Intestinal Macrophages in Resolving Inflammation

Intestinal barrier permeability: the influence of gut microbiota, nutrition, and exercise

The intestinal wall is a selectively permeable barrier between the content of the intestinal lumen and the internal environment of the body. Disturbances of intestinal wall permeability can potentially lead to unwanted activation of the enteric immune system due to excessive contact with gut microbiota and its components, and the development of endotoxemia, when the level of bacterial lipopolysaccharides increases in the blood, causing chronic low-intensity inflammation. In this review, the following aspects are covered: the structure of the intestinal wall barrier; the influence of the gut microbiota on the permeability of the intestinal wall via the regulation of functioning of tight junction proteins, synthesis/degradation of mucus and antioxidant effects; the molecular mechanisms of activation of the pro-inflammatory response caused by bacterial invasion through the TLR4-induced TIRAP/MyD88 and TRAM/TRIF signaling cascades; the influence of nutrition on intestinal permeability, and the influence of exercise with an emphasis on exercise-induced heat stress and hypoxia. Overall, this review provides some insight into how to prevent excessive intestinal barrier permeability and the associated inflammatory processes involved in many if not most pathologies. Some diets and physical exercise are supposed to be non-pharmacological approaches to maintain the integrity of intestinal barrier function and provide its efficient operation. However, at an early age, the increased intestinal permeability has a hormetic effect and contributes to the development of the immune system.

M1 and M2 Macrophages Differentially Regulate Colonic Crypt Renewal

BACKGROUND

The colonic epithelium is the most rapidly renewing tissue in the body and is organized into a single cell layer of invaginations called crypts. Crypt renewal occurs through Lgr5 + gut stem cells situated at the crypt base, which divide, produce daughter cells that proliferate, migrate, differentiate into all the cells required for normal gut function, and are finally shed into the crypt lumen. In health, this rapid renewal helps maintain barrier function next to the hostile gut microbial luminal environment. Inflammation results in an influx of immune cells including inflammatory M1 macrophages into the gut mucosa next to the crypt epithelium, but the direct effect of macrophages on crypt regeneration and renewal are poorly understood.

METHODS

Using an in vitro macrophage-crypt coculture model, we show that homeostatic M2 macrophages and inflammatory M1 macrophages confer different effects on the crypt epithelium.

RESULTS

Both M1 and M2 increase crypt cell proliferation, with M2 macrophages requiring physical contact with the crypt epithelium, whereas M1 macrophages exert their effect through a secreted factor. Only M1 macrophages reduce goblet and Tuft cell numbers and increase Lgr5 + crypt stem cell numbers, all dependent on physical contact with the crypt epithelium. Further studies showed that M1 macrophages increase the Wnt signaling pathways cyclin D1 and LEF1 through physical contact rather than a secreted factor.

CONCLUSIONS

These findings highlight the importance of understanding distinct cellular interactions and direct dialogue between cells and increase our understanding of the contribution of different immune cell subtypes on crypt cell biology during inflammation.

Bacteroides uniformis CECT 7771 requires adaptive immunity to improve glucose tolerance but not to prevent body weight gain in diet-induced obese mice

BACKGROUND

The metabolic disturbances of obesity can be mitigated by strategies modulating the gut microbiota. In this study, we sought to identify whether innate or adaptive immunity mediates the beneficial metabolic effects of the human intestinal bacterium Bacteroides uniformis CECT 7771 in obesity.

METHODS

We evaluated the effects of orally administered B. uniformis on energy homeostasis, intestinal immunity, hormone levels, and gut microbiota in wild-type and Rag1-deficient mice with diet-induced obesity. We also assessed whether B. uniformis needed to be viable to exert its beneficial effects in obesity and to directly induce immunoregulatory effects.

RESULTS

The administration of B. uniformis to obese mice improved glucose tolerance and insulin secretion, restored the caloric intake suppression after an oral glucose challenge, and reduced hyperglycemia. The pre- and post-prandial glucose-related benefits were associated with restoration of the anti-inflammatory tone mediated by type 2 macrophages and regulatory T cells (Tregs) in the lamina propria of the small intestine. Contrastingly, B. uniformis administration failed to improve glucose tolerance in obese Rag1-/- mice, but prevented the increased body weight gain and adiposity. Overall, the beneficial effects seemed to be independent of enteroendocrine effects and of major changes in gut microbiota composition. B. uniformis directly induced Tregs generation from naïve CD4+ T cells in vitro and was not required to be viable to improve glucose homeostasis but its viability was necessary to prevent body weight gain in diet-induced obese wild-type mice.

CONCLUSIONS

Here we demonstrate that B. uniformis modulates the energy homeostasis in diet-induced obese mice through different mechanisms. The bacterium improves oral glucose tolerance by adaptive immunity-dependent mechanisms that do not require cell viability and prevents body weight gain by adaptive immunity-independent mechanisms which require cell viability. Video Abstract.

Efficient enzyme-free method to assess the development and maturation of the innate and adaptive immune systems in the mouse colon

Researchers who aim to globally analyze the gastrointestinal immune system via flow cytometry have many protocol options to choose from, with specifics generally tied to gut wall layers of interest. To get a clearer idea of the approach we should use on full-thickness colon samples from mice, we first undertook a systematic comparison of three tissue dissociation techniques: two based on enzymatic cocktails and the other one based on manual crushing. Using flow cytometry panels of general markers of lymphoid and myeloid cells, we found that the presence of cell-surface markers and relative cell population frequencies were more stable with the mechanical method. Both enzymatic approaches were associated with a marked decrease of several cell-surface markers. Using mechanical dissociation, we then developed two minimally overlapping panels, consisting of a total of 26 antibodies, for serial profiling of lymphoid and myeloid lineages from the mouse colon in greater detail. Here, we highlight how we accurately delineate these populations by manual gating, as well as the reproducibility of our panels on mouse spleen and whole blood. As a proof-of-principle of the usefulness of our general approach, we also report segment- and life stage-specific patterns of immune cell profiles in the colon. Overall, our data indicate that mechanical dissociation is more suitable and efficient than enzymatic methods for recovering immune cells from all colon layers at once. Additionally, our panels will provide researchers with a relatively simple tool for detailed immune cell profiling in the murine gastrointestinal tract, regardless of life stage or experimental conditions.

Genistein alleviates dextran sulfate sodium-induced colitis in mice through modulation of intestinal microbiota and macrophage polarization

OBJECTIVES

Ulcerative colitis (UC) is a colonic immune system disorder, manifested with long duration and easy relapse. Genistein has been reported to possess various biological activities. However, it remains unclear whether genistein can ameliorate UC by modulating the homeostasis of the intestinal bacterial community.

METHODS

The dextran sodium sulfate (DSS)-induced UC mice were administrated with genistein (20 mg/kg/day) or genistein (40 mg/kg/day) for ten days. The general physical condition of the mice was monitored. After sacrifice, the changes in colon length and colonic pathological morphology were observed. The expression of intestinal barrier proteins, inflammatory cytokines, and macrophage markers in the colon was detected. The composition and metabolic products of the intestinal microbiota were analyzed.

RESULTS

Genistein treatment visibly improved body weight change and disease activity index in DSS-induced mice. Genistein treatment ameliorated colonic pathological alterations and promoted the expression of mucin-2 and tight junction proteins. Genistein administration inhibited myeloperoxidase activity and colonic inflammatory cytokines. Furthermore, genistein administration improved the structure of the intestinal microbial community, promoted the production of short-chain fatty acids, and modulated macrophage polarization.

CONCLUSIONS

These results revealed that genistein mediated macrophage polarization balance by improving intestinal microbiota and its metabolites, thereby alleviating DSS-induced colitis.

Macrophage Tim-3 maintains intestinal homeostasis in DSS-induced colitis by suppressing neutrophil necroptosis

T-cell immunoglobulin domain and mucin domain-3 (Tim-3) is a versatile immunomodulator that protects against intestinal inflammation. Necroptosis is a type of cell death that regulates intestinal homeostasis and inflammation. The mechanism(s) underlying the protective role of macrophage Tim-3 in intestinal inflammation is unclear; thus, we investigated whether specific Tim-3 knockdown in macrophages drives intestinal inflammation via necroptosis. Tim-3 protein and mRNA expression were assessed via double immunofluorescence staining and single-cell RNA sequencing (sc-RNA seq), respectively, in the colonic tissues of patients with inflammatory bowel disease (IBD) and healthy controls. Macrophage-specific Tim3-knockout (Tim-3M-KO) mice were generated to explore the function and mechanism of Tim-3 in dextran sodium sulfate (DSS)-induced colitis. Necroptosis was blocked by pharmacological inhibitors of receptor-interacting protein kinase (RIP)1, RIP3, and reactive oxygen species (ROS). Additionally, in vitro experiments were performed to assess the mechanisms of neutrophil necroptosis induced by Tim-3 knockdown macrophages. Although Tim-3 is relatively inactive in macrophages during colon homeostasis, it is highly active during colitis. Compared to those in controls, Tim-3M-KO mice showed increased susceptibility to colitis, higher colitis scores, and increased pro-inflammatory mediator expression. Following the administration of RIP1/RIP3 or ROS inhibitors, a significant reduction in intestinal inflammation symptoms was observed in DSS-treated Tim-3M-KO mice. Further analysis indicated the TLR4/NF-κB pathway in Tim-3 knockdown macrophages mediates the TNF-α-induced necroptosis pathway in neutrophils. Macrophage Tim-3 regulates neutrophil necroptosis via intracellular ROS signaling. Tim-3 knockdown macrophages can recruit neutrophils and induce neutrophil necroptosis, thereby damaging the intestinal mucosal barrier and triggering a vicious cycle in the development of colitis. Our results demonstrate a protective role of macrophage Tim-3 in maintaining gut homeostasis by inhibiting neutrophil necroptosis and provide novel insights into the pathogenesis of IBD.

Nocardia rubra cell-wall skeleton mitigates whole abdominal irradiation-induced intestinal injury via regulating macrophage function

Background

Ionizing radiation (IR)-induced intestinal injury is a major side effect and dose-limiting toxicity in patients receiving radiotherapy. There is an urgent need to identify an effective and safe radioprotectant to reduce radiation-induced intestinal injury. Immunoregulation is considered an effective strategy against IR-induced injury. The purpose of this article was to investigate the protective effect of Nocardia rubra cell wall skeleton (Nr-CWS), an immunomodulator, on radiation-induced intestinal damage and to explore its potential mechanism.

Methods

C57BL/6 J male mice exposed to 12 Gy whole abdominal irradiation (WAI) were examined for survival rate, morphology and function of the intestine and spleen, as well as the gut microbiota, to comprehensively evaluate the therapeutic effects of Nr-CWS on radiation-induced intestinal and splenetic injury. To further elucidate the underlying mechanisms of Nr-CWS-mediated intestinal protection, macrophages were depleted by clodronate liposomes to determine whether Nr-CWS-induced radioprotection is macrophage dependent, and the function of peritoneal macrophages stimulated by Nr-CWS was detected in vitro.

Results

Our data showed that Nr-CWS promoted the recovery of intestinal barrier function, enhanced leucine-rich repeat-containing G protein-coupled receptor 5+ intestinal stem cell survival and the regeneration of intestinal epithelial cells, maintained intestinal flora homeostasis, protected spleen morphology and function, and improved the outcome of mice exposed to 12 Gy WAI. Mechanistic studies indicated that Nr-CWS recruited macrophages to reduce WAI-induced intestinal damage. Moreover, macrophage depletion by clodronate liposomes blocked Nr-CWS-induced radioprotection. In vitro, we found that Nr-CWS activated the nuclear factor kappa-B signaling pathway and promoted the phagocytosis and migration ability of peritoneal macrophages.

Conclusions

Our study suggests the therapeutic effect of Nr-CWS on radiation-induced intestinal injury, and provides possible therapeutic strategy and potential preventive and therapeutic drugs to alleviate it.

The roles of tissue resident macrophages in health and cancer

As integral components of the immune microenvironment, tissue resident macrophages (TRMs) represent a self-renewing and long-lived cell population that plays crucial roles in maintaining homeostasis, promoting tissue remodeling after damage, defending against inflammation and even orchestrating cancer progression. However, the exact functions and roles of TRMs in cancer are not yet well understood. TRMs exhibit either pro-tumorigenic or anti-tumorigenic effects by engaging in phagocytosis and secreting diverse cytokines, chemokines, and growth factors to modulate the adaptive immune system. The life-span, turnover kinetics and monocyte replenishment of TRMs vary among different organs, adding to the complexity and controversial findings in TRMs studies. Considering the complexity of tissue associated macrophage origin, macrophages targeting strategy of each ontogeny should be carefully evaluated. Consequently, acquiring a comprehensive understanding of TRMs' origin, function, homeostasis, characteristics, and their roles in cancer for each specific organ holds significant research value. In this review, we aim to provide an outline of homeostasis and characteristics of resident macrophages in the lung, liver, brain, skin and intestinal, as well as their roles in modulating primary and metastatic cancer, which may inform and serve the future design of targeted therapies.

Involvement of Embryo-Derived and Monocyte-Derived Intestinal Macrophages in the Pathogenesis of Inflammatory Bowel Disease and Their Prospects as Therapeutic Targets

Inflammatory bowel disease (IBD) is a group of intestinal inflammatory diseases characterized by chronic, recurrent, remitting, or progressive inflammation, which causes the disturbance of the homeostasis between immune cells, such as macrophages, epithelial cells, and microorganisms. Intestinal macrophages (IMs) are the largest population of macrophages in the body, and the abnormal function of IMs is an important cause of IBD. Most IMs come from the replenishment of blood monocytes, while a small part come from embryos and can self-renew. Stimulated by the intestinal inflammatory microenvironment, monocyte-derived IMs can interact with intestinal epithelial cells, intestinal fibroblasts, and intestinal flora, resulting in the increased differentiation of proinflammatory phenotypes and the decreased differentiation of anti-inflammatory phenotypes, releasing a large number of proinflammatory factors and aggravating intestinal inflammation. Based on this mechanism, inhibiting the secretion of IMs' proinflammatory factors and enhancing the differentiation of anti-inflammatory phenotypes can help alleviate intestinal inflammation and promote tissue repair. At present, the clinical medication of IBD mainly includes 5-aminosalicylic acids (5-ASAs), glucocorticoid, immunosuppressants, and TNF-α inhibitors. The general principle of treatment is to control acute attacks, alleviate the condition, reduce recurrence, and prevent complications. Most classical IBD therapies affecting IMs function in a variety of ways, such as inhibiting the inflammatory signaling pathways and inducing IM2-type macrophage differentiation. This review explores the current understanding of the involvement of IMs in the pathogenesis of IBD and their prospects as therapeutic targets.

Glycolytic Activation of CD14+ Intestinal Macrophages Contributes to the Inflammatory Responses via Exosomal Membrane Tumor Necrosis Factor in Crohn's Disease

BACKGROUND

Macrophage (Mφ) activation plays a critical role in the inflammatory response. Activated Mφ go through profound reprogramming of cellular metabolism. However, changes in their intracellular energy metabolism and its effect on inflammatory responses in Crohn's disease (CD) remain currently unclear. The aim of this study is to explore metabolic signatures of CD14+ Mφ and their potential role in CD pathogenesis as well as the underlying mechanisms.

METHODS

CD14+ Mφ were isolated from peripheral blood or intestinal tissues of CD patients and control subjects. Real-time flux measurements and enzyme-linked immunosorbent assay were used to determine the inflammatory states of Mφ and metabolic signatures. Multiple metabolic routes were suppressed to determine their relevance to cytokine production.

RESULTS

Intestinal CD14+ Mφ in CD patients exhibited activated glycolysis compared with those in control patients. Specifically, macrophagic glycolysis in CD largely induced inflammatory cytokine release. The intestinal inflammatory microenvironment in CD elicited abnormal glycolysis in Mφ. Mechanistically, CD14+ Mφ derived exosomes expressed membrane tumor necrosis factor (TNF), which engaged TNFR2 and triggered glycolytic activation via TNF/nuclear factor κB autocrine and paracrine signaling. Importantly, clinically applicable anti-TNF antibodies effectively prevented exosomal membrane TNF-induced glycolytic activation in CD14+ Mφ.

CONCLUSIONS

CD14+ Mφ take part in CD pathogenesis by inducing glycolytic activation via membrane TNF-mediated exosomal autocrine and paracrine signaling. These results provide novel insights into pathogenesis of CD and enhance understanding of the mechanisms of anti-TNF agents.

Natural Products as Dietary Agents for the Prevention and Mitigation of Oxidative Damage and Inflammation in the Intestinal Barrier

Food intake is a basic need to sustain life, but foodborne pathogens and food-related xenobiotics are also the main health concerns regarding intestinal barrier homeostasis. With a predominant role in the well-being of the entire human body, intestinal barrier homeostasis is strictly regulated by epithelial and immune cells. These cells are also the main intervenients in oxidative stress and inflammation-related diseases in the intestinal tract, triggered, for example, by genetic/epigenetic factors, food additives, pesticides, drugs, pathogens, and their metabolites. Nevertheless, the human diet can also be seen as a solution for the problem, mainly via the inclusion of functional foods or nutraceuticals that may act as antioxidant/anti-inflammatory agents to prevent and mitigate acute and chronic oxidative damage and inflammation. A literature analysis of recent advances in this topic highlights the significant role of Nrf2 (nuclear factor erythroid 2-related factor 2) and NF-kB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathways in these biological processes, with many natural products and phytochemicals targeting endogenous antioxidant systems and cytokine production and balance. In this review, we summarized and discussed studies using in vitro and in vivo models of the intestinal tract used to reproduce oxidative damage and inflammatory events, as well as the role of natural products as modulators of Nrf2 and NK-kB pathways.

Immune regulation and inflammation inhibition of Arctium lappa L. polysaccharides by TLR4/NF-κB signaling pathway in cells

Arctium lappa L. polysaccharides (ALP) are important active ingredients of burdocks with various bioactivities. In the present study, a crude polysaccharide was extracted from A. lappa L. roots and purified using DEAE-52 and Sephacryl™ S-400 columns to reach 99 % purity. This neutral polysaccharide contained fructose, glucose, galactose and arabinose in a ratio of 0.675:0.265:0.023:0.016 and had a Mw of 4256 Da. The immunomodulatory activity and intestinal inflammation inhibitory effects of ALP were investigated in in vitro models, including lipopolysaccharide-induced macrophage RAW264.7 and interleukin (IL)-1β-induced colon Caco-2 cells. The results revealed that ALP possessed both antioxidant and anti-inflammatory effects by decreasing nuclear factor-E2-related factor 2 mRNA expression and reactive oxygen species. Furthermore, ALP was found to have inhibitory effects on pro-inflammatory cytokines, including IL-8, IL-6, IL-1β, and tumor necrosis factor-α, as well as inflammatory cytokines, such as intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and monocyte chemoattractant protein-1 by down-regulating the Toll-like receptor 4 (TLR4)/NF-κB (nuclear factor-kappa B signaling) pathway. It indicated that A. lappa L. was an ideal source of bioactive polysaccharides having potential to be developed as functional foods or nutraceuticals to improve immune system and prevent/treat intestinal inflammation.

The gut ecosystem and immune tolerance

The gastrointestinal tract is home to the largest microbial population in the human body. The gut microbiota plays significant roles in the development of the gut immune system and has a substantial impact on the maintenance of immune tolerance beginning in early life. These microbes interact with the immune system in a dynamic and interdependent manner. They generate immune signals by presenting a vast repertoire of antigenic determinants and microbial metabolites that influence the development, maturation and maintenance of immunological function and homeostasis. At the same time, both the innate and adaptive immune systems are involved in modulating a stable microbial ecosystem between the commensal and pathogenic microorganisms. Hence, the gut microbial population and the host immune system work together to maintain immune homeostasis synergistically. In susceptible hosts, disruption of such a harmonious state can greatly affect human health and lead to various auto-inflammatory and autoimmune disorders. In this review, we discuss our current understanding of the interactions between the gut microbiota and immunity with an emphasis on: a) important players of gut innate and adaptive immunity; b) the contribution of gut microbial metabolites; and c) the effect of disruption of innate and adaptive immunity as well as alteration of gut microbiome on the molecular mechanisms driving autoimmunity in various autoimmune diseases.

Impact of intestinal microenvironments in obesity and bariatric surgery on shaping macrophages

Obesity is associated with alterations in tissue composition, systemic cellular metabolism, and low-grade chronic inflammation. Macrophages are heterogenous innate immune cells ubiquitously localized throughout the body and are key components of tissue homeostasis, inflammation, wound healing, and various disease states. Macrophages are highly plastic and can switch their phenotypic polarization and change function in response to their local environments. Here, we discuss how obesity alters the intestinal microenvironment and potential key factors that can influence intestinal macrophages as well as macrophages in other organs, including adipose tissue and hematopoietic organs. As bariatric surgery can induce metabolic adaptation systemically, we discuss the potential mechanisms through which bariatric surgery reshapes macrophages in obesity.

The interplay between T lymphocytes and macrophages in myocardial ischemia/reperfusion injury

Acute myocardial infarction is one of the most important causes of death in the world, causing a huge health and economic burden to the world. It is still a ticklish problem how to effectively prevent reperfusion injury while recovering the blood flow of ischemic myocardium. During the process of myocardial ischemia/reperfusion injury (MI/RI), the modulation of immune cells plays an important role. Monocyte/macrophage, neutrophils and endothelial cells initiate the inflammatory response and induce the release of various inflammatory cytokines, resulting in increased vascular permeability, tissue edema and damage. Meanwhile, T cells were recruited to impaired myocardium and release pro-inflammatory and anti-inflammatory cytokines. T cells and macrophages play important roles in keeping cardiac homeostasis and orchestrate tissue repair. T cells differentiation and macrophages polarization precisely regulates the tissue microenvironment in MI/RI, and shows cross action, but the mechanism is unclear. To identify potential intervention targets and propose ideas for treatment and prevention of MI/RI, this review explores the crosstalk between T lymphocytes and macrophages in MI/RI.

Development of an Inflammation-Triggered In Vitro "Leaky Gut" Model Using Caco-2/HT29-MTX-E12 Combined with Macrophage-like THP-1 Cells or Primary Human-Derived Macrophages

The "leaky gut" syndrome describes a damaged (leaky) intestinal mucosa and is considered a serious contributor to numerous chronic diseases. Chronic inflammatory bowel diseases (IBD) are particularly associated with the "leaky gut" syndrome, but also allergies, autoimmune diseases or neurological disorders. We developed a complex in vitro inflammation-triggered triple-culture model using 21-day-differentiated human intestinal Caco-2 epithelial cells and HT29-MTX-E12 mucus-producing goblet cells (90:10 ratio) in close contact with differentiated human macrophage-like THP-1 cells or primary monocyte-derived macrophages from human peripheral blood. Upon an inflammatory stimulus, the characteristics of a "leaky gut" became evident: a significant loss of intestinal cell integrity in terms of decreased transepithelial/transendothelial electrical resistance (TEER), as well as a loss of tight junction proteins. The cell permeability for FITC-dextran 4 kDa was then increased, and key pro-inflammatory cytokines, including TNF-alpha and IL-6, were substantially released. Whereas in the M1 macrophage-like THP-1 co-culture model, we could not detect the release of IL-23, which plays a crucial regulatory role in IBD, this cytokine was clearly detected when using primary human M1 macrophages instead. In conclusion, we provide an advanced human in vitro model that could be useful for screening and evaluating therapeutic drugs for IBD treatment, including potential IL-23 inhibitors.

Immunomodulatory Properties of Vitamin D in the Intestinal and Respiratory Systems

Vitamin D plays a crucial role in modulating the innate immune response by interacting with its intracellular receptor, VDR. In this review, we address vitamin D/VDR signaling and how it contributes to the regulation of intestinal and respiratory microbiota. We additionally review some components of the innate immune system, such as the barrier function of the pulmonary and intestinal epithelial membranes and secretion of mucus, with their respective modulation by vitamin D. We also explore the mechanisms by which this vitamin D/VDR signaling mounts an antimicrobial response through the transduction of microbial signals and the production of antimicrobial peptides that constitute one of the body’s first lines of defense against pathogens. Additionally, we highlight the role of vitamin D in clinical diseases, namely inflammatory bowel disease and acute respiratory distress syndrome, where excessive inflammatory responses and dysbiosis are hallmarks. Increasing evidence suggests that vitamin D supplementation may have potentially beneficial effects on those diseases.

The modulation of macrophage subsets in celiac disease pathogenesis

BACKGROUND

So far, limited studies have focused on the role of Macrophages (MQs) in the development or progression of celiac disease (CD). Researchers believe that increasing knowledge about the function of MQs in inflammatory disorders plays a critical role in finding a new treatment for these kinds of diseases.

MAIN BODY

CD is a permanent autoimmune intestinal disorder triggered by gluten exposure in predisposed individuals. This disorder happens due to the loss of intestinal epithelial barrier integrity characterized by dysregulated innate and adaptive immune responses. MQs are known as key players of the innate immune system that link innate and adaptive immunity. MQs of human intestinal lamina propria participate in maintaining tissue homeostasis, and also intestinal inflammation development. Previous studies suggested that gliadin triggers a proinflammatory phenotype (M1 MQ) in human primary MQs. Moreover, M2-related immunosuppressive mediators are also present in CD. In fact, CD patients present an impaired transition from pro-inflammatory to anti-inflammatory responses due to inappropriate responses to gliadin peptides.

CONCLUSION

The M1/M2 MQs polarization balancing regulators can be considered novel therapeutic targets for celiac disease.

Deciphering the role of autophagy in the immunopathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a typical immune-mediated chronic inflammatory disorder. Following the industrialization and changes in lifestyle, the incidence of IBD in the world is rising, which makes health concerns and heavy burdens all over the world. However, the pathogenesis of IBD remains unclear, and the current understanding of the pathogenesis involves dysregulation of mucosal immunity, gut microbiome dysbiosis, and gut barrier defect based on genetic susceptibility and environmental triggers. In recent years, autophagy has emerged as a key mechanism in IBD development and progression because Genome-Wide Association Study revealed the complex interactions of autophagy in IBD, especially immunopathogenesis. Besides, autophagy markers are also suggested to be potential biomarkers and target treatment in IBD. This review summarizes the autophagy-related genes regulating immune response in IBD. Furthermore, we explore the evolving evidence that autophagy interacts with intestinal epithelial and immune cells to contribute to the inflammatory changes in IBD. Finally, we discuss how novel discovery could further advance our understanding of the role of autophagy and inform novel therapeutic strategies in IBD.

Suspended Collagen Hydrogels to Replicate Human Colonic Epithelial Cell Interactions with Immune Cells

The human colon plays a critical role in fluid and salt absorption and harbors the largest immune compartment. There is a widespread need for in vitro models of human colon physiology with its innate immune system. A method is described to produce a cassette with a network of struts supporting a suspended, non-chemically cross-linked collagen hydrogel scaffold compatible with the co-culture of primary gastrointestinal epithelium and migratory inflammatory cells. The epithelial monolayer cultured on the suspended collagen possesses a population of polarized and differentiated cells similar to that present in vivo. This epithelial layer displays proper barrier function with a transepithelial electrical resistance (TEER) ≥ 1,500 Ω cm2 and an apparent permeability ≤10-5 cm2 s-1 . Immune cells plated on the basal face of the scaffold transmigrated over a period of 24 h to the epithelial layer in response to epithelial production of IL-8 induced by luminal stimulation of Clostridium difficile Toxin A. These studies demonstrate that this in vitro platform possesses a functional primary colonic epithelial layer with an immune cell compartment capable of recruitment in response to pro-inflammatory cues coming from the epithelium.

Mercury toxic effects on the intestinal mucosa assayed on a bicameral in vitro model: Possible role of inflammatory response and oxidative stress

Exposure to mercury (Hg) mostly occurs through diet, where it is mainly found as inorganic Hg [Hg(II)] or methylmercury (MeHg). In vivo studies have linked its exposure with neurological and renal diseases, however, its toxic effects upon the gastrointestinal tract are largely unknown. In order to evaluate the effect of Hg on intestinal mucosa, a bicameral system was employed with co-cultures of Caco-2 and HT29-MTX intestinal epithelial cells and THP-1 macrophages. Cells were exposed to Hg(II) and MeHg (0.1, 0.5, 1 mg/L) during 11 days. The results evidenced a greater pro-inflammatory response in cells exposed to Hg with increments of IL-8 (15-126%) and IL-1β release (39-63%), mainly induced by macrophages which switched to a M1 phenotype. A pro-oxidant response was also observed in both cell types with an increase in ROS/RNS levels (44-140%) and stress proteins expression. Intestinal cells treated with Hg displayed structural abnormalities, hypersecretion of mucus and defective tight junctions. An increased paracellular permeability (123-170%) at the highest concentrations of Hg(II) and MeHg and decreased capacity to restore injuries in the cell monolayer were also observed. All these toxic effects were governed by various inflammatory signalling pathways (p38 MAPK, JNK and NF-κB).

Identification of CX3CR1+ mononuclear phagocyte subsets involved in HIV-1 and SIV colorectal transmission

The difficulty to unambiguously identify the various subsets of mononuclear phagocytes (MNPs) of the intestinal lamina propria has hindered our understanding of the initial events occurring after mucosal exposure to HIV-1.

Here, we compared the composition and function of MNP subsets at steady-state and following ex vivo and in vivo viral exposure in human and macaque colorectal tissues.

Combined evaluation of CD11c, CD64, CD103, and CX3CR1 expression allowed to differentiate lamina propria MNPs subsets common to both species. Among them, CD11c⁺ CX3CR1⁺ cells expressing CCR5 migrated inside the epithelium following ex vivo and in vivo exposure of colonic tissue to HIV-1 or SIV. In addition, the predominant population of CX3CR1^high macrophages present at steady-state partially shifted to CX3CR1^low macrophages as early as three days following in vivo SIV rectal challenge of macaques.

Our analysis identifies CX3CR1⁺ MNPs as novel players in the early events of HIV-1 and SIV colorectal transmission.

•

Human and macaque intestinal MNPs show similar phenotype, localization, and function

•

CX3CR1⁺ MNPs migrate inside the intestinal epithelium to sample HIV/SIV

•

SIV infection alters the balance between CX3CR1^high and CX3CR1^low Mφs

•

CX3CR1⁺ Mφs contribute to the breakdown of the intestinal barrier in HIV/SIV infection

NAMPT mitigates colitis severity by supporting redox-sensitive activation of phagocytosis in inflammatory macrophages

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the nicotinamide adenine dinucleotide (NAD+) salvage pathway and plays a crucial role in the maintenance of the NAD+ pool during inflammation. Considering that macrophages are essential for tissue homeostasis and inflammation, we sought to examine the functional impact of NAMPT in inflammatory macrophages, particularly in the context of inflammatory bowel disease (IBD). In this study, we show that mice with NAMPT deletion within the myeloid compartment (Namptf/fLysMCre+/-, Nampt mKO) have more pronounced colitis with lower survival rates, as well as numerous uncleared apoptotic corpses within the mucosal layer. Nampt-deficient macrophages exhibit reduced phagocytic activity due to insufficient NAD+ abundance, which is required to produce NADPH for the oxidative burst. Nicotinamide mononucleotide (NMN) treatment rescues NADPH levels in Nampt mKO macrophages and sustains superoxide generation via NADPH oxidase. Consequently, Nampt mKO mice fail to clear dead cells during tissue repair, leading to substantially prolonged chronic colitis. Moreover, systemic administration of NMN, to supply NAD+, effectively suppresses the disease severity of DSS-induced colitis. Collectively, our findings suggest that activation of the NAMPT-dependent NAD+ biosynthetic pathway, via NMN administration, is a potential therapeutic strategy for managing inflammatory diseases.

Pro-inflammatory effects of silver nanoparticles in the intestine

Nanotechnology is a promising technology of the twenty-first century, being a rapidly evolving field of research and industrial innovation widely applied in our everyday life. Silver nanoparticles (AgNP) are considered the most commercialized nanosystems worldwide, being applied in diverse sectors, from medicine to the food industry. Considering their unique physical, chemical and biological properties, AgNP have gained access into our daily life, with an exponential use in food industry, leading to an increased inevitable human oral exposure. With the growing use of AgNP, several concerns have been raised, in recent years, about their potential hazards to human health, more precisely their pro-inflammatory effects within the gastrointestinal system. Therefore a review of the literature has been undertaken to understand the pro-inflammatory potential of AgNP, after human oral exposure, in the intestine. Despite the paucity of information reported in the literature about this issue, existing studies indicate that AgNP exert a pro-inflammatory action, through generation of oxidative stress, accompanied by mitochondrial dysfunction, interference with transcription factors and production of cytokines. However, further studies are needed to elucidate the mechanistic pathways and molecular targets involved in the intestinal pro-inflammatory effects of AgNP.

Alzheimer's disease and depression in the elderly: A trajectory linking gut microbiota and serotonin signaling

The occurrence of neuropsychiatric symptoms in the elderly is viewed as an early sign of subsequent cognitive deterioration and conversion from mild cognitive impairment to Alzheimer's disease. The prognosis in terms of both the severity and progression of clinical dementia is generally aggravated by the comorbidity of neuropsychiatric symptoms and decline in cognitive function. Undeniably, aging and in particular unhealthy aging, is a silent "engine of neuropathology" over which multiple changes take place, including drastic alterations of the gut microbial ecosystem. This narrative review evaluates the role of gut microbiota changes as a possible unifying concept through which the comorbidity of neuropsychiatric symptoms and Alzheimer's disease can be considered. However, since the heterogeneity of neuropsychiatric symptoms, it is improbable to describe the same type of alterations in the bacteria population observed in patients with Alzheimer's disease, as well as it is improbable that the variety of drugs used to treat neuropsychiatric symptoms might produce changes in gut bacterial diversity similar to that observed in the pathophysiology of Alzheimer's disease. Depression seems to be another very intriguing exception, as it is one of the most frequent neuropsychiatric symptoms in dementia and a mood disorder frequently associated with brain aging. Antidepressants (i.e., serotonin reuptake inhibitors) or tryptophan dietary supplementation have been shown to reduce Amyloid β-loading, reinstate microbial diversity and reduce the abundance of bacterial taxa dominant in depression and Alzheimer's disease. This review briefly examines this trajectory by discussing the dysfunction of gut microbiota composition, selected bacterial taxa, and alteration of tryptophan and serotonin metabolism/neurotransmission as overlapping in-common mechanisms involved with depression, Alzheimer's disease, and unhealthy aging.

Macrophage Heterogeneity in the Intestinal Cells of Salmon: Hints From Transcriptomic and Imaging Data

The intestine has many types of cells that are present mostly in the epithelium and lamina propria. The importance of the intestinal cells for the mammalian mucosal immune system is well-established. However, there is no in-depth information about many of the intestinal cells in teleosts. In our previous study, we reported that adherent intestinal cells (AIC) predominantly express macrophage-related genes. To gather further evidence that AIC include macrophage-like cells, we compared their phagocytic activity and morphology with those of adherent head kidney cells (AKC), previously characterized as macrophage-like cells. We also compared equally abundant as well as differentially expressed mRNAs and miRNAs between AIC and AKC. AIC had lower phagocytic activity and were larger and more circular than macrophage-like AKC. RNA-Seq data revealed that there were 18309 mRNAs, with 59 miRNAs that were equally abundant between AIC and AKC. Integrative analysis of the mRNA and miRNA transcriptomes revealed macrophage heterogeneity in both AIC and AKC. In addition, analysis of AIC and AKC transcriptomes revealed functional characteristics of mucosal and systemic macrophages. Five pairs with significant negative correlations between miRNA and mRNAs were linked to macrophages and epithelial cells and their interaction could be pointing to macrophage activation and differentiation. The potential macrophage markers suggested in this study should be investigated under different immune conditions to understand the exact macrophage phenotypes.

There Is Strength in Numbers: Quantitation of Fc Gamma Receptors on Murine Tissue-Resident Macrophages

Many of the effector functions of antibodies rely on the binding of antibodies/immune complexes to cellular Fcγ receptors (FcγRs). Since the majority of innate immune effector cells express both activating and inhibitory Fc receptors, the outcome of the binding of immune complexes to cells of a given population is influenced by the relative affinities of the respective IgG subclasses to these receptors, as well as by the numbers of activating and inhibitory FcγRs on the cell surface. A group of immune cells that has come into focus more recently is the various subsets of tissue-resident macrophages. The central functions of FcγRs on tissue macrophages include the clearance of opsonized pathogens, the removal of small immune complexes from the circulation and the depletion of antibody-opsonized cells in the therapy of autoimmunity and cancer. Despite these essential functions of FcγRs on tissue-resident macrophages, an in-depth quantification of FcγRs is lacking. Thus, the aim of our current study was to quantify the various Fcγ receptors on macrophages in murine liver, lung, kidney, brain, skin and spleen. Our study identified a pronounced heterogeneity between FcγR expression patterns of the different tissue macrophages, which may reflect their specialized functions within their unique niches in different organ environments.

The Gut-Liver Axis in Chronic Liver Disease: A Macrophage Perspective

Chronic liver disease (CLD) is a growing health concern which accounts for two million deaths per year. Obesity, alcohol overconsumption, and progressive cholestasis are commonly characterized by persistent low-grade inflammation and advancing fibrosis, which form the basis for development of end-stage liver disease complications, including hepatocellular carcinoma. CLD pathophysiology extends to the intestinal tract and is characterized by intestinal dysbiosis, bile acid dysregulation, and gut barrier disruption. In addition, macrophages are key players in CLD progression and intestinal barrier breakdown. Emerging studies are unveiling macrophage heterogeneity and driving factors of their plasticity in health and disease. To date, in-depth investigation of how gut-liver axis disruption impacts the hepatic and intestinal macrophage pool in CLD pathogenesis is scarce. In this review, we give an overview of the role of intestinal and hepatic macrophages in homeostasis and gut-liver axis disruption in progressive stages of CLD.

Astragaloside IV Alleviates the Experimental DSS-Induced Colitis by Remodeling Macrophage Polarization Through STAT Signaling

Inflammatory bowel disease (IBD) is characterized by chronic and relapsing intestinal inflammation, which currently lacks safe and effective medicine. Some previous studies indicated that Astragaloside IV (AS-IV), a natural saponin extracted from the traditional Chinese medicine herb Ligusticum chuanxiong, alleviates the experimental colitis symptoms in vitro and in vivo. However, the mechanism of AS-IV on IBD remains unclear. Accumulating evidence suggests that M2-polarized intestinal macrophages play a pivotal role in IBD progression. Here, we found that AS-IV attenuated clinical activity of DSS-induced colitis that mimics human IBD and resulted in the phenotypic transition of macrophages from immature pro-inflammatory macrophages to mature pro-resolving macrophages. In vitro, the phenotype changes of macrophages were observed by qRT-PCR after bone marrow-derived macrophages (BMDMs) were induced to M1/M2 and incubated with AS-IV, respectively. In addition, AS-IV was effective in inhibiting pro-inflammatory macrophages and promoting the pro-resolving macrophages to ameliorate experimental colitis via the regulation of the STAT signaling pathway. Hence, we propose that AS-IV can ameliorate experimental colitis partially by modulating macrophage phenotype by remodeling the STAT signaling, which seems to have an essential function in the ability of AS-IV to alleviate the pathological progress of IBD.

The Evolving Roles of Cardiac Macrophages in Homeostasis, Regeneration, and Repair

Macrophages were first described as phagocytic immune cells responsible for maintaining tissue homeostasis by the removal of pathogens that disturb normal function. Historically, macrophages have been viewed as terminally differentiated monocyte-derived cells that originated through hematopoiesis and infiltrated multiple tissues in the presence of inflammation or during turnover in normal homeostasis. However, improved cell detection and fate-mapping strategies have elucidated the various lineages of tissue-resident macrophages, which can derive from embryonic origins independent of hematopoiesis and monocyte infiltration. The role of resident macrophages in organs such as the skin, liver, and the lungs have been well characterized, revealing functions well beyond a pure phagocytic and immunological role. In the heart, recent research has begun to decipher the functional roles of various tissue-resident macrophage populations through fate mapping and genetic depletion studies. Several of these studies have elucidated the novel and unexpected roles of cardiac-resident macrophages in homeostasis, including maintaining mitochondrial function, facilitating cardiac conduction, coronary development, and lymphangiogenesis, among others. Additionally, following cardiac injury, cardiac-resident macrophages adopt diverse functions such as the clearance of necrotic and apoptotic cells and debris, a reduction in the inflammatory monocyte infiltration, promotion of angiogenesis, amelioration of inflammation, and hypertrophy in the remaining myocardium, overall limiting damage extension. The present review discusses the origin, development, characterization, and function of cardiac macrophages in homeostasis, cardiac regeneration, and after cardiac injury or stress.

Steroid Receptor Coactivator-3 Is Required for Inhibition of the Intestinal Muscularis Inflammatory Response of Postoperative Ileus in Mice

Inflammation theory has suggested that the pathogenesis of postoperative ileus (POI) involves the steroid receptor coactivator-3 (SRC-3). Therefore, we investigated the role of SRC-3 in the muscles of the small intestine using a mouse POI model. Here, we reported that intestinal manipulation (IM) significantly reduced the extent of phenol red migration in the entire gastrointestinal tract, and the calculated geometric center (GC) value in wild-type (WT) mice at 24 h after surgery was higher than that in the knockout (KO) mice and in the sham-operated control group. The expression of SRC-3 was upregulated in the mouse intestinal muscularis at 24 h after surgical manipulation, and the mRNA and protein levels of inflammatory cytokines were upregulated compared with those in the control group. At 24 h after IM, the number of neutrophils in the experimental group was significantly higher than that in the control group; in the IM group, the number of neutrophils in the SRC-3-/- mice was markedly higher than that in the WT mice. At 24 h after IM, the myeloperoxidase (MPO) activity in the experimental group was significantly higher than that in the control group. In the IM group, the MPO activity of the SRC-3-/- mice was markedly higher than that of the WT mice. In summary, proinflammatory cytokines, the number of neutrophils, and the MPO activity were significantly increased in the muscularis of the jejunum and ileum of KO mice after IM compared with those of the WT mice, indicating that SRC-3 might play a protective role in POI.

Glucocorticoids as Regulators of Macrophage-Mediated Tissue Homeostasis

Our immune system has evolved as a complex network of cells and tissues tasked with maintaining host homeostasis. This is evident during the inflammatory responses elicited during a microbial infection or traumatic tissue damage. These responses seek to eliminate foreign material or restore tissue integrity. Even during periods without explicit disturbances, the immune system plays prominent roles in tissue homeostasis. Perhaps one of the most studied cells in this regard is the macrophage. Tissue-resident macrophages are a heterogenous group of sensory cells that respond to a variety of environmental cues and are essential for organ function. Endogenously produced glucocorticoid hormones connect external environmental stress signals with the function of many cell types, producing profound changes in immune cells, including macrophages. Here, we review the current literature which demonstrates specific effects of glucocorticoids in several organ systems. We propose that tissue-resident macrophages, through glucocorticoid signaling, may play an underappreciated role as regulators of organ homeostasis.

Inhibition of TLR4 Signaling Impedes Tumor Growth in Colitis-Associated Colon Cancer

Patients suffering from ulcerative colitis are at increased risk of developing colorectal cancer. Although the exact underlying mechanisms of inflammation-associated carcinogenesis remain unknown, the intestinal microbiota as well as pathogenic bacteria are discussed as contributors to inflammation and colitis-associated colon cancer (CAC). In the present study, we analyzed the impact of TLR4, the receptor for Gram-negative bacteria derived lipopolysaccharides, on intestinal inflammation and tumorigenesis in a murine model of CAC. During the inflammatory phases of CAC development, we observed a strong upregulation of Tlr4 expression in colonic tissues. Blocking of TLR4 signaling by a small-molecule-specific inhibitor during the inflammatory phases of CAC strongly diminished the development and progression of colonic tumors, which was accompanied by decreased numbers of infiltrating macrophages and reduced colonic pro-inflammatory cytokine levels compared to CAC control mice. Interestingly, inhibiting bacterial signaling by antibiotic treatment during the inflammatory phases of CAC also protected mice from severe intestinal inflammation and almost completely prevented tumor growth. Nevertheless, application of antibiotics involved rapid and severe body weight loss and might have unwanted side effects. Our results indicate that bacterial activation of TLR4 on innate immune cells in the colon triggers inflammation and promotes tumor growth. Thus, the inhibition of the TLR4 signaling during intestinal inflammation might be a novel approach to impede CAC development.

Phenotypic and Functional Heterogeneity of Low-Density and High-Density Human Lung Macrophages

BACKGROUND

Pulmonary macrophages are a highly heterogeneous cell population distributed in different lung compartments.

METHODS

We separated two subpopulations of macrophages from human lung parenchyma according to flotation over density gradients.

RESULTS

Two-thirds 65.4% of the lung macrophages have a density between 1.065 and 1.078 (high-density macrophages: HDMs), and the remaining one-third (34.6) had a density between 1.039 and 1.052 (low-density macrophages: LDMs). LDMs had a larger area (691 vs. 462 μm²) and cell perimeter (94 vs. 77 μm) compared to HDMs. A significantly higher percentage of HDMs expressed CD40, CD45, and CD86 compared to LDMs. In contrast, a higher percentage of LDMs expressed the activation markers CD63 and CD64. The release of TNF-α, IL-6, IL-10 and IL-12 induced by lipopolysaccharide (LPS) was significantly higher in HDMs than in LDMs.

CONCLUSION

The human lung contains two subpopulations of macrophages that differ in buoyancy, morphometric parameters, surface marker expression and response to LPS. These subpopulations of macrophages probably play distinct roles in lung inflammation and immune responses.

Three main short-chain fatty acids inhibit the activation of THP-1 cells by Mycoplasma pneumoniae

The overactivation of macrophages causes chronic inflammatory diseases. Short-chain fatty acids (SCFAs), potential drugs for clinical treatment, are modulators of macrophage inflammatory reaction. Therefore, the modulation of macrophage-mediated cell activity is expected to become a new therapeutic strategy for inflammatory diseases caused by Mycoplasma pneumoniae. In this study, 2 kinds of SCFAs (propionate and butyrate) were found to have anti-inflammatory effects in M. pneumoniae-stimulated THP-1 cells inflammatory. They inhibited the expressions of IL-4, IL-6, ROS, and NLRP3 inflammasome, while enhancing the expressions of IL-10 and IFN-γ. Our study revealed these 2 agents to repress transcriptional activities of NF-κB, which are important modulators of inflammation. Meanwhile, SCFAs can significantly enhance the autophagy induced by M. pneumoniae. Considering that SCFAs have few side effects, they might be the promising adjuvant therapy for the prevention and/or treatment of various inflammatory diseases.

Macrophage polarization: an effective approach to targeted therapy of inflammatory bowel disease

Introduction: Inflammatory bowel disease (IBD) is a systemic disease with immune abnormalities that can affect the entire digestive tract. A high percentage of patients with IBD are unresponsive to current pharmacological agents, hence the need exists for novel therapeutic approaches. There is compelling evidence that macrophage polarization plays a key role in the remission of IBD patients and that it could open up future treatment options for patients.Areas covered: This paper highlights the crucial role of macrophage polarization in IBD. The authors shed light on the phenotype and function of macrophages and potential drug targets for polarization regulation. Existing approaches for regulating macrophage polarization are discussed and potential solutions for safety concerns are considered. We performed a literature search on the IBD and macrophage polarization mainly published in PubMed January 2010-July 2020.Expert opinion: Evidence indicates that there are fewer M2 macrophages and a high proportion of M1 macrophages in the intestinal tissues of individuals who are non- responsive to treatment. Regulating macrophage polarization is a potential novel targeted option for IBD treatment. Improved mechanistic insights are required to uncover more precise and effective targets for skewing macrophages into a proper phenotype.

Unfolded Protein Response and Crohn’s Diseases: A Molecular Mechanism of Wound Healing in the Gut

Endoplasmic reticulum (ER) stress triggers a series of signaling and transcriptional events termed the unfolded protein response (UPR). Severe ER stress is associated with the development of fibrosis in different organs, including lung, liver, kidney, heart, and intestine. ER stress is an essential response of epithelial and immune cells in the pathogenesis of Inflammatory Bowel Disease (IBD), including Crohn’s disease (CD). Intestinal epithelial cells are susceptible to ER stress-mediated damage due to secretion of a large amount of proteins that are involved in mucosal defense. In other cells, ER stress is linked to myofibroblast activation, extracellular matrix production, macrophage polarization, and immune cell differentiation. This review focuses on the role of the UPR in the pathogenesis in IBD from an immunologic perspective. The roles of macrophage and mesenchymal cells in the UPR from in vitro and in vivo animal models are discussed. The links between ER stress and other signaling pathways, such as senescence and autophagy, are introduced. Recent advances in the understanding of the epigenetic regulation of the UPR signaling are also updated here. The future directions of development of the UPR research and therapeutic strategies to manipulate ER stress levels are also reviewed.

15-Deoxy-△12,14-Prostaglandin J2 Promotes Resolution of Experimentally Induced Colitis

Uncontrolled macrophage functions cause failure to resolve gut inflammation and has been implicated in the pathogenesis of inflammatory bowel disease (IBD). 15-Deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), one of endogenous lipid mediators formed from arachidonic acid during the inflammatory process, has been reported to terminate inflammation. However, the pro-resolving effect of 15d-PGJ₂ on intestinal inflammation and underlying molecular mechanisms remain largely unknown. In the present study, we examined the effects of 15d-PGJ₂ on the resolution of dextran sulfate sodium (DSS)-induced murine colitis that mimics human IBD. Pharmacologic inhibition of prostaglandin D synthase (PGDS) responsible for the synthesis of 15d-PGJ₂ hampered resolution of inflammation in the colonic mucosa of mice treated with DSS. Notably, intraperitoneal injection of 15d-PGJ₂ accelerated the resolution of experimentally induced colitis. 15d-PGJ₂ treatment reduced the number of neutrophils and M1 macrophages, while it increased the proportion of M2 macrophages. Moreover, 15d-PGJ₂ treated mice exhibited the significantly reduced proportion of macrophages expressing the pro-inflammatory cytokine, IL-6 with concomitant suppression of STAT3 phosphorylation in the colonic mucosa of mice administered 2.5% DSS in drinking water. Taken together, these findings clearly indicate that 15d-PGJ₂, endogenously generated from arachidonic acid by cyclooxygenase-2 and PGDS activities in inflamed tissue, promotes resolution of intestinal colitis.

Receptors for pro-resolving mediators as a therapeutic tool for smooth muscle remodeling-associated disorders

Respiratory airway, blood vessel and intestinal wall remodeling, in which smooth muscle remodeling plays a major role, is a key pathological event underlying the development of several associated diseases, including asthma, cardiovascular disorders (e.g., atherosclerosis, hypertension, and aneurism formation), and inflammatory bowel disease. However, the mechanisms underlying these remodeling processes remain poorly understood. We hypothesize that the creation of chronic inflammation-mediated networks that support and exacerbate the airway, as well as vascular and intestinal wall remodeling, is a crucial pathogenic mechanism governing the development of the associated diseases. The failed inflammation resolution might be one of the causal pathogenic mechanisms. Hence, it is reasonable to assume that applying specialized, pro-resolving mediators (SPMs), acting via cognate G-protein coupled receptors (GPCRs), could potentially be an effective pathway for treating these disorders. However, several obstacles, such as poor understanding of the SPM/receptor signaling pathways, SMP rapid inactivation as well as their complex and costly synthesis, limit their translational potential. In this connection, stable, small-molecule SPM mimetics and receptor agonists have emerged as new, potentially suitable drugs. It has been recently shown in preclinical studies that they can effectively attenuate the manifestations of asthma, atherosclerosis and Crohn's disease. Remarkably, some biased SPM receptor agonists, which cause a signaling response in the desired inflammation pro-resolving direction, revealed similar beneficial effects. These encouraging observations suggest that SPM mimetics and receptor agonists can be applied as a novel approach for the treatment of various chronic inflammation conditions, including airway, vascular and intestinal wall remodeling-associated disorders.

GPA peptide inhibits NLRP3 inflammasome activation to ameliorate colitis through AMPK pathway

Ulcerative colitis (UC) is a chronic and idiopathic inflammatory disease that affects the colon, resulting in immune dysregulation and the production of large amounts of pro-inflammatory cytokines. Pyroptosis and NLRP3 inflammasome are associated with various kinds of inflammatory diseases including colitis. The purpose of this study is to investigate the protective effects and regulatory mechanism of Gly-Pro-Ala (GPA) peptide on DSS-induced colitis. In vivo, we find GPA peptide could exert anti-inflammatory effects on DSS-induced mice colitis, and its anti-inflammatory effects are abolished in NLRP3^-/- mice. In macrophage, GPA suppresses the assembly of NLRP3 inflammasome and GSDMD cleavage. Furthermore, GPA maintains mitochondrial homeostasis through inhibiting ROS, mtDNA and NLRP3 mitochondrial localization, with other signals related to NLRP3 inflammasome unaffected. Furthermore, the inhibitory effects of GPA on reactive oxygen species (ROS) are found to be achieved by increasing AMPK phosphorylation. Our results suggest that GPA inhibits NLRP3 inflammasome activation through increasing AMPK phosphorylation to suppress ROS, and can be applied in the prevention of colitis through targeting NLRP3.

Serum-Free Medium Enhances the Therapeutic Effects of Umbilical Cord Mesenchymal Stromal Cells on a Murine Model for Acute Colitis

The usage of animal serum may ultimately prevent the application of ex vivo cultured mesenchymal stromal cells (MSCs) in a clinical setting due to safety concerns and batch-to-batch variability. Increasing regulatory pressure to limit use of animal serum has been issued and serum-free, xeno-free, and chemically defined media (S&XFM-CD) is encouraged to replace serum-containing media (SCM) in the stem cell preparation process. We previously developed a S&XFM-CD for the expansion of umbilical cord-derived MSCs (UCMSCs). Different culture conditions affect the function of MSCs, which may further affect the therapeutic efficiency and mechanisms of action. In this study, we compared the therapeutic effect and mechanism of UCMSCs in S&XFM-CD (UCMSC^S&XFM−CD) in experimental colitis with those in SCM (UCMSC^SCM). UCMSC^S&XFM−CD exhibited better therapeutic effects than UCMSC^SCM by body weight, disease activity index, and histological colitis score. UCMSC^S&XFM−CD or UCMSC^SCM migrated to the inflammation site of injured colon, but exhibited low levels of recruitment and persistence. Systemic depletion of endogenous macrophages impaired the therapeutic effects of UCMSC^SCM and UCMSC^S&XFM−CD. Furthermore, UCMSC^S&XFM−CD more markedly promoted intestinal macrophage polarisation from M1 to M2 phenotype to produce higher levels of IL-10 and lower levels of TNF-α in colon tissue than UCMSC^SCM, while a higher level of IL-4 was produced in UCMSC^SCM-treated group. UCMSC^S&XFM−CD cocultured with RAW264.7 cells in a transwell system promoted the release of TSG-6 and IL-6, whereas UCMSC^SCM increased PGE₂ levels. Taken together, we demonstrated that UCMSCs in S&XFM-CD exhibited improved therapeutic effects with altered cytokine secretion in an experimental acute colitis model.

Mechanistic insights into the attenuation of intestinal inflammation and modulation of the gut microbiome by krill oil using in vitro and in vivo models

BACKGROUND

The anti-inflammatory property of ω-3 polyunsaturated fatty acids (PUFA) has been exploited in the management of inflammatory bowel disease (IBD) with promising results. However, it remains unclear if PUFA play a significant role in the resolution of inflammation and promotion of mucosal healing. Krill oil (KO) is a natural product rich in PUFA and the potent antioxidant, astaxanthin. In this study, we attempted to understand the mechanisms through which KO modulates the gut microbiome and metabolome using in vitro and in vivo colitis models and a multi-omics based approach.

RESULTS

KO significantly decreased LPS-induced IL1β and TNFα expression in human macrophages in vitro in a dose-dependent manner by regulating a broad spectrum of signaling pathways, including NF-κB and NOD-like receptor signaling, and displayed a synergistic effect with COX2 and IKK2 inhibitors in attenuating inflammatory pathways. Moreover, KO was involved in the resolution of inflammation by promoting M2 polarization and enhancing macrophage-mediated intracellular bacterial killing. Parasite-dependent intestinal mucosal damage and microbial dysbiosis induced by Trichuris suis infection in pigs were partially restored by feeding KO. KO supplementation reduced the abundance of Rickettsiales and several species of Lactobacillus, which were among the important features identified by random forests analysis contributing to classification accuracy for KO supplementation. Several microbial signatures with strong predictive power for the status of both infection and supplementation were identified. The inhibitory effect of KO on histidine metabolism was identified using untargeted metabolomics. KO supplementation reduced several key metabolites related to histamine metabolism by suppressing the expression of a gene encoding L-histidine decarboxylase in the colon mucosa and reducing histamine biosynthesis of microbial origin. Moreover, the pro-resolving properties of KO were validated using a Citrobacter rodentium-induced Th1-dependent colitis murine model. Further, microbial signatures with high prediction accuracy for colitis-related pathophysiological traits were identified in mice.

CONCLUSION

The findings from this study provided a mechanistic basis for optimizing microbiome-inspired alternative therapeutics in the management of IBD. The microbial signatures identified, particularly those with strong predictive accuracy for colitis phenotypes, will facilitate the development of biomarkers associated with appropriate dietary intervention to manage intestinal inflammation. Video abstract.

The fate of myofibroblasts during the development of fibrosis in Crohn's disease

Intestinal fibrosis is a devastating complication in patients with inflammatory bowel disease. Its characteristics include the loss of regular peristalsis and nutrition absorption, excessive deposition of extracellular matrix (ECM) components, thickness of intestinal lumen due to the formation of strictures and of scar tissue. As a major cell type involved in fibrogenesis, the myofibroblasts have already been shown to have a plastic and heterogeneous function in producing abundant collagen, fibronectin and connective tissue growth factor. The primary sources of ECM-producing and vimentin-positive myofibroblasts come from different precursor cells, including bone marrow-derived mesenchymal cells, fibrocytes, pericytes, epithelial to mesenchymal transition and endothelial to mesenchymal transition. Recent immunological research findings suggest that numerous cytokines and chemokines made from macrophages, in addition to T cells and other myeloid cell types, are also important drivers of myofibroblast differentiation and hence of the activation of myofibroblast-mediated transforming growth factor and collagen production. In this review we discuss the origins, roles and cell signaling of myofibroblasts during the development of fibrosis in different organs, particularly in Crohn's disease. Finally, we suggest that the epigenetic and immunological regulation of myofibroblast differentiation may provide a novel antifibrotic strategy in the near future.

TRAF3 promotes ROS production and pyroptosis by targeting ULK1 ubiquitination in macrophages

Disrupted mitochondrial function and reactive oxygen species (ROS) generation cause cellular damage and oxidative stress-induced macrophage inflammatory cell death. It remains unclear how mitochondrial dysfunction relates to inflammasome activation and pyroptotic cell death. In this study, we demonstrated that tumor necrosis factor receptor-associated factor 3 (TRAF3) regulates mitochondrial ROS production and promotes TLR agonist LPS plus nigericin (LPS/Ng)-induced inflammasome and pyroptosis in mouse primary macrophages and human monocyte THP-1 cells. Co-IP assays confirmed that TRAF3 forms a complex with TRAF2 and cIAP1 and mediates ubiquitin and degradation of Unc-51 like autophagy activating kinase 1 (ULK1). Moreover, knockdown of ULK1 in THP-1 cells significantly promoted LPS/Ng-induced inflammasome by activating caspase 1 and mature IL-1β. Apoptosis inducing factor (AIF) translocation from mitochondrial to nuclear was observed in ULK1-deficient THP-1 cells under LPS/Ng stimulation, which mediates LPS/Ng-induced cell death in ULK1 deficient macrophages. In conclusion, this study identified a novel role of TRAF3 in regulation of ULK1 ubiquitination and inflammasome signaling and provided molecular mechanisms by which ubiquitination of ULK1 controls mitochondrial ROS production, inflammasome activity, and AIF-dependent pyroptosis.

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.

Gut macrophages: key players in intestinal immunity and tissue physiology

The mammalian gastrointestinal tract harbors a large reservoir of tissue macrophages, which, in concert with other immune cells, help to maintain a delicate balance between tolerance to commensal microbes and food antigens, and resistance to potentially harmful microbes or toxins. Beyond their roles in resistance and tolerance, recent studies have uncovered novel roles played by tissue-resident, including intestinal-resident macrophages in organ physiology. Here, we will discuss recent advances in the understanding of the origin, phenotype and function of macrophages residing in the different layers of the intestine during homeostasis and under pathological conditions.