MAIT cells come to the rescue in cancer immunotherapy?

Recent progress in immunobiology has led to the observation that, among cells classically categorized as the typical representatives of the adaptive immune system, i.e., T cells, some possess the phenotype of innate cells. Invariant T cells are characterized by T cell receptors recognizing a limited range of non-peptide antigens, presented only in the context of particular molecules. Mucosal-associated invariant T cells (MAIT cells) are an example of such unconventional cells. In humans, they constitute between 1% and 8% of the peripheral blood T lymphocytes and are further enriched in mucosal tissues, mesenteric lymph nodes, and liver, where they can account for even 40% of all the T cells. MAIT cells recognize antigens in the context of major histocompatibility complex class I-related protein (MR1). Upon activation, they instantly release pro-inflammatory cytokines and mediate cytolytic function towards bacterially infected cells. As such, they have been a rapidly evolving research topic not only in the field of infectious diseases but also in the context of many chronic inflammatory diseases and, more recently, in immuno-oncology. Novel findings suggest that MAIT cells function could also be modulated by endogenous ligands and drugs, making them an attractive target for therapeutic approaches. In this review, we summarize the current understanding of MAIT cell biology, their role in health and disease and discuss their future potential in cancer immunotherapy. This is discussed through the prism of knowledge and experiences with invariant natural killer T cells (iNKT)—another prominent unconventional T cell subset that shares many features with MAIT cells

Stabilizing short-lived Schiff base derivatives of 5-aminouracils that activate mucosal-associated invariant T cells

Mucosal-associated invariant T (MAIT) cells are activated by unstable antigens formed by reactions of 5-amino-6-D-ribitylaminouracil (a vitamin B2 biosynthetic intermediate) with glycolysis metabolites such as methylglyoxal. Here we show superior preparations of antigens in dimethylsulfoxide, avoiding their rapid decomposition in water (t1/2 1.5 h, 37 °C). Antigen solution structures, MAIT cell activation potencies (EC50 3–500 pM), and chemical stabilities are described. Computer analyses of antigen structures reveal stereochemical and energetic influences on MAIT cell activation, enabling design of a water stable synthetic antigen (EC50 2 nM). Like native antigens, this antigen preparation induces MR1 refolding and upregulates surface expression of human MR1, forms MR1 tetramers that detect MAIT cells in human PBMCs, and stimulates cytokine expression (IFNγ, TNF) by human MAIT cells. These antigens also induce MAIT cell accumulation in mouse lungs after administration with a co-stimulant. These chemical and immunological findings provide new insights into antigen properties and MAIT cell activation

Endosomal MR1 Trafficking Plays a Key Role in Presentation of Mycobacterium tuberculosis Ligands to MAIT Cells

Mucosal-Associated Invariant T (MAIT) cells, present in high frequency in airway and other mucosal tissues, have Th1 effector capacity positioning them to play a critical role in the early immune response to intracellular pathogens, including Mycobacterium tuberculosis (Mtb). MR1 is a highly conserved Class I-like molecule that presents vitamin B metabolites to MAIT cells. The mechanisms for loading these ubiquitous small molecules are likely to be tightly regulated to prevent inappropriate MAIT cell activation. To define the intracellular localization of MR1, we analyzed the distribution of an MR1-GFP fusion protein in antigen presenting cells. We found that MR1 localized to endosomes and was translocated to the cell surface upon addition of 6-formyl pterin (6-FP). To understand the mechanisms by which MR1 antigens are presented, we used a lentiviral shRNA screen to identify trafficking molecules that are required for the presentation of Mtb antigen to HLA-diverse T cells. We identified Stx18, VAMP4, and Rab6 as trafficking molecules regulating MR1-dependent MAIT cell recognition of Mtb-infected cells. Stx18 but not VAMP4 or Rab6 knockdown also resulted in decreased 6-FP-dependent surface translocation of MR1 suggesting distinct pathways for loading of exogenous ligands and intracellular mycobacterially-derived ligands. We postulate that endosome-mediated trafficking of MR1 allows for selective sampling of the intracellular environment.Career Development Award: (#IK2 CX000538); U.S. Department of Veterans Affairs Clinical Sciences Research and Development Program (MJH); U.S.Department of Veterans Affairs Biomedical Laboratory Research and Development Program (DML) Merit Award: (#I01 BX000533); American Lung Association: (RT-350058)

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.</p

Identification of phenotypically and functionally heterogeneous mouse mucosal-associated invariant T cells using MR1 tetramers

Studies on the biology of mucosal-associated invariant T cells (MAIT cells) in mice have been hampered by a lack of specific reagents. Using MR1-antigen (Ag) tetramers that specifically bind to the MR1-restricted MAIT T cell receptors (TCRs), we demonstrate that MAIT cells are detectable in a broad range of tissues in C57BL/6 and BALB/c mice. These cells include CD4(-)CD8(-), CD4(-)CD8(+), and CD4(+)CD8(-) subsets, and their frequency varies in a tissue- and strain-specific manner. Mouse MAIT cells have a CD44(hi)CD62L(lo) memory phenotype and produce high levels of IL-17A, whereas other cytokines, including IFN-gamma, IL-4, IL-10, IL-13, and GM-CSF, are produced at low to moderate levels. Consistent with high IL-17A production, most MAIT cells express high levels of retinoic acid-related orphan receptor gamma t (ROR gamma t), whereas ROR gamma t(lo) MAIT cells predominantly express T-bet and produce IFN-gamma. Most MAIT cells express the promyelocytic leukemia zinc finger (PLZF) transcription factor, and their development is largely PLZF dependent. These observations contrast with previous reports that MAIT cells from V alpha 19 TCR transgenic mice are PLZF(-) and express a naive CD44(lo) phenotype. Accordingly, MAIT cells from normal mice more closely resemble human MAIT cells than previously appreciated, and this provides the foundation for further investigations of these cells in health and disease

An overview on the identification of MAIT cell antigens

Mucosal Associated Invariant T (MAIT) cells are restricted by the monomorphic MHC class I-like molecule, MHC-related protein-1 (MR1). Until 2012, the origin of the MAIT cell antigens (Ags) was unknown, although it was established that MAIT cells could be activated by a broad range of bacteria and yeasts, possibly suggesting a conserved Ag. Using a combination of protein chemistry, mass spectrometry, cellular biology, structural biology and chemistry, we discovered MAIT cell ligands derived from folic acid (vitamin B9) and from an intermediate in the microbial biosynthesis of riboflavin (vitamin B2). While the folate derivative 6-formylpterin (6-FP) generally inhibited MAIT cell activation, two riboflavin pathway derivatives, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU) and 5-(2-oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU), were potent MAIT cell agonists. Other intermediates and derivatives of riboflavin synthesis displayed weak or no MAIT cell activation. Collectively, these studies revealed that in addition to peptide and lipid-based Ags, small molecule natural product metabolites are also ligands that can activate T cells expressing αβ T cell receptors, and here we recount this discovery. This article is protected by copyright. All rights reserved

Functional Roles of B‐Vitamins in the Gut and Gut Microbiome

The gut microbiota produce hundreds of bioactive compounds, including B-vitamins, which play significant physiological roles in hosts by supporting the fitness of symbiotic species and suppressing the growth of competitive species. B-vitamins are also essential to the host and certain gut bacterium. Although dietary B-vitamins are mainly absorbed from the small intestine, excess B-vitamins unable to be absorbed in the small intestine are supplied to the distal gut. In addition, B-vitamins are supplied from biosynthesis by distal gut microbiota. B-vitamins in the distal colon may perform many important functions in the body; they act as (1) nutrients for a host and their microbiota, (2) regulators of immune cell activity, (3) mediators of drug efficacy, (4) supporters of survival, or the fitness of certain bacterium, (5) suppressors of colonization by pathogenic bacteria, and (6) modulators of colitis. Insights into basic biophysical principles, including the bioavailability of B-vitamins and their derivatives in the distal gut are still not fully elucidated. Here we briefly review the function of single B-vitamin in the distal gut including their roles in relation to bacteria. The prospect of extending analytical methods to better understand the role of B-vitamins in the gut is also explored.journal articl

Functional Roles of B‐Vitamins in the Gut and Gut Microbiome

The gut microbiota produce hundreds of bioactive compounds, including B-vitamins, which play significant physiological roles in hosts by supporting the fitness of symbiotic species and suppressing the growth of competitive species. B-vitamins are also essential to the host and certain gut bacterium. Although dietary B-vitamins are mainly absorbed from the small intestine, excess B-vitamins unable to be absorbed in the small intestine are supplied to the distal gut. In addition, B-vitamins are supplied from biosynthesis by distal gut microbiota. B-vitamins in the distal colon may perform many important functions in the body; they act as (1) nutrients for a host and their microbiota, (2) regulators of immune cell activity, (3) mediators of drug efficacy, (4) supporters of survival, or the fitness of certain bacterium, (5) suppressors of colonization by pathogenic bacteria, and (6) modulators of colitis. Insights into basic biophysical principles, including the bioavailability of B-vitamins and their derivatives in the distal gut are still not fully elucidated. Here we briefly review the function of single B-vitamin in the distal gut including their roles in relation to bacteria. The prospect of extending analytical methods to better understand the role of B-vitamins in the gut is also explored

Evasion of MAIT cell recognition by the African Salmonella Typhimurium ST313 pathovar that causes invasive disease

Mucosal-associated invariant T (MAIT) cells are innate T lymphocytes activated by bacteria that produce vitamin B2 metabolites. Mouse models of infection have demonstrated a role for MAIT cells in antimicrobial defense. However, proposed protective roles of MAIT cells in human infections remain unproven and clinical conditions associated with selective absence of MAIT cells have not been identified. We report that typhoidal and nontyphoidal Salmonella enterica strains activate MAIT cells. However, S. Typhimurium sequence type 313 (ST313) lineage 2 strains, which are responsible for the burden of multidrug-resistant nontyphoidal invasive disease in Africa, escape MAIT cell recognition through overexpression of ribB. This bacterial gene encodes the 4-dihydroxy-2-butanone-4-phosphate synthase enzyme of the riboflavin biosynthetic pathway. The MAIT cell-specific phenotype did not extend to other innate lymphocytes. We propose that ribB overexpression is an evolved trait that facilitates evasion from immune recognition by MAIT cells and contributes to the invasive pathogenesis of S. Typhimurium ST313 lineage 2