The role of TLRs,NLRs, and RLRs in mucosal innate immunity and homeostasis

The mucosal surfaces of the gastrointestinal tract are continually exposed to an enormous antigenic load of microbial and dietary origin, yet homeostasis is maintained. Pattern recognition molecules (PRMs) have a key role in maintaining the integrity of the epithelial barrier and in promoting maturation of the mucosal immune system. Commensal bacteria modulate the expression of a broad range of genes involved in maintaining epithelial integrity, inflammatory responses, and production of antimicrobial peptides. Mice deficient in PRMs can develop intestinal inflammation, which is dependent on the microbiota, and in humans, PRM polymorphisms are associated with exacerbated inflammatory bowel disease. Innate immune responses and epithelial barrier function are regulated by PRM-induced signaling at multiple levels, from the selective expression of receptors on mucosal cells or compartments to the expression of negative regulators. Here, we describe recent advances in our understanding of innate signaling pathways, particularly by Toll-like receptors and nucleotide-binding domain and leucine-rich repeat containing receptors at mucosal surfaces.     

Inflammasomes and intestinal inflammation

The inflammasome is a cytosolic multi-protein innate immune rheostat, sensing a variety of endogenous and environmental stimuli, and regulating homeostasis or damage control. In the gastrointestinal tract, inflammasomes orchestrate immune tolerance to microbial and potentially food-related signals or drive the initiation of inflammatory responses to invading pathogens. When inadequately regulated, intestinal inflammasome activation leads to a perpetuated inflammatory response leading to immune pathology and tissue damage. In this review, we present the main features of the predominant types of inflammasomes participating in intestinal homeostasis and inflammation. We then discuss current controversies and open questions related to their functions and implications in disease, highlighting how pathological inflammasome over-activation or impaired function impact gut homeostasis, the microbiome ecosystem, and the propensity to develop gut-associated diseases. Collectively, understanding of the molecular basis of intestinal inflammasome signaling may be translated into clinical manipulation of this fundamental pathway as a potential immune modulatory therapeutic intervention.     

Pathogenic T cell subsets in allergic and chronic inflammatory bowel disorders

Homeostasis in the gastrointestinal tract relies on a sensitive equilibrium between permissive and protective functions. This is closely reflected in the regulation of the intestinal immune system and especially T cells in the gut. This balance, however, is susceptible to disturbances as demonstrated by pathological conditions like food allergy, celiac disease, or inflammatory bowel disease. In these allergic and chronic inflammatory bowel disorders, luminal antigens get access to the lamina propria where they trigger a dysregulated immune response with crucial involvement of different T cell subsets. We will begin this review with some comprehensive remarks on current concepts on the pathogenesis of these diseases before taking a closer look at the life cycle of intestinal T cells consisting of priming, homing, differentiation and proliferation and apoptosis respectively. Subsequently we will discuss the specific implication of distinct T cell subsets in allergic and chronic inflammatory conditions of the gastrointestinal tract in detail and comment on current and future approaches to targeted therapy in this context.     

Manipulation of epithelial integrity and mucosal immunity by host and microbiota-derived metabolites

The human intestinal tract contains a large number of microbes, their metabolites, and potentially harmful food antigens. The intestinal epithelium separates the mucosa where immune cells are located from luminal microbes by expressing various factors that assemble into physical and chemical barriers. In addition to epithelial cells, immune cells are essential for enforcing mucosal barriers through production of inflammatory and anti-inflammatory mediators. Intestinal microbiota, represented by gut ecological communities of living microorganisms, influences maturation and homeostasis of host immune system and contributes to the maintenance of the epithelial integrity with small molecules derived from their metabolism, termed metabolites. In turn, immune cells receive signals from microbiota, and may play key role in maintenance of a healthy bacterial composition and reinforcement of epithelial barrier functions, leading to the establishment of a host-bacterial mutualism. Alterations in the microbiota community and metabolome profiles are observed in patients with various disorders including inflammatory bowel disease. In this review, we will discuss physiological functions of the microbiota and its metabolites in regulating host immune system and reinforcing epithelial barrier functions. Further understanding of these processes will aid in identification of novel therapeutic targets and subsequent development of therapeutic interventions in a range of chronic inflammatory diseases.     

Antibiotics, microbiota, and immune defense

The gastrointestinal tract microbiota contributes to the development and differentiation of the mammalian immune system. The composition of the microbiota affects immune responses and affects susceptibility to infection by intestinal pathogens and development of allergic and inflammatory bowel diseases. Antibiotic administration, while facilitating clearance of targeted infections, also perturbs commensal microbial communities and decreases host resistance to antibiotic-resistant microbes. Here, we review recent advances that begin to define the interactions between complex intestinal microbial populations and the mammalian immune system and how this relation is perturbed by antibiotic administration. We further discuss how antibiotic-induced disruption of the microbiota and immune homeostasis can lead to disease and we review strategies to restore immune defenses during antibiotic administration.     

Anatomical localization of commensal bacteria in immune cell homeostasis and disease

The mammalian gastrointestinal (GI) tract is colonized by trillions of beneficial commensal bacteria that are essential for promoting normal intestinal physiology. While the majority of commensal bacteria are found in the intestinal lumen, many species have also adapted to colonize different anatomical locations in the intestine, including the surface of intestinal epithelial cells (IECs) and the interior of gut-associated lymphoid tissues. These distinct tissue localization patterns permit unique interactions with the mammalian immune system and collectively influence intestinal immune cell homeostasis. Conversely, dysregulated localization of commensal bacteria can lead to inappropriate activation of the immune system and is associated with numerous chronic infectious, inflammatory, and metabolic diseases. Therefore, regulatory mechanisms that control proper anatomical containment of commensal bacteria are essential to maintain tissue homeostasis and limit pathology. In this review, we propose that commensal bacteria associated with the mammalian GI tract can be anatomically defined as (i) luminal, (ii) epithelial-associated, or (iii) lymphoid tissue-resident, and we discuss the role and regulation of these microbial populations in health and disease.     

Innate immune signaling in defense against intestinal microbes

The gastrointestinal system is a common entry point for pathogenic microbes to access the inner environment of the body. Anti-microbial factors produced by the intestinal mucosa limit the translocation of both commensal and pathogenic microbes across the intestinal epithelial cell barrier. The regulation of these host defense mechanisms largely depends on the activation of innate immune receptors by microbial molecules. Under steady-state conditions, the microbiota provides constitutive signals to the innate immune system, which helps to maintain a healthy inflammatory tone within the intestinal mucosa and, thus, enhances resistance to infection with enteric pathogens. During an acute infection, the intestinal epithelial cell barrier is breached, and the detection of microbial molecules in the intestinal lamina propria rapidly stimulates innate immune signaling pathways that coordinate early defense mechanisms. Herein, we review how microbial molecules shed by both commensal and pathogenic microbes direct host defenses at the intestinal mucosa. We highlight the signaling pathways, effector molecules, and cell populations that are activated by microbial molecule recognition and, thereby, are involved in the maintenance of homeostatic levels of host defense and in the early response to acute enteric infection. Finally, we discuss how manipulation of these host defense pathways by stimulating innate immune receptors is a potential therapeutic strategy to prevent or alleviate intestinal disease.     

Immunological roles of intestinal mesenchymal cells

The intestine is continuously exposed to an enormous variety and quantity of antigens and innate immune stimuli derived from both pathogens and harmless materials, such as food and commensal bacteria. Accordingly, the intestinal immune system is uniquely adapted to ensure appropriate responses to the different kinds of challenge; maintaining tolerance to harmless antigens in the steady-state, whilst remaining poised to deal with potential pathogens. To accomplish this, leucocytes of the intestinal immune system have to adapt to a constantly changing environment and interact with many different non-leucocytic intestinal cell types, including epithelial and endothelial cells, neurons, and a heterogenous network of intestinal mesenchymal cells (iMC). These interactions are intricately involved in the generation of protective immunity, the elaboration of inflammatory responses, and the development of inflammatory conditions, such as inflammatory bowel diseases. Here we discuss recent insights into the immunological functions of iMC under homeostatic and inflammatory conditions, focusing particularly on iMC in the mucosa and submucosa, and highlighting how an appreciation of the immunology of iMC may help understand the pathogenesis and treatment of disease.     

Calcitonin gene-related peptide-alpha receptor binding sites in the gastrointestinal tract

Calcitonin gene-related peptide-alpha (CGRP alpha) is a putative neurotransmitter in the brain and in peripheral tissues. Quantitative receptor autoradiography was used to localize and quantify the distribution of specific binding sites for radiolabeled human CGRP alpha in the canine gastrointestinal tract. The canine gastrointestinal tract was chosen as a model since it is similar in both size and structure to the human gastrointestinal tract. In the stomach CGRP alpha binding sites were localized to smooth muscle cells in the muscularis mucosa and muscularis externa, the smooth muscle and endothelium of medium and small arteries, neurons in the myenteric plexus, mucosal epithelial cells and the germinal centers of lymph nodules. In the intestines, the prominent cells types expressing CGRP alpha receptors were myenteric neurons and the germinal centers of lymph nodules. Since previous studies have demonstrated that CGRP-containing sensory neurons innervate the muscularis externa in the stomach and since CGRP alpha receptors are expressed by smooth muscle cells in the muscularis externa, these results suggest that sensory neurons may directly regulate gastric motility by releasing CGRP. In correlation with previous physiological data, the present study suggests that CGRP is involved in the regulation of a variety of gastrointestinal functions including gastric motility, mucosal ion transport, hemodynamics, digestive enzyme secretion, neuronal excitability, and the inflammatory and immune response.     

Welcome to the neighborhood: epithelial cell-derived cytokines license innate and adaptive immune responses at mucosal sites

Summary: There is compelling evidence that epithelial cells (ECs) at mucosal surfaces, beyond their role in creating a physical barrier, are integral components of innate and adaptive immunity. The capacity of these cells to license the functions of specific immune cell populations in the airway and gastrointestinal tract offers the prospect of novel therapeutic strategies to target multiple inflammatory diseases in which barrier immunity is dysregulated. In this review, we discuss the critical functions of EC-derived thymic stromal lymphopoietin (TSLP), interleukin-25 (IL-25), and IL-33 in the development and regulation of T-helper 2 (Th2) cytokine-dependent immune responses. We first highlight recent data that have provided new insights into the factors that control expression of this triad of cytokines and their receptors. In addition, we review their proinflammatory and immunoregulatory functions in models of mucosal infection and inflammation. Lastly, we discuss new findings indicating that despite their diverse structural features and differential expression of their receptors, TSLP, IL-25, and IL-33 cross-regulate one another and share overlapping properties that influence Th2 cytokine-dependent responses at mucosal sites.     

Neutrophil interactions with epithelial-expressed ICAM-1 enhances intestinal mucosal wound healing

A characteristic feature of gastrointestinal tract inflammatory disorders, such as inflammatory bowel disease, is polymorphonuclear neutrophil (PMN) transepithelial migration (TEM) and accumulation in the gut lumen. PMN accumulation within the intestinal mucosa contributes to tissue injury. Although epithelial infiltration by large numbers of PMNs results in mucosal injury, we found that PMN interactions with luminal epithelial membrane receptors may also play a role in wound healing. Intercellular adhesion molecule-1 (ICAM-1) is a PMN ligand that is upregulated on apical surfaces of intestinal epithelial cells under inflammatory conditions. In our study, increased expression of ICAM-1 resulted in enhanced PMN binding to the apical epithelium, which was associated with reduced PMN apoptosis. Following TEM, PMN adhesion to ICAM-1 resulted in activation of Akt and β-catenin signaling, increased epithelial-cell proliferation, and wound healing. Such responses were ICAM-1 dependent as engagement of epithelial ICAM-1 by antibody-mediated cross-linking yielded similar results. Furthermore, using an in-vivo biopsy-based, colonic-mucosal-injury model, we demonstrated epithelial ICAM-1 has an important role in activation of epithelial Akt and β-catenin signaling and wound healing. These findings suggest that post-migrated PMNs within the intestinal lumen can regulate epithelial homeostasis, thereby identifying ICAM-1 as a potential therapeutic target for promoting mucosal wound healing.     

Role of NF-κB in epithelial biology

Since its discovery, nuclear factor-κB (NF-κB) has been recognized as a critical regulator of immune responses. While early studies focused on studying the role of NF-κB in the development and function of immune cells, more recently the function of the inhibitor of NF-κB kinase (IKK)/NF-κB pathway in non-immune cells has gained increased attention. Studies in genetic mouse models were instrumental in dissecting the cell-specific functions of NF-κB and provided experimental evidence that NF-κB signaling in epithelial cells is important for the maintenance of immune homeostasis in barrier tissues such as the skin and the intestine. Increased activation of IKK/NF-κB triggered cytokine expression by the epithelial cells, resulting in exacerbated tissue inflammatory responses. NF-κB inhibition in keratinocytes triggered severe tumor necrosis factor-dependent skin inflammation and epidermal hyperplasia, while inhibition of IKK/NF-κB signaling in intestinal epithelial cells disturbed the intestinal barrier and triggered severe chronic colon inflammation. Therefore, epithelial NF-κB signaling performs critical 'peace keeping' functions in barrier tissues at the interface with the environment by regulating cell survival, barrier integrity, and the immunological and anti-microbial responses of epithelial cells. Improved understanding of epithelial NF-κB functions may hold the key for elucidating the etiology and pathophysiology of chronic inflammatory diseases in epithelial tissues.     

NKT cells in mucosal immunity

The gastrointestinal tract allows the residence of an almost enumerable number of bacteria. To maintain homeostasis, the mucosal immune system must remain tolerant to the commensal microbiota and eradicate pathogenic bacteria. Aberrant interactions between the mucosal immune cells and the microbiota have been implicated in the pathogenesis of inflammatory disorders, such as inflammatory bowel disease (IBD). In this review, we discuss the role of natural killer T cells (NKT cells) in intestinal immunology. NKT cells are a subset of non-conventional T cells recognizing endogenous and/or exogenous glycolipid antigens when presented by the major histocompatibility complex (MHC) class I-like antigen-presenting molecules CD1d and MR1. Upon T-cell receptor (TCR) engagement, NKT cells can rapidly produce various cytokines that have important roles in mucosal immunity. Our understanding of NKT-cell-mediated pathways including the identification of specific antigens is expanding. This knowledge will facilitate the development of NKT cell-based interventions and immune therapies for human intestinal diseases.     

The physiological and pathophysiological roles of eosinophils in the gastrointestinal tract

Eosinophils and the gastrointestinal tract interact in an intimate and enigmatic relationship. Under healthy conditions, the presence of eosinophils is limited almost exclusively to the digestive tract mucosa where they exert several effector and immunoregulatory functions. While their precise function in the gastrointestinal tract is not completely understood, it is likely that, together with different T cell subsets, eosinophils are involved in maintaining the immunologic homeostastis across the mucosal barrier under resting conditions. Eosinophils also play a role in several inflammatory conditions, such as intestinal infections, hypersensitivity reactions, primary eosinophilic inflammations and several other chronic intestinal disorders. Depending on the responsible trigger, their effects may be beneficial or detrimental. Here, we discuss the available information regarding the physiological and pathological functions of eosinophils within the gastrointestinal tract.     

Normal gastrointestinal tract inflammatory cells and review of select benign hematolymphoid proliferations

The luminal gastrointestinal tract can be a site of robust immune response in which reactive lymphoproliferative processes can sometimes be difficult to distinguish from lymphoma. In this article, we review gastrointestinal tract normal resident inflammatory cells and common nonneoplastic lymphoproliferative responses with emphasis on their differential and links to lymphoma. Topics that are covered include lymphocytic esophagitis, gastric chronic inflammation, mucosa-associated lymphoid tissue, and ulceration, small intestinal lymphoid hyperplasia, celiac disease, microscopic colitis, inflammatory bowel disease, primary immunodeficiency, graft-versus-host disease, and anti-programmed cell death protein-1 effect. We additionally present the less common differential of histiocytic processes within the gastrointestinal tract. The aim of this paper is to serve as a reference for practicing pathologists facing lymphoid, lymphoplasmacytic, or histiocytic processes in the luminal gastrointestinal tract. We hope to help the practicing pathologist distinguish benign from malignant entities and identify features requiring further workup.     

The Treatment of Inflammatory Bowel Disease in Patients with Selected Primary Immunodeficiencies

The gastrointestinal tract is heavily populated with innate and adaptive immune cells that have an active role in preservation of mucosal homeostasis and prevention of inflammation. Inflammatory bowel diseases are thought to result from dysregulated immune function that is influenced by genetic background, environmental triggers, and microbiome changes. While most inflammatory bowel disease patients present in adolescent years or adulthood, in a minority of cases, the disease develops early in life, and in some of these young patients, a monogenic disease causing intestinal inflammation can be identified. Many of these conditions result from mutations in immune-mediated genes and can present with or without concomitant recurrent infections. In this review, we will discuss the treatment of patients with selected primary immunodeficiencies and inflammatory bowel diseases. We will focus on five conditions resulting from mutations in IL10/IL10 receptor, NADPH oxidase complex, XIAP, LRBA, and CTLA-4.     

Mycobacterium avium Infection of Epithelial Cells Results in Inhibition or Delay in the Release of Interleukin-8 and RANTES

Mycobacterium avium is an opportunistic pathogen in AIDS patients, who acquire the infection mainly through the gastrointestinal tract. Previous studies in vitro have shown that M. avium invades epithelial cells of both intestinal and laryngeal origin. In addition, M. avium enters the intestinal mucosa of healthy mice. Because M. avium invasion of the intestinal mucosa in vivo initially is not accompanied by significant influx of inflammatory cells, we sought to determine whether M. avium would trigger chemokine release upon entry into epithelial cells by using HT-29 intestinal and HEp-2 laryngeal epithelial cell lines. Chemokine synthesis was measured both by the presence of specific mRNA and protein secretion in the cell culture supernatant as determined by enzyme-linked immunosorbent assay. Infection of HT-29 intestinal cells with M. avium did not induce the release of interleukin-8 (IL-8) or RANTES for up to 7 days postinfection. However, infection of HEp-2 cells resulted in the release of IL-8 and RANTES at 72 h. Similar findings were observed with other AIDS M. avium isolates belonging to different serovars. Secretion of IL-8 by HEp-2 cells was dependent upon bacterial uptake. In addition, prior infection with M. avium suppressed IL-8 production by HT-29 cells infected with Salmonella typhimurium. Our results suggest that M. avium infection of epithelial cells is associated with a delay in IL-8 and RANTES production which, in the case of HT-29, is prolonged up to 1 week. These findings may explain the weak inflammatory response after intestinal mucosa invasion in mice and are probably related with the ability of the bacterium to evade the host's immune response.     

Epithelial barrier: an interface for the cross-communication between gut flora and immune system

Large numbers of environmental antigens, including commensal bacteria and food-derived antigens, constitutively interact with the epithelial layer of the gastrointestinal (GI) tract. Commensal bacteria peacefully cohabit with the host GI tract and exert multiple beneficial or destructive effects on their host. Intestinal epithelial cells (IECs) constitute the first physical and immunological protective wall against invasive pathogens and a cohabitation niche for commensal bacteria. As the physiological homeostasis of IECs is maintained by multiple biological processes such as apoptosis, autophagy, and the handling of endoplasmic reticulum stress, the aberrant kinetics of these biological events, which have genetic and environmental causes, leads to the development of host intestinal pathogenesis such as inflammatory bowel disease. In addition, IECs recognize and interact with commensal bacteria and give instructions to mucosal immune cells to initiate an immunological balance between active and quiescent conditions, eventually establishing intestinal homeostasis. The mucosal immune system regulates the homeostasis of gut microbiota by producing immunological molecules such as secretory immunoglobulin A, the production of which is mediated by IECs. IECs therefore play a central role in the creation and maintenance of a physiologically and immunologically stable intestinal environment.