The Chemical Synthesis, Stability, and Activity of MAIT Cell Prodrug Agonists That Access MR1 in Recycling Endosomes

Development of Ribityllumazine Analogue as Mucosal-Associated Invariant T Cell Ligands

Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T cells abundant in human tissues that play a significant role in defense against bacterial and viral infections and in tissue repair. MAIT cells are activated by recognizing microbial-derived small-molecule ligands presented by the MHC class I related-1 protein. Although several MAIT cell modulators have been identified in the past decade, potent and chemically stable ligands remain limited. Herein, we carried out a structure-activity relationship study of ribityllumazine derivatives and found a chemically stable MAIT cell ligand with a pteridine core and a 2-oxopropyl group as the Lys-reactive group. The ligand showed high potency in a cocultivation assay using model cell lines of antigen-presenting cells and MAIT cells. The X-ray crystallographic analysis revealed the binding mode of the ligand to MR1 and the T cell receptor, indicating that it forms a covalent bond with MR1 via Schiff base formation. Furthermore, we found that the ligand stimulated proliferation of human MAIT cells in human peripheral blood mononuclear cells and showed an adjuvant effect in mice. Our developed ligand is one of the most potent among chemically stable MAIT cell ligands, contributing to accelerating therapeutic applications of MAIT cells.

Protective effect of TCR-mediated MAIT cell activation during experimental autoimmune encephalomyelitis

Mucosal-associated invariant T (MAIT) cells express semi-invariant T cell receptors (TCR) for recognizing bacterial and yeast antigens derived from riboflavin metabolites presented on the non-polymorphic MHC class I-related protein 1 (MR1). Neuroinflammation in multiple sclerosis (MS) is likely initiated by autoreactive T cells and perpetuated by infiltration of additional immune cells, but the precise role of MAIT cells in MS pathogenesis remains unknown. Here, we use experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, and find an accumulation of MAIT cells in the inflamed central nervous system (CNS) enriched for MAIT17 (RORγt+) and MAIT1/17 (T-bet+RORγt+) subsets with inflammatory and protective features. Results from transcriptome profiling and Nur77GFP reporter mice show that these CNS MAIT cells are activated via cytokines and TCR. Blocking TCR activation with an anti-MR1 antibody exacerbates EAE, whereas enhancing TCR activation with the cognate antigen, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil, ameliorates EAE severity, potentially via the induction of amphiregulin (AREG). In summary, our findings suggest that TCR-mediated MAIT cell activation is protective in CNS inflammation, likely involving an induction of AREG.

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.

Keratinocytes stimulate MAIT cells to produce granzyme B via MR1 and cytokines in oral lichen planus

OBJECTIVE

Oral lichen planus (OLP) is a chronic inflammatory disease characterized by a dense T-cell infiltration and the degeneration of basal keratinocytes. The potential functions of mucosal associated invariant T (MAIT) cells in OLP have been analyzed in our previous study. Keratinocytes under proinflammatory conditions have been demonstrated to activate T cells. This study was aimed to investigate how keratinocytes stimulate MAIT cells in OLP, and to explore the role of activated MAIT cells on keratinocytes.

METHODS AND RESULTS

Increased MAIT cells and higher activation marker CD69 were detected in OLP lesions by flow cytometry. The enhanced expression of MHC class I-like molecule (MR1) required for MAIT cell activation in the epithelial layer of OLP lesions was determined by immunohistochemistry. Keratinocytes treated by 5-A-RU prodrug and lipopolysaccharide, respectively, exhibited higher expression of MR1 and secretion of IL-18. In direct coculture systems consisting of keratinocytes and peripheral blood mononuclear cells, both 5-A-RU prodrug-pretreated keratinocytes and lipopolysaccharide-pretreated keratinocytes activated MAIT cells to secrete granzyme B, contributing to elevated keratinocyte apoptosis.

CONCLUSIONS

Keratinocytes were capable to activate MAIT cells via MR1 and cytokines in OLP, and granzyme B produced by activated MAIT cells intensified keratinocyte apoptosis, engaging in the pathogenesis of OLP.

MR1 antigen presentation to MAIT cells and other MR1-restricted T cells

MHC antigen presentation plays a fundamental role in adaptive and semi-invariant T cell immunity. Distinct MHC molecules bind antigens that differ in chemical structure, origin and location and present them to specialized T cells. MHC class I-related protein 1 (MR1) presents a range of small molecule antigens to MR1-restricted T (MR1T) lymphocytes. The best studied MR1 ligands are derived from microbial metabolism and are recognized by a major class of MR1T cells known as mucosal-associated invariant T (MAIT) cells. Here, we describe the MR1 antigen presentation pathway: the known types of antigens presented by MR1, the location where MR1-antigen complexes form, the route followed by the complexes to the cell surface, the mechanisms involved in termination of MR1 antigen presentation and the accessory cellular proteins that comprise the MR1 antigen presentation machinery. The current road map of the MR1 antigen presentation pathway reveals potential strategies for therapeutic manipulation of MR1T cell function and provides a foundation for further studies that will lead to a deeper understanding of MR1-mediated immunity.

Delivery of loaded MR1 monomer results in efficient ligand exchange to host MR1 and subsequent MR1T cell activation

MR1-restricted T cells have been implicated in microbial infections, sterile inflammation, wound healing and cancer. Similar to other antigen presentation molecules, evidence supports multiple, complementary MR1 antigen presentation pathways. To investigate ligand exchange pathways for MR1, we used MR1 monomers and tetramers loaded with 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU) to deliver the antigen. Using MR1-deficient cells reconstituted with wild-type MR1 or MR1 molecules that cannot bind 5-OP-RU, we show that presentation of monomer-delivered 5-OP-RU is dependent on cellular MR1 and requires the transfer of ligand from the soluble molecule onto MR1 expressed by the antigen presenting cell. This mode of antigen delivery strengthens the evidence for post-ER ligand exchange pathways for MR1, which could represent an important avenue by which MR1 acquires antigens derived from endocytosed pathogens.

Calcium Signaling in MR1-Dependent Antigen Presentation of Mycobacterium tuberculosis

MR1 is a ubiquitously expressed MHC-Ib molecule that presents microbial metabolites to MR1-restricted T cells, but there are differences in the antigen presentation pathway of an intracellular microbe compared to exogenous antigen. We have shown the importance of endosomal trafficking proteins in MR1-dependent presentation of Mycobacterium tuberculosis (Mtb). Two pore channels (TPCs) are endosomal calcium channels that regulate endosomal trafficking. Due to their location on endosomes, we hypothesized that TPCs could be required for MR1-dependent presentation of antigens derived from the intracellular microbe Mtb. We found that TPCs are critical for the presentation of Mtb by MR1; inhibition of TPCs had no effect on MR1 presentation of extracellular (exogenous) antigens, HLA-B presentation, or HLA-II presentation. Finally, we found that the calcium sensitive trafficking protein Synaptotagmin 7 was also key in the presentation of Mtb by MR1. This calcium-dependent endosomal pathway is a novel mechanism by which the immune system can sample intracellular antigens.

Molecular insights into metabolite antigen recognition by mucosal-associated invariant T cells

Metabolite-based T-cell immunity is emerging as a major player in antimicrobial immunity, autoimmunity, and cancer. Here, small-molecule metabolites were identified to be captured and presented by the major histocompatibility complex class-I-related molecule (MR1) to T cells, namely mucosal-associated invariant T cells (MAIT) and diverse MR1-restricted T cells. Both MR1 and MAIT are evolutionarily conserved in many mammals, suggesting important roles in host immunity. Rational chemical modifications of these naturally occurring metabolites, termed altered metabolite ligands (AMLs), have advanced our understanding of the molecular correlates of MAIT T cell receptor (TCR)-MR1 recognition. This review provides a generalized framework for metabolite recognition and modulation of MAIT cells.

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.

A specialized tyrosine-based endocytosis signal in MR1 controls antigen presentation to MAIT cells

MR1 is a highly conserved microbial immune-detection system in mammals. It captures vitamin B-related metabolite antigens from diverse microbes and presents them at the cell surface to stimulate MR1-restricted lymphocytes including mucosal-associated invariant T (MAIT) cells. MR1 presentation and MAIT cell recognition mediate homeostasis through host defense and tissue repair. The cellular mechanisms regulating MR1 cell surface expression are critical to its function and MAIT cell recognition, yet they are poorly defined. Here, we report that human MR1 is equipped with a tyrosine-based motif in its cytoplasmic domain that mediates low affinity binding with the endocytic adaptor protein 2 (AP2) complex. This interaction controls the kinetics of MR1 internalization from the cell surface and minimizes recycling. We propose MR1 uses AP2 endocytosis to define the duration of antigen presentation to MAIT cells and the detection of a microbial metabolic signature by the immune system.

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.

MR1-dependent immune surveillance of the skin contributes to pathogenesis and is a photobiological target of UV light therapy in a mouse model of atopic dermatitis

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are unconventional T cells which recognize microbial metabolites presented by the major histocompatibility complex class I-related molecule MR1. Although MAIT cells have been shown to reside in human and murine skin, their contribution to atopic dermatitis (AD), an inflammatory skin disease associated with barrier dysfunction and microbial translocation, has not yet been determined.

METHODS

Genetic deletion of MR1 and topical treatment with inhibitory MR1 ligands, which result in the absence and functional inhibition of MAIT cells, respectively, were used to investigate the role of MR1-dependent immune surveillance in a MC903-driven murine model of AD.

RESULTS

The absence or inhibition of MR1 arrested AD disease progression through the blockade of both eosinophil activation and recruitment of IL-4- and IL-13-producing cells. In addition, the therapeutic efficacy of phototherapy against MC903-driven AD could be increased with prior application of folate, which photodegrades into the inhibitory MR1 ligand 6-formylpterin.

CONCLUSION

We identified MAIT cells as sentinels and mediators of cutaneous type 2 immunity. Their pathogenic activity can be inhibited by topical application or endogenous generation, via phototherapy, of inhibitory MR1 ligands.

Chemical Modulators of Mucosal Associated Invariant T Cells

Over the past decade, we have contributed to the chemistry of microbial natural products and synthetic ligands, related to riboflavin and uracils, that modulate immune cells called mucosal associated invariant T cells (MAIT cells). These highly abundant T lymphocytes were only discovered in 2003 and have become recognized for their importance in mammalian immunology. Unlike other T cells, MAIT cells are not activated by peptide or lipid antigens. In collaboration with immunology and structural biology research groups, we discovered that they are instead activated by unstable nitrogen-containing heterocycles synthesized by bacteria. The most potent naturally occurring activating compound (antigen) is 5-(2-oxopropylideneamino)-d-ribitylaminouracil (5-OP-RU). This compound is an imine (Schiff base) formed through condensation between an intermediate in the biosynthesis of riboflavin (vitamin B2) and a metabolic byproduct of mammalian and microbial glycolysis. Although it is very unstable in water due to intramolecular ring closure or hydrolysis, we were able to develop a non-enzymatic synthesis that yields a pure kinetically stable compound in a nonaqueous solvent. This compound has revolutionized the study of MAIT cell immunology due to its potent activation (EC₅₀ = 2 pM) of MAIT cells and its development into immunological reagents for detecting and characterizing MAIT cells in tissues. MAIT cells are now linked to key physiological processes and disease, including antibacterial defense, tissue repair, regulation of graft-vs-host disease, gastritis, inflammatory bowel diseases, and cancer. 5-OP-RU activates MAIT cells and, like a vaccine, has been shown to protect mice from bacterial infections and cancers. Mechanistic studies on the binding of 5-OP-RU to its dual protein targets, the major histocompatibility complex class I related protein (MR1) and the MAIT cell receptor (MAIT TCR), have involved synthetic chemistry, 2D ¹H NMR spectroscopy, mass spectrometry, computer modeling and molecular dynamics simulations, biochemical, cellular, and immunological assays, and protein structural biology. These combined studies have revealed structural influences for 5-OP-RU in solution on protein binding and antigen presentation and potency; informed the development of potent (EC₅₀ = 2 nM) and water stable analogues; led to fluorescent analogues for detecting and tracking binding proteins in and on cells; and enabled discovery of drugs and drug-like molecules that bind MR1 and modulate MAIT cell function. MAIT cells offer new opportunities for chemical synthesis to enhance the stability, potency, selectivity, and bioavailability of small molecule ligands for MR1 or MAIT TCR proteins, and to contribute to the understanding of T cell immunity and the development of prospective new immunomodulating medicines.

Mouse models illuminate MAIT cell biology

The field of mucosal-associated invariant T cell (MAIT) biology has grown rapidly since the identification of the vitamin-B-based antigens recognised by these specialised T cells. Over the past few years, our understanding of the complexities of MAIT cell function has developed, as they find their place among the other better known cells of the immune system. Key questions relate to understanding when MAIT cells help, when they hinder or cause harm, and when they do not matter. Exploiting mouse strains that differ in MAIT cell numbers, leveraged by specific detection of MAIT cells using MR1-tetramers, it has now been shown that MAIT cells play important immune roles in settings that include bacterial and viral infections, autoimmune diseases and cancer. We have also learnt much about their development, modes of activation and response to commensal microbiota, and begun to try ways to manipulate MAIT cells to improve disease outcomes. Here we review recent studies that have assessed MAIT cells in models of disease.

Antigen specificities and functional properties of MR1-restricted T cells

MR1 is an MHC class I-like molecule with unique structural and biological features that make it an important member among the molecules involved in antigen presentation to T cells. Distinctive features include ubiquitous expression of the MR1 gene and its monomorphism. Another relevant property is that the MR1 protein appears at very low levels on the plasma membrane and its surface expression is regulated by antigen binding. Finally, the nature of presented antigens differs from those that bind other presenting molecules and includes small metabolites of microbial and self-origin, small drugs and tumor-associated antigens. This opinion paper describes in detail some of those features and discusses recent literature in the field.

Understanding and modulating the MR1 metabolite antigen presentation pathway

The MHC class I-related protein, MR1, presents small metabolite antigens to an unusual subset of innate-like T cells. Herein, we highlight recent progress in our understanding of MR1's unique antigen presenting pathway, with features of both MHC class I and class II antigen presentation, as highlighted during the EMBO Workshop: CD1-MR1, Beyond MHC-restricted lymphocytes, Oxford, 2019. There is increasing evidence for a role of MR1 restricted T cells in several immune contexts, from cancer to autoimmunity and infections, and therapeutic harnessing of this important biological axis through generation of agonist and antagonist MR1 ligands requires a thorough understanding of the molecular mechanisms of MR1-dependent antigen presentation.

Chemical insights into the search for MAIT cells activators

Mucosal-associated invariant T cells (MAIT cells) represent a potential therapeutic target as they can tune or enhance immune responses. They recognise and become activated by antigens, presented by the monomorphic MHC-I related molecule, MR1. To assess the significance of MAIT cells in human diseases, a better understanding of the MAIT cell-MR1-antigen interaction is imperative. Easy access to MR1 ligands and MAIT cells activators can help achieve this. In this review, we summarise current literature that has identified the natural ligands and drug-like molecules that activate MAIT cells and provide insight into their key molecular interactions with MR1 and MAIT T cell receptors (TCRs). We focus on the progress made in synthesizing and isolating 5-amino-6-d-ribitylaminouracil (5-A-RU), a key precursor in the synthesis of the known natural ligands, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil(5-OP-RU) and 5-(2-oxoethylideneamino)-6-d-ribitylaminouracil (5-OE-RU), and also on the stabilisation and optimisation of the latter compounds.

The effects of 5-OP-RU stereochemistry on its stability and MAIT-MR1 axis

Mucosal-associated invariant T (MAIT) cells are an abundant subset of innate-like T lymphocytes. MAIT cells are activated by microbial riboflavin-derived antigens, such as 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), when presented by the major histocompatibility complex (MHC) class I-related protein (MR1). We have synthesized all stereoisomers of 5-OP-RU to investigate the effects of its stereochemistry on the MR1-dependent MAIT cell activation and MR1 upregulation. The analysis of MAIT cell activation by these 5-OP-RU isomers revealed that the stereocenters at the 2'- and 3'-OH groups in the ribityl tail are crucial for the recognition of MAIT-TCR, whereas that of 4'-OH group does not significantly affect the regulation of MAIT cell activity. Furthermore, kinetic analysis of complex formation between the ligands and MR1 suggested that 5-OP-RU forms a covalent bond to MR1 in cells within 1 hour. These findings provide guidelines for designing ligands that regulate MAIT cell functions.

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.

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.

The Immune Modulating Properties of Mucosal-Associated Invariant T Cells

Mucosal-associated invariant T (MAIT) cells are unconventional T lymphocytes that express a semi-invariant T cell receptor (TCR) recognizing microbial vitamin B metabolites presented by the highly conserved major histocompatibility complex (MHC) class I like molecule, MR1. The vitamin B metabolites are produced by several commensal and pathogenic bacteria and yeast, but not viruses. Nevertheless, viral infections can trigger MAIT cell activation in a TCR-independent manner, through the release of pro-inflammatory cytokines by antigen-presenting cells (APCs). MAIT cells belong to the innate like T family of cells with a memory phenotype, which allows them to rapidly release Interferon (IFN)-γ, tumor necrosis factor (TNF)-α, and in some circumstances Interleukin (IL)-17 and IL-10, exerting an immunomodulatory role on the ensuing immune response, akin to iNKT cells and γδ T cells. Recent studies implicate MAIT cells in a variety of inflammatory, autoimmune diseases, and in cancer. In addition, through the analysis of the transcriptome of MAIT cells activated in different experimental conditions, an important function in tissue repair and control of immune homeostasis has emerged, shared with other innate-like T cells. In this review, we discuss these recent findings, focussing on the understanding of the molecular mechanisms underpinning MAIT cell activation and effector function in health and disease, which ultimately will aid in clinically harnessing this unique, not donor-restricted cell subtype.

Mānuka honey-derived methylglyoxal enhances microbial sensing by mucosal-associated invariant T cells

Methylglyoxal (MGO) is the main antimicrobial determinant associated with using Mānuka Honey as a topical dressing. While direct mechanisms of Mānuka honey MGO's antimicrobial activity have been demonstrated, such as disruption of bacterial fimbria and flagella, no interaction of Mānuka honey-derived MGO with antimicrobial effector cells of the immune system, such as mucosal-associated invariant T cells (MAIT cells), has yet been reported. MAIT cells are an abundant subset of human T cells, critical for regulating a diverse range of immune functions, including antimicrobial defense mechanisms but also mucosal barrier integrity. MAIT cells become activated by recognition of an important microbial metabolite, 5-amino-6-d-ribitylaminouracil (5-A-RU), which is produced by a wide range of microbial pathogens and commensals. Recognition is afforded when 5-A-RU condenses with mammalian-cell derived MGO to form the potent MAIT cell activator, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU). Formation of 5-OP-RU and its subsequent presentation to MAIT cells by major histocompatibility (MHC)-related molecule 1 (MR1) facilitates host-pathogen and host-commensal interactions. While MGO is a metabolite naturally present in mammalian cells, it is unclear whether exogenous dietary MGO sources, such as those obtained from Mānuka honey intake, can contribute to 5-OP-RU formation and enhance MAIT cell activation. In this work, we report that endogenous MGO is the rate-limiting substrate for converting microbial 5-A-RU to 5-OP-RU and that Mānuka honey-derived MGO significantly enhances MAIT cell activation in vitro. Our findings posit a novel mechanism by which intake of a food item, such as Mānuka honey, can potentially support immune homeostasis by enhancing MAIT cell-specific microbial sensing.