Summary
The intestinal epithelium forms a barrier between the microbiota and the rest of the body. In addition, beyond acting as a physical barrier, the function of intestinal epithelial cells (IECs) ...in sensing and responding to microbial signals is increasingly appreciated and likely has numerous implications for the vast network of immune cells within and below the intestinal epithelium. IECs also respond to factors produced by immune cells, and these can regulate IEC barrier function, proliferation and differentiation, as well as influence the composition of the microbiota. The mechanisms involved in IEC–microbe–immune interactions, however, are not fully characterized. In this review, we explore the ability of IECs to direct intestinal homeostasis by orchestrating communication between intestinal microbes and mucosal innate and adaptive immune cells during physiological and inflammatory conditions. We focus primarily on the most recent findings and call attention to the numerous remaining unknowns regarding the complex crosstalk between IECs, the microbiota and intestinal immune cells.
Beyond acting as a physical barrier, the function of intestinal epithelial cells (IECs) in sensing and responding to microbial signals is increasingly appreciated and likely has numerous implications for the vast network of immune cells within and below the intestinal epithelium. In this review, we explore the ability of IECs to direct intestinal homeostasis by orchestrating communication between intestinal microbes and mucosal innate and adaptive immune cells during physiological and inflammatory conditions. We focus primarily on the most recent findings and call attention to the numerous remaining unknowns regarding the complex crosstalk between IECs, the microbiota and intestinal immune cells.
The intestinal microbiome modulates host susceptibility to enteric pathogens, but the specific protective factors and mechanisms of individual bacterial species are not fully characterized. We show ...that secreted antigen A (SagA) from Enterococcus faecium is sufficient to protect Caenorhabditis elegans against Salmonella pathogenesis by promoting pathogen tolerance. The NlpC/p60 peptidoglycan hydrolase activity of SagA is required and generates muramylpeptide fragments that are sufficient to protect C. elegans against Salmonella pathogenesis in a tol-1-dependent manner. SagA can also be heterologously expressed and secreted to improve the protective activity of probiotics against Salmonella pathogenesis in C. elegans and mice. Our study highlights how protective intestinal bacteria can modify microbialassociated molecular patterns to enhance pathogen tolerance.
Microbiota generates millimolar concentrations of short-chain fatty acids (SCFAs) that can modulate host metabolism, immunity and susceptibility to infection. Butyrate in particular can function as a ...carbon source and anti-inflammatory metabolite, but the mechanism by which it inhibits pathogen virulence has been elusive. Using chemical proteomics, we found that several virulence factors encoded by Salmonella pathogenicity island-1 (SPI-1) are acylated by SCFAs. Notably, a transcriptional regulator of SPI-1, HilA, was acylated on several key lysine residues. Subsequent incorporation of stable butyryl-lysine analogs using CRISPR-Cas9 gene editing and unnatural amino acid mutagenesis revealed that site-specific modification of HilA impacts its genomic occupancy, expression of SPI-1 genes and attenuates Salmonella enterica serovar Typhimurium invasion of epithelial cells, as well as dissemination in vivo. Moreover, a multiple-site HilA lysine acylation mutant strain of S. Typhimurium was resistant to butyrate inhibition ex vivo and microbiota attenuation in vivo. Our results suggest that prominent microbiota-derived metabolites may directly acylate virulence factors to inhibit microbial pathogenesis in vivo.
Intestinal intraepithelial lymphocytes (IELs) are located at the critical interface between the intestinal lumen, which is chronically exposed to food and microbes, and the core of the body. Using ...high-resolution microscopy techniques and intersectional genetic tools, we investigated the nature of IEL responses to luminal microbes. We observed that TCRγδ IELs exhibit unique microbiota-dependent location and movement patterns in the epithelial compartment. This behavioral pattern quickly changes upon exposure to different enteric pathogens, resulting in increased interepithelial cell (EC) scanning, expression of antimicrobial genes, and glycolysis. Both dynamic and metabolic changes to γδ IEL depend on pathogen sensing by ECs. Direct modulation of glycolysis is sufficient to change γδ IEL behavior and susceptibility to early pathogen invasion. Our results uncover a coordinated EC-IEL response to enteric infections that modulates lymphocyte energy utilization and dynamics and supports maintenance of the intestinal epithelial barrier.
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•γδ IELs display microbe-dependent localization and motility patterns•Infection induces rapid behavioral changes in γδ IELs•Coordinated epithelial cell-IEL responses to pathogens are EC-Myd88 dependent•EC responses to microbial cues trigger a protective IEL metabolic switch
Communication between epithelial cells and intraepithelial lymphocytes guides lymphocyte energy utilization and dynamic behavior, helping to support maintenance of the intestinal epithelial barrier during infections.
Conventional dendritic cells (cDCs) play an essential role in host immunity by initiating adaptive T cell responses and by serving as innate immune sensors. Although both innate and adaptive ...functions of cDCs are well documented, their relative importance in maintaining immune homeostasis is poorly understood. To examine the significance of cDC-initiated adaptive immunity in maintaining homeostasis, independent of their innate activities, we generated a cDC-specific Cre mouse and crossed it to a floxed MHC class II (MHCII) mouse. Absence of MHCII on cDCs resulted in chronic intestinal inflammation that was alleviated by antibiotic treatment and entirely averted under germ-free conditions. Uncoupling innate and adaptive functions of cDCs revealed that innate immune functions of cDCs are insufficient to maintain homeostasis and antigen presentation by cDCs is essential for a mutualistic relationship between the host and intestinal bacteria.
CTLA-4, an Ig superfamily molecule with homology to CD28, is one of the most potent negative regulators of T-cell responses. In vivo blockade of CTLA-4 exacerbates autoimmunity, enhances ...tumor-specific T-cell responses, and may inhibit the induction of T-cell anergy. Clinical trials of CTLA-4—blocking antibodies to augment T-cell responses to malignant melanoma are at an advanced stage; however, little is known about the effects of CTLA-4 blockade on memory CD8 + T-cell responses and the formation and maintenance of long-term CD8 + T-cell memory. In our studies, we show that during in vivo memory CD8 + T-cell responses to Listeria monocytogenes infection, CTLA-4 blockade enhances bacterial clearance and increases memory CD8 + T-cell expansion. This is followed by an accumulation of memory cells that are capable of producing the effector cytokines IFN-γ and TNF-α. We also demonstrate that in a vaccination setting, blocking CTLA-4 during CD8 + T-cell priming leads to increased expansion and maintenance of antigen-specific memory CD8 + T cells without adversely affecting the overall T-cell repertoire. This leads to an increase in memory cell effector function and improved protective immunity against further bacterial challenges. These results indicate that transient blockade of CTLA-4 enhances memory CD8 + T-cell responses and support the possible use of CTLA-4—blocking antibodies during vaccination to augment memory formation and maintenance.
We discovered that
(
), a ubiquitous commensal bacterium, and its secreted peptidoglycan hydrolase (SagA) were sufficient to enhance intestinal barrier function and pathogen tolerance, but the ...precise biochemical mechanism was unknown. Here we show
has unique peptidoglycan composition and remodeling activity through SagA, which generates smaller muropeptides that more effectively activates nucleotide-binding oligomerization domain-containing protein 2 (NOD2) in mammalian cells. Our structural and biochemical studies show that SagA is a NlpC/p60-endopeptidase that preferentially hydrolyzes crosslinked Lys-type peptidoglycan fragments. SagA secretion and NlpC/p60-endopeptidase activity was required for enhancing probiotic bacteria activity against
pathogenesis
. Our results demonstrate that the peptidoglycan composition and hydrolase activity of specific microbiota species can activate host immune pathways and enhance tolerance to pathogens.
Gut bacteria are known to affect immune cell development, but most intestinal lymphocytes have no direct contact with luminal bacteria. Two studies by Atarashi et al. and Sano et al. shed light on ...how bacterial adhesion can cue intestinal epithelial cells to direct differentiation of gut T cells.
Gut bacteria are known to affect immune cell development, but most intestinal lymphocytes have no direct contact with luminal bacteria. Two studies by Atarashi et al. and Sano et al. shed light on how bacterial adhesion can cue intestinal epithelial cells to direct differentiation of gut T cells.
Human health and disease have increasingly been shown to be impacted by the gut microbiota, and mouse models are essential for investigating these effects. However, the compositions of human and ...mouse gut microbiotas are distinct, limiting translation of microbiota research between these hosts. To address this, we constructed the Mouse Gastrointestinal Bacteria Catalogue (MGBC), a repository of 26,640 high-quality mouse microbiota-derived bacterial genomes. This catalog enables species-level analyses for mapping functions of interest and identifying functionally equivalent taxa between the microbiotas of humans and mice. We have complemented this with a publicly deposited collection of 223 bacterial isolates, including 62 previously uncultured species, to facilitate experimental investigation of individual commensal bacteria functions in vitro and in vivo. Together, these resources provide the ability to identify and test functionally equivalent members of the host-specific gut microbiotas of humans and mice and support the informed use of mouse models in human microbiota research.
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•MGBC allows functional translation between human and mouse microbiotas•Previously uncultured isolates expand culture collection of mouse gut microbes•Bioinformatic toolkit maps taxonomic locations of microbial functions of interest
Beresford-Jones et al. find that while less than 3% of bacterial species are shared between human and mouse gut microbiotas, they can identify the closest functionally related species between these host-specific microbiotas using the bacterial genome catalog they developed. This will facilitate translation of microbiota-related research between humans and mice.
During primary Ag encounter, T cells receive numerous positive and negative signals that control their proliferation, function, and differentiation, but how these signals are integrated to modulate T ...cell memory has not been fully characterized. In these studies, we demonstrate that combining seemingly opposite signals, CTLA-4 blockade and rapamycin-mediated mammalian target of rapamycin inhibition, during in vivo T cell priming leads to both an increase in the frequency of memory CD8(+) T cells and improved memory responses to tumors and bacterial challenges. This enhanced efficacy corresponds to increased early expansion and memory precursor differentiation of CD8(+) T cells and increased mitochondrial biogenesis and spare respiratory capacity in memory CD8(+) T cells in mice treated with anti-CTLA-4 and rapamycin during immunization. Collectively, these results reveal that mammalian target of rapamycin inhibition cooperates with rather than antagonizes blockade of CTLA-4, promoting unrestrained effector function and proliferation, and an optimal metabolic program for CD8(+) T cell memory.
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