The Intestinal Epithelium: Central Coordinator of Mucosal Immunity
Allaire JM, Crowley SM, Law HT, Chang SY, Ko HJ, Vallance BA
Trends Immunol. Sep 2018
COMMENT: The intestinal mucosa, a single layer of cells, interacts with and, at the same time, is a barrier to luminal microbiota. The communication between microbes and intestinal epithelial cells (IECs) and how these cells interacts with the immune system is a crucial matter and the main topic of this review.
This review illustrates the cellular and functional complexity of the intestinal mucosa:
Various differentiated cell types are found within the gut epithelium, and each carries out unique and specialized functions. The distribution of these cell types is also different between the small and large bowel
These cell types include: enterocytes, the most prominent cell type of the intestinal epithelium that is responsible for nutrient and water absorption, various secretory cells such as goblet cells that secrete mucins, enteroendocrine cells that secrete hormones, and Paneth cells that release antimicrobial factors to protect nearby stem cells at the base of small intestinal crypts. Finally, there are the chemosensory tuft cells which play a key role in defense against helminths, and M cells that are integral to the uptake and eventual presentation of luminal antigens to the immune system
Regarding the well known and close relationship between microbiome and Intestinal mucosa, the authors indicate:
The intestinal mucosa of germ- free mice is very thin, displaying reduced IEC proliferation as well as impaired production of mucins and other IEC-derived mediators. This thinning of the protective mucin layer makes germ-free mice highly susceptible to the direct toxic effects of the colitogenic agent dextran sodium sulfate (DSS), reinforcing the importance of gut microbes in promoting intestinal tissue protection and repair
It seems that the presence of continous antigenic stimulus at the mucosa is an important factor to maintain a normal mucosa and that this depends on an intact innate MyD88 signalling pathway.
Another important factor is the metabolic products of microbiota:
Obligate anaerobes such as Clostridium clusters IV and IXa, Faecalibacterium prausnitzii, and Bacteroides thetaiotaomicron produce the majority of butyrate, a short-chain fatty acid (SCFA) that is found in the lumen of the colon and is mostly converted through fermentation of dietary fiber
Luminal butyrate is largely consumed by differentiated colonocytes residing at the top of the crypts.
The architecture of the intestinal crypt shields the stem cells located at the crypt base from this metabolite. Disruption/damage of the crypt architecture (as occurs in DSS colitis) exposes stem cells to higher butyrate levels, which in turn suppresses their proliferative capacity by inhibiting histone deacetylase (HDAC) enzymes and inducing transcription factor Forkhead box O3 (FoxO3)
Other microbial metabolites have been found to modulate IEC function. Lactate is an important energy source for small intestinal stem cells to sustain their proliferation and differentiation capacity. Acetate, an SCFA produced by Bifidobacterium, has also been shown to influence goblet cell differentiation. The presence of acetate-producing microbes as well as acetate itself increased goblet cell secretion of mucin, and also promoted the terminal decoration of mucin glycans with sialic acid, whereas the presence of an acetate-consumer (Fecalibacterium) reduced these effects. Germ-free mice, which lack these bacterial metabolites, displayed shorter Muc2 O-glycans, and this correlated with a decrease in the expression of their respective glycosyltransferase enzymes by IECs
Bacterial metabolites within the intestinal environment are important in maintaining normal IEC physiology in the small intestine as well as in the colon. Crosstalk between the microbiota and IECs developed as a means to prevent IEC dysfunction, and recent studies have shown that disruption of this crosstalk can also lead to aberrant changes in the resident microbial populations, termed ‘dysbiosis’. There is substantial evidence that the resident intestinal microbiota works in concert with colonocytes to prevent enteric microbial dysbiosis such as that observed in IBD patients
Butyrate is an important metabolic factor modulating intestinal immune cells functions:
Butyrate can also epigenetically regulate gene expression through the inhibition of histone deacetylases.
This appears to be the mode of action for the modulating influence of commensal microbiota on peripheral regulatory T cell (Treg) differentiation.
In butyrate-treated naïve CD4+ T cells has been reported an increased histone H3 acetylation of Foxp3 at its promoter region and at the intronic enhancer ‘intragenic enhancer elements conserved noncoding DNA sequence 1’ (CNS1) and CNS3 . This increased acetylation of Foxp3 resulted in increased gene expression. CNS1-deficient mice were unresponsive to the pres- ence of butyrate and did not induce FOXP3 expression in naïve CD4+ T cells
Overall, butyrate influences the balance between intestinal pro- and anti-inflammatory mechanisms
Colonization resistance is an important feature of a healthy microbiome:
Invading microbes must compete for limited space and nutrients against entrenched microbiota–host symbiotic relationships as well as resist against potent antimicrobial molecules produced by some resident gut microbes. For example, Bacteroides thuringiensis, a bacterium found in human feces, produces a narrow-spectrum bacteriocin that is highly potent against the nosocomial pathogen Clostridium difficile. This bacteriocin, named thucin CD, is a two-component antimicrobial that acts synergistically to disrupt the cell membrane of its target
Innate Immune System molecules and IECs:
Situated at the interface between the luminal microbiota and the underlying immune system, the intestinal epithelium plays a crucial role in the detection of microbes under homeostatic as well as pathologic conditions. IECs express a variety of innate receptors that detect microbes and endogenous danger signals, including the TLRs
Enterocytes are known to express TLR2, TLR3, TLR4, TLR5, and TLR9
The polarized nature of IECs facilitates the anatomical distribution of TLRs, segregating the majority to the basolateral membrane, while TLR2 and TLR9 are also expressed at the apical surface
Once activated at the basolateral membrane, TLR signaling initiates a signaling cascade cumulating in the nuclear translocation of NF-kB. This leads to the expression and secretion of various cytokines and chemokines, including TNF-a, IL-6, IL-8, IL-18, and CCL20, which signal and prime underlying immune cells
By contrast, apical stimulation of TLR9 results in a net immune inhibitory effect through the stabilization of IkB, demonstrating the unique ability of IECs to differentially respond to microbial signals using the same receptors but at varying spatiotemporal positions
TLRs expressed by IECs play a key role in recognizing nearby microbes and their products, the responses they elicit are primarily aimed at keeping microbes at a distance (mucin secretion and antimicrobial production) or recruiting immune cells (via chemokines) to the site of infection. Upon direct invasion of IECs by intracellular pathogens, a more vigorous means of host defense is triggered, by which enterocytes can physically expel themselves from the intestinal epithelial lining while still maintaining mucosal barrier function. This process prevents intracellular pathogens from breaching the epithelial barrier, and instead they are extruded into the fecal stream, along with the sloughed enterocyte. This expulsion is independent from the homeostatic process of continuous enterocyte renewal. and instead involves activation of the inflammatory caspases (caspase-1, -4, and -11) as well as caspase-8
Not only enterocytes and colonocytes but also enterochromaffin cells make a crosstalk with microbiota:
Some members of the intestinal microbiota, particularly spore-forming Clostridia spp., can stimulate colonic entero- chromaffin cells to upregulate their production of the neurotransmitter 5-HT
In the context of an enteric infection, a limited number of studies suggest that production of 5-HT can aid in preventing incursion by microbial (S. Typhimurium) and helminthic (Trichuris muris) pathogens, possibly through the modulation of enterocyte function (such as antimicrobial peptide secretion and IL-13 receptor a1 signaling). Recent findings revealed that L cells, another type of enteroendocrine cell, can secrete glucagon-like peptide (GLP) 1 in a TLR4-dependent manner upon exposure to bacteria-derived LPS during gut injury or chemical-induced colitis in mice
And also Paneth cells with a potential very interesting relationship with Crohn’s disease:
Paneth cells, found specifically in the small intestine, have received the most attention based on their specialized role in secreting large granules filled with antimicrobial peptides and enzymes such as a-defensins, RegIIIg, sPLA2, and lysozyme. The baseline release of these antimicrobials is thought to promote normal microbial composition within the intestinal lumen. This theory is based on recent studies linking the presence of structurally and functionally abnormal Paneth cells in Crohn’s disease (CD) patients to the development of microbial dysbiosis within the ileum. Significant reductions in barrier promoting microbes such as Faecalibacterium prausnitzii were also observed, emphasizing the crucial role played by Paneth cells in regulating intestinal microbial homeostasis. In this regard, single-nucleotide polymorphisms (SNPs) in several IBD-associated genes such as NOD2, XBP1, ATG16L1, and IRGM have been linked to Paneth cell dysfunction in IBD patients. These Paneth cell abnormalities include structurally irregular granules as well as impaired autophagic secretory pathways and failure in autophagy-based removal of damaged organelles. Two different hypomorphic Atg16l mouse models suggest that ATG16L1 controls the shape and distribution of Paneth cell granules as well as their production of lysozyme. Interestingly, biopsies of CD patients possessing the relevant ATG16L1 polymorphisms, but with quiescent disease, show an increased incidence of intestinal colonization by the microbial pathobiont adherent-invasive Escherichia coli (AIEC). This suggests that defects in autophagy, presumably within Paneth cells, may increase the susceptibility of CD patients to colonization or infection by specific microbes such as AIEC, and potentially predispose the patients to disease progression that requires intestinal surgery
Although innate signaling plays an important role in the function of Paneth cells, presumably there are other pathways involved in controlling their ability to release their secretory granules, such as would occur in response to an infection of the small intestine. Interestingly, both in vivo and in vitro studies have demonstrated that exposure to IFN-g causes rapid and complete degranulation of Paneth cells, accompanied by their luminal extrusion and apoptosis
And Globet cells:
Intestinal goblet cells are best known for their production and secretion of the glycosylated mucin protein Muc2 which forms the protective mucus barrier atop the intestinal epithelium.
In its absence, Muc2-/- mice suffer exaggerated bacterial contact with their colonic epithelium, provoking inflammatory responses that eventually develop into spontaneous colitis. While Muc2 is normally released at a baseline level, in response to noxious luminal stimuli goblet cells undergo compound exocytosis, thereby releasing all of their mucin granules and other contents into the lumen to protect the epithelial barrier from direct contact with these threats. Intestinal goblet cells exposed to bacterial products or cytokines such IL-22 and IFN-g undergo this compound exocytosis
IECs communicate with the Immune System:
Early in the 1990s enterocytes were found to secrete an array of chemokines and cytokines, particularly in response to direct contact with bacteria or their products. Many of these secreted molecules are known to recruit and/or activate neutrophils, macrophages, dendritic cells, and T cells. The ability of enterocytes, and presumably other IEC types, to direct these cells and the local immune response is thought to be key to the resolution of infections that have crossed the gut barriers, as well as to the promotion mucosal repair following intestinal damage.
Renewed interest in tuft cells has been prompted recently owing to their newly discovered roles in detecting helminth parasites and as drivers of ILC2 expansion. Luminal helminths such as T. muris can be sensed by tuft cells through their GTP-binding protein a-gustducin that activates the non-selective cation channel transient receptor potential melastatin-like subtype 5 channel (Trpm5). Interestingly, in the oral cavity Trpm5 is responsible for the transduction of bitter, sweet, and umami tastes, and when colonic tuft cells are subjected to the bitter substrate denatonium this correlated with a sharp increase in intracellular calcium. This may denote chemosensors as a new class of pathogen recognition receptors if tuft cells are truly responding to chemicals secreted by invading parasites. Tuft cells constitutively secrete IL-25 to maintain ILC2 homeostasis, but upon recognizing helminths they increased their secretion of IL-25, which directly acts on ILC2 cells to release IL-13
This in turns feeds forward, with IL- 13 acting on epithelial progenitors in an IL-13Ra1/IL-4aR-dependent manner to promote tuft cell and goblet cell hyperplasia, cumulating in a dramatic increase in bulk mucin release that drives the expulsion of the parasite from the intestinal tract
In summary, this review highlights the importance and complexity of IECs for the microbiome, defense against pathogens and local modulation of the Immune System