Activated Mucosal-associated Invariant T Cells Have a Pathogenic Role in a Murine Model of Inflammatory Bowel Disease

MAIT cells: Conserved watchers on the wall

MAIT cells are innate-like T cells residing in barrier tissues such as the lung, skin, and intestine. Both the semi-invariant T cell receptor of MAIT cells and the restricting element MR1 are deeply conserved across mammals, indicating non-redundant functions linked to antigenic specificity. MAIT cells across species concomitantly express cytotoxicity and tissue-repair genes, suggesting versatile functions. Accordingly, MAIT cells contribute to antibacterial responses as well as to the repair of damaged barrier tissues. MAIT cells recognize riboflavin biosynthetic pathway-derived metabolites, which rapidly cross epithelial barriers to be presented by antigen-presenting cells. Changes in gut ecology during intestinal inflammation drive the expansion of strong riboflavin and MAIT ligand producers. Thus, MAIT cells may enable real-time surveillance of microbiota dysbiosis across intact epithelia and provide rapid and context-dependent responses. Here, we discuss recent findings regarding the origin and regulation of MAIT ligands and the role of MAIT cells in barrier tissues. We speculate on the potential reasons for MAIT cell conservation during evolution.

Potent Immunomodulators Developed from an Unstable Bacterial Metabolite of Vitamin B2 Biosynthesis

Bacterial synthesis of vitamin B2 generates a by-product, 5-(2-oxopropylideneamino)-d-ribityl-aminouracil (5-OP-RU), with potent immunological properties in mammals, but it is rapidly degraded in water. This natural product covalently bonds to the key immunological protein MR1 in the endoplasmic reticulum of antigen presenting cells (APCs), enabling MR1 refolding and trafficking to the cell surface, where it interacts with T cell receptors (TCRs) on mucosal associated invariant T lymphocytes (MAIT cells), activating their immunological and antimicrobial properties. Here, we strategically modify this natural product to understand the molecular basis of its recognition by MR1. This culminated in the discovery of new water-stable compounds with extremely powerful and distinctive immunological functions. We report their capacity to bind MR1 inside APCs, triggering its expression on the cell surface (EC50 17 nM), and their potent activation (EC50 56 pM) or inhibition (IC50 80 nM) of interacting MAIT cells. We further derivatize compounds with diazirine-alkyne, biotin, or fluorophore (Cy5 or AF647) labels for detecting, monitoring, and studying cellular MR1. Computer modeling casts new light on the molecular mechanism of activation, revealing that potent activators are first captured in a tyrosine- and serine-lined cleft in MR1 via specific pi-interactions and H-bonds, before more tightly attaching via a covalent bond to Lys43 in MR1. This chemical study advances our molecular understanding of how bacterial metabolites are captured by MR1, influence cell surface expression of MR1, interact with T cells to induce immunity, and offers novel clues for developing new vaccine adjuvants, immunotherapeutics, and anticancer drugs.

Role of mucosal-associated invariant T cells dynamics in pathogenesis of Sjögren syndrome

Sjögren syndrome (SS) is an autoimmune disease characterized by chronic inflammatory infiltrates in the salivary and lacrimal glands. Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T-cells, predominantly found in mucosal tissues with crucial role in epithelial homeostasis. Thus, MAIT cells may be implicated in mucosal alterations of SS patients. Activation markers, inflammatory and cytotoxic cytokines were examined in 23 SS patients and compared to 23 healthy controls (HC). Tissular MAIT cells in salivary gland (SG) biopsies were also analyzed. Circulating MAIT cells were decreased in SS patients with a higher expression of CD69 and a higher CD4/CD8 ratio of MAIT cells. MAIT cells showed a higher production of IFNγ, TNFα and GzB in SS compare to HC. Tissular MAIT cells were present within inflamed SG of SS patients, while they were absent in SG of HC. Overall, circulating MAIT cells are decreased in the peripheral blood of SS albeit producing higher amounts of IFNγ, TNFα, and GzB. Tissular MAIT cells are detected in salivary glands from SS with a proinflammatory tissular cytokine environment. MAIT cells with abnormal phenotype, functions and tissular homeostasis may contribute to epithelial damage in SS.

MAIT cells monitor intestinal dysbiosis and contribute to host protection during colitis

Intestinal inflammation shifts microbiota composition and metabolism. How the host monitors and responds to such changes remains unclear. Here, we describe a protective mechanism by which mucosal-associated invariant T (MAIT) cells detect microbiota metabolites produced upon intestinal inflammation and promote tissue repair. At steady state, MAIT ligands derived from the riboflavin biosynthesis pathway were produced by aerotolerant bacteria residing in the colonic mucosa. Experimental colitis triggered luminal expansion of riboflavin-producing bacteria, leading to increased production of MAIT ligands. Modulation of intestinal oxygen levels suggested a role for oxygen in inducing MAIT ligand production. MAIT ligands produced in the colon rapidly crossed the intestinal barrier and activated MAIT cells, which expressed tissue-repair genes and produced barrier-promoting mediators during colitis. Mice lacking MAIT cells were more susceptible to colitis and colitis-driven colorectal cancer. Thus, MAIT cells are sensitive to a bacterial metabolic pathway indicative of intestinal inflammation.

Lung microbiome: new insights into the pathogenesis of respiratory diseases

The lungs were long thought to be sterile until technical advances uncovered the presence of the lung microbial community. The microbiome of healthy lungs is mainly derived from the upper respiratory tract (URT) microbiome but also has its own characteristic flora. The selection mechanisms in the lung, including clearance by coughing, pulmonary macrophages, the oscillation of respiratory cilia, and bacterial inhibition by alveolar surfactant, keep the microbiome transient and mobile, which is different from the microbiome in other organs. The pulmonary bacteriome has been intensively studied recently, but relatively little research has focused on the mycobiome and virome. This up-to-date review retrospectively summarizes the lung microbiome's history, composition, and function. We focus on the interaction of the lung microbiome with the oropharynx and gut microbiome and emphasize the role it plays in the innate and adaptive immune responses. More importantly, we focus on multiple respiratory diseases, including asthma, chronic obstructive pulmonary disease (COPD), fibrosis, bronchiectasis, and pneumonia. The impact of the lung microbiome on coronavirus disease 2019 (COVID-19) and lung cancer has also been comprehensively studied. Furthermore, by summarizing the therapeutic potential of the lung microbiome in lung diseases and examining the shortcomings of the field, we propose an outlook of the direction of lung microbiome research.

Pretreatment circulating MAIT cells, neutrophils, and periostin predicted the real-world response after 1-year mepolizumab treatment in asthmatics

BACKGROUND

Mepolizumab treatment improves symptom control and quality of life and reduces exacerbations in patients with severe eosinophilic asthma. However, biomarkers that predict therapeutic effectiveness must be determined for use in precision medicine. Herein, we elucidated the dynamics of various parameters before and after treatment as well as patient characteristics predictive of clinical responsiveness to mepolizumab after 1-year treatment.

METHODS

Twenty-seven patients with severe asthma were treated with mepolizumab for one year. Asthma control test scores, pulmonary function tests, fractional exhaled nitric oxide levels, and blood samples were evaluated. Additionally, we explored the role of CD69-positive mucosal-associated invariant T (MAIT) cells as a candidate biomarker for predicting treatment effectiveness by evaluating an OVA-induced asthma murine model using MR1 knockout mice, where MAIT cells were absent.

RESULTS

The frequencies of CD69-positive group 1 innate lymphoid cells, group 3 innate lymphoid cells, natural killer cells, and MAIT cells decreased after mepolizumab treatment. The frequency of CD69-positive MAIT cells and neutrophils was lower and serum periostin levels were higher in responders than in non-responders. In the OVA-induced asthma murine model, CD69-positive MAIT cell count in the whole mouse lung was significantly higher than that in the control mice. Moreover, OVA-induced eosinophilic airway inflammation was exacerbated in the MAIT cell-deficient MR1 knockout mice.

CONCLUSIONS

This study shows that circulating CD69-positive MAIT cells, neutrophils, and serum periostin might predict the real-world response after 1-year mepolizumab treatment. Furthermore, MAIT cells potentially have a protective role against type 2 airway inflammation.

MAIT cells in bacterial infectious diseases: heroes, villains, or both?

Due to the aggravation of bacterial drug resistance and the lag in the development of new antibiotics, it is crucial to develop novel therapeutic regimens for bacterial infectious diseases. Currently, immunotherapy is a promising regimen for the treatment of infectious diseases. Mucosal-associated invariant T (MAIT) cells, a subpopulation of innate-like T cells, are abundant in humans and can mount a rapid immune response to pathogens, thus becoming a potential target of immunotherapy for infectious diseases. At the site of infection, activated MAIT cells perform complex biological functions by secreting a variety of cytokines and cytotoxic substances. Many studies have shown that MAIT cells have immunoprotective effects because they can bridge innate and adaptive immune responses, leading to bacterial clearance, tissue repair, and homeostasis maintenance. MAIT cells also participate in cytokine storm generation, tissue fibrosis, and cancer progression, indicating that they play a role in immunopathology. In this article, we review recent studies of MAIT cells, discuss their dual roles in bacterial infectious diseases and provide some promising MAIT cell-targeting strategies for the treatment of bacterial infectious diseases.

MAIT cells drive chronic inflammation in a genetically diverse murine model of spontaneous colitis

Background & aims

Lymphocytes that produce IL-17 can confer protective immunity during infections by pathogens, yet their involvement in inflammatory diseases is a subject of debate. Although these cells may perpetuate inflammation, resulting in tissue damage, they are also capable of contributing directly or indirectly to tissue repair, thus necessitating more detailed investigation. Mucosal-Associated-Invariant-T (MAIT) cells are innate-like T cells, acquiring a type III phenotype in the thymus. Here, we dissected the role of MAIT cells in vivo using a spontaneous colitis model in a genetically diverse mouse strain.

Methods

Multiparameter spectral flow cytometry and scRNAseq were used to characterize MAIT and immune cell dynamics and transcriptomic signatures respectively, in the collaborative-cross strain, CC011/Unc and CC011/Unc- Traj33 -/- .

Results

In contrast to many conventional mouse laboratory strains, the CC011 strain harbors a high baseline population of MAIT cells. We observed an age-related increase in colonic MAIT cells, Th17 cells, regulatory T cells, and neutrophils, which paralleled the development of spontaneous colitis. This progression manifested histological traits reminiscent of human IBD. The transcriptomic analysis of colonic MAIT cells from CC011 revealed an activation profile consistent with an inflammatory milieu, marked by an enhanced type-III response. Notably, IL-17A was abundantly secreted by MAIT cells in the colons of afflicted mice. Conversely, in the MAIT cell-deficient CC011-Traj33-/- mice, there was a notable absence of significant colonic histopathology. Furthermore, myeloperoxidase staining indicated a substantial decrease in colonic neutrophils.

Conclusions

Our findings suggest that MAIT cells play a pivotal role in modulating the severity of intestinal pathology, potentially orchestrating the inflammatory process by driving the accumulation of neutrophils within the colonic environment.

MAIT cells in immune-mediated tissue injury and repair

Mucosal-associated invariant T (MAIT) cells are T cells that express a semi-invariant αβ T-cell receptor (TCR), recognizing non-peptide antigens, such as microbial-derived vitamin B2 metabolites, presented by the nonpolymorphic MHC class I related-1 molecule. Like NKT cells and γδT cells, MAIT cells belong to the group of innate-like T cells that combine properties of the innate and adaptive immune systems. They account for up to 10% of the blood T-cell population in humans and are particularly abundant at mucosal sites. Beyond the emerging role of MAIT cells in antibacterial and antiviral defenses, increasing evidence suggests additional functions in noninfectious settings, including immune-mediated inflammatory diseases and tissue repair. Here, we discuss recent advances in the understanding of MAIT cell functions in sterile tissue inflammation, with a particular focus on autoimmunity, chronic inflammatory diseases, and tissue repair.

Mucosal-associated invariant T cells display both pathogenic and protective roles in patients with inflammatory bowel diseases

An important subtype of the innate-like T lymphocytes is mucosal-associated invariant T (MAIT) cells expressing a semi-invariant T cell receptor α (TCR-α) chain. MAIT cells could be activated mainly by TCR engagement or cytokines. They have been found to have essential roles in various immune mediated. There have been growing preclinical and clinical findings that show an association between MAIT cells and the physiopathology of inflammatory bowel diseases (IBD). Of note, published reports demonstrate contradictory findings regarding the role of MAIT cells in IBD patients. A number of reports suggests a protective effect, whereas others show a pathogenic impact. The present review article aimed to explore and discuss the findings of experimental and clinical investigations evaluating the effects of MAIT cells in IBD subjects and animal models. Findings indicate that MAIT cells could exert opposite effects in the course of IBD, including an anti-inflammatory protective effect of blood circulating MAIT cells and an effector pathogenic effect of colonic MAIT cells. Another important finding is that blood levels of MAIT cells can be considered as a potential biomarker in IBD patients.

Gut Inflammation in Axial Spondyloarthritis Patients is Characterized by a Marked Type 17 Skewed Mucosal Innate-like T Cell Signature

OBJECTIVE

Patients with spondyloarthritis (SpA) often present with microscopic signs of gut inflammation, a risk factor for progressive disease. We investigated whether mucosal innate-like T cells are involved in dysregulated interleukin-23 (IL-23)/IL-17 responses in the gut-joint axis in SpA.

METHODS

Ileal and colonic intraepithelial lymphocytes (IELs), lamina propria lymphocytes (LPLs), and paired peripheral blood mononuclear cells (PBMCs) were isolated from treatment-naive patients with nonradiographic axial SpA with (n = 11) and without (n = 14) microscopic gut inflammation and healthy controls (n = 15) undergoing ileocolonoscopy. The presence of gut inflammation was assessed histopathologically. Immunophenotyping of innate-like T cells and conventional T cells was performed using intracellular flow cytometry. Unsupervised clustering analysis was done by FlowSOM technology. Serum IL-17A levels were measured via Luminex.

RESULTS

Microscopic gut inflammation in nonradiographic axial SpA was characterized by increased ileal intraepithelial γδ-hi T cells, a γδ-T cell subset with elevated γδ-T cell receptor expression. γδ-hi T cells were also increased in PBMCs of patients with nonradiographic axial SpA versus healthy controls and were strongly associated with Ankylosing Spondylitis Disease Activity Score. The abundance of mucosal-associated invariant T cells and invariant natural killer T cells was unaltered. Innate-like T cells in the inflamed gut showed increased RORγt, IL-17A, and IL-22 levels with loss of T-bet, a signature that was less pronounced in conventional T cells. Presence of gut inflammation was associated with higher serum IL-17A levels. In patients treated with tumor necrosis factor blockade, the proportion of γδ-hi cells and RORγt expression in blood was completely restored.

CONCLUSION

Intestinal innate-like T cells display marked type 17 skewing in the inflamed gut mucosa of patients with nonradiographic axial SpA. γδ-hi T cells are linked to intestinal inflammation and disease activity in SpA.

Mucosal-associated invariant T cells in digestive tract: Local guardians or destroyers?

Mucosa-associated invariant T cells (MAIT) are a class of innate-like T lymphocytes mainly presenting CD8+ phenotype with a semi-invariant αβ T-cell receptor, which specifically recognises MR1-presented biosynthetic derivatives of riboflavin synthesis produced by various types of microbiomes. As innate-like T lymphocytes, MAIT can be activated by a variety of cytokines, leading to immediate immune responses to infection and tumour cues. As an organ that communicates with the external environment, the digestive tract, especially the gastrointestinal tract, contains abundant microbial populations. Communication between MAIT and local microbiomes is important for the homeostasis of mucosal immunity. In addition, accumulating evidence suggests changes in the abundance and structure of the microbial community during inflammation and tumorigenesis plays a critical role in disease progress partly through their impact on MAIT development and function. Therefore, it is essential for the understanding of MAIT response and their interaction with microbiomes in the digestive tract. Here, we summarised MAIT characteristics in the digestive tract and its alteration facing inflammation and tumour, raising that targeting MAIT can be a candidate for treatment of gastrointestinal diseases.

T Cell Repertoire Homogeneity and Blood-Gut Overlap in Patients With Inflammatory Bowel Disease

BACKGROUND & AIMS

Inflammatory bowel disease (IBD) causes a marked increase in the number of T cells in the intestinal mucosa. Debate exists about whether these excess cells arise from local clonal proliferation or recruitment from the periphery.

METHODS

CD8+ T cells were sorted from colon biopsy specimens and blood for T-cell receptor (TCR) β-chain sequencing. Biopsy specimens from inflamed or uninflamed colon from ulcerative colitis or Crohn's disease cohorts were compared with colon biopsy specimens from people without IBD, as well as with autologous blood α4β7+, α4β7- effector/memory, terminal effector/memory CD45RA+ T cell, and mucosal-associated invariant T-cell CD8 subpopulations.

RESULTS

CD8 TCR diversity in mucosa and blood did not correlate with inflammation. Repertoire overlap between any 2 distinct locations of a given person's colon was consistently high, although often lower between inflamed and uninflamed sites. CD8 TCR repertoires overlapped between the colon and each peripheral blood subpopulation studied, with the highest overlap seen for integrin α4β7+ T cells. Inflamed tissue consistently overlapped more than uninflamed tissue with each blood subpopulation.

CONCLUSIONS

CD8 T-cell clones are spread homogenously throughout the length of the colon. Although TCR repertoire overlap is greater within than between inflamed and uninflamed colon segments, a similar TCR diversity in both argues against local clonal expansion being the main source of excess cytotoxic T cells in inflamed mucosa. Rather, the increased TCR overlap observed between blood and inflamed mucosa supports the significance of T-cell trafficking in IBD pathogenesis, particularly concerning α4β7+ T-cell populations.

ScRNA-seq defines dynamic T-cell subsets in longitudinal colon and peripheral blood samples in immune checkpoint inhibitor-induced colitis

Immune checkpoint inhibitors (ICIs) are increasingly being used to manage multiple tumor types. Unfortunately, immune-related adverse events affect up to 60% of recipients, often leading to treatment discontinuation in settings where few alternative cancer therapies may be available. Checkpoint inhibitor induced colitis (ICI-colitis) is a common toxicity for which the underlying mechanisms are poorly defined. To better understand the changing colon-specific and peripheral immune environments over the course of progression and treatment of colitis, we collected blood and colon tissue from a patient with Merkel cell carcinoma who developed colitis on treatment with pembrolizumab. We performed single-cell RNA sequencing and T-cell receptor sequencing on samples collected before, during and after pembrolizumab and after various interventions to mitigate toxicity. We report T-cells populations defined by cytotoxicity, memory, and proliferation markers at various stages of colitis. We show preferential depletion of CD8+ T cells with biologic therapy and nominate both circulating and colon-resident T-cell subsets as potential drivers of inflammation and response to immune suppression. Our findings highlight the need for further exploration of the colon immune environment and rationalize future studies evaluating biologics for ICI-colitis, including in the context of ICI re-challenge.

New insights into MAIT cells in autoimmune diseases

Mucosal-associated invariant T (MAIT) cells are resident T cells that express semi-invariant TCR chains and are restricted by monomorphic major histocompatibility complex (MHC) class I-related molecules (MR1). MAIT cells can be activated by microbial-specific metabolites (MR1-dependent mode) or cytokines (MR1-independent mode). Activated MAIT cells produce chemokines, cytotoxic molecules (granzyme B and perforin), and proinflammatory cytokines (IFN-γ, TNF-α, and IL-17), to clear pathogens and target infected cells involved in the pro-inflammatory, migratory, and cytolytic properties of MAIT cells. MAIT cells produce pro-inflammatory cytokines in the target organs of autoimmune diseases and contribute to the development and progression of autoimmune diseases. This article reviews the biological characteristics, activation mechanism, dynamic migration, and dual functions of MAIT cells, and focuses on the mechanism and potential application of MAIT cells in the early diagnosis, disease activity monitoring, and therapeutic targets of autoimmune diseases, to lay a foundation for future research.

MAIT cells and the microbiome

Mucosal associated invariant T (MAIT) cells are innate-like T lymphocytes, strikingly enriched at mucosal surfaces and characterized by a semi-invariant αβ T cell receptor (TCR) recognizing microbial derived intermediates of riboflavin synthesis presented by the MHC-Ib molecule MR1. At barrier sites MAIT cells occupy a prime position for interaction with commensal microorganisms, comprising the microbiota. The microbiota is a rich source of riboflavin derived antigens required in early life to promote intra-thymic MAIT cell development and sustain a life-long population of tissue resident cells. A symbiotic relationship is thought to be maintained in health whereby microbes promote maturation and homeostasis, and in turn MAIT cells can engage a TCR-dependent "tissue repair" program in the presence of commensal organisms conducive to sustaining barrier function and integrity of the microbial community. MAIT cell activation can be induced in a MR1-TCR dependent manner or through MR1-TCR independent mechanisms via pro-inflammatory cytokines interleukin (IL)-12/-15/-18 and type I interferon. MAIT cells provide immunity against bacterial, fungal and viral pathogens. However, MAIT cells may have deleterious effects through insufficient or exacerbated effector activity and have been implicated in autoimmune, inflammatory and allergic conditions in which microbial dysbiosis is a shared feature. In this review we summarize the current knowledge on the role of the microbiota in the development and maintenance of circulating and tissue resident MAIT cells. We also explore how microbial dysbiosis, alongside changes in intestinal permeability and imbalance between pro- and anti-inflammatory components of the immune response are together involved in the potential pathogenicity of MAIT cells. Whilst there have been significant improvements in our understanding of how the microbiota shapes MAIT cell function, human data are relatively lacking, and it remains unknown if MAIT cells can conversely influence the composition of the microbiota. We speculate whether, in a human population, differences in microbiomes might account for the heterogeneity observed in MAIT cell frequency across mucosal sites or between individuals, and response to therapies targeting T cells. Moreover, we speculate whether manipulation of the microbiota, or harnessing MAIT cell ligands within the gut or disease-specific sites could offer novel therapeutic strategies.

Mucosal-Associated Invariant T Cells in the Digestive System: Defender or Destroyer?

Mucosal-associated invariant T (MAIT) cells are a subset of innate T lymphocytes that express the semi-invariant T cell receptor and recognize riboflavin metabolites via the major histocompatibility complex class I-related protein. Given the abundance of MAIT cells in the human body, their role in human diseases has been increasingly studied in recent years. MAIT cells may serve as targets for clinical therapy. Specifically, this review discusses how MAIT cells are altered in gastric, esophageal, intestinal, and hepatobiliary diseases and describes their protective or pathogenic roles. A greater understanding of MAIT cells will provide a more favorable therapeutic approach for digestive diseases in the clinical field.

MR1-dependence of unmetabolized folic acid side-effects

The fortification of flour with folic acid for the prevention of neural tube defects (NTD) is currently mandated in over eighty countries worldwide, hence compelling its consumption by the greater part of the world’s population. Notwithstanding its beneficial impact on rates of NTD, pervasive folic acid supplementation has invariably led to additive daily intakes reaching well beyond their original target, resulting in the circulation of unmetabolized folic acid. Associated idiopathic side-effects ranging from allergies to cancer have been suggested, albeit inconclusively. Herein, we hypothesize that their inconsistent detection and elusive etiology are linked to the in vivo generation of the immunosuppressive folic acid metabolite 6-formylpterin, which interferes with the still emerging and varied functions of Major Histocompatibility Complex-related molecule 1 (MR1)-restricted T cells. Accordingly, we predict that fortification-related adverse health outcomes can be eliminated by substituting folic acid with the bioequivalent folate vitamer 5-methyltetrahydrofolate, which does not break down into 6-formylpterin.

Mucosal-associated invariant T cells: a cryptic coordinator in HIV-infected immune reconstitution

Human immunodeficiency virus type 1 (HIV-1) infection causes considerable morbidity and mortality worldwide. Although antiretroviral therapy (ART) has largely transformed HIV infection from a fatal disease to a chronic condition, approximately 10%~40% of HIV-infected individuals who receive effective ART and sustain long-term viral suppression still cannot achieve optimal immune reconstitution. These patients are called immunological non-responders, a state associated with poor clinical prognosis. Mucosal-associated invariant T (MAIT) cells are an evolutionarily conserved unconventional T cell subset defined by expression of semi-invariant αβ T cell receptor (TCR), which recognizes metabolites derived from the riboflavin biosynthetic pathway presented on major histocompatibility complex (MHC)-related protein-1 (MR1). MAIT cells, which are considered to act as a bridge between innate and adaptive immunity, produce a wide range of cytokines and cytotoxic molecules upon activation through TCR-dependent and TCR-independent mechanisms, which is of major importance in defense against a variety of pathogens. In addition, MAIT cells are involved in autoimmune and immune-mediated diseases. The number of MAIT cells is dramatically and irreversibly decreased in the early stage of HIV infection and is not fully restored even after long-term suppressive ART. In light of the important role of MAIT cells in mucosal immunity and because microbial translocation is inversely associated with CD4⁺ T cell counts, we propose that MAIT cells participate in the maintenance of intestinal barrier integrity and microbial homeostasis, thus further affecting immune reconstitution in HIV-infected individuals. This article is protected by copyright. All rights reserved.

Characteristics of mucosal-associated invariant T cells and their roles in immune diseases

Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T cells that express a semi-invariant T cell receptor and are restricted by the molecule major histocompatibility complex class I-related molecule 1 (MR1). MAIT cells recognize biosynthetic derivatives of the riboflavin synthesis pathway present in microbes. MAIT cells have attracted increased interest related to various immune responses because of their unique features including their abundance in humans, nonpeptidic antigens, and ability to respond to antigenic and non-antigenic stimuli. The numbers of circulating MAIT cells are decreased in many immune diseases such as multiple sclerosis, systemic lupus erythematosus, and inflammatory bowel diseases. However, the remaining MAIT cells have an increased cytokine-producing capacity and activated status, which is related to disease activity. Additionally, MAIT cells have been observed at sites of inflammation including the kidneys, synovial fluid and intestinal mucosa. These findings suggest their involvement in the pathogenesis of immune diseases. In this mini-review, we summarize the recent findings of MAIT cells in human immune diseases and animal models, and discuss their role and potential as a therapeutic target.