Epithelial-intrinsic IKKα expression regulates group 3 innate lymphoid cell responses and antibacterial immunity

Ulcerative colitis: molecular insights and intervention therapy

Ulcerative colitis (UC) is a chronic inflammatory bowel disease characterized by abdominal pain, diarrhea, rectal bleeding, and weight loss. The pathogenesis and treatment of UC remain key areas of research interest. Various factors, including genetic predisposition, immune dysregulation, and alterations in the gut microbiota, are believed to contribute to the pathogenesis of UC. Current treatments for UC include 5-aminosalicylic acids, corticosteroids, immunosuppressants, and biologics. However, study reported that the one-year clinical remission rate is only around 40%. It is necessary to prompt the exploration of new treatment modalities. Biologic therapies, such as anti-TNF-α monoclonal antibody and JAK inhibitor, primarily consist of small molecules targeting specific pathways, effectively inducing and maintaining remission. Given the significant role of the gut microbiota, research into intestinal microecologics, such as probiotics and prebiotics, and fecal microbiota transplantation (FMT) shows promising potential in UC treatment. Additionally, medicinal herbs, such as chili pepper and turmeric, used in complementary therapy have shown promising results in UC management. This article reviews recent findings on the mechanisms of UC, including genetic susceptibility, immune cell dynamics and cytokine regulation, and gut microbiota alterations. It also discusses current applications of biologic therapy, herbal therapy, microecologics, and FMT, along with their prospects and challenges.

LTβR-RelB signaling in intestinal epithelial cells protects from chemotherapy-induced mucosal damage

The intricate immune mechanisms governing mucosal healing following intestinal damage induced by cytotoxic drugs remain poorly understood. The goal of this study was to investigate the role of lymphotoxin beta receptor (LTβR) signaling in chemotherapy-induced intestinal damage. LTβR deficient mice exhibited heightened body weight loss, exacerbated intestinal pathology, increased proinflammatory cytokine expression, reduced IL-22 expression, and proliferation of intestinal epithelial cells following methotrexate (MTX) treatment. Furthermore, LTβR-/-IL-22-/- mice succumbed to MTX treatment, suggesting that LTβR- and IL-22- dependent pathways jointly promote mucosal repair. Although both LTβR ligands LIGHT and LTβ were upregulated in the intestine early after MTX treatment, LIGHT-/- mice, but not LTβ-/- mice, displayed exacerbated disease. Further, we revealed the critical role of T cells in mucosal repair as T cell-deficient mice failed to upregulate intestinal LIGHT expression and exhibited increased body weight loss and intestinal pathology. Analysis of mice with conditional inactivation of LTβR revealed that LTβR signaling in intestinal epithelial cells, but not in Lgr5+ intestinal stem cells, macrophages or dendritic cells was critical for mucosal repair. Furthermore, inactivation of the non-canonical NF-kB pathway member RelB in intestinal epithelial cells promoted MTX-induced disease. Based on these results, we propose a model wherein LIGHT produced by T cells activates LTβR-RelB signaling in intestinal epithelial cells to facilitate mucosal repair following chemotherapy treatment.

Epithelial regulation of microbiota-immune cell dynamics

The mammalian gastrointestinal tract hosts a diverse community of trillions of microorganisms, collectively termed the microbiota, which play a fundamental role in regulating tissue physiology and immunity. Recent studies have sought to dissect the cellular and molecular mechanisms mediating communication between the microbiota and host immune system. Epithelial cells line the intestine and form an initial barrier separating the microbiota from underlying immune cells, and disruption of epithelial function has been associated with various conditions ranging from infection to inflammatory bowel diseases and cancer. From several studies, it is now clear that epithelial cells integrate signals from commensal microbes. Importantly, these non-hematopoietic cells also direct regulatory mechanisms that instruct the recruitment and function of microbiota-sensitive immune cells. In this review, we discuss the central role that has emerged for epithelial cells in orchestrating intestinal immunity and highlight epithelial pathways through which the microbiota can calibrate tissue-intrinsic immune responses.

An Evaluation of Type 1 Interferon Related Genes in Male and Female-Matched, SARS-CoV-2 Infected Individuals Early in the COVID-19 Pandemic

SARS-CoV-2 infection has claimed just over 1.1 million lives in the US since 2020. Globally, the SARS-CoV-2 respiratory infection spread to 771 million people and caused mortality in 6.9 million individuals to date. Much of the early literature showed that SARS-CoV-2 immunity was defective in the early stages of the pandemic, leading to heightened and, sometimes, chronic inflammatory responses in the lungs. This lung-associated 'cytokine storm' or 'cytokine release syndrome' led to the need for oxygen supplementation, respiratory distress syndrome, and mechanical ventilation in a relatively high number of people. In this study, we evaluated circulating PBMC from non-hospitalized, male and female, COVID-19+ individuals over the course of infection, from the day of diagnosis (day 0) to one-week post diagnosis (day 7), and finally 4 weeks after diagnosis (day 28). In our early studies, we included hospitalized and critically care patient PBMC; however, most of these individuals were lymphopenic, which limited our assessments of their immune integrity. We chose a panel of 30 interferon-stimulated genes (ISG) to evaluate by PCR and completed flow analysis for immune populations present in those PBMC. Lastly, we assessed immune activation by stimulating PBMC with common TLR ligands. We identified changes in innate cells, primarily the innate lymphoid cells (ILC, NK cells) and adaptive immune cells (CD4+ and CD8+ T cells) over this time course of infection. We found that the TLR-7 agonist, Resiquimod, and the TLR-4 ligand, LPS, induced significantly better IFNα and IFNγ responses in the later phase (day 28) of SARS-CoV-2 infection in those non-hospitalized COVID-19+ individuals as compared to early infection (day 0 and day 7). We concluded that TLR-7 and TLR-4 agonists may be effective adjuvants in COVID-19 vaccines for mounting immunity that is long-lasting against SARS-CoV-2 infection.

The NF-κB signaling system in the immunopathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is an idiopathic, chronic condition characterized by episodes of inflammation in the gastrointestinal tract. The nuclear factor κB (NF-κB) system describes a family of dimeric transcription factors. Canonical NF-κB signaling is stimulated by and enhances inflammation, whereas noncanonical NF-κB signaling contributes to immune organogenesis. Dysregulation of NF-κB factors drives various inflammatory pathologies, including IBD. Signals from many immune sensors activate NF-κB subunits in the intestine, which maintain an equilibrium between local microbiota and host responses. Genetic association studies of patients with IBD and preclinical mouse models confirm the importance of the NF-κB system in host defense in the gut. Other studies have investigated the roles of these factors in intestinal barrier function and in inflammatory gut pathologies associated with IBD. NF-κB signaling modulates innate and adaptive immune responses and the production of immunoregulatory proteins, anti-inflammatory cytokines, antimicrobial peptides, and other tolerogenic factors in the intestine. Furthermore, genetic studies have revealed critical cell type-specific roles for NF-κB proteins in intestinal immune homeostasis, inflammation, and restitution that contribute to the etiopathology of IBD-associated manifestations. Here, we summarize our knowledge of the roles of these NF-κB pathways, which are activated in different intestinal cell types by specific ligands, and their cross-talk, in fueling aberrant intestinal inflammation. We argue that an in-depth understanding of aberrant immune signaling mechanisms may hold the key to identifying predictive or prognostic biomarkers and developing better therapeutics against inflammatory gut pathologies.

IL-22: Immunity's bittersweet symphony

Epithelial cells play a crucial role in barrier defense. Here, Moniruzzaman et al. (2023. J. Exp. Med.https://doi.org/10.1084/jem.20230106) discovered that interleukin-22 (IL-22) represses MHC class II expression by epithelial cells with an opposite impact on chronic inflammatory disease and viral infection.

ILC3: a case of conflicted identity

Innate lymphoid cells type 3 (ILC3s) are the first line sentinels at the mucous tissues, where they contribute to the homeostatic immune response in a major way. Also, they have been increasingly appreciated as important modulators of chronic inflammatory and autoimmune responses, both locally and systemically. The proper identification of ILC3 is of utmost importance for meaningful studies on their role in immunity. Flow cytometry is the method of choice for the detection and characterization of ILC3. However, the analysis of ILC3-related papers shows inconsistency in ILC3 phenotypic definition, as different inclusion and exclusion markers are used for their identification. Here, we present these discrepancies in the phenotypic characterization of human and mouse ILC3s. We discuss the pros and cons of using various markers for ILC3 identification. Furthermore, we consider the possibilities for the efficient isolation and propagation of ILC3 from different organs and tissues for in-vitro and in-vivo studies. This paper calls upon uniformity in ILC3 definition, isolation, and propagation for the increased possibility of confluent interpretation of ILC3's role in immunity.

Unveiling the mystery of ILC3s: Their functions and interactions in mucosal immunity

Innate lymphoid cells (ILCs) are a recently discovered subset of immune cells that play a crucial role in preserving tissue health and combating infections. Among these, ILC3s are particularly vital in regulating mucosal immunity across multiple organs such as the gut, lungs, and skin. The purpose of this article is to present a comprehensive and detailed overview of current knowledge on ILC3s, with a specific emphasis on their intricate interactions with various components of the intestinal microenvironment. Recent research on the complex, bidirectional communication pathways between ILC3s and intestinal epithelial cells, stromal cells, immune cells, microbiota, their metabolites, and diet are highlighted. Furthermore, this review comprehensively examines the diverse functions of ILC3s, which include lymphoid tissue development, tissue repair, infection, inflammation, and metabolic diseases, as well as the effector molecules that facilitate these functions. Overall, this review provides valuable insights into the biological and functional aspects of ILC3s and underscores their potential for developing innovative therapies for immune-mediated disorders, while also acknowledging the remaining knowledge gaps and challenges that need to be addressed.

The interplay between innate lymphoid cells and microbiota

Innate lymphoid cells (ILCs) are mainly resident in mucosal tissues such as gastrointestinal tract and respiratory tract, so they are closely linked to the microbiota. ILCs can protect commensals to maintain homeostasis and increase resistance to pathogens. Moreover, ILCs also play an early role in defense against a variety of pathogenic microorganisms including pathogenic bacteria, viruses, fungi and parasites, before the intervention of adaptive immune system. Due to the lack of adaptive antigen receptors expressed on T cells and B cells, ILCs need to use other means to sense the signals of microbiota and play a role in corresponding regulation. In this review, we focus on and summarize three major mechanisms used in the interaction between ILCs and microbiota: the mediation of accessory cells represented by dendritic cells; the metabolic pathways of microbiota or diet; the participation of adaptive immune cells.

Roseburia intestinalis stimulates TLR5-dependent intestinal immunity against Crohn's disease

Background

Methods

Findings

Interpretation

Funding

Pattern Recognition Receptor Signaling and Cytokine Networks in Microbial Defenses and Regulation of Intestinal Barriers: Implications for Inflammatory Bowel Disease

Inflammatory bowel disease is characterized by defects in epithelial function and dysregulated inflammatory signaling by lamina propria mononuclear cells including macrophages and dendritic cells in response to microbiota. In this review, we focus on the role of pattern recognition receptors in the inflammatory response as well as epithelial barrier regulation. We explore cytokine networks that increase inflammation, regulate paracellular permeability, cause epithelial damage, up-regulate epithelial proliferation, and trigger restitutive processes. We focus on studies using patient samples as well as speculate on pathways that can be targeted to more holistically treat patients with inflammatory bowel disease.

Effective Barriers: The Role of NKT Cells and Innate Lymphoid Cells in the Gut

The critical role of commensal microbiota in regulating the host immune response has been established. In addition, it is known that host-microbial interactions are bidirectional, and this interplay is tightly regulated to prevent chronic inflammatory disease. Although many studies have focused on the role of classic T cell subsets, unconventional lymphocytes such as NKT cells and innate lymphoid cells also contribute to the regulation of homeostasis at mucosal surfaces and influence the composition of the intestinal microbiota. In this review, we discuss the mechanisms involved in the cross-regulation between NKT cells, innate lymphoid cells, and the gut microbiota. Moreover, we highlight how disruptions in homeostasis can lead to immune-mediated disorders.

Update: Innate Lymphoid Cells in Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic and nonspecific intestinal inflammatory condition with high relapse rate. Its pathogenesis has been linked to dysbacteriosis, genetic and environmental factors. In recent years, a new type of lymphocytes, termed innate lymphoid cells, has been described and classified into three subtypes of innate lymphoid cells-group 1, group 2 and group 3. An imbalance among these subsets' interaction with gut microbiome, and other immune cells affects intestinal mucosal homeostasis. Understanding the role of innate lymphoid cells may provide ideas for developing novel and targeted approaches for treatment of IBD.

Characteristics of tissue-resident ILCs and their potential as therapeutic targets in mucosal and skin inflammatory diseases

Discovery of innate lymphoid cells (ILCs), which are non-T and non-B lymphocytes that have no antigen-specific receptors, changed the classical concept of the mechanism of allergy, which had been explained mainly as antigen-specific acquired immunity based on IgE and Th2 cells. The discovery led to dramatic improvement in our understanding of the mechanism of non-IgE-mediated allergic inflammation. Numerous studies conducted in the past decade have elucidated the characteristics of each ILC subset in various organs and tissues and their ontogeny. We now know that each ILC subset exhibits heterogeneity. Moreover, the functions and activating/suppressing factors of each ILC subset were found to differ among both organs and types of tissue. Therefore, in this review, we summarize our current knowledge of ILCs by focusing on the organ/tissue-specific features of each subset to understand their roles in various organs. We also discuss ILCs' involvement in human inflammatory diseases in various organs and potential therapeutic/preventive strategies that target ILCs.

Combined immunodeficiency with autoimmunity caused by a homozygous missense mutation in inhibitor of nuclear factor 𝛋B kinase alpha (IKKα)

[Figure: see text].

Dietary Choline-Enhanced Skin Immune Response of Juvenile Grass Carp Might Be Related to the STAT3 and NF-kB Signaling Pathway (Ctenopharyngodon idella)

To investigate the effects and potential mechanisms of dietary choline on immune function in the skin of juvenile grass carp (Ctenopharyngodon idella), fish were fed different diets containing different levels of choline (142. 2, 407.4, 821.6, 1215.8, 1589.3, and 1996.6 mg/kg) for 70 d and then sampled after a 6-d challenge test. The results exhibited that dietary choline (1) advanced the contents of phosphatidylcholine (PC), betaine, and choline in grass carp skin (P < 0.05) and upregulated the mRNA abundance of choline transporter high-affinity choline transporter 1 (CHT1), choline transporter-like protein 1 (CTL1), and choline transporter-like protein 5 (CTL5), indicating that dietary choline could increase the contents of choline which might be connected with choline transporters in the grass carp skin; (2) receded skin rot symptom after infection with A. hydrophila (Aeromonas hydrophila), increased the levels of IgM, C4, and C3 and the activities of acid phosphatase (ACP) and lysozyme (LZ), raised mucin2, β-defensin, hepcidin, and LEAP-2B mRNA abundance (rather than LEAP-2A), downregulated pro-inflammatory cytokine mRNA abundance (IFN-γ2, IL-15, TNF-α, IL-6, IL-12P40, and IL-1β) in skin of juvenile grass carp (P < 0.05), and upregulated anti-inflammatory cytokine mRNA abundance (IL-10, IL-4/13A, TGF-β1, IL-11, and IL-4/13B) in grass carp skin (P < 0.05), demonstrating that choline enhanced the skin immune function; and (3) downregulated the mRNA abundance of IKKγ, NF-κBp52, IKKβ, c-Rel, NF-κBp65, STAT3b2, STAT3b1, JAK1, and JAK2 as well as protein level of NF-κBp65, p-STAT3 Tyr705, and p-STAT3 Ser727 in nucleus and inhibited the mRNA and protein level of IkBα (P < 0.05), indicating that choline-enhanced immune function might be relevant to the JAK1, 2 /STAT3, and NF-κB signaling pathway in fish skin. In conclusion, choline enhanced the skin immune function which might be related to JAK1, 2/STAT3, and NF-κB signaling molecules in fish. Furthermore, based on immune indices of grass carp (9.28-108.97 g) skin (C3 and IgM contents as well as ACP activities), the choline requirements were estimated to be 1475.81, 1364.24, and 1574.37 mg/kg diet, respectively.

Attribution of NF-κB Activity to CHUK/IKKα-Involved Carcinogenesis

Simple Summary

CHUK/IKKα has emerged as a novel tumor suppressor in several organs of humans and mice. In general, activation of NF-κB promotes inflammation and tumorigenesis. IKKα reduction stimulates inflammatory responses including NF-κB’s targets and NF-κB-independent pathways for tumor promotion. Specific phenomena from genetically-modified mice and human TCGA database show the crosstalk between IKKα and NF-κB although their nature paths for normal organ development and the disease and cancer pathogenesis remains largely under investigation. In this review, we focus on the interplay between IKKα and NF-κB signaling during carcinogenesis. A better understanding of their relationship will provide insight into therapeutic targets of cancer.

Abstract

Studies analyzing human cancer genome sequences and genetically modified mouse models have extensively expanded our understanding of human tumorigenesis, even challenging or reversing the dogma of certain genes as originally characterized by in vitro studies. Inhibitor-κB kinase α (IKKα), which is encoded by the conserved helix-loop-helix ubiquitous kinase (CHUK) gene, is first identified as a serine/threonine protein kinase in the inhibitor-κB kinase complex (IKK), which is composed of IKKα, IKKβ, and IKKγ (NEMO). IKK phosphorylates serine residues 32 and 36 of IκBα, a nuclear factor-κB (NF-κB) inhibitor, to induce IκBα protein degradation, resulting in the nuclear translocation of NF-κB dimers that function as transcriptional factors to regulate immunity, infection, lymphoid organ/cell development, cell death/growth, and tumorigenesis. NF-κB and IKK are broadly and differentially expressed in the cells of our body. For a long time, the idea that the IKK complex acts as a direct upstream activator of NF-κB in carcinogenesis has been predominately accepted in the field. Surprisingly, IKKα has emerged as a novel suppressor for skin, lung, esophageal, and nasopharyngeal squamous cell carcinoma, as well as lung and pancreatic adenocarcinoma (ADC). Thus, Ikkα loss is a tumor driver in mice. On the other hand, lacking the RANKL/RANK/IKKα pathway impairs mammary gland development and attenuates oncogene- and chemical carcinogen-induced breast and prostate tumorigenesis and metastasis. In general, NF-κB activation leads one of the major inflammatory pathways and stimulates tumorigenesis. Since IKKα and NF-κB play significant roles in human health, revealing the interplay between them greatly benefits the diagnosis, treatment, and prevention of human cancer. In this review, we discuss the intriguing attribution of NF-κB to CHUK/IKKα-involved carcinogenesis.

An acetate-yielding diet imprints an immune and anti-microbial programme against enteric infection

Objectives

During gastrointestinal infection, dysbiosis can result in decreased production of microbially derived short‐chain fatty acids (SCFAs). In response to the presence of intestinal pathogens, we examined whether an engineered acetate‐ or butyrate‐releasing diet can rectify the deficiency of SCFAs and lead to the resolution of enteric infection.

Methods

We tested whether a high acetate‐ or butyrate‐producing diet (HAMSA or HAMSB, respectively) condition Citrobacterrodentium infection in mice and assess its impact on host‐microbiota interactions. We analysed the adaptive and innate immune responses, changes in gut microbiome function, epithelial barrier function and the molecular mechanism via metabolite sensing G protein‐coupled receptor 43 (GPR43) and IL‐22 expression.

Results

HAMSA diet rectified the deficiency in acetate production and protected against enteric infection. Increased SCFAs affect the expression of pathogen virulence genes. HAMSA diet promoted compositional and functional changes in the gut microbiota during infection similar to healthy microbiota from non‐infected mice. Bacterial changes were evidenced by the production of proteins involved in acetate utilisation, starch and sugar degradation, amino acid biosynthesis, carbohydrate transport and metabolism. HAMSA diet also induced changes in host proteins critical in glycolysis, wound healing such as GPX1 and epithelial architecture such as EZR1 and PFN1. Dietary acetate assisted in rapid epithelial repair, as shown by increased colonic Muc‐2, Il‐22, and anti‐microbial peptides. We found that acetate increased numbers of colonic IL‐22 producing TCRαβ⁺CD8αβ⁺ and TCRγδ⁺CD8αα⁺ intraepithelial lymphocytes expressing GPR43.

Conclusion

HAMSA diet may be an effective therapeutic approach for fighting inflammation and enteric infections and offer a safe alternative that may impact on human health.

Physiological function and regulatory signal of intestinal type 3 innate lymphoid cell(s)

Of the three groups of innate lymphoid cells, the type 3 innate lymphoid cell(s) (ILC3) include the subgroup of enteric ILC3 that participates in many physiological functions of the organism, such as promoting the repair of damaged mucosa, maintaining the homeostasis of gut symbiotic microorganisms, and presenting specific antigens. ILC3 also includes splenic and decidual ILC3. Like other physiological processes in the organism, enteric ILC3 functions are precisely regulated at the endogenous and exogenous levels. However, there has been no review on the physiological functions and regulatory signals of intestinal ILC3. In this paper, based on the current research on the physiological functions of enteric ILC3 in animals and the human, we summarize the signals that regulate cytokine secretion, antigen presentation and the quantity of ILC3 under normal intestinal conditions. We discuss for the first time the classification of the promoting mechanism of secretagogues of ILC3 into direct and indirect types. We also propose that ILC3 can promote intestinal homeostasis, and intestinal homeostasis can ensure the physiological phenotype of ILC3. If homeostasis is disturbed, ILC3 may participate in intestinal pathological changes. Therefore, regulating ILC3 and maintaining intestinal homeostasis are critical to the body.

Structural and functional analysis of three Iκb kinases (IKK) in disk abalone (Haliotis discus discus): Investigating their role in the innate immune responses

The IκB kinases (IKK) are large multiprotein complexes that regulate the activation of the transcription factor NF-κB and are involved in a diverse range of biological processes, including innate immunity, inflammation, and development. To explore the potential roles of invertebrate IKKs on immunity, three IKK encoding genes have been identified from molluscan species disk abalone and designed as AbIKK1, AbIKK2 and AbIKK3 at the transcriptional level. Coding sequences of AbIKK1, AbIKK2 and AbIKK3 encode the peptides of 746, 751 and 713 amino acids with the predicted molecular mass of 86.16, 86.12 and 81.88 kDa respectively. All three AbIKKs were found to share conserved IKK family features including the kinase superfamily domain (KD), ubiquitin-like domain (ULD), and α-helical scaffold/dimerization domain (SDD), similar to their mammalian counterparts. Under normal physiological conditions, AbIKKs were ubiquitously detected in six different tissues, with the highest abundance in the digestive tract and gills. Temporal transcriptional profiles in abalone hemocytes revealed the induction of AbIKK1, AbIKK2, and AbIKK3 expression following exposure to Gram-negative (Vibrio parahemolyticus) and Gram-positive (Listeria monocytogenes) bacteria, viruses (viral hemorrhagic septicemia virus, VHSV), LPS, or poly I:C. The overexpression of AbIKKs in HEK293T or RAW264.7 murine macrophage cells induced NF-κB promoter activation independent of stimulation by TNF-α or LPS. Moreover, iNOS and COX2 expression was induced in AbIKK transfected RAW264.7 murine macrophage cells and the induced state was maintained post-LPS treatment. Furthermore, mRNA levels of three selected cytokine-encoding genes (IL-1β, IL-6, and TNF-α) were found to be elevated in abalone IKK overexpressed RAW264.7 murine macrophage cells, both with and without LPS exposure. Overall, our findings demonstrated that AbIKKs identified in this study were positively involved in eliciting innate immune responses in abalone. In addition, the data revealed the presence of an evolutionarily conserved signaling mechanism for IKK mediated NF-κB activation in mollusks.

Activation and Suppression of Group 3 Innate Lymphoid Cells in the Gut

Group 3 innate lymphoid cells (ILC3s) have emerged as master regulators of intestinal health and tissue homeostasis in mammals. Through a diverse array of cytokines and cellular interactions, ILC3s crucially orchestrate lymphoid organogenesis, promote tissue protection or regeneration, facilitate antimicrobial responses, and directly regulate adaptive immunity. Further, translational studies have found that ILC3 responses are altered in the intestine of defined patient populations with chronic infectious, inflammatory, or metabolic diseases. Therefore, it is essential to broadly understand the signals that activate, suppress, or fine-tune ILC3s in the gut. Here, we discuss recent exciting advances in this field, integrate them into our current understanding of ILC3 biology, and highlight fundamental gaps in knowledge that require additional investigation.

IL-10 and IL-22 in Mucosal Immunity: Driving Protection and Pathology

The barrier surfaces of the gastrointestinal tract are in constant contact with various microorganisms. Cytokines orchestrate the mucosal adaptive and innate immune cells in the defense against pathogens. IL-10 and IL-22 are the best studied members of the IL-10 family and play essential roles in maintaining mucosal homeostasis. IL-10 serves as an important regulator in preventing pro-inflammatory responses while IL-22 plays a protective role in tissue damage and contributes to pathology in certain settings. In this review, we focus on these two cytokines in the development of gastrointestinal diseases, including inflammatory bowel diseases (IBD) and colitis-associated cancer (CAC). We summarize the recent studies and try to gain a better understanding on how they regulate immune responses to maintain equilibrium under inflammatory conditions.

Innate lymphoid cells control signaling circuits to regulate tissue-specific immunity

The multifaceted organization of the immune system involves not only patrolling lymphocytes that constantly monitor antigen-presenting cells in secondary lymphoid organs but also immune cells that establish permanent tissue-residency. The integration in the respective tissue and the adaption to the organ milieu enable tissue-resident cells to establish signaling circuits with parenchymal cells to coordinate immune responses and maintain tissue homeostasis. Innate lymphoid cells (ILCs) are tissue-resident innate immune cells that have a similar functional diversity to T cells including lineage-specifying transcription factors that drive certain effector programs. Since their formal discovery 10 years ago, it has become clear that ILCs are present in almost every tissue but strongly enriched at barrier surfaces, where they regulate immunity to infection, chronic inflammation, and tissue maintenance. In this context, recent research has identified ILCs as key in orchestrating tissue homeostasis through their ability to sustain bidirectional interactions with epithelial cells, neurons, stromal cells, adipocytes, and many other tissue-resident cells. In this review, we provide a comprehensive discussion of recent studies that define the development and heterogeneity of ILC populations and their impact on innate and adaptive immunity. Further, we discuss emerging research on the influence of the nervous system, circadian rhythm, and developmental plasticity on ILC function. Uncovering the signaling circuits that control development and function of ILCs will provide an integrated view on how immune responses in tissues are synchronized with functional relevance far beyond the classical view of the role of the immune system in discrimination between self/non-self and host defense.

Identification and Functional Characterization of Murine Group 3 Innate Lymphoid Cell (ILC3) Subsets in the Intestinal Tract and Associated Lymphoid Tissues

Group 3 innate lymphoid cells (ILC3s) are critical mediators of innate immune responses at mucosal barriers in both health and disease. ILC3s rapidly respond to environmental cues to reinforce barrier function and foster a mutualistic microbiota. ILC3s are defined by the expression of the master transcription factor RORγt, but can be further subdivided by the surface expression of the chemokine receptor CCR6 or the natural killer cell-associated receptor NKp46, as well as through the coexpression of the transcription factor T-bet. Importantly, while these subsets exhibit overlapping functions such as the secretion of the cytokines IL-17A and IL-22, they also differ significantly transcriptionally, functionally and by their localization within tissues. Thus, it is critical that studies investigating ILC3 biology consider the heterogeneity and tissue specificities of these subsets. Here, we describe common tools used to dissect and characterize ILC3s subset phenotypes and functions by flow cytometry and strategies for cell sorting of these cells in both the gastrointestinal tract and associated lymph nodes. Together, these approaches provide a tool kit for researchers aiming to dissect ILC3 subset responses at homeostasis, during infection, or in the context of inflammation.

Bacteria That Cause Enteric Diseases Stimulate Distinct Humoral Immune Responses

Bacterial enteric pathogens individually and collectively represent a serious global health burden. Humoral immune responses following natural or experimentally-induced infections are broadly appreciated to contribute to pathogen clearance and prevention of disease recurrence. Herein, we have compared observations on humoral immune mechanisms following infection with Citrobacter rodentium, the model for enteropathogenic Escherichia coli, Vibrio cholerae, Shigella species, Salmonella enterica species, and Clostridioides difficile. A comparison of what is known about the humoral immune responses to these pathogens reveals considerable variance in specific features of humoral immunity including establishment of high affinity, IgG class-switched memory B cell and long-lived plasma cell compartments. This article suggests that such variance could be contributory to persistent and recurrent disease.

Condensed tannins decreased the growth performance and impaired intestinal immune function in on-growing grass carp (Ctenopharyngodon idella)

The present study investigated the effects of condensed tannins (CT) on intestinal immune function in on-growing grass carp (Ctenopharyngodon idella). A total of 540 healthy grass carp were fed six diets containing different levels of CT (0, 10·00, 20·00, 30·00, 40·00 and 50·00 g/kg diet) for 70 d and then challenged with Aeromonas hydrophila for 14 d. The results showed that, compared with the control group, dietary CT (1) induced intestinal histopathological lesions and aggravated enteritis; (2) decreased lysozyme and acid phosphatase activities, complement 3 (C3), C4 and IgM contents and down-regulated the Hepcidin, liver-expressed antimicrobial peptide (LEAP)-2A, LEAP-2B, Mucin2 and β-defensin-1 mRNA levels in the proximal intestine (PI), mid intestine (MI) and distal intestine (DI) (P < 0·05); (3) down-regulated the mRNA levels of anti-inflammatory cytokines transforming growth factor (TGF)-β1, TGF-β2 (not in MI and DI), IL-4/13A (not IL-4/13B), IL-10 and IL-11 partly correlated with target of rapamycin (TOR) signalling; and (4) up-regulated the mRNA levels of pro-inflammatory cytokines interferon-γ2, IL-1β, IL-6, IL-8 (not in PI), IL-12p35, IL-12p40, IL-15 and IL-17D partly related to NF-κB signalling in the intestine of on-growing grass carp. Overall, the results indicated that CT could impair the intestinal immune function, and its potential regulation mechanisms were partly associated with the TOR and NF-κB signalling pathways. Finally, based on the percentage weight gain and enteritis morbidity, the maximum allowable levels of CT for on-growing grass carp (232·22-890·11 g) were estimated to be 18·6 and 17·4 g/kg diet, respectively.

Interleukin-7 Receptor Alpha in Innate Lymphoid Cells: More Than a Marker

Innate lymphoid cells (ILCs) are a group of immune cells that are important for defense against pathogens, tissue repair, and lymphoid organogenesis. They share similar characteristics with various subsets of helper T cells but lack specific antigen receptors. Interleukin-7 (IL-7) and thymic stromal lymphopoietin (TSLP) are cytokines that engage the IL-7Rα and have major roles in dictating the fate of ILCs. Recent advances in the field have revealed transcriptional programs associated with ILC development and function. In this article, we will review recent studies of the role of IL-7 and TSLP in ILC development and function during infection and inflammation.

Critical Roles of Balanced Innate Lymphoid Cell Subsets in Intestinal Homeostasis, Chronic Inflammation, and Cancer

Innate lymphoid cells (ILCs) comprise a recently identified subset of innate immune cells that are mainly localized to mucosa-associated tissues. Although they have not yet been fully characterized, they can generally be divided into ILC1s, ILC2s, and ILC3s. ILCs and their corresponding cytokines act as important mediators of the early stages of the immune response during inflammation, tissue repair, and the maintenance of epithelial integrity. Consequently, the dysregulation of ILC subsets might promote inflammation and cancer. Numerous studies have demonstrated that these cells play an important role in maintaining the microecological balance of the small intestine; however, their specific roles in mediating inflammation in this tissue and tumorigenesis remain unclear and controversial. In this review, we focus on recent progress that has helped to gain a better understanding of the role of ILCs in intestinal homeostasis, chronic inflammation, and cancer. Further focused research on the regulation and role of ILCs in intestinal homeostasis and pathology will help to reveal valuable diagnostic and therapeutic targets for the treatment of intestinal diseases.

Tissue-resident memory T cells and their biological characteristics in the recurrence of inflammatory skin disorders

The skin is the largest organ of the body. The establishment of immunological memory in the skin is a crucial component of the adaptive immune response. Once naive T cells are activated by antigen-presenting cells, a small fraction of them differentiate into precursor memory T cells. These precursor cells ultimately develop into several subsets of memory T cells, including central memory T (T_CM) cells, effector memory T (T_EM) cells, and tissue resident memory T (T_RM) cells. T_RM cells have a unique transcriptional profile, and their most striking characteristics are their long-term survival (longevity) and low migration in peripheral tissues, including the skin. Under physiological conditions, T_RM cells that reside in the skin can respond rapidly to pathogenic challenges. However, there is emerging evidence to support the vital role of T_RM cells in the recurrence of chronic inflammatory skin disorders, including psoriasis, vitiligo, and fixed drug eruption, under pathological or uncontrolled conditions. Clarifying and characterizing the mechanisms that are involved in skin T_RM cells will help provide promising strategies for reducing the frequency and magnitude of skin inflammation recurrence. Here, we discuss recent insights into the generation, homing, retention, and survival of T_RM cells and share our perspectives on the biological characteristics of T_RM cells in the recurrence of inflammatory skin disorders.

STAT4 Directs a Protective Innate Lymphoid Cell Response to Gastrointestinal Infection

Innate lymphoid cells (ILCs) are strategically positioned at mucosal barrier surfaces where they respond quickly to infection or injury. Therefore, we hypothesized that ILCs are key contributors to the early immune response in the intestine against Listeria monocytogenes Using a modified strain of L. monocytogenes that mimics human gastrointestinal listeriosis in mice, we find ILCs to be essential for control of early replication of L. monocytogenes in the intestine as well as for restricted dissemination of bacteria to peripheral tissues. Specifically, group 1 ILCs (ILC1s) and group 3 ILCs (ILC3s) respond to infection with proliferation and IFN-γ and IL-22 production. Mechanistically, we show that the transcription factor STAT4 is required for the proliferative and IFN-γ effector response by ILC1s and ILC3s, and loss of STAT4 signaling in the innate immune compartment results in an inability to control bacterial growth and dissemination. Interestingly, STAT4 acts acutely as a transcription factor to promote IFN-γ production. Together, these data illustrate a critical role for ILCs in the early responses to gastrointestinal infection with L. monocytogenes and identify STAT4 as a central modulator of ILC-mediated protection.

Mice Deficient in Epithelial or Myeloid Cell Iκκβ Have Distinct Colonic Microbiomes and Increased Resistance to Citrobacter rodentium Infection

The colonic microenvironment, stemming from microbial, immunologic, stromal, and epithelial factors, serves as an important determinant of the host response to enteric pathogenic colonization. Infection with the enteric bacterial pathogen Citrobacter rodentium elicits a strong mucosal Th1-mediated colitis and monocyte-driven inflammation activated via the classical NF-κB pathway. Research has focused on leukocyte-mediated signaling as the main driver for C. rodentium-induced colitis, however we hypothesize that epithelial cell NF-κB also contributes to the exacerbation of infectious colitis. To test this hypothesis, compartmentalized classical NF-κB defective mice, via the deletion of IKKβ in either intestinal epithelial cells (IKKβ^ΔIEC) or myeloid-derived cells (IKKβ^ΔMY), and wild type (WT) mice were challenged with C. rodentium. Both pathogen colonization and colonic histopathology were significantly reduced in IKKβ-deficient mice compared to WT mice. Interestingly, colonic IL-10, RegIIIγ, TNF-α, and iNOS gene expression were increased in IKKβ-deficient mice in the absence of bacterial challenge. This was associated with increased p52, which is involved with activation of NF-κβ through the alternative pathway. IKKβ-deficient mice also had distinct differences in colonic tissue-associated and luminal microbiome that may confer protection against C. rodentium. Taken together, these data demonstrate that classical NF-κB signaling can lead to enhanced enteric pathogen colonization and resulting colonic histopathology.

Epithelial Cells as a Transmitter of Signals From Commensal Bacteria and Host Immune Cells

Intestinal epithelial cells (IECs) are non-hematopoietic cells that form a physical barrier against external antigens. Recent studies indicate that IECs have pleiotropic functions in the regulation of luminal microbiota and the host immune system. IECs produce various immune modulatory cytokines and chemokines in response to commensal bacteria and contribute to developing the intestinal immune system. In contrast, IECs receive cytokine signals from immune cells and produce various immunological factors against luminal bacteria. This bidirectional function of IECs is critical to regulate homeostasis of microbiota and the host immune system. Disruption of the epithelial barrier leads to detrimental host diseases such as inflammatory bowel disease, colonic cancer, and pathogenic infection. This review provides an overview of the functions and physiology of IECs and highlights their bidirectional functions against luminal bacteria and immune cells, which contribute to maintaining gut homeostasis.

Microbiota Inhibit Epithelial Pathogen Adherence by Epigenetically Regulating C-Type Lectin Expression

Numerous bacterial pathogens infect the mammalian host by initially associating with epithelial cells that line the intestinal lumen. Recent work has revealed that commensal bacteria that reside in the intestine promote defense against pathogenic infection, however whether the microbiota direct host pathways that alter pathogen adherence is not well-understood. Here, by comparing germ-free mice, we identify that the microbiota decrease bacterial pathogen adherence and dampen epithelial expression of the cell surface glycoprotein C-type lectin 2e (Clec2e). Functional studies revealed that overexpression of this lectin promotes adherence of intestinal bacterial pathogens to mammalian cells. Interestingly, microbiota-sensitive downregulation of Clec2e corresponds with decreased histone acetylation of the Clec2e gene in intestinal epithelial cells. Histone deacetylation and transcriptional regulation of Clec2e depends on expression and recruitment of the histone deacetylase HDAC3. Thus, commensal bacteria epigenetically instruct epithelial cells to decrease expression of a C-type lectin that promotes pathogen adherence, revealing a novel mechanism for how the microbiota promote innate defense against infection.

IL-10 Family Cytokines IL-10 and IL-22: from Basic Science to Clinical Translation

Cytokines are among the most important effector and messenger molecules in the immune system. They profoundly participate in immune responses during infection and inflammation, protecting against or contributing to diseases such as allergy, autoimmunity, and cancer. Manipulating cytokine pathways, therefore, is one of the most effective strategies to treat various diseases. IL-10 family cytokines exert essential functions to maintain tissue homeostasis during infection and inflammation through restriction of excessive inflammatory responses, upregulation of innate immunity, and promotion of tissue repairing mechanisms. Their important functions in diseases are supported by data from many preclinical models, human genetic studies, and clinical interventions. Despite significant efforts, however, there is still no clinically approved therapy through manipulating IL-10 family cytokines. Here, we summarize the recent progress in understanding the biology of this family of cytokines, suggesting more specific strategies to maneuver these cytokines for the effective treatment of inflammatory diseases and cancers.

Innate lymphoid cell type 3-derived interleukin-22 boosts lipocalin-2 production in intestinal epithelial cells via synergy between STAT3 and NF-κB

Escherichia coli and Klebsiella pneumoniae are opportunistic pathogens that are commonly associated with infections at mucosal surfaces, such as the lung or the gut. The host response against these types of infections includes the release of epithelial-derived antimicrobial factors such as lipocalin-2 (LCN-2), a protein that specifically inhibits the iron acquisition of Enterobacteriaceae by binding and neutralizing the bacterial iron-scavenging molecule enterobactin. Regulation of epithelial antimicrobial responses, including the release of LCN-2, has previously been shown to depend on IL-22, a cytokine produced by innate lymphoid cells type 3 (ILC3) during Enterobacteriaceae infections. However, much remains unknown about the extent to which antimicrobial responses are regulated by IL-22 and how IL-22 regulates the expression and production of LCN-2 in intestinal epithelial cells (IECs). Our study demonstrates how IL-22-induced activation of STAT3 synergizes with NF-κB-activating cytokines to enhance LCN-2 expression in human IECs and elucidates how ILC3 are involved in LCN-2-mediated host defense against Enterobacteriaceae. Together, these results provide new insight into the role of ILC3 in regulating LCN-2 expression in human IECs and could prove useful in future studies aimed at understanding the host response against Enterobacteriaceae as well as for the development of antimicrobial therapies against Enterobacteriaceae-related infections.

TSLP Protects Corneas From Pseudomonas aeruginosa Infection by Regulating Dendritic Cells and IL-23-IL-17 Pathway

Purpose

We sought to determine the role of epithelium-produced thymic stromal lymphopoietin (TSLP) and its underlying mechanisms in corneal innate immune defense against Pseudomonas (P.) aeruginosa keratitis.

Methods

The expression of TSLP and TSLPR in cultured human corneal epithelial cells (HCECs) and mouse corneas was determined by PCR, Western, and/or ELISA. Cellular localization of TSLP receptor (TSLPR) was determined by whole mount confocal microscopy. TSLP-TSLPR signaling was downregulated by neutralizing antibodies and/or small interfering (si)RNA; their effects on the severity of P. aeruginosa–keratitis and cytokine expression were assessed using clinical scoring, bacterial counting, PMN infiltration, and real-time PCR. The role of dendritic cells (DCs) in corneal innate immunity was determined by local DC depletion using CD11c-DTR mice.

Results

P. aeruginosa–infection induced the expression of TSLP and TSLPR in both cultured primary HCECs and in C57BL/6 mouse corneas. While TSLP was mostly expressed by epithelial cells, CD11c-positive cells were positive for TSLPR. Targeting TSLP or TSLPR with neutralizing antibodies or TSLPR with siRNA resulted in more severe keratitis, attributable to an increase in bacterial burden and PMN infiltration. TSLPR neutralization significantly suppressed infection-induced TSLP and interleukin (IL)-17C expression and augmented the expression of IL-23 and IL-17A. Local depletion of DCs markedly increased the severity of keratitis and exhibited no effects on TSLP and IL-23 expression while suppressing IL-17A and C expression in P. aeruginosa–infected corneas.

Conclusions

The epithelium-expressed TSLP plays a protective role in P. aeruginosa keratitis through targeting of DCs and in an IL-23/IL-17 signaling pathway-related manner.

TL1A (TNFSF15) and DR3 (TNFRSF25): A Co-stimulatory System of Cytokines With Diverse Functions in Gut Mucosal Immunity

TL1A and its functional receptor DR3 are members of the TNF/TNFR superfamilies of proteins. Binding of APC-derived TL1A to lymphocytic DR3 provides co-stimulatory signals for activated lymphocytes. DR3 signaling affects the proliferative activity of and cytokine production by effector lymphocytes, but also critically influences the development and suppressive function of regulatory T-cells. DR3 was also found to be highly expressed by innate lymphoid cells (ILCS), which respond to stimulation by TL1A. Several recent studies with transgenic and knockout mice as well as neutralizing or agonistic antibodies for these two proteins, have clearly shown that TL1A/DR3 are important mediators of several chronic immunological disorders, including Inflammatory Bowel Disease (IBD). TL1A and DR3 are abundantly localized at inflamed intestinal areas of patients with IBD and mice with experimental ileitis or colitis and actively participate in the immunological pathways that underlie mucosal homeostasis and intestinal inflammation. DR3 signaling has demonstrated a dichotomous role in mucosal immunity. On the one hand, during acute mucosal injury it exerts protective functions by ameliorating the severity of acute inflammatory responses and facilitating tissue repair. On the other hand, it critically participates in the pro-inflammatory pathways that underlie chronic inflammatory responses, such as those that take place in IBD. These effects are mediated through modulation of the relative mucosal abundance and function of Th1, Th2, Th17, Th9, and Treg lymphocytes, but also of all types of ILCs. Recently, an important role was demonstrated for TL1A/DR3 as potential mediators of intestinal fibrosis that is associated with the presence of gut inflammation. These accumulating data have raised the possibility that TL1A/DR3 pathways may represent a valid therapeutic target for chronic immunological diseases. Nevertheless, applicability of such a therapeutic approach will greatly rely on the net result of TL1A/DR3 manipulation on the various cell populations that will be affected by this approach.

Innate lymphoid cells: A potential link between microbiota and immune responses against cancer

The adaptive immune system plays a crucial role in anti-tumor surveillance. Enhancement of T cell responses through checkpoint blockade has become a major therapeutic avenue of intervention for several tumors. Because it shapes immune responses and regulates their amplitude and duration, the microbiota has a substantial impact on anti-tumor immunity. Innate lymphoid cells (ILCs) comprise a heterogeneous population of lymphocytes devoid of antigen-specific receptors that mirror T helper cells in their ability to secrete cytokines that activate immune responses. Ongoing studies suggest that ILCs contribute to anti-tumor responses. Moreover, since ILCs are present at barrier surfaces, they are stimulated by the microbiota and, reciprocally, influence the composition of the microbiota by regulating the surface barrier microenvironment. Thus, ILC-microbiota cross-talk may in part underpin the effects of the microbiota on anti-tumor responses. In this article, we review current evidence linking ILCs to cancer and discuss the potential impact of ILC-microbiota cross-talk in anti-tumor immune responses.

Dietary gossypol reduced intestinal immunity and aggravated inflammation in on-growing grass carp (Ctenopharyngodon idella)

The present study explored the effects of dietary gossypol on the gut health of on-growing grass carp. The fish were fed six diets containing different levels of free gossypol (0, 121.38, 243.94, 363.89, 759.93 and 1162.06 mg/kg diet) from gossypol-acetic acid for 60 days and then challenged with Aeromonas hydrophila for 14 days. The results showed that dietary gossypol (1) could aggravate enteritis and damage the structure of intestinal epithelial cells, (2) decreased the lysozyme (LZ) and Acid phosphatase (ACP) activities, complement 3 (C3), C4 and immunoglobulin M (IgM) contents, and it down-regulated the Hepcidin (rather than distal intestine (DI)), immunoglobulin Z (IgZ), liver-expressed antimicrobial peptide (LEAP)-2B, Mucin2 and β-defensin-1 mRNA levels in the proximal intestine (PI), mid intestine (MI) and DI, (3) up-regulated intestinal pro-inflammatory cytokines tumor necrosis factor α (TNF-α), interferon γ2 (IFN-γ2), interleukin 1β (IL-1β), IL-6 (only in PI), IL-8 and IL-12p35 mRNA levels partly related to nuclear factor kappa B (NF-κB) signalling, and (4) down-regulated the mRNA levels of anti-inflammatory cytokines such as transforming growth factor (TGF)-β1, TGF-β2, interleukin 4/13A (IL-4/13A) (except IL-4/13B), IL-10 and IL-11 partly relating to target of rapamycin (TOR) signalling in the intestines of on-growing grass carp. Moreover, the dietary gossypol had no impact on the LEAP-2A, IL-12P40, IL-17D, IL-10, NF-κBp52, IKKα and eIF4E-binding proteins 2 (4E-BP2) mRNA levels in the intestines. Finally, based on the intestinal histopathological results, enteritis morbidity, LZ activity and IgM content, the safe dose of gossypol in the diets for on-growing grass carp should be less than 103.42 mg/kg diet.

Intestinal non-canonical NFκB signaling shapes the local and systemic immune response

Microfold cells (M-cells) are specialized cells of the intestine that sample luminal microbiota and dietary antigens to educate the immune cells of the intestinal lymphoid follicles. The function of M-cells in systemic inflammatory responses are still unclear. Here we show that epithelial non-canonical NFkB signaling mediated by NFkB-inducing kinase (NIK) is highly active in intestinal lymphoid follicles, and is required for M-cell maintenance. Intestinal NIK signaling modulates M-cell differentiation and elicits both local and systemic IL-17A and IgA production. Importantly, intestinal NIK signaling is active in mouse models of colitis and patients with inflammatory bowel diseases; meanwhile, constitutive NIK signaling increases the susceptibility to inflammatory injury by inducing ectopic M-cell differentiation and a chronic increase of IL-17A. Our work thus defines an important function of non-canonical NFkB and M-cells in immune homeostasis, inflammation and polymicrobial sepsis.

Macrophage β2-Integrins Regulate IL-22 by ILC3s and Protect from Lethal Citrobacter rodentium-Induced Colitis

β2-integrins promote neutrophil recruitment to infected tissues and are crucial for host defense. Neutrophil recruitment is defective in leukocyte adhesion deficiency type-1 (LAD1), a condition caused by mutations in the CD18 (β2-integrin) gene. Using a model of Citrobacter rodentium (CR)-induced colitis, we show that CD18-/- mice display increased intestinal damage and systemic bacterial burden, compared to littermate controls, ultimately succumbing to infection. This phenotype is not attributed to defective neutrophil recruitment, as it is shared by CXCR2-/- mice that survive CR infection. CR-infected CD18-/- mice feature prominent upregulation of IL-17 and downregulation of IL-22. Exogenous IL-22 administration, but not endogenous IL-17 neutralization, protects CD18-/- mice from lethal colitis. β2-integrin expression on macrophages is mechanistically linked to Rac1/ROS-mediated induction of noncanonical-NLRP3 (nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3) inflammasome-dependent IL-1β production, which promotes ILC3-derived IL-22. Therefore, β2-integrins are required for protective IL-1β-dependent IL-22 responses in colitis, and the identified mechanism may underlie the association of human LAD1 with colitis.

Innate lymphoid cell sensing of tissue vitality

Innate lymphoid cells (ILCs) constitute a heterogeneous population of cytokine-secreting cells that colonize different tissues and are heavily reliant on cytokines and other secreted factors for their development, maintenance and effector functions. Most ILCs are tissue resident and differentiate in non-lymphoid peripheral tissues. As tissue-resident sentinels, ILCs must rapidly identify pathogens or malignancy in an effort to return the tissue to homeostasis. Here we review the mechanisms that ILCs employ to sense cytokines and other potent immunoregulatory factors that promote their development in different tissues as well as the ability to distinguish pathogenic versus healthy tissue microenvironments and highlight the importance of these pathways for human disease.

Interplay of innate lymphoid cells and the microbiota

Innate lymphoid cells (ILC) are a recently identified group of innate lymphocytes that are preferentially located at barrier surfaces. Barrier surfaces are in direct contact with complex microbial ecosystems, collectively referred to as the microbiota. It is now believed that the interplay of the microbiota with host components (i.e. epithelial cells and immune cells) promotes host fitness by regulating organ homeostasis, metabolism, and host defense against pathogens. In this review, we will give an overview of this multifaceted interplay between ILC and components of the microbiota.

The Citrobacter rodentium type III secretion system effector EspO affects mucosal damage repair and antimicrobial responses

Infection with Citrobacter rodentium triggers robust tissue damage repair responses, manifested by secretion of IL-22, in the absence of which mice succumbed to the infection. Of the main hallmarks of C. rodentium infection are colonic crypt hyperplasia (CCH) and dysbiosis. In order to colonize the host and compete with the gut microbiota, C. rodentium employs a type III secretion system (T3SS) that injects effectors into colonic intestinal epithelial cells (IECs). Once injected, the effectors subvert processes involved in innate immune responses, cellular metabolism and oxygenation of the mucosa. Importantly, the identity of the effector/s triggering the tissue repair response is/are unknown. Here we report that the effector EspO ,an orthologue of OspE found in Shigella spp, affects proliferation of IECs 8 and 14 days post C. rodentium infection as well as secretion of IL-22 from colonic explants. While we observed no differences in the recruitment of group 3 innate lymphoid cells (ILC3s) and T cells, which are the main sources of IL-22 at the early and late stages of C. rodentium infection respectively, infection with ΔespO was characterized by diminished recruitment of sub-mucosal neutrophils, which coincided with lower abundance of Mmp9 and chemokines (e.g. S100a8/9) in IECs. Moreover, mice infected with ΔespO triggered significantly lesser nutritional immunity (e.g. calprotectin, Lcn2) and expression of antimicrobial peptides (Reg3β, Reg3γ) compared to mice infected with WT C. rodentium. This overlapped with a decrease in STAT3 phosphorylation in IECs. Importantly, while the reduced CCH and abundance of antimicrobial proteins during ΔespO infection did not affect C. rodentium colonization or the composition of commensal Proteobacteria, they had a subtle consequence on Firmicutes subpopulations. EspO is the first bacterial virulence factor that affects neutrophil recruitment and secretion of IL-22, as well as expression of antimicrobial and nutritional immunity proteins in IECs.

Interleukin 22 Expands Transit-Amplifying Cells While Depleting Lgr5+ Stem Cells via Inhibition of Wnt and Notch Signaling

BACKGROUND & AIMS

Epithelial regeneration is essential for homeostasis and repair of the mucosal barrier. In the context of infectious and immune-mediated intestinal disease, interleukin (IL) 22 is thought to augment these processes. We sought to define the mechanisms by which IL22 promotes mucosal healing.

METHODS

Intestinal stem cell cultures and mice were treated with recombinant IL22. Cell proliferation, death, and differentiation were assessed in vitro and in vivo by morphometric analysis, quantitative reverse transcriptase polymerase chain reaction, and immunohistochemistry.

RESULTS

IL22 increased the size and number of proliferating cells within enteroids but decreased the total number of enteroids. Enteroid size increases required IL22-dependent up-regulation of the tight junction cation and water channel claudin-2, indicating that enteroid enlargement reflected paracellular flux-induced swelling. However, claudin-2 did not contribute to IL22-dependent enteroid loss, depletion of Lgr5+ stem cells, or increased epithelial proliferation. IL22 induced stem cell apoptosis but, conversely, enhanced proliferation within and expanded numbers of transit-amplifying cells. These changes were associated with reduced wnt and notch signaling, both in vitro and in vivo, as well as skewing of epithelial differentiation, with increases in Paneth cells and reduced numbers of enteroendocrine cells.

CONCLUSIONS

IL22 promotes transit-amplifying cell proliferation but reduces Lgr5+ stem cell survival by inhibiting notch and wnt signaling. IL22 can therefore promote or inhibit mucosal repair, depending on whether effects on transit-amplifying or stem cells predominate. These data may explain why mucosal healing is difficult to achieve in some inflammatory bowel disease patients despite markedly elevated IL22 production.

Runx3 Mediates Resistance to Intracellular Bacterial Infection by Promoting IL12 Signaling in Group 1 ILC and NCR+ILC3

Innate lymphoid cells (ILCs) are the most recently identified family of the innate immune system and are hypothesized to modulate immune functions prior to the generation of adaptive immune responses. Subsets of ILCs reside in the mucosa and regulate immune responses to external pathogens; however, their role and the mechanism by which they protect against intracellular bacterial infection is not completely understood. In this report, using S. typhimurium and L. monocytogenes, we found that the levels of group 1 ILCs and NCR⁺ ILC3s were increased upon infection and that these increases were associated with Runt-related transcription factor 3 (Runx3) expression. Runx3 ^fl/fl PLZF-cre mice were much more sensitive to infection with the intracellular bacterial pathogens S. typhimurium and L. monocytogenes partially due to abnormal Group 1 ILC and NCR⁺ILC3 function. We also found that Runx3 directly binds to the Il12Rβ2 promoter and intron 8 to accelerate the expression of Il12Rβ2 and modulates IFNγ secretion triggered by the IL12/ STAT4 axis. Therefore, we demonstrate that Runx3 influences group 1 ILC- and NCR+ILC3-mediated immune protection against intracellular bacterial infections of both the gut and liver.

The Innate Immune System in the Gastrointestinal Tract: Role of Intraepithelial Lymphocytes and Lamina Propria Innate Lymphoid Cells in Intestinal Inflammation

BACKGROUND

The gastrointestinal tract harbors the largest microbiota load in the human body, hence maintaining a delicate balance between immunity against invading pathogens and tolerance toward commensal. Such immune equilibrium, or intestinal homeostasis, is conducted by a tight regulation and cooperation of the different branches of the immune system, including the innate and the adaptive immune system. However, several factors affect this delicate equilibrium, ultimately leading to gastrointestinal disorders including inflammatory bowel disease. Therefore, here we decided to review the currently available information about innate immunity lymphocyte subsets playing a role in intestinal inflammation.

RESULTS

Intestinal innate lymphocytes are composed of intraepithelial lymphocytes (IELs) and lamina propria innate lymphoid cells (ILCs). While IELs can be divided into natural or induced, ILCs can be classified into type 1, 2, or 3, resembling, respectively, the properties of TH1, TH2, or TH17 adaptive lymphocytes. Noteworthy, the phenotype and function of both IELs and ILCs are disrupted under inflammatory conditions, where they help to exacerbate intestinal immune responses.

CONCLUSIONS

The modulation of both IELs and ILCs to control intestinal inflammatory responses represents a major challenge, as they provide tight regulation among the epithelium, the microbiota, and the adaptive immune system. An improved understanding of the innate immunity mechanisms involved in gastrointestinal inflammation would therefore aid in the diagnosis and further treatment of gastrointestinal inflammatory disorders.