Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Investigating tributyltin's toxic effects: Intestinal barrier and neuroenteric disruption in rat's jejunum

The expansion of economic activities in coastal areas has significantly increased chemical contamination, leading to major environmental challenges. Contaminants enter the human body through the food chain, particularly via seafood and water consumption, triggering biomagnification and bioaccumulation processes. The gastrointestinal tract (GIT) acts as a selective barrier, protecting against chemical pollutants and maintaining homeostasis through a complex network of cells and immune responses. This study assessed impact of tributyltin (TBT), a highly toxic organometallic compound used in antifouling coatings for ships, on the GIT and myenteric neural plasticity in young rats. TBT exposure leads to histopathological changes, including epithelial detachment and inflammatory foci, especially at lower environmental doses. The study found that TBT causes significant reductions in villi height, increases in goblet cells and intraepithelial lymphocytes, and disrupts the myenteric plexus, with higher densities of extraganglionic neurons in exposed animals.

AP-1B regulates interactions of epithelial cells and intraepithelial lymphocytes in the intestine

Intraepithelial lymphocytes (IELs) reside in the epithelial layer and protect against foreign pathogens, maintaining the epithelial barrier function in the intestine. Interactions between IEL and epithelial cells are required for IELs to function effectively; however, the underlying molecular machinery remains to be elucidated. In this study, we found that intestinal epithelium-specific deficiency of the clathrin adaptor protein (AP)-1B, which regulates basolateral protein sorting, led to a massive reduction in IELs. Quantitative proteomics demonstrated that dozens of proteins, including known IEL-interacting proteins (E-cadherin, butyrophilin-like 2, and plexin B2), were decreased in the basolateral membrane of AP-1B-deficient epithelial cells. Among these proteins, CD166 interacted with CD6 on the surface of induced IEL. CD166 knockdown, using shRNA in intestinal organoid cultures, significantly inhibited IEL recruitment to the epithelial layer. These findings highlight the essential role of AP-1B-mediated basolateral sorting in IEL maintenance and survival within the epithelial layer. This study reveals a novel function of AP-1B in the intestinal immune system.

TGF-β2, EGF and FGF21 influence the suckling rat intestinal maturation

Some of the growth factors present in breast milk, such as transforming growth factor-β (TGF-β), epidermal growth factor (EGF) and fibroblast growth factor 21 (FGF21), play important roles in the development of the intestinal tract. The aim of this study was to determine the effect of a supplementation with TGF-β2, EGF and FGF21 on suckling rats intestinal maturation. For this purpose, Wistar rats were supplemented daily with TGF-β2, EGF or FGF21 throughout the suckling period. We evaluated the functionality of the intestinal epithelial barrier through an in vivo dextran permeability assay, and by a histomorphometric and immunohistochemical study. In addition, the intestinal gene expression of tight junction-associated proteins, mucins, toll-like receptors, and maturation markers was analyzed. Moreover, the intraepithelial lymphocyte (IEL) phenotypical composition was established.. During the suckling period, the supplementation with TGF-β2, EGF and FGF21 showed important signs of intestinal maturation. These results suggest that these molecules, present in breast milk, play a modulatory role in the maturation of the intestinal barrier function and the IEL composition during the suckling period.

Contribution of gut-derived T cells to extraintestinal autoimmune diseases

The mammalian intestine harbors abundant T cells with high motility, where these cells can affect both intestinal and extraintestinal disorders. Growing evidence shows that gut-derived T cells migrate to extraintestinal organs, contributing to the pathogenesis of certain autoimmune diseases, including type 1 diabetes (T1D) and multiple sclerosis (MS). However, three key questions require further elucidation. First, how do intestinal T cells egress from the intestine? Second, how do gut-derived T cells enter organs outside the gut? Third, what is the pathogenicity of gut-derived T cells and their correlation with the gut microenvironment? In this Opinion, we propose answers to these questions. Understanding the migration and functional regulation of gut-derived T cells might inform precise targeting for achieving safe and effective approaches to treat certain extraintestinal autoimmune diseases.

Recent Advances in Gut- and Gut-Organ-Axis-on-a-Chip Models

The human gut extracts nutrients from the diet while forming the largest barrier against the outer environment. In addition, the gut actively maintains homeostasis through intricate interactions with the gut microbes, the immune system, the enteric nervous system, and other organs. These interactions influence digestive health and, furthermore, play crucial roles in systemic health and disease. Given its primary role in absorbing and metabolizing orally administered drugs, there is significant interest in the development of preclinical in vitro model systems that can accurately emulate the intestine in vivo. A gut-on-a-chip system holds great potential as a testing and screening platform because of its ability to emulate the physiological aspects of in vivo tissues and expandability to incorporate and combine with other organs. This review aims to identify the key physiological features of the human gut that need to be incorporated to build more accurate preclinical models and highlights the recent progress in gut-on-a-chip systems and competing technologies toward building more physiologically relevant preclinical model systems. Furthermore, various efforts to construct multi-organ systems with the gut, called gut-organ-axis-on-a-chip models, are discussed. In vitro gut models with physiological relevance can provide valuable platforms for bridging the gap between preclinical and clinical studies.

Induced CD8α identifies human NK cells with enhanced proliferative fitness and modulates NK cell activation

The surface receptor CD8α is present on 20%-80% of human (but not mouse) NK cells, yet its function on NK cells remains poorly understood. CD8α expression on donor NK cells was associated with a lack of therapeutic responses in patients with leukemia in prior studies, thus, we hypothesized that CD8α may affect critical NK cell functions. Here, we discovered that CD8α- NK cells had improved control of leukemia in xenograft models compared with CD8α+ NK cells, likely due to an enhanced capacity for proliferation. Unexpectedly, we found that CD8α expression was induced on approximately 30% of previously CD8α- NK cells following IL-15 stimulation. These induced CD8α+ (iCD8α+) NK cells had the greatest proliferation, responses to IL-15 signaling, and metabolic activity compared with those that sustained existing CD8α expression (sustained CD8α+) or those that remained CD8α- (persistent CD8α-). These iCD8α+ cells originated from an IL-15Rβhi NK cell population, with CD8α expression dependent on the transcription factor RUNX3. Moreover, CD8A CRISPR/Cas9 deletion resulted in enhanced responses through the activating receptor NKp30, possibly by modulating KIR inhibitory function. Thus, CD8α status identified human NK cell capacity for IL-15-induced proliferation and metabolism in a time-dependent fashion, and its presence had a suppressive effect on NK cell-activating receptors.

Revitalizing the Gut Microbiome in Chronic Kidney Disease: A Comprehensive Exploration of the Therapeutic Potential of Physical Activity

Both physical inactivity and disruptions in the gut microbiome appear to be prevalent in patients with chronic kidney disease (CKD). Engaging in physical activity could present a novel nonpharmacological strategy for enhancing the gut microbiome and mitigating the adverse effects associated with microbial dysbiosis in individuals with CKD. This narrative review explores the underlying mechanisms through which physical activity may favorably modulate microbial health, either through direct impact on the gut or through interorgan crosstalk. Also, the development of microbial dysbiosis and its interplay with physical inactivity in patients with CKD are discussed. Mechanisms and interventions through which physical activity may restore gut homeostasis in individuals with CKD are explored.

Efficacy of Laurus nobilis L. for Tight Junction Protein Imbalance in Leaky Gut Syndrome

Laurus nobilis L. (LNL) belongs to the evergreen Lauraceae family. It is native to the Mediterranean and widely distributed in the southern United States, Europe, and the Middle East. LNL is rich in active ingredients of the sesquiterpene lactone series and has been reported to have antioxidant, anti-inflammatory, and anticancer effects. And parthenolide, known as a sesquiterpene lactone-based compound, inhibits the activation of lipopolysaccharide-binding protein (LBP), which is a major trigger for leaky gut syndrome. However, the effectiveness of LNL in improving the state of increased intestinal permeability has not yet been reported. Therefore, we demonstrated the efficacy of LNL, which is known to be rich in parthenolide, in improving intestinal permeability induced by IL-13. We investigated the improvement in permeability and analyzed major tight junction proteins (TJs), permeability-related mechanisms, weight and disease activity indices, and corresponding cytokine mechanisms. LNL maintained TJs homeostasis and clinical improvement by reducing increased claudin-2 through the inhibition of IL-13/STAT6 activation in TJ-damaged conditions. These results are expected to be effective in preventing leaky gut syndrome through the TJ balance and to further improve intestinal-related diseases, such as inflammatory bowel disease.

Intestinal Mucosal Immune Barrier: A Powerful Firewall Against Severe Acute Pancreatitis-Associated Acute Lung Injury via the Gut-Lung Axis

The pathogenesis of severe acute pancreatitis-associated acute lung injury (SAP-ALI), which is the leading cause of mortality among hospitalized patients in the intensive care unit, remains incompletely elucidated. The intestinal mucosal immune barrier is a crucial component of the intestinal epithelial barrier, and its aberrant activation contributes to the induction of sustained pro-inflammatory immune responses, paradoxical intercellular communication, and bacterial translocation. In this review, we firstly provide a comprehensive overview of the composition of the intestinal mucosal immune barrier and its pivotal roles in the pathogenesis of SAP-ALI. Secondly, the mechanisms of its crosstalk with gut microbiota, which is called gut-lung axis, and its effect on SAP-ALI were summarized. Finally, a number of drugs that could enhance the intestinal mucosal immune barrier and exhibit potential anti-SAP-ALI activities were presented, including probiotics, glutamine, enteral nutrition, and traditional Chinese medicine (TCM). The aim is to offer a theoretical framework based on the perspective of the intestinal mucosal immune barrier to protect against SAP-ALI.

Pathways and mechanisms of CD4+CD8αα+ intraepithelial T cell development

The mammalian small intestine epithelium harbors a peculiar population of CD4+CD8αα+ T cells that are derived from mature CD4+ T cells through reprogramming of lineage-specific transcription factors. CD4+CD8αα+ T cells occupy a unique niche in T cell biology because they exhibit mixed phenotypes and functional characteristics of both CD4+ helper and CD8+ cytotoxic T cells. The molecular pathways driving their generation are not fully mapped. However, recent studies demonstrate the unique role of the commensal gut microbiota as well as distinct cytokine and chemokine requirements in the differentiation and survival of these cells. We review the established and newly identified factors involved in the generation of CD4+CD8αα+ intraepithelial lymphocytes (IELs) and place them in the context of the molecular machinery that drives their phenotypic and functional differentiation.

Activation of mucosal insulin receptor exacerbates intestinal inflammation by promoting tissue resident memory T cells differentiation through EZH2

BACKGROUND

Inflammatory Bowel Diseases (IBD), an autoimmune disease characterised by abnormal intestinal immunity, are related to vital morbidity around the world. However, therapeutic agents for IBD have not achieved desired benefit. Exploring new therapeutic targets for IBD, especially based on its abnormally intestinal immunity, could alleviate the flare-up and worsening of IBD. Tissue resident memory T cells (TRM) are core of multiple autoimmune diseases, including IBD. However, the mechanism of TRM differentiation remains to be investigated.

METHODS

The alterations in mRNA and lncRNA profile of intestinal intraepithelial lymphocytes (IELs), the largest component of intestinal TRM, were analyzed in DSS-induced chronic colitis. Based on it, we examined the function of rectal insulin instillation in a dextran sodium sulfate (DSS) induced chronic colitis. Furthermore, we investigated the downstream-target of the insulin pathway-EZH2 and the crucial role of EZH2 in intestinal tissue resident memory T cell differentiation by utilizing EZH2fl/flCD4cre mice.

RESULTS

Insulin receptor (INSR) expression was found to be significantly reduced. Activation of mucosal insulin pathway by rectal insulin instillation exacerbated colitis by disrupting IELs subgroups and up-regulating TNF-ɑ and IL-17 expression. Rectal insulin instillation promoted EZH2 expression and EZH2 inhibition alleviated chronic colitis. EZH2fl/flCD4cre mice restored the normal IEL subgroups and suppressed TNF-ɑ and IL-17 expression, exhibiting alleviated colitis. IELs from EZH2fl/flCD4cre mice exhibit significant changes in TRM related phenotype. CD4+TRM was significantly increased in chronic colitis and decreased in EZH2fl/flCD4cre mice.

CONCLUSION

Insulin receptor of intestinal mucosal T-cells could promote intestinal TRM differentiation via EZH2. Our discoveries suggest that therapies targeting colonic INSR and EZH2 could be potential treatment for IBD based on its regulatory effects on TRM. Insulin receptor inhibitors rather than insulin should be applied during colitis-active phase. In addition, EZH2 shows to be a downstream signal of the insulin pathway and EZH2 inhibitor could alleviating intestinal inflammation. However, the critical role of EZH2 in TRM differentiation restricts the anti-tumor effects of EZH2 inhibitor in vivo.

Gut immune microenvironment and autoimmunity

A variety of immune cells or tissues are present in the gut to form the gut immune microenvironment by interacting with gut microbiota, and to maintain the gut immune homeostasis. Accumulating evidence indicated that gut microbiota dysbiosis might break the homeostasis of the gut immune microenvironment, which was associated with many health problems including autoimmune diseases. Moreover, disturbance of the gut immune microenvironment can also induce extra-intestinal autoimmune disorders through the migration of intestinal pro-inflammatory effector cells from the intestine to peripheral inflamed sites. This review discussed the composition of the gut immune microenvironment and its association with autoimmunity. These findings are expected to provide new insights into the pathogenesis of various autoimmune disorders, as well as novel strategies for the prevention and treatment against related diseases.

Intestinal factors promoting the development of RORγt+ cells and oral tolerance

The gastrointestinal tract has to harmonize the two seemingly opposite functions of fulfilling nutritional needs and avoiding the entry of pathogens, toxins and agents that can cause physical damage. This balance requires a constant adjustment of absorptive and defending functions by sensing environmental changes or noxious substances and initiating adaptive or protective mechanisms against them through a complex network of receptors integrated with the central nervous system that communicate with cells of the innate and adaptive immune system. Effective homeostatic processes at barrier sites take the responsibility for oral tolerance, which protects from adverse reactions to food that cause allergic diseases. During a very specific time interval in early life, the establishment of a stable microbiota in the large intestine is sufficient to prevent pathological events in adulthood towards a much larger bacterial community and provide tolerance towards diverse food antigens encountered later in life. The beneficial effects of the microbiome are mainly exerted by innate and adaptive cells that express the transcription factor RORγt, in whose generation, mediated by different bacterial metabolites, retinoic acid signalling plays a predominant role. In addition, recent investigations indicate that food antigens also contribute, analogously to microbial-derived signals, to educating innate immune cells and instructing the development and function of RORγt+ cells in the small intestine, complementing and expanding the tolerogenic effect of the microbiome in the colon. This review addresses the mechanisms through which microbiota-produced metabolites and dietary antigens maintain intestinal homeostasis, highlighting the complementarity and redundancy between their functions.

Human thymic putative CD8αα precursors exhibit a biased TCR repertoire in single cell AIRR-seq

Thymic T cell development comprises T cell receptor (TCR) recombination and assessment of TCR avidity towards self-peptide-MHC complexes presented by antigen-presenting cells. Self-reactivity may lead to negative selection, or to agonist selection and differentiation into unconventional lineages such as regulatory T cells and CD8[Formula: see text] T cells. To explore the effect of the adaptive immune receptor repertoire on thymocyte developmental decisions, we performed single cell adaptive immune receptor repertoire sequencing (scAIRR-seq) of thymocytes from human young paediatric thymi and blood. Thymic PDCD1+ cells, a putative CD8[Formula: see text] T cell precursor population, exhibited several TCR features previously associated with thymic and peripheral ZNF683+ CD8[Formula: see text] T cells, including enrichment of large and positively charged complementarity-determining region 3 (CDR3) amino acids. Thus, the TCR repertoire may partially explain the decision between conventional vs. agonist selected thymocyte differentiation, an aspect of importance for the development of therapies for patients with immune-mediated diseases.

Microbiota as key factors in inflammatory bowel disease

Inflammatory Bowel Disease (IBD) is characterized by prolonged inflammation of the gastrointestinal tract, which is thought to occur due to dysregulation of the immune system allowing the host's cells to attack the GI tract and cause chronic inflammation. IBD can be caused by numerous factors such as genetics, gut microbiota, and environmental influences. In recent years, emphasis on commensal bacteria as a critical player in IBD has been at the forefront of new research. Each individual harbors a unique bacterial community that is influenced by diet, environment, and sanitary conditions. Importantly, it has been shown that there is a complex relationship among the microbiome, activation of the immune system, and autoimmune disorders. Studies have shown that not only does the microbiome possess pathogenic roles in the progression of IBD, but it can also play a protective role in mediating tissue damage. Therefore, to improve current IBD treatments, understanding not only the role of harmful bacteria but also the beneficial bacteria could lead to attractive new drug targets. Due to the considerable diversity of the microbiome, it has been challenging to characterize how particular microorganisms interact with the host and other microbiota. Fortunately, with the emergence of next-generation sequencing and the increased prevalence of germ-free animal models there has been significant advancement in microbiome studies. By utilizing human IBD studies and IBD mouse models focused on intraepithelial lymphocytes and innate lymphoid cells, this review will explore the multifaceted roles the microbiota plays in influencing the immune system in IBD.

Characterization of Bovine Intraepithelial T Lymphocytes in the Gut

Intraepithelial T lymphocytes (T-IELs), which constitute over 50% of the total T lymphocytes in the animal, patrol the mucosal epithelial lining to defend against pathogen invasion while maintaining gut homeostasis. In addition to expressing T cell markers such as CD4 and CD8, T-IELs display T cell receptors (TCR), including either TCRαβ or TCRγδ. Both humans and mice share similar T-IEL subsets: TCRγδ+, TCRαβ+CD8αα+, TCRαβ+CD4+, and TCRαβ+CD8αβ+. Among these subsets, human T-IELs are predominantly TCRαβ+ (over 80%), whereas those in mice are mostly TCRγδ+ (~60%). Of note, the majority of the TCRγδ+ subset expresses CD8αα in both species. Although T-IELs have been extensively studied in humans and mice, their profiles in cattle have not been well examined. Our study is the first to characterize bovine T-IELs using flow cytometry, where we identified several distinct features. The percentage of TCRγδ+ was comparable to that of TCRαβ+ T-IELs (both ~50% of CD3+), and the majority of bovine TCRγδ+ T-IELs did not express CD8 (CD8-) (above 60%). Furthermore, about 20% of TCRαβ+ T-IELs were CD4+CD8αβ+, and the remaining TCRαβ+ T-IELs were evenly distributed between CD4+ and CD8αβ+ (~40% of TCRαβ+ T-IELs each) with no TCRαβ+CD8αα+ identified. Despite these unique properties, bovine T-IELs, similar to those in humans and mice, expressed a high level of CD69, an activation and tissue-retention marker, and a low level of CD62L, a lymphoid adhesion marker. Moreover, bovine T-IELs produced low levels of inflammatory cytokines such as IFNγ and IL17A, and secreted small amounts of the immune regulatory cytokine TGFβ1. Hence, bovine T-IELs' composition largely differs from that of human and mouse, with the dominance of the CD8- population among TCRγδ+ T-IELs, the substantial presence of TCRαβ+CD4+CD8αβ+ cells, and the absence of TCRαβ+CD8αα+ T-IELs. These results provide the groundwork for conducting future studies to examine how bovine T-IELs respond to intestinal pathogens and maintain the integrity of the gut epithelial barrier in animals.

A novel method for isolation and flow cytometry analysis of intraepithelial lymphocytes from colon biopsies

Investigating the immune responses of the intestine in response to different insults is predominantly limited to indirect methods such as circulating markers of intestinal health or gene expression from dissections. We describe here a validated protocol for the isolation and subsequent flow cytometry analysis of intestinal intraepithelial lymphocytes (IEL) from colonic biopsy samples. Colon biopsy samples were collected with endoscopy forceps from Holstein dairy bull calves at d 2, 28, and 42 of life. The biopsies were put into an isolation solution of Hanks' balanced salt solution, and fetal bovine serum followed by digestion solution. The solution was filtered and the flow-through, containing IEL, was stained with fluorescent antibodies for flow cytometry analysis. Density gradient separation of the isolate yielded higher viability and cleaner samples for flow cytometry analysis. Anti-bovine γ chain of the T cell receptor was used to identify populations of gamma delta (γδ) T cells via flow cytometry. In addition, γδ T cell subsets were identified using an anti-bovine antibody against the coreceptor workshop cluster 1. This method allowed for the precise identification of lymphocyte populations and evaluation of the proportion of different subsets of γδ T cells from intestinal IEL over time. The technique described here will allow the research community to characterize intestinal immune function over time and improve our understanding of how different management and nutritional strategies affect intestinal health.

The intestinal immune system and gut barrier function in obesity and ageing

Obesity and ageing predispose to numerous, yet overlapping chronic diseases. For example, metabolic abnormalities, including insulin resistance (IR) and type 2 diabetes (T2D) are important causes of morbidity and mortality. Low-grade chronic inflammation of tissues, such as the liver, visceral adipose tissue and neurological tissues, is considered a significant contributor to these chronic diseases. Thus, it is becoming increasingly important to understand what drives this inflammation in affected tissues. Recent evidence, especially in the context of obesity, suggests that the intestine plays an important role as the gatekeeper of inflammatory stimuli that ultimately fuels low-grade chronic tissue inflammation. In addition to metabolic diseases, abnormalities in the intestinal mucosal barrier have been linked to a range of other chronic inflammatory conditions, such as neurodegeneration and ageing. The flow of inflammatory stimuli from the gut is in part controlled by local immunological inputs impacting the intestinal barrier. Here, we will review the impact of obesity and ageing on the intestinal immune system and its downstream consequences on gut barrier function, which is strongly implicated in the pathogenesis of obesity and age-related diseases. In particular, we will discuss the effects of age-related intestinal dysfunction on neurodegenerative diseases.

Novel T cell/organoid culture system allows ex vivo modeling of intestinal graft-versus-host disease

Acute graft-versus-host disease (GvHD) remains the biggest clinical challenge and prognosis-determining complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Donor T cells are acceptedly key mediators of alloreactivity against host tissues and here especially the gut. In support of previous studies, we found that the intestinal intra-epithelial lymphocyte (IEL) compartment was dynamically regulated in the course of MHC class I full mismatch allo-HSCT. However, while intestinal epithelial cell (IEC) damage endangers the integrity of the intestinal barrier and is a core signature of intestinal GvHD, the question whether and to what degree IELs are contributing to IEC dysregulation is poorly understood. To study lymphoepithelial interaction, we employed a novel ex vivo T cell/organoid co-culture model system. Here, allogeneic intra-epithelial T cells were superior in inducing IEC death compared to syngeneic IEL and allogeneic non-IEL T cells. The ability to induce IEC death was predominately confined to TCRβ+ T cells and was executed in a largely IFNγ-dependent manner. Alloreactivity required a diverse T cell receptor (TCR) repertoire since IELs genetically modified to express a TCR restricted to a single, non-endogenous antigen failed to mediate IEC pathology. Interestingly, minor histocompatibility antigen (miHA) mismatch was sufficient to elicit IEL-driven IEC damage. Finally, advanced live cell imaging analyses uncovered that alloreactive IELs patrolled smaller areas within intestinal organoids compared to syngeneic controls, indicating their unique migratory properties within allogeneic IECs. Together, we provide here experimental evidence for the utility of a co-culture system to model the cellular and molecular characteristics of the crosstalk between IELs and IEC in an allogeneic setting ex vivo. In the light of the emerging concept of dysregulated immune-epithelial homeostasis as a core aspect of intestinal GvHD, this approach represents a novel experimental system to e.g. screen therapeutic strategies for their potential to normalize T cell/IEC- interaction. Hence, analyses in pre-clinical in vivo allo-HSCT model systems may be restricted to hereby positively selected, promising approaches.

Glutamine alleviates intestinal injury in a murine burn sepsis model by maintaining intestinal intraepithelial lymphocyte homeostasis

Intestinal intraepithelial lymphocytes (IELs) play a sentinel role in the mucosal immune system because of their unique anatomical location in the epithelial layer. The disruption of IEL homeostasis is implicated in driving the intestinal injury of many typical inflammatory disorders, such as inflammatory bowel disease (IBD) and sepsis. Therefore, it is meaningful to alleviate intestinal injury by restoring IEL homeostasis in disease conditions. This study explores the effects of glutamine on intestinal IEL homeostasis in a murine model of burn sepsis. We report that glutamine inhibits inflammatory response and reduces injury in the small intestine of burn septic mice. This effect is attributed to the maintaining of IEL homeostasis by suppressing apoptosis and restoring the disrupted subpopulation balance induced by burn sepsis. Mechanistically, we show that glutamine does not affect the IL-15 dependent mechanisms that drive the maintenance and differentiation of IELs. Instead, glutamine sustains IEL homeostasis by upregulate aryl hydrocarbon receptor (AHR) and interleukin (IL)-22 transcription and expression. Consistently, the protective roles of glutamine in burn septic mice were repressed by further supplement with an AHR antagonist CH-223191. Collectively, our study reveals a new role of glutamine to maintain IEL homeostasis by activating the AHR signaling pathway, which in turn ameliorates intestinal injury in burn sepsis.

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.

Antibiotic-resistant bacteria originating from the gut may modulate the mucosal immune response during sepsis and septic shock

The enrichment and diversity of gut microbiota play an important role in sepsis, but the role of gut microbiota composition and early-life colonization in sepsis and septic shock has not yet been characterized. The impact of gut microbiota diversity on host immunological disorders and future treatments of inflammatory diseases are not yet fully elucidated. Further, the association between the microbiota and immune development in sepsis remains unknown, and the underlying mechanisms are not well understood. The altered composition of gut microbiota during sepsis is profoundly associated with a loss of commensal bacteria and an overgrowth of potentially pathogenic bacteria, especially AMR bacteria. Disruptions of gut microbiota diversity are directly associated with susceptibility to sepsis and a higher risk of adverse outcomes. Several studies have confirmed that a mutual association between gut microbiota and the host is important for the metabolism of essential nutrients for the organism, for gut development, and for the maturation and development of a fully functional immune system. Therefore, understanding the gut microbiota diversity, composition, and function during various inflammatory conditions and sepsis may provide a comprehensive knowledge of the mechanisms behind the pathogenesis of gut-derived infection in diseases and the design of new treatment options (e.g., probiotics or fecal microbiota transplantation).

Emerging evidence displays an important role of gut microbiota and their derived metabolites in modulating the host mucosal immune response and determining the susceptibility to, as well as outcomes of sepsis.

Immune-mediated food reactions in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder characterized by recurrent abdominal pain and an altered defecation pattern. Depending on the criteria used, it affects between 5 and 10% of the general population and has a serious impact on quality of life. Most patients with IBS show an induction or exacerbation of their symptoms, particularly abdominal pain, after eating certain foods. This raises the question of the role played by food in IBS pathophysiology. In this review, we describe the multiple risk factors of IBS, and we give an overview of the role of food as a trigger of IBS, distinguishing between immune and non-immune reactions to food. We finally highlight recent findings identifying an immune-mediated mechanism underlying food-induced abdominal pain in IBS.

Crosstalk between epithelium, myeloid and innate lymphoid cells during gut homeostasis and disease

The gut epithelium not only provides a physical barrier to separate a noxious outside from a sterile inside but also allows for highly regulated interactions between bacteria and their products, and components of the immune system. Homeostatic maintenance of an intact epithelial barrier is paramount to health, requiring an intricately regulated and highly adaptive response of various cells of the immune system. Prolonged homeostatic imbalance can result in chronic inflammation, tumorigenesis and inefficient antitumor immune control. Here we provide an update on the role of innate lymphoid cells, macrophages and dendritic cells, which collectively play a critical role in epithelial barrier maintenance and provide an important linkage between the classical innate and adaptive arm of the immune system. These interactions modify the capacity of the gut epithelium to undergo continuous renewal, safeguard against tumor formation and provide feedback to the gut microbiome, which acts as a seminal contributor to cellular homeostasis of the gut.

Aqueous extract of Paeoniae Radix Alba (Paeonia lactiflora Pall.) ameliorates DSS-induced colitis in mice by tunning the intestinal physical barrier, immune responses, and microbiota

ETHNOPHARMACOLOGICAL RELEVANCE

Ulcerative colitis (UC) is a chronic non-specific intestinal inflammatory disease, the pathogenesis of which is strongly associated with the compromised intestinal barrier. Paeoniae Radix Alba (PRA), the root of Paeonia lactiflora Pall., is a well-known traditional Chinese medicine and an adaptogen used in Hozai, exhibiting appreciable anti-inflammatory and immunomodulatory activity. Nevertheless, the role and mechanism of PRA in UC have yet to be elucidated.

AIM OF THE STUDY

This study was set out to examine the ameliorative effects of the aqueous extract of PRA (i.e., PRA dispensing granule, PRADG) on dextran sulfate sodium (DSS)-induced colitis.

MATERIALS AND METHODS

The chemical components of PRADG was analyzed by HPLC. Colitis model mice were induced by free access to water containing 2.5% DSS for 10 consecutive days, and concurrently, PRADG (0.1025 and 0.41 g/kg) or Salazosulfapyridine (SASP, 450 mg/kg) was given orally from day 1-10. Body weight, disease activity index (DAI), colon length, histologic scoring, and inflammatory response were assessed. Additionally, IL-23/IL-17 axis and tight junction (TJ) proteins, as well as gut microbiota were also investigated under the above-mentioned regimen.

RESULTS

Eight main chemical constituents of CPT were revealed with HPLC analysis. Noticeably, PRADG could effectively lower body weight loss as well as DAI scores, alleviate colon shortening, and reduce the levels of proinflammatory cytokines in mice with colitis. Further exploration found that increment of TJ proteins expression (ZO-1, occludin and claudin-1) and inhibition of IL-23/IL-17 axis-modulated inflammation were observed in PRADG-treated mice. Additionally, the diversity of gut microbiota and the relative abundance of beneficial bacteria were increased following PRADG treatment.

CONCLUSIONS

PRADG could be sufficient to ameliorate colitis by regulating the intestinal physical barrier, immune responses, and gut microbiota in mice. Our findings highlight that PRADG might be a prospective remedy for UC.

Innate and Innate-like Effector Lymphocytes in Health and Disease

Lymphocytes can be functionally partitioned into subsets belonging to the innate or adaptive arms of the immune system. Subsets of innate and innate-like lymphocytes may or may not express Ag-specific receptors of the adaptive immune system, yet they are poised to respond with innate-like speed to pathogenic insults but lack the capacity to develop classical immunological memory. These lymphocyte subsets display a number of common properties that permit them to integrate danger and stress signals dispatched by innate sensor cells to facilitate the generation of specialized effector immune responses tailored toward specific pathogens or other insults. In this review, we discuss the functions of distinct subsets of innate and innate-like lymphocytes. A better understanding of the mechanisms by which these cells are activated in different contexts, their interactions with other immune cells, and their role in health and disease may inform the development of new or improved immunotherapies.

Differential activation of chicken gamma delta T cells from different tissues by Toll-like receptor 3 or 21 ligands

Gamma delta (γδ) T cells are highly enriched in mucosal barrier sites including intestinal tissues where microbial infections and tumors often originate in mammals. Human γδ T cells recognize stress antigens and microbial signals via their T cell receptor (TCR), natural killer (NK) receptors, and pattern recognition receptors. However, little is known about antigens or ligands capable of stimulating chicken γδ T cells. The results of the present study demonstrated that polyinosinic-polycytidylic acid (poly(I:C)), a Toll-like receptor (TLR)3 ligand, significantly induced upregulation of CD8α molecules on circulating and lung γδ T cells. Moreover, poly(I:C) stimulation induced interferon (IFN)-γ production from splenic and lung CD8α⁺ γδ T cells while Cytosine-phosphate-Guanine oligodeoxynucleotides (CpG-ODN) 2007, a TLR21 ligand, stimulation induced IFN-γ production by circulating γδ T cells. Neither poly(I:C) nor CpG-ODN 2007 stimulation elicited degranulation of γδ T cells. Additionally, the results revealed that CpG-ODN 2007 induced IFN-γ production from TCR-stimulated γδ T cells sorted from spleen. In our experiments, isopentenyl pyrophosphate (IPP), 4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), or zoledronate (Zol) stimulation did not induce IFN-γ production or degranulation in γδ T cells. Taken together, a combination of CpG-ODN 2007 and anti-CD3ε monoclonal antibodies (mAbs) can stimulate chicken γδ T cells and induce production of IFN-γ by these cells while IFN-γ production by γδ T cells induced by stimulation of poly(I:C) needs signals from other cells. These results suggest that chicken γδ T cells can sense invading pathogens via TLRs and produce IFN-γ as a first line of defense.

Cellular Origins and Pathogenesis of Gastrointestinal NK- and T-Cell Lymphoproliferative Disorders

The intestinal immune system, which must ensure appropriate immune responses to both pathogens and commensal microflora, comprises innate lymphoid cells and various T-cell subsets, including intra-epithelial lymphocytes (IELs). An example of innate lymphoid cells is natural killer cells, which may be classified into tissue-resident, CD56bright NK-cells that serve a regulatory function and more mature, circulating CD56dim NK-cells with effector cytolytic properties. CD56bright NK-cells in the gastrointestinal tract give rise to indolent NK-cell enteropathy and lymphomatoid gastropathy, as well as the aggressive extranodal NK/T cell lymphoma, the latter following activation by EBV infection and neoplastic transformation. Conventional CD4+ TCRαβ+ and CD8αβ+ TCRαβ+ T-cells are located in the lamina propria and the intraepithelial compartment of intestinal mucosa as type 'a' IELs. They are the putative cells of origin for CD4+ and CD8+ indolent T-cell lymphoproliferative disorders of the gastrointestinal tract and intestinal T-cell lymphoma, NOS. In addition to such conventional T-cells, there are non-conventional T-cells in the intra-epithelial compartment that express CD8αα and innate lymphoid cells that lack TCRs. The central feature of type 'b' IELs is the expression of CD8αα homodimers, seen in monomorphic epitheliotropic intestinal T-cell lymphoma (MEITL), which primarily arises from both CD8αα+ TCRαβ+ and CD8αα+ TCRγδ+ IELs. EATL is the other epitheliotropic T-cell lymphoma in the GI tract, a subset of which arises from the expansion and reprograming of intracytoplasmic CD3+ innate lymphoid cells, driven by IL15 and mutations of the JAK-STAT pathway.

The Pathology and Physiology of Ileostomy

An ileostomy is a surgery that is commonly performed to protect low pelvic anastomoses or prevent high-risk anastomotic leakages. However, various postoperative complications remain of major concern. After an ileostomy, the distal intestinal segment is left open for an extended period and is in a non-functional state. Consequently, the intestinal mucosa, smooth muscle, and microbiota undergo significant changes that are closely related to postoperative recovery and complications. A systematic description of these changes is necessary to understand the relationship among them and take more effective measures for postoperative intervention.

Immune Tolerance vs. Immune Resistance: The Interaction Between Host and Pathogens in Infectious Diseases

The immune system is most likely developed to reduce the harmful impact of infections on the host homeostasis. This defense approach is based on the coordinated activity of innate and adaptive immune system components, which detect and target infections for containment, killing, or expulsion by the body's defense mechanisms. These immunological processes are responsible for decreasing the pathogen burden of an infected host to maintain homeostasis that is considered to be infection resistance. Immune-driven resistance to infection is connected with a second, and probably more important, defensive mechanism: it helps to minimize the amount of dysfunction imposed on host parenchymal tissues during infection without having a direct adverse effect on pathogens. Disease tolerance is a defensive approach that relies on tissue damage control systems to prevent infections from causing harm to the host. It also uncouples immune-driven resistance mechanisms from immunopathology and disease, allowing the body to fight infection more effectively. This review discussed the cellular and molecular processes that build disease tolerance to infection and the implications of innate immunity on those systems. In addition, we discuss how symbiotic relationships with microbes and their control by particular components of innate and adaptive immunity alter disease tolerance to infection.

Porcine intraepithelial lymphocytes undergo migration and produce an antiviral response following intestinal virus infection

The location of intraepithelial lymphocytes (IELs) between epithelial cells provide a first line of immune defense against enteric infection. It is assumed that IELs migrate only along the basement membrane or into the lateral intercellular space (LIS) between epithelial cells. Here, we identify a unique transepithelial migration of porcine IELs as they move to the free surface of the intestinal epithelia. The major causative agent of neonatal diarrhea in piglets, porcine epidemic diarrhea virus (PEDV), increases the number of IELs entering the LIS and free surface of the intestinal epithelia, driven by chemokine CCL2 secreted from virus-infected intestinal epithelial cells. Remarkably, only virus pre-activated IELs inhibits PEDV infection and their antiviral activity depends on the further activation by virus-infected cells. Although high levels of perforin is detected in the co-culture system, the antiviral function of activated IELs is mainly mediated by IFN-γ secretion inducing robust antiviral response in virus-infected cells. Our results uncover a unique migratory behavior of porcine IELs as well as their protective role in the defense against intestinal infection.

When piglets are infected with intestinal virus, porcine intraepithelial lymphocytes undergo intra-and trans-epithelial migration promoted by chemokines from infected epithelial cells and produce an antiviral response.

Impact of Early ARV Initiation on Relative Proportions of Effector and Regulatory CD8 T Cell in Mesenteric Lymph Nodes and Peripheral Blood During Acute SIV Infection of Rhesus Macaques

CD8 T cells are key players in the clearance of human immunodeficiency virus (HIV)-infected cells, such that CD8 T-cell dysfunction contributes to viral persistence despite antiretroviral (ARV) therapy. Mesenteric lymph nodes (MLNs) are major sites of gut mucosal immunity. While different CD8 T cell subsets such as CD8 alpha-alpha (CD8αα), CD8 alpha-beta (CD8αβ), CD8 regulatory T cells (Treg), and mucosa-associated invariant T cells (MAIT) are present in the gut and exhibit distinct functions, their dynamics remain poorly understood due to the lack of accessibility to these tissues in humans. We thus assessed CD8 T cells in MLNs versus peripheral blood in simian immunodeficiency virus (SIV)-infected rhesus macaques (RMs) following early ARV therapy initiation. SIV infection was associated with an increase over time of both CD8αβ and CD8αα T cells in the blood and MLNs, whereas early ARV initiation significantly decreased the frequencies of CD8αα but not CD8αβ T cells in MLNs. A significant decrease in the expression of chemokine receptors CCR6 and CXCR3 by CD8 T cells, which are essential for T-cell trafficking to the inflammatory sites, was observed in chronically SIV-infected RMs. Surprisingly, while MAIT cells are increased in ARV-treated RMs, their frequencies in MLN are extremely low and were not impacted by ARV. The acute infection resulted in an early CD39⁺FoxP3⁺ CD8 Tregs increase in both compartments, which was normalized after early ARV. Frequencies of CD8 Treg cells were positively correlated with frequencies of CD4 Tregs and accordingly negatively correlated with the Th17/Treg ratio in the blood but not in MLNs. Overall, our results underscore the difference in CD8 T-cell subset dynamics in the blood and MLNs.

IMPORTANCE Changes in CD8 T-cell subsets during acute SIV/HIV infections and following early ARV initiation in gut lymphoid tissues are poorly understood. Using an acute SIV infection model in rhesus macaques, we assessed the impact of early ARV, initiated 4 days postinfection, on relative proportions of CD8 T-cell subsets in MLNs compared to blood. We found that acute SIV infection and early ARV initiation differentially affect the distribution of effector CD8 T cells, CD8 MAIT cells, and CD8 Tregs in MLNs compared to blood. Overall, early ARV initiation maintains the frequency of effector CD8 T cells while reducing immunosuppressive CD39⁺ CD8 Tregs. Our study provides deeper insight into the dynamics of the CD8 T-cell compartment in gut mucosal immune surveillance during acute SIV infection and following early ARV initiation.

Immune activation in irritable bowel syndrome: what is the evidence?

Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder that is characterized by abdominal pain and an altered defecation pattern. It affects between 5 and 20% of the general population and can seriously impact quality of life. The pathophysiology of IBS is rather complex and multifactorial including, for example, altered signalling by the gut-brain axis, dysbiosis, abnormal visceral pain signalling and intestinal immune activation. The latter has gained particular interest in recent years, with growing insight into the bidirectional communication between the nervous system and the immune system. In this Review, we detail the current evidence suggesting that immune activation contributes to the pathology seen in patients with IBS and discuss the potential mechanisms involved. Moreover, we describe how immune mediators, particularly those released by mast cells, can directly activate or sensitize pain-transmitting nerves, leading to increased pain signalling and abdominal pain. Finally, we discuss the potential of interventions targeting immune activation as a new therapeutic strategy for patients suffering from IBS.

Biological Clock and Inflammatory Bowel Disease Review: From the Standpoint of the Intestinal Barrier

Inflammatory bowel disease is a group of chronic, recurrent, nonspecific inflammatory diseases of the intestine that severely affect the quality of life of patients. The pathogenesis of this disease is caused by complex and interactive neural networks composed of factors such as genetic susceptibility, external environment, immune disorders, and intestinal barrier dysfunction. It is well known that there is a strong link between environmental stressors (also known as circadian clocks) that can influence circadian changes and inflammatory bowel disease. Among them, the biological clock is involved in the pathogenesis of inflammatory bowel disease by affecting the function of the intestinal barrier. Therefore, this review is aimed at systematically summarizing the latest research progress on the role of the circadian clock in the pathogenesis of inflammatory bowel disease by affecting intestinal barrier functions (intestinal mechanical barrier, intestinal immune barrier, intestinal microecological barrier, and intestinal chemical barrier) and the potential clinical value of clock genes in the management of inflammatory bowel disease, for the application of circadian clock therapy in the management of inflammatory bowel disease and then the benefit to the majority of patients.

Enterovirus 71 Antagonizes Antiviral Effects of Type III Interferon and Evades the Clearance of Intestinal Intraepithelial Lymphocytes

Enterovirus 71 (EV71) is the major pathogen causing severe neurological complications and hand, foot, and mouth disease. The intestinal mucosal immune system has a complete immune response and immune regulation mechanism, consisting of densely arranged monolayer intestinal epithelial cells (IECs) and intestinal intraepithelial lymphocytes (iIELs) distributed among the IECs, which constitute the first line of intestinal mucosa against infection of foreign pathogens. As an enterovirus, EV71 is transmitted by the intestinal tract; however, the mechanisms it uses to evade the immunosurveillance of the intestinal mucosal immune system are still incompletely clarified. The present study investigated how EV71 evades from recognizing and eliminating IECs, iIELs, and iNK cells. We found that EV71 infection induced a higher level of type III interferons (IFN-λ) than type I interferons (IFN-β) in IECs, and the addition of IFN-λ markedly restricted EV71 replication in IECs. These results indicate that IFN-λ plays a more important role in anti-EV71 intestinal infection. However, EV71 infection could markedly attenuate the antiviral responses of IFN-λ. Mechanistically, 2A protease (2Apro) and 3C protease (3Cpro) of EV71 inhibited the IFN-λ production and IFN-λ receptor expression and further decreased the response of IECs to IFN-λ. In addition, we found that EV71-infected IECs were less susceptible to the lysis of intestinal NK (iNK) cells and CD3⁺iIELs. We revealed that the viral 2Apro and 3Cpro could significantly reduce the expression of the ligands of natural killer group 2D (NKG2D) and promote the expression of PD-L1 on IECs, rendering them to evade the recognition and killing of iNK and CD3⁺iIELs. These results provide novel evasion mechanisms of EV71 from intestinal mucosal innate immunity and may give new insights into antiviral therapy.

Therapeutic effect of baicalin on inflammatory bowel disease: A review

ETHNOPHARMACOLOGICAL RELEVANCE

Baicalin (BI) is an important biologically active flavonoid isolated from the root of Scutellaria radix (Huang Qin). Traditionally Scutellaria radix was the common drug of dysentery. As the main flavonoid compound, there is a distribution tendency of baicalin to the intestinal tract and it has a protective effect on the gastrointestinal tract.

AIM OF THE REVIEW

This review aims to compile up-to-date and comprehensive information on the efficacy of baicalin in vitro and in vivo, about treating inflammatory bowel disease. Relevant information on the therapeutic potential of baicalin against inflammatory bowel disease was collected from the Web of Science, Pubmed and so on. Additionally, a few books and magazines were also consulted to get the important information.

RESULTS

The mechanisms of baicalin against inflammatory bowel disease mainly include anti-inflammation, antioxidant, immune regulation, maintenance of intestinal barrier, maintenance of intestinal flora balance. Also, BI can relieve parts of extraintestinal manifestations (EIMs), and prevent colorectal cancer.

CONCLUSION

Baicalin determined the promising therapeutic prospects as potential supplementary medicines for the treatment of IBD.

Intestinal organoids in coculture: redefining the boundaries of gut mucosa ex vivo modeling

All-time preservation of an intact mucosal barrier is crucial to ensuring intestinal homeostasis and, hence, the organism's overall health maintenance. This complex process relies on an equilibrated signaling system between the intestinal epithelium and numerous cell populations inhabiting the gut mucosa. Any perturbations of this delicate cross talk, particularly regarding the immune cell compartment and microbiota, may sustainably debilitate the intestinal barrier function. As a final joint event, a critical rise in epithelial permeability facilitates the exposure of submucosal immunity to microbial antigens, resulting in uncontrolled inflammation, collateral tissue destruction, and dysbiosis. Organoid-derived intestinal coculture models have established themselves as convenient tools to reenact such pathophysiological events, explore interactions between selected cell populations, and assess their roles with a central focus on intestinal barrier recovery and stabilization.

The Functional Role of Lactoferrin in Intestine Mucosal Immune System and Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), encompassing ulcerative colitis (UC) and Crohn's disease (CD), is one of the main types of intestinal inflammatory diseases with intestine mucosal immune disorder. Intestine mucosal immune system plays a remarkable and important role in the etiology and pathogenesis of IBD. Therefore, understanding the intestine mucosal immune mechanism is a key step to develop therapeutic interventions for IBD. Intestine mucosal immune system and IBD are influenced by various factors, such as inflammation, gut permeability, gut microbiota, and nutrients. Among these factors, emerging evidence show that nutrients play a key role in inflammation activation, integrity of intestinal barrier, and immune cell modulation. Lactoferrin (LF), an iron-binding glycoprotein belonging to transferrin family, is a dietary bioactive component abundantly found in mammalian milk. Notably, LF has been reported to perform diverse biological functions including antibacterial activity, anti-inflammatory activity, intestinal barrier protection, and immune cell modulation, and is involved in maintaining intestine mucosal immune homeostasis. The improved understanding of the properties of LF in intestine mucosal immune system and IBD will facilitate its application in nutrition, clinical medicine, and health. Herein, this review outlines the recent advancements on LF as a potential therapeutic intervention for IBD associated with intestine mucosal immune system dysfunction. We hope this review will provide a reference for future studies and lay a theoretical foundation for LF-based therapeutic interventions for IBD by understanding the particular effects of LF on intestine mucosal immune system.

Regulatory Effect of Mesenchymal Stem Cells on T Cell Phenotypes in Autoimmune Diseases

Research on mesenchymal stem cells (MSCs) starts from the earliest assumption that cells derived from the bone marrow have the ability to repair tissues. Several scientists have since documented the crucial role of bone marrow-derived MSCs (BM-MSCs) in processes such as embryonic bone and cartilage formation, adult fracture and tissue repair, and immunomodulatory activities in therapeutic applications. In addition to BM-MSCs, several sources of MSCs have been reported to possess tissue repair and immunoregulatory abilities, making them potential treatment options for many diseases. Therefore, the therapeutic potential of MSCs in various diseases including autoimmune conditions has been explored. In addition to an imbalance of T cell subsets in most patients with autoimmune diseases, they also exhibit complex disease manifestations, overlapping symptoms among diseases, and difficult treatment. MSCs can regulate T cell subsets to restore their immune homeostasis toward disease resolution in autoimmune conditions. This review summarizes the role of MSCs in relieving autoimmune diseases via the regulation of T cell phenotypes.

Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Intestinal immune tolerance is essential for the immune system, as it prevents abnormal immune responses to large quantities of antigens from the intestinal lumen, such as antigens from commensal microorganisms, and avoids self‐injury. Intestinal intraepithelial lymphocytes (IELs), a special group of mucosal T lymphocytes, play a significant role in intestinal immune tolerance. To accomplish this, IELs exhibit a high threshold of activation and low reactivity to most antigens from the intestinal lumen. In particular, CD8αα⁺TCRαβ⁺ IELs, TCRγδ⁺ IELs, and CD4⁺CD8αα⁺ IELs show great potential for maintaining intestinal immune tolerance and regulating intestinal immunity. However, if the intestinal microenvironment becomes abnormal or intestinal tolerance is broken, IELs may be activated abnormally and become pathogenic.