Mammalian gut immunity

The mammalian intestinal tract is the largest immune organ in the body and comprises cells from non-hemopoietic (epithelia, Paneth cells, goblet cells) and hemopoietic (macrophages, dendritic cells, T-cells) origin, and is also a dwelling for trillions of microbes collectively known as the microbiota. The homeostasis of this large microbial biomass is prerequisite to maintain host health by maximizing beneficial symbiotic relationships and minimizing the risks of living in such close proximity. Both microbiota and host immune system communicate with each other to mutually maintain homeostasis in what could be called a "love-hate relationship." Further, the host innate and adaptive immune arms of the immune system cooperate and compensate each other to maintain the equilibrium of a highly complex gut ecosystem in a stable and stringent fashion. Any imbalance due to innate or adaptive immune deficiency or aberrant immune response may lead to dysbiosis and low-grade to robust gut inflammation, finally resulting in metabolic diseases.

Proneness of TLR5 deficient mice to develop colitis is microbiota dependent

Alterations in the gut microbiota have been implicated to play a role in potentiating inflammatory bowel diseases in both humans and mice. Mice lacking the flagellin receptor, toll-like receptor 5 (TLR5), are prone to develop spontaneous gut inflammation, but are significantly protected when treated with antibiotics or maintained in germ-free conditions. However, given that the incidence of spontaneous inflammation in TLR5KO mice is quite variable in conventional conditions (typically ∼10% show clear colitis), this result is far from definitive and does not rule out that TLR5KO mice might be prone to develop inflammation even in the absence of a microbiota. Herein, we demonstrate that neutralization of IL10 signaling induces colitis in 100% of TLR5KO mice which provide a more rigorous approach to evaluate the role of microbiota in gut inflammation. Mice treated with antibiotics or maintained in germ-free condition are substantially protected against IL-10R neutralization-induced colitis, underscoring that gut inflammation in TLR5KO mice is dependent upon the presence of a gut microbiota.

Interplay between enterobactin, myeloperoxidase and lipocalin 2 regulates E. coli survival in the inflamed gut

During an inflammatory response in the gut, some commensal bacteria such as E. coli can thrive and contribute to disease. Here we demonstrate that enterobactin (Ent), a catecholate siderophore released by E. coli, is a potent inhibitor of myeloperoxidase (MPO), a bactericidal enzyme of the host. Glycosylated Ent (salmochelin) and non-catecholate siderophores (yersiniabactin and ferrichrome) fail to inhibit MPO activity. An E. coli mutant (ΔfepA) that overproduces Ent, but not an Ent-deficient double mutant (ΔaroB/ΔfepA), inhibits MPO activity and exhibits enhanced survival in inflamed guts. This survival advantage is counter-regulated by lipocalin 2, a siderophore-binding host protein, which rescues MPO from Ent-mediated inhibition. Spectral analysis reveals that Ent interferes with compound I [oxoiron, Fe(IV)=O] and reverts the enzyme back to its native ferric [Fe(III)] state. These findings define a fundamental mechanism by which E. coli surpasses the host innate immune responses during inflammatory gut diseases and gains a distinct survival advantage.

Survival advantage of E. coli in the inflamed gut is mechanistically linked to enterobactin-mediated inhibition of myeloperoxidase and regulation by lipocalin 2 (MPF4P.736)

During an inflammatory response in the gut, several commensal bacteria such as E.coli not only survive but also thrive and consequently contribute to the disease pathogenesis. The mechanisms by which such opportunistic pathogens bloom despite the hostile pro-inflammatory milieu of the inflamed gut remain largely unknown. We demonstrated that enterobactin (Ent), a siderophore released by E. coli, is a potent inhibitor of a key bactericidal enzyme of the host innate immune system, myeloperoxidase (MPO). The inhibitory activity is specific as glycosylated Ent (salmochelin) and non-catecholate siderophores such as yersiniabactin and ferrichrome failed to inhibit MPO activity. An E. coli ferrienterobactin permease mutant (ΔfepA) overexpressing Ent, but not 3-dehydroquinate synthase (ΔaroB)/ΔfepA double mutant inhibited MPO activity and exhibited enhanced survival in inflamed guts. This survival advantage was counter-regulated by the host siderophore binding protein, lipocalin 2, which rescued MPO from Ent-mediated inhibition. Spectral analysis revealed that Ent interferes with compound I [oxoiron, Fe(IV)=O] and reverts the enzyme back to its native ferric [Fe(III)] state. These findings define a fundamental mechanism by which E. coli surpasses the host innate immune responses during inflammatory gut diseases and gains a distinct survival advantage.

Effector CD4+CD25- T cells from Stearoyl CoA Desaturase deficient mice drive chronic colitis in RAG-1KO mice (MUC5P.761)

Fatty acids in lipid bilayer substantially influence membrane fluidity and thus dictate cellular functions. Stearoyl CoA Desaturase (SCD-1) is a lipogenic enzyme that assists in the de novo biosynthesis of oleate (C18:1, n9), a major fatty acid in the phospholipids of lipid bilayer. Accordingly SCD-1KO mice display substantially reduced oleate in cell membranes. We investigated the colitogenic capacity of effector T-cells from SCD-1KO mice and their WT littermates in adoptive T-cell transfer colitis model. Splenic effector T-cells (CD4+CD25-) from age and gender matched WT and SCD-1KO mice were isolated by cell sorting using magnetic beads and flow cytometry. RAG-1KO (n=5) were administered 5.0x105 CD4+CD25- cells/mouse intraperitoneally and monitored for body weights and colitis development. RAG-1KO mice which received CD4+CD25- cells from SCD-1KO mice displayed colomegaly, splenomegaly, elevated colonic, systemic lipocalin-2, a general inflammatory marker and hallmarks of colitis at histologic level. Interestingly, colonic TNFα and IL-10 gene transcripts were significantly reduced in SCD-1KO CD4+CD25- recipients whereas the opposite is true for iNOS and lipocalin-2. Further, systemic sIL-1Ra was significantly reduced in SCD-1KO CD4+CD25- recipients when compared to WT T cell recipients. Our results demonstrate that effector T-cells from oleate poor environment attain colitogenic properties and may be associated with increased immunoreactivity to gut microbiotal antigens.

Inflammation and gut microbiota-induced hepatic lipogenesis drives metabolic syndrome in TLR5 deficient mice (HUM1P.310)

The gut microbiota plays a key role in host metabolism. Toll-Like Receptor 5 (TLR5), a flagellin receptor, is required for gut microbiota homeostasis. Accordingly, TLR5 deficient (T5KO) mice are prone to develop microbiota-dependent spontaneous colitis and metabolic syndrome. Herein, we investigated whether metabolic syndrome in T5KO mice correlates with hepatic dyslipidemia and inflammation. T5KO mice displayed elevated neutral lipids with substantial enrichment of C18:1 (n9) relative to wild-type littermates. Oleate enrichment of hepatic lipids was microbiota-dependent. Analyzing cecal contents via 1H NMR-based metabolomics revealed that T5KO mice exhibited elevated short chain fatty acids (SCFA) with a concomitant increase in colonic SCFA receptors and hepatic pro-inflammatory genes and lipogenic enzymes including stearoyl-CoA desaturase1 (SCD1). SCFA treatment further aggravated metabolic syndrome in T5KO mice. Interestingly, deletion of hepatic SCD1 not only prevented hepatic neutral lipid oleate enrichment but also ameliorated hepatic inflammation and metabolic syndrome in T5KO mice. Collectively, these results underscore the key role of the gut microbiota-liver axis in the pathogenesis of metabolic diseases.

Inhibition of Interleukin 10 receptor signaling induces microbiota-dependent chronic colitis in Apolipoprotein E deficient mice (MUC5P.754)

Apolipoprotein E (ApoE), a secreted protein, plays major role in cholesterol metabolism. ApoE is also known to exhibit anti-inflammatory properties. However, its role in the intestine, specifically during inflammation is not very clear. First, we verified upregulation of colonic ApoE transcripts in various models of murine acute and chronic colitis models. Age and gender matched ApoEKO mice and their WT littermates (n=5) were given either one or 4 weekly injections of IL-10 receptor (IL-10R) neutralizing mAb intraperitoneally and colitis development was monitored. ApoEKO mice given single injection of IL-10R mAb displayed acute colitis. Four weekly injections IL-10R mAb resulted in the development of chronic colitis as measured by colomegaly, splenomegaly and elevated colonic pro-inflammatory gene expression (TNFα, IFNγ) and histological score when compared to WT littermates. IL-10R neutralization also induced substantial levels of lipocalin 2, a general inflammatory marker in the feces, colon, and circulation in ApoEKO mice. Ablation of gut microbiota by broad-spectrum antibiotics substantially protected ApoEKO mice against IL-10R neutralization induced chronic colitis. In addition, cohousing ApoEKO mice with IL-10KO mice exaggerated spontaneous colitis (rectal prolapse) in IL-10KO mice when compared to WT suggesting ApoEKO harbor colitogenic microbiota. Collectively, our results demonstrate that ApoE plays a critical role in protecting the gut against inflammation.

Evidence for a link between gut microbiota and hypertension in the Dahl rat

The gut microbiota plays a critical role in maintaining physiological homeostasis. This study was designed to evaluate whether gut microbial composition affects hypertension. 16S rRNA genes obtained from cecal samples of Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats were sequenced. Bacteria of the phylum Bacteroidetes were higher in the S rats compared with the R rats. Furthermore, the family S24-7 of the phylum Bacteroidetes and the family Veillonellaceae of the phylum Firmicutes were higher in the S rats compared with the R rats. Analyses of the various phylogenetic groups of cecal microbiota revealed significant differences between S and R rats. Both strains were maintained on a high-salt diet, administered antibiotics for ablation of microbiota, transplanted with S or R rat cecal contents, and monitored for blood pressure (BP). Systolic BP of the R rats remained unaltered irrespective of S or R rat cecal transplantation. Surprisingly, compared with the S rats given S rat cecal content, systolic BP of the S rats given a single bolus of cecal content from R rats was consistently and significantly elevated during the rest of their life, and they had a shorter lifespan. A lower level of fecal bacteria of the family Veillonellaceae and increased plasma acetate and heptanoate were features associated with the increased BP observed in the S rats given R rat microbiota compared with the S rats given S rat microbiota. These data demonstrate a link between microbial content and BP regulation and, because the S and R rats differ in their genomic composition, provide the necessary basis to further examine the relationship between the host genome and microbiome in the context of BP regulation in the Dahl rats.

Viral infection. Prevention and cure of rotavirus infection via TLR5/NLRC4-mediated production of IL-22 and IL-18

Activators of innate immunity may have the potential to combat a broad range of infectious agents. We report that treatment with bacterial flagellin prevented rotavirus (RV) infection in mice and cured chronically RV-infected mice. Protection was independent of adaptive immunity and interferon (IFN, type I and II) and required flagellin receptors Toll-like receptor 5 (TLR5) and NOD-like receptor C4 (NLRC4). Flagellin-induced activation of TLR5 on dendritic cells elicited production of the cytokine interleukin-22 (IL-22), which induced a protective gene expression program in intestinal epithelial cells. Flagellin also induced NLRC4-dependent production of IL-18 and immediate elimination of RV-infected cells. Administration of IL-22 and IL-18 to mice fully recapitulated the capacity of flagellin to prevent or eliminate RV infection and thus holds promise as a broad-spectrum antiviral agent.

Innate and adaptive immunity interact to quench microbiome flagellar motility in the gut

Gut mucosal barrier breakdown and inflammation have been associated with high levels of flagellin, the principal bacterial flagellar protein. Although several gut commensals can produce flagella, flagellin levels are low in the healthy gut, suggesting the existence of control mechanisms. We find that mice lacking the flagellin receptor Toll-like receptor 5 (TLR5) exhibit a profound loss of flagellin-specific immunoglobulins (Igs) despite higher total Ig levels in the gut. Ribotyping of IgA-coated cecal microbiota showed Proteobacteria evading antibody coating in the TLR5(-/-) gut. A diversity of microbiome members overexpressed flagellar genes in the TLR5(-/-) host. Proteobacteria and Firmicutes penetrated small intestinal villi, and flagellated bacteria breached the colonic mucosal barrier. In vitro, flagellin-specific Ig inhibited bacterial motility and downregulated flagellar gene expression. Thus, innate-immunity-directed development of flagellin-specific adaptive immune responses can modulate the microbiome's production of flagella in a three-way interaction that helps to maintain mucosal barrier integrity and homeostasis.

Dextran sulfate sodium (DSS)-induced colitis in mice

Inflammatory bowel diseases (IBD), mainly comprising ulcerative colitis and Crohn's Disease, are complex and multifactorial diseases with unknown etiology. For the past 20 years, to study human IBD mechanistically, a number of murine models of colitis have been developed. These models are indispensable tools to decipher underlying mechanisms of IBD pathogenesis as well as to evaluate a number of potential therapeutics. Among various chemically induced colitis models, the dextran sulfate sodium (DSS)-induced colitis model is widely used because of its simplicity and many similarities with human ulcerative colitis. This model has both advantages and disadvantages that must be considered when employed. This protocol describes the DSS-induced colitis model, focusing on details and factors that could affect DSS-induced pathology.

Harnessing the beneficial properties of adipogenic microbes for improving human health

Obesity is associated with numerous metabolic comorbidities. Weight loss is an effective measure for alleviating many of these metabolic abnormalities. However, considering the limited success of most medical weight-management approaches in producing a sustained weight loss, approaches that improve obesity-related metabolic abnormalities independent of weight loss would be extremely attractive and of practical benefit. Metabolically healthy obesity supports the notion that a better metabolic profile is possible despite obesity. Moreover, adequate expansion of adipose tissue appears to confer protection from obesity-induced metabolic comorbidities. To this end, the 10th Stock conference examined new approaches to improve metabolic comorbidities independent of weight loss. In particular, human adenovirus 36 (Ad36) and specific gut microbes were examined for their potential to influence lipid and glucose homeostasis in animals and humans. While these microbes possess some undesirable properties, research has identified attributes of adenovirus Ad36 and gut microbes that may be selectively harnessed to improve metabolic profile without the obligatory weight loss. Furthermore, identifying the host signalling pathways that these microbes recruit to improve the metabolic profile may offer new templates and targets, which may facilitate the development of novel treatment strategies for obesity-related metabolic conditions.

Differential role of lipocalin 2 during immune complex-mediated acute and chronic inflammation in mice

OBJECTIVE

Lipocalin 2 (LCN-2) is an innate immune protein that is expressed by a variety of cells and is highly up-regulated during several pathologic conditions, including immune complex (IC)-mediated inflammatory/autoimmune disorders. However, the function of LCN-2 during IC-mediated inflammation is largely unknown. Therefore, this study was undertaken to investigate the role of LCN-2 in IC-mediated diseases.

METHODS

The up-regulation of LCN-2 was determined by enzyme-linked immunosorbent assay in 3 different mouse models of IC-mediated autoimmune disease: systemic lupus erythematosus, collagen-induced arthritis, and serum-transfer arthritis. The in vivo role of LCN-2 during IC-mediated inflammation was investigated using LCN-2-knockout mice and their wild-type littermates.

RESULTS

LCN-2 levels were significantly elevated in all 3 of the autoimmune disease models. Further, in an acute skin inflammation model, LCN-2-knockout mice exhibited a 50% reduction in inflammation, with histopathologic analysis revealing notably reduced immune cell infiltration as compared to wild-type mice. Administration of recombinant LCN-2 to LCN-2-knockout mice restored inflammation to levels observed in wild-type mice. Neutralization of LCN-2 using a monoclonal antibody significantly reduced inflammation in wild-type mice. In contrast, LCN-2-knockout mice developed more severe serum-induced arthritis compared to wild-type mice. Histologic analysis revealed extensive tissue and bone destruction, with significantly reduced neutrophil infiltration but considerably more macrophage migration, in LCN-2-knockout mice compared to wild-type mice.

CONCLUSION

These results demonstrate that LCN-2 may regulate immune cell recruitment to the site of inflammation, a process essential for the controlled initiation, perpetuation, and resolution of inflammatory processes. Thus, LCN-2 may present a promising target in the treatment of IC-mediated inflammatory/autoimmune diseases.

Regulatory T cell dysfunction in lipocalin 2 knockout mice exacerbates serum-induced arthritis (P1326)

Lipocalin 2 (Lcn2), a 25KDa innate immune protein, known to be dramatically up regulated in various inflammatory disorders including antibody-mediated arthritis, yet its biological role remains unclear. More recently our studies have suggested that the regulation might be necessary for the initiation and further resolution of inflammatory process. However, the exact mechanism involved in this process is still unknown. In this study, we report that Lcn2 is up regulated up to 5 fold during the course of serum-induced arthritis (SIA) in wild type (WT) mice. Likewise, Lcn2KO mice suffered a severe arthritic disease with elevated systemic proinflammatory cytokines as compared to their WT littermates. Immunological analysis revealed a considerable increase in granulocytic population (Gr1+, CD11b+, CD11c+), and a significant decrease (4.2 fold) in regulatory T cell (CD4+, CD25+ and FoxP3+) population in Lcn2KO arthritic mice as compared to their WT littermates. Accordingly, Lcn2KO mice exhibited a defect in T cell proliferation as compared to their WT counterparts, suggesting a novel role for Lcn2 in splenocyte proliferation and T-reg expansion. Collectively, our data suggests a crucial role of Lcn2 in resolution of arthritic inflammation via T-reg expansion, making it a promising target in designing better therapeutic strategies for the treatment of rheumatoid arthritis.

Gut microbial products regulate murine gastrointestinal motility via Toll-like receptor 4 signaling

BACKGROUND & AIMS

Altered gastrointestinal motility is associated with significant morbidity and health care costs. Toll-like receptors (TLR) regulate intestinal homeostasis. We examined the roles of TLR4 signaling in survival of enteric neurons and gastrointestinal motility.

METHODS

We assessed changes in intestinal motility by assessing stool frequency, bead expulsion, and isometric muscle recordings of colonic longitudinal muscle strips from mice that do not express TLR4 (Tlr4(Lps-d) or TLR4(-/-)) or Myd88 (Myd88(-/-)), in wild-type germ-free mice or wild-type mice depleted of the microbiota, and in mice with neural crest-specific deletion of Myd88 (Wnt1Cre(+/-)/Myd88(fl/fl)). We studied the effects of the TLR4 agonist lipopolysaccharide (LPS) on survival of cultured, immortalized fetal enteric neurons and enteric neuronal cells isolated from wild-type and Tlr4(Lps-d) mice at embryonic day 13.5.

RESULTS

There was a significant delay in gastrointestinal motility and reduced numbers of nitrergic neurons in TLR4(Lps-d), TLR4(-/-), and Myd88(-/-) mice compared with wild-type mice. A similar phenotype was observed in germ-free mice, mice depleted of intestinal microbiota, and Wnt1Cre(+/-)/Myd88(fl/fl) mice. Incubation of enteric neuronal cells with LPS led to activation of the transcription factor nuclear factor (NF)-κB and increased cell survival.

CONCLUSIONS

Interactions between enteric neurons and microbes increases neuron survival and gastrointestinal motility in mice. LPS activation of TLR4 and NF-κB appears to promote survival of enteric neurons. Factors that regulate TLR4 signaling in neurons might be developed to alter gastrointestinal motility.

Fecal lipocalin 2, a sensitive and broadly dynamic non-invasive biomarker for intestinal inflammation

Inflammation has classically been defined histopathologically, especially by the presence of immune cell infiltrates. However, more recent studies suggest a role for "low-grade" inflammation in a variety of disorders ranging from metabolic syndrome to cancer, which is defined by modest elevations in pro-inflammatory gene expression. Consequently, there is a need for cost-effective, non-invasive biomarkers that, ideally, would have the sensitivity to detect low-grade inflammation and have a dynamic range broad enough to reflect classic robust intestinal inflammation. Herein, we report that, for assessment of intestinal inflammation, fecal lipocalin 2 (Lcn-2), measured by ELISA, serves this purpose. Specifically, using a well-characterized mouse model of DSS colitis, we observed that fecal Lcn-2 and intestinal expression of pro-inflammatory cytokines (IL-1β, CXCL1, TNFα) are modestly but significantly induced by very low concentrations of DSS (0.25 and 0.5%), and become markedly elevated at higher concentrations of DSS (1.0 and 4.0%). As expected, careful histopathologic analysis noted only modest immune infiltrates at low DSS concentration and robust colitis at higher DSS concentrations. In accordance, increased levels of the neutrophil product myeloperoxidase (MPO) was only detected in mice given 1.0 and 4.0% DSS. In addition, fecal Lcn-2 marks the severity of spontaneous colitis development in IL-10 deficient mice. Unlike histopathology, MPO, and q-RT-PCR, the assay of fecal Lcn-2 requires only a stool sample, permits measurement over time, and can detect inflammation as early as 1 day following DSS administration. Thus, assay of fecal Lcn-2 by ELISA can function as a non-invasive, sensitive, dynamic, stable and cost-effective means to monitor intestinal inflammation in mice.

Transient inability to manage proteobacteria promotes chronic gut inflammation in TLR5-deficient mice

Colitis results from breakdown of homeostasis between intestinal microbiota and the mucosal immune system, with both environmental and genetic influencing factors. Flagellin receptor TLR5-deficient mice (T5KO) display elevated intestinal proinflammatory gene expression and colitis with incomplete penetrance, providing a genetically sensitized system to study the contribution of microbiota to driving colitis. Both colitic and noncolitic T5KO exhibited transiently unstable microbiotas, with lasting differences in colitic T5KO, while their noncolitic siblings stabilized their microbiotas to resemble wild-type mice. Transient high levels of proteobacteria, especially enterobacteria species including E. coli, observed in close proximity to the gut epithelium were a striking feature of colitic microbiota. A Crohn's disease-associated E. coli strain induced chronic colitis in T5KO, which persisted well after the exogenously introduced bacterial species had been eliminated. Thus, an innate immune deficiency can result in unstable gut microbiota associated with low-grade inflammation, and harboring proteobacteria can drive and/or instigate chronic colitis.

Lipocalin 2 deficiency dysregulates iron homeostasis and exacerbates endotoxin-induced sepsis

Various states of inflammation, including sepsis, are associated with hypoferremia, which limits iron availability to pathogens and reduces iron-mediated oxidative stress. Lipocalin 2 (Lcn2; siderocalin, 24p3) plays a central role in iron transport. Accordingly, Lcn2-deficient (Lcn2KO) mice exhibit elevated intracellular labile iron. In this study, we report that LPS induced systemic Lcn2 by 150-fold in wild-type mice at 24 h. Relative to wild-type littermates, Lcn2KO mice were markedly more sensitive to endotoxemia, exhibiting elevated indices of organ damage (transaminasemia, lactate dehydrogenase) and increased mortality. Such exacerbated endotoxemia was associated with substantially increased caspase-3 cleavage and concomitantly elevated immune cell apoptosis. Furthermore, cells from Lcn2KO mice were hyperresponsive to LPS ex vivo, exhibiting elevated cytokine secretion. Additionally, Lcn2KO mice exhibited delayed LPS-induced hypoferremia despite normal hepatic hepcidin expression and displayed decreased levels of the tissue redox state indicators cysteine and glutathione in liver and plasma. Desferroxamine, an iron chelator, significantly protects Lcn2KO mice from LPS-induced toxicity, including mortality, suggesting that Lcn2 may act as an antioxidant in vivo by regulating iron homeostasis. Thus, Lcn2-mediated regulation of labile iron protects the host against sepsis. Its small size and simple structure may make Lcn2 a deployable treatment for sepsis.

Lipocalin 2 mediated hypoferremia protects against endotoxin- induced sepsis in mice (112.11)

Sepsis is characterized by hypoferremia, a primitive protective mechanism to limit iron-induced oxidative stress. Lipocalin-2 (Lcn2) which is highly upregulated during inflammation and has been shown to transport iron under physiologic conditions. Accordingly, we hypothesized that such upregulation of Lcn2 serves to protect against sepsis associated oxidative stress. Lcn2 deficient mice (Lcn2KO) and their WT littermates were given E. coli LPS (20mg/kg BW) i.p. and monitored for mortality. Serum TNFα, IL-18, LDH, aminotransferases, iron, glutathione and cysteine were analyzed. Splenocyte apoptosis was assayed by TUNEL, caspase 3 cleavage and flow cytometry. In WT mice, LPS-induced systemic Lcn2 peaked at 24h and returned to basal levels by 48h. All Lcn2KO died by day 8 while only 20% mortality was observed in WT mice. In addition, Lcn2KO exhibited substantial elevation of every parameter tested including both pro- and anti-inflammatory cytokines. Further,Lcn2KO splenocytes exhibited extensive apoptosis as measured by caspase 3 cleavage, TUNEL staining and flow cytometry. More importantly, when compared to WT mice, Lcn2KO mice failed to exhibit LPS-induced hypoferremia at early time points and exhibited severe hepatic oxidative stress. Our data demonstrate that Lcn2 facilitates LPS-induced hypoferremia and serves as a host protective factor by reducing oxidative stress. Given Lcn2 multifunctionality, it may be developed as a potential therapeutic agent to treat sepsis.

Cytosolic flagellin receptor NLRC4 protects mice against mucosal and systemic challenges

Bacterial flagellin is a dominant innate immune activator of the intestine. Therefore, we examined the role of the intracellular flagellin receptor, NLRC4, in protecting the gut and/or driving inflammation. In accordance with NLRC4 acting through transcription-independent pathways, loss of NLRC4 did not reduce the rapid robust changes in intestinal gene expression induced by flagellin administration. Loss of NLRC4 did not alter basal intestinal homeostasis nor predispose mice to development of colitis upon administration of an anti-interleukin (IL)-10R monoclonal antibody. However, epithelial injury induced by dextran sulfate sodium in mice lacking NLRC4 resulted in a more severe disease, indicating a role for NLRC4 in protecting the gut. Moreover, loss of NLRC4 resulted in increased mortality in response to flagellate, but not aflagellate Salmonella infection. Thus, despite not being involved in rapid intestinal gene remodeling upon detection of flagellin, NLRC4-mediated inflammasome activation results in production of IL-1β and IL-18, two cytokines that protect mice from mucosal and systemic challenges.

Is predisposition to NAFLD and obesity communicable?

A forthcoming article in Nature reveals that inflammasome deficiency disturbs gut microbiota and predisposes to diet-induced obesity and liver disease. The aberrant microbiota and its consequences could be transferred by cohousing mutant and wild-type mice, suggesting that some aspects of metabolic syndrome can be communicable (Henao-Mejia et al., 2012).

Interleukin-1β (IL-1β) promotes susceptibility of Toll-like receptor 5 (TLR5) deficient mice to colitis

BACKGROUND

The extent to which numerous strains of genetically engineered mice, including mice lacking Toll-like receptor 5 (T5KO), display colitis is environment dependent. Gut microbiota underlie much of the variation in phenotype. Accordingly, embryonic rederivation of T5KO mice ameliorated their spontaneous colitis despite only partially correcting elevated proinflammatory gene expression. It was postulated that endogenous anti-inflammatory pathways mediated the absence of overt inflammation in these mice when their gut microbiota were reset. Consequently, it was hypothesised that neutralisation of the anti-inflammatory cytokine interleukin 10 (IL-10) might induce uniform colitis in T5KO mice, and thus provide a practical means to study mechanisms underlying their inflammation.

METHODS

Two distinct strains of non-colitic T5KO mice, as well as mice lacking MyD88, Toll-like receptor 4 (TLR4), IL-1 receptor (IL-1R) and various double knockouts (DKOs) were treated weekly for 4  weeks with 1 mg/mouse of IL-10 receptor neutralising antibody (IL-10R mAb) and colitis assayed 1 week later. The composition of the caecal microbiota was determined by 454 pyrosequencing of 16S rRNA genes.

RESULTS

Anti-IL-10R mAb treatment led to severe uniform intestinal inflammation in both strains of T5KO mice. Such neutralisation of IL-10 signalling did not cause colitis in wild-type littermates nor mice lacking TLR4, MyD88 or IL-1R. The susceptibility of T5KO mice to this colitis model was not rescued by absence of TLR4 in that T4/T5 DKO mice displayed severe colitis in response to anti-IL-10R mAb treatment. IL-1β signalling was crucial for this colitis model in that IL-1R/T5 DKOs were completely protected from colitis in response to IL-10R mAb treatment. Lastly, it was observed that blockade of IL-10R function was associated with changes in the composition of gut microbiota, which were observed in mice that were susceptible and resistant to IL-10R mAb-induced colitis.

CONCLUSION

Regardless of whether they harbour a colitogenic microbiota, loss of TLR5 predisposes mice to colitis triggered by immune dysregulation via an IL-1β-dependent pathway.