Induction of genes mediating interferon-dependent extracellular trap formation during neutrophil differentiation

Dirofilaria immitis Microfilariae and Third-Stage Larvae Induce Canine NETosis Resulting in Different Types of Neutrophil Extracellular Traps

Heartworm disease is a zoonotic vector-borne disease caused by Dirofilaria immitis mainly affecting canids. Infectious third-stage larvae (L3) are transmitted to the definitive hosts via culicid mosquitoes; adult nematodes reside in the pulmonary arteries and in the right heart releasing unsheathed first-stage larvae (microfilariae) into the bloodstream leading to chronic and sometimes fatal disease. So far, early innate immune reactions triggered by these different D. immitis stages in the canine host have scarcely been investigated. Therefore, D. immitis microfilariae and L3 were analyzed for their capacity to induce neutrophil extracellular traps (NETs) in canine polymorphonuclear neutrophils (PMN). Overall, scanning electron microscopy analysis revealed both larval stages as strong inducers of canine NETosis. Co-localization of PMN-derived extracellular DNA with granulocytic histones, neutrophil elastase, or myeloperoxidase in parasite-entrapping structures confirmed the classical characteristics of NETosis. Quantitative analyses showed that both larval stages triggered canine NETs in a time-dependent but dose-independent manner. Moreover, parasite-induced NET formation was not influenced by the parasites viability since heat-inactivated microfilariae and L3 also induced NETs. In addition, parasite/PMN confrontation promoted significant entrapment but not killing of microfilariae and L3. Both, NETosis and larval entrapment was significantly reversed via DNase I treatments while treatments with the NADPH oxidase inhibitor diphenyleneiodonium failed to significantly influence these reactions. Interestingly, different types of NETs were induced by microfilariae and L3 since microfilarial stages merely induced spread and diffuse NETs while the larger L3 additionally triggered aggregated NET formation.

Exogenous Stimulation of Type I Interferon Protects Mice with Chronic Granulomatous Disease from Aspergillosis through Early Recruitment of Host-Protective Neutrophils into the Lung

Invasive aspergillosis (IA) remains the primary cause of morbidity and mortality in chronic granulomatous disease (CGD) patients, often due to infection by Aspergillus species refractory to antifungals. This motivates the search for alternative treatments, including immunotherapy. We investigated the effect of exogenous type I interferon (IFN) activation on the outcome of IA caused by three Aspergillus species, A. fumigatus, A. nidulans, and A. tanneri, in CGD mice. The animals were treated with poly(I):poly(C) carboxymethyl cellulose poly-l-lysine (PICLC), a mimetic of double-stranded RNA, 24 h preinfection and postinfection. The survival rates and lung fungal burdens were markedly improved by PICLC immunotherapy in animals infected with any one of the three Aspergillus species. While protection from IA was remarkable, PICLC induction of type I IFN in the lungs surged 24 h posttreatment and returned to baseline levels by 48 h, suggesting that PICLC altered early events in protection against IA. Immunophenotyping of recruited leukocytes and histopathological examination of tissue sections showed that PICLC induced similar cellular infiltrates as those in untreated-infected mice, in both cases dominated by monocytic cells and neutrophils. However, the PICLC immunotherapy resulted in a marked earlier recruitment of the leukocytes. Unlike with conidia, infection with A. nidulans germlings reduced the protective effect of PICLC immunotherapy. Additionally, antibody depletion of neutrophils totally reversed the protection, suggesting that neutrophils are crucial for PICLC-mediated protection. Together, these data show that prophylactic PICLC immunotherapy prerecruits these cells, enabling them to attack the conidia and thus resulting in a profound protection from IA.IMPORTANCE Patients with chronic granulomatous disease (CGD) are highly susceptible to invasive aspergillosis (IA). While Aspergillus fumigatus is the most-studied Aspergillus species, CGD patients often suffer IA caused by A. nidulans, A. tanneri, and other rare species. These non-fumigatus Aspergillus species are more resistant to antifungal drugs and cause higher fatality rates than A. fumigatus Therefore, alternative therapies are needed to protect CGD patients. We report an effective immunotherapy of mice infected with three Aspergillus species via PICLC dosing. While protection from IA was long lasting, PICLC induction of type I IFN surged but quickly returned to baseline levels, suggesting that PICLC was altering early events in IA. Interestingly, we found responding immune cells to be similar between PICLC-treated and untreated-infected mice. However, PICLC immunotherapy resulted in an earlier recruitment of the leukocytes and suppressed fungal growth. This study highlights the value of type I IFN induction in CGD patients.

NETosis, complement, and coagulation: a triangular relationship

NETosis is a regulated form of neutrophil cell death that contributes to the host defense against pathogens and was linked to various diseases soon after its first description in 2004. During NETosis, neutrophils release neutrophil extracellular traps (NETs), which can capture and kill bacteria and other pathogens to prevent them from spreading. Although substantial progress has been made in our understanding of NETosis, the precise mechanism underlying NETosis is still a matter of debate. Research continues to elucidate the molecular pathways involved in NETosis. In recent years, interactions with the complement and coagulation systems have become increasingly apparent. Activated complement proteins can stimulate NET formation, and NETs, in turn, can serve as a platform for complement activation. In addition, NETs can act as a scaffold for thrombus formation during coagulation. While crosstalk between the coagulation and complement systems has been previously described, NETosis appears to be a third important player in this consortium to protect the host against pathogens. This review summarizes our current knowledge on the mutual interactions between NETosis, the complement system and the coagulation system, with an emerging description of their complex triangular relationship.

E-Cigarette Use Causes a Unique Innate Immune Response in the Lung, Involving Increased Neutrophilic Activation and Altered Mucin Secretion

RATIONALE

E-cigarettes have become increasingly popular and little is known about their potential adverse health effects.

OBJECTIVES

To determine the effects of e-cigarette use on the airways.

METHODS

Induced sputum samples from cigarette smokers, e-cigarette users, and nonsmokers were analyzed by quantitative proteomics, and the total and individual concentrations of mucins MUC5AC and MUC5B were determined by light scattering/refractometry and labeled mass spectrometry, respectively. Neutrophil extracellular trap (NET) formation rates were also determined for the same groups.

MEASUREMENTS AND MAIN RESULTS

E-cigarette users exhibited significant increases in aldehyde-detoxification and oxidative stress-related proteins associated with cigarette smoke compared with nonsmokers. The levels of innate defense proteins associated with chronic obstructive pulmonary disease, such as elastase and matrix metalloproteinase-9, were significantly elevated in e-cigarette users as well. E-cigarette users' sputum also uniquely exhibited significant increases in neutrophil granulocyte-related and NET-related proteins, such as myeloperoxidase, azurocidin, and protein-arginine deiminase 4, despite no significant elevation in neutrophil cell counts. Peripheral neutrophils from e-cigarette users showed increased susceptibility to phorbol 12-myristate 13-acetate-induced NETosis. Finally, a compositional change in the gel-forming building blocks of airway mucus (i.e., an elevated concentration of mucin MUC5AC) was observed in both cigarette smokers and e-cigarette users.

CONCLUSIONS

Together, our results indicate that e-cigarette use alters the profile of innate defense proteins in airway secretions, inducing similar and unique changes relative to cigarette smoking. These data challenge the concept that e-cigarettes are a healthier alternative to cigarettes.

Age is the work of art? Impact of neutrophil and organism age on neutrophil extracellular trap formation

Neutrophil extracellular traps or NETs are released by highly activated neutrophils in response to infectious agents, sterile inflammation, autoimmune stimuli and cancer. In the cells, the nuclear envelop disintegrates and decondensation of chromatin occurs that depends on peptidylarginine deiminase 4 (PAD4) and neutrophil elastase (NE). Subsequently, proteins from neutrophil granules (e.g., NE, lactoferrin and myeloperoxidase) and the nucleus (histones) bind to decondensed DNA and the whole structure is ejected from the cell. The DNA decorated with potent antimicrobials and proteases can act to contain dissemination of infection and in sterile inflammation NETs were shown to degrade cytokines and chemokines via serine proteases. On the other hand, overproduction of NETs, or their inadequate removal and prolonged presence in vasculature or tissues, can lead to bystander damage or even initiation of diseases. Considering the pros and cons of NET formation, it is of relevance if the stage of neutrophil maturation (immature, mature and senescent cells) affects the capacity to produce NETs as the cells of different age-related phenotypes dominate in given (pathological) conditions. Moreover, the immune system of neonates and elderly individuals is weaker than in adulthood. Is the same pattern followed when it comes to NETs? The overall importance of individual and neutrophil age on the capacity to release NETs is reviewed in detail and the significance of these facts is discussed.

Bottlenose dolphins (Tursiops truncatus) do also cast neutrophil extracellular traps against the apicomplexan parasite Neospora caninum

Neutrophil extracellular traps (NETs) are web-like structures composed of nuclear DNA decorated with histones and cytoplasmic peptides which antiparasitic properties have not previously been investigated in cetaceans. Polymorphonuclear neutrophils (PMN) were isolated from healthy bottlenose dolphins (Tursiops truncatus), and stimulated with Neospora caninum tachyzoites and the NETs-agonist zymosan. In vitro interactions of PMN with the tachyzoites resulted in rapid extrusion of NETs. For the demonstration and quantification of cetacean NETs, extracellular DNA was stained by using either Sytox Orange® or Pico Green®. Scanning electron microscopy (SEM) and fluorescence analyses demonstrated PMN-derived release of NETs upon exposure to tachyzoites of N. caninum. Co-localization studies of N. caninum induced cetacean NETs proved the presence of DNA adorned with histones (H1, H2A/H2B, H3, H4), neutrophil elastase (NE), myeloperoxidase (MPO) and pentraxin (PTX) confirming the molecular properties of mammalian NETosis. Dolphin-derived N. caninum-NETosis were efficiently suppressed by DNase I and diphenyleneiodonium (DPI) treatments. Our results indicate that cetacean-derived NETs represent an ancient, conserved and relevant defense effector mechanism of the host innate immune system against N. caninum and probably other related neozoan parasites circulating in the marine environment.

Type I interferon-mediated autoimmune diseases: pathogenesis, diagnosis and targeted therapy

Type I interferons (IFN-Is) are a group of molecules with pleiotropic effects on the immune system forming a crucial link between innate and adaptive immune responses. Apart from their important role in antiviral immunity, IFN-Is are increasingly recognized as key players in autoimmune CTDs such as SLE. Novel therapies that target IFN-I appear effective in SLE in early trials, but effectiveness is related to the presence of IFN-I biomarkers. IFN-I biomarkers may also act as positive or negative predictors of response to other biologics. Despite the high failure rate of clinical trials in SLE, subgroups of patients often respond better. Fully optimizing the potential of these agents is therefore likely to require stratification of patients using IFN-I and other biomarkers. This suggests the unified concept of type I IFN-mediated autoimmune diseases as a grouping including patients with a variety of different traditional diagnoses.

Neutrophils in Homeostasis, Immunity, and Cancer

Neutrophils were among the first leukocytes described and visualized by early immunologists. Prominent effector functions during infection and sterile inflammation classically placed them low in the immune tree as rapid, mindless aggressors with poor regulatory functions. This view is currently under reassessment as we uncover new aspects of their life cycle and identify transcriptional and phenotypic diversity that endows them with regulatory properties that extend beyond their lifetime in the circulation. These properties are revealing unanticipated roles for neutrophils in supporting homeostasis, as well as complex disease states such as cancer. We focus this review on these emerging functions in order to define the true roles of neutrophils in homeostasis, immunity, and disease.

Neutrophil extracellular traps (NETs) in autoimmune diseases: A comprehensive review

Neutrophil extracellular traps (NETs) are fibrous networks which protrude from the membranes of activated neutrophils. NETs are found in a variety of conditions such as infection, malignancy, atherosclerosis, and autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), anti-neutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV), psoriasis, and gout. Studies suggest that an imbalance between "NETosis," which is a process by which NETs are formed, and NET degradation may be associated with autoimmune diseases. Neutrophils, interleukin-8, ANCA and other inflammatory molecules are considered to play a key role in NET formation. Prolonged exposure to NETs-related cascades is associated with autoimmunity and increases the chance of systemic organ damage. In this review, we discuss the roles of various inflammatory molecules in relation to NETs. We also describe the role of NETs in the pathogenesis of autoimmune diseases and discuss the possibility of using targeted therapies directed to NETs and associated molecules to treat autoimmune diseases.

Immune cell profiling in cancer: molecular approaches to cell-specific identification

The immune system has many important regulatory roles in cancer development and progression. Given the emergence of effective immune therapies against many cancers, reliable predictors of response are needed. One method of determining response is by evaluating immune cell populations from treated and untreated tumor samples. The amount of material obtained from tumor biopsies can be limited; therefore, gene-based or protein-based analyses may be attractive because they require minimal tissue. Cell-specific signatures are being analyzed with use of the latest technologies, including NanoString’s nCounter technology, intracellular staining flow cytometry, cytometry by time-of-flight, RNA-Seq, and barcoding antibody-based protein arrays. These signatures provide information about the contributions of specific types of immune cells to bulk tumor samples. To date, both tumor tissue and immune cells have been analyzed for molecular expression profiles that can assess genes and proteins that are specific to immune cells, yielding results of varying specificity. Here, we discuss the importance of profiling tumor tissue and immune cells to identify immune-cell-associated genes and proteins and specific gene profiles of immune cells. We also discuss the use of these signatures in cancer treatment and the challenges faced in molecular expression profiling of immune cell populations.

Persistent Immune Stimulation Exacerbates Genetically Driven Myeloproliferative Disorders via Stromal Remodeling

Systemic immune stimulation has been associated with increased risk of myeloid malignancies, but the pathogenic link is unknown. We demonstrate in animal models that experimental systemic immune activation alters the bone marrow stromal microenvironment, disarranging extracellular matrix (ECM) microarchitecture, with downregulation of secreted protein acidic and rich in cysteine (SPARC) and collagen-I and induction of complement activation. These changes were accompanied by a decrease in Treg frequency and by an increase in activated effector T cells. Under these conditions, hematopoietic precursors harboring nucleophosmin-1 (NPM1) mutation generated myeloid cells unfit for normal hematopoiesis but prone to immunogenic death, leading to neutrophil extracellular trap (NET) formation. NET fostered the progression of the indolent NPM1-driven myeloproliferation toward an exacerbated and proliferative dysplastic phenotype. Enrichment in NET structures was found in the bone marrow of patients with autoimmune disorders and in NPM1-mutated acute myelogenous leukemia (AML) patients. Genes involved in NET formation in the animal model were used to design a NET-related inflammatory gene signature for human myeloid malignancies. This signature identified two AML subsets with different genetic complexity and different enrichment in NPM1 mutation and predicted the response to immunomodulatory drugs. Our results indicate that stromal/ECM changes and priming of bone marrow NETosis by systemic inflammatory conditions can complement genetic and epigenetic events towards the development and progression of myeloid malignancy. Cancer Res; 77(13); 3685-99. ©2017 AACR.

Molecular analyses on Neospora caninum-triggered NETosis in the caprine system

Neospora caninum is an obligate intracellular protozoan parasite causing serious reproductive disorders in large and small ruminants worldwide. Polymorphonuclear neutrophils (PMN) react against multiple invading pathogens through different mechanisms including the release of neutrophil extracellular traps (NETs). Here, in vitro interactions of caprine PMN and N. caninum tachyzoites were studied. Scanning electron microscopic- and immunofluorescence-analyses demonstrated that caprine PMN undergo NETosis upon contact with tachyzoites of N. caninum, extruding filaments that entrap parasites. Detailed co-localization studies of N. caninum tachyzoite-induced NETs revealed the presence of PMN-derived DNA being decorated with histones (H1, H2A/H2B, H3,H4) and neutrophil elastase (NE) corroborating the molecular characteristics of classical mammalian NETs. As a new result for parasite-induced NETosis, we identified pentraxin and cathepsin B in N. caninum-triggered NETs. Nonetheless, functional inhibition assays revealed that during caprine NET formation triggered by N. caninum different molecular signaling pathways are induced, when compared to other apicomplexan parasites or host species. As such, N. caninum-induced NETosis appears to be influenced by MPO but independent of NADPH oxidase, SOCE, ERK1/2 and p38 MAPK activities. Furthermore, the inhibition of PMN autophagy via blockage of the PI3K-mediated signaling pathway failed to influence tachyzoite-induced NETosis. Since N. caninum-tachyzoites induced caprine NETosis, this effector mechanism should be considered as an early host immune response during acute caprine neosporosis.

Re-Examining Neutrophil Participation in GN

Significant advances in understanding the pathogenesis of GN have occurred in recent decades. Among those advances is the finding that both innate and adaptive immune cells contribute to the development of GN. Neutrophils were recognized as key contributors in early animal models of GN, at a time when the prevailing view considered neutrophils to function as nonspecific effector cells that die quickly after performing antimicrobial functions. However, advances over the past two decades have shown that neutrophil functions are more complex and sophisticated. Specifically, research has revealed that neutrophil survival is regulated by the inflammatory milieu and that neutrophils demonstrate plasticity, mediate microbial killing through previously unrecognized mechanisms, demonstrate transcriptional activity leading to the release of cytokines and chemokines, interact with and regulate cells of the innate and adaptive immune systems, and contribute to the resolution of inflammation. Therefore, neutrophil participation in glomerular diseases deserves re-evaluation. In this review, we describe advances in understanding classic neutrophil functions, review the expanded roles of neutrophils in innate and adaptive immune responses, and summarize current knowledge of neutrophil contributions to GN.

A Role for Neutrophils in Viral Respiratory Disease

Neutrophils are immune cells that are well known to be present during many types of lung diseases associated with acute respiratory distress syndrome (ARDS) and may contribute to acute lung injury. Neutrophils are poorly studied with respect to viral infection, and specifically to respiratory viral disease. Influenza A virus (IAV) infection is the cause of a respiratory disease that poses a significant global public health concern. Influenza disease presents as a relatively mild and self-limiting although highly pathogenic forms exist. Neutrophils increase in the respiratory tract during infection with mild seasonal IAV, moderate and severe epidemic IAV infection, and emerging highly pathogenic avian influenza (HPAI). During severe influenza pneumonia and HPAI infection, the number of neutrophils in the lower respiratory tract is correlated with disease severity. Thus, comparative analyses of the relationship between IAV infection and neutrophils provide insights into the relative contribution of host and viral factors that contribute to disease severity. Herein, we review the contribution of neutrophils to IAV disease pathogenesis and to other respiratory virus infections.

Neutrophil Extracellular Traps in Atherosclerosis and Atherothrombosis

Neutrophil extracellular traps expelled from suicidal neutrophils comprise a complex structure of nuclear chromatin and proteins of nuclear, granular, and cytosolic origin. These net-like structures have also been detected in atherosclerotic lesions and arterial thrombi in humans and mice. Functionally, neutrophil extracellular traps have been shown to induce activation of endothelial cells, antigen-presenting cells, and platelets, resulting in a proinflammatory immune response. Overall, this suggests that they are not only present in plaques and thrombi but also they may play a causative role in triggering atherosclerotic plaque formation and arterial thrombosis. This review will focus on current findings of the involvement of neutrophil extracellular traps in atherogenesis and atherothrombosis.

Multicenter Systems Analysis of Human Blood Reveals Immature Neutrophils in Males and During Pregnancy

Despite clear differences in immune system responses and in the prevalence of autoimmune diseases between males and females, there is little understanding of the processes involved. In this study, we identified a gene signature of immature-like neutrophils, characterized by the overexpression of genes encoding for several granule-containing proteins, which was found at higher levels (up to 3-fold) in young (20-30 y old) but not older (60 to >89 y old) males compared with females. Functional and phenotypic characterization of peripheral blood neutrophils revealed more mature and responsive neutrophils in young females, which also exhibited an elevated capacity in neutrophil extracellular trap formation at baseline and upon microbial or sterile autoimmune stimuli. The expression levels of the immature-like neutrophil signature increased linearly with pregnancy, an immune state of increased susceptibility to certain infections. Using mass cytometry, we also find increased frequencies of immature forms of neutrophils in the blood of women during late pregnancy. Thus, our findings show novel sex differences in innate immunity and identify a common neutrophil signature in males and in pregnant women.

Utilization of cholera toxin B as a mucosal adjuvant elicits antibody-mediated protection against S. pneumoniae infection in mice

BACKGOUND

The introduction of the pneumococcal conjugate and polysaccharide vaccines have been valuable tools for combating invasive pneumococcal infection in children and healthy adults. Despite the available vaccination strategies, pneumococcal pneumonia and associated diseases continue to cause substantial morbidity and mortality, particularly in individuals with chronic disease and ageing populations. Next-generation pneumococcal vaccines will need to be highly immunogenic across patient populations providing both mucosal and systemic protective immunity. Mucosal immunization is an effective strategy for stimulating the immune response at the site of pathogen entry while increasing systemic immunity. In this study we utilized intranasal immunization with pneumococcal surface protein A (PspA), in combination with the mucosal adjuvant cholera toxin B (CTB), to characterize the immune components providing protection against S. pneumoniae challenge.

METHODS

Mice were immunized intranasally with CTB and PspA individually, and in combination, followed by lethal bacterial challenge with S. pneumoniae, strain A66.1. Animals were monitored for survival and tested for lung bacterial burden, cytokine production as well as S. pneumoniae-specific antibody titer in mouse sera. The primary immunological contributor to the observed protection was confirmed by cytokine neutralization and serum passive transfer.

RESULTS

The combination of CTB and PspA provided complete protection against bacterial challenge, which coincided with a significant decrease in lung bacterial burden. Increases in the T-helper (Th) 1 cytokines, interferon (IFN)-γ and interleukin (IL)-2 were observed in the lung 24 h post-challenge while decreases in proinflammatory mediators IL-6 and tumor necrosis factor (TNF)-α were also recorded at the same time point. The adjuvanted PspA immunization induced significant titers of S. pneumoniae-specific antibody in the serum of mice prior to infection. Serum adoptive transfer passively protected animals against subsequent challenge while IFN-γ neutralization had no impact on the outcome of immunization, suggesting a primary role for antibody-mediated protection in the context of this immunization strategy.

CONCLUSION

Mucosal immunization with CTB and PspA induced a local cellular immune response and systemic humoral immunity which resulted in effective reduction of pulmonary bacterial burden and complete protection against S. pneumoniae challenge. While induction of the pleiotropic cytokine IFN-γ likely contributes to control of infection through activation of effector pathways, it was not required for protection. Instead, immunization with PspA and CTB-induced S. pneumoniae-specific antibodies in the serum prior to infection that were sufficient to protect against mucosal challenge.

Gastropod-derived haemocyte extracellular traps entrap metastrongyloid larval stages of Angiostrongylus vasorum, Aelurostrongylus abstrusus and Troglostrongylus brevior

Background

Phagocyte-derived extracellular traps (ETs) were recently demonstrated mainly in vertebrate hosts as an important effector mechanism against invading parasites. In the present study we aimed to characterize gastropod-derived invertebrate extracellular phagocyte trap (InEPT) formation in response to larval stages of important canine and feline metastrongyloid lungworms. Gastropod haemocytes were isolated from the slug species Arion lusitanicus and Limax maximus, and the snail Achatina fulica, and exposed to larval stages of Angiostrongylus vasorum, Aelurostrongylus abstrusus and Troglostrongylus brevior and investigated for gastropod-derived InEPT formation.

Results

Phase contrast as well as scanning electron microscopy (SEM) analyses of lungworm larvae-exposed haemocytes revealed ET-like structures to be extruded by haemocytes thereby contacting and ensnaring the parasites. Co-localization studies of haemocyte-derived extracellular DNA with histones and myeloperoxidase in larvae-entrapping structures confirmed classical characteristics of ETs. In vivo exposure of slugs to A. vasorum larvae resulted in InEPTs being extruded from haemocytes in the slug mucous extrapallial space emphasizing the pivotal role of this effector mechanism against invasive larvae. Functional larval entrapment assays demonstrated that almost half of the haemocyte-exposed larvae were contacted or even immobilized by released InEPTs. Overall, as reported for mammalian-derived ETs, different types of InEPTs were here observed, i.e. aggregated, spread and diffused InEPTs.

Conclusions

To our knowledge, this study represents the first report on metastrongyloid lungworm-triggered ETosis in gastropods thereby providing evidence of early mollusc host innate immune reactions against invading larvae. These findings will contribute to the better understanding on complex parasite-intermediate host interactions since different gastropod species bear different transmitting capacities for metastrongyloid infections.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1961-z) contains supplementary material, which is available to authorized users.

The Essential Role of Type I Interferons in Differentiation and Activation of Tumor-Associated Neutrophils

Type I interferons (IFNs) were first characterized in the process of viral interference. However, since then, IFNs are found to be involved in a wide range of biological processes. In the mouse, type I IFNs comprise a large family of cytokines. At least 12 IFN-α and one IFN-β can be found and they all signal through the same receptor (IFNAR). A hierarchy of expression has been established for type I IFNs, where IFN-β is induced first and it activates in a paracrine and autocrine fashion a cascade of other type I IFNs. Besides its importance in the induction of the IFN cascade, IFN-β is also constitutively expressed in low amounts under normal non-inflammatory conditions, thus facilitating "primed" state of the immune system. In the context of cancer, type I IFNs show strong antitumor function as they play a key role in mounting antitumor immune responses through the modulation of neutrophil differentiation, activation, and migration. Owing to their plasticity, neutrophils play diverse roles during cancer development and metastasis since they possess both tumor-promoting (N2) and tumor-limiting (N1) properties. Notably, the differentiation into antitumor phenotype is strongly supported by type I IFNs. It could also be shown that these cytokines are critical for the suppression of neutrophil migration into tumor and metastasis site by regulating chemokine receptors, e.g., CXCR2 on these cells and by influencing their longevity. Type I IFNs limit the life span of neutrophils by influencing both, the extrinsic as well as the intrinsic apoptosis pathways. Such antitumor neutrophils efficiently suppress the pro-angiogenic factors expression, e.g., vascular endothelial growth factor and matrix metallopeptidase 9. This in turn restricts tumor vascularization and growth. Thus, type I IFNs appear to be the part of the natural tumor surveillance mechanism. Here we provide an up to date review of how type I IFNs influence the pro- and antitumor properties of neutrophils. Understanding these mechanisms is particularly important from a therapeutic point of view.

Propagation of thrombosis by neutrophils and extracellular nucleosome networks

Neutrophils, early mediators of the innate immune defense, are recruited to developing thrombi in different types of thrombosis. They amplify intravascular coagulation by stimulating the tissue factor-dependent extrinsic pathway via inactivation of endogenous anticoagulants, enhancing factor XII activation or decreasing plasmin generation. Neutrophil-dependent prothrombotic mechanisms are supported by the externalization of decondensed nucleosomes and granule proteins that together form neutrophil extracellular traps. These traps, either in intact or fragmented form, are causally involved in various forms of experimental thrombosis as first indicated by their role in the enhancement of both microvascular thrombosis during bacterial infection and carotid artery thrombosis. Neutrophil extracellular traps can be induced by interactions of neutrophils with activated platelets; vice versa, these traps enhance adhesion of platelets via von Willebrand factor. Neutrophil-induced microvascular thrombus formation can restrict the dissemination and survival of blood-borne bacteria and thereby sustain intravascular immunity. Dysregulation of this innate immune pathway may support sepsis-associated coagulopathies. Notably, neutrophils and extracellular nucleosomes, together with platelets, critically promote fibrin formation during flow restriction-induced deep vein thrombosis. Neutrophil extracellular traps/extracellular nucleosomes are increased in thrombi and in the blood of patients with different vaso-occlusive pathologies and could be therapeutically targeted for the prevention of thrombosis. Thus, during infections and in response to blood vessel damage, neutrophils and externalized nucleosomes are major promoters of intravascular blood coagulation and thrombosis.

Neutrophil extracellular traps are indirectly triggered by lipopolysaccharide and contribute to acute lung injury

Neutrophil extracellular traps (NETs) facilitate the extracellular killing of pathogens. However, excessive NETs formation and poor degradation are associated with exacerbated immune responses and tissue injury. In this study, we investigated the role of NETs in lipopolysaccharide (LPS)-mediated acute lung injury (ALI) and assessed the use of DNase I, for the treatment of ALI. Additionally, we focused on the controversial issue of whether LPS directly induces NETs release in vitro. NETs formation was detected in murine ALI tissue in vivo and was associated with increased NETs markers, citrullinated-histone H3 tissue levels and NET-DNA levels in BALF. Treatment with DNase I significantly degraded NETs and reduced citrullinated-histone H3 levels, which protected against ALI and ameliorated pulmonary oedema and total protein in BALF. In addition, DNase I significantly reduced IL-6 and TNF-α levels in plasma and BALF. In vitro, LPS-activated platelets rather than LPS alone efficiently induced NETs release. In conclusion, NETs formed during LPS-induced ALI, caused organ damage and initiated the inflammatory response. NETs degradation by DNase I promoted NET-protein clearance and protected against ALI in mice; thus, DNase I may be a new potential adjuvant for ALI therapy. Specifically, LPS induced NETs formation in an indirect manner via platelets activation.

Insights from Mendelian Interferonopathies: Comparison of CANDLE, SAVI with AGS, Monogenic Lupus

Autoinflammatory disorders are sterile inflammatory conditions characterized by episodes of early-onset fever and disease-specific patterns of organ inflammation. Recently, the discoveries of monogenic disorders with strong type I interferon (IFN) signatures caused by mutations in proteasome degradation and cytoplasmic RNA and DNA sensing pathways suggest a pathogenic role of IFNs in causing autoinflammatory phenotypes. The IFN response gene signature (IGS) has been associated with systemic lupus erythematosus (SLE) and other autoimmune diseases. In this review, we compare the clinical presentations and pathogenesis of two IFN-mediated autoinflammatory diseases, CANDLE and SAVI, with Aicardi Goutières syndrome (AGS) and monogenic forms of SLE (monoSLE) caused by loss-of-function mutations in complement 1 (C1q) or the DNA nucleases, DNASE1 and DNASE1L3. We outline differences in intracellular signaling pathways that fuel a pathologic type I IFN amplification cycle. While IFN amplification is caused by predominantly innate immune cell dysfunction in SAVI, CANDLE, and AGS, autoantibodies to modified RNA and DNA antigens interact with tissues and immune cells including neutrophils and contribute to IFN upregulation in some SLE patients including monoSLE, thus justifying a grouping of "autoinflammatory" and "autoimmune" interferonopathies. Understanding of the differences in the cellular sources and signaling pathways will guide new drug development and the use of emerging targeted therapies.

Neutrophil Extracellular Traps Go Viral

Neutrophils are the most numerous immune cells. Their importance as the first line of defense against bacterial and fungal pathogens is well described. In contrast, the role of neutrophils in controlling viral infections is less clear. Bacterial and fungal pathogens can stimulate neutrophils extracellular traps (NETs) in a process called NETosis. Although NETosis has previously been described as a special form of programmed cell death, there are forms of NET production that do not end with the demise of neutrophils. As an end result of NETosis, genomic DNA complexed with microbicidal proteins is expelled from neutrophils. These structures can kill pathogens or at least prevent their local spread within host tissue. On the other hand, disproportionate NET formation can cause local or systemic damage. Only recently, it was recognized that viruses can also induce NETosis. In this review, we discuss the mechanisms by which NETs are produced in the context of viral infection and how this may contribute to both antiviral immunity and immunopathology. Finally, we shed light on viral immune evasion mechanisms targeting NETs.

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

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

Neutrophil extracellular trap formation is increased in psoriasis and induces human β-defensin-2 production in epidermal keratinocytes

Neutrophil extracellular traps (NETs) have been implicated in the development of certain immune-mediated diseases, but their role in psoriasis has not been clearly defined. Human β-defensin-2 (HBD-2) is an important antimicrobial peptide overexpressed in psoriasis epidermis. We evaluated whether the amount of NETs is increased in psoriasis and determined the effect of NETs on HBD-2 production in epidermal keratinocytes. Using fluorescent microscopy, we found that patients with psoriasis (n = 48) had higher amount of NETotic cells in their peripheral blood compared to healthy controls (n = 48) and patients with eczema (n = 35). Psoriasis sera showed increased ability to induce NET formation in control neutrophils but normal NET degradation ability. The amount of NETs in the peripheral blood correlated with psoriasis disease severity. NETosis was also observed in the majority (18 of 20) of psoriasis skin specimens. Furthermore, NETs induced HBD-2 mRNA and protein production in keratinocytes, and immunohistochemical analysis confirmed strong expression of HBD-2 in psoriasis lesional skin. In summary, NET formation is increased in peripheral blood and lesional skin of psoriasis patients and correlates with disease severity. Additionally, NET-induced HBD-2 production may provide a novel mechanism for the decreased susceptibility of psoriasis plaques to microbial infections.

The type I interferon signature in leukocyte subsets from peripheral blood of patients with early arthritis: a major contribution by granulocytes

Background

The type I interferon (IFN) signature in rheumatoid arthritis (RA) has shown clinical relevance in relation to disease onset and therapeutic response. Identification of the cell type(s) contributing to this IFN signature could provide insight into the signature’s functional consequences. The aim of this study was to investigate the contribution of peripheral leukocyte subsets to the IFN signature in early arthritis.

Methods

Blood was collected from 26 patients with early arthritis and lysed directly or separated into peripheral blood mononuclear cells (PBMCs) and polymorphonuclear granulocytes (PMNs). PBMCs were sorted into CD4⁺ T cells, CD8⁺ T cells, CD19⁺ B cells, and CD14⁺ monocytes by flow cytometry. Messenger RNA expression of three interferon response genes (IRGs RSAD2, IFI44L, and MX1) and type I interferon receptors (IFNAR1 and IFNAR2) was determined in whole blood and blood cell subsets by quantitative polymerase chain reaction. IRG expression was averaged to calculate an IFN score for each sample.

Results

Patients were designated “IFN^high” (n = 8) or “IFN^low” (n = 18) on the basis of an IFN score cutoff in whole peripheral blood from healthy control subjects. The difference in IFN score between IFN^high and IFN^low patients was remarkably large for the PMN fraction (mean 25-fold) compared with the other subsets (mean 6- to 9-fold), indicating that PMNs are the main inducers of IRGs. Moreover, the relative contribution of the PMN fraction to the whole-blood IFN score was threefold higher than expected from its abundance in blood (p = 0.008), whereas it was three- to sixfold lower for the other subsets (p ≤ 0.063), implying that the PMNs are most sensitive to IFN signaling. Concordantly, IFNAR1 and IFNAR2 were upregulated compared with healthy controls selectively in patient PMNs (p ≤ 0.0077) but not in PBMCs.

Conclusions

PMNs are the main contributors to the whole-blood type I IFN signature in patients with early arthritis, which seems due to increased sensitivity of these cells to type I IFN signaling. Considering the well-established role of neutrophils in the pathology of arthritis, this suggests a role of type I IFN activity in the disease as well.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-1065-3) contains supplementary material, which is available to authorized users.

Far beyond Phagocytosis: Phagocyte-Derived Extracellular Traps Act Efficiently against Protozoan Parasites In Vitro and In Vivo

Professional mononuclear phagocytes such as polymorphonuclear neutrophils (PMN), monocytes, and macrophages are considered as the first line of defence against invasive pathogens. The formation of extracellular traps (ETs) by activated mononuclear phagocytes is meanwhile well accepted as an effector mechanism of the early host innate immune response acting against microbial infections. Recent investigations showed evidence that ETosis is a widely spread effector mechanism in vertebrates and invertebrates being utilized to entrap and kill bacteria, fungi, viruses, and protozoan parasites. ETs are released in response to intact protozoan parasites or to parasite-specific antigens in a controlled cell death process. Released ETs consist of nuclear DNA as backbone adorned with histones, antimicrobial peptides, and phagocyte-specific granular enzymes thereby producing a sticky extracellular matrix capable of entrapping and killing pathogens. This review summarizes recent data on protozoa-induced ETosis. Special attention will be given to molecular mechanisms of protozoa-induced ETosis and on its consequences for the parasites successful reproduction and life cycle accomplishment.

Iron-chelating agent desferrioxamine stimulates formation of neutrophil extracellular traps (NETs) in human blood-derived neutrophils

Here we show that iron-chelating agent desferrioxamine significantly induced the formation of neutrophil extracellular traps by human blood-derived neutrophils as visualized and quantified by immunofluorescence microscopy. Further analyses characterized biochemical mechanisms associated with the NET formation by desferrioxamine.

Neutrophil extracellular trap (NET) formation is a significant innate immune defense mechanism against microbial infection that complements other neutrophil functions including phagocytosis and degranulation of antimicrobial peptides. NETs are decondensed chromatin structures in which antimicrobial components (histones, antimicrobial peptides and proteases) are deployed and mediate immobilization of microbes. Here we describe an effect of iron chelation on the phenotype of NET formation. Iron-chelating agent desferrioxamine (DFO) showed a modest but significant induction of NETs by freshly isolated human neutrophils as visualized and quantified by immunocytochemistry against histone–DNA complexes. Further analyses revealed that NET induction by iron chelation required NADPH-dependent production of reactive oxygen species (ROS) as well as protease and peptidyl-arginine-deiminase 4 (PAD4) activities, three key mechanistic pathways previously linked to NET formation. Our results demonstrate that iron chelation by DFO contributes to the formation of NETs and suggest a target for pharmacological manipulation of NET activity.

Neutrophils of Scophthalmus maximus produce extracellular traps that capture bacteria and inhibit bacterial infection

Neutrophils constitute an essential part of the innate immune system. Recently, neutrophils have been found to produce a complex extracellular structure called neutrophil extracellular traps (NETs) that capture bacteria, fungi, and parasites. In fish, a few studies on NETs production have been reported, however, the function of fish NETs is unknown. In this study, we examined the ability of turbot (Scophthalmus maximus) neutrophils to produce NETs and investigated the effect of turbot NETs on bacterial infection. We found that upon lipopolysaccharides treatment, turbot head kidney neutrophils produced typical NETs structures that contained DNA and histones. Bacteria treatment also induced production of NETs, which in turn entrapped the bacterial cells and inhibited bacterial replication. Furthermore, when introduced into turbot, NETs-trapped bacteria exhibited significantly weakened ability of tissue dissemination and colonization. These results indicate for the first time that teleost NETs possess apparent antibacterial effect both in vitro and in vivo.

Complement factor H modulates the activation of human neutrophil granulocytes and the generation of neutrophil extracellular traps

Factor H (FH) is a major inhibitor of the alternative pathway of complement activation in plasma and on certain host surfaces. In addition to being a complement regulator, FH can bind to various cells via specific receptors, including binding to neutrophil granulocytes through complement receptor type 3 (CR3; CD11b/CD18), and modulate their function. The cellular roles of FH are, however, poorly understood. Because neutrophils are important innate immune cells in inflammatory processes and the host defense against pathogens, we aimed at studying the effects of FH on various neutrophil functions, including the generation of extracellular traps. FH co-localized with CD11b on the surface of neutrophils isolated from peripheral blood of healthy individuals, and cell-bound FH retained its cofactor activity and enhanced C3b degradation. Soluble FH supported neutrophil migration and immobilized FH induced cell spreading. In addition, immobilized but not soluble FH enhanced IL-8 release from neutrophils. FH alone did not trigger the cells to produce neutrophil extracellular traps (NETs), but NET formation induced by PMA and by fibronectin plus fungal β-glucan were inhibited by immobilized, but not by soluble, FH. Moreover, in parallel with NET formation, immobilized FH also inhibited the production of reactive oxygen species induced by PMA and by fibronectin plus β-glucan. Altogether, these data indicate that FH has multiple regulatory roles on neutrophil functions. While it can support the recruitment of neutrophils, FH may also exert anti-inflammatory effects and influence local inflammatory and antimicrobial reactions, and reduce tissue damage by modulating NET formation.

Neutrophil extracellular traps: a walk on the wild side of exercise immunology

Intense exercise evokes a rapid and transient increase in circulating cell-free DNA (cf-DNA), a phenomenon that is commonly observed in a variety of acute and chronic inflammatory conditions. While the potential value of cf-DNA for the prediction of disease outcome and therapeutic response is well documented, the release mechanisms and biological relevance of cf-DNA have long remained enigmatic. The discovery of neutrophil extracellular traps (NETs) provided a novel mechanistic explanation for increased cf-DNA levels. Now there is increasing evidence that NETs may contribute to cf-DNA in diverse infectious, non-infectious and autoinflammatory conditions, as well as in response to acute exercise. NETs have now been firmly established as a fundamental immune mechanism used by neutrophils to respond to infection and tissue injury. On the other side, aberrant formation of NETs appears to be a driving force in the pathogenesis of autoimmunity and cardiovascular disease. Thus, the emergence of NETs in the 'exercising vasculature' raises important questions considering beneficial effects, as well as occasional adverse effects, of exercise on immune homeostasis. This review gives an overview of the current state of research into the mechanisms of how NETs are released, contribute to host defence and participate in inflammatory disorders. We discuss the impact of exercise-induced NETs, considering a potentially beneficial role in the prevention of lifestyle-related diseases, as well as putative detrimental effects that may arise in elite sports. Finally, we propose that exercise-induced cf-DNA responses could be exploited for diagnostic/prognostic purposes to identify individuals who are at increased risk of cardiovascular events or autoimmunity.

The influence of hypoxic physical activity on cfDNA as a new marker of vascular inflammation

The phenomenon of circulating cell-free DNA (cfDNA) is important for many biomedical disciplines including the field of exercise biochemistry and physiology. It is likely that cfDNA is released into the plasma by apoptosis of endothelial cells and circulating endothelial progenitor cells (EPCs), and/or by NETosis of immune cells induced by strenuous exercise. Increases of cfDNA are described to be a potential hallmark for the overtraining syndrome, and might be related to aseptic vascular inflammation in athletes. Yet, the relevance of systemic inflammation and cfDNA with endothelial dysfunction in athletes still remains unclear. In this review article, we provide a current overview of exercise-induced cfDNA release to the circulation with special emphasis on its relationship with apoptosis and NETosis and the effect of hypoxic physical activity on vascular inflammation in athletes.

Host response mechanisms in periodontal diseases

Periodontal diseases usually refer to common inflammatory disorders known as gingivitis and periodontitis, which are caused by a pathogenic microbiota in the subgingival biofilm, including Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans, Tannerella forsythia and Treponema denticola that trigger innate, inflammatory, and adaptive immune responses. These processes result in the destruction of the tissues surrounding and supporting the teeth, and eventually in tissue, bone and finally, tooth loss. The innate immune response constitutes a homeostatic system, which is the first line of defense, and is able to recognize invading microorganisms as non-self, triggering immune responses to eliminate them. In addition to the innate immunity, adaptive immunity cells and characteristic cytokines have been described as important players in the periodontal disease pathogenesis scenario, with a special attention to CD4⁺ T-cells (T-helper cells). Interestingly, the T cell-mediated adaptive immunity development is highly dependent on innate immunity-associated antigen presenting cells, which after antigen capture undergo into a maturation process and migrate towards the lymph nodes, where they produce distinct patterns of cytokines that will contribute to the subsequent polarization and activation of specific T CD4+ lymphocytes. Skeletal homeostasis depends on a dynamic balance between the activities of the bone-forming osteoblasts (OBLs) and bone-resorbing osteoclasts (OCLs). This balance is tightly controlled by various regulatory systems, such as the endocrine system, and is influenced by the immune system, an osteoimmunological regulation depending on lymphocyte- and macrophage-derived cytokines. All these cytokines and inflammatory mediators are capable of acting alone or in concert, to stimulate periodontal breakdown and collagen destruction via tissue-derived matrix metalloproteinases, a characterization of the progression of periodontitis as a stage that presents a significantly host immune and inflammatory response to the microbial challenge that determine of susceptibility to develop the destructive/progressive periodontitis under the influence of multiple behavioral, environmental and genetic factors.

Leucocyte-derived extracellular trap formation significantly contributes to Haemonchus contortus larval entrapment

BACKGROUND

Polymorphonuclear neutrophil (PMN) and eosinophil extracellular trap (ETs) formation has recently been described as an important host effector mechanism against invading pathogens. So far, scarce evidence on metazoan-triggered ET formation has been published. We here describe for the first time Haemonchus contortus-triggered ETs being released by bovine PMN and ovine eosinophils in response to ensheathed and exsheathed third stage larvae (L3).

METHODS

The visualization of ETs was achieved by SEM analysis. The identification of classical ETs components was performed via fluorescence microscopy analysis. The effect of larval exsheathment and parasite integrity on ET formation was evaluated via Pico Green®- fluorescence intensities. ETs formation under acidic conditions was assessed by using media of different pH ranges. Parasite entrapment was evaluated microscopically after co-culture of PMN and L3. ET inhibition experiments were performed using inhibitors against NADPH oxidase, NE and MPO. Eosinophil-derived ETs were estimated via fluorescence microscopy analysis.

RESULTS

L3 significantly induced PMN-mediated ETs and significant parasite entrapment through ETs structures was rapidly observed after 60 min of PMN and L3 co-culture. Co-localization studies of PMN-derived extracellular DNA with histones (H3), neutrophil elastase (NE) and myeloperoxidase (MPO) in parasite-entrapping structures confirmed the classical characteristics of ETs. Haemonchus contortus-triggered ETs were significantly diminished by NADPH oxidase-, NE- and MPO-inhibition. Interestingly, different forms of ETs, i.e. aggregated (aggETs), spread (sprETs) and diffused (diffETs) ETs, were induced by L3. AggETs and sprETs firmly ensnared larvae in a time dependent manner. Significantly stronger aggETs reactions were detected upon exposure of PMN to ensheathed larvae than to exsheathed ones. Low pH conditions as are present in the abomasum did not block ETosis and led to a moderate decrease of ETs. Eosinophil-ETs were identified extruding DNA via fluorescence staining.

CONCLUSION

We postulate that ETs may limit the establishment of H. contortus within the definitive host by immobilizing the larvae and hampering them from migrating into the site of infection. Consequently, H. contortus-mediated ET formation might have an impact on the outcome of the disease. Finally, besides PMN-triggered ETs, we here present first indications of ETs being released by eosinophils upon H. contortus L3 exposure.

IFN-γ Induces Mimic Extracellular Trap Cell Death in Lung Epithelial Cells Through Autophagy-Regulated DNA Damage

Treatment of interferon-γ (IFN-γ) causes cell growth inhibition and cytotoxicity in lung epithelial malignancies. Regarding the induction of autophagy related to IFN-γ signaling, this study investigated the link between autophagy and IFN-γ cytotoxicity. In A549 human lung cancer cells, IFN-γ treatment induced concurrent apoptotic and nonapoptotic events. Unexpectedly, the nonapoptotic cells present mimic extracellular trap cell death (ETosis), which was regulated by caspase-3 and by autophagy induction through immunity-related GTPase family M protein 1 and activating transcription factor 6. Furthermore, IFN-γ signaling controlled mimic ETosis through a mechanism involving an autophagy- and Fas-associated protein with death domain-controlled caspase-8/-3 activation. Following caspase-mediated lamin degradation, IFN-γ caused DNA damage-associated ataxia telangiectasia and Rad3-related protein (ATR)/ataxia telangiectasia mutated (ATM)-regulated mimic ETosis. Upon ATR/ATM signaling, peptidyl arginine deiminase 4 (PAD4)-mediated histone 3 citrullination promoted mimic ETosis. Such IFN-γ-induced effects were defective in PC14PE6/AS2 human lung cancer cells, which were unsusceptible to IFN-γ-induced autophagy. Due to autophagy-based caspase cascade activation, IFN-γ triggers unconventional caspase-mediated DNA damage, followed by ATR/ATM-regulated PAD4-mediated histone citrullination during mimic ETosis in lung epithelial malignancy.

Yersinia enterocolitica-mediated degradation of neutrophil extracellular traps (NETs)

Neutrophil extracellular trap (NET) formation is described as a tool of the innate host defence to fight against invading pathogens. Fibre-like DNA structures associated with proteins such as histones, cell-specific enzymes and antimicrobial peptides are released, thereby entrapping invading pathogens. It has been reported that several bacteria are able to degrade NETs by nucleases and thus evade the NET-mediated entrapment. Here we studied the ability of three different Yersinia serotypes to induce and degrade NETs. We found that the common Yersinia enterocolitica serotypes O:3, O:8 and O:9 were able to induce NETs in human blood-derived neutrophils during the first hour of co-incubation. At later time points, the NET amount was reduced, suggesting that degradation of NETs has occurred. This was confirmed by NET degradation assays with phorbol-myristate-acetate-pre-stimulated neutrophils. In addition, we found that the Yersinia supernatants were able to degrade purified plasmid DNA. The absence of Ca(2+) and Mg(2+) ions, but not that of a protease inhibitor cocktail, completely abolished NET degradation. We therefore postulate that Y. enterocolitica produces Ca(2+)/Mg(2+)-dependent NET-degrading nucleases as shown for some Gram-positive pathogens.

NADPH oxidase, MPO, NE, ERK1/2, p38 MAPK and Ca2+ influx are essential for Cryptosporidium parvum-induced NET formation

Cryptosporidium parvum causes a zoonotic infection with worldwide distribution. Besides humans, cryptosporidiosis affects a wide range of animals leading to significant economic losses due to severe enteritis in neonatal livestock. Neutrophil extracellular trap (NET) formation has been demonstrated as an important host effector mechanism of PMN acting against several invading pathogens. In the present study, C. parvum-mediated NET formation was investigated in human and bovine PMN in vitro. We here demonstrate that C. parvum sporozoites indeed trigger NET formation in a time-dependent manner. Thereby, the classical characteristics of NETs were demonstrated by co-localization of extracellular DNA with histones, neutrophil elastase (NE) and myeloperoxidase (MPO). A significant reduction of NET formation was measured following treatments of PMN with NADPH oxidase-, NE- and MPO-inhibitors, confirming the key role of these enzymes in C. parvum-induced NETs. Additionally, sporozoite-triggered NETosis revealed as dependent on intracellular Ca(++) concentration and the ERK 1/2 and p38 MAPK-mediated signaling pathway. Moreover, sporozoite-triggered NET formation led to significant parasite entrapment since 15% of the parasites were immobilized in NET structures. Consequently, PMN-pre-exposed sporozoites showed significantly reduced infectivity for epithelial host cells confirming the capability of NETs to prevent active parasite invasion. Besides NETs, we here show that C. parvum significantly up-regulated CXCL8, IL6, TNF-α and of GM-CSF gene transcription upon sporozoite confrontation, indicating a pivotal role of PMN not only in the bovine and human system but most probably in other final hosts for C. parvum.

Neutrophil Extracellular Traps Identification in Tegumentary Lesions of Patients with Paracoccidioidomycosis and Different Patterns of NETs Generation In Vitro

Paracoccidioidomycosis (PCM) is a systemic mycosis, endemic in most Latin American countries, especially in Brazil. It is caused by the thermo-dimorphic fungus of the genus Paracoccidioides (Paracoccidioides brasiliensis and Paracoccidioides lutzii). Innate immune response plays a crucial role in host defense against fungal infections, and neutrophils (PMNs) are able to combat microorganisms with three different mechanisms: phagocytosis, secretion of granular proteins, which have antimicrobial properties, and the most recent described mechanism called NETosis. This new process is characterized by the release of net-like structures called Neutrophil Extracellular Traps (NETs), which is composed of nuclear (decondensed DNA and histones) and granular material such as elastase. Several microorganisms have the ability of inducing NETs formation, including gram-positive and gram-negative bacteria, viruses and some fungi. We proposed to identify NETs in tegumentary lesions of patients with PCM and to analyze the interaction between two strains of P. brasiliensis and human PMNs by NETs formation in vitro. In this context, the presence of NETs in vivo was evidenced in tegumentary lesions of patients with PCM by confocal spectrum analyzer. Furthermore, we showed that the high virulent P. brasiliensis strain 18 (Pb18) and the lower virulent strain Pb265 are able to induce different patterns of NETs formation in vitro. The quantification of extracellular DNA corroborates the idea of the ability of P. brasiliensis in inducing NETs release. In conclusion, our data show for the first time the identification of NETs in lesions of patients with PCM and demonstrate distinct patterns of NETs in cultures challenged with fungi in vitro. The presence of NETs components both in vivo and in vitro open new possibilities for the detailed investigation of immunity in PCM.

Paracoccidioidomycosis (PCM) is an infectious disease caused by fungi of genus Paracoccidioides (P. brasiliensis and P. lutzii). PCM is endemic in Latin America, with a greater incidence in Brazil, Colombia, and Argentina. Over the last years, studies are focusing on neutrophils’ (PMNs) actions against P. brasiliensis, due to the capacity of these cells to develop different defense strategies against pathogens. and especially due to constant presence of inflammatory infiltrates full of PMNs in the granuloma of the disease. As PMN release of both granular and nuclear material, identified as Neutrophil Extracellular Traps (NETs), is a spectacular action mechanism against microbes, we seek to identify whether this process would be an important mechanism triggered against P. brasiliensis. Thus, we showed for the first time the identification of NETs in tegumentary lesions of patients with PCM by viewing the individual components of NETs. Beyond that, we demonstrated the entrapment of P. brasiliensis in vitro by these structures released from human PMNs of patients with PCM and healthy donors, with different patterns, in a dependence of the evaluated strain. Our data provides important new information regarding the role of PMNs against P. brasiliensis, opening new avenues for the research on immunity of PCM.

Influence of complement on neutrophil extracellular trap release induced by bacteria

BACKGROUND AND OBJECTIVES

Neutrophil extracellular trap (NET) release has generally been studied in the absence of serum, or at low concentrations of untreated or heat-inactivated serum. The influence of serum complement on NET release therefore remains unclear. We examined the DNA release induced by Staphylococcus aureus and three oral bacteria: Actinomyces viscosus, Aggregatibacter actinomycetemcomitans and Fusobacterium nucleatum subsp. vincettii.

MATERIAL AND METHODS

Bacteria-stimulated NET release from the neutrophils of healthy donors was measured fluorometrically. Various complement containing and complement blocking conditions were used, including heat inactivation of the serum and antibody blockade of complement receptors 1 (CR1, CD35) and 3 (CR3, CD11b/CD18).

RESULTS

While the presence of serum markedly enhanced NET release induced by S. aureus, A. actinomycetemcomitans, and to a lesser extent by A. viscosus, there was no enhancement of NET release induced by F. nucleatum. The serum-mediated enhancement of NET release by A. actinomycetemcomitans was neutralized by heat inactivation of serum complement, while this was not the case for S. aureus. Blockade of CR1, significantly reduced NET release induced by S. aureus, A. actinomycetemcomitans and A. viscosus, while blockade of CR3, had no effect. However, opsonization of S. aureus with antibodies may also have contributed to the enhancing effect of serum, independently of complement, in that purified IgG promoted NET release.

CONCLUSIONS

In conclusion, complement opsonization promotes NET release induced by a variety of bacteria, including A. actinomycetemcomitans, and CR1 plays a dominant role in the process. Complement consumption or deficiency may compromise NETosis induced by some bacterial species, including A. actinomycetemcomitans. Within biofilms, the complement-inactivating abilities of some bacteria may protect other species against NETosis, while these are more vulnerable when adopting a planktonic lifestyle.

Extracellular traps and macrophages: new roles for the versatile phagocyte

MΦ are multipurpose phagocytes with a large repertoire of well-characterized abilities and functions, including regulation of inflammation, wound healing, maintenance of tissue homeostasis, as well as serving as an integral component of the innate-immune defense against microbial pathogens. Working along with neutrophils and dendritic cells, the other myeloid-derived professional phagocytes, MΦ are one of the key effector cells initiating and directing the host reaction to pathogenic organisms and resolving subsequent responses once the threat has been cleared. ETs are a relatively novel strategy of host defense involving expulsion of nuclear material and embedded proteins from immune cells to immobilize and kill bacteria, fungi, and viruses. As research on ETs expands, it has begun to encompass many immune cell types in unexpected ways, including various types of MΦ, which are not only capable of generating METs in response to various stimuli, but recent preclinical data suggest that they are an important agent in clearing ETs and limiting ET-mediated inflammation and tissue damage. This review aims to summarize historical and recent findings of biologic research regarding ET formation and function and discuss the role of MΦ in ET physiology and associated pathologies.

Neutrophil extracellular traps as a new paradigm in innate immunity: friend or foe?

The discovery of neutrophil extracellular traps in 2004 opened a fascinating new chapter in immune-mediated microbial killing. Brinkman et al. demonstrated that neutrophils, when catastrophically stimulated, undergo a novel form of programmed cell death (neutrophil extracellular trap formation) whereby they decondense their entire nuclear chromatin/DNA and release the resulting structure into the cytoplasm to mix with granule-derived antimicrobial peptides before extruding these web-like structures into the extracellular environment. The process requires the activation of the granule enzyme peptidyl arginine deiminase-4, the formation of reactive oxygen species (in particular hypochlorous acid), the neutrophil microtubular system and the actin cytoskeleton. Recent work by Yousefi et al. demonstrated that exposure to different agents for shorter stimulation periods resulted in neutrophil extracellular trap release from viable granulocytes, and that such neutrophil extracellular traps comprised mitochondrial DNA rather than nuclear DNA and were also capable of microbial entrapment and destruction. Deficiency in NADPH-oxidase production (as found in patients with chronic granulomatous disease) results in an inability to produce neutrophil extracellular traps and, along with their failure to produce antimicrobial reactive oxygen species, these patients suffer from severe, and sometimes life-threatening, infections. However, conversely the release of nuclear chromatin into tissues is also potentially autoimmunogenic and is now associated with the generation of anti-citrullinated protein antibodies in seropositive rheumatoid arthritis. Other neutrophil-derived nuclear and cytoplasmic contents are also pathogenic, either through direct effects on tissues or via autoimmune processes (e.g. autoimmune vasculitis). In this review, we discuss the plant origins of a highly conserved innate immune method of microbial killing, the history and biology of neutrophil extracellular traps and their role in defence and in human diseases. We attempt to resolve areas of controversy and propose roles for excess neutrophil extracellular trap release from hyperactive/reactive neutrophils and for the unique peptidyl arginine deiminase enzyme of Porphyromonas gingivalis in the pathogenesis of periodontitis, and subsequently a role for periodontitis/the peptidyl arginine deiminase enzyme of P. gingivalis in the causal pathway of autoimmune diseases such as rheumatoid arthritis. We propose that neutrophil extracellular trap and peptidyl arginine deiminase release may propagate tissue-destructive mechanisms rather than provide protection in susceptible individuals and that release of host-derived DNase may play an important role in the digestion and removal of neutrophil extracellular traps within tissues.

Eimeria bovis-triggered neutrophil extracellular trap formation is CD11b-, ERK 1/2-, p38 MAP kinase- and SOCE-dependent

Eimeria bovis is an important coccidian parasite that causes high economic losses in the cattle industry. We recently showed that polymorphonuclear neutrophils (PMN) react upon E. bovis sporozoite exposure by neutrophil extracellular trap (NET) formation. We focused here on the molecular mechanisms that are involved in this process. The sporozoite encounter led to an enhanced surface expression of neutrophil CD11b suggesting a potential role of this receptor in E. bovis-mediated NETosis. Antibody-mediated blockage of CD11b confirmed this assumption and led to a significantly decreased sporozoite-triggered NET. In addition, E. bovis-induced NETosis was found to be Ca²⁺-dependent since the inhibition of store-operated calcium entry (SOCE) significantly diminished NET. Furthermore, NADPH oxidase, neutrophil elastase (NE) and myeloperoxidase (MPO) were confirmed as key molecules in sporozoite-triggered NETosis, as inhibition thereof blocked parasite-triggered NET. PMN degranulation analyses revealed a significant release of matrix metalloprotease-9 containing granules upon sporozoite exposure. We further show a significantly enhanced phosphorylation of ERK1/2 and p38 MAPK in sporozoite-exposed PMN indicating a key role of this signaling pathway in E. bovis-mediated NETosis. Accordingly, ERK 1/2 and p38 MAPK inhibition led to a significant decrease in NET formation. Finally, we demonstrate that sporozoite-induced NETosis is neither a stage-, species-, nor host-specific process.

Human neutrophils secrete bioactive paucimannosidic proteins from azurophilic granules into pathogen-infected sputum

Unlike plants and invertebrates, mammals reportedly lack proteins displaying asparagine (N)-linked paucimannosylation (mannose(1-3)fucose(0-1)N-acetylglucosamine(2)Asn). Enabled by technology advancements in system-wide biomolecular characterization, we document that protein paucimannosylation is a significant host-derived molecular signature of neutrophil-rich sputum from pathogen-infected human lungs and is negligible in pathogen-free sputum. Five types of paucimannosidic N-glycans were carried by compartment-specific and inflammation-associated proteins of the azurophilic granules of human neutrophils including myeloperoxidase (MPO), azurocidin, and neutrophil elastase. The timely expressed human azurophilic granule-resident β-hexosaminidase A displayed the capacity to generate paucimannosidic N-glycans by trimming hybrid/complex type N-glycan intermediates with relative broad substrate specificity. Paucimannosidic N-glycoepitopes showed significant co-localization with β-hexosaminidase A and the azurophilic marker MPO in human neutrophils using immunocytochemistry. Furthermore, promyelocyte stage-specific expression of genes coding for paucimannosidic proteins and biosynthetic enzymes indicated a novel spatio-temporal biosynthetic route in early neutrophil maturation. The absence of bacterial exoglycosidase activities and paucimannosidic N-glycans excluded exogenous origins of paucimannosylation. Paucimannosidic proteins from isolated and sputum neutrophils were preferentially secreted upon inoculation with virulent Pseudomonas aeruginosa. Finally, paucimannosidic proteins displayed affinities to mannose-binding lectin, suggesting immune-related functions of paucimannosylation in activated human neutrophils. In conclusion, we are the first to document that human neutrophils produce, store and, upon activation, selectively secrete bioactive paucimannosidic proteins into sputum of lungs undergoing pathogen-based inflammation.

Production of inflammatory mediators and extracellular traps by carp macrophages and neutrophils in response to lipopolysaccharide and/or interferon-γ2

Neutrophilic granulocytes and macrophages are crucial for the innate immune response against infections. They migrate into the focus of inflammation, where they efficiently bind, engulf and kill bacteria by proteolytic enzymes, antimicrobial peptides, reactive oxygen (ROS) and nitrogen (RNS) species. Moreover, activated neutrophils and macrophages can form extracellular traps (ETs). Fish neutrophils and macrophages are morphologically, histochemically, and functionally similar to their mammalian counterparts, but their significance for regulation of inflammatory responses and pathogen killing needs further elucidation. We compared the activity of head kidney monocytes/macrophages and neutrophilic granulocytes of common carp and established that upon lipopolysaccharide stimulation, not only neutrophils, but also carp monocytes/macrophages release extracellular DNA and are capable to form macrophage extracellular traps (METs). To clarify whether many specific LPS functions reported for piscine phagocytes might be due to impurities in the commonly used LPS preparations we studied expression of inflammatory mediators, release of DNA, ROS and RNS in cells stimulated with LPS or its highly purified form (pLPS). Also IFN-γ2 stimulation and its synergism with LPS/pLPS in stimulating expression of pro-inflammatory mediators was studied. Results substantiate that a classical stimulation of TLR4 by LPS may indeed be absent in carp as most of the classically reported LPS effects are abolished or diminished when pLPS is used. Interestingly, we also observed a potent IL-10 expression in neutrophilic granulocytes upon LPS stimulation, which, apart from their pro-inflammatory function, clearly indicates a role in restrictive control of the inflammatory reaction.

Neutrophil extracellular traps: Their role in periodontal disease

Neutrophils are the first line of innate immune defense against infectious diseases. Since their discovery, they have always been considered tissue-destructive cells responsible for inflammatory tissue damage occurring during infections. Extensive research in the field of neutrophil cell biology and their role skewing the immune response in various infections or inflammatory disorders revealed their importance in the regulation of immune response. Neutrophils also release neutrophil extracellular traps (NETs) for the containment of infection and inflammation along with other antimicrobial molecules. Activated neutrophils provide signals for the activation and maturation of macrophages as well as dendritic cells. Neutrophils are also involved in the regulation of T-cell immune response against various pathogens and tumor antigens. Thus, the present review is intended to highlight the emerging role of neutrophil extracellular trap production in the regulation of immune response and its role in periodontal disease.

Besnoitia besnoiti tachyzoites induce monocyte extracellular trap formation

Extracellular trap (ET) formation has been demonstrated as an important novel effector mechanism of polymorphonuclear neutrophils (PMN), eosinophils, mast cells and macrophages acting extracellularly against pathogens. In the present study, we show that tachyzoites of the emerging apicomplexan parasite Besnoitia besnoiti, that have recently been reported as potent inducers of PMN-derived ETosis, also trigger the release of ETs in an additional cell type, namely in monocytes. Fluorescence illustrations as well as scanning electron microscopy analyses (SEM) showed monocyte-promoted ET formation to be rapidly induced upon exposure to viable tachyzoites of B. besnoiti. Classical characteristics of ETs were confirmed by the co-localization of extracellular DNA with histones (H3) or myeloperoxidase (MPO) in parasite-entrapping structures. Monocyte-derived ETs were efficiently abolished by DNase I treatment and significantly reduced by treatments with inhibitors of MPO and NADPH oxidase, thus strengthening the key roles of reactive oxygen species (ROS) and MPO in monocyte ET formation. For comparative reasons, we additionally tested sporozoite stages of the closely related parasite Eimeria bovis for their capacity to induce monocyte-derived ETs and showed that these stages indeed induce ETs. To our best knowledge, we here report for the first time on monocyte ETs against the apicomplexan parasites B. besnoiti and E. bovis. Our results indicate that monocyte-triggered ETs may represent an important effector mechanism of the host early innate immune response against B. besnoiti and add a new cell type to the list of cells capable to release ETs.

Impaired neutrophil extracellular trap formation: a novel defect in the innate immune system of aged individuals

Neutrophil extracellular traps (NETs) are a recently discovered addition to the defensive armamentarium of neutrophils, assisting in the immune response against rapidly dividing bacteria. Although older adults are more susceptible to such infections, no study has examined whether aging in humans influences NET formation. We report that TNF-α-primed neutrophils generate significantly more NETs than unprimed neutrophils and that lipopolysaccharide (LPS)- and interleukin-8 (IL-8)-induced NET formation exhibits a significant age-related decline. NET formation requires generation of reactive oxygen species (ROS), and this was also reduced in neutrophils from older donors identifying a mechanism for reduced NET formation. Expression of IL-8 receptors (CXCR1 and CXCR2) and the LPS receptor TLR4 was similar on neutrophils from young and old subjects, and neutrophils challenged with phorbol-12-myristate-13-acetate (PMA) showed no age-associated differences in ROS or NET production. Taken together, these data suggest a defect in proximal signalling underlies the age-related decline in NET and ROS generation. TNF-α priming involves signalling through p38 MAP kinase, but activation kinetics were comparable in neutrophils from young and old donors. In a clinical setting, we assessed the capacity of neutrophils from young and older patients with chronic periodontitis to generate NETs in response to PMA and hypochlorous acid (HOCL). Neutrophil extracellular trap generation to HOCL, but not PMA, was lower in older periodontitis patients but not in comparison with age-matched controls. Impaired NET formation is thus a novel defect of innate immunity in older adults but does not appear to contribute to the increased incidence of periodontitis in older adults.

New insights into the pathogenesis and management of lupus in children

Systemic lupus erythematosus (SLE) is the archetypal systemic autoimmune disease, characterised by inflammation causing a wide spectrum of major clinical manifestations that may affect any organ. Childhood-onset SLE (cSLE) is more severe with greater damage and drug burden than adult-onset SLE. Understanding the pathogenesis of cSLE is a key step in directing medical management. The dysregulated immune system, that in health is usually vital in protecting the body from infection, contributes significantly to the disease process. Improved knowledge of disease mechanism will help to identify potential targets for novel agents and the identification of new biomarkers of disease activity. This review will present current knowledge of the innate and adaptive immune responses in cSLE and the optimal patient management that aims to control the disease. Innate immune dysregulation includes the overexpression of interferon-α, dendritic cell activation, neutrophil extracellular traps and phagocyte abnormalities. The classical adaptive immune system is over activated in lupus with excessive autoantibody production due to abnormalities in B and T cell regulation. Novel biologic medications are being developed to specifically target these areas with the ultimate aim of improving the long-term outlook and quality of life for children living with Lupus.

The apicomplexan parasite Eimeria arloingi induces caprine neutrophil extracellular traps

As a novel effector mechanism polymorphonuclear neutrophils (PMN) release neutrophil extracellular traps (NETs), which represent protein-labeled DNA matrices capable of extracellular trapping and killing of invasive pathogens. Here, we demonstrate for the first time NET formation performed by caprine PMN exposed to different stages (sporozoites and oocysts) of the goat apicomplexan protozoan parasite Eimeria arloingi. Scanning electron microscopy as well as fluorescence microscopy of sporozoites- and oocysts-PMN co-cultures revealed a fine network of DNA fibrils partially covering the parasites. Immunofluorescence analyses confirmed the co-localization of histones (H3), neutrophil elastase (NE), and myeloperoxidase (MPO) in extracellular traps released from caprine PMN. In addition, the enzymatic activity of NE was found significantly enhanced in sporozoite-exposed caprine PMN. The treatment of caprine NET structures with deoxyribonuclease (DNase) and the NADPH oxidase inhibitor diphenylene iodondium (DPI) significantly reduced NETosis confirming the classical characteristics of NETs. Caprine NETs efficiently trapped vital sporozoites of E. arloingi since 72% of these stages were immobilized-but not killed-in NET structures. As a consequence, early infection rates were significantly reduced when PMN-pre-exposed sporozoites were allowed to infect adequate host cells. These findings suggest that NETs may play an important role in the early innate host response to E. arloingi infection in goats.