Symbiotic bacteria direct expression of an intestinal bactericidal lectin

Intestinal bacterial translocation in rats with cirrhosis is related to compromised Paneth cell antimicrobial host defense

Liver cirrhosis is associated with bacterial translocation (BT) and endotoxemia. Most translocating bacteria belong to the common intestinal microbiota, suggesting a breakdown of intestinal barrier function. We hypothesized that diminished mucosal antimicrobial host defense could predispose to BT. Two rodent models of portal hypertension with increased BT were used, CCl(4)-induced ascitic cirrhosis and 2-day portal vein-ligated (PVL) animals. BT was assessed by standard microbiological techniques on mesenteric lymph nodes. Total RNA was isolated systematically throughout the intestinal tract, and expression of Paneth cell α-cryptdins and β-defensins was determined by real-time quantitative polymerase chain reaction (qPCR). To determine functional consequences, mucosal antimicrobial activity was assessed with a fluorescence-activated cell sorting assay. BT was detectable in 40% of rats with cirrhosis. Compared with the group without BT, these animals exhibited diminished intestinal Paneth cell α-cryptdin 5 and 7 expression. In contrast, PVL was associated with BT in all animals but did not affect antimicrobial peptides. The decrease in Paneth cell antimicrobials was most pronounced in the ileum and the coecum. Other antimicrobials showed no changes or even an induction in the case of BT at different sites. Antimicrobial activity toward different commensal strains was reduced, especially in the distal ileum and the cecum in experimental cirrhosis with BT (excluding PVL).

CONCLUSION

Compromised Paneth cell antimicrobial host defense seems to predispose to BT in experimental cirrhosis. Understanding this liver-gut axis including the underlying mechanisms could help us to find new treatment avenues.

Homeland security: IgA immunity at the frontiers of the body

IgA is the most abundant immunoglobulin produced in mammals, and is mostly secreted across mucous membranes. At these frontiers, which are constantly assaulted by pathogenic and commensal microbes, IgA provides part of a layered system of immune protection. In this review, we describe how IgA induction occurs through both T-dependent and T-independent mechanisms, and how IgA is generated against the prodigious load of commensal microbes after mucosal dendritic cells (DCs) have sampled a tiny fraction of the microbial consortia in the intestinal lumen. To function in this hostile environment, IgA must be induced behind the 'firewall' of the mesenteric lymph nodes to generate responses that integrate microbial stimuli, rather than the classical prime-boost effects characteristic of systemic immunity.

Innate immunity in the small intestine

PURPOSE OF REVIEW

This article reviews the most recent publications on innate immunity in the small intestine. We will go over the innate immune receptors that act as sensors of microbial presence or cell injury, Paneth cells as the main epithelial cell type that secrete antimicrobial peptides, and mucosal production of immunoglobulin A (IgA). In addition, we will give an update on examples of imbalance of the innate immune response resulting in clinical disease with the most relevant example being Crohn's disease.

RECENT FINDINGS

Toll-like receptors (TLRs) are involved in B-cell homing to the intestine, rejection of small intestinal allografts, and recruitment of mast cells. The TLR adaptor Toll/interleukin-1 receptor domain-containing adapter-inducing interferon-β is necessary to activate innate immunity after Yersinia enterocolitica infection. Moreover, MyD88 is required to keep the intestinal microbiota under control and physically separated from the epithelium, and RegIIIγ is responsible for the bacterial segregation from the lining epithelial cells. In Crohn's disease, ATG16L1 T300A variant promotes a proinflammatory response; and miR-196 downregulates a protective immunity-related GTPase family M protein (IRGM) polymorphism leading to impaired clearance of adherent Escherichia coli in the intestine.

SUMMARY

The intestine is continuously exposed to dietary and microbial antigens. The host has to maintain intestinal homeostasis to keep the commensal and pathogenic bacteria under control. Some of the mechanisms to do so are by expression of innate immune receptors, production of antimicrobial peptides, secretion of IgA, or autophagy of intracellular bacteria. Unfortunately, in some cases the innate immune response fails to protect the host and chronic inflammation, transplant rejection, or other disorders may occur.

Mouse background strain profoundly influences Paneth cell function and intestinal microbial composition

BACKGROUND

Increasing evidence supports the central role of Paneth cells in maintaining intestinal host-microbial homeostasis. However, the direct impact of host genotype on Paneth cell function remains unclear. Here, we characterize key differences in Paneth cell function and intestinal microbial composition in two widely utilized, genetically distinct mouse strains (C57BL/6 and 129/SvEv). In doing so, we demonstrate critical influences of host genotype on Paneth cell activity and the enteric microbiota.

METHODOLOGY AND PRINCIPAL FINDINGS

Paneth cell numbers were determined by flow cytometry. Antimicrobial peptide (AMP) expression was evaluated using quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR), acid urea-polyacrylamide gel electrophoresis, and mass spectrometry. Effects of mouse background on microbial composition were assessed by reciprocal colonization of germ-free mice from both background strains, followed by compositional analysis of resultant gut bacterial communities using terminal restriction fragment length polymorphism analysis and 16 S qPCR. Our results revealed that 129/SvEv mice possessed fewer Paneth cells and a divergent AMP profile relative to C57BL/6 counterparts. Novel 129/SvEv á-defensin peptides were identified, including Defa2/18v, Defa11, Defa16, and Defa18. Host genotype profoundly affected the global profile of the intestinal microbiota, while both source and host factors were found to influence specific bacterial groups. Interestingly, ileal α-defensins from 129/SvEv mice displayed attenuated antimicrobial activity against pro-inflammatory E. coli strains, a bacterial species found to be expanded in these animals.

CONCLUSIONS AND SIGNIFICANCE

This work establishes the important impact of host genotype on Paneth cell function and the composition of the intestinal microbiota. It further identifies specific AMP and microbial alterations in two commonly used inbred mouse strains that have varying susceptibilities to a variety of disorders, ranging from obesity to intestinal inflammation. This will be critical for future studies utilizing these murine backgrounds to study the effects of Paneth cells and the intestinal microbiota on host health and disease.

Interleukin 23 production by intestinal CD103(+)CD11b(+) dendritic cells in response to bacterial flagellin enhances mucosal innate immune defense

Microbial penetration of the intestinal epithelial barrier triggers inflammatory responses that include induction of the bactericidal C-type lectin RegIIIγ. Systemic administration of flagellin, a bacterial protein that stimulates Toll-like receptor 5 (TLR5), induces epithelial expression of RegIIIγ and protects mice from intestinal colonization with antibiotic-resistant bacteria. Flagellin-induced RegIIIγ expression is IL-22 dependent, but how TLR signaling leads to IL-22 expression is incompletely defined. By using conditional depletion of lamina propria dendritic cell (LPDC) subsets, we demonstrated that CD103(+)CD11b(+) LPDCs, but not monocyte-derived CD103(-)CD11b(+) LPDCs, expressed high amounts of IL-23 after bacterial flagellin administration and drove IL-22-dependent RegIIIγ production. Maximal expression of IL-23 subunits IL-23p19 and IL-12p40 occurred within 60 min of exposure to flagellin. IL-23 subsequently induced a burst of IL-22 followed by sustained RegIIIγ expression. Thus, CD103(+)CD11b(+) LPDCs, in addition to promoting long-term tolerance to ingested antigens, also rapidly produce IL-23 in response to detection of flagellin in the lamina propria.

Peroral ciprofloxacin therapy impairs the generation of a protective immune response in a mouse model for Salmonella enterica serovar Typhimurium diarrhea, while parenteral ceftriaxone therapy does not

Nontyphoidal Salmonella (NTS) species cause self-limiting diarrhea and sometimes severe disease. Antibiotic treatment is considered only in severe cases and immune-compromised patients. The beneficial effects of antibiotic therapy and the consequences for adaptive immune responses are not well understood. We used a mouse model for Salmonella diarrhea to assess the effects of per os treatment with ciprofloxacin (15 mg/kg of body weight intragastrically 2 times/day, 5 days) or parenteral ceftriaxone (50 mg/kg intraperitoneally, 5 days), two common drugs used in human patients. The therapeutic and adverse effects were assessed with respect to generation of a protective adaptive immune response, fecal pathogen excretion, and the emergence of nonsymptomatic excreters. In the mouse model, both therapies reduced disease severity and reduced the level of fecal shedding. In line with clinical data, in most animals, a rebound of pathogen gut colonization/fecal shedding was observed 2 to 12 days after the end of the treatment. Yet, levels of pathogen shedding and frequency of appearance of nonsymptomatic excreters did not differ from those for untreated controls. Moreover, mice treated intraperitoneally with ceftriaxone developed an adaptive immunity protecting the mice from enteropathy in wild-type Salmonella enterica serovar Typhimurium challenge infections. In contrast, the mice treated intragastrically with ciprofloxacin were not protected. Thus, antibiotic treatment regimens can disrupt the adaptive immune response, but treatment regimens may be optimized in order to preserve the generation of protective immunity. It might be of interest to determine whether this also pertains to human patients. In this case, the mouse model might be a tool for further mechanistic studies.

Alternative luminal activation mechanisms for paneth cell α-defensins

Paneth cell α-defensins mediate host defense and homeostasis at the intestinal mucosal surface. In mice, matrix metalloproteinase-7 (MMP7) converts inactive pro-α-defensins (proCrps) to bactericidal forms by proteolysis at specific proregion cleavage sites. MMP7(-/-) mice lack mature α-defensins in Paneth cells, accumulating unprocessed precursors for secretion. To test for activation of secreted pro-α-defensins by host and microbial proteinases in the absence of MMP7, we characterized colonic luminal α-defensins. Protein extracts of complete (organ plus luminal contents) ileum, cecum, and colon of MMP7-null and wild-type mice were analyzed by sequential gel permeation chromatography/acid-urea polyacrylamide gel analyses. Mature α-defensins were identified by N-terminal sequencing and mass spectrometry and characterized in bactericidal assays. Abundance of specific bacterial groups was measured by qPCR using group specific 16 S rDNA primers. Intact, native α-defensins, N-terminally truncated α-defensins, and α-defensin variants with novel N termini due to alternative processing were identified in MMP7(-/-) cecum and colon, and proteinases of host and microbial origin catalyzed proCrp4 activation in vitro. Although Paneth cell α-defensin deficiency is associated with ileal microbiota alterations, the cecal and colonic microbiota of MMP7(-/-) and wild-type mice were not significantly different. Thus, despite the absence of MMP7, mature α-defensins are abundant in MMP7(-/-) cecum and colon due to luminal proteolytic activation by alternative host and microbial proteinases. MMP7(-/-) mice only lack processed α-defensins in the small intestine, and the model is not appropriate for studying effects of α-defensin deficiency in cecal or colonic infection or disease.

Role of the commensal microbiota in normal and pathogenic host immune responses

The commensal microbiota that inhabit different parts of the gastrointestinal (GI) tract have been shaped by coevolution with the host species. The symbiotic relationship of the hundreds of microbial species with the host requires a tuned response that prevents host damage, e.g., inflammation, while tolerating the presence of the potentially beneficial microbes. Recent studies have begun to shed light on immunological processes that participate in maintenance of homeostasis with the microbiota and on how disturbance of host immunity or the microbial ecosystem can result in disease-provoking dysbiosis. Our growing appreciation of this delicate host-microbe relationship promises to influence our understanding of inflammatory diseases and infection by microbial pathogens and to provide new therapeutic opportunities.

Epithelial barrier biology: good fences make good neighbours

The external surfaces of the body, such as the skin and the gastrointestinal mucosal membrane, are an important line of defence preventing the invasion of microorganisms and their products. Mucosal immune cells, especially intraepithelial lymphocytes, are involved in maintaining the integrity of these epithelial barriers. They contribute towards the tolerance to commensal organisms, which occupy these same sites, and to the immune responses against harmful organisms and their products. The composition of the microbiota is influenced by immune cells as well as external environmental factors, especially the use of antibiotics and diet. There is an increasing appreciation that the microbiota affects systemic immune responses in addition to local immunity. Failure to control the occupancy by microorganisms may result in the disruption of the delicate homeostasis between beneficial and harmful microorganisms and contribute to inflammatory pathologies. This review will discuss some of our current understanding of the impact of immune cells and diet on the microbiota.

B cell-intrinsic MyD88 signaling prevents the lethal dissemination of commensal bacteria during colonic damage

The Toll-like receptor adaptor protein MyD88 is essential for the regulation of intestinal homeostasis in mammals. In this study, we determined that Myd88-deficient mice are susceptible to colonic damage that is induced by dextran sulfate sodium (DSS) administration resulting from uncontrolled dissemination of intestinal commensal bacteria. The DSS-induced mortality of Myd88-deficient mice was completely prevented by antibiotic treatment to deplete commensal bacteria. By using cell type-specific Myd88-deficient mice, we established that B cell-intrinsic MyD88 signaling plays a central role in the resistance to DSS-induced colonic damage via the production of IgM and complement-mediated control of intestinal bacteria. Our results indicate that the lack of intact MyD88 signaling in B cells, coupled with impaired epithelial integrity, enables commensal bacteria to function as highly pathogenic organisms, causing rapid host death.

Helicobacter pylori CagA triggers expression of the bactericidal lectin REG3γ via gastric STAT3 activation

BACKGROUND

Most of what is known about the Helicobacter pylori (H. pylori) cytotoxin, CagA, pertains to a much-vaunted role as a determinant of gastric inflammation and cancer. Little attention has been devoted to potential roles of CagA in the majority of H. pylori infected individuals not showing oncogenic progression, particularly in relation to host tolerance. Regenerating islet-derived (REG)3γ encodes a secreted C-type lectin that exerts direct bactericidal activity against Gram-positive bacteria in the intestine. Here, we extend this paradigm of lectin-mediated innate immunity, showing that REG3γ expression is triggered by CagA in the H. pylori-infected stomach.

METHODOLOGY/PRINCIPAL FINDINGS

In human gastric mucosal tissues, REG3γ expression was significantly increased in CagA-positive, compared to CagA-negative H. pylori infected individuals. Using transfected CagA-inducible gastric MKN28 cells, we recapitulated REG3γ induction in vitro, also showing that tyrosine phosphorylated, not unphosphorylated CagA triggers REG3γ transcription. In concert with induced REG3γ, pro-inflammatory signalling downstream of the gp130 cytokine co-receptor via the signal transducer and activator of transcription (STAT)3 and transcription of two cognate ligands, interleukin(IL)-11 and IL-6, were significantly increased. Exogenous IL-11, but not IL-6, directly stimulated STAT3 activation and REG3γ transcription. STAT3 siRNA knockdown or IL-11 receptor blockade respectively abrogated or subdued CagA-dependent REG3γ mRNA induction, thus demonstrating a requirement for uncompromised signalling via the IL-11/STAT3 pathway. Inhibition of the gp130-related SHP2-(Ras)-ERK pathway did not affect CagA-dependent REG3γ induction, but strengthened STAT3 activation as well as augmenting transcription of mucosal innate immune regulators, IL-6, IL-8 and interferon-response factor (IRF)1.

CONCLUSIONS/SIGNIFICANCE

Our results support a model of CagA-directed REG3γ expression in gastric epithelial cells via activation of the IL-11/gp130/STAT3 pathway. This response might allow Gram-negative H. pylori to manipulate host immunity to favour its own survival, by reducing the fitness of co-habiting Gram-positive bacteria with which it competes for resources in the gastric mucosal niche.

Structural modification of the gastrointestinal epithelium during immune-dependent granulomatosis

We present results of pathomorphological study of the mucosa epithelium in various parts of the gastrointestinal tract in Crohn's disease. Particular attention was paid to structural transformation of epithelial cells. Barrier dysfunction of the gastrointestinal mucosa was shown to be one of the major pathogenetic factors for Crohn's disease.

TLR5 risk-associated haplotype for canine inflammatory bowel disease confers hyper-responsiveness to flagellin

Single nucleotide polymorphisms (SNP) in the TLR5 gene have been associated with human inflammatory bowel disease (IBD) and animal models of this disease. We recently demonstrated a significant association between three non-synonymous SNPs in the canine TLR5 gene and IBD in German shepherd dogs (GSDs). However, so far, no direct link between these SNPs and a disturbance in TLR5 function was shown. In the present study, we determined the functional significance of the canine TLR5 SNPs by transfecting the identified risk-protective and risk-associated haplotype into human embryonic kidney cells (HEK) and assessed nuclear factor-kappa B (NF-κB) activation and CXCL8 production after stimulation. In addition, a whole blood assay for TLR5 activation was developed using blood derived from carrier dogs of either haplotype. There was a significant increase in NF-kB activity when cells transfected with the risk-associated TLR5 haplotype were stimulated with flagellin compared to the cells expressing the risk-protective TLR5 haplotype. This difference in NFkB activation correlated with CXCL8 expression in the supernatant measured by ELISA. Furthermore, whole blood taken from carrier dogs of the risk-associated TLR5 haplotype produced significantly more TNF after stimulation with flagellin compared to that taken from carriers of the risk-protective haplotype. Thus, we show for the first time a direct functional impact of the canine IBD risk-associated TLR5 haplotype, which results in hyper-responsiveness to flagellin compared to the IBD risk-protective TLR5 haplotype. Our data potentially suggest that similarly to human IBD and experimental models, TLR5 may also play a role in canine IBD. Blocking the hyper-responsive receptor found in susceptible dogs with IBD may alleviate the inappropriate inflammation seen in this disease.

Intestinally secreted C-type lectin Reg3b attenuates salmonellosis but not listeriosis in mice

The Reg3 protein family, including the human member designated pancreatitis-associated protein (PAP), consists of secreted proteins that contain a C-type lectin domain involved in carbohydrate binding. They are expressed by intestinal epithelial cells. Colonization of germ-free mice and intestinal infection with pathogens increase the expression of Reg3g and Reg3b in the murine ileum. Reg3g is directly bactericidal for gram-positive bacteria, but the exact role of Reg3b in bacterial infections is unknown. To investigate the possible protective role of Reg3b in intestinal infection, Reg3b knockout (Reg3b(-/-)) mice and wild-type (WT) mice were orally infected with gram-negative Salmonella enteritidis or gram-positive Listeria monocytogenes. At day 2 after oral Listeria infection and at day 4 after oral Salmonella infection, mice were sacrificed to collect intestinal and other tissues for pathogen quantification. Protein expression of Reg3b and Reg3g was determined in intestinal mucosal scrapings of infected and noninfected mice. In addition, ex vivo binding of ileal mucosal Reg3b to Listeria and Salmonella was investigated. Whereas recovery of Salmonella or Listeria from feces of Reg3b(-/-) mice did not differ from that from feces of WT mice, significantly higher numbers of viable Salmonella, but not Listeria, bacteria were recovered from the colon, mesenteric lymph nodes, spleen, and liver of the Reg3b(-/-) mice than from those of WT mice. Mucosal Reg3b binds to both bacterial pathogens and may interfere with their mode of action. Reg3b plays a protective role against intestinal translocation of the gram-negative bacterium S. enteritidis in mice but not against the gram-positive bacterium L. monocytogenes.

The human gut microbiome: ecology and recent evolutionary changes

The human gastrointestinal tract is divided into sections, allowing digestion and nutrient absorption in the proximal region to be separate from the vast microbial populations in the large intestine, thereby reducing conflict between host and microbes. In the distinct habitats of the gut, environmental filtering and competitive exclusion between microbes are the driving factors shaping microbial diversity, and stochastic factors during colonization history and in situ evolution are likely to introduce intersubject variability. Adaptive strategies of microbes with different niches are genomically encoded: Specialists have smaller genomes than generalists, and microbes with environmental reservoirs have large accessory genomes. A shift toward a Neolithic diet increased loads of simple carbohydrates and selected for their increased breakdown and absorption in the small intestine. Humans who outcompeted microbes for the new substrates obtained more energy from their diets and prospered, an evolutionary process reflected in modern population genetics. The three-way interactions between human genetics, diet, and the microbiota fundamentally shaped modern populations and continue to affect health globally.

The microbiome in infectious disease and inflammation

The mammalian alimentary tract harbors hundreds of species of commensal microorganisms (microbiota) that intimately interact with the host and provide it with genetic, metabolic, and immunological attributes. Recent reports have indicated that the microbiota composition and its collective genomes (microbiome) are major factors in predetermining the type and robustness of mucosal immune responses. In this review, we discuss the recent advances in our understanding of host-microbiota interactions and their effect on the health and disease susceptibility of the host.

Opposing consequences of IL-23 signaling mediated by innate and adaptive cells in chemically induced colitis in mice

The interleukin-23 (IL-23) pathway has emerged as a promising therapeutic target for inflammatory bowel disease. Although the pathogenic role of IL-23 receptor (IL-23R) on T lymphocytes is well established, its function on innate immune cells has not been thoroughly examined. Here we investigate the consequence of IL-23R deletion in dextran sulfate sodium (DSS)-induced colitis. In IL23R(-/-) and IL23p19(-/-) mice, we observed decreased weight loss and reduced leukocyte infiltrate following DSS exposure. Surprisingly, when the IL-23R(-/-) allele was crossed into Rag2(-/-) mice, we observed exacerbated disease, increased epithelial damage, reduced pSTAT3 in the epithelium, and delayed recovery of IL23R(-/-)Rag2(-/-) mice. This phenotype was rescued with exogenous IL22-Fc, and epithelial pSTAT3 was restored. Depletion of Thy1(+) innate lymphoid cells eliminated the majority of IL-22 production in the colon lamina propria of DSS-treated Rag2(-/-) mice, suggesting that these are the major IL-23 responsive innate cells in this context. In summary, we provide evidence for opposing consequences of IL-23R on innate and adaptive lymphoid cells in murine colitis.

Restricting microbial exposure in early life negates the immune benefits associated with gut colonization in environments of high microbial diversity

Background

Acquisition of the intestinal microbiota in early life corresponds with the development of the mucosal immune system. Recent work on caesarean-delivered infants revealed that early microbial composition is influenced by birthing method and environment. Furthermore, we have confirmed that early-life environment strongly influences both the adult gut microbiota and development of the gut immune system. Here, we address the impact of limiting microbial exposure after initial colonization on the development of adult gut immunity.

Methodology/Principal Findings

Piglets were born in indoor or outdoor rearing units, allowing natural colonization in the immediate period after birth, prior to transfer to high-health status isolators. Strikingly, gut closure and morphological development were strongly affected by isolator-rearing, independent of indoor or outdoor origins of piglets. Isolator-reared animals showed extensive vacuolation and disorganization of the gut epithelium, inferring that normal gut closure requires maturation factors present in maternal milk. Although morphological maturation and gut closure were delayed in isolator-reared animals, these hard-wired events occurred later in development. Type I IFN, IL-22, IL-23 and Th17 pathways were increased in indoor-isolator compared to outdoor-isolator animals during early life, indicating greater immune activation in pigs originating from indoor environments reflecting differences in the early microbiota. This difference was less apparent later in development due to enhanced immune activation and convergence of the microbiota in all isolator-reared animals. This correlated with elevation of Type I IFN pathways in both groups, although T cell pathways were still more affected in indoor-reared animals.

Conclusions/Significance

Environmental factors, in particular microbial exposure, influence expression of a large number of immune-related genes. However, the homeostatic effects of microbial colonization in outdoor environments require sustained microbial exposure throughout development. Gut development in high-hygiene environments negatively impacts on normal succession of the gut microbiota and promotes innate immune activation which may impair immune homeostasis.

You AhR what you eat: linking diet and immunity

The aryl hydrocarbon receptor (AhR) is responsible for the toxic effects of environmental pollutants such as dioxin, but little is known about its normal physiological functions. Li et al. (2011) now show that specific dietary compounds present in cruciferous vegetables act through the AhR to promote intestinal immune function, revealing AhR as a critical link between diet and immunity.

Intestinal microbiota: shaping local and systemic immune responses

Recent studies have highlighted the fundamental role of commensal microbes in the maintenance of host homeostasis. For instance, commensals can play a major role in the control of host defense, metabolism and tissue development. Over the past few years, abundant experimental data also support their central role in the induction and control of both innate and adaptive responses. It is now clearly established that commensals are not equal in their capacity to trigger control regulatory or effector responses, however, the molecular basis of these differences has only recently begun to be explored. This review will discuss recent findings evaluating how commensals shape both effector and regulatory responses at steady state and during infections and the consequence of this effect on local and systemic protective and inflammatory responses.

The impact of perinatal immune development on mucosal homeostasis and chronic inflammation

The mucosal surfaces of the gut and airways have important barrier functions and regulate the induction of immunological tolerance. The rapidly increasing incidence of chronic inflammatory disorders of these surfaces, such as inflammatory bowel disease and asthma, indicates that the immune functions of these mucosae are becoming disrupted in humans. Recent data indicate that events in prenatal and neonatal life orchestrate mucosal homeostasis. Several environmental factors promote the perinatal programming of the immune system, including colonization of the gut and airways by commensal microorganisms. These complex microbial-host interactions operate in a delicate temporal and spatial manner and have an important role in the induction of homeostatic mechanisms.

Brain-gut-microbe communication in health and disease

Bidirectional signalling between the gastrointestinal tract and the brain is regulated at neural, hormonal, and immunological levels. This construct is known as the brain–gut axis and is vital for maintaining homeostasis. Bacterial colonization of the intestine plays a major role in the post-natal development and maturation of the immune and endocrine systems. These processes are key factors underpinning central nervous system (CNS) signaling. Recent research advances have seen a tremendous improvement in our understanding of the scale, diversity, and importance of the gut microbiome. This has been reflected in the form of a revised nomenclature to the more inclusive brain–gut–enteric microbiota axis and a sustained research effort to establish how communication along this axis contributes to both normal and pathological conditions. In this review, we will briefly discuss the critical components of this axis and the methodological challenges that have been presented in attempts to define what constitutes a normal microbiota and chart its temporal development. Emphasis is placed on the new research narrative that confirms the critical influence of the microbiota on mood and behavior. Mechanistic insights are provided with examples of both neural and humoral routes through which these effects can be mediated. The evidence supporting a role for the enteric flora in brain–gut axis disorders is explored with the spotlight on the clinical relevance for irritable bowel syndrome, a stress-related functional gastrointestinal disorder. We also critically evaluate the therapeutic opportunities arising from this research and consider in particular whether targeting the microbiome might represent a valid strategy for the management of CNS disorders and ponder the pitfalls inherent in such an approach. Despite the considerable challenges that lie ahead, this is an exciting area of research and one that is destined to remain the center of focus for some time to come.

The mammalian intestinal epithelium as integral player in the establishment and maintenance of host-microbial homeostasis

Only one single layer of epithelial cells separates the densely colonized and environmentally exposed intestinal lumen from the largely sterile subepithelial tissue. Together with the overlaying mucus and the subepithelial mucosal immune system the epithelium has evolved to maintain homeostasis in the presence of the enteric microbiota. It also contributes to rapid and efficient antimicrobial host defence in the event of infection with pathogenic microorganisms. Both, epithelial antimicrobial host defence and homeostasis rely on signalling pathways induced by innate immune receptors demonstrating the active role of epithelial cells in the host-microbial interplay. The interaction of epithelial cells with professional immune cells illustrates the integrated function within the mucosal tissue. In the present review we focus on structural and functional changes of the intestinal epithelium during the fetal-neonatal transition and infancy and try to delineate its role in the induction and maintenance of host-microbial homeostasis. We also address factors that impair epithelial functions and may lead to disruption of the mucosal barrier, tissue damage and the development of symptomatic disease.

The intestinal flora is required to support antibody responses to systemic immunization in infant and germ free mice

The presence of a complex and diverse intestinal flora is functionally important for regulating intestinal mucosal immune responses. However, the extent to which a balanced intestinal flora regulates systemic immune responses is still being defined. In order to specifically examine whether the acquisition of a less complex flora influences responses to immunization in the pre-weaning stages of life, we utilize a model in which infant mice acquire an intestinal flora from their mothers that has been altered by broad-spectrum antibiotics. In this model, pregnant dams are treated with a cocktail of antibiotics that alters both the density and microbial diversity of the intestinal flora. After challenge with a subcutaneous immunization, the antibiotic altered flora infant mice have lower antigen specific antibody titers compared to control age-matched mice. In a second model, we examined germ free (GF) mice to analyze how the complete lack of flora influences the ability to mount normal antibody responses following subcutaneous immunization. GF mice do not respond well to immunization and introduction of a normal flora into GF mice restores the capacity of these mice to respond. These results indicate that a gastrointestinal flora reduced in density and complexity at critical time points during development adversely impacts immune responses to systemic antigens.

Expression and functional importance of innate immune receptors by intestinal epithelial cells

Pattern recognition receptors are somatically encoded and participate in the innate immune responses of a host to microbes. It is increasingly acknowledged that these receptors play a central role both in beneficial and pathogenic interactions with microbes. In particular, these receptors participate actively in shaping the gut environment to establish a fruitful life-long relationship between a host and its microbiota. Commensal bacteria engage Toll-like receptors (TLRs) and nucleotide oligomerization domain (NOD)-like receptors (NLRs) to induce specific responses by intestinal epithelial cells such as production of antimicrobial products or of a functional mucus layer. Furthermore, a complex crosstalk between intestinal epithelial cells and the immune system is initiated leading to a mature gut-associated lymphoid tissue to secrete IgA. Impairment in NLR and TLR functionality in epithelial cells is strongly associated with chronic inflammatory diseases such as Crohn's disease, cancer, and with control of the commensal microbiota creating a more favorable environment for the emergence of new infections.

Activation of REG family proteins in colitis

AIMS

To do a genome-wide gene expression study of active and inactive ulcerative colitis and Crohn's disease (inflammatory bowel disease--IBD) and examine the most differentially expressed genes. As the study showed an extreme upregulation of all regenerating islet-derived genes (REG proteins) in active IBD, we further studied the expression of REGs on protein level in active and inactive IBD, as well as in non-IBD (pseudomembranous) colitis.

METHODS

Microarray analysis was done on a total of 100 pinch biopsy samples from healthy controls and patients with Crohn's disease or ulcerative colitis. Tissue samples from IBD and pseudomembranous colitis were examined with routine histology and immunohistochemical analysis for REGIα, REGIV, DEFA6, and serotonin.

RESULTS

REG mRNAs were up to 83 times overexpressed in diseased mucosa compared with mucosa from healthy individuals. REGIα and REGIV were overexpressed at immunohistochemistry and located to different mucosal cell types. REGIα was expressed in basal half of crypts, REGIV in mid and outer parts of crypts and in surface epithelium and seems to be stored in, and secreted from, goblets. Pseudomembranous colitis samples showed similar staining patterns, and some IBD samples stained REG positive without inflammation on routine histology.

CONCLUSIONS

All REG family mRNAs are upregulated in IBD. REGIα and REGIV have different cellular localization, possibly reflecting different biological functions. REG protein expression also in pseudomembranous colitis shows that REG family proteins are regulated in inflammatory injury and repair, not specifically for IBD as previously thought.

Establishment of intestinal homeostasis during the neonatal period

The intestinal mucosa faces the challenge of regulating the balance between immune tolerance towards commensal bacteria, environmental stimuli and food antigens on the one hand, and induction of efficient immune responses against invading pathogens on the other hand. This regulatory task is of critical importance to prevent inappropriate immune activation that may otherwise lead to chronic inflammation, tissue disruption and organ dysfunction. The most striking example for the efficacy of the adaptive nature of the intestinal mucosa is birth. Whereas the body surfaces are protected from environmental and microbial exposure during fetal life, bacterial colonization and contact with potent immunostimulatory substances start immediately after birth. In the present review, we summarize the current knowledge on the mechanisms underlying the transition of the intestinal mucosa during the neonatal period leading to the establishment of a stable, life-long host-microbial homeostasis. The environmental exposure and microbial colonization during the neonatal period, and also the influence of maternal milk on the immune protection of the mucosa and the role of antimicrobial peptides, are described. We further highlight the molecular mechanisms of innate immune tolerance in neonatal intestinal epithelium. Finally, we link the described immunoregulatory mechanisms to the increased susceptibility to inflammatory and infectious diseases during the neonatal period.

The potter's wheel: the host's role in sculpting its microbiota

Animals, ranging from basal metazoans to primates, are host to complex microbial ecosystems; engaged in a symbiotic relationship that is essential for host physiology and homeostasis. Epithelial surfaces vary in the composition of colonizing microbiota as one compares anatomic sites, developmental stages and species origin. Alterations of microbial composition likely contribute to susceptibility to several distinct diseases. The forces that shape the colonizing microbial composition are the focus of much current investigation, and it is evident that there are pressures exerted both by the host and the external environment to mold these ecosystems. The focus of this review is to discuss recent studies that demonstrate the critical importance of host factors in selecting for its microbiome. Greater insight into host-microbiome interactions will be essential for understanding homeostasis at mucosal surfaces, and developing useful interventions when homeostasis is disrupted.

Altering host resistance to infections through microbial transplantation

Host resistance to bacterial infections is thought to be dictated by host genetic factors. Infections by the natural murine enteric pathogen Citrobacter rodentium (used as a model of human enteropathogenic and enterohaemorrhagic E. coli infections) vary between mice strains, from mild self-resolving colonization in NIH Swiss mice to lethality in C3H/HeJ mice. However, no clear genetic component had been shown to be responsible for the differences observed with C. rodentium infections. Because the intestinal microbiota is important in regulating resistance to infection, and microbial composition is dependent on host genotype, it was tested whether variations in microbial composition between mouse strains contributed to differences in “host” susceptibility by transferring the microbiota of resistant mice to lethally susceptible mice prior to infection. Successful transfer of the microbiota from resistant to susceptible mice resulted in delayed pathogen colonization and mortality. Delayed mortality was associated with increased IL-22 mediated innate defense including antimicrobial peptides Reg3γ and Reg3β, and immunono-neutralization of IL-22 abrogated the beneficial effect of microbiota transfer. Conversely, depletion of the native microbiota in resistant mice by antibiotics and transfer of the susceptible mouse microbiota resulted in reduced innate defenses and greater pathology upon infection. This work demonstrates the importance of the microbiota and how it regulates mucosal immunity, providing an important factor in susceptibility to enteric infection. Transfer of resistance through microbial transplantation (bacteriotherapy) provides additional mechanisms to alter “host” resistance, and a novel means to alter enteric infection and to study host-pathogen interactions.

The germfree murine animal: an important animal model for research on the relationship between gut microbiota and the host

Scientific findings in recent decades have demonstrated that the commensal intestinal microbiota has profound effects on the physiology and diseases of the host. It is estimated that the human microbiota is composed of 10(14) bacterial cells, a number 10 times greater than the total number of human cells. The variety and the complex interactions of the intestinal microbiota are associated with physiological details that remain largely unknown. Germfree hosts, especially murine (rat or mouse) animals that have been maintained free from demonstrable microbial associates such as bacteria, viruses, fungi, and parasites throughout life, have become a powerful tool for exploring the interplay between the host and microorganisms inhabiting the human intestine. This review and survey of recent findings will argue that the germfree mouse model can produce its greatest potential benefits in the study of the metabolism and immunity of the host.

Toll-like receptor-gut microbiota interactions: perturb at your own risk!

The well-being of the intestine and its host requires that this organ execute its complex function amid colonization by a large and diverse microbial community referred to as the gut microbiota. A myriad of interacting mechanisms of mucosal immunity permit the gut to corral the microbiota in such a way as to maximize the benefits and to minimize the danger of living in close proximity to this large microbial biomass. Toll-like receptors and Nod-like receptors, collectively referred to as pattern recognition receptors (PRRs), recognize a variety of microbial components and, hence, play a central role in governing the interface between host and microbiota. This review examines mechanisms by which PRR-microbiota interactions are regulated so as to allow activation of host defense when necessary while preventing excessive inflammation, which can have a myriad of negative consequences for the host. Analysis of published studies performed in human subjects and a variety of murine disease models reveals the central theme that PRRs play a key role in maintaining a healthful stable relationship between the intestine and its microbiota. In contrast, although select genetic ablations of PRR signaling may protect against some chronic diseases, the overriding theme of studies performed to date is that perturbations of PRR-microbiota interactions are more likely to promote disease states associated with inflammation.

A novel C-type lectin identified by EST analysis in tissue migratory larvae of Ascaris suum

C-type lectins (CTLs) are a group of proteins which bind to carbohydrate epitopes in the presence of Ca(2+), which have been described in a wide range of species. In this study, a cDNA sequence coding a putative CTL has been identified from the cDNA library constructed from the pig round worm Ascaris suum lung L3 (LL3) larvae, which was designated as A. suum C-type lectin-1 (As-CTL-1). The 510 nucleotide open reading frame of As-CTL-1 cDNA encoded the predicted 169 amino acid protein including a putative signal peptide of 23 residues and C-type lectin/C-type lectin-like domain (CLECT) at residue 26 to 167. As-CTL-1 was most similar to Toxocara canis C-type lectin-1 and 4 (Tc-CTL-1 and 4), and highly homologous to namatode CTLs and mammalian CTLs as well, such as human C-type lectin domain family 4 member G (CLECG4). In addition, As-CTL-1 was strongly expressed in tissue migrating LL3 and the L4 larvae, which were developmental larvae stages within the mammalian host. These results suggest that A. suum larvae might utilize As-CTL-1 to avoid pathogen recognition mechanisms in mammalian hosts due to it is similarity to host immune cell receptors.

Expansion of Paneth cell population in response to enteric Salmonella enterica serovar Typhimurium infection

Paneth cells residing at the base of the small intestinal crypts contribute to the mucosal intestinal first line defense by secreting granules filled with antimicrobial polypeptides including lysozyme. These cells derive from the columnar intestinal stem cell located at position 0 and the transit amplifying cell located at position +4 in the crypts. We have previously shown that Salmonella enterica serovar Typhimurium (ST), a leading cause of gastrointestinal infections in humans, effects an overall reduction of lysozyme in the small intestine. To extend this work, we examined small-intestinal tissue sections at various time points after ST infection to quantify and localize expression of lysozyme and assess Paneth cell abundance, apoptosis, and the expression of Paneth cell differentiation markers. In response to infection with ST, the intestinal Paneth cell-specific lysozyme content, the number of lysozyme-positive Paneth cells, and the number of granules per Paneth cell decreased. However, this was accompanied by increases in the total number of Paneth cells and the frequency of mitotic events in crypts, by increased staining for the proliferation marker PCNA, primarily at the crypt side walls where the transit amplifying cell resides and not at the crypt base, and by apoptotic events in villi. Furthermore, we found a time-dependent upregulation of first β-catenin, followed by EphB3, and lastly Sox9 in response to ST, which was not observed after infection with a Salmonella pathogenicity island 1 mutant deficient in type III secretion. Our data strongly suggest that, in response to ST infection, a Paneth cell differentiation program is initiated that leads to an expansion of the Paneth cell population and that the transit amplifying cell is likely the main progenitor responder. Infection-induced expansion of the Paneth cell population may represent an acute intestinal inflammatory response similar to neutrophilia in systemic infection.

Intestinal microbiota promote enteric virus replication and systemic pathogenesis

Intestinal bacteria aid host health and limit bacterial pathogen colonization. However, the influence of bacteria on enteric viruses is largely unknown. We depleted the intestinal microbiota of mice with antibiotics before inoculation with poliovirus, an enteric virus. Antibiotic-treated mice were less susceptible to poliovirus disease and supported minimal viral replication in the intestine. Exposure to bacteria or their N-acetylglucosamine-containing surface polysaccharides, including lipopolysaccharide and peptidoglycan, enhanced poliovirus infectivity. We found that poliovirus binds lipopolysaccharide, and exposure of poliovirus to bacteria enhanced host cell association and infection. The pathogenesis of reovirus, an unrelated enteric virus, also was more severe in the presence of intestinal microbes. These results suggest that antibiotic-mediated microbiota depletion diminishes enteric virus infection and that enteric viruses exploit intestinal microbes for replication and transmission.

The antibacterial lectin RegIIIgamma promotes the spatial segregation of microbiota and host in the intestine

The mammalian intestine is home to ~100 trillion bacteria that perform important metabolic functions for their hosts. The proximity of vast numbers of bacteria to host intestinal tissues raises the question of how symbiotic host-bacterial relationships are maintained without eliciting potentially harmful immune responses. Here, we show that RegIIIγ, a secreted antibacterial lectin, is essential for maintaining a ~50-micrometer zone that physically separates the microbiota from the small intestinal epithelial surface. Loss of host-bacterial segregation in RegIIIγ(-/-) mice was coupled to increased bacterial colonization of the intestinal epithelial surface and enhanced activation of intestinal adaptive immune responses by the microbiota. Together, our findings reveal that RegIIIγ is a fundamental immune mechanism that promotes host-bacterial mutualism by regulating the spatial relationships between microbiota and host.

Regenerating islet-derived 3-alpha is a biomarker of gastrointestinal graft-versus-host disease

There are no plasma biomarkers specific for GVHD of the gastrointestinal (GI) tract, the GVHD target organ most associated with nonrelapse mortality (NRM) following hematopoietic cell transplantation (HCT). Using an unbiased, large-scale, quantitative proteomic discovery approach to identify candidate biomarkers that were increased in plasma from HCT patients with GI GVHD, 74 proteins were increased at least 2-fold; 5 were of GI origin. We validated the lead candidate, REG3α, by ELISA in samples from 1014 HCT patients from 3 transplantation centers. Plasma REG3α concentrations were 3-fold higher in patients at GI GVHD onset than in all other patients and correlated most closely with lower GI GVHD. REG3α concentrations at GVHD onset predicted response to therapy at 4 weeks, 1-year NRM, and 1-year survival (P ≤ .001). In a multivariate analysis, advanced clinical stage, severe histologic damage, and high REG3α concentrations at GVHD diagnosis independently predicted 1-year NRM, which progressively increased with higher numbers of onset risk factors present: 25% for patients with 0 risk factors to 86% with 3 risk factors present (P < .001). REG3α is a plasma biomarker of GI GVHD that can be combined with clinical stage and histologic grade to improve risk stratification of patients.

Colonic gene expression patterns of mucin Muc2 knockout mice reveal various phases in colitis development

BACKGROUND

Mucin Muc2 knockout (Muc2(-/-)) mice spontaneously develop colitis.

METHODS

To identify genes and biological responses which play a pivotal role during colitis development in Muc2(-/-) mice, gene expression profiles of colonic tissues from 2- and 4-week-old Muc2(-/-) and wildtype mice were determined using microarrays.

RESULTS

The majority of highly upregulated genes in 2-week-old as well as 4-week-old Muc2(-/-) mice were primarily involved in immune responses related to antigen processing/presentation, B-cell and T-cell receptor signaling, leukocyte transendothelial migration, and Jak-STAT signaling. Specifically, Muc2(-/-) mice expressed high levels of immunoglobulins, murine histocompatibility-2, proinflammatory cytokines, chemokines, and antimicrobial proteins. Additionally, in 4-week-old Muc2(-/-) mice, expression of genes involved in cell structure related pathways was significantly altered. Particularly, the tight junction-associated gene claudin-10 was upregulated, whereas claudin-1 and claudin-5 were downregulated. Furthermore, 4-week-old Muc2(-/-) mice showed increased expression of genes regulating cell growth in conjunction with increased crypt length and increased epithelial proliferation.

CONCLUSIONS

Muc2-deficiency leads to an active inflammatory response in 2- and 4-week-old Muc2(-/-) mice as demonstrated by the altered expression in immune response related genes. In addition, 4-week-old Muc2(-/-) mice also showed a decrease in epithelial barrier function and an increase in epithelial proliferation as indicated by, respectively, the altered expression in tight junction-related genes and upregulation of genes stimulating cell growth. Remarkably, upregulation of genes stimulating cell growth correlated with increased crypt length and increased epithelial proliferation in 4-week-old Muc2(-/-) mice. Together, these data demonstrate that there are distinct phases in colitis development in 2-4-week-old Muc2(-/-) mice.

Mitochondrial antiviral signaling protein (MAVS) monitors commensal bacteria and induces an immune response that prevents experimental colitis

RIG-I-like receptors (RLRs) activate host innate immune responses against virus infection through recruiting the mitochondrial adaptor protein MAVS (also known as IPS1, VISA, or CARDIF). Here we show that MAVS also plays a pivotal role in maintaining intestinal homeostasis. We found that MAVS knockout mice developed more severe mortality and morbidity than WT animals in an experimental model of colitis. Bone marrow transplantation experiments revealed that MAVS in cells of nonhematopoietic origin plays a dominant role in the protection against colitis. Importantly, RNA species derived from intestinal commensal bacteria activate the RIG-I-MAVS pathway to induce the production of multiple cytokines and antimicrobial peptides, including IFN-β and RegIIIγ. These results unveil a previously unexplored role of MAVS in monitoring intestinal commensal bacteria and maintaining tissue homeostasis.

The intestinal microbiota, gastrointestinal environment and colorectal cancer: a putative role for probiotics in prevention of colorectal cancer?

Colorectal cancer (CRC) is the third most commonly diagnosed cancer in the United States, and, even though 5-15% of the total CRC cases can be attributed to individual genetic predisposition, environmental factors could be considered major factors in susceptibility to CRC. Lifestyle factors increasing the risks of CRC include elevated body mass index, obesity, and reduced physical activity. Additionally, a number of dietary elements have been associated with higher or lower incidence of CRC. In this context, it has been suggested that diets high in fruit and low in meat might have a protective effect, reducing the incidence of colorectal adenomas by modulating the composition of the normal nonpathogenic commensal microbiota. In addition, it has been demonstrated that changes in abundance of taxonomic groups have a profound impact on the gastrointestinal physiology, and an increasing number of studies are proposing that the microbiota mediates the generation of dietary factors triggering colon cancer. High-throughput sequencing and molecular taxonomic technologies are rapidly filling the knowledge gaps left by conventional microbiology techniques to obtain a comprehensive catalog of the human intestinal microbiota and their associated metabolic repertoire. The information provided by these studies will be essential to identify agents capable of modulating the massive amount of gut bacteria in safe noninvasive manners to prevent CRC. Probiotics, defined as "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host" (219), are capable of transient modulation of the microbiota, and their beneficial effects include reinforcement of the natural defense mechanisms and protection against gastrointestinal disorders. Probiotics have been successfully used to manage infant diarrhea, food allergies, and inflammatory bowel disease; hence, the purpose of this review was to examine probiotic metabolic activities that may have an effect on the prevention of CRC by scavenging toxic compounds or preventing their generation in situ. Additionally, a brief consideration is given to safety evaluation and production methods in the context of probiotics efficacy.

NF-κB and mucosal homeostasis

NF-κB is well characterized as a primary mediator of inflammatory responses during infection and immune reactions, but it has recently become evident that NF-κB also mediates a potent cytoprotective, homeostatic function under basal conditions. This role is especially evident in the mammalian intestine, which is challenged not only with a range of microbial pathogens, but is also in constant contact with potent proinflammatory commensal bacteria and their products. Present data lead to the overall conclusion that antiapoptotic actions of NF-κB in intestinal epithelial cells dominate tissue responses to many acute inflammatory and injurious challenges, whereas proinflammatory and cell survival functions of NF-κB in macrophages and T cells govern chronic intestinal inflammation. This review focuses on the protective and homeostatic functions of NF-κB, and the importance of NF-κB in determining host-microbe interactions in the intestinal tract.

Myeloid C-type lectin receptors in pathogen recognition and host defense

C-type lectin receptors (CLRs) comprise a heterogeneous group of transmembrane proteins. Many of them are expressed in myeloid cells and signal in response to pathogen-derived or self ligands to initiate or regulate cell activation. Here, we review the properties of myeloid CLRs, highlighting how their signaling function is coordinated with that of other innate receptor families to control immunity to infection.

Metaorganisms as the new frontier

Because it appears that almost all organisms are part of an interdependent metaorganism, an understanding of the underlying host-microbe species associations, and of evolution and molecular underpinnings, has become the new frontier in zoology. The availability of novel high-throughput sequencing methods, together with the conceptual understanding that advances mostly originate at the intersection of traditional disciplinary boundaries, enable biologists to dissect the mechanisms that control the interdependent associations of species. In this review article, we outline some of the issues in inter-species interactions, present two case studies illuminating the necessity of interfacial research when addressing complex and fundamental zoological problems, and show that an interdisciplinary approach that seeks to understand co-evolved multi-species relationships will connect genomes, phenotypes, ecosystems and the evolutionary forces that have shaped them. We hope that this article inspires other collaborations of a similar nature on the diverse landscape commonly referred to as "zoology".

Paneth cell α-defensins in enteric innate immunity

Paneth cells at the base of small intestinal crypts of Lieberkühn secrete high levels of α-defensins in response to cholinergic and microbial stimuli. Paneth cell α-defensins are broad spectrum microbicides that function in the extracellular environment of the intestinal lumen, and they are responsible for the majority of secreted bactericidal peptide activity. Paneth cell α-defensins confer immunity to oral infection by Salmonella enterica serovar Typhimurium, and they are major determinants of the composition of the small intestinal microbiome. In addition to host defense molecules such as α-defensins, lysozyme, and Pla2g2a, Paneth cells also produce and release proinflammatory mediators as components of secretory granules. Disruption of Paneth cell homeostasis, with subsequent induction of endoplasmic reticulum stress, autophagy, or apoptosis, contributes to inflammation in diverse genetic and experimental mouse models.

Candida albicans infection of Caenorhabditis elegans induces antifungal immune defenses

Candida albicans yeast cells are found in the intestine of most humans, yet this opportunist can invade host tissues and cause life-threatening infections in susceptible individuals. To better understand the host factors that underlie susceptibility to candidiasis, we developed a new model to study antifungal innate immunity. We demonstrate that the yeast form of C. albicans establishes an intestinal infection in Caenorhabditis elegans, whereas heat-killed yeast are avirulent. Genome-wide, transcription-profiling analysis of C. elegans infected with C. albicans yeast showed that exposure to C. albicans stimulated a rapid host response involving 313 genes (124 upregulated and 189 downregulated, ∼1.6% of the genome) many of which encode antimicrobial, secreted or detoxification proteins. Interestingly, the host genes affected by C. albicans exposure overlapped only to a small extent with the distinct transcriptional responses to the pathogenic bacteria Pseudomonas aeruginosa or Staphylococcus aureus, indicating that there is a high degree of immune specificity toward different bacterial species and C. albicans. Furthermore, genes induced by P. aeruginosa and S. aureus were strongly over-represented among the genes downregulated during C. albicans infection, suggesting that in response to fungal pathogens, nematodes selectively repress the transcription of antibacterial immune effectors. A similar phenomenon is well known in the plant immune response, but has not been described previously in metazoans. Finally, 56% of the genes induced by live C. albicans were also upregulated by heat-killed yeast. These data suggest that a large part of the transcriptional response to C. albicans is mediated through “pattern recognition,” an ancient immune surveillance mechanism able to detect conserved microbial molecules (so-called pathogen-associated molecular patterns or PAMPs). This study provides new information on the evolution and regulation of the innate immune response to divergent pathogens and demonstrates that nematodes selectively mount specific antifungal defenses at the expense of antibacterial responses.

Despite being a part of the normal flora of healthy individuals, Candida albicans is the most common fungal pathogen of humans and can cause infections that are associated with staggeringly high mortality rates. Here we devise a model for the study of the host immune response to C. albicans infection using the nematode C. elegans. We found that infection with the yeast form of C. albicans induces rapid and robust transcriptional changes in C. elegans. Analyses of these differentially regulated genes indicate that the nematode mounts antifungal defenses that are remarkably distinct from the host responses to pathogenic bacteria and that the nematode recognizes components possessed by heat-killed C. albicans to initiate this response. Interestingly, during infection with a pathogenic fungus, the nematode downregulates antibacterial immune response genes, which may reflect an evolutionary tradeoff between bacterial and fungal defense.

Identification of an innate T helper type 17 response to intestinal bacterial pathogens

Interleukin 17 (IL-17) is a central cytokine implicated in inflammation and antimicrobial defense. After infection, both innate and adaptive IL-17 responses have been reported, but the type of cells involved in innate IL-17 induction, as well as their contribution to in vivo responses, are poorly understood. Here we found that Citrobacter and Salmonella infection triggered early IL-17 production, which was crucial for host defense and was mediated by CD4(+) T helper cells. Enteric innate T helper type 17 (iT(H)17) responses occurred principally in the cecum, were dependent on the Nod-like receptors Nod1 and Nod2, required IL-6 induction and were associated with a decrease in mucosal CD103(+) dendritic cells. Moreover, imprinting by the intestinal microbiota was fully required for the generation of iT(H)17 responses. Together, these results identify the Nod-iT(H)17 axis as a central element in controlling enteric pathogens, which may implicate Nod-driven iT(H)17 responses in the development of inflammatory bowel diseases.

Salmonella-induced mucosal lectin RegIIIβ kills competing gut microbiota

Intestinal inflammation induces alterations of the gut microbiota and promotes overgrowth of the enteric pathogen Salmonella enterica by largely unknown mechanisms. Here, we identified a host factor involved in this process. Specifically, the C-type lectin RegIIIβ is strongly upregulated during mucosal infection and released into the gut lumen. In vitro, RegIIIβ kills diverse commensal gut bacteria but not Salmonella enterica subspecies I serovar Typhimurium (S. Typhimurium). Protection of the pathogen was attributable to its specific cell envelope structure. Co-infection experiments with an avirulent S. Typhimurium mutant and a RegIIIβ-sensitive commensal E. coli strain demonstrated that feeding of RegIIIβ was sufficient for suppressing commensals in the absence of all other changes inflicted by mucosal disease. These data suggest that RegIIIβ production by the host can promote S. Typhimurium infection by eliminating inhibitory gut microbiota.

Gut microbiota and inflammation

Systemic and local inflammation in relation to the resident microbiota of the human gastro-intestinal (GI) tract and administration of probiotics are the main themes of the present review. The dominating taxa of the human GI tract and their potential for aggravating or suppressing inflammation are described. The review focuses on human trials with probiotics and does not include in vitro studies and animal experimental models. The applications of probiotics considered are systemic immune-modulation, the metabolic syndrome, liver injury, inflammatory bowel disease, colorectal cancer and radiation-induced enteritis. When the major genomic differences between different types of probiotics are taken into account, it is to be expected that the human body can respond differently to the different species and strains of probiotics. This fact is often neglected in discussions of the outcome of clinical trials with probiotics.