An inducible transgene reports activation of macrophages in live zebrafish larvae

Zebrafish tsc1 and cxcl12a increase susceptibility to mycobacterial infection

Regulation of host miRNA expression is a contested node that controls the host immune response to mycobacterial infection. The host must counter subversive efforts of pathogenic mycobacteria to launch a protective immune response. Here, we examine the role of miR-126 in the zebrafish-Mycobacterium marinum infection model and identify a protective role for infection-induced miR-126 through multiple effector pathways. We identified a putative link between miR-126 and the tsc1a and cxcl12a/ccl2/ccr2 signalling axes resulting in the suppression of non-tnfa expressing macrophage accumulation at early M. marinum granulomas. Mechanistically, we found a detrimental effect of tsc1a expression that renders zebrafish embryos susceptible to higher bacterial burden and increased cell death via mTOR inhibition. We found that macrophage recruitment driven by the cxcl12a/ccl2/ccr2 signalling axis was at the expense of the recruitment of classically activated tnfa-expressing macrophages and increased cell death around granulomas. Together, our results delineate putative pathways by which infection-induced miR-126 may shape an effective immune response to M. marinum infection in zebrafish embryos.

Animal models of silicosis: fishing for new therapeutic targets and treatments

Silicosis as an occupational lung disease has been present in our lives for centuries. Research studies have already developed and implemented many animal models to study the pathogenesis and molecular basis of the disease and enabled the search for treatments. As all experimental animal models used to date have their advantages and disadvantages, there is a continuous search for a better model, which will not only accelerate basic research, but also contribute to clinical aspects and drug development. We review here, for the first time, the main animal models developed to date to study silicosis and the unique advantages of the zebrafish model that make it an optimal complement to other models. Among the main advantages of zebrafish for modelling human diseases are its ease of husbandry, low maintenance cost, external fertilisation and development, its transparency from early life, and its amenability to chemical and genetic screening. We discuss the use of zebrafish as a model of silicosis, its similarities to other animal models and the characteristics of patients at molecular and clinical levels, and show the current state of the art of inflammatory and fibrotic zebrafish models that could be used in silicosis research.

Nrf2 in TIME: The Emerging Role of Nuclear Factor Erythroid 2-Related Factor 2 in the Tumor Immune Microenvironment

Nuclear factor erythroid 2-related factor 2 (Nrf2) mediates the cellular antioxidant response, allowing adaptation and survival under conditions of oxidative, electrophilic and inflammatory stress, and has a role in metabolism, inflammation and immunity. Activation of Nrf2 provides broad and long-lasting cytoprotection, and is often hijacked by cancer cells, allowing their survival under unfavorable conditions. Moreover, Nrf2 activation in established human tumors is associated with resistance to chemo-, radio-, and immunotherapies. In addition to cancer cells, Nrf2 activation can also occur in tumor-associated macrophages (TAMs) and facilitate an anti-inflammatory, immunosuppressive tumor immune microenvironment (TIME). Several cancer cell-derived metabolites, such as itaconate, L-kynurenine, lactic acid and hyaluronic acid, play an important role in modulating the TIME and tumor-TAMs crosstalk, and have been shown to activate Nrf2. The effects of Nrf2 in TIME are context-depended, and involve multiple mechanisms, including suppression of pro-inflammatory cytokines, increased expression of programmed cell death ligand 1 (PD-L1), macrophage colony-stimulating factor (M-CSF) and kynureninase, accelerated catabolism of cytotoxic labile heme, and facilitating the metabolic adaptation of TAMs. This understanding presents both challenges and opportunities for strategic targeting of Nrf2 in cancer.

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.

Macrophage NFATC2 mediates angiogenic signaling during mycobacterial infection

During mycobacterial infections, pathogenic mycobacteria manipulate both host immune and stromal cells to establish and maintain a productive infection. In humans, non-human primates, and zebrafish models of infection, pathogenic mycobacteria produce and modify the specialized lipid trehalose 6,6'-dimycolate (TDM) in the bacterial cell envelope to drive host angiogenesis toward the site of forming granulomas, leading to enhanced bacterial growth. Here, we use the zebrafish-Mycobacterium marinum infection model to define the signaling basis of the host angiogenic response. Through intravital imaging and cell-restricted peptide-mediated inhibition, we identify macrophage-specific activation of NFAT signaling as essential to TDM-mediated angiogenesis in vivo. Exposure of cultured human cells to Mycobacterium tuberculosis results in robust induction of VEGFA, which is dependent on a signaling pathway downstream of host TDM detection and culminates in NFATC2 activation. As granuloma-associated angiogenesis is known to serve bacterial-beneficial roles, these findings identify potential host targets to improve tuberculosis disease outcomes.

An anti-inflammatory activation sequence governs macrophage transcriptional dynamics during tissue injury in zebrafish

Macrophages are essential for tissue repair and regeneration. Yet, the molecular programs, as well as the timing of their activation during and after tissue injury are poorly defined. Using a high spatio-temporal resolution single cell analysis of macrophages coupled with live imaging after sensory hair cell death in zebrafish, we find that the same population of macrophages transitions through a sequence of three major anti-inflammatory activation states. Macrophages first show a signature of glucocorticoid activation, then IL-10 signaling and finally the induction of oxidative phosphorylation by IL-4/Polyamine signaling. Importantly, loss-of-function of glucocorticoid and IL-10 signaling shows that each step of the sequence is independently activated. Lastly, we show that IL-10 and IL-4 signaling act synergistically to promote synaptogenesis between hair cells and efferent neurons during regeneration. Our results show that macrophages, in addition to a switch from M1 to M2, sequentially and independently transition though three anti-inflammatory pathways in vivo during tissue injury in a regenerating organ.

Macrophages are important for tissue regeneration after injury. Here, using a single cell sequencing approach in a zebrafish injury model, the authors show that macrophages transition through a three stage process during tissue regeneration after hair cell damage.

Towards a new model of trained immunity: Exposure to bacteria and β-glucan protects larval zebrafish against subsequent infections

Once thought to be a feature exclusive to lymphocyte-driven adaptive immunity, immune memory has also been shown to operate as part of the innate immune system following infection to provide an elevated host response to subsequent pathogenic challenge. This evolutionarily conserved process, termed 'trained immunity', enables cells of the innate immune system to 'remember' previous pathogen encounters and mount stronger responses to the same, or different, pathogens after returning to a non-activated state. Here we show that challenging larval zebrafish, that exclusively rely on innate immunity, with live or heat-killed Salmonella typhimurium provides protection to subsequent infection with either Salmonella typhimurium or Streptococcus iniae, that lasts for at least 12 days. We also show that larvae injected with β-glucan, the well-known trigger of trained immunity, demonstrate enhanced survival to similar live bacterial infections, a phenotype supported by increased cxcl8 expression and neutrophil recruitment to the infection site. These results support the conservation of a trained immunity-like phenotype in larval zebrafish and provide a foundation to exploit the experimental attributes of larval zebrafish to further understand this form of immunological memory.

Peripheral NOD-like receptor deficient inflammatory macrophages trigger neutrophil infiltration into the brain disrupting daytime locomotion

Inflammation is known to disrupt normal behavior, yet the underlying neuroimmune interactions remain elusive. Here, we investigated whether inappropriate macrophage-evoked inflammation alters CNS control of daily-life animal locomotion using a set of zebrafish mutants selected for specific macrophage dysfunction and microglia deficiency. Large-scale genetic and computational analyses revealed that NOD-like receptor nlrc3l mutants are capable of normal motility and visuomotor response, but preferentially swim less in the daytime, suggesting possible low motivation rather than physical impairment. Examining their brain activities and structures implicates impaired dopaminergic descending circuits, where neutrophils abnormally infiltrate. Furthermore, neutrophil depletion recovered daytime locomotion. Restoring wild-type macrophages reversed behavioral and neutrophil aberrations, while three other microglia-lacking mutants failed to phenocopy nlrc3l mutants. Overall, we reveal how peripheral inflammatory macrophages with elevated pro-inflammatory cues (including il1β, tnfα, cxcl8a) in the absence of microglia co-opt neutrophils to infiltrate the brain, thereby potentially enabling local circuitry modulation affecting daytime locomotion.

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.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Zebrafish Models for the Safety and Therapeutic Testing of Nanoparticles with a Focus on Macrophages

New nanoparticles and biomaterials are increasingly being used in biomedical research for drug delivery, diagnostic applications, or vaccines, and they are also present in numerous commercial products, in the environment and workplaces. Thus, the evaluation of the safety and possible therapeutic application of these nanomaterials has become of foremost importance for the proper progress of nanotechnology. Due to economical and ethical issues, in vitro and in vivo methods are encouraged for the testing of new compounds and/or nanoparticles, however in vivo models are still needed. In this scenario, zebrafish (Danio rerio) has demonstrated potential for toxicological and pharmacological screenings. Zebrafish presents an innate immune system, from early developmental stages, with conserved macrophage phenotypes and functions with respect to humans. This fact, combined with the transparency of zebrafish, the availability of models with fluorescently labelled macrophages, as well as a broad variety of disease models offers great possibilities for the testing of new nanoparticles. Thus, with a particular focus on macrophage-nanoparticle interaction in vivo, here, we review the studies using zebrafish for toxicological and biodistribution testing of nanoparticles, and also the possibilities for their preclinical evaluation in various diseases, including cancer and autoimmune, neuroinflammatory, and infectious diseases.

Tipping the Scales With Zebrafish to Understand Adaptive Tumor Immunity

The future of improved immunotherapy against cancer depends on an in-depth understanding of the dynamic interactions between the immune system and tumors. Over the past two decades, the zebrafish has served as a valuable model system to provide fresh insights into both the development of the immune system and the etiologies of many different cancers. This well-established foundation of knowledge combined with the imaging and genetic capacities of the zebrafish provides a new frontier in cancer immunology research. In this review, we provide an overview of the development of the zebrafish immune system along with a side-by-side comparison of its human counterpart. We then introduce components of the adaptive immune system with a focus on their roles in the tumor microenvironment (TME) of teleosts. In addition, we summarize zebrafish models developed for the study of cancer and adaptive immunity along with other available tools and technology afforded by this experimental system. Finally, we discuss some recent research conducted using the zebrafish to investigate adaptive immune cell-tumor interactions. Without a doubt, the zebrafish will arise as one of the driving forces to help expand the knowledge of tumor immunity and facilitate the development of improved anti-cancer immunotherapy in the foreseeable future.

Genetic Approaches Using Zebrafish to Study the Microbiota-Gut-Brain Axis in Neurological Disorders

The microbiota-gut-brain axis (MGBA) is a bidirectional signaling pathway mediating the interaction of the microbiota, the intestine, and the central nervous system. While the MGBA plays a pivotal role in normal development and physiology of the nervous and gastrointestinal system of the host, its dysfunction has been strongly implicated in neurological disorders, where intestinal dysbiosis and derived metabolites cause barrier permeability defects and elicit local inflammation of the gastrointestinal tract, concomitant with increased pro-inflammatory cytokines, mobilization and infiltration of immune cells into the brain, and the dysregulated activation of the vagus nerve, culminating in neuroinflammation and neuronal dysfunction of the brain and behavioral abnormalities. In this topical review, we summarize recent findings in human and animal models regarding the roles of the MGBA in physiological and neuropathological conditions, and discuss the molecular, genetic, and neurobehavioral characteristics of zebrafish as an animal model to study the MGBA. The exploitation of zebrafish as an amenable genetic model combined with in vivo imaging capabilities and gnotobiotic approaches at the whole organism level may reveal novel mechanistic insights into microbiota-gut-brain interactions, especially in the context of neurological disorders such as autism spectrum disorder and Alzheimer's disease.

Asap1 Affects the Susceptibility of Zebrafish to Mycobacterium by Regulating Macrophage Migration

The ADP ribosylation factor (ARF) GTPase activation protein ASAP1 possesses multiple biological functions, including regulation of cytoskeletal dynamics, small GTP-binding protein receptor recycling, and intracellular vesicle trafficking. Recently, ASAP1 polymorphisms have been reported to be associated with human susceptibility to tuberculosis (TB) according to a large-scale genome-wide association study (GWAS); ASAP1 expression affects dendritic cell migration, which may be involved in TB predisposition. However, it remains unclear whether ASAP1 affects TB in vivo. To address this issue, we used zebrafish as a model system to examine the effects of Asap1 against Mycobacterium marinum, an organism closely related to Mycobacterium tuberculosis. Two zebrafish asap1 homologs (asap1a and asap1b) were identified and characterized. By morpholino knockdown of asap1a and asap1b as a whole, we found that the asap1 morphants showed a higher mycobacterial load than the controls, which was almost rescued by injecting asap1 mRNA that confers resistance to mycobacterial infection. These Asap1-depleted zebrafish also exhibited decreased macrophage migration in response to tail injury or upon infection with M. marinum in the hindbrain ventricle, which was also proved in THP1-derived macrophages of knockdown ASAP1. Together, these findings represent a new perspective on the role of Asap1 in resistance to mycobacterial infection.

Long-term in vivo imaging reveals tumor-specific dissemination and captures host tumor interaction in zebrafish xenografts

Understanding mechanisms mediating tumor metastasis is crucial for diagnostic and therapeutic targeting. Here, we take advantage of a transparent embryonic zebrafish xenograft model (eZXM) to visualize and track metastatic cells in real time using selective plane illumination microscopy (SPIM) for up to 30 h. Injected human leukemic and breast cancer cells exhibited cell-type specific patterns of intravascular distribution with leukemic cells moving faster than breast cancer cells. Tracking of tumor cells from high-resolution images revealed acute differences in intravascular speed and distance covered by cells. While the majority of injected breast cancer cells predominantly adhered to nearby vasculature, about 30% invaded the non-vascularized tissue, reminiscent of their metastatic phenotype. Survival of the injected tumor cells appeared to be partially inhibited and time-lapse imaging showed a possible role for host macrophages of the recipient embryos. Leukemic cell dissemination could be effectively blocked by pharmacological ROCK1 inhibition using Fasudil. These observations, and the ability to image several embryos simultaneously, support the use of eZXM and SPIM imaging as a functional screening platform to identify compounds that suppress cancer cell spread and invasion.

Drainage of inflammatory macromolecules from the brain to periphery targets the liver for macrophage infiltration

Many brain pathologies are associated with liver damage, but a direct link has long remained elusive. Here, we establish a new paradigm for interrogating brain-periphery interactions by leveraging zebrafish for its unparalleled access to the intact whole animal for in vivo analysis in real time after triggering focal brain inflammation. Using traceable lipopolysaccharides (LPS), we reveal that drainage of these inflammatory macromolecules from the brain led to a strikingly robust peripheral infiltration of macrophages into the liver independent of Kupffer cells. We further demonstrate that this macrophage recruitment requires signaling from the cytokine IL-34 and Toll-like receptor adaptor MyD88, and occurs in coordination with neutrophils. These results highlight the possibility for circulation of brain-derived substances to serve as a rapid mode of communication from brain to the liver. Understanding how the brain engages the periphery at times of danger may offer new perspectives for detecting and treating brain pathologies.

Modeling oncolytic virus dynamics in the tumor microenvironment using zebrafish

We have adapted a zebrafish (Danio rerio) tumor xenograft model for use in the study of oncolytic virotherapy. Following implantation of mammalian cancer cells into the perivitelline space of developing zebrafish embryos, both local and intravenous oncolytic virus treatments produce a tumor-specific infection with measurable antitumor effects. Tumor cells are injected at 48 h post fertilization, with oncolytic virus treatment then being administered 24 h later to allow for an initial period of tumor development and angiogenesis. Confocal fluorescent imaging is used to quantify dynamics within the tumor environment. The natural translucency of zebrafish at the embryo stage, coupled with the availability of strains with fluorescent immune and endothelial cell reporter lines, gives the model broad potential to allow for real time, in vivo investigation of important events within tumors throughout the course of virotherapy. Zebrafish xenografts offer a system with biologic fidelity to processes in human cancer development that influence oncolytic virus efficacy, and to our knowledge this is the first demonstration of the model's use in the context of virotherapy. Compared with other models, our protocol offers a powerful, inexpensive approach to evaluating novel oncolytic viruses and oncolytic virus-based combination therapies, with potential application to investigating the impacts of virotherapy on immune response, tumor vasculature, and metastatic disease.

The Diverse Roles of Phagocytes During Bacterial and Fungal Infections and Sterile Inflammation: Lessons From Zebrafish

The immediate and natural reaction to both infectious challenges and sterile insults (wounds, tissue trauma or crystal deposition) is an acute inflammatory response. This inflammatory response is mediated by activation of the innate immune system largely comprising professional phagocytes (neutrophils and macrophages). Zebrafish (danio rerio) larvae possess many advantages as a model organism, including their genetic tractability and highly conserved innate immune system. Exploiting these attributes and the live imaging potential of optically transparent zebrafish larvae has greatly contributed to our understanding of how neutrophils and macrophages orchestrate the initiation and resolution phases of inflammatory responses. Numerous bacterial and fungal infection models have been successfully established using zebrafish as an animal model and studies investigating neutrophil and macrophage behavior to sterile insults have also provided unique insights. In this review we highlight how examining the larval zebrafish response to specific bacterial and fungal pathogens has uncovered cellular and molecular mechanisms behind a variety of phagocyte responses, from those that protect the host to those that are detrimental. We also describe how modeling sterile inflammation in larval zebrafish has provided an opportunity to dissect signaling pathways that control the recruitment, and fate, of phagocytes at inflammatory sites. Finally, we briefly discuss some current limitations, and opportunities to improve, the zebrafish model system for studying phagocyte biology.

The zebrafish as a model for gastrointestinal tract-microbe interactions

The zebrafish (Danio rerio) has become a widely used vertebrate model for bacterial, fungal, viral, and protozoan infections. Due to its genetic tractability, large clutch sizes, ease of manipulation, and optical transparency during early life stages, it is a particularly useful model to address questions about the cellular microbiology of host-microbe interactions. Although its use as a model for systemic infections, as well as infections localised to the hindbrain and swimbladder having been thoroughly reviewed, studies focusing on host-microbe interactions in the zebrafish gastrointestinal tract have been neglected. Here, we summarise recent findings regarding the developmental and immune biology of the gastrointestinal tract, drawing parallels to mammalian systems. We discuss the use of adult and larval zebrafish as models for gastrointestinal infections, and more generally, for studies of host-microbe interactions in the gut.

Macrophages in Zebrafish Models of Liver Diseases

Hepatic macrophages are key components of the liver immunity and consist of two main populations. Liver resident macrophages, known as Kupffer cells in mammals, are crucial for maintaining normal liver homeostasis. Upon injury, they become activated to release proinflammatory cytokines and chemokines and recruit a large population of inflammatory monocyte-derived macrophages to the liver. During the progression of liver diseases, macrophages are highly plastic and have opposing functions depending on the signaling cues that they receive from the microenvironment. A comprehensive understanding of liver macrophages is essential for developing therapeutic interventions that target these cells in acute and chronic liver diseases. Mouse studies have provided the bulk of our current knowledge of liver macrophages. The emergence of various liver disease models and availability of transgenic tools to visualize and manipulate macrophages have made the teleost zebrafish (Danio rerio) an attractive new vertebrate model to study liver macrophages. In this review, we summarize the origin and behaviors of macrophages in healthy and injured livers in zebrafish. We highlight the roles of macrophages in zebrafish models of alcoholic and non-alcoholic liver diseases, hepatocellular carcinoma, and liver regeneration, and how they compare with the roles that have been described in mammals. We also discuss the advantages and challenges of using zebrafish to study liver macrophages.

Commensal Microbiota Regulate Vertebrate Innate Immunity-Insights From the Zebrafish

Microbial communities populate the mucosal surfaces of all animals. Metazoans have co-evolved with these microorganisms, forming symbioses that affect the molecular and cellular underpinnings of animal physiology. These microorganisms, collectively referred to as the microbiota, are found on many distinct body sites (including the skin, nasal cavity, and urogenital tract), however the most densely colonized host tissue is the intestinal tract. Although spatially confined within the intestinal lumen, the microbiota and associated products shape the development and function of the host immune system. Studies comparing gnotobiotic animals devoid of any microbes (germ free) with counterparts colonized with selected microbial communities have demonstrated that commensal microorganisms are required for the proper development and function of the immune system at homeostasis and following infectious challenge or injury. Animal model systems have been essential for defining microbiota-dependent shifts in innate immune cell function and intestinal physiology during infection and disease. In particular, the zebrafish has emerged as a powerful vertebrate model organism with unparalleled capacity for in vivo imaging, a full complement of genetic approaches, and facile methods to experimentally manipulate microbial communities. Here we review key insights afforded by the zebrafish into the impact of microbiota on innate immunity, including evidence that the perception of and response to the microbiota is evolutionarily conserved. We also highlight opportunities to strengthen the zebrafish model system, and to gain new insights into microbiota-innate immune interactions that would be difficult to achieve in mammalian models.

The Zebrafish as a Model Host for Invasive Fungal Infections

The zebrafish has become a widely accepted model host for studies of infectious disease, including fungal infections. The species is genetically tractable, and the larvae are transparent and amenable to prolonged in vivo imaging and small molecule screening. The aim of this review is to provide a thorough introduction into the published studies of fungal infection in the zebrafish and the specific ways in which this model has benefited the field. In doing so, we hope to provide potential new zebrafish researchers with a snapshot of the current toolbox and prior results, while illustrating how the model has been used well and where the unfulfilled potential of this model can be found.

Macrophages enhance Vegfa-driven angiogenesis in an embryonic zebrafish tumour xenograft model

Tumour angiogenesis has long been a focus of anti-cancer therapy; however, anti-angiogenic cancer treatment strategies have had limited clinical success. Tumour-associated myeloid cells are believed to play a role in the resistance of cancer towards anti-angiogenesis therapy, but the mechanisms by which they do this are unclear. An embryonic zebrafish xenograft model has been developed to investigate the mechanisms of tumour angiogenesis and as an assay to screen anti-angiogenic compounds. In this study, we used cell ablation techniques to remove either macrophages or neutrophils and assessed their contribution towards zebrafish xenograft angiogenesis by quantitating levels of graft vascularisation. The ablation of macrophages, but not neutrophils, caused a strong reduction in tumour xenograft vascularisation and time-lapse imaging demonstrated that tumour xenograft macrophages directly associated with the migrating tip of developing tumour blood vessels. Finally, we found that, although macrophages are required for vascularisation in xenografts that either secrete VEGFA or overexpress zebrafish vegfaa, they are not required for the vascularisation of grafts with low levels of VEGFA, suggesting that zebrafish macrophages can enhance Vegfa-driven tumour angiogenesis. The importance of macrophages to this angiogenic response suggests that this model could be used to further investigate the interplay between myeloid cells and tumour vascularisation.

Summary: Zebrafish embryonic macrophages associate with the distal tips of tumour xenograft blood vessels and are required for Vegfa-driven angiogenesis.

Blocking fatty acid-fueled mROS production within macrophages alleviates acute gouty inflammation

Gout is the most common inflammatory arthritis affecting men. Acute gouty inflammation is triggered by monosodium urate (MSU) crystal deposition in and around joints that activates macrophages into a proinflammatory state, resulting in neutrophil recruitment. A complete understanding of how MSU crystals activate macrophages in vivo has been difficult because of limitations of live imaging this process in traditional animal models. By live imaging the macrophage and neutrophil response to MSU crystals within an intact host (larval zebrafish), we reveal that macrophage activation requires mitochondrial ROS (mROS) generated through fatty acid oxidation. This mitochondrial source of ROS contributes to NF-κB-driven production of IL-1β and TNF-α, which promote neutrophil recruitment. We demonstrate the therapeutic utility of this discovery by showing that this mechanism is conserved in human macrophages and, via pharmacologic blockade, that it contributes to neutrophil recruitment in a mouse model of acute gouty inflammation. To our knowledge, this study is the first to uncover an immunometabolic mechanism of macrophage activation that operates during acute gouty inflammation. Targeting this pathway holds promise in the management of gout and, potentially, other macrophage-driven diseases.

Utilizing Zebrafish to Identify Anti-(Lymph)Angiogenic Compounds for Cancer Treatment: Promise and Future Challenges

Cancer metastasis which predominantly occurs through blood and lymphatic vessels, is the leading cause of death in cancer patients. Consequently, several anti-angiogenic agents have been approved as therapeutic agents for human cancers such as metastatic renal cell carcinoma. Also, anti-lymphangiogenic drugs such as monoclonal antibodies VGX-100 and IMC-3C5 have undergone phase I clinical trials for advanced and metastatic solid tumors. Although anti-tumor-associated angiogenesis has proven to be a promising therapeutic strategy for human cancers, this approach is fraught with toxicities and development of drug resistance. This emphasizes the need for alternative anti-(lymph)angiogenic drugs. The use of zebrafish has become accepted as an established model for high-throughput screening, vascular biology, and cancer research. Importantly, various zebrafish transgenic lines have now been generated that can readily discriminate different vascular compartments. This now enables detailed in vivo studies that are relevant to both human physiological and tumor (lymph)angiogenesis to be conducted in zebrafish. This review highlights recent advancements in the zebrafish anti-vascular screening platform and showcases promising new anti-(lymph)angiogenic compounds that have been derived from this model. In addition, this review discusses the promises and challenges of the zebrafish model in the context of anti-(lymph)angiogenic compound discovery for cancer treatment.

Modeling Infectious Diseases in the Context of a Developing Immune System

Zebrafish has been used for over a decade to study the mechanisms of a wide variety of inflammatory disorders and infections, with models ranging from bacterial, viral, to fungal pathogens. Zebrafish has been especially relevant to study the differentiation, specialization, and polarization of the two main innate immune cell types, the macrophages and the neutrophils. The optical accessibility and the early appearance of myeloid cells that can be tracked with fluorescent labels in zebrafish embryos and the ability to use genetics to selectively ablate or expand immune cell populations have permitted studying the interaction between infection, development, and metabolism. Additionally, zebrafish embryos are readily colonized by a commensal flora, which facilitated studies that emphasize the requirement for immune training by the natural microbiota to properly respond to pathogens. The remarkable conservation of core mechanisms required for the recognition of microbial and danger signals and for the activation of the immune defenses illustrates the high potential of the zebrafish model for biomedical research. This review will highlight recent insight that the developing zebrafish has contributed to our understanding of host responses to invading microbes and the involvement of the microbiome in several physiological processes. These studies are providing a mechanistic basis for developing novel therapeutic approaches to control infectious diseases.

Live imaging the earliest host innate immune response to preneoplastic cells using a zebrafish inducible KalTA4-ERT2/UAS system

As cancers develop, transformed cells hijack various host mechanisms and manipulate them to create a dynamic tumor microenvironment, which supports tumor growth. This protumorigenic microenvironment is made up of many different cell types, including transformed cells, fibroblasts, inflammatory cells, and endothelial cells, the interactions of which have been shown to play a role in sustaining tumor growth. Multiple reports implicate the inflammatory cells of the tumor microenvironment as having both pro- and antitumorigenic roles, the balance of which is vital for the progression of the tumor, and while our understanding of established cancers has vastly increased since the turn of the 21st Century, our knowledge of these cellular interactions at the earliest stages of cancer initiation and development remains relatively limited. This is largely due to difficulties in monitoring these processes in vivo and in real time. Since the late nineties, the zebrafish (Danio rerio) has emerged as a vital model organism, allowing studies of previously unattainable stages of tumor initiation in a vertebrate model system. Using genetic and live-imaging approaches, this model system can be used both independently to monitor stages of tumor progression from the earliest initiation stages and incorporated into previously established systems to investigate the interactions between cancer cells and the various cell types of the tumor microenvironment, including inflammatory cells. Here, we describe the use of an inducible KalTA4-ER^T2/UAS expression system in zebrafish, which allows spatial and temporal control of preneoplastic cell (PNC) growth and monitoring of innate immune cells in response to the developing PNC microenvironment.

Innate immune cells and bacterial infection in zebrafish

The physical attributes of the zebrafish, including optical transparency during embryogenesis, large clutch sizes, external development, and rapid organogenesis were features that initially attracted developmental biologists to use this vertebrate as an experimental model system. With the progressive development of an extensive genetic "tool kit" and an ever-growing number of transgenic reporter lines, the zebrafish model has evolved into an informative system in which to mimic and study aspects of human disease, including those associated with bacterial infections. This chapter provides detailed protocols for microinjection of bacterial strains into zebrafish larvae and subsequent experiments to investigate single-larva bacterial burdens, live imaging of specific neutrophil and macrophage bactericidal functions, and how these protocols may be applied to drug discovery approaches to uncover novel immunomodulatory drugs.

Transcriptomic Approaches in the Zebrafish Model for Tuberculosis-Insights Into Host- and Pathogen-specific Determinants of the Innate Immune Response

Mycobacterium marinum infection in zebrafish has become a well-established model of tuberculosis. Both embryonic and adult zebrafish infection studies have contributed to our knowledge of the development and function of tuberculous granulomas, which are typical of mycobacterial pathogenesis. In this review we discuss how transcriptome profiling studies have helped to characterize this infection process. We illustrate this using new RNA sequencing (RNA-Seq) data that reveals three main phases in the host response to M. marinum during the early stages of granuloma development in zebrafish embryos and larvae. The early phase shows induction of complement and transcription factors, followed by a relatively minor induction of pro-inflammatory cytokines within hours following phagocytosis of M. marinum. A minimal response is observed in the mid-phase, between 6 hours and 1day post infection, when the tissue dissemination of M. marinum begins. During subsequent larval development the granulomas expand and a late-phase response is apparent, which is characterized by progressively increasing induction of complement, transcription factors, pro-inflammatory cytokines, matrix metalloproteinases, and other defense and inflammation-related gene groups. This late-phase response shares common components with the strong and acute host transcriptome response that has previously been reported for Salmonella typhimurium infection in zebrafish embryos. In contrast, the early/mid-phase response to M. marinum infection, characterized by suppressed pro-inflammatory signaling, is strikingly different from the acute response to S. typhimurium infection. Furthermore, M. marinum infection shows a collective and strongly fluctuating regulation of lipoproteins, while S. typhimurium infection has pronounced effects on amino acid metabolism and glycolysis.

The necessity for in vivo functional analysis in human medical genetics

Approximately 50% of all congenital anomalies cannot be linked to any specific genetic etiology, but in recent years cost effective high throughput sequencing has emerged as an efficient strategy for identifying single nucleotide polymorphisms (SNPs) associated with disease. However, in many cases there is not enough evidence to determine if these SNPs underlie disease. To bridge this gap in our understanding advances in functional analyses are warranted. Several preclinical model systems are currently being utilized to provide such evidence, including the advantageous zebrafish embryo. While every system exhibits disadvantages and caveats, a new era of multidisciplinary research has evolved, which uses a broad spectrum of functional analysis tools. This approach will make it possible to identify potential therapeutic targets for both common and rare human disorders.

The Macrophage-Specific Promoter mfap4 Allows Live, Long-Term Analysis of Macrophage Behavior during Mycobacterial Infection in Zebrafish

Transgenic labeling of innate immune cell lineages within the larval zebrafish allows for real-time, in vivo analyses of microbial pathogenesis within a vertebrate host. To date, labeling of zebrafish macrophages has been relatively limited, with the most specific expression coming from the mpeg1 promoter. However, mpeg1 transcription at both endogenous and transgenic loci becomes attenuated in the presence of intracellular pathogens, including Salmonella typhimurium and Mycobacterium marinum. Here, we describe mfap4 as a macrophage-specific promoter capable of producing transgenic lines in which transgene expression within larval macrophages remains stable throughout several days of infection. Additionally, we have developed a novel macrophage-specific Cre transgenic line under the control of mfap4, enabling macrophage-specific expression using existing floxed transgenic lines. These tools enrich the repertoire of transgenic lines and promoters available for studying zebrafish macrophage dynamics during infection and inflammation and add flexibility to the design of future macrophage-specific transgenic lines.

Identification of polarized macrophage subsets in zebrafish

While the mammalian macrophage phenotypes have been intensively studied in vitro, the dynamic of their phenotypic polarization has never been investigated in live vertebrates. We used the zebrafish as a live model to identify and trail macrophage subtypes. We generated a transgenic line whose macrophages expressing tumour necrosis factor alpha (tnfa), a key feature of classically activated (M1) macrophages, express fluorescent proteins Tg(mpeg1:mCherryF/tnfa:eGFP-F). Using 4D-confocal microscopy, we showed that both aseptic wounding and Escherichia coli inoculation triggered macrophage recruitment, some of which started to express tnfa. RT-qPCR on Fluorescence Activated Cell Sorting (FACS)-sorted tnfa⁺ and tnfa⁻ macrophages showed that they, respectively, expressed M1 and alternatively activated (M2) mammalian markers. Fate tracing of tnfa⁺ macrophages during the time-course of inflammation demonstrated that pro-inflammatory macrophages converted into M2-like phenotype during the resolution step. Our results reveal the diversity and plasticity of zebrafish macrophage subsets and underline the similarities with mammalian macrophages proposing a new system to study macrophage functional dynamic.

DOI:http://dx.doi.org/10.7554/eLife.07288.001

Inflammation plays an important role in helping the body to heal wounds and fight off certain diseases. Immune cells called macrophages—which are perhaps best known for their ability to engulf and digest microbes and cell debris—help to control inflammation. In mammals, different types of macrophage exist; the most functionally extreme of which are the M1 macrophages that stimulate inflammation and M2 macrophages that reduce the inflammatory response.

Macrophages acquire different abilities through a process called polarization, which is controlled by signals produced by a macrophage's environment. Polarization has been well investigated in human and mouse cells grown in the laboratory, but less is understood about how this process occurs in live animals.

Nguyen Chi, Laplace-Builhe et al. investigated whether zebrafish larvae (which are naturally transparent) could form an experimental model in which to investigate macrophage polarization in living animals. Zebrafish were first genetically engineered to produce two fluorescent proteins: one that marks macrophages and one that marks M1 macrophages. These fluorescent proteins allow the movement and polarization of macrophages to be tracked in real time in living larvae using a technique called confocal microscopy. Nguyen Chi, Laplace-Builhe et al. also isolated macrophage cells from these zebrafish at different times during the inflammatory process to identify which macrophage subtypes form and when.

The results show that unpolarized macrophages move to the sites of inflammation (caused by wounds or bacterial infection), where they become polarized into M1 cells. Over time, these M1 macrophages progressively convert into an M2-like macrophage subtype, presumably to help clear up the inflammation.

Furthermore, Nguyen Chi, Laplace-Builhe et al. show that the M1 and M2 macrophage subtypes in zebrafish are similar to those found in mammals. Therefore, genetically engineered zebrafish larvae are likely to prove useful for studying macrophage activity and polarization in living animals.

DOI:http://dx.doi.org/10.7554/eLife.07288.002