Immobilized immune complexes induce neutrophil extracellular trap release by human neutrophil granulocytes via FcγRIIIB and Mac-1

Current Challenges and Limitations in Antibody-Based Detection of Citrullinated Histones

Studies on NETosis demand reliable and convenient markers to monitor the progress of this form of cell death. Because a determining step in the release of nuclear chromatin NETs requires the conversion of arginine residues to citrulline residues in histones by peptidylarginine deiminase, citrullinated histones can provide such a marker. Here, we evaluate antibody reagents for the detection of citrulline residues in histones and observe alarming differences between commercial antisera and mouse and rabbit monoclonal antibodies in their ability to detect their nominal target residues. Differences between antibodies that are currently used to detect citrulline residues in histones could jeopardize efforts to reach a scientific consensus and instead lead to inconsistent and even conflicting conclusions regarding the regulation of histone deimination. Our results will assist others in planning their initial or ongoing studies on peptidylarginine deiminase activity with the use of currently available antibodies. Furthermore, we argue that, along with the careful attention to experimental conditions and calcium concentrations, validated antibody reagents are urgently needed to avoid possible setbacks in the research on NETosis.

Neutrophils Discriminate between Lipopolysaccharides of Different Bacterial Sources and Selectively Release Neutrophil Extracellular Traps

The release of neutrophil extracellular traps (NETs), either during "suicidal" or "vital" NETosis, represents an important strategy of neutrophils to combat Gram-negative bacteria. Lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria, is a reported stimulus for NET formation. Although it is widely acknowledged that the structural diversity in LPS structures can elicit heterogeneous immune responses, species- and serotype-specific differences in the capacity of LPS to trigger NET formation have not yet been investigated. In the present study, we compared the NET-inducing potential of LPS derived from Escherichia coli (serotypes O55:B5, O127:B8, O128:B12, O111:B4, and O26:B6), Salmonella enterica (serotype enteritidis), and Pseudomonas aeruginosa (serotype 10), under platelet-free and platelet-rich conditions in vitro, and in whole blood ex vivo. Here, we demonstrate that under serum- and platelet-free conditions, mimicking tissue circumstances, neutrophils discriminate between LPS of different bacterial sources and selectively release NETs only in response to LPS derived from E. coli O128:B12 and P. aeruginosa 10, which both induced "suicidal" NETosis in an autophagy- and reactive oxygen species (ROS)-dependent, but TLR4-independent manner. Intriguingly, in whole blood cultures ex vivo, or in vitro in the presence of platelets, all LPS serotypes induced "vital" NET formation. This platelet-dependent release of NETs occurred rapidly without neutrophil cell death and was independent from ROS formation and autophagy but required platelet TLR4 and CD62P-dependent platelet-neutrophil interactions. Taken together, our data reveal a complex interplay between neutrophils and LPS, which can induce both "suicidal" and "vital" NETosis, depending on the bacterial origin of LPS and the presence or absence of platelets. Our findings suggest that LPS sensing by neutrophils may be a critical determinant for restricting NET release to certain Gram-negative bacteria only, which in turn may be crucial for minimizing unnecessary NET-associated immunopathology.

Milk fat globule-EGF factor 8 suppresses the aberrant immune response of systemic lupus erythematosus-derived neutrophils and associated tissue damage

Abnormal features of the systemic lupus erythematosus (SLE)-derived neutrophils, promoted aberrant immune response, have inspired new studies of the induction of autoimmunity and the development of organ damage in SLE. In this study, we explore the effect of milk fat globule-EGF factor 8 (MFG-E8) on the aberrant nitrification features in pristane-induced lupus. SLE patients and mice with pristane-induced lupus develop autoantibodies associated with MFG-E8 overproduction. However, the deletion of MFG-E8 leads to uncontrolled early pulmonary and peritoneal inflammation and tissue damage in mice with pristane-induced lupus. Consistent with these findings, MFG-E8-deficient mice that are exposed to pristane show enhanced neutrophil accumulation and increased neutrophil death, including apoptosis, necrosis and NETosis, as well as impaired phagocytosis of macrophages. The consequences are the expansion of diffuse pulmonary hemorrhage, increased anti-nuclear antibody, anti-dsDNA antibody and anti-neutrophil cytoplasmic antibody levels, and enhanced immune complexes deposition and neutrophil extracellular traps (NETs) formation in the lung and kidney tissues of MFG-E8-deficient mice exposed to pristane. In patients with SLE and mice with pristane-induced lupus, neutrophil accumulation is elevated, which depends on higher expression of the surface receptor CXCR2. After pretreatment with recombinant MFG-E8, the surface expression of CXCR2 on neutrophil is downregulated, and the MFG-E8 deletion increase CXCR2 expression by ~40%. These studies indicate that MFG-E8 reduces neutrophil migration and NETosis via downregulating surface CXCR2 expression in parallel with its role in the phagocytosis of apoptotic neutrophils, suggesting that MFG-E8 may serve as a therapeutic agent for attenuating the early inflammatory responses of SLE and protect patients from lupus-related damage.

The many faces of Mac-1 in autoimmune disease

Mac-1 (CD11b/CD18) is a β2 integrin classically regarded as a pro-inflammatory molecule because of its ability to promote phagocyte cytotoxic functions and enhance the function of several effector molecules such as FcγR, uPAR, and CD14. Nevertheless, recent reports have revealed that Mac-1 also plays significant immunoregulatory roles, and genetic variants in ITGAM, the gene that encodes CD11b, confer risk for the autoimmune disease systemic lupus erythematosus (SLE). This has renewed interest in the physiological roles of this integrin and raised new questions on how its seemingly opposing biological functions may be regulated. Here, we provide an overview of the CD18 integrins and how their activation may be regulated as this may shed light on how the opposing roles of Mac-1 may be elicited. We then discuss studies that exemplify Mac-1's pro-inflammatory versus regulatory roles particularly in the context of IgG immune complex-mediated inflammation. This includes a detailed examination of molecular mechanisms that could explain the risk-conferring effect of rs1143679, a single nucleotide non-synonymous Mac-1 polymorphism associated with SLE.

Characterization of neutrophils and macrophages from ex vivo-cultured murine bone marrow for morphologic maturation and functional responses by imaging flow cytometry

Neutrophils and macrophages differentiate from common myeloid progenitors in the bone marrow, where they undergo nuclear morphologic changes during maturation. During this process, both cell types acquire critical innate immune functions that include phagocytosis of pathogens, and for neutrophils the release of nuclear material called nuclear extracellular traps (NETs). Primary cells used to study these functions are typically purified from mature mouse tissues, but bone marrow-derived ex vivo cultures provide more abundant numbers of progenitors and functionally mature cells. Routine analyses of these cells use conventional microscopy and flow cytometry, which present limitations; microscopy is laborious and subjective, whereas flow cytometry lacks spatial resolution. Here we describe methods to generate enriched populations of neutrophils or macrophages from cryopreserved mouse bone marrow cultured ex vivo, and to use imaging flow cytometry that combines the resolution of microscopy with flow cytometry to analyze cells for morphologic features, phagocytosis, and NETosis.

Tyrosine kinase signaling pathways in neutrophils

Neutrophils play a critical role in antimicrobial host defense, but their improper activation also contributes to inflammation-induced tissue damage. Therefore, understanding neutrophil biology is important for the understanding, diagnosis, and therapy of both infectious and inflammatory diseases. Neutrophils express a large number of cell-surface receptors that sense extracellular cues and trigger various functional responses through complex intracellular signaling pathways. During the last several years, we and others have shown that tyrosine kinases play a critical role in those processes. In particular, Src-family and Syk tyrosine kinases couple Fc-receptors and adhesion receptors (integrins and selectins) to various neutrophil effector functions. This pathway shows surprising similarity to lymphocyte antigen receptor signaling and involves various other enzymes (e.g. PLCγ2), exchange factors (e.g. Vav-family members) and adapter proteins (such as ITAM-containing adapters, SLP-76, and CARD9). Those mediators trigger various antimicrobial functions and play a critical role in coordinating the inflammatory response through the release of inflammatory mediators, such as chemokines and LTB4 . Interestingly, however, tyrosine kinases have a limited direct role in the migration of neutrophils to the site of inflammation. Here, we review the role of tyrosine kinase signaling pathways in neutrophils and how those pathways contribute to neutrophil activation in health and disease.

Neutrophil extracellular traps in dermatology: Caught in the NET

Neutrophil, or polymorphonuclear granulocytes (PMN) constitute the most abundant type of leucocytes in peripheral human blood. One of the major advances in the last decade was the discovery of neutrophil extracellular trap (NET) formation: a process by which neutrophils externalize web-like chromatin strands decorated with antimicrobial peptides. These structures were soon implicated in immune defense and auto-immunity alike and now link neutrophils to the pathogenesis of a variety of diseases of dermatological relevance. Currently, NET formation is mainly subdivided into suicidal and vital NETosis. Controversy exists regarding the capacity of NETs to kill pathogens, and little is known about the way NETs are formed in vivo. Here, we discuss the current terminology, methods for NET quantification, pathways leading to NET formation, and the role of NETs in systemic and cutaneous immune defense and auto-immunity, with a focus on psoriasis and systemic lupus erythematosus.

Transforming Growth Factor-β-Activated Kinase 1 Is Required for Human FcγRIIIb-Induced Neutrophil Extracellular Trap Formation

Neutrophils (PMNs) are the most abundant leukocytes in the blood. PMN migrates from the circulation to sites of infection where they are responsible for antimicrobial functions. PMN uses phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs) to kill microbes. Several stimuli, including bacteria, fungi, and parasites, and some pharmacological compounds, such as Phorbol 12-myristate 13-acetate (PMA), are efficient inducers of NETs. Antigen-antibody complexes are also capable of inducing NET formation. Recently, it was reported that FcγRIIIb cross-linking induced NET formation similarly to PMA stimulation. Direct cross-linking of FcγRIIA or integrins did not promote NET formation. FcγRIIIb-induced NET formation presented different kinetics from PMA-induced NET formation, suggesting differences in signaling. Because FcγRIIIb also induces a strong activation of extracellular signal-regulated kinase (ERK) and nuclear factor Elk-1, and the transforming growth factor-β-activated kinase 1 (TAK1) has recently been implicated in ERK signaling, in the present report, we explored the role of TAK1 in the signaling pathway activated by FcγRIIIb leading to NET formation. FcγRIIIb was stimulated by specific monoclonal antibodies, and NET formation was evaluated in the presence or absence of pharmacological inhibitors. The antibiotic LL Z1640-2, a selective inhibitor of TAK1 prevented FcγRIIIb-induced, but not PMA-induced NET formation. Both PMA and FcγRIIIb cross-linking induced phosphorylation of ERK. But, LL Z1640-2 only inhibited the FcγRIIIb-mediated activation of ERK. Also, only FcγRIIIb, similarly to transforming growth factor-β-induced TAK1 phosphorylation. A MEK (ERK kinase)-specific inhibitor was able to prevent ERK phosphorylation induced by both PMA and FcγRIIIb. These data show for the first time that FcγRIIIb cross-linking activates TAK1, and that this kinase is required for triggering the MEK/ERK signaling pathway to NETosis.

Superoxide induces Neutrophil Extracellular Trap Formation in a TLR-4 and NOX-dependent mechanism

Neutrophils constitute the early innate immune response to perceived infectious and sterile threats. Neutrophil Extracellular Traps (NETs) are a novel mechanism to counter pathogenic invasion and sequelae of ischemia including cell death and oxidative stress. Superoxide is a radical intermediate of oxygen metabolism produced by parenchymal and non-parenchymal hepatic cells, and is a hallmark of oxidative stress after liver ischemia-reperfusion (I/R). While extracellular superoxide recruits neutrophils to the liver and initiates sterile inflammatory injury, it is unknown whether superoxide induces the formation of NETs. We hypothesize that superoxide induces NET formation through a signaling cascade involving Toll-like receptor 4 (TLR-4) and neutrophil NADPH Oxidase (NOX). We treated neutrophils with extracellular superoxide and observed NET DNA release, histone H3 citrullination, and increased levels of MPO-DNA complexes occurring in a TLR-4 dependent manner. Inhibition of superoxide generation by Allopurinol and inhibition of NOX by diphenyleneiodonium prevented NET formation. When mice were subjected to warm liver I/R, we found significant NET formation associated with liver necrosis and increased serum ALT in TLR-4 WT, but not TLR-4 KO mice. To reduce circulating superoxide we pretreated mice undergoing I/R with Allopurinol and N-acetylcysteine, which resulted in decreased NETs and ameliorated liver injury. Our study demonstrates a requirement for TLR-4 and NOX in superoxide-induced NETs, and suggests involvement of superoxide-induced NETs in pathophysiologic settings.

The role of neutrophils and NETosis in autoimmune and renal diseases

Systemic autoimmune diseases are a group of disorders characterized by a failure in self-tolerance to a wide variety of autoantigens. In genetically predisposed individuals, these diseases occur as a multistep process in which environmental factors have key roles in the development of abnormal innate and adaptive immune responses. Experimental evidence collected in the past decade suggests that neutrophils - the most abundant type of white blood cell - might have an important role in the pathogenesis of these diseases by contributing to the initiation and perpetuation of immune dysregulation through the formation of neutrophil extracellular traps (NETs), synthesis of proinflammatory cytokines and direct tissue damage. Many of the molecules externalized through NET formation are considered to be key autoantigens and might be involved in the generation of autoimmune responses in predisposed individuals. In several systemic autoimmune diseases, the imbalance between NET formation and degradation might increase the half-life of these lattices, which could enhance the exposure of the immune system to modified autoantigens and increase the capacity for NET-induced organ damage. This Review details the role of neutrophils and NETs in the pathophysiology of systemic autoimmune diseases, including their effect on renal damage, and discusses neutrophil targets as potential novel therapies for these diseases.

Dimethylfumarate Impairs Neutrophil Functions

Host defense against pathogens relies on neutrophil activation. Inadequate neutrophil activation is often associated with chronic inflammatory diseases. Neutrophils also constitute a significant portion of infiltrating cells in chronic inflammatory diseases, for example, psoriasis and multiple sclerosis. Fumarates improve the latter diseases, which so far has been attributed to the effects on lymphocytes and dendritic cells. Here, we focused on the effects of dimethylfumarate (DMF) on neutrophils. In vitro, DMF inhibited neutrophil activation, including changes in surface marker expression, reactive oxygen species production, formation of neutrophil extracellular traps, and migration. Phagocytic ability and autoantibody-induced, neutrophil-dependent tissue injury ex vivo was also impaired by DMF. Regarding the mode of action, DMF modulates-in a stimulus-dependent manner-neutrophil activation using the phosphoinositide 3-kinase/Akt-p38 mitogen-activated protein kinase and extracellular signal-regulated kinase 1/2 pathways. For in vivo validation, mouse models of epidermolysis bullosa acquisita, an organ-specific autoimmune disease caused by autoantibodies to type VII collagen, were employed. In the presence of DMF, blistering induced by injection of anti-type VII collagen antibodies into mice was significantly impaired. DMF treatment of mice with clinically already-manifested epidermolysis bullosa acquisita led to disease improvement. Collectively, we demonstrate a profound inhibitory activity of DMF on neutrophil functions. These findings encourage wider use of DMF in patients with neutrophil-mediated diseases.

Neutrophil extracellular traps release induced by Leishmania: role of PI3Kγ, ERK, PI3Kσ, PKC, and [Ca2+]

Upon in vitro stimulation, neutrophils undergo a cell death named netosis. This process is characterized by extracellular release of chromatin scaffold associated with granular and cytoplasmic proteins, which together, ensnare and kill microbes. We have previously described that interaction of Leishmania amazonensis with human neutrophils leads to the release of neutrophil extracellular traps, which trap and kill the parasite. However, the signaling leading to Leishmania induced netosis is still unknown. Thus, we sought to evaluate signaling events that drive L. amazonensis induced neutrophil extracellular trap release from human neutrophils. Here, we found that PI3K, independently of protein kinase B, has a role in parasite-induced netosis. We also described that the main isoforms involved are PI3Kγ and PI3Kδ, which work in reactive oxygen species-dependent and -independent ways, respectively. We demonstrated that activation of ERK downstream of PI3Kγ is important to trigger reactive oxygen species-dependent, parasite-induced netosis. Pharmacological inhibition of protein kinase C also significantly decreased parasite-induced neutrophil extracellular trap release. Intracellular calcium, regulated by PI3Kδ, represents an alternative reactive oxygen species-independent pathway of netosis stimulated by L. amazonensis Finally, intracellular calcium mobilization and reactive oxygen species generation are the major regulators of parasite-induced netosis. Our results contribute to a better understanding of the signaling behind netosis induced by interactions between Leishmania and neutrophils.

Placental histology and neutrophil extracellular traps in lupus and pre-eclampsia pregnancies

Objective

Systemic lupus erythematosus (SLE) is associated with increased risk of adverse pregnancy outcomes, including pre-eclampsia, particularly in association with antiphospholipid antibody syndrome (APS). While significant placental abnormalities are expected in pre-eclampsia, less is known about how lupus activity and APS in pregnancy affect the placenta. We describe placental pathology from a population of lupus pregnancies, several of which were complicated by APS-related thromboses, in which pre-eclampsia and other complications developed. We performed standard histopathological placental review and quantified neutrophils and neutrophil extracellular traps (NETs) in the intervillous space, given the recognised association of NETs with lupus, APS and pre-eclampsia.

Methods

Pre-eclampsia, SLE and control placentas were scored for histological features, and neutrophils were quantified on H&E and immunohistochemical staining for the granular protein myeloperoxidase. NETs were identified by extracellular myeloperoxidase staining in the setting of decondensed nuclei. Non-parametric analysis was used to evaluate differences in netting and intact neutrophils between groups, with Kruskal–Wallis testing for associations between histological findings and neutrophils.

Results

Placentas were evaluated from 35 pregnancies: 10 controls, 11 pre-eclampsia, 4 SLE+pre-eclampsia and 10 SLE, including one complicated by catastrophic APS and one complicated by hepatic and splenic vein thromboses during pregnancy. Intrauterine growth restriction and oligohydramnios were observed in lupus cases but not controls. Significantly more NETs were found infiltrating placental intervillous spaces in pre-eclampsia, SLE+pre-eclampsia and all 10 SLE non-pre-eclampsia cases. The ratio of NETs to total neutrophils was significantly increased in all case groups compared with controls. When present, NETs were associated with maternal vasculitis, laminar decidual necrosis, maternal–fetal interface haemorrhage and non-occlusive fetal thrombotic vasculopathy.

Conclusions

In this pilot study of placental tissue from lupus pregnancies, outcomes were more complicated, particularly if associated with APS. Placental tissue revealed marked inflammatory and vascular changes that were essentially indistinguishable from placental tissue of pre-eclampsia pregnancies.

Oxidized LDL induced extracellular trap formation in human neutrophils via TLR-PKC-IRAK-MAPK and NADPH-oxidase activation

Neutrophil extracellular traps (NETs) formation was initially linked with host defence and extracellular killing of pathogens. However, recent studies have highlighted their inflammatory potential. Oxidized low density lipoprotein (oxLDL) has been implicated as an independent risk factor in various acute or chronic inflammatory diseases including systemic inflammatory response syndrome (SIRS). In the present study we investigated effect of oxLDL on NETs formation and elucidated the underlying signalling mechanism. Treatment of oxLDL to adhered PMNs led to a time and concentration dependent ROS generation and NETs formation. OxLDL induced free radical formation and NETs release were significantly prevented in presence of NADPH oxidase (NOX) inhibitors suggesting role of NOX activation in oxLDL induced NETs release. Blocking of both toll like receptor (TLR)-2 and 6 significantly reduced oxLDL induced NETs formation indicating requirement of both the receptors. We further identified Protein kinase C (PKC), Interleukin-1 receptor associated kinase (IRAKs), mitogen-activated protein kinase (MAPK) pathway as downstream intracellular signalling mediators involved in oxLDL induced NETs formation. OxLDL components such as oxidized phospholipids (lysophosphatidylcholine (LPC) and oxidized 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (oxPAPC)) were most potent NETs inducers and might be crucial for oxLDL mediating NETs release. Other components like, oxysterols, malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) were however less potent as compared to oxidized phospholipids. This study thus demonstrates for the first time that treatment of human PMNs with oxLDL or its various oxidized phopholipid component mediated NETs release, implying their role in the pathogenesis of inflammatory diseases such as SIRS.

Neutrophil Isolation and Analysis to Determine their Role in Lymphoma Cell Sensitivity to Therapeutic Agents

Neutrophils are the most abundant (40% to 75%) type of white blood cells and among the first inflammatory cells to migrate towards the site of inflammation. They are key players in the innate immune system and play major roles in cancer biology. Neutrophils have been proposed as key mediators of malignant transformation, tumor progression, angiogenesis and in the modulation of the antitumor immunity; through their release of soluble factors or their interaction with tumor cells. To characterize the specific functions of neutrophils, a fast and reliable method is coveted for in vitro isolation of neutrophils from human blood. Here, a density gradient separation method is demonstrated to isolate neutrophils as well as mononuclear cells from the blood. The procedure consists of layering the density gradient solution such as Ficoll carefully above the diluted blood obtained from patients diagnosed with chronic lymphocytic leukemia (CLL), followed by centrifugation, isolation of mononuclear layer, separation of neutrophils from RBCsby dextran then lysis of residual erythrocytes. This method has been shown to isolate neutrophils ≥ 90 % pure. To mimic the tumor microenvironment, 3-dimensional (3D) experiments were performed using basement membrane matrix such as Matrigel. Given the short half-life of neutrophils in vitro, 3D experiments with fresh human neutrophils cannot be performed. For this reason promyelocytic HL60 cells are differentiated along the granulocytic pathway using the differentiation inducers dimethyl sulfoxide (DMSO) and retinoic acid (RA). The aim of our experiments is to study the role of neutrophils on the sensitivity of lymphoma cells to anti-lymphoma agents. However these methods can be generalized to study the interactions of neutrophils or neutrophil-like cells with a large range of cell types in different situations.

Immunoreceptors on neutrophils

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Neutrophil activities must be tightly controlled to maintain immune homeostasis.

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Activating and inhibitory receptors balance the outcome of immune cell activation.

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Immunoreceptors contain Ig-like extracellular domains and signal via ITAMs or ITIMs.

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Syk or SHP/SHIP mediate downstream signaling after immunoreceptor activation.

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Targeting immunoreceptors provides opportunities for therapeutic interventions.

Neutrophils play a critical role in the host defense against infection, and they are able to perform a variety of effector mechanisms for this purpose. However, there are also a number of pathological conditions, including autoimmunity and cancer, in which the activities of neutrophils can be harmful to the host. Thus the activities of neutrophils need to be tightly controlled. As in the case of other immune cells, many of the neutrophil effector functions are regulated by a series of immunoreceptors on the plasma membrane. Here, we review what is currently known about the functions of the various individual immunoreceptors and their signaling in neutrophils. While these immunoreceptors allow for the recognition of a diverse range of extracellular ligands, such as cell surface structures (like proteins, glycans and lipids) and extracellular matrix components, they commonly signal via conserved ITAM or ITIM motifs and their associated downstream pathways that depend on the phosphorylation of tyrosine residues in proteins and/or inositol lipids. This allows for a balanced homeostatic regulation of neutrophil effector functions. Given the number of available immunoreceptors and their fundamental importance for neutrophil behavior, it is perhaps not surprising that pathogens have evolved means to evade immune responses through some of these pathways. Inversely, some of these receptors evolved to specifically recognize these pathogens. Finally, some interactions mediated by immunoreceptors in neutrophils have been identified as promising targets for therapeutic intervention.

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.

A novel method for high-throughput detection and quantification of neutrophil extracellular traps reveals ROS-independent NET release with immune complexes

A newly-described first-line immune defence mechanism of neutrophils is the release of neutrophil extracellular traps (NETs). Immune complexes (ICxs) induce low level NET release. As such, the in vitro quantification of NETs is challenging with current methodologies. In order to investigate the role of NET release in ICx-mediated autoimmune diseases, we developed a highly sensitive and automated method for quantification of NETs. After labelling human neutrophils with PKH26 and extracellular DNA with Sytox green, cells are fixed and automatically imaged with 3-dimensional confocal laser scanning microscopy (3D-CLSM). NET release is then quantified with digital image analysis whereby the NET amount (Sytox green area) is corrected for the number of imaged neutrophils (PKH26 area). A high sensitivity of the assay is achieved by a) significantly augmenting the area of the well imaged (11%) as compared to conventional assays (0.5%) and b) using a 3D imaging technique for optimal capture of NETs, which are topologically superimposed on neutrophils. In this assay, we confirmed low levels of NET release upon human ICx stimulation which were positive for citrullinated histones and neutrophil elastase. In contrast to PMA-induced NET release, ICx-induced NET release was unchanged when co-incubated with diphenyleneiodonium (DPI). We were able to quantify NET release upon stimulation with serum from RA and SLE patients, which was not observed with normal human serum. To our knowledge, this is the first semi-automated assay capable of sensitive detection and quantification of NET release at a low threshold by using 3D CLSM. The assay is applicable in a high-throughput manner and allows the in vitro analysis of NET release in ICx-mediated autoimmune diseases.

Differential Use of Human Neutrophil Fcγ Receptors for Inducing Neutrophil Extracellular Trap Formation

Neutrophils (PMN) are the most abundant leukocytes in the blood. PMN migrate from the circulation to sites of infection, where they are responsible for antimicrobial functions. PMN use phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs) to kill microbes. NETs are fibers composed of chromatin and neutrophil-granule proteins. Several pathogens, including bacteria, fungi, and parasites, and also some pharmacological stimuli such as phorbol 12-myristate 13-acetate (PMA) are efficient inducers of NETs. Antigen-antibody complexes are also capable of inducing NET formation. However the particular Fcγ receptor involved in triggering this function is a matter of controversy. In order to provide some insight into what Fcγ receptor is responsible for NET formation, each of the two human Fcγ receptors was stimulated individually by specific monoclonal antibodies and NET formation was evaluated. FcγRIIa cross-linking did not promote NET formation. Cross-linking other receptors such as integrins also did not promote NET formation. In contrast FcγRIIIb cross-linking induced NET formation similarly to PMA stimulation. NET formation was dependent on NADPH-oxidase, PKC, and ERK activation. These data show that cross-linking FcγRIIIb is responsible for NET formation by the human neutrophil.

Roflumilast inhibits leukocyte-platelet interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes and monocytes

ESSENTIALS: Thrombosis is a major comorbidity in patients with chronic obstructive pulmonary disease (COPD). Roflumilast is a selective phosphodiesterase type-4 (PDE4) inhibitor approved for treatment of severe COPD. PDE4 blockade by roflumilast inhibits prothrombotic functions of neutrophils and monocytes. PDE4 inhibitors may reduce thrombotic risk in COPD as well as in other vascular diseases.

BACKGROUND

Roflumilast, an oral selective phosphodiesterase type 4 inhibitor, is approved for the treatment of severe chronic obstructive pulmonary disease (COPD). A recent meta-analysis of trials on COPD revealed that treatment with roflumilast was associated with a significant reduction in the rate of major cardiovascular events. The mechanisms of this effect remain unknown.

OBJECTIVES

We tested the hypothesis that roflumilast N-oxide (RNO), the active metabolite of roflumilast, curbs the molecular mechanisms required for leukocyte-platelet (PLT) interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes (PMNs) and monocytes (MNs).

METHODS

Using well-characterized in vitro models, we analysed the effects of RNO on: (i) PMN adhesiveness; (ii) the release of neutrophil extracellular traps (NETs); and (iii) tissue factor expression in MNs. Key biochemical events underlying the inhibitory effects of RNO were defined.

RESULTS AND CONCLUSIONS

In PMNs, RNO prevented phosphoinositide 3-kinase (PI3K)-dependent phosphorylation of Akt on Ser473, and Src family kinase (SFK)-mediated Pyk2 phosphorylation on Tyr579-580, while inducing protein kinase A-mediated phosphorylation of C-terminal Src kinase, the major negative regulator of SFKs. Modulation of these signaling pathways by RNO resulted in a significant impairment of PMN adhesion to activated PLTs or human umbilical vein endothelial cells, mainly mediated by inhibition of the adhesive function of Mac-1. Moreover RNO curbed SFK/PI3K-mediated NET release by PMNs adherent on fibrinogen-coated surfaces. In MNs interacting with activated PLTs, RNO curbed PI3K-mediated expression of tissue factor. The efficacy of RNO was significantly potentiated by formoterol, a long acting β-adrenergic receptor agonist. This study reveals novel antithrombotic activities by which roflumilast may exert protective effects against cardiovascular comorbodities in COPD.

Do neutrophil extracellular traps contribute to the heightened risk of thrombosis in inflammatory diseases?

Thrombotic events, both arterial and venous, are a major health concern worldwide. Further, autoimmune diseases, such as systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, and antiphospholipid syndrome, predispose to thrombosis, and thereby push the risk for these morbid events even higher. In recent years, neutrophils have been identified as important players in both arterial and venous thrombosis. Specifically, chromatin-based structures called neutrophil extracellular traps (NETs) play a key role in activating the coagulation cascade, recruiting platelets, and serving as scaffolding upon which the thrombus can be assembled. At the same time, neutrophils and NETs are emerging as important mediators of pathogenic inflammation in the aforementioned autoimmune diseases. Here, we first review the general role of NETs in thrombosis. We then posit that exaggerated NET release contributes to the prothrombotic diatheses of systemic lupus erythematosus, ANCA-associated vasculitis, and antiphospholipid syndrome.

Neutrophils in Cancer: Two Sides of the Same Coin

Neutrophils are the most abundant leukocytes in blood and are considered to be the first line of defense during inflammation and infections. In addition, neutrophils are also found infiltrating many types of tumors. Tumor-associated neutrophils (TANs) have relevant roles in malignant disease. Indeed neutrophils may be potent antitumor effector cells. However, increasing clinical evidence shows TANs correlate with poor prognosis. The tumor microenvironment controls neutrophil recruitment and in turn TANs help tumor progression. Hence, TANs can be beneficial or detrimental to the host. It is the purpose of this review to highlight these two sides of the neutrophil coin in cancer and to describe recent studies that provide some light on the mechanisms for neutrophil recruitment to the tumor, for neutrophils supporting tumor progression, and for neutrophil activation to enhance their antitumor functions.

The NET, the trap and the pathogen: neutrophil extracellular traps in cutaneous immunity

Neutrophil extracellular traps (NETs), large chromatin structures casted with various proteins, are externalized by neutrophils upon induction by both self- and non-self-stimuli. It has become clear that NETs are potent triggers of inflammation in autoimmune skin diseases. Moreover, the ability of NETs to trap pathogens suggests a crucial role in innate host defense. However, the outcome of the encounter between pathogens and NETs remains highly controversial. Here, we discuss recent insights into the morphology and formation of NETs, their role in skin inflammation and how NETs might contribute to host protection in skin infection.

Plasmacytoid Dendritic Cell Activation and IFN-α Production Are Prominent Features of Murine Autoimmune Pancreatitis and Human IgG4-Related Autoimmune Pancreatitis

The abnormal immune response accompanying IgG4-related autoimmune pancreatitis (AIP) is presently unclear. In this study, we examined the role of plasmacytoid dendritic cell (pDC) activation and IFN-α production in this disease as well as in a murine model of AIP (MRL/Mp mice treated with polyinosinic-polycytidylic acid). We found that the development of AIP in treated MRL/Mp mice occurred in parallel with pancreatic accumulation of pDCs producing IFN-α, and with pDC depletion and IFN-α-blocking studies, we showed that such accumulation was necessary for AIP induction. In addition, we found that the pancreas of treated MRL/Mp mice contained neutrophil extracellular traps (NETs) shown previously to stimulate pDCs to produce IFN-α. Consistent with these findings, we found that patients with IgG4-related AIP also exhibited pancreatic tissue localization of IFN-α-expressing pDCs and had significantly higher serum IFN-α levels than healthy controls. In addition, the inflamed pancreas of these patients but not controls also contained NETs that were shown to be capable of pDC activation. More importantly, patient pDCs cultured in the presence of NETs produced greatly increased levels of IFN-α and induced control B cells to produce IgG4 (but not IgG1) as compared with control pDCs. These data suggest that pDC activation and production of IFN-α is a major cause of murine AIP; in addition, the increased pDC production of IFN-α and its relation to IgG4 production observed in IgG4-related AIP suggest that this mechanism also plays a role in the human disease.

How neutrophil extracellular traps orchestrate the local immune response in gout

Neutrophil granulocytes possess a large arsenal of pro-inflammatory substances and mechanisms that empower them to drive local acute immune reactions to invading microorganisms or endogenous inflammatory triggers. The use of this armory needs to be tightly controlled to avoid chronic inflammation and collateral tissue damage. In gout, inflammation arises from precipitation of uric acid in the form of needle-shaped monosodium urate crystals. Inflammasome activation by these crystals in local immune cells results in a rapid and dramatic recruitment of neutrophils. This neutrophil influx is accompanied by the infamously intense clinical symptoms of inflammation during an acute gout attack. Neutrophilic inflammation however is equipped with a built-in safeguard; activated neutrophils form neutrophil extracellular traps (NETs). At the very high neutrophil densities that occur at the site of inflammation, NETs build aggregates that densely pack the monosodium urate (MSU) crystals and trap and degrade pro-inflammatory mediators by inherent proteases. Local removal of cytokines and chemokines by aggregated NETs explains how acute inflammation can stop in the consistent presence of the inflammatory trigger. Aggregated NETs resemble early stages of the typical large MSU deposits that constitute the pathognomonic structures of gout, tophi. Although tophi contribute to muscosceletal damage and mortality in patients with chronic gout, they can therefore be considered as a payoff that is necessary to silence the intense inflammatory response during acute gout.

The Role of Reactive Oxygen Species (ROS) in the Formation of Extracellular Traps (ETs) in Humans

Extracellular traps (ETs) are reticulate structures of extracellular DNA associated with antimicrobial molecules. Their formation by phagocytes (mainly by neutrophils: NETs) has been identified as an essential element of vertebrate innate immune defense. However, as ETs are also toxic to host cells and potent triggers of autoimmunity, their role between pathogen defense and human pathogenesis is ambiguous, and they contribute to a variety of acute and chronic inflammatory diseases. Since the discovery of ET formation (ETosis) a decade ago, evidence has accumulated that most reaction cascades leading to ET release involve ROS. An important new facet was added when it became apparent that ETosis might be directly linked to, or be a variant of, the autophagy cell death pathway. The present review analyzes the evidence to date on the interplay between ROS, autophagy and ETosis, and highlights and discusses several further aspects of the ROS-ET relationship that are incompletely understood. These aspects include the role of NADPH oxidase-derived ROS, the molecular requirements of NADPH oxidase-dependent ETosis, the roles of NADPH oxidase subtypes, extracellular ROS and of ROS from sources other than NADPH oxidase, and the present evidence for ROS-independent ETosis. We conclude that ROS interact with ETosis in a multidimensional manner, with influence on whether ETosis shows beneficial or detrimental effects.

Platelet-neutrophil interactions under thromboinflammatory conditions

Platelets primarily mediate hemostasis and thrombosis, whereas leukocytes are responsible for immune responses. Since platelets interact with leukocytes at the site of vascular injury, thrombosis and vascular inflammation are closely intertwined and occur consecutively. Recent studies using real-time imaging technology demonstrated that platelet-neutrophil interactions on the activated endothelium are an important determinant of microvascular occlusion during thromboinflammatory disease in which inflammation is coupled to thrombosis. Although the major receptors and counter receptors have been identified, it remains poorly understood how heterotypic platelet-neutrophil interactions are regulated under disease conditions. This review discusses our current understanding of the regulatory mechanisms of platelet-neutrophil interactions in thromboinflammatory disease.

Polymorphism of FCGR2A, FCGR2C, and FCGR3B Genes in the Pathogenesis of Sarcoidosis

We have previously presented evidence that the polymorphism of the FCGR3A gene, encoding the receptor for Fc fragment of immunoglobulin G IIIa (FcγRIIIa) plays a role in the enhancement of circulating immune complexes (CIs) with the occurrence of Mycobacterium tuberculosis heat shock proteins in patients with sarcoidosis (SA). The immunocomplexemia might be caused by decreased affinity of CIs to Fcγ receptors, with the subsequently decreased receptor clearance by immune cells. In the present study we examined whether the polymorphisms of other related genes (FCGR2A, FCGR2C, FCGR3B) encoding other activatory Fcγ receptors, could have a similar effect. To this end, we genotyped 124 patients with sarcoidosis and 148 healthy volunteers using polymerase chain reaction with sequence-specific primers. We revealed a significant decrease in the percentage of the FCGR2A and FCGR2C variants that ensure effective CIs clearance, with a concomitant increase of less functional variants of these genes in Stages I/II, compared with Stages III/IV of SA. There was no aberration in FCGR3B allele/genotype frequencies. We conclude that the FCGR2A and FCGR2C polymorphisms may also contribute to immunocomplexemia present in SA. The assessment of FCGR genes could become a tool in presaging a clinical course of sarcoidosis and in its personalized therapy.