Circulating and tumor-infiltrating mucosal associated invariant T (MAIT) cells in colorectal cancer patients

Does exercise attenuate age- and disease-associated dysfunction in unconventional T cells? Shining a light on overlooked cells in exercise immunology

Unconventional T Cells (UTCs) are a unique population of immune cells that links innate and adaptive immunity. Following activation, UTCs contribute to a host of immunological activities, rapidly responding to microbial and viral infections and playing key roles in tumor suppression. Aging and chronic disease both have been shown to adversely affect UTC numbers and function, with increased inflammation, change in body composition, and physical inactivity potentially contributing to the decline. One possibility to augment circulating UTCs is through increased physical activity. Acute exercise is a potent stimulus leading to the mobilization of immune cells while the benefits of exercise training may include anti-inflammatory effects, reductions in fat mass, and improved fitness. We provide an overview of age-related changes in UTCs, along with chronic diseases that are associated with altered UTC number and function. We summarize how UTCs respond to acute exercise and exercise training and discuss potential mechanisms that may lead to improved frequency and function.

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.

Virus-Mediated Suppression of the Antigen Presentation Molecule MR1

The antigen-presenting molecule MR1 presents microbial metabolites related to vitamin B2 biosynthesis to mucosal-associated invariant T cells (MAIT cells). Although bacteria and fungi drive the MR1 biosynthesis pathway, viruses have not previously been implicated in MR1 expression or its antigen presentation. We demonstrate that several herpesviruses inhibit MR1 cell surface upregulation, including a potent inhibition by herpes simplex virus type 1 (HSV-1). This virus profoundly suppresses MR1 cell surface expression and targets the molecule for proteasomal degradation, whereas ligand-induced cell surface expression of MR1 prior to infection enables MR1 to escape HSV-1-dependent targeting. HSV-1 downregulation of MR1 is dependent on de novo viral gene expression, and we identify the Us3 viral gene product as functioning to target MR1. Furthermore, HSV-1 downregulation of MR1 disrupts MAIT T cell receptor (TCR) activation. Accordingly, virus-mediated targeting of MR1 defines an immunomodulatory strategy that functionally disrupts the MR1-MAIT TCR axis.

An Unconventional View of T Cell Reconstitution After Allogeneic Hematopoietic Cell Transplantation

Allogeneic hematopoietic cell transplantation (allo-HCT) is performed as curative-intent therapy for hematologic malignancies and non-malignant hematologic, immunological and metabolic disorders, however, its broader implementation is limited by high rates of transplantation-related complications and a 2-year mortality that approaches 50%. Robust reconstitution of a functioning innate and adaptive immune system is a critical contributor to good long-term patient outcomes, primarily to prevent and overcome post-transplantation infectious complications and ensure adequate graft-versus-leukemia effects. There is increasing evidence that unconventional T cells may have an important immunomodulatory role after allo-HCT, which may be at least partially dependent on the post-transplantation intestinal microbiome. Here we discuss the role of immune reconstitution in allo-HCT outcome, focusing on unconventional T cells, specifically mucosal-associated invariant T (MAIT) cells, γδ (gd) T cells, and invariant NK T (iNKT) cells. We provide an overview of the mechanistic preclinical and associative clinical studies that have been performed. We also discuss the emerging role of the intestinal microbiome with regard to hematopoietic function and overall immune reconstitution.

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.

The burgeoning role of MR1-restricted T-cells in infection, cancer and autoimmune disease

MR1 is a ubiquitously expressed, monomorphic antigen presenting molecule that has been largely preserved throughout mammalian evolution. The primary role of MR1 is to present conserved microbial metabolites to highly abundant mucosal-associated invariant T (MAIT) cells. The role of MAIT cells and other MR1-restricted T cells (MR1T) has been recently extended to immunomodulation during cancer. MR1Ts have also been implicated in autoimmune disease. The highly conserved nature of MR1 across the human population is in stark contrast to the MHC molecules recognised by conventional αβ T-cells, therefore MR1Ts may form fertile ground for the development of pan-population T-cell immunotherapeutics for a wide range of important morbidities.

Decrease of peripheral blood mucosal-associated 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.

Translating Unconventional T Cells and Their Roles in Leukemia Antitumor Immunity

Recently, cell-mediated immune response in malignant neoplasms has become the focus in immunotherapy against cancer. However, in leukemia, most studies on the cytotoxic potential of T cells have concentrated only on T cells that recognize peptide antigens (Ag) presented by polymorphic molecules of the major histocompatibility complex (MHC). This ignores the great potential of unconventional T cell populations, which include gamma-delta T cells (γδ), natural killer T cells (NKT), and mucosal-associated invariant T cells (MAIT). Collectively, these T cell populations can recognize lipid antigens, specially modified peptides and small molecule metabolites, in addition to having several other advantages, which can provide more effective applications in cancer immunotherapy. In recent years, these cell populations have been associated with a repertoire of anti- or protumor responses and play important roles in the dynamics of solid tumors and hematological malignancies, thus, encouraging the development of new investigations in the area. This review focuses on the current knowledge regarding the role of unconventional T cell populations in the antitumor immune response in leukemia and discusses why further studies on the immunotherapeutic potential of these cells are needed.

MAIT cells, guardians of skin and mucosa?

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

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

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

The 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.

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.

Re-education of the Tumor Microenvironment With Targeted Therapies and Immunotherapies

The clinical success of cancer immunotherapies targeting PD-1 and CTLA-4 has ignited a substantial research effort to improve our understanding of tumor immunity. Recent studies have revealed that the immune contexture of a tumor influences therapeutic response and survival benefit for cancer patients. Identifying treatment modalities that limit immunosuppression, relieve T cell exhaustion, and potentiate effector functions in the tumor microenvironment (TME) is of much interest. In particular, combinatorial therapeutic approaches that re-educate the TME by limiting the accumulation of immunosuppressive immune cells, such as Foxp3 regulatory T cells (Tregs) and tumor-associated macrophages (TAMs), while promoting CD8+ and CD4+ effector T cell activity is critical. Here, we review key approaches to target these immunosuppressive immune cell subsets and signaling molecules and define the impact of these changes to the tumor milieu. We will highlight the preclinical and clinical evidence for their ability to improve anti-tumor immune responses as well as strategies and challenges for their implementation. Together, this review will provide understanding of therapeutic approaches to efficiently shape the TME and reinvigorate the immune response against cancer.

Absence of mucosal-associated invariant T cells in a person with a homozygous point mutation in MR1

The role unconventional T cells play in protective immunity in humans is unclear. Mucosal-associated invariant T (MAIT) cells are an unconventional T cell subset restricted to the antigen-presenting molecule MR1. Here, we report the discovery of a patient homozygous for a rare Arg31His (R9H in the mature protein) mutation in MR1 who has a history of difficult-to-treat viral and bacterial infections. MR1^R9H was unable to present the potent microbially derived MAIT cell stimulatory ligand. The MR1^R9H crystal structure revealed that the stimulatory ligand cannot bind due to the mutation lying within, and causing structural perturbation to, the ligand-binding domain of MR1. While MR1^R9H could bind and be up-regulated by a MAIT cell inhibitory ligand, the patient lacked circulating MAIT cells. This shows the importance of the stimulatory ligand for MAIT cell selection in humans. The patient had an expanded γδ T cell population, indicating a compensatory interplay between these unconventional T cell subsets.

Investigation of mucosal-associated invariant T (MAIT) cells expressing immune checkpoint receptors (TIGIT and CD226) in early-onset preeclampsia

OBJECTIVE

During our work, we examined the possible contribution of MAIT cells in the pathogenesis of the clinical phase of early-onset preeclampsia and how this could be influenced by TIGIT and CD226 immune checkpoint molecules.

STUDY DESIGN

37 pregnant women diagnosed with early-onset preeclampsia and 36 healthy, age-matched control women were involved in this study. Peripheral blood mononuclear cells were isolated by density gradient and frozen. After thawing, cells were stained with monoclonal antibodies to characterize MAIT, MAIT-like, and non-MAIT cells. Flow cytometric analyses were used to measure TIGIT, CD226, intracellular perforin, and granzyme B expression.

RESULTS

MAIT (CD3+ CD8+ Vα7.2+ CD161++), MAIT-like (CD3+ CD8+ Vα7.2+ CD161+) and non-MAIT (CD3+ CD8+ Vα7.2+ CD161-) cell population were identified based on their CD161 receptor positivity. MAIT cells markedly differed in proportion, TIGIT expression, granzyme B, and perforin content compared to MAIT-like and non-MAIT cells. A significant difference was determined in TIGIT expression by non-MAIT cells and in CD8/CD226 positive relationship between the preeclamptic and healthy condition.

CONCLUSIONS

Considering that we did not detect a notable difference between early-onset preeclampsia and healthy pregnancy, we suppose that peripheral MAIT cells expressing TIGIT and CD226 immune checkpoint molecules have a marginal role in the pathogenesis of early-onset preeclampsia.

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.

Perplexing Role of P-Glycoprotein in Tumor Microenvironment

Development of multidrug resistance (MDR) still remains a major obstacle to the long-term success of cancer therapy. P-glycoprotein (P-gp) is a well-identified membrane transporter with capability to efflux drug molecules out of the cancer cell leading to reduced efficiency of chemotherapy. Cancer cells upregulate P-gp expression as an adaptive response to evade chemotherapy mediated cell death. While several P-gp inhibitors have been discovered by in silico and pre-clinical studies, very few have successfully passed all phases of the clinical trials. Studies show that application of P-gp inhibitors in cancer therapy regimen following development of MDR achieved limited beneficial outcomes. While, the non-specific substrate binding to P-gp has made the drug-design a challenge, a bigger perplexing challenge comes from its role in tumor immunology. Expression of P-gp was noted immune cell phenotypes with apparently antagonistic functionality. Both pro-tumor MΦ2-macrophages and, anti-tumor NK-cell and Th17/CD4⁺T cell subsets have shown enhanced expression of P-gp. While drug based inhibition of P-gp in pro-tumor immune cell phenotypes could promote tumor elimination, however, it would not be a rational choice to exert inhibition of P-gp on anti-tumor immune cell phenotypes. This mutually exclusive paradigm of P-gp functionality requires a more comprehensive and detailed understanding of its role in tumor microenvironment with active interplay of cancer and immune cells in the tumor mileu. In this review, we focus on the current understanding of the role of P-gp in cancer cells and immune cells and finally attempt to highlight some caveats in the current understanding of its role in comprehensive tumor microenvironment along with challenges in the development of P-gp inhibitors toward anti-cancer therapy.

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.

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.

Immunological Differences Between Right-Sided and Left-Sided Colorectal Cancers: A Comparison of Embryologic Midgut and Hindgut

Purpose

There are known differences in embryology, clinical symptoms, incidences, molecular pathways involved, and oncologic outcomes of right-sided and left-sided colorectal cancers. However, immunologic study has only been characterized for healthy adults. The present study was designed to identify differences in immune cell populations in patients with right-sided and left-sided colorectal cancers.

Methods

A total of 35 patients who underwent colorectal resection for cancer between November 2016 and August 2017 at a tertiary teaching hospital were enrolled in this study. Patients were excluded if they had a disease affecting their immune system. Populations of immune cells, including mucosal-associated invariant T (MAIT), gamma delta T, invariant natural killer T, T, natural killer, and B cells, were measured in the peripheral blood and cancer tissues using flow cytometry, and then assessed based on the origin of the colorectal cancer.

Results

Fifteen had right-side and 20 had left-side colorectal cancer. There were no significant differences between the 2 cohorts for patient characteristics including pathologic stage. Peripheral blood from patients with right-side colon cancers contained fewer MAIT (0.87% right-side vs. 1.74% left-side, P = 0.028) and gamma delta T cells (1.10% right-side vs. 3.05% left-side, P = 0.002). Although the group with right-side colorectal cancer had more MAIT cells in cancer tissues (1.71% vs. 1.00%), this difference was not statistically significant.

Conclusion

There is a difference in population sizes of immune cells in blood between patients with right-sided and leftsided colon cancers. The immune cell composition was determined to be distinct based on embryologic origin.

MAIT Cells Promote Tumor Initiation, Growth, and Metastases via Tumor MR1

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that require MHC class I-related protein 1 (MR1) for their development. The role of MAIT cells in cancer is unclear, and to date no study has evaluated these cells in vivo in this context. Here, we demonstrated that tumor initiation, growth, and experimental lung metastasis were significantly reduced in Mr1 ^-/- mice, compared with wild-type mice. The antitumor activity observed in Mr1 ^-/- mice required natural killer (NK) and/or CD8⁺ T cells and IFNγ. Adoptive transfer of MAIT cells into Mr1 ^-/- mice reversed metastasis reduction. Similarly, MR1-blocking antibodies decreased lung metastases and suppressed tumor growth. Following MR1 ligand exposure, some, but not all, mouse and human tumor cell lines upregulated MR1. Pretreatment of tumor cells with the stimulatory ligand 5-OP-RU or inhibitory ligand Ac-6-FP increased or decreased lung metastases, respectively. MR1-deleted tumors resulted in fewer metastases compared with parental tumor cells. MAIT cell suppression of NK-cell effector function was tumor-MR1-dependent and partially required IL17A. Our studies indicate that MAIT cells display tumor-promoting function by suppressing T and/or NK cells and that blocking MR1 may represent a new therapeutic strategy for cancer immunotherapy. SIGNIFICANCE: Contradicting the perception that MAIT cells kill tumor cells, here MAIT cells promoted tumor initiation, growth, and metastasis. MR1-expressing tumor cells activated MAIT cells to reduce NK-cell effector function, partly in a host IL17A-dependent manner. MR1-blocking antibodies reduced tumor metastases and growth, and may represent a new class of cancer therapeutics.This article is highlighted in the In This Issue feature, p. 1.

Chronically stimulated human MAIT cells are unexpectedly potent IL-13 producers

Mucosal‐associated invariant T (MAIT) cells are unconventional T cells that recognize antigens derived from riboflavin biosynthesis. In addition to anti‐microbial functions, human MAIT cells are associated with cancers, autoimmunity, allergies and inflammatory disorders, although their role is poorly understood. Activated MAIT cells are well known for their rapid release of Th1 and Th17 cytokines, but we have discovered that chronic stimulation can also lead to potent interleukin (IL)‐13 expression. We used RNA‐seq and qRT‐PCR to demonstrate high expression of the IL‐13 gene in chronically stimulated MAIT cells, and directly identify IL‐13 using intracellular flow cytometry and multiplex bead analysis of MAIT cell cultures. This unexpected finding has important implications for IL‐13‐dependent diseases, such as colorectal cancer (CRC), that occur in mucosal areas where MAIT cells are abundant. We identify MAIT cells near CRC tumors and show that these areas and precancerous polyps express high levels of the IL‐13 receptor, which promotes tumor progression and metastasis. Our data suggest that MAIT cells have a more complicated role in CRC than currently realized and that they represent a promising new target for immunotherapies where IL‐13 can be a critical factor.

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

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

Bifidobacteria and Mucosal-Associated Invariant T (MAIT) Cells: A New Approach to Colorectal Cancer Prevention?

Colorectal cancer is the most preventable form of cancer worldwide. The pathogenesis of colorectal cancer includes gut inflammation, genetic and microbial composition factors. İmpairment of the gut microbiota has been associated with development of colorectal cancer. The genus Bifidobacterium is an important component of the commensal gut microbiota. Bifidobacteria are considered to have important roles in multiple homeostatic functions: immunologic, hormonal and metabolic. Mucosal-associated invariant T cells (MAIT) are components of the immune system involved in protection against infectious pathogens and regulate the pathogenesis of various inflammatory diseases and, potentially, colorectal cancer. Engagement between Bifidobacterium and MAIT cells could exert a beneficial effect on colorectal cancer prevention and treatment.

Differences in the molecular signatures of mucosal-associated invariant T cells and conventional T cells

Mucosal-associated invariant T (MAIT) cells exhibit different characteristics from those of TCRα7.2⁻ conventional T cells. They play important roles in various inflammatory diseases, including rheumatoid arthritis and inflammatory bowel disease. MAIT cells express a single T cell receptor alpha chain, TCRα7.2 segment associated with Jα33 and CDR3 with fixed length, which recognizes bacteria-derived vitamin B metabolites. However, the characteristics of MAIT cells and TCRα7.2⁺ CD161⁻ T cells have never been compared. Here, we performed RNA sequencing to compare the properties of MAIT cells, TCRα7.2⁻ conventional T cells and TCRα7.2⁺ CD161⁻ T cells. Genome-wide transcriptomes of MAIT cells, TCRα7.2⁻ conventional T cells, and TCRα7.2⁺ CD161⁻ T cells were compared and analyzed using causal network analysis. This is the first report comparing the transcriptomes of MAIT cells, TCRα7.2⁻ conventional T cells and TCRα7.2⁺ CD161⁻ T cells. We also identified the predominant signaling pathways of MAIT cells, which differed from those of TCRα7.2⁻ conventional T cells and TCRα7.2⁺ CD161⁻ T cells, through a gene set enrichment test and upstream regulator analysis and identified the genes responsible for the characteristic MAIT cell phenotypes. Our study advances the complete understanding of MAIT biology.

Different distribution of mucosal-associated invariant T cells within the human cecum and colon

INTRODUCTION

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that are involved in anti-bacterial immunity. MAIT cells are found in the intestines, but their role and distribution within the large intestine have not been fully elucidated. Therefore, we investigated the distribution of MAIT cells within the cecum and colon.

MATERIAL AND METHODS

Surgically resected tissues of the cecum and colon were obtained from 4 patients with cecal appendix cancer and 8 patients with colorectal cancer, respectively. Lymphocytes were isolated from the intestinal epithelium (intraepithelial lymphocytes - IELs) and the underlying lamina propria (lamina propria lymphocytes - LPLs), and then, MAIT cells were analyzed by flow cytometry.

RESULTS

Compared with the colon, the cecum showed a significantly increased frequency of MAIT cells among IELs (p < 0.01). CD69 expression on MAIT cells was significantly increased in the cecum and colon compared with that in the blood, and the frequency of natural killer group 2, member A+ cells among MAIT cells was significantly increased in the cecum.

CONCLUSIONS

These results suggest that the distribution of MAIT cells was different between the cecum and colon and that MAIT cells were more likely to be activated, especially in the intestinal epithelium of the cecum than in the colon and blood.

Less circulating mucosal-associated invariant T cells in patients with cervical cancer

OBJECTIVE

Mucosal-associated invariant T cells (MAITs) are important for immune defense against infectious pathogens and regulation of various inflammatory diseases. However, their roles in cancer are rarely reported. Since cervical cancer is one of the diseases involving mucosal tissue, we try to investigate the association between circulating MAITs and cervical cancer.

MATERIALS AND METHODS

Blood samples were obtained from patients with cervical cancer (n = 47) and healthy individuals (n = 39). We determined phenotypic MAITs in peripheral blood mononuclear cells (PBMCs) and evaluated the percentage of MAITs in CD3+ cells by flow cytometry. The percentage of MAITs was stratified according to Federation of Gynecology and Obstetrics (FIGO) staging system in patients with cervical cancer. Progression-free survival (PFS) with respect to the amount of MAITs was also analyzed.

RESULTS

The percentage of circulating MAITs in patients with cervical cancer was significantly lower than in healthy group (0.987% vs. 4.008%, p < 0.0001). In subgroup analysis, though not statistically significant, it showed a trend of lower percentage of circulating MAITs in cervical cancer patients with FIGO stage II-IV disease than in patients with FIGO stage I disease (0.4045% vs. 1.098%, p = 0.11). A trend of poor PFS in patients with lower circulating MAITs was also noted.

CONCLUSION

MAITs play a crucial role in cancer immunity. The decrease of MAITs in peripheral blood is related to cervical cancer. There is a trend of lower percentage of MAITs in advanced stages and lower percentage of MAITs towards poor PFS in patients with cervical cancer.

Circulating Mucosal-Associated Invariant T Cells in a Large Cohort of Healthy Chinese Individuals From Newborn to Elderly

Mucosal-associated invariant T (MAIT) cells, which are enriched in human blood and express a semi-invariant TCR chain, play important roles in conditions such as infectious diseases and cancer. The influence of age on levels and functional characteristics of circulating MAIT cells have not been fully addressed. Here we have collected blood samples from a large cohort of healthy Chinese individuals from newborn (cord blood) to the elderly and assessed the levels of circulating MAIT cells as well as their phenotype, activation and apoptosis status, and cytokine expression profiles after in vitro stimulation. We found that the frequencies of circulating MAIT cells gradually increased in blood from newborns as they progressed into adulthood (20–40 years old) but then decreased during further progression toward old age (>60 years old). The lowered numbers of circulating MAIT cells in the elderly was correlated with a gradual increase of apoptosis. A majority of circulating MAIT cells expressed the chemokine receptors CCR5 and CCR6, and most also expressed CD8 and CD45RO. Few expressed CD69 in cord blood, but the frequency increased with age. Upon in vitro activation with PMA plus ionomycin or IL12 plus IL18, fewer MAIT cells isolated from the young adult group expressed IFN-γ, IL17A and Granzyme B then cells from other age groups while the proportion of cells that expressed TNF-α was similar. Taken together, our data provide information for guiding the assessment of normal levels and phenotypes of MAIT cells at different ages in healthy individuals and patients.

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

PURPOSE

Innate immunity is an indispensable arm of tumor immune surveillance, and the liver is an organ with a predominance of innate immunity, where mucosal-associated invariant T (MAIT) cells are enriched. However, little is known about the phenotype, functions, and immunomodulatory role of MAIT cells in hepatocellular carcinoma (HCC).Experimental Design: The distribution, phenotype, and function of MAIT cells in patients with HCC were evaluated by both flow cytometry (FCM) and in vitro bioassays. Transcriptomic analysis of MAIT cells was also performed. Prognostic significance of tumor-infiltrating MAIT cells was validated in four independent cohorts of patients with HCC.

RESULTS

Despite their fewer densities in HCC tumor than normal liver, MAIT cells were significantly enriched in the HCC microenvironment compared with other mucosa-associated organs. Tumor-derived MAIT cells displayed a typical CCR7^-CD45RA^-CD45RO⁺CD95⁺ effector memory phenotype with lower costimulatory and effector capabilities. Tumor-educated MAIT cells significantly upregulated inhibitory molecules like PD-1, CTLA-4, TIM-3, secreted significantly less IFNγ and IL17, and produced minimal granzyme B and perforin while shifting to produce tumor-promoting cytokines like IL8. Transcriptome sequencing confirmed that tumor-derived MAIT cells were reprogrammed toward a tumor-promoting direction by downregulating genes enriched in pathways of cytokine secretion and cytolysis effector function like NFKB1 and STAT5B and by upregulating genes like IL8, CXCL12, and HAVCR2 (TIM-3). High infiltration of MAIT cells in HCC significantly correlated with an unfavorable clinical outcome, revealed by FCM, qRT-PCR, and multiplex IHC analyses, respectively.

CONCLUSIONS

HCC-infiltrating MAIT cells were functionally impaired and even reprogrammed to shift away from antitumor immunity and toward a tumor-promoting direction.See related commentary by Carbone, p. 3199.

Mucosa-associated invariant T cells in malignancies: a faithful friend or formidable foe?

Mucosa-associated invariant T (MAIT) cells are a subset of innate-like T lymphocytes known for their ability to respond to MHC-related protein 1 (MR1)-restricted stimuli and select cytokine signals. They are abundant in humans and especially enriched in mucosal layers, common sites of neoplastic transformation. MAIT cells have been found within primary and metastatic tumors. However, whether they promote malignancy or contribute to anticancer immunity is unclear. On the one hand, MAIT cells produce IL-17A in certain locations and under certain circumstances, which could in turn facilitate neoangiogenesis, intratumoral accumulation of immunosuppressive cell populations, and cancer progression. On the other hand, they can express a potent arsenal of cytotoxic effector molecules, NKG2D and IFN-γ, all of which have established roles in cancer immune surveillance. In this review, we highlight MAIT cells' characteristics as they might pertain to cancer initiation, progression, or control. We discuss recent findings, including our own, that link MAIT cells to cancer, with a focus on colorectal carcinoma, as well as some of the outstanding questions in this active area of research. Finally, we provide a hypothetical picture in which MAIT cells constitute attractive targets in cancer immunotherapy.

Unconventional T Cell Targets for Cancer Immunotherapy

Most studies on the immunotherapeutic potential of T cells have focused on CD8 and CD4 T cells that recognize peptide antigens (Ag) presented by polymorphic major histocompatibility complex (MHC) class I and MHC class II molecules, respectively. However, unconventional T cells, which interact with MHC class Ib and MHC-I like molecules, are also implicated in tumor immunity, although their role therein is unclear. These include unconventional T cells targeting MHC class Ib molecules such as HLA-E and its murine ortholog Qa-1b, natural killer T (NKT) cells, mucosal associated invariant T (MAIT) cells, and γδ T cells. Here, we review the current understanding of the roles of these unconventional T cells in tumor immunity and discuss why further studies into the immunotherapeutic potential of these cells is warranted.

Altered Populations of Unconventional T Cell Lineages in Patients with Langerhans Cell Histiocytosis

Langerhans cell histiocytosis (LCH) lesions are defined by the presence of CD1a⁺/CD207⁺ myeloid cells, but many other immune cells are present including unconventional T cells, which have powerful immunoregulatory functions. Unconventional T cell lineages include mucosal-associated invariant T (MAIT) cells, type I natural killer T (NKT) cells and gamma-delta (γδ) T cells, which are associated with many inflammatory conditions, although their importance has not been studied in LCH. We characterized their phenotype and function in blood and lesions from patients with LCH, and identified a deficiency in MAIT cell frequency and abnormalities in the subset distributions of γδ T cells and NKT cells. Such abnormalities are associated with immune dysregulation in other disease settings and are therefore potentially important in LCH. Our study is the first to recognize alterations to MAIT cell proportions in patients with LCH. This finding along with other abnormalities identified amongst unconventional T cells could potentially influence the onset and progression of LCH, thereby highlighting potential targets for new immune based therapies.

Innate Lymphocytes in Adipose Tissue Homeostasis and Their Alterations in Obesity and Colorectal Cancer

Colorectal cancer (CRC) is the third most common cancer worldwide and a leading cause of death, with burden expected to increase in the coming years. Enhanced adiposity, particularly visceral fat, is associated with increased cancer incidence representing an important indicator of survival, prognosis, recurrence rates, and response to therapy for several tumors including CRC. Compelling evidence has been achieved that the low-grade chronic inflammation characterizing obesity represents a main factor that can favor carcinogenesis. Adipocytes and adipose tissue (AT) infiltrating immune cells contribute to obesity-related inflammation by releasing soluble factors affecting, both locally and systemically, the function of several cell types, including immune and cancer cells. The unbalanced production of immune mediators as well as the profound changes in the repertoire and activation state of immune cells in AT of obese subjects represent key events in the processes that set the basis for a pro-tumorigenic microenvironment. AT harbors a unique profile of immune cells of different origin that play an important role in tissue homeostasis. Among these, tissue-resident innate lymphocytes are emerging as important AT components whose depletion/aberrant activation occurring in obesity could have an impact on inflammation and immune-surveillance against tumors. However, a direct link between obesity-induced dysfunction and cancer development has not been demonstrated yet. In this review, we provide an overview of human obesity- and CRC-induced alterations of blood and adipose tissue-associated innate lymphocytes, and discuss how the adipose tissue microenvironment in obesity might influence the development of CRC.

The Role of CD1d and MR1 Restricted T Cells in the Liver

The liver is one of the most important immunological organs that remains tolerogenic in homeostasis yet promotes rapid responses to pathogens in the presence of a systemic infection. The composition of leucocytes in the liver is highly distinct from that of the blood and other lymphoid organs, particularly with respect to enrichment of innate T cells, i.e., invariant NKT cells (iNKT cells) and Mucosal-Associated Invariant T cells (MAIT cells). In recent years, studies have revealed insights into their biology and potential roles in maintaining the immune-environment in the liver. As the primary liver-resident immune cells, they are emerging as significant players in the human immune system and are associated with an increasing number of clinical diseases. As such, innate T cells are promising targets for modifying host defense and inflammation of various liver diseases, including viral, autoimmune, and those of tumor origin. In this review, we emphasize and discuss some of the recent discoveries and advances in the biology of innate T cells, their recruitment and diversity in the liver, and their role in various liver diseases, postulating on their potential application in immunotherapy.

Positive & Negative Roles of Innate Effector Cells in Controlling Cancer Progression

Innate immune cells are active at the front line of host defense against pathogens and now appear to play a range of roles under non-infectious conditions as well, most notably in cancer. Establishing the balance of innate immune responses is critical for the “flavor” of these responses and subsequent adaptive immunity and can be either “good or bad” in controlling cancer progression. The importance of innate NK cells in tumor immune responses has already been extensively studied over the last few decades, but more recently several relatively mono- or oligo-clonal [i.e., (semi-) invariant] innate T cell subsets received substantial interest in tumor immunology including invariant natural killer T (iNKT), γδ-T and mucosal associated invariant T (MAIT) cells. These subsets produce high levels of various pro- and/or anti-inflammatory cytokines/chemokines reflecting their capacity to suppress or stimulate immune responses. Survival of patients with cancer has been linked to the frequencies and activation status of NK, iNKT, and γδ-T cells. It has become clear that NK, iNKT, γδ-T as well as MAIT cells all have physiological roles in anti-tumor responses, which emphasize their possible relevance for tumor immunotherapy. A variety of clinical trials has focused on manipulating NK, iNKT, and γδ-T cell functions as a cancer immunotherapeutic approach demonstrating their safety and potential for achieving beneficial therapeutic effects, while the exploration of MAIT cell related therapies is still in its infancy. Current issues limiting the full therapeutic potential of these innate cell subsets appear to be related to defects and suppressive properties of these subsets that, with the right stimulus, might be reversed. In general, how innate lymphocytes are activated appears to control their subsequent abilities and consequent impact on adaptive immunity. Controlling these potent regulators and mediators of the immune system should enable their protective roles to dominate and their deleterious potential (in the specific context of cancer) to be mitigated.

High MDR-1 expression by MAIT cells confers resistance to cytotoxic but not immunosuppressive MDR-1 substrates

High expression of the ATP-binding cassette-multi-drug efflux protein 1 (MDR1) is a striking feature of mucosal-associated invariant T (MAIT) cells, a prominent human innate-like T cell subset. We demonstrate significantly higher MDR1 expression by CD8 ⁺ CD161 ⁺⁺ Vα7.2 ⁺ MAIT cells than the phenotypically and functionally related CD8 ⁺ CD161 ⁺⁺ Vα7.2-subset and show MDR1 expression to be similarly high throughout MAIT CD4 ⁺ , CD8 ⁺ , double-negative (DN) and double-positive (DP) cell subsets. We demonstrate the MAIT cell-predominant CD8⁺ CD161⁺⁺ subset to uniquely and efficiently efflux the cytotoxic anthracycline daunorubicin, retain function on daunorubicin exposure and demonstrate MDR1-dependent protection from daunorubicin-induced apoptosis. By contrast, CD8⁺ CD161⁺⁺ Vα7.2⁺ MAIT cells were not protected from the anti-proliferative and cytotoxic effects of the immunosuppressive MDR1 substrates tacrolimus and mycophenoic acid, although function following MAIT cell-specific T cell receptor (TCR)-dependent and -independent stimulation was preserved on in-vitro exposure to these agents. Overall, our data further define MDR1 expression by CD161⁺⁺ T and MAIT cells and demonstrate the potential for high MDR1 expression by MAIT cells to confer resistance to cytotoxic MDR1 substrates in vivo . As our understanding of the importance of MAIT cells in human immunity and immunopathology grows, this is an important observation for clinical contexts such as the treatment of malignancy, autoimmunity and post-transplant immunosuppression.

Mucosal associated invariant T cells from human breast ducts mediate a Th17-skewed response to bacterially exposed breast carcinoma cells

BACKGROUND

Antimicrobial T cells play key roles in the disease progression of cancers arising in mucosal epithelial tissues, such as the colon. However, little is known about microbe-reactive T cells within human breast ducts and whether these impact breast carcinogenesis.

METHODS

Epithelial ducts were isolated from primary human breast tissue samples, and the associated T lymphocytes were characterized using flow cytometric analysis. Functional assays were performed to determine T-cell cytokine secretion in response to bacterially treated human breast carcinoma cells.

RESULTS

We show that human breast epithelial ducts contain mucosal associated invariant T (MAIT) cells, an innate T-cell population that recognizes specific bacterial metabolites presented by nonclassical MR1 antigen-presenting molecules. The MAIT cell population from breast ducts resembled that of peripheral blood in its innate lymphocyte phenotype (i.e., CD161, PLZF, and interleukin [IL]-18 receptor coexpression), but the breast duct MAIT cell population had a distinct T-cell receptor Vβ use profile and was markedly enriched for IL-17-producing cells compared with blood MAIT cells. Breast carcinoma cells that had been exposed to Escherichia coli activated MAIT cells in an MR1-dependent manner. However, whereas phorbol 12-myristate 13-acetate/ionomycin stimulation induced the production of both interferon-γ and IL-17 by breast duct MAIT cells, bacterially exposed breast carcinoma cells elicited a strongly IL-17-biased response. Breast carcinoma cells also showed upregulated expression of natural killer group 2 member D (NKG2D) ligands compared with primary breast epithelial cells, and the NKG2D receptor contributed to MAIT cell activation by the carcinoma cells.

CONCLUSIONS

These results demonstrate that MAIT cells from human breast ducts mediate a selective T-helper 17 cell response to human breast carcinoma cells that were exposed to E. coli. Thus, cues from the breast microbiome and the expression of stress-associated ligands by neoplastic breast duct epithelial cells may shape MAIT cell responses during breast carcinogenesis.

MDR1 in immunity: friend or foe?

MDR1 is an ATP-dependent transmembrane transporter primarily studied for its role in the detoxification of tissues and for its implication in resistance of tumor cells to chemotherapy treatment. Several studies also report on its expression on immune cells where it plays a protective role from xenobiotics and toxins. This review provides an overview of what is known on MDR1 expression in immune cells in human, and its implications in different pathologies and their treatment options.