Neutrophil extracellular traps in immunity and disease.Nat Rev Immunol. 2018;18:134-147. [PMID: 28990587 DOI: 10.1038/nri.2017.105]

Caught in a Trap? Proteomic Analysis of Neutrophil Extracellular Traps in Rheumatoid Arthritis and Systemic Lupus Erythematosus

Neutrophil Extracellular Traps (NETs) are implicated in the development of auto-immunity in diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) through the externalization of intracellular neoepitopes e.g., dsDNA and nuclear proteins in SLE and citrullinated peptides in RA. The aim of this work was to use quantitative proteomics to identify and measure NET proteins produced by neutrophils from healthy controls, and from patients with RA and SLE to determine if NETs can be differentially-generated to expose different sets of neoepitopes. Ultra-pure neutrophils (>99%) from healthy individuals (n = 3) and patients with RA or SLE (n = 6 each) were incubated ± PMA (50 nM, PKC super-activator) or A23187 (3.8 μM, calcium ionophore) for 4 h. NETs were liberated by nuclease digestion and concentrated onto Strataclean beads prior to on-bead digestion with trypsin. Data-dependent LC-MS/MS analyses were conducted on a QExactive HF quadrupole-Orbitrap mass spectrometer, and label-free protein quantification was carried out using Progenesis QI. PMA-induced NETs were decorated with annexins, azurocidin and histone H3, whereas A23187-induced NETs were decorated with granule proteins including CAMP/LL37, CRISP3, lipocalin and MMP8, histones H1.0, H1.4, and H1.5, interleukin-8, protein-arginine deiminase-4 (PADI4), and α-enolase. Four proteins were significantly different between PMA-NETs from RA and SLE neutrophils (p < 0.05): RNASE2 was higher in RA, whereas MPO, leukocyte elastase inhibitor and thymidine phosphorylase were higher in SLE. For A23187-NETs, six NET proteins were higher in RA (p < 0.05), including CAMP/LL37, CRISP3, interleukin-8, MMP8; Thirteen proteins were higher in SLE, including histones H1.0, H2B, and H4. This work provides the first, direct comparison of NOX2-dependent (PMA) and NOX2-independent (A23187) NETs using quantitative proteomics, and the first direct comparison of RA and SLE NETs using quantitative proteomics. We show that it is the nature of the stimulant rather than neutrophil physiology that determines NET protein profiles in disease, since stimulation of NETosis in either a NOX2-dependent or a NOX2-independent manner generates broadly similar NET proteins irrespective of the disease background. We also use our proteomics pipeline to identify an extensive range of post-translationally modified proteins in RA and SLE, including histones and granule proteins, many of which are known targets of auto-antibodies in each disease.

Pinniped- and Cetacean-Derived ETosis Contributes to Combating Emerging Apicomplexan Parasites (Toxoplasma gondii, Neospora caninum) Circulating in Marine Environments

Leukocytes play a major role in combating infections either by phagocytosis, release of antimicrobial granules, or extracellular trap (ET) formation. ET formation is preceded by a certain leukocyte cell death form, known as ETosis, an evolutionarily conserved mechanism of the innate immune system also observed in marine mammals. Besides several biomolecules and microbial stimuli, marine mammal ETosis is also trigged by various terrestrial protozoa and metazoa, considered nowadays as neozoan parasites, which are circulating in oceans worldwide and causing critical emerging marine diseases. Recent studies demonstrated that pinniped- and cetacean-derived polymorphonuclear neutrophils (PMNs) and monocytes are able to form different phenotypes of ET structures composed of nuclear DNA, histones, and cytoplasmic peptides/proteases against terrestrial apicomplexan parasites, e.g., Toxoplasma gondii and Neospora caninum. Detailed molecular analyses and functional studies proved that marine mammal PMNs and monocytes cast ETs in a similar way as terrestrial mammals, entrapping and immobilizing T. gondii and N. caninum tachyzoites. Pinniped- and cetacean leukocytes induce vital and suicidal ETosis, with highly reliant actions of nicotinamide adenine dinucleotide phosphate oxidase (NOX), generation of reactive oxygen species (ROS), and combined mechanisms of myeloperoxidase (MPO), neutrophil elastase (NE), and DNA citrullination via peptidylarginine deiminase IV (PAD4).This scoping review intends to summarize the knowledge on emerging protozoans in the marine environment and secondly to review limited data about ETosis mechanisms in marine mammalian species.

Regulation of neutrophil pro-inflammatory functions sheds new light on the pathogenesis of rheumatoid arthritis

For more than two centuries now, rheumatoid arthritis (RA) is under investigation intending to discover successful treatment. Despite decades of scientific advances, RA is still representing a challenge for contemporary medicine. Current drug therapies allow to improve significantly the quality of life of RA patients; however, they are still insufficient to reverse tissue injury and are often generating side-effects. The difficulty arises from the considerable fluctuation of the clinical course of RA among patients, making the predictive prognosis difficult. More and more studies underline the profound influence of the neutrophil multifaceted functions in the pathogenesis of RA. This renewed interest in the complexity of neutrophil functions in RA offers new exciting opportunities for valuable therapeutic targets as well as for safe and well-tolerated RA treatments. In this review, we aim to update the recent findings on the multiple facets of neutrophils in RA, in particular their impact in promoting the RA-based inflammation through the release of the cytokine-like S100A8/A9 protein complex, as well as the importance of NETosis in the disease progression and development. Furthermore, we delve into the complex question of neutrophil heterogeneity and plasticity and discuss the emerging role of miRNAs and epigenetic markers influencing the inflammatory response of neutrophils in RA and how they could constitute the starting point for novel attractive targets in RA therapy.

Stimulation of the class-A scavenger receptor induces neutrophil extracellular traps (NETs) by ERK dependent NOX2 and ROMO1 activation

Neutrophil extracellular traps (NETs) play a critical role in host antimicrobial response whereas they are also implicated in the pathogenesis of inflammatory and autoimmunediseases. Generation of reactiveoxygen species (ROS) is key to NETs formation. A variety of stimulatory ligands have been found to enhance ROS production and thus trigger NETs. However, the mechanisms that connect receptor stimuli with ROS production and NETs formation remain unclear. In this study, we described a new mechanism of NETs generation in neutrophils triggered by stimulation of the class A scavenger receptor (SRA), a major subtype of scavenger receptors in response to various stimuli during infection and inflammatory disorders. By using polyinosinic acid (Poly I), a ribonucleotide ligand of SRA, we demonstrated that SRA stimulation lead to selective ERK phosphorylation, which upregulated cytosol ROS levels and induced canonical NETs formation by activating NADPH oxidase 2 (NOX2). Interestingly, our results showed that mitochondrial ROS (mtROS) production was also enhanced by the SRA dependent ERK activation through upregulation and activation of reactive oxygen species modulator 1(ROMO1), a mitochondrial membrane protein and a key mediator of mtROS. Moreover, inhibition of the SRA elicited ROMO1 activation dampened NETs release upon SRA stimulation. Overall, our study describes a new insight into the NETs release triggered by membrane SRA stimulation and mediated by ERK dependent NOX2 and ROMO1 activation.

Iron and iron-dependent reactive oxygen species in the regulation of macrophages and fibroblasts in non-healing chronic wounds

Chronic wounds pose a stern challenge to health care systems with growing incidence especially in the aged population. In the presence of increased iron concentrations, recruitment of monocytes from the circulation and activation towards ROS and RNS releasing M1 macrophages together with the persistence of senescent fibroblasts at the wound site are significantly enhanced. This unrestrained activation of pro-inflammatory macrophages and senescent fibroblasts has increasingly been acknowledged as main driver causing non-healing wounds. In a metaphor, macrophages act like stage directors of wound healing, resident fibroblasts constitute main actors and increased iron concentrations are decisive parts of the libretto, and - if dysregulated - are responsible for the development of non-healing wounds. This review will focus on recent cellular and molecular findings from chronic venous leg ulcers and diabetic non-healing wounds both constituting the most common pathologies often resulting in limb amputations of patients. This not only causes tremendous suffering and loss of life quality, but is also associated with an increase in mortality and a major socio-economic burden. Despite recent advances, the underlying molecular mechanisms are not completely understood. Overwhelming evidence shows that reactive oxygen species and the transition metal and trace element iron at pathological concentrations are crucially involved in a complex interplay between cells of different histogenetic origin and their extracellular niche environment. This interplay depends on a variety of cellular, non-cellular biochemical and cell biological mechanisms. Here, we will highlight recent progress in the field of iron-dependent regulation of macrophages and fibroblasts and related pathologies linked to non-healing chronic wounds.

Excessive neutrophil levels in the lung underlie the age-associated increase in influenza mortality

Neutrophils clear viruses, but excessive neutrophil responses induce tissue injury and worsen disease. Aging increases mortality to influenza infection; however, whether this is due to impaired viral clearance or a pathological host immune response is unknown. Here we show that aged mice have higher levels of lung neutrophils than younger mice after influenza viral infection. Depleting neutrophils after, but not before, infection substantially improves the survival of aged mice without altering viral clearance. Aged alveolar epithelial cells (AECs) have a higher frequency of senescence and secrete higher levels of the neutrophil-attracting chemokines CXCL1 and CXCL2 during influenza infection. These chemokines are required for age-enhanced neutrophil chemotaxis in vitro. Our work suggests that aging increases mortality from influenza in part because senescent AECs secrete more chemokines, leading to excessive neutrophil recruitment. Therapies that mitigate this pathological immune response in the elderly might improve outcomes of influenza and other respiratory infections.

Pathology of spontaneous and immunotherapy-induced tumor regression in a murine model of melanoma

We evaluated the spontaneous and immunotherapy-induced histological changes in the tumor microenvironment of a mouse melanoma regression model consisting of immunocompetent C57BL/6J mice implanted with syngeneic YUMMER1.7 melanoma cells. We focused on tumor regression phenotypes and spatial relationships of melanoma cells with B cells and neutrophils since this was not previously described. We found common themes to the host response to cancer irrespective of the mode of tumor regression. In nonregression tumors, melanoma cells were epithelioid shaped and tightly packed. In regression tumors, melanoma cells were spindle shaped and discohesive. B cells including plasmablasts and plasma cells were numerous and were increased with immunotherapy. Neutrophils were in direct contact with dead or dying melanoma cells. Immunotherapy increased neutrophil counts and induced neutrophil extracellular traps (NETs)-like formations and geographic necrosis. Beyond tumor regression, the increase in the B cell and neutrophil response could play a role in immunotherapy-induced adverse reactions.

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

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

Toll-like Receptor 2 Prevents Neutrophil-Driven Immunopathology during Infection with Mycobacterium tuberculosis by Curtailing CXCL5 Production

The W-Beijing strain family is globally distributed and is associated with multidrug-resistant tuberculosis (TB) and treatment failure. Therefore, in this study, we examined the contribution of Toll-like receptor 2 (TLR2) to host resistance against Mycobacterium tuberculosis HN878, a clinical isolate belonging to the W-Beijing family. We show that TLR2 knockout (TLR2KO) mice infected with M. tuberculosis HN878 exhibit increased bacterial burden and are unable to control tissue-damaging, pulmonary neutrophilic inflammation. Consistent with a critical role for CXCL5 in regulating neutrophil influx, expression of epithelial cell-derived CXCL5 is significantly enhanced in TLR2KO mice prior to their divergence from wild-type (WT) mice in M. tuberculosis replication and neutrophilic inflammation. Depletion of neutrophils in TLR2KO mice by targeting Ly6G reverts lung inflammation and bacterial burden to levels comparable to those of WT mice. Together, the results establish that TLR2 controls neutrophil-driven immunopathology during infection with M. tuberculosis HN878 infection, likely by curtailing CXCL5 production.

NLRP3 activation induced by neutrophil extracellular traps sustains inflammatory response in the diabetic wound

Persistent inflammatory response in the diabetic wound impairs the healing process, resulting in significant morbidity and mortality. Mounting evidence indicate that the activation of Nod-like receptor protein (NLRP) 3 inflammasome in macrophages (MΦ) contributes to the sustained inflammatory response and impaired wound healing associated with diabetes. However, the main trigger of NLRP3 inflammasome in the wounds is not known. Neutrophils, as sentinels of the innate immune system and key stimulators of MΦ, are immune cells that play the main role in the early phase of healing. Neutrophils release extracellular traps (NETs) as defense against pathogens. On the other hand, NETs induce tissue damage. NETs have been detected in the diabetic wound and implicated in the impaired healing process, but the mechanism of NETs suspend wound healing and its role in fostering inflammatory dysregulation are elusive. Here, we report that NLRP3 and NETs production are elevated in human and rat diabetic wounds. NETs overproduced in the diabetic wounds triggered NLRP3 inflammasome activation and IL-1β release in MΦ. Furthermore, NETs up-regulated NLRP3 and pro-IL-1β levels via the TLR-4/TLR-9/NF-κB signaling pathway. They also elicited the generation of reactive oxygen species, which facilitated the association between NLRP3 and thioredoxin-interacting protein, and activated the NLRP3 inflammasome. In addition, NET digestion by DNase I alleviated the activation of NLRP3 inflammasome, regulated the immune cell infiltration, and accelerated wound healing in diabetic rat model. These findings illustrate a new mechanism by which NETs contribute to the activation of NLRP3 inflammasome and sustained inflammatory response in the diabetic wound.

IL-4 receptor engagement in human neutrophils impairs their migration and extracellular trap formation

BACKGROUND

Type 2 immunity serves to resist parasitic helminths, venoms, and toxins, but the role and regulation of neutrophils during type 2 immune responses are controversial. Helminth models suggested a contribution of neutrophils to type 2 immunity, whereas neutrophils are associated with increased disease severity during type 2 inflammatory disorders, such as asthma.

OBJECTIVE

We sought to evaluate the effect of the prototypic type 2 cytokines IL-4 and IL-13 on human neutrophils.

METHODS

Human neutrophils from peripheral blood were assessed without or with IL-4 or IL-13 for (1) expression of IL-4 receptor subunits, (2) neutrophil extracellular trap (NET) formation, (3) migration toward CXCL8 in vitro and in humanized mice, and (4) CXCR1, CXCR2, and CXCR4 expression, as well as (5) in nonallergic versus allergic subjects.

RESULTS

Human neutrophils expressed both types of IL-4 receptors, and their stimulation through IL-4 or IL-13 diminished their ability to form NETs and migrate toward CXCL8 in vitro. Likewise, in vivo chemotaxis in NOD-scid-Il2rg^-/- mice was reduced in IL-4-stimulated human neutrophils compared with control values. These effects were accompanied by downregulation of the CXCL8-binding chemokine receptors CXCR1 and CXCR2 on human neutrophils on IL-4 or IL-13 stimulation in vitro. Ex vivo analysis of neutrophils from allergic patients or exposure of neutrophils from nonallergic subjects to allergic donor serum in vitro impaired their NET formation and migration toward CXCL8, thereby mirroring IL-4/IL-13-stimulated neutrophils.

CONCLUSION

IL-4 receptor signaling in human neutrophils affects several neutrophil effector functions, which bears important implications for immunity in type 2 inflammatory disorders.

Phosphoinositides: multipurpose cellular lipids with emerging roles in cell death

Phosphorylated phosphatidylinositol lipids, or phosphoinositides, critically regulate diverse cellular processes, including signalling transduction, cytoskeletal reorganisation, membrane dynamics and cellular trafficking. However, phosphoinositides have been inadequately investigated in the context of cell death, where they are mainly regarded as signalling secondary messengers. However, recent studies have begun to highlight the importance of phosphoinositides in facilitating cell death execution. Here, we cover the latest phosphoinositide research with a particular focus on phosphoinositides in the mechanisms of cell death. This progress article also raises key questions regarding the poorly defined role of phosphoinositides, particularly during membrane-associated events in cell death such as apoptosis and secondary necrosis. The review then further discusses important future directions for the phosphoinositide field, including therapeutically targeting phosphoinositides to modulate cell death.

Fibrin Modulates Shear-Induced NETosis in Sterile Occlusive Thrombi Formed under Haemodynamic Flow

Neutrophils can release extracellular traps (NETs) in infectious, inflammatory and thrombotic diseases. NETs have been detected in deep vein thrombosis, atherothrombosis, stroke, disseminated intravascular coagulation and trauma. We have previously shown that haemodynamic forces trigger rapid NETosis within sterile occlusive thrombi in vitro. Here, we tested the effects of thrombin, fibrin and fibrinolysis on shear-induced NETosis by imaging NETs with Sytox Green during microfluidic perfusion of factor XIIa-inhibited or thrombin-inhibited human whole blood over fibrillar collagen (±tissue factor). For perfusions under venous pressure drops (19 mm Hg/mm-clot), thrombin generation did not alter the near-zero level of NET generation. In contrast, production of thrombin/fibrin led to a twofold reduction in neutrophil accumulation and a sixfold reduction in NET generation after 30 minutes of arterial perfusion (163 mm Hg/mm-clot). Exogenously added tissue type plasminogen activator (tPA) drove robust fibrinolysis; however, tPA did not trigger NETosis under venous flow. In contrast, tPA did enhance NET generation in clots subjected to arterial pressure drops. After 45 minutes of arterial perfusion, clots treated with 30 nM tPA had a threefold increase in total NET production and a twofold increase in normalized NET generation (measured as deoxyribonucleic acid:neutrophil) compared with fibrin-rich clots. Blocking fibrin polymerization resulted in similar level of NET release seen in tPA-treated clots, whereas ε-aminocaproic acid abolished the NET-enhancing effect of tPA. Therefore, fibrin suppresses NET generation and the absence of fibrin promotes NETs. We demonstrated that shear-induced NETosis was strongly inversely correlated with fibrin in sterile occlusive clots.

Clinical and etiological meaning of anti-carbamylated protein antibodies in rheumatoid arthritis

Several autoantibodies against proteins with post-translational modifications have been detected in patients with rheumatoid arthritis (RA) and are called anti-modified protein antibodies (AMPAs). Anti-carbamylated protein antibodies (Anti-CarP Ab) are the second most vigorously researched AMPAs following anti-citrullinated protein/peptide antibodies (ACPA). Anti-CarP Ab and ACPA show cross-reactivity to some extent and frequently co-exist with each other in RA, but are two distinct antibodies. Although the diagnostic efficacy of anti-CarP Ab is inferior to that of ACPA, the diagnostic specificity of RA may improve when used in combination with ACPA and rheumatoid factor. Anti-CarP Ab and ACPA are also useful for identifying patients at high risk of more severe joint destruction and cardiovascular diseases. The high prevalence of the co-existence of both antibodies suggests a common factor in their production, and this is important for the development of RA because both antibodies emerge before the onset of clinical symptoms. Neutrophils may also be crucially involved. It is important to distinguish citrullinated antigens from carbamylated antigens because the methods commonly used to detect the former are now known to be cross-reactive with the latter. Research on anti-CarP Ab will provide novel insights into the pathology and etiology of RA.

Protective Features of Autophagy in Pulmonary Infection and Inflammatory Diseases

Autophagy is a highly conserved catabolic process involving autolysosomal degradation of cellular components, including protein aggregates, damaged organelles (such as mitochondria, endoplasmic reticulum, and others), as well as various pathogens. Thus, the autophagy pathway represents a major adaptive response for the maintenance of cellular and tissue homeostasis in response to numerous cellular stressors. A growing body of evidence suggests that autophagy is closely associated with diverse human diseases. Specifically, acute lung injury (ALI) and inflammatory responses caused by bacterial infection or xenobiotic inhalation (e.g., chlorine and cigarette smoke) have been reported to involve a spectrum of alterations in autophagy phenotypes. The role of autophagy in pulmonary infection and inflammatory diseases could be protective or harmful dependent on the conditions. In this review, we describe recent advances regarding the protective features of autophagy in pulmonary diseases, with a focus on ALI, idiopathic pulmonary fibrosis (IPF), chronic obstructive pulmonary disease (COPD), tuberculosis, pulmonary arterial hypertension (PAH) and cystic fibrosis.

DNA-Based Biomaterials for Immunoengineering

Man-made DNA materials hold the potential to modulate specific immune pathways toward immunoactivating or immunosuppressive cascades. DNA-based biomaterials introduce DNA into the extracellular environment during implantation or delivery, and subsequently intracellularly upon phagocytosis or degradation of the material. Therefore, the immunogenic functionality of biological and synthetic extracellular DNA should be considered to achieve desired immune responses. In vivo, extracellular DNA from both endogenous and exogenous sources holds immunoactivating functions which can be traced back to the molecular features of DNA, such as sequence and length. Extracellular DNA is recognized as damage-associated molecular patterns (DAMPs), or pathogen-associated molecular patterns (PAMPs), by immune cell receptors, activating either proinflammatory signaling pathways or immunosuppressive cell functions. Although extracellular DNA promotes protective immune responses during early inflammation such as bacterial killing, recent advances demonstrate that unresolved and elevated DNA concentrations may contribute to the pathogenesis of autoimmune diseases, cancer, and fibrosis. Therefore, addressing the immunogenicity of DNA enables immune responses to be engineered by optimizing their activating and suppressive performance per application. To this end, emerging biology relevant to the generation of extracellular DNA, DNA sensors, and its role concerning existing and future synthetic DNA biomaterials are reviewed.

Transforming Weakness into Strength: Photothermal-Therapy-Induced Inflammation Enhanced Cytopharmaceutical Chemotherapy as a Combination Anticancer Treatment

A new synergistic treatment that combines photothermal therapy (PTT) and inflammation-mediated active targeting (IMAT) chemotherapy based on cytopharmaceuticals is developed. During PTT, the photothermal tumor ablation is accompanied by an inflammatory effect and upregulation of inflammatory factors at the tumor site, which may accelerate tumor regeneration. Moreover, PTT-induced inflammation can also recruit neutrophils (NEs) to the tumor site. To convert the disadvantages of PTT-induced inflammation into strengths, NEs are investigated as cytopharmaceuticals for IMAT chemotherapy to further inhibit the tumor recurrence after PTT due to the chemotaxis of NEs to the inflammatory sites. In this study, PEGylated gold nanorods (PEG-GNRs) are explored as the photothermal agent and paclitaxel-loaded cytopharmaceuticals of NEs as the IMAT chemotherapeutic agent. PTT is conducted at 72 h postinjection of PEG-GNRs, followed by cytopharmaceuticals for IMAT chemotherapy. It is demonstrated that the cytopharmaceuticals effectively accumulate in the tumor sites after PTT, which leads to a significant enhancement of antitumor efficacy and a reduction in systemic toxicity. These studies suggest that PTT-induced inflammation further enhances the chemotherapy of cytopharmaceuticals, and the combination of PTT and IMAT chemotherapy may be a promising synergistic strategy for targeted cancer therapy.

Vitamin C in Cancer Therapeutics and Metastasis

There have been significant increased publications of preclinical studies and clinical trials of vitamin C (ascorbate) on cancer therapeutics in the past a few years. In this communication reflecting my personal opinions, I will highlight the main points of current research status; discuss impacts of vitamin C on immune cell function and inflammation at tumor microenvironment, and tumor metastasis; and propose stimulating questions and direction for future research in this area.

Serum and Serum Albumin Inhibit in vitro Formation of Neutrophil Extracellular Traps (NETs)

The formation of neutrophil extracellular traps (NETs) is an immune defense mechanism of neutrophilic granulocytes. Moreover, it is also involved in the pathogenesis of autoimmune, inflammatory, and neoplastic diseases. For that reason, the process of NET formation (NETosis) is subject of intense ongoing research. In vitro approaches to quantify NET formation are commonly used and involve neutrophil stimulation with various activators such as phorbol 12-myristate 13-acetate (PMA), lipopolysaccharides (LPS), or calcium ionophores (CaI). However, the experimental conditions of these experiments, particularly the media and media supplements employed by different research groups, vary considerably, rendering comparisons of results difficult. Here, we present the first standardized investigation of the influence of different media supplements on NET formation in vitro. The addition of heat-inactivated (hi) fetal calf serum (FCS), 0.5% human serum albumin (HSA), or 0.5% bovine serum albumin (BSA) efficiently prevented NET formation of human neutrophils following stimulation with LPS and CaI, but not after stimulation with PMA. Thus, serum components such as HSA, BSA and hiFCS (at concentrations typically found in the literature) inhibit NET formation to different degrees, depending on the NETosis inducer used. In contrast, in murine neutrophils, NETosis was inhibited by FCS and BSA, regardless of the inducer employed. This shows that mouse and human neutrophils have different susceptibilities toward the inhibition of NETosis by albumin or serum components. Furthermore, we provide experimental evidence that albumin inhibits NETosis by scavenging activators such as LPS. We also put our results into the context of media supplements most commonly used in NET research. In experiments with human neutrophils, either FCS (0.5–10%), heat-inactivated (hiFCS, 0.1–10%) or human serum albumin (HSA, 0.05–2%) was commonly added to the medium. For murine neutrophils, serum-free medium was used in most cases for stimulation with LPS and CaI, reflecting the different sensitivities of human and murine neutrophils to media supplements. Thus, the choice of media supplements greatly determines the outcome of experiments on NET-formation, which must be taken into account in NETosis research.

Enhanced Neutrophil Extracellular Trap Formation in Acute Pancreatitis Contributes to Disease Severity and Is Reduced by Chloroquine

Background: Neutrophil extracellular traps (NETs) are generated when activated neutrophils, driven by PAD4, release their DNA, histones, HMGB1, and other intracellular granule components. NETs play a role in acute pancreatitis, worsening pancreatic inflammation, and promoting pancreatic duct obstruction. The autophagy inhibitor chloroquine (CQ) inhibits NET formation; therefore, we investigated the impact of CQ mediated NET inhibition in murine models of pancreatitis and human correlative studies.

Methods: L-arginine and choline deficient ethionine supplemented (CDE) diet models of acute pancreatitis were studied in wild type and PAD4^−/− mice, incapable of forming NETs. Isolated neutrophils were stimulated to induce NET formation and visualized with fluorescence microscopy. CQ treatment (0.5 mg/ml PO) was initiated after induction of pancreatitis. Biomarkers of NET formation, including cell-free DNA, citrullinated histone H3 (CitH3), and MPO-DNA conjugates were measured in murine serum and correlative human patient serum samples.

Results: We first confirmed the role of NETs in the pathophysiology of acute pancreatitis by demonstrating that PAD4^−/− mice had decreased pancreatitis severity and improved survival compared to wild-type controls. Furthermore, patients with severe acute pancreatitis had elevated levels of cell-free DNA and MPO-DNA conjugates, consistent with NET formation. Neutrophils from mice with pancreatitis were more prone to NET formation and CQ decreased this propensity to form NETs. CQ significantly reduced serum cell-free DNA and citrullinated histone H3 in murine models of pancreatitis, increasing survival in both models.

Conclusions: Inhibition of NETs with CQ decreases the severity of acute pancreatitis and improves survival. Translating these findings into clinical trials of acute pancreatitis is warranted.

Imaging the neutrophil: Intravital microscopy provides a dynamic view of neutrophil functions in host immunity

Neutrophils are the first cellular responders of the immune system. They employ their impressive arsenal of microbicidal molecules to provide rapid and efficient defense against pathogens. However, the role of neutrophils extends far beyond microbial destruction to include tissue repair and remodeling, provision of signals to the adaptive immune system and body homeostasis. Intravital imaging has allowed the visualization of neutrophils in their native environment in both health and disease and provided crucial insights into their mechanisms of action. In the last few years the power of intravital imaging has been considerably extended by the introduction of photoconvertible proteins and intracellular signaling reporter mice. This review will highlight recent advances in our understanding of neutrophil biology based on the use of intravital microscopy to visualize their modus operandi in vivo including migration in and out of inflamed tissues, host-pathogen interactions and cell fate.

Hemolysis and immune regulation

PURPOSE OF REVIEW

Hemolytic anemias caused by premature destruction of red blood cells occur in many disorders including hemoglobinopathies, autoimmune conditions, during infection or following reaction to drugs or transfusions. Recent studies which will be reviewed here have uncovered several novel mechanisms by which hemolysis can alter immunological functions and increase the risk of severe complications in hemolytic disorders.

RECENT FINDINGS

Plasma-free heme can induce the formation of neutrophil extracellular traps (NETs) through reactive oxygen species signaling. Although NETs protect the host against infections, in patients with sickle disease, they are associated with vaso-occlusive crises. Heme may increase host susceptibility to infections by inducing heme oxygenase 1 (HO-1) in immature neutrophils, thereby inhibiting oxidative burst required for clearance of engulfed bacteria. In addition, heme impairs macrophage phagocytosis and microbial clearance through inhibition of cytoskeletal remodeling. Hemolysis can also favor anti-inflammatory immune cell polarization by inhibiting dendritic cell maturation necessary for effector T-cell responses, inducing differentiation of monocytes into red pulp macrophages, important for iron recycling from senescent erythrocytes, and driving regulatory T-cell expansion through modulation of HO-1 expression in nonclassical monocytes.

SUMMARY

Hemolysis breakdown products show remarkable effects on the regulation of immune cell differentiation and function.

1,25-Dihydroxyvitamin D3 induces formation of neutrophil extracellular trap-like structures and modulates the transcription of genes whose products are neutrophil extracellular trap-associated proteins: A pilot study

Neutrophils are components of the innate immune system that participate in controlling infectious diseases through microbicidal mechanisms such as phagocytosis, degranulation and the release of neutrophil extracellular traps (NETs). NETs are DNA structures that are released through the decondensation and spreading of chromatin and the adherence of various proteins, including neutrophil elastase (NE), myeloperoxidase (MPO) and peptidyl arginine deiminase 4 (PDA4). Since NETs recovered after treatment of activated polymorphonuclear neutrophils can enhance IL-1β and IFN-α production by LPS-activated macrophages, they are thought to be keys to the host's defenses and inflammation. 1,25(OH)₂D₃ has been shown to play an important role in modulating neutrophils activation and in preventing infections. Therefore, the aim of this study was to assess the effect of 1,25(OH)₂D₃ in modulating induction of the release of NETs and in regulating the transcription of genes whose products in human neutrophils are NETs-associated proteins, TLRs and interferon. We observed that 1,25(OH)₂D₃ induced production of NETs-like structures while also upregulating NE/PAD4/COX-3/GAPDH mRNA levels. Additionally, we found an increase in TLR7 and type I interferon (IFN) mRNA levels as a result of neutrophil activation by 1,25(OH)₂D₃. Since the transcription of genes whose products constitute NETs-associated proteins are differentially-regulated by 1,25(OH)₂D₃, we proposed that this might restrict the spread of pathogens, such as virus, by inducing NETs, the expression of TLR7 and secretion of IFN-α. Our results suggest the potential importance of this hormone in preventing infections by inducing NETs formation.

Neutrophil recruitment and activation are differentially dependent on MyD88/TRIF and MAVS signaling during RSV infection

Respiratory syncytial virus (RSV) is a leading cause of severe lower respiratory tract infections, especially in infants. Lung neutrophilia is a hallmark of RSV disease but the mechanism by which neutrophils are recruited and activated is unclear. Here, we investigate the innate immune signaling pathways underlying neutrophil recruitment and activation in RSV-infected mice. We show that MyD88/TRIF signaling is essential for lung neutrophil recruitment while MAVS signaling, leading to type I IFN production, is necessary for neutrophil activation. Consistent with that notion, administration of type I IFNs to the lungs of RSV-infected Mavs^-/- mice partially activates lung neutrophils recruited via the MyD88/TRIF pathway. Conversely, lack of neutrophil recruitment to the lungs of RSV-infected Myd88/Trif^-/- mice can be corrected by administration of chemoattractants and those neutrophils become fully activated. Interestingly, Myd88/Trif^-/- mice did not have increased lung viral loads during RSV infection, suggesting that neutrophils are dispensable for viral control. Thus, two distinct pathogen sensing pathways collaborate for neutrophil recruitment and full activation during RSV infection.

Neutrophil extracellular traps (NETs) are increased in the alveolar spaces of patients with ventilator-associated pneumonia

Background

Neutrophils release neutrophil extracellular traps (NETs) in response to invading pathogens. Although NETs play an important role in host defense against microbial pathogens, they have also been shown to play a contributing mechanistic role in pathologic inflammation in the absence of infection. Although a role for NETs in bacterial pneumonia and acute respiratory distress syndrome (ARDS) is emerging, a comprehensive evaluation of NETs in the alveolar space of critically ill patients has yet to be reported. In this study, we evaluated whether markers of NET formation in mechanically ventilated patients are associated with ventilator-associated pneumonia (VAP).

Methods

We collected bronchoalveolar lavage fluid from 100 critically ill patients undergoing bronchoscopy for clinically suspected VAP. Subjects were categorized by the absence or presence of VAP and further stratified by ARDS status. NETs (myeloperoxidase (MPO)-DNA complexes) and the NET-associated markers peroxidase activity and cell-free DNA were analyzed by enzyme-linked immunosorbent assay and colorimetric assays, respectively. Quantitative polymerase chain reaction of nuclear and mitochondrial DNA was used to determine the origin of the extruded DNA. Interleukin (IL)-8 and calprotectin were assayed as measures of alveolar inflammation and neutrophil activation. Correlations between NETs and markers of neutrophil activation were determined using Spearman’s correlation. We tested for associations with VAP and bacterial burden by logistic and linear regression, respectively, using log₁₀-transformed NETs.

Results

MPO-DNA concentrations were highly correlated with other measures of NET formation in the alveolar space, including cell-free DNA and peroxidase activity (r = 0.95 and r = 0.87, p < 0.0001, respectively). Alveolar concentrations of MPO-DNA were higher in subjects with VAP and ARDS compared with those with ARDS alone (p < 0.0001), and higher MPO-DNA was associated with increased odds of VAP (odds ratio 3.03, p < 0.0001). In addition, NET concentrations were associated with bacterial burden (p < 0.0001) and local alveolar inflammation as measured by IL-8 (r = 0.89, p < 0.0001).

Conclusions

Alveolar NETs measured by MPO-DNA complex are associated with VAP, and markers of NETosis are associated with local inflammation and bacterial burden in the lung. These results suggest that NETs contribute to inflammatory responses involved in the pathogenesis of VAP.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-2290-8) contains supplementary material, which is available to authorized users.

Can uterine secretion of modified histones alter blastocyst implantation, embryo nutrition, and transgenerational phenotype?

Extracellular histones support rodent and human embryo development in at least two ways. First, these molecules in uterine secretions protect embryos from inflammation caused by pathogens that gain access to the reproductive tract. Also, histones in uterine secretions likely support penetration of the uterine epithelium by blastocysts during embryo implantation. Extracellular histones seem to preserve amino acid transport system B0,+ in blastocysts by inhibiting its activity. Preservation of system B0,+ is needed because, at the time of invasion of the uterine epithelium by motile trophoblasts, system B0,+ is likely reactivated to help remove tryptophan from the implantation chamber. If tryptophan is not removed, T-cells proliferate and reject the implanting blastocyst. Epigenetic modification of histones could alter their promotion of normal implantation through, say, incomplete tryptophan removal and, thus, allow partial T-cell rejection of the conceptus. Such partial rejection could impair placental development, embryonal/fetal nutrition, and weight gain prior to birth. Small-for-gestational-age offspring are predisposed to developing metabolic syndrome, obesity, and associated complications as adults. Shifting expression of these phenotypes might contribute to transgenerational variation and evolution. The spectrum of possible extracellular histone targets in early development warrant new research, especially since the effects of epigenetic histone modifications might be transgenerational.

Resistance of Leishmania (Viannia) Panamensis to Meglumine Antimoniate or Miltefosine Modulates Neutrophil Effector Functions

Leishmania (Viannia) panamensis (L. (V.) p.) is the main causative agent of cutaneous leishmaniasis in Colombia and is usually treated with either meglumine antimoniate (MA) or miltefosine (MIL). In recent years, there has been increasing evidence of the emergence of drug-resistance against these compounds. Neutrophils are known to play an important role in immunity against Leishmania. These cells are rapidly recruited upon infection and are also present in chronic lesions. However, their involvement in the outcome of infection with drug-resistant Leishmania has not been examined. In this study, human and murine neutrophils were infected in vitro with MA or MIL drug-resistant L. (V.) p. lines derived from a parental L. (V.) p. drug-susceptible strain. Neutrophil effector functions were assessed analyzing the production of reactive oxygen species (ROS), the formation of neutrophil extracellular trap (NET) and the expression of cell surface activation markers. Parasite killing by neutrophils was assessed using L. (V.) p. transfected with a luciferase reporter. We show here that MA and MIL-resistant L. (V.) p. lines elicited significantly increased NET formation and MA-resistant L. (V.) p. induced significantly increased ROS production in both murine and human neutrophils, compared to infections with the parental MIL and MA susceptible strain. Furthermore, neutrophils exposed to drug-resistant lines showed increased activation, as revealed by decreased expression of CD62L and increased expression of CD66b in human neutrophils yet presented higher survival within neutrophils than the drug-susceptible strain. These results provide evidence that parasite drug-susceptibility may influences neutrophil activation and function as well as parasite survival within neutrophils. Further investigaton of the inter-relationship of drug susceptibility and neutrophil effector function should contribute to better understanding of the factors involved in susceptibility to anti-Leishmania drugs.

Streptococcus Suis Serotype 2 Stimulates Neutrophil Extracellular Traps Formation via Activation of p38 MAPK and ERK1/2

Streptococcus suis serotype 2 is a major pathogen of swine streptococcicosis, which result in serious economic loss worldwide. SS2 is an important zoonosis causing meningitis and even death in humans. Neutrophil extracellular traps (NETs) constitute a significant bactericidal strategy of innate immune. The battle between SS2 and NETs may account for the pathogenicity of SS2. However, the molecular mechanism underlying release of SS2-induced NETs remains unclear. In this study, SS2 was found to induce NETs within 2–4 h, and was dependent on reactive oxygen species (ROS) from NADPH oxidase. Moreover, SS2 could activate neutrophil p38 MAPK and ERK1/2. Blockage of p38 MAPK or ERK1/2 activation decreased SS2-induced NETs formation by 65 and 85%, respectively. In addition, NADPH oxidase derived ROS inhibition negatively affected phosphorylation of p38 MAPK and ERK1/2 in SS2 induced neutrophils. Both TLR2 and TLR4 were significantly up-regulated by SS2 infection in blood cells in vivo and neutrophils in vitro, which indicates these two receptors are involved in SS2 recognition. Blocking TLR4 signaling could further inhibit the activation of ERK1/2, but not p38 MAPK; however, TLR4 signaling inhibition reduced NETs formation induced by SS2. In conclusion, SS2 could be recognized by TLR2 and/or TLR4, initiating NETs formation signaling pathways in a NADPH oxidase derived ROS dependent manner. ROS will activate p38 MAPK and ERK1/2, which ultimately induces NETs formation.

Virulent Pseudorabies Virus Infection Induces a Specific and Lethal Systemic Inflammatory Response in Mice

Pseudorabies virus (PRV) is an alphaherpesvirus that infects the peripheral nervous system (PNS). The natural host of PRV is the swine, but it can infect most mammals, including cattle, rodents, and dogs. In these nonnatural hosts, PRV always causes a severe acute and lethal neuropathy called the "mad itch," which is uncommon in swine. Thus far, the pathophysiological and immunological processes leading to the development of the neuropathic itch and the death of the animal are unclear. Using a footpad inoculation model, we established that mice inoculated with PRV-Becker (virulent strain) develop a severe pruritus in the foot and become moribund at 82 h postinoculation (hpi). We found necrosis and inflammation with a massive neutrophil infiltration only in the footpad and dorsal root ganglia (DRGs) by hematoxylin and eosin staining. PRV load was detected in the foot, PNS, and central nervous system tissues by quantitative reverse transcription-PCR. Infected mice had elevated plasma levels of proinflammatory cytokines (interleukin-6 [IL-6] and granulocyte colony-stimulating factor [G-CSF]) and chemokines (Gro-1 and monocyte chemoattractant protein 1). Significant IL-6 and G-CSF levels were detected in several tissues at 82 hpi. High plasma levels of C-reactive protein confirmed the acute inflammatory response to PRV-Becker infection. Moreover, mice inoculated with PRV-Bartha (attenuated, live vaccine strain) did not develop pruritus at 82 hpi. PRV-Bartha also replicated in the PNS, and the infection spread further in the brain than PRV-Becker. PRV-Bartha infection did not induce the specific and lethal systemic inflammatory response seen with PRV-Becker. Overall, we demonstrated the importance of inflammation in the clinical outcome of PRV infection in mice and provide new insights into the process of PRV-induced neuroinflammation.IMPORTANCE Pseudorabies virus (PRV) is an alphaherpesvirus related to human pathogens such as herpes simplex virus 1 and varicella-zoster virus (VZV). The natural host of PRV is the swine, but it can infect most mammals. In susceptible animals other than pigs, PRV infection always causes a characteristic lethal pruritus known as the "mad itch." The role of the immune response in the clinical outcome of PRV infection is still poorly understood. Here, we show that a systemic host inflammatory response is responsible for the severe pruritus and acute death of mice infected with virulent PRV-Becker but not mice infected with attenuated strain PRV-Bartha. In addition, we identified IL-6 and G-CSF as two main cytokines that play crucial roles in the regulation of this process. Our findings give new insights into neuroinflammatory diseases and strengthen further the similarities between VZV and PRV infections at the level of innate immunity.

Microfluidic Assay Measures Increased Neutrophil Extracellular Traps Circulating in Blood after Burn Injuries

Cell-free DNA (cf-DNA) concentration in human plasma is often increased after burn and trauma injuries. Two major sources of cf-DNA are the parenchymal cells damaged by the injury and various circulating cells indirectly altered by the response to injury. The cf-DNA originating from neutrophils, also known as circulating neutrophil extracellular traps (cNETs), is of notable interest because cNETs have been associated with pathological processes in other conditions, including cancer, autoimmunity, etc. Both intact chromatin and oligonucleotides, which are the by-product of cf-DNA degradation, are assumed to contribute to the cf-DNA in patients. However, traditional assays for cf-DNA quantification do not distinguish between cNETs and cf-DNA of other origins and do not differentiate between intact chromatin and oligonucleotides. Here we measure the amount of intact cNETs in the circulation, using a microfluidic device that mechanically traps chromatin fibers directly from blood and an immunofluorescence protocol that detects neutrophil-specific proteins associated with chromatin. In a rat model of burn injury, we determined that the chromatin fibers in the circulation after injury originate exclusively from neutrophils and are cNETs. We found that the concentration of cNETs surges the first day after injury and then decreases slowly over several days. In a secondary sepsis model, which involved a burn injury followed by cecal-ligation-puncture, we measured additional increases in cNETs in the days after sepsis was induced. These results validate a microfluidic assay for the quantification of cNETs and will facilitate fruther studies probing the contribution of cNETs to complications after burns and sepsis.

Mechanisms and disease relevance of neutrophil extracellular trap formation

While the microscopic appearance of neutrophil extracellular traps (NETs) has fascinated basic researchers since its discovery, the (patho)physiological mechanisms triggering NET release, the disease relevance and clinical translatability of this unconventional cellular mechanism remained poorly understood. Here, we summarize and discuss current concepts of the mechanisms and disease relevance of NET formation.

The Role of Leukocytes in Diabetic Cardiomyopathy

Diabetes is predominant risk factor for cardiovascular diseases such as myocardial infarction and heart failure. Recently, leukocytes, particularly neutrophils, macrophages, and lymphocytes, have become targets of investigation for their potential role in a number of chronic inflammatory diseases such as diabetes and heart failure. While leukocytes contribute significantly to the progression of diabetes and heart failure individually, understanding their participation in the pathogenesis of diabetic heart failure is much less understood. The present review summarizes the role of leukocytes in the complex interplay between diabetes and heart failure, which is critical to the discovery of new targeted therapies for diabetic cardiomyopathy.

Roles of the immune system in cancer: from tumor initiation to metastatic progression

In this review, Gonzelez et al. provide an update of recent accomplishments, unifying concepts, and futures challenges to study tumor-associated immune cells, with an emphasis on metastatic carcinomas.

The presence of inflammatory immune cells in human tumors raises a fundamental question in oncology: How do cancer cells avoid the destruction by immune attack? In principle, tumor development can be controlled by cytotoxic innate and adaptive immune cells; however, as the tumor develops from neoplastic tissue to clinically detectable tumors, cancer cells evolve different mechanisms that mimic peripheral immune tolerance in order to avoid tumoricidal attack. Here, we provide an update of recent accomplishments, unifying concepts, and future challenges to study tumor-associated immune cells, with an emphasis on metastatic carcinomas.

A Secreted Bacterial Peptidylarginine Deiminase Can Neutralize Human Innate Immune Defenses

The keystone oral pathogen Porphyromonas gingivalis is associated with severe periodontitis. Intriguingly, this bacterium is known to secrete large amounts of an enzyme that converts peptidylarginine into citrulline residues. The present study was aimed at identifying possible functions of this citrullinating enzyme, named Porphyromonas peptidylarginine deiminase (PPAD), in the periodontal environment. The results show that PPAD is detectable in the gingiva of patients with periodontitis, and that it literally neutralizes human innate immune defenses at three distinct levels, namely bacterial phagocytosis, capture in neutrophil extracellular traps (NETs), and killing by the lysozyme-derived cationic antimicrobial peptide LP9. As shown by mass spectrometry, exposure of neutrophils to PPAD-proficient bacteria reduces the levels of neutrophil proteins involved in phagocytosis and the bactericidal histone H2. Further, PPAD is shown to citrullinate the histone H3, thereby facilitating the bacterial escape from NETs. Last, PPAD is shown to citrullinate LP9, thereby restricting its antimicrobial activity. The importance of PPAD for immune evasion is corroborated in the infection model Galleria mellonella, which only possesses an innate immune system. Together, the present observations show that PPAD-catalyzed protein citrullination defuses innate immune responses in the oral cavity, and that the citrullinating enzyme of P. gingivalis represents a new type of bacterial immune evasion factor.IMPORTANCE Bacterial pathogens do not only succeed in breaking the barriers that protect humans from infection, but they also manage to evade insults from the human immune system. The importance of the present study resides in the fact that protein citrullination is shown to represent a new bacterial mechanism for immune evasion. In particular, the oral pathogen P. gingivalis employs this mechanism to defuse innate immune responses by secreting a protein-citrullinating enzyme. Of note, this finding impacts not only the global health problem of periodontitis, but it also extends to the prevalent autoimmune disease rheumatoid arthritis, which has been strongly associated with periodontitis, PPAD activity, and loss of tolerance against citrullinated proteins, such as the histone H3.

Dead or alive: how the immune system detects microbial viability

Immune detection of microbial viability is increasingly recognized as a potent driver of innate and adaptive immune responses. Here we describe recent mechanistic insights into the process of how the immune system discriminates between viable and non-viable microbial matter. Accumulating evidence suggests a key role for microbial RNA as a widely conserved viability associated PAMP (vita-PAMP) and a molecular signal of increased infectious threat. Toll-like receptor 8 (TLR8) has recently emerged as a critical sensor for viable bacteria, ssRNA viruses, and archaea in human antigen presenting cells (APC). We discuss the role of microbial RNA, and other potential vita-PAMPs in antimicrobial immunity and vaccine responses.

Programmed Necrosis and Disease:We interrupt your regular programming to bring you necroinflammation

Compared to the tidy and immunologically silent death during apoptosis, necrosis seems like a chaotic and unorganized demise. However, we now recognize that there is a method to its madness, as many forms of necrotic cell death are indeed programmed and function beyond lytic cell death to support homeostasis and immunity. Inherently more immunogenic than their apoptotic counterpart, programmed necrosis, such as necroptosis, pyroptosis, ferroptosis, and NETosis, releases inflammatory cytokines and danger-associated molecular patterns (DAMPs), skewing the milieu to a pro-inflammatory state. Moreover, impaired clearance of dead cells often leads to inflammation. Importantly, these pathways have all been implicated in inflammatory and autoimmune diseases, therefore careful understanding of their molecular mechanisms can have long lasting effects on how we interpret their role in disease and how we translate these mechanisms into therapy.

Platelets, NETs and cancer

In addition to the central role of platelets in hemostasis, they contribute to pathological conditions such as inflammation and tumor progression. Aberrant expression and/or exposure of pro-coagulant factors in the tumor microenvironment induce platelet activation and subsequent release of growth factors from platelet granules. Cancer patients are commonly affected by thrombotic events, as a result of tumor-induced platelet activation. A novel player potentially contributing to cancer-associated thrombosis is the formation of neutrophil extracellular traps (NETs). NETs are composed of externalized DNA of nuclear or mitochondrial origin, bound to histones and granular proteases such as neutrophil elastase (NE) and myeloperoxidase (MPO). These extracellular traps help neutrophils to catch and kill pathogens such as bacteria, virus and fungi. It is now clear that NETs form also under conditions of sterile inflammation such as cancer and autoimmunity and can promote thrombosis. Recent data show that platelets play a key role in determining when and where NETs should form. This review will highlight our current insight in the role of platelets as regulators of NET formation, both during infection and sterile inflammation.

Evidence for a direct link between PAD4-mediated citrullination and the oxidative burst in human neutrophils

Neutrophils are critical for the defense against pathogens, in part through the extrusion of extracellular DNA traps, phagocytosis, and the production of reactive oxygen species. Neutrophils may also play an important role in the pathogenesis of rheumatoid arthritis (RA) through the activation of protein arginine deiminases (PADs) that citrullinate proteins that subsequently act as autoantigens. We report that PAD4 is physically associated with the cytosolic subunits of the oxidative burst machinery, p47^phox (also known as neutrophil cytosol factor 1, NCF1) and p67^phox (NCF2). Activation of PAD4 by membranolytic insults that result in high levels of intracellular calcium (higher than physiological neutrophil activation) leads to rapid citrullination of p47^phox/NCF1 and p67^phox/NCF2, as well as their dissociation from PAD4. This dissociation prevents the assembly of an active NADPH oxidase complex and an oxidative burst in neutrophils stimulated by phorbol-ester or immune complexes. In further support of a substrate-to-inactive enzyme interaction, small-molecule PAD inhibitors also disrupt the PAD4-NCF complex and reduce oxidase activation and phagocytic killing of Staphylococcus aureus. This novel role of PAD4 in the regulation of neutrophil physiology suggests that targeting PAD4 with active site inhibitors for the treatment of RA may have a broader impact on neutrophil biology than just inhibition of citrullination.

Impaired neutrophil extracellular traps and inflammatory responses in the peritoneal fluid of patients with liver cirrhosis

Liver cirrhosis (LC) is an inflammatory process associated with impaired functions in adaptive and innate immune responses at both systemic and local levels, also referred as Cirrhosis-Associated Immune Dysfunction. In this study, we evaluated the functionality of neutrophils from ascitic fluid (AF) of patients with hepatic cirrhosis by testing their ability to generate neutrophil extracellular traps (NETs) in vitro. To further determine the activation state of neutrophils, expression of the activation markers CD66b, CD69, and CD80 on these cells was analysed by flow cytometry. The inflammatory environment in AF was assessed by measured concentration of pro- and anti-inflammatory cytokines. Samples were collected from 40 patients with LC, 20 of them with uncomplicated ascites (ASC) and 20 with spontaneous bacterial peritonitis (SBP). Peripheral blood (PB) neutrophils from healthy individuals were used as control (HC). Our results revealed a significant decrease in the release of NETs in neutrophils from the SBP group compared with HC. Low expression of CD69 and CD80 on neutrophils from AF of SBP patients was also observed. Comparisons of inflammatory cytokine levels in AF from the different study groups (SBP and ASC) revealed significant differences. In conclusion, we demonstrate that the development of complications, such as SBP, increases initially the inflammatory status, but chronically results in impaired neutrophil function as demonstrated by the decreased capability of NETs formation. There is also an increase in both pro-inflammatory and anti-inflammatory cytokines, thus predisposing for new episodes of SPB and increasing morbidity and mortality in cirrhotic patients.

Recent advances on the crosstalk between neutrophils and B or T lymphocytes

An increasing body of literature supports a role for neutrophils as players in the orchestration of adaptive immunity. During acute and chronic inflammatory conditions, neutrophils rapidly migrate not only to sites of inflammation, but also to draining lymph nodes and spleen, where they engage bidirectional interactions with B- and T-lymphocyte subsets. Accordingly, a relevant role of neutrophils in modulating B-cell responses under homeostatic conditions has recently emerged. Moreover, specialized immunoregulatory properties towards B or T cells acquired by distinct neutrophil populations, originating under pathological conditions, have been consistently described. In this article, we summarize the most recent data from human studies and murine models on the ability of neutrophils to modulate adaptive immune responses under physiological and pathological conditions and the mechanisms behind these processes.

Characterization and comparison of neutrophil extracellular traps in gingival samples of periodontitis and gingivitis: A pilot study

BACKGROUND AND OBJECTIVE

Neutrophil extracellular traps (NETs) are a recently discovered antimicrobial mechanism used by neutrophils that have been proposed as an intervention in the pathogenesis of periodontitis. The objective of our study was to characterize the expression of NETs in gingival tissues with periodontitis and controls and to compare the expression of these traps in gingival tissue samples of patients with gingivitis and periodontitis.

MATERIAL AND METHODS

An observational cross-sectional study was conducted on patients with periodontitis, gingivitis, and controls that needed tooth extraction. Gingival tissue biopsies were gathered after clinical examination and tooth extraction. Electron microscopy and immunofluorescence were performed to characterize NETs, comparing periodontitis and control patients. Immunohistochemical analysis was performed to quantify neutrophil extracellular trap expression through extracellular citrullinated histone H3 and myeloperoxidase in biopsies from patients with gingivitis and periodontitis.

RESULTS

Thirteen biopsies were gathered from 13 patients: five gingivitis, six periodontitis, and two controls. Electron microscopy and immunofluorescence imaging showed greater expression of neutrophils present in periodontal inflamed tissue compared with controls. Release of nuclear content to the extracellular space was observed, compatible with the formation of NETs. The expression of citrullinated histone H3 was higher in gingivitis samples than periodontitis samples (P = 0.0106). Myeloperoxidase expression was higher in periodontitis than gingivitis, but without achieving statistical significance.

CONCLUSION

Neutrophil extracellular traps were found in tissue samples of periodontitis as extracellular components of chromatin, along with neutrophil enzymes, that were not present in healthy controls. The comparison of NETs expression in periodontitis and gingivitis showed higher expression in gingivitis, associating them to acute phases of the periodontal inflammatory process.

Mast cells and neutrophils mediate peripheral motor pathway degeneration in ALS

Neuroinflammation is a recognized pathogenic mechanism underlying motor neuron degeneration in amyotrophic lateral sclerosis (ALS), but the inflammatory mechanisms influencing peripheral motor axon degeneration remain largely unknown. A recent report showed a pathogenic role for c-Kit-expressing mast cells mediating inflammation and neuromuscular junction denervation in muscles from SOD1G93A rats. Here, we have explored whether mast cells infiltrate skeletal muscles in autopsied muscles from ALS patients. We report that degranulating mast cells were abundant in the quadriceps muscles from ALS subjects but not in controls. Mast cells were associated with myofibers and motor endplates and, remarkably, interacted with neutrophils forming large extracellular traps. Mast cells and neutrophils were also abundant around motor axons in the extensor digitorum longus muscle, sciatic nerve, and ventral roots of symptomatic SOD1G93A rats, indicating that immune cell infiltration extends along the entire peripheral motor pathway. Postparalysis treatment of SOD1G93A rats with the tyrosine kinase inhibitor drug masitinib prevented mast cell and neutrophil infiltration, axonal pathology, secondary demyelination, and the loss of type 2B myofibers, compared with vehicle-treated rats. These findings provide further evidence for a yet unrecognized contribution of immune cells in peripheral motor pathway degeneration that can be therapeutically targeted by tyrosine kinase inhibitors.

Update on Neutrophil Function in Severe Inflammation

Neutrophils are main players in the effector phase of the host defense against micro-organisms and have a major role in the innate immune response. Neutrophils show phenotypic heterogeneity and functional flexibility, which highlight their importance in regulation of immune function. However, neutrophils can play a dual role and besides their antimicrobial function, deregulation of neutrophils and their hyperactivity can lead to tissue damage in severe inflammation or trauma. Neutrophils also have an important role in the modulation of the immune system in response to severe injury and trauma. In this review we will provide an overview of the current understanding of neutrophil subpopulations and their function during and post-infection and discuss the possible mechanisms of immune modulation by neutrophils in severe inflammation.

Neutrophil extracellular traps promote inflammation and development of hepatocellular carcinoma in nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a progressive, inflammatory form of fatty liver disease. It is the most rapidly rising risk factor for the development of hepatocellular carcinoma (HCC), which can arise in NASH with or without cirrhosis. The inflammatory signals promoting the progression of NASH to HCC remain largely unknown. The propensity of neutrophils to expel decondensed chromatin embedded with inflammatory proteins, known as neutrophil extracellular traps (NETs), has been shown to be important in chronic inflammatory conditions and in cancer progression. In this study, we asked whether NET formation occurs in NASH and contributes to the progression of HCC. We found elevated levels of a NET marker in serum of patients with NASH. In livers from STAM mice (NASH induced by neonatal streptozotocin and high-fat diet), early neutrophil infiltration and NET formation were seen, followed by an influx of monocyte-derived macrophages, production of inflammatory cytokines, and progression of HCC. Inhibiting NET formation, through treatment with deoxyribonuclease (DNase) or using mice knocked out for peptidyl arginine deaminase type IV (PAD4^-/- ), did not affect the development of a fatty liver but altered the consequent pattern of liver inflammation, which ultimately resulted in decreased tumor growth. Mechanistically, we found that commonly elevated free fatty acids stimulate NET formation in vitro.

CONCLUSION

Our findings implicate NETs in the protumorigenic inflammatory environment in NASH, suggesting that their elimination may reduce the progression of liver cancer in NASH. (Hepatology 2018).

Cooperative PSGL-1 and CXCR2 signaling in neutrophils promotes deep vein thrombosis in mice

Inflammation is a major contributor to deep vein thrombosis (DVT). Flow restriction of the inferior vena cava (IVC) in mice induces DVT like that in humans. In this model, P-selectin-dependent adhesion of neutrophils and monocytes leads to release of neutrophil extracellular traps (NETs) and expression of tissue factor. However, it is not known what signals cause myeloid cells to generate these procoagulant effectors. Using ultrasonography and spinning-disk intravital microscopy in genetically engineered mice, we found that engagement of P-selectin glycoprotein ligand-1 (PSGL-1) and the chemokine receptor CXCR2 on rolling neutrophils propagated signals that cooperated to induce β2 integrin-dependent arrest in flow-restricted IVCs. Unlike previous reports, PSGL-1 signaling in neutrophils did not require L-selectin, and it used tyrosine 145 rather than tyrosines 112 and 128 on the adaptor Src homology domain-containing leukocyte phosphoprotein of 76 kDa. PSGL-1 and CXCR2 signaling cooperated to increase the frequency and size of thrombi, in part by stimulating release of NETs. Unlike in neutrophils, blocking PSGL-1 or CXCR2 signaling in monocytes did not affect their recruitment into thrombi or their expression of tissue factor. Our results demonstrate that neutrophils cooperatively signal through PSGL-1 and CXCR2 to promote DVT.

Recent advances in understanding rhinovirus immunity

Rhinoviruses are the most common cause of upper respiratory tract infections. However, they can induce exacerbations of chronic obstructive pulmonary disease and asthma, bronchiolitis in infants, and significant lower respiratory tract infections in children, the immunosuppressed, and the elderly. The large number of rhinovirus strains (currently about 160) and their antigenic diversity are significant obstacles in vaccine development. The phenotype of immune responses induced during rhinovirus infection can affect disease severity. Recognition of rhinovirus and a balance of innate responses are important factors in rhinovirus-induced morbidity. Immune responses to rhinovirus infections in healthy individuals are typically of the T helper type 1 (Th1) phenotype. However, rhinovirus-driven asthma exacerbations are additionally characterised by an amplified Th2 immune response and airway neutrophilia. This commentary focuses on recent advances in understanding immunity toward rhinovirus infection and how innate and adaptive immune responses drive rhinovirus-induced asthma exacerbations.