Human mucosal-associated invariant T (MAIT) cells possess capacity for B cell help

Role of mucosal-associated invariant T cells in coronavirus disease 2019 vaccine immunogenicity

Mucosal-associated invariant T (MAIT) cells are an unconventional T cell population that are highly abundant in humans. They possess a semi-invariant T cell receptor (TCR) that recognises microbial metabolites formed during riboflavin biosynthesis, presented on a nonpolymorphic MHC-like molecule MR1. MAIT cells possess an array of effector functions, including type 1, type 17, and tissue repair activity. Deployment of these functions depends on the stimuli they receive through their TCR and/or cytokine receptors. Strong cytokine signalling, such as in response to vaccination, can bypass TCR triggering and provokes a strong proinflammatory response. Although data are still emerging, multiple aspects of MAIT cell biology are associated with modulation of immunity induced by the coronavirus disease 2019 mRNA and adenovirus vector vaccines. In this review, we will address how MAIT cells may play a role in immunogenicity of vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and how these cells can be harnessed as cellular adjuvants.

Mucosal-associated invariant T cells modulate innate immune cells and inhibit colon cancer growth

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that can be activated by microbial antigens and cytokines and are abundant in mucosal tissues including the colon. MAIT cells have cytotoxic and pro-inflammatory functions and have potentials for use as adoptive cell therapy. However, studies into their anti-cancer activity, including their role in colon cancer, are limited. Using an animal model of colon cancer, we showed that peritumoral injection of in vivo-expanded MAIT cells into RAG1-/- mice with MC38-derived tumours inhibits tumour growth compared to control. Multiplex cytokine analyses showed that tumours from the MAIT cell-treated group have higher expression of markers for eosinophil-activating cytokines, suggesting a potential association between eosinophil recruitment and tumour inhibition. In a human peripheral leukocyte co-culture model, we showed that leukocytes stimulated with MAIT ligand showed an increase in eotaxin-1 production and activation of eosinophils, associated with increased cancer cell killing. In conclusion, we showed that MAIT cells have a protective role in a murine colon cancer model, associated with modulation of the immune response to cancer, potentially involving eosinophil-associated mechanisms. Our results highlight the potential of MAIT cells for non-donor restricted colon cancer immunotherapy.

IL-21 conditions antigen-presenting human γδ T-cells to promote IL-10 expression in naïve and memory CD4+ T-cells

Direct interaction between T-cells exerts a major influence on tissue immunity and inflammation across multiple body sites including the human gut, which is highly enriched in 'unconventional' lymphocytes such as γδ T-cells. We previously reported that microbial activation of human Vγ9/Vδ2+ γδ T-cells in the presence of the mucosal damage-associated cytokine IL-15 confers the ability to promote epithelial barrier defence, specifically via induction of IL-22 expression in conventional CD4+ T-cells. In the current report, we assessed whether other cytokines enriched in the gut milieu also functionally influence microbe-responsive Vγ9/Vδ2 T-cells. When cultured in the presence of IL-21, Vγ9/Vδ2 T-cells acquired the ability to induce expression of the immunoregulatory cytokine IL-10 in both naïve and memory CD4+ T-cells, at levels surpassing those induced by monocytes or monocyte-derived DCs. These findings identify an unexpected influence of IL-21 on Vγ9/Vδ2 T-cell modulation of CD4+ T-cell responses. Further analyses suggested a possible role for CD30L and/or CD40L reverse signalling in mediating IL-10 induction by IL-21 conditioned Vγ9/Vδ2 T-cells. Our findings indicate that the local microenvironment exerts a profound influence on Vγ9/Vδ2 T-cell responses to microbial challenge, leading to induction of distinct functional profiles among CD4+ T-cells that may influence inflammatory events at mucosal surfaces. Targeting these novel pathways may offer therapeutic benefit in disorders such as inflammatory bowel disease.

Signals that control MAIT cell function in healthy and inflamed human tissues

Mucosal-associated invariant T (MAIT) cells have a semi-invariant T-cell receptor that allows recognition of antigen in the context of the MHC class I-related (MR1) protein. Metabolic intermediates of the riboflavin synthesis pathway have been identified as MR1-restricted antigens with agonist properties. As riboflavin synthesis occurs in many bacterial species, but not human cells, it has been proposed that the main purpose of MAIT cells is antibacterial surveillance and protection. The majority of human MAIT cells secrete interferon-gamma (IFNg) upon activation, while some MAIT cells in tissues can also express IL-17. Given that MAIT cells are present in human barrier tissues colonized by a microbiome, MAIT cells must somehow be able to distinguish colonization from infection to ensure effector functions are only elicited when necessary. Importantly, MAIT cells have additional functional properties, including the potential to contribute to restoring tissue homeostasis by expression of CTLA-4 and secretion of the cytokine IL-22. A recent study provided compelling data indicating that the range of human MAIT cell functional properties is explained by plasticity rather than distinct lineages. This further underscores the necessity to better understand how different signals regulate MAIT cell function. In this review, we highlight what is known in regards to activating and inhibitory signals for MAIT cells with a specific focus on signals relevant to healthy and inflamed tissues. We consider the quantity, quality, and the temporal order of these signals on MAIT cell function and discuss the current limitations of computational tools to extrapolate which signals are received by MAIT cells in human tissues. Using lessons learned from conventional CD8 T cells, we also discuss how TCR signals may integrate with cytokine signals in MAIT cells to elicit distinct functional states.

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.

MAIT Cells Modulate Innate Immune Cells and Inhibit Colon Cancer Growth

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that can be activated by microbial antigens and cytokines and are abundant in mucosal tissues including the colon. MAIT cells have cytotoxic and pro-inflammatory functions and have potentials for use as adoptive cell therapy. However, studies into their anti-cancer activity, including their role in colon cancer, are limited. Using an animal model of colon cancer, we show that peritumoral injection of in vivo-expanded MAIT cells into RAG1-/- mice with MC38-derived tumors inhibits tumor growth compared to control. Multiplex cytokine analyses show that tumors from the MAIT cell-treated group have higher expression of markers for eosinophil-activating cytokines, suggesting an association between eosinophil recruitment and tumor inhibition. In a human peripheral leukocyte co-culture model, we show that leukocytes stimulated with MAIT ligand show an increase in eotaxin-1 production and activation of eosinophils, associated with increased cancer cell killing. In conclusion, we show that MAIT cells have a protective role in a murine colon cancer model, associated with modulation of the immune response to cancer, potentially involving eosinophil-associated mechanisms. Our results highlight the potential of MAIT cells for non-donor restricted colon cancer immunotherapy.

IL-4-secreting CD40L+ MAIT cells support antibody production in the peripheral blood of Heonch-Schönlein purpura patients

OBJECTIVE

Here, we explored the phenotype and function of MAIT cells in the peripheral blood of patients with HSP.

METHODS

Blood samples from HSP patients and HDs were assessed by flow cytometry and single-cell RNA sequencing to analyze the proportion, phenotype, and function of MAIT cells. Th-cytokines in the serum of HSP patients were analyzed by CBA. IgA in cocultured supernatant was detected by CBA to analyze antibody production by B cells.

RESULTS

The percentage of MAIT cells in HSP patients was significantly reduced compared with that in HDs. Genes related to T cell activation and effector were up-regulated in HSP MAIT cells, indicating a more activated phenotype. In addition, HSP MAIT cells displayed a Th2-like profile with the capacity to produce more IL-4 and IL-5, and IL-4 was correlated with IgA levels in the serum of HSP patients. Furthermore, CD40L was up-regulated in HSP MAIT cells, and CD40L+ MAIT cells showed an increased ability to produce IL-4 and to enhance IgA production by B cells.

CONCLUSION

Our data demonstrate that MAIT cells in HSP patients exhibit an activated phenotype. The enhanced IL-4 production and CD40L expression of MAIT cells in HSP patients could take part in the pathogenesis of HSP.

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.

Transcriptomic profile of TNFhigh MAIT cells is linked to B cell response following SARS-CoV-2 vaccination

Introduction

Higher frequencies of mucosal-associated invariant T (MAIT) cells were associated with an increased adaptive response to mRNA BNT162b2 SARS-CoV-2 vaccine, however, the mechanistic insights into this relationship are unknown. In the present study, we hypothesized that the TNF response of MAIT cells supports B cell activation following SARS-CoV-2 immunization.

Methods

To investigate the effects of repeated SARS-CoV-2 vaccinations on the peripheral blood mononuclear cells (PBMCs), we performed a longitudinal single cell (sc)RNA-seq and scTCR-seq analysis of SARS-CoV-2 vaccinated healthy adults with two doses of the Pfizer-BioNTech BNT162b2 mRNA vaccine. Collection of PBMCs was performed 1 day before, 3 and 17 days after prime vaccination, and 3 days and 3 months following vaccine boost. Based on scRNA/TCR-seq data related to regulatory signals induced by the vaccine, we used computational approaches for the functional pathway enrichment analysis (Reactome), dynamics of the effector cell-polarization (RNA Velocity and CellRank), and cell-cell communication (NicheNet).

Results

We identified MAIT cells as an important source of TNF across circulating lymphocytes in response to repeated SARS-CoV-2 BNT162b2 vaccination. The TNFhigh signature of MAIT cells was induced by the second administration of the vaccine. Notably, the increased TNF expression was associated with MAIT cell proliferation and efficient anti-SARS-CoV-2 antibody production. Finally, by decoding the ligand-receptor interactions and incorporating intracellular signaling, we predicted TNFhigh MAIT cell interplay with different B cell subsets. In specific, predicted TNF-mediated activation was selectively directed to conventional switched memory B cells, which are deputed to high-affinity long-term memory.

Discussion

Overall, our results indicate that SARS-CoV-2 BNT162b2 vaccination influences MAIT cell frequencies and their transcriptional effector profile with the potential to promote B cell activation. This research also provides a blueprint for the promising use of MAIT cells as cellular adjuvants in mRNA-based vaccines.

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.

Targeting MAIT cells as a cellular adjuvant for humoral immunity: a new player in a very old game

In this article, I discuss recent work by Pankhurst et al. They found that MAIT cells can serve as a cellular adjuvant to boost immunity to a protein adjuvant. Intranasal co-administration of protein antigen with a strong MAIT cell ligand results in the the production of mucosal IgA and IgG antibody responses. This process is driven by MAIT cell-mediated maturation of migratory dendritic cells.

MAIT cells activate dendritic cells to promote TFH cell differentiation and induce humoral immunity

Protective immune responses against respiratory pathogens, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus, are initiated by the mucosal immune system. However, most licensed vaccines are administered parenterally and are largely ineffective at inducing mucosal immunity. The development of safe and effective mucosal vaccines has been hampered by the lack of a suitable mucosal adjuvant. In this study we explore a class of adjuvant that harnesses mucosal-associated invariant T (MAIT) cells. We show evidence that intranasal immunization of MAIT cell agonists co-administered with protein, including the spike receptor binding domain from SARS-CoV-2 virus and hemagglutinin from influenza virus, induce protective humoral immunity and immunoglobulin A production. MAIT cell adjuvant activity is mediated by CD40L-dependent activation of dendritic cells and subsequent priming of T follicular helper cells. In summary, we show that MAIT cells are promising vaccine targets that can be utilized as cellular adjuvants in mucosal vaccines.

How do nuclear factor kappa B (NF-κB)1 and NF-κB2 defects lead to the incidence of clinical and immunological manifestations of inborn errors of immunity?

INTRODUCTION

Genetic defects affect the manner of the immune system's development, activation, and function. Nuclear factor-kappa B subunit 1 (NF-κB1) and NF-κB2 are involved in different biological processes, and deficiency in these transcription factors may reveal clinical and immunological difficulties.

AREAS COVERED

This review article gathers the most frequent clinical and immunological remarkable characteristics of NF-κB1 and NF-κB2 deficiencies. Afterward, an effort is made to describe the biological mechanism, which is likely to be the cause of these clinical and immunological abnormalities.

EXPERT OPINION

The present review article has explained the mechanism of contributions of the NF-κB1 and NF-κB2 deficiency in revealing immunodeficiency symptoms, specifically immunological and clinical manifestations. These mechanisms demonstrate the importance of NF-κB1 and NF-κB2 signaling pathways for B and T cell development, activation, antibody production, and immunotolerance. The manifestation of a mutation can range from no symptoms to severe complications in a family.

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.

In patients with primary Sjögren's syndrome innate-like MAIT cells display upregulated IL-7R, IFN-γ, and IL-21 expression and have increased proportions of CCR9 and CXCR5-expressing cells

Introduction

Mucosal-associated invariant T (MAIT) cells might play a role in B cell hyperactivity and local inflammation in primary Sjögren's syndrome (pSS), just like previously studied mucosa-associated CCR9+ and CXCR5+ T helper cells. Here, we investigated expression of CCR9, CXCR5, IL-18R and IL-7R on MAIT cells in pSS, and assessed the capacity of DMARDs to inhibit the activity of MAIT cells.

Methods

Circulating CD161+ and IL-18Rα+ TCRVα7.2+ MAIT cells from pSS patients and healthy controls (HC) were assessed using flow cytometry, and expression of CCR9, CXCR5, and IL-7R on MAIT cells was studied. Production of IFN-γ and IL-21 by MAIT cells was measured upon IL-7 stimulation in the presence of leflunomide (LEF) and hydroxychloroquine (HCQ).

Results

The numbers of CD161+ and IL-18Rα+ MAIT cells were decreased in pSS patients compared to HC. Relative increased percentages of CD4 MAIT cells in pSS patients caused significantly higher CD4/CD8 ratios in MAIT cells. The numbers of CCR9 and CXCR5-expressing MAIT cells were significantly higher in pSS patients. IL-7R expression was higher in CD8 MAIT cells as compared to all CD8 T cells, and changes in IL-7R expression correlated to several clinical parameters. The elevated production of IL-21 by MAIT cells was significantly inhibited by LEF/HCQ treatment.

Conclusion

Circulating CD161+ and IL-18Rα+ MAIT cell numbers are decreased in pSS patients. Given their enriched CCR9/CXCR5 expression this may facilitate migration to inflamed salivary glands known to overexpress CCL25/CXCL13. Given the pivotal role of IL-7 and IL-21 in inflammation in pSS this indicates a potential role for MAIT cells in driving pSS immunopathology.

Adenovirus vector and mRNA vaccines: Mechanisms regulating their immunogenicity

Replication-incompetent adenovirus (Ad) vector and mRNA-lipid nanoparticle (LNP) constructs represent two modular vaccine platforms that have attracted substantial interest over the past two decades. Due to the COVID-19 pandemic and the rapid development of multiple successful vaccines based on these technologies, there is now clear real-world evidence of the utility and efficacy of these platforms. Considerable optimization and refinement efforts underpin the successful application of these technologies. Despite this, our understanding of the specific pathways and processes engaged by these vaccines to stimulate the immune response remains incomplete. This review will synthesize our current knowledge of the specific mechanisms by which CD8⁺ T cell and antibody responses are induced by each of these vaccine platforms, and how this can be impacted by specific vaccine construction techniques. Key gaps in our knowledge are also highlighted, which can hopefully focus future studies.

Symbiotic bacteria-dependent expansion of MR1-reactive T cells causes autoimmunity in the absence of Bcl11b

MHC class I-related protein 1 (MR1) is a metabolite-presenting molecule that restricts MR1-reactive T cells including mucosal-associated invariant T (MAIT) cells. In contrast to MAIT cells, the function of other MR1-restricted T cell subsets is largely unknown. Here, we report that mice in which a T cell-specific transcription factor, B-cell lymphoma/leukemia 11B (Bcl11b), was ablated in immature thymocytes (Bcl11b^∆iThy mice) develop chronic inflammation. Bcl11b^∆iThy mice lack conventional T cells and MAIT cells, whereas CD4⁺IL-18R⁺ αβ T cells expressing skewed Traj33 (Jα33)⁺ T cell receptors (TCR) accumulate in the periphery, which are necessary and sufficient for the pathogenesis. The disorders observed in Bcl11b^∆iThy mice are ameliorated by MR1-deficiency, transfer of conventional T cells, or germ-free conditions. We further show the crystal structure of the TCR expressed by Traj33⁺ T cells expanded in Bcl11b^∆iThy mice. Overall, we establish that MR1-reactive T cells have pathogenic potential.

Emerging features of MAIT cells and other unconventional T cell populations in human viral disease and vaccination

MAIT cells are one representative of a group of related unconventional or pre-set T cells, and are particularly abundant in humans. While these unconventional T cell types, which also include populations of Vδ2 cells and iNKT cells, recognise quite distinct ligands, they share functional features including the ability to sense "danger" by integration of cytokine signals. Since such signals are common to many human pathologies, activation of MAIT cells in particular has been widely observed. In this review we will discuss recent trends in these data, for example the findings from patients with Covid-19 and responses to novel vaccines. Covid-19 is an example where MAIT cell activation has been correlated with disease severity by several groups, and the pathways leading to activation are being clarified, but the overall role of the cells in vivo requires further exploration. Given the potential wide functional responsiveness of these cells, which ranges from tissue repair to cytotoxicity, and likely impacts on the activity of many other cell populations, defining the role of these cells - not only as sensitive biomarkers but also as mediators - across human disease remains an important task.

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.

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.

MAIT cells and their implication in human oral diseases

OBJECTIVE

Mucosal-associated invariant T (MAIT) cells are unique innate-like T cells that are abundant in humans, accounting for 1-10% of circulating T cells and about 2% of total T cells in human oral cavity. MAIT cells can mount a strong immune response quickly without exogenous antigens and undergo a phenotypic transformation in the development of diseases. They produce cytokines involved in the Th1 and Th17 immune response and cytotoxic proteins, promote the dysfunction of autoreactive B cell and inhibit the function of NK cells. MAIT cells have been widely explored in autoimmune diseases, inflammatory diseases and tumors, and these mechanisms may also be involved in the pathogenesis of some oral diseases, while MAIT cells have not been systematically discussed in oral diseases.

METHODS

We searched PubMed/MEDLINE, EMBASE and Microsoft Bing databases to review and analyze relevant literatures on the impact of MAIT cells in the pathogenesis of human oral diseases.

CONCLUSION

Collected evidence elucidated the characteristics of MAIT cells and emphasized the potential roles of MAIT cells in oral lichen planus (OLP), chronic graft-versus-host disease (cGVHD), oral squamous cell carcinoma (OSCC), apical periodontitis (AP) and primary Sjogren's syndrome (pSS).

MAIT cell compartment characteristics are associated with the immune response magnitude to the BNT162b2 mRNA anti-SARS-CoV-2 vaccine

Mucosa-associated invariant T (MAIT) cells are unconventional T cells with innate-like capacity to rapidly respond to microbial infection via MR1-restricted antigen recognition. Emerging evidence indicate that they can also act as rapid sensors of viral infection via innate cytokine activation. However, their possible role in the immune response to mRNA vaccination is unknown. Here, we evaluated the involvement of MAIT cells in individuals vaccinated with the BNT162b2 mRNA SARS-CoV-2 vaccine. MAIT cell levels, phenotype and function in circulation were preserved and unperturbed through day 35 post-vaccination in healthy donor (HD) vaccinees, as well as people living with HIV (PLWH) or with primary immunodeficiency (PID). Unexpectedly, pre-vaccination and post-vaccination levels of MAIT cells correlated positively with the magnitude of the SARS-CoV-2 spike protein-specific CD4 T cell and antibody responses in the HD vaccinees. This pattern was largely preserved in the PID group, but less so in the PLWH group. Furthermore, in the HD vaccinees levels of MAIT cell activation and cytolytic potential correlated negatively to the adaptive antigen-specific immune responses. These findings indicate an unexpected association between MAIT cell compartment characteristics and the immune response magnitude to the BNT162b2 mRNA vaccine.

Emerging Roles of Mucosal-Associated Invariant T Cells in Rheumatology

Mucosal-associated invariant T (MAIT) cells are an unconventional T cell subset expressing a semi-invariant TCR and recognize microbial riboflavin metabolites presented by major histocompatibility complex class 1-related molecule (MR1). MAIT cells serve as innate-like T cells bridging innate and adaptive immunity, which have attracted increasing attention in recent years. The involvement of MAIT cells has been described in various infections, autoimmune diseases and malignancies. In this review, we first briefly introduce the biology of MAIT cells, and then summarize their roles in rheumatic diseases including systemic lupus erythematosus, rheumatoid arthritis, primary Sjögren’s syndrome, psoriatic arthritis, systemic sclerosis, vasculitis and dermatomyositis. An increased knowledge of MAIT cells will inform the development of novel biomarkers and therapeutic approaches in rheumatology.

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.

A subset of follicular helper-like MAIT cells can provide B cell help and support antibody production in the mucosa

[Figure: see text].

Oral Selective TLR8 Agonist Selgantolimod Induces Multiple Immune Cell Responses in Humans

TLR8 agonists have the potential for use as immunomodulatory components in therapeutic modalities for viral infections such as chronic HBV (CHB) and HIV. In this study, using peripheral blood samples from a phase 1a clinical trial, we examined the acute effects of a single oral administration of a selective TLR8 agonist on immune cell phenotypes. Administration of the TLR8 agonist selgantolimod (SLGN) in healthy individuals resulted in alteration in frequencies of peripheral blood monocytes, pDCs, mDCs and MAIT cells. Frequencies of mDCs and lymphoid cells significantly reduced after 8 h of SLGN administration, whereas pDC frequencies significantly increased, with changes possibly reflecting migration of different cell types between peripheral and tissue compartments in response to the agonist. Myeloid cell activation was evident by an upregulated expression of co-stimulatory molecules CD40 and CD86 accompanied by the production of IL-6 and IL-18 from these cells. Concomitantly, there was induction of the early activation marker CD69 on innate and adaptive lymphoid cells, including MAIT and NK cell subsets. Further, these activated lymphoid cells had enhanced expression of the effector molecules granzyme B and perforin. Microarray analysis of isolated lymphocytes and monocytes from baseline and post-SLGN treatment revealed changes in expression of genes involved in cellular response to cytokine stimulus, innate immune response, myeloid cell differentiation and antigen receptor-mediated signaling pathway. In a preliminary analysis of samples from CHB patients treated with selgantolimod, activation of innate and adaptive lymphocytes was evident. In conclusion, this first in-human study shows that selgantolimod administration in humans results in activation of multiple immune cell responses with antiviral potential.

B Cells Control Mucosal-Associated Invariant T Cell Responses to Salmonella enterica Serovar Typhi Infection Through the CD85j HLA-G Receptor

Mucosal-associated invariant T (MAIT) cells are an innate-like population of T cells that display a TCR Vα7.2+ CD161+ phenotype and are restricted by the nonclassical MHC-related molecule 1 (MR1). Although B cells control MAIT cell development and function, little is known about the mechanisms underlying their interaction(s). Here, we report, for the first time, that during Salmonella enterica serovar Typhi (S. Typhi) infection, HLA-G expression on B cells downregulates IFN-γ production by MAIT cells. In contrast, blocking HLA-G expression on S. Typhi-infected B cells increases IFN-γ production by MAIT cells. After interacting with MAIT cells, kinetic studies show that B cells upregulate HLA-G expression and downregulate the inhibitory HLA-G receptor CD85j on MAIT cells resulting in their loss. These results provide a new role for HLA-G as a negative feedback loop by which B cells control MAIT cell responses to antigens.

Immunoprofiles associated with controlled human malaria infection and naturally acquired immunity identify a shared IgA pre-erythrocytic immunoproteome

Knowledge of the Plasmodium falciparum antigens that comprise the human liver stage immunoproteome is important for pre-erythrocytic vaccine development, but, compared with the erythrocytic stage immunoproteome, more challenging to classify. Previous studies of P. falciparum antibody responses report IgG and rarely IgA responses. We assessed IgG and IgA antibody responses in adult sera collected during two controlled human malaria infection (CHMI) studies in malaria-naïve volunteers and in 1- to 6-year-old malaria-exposed Malian children on a 251 P. falciparum antigen protein microarray. IgG profiles in the two CHMI groups were equivalent and differed from Malian children. IgA profiles were robust in the CHMI groups and a subset of Malian children. We describe immunoproteome differences in naïve vs. exposed individuals and report pre-erythrocytic proteins recognized by the immune system. IgA responses detected in this study expand the list of pre-erythrocytic antigens for further characterization as potential vaccine candidates.

Viral Respiratory Pathogens and Lung Injury

Several viruses target the human respiratory tract, causing different clinical manifestations spanning from mild upper airway involvement to life-threatening acute respiratory distress syndrome (ARDS). As dramatically evident in the ongoing SARS-CoV-2 pandemic, the clinical picture is not always easily predictable due to the combined effect of direct viral and indirect patient-specific immune-mediated damage. In this review, we discuss the main RNA (orthomyxoviruses, paramyxoviruses, and coronaviruses) and DNA (adenoviruses, herpesviruses, and bocaviruses) viruses with respiratory tropism and their mechanisms of direct and indirect cell damage. We analyze the thin line existing between a protective immune response, capable of limiting viral replication, and an unbalanced, dysregulated immune activation often leading to the most severe complication. Our comprehension of the molecular mechanisms involved is increasing and this should pave the way for the development and clinical use of new tailored immune-based antiviral strategies.

Monkeying around with MAIT Cells: Studying the Role of MAIT Cells in SIV and Mtb Co-Infection

There were an estimated 10 million new cases of tuberculosis (TB) disease in 2019. While over 90% of individuals successfully control Mycobacterium tuberculosis (Mtb) infection, which causes TB disease, HIV co-infection often leads to active TB disease. Despite the co-endemic nature of HIV and TB, knowledge of the immune mechanisms contributing to the loss of control of Mtb replication during HIV infection is lacking. Mucosal-associated invariant T (MAIT) cells are innate-like T cells that target and destroy bacterially-infected cells and may contribute to the control of Mtb infection. Studies examining MAIT cells in human Mtb infection are commonly performed using peripheral blood samples. However, because Mtb infection occurs primarily in lung tissue and lung-associated lymph nodes, these studies may not be fully translatable to the tissues. Additionally, studies longitudinally examining MAIT cell dynamics during HIV/Mtb co-infection are rare, and lung and lymph node tissue samples from HIV+ patients are typically unavailable. Nonhuman primates (NHP) provide a model system to characterize MAIT cell activity during Mtb infection, both in Simian Immunodeficiency Virus (SIV)-infected and SIV-naïve animals. Using NHPs allows for a more comprehensive understanding of tissue-based MAIT cell dynamics during infection with both pathogens. NHP SIV and Mtb infection is similar to human HIV and Mtb infection, and MAIT cells are phenotypically similar in humans and NHPs. Here, we discuss current knowledge surrounding MAIT cells in SIV and Mtb infection, how SIV infection impairs MAIT cell function during Mtb co-infection, and knowledge gaps to address.

Alcohol-associated intestinal dysbiosis alters mucosal-associated invariant T-cell phenotype and function

BACKGROUND

Chronic alcohol consumption is associated with a compromised innate and adaptive immune responses to infectious disease. Mucosa-associated invariant T (MAIT) cells play a critical role in antibacterial host defense. However, whether alcohol-associated deficits in innate and adaptive immune responses are mediated by alterations in MAIT cells remains unclear.

METHODS

To investigate the impact of alcohol on MAIT cells, mice were treated with binge-on-chronic alcohol for 10 days and sacrificed at day 11. MAIT cells in the barrier organs (lung, liver, and intestine) were characterized by flow cytometry. Two additional sets of animals were used to examine the involvement of gut microbiota on alcohol-induced MAIT cell changes: (1) Cecal microbiota from alcohol-fed (AF) mice were adoptive transferred into antibiotic-pretreated mice and (2) AF mice were treated with antibiotics during the experiment. MAIT cells in the barrier organs were measured via flow cytometry.

RESULTS

Binge-on-chronic alcohol feeding led to a significant reduction in the abundance of MAIT cells in the barrier tissues. However, CD69 expression on tissue-associated MAIT cells was increased in AF mice compared with pair-fed (PF) mice. The expression of Th1 cytokines and the corresponding transcriptional factor was tissue specific, showing downregulation in the intestine and increases in the lung and liver in AF animals. Transplantation of fecal microbiota from AF mice resulted in a MAIT cell profile aligned to that of AF mouse donor. Antibiotic treatment abolished the MAIT cell differences between AF and PF animals.

CONCLUSION

MAIT cells in the intestine, liver, and lung are perturbed by alcohol use and these changes are partially attributable to alcohol-associated dysbiosis. MAIT cell dysfunction may contribute to alcohol-induced innate and adaptive immunity and consequently end-organ pathophysiology.

Does exercise attenuate age- and disease-associated dysfunction in unconventional T cells? Shining a light on overlooked cells in exercise immunology

Unconventional T Cells (UTCs) are a unique population of immune cells that links innate and adaptive immunity. Following activation, UTCs contribute to a host of immunological activities, rapidly responding to microbial and viral infections and playing key roles in tumor suppression. Aging and chronic disease both have been shown to adversely affect UTC numbers and function, with increased inflammation, change in body composition, and physical inactivity potentially contributing to the decline. One possibility to augment circulating UTCs is through increased physical activity. Acute exercise is a potent stimulus leading to the mobilization of immune cells while the benefits of exercise training may include anti-inflammatory effects, reductions in fat mass, and improved fitness. We provide an overview of age-related changes in UTCs, along with chronic diseases that are associated with altered UTC number and function. We summarize how UTCs respond to acute exercise and exercise training and discuss potential mechanisms that may lead to improved frequency and function.

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.

Efficient 5-OP-RU-Induced Enrichment of Mucosa-Associated Invariant T Cells in the Murine Lung Does Not Enhance Control of Aerosol Mycobacterium tuberculosis Infection

Mucosa-associated invariant T (MAIT) cells are an innate-like T cell subset in mammals that recognize microbial vitamin B metabolites presented by the evolutionarily conserved major histocompatibility complex class I (MHC I)-related molecule, MR1. Emerging data suggest that MAIT cells may be an attractive target for vaccine-induced protection against bacterial infections because of their rapid cytotoxic responses at mucosal services to a widely conserved bacterial ligand. In this study, we tested whether a MAIT cell priming strategy could protect against aerosol Mycobacterium tuberculosis infection in mice. Intranasal costimulation with the lipopeptide Toll-like receptor (TLR)2/6 agonist, Pam2Cys (P2C), and the synthetic MR1 ligand, 5-OP-RU, resulted in robust expansion of MAIT cells in the lung. Although MAIT cell priming significantly enhanced MAIT cell activation and expansion early after M. tuberculosis challenge, these MAIT cells did not restrict M. tuberculosis bacterial load. MAIT cells were depleted by the onset of the adaptive immune response, with decreased detection of granzyme B⁺ and gamma interferon (IFN-γ)⁺ MAIT cells relative to that in uninfected P2C/5-OP-RU-treated mice. Decreasing the infectious inoculum, varying the time between priming and aerosol infection, and testing MAIT cell priming in nitric oxide synthase 2 (NOS2)-deficient mice all failed to reveal an effect of P2C/5-OP-RU-induced MAIT cells on M. tuberculosis control. We conclude that intranasal MAIT cell priming in mice induces early MAIT cell activation and expansion after M. tuberculosis exposure, without attenuating M. tuberculosis growth, suggesting that MAIT cell enrichment in the lung is not sufficient to control M. tuberculosis infection.

Mucosal-associated invariant T (MAIT) cells mediate protective host responses in sepsis

Sepsis is a systemic inflammatory response to infection and a leading cause of death. Mucosal-associated invariant T (MAIT) cells are innate-like T cells enriched in mucosal tissues that recognize bacterial ligands. We investigated MAIT cells during clinical and experimental sepsis, and their contribution to host responses. In experimental sepsis, MAIT-deficient mice had significantly increased mortality and bacterial load, and reduced tissue-specific cytokine responses. MAIT cells of WT mice expressed lower levels of IFN-γ and IL-17a during sepsis compared to sham surgery, changes not seen in non-MAIT T cells. MAIT cells of patients at sepsis presentation were significantly reduced in frequency compared to healthy donors, and were more activated, with decreased IFN-γ production, compared to both healthy donors and paired 90-day samples. Our data suggest that MAIT cells are highly activated and become dysfunctional during clinical sepsis, and contribute to tissue-specific cytokine responses that are protective against mortality during experimental sepsis.

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.

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.

Mucosal-associated invariant T (MAIT) cells provide B-cell help in vaccinated and subsequently SIV-infected Rhesus Macaques

Mucosal-associated invariant T (MAIT) cells help combat opportunistic infections. Thus, MAIT cells are of interest in HIV/SIV vaccination and infection. We investigated MAIT cell dynamics and function in rhesus macaque blood and bronchoalveolar lavage (BAL) following mucosal adenovirus (Ad)-SIV recombinant priming, intramuscular SIV envelope boosting and infection following repeated low-dose intravaginal SIV exposures. Increased frequencies of blood MAIT cells over the course of vaccination were observed, which were maintained even 12-weeks post-SIV infection. BAL MAIT cells only increased after the first Ad immunization. Vaccination increased MAIT cell levels in blood and BAL expressing the antiviral cytokine IFN-γ and TNF-α and the proliferation marker Ki67. Upon T cell-specific α-CD3, α-CD28 stimulation, MAIT cells showed a greater capacity to secrete cytokines/chemokines associated with help for B cell activation, migration and regulation compared to CD3⁺MR1⁻ cells. Culture of MAIT cell supernatants with B cells led to greater tissue like memory B cell frequencies. MAIT cell frequencies in blood and BAL correlated with SIV-specific antibody levels in rectal secretions and with SIV-specific tissue resident memory B cells. Overall, SIV vaccination influenced MAIT cell frequency and functionality. The potential for MAIT cells to provide help to B cells was evident during both vaccination and infection.

Opsonization-Enhanced Antigen Presentation by MR1 Activates Rapid Polyfunctional MAIT Cell Responses Acting as an Effector Arm of Humoral Antibacterial Immunity

Mucosa-associated invariant T (MAIT) cells are innate-like antimicrobial T cells recognizing a breadth of important pathogens via presentation of microbial riboflavin metabolite Ags by MHC class Ib-related (MR1) molecules. However, the interaction of human MAIT cells with adaptive immune responses and the role they may play in settings of vaccinology remain relatively little explored. In this study we investigated the interplay between MAIT cell-mediated antibacterial effector functions and the humoral immune response. IgG opsonization of the model microbe Escherichia coli with pooled human sera markedly enhanced the capacity of monocytic APC to stimulate MAIT cells. This effect included greater sensitivity of recognition and faster response kinetics, as well as a markedly higher polyfunctionality and magnitude of MAIT cell responses involving a range of effector functions. The boost of MAIT cell responses was dependent on strongly enhanced MR1-mediated Ag presentation via increased FcγR-mediated uptake and signaling primarily mediated by FcγRI. To investigate possible translation of this effect to a vaccine setting, sera from human subjects before and after vaccination with the 13-valent-conjugated Streptococcus pneumoniae vaccine were assessed in a MAIT cell activation assay. Interestingly, vaccine-induced Abs enhanced Ag presentation to MAIT cells, resulting in more potent effector responses. These findings indicate that enhancement of Ag presentation by IgG opsonization allows innate-like MAIT cells to mount a faster, stronger, and qualitatively more complex response and to function as an effector arm of vaccine-induced humoral adaptive antibacterial immunity.

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 are functionally impaired in a Mauritian cynomolgus macaque model of SIV and Mtb co-infection

Mucosal-associated invariant T (MAIT) cells can recognize and respond to some bacterially infected cells. Several in vitro and in vivo models of Mycobacterium tuberculosis (Mtb) infection suggest that MAIT cells can contribute to control of Mtb, but these studies are often cross-sectional and use peripheral blood cells. Whether MAIT cells are recruited to Mtb-affected granulomas and lymph nodes (LNs) during early Mtb infection and what purpose they might serve there is less well understood. Furthermore, whether HIV/SIV infection impairs MAIT cell frequency or function at the sites of Mtb replication has not been determined. Using Mauritian cynomolgus macaques (MCM), we phenotyped MAIT cells in the peripheral blood and bronchoalveolar lavage (BAL) before and during infection with SIVmac239. To test the hypothesis that SIV co-infection impairs MAIT cell frequency and function within granulomas, SIV+ and -naïve MCM were infected with a low dose of Mtb Erdman, and necropsied at 6 weeks post Mtb-challenge. MAIT cell frequency and function were examined within the peripheral blood, BAL, and Mtb-affected lymph nodes (LN) and granulomas. MAIT cells did not express markers indicative of T cell activation in response to Mtb in vivo within granulomas in animals infected with Mtb alone. SIV and Mtb co-infection led to increased expression of the activation/exhaustion markers PD-1 and TIGIT, and decreased ability to secrete TNFα when compared to SIV-naïve MCM. Our study provides evidence that SIV infection does not prohibit the recruitment of MAIT cells to sites of Mtb infection, but does functionally impair those MAIT cells. Their impaired function could have impacts, either direct or indirect, on the long-term containment of TB disease.

Impact of IgA isoforms on their ability to activate dendritic cells and to prime T cells

Human IgA could be from different isotypes (IgA1/IgA2) and/or isoforms (monomeric, dimeric, or secretory). Monomeric IgA mainly IgA1 are considered as an anti-inflammatory isotype whereas dimeric/secretory IgA have clearly dual pro- and anti-inflammatory effects. Here, we show that IgA isotypes and isoforms display different binding abilities to FcαRI, Dectin-1, DC-SIGN, and CD71 on monocyte-derived dendritic cells (moDC). We describe that IgA regulate the expression of their own receptors and trigger modulation of moDC maturation. We also demonstrate that dimeric IgA2 and IgA1 induce different inflammatory responses leading to cytotoxic CD8⁺ T cells activation. moDC stimulation by dimeric IgA2 was followed by a strong pro-inflammatory effect. Our study highlights differences regarding IgA isotypes and isoforms in the context of DC conditioning. Further investigations are needed on the activation of adaptive immunity by IgA in the context of microbiota/IgA complexes during antibody-mediated immune selection.

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.

T Cell Responses to Mycobacterial Glycolipids: On the Spectrum of "Innateness"

Diseases due to mycobacteria, including tuberculosis, leprosy, and Buruli ulcer, rank among the top causes of death and disability worldwide. Animal studies have revealed the importance of T cells in controlling these infections. However, the specific antigens recognized by T cells that confer protective immunity and their associated functions remain to be definitively established. T cells that respond to mycobacterial peptide antigens exhibit classical features of adaptive immunity and have been well-studied in humans and animal models. Recently, innate-like T cells that recognize lipid and metabolite antigens have also been implicated. Specifically, T cells that recognize mycobacterial glycolipid antigens (mycolipids) have been shown to confer protection to tuberculosis in animal models and share some biological characteristics with adaptive and innate-like T cells. Here, we review the existing data suggesting that mycolipid-specific T cells exist on a spectrum of “innateness,” which will influence how they can be leveraged to develop new diagnostics and vaccines for mycobacterial diseases.

Neutrophils suppress mucosal-associated invariant T cells in humans

Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes that are abundant in mucosal tissues and the liver where they can respond rapidly to a broad range of riboflavin producing bacterial and fungal pathogens. Neutrophils, which are recruited early to sites of infection, play a nonredundant role in pathogen clearance and are crucial for controlling infection. The interaction of these two cell types is poorly studied. Here, we investigated both the effect of neutrophils on MAIT cell activation and the effect of activated MAIT cells on neutrophils. We show that neutrophils suppress the activation of MAIT cells by a cell-contact and hydrogen peroxide dependent mechanism. Moreover, highly activated MAIT cells were able to produce high levels of TNF-α that induced neutrophil death. We therefore provide evidence for a negative regulatory feedback mechanism in which neutrophils prevent overactivation of MAIT cells and, in turn, MAIT cells limit neutrophil survival.

Inflammatory Cytokines Induce Sustained CTLA-4 Cell Surface Expression on Human MAIT Cells

Mucosal-associated invariant T (MAIT) cells acquire effector function in response to proinflammatory signals, which synergize with TCR-mediated signals. We asked if cell-intrinsic regulatory mechanisms exist to curtail MAIT cell effector function akin to the activation-induced expression of inhibitory receptors by conventional T cells. We examined human MAIT cells from blood and oral mucosal tissues by RNA sequencing and found differential expression of immunoregulatory genes, including CTLA-4, by MAIT cells isolated from tissue. Using an ex vivo experimental setup, we demonstrate that inflammatory cytokines were sufficient to induce CTLA-4 expression on the MAIT cell surface in the absence of TCR signals. Even brief exposure to the cytokines IL-12, IL-15, and IL-18 was sufficient for sustained CTLA-4 expression by MAIT cells. These data suggest that control of CTLA-4 expression is fundamentally different between MAIT cells and conventional T cells. We propose that this mechanism serves to limit MAIT cell-mediated tissue damage.