The multisensory regulation of unconventional T cell homeostasis

Unconventional T cells typically group γδ T cells, invariant Natural Killer T cells (NKT) and Mucosal Associated Invariant T (MAIT) cells. With their pre-activated status and biased tropism for non-lymphoid organs, they provide a rapid (innate-like) and efficient first line of defense against pathogens at strategical barrier sites, while they can also trigger chronic inflammation, and unexpectedly contribute to steady state physiology. Thus, a tight control of their homeostasis is critical to maintain tissue integrity. In this review, we discuss the recent advances of our understanding of the factors, from neuroimmune to inflammatory regulators, shaping the size and functional properties of unconventional T cell subsets in non-lymphoid organs. We present a general overview of the mechanisms common to these populations, while also acknowledging specific aspects of their diversity. We mainly focus on their maintenance at steady state and upon inflammation, highlighting some key unresolved issues and raising upcoming technical, fundamental and translational challenges.

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.

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

Background

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

Methods

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

Results

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

Conclusions

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

Lymphatic migration of unconventional T cells promotes site-specific immunity in distinct lymph nodes

Lymphatic transport of molecules and migration of myeloid cells to lymph nodes (LNs) continuously inform lymphocytes on changes in drained tissues. Here, using LN transplantation, single-cell RNA-seq, spectral flow cytometry, and a transgenic mouse model for photolabeling, we showed that tissue-derived unconventional T cells (UTCs) migrate via the lymphatic route to locally draining LNs. As each tissue harbored a distinct spectrum of UTCs with locally adapted differentiation states and distinct T cell receptor repertoires, every draining LN was thus populated by a distinctive tissue-determined mix of these lymphocytes. By making use of single UTC lineage-deficient mouse models, we found that UTCs functionally cooperated in interconnected units and generated and shaped characteristic innate and adaptive immune responses that differed between LNs that drained distinct tissues. Lymphatic migration of UTCs is, therefore, a key determinant of site-specific immunity initiated in distinct LNs with potential implications for vaccination strategies and immunotherapeutic approaches.

MAIT cells and their implication in human oral diseases

OBJECTIVE

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

METHODS

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

CONCLUSION

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

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.

Mucosal-Associated Invariant T Cells in T-Cell Non-Hodgkin Lymphomas: A Case Series

Simple Summary

Mucosal-associated invariant T (MAIT) cells are a subgroup of T lymphocytes whose role has recently been investigated in several types of diseases, including cancer. However, little is known about these cells in lymphomas. In this case series, we investigated the presence of MAIT cells in biopsies obtained from patients diagnosed with T-cell non-Hodgkin lymphomas, uncommon hematological malignancies with often not clearly defined etiopathology.

Abstract

Background: Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T lymphocytes expressing a semi-invariant α/β T-cell receptor (TCR). The physiological functions of these cells, which are particularly abundant in normal liver and mucosal sites, have become clear only in recent years, but their role in most human diseases is still unknown. Since the cellular origin and etiopathogenesis of most T-lymphomas are still elusive, we decided to explore the presence of MAIT cells in biopsies from these neoplasms. Methods: Sixteen biopsies obtained from patients with a T-cell lymphoma diagnosis were analyzed via immunofluorescence staining using an anti-Vα7.2 antibody and the MR1-antigen tetramer. Positive cases were subjected to a polymerase chain reaction for the detection of Vα7.2–Jα33, Vα7.2–Jα20, or Vα7.2–Jα12 rearrangements, followed by sequencing of the CDR3α region. Results: CD3+/Vα7.2+ and CD3+/MR1-Ag-tetramer+ cells were found in 4 of 16 samples analyzed. The identification of specific TCR rearrangements confirmed the presence of these cells in all four samples. PCR and sequencing results documented the presence of multiple clones of MAIT cells in each positive sample. Conclusions: MAIT cells are frequently found in T-cell lymphomas. More in-depth studies and a larger number of samples are needed to better clarify the contribution of MAIT cells to this rare neoplasm.

Differential Effects of Dietary versus Exercise Intervention on Intrahepatic MAIT Cells and Histological Features of NAFLD

Background: Mucosal-associated invariant T (MAIT) cells promote inflammation in obesity and are implicated in the progression of non-alcoholic fatty liver disease (NAFLD). However, as the intrahepatic MAIT cell response to lifestyle intervention in NAFLD has not been investigated, this work aimed to examine circulating and intrahepatic MAIT cell populations in patients with NAFLD, after either 12 weeks of dietary intervention (DI) or aerobic exercise intervention (EI). Methods: Multicolour flow cytometry was used to immunophenotype circulating and intrahepatic MAIT cells and measure MAIT cell expression (median fluorescence intensity, MFI) of the activation marker CD69 and apoptotic marker CD95. Liver histology, clinical parameters, and MAIT cell populations were assessed at baseline (T0) and following completion (T1) of DI or EI. Results: Forty-five patients completed the study. DI participants showed decreased median (interquartile range) expression of the activation marker CD69 on circulating MAIT cells (T0: 104 (134) versus T1 27 (114) MFI; p = 0.0353) and improvements in histological steatosis grade post-intervention. EI participants showed increased expression of the apoptotic marker CD95, both in circulating (T0: 1549 (888) versus T1: 2563 (1371) MFI; p = 0.0043) and intrahepatic MAIT cells (T0: 2724 (862) versus T1: 3117 (1622) MFI; p = 0.0269). Moreover, the percentage of intrahepatic MAIT cells significantly decreased after EI (T0: 11.1 (14.4) versus T1: 5.3 (9.3)%; p = 0.0029), in conjunction with significant improvements in fibrosis stage and hepatocyte ballooning. Conclusions: These data demonstrate independent benefits from dietary and exercise intervention and suggest a role for intrahepatic MAIT cells in the observed histological improvements in NAFLD.

MAIT cell counts are associated with the risk of hospitalization in COPD

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is characterized by persistent airflow limitation associated with chronic inflammation in the airways. Mucosal-associated invariant T (MAIT) cells are unconventional, innate-like T cells highly abundant in mucosal tissues including the lung. We hypothesized that the characteristics of MAIT cells in circulation may be prospectively associated with COPD morbidity.

METHODS

COPD subjects (n = 61) from the Tools for Identifying Exacerbations (TIE) study were recruited when in stable condition. At study entry, forced expiratory volume in 1 s (FEV₁) was measured and peripheral blood mononuclear cells were cryopreserved for later analysis by flow cytometry. Patients were followed for 3 years to record clinically meaningful outcomes.

RESULTS

Patients who required hospitalization at one or more occasions during the 3-year follow-up (n = 21) had lower MAIT cell counts in peripheral blood at study inclusion, compared with patients who did not get hospitalized (p = 0.036). In contrast, hospitalized and never hospitalized patients did not differ in CD8 or CD4 T cell counts (p = 0.482 and p = 0.221, respectively). Moreover, MAIT cells in hospitalized subjects showed a more activated phenotype with higher CD38 expression (p = 0.014), and there was a trend towards higher LAG-3 expression (p = 0.052). Conventional CD4 and CD8 T cells were similar between the groups. Next we performed multi-variable logistic regression analysis with hospitalizations as dependent variable, and FEV₁, GOLD 2017 group, and quantity or activation of MAIT and conventional T cells as independent variables. MAIT cell count, CD38 expression on MAIT cells, and LAG-3 expression on both MAIT and CD8 T cells were all independently associated with the risk of hospitalization.

CONCLUSIONS

These findings suggest that MAIT cells might reflect a novel, FEV₁-independent immunological dimension in the complexity of COPD. The potential implication of MAIT cells in COPD pathogenesis and MAIT cells' prognostic potential deserve further investigation.

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

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

The duplexity of unconventional T cells in cancer

Unconventional T cells and their involvement in cancer are understudied in comparison to conventional T cells, but recent findings indicate that these cells play important roles in both cancer progression and inhibition. Here, we briefly review the dichotomous role of three unconventional T cell lineages: γδ T cells, MAIT cells and NKT cells. Studies using mouse models of cancer show how this unconventional trilogy interacts with cancer epithelial cells and other immune cell populations during tumour evolution. These reports highlight various potential avenues for therapeutic intervention that may be exploited for cancer immunotherapy.

T Cell Responses to the Microbiota

The immune system employs recognition tools to communicate with its microbial evolutionary partner. Among all the methods of microbial perception, T cells enable the widest spectrum of microbial recognition resolution, ranging from the crudest detection of whole groups of microbes to the finest detection of specific antigens. The application of this recognition capability to the crucial task of combatting infections has been the focus of classical immunology. We now appreciate that the coevolution of the immune system and the microbiota has led to development of a lush immunological decision tree downstream of microbial recognition, of which an inflammatory response is but one branch. In this review we discuss known T cell-microbe interactions in the gut and place them in the context of an algorithmic framework of recognition, context-dependent interpretation, and response circuits across multiple levels of microbial recognition resolution. The malleability of T cells in response to the microbiota presents an opportunity to edit immune response cellularity, identity, and functionality by utilizing microbiota-controlled pathways to promote human health.

T Cell Responses to the Microbiota

The immune system employs recognition tools to communicate with its microbial evolutionary partner. Among all the methods of microbial perception, T cells enable the widest spectrum of microbial recognition resolution, ranging from the crudest detection of whole groups of microbes to the finest detection of specific antigens. The application of this recognition capability to the crucial task of combatting infections has been the focus of classical immunology. We now appreciate that the coevolution of the immune system and the microbiota has led to development of a lush immunological decision tree downstream of microbial recognition, of which an inflammatory response is but one branch. In this review we discuss known T cell-microbe interactions in the gut and place them in the context of an algorithmic framework of recognition, context-dependent interpretation, and response circuits across multiple levels of microbial recognition resolution. The malleability of T cells in response to the microbiota presents an opportunity to edit immune response cellularity, identity, and functionality by utilizing microbiota-controlled pathways to promote human health.

Reprogramming and redifferentiation of mucosal-associated invariant T cells reveal tumor inhibitory activity

Mucosal-associated invariant T (MAIT) cells belong to a family of innate-like T cells that bridge innate and adaptive immunities. Although MAIT cells have been implicated in tumor immunity, it currently remains unclear whether they function as tumor-promoting or inhibitory cells. Therefore, we herein used induced pluripotent stem cell (iPSC) technology to investigate this issue. Murine MAIT cells were reprogrammed into iPSCs and redifferentiated towards MAIT-like cells (m-reMAIT cells). m-reMAIT cells were activated by an agonist in the presence and absence of antigen-presenting cells and MR1-tetramer, a reagent to detect MAIT cells. This activation accompanied protein tyrosine phosphorylation and the production of T helper (Th)1, Th2, and Th17 cytokines and inflammatory chemokines. Upon adoptive transfer, m-reMAIT cells migrated to different organs with maturation in mice. Furthermore, m-reMAIT cells inhibited tumor growth in the lung metastasis model and prolonged mouse survival upon tumor inoculation through the NK cell-mediated reinforcement of cytolytic activity. Collectively, the present results demonstrated the utility and role of m-reMAIT cells in tumor immunity and provide insights into the function of MAIT cells in immunity.

The role of unconventional T cells in COVID-19

COVID-19 is a respiratory disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It was first documented in late 2019, but within months, a worldwide pandemic was declared due to the easily transmissible nature of the virus. Research to date on the immune response to SARS-CoV-2 has focused largely on conventional B and T lymphocytes. This review examines the emerging role of unconventional T cell subsets, including γδ T cells, invariant natural killer T (iNKT) cells and mucosal associated invariant T (MAIT) cells in human SARS-CoV-2 infection.Some of these T cell subsets have been shown to play protective roles in anti-viral immunity by suppressing viral replication and opsonising virions of SARS-CoV. Here, we explore whether unconventional T cells play a protective role in SARS-CoV-2 infection as well. Unconventional T cells are already under investigation as cell-based immunotherapies for cancer. We discuss the potential use of these cells as therapeutic agents in the COVID-19 setting. Due to the rapidly evolving situation presented by COVID-19, there is an urgent need to understand the pathogenesis of this disease and the mechanisms underlying its immune response. Through this, we may be able to better help those with severe cases and lower the mortality rate by devising more effective vaccines and novel treatment strategies.

NK Cells and Innate-Like T Cells After Autologous Hematopoietic Stem Cell Transplantation in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system, in which autoreactive T and B cells play important roles. Other lymphocytes such as NK cells and innate-like T cells appear to be involved as well. To name a few examples, CD56^bright NK cells were described as an immunoregulatory NK cell subset in MS while innate-like T cells in MS were described in brain lesions and with proinflammatory signatures. Autologous hematopoietic stem cell transplantation (aHSCT) is a procedure used to treat MS. This procedure includes hematopoietic stem/progenitor cell (HSPC) mobilization, then high-dose chemotherapy combined with anti-thymocyte globulin (ATG) and subsequent infusion of the patients own HSPCs to reconstitute a functional immune system. aHSCT inhibits MS disease activity very effectively and for long time, presumably due to elimination of autoreactive T cells. Here, we performed multidimensional flow cytometry experiments in peripheral blood lymphocytes of 27 MS patients before and after aHSCT to address its potential influence on NK and innate-like T cells. After aHSCT, the relative frequency and absolute numbers of CD56^bright NK cells rise above pre-aHSCT levels while all studied innate-like T cell populations decrease. Hence, our data support an enhanced immune regulation by CD56^bright NK cells and the efficient reduction of proinflammatory innate-like T cells by aHSCT in MS. These observations contribute to our current understanding of the immunological effects of aHSCT in MS.

Age-dependent frequency of unconventional T cells in a healthy adult Caucasian population: a combinational study of invariant natural killer T cells, γδ T cells, and mucosa-associated invariant T cells

Unconventional T cells show distinct and unique features during antigen recognition as well as other immune responses. Their decrease in frequency is associated with various autoimmune disorders, allergy, inflammation, and cancer. The landscape frequency of the unconventional T cells altogether (iNKT, γδ T, and MAIT) is largely unestablished leading to various challenges affecting diagnosis and research in this field. In this study, we have established the age group–wise frequency of iNKT, γδ T, and MAIT cells altogether on a total of 203 healthy adult samples of the Caucasian population. The results revealed that iNKT cells were 0.095%, γδ T cells were 2.175%, and MAIT cells were 2.99% of the total T cell population. γδ and MAIT cell frequency is higher in younger age groups than elderly; however, there is no statistically significant difference in the frequency of iNKT cells. Furthermore, γδ and MAIT cells were negatively correlating with age, supporting immunosenescence, unlike iNKT cells. Our finding could be used for further age-wise investigation of various pathological conditions such as cancer and their prognosis, autoimmune diseases and their pathogenicity.

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

Objectives

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

Methods

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

Results

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

Conclusion

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

Clostridioides difficile Toxin CDT Induces Cytotoxic Responses in Human Mucosal-Associated Invariant T (MAIT) Cells

Clostridioides difficile is the major cause of antibiotic-associated colitis (CDAC) with increasing prevalence in morbidity and mortality. Severity of CDAC has been attributed to hypervirulent C. difficile strains, which in addition to toxin A and B (TcdA, TcdB) produce the binary toxin C. difficile transferase (CDT). However, the link between these toxins and host immune responses as potential drivers of immunopathology are still incompletely understood. Here, we provide first experimental evidence that C. difficile toxins efficiently activate human mucosal-associated invariant T (MAIT) cells. Among the tested toxins, CDT and more specifically, the substrate binding and pore-forming subunit CDTb provoked significant MAIT cell activation resulting in selective MAIT cell degranulation of the lytic granule components perforin and granzyme B. CDT-induced MAIT cell responses required accessory immune cells, and we suggest monocytes as a potential CDT target cell population. Within the peripheral blood mononuclear cell fraction, we found increased IL-18 levels following CDT stimulation and MAIT cell response was indeed partly dependent on this cytokine. Surprisingly, CDT-induced MAIT cell activation was found to be partially MR1-dependent, although bacterial-derived metabolite antigens were absent. However, the role of antigen presentation in this process was not analyzed here and needs to be validated in future studies. Thus, MR1-dependent induction of MAIT cell cytotoxicity might be instrumental for hypervirulent C. difficile to overcome cellular barriers and may contribute to pathophysiology of CDAC.

MAIT cell activation is reduced by direct and microbiota-mediated exposure to bisphenols

Oral uptake is the primary route of human bisphenol exposure, resulting in an exposure of the intestinal microbiota and intestine-associated immune cells. Therefore, we compared the impact of bisphenol A (BPA), bisphenol F (BPF) and bisphenol S (BPS) on (i) intestinal microbiota, (ii) microbiota-mediated immunomodulatory effects and (iii) direct effects on mucosal-associated invariant T (MAIT) cells in vitro. We acutely exposed human fecal microbiota, Bacteroides thetaiotaomicron and Escherichia coli to BPA and its analogues BPF and BPS referring to the European tolerable daily intake (TDI), i.e. 2.3 µg/mL, 28.3 µg/mL and 354.0 µg/mL. Growth and viability of E. coli was most susceptible to BPF, whereas B.thetaiotaomicron and fecal microbiota were affected by BPA > BPF > BPS. At 354.0 µg/mL bisphenols altered microbial diversity in compound-specific manner and modulated microbial metabolism, with BPA already acting on metabolism at 28.3 µg/mL. Microbiota-mediated effects on MAIT cells were observed for the individual bacteria at 354.0 µg/mL only. However, BPA and BPF directly modulated MAIT cell responses at low concentrations, whereby bisphenols at concentrations equivalent for the current TDI had no modulatory effects for microbiota or for MAIT cells. Our findings indicate that acute bisphenol exposure may alter microbial metabolism and impact directly on immune cells.

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.

Mucosal-associated invariant T (MAIT) cells, a new source of universal immune cells for chimeric antigen receptor (CAR)-cell therapy

Treatment of hematological malignancies by autologous T cells expressing a chimeric antigen receptor (CAR) is a breakthrough in the field of cancer immunotherapy. As CAR-T cells are entering advanced phases of clinical development, there is a need to develop universal, ready-to-use products using immune cells from healthy donors, to reduce time to treatment, improve response rate and finally reduce the cost of production. Mucosal-associated invariant T cells (MAIT) are unconventional T cells which recognize microbial-derived riboflavin derivatives presented by the conserved MR1 molecule and are endowed with potent effector functions. Because they are not selected by classical MHC/peptide complexes and express a semi-invariant T cell receptor, MAIT cells do not mediate alloreactivity, prompting their use as a new source of universal effector cells for allogeneic CAR-T cell therapy without the need to inactivate their endogenous TCR. We produced CD19-CAR MAIT cells as proof-of-concept allowing subsequent head-to-head comparison with currently used CD19-CAR T cells. We demonstrated their anti-tumor efficacy in vitro and their capacity to engraft without mediating GVHD in preclinical immunodeficient mouse models. Universal, off-the-shelf CAR-MAIT cells could provide a suitable alternative to current autologous CAR-T cells to treat patients regardless of HLA disparity, without production delay, enabling a cost-effective manufacturing model for large-scale clinical application.

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.

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.