Bidirectional communication between the Aryl hydrocarbon Receptor (AhR) and the microbiome tunes host metabolism.

The ligand-induced transcription factor, aryl hydrocarbon receptor (AhR) is known for its capacity to tune adaptive immunity and xenobiotic metabolism-biological properties subject to regulation by the indigenous microbiome. The objective of this study was to probe the postulated microbiome-AhR crosstalk and whether such an axis could influence metabolic homeostasis of the host. Utilising a systems-biology approach combining in-depth 1H-NMR-based metabonomics (plasma, liver and skeletal muscle) with microbiome profiling (small intestine, colon and faeces) of AhR knockout (AhR-/-) and wild-type (AhR+/+) mice, we assessed AhR function in host metabolism. Microbiome metabolites such as short-chain fatty acids were found to regulate AhR and its target genes in liver and intestine. The AhR signalling pathway, in turn, was able to influence microbiome composition in the small intestine as evident from microbiota profiling of the AhR+/+ and AhR-/- mice fed with diet enriched with a specific AhR ligand or diet depleted of any known AhR ligands. The AhR-/- mice also displayed increased levels of corticosterol and alanine in serum. In addition, activation of gluconeogenic genes in the AhR-/- mice was indicative of on-going metabolic stress. Reduced levels of ketone bodies and reduced expression of genes involved in fatty acid metabolism in the liver further underscored this observation. Interestingly, exposing AhR-/- mice to a high-fat diet showed resilience to glucose intolerance. Our data suggest the existence of a bidirectional AhR-microbiome axis, which influences host metabolic pathways.

Modulatory activity of Lactobacillus rhamnosus OLL2838 in a mouse model of intestinal immunopathology.

Gut microbiota and probiotic strains play an important role in oral tolerance by modulating regulatory and effector cell components of the immune system. We have previously described the ability of Lactobacilli to influence both the innate and adaptive immunity to wheat gluten, a food antigen, in mouse. In this study, we further explored the immunomodulatory mechanisms elicited in this model by testing three specific probiotic strains, namely L. rhamnosus OLL2838, B. infantis ATCC15697 and S. thermophilus Sfi39. In vitro analysis showed the all tested strains induced maturation of bone marrow derived dendritic cells (DCs). However, only L. rhamnosus induced appreciable levels of IL-10 and nitric oxide productions, whereas S. thermophilus essentially elicited IL-12 and TNF-α. The anti-inflammatory ability of OLL2838 was then tested in vivo by adopting mice that develop a gluten-specific enteropathy. This model is characterized by villus blunting, crypt hyperplasia, high levels of intestinal IFN-γ, increased cell apoptosis in lamina propria, and reduced intestinal total glutathione (GSHtot) and glutathione S-transferase (GST) activity. We found that, following administration of OLL2838, GSHtot and GST activity were enhanced, whereas caspase-3 activity was reduced. On the contrary, this probiotic strain failed in recovering the normal histology and further increased intestinal IFN-γ. Confocal microscopy revealed the inability of the probiotic strain to appropriately interact with enterocytes of the small intestine and with Peyer's patches in treated mice. In conclusion, these data highlighted the potential of L. rhamnosus OLL2838 to recover specific toxicity parameters induced by gluten in enteropathic mice through mechanisms that involve induction of low levels of reactive oxygen species (ROS).

Absence of intestinal PPARγ aggravates acute infectious colitis in mice through a lipocalin-2-dependent pathway.

To be able to colonize its host, invading Salmonella enterica serovar Typhimurium must disrupt and severely affect host-microbiome homeostasis. Here we report that S. Typhimurium induces acute infectious colitis by inhibiting peroxisome proliferator-activated receptor gamma (PPARγ) expression in intestinal epithelial cells. Interestingly, this PPARγ down-regulation by S. Typhimurium is independent of TLR-4 signaling but triggers a marked elevation of host innate immune response genes, including that encoding the antimicrobial peptide lipocalin-2 (Lcn2). Accumulation of Lcn2 stabilizes the metalloproteinase MMP-9 via extracellular binding, which further aggravates the colitis. Remarkably, when exposed to S. Typhimurium, Lcn2-null mice exhibited a drastic reduction of the colitis and remained protected even at later stages of infection. Our data suggest a mechanism in which S. Typhimurium hijacks the control of host immune response genes such as those encoding PPARγ and Lcn2 to acquire residence in a host, which by evolution has established a symbiotic relation with its microbiome community to prevent pathogen invasion.

Immunomodulation of gut-associated lymphoid tissue: current perspectives.

The gut-associated lymphoid tissue is deputed both to protect from infectious diseases and to evoke immune tolerance. Efficient responses need mucosal adjuvants: starting from cholera toxin, new variants of cholera toxin were developed depleted of toxicity. In addition, lipid colloidal particles, bacterial DNA, and probiotics have been experimented. Tolerance is currently induced by means of the B subunit of cholera toxin, whereas new strategies encompass the use of probiotics, expansion of regulatory T cells and blocking of paracellular entry of antigens. Finally, we report different approaches developed for celiac disease, an immune-mediated disease whose triggering antigen is known.

A deregulated immune response to gliadin causes a decreased villus height in DQ8 transgenic mice.

Celiac disease (CD) is an enteropathy triggered by gluten and mediated by CD4+ T cells. A complete understanding of CD immunopathogenesis has been hindered due to the lack of adequate in vivo models. Here, we explored the effect of the inhibition of COX by indomethacin in wheat gliadin-sensitized transgenic mice expressing the HLA-DQ8 heterodimer, a molecule associated with CD. Treated mice showed a gliadin-specific immune response with a significant reduction of villus height, not linked to crypt hyperplasia and to expansion of intraepithelial T cells. Notably, treated mice showed increased numbers of CD25+ and apoptotic cells in the lamina propria, whereas high basal levels of IFN-gamma secretion, along with a reduced gliadin-specific IL-2 expression were detected in MLN. Biochemical assessment of the lesion revealed increased mRNA of Lamb3 and Adamts2, encoding for ECM proteins, and enhanced activities of metalloproteinases MMP1, 2 and 7. We conclude that an intestinal sensitivity to gliadin, in connection with COX inhibition, caused a decreased villus height in DQ8 tg mice. The lesion was induced by a deregulated mucosal cell immunity to gliadin, thus triggering activation of a specific ECM protein pathway responsible for lamina propria remodeling.

Modulation of the immune response by probiotic strains in a mouse model of gluten sensitivity.

Probiotic strains play an important role in modulating activities in the gut-associated lymphoid tissue. Elucidation of the mechanisms that mediate probiotic-driven immunomodulation may facilitate their therapeutic application for specific immune-mediated diseases or for prophylaxis. In this study, we explored the effect of different Lactobacillus spp. and Bifidobacterium lactis in transgenic mice expressing the human DQ8 heterodimer, a HLA molecule linked to Celiac Disease (CD). In vitro analysis on immature bone marrow-derived dendritic cells (iBMDCs) showed that all strains up-regulated surface B7-2 (CD86), indicative of DC maturation, however, with different intensity. No strain induced appreciable levels of IL-10 or IL-12 in iBMDCs, whereas TNF-alpha expression was essentially elicited by Lactobacillus paracasei and Lactobacillus fermentum. Interestingly, these strains were found also to increase the antigen-specific TNF-alpha secretion in vivo, following co-administration of probiotic bacteria in mice mucosally immunized with the gluten component gliadin. Together these findings highlighted the ability of probiotics to exert strain-specific inductive rather than suppressive effects both on the innate and adaptive immunity in a mouse model of food antigen sensitivity.

Adjuvant effect of Lactobacillus casei in a mouse model of gluten sensitivity.

Probiotic strains have been reported to exert immunomodulatory activities in the gut-associated lymphoid tissue. In this study we explored the effect of Lactobacillus casei in transgenic mice expressing the human DQ8 heterodimer, a HLA molecule linked to Celiac Disease (CD). DQ8 mice, mucosally immunized with the gluten component gliadin, mounted an intestinal Th1-like response as observed in CD, without developing enteropathy. Co-administration of L. casei in sensitized mice specifically enhanced the gliadin-specific response mediated by CD4(+) T cells. Notably, both a strong increase of the gliadin-specific IFNgamma expression and a pro-inflammatory polarization of the cytokine milieu in the small intestinal mucosa were associated to the presence of the probiotic strain. However, this condition did not bring on any mucosal alteration. These findings suggest that the gliadin-specific enteropathy is not merely related to the HLA DQ8-restricted massive production of IFNgamma, but additional parameters are involved. Moreover, our data imply that the intrinsic adjuvanticity of L. casei can be exploited to further enhance both mucosal and systemic T cell-mediated responses.

Association between activation of phase 2 enzymes and down-regulation of dendritic cell maturation by c9,t11-conjugated linoleic acid.

Antioxidant and cytoprotective enzymes (phase 2) exert protective activity against reactive oxygen species (ROS)-induced injury. We have recently shown how the beneficial effects of conjugated linoleic acid (CLA) in a mouse model of an autoimmune disease are parallel with the activation of phase 2 enzymes. In the present study we found that c9,t11-CLA isomer activates cytoprotective enzymes and down-regulates LPS- or gliadin-induced maturation in dendritic cells (DCs) obtained from a murine model of celiac disease. As expected, the enhancement of LPS-induced maturation (increased NFkappaB p65 nuclear translocation, CD86 expression and decreased CD11c+ cell number) was exacerbated by specific glutathione (GSH) inhibitor (buthionine sulphoximine; BSO). Conversely, the down-regulation of DC maturation by antioxidant N-acetylcysteine (NAC) was associated with the marked increase of intracellular thiol concentration. c9,t11-CLA activation of phase 2 enzymes in mouse DCs was observed first. Next, we found that the significant reduction of LPS- and gliadin-induced DC maturation in cultures pre-treated with c9,t11-CLA improved cellular redox status (decreased ROS and higher antioxidant defenses). Finally, the process of DC maturation triggered by gliadin, in contrast with that exhibited by LPS, was not associated with enhanced NFkappaB nuclear translocation and pro-inflammatory cytokines synthesis. These results demonstrate that c9,t11-CLA renders DCs more resistant to gliadin- or LPS-induced maturation, thus indicating that a cytoprotective mechanism elicited by c9,t11-CLA may modulate DC responsiveness.

Bacillus subtilis spores reduce susceptibility to Citrobacter rodentium-mediated enteropathy in a mouse model.

The present work was aimed at investigating whether Bacillus subtilis spores, widely used in probiotic as well as pharmaceutical preparations for mild gastrointestinal disorders, can suppress enteric infections. To address this issue, we developed a mouse model of infection using the mouse enteropathogen Citrobacter rodentium, a member of a family of human and animal pathogens which includes the clinically significant enteropathogenic (EPEC) and enterohemorrhagic (EHEC) Escherichia coli strains. This group of pathogens causes transmissible colonic hyperplasia by using attaching and effacing (A/E) lesions to colonize the host colon. Because of its similarities to human enteropathogens, C. rodentium is now widely used as an in vivo model for gastrointestinal infections. Swiss NIH mice were orally administered B. subtilis spores one day before infection with C. rodentium. Mice were sacrificed on day 15 after infection, and distal colon, liver and mesenteric lymph nodes were removed for bacteria counts, morphology, immunohistology and IFNgamma mRNA analysis. We observed that spore predosing was effective in significantly decreasing infection and enteropathy in suckling mice infected with a dose of C. rodentium sufficient to cause colon colonization, crypt hyperplasia and high mortality rates. Moreover, in mice predosed with spores, the number of CD4(+) cells and IFNgamma transcript levels remained high. These results thus indicate that our newly established model of C. rodentium infection is a suitable system for analyzing the effects of probiotic bacteria on enteroinfections and that B. subtilis spores are efficient at reducing C. rodentium infection in mice, leaving unaltered the immune response against the pathogen.