Regulation of immune responses by tuft cells

The gut microbiome and hypertension

A large body of evidence has emerged in the past decade supporting a role for the gut microbiome in the regulation of blood pressure. The field has moved from association to causation in the last 5 years, with studies that have used germ-free animals, antibiotic treatments and direct supplementation with microbial metabolites. The gut microbiome can regulate blood pressure through several mechanisms, including through gut dysbiosis-induced changes in microbiome-associated gene pathways in the host. Microbiota-derived metabolites are either beneficial (for example, short-chain fatty acids and indole-3-lactic acid) or detrimental (for example, trimethylamine N-oxide), and can activate several downstream signalling pathways via G protein-coupled receptors or through direct immune cell activation. Moreover, dysbiosis-associated breakdown of the gut epithelial barrier can elicit systemic inflammation and disrupt intestinal mechanotransduction. These alterations activate mechanisms that are traditionally associated with blood pressure regulation, such as the renin-angiotensin-aldosterone system, the autonomic nervous system, and the immune system. Several methodological and technological challenges remain in gut microbiome research, and the solutions involve minimizing confounding factors, establishing causality and acting globally to improve sample diversity. New clinical trials, precision microbiome medicine and computational methods such as Mendelian randomization have the potential to enable leveraging of the microbiome for translational applications to lower blood pressure.

Deoxycholic acid exacerbates intestinal inflammation by modulating interleukin-1β expression and tuft cell proportion in dextran sulfate sodium-induced murine colitis

Background

The etiology of inflammatory bowel disease (IBD) remains unclear. However, intestinal metabolism is known to be critical in the pathogenesis of IBD. Bile acid is one of the main intestinal metabolites, and its role in the pathogenesis of IBD is worthy of investigation. This study investigated the role of deoxycholic acid (DCA), a bile acid, in the pathogenesis of IBD.

Methods

Peripheral serum metabolomics, fecal metabolomics, and microbiome analyses were performed on patients with IBD and healthy controls. Flow cytometry, real-time quantitative polymerase chain reaction, western blotting, enzyme-linked immunosorbent assay, immunohistochemical staining, and immunofluorescence analysis were used to evaluate cytokines in the inflamed colonic mucosa and immune cells and tuft cells in the intestine of mice with dextran sulfate sodium (DSS)-induced colitis.

Results

In total, 156 patients with IBD and 58 healthy controls were enrolled. DCA levels in the serum and feces of patients with IBD were significantly decreased compared to the controls. This decrease was associated with a decrease in the abundance of intestinal flora, including Firmicutes, Clostridia, Ruminnococcaceae, and Lachnospiraceae. Additionally, interleukin (IL)-1β levels in the serum of patients with active Crohn's disease were significantly increased compared with the healthy controls. Moreover, in DCA-treated DSS-induced mice, the expression of IL-1β and the proportion of CD3⁺ and CD4⁺ T cells increased while the number of intestinal tuft cells decreased, compared with the DSS group.

Conclusion

In IBD patients, the decreased DCA levels in serum and fecal samples are associated with disturbances in gut microflora diversity and abundance. Possible mechanisms by which DCA affects immunity in DSS-induced murine colitis include increasing IL-1β secretion, reducing the number of tuft cells in the mucosa, and activating CD4⁺ and CD3⁺ T cells to exaggerate immune responses, consequently worsening intestinal inflammation.

Inflammation induces bitter taste oversensitization via epigenetic changes in Tas2r gene clusters

T2R bitter receptors, encoded by Tas2r genes, are not only critical for bitter taste signal transduction but also important for defense against bacteria and parasites. However, little is known about whether and how Tas2r gene expression are regulated. Here we show that, in an inflammation model mimicking bacterial infection, the expression of many Tas2rs are significantly up-regulated and mice displayed markedly increased neural and behavioral responses to bitter compounds. Using single-cell assays for transposase-accessible chromatin with sequencing (scATAC-seq), we found that the chromatin accessibility of Tas2rs was highly cell type specific and inflammation increased the accessibility of many Tas2rs . scATAC-seq also revealed substantial chromatin remodeling in immune response genes in taste tissue stem cells, suggesting potential long-term effects. Together, our results suggest an epigenetic mechanism connecting inflammation, Tas2r gene regulation, and altered bitter taste, which may explain heightened bitter taste that can occur with infections and cancer treatments.

Interleukin 25 and its biological features and function in intestinal diseases

Interleukin 25 (IL-25), also known as IL-17E, is a member of the IL-17 cytokine family and an important regulator of the type 2 immune response. Accumulating evidence suggests that IL-25 interacts with diverse immune as well as non-immune cells and plays a rather complicated role in different backgrounds of multiple organs. IL-25 has been studied in the physiology and pathology of the intestine to some extent. With epithelial cells being an important source in the intestine, IL-25 plays a key role in intestinal immune responses and is associated with inappropriate allergic reactions, autoimmune diseases, and cancer tumorigenesis. In this review, we discuss the emerging comprehension of the biology of IL-25, as well as its cellular sources, targets, and signaling transduction. In particular, we discuss how IL-25 participates in the development of intestinal diseases including helminth infection, inflammatory bowel diseases, food allergy and colorectal cancer, as well as its underlying role in future therapy.

Tuft Cells: Context- and Tissue-Specific Programming for a Conserved Cell Lineage

Tuft cells are found in tissues with distinct stem cell compartments, tissue architecture, and luminal exposures but converge on a shared transcriptional program, including expression of taste transduction signaling pathways. Here, we summarize seminal and recent findings on tuft cells, focusing on major categories of function-instigation of type 2 cytokine responses, orchestration of antimicrobial responses, and emerging roles in tissue repair-and describe tuft cell-derived molecules used to affect these functional programs. We review what is known about the development of tuft cells from epithelial progenitors under homeostatic conditions and during disease. Finally, we discuss evidence that immature, or nascent, tuft cells with potential for diverse functions are driven toward dominant effector programs by tissue- or perturbation-specific contextual cues, which may result in heterogeneous mature tuft cell phenotypes both within and between tissues.

The intestine: A highly dynamic microenvironment for IgA plasma cells

To achieve longevity, IgA plasma cells require a sophisticated anatomical microenvironment that provides cytokines, cell-cell contacts, and nutrients as well as metabolites. The intestinal epithelium harbors cells with distinct functions and represents an important defense line. Anti-microbial peptide-producing paneth cells, mucus-secreting goblet cells and antigen-transporting microfold (M) cells cooperate to build a protective barrier against pathogens. In addition, intestinal epithelial cells are instrumental in the transcytosis of IgA to the gut lumen, and support plasma cell survival by producing the cytokines APRIL and BAFF. Moreover, nutrients are sensed through specialized receptors such as the aryl hydrocarbon receptor (AhR) by both, intestinal epithelial cells and immune cells. However, the intestinal epithelium is highly dynamic with a high cellular turn-over rate and exposure to changing microbiota and nutritional factors. In this review, we discuss the spatial interplay of the intestinal epithelium with plasma cells and its potential contribution to IgA plasma cell generation, homing, and longevity. Moreover, we describe the impact of nutritional AhR ligands on intestinal epithelial cell-IgA plasma cell interaction. Finally, we introduce spatial transcriptomics as a new technology to address open questions in intestinal IgA plasma cell biology.

Revelations in Thymic Epithelial Cell Biology and Heterogeneity from Single-Cell RNA Sequencing and Lineage Tracing Methodologies

Thymic epithelial cells (TECs) make up the thymic microenvironments that support the generation of a functionally competent and self-tolerant T-cell repertoire. Cortical (c)TECs, present in the cortex, are essential for early thymocyte development including selection of thymocytes expressing functional TCRs (positive selection). Medullary (m)TECs, located in the medulla, play a key role in late thymocyte development, including depletion of self-reactive T cells (negative selection) and selection of regulatory T cells. In recent years, transcriptomic analysis by single-cell (sc)RNA sequencing (Seq) has revealed TEC heterogeneity previously masked by population-level RNA-Seq or phenotypic studies. We summarize the discoveries made possible by scRNA-Seq, including the identification of novel mTEC subsets, advances in understanding mTEC promiscuous gene expression, and TEC alterations from embryonic to adult stages. Whereas pseudotime analyses of scRNA-Seq data can suggest relationships between TEC subsets, experimental methods such as lineage tracing and reaggregate thymic organ culture (RTOC) are required to test these hypotheses. Lineage tracing - namely, of β5t or Aire expressing cells - has exposed progenitor and parent-daughter cellular relationships within TEC.

The shaping of gut immunity in cirrhosis

Cirrhosis is the common end-stage of chronic liver diseases of different etiology. The altered bile acids metabolism in the cirrhotic liver and the increase in the blood-brain barrier permeability, along with the progressive dysbiosis of intestinal microbiota, contribute to gut immunity changes, from compromised antimicrobial host defense to pro-inflammatory adaptive responses. In turn, these changes elicit a disruption in the epithelial and gut vascular barriers, promoting the increased access of potential pathogenic microbial antigens to portal circulation, further aggravating liver disease. After summarizing the key aspects of gut immunity during homeostasis, this review is intended to update the contribution of liver and brain metabolites in shaping the intestinal immune status and, in turn, to understand how the loss of homeostasis in the gut-associated lymphoid tissue, as present in cirrhosis, cooperates in the advanced chronic liver disease progression. Finally, several therapeutic approaches targeting the intestinal homeostasis in cirrhosis are discussed.

The critical roles and therapeutic implications of tuft cells in cancer

Tuft cells are solitary chemosensory epithelial cells with microvilli at the top, which are found in hollow organs such as the gastrointestinal tract, pancreas, and lungs. Recently, an increasing number of studies have revealed the chemotactic abilities and immune function of the tuft cells, and numerous efforts have been devoted to uncovering the role of tuft cells in tumors. Notably, accumulating evidence has shown that the specific genes (POU2F3, DCLK1) expressed in tuft cells are involved in vital processes related with carcinogenesis and cancer development. However, the interaction between the tuft cells and cancer remains to be further elucidated. Here, based on an introduction of biological functions and specific markers of the tuft cells, we have summarized the functional roles and potential therapeutic implications of tuft cells in cancers, including pancreatic cancer, lung cancer, gastric cancer, colon cancer, and liver cancer, which is in the hope of inspiring the future research in validating tuft cells as novel strategies for cancer therapies.

c-MAF coordinates enterocyte zonation and nutrient uptake transcriptional programs

Small intestinal villi are structural and functional units present in higher vertebrates and uniquely adapted to nutrient absorption. Villus enterocytes are organized in transcriptional "zones" dedicated to specialized tasks such as absorption of specific nutrients. We report that the transcription factor c-MAF is expressed in differentiated lower and mid-villus enterocytes and is a target of BMP signaling. Maf inactivation perturbed the villus zonation program by increasing carbohydrate-related transcripts while suppressing transcripts linked to amino-acid and lipid absorption. The formation of cytoplasmic lipid droplets, shuttling dietary fat to chylomicrons, was impaired upon Maf loss indicating its role in dietary lipid handling. Maf inactivation under homeostatic conditions expanded tuft cells and led to compensatory gut lengthening, preventing weight loss. However, delayed Maf-/- enterocyte maturation impaired weight recovery after acute intestinal injury, resulting in reduced survival. Our results identify c-MAF as a regulator of the intestinal villus zonation program, while highlighting the importance of coordination between stem/progenitor and differentiation programs for intestinal regeneration.

Pulmonary cancers across different histotypes share hybrid tuft cell/ionocyte-like molecular features and potentially druggable vulnerabilities

Tuft cells are chemosensory epithelial cells in the respiratory tract and several other organs. Recent studies revealed tuft cell-like gene expression signatures in some pulmonary adenocarcinomas, squamous cell carcinomas (SQCC), small cell carcinomas (SCLC), and large cell neuroendocrine carcinomas (LCNEC). Identification of their similarities could inform shared druggable vulnerabilities. Clinicopathological features of tuft cell-like (tcl) subsets in various lung cancer histotypes were studied in two independent tumor cohorts using immunohistochemistry (n = 674 and 70). Findings were confirmed, and additional characteristics were explored using public datasets (RNA seq and immunohistochemical data) (n = 555). Drug susceptibilities of tuft cell-like SCLC cell lines were also investigated. By immunohistochemistry, 10-20% of SCLC and LCNEC, and approximately 2% of SQCC expressed POU2F3, the master regulator of tuft cells. These tuft cell-like tumors exhibited "lineage ambiguity" as they co-expressed NCAM1, a marker for neuroendocrine differentiation, and KRT5, a marker for squamous differentiation. In addition, tuft cell-like tumors co-expressed BCL2 and KIT, and tuft cell-like SCLC and LCNEC, but not SQCC, also highly expressed MYC. Data from public datasets confirmed these features and revealed that tuft cell-like SCLC and LCNEC co-clustered on hierarchical clustering. Furthermore, only tuft cell-like subsets among pulmonary cancers significantly expressed FOXI1, the master regulator of ionocytes, suggesting their bidirectional but immature differentiation status. Clinically, tuft cell-like SCLC and LCNEC had a similar prognosis. Experimentally, tuft cell-like SCLC cell lines were susceptible to PARP and BCL2 co-inhibition, indicating synergistic effects. Taken together, pulmonary tuft cell-like cancers maintain histotype-related clinicopathologic characteristics despite overlapping unique molecular features. From a therapeutic perspective, identification of tuft cell-like LCNECs might be crucial given their close kinship with tuft cell-like SCLC.

Ischemic Stroke Impacts the Gut Microbiome, Ileal Epithelial and Immune Homeostasis

Ischemic stroke critically impacts neurovascular homeostasis, potentially resulting in neurological disorders. However, the mechanisms through which stroke-induced inflammation modifies the molecular and metabolic circuits, particularly in ileal epithelial cells (iECs), currently remain elusive. Using multiomic approaches, we illustrated that stroke impaired the ileal microbiome and associated metabolites, leading to increased inflammatory signals and altered metabolites, potentially deteriorating the iEC homeostasis. Bulk transcriptomic and metabolomic profiling demonstrated that stroke enhanced fatty acid oxidation while reducing the tricarboxylic acid (TCA) cycle in iECs within the first day after stroke. Intriguingly, single-cell RNA sequencing analysis revealed that stroke dysregulated cell-type-specific gene responses within iECs and reduced frequencies of goblet and tuft cells. Additionally, stroke augmented interleukin-17A+ γδ T cells but decreased CD4+ T cells in the ileum. Collectively, our findings provide a comprehensive overview of stroke-induced intestinal dysbiosis and unveil responsive gene programming within iECs with implications for disease development.

Tuft cell-like carcinomas: novel cancer subsets present in multiple organs sharing a unique gene expression signature

BACKGROUND

Tuft cells are chemosensory epithelial cells playing a role in innate immunity. Recent studies revealed cancers with a tuft cell-like gene expression signature in the thorax. We wondered whether this signature might also occur in extrathoracic cancers.

METHODS

We examined mRNA expression of tuft cell markers (POU2F3, GFI1B, TRPM5, SOX9, CHAT, and AVIL) in 19 different types of cancers in multiple extrathoracic organs with The Cancer Genome Atlas (TCGA) (N = 6322). Four different extrathoracic cancers in our local archives (N = 909) were analysed by immunohistochemistry.

RESULTS

Twenty-two (0.35%) extrathoracic tumours with co-expression of POU2F3 and other tuft cell markers were identified in various TCGA datasets. Twelve of the 22 "tuft cell-like tumours" shared poor differentiation and a gene expression pattern, including KIT, anti-apoptotic BCL2, and ionocyte-associated genes. In our archival cases, eleven (1.21%) tumours co-expressing POU2F3, KIT, and BCL2 on immunohistochemistry, i.e., were presumable tuft cell-like cancers. In three among five TCGA cohorts, the tuft cell-like cancer subsets expressed SLFN11, a promising biomarker of PARP inhibitor susceptibility.

CONCLUSIONS

Tuft cell-like carcinomas form distinct subsets in cancers of many organs. It appears warranted to investigate their shared gene expression signature as a predictive biomarker for novel therapeutic strategies.

Duodenal cholinergic tuft cell number is increased in functional dyspepsia

BACKGROUND

Low-grade duodenal inflammation has recently been identified in patients with functional dyspepsia (FD). Chemosensory tuft cells were reported to be associated with gastrointestinal diseases. We therefore assessed duodenal tuft cell density and microinflammation in patients with FD to determine whether these measures could serve as useful biomarkers, and also correlated tuft cell density and microinflammation in FD patients.

METHODS

Duodenal biopsy specimens were obtained from patients with FD and from controls. Tuft cells, eosinophils, and mast cells were immunochemically stained with specific antibodies. Tuft cells were identified by immunostaining for doublecortin-like kinase 1 (DCLK1); cholinergic tuft cells were assessed by double staining for choline acetyltransferase (ChAT) and DCLK1. Immune-type tuft cells were assessed by IL-25 mRNA expression using real-time PCR.

KEY RESULTS

The density of intramucosal eosinophils and mast cells was significantly higher in the duodenum of FD patients than in controls. The density of tuft cells was significantly higher in the duodenum of FD patients compared with controls, and significantly correlated with eosinophil density in the duodenum of FD patients and controls. Moreover, a fraction of ChAT-positive cells was DCLK1 positive; all duodenal DCLK1+ tuft cells were ChAT-immunoreactive in FD and in control subjects.

CONCLUSIONS AND INFERENCES

Cholinergic tuft cell density was higher in the duodenum of patients with FD and significantly correlated with eosinophil density. Further studies are needed to investigate the pathophysiological significance of tuft cells in FD and may provide valuable clues to the pathophysiology of FD.

Intestinal cellular heterogeneity and disease development revealed by single-cell technology

The intestinal epithelium is responsible for food digestion and nutrient absorption and plays a critical role in hormone secretion, microorganism defense, and immune response. These functions depend on the integral single-layered intestinal epithelium, which shows diversified cell constitution and rapid self-renewal and presents powerful regeneration plasticity after injury. Derailment of homeostasis of the intestine epithelium leads to the development of diseases, most commonly including enteritis and colorectal cancer. Therefore, it is important to understand the cellular characterization of the intestinal epithelium at the molecular level and the mechanisms underlying its homeostatic maintenance. Single-cell technologies allow us to gain molecular insights at the single-cell level. In this review, we summarize the single-cell RNA sequencing applications to understand intestinal cell characteristics, spatiotemporal evolution, and intestinal disease development.

Store-operated calcium entry controls innate and adaptive immune cell function in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by dysregulated intestinal immune responses. Using mass cytometry (CyTOF) to analyze the immune cell composition in the lamina propria (LP) of patients with ulcerative colitis (UC) and Crohn's disease (CD), we observed an enrichment of CD4⁺ effector T cells producing IL‐17A and TNF, CD8⁺ T cells producing IFNγ, T regulatory (Treg) cells, and innate lymphoid cells (ILC). The function of these immune cells is regulated by store‐operated Ca²⁺ entry (SOCE), which results from the opening of Ca²⁺ release‐activated Ca²⁺ (CRAC) channels formed by ORAI and STIM proteins. We observed that the pharmacologic inhibition of SOCE attenuated the production of proinflammatory cytokines including IL‐2, IL‐4, IL‐6, IL‐17A, TNF, and IFNγ by human colonic T cells and ILCs, reduced the production of IL‐6 by B cells and the production of IFNγ by myeloid cells, but had no effect on the viability, differentiation, and function of intestinal epithelial cells. T cell‐specific deletion of CRAC channel genes in mice showed that Orai1, Stim1, and Stim2‐deficient T cells have quantitatively distinct defects in SOCE, which correlate with gradually more pronounced impairment of cytokine production by Th1 and Th17 cells and the severity of IBD. Moreover, the pharmacologic inhibition of SOCE with a selective CRAC channel inhibitor attenuated IBD severity and colitogenic T cell function in mice. Our data indicate that SOCE inhibition may be a suitable new approach for the treatment of IBD.

OCA-T1 and OCA-T2 are coactivators of POU2F3 in the tuft cell lineage

Tuft cells are a rare chemosensory lineage that coordinates immune and neural responses to foreign pathogens in mucosal tissues1. Recent studies have also revealed tuft-cell-like human tumours2,3, particularly as a variant of small-cell lung cancer. Both normal and neoplastic tuft cells share a genetic requirement for the transcription factor POU2F3 (refs. 2,4), although the transcriptional mechanisms that generate this cell type are poorly understood. Here we show that binding of POU2F3 to the uncharacterized proteins C11orf53 and COLCA2 (renamed here OCA-T1/POU2AF2 and OCA-T2/POU2AF3, respectively) is critical in the tuft cell lineage. OCA-T1 and OCA-T2 are paralogues of the B-cell-specific coactivator OCA-B; all three proteins are encoded in a gene cluster and contain a conserved peptide that binds to class II POU transcription factors and a DNA octamer motif in a bivalent manner. We demonstrate that binding between POU2F3 and OCA-T1 or OCA-T2 is essential in tuft-cell-like small-cell lung cancer. Moreover, we generated OCA-T1-deficient mice, which are viable but lack tuft cells in several mucosal tissues. These findings reveal that the POU2F3-OCA-T complex is the master regulator of tuft cell identity and a molecular vulnerability of tuft-cell-like small-cell lung cancer.

Therapeutic Effect of Renifolin F on Airway Allergy in an Ovalbumin-Induced Asthma Mouse Model In Vivo

Renifolin F is a prenylated chalcone isolated from Shuteria involucrata, a traditional minority ethnic medicine used to treat the respiratory diseases and asthma. Based on the effects of the original medicine plant, we established an in vivo mouse model of allergic asthma using ovalbumin (OVA) as an inducer to evaluate the therapeutic effects of Renifolin F. In the research, mice were sensitized and challenged with OVA to establish an allergic asthma model to evaluate the effects of Renifolin F on allergic asthma. The airway hyper-reactivity (AHR) to methacholine, cytokine levels, ILC2s quantity and mircoRNA-155 expression were assessed. We discovered that Renifolin F attenuated AHR and airway inflammation in the OVA-induced asthmatic mouse model by inhibiting the regulation of ILC2s in the lung, thereby, reducing the upstream inflammatory cytokines IL-25, IL-33 and TSLP; the downstream inflammatory cytokines IL-4, IL-5, IL-9 and IL-13 of ILC2s; and the co-stimulatory factors IL-2 and IL-7; as well as the expression of microRNA-155 in the lung. The findings suggest a therapeutic potential of Renifolin F on OVA-induced airway inflammation.

The IL-25-dependent tuft cell circuit driven by intestinal helminths requires macrophage migration inhibitory factor (MIF)

Macrophage migration inhibitory factor (MIF) is a key innate immune mediator with chemokine- and cytokine-like properties in the inflammatory pathway. While its actions on macrophages are well-studied, its effects on other cell types are less understood. Here we report that MIF is required for expansion of intestinal tuft cells during infection with the helminth Nippostrongylus brasiliensis. MIF-deficient mice show defective innate responses following infection, lacking intestinal epithelial tuft cell hyperplasia or upregulation of goblet cell RELMβ, and fail to expand eosinophil, type 2 innate lymphoid cell (ILC2) and macrophage (M2) populations. Similar effects were observed in MIF-sufficient wild-type mice given the MIF inhibitor 4-IPP. MIF had no direct effect on epithelial cells in organoid cultures, and MIF-deficient intestinal stem cells could generate tuft cells in vitro in the presence of type 2 cytokines. In vivo the lack of MIF could be fully compensated by administration of IL-25, restoring tuft cell differentiation and goblet cell expression of RELM-β, demonstrating its requirement upstream of the ILC2-tuft cell circuit. Both ILC2s and macrophages expressed the MIF receptor CXCR4, indicating that MIF may act as an essential co-factor on both cell types to activate responses to IL-25 in helminth infection.

Heterogeneity of ILC2s in the Intestine; Homeostasis and Pathology

Group 2 innate lymphoid cells (ILC2s) were identified in 2010 as a novel lymphocyte subset lacking antigen receptors, such as T-cell or B-cell receptors. ILC2s induce local immune responses characterized by producing type 2 cytokines and play essential roles for maintaining tissue homeostasis. ILC2s are distributed across various organs, including the intestine where immune cells are continuously exposed to external antigens. Followed by luminal antigen stimulation, intestinal epithelial cells produce alarmins, such as IL-25, IL-33, and thymic stromal lymphopoietin, and activate ILC2s to expand and produce cytokines. In the context of parasite infection, the tuft cell lining in the epithelium has been revealed as a dominant source of intestinal IL-25 and possesses the capability to regulate ILC2 homeostasis. Neuronal systems also regulate ILC2s through neuropeptides and neurotransmitters, and interact with ILC2s bidirectionally, a process termed “neuro-immune crosstalk”. Activated ILC2s produce type 2 cytokines, which contribute to epithelial barrier function, clearance of luminal antigens and tissue repair, while ILC2s are also involved in chronic inflammation and tissue fibrosis. Recent studies have shed light on the contribution of ILC2s to inflammatory bowel diseases, mainly comprising ulcerative colitis and Crohn’s disease, as defined by chronic immune activation and inflammation. Modern single-cell analysis techniques provide a tissue-specific picture of ILC2s and their roles in regulating homeostasis in each organ. Particularly, single-cell analysis helps our understanding of the uniqueness and commonness of ILC2s across tissues and opens the novel research area of ILC2 heterogeneity. ILC2s are classified into different phenotypes depending on tissue and phase of inflammation, mainly inflammatory and natural ILC2 cells. ILC2s can also switch phenotype to ILC1- or ILC3-like subsets. Hence, recent studies have revealed the heterogeneity and plasticity of ILC2, which indicate dynamicity of inflammation and the immune system. In this review, we describe the regulatory mechanisms, function, and pathological roles of ILC2s in the intestine.

Mechanisms involved in controlling RNA virus-induced intestinal inflammation

Gastroenteritis is inflammation of the lining of stomach and intestines and causes significant morbidity and mortality worldwide. Many viruses, especially RNA viruses are the most common cause of enteritis. Innate immunity is the first line of host defense against enteric RNA viruses and virus-induced intestinal inflammation. The first layer of defense against enteric RNA viruses in the intestinal tract is intestinal epithelial cells (IECs), dendritic cells and macrophages under the intestinal epithelium. These innate immune cells express pathogen-recognition receptors (PRRs) for recognizing enteric RNA viruses through sensing viral pathogen-associated molecular patterns (PAMPs). As a result of this recognition type I interferon (IFN), type III IFN and inflammasome activation occurs, which function cooperatively to clear infection and reduce viral-induced intestinal inflammation. In this review, we summarize recent findings about mechanisms involved in enteric RNA virus-induced intestinal inflammation. We will provide an overview of the enteric RNA viruses, their RNA sensing mechanisms by host PRRs, and signaling pathways triggered by host PRRs, which shape the intestinal immune response to maintain intestinal homeostasis.

BMP signaling in the intestinal epithelium drives a critical feedback loop to restrain IL-13-driven tuft cell hyperplasia

The intestinal tract is a common site for various types of infections including viruses, bacteria, and helminths, each requiring specific modes of immune defense. The intestinal epithelium has a pivotal role in both immune initiation and effector stages, which are coordinated by lymphocyte cytokines such as IFNγ, IL-13, and IL-22. Here, we studied intestinal epithelial immune responses using organoid image analysis based on a convolutional neural network, transcriptomic analysis, and in vivo infection models. We found that IL-13 and IL-22 both induce genes associated with goblet cells, but the resulting goblet cell phenotypes are dichotomous. Moreover, only IL-13-driven goblet cells are associated with classical NOTCH signaling. We further showed that IL-13 induces the bone morphogenetic protein (BMP) pathway, which acts in a negative feedback loop on immune type 2-driven tuft cell hyperplasia. This is associated with inhibiting Sox4 expression to putatively limit the tuft cell progenitor population. Blocking ALK2, a BMP receptor, with the inhibitor dorsomorphin homolog 1 (DMH1) interrupted the feedback loop, resulting in greater tuft cell numbers both in vitro and in vivo after infection with Nippostrongylus brasiliensis. Together, this investigation of cytokine effector responses revealed an unexpected and critical role for the BMP pathway in regulating type 2 immunity, which can be exploited to tailor epithelial immune responses.

Bitter taste signaling in tracheal epithelial brush cells elicits innate immune responses to bacterial infection

Constant exposure of the airways to inhaled pathogens requires efficient early immune responses protecting against infections. How bacteria on the epithelial surface are detected and first-line protective mechanisms are initiated are not well understood. We have recently shown that tracheal brush cells (BCs) express functional taste receptors. Here we report that bitter taste signaling in murine BCs induces neurogenic inflammation. We demonstrate that BC signaling stimulates adjacent sensory nerve endings in the trachea to release the neuropeptides CGRP and substance P that mediate plasma extravasation, neutrophil recruitment, and diapedesis. Moreover, we show that bitter tasting quorum-sensing molecules from Pseudomonas aeruginosa activate tracheal BCs. BC signaling depends on the key taste transduction gene Trpm5, triggers secretion of immune mediators, among them the most abundant member of the complement system, and is needed to combat P. aeruginosa infections. Our data provide functional insight into first-line defense mechanisms against bacterial infections of the lung.

Critical roles of G protein-coupled receptors in regulating intestinal homeostasis and inflammatory bowel disease

G protein-coupled receptors (GPCRs) are a group of membrane proteins that mediate most of the physiological responses to various signaling molecules such as hormones, neurotransmitters, and environmental stimulants. Inflammatory bowel disease (IBD) is a chronic relapsing disorder of the gastrointestinal tract and presents a spectrum of heterogeneous disorders falling under two main clinical subtypes including Crohn's disease (CD) and ulcerative colitis (UC). The pathogenesis of IBD is multifactorial and is related to a genetically dysregulated mucosal immune response to environmental drivers, mainly microbiotas. Although many drugs, such as 5-aminosalicylic acid, glucocorticoids, immunosuppressants, and biological agents, have been approved for IBD treatment, none can cure IBD permanently. Emerging evidence indicates significant associations between GPCRs and the pathogenesis of IBD. Here, we provide an overview of the essential physiological functions and signaling pathways of GPCRs and their roles in mucosal immunity and IBD regulation.

Not Only COVID-19: Involvement of Multiple Chemosensory Systems in Human Diseases

Chemosensory systems are deemed marginal in human pathology. In appraising their role, we aim at suggesting a paradigm shift based on the available clinical and experimental data that will be discussed. Taste and olfaction are polymodal sensory systems, providing inputs to many brain structures that regulate crucial visceral functions, including metabolism but also endocrine, cardiovascular, respiratory, and immune systems. Moreover, other visceral chemosensory systems monitor different essential chemical parameters of “milieu intérieur,” transmitting their data to the brain areas receiving taste and olfactory inputs; hence, they participate in regulating the same vital functions. These chemosensory cells share many molecular features with olfactory or taste receptor cells, thus they may be affected by the same pathological events. In most COVID-19 patients, taste and olfaction are disturbed. This may represent only a small portion of a broadly diffuse chemosensory incapacitation. Indeed, many COVID-19 peculiar symptoms may be explained by the impairment of visceral chemosensory systems, for example, silent hypoxia, diarrhea, and the “cytokine storm”. Dysregulation of chemosensory systems may underlie the much higher mortality rate of COVID-19 Acute Respiratory Distress Syndrome (ARDS) compared to ARDSs of different origins. In chronic non-infectious diseases like hypertension, diabetes, or cancer, the impairment of taste and/or olfaction has been consistently reported. This may signal diffuse chemosensory failure, possibly worsening the prognosis of these patients. Incapacitation of one or few chemosensory systems has negligible effects on survival under ordinary life conditions but, under stress, like metabolic imbalance or COVID-19 pneumonia, the impairment of multiple chemosensory systems may lead to dire consequences during the course of the disease.

The Role of Defective Epithelial Barriers in Allergic Lung Disease and Asthma Development

The respiratory epithelium constitutes the physical barrier between the human body and the environment, thus providing functional and immunological protection. It is often exposed to allergens, microbial substances, pathogens, pollutants, and environmental toxins, which lead to dysregulation of the epithelial barrier and result in the chronic inflammation seen in allergic diseases and asthma. This epithelial barrier dysfunction results from the disturbed tight junction formation, which are multi-protein subunits that promote cell-cell adhesion and barrier integrity. The increasing interest and evidence of the role of impaired epithelial barrier function in allergy and asthma highlight the need for innovative approaches that can provide new knowledge in this area. Here, we review and discuss the current role and mechanism of epithelial barrier dysfunction in developing allergic diseases and the effect of current allergy therapies on epithelial barrier restoration.

Immune cell-stem cell interactions in regeneration and repair: who's calling the shots?

In November 2021, the Institute for Regenerative Medicine (IRM) and the Institute for Immunology (IFI) at the University of Pennsylvania, USA, joined forces and organized a symposium featuring external speakers as well as locally based scientists to discuss how the immune system influences tissue stem cell biology. As we review here, the presentations highlighted emerging concepts in the field, revealing how tissue-specific immune cell activation can guide stem cells in regeneration and repair.

Expelliarmus helminthus! Harry Helminth and the Goblet of Alarmins

Epithelial cells (tuft and goblet cells) interact with immune cells on the "inside" while secreting effector molecules into the topological "outside." In this issue of Immunity, Zhao et al. investigate an interleukin-33 (IL-33) secretion mechanism in goblet cells dependent on O-GlcNAcylation and gasdermin pores facilitating worm expulsion.

Epithelial STAT6 O-GlcNAcylation drives a concerted anti-helminth alarmin response dependent on tuft cell hyperplasia and Gasdermin C

The epithelium is an integral component of mucosal barrier and host immunity. Following helminth infection, the intestinal epithelial cells secrete "alarmin" cytokines, such as interleukin-25 (IL-25) and IL-33, to initiate the type 2 immune responses for helminth expulsion and tolerance. However, it is unknown how helminth infection and the resulting cytokine milieu drive epithelial remodeling and orchestrate alarmin secretion. Here, we report that epithelial O-linked N-Acetylglucosamine (O-GlcNAc) protein modification was induced upon helminth infections. By modifying and activating the transcription factor STAT6, O-GlcNAc transferase promoted the transcription of lineage-defining Pou2f3 in tuft cell differentiation and IL-25 production. Meanwhile, STAT6 O-GlcNAcylation activated the expression of Gsdmc family genes. The membrane pore formed by GSDMC facilitated the unconventional secretion of IL-33. GSDMC-mediated IL-33 secretion was indispensable for effective anti-helminth immunity and contributed to induced intestinal inflammation. Protein O-GlcNAcylation can be harnessed for future treatment of type 2 inflammation-associated human diseases.

ILC2s - development, divergence, dispersal

Over the last decade, we have come to appreciate group 2 innate lymphoid cells (ILC2s) as important players in host and tissue immunity. New studies of ILC2s and their precursors using novel reporter mice, advanced microscopy, and multi-omics approaches have expanded our knowledge on how these cells contribute to tissue physiology and function. This review highlights recent literature on this enigmatic cell, and we organize our discussion across three important paradigms in ILC2 biology: development, divergence, and dispersal. In addition, we frame our discussion in the context of other innate and adaptive immune cells to emphasize the relevance of expanding knowledge of ILC2s and tissue immunity.

Oral Microbiota-Host Interaction Mediated by Taste Receptors

Taste receptors, originally identified in taste buds, function as the periphery receptors for taste stimuli and play an important role in food choice. Cohort studies have revealed that single nucleotide polymorphisms of taste receptors such as T1R1, T1R2, T2R38 are associated with susceptibility to oral diseases like dental caries. Recent studies have demonstrated the wide expression of taste receptors in various tissues, including intestinal epithelia, respiratory tract, and gingiva, with an emerging role of participating in the interaction between mucosa surface and microorganisms via monitoring a wide range of metabolites. On the one hand, individuals with different oral microbiomes exhibited varied taste sensitivity, suggesting a potential impact of the oral microbiota composition on taste receptor function. On the other hand, animal studies and in vitro studies have uncovered that a variety of oral cells expressing taste receptors such as gingival solitary chemosensory cells, gingival epithelial cells (GECs), and gingival fibroblasts can detect bacterial signals through bitter taste receptors to trigger host innate immune responses, thus regulating oral microbial homeostasis. This review focuses on how taste receptors, particularly bitter and sweet taste receptors, mediate the oral microbiota-host interaction as well as impact the occurrence and development of oral diseases. Further studies delineating the role of taste receptors in mediating oral microbiota-host interaction will advance our knowledge in oral ecological homeostasis establishment, providing a novel paradigm and treatment target for the better management of dental infectious diseases.

Intestinal Tuft-2 cells exert antimicrobial immunity via sensing bacterial metabolite N-undecanoylglycine

Tuft cells are a type of intestinal epithelial cells that exist in epithelial barriers and play a critical role in immunity against parasite infection. It remains insufficiently clear whether Tuft cells participate in bacterial eradication. Here, we identified Sh2d6 as a signature marker for CD45⁺ Tuft-2 cells. Depletion of Tuft-2 cells resulted in susceptibility to bacterial infection. Tuft-2 cells quickly expanded in response to bacterial infection and sensed the bacterial metabolite N-undecanoylglycine through vomeronasal receptor Vmn2r26. Mechanistically, Vmn2r26 engaged with N-undecanoylglycine activated G-protein-coupled receptor-phospholipase C gamma2 (GPCR-PLCγ2)-Ca²⁺ signaling axis, which initiated prostaglandin D2 (PGD2) production. PGD2 enhanced the mucus secretion of goblet cells and induced antibacterial immunity. Moreover, Vmn2r26 signaling also promoted SpiB transcription factor expression, which is responsible for Tuft-2 cell development and expansion in response to bacterial challenge. Our findings reveal an additional function of Tuft-2 cells in immunity against bacterial infection through Vmn2r26-mediated recognition of bacterial metabolites.

Cysteinyl leukotrienes and acetylcholine are biliary tuft cell cotransmitters

The gallbladder stores bile between meals and empties into the duodenum upon demand and is thereby exposed to the intestinal microbiome. This exposure raises the need for antimicrobial factors, among them, mucins produced by cholangiocytes, the dominant epithelial cell type in the gallbladder. The role of the much less frequent biliary tuft cells is still unknown. We here show that propionate, a major metabolite of intestinal bacteria, activates tuft cells via the short-chain free fatty acid receptor 2 and downstream signaling involving the cation channel transient receptor potential cation channel subfamily M member 5. This results in corelease of acetylcholine and cysteinyl leukotrienes from tuft cells and evokes synergistic paracrine effects upon the epithelium and the gallbladder smooth muscle, respectively. Acetylcholine triggers mucin release from cholangiocytes, an epithelial defense mechanism, through the muscarinic acetylcholine receptor M3. Cysteinyl leukotrienes cause gallbladder contraction through their cognate receptor CysLTR1, prompting emptying and closing. Our results establish gallbladder tuft cells as sensors of the microbial metabolite propionate, initiating dichotomous innate defense mechanisms through simultaneous release of acetylcholine and cysteinyl leukotrienes.

Tuft cells are key mediators of interkingdom interactions at mucosal barrier surfaces

Although tuft cells were discovered over 60 years ago, their functions have long been enigmatic, especially in human health. Nonetheless, tuft cells have recently emerged as key orchestrators of the host response to diverse microbial infections in the gut and airway. While tuft cells are epithelial in origin, they exhibit functions akin to immune cells and mediate important interkingdom interactions between the host and helminths, protists, viruses, and bacteria. With broad intra- and intertissue heterogeneity, tuft cells sense and respond to microbes with exquisite specificity. Tuft cells can recognize helminth and protist infection, driving a type 2 immune response to promote parasite expulsion. Tuft cells also serve as the primary physiologic target of persistent murine norovirus (MNV) and promote immune evasion. Recently, tuft cells were also shown to be infected by rotavirus. Other viral infections, such as influenza A virus, can induce tuft cell–dependent tissue repair. In the context of coinfection, tuft cells promote neurotropic flavivirus replication by dampening antiviral adaptive immune responses. Commensal and pathogenic bacteria can regulate tuft cell abundance and function and, in turn, tuft cells are implicated in modulating bacterial infiltration and mucosal barrier integrity. However, the contribution of tuft cells to microbial sensing in humans and their resulting effector responses are poorly characterized. Herein, we aim to provide a comprehensive overview of microbial activation of tuft cells with an emphasis on tuft cell heterogeneity and differences between mouse and human tuft cell biology as it pertains to human health and disease.

Irradiation Induces Tuft Cell Hyperplasia and Myenteric Neuronal Loss in the Absence of Dietary Fiber in a Mouse Model of Pelvic Radiotherapy

Pelvic radiotherapy is associated with chronic intestinal dysfunction. Dietary approaches, such as fiber enrichment during and after pelvic radiotherapy, have been suggested to prevent or reduce dysfunctions. In the present paper, we aimed to investigate whether a diet rich in fermentable fiber could have a positive effect on radiation-induced intestinal damage, especially focusing on tuft cells and enteric neurons. Male C57BL/6 mice were fed either a purified non-fiber diet or the same purified diet with 5% or 15% oat fiber added, starting two weeks prior to sham-irradiation or irradiation with four fractions of 8 Gray. The animals continued on the diets for 1, 6 or 18 weeks, after which the gross morphology of the colorectum was assessed together with the numbers of enteric neurons, tuft cells and crypt-surface units. The results showed that dietary fiber significantly affected the intestinal morphometrics, both in the short and long-term. The presence of dietary fiber stimulated the re-emergence of crypt-surface unit structures after irradiation. At 18 weeks, the animals fed with the non-fiber diet displayed more myenteric neurons than the animals fed with the dietary fibers, but irradiation resulted in a loss of neurons in the non-fiber fed animals. Irradiation, but not diet, affected the tuft cell numbers, and a significant increase in tuft cells was found 6 and 18 weeks after irradiation. In conclusion, dietary fiber intake has the potential to modify neuronal pathogenesis in the colorectum after irradiation. The long-lasting increase in tuft cells induced by irradiation may reflect an as yet unknown role in the mucosal pathophysiology after pelvic irradiation.

A Proximal-to-Distal Survey of Healthy Adult Human Small Intestine and Colon Epithelium by Single-Cell Transcriptomics

BACKGROUND & AIMS

Single-cell transcriptomics offer unprecedented resolution of tissue function at the cellular level, yet studies analyzing healthy adult human small intestine and colon are sparse. Here, we present single-cell transcriptomics covering the duodenum, jejunum, ileum, and ascending, transverse, and descending colon from 3 human beings.

METHODS

A total of 12,590 single epithelial cells from 3 independently processed organ donors were evaluated for organ-specific lineage biomarkers, differentially regulated genes, receptors, and drug targets. Analyses focused on intrinsic cell properties and their capacity for response to extrinsic signals along the gut axis across different human beings.

RESULTS

Cells were assigned to 25 epithelial lineage clusters. Multiple accepted intestinal stem cell markers do not specifically mark all human intestinal stem cells. Lysozyme expression is not unique to human Paneth cells, and Paneth cells lack expression of expected niche factors. Bestrophin 4 (BEST4)⁺ cells express Neuropeptide Y (NPY) and show maturational differences between the small intestine and colon. Tuft cells possess a broad ability to interact with the innate and adaptive immune systems through previously unreported receptors. Some classes of mucins, hormones, cell junctions, and nutrient absorption genes show unappreciated regional expression differences across lineages. The differential expression of receptors and drug targets across lineages show biological variation and the potential for variegated responses.

CONCLUSIONS

Our study identifies novel lineage marker genes, covers regional differences, shows important differences between mouse and human gut epithelium, and reveals insight into how the epithelium responds to the environment and drugs. This comprehensive cell atlas of the healthy adult human intestinal epithelium resolves likely functional differences across anatomic regions along the gastrointestinal tract and advances our understanding of human intestinal physiology.

How Gut Microbes Nurture Intestinal Stem Cells: A Drosophila Perspective

Host-microbiota interactions are key modulators of host physiology and behavior. Accumulating evidence suggests that the complex interplay between microbiota, diet and the intestine controls host health. Great emphasis has been given on how gut microbes have evolved to harvest energy from the diet to control energy balance, host metabolism and fitness. In addition, many metabolites essential for intestinal homeostasis are mainly derived from gut microbiota and can alleviate nutritional imbalances. However, due to the high complexity of the system, the molecular mechanisms that control host-microbiota mutualism, as well as whether and how microbiota affects host intestinal stem cells (ISCs) remain elusive. Drosophila encompasses a low complexity intestinal microbiome and has recently emerged as a system that might uncover evolutionarily conserved mechanisms of microbiota-derived nutrient ISC regulation. Here, we review recent studies using the Drosophila model that directly link microbiota-derived metabolites and ISC function. This research field provides exciting perspectives for putative future treatments of ISC-related diseases based on monitoring and manipulating intestinal microbiota.

Cholinergic immunomodulation in inflammatory bowel diseases

Inflammatory bowel diseases (IBD) are chronic intestinal disorders characterized by dysregulated immune responses to resident microbiota in genetically susceptible hosts. The activation of the cholinergic system has been proposed for the treatment of IBD patients according to its potential anti-inflammatory effect in vivo. The α-7-nicotinic-acetylcholine receptor (α7nAChR) is involved in the inhibition of inflammatory processes, modulating the production of cytokines, suppressing dendritic cells and macrophage activity, leading to the suppression of T cells. In this review, we address the most recent studies and clinical trials concerning cholinergic signaling and its therapeutic potential for inflammatory bowel diseases.

POU2F3 beyond thymic carcinomas: expression across the spectrum of thymomas hints to medullary differentiation in type A thymoma

The thymic medulla comprises various cell types, including tuft cells that are involved in innate immunity. We recently reported that in Western cohorts of patients, most thymic squamous cell carcinomas (TSQCCs), in contrast to thymomas, exhibit strong and extensive expression of tuft cell markers, including the tuft cell master regulator, POU2F3. On closer inspection of 94 thymomas that cover the full spectrum of thymoma histotypes, we now find by immunohistochemistry that approximately half of types A, AB, and B1 thymomas contain small numbers (< 10%) of cells expressing POU2F3, while most types B2 and B3 thymomas do not (p < 0.05). Further, in rarer types A and AB thymomas with adenoid growth pattern, POU2F3( +) cells formed aggregates and co-expressed KIT, as did the tumor cells in 100% (9/9) of TSQCCs expressing POU2F3. However, the expression of another tuft cell marker, L1CAM, still distinguished TSQCC from the spectrum of thymomas that were all L1CAM-negative. This study is the first to demonstrate the high frequency of POU2F3 expression in an Asian cohort of TSQCCs. The common occurrence of scattered POU2F3( +) cells in types A and AB thymomas hints at their variable degree of medullary differentiation and supports the historical hypothesis of the medullary nature of type A thymomas. Immunohistochemistry of L1CAM may be a valuable tool to differentiate TSQCCs from thymomas.

Emerging Functions of IL-33 in Homeostasis and Immunity

Our understanding of the functions of the IL-1 superfamily cytokine and damage-associated molecular pattern IL-33 continues to evolve with our understanding of homeostasis and immunity. The early findings that IL-33 is a potent driver of type 2 immune responses promoting parasite expulsion, but also inflammatory diseases like allergy and asthma, have been further supported. Yet, as the importance of a type 2 response in tissue repair and homeostasis has emerged, so has the fundamental importance of IL-33 to these processes. In this review, we outline an evolving understanding of IL-33 immunobiology, paying particular attention to how IL-33 directs a network of ST2+ regulatory T cells, reparative and regulatory macrophages, and type 2 innate lymphoid cells that are fundamental to tissue development, homeostasis, and repair.

Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Injury-induced pulmonary tuft cells are heterogenous, arise independent of key Type 2 cytokines, and are dispensable for dysplastic repair

While the lung bears significant regenerative capacity, severe viral pneumonia can chronically impair lung function by triggering dysplastic remodeling. The connection between these enduring changes and chronic disease remains poorly understood. We recently described the emergence of tuft cells within Krt5⁺ dysplastic regions after influenza injury. Using bulk and single-cell transcriptomics, we characterized and delineated multiple distinct tuft cell populations that arise following influenza clearance. Distinct from intestinal tuft cells which rely on Type 2 immune signals for their expansion, neither IL-25 nor IL-4ra signaling are required to drive tuft cell development in dysplastic/injured lungs. In addition, tuft cell expansion occurred independently of type I or type III interferon signaling. Furthermore, tuft cells were also observed upon bleomycin injury, suggesting that their development may be a general response to severe lung injury. While intestinal tuft cells promote growth and differentiation of surrounding epithelial cells, in the lungs of tuft cell deficient mice, Krt5⁺ dysplasia still occurs, goblet cell production is unchanged, and there remains no appreciable contribution of Krt5⁺ cells into more regionally appropriate alveolar Type 2 cells. Together, these findings highlight unexpected differences in signals necessary for murine lung tuft cell amplification and establish a framework for future elucidation of tuft cell functions in pulmonary health and disease.

Redefining intestinal immunity with single-cell transcriptomics

The intestinal immune system represents the largest collection of immune cells in the body and is continually exposed to antigens from food and the microbiota. Here we discuss the contribution of single-cell transcriptomics in shaping our understanding of this complex system. We consider the impact on resolving early intestine development, engagement with the neighbouring microbiota, diversity of intestinal immune cells, compartmentalisation within the intestines and interactions with non-immune cells. Finally, we offer a perspective on open questions about gut immunity that evolving single-cell technologies are well placed to address.

Human intestinal bitter taste receptors regulate innate immune responses and metabolic regulators in obesity

Bitter taste receptors (taste 2 receptors, TAS2Rs) serve as warning sensors in the lingual system against the ingestion of potentially poisonous food. Here, we investigated the functional role of TAS2Rs in the human gut and focused on their potential to trigger an additional host defense pathway in the intestine. Human jejunal crypts, especially those from individuals with obesity, responded to bitter agonists by inducing the release of antimicrobial peptides (α-defensin 5 and regenerating islet–derived protein 3 α [REG3A]) but also regulated the expression of other innate immune factors (mucins, chemokines) that affected E. coli growth. We found that the effect of aloin on E. coli growth and on the release of the mucus glycoprotein CLCA1, identified via proteomics, was affected by TAS2R43 deletion polymorphisms and thus confirmed a role for TAS2R43. RNA-Seq revealed that denatonium benzoate induced an NRF2-mediated nutrient stress response and an unfolded protein response that increased the expression of the mitokine GDF15 but also ADM2 and LDLR, genes that are involved in anorectic signaling and lipid homeostasis. In conclusion, TAS2Rs in the intestine constitute a promising target for treating diseases that involve disturbances in the innate immune system and body weight control. TAS2R polymorphisms may be valuable genetic markers to predict therapeutic responses.

Single-cell sequencing of rotavirus-infected intestinal epithelium reveals cell-type specific epithelial repair and tuft cell infection

Intestinal epithelial damage is associated with most digestive diseases and results in detrimental effects on nutrient absorption and production of hormones and antimicrobial defense molecules. Thus, understanding epithelial repair and regeneration following damage is essential in developing therapeutics that assist in rapid healing and restoration of normal intestinal function. Here we used a well-characterized enteric virus (rotavirus) that damages the epithelium at the villus tip but does not directly damage the intestinal stem cell, to explore the regenerative transcriptional response of the intestinal epithelium at the single-cell level. We found that there are specific Lgr5+ cell subsets that exhibit increased cycling frequency associated with significant expansion of the epithelial crypt. This was accompanied by an increase in the number of immature enterocytes. Unexpectedly, we found rotavirus infects tuft cells. Transcriptional profiling indicates tuft cells respond to viral infection through interferon-related pathways. Together these data provide insights as to how the intestinal epithelium responds to insults by providing evidence of stimulation of a repair program driven by stem cells with involvement of tuft cells that results in the production of immature enterocytes that repair the damaged epithelium.

Distribution of α-synuclein in normal human jejunum and its relations with the chemosensory and neuroendocrine system

Alpha-synuclein (α-syn) is a presynaptic neuronal protein and its structural alterations play an important role in the pathogenesis of neurodegenerative diseases, such as Parkinson’s disease (PD). It has been originally described in the brain and aggregated α-syn has also been found in the peripheral nerves including the enteric nervous system (ENS) of PD patients. ENS is a network of neurons and glia found in the gut wall which controls gastrointestinal function independently from the central nervous system. Moreover, two types of epithelial cells are crucial in the creation of an interface between the lumen and the ENS: they are the tuft cells and the enteroendocrine cells (EECs). In addition, the abundant enteric glial cells (EGCs) in the intestinal mucosa play a key role in controlling the intestinal epithelial barrier. Our aim was to localize and characterize the presence of α-syn in the normal human jejunal wall. Surgical specimens of proximal jejunum were collected from patients submitted to pancreaticoduodenectomy and intestinal sections underwent immunohistochemical procedure. Alpha-syn has been found both at the level of the ENS and the epithelial cells. To characterize α-syn immunoreactive epithelial cells, we used markers such as choline acetyltransferase (ChAT), useful for the identification of tuft cells. Then we evaluated the co-presence of α-syn with serotonin (5-HT), expressed in EECs. Finally, we used the low-affinity nerve growth factor receptor (p75NTR), to detect peripheral EGCs. The presence of α-syn has been demonstrated in EECs, but not in the tuft cells. Additionally, p75NTR has been highlighted in EECs of the mucosal layer and co-localized with α-syn in EECs but not with ChAT-positive cells. These findings suggest that α-syn could play a possible role in synaptic transmission of the ENS and may contribute to maintain the integrity of the epithelial barrier of the small intestine through EECs.

Regulation of airway immunity by epithelial miRNAs

The airway epithelium is essential to protect the host from inhaled pathogens and particles. It maintains immune homeostasis and mediates tissue repair after injury. Inflammatory diseases of the airways are associated with failure of epithelial functions, including loss of barrier integrity that results in increased tissue permeability and immune activation; excessive mucus secretion and impaired mucociliary clearance that leads to airflow obstruction and microbial overgrowth; and dysregulation of cellular signals that promotes inflammation and alters tissue structure and airway reactivity. MicroRNAs play crucial roles in mounting appropriate cellular responses to environmental stimuli and preventing disease, using a common machinery and mechanism to regulate gene expression in epithelial cells, immune cells of hematopoietic origin, and other cellular components of the airways. Respiratory diseases are accompanied by dramatic changes in epithelial miRNA expression that drive persistent immune dysregulation. In this review, we discuss responses of the epithelium that promote airway immunopathology, with a focus on miRNAs that contribute to the breakdown of essential epithelial functions. We emphasize the emerging role of miRNAs in regulation of epithelial responses in respiratory health and their value as diagnostic and therapeutic targets.

Single-Cell Transcriptome Sequencing and Proteomics Reveal Neonatal Ileum Dynamic Developmental Potentials

The neonatal period is a crucial time during development of the mammalian small intestine. Moreover, neonatal development and maturation of the small intestine are exceptionally important for early growth, successful weaning, and postweaning growth and development, in order to achieve species-specific milestones. Although several publications recently characterized intestinal epithelial cell diversity at the single-cell level, it remains unclear how differentiation and molecular interactions take place between types and subtypes of epithelial cells during the neonatal period. A single-cell RNA sequencing (scRNA-seq) survey of 40,186 ileal epithelial cells and proteomics analysis of ileal samples at 6 time points in the swine neonatal period were performed. The results revealed previously unknown developmental changes: specific increases in undifferentiated cells, unique enterocyte differentiation, and time-dependent reduction in secretory cells. Moreover, we observed specific transcriptional factors, ligand-receptor pairs, G protein-coupled receptors, transforming growth factor β, bone morphogenetic protein signaling pathways, and gut mucosal microbiota playing vital roles in ileal development during the neonatal window. This work offers new comprehensive information regarding ileal development throughout the neonatal period. Reference to this data set may assist in the creation of novel interventions for inflammation-, metabolism-, and proliferation-related gut pathologies. IMPORTANCE We found previously unknown neonatal ileum developmental potentials: specific increases in undifferentiated cells, unique enterocyte differentiation, and time dependent reduction in secretory cells. Specific transcriptional factors (TFs), ligand-receptor pairs, G protein-coupled receptors, transforming growth factor β, bone morphogenetic protein signaling pathways, and the gut mucosal microbiota are involved in this process. Our results may assist in the creation of novel interventions for inflammation-, metabolism-, and proliferation-related gut pathologies.