Seminal DNase frees spermatozoa entangled in neutrophil extracellular traps

Carp neutrophilic granulocytes form extracellular traps via ROS-dependent and independent pathways

Neutrophil extracellular traps (NETs) have recently been described as an important innate defense mechanism that leads to immobilization and killing of invading pathogens. NETs have been identified in several species, but the mechanisms involved in NET formation and their role in infection have not been well determined yet. Here we show that upon in vitro stimulation with different immunostimulants of bacterial, fungal or viral origin, carp neutrophilic granulocytes rapidly release NET structures. We analyzed the composition of these structures and the kinetics of their formation by confocal microscopy, by quantifying the levels of extracellular DNA and the release of enzymes originating from neutrophilic granules: myeloperoxidase, neutrophil elastase and matrix metalloproteinase 9 (MMP-9). Profiles of NET release by carp neutrophils as well as their enzyme composition are stimulus- and time-dependent. This study moreover provides evidence for a stimulus-dependent selective requirement of reactive oxygen species in the process of NET formation. Collectively the results support an evolutionary conserved and strictly regulated mechanism of NET formation in teleost fish.

NETosis

Neutrophils are the first line of defense of the immune system against infection. Among their weaponry, they have the ability to mix and extrude their DNA and bactericidal molecules creating NET-like structures in a unique type of cell death called NETosis. This process is important in order to control extracellular infections limiting collateral damage. Its aberrant function has been implicated in several human diseases including sepsis and autoimmune disease. The purpose of the present paper is to give a general introduction to this concept.

Neutrophils cast extracellular traps in response to protozoan parasites

Release of extracellular traps by neutrophils is a now well-established phenomenon that contributes to the innate response to extracellular bacterial and fungal pathogens. The importance of NETs during protozoan infection has been less explored, but recent findings suggest an emerging role for release of neutrophil-derived extracellular DNA in response to this class of microbial pathogens. The present review summarizes findings to date regarding elicitation of NETs by Toxoplasma gondii, Plasmodium falciparum, Eimeria bovis, and Leishmania spp.

Neutrophil NETs in reproduction: from infertility to preeclampsia and the possibility of fetal loss

The intention of this review is to provide an overview of the potential role of neutrophil extracellular traps (NETs) in mammalian reproduction. Neutrophil NETs appear to be involved in various stages of the reproductive cycle, starting with fertility and possibly ending with fetal loss. The first suggestion that NETs may play a role in pregnancy-related disorders was in preeclampsia, where vast numbers were detected in the intervillous space of affected placentae. The induction of NETosis involved an auto-inflammatory component, mediated by the increased release of placental micro-debris in preeclampsia. This report was the first indicating that NETs may be associated with a human pathology not involving infection. Subsequently, NETs have since then been implicated in bovine or equine infertility, in that semen may become entrapped in the female reproductive tract during their passage to the oocyte. In this instance interesting species-specific differences are apparent, in that equine sperm evade entrapment via expression of a DNAse-like molecule, whereas highly motile bovine sperm, once free from seminal plasma (SP) that promotes interaction with neutrophils, appear impervious to NETs entrapment. Although still in the realm of speculation it is plausible that NETs may be involved in recurrent fetal loss mediated by anti-phospholipid antibodies, or perhaps even in fetal abortion triggered by infections with microorganisms such as L. monocytogenes or B. abortus.

ETosis: A Microbicidal Mechanism beyond Cell Death

Netosis is a recently described type of neutrophil death occurring with the release to the extracellular milieu of a lattice composed of DNA associated with histones and granular and cytoplasmic proteins. These webs, initially named neutrophil extracellular traps (NETs), ensnare and kill microorganisms. Similarly, other cell types, such as eosinophils, mast cells, and macrophages, can also dye by this mechanism; thus, it was renamed as ETosis, meaning death with release of extracellular traps (ETs). Here, we review the mechanism of NETosis/etosis, emphasizing its role in diseases caused by protozoan parasites, fungi, and viruses.

Extracellular deoxyribonuclease production by periodontal bacteria

BACKGROUND AND OBJECTIVE

Whilst certain bacteria have long been known to secrete extracellular deoxyribonuclease (DNase), the purpose in microbial physiology was unclear. Recently, however, this enzyme has been demonstrated to confer enhanced virulence, enabling bacteria to evade the host's immune defence of extruded DNA/chromatin filaments, termed neutrophil extracellular traps (NETs). As NETs have recently been identified in infected periodontal tissue, the aim of this study was to screen periodontal bacteria for extracellular DNase activity.

MATERIAL AND METHODS

To determine whether DNase activity was membrane bound or secreted, 34 periodontal bacteria were cultured in broth and on agar plates. Pelleted bacteria and supernatants from broth cultures were analysed for their ability to degrade DNA, with relative activity levels determined using an agarose gel electrophoresis assay. Following culture on DNA-supplemented agar, expression was determined by the presence of a zone of hydrolysis and DNase activity related to colony size.

RESULTS

Twenty-seven bacteria, including red and orange complex members Porphyromonas gingivalis, Tannerella forsythia, Fusobacterium nucleatum, Parvimonas micra, Prevotella intermedia, Streptococcus constellatus, Campylobacter rectus and Prevotella nigrescens, were observed to express extracellular DNase activity. Differences in DNase activity were noted, however, when bacteria were assayed in different culture states. Analysis of the activity of secreted DNase from bacterial broth cultures confirmed their ability to degrade NETs.

CONCLUSION

The present study demonstrates, for the first time, that DNase activity is a relatively common property of bacteria associated with advanced periodontal disease. Further work is required to determine the importance of this bacterial DNase activity in the pathogenesis of periodontitis.

Neutrophil extracellular trap (NET)-mediated killing of Pseudomonas aeruginosa: evidence of acquired resistance within the CF airway, independent of CFTR

The inability of neutrophils to eradicate Pseudomonas aeruginosa within the cystic fibrosis (CF) airway eventually results in chronic infection by the bacteria in nearly 80 percent of patients. Phagocytic killing of P. aeruginosa by CF neutrophils is impaired due to decreased cystic fibrosis transmembrane conductance regulator (CFTR) function and virulence factors acquired by the bacteria. Recently, neutrophil extracellular traps (NETs), extracellular structures composed of neutrophil chromatin complexed with granule contents, were identified as an alternative mechanism of pathogen killing. The hypothesis that NET-mediated killing of P. aeruginosa is impaired in the context of the CF airway was tested. P. aeruginosa induced NET formation by neutrophils from healthy donors in a bacterial density dependent fashion. When maintained in suspension through continuous rotation, P. aeruginosa became physically associated with NETs. Under these conditions, NETs were the predominant mechanism of killing, across a wide range of bacterial densities. Peripheral blood neutrophils isolated from CF patients demonstrated no impairment in NET formation or function against P. aeruginosa. However, isogenic clinical isolates of P. aeruginosa obtained from CF patients early and later in the course of infection demonstrated an acquired capacity to withstand NET-mediated killing in 8 of 9 isolates tested. This resistance correlated with development of the mucoid phenotype, but was not a direct result of the excess alginate production that is characteristic of mucoidy. Together, these results demonstrate that neutrophils can kill P. aeruginosa via NETs, and in vitro this response is most effective under non-stationary conditions with a low ratio of bacteria to neutrophils. NET-mediated killing is independent of CFTR function or bacterial opsonization. Failure of this response in the context of the CF airway may occur, in part, due to an acquired resistance against NET-mediated killing by CF strains of P. aeruginosa.

Coming of age: reconstruction of heterosexual HIV-1 transmission in human ex vivo organ culture systems

Heterosexual transmission of human immunodeficiency virus-1 (HIV-1), from men to women, involves exposure to infectious HIV-1 in semen. Therefore, the cellular and molecular processes that underlie HIV-1 transmission are closely interconnected with fundamental principles of human reproductive biology. Human ex vivo organ culture systems allow experimental reconstruction of HIV-1 transmission, using human semen and premenopausal cervicovaginal mucosal tissue, with specific emphasis on the progression from exposure to development of primary HIV-1 infection. Clearly, an isolated piece of human tissue cannot duplicate the full complexity of events in natural infections, but with correct observation of conventional medical and ethical standards, there is no opportunity to study HIV-1 exposure and primary infection in young women. Human mucosal organ cultures allow direct study of HIV-1 infection in a reproducible format while retaining major elements of complexity and variability that typify community-based HIV-1 transmission. Experimental manipulation of human mucosal tissue both allows and requires acquisition of new insights into basic processes of human mucosal immunology. Expanding from the current foundations, we believe that human organ cultures will become increasingly prominent in experimental studies of HIV-1 transmission and continuing efforts to prevent HIV-1 infection at human mucosal surfaces.

Identification of ejaculated proteins in the house mouse (Mus domesticus) via isotopic labeling

BACKGROUND

Seminal fluid plays an important role in successful fertilization, but knowledge of the full suite of proteins transferred from males to females during copulation is incomplete. The list of ejaculated proteins remains particularly scant in one of the best-studied mammalian systems, the house mouse (Mus domesticus), where artificial ejaculation techniques have proven inadequate. Here we investigate an alternative method for identifying ejaculated proteins, by isotopically labeling females with 15N and then mating them to unlabeled, vasectomized males. Proteins were then isolated from mated females and identified using mass spectrometry. In addition to gaining insights into possible functions and fates of ejaculated proteins, our study serves as proof of concept that isotopic labeling is a powerful means to study reproductive proteins.

RESULTS

We identified 69 male-derived proteins from the female reproductive tract following copulation. More than a third of all spectra detected mapped to just seven genes known to be structurally important in the formation of the copulatory plug, a hard coagulum that forms shortly after mating. Seminal fluid is significantly enriched for proteins that function in protection from oxidative stress and endopeptidase inhibition. Females, on the other hand, produce endopeptidases in response to mating. The 69 ejaculated proteins evolve significantly more rapidly than other proteins that we previously identified directly from dissection of the male reproductive tract.

CONCLUSION

Our study attempts to comprehensively identify the proteins transferred from males to females during mating, expanding the application of isotopic labeling to mammalian reproductive genomics. This technique opens the way to the targeted monitoring of the fate of ejaculated proteins as they incubate in the female reproductive tract.

Dying for a cause: NETosis, mechanisms behind an antimicrobial cell death modality

Neutrophil extracellular traps (NETs) are chromatin structures loaded with antimicrobial molecules. They can trap and kill various bacterial, fungal and protozoal pathogens, and their release is one of the first lines of defense against pathogens. In vivo, NETs are released during a form of pathogen-induced cell death, which was recently named NETosis. Ex vivo, both dead and viable neutrophils can be stimulated to release NETs composed of either nuclear or mitochondrial chromatin, respectively. In certain pathological conditions, NETs are associated with severe tissue damage or certain auto-immune diseases. This review describes the recent progress made in the identification of the mechanisms involved in NETosis and discusses its interplay with autophagy and apoptosis.

Selection against spermatozoa with fragmented DNA after postovulatory mating depends on the type of damage

BACKGROUND

Before ovulation, sperm-oviduct interaction mechanisms may act as checkpoint for the selection of fertilizing spermatozoa in mammals. Postovulatory mating does not allow the sperm to attach to the oviduct, and spermatozoa may only undergo some selection processes during the transport through the female reproductive tract and/or during the zona pellucida (ZP) binding/penetration.

METHODS

We have induced DNA damage in spermatozoa by two treatments, (a) a scrotal heat treatment (42 degrees C, 30 min) and (b) irradiation with 137Cs gamma-rays (4 Gy, 1.25 Gy/min). The effects of the treatments were analyzed 21-25 days post heat stress or gamma-radiation. Postovulatory females mated either with treated or control males were sacrificed at Day 14 of pregnancy, and numbers of fetuses and resorptions were recorded.

RESULTS

Both treatments decreased significantly implantation rates however, the proportion of fetuses/resorptions was only reduced in those females mated to males exposed to radiation, indicating a selection favoring fertilization of sperm with unfragmented DNA on the heat treatment group. To determine if DNA integrity is one of the keys of spermatozoa selection after postovulatory mating, we analyzed sperm DNA fragmentation by COMET assay in: a) sperm recovered from mouse epididymides; b) sperm recovered from three different regions of female uterine horns after mating; and c) sperm attached to the ZP after in vitro fertilization (IVF). Similar results were found for control and both treatments, COMET values decreased significantly during the transit from the uterine section close to the uterotubal junction to the oviduct, and in the spermatozoa attached to ZP. However, fertilization by IVF and intracytoplasmatic sperm injection (ICSI) showed that during sperm ZP-penetration, a stringent selection against fragmented-DNA sperm is carried out when the damage was induced by heat stress, but not when DNA fragmentation was induced by radiation.

CONCLUSION

Our results indicate that in postovulatory mating there is a preliminary general selection mechanism against spermatozoa with low motility and fragmented-DNA during the transport through the female reproductive tract and in the ZP binding, but the ability of the ZP to prevent fertilization by fragmented-DNA spermatozoa is achieved during sperm-ZP penetration, and depends on the source of damage.

Diagnostic accuracy of neutrophil-derived circulating free DNA (cf-DNA/NETs) for septic arthritis

The release of "neutrophil extracellular traps" (NETs) has been identified as a novel immune response in innate immunity. NETs are composed of neutrophil-derived circulating free DNA (cf-DNA) and neutrophil cytoplasm-derived proteins such as proteases. In this study, we analyzed the putative diagnostic value of synovial cf-DNA/NETs for identification of septic arthritis. Forty-two patients with a joint effusion who had undergone arthrocentesis were included. From synovial fluid, cf-DNA/NETs (j-cf-DNA) levels were directly quantified. Diagnostic value of j-cf-DNA was compared with white blood cells (WBC), synovial white blood cells (j-WBC), C-reactive protein (CRP), j-IL-6, j-TNF alpha, j-IL-1 beta, and myeloperoxidase (j-MPO). Sensitivity, specificity, positive and negative predictive value, as well as ROC-curves for each parameter were calculated. Synovial fluid cf-DNA/NETs values from patients with septic arthritis (3,286 +/- 386 ng/ml, n = 9) were significantly increased compared to patients with noninfectious joint inflammation (1,040 +/- 208 ng/ml, n = 17) or osteoarthritis (278 +/- 34 ng/ml, n = 16, p < 0.01). In conjunction with j-cf-DNA, j-IL-6 and j-IL-1 beta were significantly elevated (p < 0.01), but WBC, CRP, and j-WBC were not. At a cut-off of 300 ng/ml, j-cf-DNA had a sensitivity of 0.89, a specificity of 1.0, a positive predictive value of 1.0, and a negative predictive value of 0.97. Receiver operation curves revealed largest areas under the curve for cf-DNA/NETs (0.933) and j-IL-6 (0.951). cf-DNA/NETs seem to be a valuable additional marker for the diagnosis of septic arthritis or periprosthetic infections. However, this result should be confirmed in a large clinical trial.

Neutrophil extracellular traps contain calprotectin, a cytosolic protein complex involved in host defense against Candida albicans

Neutrophils are the first line of defense at the site of an infection. They encounter and kill microbes intracellularly upon phagocytosis or extracellularly by degranulation of antimicrobial proteins and the release of Neutrophil Extracellular Traps (NETs). NETs were shown to ensnare and kill microbes. However, their complete protein composition and the antimicrobial mechanism are not well understood. Using a proteomic approach, we identified 24 NET-associated proteins. Quantitative analysis of these proteins and high resolution electron microscopy showed that NETs consist of modified nucleosomes and a stringent selection of other proteins. In contrast to previous results, we found several NET proteins that are cytoplasmic in unstimulated neutrophils. We demonstrated that of those proteins, the antimicrobial heterodimer calprotectin is released in NETs as the major antifungal component. Absence of calprotectin in NETs resulted in complete loss of antifungal activity in vitro. Analysis of three different Candida albicans in vivo infection models indicated that NET formation is a hitherto unrecognized route of calprotectin release. By comparing wild-type and calprotectin-deficient animals we found that calprotectin is crucial for the clearance of infection. Taken together, the present investigations confirmed the antifungal activity of calprotectin in vitro and, moreover, demonstrated that it contributes to effective host defense against C. albicans in vivo. We showed for the first time that a proportion of calprotectin is bound to NETs in vitro and in vivo.

Proteomics and comparative genomic investigations reveal heterogeneity in evolutionary rate of male reproductive proteins in mice (Mus domesticus)

Male reproductive fitness is strongly affected by seminal fluid. In addition to interacting with the female environment, seminal fluid mediates important physiological characteristics of sperm, including capacitation and motility. In mammals, the male reproductive tract shows a striking degree of compartmentalization, with at least six distinct tissue types contributing material that is combined with sperm in an ejaculate. Although studies of whole ejaculates have been undertaken in some species, we lack a comprehensive picture of the specific proteins produced by different accessory tissues. Here, we perform proteomic investigations of six regions of the male reproductive tract in mice -- seminal vesicles, anterior prostate, dorsolateral prostate, ventral prostate, bulbourethral gland, and bulbourethral diverticulum. We identify 766 proteins that could be mapped to 506 unique genes and compare them with a high-quality human seminal fluid data set. We find that Gene Ontology functions of seminal proteins are largely conserved between mice and humans. By placing these data in an evolutionary framework, we show that seminal vesicle proteins have experienced a significantly higher rate of nonsynonymous substitution compared with the genome, which could be the result of adaptive evolution. In contrast, proteins from the other five tissues showed significantly lower nonsynonymous substitution, revealing a previously unappreciated level of evolutionary constraint acting on the majority of male reproductive proteins.

Neutrophil extracellular trap formation as innate immune reactions against the apicomplexan parasite Eimeria bovis

Eimeria bovis infections are under immunological control and recent studies have emphasized the role of early PMN-mediated innate immune responses in infected calves. Neutrophil extracellular traps (NETs) have recently been demonstrated to act as a killing mechanism of PMN against several pathogens. In the present study, the interactions of bovine PMN with sporozoites of E. bovis were investigated in this respect in vitro. For demonstration and quantification of NET formation, extracellular DNA was stained by Sytox Orange. Fluorescence images after Sytox Orange staining as well as scanning electron microscopy (SEM) showed NET formation to occur upon contact with E. bovis sporozoites. Exposure of PMN to viable sporozoites induced stronger NET formation than to dead or homogenized parasites. NET formation was abolished by treatment with DNase and could be reduced by diphenylene iodonium, which is described as a potent inhibitor of NADPH oxidase. After sporozoite and PMN co-culture, extracellular fibres were found attached to sporozoites and seemed to trap them, strongly suggesting that NETs immobilize E. bovis sporozoites and thereby prevent them from infecting host cells. Thus, transfer of sporozoites, previously being confronted with PMN, to adequate host cells resulted in clearly reduced infection rates when compared to PMN-free controls. NET formation by PMN may therefore represent an effector mechanism in early innate immune reactions against E. bovis. This is the first report indicating Eimeria-induced NET formation.

Innate immunity turned inside-out: antimicrobial defense by phagocyte extracellular traps

The formation of extracellular traps (ETs) by phagocytic cells has been recognized as a novel and important mechanism of the host innate immune response against infections. ETs are formed by different host immune cells such as neutrophils, mast cells, and eosinophils after stimulation with mitogens, cytokines, or pathogens themselves, in a process dependent upon induction of a reactive-oxygen-species-mediated signaling cascade. ETs consist of nuclear or mitochondrial DNA as a backbone with embedded antimicrobial peptides, histones, and cell-specific proteases and thereby provide a matrix to entrap and kill microbes and to induce the contact system. This review summarizes the latest research on ETs and their role in innate immunity and host innate defense. Attention is also given to mechanisms by which certain leading bacterial pathogens have evolved to avoid entrapment and killing in these specialized structures.

Human prostasomes contain chromosomal DNA

BACKGROUND

The aim of this study was to perform a comprehensive evaluation of the occurrence of DNA in human prostasomes.

METHODS

Prostasomes were purified from seminal fluid (seminal prostasomes) and from PC-3-cells (PC-3 cell prostasomes). DNA extracted from both sources of prostasomes was visualized on agarose gels. Further, the DNA was cloned and sequenced (13 clones from seminal prostasomal DNA and 16 clones from PC-3 cell prostasomal DNA) and identified by alignment in the BLAST-nucleotide search database. In order to decide if the DNA was internally or externally located in/on prostasomes, prostasomes were treated with nuclease (DNase) and A(260) was measured before and after treatment. Additionally, flow cytometric studies were performed with membrane permeable and membrane impermeable DNA stains.

RESULTS

We identified human chromosomal DNA in purified prostasomes from both sources and treatment with DNase demonstrated that the prostasome-shielded DNA was protected from enzyme attack. Membrane-permeable DNA stain raised the fluorescence contrary to membrane-impermeable stain. Clearly discernible nucleic acid of prostasomes was separated on 1% agarose gel yielding DNA fragments of about 13 kbp and below with a marked band at about 1 kbp. Cloning and sequencing of 13 fragments from seminal prostasomes and 16 from PC-3 cell prostasomes revealed a chromosomal origin of the DNA. In purified seminal prostasomes, 4 out of 13 DNA clones featured gene sequences (31%). The corresponding figure for PC3-derived prostasomes was 4 out of 16 clones featuring gene sequences (25%).

CONCLUSION

Human prostasomes contain chromosomal DNA. Both nuclease treatment and differential DNA stainings indicated an inside location of the prostasomal DNA. Our findings suggest a DNA-delivery function of prostasomes to sperm cells.

Mouse neutrophil extracellular traps in microbial infections

Neutrophil extracellular traps (NETs) play an important role in innate immunity to microbial infections. NETs have been described in several species, but the molecular details of NET formation and their role in infection has not been addressed, partly because we lack optimal experimental models. Here we describe tools to investigate NET formation in neutrophils isolated from mice. Upon in vitro stimulation of wild-type mouse neutrophils with PMA, we analyzed 3 important steps in the process of NET formation: reactive oxygen species (ROS) production, NET cell death and NET release. As expected, neutrophils from NADPH oxidase-deficient mice failed to produce ROS and did not die nor release NETs upon stimulation. We found that neutrophils from several mouse strains produced NETs with different efficiency and that NET formation correlated with the amount of ROS produced. Activation with Candida albicans also resulted in ROS production and NET cell death. The hyphal form of this fungus induced NETs more effectively than the yeast form. With this work, we provide tools to study in vitro NET assembly in the mouse system.

Endometrial explant culture to study the response of equine endometrium to insemination

Mating-induced endometritis (MIE) is ubiquitous in the horse after natural mating and artificial insemination with frozen/thawed semen causing the most aggressive response. The majority of mares eliminate MIE 24-48 h after insemination. An endometrial explant culture was tested as a potential in vitro exemplar for sperm-induced MIE. Endometrial prostaglandin F(2alpha) (PGF(2alpha)) secretion and expression of interleukin-8 (IL-8) were used as markers of inflammation. Endometrial explants were cultured from uteri collected from follicular phase mares. Explants were challenged with 1 or 10 x 10(6) sperm/ml frozen/thawed semen, chilled semen, washed sperm or seminal plasma. Medium was collected 24 and 72 h after challenge and assayed for PGF(2alpha) by radioimmunoassay. Treatment of endometrial explants with frozen/thawed, chilled semen or washed sperm did not change the secretion of PGF(2alpha) compared with untreated controls. However, 24 h after challenge cultured explants expressed IL-8. The in vitro endometrial explant system did not represent the in vivo response to semen when PGF(2alpha) was used as a marker of inflammation, yet the use of gene expression as an inflammatory marker warrants further investigation.

Immunological responses to semen in the female genital tract

When spermatozoa, seminal plasma and semen extender reach the uterus and interact with local leukocytes and endometrial cells, several immune mechanisms are initiated which have immediate, mid-term and long-term effects on ovulation, sperm cell selection, fertilization and pregnancy success by assuring the acceptance of fetal tissues. This report gives an overview on relevant key immune mechanisms following roughly the time axis after insemination. Detailed knowledge regarding these mechanisms will aid maximizing reproductive efficiency in livestock production. In the future, the many species involved will require a more comparative approach, since evidence is growing that endometrial physiology and the response to varying amounts and compositions of seminal plasma, various semen extenders, and variable numbers of spermatozoa also provoke different immune responses.

Breeding-induced endometritis in mares

Endometritis is a common cause of infertility in broodmares. In the past, the condition was believed to be exclusively the result of bacterial contamination of the uterus. Treatment strategies were focused on preventing bacteria from entering the uterus and on treating mares with signs of endometritis with antibiotics. More recent research on uterine defense mechanisms has increased our understanding of the pathophysiology of equine endometritis. Additional causative agents have been identified, and we have learned to separate uterine infections and a physiologic breeding-induced endometritis resulting from uterine exposure to semen.vi

Beneficial suicide: why neutrophils die to make NETs

Neutrophils are one of the main types of effector cell in the innate immune system and were first shown to effectively kill microorganisms by phagocytosis more than 100 years ago. Recently, however, it has been found that stimulated neutrophils can also produce extracellular structures called neutrophil extracellular traps (NETs) that capture and kill microorganisms. This Progress article gives an overview of the structure, function and generation of NETs, and their role in infections.

Platelet TLR4 activates neutrophil extracellular traps to ensnare bacteria in septic blood

It has been known for many years that neutrophils and platelets participate in the pathogenesis of severe sepsis, but the inter-relationship between these players is completely unknown. We report several cellular events that led to enhanced trapping of bacteria in blood vessels: platelet TLR4 detected TLR4 ligands in blood and induced platelet binding to adherent neutrophils. This led to robust neutrophil activation and formation of neutrophil extracellular traps (NETs). Plasma from severely septic humans also induced TLR4-dependent platelet-neutrophil interactions, leading to the production of NETs. The NETs retained their integrity under flow conditions and ensnared bacteria within the vasculature. The entire event occurred primarily in the liver sinusoids and pulmonary capillaries, where NETs have the greatest capacity for bacterial trapping. We propose that platelet TLR4 is a threshold switch for this new bacterial trapping mechanism in severe sepsis.

Neutrophil extracellular traps: casting the NET over pathogenesis

Neutrophil extracellular traps (NETs) are considered to be part of the human innate immunity because they trap and kill pathogens. NETs are formed by activated neutrophils and consist of a DNA backbone with embedded antimicrobial peptides and enzymes. They are involved in host defense during pneumococcal pneumonia, streptococcal necrotizing fasciitis, appendicitis and insemination. Recently, bacterial virulence factors that counteract NETs have been identified. These include the degradation of the NET-backbone by DNases enabling the liberation of bacteria from NETs, as well as capsule formation, which reduces bacterial trapping. Furthermore, pathogens can resist NET-mediated killing by adding positive charge to their cell surface.

Zebrafish (Danio rerio) whole kidney assays to measure neutrophil extracellular trap release and degranulation of primary granules

The zebrafish (Danio rerio) is an excellent model system for studies in developmental biology, genetics, and toxicology, and is increasingly gaining importance in the field of immunology. The use of whole zebrafish kidneys as source of neutrophils for degranulation assays and detection of neutrophil extracellular traps is described for the first time. Neutrophils from zebrafish kidneys released neutrophil extracellular traps (NETs) and myeloperoxidase (MPO) upon stimulation with calcium ionophore, phorbol myristate acetate, and beta-glucan. Immunocytochemical study of zebrafish kidney cells revealed that NETs are made of DNA fibers associated with neutrophil granular proteins, but not with cytoskeleton. Rapid, direct MPO and extracellular DNA detection assays were developed to quantify NET release and degranulation of neutrophil primary granules from whole zebrafish kidneys. The assays were used to measure the effects of acute crowding and handling stress on neutrophils, and to examine the potential for use of zebrafish whole kidney assays in evaluation of neutrophil function under different conditions in vivo. The whole kidney NET release and degranulation assays are quantitative, can rapidly measure a large number of samples, and are capable of detecting inhibition of neutrophil activity in stressed fish, overcoming the limitations that prevented use of zebrafish in the investigations of cellular innate immune function. The assays can be used as a new research model to study effects of stress, immunomodulators, toxicants, and diseases on fish neutrophil biology.

Fish cast NETs: neutrophil extracellular traps are released from fish neutrophils

Neutrophil extracellular traps (NETs), which are extracellular DNA structures released from neutrophils, are described and characterized for the first time in fish using fluorescent confocal microscopy. Confocal images of fish neutrophil suspensions stained with 6'-diamino-2-phenylindole, dihydrochloride DNA fluorescent stain (DAPI) revealed the presence of NETs which appeared as fibrous structures connecting several cells. Co-localization of NETs with neutrophil granular proteins and actin was investigated using specific antibodies and probes. Double staining of neutrophils with SYTOX green and DAPI revealed that SYTOX stain applied to living cells stained extracellular DNA, but not nuclei. NETs are actively released from stimulated living cells, associated with granular proteins, but not with cytoskeleton, and are not a product of nuclear degradation seen in late apoptotic stages. Additionally, a fluorometric microtiter plate assay to quantify the release of NETs was adopted for use with fish neutrophils, and the effect of stress on NETs release was studied. This assay detected the inhibition of DNA release during stress conditions. In summary, NETs were released from living fish kidney neutrophils upon stimulation, characterized using fluorescence DNA-binding dyes, specific antibodies and probes, and quantified using a microtiter plate fluorometric assay that can rapidly measure a large number of samples. Detection of NETs can be used as an additional assay to an existing battery of functional tests, and as a new research model to study the effects of stress, immunomodulators, and diseases.

Topoisomerase IIB and an Extracellular Nuclease Interact to Digest Sperm DNA in an Apoptotic-Like Manner1

We previously demonstrated that mammalian spermatozoa contain a nuclease activity that cleaves DNA into loop-sized fragments. We show here that this activity is mediated by a nuclear matrix-associated topoisomerase IIB (TOP2B) interacting with an extracellular Mn2+/Ca2+-dependent nuclease. Together, these enzymes cleave all of the DNA into fragments of 50 kb, and this cleavage can be reversed by EDTA. If dithiothreitol is included, the nuclease digests the DNA, and if the protamines are removed the DNA is completely digested. A similar, TOP2B-mediated, chromatin fragmentation, which is reversible, followed by digestion of the DNA by an intracellular nuclease occurs in somatic cells during apoptosis. The extracellular location of the sperm nuclease made it possible to reconstitute the fragmentation activity in isolated spermatozoa, thus allowing us to identify two novel aspects of the mechanism. First, the fragmentation of all of the DNA to 50 kb by TOP2B required the addition of the extracellular nuclease or factor. Second, the subsequent, complete digestion of the DNA by the nuclease could be inhibited by etoposide, suggesting that the nuclease digestion requires TOP2B religation of the cleaved DNA. These data are the first demonstration of an active TOP2B in spermatozoa, suggesting this inert chromatin may be more active than previously thought. They also show that the unique chromatin structure of spermatozoa may provide an important model to study the regulated degradation of chromatin by TOP2B and associated nucleases.