Arming of MAIT Cell Cytolytic Antimicrobial Activity Is Induced by IL-7 and Defective in HIV-1 Infection

Inflammation and Microbial Translocation Correlate with Reduced MAIT Cells in People with HIV

Background

Optimal control of microbial infections requires mucosal-associated invariant T (MAIT) cells. People living with HIV (PWH) on antiretroviral therapy (ART) can be divided into 2 groups: immune responders (IR) who recover or retain CD4 T cell numbers, and immune non-responders (INR) who do not. Compared to IR, INR have fewer MAIT cells and increased systemic inflammation and microbial translocation, but how these factors affect MAIT cells is unknown.

Methods

MAIT cells from IR, INR, and from controls without HIV were enumerated and characterized by flow cytometry. To determine the links among MAIT cells, inflammation, and microbial translocation, the correlations of MAIT cell numbers to previously published soluble inflammatory markers and plasma microbial genetic sequences were assessed by Spearman analysis. In vitro assays were used to support our findings.

Results

MAIT cell numbers were significantly negatively correlated with levels of IL-6 and IP-10 (markers of inflammation); CD14, LPS, and FABP2 (markers of microbial translocation); and with abundance of Serratia and other Proteobacteria genetic sequences in plasma. In a separate analysis of PWH on ART receiving the IL-6 receptor antagonist tocilizumab (TCZ), we found that blocking IL-6 signaling with TCZ increased IL-7 receptor expression on MAIT cells and reduced plasma IL-7 levels, consistent with improved uptake of IL-7 in vivo.

Conclusions

Our findings suggest inflammation and microbial translocation in PWH on ART lead to a loss of MAIT cells via impaired IL-7 responsiveness, resulting in further increased microbial translocation and inflammation.

Protective effect of TCR-mediated MAIT cell activation during experimental autoimmune encephalomyelitis

Mucosal-associated invariant T (MAIT) cells express semi-invariant T cell receptors (TCR) for recognizing bacterial and yeast antigens derived from riboflavin metabolites presented on the non-polymorphic MHC class I-related protein 1 (MR1). Neuroinflammation in multiple sclerosis (MS) is likely initiated by autoreactive T cells and perpetuated by infiltration of additional immune cells, but the precise role of MAIT cells in MS pathogenesis remains unknown. Here, we use experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, and find an accumulation of MAIT cells in the inflamed central nervous system (CNS) enriched for MAIT17 (RORγt+) and MAIT1/17 (T-bet+RORγt+) subsets with inflammatory and protective features. Results from transcriptome profiling and Nur77GFP reporter mice show that these CNS MAIT cells are activated via cytokines and TCR. Blocking TCR activation with an anti-MR1 antibody exacerbates EAE, whereas enhancing TCR activation with the cognate antigen, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil, ameliorates EAE severity, potentially via the induction of amphiregulin (AREG). In summary, our findings suggest that TCR-mediated MAIT cell activation is protective in CNS inflammation, likely involving an induction of AREG.

MAIT cell heterogeneity across paired human tissues reveals specialization of distinct regulatory and enhanced effector profiles

Mucosal-associated invariant T (MAIT) cells are unconventional T cells that recognize microbial riboflavin pathway metabolites presented by evolutionarily conserved MR1 molecules. We explored the human MAIT cell compartment across organ donor-matched blood, barrier, and lymphoid tissues. MAIT cell population size was donor dependent with distinct tissue compartmentalization patterns and adaptations: Intestinal CD103+ resident MAIT cells presented an immunoregulatory CD39highCD27low profile, whereas MAIT cells expressing NCAM1/CD56 dominated in the liver and exhibited enhanced effector capacity with elevated response magnitude and polyfunctionality. Both intestinal CD39high and hepatic CD56+ adaptations accumulated with donor age. CD56+ MAIT cells displayed limited T cell receptor-repertoire breadth, elevated MR1 binding, and a transcriptional profile skewed toward innate activation pathways. Furthermore, CD56 was dynamically up-regulated to a persistent steady-state equilibrium after exposure to antigen or IL-7. In summary, we demonstrate functional heterogeneity and tissue site adaptation in resident MAIT cells across human barrier tissues with distinct regulatory and effector signatures.

Mucosal T-cell responses to chronic viral infections: Implications for vaccine design

Mucosal surfaces that line the respiratory, gastrointestinal and genitourinary tracts are the major interfaces between the immune system and the environment. Their unique immunological landscape is characterized by the necessity of balancing tolerance to commensal microorganisms and other innocuous exposures against protection from pathogenic threats such as viruses. Numerous pathogenic viruses, including herpesviruses and retroviruses, exploit this environment to establish chronic infection. Effector and regulatory T-cell populations, including effector and resident memory T cells, play instrumental roles in mediating the transition from acute to chronic infection, where a degree of viral replication is tolerated to minimize immunopathology. Persistent antigen exposure during chronic viral infection leads to the evolution and divergence of these responses. In this review, we discuss advances in the understanding of mucosal T-cell immunity during chronic viral infections and how features of T-cell responses develop in different chronic viral infections of the mucosa. We consider how insights into T-cell immunity at mucosal surfaces could inform vaccine strategies: not only to protect hosts from chronic viral infections but also to exploit viruses that can persist within mucosal surfaces as vaccine vectors.

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.

MR1 blockade drives differential impact on integrative signatures based on circuits of circulating immune cells and soluble mediators in visceral leishmaniasis

Introduction

Visceral leishmaniasis (VL) is an important tropical and neglected disease and represents a serious global health problem. The initial interaction between the phagocytes and the parasite is crucial to determine the pathogen's capacity to initiate infection and it shapes the subsequent immune response that will develop. While type-1 T-cells induce IL-6, IL-1β, TNF-α, and IL-12 production by monocytes/macrophages to fight the infection, type-2 T-cells are associated with a regulatory phenotype (IL-10 and TGF-β) and successful infection establishment. Recently, our group demonstrated the role of an important Th1/Th17 T-cell population, the mucosal-associated invariant T (MAIT) cells, in VL. MAIT cells can respond to L. infantum by producing TNF-α and IFN-γ upon MR1-dependent activation.

Objective and methods

Here, we describe the impact of the MR1-blockage on L. infantum internalization on the functional profile of circulating neutrophils and monocytes as well as the impact of the MR1-blockage on the soluble mediator signatures of in vitro whole blood cultures.

Results

Overall, our data showed that VL patients presents higher percentage of activated neutrophils than asymptomatic and non-infected controls. In addition, MR1 blockade led to lower TNF-α and TGF-β production by non-activated neutrophils from asymptomatic individuals. Moreover, TNF-α and IL-10 production by monocytes was higher in VL patients. In the analysis of soluble mediators produced in vitro, MR1-blockade induced a decrease of IFN-γ and an increase of IL-10, IL-27 and IL-33 in the cell cultures of AS group, a cytokine pattern associated with type 2 deleterious response.

Discussion and conclusion

These data corroborate the hypothesis that MR1-restricted responses are associated to a protective role during Leishmania infection.

Varicella Zoster Virus disrupts MAIT cell polyfunctional effector responses

Mucosal-associated invariant T (MAIT) cells are unconventional T cells that respond to riboflavin biosynthesis and cytokines through TCR-dependent and -independent pathways, respectively. MAIT cell activation plays an immunoprotective role against several pathogens, however the functional capacity of MAIT cells following direct infection or exposure to infectious agents remains poorly defined. We investigated the impact of Varicella Zoster Virus (VZV) on blood-derived MAIT cells and report virus-mediated impairment of activation, cytokine production, and altered transcription factor expression by VZV infected (antigen+) and VZV exposed (antigen-) MAIT cells in response to TCR-dependent and -independent stimulation. Furthermore, we reveal that suppression of VZV exposed (antigen-) MAIT cells is not mediated by a soluble factor from neighbouring VZV infected (antigen+) MAIT cells. Finally, we demonstrate that VZV impairs the cytolytic potential of MAIT cells in response to riboflavin synthesising bacteria. In summary, we report a virus-mediated immune-evasion strategy that disarms MAIT cell responses.

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.

Multi-targeted loss of the antigen presentation molecule MR1 during HSV-1 and HSV-2 infection

The major histocompatibility complex (MHC), Class-I-related (MR1) molecule presents microbiome-synthesized metabolites to Mucosal-associated invariant T (MAIT) cells, present at sites of herpes simplex virus (HSV) infection. During HSV type 1 (HSV-1) infection there is a profound and rapid loss of MR1, in part due to expression of unique short 3 protein. Here we show that virion host shutoff RNase protein downregulates MR1 protein, through loss of MR1 transcripts. Furthermore, a third viral protein, infected cell protein 22, also downregulates MR1, but not classical MHC-I molecules. This occurs early in the MR1 trafficking pathway through proteasomal degradation. Finally, HSV-2 infection results in the loss of MR1 transcripts, and intracellular and surface MR1 protein, comparable to that seen during HSV-1 infection. Thus HSV coordinates a multifaceted attack on the MR1 antigen presentation pathway, potentially protecting infected cells from MAIT cell T cell receptor-mediated detection at sites of primary infection and reactivation.

A conserved transcriptional program for MAIT cells across mammalian evolution

Mucosal-associated invariant T (MAIT) cells harbor evolutionarily conserved TCRs, suggesting important functions. As human and mouse MAIT functional programs appear distinct, the evolutionarily conserved MAIT functional features remain unidentified. Using species-specific tetramers coupled to single-cell RNA sequencing, we characterized MAIT cell development in six species spanning 110 million years of evolution. Cross-species analyses revealed conserved transcriptional events underlying MAIT cell maturation, marked by ZBTB16 induction in all species. MAIT cells in human, sheep, cattle, and opossum acquired a shared type-1/17 transcriptional program, reflecting ancestral features. This program was also acquired by human iNKT cells, indicating common differentiation for innate-like T cells. Distinct type-1 and type-17 MAIT subsets developed in rodents, including pet mice and genetically diverse mouse strains. However, MAIT cells further matured in mouse intestines to acquire a remarkably conserved program characterized by concomitant expression of type-1, type-17, cytotoxicity, and tissue-repair genes. Altogether, the study provides a unifying view of the transcriptional features of innate-like T cells across evolution.

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.

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 immune-mediated tissue injury and repair

Mucosal-associated invariant T (MAIT) cells are T cells that express a semi-invariant αβ T-cell receptor (TCR), recognizing non-peptide antigens, such as microbial-derived vitamin B2 metabolites, presented by the nonpolymorphic MHC class I related-1 molecule. Like NKT cells and γδT cells, MAIT cells belong to the group of innate-like T cells that combine properties of the innate and adaptive immune systems. They account for up to 10% of the blood T-cell population in humans and are particularly abundant at mucosal sites. Beyond the emerging role of MAIT cells in antibacterial and antiviral defenses, increasing evidence suggests additional functions in noninfectious settings, including immune-mediated inflammatory diseases and tissue repair. Here, we discuss recent advances in the understanding of MAIT cell functions in sterile tissue inflammation, with a particular focus on autoimmunity, chronic inflammatory diseases, and tissue repair.

Transcriptomes and metabolism define mouse and human MAIT cell populations

Mucosal-associated invariant T (MAIT) cells are a subset of T lymphocytes that respond to microbial metabolites. We defined MAIT cell populations in different organs and characterized the developmental pathway of mouse and human MAIT cells in the thymus using single-cell RNA sequencing and phenotypic and metabolic analyses. We showed that the predominant mouse subset, which produced IL-17 (MAIT17), and the subset that produced IFN-γ (MAIT1) had not only greatly different transcriptomes but also different metabolic states. MAIT17 cells in different organs exhibited increased lipid uptake, lipid storage, and mitochondrial potential compared with MAIT1 cells. All these properties were similar in the thymus and likely acquired there. Human MAIT cells in lung and blood were more homogeneous but still differed between tissues. Human MAIT cells had increased fatty acid uptake and lipid storage in blood and lung, similar to human CD8 T resident memory cells, but unlike mouse MAIT17 cells, they lacked increased mitochondrial potential. Although mouse and human MAIT cell transcriptomes showed similarities for immature cells in the thymus, they diverged more strikingly in the periphery. Analysis of pet store mice demonstrated decreased lung MAIT17 cells in these so-called "dirty" mice, indicative of an environmental influence on MAIT cell subsets and function.

Colonic mucosal associated invariant T cells in Crohn's disease have a diverse and non-public T cell receptor beta chain repertoire

OBJECTIVES

Mucosal-Associated Invariant T (MAIT) cells are T cells with a semi-invariant T cell receptor (TCR), recognizing riboflavin precursors presented by a non-polymorphic MR1 molecule. As these precursors are produced by the gut microbiome, we characterized the frequency, phenotype and clonality of MAIT cells in human colons with and without Crohn's disease (CD).

METHODS

The transcriptome of MAIT cells sorted from blood and intestinal lamina propria cells from colectomy recipients were compared with other CD8+ T cells. Colon biopsies from an additional ten CD patients and ten healthy controls (HC) were analyzed by flow cytometry. TCR genes were sequenced from individual MAIT cells from these biopsies and compared with those of MAIT cells from autologous blood.

RESULTS

MAIT cells in the blood and colon showed a transcriptome distinct from other CD8 T cells, with more expression of the IL-23 receptor. MAIT cells were enriched in the colons of CD patients, with less NKG2D in inflamed versus uninflamed segments. Regardless of disease, most MAIT cells expressed integrin α4β7 in the colon but not in the blood, where they were enriched for α4β7 expression. TCR sequencing revealed heterogeneity in the colon and blood, with few public sequences associated with cohorts.

CONCLUSION

MAIT cells are enriched in the colons of CD patients and disproportionately express molecules (IL-23R, integrin α4β7) targeted by CD therapeutics, to suggest a pathogenic role for them in CD. Public TCR sequences were neither common nor sufficiently restricted to a cohort to suggest protective or pathogenic antigen-specificities.

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.

Mucosal-associated invariant T cells in digestive tract: Local guardians or destroyers?

Mucosa-associated invariant T cells (MAIT) are a class of innate-like T lymphocytes mainly presenting CD8+ phenotype with a semi-invariant αβ T-cell receptor, which specifically recognises MR1-presented biosynthetic derivatives of riboflavin synthesis produced by various types of microbiomes. As innate-like T lymphocytes, MAIT can be activated by a variety of cytokines, leading to immediate immune responses to infection and tumour cues. As an organ that communicates with the external environment, the digestive tract, especially the gastrointestinal tract, contains abundant microbial populations. Communication between MAIT and local microbiomes is important for the homeostasis of mucosal immunity. In addition, accumulating evidence suggests changes in the abundance and structure of the microbial community during inflammation and tumorigenesis plays a critical role in disease progress partly through their impact on MAIT development and function. Therefore, it is essential for the understanding of MAIT response and their interaction with microbiomes in the digestive tract. Here, we summarised MAIT characteristics in the digestive tract and its alteration facing inflammation and tumour, raising that targeting MAIT can be a candidate for treatment of gastrointestinal diseases.

HLA-DR+ mucosal-associated invariant T cells predict poor prognosis in patients with sepsis: A prospective observational study

Mucosal-associated invariant T (MAIT) cells are important in antibacterial immune responses; however, during sepsis, they are few in number and exhibit highly activated phenotypes. The relationship between MAIT cells in peripheral blood and the prognosis of sepsis is not well understood. Thus, this study aimed to examine the levels and phenotypes of MAIT cells in early sepsis, evaluate their clinical relevance, and investigate their association with patient prognosis. This prospective observational study enrolled 72 septic patients defined according to the Sepsis 3.0 criteria and 21 healthy controls matched for age and sex. Their peripheral blood samples were used to assay the expression of immune activation (CD69 and HLA-DR) and immune checkpoint (PD-1 and PD-L1) markers on MAIT cells. The systemic inflammatory response syndrome, acute physiology and chronic health evaluation (APACHE) II, and sequential organ failure assessment scores were recorded. Subsequently, the association between MAIT cell characteristics and clinical indicators was assessed using Spearman's rank correlation analysis, and binary logistic regression analysis with a forward stepwise approach assessed independent risk factors for 28-day mortality. We noted a decrease in the percentage of MAIT cells in the patients' peripheral blood, which exhibited an activated phenotype. Besides, HLA-DR+ MAIT cell percentage and the APACHE II score were independently associated with the 28-day mortality and, in combination, were the best indicators of mortality. Thus, the percentage of HLA-DR+ MAIT cells in early sepsis serves as a novel prognostic biomarker for predicting mortality and improves the predictive capacity of the APACHE II score.

Aging unconventionally: γδ T cells, iNKT cells, and MAIT cells in aging

Unconventional T cells include γδ T cells, invariant Natural Killer T cells (iNKT) cells and Mucosal Associated Invariant T (MAIT) cells, which are distinguished from conventional T cells by their recognition of non-peptide ligands presented by non-polymorphic antigen presenting molecules and rapid effector functions that are pre-programmed during their development. Here we review current knowledge of the effect of age on unconventional T cells, from early life to old age, in both mice and humans. We then discuss the role of unconventional T cells in age-associated diseases and infections, highlighting the similarities between members of the unconventional T cell family in the context of aging.

The Emerging Role of MAIT Cell Responses in Viral Infections

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

Expansion of MAIT cells in the combined absence of NKT and γδ-T cells

Mucosal-associated invariant T (MAIT) cells, natural killer T (NKT) cells, and γδT cells are collectively referred to as 'unconventional T cells' due to their recognition of non-peptide antigens and restriction to MHC-I-like molecules. However, the factors controlling their widely variable frequencies between individuals and organs are poorly understood. We demonstrated that MAIT cells are increased in NKT or γδT cell-deficient mice and highly expand in mice lacking both cell types. TCRα repertoire analysis of γδT cell-deficient thymocytes revealed altered Trav segment usage relative to wild-type thymocytes, highlighting retention of the Tcra-Tcrd locus from the 129 mouse strain used to generate Tcrd-/- mice. This resulted in a moderate increase in distal Trav segment usage, including Trav1, potentially contributing to increased generation of Trav1-Traj33+ MAIT cells in the Tcrd-/- thymus. Importantly, adoptively transferred MAIT cells underwent increased homeostatic proliferation within NKT/gdT cell-deficient tissues, with MAIT cell subsets exhibiting tissue-specific homing patterns. Our data reveal a shared niche for unconventional T cells, where competition for common factors may be exploited to collectively modulate these cells in the immune response. Lastly, our findings emphasise careful assessment of studies using NKT or γδT cell-deficient mice when investigating the role of unconventional T cells in disease.

Regulatory mucosa-associated invariant T cells controlled by β1 adrenergic receptor signaling contribute to hepatocellular carcinoma progression

BACKGROUND AND AIMS

The innate-like mucosa-associated invariant T (MAIT) cells are enriched in human liver and have been linked to human HCC. However, their contributions to the progression of HCC are controversial due to the heterogeneity of MAIT cells, and new MAIT cell subsets remain to be explored.

APPROACH AND RESULTS

Combining single cell RNA sequencing (scRNA-seq) and flow cytometry analysis, we performed phenotypic and functional studies and found that FOXP3 + CXCR3 + MAIT cells in HCC patients were regulatory MAIT cells (MAITregs) with high immunosuppressive potential. These MAITregs were induced under Treg-inducing condition and predominantly from FOXP3 - CXCR3 + MAIT cells, which displayed mild Treg-related features and represented a pre-MAITreg reservoir. In addition, the induction and function of MAITregs were promoted by β1 adrenergic receptor signaling in pre-MAITregs and MAITregs, respectively. In HCC patients, high proportion of the intratumoral MAITregs inhibited antitumor immune responses and was associated with poor clinical outcomes.

CONCLUSIONS

Together, we reveal an immunosuppressive subset of MAIT cells in HCC patients that contributes to HCC progression, and propose a control through neuroimmune crosstalk.

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.

Single-cell profiling identifies a novel human polyclonal unconventional T cell lineage

In the human thymus, a CD10+ PD-1+ TCRαβ+ differentiation pathway diverges from the conventional single positive T cell lineages at the early double-positive stage. Here, we identify the progeny of this unconventional lineage in antigen-inexperienced blood. These unconventional T cells (UTCs) in thymus and blood share a transcriptomic profile, characterized by hallmark transcription factors (i.e., ZNF683 and IKZF2), and a polyclonal TCR repertoire with autoreactive features, exhibiting a bias toward early TCRα chain rearrangements. Single-cell RNA sequencing confirms a common developmental trajectory between the thymic and blood UTCs and clearly delineates this unconventional lineage in blood. Besides MME+ recent thymic emigrants, effector-like clusters are identified in this heterogeneous lineage. Expression of Helios and KIR and a decreased CD8β expression are characteristics of this lineage. This UTC lineage could be identified in adult blood and intestinal tissues. In summary, our data provide a comprehensive characterization of the polyclonal unconventional lineage in antigen-inexperienced blood and identify the adult progeny.

Comparative transcriptomes reveal pro-survival and cytotoxic programs of mucosal-associated invariant T cells upon Bacillus Calmette-Guérin stimulation

Mucosal-associated invariant T (MAIT) cells are protective against tuberculous and non-tuberculous mycobacterial infections with poorly understood mechanisms. Despite an innate-like nature, MAIT cell responses remain heterogeneous in bacterial infections. To comprehensively characterize MAIT activation programs responding to different bacteria, we stimulated MAIT cells with E. coli to compare with Bacillus Calmette-Guérin (BCG), which remains the only licensed vaccine and a feasible tool for investigating anti-mycobacterial immunity in humans. Upon sequencing mRNA from the activated and inactivated CD8+ MAIT cells, results demonstrated the altered MAIT cell gene profiles by each bacterium with upregulated expression of activation markers, transcription factors, cytokines, and cytolytic mediators crucial in anti-mycobacterial responses. Compared with E. coli, BCG altered more MAIT cell genes to enhance cell survival and cytolysis. Flow cytometry analyses similarly displayed a more upregulated protein expression of B-cell lymphoma 2 and T-box transcription factor Eomesodermin in BCG compared to E.coli stimulations. Thus, the transcriptomic program and protein expression of MAIT cells together displayed enhanced pro-survival and cytotoxic programs in response to BCG stimulation, supporting BCG induces cell-mediated effector responses of MAIT cells to fight mycobacterial infections.

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.

IL7RA single nucleotide polymorphisms are associated with the size and function of the MAIT cell population in treated HIV-1 infection

MAIT cells are persistently depleted and functionally exhausted in HIV-1-infected patients despite long-term combination antiretroviral therapy (cART). IL-7 treatment supports MAIT cell reconstitution in vivo HIV-1-infected individuals and rescues their functionality in vitro. Single-nucleotide polymorphisms (SNPs) of the IL-7RA gene modulate the levels of soluble(s)IL-7Rα (sCD127) levels and influence bioavailability of circulating IL-7. Here we evaluate the potential influence of IL-7RA polymorphisms on MAIT cell numbers and function in healthy control (HC) subjects and HIV-1-infected individuals on long-term cART. Our findings indicate that IL-7RA haplotype 2 (H2*T), defined as T-allele carriers at the tagging SNP rs6897932, affects the size of the peripheral blood MAIT cell pool, as well as their production of cytokines and cytolytic effector proteins in response to bacterial stimulation. H2*T carriers had lower sIL-7Rα levels and higher MAIT cell frequency with enhanced functionality linked to higher expression of MAIT cell-associated transcription factors. Despite an average of 7 years on suppressive cART, MAIT cell levels and function in HIV-1-infected individuals were still significantly lower than those of HC. Notably, we observed a significant correlation between MAIT cell levels and cART duration only in HIV-1-infected individuals carrying IL-7RA haplotype 2. Interestingly, treatment with sIL-7Rα in vitro suppressed IL-7-dependent MAIT cell proliferation and function following cognate stimulations. These observations suggest that sIL-7Rα levels may influence MAIT cell numbers and function in vivo by limiting IL-7 bioavailability to MAIT cells. Collectively, these observations suggest that IL-7RA polymorphisms may play a significant role in MAIT cell biology and influence MAIT cells recovery in HIV-1 infection. The potential links between IL7RA polymorphisms, MAIT cell immunobiology, and HIV-1 infection warrant further studies going forward.

IL-15 Superagonist N-803 Enhances IFN-γ Production of MAIT Cells in SIV+ Macaques

Mucosal associated invariant T (MAIT) cells are innate T cells that recognize bacterial metabolites and secrete cytokines and cytolytic enzymes to destroy infected target cells. This makes MAIT cells promising targets for immunotherapy to combat bacterial infections. Here, we analyzed the effects of an immunotherapeutic agent, the IL-15 superagonist N-803, on MAIT cell activation, trafficking, and cytolytic function in macaques. We found that N-803 could activate MAIT cells in vitro and increase their ability to produce IFN-γ in response to bacterial stimulation. To expand upon this, we examined the phenotypes and functions of MAIT cells present in samples collected from PBMC, airways (bronchoalveolar lavage [BAL] fluid), and lymph nodes (LN) from rhesus macaques that were treated in vivo with N-803. N-803 treatment led to a transient 6 to 7-fold decrease in the total number of MAIT cells in the peripheral blood, relative to pre N-803 time points. Concurrent with the decrease in cells in the peripheral blood, we observed a rapid decline in the frequency of CXCR3⁺CCR6⁺ MAITs. This corresponded with an increase in the frequency of CCR6⁺ MAITs in the BAL fluid, and higher frequencies of ki-67⁺ and granzyme B⁺ MAITs in the blood, LN, and BAL fluid. Finally, N-803 improved the ability of MAIT cells collected from PBMC and airways to produce IFN-γ in response to bacterial stimulation. Overall, N-803 shows the potential to transiently alter the phenotypes and functions of MAIT cells, which could be combined with other strategies to combat bacterial infections.

Human lung-resident mucosal-associated invariant T cells are abundant, express antimicrobial proteins, and are cytokine responsive

Mucosal-associated Invariant T (MAIT) cells are an innate-like T cell subset that recognize a broad array of microbial pathogens, including respiratory pathogens. Here we investigate the transcriptional profile of MAIT cells localized to the human lung, and postulate that MAIT cells may play a role in maintaining homeostasis at this mucosal barrier. Using the MR1/5-OP-RU tetramer, we identified MAIT cells and non-MAIT CD8+ T cells in lung tissue not suitable for transplant from human donors. We used RNA-sequencing of MAIT cells compared to non-MAIT CD8+ T cells to define the transcriptome of MAIT cells in the human lung. We show that, as a population, lung MAIT cells are polycytotoxic, secrete the directly antimicrobial molecule IL-26, express genes associated with persistence, and selectively express cytokine and chemokine- related molecules distinct from other lung-resident CD8+ T cells, such as interferon-γ- and IL-12- receptors. These data highlight MAIT cells' predisposition to rapid pro-inflammatory cytokine responsiveness and antimicrobial mechanisms in human lung tissue, concordant with findings of blood-derived counterparts, and support a function for MAIT cells as early sensors in the defense of respiratory barrier function.

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

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

γδ T, NKT, and MAIT Cells During Evolution: Redundancy or Specialized Functions?

Innate-like T cells display characteristics of both innate lymphoid cells (ILCs) and mainstream αβ T cells, leading to overlapping functions of innate-like T cells with both subsets. In this review, we show that although innate-like T cells are probably present in all vertebrates, their main characteristics are much better known in amphibians and mammals. Innate-like T cells encompass both γδ and αβ T cells. In mammals, γδ TCRs likely coevolved with molecules of the butyrophilin family they interact with, whereas the semi-invariant TCRs of iNKT and mucosal-associated invariant T cells are evolutionarily locked with their restricting MH1b molecules, CD1d and MR1, respectively. The strong conservation of the Ag recognition systems of innate-like T cell subsets despite similar effector potentialities supports that each one fulfills nonredundant roles related to their Ag specificity.

Viral Delivery of IL-7 Is a Potent Immunotherapy Stimulating Innate and Adaptive Immunity and Confers Survival in Sepsis Models

Persistence of an immunosuppressive state plays a role in septic patient morbidity and late mortality. Both innate and adaptive pathways are impaired, pointing toward the need for immune interventions targeting both arms of the immune system. We developed a virotherapy using the nonpropagative modified vaccinia virus Ankara (MVA), which harbors the intrinsic capacity to stimulate innate immunity, to deliver IL-7, a potent activator of adaptive immunity. The rMVA-human IL-7 (hIL-7)-Fc encoding the hIL-7 fused to the human IgG2-Fc was engineered and shown to express a dimeric, glycosylated, and biologically active cytokine. Following a single i.v. injection in naive mice, the MVA-hIL-7-Fc increased the number of total and activated B, T, and NK cells but also myeloid subpopulations (Ly6C^high, Ly6C^int, and Ly6C^neg cells) in both lung and spleen. It triggered differentiation of T cells in central memory, effector memory, and acute effector phenotypes and enhanced polyfunctionality of T cells, notably the number of IFN-γ-producing cells. The MVA vector contributed significantly to immune cell activation, particularly of NK cells. The MVA-hIL-7-Fc conferred a significant survival advantage in the cecal ligation and puncture (CLP) and Candida albicans sepsis models. It significantly increased cell numbers and activation in both spleen and lung of CLP mice. Comparatively, in naive and CLP mice, the rhIL-7-Fc soluble counterpart overall induced less vigorous, shorter lasting, and narrower immune activities than did the MVA-hIL-7-Fc and favored TNF-α-producing cells. The MVA-hIL-7-Fc represents a novel class of immunotherapeutic with clinical potential for treatment of septic patients.

Mucosal-Associated Invariant T Cells Accumulate in the Lungs during Murine Pneumocystis Infection but Are Not Required for Clearance

Pneumocystis is a fungal pathogen that can cause pneumonia in immunosuppressed hosts and subclinical infection in immunocompetent hosts. Mucosal-associated invariant T (MAIT) cells are unconventional lymphocytes with a semi-invariant T-cell receptor that are activated by riboflavin metabolites that are presented by the MHC-1b molecule MR1. Although Pneumocystis can presumably synthesize riboflavin metabolites based on whole-genome studies, the role of MAIT cells in controlling Pneumocystis infection is unknown. We used a co-housing mouse model of Pneumocystis infection, combined with flow cytometry and qPCR, to characterize the response of MAIT cells to infection in C57BL/6 mice, and, using MR1^−/− mice, which lack MAIT cells, to examine their role in clearing the infection. MAIT cells accumulated in the lungs of C57BL/6 mice during Pneumocystis infection and remained at increased levels for many weeks after clearance of infection. In MR1^−/− mice, Pneumocystis infection was cleared with kinetics similar to C57BL/6 mice. Thus, MAIT cells are not necessary for control of Pneumocystis infection, but the prolonged retention of these cells in the lungs following clearance of infection may allow a more rapid future response to other pathogens.

Does Dementia Have a Microbial Cause?

The potential contribution of pathogenic microbes to dementia-inducing disease is a subject of considerable importance. Alzheimer's disease (AD) is a neurocognitive disease that slowly destroys brain function, leading to cognitive decline and behavioral and psychiatric disorders. The histopathology of AD is associated with neuronal loss and progressive synaptic dysfunction, accompanied by the deposition of amyloid-β (Aβ) peptide in the form of parenchymal plaques and abnormal aggregated tau protein in the form of neurofibrillary tangles. Observational, epidemiological, experimental, and pathological studies have generated evidence for the complexity and possible polymicrobial causality in dementia-inducing diseases. The AD pathogen hypothesis states that pathogens and microbes act as triggers, interacting with genetic factors to initiate the accumulation of Aβ, hyperphosphorylated tau protein (p-tau), and inflammation in the brain. Evidence indicates that Borrelia sp., HSV-1, VZV (HHV-2), HHV-6/7, oral pathogens, Chlamydophila pneumoniae, and Candida albicans can infect the central nervous system (CNS), evade the immune system, and consequently prevail in the AD brain. Researchers have made significant progress in understanding the multifactorial and overlapping factors that are thought to take part in the etiopathogenesis of dementia; however, the cause of AD remains unclear.

Activation-induced pyroptosis contributes to the loss of MAIT cells in chronic HIV-1 infected patients

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are systemically depleted in human immunodeficiency virus type 1 (HIV-1) infected patients and are not replenished even after successful combined antiretroviral therapy (cART). This study aimed to identify the mechanism underlying MAIT cell depletion.

METHODS

In the present study, we applied flow cytometry, single-cell RNA sequencing and immunohistochemical staining to evaluate the characteristics of pyroptotic MAIT cells in a total of 127 HIV-1 infected individuals, including 69 treatment-naive patients, 28 complete responders, 15 immunological non-responders, and 15 elite controllers, at the Fifth Medical Center of Chinese PLA General Hospital, Beijing, China.

RESULTS

Single-cell transcriptomic profiles revealed that circulating MAIT cells from HIV-1 infected subjects were highly activated, with upregulation of pyroptosis-related genes. Further analysis revealed that increased frequencies of pyroptotic MAIT cells correlated with markers of systemic T-cell activation, microbial translocation, and intestinal damage in cART-naive patients and poor CD4⁺ T-cell recovery in long-term cART patients. Immunohistochemical staining revealed that MAIT cells in the gut mucosa of HIV-1 infected patients exhibited a strong active gasdermin-D (GSDMD, marker of pyroptosis) signal near the cavity side, suggesting that these MAIT cells underwent active pyroptosis in the colorectal mucosa. Increased levels of the proinflammatory cytokines interleukin-12 (IL-12) and IL-18 were observed in HIV-1 infected patients. In addition, activated MAIT cells exhibited an increased pyroptotic phenotype after being triggered by HIV-1 virions, T-cell receptor signals, IL-12 plus IL-18, and combinations of these factors, in vitro.

CONCLUSIONS

Activation-induced MAIT cell pyroptosis contributes to the loss of MAIT cells in HIV-1 infected patients, which could potentiate disease progression and poor immune reconstitution.

Mucosal-associated invariant T-cell tumor infiltration predicts long-term survival in cholangiocarcinoma

BACKGROUND AND AIMS

Cholangiocarcinoma (CCA) is a malignancy arising from biliary epithelial cells of intra- and extrahepatic bile ducts with dismal prognosis and few nonsurgical treatments available. Despite recent success in the immunotherapy-based treatment of many tumor types, this has not been successfully translated to CCA. Mucosal-associated invariant T (MAIT) cells are cytotoxic innate-like T cells highly enriched in the human liver, where they are located in close proximity to the biliary epithelium. Here, we aimed to comprehensively characterize MAIT cells in intrahepatic (iCCA) and perihilar CCA (pCCA).

APPROACH AND RESULTS

Liver tissue from patients with CCA was used to study immune cells, including MAIT cells, in tumor-affected and surrounding tissue by immunohistochemistry, RNA-sequencing, and multicolor flow cytometry. The iCCA and pCCA tumor microenvironment was characterized by the presence of both cytotoxic T cells and high numbers of regulatory T cells. In contrast, MAIT cells were heterogenously lost from tumors compared to the surrounding liver tissue. This loss possibly occurred in response to increased bacterial burden within tumors. The residual intratumoral MAIT cell population exhibited phenotypic and transcriptomic alterations, but a preserved receptor repertoire for interaction with tumor cells. Finally, the high presence of MAIT cells in livers of iCCA patients predicted long-term survival in two independent cohorts and was associated with a favorable antitumor immune signature.

CONCLUSIONS

MAIT cell tumor infiltration associates with favorable immunological fitness and predicts survival in CCA.

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.

Mucosa-Associated Invariant T Cell Hypersensitivity to Staphylococcus aureus Leukocidin ED and Its Modulation by Activation

Mucosa-associated invariant T (MAIT) cells recognize bacterial riboflavin metabolite Ags presented by MHC class Ib-related protein (MR1) and play important roles in immune control of microbes that synthesize riboflavin. This includes the pathobiont Staphylococcus aureus, which can also express a range of virulence factors, including the secreted toxin leukocidin ED (LukED). In this study, we found that human MAIT cells are hypersensitive to LukED-mediated lysis and lost on exposure to the toxin, leaving a T cell population devoid of MAIT cells. The cytolytic effect of LukED on MAIT cells was rapid and occurred at toxin concentrations lower than those required for toxicity against conventional T cells. Furthermore, this coincided with high MAIT cell expression of CCR5, and loss of these cells was efficiently inhibited by the CCR5 inhibitor maraviroc. Interestingly, exposure and preactivation of MAIT cells with IL-12 and IL-18, or activation via TCR triggering, partially protected from LukED toxicity. Furthermore, analysis of NK cells indicated that LukED targeted the mature cytotoxic CD57⁺ NK cell subset in a CCR5-independent manner. Overall, these results indicate that LukED efficiently eliminates immune cells that can respond rapidly to S. aureus in an innate fashion without the need for clonal expansion, and that MAIT cells are exceptionally vulnerable to this toxin. Thus, the findings support a model where LukED secretion may allow S. aureus to avoid recognition by the rapid cell-mediated responses mediated by MAIT cells and NK cells.

Single-Cell Transcriptional Profiling Reveals Signatures of Helper, Effector, and Regulatory MAIT Cells during Homeostasis and Activation

Mucosal-associated invariant T (MAIT) cells are innate-like lymphocytes that recognize microbial vitamin B metabolites and have emerging roles in infectious disease, autoimmunity, and cancer. Although MAIT cells are identified by a semi-invariant TCR, their phenotypic and functional heterogeneity is not well understood. Here we present an integrated single cell transcriptomic analysis of over 76,000 human MAIT cells during early and prolonged Ag-specific activation with the MR1 ligand 5-OP-RU and nonspecific TCR stimulation. We show that MAIT cells span a broad range of homeostatic, effector, helper, tissue-infiltrating, regulatory, and exhausted phenotypes, with distinct gene expression programs associated with CD4+ or CD8+ coexpression. During early activation, MAIT cells rapidly adopt a cytotoxic phenotype characterized by high expression of GZMB, IFNG and TNF In contrast, prolonged stimulation induces heterogeneous states defined by proliferation, cytotoxicity, immune modulation, and exhaustion. We further demonstrate a FOXP3 expressing MAIT cell subset that phenotypically resembles conventional regulatory T cells. Moreover, scRNAseq-defined MAIT cell subpopulations were also detected in individuals recently exposed to Mycobacterium tuberculosis, confirming their presence during human infection. To our knowledge, our study provides the first comprehensive atlas of human MAIT cells in activation conditions and defines substantial functional heterogeneity, suggesting complex roles in health and disease.

Human MAIT Cells Respond to Staphylococcus aureus with Enhanced Anti-Bacterial Activity

Mucosal-Associated Invariant T (MAIT) cells have been shown to play protective roles during infection with diverse pathogens through their propensity for rapid innate-like cytokine production and cytotoxicity. Among the potential applications for MAIT cells is to defend against Staphylococcus aureus, a pathogen of serious clinical significance. However, it is unknown how MAIT cell responses to S. aureus are elicited, nor has it been investigated whether MAIT cell cytotoxicity is mobilized against intracellular S. aureus. In this study, we investigate the capacity of human MAIT cells to respond directly to S. aureus. MAIT cells co-cultured with dendritic cells (DCs) infected with S. aureus rapidly upregulate CD69, express IFNγ and Granzyme B and degranulate. DC secretion of IL-12, but not IL-18, was implicated in this immune response, while TCR binding of MR1 is required to commence cytokine production. MAIT cell cytotoxicity resulted in apoptosis of S. aureus-infected cells, and reduced intracellular persistence of S. aureus. These findings implicate these unconventional T cells in important, rapid anti-S. aureus responses that may be of great relevance to the ongoing development of novel anti-S. aureus treatments.

Human MAIT cells respond to and suppress HIV-1

Human MAIT cells sit at the interface between innate and adaptive immunity, are polyfunctional and are capable of killing pathogen infected cells via recognition of the Class IB molecule MR1. MAIT cells have recently been shown to possess an antiviral protective role in vivo and we therefore sought to explore this in relation to HIV-1 infection. There was marked activation of MAIT cells in vivo in HIV-1-infected individuals, which decreased following ART. Stimulation of THP1 monocytes with R5 tropic HIVBAL potently activated MAIT cells in vitro. This activation was dependent on IL-12 and IL-18 but was independent of the TCR. Upon activation, MAIT cells were able to upregulate granzyme B, IFNγ and HIV-1 restriction factors CCL3, 4, and 5. Restriction factors produced by MAIT cells inhibited HIV-1 infection of primary PBMCs and immortalized target cells in vitro. These data reveal MAIT cells to be an additional T cell population responding to HIV-1, with a potentially important role in controlling viral replication at mucosal sites.

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.

Germline biallelic mutation affecting the transcription factor Helios causes pleiotropic defects of immunity

[Figure: see text].

MAIT cell alterations in adults with recent-onset and long-term type 1 diabetes

AIMS/HYPOTHESIS

Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes expressing an αβ T cell antigen receptor that recognises the MHC-related 1 molecule. MAIT cells are altered in children at risk for and with type 1 diabetes, and mouse model studies have shown MAIT cell involvement in type 1 diabetes development. Since several studies support heterogeneity in type 1 diabetes physiopathology according to the age of individuals, we investigated whether MAIT cells were altered in adults with type 1 diabetes.

METHODS

MAIT cell frequency, phenotype and function were analysed by flow cytometry, using fresh peripheral blood from 21 adults with recent-onset type 1 diabetes (2-14 days after disease onset) and 47 adults with long-term disease (>2 years after diagnosis) compared with 55 healthy blood donors. We also separately analysed 17 women with long-term type 1 diabetes and an associated autoimmune disease, compared with 30 healthy women and 27 women with long-term type 1 diabetes.

RESULTS

MAIT cells from adults with recent-onset type 1 diabetes, compared with healthy adult donors, harboured a strongly activated phenotype indicated by an elevated CD25⁺ MAIT cell frequency. In adults with long-term type 1 diabetes, MAIT cells displayed an activated and exhausted phenotype characterised by high CD25 and programmed cell death 1 (PD1) expression and a decreased production of proinflammatory cytokines, IL-2, IFN-γ and TNF-α. Even though MAIT cells from these patients showed upregulated IL-17 and IL-4 production, the polyfunctionality of MAIT cells was decreased (median 4.8 vs 13.14% of MAIT cells, p < 0.001) and the frequency of MAIT cells producing none of the effector molecules analysed increased (median 34.40 vs 19.30% of MAIT cells, p < 0.01). Several MAIT cell variables correlated with HbA_1c level and more particularly in patients with recent-onset type 1 diabetes. In women with long-term type 1 diabetes, MAIT cell alterations were more pronounced in those with an associated autoimmune disease than in those without another autoimmune disease. In women with long-term type 1 diabetes and an associated autoimmune disease, there was an increase in CD69 expression and a decrease in the survival B-cell lymphoma 2 (BCL-2) (p < 0.05) and CD127 (IL-7R) (p < 0.01) marker expression compared with women without a concomitant autoimmune disorder. Concerning effector molecules, TNF-α and granzyme B production by MAIT cells was decreased.

CONCLUSIONS/INTERPRETATION

Alterations in MAIT cell frequency, phenotype and function were more pronounced in adults with long-term type 1 diabetes compared with adults with recent-onset type 1 diabetes. There were several correlations between MAIT cell variables and clinical characteristics. Moreover, the presence of another autoimmune disease in women with long-term type 1 diabetes further exacerbated MAIT cell alterations. Our results suggest that MAIT cell alterations in adults with type 1 diabetes could be associated with two aspects of the disease: impaired glucose homeostasis; and autoimmunity.

The promise of endogenous and exogenous riboflavin in anti-infection

To resolve the growing problem of drug resistance in the treatment of bacterial and fungal pathogens, specific cellular targets and pathways can be used as targets for new antimicrobial agents. Endogenous riboflavin biosynthesis is a conserved pathway that exists in most bacteria and fungi. In this review, the roles of endogenous and exogenous riboflavin in infectious disease as well as several antibacterial agents, which act as analogues of the riboflavin biosynthesis pathway, are summarized. In addition, the effects of exogenous riboflavin on immune cells, cytokines, and heat shock proteins are described. Moreover, the immune response of endogenous riboflavin metabolites in infectious diseases, recognized by MHC-related protein-1, and then presented to mucosal associated invariant T cells, is highlighted. This information will provide a strategy to identify novel drug targets as well as highlight the possible clinical use of riboflavin.

In vitro Interleukin-7 treatment partially rescues MAIT cell dysfunction caused by SARS-CoV-2 infection

MAIT cells have been shown to be activated upon several viral infections in a TCR-independent manner by responding to inflammatory cytokines secreted by antigen-presenting cells. Recently, a few studies have shown a similar activation of MAIT cells in response to severe acute respiratory coronavirus 2 (SARS-CoV-2) infection. In this study, we investigate the effect of SARS-CoV-2 infection on the frequency and phenotype of MAIT cells by flow cytometry, and we test in vitro stimulation conditions on the capacity to enhance or rescue the antiviral function of MAIT cells from patients with coronavirus disease 2019 (COVID-19). Our study, in agreement with recently published studies, confirmed the decline in MAIT cell frequency of hospitalized donors in comparison to healthy donors. MAIT cells of COVID-19 patients also had lower expression levels of TNF-alpha, perforin and granzyme B upon stimulation with IL-12 + IL-18. 24 h’ incubation with IL-7 successfully restored perforin expression levels in COVID-19 patients. Combined, our findings support the growing evidence that SARS-CoV-2 is dysregulating MAIT cells and that IL-7 treatment might improve their function, rendering them more effective in protecting the body against the virus.

Incorporating mucosal-associated invariant T cells into the pathogenesis of chronic liver disease

Mucosal-associated invariant T (MAIT) cells have been described in liver and non-liver diseases, and they have been ascribed antimicrobial, immune regulatory, protective, and pathogenic roles. The goals of this review are to describe their biological properties, indicate their involvement in chronic liver disease, and encourage investigations that clarify their actions and therapeutic implications. English abstracts were identified in PubMed by multiple search terms, and bibliographies were developed. MAIT cells are activated by restricted non-peptides of limited diversity and by multiple inflammatory cytokines. Diverse pro-inflammatory, anti-inflammatory, and immune regulatory cytokines are released; infected cells are eliminated; and memory cells emerge. Circulating MAIT cells are hyper-activated, immune exhausted, dysfunctional, and depleted in chronic liver disease. This phenotype lacks disease-specificity, and it does not predict the biological effects. MAIT cells have presumed protective actions in chronic viral hepatitis, alcoholic hepatitis, non-alcoholic fatty liver disease, primary sclerosing cholangitis, and decompensated cirrhosis. They have pathogenic and pro-fibrotic actions in autoimmune hepatitis and mixed actions in primary biliary cholangitis. Local factors in the hepatic microenvironment (cytokines, bile acids, gut-derived bacterial antigens, and metabolic by-products) may modulate their response in individual diseases. Investigational manipulations of function are warranted to establish an association with disease severity and outcome. In conclusion, MAIT cells constitute a disease-nonspecific, immune response to chronic liver inflammation and infection. Their pathological role has been deduced from their deficiencies during active liver disease, and future investigations must clarify this role, link it to outcome, and explore therapeutic interventions.

Activating Mucosal-Associated Invariant T Cells Induces a Broad Antitumor Response

Mucosal-associated invariant T (MAIT) cells are MR1-restricted innate-like T cells that recognize non-peptide antigens including riboflavin derivates. Although in vitro-activated MAIT cells show antitumor activity, the in vivo role of MAIT cells in cancer is still unclear. Here, we have shown that MAIT cells have antitumor function in vivo when activated by a combination of the synthetic riboflavin synthesis pathway-derived antigen 5-OP-RU [5-(2-oxopropylideneamino)-6-D-ribitylaminouracil] and the Toll-like receptor 9 (TLR9) agonist CpG. Coadministration of 5-OP-RU and CpG induced strong systemic in vivo expansion and activation of MAIT cells with high CD69 expression, pronounced effector memory phenotype, and upregulated levels of effector molecules including IFNγ, granzyme B, and perforin. Activated and expanded MAITs induced a potent and broad antitumor immune response in murine models of liver metastasis and hepatocellular carcinoma, lung metastasis, and subcutaneous tumors in two different mouse strains. Such tumor inhibition was absent in MAIT-deficient Mr1 ^-/- mice. CRISPR/Cas9-mediated MR1 knockout in tumor cells did not affect efficacy of this MAIT-directed immunotherapy, pointing toward an indirect mechanism of action. Our findings suggest that MAIT cells are an attractive target for cancer immunotherapy.See related Spotlight by Lantz, p. 996.

High CD26 and Low CD94 Expression Identifies an IL-23 Responsive Vδ2+ T Cell Subset with a MAIT Cell-like Transcriptional Profile

Vδ2⁺ T cells play a critical role in immunity to micro-organisms and cancer but exhibit substantial heterogeneity in humans. Here, we demonstrate that CD26 and CD94 define transcriptionally, phenotypically, and functionally distinct Vδ2⁺ T cell subsets. Despite distinct antigen specificities, CD26^hiCD94^lo Vδ2⁺ cells exhibit substantial similarities to CD26^hi mucosal-associated invariant T (MAIT) cells, although CD26^- Vδ2⁺ cells exhibit cytotoxic, effector-like profiles. At birth, the Vδ2⁺Vγ9⁺ population is dominated by CD26^hiCD94^lo cells; during adolescence and adulthood, Vδ2⁺ cells acquire CD94/NKG2A expression and the relative frequency of the CD26^hiCD94^lo subset declines. Critically, exposure of the CD26^hiCD94^lo subset to phosphoantigen in the context of interleukin-23 (IL-23) and CD26 engagement drives the acquisition of a cytotoxic program and concurrent loss of the MAIT cell-like phenotype. The ability to modulate the cytotoxic potential of CD26^hiCD94^lo Vδ2⁺ cells, combined with their adenosine-binding capacity, may make them ideal targets for immunotherapeutic expansion and adoptive transfer.

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.