in relation to the number of crystals in synovial fluid and irrespective of neutrophil density and type of crystal. This release was accompanied by necroptosis through the activation of the MLKL pathway.

CONCLUSIONS

Our findings suggest that a role of NETs in crystal-induced arthritis is to "trap extracellular particles", including microcrystals. Embedding crystals in aggregates of NETs may be the basis of tophi and CPPD deposition and may have implications for disease evolution, rather than for spontaneous resolution of the acute attack.

Studying Neutrophil Function in vitro: Cell Models and Environmental Factors

Neutrophils are the most abundant immune cell type in the blood and constitute the first line of defense against invading pathogens. Despite their important role in many diseases, they are challenging to study due to their short life span and the inability to cryopreserve or expand them in vitro. Thus, research into neutrophils has to rely on cells freshly isolated from peripheral blood of human donors, introducing donor-dependent variation in the experimental data. To counteract these problems, researchers tried to develop adequate cell models, such as cell lines. For those functional studies that cannot rely on cell models, a standardization of protocols regarding neutrophil purification and culturing could be a solution. In this review, we provide an overview of the most commonly used models for neutrophil function (HL-60, PLB-985, NB4, Kasumi-1 and induced pluripotent stem cells). In addition, we describe the effects of glucose concentration, pH, oxygen tension and temperature on neutrophil function.

Role of Neutrophils in Systemic Vasculitides

Neutrophils and neutrophil extracellular traps (NETs) contribute to the pathogenesis of many autoimmune diseases, including vasculitis. Though neutrophils, and NETs, can break self-tolerance by being a source of autoantigens for autoantibodies in anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, playing a key role in driving the autoimmune response, the role of neutrophils and NETs in large vessel vasculitis, including giant cell arteritis (GCA), is not well understood. In this review, we summarize the current insight into molecular mechanisms contributing to neutrophil-mediated pathology in small and medium vessel vasculitis, as well as provide potential translational perspectives on how neutrophils, and NETs, may partake in large vessel vasculitis, a rare disease entity of unclear pathogenesis.

NETosis: Molecular Mechanisms, Role in Physiology and Pathology

NETosis is a program for formation of neutrophil extracellular traps (NETs), which consist of modified chromatin decorated with bactericidal proteins from granules and cytoplasm. Various pathogens, antibodies and immune complexes, cytokines, microcrystals, and other physiological stimuli can cause NETosis. Induction of NETosis depends on reactive oxygen species (ROS), the main source of which is NADPH oxidase. Activation of NADPH oxidase depends on increase in the concentration of Ca²⁺ in the cytoplasm and in some cases on the generation of ROS in mitochondria. NETosis includes release of the granule components into the cytosol, modification of histones leading to chromatin decondensation, destruction of the nuclear envelope, as well as formation of pores in the plasma membrane. In this review, basic mechanisms of NETosis, as well as its role in the pathogenesis of some diseases including COVID-19 are discussed.

"NETs and EETs, a Whole Web of Mess"

Neutrophils and eosinophils are granulocytes that have very distinct functions. Neutrophils are first responders to external threats, and they use different mechanisms to control pathogens. Phagocytosis, reactive oxygen species, and neutrophil extracellular traps (NETs) are some of the mechanisms that neutrophils utilize to fight pathogens. Although there is some controversy as to whether NETs are in fact beneficial or detrimental to the host, it mainly depends on the biological context. NETs can contribute to disease pathogenesis in certain types of diseases, while they are also undeniably critical components of the innate immune response. On the contrary, the role of eosinophils during host immune responses remains to be better elucidated. Eosinophils play an important role during helminthic infections and allergic responses. Eosinophils can function as effector cells in viral respiratory infections, gut bacterial infections, and as modulators of immune responses by driving the balance between Th1 and Th2 responses. In particular, eosinophils have biological activities that appear to be quite similar to those of neutrophils. Both possess bactericidal activity, can activate proinflammatory responses, can modulate adaptive immune responses, can form extracellular traps, and can be beneficial or detrimental to the host according to the underlying pathology. In this review we compare these two cell types with a focus on highlighting their numerous similarities related to extracellular traps.

Neutrophil Extracellular Traps Promote the Development and Growth of Human Salivary Stones

Salivary gland stones, or sialoliths, are the most common cause of the obstruction of salivary glands. The mechanism behind the formation of sialoliths has been elusive. Symptomatic sialolithiasis has a prevalence of 0.45% in the general population, is characterized by recurrent painful periprandial swelling of the affected gland, and often results in sialadenitis with the need for surgical intervention. Here, we show by the use of immunohistochemistry, immunofluorescence, computed tomography (CT) scans and reconstructions, special dye techniques, bacterial genotyping, and enzyme activity analyses that neutrophil extracellular traps (NETs) initiate the formation and growth of sialoliths in humans. The deposition of neutrophil granulocyte extracellular DNA around small crystals results in the dense aggregation of the latter, and the subsequent mineralization creates alternating layers of dense mineral, which are predominantly calcium salt deposits and DNA. The further agglomeration and appositional growth of these structures promotes the development of macroscopic sialoliths that finally occlude the efferent ducts of the salivary glands, causing clinical symptoms and salivary gland dysfunction. These findings provide an entirely novel insight into the mechanism of sialolithogenesis, in which an immune system-mediated response essentially participates in the physicochemical process of concrement formation and growth.

Alum triggers infiltration of human neutrophils ex vivo and causes lysosomal destabilization and mitochondrial membrane potential-dependent NET-formation

Aluminium salts have been used in vaccines for decades. However, the mechanisms underlying their adjuvant effect are still unclear. Neutrophils, the first immune cells at the injection site, can release cellular DNA together with granular material, so‐called neutrophil extracellular traps (NETs). In mice, NETs apparently play a role in aluminium hydroxide (alum)‐adjuvant immune response to vaccines. Although no experimental data exist, this effect is assumed to be operative also in humans. As a first step to verify this knowledge in humans, we demonstrate that the injection of alum particles into human skin biopsies ex vivo leads to similar tissue infiltration of neutrophils and NET‐formation. Moreover, we characterized the mechanism leading to alum‐induced NET‐release in human neutrophils as rapid, NADPH oxidase‐independent process involving charge, phagocytosis, phagolysosomal rupture, Ca²⁺‐flux, hyperpolarization of the mitochondrial membrane, and mitochondrial ROS. Extracellular flow and inhibition experiments suggested that no additional energy from oxidative phosphorylation or glycolysis is required for NET‐release. This study suggests a so far unappreciated role for neutrophils in the initial phase of immune responses to alum‐containing vaccines in humans and provides novel insights into bioenergetic requirements of NET‐formation.

In Vivo Transmigrated Human Neutrophils Are Highly Primed for Intracellular Radical Production Induced by Monosodium Urate Crystals

Gout is an inflammatory disease caused by monosodium urate (MSU) crystals. The role of neutrophils in gout is less clear, although several studies have shown neutrophil extracellular trap (NET) formation in acutely inflamed joints of gout patients. MSU crystals are known to induce the production of reactive oxygen species (ROS) and NET formation in neutrophils isolated from blood, but there is inconclusive knowledge on the localization of ROS production as well as whether the ROS are required for NET formation. In this report we demonstrate that MSU crystals activate human neutrophils to produce ROS exclusively in intracellular compartments. Additionally, in vivo transmigrated neutrophils derived from experimental skin chambers displayed markedly increased ROS production as compared to resting blood neutrophils. We also confirmed that MSU stimulation potently induced NET formation, but this response was not primed in in vivo transmigrated neutrophils. In line with this we found that MSU-triggered NET formation was independent of ROS production and proceeded normally in neutrophils from patients with dysfunctional respiratory burst (chronic granulomatous disease (CGD) and complete myeloperoxidase (MPO) deficiency). Our data indicate that in vivo transmigrated neutrophils are markedly primed for oxidative responses to MSU crystals and that MSU triggered NET formation is independent of ROS production.

Mitochondrial permeability transition pore is involved in oxidative burst and NETosis of human neutrophils

Neutrophils release neutrophil extracellular traps (NETs) in response to numerous pathogenic microbes as the last suicidal resource (NETosis) in the fight against infection. Apart from the host defense function, NETs play an essential role in the pathogenesis of various autoimmune and inflammatory diseases. Therefore, understanding the molecular mechanisms of NETosis is important for regulating aberrant NET release. The initiation of NETosis after the recognition of pathogens by specific receptors is mediated by an increase in intracellular Ca²⁺ concentration, therefore, the use of Ca²⁺ ionophore A23187 can be considered a semi-physiological model of NETosis. Induction of NETosis by various stimuli depends on reactive oxygen species (ROS) produced by NADPH oxidase, however, NETosis induced by Ca²⁺ ionophores was suggested to be mediated by ROS produced in mitochondria (mtROS). Using the mitochondria-targeted antioxidant SkQ1 and specific inhibitors of NADPH oxidase, we showed that both sources of ROS, mitochondria and NADPH oxidase, are involved in NETosis induced by A23187 in human neutrophils. In support of the critical role of mtROS, SkQ1-sensitive NETosis was demonstrated to be induced by A23187 in neutrophils from patients with chronic granulomatous disease (CGD). We assume that Ca²⁺-triggered mtROS production contributes to NETosis either directly (CGD neutrophils) or by stimulating NADPH oxidase. The opening of the mitochondrial permeability transition pore (mPTP) in neutrophils treated by A23187 was revealed using the electron transmission microscopy as a swelling of the mitochondrial matrix. Using specific inhibitors, we demonstrated that the mPTP is involved in mtROS production, NETosis, and the oxidative burst induced by A23187.

Caspase-11 Mediates Neutrophil Chemotaxis and Extracellular Trap Formation During Acute Gouty Arthritis Through Alteration of Cofilin Phosphorylation

Gout is characterized by attacks of arthritis with hyperuricemia and monosodium urate (MSU) crystal-induced inflammation within joints. Innate immune responses are the primary drivers for tissue destruction and inflammation in gout. MSU crystals engage the Nlrp3 inflammasome, leading to the activation of caspase-1 and production of IL-1β and IL-18 within gout-affected joints, promoting the influx of neutrophils and monocytes. Here, we show that caspase-11^−/− mice and their derived macrophages produce significantly reduced levels of gout-specific cytokines including IL-1β, TNFα, IL-6, and KC, while others like IFNγ and IL-12p70 are not altered. IL-1β induces the expression of caspase-11 in an IL-1 receptor-dependent manner in macrophages contributing to the priming of macrophages during sterile inflammation. The absence of caspase-11 reduced the ability of macrophages and neutrophils to migrate in response to exogenously injected KC in vivo. Notably, in vitro, caspase-11^−/− neutrophils displayed random migration in response to a KC gradient when compared to their WT counterparts. This phenotype was associated with altered cofilin phosphorylation. Unlike their wild-type counterparts, caspase-11^−/− neutrophils also failed to produce neutrophil extracellular traps (NETs) when treated with MSU. Together, this is the first report demonstrating that caspase-11 promotes neutrophil directional trafficking and function in an acute model of gout. Caspase-11 also governs the production of inflammasome-dependent and -independent cytokines from macrophages. Our results offer new, previously unrecognized functions for caspase-11 in macrophages and neutrophils that may apply to other neutrophil-mediated disease conditions besides gout.

The role of neutrophil extracellular traps in rheumatic diseases

Rheumatic diseases are a collection of disorders defined by the presence of inflammation and destruction of joints and internal organs. A common feature of these diseases is the presence of autoantibodies targeting molecules commonly expressed in neutrophils. These preformed mediators are released by neutrophils but not by other immune cells such as macrophages. Neutrophils, major players in the host innate immune response, initiate a cell death mechanism termed neutrophil extracellular trap (NET) formation as a way to ensnare pathogens. NETs are also a source of released self-molecules found in rheumatic diseases. Subsequently, research on the role of NETs in the onset, progression and resolution of inflammation in rheumatic diseases has intensified. This Review has two aims. First, it aims to highlight the mechanisms required for the generation of NETs, the research landscape of which is rapidly changing. Second, it aims to discuss the role of neutrophils and NETs in systemic lupus erythematosus, vasculitis (specifically anti-neutrophil cytoplasmic autoantibody-associated vasculitis), rheumatoid arthritis and gout. Our goal is to clarify the field of NET research in rheumatic diseases in the hope of improving the therapeutic approaches utilized for these diseases.

Pro-inflammatory mediators and neutrophils are increased in synovial fluid from heifers with acute ruminal acidosis

BACKGROUND

Acute ruminal acidosis (ARA) is a metabolic disease of cattle characterized by an aseptic synovitis. ARA is the result of an increased intake of highly fermentable carbohydrates that frequently occurs in dairy cattle subjected to high production requirements. In human joint diseases such as rheumatoid arthritis and gout, several pro-inflammatory molecules are increased in the synovial fluid, including cytokines, prostaglandin E₂ (PGE₂), metalloproteinases, and neutrophil extracellular traps (NETs). The aim of this study was to identify the presence of proinflammatory mediators and neutrophils in the synovial fluid of heifers with ARA, induced by an oligofructose overload. Five heifers were challenged with an oligofructose overload (13 g/kg BW) dissolved in water. As a control, a similar vehicle volume was used in four heifers. Synovial fluid samples were collected from the tarso-crural joint and PGE₂, IL-6, IL-1β, ATP, lactate dehydrogenase (LDH), albumin, glucose, matrix metalloproteinase-9 (MMP-9), cellular free DNA, NETs, and serpin B1 were analyzed at 0, 9, and 24 h post treatment.

RESULTS

At 9 h post oligofructose overload, an increase of IL-1β, IL-6, PGE₂, serpin B1 and LDH was detected in the joints when compared to the control group. At 24 h, the synovial fluid was yellowish, viscous, turbid, and contained abundant neutrophils. An increase of DNA-backbone-like traps, histone 3 (H₃cit), aggregated neutrophil extracellular traps (aggNETs), and serpin B1 were observed 24 h post treatment. Furthermore, albumins, LDH, ATP, MMP-9, IL-6, and IL-1β were increased after 24 h.

CONCLUSIONS

The overall results indicate that IL-1β, IL-6 and PGE₂, were the earliest proinflammatory parameters that increased in the synovial fluid of animals with ARA. Furthermore, the most sever inflammatory response in the joint was observed after 24 h and could be associated with a massive presence of neutrophils and release of aggNETs.

Extracellular DNA traps in inflammation, injury and healing

Following strong activation signals, several types of immune cells reportedly release chromatin and granular proteins into the extracellular space, forming DNA traps. This process is especially prominent in neutrophils but also occurs in other innate immune cells such as macrophages, eosinophils, basophils and mast cells. Initial reports demonstrated that extracellular traps belong to the bactericidal and anti-fungal armamentarium of leukocytes, but subsequent studies also linked trap formation to a variety of human diseases. These pathological roles of extracellular DNA traps are now the focus of intensive biomedical research. The type of pathology associated with the release of extracellular DNA traps is mainly determined by the site of trap formation and the way in which these traps are further processed. Targeting the formation of aberrant extracellular DNA traps or promoting their efficient clearance are attractive goals for future therapeutic interventions, but the manifold actions of extracellular DNA traps complicate these approaches.

Neutrophil Extracellular Traps

Neutrophil extracellular traps (NETs) are made of a network of extracellular strings of DNA that bind pathogenic microbes. Histones and several neutrophil granule proteins associated with the DNA framework damage entrapped microorganisms. Reactive oxygen species generated by the neutrophil NADPH oxidase have been shown to be essential to mediate NET release by several stimuli including numerous pathogenic bacteria. Although several methods have been used in the literature to detect NETs in vitro and in vivo, a consensus is urgently needed on the field to standardize the best NET-specific assays. In this chapter, two methods are described in details that can be used to detect NETs and to distinguish them from other mechanisms of neutrophil cell death. While NET-specific, these assays are also relatively simple and straightforward enabling their potential use by a wide audience.

Two-in-one: UV radiation simultaneously induces apoptosis and NETosis

NETosis is a unique form of neutrophil death that differs from apoptosis and necrosis. However, whether NETosis and apoptosis can occur simultaneously in the same neutrophil is unknown. In this paper, we show that increasing doses of ultraviolet (UV) irradiation increases NETosis, which is confirmed by myeloperoxidase colocalisation to neutrophil extracellular DNA. Increasing UV irradiation increases caspase 3 activation, mitochondrial reactive oxygen species (ROS) generation and p38, but not ERK, phosphorylation. Inhibition of mitochondrial ROS production and p38 activation, but not NADPH oxidase (NOX) activity, suppresses UV-induced NETosis, indicating that UV induces NOX-independent NETosis. Like classical NOX-dependent and -independent NETosis, UV-induced NETosis requires transcriptional firing for chromatin decondensation. Cell death-specific inhibitor studies indicate that UV-mediated NETosis is not apoptosis, necrosis or necroptosis. Collectively, these studies indicate that increasing doses of UV irradiation induce both apoptosis and NETosis simultaneously, but the ultimate outcome is the induction of a novel form of NOX-independent NETosis, or “ApoNETosis”.

Particles of different sizes and shapes induce neutrophil necroptosis followed by the release of neutrophil extracellular trap-like chromatin

The human body is exposed to a wide range of particles of industrial, environmental or internal origin such as asbestos, alum, silica or crystals of urate, calcium phosphate, calcium oxalate, cystine or cholesterol. Phagocytic clearance of such particles involves neutrophils and macrophages. Here we report that neutrophils encountering such particles of diverse sizes and shapes undergo necrotic cell death, a process associated with the formation of neutrophil extracellular trap (NET)-like extracellular DNA. In human neutrophils receptor-interacting protein kinase (RIPK)-1 inhibition with necrostatin-1s or mixed lineage kinase domain-like (MLKL) inhibition with necrosulfonamide abrogated cell death and associated-neutrophil extracellular DNA release induced by all of the aforementioned particles. Similar results were obtained with Mlkl-deficient mice neutrophils for all particles in vitro. Furthermore, Mlkl-deficient mice lacked tophus formation upon injection of MSU crystals into subcutaneous air pouches. These findings imply that nano- or microparticle-induced neutrophil extracellular DNA release is the consequence of neutrophil necroptosis, a regulated form of cell necrosis defined by RIPK1-RIPK3-MLKL signaling. Interestingly, this finding was consistent across different particle sizes and shapes. The RIPK1-RIPK3-MLKL signaling pathway may represent a potential therapeutic target in nano- or microparticle-related diseases (crystallopathies).

Artificial Polysialic Acid Chains as Sialidase-Resistant Molecular-Anchors to Accumulate Particles on Neutrophil Extracellular Traps

Neutrophils are involved in numerous immunological events. One mechanism of neutrophils to combat pathogens is the formation of neutrophil extracellular traps (NETs). Thereby, neutrophils use DNA fibers to form a meshwork of DNA and histones as well as several antimicrobial components to trap and kill invaders. However, the formation of NETs can lead to pathological conditions triggering among other things (e.g., sepsis or acute lung failure), which is mainly a consequence of the cytotoxic characteristics of accumulated extracellular histones. Interestingly, the carbohydrate polysialic acid represents a naturally occurring antagonist of the cytotoxic properties of extracellular histones. Inspired by polysialylated vesicles, we developed polysialylated nanoparticles. Since sialidases are frequently present in areas of NET formation, we protected the sensitive non-reducing end of these homopolymers. To this end, the terminal sialic acid residue of the non-reducing end was oxidized and directly coupled to nanoparticles. The covalently linked sialidase-resistant polysialic acid chains are still able to neutralize histone-mediated cytotoxicity and to initiate binding of these polysialylated particles to NET filaments. Furthermore, polysialylated fluorescent microspheres can be used as a bioanalytical tool to stain NET fibers. Thus, polySia chains might not only be a useful agent to reduce histone-mediated cytotoxicity but also an anchor to accumulate nanoparticles loaded with active substances in areas of NET formation.