Vital NETosis Research Articles

Highly Motile Neutrophil Cytoplast ameliorate Bacterial Pneumonia

Abstract Polymorphonuclear neutrophils (PMN) function as innate immune cell through rapid mobilization at site of infection and prompt phagocytosis and bactericidal activity. We addressed in vivo in mouse lungs the function of cytoplasts (PMNcyt), the PMN generated following release of extracellular traps (NETs). We found ~50% of PMN lost their nucleus post-NETosis following pneumonia induced by Pseudomonas aeruginosa (PA), and become PMNcyt. Interestingly, , PMNcyt in lung microvessels showed >2 greater velocity and > 3 greater deformability as compared to PMN. PMNcyt were also 9x more efficient in killing airway PA as compared to PMN. Using the protein arginine deiminase 4 (PAD4) inhibitor (Cl-amidine) to prevent NET formation in mice, we prevented the generation of PMNcyt demonstrating that PMNcyt were derived post-vital NETosis. We also showed PMNcyt retained the ability to phagocytose PA. Moreover, the inhibition of PMNcyt generation by PAD4 blocker induced severe lung injury in the pneumonia model, indicating an important anti-bacterial protective role of PMNcyt. Adoptive transfer of PMNcyt in PA infected mice also significantly reduced bacterial load and prevented lung tissue injury in mice. The results suggest a crucial host-defense function of PMNcyt generated post vital NETosis and their usefulness as cell-based therapy in inflammatory diseases such as pneumonia.

Extracellular neutrophil traps in cardiovascular diseases: problems and prospects of research

One of the links in the pathogenesis of cardiovascular diseases is chronic low-intensity systemic inflammation. In 2004, a previously unknown process was discovered — the formation of extracellular neutrophil traps (NETs) — NETosis (n eutrophil extracellular traps). NETs play a role in antimicrobial immunity though in certain cases they become a factor in the development of pathology. This review presents data on the effect of extracellular neutrophil traps on individual pathologies of the cardiovascular system (atherosclerosis, atrial fibrillation, thrombosis). The authors describe the mechanisms of NET formation (vital NETosis, suicidal NETosis) and their role in thrombus formation (as a framework for thrombus formation, initiation of coagulation), in the development of endothelial dysfunction, and electrical heterogeneity of the atrial myocardium. Data are presented indicating the connection between atherosclerosis, thrombosis, and atrial fibrillation with the activity of NETosis. Most studies demonstrate existing correlations on laboratory models, while the determination of NETs in patients with cardiovascular pathology in real clinical practice is almost absent. At the same time, understanding the processes associated with NETosis can help to identify specific markers and further strategies for the therapy of cardiovascular diseases.

Composition and Function of Neutrophil Extracellular Traps.

Neutrophil extracellular traps (NETs) are intricate fibrous structures released by neutrophils in response to specific stimuli. These structures are composed of depolymerized chromatin adorned with histones, granule proteins, and cytosolic proteins. NETs are formed via two distinct pathways known as suicidal NETosis, which involves NADPH oxidase (NOX), and vital NETosis, which is independent of NOX. Certain proteins found within NETs exhibit strong cytotoxic effects against both pathogens and nearby host cells. While NETs play a defensive role against pathogens, they can also contribute to tissue damage and worsen inflammation. Despite extensive research on the pathophysiological role of NETs, less attention has been paid to their components, which form a unique structure containing various proteins that have significant implications in a wide range of diseases. This review aims to elucidate the components of NETs and provide an overview of their impact on host defense against invasive pathogens, autoimmune diseases, and cancer.

Patients who develop insulin resistance 4-months post-COVID-19 exhibited higher NETosis and miR-21-5p expression than COVID-19 patients without metabolic alterations.

Abstract Post COVID-19 has been associated with alterations in the glucidic metabolism and the presence of metabolic-related microRNAs (miRNAs). Since neutrophils response depends on glycolysis, it is unclear whether the formation of neutrophil extracellular traps (NETosis) is also affected in post COVID-19 patients that developed insulin resistance (IR) post infection. In addition, miR-21-5p has been described as a direct gene regulator of the innate immune response against COVID-19 and the presence of IR. In this study, we evaluated NETosis at 4-months post-COVID-19 and its association with the development of IR and the expression of circulating miR-21-5p. Sixty post-COVID-19 patients were recruited from Víctor-Rios-Ruiz Hospital, and post COVID-19 was evaluated 4-months after acute phase. Serum samples were used to determine biochemical parameters. NETosis was measured in patients at basal condition and in response to TLR agonists using flow cytometry. miR-21-5p expression was measured with RT-qPCR. From our cohort, 19 patients were excluded because of they had IR or type 2 diabetes mellitus before COVID-19, 25 patients developed new IR and 16 remained without glucidic alteration. Our analysis showed that COVID-19 patients with IR-Post-COVID-19 had a significant increment of NETosis in basal condition, reducing the capacity to respond efficiently to TRL3/7/8 agonists. We also found a significant increment of miR-21-5p expression in IR-Post-COVID-19 patients and a significant and positive correlation between miR-21-5p and vital NETosis. These results demonstrated that patients who developed IR post-COVID-19 exhibited higher basal NETosis and higher miR-21-5p expression in comparison with patients without alterations. Supported by grants from PROYECTO COVID1005, ACT210085, Fondecyt Regular 1211480, Instituto de Salud Carlos III (COV20/00110, CP20/00041), Programa de donaciones "estar preparados" UNESPA (Madrid, Spain), Fundación Francisco Soria Melguizo (Madrid, Spain).

Neutrophil Extracellular Traps and Cancer: Trapping Our Attention with Their Involvement in Ovarian Cancer.

Neutrophils, the most abundant circulating leukocytes, play a well-known role in defense against pathogens through phagocytosis and degranulation. However, a new mechanism involving the release of neutrophil extracellular traps (NETs) composed of DNA, histones, calprotectin, myeloperoxidase, and elastase, among others, has been described. The so-called NETosis process can occur through three different mechanisms: suicidal, vital, and mitochondrial NETosis. Apart from their role in immune defense, neutrophils and NETs have been involved in physiopathological conditions, highlighting immunothrombosis and cancer. Notably, neutrophils can either promote or inhibit tumor growth in the tumor microenvironment depending on cytokine signaling and epigenetic modifications. Several neutrophils' pro-tumor strategies involving NETs have been documented, including pre-metastatic niche formation, increased survival, inhibition of the immune response, and resistance to oncologic therapies. In this review, we focus on ovarian cancer (OC), which remains the second most incidental but the most lethal gynecologic malignancy, partly due to the presence of metastasis, often omental, at diagnosis and the resistance to treatment. We deepen the state-of-the-art on the participation of NETs in OC metastasis establishment and progression and their involvement in resistance to chemo-, immuno-, and radiotherapies. Finally, we review the current literature on NETs in OC as diagnostic and/or prognostic markers, and their contribution to disease progression at early and advanced stages. The panoramic view provided in this article might pave the way for enhanced diagnostic and therapeutic strategies to improve the prognosis of cancer patients and, specifically, OC patients.

Vital NETosis vs. suicidal NETosis during normal pregnancy and preeclampsia.

Background: NETosis occurs in the context of infection or inflammation and results in the expulsion of decondensed DNA filaments called NETs (Neutrophil Extracellular Traps) into the extracellular environment. NETosis activates coagulation and contributes to the thrombotic risk of inflammatory diseases. To date, two mechanisms of NETosis have been identified: suicidal NETosis, in which neutrophils die after expelling the filaments; and vital NETosis, in which expulsion appears without altering the membrane. Human pregnancy is associated with a mild pro-inflammatory state, which is increased in the event of complications such as preeclampsia (PE). NETosis has been observed in these situations, but the mechanism of its production has not yet been studied. The aim of our study was to evaluate the balance of vital vs. suicidal NETosis in normal pregnancy and in PE. Patients/Methods: Neutrophils from healthy volunteers were stimulated with plasma from normal pregnancies (n = 13) and from women developing preeclampsia (n = 13). Immunofluorescent labelling was performed to determine the percentages and origin of NETs in both groups. Inhibition with suicidal or vital NETosis inhibitors was also performed to validate our results. Results: We found a significant increase in NETs in women with PE compared to women with normal pregnancies. We showed that vital and non-vital NETosis are present in normal and preeclamptic pregnancies. We demonstrated that the higher proportion of NETs observed in PE was due to non-vital NETosis whose main component is represented by suicidal NETosis. Discussion: These results suggest the important part of non-vital NETosis in the pathophysiology of PE.

Extracellular vesicles from A23187-treated neutrophils cause cGAS-STING-dependent IL-6 production by macrophages.

In response to several types of bacteria, as well as pharmacological agents, neutrophils produce extracellular vesicles (EVs) and release DNA in the form of neutrophil extracellular traps (NETs). However, it is unknown whether these two neutrophil products cooperate to modulate inflammation. Consistent with vital NETosis, neutrophils challenged with S. aureus, as well as those treated with A23187, released significantly more DNA relative to untreated or fMLF-treated neutrophils, with no lysis occurring for any condition. To test the hypothesis that EVs generated during NETosis caused macrophage inflammation, we isolated and characterized EVs from A23187-treated neutrophils (A23187-EVs). A23187-EVs associated with neutrophil granule proteins, histone H3, transcription factor A, mitochondrial (TFAM), and nuclear and mitochondrial DNA (mtDNA). We showed that DNA from A23187-EVs, when transfected into macrophages, led to production of IL-6 and IFN-α2, and this response was blunted by pre-treatment with the STING inhibitor H151. Next, we confirmed that A23187-EVs were engulfed by macrophages, and showed that they induced cGAS-STING-dependent IL-6 production. In contrast, neither EVs from untreated or fMLF-treated cells exhibited pro-inflammatory activity. Although detergent-mediated lysis of A23187-EVs diminished IL-6 production, removal of surface-associated DNA with DNase I treatment had no effect, and A23187-EVs did not induce IFN-α2 production. Given these unexpected results, we investigated whether macrophage mtDNA activated the cGAS-STING signaling axis. Consistent with mitochondrial outer membrane permeabilization (MOMP), a defined mechanism of mtDNA release, we observed macrophage mitochondrial membrane depolarization, a decrease in cytosolic Bax, and a decrease in mitochondrial cytochrome c, suggesting that macrophage mtDNA may initiate this EV-dependent signaling cascade. All together, these data demonstrate that A23187-EVs behave differently than transfected NET- or EV-DNA, and that neutrophil-derived EVs could be used as a model to study NF-κB-dependent STING activation.

Mitochondria in human neutrophils mediate killing of Staphylococcus aureus

BackgroundNeutrophils play a role in innate immunity and are critical for clearance of Staphylococcus aureus. Current understanding of neutrophil bactericidal effects is that NADPH oxidase produces reactive oxygen species (ROS), mediating bacterial killing. Neutrophils also contain numerous mitochondria; since these organelles lack oxidative metabolism, their function is unclear. We hypothesize that mitochondria in human neutrophils contribute to the bactericidal capacity of S. aureus. Methodsand Findings: Using human neutrophils isolated from healthy volunteers (n = 13; 7 females, 6 males), we show that mitochondria are critical in the immune response to S. aureus. Using live-cell and fixed confocal, and transmission electron microscopy, we show mitochondrial tagging of bacteria prior to ingestion and surrounding of phagocytosed bacteria immediately upon engulfment. Further, we demonstrate that mitochondria are ejected from intact neutrophils and engage bacteria during vital NETosis. Inhibition of the mitochondrial electron transport chain at Complex III, but not Complex I, attenuates S. aureus killing by 50 ± 7%, comparable to the NADPH oxidase inhibitor apocynin. Similarly, mitochondrial ROS scavenging using MitoTEMPO attenuates bacterial killing 112 ± 60% versus vehicle control. Antimycin A treatment also reduces mitochondrial ROS production by 50 ± 12% and NETosis by 53 ± 5%. ConclusionsWe identify a previously unrecognized role for mitochondria in human neutrophils in the killing of S. aureus. Inhibition of electron transport chain Complex III significantly impairs antimicrobial activity. This is the first demonstration that vital NETosis, an early event in the antimicrobial response, occurring within 5 min of bacterial exposure, depends on the function of mitochondrial Complex III. Mitochondria join NADPH oxidase as bactericidal ROS generators that mediate the bactericidal activities of human neutrophils.

Effects of platelets on extracellular traps of neutrophils in patients with systemic lupus erythematosus

Platelets are central participants in hemostasis, and also contribute to the host inflammatory and immune responses. Platelets are known to have a direct effect on the formation of neutrophil extracellular traps. Moreover, the patients with systemic lupus erythematosus exhibit multidirectional disturbances in the functional activity of platelets and neutrophils. Changes in inflammatory and thrombotic events can be considered predictors for adverse clinical course in systemic pathology. The aim of present study was to evaluate the possible role of platelets in maintaining increased netosis in patients with systemic lupus erythematosus. Blood platelets and white blood cells from 29 patients with systemic lupus erythematosus (SLE) were subject to the study. We have registered the in vitro effects of platelets upon formation of extracellular traps by autologous neutrophils under the conditions of co-cultivation for 30 minutes (vital NETosis) and 150 minutes (suicidal NETosis), as well as the relationships between the platelet counts, their activity and the number of NETs observed. It was found that the severity and direction of the platelets effect upon NETosis in vitro cultures depends on the degree of activity of disease: in the 1st degree of SLE, the effect of platelets did not differ from healthy individuals, i.e., intact platelets suppress NETosis (p = 0.002), whereas ADP-induced patelets did not exert any effect); at the 2nd degree of activity, both intact and activated platelets increase NETotic activity (p = 0.03 and p = 0.04 for intact and activated platelets, respectively). In the patients with 3rd degree of the disease activity, platelets did not affect formation of NETs. Hyperactivation of platelets was detected in SLE patients, mostly pronounced in the cases with 2nd degree of activity. However, we have not revealed any significant relationships between the count of platelets, their functional activity (according to results of ADP-test aggregation), and the indexes of NETosis. At the same time, the counts of neutrophil extracellular traps in bloodstream depended on the concentration of C-reactive protein (r = 0.58; p = 0.02), the titer of autoantibodies (anti-SS-A and anti-SS-B) (r = 0.66; p = 0.04 and r = 0.76; p = 0.02, respectively), rheumatoid factor (r = 0.73; p = 0.007) and circulating immune complexes (r = 0.68; p = 0.02). The obtained results indicate that the platelet/neutrophil interactions are not the leading cause for increased NETs numbers in SLE, compared to significantly higher effects of soluble autoagressive factors.

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

BackgroundNeutrophils are the first line defense cells of the innate immunity. As a final defense, they discharge their de-condensed chromatin/DNA fibers, the NETs (Neutrophil Extracellular Traps), by a process called NETosis. Two types of NETosis have been currently described: the suicidal/delayed/classical-type, which is ROS dependent that results in the ejection of nuclear DNA, and the vital/rapid/early-type, which may or may not require ROS but, eject nuclear/mitochondrial DNA or both. Thus, Echis carinatus and Naja naja venoms are comparatively studied for their NET inducing property. MethodsFormation of NETs, cell viability, ROS, and Ca2+ levels are estimated. An in vivo toxicity study and possible cellular signaling have been addressed using immunoblots and pharmacological inhibitors. ResultsE. carinatus and N. naja venoms respectively induce suicidal and vital NETosis. E. carinatus venom induces NETosis by activating NOX and PAD-4 enzymes in a ROS dependent manner via PKC/ERK/JNK signaling axis, while N. naja venom does it by activating PAD-4 enzyme, but independent of ROS requirement and as well as PKC/ERK/JNK activation. ConclusionFor the first time our study demonstrates the distinct action of E. carinatus and N. naja venoms on the process of NETosis. NETosis being a newly explored area in snake venom pharmacodynamics, it is important to study its impact on the various pathophysiological properties induced by snake venoms. SignificanceUnderstanding the varied actions of snake venoms on neutrophils/blood cells and the role of DNase are likely to provide insights for better management of snakebite pathophysiology.

NETosis in cancer: a critical analysis of the impact of cancer on neutrophil extracellular trap (NET) release in lung cancer patients vs. mice.

Neutrophils play a major role in tumor biology. Among other functions, neutrophils can release extracellular traps (NETs), mesh-like structures of decondensed chromatin fibers, in a process termed NETosis. Originally characterized as an antimicrobial mechanism, NETosis has been described in cancer, but cancer-related predisposition is not clear. In the current study, we investigatedthe predisposition of circulating neutrophils to release NETs in lung cancer and the impact of G-CSF on this function, comparing circulating neutrophils isolated from cancer patients to the LLC and AB12 mouse models. We find that neutrophils from both healthy donors and cancer patients display high NETotic potential, with 30-60% of cells undergoing NETosis upon PMA stimulation. In contrast, neutrophils isolated from tumor-bearing mice displayed only 4-5% NETotic cells, though significantly higher than naive controls (1-2%). Despite differential mechanisms of activation described, Ionomycin and PMA mainly triggered suicidal rather than vital NETosis. G-CSF secreting tumors did not increase NETotic rates in murine neutrophils, and direct G-CSF stimulation did not promote their NET release. In contrast, human neutrophils strongly responded to G-CSF stimulation resulting also in a higher response to PMA + G-CSF stimulation. Our data show clear differences in NETotic potentials between human and murine neutrophils. We do not find a predisposition of neutrophils to release NETs in lung cancer patients compared to healthy controls, whereas cancer may modulate neutrophils' NETotic potential in mice. G-CSF secreted from tumors differentially affects murine and human NETosis in cancer. These important differences should be considered in future studies of NETosis in cancer.

Besnoitia besnoiti bradyzoite stages induce suicidal- and rapid vital-NETosis.

Besnoitia besnoiti is an obligate intracellular apicomplexan protozoan parasite, which causes bovine besnoitiosis. Recently increased emergence within Europe was responsible for significant economic losses in the cattle industry due to the significant reduction of productivity. However, still limited knowledge exists on interactions between B. besnoiti and host innate immune system. Here, B. besnoiti bradyzoites were successfully isolated from tissue cysts located in skin biopsies of a naturally infected animal, and we aimed to investigate for the first time reactions of polymorphonuclear neutrophils (PMN) exposed to these vital bradyzoites. Freshly isolated bovine PMN were confronted to B. besnoiti bradyzoites. Scanning electron microscopy (s.e.m.)- and immunofluorescence microscopy-analyses demonstrated fine extracellular networks released by exposed bovine PMN resembling suicidal NETosis. Classical NETosis components were confirmed via co-localization of extracellular DNA decorated with histone 3 (H3) and neutrophil elastase (NE). Live cell imaging by 3D holotomographic microscopy (Nanolive®) unveiled rapid vital NETosis against this parasite. A significant increase of autophagosomes visualized by specific-LC3B antibodies and confocal microscopy was observed in B. besnoiti-stimulated bovine PMN when compared to non-stimulated group. As such, a significant positive correlation (r = 0.37; P = 0.042) was found between B. besnoiti-triggered suicidal NETosis and autophagy. These findings suggest that vital- as well as suicidal-NETosis might play a role in early innate host defence mechanisms against released B. besnoiti bradyzoites from tissue cysts, and possibly hampering further parasitic replication. Our data generate first hints on autophagy being associated with B. besnoiti bradyzoite-induced suicidal NETosis and highlighting for first time occurrence of parasite-mediated vital NETosis.

Immunofluorescence Labelling of Human and Murine Neutrophil Extracellular Traps in Paraffin-Embedded Tissue.

Neutrophil granulocytes, also called polymorphonuclear leukocytes (PMN) due to their lobulated nucleus, are the most abundant type of leukocytes. They mature in the bone marrow and are released into the peripheral blood, where they circulate for about 6-8 h; however, in tissue, they can survive for days. By diapedesis through the endothelium, they leave the blood stream, enter tissues, and migrate towards the site of an infection following chemotactic gradients.Neutrophils can combat invading microorganisms by phagocytosis,degranulation, and generation of neutrophil extracellular traps (NETs). This protocol will help to detect NETs in paraffin-embedded tissue. NETs are the result of a process called NETosis, which leads to the release of nuclear, granular, and cytoplasmic components either from living (vital NETosis) or dying (suicidal NETosis) neutrophils. In vitro, NETs form cloud-like structures, which occupy a space several times larger than that of the cells from which they descended. The backbone of NETs is chromatin, to which a selection of proteins and peptides originating from granules and cytoplasm are bound. Thereby, a high local concentration of toxic compounds is maintained so that NETs can capture and inactivate a variety of pathogens including bacteria, fungi, viruses, and parasites, while diffusion of the highly active NET components leading to damage in neighboring tissue is limited. Nevertheless, in recent years it has become apparent that NETs, if generated in abundance or cleared insufficiently, do have pathological potential ranging from autoimmune diseases to cancer. Thus, detection of NETs in tissue samples may have diagnostic significance, and the detection of NETs in diseased tissue can influence the treatment of patients. Since paraffin-embedded tissue samples are the standard specimen used for pathological analysis, it was sought to establish a protocol for fluorescent staining of NET components in paraffin-embedded tissue using commercially available antibodies.

Prognostic Role of Blood NETosis in the Progression of Head and Neck Cancer.

Neutrophil extracellular traps (NETs) represent web-like structures consisting of externalized DNA decorated with granule proteins that are responsible for trapping and killing bacteria. However, undesirable effects of NET formation during carcinogenesis, such as metastasis support, have been described. In the present study, we evaluated the correlation between NETosis and disease progression in head and neck cancer (HNC) patients in order to establish a valid biomarker for an early detection and monitoring of HNC progression. Moreover, factors influencing NET release in HNC patients were revealed. We showed a significantly elevated vital NETosis in neutrophils isolated from early T1–T2 and N0–N2 stage patients, as compared to healthy controls. Additionally, in our experimental setting, we confirmed the involvement of tumor cells in the stimulation of NET formation. Interestingly, in advanced cancer stages (T3–4, N3) NETosis was reduced. This also correlated with the levels of granulocyte colony-stimulating factor (G-CSF) in plasma and tumor tissue. Altogether, we suggest that the elevated NETosis in blood can be used as a biomarker to detect early HNC and to predict patients at risk to develop tumor metastasis. Therapeutic disruption of NET formation may offer new roads for successful treatment of HNC patients in order to prevent metastasis.

Neutrophil-Extracellular Traps, Cell-Free DNA, and Immunothrombosis in Companion Animals: A Review

Immunothrombosis is a potentially beneficial physiological process that aids innate immunity and host defense against pathogen invasion. However, this process can also be damaging when it occurs to excess or in critical blood vessels. Formation of extracellular traps by leukocytes, particularly neutrophils, is central to our understanding of immunothrombosis. In addition to degranulation and phagocytosis, extracellular traps are the third mechanism by which neutrophils combat potential pathogens. These traps consist of extracellular DNA decorated with bactericidal cellular proteins, including elastase, myeloperoxidase, and cathepsins. Neutrophils can release these structures as part of a controlled cell-death process or via a process termed vital NETosis that enables the cells to extrude DNA but remain viable. There is accumulating evidence that NETosis occurs in companion animals, including dogs, horses, and cats, and that it actively contributes to pathogenesis. Numerous studies have been published detailing various methods for identification and quantification of extracellular trap formation, including cell-free DNA, measurements of histones and proteins such as high-mobility group box-1, and techniques involving microscopy and flow cytometry. Here, we outline the present understanding of these phenomena and the mechanisms of extracellular trap formation. We critically review the data regarding measurement of NETosis in companion animals, summarize the existing literature on NETosis in veterinary species, and speculate on what therapeutic options these insights might present to clinicians in the future.

Ovine vital neutrophil extracellular traps bind and impair Haemonchuscontortus L3 in a breed-dependent manner.

This study aimed to characterize neutrophil response to Haemonchus contortus (Hc) in vitro using cells from parasite-resistant St. Croix (STC) and parasite-susceptible Suffolk (SUF) sheep. Neutrophils from Hc-primed and naive STC and SUF sheep were incubated with Hc larval antigen (HcLA), Hc worm antigen (HcWA) or complete media (CM). After HcLA exposure, neutrophils from STC and SUF formed extracellular traps composed of DNA. Stimulation with HcLA induced a 35-fold increase in extracellular DNA compared to CM controls. However, extracellular DNA was not found when neutrophils were cultured with HcWA. The formation of neutrophil extracellular traps (NET) in response to HcLA yields a low percentage of necrotic cells indicating a form of vital NETosis. Neutrophils from primed and naïve STC bound Hc L3 greater (93% and 68%) than SUF (78% and 45%; P<0.001). Furthermore, STC neutrophils significantly reduced larval ATP levels compared to SUF neutrophils (0.05μmol/L vs 0.1μmol/L ATP, P<0.001). These data indicate that ovine neutrophils bind, form vital NET and reduce ATP to Hc L3 in a breed and infection status-dependent manner.

Neutrophil Extracellular Traps and Its Implications in Inflammation: An Overview.

In addition to physical barriers, neutrophils are considered a part of the first line of immune defense. They can be found in the bloodstream, with a lifespan of 6–8 h, and in tissue, where they can last up to 7 days. The mechanisms that neutrophils utilize for host defense are phagocytosis, degranulation, cytokine production, and, the most recently described, neutrophil extracellular trap (NET) production. NETs are DNA structures released due to chromatin decondensation and spreading, and they thus occupy three to five times the volume of condensed chromatin. Several proteins adhere to NETs, including histones and over 30 components of primary and secondary granules, among them components with bactericidal activity such as elastase, myeloperoxidase, cathepsin G, lactoferrin, pentraxin 3, gelatinase, proteinase 3, LL37, peptidoglycan-binding proteins, and others with bactericidal activity able to destroy virulence factors. Three models for NETosis are known to date. (a) Suicidal NETosis, with a duration of 2–4 h, is the best described model. (b) In vital NETosis with nuclear DNA release, neutrophils release NETs without exhibiting loss of nuclear or plasma membrane within 5–60 min, and it is independent of reactive oxygen species (ROS) and the Raf/MERK/ERK pathway. (c) The final type is vital NETosis with release of mitochondrial DNA that is dependent on ROS and produced after stimuli with GM-CSF and lipopolysaccharide. Recent research has revealed neutrophils as more sophisticated immune cells that are able to precisely regulate their granular enzymes release by ion fluxes and can release immunomodulatory cytokines and chemokines that interact with various components of the immune system. Therefore, they can play a key role in autoimmunity and in autoinflammatory and metabolic diseases. In this review, we intend to show the two roles played by neutrophils: as a first line of defense against microorganisms and as a contributor to the pathogenesis of various illnesses, such as autoimmune, autoinflammatory, and metabolic diseases.

Bet on NETs! Or on How to Translate Basic Science into Clinical Practice.

Neutrophil extracellular traps (NETs) comprise neutrophil DNA, citrullinated (deiminated) histones, and proteases (1, 2). NET formation in vivo mostly occurs at the sites of inflammation. Neutrophils, adhering to the endothelium or after extravasation, generate NETs upon interaction with microbes, activated platelets, cytokines, alarmins, such as high-mobility group box 1 (HMGB1), or uric acid. Environmental cues, such as hyperosmolarity or hyperglycemia, neurotransmitters, and some autoantibodies also trigger NET generation (3–7). In vitro, two main pathways are activated: suicidal and vital NETosis (8). In the former, membrane integrity is disrupted and neutrophils die. In contrast, after “vital” NETosis neutrophils still migrate, chase bacteria, and extrude residual nuclear material through exocytosis (9). Generation of reactive oxygen species (ROS) and fusion of neutrophil primary granules with the nuclear membrane that promote interactions between elastase, myeloperoxidase (MPO), and DNA are features of suicidal NETosis. Their role in vital NETosis is debated (8, 10–12). Activation of the autophagic pathway is intermingled with NET generation. Autophagy sustains the metabolic requirements of the extensive intracellular vesicular formation, transport, and fusion associated with NET generation. It also sustains neutrophil survival (5).

New Insights into Neutrophil Extracellular Traps: Mechanisms of Formation and Role in Inflammation.

Recent data suggest that NETosis plays a crucial role in the innate immune response and disturbs the homeostasis of the immune system. NETosis is a form of neutrophil-specific cell death characterized by the release of large web-like structures referred to as neutrophil extracellular traps (NETs). NETs are composed of DNA strands associated with histones and decorated with about 20 different proteins, including neutrophil elastase, myeloperoxidase, cathepsin G, proteinase 3, high mobility group protein B1, and LL37. Reportedly, NETosis can be induced by several microbes, and particulate matter including sterile stimuli, via distinct cellular mechanisms. Meanwhile, suicidal NETosis and vital NETosis are controversial. As we enter the second decade of research on NETosis, we have partly understood NETs as double-edged swords of innate immunity. In this review, we will discuss the mechanisms of NETosis, its antimicrobial action, and role in autoimmune diseases, as well as the relatively new field of NET-associated mitochondrial DNA.

NETosis: At the crossroads of rheumatoid arthritis, lupus, and vasculitis

Suicidal NETosis differs from other mechanisms of cell death by the release of a lattice, composed of DNA associated with proteins citrullinated by protein-arginine deiminase 4, from neutrophils. These ‘NETs’ are composed of granule-derived proteins with microbicidal activity. A similar type of release occurs during vital NETosis, in which anuclear neutrophils maintain their chemotactic ability and imprison live bacteria, even after NET extrusion. Mitochondrial NETosis is limited to the expulsion of oxidised mitochondrial DNA and cytoplasmic enzymes. NETs include the targets of most autoantibodies found in rheumatoid arthritis, lupus, and vasculitis. The clinical and biological overlaps sometimes observed between bronchectiasis and RA, RA and SLE, or SLE and vasculitis, implicate NETosis as a major triggering event common to these disorders. NETosis increases the possibility of association between autoantigens and infectious antigens in mucosal biofilms, impairing the clearance of pathogens and possibly triggering autoimmune reactions. NETosis aggravates these three conditions and increases endothelial damage and the risk of thrombosis. However, the pathogenesis of RA, SLE, and vasculitis is not confined to autoantibodies against NET components, and other mechanisms have been suggested to explain the breakdown of tolerance to NET autoantigens, such as hypercitrullination. The question of whether continuous presentation of autoantigens mixed with antigens from dormant intracellular pathogens (released following suicidal, vital, or mitochondrial NETosis) is required to induce and sustain autoimmunity must be addressed. Inhibiting NETois may not be sufficient to improve autoimmune disorders whereas such latent infections remain uncontrolled.