HOIL1 Regulates Group 3 Innate Lymphoid Cells in the Colon and Protects against Systemic Dissemination, Colonic Ulceration, and Lethality from Citrobacter rodentium Infection

Heme-oxidized IRP2 ubiquitin ligase-1 (HOIL1)-deficient patients experience chronic intestinal inflammation and diarrhea as well as increased susceptibility to bacterial infections. HOIL1 is a component of the linear ubiquitin chain assembly complex that regulates immune signaling pathways, including NF-κB-activating pathways. We have shown previously that HOIL1 is essential for survival following Citrobacter rodentium gastrointestinal infection of mice, but the mechanism of protection by HOIL1 was not examined. C. rodentium is an important murine model for human attaching and effacing pathogens, enteropathogenic and enterohemorrhagic Escherichia coli that cause diarrhea and foodborne illnesses and lead to severe disease in children and immunocompromised individuals. In this study, we found that C. rodentium infection resulted in severe colitis and dissemination of C. rodentium to systemic organs in HOIL1-deficient mice. HOIL1 was important in the innate immune response to limit early replication and dissemination of C. rodentium. Using bone marrow chimeras and cell type-specific knockout mice, we found that HOIL1 functioned in radiation-resistant cells and partly in radiation-sensitive cells and in myeloid cells to limit disease, but it was dispensable in intestinal epithelial cells. HOIL1 deficiency significantly impaired the expansion of group 3 innate lymphoid cells and their production of IL-22 during C. rodentium infection. Understanding the role HOIL1 plays in type 3 inflammation and in limiting the pathogenesis of attaching and effacing lesion-forming bacteria will provide further insight into the innate immune response to gastrointestinal pathogens and inflammatory disorders.

Muc2-dependent microbial colonization of the jejunal mucus layer is diet sensitive and confers local resistance to enteric pathogen infection

Intestinal mucus barriers normally prevent microbial infections but are sensitive to diet-dependent changes in the luminal environment. Here we demonstrate that mice fed a Western-style diet (WSD) suffer regiospecific failure of the mucus barrier in the small intestinal jejunum caused by diet-induced mucus aggregation. Mucus barrier disruption due to either WSD exposure or chromosomal Muc2 deletion results in collapse of the commensal jejunal microbiota, which in turn sensitizes mice to atypical jejunal colonization by the enteric pathogen Citrobacter rodentium. We illustrate the jejunal mucus layer as a microbial habitat, and link the regiospecific mucus dependency of the microbiota to distinctive properties of the jejunal niche. Together, our data demonstrate a symbiotic mucus-microbiota relationship that normally prevents jejunal pathogen colonization, but is highly sensitive to disruption by exposure to a WSD.

Mouse models for bacterial enteropathogen infections: insights into the role of colonization resistance

Globally, enteropathogenic bacteria are a major cause of morbidity and mortality.1-3 Campylobacter, Salmonella, Shiga-toxin-producing Escherichia coli, and Listeria are among the top five most commonly reported zoonotic pathogens in the European Union.4 However, not all individuals naturally exposed to enteropathogens go on to develop disease. This protection is attributable to colonization resistance (CR) conferred by the gut microbiota, as well as an array of physical, chemical, and immunological barriers that limit infection. Despite their importance for human health, a detailed understanding of gastrointestinal barriers to infection is lacking, and further research is required to investigate the mechanisms that underpin inter-individual differences in resistance to gastrointestinal infection. Here, we discuss the current mouse models available to study infections by non-typhoidal Salmonella strains, Citrobacter rodentium (as a model for enteropathogenic and enterohemorrhagic E. coli), Listeria monocytogenes, and Campylobacter jejuni. Clostridioides difficile is included as another important cause of enteric disease in which resistance is dependent upon CR. We outline which parameters of human infection are recapitulated in these mouse models, including the impact of CR, disease pathology, disease progression, and mucosal immune response. This will showcase common virulence strategies, highlight mechanistic differences, and help researchers from microbiology, infectiology, microbiome research, and mucosal immunology to select the optimal mouse model.

Selected commensals educate the intestinal vascular and immune system for immunocompetence

BACKGROUND

The intestinal microbiota fundamentally guides the development of a normal intestinal physiology, the education, and functioning of the mucosal immune system. The Citrobacter rodentium-carrier model in germ-free (GF) mice is suitable to study the influence of selected microbes on an otherwise blunted immune response in the absence of intestinal commensals.

RESULTS

Here, we describe that colonization of adult carrier mice with 14 selected commensal microbes (OMM¹² + MC²) was sufficient to reestablish the host immune response to enteric pathogens; this conversion was facilitated by maturation and activation of the intestinal blood vessel system and the step- and timewise stimulation of innate and adaptive immunity. While the immature colon of C. rodentium-infected GF mice did not allow sufficient extravasation of neutrophils into the gut lumen, colonization with OMM¹² + MC² commensals initiated the expansion and activation of the visceral vascular system enabling granulocyte transmigration into the gut lumen for effective pathogen elimination.

CONCLUSIONS

Consortium modeling revealed that the addition of two facultative anaerobes to the OMM¹² community was essential to further progress the intestinal development. Moreover, this study demonstrates the therapeutic value of a defined consortium to promote intestinal maturation and immunity even in adult organisms. Video Abstract.

A blend of broadly-reactive and pathogen-selected Vγ4 Vδ1 T cell receptors confer broad bacterial reactivity of resident memory γδ T cells

Although murine γδ T cells are largely considered innate immune cells, they have recently been reported to form long-lived memory populations. Much remains unknown about the biology and specificity of memory γδ T cells. Here, we interrogated intestinal memory Vγ4 Vδ1 T cells generated after foodborne Listeria monocytogenes (Lm) infection to uncover an unanticipated complexity in the specificity of these cells. Deep TCR sequencing revealed that a subset of non-canonical Vδ1 clones are selected by Lm infection, consistent with antigen-specific clonal expansion. Ex vivo stimulations and in vivo heterologous challenge infections with diverse pathogenic bacteria revealed that Lm-elicited memory Vγ4 Vδ1 T cells are broadly reactive. The Vγ4 Vδ1 T cell recall response to Lm, Salmonella enterica serovar Typhimurium (STm) and Citrobacter rodentium was largely mediated by the γδTCR as internalizing the γδTCR prevented T cell expansion. Both broadly-reactive canonical and pathogen-selected non-canonical Vδ1 clones contributed to memory responses to Lm and STm. Interestingly, some non-canonical γδ T cell clones selected by Lm infection also responded after STm infection, suggesting some level of cross-reactivity. These findings underscore the promiscuous nature of memory γδ T cells and suggest that pathogen-elicited memory γδ T cells are potential targets for broad-spectrum anti-infective vaccines.

Dietary Interventions Ameliorate Infectious Colitis by Restoring the Microbiome and Promoting Stem Cell Proliferation in Mice

Decreases in short-chain-fatty-acids (SCFAs) are linked to inflammatory bowel disease (IBD). Yet, the mechanisms through which SCFAs promote wound healing, orchestrated by intestinal stem cells, are poorly understood. We discovered that, in mice with Citrobacter rodentium (CR)-induced infectious colitis, treatment with Pectin and Tributyrin diets reduced the severity of colitis by restoring Firmicutes and Bacteroidetes and by increasing mucus production. RNA-seq in young adult mouse colon (YAMC) cells identified higher expression of Lgr4, Lgr6, DCLK1, Muc2, and SIGGIR after Butyrate treatment. Lineage tracing in CR-infected Lgr5-EGFP-IRES-CreERT2/ROSA26-LacZ (Lgr5-R) mice also revealed an expansion of LacZ-labeled Lgr5(+) stem cells in the colons of both Pectin and Tributyrin-treated mice compared to control. Interestingly, gut microbiota was required for Pectin but not Tributyrin-induced Lgr5(+) stem cell expansion. YAMC cells treated with sodium butyrate exhibited increased Lgr5 promoter reporter activity due to direct Butyrate binding with Lgr5 at -4.0 Kcal/mol, leading to thermal stabilization. Upon ChIP-seq, H3K4me3 increased near Lgr5 transcription start site that contained the consensus binding motif for a transcriptional activator of Lgr5 (SPIB). Thus, a multitude of effects on gut microbiome, differential gene expression, and/or expansion of Lgr5(+) stem cells seem to underlie amelioration of colitis following dietary intervention.

Geographic differences in gut microbiota composition impact susceptibility to enteric infection

Large-scale studies of human gut microbiomes have revealed broad differences in composition across geographically distinct populations. Yet, studies examining impacts of microbiome composition on various health outcomes typically focus on single populations, posing the question of whether compositional differences between populations translate into differences in susceptibility. Using germ-free mice humanized with microbiome samples from 30 donors representing three countries, we observe robust differences in susceptibility to Citrobacter rodentium, a model for enteropathogenic Escherichia coli infections, according to geographic origin. We do not see similar responses to Listeria monocytogenes infections. We further find that cohousing the most susceptible and most resistant mice confers protection from C. rodentium infection. This work underscores the importance of increasing global participation in microbiome studies related to health outcomes. Diverse cohorts are needed to identify both population-specific responses to specific microbiome interventions and to achieve broader-reaching biological conclusions that generalize across populations.

Western-style diet impedes colonization and clearance of Citrobacter rodentium

Western-style diet (WSD), which is high in fat and low in fiber, lacks nutrients to support gut microbiota. Consequently, WSD reduces microbiota density and promotes microbiota encroachment, potentially influencing colonization resistance, immune system readiness, and thus host defense against pathogenic bacteria. Here we examined the impact of WSD on infection and colitis in response to Citrobacter rodentium. We observed that, relative to mice consuming standard rodent grain-based chow (GBC), feeding WSD starkly altered the dynamics of Citrobacter infection, reducing initial colonization and inflammation but frequently resulting in persistent infection that associated with low-grade inflammation and insulin resistance. WSD's reduction in initial Citrobacter virulence appeared to reflect that colons of GBC-fed mice contain microbiota metabolites, including short-chain fatty acids, especially acetate, that drive Citrobacter growth and virulence. Citrobacter persistence in WSD-fed mice reflected inability of resident microbiota to out-compete it from the gut lumen, likely reflecting the profound impacts of WSD on microbiota composition. These studies demonstrate potential of altering microbiota and their metabolites by diet to impact the course and consequence of infection following exposure to a gut pathogen.

Overview of the Effect of Citrobacter rodentium Infection on Host Metabolism and the Microbiota

Citrobacter rodentium is a natural enteric mouse pathogen that models human intestinal diseases, such as pathogenic E. coli infections, ulcerative colitis, and colon cancer. Upon reaching the monolayer of intestinal epithelial cells (IECs) lining the gut, a complex web of interactions between the host, the pathogen, and the microbiota ensues. A number of studies revealed surprisingly rapid changes in IEC bioenergetics upon infection, involving a switch from oxidative phosphorylation to aerobic glycolysis, leading to mucosal oxygenation and subsequent changes in microbiota composition. Microbiome studies have revealed a bloom in Enterobacteriaceae during C. rodentium infection in both resistant (i.e., C57BL/6) and susceptible (i.e., C3H/HeN) strains of mice concomitant with a depletion of butyrate-producing Clostridia. The emerging understanding that dysbiosis of cholesterol metabolism is induced by enteric infection further confirms the pivotal role immunometabolism plays in disease outcome. Inversely, the host and microbiota also impact upon the progression of infection, from the susceptibility of the distal colon to C. rodentium colonization to clearance of the pathogen, both via opsonization from the host adaptive immune system and out competition by the resident microbiota. Further complicating this compendium of interactions, C. rodentium exploits microbiota metabolites to fine-tune virulence gene expression and promote colonization. This chapter summarizes the current knowledge of the myriad of pathogen-host-microbiota interactions that occur during the progression of C. rodentium infection in mice and the broader implications of these findings on our understanding of enteric disease.

Host immunity and the colon microbiota of mice infected with Citrobacter rodentium are beneficially modulated by lipid-soluble extract from late-cutting alfalfa in the early stages of infection

Alfalfa is a forage legume commonly associated with ruminant livestock production that may be a potential source of health-promoting phytochemicals. Anecdotal evidence from producers suggests that later cuttings of alfalfa may be more beneficial to non-ruminants; however, published literature varies greatly in measured outcomes, supplement form, and cutting. The objective of this study was to measure body weight, average daily feed intake, host immunity, and the colon microbiota composition in mice fed hay, aqueous, and chloroform extracts of early (1^st) and late (5^th) cutting alfalfa before and after challenge with Citrobacter rodentium. Prior to inoculation, alfalfa supplementation did not have a significant impact on body weight or feed intake, but 5^th cutting alfalfa was shown to improve body weight at 5- and 6-days post-infection compared to 1^st cutting alfalfa (P = 0.02 and 0.01). Combined with the observation that both chloroform extracts improved mouse body weight compared to control diets in later stages of C. rodentium infection led to detailed analyses of the immune system and colon microbiota in mice fed 1^st and 5^th cutting chloroform extracts. Immediately following inoculation, 5^th cutting chloroform extracts significantly reduced the relative abundance of C. rodentium (P = 0.02) and did not display the early lymphocyte recruitment observed in 1^st cutting extract. In later timepoints, both chloroform extracts maintained lower splenic B-cell and macrophage populations while increasing the relative abundance of potentially beneficially genera such as Turicibacter (P = 0.02). At 21dpi, only 5^th cutting chloroform extracts increased the relative abundance of beneficial Akkermansia compared to the control diet (P = 0.02). These results suggest that lipid soluble compounds enriched in late-cutting alfalfa modulate pathogen colonization and early immune responses to Citrobacter rodentium, contributing to protective effects on body weight.

GM-CSF Calibrates Macrophage Defense and Wound Healing Programs during Intestinal Infection and Inflammation

Macrophages play a central role in intestinal immunity, but inappropriate macrophage activation is associated with inflammatory bowel disease (IBD). Here, we identify granulocyte-macrophage colony stimulating factor (GM-CSF) as a critical regulator of intestinal macrophage activation in patients with IBD and mice with dextran sodium sulfate (DSS)-induced colitis. We find that GM-CSF drives the maturation and polarization of inflammatory intestinal macrophages, promoting anti-microbial functions while suppressing wound-healing transcriptional programs. Group 3 innate lymphoid cells (ILC3s) are a major source of GM-CSF in intestinal inflammation, with a strong positive correlation observed between ILC or CSF2 transcripts and M1 macrophage signatures in IBD mucosal biopsies. Furthermore, GM-CSF-dependent macrophage polarization results in a positive feedback loop that augmented ILC3 activation and type 17 immunity. Together, our data reveal an important role for GM-CSF-mediated ILC-macrophage crosstalk in calibrating intestinal macrophage phenotype to enhance anti-bacterial responses, while inhibiting pro-repair functions associated with fibrosis and stricturing, with important clinical implications.

IFN-I and IL-22 mediate protective effects of intestinal viral infection

Products derived from bacterial members of the gut microbiota evoke immune signalling pathways of the host that promote immunity and barrier function in the intestine. How immune reactions to enteric viruses support intestinal homeostasis is unknown. We recently demonstrated that infection by murine norovirus (MNV) reverses intestinal abnormalities following depletion of bacteria, indicating that an intestinal animal virus can provide cues to the host that are typically attributed to the microbiota. Here, we elucidate mechanisms by which MNV evokes protective responses from the host. We identify an important role for the viral protein NS1/2 in establishing local replication and a type I interferon (IFN-I) response in the colon. We further show that IFN-I acts on intestinal epithelial cells to increase the proportion of CCR2-dependent macrophages and interleukin (IL)-22-producing innate lymphoid cells, which in turn promote pSTAT3 signalling in intestinal epithelial cells and protection from intestinal injury. In addition, we demonstrate that MNV provides a striking IL-22-dependent protection against early-life lethal infection by Citrobacter rodentium. These findings demonstrate novel ways in which a viral member of the microbiota fortifies the intestinal barrier during chemical injury and infectious challenges.

Epigenetic initiation of the TH17 differentiation program is promoted by Cxxc finger protein 1

IL-6/STAT3 signaling is known to initiate the T_H17 differentiation program, but the upstream regulatory mechanisms remain minimally explored. Here, we show that Cxxc finger protein 1 (Cxxc1) promoted the generation of T_H17 cells as an epigenetic regulator and prevented their differentiation into T_reg cells. Mice with a T cell-specific deletion of Cxxc1 were protected from experimental autoimmune encephalomyelitis and were more susceptible to Citrobacter rodentium infection. Cxxc1 deficiency decreased IL-6Rα expression and impeded IL-6/STAT3 signaling, whereas the overexpression of IL-6Rα could partially reverse the defects in Cxxc1-deficient T_H17 cells in vitro and in vivo. Genome-wide occupancy analysis revealed that Cxxc1 bound to Il6rα gene loci by maintaining the appropriate H3K4me3 modification of its promoter. Therefore, these data highlight that Cxxc1 as a key regulator governs the balance between T_H17 and T_reg cells by controlling the expression of IL-6Rα, which affects IL-6/STAT3 signaling and has an impact on T_H17-related autoimmune diseases.

PAD4-dependent NETs generation are indispensable for intestinal clearance of Citrobacter rodentium

Peptidyl arginine deiminase-4 (PAD4) is indispensable for generation of neutrophil extracellular traps (NETs), which can provide antimicrobial effects during host innate immune response; however, the role of PAD4 against gastrointestinal infection is largely unknown. Herein, we challenged PAD4-deficient (Pad4-/-) mice and wild-type (WT) littermates with Citrobacter rodentium (CR), and investigated bacteria clearance and gut pathology. Luminal colonization of CR in Pad4-/- mice peaked between 11-14 days post-infection, whereas WT mice suppressed the infection by 14 days. We demonstrated that Pad4-/- mice were unable to form NETs, whereas WT mice showed increased NETs formation in the colon during infection. Pad4-/- mice showed aggravated CR-associated inflammation as indicated by elevated systemic and colonic pro-inflammatory markers. Histological analysis revealed that transmissible colonic hyperplasia, goblet cell depletion, and apoptotic cell death were more pronounced in the colon of CR-infected Pad4-/- mice. Treating WT mice with deoxyribonuclease I, which can disrupt NETs generation, recapitulated the exacerbated CR infection and gut pathology associated with the loss of PAD4. Administration of the PAD4 inhibitor, Cl-amidine also aggravated CR infection, but to a lesser extent. Taken together, our findings highlight the importance of PAD4 in the mucosal clearance of CR and in resolving gut-associated inflammation.

Vitamin D Is Required for ILC3 Derived IL-22 and Protection From Citrobacter rodentium Infection

Citrobacter rodentium is a gastrointestinal infection that requires early IL-22 from group 3 innate lymphoid cells (ILC3) for resistance. The role of vitamin D in the clearance of C. rodentium infection was tested in vitamin D sufficient (D+) and vitamin D deficient (D-) wildtype (WT) and Cyp27B1 (Cyp) KO mice (unable to produce the high affinity vitamin D ligand 1,25(OH)2D, 1,25D). Feeding Cyp KO mice D- diets reduced vitamin D levels and prevented synthesis of 1,25D. D- (WT and Cyp KO) mice had fewer ILC3 cells and less IL-22 than D+ mice. D- Cyp KO mice developed a severe infection that resulted in the lethality of the mice by d14 post-infection. T and B cell deficient D- Rag KO mice also developed a severe and lethal infection with C. rodentium compared to D+ Rag KO mice. D- WT mice survived the infection but took significantly longer to clear the C. rodentium infection than D+ WT or D+ Cyp KO mice. Treating infected D- Cyp KO mice with IL-22 protected the mice from lethality. Treating the D- WT mice with 1,25D reconstituted the ILC3 cells in the colon and protected the mice from C. rodentium. IL-22 treatment of D- WT mice eliminated the need for vitamin D to clear the C. rodentium infection. Vitamin D is required for early IL-22 production from ILC3 cells and protection from enteric infection with C. rodentium.

miR-34a is a microRNA safeguard for Citrobacter-induced inflammatory colon oncogenesis

Inflammation often induces regeneration to repair the tissue damage. However, chronic inflammation can transform temporary hyperplasia into a fertile ground for tumorigenesis. Here, we demonstrate that the microRNA miR-34a acts as a central safeguard to protect the inflammatory stem cell niche and reparative regeneration. Although playing little role in regular homeostasis, miR-34a deficiency leads to colon tumorigenesis after Citrobacter rodentium infection. miR-34a targets both immune and epithelial cells to restrain inflammation-induced stem cell proliferation. miR-34a targets Interleukin six receptor (IL-6R) and Interleukin 23 receptor (IL-23R) to suppress T helper 17 (Th17) cell differentiation and expansion, targets chemokine CCL22 to hinder Th17 cell recruitment to the colon epithelium, and targets an orphan receptor Interleukin 17 receptor D (IL-17RD) to inhibit IL-17-induced stem cell proliferation. Our study highlights the importance of microRNAs in protecting the stem cell niche during inflammation despite their lack of function in regular tissue homeostasis.

Distinct Immunomodulatory Effects of Spermine Oxidase in Colitis Induced by Epithelial Injury or Infection

Polyamines have been implicated in numerous biological processes, including inflammation and carcinogenesis. Homeostatic regulation leads to interconversion of the polyamines putrescine and the downstream metabolites spermidine and spermine. The enzyme spermine oxidase (SMOX), which back-converts spermine to spermidine, contributes to regulation of polyamine levels, but can also have other effects. We have implicated SMOX in gastric inflammation and carcinogenesis due to infection by the pathogen Helicobacter pylori. In addition, we reported that SMOX can be upregulated in humans with inflammatory bowel disease. Herein, we utilized Smox-deficient mice to examine the role of SMOX in two murine colitis models, Citrobacter rodentium infection and dextran sulfate sodium (DSS)-induced epithelial injury. In C. rodentium-infected wild-type (WT) mice, there were marked increases in colon weight/length and histologic injury, with mucosal hyperplasia and inflammatory cell infiltration; these changes were ameliorated in Smox-/- mice. In contrast, with DSS, Smox-/- mice exhibited substantial mortality, and increased body weight loss, colon weight/length, and histologic damage. In C. rodentium-infected WT mice, there were increased colonic levels of the chemokines CCL2, CCL3, CCL4, CXCL1, CXCL2, and CXCL10, and the cytokines IL-6, TNF-α, CSF3, IFN-γ, and IL-17; each were downregulated in Smox-/- mice. In DSS colitis, increased levels of IL-6, CSF3, and IL-17 were further increased in Smox-/- mice. In both models, putrescine and spermidine were increased in WT mice; in Smox-/- mice, the main effect was decreased spermidine and spermidine/spermine ratio. With C. rodentium, polyamine levels correlated with histologic injury, while with DSS, spermidine was inversely correlated with injury. Our studies indicate that SMOX has immunomodulatory effects in experimental colitis via polyamine flux. Thus, SMOX contributes to the immunopathogenesis of C. rodentium infection, but is protective in DSS colitis, indicating the divergent effects of spermidine.

Colitis susceptibility in p47(phox-/-) mice is mediated by the microbiome

BACKGROUND

Chronic granulomatous disease (CGD) is caused by defects in nicotinamide adenine dinucleotide phosphate oxidase 2 (NOX2) complex subunits (gp91(phox) (a.k.a. Nox2), p47(phox), p67(phox), p22(phox), p40(phox)) leading to reduced phagocyte-derived reactive oxygen species production. Almost half of patients with CGD develop inflammatory bowel disease, and the involvement of the intestinal microbiome in relation to this predisposing immunodeficiency has not been explored.

RESULTS

Although CGD mice do not spontaneously develop colitis, we demonstrate that p47(phox-/-) mice have increased susceptibility to dextran sodium sulfate colitis in association with a distinct colonic transcript and microbiome signature. Neither restoring NOX2 reactive oxygen species production nor normalizing the microbiome using cohoused adult p47(phox-/-) with B6Tac (wild type) mice reversed this phenotype. However, breeding p47(phox+/-) mice and standardizing the microflora between littermate p47(phox-/-) and B6Tac mice from birth significantly reduced dextran sodium sulfate colitis susceptibility in p47(phox-/-) mice. We found similarly decreased colitis susceptibility in littermate p47(phox-/-) and B6Tac mice treated with Citrobacter rodentium.

CONCLUSIONS

Our findings suggest that the microbiome signature established at birth may play a bigger role than phagocyte-derived reactive oxygen species in mediating colitis susceptibility in CGD mice. These data further support bacteria-related disease in CGD colitis.

Interleukin-1 (IL-1) signaling in intestinal stromal cells controls KC/ CXCL1 secretion, which correlates with recruitment of IL-22- secreting neutrophils at early stages of Citrobacter rodentium infection

Attaching and effacing pathogens, including enterohemorrhagic Escherichia coli in humans and Citrobacter rodentium in mice, raise serious public health concerns. Here we demonstrate that interleukin-1 receptor (IL-1R) signaling is indispensable for protection against C. rodentium infection in mice. Four days after infection with C. rodentium, there were significantly fewer neutrophils (CD11b+ Ly6C+ Ly6G+) in the colons of IL-1R−/− mice than in wild-type mice. Levels of mRNA and protein of KC/CXCL1 were also significantly reduced in colon homogenates of infected IL-1R−/− mice relative to wild-type mice. Of note, infiltrated CD11b+ Ly6C+ Ly6G+ neutrophils were the main source of IL-22 secretion after C. rodentium infection. Interestingly, intestinal stromal cells isolated from IL-1R−/− mice secreted lower levels of KC/CXCL1 than stromal cells from wild-type mice during C. rodentium infection. Similar effects were found when mouse intestinal stromal cells and human nasal polyp stromal cells were treated with IL-1R antagonists (i.e., anakinra) in vitro. These results suggest that IL-1 signaling plays a pivotal role in activating mucosal stromal cells to secrete KC/CXCL1, which is essential for infiltration of IL-22-secreting neutrophils upon bacterial infection.

Interleukin-22 alleviates metabolic disorders and restores mucosal immunity in diabetes

The connection between an altered gut microbiota and metabolic disorders such as obesity, diabetes, and cardiovascular disease is well established. Defects in preserving the integrity of the mucosal barriers can result in systemic endotoxaemia that contributes to chronic low-grade inflammation, which further promotes the development of metabolic syndrome. Interleukin (IL)-22 exerts essential roles in eliciting antimicrobial immunity and maintaining mucosal barrier integrity within the intestine. Here we investigate the connection between IL-22 and metabolic disorders. We find that the induction of IL-22 from innate lymphoid cells and CD4(+) T cells is impaired in obese mice under various immune challenges, especially in the colon during infection with Citrobacter rodentium. While innate lymphoid cell populations are largely intact in obese mice, the upregulation of IL-23, a cytokine upstream of IL-22, is compromised during the infection. Consequently, these mice are susceptible to C. rodentium infection, and both exogenous IL-22 and IL-23 are able to restore the mucosal host defence. Importantly, we further unveil unexpected functions of IL-22 in regulating metabolism. Mice deficient in IL-22 receptor and fed with high-fat diet are prone to developing metabolic disorders. Strikingly, administration of exogenous IL-22 in genetically obese leptin-receptor-deficient (db/db) mice and mice fed with high-fat diet reverses many of the metabolic symptoms, including hyperglycaemia and insulin resistance. IL-22 shows diverse metabolic benefits, as it improves insulin sensitivity, preserves gut mucosal barrier and endocrine functions, decreases endotoxaemia and chronic inflammation, and regulates lipid metabolism in liver and adipose tissues. In summary, we identify the IL-22 pathway as a novel target for therapeutic intervention in metabolic diseases.

Epithelial IL-22RA1-mediated fucosylation promotes intestinal colonization resistance to an opportunistic pathogen

Our intestinal microbiota harbors a diverse microbial community, often containing opportunistic bacteria with virulence potential. However, mutualistic host-microbial interactions prevent disease by opportunistic pathogens through poorly understood mechanisms. We show that the epithelial interleukin-22 receptor IL-22RA1 protects against lethal Citrobacter rodentium infection and chemical-induced colitis by promoting colonization resistance against an intestinal opportunistic bacterium, Enterococcus faecalis. Susceptibility of Il22ra1(-/-) mice to C. rodentium was associated with preferential expansion and epithelial translocation of pathogenic E. faecalis during severe microbial dysbiosis and was ameloriated with antibiotics active against E. faecalis. RNA sequencing analyses of primary colonic organoids showed that IL-22RA1 signaling promotes intestinal fucosylation via induction of the fucosyltransferase Fut2. Additionally, administration of fucosylated oligosaccharides to C. rodentium-challenged Il22ra1(-/-) mice attenuated infection and promoted E. faecalis colonization resistance by restoring the diversity of anaerobic commensal symbionts. These results support a model whereby IL-22RA1 enhances host-microbiota mutualism to limit detrimental overcolonization by opportunistic pathogens.

CX₃CR1⁺ mononuclear phagocytes support colitis-associated innate lymphoid cell production of IL-22

Interleukin (IL)-22-producing group 3 innate lymphoid cells (ILC3) promote mucosal healing and maintain barrier integrity, but how microbial signals are integrated to regulate mucosal protection offered by these cells remains unclear. Here, we show that in vivo depletion of CX₃CR1⁺ mononuclear phagocytes (MNPs) resulted in more severe colitis and death after infection with Citrobacter rodentium. This phenotype was rescued by exogenous IL-22, which was endogenously produced by ILC3 in close spatial proximity to CX₃CR1⁺ MNPs that were dependent on MyD88 signaling. CX₃CR1⁺MNPs from both mouse and human tissue produced more IL-23 and IL-1β than conventional CD103(+) dendritic cells (cDCs) and were more efficient than cDCs in supporting IL-22 production in ILC3 in vitro and in vivo. Further, colonic ILC3 from patients with mild to moderate ulcerative colitis or Crohn's disease had increased IL-22 production. IBD-associated SNP gene set analysis revealed enrichment for genes selectively expressed in human intestinal MNPs. The product of one of these, TL1A, potently enhanced IL-23- and IL-1β-induced production of IL-22 and GM-CSF by ILC3. Collectively, these results reveal a critical role for CX₃CR1⁺ mononuclear phagocytes in integrating microbial signals to regulate colonic ILC3 function in IBD.

Cytokine IL-6 is required in Citrobacter rodentium infection-induced intestinal Th17 responses and promotes IL-22 expression in inflammatory bowel disease

Citrobacter rodentium (C. rodentium) infection is a widely used murine model to mimic human enteric bacteria infection and inflammatory bowel disease (IBD). In this model, interleukin (IL)‑17A plays critical roles in increasing chemokine and cytokine production in various tissues to recruit innate cells, including monocytes and neutrophils, to the local site of infection. However, the source of IL‑17A remains unclear, as the majority of cell types produce IL‑17A, including intestinal endothelium cells, innate immune cells and CD4+ T cells in disease development. In the current study, wild‑type B6 mice were treated with C. rodentium and the CD4+ Th17 cell subset was observed as being specifically increased in Peyer's patches (PP), but not in mesenteric draining lymph nodes. Furthermore, the research suggested that the differentiation and activation of Th17 cells in PP were dependent on the inflammatory cytokine IL‑6, as blocking IL‑6 signaling with neutralizing antibodies decreased Th17 cells and resulted in the mice being more susceptible to C. rodentium infection. These results confirmed that the Th17 cell subset was specifically activated in PP and demonstrated that IL‑6 is required in Th17 cell activation, which are important to the clinical treatment of IBD.

Dominant effects of the diet on the microbiome and the local and systemic immune response in mice

Outside the nutrition community the effects of diet on immune-mediated diseases and experimental outcomes have not been appreciated. Investigators that study immune-mediated diseases and/or the microbiome have overlooked the potential of diet to impact disease phenotype. We aimed to determine the effects of diet on the bacterial microbiota and immune-mediated diseases. Three different laboratory diets were fed to wild-type mice for 2 weeks and resulted in three distinct susceptibilities to dextran sodium sulfate (DSS)-induced colitis. Examination of the fecal microbiota demonstrated a diet-mediated effect on the bacteria found there. Broad-spectrum antibiotics disturbed the gut microbiome and partially eliminated the diet-mediated changes in DSS susceptibility. Dietary changes 2 days after DSS treatment were protective and suggested that the diet-mediated effect occurred quickly. There were no diet-mediated effects on DSS susceptibility in germ-free mice. In addition, the diet-mediated effects were evident in a gastrointestinal infection model (Citrobacter rodentium) and in experimental autoimmune encephalomyelitis. Taken together, our study demonstrates a dominant effect of diet on immune-mediated diseases that act rapidly by changing the microbiota. These findings highlight the potential of using dietary manipulation to control the microbiome and prevent/treat immune-mediated disease.

Defining the range of pathogens susceptible to Ifitm3 restriction using a knockout mouse model

The interferon-inducible transmembrane (IFITM) family of proteins has been shown to restrict a broad range of viruses in vitro and in vivo by halting progress through the late endosomal pathway. Further, single nucleotide polymorphisms (SNPs) in its sequence have been linked with risk of developing severe influenza virus infections in humans. The number of viruses restricted by this host protein has continued to grow since it was first demonstrated as playing an antiviral role; all of which enter cells via the endosomal pathway. We therefore sought to test the limits of antimicrobial restriction by Ifitm3 using a knockout mouse model. We showed that Ifitm3 does not impact on the restriction or pathogenesis of bacterial (Salmonella typhimurium, Citrobacter rodentium, Mycobacterium tuberculosis) or protozoan (Plasmodium berghei) pathogens, despite in vitro evidence. However, Ifitm3 is capable of restricting respiratory syncytial virus (RSV) in vivo either through directly restricting RSV cell infection, or by exerting a previously uncharacterised function controlling disease pathogenesis. This represents the first demonstration of a virus that enters directly through the plasma membrane, without the need for the endosomal pathway, being restricted by the IFITM family; therefore further defining the role of these antiviral proteins.

A type III effector antagonizes death receptor signalling during bacterial gut infection

Successful infection by enteric bacterial pathogens depends on the ability of the bacteria to colonize the gut, replicate in host tissues and disseminate to other hosts. Pathogens such as Salmonella, Shigella and enteropathogenic and enterohaemorrhagic (EPEC and EHEC, respectively) Escherichia coli use a type III secretion system (T3SS) to deliver virulence effector proteins into host cells during infection that promote colonization and interfere with antimicrobial host responses. Here we report that the T3SS effector NleB1 from EPEC binds to host cell death-domain-containing proteins and thereby inhibits death receptor signalling. Protein interaction studies identified FADD, TRADD and RIPK1 as binding partners of NleB1. NleB1 expressed ectopically or injected by the bacterial T3SS prevented Fas ligand or TNF-induced formation of the canonical death-inducing signalling complex (DISC) and proteolytic activation of caspase-8, an essential step in death-receptor-induced apoptosis. This inhibition depended on the N-acetylglucosamine transferase activity of NleB1, which specifically modified Arg 117 in the death domain of FADD. The importance of the death receptor apoptotic pathway to host defence was demonstrated using mice deficient in the FAS signalling pathway, which showed delayed clearance of the EPEC-like mouse pathogen Citrobacter rodentium and reversion to virulence of an nleB mutant. The activity of NleB suggests that EPEC and other attaching and effacing pathogens antagonize death-receptor-induced apoptosis of infected cells, thereby blocking a major antimicrobial host response.

Genome-wide analysis of the Pho regulon in a pstCA mutant of Citrobacter rodentium

The phosphate-specific transport operon, pstSCAB-phoU, of Gram-negative bacteria is an essential part of the Pho regulon. Its key roles are to encode a high-affinity inorganic phosphate transport system and to prevent activation of PhoB in phosphate-rich environments. In general, mutations in pstSCAB-phoU lead to the constitutive expression of the Pho regulon. Previously, we constructed a pstCA deletion mutant of Citrobacter rodentium and found it to be attenuated for virulence in mice, its natural host. This attenuation was dependent on PhoB or PhoB-regulated gene(s) because a phoB mutation restored virulence for mice to the pstCA mutant. To investigate how downstream genes may contribute to the virulence of C. rodentium, we used microarray analysis to investigate global gene expression of C. rodentium strain ICC169 and its isogenic pstCA mutant when grown in phosphate-rich medium. Overall 323 genes of the pstCA mutant were differentially expressed by at least 1.5-fold compared to the wild-type C. rodentium. Of these 145 were up-regulated and 178 were down-regulated. Differentially expressed genes included some involved in phosphate homoeostasis, cellular metabolism and protein metabolism. A large number of genes involved in stress responses and of unknown function were also differentially expressed, as were some virulence-associated genes. Up-regulated virulence-associated genes in the pstCA mutant included that for DegP, a serine protease, which appeared to be directly regulated by PhoB. Down-regulated genes included those for the production of the urease, flagella, NleG8 (a type III-secreted protein) and the tad focus (which encodes type IVb pili in Yersinia enterocolitica). Infection studies using C57/BL6 mice showed that DegP and NleG8 play a role in bacterial virulence. Overall, our study provides evidence that Pho is a global regulator of gene expression in C. rodentium and indicates the presence of at least two previously unrecognized virulence determinants of C. rodentium, namely, DegP and NleG8.

Medicinal lavender modulates the enteric microbiota to protect against Citrobacter rodentium-induced colitis

Inflammatory bowel disease, inclusive of Crohn's disease and ulcerative colitis, consists of immunologically mediated disorders involving the microbiota in the gastrointestinal tract. Lavender oil is a traditional medicine used to relieve many gastrointestinal disorders. The goal of this study was to examine the therapeutic effects of the essential oil obtained from a novel lavender cultivar, Lavandula×intermedia cultivar Okanagan lavender (OLEO), in a mouse model of acute colitis caused by Citrobacter rodentium. In colitic mice, oral gavage with OLEO resulted in less severe disease, including decreased morbidity and mortality, reduced intestinal tissue damage, and decreased infiltration of neutrophils and macrophages, with reduced levels of TNF-α, IFN-γ, IL-22, macrophage inflammatory protein-2α, and inducible nitric oxide synthase expression. This was associated with increased levels of regulatory T cell populations compared with untreated colitic mice. Recently, we demonstrated that the composition of the enteric microbiota affects susceptibility to C. rodentium-induced colitis. Here, we found that oral administration of OLEO induced microbiota enriched with members of the phylum Firmicutes, including segmented filamentous bacteria, which are known to protect against the damaging effects of C. rodentium. Additionally, during infection, OLEO treatment promoted the maintenance of microbiota loads, with specific increases in Firmicutes bacteria and decreases in γ-Proteobacteria. We observed that Firmicutes bacteria were intimately associated with the apical region of the intestinal epithelial cells during infection, suggesting that their protective effect was through contact with the gut wall. Finally, we show that OLEO inhibited C. rodentium growth and adherence to Caco-2 cells, primarily through the activities of 1,8-cineole and borneol. These results indicate that while OLEO promoted Firmicutes populations, it also controlled pathogen load through antimicrobial activity. Overall, our results reveal that OLEO can protect against colitis through the microbial-immunity nexus and that a pharmacological agent, in this case OLEO, alters the normal enteric microbiota.

The aryl hydrocarbon receptor regulates gut immunity through modulation of innate lymphoid cells

Innate lymphoid cells (ILCs) expressing the nuclear receptor RORγt are essential for gut immunity presumably through production of interleukin-22 (IL-22). The molecular mechanism underlying the development of RORγt(+) ILCs is poorly understood. Here, we have shown that the aryl hydrocarbon receptor (Ahr) plays an essential role in RORγt(+) ILC maintenance and function. Expression of Ahr in the hematopoietic compartment was important for accumulation of adult but not fetal intestinal RORγt(+) ILCs. Without Ahr, RORγt(+) ILCs had increased apoptosis and less production of IL-22. RORγt interacted with Ahr and promoted Ahr binding at the Il22 locus. Upon IL-23 stimulation, Ahr-deficient RORγt(+) ILCs had reduced IL-22 expression, consistent with downregulation of IL-23R in those cells. Ahr-deficient mice succumbed to Citrobacter rodentium infection, whereas ectopic expression of IL-22 protected animals from early mortality. Our data uncover a previously unrecognized physiological role for Ahr in promoting innate gut immunity by regulating RORγt(+) ILCs.

Mucin dynamics in intestinal bacterial infection

BACKGROUND

Bacterial gastroenteritis causes morbidity and mortality in humans worldwide. Murine Citrobacter rodentium infection is a model for gastroenteritis caused by the human pathogens enteropathogenic Escherichia coli and enterohaemorrhagic E. coli. Mucin glycoproteins are the main component of the first barrier that bacteria encounter in the intestinal tract.

METHODOLOGY/PRINCIPAL FINDINGS

Using Immunohistochemistry, we investigated intestinal expression of mucins (Alcian blue/PAS, Muc1, Muc2, Muc4, Muc5AC, Muc13 and Muc3/17) in healthy and C. rodentium infected mice. The majority of the C. rodentium infected mice developed systemic infection and colitis in the mid and distal colon by day 12. C. rodentium bound to the major secreted mucin, Muc2, in vitro, and high numbers of bacteria were found in secreted MUC2 in infected animals in vivo, indicating that mucins may limit bacterial access to the epithelial surface. In the small intestine, caecum and proximal colon, the mucin expression was similar in infected and non-infected animals. In the distal colonic epithelium, all secreted and cell surface mucins decreased with the exception of the Muc1 cell surface mucin which increased after infection (p<0.05). Similarly, during human infection Salmonella St Paul, Campylobacter jejuni and Clostridium difficile induced MUC1 in the colon.

CONCLUSION

Major changes in both the cell-surface and secreted mucins occur in response to intestinal infection.

Combined challenge of mice with Citrobacter rodentium and ionizing radiation promotes bacterial translocation

PURPOSE

Both enteric infection and exposure to ionizing radiation are associated with increased intestinal permeability. However, the combined effect of irradiation and enteric infection has not been described. We combined infection of mice with the enteric pathogen, Citrobacter rodentium, with exposure to ionizing radiation and assessed the impact on colonic epithelial ion transport, permeability and bacterial translocation.

MATERIALS AND METHODS

Mice were infected with C. rodentium and then received whole-body exposure to 5 Gray gamma-radiation 7 days later. Three days post-irradiation, mice were euthanized and colons removed. Control groups included sham-infected mice that were irradiated and mice that were infected, but not irradiated.

RESULTS

Macroscopic damage score and colonic wall thickness were increased by C. rodentium infection, but these parameters were not exacerbated by irradiation. Infection caused an increase in myeloperoxidase activity that was reduced by irradiation. Irradiation reduced the secretory response to electrical field stimulation, forskolin and carbachol; these changes were not altered by infection with C. rodentium. None of the treatments caused an increase in permeability to 51Cr-ethylenediaminetetraacetic acid (EDTA). However, combined infection and irradiation synergistically increased bacterial translocation to mesenteric lymph nodes, liver, spleen and blood.

CONCLUSIONS

Although the combination of irradiation and infection did not exacerbate the individual effects of these challenges on ion secretion and mucosal permeability to 51Cr-EDTA, it dramatically increased susceptibility to bacterial translocation and bacteremia. These results have important implications for patients who develop an enteric infection during the course of abdominopelvic radiotherapy.

Cell attachment properties and infectivity of host-adapted and environmentally adapted Citrobacter rodentium

Citrobacter rodentium belongs to a family of extracellular enteric pathogens that include enterohaemorrhagic and enteropathogenic Escherichia coli, which colonises the gastrointestinal mucosa by the attaching and effacing (A/E) mechanism. We previously described the appearance of a 'hyper-infectious' state after passage of C. rodentium through the murine gastrointestinal tract. Here we report that host-adapted C. rodentium is able to efficiently adhere and trigger actin polymerisation on cultured epithelial cells. Consistent with these observations we recorded higher levels of expression of genes carried on the LEE pathogenicity island and type III secretion system effector genes carried on prophages compared with in vitro-grown bacteria; importantly, the level of ler gene expression was unchanged. These phenotypes were lost after shed C. rodentium was adapted to the external environment. Upon exposure of C57Bl/6 mice, environmentally adapted C. rodentium was no longer infectious at the low doses associated with host-adapted bacteria and the bacteria were found to be localised in the caecal patch in a similar way to C. rodentium cultured in laboratory media. Thus, the 'hyper-infectious' host-adapted state, allowing efficient transmission and colonisation of naive hosts, is transient in nature and gradually lost after shedding into the environment.

Citrobacter rodentium virulence in mice associates with bacterial load and the type III effector NleE

Severe disease caused by Shiga toxin-producing Escherichia coli (STEC) has been associated with a pathogenicity island, O-Island 122, which encodes the type III secretion system-effector NleE. Here we show that full virulence of the related attaching and effacing mouse pathogen Citrobacter rodentium requires NleE. Relative to wild-type bacteria, nleE-mutant C. rodentium are attenuated for colonisation in mice in both single and mixed infections. Examination of the ability of nleE-mutant bacteria to induce pathologic change in vivo revealed that nleE-mutant bacteria induce significantly less pathologic change than wild-type bacteria in susceptible mice. Consistent with these results, mice infected with nleE-mutant bacteria exhibit delayed mortality. These results suggested that pathologic change during attaching and effacing pathogen infection may associate with the degree of pathogen colonisation. Using mutants of 23 type III secretion genes, including the type III effectors nleC, nleD, nleE and nleF, the association of pathologic change with the ability of these mutants to colonise mice was examined. The induction of in vivo disease correlates strongly with the degree of colonisation, suggesting that the colonisation advantage type III secretion genes afford the bacteria, contribute to, and are required for, full virulence.

Citrobacter rodentium infection causes both mitochondrial dysfunction and intestinal epithelial barrier disruption in vivo: role of mitochondrial associated protein (Map)

Enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli are non-invasive attaching/effacing (A/E) bacterial pathogens that infect their host's intestinal epithelium, causing severe diarrhoeal disease. These bacteria utilize a type III secretion apparatus to deliver effector molecules into host cells, subverting cellular function. Mitochondrial associated protein (Map) is a multifunctional effector protein that targets host cell mitochondria and contributes to infection-induced epithelial barrier dysfunction in vitro. Unfortunately, the relevance of these actions to the pathogenesis of EPEC-induced disease is uncertain. Using Citrobacter rodentium, a mouse-adapted A/E bacterium, we found that Map colocalized with host cell mitochondria, and that in vivo infection led to a disruption of mitochondrial morphology in infected colonocytes as assessed by electron microscopy. Histochemical staining for the mitochondrial enzyme succinate dehydrogenase also revealed a significant loss of mitochondrial respiratory function in the infected intestinal epithelium; however, both pathologies were attenuated in mice infected with a Deltamap strain. C. rodentium Map was also implicated in the disruption of epithelial barrier function both in vitro and in vivo. These studies thus advance our understanding of how A/E pathogens subvert host cell functions and cause disease, demonstrating that Map contributes to the functional disruption of the intestinal epithelium during enteric infection by C. rodentium.

Attaching and effacing pathogen-induced tight junction disruption in vivo

Diarrhoea is a hallmark of infections by the human attaching and effacing (A/E) pathogens, enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). Although the mechanisms underlying diarrhoea induced by these pathogens remain unknown, cell culture results have suggested that these pathogens may target tight junctions. Tight junctions in the colon function as physical intercellular barriers that separate and prevent mixing of the luminal contents with adlumenal regions of the epithelium. Consequently, it is thought that the disruption of intestinal epithelial tight junctions by A/E pathogens could result in a loss of barrier function in the alimentary tract; however, this remains unexamined. Here we demonstrate for the first time that A/E pathogen infection results in the morphological alteration of tight junctions during natural disease. Tight junction alteration, characterized by relocalization of the transmembrane tight junction proteins claudin 1, 3 and 5, is a functional disruption; molecular tracers, which do not normally penetrate uninfected epithelia, pass across pathogen-infected epithelia. Functional junction disruption occurs with a concomitant increase in colon luminal water content. The effects on tissue are dependent upon the bacterial type III effector EspF (E. coli secreted protein F), because bacteria lacking EspF, while able to colonize, are defective for junction disruption and result in decreased proportions of water in the colon compared with wild-type infection. These results suggest that the diarrhoea induced by A/E pathogens occurs as part of functional tight junction disruption.

Citrobacter rodentium of mice and man

The major classes of enteric bacteria harbour a conserved core genomic structure, common to both commensal and pathogenic strains, that is most likely optimized to a life style involving colonization of the host intestine and transmission via the environment. In pathogenic bacteria this core genome framework is decorated with novel genetic islands that are often associated with adaptive phenotypes such as virulence. This classical genome organization is well illustrated by a group of extracellular enteric pathogens, which includes enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli (EHEC) and Citrobacter rodentium, all of which use attaching and effacing (A/E) lesion formation as a major mechanism of tissue targeting and infection. Both EHEC and EPEC are poorly pathogenic in mice but infect humans and domestic animals. In contrast, C. rodentium is a natural mouse pathogen that is related to E. coli, hence providing an excellent in vivo model for A/E lesion forming pathogens. C. rodentium also provides a model of infections that are mainly restricted to the lumen of the intestine. The mechanism's by which the immune system deals with such infections has become a topic of great interest in recent years. Here we review the literature of C. rodentium from its emergence in the mid-1960s to the most contemporary reports of colonization, pathogenesis, transmission and immunity.

Emergence of a 'hyperinfectious' bacterial state after passage of Citrobacter rodentium through the host gastrointestinal tract

Citrobacter rodentium belongs to a family of human and animal enteric pathogens that includes the clinically significant enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens exploit attaching and effacing (A/E) lesions to colonize the host gastrointestinal tract. However, both EHEC and EPEC are poorly pathogenic in mice. In contrast, C. rodentium, which is genetically highly related to E. coli, relies on A/E lesion formation as an essential step in both colonization and infection of the murine mucosa, providing an excellent in vivo model. In this study we have used bioluminescence imaging (BLI) to investigate the organ specificity and dynamics of colonization of mice by LB-grown and mouse-passaged C. rodentium in situ and in real time. We have demonstrated the appearance of a 'hyperinfectious' state after passage of C. rodentium through the murine gastrointestinal tract. The 'hyperinfectious' state was found to dramatically reduce the dose required to infect secondary individuals, and also influenced the tissue distribution of colonizing bacteria, removing the requirement for primary colonization of the caecal patch. In addition, the 'hyperinfectious' phenotype was found to be transient with one overnight passage in rich medium sufficient to return C. rodentium to 'culture' infectivity.

The lrp gene and its role in type I fimbriation in Citrobacter rodentium

Citrobacter rodentium is a murine pathogen that is now widely used as an in vivo model for gastrointestinal infections due to its similarities with human enteropathogens, such as the possession of a locus for enterocyte effacement (the LEE island). We studied the lrp gene of C. rodentium and found that it encodes a product highly similar to members of the Lrp (leucine-responsive regulatory protein) family of transcriptional regulators, able to recognize leucine as an effector and to repress the expression of its own structural gene. In enterobacteria, Lrp is a global regulator of gene expression, as it controls a large variety of genes, including those coding for cell appendages and other potential virulence factors. Based on the well-established role of Lrp on the expression of pilus genes in Escherichia coli, we also studied the role of Lrp in controlling the formation of the type I pilus in C. rodentium. Type I pili, produced by the fim system, are virulence factors of uropathogens, involved in mediating bacterial adhesion to bladder epithelial cells. Yeast agglutination assays showed that Lrp is needed for type I pilus formation and real-time PCR experiments indicated that Lrp has a strong leucine-mediated effect on the expression of the fimAICDFGH operon. Mutant studies indicated that this positive action is exerted mainly through a positive control of Lrp on the phase variation mechanism that regulates fimAICDFGH expression. A quantitative analysis of its expression suggested that this operon may also be negatively regulated at the level of transcription.

A major role for proteolytic activity and proteinase-activated receptor-2 in the pathogenesis of infectious colitis

Citrobacter rodentium is a bacterial pathogen that causes a murine infectious colitis equivalent to enterohemorrhagic Escherichia coli infection in humans. Colonic luminal fluid from C. rodentium-infected mice, but not from sham-infected mice, contains active serine proteinases that can activate proteinase-activated receptor-2 (PAR2). We have identified granzyme A and murine trypsins to be present in C. rodentium-infected luminal fluid, as determined by mass spectrometry and Western blot analysis. Inflammatory indices (colonic mucosa macroscopic damage score, increased intestinal wall thickness, granulocyte infiltration, and bacterial translocation from the colonic lumen to peritoneal organs) were all increased in C. rodentium-infected mice, compared with sham-infected mice. Soybean trypsin inhibitor-treated wild-type mice and untreated PAR2-deficient (PAR2-/-) mice (compared with their wild-type littermates) both had substantially reduced levels of C. rodentium-induced inflammation. These data point to an important role for both pathogen-induced host serine proteinases and PAR2 in the setting of infectious colitis.

Modulation of host cytoskeleton function by the enteropathogenic Escherichia coli and Citrobacter rodentium effector protein EspG

EspG is a conserved protein encoded by the locus of enterocyte effacement (LEE) of attaching and effacing (A/E) pathogens, including enteropathogenic and enterohemorrhagic Escherichia coli and Citrobacter rodentium. EspG is delivered into infected host cells by a type III secretion system. The role of EspG in virulence has not yet been defined. Here we describe experiments that probe the virulence characteristics and biological activities of EspG in vitro and in vivo. A C. rodentium espG mutant displayed a significantly reduced ability to colonize C57BL/6 mice and to cause colonic hyperplasia. Epitope-tagged EspG was detected in the apical regions of infected colonic epithelial cells in infected mice, partially localizing with another LEE-encoded effector protein, Tir. EspG was found to interact with mammalian tubulin in both genetic screens and gel overlay assays. Binding to tubulin by EspG caused localized microtubule depolymerization, resulting in actin stress fiber formation through an undefined mechanism. Heterologous expression of EspG in yeast resulted in loss of cytoplasmic microtubule structure and function, preventing coordination between bud development and nuclear division. Yeast expressing EspG were also unable to control cortical actin polarity. We suggest that EspG contributes to the ability of A/E pathogens to establish infection through a modulation of the host cytoskeleton involving transient microtubule destruction and actin polymerization in a manner akin to the Shigella flexneri VirA protein.