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

Physiopathologic mechanisms involved in mare endometrosis

Endometrosis is a degenerative chronic process, characterized by paramount fibrosis development in mare endometrium. This condition is one of the major causes of subfertility/infertility in mares. As in other organs, fibrosis might be a pathologic sequel of many chronic inflammatory diseases. However, aetiology and physiopathologic mechanisms involved in endometrial fibrosis are still controversial. This review presents new hypotheses based on our newest data. As the first line of innate immune defence, systemic neutrophils arrive in the uterus at mating or in the presence of pathogens. A novel paradigm is that neutrophils cast out their DNA in response to infectious stimuli and form neutrophil extracellular traps (NETs). We have shown that bacterial strains of Streptococcus zooepidemicus, Escherichia coli or Staphylococcus capitis, known to cause endometritis in mares were able to induce NETs release in vitro by equine PMN to different extents. An intriguing dilemma is the dual action of NETs. While NETs play a desirable role fighting micro-organisms in mare uterus, they may also contribute to endometrial fibrosis. A long-term in vitro exposure of mare endometrium explants to NETs components (myeloperoxidase, elastase and cathepsin G) up-regulated fibrosis markers TGFβ and Tissue inhibitor of metalloproteinase (TIMP-1). Also, pro-fibrotic cytokines regulated collagen deposition and fibrosis. Changes in expression of connective tissue growth factor (CTGF), interleukins (IL)1-α, IL-1β, IL-6 and receptors in endometrium with different degrees of fibrosis and/or inflammation were observed. A putative role of CTGF, IL and NETs components in endometrosis development should be considered. Additionally, we speculate that in sustained endometritis in mares, prostaglandins may not only cause early luteolysis or early pregnancy loss, but may also be related to endometrial fibrosis pathogenesis by stimulating collagen deposition.

A Metabolic Shift toward Pentose Phosphate Pathway Is Necessary for Amyloid Fibril- and Phorbol 12-Myristate 13-Acetate-induced Neutrophil Extracellular Trap (NET) Formation

Neutrophils are the main defense cells of the innate immune system. Upon stimulation, neutrophils release their chromosomal DNA to trap and kill microorganisms and inhibit their dissemination. These chromatin traps are termed neutrophil extracellular traps (NETs) and are decorated with granular and cytoplasm proteins. NET release can be induced by several microorganism membrane components, phorbol 12-myristate 13-acetate as well as by amyloid fibrils, insoluble proteinaceous molecules associated with more than 40 different pathologies among other stimuli. The intracellular signaling involved in NET formation is complex and remains unclear for most tested stimuli. Herein we demonstrate that a metabolic shift toward the pentose phosphate pathway (PPP) is necessary for NET release because glucose-6-phosphate dehydrogenase (G6PD), an important enzyme from PPP, fuels NADPH oxidase with NADPH to produce superoxide and thus induce NETs. In addition, we observed that mitochondrial reactive oxygen species, which are NADPH-independent, are not effective in producing NETs. These data shed new light on how the PPP and glucose metabolism contributes to NET formation.

Peripheral blood neutrophil cytokine hyper-reactivity in chronic periodontitis

Pro-inflammatory cytokine release (IL-8, IL-6, TNF-α, IL-1β) by peripheral blood neutrophils, isolated from periodontitis patients (before/after therapy) and matched controls, was determined after 18 h culture in the presence/absence of Escherichia coli LPS, opsonised Staphylococcus aureus, heat-killed Fusobacterium nucleatum and Porphyromonas gingivalis. All cultures demonstrated differences in the amounts of each cytokine detected (P < 0.0001), with a clear release pattern (IL-8 > IL-6 > TNF-α = IL-1β). Median cytokine release from unstimulated patient neutrophils was consistently, but non-significantly, higher than from control cells. Stimulated cytokine release from untreated patient neutrophils was also consistently higher than from control cells. This hyper-reactivity was significant for all tested cytokines when data for all stimuli were combined (P < 0.016). In terms of individual stimuli, significant hyper-reactivity was detected with LPS (IL-8), F. nucleatum (IL-8, TNF-α), opsonised S. aureus (IL-8, TNF-α, IL-1β) and P. gingivalis (IL-8, IL-1β). Cytokine production by patient neutrophils did not reduce following successful non-surgical periodontal therapy and, except for responses to F. nucleatum, the cytokine hyper-reactivity detected pre-therapy was retained. These data demonstrate that chronic periodontitis is characterised by neutrophils that constitutively exhibit cytokine hyper-reactivity, the effects of which could modulate local and systemic inflammatory-immune responses and influence the risk and severity of periodontitis-associated systemic inflammatory diseases.

Periodontitis and Rheumatoid Arthritis: What Do We Know?

BACKGROUND

Currently, in the field of rheumatology, there is much attention given towards the possible causality between periodontitis and rheumatoid arthritis (RA), specifically regarding the role of Porphyromonas gingivalis (Pg). This bacterium is unique, having a citrullinating enzyme. Antibodies against citrullinated proteins are rather specific for RA.

METHODS

Because causality is ultimately tested in longitudinal cohort studies which currently do not exist for periodontitis and RA, this commentary applied Bradford Hill criteria on the existing literature to assess causality as the most likely interpretation of this association.

CONCLUSIONS

From an epidemiologic point of view, patients with RA have a higher incidence of periodontal disease than individuals without RA. In addition, there is a dose-response pattern in the association between the severity of periodontitis and RA disease activity. There are indications that periodontitis precedes RA, but there is no evidence yet available to show that Pg plays a direct role in this temporal relationship. The role of the unique characteristic of citrullination by Pg remains unexplained. However, in animal models, citrullination by Pg plays a distinct role in the development and aggravation of experimental arthritis. Although the role of Pg in RA remains speculative, a causative role for periodontitis as a chronic inflammatory disease caused by infectious agents in RA seems biologically plausible.

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

Increasing evidence indicates that aberrant neutrophil extracellular trap (NET) formation could contribute to the pathogenesis of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). Recent research has provided evidence that a novel type of ANCA autoantibody, anti-lysosomal membrane protein-2 (LAMP-2) antibody, may have a pathogenic role in AAV. We have shown previously that anti-LAMP-2 antibody-stimulated NET formation contains autoantigens and anti-microbial peptides. The current study sought to determine whether LAMP-2, as a novel antigen of ANCA, was present on NETs in AAV patients, the influence of the anti-LAMP-2 antibody on the neutrophil apoptosis rate and the role of autophagy in anti-LAMP-2 antibody-induced NET formation. NET formation was assessed using immunofluorescence microscopy, scanning electron microscopy or live cell imaging. The neutrophil apoptosis rate was analysed using fluorescence activated cell sorting (FACS). Autophagy was detected using LC3B accumulation and transmission electron microscopy. The results showed that enhanced NET formation, which contains LAMP-2, was observed in kidney biopsies and neutrophils from AAV patients. The apoptosis rate decreased significantly in human neutrophils stimulated with anti-LAMP-2 antibody, and this effect was attenuated by the inhibitors of autophagy 3-methyladenine (3MA) and 2-morpholin-4-yl-8-phenylchromen-4-one (LY294002). The anti-LAMP-2 antibody-stimulated NET formation was unaffected by benzyloxycarbonyl-Val- Ala-Asp (OMe)-fluoromethylketone (zVAD-fmk) and necrostatin-1 (Nec-1), which are inhibitors of apoptosis and necrosis, respectively, but was inhibited by 3MA and LY294002. Moreover, the proportion of LC3BI that was converted to LC3BII increased significantly (P=0.0057), and massive vacuolizations that exhibited characteristics typical of autophagy were detected in neutrophils stimulated with anti-LAMP-2 antibody. Our results provide further evidence that autophagy is involved in ANCA-induced NET formation in human neutrophils.

Information generation and processing systems that regulate periodontal structure and function

The periodontium is a very dynamic organ that responds rapidly to mechanical and chemical stimuli. It is very complex in that it is composed of two hard tissues (cementum and bone) and two soft connective tissues (periodontal ligament and gingiva). Together these tissues are defined by the molecules expressed by the resident periodontal cells in each compartment and this determines not only the structure and function of the periodontium but also how it responds to infection and inflammation. The biological activity of these molecules is tightly regulated in time and space to preserve tissue homeostasis, influence inflammatory responses and participate in tissue regeneration. In this issue of Periodontology 2000 we explore new experimental approaches and data sets which help to understand the molecules and cells that regulate tissue form and structure in health, disease and regeneration.

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.

Impaired neutrophil directional chemotactic accuracy in chronic periodontitis patients

AIM

To investigate the chemotactic accuracy of peripheral blood neutrophils from patients with chronic periodontitis compared with matched healthy controls, before and after non-surgical periodontal therapy.

MATERIAL & METHODS

Neutrophils were isolated from patients and controls (n = 18) by density centrifugation. Using the Insall chamber and video microscopy, neutrophils were analysed for directional chemotaxis towards N-formyl-methionyl-leucyl-phenylalanine [fMLP (10 nM), or CXCL8 (200 ng/ml)]. Circular statistics were utilized for the analysis of cell movement.

RESULTS

Prior to treatment, neutrophils from patients with chronic periodontitis had significantly reduced speed, velocity and chemotactic accuracy compared to healthy controls for both chemoattractants. Following periodontal treatment, patient neutrophils continued to display reduced speed in response to both chemoattractants. However, velocity and accuracy were normalized for the weak chemoattractant CXCL8 while they remained significantly reduced for fMLP.

CONCLUSIONS

Chronic periodontitis is associated with reduced neutrophil chemotaxis, and this is only partially restored by successful treatment. Dysfunctional neutrophil chemotaxis may predispose patients with periodontitis to their disease by increasing tissue transit times, thus exacerbating neutrophil-mediated collateral host tissue damage.

Hyperosmolar stress induces neutrophil extracellular trap formation: implications for dry eye disease

PURPOSE

To determine if hyperosmolar stress can stimulate human neutrophils to form neutrophil extracellular traps (NETs) and to investigate potential strategies to reduce formation of NETs (NETosis) in a hyperosmolar environment.

METHODS

Neutrophils were isolated from peripheral venous blood of healthy subjects and incubated in iso-osmolar (280 mOsM) or hyperosmolar (420 mOsM) media for 4 hours. Neutrophil extracellular traps were quantified using a PicoGreen dye assay to measure extracellular DNA. Two known inhibitors of NETosis, staurosporine and anti-β2 integrin blocking antibody, and two proresolution formyl peptide receptor 2 (FPR2) agonists, annexin/lipocortin-1 mimetic peptide and 15-epi-lipoxin A4, were evaluated as possible strategies to reduce hyperosmolarity-induced NETosis.

RESULTS

The amount of NETs induced by hyperosmolar medium (420 mOsM) increased linearly over time to 3.2 ± 0.3 times that induced by iso-osmolar medium at 4 hours (P < 0.05). NETosis increased exponentially with increasing osmolarity and was independent of the stimulus used to increase osmolarity. Upon neutrophil exposure to hyperosmolar stress, restoration of iso-osmolar conditions decreased NET formation by 52.7% ± 5% (P < 0.05) but did not completely abrogate it. Among the strategies tested to reduce NETosis in a hyperosmolar environment, annexin-1 peptide was the most efficacious.

CONCLUSIONS

Hyperosmolarity induces formation of NETs by neutrophils. This NETosis mechanism may explain the presence of excessive NETs on the ocular surface of patients with dry eye disease. Because they reduce hyperosmolarity-induced NETosis, FPR2 agonists may have therapeutic potential in these patients.

Neutrophil extracellular traps formation by bacteria causing endometritis in the mare

Besides the classical functions, neutrophils (PMNs) are able to release DNA in response to infectious stimuli, forming neutrophil extracellular traps (NETs) and killing pathogens. The pathogenesis of endometritis in the mare is not completely understood. The aim was to evaluate the in vitro capacity of equine PMNs to secrete NETs by chemical activation, or stimulated with Streptococcus equi subspecies zooepidemicus (Szoo), Escherichia coli (Ecoli) or Staphylococcus capitis (Scap) strains obtained from mares with endometritis. Ex vivo endometrial mucus from mares with bacterial endometritis were evaluated for the presence of NETs. Equine blood PMNs were used either without or with stimulation by phorbol-myristate-acetate (PMA), a strong inducer of NETs, for 1-3h. To evaluate PMN ability to produce NETs when phagocytosis was impaired, the phagocytosis inhibitor cytochalasin (Cyt) was added after PMA. After the addition of bacteria, a subsequent 1-h incubation was carried out in seven groups. NETs were visualized by 4',6-diamidino-2-phenylindole (DAPI) and anti-histone. Ex vivo samples were immunostained for myeloperoxidase and neutrophil elastase. A 3-h incubation period of PMN + PMA increased NETs (p < 0.05). Bacteria + 25 nM PMA and bacteria + PMA + Cyt increased NETs (p<0.05). Szoo induced fewer NETs than Ecoli or Scap (p < 0.05). Ex vivo NETs were present in mares with endometritis. Scanning electron microscopy showed the spread of NETs formed by smooth fibers and globules that can be aggregated in thick bundles. Formation of NETs and the subsequent entanglement of bacteria suggest that equine NETs might be a complementary mechanism in fighting some of the bacteria causing endometritis in the mare.

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.

The multifactorial role of neutrophils in rheumatoid arthritis

Of all cells implicated in the pathology of rheumatoid arthritis (RA), neutrophils possess the greatest cytotoxic potential, owing to their ability to release degradative enzymes and reactive oxygen species. Neutrophils also contribute to the cytokine and chemokine cascades that accompany inflammation, and regulate immune responses via cell-cell interactions. Emerging evidence suggests that neutrophils also have a previously unrecognised role in autoimmune diseases: neutrophils can release neutrophil extracellular traps (NETs) containing chromatin associated with granule enzymes, which not only kill extracellular microorganisms but also provide a source of autoantigens. For example, citrullinated proteins that can act as neoepitopes in loss of immune tolerance are generated by peptidylarginine deiminases, which replace arginine with citrulline residues, within neutrophils. Indeed, antibodies to citrullinated proteins can be detected before the onset of symptoms in patients with RA, and are predictive of erosive disease. Neutrophils from patients with RA have an increased tendency to form NETs containing citrullinated proteins, and sera from such patients contain autoantibodies that recognize these proteins. Thus, in addition to their cytotoxic and immunoregulatory role in RA, neutrophils may be a source of the autoantigens that drive the autoimmune processes underlying this disease.

Pathogenicity of ocular isolates of Acinetobacter baumannii in a mouse model of bacterial endophthalmitis

PURPOSE

To determine the virulence properties of ocular isolates of Acinetobacter baumannii in causing endophthalmitis in a mouse model.

METHODS

Endophthalmitis was induced by intravitreal injections of the bacteria into C57BL/6 (B6) mouse eyes. The disease progression was monitored by ophthalmoscopic, electroretinography (ERG), histologic, cell death (TUNEL labeling), and microbiological parameters. The expression of cytokines/chemokines was checked by quantitative RT-PCR (qRT-PCR) and ELISA. Flow cytometry was used to determine cellular infiltration. The role of neutrophils was determined using neutropenic mice. The virulence traits (biofilm formation, adherence, and cytotoxicity) of the ocular isolates were tested using corneal epithelial cells.

RESULTS

Among the three clinical isolates and a standard ATCC 19606 strain tested, a biofilm producing multidrug resistant (MDR) strain of A. baumannii AB12 caused severe endophthalmitis (100% destruction of the eyes) leading to the loss of retinal function as assessed by ERG analysis. Elevated levels of inflammatory mediators (TNF-α, IL-1β, CXCL2, and IL-6) were detected in AB12-infected eyes. Histologic and TUNEL staining revealed increased retinal cell death and the flow cytometry data showed the presence of inflammatory cells, primarily neutrophils (CD45(+)/Ly6G(+)). Neutropenic mice showed an increased bacterial burden, reduced inflammatory response, and severe tissue destruction.

CONCLUSIONS

These results indicate that A. baumannii causes severe intraocular inflammation and retinal damage. Furthermore, neutrophils play an important role in the pathogenesis of A. baumannii endophthalmitis.

Tear fluid extracellular DNA: diagnostic and therapeutic implications in dry eye disease

PURPOSE

To determine the abundance of extracellular DNA (eDNA) in tear fluid of patients with dry eye disease (DED) and to report clinical outcomes after DNase I eyedrops use to reduce excessive tear fluid eDNA.

METHODS

Tear fluid was collected from healthy control subjects and patients with DED. The eDNA abundance was determined with the PicoGreen dye assay. The DED symptoms and clinical signs were recorded and correlated with eDNA abundance. Two patients with DED having excessive eDNA in tear fluid were treated with DNase I eyedrops.

RESULTS

The PicoGreen dye assay measures tear fluid eDNA abundance after a 2-minute incubation time. With longer incubations, admixed cells also contribute to eDNA measurements. The mean (SE) eDNA abundance in healthy control subjects' tear fluid was 1.4 (0.2) μg/mL. The mean (SE) eDNA abundance in tear fluid of patients with nonautoimmune DED, autoimmune DED, and graft versus host disease was significantly higher: the values were 2.9 (0.6), 5.2 (1.2), and 9.1 (2.3) μg/mL, respectively (P < 0.05). In most of these patients, the PicoGreen dye kinetic assay of tear fluid showed an increase in fluorescence signal due to the presence of viable cells in tear fluid. Tear fluid eDNA had the best correlation with corneal Rose Bengal staining (r = 0.55). Treatment of patients having DED with DNase I eyedrops reduced eDNA abundance, abrogated signal increase, and improved comfort.

CONCLUSIONS

Excessive eDNA is present in tear fluid of patients with dry eyes. A novel therapeutic approach for managing DED may be to measure eDNA abundance in tear fluid with the PicoGreen dye assay and reduce excessive amounts with DNase I eyedrops.

The role of osteoimmunology in periodontal disease

Periodontal disease is a pathological condition that involves inflammation of the tooth supporting structures. It occurs in response to the presence of bacterial plaque on the tooth structure. The host defense system, including innate and adaptive immunity, is responsible for combating the pathologic bacteria invading the periodontal tissue. Failure to eradicate the invading pathogens will result in a continuous state of inflammation where inflammatory cells such as lymphocytes, PMNs, and macrophages will continue to produce inflammatory mediators in an effort to destroy the invaders. Unfortunately, these inflammatory mediators have a deleterious effect on the host tissue as well as foreign microbes. One of the effects of these mediators on the host is the induction of matrix degradation and bone resorption through activation of proteases and other inflammatory mediators that activate osteoclasts.