Intestinal epithelial cells: regulators of barrier function and immune homeostasis.

F O C U S O N H o m e os tat i c I m m un e R e s p ons e s

REVIEWS Intestinal epithelial cells: regulators of barrier function and immune homeostasis Lance W. Peterson1 and David Artis1,2

Abstract | The abundance of innate and adaptive immune cells that reside together with trillions of beneficial commensal microorganisms in the mammalian gastrointestinal tract requires barrier and regulatory mechanisms that conserve host–microbial interactions and tissue homeostasis. This homeostasis depends on the diverse functions of intestinal epithelial cells (IECs), which include the physical segregation of commensal bacteria and the integration of microbial signals. Hence, IECs are crucial mediators of intestinal homeostasis that enable the establishment of an immunological environment permissive to colonization by commensal bacteria. In this Review, we provide a comprehensive overview of how IECs maintain host–commensal microbial relationships and immune cell homeostasis in the intestine.

Inflammatory bowel disease (IBD). A chronic condition of the intestine characterized by severe inflammation and mucosal destruction. The most common forms of IBD in humans are ulcerative colitis and Crohn’s disease, which have both distinct and overlapping pathological and clinical characteristics.

Mucins Heavily glycosylated proteins that are the major component of the mucus that coats epithelial barrier surfaces.

Department of Microbiology and Institute for Immunology, Perelman School of Medicine, University of Pennsylvania. 2 Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. e‑mails: [email protected]. upenn.edu; [email protected] doi:10.1038/nri3608 1

Specialized epithelial cells constitute barrier surfaces that separate mammalian hosts from the external environment. The gastrointestinal tract is the largest of these barriers and is specially adapted to colonization by commensal bacteria that aid in digestion and markedly influence the development and function of the mucosal immune system. However, microbial colonization carries with it the risk of infection and inflammation if epithelial or immune cell homeostasis is disrupted. Key to the coexistence of commensal microbial communities and mucosal immune cells is the capacity to maintain the segregation between host and microorganism. The intestinal epithelium accomplishes this by forming a physical and biochemical barrier to commensal and pathogenic microorganisms. Furthermore, intestinal epithelial cells (IECs) can sense and respond to microbial stimuli to reinforce their barrier function and to participate in the coordination of appropriate immune responses, ranging from tolerance to anti-pathogen immunity. Thus, IECs maintain a fundamental immunoregulatory function that influences the development and homeostasis of mucosal immune cells. The association between increased bacterial trans location and risk of developing inflammatory bowel disease (IBD) suggests a central role for dysregulated epithelial barrier function in either the aetiology or the pathology of intestinal inflammation and IBD1. Increasing evidence also indicates that the loss of intestinal barrier

function contributes to systemic immune activation, which promotes the progression of chronic viral infections, including infection with HIV and hepatitis virus2,3, and metabolic disease4,5. Furthermore, host–microbial interactions that occur at the IEC barrier contribute to a broad range of extra-intestinal autoimmune and inflammatory diseases, including type 1 diabetes, rheumatoid arthritis and multiple sclerosis6–9. Hence, a comprehensive understanding of the barrier and immunoregulatory properties of IECs could aid in the development of new strategies to prevent and treat multiple human infectious, inflammatory and metabolic diseases. The topics of commensal bacterial diversity, microbial regulation of immune cell development and host–viral interactions in the intestine have been reviewed extensively elsewhere10–14. Therefore, in this Review, we discuss the role of IECs in promoting intestinal homeostasis through the segregation and regulation of commensal microorganisms and the host immune system. Recent advances in the understanding of the barrier, microbialsensing and immunoregulatory functions of IECs are reviewed, with a particular focus on their relationship to intestinal health and disease. We discuss the barrier function maintained by IEC-derived mucins and antimicrobial proteins, the pathways through which IECs regulate innate and adaptive immune cells present at the intestinal barrier and the contribution of IEC recognition of microbial colonization to IEC function and homeostasis.

NATURE REVIEWS | IMMUNOLOGY

VOLUME 14 | MARCH 2014 | 141 © 2014 Macmillan Publishers Limited. All rights reserved

R E V IE W S Small intestine

Follicle-associated epithelium

Colon

Apoptotic IECs

Commensal bacteria

Mucus Secondlayer mucus

TFF3

sIgA

AMPs

M cell

Mucus

Enteroendocrine cell

Goblet cell

Enterocyte

B cell

Stromal cell

Lymphoid follicle Paneth cell

IESC

Crypts Tubular invaginations of the intestinal epithelium. Lining the base of the crypts are small intestinal Paneth cells, which produce numerous antimicrobial proteins, and stem cells, which continuously divide to give rise to the entire intestinal epithelium.

Villi Projections of the intestinal epithelium into the lumen of the small intestine that have an outer layer consisting of mature, absorptive enterocytes, mucus-secreting goblet cells and enteroendocrine cells.

Pluripotent intestinal epithelial stem cells (Pluripotent IESCs). Tissue-resident stem cells that undergo continuous self-renewal and are responsible for regenerating all lineages of mature intestinal epithelial cells, including enterocytes, enteroendocrine cells, goblet cells and Paneth cells.

Macrophage

DC

Figure 1 | The IEC barrier. Intestinal epithelial cells (IECs) form a biochemical and physical barrier that maintains segregation between luminal microbial communities and the mucosal immune system. The intestinal epithelial stem Nature Reviews | Immunology cell (IESC) niche, containing epithelial, stromal and haematopoietic cells, controls the continuous renewal of the epithelial cell layer by crypt-resident stem cells. Differentiated IECs — with the exception of Paneth cells — migrate up the crypt–villus axis, as indicated by the dashed arrows. Secretory goblet cells and Paneth cells secrete mucus and antimicrobial proteins (AMPs) to promote the exclusion of bacteria from the epithelial surface. The transcytosis and luminal release of secretory IgA (sIgA) further contribute to this barrier function. Microfold cells (M cells) and goblet cells mediate transport of luminal antigens and live bacteria across the epithelial barrier to dendritic cells (DCs), and intestine-resident macrophages sample the lumen through transepithelial dendrites. TFF3, trefoil factor 3.

IEC regulation of barrier function The intestinal epithelium is the largest of the body’s mucosal surfaces, covering ~400 m2 of surface area with a single layer of cells organized into crypts and villi (FIG. 1) . This surface is continually renewed by pluripotent intestinal epithelial stem cells (pluripotent IESCs) that reside in the base of crypts, where the proliferation, differentiation and functional potential of epithelial cell progenitors is regulated by the local stem cell niche15,16 (BOX 1). Although the majority of cells bordering the intestinal lumen are absorptive enterocytes, which are adapted for metabolic and digestive function, the diversity of functions that the intestinal epithelium carries out is reflected by the presence of additional specialized IEC lineages. Secretory IECs, including enteroendocrine cells, goblet cells and Paneth cells, are specialized for maintaining the digestive or barrier function of the epithelium. Enteroendocrine cells represent a link between the central and enteric neuroendocrine systems through the secretion of numerous hormone regulators of

digestive function. The luminal secretion of mucins and antimicrobial proteins (AMPs) by goblet cells and Paneth cells, respectively, establishes a physical and biochemical barrier to microbial contact with the epithelial surface and underlying immune cells17,18 (FIG. 1). Collectively, the diverse functions of IECs result in a dynamic barrier to the environment, which protects the host from infection and continuous exposure to potentially inflammatory stimuli. Keeping the bugs at bay — IEC secretory defences. The secretion of highly glycosylated mucins into the intestinal lumen by goblet cells creates the first line of defence against microbial encroachment. The most abundant of these mucins, mucin 2 (MUC2), plays an essential part in the organization of the intestinal mucous layers at the epithelial surface of the colon19. The importance of mucin production by goblet cells is emphasized by the spontaneous development of colitis and the predisposition to inflammation-induced colorectal cancers observed in MUC2‑deficient mice20,21. Additional goblet

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F O C U S O N H o m e os tat i c I m m un e R e sRpEons eS s V IE W Box 1 | The IESC niche Along the crypt–villus axis of the epithelium, pluripotent intestinal epithelial stem cells (IESCs) residing in the base of crypts give rise to a transit-amplifying population of cells that undergo rapid proliferation and differentiation into the various intestinal epithelial cell (IEC) subsets. Terminally differentiated cells — with the exception of Paneth cells — migrate up the crypt–villus axis until they are lost from the epithelial layer. For this process to be maintained, epithelial stem cells must be able to undergo repeated rounds of replication and possess the capacity for continuous self-renewal16. Recent advances in stem cell biology have identified markers of IESCs that have contributed to the understanding of epithelial self-renewal and differentiation16,183–185. The patterning and distribution of proliferating crypt units in the intestine depend on paracrine signalling between the epithelium and the underlying mesenchyme. A balance between bone morphogenetic protein signals and antagonists, such as noggin and gremlin, provides a niche for proliferating stem cells while limiting ectopic crypt formation15. IESCs further rely on signalling through both the WNT–β‑catenin and the Notch pathways for promoting self-renewal and directing differentiation towards secretory versus non-secretory lineage IEC fates16. The responsiveness of epithelial progenitors to external regulation in settings of inflammation or infection remains less well understood. In particular, how immune system-mediated signalling integrates into the homeostatic pathways described above or acts through alternative pathways for altering stem cell function is poorly defined. However, several recent studies have given insight into the regulation of WNT–β‑catenin signalling by the pro-inflammatory cytokines interferon‑γ and tumour necrosis factor, offering an example of how immune signalling and homeostatic pathways for regulating the stem cell niche can converge186,187. Furthermore, cell-intrinsic mechanisms of integrating host–commensal microorganism interactions into IEC homeostasis have been recently described188.

Autophagy A cellular process by which cytoplasmic organelles and macromolecular complexes are engulfed by double membrane-bound vesicles for delivery to lysosomes and subsequent degradation. This process is involved in constitutive turnover of proteins and organelles and is central to cellular activities that maintain a balance between the synthesis and breakdown of various proteins.

Unfolded protein response (UPR). A response that increases the ability of the endoplasmic reticulum to fold and translocate proteins, decreases the synthesis of proteins, causes the arrest of the cell cycle and promotes apoptosis.

Plasma cells Terminally differentiated cells of the B cell lineage that secrete large amounts of antibodies.

Lamina propria Connective tissue that underlies the epithelium of the mucosa and contains stromal and haematopoietic cells.

cell-derived products, such as trefoil factor 3 (TFF3) and resistin-like molecule‑β (RELMβ), further contribute to the regulation of a physical barrier in the intestine. TFF3 provides structural integrity to mucus through mucin crosslinking and acts as a signal that promotes epithelial repair, migration of IECs and resistance to apoptosis22,23. RELMβ functions to promote MUC2 secretion, regulate macrophage and adaptive T cell responses during inflammation and, in the setting of nematode infection, directly inhibit parasite chemotaxis24,25. Intestinal barrier function is further reinforced by the secretion of AMPs by IECs. Enterocytes are capable of producing some AMPs, such as the C‑type lectin regenerating islet-derived protein IIIγ (REGIIIγ), throughout the small intestine and colon. By contrast, Paneth cells are uniquely adapted for the secretion of many additional AMPs, including defensins (cryptdins in mice), cathelicidins and lysozyme, in the crypts of the small intestine18,26. These AMPs disrupt highly conserved and essential features of bacterial biology, such as surface membranes, which are targeted by pore-forming defensins and cathelicidins, and Gram-positive cell wall peptidoglycans, which are targeted by C‑type lectins18,27. This strategy enables the broad regulation of both commensal and pathogenic bacteria and limits resistance of bacteria to antimicrobial responses. Regional variation in AMP production exists along the longitudinal axis of the intestinal tract 28. Although further analysis is required, this distribution may reflect anatomically restricted host–commensal bacteria interactions that drive the differential regulation of IEC responses or serve to shape heterogeneity in the composition and localization of microbial communities.

Paneth cell- and enterocyte-derived REGIIIγ has recently been described as a mediator of host–microbial segregation in the gut 29. Similar to the function and regulation of MUC2 in the colon, REGIIIγ acts to exclude bacteria from the epithelial surface of the small intestine, and its production is dependent on IEC-intrinsic recognition of commensal microbial signals29. Interactions between AMPs, including REGIIIγ, and mucins lead to concentrated antimicrobial activity at the epithelial surface30. Thus, the combined functions of secretory IECs seem to limit the quantity and diversity of live bacteria that can reach the epithelial surface or interact with the underlying mucosal immune system. The importance of maintaining the health of secretory IECs is reflected in human IBD and models of murine intestinal inflammation, in which genetic defects in autophagy and the unfolded protein response (UPR) disrupt the function of Paneth and goblet cells and promote disease susceptibility 31–37. Autophagy in IECs has been shown to act in an innate immune capacity to limit the dissemination of invasive bacteria passing through the epithelium38, but it also supports the packaging and exocytosis of Paneth cell granules33. When autophagy is disrupted in mice they become susceptible to a form of experimental colitis33. Notably, this susceptibility is dependent on exposure to a common strain of an enteric virus (murine norovirus), providing an example of the compound genetic and environmental interactions that contribute to disease pathogenesis36. Disruption of UPR genes results in endoplasmic reticulum stress in secretory cells and spontaneous intestinal inflammation34. Notably, disruption of either autophagy or the UPR leads to the compensatory engagement of the other, supporting a model in which the two are interrelated39. Furthermore, the engagement of these pathways by Paneth cells is required for maintaining intestinal homeostasis in mice, and their combined absence leads to the development of a spontaneous disease resembling human Crohn’s disease39. These findings, coupled with genetic evidence from patients with IBD for the role of autophagy and the UPR in disease pathogenesis31,32,34,37, support an important link between the disruption of Paneth cell function and the potential origins of intestinal inflammation. Finally, IECs directly transport secretory immunoglobulins across the epithelial barrier. Following their production by plasma cells in the lamina propria, dimeric IgA complexes are bound by the polymeric immunoglobulin receptor (pIgR) on the basolateral membrane of IECs and actively transcytosed into the intestinal lumen 40. The collaboration between IgA-secreting B cells and IECs provides an adaptive immune component to the epithelial barrier that regulates commensal bacterial populations to maintain IEC and immune cell homeostasis41–43. Future studies to better understand how mucus, AMP and secretory immunoglobulin dynamics can be regulated to support barrier function will enable the development of therapeutic interventions for preserving intestinal homeostasis.



R E V IE W S Box 2 | IEC tight junctions and turnover Below the mucous layers, intestinal epithelial cells (IECs) form a continuous physical barrier. Tight junctions connect adjacent IECs and are associated with cytoplasmic actin and myosin networks that regulate intestinal permeability. In the setting of inflammatory bowel disease (IBD), dysregulation of these interactions, mediated by tumour necrosis factor signalling and by myosin light chain kinase activity, leads to IEC cytoskeletal rearrangements that disrupt tight junctions and increase permeability189,190. These findings suggest that IEC tight junctions could be important targets for enhancing the integrity of the intestinal barrier in IBD. As the IEC barrier is continuously renewed, the turnover of IECs provides an additional challenge to the maintenance of epithelial continuity. Recent studies have described pathways by which adjacent cells seal potential voids created during the extrusion of either apoptotic or live cells from the single-cell layer191,192. As dysregulated epithelial cell turnover and apoptosis are associated with intestinal inflammation, the contribution of these mechanisms to the limiting of barrier breaches and further inflammation is of relevance to our understanding of epithelial cells as an efficient physical barrier. Although increased intestinal permeability has been correlated with IBD1,193,194, it remains unclear whether the loss of barrier function is a cause or a consequence of intestinal inflammation in human disease. Evidence from mouse models with genetic defects in tight-junction-associated proteins suggests that disruption of barrier function alone is not always sufficient to cause disease195,196. Notably, in mice with a deletion of the tight-junction protein junctional adhesion molecule A, the secretion of commensal bacteria-specific IgA can compensate for the loss of barrier function and limit disease severity following chemically induced colitis195. Thus, compensatory immune mechanisms can act to protect against the development of colitis, even in the setting of barrier disruption, supporting a multi-hit model of disease susceptibility195.

Peyer’s patches Groups of lymphoid aggregates located in the submucosa of the small intestine that contain many immune cells, including B cells, T cells and dendritic cells. They have a luminal barrier consisting of specialized epithelial cells, called microfold cells, which sample the lumen and transport antigens.

Pattern-recognition receptors (PRRs). Receptors that recognize structures shared by foreign microorganisms or endogenous molecules associated with pathogenesis. Signalling through these receptors promotes tissue-specific innate immune responses including the production of cytokines.

Toll-like receptor (TLR). An evolutionarily conserved pattern-recognition receptor located at the cell surface or at intracellular membranes. The natural ligands of TLRs are conserved molecular structures found in bacteria, viruses and fungi.

Sampling of luminal contents by IECs. Despite the barrier function supported by IECs (BOX 2), the intestinal epithelium includes specialized adaptations that conflict with the concept of complete segregation between host immune cells and microorganisms. Specialized IECs, called microfold cells (M cells), mediate the sampling of luminal antigens and intact microorganisms for presentation to the underlying mucosal immune system44. These specialized IECs are concentrated in the follicleassociated epithelium overlaying the luminal surface of intestinal lymphoid structures, including Peyer’s patches and isolated lymphoid follicles44,45. Although nonspecific uptake and transcytosis of antigens represents a well-established mechanism of sampling by M cells, it has recently been demonstrated that more efficient mechanisms of receptor-mediated transport also exist. The surface glycoprotein GP2 acts as a receptor for the bacterial pilus protein FimH, and the M cell-mediated transport of the pathogen Salmonella enterica across the epithelial barrier depends on GP2–FimH interaction46. This suggests that M cells are capable of both specific receptor-mediated microbial uptake and nonspecific antigen uptake from the intestinal lumen. Although the active transport of luminal contents across the epithelial barrier was thought to be a unique function of M cells, it was recently shown that small-intestinal goblet cells also contribute to this process through the delivery of soluble luminal antigens to subepithelial dendritic cells (DCs)47. Although both M cells and goblet cells seem to be capable of antigen delivery to the lamina propria, the functional importance and contribution of these two pathways to the development of anti-pathogen responses or to the maintenance of immune tolerance remains incompletely understood.

In addition to luminal antigen sampling by IECs, subepithelial mononuclear phagocytes, through interactions with IECs, sample luminal contents through transepithelial dendrites 48,49 (discussed below). The adaptation of the epithelial barrier for the sampling of luminal contents accommodates limited and controlled bacterial and antigen translocation to direct appropriate tolerogenic or anti-pathogen responses. The influence of these transport pathways on the immune response is not well understood, but distinct pathways of acquiring antigens may influence the context in which immune cells interpret microbial signals50. Furthermore, transport through these pathways may alter bacteria and antigens to enable controlled transport of antigens to be differentiated from dysregulated bacterial translocation50. Harnessing the functions of IECs in this sampling process holds promise for the development of mucosal vaccines and the regulation of intestinal inflammation.

IECs — sentinels in intestinal homeostasis Central to the capacity of IECs to maintain barrier and immunoregulatory functions is their ability to act as frontline sensors for microbial encounters and to integrate commensal bacteria-derived signals into anti microbial and immunoregulatory responses (FIG. 2). IECs express pattern-recognition receptors (PRRs) that enable them to act as dynamic sensors of the microbial environment and as active participants in the directing of mucosal immune cell responses (see Supplementary information S1 (table)). Members of the Toll-like receptor (TLR)51, NOD-like receptor (NLR)52,53 and RIG‑I‑like receptor (RLR) families54,55 provide distinct pathways for the recognition of microbial ligands or endogenous signals associated with pathogenesis. Unlike sterile sites in the body where recognition of foreign microorganisms initiates highly inflammatory cascades, the abundance of symbiotic commensals in the intestine necessitates that IECs maintain a state of altered responsiveness (discussed below). Although the study of PRR pathways in haematopoietic cells has mostly focused on their pro-inflammatory properties in antigen-presenting and effector immune cell populations, their role in regulating tissue homeostasis and immune tolerance has emerged as a major component of their function in IECs (see Supplementary information S1 (table)). The homeostatic role of microbial recognition by IECs. Evidence of a role for PRRs in the protection against intestinal inflammation and repair of epithelial damage emerged from studies of mice deficient in TLRs and signalling adaptors or depleted of key commensal microorganisms56. Landmark work by Medzhitov and colleagues demonstrated, through the use of TLR- and MYD88‑deficient and broad-spectrum antibiotic-treated mice, that commensal bacteria-derived signals contribute to epithelial homeostasis and repair in a model of chemically induced colitis using dextran sodium sulphate (DSS)56. This and other studies defined beneficial roles of IEC-intrinsic TLR signalling that include the expression of cytoprotective heat-shock proteins, epidermal growth factor receptor ligands56,57, and TFF3 (REF. 58), and the

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F O C U S O N H o m e os tat i c I m m un e R e sRpEons eS s V IE W a

b

Commensal bacteria

Mucin

AMPs

TFF3

TLR9 (apical)

ROS Tight junction

TLR3, TLR7, TLR8

Endosome

MYD88 Ub Ub Ub Iκ B NLRP3, NLRP6, NLRC4

Heat-shock proteins IκB p50 p65

NF-κB

NF-κB

p50 p65

p50 p65

NOD1, NOD2

Inflammasome

RIP2 FRMPD2

IL-1β and IL-18

(NLR). A pattern-recognition receptor located in the cytosol. NLRs recognize a wide range of foreign structures and patterns associated with pathogenesis. Some members of this family form multiprotein complexes known as inflammasomes, which regulate the processing and secretion of pro-inflammatory cytokines.

RIG‑I‑like receptor (RLR). A pattern-recognition receptor located in the cytosol that responds to viral RNA.

IKKγ IKKα IKKβ

NF-κB

Pro-caspase 1

NOD-like receptor

TRIF

TLR2, TLR4, TLR5 or TLR9

APRIL, BAFF, IL-25, retinoic acid, TGFβ and TSLP

EGFR EGFR ligands

Figure 2 | Microbial recognition promotes IEC health and function. a | Pattern-recognition receptors (PRRs), including intestinal epithelial cell (IEC)-expressed Toll-like receptors (TLRs) and NOD-like receptors (NLRs), recognize Nature | Immunology conserved microbial-associated molecular motifs and pathogen-specific virulence properties. TLRsReviews recruit the signalling adaptors MYD88 and TIR-domain-containing adaptor protein inducing interferon‑β (TRIF) on ligation to signal molecules via nuclear factor‑κB (NF‑κB), p50 and p65 subunit activation and the mitogen-activated protein kinase (MAPK) pathway (not shown). Nucleotide-binding oligomerization domain 1 (NOD1) and NOD2 signal through receptor-interacting protein 2 (RIP2) to activate NF‑κB and MAPKs, whereas other IEC-expressed NLRs, including NOD-, LRR- and pyrin domain-containing 3 (NLRP3), NLRP6 and NOD-, LRR- and CARD-containing 4 (NLRC4), form inflammasome complexes with pro-caspase 1 for the cleavage and activation of interleukin‑1β (IL‑1β) and IL‑18. Polarized expression of PRRs by IECs at either the apical or basolateral membrane may contribute to the discrimination between commensal and pathogen microbial signals. For example, signalling through surface or endosomal TLR9 at the apical pole of IECs promotes the inhibition of NF-κB signalling, whereas TLR signalling from the basolateral pole promotes NF-κB activation. b | Microbial recognition is integrated by IECs. This promotes cell survival and repair (mediated by trefoil factor 3 (TFF3), heat-shock proteins and epidermal growth factor receptor (EGFR) ligand expression), barrier function (mediated by increased mucin and antimicrobial peptide (AMP) producton) and immunoregulatory responses (mediated by a proliferation-inducing ligand (APRIL), B cell-activating factor (BAFF), IL-25, retinoic acid, transforming growth factor‑β (TGFβ) and thymic stromal lymphopoietin (TSLP)), FRMPD2, FERM and PDZ domain-containing 2; IκB, inhibitor of NF‑κB; IKK, IκB kinase; ROS, reactive oxygen species; Ub, ubiquitin.

Dextran sodium sulphate (DSS). A large polysaccharide that causes epithelial injury and inflammation in the intestinal tract and is commonly used in models of experimentally induced colitis for studying the response to intestinal injury.

enhanced integrity of apical tight-junction complexes59 (FIG. 2). Furthermore, IEC-specific deletion of elements necessary for the activation of the transcription factor complex nuclear factor-κB (NF‑κB) downstream of TLR signalling in mice, including the inhibitor of NF‑κB

(IκB) kinase (IKK) complex or NF‑κB essential modulator (NEMO), results in enhanced DSS-induced or spontaneous colitis60,61. These studies establish an essential role for TLRs, in addition to other NF‑κB signalling pathways, in epithelial homeostasis and repair.



R E V IE W S

Nuclear factor-κB (NF‑κB). A family of transcription factors important for pro-inflammatory and anti-apoptotic responses that are activated by the ubiquitindependent degradation of their respective inhibitors, members of the inhibitor of NF‑κB (IκB) family. This process is mediated by the kinases, IκB kinase 1 (IKK1) and IKK2.

Inflammasomes Multiprotein complexes that contain a member of the NOD-like receptor family, adaptor proteins and the protease caspase 1. These complexes regulate the catalytic processing and secretion of pro-inflammatory cytokines, including interleukin‑1β (IL‑1β) and IL‑18.

Reactive oxygen species (ROS). Chemically reactive molecules containing oxygen that, when produced in large amounts, have pro-inflammatory and antimicrobial effects. Physiological levels of ROS have been shown to regulate cellular signalling pathways.

As additional families of TLRs, such as the NLRs and RLRs, have been ascribed roles in regulating inflammatory immune cell responses, they have also been shown to be important in IECs for the regulation of intestinal homeostasis52–55. The identification of nucleotide-binding oligomerization domain 2 (NOD2), an NLR family member that recognizes bacterial muramyl dipeptide (MDP), as the first genetic susceptibility locus for Crohn’s disease has fuelled interest in the role of this PRR and the related protein NOD1 in both immune cells and the intestinal epithelium37,62,63. Moreover, inflammasomes formed by caspase 1 and NLRs, including IEC-expressed NOD-, LRR- and pyrin domaincontaining 3 (NLRP3), NLRP6, NLRP12 and NOD-, LRR- and CARD-containing 4 (NLRC4), have a complex influence over inflammation and epithelial repair, as demonstrated by both pathological and protective roles in constitutive knockout mouse models52,53,64 (see Supplementary information S1 (table)). As NLRs are expressed by several cell populations in the intestine, conditional knockout models will be required to elucidate the precise haematopoietic, epithelial and stromal contributions of these PRRs during inflammation and repair. Finally, reactive oxygen species (ROS) produced in response to commensal or pathogenic bacteria have a role in IEC-intrinsic signalling that acts to promote epithelial repair, independently of their microbicidal effects65,66. Through the inactivation of cellular redoxsensitive tyrosine phosphatases, ROS promote the form ation by IECs of focal adhesions, which are necessary for cell migration and wound healing 65,66. Strikingly, these findings show remarkable symmetry with studies in Drosophila melanogaster, in which ROS also promote epithelial homeostasis, suggesting an evolutionarily conserved role for ROS in mediating protective effects of commensal microorganism-dependent cellular responses67–69. The protective effects of microbial recognition by IECs may come at a cost. Although commensal microbial signals are protective in settings of tissue damage or infection, they can drive tumorigenesis and cancer when homeostatic responses become dysregulated60,70. Epithelial cell-intrinsic TLR, MYD88 and NF‑κB signalling have all been implicated in promoting tumour development and progression in multiple genetic70–73 and inflammationinduced60,72,74,75 mouse models of colorectal cancer. The convergence between PRR signalling and pro-oncogenic signalling pathways could partly explain the tumorigenic effects of microbial stimulation. The stabilization of key oncogenic proteins, such as MYC, has been shown to be promoted by MYD88 signalling 71. Furthermore, NF‑κB can enhance WNT signalling in terminally differentiated IECs to promote their dedifferentiation into stem cell‑like tumour initiators73. Paradoxically, some NLR signalling pathways protect against tumorigenesis, partly through the regulation of cell death and proliferation in damaged or transformed IECs76–80 and through the regulation of tissue repair responses mediated by interleukin‑18 (IL‑18) signalling 53,81,82. The complexity of the multiple roles of microbial recognition by IECs serves to further highlight the

delicate nature of the balance that exists between homeostasis and inflammation and its importance in maintaining healthy host–microorganism symbiosis. Specialized regulation of PRR pathways in IECs. The proximity of IECs to an abundance of luminal microbial signals necessitates specialized mechanisms for maintaining altered or hyporesponsive PRR signalling in response to commensal bacteria-dependent stimuli83,84. In support of this, IECs express negative regulators of PRR-dependent pro-inflammatory signalling 75,83,85,86 (see Supplementary information S1 (table)). The disruption of these regulatory pathways or constitutive activation of NF‑κB predispose mice to dysregulated epithelial homeostasis and exaggerated inflammation72,75,85,87. Furthermore, it has been appreciated that commensal bacteria-dependent production of ROS by IECs can attenuate the activation of NF‑κB, broadly tolerizing IECs to microbial stimulation through PRR signalling 88,89. Although additional mechanisms exist for the negative regulation of PRR signalling pathways90, in most cases the extent to which they are active in IECs and their contributions to intestinal homeostasis remain to be determined. In addition to maintaining the hyporesponsiveness of IECs, innate immune pathways must differentiate between signals derived from commensal and pathogenic microorganisms for the scaling of an appropriate inflammatory response91. The polarized nature of the intestinal epithelium allows for the anatomical segregation of PRRs (FIG. 2). In vitro and in vivo models demonstrate differential responsiveness of IECs to apical versus basolateral stimulation with multiple TLR ligands92–94. For example, although basolateral exposure of IECs to TLR9 ligands results in canonical activation and nuclear translocation of NF‑κB, apical exposure results in a net inhibitory effect through the stabilization of IκB94. This apical signal induces tolerance to subsequent TLR stimulation, demonstrating a unique adaptation for the cross-tolerance of microbial recognition pathways and a differential response to microbial signals based on anatomical location94 (FIG. 2). This concept of subcellular segregation and polarized distribution of TLRs has been translated to the regulation of additional PRR pathways95,96. Through a series of elegant genetic screens, FERM and PDZ domaincontaining 2 (FRMPD2) — which is a positive regulator of NOD2‑mediated NF‑κB activation in response to MDP recognition — was recently identified to act as a scaffold protein that promotes basolateral membrane localization and selective basolateral activation through interactions with the leucine-rich repeat (LRR) domain of NOD2 (REF. 96) (FIG. 2). Common Crohn’s diseaseassociated variants of NOD2 contain mutations in this LRR domain. These NOD2-mutant proteins were shown in vitro to lack the ability to interact with FRMPD2, to colocalize at the basolateral membrane of epithelial cells and to respond to stimulation with NOD2 ligands62,63,96. These studies give insight into the mechanism of NOD2 dysfunction associated with IBD and how IECs may spatially regulate the activation of PRR signals at the intestinal barrier.

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F O C U S O N H o m e os tat i c I m m un e R e sRpEons eS s V IE W Finally, mechanisms by which IECs may break their relative tolerance to microbial signals in settings of pathogen infection are poorly defined. In contrast to sterile sites in the body, control of inflammation in the intestine may be more adapted to relying on the recognition of ‘danger’ signals associated with pathogenesis, rather than on the presence of microbial signals alone97. The recognition of danger has been proposed to be mediated through the detection of properties associated with microbial viability, termed viability-associated PAMPs (vita-PAMPs), that distinguish living pathogens from inert microbial debris, as well as through the detection of conserved virulence factors of pathogens, such as bacterial secretion systems and toxins that penetrate into the cellular cytosol91,98. Although these mechanisms for scaling microbial threats have been studied and identified in phagocytes and antigen-presenting cells, their function and relevance in IECs are less well understood. Commensal microorganism-dependent regulation of barrier function. In addition to the homeostatic role of microbial recognition by IECs, the intestinal epithelium acts as an essential integrator of environmental signals for the regulation of microbial colonization, barrier function and mucosal immune responses. As previously discussed, the production of an apical mucous layer, the secretion of broadly targeted AMPs and the transcytosis of secretory IgA contribute to epithelial barrier function. Reduced mucous layer thickness in germ-free mice can be reversed by treatment with TLR ligands, indicating that commensal bacteria-dependent signals regulate mucus production by goblet cells17. Similarly, the expression of many epithelial cell-derived AMPs is enhanced by, or dependent on, the presence of commensal microbial signals18,29,99–101. As cells with specialized antimicrobial function, Paneth cells play a particularly important part in the regulation of AMP production through cell-intrinsic expression of MYD88 and NOD2 (REFS 100,101). The transport of IgA across the epithelial barrier is regulated, in part, by the expression of pIgR on the basolateral membrane of IECs, which is promoted by MYD88- and NF‑κB‑dependent signalling in response to commensal microbial signals40,102. Finally, the integrity of tight junctions and transepithelial permeability are regulated by commensal microbial signals, including TLR2‑dependent redistribution of the tight-junction proteins to apical cell–cell contacts59. Thus, the ability of IECs to sense their microbial surroundings has an integral role in regulating their barrier function. Viability-associated PAMPs (Vita-PAMPs). Members of a special class of pathogenassociated molecular patterns recognized by the innate immune system to signify microbial life. These patterns differentiate dead and living microorganisms to allow for scaling of appropriate immune responses based on the level of threat the microbial signals represents.

Regulation of immune cells by IECs IECs produce numerous immunoregulatory signals that are necessary for tolerizing immune cells, limiting steadystate inflammation and directing appropriate innate and adaptive immune cell responses against pathogens and commensal bacteria. Many of these responses depend on the translation of commensal bacteria-derived signals by IECs to mucosal immune cells. The production of the cytokines thymic stromal lymphopoietin (TSLP)103–105, transforming growth factor‑β (TGFβ)104,106 and IL‑25 (REF. 107) and the B cell-stimulating factors

a proliferation-inducing ligand (APRIL; also known as TNFSF13) and B cell-activating factor (BAFF; also known as TNFSF13B)108,109 by IECs is promoted by commensal bacteria via PRR signalling (FIG. 2). We discuss below the immunoregulatory functions of IECs, describing their contribution to the priming of adaptive immune cell responses, regulation of innate effector responses and homeostasis of adaptive immune cell function in the intestinal environment. Mononuclear phagocytes and antigen presentation. IECs exert their influence over the priming of both cellular and humoral adaptive immune responses via a continuous dialogue with antigen-presenting mononuclear phagocytes (FIG. 3). IEC-derived TSLP, TGFβ and retinoic acid, produced in response to commensal bacteria-derived signals, promote the development of DCs and macrophages with tolerogenic properties, including the production of IL‑10 and retinoic acid103,104,110. Considerable hetero geneity exists among intestinal mononuclear phagocytes, the classification of which has been previously complicated by conflicting nomenclature, as well as phenotypical and functional plasticity in settings of inflammation111. However, two distinct populations that have been characterized are the pre‑DC‑derived CD11c+CD103+ DCs and monocyte-derived CD11clowF4/80+CX3CR1hi intestine‑resident macrophages112–115. CD103+ DCs act as migratory antigen-presenting cells and upon activation traffic to secondary lymphoid tissues, including the mesenteric lymph nodes and Peyer’s patches, carrying with them antigenic material and live bacteria for presentation to adaptive immune cells116,117. Influenced by their previous interactions with IECs at the intestinal barrier, these migratory DCs promote immune tolerance through the differentiation of forkhead box P3 (FOXP3+) regulatory T cells by a TGFβ- and retinoic acid‑dependent mechanism116,118,119. Furthermore, the production of retinoic acid by IEC-conditioned CD103+ DCs is responsible for the imprinting of gut-homing properties on T cells, allowing for the targeting of recirculating mature cells to the original site of antigen encounter in the intestinal lamina propria120–124. Thus, in addition to promoting naive T cell maturation based on antigen specificity, CD103+ DCs relay the original context of antigenic encounter at the intestinal epithelial barrier. In contrast to CD103+ DCs, CX3CR1hi intestineresident macrophages lack migratory properties in the steady state and instead persist in close physical contact with IECs, where they act as avid phagocytes to mediate clearance of pathogens and commensal bacteria that traverse the epithelial barrier 116,125. Their expression of tight-junction proteins allows the formation of transepithelial dendrites that penetrate into the lumen of the intestine for sampling of exogenous antigens48,125. Reflecting the functional dependence of these cells on the epithelium, the extension of these transepithelial dendrites is initiated by TLR signalling, not in myeloid cells themselves, but in IECs49. The CX3CR1hi intestine‑ resident macrophage population has also been implicated in the maintenance of mucosal tolerance, as they have been shown to promote the survival and local



R E V IE W S Innate immune regulation

IL-25, IL-33, TSLP IL-13, amphiregulin

Adaptive immune regulation SEMA7A TLA

Commensal bacterium

IL-25

IFNγ, TNF

IL-1β, IL-23

IL-17, IL-22

TSLP, TGFβ, RA

IEL

sIgA

TSLP APRIL, BAFF

IL-7, IL-15

IL-12 IL-10

ILC2

IL-25

IgA+ plasma cell ILC3

ILC1

TSLP

TReg cell

DC

Lamina propria

TCR MHC Macrophage Monocyte Basophil progenitor

Peyer’s patch or mesenteric lymph node

Naive T cell

Type 2 MPP Mast cell

Basophil

RA, TGFβ

B cell

Basophil

IL-10, RA, TGFβ

TReg cell Direct IEC effect Indirect IEC effect Immune response Differentiation

Figure 3 | IECs regulate innate and adaptive immunity. Intestinal epithelial cell (IEC)-derived cytokines interleukin‑25 Nature progenitors Reviews | Immunology (IL‑25) and thymic stromal lymphopoietin (TSLP) elicit the expansion and differentiation of basophil and multipotent progenitor type 2 (type 2 MPP) cells, respectively. IL‑25, IL‑33 and TSLP stimulate group 2 innate lymphoid cells (ILC2s), whereas IL‑25 suppresses innate lymphoid cell subset 1 (ILC1) and ILC3 function by limiting macrophage production of pro-inflammatory cytokines IL‑1β, IL‑12 and IL‑23. IECs condition dendritic cells (DCs) and macrophages towards a tolerogenic phenotype through the production of TSLP, transforming growth factor‑β (TGFβ) and retinoic acid (RA). These DCs promote the differentiation of naive CD4+ T cells into regulatory T (TReg) cells and the maturation of B cells into IgA-secreting plasma cells. Mucosal cell-derived DCs also imprint a gut-homing phenotype on primed B cells and T cells through the production of RA. After trafficking to the intestine, TReg cells are expanded in number by macrophages that are conditioned to produce IL‑10 by TSLP-mediated stimulation and through contact-dependent interactions with IEC-expressed semaphorin 7A (SEMA7A). The production of a proliferation-inducing ligand (APRIL) and B cell-activating factor (BAFF) by IECs and by TSLP-stimulated macrophages and DCs promotes class-switch recombination and the production of IgA by B cells in the intestinal lamina propria. IEL, intra-epithelial lymphocyte; IFNγ, interferon‑γ; sIgA, secretory IgA; TCR, T cell receptor; TLA, thymus leukaemia antigen; TNF, tumour necrosis factor.

Innate lymphoid cells (ILCs). A group of innate immune cells that are lymphoid in morphology and developmental origin, but lack properties of adaptive B cells and T cells such as recombined antigen-specific receptors. They function in the regulation of immunity, tissue homeostasis and inflammation in response to cytokine stimulation.

expansion of previously primed regulatory T cells 126. CX3CR1hi macrophages promote tolerance in the intestinal lamina propria through the production of IL‑10, which leads to suppression of inflammatory cytokine production by colitogenic T cells and promotion of regulatory T cell function127,128. IECs maintain this tolerogenic function through their production of soluble factors, such as TSLP, TGFβ and retinoic acid103,104,110, as well as through contact-dependent interactions involving IEC expression of the integrin ligand semaphorin 7A, which induces IL‑10 expression by CX3CR1hi macrophages and promotes intestinal homeostasis129. IECs also play an important part in the induction of T helper 2 (TH2) cell responses during helminth infection. In this setting, the IEC-derived cytokines TSLP and IL‑25 promote the expansion and differentiation of haematopoietic progenitor cells towards mononuclear

and myeloid cell phenotypes that promote the development of type 2 cytokine responses at mucosal sites130–133. These cells include a distinct population of basophil progenitors and a population of multipotent progenitor cells, which undergo extramedullary haematopoiesis and represent an innate link between IEC-derived signals and the polarization of TH2 cell immune responses to helminths and allergens132,133. Innate lymphocyte function. In addition to the myeloid cell and granulocyte populations, a recently identified innate immune cell population of innate lymphoid cells (ILCs) plays a crucial part in intestinal immune homeostasis. ILCs lack properties of adaptive lymphocytes, such as recombined antigen-specific receptors134. They are found at barrier surfaces, including mouse and human lung 135, skin136 and intestine137, where they function

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Natural killer cells (NK cells). A subset of innate lymphoid cells originally defined on the basis of their cytolytic activity against tumour targets but now recognized to serve a broader role in host defence and inflammation through the production of cytokines.

as regulators of tissue homeostasis, inflammation and early innate response to infection. ILCs are regulated, in part, by epithelial cell-derived immunoregulatory signals (FIG. 3). ILCs display phenotypical and functional heterogeneity, which has been reviewed extensively elsewhere134,138–140. ILCs are characterized by their developmental requirements and differential cytokine expression into group 1, group 2 and group 3 ILCs, which share functional similarities with the adaptive CD4+ TH1, TH2 and TH17 cell populations, respectively. Group 1 ILCs include classical natural killer cells (NK cells) and innate lymphoid cell subset 1 (ILC1) cells, and are characterized by the production of the TH1 cell‑associated cytokines interferon-γ (IFNγ) and tumour necrosis factor (TNF) in response to IL‑12 and/or IL‑15 (REF. 140). Although NK cells can directly kill target cells through cytotoxic activity, other ILC1s are limited to cytokine production in response to stimulation. Although these ILC1s have a less well-understood function than NK cells, several recent reports suggest a possible role in mediating intestinal inflammation in murine colitis models and human IBD141,142. Group 2 ILCs (collectively termed ILC2s) produce the TH2 cell‑associated cytokines IL‑5 and IL‑13 (REF. 140). These factors contribute to an early innate response to intestinal helminth infection and invoke a protective epithelial response, including goblet cell hyperplasia and enhanced mucus secretion143–145. Furthermore, ILC2s present in the lung promote airway hyperresponsiveness or tissue repair in mouse models of allergy and influenza virus infection146–149. This suggests that ILC2s may have analogous functions in the intestine, perhaps during food allergy or wound repair; however, evidence for these roles has yet to be described. The proliferation and activation of ILC2s is supported by the predominantly epithelial cell-derived cytokines IL‑25, IL‑33 and TSLP143–145,150. The contribution of microbial stimulation to these signals reinforces the idea of the epithelium as an integrator of environmental signals for the regulation of immune cell function103,104,107. Finally, group 3 ILCs produce TH17 and TH22 cellassociated cytokines, including IL‑17A and IL‑22, in response to stimulation by IL‑23 (REF. 140). This group includes ILC3s, as well as lymphoid tissue inducer (LTi) cells, which have a well-established role in secondary lymphoid tissue organogenesis, mediated by interactions with stromal cells during embryonic development 151. IL‑22 has an important role in protecting the intestinal epithelium following injury or infection by bacterial pathogens152,153. In addition, ILC3‑derived IL‑22 supports the anatomical containment of gut-associated lymphoid tissue-resident commensal bacteria and the protection of IESCs in models of graft-versus-host disease154–156. These tissue-protective functions of IL‑22 are balanced by detrimental effects in certain inflammatory settings and in the initiation of inflammation-induced cancer 82,156,157. Collectively, these studies demonstrate the context-dependent nature of IL‑22 function. By contrast, ILC3‑derived IL‑17 is thought to have a primarily pro-inflammatory effect in the intestine and has been implicated in both mouse colitis and human IBD158–160.

IECs play an indirect part in the regulation of ILC3s in response to commensal bacteria-derived signals. For example, IEC-derived IL‑25 leads to the suppression of IL‑23 production by macrophages and decreased IL‑22 production by ILC3s161. By contrast, commensal bacteriadependent signals have also been shown to stimulate the production of IL‑7 by IECs162, which supports the production of IL‑22 by ILC3s through the stabilization of the transcription factor retinoid-related orphan receptor-γt (RORγt; encoded by RORC)162,163. These seemingly conflicting roles for IECs in regulating ILC3 function in response to commensal bacterial stimulation may be explained by heterogeneity among intestinal ILCs and by differential targeting of cell types capable of producing pro-inflammatory versus tissue-protective cytokines139. Although the function of ILCs has been appreciated in numerous mouse models, the importance and relative contribution of these cells to inflammation in settings of human disease remain incompletely defined. Future work in this field will be required to further characterize the heterogeneity and tissue-specific functions of these cells, elaborate our understanding of their contributions to human disease and develop means of clinically targeting their protective or detrimental functions. Tissue-resident T cells. Following priming by intestinederived antigen-presenting cells in secondary lymphoid tissues, conventional effector T cells recirculate through the body before settling in the intestine, where they exert their tolerogenic or inflammatory effect on the local environment (FIG. 3). Here, mature T cells are subject to the direct influence of IECs for their functional maintenance and survival in the lamina propria. Specialized cells known as intraepithelial lymphocytes (IELs) exist in intimate contact with the IEC layer, and bidirectional interactions between IELs and IECs maintain immune homeostasis at the intestinal barrier 164–166. IELs display an activated phenotype and include conventional T cells, as well as subsets of cells expressing a restricted repertoire of T cell receptor specificities and specialized properties, including γδ T cells and NKT cells165,167. Recent studies have advanced the understanding of the developmental origin of these cells and the functions that they have at the intestinal barrier 165. These include the demonstration that committed CD4+ T cells can undergo transcriptional reprogramming when they become IELs to develop a distinct phenotype resembling that of CD8+ cytotoxic T cells168. Although the influence of the local environment in promoting this developmental change has not been explored, the intimate interactions that these cells have with IECs suggest that epithelial cell-derived signals may promote their maintenance and function. Tissue-resident conventional T cells primed to act as rapidly responsive effectors are important during ongoing inflammation and infection, as well as for the protection of the mucosal barrier against future challenge. This is thought to be particularly important in CD8+ T cell-dependent memory responses169. As such, CD8+ T cells with a tissue-resident memory phenotype are uniquely enriched among αβ T cells present in the intestinal IEL compartment of mice and humans169,170.



R E V IE W S Class-switch recombination (CSR). The process by which proliferating B cells rearrange their DNA to switch from expressing IgM (or another class of immunoglobulin) to expressing a different immunoglobulin heavy-chain constant region, thereby producing antibody with different effector functions.

These tissue-resident memory T (TRM) cells interact with IECs through CD103 (also known as αEβ7 integrin), which binds the adhesion molecule E-cadherin on IECs171,172. This may promote retention of these and other cells at the intestinal epithelium. Mouse IECs were recently demonstrated to contribute to the refinement of the CD8+ TRM cell pool in favour of high-affinity precursors that allow for a more efficient memory response to secondary mucosal challenge173. This occurs through the contact-dependent selective expansion and survival of high-affinity or high-avidity CD8+ T cell populations expressing homodimers of the co-receptor subunit CD8α (known as CD8αα+ IELs), which interact with the IEC-expressed MHC class I‑like molecule, thymus leukaemia antigen (TLA) 173 . Understanding how such memory cell populations are maintained is of particular interest in the design of efficient vaccines against pathogens that invade mucosal surfaces. Strategies have been explored for generating CD8+ TRM cells with protective effects at extra-intestinal sites174,175. Through an improved understanding of how IELs are maintained within the intestinal epithelium, we can hope to improve vaccine strategies for preventing infections with pathogens such as HIV and enteric viruses176. IgA-secreting plasma cells. The maturation of naive B cells into mature IgA-secreting plasma cells through heavy chain class-switch recombination (CSR) depends on priming by mucosal DCs carrying antigen and live bacteria from the intestinal epithelium117,177. Similar to the priming of a T cell mucosal phenotype, these DCs are conditioned by IEC-derived signals to promote IgA class switching and a gut-homing phenotype through the production of nitric oxide (NO), IL‑10 and retinoic acid, in conjunction with TGFβ signalling 117,124,178 (FIG. 3).

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In the presence of a cognate CD4+ T cell response, T cell expression of CD40L acts as a necessary signal for B cell CSR. In the absence of help from T cells, CSR can occur through the stimulation of B cells by APRIL and BAFF, and signalling through transmembrane activator and CAML interactor (TACI) and BAFF receptor (BAFFR; also known as TNFRSF13C)108,109,179,180. This process is directly supported by IECs through the production of APRIL and BAFF in response to commensal bacteriainduced NF‑κB signalling 108,109. Moreover, IECs induce APRIL and BAFF production by mucosal DCs through TSLP signalling, which acts to amplify the effect on B cell stimulation108,109. This pathway is of clinical relevance to the most prevalent human primary immunodeficiencies, common variable immunodeficiency and IgA deficiency, in which a subset of patients have mutations in the gene encoding the TACI receptor that lead to defects in CSR and IgA production181,182.

Concluding remarks Collectively, the studies highlighted in this Review demonstrate the diverse and multifaceted roles that IECs have in the continuous maintenance of intestinal homeostasis. Through secretory epithelial cell responses and the maintenance of a continuous cell layer, IECs effectively sustain a physical and biochemical barrier between hosts and their environment. As cells forming a uniquely adapted barrier surface, IECs actively respond to their local environment through regulatory mechanisms that earn IECs recognition as central mediators of microbial and immune homeostasis in the intestine. As much of what is understood of IEC function has been derived from studies using mouse models, a future challenge lies in the translation of this understanding into human systems and the development of novel therapeutics for targeting the pathways that contribute to human health.

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Acknowledgements

The authors thank all members of the Artis laboratory for discussions and critical reading of the manuscript. This work is supported by US National Institutes of Health grants (AI061570, AI095608, AI087990, AI074878, AI095466, AI106697, AI102942 and AI097333 to D.A.; T32AI00744 to L.W.P.), the Burroughs Wellcome Fund Investigator in Pathogenesis of Infectious Disease Award (D.A.) and the Crohn’s and Colitis Foundation of America (D.A.).

Competing interests statement

The authors declare no competing interests.

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