IL-17A expressed on neutrophil extracellular traps promotes mesenchymal stem cell differentiation toward bone-forming cells in ankylosing spondylitis

Ankylosing spondylitis (AS) is an inflammatory disease characterized by excessive bone formation. We investigated the presence of neutrophil extracellular traps (NETs) in AS and how they are involved in the osteogenic capacity of bone marrow mesenchymal stem cells (MSCs) through interleukin-17A (IL-17A). Peripheral neutrophils and sera were obtained from patients with active AS and healthy controls. NET formation and neutrophil/NET-associated proteins were studied using immunofluorescence, immunoblotting, qPCR, and ELISA. In vitro co-culture systems of AS NET structures and MSCs isolated from controls were deployed to examine the role of NETs in the differentiation of MSCs toward osteogenic cells. Analysis was performed using specific staining and qPCR. Neutrophils from patients with AS were characterized by enhanced formation of NETs carrying bioactive IL-17A and IL-1β. IL-17A-enriched AS NETs mediated the differentiation of MSCs toward bone-forming cells. The neutrophil expression of IL-17A was positively regulated by IL-1β. Blocking IL-1β signaling on neutrophils with anakinra or dismantling NETs using DNase-I disrupted osteogenesis driven by IL-17A-bearing NETs. These findings propose a novel role of neutrophils in AS-related inflammation, linking IL-17A-decorated NETs with the differentiation of MSCs toward bone-forming cells. Moreover, IL-1β triggers the expression of IL-17A on NETs offering an additional therapeutic target in AS.     

Carrageenan‐induced inflammation promotes ROS generation and neutrophil extracellular trap formation in a mouse model of peritonitis

Neutrophil extracellular traps (NETs) are a combination of DNA fibers and granular proteins, such as neutrophil elastase (NE). NETs are released in the extracellular space in response to different stimuli. Carrageenan is a sulfated polysaccharide extracted from Chondrus crispus, a marine algae, used for decades in research for its potential to induce inflammation in different animal models. In this study, we show for the first time that carrageenan injection can induce NET release in a mouse model of acute peritonitis. Carrageenan induced NET release by viable neutrophils with NE and myeloperoxidase (MPO) expressed on DNA fibers. Furthermore, although this polysaccharide was able to stimulate reactive oxygen species (ROS) generation by peritoneal neutrophils, NADPH oxidase derived ROS were dispensable for NET formation by carrageenan. In conclusion, our results show that carrageenan‐induced inflammation in the peritoneum of mice can induce NET formation in an ROS‐independent manner. These results may add important information to the field of inflammation and potentially lead to novel anti‐inflammatory agents targeting the production of NETs.     

Interferon lambda1/IL‐29 and inorganic polyphosphate are novel regulators of neutrophil‐driven thromboinflammation

Neutrophils and neutrophil‐released meshwork structures termed neutrophil extracellular traps (NETs) are major mediators of thromboinflammation and emerging targets for therapy, yet the mechanisms and pathways that control the role of neutrophils in thromboinflammation remain poorly understood. Here, we explored the role of IFN‐λ1/IL‐29, a major antiviral cytokine recently shown to suppress the neutrophil migratory capacity, in prothrombotic and proNETotic functions of neutrophils. In an ex vivo human experimental setting of acute ST‐segment elevation myocardial infarction (STEMI), we show that IFN‐λ1/IL‐29 hinders NET release and diminishes the amount of cytoplasmic TF in neutrophils. Since platelet–neutrophil interaction plays a major role in NET‐induced thromboinflammation, we further studied how IFN‐λ1/IL‐29 may interrupt this interaction. In this context, we identified inorganic polyphosphate (polyP) as a platelet‐derived NET inducer in STEMI. In arterial STEMI thrombi, polyP was present in platelets and in close proximity to NET remnants. PolyP release from activated platelets was dependent on thrombin present in infarcted artery plasma, resulting in NET formation by promoting mTOR inhibition and autophagy induction. The effect of polyP on mTOR inhibition was counteracted by IFN‐λ1/IL‐29 treatment, leading to inhibition of NET formation. Consistently, we show in an in vivo model of FeCl₃‐induced arterial thrombosis that IFN‐λ2/IL‐28A exerts strong antithrombotic potential. Taken together, these findings reveal a novel function of IFN‐λ1/IL‐29 in the suppression of thromboinflammation. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Aging‐related Atg5 defect impairs neutrophil extracellular traps formation

Formation of neutrophil extracellular traps (NETs) is an important function of the innate immune system against infections. It has been proven that aging dysregulates immunity and impairs neutrophil function. However, the influence of aging on the ability to produce NETs has yet to be fully addressed. In this study, we tested the hypothesis that a lower level of autophagy in neutrophils from aged mice was responsible for the decrease in NET formation. We demonstrated that a broad range of Toll‐like receptor 2 (TLR2) ligands could efficiently induce reactive oxygen species (ROS) ‐dependent NET release in young mice, but not in aged ones. We further explored that the difference between young and aged mice in TLR2 ligand‐induced NETosis is the result of an Atg5 defect and subsequent impaired autophagy. Furthermore, we found that lower autophagy capacity led to not only reduced NET formation, but also increased apoptosis. Our results suggest an important role of Atg5 and autophagy in maintaining the function of NETs formation in response to infection and in regulating neutrophil death. Targeting autophagy‐promoted NETs may present a therapeutic strategy to improve infection defence in an aged population.     

IL‐17A produced by both γδ T and Th17 cells promotes renal fibrosis via RANTES‐mediated leukocyte infiltration after renal obstruction

IL‐17A‐producing T lymphocytes play a crucial role in inflammatory kidney diseases, but their role in renal fibrosis remains to be explored. Here, we demonstrated that up‐regulation of IL‐17A was associated with the development of obstructive kidney injury. The primary source of IL‐17A production in obstructed kidneys was infiltrating γδ T lymphocytes and CD4⁺ T cells. IL‐17A‐deficient mice were protected from myofibroblast activation and extracellular matrix deposition, leading to reduced kidney fibrosis in response to obstructive injury. Mechanistically, IL‐17A deficiency suppressed the expression of the chemokine RANTES in infiltrated CD3⁺ T cells and peritubular inflammation following renal obstruction. Administration of RANTES‐neutralizing antibody significantly reduced the accumulation of T cells and macrophages, and of collagen deposition in obstructed kidneys. Taken together, our results indicate that IL‐17A contributes significantly to the pathogenesis of renal fibrosis by regulating RANTES‐mediated inflammatory cell infiltration. Copyright © 2014 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd     

The effects of interleukin‐33 on airways collagen deposition and matrix metalloproteinase expression in a murine surrogate of asthma

It has been suggested that interleukin‐33 (IL‐33) plays an important role in the pathogenesis of asthma through a variety of pathways, but its role in airways fibrosis in asthma has not been fully elucidated. In the present study we evaluated changes in the expression of extracellular matrix proteins (ECMs) as well as matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) in an IL‐33‐induced, antigen‐independent murine surrogate of asthma as well as a conventional surrogate employing per‐nasal challenge of mice previously sensitized to produce an IgE response to ovalbumin (OVA). In addition, in in vitro experiments we explored the direct effects of IL‐33 on the proliferation and function of murine fibroblasts. Per‐nasal administration of IL‐33 alone was sufficient to induce airways deposition of ECMs, including collagens I, III, V and fibronectin, to a degree comparable with that observed in the OVA‐sensitized and challenged mice. These changes were associated with a local imbalance between the expression of extracellular MMPs and TIMPs. Per‐nasal challenge of mice with IL‐33 also induced elevated airways expression of connective tissue growth factor and fibroblast growth factor receptor 4, two key facilitators of local fibrosis, again to a degree compatible with that observed in OVA‐sensitized and challenged mice. Deletion of the ST2 gene, which encodes the IL‐33 receptor, abrogated these fibrotic changes in the airways in the OVA surrogate. In vitro, IL‐33 significantly increased the proliferation and expression of collagen III by murine lung fibroblasts. These data suggest that direct exposure of murine airways to IL‐33 is able to induce local fibrotic changes, at least partially through effects of signalling through the IL‐33/ST2 axis on fibroblast function and local expression of MMPs and their inhibitors, and other fibrosis‐related proteins.     

The Effect of Antifibrotic Drug Halofugine on Th17 Cells in Concanavalin A‐Induced Liver Fibrosis

Anti‐inflammation strategy is one of the proposed therapeutic approaches to hepatic fibrosis. T helper (Th) 17 cells, which play a detrimental role in experimental murine models of inflammatory diseases, have been demonstrated to participate in the pathogenesis of liver damage. The inhibitory effect of halofuginone (HF), an active component of extracts derived from the plant alkaloid febrifugine, on collagen synthesis has been shown in animal models of the fibrotic disease. The aim of this study was to clarify the in vivo effect of HF on Th17 cells in concanavalin A‐induced fibrosis rats. Haematoxylin–eosin (HE) staining and Masson staining were performed to observe collagen deposition. The presence of INF‐gamma, TNF‐alpha, IL‐6, IL‐17, IL‐1beta, IL‐33 and IL‐10 in serum and the presence of ROR‐γt, IL‐17, TGF‐β1 and α‐SMA in liver tissue were detected. Flow cytometry was performed to analyse the percentage of Th17 cells. We observed significantly lower levels of INF‐gamma, TNF‐alpha, IL‐6, IL‐17, IL‐1beta, TGF‐β1 and α‐SMA in HF‐treated group of rats, and the percentage of Th17 cells in splenic lymphocyte was decreased well. Histological examination demonstrated that HF significantly reduced the severity of liver fibrosis in HF‐treated rats. We concluded that HF (10 mg/kg) exerts an antifibrotic impact on Th17 cells and its relative cytokines in rats with ConA‐induced fibrosis.     

Instructive influences of phagocytic clearance of dying cells on neutrophil extracellular trap generation

Coordinated programmes of resolution are thought to initiate early after an inflammatory response begins, actively terminating leucocyte recruitment, allowing their demise via apoptosis and their clearance by phagocytosis. In this review we describe an event that could be implicated in the resolution of inflammation, i.e. the establishment of a refractory state in human neutrophils that had phagocytosed apoptotic cells. Adherent neutrophils challenged with apoptotic cells generate neutrophil extracellular traps (NETs), filaments of decondensed chromatin decorated with bioactive molecules that are involved in the capture of various microbes and in persistent sterile inflammation. In contrast, neutrophils that had previously phagocytosed apoptotic cells lose their capacity to up-regulate β2 integrins and to respond to activating stimuli that induce NET generation, such as interleukin (IL)-8. A defective regulation of NET generation might contribute to the persistent inflammation and tissue injury in diseases in which the clearance of apoptotic cells is jeopardized, including systemic lupus erythematosus and anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis.     

Interleukin-13 acts as an apoptotic effector on lung epithelial cells and induces pro-fibrotic gene expression in lung fibroblasts

Background IL‐13 promotes acute allergic asthma and is discussed to play a role in late asthmatic features such as fibrotic processes and airway remodelling. The contributions of IL‐13‐mediated mechanisms to subepithelial events related to fibrosis are not yet settled. Objective We investigated the impact of IL‐13 on lung epithelial cells as apoptotic effector and on lung fibroblasts as inducer of pro‐fibrotic gene expression. Methods Using the two lung epithelial cell lines A549 and BEAS‐2B as well as primary lung epithelial cells, we investigated the capability of IL‐13 to induce apoptosis by both flow‐cytometry and ELISA. The ability of IL‐13 to increase the expression of pro‐fibrotic genes and to exert influence on the expression of its own receptor was investigated by real‐time quantitative PCR measurement of mRNAs encoding collagen I, collagen III, basic fibroblast growth factor (bFGF), α‐smooth muscle actin (α‐SMA) and the IL‐13 receptor α1 (IL‐13Rα1) chain in human primary lung fibroblasts. The specificity of IL‐13‐mediated cellular responses was confirmed by means of an inhibitory monoclonal antibody directed to the IL‐13 receptor. Results IL‐13 induces apoptosis in lung epithelial cell lines as well as in primary lung epithelial cells. Furthermore, IL‐13 increases the expression of mRNA for α‐SMA and collagen III, but not for bFGF in human primary lung fibroblasts. The susceptibility of lung fibroblasts to IL‐13‐induced up‐regulation of pro‐fibrotic genes is associated with the regulation of IL‐13 receptor expression. IL‐13‐dependent fibrosis‐associated effects could be inhibited by antibody‐mediated blockade of the IL‐13Rα1 subunit. Conclusion Our findings indicate a function of IL‐13 as a mediator in fibrotic processes leading to loss of functional airway tissue in asthma. They also highlight the therapeutic potential of specifically targeting the interaction between IL‐13 and its receptor.     

Induction of autophagy in Cx3cr1+ mononuclear cells limits IL-23/IL-22 axis-mediated intestinal fibrosis

Intestinal fibrosis is an excessive proliferation of myofibroblasts and deposition of collagen, a condition frequently seen in Crohn's disease (CD). The mechanism underlying myofibroblast hyper-proliferation in CD needs to be better understood. In this report, we found that mTOR inhibitor rapamycin or mTOR deletion in CX3Cr1⁺ mononuclear phagocytes inhibits expression of interleukin (IL)−23, accompanied by reduced intestinal production of IL-22 and ameliorated fibrosis in the TNBS-induced fibrosis mouse model. This inhibition of IL-23 expression is associated with elevated autophagy activity. Ablating the autophagy gene Atg7 increases the expression of IL-23, leading to increased expression of IL-22 and increased fibrosis. Both induction of IL-22 and intestinal fibrosis occurred in RAG^−/− mice and depletion of innate lymphoid cells (ILCs) attenuates the fibrotic reaction, suggesting that the pro-fibrotic process is independent of T and B cells. Moreover, IL-22 facilitates the transformation of fibroblasts into myofibroblasts. Finally, the fibrotic reaction was attenuated upon neutralization of either IL-23 or IL-22. Altogether, this study elucidated a signaling cascade underlying intestinal fibrosis in which altered mTOR/autophagy in CX3Cr1⁺ mononuclear phagocytes up-regulates the IL-23/IL-22 axis, leading to an excessive fibrotic response. Thus, our findings suggest that this cascade could be a therapeutic target for alleviation of CD fibrosis.     

Neutrophil extracellular trap formation is associated with autophagy‐related signalling in ANCA‐associated vasculitis

Increasing evidence indicates that aberrant neutrophil extracellular trap (NET) formation could contribute to the pathogenesis of anti‐neutrophil cytoplasmic antibody (ANCA)‐associated vasculitis (AAV). Recent research has provided evidence that a novel type of ANCA autoantibody, anti‐lysosomal membrane protein‐2 (LAMP‐2) antibody, may have a pathogenic role in AAV. We have shown previously that anti‐LAMP‐2 antibody‐stimulated NET formation contains autoantigens and anti‐microbial peptides. The current study sought to determine whether LAMP‐2, as a novel antigen of ANCA, was present on NETs in AAV patients, the influence of the anti‐LAMP‐2 antibody on the neutrophil apoptosis rate and the role of autophagy in anti‐LAMP‐2 antibody‐induced NET formation. NET formation was assessed using immunofluorescence microscopy, scanning electron microscopy or live cell imaging. The neutrophil apoptosis rate was analysed using fluorescence activated cell sorting (FACS). Autophagy was detected using LC3B accumulation and transmission electron microscopy. The results showed that enhanced NET formation, which contains LAMP‐2, was observed in kidney biopsies and neutrophils from AAV patients. The apoptosis rate decreased significantly in human neutrophils stimulated with anti‐LAMP‐2 antibody, and this effect was attenuated by the inhibitors of autophagy 3‐methyladenine (3MA) and 2‐morpholin‐4‐yl‐8‐phenylchromen‐4‐one (LY294002). The anti‐LAMP‐2 antibody‐stimulated NET formation was unaffected by benzyloxycarbonyl‐Val‐ Ala‐Asp (OMe)‐fluoromethylketone (zVAD‐fmk) and necrostatin‐1 (Nec‐1), which are inhibitors of apoptosis and necrosis, respectively, but was inhibited by 3MA and LY294002. Moreover, the proportion of LC3BI that was converted to LC3BII increased significantly (P = 0·0057), and massive vacuolizations that exhibited characteristics typical of autophagy were detected in neutrophils stimulated with anti‐LAMP‐2 antibody. Our results provide further evidence that autophagy is involved in ANCA‐induced NET formation in human neutrophils.     

Neutrophils releasing IL-17A into NETs are essential to plasma cell differentiation in inflamed tissue dependent on IL-1R

Interleukin (IL) 17A in chronic inflammation is also produced by innate immune cells as neutrophils. Mice with chronic humoral response induced by venom of Thalassophryne nattereri (VTn) proved to be a good tool for evaluating the impact of IL-17A on the development of long-lived plasma cells in the inflamed peritoneal cavity. Here, we report that VTn induces IL-17A production by neutrophils accumulating in the peritoneal cavity and triggers the extrusion of IL-17A along with neutrophil extracellular traps (NETs). Neutrophil depletion reduced the number of IL17A-producing cells in VTn-immunized mice and blocked the differentiation of long-lived plasma cells. Specific antibody production and survival of long-lived plasma cells was ablated in VTn-immunized mice deficient in CD4, while CD28 signaling had the opposite effect on differentiation of long-lived plasma cells. Further, maturation of long-lived plasma cells in inflamed peritoneal cavity was IL-1R1 and COX-2 dependent. Finally, when both the Raf-MEK-ERK pathway and the IL-17A or IL-1R1 activities were blocked, neutrophils were unable to promote the differentiation of memory B cells into long-lived plasma cells, confirming the essential role of neutrophils and IL-17A along with NETs in an IL-1/IL-1R-dependent manner as the novel helping partner for plasma cell differentiation in chronically inflamed tissues.     

Serum amyloid A induces interleukin‐6 in dermal fibroblasts via Toll‐like receptor 2, interleukin‐1 receptor‐associated kinase 4 and nuclear factor‐κB

Systemic sclerosis is an autoimmune idiopathic connective tissue disease, characterized by vasculopathy, inflammation and fibrosis. There appears to be a link between inflammation and fibrosis, although the exact nature of the relationship is unknown. Serum amyloid A (SAA) is an acute‐phase protein that is elevated up to 1000‐fold in times of infection or inflammation. This acute‐phase reactant, as well as being a marker of inflammation, may initiate signals in a cytokine‐like manner, possibly through toll‐like receptors (TLRs) promoting inflammation. This study addressed the role of SAA in initiating interleukin‐6 (IL‐6) production in dermal fibroblasts and the role of TLR2 in this system. We show that SAA induces IL‐6 secretion in healthy dermal fibroblasts and that blockade of TLR2 with a neutralizing antibody to TLR2 or specific small interfering RNA attenuated the SAA‐induced IL‐6 secretion and that this was also mediated through the TLR adaptor protein IL‐1 receptor‐associated kinase 4. The effect is nuclear factor‐κB‐mediated because blockade of nuclear factor‐κB reduced the induction. We also demonstrate that dermal fibroblasts express TLR2; this is functional and over‐expressed in the fibroblasts of patients with systemic sclerosis. Taken together these data suggest that SAA is a danger signal that initiates IL‐6 signalling in systemic sclerosis via enhanced TLR2 signalling.     

Neutrophil extracellular traps exacerbate Th1‐mediated autoimmune responses in rheumatoid arthritis by promoting DC maturation

Aberrant formation of neutrophil extracellular traps (NETs) is a key feature in rheumatoid arthritis (RA) and plays a pivotal role in disease pathogenesis. However, the mechanism through which NETs shape the autoimmune response in RA remains elusive. In this study, we demonstrate that inhibition of peptidylarginine deiminases activity in collagen‐induced arthritis (CIA) mouse model significantly reduces NET formation, attenuates clinical disease activity, and prevents joint destruction. Importantly, peptidylarginine deiminase 4 blocking markedly reduces the frequency of collagen‐specific IFN‐γ‐producing T helper 1 (Th1) cells in the draining lymph nodes of immunized mice. Exposure of dendritic cells (DCs) to CIA‐derived NETs induces DC maturation characterized by significant upregulation of costimulatory molecules, as well as elevated secretion of IL‐6. Moreover, CIA‐NET‐treated DCs promote the induction of antigen‐specific Th1 cells in vitro. Finally, NETs from RA patients show an increased potential to induce the maturation of DCs from healthy individuals, corroborating the findings obtained in CIA mouse model. Collectively, our findings delineate an important role of NETs in the induction and expansion of Th1 pathogenic cells in CIA through maturation of DCs and reveal a novel role of NETs in shaping the RA‐autoimmune response that could be exploited therapeutically.     

Neutrophil extracellular traps cause airway obstruction during respiratory syncytial virus disease

Human respiratory syncytial virus (RSV) is the most important cause of severe lower respiratory tract disease (LRTD) in young children worldwide. Extensive neutrophil accumulation in the lungs and occlusion of small airways by DNA‐rich mucus plugs are characteristic features of severe RSV–LRTD. Activated neutrophils can release neutrophil extracellular traps (NETs), extracellular networks of DNA covered with antimicrobial proteins, as part of the first‐line defence against pathogens. NETs can trap and eliminate microbes; however, abundant NET formation may also contribute to airway occlusion. In this study, we investigated whether NETs are induced by RSV and explored their potential anti‐viral effect in vitro. Second, we studied NET formation in vivo during severe RSV–LRTD in infants and bovine RSV–LRTD in calves, by examining bronchoalveolar lavage fluid and lung tissue sections, respectively. NETs were visualized in lung cytology and tissue samples by DNA and immunostaining, using antibodies against citrullinated histone H3, elastase and myeloperoxidase. RSV was able to induce NET formation by human neutrophils in vitro. Furthermore, NETs were able to capture RSV, thereby precluding binding of viral particles to target cells and preventing infection. Evidence for the formation of NETs in the airways and lungs was confirmed in children with severe RSV–LRTD. Detailed histopathological examination of calves with RSV–LRTD showed extensive NET formation in dense plugs occluding the airways, either with or without captured viral antigen. Together, these results suggest that, although NETs trap viral particles, their exaggerated formation during severe RSV–LRTD contributes to airway obstruction. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Fibrin Mimics Neutrophil Extracellular Traps in SEM

Neutrophil extracellular traps (NETs) are extracellular web-like structures produced by activated polymorphonuclear neutrophils. NETs kill bacteria extracellularly, but their role in human pathology remains largely unclear. One possible way of studying NETs is through the SEM approach. However, web-like structures observed with SEM in sites of inflammation have been interpreted either as NETs or as fibrin. Thus, the question arises whether a reliable SEM discrimination between NETs and fibrin is at all possible. NET samples were collected as purulent crevicular exudate from periodontal pockets. DNase-digested controls for SEM were employed to demonstrate the DNA backbone and immuno-staining for confocal laser scanning microscopy was used to show the citrullinated histones of NETs. Blood clot samples were treated in the same way as the exudate samples to demonstrate that fibrin and fibrinolysis can mimic NETs and DNA digestion, respectively. No discrimination between fibrin and NETs based on morphological criteria in SEM was possible. Furthermore, only a vague distinction between DNA digestion and fibrinolysis could be made. These findings unambiguously indicate that the discrimination between NETs and fibrin by means of SEM is untrustworthy for samples of inflammatory exudate.     

Metabolic requirements for neutrophil extracellular traps formation

As part of the innate immune response, neutrophils are at the forefront of defence against infection, resolution of inflammation and wound healing. They are the most abundant leucocytes in the peripheral blood, have a short lifespan and an estimated turnover of 10¹⁰ to 10¹¹ cells per day. Neutrophils efficiently clear microbial infections by phagocytosis and by oxygen‐dependent and oxygen‐independent mechanisms. In 2004, a new neutrophil anti‐microbial mechanism was described, the release of neutrophil extracellular traps (NETs) composed of DNA, histones and anti‐microbial peptides. Several microorganisms, bacterial products, as well as pharmacological stimuli such as PMA, were shown to induce NETs. Neutrophils contain relatively few mitochondria, and derive most of their energy from glycolysis. In this scenario we aimed to analyse some of the metabolic requirements for NET formation. Here it is shown that NETs formation is strictly dependent on glucose and to a lesser extent on glutamine, that Glut‐1, glucose uptake, and glycolysis rate increase upon PMA stimulation, and that NET formation is inhibited by the glycolysis inhibitor, 2‐deoxy‐glucose, and to a lesser extent by the ATP synthase inhibitor oligomycin. Moreover, when neutrophils were exposed to PMA in glucose‐free medium for 3 hr, they lost their characteristic polymorphic nuclei but did not release NETs. However, if glucose (but not pyruvate) was added at this time, NET release took place within minutes, suggesting that NET formation could be metabolically divided into two phases; the first, independent from exogenous glucose (chromatin decondensation) and, the second (NET release), strictly dependent on exogenous glucose and glycolysis.     

NET formation can occur independently of RIPK3 and MLKL signaling

The importance of neutrophil extracellular traps (NETs) in innate immunity is well established but the molecular mechanisms responsible for their formation are still a matter of scientific dispute. Here, we aim to characterize a possible role of the receptor‐interacting protein kinase 3 (RIPK3) and the mixed lineage kinase domain‐like (MLKL) signaling pathway, which are known to cause necroptosis, in NET formation. Using genetic and pharmacological approaches, we investigated whether this programmed form of necrosis is a prerequisite for NET formation. NETs have been defined as extracellular DNA scaffolds associated with the neutrophil granule protein elastase that are capable of killing bacteria. Neither Ripk3‐deficient mouse neutrophils nor human neutrophils in which MLKL had been pharmacologically inactivated, exhibited abnormalities in NET formation upon physiological activation or exposure to low concentrations of PMA. These data indicate that NET formation occurs independently of both RIPK3 and MLKL signaling.     

Therapeutic ACPA inhibits NET formation: a potential therapy for neutrophil-mediated inflammatory diseases

Excessive release of neutrophil extracellular traps (NETs) is associated with disease severity and contributes to tissue injury, followed by severe organ damage. Pharmacological or genetic inhibition of NET release reduces pathology in multiple inflammatory disease models, indicating that NETs are potential therapeutic targets. Here, we demonstrate using a preclinical basket approach that our therapeutic anti-citrullinated protein antibody (tACPA) has broad therapeutic potential. Treatment with tACPA prevents disease symptoms in various mouse models with plausible NET-mediated pathology, including inflammatory arthritis (IA), pulmonary fibrosis, inflammatory bowel disease and sepsis. We show that citrulline residues in the N-termini of histones 2A and 4 are specific targets for therapeutic intervention, whereas antibodies against other N-terminal post-translational histone modifications have no therapeutic effects. Because citrullinated histones are generated during NET release, we investigated the ability of tACPA to inhibit NET formation. tACPA suppressed NET release from human neutrophils triggered with physiologically relevant human disease-related stimuli. Moreover, tACPA diminished NET release and potentially initiated NET uptake by macrophages in vivo, which was associated with reduced tissue damage in the joints of a chronic arthritis mouse model of IA. To our knowledge, we are the first to describe an antibody with NET-inhibiting properties and thereby propose tACPA as a drug candidate for NET-mediated inflammatory diseases, as it eliminates the noxious triggers that lead to continued inflammation and tissue damage in a multidimensional manner.