The mononuclear phagocyte system of the mouse defined by immunohistochemical localisation of antigen F4/80: macrophages associated with epithelia

Mononuclear Phagocytes, Cellular Immunity, and Nobel Prizes: A Historic Perspective

The mononuclear phagocyte system includes monocytes, macrophages, some dendritic cells, and multinuclear giant cells. These cell populations display marked heterogeneity depending on their differentiation from embryonic and bone marrow hematopoietic progenitors, tissue location, and activation. They contribute to tissue homeostasis by interacting with local and systemic immune and non-immune cells through trophic, clearance, and cytocidal functions. During evolution, they contributed to the innate host defense before effector mechanisms of specific adaptive immunity emerged. Mouse macrophages appear at mid-gestation and are distributed throughout the embryo to facilitate organogenesis and clear cells undergoing programmed cell death. Yolk sac, AGM, and fetal liver-derived tissue-resident macrophages persist throughout postnatal and adult life, supplemented by bone marrow-derived blood monocytes, as required after injury and infection. Nobel awards to Elie Metchnikoff and Paul Ehrlich in 1908 drew attention to cellular phagocytic and humoral immunity, respectively. In 2011, prizes were awarded to Jules Hoffmann and Bruce Beutler for contributions to innate immunity and to Ralph Steinman for the discovery of dendritic cells and their role in antigen presentation to T lymphocytes. We trace milestones in the history of mononuclear phagocyte research from the perspective of Nobel awards bearing directly and indirectly on their role in cellular immunity.

Synthetic Biomimetic Liposomes Harness Efferocytosis Machinery for Highly Efficient Macrophages-Targeted Drug Delivery to Alleviate Inflammation

Macrophages play pivotal roles in the regulation of inflammatory responses and tissue repair, making them a prime target for inflammation alleviation. However, the accurate and efficient macrophages targeting is still a challenging task. Motivated by the efficient and specific removal of apoptotic cells by macrophages efferocytosis, a novel biomimetic liposomal system called Effero-RLP (Efferocytosis-mediated Red blood cell hybrid Liposomes) is developed which incorporates the membrane of apoptotic red blood cells (RBCs) with liposomes for the purpose of highly efficient macrophages targeting. Rosiglitazone (ROSI), a PPARγ agonist known to attenuate macrophage inflammatory responses, is encapsulated into Effero-RLP as model drug to regulate macrophage functions in DSS-induced colitis mouse model. Intriguingly, the Effero-RLP exhibits selective and efficient uptake by macrophages, which is significantly inhibited by the efferocytosis blocker Annexin V. In animal models, the Effero-RLP demonstrates rapid recognition by macrophages, leading to enhanced accumulation at inflammatory sites. Furthermore, ROSI-loaded Effero-RLP effectively alleviates inflammation and protects colon tissue from injury in the colitis mouse model, which is abolished by deletion of macrophages from mice model. In conclusion, the study highlights the potential of macrophage targeting using efferocytosis biomimetic liposomes. The development of Effero-RLP presents novel and promising strategies for alleviating inflammation.

Epithelial-immune cell crosstalk for intestinal barrier homeostasis

The intestinal barrier is mainly formed by a monolayer of epithelial cells, which forms a physical barrier to protect the gut tissues from external insults and provides a microenvironment for commensal bacteria to colonize while ensuring immune tolerance. Moreover, various immune cells are known to significantly contribute to intestinal barrier function by either directly interacting with epithelial cells or by producing immune mediators. Fulfilling this function of the gut barrier for mucosal homeostasis requires not only the intrinsic regulation of intestinal epithelial cells (IECs) but also constant communication with immune cells and gut microbes. The reciprocal interactions between IECs and immune cells modulate mucosal barrier integrity. Dysregulation of barrier function could lead to dysbiosis, inflammation, and tumorigenesis. In this overview, we provide an update on the characteristics and functions of IECs, and how they integrate their functions with tissue immune cells and gut microbiota to establish gut homeostasis.

The intestinal epithelial-macrophage-crypt stem cell axis plays a crucial role in regulating and maintaining intestinal homeostasis

The intestinal tract plays a vital role in both digestion and immunity, making its equilibrium crucial for overall health. This equilibrium relies on the dynamic interplay among intestinal epithelial cells, macrophages, and crypt stem cells. Intestinal epithelial cells play a pivotal role in protecting and regulating the gut. They form vital barriers, modulate immune responses, and engage in pathogen defense and cytokine secretion. Moreover, they supervise the regulation of intestinal stem cells. Macrophages, serving as immune cells, actively influence the immune response through the phagocytosis of pathogens and the release of cytokines. They also contribute to regulating intestinal stem cells. Stem cells, known for their self-renewal and differentiation abilities, play a vital role in repairing damaged intestinal epithelium and maintaining homeostasis. Although research has primarily concentrated on the connections between epithelial and stem cells, interactions with macrophages have been less explored. This review aims to fill this gap by exploring the roles of the intestinal epithelial-macrophage-crypt stem cell axis in maintaining intestinal balance. It seeks to unravel the intricate dynamics and regulatory mechanisms among these essential players. A comprehensive understanding of these cell types' functions and interactions promises insights into intestinal homeostasis regulation. Moreover, it holds potential for innovative approaches to manage conditions like radiation-induced intestinal injury, inflammatory bowel disease, and related diseases.

Diet-induced obesity alters intestinal monocyte-derived and tissue-resident macrophages and increases intestinal permeability in female mice independent of tumor necrosis factor

Macrophages are essential for homeostatic maintenance of the anti-inflammatory and tolerogenic intestinal environment, yet monocyte-derived macrophages can promote local inflammation. Proinflammatory macrophage accumulation within the intestines may contribute to the development of systemic chronic inflammation and immunometabolic dysfunction in obesity. Using a model of high-fat diet-induced obesity in C57BL/6J female mice, we assessed intestinal paracellular permeability by in vivo and ex vivo assays and quantitated intestinal macrophages in ileum and colon tissues by multicolor flow cytometry after short (6 wk), intermediate (12 wk), and prolonged (18 wk) diet allocation. We characterized monocyte-derived CD4^-TIM4^- and CD4⁺TIM4^- macrophages, as well as tissue-resident CD4⁺TIM4⁺ macrophages. Diet-induced obesity had tissue- and time-dependent effects on intestinal permeability, as well as monocyte and macrophage numbers, surface marker phenotype, and intracellular production of the cytokines IL-10 and tumor necrosis factor (TNF). We found that obese mice had increased paracellular permeability, in particular within the ileum, but this did not elicit recruitment of monocytes nor a local proinflammatory response by monocyte-derived or tissue-resident macrophages in either the ileum or colon. Proliferation of monocyte-derived and tissue-resident macrophages was also unchanged. Wild-type and TNF^-/- littermate mice had similar intestinal permeability and macrophage population characteristics in response to diet-induced obesity. These data are unique from reported effects of diet-induced obesity on macrophages in metabolic tissues, as well as outcomes of acute inflammation within the intestines. These experiments also collectively indicate that TNF does not mediate effects of diet-induced obesity on paracellular permeability or intestinal monocyte-derived and tissue-resident intestinal macrophages in young female mice.NEW & NOTEWORTHY We found that diet-induced obesity in female mice has tissue- and time-dependent effects on intestinal paracellular permeability as well as monocyte-derived and tissue-resident macrophage numbers, surface marker phenotype, and intracellular production of the cytokines IL-10 and TNF. These changes were not mediated by TNF.

MicroRNA-124 expression in Kupffer cells modulates liver injury by targeting IL-6/STAT3 signaling

MicroRNA-124 (miR-124) is related to liver injury due to chronic hepatitis B (CHB) and hepatitis B virus-related acute-on-chronic liver failure (HBV-ACLF). However, the mechanism whereby miR-124 regulates liver inflammation remains unknown. In this study, we show that serum miR-124 serves as a compensatory predictive factor for organ failure and the 28-day prognosis of patients with HBV-ACLF. Moreover, within a mouse model of concanavalin A-induced acute liver injury, miR-124 is highly expressed in Kupffer cells. Overexpression of miR-124 significantly decreases interleukin-6 (IL-6) secretion, and relieves pathological liver necrosis to a great extent. Mechanistically, miR-124 directly targets the 3'-untranslated region of signal transducer and activator of transcription 3 (STAT3) and inhibits IL-6/STAT3 signaling, which reduces pro-inflammatory Kupffer cell polarization. Collectively, our findings suggest that miR-124 can potentially serve as a predictive biomarker for HBV-ACLF prognosis and may represent a promising therapeutic target for relieving severe liver injury resulting from cytokine storms.

Pathophysiology of Inflammatory Bowel Disease: Innate Immune System

Inflammatory bowel disease (IBD), comprising Crohn's disease (CD) and ulcerative colitis (UC), is a heterogeneous state of chronic intestinal inflammation with no exact known cause. Intestinal innate immunity is enacted by neutrophils, monocytes, macrophages, and dendritic cells (DCs), and innate lymphoid cells and NK cells, characterized by their capacity to produce a rapid and nonspecific reaction as a first-line response. Innate immune cells (IIC) defend against pathogens and excessive entry of intestinal microorganisms, while preserving immune tolerance to resident intestinal microbiota. Changes to this equilibrium are linked to intestinal inflammation in the gut and IBD. IICs mediate host defense responses, inflammation, and tissue healing by producing cytokines and chemokines, activating the complement cascade and phagocytosis, or presenting antigens to activate the adaptive immune response. IICs exert important functions that promote or ameliorate the cellular and molecular mechanisms that underlie and sustain IBD. A comprehensive understanding of the mechanisms underlying these clinical manifestations will be important for developing therapies targeting the innate immune system in IBD patients. This review examines the complex roles of and interactions among IICs, and their interactions with other immune and non-immune cells in homeostasis and pathological conditions.

Targeting macrophage autophagy for inflammation resolution and tissue repair in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic, non-specific, recurrent inflammatory disease, majorly affecting the gastrointestinal tract. Due to its unclear pathogenesis, the current therapeutic strategy for IBD is focused on symptoms alleviation. Autophagy is a lysosome-mediated catabolic process for maintaining cellular homeostasis. Genome-wide association studies and subsequent functional studies have highlighted the critical role of autophagy in IBD via a number of mechanisms, including modulating macrophage function. Macrophages are the gatekeepers of intestinal immune homeostasis, especially involved in regulating inflammation remission and tissue repair. Interestingly, many autophagic proteins and IBD-related genes have been revealed to regulate macrophage function, suggesting that macrophage autophagy is a potentially important process implicated in IBD regulation. Here, we have summarized current understanding of macrophage autophagy function in pathogen and apoptotic cell clearance, inflammation remission and tissue repair regulation in IBD, and discuss how this knowledge can be used as a strategy for IBD treatment.

Mucosal Immunity and the Gut-Microbiota-Brain-Axis in Neuroimmune Disease

Recent advances in next-generation sequencing (NGS) technologies have opened the door to a wellspring of information regarding the composition of the gut microbiota. Leveraging NGS technology, early metagenomic studies revealed that several diseases, such as Alzheimer's disease, Parkinson's disease, autism, and myalgic encephalomyelitis, are characterized by alterations in the diversity of gut-associated microbes. More recently, interest has shifted toward understanding how these microbes impact their host, with a special emphasis on their interactions with the brain. Such interactions typically occur either systemically, through the production of small molecules in the gut that are released into circulation, or through signaling via the vagus nerves which directly connect the enteric nervous system to the central nervous system. Collectively, this system of communication is now commonly referred to as the gut-microbiota-brain axis. While equally important, little attention has focused on the causes of the alterations in the composition of gut microbiota. Although several factors can contribute, mucosal immunity plays a significant role in shaping the microbiota in both healthy individuals and in association with several diseases. The purpose of this review is to provide a brief overview of the components of mucosal immunity that impact the gut microbiota and then discuss how altered immunological conditions may shape the gut microbiota and consequently affect neuroimmune diseases, using a select group of common neuroimmune diseases as examples.

Macrophages in intestinal fibrosis and regression

Intestinal macrophages are heterogenous cell populations with different developmental ontogeny and tissue anatomy. The concerted actions of intestinal macrophage subsets are critical to maintaining tissue homeostasis. However, the dysregulation of macrophages following tissue injury or chronic inflammation could also lead to intestinal fibrosis, with few treatment options in the clinic. In this review, we will characterize the features of intestinal macrophages in light of the latest advances in lineage tracing and single-cell sequencing technology. The roles of macrophages in distinct stages of intestinal fibrosis would be also elaborated. Finally, based on the reciprocal interaction between macrophages and intestinal fibrosis, we will propose the potential macrophage targeting anti-intestinal fibrosis therapies.

Crosstalk between epithelium, myeloid and innate lymphoid cells during gut homeostasis and disease

The gut epithelium not only provides a physical barrier to separate a noxious outside from a sterile inside but also allows for highly regulated interactions between bacteria and their products, and components of the immune system. Homeostatic maintenance of an intact epithelial barrier is paramount to health, requiring an intricately regulated and highly adaptive response of various cells of the immune system. Prolonged homeostatic imbalance can result in chronic inflammation, tumorigenesis and inefficient antitumor immune control. Here we provide an update on the role of innate lymphoid cells, macrophages and dendritic cells, which collectively play a critical role in epithelial barrier maintenance and provide an important linkage between the classical innate and adaptive arm of the immune system. These interactions modify the capacity of the gut epithelium to undergo continuous renewal, safeguard against tumor formation and provide feedback to the gut microbiome, which acts as a seminal contributor to cellular homeostasis of the gut.

Tissue Immunity in the Bladder

The bladder is a major component of the urinary tract, an organ system that expels metabolic waste and excess water, which necessitates proximity to the external environment and its pathogens. It also houses a commensal microbiome. Therefore, its tissue immunity must resist pathogen invasion while maintaining tolerance to commensals. Bacterial infection of the bladder is common, with half of women globally experiencing one or more episodes of cystitis in their lifetime. Despite this, our knowledge of bladder immunity, particularly in humans, is incomplete. Here we consider the current view of tissue immunity in the bladder, with a focus on defense against infection. The urothelium has robust immune functionality, and its defensive capabilities are supported by resident immune cells, including macrophages, dendritic cells, natural killer cells, and γδ T cells. We discuss each in turn and consider why adaptive immune responses are often ineffective in preventing recurrent infection, as well as areas of priority for future research.

Macrophages play a role in inflammatory transformation of colorectal cancer

Colorectal cancer (CRC) is one of the most common and fatal cancers worldwide, and it is also a typical inflammatory cancer. The function of macrophages is very important in the tissue immune microenvironment during inflammatory and carcinogenic transformation. Here, we evaluated the function and mechanism of macrophages in intestinal physiology and in different pathological stages. Furthermore, the role of macrophages in the immune microenvironment of CRC and the influence of the intestinal population and hypoxic environment on macrophage function are summarized. In addition, in the era of tumor immunotherapy, CRC currently has a limited response rate to immune checkpoint inhibitors, and we summarize potential therapeutic strategies for targeting tumor-associated macrophages.

Regional specialization of macrophages along the gastrointestinal tract

The tissue microenvironment is a major driver in imprinting tissue-specific macrophage functions in various mammalian tissues. As monocytes are recruited into the gastrointestinal (GI) tract at steady state and inflammation, they rapidly adopt a tissue-specific and distinct transcriptome. However, the GI tract varies significantly along its length, yet most studies of intestinal macrophages do not directly compare the phenotype and function of these macrophages in the small and large intestine, thus leading to disparities in data interpretations. This review highlights differences along the GI tract that are likely to influence macrophage function, with a specific focus on diet and microbiota. This analysis may fuel further investigation regarding the interplay between the intestinal immune system and GI tissue microenvironments, ideally providing unique therapeutic targets to modulate specific intestinal macrophage populations and/or functions.

NLRP3 at the crossroads between immune/inflammatory responses and enteric neuroplastic remodelling in a mouse model of diet-induced obesity

BACKGROUND AND PURPOSE

Enteric neurogenic/inflammation contributes to bowel dysmotility in obesity. We examined the role of NLRP3 in colonic neuromuscular dysfunctions in mice with high-fat diet (HFD)-induced obesity.

EXPERIMENTAL APPROACH

Wild-type C57BL/6J and NLRP3-KO (Nlrp3^-/- ) mice were fed with HFD or standard diet for 8 weeks. The activation of inflammasome pathways in colonic tissues from obese mice was assessed. The role of NLRP3 in in vivo colonic transit and in vitro tachykininergic contractions and substance P distribution was evaluated. The effect of substance P on NLRP3 signalling was tested in cultured cells.

KEY RESULTS

HFD mice displayed increased body and epididymal fat weight, cholesterol levels, plasma resistin levels and plasma and colonic IL-1β levels, colonic inflammasome adaptor protein apoptosis-associated speck-like protein containing caspase-recruitment domain (ASC) and caspase-1 mRNA expression and ASC immunopositivity in macrophages. Colonic tachykininergic contractions were enhanced in HFD mice. HFD NLRP3^-/- mice developed lower increase in body and epididymal fat weight, cholesterol levels, systemic and bowel inflammation. In HFD Nlrp3^-/- mice, the functional alterations of tachykinergic pathways and faecal output were normalized. In THP-1 cells, substance P promoted IL-1β release. This effect was inhibited upon incubation with caspase-1 inhibitor or NK₁ antagonist and not observed in ASC^-/- cells.

CONCLUSION AND IMPLICATIONS

In obesity, NLRP3 regulates an interplay between the shaping of enteric immune/inflammatory responses and the activation of substance P/NK₁ pathways underlying the onset of colonic dysmotility. Identifying NLRP3 as a therapeutic target for the treatment of bowel symptoms related to obesity.

From Species to Regional and Local Specialization of Intestinal Macrophages

Initially intended for nutrient uptake, phagocytosis represents a central mechanism of debris removal and host defense against invading pathogens through the entire animal kingdom. In vertebrates and also many invertebrates, macrophages (MFs) and MF-like cells (e.g., coelomocytes and hemocytes) are professional phagocytic cells that seed tissues to maintain homeostasis through pathogen killing, efferocytosis and tissue shaping, repair, and remodeling. Some MF functions are common to all species and tissues, whereas others are specific to their homing tissue. Indeed, shaped by their microenvironment, MFs become adapted to perform particular functions, highlighting their great plasticity and giving rise to high population diversity. Interestingly, the gut displays several anatomic and functional compartments with large pools of strikingly diversified MF populations. This review focuses on recent advances on intestinal MFs in several species, which have allowed to infer their specificity and functions.

Neural control of gut homeostasis

The gut-brain axis is a coordinated communication system that not only maintains homeostasis, but significantly influences higher cognitive functions and emotions, as well as neurological and behavioral disorders. Among the large populations of sensory and motor neurons that innervate the gut, insights into the function of primary afferent nociceptors, whose cell bodies reside in the dorsal root ganglia and nodose ganglia, have revealed their multiple crosstalk with several cell types within the gut wall, including epithelial, vascular, and immune cells. These bidirectional communications have immunoregulatory functions, control host response to pathogens, and modulate sensations associated with gastrointestinal disorders, through activation of immune cells and glia in the peripheral and central nervous system, respectively. Here, we will review the cellular and neurochemical basis of these interactions at the periphery, in dorsal root ganglia, and in the spinal cord. We will discuss the research gaps that should be addressed to get a better understanding of the multifunctional role of sensory neurons in maintaining gut homeostasis and regulating visceral sensitivity.

The equine mononuclear phagocyte system: The relevance of the horse as a model for understanding human innate immunity

The mononuclear phagocyte system (MPS) is a family of cells of related function that includes bone marrow progenitors, blood monocytes and resident tissue macrophages. Macrophages are effector cells in both innate and acquired immunity. They are a major resident cell population in every organ and their numbers increase in response to proinflammatory stimuli. Their function is highly regulated by a wide range of agonists, including lymphokines, cytokines and products of microorganisms. Macrophage biology has been studied most extensively in mice, yet direct comparisons of rodent and human macrophages have revealed many functional differences. In this review, we provide an overview of the equine MPS, describing the variation in the function and phenotype of macrophages depending on their location and the similarities and differences between the rodent, human and equine immune response. We discuss the use of the horse as a large animal model in which to study macrophage biology and pathological processes shared with humans. Finally, following the recent update to the horse genome, facilitating further comparative analysis of regulated gene expression between the species, we highlight the importance of future transcriptomic macrophage studies in the horse, the findings of which may also be applicable to human as well as veterinary research.

At the Forefront of the Mucosal Barrier: The Role of Macrophages in the Intestine

Macrophages are part of the innate immunity and are key players for the maintenance of intestinal homeostasis. They belong to the group of mononuclear phagocytes, which exert bactericidal functions and help to clear apoptotic cells. Moreover, they play essential roles for the maintenance of epithelial integrity and tissue remodeling during wound healing processes and might be implicated in intestinal tumor development. Macrophages are antigen-presenting cells and secrete immune-modulatory factors, like chemokines and cytokines, which are necessary to activate other intestinal immune cells and therefore to shape immune responses in the gut. However, overwhelming activation or increased secretion of pro-inflammatory cytokines might also contribute to the pathogenesis of inflammatory bowel disease. Presently, intestinal macrophages are in the center of intense studies, which might help to develop new therapeutic strategies to counteract the development or treat already existing inflammatory diseases in the gut. In this review, we focus on the origin of intestinal macrophages and, based on current knowledge, discuss their role in the gut during homeostasis and inflammation, as well as during intestinal wound healing and tumor development.

Intestinal resident macrophages: Multitaskers of the gut

BACKGROUND

Intestinal resident macrophages play a crucial role in homeostasis and have been implicated in numerous gastrointestinal diseases. While historically believed to be largely of hematopoietic origin, recent advances in fate-mapping technology have unveiled the existence of long-lived, self-maintaining populations located in specific niches throughout the gut wall. Furthermore, the advent of single-cell technology has enabled an unprecedented characterization of the functional specialization of tissue-resident macrophages throughout the gastrointestinal tract.

PURPOSE

The purpose of this review was to provide a panorama on intestinal resident macrophages, with particular focus to the recent advances in the field. Here, we discuss the functions and phenotype of intestinal resident macrophages and, where possible, the functional specialization of these cells in response to the niche they occupy. Furthermore, we will discuss their role in gastrointestinal diseases.

Ninjurin1 deficiency aggravates colitis development by promoting M1 macrophage polarization and inducing microbial imbalance

Disruption of colonic homeostasis caused by aberrant M1/M2 macrophage polarization and dysbiosis contributes to inflammatory bowel disease (IBD) pathogenesis. However, the molecular factors mediating colonic homeostasis are not well characterized. Here, we found that Ninjurin1 (Ninj1) limits colon inflammation by regulating macrophage polarization and microbiota composition under homeostatic conditions and during colitis development. Ninj1 deletion in mice induced hypersusceptibility to colitis, with increased prevalence of colitogenic Prevotellaceae strains and decreased immunoregulatory Lachnospiraceae strains. Upon co-housing (CoH) with WT mice, Ninj1^-/- mice showed increased Lachnospiraceae and decreased Prevotellaceae abundance, with subsequent improvement of colitis. Under homeostatic conditions, M1 macrophage frequency was higher in the Ninj1^-/- mouse colons than wild-type (WT) mouse colons, which may contribute to increased basal colonic inflammation and microbial imbalance. Following colitis induction, Ninj1 expression was increased in macrophages; meanwhile Ninj1^-/- mice showed severe colitis development and impaired recovery, associated with decreased M2 macrophages and escalated microbial imbalance. In vitro, Ninj1 knockdown in mouse and human macrophages activated M1 polarization and restricted M2 polarization. Finally, the transfer of WT macrophages ameliorated severe colitis in Ninj1^-/- mice. These findings suggest that Ninj1 mediates colonic homeostasis by modulating M1/M2 macrophage balance and preventing extensive dysbiosis, with implications for IBD prevention and therapy.

In-Vitro Cell Culture for Efficient Assessment of Mycotoxin Exposure, Toxicity and Risk Mitigation

Mycotoxins are toxic secondary fungal metabolites that commonly contaminate crops and food by-products and thus, animal feed. Ingestion of mycotoxins can lead to mycotoxicosis in both animals and humans, and at subclinical concentrations may affect animal production and adulterate feed and animal by-products. Mycotoxicity mechanisms of action (MOA) are largely unknown, and co-contamination, which is often the case, raises the likelihood of mycotoxin interactions. Mitigation strategies for reducing the risk of mycotoxicity are diverse and may not necessarily provide protection against all mycotoxins. These factors, as well as the species-specific risk of toxicity, collectively make an assessment of exposure, toxicity, and risk mitigation very challenging and costly; thus, in-vitro cell culture models provide a useful tool for their initial assessment. Since ingestion is the most common route of mycotoxin exposure, the intestinal epithelial barrier comprised of epithelial cells (IECs) and immune cells such as macrophages, represents ground zero where mycotoxins are absorbed, biotransformed, and elicit toxicity. This article aims to review different in-vitro IEC or co-culture models that can be used for assessing mycotoxin exposure, toxicity, and risk mitigation, and their suitability and limitations for the safety assessment of animal foods and food by-products.

The Macrophages and Intestinal Symbiosis

The human intestinal tract is inhabited by trillions of microorganisms and houses the largest pool of macrophages in the human body. Being a part of the innate immune system, the macrophages, the professional phagocytes, vigorously respond to the microbial and dietary antigens present in the intestine. Because such a robust immune response poses the danger to the survival of the non-harmful and beneficial gut microbiota, the macrophages developed mechanisms of recognition and hyposensitivity toward the non-harmful/beneficial inhabitants of the gut. We will discuss the evolution and identity of some of these mechanisms in the following chapter.

Spatial proteomics revealed a CX3CL1-dependent crosstalk between the urothelium and relocated macrophages through IL-6 during an acute bacterial infection in the urinary bladder

The urothelium of the urinary bladder represents the first line of defense. However, uropathogenic E. coli (UPEC) damage the urothelium and cause acute bacterial infection. Here, we demonstrate the crosstalk between macrophages and the urothelium stimulating macrophage migration into the urothelium. Using spatial proteomics by MALDI-MSI and LC-MS/MS, a novel algorithm revealed the spatial activation and migration of macrophages. Analysis of the spatial proteome unravelled the coexpression of Myo9b and F4/80 in the infected urothelium, indicating that macrophages have entered the urothelium upon infection. Immunofluorescence microscopy additionally indicated that intraurothelial macrophages phagocytosed UPEC and eliminated neutrophils. Further analysis of the spatial proteome by MALDI-MSI showed strong expression of IL-6 in the urothelium and local inhibition of this molecule reduced macrophage migration into the urothelium and aggravated the infection. After IL-6 inhibition, the expression of matrix metalloproteinases and chemokines, such as CX₃CL1 was reduced in the urothelium. Accordingly, macrophage migration into the urothelium was diminished in the absence of CX₃CL1 signaling in Cx₃cr1^gfp/gfp mice. Conclusively, this study describes the crosstalk between the infected urothelium and macrophages through IL-6-induced CX₃CL1 expression. Such crosstalk facilitates the relocation of macrophages into the urothelium and reduces bacterial burden in the urinary bladder.

Tenophages: a novel macrophage-like tendon cell population expressing CX3CL1 and CX3CR1

Tendon disorders frequently occur and recent evidence has clearly implicated the presence of immune cells and inflammatory events during early tendinopathy. However, the origin and properties of these cells remain poorly defined. Therefore, the aim of this study was to determine the presence of cells in healthy rodent and human tendon tissue fulfilling macrophage-like functions. Using various transgenic reporter mouse models, we demonstrate the presence of tendon-resident cells in the dense matrix of the tendon core expressing the fractalkine (Fkn) receptor CX3CR1 and its cognate ligand CX3CL1/Fkn. Pro-inflammatory stimulation of 3D tendon-like constructs in vitro resulted in a significant increase in the expression of IL-1β, IL-6, Mmp3, Mmp9, CX3CL1 and epiregulin, which has been reported to contribute to inflammation, wound healing and tissue repair. Furthermore, we demonstrate that inhibition of the Fkn receptor blocked tendon cell migration in vitro, and show the presence of CX3CL1/CX3CR1/EREG-expressing cells in healthy human tendons. Taken together, we demonstrate the presence of CX3CL1⁺/CX3CR1⁺ 'tenophages' within the healthy tendon proper, which potentially fulfill surveillance functions in tendons.This article has an associated First Person interview with the first author of the paper.

Developmental Stage-Specific Distribution of Macrophages in Mouse Mammary Gland

Mammary gland development begins in the embryo and continues throughout the reproductive life of female mammals. Tissue macrophages (Mϕs), dependent on signals from the Mϕ colony stimulating factor 1 receptor (CSF1R), have been shown to regulate the generation, regression and regeneration of this organ, which is central for mammalian offspring survival. However, the distribution of Mϕs in the pre- and post-natal mammary gland, as it undergoes distinct phases of development and regression, is unknown or has been inferred from immunostaining of thin tissue sections. Here, we used optical tissue clearing and 3-dimensional imaging of mammary tissue obtained from Csf1r-EGFP mice. Whilst tissue Mϕs were observed at all developmental phases, their abundance, morphology, localization and association with luminal and basal epithelial cells exhibited stage-specific differences. Furthermore, sexual dimorphism was observed at E14.5, when the male mammary bud is severed from the overlying epidermis. These findings provide new insights into the localization and possible functions of heterogeneous tissue Mϕ populations in mammogenesis.

Eosinophils and Macrophages within the Th2-Induced Granuloma: Balancing Killing and Healing in a Tight Space

Granuloma formation is a key host immune response generated to confine invading pathogens and limit extensive host damage. It consists of an accumulation of host immune cells around a pathogen. This host response has been extensively studied in the context of inflammatory diseases. However, there is much less known about Th2-type granulomas generated in response to parasitic worms. Based on in vitro data, innate immune cells within the granuloma are thought to immobilize and kill parasites but also act to repair damaged tissue. Understanding this dual function is key. The two billion people and many livestock/wild animals infected with helminths demonstrate that granulomas are not effective at clearing infection. However, the lack of high mortality highlights their importance in ensuring that parasite migration/tissue damage is restricted and wound healing is effective. In this review, we define two key cellular players (macrophages and eosinophils) and their associated molecular players involved in Th2 granuloma function. To date, the underlying mechanisms remain poorly understood, which is in part due to a lack of conclusive studies. Most have been performed in vitro rather than in vivo, using cells that have not been obtained from granulomas. Experiments using genetically modified mouse strains and/or antibody/chemical-mediated cell depletion have also generated conflicting results depending on the model. We discuss the caveats of previous studies and the new tools available that will help fill the gaps in our knowledge and allow a better understanding of the balance between immune killing and healing.

Long-term mucosal injury and repair in a murine model of pelvic radiotherapy

Chronic intestinal injury after pelvic radiotherapy affects countless cancer survivors worldwide. A comprehensive understanding of the long-term injury dynamics is prevented in available animal models. With linear accelerators that are used to treat cancer in patients, we irradiated a small volume encompassing the colorectum in mice with four fractions of 8 Gy per fraction. We then determined the long-term dynamics of mucosal injury, repair, and the duration of inflammation. We show that crypt fission, not cell proliferation, is the main long-term mechanism for rescuing crypt density after irradiation, and provides a potentially wide window for clinical interventions. Persisting macrophage aggregations indicate a chronic mucosal inflammation. A better understanding as to how crypt fission is triggered and why it fails to repair fully the mucosa may help restore bowel health after pelvic radiotherapy. Moreover, anti-inflammatory interventions, even if implemented long after completed radiotherapy, could promote bowel health in pelvic cancer survivors.

Mumps virus infection disrupts blood-testis barrier through the induction of TNF-α in Sertoli cells

Mumps virus (MuV) has high tropism to the testis and may lead to male infertility. Sertoli cells are the major targets of MuV infection. However, the mechanisms by which MuV infection impairs male fertility and Sertoli cell function remain unclear. The present study elucidated the effect of MuV infection on the blood-testis barrier (BTB). The transepithelial electrical resistance of MuV-infected mouse Sertoli cells was monitored, and the expression of major proteins of the BTB was examined. We demonstrated that MuV infection disrupted the BTB by reducing the levels of occludin and zonula occludens 1. Sertoli cells derived from Tlr2^-/- and Tnfa^-/- mice were analyzed for mediating MuV-induced impairment. TLR2-mediated TNF-α production by Sertoli cells in response to MuV infection impaired BTB integrity. MuV-impaired BTB was not observed in Tlr2^-/- and Tnfa^-/- Sertoli cells. Moreover, an inhibitor of TNF-α, pomalidomide, prevents the disruption of BTB in response to MuV infection. FITC-labeled biotin tracing assay confirmed that BTB permeability and spermatogenesis were transiently impaired by MuV infection in vivo. These findings suggest that the disruption of the BTB could be one of the mechanisms underlying MuV-impaired male fertility, in which TNF-α could play a critical role.-Wu, H., Jiang, X., Gao, Y., Liu, W., Wang, F., Gong, M., Chen, R., Yu, X., Zhang, W., Gao, B., Song, C., Han, D. Mumps virus infection disrupts blood-testis barrier through the induction of TNF-α in Sertoli cells.

A three-dimensional immunocompetent intestine-on-chip model as in vitro platform for functional and microbial interaction studies

Alterations of the microbial composition in the gut and the concomitant dysregulation of the mucosal immune response are associated with the pathogenesis of opportunistic infections, chronic inflammation, and inflammatory bowel disease. To create a platform for the investigation of the underlying mechanisms, we established a three-dimensional microphysiological model of the human intestine. This model resembles organotypic microanatomical structures and includes tissue resident innate immune cells exhibiting features of mucosal macrophages and dendritic cells. The model displays the physiological immune tolerance of the intestinal lumen to microbial-associated molecular patterns and can, therefore, be colonised with living microorganisms. Functional studies on microbial interaction between probiotic Lactobacillus rhamnosus and the opportunistic pathogen Candida albicans show that pre-colonization of the intestinal lumen of the model by L. rhamnosus reduces C. albicans-induced tissue damage, lowers its translocation, and limits fungal burden. We demonstrate that microbial interactions can be efficiently investigated using the in vitro model creating a more physiological and immunocompetent microenvironment. The intestinal model allows a detailed characterisation of the immune response, microbial pathogenicity mechanisms, and quantification of cellular dysfunction attributed to alterations in the microbial composition.

Origin, Differentiation, and Function of Intestinal Macrophages

Macrophages are increasingly recognized as essential players in the maintenance of intestinal homeostasis and as key sentinels of the intestinal immune system. However, somewhat paradoxically, they are also implicated in chronic pathologies of the gastrointestinal tract, such as inflammatory bowel disease (IBD) and are therefore considered potential targets for novel therapies. In this review, we will discuss recent advances in our understanding of intestinal macrophage heterogeneity, their ontogeny and the potential factors that regulate their origin. We will describe how the local environment of the intestine imprints the phenotypic and functional identity of the macrophage compartment, and how this changes during intestinal inflammation and infection. Finally, we highlight key outstanding questions that should be the focus of future research.

Tributyrin Supplementation Protects Immune Responses and Vasculature and Reduces Oxidative Stress in the Proximal Colon of Mice Exposed to Chronic-Binge Ethanol Feeding

Excessive ethanol consumption causes adverse effects and contributes to organ dysfunction. Ethanol metabolism triggers oxidative stress, altered immune function, and gut dysbiosis. The gut microbiome is known to contribute to the maintenance of intestinal homeostasis, and disturbances are associated with pathology. A consequence of gut dysbiosis is also alterations in its metabolic and fermentation byproducts. The gut microbiota ferments undigested dietary polysaccharides to yield short-chain fatty acids, predominantly acetate, propionate, and butyrate. Butyrate has many biological mechanisms of action including anti-inflammatory and immunoprotective effects, and its depletion is associated with intestinal injury. We previously showed that butyrate protects gut-liver injury during ethanol exposure. While the intestine is the largest immune organ in the body, little is known regarding the effects of ethanol on intestinal immune function. This work is aimed at investigating the effects of butyrate supplementation, in the form of the structured triglyceride tributyrin, on intestinal innate immune responses and oxidative stress following chronic-binge ethanol exposure in mice. Our work suggests that tributyrin supplementation preserved immune responses and reduced oxidative stress in the proximal colon during chronic-binge ethanol exposure. Our results also indicate a possible involvement of tributyrin in maintaining the integrity of intestinal villi vasculature disrupted by chronic-binge ethanol exposure.

Autocrine GABA signaling distinctively regulates phenotypic activation of mouse pulmonary macrophages

In response to micro-environmental cues such as microbial infections or T-helper 1 and 2 (T_H1 and T_H2) cytokines, macrophages (Mϕs) develop into M1- or M2-like phenotypes. Phenotypic polarization/activation of Mϕs are also essentially regulated by autocrine signals. Type-A γ-aminobutyric acid receptor (GABA_AR)-mediated autocrine signaling is critical for phenotypic differentiation and transformation of various cell types. The present study explored whether GABA_AR signaling regulates lung Mϕ (LMϕ) phenotypic activation under M1/T_H1 and M2/T_H2 environments. Results showed that GABA_AR subunits were expressed by primary LMϕ of mice and the mouse Mϕ cell line RAW264.7. The expression levels of GABA_AR subunits in mouse LMϕs and RAW264.7 cells decreased or increased concurrently with classical (M1) or alternative (M2) activation, respectively. Moreover, activation or blockade of GABA_ARs distinctively influenced the phenotypic characteristics of Mϕ. These results suggested that microenvironments leading to LMϕ phenotypic polarization concurrently modulates autocrine GABA signaling and its role in Mϕ activation.

NAD(P)H Oxidase Activity in the Small Intestine Is Predominantly Found in Enterocytes, Not Professional Phagocytes

The balance between various cellular subsets of the innate and adaptive immune system and microbiota in the gastrointestinal tract is carefully regulated to maintain tolerance to the normal flora and dietary antigens, while protecting against pathogens. The intestinal epithelial cells and the network of dendritic cells and macrophages in the lamina propria are crucial lines of defense that regulate this balance. The complex relationship between the myeloid compartment (dendritic cells and macrophages) and lymphocyte compartment (T cells and innate lymphoid cells), as well as the impact of the epithelial cell layer have been studied in depth in recent years, revealing that the regulatory and effector functions of both innate and adaptive immune compartments exhibit more plasticity than had been previously appreciated. However, little is known about the metabolic activity of these cellular compartments, which is the basic function underlying all other additional tasks the cells perform. Here we perform intravital NAD(P)H fluorescence lifetime imaging in the small intestine of fluorescent reporter mice to monitor the NAD(P)H-dependent metabolism of epithelial and myeloid cells. The majority of myeloid cells which comprise the surveilling network in the lamina propria have a low metabolic activity and remain resting even upon stimulation. Only a few myeloid cells, typically localized at the tip of the villi, are metabolically active and are able to activate NADPH oxidases upon stimulation, leading to an oxidative burst. In contrast, the epithelial cells are metabolically highly active and, although not considered professional phagocytes, are also able to activate NADPH oxidases, leading to massive production of reactive oxygen species. Whereas the oxidative burst in myeloid cells is mainly catalyzed by the NOX2 isotype, in epithelial cells other isotypes of the NADPH oxidases family are involved, especially NOX4. They are constitutively expressed by the epithelial cells, but activated only on demand to ensure rapid defense against pathogens. This minimizes the potential for inadvertent damage from resting NOX activation, while maintaining the capacity to respond quickly if needed.

Adult hepatocytes direct liver organogenesis through non-parenchymal cell recruitment in the kidney

BACKGROUND & AIMS

Since the first account of the myth of Prometheus, the amazing regenerative capacity of the liver has fascinated researchers because of its enormous medical potential. Liver regeneration is promoted by multiple types of liver cells, including hepatocytes and liver non-parenchymal cells (NPCs), through complex intercellular signaling. However, the mechanism of liver organogenesis, especially the role of adult hepatocytes at ectopic sites, remains unknown. In this study, we demonstrate that hepatocytes alone spurred liver organogenesis to form an organ-sized complex 3D liver that exhibited native liver architecture and functions in the kidneys of mice.

METHODS

Isolated hepatocytes were transplanted under the kidney capsule of monocrotaline (MCT) and partial hepatectomy (PHx)-treated mice. To determine the origin of NPCs in neo-livers, hepatocytes were transplanted into MCT/PHx-treated green fluorescent protein transgenic mice or wild-type mice transplanted with bone marrow cells isolated from green fluorescent protein-mice.

RESULTS

Hepatocytes engrafted at the subrenal space of mice underwent continuous growth in response to a chronic hepatic injury in the native liver. More than 1.5 years later, whole organ-sized liver tissues with greater mass than those of the injured native liver had formed. Most remarkably, we revealed that at least three types of NPCs with similar phenotypic features to the liver NPCs were recruited from the host tissues including bone marrow. The neo-livers in the kidney exhibited liver-specific functions and architectures, including sinusoidal vascular systems, zonal heterogeneity, and emergence of bile duct cells. Furthermore, the neo-livers successfully rescued the mice with lethal liver injury.

CONCLUSION

Our data clearly show that adult hepatocytes play a leading role as organizer cells in liver organogenesis at ectopic sites via NPC recruitment.

LAY SUMMARY

The role of adult hepatocytes at ectopic locations has not been clarified. In this study, we demonstrated that engrafted hepatocytes in the kidney proliferated, recruited non-parenchymal cells from host tissues including bone marrow, and finally created an organ-sized, complex liver system that exhibited liver-specific architectures and functions. Our results revealed previously undescribed functions of hepatocytes to direct liver organogenesis through non-parenchymal cell recruitment and organize multiple cell types into a complex 3D liver at ectopic sites. Transcript profiling: Microarray data are deposited in GEO (GEO accession: GSE99141).

Macrophage-Mediated Glial Cell Elimination in the Postnatal Mouse Cochlea

Hearing relies on the transmission of auditory information from sensory hair cells (HCs) to the brain through the auditory nerve. This relay of information requires HCs to be innervated by spiral ganglion neurons (SGNs) in an exclusive manner and SGNs to be ensheathed by myelinating and non-myelinating glial cells. In the developing auditory nerve, mistargeted SGN axons are retracted or pruned and excessive cells are cleared in a process referred to as nerve refinement. Whether auditory glial cells are eliminated during auditory nerve refinement is unknown. Using early postnatal mice of either sex, we show that glial cell numbers decrease after the first postnatal week, corresponding temporally with nerve refinement in the developing auditory nerve. Additionally, expression of immune-related genes was upregulated and macrophage numbers increase in a manner coinciding with the reduction of glial cell numbers. Transient depletion of macrophages during early auditory nerve development, using transgenic CD11bDTR/EGFP mice, resulted in the appearance of excessive glial cells. Macrophage depletion caused abnormalities in myelin formation and transient edema of the stria vascularis. Macrophage-depleted mice also showed auditory function impairment that partially recovered in adulthood. These findings demonstrate that macrophages contribute to the regulation of glial cell number during postnatal development of the cochlea and that glial cells play a critical role in hearing onset and auditory nerve maturation.

C-X-C motif chemokine ligand 10 produced by mouse Sertoli cells in response to mumps virus infection induces male germ cell apoptosis

Mumps virus (MuV) infection usually results in germ cell degeneration in the testis, which is an etiological factor for male infertility. However, the mechanisms by which MuV infection damages male germ cells remain unclear. The present study showed that C-X-C motif chemokine ligand 10 (CXCL10) is produced by mouse Sertoli cells in response to MuV infection, which induces germ cell apoptosis through the activation of caspase-3. CXC chemokine receptor 3 (CXCR3), a functional receptor of CXCL10, is constitutively expressed in male germ cells. Neutralizing antibodies against CXCR3 and an inhibitor of caspase-3 activation significantly inhibited CXCL10-induced male germ cell apoptosis. Furthermore, the tumor necrosis factor-α (TNF-α) upregulated CXCL10 production in Sertoli cells after MuV infection. The knockout of either CXCL10 or TNF-α reduced germ cell apoptosis in the co-cultures of germ cells and Sertoli cells in response to MuV infection. Local injection of MuV into the testes of mice confirmed the involvement of CXCL10 in germ cell apoptosis in vivo. These results provide novel insights into MuV-induced germ cell apoptosis in the testis.

The Complement System Component C5a Produces Thermal Hyperalgesia via Macrophage-to-Nociceptor Signaling That Requires NGF and TRPV1

The complement cascade is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other components of the complement system in inflammatory and neuropathic pain, although the underlying mechanisms are largely unknown. In particular, it is unclear how the complement system communicates with nociceptors and which ion channels and receptors are involved. Here we demonstrate that inflammatory thermal and mechanical hyperalgesia induced by complete Freund's adjuvant was accompanied by C5a upregulation and was markedly reduced by C5a receptor (C5aR1) knock-out or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong thermal hyperalgesia, an effect that was absent in TRPV1 knock-out mice, and was blocked by the TRPV1 antagonist AMG9810. Immunohistochemistry of mouse plantar skin showed prominent expression of C5aR1 in macrophages. Additionally, C5a evoked strong Ca(2+) mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis mice abolished C5a-dependent thermal hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of NGF, a mediator known to sensitize TRPV1. Preinjection of an NGF-neutralizing antibody or Trk inhibitor GNF-5837 prevented C5a-induced thermal hyperalgesia. Notably, NGF-induced thermal hyperalgesia was unaffected by macrophage depletion. Collectively, these results suggest that complement fragment C5a induces thermal hyperalgesia by triggering macrophage-dependent signaling that involves mobilization of NGF and NGF-dependent sensitization of TRPV1. Our findings highlight the importance of macrophage-to-neuron signaling in pain processing and identify C5a, NGF, and TRPV1 as key players in this cross-cellular communication.

SIGNIFICANCE STATEMENT

This study provides mechanistic insight into how the complement system, a key component of innate immunity, regulates the development of pain hypersensitivity. We demonstrate a crucial role of the C5a receptor, C5aR1, in the development of inflammatory thermal and mechanical sensitization. By focusing on the mechanisms of C5a-induced thermal hyperalgesia, we show that this process requires recruitment of macrophages and initiation of macrophage-to-nociceptor signaling. At the molecular level, we demonstrate that this signaling depends on NGF and is mediated by the heat-sensitive nociceptive channel TRPV1. This deeper understanding of how immune cells and neurons interact to regulate pain processing is expected to facilitate mechanism-based approaches in the development of new analgesics.

Inhibition of Oxidative Stress in Renal Ischemia-Reperfusion Injury

BACKGROUND

Superoxide, nitric oxide (NO), and peroxynitrite are important mediators in the pathogenesis of ischemia-reperfusion (I/R) injury. We tested the renoprotective effects of allopurinol (ALP), a xanthine oxidase inhibitor, N-nitro-L-arginine methyl ester (L-NAME), and 5,10,15,20-tetrakis (N-methyl-4-pyridyl) porphyrinato iron (III) (FeTMPyP) by selective inhibition of superoxide, NO, and peroxynitrite, respectively.

METHODS

Male Sprague-Dawley rats were randomly assigned to 5 groups (n = 6 per group). Group 1 was a sham-operated group. Group 2 was the renal I/R group (30-minute ischemia followed by 24-hour reperfusion). Rats in groups 3, 4, and 5 received ALP, L-NAME, or FeTMPyP, respectively, at 5 minutes before the reperfusion. Serum creatinine (Cr), blood urea nitrogen (BUN), renal tissue malondialdehyde, superoxide dismutase, histological changes, apoptosis, and monocyte infiltration were evaluated. In addition, the combined treatment with ALP and L-NAME was compared with FeTMPyP in a second independent experiment.

RESULTS

The administration of ALP, L-NAME, and FeTMPyP diminished the increase in Cr (P = .0066 for all) and BUN (P = .0066 for ALP; and P = .013 for L-NAME) induced by I/R injury and decreased the histological damage (P = .0066 for all). In addition, ALP, L-NAME, and FeTMPyP attenuated the oxidative stress response as determined by a decrease in malondialdehyde level (P = .0066 for all), apoptotic renal tubular cells (P = .0066 for all), and monocyte infiltration (P = .0066 for all). The combined treatment of ALP and L-NAME decreased Cr and BUN levels to a greater degree than FeTMPyP (P = .016 for Cr; P = .0079 for BUN).

CONCLUSIONS

Superoxide, NO, and peroxynitrite are involved in renal I/R injury. The reduction of peroxynitrite formation, via inhibition of superoxide or NO, or the induction of peroxynitrite decomposition may be beneficial in renal I/R injury.

Diversity and functions of intestinal mononuclear phagocytes

The intestinal lamina propria (LP) contains a diverse array of mononuclear phagocyte (MNP) subsets, including conventional dendritic cells (cDC), monocytes and tissue-resident macrophages (mφ) that collectively play an essential role in mucosal homeostasis, infection and inflammation. In the current review we discuss the function of intestinal cDC and monocyte-derived MNP, highlighting how these subsets play several non-redundant roles in the regulation of intestinal immune responses. While much remains to be learnt, recent findings also underline how the various populations of MNP adapt to deal with the challenges specific to their environment. Understanding these processes should help target individual subsets for 'fine tuning' immunological responses within the intestine, a process that may be of relevance both for the treatment of inflammatory bowel disease (IBD) and for optimized vaccine design.

Tissue macrophages: heterogeneity and functions

Macrophages are present in all vertebrate tissues, from mid-gestation throughout life, constituting a widely dispersed organ system. They promote homeostasis by responding to internal and external changes within the body, not only as phagocytes in defence against microbes and in clearance of dead and senescent cells, but also through trophic, regulatory and repair functions. In this review, we describe macrophage phenotypic heterogeneity in different tissue environments, drawing particular attention to organ-specific functions.

Macrophages in gastrointestinal homeostasis and inflammation

Monocyte-derived mononuclear phagocytes, particularly macrophages, are crucial to maintain gastrointestinal homeostasis in the steady state but are also important for protection against certain pathogens. However, when uncontrolled, they can promote immunopathology. Broadly two subsets of macrophages can be considered to perform the vast array of functions to complete these complex tasks: resident macrophages that dominate in the healthy gut and inflammation-elicited (inflammatory) macrophages that derive from circulating monocytes infiltrating inflamed tissue. Here, we discuss the features of resident and inflammatory intestinal macrophages, complexities in identifying and defining these populations and the mechanisms involved in their differentiation. In particular, focus will be placed on describing their unique ontogeny as well as local gastrointestinal signals that instruct specialisation of resident macrophages in healthy tissue. We then explore the very different roles of inflammatory macrophages and describe new data suggesting that they may be educated not only by the gut microenvironment but also by signals they receive during development in the bone marrow. Given the high degree of plasticity of gut macrophages and their multifaceted roles in both healthy and inflamed tissue, understanding the mechanisms controlling their differentiation could inform development of improved therapies for inflammatory diseases such as inflammatory bowel disease (IBD).

Mouse Testicular Cell Type-Specific Antiviral Response against Mumps Virus Replication

Mumps virus (MuV) infection has high tropism to the testis and usually leads to orchitis, an etiological factor in male infertility. However, MuV replication in testicular cells and the cellular antiviral responses against MuV are not fully understood. The present study showed that MuV infected the majority of testicular cells, including Leydig cells (LC), testicular macrophages, Sertoli cells (SC), and male germ cells (GC). MuV was replicated at relatively high efficiencies in SC compared with LC and testicular macrophages. In contrast, MuV did not replicate in male GC. Notably, testicular cells exhibited different innate antiviral responses against MuV replication. We showed that interferon β (IFN-β) inhibited MuV replication in LC, macrophages, and SC, which were associated with the upregulation of major antiviral proteins. We provided primary evidence that autophagy plays a role in blocking MuV replication in male GC. Autophagy was also involved in limiting MuV replication in testicular macrophages but not in Leydig and SC. These findings indicate the involvement of the innate defense against MuV replication in testicular cells.

Intestinal Autophagy and Its Pharmacological Control in Inflammatory Bowel Disease

Intestinal mucosal barrier, mainly composed of the intestinal mucus layer and the epithelium, plays a critical role in nutrient absorption as well as protection from pathogenic microorganisms. It is widely acknowledged that the damage of intestinal mucosal barrier or the disturbance of microorganism balance in the intestinal tract contributes greatly to the pathogenesis and progression of inflammatory bowel disease (IBD), which mainly includes Crohn’s disease and ulcerative colitis. Autophagy is an evolutionarily conserved catabolic process that involves degradation of protein aggregates and damaged organelles for recycling. The roles of autophagy in the pathogenesis and progression of IBD have been increasingly studied. This present review mainly describes the roles of autophagy of Paneth cells, macrophages, and goblet cells in IBD, and finally, several potential therapeutic strategies for IBD taking advantage of autophagy.

F4/80 as a Major Macrophage Marker: The Case of the Peritoneum and Spleen

Tissue macrophages are a heterogeneous cell population residing in all body tissues that contribute to the maintenance of homeostasis and trigger immune activation in response to injurious stimuli. This heterogeneity may be associated with tissue-specific functions; however, the presence of distinct macrophage populations within the same microenvironment indicates that macrophage heterogeneity may also be influenced outside of tissue specialization. The F4/80 molecule was established as a unique marker of murine macrophages when a monoclonal antibody was found to recognize an antigen exclusively expressed by these cells. However, recent research has shown that F4/80 is expressed by other immune cells and is not equivalently expressed across tissue-specific macrophage lineages, including those residing in the same microenvironment, such as the peritoneum and spleen. In this context, two murine macrophage subtypes with distinct F4/80 expression patterns were recently found to coexist in the peritoneum, termed large peritoneal macrophages (LPMs) and small peritoneal macrophages (SPMs). However, the presence of phenotypic and functional heterogeneous macrophage subpopulations in the spleen was already known. Thus, although F4/80 surface expression continues to be the best method to identify tissue macrophages, additional molecules must also be examined to distinguish these cells from other immune cells.

Polyinosinic-polycytidylic acid inhibits the differentiation of mouse preadipocytes through pattern recognition receptor-mediated secretion of tumor necrosis factor-α

Viral infections can disturb the functions of adipose tissues and thus result in metabolic diseases. Polyinosinic-polycytidylic acid (poly(I:C)), a synthetic analog of viral double-stranded RNA, induces innate antiviral responses by mimicking viral infection through the activation of pattern recognition receptors (PRRs) such as Toll-like receptor 3 (TLR3), retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). Poly(I:C) also inhibits the differentiation of mouse preadipocytes but the mechanism underlying this process remains unclear. In this study, poly(I:C) inhibited preadipocyte differentiation in a dose-dependent manner, but not in a time-dependent manner. Endogenously transfected poly(I:C) severely impaired the adipogenesis of preadipocytes compared with exogenously added poly(I:C). Low concentration of tumor necrosis factor-α (TNF-α) could effectively inhibit the preadipocyte differentiation. The effect of exogenously added poly(I:C) on inhibition of differentiation was significantly diminished in the preadipocytes of TLR3 knockout mice. By contrast, endogenously transfected poly(I:C) still inhibited the differentiation of TLR3-deficient preadipocytes. Hence, MDA5/RIG-I signaling was involved in the poly(I:C)-induced inhibition of preadipocyte differentiation. The effect of poly(I:C) stimulation, either through endogenous transfection or exogenous addition, on inhibition of differentiation was significantly diminished in the preadipocytes of TNF-α knockout mice. These results confirmed the evidence that poly(I:C) inhibited the differentiation of mouse preadipocytes through PRR-mediated secretion of TNF-α.

Intrinsic Gastrointestinal Macrophages: Their Phenotype and Role in Gastrointestinal Motility

There is an increasing awareness of the role of macrophages in the regulation and maintenance of gastrointestinal function in health and disease. This work has proceeded in the context of an increased understanding of the complex phenotypic variation in macrophages throughout the body and has revealed previously un-identified roles for macrophages in diseases like gastroparesis, post-operative ileus and inflammatory bowel disease. Opportunities for exploiting the phenotypic modulation of tissue resident macrophages have been identified as possible therapies for some of these diseases. In addition, macrophages are an established component of the innate immune system that can respond to variations and changes in the intestinal microbiome and potentially mediate part of the impact of the microbiota on intestinal health. We reviewed the latest work on novel concepts in defining macrophage phenotype, discuss possible mechanisms of action for tissue-resident macrophages in the gut, address the significance of microbiome effects on macrophage phenotype and review the known and possible roles of macrophages in motility disorders of the gastrointestinal tract.

Bardoxolone Methyl Prevents High-Fat Diet-Induced Colon Inflammation in Mice

Obesity induces chronic, low-grade inflammation, which increases the risk of colon cancer. We investigated the preventive effects of Bardoxolone methyl (BARD) on high-fat diet (HFD)-induced inflammation in a mouse colon. Male C57BL/6J mice (n=7) were fed a HFD (HFD group), HFD plus BARD (10 mg/kg) in drinking water (HFD/BARD group), or normal laboratory chow diet (LFD group) for 21 weeks. In HFD mice, BARD reduced colon thickness and decreased colon weight per length. This was associated with an increase in colon crypt depth and the number of goblet cells per crypt. BARD reduced the expression of F4/80 and CD11c but increased CD206 and IL-10, indicating an anti-inflammatory effect. BARD prevented an increase of the intracellular pro-inflammatory biomarkers (NF-қB, p NF-қB, IL-6, TNF-α) and cell proliferation markers (Cox2 and Ki67). BARD prevented fat deposition in the colon wall and prevented microbial population changes. Overall, we report the preventive effects of BARD on colon inflammation in HFD-fed mice through its regulation of macrophages, NF-қB, cytokines, Cox2 and Ki67, fat deposition and microflora.