IL-22BP dictates characteristics of Peyer's patch follicle-associated epithelium for antigen uptake

A comprehensive immunobiology review of IBD: With a specific glance to Th22 lymphocytes development, biology, function, and role in IBD

The two primary forms of inflammatory disorders of the small intestine andcolon that make up inflammatory bowel disease (IBD) are ulcerative colitis (UC) and Crohn's disease (CD). While ulcerative colitis primarily affects the colon and the rectum, CD affects the small and large intestines, as well as the esophagus,mouth, anus, andstomach. Although the etiology of IBD is not completely clear, and there are many unknowns about it, the development, progression, and recurrence of IBD are significantly influenced by the activity of immune system cells, particularly lymphocytes, given that the disease is primarily caused by the immune system stimulation and activation against gastrointestinal (GI) tract components due to the inflammation caused by environmental factors such as viral or bacterial infections, etc. in genetically predisposed individuals. Maintaining homeostasis and the integrity of the mucosal barrier are critical in stopping the development of IBD. Specific immune system cells and the quantity of secretory mucus and microbiome are vital in maintaining this stability. Th22 cells are helper T lymphocyte subtypes that are particularly important for maintaining the integrity and equilibrium of the mucosal barrier. This review discusses the most recent research on these cells' biology, function, and evolution and their involvement in IBD.

Deficiency of IL-22-binding protein enhances the ability of the gut microbiota to protect against enteric pathogens

Interleukin 22 (IL-22) promotes intestinal barrier integrity, stimulating epithelial cells to enact defense mechanisms against enteric infections, including the production of antimicrobial peptides. IL-22 binding protein (IL-22BP) is a soluble decoy encoded by the Il22ra2 gene that decreases IL-22 bioavailability, attenuating IL-22 signaling. The impact of IL-22BP on gut microbiota composition and functioning is poorly understood. We found that Il22ra2-/- mice are better protected against Clostridioides difficile and Citrobacter rodentium infections. This protection relied on IL-22-induced antimicrobial mechanisms before the infection occurred, rather than during the infection itself. Indeed, the gut microbiota of Il22ra2-/- mice mitigated infection of wild-type (WT) mice when transferred via cohousing or by cecal microbiota transplantation. Indicator species analysis of WT and Il22ra2-/- mice with and without cohousing disclosed that IL22BP deficiency yields a gut bacterial composition distinct from that of WT mice. Manipulation of dietary fiber content, measurements of intestinal short-chain fatty acids and oral treatment with acetate disclosed that resistance to C. difficile infection is related to increased production of acetate by Il22ra2-/--associated microbiota. Together, these findings suggest that IL-22BP represents a potential therapeutic target for those at risk for or with already manifest infection with this and perhaps other enteropathogens.

Discrimination of distinct chicken M cell subsets based on CSF1R expression

In mammals, a subset of follicle-associated epithelial (FAE) cells, known as M cells, conduct the transcytosis of antigens across the epithelium into the underlying lymphoid tissues. We previously revealed that M cells in the FAE of the chicken lung, bursa of Fabricius (bursa), and caecum based on the expression of CSF1R. Here, we applied RNA-seq analysis on highly enriched CSF1R-expressing bursal M cells to investigate their transcriptome and identify novel chicken M cell-associated genes. Our data show that, like mammalian M cells, those in the FAE of the chicken bursa also express SOX8, MARCKSL1, TNFAIP2 and PRNP. Immunohistochemical analysis also confirmed the expression of SOX8 in CSF1R-expressing cells in the lung, bursa, and caecum. However, we found that many other mammalian M cell-associated genes such as SPIB and GP2 were not expressed by chicken M cells or represented in the chicken genome. Instead, we show bursal M cells express high levels of related genes such as SPI1. Whereas our data show that bursal M cells expressed CSF1R-highly, the M cells in the small intestine lacked CSF1R and both expressed SOX8. This study offers insights into the transcriptome of chicken M cells, revealing the expression of CSF1R in M cells is tissue-specific.

C5aR+ dendritic cells fine-tune the Peyer's patch microenvironment to induce antigen-specific CD8+ T cells

The mucosal delivery route is considered ideal for immunization. However, induction of antigen-specific mucosal immunity is difficult due to the tolerogenic environment. Therefore, developing an immunogenic mucosal dendritic cell (DC)-targeting strategy is required. Herein, we investigated the characteristics and immunogenic potential of Peyer's patch (PP) DCs as an oral vaccination-targeting strategy. Single-cell RNA sequencing analysis of the PP DCs showed that complement C5a receptor- and lysozyme-expressing DCs exhibit increased expression of genes related to chemotaxis. Administration of the Co1 peptide, a C5aR ligand, increased CD8+ T cell infiltration and response to the co-delivered model antigen in mice. Furthermore, in the SARS-CoV-2 vaccine model, vaccination with Co1 elicited both systemic and mucosal immunity. Collectively, these findings demonstrate that C5aR signaling in mucosal DCs plays a role in regulating adjuvant activity by modulating the tissue microenvironment.

Peyer's patch phagocytes acquire specific transcriptional programs that influence their maturation and activation profiles

Peyer's patches (PPs) are secondary lymphoid organs in contact with the external environment via the intestinal lumen, thus combining antigen sampling and immune response initiation sites. Therefore, they provide a unique opportunity to study the entire process of phagocyte differentiation and activation in vivo. Here, we deciphered the transcriptional and spatial landscape of PP phagocyte populations from their emergence in the tissue to their final maturation state at homeostasis and under stimulation. Activation of monocyte-derived Lysozyme-expressing dendritic cells (LysoDCs) differs from that of macrophages by their upregulation of conventional DC (cDC) signature genes such as Ccr7 and downregulation of typical monocyte-derived cell genes such as Cx3cr1. We identified gene sets that distinguish PP cDCs from the villus ones and from LysoDCs. We also identified key immature, early, intermediate, and late maturation markers of PP phagocytes. Finally, exploiting the ability of the PP interfollicular region to host both villous and subepithelial dome emigrated cDCs, we showed that the type of stimulus, the subset, but also the initial location of cDCs shape their activation profile and thus direct the immune response. Our study highlights the importance of targeting the right phagocyte subset at the right place and time to manipulate the immune response.

Effects of Royal Jelly on Gut Dysbiosis and NAFLD in db/db Mice

Royal jelly (RJ) is a naturally occurring substance synthesized by honeybees and has various health benefits. Herein, we focused on the medium-chain fatty acids (MCFAs) unique to RJ and evaluated their therapeutic efficacy in treating non-alcoholic fatty liver disease (NAFLD). We examined db/m mice that were exclusively fed a normal diet, db/db mice exclusively fed a normal diet, and db/db mice fed varying RJ quantities (0.2, 1, and 5%). RJ improved NAFLD activity scores and decreased gene expression related to fatty acid metabolism, fibrosis, and inflammation in the liver. RJ regulated innate immunity-related inflammatory responses in the small intestine and decreased the expression of genes associated with inflammation and nutrient absorption transporters. RJ increased the number of operational taxonomic units, the abundance of Bacteroides, and seven taxa, including bacteria that produce short-chain fatty acids. RJ increased the concentrations of RJ-related MCFAs (10-hidroxy-2-decenoic acid, 10-hydroxydecanoic acid, 2-decenedioic acid, and sebacic acid) in the serum and liver. These RJ-related MCFAs decreased saturated fatty acid deposition in HepG2 cells and decreased the gene expression associated with fibrosis and fatty acid metabolism. RJ and RJ-related MCFAs improved dysbiosis and regulated the expression of inflammation-, fibrosis-, and nutrient absorption transporter-related genes, thereby preventing NAFLD.

Gut-liver axis: barriers and functional circuits

The gut and the liver are characterized by mutual interactions between both organs, the microbiome, diet and other environmental factors. The sum of these interactions is conceptualized as the gut-liver axis. In this Review we discuss the gut-liver axis, concentrating on the barriers formed by the enterohepatic tissues to restrict gut-derived microorganisms, microbial stimuli and dietary constituents. In addition, we discuss the establishment of barriers in the gut and liver during development and their cooperative function in the adult host. We detail the interplay between microbial and dietary metabolites, the intestinal epithelium, vascular endothelium, the immune system and the various host soluble factors, and how this interplay establishes a homeostatic balance in the healthy gut and liver. Finally, we highlight how this balance is disrupted in diseases of the gut and liver, outline the existing therapeutics and describe the cutting-edge discoveries that could lead to the development of novel treatment approaches.

CD4 T-Cell Subsets and the Pathophysiology of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is an umbrella term for the chronic immune-mediated idiopathic inflammation of the gastrointestinal tract, manifesting as Crohn's disease (CD) or ulcerative colitis (UC). IBD is characterized by exacerbated innate and adaptive immunity in the gut in association with microbiota dysbiosis and the disruption of the intestinal barrier, resulting in increased bacterial exposure. In response to signals from microorganisms and damaged tissue, innate immune cells produce inflammatory cytokines and factors that stimulate T and B cells of the adaptive immune system, and a prominent characteristic of IBD patients is the accumulation of inflammatory T-cells and their proinflammatory-associated cytokines in intestinal tissue. Upon antigen recognition and activation, CD4 T-cells differentiate towards a range of distinct phenotypes: T helper(h)1, Th2, Th9, Th17, Th22, T follicular helper (Tfh), and several types of T-regulatory cells (Treg). T-cells are generated according to and adapt to microenvironmental conditions and participate in a complex network of interactions among other immune cells that modulate the further progression of IBD. This review examines the role of the CD4 T-cells most relevant to IBD, highlighting how these cells adapt to the environment and interact with other cell populations to promote or inhibit the development of IBD.

The intestinal microenvironment shapes macrophage and dendritic cell identity and function

The gut comprises the largest body interface with the environment and is continuously exposed to nutrients, food antigens, and commensal microbes, as well as to harmful pathogens. Subsets of both macrophages and dendritic cells (DCs) are present throughout the intestinal tract, where they primarily inhabit the gut-associate lymphoid tissue (GALT), such as Peyer's patches and isolated lymphoid follicles. In addition to their role in taking up and presenting antigens, macrophages and DCs possess extensive functional plasticity and these cells play complementary roles in maintaining immune homeostasis in the gut by preventing aberrant immune responses to harmless antigens and microbes and by promoting host defense against pathogens. The ability of macrophages and DCs to induce either inflammation or tolerance is partially lineage imprinted, but can also be dictated by their activation state, which in turn is determined by their specific microenvironment. These cells express several surface and intracellular receptors that detect danger signals, nutrients, and hormones, which can affect their activation state. DCs and macrophages play a fundamental role in regulating T cells and their effector functions. Thus, modulation of intestinal mucosa immunity by targeting antigen presenting cells can provide a promising approach for controlling pathological inflammation. In this review, we provide an overview on the characteristics, functions, and origins of intestinal macrophages and DCs, highlighting the intestinal microenvironmental factors that influence their functions during homeostasis. Unraveling the mechanisms by which macrophages and DCs regulate intestinal immunity will deepen our understanding on how the immune system integrates endogenous and exogenous signals in order to maintain the host's homeostasis.

IL-22BP production is heterogeneously distributed in Crohn’s disease

Crohn’s disease (CD), a form of inflammatory bowel disease (IBD), is characterized by impaired epithelial barrier functions and dysregulated mucosal immune responses. IL-22 binding protein (IL-22BP) is a soluble inhibitor regulating IL-22 bioactivity, a cytokine proposed to play protective roles during CD. We and others have shown that IL-22BP is produced in IBD inflamed tissues, hence suggesting a role in CD. In this work, we extended the characterization of IL-22BP production and distribution in CD tissues by applying enzyme-linked immunosorbent assays to supernatants obtained from the culture of endoscopic biopsies of patients, and reverse transcription-quantitative polymerase chain reaction on sorted immune cell subsets. We reveal that IL-22BP levels are higher in inflamed ileums than colons. We observe that in a cell-intrinsic fashion, populations of mononuclear phagocytes and eosinophils express IL-22BP at the highest levels in comparison to other sources of T cells. We suggest the enrichment of intestinal eosinophils could explain higher IL-22BP levels in the ileum. In inflamed colon, we reveal the presence of increased IL-22/IL22BP ratios compared to controls, and a strong correlation between IL-22BP and CCL24. We identify monocyte-derived dendritic cells (moDC) as a cellular subtype co-expressing both cytokines and validate our finding using in vitro culture systems. We also show that retinoic acid induces the secretion of both IL-22BP and CCL24 by moDC. Finally, we report on higher IL-22BP levels in active smokers. In conclusion, our work provides new information relevant to therapeutic strategies modulating IL-22 bioactivity in CD, especially in the context of disease location.

A potential network structure of symbiotic bacteria involved in carbon and nitrogen metabolism of wood-utilizing insect larvae

Effective biological utilization of wood biomass is necessary worldwide. Since several insect larvae can use wood biomass as a nutrient source, studies on their digestive microbial structures are expected to reveal a novel rule underlying wood biomass processing. Here, structural inferences for inhabitant bacteria involved in carbon and nitrogen metabolism for beetle larvae, an insect model, were performed to explore the potential rules. Bacterial analysis of larval feces showed enrichment of the phyla Chroloflexi, Gemmatimonadetes, and Planctomycetes, and the genera Bradyrhizobium, Chonella, Corallococcus, Gemmata, Hyphomicrobium, Lutibacterium, Paenibacillus, and Rhodoplanes, as bacteria potential involved in plant growth promotion, nitrogen cycle modulation, and/or environmental protection. The fecal abundances of these bacteria were not necessarily positively correlated with their abundances in the habitat, indicating that they were selectively enriched in the feces of the larvae. Correlation and association analyses predicted that common fecal bacteria might affect carbon and nitrogen metabolism. Based on these hypotheses, structural equation modeling (SEM) statistically estimated that inhabitant bacterial groups involved in carbon and nitrogen metabolism were composed of the phylum Gemmatimonadetes and Planctomycetes, and the genera Bradyrhizobium, Corallococcus, Gemmata, and Paenibacillus, which were among the fecal-enriched bacteria. Nevertheless, the selected common bacteria, i.e., the phyla Acidobacteria, Armatimonadetes, and Bacteroidetes and the genera Candidatus Solibacter, Devosia, Fimbriimonas, Gemmatimonas Opitutus, Sphingobium, and Methanobacterium, were necessary to obtain good fit indices in the SEM. In addition, the composition of the bacterial groups differed depending upon metabolic targets, carbon and nitrogen, and their stable isotopes, δ¹³C and δ¹⁵N, respectively. Thus, the statistically derived causal structural models highlighted that the larval fecal-enriched bacteria and common symbiotic bacteria might selectively play a role in wood biomass carbon and nitrogen metabolism. This information could confer a new perspective that helps us use wood biomass more efficiently and might stimulate innovation in environmental industries in the future.

Development of a novel feeding method for Japanese black calves with thermophile probiotics at postweaning

AIMS

Probiotic effects of compost containing thermophiles on productivity have been reported in domestic animals, although not cattle. We evaluated the effects of administering Caldibacillus hisashii, a thermophile contained in compost, on growth, blood components, faecal organic acid concentrations and microbiota population in Japanese black calves.

METHODS AND RESULTS

Calves were administered C. hisashii from 3 to 5 mo of age. Administering C. hisashii decreased feed intake without affecting body weight, indicating that feed efficiency is improved by administration. Administering C. hisashii decreased plasma insulin concentration without affecting glucose and nonesterified fatty acid concentrations. Chao1 was decreased by exposure at 5 mo of age. Similarly, weighted- and unweighted UniFrac distances were affected by treatment at 5 mo of age. Faecal abundance of the phylum Bacteroidetes tended to be increased by exposure. Faecal propionic acid concentration was correlated positively with faecal abundance of phylum Bacteroidetes but negatively with that of Firmicutes. Interestingly, the population of the genus Methanobrevibacter, representing the majority of methanogens, was lowered by exposure and was negatively correlated with faecal propionic acid concentration.

CONCLUSION

Administration of C. hisashii has the potential to improve growth performance of Japanese black calves and to contribute to reducing environmental load, which may be associated with altered endocrine kinetics and gut microbial populations.

SIGNIFICANCE AND IMPACTS OF THE STUDY

This study revealed that isolated thermophiles included in compost may exert probiotic effects on calves.

Interleukin-1β secretion induced by mucosa-associated gut commensal bacteria promotes intestinal barrier repair

The gut microbiota is essential for maintenance and repair of the intestinal epithelial barrier. As shifts in both intestinal epithelial barrier function and microbiota composition are found in inflammatory bowel disease patients, it is critical to understand the role of distinct bacteria in regulating barrier repair. We identified a mouse commensal E. coli isolate, GDAR2-2, that protects mice from Citrobacter rodentium infection and dextran sulfate sodium-induced colitis. Colonization with GDAR2-2 in mice resulted in expansion of CX3CR1+ mononuclear phagocytes, including CX3CR1+ macrophages/dendritic cells and monocytes, along with IL-22-secreting type 3 innate lymphoid cells and improved epithelial barrier function. In vitro co-culture of macrophages with GDAR2-2 resulted in IL-1β production. In vivo, protection after GDAR2-2 colonization was lost after depletion of CX3CR1+ MNPs, or blockade of IL-1β or IL-22. We further identified human commensal E. coli isolates that similarly protect mice from C. rodentium infection through CX3CR1+ MNP and IL-1β production. Together, these findings demonstrate an unexpected role for commensal bacteria in promoting IL-1β secretion to support intestinal barrier repair.

Dendritic cell functions in the inductive and effector sites of intestinal immunity

The intestine is constantly exposed to foreign antigens, which are mostly innocuous but can sometimes be harmful. Therefore, the intestinal immune system has the delicate task of maintaining immune tolerance to harmless food antigens while inducing tailored immune responses to pathogens and regulating but tolerating the microbiota. Intestinal dendritic cells (DCs) play a central role in these functions as sentinel cells able to prime and polarize the T cell responses. DCs are deployed throughout the intestinal mucosa but with local specializations along the gut length and between the diffuse effector sites of the gut lamina propria (LP) and the well-organized immune inductive sites comprising isolated lymphoid follicles (ILFs), Peyer's patches (PPs), and other species-specific gut-associated lymphoid tissues (GALTs). Understanding the specificities of each intestinal DC subset, how environmental factors influence DC functions, and how these can be modulated is key to harnessing the therapeutic potential of mucosal adaptive immune responses, whether by enhancing the efficacy of mucosal vaccines or by increasing tolerogenic responses in inflammatory disorders. In this review, we summarize recent findings related to intestinal DCs in steady state and upon inflammation, with a special focus on their functional specializations, highly dependent on their microenvironment.

IL-22 Binding Protein (IL-22BP) in the Regulation of IL-22 Biology

Cytokines are powerful mediators of inflammation. Consequently, their potency is regulated in many ways to protect the host. Several cytokines, including IL-22, have coordinating binding proteins or soluble receptors that bind to the cytokine, block the interaction with the cellular receptor, and thus prevent cellular signaling. IL-22 is a critical cytokine in the modulation of tissue responses during inflammation and is highly upregulated in many chronic inflammatory disease patients, including those with psoriasis, rheumatoid arthritis, and inflammatory bowel disease (IBD). In healthy individuals, low levels of IL-22 are secreted by immune cells, mainly in the gastrointestinal (GI) tract. However, much of this IL-22 is likely not biologically active due to the high levels of IL-22 binding protein (IL-22BP) produced by intestinal dendritic cells (DCs). IL-22BP is a soluble receptor homolog that binds to IL-22 with greater affinity than the membrane spanning receptor. Much is known regarding the regulation and function of IL-22 in health and disease. However, less is known about IL-22BP. In this review, we will focus on IL-22BP, including its regulation, role in IL-22 biology and inflammation, and promise as a therapeutic. IL-22 can be protective or pathogenic, depending on the context of inflammation. IL-22BP also has divergent roles. Ongoing and forthcoming studies will expand our knowledge of IL-22BP and IL-22 biology, and suggest that IL-22BP holds promise as a way to regulate IL-22 biology in patients with chronic inflammatory disease.

From digestion and absorption to innate immunity and health care: water and food intake may contribute to IL-22 in ILC3-dependent mucosal immunity in the jejunum

In this review, with our current studies we demonstrated medical evidence that water and food intake are useful for IL-22-related mucosal immunity-dependent maintenance of health care. The traditional Japanese health care practices recommend daily consumption of suitable volume of water. However, immunological mechanisms that support of the traditional practices are still unsolved. We focused on type 3 innate lymphoid cells (ILC3s), because the ILC3s are mainly housed in the lamina propria of the jejunum. IL-22 released from the ILC3 is transported through mesenteric lymph in collaboration with the albumin-mediated movement of consumed water. Thus, water intake-mediated upregulation of IL-22-dependent mucosal immunity contributes to the traditional Japanese health care practices. We also reviewed current studies that food intake-mediated increase in VIP-dependent neuronal activity in the small intestine and the food intake included with tryptophan-derived metabolites may accelerate the IL-22 in ILC3s-dependent mucosal immunity and then contribute in keeping health care.

Musculin Deficiency Aggravates Colonic Injury and Inflammation in Mice with Inflammatory Bowel Disease

Intestinal inflammatory reactions and resulting tissue injuries are two major aspects of inflammatory bowel disease (IBD). The regulatory factors involved in the pathogenesis of IBD remain unclear. Recent studies showed that musculin (MSC) as a transcription suppressor participates in the regulation of certain immune functions. The purpose of this study was to determine the impact of MSC deficiency on colonic injury and inflammatory reaction under IBD, where wild-type (WT, +/+) and MSC-knockout (MSCKO, MSC-/-) mice were induced for disease by dextran sulfate sodium (DSS) in drinking water. Immunohistochemistry hematoxylin-eosin (H&E) staining, enzyme-linked immunosorbent assay (ELISA), and quantitative real-time polymerase chain reaction (qRT-PCR) were used to analyze the matching samples from groups of different genotypes. The colonic epithelial injury in the MSC-/- IBD group was much severer than that in the +/+ IBD group, concurrent with higher IL-22 levels from the supernatant of ex vivo cultured colon tissues in the MSC-/- IBD group than those in the +/+ IBD group. The mRNA levels of IL-22 in mesenteric lymph nodes (MLN) also manifested similar tendency. MSC deficiency may enhance the inflammatory reactions in the gut via excessive secretion of IL-22, leading to aggravated colonic epithelial injury under IBD.

Spatial and temporal key steps in early-life intestinal immune system development and education

Education of our intestinal immune system early in life strongly influences adult health. This education strongly relies on series of events that must occur in well-defined time windows. From initial colonization by maternal-derived microbiota during delivery to dietary changes from mother's milk to solid foods at weaning, these early-life events have indeed long-standing consequences on our immunity, facilitating tolerance to environmental exposures or, on the contrary, increasing the risk of developing noncommunicable diseases such as allergies, asthma, obesity, and inflammatory bowel diseases. In this review, we provide an outline of the recent advances in our understanding of these events and how they are mechanistically related to intestinal immunity development and education. First, we review the susceptibility of neonates to infections and inflammatory diseases, related to their immune system and microbiota changes. Then, we highlight the maternal factors involved in protection and education of the mucosal immune system of the offspring, the role of the microbiota, and the nature of neonatal immune system until weaning. We also present how the development of some immune responses is intertwined in temporal and spatial windows of opportunity. Finally, we discuss pending questions regarding the neonate particular immune status and the activation of the intestinal immune system at weaning.

Group 3 Innate Lymphoid Cells Program a Distinct Subset of IL-22BP-Producing Dendritic Cells Demarcating Solitary Intestinal Lymphoid Tissues

Solitary intestinal lymphoid tissues such as cryptopatches (CPs) and isolated lymphoid follicles (ILFs) constitute steady-state activation hubs containing group 3 innate lymphoid cells (ILC3) that continuously produce interleukin (IL)-22. The outer surface of CPs and ILFs is demarcated by a poorly characterized population of CD11c⁺ cells. Using genome-wide single-cell transcriptional profiling of intestinal mononuclear phagocytes and multidimensional flow cytometry, we found that CP- and ILF-associated CD11c⁺ cells were a transcriptionally distinct subset of intestinal cDCs, which we term CIA-DCs. CIA-DCs required programming by CP- and ILF-resident CCR6⁺ ILC3 via lymphotoxin-β receptor signaling in cDCs. CIA-DCs differentially expressed genes associated with immunoregulation and were the major cellular source of IL-22 binding protein (IL-22BP) at steady state. Mice lacking CIA-DC-derived IL-22BP exhibited diminished expression of epithelial lipid transporters, reduced lipid resorption, and changes in body fat homeostasis. Our findings provide insight into the design principles of an immunoregulatory checkpoint controlling nutrient absorption.

IL-22 receptor signaling in Paneth cells is critical for their maturation, microbiota colonization, Th17-related immune responses, and anti-Salmonella immunity

Interleukin-22 (IL-22) signaling in the intestines is critical for promoting tissue-protective functions. However, since a diverse array of cell types (absorptive and secretory epithelium as well as stem cells) express IL-22Ra1, a receptor for IL-22, it has been difficult to determine what cell type(s) specifically respond to IL-22 to mediate intestinal mucosal host defense. Here, we report that IL-22 signaling in the small intestine is positively correlated with Paneth cell differentiation programs. Our Il22Ra1fl/fl;Lgr5-EGFP-creERT2-specific knockout mice and, independently, our lineage-tracing findings rule out the involvement of Lgr5+ intestinal stem cell (ISC)-dependent IL-22Ra1 signaling in regulating the lineage commitment of epithelial cells, including Paneth cells. Using novel Paneth cell-specific IL-22Ra1 knockout mice (Il22Ra1fl/fl;Defa6-cre), we show that IL-22 signaling in Paneth cells is required for small intestinal host defense. We show that Paneth cell maturation, antimicrobial effector function, expression of specific WNTs, and organoid morphogenesis are dependent on cell-intrinsic IL-22Ra1 signaling. Furthermore, IL-22 signaling in Paneth cells regulates the intestinal commensal bacteria and microbiota-dependent IL-17A immune responses. Finally, we show ISC and, independently, Paneth cell-specific IL-22Ra1 signaling are critical for providing immunity against Salmonella enterica serovar Typhimurium. Collectively, our findings illustrate a previously unknown role of IL-22 in Paneth cell-mediated small intestinal host defense.

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.

Recent Insights into Cellular Crosstalk in Respiratory and Gastrointestinal Mucosal Immune Systems

The human body is continuously threatened by pathogens, and the immune system must maintain a balance between fighting infection and becoming over-activated. Mucosal surfaces cover several anatomically diverse organs throughout the body, such as the respiratory and gastrointestinal tracts, and are directly exposed to the external environment. Various pathogens invade the body through mucosal surfaces, making the mucosa the frontline of immune defense. The immune systems of various mucosal tissues display distinctive features that reflect the tissues' anatomical and functional characteristics. This review discusses the cellular components that constitute the respiratory and gastrointestinal tracts; in particular, it highlights the complex interactions between epithelial and immune cells to induce Ag-specific immune responses in the lung and gut. This information on mucosal immunity may facilitate understanding of the defense mechanisms against infectious agents that invade mucosal surfaces, such as severe acute respiratory syndrome coronavirus 2, and provide insight into effective vaccine development.

Microbiota-Dependent Effects of IL-22

Cytokines are important contributors to immune responses against microbial and environmental threats and are of particular importance at epithelial barriers. These interfaces are continuously exposed to external factors and thus require immune components to both protect the host from pathogen invasion and to regulate overt inflammation. Recently, substantial efforts have been devoted to understanding how cytokines act on certain cells at barrier sites, and why the dysregulation of immune responses may lead to pathogenesis. In particular, the cytokine IL-22 is involved in preserving an intact epithelium, maintaining a balanced microbiota and a functioning defense system against external threats. However, a tight regulation of IL-22 is generally needed, since uncontrolled IL-22 production can lead to the progression of autoimmunity and cancer. Our aim in this review is to summarize novel findings on IL-22 and its interactions with specific microbial stimuli, and subsequently, to understand their contributions to the function of IL-22 and the clinical outcome. We particularly focus on understanding the detrimental effects of dysregulated control of IL-22 in certain disease contexts.

Retinoic Acid and Lymphotoxin Signaling Promote Differentiation of Human Intestinal M Cells

BACKGROUND & AIMS

Intestinal microfold (M) cells are a unique subset of intestinal epithelial cells in the Peyer's patches that regulate mucosal immunity, serving as portals for sampling and uptake of luminal antigens. The inability to efficiently develop human M cells in cell culture has impeded studies of the intestinal immune system. We aimed to identify signaling pathways required for differentiation of human M cells and establish a robust culture system using human ileum enteroids.

METHODS

We analyzed transcriptome data from mouse Peyer's patches to identify cell populations in close proximity to M cells. We used the human enteroid system to determine which cytokines were required to induce M-cell differentiation. We performed transcriptome, immunofluorescence, scanning electron microscope, and transcytosis experiments to validate the development of phenotypic and functional human M cells.

RESULTS

A combination of retinoic acid and lymphotoxin induced differentiation of glycoprotein 2-positive human M cells, which lack apical microvilli structure. Upregulated expression of innate immune-related genes within M cells correlated with a lack of viral antigens after rotavirus infection. Human M cells, developed in the enteroid system, internalized and transported enteric viruses, such as rotavirus and reovirus, across the intestinal epithelium barrier in the enteroids.

CONCLUSIONS

We identified signaling pathways required for differentiation of intestinal M cells, and used this information to create a robust culture method to develop human M cells with capacity for internalization and transport of viruses. Studies of this model might increase our understanding of antigen presentation and the systemic entry of enteric pathogens in the human intestine.

Organoids in immunological research

Much of our knowledge regarding the interactions between epithelial tissues and the immune system has been gathered from animal models and co-cultures with cell lines. However, unique features of human cells cannot be modelled in mice, and cell lines are often transformed or genetically immortalized. Organoid technology has emerged as a powerful tool to maintain epithelial cells in a near-native state. In this Review, we discuss how organoids are being used in immunological research to understand the role of epithelial cell-immune cell interactions in tissue development and homeostasis, as well as in diseases such as cancer.

SOD1 deficiency alters gastrointestinal microbiota and metabolites in mice

Redox imbalance induces oxidative damage and causes age-related pathologies. Mice lacking the antioxidant enzyme SOD1 (Sod1^-/-) exhibit various aging-like phenotypes throughout the body and are used as aging model mice. Recent reports suggested that age-related changes in the intestinal environment are involved in various diseases. We investigated cecal microbiota profiles and gastrointestinal metabolites in wild-type (Sod1^+/+) and Sod1^-/- mice. Firmicutes and Bacteroidetes were dominant in Sod1^+/+ mice, and most of the detected bacterial species belong to these two phyla. Meanwhile, the Sod1^-/- mice had an altered Firmicutes and Bacteroidetes ratio compared to Sod1^+/+ mice. Among the identified genera, Paraprevotella, Prevotella, Ruminococcus, and Bacteroides were significantly increased, but Lactobacillus was significantly decreased in Sod1^-/- mice compared to Sod1^+/+ mice. The correlation analyses between cecal microbiota and liver metabolites showed that Bacteroides and Prevotella spp. were grouped into the same cluster, and Paraprevotella and Ruminococcus spp. were also grouped as another cluster. These four genera showed a positive and a negative correlation with increased and decreased liver metabolites in Sod1^-/- mice, respectively. In contrast, Lactobacillus spp. showed a negative correlation with increased liver metabolites and a positive correlation with decreased liver metabolites in Sod1^-/- mice. These results suggest that the redox imbalance induced by Sod1 loss alters gastrointestinal microflora and metabolites.

Disseminated Mycobacterium Avium Infection in a Child with Complete Interferon-γ Receptor 1 Deficiency due to Compound Heterozygosis of IFNGR1 for a Subpolymorphic Copy Number Variation and a Novel Splice-Site Variant

Complete interferon-γ receptor 1 deficiency is a monogenic primary immunodeficiency caused by IFNGR1 germline defects, with autosomal dominant or recessive inheritance, which results in invasive mycobacterial diseases with varying degrees of severity. Most of the autosomal recessive IFNGR1 mutations are homozygous loss-of-function single-nucleotide variants, whereas large genomic deletions and compound heterozygosity have been very rarely reported. Herein we describe the clinical presentation, diagnosis, and successful treatment with hematopoietic stem cell transplantation of a child with disseminated Mycobacterium avium infection due to compound heterozygosity for a subpolymorphic copy number variation and a novel splice-site variant.

Interleukin-22 (IL-22) Binding Protein Constrains IL-22 Activity, Host Defense, and Oxidative Phosphorylation Genes during Pneumococcal Pneumonia

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia worldwide, and interleukin-22 (IL-22) helps contain pneumococcal burden in lungs and extrapulmonary tissues. Administration of IL-22 increases hepatic complement 3 and complement deposition on bacteria and improves phagocytosis by neutrophils. The effects of IL-22 can be tempered by a secreted natural antagonist, known as IL-22 binding protein (IL-22BP), encoded by Il22ra2 To date, the degree to which IL-22BP controls IL-22 in pulmonary infection is not well defined. Here, we show that Il22ra2 inhibits IL-22 during S. pneumoniae lung infection and that Il22ra2 deficiency favors downregulation of oxidative phosphorylation (OXPHOS) genes in an IL-22-dependent manner. Il22ra2-/- mice are more resistant to S. pneumoniae infection, have increased IL-22 in lung tissues, and sustain longer survival upon infection than control mice. Transcriptome sequencing (RNA-seq) analysis of infected Il22ra2-/- mouse lungs revealed downregulation of genes involved in OXPHOS. Downregulation of this metabolic process is necessary for increased glycolysis, a crucial step for transitioning to a proinflammatory phenotype, in particular macrophages and dendritic cells (DCs). Accordingly, we saw that macrophages from Il22ra2-/- mice displayed reduced OXPHOS gene expression upon infection with S. pneumoniae, changes that were IL-22 dependent. Furthermore, we showed that macrophages express IL-22 receptor subunit alpha-1 (IL-22Ra1) during pneumococcal infection and that Il22ra2-/- macrophages rely more on the glycolytic pathway than wild-type (WT) controls. Together, these data indicate that IL-22BP deficiency enhances IL-22 signaling in the lung, thus contributing to resistance to pneumococcal pneumonia by downregulating OXPHOS genes and increasing glycolysis in macrophages.

IL-18/IL-18BP and IL-22/IL-22BP: Two interrelated couples with therapeutic potential

Interleukin (IL)-18 and IL-22 are key components of cytokine networks that play a decisive role in (pathological) inflammation, host defense, and tissue regeneration. Tight regulation of cytokine-driven signaling, inflammation, and immunoactivation is supposed to enable nullification of a given deleterious trigger without mediating overwhelming collateral tissue damage or even activating a cancerous face of regeneration. In fact, feedback regulation by specific cytokine opponents is regarded as a major means by which the immune system is kept in balance. Herein, we shine a light on the interplay between IL-18 and IL-22 and their opponents IL-18 binding protein (IL-18BP) and IL-22BP in order to provide integrated information on their biology, pathophysiological significance, and prospect as targets and/or instruments of therapeutic intervention.

The Roles of Peyer's Patches and Microfold Cells in the Gut Immune System: Relevance to Autoimmune Diseases

Microfold (M) cells are located in the epithelium covering mucosa-associated lymphoid tissues, such as the Peyer's patches (PPs) of the small intestine. M cells actively transport luminal antigens to the underlying lymphoid follicles to initiate an immune response. The molecular machinery of M-cell differentiation and function has been vigorously investigated over the last decade. Studies have shed light on the role of M cells in the mucosal immune system and have revealed that antigen uptake by M cells contributes to not only mucosal but also systemic immune responses. However, M-cell studies usually focus on infectious diseases; the contribution of M cells to autoimmune diseases has remained largely unexplored. Accumulating evidence suggests that dysbiosis of the intestinal microbiota is implicated in multiple systemic diseases, including autoimmune diseases. This implies that the uptake of microorganisms by M cells in PPs may play a role in the pathogenesis of autoimmune diseases. We provide an outline of the current understanding of M-cell biology and subsequently discuss the potential contribution of M cells and PPs to the induction of systemic autoimmunity, beyond the mucosal immune response.

Innate lymphoid cells, mediators of tissue homeostasis, adaptation and disease tolerance

Innate lymphoid cells (ILC) are a recently identified group of tissue-resident innate lymphocytes. Available data support the view that ILC or their progenitors are deposited and retained in tissues early during ontogeny. Thereby, ILC become an integral cellular component of tissues and organs. Here, we will review the intriguing relationships between ILC and basic developmental and homeostatic processes within tissues. Studying ILC has already led to the appreciation of the integral roles of immune cells in tissue homeostasis, morphogenesis, metabolism, regeneration, and growth. This area of immunology has not yet been studied in-depth but is likely to reveal important networks contributing to disease tolerance and may be harnessed for future therapeutic approaches.

Lactobacillus sp. improved microbiota and metabolite profiles of aging rats

Aging is closely associated with altered gut function and composition, in which elderly were reported with reduced gut microbiota diversity and increased incidence of age-related diseases. Probiotics have been shown to exert beneficial health-promoting effects through modulation of intestinal microflora biodiversity, thus the effects of probiotics administration on D-galactose (D-gal) senescence-induced rat were evaluated based on the changes in gut microbiota and metabolomic profiles. Upon senescence induction, the ratio of Firmicutes/ Bacteroidetes was significantly lowered, while treatment with Lactobacillus helveticus OFS 1515 and L. fermentum DR9 increased the ratio at the phylum level (P < 0.05). Study on the genus level showed that L. paracasei OFS 0291 and L. helveticus OFS 1515 administration reduced Bacteroides, which are prominently opportunistic pathogens while L. fermentum DR9 treated rats promoted the proliferation of Lactobacillus compared to the aged rats (P < 0.05). Probiotics treatment did not alter fecal short-chain fatty acid (SCFA) profile, but an increase in acetate was observed in the D-gal rats. The analysis of fecal water-soluble metabolites showed that D-gal induced senescence caused great impact on amino acids metabolism such as urocanic acid, citrulline, cystamine and 5-oxoproline, which could serve as potential aging biomarkers. Treatment with probiotics ameliorated these metabolites in a strain-specific manner, whereby L. fermentum DR9 promoted antioxidative effect through upregulation of oxoproline, whereas both L. paracasei OFS 0291 and L. helveticus OFS 1515 restored the levels of reducing sugars, arabinose and ribose similar to the young rats. D-gal induced senescence did cause significant immunological alteration in the colon of aged rats however, all probiotic strains demonstrated immunomodulatory properties as L. paracasei OFS 0291, L. helveticus OFS 1515 and L. fermentum DR9 alleviated proinflammatory cytokines TNF-α, IFN-γ and IL-1β as well as IL-4 compared to the aged control (P < 0.05). Our study highlights the potential of probiotics as an anti-aging therapy through healthy gut modulation.

Pharmacological Targeting of the ER-Resident Chaperones GRP94 or Cyclophilin B Induces Secretion of IL-22 Binding Protein Isoform-1 (IL-22BPi1)

Of the three interleukin-22 binding protein (IL-22BP) isoforms produced by the human IL22RA2 gene, IL-22BPi2 and IL-22BPi3 are capable of neutralizing IL-22. The longest isoform, IL-22BPi1, does not bind IL-22, is poorly secreted, and its retention within the endoplasmic reticulum (ER) is associated with induction of an unfolded protein response (UPR). Therapeutic modulation of IL-22BPi2 and IL-22BPi3 production may be beneficial in IL-22-dependent disorders. Recently, we identified the ER chaperones GRP94 and cyclophilin B in the interactomes of both IL-22BPi1 and IL-22BPi2. In this study, we investigated whether secretion of the IL-22BP isoforms could be modulated by pharmacological targeting of GRP94 and cyclophilin B, either by means of geldanamycin, that binds to the ADP/ATP pocket shared by HSP90 paralogs, or by cyclosporin A, which causes depletion of ER cyclophilin B levels through secretion. We found that geldanamycin and its analogs did not influence secretion of IL-22BPi2 or IL-22BPi3, but significantly enhanced intracellular and secreted levels of IL-22BPi1. The secreted protein was heterogeneously glycosylated, with both high-mannose and complex-type glycoforms present. In addition, cyclosporine A augmented the secretion of IL-22BPi1 and reduced that of IL-22BPi2 and IL-22BPi3. Our data indicate that the ATPase activity of GRP94 and cyclophilin B are instrumental in ER sequestration and degradation of IL-22BPi1, and that blocking these factors mobilizes IL-22BPi1 toward the secretory route.

Long Interleukin-22 Binding Protein Isoform-1 Is an Intracellular Activator of the Unfolded Protein Response

The human IL22RA2 gene co-produces three protein isoforms in dendritic cells [IL-22 binding protein isoform-1 (IL-22BPi1), IL-22BPi2, and IL-22BPi3]. Two of these, IL-22BPi2 and IL-22BPi3, are capable of neutralizing the biological activity of IL-22. The function of IL-22BPi1, which differs from IL-22BPi2 through an in-frame 32-amino acid insertion provided by an alternatively spliced exon, remains unknown. Using transfected human cell lines, we demonstrate that IL-22BPi1 is secreted detectably, but at much lower levels than IL-22BPi2, and unlike IL-22BPi2 and IL-22BPi3, is largely retained in the endoplasmic reticulum (ER). As opposed to IL-22BPi2 and IL-22BPi3, IL-22BPi1 is incapable of neutralizing or binding to IL-22 measured in bioassay or assembly-induced IL-22 co-folding assay. We performed interactome analysis to disclose the mechanism underlying the poor secretion of IL-22BPi1 and identified GRP78, GRP94, GRP170, and calnexin as main interactors. Structure-function analysis revealed that, like IL-22BPi2, IL-22BPi1 binds to the substrate-binding domain of GRP78 as well as to the middle domain of GRP94. Ectopic expression of wild-type GRP78 enhanced, and ATPase-defective GRP94 mutant decreased, secretion of both IL-22BPi1 and IL-22BPi2, while neither of both affected IL-22BPi3 secretion. Thus, IL-22BPi1 and IL-22BPi2 are bona fide clients of the ER chaperones GRP78 and GRP94. However, only IL-22BPi1 activates an unfolded protein response (UPR) resulting in increased protein levels of GRP78 and GRP94. Cloning of the IL22RA2 alternatively spliced exon into an unrelated cytokine, IL-2, bestowed similar characteristics on the resulting protein. We also found that CD14++/CD16+ intermediate monocytes produced a higher level of IL22RA2 mRNA than classical and non-classical monocytes, but this difference disappeared in immature dendritic cells (moDC) derived thereof. Upon silencing of IL22RA2 expression in moDC, GRP78 levels were significantly reduced, suggesting that native IL22RA2 expression naturally contributes to upregulating GRP78 levels in these cells. The IL22RA2 alternatively spliced exon was reported to be recruited through a single mutation in the proto-splice site of a Long Terminal Repeat retrotransposon sequence in the ape lineage. Our work suggests that positive selection of IL-22BPi1 was not driven by IL-22 antagonism as in the case of IL-22BPi2 and IL-22BPi3, but by capacity for induction of an UPR response.

Pleiotropic Impacts of Macrophage and Microglial Deficiency on Development in Rats with Targeted Mutation of the Csf1r Locus

We have produced Csf1r-deficient rats by homologous recombination in embryonic stem cells. Consistent with the role of Csf1r in macrophage differentiation, there was a loss of peripheral blood monocytes, microglia in the brain, epidermal Langerhans cells, splenic marginal zone macrophages, bone-associated macrophages and osteoclasts, and peritoneal macrophages. Macrophages of splenic red pulp, liver, lung, and gut were less affected. The pleiotropic impacts of the loss of macrophages on development of multiple organ systems in rats were distinct from those reported in mice. Csf1r^-/- rats survived well into adulthood with postnatal growth retardation, distinct skeletal and bone marrow abnormalities, infertility, and loss of visceral adipose tissue. Gene expression analysis in spleen revealed selective loss of transcripts associated with the marginal zone and, in brain regions, the loss of known and candidate novel microglia-associated transcripts. Despite the complete absence of microglia, there was little overt phenotype in brain, aside from reduced myelination and increased expression of dopamine receptor-associated transcripts in striatum. The results highlight the redundant and nonredundant functions of CSF1R signaling and of macrophages in development, organogenesis, and homeostasis.

M cell-dependent antigen uptake on follicle-associated epithelium for mucosal immune surveillance

The follicle-associated epithelium (FAE) covering mucosa-associated lymphoid tissue is distinct from the villous epithelium in cellular composition and functions. Interleukin-22 binding protein (IL-22BP), provided by dendritic cells at the sub-epithelial dome region, inhibits the IL-22-mediated secretion of antimicrobial peptides by the FAE. The Notch signal from stromal cells underneath the FAE diminishes goblet cell differentiation. These events dampen the mucosal barrier functions to allow luminal microorganisms to readily gain access to the luminal surface of the FAE. Furthermore, receptor activator of nucleic factor-kappa B ligand (RANKL) from a certain stromal cell type induces differentiation into microfold (M) cells that specialize in antigen uptake in the mucosa. Microfold (M) cells play a key role in mucosal immune surveillance by actively transporting external antigens from the gut lumen to the lymphoid follicle. The molecular basis of antigen uptake by M cells has been gradually identified in the last decade. For example, GPI-anchored molecules (e.g., glycoprotein 2 (GP2) and cellular prion protein (PrP^C)) and β1-integrin facilitate the transport of specific types of xenobiotics. The antigen transport by M cells initiates antigen-specific mucosal immune responses represented by the induction of secretory immunoglobulin A (S-IgA). Meanwhile, several invasive pathogens exploit M cells as a portal to establish a systemic infection. Recent findings have uncovered the molecular machinery of differentiation and functions of M cells.

IL-22: There Is a Gap in Our Knowledge

IL-22 is a critical cytokine in modulating tissue responses during inflammation. IL-22 is upregulated in many chronic inflammatory diseases, making IL-22 biology a potentially rewarding therapeutic target. However, this is complicated by the dual-natured role of IL-22 in inflammation, as the cytokine can be protective or inflammatory depending on the disease model. Although scientific interest in IL-22 has increased considerably in the past 10 y, there is still much we do not know about the environmental, cellular, and molecular factors that regulate the production and function of this cytokine. A better understanding of IL-22 biology will allow us to develop new or improved therapeutics for treating chronic inflammatory diseases. In this article, I will highlight some of the outstanding questions in IL-22 biology.

Pivotal Role of IL-22 Binding Protein in the Epithelial Autoregulation of Interleukin-22 Signaling in the Control of Skin Inflammation

Disruption of skin homeostasis can lead to inflammatory cutaneous diseases resulting from the dysregulated interplay between epithelial keratinocytes and immune cells. Interleukin (IL)-22 signaling through membrane-bound IL-22 receptor 1 (IL-22R1) is crucial to maintain cutaneous epithelial integrity, and its malfunction mediates deleterious skin inflammation. While IL-22 binding protein (IL-22BP) binds IL-22 to suppress IL-22 signaling, how IL-22BP controls epithelial functionality to prevent skin inflammation remains unclear. Here, we describe the pivotal role of IL-22BP in mediating epithelial autoregulation of IL-22 signaling for the control of cutaneous pathogenesis. Unlike prominent expression of IL-22BP in dendritic cells in lymphoid tissues, epidermal keratinocytes predominantly expressed IL-22BP in the skin in the steady state, whereas its expression decreased during the development of psoriatic inflammation. Deficiency in IL-22BP aggravates psoriasiform dermatitis, accompanied by abnormal hyperproliferation of keratinocytes and excessive cutaneous inflammation as well as enhanced dermal infiltration of granulocytes and γδT cells. Furthermore, IL-22BP abrogates the functional alternations of keratinocytes upon stimulation with IL-22. On the other hand, treatment with IL-22BP alleviates the severity of cutaneous pathology and inflammation in psoriatic mice. Thus, the fine-tuning of IL-22 signaling through autocrine IL-22BP production in keratinocytes is instrumental in the maintenance of skin homeostasis.

Effector T Helper Cell Subsets in Inflammatory Bowel Diseases

The gastrointestinal tract is a site of high immune challenge, as it must maintain a delicate balance between tolerating luminal contents and generating an immune response toward pathogens. CD4⁺ T cells are key in mediating the host protective and homeostatic responses. Yet, CD4⁺ T cells are also known to be the main drivers of inflammatory bowel disease (IBD) when this balance is perturbed. Many subsets of CD4⁺ T cells have been identified as players in perpetuating chronic intestinal inflammation. Over the last few decades, understanding of how each subset of Th cells plays a role has dramatically increased. Simultaneously, this has allowed development of therapeutic innovation targeting specific molecules rather than broad immunosuppressive agents. Here, we review the emerging evidence of how each subset functions in promoting and sustaining the chronic inflammation that characterizes IBD.

Clinical importance of IL-22 cascade in IBD

IL-22 is a relatively new cytokine that is characterized by several unique biological properties. In the intestines, the effect of IL-22 is restricted mainly to non-lymphoid cells such as epithelial cells. Interestingly, the expression pattern and major cellular source of IL-22 have distinct difference between large and small intestines. IL-22 possesses an ability to constitutively activate STAT3 for promoting epithelial cell regeneration and reinforcing mucosal barrier integrity through stimulating the expression of anti-bacterial peptide and mucins. Of note, IL-22 is characterized as a two-faced cytokine that can play not only protective but also deleterious roles in the intestinal inflammation depending on the cytokine environment such as the expression levels of IL-23, T-bet, and IL-22 binding protein. Most importantly, clinical relevance of IL-22 to inflammatory bowel disease has been well highlighted. Mucosal healing, which represents the current therapeutic goal for IBD, can be induced by IL-22. Indeed, indigo naturalis, which can activate IL-22 pathway through Ahr, has been shown in a clinical trial to exhibit a strong therapeutic effect on ulcerative colitis. Despite the beneficial effect of IL-22, continuous activation of the IL-22 pathway increases the risk of colitis-associated cancer, particularly in patients with an extended history of IBD. This review article discusses how IL-22 regulates colitis, how beneficial versus deleterious effects of IL-22 is determined, and why IL-22 represents a promising target for IBD therapy.

Some news from the unknown soldier, the Peyer's patch macrophage

In mammals, macrophages (MF) are present in virtually all tissues where they serve many different functions linked primarily to the maintenance of homeostasis, innate defense against pathogens, tissue repair and metabolism. Although some of these functions appear common to all tissues, others are specific to the homing tissue. Thus, MF become adapted to perform particular functions in a given tissue. Accordingly, MF express common markers but also sets of tissue-specific markers linked to dedicated functions. One of the largest pool of MF in the body lines up the wall of the gut. Located in the small intestine, Peyer's patches (PP) are primary antigen sampling and mucosal immune response inductive sites. Surprisingly, although markers of intestinal MF, such as F4/80, have been identified more than 30 years ago, MF of PP escaped any kind of phenotypic description and remained "unknown" for decades. In absence of MF identification, the characterization of the PP mononuclear phagocyte system (MPS) functions has been impaired. However, taking into account that PP are privileged sites of entry for pathogens, it is important to understand how the latter are handled by and/or escape the PP MPS, especially MF, which role in killing invaders is well known. This review focuses on recent advances on the PP MPS, which have allowed, through new criteria of PP phagocyte subset identification, the characterization of PP MF origin, diversity, specificity, location and functions.

Development of intestinal M cells and follicle-associated epithelium is regulated by TRAF6-mediated NF-κB signaling

TRAF6 is essential for RANK-mediated NF-κB activation and is involved in the development of several types of cells. Kanaya et al. demonstrate that RANK–TRAF6-mediated NF-κB is essential for the development of M cells and FAE.

M cells are located in the follicle-associated epithelium (FAE) that covers Peyer’s patches (PPs) and are responsible for the uptake of intestinal antigens. The differentiation of M cells is initiated by receptor activator of NF-κB. However, the intracellular pathways involved in M cell differentiation are still elusive. In this study, we demonstrate that the NF-κB pathway activated by RANK is essential for M cell differentiation using in vitro organoid culture. Overexpression of NF-κB transcription factors enhances the expression of M cell–associated molecules but is not sufficient to complete M cell differentiation. Furthermore, we evaluated the requirement for tumor necrosis factor receptor–associated factor 6 (TRAF6). Conditional deletion of TRAF6 in the intestinal epithelium causes a complete loss of M cells in PPs, resulting in impaired antigen uptake into PPs. In addition, the expression of FAE-associated genes is almost silenced in TRAF6-deficient mice. This study thus demonstrates the crucial role of TRAF6-mediated NF-κB signaling in the development of M cells and FAE.

The Peyer's Patch Mononuclear Phagocyte System at Steady State and during Infection

The gut represents a potential entry site for a wide range of pathogens including protozoa, bacteria, viruses, or fungi. Consequently, it is protected by one of the largest and most diversified population of immune cells of the body. Its surveillance requires the constant sampling of its encounters by dedicated sentinels composed of follicles and their associated epithelium located in specialized area. In the small intestine, Peyer’s patches (PPs) are the most important of these mucosal immune response inductive sites. Through several mechanisms including transcytosis by specialized epithelial cells called M-cells, access to the gut lumen is facilitated in PPs. Although antigen sampling is critical to the initiation of the mucosal immune response, pathogens have evolved strategies to take advantage of this permissive gateway to enter the host and disseminate. It is, therefore, critical to decipher the mechanisms that underlie both host defense and pathogen subversive strategies in order to develop new mucosal-based therapeutic approaches. Whereas penetration of pathogens through M cells has been well described, their fate once they have reached the subepithelial dome (SED) remains less well understood. Nevertheless, it is clear that the mononuclear phagocyte system (MPS) plays a critical role in handling these pathogens. MPS members, including both dendritic cells and macrophages, are indeed strongly enriched in the SED, interact with M cells, and are necessary for antigen presentation to immune effector cells. This review focuses on recent advances, which have allowed distinguishing the different PP mononuclear phagocyte subsets. It gives an overview of their diversity, specificity, location, and functions. Interaction of PP phagocytes with the microbiota and the follicle-associated epithelium as well as PP infection studies are described in the light of these new criteria of PP phagocyte identification. Finally, known alterations affecting the different phagocyte subsets during PP stimulation or infection are discussed.

Vigilance or Subversion? Constitutive and Inducible M Cells in Mucosal Tissues

Microfold (M) cells are epithelial cells present in mucosal tissues and specialized for the capture of luminal microparticles and their delivery to underlying immune cells; thus, they are crucial participants in mucosal immune surveillance. Multiple phenotypic subsets of M cells have now been described, all sharing a unique apical morphology that provides clues to their ability to capture microbial particles. The existence of diverse M cell phenotypes, especially inflammation-inducible M cells, provides an intriguing puzzle: some variants may augment luminal surveillance to boost mucosal immunity, while others may promote microbial access to tissues. Here, I consider the unique induction requirements of each M cell subset and functional differences, highlighting the potentially distinct consequences in mucosal immunity.

The Development of Steady-State Activation Hubs between Adult LTi ILC3s and Primed Macrophages in Small Intestine

Group 3 innate lymphoid cells (ILC3s) are important for intestinal health, particularly in controlling inflammation in response to epithelial dysregulation, but their role during homeostasis remains less well understood. We generated IL-22 reporter mice to assess production of this key cytokine by ILC3s in the small intestine during development and under basal conditions. Although IL-22 is produced by a variety of lymphocyte populations, constitutively high IL-22 expression was limited to lymphoid-tissue inducer (LTi) cells residing in lymph node-like structures in the gut called solitary intestinal lymphoid tissues (SILT). Constitutive IL-22 expression was dependent on the microbiota and MyD88 signaling, appeared upon weaning, and was present across the spectrum of SILT, including in cryptopatches. Activated SILT LTi cells colocalized with a rare subpopulation of activated macrophages constitutively positive for IL-12/23 p40 and capable of activating neonatal LTi cells in response to TLR stimulus. Thus, weaning leads to the organization of innate immune activation hubs at SILT that mature and are continuously sustained by signals from the microbiota. This functional and anatomic organization constitutes a significant portion of the steady-state IL-23/IL-22 axis.