Association between OX40L polymorphism and type 2 diabetes mellitus in Iranians

INTRODUCTION

Diabetes mellitus (DM) is one of the leading causes of morbidity and mortality worldwide. It is a multifactorial disease that genetic and environmental factors contribute to its development. The aim of the study was to investigate the association of OX40L promoter gene polymorphisms with type 2 diabetes mellitus (T2DM) in Iranians.

MATERIALS AND METHODS

Three hundred and sixty-eight subjects including 184 healthy subjects and 184 T2DM patients were enrolled in our study. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) was applied to detect genotype and allele frequencies of rs3850641, rs1234313 and rs10912580. In addition, SNPStats web tool was applied to estimate haplotype frequency and linkage disequilibrium (LD).

RESULTS

The distribution of tested polymorphisms was statistically different between the T2DM patients and healthy subjects (P < 0.01). rs1234313 AG (OR = 0.375, 95% CI = 0.193-0.727, P = 0.004) and rs10912580 AG (OR = 0.351, 95% CI = 0.162-0.758, P = 0.008) genotypes were associated with the decreased risk of T2DM in Iranians. Moreover, our prediction revealed that AAG (OR = 0.46, 95% CI= (0.28-0.76), P = 0.0028) and GAG (OR = 0.24, 95% CI= (0.13-0.45), P < 0.0001) haplotypes were related to the reduced risk of the disease. However, the tested polymorphisms had no effect on biochemical parameters and body mass index (BMI) in the patient group (P > 0.05).

CONCLUSION

Our findings revealed that OX40L promoter gene polymorphisms are associated with T2DM. Moreover, genotype and allelic variations were related to the decreased risk of T2DM in Iranians. Further studies are recommended to show whether these polymorphic variations could affect OX40/OX40L interaction or OX40L phenotype.

Oral histidine affects gut microbiota and MAIT cells improving glycemic control in type 2 diabetes patients

Amino acids, metabolized by host cells as well as commensal gut bacteria, have signaling effects on host metabolism. Oral supplementation of the essential amino acid histidine has been shown to exert metabolic benefits. To investigate whether dietary histidine aids glycemic control, we performed a case-controlled parallel clinical intervention study in participants with type 2 diabetes (T2D) and healthy controls. Participants received oral histidine for seven weeks. After 2 weeks of histidine supplementation, the microbiome was depleted by antibiotics to determine the microbial contribution to histidine metabolism. We assessed glycemic control, immunophenotyping of peripheral blood mononucelar cells (PBMC), DNA methylation of PBMCs and fecal gut microbiota composition. Histidine improves several markers of glycemic control, including postprandial glucose levels with a concordant increase in the proportion of MAIT cells after two weeks of histidine supplementation. The increase in MAIT cells was associated with changes in gut microbial pathways such as riboflavin biosynthesis and epigenetic changes in the amino acid transporter SLC7A5. Associations between the microbiome and MAIT cells were replicated in the MetaCardis cohort. We propose a conceptual framework for how oral histidine may affect MAIT cells via altered gut microbiota composition and SLC7A5 expression in MAIT cells directly and thereby influencing glycemic control. Future studies should focus on the role of flavin biosynthesis intermediates and SLC7A5 modulation in MAIT cells to modulate glycemic control.

Tissue-resident and innate-like T cells in patients with advanced chronic liver disease

Chronic liver disease results from the orchestrated interplay of components of innate and adaptive immunity in response to liver tissue damage. Recruitment, positioning, and activation of immune cells can contribute to hepatic cell death, inflammation, and fibrogenesis. With disease progression and increasing portal pressure, repeated translocation of bacterial components from the intestinal lumen through the epithelial and vascular barriers leads to persistent mucosal, hepatic, and systemic inflammation which contributes to tissue damage, immune dysfunction, and microbial infection. It is increasingly recognised that innate-like and adaptive T-cell subsets located in the liver, mucosal surfaces, and body cavities play a critical role in the progression of advanced liver disease and inflammatory complications of cirrhosis. Mucosal-associated invariant T cells, natural killer T cells, γδ T cells, and tissue-resident memory T cells in the gut, liver, and ascitic fluid share certain characteristic features, which include that they recognise microbial products, tissue alarmins, cytokines, and stress ligands in tissues, and perform effector functions in chronic liver disease. This review highlights recent advances in the comprehension of human tissue-resident and unconventional T-cell populations and discusses the mechanisms by which they contribute to inflammation, fibrosis, immunosuppression, and antimicrobial surveillance in patients with cirrhosis. Understanding the complex interactions of immune cells in different compartments and their contribution to disease progression will provide further insights for effective diagnostic interventions and novel immunomodulatory strategies in patients with advanced chronic liver disease.

Classic costimulatory interactions in MAIT cell responses: from gene expression to immune regulation

Mucosa-associated invariant T (MAIT) cells are evolutionarily conserved, innate-like T lymphocytes with enormous immunomodulatory potentials. Due to their strategic localization, their invariant T cell receptor (iTCR) specificity for major histocompatibility complex-related protein 1 (MR1) ligands of commensal and pathogenic bacterial origin, and their sensitivity to infection-elicited cytokines, MAIT cells are best known for their antimicrobial characteristics. However, they are thought to also play important parts in the contexts of cancer, autoimmunity, vaccine-induced immunity, and tissue repair. While cognate MR1 ligands and cytokine cues govern MAIT cell maturation, polarization, and peripheral activation, other signal transduction pathways, including those mediated by costimulatory interactions, regulate MAIT cell responses. Activated MAIT cells exhibit cytolytic activities and secrete potent inflammatory cytokines of their own, thus transregulating the biological behaviors of several other cell types, including dendritic cells, macrophages, natural killer cells, conventional T cells, and B cells, with significant implications in health and disease. Therefore, an in-depth understanding of how costimulatory pathways control MAIT cell responses may introduce new targets for optimized MR1/MAIT cell-based interventions. Herein, we compare and contrast MAIT cells and mainstream T cells for their expression of classic costimulatory molecules belonging to the immunoglobulin superfamily and the tumor necrosis factor (TNF)/TNF receptor superfamily, based not only on the available literature but also on our transcriptomic analyses. We discuss how these molecules participate in MAIT cells' development and activities. Finally, we introduce several pressing questions vis-à-vis MAIT cell costimulation and offer new directions for future research in this area.

MAITabolism2 - the emerging understanding of MAIT cell metabolism and their role in metabolic disease

Mucosal associated invariant T (MAIT) cells are a population of unconventional innate T cells due to their non-MHC restriction and rapid effector responses. MAIT cells can recognise bacterial derived antigens presented on the MHC-like protein via their semi-restricted T cell receptor (TCR). Upon TCR triggering MAIT cells rapidly produce a range of effector molecules including cytokines, lytic granules and chemokines. This rapid and robust effector response makes MAIT cells critical in host responses against many bacterial pathogens. MAIT cells can also respond independent of their TCR via innate cytokines such as interleukin (IL)-18, triggering cytokine production, and are important in anti-viral responses. In addition to their protective role, MAIT cells have been implicated in numerous inflammatory diseases, including metabolic diseases often contributing to the pathogenesis via their robust cytokine production. Effector cells such as MAIT cells require significant amounts of energy to support their potent responses, and the type of nutrients available can dictate the functionality of the cell. Although data on MAIT cell metabolism is just emerging, several recent studies are starting to define the intrinsic metabolic requirements and regulators of MAIT cells. In this review we will outline our current understanding of MAIT cell metabolism, and outline their role in metabolic disease, and how disease-related changes in extrinsic metabolism can alter MAIT cell responses.

Mucosal-associated invariant T cells predict increased acute graft-versus-host-disease incidence in patients receiving allogeneic hematopoietic stem cell transplantation

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are innate-like T cells, some studies have reported that the number of circulating MAIT cells reduced in patients with acute graft-versus-host-disease (aGVHD) development. However, the role of donor MAIT cells on aGVHD development and subsequent functional change still remain unclear.

METHODS

The study recruited 86 patients with hematological malignancies who underwent allogeneic hematopoietic cell transplantation (HCT) from May 1, 2018 to June 30, 2019. MAIT cells, their subset, and cytokine levels were measured by flow cytometry. Gray's test was used to assess the impact of graft MAIT cell proportion and number on aGVHD incidence. The Cox proportional hazard model was used in the multivariate analysis. The comparison for continuous variables was assessed using Mann-Whitney analysis. RNA-sequencing was performed to investigate the possible molecular pathway changes.

RESULTS

Our study showed that the proportion of MAIT cells in grafts was not different from normal controls, but the CD4/8 subsets were altered. Taking the median of the proportion and number of MAIT cells in the graft as the threshold, the results showed that the incidence of grade B-D aGVHD in patients with MAIT cell proportion ≥ 3.03% was significantly higher than that in patients with MAIT cell proportion < 3.03% (56.3%, 95% CI 37.1-71.2 versus 23.1%, 95% CI 13.8-46.2; P = 0.038).The number of MAIT cells in the graft was not associated with aGVHD development (P = 0.173), however, when the graft contained more CD4 positive, CD8 positive, and CD4/CD8 double-positive MAIT cells, the incidence of aGVHD was significantly increased (P = 0.019, P = 0.035 and P = 0.027, respectively). Besides, reduced frequencies and counts of circulating MAIT cells were observed in patients with aGVHD when compared to patients without aGVHD, accompanied by enhanced production of Tumor necrosis factor-α, Interferon-γ and upregulated programmed death-1, CXC Chemokine Receptor-6 (CXCR6) and CD38 expression. Gene set enrichment analysis of MAIT cell RNA-seq data showed interferon-α response pathway upregulated in aGVHD patients when compared with patients without aGVHD and healthy controls.

CONCLUSIONS

Our study shows that MAIT cells in grafts and peripheral blood are both closely related to the aGVHD development post allogeneic HCT. Interferon-α response pathway perhaps is a critical regulation mechanism for the MAIT cell involvement in aGVHD development.

Activation-induced pyroptosis contributes to the loss of MAIT cells in chronic HIV-1 infected patients

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are systemically depleted in human immunodeficiency virus type 1 (HIV-1) infected patients and are not replenished even after successful combined antiretroviral therapy (cART). This study aimed to identify the mechanism underlying MAIT cell depletion.

METHODS

In the present study, we applied flow cytometry, single-cell RNA sequencing and immunohistochemical staining to evaluate the characteristics of pyroptotic MAIT cells in a total of 127 HIV-1 infected individuals, including 69 treatment-naive patients, 28 complete responders, 15 immunological non-responders, and 15 elite controllers, at the Fifth Medical Center of Chinese PLA General Hospital, Beijing, China.

RESULTS

Single-cell transcriptomic profiles revealed that circulating MAIT cells from HIV-1 infected subjects were highly activated, with upregulation of pyroptosis-related genes. Further analysis revealed that increased frequencies of pyroptotic MAIT cells correlated with markers of systemic T-cell activation, microbial translocation, and intestinal damage in cART-naive patients and poor CD4⁺ T-cell recovery in long-term cART patients. Immunohistochemical staining revealed that MAIT cells in the gut mucosa of HIV-1 infected patients exhibited a strong active gasdermin-D (GSDMD, marker of pyroptosis) signal near the cavity side, suggesting that these MAIT cells underwent active pyroptosis in the colorectal mucosa. Increased levels of the proinflammatory cytokines interleukin-12 (IL-12) and IL-18 were observed in HIV-1 infected patients. In addition, activated MAIT cells exhibited an increased pyroptotic phenotype after being triggered by HIV-1 virions, T-cell receptor signals, IL-12 plus IL-18, and combinations of these factors, in vitro.

CONCLUSIONS

Activation-induced MAIT cell pyroptosis contributes to the loss of MAIT cells in HIV-1 infected patients, which could potentiate disease progression and poor immune reconstitution.

Characterization of cord blood CD3+ TCRVα7.2+ CD161high T and innate lymphoid cells in the pregnancies with gestational diabetes, morbidly adherent placenta, and pregnancy hypertension diseases

PROBLEM

Although pregnant women with gestational diabetes (GD), morbidly adherent placenta (MAP), and pregnancy hypertension (pHT) diseases lead to intrauterine growth restriction (IUGR), little is known about their effect on mucosal-associated invariant T (MAIT) and innate lymphoid cells (ILC) in the umbilical cord. This study aimed to quantify and characterize MAIT cells and ILCs in the cord blood of pregnant women with GD, MAP, and pHT diseases.

METHOD OF STUDY

Cord blood mononuclear cells (CBMCs) were isolated by Ficoll-Paque gradient. CD3⁺ TCRVα7.2⁺ CD161^high cells and ILC subsets were quantified by flow cytometry. CBMCs were stimulated with PMA/Ionomycin and Golgi Plug for 4 h and stained for IFN-γ, TNF-α, and granzyme B. The stained cells were analyzed on FACS ARIA III.

RESULTS

Compared with healthy pregnancies, in the cord blood of the pHT group, elevated number of lymphocytes was observed. Moreover, the absolute number of IFN-γ producing CD4⁺ or CD4^- subsets of CD3⁺ TCRVα7.2⁺ CD161^high cells as well as those producing granzyme B were significantly elevated in the pHT group compared to healthy controls suggesting increased MAIT cell activity in the pHT cord blood. Similarly, in the MAP group, the absolute number of total CD3⁺ TCRVα7.2⁺ CD161^high cells, but not individual CD4⁺ or negative subsets, were significantly increased compared with healthy controls' cord blood. Absolute numbers of total CD3⁺ TCRVα7.2⁺ CD161^high cells and their subsets were comparable in the cord blood of the GD group compared with healthy controls. Finally, the absolute number of total ILCs and ILC3 subset were significantly elevated in only pHT cord blood compared with healthy controls. Our data also reveal that IFN-γ⁺ or granzyme B⁺ cell numbers negatively correlated with fetal birth weight.

CONCLUSIONS

CD3⁺ TCRVα7.2⁺ CD161^high cells and ILCs show unique expansion and activity in the cord blood of pregnant women with distinct diseases causing IUGR and may play roles in fetal growth restriction.

Modulating Oxidative Stress in B Cells Promotes Immunotherapy in Food Allergy

Allergen-specific immunotherapy (SIT) is the mainstay in the treatment of allergic diseases; its therapeutic efficacy is to be improved. Bacterial flagellin (FGN) has immune regulatory functions. This study investigates the role of FGN in promoting immunotherapy efficacy through modulating oxidative stress in regulatory B cells (Bregs). Blood samples were collected from patients with food allergy (FA) and healthy control (HC) subjects. CD19+ CD5+ Bregs were purified from blood samples by flow cytometry cell sorting. A murine FA model was developed with ovalbumin as the specific antigen. The results showed that peripheral Bregs from FA patients showed lower TLR5-related signals and higher apoptotic activities. The peripheral Breg frequency was negatively correlated with serum FGN levels in FA patients. Exposure to a specific antigen in culture induced antigen-specific Breg apoptosis that was counteracted by the presence of FGN. FGN diminished specific antigen-induced oxidative stress in Bregs. The STAT3/MAPKp38/NF-κB signal pathway was involved in the FGN/TLR5 signal-promoted superoxide dismutase expression in Bregs. Administration of FGN promotes the SIT efficacy in suppressing experimental FA. In summary, administration of FGN promotes SIT efficacy on FA, suggesting that the combination of FGN and SIT can be a novel therapy that has the translational potential to be employed in the treatment of allergic diseases.

Clostridioides difficile Toxin CDT Induces Cytotoxic Responses in Human Mucosal-Associated Invariant T (MAIT) Cells

Clostridioides difficile is the major cause of antibiotic-associated colitis (CDAC) with increasing prevalence in morbidity and mortality. Severity of CDAC has been attributed to hypervirulent C. difficile strains, which in addition to toxin A and B (TcdA, TcdB) produce the binary toxin C. difficile transferase (CDT). However, the link between these toxins and host immune responses as potential drivers of immunopathology are still incompletely understood. Here, we provide first experimental evidence that C. difficile toxins efficiently activate human mucosal-associated invariant T (MAIT) cells. Among the tested toxins, CDT and more specifically, the substrate binding and pore-forming subunit CDTb provoked significant MAIT cell activation resulting in selective MAIT cell degranulation of the lytic granule components perforin and granzyme B. CDT-induced MAIT cell responses required accessory immune cells, and we suggest monocytes as a potential CDT target cell population. Within the peripheral blood mononuclear cell fraction, we found increased IL-18 levels following CDT stimulation and MAIT cell response was indeed partly dependent on this cytokine. Surprisingly, CDT-induced MAIT cell activation was found to be partially MR1-dependent, although bacterial-derived metabolite antigens were absent. However, the role of antigen presentation in this process was not analyzed here and needs to be validated in future studies. Thus, MR1-dependent induction of MAIT cell cytotoxicity might be instrumental for hypervirulent C. difficile to overcome cellular barriers and may contribute to pathophysiology of CDAC.

TARM-1 Is Critical for Macrophage Activation and Th1 Response in Mycobacterium tuberculosis Infection

T cell-interacting activating receptor on myeloid cells 1 (TARM-1) is a novel leukocyte receptor expressed in neutrophils and macrophages. It plays an important role in proinflammatory response in acute bacterial infection, but its immunomodulatory effects on chronic Mycobacterium tuberculosis infections remain unclear. TARM-1 expression was significantly upregulated on CD14^high monocytes from patients with active pulmonary tuberculosis (TB) as compared that on cells from patients with latent TB or from healthy control subjects. Small interfering RNA knockdown of TARM-1 reduced expression levels of proinflammatory cytokines IL-12, IL-18, IL-1β, and IL-8 in M. tuberculosis-infected macrophages, as well as that of HLA-DR and costimulatory molecules CD83, CD86, and CD40. Moreover, TARM-1 enhanced phagocytosis and intracellular killing of M. tuberculosis through upregulating reactive oxygen species. In an in vitro monocyte and T cell coculture system, blockade of TARM-1 activity by TARM-1 blocking peptide suppressed CD4⁺ T cell activation and proliferation. Finally, administration of TARM-1 blocking peptide in a mouse model of M. tuberculosis infection increased bacterial load and lung pathology, which was associated with decreased macrophage activation and IFN-γ production by T cell. Taken together, these results, to our knowledge, demonstrate a novel immune protective role of TARM-1 in M. tuberculosis infection and provide a potential therapeutic target for TB disease.

OX40L/OX40 Signal Promotes IL-9 Production by Mucosal MAIT Cells During Helicobacter pylori Infection

Mucosal associated invariant T (MAIT) cells play a critical role in Helicobacter pylori (H. pylori)-induced gastritis by promoting mucosal inflammation and aggravating mucosal injuries (1, 2). However, the underlying mechanism and key molecules involved are still uncertain. Here we identified OX40, a co-stimulatory molecule mainly expressed on T cells, as a critical regulator to promote proliferation and IL-9 production by MAIT cells and facilitate mucosal inflammation in H. pylori-positive gastritis patients. Serum examination revealed an increased level of IL-9 in gastritis patients. Meanwhile, OX40 expression was increased in mucosal MAIT cells, and its ligand OX40L was also up-regulated in mucosal dendritic cells (DCs) of gastritis patients, compared with healthy controls. Further results demonstrated that activation of the OX40/OX40L pathway promoted IL-9 production by MAIT cells, and MAIT cells displayed a highly-activated phenotype after the cross-linking of OX40 and OX40L. Moreover, the level of IL-9 produced by MAIT cells was positively correlated with inflammatory indexes in the gastric mucosa, suggesting the potential role of IL-9-producing MAIT cells in mucosal inflammation. Taken together, we elucidated that OX40/OX40L axis promoted mucosal MAIT cell proliferation and IL-9 production in H. pylori-induced gastritis, which may provide potential targeting strategies for gastritis treatment.

Role of MAIT cells in metabolic diseases

MAIT cells are innate-like T cells that are enriched in mucosal sites and tissues including adipose tissue and liver. They play an important role in immunity against microbial pathogens. Recently, it has been reported that MAIT cells could also be important in metabolic diseases and can be involved in setting up and maintaining chronic inflammation. In this review, we give an overview of recent advances in understanding MAIT cells role in the ethology of this diseases.

Efficient 5-OP-RU-Induced Enrichment of Mucosa-Associated Invariant T Cells in the Murine Lung Does Not Enhance Control of Aerosol Mycobacterium tuberculosis Infection

Mucosa-associated invariant T (MAIT) cells are an innate-like T cell subset in mammals that recognize microbial vitamin B metabolites presented by the evolutionarily conserved major histocompatibility complex class I (MHC I)-related molecule, MR1. Emerging data suggest that MAIT cells may be an attractive target for vaccine-induced protection against bacterial infections because of their rapid cytotoxic responses at mucosal services to a widely conserved bacterial ligand. In this study, we tested whether a MAIT cell priming strategy could protect against aerosol Mycobacterium tuberculosis infection in mice. Intranasal costimulation with the lipopeptide Toll-like receptor (TLR)2/6 agonist, Pam2Cys (P2C), and the synthetic MR1 ligand, 5-OP-RU, resulted in robust expansion of MAIT cells in the lung. Although MAIT cell priming significantly enhanced MAIT cell activation and expansion early after M. tuberculosis challenge, these MAIT cells did not restrict M. tuberculosis bacterial load. MAIT cells were depleted by the onset of the adaptive immune response, with decreased detection of granzyme B⁺ and gamma interferon (IFN-γ)⁺ MAIT cells relative to that in uninfected P2C/5-OP-RU-treated mice. Decreasing the infectious inoculum, varying the time between priming and aerosol infection, and testing MAIT cell priming in nitric oxide synthase 2 (NOS2)-deficient mice all failed to reveal an effect of P2C/5-OP-RU-induced MAIT cells on M. tuberculosis control. We conclude that intranasal MAIT cell priming in mice induces early MAIT cell activation and expansion after M. tuberculosis exposure, without attenuating M. tuberculosis growth, suggesting that MAIT cell enrichment in the lung is not sufficient to control M. tuberculosis infection.

Antigen Recognition by MR1-Reactive T Cells; MAIT Cells, Metabolites, and Remaining Mysteries

Mucosal-associated Invariant T (MAIT) cells recognize vitamin B-based antigens presented by the non-polymorphic MHC class I related-1 molecule (MR1). Both MAIT T cell receptors (TCR) and MR1 are highly conserved among mammals, suggesting an important, and conserved, immune function. For many years, the antigens they recognize were unknown. The discovery that MR1 presents vitamin B-based small molecule ligands resulted in a rapid expansion of research in this area, which has yielded information on the role of MAIT cells in immune protection, autoimmune disease and recently in homeostasis and cancer. More recently, we have begun to appreciate the diverse nature of the small molecule ligands that can bind MR1, with several less potent antigens and small molecule drugs that can bind MR1 being identified. Complementary structural information has revealed the complex nature of interactions defining antigen recognition. Additionally, we now view MAIT cells (defined here as MR1-riboflavin-Ag reactive, TRAV1-2+ cells) as one subset of a broader family of MR1-reactive T cells (MR1T cells). Despite these advances, we still lack a complete understanding of how MR1 ligands are generated, presented and recognized in vivo. The biological relevance of these MR1 ligands and the function of MR1T cells in infection and disease warrants further investigation with new tools and approaches.

Mucosal-associated invariant T cells promote inflammation and intestinal dysbiosis leading to metabolic dysfunction during obesity

Obesity is associated with low-grade chronic inflammation promoting insulin-resistance and diabetes. Gut microbiota dysbiosis is a consequence as well as a driver of obesity and diabetes. Mucosal-associated invariant T cells (MAIT) are innate-like T cells expressing a semi-invariant T cell receptor restricted to the non-classical MHC class I molecule MR1 presenting bacterial ligands. Here we show that during obesity MAIT cells promote inflammation in both adipose tissue and ileum, leading to insulin resistance and impaired glucose and lipid metabolism. MAIT cells act in adipose tissue by inducing M1 macrophage polarization in an MR1-dependent manner and in the gut by inducing microbiota dysbiosis and loss of gut integrity. Both MAIT cell-induced tissue alterations contribute to metabolic dysfunction. Treatment with MAIT cell inhibitory ligand demonstrates its potential as a strategy against inflammation, dysbiosis and metabolic disorders.

Inflammation, immune cells and the host microbiota are intimately linked in the pathophysiology of obesity and diabetes. Here the authors show mucosal-associated invariant T cells fuel inflammation in the tissues and serve a function in promoting metabolic breakdown, polarising macrophage populations and inducing dysbiosis of the intestinal microbiota.

MAIT Cell Activation and Functions

Mucosal associated invariant T (MAIT) cells are striking in their abundance and their strict conservation across 150 million years of mammalian evolution, implying they must fulfill critical immunological function(s). MAIT cells are defined by their expression of a semi-invariant αβ TCR which recognizes biosynthetic derivatives of riboflavin synthesis presented on MR1. Initial studies focused on their role in detecting predominantly intracellular bacterial and mycobacterial infections. However, it is now recognized that there are several modes of MAIT cell activation and these are related to activation of distinct transcriptional programmes, each associated with distinct functional roles. In this minireview, we summarize current knowledge from human and animal studies of MAIT cell activation induced (1) in an MR1-TCR dependent manner in the context of inflammatory danger signals and associated with antibacterial host defense; (2) in an MR1-TCR independent manner by the cytokines interleukin(IL)-12/-15/-18 and type I interferon, which is associated with antiviral responses; and (3) a recently-described TCR-dependent “tissue repair” programme which is associated with accelerated wound healing in the context of commensal microbiota. Because of this capability for diverse functional responses in diverse immunological contexts, these intriguing cells now appear to be multifunctional effectors central to the interface of innate and adaptive immunity.