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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. Godfrey and colleagues review the basic biology, development, role in disease and immunotherapeutic potential of MAIT cells.

T cell lineages are defined by specialized functions and differential expression of surface antigens, cytokines and transcription factors. Conventional CD4+ and CD8+ T cells are the best studied of the T cell subsets, but ‘unconventional’ T cells have emerged as being more abundant and influential than has previously been appreciated. Key subsets of unconventional T cells include natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells and γδ T cells; collectively, these make up ~10% of circulating T cells, and often they are the majority of T cells in tissues such as the liver and gut mucosa. Defects and deficiencies in unconventional T cells are associated with autoimmunity, chronic inflammation and cancer, so it is important to understand how their development is regulated. In this Review, we describe the thymic development of NKT cells, MAIT cells and γδ T cells and highlight some of the key differences between conventional and unconventional T cell development.The authors compare the thymic development of several innate-like T cell populations, including natural killer T cells, mucosal-associated invariant T cells and γδ T cells. They focus on the cytokines, surface molecules and transcription factors that are necessary for the development of these cells and highlight some of the key differences from conventional T cell development.

Mucosal‐associated invariant T (MAIT) cells represent up to 10% of circulating human T cells. They are usually defined using combinations of non‐lineage‐specific (surrogate) markers such as anti‐TRAV1‐2, CD161, IL‐18Rα and CD26. The development of MR1‐Ag tetramers now permits the specific identification of MAIT cells based on T‐cell receptor specificity. Here, we compare these approaches for identifying MAIT cells and show that surrogate markers are not always accurate in identifying these cells, particularly the CD4+ fraction. Moreover, while all MAIT cell subsets produced comparable levels of IFNγ, TNF and IL‐17A, the CD4+ population produced more IL‐2 than the other subsets. In a human ontogeny study, we show that the frequencies of most MR1 tetramer+ MAIT cells, with the exception of CD4+ MAIT cells, increased from birth to about 25 years of age and declined thereafter. We also demonstrate a positive association between the frequency of MAIT cells and other unconventional T cells including Natural Killer T (NKT) cells and Vδ2+ γδ T cells. Accordingly, this study demonstrates that MAIT cells are phenotypically and functionally diverse, that surrogate markers may not reliably identify all of these cells, and that their numbers are regulated in an age‐dependent manner and correlate with NKT and Vδ2+ γδ T cells. This study uses MR1 tetramers to enumerate and phenotypically characterize human blood MAIT cells, and subsets thereof based on CD4 and CD8 expression. Furthermore MR1 tetramers are compared to the commonly used mAb‐based MAIT cell identification techniques.

Mucosal-associated invariant T (MAIT) cells express an invariant TRAV1/TRAJ33 TCR-α chain and are restricted to the MHC-I-like molecule, MR1. Whether MAIT cell development depends on this invariant TCR-α chain is unclear. Here we generate Traj33 -deficient mice and show that they are highly depleted of MAIT cells; however, a residual population remains and can respond to exogenous antigen in vitro or pulmonary Legionella challenge in vivo. These residual cells include some that express Trav1 + TCRs with conservative Traj -gene substitutions, and others that express Trav1 - TCRs with a broad range of Traj genes. We further report that human TRAV1-2 - MR1-restricted T cells contain both MAIT-like and non-MAIT-like cells, as judged by their TCR repertoire, antigen reactivity and phenotypic features. These include a MAIT-like population that expresses a public, canonical TRAV36 + TRBV28 + TCR. Our findings highlight the TCR diversity and the resulting potential impact on antigen recognition by MR1-restricted T cells. Mucosal-associated invariant T (MAIT) cells express invariant TRAV1/TRAJ33 TCR-α gene segments and detect antigens presented by MR1. Here the authors show that atypical, MR1-restricted MAIT populations that include both Trav1 + and Trav1- cells are found in both Traj33 -deficient mice and human peripheral blood.

i NKT cells in adipose tissue are anti-inflammatory. Brenner and colleagues show that adipose i NKT cells have a unique transcriptional program, produce IL-2 and IL-10 and lack expression of the transcription factor PLZF. Invariant natural killer T cells ( i NKT cells) are lipid-sensing innate T cells that are restricted by the antigen-presenting molecule CD1d and express the transcription factor PLZF. i NKT cells accumulate in adipose tissue, where they are anti-inflammatory, but the factors that contribute to their anti-inflammatory nature, as well as their targets in adipose tissue, are unknown. Here we found that i NKT cells in adipose tissue had a unique transcriptional program and produced interleukin 2 (IL-2) and IL-10. Unlike other i NKT cells, they lacked PLZF but expressed the transcription factor E4BP4, which controlled their IL-10 production. The adipose i NKT cells were a tissue-resident population that induced an anti-inflammatory phenotype in macrophages and, through the production of IL-2, controlled the number, proliferation and suppressor function of regulatory T cells (T reg cells) in adipose tissue. Thus, i NKT cells in adipose tissue are unique regulators of immunological homeostasis in this tissue.

Mucosal‐associated invariant T (MAIT) cells develop in the thymus and migrate into the periphery to become the largest antigen‐specific αβ T‐cell population in the human immune system. However, the frequency of MAIT cells varies widely between human individuals, and the basis for this is unclear. While MAIT cells are highly conserved through evolution and are phenotypically similar between humans and mice, they represent a much smaller proportion of total T cells in mice. In this review, we discuss how MAIT cells transition through a three‐stage development pathway in both mouse and human thymus, and continue to mature and expand after they leave the thymus. Moreover, we will explore and speculate on how specific factors regulate different stages of this process. Mucosal‐associated invariant T cells represent the largest population of T cells with a single specificity in the human immune system. This article reviews how MAIT cells develop in mice and humans.

The function of MR1-restricted mucosal-associated invariant T (MAIT) cells in tumor immunity is unclear. Here we show that MAIT cell-deficient mice have enhanced NK cell-dependent control of metastatic B16F10 tumor growth relative to control mice. Analyses of this interplay in human tumor samples reveal that high expression of a MAIT cell gene signature negatively impacts the prognostic significance of NK cells. Paradoxically, pre-pulsing tumors with MAIT cell antigens, or activating MAIT cells in vivo, enhances anti-tumor immunity in B16F10 and E0771 mouse tumor models, including in the context of established metastasis. These effects are associated with enhanced NK cell responses and increased expression of both IFN-γ-dependent and inflammatory genes in NK cells. Importantly, activated human MAIT cells also promote the function of NK cells isolated from patient tumor samples. Our results thus describe an activation-dependent, MAIT cell-mediated regulation of NK cells, and suggest a potential therapeutic avenue for cancer treatment. Mucosal-associated invariant T (MAIT) cells facilitate anti-microbial responses, but their functions in cancer protection is unclear. Here the authors show that activated MAIT cells induce an IFN-γ transcriptome in natural killer (NK) cells and enhance NK-dependent anti-cancer immunity in mice, thereby hinting a new avenue for cancer therapy.

There are reports of poor working conditions for early and mid-career academics (EMCAs) in universities, however, empirical data using validated tools are scarce. We conducted an online, cross-sectional survey using validated tools to assess workplace satisfaction, exposure to workplace abuse, and mental health. Participants included employees of medical and health faculties of two of the largest Australian universities, surveyed between October 2020 and January 2021. Overall, 284 participants responded. Many reported job insecurity: half (50.7%) working on contracts with less than one remaining year. Workloads were considerable, with 89.5% of participants working overtime and 54.8% reporting burnout. Workplace abuse in the forms of bullying (46.6%), sexual harassment (25.3%), sexism (49.8%) and racism (22.5%) were commonly reported. Clinically significant symptoms of depression (28.0%), anxiety (21.7%) and suicidal ideation or self-harm (13.6%) were reported; with a higher prevalence among those working more overtime, and those exposed to workplace abuse. Priorities include providing a stable and safe workplace, increasing accountability and transparency in addressing workplace abuse, and supporting professional development. In summary, EMCAs in our study were commonly exposed to precarious employment conditions and workplace abuse. Our findings provide empirical evidence on where universities and funding bodies should direct resources and change organisational risk factors, to improve workplace culture.

Innate T cells, including CD1d-restricted invariant natural killer T (iNKT) cells, are characterized by their rapid activation in response to non-peptide antigens, such as lipids. While the transcriptional profiles of naive, effector, and memory adaptive T cells have been well studied, less is known about the transcriptional regulation of different iNKT cell activation states. Here, using single-cell RNA-sequencing, we performed longitudinal profiling of activated murine iNKT cells, generating a transcriptomic atlas of iNKT cell activation states. We found that transcriptional signatures of activation are highly conserved among heterogeneous iNKT cell populations, including NKT1, NKT2, and NKT17 subsets, and human iNKT cells. Strikingly, we found that regulatory iNKT cells, such as adipose iNKT cells, undergo blunted activation and display constitutive enrichment of memory-like cMAF + and KLRG1 + populations. Moreover, we identify a conserved cMAF-associated transcriptional network among NKT10 cells, providing novel insights into the biology of regulatory and antigen-experienced iNKT cells.

There is significant clinical interest in targeting adenosine-mediated immunosuppression, with several small molecule inhibitors having been developed for targeting the A 2A R receptor. Understanding of the mechanism by which A 2A R is regulated has been hindered by difficulty in identifying the cell types that express A 2A R due to a lack of robust antibodies for these receptors. To overcome this limitation, here an A 2A R eGFP reporter mouse is developed, enabling the expression of A 2A R during ongoing anti-tumor immune responses to be assessed. This reveals that A 2A R is highly expressed on all tumor-infiltrating lymphocyte subsets including Natural Killer (NK) cells, NKT cells, γδ T cells, conventional CD4 + and CD8 + T lymphocytes and on a MHCII hi CD86 hi subset of type 2 conventional dendritic cells. In response to PD-L1 blockade, the emergence of PD-1 + A 2A R - cells correlates with successful therapeutic responses, whilst IL-18 is identified as a cytokine that potently upregulates A 2A R and synergizes with A 2A R deficiency to improve anti-tumor immunity. These studies provide insight into the biology of A 2A R in the context of anti-tumor immunity and reveals potential combination immunotherapy approaches. Adenosine is an immunosuppressive metabolite known to limit anti-tumor immune responses. Here the authors report the characterization of an adenosine A2A receptor (A2AR) eGFP reporter mouse, providing immunological insights into the biology of A2AR expression in the context of anti-tumor immunity.