Activation of Human Mucosal-Associated Invariant T Cells Induces CD40L-Dependent Maturation of Monocyte-Derived and Primary Dendritic Cells

Insights into the tissue repair features of MAIT cells

Mucosa-associated invariant T (MAIT) cells are a subset of innate-like non-conventional T cells characterized by multifunctionality. In addition to their well-recognized antimicrobial activity, increasing attention is being drawn towards their roles in tissue homeostasis and repair. However, the precise mechanisms underlying these functions remain incompletely understood and are still subject to ongoing exploration. Currently, it appears that the tissue localization of MAIT cells and the nature of the diseases or stimuli, whether acute or chronic, may induce a dynamic interplay between their pro-inflammatory and anti-inflammatory, or pathogenic and reparative functions. Therefore, elucidating the conditions and mechanisms of MAIT cells' reparative functions is crucial for fully maximizing their protective effects and advancing future MAIT-related therapies. In this review, we will comprehensively discuss the establishment and potential mechanisms of their tissue repair functions as well as the translational application prospects and current challenges in this field.

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.

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.

Innate immune cell activation after HIV-1 vaccine administration is associated with increased antibody production

The RV144 Thai phase III clinical trial's canarypox-protein HIV vaccine regimen showed modest efficacy in reducing infection. We therefore sought to determine the effects of vaccine administration on innate cell activation and subsequent associations with vaccine-induced immune responses. RV306 was a randomized, double-blind clinical trial in HIV-uninfected Thai adults that tested delayed boosting following the RV144 regimen. PBMC collected from RV306 participants prior to and 3 days after the last boost were used to investigate innate immune cell activation. Our analysis showed an increase in CD38+ mucosal associated invariant T (MAIT) cells, CD38+ invariant natural killer T (iNKT) cells, CD38+ γδ T cells, CD38+, CD69+ and HLA-DR+ NK cells 3 days after vaccine administration. An increase in CD14-CD16+ non-classical monocytes and CD14+CD16+ intermediate monocytes accompanied by a decrease in CD14+CD16- classical monocytes was also associated with vaccine administration. Inclusion of ALVAC-HIV in the boost did not further increase MAIT, iNKT, γδ T, and NK cell activation or increase the proportion of non-classical monocytes. Additionally, NK cell activation 3 days after vaccination was positively associated with antibody titers of HIV Env-specific total IgG and IgG1. Vδ1 T cell activation 3 days after vaccine administration was associated with HIV Env-specific IgG3 titers. Finally, we observed trending associations between MAIT cell activation and Env-specific IgG3 titers and between NK cell activation and TH023 pseudovirus neutralization titers. Our study identifies a potential role for innate cells, specifically NK, MAIT, and γδ T cells, in promoting antibody responses following HIV-1 vaccine administration.

Dysfunctional states of unconventional T-cell subsets in cancer

Unconventional T cells represent a promising therapeutic agent to overcome the current limitations of immunotherapies due to their universal T-cell receptors, ability to respond directly to cytokine stimulation, and capacity to recruit and modulate conventional immune cells in the tumor microenvironment. Like conventional T cells, unconventional T cells can enter a dysfunctional state, and the functional differences associated with this state may provide insight into the discrepancies observed in their role in antitumor immunity in various cancers. The exhaustive signature of unconventional T cells differs from conventional αβ T cells, and understanding the differences in the mechanisms underlying exhaustive differentiation in these cell types may aid in the discovery of new treatments to improve sustained antitumor responses. Ongoing clinical trials investigating therapies that leverage unconventional T-cell populations have shown success in treating hematologic malignancies and reducing the immunosuppressive tumor environment. However, several hurdles remain to extend these promising results into solid tumors. Here we discuss the current knowledge on unconventional T-cell function/dysfunction and consider how the incorporation of therapies that modulate unconventional T-cell exhaustion may aid in overcoming the current limitations of immunotherapy. Additionally, we discuss how components of the tumor microenvironment alter the functions of unconventional T cells and how these changes can affect tumor infiltration by lymphocytes and alter conventional T-cell responses.

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.

Current Perspectives and Challenges of MAIT Cell-Directed Therapy for Tuberculosis Infection

Mucosal-associated invariant T (MAIT) cells are a distinct population of non-conventional T cells that have been preserved through evolution and possess properties of both innate and adaptive immune cells. They are activated through the recognition of antigens presented by non-polymorphic MR1 proteins or, alternately, can be stimulated by specific cytokines. These cells are multifaceted and exert robust antimicrobial activity against bacterial and viral infections, direct the immune response through the modulation of other immune cells, and exhibit a specialized tissue homeostasis and repair function. These distinct characteristics have instigated interest in MAIT cell biology for immunotherapy and vaccine development. This review describes the current understanding of MAIT cell activation, their role in infections and diseases with an emphasis on tuberculosis (TB) infection, and perspectives on the future use of MAIT cells in immune-mediated therapy.

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.

Promiscuous recognition of MR1 drives self-reactive mucosal-associated invariant T cell responses

Mucosal-associated invariant T (MAIT) cells use canonical semi-invariant T cell receptors (TCR) to recognize microbial riboflavin precursors displayed by the antigen-presenting molecule MR1. The extent of MAIT TCR crossreactivity toward physiological, microbially unrelated antigens remains underexplored. We describe MAIT TCRs endowed with MR1-dependent reactivity to tumor and healthy cells in the absence of microbial metabolites. MAIT cells bearing TCRs crossreactive toward self are rare but commonly found within healthy donors and display T-helper-like functions in vitro. Experiments with MR1-tetramers loaded with distinct ligands revealed significant crossreactivity among MAIT TCRs both ex vivo and upon in vitro expansion. A canonical MAIT TCR was selected on the basis of extremely promiscuous MR1 recognition. Structural and molecular dynamic analyses associated promiscuity to unique TCRβ-chain features that were enriched within self-reactive MAIT cells of healthy individuals. Thus, self-reactive recognition of MR1 represents a functionally relevant indication of MAIT TCR crossreactivity, suggesting a potentially broader role of MAIT cells in immune homeostasis and diseases, beyond microbial immunosurveillance.

Classic costimulatory interactions in MAIT cell responses: from gene expression to immune regulation

Mucosa-associated invariant T (MAIT) cells are evolutionarily conserved, innate-like T lymphocytes with enormous immunomodulatory potentials. Due to their strategic localization, their invariant T cell receptor (iTCR) specificity for major histocompatibility complex-related protein 1 (MR1) ligands of commensal and pathogenic bacterial origin, and their sensitivity to infection-elicited cytokines, MAIT cells are best known for their antimicrobial characteristics. However, they are thought to also play important parts in the contexts of cancer, autoimmunity, vaccine-induced immunity, and tissue repair. While cognate MR1 ligands and cytokine cues govern MAIT cell maturation, polarization, and peripheral activation, other signal transduction pathways, including those mediated by costimulatory interactions, regulate MAIT cell responses. Activated MAIT cells exhibit cytolytic activities and secrete potent inflammatory cytokines of their own, thus transregulating the biological behaviors of several other cell types, including dendritic cells, macrophages, natural killer cells, conventional T cells, and B cells, with significant implications in health and disease. Therefore, an in-depth understanding of how costimulatory pathways control MAIT cell responses may introduce new targets for optimized MR1/MAIT cell-based interventions. Herein, we compare and contrast MAIT cells and mainstream T cells for their expression of classic costimulatory molecules belonging to the immunoglobulin superfamily and the tumor necrosis factor (TNF)/TNF receptor superfamily, based not only on the available literature but also on our transcriptomic analyses. We discuss how these molecules participate in MAIT cells' development and activities. Finally, we introduce several pressing questions vis-à-vis MAIT cell costimulation and offer new directions for future research in this area.

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.

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.

Longitudinal analysis of mucosa-associated invariant T cells in sepsis reveals their early numerical decline with prognostic implications and a progressive loss of antimicrobial functions

Sepsis-elicited immunosuppression elevates the risk of secondary infections. We used a clinically relevant mouse model and serial peripheral blood samples from patients to assess the antimicrobial activities of mucosa-associated invariant T (MAIT) cells in sepsis. Hepatic and splenic MAIT cells from B6-MAIT^CAST mice displayed increased CD69 expression and a robust interferon-γ (IFNγ) production capacity shortly after sublethal cecal ligation and puncture, but not at a late timepoint. Peripheral blood MAIT cell frequencies were reduced in septic patients at the time of intensive care unit (ICU) admission, and more dramatically so among nonsurvivors, suggesting the predictive usefulness of early MAIT cell enumeration. In addition, at ICU admission, MAIT cells from sepsis survivors launched stronger IFNγ responses to several bacterial species compared with those from patients who subsequently died of sepsis. Of note, while low human leukocyte antigen (HLA)-DR⁺ monocyte frequencies, widely regarded as a surrogate indicator of sepsis-induced immunosuppression, were gradually corrected, the numerical insufficiency of MAIT cells was not resolved over time, and their CD69 expression continued to decline. MAIT cell responses to bacterial pathogens, a major histocompatibility complex-related protein 1 (MR1) ligand, and interleukin (IL)-12 and IL-18 were also progressively lost during sepsis and did not recover by the time of ICU/hospital discharge. We propose that MAIT cell dysfunctions contribute to post-sepsis immunosuppression.

Mucosal-associated invariant T cells in hematological malignancies: Current knowledge, pending questions

Non-classical HLA restricted T cell subsets such as γδ T and NK-T cells are showing promises for immune-based therapy of hematological malignancies. Mucosal-Associated Invariant T cells (MAIT) belong to this family of innate-like T cell subsets and are the focus of many studies on infectious diseases, owing to their unusual recognition of bacterial/fungal metabolites. Their ability to produce type 1 cytokines (IFNγ, TNFα) as well as cytotoxic effector molecules endows them with potential anti-tumor functions. However, their contribution to tumor surveillance in solid cancers is unclear, and only few studies have specifically focused on MAIT cells in blood cancers. In this review, we wish to recapitulate our current knowledge on MAIT cells biology in hematological neoplasms, at diagnosis and/or during treatment, as well as tentative approaches to target them as therapeutic tools. We also wish to take this opportunity to briefly elaborate on what we think are important question to address in this field, as well as potential limitations to overcome in order to make MAIT cells the basis of future, novel therapies for hematological cancers.

Development of Adaptive Immunity and Its Role in Lung Remodeling

Asthma is characterized by airflow limitations resulting from bronchial closure, which can be either reversible or fixed due to changes in airway tissue composition and structure, also known as remodeling. Airway remodeling is defined as increased presence of mucins-producing epithelial cells, increased thickness of airway smooth muscle cells, angiogenesis, increased number and activation state of fibroblasts, and extracellular matrix (ECM) deposition. Airway inflammation is believed to be the main cause of the development of airway remodeling in asthma. In this chapter, we will review the development of the adaptive immune response and the impact of its mediators and cells on the elements defining airway remodeling in asthma.

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.

Use of a MAIT-Activating Ligand, 5-OP-RU, as a Mucosal Adjuvant in a Murine Model of Vibrio cholerae O1 Vaccination

Background

Mucosal-associated invariant T (MAIT) cells are innate-like T cells enriched in the mucosa with capacity for B-cell help. We hypothesize that targeting MAIT cells, using a MAIT-activating ligand as an adjuvant, could improve mucosal vaccine responses to bacterial pathogens such as Vibrio cholerae.

Methods

We utilized murine models of V. cholerae vaccination to test the adjuvant potential of the MAIT-activating ligand, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU). We measured V. cholerae-specific antibody and antibody-secreting cell responses and used flow cytometry to examine MAIT-cell and B-cell phenotype, in blood, bronchoalveolar lavage fluid (BALF), and mucosal tissues, following intranasal vaccination with live V. cholerae O1 or a V. cholerae O1 polysaccharide conjugate vaccine.

Results

We report significant expansion of MAIT cells in the lungs (P < 0.001) and BALF (P < 0.001) of 5-OP-RU treated mice, and higher mucosal (BALF, P = 0.045) but not systemic (serum, P = 0.21) V. cholerae O-specific-polysaccharide IgG responses in our conjugate vaccine model when adjuvanted with low-dose 5-OP-RU. In contrast, despite significant MAIT cell expansion, no significant differences in V. cholerae-specific humoral responses were found in our live V. cholerae vaccination model.

Conclusions

Using a murine model, we demonstrate the potential, as well as the limitations, of targeting MAIT cells to improve antibody responses to mucosal cholera vaccines. Our study highlights the need for future research optimizing MAIT-cell targeting for improving mucosal vaccines.

Mucosal-associated invariant T cells: a cryptic coordinator in HIV-infected immune reconstitution

Human immunodeficiency virus type 1 (HIV-1) infection causes considerable morbidity and mortality worldwide. Although antiretroviral therapy (ART) has largely transformed HIV infection from a fatal disease to a chronic condition, approximately 10%~40% of HIV-infected individuals who receive effective ART and sustain long-term viral suppression still cannot achieve optimal immune reconstitution. These patients are called immunological non-responders, a state associated with poor clinical prognosis. Mucosal-associated invariant T (MAIT) cells are an evolutionarily conserved unconventional T cell subset defined by expression of semi-invariant αβ T cell receptor (TCR), which recognizes metabolites derived from the riboflavin biosynthetic pathway presented on major histocompatibility complex (MHC)-related protein-1 (MR1). MAIT cells, which are considered to act as a bridge between innate and adaptive immunity, produce a wide range of cytokines and cytotoxic molecules upon activation through TCR-dependent and TCR-independent mechanisms, which is of major importance in defense against a variety of pathogens. In addition, MAIT cells are involved in autoimmune and immune-mediated diseases. The number of MAIT cells is dramatically and irreversibly decreased in the early stage of HIV infection and is not fully restored even after long-term suppressive ART. In light of the important role of MAIT cells in mucosal immunity and because microbial translocation is inversely associated with CD4⁺ T cell counts, we propose that MAIT cells participate in the maintenance of intestinal barrier integrity and microbial homeostasis, thus further affecting immune reconstitution in HIV-infected individuals. This article is protected by copyright. All rights reserved.

The P5-type ATPase ATP13A1 modulates major histocompatibility complex I-related protein 1 (MR1)-mediated antigen presentation

The monomorphic antigen-presenting molecule major histocompatibility complex-I-related protein 1 (MR1) presents small-molecule metabolites to mucosal-associated invariant T (MAIT) cells. The MR1-MAIT cell axis has been implicated in a variety of infectious and noncommunicable diseases, and recent studies have begun to develop an understanding of the molecular mechanisms underlying this specialized antigen presentation pathway. However, proteins regulating MR1 folding, loading, stability, and surface expression remain to be identified. Here, we performed a gene trap screen to discover novel modulators of MR1 surface expression through insertional mutagenesis of an MR1-overexpressing clone derived from the near-haploid human cell line HAP1 (HAP1.MR1). The most significant positive regulators identified included β2-microglobulin, a known regulator of MR1 surface expression, and ATP13A1, a P5-type ATPase in the endoplasmic reticulum (ER) not previously known to be associated with MR1-mediated antigen presentation. CRISPR/Cas9-mediated knockout of ATP13A1 in both HAP1.MR1 and THP-1 cell lines revealed a profound reduction in MR1 protein levels and a concomitant functional defect specific to MR1-mediated antigen presentation. Collectively, these data are consistent with the ER-resident ATP13A1 being a key posttranscriptional determinant of MR1 surface expression.

Circulating CD8+ mucosal-associated invariant T cells correlate with improved treatment responses and overall survival in anti-PD-1-treated melanoma patients

Objectives

While much of the research concerning factors associated with responses to immune checkpoint inhibitors (ICIs) has focussed on the contributions of conventional peptide‐specific T cells, the role of unconventional T cells, such as mucosal‐associated invariant T (MAIT) cells, in human melanoma remains largely unknown. MAIT cells are an abundant population of innate‐like T cells expressing a semi‐invariant T‐cell receptor restricted to the MHC class I‐like molecule, MR1, presenting vitamin B metabolites derived from bacteria. We sought to characterise MAIT cells in melanoma patients and determined their association with treatment responses and clinical outcomes.

Methods

In this prospective clinical study, we analysed the frequency and functional profile of circulating and tumor‐infiltrating MAIT cells in human melanoma patients. Using flow cytometry, we compared these across metastatic sites and between ICI responders vs. non‐responders as well as healthy donors.

Results

We identified tumor‐infiltrating MAIT cells in melanomas across metastatic sites and found that the number of circulating MAIT cells is reduced in melanoma patients compared to healthy donors. However, circulating MAIT cell frequencies are restored by ICI treatment in responding patients, correlating with treatment responses, in which patients with high frequencies of MAIT cells exhibited significantly improved overall survival.

Conclusion

Our results suggest that MAIT cells may be a potential predictive marker of responses to immunotherapies and provide rationale for testing MAIT cell‐directed therapies in combination with current and next‐generation ICIs.

MAIT cells accumulate in ovarian cancer-elicited ascites where they retain their capacity to respond to MR1 ligands and cytokine cues

The low mutational burden of epithelial ovarian cancer (EOC) is an impediment to immunotherapies that rely on conventional MHC-restricted, neoantigen-reactive T lymphocytes. Mucosa-associated invariant T (MAIT) cells are MR1-restricted T cells with remarkable immunomodulatory properties. We sought to characterize intratumoral and ascitic MAIT cells in EOC. Single-cell RNA sequencing of six primary human tumor specimens demonstrated that MAIT cells were present at low frequencies within several tumors. When detectable, these cells highly expressed CD69 and VSIR, but otherwise exhibited a transcriptomic signature inconsistent with overt cellular activation and/or exhaustion. Unlike mainstream CD8⁺ T cells, CD8⁺ MAIT cells harbored high transcript levels of TNF, PRF1, GZMM and GNLY, suggesting their arming and cytotoxic potentials. In a congenic, MAIT cell-sufficient mouse model of EOC, MAIT and invariant natural killer T cells amassed in the peritoneal cavity where they showed robust IL-17A and IFN-γ production capacities, respectively. However, they gradually lost these functions with tumor progression. In a cohort of 23 EOC patients, MAIT cells were readily detectable in all ascitic fluids examined. In a sub-cohort in which we interrogated ascitic MAIT cells for functional impairments, several exhaustion markers, most notably VISTA, were present on the surface. However, ascitic MAIT cells were capable of producing IFN-γ, TNF-α and granzyme B, but neither IL-17A nor IL-10, in response to an MR1 ligand, bacterial lysates containing MR1 ligands, or a combination of IL-12 and IL-18. In conclusion, ascitic MAIT cells in EOC possess inducible effector functions that may be modified in future immunotherapeutic strategies.

A Multi-Omics Analysis of Mucosal-Associated-Invariant T Cells Reveals Key Drivers of Distinct Modes of Activation

The function of mucosal-associated invariant T (MAIT) cells highly depends on the mode of activation, either by recognition of bacterial metabolites via their T cell receptor (TCR) or in a TCR-independent manner via cytokines. The underlying molecular mechanisms are not entirely understood. To define the activation of MAIT cells on the molecular level, we applied a multi-omics approach with untargeted transcriptomics, proteomics and metabolomics. Transcriptomic analysis of E. coli- and TCR-activated MAIT cells showed a distinct transcriptional reprogramming, including altered pathways, transcription factors and effector molecules. We validated the consequences of this reprogramming on the phenotype by proteomics and metabolomics. Thus, and to distinguish between TCR-dependent and -independent activation, MAIT cells were stimulated with IL12/IL18, anti-CD3/CD28 or both. Only a combination of both led to full activation of MAIT cells, comparable to activation by E. coli. Using an integrated network-based approach, we identified key drivers of the distinct modes of activation, including cytokines and transcription factors, as well as negative feedback regulators like TWIST1 or LAG3. Taken together, we present novel insights into the biological function of MAIT cells, which may represent a basis for therapeutic approaches to target MAIT cells in pathological conditions.

Chronic stress physically spares but functionally impairs innate-like invariant T cells

The deleterious effects of psychological stress on mainstream T lymphocytes are well documented. However, how stress impacts innate-like T cells is unclear. We report that long-term stress surprisingly abrogates both T helper 1 (TH1)- and TH2-type responses orchestrated by invariant natural killer T (iNKT) cells. This is not due to iNKT cell death because these cells are unusually refractory to stress-inflicted apoptosis. Activated iNKT cells in stressed mice exhibit a "split" inflammatory signature and trigger sudden serum interleukin-10 (IL-10), IL-23, and IL-27 spikes. iNKT cell dysregulation is mediated by cell-autonomous glucocorticoid receptor signaling and corrected upon habituation to predictable stressors. Importantly, under stress, iNKT cells fail to potentiate cytotoxicity against lymphoma or to reduce the burden of metastatic melanoma. Finally, stress physically spares mouse mucosa-associated invariant T (MAIT) cells but hinders their TH1-/TH2-type responses. The above findings are corroborated in human peripheral blood and hepatic iNKT/MAIT cell cultures. Our work uncovers a mechanism of stress-induced immunosuppression.

MR1-restricted T cells: the new dawn of cancer immunotherapy

Cancer immunotherapy has recently undergone rapid development into a validated therapy for clinical use. The adoptive transfer of engineered autologous T cells, such as chimeric antigen receptor (CAR) T cells, has been remarkably successful in patients with leukemia and lymphoma with cluster of differentiation (CD)19 expression. Because of the higher number of antigen choices and reduced incidence of cytokine release syndrome (CRS) than CAR-T cells, T cell receptor (TCR)-T cells are also considered a promising immunotherapy. More therapeutic targets for other cancers need to be explored due to the human leukocyte antigen (HLA)-restricted recognition of TCR-T. Major histocompatibility complex (MHC), class I-related (MR1)-restricted T cells can recognize metabolites presented by MR1 in the context of host cells infected with pathogens. MR1 is expressed by all types of human cells. Recent studies have shown that one clone of a MR1-restricted T (MR1-T) cell can recognize many types of cancer cells without HLA-restriction. These studies provide additional information on MR1-T cells for cancer immunotherapy. This review describes the complexity of MR1-T cell TCR in diseases and the future of cancer immunotherapy.

Augmentation of the Riboflavin-Biosynthetic Pathway Enhances Mucosa-Associated Invariant T (MAIT) Cell Activation and Diminishes Mycobacterium tuberculosis Virulence

Mucosa-associated invariant T (MAIT) cells play a critical role in antimicrobial defense. Despite increased understanding of their mycobacterial ligands and the clinical association of MAIT cells with tuberculosis (TB), their function in protection against Mycobacterium tuberculosis infection remains unclear. Here, we show that overexpressing key genes of the riboflavin-biosynthetic pathway potentiates MAIT cell activation and results in attenuation of M. tuberculosis virulence in vivo. Further, we observed greater control of M. tuberculosis infection in MAIT^hi CAST/EiJ mice than in MAIT^lo C57BL/6J mice, highlighting the protective role of MAIT cells against TB. We also endogenously adjuvanted Mycobacterium bovis BCG with MR1 ligands via overexpression of the lumazine synthase gene ribH and evaluated its protective efficacy in the mouse model of M. tuberculosis infection. Altogether, our findings demonstrate that MAIT cells confer host protection against TB and that overexpression of genes in the riboflavin-biosynthetic pathway attenuates M. tuberculosis virulence. Enhancing MAIT cell-mediated immunity may also offer a novel approach toward improved vaccines against TB. IMPORTANCE Mucosa-associated invariant T (MAIT) cells are an important subset of innate lymphocytes that recognize microbial ligands derived from the riboflavin biosynthesis pathway and mediate antimicrobial immune responses. Modulated MAIT cell responses have been noted in different forms of tuberculosis. However, it has been unclear if increased MAIT cell abundance is protective against TB disease. In this study, we show that augmentation of the mycobacterial MAIT cell ligands leads to higher MAIT cell activation with reduced M. tuberculosis virulence and that elevated MAIT cell abundance confers greater control of M. tuberculosis infection. Our study also highlights the potential of endogenously adjuvanting the traditional BCG vaccine with MR1 ligands to augment MAIT cell activation. This study increases current knowledge on the roles of the riboflavin-biosynthetic pathway and MAIT cell activation in M. tuberculosis virulence and host immunity against TB.

Translating Unconventional T Cells and Their Roles in Leukemia Antitumor Immunity

Recently, cell-mediated immune response in malignant neoplasms has become the focus in immunotherapy against cancer. However, in leukemia, most studies on the cytotoxic potential of T cells have concentrated only on T cells that recognize peptide antigens (Ag) presented by polymorphic molecules of the major histocompatibility complex (MHC). This ignores the great potential of unconventional T cell populations, which include gamma-delta T cells (γδ), natural killer T cells (NKT), and mucosal-associated invariant T cells (MAIT). Collectively, these T cell populations can recognize lipid antigens, specially modified peptides and small molecule metabolites, in addition to having several other advantages, which can provide more effective applications in cancer immunotherapy. In recent years, these cell populations have been associated with a repertoire of anti- or protumor responses and play important roles in the dynamics of solid tumors and hematological malignancies, thus, encouraging the development of new investigations in the area. This review focuses on the current knowledge regarding the role of unconventional T cell populations in the antitumor immune response in leukemia and discusses why further studies on the immunotherapeutic potential of these cells are needed.

MAIT cells, guardians of skin and mucosa?

Mucosal Associated Invariant T (MAIT) cells are evolutionary conserved innate-like T cells able to recognize bacterial and fungal ligands derived from vitamin B biosynthesis. These cells are particularly present in liver and blood but also populate mucosal sites including skin, oral, intestinal, respiratory, and urogenital tracts that are in contact with the environment and microbiota of their host. Growing evidence suggests important involvement of MAIT cells in safeguarding the mucosa against external microbial threats. Simultaneously, mucosal MAIT cells have been implicated in immune and inflammatory pathologies affecting these organs. Here, we review the specificities of mucosal MAIT cells, their functions in the protection and maintenance of mucosal barriers, and their interactions with other mucosal cells.

Emerging Role for MAIT Cells in Control of Antimicrobial Resistance

Antimicrobial resistance is a serious threat to global public health as antibiotics are losing effectiveness due to rapid development of resistance. The human immune system facilitates control and clearance of resistant bacterial populations during the course of antimicrobial therapy. Here we review current knowledge of mucosa-associated invariant T (MAIT) cells, an arm of the immune system on the border between innate and adaptive, and their critical place in human antibacterial immunity. We propose that MAIT cells play important roles against antimicrobial-resistant infections through their capacity to directly clear multidrug-resistant bacteria and overcome mechanisms of antimicrobial resistance. Finally, we discuss outstanding questions pertinent to the possible advancement of host-directed therapy as an alternative intervention strategy for antimicrobial-resistant bacterial infections.

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.

Differential Skewing of Circulating MR1-Restricted and γδ T Cells in Human Psoriasis Vulgaris

Psoriasis vulgaris (PV) is a chronic, recurrent inflammatory dermatosis mediated by aberrantly activated immune cells. The role of the innate-like T cells, particularly gammadelta T (γδT) cells and MR1-restricted T lymphocytes, is incompletely explored, mainly through animal models, or by use of surrogate lineage markers, respectively. Here, we used case-control settings, multiparameter flow cytometry, 5-OP-RU-loaded MR1-tetramers, Luminex technology and targeted qRT-PCR to dissect the cellular and transcriptional landscape of γδ and MR1-restricted blood T cells in untreated PV cases (n=21, 22 matched controls). High interpersonal differences in cell composition were observed, fueling transcriptional variability at healthy baseline. A minor subset of canonical CD4+CD8+MR1-tet+TCRVα7.2+ and CD4+CD8-MR1-tet+TCRVα7.2+ T cells was the most significantly underrepresented community in male PV individuals, whereas Vδ2+ γδ T cells expressing high levels of TCR and Vδ1-δ2- γδ T cells expressing intermediate levels of TCR were selectively enriched in affected males, partly reflecting disease severity. Our findings highlight a formerly unappreciated skewing of human circulating MAIT and γδ cytomes during PV, and reveal their compositional changes in relation to sex, CMV exposure, serum cytokine content, BMI, and inflammatory burden. Complementing numerical alterations, we finally show that flow-sorted, MAIT and γδ populations exhibit divergent transcriptional changes in mild type I psoriasis, consisting of differential bulk expression for signatures of cytotoxicity/type-1 immunity (EOMES, RUNX3, IL18R), type-3 immunity (RORC, CCR6), and T cell innateness (ZBTB16).

MAIT Cells in Barrier Tissues: Lessons from Immediate Neighbors

Mucosal-associated invariant T (MAIT) cells are innate-like T cells present at considerable frequencies in human blood and barrier tissues, armed with an expanding array of effector functions in response to homeostatic perturbations. Analogous to other barrier immune cells, their phenotype and function is driven by crosstalk with host and dynamic environmental factors, most pertinently the microbiome. Given their distribution, they must function in diverse extracellular milieus. Tissue-specific and adapted functions of barrier immune cells are shaped by transcriptional programs and regulated through a blend of local cellular, inflammatory, physiological, and metabolic mediators unique to each microenvironment. This review compares the phenotype and function of MAIT cells with other barrier immune cells, highlighting potential areas for future exploration. Appreciation of MAIT cell biology within tissues is crucial to understanding their niche in health and disease.

Interleukin (IL)-12 and IL-18 Synergize to Promote MAIT Cell IL-17A and IL-17F Production Independently of IL-23 Signaling

IL-23 is considered a critical regulator of IL-17 in Th17 cells; however, its requirement for inducing IL-17 production in other human immune subsets remains incompletely understood. Mucosal associated invariant T (MAIT) cells uniformly express retinoic acid receptor-related orphan receptor gamma t (RORγt) but only a minor population have been shown to produce IL-17A. Here we show that IL-17F is the dominant IL-17 isoform produced by MAIT cells, not IL-17A. For optimal MAIT cell derived IL-17A and IL-17F production, T cell receptor (TCR) triggering, IL-18 and monocyte derived IL-12 signaling is required. Unlike Th17 cells, this process is independent of IL-23 signaling. Using an in vitro skin cell activation assay, we demonstrate that dual neutralization of both IL-17A and IL-17F resulted in greater suppression of inflammatory proteins than inhibition of IL-17A alone. Finally, we extend our findings by showing that other innate-like lymphocytes such as group 3 innate lymphoid cells (ILC3) and gamma delta (γδ) T cells are also capable of IL-23 independent IL-17A and IL-17F production. These data indicate both IL-17F and IL-17A production from MAIT cells may contribute to tissue inflammation independently of IL-23, in part explaining the therapeutic disconnect between targeting IL-17 or IL-23 in certain inflammatory diseases.

MAIT Cells Display a Specific Response to Type 1 IFN Underlying the Adjuvant Effect of TLR7/8 Ligands

Mucosal-associated invariant T (MAIT) cells constitute a highly conserved subset of effector T cells with innate-like recognition of a wide array of bacteria and fungi in humans. Harnessing the potential of these cells could represent a major advance as a new immunotherapy approach to fight difficult-to-treat bacterial infections. However, despite recent advances in the design of potent agonistic ligands for MAIT cells, it has become increasingly evident that adjuvants are required to elicit potent antimicrobial effector functions by these cells, such as IFNγ production and cytotoxicity. Indeed, TCR triggering alone elicits mostly barrier repair functions in MAIT cells, whereas an inflammatory milieu is required to drive the antibacterial functions. Cytokines such as IL-7, IL-12 and IL-18, IL-15 or more recently type 1 IFN all display an apparently similar ability to synergize with TCR stimulation to induce IFNγ production and/or cytotoxic functions in vitro, but their mechanisms of action are not well established. Herein, we show that MAIT cells feature a build-in mechanism to respond to IFNα. We confirm that IFNα acts directly and specifically on MAIT cells and synergizes with TCR/CD3 triggering to induce maximum cytokine production and cytotoxic functions. We provide evidences suggesting that the preferential activation of the Stat4 pathway is involved in the high sensitivity of MAIT cells to IFNα stimulation. Finally, gene expression data confirm the specific responsiveness of MAIT cells to IFNα and pinpoints specific pathways that could be the target of this cytokine. Altogether, these data highlight the potential of IFNα-inducing adjuvants to maximize MAIT cells responsiveness to purified ligands in order to induce potent anti-infectious responses.

Harnessing Unconventional T Cells for Immunotherapy of Tuberculosis

Even if the incidence of tuberculosis (TB) has been decreasing over the last years, the number of patients with TB is increasing worldwide. The emergence of multidrug-resistant and extensively drug-resistant TB is making control of TB more difficult. Mycobacterium bovis bacillus Calmette–Guérin vaccine fails to prevent pulmonary TB in adults, and there is an urgent need for a vaccine that is also effective in patients with human immunodeficiency virus (HIV) coinfection. Therefore, TB control may benefit on novel therapeutic options beyond antimicrobial treatment. Host-directed immunotherapies could offer therapeutic strategies for patients with drug-resistant TB or with HIV and TB coinfection. In the last years, the use of donor lymphocytes after hematopoietic stem cell transplantation has emerged as a new strategy in the cure of hematologic malignancies in order to induce graft-versus leukemia and graft-versus-infection effects. Moreover, adoptive therapy has proven to be effective in controlling cytomegalovirus and Epstein-Barr virus reactivation in immunocompromised patients with ex vivo expanded viral antigen-specific T cells. Unconventional T cells are a heterogeneous group of T lymphocytes with limited diversity. One of their characteristics is that antigen recognition is not restricted by the classical major histocompatibility complex (MHC). They include CD1 (cluster of differentiation 1)–restricted T cells, MHC-related protein-1–restricted mucosal-associated invariant T (MAIT) cells, MHC class Ib–reactive T cells, and γδ T cells. Because these T cells are genotype-independent, they are also termed “donor unrestricted” T cells. The combined features of low donor diversity and the lack of genetic restriction make these cells suitable candidates for T cell–based immunotherapy of TB.

Hizikia fusiforme extract enhances dendritic cell maturation in vitro and in vivo

Dendritic cells (DCs) are play critical roles in the priming and regulation of immune responses. DCs rapidly process and convey these antigens to prime antigen-specific T cells. Therefore, regulation of DCs functions is important for immunity and immunotherapies. Immune adjuvants for DCs activation are needed to improve the efficacy of vaccines against tumors and many infectious diseases. Therefore, we demonstrate that H. fusiformis extract can regulate DCs maturation and activation. H. fusiformis extract induced costimulatory molecules (CD 80 and CD86), antigen-presenting molecules (major histocompatibility complex (MHC) I and II), CCR7 expression, and interleukin (IL)-12 production in DCs. These effects are associated with upregulation of mitogen-activated protein kinase (MAPK) signaling pathway. In addition, H. fusiformis extract induces costimulatory molecules on splenic DCs and activated CD8+ T cells in vivo. Taken together, these findings suggest that H. fusiformis extract may be a potential efficient immune therapeutic compound in DCs-mediated immunotherapies.

ABBREVIATIONS

CTL: cytotoxic T lymphocytes; DCs: dendritic cells; ERK: extracellular signal-regulated kinases; IL: interleukini; JNK: c-Jun N-terminal kinase; MAPK: mitogen-activated protein kinase; MHC: major histocompatibility complex.

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.

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.

Tissue-resident mucosal-associated invariant T (MAIT) cells in the human kidney represent a functionally distinct subset

Mucosal‐associated invariant T (MAIT) cells are innate‐like T‐cells that recognize bacterial riboflavin metabolites. They are present in human blood but are abundant at barrier sites, including the liver, lungs, and kidneys, where they possess a CD69⁺/CD103^+/− tissue‐resident phenotype. In renal tissue, MAIT cells likely defend against the ascending uropathogens responsible for urinary tract infections (UTIs), which are common, especially among renal transplant recipients (RTRs). Nevertheless, the functional role for MAIT cells in renal tissue and the influence of renal transplantation on MAIT cells remains unclear. Using multiparameter flow cytometry and the MR1‐tetramer, we characterized MAIT cell phenotype and function in healthy renal tissue (n = 6), renal transplants explanted after allograft failure (n = 14) and in blood from healthy controls (n = 20) and RTRs before and 1‐year after transplantation (n = 21). MAIT cells in renal tissue constitute a distinct CD69⁺CD103^+/− population that displays typical phenotypic features of tissue‐resident T‐cells and is skewed toward IL‐2, GM‐CSF, and IL‐17A production upon stimulation. The circulating MAIT cell population was not decreased in number in RTRs pre‐ or post‐transplantation. Tissue‐resident MAIT cells in the kidney represent a functionally distinct population. This shows how MAIT cells in the kidney may be involved in the protection against microorganisms.

Characterization of Human Mucosal-associated Invariant T (MAIT) Cells

Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells restricted by the major histocompatibility complex (MHC) class I-like molecule MHC-related protein 1 (MR1). MAIT cells are found throughout the body, especially in human blood and liver. Unlike conventional T cells, which are stimulated by peptide antigens presented by MHC molecules, MAIT cells recognize metabolite antigens derived from an intermediate in the microbial biosynthesis of riboflavin. MAIT cells mediate protective immunity to infections by riboflavin-producing microbes via the production of cytokines and cytotoxicity. The discovery of stimulating MAIT cell antigens allowed for the development of an analytical tool, the MR1 tetramer, that binds specifically to the MAIT T cell receptor (TCR) and is becoming the gold standard for identification of MAIT cells by flow cytometry. This article describes protocols to characterize the phenotype of human MAIT cells in blood and tissues by flow cytometry using fluorescently labeled human MR1 tetramers alongside antibodies specific for MAIT cell markers. © 2019 by John Wiley & Sons, Inc. The main protocols include: Basic Protocol 1: Determining the frequency and steady-state surface phenotype of human MAIT cells Basic Protocol 2: Determining the activation phenotype of human MAIT cells in blood Basic Protocol 3: Characterizing MAIT cell TCRs using TCR-positive reporter cell lines Alternate protocols are provided for determining the absolute number, transcription factor phenotype, and TCR usage of human MAIT cells; and determining activation phenotype by staining for intracellular markers, measuring secreted cytokines, and measuring fluorescent dye dilution due to proliferation. Additional methods are provided for determining the capacity of MAIT cells to produce cytokine independently of antigen using plate-bound or bead-immobilized CD3/CD28 stimulation; and determining the MR1-Ag dependence of MAIT cell activation using MR1-blocking antibody or competitive inhibition. For TCR-positive reporter cell lines, methods are also provided for evaluating the MAIT TCR-mediated MR1-Ag response, determining the capacity of the reporter lines to produce cytokine independently of antigen, determining the MR1-Ag dependence of the reporter lines, and evaluating the MR1-Ag response of the reporter lines using IL-2 secretion. Support Protocols describe the preparation of PBMCs from human blood, the preparation of single-cell suspensions from tissue, the isolation of MAIT cells by FACS and MACS, cloning MAIT TCRα and β chain genes and MR1 genes for transduction, generating stably and transiently transfected cells lines, generating a stable MR1 knockout antigen-presenting cell line, and generating monocyte-derived dendritic cells.

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.

Roles of IL-2 in bridging adaptive and innate immunity, and as a tool for cellular immunotherapy

IL-2 was initially characterized as a T cell growth factor in the 1970s, and has been studied intensively ever since. Decades of research have revealed multiple and diverse roles for this potent cytokine, indicating a unique linking role between adaptive and innate arms of the immune system. Here, we review the literature showing that IL-2 is expressed in a plethora of cell types across the immune system, where it has indispensable functions in orchestrating cellular interactions and shaping the nature and magnitude of immune responses. Emerging from the basic research that has revealed the molecular mechanisms and the complexity of the biologic actions of IL-2, several immunotherapeutic approaches have now focused on manipulating the levels of this cytokine in patients. These strategies range from inhibition of IL-2 to achieve immunosuppression, to the application of IL-2 as a vaccine adjuvant and in cancer therapies. This review will systematically summarize the major findings in the field and identify key areas requiring further research in order to realize the potential of IL-2 in the treatment of human diseases.

Mucosal-Associated Invariant T Cells Develop an Innate-Like Transcriptomic Program in Anti-mycobacterial Responses

Conventional T cells exhibit a delayed response to the initial priming of peptide antigens presented by major histocompatibility complex (MHC) proteins. Unlike conventional T cells, mucosal-associated invariant T (MAIT) cells quickly respond to non-peptidic metabolite antigens presented by MHC-related protein 1 (MR1). To elucidate the MR1-dependent activation program of MAIT cells in response to mycobacterial infections, we determined the surface markers, transcriptomic profiles, and effector responses of activated human MAIT cells. Results revealed that mycobacterial-incubated antigen-presenting cells stimulated abundant human CD8⁺ MAIT cells to upregulate the co-expression of CD69 and CD26, as a combinatorial activation marker. Further transcriptomic analyses demonstrated that CD69⁺CD26⁺⁺ CD8⁺MAIT cells highly expressed numerous genes for mediating anti-mycobacterial immune responses, including pro-inflammatory cytokines, cytolytic molecules, NK cell receptors, and transcription factors, in contrast to inactivated counterparts CD69^+/−CD26^+/− CD8⁺MAIT cells. Gene co-expression, enrichment, and pathway analyses yielded high statistical significance to strongly support that activated CD8⁺ MAIT cells shared gene expression and numerous pathways with NK and CD8⁺ T cells in activation, cytokine production, cytokine signaling, and effector functions. Flow cytometry detected that activated CD8⁺MAIT cells produced TNFα, IFNγ, and granulysin to inhibit mycobacterial growth and fight mycobacterial infection. Together, results strongly support that the combinatorial activation marker CD69⁺CD26⁺⁺ labels the activated CD8⁺MAIT cells that develop an innate-like activation program in anti-mycobacterial immune responses. We speculate that the rapid production of anti-mycobacterial effector molecules facilitates MAIT cells to fight early mycobacterial infection in humans.

Ligand-dependent downregulation of MR1 cell surface expression

The antigen-presenting molecule MR1 presents riboflavin-based metabolites to Mucosal-Associated Invariant T (MAIT) cells. While MR1 egress to the cell surface is ligand-dependent, the ability of small-molecule ligands to impact on MR1 cellular trafficking remains unknown. Arising from an in silico screen of the MR1 ligand-binding pocket, we identify one ligand, 3-([2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl]formamido)propanoic acid, DB28, as well as an analog, methyl 3-([2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl]formamido)propanoate, NV18.1, that down-regulate MR1 from the cell surface and retain MR1 molecules in the endoplasmic reticulum (ER) in an immature form. DB28 and NV18.1 compete with the known MR1 ligands, 5-OP-RU and acetyl-6-FP, for MR1 binding and inhibit MR1-dependent MAIT cell activation. Crystal structures of the MAIT T cell receptor (TCR) complexed with MR1-DB28 and MR1-NV18.1, show that these two ligands reside within the A'-pocket of MR1. Neither ligand forms a Schiff base with MR1 molecules; both are nevertheless sequestered by a network of hydrophobic and polar contacts. Accordingly, we define a class of compounds that inhibits MR1 cellular trafficking.

MAIT Cells in Health and Disease

Mucosal-associated invariant T (MAIT) cells have been attracting increasing attention over the last few years as a potent unconventional T cell subset. Three factors largely account for this emerging interest. Firstly, these cells are abundant in humans, both in circulation and especially in some tissues such as the liver. Secondly is the discovery of a ligand that has uncovered their microbial targets, and also allowed for the development of tools to accurately track the cells in both humans and mice. Finally, it appears that the cells not only have a diverse range of functions but also are sensitive to a range of inflammatory triggers that can enhance or even bypass T cell receptor–mediated signals—substantially broadening their likely impact in health and disease. In this review we discuss how MAIT cells display antimicrobial, homeostatic, and amplifier roles in vivo, and how this may lead to protection and potentially pathology.

Distinct Dysfunctional States of Circulating Innate-Like T Cells in Metabolic Disease

The immune system plays a significant role in controlling systemic metabolism. Innate-like T (ILT) cells in particular, such as mucosal-associated invariant T (MAIT) cells, invariant natural killer T (iNKT) cells and γδ T cell receptor expressing cells, have been reported to promote metabolic homeostasis. However, these different ILT cell subsets have, to date, been generally studied in isolation. Here we conducted a pilot study assessing the phenotype and function of circulating MAIT, iNKT, and Vδ2⁺ T cells in a small cohort of 10 people with obesity and type 2 diabetes (T2D), 10 people with obesity but no diabetes, and 12 healthy individuals. We conducted phenotypic analysis by flow cytometry ex vivo, and then functional analysis after in vitro stimulation using either PMA/ionomycin or synthetic agonists, or precursors thereof, for each of the cell-types; use of the latter may provide important knowledge for the development of novel therapeutics aimed at activating human ILT cells. The results of our pilot study, conducted on circulating cells, show clear dysfunction of all three ILT cell subsets in obese and obese T2D patients, as compared to healthy controls. Importantly, while both iNKT and Vδ2⁺ T cell dysfunctions were characterized by diminished IL-2 and interferon-γ production, the distinct dysfunctional state of MAIT cells was instead defined by skewed subset composition, heightened sensitivity to T cell receptor engagement and unchanged production of all measured cytokines.

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

Mucosal-associated invariant T (MAIT) cells are antibacterial effector T cells that react to pyrimidines derived from bacterial riboflavin synthesis presented by the monomorphic molecule MR1. A major challenge in MAIT cell research is that the commonly used MAIT agonist precursor, 5-amino-6-d-ribitylaminouracil (5-A-RU), is labile to autoxidation, resulting in a loss of biological activity. Here, we characterize two independent autoxidation processes by LCMS. To overcome the marked instability, we report the synthesis of a 5-A-RU prodrug generated by modification of the 5-amino group with a cleavable valine-citrulline-p-aminobenzyl carbamate. The compound is stable in prodrug form, with the parent amine (i.e., 5-A-RU) released only after enzymatic cleavage. Analysis of the prodrug in vitro and in vivo showed an enhanced MAIT cell activation profile compared to 5-A-RU, which was associated with preferential loading within recycling endosomes, a route used by some natural agonists. This prodrug design therefore overcomes the difficulties associated with 5-A-RU in biological studies and provides an opportunity to explore different presentation pathways.