Resolution of inflammation by retrograde chemotaxis of neutrophils in transgenic zebrafish

IL-1β disrupts the initiation of blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo initiation of BBB development. We generated doxycycline-inducible transgenic zebrafish to secrete Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR-Cas9. Mechanistically, we determined that Il-1β disrupts the initiation of BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

LECs regulate neutrophil clearance through IL-17RC/CMTM4/NF-κB axis at sites of inflammation or infection

The lymphatic system plays a vital role in the regulation of tissue fluid balance and the immune response to inflammation or infection. The effects of lymphatic endothelial cells (LECs) on the regulation of neutrophil migration have not been well-studied. In three murine models: imiquimod-induced skin inflammation, Staphylococcus aureus-induced skin infection, and ligature-induced periodontitis, we show that numerous neutrophils migrate from inflamed or infected tissues to the draining lymph nodes via lymphatic vessels. Moreover, inflamed or infected tissues express a high level of interleukin (IL)-17A and tumor necrosis factor (TNF)-α, simultaneously with a significant increase in the release of neutrophil attractors, including CXCL1, CXCL2, CXCL3, and CXCL5. Importantly, in vitro stimulation of LECs with IL-17A plus TNF-α synergistically promoted these chemokine secretions. Mechanistically, tetra-transmembrane protein CMTM4 directly binds to IL-17RC in LECs. IL-17A plus TNF-α stimulates CXC chemokine secretion by promoting nuclear factor-kappa B signaling. In contrast, knockdown of CMTM4 abrogates IL-17A plus TNF-α activated nuclear factor-kappa B signaling pathways. Lastly, the local administration of adeno-associated virus for CMTM4 in Prox1-CreERT2 mice, mediating LEC-specific overexpression of CMTM4, promotes the drainage of neutrophils by LECs and alleviates immune pathological responses. Thus, our findings reveal the vital role of LECs-mediated neutrophil attraction and clearance at sites of inflammation or infection.

ILC2s navigate tissue redistribution during infection using stage-specific S1P receptors

Lymphocytes can circulate as well as take residence within tissues. While the mechanisms by which circulating populations are recruited to infection sites have been extensively characterized, the molecular basis for the recirculation of tissue-resident cells is less understood. Here, we show that helminth infection- or IL-25-induced redistribution of intestinal group 2 innate lymphoid cells (ILC2s) requires access to the lymphatic vessel network. Although the secondary lymphoid structure is an essential signal hub for adaptive lymphocyte differentiation and dispatch, it is redundant for ILC2 migration and effector function. Upon IL-25 stimulation, a dramatic change in epigenetic landscape occurs in intestinal ILC2s, leading to the expression of sphingosine-1-phosphate receptors (S1PRs). Among the various S1PRs, we found that S1PR5 is critical for ILC2 exit from intestinal tissue to lymph. By contrast, S1PR1 plays a dominant role in ILC2 egress from mesenteric lymph nodes to blood circulation and then to distal tissues including the lung where the redistributed ILC2s contribute to tissue repair. The requirement of two S1PRs for ILC2 migration is largely due to the dynamic expression of the tissue-retention marker CD69, which mediates S1PR1 internalization. Thus, our study demonstrates a stage-specific requirement of different S1P receptors for ILC2 redistribution during infection. We therefore propose a fundamental paradigm that innate and adaptive lymphocytes utilize a shared vascular network frame and specialized navigation cues for migration.

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

Loss of developmentally derived Irf8+ macrophages promotes hyperinnervation and arrhythmia in the adult zebrafish heart

Recent developments in cardiac macrophage biology have broadened our understanding of the critical functions of macrophages in the heart. As a result, there is further interest in understanding the independent contributions of distinct subsets of macrophage to cardiac development and function. Here, we demonstrate that genetic loss of interferon regulatory factor 8 (Irf8)-positive embryonic-derived macrophages significantly disrupts cardiac conduction, chamber function, and innervation in adult zebrafish. At 4 months post-fertilization (mpf), homozygous irf8st96/st96 mutants have significantly shortened atrial action potential duration and significant differential expression of genes involved in cardiac contraction. Functional in vivo assessments via electro- and echocardiograms at 12 mpf reveal that irf8 mutants are arrhythmogenic and exhibit diastolic dysfunction and ventricular stiffening. To identify the molecular drivers of the functional disturbances in irf8 null zebrafish, we perform single cell RNA sequencing and immunohistochemistry, which reveal increased leukocyte infiltration, epicardial activation, mesenchymal gene expression, and fibrosis. Irf8 null hearts are also hyperinnervated and have aberrant axonal patterning, a phenotype not previously assessed in the context of cardiac macrophage loss. Gene ontology analysis supports a novel role for activated epicardial-derived cells (EPDCs) in promoting neurogenesis and neuronal remodeling in vivo. Together, these data uncover significant cardiac abnormalities following embryonic macrophage loss and expand our knowledge of critical macrophage functions in heart physiology and governing homeostatic heart health.

Screening of Microalgae for Bioactivity with Antiviral, Antibacterial, Anti-Inflammatory and Anti-Cancer Assays

Marine microalgae are a rich reservoir of natural compounds, including bioactives. Nonetheless, these organisms remain fairly unexplored despite their potential biotechnological applications. Culture collections with diverse taxonomic groups and lifestyles are a good source to unlock this potential and discover new molecules for multiple applications such as the treatment of human pathologies or the production of aquaculture species. In the present work extracts from thirty-three strains (including twenty dinoflagellates, four diatoms and nine strains from seven other algal classes), cultivated under identical conditions, were examined for their antiviral, antibacterial, anti-inflammatory and anti-cancer activities. Among these, antiviral and anti-inflammatory activities were detected in a few strains while the antibacterial tests showed positive results in most assays. In turn, most trials did not show any anti-cancer activity. Significant differences were observed between species within the same class, in particular dinoflagellates, which were better represented in this study. These preliminary findings pave the way for an in-depth characterization of the extracts with highest signals in each test, the identification of the compounds responsible for the biological activities found and a further screening of the CCVIEO culture collection.

A review of cumulative toxic effects of environmental endocrine disruptors on the zebrafish immune system: Characterization methods, toxic effects and mechanisms

Environmental endocrine disrupting chemicals (EDCs), are a type of exogenous organic pollutants, are ubiquitous in natural aquatic environments. Currently, in addition to neurological, endocrine, developmental and reproductive toxicity, ecotoxicology studies on immunotoxicity are receiving increasing attention. In this review, the composition of immune system of zebrafish, the common indicators of immunotoxicity, the immunotoxicity of EDCs and their molecular mechanism were summarized. We reviewed the immunotoxicity of EDCs on zebrafish mainly in terms of immune organs, immunocytes, immune molecules and immune functions, meanwhile, the possible molecular mechanisms driving these effects were elucidated in terms of endocrine disruption, dysregulation of signaling pathways, and oxidative damage. Hopefully, this review will provide a reference for further investigation of the immunotoxicity of EDCs.

Cellular and molecular mechanisms of skin wound healing

Wound healing is a complex process that involves the coordinated actions of many different tissues and cell lineages. It requires tight orchestration of cell migration, proliferation, matrix deposition and remodelling, alongside inflammation and angiogenesis. Whereas small skin wounds heal in days, larger injuries resulting from trauma, acute illness or major surgery can take several weeks to heal, generally leaving behind a fibrotic scar that can impact tissue function. Development of therapeutics to prevent scarring and successfully repair chronic wounds requires a fuller knowledge of the cellular and molecular mechanisms driving wound healing. In this Review, we discuss the current understanding of the different phases of wound healing, from clot formation through re-epithelialization, angiogenesis and subsequent scar deposition. We highlight the contribution of different cell types to skin repair, with emphasis on how both innate and adaptive immune cells in the wound inflammatory response influence classically studied wound cell lineages, including keratinocytes, fibroblasts and endothelial cells, but also some of the less-studied cell lineages such as adipocytes, melanocytes and cutaneous nerves. Finally, we discuss newer approaches and research directions that have the potential to further our understanding of the mechanisms underpinning tissue repair.

Beyond host defense and tissue injury: the emerging role of neutrophils in tissue repair

Neutrophils, the most abundant immune cells in human blood, play a fundamental role in host defense against invading pathogens and tissue injury. Neutrophils carry potentially lethal weaponry to the affected site. Inadvertent and perpetual neutrophil activation could lead to nonresolving inflammation and tissue damage, a unifying mechanism of many common diseases. The prevailing view emphasizes the dichotomy of their function, host defense versus tissue damage. However, tissue injury may also persist during neutropenia, which is associated with disease severity and poor outcome. Numerous studies highlight neutrophil phenotypic heterogeneity and functional versatility, indicating that neutrophils play more complex roles than previously thought. Emerging evidence indicates that neutrophils actively orchestrate resolution of inflammation and tissue repair and facilitate return to homeostasis. Thus, neutrophils mobilize multiple mechanisms to limit the inflammatory reaction, assure debris removal, matrix remodeling, cytokine scavenging, macrophage reprogramming, and angiogenesis. In this review, we will summarize the homeostatic and tissue-reparative functions and mechanisms of neutrophils across organs. We will also discuss how the healing power of neutrophils might be harnessed to develop novel resolution and repair-promoting therapies while maintaining their defense functions.

IL-1β disrupts blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine Interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo development of the BBB. We generated a doxycycline-inducible transgenic zebrafish model that drives secretion of Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR/Cas9. Mechanistically, we determined that Il-1β disrupts BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

Pharmacological potential of Jussiaea repens L. against CuSO4 and bacterial lipopolysaccharide O55:B5 induced inflammation using in-vivo zebrafish models

ETHNOPHARMACOLOGICAL RELEVANCE

The Northeastern state of India is known for its remarkable biodiversity and untapped medicinal resources. Jussiaea repens L., commonly known as water primrose, is a plant found in this region that has been traditionally used by indigenous communities for various purposes. It has been employed to treat skin ulcerations, bone fractures, rheumatism, stomach pain, and intestinal worms. Despite its long-standing ethnopharmacological usage, there is limited scientific research on the bioactivity of Jussiaea repens L. However, preliminary studies have shown its potential antioxidant properties and cytotoxicity against cancer cells. Further exploration of its medicinal properties, particularly its potential as an anti-inflammatory agent, is warranted.

AIM OF THE STUDY

This study aimed to investigate the anti-inflammatory properties of Jussiaea repens L., a plant species found in the biodiverse Northeastern region of India. The plant has been traditionally used by indigenous communities for various ailments. By utilizing zebrafish as an animal model and evaluating its effects in different inflammation models, the study aimed to uncover the plant's potential as an anti-inflammatory agent. The research contributes to the scientific understanding of this traditional remedy and its potential therapeutic applications.

METHODS

Jussiaea repens L. extract was obtained from the stem and leaves using methanol as the solvent. Zebrafish embryos were used for in vivo assays. The anti-inflammatory study included two models: CuSO4-induced inflammation and tail wounding followed by bacterial lipopolysaccharide-induced inflammation. The activities of catalase (CAT) and superoxide dismutase (SOD) were measured in CuSO4-induced inflammation. Leukocyte migration at the injury site was observed in the tail wounding model. The extract's inhibition of the 15-LOX enzyme was assessed. All procedures followed established protocols and ethical guidelines.

RESULTS AND CONCLUSION

Jussiaea repens L. extract exhibited anti-inflammatory activity in two in vivo zebrafish models: CuSO4-induced inflammation and tail wounding combined with bacterial lipopolysaccharide-induced inflammation. The extract reduced mortality rates and showed antioxidant effects by increasing catalase (CAT) and superoxide dismutase (SOD) activities in the CuSO4 model. In the tail wounding model, the extract reduced leukocyte migration in a concentration-dependent manner. Additionally, the extract demonstrated dose-dependent inhibition of the 15-LOX enzyme in the in vitro assay. These results suggest that Jussiaea repens L. extract possesses anti-inflammatory properties and inhibits the 15-LOX enzyme.

Exploring the dynamic behavior of leukocytes with zebrafish

Cell migration is a complex and intricate network of physical, chemical, and molecular events that ultimately leads to cell motility. This phenomenon is involved in both physiological and pathological processes such as proper immune and inflammatory responses. Dysregulation of cell migration machinery in immune cells can have a tremendous impact on the trajectory of inflammation, infection, and resolution. The small vertebrate, the zebrafish, has a remarkable capacity for genetic and pharmacological manipulation aligned to transparency that enables modulation and visualization of cell migration in vivo noninvasively. Such characteristics revolutionized the field of leukocyte biology, particularly neutrophils. In this review, we will focus on leukocyte migration and highlight findings made in the zebrafish that demonstrate how this small vertebrate system is a unique model to perform in vivo imaging and study mechanisms that regulate the dynamic behavior of immune cells in their native environment under homeostasis or upon challenge.

Effects of neutrophil fate on inflammation

INTRODUCTION

Neutrophils are important participants in the innate immune response. They rapidly and efficiently identify and clear infectious agents by expressing large numbers of membrane receptors. Upon tissue injury or pathogen invasion, neutrophils are the first immune cells to reach the site of injury and participate in the inflammatory response.

MATERIALS AND METHODS

A thorough search on PubMed related to neutrophil death or clearance pathways was performed.

CONCLUSION

Inflammatory response and tissue damage can be aggravated when neutrophils are not removed rapidly from the site of injury. Recent studies have shown that neutrophils can be cleared through a variety of pathways, including non-inflammatory and inflammatory death, as well as reverse migration. Non-inflammatory death pathways include apoptosis and autophagy. Inflammatory death pathways include necroptosis, pyroptosis and NETosis. This review highlights the basic properties of neutrophils and the impact of their clearance pathways on the inflammatory response.

Neutrophils' Contribution to Periodontitis and Periodontitis-Associated Cardiovascular Diseases

Neutrophils represent the primary defense against microbial threats playing a pivotal role in maintaining tissue homeostasis. This review examines the multifaceted involvement of neutrophils in periodontitis, a chronic inflammatory condition affecting the supporting structures of teeth summarizing the contribution of neutrophil dysfunction in periodontitis and periodontal-related comorbidities. Periodontitis, a pathological condition promoted by dysbiosis of the oral microbiota, is characterized by the chronic inflammation of the gingiva and subsequent tissue destruction. Neutrophils are among the first immune cells recruited to the site of infection, releasing antimicrobial peptides, enzymes, and reactive oxygen species to eliminate pathogens. The persistent inflammatory state in periodontitis can lead to aberrant neutrophil activation and a sustained release of proinflammatory mediators, finally resulting in tissue damage, bone resorption, and disease progression. Growing evidence now points to the correlation between periodontitis and systemic comorbidities. Indeed, the release of inflammatory mediators, immune complexes, and oxidative stress by neutrophils, bridge the gap between local and systemic immunity, thus highlighting neutrophils as key players in linking periodontal inflammation to chronic conditions, including cardiovascular diseases, diabetes mellitus, and rheumatoid arthritis. This review underscores the crucial role of neutrophils in the pathogenesis of periodontitis and the complex link between neutrophil dysfunction, local inflammation, and systemic comorbidities. A comprehensive understanding of neutrophil contribution to periodontitis development and their impact on periodontal comorbidities holds significant implications for the management of oral health. Furthermore, it highlights the need for the development of novel approaches aimed at limiting the persistent recruitment and activation of neutrophils, also reducing the impact of periodontal inflammation on broader health contexts, offering promising avenues for improved disease management and patient care.

Expanded population of low-density neutrophils in juvenile idiopathic arthritis

Introduction

Juvenile idiopathic arthritis (JIA), a clinically variable disease characterized by autoimmune arthritis, affects children, and its immunopathology remains elusive. Alterations in neutrophil biology play an important role in this disease. In the present study, we aimed to explore the features of low-density neutrophils (LDNs) in patients with JIA.

Methods

Gene expression of peripheral blood mononuclear cells (PBMCs) from children with distinct subtypes of JIA was analyzed by NanoString Immunology panel. Presence of LDNs was ascertained by flow cytometry and the release of neutrophil-associated products were analyzed by LUMINEX.

Results

LDNs were detected in patients' peripheral blood mononuclear cells (PBMCs) after density gradient centrifugation. Transcriptomic analysis of JIA PBMCs revealed that genes related to neutrophil degranulation were markedly upregulated. The number of LDNs and level of their degranulation products increased in patients' PBMCs and correlated with serum calprotectin, but not with disease activity, sedimentation rate and C-reactive protein (CRP) levels. The phenotypes of LDNs varied from those of normal-density neutrophils and healthy donor LDNs. Phenotypical analysis revealed LDNs are immature and primed population with decreased suppressive capacity. A negative correlation between surface proteins CD62L, CD66b, and CD11b and the number of inflamed joints/JADAS was established.

Conclusion

Our results describe LDNs as primed, degranulated, immature cells with impaired suppressive activities. This work thus contributes to the increasing body of evidence that LDNs in JIA are altered and their role in the disease immunopathogenesis and possible clinical associations should be investigated further.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Germ cells do not progress through spermatogenesis in the infertile zebrafish testis

Vertebrate spermatogonial stem cells maintain sperm production over the lifetime of an animal but fertility declines with age. While morphological studies have greatly informed our understanding of typical spermatogenesis, the molecular and cellular mechanisms underlying spermatogenesis are not yet understood, particularly with respect to the onset of fertility. We used single-cell RNA sequencing to generate a developmental atlas of the zebrafish testis. Using 5 timepoints across the adult life of a zebrafish, we described cellular profiles in the testis during and after fertility. While all germ cell stages of spermatogenesis are detected in testes from fertile adult zebrafish, testes from older infertile males only contained spermatogonia and a reduced population of spermatocytes. These remaining germ cells are transcriptionally distinct from fertile spermatogonia. Immune cells including macrophages and lymphocytes drastically increase in abundance in infertile testes. Our developmental atlas reveals the cellular changes as the testis ages and defines a molecular roadmap for the regulation of male fertility.

Neutrophil biology in injuries and diseases of the central and peripheral nervous systems

The role of inflammation in nervous system injury and disease is attracting increased attention. Much of that research has focused on microglia in the central nervous system (CNS) and macrophages in the peripheral nervous system (PNS). Much less attention has been paid to the roles played by neutrophils. Neutrophils are part of the granulocyte subtype of myeloid cells. These cells, like macrophages, originate and differentiate in the bone marrow from which they enter the circulation. After tissue damage or infection, neutrophils are the first immune cells to infiltrate into tissues and are directed there by specific chemokines, which act on chemokine receptors on neutrophils. We have reviewed here the basic biology of these cells, including their differentiation, the types of granules they contain, the chemokines that act on them, the subpopulations of neutrophils that exist, and their functions. We also discuss tools available for identification and further study of neutrophils. We then turn to a review of what is known about the role of neutrophils in CNS and PNS diseases and injury, including stroke, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, spinal cord and traumatic brain injuries, CNS and PNS axon regeneration, and neuropathic pain. While in the past studies have focused on neutrophils deleterious effects, we will highlight new findings about their benefits. Studies on their actions should lead to identification of ways to modify neutrophil effects to improve health.

The double-edged role of neutrophil heterogeneity in inflammatory diseases and cancers

Neutrophils are important immune cells act as the body's first line of defense against infection and respond to diverse inflammatory cues. Many studies have demonstrated that neutrophils display plasticity in inflammatory diseases and cancers. Clarifying the role of neutrophil heterogeneity in inflammatory diseases and cancers will contribute to the development of novel treatment strategies. In this review, we have presented a review on the development of the understanding on neutrophil heterogeneity from the traditional perspective and a high-resolution viewpoint. A growing body of evidence has confirmed the double-edged role of neutrophils in inflammatory diseases and tumors. This may be due to a lack of precise understanding of the role of specific neutrophil subsets in the disease. Thus, elucidating specific neutrophil subsets involved in diseases would benefit the development of precision medicine. Thusly, we have summarized the relevance and actions of neutrophil heterogeneity in inflammatory diseases and cancers comprehensively. Meanwhile, we also discussed the potential intervention strategy for neutrophils. This review is intended to deepen our understanding of neutrophil heterogeneity in inflammatory diseases and cancers, while hold promise for precise treatment of neutrophil-related diseases.

Comparative single-cell profiling reveals distinct cardiac resident macrophages essential for zebrafish heart regeneration

Zebrafish exhibit a robust ability to regenerate their hearts following injury, and the immune system plays a key role in this process. We previously showed that delaying macrophage recruitment by clodronate liposome (-1d_CL, macrophage-delayed model) impairs neutrophil resolution and heart regeneration, even when the infiltrating macrophage number was restored within the first week post injury (Lai et al., 2017). It is thus intriguing to learn the regenerative macrophage property by comparing these late macrophages vs. control macrophages during cardiac repair. Here, we further investigate the mechanistic insights of heart regeneration by comparing the non-regenerative macrophage-delayed model with regenerative controls. Temporal RNAseq analyses revealed that -1d_CL treatment led to disrupted inflammatory resolution, reactive oxygen species homeostasis, and energy metabolism during cardiac repair. Comparative single-cell RNAseq profiling of inflammatory cells from regenerative vs. non-regenerative hearts further identified heterogeneous macrophages and neutrophils, showing alternative activation and cellular crosstalk leading to neutrophil retention and chronic inflammation. Among macrophages, two residential subpopulations (hbaa+ Mac and timp4.3+ Mac 3) were enriched only in regenerative hearts and barely recovered after +1d_CL treatment. To deplete the resident macrophage without delaying the circulating macrophage recruitment, we established the resident macrophage-deficient model by administrating CL earlier at 8 d (-8d_CL) before cryoinjury. Strikingly, resident macrophage-deficient zebrafish still exhibited defects in revascularization, cardiomyocyte survival, debris clearance, and extracellular matrix remodeling/scar resolution without functional compensation from the circulating/monocyte-derived macrophages. Our results characterized the diverse function and interaction between inflammatory cells and identified unique resident macrophages prerequisite for zebrafish heart regeneration.

Fusobacterium nucleatum dissemination by neutrophils

Recent studies uncovered that Fusobacterium nucleatum (Fn), a common, opportunistic bacterium in the oral cavity, is associated with a growing number of systemic diseases, ranging from colon cancer to Alzheimer's disease. However, the pathological mechanisms responsible for this association are still poorly understood. Here, we leverage recent technological advances to study the interactions between Fn and neutrophils. We show that Fn survives within human neutrophils after phagocytosis. Using in vitro microfluidic devices, we determine that human neutrophils can protect and transport Fn over large distances. Moreover, we validate these observations in vivo by showing that neutrophils disseminate Fn using a zebrafish model. Our data support the emerging hypothesis that bacterial dissemination by neutrophils is a mechanistic link between oral and systemic diseases. Furthermore, our results may ultimately lead to therapeutic approaches that target specific host-bacteria interactions, including the dissemination process.

Neutrophils in host defense, healing, and hypersensitivity: Dynamic cells within a dynamic host

Neutrophils are cells of the innate immune system that are extremely abundant in vivo and respond quickly to infection, injury, and inflammation. Their constant circulation throughout the body makes them some of the first responders to infection, and indeed they play a critical role in host defense against bacterial and fungal pathogens. It is now appreciated that neutrophils also play an important role in tissue healing after injury. Their short life cycle, rapid response kinetics, and vast numbers make neutrophils a highly dynamic and potentially extremely influential cell population. It has become clear that they are highly integrated with other cells of the immune system and can thus exert critical effects on the course of an inflammatory response; they can further impact tissue homeostasis and recovery after challenge. In this review, we discuss the fundamentals of neutrophils in host defense and healing; we explore the relationship between neutrophils and the dynamic host environment, including circadian cycles and the microbiome; we survey the field of neutrophils in asthma and allergy; and we consider the question of neutrophil heterogeneity-namely, whether there could be specific subsets of neutrophils that perform different functions in vivo.

Biomineralized Nanoscavenger Abrogates Proinflammatory Macrophage Polarization and Induces Neutrophil Clearance through Reverse Migration during Gouty Arthritis

The deposition of monosodium urate (MSU) crystals induces the overexpression of reactive oxygen species (ROS) and proinflammatory cytokines in residential macrophages, further promoting the infiltration of inflammatory leukocytes in the joints of gouty arthritis. Herein, a peroxidase-mimicking nanoscavenger was developed by forming manganese dioxide over albumin nanoparticles loaded with an anti-inflammatory drug, indomethacin (BIM), to block the secretion of ROS and COX2-induced proinflammatory cytokines in the MSU-induced gouty arthritis model. In the MSU-induced arthritis mouse model, the BIM nanoparticles alleviated joint swelling, which is attributed to the abrogation of ROS and inflammatory cytokine secretions from proinflammatory macrophages that induces neutrophil infiltration and fluid building up in the inflammation site. Further, the BIM nanoparticle treatment reduced the influx of macrophages and neutrophils in the injured region by blocking migration and inducing reverse migration in the zebrafish larva tail amputation model as well as in MSU-induced peritonitis and air pouch mouse models. Overall, the current strategy of employing biomineralized nanoscavengers for arthritis demonstrates clinical significance in dual blocking of peroxides and COX2 to prevent influx of inflammatory cells into the sites of inflammation.

Molecular Actors of Inflammation and Their Signaling Pathways: Mechanistic Insights from Zebrafish

Inflammation is a hallmark of the physiological response to aggressions. It is orchestrated by a plethora of molecules that detect the danger, signal intracellularly, and activate immune mechanisms to fight the threat. Understanding these processes at a level that allows to modulate their fate in a pathological context strongly relies on in vivo studies, as these can capture the complexity of the whole process and integrate the intricate interplay between the cellular and molecular actors of inflammation. Over the years, zebrafish has proven to be a well-recognized model to study immune responses linked to human physiopathology. We here provide a systematic review of the molecular effectors of inflammation known in this vertebrate and recapitulate their modes of action, as inferred from sterile or infection-based inflammatory models. We present a comprehensive analysis of their sequence, expression, and tissue distribution and summarize the tools that have been developed to study their function. We further highlight how these tools helped gain insights into the mechanisms of immune cell activation, induction, or resolution of inflammation, by uncovering downstream receptors and signaling pathways. These progresses pave the way for more refined models of inflammation, mimicking human diseases and enabling drug development using zebrafish models.

A tessellated lymphoid network provides whole-body T cell surveillance in zebrafish

Homeostatic trafficking to lymph nodes allows T cells to efficiently survey the host for cognate antigen. Non-mammalian jawed vertebrates lack lymph nodes but maintain similarly diverse T cell pools. Here, we exploit in vivo imaging of transparent zebrafish to investigate how T cells organize and survey for antigen in an animal devoid of lymph nodes. We find that naïve-like T cells in zebrafish organize into a previously undescribed whole-body lymphoid network that supports streaming migration and coordinated trafficking through the host. This network has the cellular hallmarks of a mammalian lymph node, including naïve T cells and CCR7-ligand expressing non-hematopoietic cells, and facilitates rapid collective migration. During infection, T cells transition to a random walk that supports antigen presenting cell interactions and subsequent activation. Our results reveal that T cells can toggle between collective migration and individual random walks to prioritize either large-scale trafficking or antigen search in situ . This novel lymphoid network thus facilitates whole-body T cell trafficking and antigen surveillance in the absence of a lymph node system.

Significance Statement

In mammals, lymph nodes play a critical role in the initiation of adaptive immune responses by providing a dedicated place for T cells to scan antigen-presenting cells. Birds, reptiles, amphibians, and fish all maintain diverse repertoires of T cells but lack lymph nodes, raising questions about how adaptive immunity functions in lower jawed vertebrates. Here, we describe a novel network of lymphocytes in zebrafish that supports whole-body T cell trafficking and provides a site for antigen search, mirroring the function of mammalian lymph nodes. Within this network, T cells can prioritize large-scale trafficking or antigen scanning by toggling between two distinct modes of migration. This network provides valuable insights into the evolution of adaptive immunity.

Neutrophil dynamics and inflammaging in acute ischemic stroke: A transcriptomic review

Stroke is among the leading causes of death and disability worldwide. Restoring blood flow through recanalization is currently the only acute treatment for cerebral ischemia. Unfortunately, many patients that achieve a complete recanalization fail to regain functional independence. Recent studies indicate that activation of peripheral immune cells, particularly neutrophils, may contribute to microcirculatory failure and futile recanalization. Stroke primarily affects the elderly population, and mortality after endovascular therapies is associated with advanced age. Previous analyses of differential gene expression across injury status and age identify ischemic stroke as a complex age-related disease. It also suggests robust interactions between stroke injury, aging, and inflammation on a cellular and molecular level. Understanding such interactions is crucial in developing effective protective treatments. The global stroke burden will continue to increase with a rapidly aging human population. Unfortunately, the mechanisms of age-dependent vulnerability are poorly defined. In this review, we will discuss how neutrophil-specific gene expression patterns may contribute to poor treatment responses in stroke patients. We will also discuss age-related transcriptional changes that may contribute to poor clinical outcomes and greater susceptibility to cerebrovascular diseases.

Neutrophils in autoimmunity: when the hero becomes the villain

Neutrophils were long considered to be a short-lived homogenous cell population, limited to their role as first responders in anti-bacterial and -fungal immunity. While it is true that neutrophils are first to infiltrate the site of infection to eliminate pathogens, growing evidence suggests their functions could extend beyond those of basic innate immune cells. Along with their well-established role in pathogen elimination, utilizing effector functions such as phagocytosis, degranulation, and the deployment of neutrophil extracellular traps (NETs), neutrophils have recently been shown to possess antigen-presenting capabilities. Moreover, the identification of different subtypes of neutrophils points to a multifactorial heterogeneous cell population with great plasticity in which some subsets have enhanced pro-inflammatory characteristics, while others seem to behave as immunosuppressors. Interestingly, the aberrant presence of activated neutrophils with a pro-inflammatory profile in several systemic and organ-specific autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), multiple sclerosis (MS), and type 1 diabetes (T1D) could potentially be exploited in novel therapeutic strategies. The full extent of the involvement of neutrophils, and more specifically that of their various subtypes, in the pathophysiology of autoimmune diseases is yet to be elucidated.

Advances in microRNA from adipose-derived mesenchymal stem cell-derived exosome: focusing on wound healing

Skin wounds are a common condition causing economic burden and they represent an urgent clinical need, especially chronic wounds. Numerous studies have been conducted on the applications of stem cell therapy in wound healing, with adipose-derived mesenchymal stem cells (ADMSCs) playing a major role since they can be isolated easily, yielding a high number of cells, the less invasive harvesting required, the longer life span and no ethical issues. However, the lack of standardized doses and protocols, the heterogeneity of clinical trials, as well as the incompatibility of the immune system limit its application. Recent studies have demonstrated that specific stem cell functions depend on paracrine factors, including extracellular vesicles, in which microRNAs in exosomes (Exo-miRNAs) are essential in controlling their functions. This paper describes the application and mechanism whereby ADMSC-Exo-miRNA regulates wound healing. ADMSC-Exo-miRNA is involved in various stages in wounds, including modulating the immune response and inflammation, accelerating skin cell proliferation and epithelialization, promoting vascular repair, and regulating collagen remodeling thereby reducing scar formation. In summary, this acellular therapy based on ADMSC-Exo-miRNA has considerable clinical potential, and provides reference values for developing new treatment strategies for wound healing.

Neutrophil reverse migration

The behavior of neutrophils is very important for the resolution of inflammation and tissue repair. People have used advanced imaging techniques to observe the phenomenon of neutrophils leaving the injured or inflammatory site and migrating back into blood vessels in transgenic zebrafish and mice, which is called neutrophil reverse migration. Numerous studies have shown that neutrophil reverse migration is a double-edged sword. On the one hand, neutrophil reverse migration can promote the resolution of local inflammation by accelerating the clearance of neutrophils from local wounds. On the other hand, neutrophils re-enter the circulatory system may lead to the spread of systemic inflammation. Therefore, accurate regulation of neutrophil reverse migration is of great significance for the treatment of various neutrophil- mediated diseases. However, the mechanism of neutrophil reverse migration and its relationship with inflammation resolution is unknown. In this review, we reviewed the relevant knowledge of neutrophil reverse migration to elucidate the potential mechanisms and factors influencing reverse migration and its impact on inflammation in different disease processes.

Macrophages undergo a behavioural switch during wound healing in zebrafish

In response to wound signals, macrophages are immediately recruited to the injury where they acquire distinct phenotypes and functions, playing crucial roles both in host defense and healing process. Although macrophage phenotypes have been intensively studied during wound healing, mostly using markers and expression profiles, the impact of the wound environment on macrophage shape and behaviour, and the underlying mechanisms deserve more in-depth investigation. Here, we sought to characterize the dynamics of macrophage recruitment and behaviour during aseptic wounding of the caudal fin fold of the zebrafish larva. Using a photo-conversion approach, we demonstrated that macrophages are recruited to the wounded fin fold as a single wave where they switch their phenotype. Intravital imaging of macrophage shape and trajectories revealed that wound-macrophages display a highly stereotypical set of behaviours and change their shape from amoeboid to elongated shape as wound healing proceeds. Using a pharmacological inhibitor of 15-lipoxygenase and protectin D1, a specialized pro-resolving lipid, we investigated the role of polyunsaturated fatty acid metabolism in macrophage behaviour. While inhibition of 15-lipoxygenase using PD146176 or Nordihydroguaiaretic acid (NDGA) decreases the switch from amoeboid to elongated shape, protectin D1 accelerates macrophage reverse migration and favours elongated morphologies. Altogether, our findings suggest that individual macrophages at the wound switch their phenotype leading to important changes in behaviour and shape to adapt to changing environment, and highlight the crucial role of lipid metabolism in the control of macrophage behaviour plasticity during inflammation in vivo.

Functional identification of the zebrafish Interleukin-1 receptor in an embryonic model of Il-1β-induced systemic inflammation

Interleukin-1β (IL-1β) is a potent proinflammatory cytokine that plays a vital role in the innate immune system. To observe the innate immune response in vivo, several transgenic zebrafish lines have been developed to model IL-1β-induced inflammation and to visualize immune cell migration and proliferation in real time. However, our understanding of the IL-1β response in zebrafish is limited due to an incomplete genome annotation and a lack of functional data for the cytokine receptors involved in the inflammatory process. Here, we use a combination of database mining, genetic analyses, and functional assays to identify zebrafish Interleukin-1 receptor, type 1 (Il1r1). We identified putative zebrafish il1r1 candidate genes that encode proteins with predicted structures similar to human IL1R1. To examine functionality of these candidates, we designed highly effective morpholinos to disrupt gene expression in a zebrafish model of embryonic Il-1β-induced systemic inflammation. In this double transgenic model, ubb:Gal4-EcR, uas:il1βmat , the zebrafish ubiquitin b (ubb) promoter drives expression of the modified Gal4 transcription factor fused to the ecdysone receptor (EcR), which in turn drives the tightly-regulated expression and secretion of mature Il-1β only in the presence of the ecdysone analog tebufenozide (Teb). Application of Teb to ubb:Gal4-EcR, uas:il1βmat embryos causes premature death, fin degradation, substantial neutrophil expansion, and generation of reactive oxygen species (ROS). To rescue these deleterious phenotypes, we injected ubb:Gal4-EcR, uas:il1βmat embryos with putative il1r1 morpholinos and found that knockdown of only one candidate gene prevented the adverse effects caused by Il-1β. Mosaic knockout of il1r1 using the CRISPR/Cas9 system phenocopied these results. Taken together, our study identifies the functional zebrafish Il1r1 utilizing a genetic model of Il-1β-induced inflammation and provides valuable new insights to study inflammatory conditions specifically driven by Il-1β or related to Il1r1 function in zebrafish.

The Fate of Erythrocytes after Cerebral Hemorrhage

After a cerebral hemorrhage (intracerebral, subarachnoid, and intraventricular), extravasated blood contributes to both initial brain injury, via physical disruption and mass effect, and secondary injury, through the release of potentially neurotoxic and pro-inflammatory factors such as hemoglobin, iron, and peroxiredoxin-2. Erythrocytes are a major blood component and are a source of such damaging factors. Erythrolysis after cerebral hemorrhage releases potential neurotoxins, contributing to brain injury and edema. Alternatively, erythrocyte phagocytosis via microglia or macrophages may limit the spill of neurotoxins therefore limiting subsequent brain injury. The aim of this review is to discuss the process of phagocytosis of erythrocytes by microglia or macrophages after cerebral hemorrhage, the effect of erythrolysis on brain injury, novel mechanisms of erythrocyte and phagocyte egress from the brain, and exciting new targets in this pathway to attenuate brain injury. Understanding the fate of erythrocytes after cerebral hemorrhage may uncover additional potential interventions for clinical translational research.

Zebrafish Models of Paediatric Brain Tumours

Paediatric brain cancer is the second most common childhood cancer and is the leading cause of cancer-related deaths in children. Despite significant advancements in the treatment modalities and improvements in the 5-year survival rate, it leaves long-term therapy-associated side effects in paediatric patients. Addressing these impairments demands further understanding of the molecularity and heterogeneity of these brain tumours, which can be demonstrated using different animal models of paediatric brain cancer. Here we review the use of zebrafish as potential in vivo models for paediatric brain tumour modelling, as well as catalogue the currently available zebrafish models used to study paediatric brain cancer pathophysiology, and discuss key findings, the unique attributes that these models add, current challenges and therapeutic significance.

Opportunities presented by zebrafish larval models to study neutrophil function in tissues

Appropriate neutrophil function is essential for innate immune defence and to avoid inflammatory pathology. Neutrophils can adapt their responses according to their environment and recently, the existence of multiple distinct neutrophil populations has been confirmed in both health and disease. However, the study of neutrophil functions in their tissue environment has remained challenging, and for instance, the relationship between neutrophil maturity and function is not fully understood. Many neutrophil morphological and functional features are highly conserved between mammals and non-mammalian vertebrates. This enables the use of the transparent and genetically tractable zebrafish larvae to study neutrophil biology. We review data on the development and function of zebrafish larval neutrophils and advances zebrafish have brought to studies of neutrophil biology. In addition, we discuss opportunities and aspects to be considered when using the larval zebrafish model to further enhance our understanding of neutrophil function in health and disease.

Using zebrafish to understand reciprocal interactions between the nervous and immune systems and the microbial world

Animals rely heavily on their nervous and immune systems to perceive and survive within their environment. Despite the traditional view of the brain as an immunologically privileged organ, these two systems interact with major consequences. Furthermore, microorganisms within their environment are major sources of stimuli and can establish relationships with animal hosts that range from pathogenic to mutualistic. Research from a variety of human and experimental animal systems are revealing that reciprocal interactions between microbiota and the nervous and immune systems contribute significantly to normal development, homeostasis, and disease. The zebrafish has emerged as an outstanding model within which to interrogate these interactions due to facile genetic and microbial manipulation and optical transparency facilitating in vivo imaging. This review summarizes recent studies that have used the zebrafish for analysis of bidirectional control between the immune and nervous systems, the nervous system and the microbiota, and the microbiota and immune system in zebrafish during development that promotes homeostasis between these systems. We also describe how the zebrafish have contributed to our understanding of the interconnections between these systems during infection in fish and how perturbations may result in pathology.

Regulation of TLR10 Expression and Its Role in Chemotaxis of Human Neutrophils

Toll-like receptors are innate immune receptors that play a critical role in pathogen-associated molecular pattern recognition. TLR10 was recently identified and very limited data are available on its expression, mechanisms that regulate its expression, and its role in primary immune cells. To study the expression pattern of TLR10 in primary immune cells, we examined TLR10 protein expression in naive and Escherichia coli lipopolysaccharide (LPS)-activated human neutrophils. Human neutrophils challenged with LPS showed a decrease in total and surface TLR10 expression at 90 min. TLR10 in LPS-activated neutrophils colocalized with flotallin-1, a lipid raft marker, and EEA-1, an early endosomal marker, to suggest its endocytosis. There was increased colocalization of TLR10 with TLR4 at LPS 60 min followed by decrease at later LPS treatment times. Treatment with TLR4 neutralizing antibody decreased cytoplasmic localization of TLR10 in LPS-treated neutrophils. Reactive oxygen species (ROS) depletion and neutralization of p65 subunit of NF-κB in LPS-treated neutrophils decreased TLR10 expression. Live cell imaging of LPS-activated neutrophils showed TLR10 translocation in the leading edge and TLR10 knockdown in neutrophils reduced their fMLP-induced chemotaxis and the number of neutrophils with pseudopodia but without affecting the expression of key proteins of actin nucleation process, ARP-3 and Diap1. Taken together, our findings show that neutrophil activation alters TLR10 expression through ROS production and NF-κB regulation, and TLR10 knockdown reduced neutrophil chemotaxis.

Laser-mediated osteoblast ablation triggers a pro-osteogenic inflammatory response regulated by reactive oxygen species and glucocorticoid signaling in zebrafish

In zebrafish, transgenic labeling approaches, robust regenerative responses and excellent in vivo imaging conditions enable precise characterization of immune cell behavior in response to injury. Here, we monitored osteoblast-immune cell interactions in bone, a tissue which is particularly difficult to in vivo image in tetrapod species. Ablation of individual osteoblasts leads to recruitment of neutrophils and macrophages in varying numbers, depending on the extent of the initial insult, and initiates generation of cathepsin K+ osteoclasts from macrophages. Osteoblast ablation triggers the production of pro-inflammatory cytokines and reactive oxygen species, which are needed for successful macrophage recruitment. Excess glucocorticoid signaling as it occurs during the stress response inhibits macrophage recruitment, maximum speed and changes the macrophage phenotype. Although osteoblast loss is compensated for within a day by contribution of committed osteoblasts, macrophages continue to populate the region. Their presence is required for osteoblasts to fill the lesion site. Our model enables visualization of bone repair after microlesions at single-cell resolution and demonstrates a pro-osteogenic function of tissue-resident macrophages in non-mammalian vertebrates.

Neutrophils in acute inflammation: current concepts and translational implications

Modulation of neutrophil recruitment and function is crucial for targeting inflammatory cells to sites of infection to combat invading pathogens while, at the same time, limiting host tissue injury or autoimmunity. The underlying mechanisms regulating recruitment of neutrophils, 1 of the most abundant inflammatory cells, have gained increasing interest over the years. The previously described classical recruitment cascade of leukocytes has been extended to include capturing, rolling, adhesion, crawling, and transmigration, as well as a reverse-transmigration step that is crucial for balancing immune defense and control of remote organ endothelial leakage. Current developments in the field emphasize the importance of cellular interplay, tissue environmental cues, circadian rhythmicity, detection of neutrophil phenotypes, differential chemokine sensing, and contribution of distinct signaling components to receptor activation and integrin conformations. The use of therapeutics modulating neutrophil activation responses, as well as mutations causing dysfunctional neutrophil receptors and impaired signaling cascades, have been defined in translational animal models. Human correlates of such mutations result in increased susceptibility to infections or organ damage. This review focuses on current advances in the understanding of the regulation of neutrophil recruitment and functionality and translational implications of current discoveries in the field with a focus on acute inflammation and sepsis.

Targeting Neutrophils for Promoting the Resolution of Inflammation

Acute inflammation is a localized and self-limited innate host-defense mechanism against invading pathogens and tissue injury. Neutrophils, the most abundant immune cells in humans, play pivotal roles in host defense by eradicating invading pathogens and debris. Ideally, elimination of the offending insult prompts repair and return to homeostasis. However, the neutrophils` powerful weaponry to combat microbes can also cause tissue damage and neutrophil-driven inflammation is a unifying mechanism for many diseases. For timely resolution of inflammation, in addition to stopping neutrophil recruitment, emigrated neutrophils need to be disarmed and removed from the affected site. Accumulating evidence documents the phenotypic and functional versatility of neutrophils far beyond their antimicrobial functions. Hence, understanding the receptors that integrate opposing cues and checkpoints that determine the fate of neutrophils in inflamed tissues provides insight into the mechanisms that distinguish protective and dysregulated, excessive inflammation and govern resolution. This review aims to provide a brief overview and update with key points from recent advances on neutrophil heterogeneity, functional versatility and signaling, and discusses challenges and emerging therapeutic approaches that target neutrophils to enhance the resolution of inflammation.

Neutrophils and aquatic pathogens

Introduction

Neutrophilic granulocytes are short‐lived cells continuously circulating in the vascular system of vertebrates. They play a basic and decisive role in the innate immune defence of the hosts against all types of pathogenic microorganisms.

Methods

Based on a literature study, the functions of neutrophils and cells with similar functions are described. The study places special emphasis on organisms in the aquatic environment and the pathogens occurring in that particular environment.

Results

The evolutionary origin of this specific cell type is not clear, but its most basic traits (recognition of foreign elements, extracellular trap release, phagocytosis and elimination of ingested material) are found in phagocytes in members of evolutionary ancient invertebrate groups spanning from amoebae, sponges, sea‐anemones, mollusks (snails and mussels), arthropods (crustaceans and insects) to echinoderms (sea stars and sea urchins). Their functions as innate immune sentinels and effector cells in these groups are well described. Neutrophilic granulocytes with elongated and lobed nuclei (possibly allowing cell movements through narrow extracellular spaces and leaving space for phagosomes) occur in vertebrates including fish, amphibians, reptiles, birds and mammals although the morphology of the nucleus, stainability of cytoplasmic granula, and the antimicrobial armament vary among groups. Following the pathogen invasion of a fish host, the neutrophils migrates from the vascular system into the infection focus. They apply their PRRs (including TLRs) to recognize the invader as non‐self, produce netosis by casting extracellular chromatin containing traps in the microenvironment. These nets assist the immobilization of invading microbes and prevents their further spread. The cells attach to and engulf the microbes by phagocytosis, whereafter they eliminate the pathogen in phagolysosomes equipped with a range of killing mechanisms and attract, by release of chemokines, additional immune cells (monocytes, macrophages and lymphocytes) to the site of invasion. Their role in innate immunity of fish hosts towards aquatic pathogens has been elucidated by in vivo and in vitro studies. Neutrophils interact with virus (e.g. IPNV and VHSV), bacteria (e.g. Aeromonas, Vibrio, Edwardsiella, Mycobacterium and Renibacterium) and parasites, including monogeneans (Gyrodactylus), cestodes (Diphyllobothrium), trematodes (Diplostomum) and ciliates (Ichthyophthirius and Philasterides). Despite the decisive function of neutrophils in innate immunity and early protection, the excessive production of ROS, RNS and NETs may lead to pathological disturbances in the host, which are exacerbated if the pathogens evolve immune evasion mechanisms.

Conclusion

Neutrophils in aquatic organisms play a central role in innate immunity but may serve as a toll and a support in acquired protection. The strong impact of the cellular reactions not only on pathogen but also on host tissues emphasizes that an optimal immune reaction is balanced, involves targeted and specific effector mechanisms, which leaves a minimum of collateral damage in host organs.

Neutrophil Migratory Patterns: Implications for Cardiovascular Disease

The body's inflammatory response involves a series of processes that are necessary for the immune system to mitigate threats from invading pathogens. Leukocyte migration is a crucial process in both homeostatic and inflammatory states. The mechanisms involved in immune cell recruitment to the site of inflammation are numerous and require several cascades and cues of activation. Immune cells have multiple origins and can be recruited from primary and secondary lymphoid, as well as reservoir organs within the body to generate an immune response to certain stimuli. However, no matter the origin, an important aspect of any inflammatory response is the web of networks that facilitates immune cell trafficking. The vasculature is an important organ for this trafficking, especially during an inflammatory response, mainly because it allows cells to migrate towards the source of insult/injury and serves as a reservoir for leukocytes and granulocytes under steady state conditions. One of the most active and vital leukocytes in the immune system's arsenal are neutrophils. Neutrophils exist under two forms in the vasculature: a marginated pool that is attached to the vessel walls, and a demarginated pool that freely circulates within the blood stream. In this review, we seek to present the current consensus on the mechanisms involved in leukocyte margination and demargination, with a focus on the role of neutrophil migration patterns during physio-pathological conditions, in particular diabetes and cardiovascular disease.

Real-time imaging of inflammation and its resolution: It's apparent because it's transparent

The ability to directly observe leukocyte behavior in vivo has dramatically expanded our understanding of the immune system. Zebrafish are particularly amenable to the high-resolution imaging of leukocytes during both homeostasis and inflammation. Due to its natural transparency, intravital imaging in zebrafish does not require any surgical manipulation. As a result, zebrafish are particularly well-suited for the long-term imaging required to observe the temporal and spatial events during the onset and resolution of inflammation. Here, we review major insights about neutrophil and macrophage function gained from real-time imaging of zebrafish. We discuss neutrophil reverse migration, the process whereby neutrophils leave sites of tissue damage and resolve local inflammation. Further, we discuss the current tools available for investigating immune function in zebrafish and how future studies that simultaneously image multiple leukocyte subsets can be used to further dissect mechanisms that regulate both the onset and resolution of inflammation.

Neutrophils-From Bone Marrow to First-Line Defense of the Innate Immune System

Neutrophils (polymorphonuclear cells; PMNs) form a first line of defense against pathogens and are therefore an important component of the innate immune response. As a result of poorly controlled activation, however, PMNs can also mediate tissue damage in numerous diseases, often by increasing tissue inflammation and injury. According to current knowledge, PMNs are not only part of the pathogenesis of infectious and autoimmune diseases but also of conditions with disturbed tissue homeostasis such as trauma and shock. Scientific advances in the past two decades have changed the role of neutrophils from that of solely immune defense cells to cells that are responsible for the general integrity of the body, even in the absence of pathogens. To better understand PMN function in the human organism, our review outlines the role of PMNs within the innate immune system. This review provides an overview of the migration of PMNs from the vascular compartment to the target tissue as well as their chemotactic processes and illuminates crucial neutrophil immune properties at the site of the lesion. The review is focused on the formation of chemotactic gradients in interaction with the extracellular matrix (ECM) and the influence of the ECM on PMN function. In addition, our review summarizes current knowledge about the phenomenon of bidirectional and reverse PMN migration, neutrophil microtubules, and the microtubule organizing center in PMN migration. As a conclusive feature, we review and discuss new findings about neutrophil behavior in cancer environment and tumor tissue.

Genetically inducible and reversible zebrafish model of systemic inflammation

The inflammatory response is a vital defense mechanism against trauma and pathogen induced damage, but equally important is its appropriate resolution. In some instances of severe trauma or sustained infection, inappropriate and persistent activation of the immune response can occur resulting in a dangerous systemic inflammatory response. Untreated, this systemic inflammatory response can lead to tissue damage, organ shutdown, and death. Replicating this condition in tractable model organisms can provide insight into the mechanisms involved in the induction, maintenance, and resolution of inflammation. To that end, we developed a non-invasive, inducible, and reversible model of systemic inflammation in zebrafish. Using the Gal4-EcR/UAS system activated by the ecdysone analog tebufenozide, we generated transgenic zebrafish that allow for chemically-induced, ubiquitous secretion of the mature form of zebrafish interleukin-1β (Il-1βmat) in both larval and adult developmental stages. To ensure a robust immune response, we attached a strong signal peptide from the Gaussia princeps luciferase enzyme to promote active secretion of the cytokine. We observe a dose-dependent inflammatory response involving neutrophil expansion accompanied by tissue damage and reduced survival. Washout of tebufenozide permits inflammation resolution. We also establish the utility of this model for the identification of small molecule anti-inflammatory compounds by treatment with the immunosuppressant rapamycin. Taken together, these features make this model a valuable new tool that can aid in identifying potential new therapies while broadening our understanding of systemic inflammation, its impact on the immune system and its resolution.

Vascular endothelial growth factor c regulates hematopoietic stem cell fate in the dorsal aorta

Hematopoietic stem and progenitor cells (HSPCs) are multipotent cells that self-renew or differentiate to establish the entire blood hierarchy. HSPCs arise from the hemogenic endothelium of the dorsal aorta (DA) during development in a process called endothelial-to-hematopoietic transition. The factors and signals that control HSPC fate decisions from the hemogenic endothelium are not fully understood. We found that Vegfc has a role in HSPC emergence from the zebrafish DA. Using time-lapse live imaging, we show that some HSPCs in the DA of vegfc loss-of-function embryos display altered cellular behavior. Instead of typical budding from the DA, emergent HSPCs exhibit crawling behavior similar to myeloid cells. This was confirmed by increased myeloid cell marker expression in the ventral wall of the DA and the caudal hematopoietic tissue. This increase in myeloid cells corresponded with a decrease in HSPCs that persisted into larval stages. Together, our data suggest that Vegfc regulates HSPC emergence in the hemogenic endothelium, in part by suppressing a myeloid cell fate. Our study provides a potential signal for modulation of HSPC fate in stem cell differentiation protocols.

The EGFR/ErbB inhibitor neratinib modifies the neutrophil phosphoproteome and promotes apoptosis and clearance by airway macrophages

Dysregulated neutrophilic inflammation can be highly destructive in chronic inflammatory diseases due to prolonged neutrophil lifespan and continual release of histotoxic mediators in inflamed tissues. Therapeutic induction of neutrophil apoptosis, an immunologically silent form of cell death, may be beneficial in these diseases, provided that the apoptotic neutrophils are efficiently cleared from the tissue. Previous research in our group identified ErbB inhibitors as able to induce neutrophil apoptosis and reduce neutrophilic inflammation both in vitro and in vivo. Here, we extend that work using a clinical ErbB inhibitor, neratinib, which has the potential to be repurposed in inflammatory diseases. We show that neratinib reduces neutrophilic migration o an inflammatory site in zebrafish larvae. Neratinib upregulates efferocytosis and reduces the number of persisting neutrophil corpses in mouse models of acute, but not chronic, lung injury, suggesting that the drug may have therapeutic benefits in acute inflammatory settings. Phosphoproteomic analysis of human neutrophils shows that neratinib modifies the phosphorylation of proteins regulating apoptosis, migration, and efferocytosis. This work identifies a potential mechanism for neratinib in treating acute lung inflammation by upregulating the clearance of dead neutrophils and, through examination of the neutrophil phosphoproteome, provides important insights into the mechanisms by which this may be occurring.

A Single-Cell Transcriptome Profiling of Anterior Kidney Leukocytes From Nile Tilapia (Oreochromis niloticus)

Teleost fish anterior kidney (AK) is an important hematopoietic organ with multifarious immune cells, which have immune functions comparable to mammalian bone marrow. Myeloid and lymphoid cells locate in the AK, but the lack of useful specific gene markers and antibody-based reagents for the cell subsets makes the identification of the different cell types difficult. Single-cell transcriptome sequencing enables single-cell capture and individual library construction, making the study on the immune cell heterogeneity of teleost fish AK possible. In this study, we examined the transcriptional patterns of 11,388 AK leukocytes using 10× Genomics single-cell RNA sequencing (scRNA-seq). A total of 22 clusters corresponding to five distinct immune cell subsets were identified, which included B cells, T cells, granulocytes, macrophages, and dendritic cells (DCs). However, the subsets of myeloid cells (granulocytes, macrophages, and DCs) were not identified in more detail according to the known specific markers, even though significant differences existed among the clusters. Thereafter, we highlighted the B-cell subsets and identified them as pro/pre B cells, immature/mature B cells, activated B/plasmablasts, or plasma cells based on the different expressions of the transcription factors (TFs) and cytokines. Clustering of the differentially modulated genes by pseudo-temporal trajectory analysis of the B-cell subsets showed the distinct kinetics of the responses of TFs to cell conversion. Moreover, we classified the T cells and discovered that CD3+CD4-CD8-, CD3+CD4+CD8+, CD4+CD8-, and CD4-CD8+ T cells existed in AK, but neither CD4+CD8- nor CD4-CD8+ T cells can be further classified into subsets based on the known TFs and cytokines. Pseudotemporal analysis demonstrated that CD4+CD8- and CD4-CD8+ T cells belonged to different states with various TFs that might control their differentiation. The data obtained above provide a valuable and detailed resource for uncovering the leukocyte subsets in Nile tilapia AK, as well as more potential markers for identifying the myeloid and lymphoid cell types.

Microfluidic Systems to Study Neutrophil Forward and Reverse Migration

During infection, neutrophils are the most abundantly recruited innate immune cells at sites of infection, playing critical roles in the elimination of local infection and healing of the injury. Neutrophils are considered to be short-lived effector cells that undergo cell death at infection sites and in damaged tissues. However, recent in vitro and in vivo evidence suggests that neutrophil behavior is more complex and that they can migrate away from the inflammatory site back into the vasculature following the resolution of inflammation. Microfluidic devices have contributed to an improved understanding of the interaction and behavior of neutrophils ex vivo in 2D and 3D microenvironments. The role of reverse migration and its contribution to the resolution of inflammation remains unclear. In this review, we will provide a summary of the current applications of microfluidic devices to investigate neutrophil behavior and interactions with other immune cells with a focus on forward and reverse migration in neutrophils.