Peripheral Blood Mucosal-associated Invariant T Cells Research Articles

Peripheral blood MR1 tetramer-positive mucosal-associated invariant T-cell function is modulated by mammalian target of rapamycin complex 1 in patients with active tuberculosis.

Tuberculosis (TB) is still an urgent global public health problem. Notably, mucosal-associated invariant T (MAIT) cells play an important role in early anti-TB immune response. Targeted control of them may be an effective method to improve vaccine efficacy and TB treatment. However, the biology and signal regulation mechanisms of MAIT cells in TB patients are still poorly understood. Previous studies have been limited by the lack of reagents to specifically identify MAIT cells. In addition, the use of alternative markers may subsume non-MAIT cell into MAIT cell populations. In this study, the human MR1 tetramer which can specifically identify MAIT cells was used to further explore the effect and mechanism of MAIT cells in anti-TB immune response. Our results showed that the tetramer+ MAIT cells in peripheral blood of TB patients were mainly CD8+ or CD4-CD8- cells, and very few were CD4+ cells. After BCG infecting autologous antigen-presenting cells, MAIT cells in patients produced significantly higher levels of cytokines, lysis and proliferation compared with healthy controls. After suppression of mTORC1 by the mTORC1-specific inhibitor rapamycin, the immune response of MAIT cells in patients was significantly reduced. This study demonstrates that peripheral blood tetramer+ MAIT cells from TB patients have significant anti-TB immune effect, which is regulated by mTORC1. This could provide ideas and potential therapeutic targets for the development of novel anti-TB immunotherapy.

Evaluation of mucosal-associated invariant T-cells as a potential biomarker to predict infection risk in liver cirrhosis.

Infection is a serious complication in patients with cirrhosis. Mucosal-associated invariant T (MAIT) cells are involved in the immune defense against infections and known to be impaired in several chronic conditions, including cirrhosis. Here, we evaluated if MAIT cell levels in peripheral blood are associated with risk of bacterial infections in patients with cirrhosis. Patients with cirrhosis seen at the Karolinska University Hospital, Stockholm, Sweden, between 2016 and 2019 were included. Levels of MAIT cells in peripheral blood were determined using flow cytometry. Baseline and follow-up data after at least two years of follow-up were collected by chart review for the primary outcome (bacterial infection) and secondary outcomes (decompensation and death). Competing risk and Cox regression were performed. We included 106 patients with cirrhosis. The median MAIT cells fraction in the circulation was 0.8% in cirrhosis compared to 6.1% in healthy controls. In contrast to our hypothesis, we found an association in the adjusted analysis between relatively preserved MAIT cell levels, and a slightly higher risk to develop bacterial infections (adjusted subdistribution hazard ratio (aSHR) 1.15 (95%CI = 1.01-1.31). However, MAIT cell levels were not associated with the risk of hepatic decompensation (aSHR 1.19 (95%CI = 0.91-1.56)) nor with death (adjusted hazard ratio 1.10 (95%CI = 0.97-1.22)). Relatively preserved MAIT cell levels in blood of patients with cirrhosis were associated with a somewhat higher risk of bacterial infections. The clinical relevance of this might not be strong. MAIT cells might however be an interesting biomarker to explore in future studies.

Phenotype and function of MAIT cells in patients with alveolar echinococcosis.

Mucosal-associated invariant T (MAIT) cells are a subpopulation of unconventional T cells widely involved in chronic liver diseases. However, the potential role and regulating factors of MAIT cells in alveolar echinococcosis (AE), a zoonotic parasitic disease by Echinococcus multilocularis (E. multilocularis) larvae chronically parasitizing liver organs, has not yet been studied. Blood samples (n=29) and liver specimens (n=10) from AE patients were enrolled. The frequency, phenotype, and function of MAIT cells in peripheral blood and liver tissues of AE patients were detected by flow cytometry. The morphology and fibrosis of liver tissue were examined by histopathology and immunohistochemistry. The correlation between peripheral MAIT cell frequency and serologic markers was assessed by collecting clinicopathologic characteristics of AE patients. And the effect of in vitro stimulation with E. multilocularis antigen (Emp) on MAIT cells. In this study, MAIT cells are decreased in peripheral blood and increased in the close-to-lesion liver tissues, especially in areas of fibrosis. Circulating MAIT exhibited activation and exhaustion phenotypes, and intrahepatic MAIT cells showed increased activation phenotypes with increased IFN-γ and IL-17A, and high expression of CXCR5 chemokine receptor. Furthermore, the frequency of circulating MAIT cells was correlated with the size of the lesions and liver function in patients with AE. After excision of the lesion site, circulating MAIT cells returned to normal levels, and the serum cytokines IL-8, IL-12, and IL-18, associated with MAIT cell activation and apoptosis, were altered. Our results demonstrate the status of MAIT cell distribution, functional phenotype, and migration in peripheral blood and tissues of AE patients, highlighting their potential as biomarkers and therapeutic targets.

MAIT Cell Proliferation Is a Potential Early Diagnosis Marker for Blood Stream Infection in AML Patients

Objectives To explore the role and dynamics of MAIT (Mucosal-associated invariant T) cells in blood stream infection of AML (Acute myeloid leukemia) patients. Methods Healthy adults and hospitalized adult AML patients that with blood stream infection, non-infection, or in completely remission (CR) were prospectively enrolled. Characteristics of MAIT cells in peripheral blood of enrolled subjects were analyzed by flow cytometry. Results Both gram negative and gram positive bacteria could induce sepsis in AML patients. MAIT cells significantly increased in AML patients with blood stream infection comparing with non-infected AML patients, healthy subjects and AML patients in completely remission. Increased MAIT cells were mainly consists of CD8+ T cells and were activated. There is no significant difference in the recruitment and death of MAIT cells, while their proliferation is significantly increased. The proliferation of MAIT cells occurs before traditional infection indicators such as CRP, PCT, endotoxin in plasma and fever. MAIT cells in neutropenic blood stream infected mice were increased as we observed in AML patients. Conclusions MAIT cells increased without bacterial specificity, and were activated and proliferated in AML patients with blood stream infection that caused by various bacteria. They may enhance host defense and play a vital role in fighting against pathogenic microbial infections. Proliferation of MAIT cells can be applied as a monitoring indicator for blood stream infection in AML patients, which helps to reduce the mortality rate of them.

MAIT cells altered phenotype and cytotoxicity in lupus patients are linked to renal disease severity and outcome.

Systemic lupus erythematosus (SLE) is an autoimmune disease in which circulating immune complexes can cause different types of glomerulonephritis, according to immune deposits and to the type of glomerular cell injury. Proliferative lesions represent the most severe form of lupus nephritis (LN) and often lead to kidney failure and death. Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T cells that recognize microbial-derived ligands from the riboflavin synthesis pathway. Although abundant in peripheral blood, MAIT cells are enriched in mucosal and inflamed tissues. While previous studies have reported concordant results concerning lower MAIT cell frequencies in the blood of SLE patients, no information is known about MAIT cell function and LN severity and outcome. In the current study, we analyzed the baseline phenotype and function of peripheral blood MAIT cells by flow cytometry in 26 patients with LN and in a control group of 16 healthy individuals. We observe that MAIT cell frequencies are markedly reduced in blood of LN patients. MAIT cells from patients have an altered phenotype in terms of migration, proliferation and differentiation markers, notably in most severe forms of LN. Frequencies of PMA/ionomycin stimulated MAIT cells secreting effector molecules, such as proinflammatory IL-17 and cytotoxic protein granzyme B, are higher in LN patients. Patients undergoing a complete renal remission after immunosuppressive therapy had higher MAIT cell frequency, lower expression of proliferation marker Ki-67 and granzyme B (GzB) at inclusion. Remarkably, GzB production defines a predictive model for complete remission. We report here that blood MAIT cells display proinflammatory and cytotoxic function in severe lupus nephritis which may play a pathogenesis role, but without association with systemic lupus activity. Finally, low cytotoxic profile of MAIT cells may represent a promising prognostic factor of lupus nephritis remission one year after induction therapy.

Effects of interleukin-23 on the activation of mucosal-associated invariant T cells from oral lichen planus.

Oral lichen planus (OLP) is a T cell-mediated inflammatory condition in the oral cavity. Mucosal-associated invariant T (MAIT) cells are gaining more relevance in immune diseases because they can be activated by cytokines without T cell receptor stimulation. Herein, we tested the effect of interleukin-23 (IL-23) on the activation status of OLP MAIT cells. Peripheral blood mononuclear cells (PBMCs) isolated from OLP patients were stimulated by IL-23 in the absence or presence of phorbol myristate acetate (PMA) and ionomycin. The activation status of MAIT cells was analyzed by flow cytometry after staining for CD3, CD4, CD8, CD161, TCR Vα7.2, and CD69. The fraction of MAIT cells in OLP peripheral blood was approximately 0.38% to 3.97%, and CD8+ subpopulations overwhelmed CD4+ cells. The mean percentages of OLP MAIT cells in PBMCs and CD8+MAIT cells in MAIT cells were approximately 40%. PMA and ionomycin significantly increased CD69 expression on OLP T cells, MAIT cells, and CD8+MAIT cells. Cells with enhanced activation had different responsiveness to exogenous IL-23, showing increased CD69 expression on OLP T cells, decreased CD69 on OLP CD8+MAIT cells, and no significant change on OLP MAIT cells. IL-23 showed different effects on the activation status of OLP MAIT cells and CD8+MAIT cells.

Clinicopathological and predictive value of MAIT cells in non-small cell lung cancer for immunotherapy

BackgroundImmune-checkpoint inhibitors (ICIs) remain ineffective in a large group of non-small cell lung cancer (NSCLC) patients. Mucosal-associated invariant T (MAIT) cells, a population of unconventional innate-like T lymphocytes abundant in...

Characterization of Blood Mucosal-Associated Invariant T Cells in Patients With Axial Spondyloarthritis and of Resident Mucosal-Associated Invariant T Cells From the Axial Entheses of Non-Axial Spondyloarthritis Control Patients.

The importance of interleukin-17A (IL-17A) in the pathogenesis of axial spondyloarthritis (SpA) has been demonstrated by the success of IL-17A blockade. However, the nature of the cell populations that produce this important proinflammatory cytokine remains poorly defined. We undertook this study to characterize the major IL-17A-producing blood cell populations in the peripheral blood of patients with axial SpA, with a focus on mucosal-associated invariant T (MAIT) cells, a population known to be capable of producing IL-17. We evaluated IL-17A production from 5 sorted peripheral blood cell populations, namely, MAIT cells, γδ T cells, CD4+ T cells, CD8+ T cells, and neutrophils, before and after stimulation with phorbol myristate acetate, the calcium ionophore A23187, and β-1,3-glucan. Expression of IL-17A transcripts and protein were determined using nCounter and ultra-sensitive Simoa technology, respectively. MAIT cells from the axial entheses of non-axial SpA control patients (n=5) were further characterized using flow cytometric immunophenotyping and quantitative polymerase chain reaction, and the production of IL-17 was assessed following stimulation. On a per-cell basis, MAIT cells from peripheral blood produced the most IL-17A compared to CD4+ T cells (P < 0.01), CD8+ T cells (P < 0.0001), and γδ T cells (P < 0.0001). IL-17A was not produced by neutrophils. Gene expression analysis also revealed significantly higher expression of IL17A and IL23R in MAIT cells. Stimulation of peripheral blood MAIT cells with anti-CD3/CD28 and IL-7 and/or IL-18 induced strong expression of IL17F. MAIT cells were present in the normal, unaffected entheses of control patients who did not have axial SpA and showed elevated AHR, JAK1, STAT4, and TGFB1 transcript expression with inducible IL-17A protein. IL-18 protein expression was evident in spinal enthesis digests. Both peripheral blood MAIT cells and resident MAIT cells in normal axial entheses contribute to the production of IL-17 and may play important roles in the pathogenesis of axial SpA.

The role of mucosal-associated invariant T cells in visceral leishmaniasis.

Mucosal-associated invariant T (MAIT) cells are restricted by MR1 and are known to protect against bacterial and viral infections. Our understanding of the role of MAIT cells in parasitic infections, such as visceral leishmaniasis (VL) caused by protozoan parasites of Leishmania donovani, is limited. This study showed that in response to L. infantum, human peripheral blood MAIT cells from children with leishmaniasis produced TNF and IFN-γ in an MR1-dependent manner. The overall frequency of MAIT cells was inversely correlated with alanine aminotransferase levels, a specific marker of liver damage strongly associated with severe hepatic involvement in VL. In addition, there was a positive correlation between total protein levels and the frequency of IL-17A+ CD8+ MAIT cells, whereby reduced total protein levels are a marker of liver and kidney damage. Furthermore, the frequencies of IFN-γ+ and IL-10+ MAIT cells were inversely correlated with hemoglobin levels, a marker of severe anemia. In asymptomatic individuals and VL patients after treatment, MAIT cells also produced IL-17A, a cytokine signature associated with resistance to visceral leishmaniasis, suggesting that MAIT cells play important role in protecting against VL. In summary, these results broaden our understanding of MAIT-cell immunity to include protection against parasitic infections, with implications for MAIT-cell-based therapeutics and vaccines. At last, this study paves the way for the investigation of putative MAIT cell antigens that could exist in the context of Leishmania infection.

CD8 coreceptor engagement of MR1 enhances antigen responsiveness by human MAIT and other MR1-reactive T cells.

Mucosal-associated invariant T (MAIT) cells detect microbial infection via recognition of riboflavin-based antigens presented by the major histocompatibility complex class I (MHC-I)-related protein 1 (MR1). Most MAIT cells in human peripheral blood express CD8αα or CD8αβ coreceptors, and the binding site for CD8 on MHC-I molecules is relatively conserved in MR1. Yet, there is no direct evidence of CD8 interacting with MR1 or the functional consequences thereof. Similarly, the role of CD8αα in lymphocyte function remains ill-defined. Here, using newly developed MR1 tetramers, mutated at the CD8 binding site, and by determining the crystal structure of MR1-CD8αα, we show that CD8 engaged MR1, analogous to how it engages MHC-I molecules. CD8αα and CD8αβ enhanced MR1 binding and cytokine production by MAIT cells. Moreover, the CD8-MR1 interaction was critical for the recognition of folate-derived antigens by other MR1-reactive T cells. Together, our findings suggest that both CD8αα and CD8αβ act as functional coreceptors for MAIT and other MR1-reactive T cells.

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.

Participation of Increased Circulating MAIT Cells in Lung Cancer: a Pilot Study

Mucosal-associated invariant T (MAIT) cells are a subset of innate-like T cells that regulate the immune response via rapidly releasing inflammatory factors, including during progression of some tumors. However, the immunological role of MAIT cells is still unclear in lung cancer. We measured percentage, partial function, and clinical correlation of circulating MAIT cells from lung cancer patients through flow cytometry. Lung cancer patients displayed a high concentration of CD4+, CD8+, and activated CD38+CD8+MAIT cells, and a decrease of PD1+ double negative (DN) MAIT cells in peripheral blood. Meanwhile, increased levels of interferon-γ, interleukin (IL)-6, and 8 were examined in the serum of lung cancer patients. Importantly, we discovered a statistically positive association between accumulation of CD38+CD8+MAIT cells and reduced progression-free survival of lung cancer patients. While preliminary, the altering frequency of MAIT cells might be involved in dysfunctional immune response in lung cancer.

IL-17A–producing sinonasal MAIT cells in patients with chronic rhinosinusitis with nasal polyps

Diverse immune cells contribute to the pathogenesis of chronic rhinosinusitis (CRS), an inflammatory disease of the nasal cavity and paranasal sinuses. However, whether mucosal-associated invariant T (MAIT) cells are present in human sinonasal tissues remains unclear. Furthermore, the characteristics of sinonasal MAIT cells have not been studied in patients with CRS. We investigated the phenotype, function, and clinical implications of MAIT cells in patients with CRS. Peripheral blood and sinonasal tissue were obtained from patients with CRS with (CRSwNP) or without nasal polyps (CRSsNP) and healthy controls. MAIT cells were analyzed by flow cytometry. We found that MAIT cells are present in human sinonasal tissues from healthy controls and patients with CRS. The sinonasal MAIT cell population, but not peripheral blood MAIT cells, from patients with CRSsNP, noneosinophilic CRSwNP (NE-NP), or eosinophilic CRSwNP (E-NP) had a significantly higher frequency of activated cells marked by CD38 expression. In functional analysis, the sinonasal MAIT cell population from NE-NP and E-NP had a significantly higher frequency of IL-17A+ cells but lower frequency of IFN-γ+ or TNF+ cells than control sinonasal tissues. Furthermore, CD38 expression and IL-17A production by sinonasal MAIT cells significantly correlated with disease extent evaluated by the Lund-Mackay computed tomography score in patients with E-NP. Sinonasal MAIT cells exhibit an activated phenotype and produce higher levels of IL-17A in patients with CRSwNP. These alterations are associated with the extent of disease in patients with E-NP.

MR1-Restricted MAIT Cells From The Human Lung Mucosal Surface Have Distinct Phenotypic, Functional, and Transcriptomic Features That Are Preserved in HIV Infection.

Mucosal associated invariant T (MAIT) cells are a class of innate-like T cells that utilize a semi-invariant αβ T cell receptor to recognize small molecule ligands produced by bacteria and fungi. Despite growing evidence that immune cells at mucosal surfaces are often phenotypically and functionally distinct from those in the peripheral circulation, knowledge about the characteristics of MAIT cells at the lung mucosal surface, the site of exposure to respiratory pathogens, is limited. HIV infection has been shown to have a profound effect on the number and function of MAIT cells in the peripheral blood, but its effect on lung mucosal MAIT cells is unknown. We examined the phenotypic, functional, and transcriptomic features of major histocompatibility complex (MHC) class I-related (MR1)-restricted MAIT cells from the peripheral blood and bronchoalveolar compartments of otherwise healthy individuals with latent Mycobacterium tuberculosis (Mtb) infection who were either HIV uninfected or HIV infected. Peripheral blood MAIT cells consistently co-expressed typical MAIT cell surface markers CD161 and CD26 in HIV-negative individuals, while paired bronchoalveolar MAIT cells displayed heterogenous expression of these markers. Bronchoalveolar MAIT cells produced lower levels of pro-inflammatory cytokine IFN-γ and expressed higher levels of co-inhibitory markers PD-1 and TIM-3 than peripheral MAIT cells. HIV infection resulted in decreased frequencies and pro-inflammatory function of peripheral blood MAIT cells, while in the bronchoalveolar compartment MAIT cell frequency was decreased but phenotype and function were not significantly altered. Single-cell transcriptomic analysis demonstrated greater heterogeneity among bronchoalveolar compared to peripheral blood MAIT cells and suggested a distinct subset in the bronchoalveolar compartment. The transcriptional features of this bronchoalveolar subset were associated with MAIT cell tissue repair functions. In summary, we found previously undescribed phenotypic and transcriptional heterogeneity of bronchoalveolar MAIT cells in HIV-negative people. In HIV infection, we found numeric depletion of MAIT cells in both anatomical compartments but preservation of the novel phenotypic and transcriptional features of bronchoalveolar MAIT cells.

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.

Decrease of peripheral blood mucosal\u2010associated invariant T cells and impaired serum Granzyme-B production in patients with gastric cancer

Mucosal-associated invariant T (MAIT) cells are an invariant T cell subset, which have been reported to play an antimicrobial role in infectious diseases. However, little is known about it in malignant diseases and tumors, especially in gastric cancer (GC). So in this study, we aim to examine the frequency, phenotype, partial functional capacity and clinical relevance of this cells from GC patients’ peripheral blood by flow cytometry. It was shown that the frequency of peripheral blood MAIT cells was negatively correlated with their increasing age in healthy adults. Importantly, comparing to the healthy controls (HC), the frequency and the absolute number of MAIT cells from GC patients’ peripheral blood with or without chemotherapy were both significantly lower than those. For the phenotype, the proportion of CD4−MAIT cell subset in GC patients without chemotherapy was lower than in HC, but higher than in GC patients with chemotherapy. Whereas, the proportion of CD4−CD8+MAIT cell subset in GC patients without chemotherapy was significantly lower than that in HC. Finally, the level of Granzyme-B (GrB), a molecule associated with MAIT cells was markedly lower in GC patients. But the correlation between the serum levels of GC-associated tumor antigens and the percentages of MAIT cells in GC patients was not observed. In conclusion, our study shows the decreased frequency, changed phenotypes and partial potentially impaired function of MAIT cells in GC patients, suggesting a possible MAIT cell-based immunological surveillance of GC.

Methods for High-Dimensional Flow Cytometry Analysis of Human MAIT Cells in Tissues and Peripheral Blood.

Mucosal-associated invariant T (MAIT) cells can be found throughout the human body, in peripheral blood, at mucosal sites, and, among other organs, in the liver. As unconventional T cells, MAIT cells have the capacity to readily respond to bacterial infections and are also engaged during anti-viral responses. To thoroughly investigate the MAIT cell phenotype and function in such conditions, multi-color flow cytometry is an appropriate and powerful tool. Yet, the recent rapid technological development within this methodology, with generation of highly complex data, has increased the need for downstream dimensionality reducing methods to fully interpret obtained results. Among such methods, stochastic neighbor embedding (SNE) analysis stands out as it provides intuitive low-dimensional representations of complex data. Here, we describe techniques and workflow for high-dimensional state-of-the-art investigation and analysis of human MAIT cells from blood and peripheral tissues.

Isolation and Characterization of MAIT Cells from Human Tissue Biopsies.

Human mucosal-associated invariant T (MAIT) cells are unconventional T cells highly enriched in tissues exposed to microbial antigens including the oral, gastrointestinal and genital mucosae, liver, and lung. Here we describe a protocol for isolation and characterization of peripheral blood and tissue-infiltrating MAIT cells by using multicolor flow cytometry. This technology allows the analysis of multiple markers in a single sample at a single-cell level. Study of human samples requires particular care since the sample amount is often limited. We present a protocol optimized for the isolation and characterization of human MAIT cells and the identification of MAIT cell populations detected by simultaneous expression of multiple activation markers and inhibitory receptors.

MAIT Cells Are Major Contributors to the Cytokine Response in Group A Streptococcal Toxic Shock Syndrome

Streptococcal toxic shock syndrome (STSS) is a rapidly progressing, life-threatening, systemic reaction to invasive infection caused by group A streptococci (GAS). GAS superantigens are key mediators of STSS through their potent activation of T cells leading to a cytokine storm and consequently vascular leakage, shock, and multiorgan failure. Mucosal-associated invariant T (MAIT) cells recognize MR1-presented antigens derived from microbial riboflavin biosynthesis and mount protective innate-like immune responses against the microbes producing such metabolites. GAS lack de novo riboflavin synthesis, and the role of MAIT cells in STSS has therefore so far been overlooked. Here we have conducted a comprehensive analysis of human MAIT cell responses to GAS, aiming to understand the contribution of MAIT cells to the pathogenesis of STSS. We show that MAIT cells are strongly activated and represent the major T cell source of IFNγ and TNF in the early stages of response to GAS. MAIT cell activation is biphasic with a rapid TCR Vβ2-specific, TNF-dominated response to superantigens and a later IL-12- and IL-18-dependent, IFNγ-dominated response to both bacterial cells and secreted factors. Depletion of MAIT cells from PBMC resulted in decreased total production of IFNγ, IL-1β, IL-2, and TNFβ. Peripheral blood MAIT cells in patients with STSS expressed elevated levels of the activation markers CD69, CD25, CD38, and HLA-DR during the acute compared with the convalescent phase. Our data demonstrate that MAIT cells are major contributors to the early cytokine response to GAS, and are therefore likely to contribute to the pathological cytokine storm underlying STSS.

Immune Profile of Mucosal-Associated Invariant T Cells in Chronic Viral Hepatitis: A Systematic Review and Meta-Analysis

Context: Chronic viral hepatitis (CVH), a world health problem, is the leading cause of hepatocellular carcinoma (HCC). Host immunity plays a critical role in viral clearance, development and progression of the disease. Mucosal-associated invariant T (MAIT) cells represent an abundant form of innate T cells, which plays an essential role in infectious diseases with releasing cytokine, lysing target cells, and shaping adaptive immunity. Objectives: Although numerous studies showed the immune profiles of MAIT cells in CVH, the results are inconsistent. Thus, we performed a meta-analysis to analyze the immune profiles of MAIT cells in CVH. Evidence Acquisition: PubMed, Embase, the Cochrane Library, Google Scholar, ScienceDirect, Web of Science, and the China National Knowledge Infrastructure (CNKI) were searched and 10 studies were included. Data from each study were compared using the standardized mean difference (SMD) with 95% confidence interval (CI). The quality assessment of studies and publication bias were evaluated by Newcastle-Ottawa scale and Begg’s and Egger’s tests, respectively, and a P value of < 0.05 was considered statistically significant. Results: Meta-analysis of the enrolled studies showed that the frequency of MAIT cells was significantly lower in patients with CVH as compared to healthy controls (SMD = -0.90, 95% CI: -1.32 to -0.48, P < 0.0001). In addition, MAIT cells displayed an activated and exhausted phenotype (CD38: SMD = 0.75, 95% CI: 0.38 to 1.13, P < 0.0001; HLA-DR: SMD = 1.42, 95% CI: 1.02 to 1.83, P < 0.00001; PD-1: SMD = 0.69, 95% CI: 0.13 to 1.26, P = 0.02; CTLA-4: SMD = 0.97, 95% CI: 0.40 to 1.54, P = 0.0008) but not impaired function during CVH (IFN-γ: SMD = 0.04, 95% CI: -0.28 to 0.37, P = 0.79; TNF-α: SMD = -0.80, 95% CI: -1.77 to 0.16, P = 0.10; Granzyme B: SMD = -0.14, 95% CI: -0.58 to 0.29, P = 0.53; Perforin: SMD = -0.27, 95% CI: -0.87 to 0.33, P = 0.38). Conclusions: In CVH patients, MAIT cells are significantly depleted in the peripheral bloodstream and displayed an activated and exhausted phenotype; however, the reduction of peripheral blood MAIT cells accompanied by activated and exhausted phenotypes may not impair the cytolytic function and cytokine production of these cells.