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Much of the research on lung immunology has concentrated on classic hematopoietically derived cells of the immune system. In this Focus Review, Alenghat and Whitsett discuss the key innate immunological functions of the respiratory epithelium. The epithelial surfaces of the lungs are in direct contact with the environment and are subjected to dynamic physical forces as airway tubes and alveoli are stretched and compressed during ventilation. Mucociliary clearance in conducting airways, reduction of surface tension in the alveoli, and maintenance of near sterility have been accommodated by the evolution of a multi-tiered innate host-defense system. The biophysical nature of pulmonary host defenses are integrated with the ability of respiratory epithelial cells to respond to and 'instruct' the professional immune system to protect the lungs from infection and injury.

The gastrointestinal tract harbors trillions of microbial species, collectively termed the microbiota, which establish a symbiotic relationship with the host. Decades of research have emphasized the necessity of microbial signals in the development, maturation, and function of host physiology. However, changes in the composition or containment of the microbiota have been linked to the development of several chronic inflammatory diseases, including inflammatory bowel diseases. Intestinal epithelial cells (IECs) are in constant contact with the microbiota and are critical for maintaining intestinal homeostasis. Signals from the microbiota are directly sensed by IECs and influence intestinal health by calibrating immune cell responses and fortifying intestinal barrier function. IECs detect commensal microbes through engagement of common pattern recognition receptors or by sensing the production of microbial-derived metabolites. Deficiencies in these microbial-detecting pathways in IECs leads to impaired epithelial barrier function and altered intestinal homeostasis. This Review aims to highlight the pathways by which IECs sense microbiota-derived signals and the necessity of these detection pathways in maintaining epithelial barrier integrity.

Aberrant immune responses to resident microbes promote inflammatory bowel disease and other chronic inflammatory conditions. However, how microbiota-specific immunity is controlled in mucosal tissues remains poorly understood. Here, we found that mice lacking epithelial expression of microbiota-sensitive histone deacetylase 3 (HDAC3) exhibited increased accumulation of commensal-specific CD4+ T cells in the intestine, provoking the hypothesis that epithelial HDAC3 may instruct local microbiota-specific immunity. Consistent with this, microbiota-specific CD4+ T cells and epithelial HDAC3 expression were concurrently induced following early-life microbiota colonization. Further, epithelium-intrinsic ablation of HDAC3 decreased commensal-specific Tregs, increased commensal-specific Th17 cells, and promoted T cell-driven colitis. Mechanistically, HDAC3 was essential for NF-κB-dependent regulation of epithelial MHC class II (MHCII). Epithelium-intrinsic MHCII dampened local accumulation of commensal-specific Th17 cells in adult mice and protected against microbiota-triggered inflammation. Remarkably, HDAC3 enabled the microbiota to induce MHCII expression on epithelial cells and limit the number of commensal-specific T cells in the intestine. Collectively, these data reveal a central role for an epithelial histone deacetylase in directing the dynamic balance of tissue-intrinsic CD4+ T cell subsets that recognize commensal microbes and control inflammation.

Disruption of the circadian clock exacerbates metabolic diseases, including obesity and diabetes. We show that histone deacetylase 3 (HDAC3) recruitment to the genome displays a circadian rhythm in mouse liver. Histone acetylation is inversely related to HDAC3 binding, and this rhythm is lost when HDAC3 is absent. Although amounts of HDAC3 are constant, its genomic recruitment in liver corresponds to the expression pattern of the circadian nuclear receptor Rev-erbα. Rev-erbα colocalizes with HDAC3 near genes regulating lipid metabolism, and deletion of HDAC3 or Rev-erbα in mouse liver causes hepatic steatosis. Thus, genomic recruitment of HDAC3 by Rev-erbα directs a circadian rhythm of histone acetylation and gene expression required for normal hepatic lipid homeostasis.

The mammalian intestinal tract is colonized by trillions of beneficial commensal bacteria that are anatomically restricted to specific niches. However, the mechanisms that regulate anatomical containment remain unclear. Here, we show that interleukin-22 (IL-22)-producing innate lymphoid cells (ILCs) are present in intestinal tissues of healthy mammals. Depletion of ILCs resulted in peripheral dissemination of commensal bacteria and systemic inflammation, which was prevented by administration of IL-22. Disseminating bacteria were identified as Alcaligenes species originating from host lymphoid tissues. Alcaligenes was sufficient to promote systemic inflammation after ILC depletion in mice, and Alcaligenes-specific systemic immune responses were associated with Crohn's disease and progressive hepatitis C virus infection in patients. Collectively, these data indicate that ILCs regulate selective containment of lymphoid-resident bacteria to prevent systemic inflammation associated with chronic diseases.

Background and Aims Colorectal cancer (CRC) incidence is increasing in young patients without a clear etiology. Emerging data have implicated the fecal microbiome in CRC carcinogenesis. However, its impact on young onset CRC is poorly defined. Methods We performed a meta‐analysis of fecal metagenomics sequencing data from n = 692 patients with CRC and n = 602 healthy controls from eleven studies to evaluate features of the fecal metagenome associated with CRC. We hypothesized that known carcinogenic virulence factors (colibactin, fadA) and species abundance may be differentially enriched in young CRC patients relative to older CRC patients and controls. Results Summary odds ratios (OR) for CRC were increased with the presence of colibactin (OR 1.92 95% CI 1.08–3.38), fadA (OR 4.57 95% CI 1.63–12.85), and F. nucleatum (OR 6.93 95% CI 3.01–15.96) in meta‐analysis models adjusted for age, gender, and body mass index. The OR for CRC for the presence of E.coli was 2.02 (0.92–4.45). An increase in the prevalence of Fusobacterium nucleatum (OR = 1.40 [1.18; 1.65]) and Escherichia coli (OR = 1.14 [1.02; 1.28]) per 10‐year increase in age was observed in models including samples from both CRC and healthy controls. Species relative abundance was differentially enriched in young CRC patients for five species‐Intestinimonas butyriciproducens, Holdemania filiformis, Firimicutues bacterium CAG 83, Bilophilia wadsworthia, and Alistipes putredinis. Conclusion In this study, we observed strong associations with CRC status for colibactin, fadA, and Fusobacterium nucleatum with CRC relative to controls. In addition, we identified several microbial species differentially enriched in young colorectal cancer patients. Studies targeting the young CRC patients are warranted to elucidate underlying preclinical mechanisms. Differentially enriched bacterial species in young onset colorectal cancer patients.

Lean and quick Nuclear receptor corepressor 1 (Ncor1) is an activator for the enzyme histone deacetylase 3 (Hdac3) that is required for embryogenesis, but its physiological functions are unknown. Now experiments in knock-out mice lacking Ncor1 show that disruption of the Ncor1–Hdac3 interaction causes aberrant regulation of clock genes and results in abnormal circadian behaviour — with a sleep–wake cycle closer to 23 hours than the normal 24. These mice are also leaner than normal and more insulin sensitive as a result of increased energy expenditure. Loss of a functional Ncor1–Hdac3 complex in vivo changes the oscillatory patterns of several metabolic genes, demonstrating that circadian regulation of metabolism is critical for normal energy balance. Targeting of the Ncor1–Hdac3 enzyme could be a highly specific intervention in diseases of nutritional stress such as obesity and diabetes. This paper shows that specific genetic disruption of the Ncor–HdaC3 interaction in mice causes aberrant regulation of clock genes and results in abnormal circadian behaviour. These mice are also leaner and more insulin sensitive due to increased energy expenditure. Loss of a functional Ncor–HdaC3 complex in vivo changes the oscillatory patterns of several metabolic genes, demonstrating that circadian regulation of metabolism is critical for normal energy balance. Rhythmic changes in histone acetylation at circadian clock genes suggest that temporal modulation of gene expression is regulated by chromatin modifications 1 , 2 , 3 . Furthermore, recent studies demonstrate a critical relationship between circadian and metabolic physiology 4 , 5 , 6 , 7 . The nuclear receptor corepressor 1 (Ncor1) functions as an activating subunit for the chromatin modifying enzyme histone deacetylase 3 (Hdac3) 8 . Lack of Ncor1 is incompatible with life, and hence it is unknown whether Ncor1, and particularly its regulation of Hdac3, is critical for adult mammalian physiology 9 . Here we show that specific, genetic disruption of the Ncor1–Hdac3 interaction in mice causes aberrant regulation of clock genes and results in abnormal circadian behaviour. These mice are also leaner and more insulin-sensitive owing to increased energy expenditure. Unexpectedly, loss of a functional Ncor1–Hdac3 complex in vivo does not lead to sustained increases in known catabolic genes, but instead significantly alters the oscillatory patterns of several metabolic genes, demonstrating that circadian regulation of metabolism is critical for normal energy balance. These findings indicate that activation of Hdac3 by Ncor1 is a nodal point in the epigenetic regulation of circadian and metabolic physiology.

Although diet has long been associated with susceptibility to infection, the dietary components that regulate host defense remain poorly understood. Here, we demonstrate that consuming rice bran decreases susceptibility to intestinal infection with Citrobacter rodentium , a murine pathogen that is similar to enteropathogenic E. coli infection in humans. Rice bran naturally contains high levels of the substance phytate. Interestingly, phytate supplementation also protected against intestinal infection, and enzymatic metabolism of phytate by commensal bacteria was necessary for phytate-induced host defense. Mechanistically, phytate consumption induced mammalian intestinal epithelial expression of STAT3-regulated antimicrobial pathways and increased phosphorylated STAT3, suggesting that dietary phytate promotes innate defense through epithelial STAT3 activation. Further, phytate regulation of epithelial STAT3 was mediated by the microbiota-sensitive enzyme histone deacetylase 3 (HDAC3). Collectively, these data demonstrate that metabolism of dietary phytate by microbiota decreases intestinal infection and suggests that consuming bran and other phytate-enriched foods may represent an effective dietary strategy for priming host immunity.

Numerous bacterial pathogens infect the mammalian host by initially associating with epithelial cells that line the intestinal lumen. Recent work has revealed that commensal bacteria that reside in the intestine promote defense against pathogenic infection, however whether the microbiota direct host pathways that alter pathogen adherence is not well-understood. Here, by comparing germ-free mice, we identify that the microbiota decrease bacterial pathogen adherence and dampen epithelial expression of the cell surface glycoprotein C-type lectin 2e (Clec2e). Functional studies revealed that overexpression of this lectin promotes adherence of intestinal bacterial pathogens to mammalian cells. Interestingly, microbiota-sensitive downregulation of Clec2e corresponds with decreased histone acetylation of the Clec2e gene in intestinal epithelial cells. Histone deacetylation and transcriptional regulation of Clec2e depends on expression and recruitment of the histone deacetylase HDAC3. Thus, commensal bacteria epigenetically instruct epithelial cells to decrease expression of a C-type lectin that promotes pathogen adherence, revealing a novel mechanism for how the microbiota promote innate defense against infection.

Unliganded thyroid hormone receptor (TR) actively represses transcription via the nuclear receptor corepressor (N‐CoR)/histone deacetylase 3 (HDAC3) complex. Although transcriptional activation by liganded receptors involves chromatin remodeling, the role of ATP‐dependent remodeling in receptor‐mediated repression is unknown. Here we report that SNF2H, the mammalian ISWI chromatin remodeling ATPase, is critical for repression of a genomically integrated, TR‐regulated reporter gene. N‐CoR and HDAC3 are both required for recruitment of SNF2H to the repressed gene. SNF2H does not interact directly with the N‐CoR/HDAC3 complex, but binds to unacetylated histone H4 tails, suggesting that deacetylase activity of the corepressor complex is critical to SNF2H function. Indeed, HDAC3 as well as SNF2H are required for nucleosomal organization on the TR target gene. Consistent with these findings, reduction of SNF2H induces expression of an endogenous TR‐regulated gene, dio1 , in liver cells. Thus, although not apparent from studies of transiently transfected reporter genes, gene repression by TR involves the targeting of chromatin remodeling factors to repressed genes by the HDAC activity of nuclear receptor corepressors.