Escherichia coli and Candida albicans induced macrophage extracellular trap-like structures with limited microbicidal activity

Targeting Macrophage Polarization in Infectious Diseases: M1/M2 Functional Profiles, Immune Signaling and Microbial Virulence Factors

INTRODUCTION

An event of increasing interest during host-pathogen interactions is the polarization of patrolling/naive monocytes (MOs) into macrophage subsets (MФs). Therapeutic strategies aimed at modulating this event are under investigation.

METHODS

This review focuses on the mechanisms of induction/development and profile of MФs polarized toward classically proinflammatory (M1) or alternatively anti-inflammatory (M2) phenotypes in response to bacteria, fungi, parasites, and viruses.

RESULTS AND DISCUSSION

It highlights nuclear, cytoplasmic, and cell surface receptors (pattern recognition receptors/PPRs), microenvironmental mediators, and immune signaling. MФs polarize into phenotypes: M1 MФs, activated by IFN-γ, pathogen-associated molecular patterns (PAMPs, e.g. lipopolysaccharide) and membrane-bound PPRs ligands (TLRs/CLRs ligands); or M2 MФs, induced by interleukins (ILs-4, -10 and -13), antigen-antibody complexes, and helminth PAMPs. Polarization toward M1 and M2 profiles evolve in a pathogen-specific manner, with or without canonicity, and can vary widely. Ultimately, this can result in varying degrees of host protection or more severe disease outcome. On the one hand, the host is driving effective MФs polarization (M1 or M2); but on the other hand, microorganisms may skew the polarization through virulence factors to increase pathogenicity. Cellular/genomic reprogramming also ensures plasticity of M1/M2 phenotypes. Because modulation of polarization can occur at multiple points, new insights and emerging perspectives may have clinical implications during the inflammation-to-resolution transition; translated into practical applications as for therapeutic/vaccine design target to boost microbicidal response (M1, e.g. triggering oxidative burst) with specifics PAMPs/IFN-γ or promote tissue repair (M2, increasing arginase activity) via immunotherapy.

Macrophage Extracellular Traps Suppress Particulate Matter-Induced Airway Inflammation

Accumulating evidence has substantiated the potential of ambient particulate matter (PM) to elicit detrimental health consequences in the respiratory system, notably airway inflammation. Macrophages, a pivotal component of the innate immune system, assume a crucial function in responding to exogenous agents. However, the roles and detailed mechanisms in regulating PM-induced airway inflammation remain unclear. The current study revealed that PM had the ability to stimulate the formation of macrophage extracellular traps (METs) both in vitro and in vivo. This effect was dependent on peptidylarginine deiminase type 4 (PAD4)-mediated histone citrullination. Additionally, reactive oxygen species were involved in the formation of PM-induced METs, in parallel with PAD4. Genetic deletion of PAD4 in macrophages resulted in an up-regulation of inflammatory cytokine expression. Moreover, mice with PAD4-specific knockout in myeloid cells exhibited exacerbated PM-induced airway inflammation. Mechanistically, inhibition of METs suppressed the phagocytic ability in macrophages, leading to airway epithelial injuries and an aggravated PM-induced airway inflammation. The present study demonstrates that METs play a crucial role in promoting the phagocytosis and clearance of PM by macrophages, thereby suppressing airway inflammation. Furthermore, it suggests that activation of METs may represent a novel therapeutic strategy for PM-related airway disorders.

The role of extracellular traps released by neutrophils, eosinophils, and macrophages in asthma

Extracellular traps (ETs) are a specialized form of innate immune defense in which leukocytes release ETs composed of chromatin and active proteins to eliminate pathogenic microorganisms. In addition to the anti-infection effect of ETs, researchers have also discovered their involvement in the pathogenesis of inflammatory disease, tumors, autoimmune disease, and allergic disease. Asthma is a chronic airway inflammatory disease involving multiple immune cells. The increased level of ETs in asthma patients suggests that ETs play an important role in the pathogenesis of asthma. Here we review the research work on the formation mechanism, roles, and therapeutic strategies of ETs released by neutrophils, eosinophils, and macrophages in asthma.

Plancitoxin-1 mediates extracellular trap evasion by the parasitic helminth Trichinella spiralis

BACKGROUND

Trichinella spiralis (T. spiralis) is a parasitic helminth that causes a globally prevalent neglected zoonotic disease, and worms at different developmental stages (muscle larvae, adult worms, newborn larvae) induce immune attack at different infection sites, causing serious harm to host health. Several innate immune cells release extracellular traps (ETs) to entrap and kill most pathogens that invade the body. In response, some unicellular pathogens have evolved a strategy to escape capture by ETs through the secretion of nucleases, but few related studies have investigated multicellular helminths.

RESULTS

In the present study, we observed that ETs from neutrophils capture adult worms of T. spiralis, while ETs from macrophages trap muscle larvae and newborn larvae, and ETs had a killing effect on parasites in vitro. To defend against this immune attack, T. spiralis secretes plancitoxin-1, a DNase II-like protein, to degrade ETs and escape capture, which is essential for the survival of T. spiralis in the host.

CONCLUSIONS

In summary, these findings demonstrate that T. spiralis escapes ET-mediated capture by secreting deoxyribonuclease as a potential conserved immune evasion mechanism, and plancitoxin-1 could be used as a potential vaccine candidate.

Necroptosis enhances 'don't eat me' signal and induces macrophage extracellular traps to promote pancreatic cancer liver metastasis

Pancreatic ductal adenocarcinoma (PDAC) is a devastating cancer with dismal prognosis due to distant metastasis, even in the early stage. Using RNA sequencing and multiplex immunofluorescence, here we find elevated expression of mixed lineage kinase domain-like pseudo-kinase (MLKL) and enhanced necroptosis pathway in PDAC from early liver metastasis T-stage (T1M1) patients comparing with non-metastatic (T1M0) patients. Mechanistically, MLKL-driven necroptosis recruits macrophages, enhances the tumor CD47 'don't eat me' signal, and induces macrophage extracellular traps (MET) formation for CXCL8 activation. CXCL8 further initiates epithelial-mesenchymal transition (EMT) and upregulates ICAM-1 expression to promote endothelial adhesion. METs also degrades extracellular matrix, that eventually supports PDAC liver metastasis. Meanwhile, targeting necroptosis and CD47 reduces liver metastasis in vivo. Our study thus reveals that necroptosis facilitates PDAC metastasis by evading immune surveillance, and also suggest that CD47 blockade, combined with MLKL inhibitor GW806742X, may be a promising neoadjuvant immunotherapy for overcoming the T1M1 dilemma and reviving the opportunity for radical surgery.

Subinhibitory concentrations of glabridin from Glycyrrhiza glabra L. reduce Listeria monocytogenes motility and hemolytic activity but do not exhibit antimicrobial activity

Increases in the virulence and survival of some pathogens in the presence of subinhibitory concentrations of antibiotics have been reported. However, research on the effects of subinhibitory concentrations of antimicrobial substances derived from traditional Chinese medicine on pathogens is still insufficient. Glabridin is a well-known active isoflavone found in licorice roots that possesses a wide range of biological activities. Therefore, in this study, Listeria monocytogenes (L. monocytogenes) exposed to subinhibitory concentrations of glabridin was used as the research object. The minimum inhibitory concentration (MIC) was determined for L. monocytogenes. We investigated the impacts of subinhibitory concentrations of glabridin on the morphology, motility, biofilm formation, adherence, and survival of L. monocytogenes. The results indicated that the MIC of glabridin for L. monocytogenes was 31.25 μg/mL. At 1/8, 1/4, or 1/2 of the MIC, glabridin did not affect the growth, morphology, flagellar production, or biofilm formation of L. monocytogenes. However, subinhibitory concentrations of glabridin inhibited bacterial swimming and swarming motility and decreased the hemolytic activity of L. monocytogenes. Glabridin reduced the hemolytic activity of L. monocytogenes culture supernatants. The results also showed that subinhibitory concentrations of glabridin had no toxic effect on RAW264.7 cells but decreased the intracellular growth of L. monocytogenes in RAW264.7 cells. Furthermore, subinhibitory concentrations of glabridin triggered ROS production but did not induce MET formation in macrophages. In addition, glabridin did not enhance the capacity of L. monocytogenes to trigger METs or the extracellular killing of macrophages by METs. Thus, we conclude that subinhibitory concentrations of glabridin reduce L. monocytogenes motility and hemolytic activity but do not exhibit antimicrobial activity. Glabridin could be an interesting food additive as a bacteriostatic agent with anti-Listeria activity.

Emerging insights into macrophage extracellular traps in bacterial infections

Macrophages possess a diverse range of well-defined capabilities and roles as phagocytes, encompassing the regulation of inflammation, facilitation of wound healing, maintenance of tissue homeostasis, and serving as a crucial element in the innate immune response against microbial pathogens. The emergence of extracellular traps is a novel strategy of defense that has been observed in several types of innate immune cells. In response to infection, macrophages are stimulated and produce macrophage extracellular traps (METs), which take the form of net-like structures, filled with strands of DNA and adorned with histones and other cellular proteins. METs not only capture and eliminate microorganisms but also play a role in the development of certain diseases such as inflammation and autoimmune disorders. The primary objective of this study is to examine the latest advancements in METs for tackling bacterial infections. We also delve into the current knowledge and tactics utilized by bacteria to elude or endure the effects of METs. Through this investigation, we hope to shed light on the intricate interactions between bacteria and the host's immune system, particularly in the context of microbicidal effector mechanisms of METs. The continued exploration of METs and their impact on host defense against various pathogens opens up new avenues for understanding and potentially manipulating the immune system's response to infections.

Macrophage extracellular traps require peptidylarginine deiminase 2 and 4 and are a source of citrullinated antigens bound by rheumatoid arthritis autoantibodies

Introduction

Anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis, but the sources of citrullinated antigens as well as which peptidylarginine deiminases (PADs) are required for their production remain incompletely defined. Here, we investigated if macrophage extracellular traps (METs) could be a source of citrullinated proteins bound by APCAs, and if their formation requires PAD2 or PAD4.

Methods

Thioglycolate-induced peritoneal macrophages from wild-type, PAD2-/-, and PAD4-/- mice or human peripheral blood-derived M1 macrophages were activated with a variety of stimulants, then fixed and stained with DAPI and either anti-citrullinated histone H4 (citH4) antibody or sera from ACPA+ or ACPA- rheumatoid arthritis subjects. METs were visualized by immunofluorescence, confirmed to be extracellular using DNase, and quantified.

Results

We found that ionomycin and monosodium urate crystals reliably induced murine citH4+ METs, which were reduced in the absence of PAD2 and lost in the absence of PAD4. Also, IgG from ACPA+, but not ACPA-, rheumatoid arthritis sera bound to murine METs, and in the absence of PAD2 or PAD4, ACPA-bound METs were lost. Finally, ionomycin induced human METs that are citH4+ and ACPA-bound.

Discussion

Thus, METs may contribute to the pool of citrullinated antigens bound by ACPAs in a PAD2- and PAD4-dependent manner, providing new insights into the targets of immune tolerance loss in rheumatoid arthritis.

Top Five Stories of the Cellular Landscape and Therapies of Atherosclerosis: Current Knowledge and Future Perspectives

Atherosclerosis (AS) is characterized by impairment and apoptosis of endothelial cells, continuous systemic and focal inflammation and dysfunction of vascular smooth muscle cells, which is documented as the traditional cellular paradigm. However, the mechanisms appear much more complicated than we thought since a bulk of studies on efferocytosis, transdifferentiation and novel cell death forms such as ferroptosis, pyroptosis, and extracellular trap were reported. Discovery of novel pathological cellular landscapes provides a large number of therapeutic targets. On the other side, the unsatisfactory therapeutic effects of current treatment with lipid-lowering drugs as the cornerstone also restricts the efforts to reduce global AS burden. Stem cell- or nanoparticle-based strategies spurred a lot of attention due to the attractive therapeutic effects and minimized adverse effects. Given the complexity of pathological changes of AS, attempts to develop an almighty medicine based on single mechanisms could be theoretically challenging. In this review, the top stories in the cellular landscapes during the initiation and progression of AS and the therapies were summarized in an integrated perspective to facilitate efforts to develop a multi-targets strategy and fill the gap between mechanism research and clinical translation. The future challenges and improvements were also discussed.

The role of platelets in sepsis: A review

Sepsis, a life-threatening condition characterized by organ dysfunction, results from a complex series of pathophysiological mechanisms including immune dysfunction, an uncontrolled inflammatory response, and coagulation abnormalities. It is a major contributor to global mortality and severe disease development. Platelets, abundant in the circulatory system, are sensitive to changes in the body's internal environment and are among the first cells to respond to dysregulated pro-inflammatory and pro-coagulant reactions at the onset of sepsis. In the initial stages of sepsis, the coagulation cascade, inflammatory response, and endothelial tissue damage perpetually trigger platelet activation. These activated platelets then engage in complex inflammatory and immune reactions, potentially leading to organ dysfunction. Therefore, further research is essential to fully understand the role of platelets in sepsis pathology and to develop effective therapeutic strategies targeting the associated pathogenic pathways. This review delves into the involvement of platelets in sepsis and briefly outlines the clinical applications of associated biomarkers.

Candidiasis in breast cancer: Tumor progression or not?

Candida albicans is an "opportunistic fungal agent" in cancer patients that can become colonized in both mucosal and deep tissues and cause severe infections. Most evidence has shown that C. albicans can enhance the progress of different cancers by several mechanisms such as generating virulence factors, participation in endogenous production of pro-inflammatory mediators, and stimulating a wide range of immune cells in the host. The main idea of this review is to describe a range of Candida-used mechanisms that are important in candidiasis-associated malignant processes and cancer development, particularly breast cancer. This review intends to provide a detailed discussion on different regulatory mechanisms of C. albicans that undoubtedly help to open new therapeutic horizons of cancer therapy in patients with fungal infection. The current therapeutic approach is not fully effective in immunocompromised and cancer patients, and further studies are required to find new products with effective antifungal properties and minimal side effects to increase the susceptibility of opportunistic fungal infections to conventional antifungal agents. So, in this situation, a special therapy should be considered to control the infection and simultaneously have the most therapeutic index on tumor patients.

Schistosome egg-derived extracellular vesicles deliver Sja-miR-71a inhibits host macrophage and neutrophil extracellular traps via targeting Sema4D

BACKGROUND

Macrophages and neutrophils are rapidly recruited around Schistosome eggs to form granulomas. Extracellular traps (ETs) of macrophages and neutrophils are part of the pathogen clearance armamentarium of leukocytes. Schistosome eggs possess the ability to resist attack by the host's immune cells and survive by employing various immune evasion mechanisms, including the release of extracellular vesicles (EVs). However, the specific mechanisms by which Schistosome egg-derived EVs (E-EVs) evade the immune response and resist attack from macrophage and neutrophil ETs remain poorly understood. In this study, we aimed to investigate the association between E-EVs and macrophage/neutrophil ETs.

METHODS

EVs were isolated from the culture supernatant of S. japonicum eggs and treated macrophages and neutrophils with E-EVs and Sja-miR-71a. The formation of ETs was then observed. Additionally, we infected mice with S. japonicum, administered HBAAV2/9-Sja-miR-71a, and the formation of macrophage ETs (METs) and neutrophil ETs (NETs) in the livers was measured. Sema4D-knockout mice, RNA sequencing, and trans-well assay were used to clarify Sja-miR-71a in E-EVs inhibits METs and NETs formation via the Sema4D/ PPAR-γ/ IL-10 axis.

RESULTS

Our findings revealed that E-EVs were internalized by macrophages and neutrophils, leading to the inhibition of METs and NETs formation. The highly expressed Sja-miR-71a in E-EVs targeted Sema4D, resulting in the up-regulation of IL-10 and subsequent inhibition of METs and NETs formation. Sema4D knockout up-regulated IL-10 expression and inhibited the formation of METs and NETs. Furthermore, we further demonstrated that Sja-miR-71a inhibits METs and NETs formation via the Sema4D/ PPAR-γ/ IL-10 axis.

CONCLUSIONS

In summary, our findings provide new insights into the immune evasion abilities of Schistosome eggs by demonstrating their ability to inhibit the formation of METs and NETs through the secretion of EVs. This study enhances our understanding of the host-pathogen interaction and may have implications for the development of novel therapeutic approaches. Video Abstract.

Impact of endogenous and exogenous nitrogen species on macrophage extracellular trap (MET) formation by bone marrow-derived macrophages

Macrophage extracellular traps (METs) represent a novel defense mechanism in the antimicrobial arsenal of macrophages. However, mechanisms of MET formation are still poorly understood and this is at least partially due to the lack of reliable and reproducible models. Thus, we aimed at establishing a protocol of MET induction by bone marrow-derived macrophages (BMDMs) obtained from cryopreserved and then thawed bone marrow (BM) mouse cells. We report that BMDMs obtained in this way were morphologically (F4/80+) and functionally (expression of inducible nitric oxide (NO) synthase and NO production) differentiated and responded to various stimuli of bacterial (lipopolysaccharide, LPS), fungal (zymosan) and chemical (PMA) origin. Importantly, BMDMs were successfully casting METs composed of extracellular DNA (extDNA) serving as their backbone to which proteins such as H2A.X histones and matrix metalloproteinase 9 (MMP-9) were attached. In rendered 3D structure of METs, extDNA and protein components were embedded in each other. Since studies had shown the involvement of oxygen species in MET release, we aimed at studying if reactive nitrogen species (RNS) such as NO are also involved in MET formation. By application of NOS inhibitor - L-NAME or nitric oxide donor (SNAP), we studied the involvement of endogenous and exogenous RNS in traps release. We demonstrated that L-NAME halted MET formation upon stimulation with LPS while SNAP alone induced it. The latter phenomenon was further enhanced in the presence of LPS. Taken together, our findings demonstrate that BMDMs obtained from cryopreserved BM cells are capable of forming METs in an RNS-dependent manner.

Sperm induce macrophage extracellular trap formation via phagocytosis-dependent mechanism

Infertility is a public health concern worldwide. Asthenozoospermia is a common cause of male infertility and is characterized by decreased motility. Sperm motility ensures that sperm migrate to complete fertilization. Macrophages are an essential component of innate immunity in the female reproductive tract. Macrophage extracellular traps are induced by various microorganisms to capture and mediate the clearance of microorganisms. The relationship between sperm and macrophage extracellular traps is unclear. The human monocyte leukemia (THP-1) cells differentiated by phorbol myristate acetate (PMA) are widely used as surrogate of human macrophages. This study investigated sperm-induced macrophage extracellular trap formation and clarified some of the mechanisms affecting macrophage extracellular trap production. Sperm-induced macrophage extracellular traps were visualized and components of macrophage extracellular traps were identified by immunofluorescence analyses and scanning electron microscopy. By inhibiting macrophage extracellular trap production and macrophage phagocytosis, the relationship between macrophage phagocytosis and macrophage extracellular trap production was analyzed. Sperm could trigger PMA-differentiated THP-1 macrophages to produce extracellular traps. Sperm-triggered macrophage extracellular traps are dependent on phagocytosis and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Sperm from asthenozoospermia donors are more likely to be phagocytosed by macrophages than sperm from healthy donors, which induce more macrophage extracellular trap release. These data confirm the phenomenon and partial mechanism of sperm-induced macrophage extracellular trap formation in vitro. These may partly provide evidence to explain the mechanisms of clearing abnormally morphological or hypomotile sperm in the female reproductive tract and the rationale for the decreased probability of successful fertilization in asthenozoospermia.

Macrophages in immunoregulation and therapeutics

Macrophages exist in various tissues, several body cavities, and around mucosal surfaces and are a vital part of the innate immune system for host defense against many pathogens and cancers. Macrophages possess binary M1/M2 macrophage polarization settings, which perform a central role in an array of immune tasks via intrinsic signal cascades and, therefore, must be precisely regulated. Many crucial questions about macrophage signaling and immune modulation are yet to be uncovered. In addition, the clinical importance of tumor-associated macrophages is becoming more widely recognized as significant progress has been made in understanding their biology. Moreover, they are an integral part of the tumor microenvironment, playing a part in the regulation of a wide variety of processes including angiogenesis, extracellular matrix transformation, cancer cell proliferation, metastasis, immunosuppression, and resistance to chemotherapeutic and checkpoint blockade immunotherapies. Herein, we discuss immune regulation in macrophage polarization and signaling, mechanical stresses and modulation, metabolic signaling pathways, mitochondrial and transcriptional, and epigenetic regulation. Furthermore, we have broadly extended the understanding of macrophages in extracellular traps and the essential roles of autophagy and aging in regulating macrophage functions. Moreover, we discussed recent advances in macrophages-mediated immune regulation of autoimmune diseases and tumorigenesis. Lastly, we discussed targeted macrophage therapy to portray prospective targets for therapeutic strategies in health and diseases.

Polystyrene microplastics-induced macrophage extracellular traps contributes to liver fibrotic injury by activating ROS/TGF-β/Smad2/3 signaling axis

Microplastics (MPs) are a type of emerging pollutant, posing a great threat to human and animal health. While recent studies have revealed the link between MPs exposure and liver injury of organisms, the effect of particle size on the level of MPs-induced hepatotoxicity and the intrinsic mechanism remain to be explored. Here, we established a mouse model exposed to two-diameter polystyrene MPs (PS-MPs, 1-10 μm or 50-100 μm) for 30 days. The in vivo results revealed that PS-MPs caused liver fibrotic injury in mice, accompanied with macrophages recruitment and macrophage extracellular traps (METs) formation, which were negatively correlated with particle size. The data in vitro showed that PS-MPs treatment could induce macrophages to release METs in a reactive oxygen species (ROS)-independent manner, and the METs formation level caused by large-size particles was higher than small-size particles. Further mechanistic analysis of a cell co-culture system revealed that PS-MPs-induced METs release led to a hepatocellular inflammatory response and epithelial-mesenchymal transition (EMT) via activating the ROS/TGF-β/Smad2/3 signaling axis, and this biological crosstalk could be relieved by DNase I. Overall, this findings demonstrates the key role of the action mechanism of METs in aggravating MPs-caused liver injury.

1,4-Naphthoquinone-Coated Black Carbon, a Kind of Atmospheric Fine Particulate Matter, Affects Macrophage Fate: New Insights into Crosstalk between Necroptosis and Macrophage Extracellular Traps

1,4-Naphthoquinone-coated BC (1,4 NQ-BC) is an important component of PM_2.5 and a representative secondary particle. However, there is no research on the crosstalk mechanism between necroptosis and macrophage extracellular traps (METs) after 1,4 NQ-BC exposure. In this study, we treated RAW264.7 cells with 50, 100, and 200 mg/L 1,4 NQ-BC for 24 h, with 10 μM necrostatin-1 for 24 h, and with 2.5 μM phorbol 12-myristate 13-acetate (PMA) for 3 h. Our experiment revealed that under normal physiological conditions, when macrophages receive external stimuli (such as pathogens; in this experiment, PMA), they will form METs and capture and kill pathogens, thus exerting innate immune function. However, exposure to 1,4 NQ-BC can cause necroptosis in macrophages, accompanied by increased levels of reactive oxygen species (ROS) and cytosolic calcium ions, as well as the expression disorder of inflammatory factors and chemokines, prevent the formation of METs, lead to loss of the function of capturing and killing pathogens, and weaken the innate immune function. Notably, inhibition of necroptosis restored the formation of METs, indicating that necroptosis inhibited the formation of METs. Our study was the first to explore the crosstalk mechanism between necroptosis and METs. This experiment will enrich the mechanism of macrophage injury caused by 1,4 NQ-BC exposure.

New insight into the composition of extracellular traps released by macrophages exposed to different types of inducers

Neutrophil extracellular trap (NET) release plays a key role in many chronic disease settings, including atherosclerosis. They are critical to innate immune defence, but also contribute to disease by promoting thrombosis and inflammation. Macrophages are known to release extracellular traps or "METs", but their composition and role in pathological processes are less well defined. In this study, we examined MET release from human THP-1 macrophages exposed to model inflammatory and pathogenic stimuli, including tumour necrosis factor α (TNFα), hypochlorous acid (HOCl) and nigericin. In each case, there was release of DNA from the macrophages, as visualized by fluorescence microscopy with the cell impermeable DNA binding dye SYTOX green, consistent with MET formation. Proteomic analysis on METs released from macrophages exposed to TNFα and nigericin reveals that they are composed of linker and core histones, together with a range of cytosolic and mitochondrial proteins. These include proteins involved in DNA binding, stress responses, cytoskeletal organisation, metabolism, inflammation, anti-microbial activity, and calcium binding. Quinone oxidoreductase in particular, was highly abundant in all METs but has not been reported previously in NETs. Moreover, there was an absence of proteases in METs in contrast to NETs. Some of the MET histones, contained post-translational modifications, including acetylation and methylation of Lys but not citrullination of Arg. These data provide new insight into the potential implications of MET formation in vivo and their contributions to immune defence and pathology.

Neutrophil Extracellular Traps and Cancer: Trapping Our Attention with Their Involvement in Ovarian Cancer

Neutrophils, the most abundant circulating leukocytes, play a well-known role in defense against pathogens through phagocytosis and degranulation. However, a new mechanism involving the release of neutrophil extracellular traps (NETs) composed of DNA, histones, calprotectin, myeloperoxidase, and elastase, among others, has been described. The so-called NETosis process can occur through three different mechanisms: suicidal, vital, and mitochondrial NETosis. Apart from their role in immune defense, neutrophils and NETs have been involved in physiopathological conditions, highlighting immunothrombosis and cancer. Notably, neutrophils can either promote or inhibit tumor growth in the tumor microenvironment depending on cytokine signaling and epigenetic modifications. Several neutrophils' pro-tumor strategies involving NETs have been documented, including pre-metastatic niche formation, increased survival, inhibition of the immune response, and resistance to oncologic therapies. In this review, we focus on ovarian cancer (OC), which remains the second most incidental but the most lethal gynecologic malignancy, partly due to the presence of metastasis, often omental, at diagnosis and the resistance to treatment. We deepen the state-of-the-art on the participation of NETs in OC metastasis establishment and progression and their involvement in resistance to chemo-, immuno-, and radiotherapies. Finally, we review the current literature on NETs in OC as diagnostic and/or prognostic markers, and their contribution to disease progression at early and advanced stages. The panoramic view provided in this article might pave the way for enhanced diagnostic and therapeutic strategies to improve the prognosis of cancer patients and, specifically, OC patients.

No NETs no TIME: Crosstalk between neutrophil extracellular traps and the tumor immune microenvironment

The tumor immune microenvironment (TIME) controls tumorigenesis. Neutrophils are important components of TIME and control tumor progression and therapy resistance. Neutrophil extracellular traps (NETs) ejected by activated neutrophils are net-like structures composed of decondensed extracellular chromatin filaments decorated with a plethora of granules as well as cytoplasmic proteins. Many of these harbour post translational modifications. Cancer cells reportedly trigger NET formation, and conversely, NETs alter the TIME and promote tumor cell proliferation and migration. The specific interactions between NETs and TIME and the respective effects on tumor progression are still elusive. In certain tumors, a CD4+ T helper (Th) 2 cell-associated TIME induces NETs and exerts immunosuppressive functions via programmed death 1 (PD-1)/PD-L1, both associated with poorer prognosis. In other cases, NETs induce the proliferation of Th1 cells, associated with an improved prognosis in cancer. In addition, NETs can drive macrophage polarization and often rely on macrophages to promote cancer cell invasion and metastasis. In turn, macrophages can swiftly clear NETs in an immunologically silent manner. The aim of this review is to summarize the knowledge about the mutual interaction between NETs and TIME and its impact on tumor growth and therapy.

Basophils from allergy to cancer

Human basophils, first identified over 140 years ago, account for just 0.5-1% of circulating leukocytes. While this scarcity long hampered basophil studies, innovations during the past 30 years, beginning with their isolation and more recently in the development of mouse models, have markedly advanced our understanding of these cells. Although dissimilarities between human and mouse basophils persist, the overall findings highlight the growing importance of these cells in health and disease. Indeed, studies continue to support basophils as key participants in IgE-mediated reactions, where they infiltrate inflammatory lesions, release pro-inflammatory mediators (histamine, leukotriene C₄: LTC₄) and regulatory cytokines (IL-4, IL-13) central to the pathogenesis of allergic diseases. Studies now report basophils infiltrating various human cancers where they play diverse roles, either promoting or hampering tumorigenesis. Likewise, this activity bears remarkable similarity to the mounting evidence that basophils facilitate wound healing. In fact, both activities appear linked to the capacity of basophils to secrete IL-4/IL-13, with these cytokines polarizing macrophages toward the M2 phenotype. Basophils also secrete several angiogenic factors (vascular endothelial growth factor: VEGF-A, amphiregulin) consistent with these activities. In this review, we feature these newfound properties with the goal of unraveling the increasing importance of basophils in these diverse pathobiological processes.

T-Cell Mediated Immunity in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare and frequently lethal skin cancer with neuroendocrine characteristics. MCC can originate from either the presence of MCC polyomavirus (MCPyV) DNA or chronic ultraviolet (UV) exposure that can cause DNA mutations. MCC is predominant in sun-exposed regions of the body and can metastasize to regional lymph nodes, liver, lungs, bone, and brain. Older, light-skinned individuals with a history of significant sun exposure are at the highest risk. Previous studies have shown that tumors containing a high number of tumor-infiltrating T-cells have favorable survival, even in the absence of MCPyV DNA, suggesting that MCPyV infection enhances T-cell infiltration. However, other factors may also play a role in the host antitumor response. Herein, we review the impact of tumor infiltrating lymphocytes (TILs), mainly the CD4⁺, CD8⁺, and regulatory T-cell (Tregs) responses on the course of MCC, including their role in initiating MCPyV-specific immune responses. Furthermore, potential research avenues related to T-cell biology in MCC, as well as relevant immunotherapies are discussed.

5'-Nucleotidase is dispensable for the growth of Salmonella Typhimurium but inhibits the bactericidal activity of macrophage extracellular traps

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a zoonotic pathogen that causes severe gastroenteritis. The 5'-nucleotidases of pathogens can dephosphorylate adenosine phosphates, boost adenosine levels and suppress the pro-inflammatory immune response. In our previous study, an extracellular nuclease, 5'-nucleotidase, was identified in the extracellular proteins of S. Typhimurium. However, the nuclease activity and the function of the 5'-nucleotidase of S. Typhimurium have not been explored. In the present study, deletion of the 5'-nucleotidase gene is dispensable for S. Typhimurium growth, even under environmental stress. Fluorescence microscopy revealed that the 5'-nucleotidase mutant induced more macrophage extracellular traps (METs) than the wild type did. Furthermore, recombinant 5'-nucleotidase protein (r5Nuc) could degrade λDNA, and the nuclease activity of r5Nuc was optimum at 37 °C and pH 6.0-7.0. The Mg²⁺ enhanced the nuclease activity of r5Nuc, whereas Zn²⁺ inhibited it. Meanwhile, deletion of the 5'-nucleotidase gene increased the bactericidal activity of METs, and r5Nuc could degrade METs and inhibit the bactericidal activity of METs. In conclusion, S. Typhimurium growth was independent of 5'-nucleotidase, but the nuclease activity of 5'-nucleotidase assisted S. Typhimurium to evade macrophage-mediated extracellular killing through degrading METs.

Extracellular traps from activated vascular smooth muscle cells drive the progression of atherosclerosis

Extracellular DNA traps (ETs) represent an immune response by which cells release essential materials like chromatin and granular proteins. Previous studies have demonstrated that the transdifferentiation of vascular smooth muscle cells (VSMCs) plays a crucial role in atherosclerosis. This study seeks to investigate the interaction between CD68⁺ VSMCs and the formation of ETs and highlight its function in atherosclerosis. Here we show that ETs are inhibited, and atherosclerotic plaque formation is alleviated in male Myh11^CrePad4^flox/flox mice undergoing an adeno-associated-virus-8 (AAV8) mediating overexpression of proprotein convertase subtilisin/kexin type 9 mutation (PCSK9) injection and being challenged with a high-fat diet. Obvious ETs generated from CD68⁺ VSMCs are inhibited by Cl-amidine and DNase I in vitro. By utilizing VSMCs-lineage tracing technology and single-cell RNA sequencing (scRNA-seq), we demonstrate that the ETs from CD68⁺ VSMCs influence the progress of atherosclerosis by regulating the direction of VSMCs' transdifferentiation through STING-SOCS1 or TLR4 signaling pathway.

Neutrophil Extracellular Traps in Asthma: Friends or Foes?

Asthma is a chronic inflammatory disease characterized by variable airflow limitation and airway hyperresponsiveness. A plethora of immune and structural cells are involved in asthma pathogenesis. The roles of neutrophils and their mediators in different asthma phenotypes are largely unknown. Neutrophil extracellular traps (NETs) are net-like structures composed of DNA scaffolds, histones and granular proteins released by activated neutrophils. NETs were originally described as a process to entrap and kill a variety of microorganisms. NET formation can be achieved through a cell-death process, termed NETosis, or in association with the release of DNA from viable neutrophils. NETs can also promote the resolution of inflammation by degrading cytokines and chemokines. NETs have been implicated in the pathogenesis of various non-infectious conditions, including autoimmunity, cancer and even allergic disorders. Putative surrogate NET biomarkers (e.g., double-strand DNA (dsDNA), myeloperoxidase-DNA (MPO-DNA), and citrullinated histone H3 (CitH3)) have been found in different sites/fluids of patients with asthma. Targeting NETs has been proposed as a therapeutic strategy in several diseases. However, different NETs and NET components may have alternate, even opposite, consequences on inflammation. Here we review recent findings emphasizing the pathogenic and therapeutic potential of NETs in asthma.

The escape of Candida albicans from macrophages is enabled by the fungal toxin candidalysin and two host cell death pathways

The egress of Candida hyphae from macrophages facilitates immune evasion, but it also alerts macrophages to infection and triggers inflammation. To better define the mechanisms, here we develop an imaging assay to directly and dynamically quantify hyphal escape and correlate it to macrophage responses. The assay reveals that Candida escapes by using two pore-forming proteins to permeabilize macrophage membranes: the fungal toxin candidalysin and Nlrp3 inflammasome-activated Gasdermin D. Candidalysin plays a major role in escape, with Nlrp3 and Gasdermin D-dependent and -independent contributions. The inactivation of Nlrp3 does not reduce hyphal escape, and we identify ETosis via macrophage extracellular trap formation as an additional pathway facilitating hyphal escape. Suppressing hyphal escape does not reduce fungal loads, but it does reduce inflammatory activation. Our findings explain how Candida escapes from macrophages by using three strategies: permeabilizing macrophage membranes via candidalysin and engaging two host cell death pathways, Gasdermin D-mediated pyroptosis and ETosis.

NETosis and thrombosis in vaccine-induced immune thrombotic thrombocytopenia

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare yet serious adverse effect of the adenoviral vector vaccines ChAdOx1 nCoV-19 (AstraZeneca) and Ad26.COV2.S (Janssen) against COVID-19. The mechanisms involved in clot formation and thrombocytopenia in VITT are yet to be fully determined. Here we show neutrophils undergoing NETosis and confirm expression markers of NETs in VITT patients. VITT antibodies directly stimulate neutrophils to release NETs and induce thrombus formation containing abundant platelets, neutrophils, fibrin, extracellular DNA and citrullinated histone H3 in a flow microfluidics system and in vivo. Inhibition of NETosis prevents VITT-induced thrombosis in mice but not thrombocytopenia. In contrast, in vivo blockage of FcγRIIa abrogates both thrombosis and thrombocytopenia suggesting these are distinct processes. Our findings indicate that anti-PF4 antibodies activate blood cells via FcγRIIa and are responsible for thrombosis and thrombocytopenia in VITT. Future development of NETosis and FcγRIIa inhibitors are needed to treat VITT and similar immune thrombotic thrombocytopenia conditions more effectively, leading to better patient outcomes.

The mechanisms underlying the pathogenesis of vaccine-induced immune thrombotic thrombocytopenia (VITT) remain unclear. Here the authors show that anti-PF4 antibodies are responsible for the activation of platelets and neutrophils, and blockage of FcγRIIa or NETosis in vivo can prevent thrombosis.

Morphologic Analysis of M2 Macrophage in Glioblastoma: Involvement of Macrophage Extracellular Traps (METs)

Macrophages are classified into two phenotypes, M1 and M2, based on their roles. M2 macrophages suppress inflammation and increase in proportion to the malignancy of brain tumors. Recently, macrophage extracellular traps (METs), which change into a network, have been reported as a unique form of macrophage cell death. In this study, immunohistochemical analysis of macrophages in METs in human glioblastoma was performed. To distinguish between M1 and M2 macrophages, multiple immunostainings with Iba1 combined with CD163 or CD204 were performed. M2 macrophages were present in small amounts in normal and borderline areas but showed an increasing trend as they shifted to tumor areas, and most of them were the activated- or phagocytic-type. We also successfully detected METs coexisting with fibrin and lactoferrin near the border between the tumor and necrotic area. M2 macrophages not only suppressed inflammation but also were involved in the formation of METs. This study found that M2 macrophages play various roles in unstable situations.

The emerging role of neutrophil extracellular traps in fungal infection

Fungal infections are global public health problems and can lead to substantial human morbidity and mortality. Current antifungal therapy is not satisfactory, especially for invasive, life-threatening fungal infections. Modulating the antifungal capacity of the host immune system is a feasible way to combat fungal infections. Neutrophils are key components of the innate immune system that resist fungal pathogens by releasing reticular extracellular structures called neutrophil extracellular traps (NETs). When compared with phagocytosis and oxidative burst, NETs show better capability in terms of trapping large pathogens, such as fungi. This review will summarize interactions between fungal pathogens and NETs. Molecular mechanisms of fungi-induced NETs formation and defensive strategies used by fungi are also discussed.

Pathogen-Derived Nucleases: An Effective Weapon for Escaping Extracellular Traps

Since the 2004 publication of the first study describing extracellular traps (ETs) from human neutrophils, several reports have shown the presence of ETs in a variety of different animals and plants. ETs perform two important functions of immobilizing and killing invading microbes and are considered a novel part of the phagocytosis-independent, innate immune extracellular defense system. However, several pathogens can release nucleases that degrade the DNA backbone of ETs, reducing their effectiveness and resulting in increased pathogenicity. In this review, we examined the relevant literature and summarized the results on bacterial and fungal pathogens and parasites that produce nucleases to evade the ET-mediated host antimicrobial mechanism.

T Cell Extracellular Traps: Tipping the Balance Between Skin Health and Disease

The role of extracellular traps (ETs) in the innate immune response against pathogens is well established. ETs were first identified in neutrophils and have since been identified in several other immune cells. Although the mechanistic details are not yet fully understood, recent reports have described antigen-specific T cells producing T cell extracellular traps (TETs). Depending on their location within the cutaneous environment, TETs may be beneficial to the host by their ability to limit the spread of pathogens and provide protection against damage to body tissues, and promote early wound healing and degradation of inflammatory mediators, leading to the resolution of inflammatory responses within the skin. However, ETs have also been associated with worse disease outcomes. Here, we consider host-microbe ET interactions by highlighting how cutaneous T cell-derived ETs aid in orchestrating host immune responses against Cutibacterium acnes (C. acnes), a commensal skin bacterium that contributes to skin health, but is also associated with acne vulgaris and surgical infections following joint-replacement procedures. Insights on the role of the skin microbes in regulating T cell ET formation have broad implications not only in novel probiotic design for acne treatment, but also in the treatment for other chronic inflammatory skin disorders and autoimmune diseases.

The "Self-Sacrifice" of ImmuneCells in Sepsis

Sepsis is a life-threatening organ dysfunction caused by the host’s malfunctioning response to infection. Due to its high mortality rate and medical cost, sepsis remains one of the world’s most intractable diseases. In the early stage of sepsis, the over-activated immune system and a cascade of inflammation are usually accompanied by immunosuppression. The core pathogenesis of sepsis is the maladjustment of the host’s innate and adaptive immune response. Many immune cells are involved in this process, including neutrophils, mononuclear/macrophages and lymphocytes. The immune cells recognize pathogens, devour pathogens and release cytokines to recruit or activate other cells in direct or indirect manner. Pyroptosis, immune cell-extracellular traps formation and autophagy are several novel forms of cell death that are different from apoptosis, which play essential roles in the progress of sepsis. Immune cells can initiate “self-sacrifice” through the above three forms of cell death to protect or kill pathogens. However, the exact roles and mechanisms of the self-sacrifice in the immune cells in sepsis are not fully elucidated. This paper mainly analyzes the self-sacrifice of several representative immune cells in the forms of pyroptosis, immune cell-extracellular traps formation and autophagy to reveal the specific roles they play in the occurrence and progression of sepsis, also to provide inspiration and references for further investigation of the roles and mechanisms of self-sacrifice of immune cells in the sepsis in the future, meanwhile, through this work, we hope to bring inspiration to clinical work.

Extracellular DNA Traps: Origin, Function and Implications for Anti-Cancer Therapies

Extracellular DNA may serve as marker in liquid biopsies to determine individual diagnosis and prognosis in cancer patients. Cell death or active release from various cell types, including immune cells can result in the release of DNA into the extracellular milieu. Neutrophils are important components of the innate immune system, controlling pathogens through phagocytosis and/or the release of neutrophil extracellular traps (NETs). NETs also promote tumor progression and metastasis, by modulating angiogenesis, anti-tumor immunity, blood clotting and inflammation and providing a supportive niche for metastasizing cancer cells. Besides neutrophils, other immune cells such as eosinophils, dendritic cells, monocytes/macrophages, mast cells, basophils and lymphocytes can also form extracellular traps (ETs) during cancer progression, indicating possible multiple origins of extracellular DNA in cancer. In this review, we summarize the pathomechanisms of ET formation generated by different cell types, and analyze these processes in the context of cancer. We also critically discuss potential ET-inhibiting agents, which may open new therapeutic strategies for cancer prevention and treatment.

The Expanding Role of Extracellular Traps in Inflammation and Autoimmunity: The New Players in Casting Dark Webs

The first description of a new form of neutrophil cell death distinct from that of apoptosis or necrosis was discovered in 2004 and coined neutrophil extracellular traps "(NETs)" or "NETosis". Different stimuli for NET formation, and pathways that drive neutrophils to commit to NETosis have been elucidated in the years that followed. Critical enzymes required for NET formation have been discovered and targeted therapeutically. NET formation is no longer restricted to neutrophils but has been discovered in other innate cells: macrophages/monocytes, mast Cells, basophils, dendritic cells, and eosinophils. Furthermore, extracellular DNA can also be extruded from both B and T cells. It has become clear that although this mechanism is thought to enhance host defense by ensnaring bacteria within large webs of DNA to increase bactericidal killing capacity, it is also injurious to innocent bystander tissue. Proteases and enzymes released from extracellular traps (ETs), injure epithelial and endothelial cells perpetuating inflammation. In the context of autoimmunity, ETs release over 70 well-known autoantigens. ETs are associated with pathology in multiple diseases: lung diseases, vasculitis, autoimmune kidney diseases, atherosclerosis, rheumatoid arthritis, cancer, and psoriasis. Defining these pathways that drive ET release will provide insight into mechanisms of pathological insult and provide potential therapeutic targets.

Stimulator of Interferon Genes (STING) Promotes Staphylococcus aureus-Induced Extracellular Traps Formation via the ROS-ERK Signaling Pathway

Stimulator of interferon genes (STING) is a cytosolic DNA sensor or directly recognizes bacterial cyclic dinucleotides, which is required for the detection of microbial infection. Extracellular traps (ETs) are known to be part of the antimicrobial defense system. However, the implication of STING in ETs formation during microbial infection remains unknown. Here, we showed that STING contributed to Staphylococcus aureus (S. aureus)-induced ETs formation through the ROS-ERK signaling. STING deficiency exhibited decreased cell-free DNA (cfDNA) level, reduced expression of citrullinated histone H3 (CitH3), and diminished DNA colocalization with CitH3 and myeloperoxidase (MPO). Interestingly, NADPH oxidase-derived reactive oxygen species (ROS) promoted ETs formation, accompanied by increased activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) in S. aureus-stimulated bone marrow-derived macrophages (BMDMs). Corresponding to less ROS production, decreased ERK1/2 activation was shown in STING^-/- BMDMs after S. aureus infection. Importantly, inhibiting the ROS-ERK signal reduced the ETs formation and the differences disappeared between WT and STING^-/- BMDMs after S. aureus infection. Moreover, STING^-/- BMDMs exhibited significantly increased levels of extracellular bacteria compared to WT BMDMs regardless of phagocytosis. In addition, such differences disappeared after DNase I treatment. DNase I treatment also facilitated pathogen colonization without affecting the inflammatory cells infiltration and pro-inflammatory factors secretion following pulmonary S. aureus infection. Furthermore, STING^-/- mice presented decreased levels of cfDNA and CitH3, along with increased bacterial colonization compared to WT mice. Altogether, these findings highlighted that STING promoted ETs formation via the ROS-ERK signal for host defense against S. aureus infection.

Role of macrophage extracellular traps in innate immunity and inflammatory disease

Macrophages play an integral role in initiating innate immune defences and regulating inflammation. They are also involved in maintaining homeostasis and the resolution of inflammation, by promoting tissue repair and wound healing. There is evidence that like neutrophils, macrophages can release extracellular traps following exposure to a range of pathogenic and pro-inflammatory stimuli. Extracellular traps are released by a specialised cell death pathway termed 'ETosis', and consist of a backbone of DNA and histones decorated with a range of other proteins. The composition of extracellular trap proteins can be influenced by both the cell type and the local environment in which the traps are released. In many cases, these proteins have an antimicrobial role and assist with pathogen killing. Therefore, the release of extracellular traps serves as a means to both immobilise and destroy invading pathogens. In addition to their protective role, extracellular traps are also implicated in disease pathology. The release of neutrophil extracellular traps (NETs) is causally linked to the development of wide range of human diseases. However, whether macrophage extracellular traps (METs) play a similar role in disease pathology is less well established. Moreover, macrophages are also involved in the clearance of extracellular traps, which could assist in the resolution of tissue damage associated with the presence of extracellular traps. In this review, we will provide an overview of the pathways responsible for macrophage extracellular trap release, and discuss the role of these structures in innate immunity and disease pathology and possible therapeutic strategies.

Role of Extracellular Trap Release During Bacterial and Viral Infection

Neutrophils are innate immune cells that play an essential role during the clearance of pathogens that can release chromatin structures coated by several cytoplasmatic and granular antibacterial proteins, called neutrophil extracellular traps (NETs). These supra-molecular structures are produced to kill or immobilize several types of microorganisms, including bacteria and viruses. The contribution of the NET release process (or NETosis) to acute inflammation or the prevention of pathogen spreading depends on the specific microorganism involved in triggering this response. Furthermore, studies highlight the role of innate cells different from neutrophils in triggering the release of extracellular traps during bacterial infection. This review summarizes the contribution of NETs during bacterial and viral infections, explaining the molecular mechanisms involved in their formation and the relationship with different components of such pathogens.

Macrophage Extracellular Traps: Current Opinions and the State of Research regarding Various Diseases

Macrophages are an important component of the human immune system and play a key role in the immune response, which can protect the body against infection and regulate the development of tissue inflammation. Some studies found that macrophages can produce extracellular traps (ETs) under various conditions of stimulation. ETs are web-like structures that consist of proteins and DNA. ETs are thought to immobilize and kill microorganisms, as well as play an important role in tissue damage, inflammatory progression, and autoimmune diseases. In this review, the structure, identification, mechanism, and research progress of macrophage extracellular traps (METs) in related diseases are reviewed.

Role of the Extracellular Traps in Central Nervous System

It has been reported that several immune cells can release chromatin and granular proteins into extracellular space in response to the stimulation, forming extracellular traps (ETs). The cells involved in the extracellular trap formation are recognized including neutropils, macrophages, basophils, eosinophils, and mast cells. With the development of research related to central nervous system, the role of ETs has been valued in neuroinflammation, blood–brain barrier, and other fields. Meanwhile, it has been found that microglial cells as the resident immune cells of the central nervous system can also release ETs, updating the original understanding. This review aims to clarify the role of the ETs in the central nervous system, especially in neuroinflammation and blood–brain barrier.

Release of chromatin extracellular traps by phagocytes of Atlantic salmon, Salmo salar (Linnaeus, 1758)

Neutrophils release chromatin extracellular traps (ETs) as part of the fish innate immune response to counter the threats posed by microbial pathogens. However, relatively little attention has been paid to this phenomenon in many commercially farmed species, despite the importance of understanding host-pathogen interactions and the potential to influence ET release to reduce disease outbreaks. The aim of this present study was to investigate the release of ETs by Atlantic salmon (Salmo salar L.) immune cells. Extracellular structures resembling ETs of different morphology were observed by fluorescence microscopy in neutrophil suspensions in vitro, as these structures stained positively with Sytox Green and were digestible with DNase I. Immunofluorescence studies confirmed the ET structures to be decorated with histones H1 and H2A and neutrophil elastase, which are characteristic for ETs in mammals and other organisms. Although the ETs were released spontaneously, release in neutrophil suspensions was stimulated most significantly with 5 μg/ml calcium ionophore (CaI) for 1 h, whilst the fish pathogenic bacterium Aeromonas salmonicida (isolates 30411 and Hooke) also exerted a stimulatory effect. Microscopic observations revealed bacteria in association with ETs, and fewer bacterial colonies of A. salmonicida Hooke were recovered at 3 h after co-incubation with neutrophils that had been induced to release ETs. Interestingly, spontaneous release of ETs was inversely associated with fish mass (p < 0.05), a surrogate for age. Moreover, suspensions enriched for macrophages and stimulated with 5 μg/ml CaI released ET-like structures that occasionally led to the formation of large clumps of cells. A deeper understanding for the roles and functions of ETs within innate immunity of fish hosts, and their interaction with microbial pathogens, may open new avenues towards protecting cultured stocks against infectious diseases.

Interaction Between Macrophage Extracellular Traps and Colon Cancer Cells Promotes Colon Cancer Invasion and Correlates With Unfavorable Prognosis

Background

Macrophage extracellular traps (METs) and tumor-infiltrating macrophages contribute to the progression of several diseases. But the role of METs and tumor-infiltrating macrophages in colon cancer (CC) has not been illuminated. In this study, we aimed to clarify the prognostic value of METs for CC patients and to explore the interaction between CC cells and METs in vitro and in vivo.

Methods

A training cohort consisting of 116 patients and a validation cohort of 94 patients were enrolled in this study. Immunofluorescence (IF) staining was conducted to determine METs formation in CC patients. Cox regression was used to perform prognostic analysis and screen out the best prognostic model. A nomogram was established to predict 5-year overall survival (OS). The correlation between METs with clinicopathological features and inflammatory markers was analyzed. The formation of METs in vitro was detected by SYTOX® green and IF staining, and the effect of METs on CC cells was detected by transwell assays. PAD2-IN-1, a selective inhibitor of peptidylarginine deiminase 2 (PAD2), was introduced to destroy the crosstalk between CC cells and METs in vitro and in vivo.

Results

METs levels were higher in CC tissues and were an independent prognostic factor for CC patients. The prognostic model consisting of age, tumors local invasion, lymph node metastasis and METs were confirmed to be consistent and accurate for predicting the 5-year OS of CC patients. Besides, METs were correlated with distant metastasis and inflammation. Through in vitro experiments, we confirmed that there was a positive feedback loop between CC cells and METs, in that METs promoted the invasion of CC cells and CC cells enhanced the production of METs, in turn. This interaction could be blocked by PAD2-IN-1 inhibitors. More importantly, animal experiments revealed that PAD2-IN-1 inhibited METs formation and CC liver metastasis in vivo.

Conclusions

METs were the potential biomarker of CC patient prognosis. PAD2-IN-1 inhibited the crosstalk between CC cells and METs in vitro and in vivo, which should be emphasized in CC therapy.

Imaging innate immunity

Innate immunity is the first line of defense against infectious intruders and also plays a major role in the development of sterile inflammation. Direct microscopic imaging of the involved immune cells, especially neutrophil granulocytes, monocytes, and macrophages, has been performed since more than 150 years, and we still obtain novel insights on a frequent basis. Initially, intravital microscopy was limited to small-sized animal species, which were often invertebrates. In this review, we will discuss recent results on the biology of neutrophils and macrophages that have been obtained using confocal and two-photon microscopy of individual cells or subcellular structures as well as light-sheet microscopy of entire organs. This includes the role of these cells in infection defense and sterile inflammation in mammalian disease models relevant for human patients. We discuss their protective but also disease-enhancing activities during tumor growth and ischemia-reperfusion damage of the heart and brain. Finally, we provide two visions, one experimental and one applied, how our knowledge on the function of innate immune cells might be further enhanced and also be used in novel ways for disease diagnostics in the future.

Pulmonary fibrosis from molecular mechanisms to therapeutic interventions: lessons from post-COVID-19 patients

Pulmonary fibrosis (PF) is characterised by several grades of chronic inflammation and collagen deposition in the interalveolar space and is a hallmark of interstitial lung diseases (ILDs). Recently, infectious agents have emerged as driving causes for PF development; however, the role of viral/bacterial infections in the initiation and propagation of PF is still debated. In this context, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for the current coronavirus disease 2019 (COVID-19) pandemic, has been associated with acute respiratory distress syndrome (ARDS) and PF development. Although the infection by SARS-CoV-2 can be eradicated in most cases, the development of fibrotic lesions cannot be precluded; furthermore, whether these lesions are stable or progressive fibrotic events is still unknown. Herein, an overview of the main molecular mechanisms driving the fibrotic process together with the currently approved and newly proposed therapeutic solutions was given. Then, the most recent data that emerged from post-COVID-19 patients was discussed, in order to compare PF and COVID-19-dependent PF, highlighting shared and specific mechanisms. A better understanding of PF aetiology is certainly needed, also to develop effective therapeutic strategies and COVID-19 pathology is offering one more chance to do it. Overall, the work reported here could help to define new approaches for therapeutic intervention in the diversity of the ILD spectrum.

Bacillus licheniformis and Bacillus subtilis, Probiotics That Induce the Formation of Macrophage Extracellular Traps

Probiotics are considered living microorganisms that help preserve the health of the host who uses them. Bacillus are a genus of Gram-positive bacteria used as probiotics for animal and human consumption. They are currently distributed in various commercial forms. Two of the species used as probiotics are B. licheniformis and B. subtilis. Macrophages are central cells in the immune response, being fundamental in the elimination of microbial pathogens, for which they use various mechanisms, including the formation of extracellular traps (METs). There have been very few studies carried out on the participation of macrophages in response to the interaction of probiotics of the genus Bacillus with the host. In this work, we used macrophages from the J774A mouse cell line.1, and we found that they are susceptible to infection by the two Bacillus species. However, both species were eliminated as the infection progressed. Using confocal microscopy, we identified the formation of METs from the first hours of infection, which were characterized by the presence of myeloperoxidase (MPO) and citrullinated histone (Hit3Cit). Quantitative data on extracellular DNA release were also obtained; release was observed starting in the first hour of infection. The induction of METs by B. licheniformis caused a significant decrease in the colony-forming units (CFU) of Staphylococcus aureus. The induction of METS by bacteria of the Bacillus genus is a mechanism that participates in controlling the probiotic and potentially pathogenic bacteria such as S. aureus. The induction of METs to control pathogens may be a novel mechanism that could explain the beneficial effects of probiotics of the genus Bacillus.

The Immune System Throws Its Traps: Cells and Their Extracellular Traps in Disease and Protection

The first formal description of the microbicidal activity of extracellular traps (ETs) containing DNA occurred in neutrophils in 2004. Since then, ETs have been identified in different populations of cells involved in both innate and adaptive immune responses. Much of the knowledge has been obtained from in vitro or ex vivo studies; however, in vivo evaluations in experimental models and human biological materials have corroborated some of the results obtained. Two types of ETs have been described—suicidal and vital ETs, with or without the death of the producer cell. The studies showed that the same cell type may have more than one ETs formation mechanism and that different cells may have similar ETs formation mechanisms. ETs can act by controlling or promoting the mechanisms involved in the development and evolution of various infectious and non-infectious diseases, such as autoimmune, cardiovascular, thrombotic, and neoplastic diseases, among others. This review discusses the presence of ETs in neutrophils, macrophages, mast cells, eosinophils, basophils, plasmacytoid dendritic cells, and recent evidence of the presence of ETs in B lymphocytes, CD4+ T lymphocytes, and CD8+ T lymphocytes. Moreover, due to recently collected information, the effect of ETs on COVID-19 is also discussed.

Significance of Mast Cell Formed Extracellular Traps in Microbial Defense

Mast cells (MCs) are critically involved in microbial defense by releasing antimicrobial peptides (such as cathelicidin LL-37 and defensins) and phagocytosis of microbes. In past years, it has become evident that in addition MCs may eliminate invading pathogens by ejection of web-like structures of DNA strands embedded with proteins known together as extracellular traps (ETs). Upon stimulation of resting MCs with various microorganisms, their products (including superantigens and toxins), or synthetic chemicals, MCs become activated and enter into a multistage process that includes disintegration of the nuclear membrane, release of chromatin into the cytoplasm, adhesion of cytoplasmic granules on the emerging DNA web, and ejection of the complex into the extracellular space. This so-called ETosis is often associated with cell death of the producing MC, and the type of stimulus potentially determines the ratio of surviving vs. killed MCs. Comparison of different microorganisms with specific elimination characteristics such as S pyogenes (eliminated by MCs only through extracellular mechanisms), S aureus (removed by phagocytosis), fungi, and parasites has revealed important aspects of MC extracellular trap (MCET) biology. Molecular studies identified that the formation of MCET depends on NADPH oxidase-generated reactive oxygen species (ROS). In this review, we summarize the present state-of-the-art on the biological relevance of MCETosis, and its underlying molecular and cellular mechanisms. We also provide an overview over the techniques used to study the structure and function of MCETs, including electron microscopy and fluorescence microscopy using specific monoclonal antibodies (mAbs) to detect MCET-associated proteins such as tryptase and histones, and cell-impermeant DNA dyes for labeling of extracellular DNA. Comparing the type and biofunction of further MCET decorating proteins with ETs produced by other immune cells may help provide a better insight into MCET biology in the pathogenesis of autoimmune and inflammatory disorders as well as microbial defense.

Salmonella Typhimurium Triggers Extracellular Traps Release in Murine Macrophages

Salmonella comprises two species and more than 2500 serovars with marked differences in host specificity, and is responsible for a wide spectrum of diseases, ranging from localized gastroenteritis to severe life-threatening invasive disease. The initiation of the host inflammatory response, triggered by many Pathogen-Associated Molecular Patterns (PAMPs) that Salmonella possesses, recruits innate immune cells in order to restrain the infection at the local site. Neutrophils are known for killing bacteria through oxidative burst, amid other mechanisms. Amongst those mechanisms for controlling bacteria, the release of Extracellular Traps (ETs) represents a newly described pathway of programmed cell death known as ETosis. Particularly, Neutrophil Extracellular Traps (NETs) were first described in 2004 and since then, a number of reports have demonstrated their role as a novel defense mechanism against different pathogens. This released net-like material is composed of cellular DNA decorated with histones and cellular proteins. These structures have shown ability to trap, neutralize and kill different kinds of microorganisms, ranging from viruses and bacteria to fungi and parasites. Salmonella was one of the first microorganisms that were reported to be killed by NETs and several studies have confirmed the observation and deepened into its variants. Nevertheless, much less is known about their counterparts in other immune cells, e.g. Macrophage Extracellular Traps (METs) and Salmonella-induced MET release has never been reported so far. In this work, we observed the production of METs induced by Salmonella enterica serovar Typhimurium and recorded their effect on bacteria, showing for the first time that macrophages can also release extracellular DNA traps upon encounter with Salmonella Typhimurium. Additionally we show that METs effectively immobilize and reduce Salmonella survival in a few minutes, suggesting METs as a novel immune-mediated defense mechanism against Salmonella infection. Of note, this phenomenon was confirmed in primary macrophages, since MET release was also observed in bone marrow-derived macrophages infected with Salmonella. The evidence of this peculiar mechanism provides new incipient insights into macrophages´ role against Salmonella infection and can help to design new strategies for the clinical control of this transcendental pathogen.