Mucosal-associated invariant T cells in cancer: dual roles, complex interactions and therapeutic potential

Mucosal-associated invariant T (MAIT) cells play diverse roles in cancer, infectious diseases, and immunotherapy. This review explores their intricate involvement in cancer, from early detection to their dual functions in promoting inflammation and mediating anti-tumor responses. Within the solid tumor microenvironment (TME), MAIT cells can acquire an 'exhausted' state and secrete tumor-promoting cytokines. On the other hand, MAIT cells are highly cytotoxic, and there is evidence that they may have an anti-tumor immune response. The frequency of MAIT cells and their subsets has also been shown to have prognostic value in several cancer types. Recent innovative approaches, such as programming MAIT cells with chimeric antigen receptors (CARs), provide a novel and exciting approach to utilizing these cells in cell-based cancer immunotherapy. Because MAIT cells have a restricted T cell receptor (TCR) and recognize a common antigen, this also mitigates potential graft-versus-host disease (GVHD) and opens the possibility of using allogeneic MAIT cells as off-the-shelf cell therapies in cancer. Additionally, we outline the interactions of MAIT cells with the microbiome and their critical role in infectious diseases and how this may impact the tumor responses of these cells. Understanding these complex roles can lead to novel therapeutic strategies harnessing the targeting capabilities of MAIT cells.

Immune checkpoint blockade improves the activation and function of circulating mucosal-associated invariant T (MAIT) cells in patients with non-small cell lung cancer

Mucosal-associated invariant T (MAIT) cells constitute one of the most numerous unconventional T cell subsets, and are characterized by rapid release of Th1- and Th17-associated cytokines and increased cytotoxic functions following activation. MAIT cells accumulate in tumor tissue but show an exhausted phenotype. Here, we investigated if immune checkpoint blockade (ICB) with antibodies to PD-1 or PD-L1 affects the function of circulating MAIT cells from non-small cell lung cancer patients. ICB increased the proliferation and co-expression of the activation markers HLA-DR and CD38 on MAIT cells in most patients after the first treatment cycle, irrespective of treatment outcome. Furthermore, production of cytokines, especially TNF and IL-2, also increased after treatment, as did MAIT cell polyfunctionality. These results indicate that MAIT cells respond to ICB, and that MAIT cell reinvigoration may contribute to tumor regression in patients undergoing immune checkpoint therapy.

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.

MAIT cells altered phenotype and cytotoxicity in lupus patients are linked to renal disease severity and outcome

Introduction

Systemic lupus erythematosus (SLE) is an autoimmune disease in which circulating immune complexes can cause different types of glomerulonephritis, according to immune deposits and to the type of glomerular cell injury. Proliferative lesions represent the most severe form of lupus nephritis (LN) and often lead to kidney failure and death. Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T cells that recognize microbial-derived ligands from the riboflavin synthesis pathway. Although abundant in peripheral blood, MAIT cells are enriched in mucosal and inflamed tissues. While previous studies have reported concordant results concerning lower MAIT cell frequencies in the blood of SLE patients, no information is known about MAIT cell function and LN severity and outcome.

Methods

In the current study, we analyzed the baseline phenotype and function of peripheral blood MAIT cells by flow cytometry in 26 patients with LN and in a control group of 16 healthy individuals.

Results

We observe that MAIT cell frequencies are markedly reduced in blood of LN patients. MAIT cells from patients have an altered phenotype in terms of migration, proliferation and differentiation markers, notably in most severe forms of LN. Frequencies of PMA/ionomycin stimulated MAIT cells secreting effector molecules, such as proinflammatory IL-17 and cytotoxic protein granzyme B, are higher in LN patients. Patients undergoing a complete renal remission after immunosuppressive therapy had higher MAIT cell frequency, lower expression of proliferation marker Ki-67 and granzyme B (GzB) at inclusion. Remarkably, GzB production defines a predictive model for complete remission.

Discussion

We report here that blood MAIT cells display proinflammatory and cytotoxic function in severe lupus nephritis which may play a pathogenesis role, but without association with systemic lupus activity. Finally, low cytotoxic profile of MAIT cells may represent a promising prognostic factor of lupus nephritis remission one year after induction therapy.

Unconventional T cells in brain homeostasis, injury and neurodegeneration

The interaction between peripheral immune cells and the brain is an important component of the neuroimmune axis. Unconventional T cells, which include natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells, γδ T cells, and other poorly defined subsets, are a special group of T lymphocytes that recognize a wide range of nonpolymorphic ligands and are the connection between adaptive and innate immunity. Recently, an increasing number of complex functions of these unconventional T cells in brain homeostasis and various brain disorders have been revealed. In this review, we describe the classification and effector function of unconventional T cells, review the evidence for the involvement of unconventional T cells in the regulation of brain homeostasis, summarize the roles and mechanisms of unconventional T cells in the regulation of brain injury and neurodegeneration, and discuss immunotherapeutic potential as well as future research goals. Insight of these processes can shed light on the regulation of T cell immunity on brain homeostasis and diseases and provide new clues for therapeutic approaches targeting brain injury and neurodegeneration.

The emerging paradigm of Unconventional T cells as a novel therapeutic target for celiac disease

Celiac disease (CD) is an organ-specific autoimmune disorder that occurs in genetically predisposed individuals when exposed to exogenous dietary gluten. This exposure to wheat gluten and related proteins from rye and barley triggers an immune response which leads to the development of enteropathy associated with symptoms of bloating, diarrhea, or malabsorption. The sole current treatment is to follow a gluten-free diet for the rest of one's life. Intestinal barriers are enriched with Unconventional T cells such as iNKT, MAIT, and γδ T cells, which lack or express only a limited range of rearranged antigen receptors. Unconventional T cells play a crucial role in regulating mucosal barrier function and microbial colonization. Unconventional T cell populations are widely represented in diseased conditions, where changes in disease activity related to iNKT and MAIT cell reduction, as well as γδ T cell expansion, are demonstrated. In this review, we discuss the role and potential employment of Unconventional T cells as a therapeutic target in the pathophysiology of celiac disease.

MAITs and their mates: "Innate-like" behaviors in conventional and unconventional T cells

Most CD4 and CD8 T cells are restricted by conventional major histocompatibility complex (MHC) molecules and mount TCR-dependent adaptive immune responses. In contrast, MAIT, iNKT, and certain γδ TCR bearing cells are characterized by their abilities to recognize antigens presented by unconventional antigen-presenting molecules and to mount cytokine-mediated TCR-independent responses in an "innate-like" manner. In addition, several more diverse T-cell subsets have been described that in a similar manner are restricted by unconventional antigen-presenting molecules but mainly depend on their TCRs for activation. Vice versa, innate-like behaviour was reported in defined subpopulations of conventional T cells, particularly in barrier sites, showing that these two features are not necessarily linked. The abilities to recognize antigens presented by unconventional antigen-presenting molecules or to mount TCR-independent responses creates unique niches for these T cells and is linked to wide range of functional capabilities. This is especially exemplified by unconventional and innate-like T cells present at barrier sites where they are involved in pathogen defense, tissue homeostasis as well as in pathologic processes.

MAIT cells activate dendritic cells to promote TFH cell differentiation and induce humoral immunity

Protective immune responses against respiratory pathogens, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus, are initiated by the mucosal immune system. However, most licensed vaccines are administered parenterally and are largely ineffective at inducing mucosal immunity. The development of safe and effective mucosal vaccines has been hampered by the lack of a suitable mucosal adjuvant. In this study we explore a class of adjuvant that harnesses mucosal-associated invariant T (MAIT) cells. We show evidence that intranasal immunization of MAIT cell agonists co-administered with protein, including the spike receptor binding domain from SARS-CoV-2 virus and hemagglutinin from influenza virus, induce protective humoral immunity and immunoglobulin A production. MAIT cell adjuvant activity is mediated by CD40L-dependent activation of dendritic cells and subsequent priming of T follicular helper cells. In summary, we show that MAIT cells are promising vaccine targets that can be utilized as cellular adjuvants in mucosal vaccines.

Mucosal-associated invariant T cells for cancer immunotherapy

Human mucosal-associated invariant T (MAIT) cells are characterized by their expression of an invariant TCR α chain Vα7.2-Jα33/Jα20/Jα12 paired with a restricted TCR β chain. MAIT cells recognize microbial peptides presented by the highly conserved MHC class I-like molecule MR1 and bridge the innate and acquired immune systems to mediate augmented immune responses. Upon activation, MAIT cells rapidly proliferate, produce a variety of cytokines and cytotoxic molecules, and trigger efficient antitumor immunity. Administration of a representative MAIT cell ligand 5-OP-RU effectively activates MAIT cells and enhances their antitumor capacity. In this review, we introduce MAIT cell biology and their importance in antitumor immunity, summarize the current development of peripheral blood mononuclear cell-derived and stem cell-derived MAIT cell products for cancer treatment, and discuss the potential of genetic engineering of MAIT cells for off-the-shelf cancer immunotherapy.

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.

Translatome analyses by bio-orthogonal non-canonical amino acid labeling reveal that MR1-activated MAIT cells induce an M1 phenotype and antiviral programming in antigen-presenting monocytes

MAIT cells are multifunctional innate-like effector cells recognizing bacterial-derived vitamin B metabolites presented by the non-polymorphic MHC class I related protein 1 (MR1). However, our understanding of MR1-mediated responses of MAIT cells upon their interaction with other immune cells is still incomplete. Here, we performed the first translatome study of primary human MAIT cells interacting with THP-1 monocytes in a bicellular system. We analyzed the interaction between MAIT and THP-1 cells in the presence of the activating 5-OP-RU or the inhibitory Ac-6-FP MR1-ligand. Using bio-orthogonal non-canonical amino acid tagging (BONCAT) we were able to enrich selectively those proteins that were newly translated during MR1-dependent cellular interaction. Subsequently, newly translated proteins were measured cell-type-specifically by ultrasensitive proteomics to decipher the coinciding immune responses in both cell types. This strategy identified over 2,000 MAIT and 3,000 THP-1 active protein translations following MR1 ligand stimulations. Translation in both cell types was found to be increased by 5-OP-RU, which correlated with their conjugation frequency and CD3 polarization at MAIT cell immunological synapses in the presence of 5-OP-RU. In contrast, Ac-6-FP only regulated a few protein translations, including GSK3B, indicating an anergic phenotype. In addition to known effector responses, 5-OP-RU-induced protein translations uncovered type I and type II Interferon-driven protein expression profiles in both MAIT and THP-1 cells. Interestingly, the translatome of THP-1 cells suggested that activated MAIT cells can impact M1/M2 polarization in these cells. Indeed, gene and surface expression of CXCL10, IL-1β, CD80, and CD206 confirmed an M1-like phenotype of macrophages being induced in the presence of 5-OP-RU-activated MAIT cells. Furthermore, we validated that the Interferon-driven translatome was accompanied by the induction of an antiviral phenotype in THP-1 cells, which were found able to suppress viral replication following conjugation with MR1-activated MAIT cells. In conclusion, BONCAT translatomics extended our knowledge of MAIT cell immune responses at the protein level and discovered that MR1-activated MAIT cells are sufficient to induce M1 polarization and an anti-viral program of macrophages.

Innate immunity and early liver inflammation

The innate system constitutes a first-line defence mechanism against pathogens. 80% of the blood supply entering the human liver arrives from the splanchnic circulation through the portal vein, so it is constantly exposed to immunologically active substances and pathogens from the gastrointestinal tract. Rapid neutralization of pathogens and toxins is an essential function of the liver, but so too is avoidance of harmful and unnecessary immune reactions. This delicate balance of reactivity and tolerance is orchestrated by a diverse repertoire of hepatic immune cells. In particular, the human liver is enriched in many innate immune cell subsets, including Kupffer cells (KCs), innate lymphoid cells (ILCs) like Natural Killer (NK) cells and ILC-like unconventional T cells - namely Natural Killer T cells (NKT), γδ T cells and Mucosal-associated Invariant T cells (MAIT). These cells reside in the liver in a memory-effector state, so they respond quickly to trigger appropriate responses. The contribution of aberrant innate immunity to inflammatory liver diseases is now being better understood. In particular, we are beginning to understand how specific innate immune subsets trigger chronic liver inflammation, which ultimately results in hepatic fibrosis. In this review, we consider the roles of specific innate immune cell subsets in early inflammation in human liver disease.

Mucosa associated invariant T and natural killer cells in active and budesonide treated collagenous colitis patients

Introduction

Collagenous colitis (CC) is an inflammatory bowel disease, which usually responds to budesonide treatment. Our aim was to study the immunological background of the disease.

Methods

Analyses of peripheral and mucosal MAIT (mucosa associated invariant T cells) and NK (natural killer) cells were performed with flow cytometry. Numbers of mucosal cells were calculated using immunohistochemistry. We studied the same patients with active untreated CC (au-CC) and again while in remission on budesonide treatment. Budesonide refractory patients and healthy controls were also included. The memory marker CD45R0 and activation marker CD154 and CD69 were used to further study the cells. Finally B cells, CD4⁺ and CD8⁺ T cells were also analysed.

Results

The percentages of circulating CD56^dimCD16⁺ NK cells as well as MAIT cells (CD3⁺TCRVa7.2⁺CD161⁺) were decreased in au-CC compared to healthy controls. This difference was not seen in the mucosa; where we instead found increased numbers of mucosal CD4⁺ T cells and CD8⁺ T cells in au-CC. Mucosal immune cell numbers were not affected by budesonide treatment. In refractory CC we found increased mucosal numbers of MAIT cells, CD4⁺ and CD8⁺ T cells compared to au-CC.

Discussion

Patients with active collagenous colitis have lower percentages of circulating MAIT and NK cells. However, there was no change of these cells in the colonic mucosa. Most mucosal cell populations were increased in budesonide refractory as compared to au-CC patients, particularly the number of MAIT cells. This may indicate that T cell targeting therapy could be an alternative in budesonide refractory CC.

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.

MAIT cells in liver inflammation and fibrosis

Mucosal-associated invariant T cells or MAIT cells are an abundant cell type in humans and especially so in the liver. MAIT cells are a subset of T lymphocytes that sit at a bridge between innate and adaptive immunity, so-called innate-like or "unconventional" T cells. The specificity of their antigen receptor (T cell receptor or TCR) is for the conserved major histocompatibility complex (MHC)-related molecule MR1, which presents a modified bacterial metabolite from the vitamin B2 biosynthesis pathway - this allows them to respond in the presence of many bacteria or yeast. MAIT cells also possess an array of cytokine receptors, which allows triggering independently of the TCR. The combination of such signals drives their functionality - this means they can respond to a range of stimuli and likely play a role not only in infection or inflammation, but also under homeostatic conditions.In this review, we will look at the question of what MAIT cells are doing in the normal liver and how they behave in the setting of disease. These questions are of relevance because MAIT cells are such a distinctive cell type enriched in the liver under normal conditions, and their modulation could be of therapeutic benefit. The recent discovery that they appear to be involved in liver fibrosis is particularly of interest in this context.

Durable Expansion of TCR-δ Meta-Clonotypes After BCG Revaccination in Humans

Mycobacterium bovis bacille Calmette-Guérin (BCG) has been used for 100 years and prevents disseminated tuberculosis and death in young children. However, it shows only partial efficacy against pulmonary tuberculosis (TB) in adults, so new vaccines are urgently needed. The protective efficacy of BCG depends on T cells, which are typically activated by pathogen-derived protein antigens that bind to highly polymorphic major histocompatibility complex (MHC) molecules. Some T cells recognize non-protein antigens via antigen presenting systems that are independent of genetic background, leading to their designation as donor-unrestricted T (DURT) cells. Whether live whole cell vaccines, like BCG, can induce durable expansions of DURT cells in humans is not known. We used combinatorial tetramer staining, multi-parameter flow cytometry, and immunosequencing to comprehensively characterize the effect of BCG on activation and expansion of DURT cell subsets. We examined peripheral blood mononuclear cells (PBMC) derived from a Phase I study of South African adults in which samples were archived at baseline, 3 weeks, and 52 weeks post-BCG revaccination. We did not observe a change in the frequency of total mucosal-associated invariant T (MAIT) cells, invariant natural killer T (iNKT) cells, germline encoded mycolyl-reactive (GEM) T cells, or γδ T cells at 52 weeks post-BCG. However, immunosequencing revealed a set of TCR-δ clonotypes that were expanded at 52 weeks post-BCG revaccination. These expanded clones expressed the Vδ2 gene segment and could be further defined on the basis of biochemical similarity into several 'meta-clonotypes' that likely recognize similar epitopes. Our data reveal that BCG vaccination leads to durable expansion of DURT cell clonotypes despite a limited effect on total circulating frequencies in the blood and have implications for defining the immunogenicity of candidate whole cell TB vaccines.

Laparoscopic sleeve gastrectomy for morbid obesity improves gut microbiota balance, increases colonic mucosal-associated invariant T cells and decreases circulating regulatory T cells

BACKGROUND

Laparoscopic sleeve gastrectomy (LSG) for morbid obesity may improve gut microbiota balance and decrease chronic inflammation. This study examines the changes in gut microbiota and immune environment, including mucosal-associated invariant T cells (MAIT cells) and regulatory T cells (Treg cells) caused by LSG.

METHODS

Ten morbidly obese patients underwent LSG at our institution between December 2018 and March 2020. Flow cytometry for Th1/Th2/Th17 cells, Treg cells and MAIT cells in peripheral blood and colonic mucosa and 16S rRNA analysis of gut microbiota were performed preoperatively and then 12 months postoperatively.

RESULTS

Twelve months after LSG, the median percent total weight loss was 30.3% and the median percent excess weight loss was 66.9%. According to laboratory data, adiponectin increased, leptin decreased, and chronic inflammation improved after LSG. In the gut microbiota, Bacteroidetes and Fusobacteria increased after LSG, and indices of alpha diversity increased after LSG. In colonic mucosa, the frequency of MAIT cells increased after LSG. In peripheral blood, the frequency of Th1 cells and effector Treg cells decreased after LSG.

CONCLUSIONS

After LSG for morbid obesity, improvement in chronic inflammation in obesity is suggested by change in the constituent bacterial species, increase in the diversity of gut microbiota, increase in MAIT cells in the colonic mucosa, and decrease in effector Treg cells in the peripheral blood.

Human MAIT Cells Respond to Staphylococcus aureus with Enhanced Anti-Bacterial Activity

Mucosal-Associated Invariant T (MAIT) cells have been shown to play protective roles during infection with diverse pathogens through their propensity for rapid innate-like cytokine production and cytotoxicity. Among the potential applications for MAIT cells is to defend against Staphylococcus aureus, a pathogen of serious clinical significance. However, it is unknown how MAIT cell responses to S. aureus are elicited, nor has it been investigated whether MAIT cell cytotoxicity is mobilized against intracellular S. aureus. In this study, we investigate the capacity of human MAIT cells to respond directly to S. aureus. MAIT cells co-cultured with dendritic cells (DCs) infected with S. aureus rapidly upregulate CD69, express IFNγ and Granzyme B and degranulate. DC secretion of IL-12, but not IL-18, was implicated in this immune response, while TCR binding of MR1 is required to commence cytokine production. MAIT cell cytotoxicity resulted in apoptosis of S. aureus-infected cells, and reduced intracellular persistence of S. aureus. These findings implicate these unconventional T cells in important, rapid anti-S. aureus responses that may be of great relevance to the ongoing development of novel anti-S. aureus treatments.

Activation of human γδ T cells and NK cells by Staphylococcal enterotoxins requires both monocytes and conventional T cells

Staphylococcal enterotoxins (SE) pose a great threat to human health due to their ability to bypass antigen presentation and activate large amounts of conventional T cells resulting in a cytokine storm potentially leading to toxic shock syndrome. Unconventional T- and NK cells are also activated by SE but the mechanisms remain poorly understood. In this study, the authors aimed to explore the underlying mechanism behind SE-mediated activation of MAIT-, γδ T-, and NK cells in vitro. CBMC or PBMC were stimulated with the toxins SEA, SEH, and TSST-1, and cytokine and cytotoxic responses were analyzed with ELISA and flow cytometry. All toxins induced a broad range of cytokines, perforin and granzyme B, although SEH was not as potent as SEA and TSST-1. SE-induced IFN-γ expression in MAIT-, γδ T-, and NK cells was clearly reduced by neutralization of IL-12, while cytotoxic compounds were not affected at all. Kinetic assays showed that unconventional T cell and NK cell-responses are secondary to the response in conventional T cells. Furthermore, co-cultures of isolated cell populations revealed that the ability of SEA to activate γδ T- and NK cells was fully dependent on the presence of both monocytes and αβ T cells. Lastly, it was found that SE provoked a reduced and delayed cytokine response in infants, particularly within the unconventional T and NK cell populations. This study provides novel insights regarding the activation of unconventional T- and NK cells by SE, which contribute to understanding the vulnerability of young children towards Staphylococcus aureus infections.

Exhaustion in tumor-infiltrating Mucosal-Associated Invariant T (MAIT) cells from colon cancer patients

Mucosal-associated invariant T (MAIT) cells are unconventional T cells recognizing microbial metabolites, presented by the invariant MR1 protein. Upon activation, MAIT cells rapidly secrete cytokines and exert cytotoxic functions, and may thus be highly relevant also in tumor immunity. MAIT cells accumulate in colon tumors, but in contrast to other cytotoxic T cell subsets, their presence in tumors has been associated with worse patient outcome. Here we investigated if exhaustion may contribute to reduced anti-tumor immunity by MAIT cells. Freshly isolated lymphocytes from colon tumors, unaffected tissue and blood from the same patients were analyzed by flow cytometry to detect MAIT cells with effector functions that are relevant for tumor immunity, and their expression of inhibitory receptors and other exhaustion markers. Our studies show that MAIT cells with a PD-1^highTim-3⁺CD39⁺ terminally exhausted phenotype and an increased proliferation accumulate in colon tumors. The exhausted MAIT cells have reduced polyfunctionality with regard to production of important anti-tumor effector molecules, and blocking antibodies to PD-1 partly improved activation of tumor-infiltrating MAIT cells in vitro. We conclude that the tumor microenvironment leads to exhaustion not only of conventional T cells, but also MAIT cells, and that checkpoint blockade therapy may be useful also to reinvigorate tumor-infiltrating MAIT cells.

Preliminary Report on Interleukin-22, GM-CSF, and IL-17F in the Pathogenesis of Acute Anterior Uveitis

Purpose:Anterior uveitis is the most common anatomic subset of uveitis. We developed a novel multi-parametric flow cytometry panel to identify immune dysregulation signatures in HLA B27-associated acute anterior uveitis (AAU) and axial spondyloarthritis (AxSpA).Methods: We used fluorescence activated cell sorting to characterize T cell cytokine expression in stimulated T cell subsets from patients with AAU (n = 4) compared to healthy controls (n = 14) or subjects with AxSpA (n = 6).Results: Positive findings among subjects with AAU included a statistically significant increase in stimulated granulocyte-macrophage colony stimulating factor (GM-CSF), IL-17, and IL-22 synthesized by CD8 cells, a trend for stimulated ILC (innate lymphoid cells)-3 cells to synthesize more IL-22 (p = .07), and stimulated MAIT (mucosa associated innate lymphoid cells)-like cells that express the T cell receptor V alpha 7.2 to express IL-17A, IL-17F, and IL-22 in a greater percentage of cells relative to controls. IL-17F, GM- CSF, and IL-22 represent potentially novel targets in AAU.Conclusion: Our report is arguably the first to implicate IL-17F or ILC-3 and MAIT cells in the pathogenesis of AAU.Abbreviations AAU: acute anterior uveitis; AxSpA: axial spondyloarthritis; BASDAI: Bath ankylosing spondylitis disease activity index; CCR: chemokine receptor; DMSO: dimethylsulfoxide; EULAR:European League Against Rheumatism; FACS: fluorescence activated cell sorter; FBS: fetal bovine serum; FSC: orward light scatter; GM-CSF: granulocyte-macrophage colony stimulating factor; HC: healthy control; ILC: innate lymphoid cell; KIR: killer immunoglobulin receptor; MAIT: mucosal associated immune T cell; ND: not detected; NK: natural killer cell; OHSU-Oregon Health & Science University; PBMC: peripheral blood mononuclear cell; SSC: side light scatter; TCR: T cell receptor.

MAIT Cells: Partners or Enemies in Cancer Immunotherapy?

Simple Summary

Unconventional T cells have recently come under intense scrutiny because of their innate-like effector functions and unique antigen specificity, suggesting their potential importance in antitumor immunity. MAIT cells, one such population of unconventional T cell, have been shown to significantly influence bacterial infections, parasitic and fungal infections, viral infections, autoimmune and other inflammatory diseases, and, as discussed thoroughly in this review, various cancers. This review aims to merge accumulating evidence, tease apart the complexities of MAIT cell biology in different malignancies, and discuss how these may impact clinical outcomes. While it is clear that MAIT cells can impact the tumor microenvironment, the nature of these interactions varies depending on the type of cancer, subset of MAIT cell, patient demographic, microbiome composition, and the type of therapy administered. This review examines the impact of these variables on MAIT cells and discusses outstanding questions within the field.

Abstract

A recent boom in mucosal-associated invariant T (MAIT) cell research has identified relationships between MAIT cell abundance, function, and clinical outcomes in various malignancies. As they express a variety of immune checkpoint receptors and ligands, and possess strong cytotoxic functions, MAIT cells are an attractive new subject in the field of tumor immunology. MAIT cells are a class of innate-like T cells that express a semi-invariant T cell antigen receptor (TCR) that recognizes microbially derived non-peptide antigens presented by the non-polymorphic MHC class-1 like molecule, MR1. In this review, we outline the current (and often contradictory) evidence exploring MAIT cell biology and how MAIT cells impact clinical outcomes in different human cancers, as well as what role they may have in cancer immunotherapy.

Virus-Mediated Suppression of the Antigen Presentation Molecule MR1

The antigen-presenting molecule MR1 presents microbial metabolites related to vitamin B2 biosynthesis to mucosal-associated invariant T cells (MAIT cells). Although bacteria and fungi drive the MR1 biosynthesis pathway, viruses have not previously been implicated in MR1 expression or its antigen presentation. We demonstrate that several herpesviruses inhibit MR1 cell surface upregulation, including a potent inhibition by herpes simplex virus type 1 (HSV-1). This virus profoundly suppresses MR1 cell surface expression and targets the molecule for proteasomal degradation, whereas ligand-induced cell surface expression of MR1 prior to infection enables MR1 to escape HSV-1-dependent targeting. HSV-1 downregulation of MR1 is dependent on de novo viral gene expression, and we identify the Us3 viral gene product as functioning to target MR1. Furthermore, HSV-1 downregulation of MR1 disrupts MAIT T cell receptor (TCR) activation. Accordingly, virus-mediated targeting of MR1 defines an immunomodulatory strategy that functionally disrupts the MR1-MAIT TCR axis.

Functional and Activation Profiles of Mucosal-Associated Invariant T Cells in Patients With Tuberculosis and HIV in a High Endemic Setting

Background: MAIT cells are non-classically restricted T lymphocytes that recognize and rapidly respond to microbial metabolites or cytokines and have the capacity to kill bacteria-infected cells. Circulating MAIT cell numbers generally decrease in patients with active TB and HIV infection, but findings regarding functional changes differ. Methods: We conducted a cross-sectional study on the effect of HIV, TB, and HIV-associated TB (HIV-TB) on MAIT cell frequencies, activation and functional profile in a high TB endemic setting in South Africa. Blood was collected from (i) healthy controls (HC, n = 26), 24 of whom had LTBI, (ii) individuals with active TB (aTB, n = 36), (iii) individuals with HIV infection (HIV, n = 50), 37 of whom had LTBI, and (iv) individuals with HIV-associated TB (HIV-TB, n = 26). All TB participants were newly diagnosed and sampled before treatment, additional samples were also collected from 18 participants in the aTB group after 10 weeks of TB treatment. Peripheral blood mononuclear cells (PBMC) stimulated with BCG-expressing GFP (BCG-GFP) and heat-killed (HK) Mycobacterium tuberculosis (M.tb) were analyzed using flow cytometry. MAIT cells were defined as CD3⁺ CD161⁺ Vα7.2⁺ T cells. Results: Circulating MAIT cell frequencies were depleted in individuals with HIV infection (p = 0.009). MAIT cells showed reduced CD107a expression in aTB (p = 0.006), and reduced IFNγ expression in aTB (p < 0.001) and in HIV-TB (p < 0.001) in response to BCG-GFP stimulation. This functional impairment was coupled with a significant increase in activation (defined by HLA-DR expression) in resting MAIT cells from HIV (p < 0.001), aTB (p = 0.019), and HIV-TB (p = 0.005) patients, and higher HLA-DR expression in MAIT cells expressing IFNγ in aTB (p = 0.009) and HIV-TB (p = 0.002) after stimulation with BCG-GFP and HK-M.tb. After 10 weeks of TB treatment, there was reversion in the observed functional impairment in total MAIT cells, with increases in CD107a (p = 0.020) and IFNγ (p = 0.010) expression. Conclusions: Frequencies and functional profile of MAIT cells in response to mycobacterial stimulation are significantly decreased in HIV infected persons, active TB and HIV-associated TB, with a concomitant increase in MAIT cell activation. These alterations may reduce the capacity of MAIT cells to play a protective role in the immune response to these two pathogens.

Pathophysiological Roles of Mucosal-Associated Invariant T Cells in the Context of Gut Microbiota-Liver Axis

Mucosal-associated invariant T (MAIT) cells are a subset of T lymphocytes expressing a semi-invariant T-cell receptor (TCR) present as TCR Vα7.2-Jα33 in humans and TCR Vα19-Jα33 in mice. They are activated by ligands produced during microbial biosynthesis of riboflavin that is presented by major histocompatibility complex class I-related (MR1) molecules on antigen-presenting cells. MAIT cells also possess interleukin (IL)-12 and IL-18 receptors and can be activated by the respective cytokines released from microbially stimulated antigen-presenting cells. Therefore, MAIT cells can be involved in bacterial and viral defenses and are a significant part of the human immune system. They are particularly abundant in the liver, an organ serving as the second firewall of gut microbes next to the intestinal barrier. Therefore, the immune functions of MAIT cells are greatly impacted by changes in the gut-microbiota and play important roles in the gut-liver pathogenesis axis. In this review, we discuss the nature and mechanisms of MAIT cell activation and their dynamics during different types of liver pathogenesis conditions. We also share our perspectives on important aspects that should be explored further to reveal the exact roles that MAIT cells play in liver pathogenesis in the context of the gut microbiota.

Biological functions of MAIT cells in tissues

Mucosal associated invariant T (MAIT) cells have a recognised innate-like capacity for antibacterial host defence, consequent on the specificity of their T cell receptor (TCR) for small molecule metabolites produced by a range of prokaryotic and fungal species, their effector memory phenotype, and their expression of cytotoxic molecules. However, recent studies have identified at least two other important functions of MAIT cells in antiviral immunity and in tissue homeostasis and repair. Each are related to distinct transcriptional programmes, which are activated differentially according to the specific immune context. Here we discuss these diverse functions, we review the evidence for the newly identified role of MAIT cells in promoting tissue repair, and we discuss emerging data pointing to the future directions of MAIT cell research including roles in cancer, in antiviral immunity and recent studies in the immune response to SARS-CoV-2 infection. Overall these studies have made us aware of the potential for pleiotropic roles of MAIT cells and related cell populations in micee and humans, and have created a simple and attractive new paradigm for regulation in barrier tissues, where antigen and tissue damage are sensed, integrated and interpreted.

Human mucosal-associated invariant T cells respond to Mucorales species in a MR1-dependent manner

Activation of mucosal-associated invariant T cells (MAIT cells) by certain bacteria, viruses, and yeast is well studied, but the activation potential of filamentous moulds from the order Mucorales is not known. Here, we show a rapid response of human MAIT cells against the Mucorales species Mucor circinelloides, Rhizopus arrhizus, and Rhizopus microsporus. This activation included upregulation of CD69 and degranulation marked by increased CD107a expression, while intracellular perforin and granzyme A expression were reduced. Furthermore, blocking of the antigen-presenting molecule major histocompatibility complex class I-related abrogated MAIT cell activation demonstrating a T cell receptor-dependent stimulation by Mucorales.

Reversal of Immunity After Clearance of Chronic HCV Infection-All Reset?

Chronic viral infections cause deterioration of our immune system. However, since persistent infections rarely can be eliminated, the reinvigoration capacity of an exhausted immune system has remained largely elusive. Chronic hepatitis C virus (HCV) infection can since some years be effectively cured with novel direct acting antiviral agents. Thus, it is now possible to study reversal of immunity in patients that are cured from a long-lasting chronic infection. We here highlight recent developments in the analysis of various immune cell populations during and after clearance of HCV infection. Surprisingly, whereas reinvigoration of certain immune traits clearly can be seen, many features of immune exhaustion persist over time after viral elimination. Thus, a long-term chronic insult might result in irreversible damage to our immune system. This will be important to consider in therapeutic vaccination efforts against chronic infection and in the development of immunotherapy based strategies against cancer.

Covering All the Bases: Complementary MR1 Antigen Presentation Pathways Sample Diverse Antigens and Intracellular Compartments

The ubiquitously expressed, monomorphic MHC class Ib molecule MHC class I-related protein 1 (MR1) presents microbial metabolites to mucosal-associated invariant T (MAIT) cells. However, recent work demonstrates that both the ligands bound by MR1 and the T cells restricted by it are more diverse than originally thought. It is becoming increasingly clear that MR1 is capable of presenting a remarkable variety of both microbial and non-microbial small molecule antigens to a diverse group of MR1-restricted T cells (MR1Ts) and that the antigen presentation pathway differs between exogenously delivered antigen and intracellular microbial infection. These distinct antigen presentation pathways suggest that MR1 shares features of both MHC class I and MHC class II antigen presentation, enabling it to sample diverse intracellular compartments and capture antigen of both intracellular and extracellular origin. Here, we review recent developments and new insights into the cellular mechanisms of MR1-dependent antigen presentation with a focus on microbial MR1T cell antigens.

Influenza A Virus-Infected Lung Epithelial Cell Co-Culture with Human Peripheral Blood Mononuclear Cells

Sensing of influenza A virus (IAV) infection by pattern recognition receptors can occur by either direct infection of lung epithelial cells or uptake of virus-infected cells by innate cells such as dendritic cells/monocytes. This triggers a series of downstream events including activation of the inflammasome, the production of cytokines, chemokines, and the upregulation of stress-induced ligands that can lead to the activation of innate cells. These cells include innate lymphocytes such as MAIT, NKT, NK, and γδ T cells. Here we describe a method used to allow activation of human innate lymphocytes in co-culture with an IAV-infected human lung epithelial cell line (A549) to measure ex vivo effector functions (TNF and IFNγ) in a mixed culture environment. We describe (1) infection of the human lung epithelial cell line, (2) co-culture with PBMC, and (3) measurement of activation using intracellular cytokine staining.

Isolation and Characterization of MAIT Cells from Tumor Tissues

Mucosal-associated invariant T (MAIT) cell infiltration has been demonstrated in colorectal and hepatocellular carcinoma, and their ability to produce Th1- and Th17-associated cytokines, as well as their cytotoxic function, suggests that MAIT cells may have important functions in both reducing and promoting protective tumor immunity. Here, we describe enzymatic methods to isolate intraepithelial and lamina propria lymphocyte single cell suspensions from colon tissue and tumors containing viable MAIT cells, which can be used for further purification, flow cytometry analysis, or culture.

MAIT Cell Activation by Fungal Pathogens

MAIT cells can recognize and respond to several pathogens in a distinct manner. However, many studies are focused on activation by bacteria and by viruses, while reports about molds and MAIT cells are rare. Here, we describe a method to investigate MAIT cell interaction with filamentous fungi from Aspergillus and Mucorales species. This includes preparation of fungal conidia, cell isolation, and analysis by flow cytometry.

Study of MAIT Cell Activation in Viral Infections In Vivo

MAIT cells are abundant, highly evolutionarily conserved innate-like lymphocytes expressing a semi-invariant T cell receptor (TCR), which recognizes microbially derived small intermediate molecules from the riboflavin biosynthetic pathway. However, in addition to their TCR-mediated functions they can also be activated in a TCR-independent manner via cytokines including IL-12, -15, -18, and type I interferon. Emerging data suggest that they are expanded and activated by a range of viral infections, and significantly that they can contribute to a protective anti-viral response. Here we describe methods used to investigate these anti-viral functions in vivo in murine models. To overcome the technical challenge that MAIT cells are rare in specific pathogen-free laboratory mice, we describe how pulmonary MAIT cells can be expanded using intranasal bacterial infection or a combination of synthetic MAIT cell antigen and TLR agonists. We also describe protocols for adoptive transfer of MAIT cells, methods for lung homogenization for plaque assays, and surface and intracellular cytokine staining to determine MAIT cell activation.

A Critical Role for Mucosal-Associated Invariant T Cells as Regulators and Therapeutic Targets in Systemic Lupus Erythematosus

Mucosal-associated invariant T (MAIT) cells are a subset of innate-like lymphocytes that are restricted by major histocompatibility complex-related molecule 1 (MR1). In this study, we investigated the role of MAIT cells in the pathogenesis of lupus in FcγRIIb^−/−Yaa mice, a spontaneous animal model of lupus. Using two approaches of MAIT cell deficiency, MR1 knockout animals and a newly synthesized inhibitory MR1 ligand, we demonstrate that MAIT cells augment the disease course of lupus by enhancing autoantibody production and tissue inflammation. MR1 deficiency reduced germinal center responses and T cell responses in these mice. Suppression of MAIT cell activation by the inhibitory MR1 ligand reduced autoantibody production and lupus nephritis in FcγRIIb^−/−Yaa mice. MAIT cells directly enhanced autoantibody production by B cells in vitro. Our results indicate the contribution of MAIT cells to lupus pathology and the potential of these cells as novel therapeutic targets for autoimmune diseases such as lupus.

Development of Unconventional T Cells Controlled by MicroRNA

Post-transcriptional gene regulation through microRNA (miRNA) has emerged as a major control mechanism of multiple biological processes, including development and function of T cells. T cells are vital components of the immune system, with conventional T cells playing a central role in adaptive immunity and unconventional T cells having additional functions reminiscent of both innate and adaptive immunity, such as involvement in stress responses and tissue homeostasis. Unconventional T cells encompass cells expressing semi-invariant T cell receptors (TCRs), such as invariant Natural Killer T (iNKT) and Mucosal-Associated Invariant T (MAIT) cells. Additionally, some T cells with diverse TCR repertoires, including γδT cells, intraepithelial lymphocytes (IEL) and regulatory T (Treg) cells, share some functional and/or developmental features with their semi-invariant unconventional counterparts. Unconventional T cells are particularly sensitive to disruption of miRNA function, both globally and on the individual miRNA level. Here, we review the role of miRNA in the development and function of unconventional T cells from an iNKT-centric point of view. The function of single miRNAs can provide important insights into shared and individual pathways for the formation of different unconventional T cell subsets.

Innate T cells in the intensive care unit

Rapid onset of acute inflammation is a hallmark of critical illnesses that bring patients to the intensive care unit (ICU). In critical illness, innate T cells rapidly reach full activation and drive a robust acute inflammatory response. As "cellular adjuvants," innate T cells worsen inflammation and mortality in several common critical illnesses including sepsis, ischemia-reperfusion injury, stroke, and exacerbations of respiratory disease. Interestingly, innate T cell subsets can also promote a protective and anti-inflammatory response in sepsis, ischemia-reperfusion injury, and asthma. Therapies that target innate T cells have been validated in several models of critical illness. Here, we review the role of natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells and γδ T cells in clinical and experimental critical illness.

Steroid-induced Deficiency of Mucosal-associated Invariant T Cells in the Chronic Obstructive Pulmonary Disease Lung. Implications for Nontypeable Haemophilus influenzae Infection

RATIONALE

Mucosal-associated invariant T (MAIT) cells are a recently described abundant, proinflammatory T-cell subset with unknown roles in pulmonary immunity. Nontypeable Haemophilus influenzae (NTHi) is the leading bacterial pathogen during chronic obstructive pulmonary disease (COPD) exacerbations and is a plausible target for MAIT cells.

OBJECTIVES

To investigate whether MAIT cells respond to NTHi and the effects of inhaled corticosteroids (ICS) on their frequency and function in COPD.

METHODS

Eleven subjects with COPD receiving ICS, 8 steroid-naive subjects with COPD, and 21 healthy control subjects underwent phlebotomy, sputum induction, bronchoalveolar lavage, and endobronchial biopsy. Pulmonary and monocyte-derived macrophages were cultured in vitro with NTHi.

MEASUREMENTS AND MAIN RESULTS

Frequencies of Vα7.2⁺CD161⁺ MAIT cells, surface expression of the major histocompatibility complex-related protein 1 (MR1), and intracellular IFN-γ expression were measured by flow cytometry. MAIT-cell frequencies were reduced in peripheral blood of ICS-treated subjects with COPD (median 0.38%; interquartile range [IQR], 0.25-0.96) compared with healthy control subjects (1.8%; IQR, 1.4-2.5; P = 0.001) or steroid-naive patients with COPD (1.8%; IQR, 1.2-2.3; P = 0.04). MAIT cells were reduced in bronchial biopsies from subjects with COPD treated with steroids (0.73%; IQR, 0.46-1.3) compared with healthy control subjects (4.0%; IQR, 1.6-5.0; P = 0.02). Coculture of live NTHi increased macrophage surface expression of MR1 and induced IFN-γ from CD4 cells and CD8 cells, but most potently from MAIT cells (median IFN-γ-positive frequencies, 2.9, 8.6, and 27.6%, respectively). In vitro fluticasone and budesonide reduced MR1 surface expression twofold and decreased NTHi-induced IFN-γ secretion eightfold.

CONCLUSIONS

MAIT cells are deficient in blood and bronchial tissue in steroid-treated, but not steroid-naive, COPD. NTHi constitutes a target for pulmonary MAIT-cell immune responses, which are significantly impaired by corticosteroids.

CD1d- and MR1-Restricted T Cells in Sepsis

Dysregulated immune responses to infection, such as those encountered in sepsis, can be catastrophic. Sepsis is typically triggered by an overwhelming systemic response to an infectious agent(s) and is associated with high morbidity and mortality even under optimal critical care. Recent studies have implicated unconventional, innate-like T lymphocytes, including CD1d- and MR1-restricted T cells as effectors and/or regulators of inflammatory responses during sepsis. These cell types are typified by invariant natural killer T (iNKT) cells, variant NKT (vNKT) cells, and mucosa-associated invariant T (MAIT) cells. iNKT and vNKT cells are CD1d-restricted, lipid-reactive cells with remarkable immunoregulatory properties. MAIT cells participate in antimicrobial defense, and are restricted by major histocompatibility complex-related protein 1 (MR1), which displays microbe-derived vitamin B metabolites. Importantly, NKT and MAIT cells are rapid and potent producers of immunomodulatory cytokines. Therefore, they may be considered attractive targets during the early hyperinflammatory phase of sepsis when immediate interventions are urgently needed, and also in later phases when adjuvant immunotherapies could potentially reverse the dangerous state of immunosuppression. We will highlight recent findings that point to the significance or the therapeutic potentials of NKT and MAIT cells in sepsis and will also discuss what lies ahead in research in this area.

Harnessing the antibacterial and immunological properties of mucosal-associated invariant T cells in the development of novel oral vaccines against enteric infections

Enteric infections are a major cause of mortality and morbidity with significant social and economic implications worldwide and particularly in developing countries. An attractive approach to minimizing the impact of these diseases is via the development of oral vaccination strategies. However, oral vaccination is challenging due to the tolerogenic and hyporesponsive nature of antigen presenting cells resident in the gastrointestinal tract. The inclusion of adjuvants in oral vaccine formulations has the potential to overcome this challenge. To date no oral adjuvants have been licenced for human use and thus oral adjuvant discovery remains a key goal in improving the potential for oral vaccine development. Mucosal-associated invariant T (MAIT) cells are a recently discovered population of unconventional T cells characterized by an evolutionarily conserved αβ T cell receptor (TCR) that recognizes antigens presented by major histocompatibility complex (MHC) class I-related (MR1) molecule. MAIT cells are selected intra-thymically by MR1 expressing double positive thymocytes and enter the circulation with a naïve phenotype. In the circulation they develop a memory phenotype and are programmed to home to mucosal tissues and the liver. Once resident in these tissues, MAIT cells respond to bacterial and yeast infections through the production of chemokines and cytokines that aid in the induction of an adaptive immune response. Their abundance in the gastrointestinal tract and ability to promote adaptive immunity suggests that MAIT cell activators may represent attractive novel adjuvants for use in oral vaccination.