Francisella tularensis induces Th1 like MAIT cells conferring protection against systemic and local infection

Mucosal associated invariant T cells: Powerhouses of the lung

The lungs face constant environmental challenges from harmless molecules, airborne pathogens and harmful agents that can damage the tissue. The lungs' immune system includes numerous tissue-resident lymphocytes that contribute to maintain tissue homeostasis and to the early initiation of immune responses. Amongst tissue-resident lymphocytes, Mucosal Associated Invariant T (MAIT) cells are present in human and murine lungs and emerging evidence supports their contribution to immune responses during infections, chronic inflammatory disorders and cancer. This review explores the mechanisms underpinning MAIT cell functions in the airways, their impact on lung immunity and the potential for targeting pulmonary MAIT cells in a therapeutic context.

MAIT cell activation and recruitment in inflammation and tissue damage in acute appendicitis

Mucosal-associated invariant T (MAIT) cells are antimicrobial T cells abundant in the gut, but mechanisms for their migration into tissues during inflammation are poorly understood. Here, we used acute pediatric appendicitis (APA), a model of acute intestinal inflammation, to examine these migration mechanisms. MAIT cells were lower in numbers in circulation of patients with APA but were enriched in the inflamed appendix with increased production of proinflammatory cytokines. Using the patient-derived appendix organoid (PDAO) model, we found that circulating MAIT cells treated with inflammatory cytokines elevated in APA up-regulated chemokine receptors, including CCR1, CCR3, and CCR4. They exhibited enhanced infiltration of Escherichia coli-pulsed PDAO in a CCR1-, CCR2-, and CCR4-dependent manner. Close interactions of MAIT cells with infected organoids led to the PDAO structural destruction and death. These findings reveal a previously unidentified mechanism of MAIT cell tissue homing, their participation in tissue damage in APA, and their intricate relationship with mucosal tissues during acute intestinal inflammation in humans.

MAIT cell-MR1 reactivity is highly conserved across multiple divergent species

Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells that recognize small molecule metabolites presented by major histocompatibility complex class I related protein 1 (MR1), via an αβ T cell receptor (TCR). MAIT TCRs feature an essentially invariant TCR α-chain, which is highly conserved between mammals. Similarly, MR1 is the most highly conserved major histocompatibility complex-I-like molecule. This extreme conservation, including the mode of interaction between the MAIT TCR and MR1, has been shown to allow for species-mismatched reactivities unique in T cell biology, thereby allowing the use of selected species-mismatched MR1-antigen (MR1-Ag) tetramers in comparative immunology studies. However, the pattern of cross-reactivity of species-mismatched MR1-Ag tetramers in identifying MAIT cells in diverse species has not been formally assessed. We developed novel cattle and pig MR1-Ag tetramers and utilized these alongside previously developed human, mouse, and pig-tailed macaque MR1-Ag tetramers to characterize cross-species tetramer reactivities. MR1-Ag tetramers from each species identified T cell populations in distantly related species with specificity that was comparable to species-matched MR1-Ag tetramers. However, there were subtle differences in staining characteristics with practical implications for the accurate identification of MAIT cells. Pig MR1 is sufficiently conserved across species that pig MR1-Ag tetramers identified MAIT cells from the other species. However, MAIT cells in pigs were at the limits of phenotypic detection. In the absence of sheep MR1-Ag tetramers, a MAIT cell population in sheep blood was identified phenotypically, utilizing species-mismatched MR1-Ag tetramers. Collectively, our results validate the use and define the limitations of species-mismatched MR1-Ag tetramers in comparative immunology studies.

Antibiotic use in early life subsequently impairs MAIT cell-mediated immunity

Mucosal-associated invariant T (MAIT) cells are predominantly located in barrier tissues where they rapidly respond to pathogens and commensals by recognizing microbial derivatives of riboflavin synthesis. Early-life exposure to these metabolites imprints the abundance of MAIT cells within tissues, so we hypothesized that antibiotic use during this period may abrogate their development. We identified antibiotics that deplete riboflavin-synthesizing commensals and revealed an early period of susceptibility during which antibiotic administration impaired MAIT cell development. The reduction in MAIT cell abundance rendered mice more susceptible to pneumonia, while MAIT cell-deficient mice were unaffected by early-life antibiotics. Concomitant administration of a riboflavin-synthesizing commensal during antibiotic treatment was sufficient to restore MAIT cell development and immunity. Our work demonstrates that transient depletion of riboflavin-synthesizing commensals in early life can adversely affect responses to subsequent infections.

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.

MAIT cells in bacterial infectious diseases: heroes, villains, or both?

Due to the aggravation of bacterial drug resistance and the lag in the development of new antibiotics, it is crucial to develop novel therapeutic regimens for bacterial infectious diseases. Currently, immunotherapy is a promising regimen for the treatment of infectious diseases. Mucosal-associated invariant T (MAIT) cells, a subpopulation of innate-like T cells, are abundant in humans and can mount a rapid immune response to pathogens, thus becoming a potential target of immunotherapy for infectious diseases. At the site of infection, activated MAIT cells perform complex biological functions by secreting a variety of cytokines and cytotoxic substances. Many studies have shown that MAIT cells have immunoprotective effects because they can bridge innate and adaptive immune responses, leading to bacterial clearance, tissue repair, and homeostasis maintenance. MAIT cells also participate in cytokine storm generation, tissue fibrosis, and cancer progression, indicating that they play a role in immunopathology. In this article, we review recent studies of MAIT cells, discuss their dual roles in bacterial infectious diseases and provide some promising MAIT cell-targeting strategies for the treatment of bacterial infectious diseases.

Mucosal-associated invariant T cells contribute to suppression of inflammatory myeloid cells in immune-mediated kidney disease

Mucosal-associated invariant T (MAIT) cells have been implicated in various inflammatory diseases of barrier organs, but so far, their role in kidney disease is unclear. Here we report that MAIT cells that recognize their prototypical ligand, the vitamin B2 intermediate 5-OP-RU presented by MR1, reside in human and mouse kidneys. Single cell RNAseq analysis reveals several intrarenal MAIT subsets, and one, carrying the genetic fingerprint of tissue-resident MAIT17 cells, is activated and expanded in a murine model of crescentic glomerulonephritis (cGN). An equivalent subset is also present in kidney biopsies of patients with anti-neutrophil cytoplasmatic antibody (ANCA)-associated cGN. MAIT cell-deficient MR1 mice show aggravated disease, whereas B6-MAITCAST mice, harboring higher MAIT cell numbers, are protected from cGN. The expanded MAIT17 cells express anti-inflammatory mediators known to suppress cGN, such as CTLA-4, PD-1, and TGF-β. Interactome analysis predicts CXCR6 - CXCL16-mediated cross-talk with renal mononuclear phagocytes, known to drive cGN progression. In line, we find that cGN is aggravated upon CXCL16 blockade. Finally, we present an optimized 5-OP-RU synthesis method which we apply to attenuating cGN in mice. In summary, we propose that CXCR6+ MAIT cells might play a protective role in cGN, implicating them as a potential target for anti-inflammatory therapies.

Role of MAIT cells in gastrointestinal tract bacterial infections in humans: More than a gut feeling

Mucosa-associated invariant T (MAIT) cells are the largest population of unconventional T cells in humans. These antimicrobial T cells are poised with rapid effector responses following recognition of the cognate riboflavin (vitamin B2)-like metabolite antigens derived from microbial riboflavin biosynthetic pathway. Presentation of this unique class of small molecule metabolite antigens is mediated by the highly evolutionarily conserved major histocompatibility complex class I-related protein. In humans, MAIT cells are widely found along the upper and lower gastrointestinal tracts owing to their high expression of chemokine receptors and homing molecules directing them to these tissue sites. In this review, we discuss recent findings regarding the roles MAIT cells play in various gastrointestinal bacterial infections, and how their roles appear to differ depending on the etiological agents and the anatomical location. We further discuss the potential mechanisms by which MAIT cells contribute to pathogen control, orchestrate adaptive immunity, as well as their potential contribution to inflammation and tissue damage during gastrointestinal bacterial infections, and the ensuing tissue repair following resolution. Finally, we propose and discuss the use of the emerging three-dimensional organoid technology to test different hypotheses regarding the role of MAIT cells in gastrointestinal bacterial infections, inflammation, and immunity.

Synthetic 5-amino-6-D-ribitylaminouracil paired with inflammatory stimuli facilitates MAIT cell expansion in vivo

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 three methods 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.

The Emerging Role of MAIT Cell Responses in Viral Infections

Mucosal-associated invariant T (MAIT) cells are unconventional T cells with innate-like antimicrobial responsiveness. MAIT cells are known for MR1 (MHC class I-related protein 1)-restricted recognition of microbial riboflavin metabolites giving them the capacity to respond to a broad range of microbes. However, recent progress has shown that MAIT cells can also respond to several viral infections in humans and in mouse models, ranging from HIV-1 and hepatitis viruses to influenza virus and SARS-CoV-2, in a primarily cognate Ag-independent manner. Depending on the disease context MAIT cells can provide direct or indirect antiviral protection for the host and may help recruit other immune cells, but they may also in some circumstances amplify inflammation and aggravate immunopathology. Furthermore, chronic viral infections are associated with varying degrees of functional and numerical MAIT cell impairment, suggesting secondary consequences for host defense. In this review, we summarize recent progress and highlight outstanding questions regarding the emerging role of MAIT cells in antiviral immunity.

Targeting the MR1-MAIT cell axis improves vaccine efficacy and affords protection against viral pathogens

Mucosa-associated invariant T (MAIT) cells are MR1-restricted, innate-like T lymphocytes with tremendous antibacterial and immunomodulatory functions. Additionally, MAIT cells sense and respond to viral infections in an MR1-independent fashion. However, whether they can be directly targeted in immunization strategies against viral pathogens is unclear. We addressed this question in multiple wild-type and genetically altered but clinically relevant mouse strains using several vaccine platforms against influenza viruses, poxviruses and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We demonstrate that 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU), a riboflavin-based MR1 ligand of bacterial origin, can synergize with viral vaccines to expand MAIT cells in multiple tissues, reprogram them towards a pro-inflammatory MAIT1 phenotype, license them to bolster virus-specific CD8+ T cell responses, and potentiate heterosubtypic anti-influenza protection. Repeated 5-OP-RU administration did not render MAIT cells anergic, thus allowing for its inclusion in prime-boost immunization protocols. Mechanistically, tissue MAIT cell accumulation was due to their robust proliferation, as opposed to altered migratory behavior, and required viral vaccine replication competency and Toll-like receptor 3 and type I interferon receptor signaling. The observed phenomenon was reproducible in female and male mice, and in both young and old animals. It could also be recapitulated in a human cell culture system in which peripheral blood mononuclear cells were exposed to replicating virions and 5-OP-RU. In conclusion, although viruses and virus-based vaccines are devoid of the riboflavin biosynthesis machinery that supplies MR1 ligands, targeting MR1 enhances the efficacy of vaccine-elicited antiviral immunity. We propose 5-OP-RU as a non-classic but potent and versatile vaccine adjuvant against respiratory viruses.

Divergent metabolic programmes control two populations of MAIT cells that protect the lung

Although mucosal-associated invariant T (MAIT) cells provide rapid, innate-like responses, they are not pre-set, and memory-like responses have been described for MAIT cells following infections. The importance of metabolism for controlling these responses, however, is unknown. Here, following pulmonary immunization with a Salmonella vaccine strain, mouse MAIT cells expanded as separate CD127-Klrg1+ and CD127+Klrg1- antigen-adapted populations that differed in terms of their transcriptome, function and localization in lung tissue. These populations remained altered from steady state for months as stable, separate MAIT cell lineages with enhanced effector programmes and divergent metabolism. CD127+ MAIT cells engaged in an energetic, mitochondrial metabolic programme, which was critical for their maintenance and IL-17A synthesis. This programme was supported by high fatty acid uptake and mitochondrial oxidation and relied on highly polarized mitochondria and autophagy. After vaccination, CD127+ MAIT cells protected mice against Streptococcus pneumoniae infection. In contrast, Klrg1+ MAIT cells had dormant but ready-to-respond mitochondria and depended instead on Hif1a-driven glycolysis to survive and produce IFN-γ. They responded antigen independently and participated in protection from influenza virus. These metabolic dependencies may enable tuning of memory-like MAIT cell responses for vaccination and immunotherapies.

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.

Die Kämpfe únd schláchten-the struggles and battles of innate-like effector T lymphocytes with microbes

The large majority of lymphocytes belong to the adaptive immune system, which are made up of B2 B cells and the αβ T cells; these are the effectors in an adaptive immune response. A multitudinous group of lymphoid lineage cells does not fit the conventional lymphocyte paradigm; it is the unconventional lymphocytes. Unconventional lymphocytes-here called innate/innate-like lymphocytes, include those that express rearranged antigen receptor genes and those that do not. Even though the innate/innate-like lymphocytes express rearranged, adaptive antigen-specific receptors, they behave like innate immune cells, which allows them to integrate sensory signals from the innate immune system and relay that umwelt to downstream innate and adaptive effector responses. Here, we review natural killer T cells and mucosal-associated invariant T cells-two prototypic innate-like T lymphocytes, which sense their local environment and relay that umwelt to downstream innate and adaptive effector cells to actuate an appropriate host response that confers immunity to infectious agents.

RIPK3 controls MAIT cell accumulation during development but not during infection

Cell death mechanisms in T lymphocytes vary according to their developmental stage, cell subset and activation status. The cell death control mechanisms of mucosal-associated invariant T (MAIT) cells, a specialized T cell population, are largely unknown. Here we report that MAIT cells express key necroptotic machinery; receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL) protein, in abundance. Despite this, we discovered that the loss of RIPK3, but not necroptotic effector MLKL or apoptotic caspase-8, specifically increased MAIT cell abundance at steady-state in the thymus, spleen, liver and lungs, in a cell-intrinsic manner. In contrast, over the course of infection with Francisella tularensis, RIPK3 deficiency did not impact the magnitude of the expansion nor contraction of MAIT cell pools. These findings suggest that, distinct from conventional T cells, the accumulation of MAIT cells is restrained by RIPK3 signalling, likely prior to thymic egress, in a manner independent of canonical apoptotic and necroptotic cell death pathways.

T-cell receptor αβ+ double-negative T cells in the kidney are predominantly extravascular and increase in abundance in response to ischemia-reperfusion injury

T-cell receptor+ CD4- CD8- double-negative (DN) T cells are a population of T cells present in low abundance in blood and lymphoid organs, but enriched in various organs including the kidney. Despite burgeoning interest in these cells, studies examining their abundance in the kidney have reported conflicting results. Here we developed a flow cytometry strategy to clearly segregate DN T cells from other immune cells in the mouse kidney and used it to characterize their phenotype and response in renal ischemia-reperfusion injury (IRI). These experiments revealed that in the healthy kidney, most DN T cells are located within the renal parenchyma and exhibit an effector memory phenotype. In response to IRI, the number of renal DN T cells is unaltered after 24 h, but significantly increased by 72 h. This increase is not related to alterations in proliferation or apoptosis. By contrast, adoptive transfer studies indicate that circulating DN T cells undergo preferential recruitment to the postischemic kidney. Furthermore, DN T cells show the capacity to upregulate CD8, both in vivo following adoptive transfer and in response to ex vivo activation. Together, these findings provide novel insights regarding the phenotype of DN T cells in the kidney, including their predominant extravascular location, and show that increases in their abundance in the kidney following IRI occur in part as a result of increased recruitment from the circulation. Furthermore, the observation that DN T cells can upregulate CD8 in vivo has important implications for detection and characterization of DN T cells in future studies.

Deaza-modification of MR1 ligands modulates recognition by MR1-restricted T cells

MR1-restricted T (MR1T) cells recognize microbial small molecule metabolites presented on the MHC Class I-like molecule MR1 and have been implicated in early effector responses to microbial infection. As a result, there is considerable interest in identifying chemical properties of metabolite ligands that permit recognition by MR1T cells, for consideration in therapeutic or vaccine applications. Here, we made chemical modifications to known MR1 ligands to evaluate the effect on MR1T cell activation. Specifically, we modified 6,7-dimethyl-8-D-ribityllumazine (DMRL) to generate 6,7-dimethyl-8-D-ribityldeazalumazine (DZ), and then further derivatized DZ to determine the requirements for retaining MR1 surface stabilization and agonistic properties. Interestingly, the IFN-γ response toward DZ varied widely across a panel of T cell receptor (TCR)-diverse MR1T cell clones; while one clone was agnostic toward the modification, most displayed either an enhancement or depletion of IFN-γ production when compared with its response to DMRL. To gain insight into a putative mechanism behind this phenomenon, we used in silico molecular docking techniques for DMRL and its derivatives and performed molecular dynamics simulations of the complexes. In assessing the dynamics of each ligand in the MR1 pocket, we found that DMRL and DZ exhibit differential dynamics of both the ribityl moiety and the aromatic backbone, which may contribute to ligand recognition. Together, our results support an emerging hypothesis for flexibility in MR1:ligand-MR1T TCR interactions and enable further exploration of the relationship between MR1:ligand structures and MR1T cell recognition for downstream applications targeting MR1T cells.

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.

Unconventional MAIT cell responses to bacterial infections

Mucosal Associated Invariant T cells (MAIT) exert potent antimicrobial activity through direct recognition of metabolite-MR1 complexes and indirect activation by inflammatory cytokines. Additionally, via licensing of antigen presenting cells, MAIT cells orchestrate humoral and cellular adaptive immunity. Our recent understanding of molecular mechanisms of MAIT cell activation, and of the signals required to differentiate them in polarised subsets, pave the way for harnessing their functionality through small molecules or adoptive cell therapy.

CD8 coreceptor engagement of MR1 enhances antigen responsiveness by human MAIT and other MR1-reactive T cells

Mucosal-associated invariant T (MAIT) cells detect microbial infection via recognition of riboflavin-based antigens presented by the major histocompatibility complex class I (MHC-I)-related protein 1 (MR1). Most MAIT cells in human peripheral blood express CD8αα or CD8αβ coreceptors, and the binding site for CD8 on MHC-I molecules is relatively conserved in MR1. Yet, there is no direct evidence of CD8 interacting with MR1 or the functional consequences thereof. Similarly, the role of CD8αα in lymphocyte function remains ill-defined. Here, using newly developed MR1 tetramers, mutated at the CD8 binding site, and by determining the crystal structure of MR1-CD8αα, we show that CD8 engaged MR1, analogous to how it engages MHC-I molecules. CD8αα and CD8αβ enhanced MR1 binding and cytokine production by MAIT cells. Moreover, the CD8-MR1 interaction was critical for the recognition of folate-derived antigens by other MR1-reactive T cells. Together, our findings suggest that both CD8αα and CD8αβ act as functional coreceptors for MAIT and other MR1-reactive T cells.

The balance of interleukin‐12 and interleukin‐23 determines the bias of MAIT1 versus MAIT17 responses during bacterial infection

Mucosal‐associated invariant T (MAIT) cells are a major subset of innate‐like T cells mediating protection against bacterial infection through recognition of microbial metabolites derived from riboflavin biosynthesis. Mouse MAIT cells egress from the thymus as two main subpopulations with distinct functions, namely, T‐bet‐expressing MAIT1 and RORγt‐expressing MAIT17 cells. Previously, we reported that inducible T‐cell costimulator and interleukin (IL)‐23 provide essential signals for optimal MHC‐related protein 1 (MR1)‐dependent activation and expansion of MAIT17 cells in vivo. Here, in a model of tularemia, in which MAIT1 responses predominate, we demonstrate that IL‐12 and IL‐23 promote MAIT1 cell expansion during acute infection and that IL‐12 is indispensable for MAIT1 phenotype and function. Furthermore, we showed that the bias toward MAIT1 or MAIT17 responses we observed during different bacterial infections was determined and modulated by the balance between IL‐12 and IL‐23 and that these responses could be recapitulated by cytokine coadministration with antigen. Our results indicate a potential for tailored immunotherapeutic interventions via MAIT cell manipulation.

The intracellular pathogen Francisella tularensis escapes from adaptive immunity by metabolic adaptation

This study shows that metabolic adaptation allows the intracellular bacterial pathogen Francisella tularensis to escape recognition by the host adaptive immunity.

Intracellular pathogens lose many metabolic genes during their evolution from free-living bacteria, but the pathogenic consequences of their altered metabolic programs on host immunity are poorly understood. Here, we show that a pathogenic strain of Francisella tularensis subsp. tularensis (FT) has five amino acid substitutions in RibD, a converting enzyme of the riboflavin synthetic pathway responsible for generating metabolites recognized by mucosal-associated invariant T (MAIT) cells. Metabolites from a free-living strain, F. tularensis subsp. novicida (FN), activated MAIT cells in a T-cell receptor (TCR)–dependent manner, whereas introduction of FT-type ribD to the free-living strain was sufficient to attenuate this activation in both human and mouse MAIT cells. Intranasal infection in mice showed that the ribD^FT-expressing FN strain induced impaired Th1-type MAIT cell expansion and resulted in reduced bacterial clearance and worsened survival compared with the wild-type free-living strain FN. These results demonstrate that F. tularensis can acquire immune evasion capacity by alteration of metabolic programs during evolution.

Immune responses to bacterial lung infections and their implications for vaccination

The pulmonary immune system plays a vital role in protecting the delicate structures of gaseous exchange against invasion from bacterial pathogens. With antimicrobial resistance becoming an increasing concern, finding novel strategies to develop vaccines against bacterial lung diseases remains a top priority. In order to do so, a continued expansion of our understanding of the pulmonary immune response is warranted. Whilst some aspects are well characterised, emerging paradigms such as the importance of innate cells and inducible immune structures in mediating protection provide avenues of potential to rethink our approach to vaccine development. In this review, we aim to provide a broad overview of both the innate and adaptive immune mechanisms in place to protect the pulmonary tissue from invading bacterial organisms. We use specific examples from several infection models and human studies to depict the varying functions of the pulmonary immune system that may be manipulated in future vaccine development. Particular emphasis has been placed on emerging themes that are less reviewed and underappreciated in vaccine development studies.

Determination of Sample Concentrations by PULCON NMR Spectroscopy

PULCON (Pulse Length Based Concentration Determination) is a powerful, versatile, non-invasive, and accurate technique for measuring solution concentrations during routine NMR spectroscopy. As solutes are quantified directly by their unique resonances, this technique avoids weight-based errors caused by contaminants (e.g. moisture), allows NMR samples to be directly employed in biological assays, and is particularly useful for quantifying small molecules, peptides, unstable molecules, and other materials that are difficult to weigh or handle. This article provides an introductory guide for biological and medicinal chemists, and highlights the diversity of applications.

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.