Innate immunity in the small intestine

Microvasculature remodeling in the mouse lower gut during inflammaging

Inflammaging is defined as low-grade, chronic, systemic inflammation in aging, in the absence of overt infection. Age-associated deterioration of gastrointestinal function could be ascribed to the inflammaging, although evidence is yet to emerge. Here we show that microvessels in aging mouse intestine were progressively deprived of supportive structures, microvessel-associated pericytes and adherens junction protein vascular endothelial (VE)-cadherin, and became leaky. This alteration was ascribed to up-regulation of angiopoetin-2 in microvascular endothelial cells. Up-regulation of the angiopoietin-2 was by TNF-α, originated from M2-like residential CD206⁺ macrophages, proportion of which increases as animal ages. It was concluded that antigenic burdens encountered in intestine throughout life create the condition of chronic stage of inflammation, which accumulates M2-like macrophages expressing TNF-α. The TNF-α induces vascular leakage to facilitate recruitment of immune cells into intestine under the chronic inflammatory setting.

The Role of Mucosal Immunity in the Pathogenesis of Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is the most devastating gastrointestinal disease of prematurity. Although the precise cause is not well understood, the main risk factors thought to contribute to NEC include prematurity, formula feeding, and bacterial colonization. Recent evidence suggests that NEC develops as a consequence of intestinal hyper-responsiveness to microbial ligands upon bacterial colonization in the preterm infant, initiating a cascade of aberrant signaling events, and a robust pro-inflammatory mucosal immune response. We now have a greater understanding of important mechanisms of disease pathogenesis, such as the role of cytokines, immunoglobulins, and immune cells in NEC. In this review, we will provide an overview of the mucosal immunity of the intestine and the relationship between components of the mucosal immune system involved in the pathogenesis of NEC, while highlighting recent advances in the field that have promise as potential therapeutic targets. First, we will describe the cellular components of the intestinal epithelium and mucosal immune system and their relationship to NEC. We will then discuss the relationship between the gut microbiota and cell signaling that underpins disease pathogenesis. We will conclude our discussion by highlighting notable therapeutic advancements in NEC that target the intestinal mucosal immunity.

Defensins, lectins, mucins, and secretory immunoglobulin A: microbe-binding biomolecules that contribute to mucosal immunity in the human gut

In the intestine, the mucosal immune system plays essential roles in maintaining homeostasis between the host and microorganisms, and protecting the host from pathogenic invaders. Epithelial cells produce and release a variety of biomolecules into the mucosa and lumen that contribute to immunity. In this review, we focus on a subset of these remarkable host-defense factors - enteric α-defensins, select lectins, mucins, and secretory immunoglobulin A - that have the capacity to bind microbes and thereby contribute to barrier function in the human gut. We provide an overview of the intestinal epithelium, describe specialized secretory cells named Paneth cells, and summarize our current understanding of the biophysical and functional properties of these select microbe-binding biomolecules. We intend for this compilation to complement prior reviews on intestinal host-defense factors, highlight recent advances in the field, and motivate investigations that further illuminate molecular mechanisms as well as the interplay between these molecules and microbes.

Cryptosporidium infection in solid organ transplantation

Diarrhea is a common complication in solid organ transplant (SOT) recipients and may be attributed to immunosuppressive drugs or infectious organisms such as bacteria, viruses or parasites. Cryptosporidium usually causes self-limited diarrhea in immunocompetent hosts. Although it is estimated that cryptosporidium is involved in about 12% of cases of infectious diarrhea in developing countries and causes approximately 748000 cases each year in the United States, it is still an under recognized and important cause of infectious diarrhea in SOT recipients. It may run a protracted course with severe diarrhea, fluid and electrolyte depletion and potential for organ failure. Although diagnostic methodologies have improved significantly, allowing for fast and accurate identification of the parasite, treatment of the disease is difficult because antiparasitic drugs have modest activity at best. Current management includes fluid and electrolyte replacement, reduction of immunosuppression and single therapy with Nitazoxanide or combination therapy with Nitazoxanide and other drugs. Future drug and vaccine development may add to the currently poor armamentarium to manage the disease. The current review highlights key epidemiological, diagnostic and management issues in the SOT population.

TLR ligands and butyrate increase Pyy expression through two distinct but inter-regulated pathways

The intestinal epithelium is an active barrier separating the host from its microbiota. It senses microbial compounds through expression of a wide range of receptors including the Toll-like receptors (TLRs). TLRs have been shown to regulate epithelium permeability or secretion of defensin by Paneth cells. However, the expression and function of TLRs in enteroendocrine L-cells, a specific subtype of intestinal cells secreting PYY and GLP-1, have not yet been assessed. PYY and GLP-1 are implicated in regulation of gut motility, food intake and insulin secretion, and are of great interest regarding obesity and type 2 diabetes. Using a cellular model of human L-cells and a reporter system for NF-κB activation pathway, we reported functional expression of TLRs in these cells. Stimulation with specific TLR-agonists increased expression of Pyy but not Proglucagon in an NF-κB-dependent manner. Moreover, the effect of TLR stimulation was additive to butyrate, a product of bacterial fermentation, on Pyy expression. Additionally, butyrate also increased Tlr expression, including Tlr4, and the NF-κB response to TLR stimulation. Altogether, our results demonstrated a role of TLRs in the modulation of Pyy expression and the importance of butyrate, a product of bacterial fermentation in regulation of microbial TLR-dependent sensing.

Effect of bowel resection on TLR signaling during intestinal adaptation in a rat model

BACKGROUND

Bacterial overgrowth is common complication of short bowel syndrome (SBS) and is a result of an impaired gut barrier function. Toll-like receptor 4 (TLR4) is crucial in maintaining intestinal epithelial homeostasis, participates in a vigorous signaling process and heightens inflammatory cytokine output. The objective of this study was to determine the effects of bowel resection on TLR4 signaling in intestinal mucosa in a rat model.

METHODS

Male Sprague-Dawley rats were randomly assigned to one of the two experimental groups of eight rats each: Sham rats underwent bowel transection and re-anastomosis and SBS rats underwent 75 % small bowel resection. Rats were killed on day 14. Bacterial translocation (BT) to mesenteric lymph nodes, liver, portal blood and peripheral blood was determined at the kill. The expression of TLR4, MyD88 and TRAF6 in the intestinal mucosa was determined using real-time PCR, Western blot and immunohistochemistry.

RESULTS

SBS rats demonstrated a 100 % BT to lymph nodes and to liver (Level I), 80 % translocation to portal blood (Level II) and 60 % translocation to peripheral blood (Level III) at day 7 as well as a 100 % BT to lymph nodes and liver, and 40 % translocation to peripheral blood at day 14. Microarray expression profiling demonstrated that most of the TLR signaling-related genes were up-regulated in resected rats compared to control animals. SBS rats showed a significant increase in TLR4 and TRAF6 mRNA in jejunum and ileum, TLR4 and MyD88 protein expression in jejunum and ileum, and a significant increase in the number of TLR4 and TRAF6 positive cells (immunohistochemistry) compared to sham animals.

CONCLUSIONS

In a rat model of SBS, elevated intestinal BT is associated with a stimulated TLR4 signaling.

Branched-chain Amino Acids are Beneficial to Maintain Growth Performance and Intestinal Immune-related Function in Weaned Piglets Fed Protein Restricted Diet

As a novel approach for disease control and prevention, nutritional modulation of the intestinal health has been proved. However, It is still unknown whether branched-chain amino acid (BCAA) is needed to maintain intestinal immune-related function. The objective of this study was to determine whether BCAA supplementation in protein restricted diet affects growth performance, intestinal barrier function and modulates post-weaning gut disorders. One hundred and eight weaned piglets (7.96±0.26 kg) were randomly fed one of the three diets including a control diet (21% crude protein [CP], CON), a protein restricted diet (17% CP, PR) and a BCAA diet (BCAA supplementation in the PR diet) for 14 d. The growth performance, plasma amino acid concentrations, small intestinal morphology and intestinal immunoglobulins were tested. First, average daily gain (ADG) (p<0.05) and average daily feed intake (ADFI) (p<0.05) of weaned pigs in PR group were lower, while gain:feed ratio was lower than the CON group (p<0.05). Compared with PR group, BCAA group improved ADG (p<0.05), ADFI (p<0.05) and feed:gain ratio (p<0.05) of piglets. The growth performance data between CON and BCAA groups was not different (p>0.05). The PR and BCAA treatments had a higher (p<0.05) plasma concentration of methionine and threonine than the CON treatment. The level of some essential and functional amino acids (such as arginine, phenylalanine, histidine, glutamine etc.) in plasma of the PR group was lower (p<0.05) than that of the CON group. Compared with CON group, BCAA supplementation significantly increased BCAA concentrations (p<0.01) and decreased urea concentration (p<0.01) in pig plasma indicating that the efficiency of dietary nitrogen utilization was increased. Compared with CON group, the small intestine of piglets fed PR diet showed villous atrophy, increasing of intra-epithelial lymphocytes (IELs) number (p<0.05) and declining of the immunoglobulin concentration, including jejunal immunoglobulin A (IgA) (p = 0.04), secreted IgA (sIgA) (p = 0.03) and immunoglobulin M (p = 0.08), and ileal IgA (p = 0.01) and immunoglobulin G (p = 0.08). The BCAA supplementation increased villous height in the duodenum (p<0.01), reversed the trend of an increasing IELs number. Notably, BCAA supplementation increased levels of jejunal and ileal immunoglobulin mentioned above. In conclusion, BCAA supplementation to protein restricted diet improved intestinal immune defense function by protecting villous morphology and by increasing levels of intestinal immunoglobulins in weaned piglets. Our finding has the important implication that BCAA may be used to reduce the negative effects of a protein restricted diet on growth performance and intestinal immunity in weaned piglets.

Impact of T300A Variant of ATG16L1 on Antibacterial Response, Risk of Culture Positive Infections, and Clinical Course of Crohn's Disease

OBJECTIVES:

Autophagy-related 16-like 1 (ATG16L1) deficiency leads to impaired cellular autophagy and bacterial degradation as well as an altered cytokine production. The single-nucleotide polymorphism rs2241880 (T300A) is associated with an increased risk for Crohn's disease (CD). ATG16L1 polymorphisms could therefore have an impact on the risk of infectious complications and disease course in CD. We examined the impact of the T300A genotype on the antibacterial response toward a panel of pathogenic bacteria in vitro, as well as clinical infectious complications in vivo and the disease course in a Danish cohort of patients with CD.

METHODS:

A total of 236 CD patients were genotyped for ATG16L1^T300A; their clinical records were reviewed, and microbial, radiological, and surgical data were scrutinized. Peripheral blood mononuclear cells (PBMCs) were isolated from healthy controls and CD patients carrying the different ATG16L1 genotypes, and the production of tumor necrosis factor (TNF)-α and interleukin (IL)-1β was measured by enzyme-linked immunosorbent assay after stimulation with a panel of pathogenic bacteria of clinical relevance for the gastrointestinal tract, e.g., enteroinvasive Escherichia coli (EIEC), Listeria monocytogenes, Salmonella typhimurium, Staphylococcus aureus, or Mycobacterium avium paratuberculosis.

RESULTS:

Fifty-seven healthy controls (15, 29, 13) and 236 patients with CD (50, 108, 78) were genotyped for the T300A ATG16L1 polymorphism (AA homozygous, GG homozygous risk variant, AG heterozygous variant, respectively). The median duration of disease was 128 months (range, 30–175). The cumulative follow-up of this cohort was 2,366 patient-years. ATG16L1 gene variations interfered with the production of IL-1β, which was significantly increased in PBMCs from GG patients in response to all tested bacteria, whereas the TNF-α production was decreased in PBMCs from GG patients stimulated with EIEC, L. monocytogenes, and S. typhimurium, but unaffected by the other bacteria tested. Moreover, the GG variant showed a nonsignificant increase in the risk of bowel resections (P=0.07) and postsurgical infections (P=0.08), whereas the risk of non-disease-related infections was unaffected by genotype in the observation period. In addition, patients with AA and AG variants had a higher frequency of complicated fistulizing disease (P=0.03) with an overall more aggravated disease course with an increased number of surgical procedures for fistulous disease from a median 6.5 operations (2.0 in GG patients; P=0.002). This risk was independent on disease phenotype (penetrating vs. non-penetrating) and immunomodulating medication.

CONCLUSIONS:

The T300A variant in patients with CD strongly increases the risk for complicated fistulizing disease, and significantly affects antibacterial responses in vitro, but the latter effect seems to have a minor role for the infectious risk in CD.

The mRNA and Protein Levels of Tubulin and β-Actin Are Greatly Reduced in the Proximal Duodenum of Mice Relative to the Rest of the Small Intestines

To accurately quantify mRNA and protein levels, it is critical to choose appropriate internal standards. As the expression of housekeeping genes is assumed to remain constant, they are often employed to normalize signals to correct for sample-to-sample variations. However, recent studies have documented that β-actin and Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) expression levels change in response to various stimuli during proliferation, activation, and differentiation. We investigated levels of α-, β-, γ-tubulin, β-actin, and GAPDH vary across the gastrointestinal tract of mice. We found that different regions of the small intestines had dramatically different expression profiles, as measured by western blot, quantitative Reverse transcription polymerase chain reaction (RT-PCR), and immunohistochemical staining. These results revealed that the expression levels of tubulins and β-actin were dramatically lower in the proximal duodenum, relative to the rest of the small intestines. These varying levels of housekeeping genes may reflect differences in the activities of specialized tissues and suggest unique requirements for tubulins in these tissue types. We conclude that the use of a single housekeeping gene to normalize gene expression in the gastrointestinal tracts of mice may introduce errors, as measured differences in gene expression may reflect regulation of the internal control rather than the mRNA or protein under investigation.

Amyloid precursor protein mediated changes in intestinal epithelial phenotype in vitro

Background

Although APP and its proteolytic metabolites have been well examined in the central nervous system, there remains limited information of their functions outside of the brain. For example, amyloid precursor protein (APP) and amyloid beta (Aβ) immunoreactivity have both been demonstrated in intestinal epithelial cells. Based upon the critical role of these cells in absorption and secretion, we sought to determine whether APP or its metabolite amyloid β (Aβ), had a definable function in these cells.

Methodology/Principal Findings

The human colonic epithelial cell line, Caco-2 cells, were cultured to examine APP expression and Aβ secretion, uptake, and stimulation. Similar to human colonic epithelium stains, Caco-2 cells expressed APP. They also secreted Aβ 1-40 and Aβ 1-42, with LPS stimulating higher concentrations of Aβ 1-40 secretion. The cells also responded to Aβ 1-40 stimulation by increasing IL-6 cytokine secretion and decreasing cholesterol uptake. Conversely, stimulation with a sAPP-derived peptide increased cholesterol uptake. APP was associated with CD36 but not FATP4 in co-IP pull down experiments from the Caco-2 cells. Moreover, stimulation of APP with an agonist antibody acutely decreased CD36-mediated cholesterol uptake.

Conclusions/Significance

APP exists as part of a multi-protein complex with CD36 in human colonic epithelial cells where its proteolytic fragments have complex, reciprocal roles in regulating cholesterol uptake. A biologically active peptide fragment from the N-terminal derived, sAPP, potentiated cholesterol uptake while the β secretase generated product, Aβ1-40, attenuated it. These data suggest that APP is important in regulating intestinal cholesterol uptake in a fashion dependent upon specific proteolytic pathways. Moreover, this biology may be applicable to cells beyond the gastrointestinal tract.

High-protein diet differently modifies intestinal goblet cell characteristics and mucosal cytokine expression in ileum and colon

We have previously shown that high-protein (HP) diet ingestion causes marked changes in the luminal environment of the colonic epithelium. This study aimed to evaluate the impact of such modifications on small intestinal and colonic mucosa, two segments with different transit time and physiological functions. Rats were fed with either normal protein (NP; 14% protein) or HP (53% protein) isocaloric diet for 2 weeks, and parameters related to intestinal mucous-secreting cells and to several innate/adaptive immune characteristics (myeloperoxidase activity, cytokine and epithelial TLR expression, proportion of immune cells in gut-associated lymphoid tissues) were measured in the ileum and colon. In ileum from HP animals, we observed hyperplasia of mucus-producing cells concomitant with an increased expression of Muc2 at both gene and protein levels, reduction of mucosal myeloperoxidase activity, down-regulation of Tlr4 gene expression in enterocytes and down-regulation of mucosal Th cytokines associated with CD4+ lymphocyte reduction in mesenteric lymph nodes. These changes coincided with an increased amount of acetate in the ileal luminal content. In colon, HP diet ingestion resulted in a lower number of goblet cells at the epithelial surface but increased goblet cell number in colonic crypts together with an increased Muc3 and a slight reduction of Il-6 gene expression. Our data suggest that HP diet modifies the goblet cell distribution in colon and, in ileum, increases goblet cell activity and decreases parameters related to basal gut inflammatory status. The impact of HP diet on intestinal mucosa in terms of beneficial or deleterious effects is discussed.

TRIF signaling drives homeostatic intestinal epithelial antimicrobial peptide expression

Recent results indicate a significant contribution of innate immune signaling to maintain mucosal homeostasis, but the precise underlying signal transduction pathways are ill-defined. By comparative analysis of intestinal epithelial cells isolated from conventionally raised and germ-free mice, as well as animals deficient in the adaptor molecules MyD88 and TRIF, the TLR3 and TLR4, as well as the type I and III IFN receptors, we demonstrate significant TLR-mediated signaling under homeostatic conditions. Surprisingly, homeostatic expression of Reg3γ and Paneth cell enteric antimicrobial peptides critically relied on TRIF and, in part, TLR3 but was independent of IFN receptor signaling. Reduced antimicrobial peptide expression was associated with significantly lower numbers of Paneth cells and a reduced Paneth cell maturation and differentiation factor expression in TRIF mutant compared with wild-type epithelium. This phenotype was not transferred to TRIF-sufficient germ-free animals during cohousing. Low antimicrobial peptide expression in TRIF-deficient mice caused reduced immediate killing of orally administered bacteria but was not associated with significant alterations in the overall composition of the enteric microbiota. The phenotype was rapidly restored in a TRIF-independent fashion after transient epithelial damage. Our results identify TRIF signaling as a truly homeostatic pathway to maintain intestinal epithelial barrier function revealing fundamental differences in the innate immune signaling between mucosal homeostasis and tissue repair.

Augmented hepatic Toll-like receptors by fatty acids trigger the pro-inflammatory state of non-alcoholic fatty liver disease in mice

AIM

There is considerable evidence that intestinal microbiota are involved in the development of metabolic syndromes and, consequently, with the development of non-alcoholic fatty liver disease (NAFLD). Toll-like receptors (TLRs) are essential for the recognition of microbiota. However, the induction mechanism of TLR signals through the gut-liver axis for triggering the development of non-alcoholic steatohepatitis (NASH) or NAFLD remains unclear. In this study, we investigated the role of palmitic acid (PA) in triggering the development of a pro-inflammatory state of NAFLD.

METHODS

Non-alcoholic fatty liver disease was induced in mice fed a high fat diet (HFD). The mice were killed and the expression of TLRs, tumor necrosis factor (TNF), interleukin (IL)-1β, and phospho-interleukin-1 receptor-associated kinase 1 in the liver and small intestine were assessed. In addition, primary hepatocytes and Kupffer cells were treated with PA, and the direct effects of PA on TLRs induction by these cells were evaluated.

RESULTS

The expression of inflammatory cytokines such as TNF, IL-1β, and TLR-2, -4, -5, and -9 was increased in the liver, but decreased in the small intestine of HFD-fed mice in vivo. In addition, the expression of TLRs in primary hepatocytes and Kupffer cells was increased by treatment with PA.

CONCLUSION

In the development of the pro-inflammatory state of NAFLD, PA triggers the expression of TLRs, which contribute to the induction of inflammatory cytokines through TLR signals by intestinal microbiota.

Leucine stimulates ASCT2 amino acid transporter expression in porcine jejunal epithelial cell line (IPEC-J2) through PI3K/Akt/mTOR and ERK signaling pathways

Leucine has been shown to influence intestinal protein metabolism, cell proliferation and migration. Furthermore, our previous study demonstrated that branched-chain amino acids could modulate the intestinal amino acid and peptide transporters in vivo. As the possible mechanisms are still largely unknown, in the present work, we studied the transcriptional and translational regulation of leucine on amino acid transporter production in IPEC-J2 cells and the signaling pathways involved. Treatment of IPEC-J2 cells with 7.5 mM leucine enhanced the mRNA expression of the Na(+)-neutral AA exchanger 2 (ASCT2) and 4F2 heavy chain (4F2hc) and caused an increase in ASCT2 protein expression. Leucine also activated phosphorylation of 4E-BP1 and eIF4E through the phosphorylation of mTOR, Akt and ERK signaling pathways in IPEC-J2 cells. Pre-treatment of IPEC-J2 cells with inhibitors of mTOR and Akt (rapamycin and wortmannin) or an inhibitor of ERK (PD098059) for 30 min before leucine treatment attenuated the positive effect of leucine in enhancing the protein abundance of ASCT2. These results demonstrate that leucine could up-regulate the expression of the amino acid transporters (ASCT2) through transcriptional and translational regulation by ERK and PI3K/Akt/mTOR activation.

Amino acids modulates the intestinal proteome associated with immune and stress response in weaning pig

The objective of this study was to investigate the effects of free amino acids supplementation to protein restricted diet on the intestinal morphology and proteome composition in weaning pigs. Weanling piglets were randomly fed one of the three diets including a corn-soybean based control diet and two lower protein diets with or without free amino acids supplementation for 2 weeks. The jejunum samples of piglets were collected for morphology and proteome analysis. Compared with the control diet, the protein restricted diet had a significant lower average daily gain and higher feed conversion rate. Free amino acids supplementation to the protein restricted diet significantly improved average daily gain and higher feed conversion rate, compared with the protein restricted diet. The villous height in pigs fed the protein restricted diet was lower than that of the control and free amino acids diet. Using two-dimensional gel electrophoresis and mass spectrometry, we identified 16 differentially expressed protein spots in the jejunum of the weaning piglet. These proteins were related to stress and immune response, the metabolism of carbohydrates and lipids, and tissue structure. Based on the proteome and ELISA analysis, free amino acids diet significantly down-regulated the jejunal expression of stress protein heat shock 60 kDa protein. Our results indicated that amino acids supplementation to the protein restricted diet could enhance weight gain and feed efficiency in weanling pigs through improving intestinal nutrient absorption and transportation, gut health, and mucosal immunity.

Interferon regulatory factor-1 in flagellin-induced reprogramming: potential protective role of CXCL10 in cornea innate defense against Pseudomonas aeruginosa infection

PURPOSE

We previously showed that pre-exposure of the cornea to Toll-like receptor (TLR)5 ligand flagellin induces strong protective innate defense against microbial pathogens and hypothesized that flagellin modulates gene expression at the transcriptional levels. Thus, we sought to determine the role of one transcription factor, interferon regulatory factor (IRF1), and its target gene CXCL10 therein.

METHODS

Superarray was used to identify transcription factors differentially expressed in Pseudomonas aeruginosa-challenged human corneal epithelial cells (CECs) with or without flagellin pretreatment. The expression of CXCL10, IRF1, LI-8(CXCL2), and IFNγ was determined by PCR, immunohistochemistry, Western/dot blotting, and/or ELISA. IRF1 knockout mice, CXCL10 and IFNγ neutralization, and NK cell depletion were used to define in vivo regulation and function of CXCL10. The severity of P. aeruginosa was assessed using clinical scoring, slit-lamp microscopy, bacterial counting, polymorphonuclear leukocytes (PMN) infiltration, and macrophage inflammatory protein 2/Chemokine (C-X-C motif) ligand 2 (MIP-2/CXCL2) expression.

RESULTS

Flagellin pretreatment drastically affected P. aeruginosa-induced IRF1 expression in human CECs. However, flagellin pretreatment augmented the P. aeruginosa-induced expression of Irf1 and its target gene Cxcl10 in B6 mouse corneas. Irf1 deficiency reduced infection-triggered CXCL10 expression, increased keratitis severity, and attenuated flagellin-elicited protection compared to values in wild-type (WT) controls. CXCL10 neutralization in the cornea of WT mice displayed pathogenesis similar to that of IRF1⁻/⁻ mice. IFNγ receptor neutralization and NK cell depletion prevented flagellin-augmented IRF1 and CXCL10 expression and increased the susceptibility to P. aeruginosa infection in mouse corneas.

CONCLUSIONS

IRF1 plays a role in the corneal innate immune response by regulating CXCL10 expression. IFNγ-producing NK cells augment the epithelial expression of IRF1 and CXCL10 and thus contribute to the innate defense of the cornea against P. aeruginosa infection.

A fresh look at the hygiene hypothesis: how intestinal microbial exposure drives immune effector responses in atopic disease

There currently is no consensus on which immunological mechanisms can best explain the rise in atopic disease post industrialization. The hygiene hypothesis lays groundwork for our understanding of how altered microbial exposures can drive atopy; yet since its introduction increasing evidence suggests the exposure of our immune system to the intestinal microbiota plays a key role in development of atopic disease. As societal change shifts our microbial exposure, concordant shifts in the tolerant and effector functions of our immune systems give rise to more hypersensitive responses to external antigens. This is contrasted with the greater immune tolerant capabilities of individuals still living in regions with lifestyles more representative of our evolutionary history. Recent findings, buoyed by technological advances in the field, suggest a direct role for the intestinal microbiota-immune system interplay in the development of atopic disease mechanisms. Overall, harnessing current mechanistic studies for translational research into microbiota composition and function in relation to atopy have potential for the design of therapeutics that could moderate these diseases.

The role of IL-15 in gastrointestinal diseases: a bridge between innate and adaptive immune response

IL-15 is a member of the IL-2 family of cytokines whose signaling pathways are a bridge between innate and adaptive immune response. IL-15 is part of the intestinal mucosal barrier, and functions to modulate gut homeostasis. IL-15 has pivotal roles in the control of development, proliferation and survival of both innate and adaptive immune cells. IL-15 becomes up-regulated in the inflamed tissue of intestinal inflammatory disease, such as IBD, Celiac Disease and related complications. Indeed, several studies have reported that IL-15 may participate to the pathogenesis of these diseases. Furthermore, although IL-15 seems to be responsible for inflammation and autoimmunity, it also may increase the immune response against cancer. For these reasons, we decided to study the intestinal mucosa as an 'immunological niche', in which immune response, inflammation and local homeostasis are modulated. Understanding the role of the IL-15/IL-15R system will provide a scientific basis for the development of new approaches that use IL-15 for immunotherapy of autoimmune diseases and malignancies. Indeed, a better understanding of the complexity of the mucosal immune system will contribute to the general understanding of immuno-pathology, which could lead to new therapeutical tools for widespread immuno-mediated diseases.

Dendritic cell expression of the signaling molecule TRAF6 is critical for gut microbiota-dependent immune tolerance

The intracellular signaling molecule TRAF6 is critical for Toll-like receptor (TLR)-mediated activation of dendritic cells (DCs). We now report that DC-specific deletion of TRAF6 (TRAF6ΔDC) resulted, unexpectedly, in loss of mucosal tolerance, characterized by spontaneous development of T helper 2 (Th2) cells in the lamina propria and eosinophilic enteritis and fibrosis in the small intestine. Loss of tolerance required the presence of gut commensal microbiota but was independent of DC-expressed MyD88. Further, TRAF6ΔDC mice exhibited decreased regulatory T (Treg) cell numbers in the small intestine and diminished induction of iTreg cells in response to model antigen. Evidence suggested that this defect was associated with diminished DC expression of interleukin-2 (IL-2). Finally, we demonstrate that aberrant Th2 cell-associated responses in TRAF6ΔDC mice could be mitigated via restoration of Treg cell activity. Collectively, our findings reveal a role for TRAF6 in directing DC maintenance of intestinal immune tolerance through balanced induction of Treg versus Th2 cell immunity.

Physiology of the small intestine after resection and transplant

PURPOSE OF REVIEW

Recent studies have evaluated intestinal physiology following bowel resection; understanding changes in small bowel physiology after intestinal transplantation has received less attention. In this review, we will examine recent studies focused on changes in intestinal physiology following resection and intestinal transplantation.

RECENT FINDINGS

Absorption, immunity, and motility are fundamental components of small bowel physiology. Absorption after resection or transplantation is mediated by adaptation and enterocyte function. After resection, adaptation results in increased villus height and crypt depth. Hepatocyte growth factor and epidermal growth factors cause enterocyte hypertrophy and hyperplasia, allowing greater peptide uptake. Little is known about intestinal adaptation after transplant, but enteral autonomy is attainable. Immunity in small bowel after transplantation relies on a balance of innate and adaptive immune responses in the presence of the luminal microbiota. Intraepithelial lymphocytes are decreased following small bowel resection. After small bowel transplant, the number and the ratio of intraepithelial lymphocytes to enterocytes, as well as changes in the microbiota, can be used to identify rejection. After intestinal transplant, immune-mediated dysmotility is common. Perioperative infliximab in addition to tacrolimus may decrease the inflammation that contributes to dysmotility.

SUMMARY

As intestinal transplantation becomes more successful, understanding how absorption, immunity, and motility changes will allow for optimization of bowel function.

Toll-like receptor signaling and regulation of intestinal immunity

The intestine is a complex organ that must maintain tolerance to innocuous food antigens and commensal microbiota while being also able to mount inflammatory responses against invading pathogenic microorganisms. The ability to restrain tolerogenic responses while permitting inflammatory responses requires communication between commensal bacteria, intestinal epithelial cells and immune cells. Disruption or improper signaling between any of these factors may lead to uncontrolled inflammation and the development of inflammatory diseases. Toll-like receptors (TLR) recognize conserved molecular motifs of microorganisms and, not surprisingly, are important for maintaining tolerance to commensal microbiota, as well as inducing inflammation against pathogens. Perturbations in individual TLR signaling can lead to a number of different outcomes and illustrate a system of regulation within the intestine in which each TLR plays a largely non-redundant role in mucosal immunity. This review will discuss recent findings on the roles of individual TLRs and intestinal homeostasis.

Autophagy and intestinal homeostasis

Nutrient absorption is the basic function that drives mammalian intestinal biology. To facilitate nutrient uptake, the host's epithelial barrier is composed of a single layer of cells. This constraint is problematic, as a design of this type can be easily disrupted. The solution during the course of evolution was to add numerous host defense mechanisms that can help prevent local and systemic infection. These mechanisms include specialized epithelial cells that produce a physiochemical barrier overlying the cellular barrier, robust and organized adaptive and innate immune cells, and the ability to mount an inflammatory response that is commensurate with a specific threat level. The autophagy pathway is a critical cellular process that strongly influences all these functions. Therefore, a fundamental understanding of the components of this pathway and their influence on inflammation, immunity, and barrier function will facilitate our understanding of homeostasis in the gastrointestinal tract.

Structural, functional and molecular analysis of the effects of aging in the small intestine and colon of C57BL/6J mice

BACKGROUND

By regulating digestion and absorption of nutrients and providing a barrier against the external environment the intestine provides a crucial contribution to the maintenance of health. To what extent aging-related changes in the intestinal system contribute to the functional decline associated with aging is still under debate.

METHODS

Young (4 M) and old (21 M) male C57BL/6J mice were fed a control low-fat (10E%) or a high-fat diet (45E%) for 2 weeks. During the intervention gross energy intake and energy excretion in the feces were measured. After sacrifice the small and large intestine were isolated and the small intestine was divided in three equal parts. Swiss rolls were prepared of each of the isolated segments for histological analysis and the luminal content was isolated to examine alterations in the microflora with 16S rRNA Q-PCR. Furthermore, mucosal scrapings were isolated from each segment to determine differential gene expression by microarray analysis and global DNA methylation by pyrosequencing.

RESULTS

Digestible energy intake was similar between the two age groups on both the control and the high-fat diet. Microarray analysis on RNA from intestinal scrapings showed no marked changes in expression of genes involved in metabolic processes. Decreased expression of Cubilin was observed in the intestine of 21-month-old mice, which might contribute to aging-induced vitamin B12 deficiency. Furthermore, microarray data analysis revealed enhanced expression of a large number of genes involved in immune response and inflammation in the colon, but not in the small intestine of the 21-month-old mice. Aging-induced global hypomethylation was observed in the colon and the distal part of the small intestine, but not in the first two sections of the small intestine.

CONCLUSION

In 21-month old mice the most pronounced effects of aging were observed in the colon, whereas very few changes were observed in the small intestine.

Transfusion-related acute gut injury: feeding, flora, flow, and barrier defense

TRAGI (transfusion-related acute gut injury) is an acronym we proposed to characterize a severe neonatal gastrointestinal reaction temporally related to a transfusion of packed blood red cells (PRBCs) for anemia in very low birth weights. The following are in support of a causative relationship: (1) the timing of necrotizing enterocolitis after a PRBC transfusion not being random, (2) traditional risk factors for necrotizing enterocolitis are often absent, (3) significant anemia appears to be a universal finding, (4) the age of donor blood is often slightly older than controls, (5) TRAGI is not postnatal age dependent, and (6) TRAGI does not show a centering at 31 weeks' postconceptual age as does nontransfusion-related NEC. Although TRAGI is linked to the timing of PRBC transfusions, we propose a novel hypothesis that the convergence at 31 weeks' postconceptual age for classic NEC approximates the age of presentation of other oxygen delivery and neovascularization syndromes (eg, retinopathy of prematurity), suggesting its etiologic link to a generalized systemic maturational mechanism or another common developmental theme. This report will begin by reviewing the history of the clinical presentation and discovery of TRAGI and will then analyze various pathophysiologic mechanisms that may account for the phenomenon when clinicians render therapies. We will end by a call to action for randomized clinical trials to test various etiologic theories.

Growth and Development Symposium: Endotoxin, inflammation, and intestinal function in livestock

Endotoxin, also referred to as lipopolysaccharide (LPS), can stimulate localized or systemic inflammation via the activation of pattern recognition receptors. Additionally, endotoxin and inflammation can regulate intestinal epithelial function by altering integrity, nutrient transport, and utilization. The gastrointestinal tract is a large reservoir of both gram-positive and gram-negative bacteria, of which the gram-negative bacteria serve as a source of endotoxin. Luminal endotoxin can enter circulation via two routes: 1) nonspecific paracellular transport through epithelial cell tight junctions, and 2) transcellular transport through lipid raft membrane domains involving receptor-mediated endocytosis. Paracellular transport of endotoxin occurs through dissociation of tight junction protein complexes resulting in reduced intestinal barrier integrity, which can be a result of enteric disease, inflammation, or environmental and metabolic stress. Transcellular transport, via specialized membrane regions rich in glycolipids, sphingolipids, cholesterol, and saturated fatty acids, is a result of raft recruitment of endotoxin-related signaling proteins leading to endotoxin signaling and endocytosis. Both transport routes and sensitivity to endotoxin may be altered by diet and environmental and metabolic stresses. Intestinal-derived endotoxin and inflammation result in suppressed appetite, activation of the immune system, and partitioning of energy and nutrients away from growth toward supporting the immune system requirements. In livestock, this leads to the suppression of growth, particularly suppression of lean tissue accretion. In this paper, we summarize the evidence that intestinal transport of endotoxin and the subsequent inflammation leads to decrease in the production performance of agricultural animals and we present an overview of endotoxin detoxification mechanisms in livestock.

Amyloid precursor protein expression modulates intestine immune phenotype

Amyloid precursor protein (APP) is widely expressed across many tissue and cell types. Proteolytic processing of the protein gives rise to a plethora of protein fragments with varied biological activities. Although a large amount of data has been generated describing the metabolism of the protein in neurons, its role in regulating the phenotype of other cells remains unclear. Based upon prior work demonstrating that APP regulates the activation phenotype of monocytic lineage cells, we hypothesized that APP can regulate macrophage activation phenotype in tissues other than brain. Ileums of the small intestines from C57BL6/J wild type and APP(-/-) mice were compared as a representative tissue normally associated with abundant macrophage infiltration. APP(-/-) intestines demonstrated diminished CD68 immunoreactivity compared to wild type mice. This correlated with significantly less cyclooxygenase-2 (cox-2), CD68, CD40, CD11c, and βIII-tubulin protein levels. Peritoneal macrophages from APP(-/-) mice demonstrated decreased in vitro migratory ability compared to wild type cells and diminished basal KC cytokine secretion. Whereas, APP(-/-) intestinal macrophages had an increase in basal KC cytokine secretion compared to wild type cells. Conversely, there was a significant decrease in multiple cytokine levels in APP(-/-) compared to wild type ileums. Finally, APP(-/-) mice demonstrated impaired absorption and increased motility compared to wild type mice. These data demonstrate the APP expression regulates immune cell secretions and phenotype and intestinal function. This data set describes a novel function for this protein or its metabolites that may be relevant not only for Alzheimer's disease but a range of immune-related disorders.

The unfolded protein response and its role in intestinal homeostasis and inflammation

The unfolded protein response (UPR) is a signaling pathway from the endoplasmic reticulum (ER) to the nucleus that protects cells from the stress caused by misfolded or unfolded proteins [1, 2]. As such, ER stress is an ongoing challenge for all cells given the central biologic importance of secretion as part of normal physiologic functions. This is especially the case for cells that are highly dependent upon secretory function as part of their major duties. Within mucosal tissues, the intestinal epithelium is especially dependent upon an intact UPR for its normal activities [3]. This review will discuss the UPR and the special role that it provides in the functioning of the intestinal epithelium and, when dysfunctional, its implications for understanding mucosal homeostasis and intestinal inflammation, as occurs in inflammatory bowel disease (IBD).

Innate Immunity in the Small Intestine of the Preterm Infant

The gastrointestinal tract comprises the largest surface area of the human body. This area is constantly exposed to myriad antigens as well as the large number of bacteria that coexist in the intestinal lumen. To protect against this exposure and help distinguish "self " from "foreign," the intestinal tract has evolved a sophisticated barrier defense system that includes both innate and adaptive immune systems. However, infants who are born preterm do not have the benefit of an adequate immune response and, therefore, are more susceptible to bacterial injury, inflammation, and intestinal diseases such as necrotizing enterocolitis. In this review, we discuss the components of innate immunity that help to protect the small intestine as well as current knowledge about the role of these components in the pathophysiology of necrotizing enterocolitis.