Human MAIT cells exit peripheral tissues and recirculate via lymph in steady state conditions

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.

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.

Bacterial heat shock protein: A new crosstalk between T lymphocyte and macrophage via JAK2/STAT1 pathway in bloodstream infection

Bloodstream infection (BSI) refers to the infection of blood by pathogens. Severe immune response to BSI can lead to sepsis, a systemic infection leading to multiple organ dysfunction, coupled with drug resistance, mortality, and limited clinical treatment options. This work aims to further investigate the new interplay between bacterial exocrine regulatory protein and host immune cells in the context of highly drug-resistant malignant BSI. Whether interfering with related regulatory signaling pathways can reverse the inflammatory disorder of immune cells. In-depth analysis of single-cell sequencing results in Septic patients for potential immunodeficiency factors. Analysis of key proteins enriched by host cells and key pathways using proteomics. Cell models and animal models validate the pathological effects of DnaK on T cells, MAITs, macrophages, and osteoclasts. The blood of patients was analyzed for the immunosuppression of T cells and MAITs. We identified that S. maltophilia-DnaK was enriched in immunodeficient T cells. The activation of the JAK2/STAT1 axis initiated the exhaustion of T cells. Septic patients with Gram-negative bacterial infections exhibited deficiencies in MAITs, which correspond to IFN-γ. Cellular and animal experiments confirmed that DnaK could facilitate MAIT depletion and M1 polarization of macrophages. Additionally, Fludarabine mitigated M1 polarization of blood, liver, and spleen in mice. Interestingly, DnaK also repressed osteoclastogenesis of macrophages stimulated by RANKL. S.maltophilia-DnaK prompts the activation of the JAK2/STAT1 axis in T cells and the M1 polarization of macrophages. Targeting the DnaK's crosstalk can be a potentially effective approach for treating the inflammatory disorder in the broad-spectrum drug-resistant BSI.

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.

Type I interferons drive MAIT cell functions against bacterial pneumonia

Mucosal-associated invariant T (MAIT) cells are abundant in the lung and contribute to host defense against infections. During bacterial infections, MAIT cell activation has been proposed to require T cell receptor (TCR)-mediated recognition of antigens derived from the riboflavin synthesis pathway presented by the antigen-presenting molecule MR1. MAIT cells can also be activated by cytokines in an MR1-independent manner, yet the contribution of MR1-dependent vs. -independent signals to MAIT cell functions in vivo remains unclear. Here, we use Klebsiella pneumoniae as a model of bacterial pneumonia and demonstrate that MAIT cell activation is independent of MR1 and primarily driven by type I interferons (IFNs). During Klebsiella infection, type I IFNs stimulate activation of murine and human MAIT cells, induce a Th1/cytotoxic transcriptional program, and modulate MAIT cell location within the lungs. Consequently, adoptive transfer or boosting of pulmonary MAIT cells protect mice from Klebsiella infection, with protection being dependent on direct type I IFN signaling on MAIT cells. These findings reveal type I IFNs as new molecular targets to manipulate MAIT cell functions during bacterial infections.

Tissue-resident and innate-like T cells in patients with advanced chronic liver disease

Chronic liver disease results from the orchestrated interplay of components of innate and adaptive immunity in response to liver tissue damage. Recruitment, positioning, and activation of immune cells can contribute to hepatic cell death, inflammation, and fibrogenesis. With disease progression and increasing portal pressure, repeated translocation of bacterial components from the intestinal lumen through the epithelial and vascular barriers leads to persistent mucosal, hepatic, and systemic inflammation which contributes to tissue damage, immune dysfunction, and microbial infection. It is increasingly recognised that innate-like and adaptive T-cell subsets located in the liver, mucosal surfaces, and body cavities play a critical role in the progression of advanced liver disease and inflammatory complications of cirrhosis. Mucosal-associated invariant T cells, natural killer T cells, γδ T cells, and tissue-resident memory T cells in the gut, liver, and ascitic fluid share certain characteristic features, which include that they recognise microbial products, tissue alarmins, cytokines, and stress ligands in tissues, and perform effector functions in chronic liver disease. This review highlights recent advances in the comprehension of human tissue-resident and unconventional T-cell populations and discusses the mechanisms by which they contribute to inflammation, fibrosis, immunosuppression, and antimicrobial surveillance in patients with cirrhosis. Understanding the complex interactions of immune cells in different compartments and their contribution to disease progression will provide further insights for effective diagnostic interventions and novel immunomodulatory strategies in patients with advanced chronic liver disease.

Single-cell analysis of human MAIT cell transcriptional, functional and clonal diversity

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that recognize microbial metabolites through a semi-invariant T cell receptor (TCR). Major questions remain regarding the extent of human MAIT cell functional and clonal diversity. To address these, we analyzed the single-cell transcriptome and TCR repertoire of blood and liver MAIT cells and developed functional RNA-sequencing, a method to integrate function and TCR clonotype at single-cell resolution. MAIT cell clonal diversity was comparable to conventional memory T cells, with private TCR repertoires shared across matched tissues. Baseline functional diversity was low and largely related to tissue site. MAIT cells showed stimulus-specific transcriptional responses in vitro, with cells positioned along gradients of activation. Clonal identity influenced resting and activated transcriptional profiles but intriguingly was not associated with the capacity to produce IL-17. Overall, MAIT cells show phenotypic and functional diversity according to tissue localization, stimulation environment and clonotype.

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.

MAIT cells and the microbiome

Mucosal associated invariant T (MAIT) cells are innate-like T lymphocytes, strikingly enriched at mucosal surfaces and characterized by a semi-invariant αβ T cell receptor (TCR) recognizing microbial derived intermediates of riboflavin synthesis presented by the MHC-Ib molecule MR1. At barrier sites MAIT cells occupy a prime position for interaction with commensal microorganisms, comprising the microbiota. The microbiota is a rich source of riboflavin derived antigens required in early life to promote intra-thymic MAIT cell development and sustain a life-long population of tissue resident cells. A symbiotic relationship is thought to be maintained in health whereby microbes promote maturation and homeostasis, and in turn MAIT cells can engage a TCR-dependent "tissue repair" program in the presence of commensal organisms conducive to sustaining barrier function and integrity of the microbial community. MAIT cell activation can be induced in a MR1-TCR dependent manner or through MR1-TCR independent mechanisms via pro-inflammatory cytokines interleukin (IL)-12/-15/-18 and type I interferon. MAIT cells provide immunity against bacterial, fungal and viral pathogens. However, MAIT cells may have deleterious effects through insufficient or exacerbated effector activity and have been implicated in autoimmune, inflammatory and allergic conditions in which microbial dysbiosis is a shared feature. In this review we summarize the current knowledge on the role of the microbiota in the development and maintenance of circulating and tissue resident MAIT cells. We also explore how microbial dysbiosis, alongside changes in intestinal permeability and imbalance between pro- and anti-inflammatory components of the immune response are together involved in the potential pathogenicity of MAIT cells. Whilst there have been significant improvements in our understanding of how the microbiota shapes MAIT cell function, human data are relatively lacking, and it remains unknown if MAIT cells can conversely influence the composition of the microbiota. We speculate whether, in a human population, differences in microbiomes might account for the heterogeneity observed in MAIT cell frequency across mucosal sites or between individuals, and response to therapies targeting T cells. Moreover, we speculate whether manipulation of the microbiota, or harnessing MAIT cell ligands within the gut or disease-specific sites could offer novel therapeutic strategies.

Role of MR1-driven signals and amphiregulin on the recruitment and repair function of MAIT cells during skin wound healing

Tissue repair processes maintain proper organ function following mechanical or infection-related damage. In addition to antibacterial properties, mucosal associated invariant T (MAIT) cells express a tissue repair transcriptomic program and promote skin wound healing when expanded. Herein, we use a human-like mouse model of full-thickness skin excision to assess the underlying mechanisms of MAIT cell tissue repair function. Single-cell RNA sequencing analysis suggested that skin MAIT cells already express a repair program at steady state. Following skin excision, MAIT cells promoted keratinocyte proliferation, thereby accelerating healing. Using skin grafts, parabiosis, and adoptive transfer experiments, we show that MAIT cells migrated into the wound in a T cell receptor (TCR)-independent but CXCR6 chemokine receptor-dependent manner. Amphiregulin secreted by MAIT cells following excision promoted wound healing. Expression of the repair function was probably independent of sustained TCR stimulation. Overall, our study provides mechanistic insights into MAIT cell wound healing function in the skin.

Increased Number of Mucosal-Associated Invariant T Cells Is Associated with the Inhibition of Nonalcoholic Fatty Liver Disease in High Fat Diet-Fed Mice

Nonalcoholic fatty liver disease (NAFLD) is an emerging worldwide health concern. The disease may involve immune cells including T cells, but little is known about the role(s) of the innate-like T cells in the liver. Furthermore, the most abundant innate-like T cells in the human liver are mucosal-associated invariant T (MAIT) cells, but the involvement of MAIT cells in NAFLD remains largely unexplored because of their paucity in mice. In this study, we used a novel mouse line, Vα19, in which the number of MAIT cells is equivalent to or greater than that in humans. Compared with the control mice, Vα19 mice fed a high-fat diet (HFD) exhibited a reduction in lipid accumulation, NAFLD activity score, and transcripts relevant to lipogenesis. In addition, serum triglyceride and non-esterified fatty acids were lower in Vα19 mice fed normal chow or HFD. In contrast, the Vα19 mice showed little or no change in glucose tolerance, insulin sensitivity, inflammation in adipose tissues, or intestinal permeability compared with the controls, irrespective of diet. These results suggest that the presence of MAIT cells is associated with reduced lipogenesis and lipid accumulation in the liver; however, further studies are needed to clarify the role of MAIT cells in hepatic lipid metabolism.

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.

Mucosal-associated invariant T cells predict increased acute graft-versus-host-disease incidence in patients receiving allogeneic hematopoietic stem cell transplantation

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are innate-like T cells, some studies have reported that the number of circulating MAIT cells reduced in patients with acute graft-versus-host-disease (aGVHD) development. However, the role of donor MAIT cells on aGVHD development and subsequent functional change still remain unclear.

METHODS

The study recruited 86 patients with hematological malignancies who underwent allogeneic hematopoietic cell transplantation (HCT) from May 1, 2018 to June 30, 2019. MAIT cells, their subset, and cytokine levels were measured by flow cytometry. Gray's test was used to assess the impact of graft MAIT cell proportion and number on aGVHD incidence. The Cox proportional hazard model was used in the multivariate analysis. The comparison for continuous variables was assessed using Mann-Whitney analysis. RNA-sequencing was performed to investigate the possible molecular pathway changes.

RESULTS

Our study showed that the proportion of MAIT cells in grafts was not different from normal controls, but the CD4/8 subsets were altered. Taking the median of the proportion and number of MAIT cells in the graft as the threshold, the results showed that the incidence of grade B-D aGVHD in patients with MAIT cell proportion ≥ 3.03% was significantly higher than that in patients with MAIT cell proportion < 3.03% (56.3%, 95% CI 37.1-71.2 versus 23.1%, 95% CI 13.8-46.2; P = 0.038).The number of MAIT cells in the graft was not associated with aGVHD development (P = 0.173), however, when the graft contained more CD4 positive, CD8 positive, and CD4/CD8 double-positive MAIT cells, the incidence of aGVHD was significantly increased (P = 0.019, P = 0.035 and P = 0.027, respectively). Besides, reduced frequencies and counts of circulating MAIT cells were observed in patients with aGVHD when compared to patients without aGVHD, accompanied by enhanced production of Tumor necrosis factor-α, Interferon-γ and upregulated programmed death-1, CXC Chemokine Receptor-6 (CXCR6) and CD38 expression. Gene set enrichment analysis of MAIT cell RNA-seq data showed interferon-α response pathway upregulated in aGVHD patients when compared with patients without aGVHD and healthy controls.

CONCLUSIONS

Our study shows that MAIT cells in grafts and peripheral blood are both closely related to the aGVHD development post allogeneic HCT. Interferon-α response pathway perhaps is a critical regulation mechanism for the MAIT cell involvement in aGVHD development.

Molecular and cellular regulation of psoriatic inflammation

This review highlights the molecular and cellular mechanisms underlying psoriatic inflammation with an emphasis on recent developments which may impact on treatment approaches for this chronic disease. We consider both the skin and the musculoskeletal compartment and how different manifestations of psoriatic inflammation are linked. This review brings a focus to the importance of inflammatory feedback loops that exist in the initiation and chronic stages of the condition, and how close interaction between the epidermis and both innate and adaptive immune compartments drives psoriatic inflammation. Furthermore, we highlight work done on biomarkers to predict the outcome of therapy as well as the transition from psoriasis to psoriatic arthritis.

Mucosal-Associated Invariant T (MAIT) cells are highly activated in duodenal tissue of humans with Vibrio cholerae O1 infection: A preliminary report

Mucosal-associated invariant T (MAIT) cells are unconventional T lymphocytes with a semi-conserved TCRα, activated by the presentation of vitamin B metabolites by the MHC-I related protein, MR1, and with diverse innate and adaptive effector functions. The role of MAIT cells in acute intestinal infections, especially at the mucosal level, is not well known. Here, we analyzed the presence and phenotype of MAIT cells in duodenal biopsies and paired peripheral blood samples, in patients during and after culture-confirmed Vibrio cholerae O1 infection. Immunohistochemical staining of duodenal biopsies from cholera patients (n = 5, median age 32 years, range 26-44, 1 female) identified MAIT cells in the lamina propria of the crypts, but not the villi. By flow cytometry (n = 10, median age 31 years, range 23-36, 1 female), we showed that duodenal MAIT cells are more activated than peripheral MAIT cells (p < 0.01 across time points), although there were no significant differences between duodenal MAIT cells at day 2 and day 30. We found fecal markers of intestinal permeability and inflammation to be correlated with the loss of duodenal (but not peripheral) MAIT cells, and single-cell sequencing revealed differing T cell receptor usage between the duodenal and peripheral blood MAIT cells. In this preliminary report limited by a small sample size, we show that MAIT cells are present in the lamina propria of the duodenum during V. cholerae infection, and more activated than those in the blood. Future work into the trafficking and tissue-resident function of MAIT cells is warranted.

Changes in the phenotype and function of mucosal-associated invariant T cells in neutrophilic asthma

Asthma is a chronic heterogeneous inflammatory disease. Most neutrophilic asthma (NA) cases are severe asthma involving many inflammatory cells and mediators, although the specific pathogenesis is not clear. Mucosal-associated invariant T (MAIT) cells as innate-like T lymphocytes play an important role in the immune response in asthma by producing cytokines. In this study, we evaluated the phenotype and function of circulating MAIT cells in patients with NA and inflammatory-related cytokines in plasma and induced sputum supernatants using flow cytometry. The results showed that the frequency of circulating MAIT cells in asthma patients, particularly NA patients, decreased significantly, and CD8⁺ MAIT and MAIT Temra cells also decreased significantly. Increased expression of CD69 and PD-1 on MAIT cells indicated excessive activation and depletion, leading to the decrease in MAIT cells. Levels of IL-17A and TNF-α secreted by MAIT cells of NA patients increased, whereas IFN-γ levels decreased, indicating that MAIT cells in NA are biased to the Th17 subtype. MAIT cells were also negatively correlated with clinical parameters, indicating that these cells are related to asthma severity. Pro-inflammatory cytokines in plasma and sputum supernatant increased to varying degrees, whereas IL-10 declined, corresponding with asthma severity. We speculate that increased IL-17A and TNF-α synergistically stimulated respiratory epithelial cells to secrete IL-6 and IL-8, thereby recruiting neutrophils to inflammatory sites and aggravating asthma symptoms. Therefore, MAIT cells could serve as a potential therapeutic target in NA immunity, thus providing a new strategy for the treatment of asthma.

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

[Figure: see text].

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.

Multiomic analysis reveals decidual-specific transcriptional programing of MAIT cells

PROBLEM

Mucosal-Associated Invariant T (MAIT) cells have been recently identified at the maternal-fetal interface. However, transcriptional programming of decidual MAIT cells in pregnancy remains poorly understood.

METHOD OF STUDY

We employed a multiomic approach to address this question. Mononuclear cells from the decidua basalis and parietalis, and control PBMCs, were analyzed via flow cytometry to investigate MAIT cells in the decidua and assess their transcription factor expression. In a separate study, both decidual and matched peripheral MAIT cells were analyzed using Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITE-seq) coupled with gene expression analysis. Lastly, decidual MAIT cells were stimulated with E.coli and expression of MR1 by antigen presenting cells was measured to evaluate decidual MAIT cell function.

RESULTS

First, we identified MAIT cells in both the decidua basalis and parietalis. CITE-seq, coupled with scRNA-seq gene expression analysis, highlighted transcriptional programming differences between decidual and matched peripheral MAIT cells at a single cell resolution. Transcription factor expression analysis further highlighted transcriptional differences between decidual MAIT cells and non-matched peripheral MAIT cells. Functionally, MAIT cells are skewed towards IFNγ and TNFα production upon stimulation, with E.coli leading to IFNγ production. Lastly, we demonstrate that MR1, the antigen presenting molecule restricting MAIT cells, is expressed by decidual APCs.

CONCLUSION

MAIT cells are present in the decidua basalis and obtain a unique gene expression profile. The presence of MR1 on APCs coupled with in vitro activation by E.coli suggests that MAIT cells might be involved in tissue-repair mechanisms at the maternal-fetal interface.

Inflammatory signals are sufficient to elicit TOX expression in mouse and human CD8+ T cells

T cell receptor (TCR) stimulation leads to the expression of the transcription factor thymocyte selection–associated high-mobility group box (TOX). Prolonged TCR signaling, such as encountered during chronic infections or in tumors, leads to sustained TOX expression, which is required for the induction of a state of exhaustion or dysfunction. Although CD8⁺ memory T (Tmem) cells in mice typically do not express TOX at steady state, some human Tmem cells express TOX but appear fully functional. This seeming discrepancy between mouse and human T cells has led to the speculation that TOX is differentially regulated between these species, which could complicate the interpretation of preclinical mouse model studies. We report here that, similar to TCR-mediated signals, inflammatory cytokines are also sufficient to increase TOX expression in human and mouse Tmem cells. Thus, TOX expression is controlled by the environment, which provides an explanation for the different TOX expression patterns encountered in T cells isolated from specific pathogen–free laboratory mice versus humans. Finally, we report that TOX is not necessary for cytokine-driven expression of programmed cell death 1. Overall, our data highlight that the mechanisms regulating TOX expression are conserved across species and indicate that TOX expression reflects a T cell’s activation state and does not necessarily correlate with T cell dysfunction.

The human memory T cell compartment changes across tissues of the female reproductive tract

Memory CD4 T cells in tissues fulfill numerous functions that are critical for local immune homeostasis and protection against pathogens. Previous studies have highlighted the phenotypic and functional heterogeneity of circulating and tissue-resident memory CD4 T cells across different human tissues such as skin, lung, liver, and colon. Comparatively little is known in regard to memory CD4 T cells across tissues of the female reproductive tract (FRT). We examined CD4 T cells in donor-matched vaginal, ecto- and endocervical tissues, which differ in mucosal structure and exposure to external environmental stimuli. We hypothesized that this could be reflected by tissue-specific differences in the memory CD4 T cell compartment. We found differences in CD4 subset distribution across these tissues. Specifically, CD69⁺CD103⁺ CD4 T cells were significantly more abundant in vaginal than cervical tissues. In contrast, the transcriptional profiles of CD4 subsets were fairly conserved across FRT tissues. CD69⁺CD103⁺ CD4 T cells showed a T_H17 bias independent of tissue niche. Our data suggest that FRT tissues affect T cell subset distribution but have limited effects on the transcriptome of each subset. We discuss the implications for barrier immunity in the FRT.

Title of article: Mucosal-associated invariant T cells in lung diseases

The lungs are directly connected to the external environment, which makes them more vulnerable to infection and injury. They are protected by the respiratory epithelium and immune cells to maintain a dynamic balance. Both innate and adaptive immune cells are involved in the pathogenesis of lung diseases. Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells, which have attracted increasing attention in recent years. Although MAIT cells account for a small part of the total immune cells in the lungs, evidence suggests that these cells are activated by T cell receptors and/or cytokine receptors and mediate immune response. They play an important role in immunosurveillance and immunity against microbial infection, and recent studies have shown that subsets of MAIT cells play a role in promoting pulmonary inflammation. Emerging data indicate that MAIT cells are involved in the immune response against SARS-CoV-2 and possible immunopathogenesis in COVID-19. Here, we introduce MAIT cell biology to clarify their role in the immune response. Then we review MAIT cells in human and murine lung diseases, including asthma, chronic obstructive pulmonary disease, pneumonia, pulmonary tuberculosis and lung cancer, and discuss their possible protective and pathological effects. MAIT cells represent an attractive marker and potential therapeutic target for disease progression, thus providing new strategies for the treatment of lung diseases.

MAIT Cell Development and Functions: the Microbial Connection

Mucosal-associated invariant T (MAIT) cells are an evolutionarily conserved T cell subset, which reacts to most bacteria through T cell receptor (TCR)-mediated recognition of metabolites derived from the vitamin B2 biosynthetic pathway. Microbiota-derived signals affect all stages of MAIT cell biology including intra-thymic development, peripheral expansion, and functions in specific organs. In tissues, MAIT cells can integrate multiple signals and display effector functions involved in the defense against infectious pathogens. In addition to anti-bacterial activity, MAIT cells improve wound healing in the skin, suggesting a role in epithelium homeostasis through bi-directional interactions with the local microbiota. In humans, blood MAIT cell frequency is modified during several auto-immune diseases, which are often associated with microbiota dysbiosis, further emphasizing the potential interplay of MAIT cells with the microbiota. Here, we will review how microbes interact with MAIT cells, from initial intra-thymic development to tissue colonization and functions.

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

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

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.

Circulating mucosal-associated invariant T cells identify patients responding to anti-PD-1 therapy

Immune checkpoint inhibitors are used for treating patients with metastatic melanoma. Since the response to treatment is variable, biomarkers are urgently needed to identify patients who may benefit from such therapy. Here, we combine single-cell RNA-sequencing and multiparameter flow cytometry to assess changes in circulating CD8⁺ T cells in 28 patients with metastatic melanoma starting anti-PD-1 therapy, followed for 6 months: 17 responded to therapy, whilst 11 did not. Proportions of activated and proliferating CD8⁺ T cells and of mucosal-associated invariant T (MAIT) cells are significantly higher in responders, prior to and throughout therapy duration. MAIT cells from responders express higher level of CXCR4 and produce more granzyme B. In silico analysis support MAIT presence in the tumor microenvironment. Finally, patients with >1.7% of MAIT among peripheral CD8⁺ population show a better response to treatment. Our results thus suggest that MAIT cells may be considered a biomarker for patients responding to anti-PD-1 therapy.

Identification and Phenotype of MAIT Cells in Cattle and Their Response to Bacterial Infections

Mucosal-associated invariant T (MAIT) cells are a population of innate-like T cells that utilize a semi-invariant T cell receptor (TCR) α chain and are restricted by the highly conserved antigen presenting molecule MR1. MR1 presents microbial riboflavin biosynthesis derived metabolites produced by bacteria and fungi. Consistent with their ability to sense ligands derived from bacterial sources, MAIT cells have been associated with the immune response to a variety of bacterial infections, such as Mycobacterium spp., Salmonella spp. and Escherichia coli. To date, MAIT cells have been studied in humans, non-human primates and mice. However, they have only been putatively identified in cattle by PCR based methods; no phenotypic or functional analyses have been performed. Here, we identified a MAIT cell population in cattle utilizing MR1 tetramers and high-throughput TCR sequencing. Phenotypic analysis of cattle MAIT cells revealed features highly analogous to those of MAIT cells in humans and mice, including expression of an orthologous TRAV1-TRAJ33 TCR α chain, an effector memory phenotype irrespective of tissue localization, and expression of the transcription factors PLZF and EOMES. We determined the frequency of MAIT cells in peripheral blood and multiple tissues, finding that cattle MAIT cells are enriched in mucosal tissues as well as in the mesenteric lymph node. Cattle MAIT cells were responsive to stimulation by 5-OP-RU and riboflavin biosynthesis competent bacteria in vitro. Furthermore, MAIT cells in milk increased in frequency in cows with mastitis. Following challenge with virulent Mycobacterium bovis, a causative agent of bovine tuberculosis and a zoonosis, peripheral blood MAIT cells expressed higher levels of perforin. Thus, MAIT cells are implicated in the immune response to two major bacterial infections in cattle. These data suggest that MAIT cells are functionally highly conserved and that cattle are an excellent large animal model to study the role of MAIT cells in important zoonotic infections.

Human liver-derived MAIT cells differ from blood MAIT cells in their metabolism and response to TCR-independent activation

Mucosal associated invariant T (MAIT) cells are anti-microbial innate-like T cells that are abundant in blood and liver. MAIT cells express a semi-invariant T-cell receptor (TCR) that recognizes a pyrimidine ligand, derived from microbial riboflavin synthesis, bound to MR1. Both blood and liver derived (ld)-MAIT cells can be robustly stimulated via TCR or by cytokines produced during bacterial or viral infection. In this study, we compared the functional and transcriptomic response of human blood and ld-MAIT cells to TCR signals (Escherichia coli or the pyrimidine ligand) and cytokines (IL-12 + IL-18). While the response of blood and ld-MAIT cells to TCR signals were comparable, following cytokine stimulation ld-MAIT cells were more polyfunctional than blood MAIT cells. Transcriptomic analysis demonstrated different effector programmes of ld-MAIT cells with the two modes of activation, including the enrichment of a tissue repair signature in TCR-stimulated MAIT cells. Interestingly, we observed enhancement of IL-12 signaling and fatty acid metabolism in untreated ld-MAIT cells compared with blood MAIT cells. Additionally, MAIT cells from blood and liver were modulated similarly by TCR and cytokine signals. Therefore, we report that blood and ld-MAIT cells are fundamentally different but undergo conserved changes following activation via TCR or by cytokines.

MAIT cells, guardians of skin and mucosa?

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

Immunobiology and immunotherapy of HCC: spotlight on innate and innate-like immune cells

Immune-based therapies such as immune checkpoint inhibitors have revolutionized the systemic treatment of various cancer types. The therapeutic application of monoclonal antibodies targeting inhibitory pathways such as programmed cell death-1(PD-1)/programmed cell death ligand 1 (PD-L1) and CTLA-4 to cells of the adaptive immune system has recently been shown to generate meaningful improvement in the clinical outcome of hepatocellular carcinoma (HCC). Nevertheless, current immunotherapeutic approaches induce durable responses in only a subset of HCC patients. Since immunologic mechanisms such as chronic inflammation due to chronic viral hepatitis or alcoholic and nonalcoholic fatty liver disease play a crucial role in the initiation, development, and progression of HCC, it is important to understand the underlying mechanisms shaping the unique tumor microenvironment of liver cancer. The liver is an immunologic organ with large populations of innate and innate-like immune cells and is exposed to bacterial, viral, and fungal antigens through the gut-liver axis. Here, we summarize and highlight the role of these cells in liver cancer and propose strategies to therapeutically target them. We also discuss current immunotherapeutic strategies in HCC and outline recent advances in our understanding of how the therapeutic potential of these agents might be enhanced.

The Identity of Human Tissue-Emigrant CD8+ T Cells

Lymphocyte migration is essential for adaptive immune surveillance. However, our current understanding of this process is rudimentary, because most human studies have been restricted to immunological analyses of blood and various tissues. To address this knowledge gap, we used an integrated approach to characterize tissue-emigrant lineages in thoracic duct lymph (TDL). The most prevalent immune cells in human and non-human primate efferent lymph were T cells. Cytolytic CD8⁺ T cell subsets with effector-like epigenetic and transcriptional signatures were clonotypically skewed and selectively confined to the intravascular circulation, whereas non-cytolytic CD8⁺ T cell subsets with stem-like epigenetic and transcriptional signatures predominated in tissues and TDL. Moreover, these anatomically distinct gene expression profiles were recapitulated within individual clonotypes, suggesting parallel differentiation programs independent of the expressed antigen receptor. Our collective dataset provides an atlas of the migratory immune system and defines the nature of tissue-emigrant CD8⁺ T cells that recirculate via TDL.

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

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

MAIT Cells in Barrier Tissues: Lessons from Immediate Neighbors

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

MAIT Cells Are Enriched and Highly Functional in Ascites of Patients With Decompensated Liver Cirrhosis

BACKGROUND AND AIMS

Patients with advanced liver cirrhosis have an increased susceptibility to infections. As part of the cirrhosis-associated immune dysfunction, mucosal-associated invariant T (MAIT) cells, which have the capacity to respond to bacteria, are severely diminished in circulation and liver tissue. However, MAIT cell presence and function in the peritoneal cavity, a common anatomical site for infections in cirrhosis, remain elusive. In this study, we deliver a comprehensive investigation of the immune compartment present in ascites of patients with decompensated liver cirrhosis, and focus especially on MAIT cells.

APPROACH AND RESULTS

To study this, matched peripheral blood and ascites fluid were collected from 35 patients with decompensated cirrhosis, with or without spontaneous bacterial peritonitis (SBP). MAIT cell phenotype and function were analyzed using high-dimensional flow cytometry, and the obtained data were compared with the blood samples of healthy controls (n = 24) and patients with compensated cirrhosis (n = 11). We found circulating MAIT cells to be severely decreased in patients with cirrhosis as compared with controls. In contrast, in ascites fluid, MAIT cells were significantly increased together with CD14⁺ CD16⁺ monocytes, innate lymphoid cells, and natural killer cells. This was paralleled by elevated levels of several pro-inflammatory cytokines and chemokines in ascites fluid as compared with plasma. Peritoneal MAIT cells displayed an activated tissue-resident phenotype, and this was corroborated by increased functional responses following stimulation with E. coli or interleukin (lL)-12 + IL-18 as compared with circulating MAIT cells. During SBP, peritoneal MAIT cell frequencies increased most among all major immune cell subsets, suggestive of active homing of MAIT cells to the site of infection.

CONCLUSIONS

Despite severely diminished MAIT cell numbers and impaired phenotype in circulation, peritoneal MAIT cells remain abundant, activated, and highly functional in decompensated cirrhosis and are further enriched in SBP. This suggests that peritoneal MAIT cells could be of interest for immune-intervention strategies in patients with decompensated liver cirrhosis and SBP.

The human tissue-resident CCR5+ T cell compartment maintains protective and functional properties during inflammation

CCR5 is thought to play a central role in orchestrating migration of cells in response to inflammation. CCR5 antagonists can reduce inflammatory disease processes, which has led to an increased interest in using CCR5 antagonists in a wide range of inflammation-driven diseases. Paradoxically, these antagonists appear to function without negatively affecting host immunity at barrier sites. We reasoned that the resolution to this paradox may lie in the CCR5⁺ T cell populations that permanently reside in tissues. We used a single-cell analysis approach to examine the human CCR5⁺ T cell compartment in the blood, healthy, and inflamed mucosal tissues to resolve these seemingly contradictory observations. We found that 65% of the CD4 tissue-resident memory T (TRM) cell compartment expressed CCR5. These CCR5⁺ T_RM cells were enriched in and near the epithelial layer and not only limited to T_H1-type cells but also contained a large T_H17-producing and a stable regulatory T cell population. The CCR5⁺ T_RM compartment was stably maintained even in inflamed tissues including the preservation of T_H17 and regulatory T cell populations. Further, using tissues from the CHARM-03 clinical trial, we found that CCR5⁺ T_RM are preserved in human mucosal tissue during treatment with the CCR5 antagonist Maraviroc. Our data suggest that the human CCR5⁺ T_RM compartment is functionally and spatially equipped to maintain barrier immunity even in the absence of CCR5-mediated, de novo T cell recruitment from the periphery.

Understanding the Role of Mucosal-Associated Invariant T-Cells in Non-human Primate Models of HIV Infection

Chronic HIV infection causes systemic immune activation and dysregulation, resulting in the impairment of most T-cell subsets including MAIT cells. Multiple human cohort studies demonstrate MAIT cells are selectively depleted in the peripheral blood and lymphoid tissues during HIV infection, with incomplete restoration during suppressive antiretroviral therapy. Because MAIT cells play an important role in mucosal defense against a wide array of pathogens, fully reconstituting the MAIT cell compartment in ART-treated populations could improve immunity against co-infections. Non-human primates (NHPs) are a valuable, well-described animal model for HIV infection in humans. NHPs also maintain MAIT cell frequencies more comparable to humans, compared to other common animal models, and provide a unique opportunity to study MAIT cells in the circulation and mucosal tissues in a longitudinal manner. Only recently, however, have NHP MAIT cells been thoroughly characterized using macaque-specific MR1 tetramer reagents. Here we review the similarities and differences between MAIT cells in humans and NHPs as well as the impact of SIV/SHIV infection on MAIT cells and the potential implications for future research.

Human CD4+CD103+ cutaneous resident memory T cells are found in the circulation of healthy individuals

Tissue-resident memory T cells (T_RM) persist locally in nonlymphoid tissues where they provide frontline defense against recurring insults. T_RM at barrier surfaces express the markers CD103 and/or CD69, which function to retain them in epithelial tissues. In humans, neither the long-term migratory behavior of T_RM nor their ability to reenter the circulation and potentially migrate to distant tissue sites has been investigated. Using tissue explant cultures, we found that CD4⁺CD69⁺CD103⁺ T_RM in human skin can down-regulate CD69 and exit the tissue. In addition, we identified a skin-tropic CD4⁺CD69^-CD103⁺ population in human lymph and blood that is transcriptionally, functionally, and clonally related to the CD4⁺CD69⁺CD103⁺ T_RM population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4⁺CD103⁺ T_RM population can reenter circulation and migrate to secondary human skin sites where they reassume a T_RM phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4⁺ T cell memory in humans.

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.

Expansion of IL-17A-secreting CD8+ mucosa-associated invariant T cells in peripheral blood following stem cell mobilization

Stem cell mobilization with G-CSF promotes IL-17A secretion by donor CD8+ MAIT cells. Tbet and RORγt coexpression identifies potential IL-17A–secreting proinflammatory populations after allogeneic stem cell transplantation.

Defining T Cell Tissue Residency in Humans: Implications for HIV Pathogenesis and Vaccine Design

PURPOSE OF REVIEW

This review summarizes recent literature defining tissue-resident memory T cells (TRM) and discusses implications for HIV pathogenesis, vaccines, and eradication efforts.

RECENT FINDINGS

Investigations using animal models and human tissues have identified a TRM transcriptional profile and elucidated signals within the tissue microenvironment leading to TRM development and maintenance. TRM are major contributors to host response in infectious diseases and cancer; in addition, TRM contribute to pathogenic inflammation in a variety of settings. Although TRM are daunting to study in HIV infection, recent work has helped define their molecular signatures and effector functions and tested strategies for their mobilization. Exclusive reliance on blood sampling to gain an understanding of host immunity overlooks the contribution of TRM, which differ in significant ways from their counterparts in circulation. It is hoped that greater understanding of these cells will lead to novel approaches to prevent and/or eradicate HIV infection.

Mucosal-associated invariant T cells and disease

Mucosal-associated invariant T (MAIT) cells are unique innate-like T cells that bridge innate and adaptive immunity. They are activated by conserved bacterial ligands derived from vitamin B biosynthesis and have important roles in defence against bacterial and viral infections. However, they can also have various deleterious and protective functions in autoimmune, inflammatory and metabolic diseases. MAIT cell involvement in a large spectrum of pathological conditions makes them attractive targets for potential therapeutic approaches.

Reconstitution of Circulating Mucosal-Associated Invariant T Cells after Allogeneic Hematopoietic Cell Transplantation: Its Association with the Riboflavin Synthetic Pathway of Gut Microbiota in Cord Blood Transplant Recipients

Mucosal-associated invariant T (MAIT) cells are a type of innate lymphocyte and recognize riboflavin (vitamin B2) synthesis products presented by MHC-related protein 1. We investigated long-term reconstitution of MAIT cells and its association with chronic graft-versus-host disease (cGVHD) in a cross-sectional cohort of 173 adult patients after allogeneic hematopoietic cell transplantation. According to donor source, the number of MAIT cells significantly correlated with time after cord blood transplantation (CBT) but not with time after bone marrow transplantation or peripheral blood stem cell transplantation. The number of MAIT cells was significantly lower in patients with cGVHD compared with patients without cGVHD. We also examined the association between MAIT cell reconstitution and gut microbiota as evaluated by 16S ribosomal sequencing of stool samples 1 mo post-CBT in 27 adult patients undergoing CBT. The diversity of gut microbiota was positively correlated with better MAIT cell reconstitution after CBT. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States analysis indicated that amounts of ribB and ribA genes were significantly higher in the microbiomes of patients with subsequent MAIT cell reconstitution after CBT. In conclusion, long-term MAIT cell reconstitution is dependent on the type of donor source. Our data also unveiled an important role for the interaction of circulating MAIT cells with gut microbiota in humans.

Human Mucosal-Associated Invariant T Cells in Older Individuals Display Expanded TCRαβ Clonotypes with Potent Antimicrobial Responses

Mucosal-associated invariant T (MAIT) cells are important for immune responses against microbial infections. Although known to undergo marked numerical changes with age in humans, our understanding of how MAIT cells are altered during different phases across the human life span is largely unknown. Although also abundant in the tissues, our study focuses on MAIT cell analyses in blood. Across the human life span, we show that naive-like MAIT cells in umbilical cord blood switch to a central/effector memory-like profile that is sustained into older age. Whereas low-grade levels of plasma cytokine/chemokine were apparent in older donors (>65 y old), surprisingly, they did not correlate with the ex vivo MAIT hyperinflammatory cytokine profile observed in older adults. Removal of MAIT cells from older individuals and an aged environment resulted in the reversal of the baseline effector molecule profile comparable with MAIT cells from younger adults. An upregulated basal inflammatory profile accounted for reduced Escherichia coli-specific responses in aged MAIT cells compared with their young adult counterparts when fold change in expression levels of GzmB, CD107a, IFN-γ, and TNF was examined. However, the magnitude of antimicrobial MR1-dependent activation remained as potent and polyfunctional as with younger adults. Paired TCRαβ analyses of MAIT cells revealed large clonal expansions in older adults and tissues that rivalled, remarkably, the TCRαβ repertoire diversity of virus-specific CD8⁺ T cells. These data suggest that MAIT cells in older individuals, although associated with large clonal TCRαβ expansions and increased baseline inflammatory potential, demonstrate plasticity and provide potent antimicrobial immunity.

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.

Mucosal-Associated Invariant T Cells in Tumors of Epithelial Origin

Mucosal-associated invariant T (MAIT) cells are innate T lymphocytes that circulate in blood and also reside in mucosal tissues. Blood MAIT cells are typically highly Th1-polarized, while those in mucosal tissues include both Th1- and Th17-polarized subsets. MAIT cells mount cytokine and cytolytic responses as a result of T cell receptor (TCR)-mediated recognition of microbially derived metabolites of riboflavin (vitamin B₂) presented by the MR1 antigen-presenting molecule. Additionally, MAIT cells can be activated by inflammatory cytokines produced by antigen-presenting cells (APCs) that have been exposed to pathogen-associated molecular patterns (PAMPs). Since the antigenic metabolites of riboflavin recognized by MAIT cells are produced by many microorganisms, including pathogens as well as non-pathogenic colonists, the inflammatory state of the tissue may be a key feature that determines the nature of MAIT cell responses. Under normal conditions where inflammatory cytokines are not produced, MAIT cell responses to microbial metabolites may simply serve to help maintain a healthy balance between epithelial cells and microbial colonists. In contrast, in situations where inflammatory cytokines are produced (e.g., pathogenic infection or damage to epithelial tissue), MAIT cell responses may be more potently pro-inflammatory. Since chronic inflammation and microbial drivers are associated with tumorigenesis and also trigger MAIT cell responses, the nexus of MAIT cells, local microbiomes, and epithelial cells may play an important role in epithelial carcinogenesis. This chapter reviews current information about MAIT cells and epithelial tumors, where the balance of evidence suggests that enrichment of Th17-polarized MAIT cells at tumor sites associates with poor patient prognosis. Studying the role of MAIT cells and their interactions with resident microbes offers a novel view of the biology of epithelial tumor progression and may ultimately lead to new approaches to target MAIT cells clinically.

Prostaglandin regulation of T cell biology

Prostaglandins (PG) are pleiotropic bioactive lipids involved in the control of many physiological processes, including key roles in regulating inflammation. This links PG to the modulation of the quality and magnitude of immune responses. T cells, as a core part of the immune system, respond readily to inflammatory cues from their environment, and express a diverse array of PG receptors that contribute to their function and phenotype. Here we put in context our knowledge about how PG affect T cell biology, and review advances that bring light into how specific T cell functions that have been newly discovered are modulated through PG. We will also comment on drugs that target PG metabolism and sensing, their effect on T cell function during disease, and we will finally discuss how we can design new approaches that modulate PG in order to maximize desired therapeutic T cell effects.

The biology and functional importance of MAIT cells

In recent years, a population of unconventional T cells called 'mucosal-associated invariant T cells' (MAIT cells) has captured the attention of immunologists and clinicians due to their abundance in humans, their involvement in a broad range of infectious and non-infectious diseases and their unusual specificity for microbial riboflavin-derivative antigens presented by the major histocompatibility complex (MHC) class I-like protein MR1. MAIT cells use a limited T cell antigen receptor (TCR) repertoire with public antigen specificities that are conserved across species. They can be activated by TCR-dependent and TCR-independent mechanisms and exhibit rapid, innate-like effector responses. Here we review evidence showing that MAIT cells are a key component of the immune system and discuss their basic biology, development, role in disease and immunotherapeutic potential.

Mucosal-Associated Invariant T Cells Display Diminished Effector Capacity in Oesophageal Adenocarcinoma

Oesophageal adenocarcinoma (OAC) is an aggressive malignancy with poor prognosis, and incidence is increasing rapidly in the Western world. Mucosal-associated invariant T (MAIT) cells recognize bacterial metabolites and kill infected cells, yet their role in OAC is unknown. We aimed to elucidate the role of MAIT cells during cancer development by characterizing the frequency, phenotype, and function of MAIT cells in human blood and tissues, from OAC and its pre-malignant inflammatory condition Barrett's oesophagus (BO). Blood and tissues were phenotyped by flow cytometry and conditioned media from explanted tissue was used to model the effects of the tumor microenvironment on MAIT cell function. Associations were assessed between MAIT cell frequency, circulating inflammatory markers, and clinical parameters to elucidate the role of MAIT cells in inflammation driven cancer. MAIT cells were decreased in BO and OAC blood compared to healthy controls, but were increased in oesophageal tissues, compared to BO-adjacent tissue, and remained detectable after neo-adjuvant treatment. MAIT cells in tumors expressed CD8, PD-1, and NKG2A but lower NKG2D than BO cohorts. MAIT cells produced less IFN-γ and TNF-α in the presence of tumor-conditioned media. OAC cell line viability was reduced upon exposure to expanded MAIT cells. Serum levels of chemokine IP-10 were inversely correlated with MAIT cell frequency in both tumors and blood. MAIT cells were higher in the tumors of node-negative patients, but were not significantly associated with other clinical parameters. This study demonstrates that OAC tumors are infiltrated by MAIT cells, a type of CD8 T cell featuring immune checkpoint expression and cytotoxic potential. These findings may have implications for immunotherapy and immune scoring approaches.

T follicular helper cells in human efferent lymph retain lymphoid characteristics

T follicular helper cells (Tfh), a subset of CD4+ T cells, provide requisite help to B cells in the germinal centers (GC) of lymphoid tissue. GC Tfh are identified by high expression of the chemokine receptor CXCR5 and the inhibitory molecule PD-1. Although more accessible, blood contains lower frequencies of CXCR5+ and PD-1+ cells that have been termed circulating Tfh (cTfh). However, it remains unclear whether GC Tfh exit lymphoid tissues and populate this cTfh pool. To examine exiting cells, we assessed the phenotype of Tfh present within the major conduit of efferent lymph from lymphoid tissues into blood, the human thoracic duct. Unlike what was found in blood, we consistently identified a CXCR5-bright PD-1-bright (CXCR5BrPD-1Br) Tfh population in thoracic duct lymph (TDL). These CXCR5BrPD-1Br TDL Tfh shared phenotypic and transcriptional similarities with GC Tfh. Moreover, components of the epigenetic profile of GC Tfh could be detected in CXCR5BrPD-1Br TDL Tfh and the transcriptional imprint of this epigenetic signature was enriched in an activated cTfh subset known to contain vaccine-responding cells. Together with data showing shared TCR sequences between the CXCR5BrPD-1Br TDL Tfh and cTfh, these studies identify a population in TDL as a circulatory intermediate connecting the biology of Tfh in blood to Tfh in lymphoid tissue.