Immunoresponsive gene 1 augments bactericidal activity of macrophage-lineage cells by regulating β-oxidation-dependent mitochondrial ROS production

Myeloid-Derived Suppressor Cells in COVID-19: The Paradox of Good

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the ongoing coronavirus disease 2019 (COVID-19) pandemic. Viral replication in the respiratory tract induces the death of infected cells and the release of pathogen- associated molecular patterns (PAMPs). PAMPs give rise to local inflammation, increasing the secretion of pro- inflammatory cytokines and chemokines, which attract immune cells from the blood into the infected lung. In most individuals, lung-recruited cells clear the infection, and the immune response retreats. However, in some cases, a dysfunctional immune response occurs, which triggers a cytokine storm in the lung, leading to acute respiratory distress syndrome (ARDS). Severe COVID-19 is characterized by an impaired innate and adaptive immune response and by a massive expansion of myeloid-derived suppressor cells (MDSCs). MDSCs function as protective regulators of the immune response, protecting the host from over-immunoreactivity and hyper-inflammation. However, under certain conditions, such as chronic inflammation and cancer, MDSCs could exert a detrimental role. Accordingly, the early expansion of MDSCs in COVID-19 is able to predict the fatal outcome of the infection. Here, we review recent data on MDSCs during COVID-19, discussing how they can influence the course of the disease and whether they could be considered as biomarker and possible targets for new therapeutic approaches.

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.

The IRG1-Itaconate axis: A regulatory hub for immunity and metabolism in macrophages

Metabolism could be served as a guiding force for immunity, and macrophages undergo drastic metabolic reprogramming during inflammatory processes, including enhancing glycolysis and reshaping the tricarboxylic acid cycle (TCA) cycle. The disrupted TCA cycle facilitates itaconate accumulation, consistent with the significant up-regulation of immune response gene 1 (IRG1) in activated macrophages. IRG1 catalyzes the decarboxylation of cis-aconitate to synthesize itaconate, and notably, the IRG1-Itaconate axis has excellent potential to link macrophages' immunity and metabolism. Here, we review vital molecules that affect the activation of the IRG1-Itaconate axis, including interferon regulatory factor 1/9 (IRF1/9), transcription 1 and 3 (STAT1/3), CCAAT enhancer-binding protein β (C/EBPβ), and the protein kinase C (PKC). We then focus on how the IRG1-Itaconate axis regulates the inflammatory pathway in macrophages, proposed to involve kelch-like ECH-associated protein 1 (Keap1), NOD-, LRR- and pyrin domain-containing 3 (NLRP3), gasdermin D (GSDMD), activating transcription factor 3 (ATF3), receptor-interacting protein kinase-3 (RIPK3), et al. In addition, we provide an overview of the way the axis participates in the metabolism of macrophages. Eventually, we summarize current connections between the IRG1-Itaconate axis and inflammatory diseases, bringing light to new therapeutic opportunities in inflammatory diseases.

Safety assessment of graphene oxide and microcystin-LR complex: a toxicological scenario beyond physical mixture

Background

Nanomaterials have been widely used in electrochemistry, sensors, medicine among others applications, causing its inevitable environmental exposure. A raising question is the “carrier” effect due to unique surface properties of nanomaterials, which may collectively impact the bioavailability, toxicokinetic, distribution and biological effects of classic toxicants. Noteworthy, this aspect of information remains largely unexplored.

Methods

Here, we deliberately selected two entities to mimic this scenario. One is graphene oxide (GO), which is made in ton quantity with huge surface-area that provides hydrophilicity and π–π interaction to certain chemicals of unique structures. The other is Microcystin-LR (MCLR), a representative double-bond rich liver-toxic endotoxin widely distributed in aquatic-system. Firstly, the adsorption of GO and MCLR after meeting under environmental conditions was explored, and then we focused on the toxicological effect and related mechanism of GO-MCLR complex on human skin cutin forming cells (HaCaT cells) and normal liver cells (L02 cells).

Results

Abiotically, our study demonstrated that GO could effectively adsorb MCLR through hydrogen bonding and π–π interaction, the oxidation degree of GO-MCLR decreased significantly and surface defect level raised. Compared to GO or MCLR, GO-MCLR was found to induce more remarkable apoptosis and ferroptosis in both HaCaT and L02 cells. The underlying mechanism was that GO-MCLR induced stronger intracellular reactive oxygen species (ROS) and mtROS generation, followed by Fe²⁺ accumulation, mitochondrial dysfunction and cytoskeletal damage.

Conclusions

These results suggest that the GO-MCLR complex formed by GO adsorption of MCLR may exhibit more toxic effects than the single material, which demonstrates the necessity for assessing nano-toxicant complexity. Our discovery may serve as a new toxicological paradigm in which nanomaterial mediated surface adsorption effects could impact the degree of cytotoxicity and toxicological mechanisms of classic toxins.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00466-x.

Melatonin alleviates PM2.5-triggered macrophage M1 polarization and atherosclerosis via regulating NOX2-mediated oxidative stress homeostasis

It is reported that oxidative stress homeostasis was involved in PM_2.5-induced foam cell formation and progression of atherosclerosis, but the exact molecular mechanism is still unclear. Melatonin is an effective antioxidant that could reverse the cardiopulmonary injury. The main purpose of this study is to investigate the latent mechanism of PM_2.5-triggered atherosclerosis development and the protective role of melatonin administration. Vascular Doppler ultrasound showed that PM_2.5 exposure reduced aortic elasticity in ApoE^-/- mice. Meanwhile, blood biochemical and pathological analysis demonstrated that PM_2.5 exposure caused dyslipidemia, elicited oxidative damage of aorta and was accompanied by an increase in atherosclerotic plaque area; while the melatonin administration could effectively alleviate PM_2.5-induced macrophage M1 polarization and atherosclerosis in mice. Further investigation verified that NADPH oxidase 2 (NOX2) and mitochondria are two prominent sources of PM_2.5-induced ROS production in vascular macrophages. Whereas, the combined use of two ROS-specific inhibitors and adopted with melatonin markedly rescued PM_2.5-triggered macrophage M1 polarization and foam cell formation by inhibiting NOX2-mediated crosstalk of Keap1/Nrf2/NF-κB and TLR4/TRAF6/NF-κB signaling pathways. Our results demonstrated that NOX2-mediated oxidative stress homeostasis is critical for PM_2.5-induced atherosclerosis and melatonin might be a potential treatment for air pollution-related cardiovascular diseases.

Intertwined Relationship of Mitochondrial Metabolism, Gut Microbiome and Exercise Potential

The microbiome has emerged as a key player contributing significantly to the human physiology over the past decades. The potential microbial niche is largely unexplored in the context of exercise enhancing capacity and the related mitochondrial functions. Physical exercise can influence the gut microbiota composition and diversity, whereas a sedentary lifestyle in association with dysbiosis can lead to reduced well-being and diseases. Here, we have elucidated the importance of diverse microbiota, which is associated with an individual's fitness, and moreover, its connection with the organelle, the mitochondria, which is the hub of energy production, signaling, and cellular homeostasis. Microbial by-products, such as short-chain fatty acids, are produced during regular exercise that can enhance the mitochondrial capacity. Therefore, exercise can be employed as a therapeutic intervention to circumvent or subside various metabolic and mitochondria-related diseases. Alternatively, the microbiome-mitochondria axis can be targeted to enhance exercise performance. This review furthers our understanding about the influence of microbiome on the functional capacity of the mitochondria and exercise performance, and the interplay between them.

Mitochondrial ACOD1/IRG1 in infection and sterile inflammation

Immunometabolism is a dynamic process involving the interplay of metabolism and immune response in health and diseases. Increasing evidence suggests that impaired immunometabolism contributes to infectious and inflammatory diseases. In particular, the mitochondrial enzyme aconitate decarboxylase 1 (ACOD1, best known as immunoresponsive gene 1 [IRG1]) is upregulated under various inflammatory conditions and serves as a pivotal regulator of immunometabolism involved in itaconate production, macrophage polarization, inflammasome activation, and oxidative stress. Consequently, the activation of the ACOD1 pathway is implicated in regulating the pathogenic process of sepsis and septic shock, which are part of a clinical syndrome of life-threatening organ failure caused by a dysregulated host response to pathogen infection. In this review, we discuss the latest research advances in ACOD1 expression and function, with particular attention to how the ACOD1-itaconate pathway affects infection and sterile inflammation diseases. These new insights may give us a deeper understanding of the role of immunometabolism in innate immunity.

Commensal and Opportunistic Bacteria Present in the Microbiota in Atlantic Cod (Gadus morhua) Larvae Differentially Alter the Hosts' Innate Immune Responses

The roles of host-associated bacteria have gained attention lately, and we now recognise that the microbiota is essential in processes such as digestion, development of the immune system and gut function. In this study, Atlantic cod larvae were reared under germ-free, gnotobiotic and conventional conditions. Water and fish microbiota were characterised by 16S rRNA gene analyses. The cod larvae’s transcriptional responses to the different microbial conditions were analysed by a custom Agilent 44 k oligo microarray. Gut development was assessed by transmission electron microscopy (TEM). Water and fish microbiota differed significantly in the conventional treatment and were dominated by different fast-growing bacteria. Our study indicates that components of the innate immune system of cod larvae are downregulated by the presence of non-pathogenic bacteria, and thus may be turned on by default in the early larval stages. We see indications of decreased nutrient uptake in the absence of bacteria. The bacteria also influence the gut morphology, reflected in shorter microvilli with higher density in the conventional larvae than in the germ-free larvae. The fact that the microbiota alters innate immune responses and gut morphology demonstrates its important role in marine larval development.

Mitochondrial connections with immune system in Zebrafish

Mitochondria are organelles commonly associated with adenosine triphosphate (ATP) formation through the oxidative phosphorylation (OXPHOS) process. However, mitochondria are also responsible for functions such as calcium homeostasis, apoptosis, autophagy, and production of reactive oxygen species (ROS) that, in conjunction, can lead to different cell fate decisions. Mitochondrial morphology changes rely on nutrients' availability and the bioenergetics demands of the cells, in a process known as mitochondrial dynamics, which includes both fusion and fission. This organelle senses the microenvironment and can modify the cells to either a pro or anti-inflammatory profile. The zebrafish has been increasingly used to research mitochondrial dynamics and its connection with the immune system since the pathways and molecules involved in these processes are conserved on this fish. Several genetic tools and technologies are currently available to analyze the behavior of mitochondria in zebrafish. However, even though zebrafish presents several similar processes known in mammals, the effect of the mitochondria in the immune system has not been so broadly studied in this model. In this review, we summarize the current knowledge in zebrafish studies regarding mitochondrial function and immuno metabolism.

Role of myeloid-derived suppressor cells in viral respiratory infections; Hints for discovering therapeutic targets for COVID-19

The expansion of myeloid-derived suppressor cells (MDSCs), known as heterogeneous population of immature myeloid cells, is enhanced during several pathological conditions such as inflammatory or viral respiratory infections. It seems that the way MDSCs behave in infection depends on the type and the virulence mechanisms of the invader pathogen, the disease stage, and the infection-related pathology. Increasing evidence showing that in correlation with the severity of the disease, MDSCs are accumulated in COVID-19 patients, in particular in those at severe stages of the disease or ICU patients, contributing to pathogenesis of SARS-CoV2 infection. Based on the involved subsets, MDSCs delay the clearance of the virus through inhibiting T-cell proliferation and responses by employing various mechanisms such as inducing the secretion of anti-inflammatory cytokines, inducible nitric oxide synthase (iNOS)-mediated hampering of IFN-γ production, or forcing arginine shortage. While the immunosuppressive characteristic of MDSCs may help to preserve the tissue homeostasis and prevent hyperinflammation at early stages of the infection, hampering of efficient immune responses proved to exert significant pathogenic effects on severe forms of COVID-19, suggesting the targeting of MDSCs as a potential intervention to reactivate T-cell immunity and thereby prevent the infection from developing into severe stages of the disease. This review tried to compile evidence on the roles of different subsets of MDSCs during viral respiratory infections, which is far from being totally understood, and introduce the promising potential of MDSCs for developing novel diagnostic and therapeutic approaches, especially against COVID-19 disease.

The Tumor Necrosis Factor Alpha and Interleukin 6 Auto-paracrine Signaling Loop Controls Mycobacterium avium Infection via Induction of IRF1/IRG1 in Human Primary Macrophages

Macrophages sense and respond to pathogens by induction of antimicrobial and inflammatory programs to alert other immune cells and eliminate the infectious threat. We have previously identified the transcription factor IRF1 to be consistently activated in macrophages during Mycobacterium avium infection, but its precise role during infection is not clear. Here, we show that tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) autocrine/paracrine signaling contributes to controlling the intracellular growth of M. avium in human primary macrophages through activation of IRF1 nuclear translocation and expression of IRG1, a mitochondrial enzyme that produces the antimicrobial metabolite itaconate. Small interfering RNA (siRNA)-mediated knockdown of IRF1 or IRG1 increased the mycobacterial load, whereas exogenously provided itaconate was bacteriostatic at high concentrations. While the overall level of endogenous itaconate was low in M. avium-infected macrophages, the repositioning of mitochondria to M. avium phagosomes suggests a mechanism by which itaconate can be delivered directly to M. avium phagosomes in sufficient quantities to inhibit growth. Using mRNA hybridization, we further show that uninfected bystander cells actively contribute to the resolution of infection by producing IL-6 and TNF-α, which, via paracrine signaling, activate IRF1/IRG1 and strengthen the antimicrobial activity of infected macrophages. This mechanism contributes to the understanding of why patients on anti-inflammatory treatment, e.g., with tocilizumab or infliximab, can be more susceptible to mycobacterial disease.

Itaconic acid exerts anti-inflammatory and antibacterial effects via promoting pentose phosphate pathway to produce ROS

Itaconic acid is produced by immune responsive gene 1 (IRG1)-coded enzyme in activated macrophages and known to play an important role in metabolism and immunity. In this study, mechanism of itaconic acid functioning as an anti-inflammatory metabolite was investigated with molecular biology and immunology techniques, by employing IRG1-null (prepared with CRISPR) and wild-type macrophages. Experimental results showed that itaconic acid significantly promoted the pentose phosphate pathway (PPP), which subsequently led to significantly higher NADPH oxidase activity and more reactive oxygen species (ROS) production. ROS production increased the expression of anti-inflammatory gene A20, which in turn decreased the production of inflammatory cytokines IL-6, IL-1β and TNF-α. NF-κB, which can up-regulate A20, was also vital in controlling IRG1 and itaconic acid involved immune-modulatory responses in LPS-stimulated macrophage in this study. In addition, itaconic acid inhibited the growth of Salmonella typhimurium in cell through increasing ROS production from NADPH oxidase and the hatching of Schistosoma japonicum eggs in vitro. In short, this study revealed an alternative mechanism by which itaconic acid acts as an anti-inflammatory metabolite and confirmed the inhibition of bacterial pathogens with itaconic acid via ROS in cell. These findings provide the basic knowledge for future biological applications of itaconic acid in anti-inflammation and related pathogens control.

Single-cell RNA-seq reveals functionally distinct biomaterial degradation-related macrophage populations

Macrophages play crucial roles in host tissue reaction to biomaterials upon implantation in vivo. However, the complexity of biomaterial degradation-related macrophage subpopulations that accumulate around the implanted biomaterials in situ is not fully understood. Here, using single cell RNA-seq, we analyze the transcriptome profiles of the various cell types around the scaffold to map the scaffold-induced reaction, in an unbiased approach. This enables mapping of all biomaterial degradation-associated cells at high resolution, revealing distinct subpopulations of tissue-resident macrophages as the major cellular sources of biomaterial degradation in situ. We also find that scaffold architecture can affect the mechanotransduction and catabolic activity of specific material degradation-related macrophage subpopulations in an Itgav-Mapk1-Stat3 dependent manner, eventually leading to differences in scaffold degradation rate in vivo. Our work dissects unanticipated aspects of the cellular and molecular basis of biomaterial degradation at the single-cell level, and provides a conceptual framework for developing functional tissue engineering scaffolds in future.

Modulating Repolarization of Tumor-Associated Macrophages with Targeted Therapeutic Nanoparticles as a Potential Strategy for Cancer Therapy

There are always some components in the tumor microenvironment (TME), such as tumor-associated macrophages (TAMs), that help tumor cells escape the body's immune surveillance. Therefore, this situation can lead to tumor growth, progression, and metastasis, resulting in low response rates for cancer therapy. Macrophages play an important role with strong plasticity and functional diversity. Facing different microenvironmental stimulations, macrophages undergo a dynamic change in phenotype and function into two major macrophage subpopulations, namely classical activation/inflammation (M1) and alternative activation/regeneration (M2) type. Through various signaling pathways, macrophages polarize into complex groups, which can perform different immune functions. In this review, we emphasize the use of nanopreparations for macrophage related immunotherapy based on the pathological knowledge of TAMs phenotype. These macrophages targeted nanoparticles re-edit and re-educate macrophages by attenuating M2 macrophages and reducing aggregation to the TME, thereby relieving or alleviating immunosuppression. Among them, we describe in detail the cellular mechanisms and regulators of several major signaling pathways involved in the plasticity and polarization functions of macrophages. The advantages and challenges of those nanotherapeutics for these pathways have been elucidated, providing the basis and insights for the diagnosis and treatment strategies of various diseases centered on macrophages.

Macrophage Responses to Environmental Stimuli During Homeostasis and Disease

Work over the last 40 years has described macrophages as a heterogeneous population that serve as the frontline surveyors of tissue immunity. As a class, macrophages are found in almost every tissue in the body and as distinct populations within discrete microenvironments in any given tissue. During homeostasis, macrophages protect these tissues by clearing invading foreign bodies and/or mounting immune responses. In addition to varying identities regulated by transcriptional programs shaped by their respective environments, macrophage metabolism serves as an additional regulator to temper responses to extracellular stimuli. The area of research known as "immunometabolism" has been established within the last decade, owing to an increase in studies focusing on the crosstalk between altered metabolism and the regulation of cellular immune processes. From this research, macrophages have emerged as a prime focus of immunometabolic studies, although macrophage metabolism and their immune responses have been studied for centuries. During disease, the metabolic profile of the tissue and/or systemic regulators, such as endocrine factors, become increasingly dysregulated. Owing to these changes, macrophage responses can become skewed to promote further pathophysiologic changes. For instance, during diabetes, obesity, and atherosclerosis, macrophages favor a proinflammatory phenotype; whereas in the tumor microenvironment, macrophages elicit an anti-inflammatory response to enhance tumor growth. Herein we have described how macrophages respond to extracellular cues including inflammatory stimuli, nutrient availability, and endocrine factors that occur during and further promote disease progression.

Mitochondrial metabolism regulates macrophage biology

Mitochondria are critical for regulation of the activation, differentiation, and survival of macrophages and other immune cells. In response to various extracellular signals, such as microbial or viral infection, changes to mitochondrial metabolism and physiology could underlie the corresponding state of macrophage activation. These changes include alterations of oxidative metabolism, mitochondrial membrane potential, and tricarboxylic acid (TCA) cycling, as well as the release of mitochondrial reactive oxygen species (mtROS) and mitochondrial DNA (mtDNA) and transformation of the mitochondrial ultrastructure. Here, we provide an updated review of how changes in mitochondrial metabolism and various metabolites such as fumarate, succinate, and itaconate coordinate to guide macrophage activation to distinct cellular states, thus clarifying the vital link between mitochondria metabolism and immunity. We also discuss how in disease settings, mitochondrial dysfunction and oxidative stress contribute to dysregulation of the inflammatory response. Therefore, mitochondria are a vital source of dynamic signals that regulate macrophage biology to fine-tune immune responses.

Mitochondrial Reactive Oxygen Species Enhance Alveolar Macrophage Activity against Aspergillus fumigatus but Are Dispensable for Host Protection

Aspergillus fumigatus is the most common cause of mold pneumonia worldwide, and a significant cause of infectious morbidity and mortality in immunocompromised individuals. The oxidative burst, which generates reactive oxidative species (ROS), plays a pivotal role in host defense against aspergillosis and induces regulated cell death in Aspergillus conidia, the infectious propagules. Beyond the well-established role of NADP (NADPH) oxidase in ROS generation by neutrophils and other innate effector cells, mitochondria represent a major ROS production site in many cell types, though it is unclear whether mitochondrial ROS (mtROS) contribute to antifungal activity in the lung. Following A. fumigatus infection, we observed that innate effector cells, including alveolar macrophages (AMs), monocyte-derived dendritic cells (Mo-DCS), and neutrophils, generated mtROS, primarily in fungus-infected cells. To examine the functional role of mtROS, specifically the H₂O₂ component, in pulmonary host defense against A. fumigatus, we infected transgenic mice that expressed a mitochondrion-targeted catalase. Using a reporter of fungal viability during interactions with leukocytes, mitochondrial H₂O₂ (mtH₂O₂) was essential for optimal AM, but not for neutrophil phagocytic and conidiacidal activity in the lung. Catalase-mediated mtH₂O₂ neutralization did not lead to invasive aspergillosis in otherwise immunocompetent mice and did not shorten survival in mice that lack NADPH oxidase function. Collectively, these studies indicate that mtROS-associated defects in AM antifungal activity can be functionally compensated by the action of NADPH oxidase and by nonoxidative effector mechanisms during murine A. fumigatus lung infection. IMPORTANCE Aspergillus fumigatus is a fungal pathogen that causes invasive disease in humans with defects in immune function. Airborne conidia, the infectious propagules, are ubiquitous and inhaled on a daily basis. In the respiratory tree, conidia are killed by the coordinated actions of phagocytes, including alveolar macrophages, neutrophils, and monocyte-derived dendritic cells. The oxidative burst represents a central killing mechanism and relies on the assembly of the NADPH oxidase complex on the phagosomal membrane. However, NADPH oxidase-deficient leukocytes have significant residual fungicidal activity in vivo, indicating the presence of alternative effector mechanisms. Here, we report that murine innate immune cells produce mitochondrial reactive oxygen species (mtROS) in response to fungal interactions. Neutralizing the mtROS constituent hydrogen peroxide (H₂O₂) via a catalase expressed in mitochondria of innate immune cells substantially diminished fungicidal properties of alveolar macrophages, but not of other innate immune cells. These data indicate that mtH₂O₂ represent a novel AM killing mechanism against Aspergillus conidia. mtH₂O₂ neutralization is compensated by other killing mechanisms in the lung, demonstrating functional redundancy at the level of host defense in the respiratory tree. These findings have important implications for the development of host-directed therapies against invasive aspergillosis in susceptible patient populations.

Damage-Induced Calcium Signaling and Reactive Oxygen Species Mediate Macrophage Activation in Zebrafish

Immediately after a wound, macrophages are activated and change their phenotypes in reaction to danger signals released from the damaged tissues. The cues that contribute to macrophage activation after wounding in vivo are still poorly understood. Calcium signaling and Reactive Oxygen Species (ROS), mainly hydrogen peroxide, are conserved early wound signals that emanate from the wound and guide neutrophils within tissues up to the wound. However, the role of these signals in the recruitment and the activation of macrophages is elusive. Here we used the transparent zebrafish larva as a tractable vertebrate system to decipher the signaling cascade necessary for macrophage recruitment and activation after the injury of the caudal fin fold. By using transgenic reporter lines to track pro-inflammatory activated macrophages combined with high-resolutive microscopy, we tested the role of Ca²⁺ and ROS signaling in macrophage activation. By inhibiting intracellular Ca²⁺ released from the ER stores, we showed that macrophage recruitment and activation towards pro-inflammatory phenotypes are impaired. By contrast, ROS are only necessary for macrophage activation independently on calcium. Using genetic depletion of neutrophils, we showed that neutrophils are not essential for macrophage recruitment and activation. Finally, we identified Src family kinases, Lyn and Yrk and NF-κB as key regulators of macrophage activation in vivo, with Lyn and ROS presumably acting in the same signaling pathway. This study describes a molecular mechanism by which early wound signals drive macrophage polarization and suggests unique therapeutic targets to control macrophage activity during diseases.

From Flies to Men: ROS and the NADPH Oxidase in Phagocytes

The cellular formation of reactive oxygen species (ROS) represents an evolutionary ancient antimicrobial defense system against microorganisms. The NADPH oxidases (NOX), which are predominantly localized to endosomes, and the electron transport chain in mitochondria are the major sources of ROS. Like any powerful immunological process, ROS formation has costs, in particular collateral tissue damage of the host. Moreover, microorganisms have developed defense mechanisms against ROS, an example for an arms race between species. Thus, although NOX orthologs have been identified in organisms as diverse as plants, fruit flies, rodents, and humans, ROS functions have developed and diversified to affect a multitude of cellular properties, i.e., far beyond direct antimicrobial activity. Here, we focus on the development of NOX in phagocytic cells, where the so-called respiratory burst in phagolysosomes contributes to the elimination of ingested microorganisms. Yet, NOX participates in cellular signaling in a cell-intrinsic and -extrinsic manner, e.g., via the release of ROS into the extracellular space. Accordingly, in humans, the inherited deficiency of NOX components is characterized by infections with bacteria and fungi and a seemingly independently dysregulated inflammatory response. Since ROS have both antimicrobial and immunomodulatory properties, their tight regulation in space and time is required for an efficient and well-balanced immune response, which allows for the reestablishment of tissue homeostasis. In addition, distinct NOX homologs expressed by non-phagocytic cells and mitochondrial ROS are interlinked with phagocytic NOX functions and thus affect the overall redox state of the tissue and the cellular activity in a complex fashion. Overall, the systematic and comparative analysis of cellular ROS functions in organisms of lower complexity provides clues for understanding the contribution of ROS and ROS deficiency to human health and disease.

Metabolic programs define dysfunctional immune responses in severe COVID-19 patients

It is unclear why some SARS-CoV-2 patients readily resolve infection while others develop severe disease. By interrogating metabolic programs of immune cells in severe and recovered coronavirus disease 2019 (COVID-19) patients compared with other viral infections, we identify a unique population of T cells. These T cells express increased Voltage-Dependent Anion Channel 1 (VDAC1), accompanied by gene programs and functional characteristics linked to mitochondrial dysfunction and apoptosis. The percentage of these cells increases in elderly patients and correlates with lymphopenia. Importantly, T cell apoptosis is inhibited in vitro by targeting the oligomerization of VDAC1 or blocking caspase activity. We also observe an expansion of myeloid-derived suppressor cells with unique metabolic phenotypes specific to COVID-19, and their presence distinguishes severe from mild disease. Overall, the identification of these metabolic phenotypes provides insight into the dysfunctional immune response in acutely ill COVID-19 patients and provides a means to predict and track disease severity and/or design metabolic therapeutic regimens.

Functions of ROS in Macrophages and Antimicrobial Immunity

Reactive oxygen species (ROS) are a chemically defined group of reactive molecules derived from molecular oxygen. ROS are involved in a plethora of processes in cells in all domains of life, ranging from bacteria, plants and animals, including humans. The importance of ROS for macrophage-mediated immunity is unquestioned. Their functions comprise direct antimicrobial activity against bacteria and parasites as well as redox-regulation of immune signaling and induction of inflammasome activation. However, only a few studies have performed in-depth ROS analyses and even fewer have identified the precise redox-regulated target molecules. In this review, we will give a brief introduction to ROS and their sources in macrophages, summarize the versatile roles of ROS in direct and indirect antimicrobial immune defense, and provide an overview of commonly used ROS probes, scavengers and inhibitors.

Modeling Inflammation in Zebrafish for the Development of Anti-inflammatory Drugs

Dysregulation of the inflammatory response in humans can lead to various inflammatory diseases, like asthma and rheumatoid arthritis. The innate branch of the immune system, including macrophage and neutrophil functions, plays a critical role in all inflammatory diseases. This part of the immune system is well-conserved between humans and the zebrafish, which has emerged as a powerful animal model for inflammation, because it offers the possibility to image and study inflammatory responses in vivo at the early life stages. This review focuses on different inflammation models established in zebrafish, and how they are being used for the development of novel anti-inflammatory drugs. The most commonly used model is the tail fin amputation model, in which part of the tail fin of a zebrafish larva is clipped. This model has been used to study fundamental aspects of the inflammatory response, like the role of specific signaling pathways, the migration of leukocytes, and the interaction between different immune cells, and has also been used to screen libraries of natural compounds, approved drugs, and well-characterized pathway inhibitors. In other models the inflammation is induced by chemical treatment, such as lipopolysaccharide (LPS), leukotriene B4 (LTB4), and copper, and some chemical-induced models, such as treatment with trinitrobenzene sulfonic acid (TNBS), specifically model inflammation in the gastro-intestinal tract. Two mutant zebrafish lines, carrying a mutation in the hepatocyte growth factor activator inhibitor 1a gene (hai1a) and the cdp-diacylglycerolinositol 3-phosphatidyltransferase (cdipt) gene, show an inflammatory phenotype, and they provide interesting model systems for studying inflammation. These zebrafish inflammation models are often used to study the anti-inflammatory effects of glucocorticoids, to increase our understanding of the mechanism of action of this class of drugs and to develop novel glucocorticoid drugs. In this review, an overview is provided of the available inflammation models in zebrafish, and how they are used to unravel molecular mechanisms underlying the inflammatory response and to screen for novel anti-inflammatory drugs.

Exploring the evolutionary roots and physiological function of itaconate

New small molecules are continuing to emerge as metabolically derived regulators of cell function. Itaconate is a recent example where endogenous mammalian synthesis was demonstrated only seven years ago. Since then, interest in the biochemistry and therapeutic potential of itaconate has grown dramatically. Itaconate is an unsaturated dicarboxylic acid that has antimicrobial properties and modulates metabolic pathways throughout the cell. Naturally occurring mutations of enzymes involved in human itaconate synthesis and degradation pathways are associated with disease susceptibility and immunity. Here, we highlight recent discoveries on itaconate metabolism and discuss the relevance of its evolutionary origin to its function in mammals. We also consider the therapeutic relevance of itaconate metabolism and its derivatives for treating metabolic and inflammatory diseases.

Transcriptome Analyses of Immune System Behaviors in Primary Polyp of Coral Acropora digitifera Exposed to the Bacterial Pathogen Vibrio coralliilyticus under Thermal Loading

Elevated sea surface temperature associated with global warming is a serious threat to coral reefs. Elevated temperatures directly or indirectly alter the distribution of coral-pathogen interactions and thereby exacerbate infectious coral diseases. The pathogenic bacterium Vibrio coralliilyticus is well-known as a causative agent of infectious coral disease. Rising sea surface temperature promotes the infection of corals by this bacterium, which causes several coral pathologies, such as bacterial bleaching, tissue lysis, and white syndrome. However, the effects of thermal stress on coral immune responses to the pathogen are poorly understood. To delineate the effects of thermal stress on coral immunity, we performed transcriptome analysis of aposymbiotic primary polyps of the reef-building coral Acropora digitifera exposed to V. coralliilyticus under thermal stress conditions. V. coralliilyticus infection of coral that was under thermal stress had negative effects on various molecular processes, including suppression of gene expression related to the innate immune response. In response to the pathogen, the coral mounted various responses including changes in protein metabolism, exosome release delivering signal molecules, extracellular matrix remodeling, and mitochondrial metabolism changes. Based on these results, we provide new insights into innate immunity of A. digitifera against pathogen infection under thermal stress conditions.

Transcriptomic Profiling of the Adaptive and Innate Immune Responses of Atlantic Salmon to Renibacterium salmoninarum Infection

Bacterial Kidney Disease (BKD), which is caused by a Gram-positive, intracellular bacterial pathogen (Renibacterium salmoninarum), affects salmonids including Atlantic salmon (Salmo salar). However, the transcriptome response of Atlantic salmon to BKD remained unknown before the current study. We used a 44K salmonid microarray platform to characterise the global gene expression response of Atlantic salmon to BKD. Fish (~54 g) were injected with a dose of R. salmoninarum (H-2 strain, 2 × 10⁸ CFU per fish) or sterile medium (control), and then head kidney samples were collected at 13 days post-infection/injection (dpi). Firstly, infection levels of individuals were determined through quantifying the R. salmoninarum level by RNA-based TaqMan qPCR assays. Thereafter, based on the qPCR results for infection level, fish (n = 5) that showed no (control), higher (H-BKD), or lower (L-BKD) infection level at 13 dpi were subjected to microarray analyses. We identified 6,766 and 7,729 differentially expressed probes in the H-BKD and L-BKD groups, respectively. There were 357 probes responsive to the infection level (H-BKD vs. L-BKD). Several adaptive and innate immune processes were dysregulated in R. salmoninarum-infected Atlantic salmon. Adaptive immune pathways associated with lymphocyte differentiation and activation (e.g., lymphocyte chemotaxis, T-cell activation, and immunoglobulin secretion), as well as antigen-presenting cell functions, were shown to be differentially regulated in response to BKD. The infection level-responsive transcripts were related to several mechanisms such as the JAK-STAT signalling pathway, B-cell differentiation and interleukin-1 responses. Sixty-five microarray-identified transcripts were subjected to qPCR validation, and they showed the same fold-change direction as microarray results. The qPCR-validated transcripts studied herein play putative roles in various immune processes including pathogen recognition (e.g., tlr5), antibacterial activity (e.g., hamp and camp), regulation of immune responses (e.g., tnfrsf11b and socs1), T-/B-cell differentiation (e.g., ccl4, irf1 and ccr5), T-cell functions (e.g., rnf144a, il13ra1b and tnfrsf6b), and antigen-presenting cell functions (e.g., fcgr1). The present study revealed diverse immune mechanisms dysregulated by R. salmoninarum in Atlantic salmon, and enhanced the current understanding of Atlantic salmon response to BKD. The identified biomarker genes can be used for future studies on improving the resistance of Atlantic salmon to BKD.

Metabolic programs define dysfunctional immune responses in severe COVID-19 patients

It remains unclear why some patients infected with SARS-CoV-2 readily resolve infection while others develop severe disease. To address this question, we employed a novel assay to interrogate immune-metabolic programs of T cells and myeloid cells in severe and recovered COVID-19 patients. Using this approach, we identified a unique population of T cells expressing high H3K27me3 and the mitochondrial membrane protein voltage-dependent anion channel (VDAC), which were expanded in acutely ill COVID-19 patients and distinct from T cells found in patients infected with hepatitis c or influenza and in recovered COVID-19. Increased VDAC was associated with gene programs linked to mitochondrial dysfunction and apoptosis. High-resolution fluorescence and electron microscopy imaging of the cells revealed dysmorphic mitochondria and release of cytochrome c into the cytoplasm, indicative of apoptosis activation. The percentage of these cells was markedly increased in elderly patients and correlated with lymphopenia. Importantly, T cell apoptosis could be inhibited in vitro by targeting the oligomerization of VDAC or blocking caspase activity. In addition to these T cell findings, we also observed a robust population of Hexokinase II+ polymorphonuclear-myeloid derived suppressor cells (PMN-MDSC), exclusively found in the acutely ill COVID-19 patients and not the other viral diseases. Finally, we revealed a unique population of monocytic MDSC (M-MDSC) expressing high levels of carnitine palmitoyltransferase 1a (CPT1a) and VDAC. The metabolic phenotype of these cells was not only highly specific to COVID-19 patients but the presence of these cells was able to distinguish severe from mild disease. Overall, the identification of these novel metabolic phenotypes not only provides insight into the dysfunctional immune response in acutely ill COVID-19 patients but also provide a means to predict and track disease severity as well as an opportunity to design and evaluate novel metabolic therapeutic regimens.

Acute Iron Deprivation Reprograms Human Macrophage Metabolism and Reduces Inflammation In Vivo

Iron is an essential metal that fine-tunes the innate immune response by regulating macrophage function, but an integrative view of transcriptional and metabolic responses to iron perturbation in macrophages is lacking. Here, we induced acute iron chelation in primary human macrophages and measured their transcriptional and metabolic responses. Acute iron deprivation causes an anti-proliferative Warburg transcriptome, characterized by an ATF4-dependent signature. Iron-deprived human macrophages show an inhibition of oxidative phosphorylation and a concomitant increase in glycolysis, a large increase in glucose-derived citrate pools associated with lipid droplet accumulation, and modest levels of itaconate production. LPS polarization increases the itaconate:succinate ratio and decreases pro-inflammatory cytokine production. In rats, acute iron deprivation reduces the severity of macrophage-dependent crescentic glomerulonephritis by limiting glomerular cell proliferation and inducing lipid accumulation in the renal cortex. These results suggest that acute iron deprivation has in vivo protective effects mediated by an anti-inflammatory immunometabolic switch in macrophages.

Impact of Immunometabolism on Cancer Metastasis: A Focus on T Cells and Macrophages

Despite improved treatment options, cancer remains the leading cause of morbidity and mortality worldwide, with 90% of this mortality correlated to the development of metastasis. Since metastasis has such an impact on treatment success, disease outcome, and global health, it is important to understand the different steps and factors playing key roles in this process, how these factors relate to immune cell function and how we can target metabolic processes at different steps of metastasis in order to improve cancer treatment and patient prognosis. Recent insights in immunometabolism direct to promising therapeutic targets for cancer treatment, however, the specific contribution of metabolism on antitumor immunity in different metastatic niches warrant further investigation. Here, we provide an overview of what is so far known in the field of immunometabolism at different steps of the metastatic cascade, and what may represent the next steps forward. Focusing on metabolic checkpoints in order to translate these findings from in vitro and mouse studies to the clinic has the potential to revolutionize cancer immunotherapy and greatly improve patient prognosis.

Macrophage polarization and its role in the pathogenesis of acute lung injury/acute respiratory distress syndrome

PURPOSE

Macrophages are highly plastic cells. Under different stimuli, macrophages can be polarized into several different subsets. Two main macrophage subsets have been suggested: classically activated or inflammatory (M1) macrophages and alternatively activated or anti-inflammatory (M2) macrophages. Macrophage polarization is governed by a highly complex set of regulatory networks. Many recent studies have shown that macrophages are key orchestrators in the pathogenesis of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) and that regulation of macrophage polarization may improve the prognosis of ALI/ARDS. A further understanding of the mechanisms of macrophage polarization is expected to be helpful in the development of novel therapeutic targets to treat ALI/ARDS. Therefore, we performed a literature review to summarize the regulatory mechanisms of macrophage polarization and its role in the pathogenesis of ALI/ARDS.

METHODS

A computer-based online search was performed using the PubMed database and Web of Science database for published articles concerning macrophages, macrophage polarization, and ALI/ARDS.

RESULTS

In this review, we discuss the origin, polarization, and polarization regulation of macrophages as well as the role of macrophage polarization in various stages of ARDS. According to the current literature, regulating the polarized state of macrophages might be a potential therapeutic strategy against ALI/ARDS.

Inhibition of Fatty Acid Oxidation Promotes Macrophage Control of Mycobacterium tuberculosis

Macrophage activation involves metabolic reprogramming to support antimicrobial cellular functions. How these metabolic shifts influence the outcome of infection by intracellular pathogens remains incompletely understood. Mycobacterium tuberculosis (Mtb) modulates host metabolic pathways and utilizes host nutrients, including cholesterol and fatty acids, to survive within macrophages. We found that intracellular growth of Mtb depends on host fatty acid catabolism: when host fatty acid β-oxidation (FAO) was blocked chemically with trimetazidine, a compound in clinical use, or genetically by deletion of the mitochondrial fatty acid transporter carnitine palmitoyltransferase 2 (CPT2), Mtb failed to grow in macrophages, and its growth was attenuated in mice. Mechanistic studies support a model in which inhibition of FAO generates mitochondrial reactive oxygen species, which enhance macrophage NADPH oxidase and xenophagy activity to better control Mtb infection. Thus, FAO inhibition promotes key antimicrobial functions of macrophages and overcomes immune evasion mechanisms of Mtb.IMPORTANCEMycobacterium tuberculosis (Mtb) is the leading infectious disease killer worldwide. We discovered that intracellular Mtb fails to grow in macrophages in which fatty acid β-oxidation (FAO) is blocked. Macrophages treated with FAO inhibitors rapidly generate a burst of mitochondria-derived reactive oxygen species, which promotes NADPH oxidase recruitment and autophagy to limit the growth of Mtb. Furthermore, we demonstrate the ability of trimetazidine to reduce pathogen burden in mice infected with Mtb. These studies will add to the knowledge of how host metabolism modulates Mtb infection outcomes.

ACOD1 in immunometabolism and disease

Immunometabolism plays a fundamental role in health and diseases and involves multiple genes and signals. Aconitate decarboxylase 1 (ACOD1; also known as IRG1) is emerging as a regulator of immunometabolism in inflammation and infection. Upregulation of ACOD1 expression occurs in activated immune cells (e.g., macrophages and monocytes) in response to pathogen infection (e.g., bacteria and viruses), pathogen-associated molecular pattern molecules (e.g., LPS), cytokines (e.g., TNF and IFNs), and damage-associated molecular patterns (e.g., monosodium urate). Mechanistically, several immune receptors (e.g., TLRs and IFNAR), adapter proteins (e.g., MYD88), ubiquitin ligases (e.g., A20), and transcription factors (e.g., NF-κB, IRFs, and STATs) form complex signal transduction networks to control ACOD1 expression in a context-dependent manner. Functionally, ACOD1 mediates itaconate production, oxidative stress, and antigen processing and plays dual roles in immunity and diseases. On the one hand, activation of the ACOD1 pathway may limit pathogen infection and promote embryo implantation. On the other hand, abnormal ACOD1 expression can lead to tumor progression, neurodegenerative disease, and immune paralysis. Further understanding of the function and regulation of ACOD1 is important for the application of ACOD1-based therapeutic strategies in disease.

Illuminating Macrophage Contributions to Host-Pathogen Interactions In Vivo: the Power of Zebrafish

Macrophages are a key cell type in innate immunity. Years of in vitro cell culture studies have unraveled myriad macrophage pathways that combat pathogens and demonstrated how pathogen effectors subvert these mechanisms. However, in vitro cell culture studies may not accurately reflect how macrophages fit into the context of an innate immune response in whole animals with multiple cell types and tissues. Larval zebrafish have emerged as an intermediate model of innate immunity and host-pathogen interactions to bridge the gap between cell culture studies and mammalian models. These organisms possess an innate immune system largely conserved with that of humans and allow state-of-the-art genetic and imaging techniques, all in the context of an intact organism. Using larval zebrafish, researchers are elucidating the function of macrophages in response to many different infections, including both bacterial and fungal pathogens. The goal of this review is to highlight studies in zebrafish that utilized live-imaging techniques to analyze macrophage activities in response to pathogens. Recent studies have explored the roles of specific pathways and mechanisms in macrophage killing ability, explored how pathogens subvert these responses, identified subsets of macrophages with differential microbicidal activities, and implicated macrophages as an intracellular niche for pathogen survival and trafficking. Research using this model continues to advance our understanding of how macrophages, and specific pathways inside these cells, fit into complex multicellular innate immune responses in vivo, providing important information on how pathogens evade these pathways and how we can exploit them for development of treatments against microbial infections.

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.

Gut bacteria signaling to mitochondria in intestinal inflammation and cancer

The gastrointestinal microbiome plays a pivotal role in physiological homeostasis of the intestine as well as in the pathophysiology of diseases including inflammatory bowel diseases (IBD) and colorectal cancer (CRC). Emerging evidence suggests that gut microbiota signal to the mitochondria of mucosal cells, including epithelial cells and immune cells. Gut microbiota signaling to mitochondria has been shown to alter mitochondrial metabolism, activate immune cells, induce inflammasome signaling, and alter epithelial barrier function. Both dysbiosis of the gut microbiota and mitochondrial dysfunction are associated with chronic intestinal inflammation and CRC. This review discusses mitochondrial metabolism of gut mucosal cells, mitochondrial dysfunction, and known gut microbiota-mediated mitochondrial alterations during IBD and CRC.

Effects of new generation progestins, including as mixtures and in combination with other classes of steroid hormones, on zebrafish early life stages

Fish are exposed to progestins and steroid mixtures in contaminated waters but the ecotoxicological implications are not sufficiently known. Here we analyze effects of the new generation progestin dienogest (DNG) followed by investigating effects of mixtures of new generation progestins containing DNG, cyproterone acetate and drospirenone and the hormone progesterone. Furthermore, effects of this mixture were studied after adding 17β-estradiol (E2) and clobetasol propionate (CLO) in zebrafish embryos and larvae at concentrations between 0.01 and 10 μg/L. DNG showed only very minor transcriptional alterations among the 24 assessed genes with downregulation of the fshb transcript only. The progestin mixture caused weak induction of the lhb, cyp2k22 and sult2st3 transcripts. Addition of E2 to the mixture caused strong induction vtg1, cyp19b, esr1 and lhb, as well as downregulation of fshb from 0.01 μg/L onwards. Besides altering the same transcripts, addition of CLO altered glucocorticoid regulated genes mmp-9, mmp-13, g6pca, fkbp5 and irg1l. While each steroid class exhibited its specific activity independently in the mixture, sult2st3 and cyp2k22 were regulated by both E2 and CLO. Furthermore, CLO alone and in mixtures decreased spontaneous muscle contractions, increased heartrate and induced edema. Our study highlights the prominent effects of E2 and CLO in environmental steroid mixtures, while new generation progestins show relatively low activity.

Role of IRG1 in Regulating Pro-inflammatory and Pro-labor Mediators in Human Myometrium

Preterm birth is a major contributor to neonatal deaths and associated long-term morbidities for the survivors, yet therapies remain elusive, given our incomplete understanding of the mechanisms driving human labor and delivery. Human labor is an inflammatory process, and we investigated whether IRG1 (immunoresponsive gene-1) plays a role in these processes. We demonstrate that IRG1 mRNA and protein expression is significantly increased in myometrium with human term labor, compared to no labor samples, and with preterm (LPS) labor in a mouse model. Pro-labor mediators such as pro-inflammatory cytokines TNF and IL1B, and TLR ligands fsl-1, flagellin, LPS, and poly(I:C) also increased IRG1 mRNA expression in myometrial explants. IRG1 silencing, using siRNA in primary myometrial cells, displayed a decrease in the expression of inflammation-induced pro-inflammatory cytokines (IL1A, IL6), chemokines (CCL2, CXCL1, CXCL8), adhesion molecules (ICAM1, VCAM1), and contractility (PTGFR mRNA expression, prostaglandin F_2α release, and in situ gel contraction assay). Our results suggest that IRG1 is involved when pro-labor mediators activate the inflammatory processes of human labor, warranting further investigation.

Chronic stress, physiological adaptation and developmental programming of the neuroendocrine stress system

Chronic stress undermines physical and mental health, in part via dysregulation of the neuroendocrine stress system. Key to understand this dysregulation is recognizing that the problem is not stress per se, but rather its chronicity. The optimally functioning stress system is highly dynamic, and negative feedback regulation enforces transient responses to acute stressors. Chronic stress overrides this, and adaptation to the chronicity can result in persistent dysregulation by altering sensitivity thresholds critical for control of system dynamics. Such adaptation involves plasticity within the central nervous system (CNS) as well as epigenetic regulation. When it occurs during development, it can have persistent effects on neuroendocrine regulation. Understanding how chronic stress programs development of the neuroendocrine stress system requires elucidation of stress-responsive gene regulatory networks that control CNS plasticity and development.

The Interplay Between Tissue Niche and Macrophage Cellular Metabolism in Obesity

Obesity is associated with the development of metabolic diseases such as type 2 diabetes and non-alcoholic fatty liver disease. The presence of chronic, low-grade inflammation appears to be an important mechanistic link between excess nutrients and clinical disease. The onset of these metabolic disorders coincides with changes in the number and phenotype of macrophages in peripheral organs, particularly in the liver and adipose tissue. Macrophage accumulation in these tissues has been implicated in tissue inflammation and fibrosis, contributing to metabolic disease progression. Recently, the concept has emerged that changes in macrophage metabolism affects their functional phenotype, possibly triggered by distinct environmental metabolic cues. This may be of particular importance in the setting of obesity, where both liver and adipose tissue are faced with a high metabolic burden. In the first part of this review we will discuss current knowledge regarding macrophage dynamics in both adipose tissue and liver in obesity. Then in the second part, we will highlight data linking macrophage metabolism to functional phenotype with an emphasis on macrophage activation in metabolic disease. The importance of understanding how tissue niche influences macrophage function in obesity will be highlighted. In addition, we will identify important knowledge gaps and outstanding questions that are relevant for future research in this area and will facilitate the identification of novel targets for therapeutic intervention in associated metabolic diseases.

New Insights on the Role of Lipid Metabolism in the Metabolic Reprogramming of Macrophages

Macrophage activation is intimately linked to metabolic reprogramming. Inflammatory (M1) macrophages are able to sustain inflammatory responses and to kill pathogens, mostly by relying on aerobic glycolysis and fatty acid biosynthesis. Glycolysis is a fast way of producing ATP, and fatty acids serve as precursors for the synthesis of inflammatory mediators. On the opposite side, anti-inflammatory (M2) macrophages mediate the resolution of inflammation and tissue repair, switching their metabolism to fatty acid oxidation and oxidative phosphorylation. Over the years, this classical view has been challenged by recent discoveries pointing to a more complex metabolic network during macrophage activation. Lipid metabolism plays a critical role in the activation of both M1 and M2 macrophages. Recent evidence shows that fatty acid oxidation is also essential for inflammasome activation in M1 macrophages, and glycolysis is now known to fuel fatty acid oxidation in M2 macrophages. Ultimately, targeting lipid metabolism in macrophages can improve the outcome of metabolic diseases. Here, we review the main aspects of macrophage immunometabolism from the perspective of the metabolism of lipids. Building a reliable metabolic network during macrophage activation will bring us closer to targeting macrophages for improving human health.

Isolation of Neutrophils from Larval Zebrafish and Their Transplantation into Recipient Larvae for Functional Studies

Live imaging of neutrophils within optically transparent larval zebrafish has proved a powerful technique to investigate how specific gene products control neutrophil function. To resolve whether a gene contributes to neutrophil function in a cell-autonomous manner necessitates a way to examine gene-deficient neutrophils in an otherwise wild type background. To this end, here we describe methods to harvest fluorescent neutrophils from larval donor zebrafish and transplant them into age-matched recipients. We show that transplanted neutrophils can survive in recipient larvae for at least 3 days providing ample opportunity for functional studies. Focusing on bactericidal activity, we show that transplanted neutrophils phagocytose and kill live bacteria with similar kinetics to nontransplanted neutrophils, indicating that the transplantation process does not influence these neutrophil effector functions. Following the methods described here to transplant neutrophils between gene-deficient and wild type larval zebrafish will enable investigations into whether a gene's contribution to neutrophil function is cell-autonomous.

The promise and peril of targeting cell metabolism for cancer therapy

A major challenge of cancer immunotherapy is the potential for undesirable effects on bystander cells and tumor-associated immune cells. Fundamentally, we need to understand what effect targeting tumor metabolism has upon the metabolism and phenotype of tumor-associated leukocytes, whose function can be critical for effective cancer therapeutic strategies. Undesirable effects of cancer therapeutics are a major reason for drug-associated toxicity, which confounds drug dosing and efficacy. As with any chemotherapeutic agent, drugs targeting tumor metabolism will exert potent effects on host stromal cells and tumor-associated leukocytes. Any drug targeting glycolysis, for example, could metabolically starve tumor-infiltrating T cells, inhibit their effector function and enable tumor progression. The targeting of oxidative phosphorylation in tumors will have complex effects on the polarization and function of tumor-associated macrophages. In short, we need to improve our understanding of tumor and immune cell metabolism and devise ways to specifically target tumors without compromising necessary host metabolism. Exploiting cell-specific metabolic pathways to directly target tumor cells may minimize detrimental effects on tumor-associated leukocytes.

Copper Regulates the Susceptibility of Zebrafish Larvae to Inflammatory Stimuli by Controlling Neutrophil/Macrophage Survival

Copper has been revealed to negatively affect the hematopoietic system, which has an important function in immune pathogen defense, but little is known about the potential mechanism. In this study, copper-stressed larvae exhibited significantly increased mortality as well as reduced percentages of GFP-labeled macrophages and neutrophils after Aeromonas hydrophila (A. hydrophila) infection. However, those copper-stressed GFP-labeled macrophages and neutrophils showed more rapid responses to A. hydrophila infection. The transcriptional profiles in copper-stressed macrophages or neutrophils were unveiled by RNA-Sequencing, and KEGG pathway analysis revealed enrichment of differentially expressed genes (DEGs) in lysosome, apoptosis, oxidative phosphorylation, phagosome, etc. The copper-stressed macrophages or neutrophils were revealed to have an increase in reactive oxygen species (ROS) and mitochondria ROS (mROS)-mediated apoptosis, and a reduction in phagocytosis. Furthermore, the A. hydrophila-infected copper-stressed macrophages or neutrophils were found to be unable to maintain a consistently increased expression in immune responsive genes. This study demonstrated for the first time that copper might induce the susceptibility of fish larvae to inflammatory stimuli via triggering macrophage or neutrophil apoptosis, leading to reduced phagocytic activities and non-sustainable immune responses in immune macrophages or neutrophils.

Zebrafish are Resistant to Staphylococcus aureus Endophthalmitis

Gram-positive bacteria remain the leading cause of endophthalmitis, a blinding infectious disease of the eye. Murine models have been widely used for understanding the pathogenesis of bacterial endophthalmitis. In this study, we sought to develop an alternative zebrafish (Danio rerio) model for Staphylococcus aureus and compare the disease pathobiology to a murine model. Endophthalmitis was induced in zebrafish and C57BL/6 mice through the intravitreal injection of S. aureus. Disease progression was monitored by assessing corneal haze, opacity, bacterial burden, and retinal histology. Our results demonstrated that, unlike the murine models, zebrafish maintained ocular integrity, corneal transparency, and retinal architecture. We found that the zebrafish was capable of clearing S. aureus from the eye via transport through retinal vessels and the optic nerve and by mounting a monocyte/macrophage response beginning at 8 hour post-infection (hpi). The bacterial burden increased up to 8 hpi and significantly decreased thereafter. An assessment of the innate retinal response revealed the induced expression of Il-1β and Il-6 transcripts. Collectively, our study shows that unlike the murine model, zebrafish do not develop endophthalmitis and rapidly clear the pathogen. Hence, a better understanding of the zebrafish protective ocular innate response may provide new insights into the pathobiology of bacterial endophthalmitis.

Pivotal Role of Mitochondria in Macrophage Response to Bacterial Pathogens

Mitochondria are essential organelles that act as metabolic hubs and signaling platforms within the cell. Numerous mitochondrial functions, including energy metabolism, lipid synthesis, and autophagy regulation, are intimately linked to mitochondrial dynamics, which is shaped by ongoing fusion and fission events. Recently, several intracellular bacterial pathogens have been shown to modulate mitochondrial functions to maintain their replicative niche. Through selected examples of human bacterial pathogens, we will discuss how infection induces mitochondrial changes in infected macrophages, triggering modifications of the host metabolism that lead to important immunological reprogramming.

Crystal structure of cis-aconitate decarboxylase reveals the impact of naturally occurring human mutations on itaconate synthesis

cis-Aconitate decarboxylase (CAD, also known as ACOD1 or Irg1) converts cis-aconitate to itaconate and plays central roles in linking innate immunity with metabolism and in the biotechnological production of itaconic acid by Aspergillus terreus We have elucidated the crystal structures of human and murine CADs and compared their enzymological properties to CAD from A. terreus Recombinant CAD is fully active in vitro without a cofactor. Murine CAD has the highest catalytic activity, whereas Aspergillus CAD is best adapted to a more acidic pH. CAD is not homologous to any known decarboxylase and appears to have evolved from prokaryotic enzymes that bind negatively charged substrates. CADs are homodimers, the active center is located in the interface between 2 distinct subdomains, and structural modeling revealed conservation in zebrafish and Aspergillus We identified 8 active-site residues critical for CAD function and rare naturally occurring human mutations in the active site that abolished CAD activity, as well as a variant (Asn152Ser) that increased CAD activity and is common (allele frequency 20%) in African ethnicity. These results open the way for 1) assessing the potential impact of human CAD variants on disease risk at the population level, 2) developing therapeutic interventions to modify CAD activity, and 3) improving CAD efficiency for biotechnological production of itaconic acid.

Aspergillus fumigatus establishes infection in zebrafish by germination of phagocytized conidia, while Aspergillus niger relies on extracellular germination

Among opportunistically pathogenic filamentous fungi of the Aspergillus genus, Aspergillus fumigatus stands out as a drastically more prevalent cause of infection than others. Utilizing the zebrafish embryo model, we applied a combination of non-invasive real-time imaging and genetic approaches to compare the infectious development of A. fumigatus with that of the less pathogenic A. niger. We found that both species evoke similar immune cell migratory responses, but A. fumigatus is more efficiently phagocytized than A. niger. Though efficiently phagocytized, A. fumigatus conidia retains the ability to germinate and form hyphae from inside macrophages leading to serious infection even at relatively low infectious burdens. By contrast, A. niger appears to rely on extracellular germination, and rapid hyphal growth to establish infection. Despite these differences in the mechanism of infection between the species, galactofuranose mutant strains of both A. fumigatus and A. niger display attenuated pathogenesis. However, deficiency in this cell wall component has a stronger impact on A. niger, which is dependent on rapid extracellular hyphal growth. In conclusion, we uncover differences in the interaction of the two fungal species with innate immune cells, noticeable from very early stages of infection, which drive a divergence in their route to establishing infections.

Targeting metabolism to regulate immune responses in autoimmunity and cancer

Metabolic programming is emerging as a critical mechanism to alter immune cell activation, differentiation and function. Targeting metabolism does not completely suppress or activate the immune system but selectively regulates immune responses. The different metabolic requirements of the diverse cells that constitute an immune response provide a unique opportunity to separate effector functions from regulatory functions. Likewise, cells can be metabolically reprogrammed to promote either their short-term effector functions or long-term memory capacity. Studies in the growing field of immunometabolism support a paradigm of 'cellular selectivity based on demand', in which generic inhibitors of ubiquitous metabolic processes selectively affect cells with the greatest metabolic demand and have few effects on other cells of the body. Targeting metabolism, rather than particular cell types or cytokines, in metabolically demanding processes such as autoimmunity, graft rejection, cancer and uncontrolled inflammation could lead to successful strategies in controlling the pathogenesis of these complex disorders.

Indoleamine 2,3-Dioxygenase Activity in Chlamydia muridarum and Chlamydia pneumoniae Infected Mouse Lung Tissues

Chlamydia trachomatis infections are the most prevalent sexually transmitted infections with potentially debilitating sequelae, such as infertility. Mouse models are generally used for vaccine development, to study the immune response and histopathology associated with Chlamydia infection. An important question regarding murine models is the in vivo identification of murine host genes responsible for the elimination of the murine and human Chlamydia strains. RNA sequencing of the Chlamydia muridarum infected BALB/c lung transcriptome revealed that several genes with direct antichlamydial functions were induced at the tissue level, including the already described and novel members of the murine interferon-inducible GTPase family, the CXCL chemokines CXCL9, CXCL11, immunoresponsive gene 1, nitric oxide synthase-2 (iNOS), and lipocalin-2. Indoleamine 2,3-dioxygenase 1-2 (IDO1-2) previously described potent antichlamydial host enzymes were also highly expressed in the infected murine lungs. This finding was novel, since IDO was considered as a unique human antichlamydial defense gene. Besides a lower level of epithelial cell positivity, immunohistochemistry showed that IDO1-2 proteins were expressed prominently in macrophages. Detection of the tryptophan degradation product kynurenine and the impact of IDO inhibition on Chlamydia muridarum growth proved that the IDO1-2 proteins were functionally active. IDO1-2 activity also increased in Chlamydia muridarum infected C57BL/6 lung tissues, indicating that this phenomenon is not mouse strain specific. Our study shows that the murine antichlamydial response includes a variety of highly up-regulated defense genes in vivo. Among these genes the antichlamydial effectors IDO1-2 were identified. The potential impact of murine IDO1-2 expression on Chlamydia propagation needs further investigation.

Intestinal dysbiosis compromises alveolar macrophage immunity to Mycobacterium tuberculosis

Current treatments for tuberculosis (TB) are effective in controlling Mycobacterium tuberculosis (Mtb) growth, yet have significant side effects and do not prevent reinfection. Therefore, it is critical to understand why our host defense system is unable to generate permanent immunity to Mtb despite prolonged anti-tuberculosis therapy (ATT). Here, we demonstrate that treatment of mice with the most widely used anti-TB drugs, rifampicin (RIF) or isoniazid (INH) and pyrazinamide (PYZ), significantly altered the composition of the gut microbiota. Unexpectedly, treatment of mice with the pro-Mtb drugs INH and PYZ, but not RIF, prior to Mtb infection resulted in an increased bacterial burden, an effect that was reversible by fecal transplantation from untreated animals. Mechanistically, susceptibility of INH/PYZ-treated mice was associated with impaired metabolism of alveolar macrophages and defective bactericidal activity. Collectively, these data indicate that dysbiosis induced by ATT administered to millions of individuals worldwide may have adverse effects on the anti-Mtb response of alveolar macrophages.