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Allergic contact dermatitis is the most frequent occupational disease in industrialized countries. It is caused by CD8(+) T cell-mediated contact hypersensitivity (CHS) reactions triggered at the site of contact by a variety of chemicals, also known as weak haptens, present in fragrances, dyes, metals, preservatives, and drugs. Despite the myriad of potentially allergenic substances that can penetrate the skin, sensitization is relatively rare and immune tolerance to the substance is often induced by as yet poorly understood mechanisms. Here we show, using the innocuous chemical 2,4-dinitrothiocyanobenzene (DNTB), that cutaneous immune tolerance in mice critically depends on epidermal Langerhans cells (LCs), which capture DNTB and migrate to lymph nodes for direct presentation to CD8(+) T cells. Depletion and adoptive transfer experiments revealed that LCs conferred protection from development of CHS by a mechanism involving both anergy and deletion of allergen-specific CD8(+) T cells and activation of a population of T cells identified as ICOS(+)CD4(+)Foxp3(+) Tregs. Our findings highlight the critical role of LCs in tolerance induction in mice to the prototype innocuous hapten DNTB and suggest that strategies targeting LCs might be valuable for prevention of cutaneous allergy. [ABSTRACT FROM AUTHOR]

Antimicrobial peptides (AMPs) are crucial components of the innate immune system in various organisms, including humans. Beyond their direct antimicrobial effects, AMPs play essential roles in various physiological processes. They induce angiogenesis, promote wound healing, modulate immune responses, and serve as chemoattractants for immune cells. AMPs regulate the microbiome and combat microbial infections on the skin, lungs, and gastrointestinal tract. Produced in response to microbial signals, AMPs help maintain a balanced microbial community and provide a first line of defense against infection. In preterm infants, alterations in microbiome composition have been linked to various health outcomes, including sepsis, necrotizing enterocolitis, atopic dermatitis, and respiratory infections. Dysbiosis, or an imbalance in the microbiome, can alter AMP profiles and potentially lead to inflammation-mediated diseases such as chronic lung disease and obesity. In the following review, we summarize what is known about the vital role of AMPs as multifunctional peptides in protecting newborn infants against infections and modulating the microbiome and immune response. Understanding their roles in preterm infants and high-risk populations offers the potential for innovative approaches to disease prevention and treatment.

Natural killer (NK) cells are present in the circulation and can also be found residing in tissues, and these populations exhibit distinct developmental requirements and are thought to differ in terms of ontogeny. Here, we investigate whether circulating conventional NK (cNK) cells can develop into long-lived tissue-resident NK (trNK) cells following acute infections. We found that viral and bacterial infections of the skin triggered the recruitment of cNK cells and their differentiation into Tcf1 hi CD69 hi trNK cells that share transcriptional similarity with CD56 bright TCF1 hi NK cells in human tissues. Skin trNK cells arose from interferon (IFN)-γ-producing effector cells and required restricted expression of the transcriptional regulator Blimp1 to optimize Tcf1-dependent trNK cell formation. Upon secondary infection, trNK cells rapidly gained effector function and mediated an accelerated NK cell response. Thus, cNK cells redistribute and permanently position at sites of previous infection via a mechanism promoting tissue residency that is distinct from Hobit-dependent developmental paths of NK cells and ILC1 seeding tissues during ontogeny., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Mouse atonal homolog 1 ( Math1/Atoh1 ) is a basic helix-loop-helix transcription factor important for the differentiation of secretory cells within the intestinal epithelium. The analysis of Paneth depletion efficiency on Math1 lox/lox VilCreER T2 ( Math1 ΔIEC ) mice treatment with tamoxifen in the presence or absence of intestinal microbiota showed a failure on Paneth cell depletion in germ-free mice as compared with specific pathogen-free (SPF) mice. However, goblet cells were efficiently depleted in Math1 ΔIEC germ-free mice. The gene expression of Math1 was significantly reduced in the ileum of germ-free Math1 ΔIEC mice 5 days after tamoxifen injection as compared with germ-free control, but its protein expression was still detectable in the nuclei of epithelial cells in the crypts. Germ-free mice showed low proliferative ileal crypts and apoptotic cells that were mainly detected in the tip of the villus, consistent with a slow turnover rate of epithelial cells. Although Paneth cells were not depleted in germ-free Math1 ΔIEC mice for the first 7 wk after the last tamoxifen injection, far already from the 5 days time-laps observed in SPF conditions, an incomplete depletion of Paneth cells was observed 14 wk after the last tamoxifen injection. Colonization of germ-free mice restored the phenotype observed in SPF mice, highlighting the regulatory role of gut microbes in our model. We conclude that absence of intestinal microbiota in Math1 ΔIEC mice is associated with reduced epithelial cell renewal and delays the depletion of preexisting Paneth cells. NEW & NOTEWORTHY Cre-lox system is a powerful and widely used research tool developed to understand the specific role of genes. It allows to control the spatial and temporal expression of genes in experimental models. Several limitations including toxicity of Cre recombinase or incomplete excision of floxed loci have been reported in the past. To date, this is the first research study reporting that gut microbes also influence the expected phenotype of Paneth cell depletion in the genetically modified Math1 lox/lox VilCreER T2 mouse model.

The limitations of 2D microscopy constrain our ability to observe and understand tissue-wide networks that are, by nature, 3-dimensional. Optical projection tomography (OPT) enables the acquisition of large volumes (ranging from micrometres to centimetres) in various tissues. We present a multi-modal workflow for the characterization of both structural and quantitative parameters of the mouse small intestine. As proof of principle, we evidence its applicability for imaging the mouse intestinal immune compartment and surrounding mucosal structures. We quantify the volumetric size and spatial distribution of Isolated Lymphoid Follicles (ILFs) and quantify the density of villi throughout centimetre-long segments of intestine. Furthermore, we exhibit the age and microbiota dependence for ILF development, and leverage a technique that we call reverse-OPT for identifying and homing in on regions of interest. Several quantification capabilities are displayed, including villous density in the autofluorescent channel and the size and spatial distribution of the signal of interest at millimetre-scale volumes. The concatenation of 3D imaging with reverse-OPT and high-resolution 2D imaging allows accurate localisation of ROIs and adds value to interpretations made in 3D. Importantly, OPT may be used to identify sparsely-distributed regions of interest in large volumes whilst retaining compatibility with high-resolution microscopy modalities, including confocal microscopy. We believe this pipeline to be approachable for a wide-range of specialties, and to provide a new method for characterisation of the mouse intestinal immune compartment., (© 2023. The Author(s).)

It is generally believed that environmental or cutaneous bacteria are the main origin of surgical infections. Therefore, measures to prevent postoperative infections focus on optimizing hygiene and improving asepsis and antisepsis. In a large cohort of patients with infections following major surgery, we identified that the causative bacteria are mainly of intestinal origin. Postoperative infections of intestinal origin were also found in mice undergoing partial hepatectomy. CCR6 + group 3 innate lymphoid cells (ILC3s) limited systemic bacterial spread. Such bulwark function against host invasion required the production of interleukin-22 (IL-22), which controlled the expression of antimicrobial peptides in hepatocytes, thereby limiting bacterial spread. Using genetic loss-of-function experiments and punctual depletion of ILCs, we demonstrate that the failure to restrict intestinal commensals by ILC3s results in impaired liver regeneration. Our data emphasize the importance of endogenous intestinal bacteria as a source for postoperative infection and indicate ILC3s as potential new targets., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The skin of preterm infants is a delicate organ with critical structural and functional differences as compared to term born infants. Unique features contribute to an increased susceptibility to injury, infection, thermal instability, and water loss. During rapid, often accelerated adaption of the physical barrier function of preterm skin, a parallel and mutual development of host skin immunity and skin microbiome seem to be crucial for skin homeostasis. Recent advances in molecular biology have enabled researchers to gain a deeper understanding of the microbial community composition of preterm skin and the important relationship with microbiome composition of other body sites. Nevertheless, several questions remain to be answered, including niche factors and environmental influences on skin maturation. In line with that, evidence-based guidelines on skin care practice in preterm infants are missing. This review articles aims to provide an overview of the current knowledge of preterm infant skin development including immune and barrier function, host-microbial interactions, and potential clinical implications., (© 2023 The Author(s). Published by S. Karger AG, Basel.)

Lymphatic transport of molecules and migration of myeloid cells to lymph nodes (LNs) continuously inform lymphocytes on changes in drained tissues. Here, using LN transplantation, single-cell RNA-seq, spectral flow cytometry, and a transgenic mouse model for photolabeling, we showed that tissue-derived unconventional T cells (UTCs) migrate via the lymphatic route to locally draining LNs. As each tissue harbored a distinct spectrum of UTCs with locally adapted differentiation states and distinct T cell receptor repertoires, every draining LN was thus populated by a distinctive tissue-determined mix of these lymphocytes. By making use of single UTC lineage-deficient mouse models, we found that UTCs functionally cooperated in interconnected units and generated and shaped characteristic innate and adaptive immune responses that differed between LNs that drained distinct tissues. Lymphatic migration of UTCs is, therefore, a key determinant of site-specific immunity initiated in distinct LNs with potential implications for vaccination strategies and immunotherapeutic approaches., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Interactions between pathogens, host microbiota and the immune system influence many physiological and pathological processes. In the 20th century, widespread dermal vaccination with vaccinia virus (VACV) led to the eradication of smallpox but how VACV interacts with the microbiota and whether this influences the efficacy of vaccination are largely unknown. Here we report that intradermal vaccination with VACV induces a large increase in the number of commensal bacteria in infected tissue, which enhance recruitment of inflammatory cells, promote tissue damage and influence the host response. Treatment of vaccinated specific-pathogen-free (SPF) mice with antibiotic, or infection of genetically-matched germ-free (GF) animals caused smaller lesions without alteration in virus titre. Tissue damage correlated with enhanced neutrophil and T cell infiltration and levels of pro-inflammatory tissue cytokines and chemokines. One month after vaccination, GF and both groups of SPF mice had equal numbers of VACV-specific CD8+ T cells and were protected from disease induced by VACV challenge, despite lower levels of VACV-neutralising antibodies observed in GF animals. Thus, skin microbiota may provide an adjuvant-like stimulus during vaccination with VACV and influence the host response to vaccination., Competing Interests: The authors have declared that no competing interests exist.

Abdominal surgeries are lifesaving procedures but can be complicated by the formation of peritoneal adhesions, intra-abdominal scars that cause intestinal obstruction, pain, infertility, and significant health costs. Despite this burden, the mechanisms underlying adhesion formation remain unclear and no cure exists. Here, we show that contamination of gut microbes increases post-surgical adhesion formation. Using genetic lineage tracing we show that adhesion myofibroblasts arise from the mesothelium. This transformation is driven by epidermal growth factor receptor (EGFR) signaling. The EGFR ligands amphiregulin and heparin-binding epidermal growth factor, are sufficient to induce these changes. Correspondingly, EGFR inhibition leads to a significant reduction of adhesion formation in mice. Adhesions isolated from human patients are enriched in EGFR positive cells of mesothelial origin and human mesothelium shows an increase of mesothelial EGFR expression during bacterial peritonitis. In conclusion, bacterial contamination drives adhesion formation through mesothelial EGFR signaling. This mechanism may represent a therapeutic target for the prevention of adhesions after intra-abdominal surgery., (© 2021. The Author(s).)

The immune cells of the central nervous system (CNS) comprise parenchymal microglia and at the CNS border regions meningeal, perivascular, and choroid plexus macrophages (collectively called CNS-associated macrophages, CAMs). While previous work has shown that microglial properties depend on environmental signals from the commensal microbiota, the effects of microbiota on CAMs are unknown. By combining several microbiota manipulation approaches, genetic mouse models, and single-cell RNA-sequencing, we have characterized CNS myeloid cell composition and function. Under steady-state conditions, the transcriptional profiles and numbers of choroid plexus macrophages were found to be tightly regulated by complex microbiota. In contrast, perivascular and meningeal macrophages were affected to a lesser extent. An acute perturbation through viral infection evoked an attenuated immune response of all CAMs in germ-free mice. We further assessed CAMs in a more chronic pathological state in 5xFAD mice, a model for Alzheimer's disease, and found enhanced amyloid beta uptake exclusively by perivascular macrophages in germ-free 5xFAD mice. Our results aid the understanding of distinct microbiota-CNS macrophage interactions during homeostasis and disease, which could potentially be targeted therapeutically., (© 2021 The Authors. Published under the terms of the CC BY NC ND 4.0 license.)

As tissue macrophages of the central nervous system (CNS), microglia constitute the pivotal immune cells of this organ. Microglial features are strongly dependent on environmental cues such as commensal microbiota. Gut bacteria are known to continuously modulate microglia maturation and function by the production of short-chain fatty acids (SCFAs). However, the precise mechanism of this crosstalk is unknown. Here we determined that the immature phenotype of microglia from germ-free (GF) mice is epigenetically imprinted by H3K4me3 and H3K9ac on metabolic genes associated with substantial functional alterations including increased mitochondrial mass and specific respiratory chain dysfunctions. We identified acetate as the essential microbiome-derived SCFA driving microglia maturation and regulating the homeostatic metabolic state, and further showed that it is able to modulate microglial phagocytosis and disease progression during neurodegeneration. These findings indicate that acetate is an essential bacteria-derived molecule driving metabolic pathways and functions of microglia during health and perturbation., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Innate lymphoid cells (ILCs) participate in tissue homeostasis, inflammation, and early immunity against infection. It is unclear how ILCs acquire effector function and whether these mechanisms differ between organs. Through multiplexed single-cell mRNA sequencing, we identified cKit + CD127 hi TCF-1 hi early differentiation stages of T-bet + ILC1s. These cells were present across different organs and had the potential to mature toward CD127 int TCF-1 int and CD127 - TCF-1 - ILC1s. Paralleling a gradual loss of TCF-1, differentiating ILC1s forfeited their expansion potential while increasing expression of effector molecules, reminiscent of T cell differentiation in secondary lymphoid organs. The transcription factor Hobit was induced in TCF-1 hi ILC1s and was required for their effector differentiation. These findings reveal sequential mechanisms of ILC1 lineage commitment and effector differentiation that are conserved across tissues. Our analyses suggest that ILC1s emerge as TCF-1 hi cells in the periphery and acquire a spectrum of organ-specific effector phenotypes through a uniform Hobit-dependent differentiation pathway driven by local cues., (© 2021. The Author(s), under exclusive licence to Springer Nature America, Inc.)

The gut microbiome has been implicated as a key regulator of brain function in health and disease. But the impact of gut microbiota on functional brain connectivity is unknown. We used resting-state functional magnetic resonance imaging in germ-free and normally colonized mice under naive conditions and after ischemic stroke. We observed a strong, brain-wide increase of functional connectivity in germ-free animals. Graph theoretical analysis revealed significant higher values in germ-free animals, indicating a stronger and denser global network but with less structural organization. Breakdown of network function after stroke equally affected germ-free and colonized mice. Results from histological analyses showed changes in dendritic spine densities, as well as an immature microglial phenotype, indicating impaired microglia-neuron interaction in germ-free mice as potential cause of this phenomenon. These results demonstrate the substantial impact of bacterial colonization on brain-wide function and extend our so far mainly (sub) cellular understanding of the gut-brain axis., Competing Interests: All authors declare no competing interests., (© 2021.)

Isobiotic mice, with an identical stable microbiota composition, potentially allow models of host-microbial mutualism to be studied over time and between different laboratories. To understand microbiota evolution in these models, we carried out a 6-year experiment in mice colonized with 12 representative taxa. Increased non-synonymous to synonymous mutation rates indicate positive selection in multiple taxa, particularly for genes annotated for nutrient acquisition or replication. Microbial sub-strains that evolved within a single taxon can stably coexist, consistent with niche partitioning of ecotypes in the complex intestinal environment. Dietary shifts trigger rapid transcriptional adaptation to macronutrient and micronutrient changes in individual taxa and alterations in taxa biomass. The proportions of different sub-strains are also rapidly altered after dietary shift. This indicates that microbial taxa within a mouse colony adapt to changes in the intestinal environment by long-term genomic positive selection and short-term effects of transcriptional reprogramming and adjustments in sub-strain proportions., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Staphylococcus aureus (S. aureus) is a common colonizer of healthy skin and mucous membranes. At the same time, S. aureus is the most frequent cause of skin and soft tissue infections. Dermal macrophages (Mφ) are critical for the coordinated defense against invading S. aureus, yet they have a limited life span with replacement by bone marrow derived monocytes. It is currently poorly understood whether localized S. aureus skin infections persistently alter the resident Mφ subset composition and resistance to a subsequent infection. In a strictly dermal infection model we found that mice, which were previously infected with S. aureus , showed faster monocyte recruitment, increased bacterial killing and improved healing upon a secondary infection. However, skin infection decreased Mφ half-life, thereby limiting the duration of memory. In summary, resident dermal Mφ are programmed locally, independently of bone marrow-derived monocytes during staphylococcal skin infection leading to transiently increased resistance against a second infection., Competing Interests: RF, AF, FL, JK, NR, JZ, MG, PH No competing interests declared, (© 2020, Feuerstein et al.)

Background & Aims: Intestinal epithelial homeostasis depends on a tightly regulated balance between intestinal epithelial cell (IEC) death and proliferation. While the disruption of several IEC death regulating factors result in intestinal inflammation, the loss of the anti-apoptotic BCL2 family members BCL2 and BCL2L1 has no effect on intestinal homeostasis in mice. We investigated the functions of the antiapoptotic protein MCL1, another member of the BCL2 family, in intestinal homeostasis in mice., Methods: We generated mice with IEC-specific disruption of Mcl1 (Mcl1 ΔIEC mice) or tamoxifen-inducible IEC-specific disruption of Mcl1 (i-Mcl1 ΔIEC mice); these mice and mice with full-length Mcl1 (controls) were raised under normal or germ-free conditions. Mice were analyzed by endoscopy and for intestinal epithelial barrier permeability. Intestinal tissues were analyzed by histology, in situ hybridization, proliferation assays, and immunoblots. Levels of calprotectin, a marker of intestinal inflammation, were measured in intestinal tissues and feces., Results: Mcl1 ΔIEC mice spontaneously developed apoptotic enterocolopathy, characterized by increased IEC apoptosis, hyperproliferative crypts, epithelial barrier dysfunction, and chronic inflammation. Loss of MCL1 retained intestinal crypts in a hyperproliferated state and prevented the differentiation of intestinal stem cells. Proliferation of intestinal stem cells in MCL1-deficient mice required WNT signaling and was associated with DNA damage accumulation. By 1 year of age, Mcl1 ΔIEC mice developed intestinal tumors with morphologic and genetic features of human adenomas and carcinomas. Germ-free housing of Mcl1 ΔIEC mice reduced markers of microbiota-induced intestinal inflammation but not tumor development., Conclusion: The antiapoptotic protein MCL1, a member of the BCL2 family, is required for maintenance of intestinal homeostasis and prevention of carcinogenesis in mice. Loss of MCL1 results in development of intestinal carcinomas, even under germ-free conditions, and therefore does not involve microbe-induced chronic inflammation. Mcl1 ΔIEC mice might be used to study apoptotic enterocolopathy and inflammatory bowel diseases., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

There is the notion that infection with a virulent intestinal pathogen induces generally stronger mucosal adaptive immunity than the exposure to an avirulent strain. Whether the associated mucosal inflammation is important or redundant for effective induction of immunity is, however, still unclear. Here we use a model of auxotrophic Salmonella infection in germ-free mice to show that live bacterial virulence factor-driven immunogenicity can be uncoupled from inflammatory pathogenicity. Although live auxotrophic Salmonella no longer causes inflammation, its mucosal virulence factors remain the main drivers of protective mucosal immunity; virulence factor-deficient, like killed, bacteria show reduced efficacy. Assessing the involvement of innate pathogen sensing mechanisms, we show MYD88/TRIF, Caspase-1/Caspase-11 inflammasome, and NOD1/NOD2 nodosome signaling to be individually redundant. In colonized animals we show that microbiota metabolite cross-feeding may recover intestinal luminal colonization but not pathogenicity. Consequent immunoglobulin A immunity and microbial niche competition synergistically protect against Salmonella wild-type infection.

Although group 3 innate lymphoid cells (ILC3s) are efficient inducers of T cell responses in the spleen, they fail to induce CD4 + T cell proliferation in the gut. The signals regulating ILC3-T cell responses remain unknown. Here, we show that transcripts associated with MHC II antigen presentation are down-modulated in intestinal natural cytotoxicity receptor (NCR) - ILC3s. Further data implicate microbiota-induced IL-23 as a crucial signal for reversible silencing of MHC II in ILC3s, thereby reducing the capacity of ILC3s to present antigen to T cells in the intestinal mucosa. Moreover, IL-23-mediated MHC II suppression is dependent on mTORC1 and STAT3 phosphorylation in NCR - ILC3s. By contrast, splenic interferon-γ induces MHC II expression and CD4 + T cell stimulation by NCR - ILC3s. Our results thus identify biological circuits for tissue-specific regulation of ILC3-dependent T cell responses. These pathways may have implications for inducing or silencing T cell responses in human diseases.

The nucleotide-binding oligomerization domain (NOD)-like receptors belong to the family of pattern recognition receptors (PRRs). NOD-like receptors play a role in regulation of innate immune response by recognition of both pathogen-associated molecular patterns that are engulfed during phagocytic process and danger-associated molecular patterns that are mainly byproducts of cell stress mediated response. NOD-like family pyrin domain containing 6 (NLRP6) is one of the 14 pyrin domain-containing receptors. NLRP6 is highly expressed by epithelial and goblet cells to regulate epithelial renewal and mucus production in mice and humans, but its function in T cells is rather unknown. Increased caspase-1 activation and cell death were observed in mouse Nlrp6 -deficient T cells following adoptive transfer into Rag2 -deficient mice, indicating that Nlrp6 deficiency in CD4 + T cells led to decreased survival., (Copyright © 2019 by The American Association of Immunologists, Inc.)

Caloric restriction (CR) stimulates development of functional beige fat and extends healthy lifespan. Here we show that compositional and functional changes in the gut microbiota contribute to a number of CR-induced metabolic improvements and promote fat browning. Mechanistically, these effects are linked to a lower expression of the key bacterial enzymes necessary for the lipid A biosynthesis, a critical lipopolysaccharide (LPS) building component. The decreased LPS dictates the tone of the innate immune response during CR, leading to increased eosinophil infiltration and anti-inflammatory macrophage polarization in fat of the CR animals. Genetic and pharmacological suppression of the LPS-TLR4 pathway or transplantation with Tlr4 -/- bone-marrow-derived hematopoietic cells increases beige fat development and ameliorates diet-induced fatty liver, while Tlr4 -/- or microbiota-depleted mice are resistant to further CR-stimulated metabolic alterations. These data reveal signals critical for our understanding of the microbiota-fat signaling axis during CR and provide potential new anti-obesity therapeutics., (Copyright © 2018 The Author(s). Published by Elsevier Inc. All rights reserved.)

Although the mammalian microbiota is well contained within the intestine, it profoundly shapes development and metabolism of almost every host organ. We questioned the range and depth of microbial metabolite penetration into the host, and how this is modulated by intestinal immunity. Chemically identical microbial and host metabolites were distinguished by stable isotope tracing from 13 C-labeled live non-replicating Escherichia coli, differentiating 12 C host isotopes with high-resolution mass spectrometry. Hundreds of endogenous microbial compounds penetrated 23 host tissues and fluids after intestinal exposure: subsequent 12 C host metabolome signatures included lipidemia, reduced glycolysis, and inflammation. Penetrant bacterial metabolites from the small intestine were rapidly cleared into the urine, whereas induced antibodies curtailed microbial metabolite exposure by accelerating intestinal bacterial transit into the colon where metabolite transport mechanisms are limiting. Pervasive penetration of microbial molecules can cause extensive host tissue responses: these are limited by immune and non-immune intestinal mucosal adaptations to the microbiota., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

Considering the increasing list of diseases linked to the commensal microbiota, experimental studies of host-microbe interactions are of growing interest. Axenic and differently colonized animal models are inalienable tools to study these interactions. Factors, such as host genetics, diet, antibiotics and litter affect microbiota composition and can be confounding factors in many experimental settings. The use of gnotobiotic mice harboring defined microbiotas of different complexity plus additional housing standardization have thus become a gold standard to study the influence of the microbiome on the host. We highlight here the recent advances, challenges and outstanding goals in gnotobiology with the ambition to contribute to the generation of reliable, reproducible and transferrable results, which form the basis for advances in biomedical research., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Biological redundancy ensures robustness in living organisms at several levels, from genes to organs. In this review, we explore the concept of redundancy and robustness through an analysis of the caecal appendix, an organ that is often considered to be a redundant remnant of evolution. However, phylogenic data show that the Appendix was selected during evolution and is unlikely to disappear once it appeared. In humans, it is highly conserved and malformations are extremely rare, suggesting a role for that structure. The Appendix could perform a dual role. First, it is a concentrate of lymphoid tissue resembling Peyer's patches and is the primary site for immunoglobulin A production which is crucial to regulate the density and quality of the intestinal flora. Second, given its shape and position, the Appendix could be a unique niche for commensal bacteria in the body. It is extremely rich in biofilms that continuously shed bacteria into the intestinal lumen. The Appendix contains a microbiota as diverse as that found in the colon and could replenish the large intestine with healthy flora after a diarrhea episode. In conditions of modern medicine hygiene, and people live healthy without their appendix. However, several reports suggest that the effects of appendectomy could be subtler and associated with the development of inflammatory conditions such as inflammatory bowel disease (IBD), heart disease but also in less expected disorders such as Parkinson's disease. Lack of an Appendix also predicts a worsen outcome for recurrent Clostridium difficile infection, which is the first nosocomial infection in hospitals. Here, we review the literature and in combination with our own data, we suggest that the Appendix might be redundant in its immunological function but unique as a reservoir of microbiota., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Background: Induction of oral tolerance to haptens is an efficient way to prevent allergic contact dermatitis (ACD) in mice. Toll-like receptor (TLR)-mediated sensing of the microbiota contributes to gut homeostasis, yet whether it contributes to induction of oral tolerance has not been documented., Objective: We examined whether oral tolerance to the contact sensitizer 2,4-dinitro-fluorobenzene (DNFB) depends on microbiota/TLRs and evaluated the role of TLR4 on the tolerogenic function of intestinal dendritic cells (DCs)., Methods: Oral tolerance was induced by DNFB gavage in germ-free and mice deficient in several TLRs. Tolerance was assessed by means of suppression of contact hypersensitivity and hapten-specific IFN-γ-producing effector T cells. The tolerogenic function of intestinal DCs was tested by adoptive transfer experiments, ex vivo hapten presentation, and forkhead box p3 regulatory T-cell conversion., Results: Oral tolerance induced by DNFB gavage was impaired in germ-free mice and TLR4-deficient mice. Bone marrow chimeras revealed that TLR4 expression on hematopoietic cells was necessary for oral tolerance induction. TLR4 appeared to be essential for the ability of intestinal dendritic cells from DNFB-fed mice to inhibit ACD on adoptive transfer. Indeed, TLR4 conditioned the in vivo mobilization to mesenteric lymph nodes of intestinal migratory CD103 + DCs carrying oral DNFB, especially the CD103 + CD11b + DC subset expressing the vitamin A-converting enzyme retinaldehyde dehydrogenase and specialized in forkhead box p3-positive regulatory T-cell conversion., Conclusions: Our data demonstrate that TLR4 conditions induction of oral tolerance to DNFB through licensing tolerogenic gut DCs. Oral biotherapy with TLR4 ligands might be useful to potentiate oral tolerance to haptens and alleviate ACD in human subjects., (Copyright © 2017 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Mammalian body surfaces are inhabited by vast numbers of microbes, the commensal microbiota, which help the host to digest food, provide nutrients, and mature its immune system. For a long time, postnatal colonization was believed to be the main stimulus for microbial-induced immune development. Using a model of reversible colonization of germ-free mice during gestation, we recently showed that the microbial shaping of the neonatal immune system begins even before birth through molecular signals derived from the microbiota of the mother. Maternal microbiota was important to mature intestinal innate immune cells and to alter intestinal gene expression profiles in the offspring. These changes prepare the newborn for postnatal colonization. The majority of the gestational colonization-dependent effects required maternal antibodies. Here, we discuss and provide further evidence how maternal antibodies are important players in transferring a signal originating from the maternal intestinal microbiota to the offspring.

Postnatal colonization of the body with microbes is assumed to be the main stimulus to postnatal immune development. By transiently colonizing pregnant female mice, we show that the maternal microbiota shapes the immune system of the offspring. Gestational colonization increases intestinal group 3 innate lymphoid cells and F4/80(+)CD11c(+) mononuclear cells in the pups. Maternal colonization reprograms intestinal transcriptional profiles of the offspring, including increased expression of genes encoding epithelial antibacterial peptides and metabolism of microbial molecules. Some of these effects are dependent on maternal antibodies that potentially retain microbial molecules and transmit them to the offspring during pregnancy and in milk. Pups born to mothers transiently colonized in pregnancy are better able to avoid inflammatory responses to microbial molecules and penetration of intestinal microbes., (Copyright © 2016, American Association for the Advancement of Science.)

Background & Aims: Orally induced tolerance to environmental allergens prevents deleterious, systemic, delayed-type hypersensitivity responses via immune suppression mechanisms believed to include either anergy/deletion of specific effector T cells or active suppression by CD4(+)CD25(+) regulatory T cells (Tregs). The aim of this study was to investigate whether and how antigen (Ag) penetration through the gut orchestrates these 2 distinct mechanisms., Methods: Using a model of allergic contact dermatitis (ACD) mediated by hapten-specific cytolytic CD8(+) T cells and a T-cell transfer model of contact hypersensitivity in CD3epsilon-deficient mice, we studied the outcome of Ag gavage on CD8(+) effectors and Tregs., Results: Full protection from ACD by gavage with the relevant allergen required 2 coordinated events taking place first in gut-associated lymphoid tissues and then systemically. Allergen gavage induced deletion by plasmacytoid dendritic cells of a large fraction of Ag-specific CD8(+) T cells in liver and mesenteric lymph nodes and also triggered the suppressive function of Treg of secondary lymphoid organs. Residual Ag-specific CD8(+) T cells conditioned during this mucosal step are fully susceptible to suppression by activated Treg, which completely prevented their differentiation into ACD effectors, upon re-exposure to the allergen via the skin., Conclusions: Thus, oral tolerance initiated in gut-associated lymphoid tissues by the plasmacytoid dendritic cells-mediated deletion of Ag-specific T cells is completed systemically by CD4(+)CD25(+) T cells. Biotherapies able to increase the susceptibility of effector T cells to the suppressive function of Treg may be valuable for the treatment of autoimmune and inflammatory diseases.