Activated and Exhausted MAIT Cells Foster Disease Progression and Indicate Poor Outcome in Hepatocellular Carcinoma

Spatiotemporal Immune Landscape of Colorectal Cancer Liver Metastasis at Single-Cell Level

Liver metastasis, the leading cause of colorectal cancer mortality, exhibits a highly heterogeneous and suppressive immune microenvironment. Here, we sequenced 97 matched samples by using single-cell RNA-seq and Spatial Transcriptomics. Strikingly, metastatic microenvironment underwent remarkable spatial reprogramming of immunosuppressive cells such as MRC1+ CCL18+ M2-like macrophages. We further developed scMetabolism, a computational pipeline for quantifying single-cell metabolism, and observed that those macrophages harbored enhanced metabolic activity. Interestingly, neoadjuvant chemotherapy could block this status and restore the antitumor immune balance in responsive patients, while the non-responsive patients deteriorated into a more suppressive one. Our work described the immune evolution of metastasis and uncovered the black box of how tumors respond to neoadjuvant chemotherapy.

Persistent deficiency of mucosa-associated invariant T (MAIT) cells during alcohol-related liver disease

BACKGROUND

Alcohol-related liver disease (ALD) is a major cause of chronic liver diseases. Inflammatory response is a basic pathological feature of ALD. Mucosal-associated invariant T(MAIT) cells are a novel population of innate immune cells, which may be depleted in various inflammatory diseases. However, the changes of MAIT cell in ALD remains unclear.

RESULTS

In this study, the levels of MAIT cell were significantly decreased in patients with alcoholic fatty liver disease, alcoholic cirrhosis, and mixed cirrhosis (alcoholic + viral). Furthermore, the reduction of circulating MAIT cells was correlated with liver function in patients with cirrhosis. Functional changes among circulating MAIT cells in patients with alcoholic cirrhosis, including increased production of IL-17A and perforin, and reduced production of TNF-α. Plasma cytokine and chemokine levels were quantified using multiple immunoassays and ELISA. Serum levels of chemokine IL-8 were correlated with MAIT cell frequency in patients with alcoholic cirrhosis. Moreover, no differences were observed in the expression of CCR6, CXCR6, and PD-1 in circulating MAIT cells of patients with alcoholic cirrhosis. The MAIT cells in patients with alcoholic cirrhosis were prone to apoptosis, which was promoted by IL-12, IL-18, and IL-8.

CONCLUSIONS

Our findings indicate persistent MAIT cell loss during alcohol-related liver disease and suggest that MAIT cells can be promising indicator and therapeutic targets in ALD.

Regulation and Functions of Protumoral Unconventional T Cells in Solid Tumors

The vast majority of studies on T cell biology in tumor immunity have focused on peptide-reactive conventional T cells that are restricted to polymorphic major histocompatibility complex molecules. However, emerging evidence indicated that unconventional T cells, including γδ T cells, natural killer T (NKT) cells and mucosal-associated invariant T (MAIT) cells are also involved in tumor immunity. Unconventional T cells span the innate-adaptive continuum and possess the unique ability to rapidly react to nonpeptide antigens via their conserved T cell receptors (TCRs) and/or to activating cytokines to orchestrate many aspects of the immune response. Since unconventional T cell lineages comprise discrete functional subsets, they can mediate both anti- and protumoral activities. Here, we review the current understanding of the functions and regulatory mechanisms of protumoral unconventional T cell subsets in the tumor environment. We also discuss the therapeutic potential of these deleterious subsets in solid cancers and why further feasibility studies are warranted.

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

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

Exhaustion in tumor-infiltrating Mucosal-Associated Invariant T (MAIT) cells from colon cancer patients

Mucosal-associated invariant T (MAIT) cells are unconventional T cells recognizing microbial metabolites, presented by the invariant MR1 protein. Upon activation, MAIT cells rapidly secrete cytokines and exert cytotoxic functions, and may thus be highly relevant also in tumor immunity. MAIT cells accumulate in colon tumors, but in contrast to other cytotoxic T cell subsets, their presence in tumors has been associated with worse patient outcome. Here we investigated if exhaustion may contribute to reduced anti-tumor immunity by MAIT cells. Freshly isolated lymphocytes from colon tumors, unaffected tissue and blood from the same patients were analyzed by flow cytometry to detect MAIT cells with effector functions that are relevant for tumor immunity, and their expression of inhibitory receptors and other exhaustion markers. Our studies show that MAIT cells with a PD-1^highTim-3⁺CD39⁺ terminally exhausted phenotype and an increased proliferation accumulate in colon tumors. The exhausted MAIT cells have reduced polyfunctionality with regard to production of important anti-tumor effector molecules, and blocking antibodies to PD-1 partly improved activation of tumor-infiltrating MAIT cells in vitro. We conclude that the tumor microenvironment leads to exhaustion not only of conventional T cells, but also MAIT cells, and that checkpoint blockade therapy may be useful also to reinvigorate tumor-infiltrating MAIT cells.

MicroRNA-155 Regulates MAIT1 and MAIT17 Cell Differentiation

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that develop in the thymus through three maturation stages to acquire effector function and differentiate into MAIT1 (T-bet⁺) and MAIT17 (RORγt⁺) subsets. Upon activation, MAIT cells release IFN-γ and IL-17, which modulate a broad spectrum of diseases. Recent studies indicate defective MAIT cell development in microRNA deficient mice, however, few individual miRNAs have been identified to regulate MAIT cells. MicroRNA-155 (miR-155) is a key regulator of numerous cellular processes that affect some immune cell development, but its role in MAIT cell development remains unclear. To address whether miR-155 is required for MAIT cell development, we performed gain-of-function and loss-of-function studies. We first generated a CD4Cre.miR-155 knock-in mouse model, in which miR-155 is over-expressed in the T cell lineage. We found that overexpression of miR-155 significantly reduced numbers and frequencies of MAIT cells in all immune organs and lungs and blocked thymic MAIT cell maturation through downregulating PLZF expression. Strikingly, upregulated miR-155 promoted MAIT1 differentiation and blocked MAIT17 differentiation, and timely inducible expression of miR-155 functionally inhibited peripheral MAIT cells secreting IL-17. miR-155 overexpression also increased CD4^–CD8⁺ subset and decreased CD4^–CD8^– subset of MAIT cells. We further analyzed MAIT cells in conventional miR-155 knockout mice and found that lack of miR-155 also promoted MAIT1 differentiation and blocked MAIT17 differentiation but without alteration of their overall frequency, maturation and function. Overall, our results indicate that adequate miR-155 expression is required for normal MAIT1 and MAIT17 cell development and function.

Mucosal Associated Invariant T Cells in Cancer-Friend or Foe?

Mucosal associated invariant T (MAIT) cells are a population of unconventional T cells which can bridge the innate and adaptive immune systems. Well-described roles for MAIT cells include host protection against invading bacteria, fungi and viruses. Upon activation, MAIT cells become prolific effector cells, capable of producing a range of cytokines and lytic molecules. In addition to their anti-microbial role, MAIT cells have been implicated in immune responses to cancer, with opposing beneficial and pathogenic roles reported. On the one hand, MAIT cells can home to the site of the tumour in many human cancers and can produce anti-tumour molecules. On the other, MAIT cells can display defective phenotypes in certain cancers and produce pro-tumour molecules. In this review, we discuss the current literature on the diverse roles for MAIT cells in cancer, outlining their frequencies, functions and associations with N staging and prognosis. We also discuss potential mechanisms underpinning cancer-related alterations in MAIT cells and highlight therapeutic approaches to harness or target MAIT cells in cancer.

MAIT Cells: Partners or Enemies in Cancer Immunotherapy?

Simple Summary

Unconventional T cells have recently come under intense scrutiny because of their innate-like effector functions and unique antigen specificity, suggesting their potential importance in antitumor immunity. MAIT cells, one such population of unconventional T cell, have been shown to significantly influence bacterial infections, parasitic and fungal infections, viral infections, autoimmune and other inflammatory diseases, and, as discussed thoroughly in this review, various cancers. This review aims to merge accumulating evidence, tease apart the complexities of MAIT cell biology in different malignancies, and discuss how these may impact clinical outcomes. While it is clear that MAIT cells can impact the tumor microenvironment, the nature of these interactions varies depending on the type of cancer, subset of MAIT cell, patient demographic, microbiome composition, and the type of therapy administered. This review examines the impact of these variables on MAIT cells and discusses outstanding questions within the field.

Abstract

A recent boom in mucosal-associated invariant T (MAIT) cell research has identified relationships between MAIT cell abundance, function, and clinical outcomes in various malignancies. As they express a variety of immune checkpoint receptors and ligands, and possess strong cytotoxic functions, MAIT cells are an attractive new subject in the field of tumor immunology. MAIT cells are a class of innate-like T cells that express a semi-invariant T cell antigen receptor (TCR) that recognizes microbially derived non-peptide antigens presented by the non-polymorphic MHC class-1 like molecule, MR1. In this review, we outline the current (and often contradictory) evidence exploring MAIT cell biology and how MAIT cells impact clinical outcomes in different human cancers, as well as what role they may have in cancer immunotherapy.

A novel panel based on immune infiltration and tumor mutational burden for prognostic prediction in hepatocellular carcinoma

Tumor mutation burden (TMB) has been associated with prognosis in various malignancies, but it has yet to be elucidated in hepatocellular carcinoma (HCC). We aimed to investigate the prognostic effects of TMB and its relationship with immune infiltration through multiple databases and whole-exome sequencing, so as to establish a panel model capable of predicting prognosis. The results demonstrated that the prognosis of high TMB group was worse than that of low TMB group, with a cutoff TMB value of 4.9. Enrichment analysis demonstrated that differentially expressed genes were mainly related to T cell activation, cell membrane localization and matrix composition. Tumor immune infiltration analysis revealed the infiltrations of Th2, Th17, and Tgd were up-regulated in high TMB group, while those of Tr1, MAIT, and DC were up-regulated in low TMB group. TMB-Infiltration model fit well with the actual survival observation, with a C-index 0.785 (0.700-0.870), which verified in ICGC-LIRI-JP was 0.650 (0.553-0.747). Additionally, these screened immune genes performed well in predicting tumor vascular invasion with a C-index of 0.847 (0.778-0.916). Overall, these results indicated that patients with high mutation frequency of immune-related genes and high TMB were prone to have worse prognosis and relapse after radical treatment.

Understanding tumour cell heterogeneity and its implication for immunotherapy in liver cancer using single-cell analysis

Over the last decade, precision medicine and immunotherapeutic approaches have become increasingly popular in oncology. Early clinical trials reported promising results, but response rates in phase III clinical trials have been suboptimal. Knowledge gained from subsequent translational studies indicates the importance of targeting the tumour microenvironment to overcome resistance to immunotherapy. In this era of precision medicine, it is crucial to consider inter- as well as intratumoural heterogeneity. Single-cell analysis is a cutting-edge technology that enables us to better define the tumour cell community and to identify potential targets for immunotherapy or combination treatments. This review focuses on single-cell analysis in the context of immunotherapy in liver cancer, including the rationale behind studying hepatocellular carcinoma biology at a single-cell level. Single-cell technologies have the potential to revolutionise our understanding of resistance mechanisms and to guide drug discovery efforts, leading to further advances in personalised medicine.

Augmentation of the Riboflavin-Biosynthetic Pathway Enhances Mucosa-Associated Invariant T (MAIT) Cell Activation and Diminishes Mycobacterium tuberculosis Virulence

Mucosa-associated invariant T (MAIT) cells play a critical role in antimicrobial defense. Despite increased understanding of their mycobacterial ligands and the clinical association of MAIT cells with tuberculosis (TB), their function in protection against Mycobacterium tuberculosis infection remains unclear. Here, we show that overexpressing key genes of the riboflavin-biosynthetic pathway potentiates MAIT cell activation and results in attenuation of M. tuberculosis virulence in vivo. Further, we observed greater control of M. tuberculosis infection in MAIT^hi CAST/EiJ mice than in MAIT^lo C57BL/6J mice, highlighting the protective role of MAIT cells against TB. We also endogenously adjuvanted Mycobacterium bovis BCG with MR1 ligands via overexpression of the lumazine synthase gene ribH and evaluated its protective efficacy in the mouse model of M. tuberculosis infection. Altogether, our findings demonstrate that MAIT cells confer host protection against TB and that overexpression of genes in the riboflavin-biosynthetic pathway attenuates M. tuberculosis virulence. Enhancing MAIT cell-mediated immunity may also offer a novel approach toward improved vaccines against TB. IMPORTANCE Mucosa-associated invariant T (MAIT) cells are an important subset of innate lymphocytes that recognize microbial ligands derived from the riboflavin biosynthesis pathway and mediate antimicrobial immune responses. Modulated MAIT cell responses have been noted in different forms of tuberculosis. However, it has been unclear if increased MAIT cell abundance is protective against TB disease. In this study, we show that augmentation of the mycobacterial MAIT cell ligands leads to higher MAIT cell activation with reduced M. tuberculosis virulence and that elevated MAIT cell abundance confers greater control of M. tuberculosis infection. Our study also highlights the potential of endogenously adjuvanting the traditional BCG vaccine with MR1 ligands to augment MAIT cell activation. This study increases current knowledge on the roles of the riboflavin-biosynthetic pathway and MAIT cell activation in M. tuberculosis virulence and host immunity against TB.

Pathophysiological Roles of Mucosal-Associated Invariant T Cells in the Context of Gut Microbiota-Liver Axis

Mucosal-associated invariant T (MAIT) cells are a subset of T lymphocytes expressing a semi-invariant T-cell receptor (TCR) present as TCR Vα7.2-Jα33 in humans and TCR Vα19-Jα33 in mice. They are activated by ligands produced during microbial biosynthesis of riboflavin that is presented by major histocompatibility complex class I-related (MR1) molecules on antigen-presenting cells. MAIT cells also possess interleukin (IL)-12 and IL-18 receptors and can be activated by the respective cytokines released from microbially stimulated antigen-presenting cells. Therefore, MAIT cells can be involved in bacterial and viral defenses and are a significant part of the human immune system. They are particularly abundant in the liver, an organ serving as the second firewall of gut microbes next to the intestinal barrier. Therefore, the immune functions of MAIT cells are greatly impacted by changes in the gut-microbiota and play important roles in the gut-liver pathogenesis axis. In this review, we discuss the nature and mechanisms of MAIT cell activation and their dynamics during different types of liver pathogenesis conditions. We also share our perspectives on important aspects that should be explored further to reveal the exact roles that MAIT cells play in liver pathogenesis in the context of the gut microbiota.

Mouse models illuminate MAIT cell biology

The field of mucosal-associated invariant T cell (MAIT) biology has grown rapidly since the identification of the vitamin-B-based antigens recognised by these specialised T cells. Over the past few years, our understanding of the complexities of MAIT cell function has developed, as they find their place among the other better known cells of the immune system. Key questions relate to understanding when MAIT cells help, when they hinder or cause harm, and when they do not matter. Exploiting mouse strains that differ in MAIT cell numbers, leveraged by specific detection of MAIT cells using MR1-tetramers, it has now been shown that MAIT cells play important immune roles in settings that include bacterial and viral infections, autoimmune diseases and cancer. We have also learnt much about their development, modes of activation and response to commensal microbiota, and begun to try ways to manipulate MAIT cells to improve disease outcomes. Here we review recent studies that have assessed MAIT cells in models of disease.

Predicting Non-Alcoholic Fatty Liver Disease Progression and Immune Deregulations by Specific Gene Expression Patterns

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide with rising rates in parallel to obesity, type 2 diabetes, and metabolic syndrome. NAFLD includes pathologies ranging from simple steatosis (NAFL) to non-alcoholic steatohepatitis and cirrhosis (NASH), which may eventually develop into hepatocellular carcinoma (HCC). Mechanically, lipids accumulation and insulin resistance act as the first hit, inflammation and fibrosis serve as the second hit. Currently, the diagnosis of NAFLD mainly depends on pathology examination and medical imaging, whereas proper gene signature classifiers are necessary for the evaluation of disease status. Here, we developed three signature classifiers to distinguish different NAFLD disease states (NAFL and NASH). Moreover, we found that B cells, DCs, and MAIT cells are key deregulated immune cells in NAFLD, which are associated with NAFLD and NAFLD-HCC progression. Meanwhile, AKR1B10 and SPP1 are closely related to the above three immune cell infiltrations and immunosuppressive cytokines expressions in NAFLD and NAFLD-HCC. Subsequently, we screened out AKR1B10 and SPP1 sensitive molecules TGX-221, which may provide a possible therapy for NAFLD and NAFLD-HCC.

Identification of Hub Genes Associated With Immune Infiltration and Predict Prognosis in Hepatocellular Carcinoma via Bioinformatics Approaches

Aims

In the cancer-related research field, there is currently a major need for a greater number of valuable biomarkers to predict the prognosis of hepatocellular carcinoma (HCC). In this study, we aimed to screen hub genes related to immune cell infiltration and explore their prognostic value for HCC.

Methods

We analyzed five datasets (GSE46408, GSE57957, GSE74656, GSE76427, and GSE87630) from the Gene Expression Omnibus database to screen the differentially expressed genes (DEGs). A protein-protein interaction network of the DEGs was constructed using the Search Tool for the Retrieval of Interacting Genes; then, the hub genes were identified. Functional enrichment of the genes was performed on the Metascape website. Next, the expression of these hub genes was validated in several databases, including Oncomine, Gene Expression Profiling Interactive Analysis 2 (GEPIA2), and Human Protein Atlas. We explored the correlations between the hub genes and infiltrated immune cells in the TIMER2.0 database. The survival curves were generated in GEPIA2, and the univariate and multivariate Cox regression analyses were performed using TIMER2.0.

Results

The top ten hub genes [DNA topoisomerase II alpha (TOP2A), cyclin B2 (CCNB2), protein regulator of cytokinesis 1 (PRC1), Rac GTPase-activating protein 1 (RACGAP1), aurora kinase A (AURKA), cyclin-dependent kinase inhibitor 3 (CDKN3), nucleolar and spindle-associated protein 1 (NUSAP1), cell division cycle-associated 5 (CDCA5), abnormal spindle microtubule assembly (ASPM), and non-SMC condensin I complex subunit G (NCAPG)] were identified in subsequent analysis. These genes are most markedly enriched in cell division, suggesting their close association with tumorigenesis. Multi-database analyses validated that the hub genes were upregulated in HCC tissues. All hub genes positively correlated with several types of immune infiltration, including B cells, CD4⁺ T cells, macrophages, and dendritic cells. Furthermore, these hub genes served as independent prognostic factors, and the expression of these hub genes combing with the macrophage levels could help predict an unfavorable prognosis of HCC.

Conclusion

In sum, these hub genes (TOP2A, CCNB2, PRC1, RACGAP1, AURKA, CDKN3, NUSAP1, CDCA5, ASPM, and NCAPG) may be pivotal markers for prognostic prediction as well as potentially work as targets for immune-based intervention strategies in HCC.

Functional inactivation of pulmonary MAIT cells following 5-OP-RU treatment of non-human primates

Targeting MAIT cells holds promise for the treatment of different diseases and infections. We previously showed that treatment of Mycobacterium tuberculosis infected mice with 5-OP-RU, a major antigen for MAIT cells, expands MAIT cells and enhances bacterial control. Here we treated M. tuberculosis infected rhesus macaques with 5-OP-RU intratracheally but found no clinical or microbiological benefit. In fact, after 5-OP-RU treatment MAIT cells did not expand, but rather upregulated PD-1 and lost the ability to produce multiple cytokines, a phenotype resembling T cell exhaustion. Furthermore, we show that vaccination of uninfected macaques with 5-OP-RU+CpG instillation into the lungs also drives MAIT cell dysfunction, and PD-1 blockade during vaccination partly prevents the loss of MAIT cell function without facilitating their expansion. Thus, in rhesus macaques MAIT cells are prone to the loss of effector functions rather than expansion after TCR stimulation in vivo, representing a significant barrier to therapeutically targeting these 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.

Tissue-Resident Lymphocytes: Implications in Immunotherapy for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a hard-to-treat cancer. The recent introduction of immune checkpoint inhibitors (ICIs) provided viable options to treat HCC, but the response rate is currently not sufficient. Thus, a better understanding of ICI-responding cells within tumors is needed to improve outcomes of ICI treatment in HCC. Recently, tissue-resident memory T (T_RM) cells were defined as a subset of the memory T cell population; this cell population is actively under investigation to elucidate its role in anti-tumor immunity. In addition, the role of other tissue-resident populations such as tissue resident regulatory T (Treg) cells, mucosal associated invariant T (MAIT) cells, γδ T cells, and invariant natural killer T (iNKT) cells in anti-tumor immunity is also actively being investigated. However, there is no study that summarizes recent studies and discusses future perspectives in terms of tissue resident lymphocytes in HCC. In this review, we summarize key features of tissue-resident lymphocytes and their role in the anti-tumor immunity. Additionally, we review recent studies regarding the characteristics of tissue-resident lymphocytes in HCC and their role in ICI treatment and other immunotherapeutic strategies.

The combined action of monocytic myeloid-derived suppressor cells and mucosal-associated invariant T cells promotes the progression of cervical cancer

One of the most common promoters of the initiation and growth of the tumor is an immune disturbance. Numerous immune cells and inflammatory factors play a role in the tumor-immune microenvironment. However, few studies have investigated the correlation between these immunological events and clinical consequences in cervical cancer. We measured the levels of numerous inflammatory mediators and frequencies of regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs) and mucosal-associated invariant T (MAIT) cells in peripheral blood (PB) of cervical cancer patients. Cervical cancer patients showed elevated production of interleukin (IL)-18 and plasma C-C chemokine ligand (CCL) 3/5. Meanwhile, an accumulation of C-C chemokine receptor 5 (CCR5) monocytic (Mo)-MDSCs and Tregs was observed. The cervical cancer group displayed increased frequencies of CD8⁺ , CD4⁺ and highly activated CD38⁺ CD8⁺ MAIT cells, and reduction of double-negative (DN) and PD1(CD279⁺ ) DN MAIT cells. Importantly, it was demonstrated that MAIT cells were positively related to Mo-MDSCs. Furthermore, an elevated concentration of PD1(CD279⁺ ) DN MAIT cells was significantly related to increased progression-free survival of patients with cervical cancer. In conclusion, our study suggests that the combined action of Mo-MDSCs and MAIT cells might be associated with the progression of cervical cancer, and the frequency of DN MAIT cells in the peripheral blood mononuclear cells was associated with the survival benefit of patients.

Off-the-Shelf Allogeneic T Cell Therapies for Cancer: Opportunities and Challenges Using Naturally Occurring "Universal" Donor T Cells

Chimeric antigen receptor (CAR) engineered T cell therapies individually prepared for each patient with autologous T cells have recently changed clinical practice in the management of B cell malignancies. Even though CARs used to redirect polyclonal T cells to the tumor are not HLA restricted, CAR T cells are also characterized by their endogenous T cell receptor (TCR) repertoire. Tumor-antigen targeted TCR-based T cell therapies in clinical trials are thus far using “conventional” αβ-TCRs that recognize antigens presented as peptides in the context of the major histocompatibility complex. Thus, both CAR- and TCR-based adoptive T cell therapies (ACTs) are dictated by compatibility of the highly polymorphic HLA molecules between donors and recipients in order to avoid graft-versus-host disease and rejection. The development of third-party healthy donor derived well-characterized off-the-shelf cell therapy products that are readily available and broadly applicable is an intensive area of research. While genome engineering provides the tools to generate “universal” donor cells that can be redirected to cancers, we will focus our attention on third-party off-the-shelf strategies with T cells that are characterized by unique natural features and do not require genome editing for safe administration. Specifically, we will discuss the use of virus-specific T cells, lipid-restricted (CD1) T cells, MR1-restricted T cells, and γδ-TCR T cells. CD1- and MR1-restricted T cells are not HLA-restricted and have the potential to serve as a unique source of universal TCR sequences to be broadly applicable in TCR-based ACT as their targets are presented by the monomorphic CD1 or MR1 molecules on a wide variety of tumor types. For each cell type, we will summarize the stage of preclinical and clinical development and discuss opportunities and challenges to deliver off-the-shelf targeted cellular therapies against cancer.

Innate lymphocytes: pathogenesis and therapeutic targets of liver diseases and cancer

The liver is a lymphoid organ with unique immunological properties, particularly, its predominant innate immune system. The balance between immune tolerance and immune activity is critical to liver physiological functions and is responsible for the sensitivity of this organ to numerous diseases, including hepatotropic virus infection, alcoholic liver disease, nonalcoholic fatty liver disease, autoimmune liver disease, and liver cancer, which are major health problems globally. In the past decade, with the discovery of liver-resident natural killer cells, the importance of innate lymphocytes with tissue residency has gradually become the focus of research. In this review, we address the current knowledge regarding hepatic innate lymphocytes with unique characteristics, including NK cells, ILC1/2/3s, NKT cells, γδ T cells, and MAIT cells, and their potential roles in liver homeostasis maintenance and the progression of liver diseases and cancer. A better understanding of the immunopathogenesis of hepatic innate lymphocytes will be helpful for proposing effective treatments for liver diseases and cancer.

Functional and Clinical Characterization of Tumor-Infiltrating T Cell Subpopulations in Hepatocellular Carcinoma

Tumor-infiltrating T-lymphocytes are defined as T-lymphocytes that infiltrated into tumor tissues; however, their composition, clinical significance, and underlying mechanism in hepatocellular carcinoma (HCC) and adjacent non-tumor tissues are still not completely understood. Herein, we collected marker genes of T cell subpopulations from a previous study and estimated their relative infiltrating levels in HCC and adjacent non-tumor tissues. Specifically, the infiltrating levels of all the T cells were significantly reduced in HCC as compared with non-tumor tissues. Unsupervised clustering of the HCC samples by the T cell infiltrating levels revealed that the HCC samples could be clearly classified into two groups. The driver genes, including PTK2B, ATM, PIK3C2B, and KIT, and several CNAs were observed to be associated with reduced T cell infiltrating levels. Particularly, deletion of TP53 more frequently occurred in low T cell infiltration HCC samples and resulted in its downregulation and cell cycle progression, indicating that cell cycle progression was closely associated with reduced T cell infiltration. In contrast, for the samples with high infiltration T cells, its immune evasion might be regulated by the immune checkpoint regulators, such as PD-1/PD-L1 and CTLA4. Moreover, Olaparib, one of the PARP inhibitors, and immune checkpoint inhibitors might be therapeutic candidates for the samples from the two T cell infiltrating clusters. Clinically, the tumor-infiltrating levels of cytotoxic CD4 cell, Mucosal associated invariant T (MAIT) cell, and exhausted CD8⁺ T cell might be used as predictors for vascular invasion, recurrence, and overall survival. Collectively, we systematically evaluated the clinical significance and potential molecular mechanisms of tumor-infiltrating T cell subpopulations in hepatocellular carcinoma, which might broaden our insights into the immunological features of HCC and provide potential immunotherapeutic targets.

Mucosal-associated invariant T cells in hepatitis B virus-related liver failure

BACKGROUND

Liver failure has high mortality and poor prognosis, and establishing new reliable markers for predicting its prognosis is necessary. Mucosal-associated invariant T (MAIT) cells are a novel population of innate-like lymphocytes involved in inflammatory liver disease, and their potential role in liver failure remains unclear.

AIM

To investigate alteration of circulating MAIT cells and assess its prognostic value in patients with hepatitis B virus (HBV)-related liver failure.

METHODS

We recruited 55 patients with HBV-related liver failure, 48 patients with chronic hepatitis B and 40 healthy controls (HCs) from Nantong Third People’s Hospital Affiliated to Nantong University. Peripheral blood mononuclear cells were isolated, and the percentage and number of circulating MAIT cells were detected by flow cytometry. Plasma levels of interleukin (IL)-7, IL-12p70, IL-18 and interferon-α were measured by Luminex assay.

RESULTS

Circulating MAIT cells were significantly decreased in HBV-related liver failure patients (percentage: 2.00 ± 1.22 vs 5.19 ± 1.27%, P < 0.0001; number: 5.47 ± 4.93 vs 84.43 ± 19.59, P < 0.0001) compared with HCs. More importantly, there was a significant reduction of MAIT cells in patients with middle/late-stage compared with early-stage liver failure. Circulating MAIT cells partially recovered after disease improvement, both in percentage (4.01 ± 1.21 vs 2.04 ± 0.95%, P < 0.0001) and in cell count (17.24 ± 8.56 vs 7.41 ± 4.99, P < 0.0001). The proportion (2.29 ± 1.01 vs 1.58 ± 1.38%, P < 0.05) and number (7.30 ± 5.70 vs 2.94 ± 1.47, P < 0.001) of circulating MAIT cells were significantly higher in the survival group than in the dead/liver transplantation group, and the Kaplan–Meier curve showed that lower expression of circulating MAIT cells (both percentage and cell count) predicted poor overall survival (P < 0.01). Also, the levels of IL-12 (20.26 ± 5.42 pg/mL vs 17.76 ± 2.79 pg/mL, P = 0.01) and IL-18 (1470.05 ± 1525.38 pg/mL vs 362.99 ± 109.64 pg/mL, P < 0.0001) were dramatically increased in HBV-related liver failure patients compared with HCs.

CONCLUSION

Circulating MAIT cells may play an important role in the process of HBV-related liver failure and can be an important prognostic marker.

Biased MAIT TCR Usage Poised for Limited Antigen Diversity?

Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells that recognize the evolutionarily conserved major histocompatibility complex (MHC) class I-like antigen-presenting molecule known as MHC class I related protein 1 (MR1). Since their rise from obscurity in the early 1990s, the study of MAIT cells has grown substantially, accelerating our fundamental understanding of these cells and their possible roles in immunity. In the context of recent advances, we review here the relationship between MR1, antigen, and TCR usage among MAIT and other MR1-reactive T cells and provide a speculative discussion.

MR1-Restricted T Cells in Cancer Immunotherapy

Major histocompatibility complex class I-related (MR1) was first identified as a cell membrane protein involved in the development and expansion of a unique set of T cells expressing an invariant T-cell receptor (TCR) α-chain. These cells were initially discovered in mucosal tissues, such as the intestinal mucosa, so they are called mucosal-associated invariant T (MAIT) cells. MR1 senses the presence of intermediate metabolites of riboflavin and folic acid synthesis that have been chemically modified by the side-products of glycolysis, glyoxal or methylglyoxal. These modified metabolites form complexes with MR1 and translocate from the endoplasmic reticulum to the plasma membrane where MAIT cells’ TCRs recognize them. Recent publications report that atypical MR1-restricted cytotoxic T cells, differing from MAIT cells in TCR usage, antigen, and transcription factor profile, recognize an as yet unknown cancer-specific metabolite presented by MR1 in cancer cells. This metabolite may represent another class of neoantigens, beyond the neo-peptides arising from altered tumor proteins. In an MR1-dependent manner, these MR1-restricted T cells, while sparing noncancerous cells, kill many cancer cell lines and attenuate cell-line-derived and patient-derived xenograft tumors. As MR1 is monomorphic and expressed in a wide range of cancer tissues, these findings raise the possibility of universal pan-cancer immunotherapies that are dependent on cancer metabolites.

Leveraging Public Single-Cell and Bulk Transcriptomic Datasets to Delineate MAIT Cell Roles and Phenotypic Characteristics in Human Malignancies

Mucosa-associated invariant T (MAIT) cells are unconventional, innate-like T lymphocytes that recognize vitamin B metabolites of microbial origin among other antigens displayed by the monomorphic molecule MHC class I-related protein 1 (MR1). Abundant in human tissues, reactive to local inflammatory cues, and endowed with immunomodulatory and cytolytic functions, MAIT cells are likely to play key roles in human malignancies. They accumulate in various tumor microenvironments (TMEs) where they often lose some of their functional capacities. However, the potential roles of MAIT cells in anticancer immunity or cancer progression and their significance in shaping clinical outcomes remain largely unknown. In this study, we analyzed publicly available bulk and single-cell tumor transcriptomic datasets to investigate the tissue distribution, phenotype, and prognostic significance of MAIT cells across several human cancers. We found that expanded MAIT cell clonotypes were often shared between the blood, tumor tissue and adjacent healthy tissue of patients with colorectal, hepatocellular, and non-small cell lung carcinomas. Gene expression comparisons between tumor-infiltrating and healthy tissue MAIT cells revealed the presence of activation and/or exhaustion programs within the TMEs of primary hepatocellular and colorectal carcinomas. Interestingly, in basal and squamous cell carcinomas of the skin, programmed cell death-1 (PD-1) blockade upregulated the expression of several effector genes in tumor-infiltrating MAIT cells. We derived a signature comprising stable and specific MAIT cell gene markers across several tissue compartments and cancer types. By applying this signature to estimate MAIT cell abundance in pan-cancer gene expression data, we demonstrate that a heavier intratumoral MAIT cell presence is positively correlated with a favorable prognosis in esophageal carcinoma but predicts poor overall survival in colorectal and squamous cell lung carcinomas. Finally, in colorectal carcinoma and four other cancer types, we found a positive correlation between MR1 expression and estimated MAIT cell abundance. Collectively, our findings indicate that MAIT cells serve important but diverse roles in human cancers. Our work provides useful models and resources that employ gene expression data platforms to enable future studies in the realm of MAIT cell biology.

Single-cell mass cytometry on peripheral blood identifies immune cell subsets associated with primary biliary cholangitis

The relationship between primary biliary cholangitis (PBC), a chronic cholestatic autoimmune liver disease, and the peripheral immune system remains to be fully understood. Herein, we performed the first mass cytometry (CyTOF)-based, immunophenotyping analysis of the peripheral immune system in PBC at single-cell resolution. CyTOF was performed on peripheral blood mononuclear cells (PBMCs) from PBC patients (n = 33) and age-/sex-matched healthy controls (n = 33) to obtain immune cell abundance and marker expression profiles. Hierarchical clustering methods were applied to identify immune cell types and subsets significantly associated with PBC. Subsets of gamma-delta T cells (CD3⁺TCRgd⁺), CD8⁺ T cells (CD3⁺CD8⁺CD161⁺PD1⁺), and memory B cells (CD3⁻CD19⁺CD20⁺CD24⁺CD27⁺) were found to have lower abundance in PBC than in control. In contrast, higher abundance of subsets of monocytes and naïve B cells were observed in PBC compared to control. Furthermore, several naïve B cell (CD3⁻CD19⁺CD20⁺CD24⁻CD27⁻) subsets were significantly higher in PBC patients with cirrhosis (indicative of late-stage disease) than in those without cirrhosis. Alternatively, subsets of memory B cells were lower in abundance in cirrhotic relative to non-cirrhotic PBC patients. Future immunophenotyping investigations could lead to better understanding of PBC pathogenesis and progression, and also to the discovery of novel biomarkers and treatment strategies.

Human Tumor-Infiltrating MAIT Cells Display Hallmarks of Bacterial Antigen Recognition in Colorectal Cancer

Growing evidence indicates a role for the gut microbiota in modulating anti-tumor treatment efficacy in human cancer. Here we study mucosa-associated invariant T (MAIT) cells to look for evidence of bacterial antigen recognition in human colon, lung, and kidney carcinomas. Using mass cytometry and single-cell mRNA sequencing, we identify a tumor-infiltrating MAIT cell subset expressing CD4 and Foxp3 and observe high expression of CD39 on MAIT cells from colorectal cancer (CRC) only, which we show in vitro to be expressed specifically after TCR stimulation. We further reveal that these cells are phenotypically and functionally exhausted. Sequencing data show high bacterial infiltration in CRC tumors and highlight an enriched species, Fusobacteria nucleatum, with capability to activate MAIT cells in a TCR-dependent way. Our results provide evidence of a MAIT cell response to microbial antigens in CRC and could pave the way for manipulating MAIT cells or the microbiome for cancer therapy.

Chronic Viral Liver Diseases: Approaching the Liver Using T Cell Receptor-Mediated Gene Technologies

Chronic infection with viral hepatitis is a major risk factor for liver injury and hepatocellular carcinoma (HCC). One major contributing factor to the chronicity is the dysfunction of virus-specific T cell immunity. T cells engineered to express virus-specific T cell receptors (TCRs) may be a therapeutic option to improve host antiviral responses and have demonstrated clinical success against virus-associated tumours. This review aims to give an overview of TCRs identified from viral hepatitis research and discuss how translational lessons learned from cancer immunotherapy can be applied to the field. TCR isolation pipelines, liver homing signals, cell type options, as well as safety considerations will be discussed herein.

A Role for MAIT Cells in Colorectal Cancer

MAIT cells are MR1-restricted T cells that are well-known for their anti-microbial properties, but they have recently been associated with different forms of cancer. Several studies have reported activated MAIT cells within the microenvironment of colorectal tumors, but there is conjecture about the nature of their response and whether they are contributing to anti-tumor immunity, or to the progression of the disease. We have reviewed the current state of knowledge about the role of MAIT cells in colorectal cancer, including their likely influence when activated and potential sources of stimulation in the tumor microenvironment. The prospects for MAIT cells being used in clinical settings as biomarkers or as targets of new immunotherapies designed to harness their function are discussed.

MR1-Restricted T Cells Are Unprecedented Cancer Fighters

Non-polymorphic MHC class I-related molecule MR1 presents antigenic bacterial metabolites to mucosal-associated invariant T (MAIT) cells and self-antigens to MR1-restricted T (MR1T) cells. Both MR1-restricted T cell populations are readily identified in healthy individuals, with MAIT cells accounting for 1-10% of circulating T cells, while MR1T cells have frequencies comparable to peptide-specific T cells (<0.1%). Self-reactive MR1T cells display a heterogeneous phenotype, and are capable of releasing both T_H1 and T_H2 cytokines, supporting not only activation of inflammation but also contributing to its regulation. Importantly, MR1T cells recognize and kill a diverse range of MR1-expressing tumor cells. On the other hand, evidence suggests MAIT cells augment cancer growth and metastases. This review addresses the potential role of MR1-restricted T cells in controlling tumor cells, facilitating their elimination and regulating cancer immunity. We also discuss therapeutic opportunities surrounding MR1-restricted T cells in cancer.

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.

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.

Promising key genes associated with tumor microenvironments and prognosis of hepatocellular carcinoma

BACKGROUND

Despite significant advances in multimodality treatments, hepatocellular carcinoma (HCC) remains one of the most common malignant tumors. Identification of novel prognostic biomarkers and molecular targets is urgently needed.

AIM

To identify potential key genes associated with tumor microenvironments and the prognosis of HCC.

METHODS

The infiltration levels of immune cells and stromal cells were calculated and quantified based on the ESTIMATE algorithm. Differentially expressed genes (DEGs) between high and low groups according to immune or stromal scores were screened using the gene expression profile of HCC patients in The Cancer Genome Atlas and were further linked to the prognosis of HCC. These genes were validated in four independent HCC cohorts. Survival-related key genes were identified by a LASSO Cox regression model.

RESULTS

HCC patients with a high immune/stromal score had better survival benefits than patients with a low score. A total of 899 DEGs were identified and found to be involved in immune responses and extracellular matrices, 147 of which were associated with overall survival. Subsequently, 52 of 147 survival-related DEGs were validated in additional cohorts. Finally, ten key genes (STSL2, TMC5, DOK5, RASGRP2, NLRC3, KLRB1, CD5L, CFHR3, ADH1C, and UGT2B15) were selected and used to construct a prognostic gene signature, which presented a good performance in predicting overall survival.

CONCLUSION

This study extracted a list of genes associated with tumor microenvironments and the prognosis of HCC, thereby providing several valuable directions for the prognostic prediction and molecular targeted therapy of HCC in the future.

MAIT Cells Come to the Rescue in Cancer Immunotherapy?

Recent progress in immunobiology has led to the observation that, among cells classically categorized as the typical representatives of the adaptive immune system, i.e., T cells, some possess the phenotype of innate cells. Invariant T cells are characterized by T cell receptors recognizing a limited range of non-peptide antigens, presented only in the context of particular molecules. Mucosal-associated invariant T cells (MAIT cells) are an example of such unconventional cells. In humans, they constitute between 1% and 8% of the peripheral blood T lymphocytes and are further enriched in mucosal tissues, mesenteric lymph nodes, and liver, where they can account for even 40% of all the T cells. MAIT cells recognize antigens in the context of major histocompatibility complex class I-related protein (MR1). Upon activation, they instantly release pro-inflammatory cytokines and mediate cytolytic function towards bacterially infected cells. As such, they have been a rapidly evolving research topic not only in the field of infectious diseases but also in the context of many chronic inflammatory diseases and, more recently, in immuno-oncology. Novel findings suggest that MAIT cells function could also be modulated by endogenous ligands and drugs, making them an attractive target for therapeutic approaches. In this review, we summarize the current understanding of MAIT cell biology, their role in health and disease and discuss their future potential in cancer immunotherapy. This is discussed through the prism of knowledge and experiences with invariant natural killer T cells (iNKT)—another prominent unconventional T cell subset that shares many features with MAIT cells.

The immunobiology of hepatocellular carcinoma in humans and mice: Basic concepts and therapeutic implications

Basic and clinical studies have demonstrated the efficacy of immunotherapy, a technical and conceptual breakthrough that has revolutionised cancer treatment. Hepatocellular carcinoma (HCC), a deadly malignancy with aetiologic diversity and a chronic course, is strongly influenced by the immune system, and was recently found to partially benefit from immune-checkpoint inhibitor therapy. Notably, HCC onco-immunology depends on diverse genetic and environmental factors that together shape cancer-promoting inflammation and immune dysfunction - critical processes that control HCC malignant progression and response to therapy. Herein, we summarise the current understanding of liver and HCC onco-immunology obtained through basic studies with mouse models and clinical practice in humans. In particular, we discuss preclinical and clinical findings that implicate immunomodulation as a major factor in HCC development and explain the basis for HCC-targeting immunotherapy.

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.

MAIT Cells in Health and Disease

Mucosal-associated invariant T (MAIT) cells are a newly described subset of T cells that are found in the blood and are enriched in many tissues, particularly in the liver. MAIT cells express a semi-invariant T cell receptor restricted by the MHC class I-related (MR1) molecule. MAIT cells are activated in a MR1-dependent manner in response to microbial-derived riboflavin metabolites which leads to rapid effector functions, but they can also be activated in a MR1-independent manner by cytokines and viruses. The use of mice models and MR1 tetramers, among other recent methodological advances, have provided more insight into the development, mode of activation, characterization in different diseases and tissues of MAIT cells. In this chapter, we provide an overview of MAIT cells and yet remaining questions about their potential therapeutic role.

Isolation and Characterization of MAIT Cells from Tumor Tissues

Mucosal-associated invariant T (MAIT) cell infiltration has been demonstrated in colorectal and hepatocellular carcinoma, and their ability to produce Th1- and Th17-associated cytokines, as well as their cytotoxic function, suggests that MAIT cells may have important functions in both reducing and promoting protective tumor immunity. Here, we describe enzymatic methods to isolate intraepithelial and lamina propria lymphocyte single cell suspensions from colon tissue and tumors containing viable MAIT cells, which can be used for further purification, flow cytometry analysis, or culture.

Mucosal-Associated Invariant T cell in liver diseases

Mucosal-associated invariant T cells (MAIT cells) are a new population of innate immune cells, which are abundant in the liver and play complex roles in various liver diseases. In this review, we summarize MAIT cells in the liver diseases in recent studies, figure out the role of MAIT cells in various liver disease, including Alcoholic liver disease, Non-alcoholic liver disease, Autoimmune liver diseases, Viral hepatitis and Liver Cancer. Briefly, MAIT cells are involved in anti-bacteria responses in the alcoholic liver diseases. Besides, the activated MAIT cells promote the liver inflammation by secreting inflammatory cytokines and produce regulatory cytokines, which induces anti-inflammatory macrophage polarization. MAIT cells participate in the liver fibrosis via enhancing hepatic stellate cell activation. In viral hepatitis, MAIT cells exhibit a flawed and exhausted phenotype, which results in little effect on controlling the virus and bacteria. In liver cancer, MAIT cells indicate the disease progression and the outcome of therapy. In summary, MAIT cells are attractive biomarkers and therapeutic targets for liver disease.

Liver-Mediated Adaptive Immune Tolerance

The liver is an immunologically tolerant organ that is uniquely equipped to limit hypersensitivity to food-derived antigens and bacterial products through the portal vein and can feasibly accept liver allografts. The adaptive immune response is a major branch of the immune system that induces organ/tissue-localized and systematic responses against pathogens and tumors while promoting self-tolerance. Persistent infection of the liver with a virus or other pathogen typically results in tolerance, which is a key feature of the liver. The liver's immunosuppressive microenvironment means that hepatic adaptive immune cells become readily tolerogenic, promoting the death of effector cells and the “education” of regulatory cells. The above mechanisms may result in the clonal deletion, exhaustion, or inhibition of peripheral T cells, which are key players in the adaptive immune response. These tolerance mechanisms are believed to be responsible for almost all liver diseases. However, optimal protective adaptive immune responses may be achieved through checkpoint immunotherapy and the modulation of hepatic innate immune cells in the host. In this review, we focus on the mechanisms involved in hepatic adaptive immune tolerance, the liver diseases caused thereby, and the therapeutic strategies needed to overcome this tolerance.

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.

The Dark Side of Innate Immunity Fosters Tumor Growth and Offers New Diagnostic

So far, the tumor's immune landscape has been related mainly to the adaptive immune cell infiltrate. The possibility of using tumor-infiltrating innate immune cells as the source for prognostic markers and their role in immune checkpoint therapy have been neglected. Unraveling these aspects may lead to new immune therapy interventions.See related article by Duan et al., p. 3304.