The role of epithelial cells in immune regulation in the gut

A variety of mechanisms contribute to the ability of the gut to either react or remain tolerant to antigens present in the intestinal lumen. Intestinal epithelial cells can control the uptake, transmission and presentation of luminal antigens through an astonishingly diverse set of pathways. Antigens can cross the epithelial barrier via non-specific pinocytotic, specific receptor mediated, or intracellular/paracellular bypass pathways. The differential processing and presentation by a variety of restriction elements may result in the activation of functionally distinct target cell populations which have the capacity to regulate the predominant trend of immune unresponsiveness within the gut.     

T cell‐mediated immunoregulation in the gastrointestinal tract

In the intestinal tract, only a single layer of epithelial cells separates innate and adaptive immune effector cells from a vast amount of antigens. Here, the immune system faces a considerable challenge in tolerating commensal flora and dietary antigens while preventing the dissemination of potential pathogens. Failure to tightly control immune reactions may result in detrimental inflammation. In this respect, ‘conventional’ regulatory CD4⁺ T cells, including naturally occurring and adaptive CD4⁺ CD25⁺ Foxp3⁺ T cells, Th3 and Tr1 cells, have recently been the focus of considerable attention. However, regulatory mechanisms in the intestinal mucosa are highly complex, including adaptations of nonhaematopoietic cells and innate immune cells as well as the presence of unconventional T cells with regulatory properties such as resident TCRγδ or TCRαβ CD8⁺ intraepithelial lymphocytes. This review aims to summarize the currently available knowledge on conventional and unconventional regulatory T cell subsets (Tregs), with special emphasis on clinical data and the potential role or malfunctioning of Tregs in four major human gastrointestinal diseases, i.e. inflammatory bowel diseases, coeliac disease, food allergy and colorectal cancer. We conclude that the clinical data confirms some but not all of the findings derived from experimental animal models.     

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.     

Mucosal innate immune cells regulate both gut homeostasis and intestinal inflammation

Continuous exposure of intestinal mucosal surfaces to diverse microorganisms and their metabolites reflects the biological necessity for a multifaceted, integrated epithelial and immune cell‐mediated regulatory system. The development and function of the host cells responsible for the barrier function of the intestinal surface (e.g., M cells, Paneth cells, goblet cells, and columnar epithelial cells) are strictly regulated through both positive and negative stimulation by the luminal microbiota. Stimulation by damage‐associated molecular patterns and commensal bacteria‐derived microbe‐associated molecular patterns provokes the assembly of inflammasomes, which are involved in maintaining the integrity of the intestinal epithelium. Mucosal immune cells located beneath the epithelium play critical roles in regulating both the mucosal barrier and the relative composition of the luminal microbiota. Innate lymphoid cells and mast cells, in particular, orchestrate the mucosal regulatory system to create a mutually beneficial environment for both the host and the microbiota. Disruption of mucosal homeostasis causes intestinal inflammation such as that seen in inflammatory bowel disease. Here, we review the recent research on the biological interplay among the luminal microbiota, epithelial cells, and mucosal innate immune cells in both healthy and pathological conditions.     

IgG transport across mucosal barriers by neonatal Fc receptor for IgG and mucosal immunity

Mucosal secretions of the human gastrointestinal, respiratory, and genital tracts contain significant quantities of IgG. The neonatal Fc receptor for IgG (FcRn) plays a major role in regulating host IgG levels and transporting IgG and associated antigens across polarized epithelial barriers. The FcRn can then recycle the IgG/antigen complex back across the intestinal barrier into the lamina propria for processing by dendritic cells and presentation to CD4⁺ T cells in regional organized lymphoid structures. FcRn, through its ability to secrete and absorb IgG, thus integrates luminal antigen encounters with systemic immune compartments and, as such, provides essential host defense and immunoregulatory functions at the mucosal surfaces.     

Epithelial cells in the female reproductive tract: a central role as sentinels of immune protection

The continued presence of bacterial and viral antigens in the lumen of the vagina coupled with the periodic presence of antigens in the lumen of the upper reproductive tract provide an ongoing challenge that can compromise female reproductive health and threaten life. Separating underlying tissues from luminal antigens, polarized epithelial cells of the cervix, uterus and Fallopian tubes have evolved to protect against potential pathogens. Once thought to function exclusively by providing a crucial barrier, mucosal epithelial cells are now known to function as sentinels that recognize antigens, respond in ways that lead to bacterial and viral killing, as well as signal to underlying immune cells when pathogenic challenge exceeds their protective capacity. Unique to epithelial cells of the female reproductive tract is the regulatory control of the female sex hormones. Acting both directly and indirectly through underlying stromal cells, estradiol and progesterone regulate epithelial cell innate and adaptive immune functions to protect against potential pathogens while providing an environment that supports an allogeneic fetus. In this article, we will outline how polarized epithelial cells function as the first line of defense against potential pathogens in the female reproductive tract.     

Oral Tolerance Failure upon Neonatal Gut Colonization with Escherichia coli Producing the Genotoxin Colibactin

The intestinal barrier controls the balance between tolerance and immunity to luminal antigens. When this finely tuned equilibrium is deregulated, inflammatory disorders can occur. There is a concomitant increase, in urban populations of developed countries, of immune-mediated diseases along with a shift in Escherichia coli population from the declining phylogenetic group A to the newly dominant group B2, including commensal strains producing a genotoxin called colibactin that massively colonized the gut of neonates. Here, we showed that mother-to-offspring early gut colonization by colibactin-producing E. coli impairs intestinal permeability and enhances the transepithelial passage of luminal antigen, leading to an increased immune activation. Functionally, this was accompanied by a dramatic increase in local and systemic immune responses against a fed antigen, decreased regulatory T cell population, tolerogenic dendritic cells, and enhanced mucosal delayed-type hypersensitivity response. Conversely, the abolition of colibactin expression by mutagenesis abrogates the alteration of oral tolerance induced by neonatal colonization by E. coli. In conclusion, the vertical colonization by E. coli producing the genotoxin colibactin enhances intestinal translocation and subsequently alters oral tolerance. Thus, early colonization by E. coli from the newly dominant phylogenetic group B2, which produces colibactin, may represent a risk factor for the development of immune-mediated diseases.     

Gut permeability and mucosal inflammation: bad,good or context dependent

Inflammatory bowel disease (IBD) is a multifactorial disease. A breach in the mucosal barrier, otherwise known as “leaky gut,” is alleged to promote mucosal inflammation by intensifying immune activation. However, interaction between the luminal antigen and mucosal immune system is necessary to maintain mucosal homeostasis. Furthermore, manipulations leading to deregulated gut permeability have resulted in susceptibility in mice to colitis as well as to creating adaptive immunity. These findings implicate a complex but dynamic association between mucosal permeability and immune homeostasis; however, they also emphasize that compromised gut permeability alone may not be sufficient to induce colitis. Emerging evidence further supports the role(s) of proteins associated with the mucosal barrier in epithelial injury and repair: manipulations of associated proteins also modified epithelial differentiation, proliferation, and apoptosis. Taken together, the role of gut permeability and proteins associated in regulating mucosal inflammatory diseases appears to be more complex than previously thought. Herein, we review outcomes from recent mouse models where gut permeability was altered by direct and indirect effects of manipulating mucosal barrier-associated proteins, to highlight the significance of mucosal permeability and the non-barrier-related roles of these proteins in regulating chronic mucosal inflammatory conditions.     

CCR6 promotes steady‐state mononuclear phagocyte association with the intestinal epithelium,imprinting and immune surveillance

The intestinal lamina propria (LP) contains antigen‐presenting cells with features of dendritic cells and macrophages, collectively referred to as mononuclear phagocytes (MNPs). Association of MNPs with the epithelium is thought to play an important role in multiple facets of intestinal immunity including imprinting MNPs with the ability to induce IgA production, inducing the expression of gut homing molecules on T cells, facilitating the capture of luminal antigens and microbes, and subsequent immune responses in the mesenteric lymph node (MLN). However, the factors promoting this process in the steady state are largely unknown, and in vivo models to test and confirm the importance of LP‐MNP association with the epithelium for these outcomes are unexplored. Evaluation of epithelial expression of chemoattractants in mice where MNP–epithelial associations were impaired suggested CCL20 as a candidate promoting epithelial association. Expression of CCR6, the only known receptor for CCL20, was required for MNPs to associate with the epithelium. LP‐MNPs from CCR6^−/− mice did not display defects in acquiring antigen and stimulating T‐cell responses in ex vivo assays or in responses to antigen administered systemically. However, LP‐MNPs from CCR6‐deficient mice were impaired at acquiring luminal and epithelial antigens, inducing IgA production in B cells, inducing immune responses in the MLN, and capturing and trafficking luminal commensal bacteria to the MLN. These findings identify a crucial role for CCR6 in promoting LP‐MNPs to associate with the intestinal epithelium in the steady state to perform multiple functions promoting gut immune homeostasis.     

Goblet cells: multifaceted players in immunity at mucosal surfaces

Goblet cells (GCs) are specialized epithelial cells that line multiple mucosal surfaces and have a well-appreciated role in barrier maintenance through the secretion of mucus. Moreover, GCs secrete anti-microbial proteins, chemokines, and cytokines demonstrating functions in innate immunity beyond barrier maintenance. Recently it was appreciated that GCs can form goblet cell-associated antigen passages (GAPs) and deliver luminal substances to underlying lamina propria (LP) antigen-presenting cells (APCs) in a manner capable of inducing adaptive immune responses. GCs at other mucosal surfaces share characteristics with the GAP forming intestinal GCs, suggesting that GAP formation may not be restricted to the gut, and that GCs may perform this gatekeeper function at other mucosal surfaces. Here we review observations of how GCs contribute to immunity at mucosal surfaces through barrier maintenance, the delivery of luminal substances to APCs, interactions with APCs, and secretion of factors modulating immune responses.     

The gut mucus network: A dynamic liaison between microbes and the immune system

A complex mucus network made up of large polymers of the mucin-family glycoprotein MUC2 exists between the large intestinal microbial mass and epithelial and immune cells. This has long been understood as an innate immune defense barrier against the microbiota and other luminal threats that reinforces the barrier function of the epithelium and limits microbiota contact with the tissues. However, past and recent studies have provided new evidence of how critical the mucus network is to act as a ‘liaison’ between host and microbe to mediate anti-inflammatory, mutualistic interactions with the microbiota and protection from pathogens. This review summarizes historical and recent insights into the formation of the gut mucus network, how the microbes and immune system influence mucus, and in turn, how the mucus influences immune responses to the microbiota.     

Direct interaction between cholera toxin and dendritic cells is required for oral adjuvant activity

Cholera toxin (CT) binds to GM1‐ganglioside receptors present on all nucleated cells. Despite this, it is a very potent mucosal adjuvant that has a dramatic impact on immune cells, as well as nerve and epithelial cells, causing diarrhea. This fact has hampered our understanding of whether the adjuvanticity of CT is direct or indirect, as cells that bind CT may or may not be involved in its adjuvant function. The mucosal barrier is maintained by tight junctions between epithelial cells but dendritic cells (DCs) can protrude luminal dendrites. Here we investigated which cells are involved in the immune augmenting effect of CT. We explored oral immunizations with ovalbumin (OVA) and CT in bone marrow chimeric mice deficient in GM1‐ganglioside in defined cellular subsets. We found that chimeric mice lacking GM1 in nonhematopoietic cells, including epithelial cells, mounted an unaltered intestinal IgA response. In contrast, chimeric mice lacking GM1‐expressing hematopoietic cells in general, or specifically GM1‐expressing conventional DCs (cDCs), largely failed to elicit anti‐OVA adaptive immune responses. Therefore, the adjuvanticity of CT does not require epithelial activation, but is directly dependent on the binding of CT to gut cDCs via GM1‐ganglioside. These results could have important implications for the generation of novel oral adjuvants.     

Human neonatal Fc receptor mediates transport of IgG into luminal secretions for delivery of antigens to mucosal dendritic cells

Mucosal secretions of the human gastrointestinal, respiratory, and genital tracts contain significant quantities of IgG. The mechanism by which IgG reaches luminal secretions and the function of IgG in these locations are unknown. Here, we find that the human neonatal Fc receptor (FcRn) is the vehicle that transports IgG across the intestinal epithelial barrier into the lumen where the IgG can bind cognate antigen. The FcRn can then recycle the IgG/antigen complex back across the intestinal barrier into the lamina propria for processing by dendritic cells and presentation to CD4(+) T cells in regional organized lymphoid structures. These results explain how IgG is secreted onto mucosal surfaces and scavenges luminal antigens for recognition by the immune system.     

Innate and adaptive immunity in inflammatory bowel disease

Inflammatory bowel disease (IBD) includes Crohn's disease (CD) and ulcerative colitis (UC). The exact cause of IBD remains unknown. Available evidence suggests that an abnormal immune response against the microorganisms of the intestinal flora is responsible for the disease in genetically susceptible individuals. The adaptive immune response has classically been considered to play a major role in the pathogenesis of IBD. However, recent advances in immunology and genetics have clarified that the innate immune response is equally as important in inducing gut inflammation in these patients. In particular, an altered epithelial barrier function contributes to intestinal inflammation in patients with UC, while aberrant innate immune responses, such as antimicrobial peptide production, innate microbial sensing and autophagy are particularly associated to CD pathogenesis. On the other hand, besides T helper cell type (Th)1 and Th2 immune responses, other subsets of T cells, namely Th17 and regulatory T (Treg) cells, are likely to play a role in IBD. However, given the complexity and probably the redundancy of pathways leading to IBD lesions, and the fact that Th17 cells may also have protective functions, neutralization of IL-17A failed to induce any improvement in CD. Studying the interactions between various constituents of the innate and adaptive immune systems will certainly open new horizons in the knowledge about the immunologic mechanisms implicated in gut inflammation.     

The intestinal epithelial barrier in the control of homeostasis and immunity

In the intestine, multiple interactions occur with the external world. Thus, the intestinal mucosal barrier has to tolerate millions of microorganisms that commonly inhabit the gut, degrade and absorb food, and establish tolerance or immunity, depending on the nature of the encountered antigens. Recent findings have highlighted that intestinal epithelial cells are not simply a barrier, but also are crucial for integrating these external and internal signals and for coordinating the ensuing immune response. Here, I review these findings and show how epithelial cells harmonize information that comes from inflammatory and non-inflammatory components of the microbiota to preserve intestinal homeostasis. If dysregulated, this immunomodulatory function of epithelial cells might contribute to the development of intestinal inflammation.     

Probiotics and down-regulation of the allergic response

The first clinical trials with probiotics, especially in the treatment of atopic eczema, have yielded encouraging results. Experimental studies have found that probiotics exert strain-specific effects in the intestinal lumen and on epithelial cells and immune cells with anti-allergic potential. These effects include enhancement in antigen degradation and gut barrier function and induction of regulatory and proinflammatory immune responses, the latter of which occurs more likely beyond the intestinal epithelium. Future studies should address more accurately how these and other possible mechanisms operate in the complex gastrointestinal macroenvironment in vivo and how these mechanisms are related to the clinical effects in a dose-dependent manner.     

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.     

Regulatory immune cells in regulation of intestinal inflammatory response to microbiota

The intestinal lumen harbors nearly 100 trillion commensal bacteria that exert crucial function for health. An elaborate balance between immune responses and tolerance to intestinal microbiota is required to maintain intestinal homeostasis. This process depends on diverse regulatory mechanisms, including both innate and adaptive immunity. Dysregulation of the homeostasis between intestinal immune systems and microbiota has been shown to be associated with the development of inflammatory bowel diseases (IBD) in genetically susceptible populations. In this review, we discuss the recent progress reported in studies of distinct types of regulatory immune cells in the gut, including intestinal intraepithelial lymphocytes, Foxp3⁺ regulatory T cells, regulatory B cells, alternatively activated macrophages, dendritic cells, and innate lymphoid cells, and how dysfunction of this immune regulatory system contributes to intestinal diseases such as IBD. Moreover, we discuss the manipulation of these regulatory immune cells as a potential therapeutic method for management of intestinal inflammatory disorders.     

A porous defense: the leaky epithelial barrier in intestinal disease

A critical function of the intestinal mucosa is to form a barrier that separates luminal contents from the interstitium. This intestinal barrier is compromised in a number of intestinal diseases, most notably inflammatory bowel disease. In vitro studies have demonstrated that cytokines elaborated by immune cells can cause the mucosal barrier to become leaky; these cytokines are known to be increased in intestinal mucosa involved in inflammatory bowel disease. Detailed information describing the mechanisms by which altered cytokine signaling occurs is not available, but recent data implicate the cytoskeleton within epithelial cells as a critical regulator of the mucosal barrier under physiological and pathophysiological conditions. Using available data, we describe a model of intestinal disease where an initial insult to the epithelial barrier may trigger a self-amplifying cycle of immune activation, cytokine release, and further barrier dysfunction. This model is supported by the observation that pharmacological abrogation of cytokine signaling corrects both barrier defects and clinical disease in animal models and human patients, although such therapy clearly has multiple mechanisms. Other therapeutic targets that represent strategies to prevent or reverse disease processes are also considered. The overarching hypothesis is that modulation of the mucosal epithelial barrier plays a critical role in the initiation and propogation of inflammatory intestinal diseases.     

Duodenal helminth infection alters barrier function of the colonic epithelium via adaptive immune activation

Chronic infection with intestinal helminth parasites is a major public health problem, particularly in the developing world, and can have significant effects on host physiology and the immune response to other enteric infections and antigens. The mechanisms underlying these effects are not well understood. In the current study, we investigated the impact of infection with the murine nematode parasite Heligmosomoides polygyrus, which resides in the duodenum, on epithelial barrier function in the colon. We found that H. polygyrus infection produced a significant increase in colonic epithelial permeability, as evidenced by detection of elevated serum levels of the tracer horseradish peroxidase following rectal administration. This loss of normal barrier function was associated with clear ultrastructural changes in the tight junctions of colonic epithelial cells and an alteration in the expression and distribution of the junctional protein E-cadherin. These parasite-induced abnormalities were not observed in SCID mice but did occur in SCID mice that were adoptively transferred with wild-type T cells, indicating a requirement for adaptive immunity. Furthermore, the helminth-induced increase in gut permeability was not seen in STAT6 knockout (KO) mice. Taken together, the results demonstrate that one of the mechanisms by which helminths exert their effects involves the lymphocyte- and STAT6-dependent breakdown of the intestinal epithelial barrier. This increase in epithelial permeability may facilitate the movement of lumenal contents across the mucosa, thus helping to explain how helminth infection can alter the immune response to enteric antigens.