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Mucosal-associated invariant T (MAIT) cells are unconventional T cells that respond to riboflavin biosynthesis and cytokines through TCR-dependent and -independent pathways, respectively. MAIT cell activation plays an immunoprotective role against several pathogens, however the functional capacity of MAIT cells following direct infection or exposure to infectious agents remains poorly defined. We investigated the impact of Varicella Zoster Virus (VZV) on blood-derived MAIT cells and report virus-mediated impairment of activation, cytokine production, and altered transcription factor expression by VZV infected (antigen+) and VZV exposed (antigen-) MAIT cells in response to TCR-dependent and -independent stimulation. Furthermore, we reveal that suppression of VZV exposed (antigen-) MAIT cells is not mediated by a soluble factor from neighbouring VZV infected (antigen+) MAIT cells. Finally, we demonstrate that VZV impairs the cytolytic potential of MAIT cells in response to riboflavin synthesising bacteria. In summary, we report a virus-mediated immune-evasion strategy that disarms MAIT cell responses. Author summary: Mucosal-associated invariant T (MAIT) cells are a uniquely specialised and substantial innate-T cell population that can rapidly respond to diverse bacterial and fungal pathogens through T cell receptor dependent recognition of riboflavin synthesis derived metabolite antigens. Additionally, MAIT cells can be triggered by local pro-inflammatory cues such as cytokines; therefore extending their functionality to non-riboflavin pathogens such as viral infections. Despite the capacity of MAIT cells to play a protective role against several classes of pathogens, there remains a dearth of studies investigating direct pathogenic suppression of MAIT cell functionality. Here, we investigate a previously uncharacterised interplay between MAIT cells and the causative agent of varicella (chickenpox) and shingles (zoster): Varicella Zoster Virus (VZV). VZV successfully infects and establishes lifelong latency within the host; in part to their ability to effectively manipulate several innate and adaptive axes of the host immune response. In this study, we report that VZV profoundly impairs MAIT cell activation in response to both riboflavin synthesis and cytokine stimulation, therefore resulting in a downstream paralysis of several effector functions such as cytokine production and cytotoxic potential. This work highlights a previously uncharacterised strategy of viral pathogens to effectively target and restrict the MAIT cell effector response. [ABSTRACT FROM AUTHOR]

Mucosal-associated invariant T (MAIT) cells can elicit immune responses against riboflavin-based antigens presented by the evolutionary conserved MHC class I related protein, MR1. While we have an understanding of the structural basis of human MAIT cell receptor (TCR) recognition of human MR1 presenting a variety of ligands, how the semi-invariant mouse MAIT TCR binds mouse MR1-ligand remains unknown. Here, we determine the crystal structures of 2 mouse TRAV1- TRBV13-2+ MAIT TCR-MR1-5-OP-RU ternary complexes, whose TCRs differ only in the composition of their CDR3β loops. These mouse MAIT TCRs mediate high affinity interactions with mouse MR1-5-OP-RU and cross-recognize human MR1-5-OP-RU. Similarly, a human MAIT TCR could bind mouse MR1-5-OP-RU with high affinity. This crossspecies recognition indicates the evolutionary conserved nature of this MAIT TCR–MR1 axis. Comparing crystal structures of the mouse versus human MAIT TCR-MR1-5-OP-RU complexes provides structural insight into the conserved nature of this MAIT TCR–MR1 interaction and conserved specificity for the microbial antigens, whereby key germlineencoded interactions required for MAIT activation are maintained. This is an important consideration for the development of MAIT cell-based therapeutics that will rely on preclinical mouse models of disease. [ABSTRACT FROM AUTHOR]

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that recognize bacterial riboflavin–based metabolites as activating antigens. Although MAIT cells are found in tissues, it is unknown whether any host tissue–derived antigens exist. Here, we report that a sulfated bile acid, cholic acid 7-sulfate (CA7S), binds the nonclassical MHC class I protein MR1 and is recognized by MAIT cells. CA7S is a host-derived metabolite whose levels were reduced by more than 98% in germ-free mice. Deletion of the sulfotransferase 2a family of enzymes (Sult2a1-8) responsible for CA7S synthesis reduced the number of thymic MAIT cells in mice. Moreover, recognition of CA7S induced MAIT cell survival and the expression of a homeostatic gene signature. By contrast, recognition of a previously described foreign antigen, 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), drove MAIT cell proliferation and the expression of inflammatory genes. Thus, CA7S is an endogenous antigen for MAIT cells, which promotes their development and function. Editor's summary: Mucosal-associated invariant T (MAIT) cells are an innate-like T cell subset important for mucosal homeostasis, which recognize microbiota-derived vitamin B metabolites presented by the MHC-Ib molecule MR1. It is unclear, however, whether any host-derived ligands are also important for MAIT cell function and survival. Ito et al. report that the host-derived bile acid metabolite cholic acid 7-sulfate (CA7S) binds MR1 and is recognized by MAIT cells in mice. Moreover, genetic deletion of sulfotransferase 2a enzymes (Sult2a1-8) needed for its synthesis in mice reduced thymic MAIT cell numbers. A previously characterized vitamin B metabolite, 5-OP-RU, triggered human MAIT cell proliferation and the expression of inflammatory genes. By contrast, CA7S promoted human MAIT cell survival and a gene program associated with wound healing and immunoregulation. —Seth Thomas Scanlon [ABSTRACT FROM AUTHOR]

Introduction: Mucosal-associated invariant T (MAIT) cells are a population of innate-like T cells, which mediate host immunity to microbial infection by recognizing metabolite antigens derived from microbial riboflavin synthesis presented by the MHC-I-related protein 1 (MR1). Namely, the potent MAIT cell antigens, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU) and 5-(2- oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU), form via the condensation of the riboflavin precursor 5-amino-6-D-ribitylaminouracil (5-A-RU) with the reactive carbonyl species (RCS) methylglyoxal (MG) and glyoxal (G), respectively. Although MAIT cells are abundant in humans, they are rare in mice, and increasing their abundance using expansion protocols with antigen and adjuvant has been shown to facilitate their study in mouse models of infection and disease. Methods: Here,we outline threemethods to increase the abundance of MAIT cells in C57BL/6 mice using a combination of inflammatory stimuli, 5-A-RU and MG. Results: Our data demonstrate that the administration of synthetic 5-A-RU in combination with one of three different inflammatory stimuli is sufficient to increase the frequency and absolute numbers of MAIT cells in C57BL/6 mice. The resultant boosted MAIT cells are functional and can provide protection against a lethal infection of Legionella longbeachae. Conclusion: These results provide alternative methods for expanding MAIT cells with high doses of commercially available 5-A-RU (± MG) in the presence of various danger signals. [ABSTRACT FROM AUTHOR]

Histone deacetylases (HDACs) catalyse removal of acetyl groups from lysine residues on both histone and non‐histone proteins to control numerous cellular processes. Of the 11 zinc‐dependent classical HDACs, HDAC4, 5, 7 and 9 are class IIa HDAC enzymes that regulate cellular and developmental processes through both enzymatic and non‐enzymatic mechanisms. Over the last two decades, HDAC7 has been associated with key roles in numerous physiological and pathological processes. Molecular, cellular, in vivo and disease association studies have revealed that HDAC7 acts through multiple mechanisms to control biological processes in immune cells, osteoclasts, muscle, the endothelium and epithelium. This HDAC protein regulates gene expression, cell proliferation, cell differentiation and cell survival and consequently controls development, angiogenesis, immune functions, inflammation and metabolism. This review focuses on the cell biology of HDAC7, including the regulation of its cellular localisation and molecular mechanisms of action, as well as its associative and causal links with cancer and inflammatory, metabolic and fibrotic diseases. We also review the development status of small molecule inhibitors targeting HDAC7 and their potential for intervention in different disease contexts. [ABSTRACT FROM AUTHOR]

PULCON (Pulse Length-Based Concentration Determination) is a powerful, versatile, non-invasive, and accurate technique for measuring solution concentrations via routine NMR spectroscopy. It is particularly useful for preparing accurate solutions of compounds for biological assays, and avoids weight-based errors caused by contaminants (e.g. moisture). This article provides an introductory guide for biological and medicinal chemists, and highlights the diversity of applications. [ABSTRACT FROM AUTHOR]

Mucosal-associated invariant T (MAIT) cells are activated in a TCR-dependent manner by antigens derived from the riboflavin synthesis pathway, including 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), bound to MHC-related protein-1 (MR1). However, MAIT cell activation in vivo has not been studied in detail. Here, we have found and characterized additional molecular signals required for optimal activation and expansion of MAIT cells after pulmonary Legionella or Salmonella infection in mice. We show that either bone marrow–derived APCs or non–bone marrow–derived cells can activate MAIT cells in vivo, depending on the pathogen. Optimal MAIT cell activation in vivo requires signaling through the inducible T cell costimulator (ICOS), which is highly expressed on MAIT cells. Subsequent expansion and maintenance of MAIT-17/1-type responses are dependent on IL-23. Vaccination with IL-23 plus 5-OP-RU augments MAIT cell–mediated control of pulmonary Legionella infection. These findings reveal cellular and molecular targets for manipulating MAIT cell function under physiological conditions. [ABSTRACT FROM AUTHOR]

Mucosal associated invariant T (MAIT) cells recognise conserved microbial metabolites from riboflavin synthesis. Striking evolutionary conservation and pulmonary abundance implicate them in antibacterial host defence, yet their functions in protection against clinically important pathogens are unknown. Here we show that mouse Legionella longbeachae infection induces MR1-dependent MAIT cell activation and rapid pulmonary accumulation of MAIT cells associated with immune protection detectable in immunocompetent host animals. MAIT cell protection is more evident in mice lacking CD4+ cells, and adoptive transfer of MAIT cells rescues immunodeficient Rag2-/-γC-/- mice from lethal Legionella infection. Protection is dependent on MR1, IFN-γ and GM-CSF, but not IL-17A, TNF or perforin, and enhanced protection is detected earlier after infection of mice antigen-primed to boost MAIT cell numbers before infection. Our findings define a function for MAIT cells in protection against a major human pathogen and indicate a potential role for vaccination to enhance MAIT cell immunity. [ABSTRACT FROM AUTHOR]

Abstract: 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 small molecule chemistry, we discovered MR1 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 generally inhibited MAIT cell activation, two riboflavin pathway derivatives, 5‐(2‐oxopropylideneamino)‐6‐D‐ribitylaminouracil and 5‐(2‐oxoethylideneamino)‐6‐D‐ribitylaminouracil, 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. [ABSTRACT FROM AUTHOR]