Ontogeny of human mucosal-associated invariant T cells and related T cell subsets

MAIT cell activation and recruitment in inflammation and tissue damage in acute appendicitis

Mucosal-associated invariant T (MAIT) cells are antimicrobial T cells abundant in the gut, but mechanisms for their migration into tissues during inflammation are poorly understood. Here, we used acute pediatric appendicitis (APA), a model of acute intestinal inflammation, to examine these migration mechanisms. MAIT cells were lower in numbers in circulation of patients with APA but were enriched in the inflamed appendix with increased production of proinflammatory cytokines. Using the patient-derived appendix organoid (PDAO) model, we found that circulating MAIT cells treated with inflammatory cytokines elevated in APA up-regulated chemokine receptors, including CCR1, CCR3, and CCR4. They exhibited enhanced infiltration of Escherichia coli-pulsed PDAO in a CCR1-, CCR2-, and CCR4-dependent manner. Close interactions of MAIT cells with infected organoids led to the PDAO structural destruction and death. These findings reveal a previously unidentified mechanism of MAIT cell tissue homing, their participation in tissue damage in APA, and their intricate relationship with mucosal tissues during acute intestinal inflammation in humans.

High pre-transplant Mucosal Associated Invariant T Cell (MAIT) count predicts favorable course of myeloid aplasia

AIMS

Mucosal Associated Invariant T (MAIT) cells are unconventional T cells with anti-infective potential. MAIT cells detect and fight against microbes on mucosal surfaces and in peripheral tissues. Previous works suggested that MAIT cells survive exposure to cytotoxic drugs in these locations. We sought to determine if they maintain their anti-infective functions after myeloablative chemotherapy.

METHODS

We correlated the amount of MAIT cells (measured by flow cytometry) in the peripheral blood of 100 adult patients before the start of myeloablative conditioning plus autologous stem cell transplantation with the clinical and laboratory outcomes of aplasia.

RESULTS

The amount of MAIT cells negatively correlated with peak C-reactive protein level and the amount of red blood cell transfusion units resulting in earlier discharge of patients with the highest amount of MAIT cells.

CONCLUSION

This work suggests the anti-infectious potential of MAIT cells is maintained during myeloid aplasia.

Antibiotic use in early life subsequently impairs MAIT cell-mediated immunity

Mucosal-associated invariant T (MAIT) cells are predominantly located in barrier tissues where they rapidly respond to pathogens and commensals by recognizing microbial derivatives of riboflavin synthesis. Early-life exposure to these metabolites imprints the abundance of MAIT cells within tissues, so we hypothesized that antibiotic use during this period may abrogate their development. We identified antibiotics that deplete riboflavin-synthesizing commensals and revealed an early period of susceptibility during which antibiotic administration impaired MAIT cell development. The reduction in MAIT cell abundance rendered mice more susceptible to pneumonia, while MAIT cell-deficient mice were unaffected by early-life antibiotics. Concomitant administration of a riboflavin-synthesizing commensal during antibiotic treatment was sufficient to restore MAIT cell development and immunity. Our work demonstrates that transient depletion of riboflavin-synthesizing commensals in early life can adversely affect responses to subsequent infections.

Functions of mucosal associated invariant T cells in eye diseases

Mucosal-associated invariant T (MAIT) cells are a unique subset of T cells that recognizes metabolites derived from the vitamin B2 biosynthetic pathway. Since the identification of cognate antigens for MAIT cells, knowledge of the functions of MAIT cells in cancer, autoimmunity, and infectious diseases has been rapidly expanding. Recently, MAIT cells have been found to contribute to visual protection against autoimmunity in the eye. The protective functions of MAIT cells are induced by T-cell receptor (TCR)-mediated activation. However, the underlying mechanisms remain unclear. Thus, this mini-review aims to discuss our findings and the complexity of MAIT cell-mediated immune regulation in the eye.

Role of innate T cells in necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a destructive gastrointestinal disease primarily affecting preterm babies. Despite advancements in neonatal care, NEC remains a significant cause of morbidity and mortality in neonatal intensive care units worldwide and the etiology of NEC is still unclear. Risk factors for NEC include prematurity, very low birth weight, feeding with formula, intestinal dysbiosis and bacterial infection. A review of the literature would suggest that supplementation of prebiotics and probiotics prevents NEC by altering the immune responses. Innate T cells, a highly conserved subpopulation of T cells that responds quickly to stimulation, develops differently from conventional T cells in neonates. This review aims to provide a succinct overview of innate T cells in neonates, encompassing their phenotypic characteristics, functional roles, likely involvement in the pathogenesis of NEC, and potential therapeutic implications.

A conserved transcriptional program for MAIT cells across mammalian evolution

Mucosal-associated invariant T (MAIT) cells harbor evolutionarily conserved TCRs, suggesting important functions. As human and mouse MAIT functional programs appear distinct, the evolutionarily conserved MAIT functional features remain unidentified. Using species-specific tetramers coupled to single-cell RNA sequencing, we characterized MAIT cell development in six species spanning 110 million years of evolution. Cross-species analyses revealed conserved transcriptional events underlying MAIT cell maturation, marked by ZBTB16 induction in all species. MAIT cells in human, sheep, cattle, and opossum acquired a shared type-1/17 transcriptional program, reflecting ancestral features. This program was also acquired by human iNKT cells, indicating common differentiation for innate-like T cells. Distinct type-1 and type-17 MAIT subsets developed in rodents, including pet mice and genetically diverse mouse strains. However, MAIT cells further matured in mouse intestines to acquire a remarkably conserved program characterized by concomitant expression of type-1, type-17, cytotoxicity, and tissue-repair genes. Altogether, the study provides a unifying view of the transcriptional features of innate-like T cells across evolution.

Human MAIT cells inhibit alloreactive T cell responses and protect against acute graft-versus-host disease

Adoptive transfer of immunoregulatory cells can prevent or ameliorate graft-versus-host disease (GVHD), which remains the main cause of nonrelapse mortality after allogeneic hematopoietic stem cell transplantation. Mucosal-associated invariant T (MAIT) cells were recently associated with tissue repair capacities and with lower rates of GVHD in humans. Here, we analyzed the immunosuppressive effect of MAIT cells in an in vitro model of alloreactivity and explored their adoptive transfer in a preclinical xenogeneic GVHD model. We found that MAIT cells, whether freshly purified or short-term expanded, dose-dependently inhibited proliferation and activation of alloreactive T cells. In immunodeficient mice injected with human PBMCs, MAIT cells greatly delayed GVHD onset and decreased severity when transferred early after PBMC injection but could also control ongoing GVHD when transferred at delayed time points. This effect was associated with decreased proliferation and effector function of human T cells infiltrating tissues of diseased mice and was correlated with lower circulating IFN-γ and TNF-α levels and increased IL-10 levels. MAIT cells acted partly in a contact-dependent manner, which likely required direct interaction of their T cell receptor with MHC class I-related molecule (MR1) induced on host-reactive T cells. These results support the setup of clinical trials using MAIT cells as universal therapeutic tools to control severe GVHD or mucosal inflammatory disorders.

The MR1/MAIT cell axis in CNS diseases

Mucosal-associated invariant T (MAIT) cells are a subpopulation of innate-like T cells that can be found throughout the body, predominantly in mucosal sites, the lungs and in the peripheral blood. MAIT cells recognize microbial-derived vitamin B (e.g., riboflavin) metabolite antigens that are presented by the major histocompatibility complex class I-like protein, MR1, found on a variety of cell types in the periphery and the CNS. Since their original discovery, MAIT cells have been studied predominantly in their roles in diseases in the periphery; however, it was not until the early 2000s that these cells were first examined for their contributions to disorders of the CNS, with the bulk of the work being done within the past few years. Currently, the MR1/MAIT cell axis has been investigated in only a few neurological diseases including, multiple sclerosis and experimental autoimmune encephalomyelitis, brain cancer/tumors, ischemia, cerebral palsy, general aging and, most recently, Alzheimer's disease. Each of these diseases demonstrates a role for this under-studied innate immune axis in its neuropathology. Together, they highlight the importance of studying the MR1/MAIT cell axis in CNS disorders. Here, we review the contributions of the MR1/MAIT cell axis in the progression or remission of these neurological diseases. This work has shed some light in terms of potentially exploiting the MR1/MAIT cell axis in novel therapeutic applications.

Natural killer cells and innate lymphoid cells but not NKT cells are mature in their cytokine production at birth

Early life is a time of increased susceptibility to infectious diseases and development of allergy. Innate lymphocytes are crucial components of the initiation and regulation of immune responses at mucosal surfaces, but functional differences in innate lymphocytes early in life are not fully described. We aimed to characterize the abundance and function of different innate lymphocyte cell populations in cord blood in comparison to that of adults. Blood was collected from adult donors and umbilical vessels at birth. Multicolor flow cytometry panels were used to identify and characterize lymphocyte populations and their capacity to produce hallmark cytokines. Lymphocytes were more abundant in cord blood compared to adults, however, mucosal-associated invariant T cells and natural killer T (NKT)-like cells, were far less abundant. The capacity of NKT-like cells to produce cytokines and their expression of the cytotoxic granule protein granzyme B and the marker of terminal differentiation CD57 were much lower in cord blood than in adults. In contrast, natural killer (NK) cells were as abundant in cord blood as in adults, they could produce IFNγ, and their expression of granzyme B was not significantly different from that of adult NK cells, although CD57 expression was lower. All innate lymphoid cell (ILC) subsets were more abundant in cord blood, and ILC1 and ILC2 were capable of production of IFNγ and IL-13, respectively. In conclusion, different innate lymphoid cells differ in both abundance and function in peripheral blood at birth and with important implications for immunity in early life.

Interleukin 23 receptor: Expression and regulation in immune cells

The importance of IL-23 and its specific receptor, IL-23R, in the pathogenesis of several chronic inflammatory diseases has been established, but the underlying pathological mechanisms are not fully understood. This review focuses on IL-23R expression and regulation in immune cells.

MAIT cells drive chronic inflammation in a genetically diverse murine model of spontaneous colitis

Background & aims

Lymphocytes that produce IL-17 can confer protective immunity during infections by pathogens, yet their involvement in inflammatory diseases is a subject of debate. Although these cells may perpetuate inflammation, resulting in tissue damage, they are also capable of contributing directly or indirectly to tissue repair, thus necessitating more detailed investigation. Mucosal-Associated-Invariant-T (MAIT) cells are innate-like T cells, acquiring a type III phenotype in the thymus. Here, we dissected the role of MAIT cells in vivo using a spontaneous colitis model in a genetically diverse mouse strain.

Methods

Multiparameter spectral flow cytometry and scRNAseq were used to characterize MAIT and immune cell dynamics and transcriptomic signatures respectively, in the collaborative-cross strain, CC011/Unc and CC011/Unc- Traj33 -/- .

Results

In contrast to many conventional mouse laboratory strains, the CC011 strain harbors a high baseline population of MAIT cells. We observed an age-related increase in colonic MAIT cells, Th17 cells, regulatory T cells, and neutrophils, which paralleled the development of spontaneous colitis. This progression manifested histological traits reminiscent of human IBD. The transcriptomic analysis of colonic MAIT cells from CC011 revealed an activation profile consistent with an inflammatory milieu, marked by an enhanced type-III response. Notably, IL-17A was abundantly secreted by MAIT cells in the colons of afflicted mice. Conversely, in the MAIT cell-deficient CC011-Traj33-/- mice, there was a notable absence of significant colonic histopathology. Furthermore, myeloperoxidase staining indicated a substantial decrease in colonic neutrophils.

Conclusions

Our findings suggest that MAIT cells play a pivotal role in modulating the severity of intestinal pathology, potentially orchestrating the inflammatory process by driving the accumulation of neutrophils within the colonic environment.

Characterization of the TCRβ repertoire of peripheral MR1-restricted MAIT cells in psoriasis vulgaris patients

Psoriasis vulgaris (PV) is an inflammatory skin disease largely driven by aberrant αβT cells. Mucosal-associated invariant T (MAIT) cells, which constitute the largest circulating innate-like αβT cell community in human adults, are characterized by a semi-invariant TCRVα7.2 receptor and MR1-restricted affinity toward microbial metabolites. Limited MAIT TCRα diversity is complemented by a more variable TCRβ repertoire, but its footprint in the MAIT repertoire of PV patients has never been tested. Here, we used bulk TCRSeq, MiXCR, VDJTools, and Immunarch pipelines to decipher and compare TCRβ clonotypes from flow-sorted, peripheral TCRVα7.2+MR1-5-OP-RU-tet+MAIT cells from 10 PV patients and 10 healthy, matched controls. The resulting TCRβ collections were highly private and individually unique, with small public clonotype content and high CDR3β amino acid length variability in both groups. The age-related increase in the 'hyperexpanded' clonotype compartment was observed in PV, but not in healthy MAIT repertoires. The TCRβ repertoires of PV patients were also marked by skewed TRBV/TRBJ pairing, and the emergence of PV-specific, public CDR3β peptide sequences closely matching the published CDR3β record from psoriatic skin. Overall, our study provides preliminary insight into the peripheral MAIT TCRβ repertoire in psoriasis and warrants further evaluation of its diagnostic and clinical significance.

Intravenous Bacille Calmette-Guérin vaccination protects simian immunodeficiency virus-infected macaques from tuberculosis

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), is the most common cause of death in people living with human immunodeficiency virus (HIV). Intra-dermal Bacille Calmette-Guérin (BCG) delivery is the only licensed vaccine against tuberculosis; however, it offers little protection from pulmonary tuberculosis in adults and is contraindicated in people living with HIV. Intravenous BCG confers protection against Mtb infection in rhesus macaques; we hypothesized that it might prevent tuberculosis in simian immunodeficiency virus (SIV)-infected macaques, a model for HIV infection. Here intravenous BCG-elicited robust airway T cell influx and elevated plasma and airway antibody titres in both SIV-infected and naive animals. Following Mtb challenge, all 7 vaccinated SIV-naive and 9 out of 12 vaccinated SIV-infected animals were protected, without any culturable bacteria detected from tissues. Peripheral blood mononuclear cell responses post-challenge indicated early clearance of Mtb in vaccinated animals, regardless of SIV infection. These data support that intravenous BCG is immunogenic and efficacious in SIV-infected animals.

Role of MAIT cells in gastrointestinal tract bacterial infections in humans: More than a gut feeling

Mucosa-associated invariant T (MAIT) cells are the largest population of unconventional T cells in humans. These antimicrobial T cells are poised with rapid effector responses following recognition of the cognate riboflavin (vitamin B2)-like metabolite antigens derived from microbial riboflavin biosynthetic pathway. Presentation of this unique class of small molecule metabolite antigens is mediated by the highly evolutionarily conserved major histocompatibility complex class I-related protein. In humans, MAIT cells are widely found along the upper and lower gastrointestinal tracts owing to their high expression of chemokine receptors and homing molecules directing them to these tissue sites. In this review, we discuss recent findings regarding the roles MAIT cells play in various gastrointestinal bacterial infections, and how their roles appear to differ depending on the etiological agents and the anatomical location. We further discuss the potential mechanisms by which MAIT cells contribute to pathogen control, orchestrate adaptive immunity, as well as their potential contribution to inflammation and tissue damage during gastrointestinal bacterial infections, and the ensuing tissue repair following resolution. Finally, we propose and discuss the use of the emerging three-dimensional organoid technology to test different hypotheses regarding the role of MAIT cells in gastrointestinal bacterial infections, inflammation, and immunity.

Single-cell analysis of human MAIT cell transcriptional, functional and clonal diversity

Mucosal-associated invariant T (MAIT) cells are innate-like T cells that recognize microbial metabolites through a semi-invariant T cell receptor (TCR). Major questions remain regarding the extent of human MAIT cell functional and clonal diversity. To address these, we analyzed the single-cell transcriptome and TCR repertoire of blood and liver MAIT cells and developed functional RNA-sequencing, a method to integrate function and TCR clonotype at single-cell resolution. MAIT cell clonal diversity was comparable to conventional memory T cells, with private TCR repertoires shared across matched tissues. Baseline functional diversity was low and largely related to tissue site. MAIT cells showed stimulus-specific transcriptional responses in vitro, with cells positioned along gradients of activation. Clonal identity influenced resting and activated transcriptional profiles but intriguingly was not associated with the capacity to produce IL-17. Overall, MAIT cells show phenotypic and functional diversity according to tissue localization, stimulation environment and clonotype.

Early gut microbiota intervention in premature infants: Application perspectives

BACKGROUND

Preterm birth is the leading cause of death in children under the age of five. One of the major factors contributing to the high risk of diseases and deaths in premature infants is the incomplete development of the intestinal immune system. The gut microbiota has been widely recognized as a critical factor in promoting the development and function of the intestinal immune system after birth. However, the gut microbiota of premature infants is at high risk of dysbiosis, which is highly associated with adverse effects on the development and education of the early life immune system. Early intervention can modulate the colonization and development of gut microbiota and has a long-term influence on the development of the intestinal immune system.

AIM OF REVIEW

This review aims to summarize the characterization, interconnection, and underlying mechanism of gut microbiota and intestinal innate immunity in premature infants, and to discuss the status, applicability, safety, and prospects of different intervention strategies in premature infants, thus providing an overview and outlook of the current applications and remaining gaps of early intervention strategies in premature infants.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review is focused on three key concepts. Firstly, the gut microbiota of premature infants is at high risk of dysbiosis, resulting in dysfunctional intestinal immune system processes. Secondly, contributing roles of early intervention have been observed in improving the intestinal environment and promoting gut microbiota colonization, which is significant in the development and function of gut immunity in premature infants. Thirdly, different strategies of early intervention, such as probiotics, fecal microbiota transplantation, and nutrients, show different safety, applicability, and outcome in premature infants, and the underlying mechanism is complex and poorly understood.

Aging unconventionally: γδ T cells, iNKT cells, and MAIT cells in aging

Unconventional T cells include γδ T cells, invariant Natural Killer T cells (iNKT) cells and Mucosal Associated Invariant T (MAIT) cells, which are distinguished from conventional T cells by their recognition of non-peptide ligands presented by non-polymorphic antigen presenting molecules and rapid effector functions that are pre-programmed during their development. Here we review current knowledge of the effect of age on unconventional T cells, from early life to old age, in both mice and humans. We then discuss the role of unconventional T cells in age-associated diseases and infections, highlighting the similarities between members of the unconventional T cell family in the context of aging.

Expansion of MAIT cells in the combined absence of NKT and γδ-T cells

Mucosal-associated invariant T (MAIT) cells, natural killer T (NKT) cells, and γδT cells are collectively referred to as 'unconventional T cells' due to their recognition of non-peptide antigens and restriction to MHC-I-like molecules. However, the factors controlling their widely variable frequencies between individuals and organs are poorly understood. We demonstrated that MAIT cells are increased in NKT or γδT cell-deficient mice and highly expand in mice lacking both cell types. TCRα repertoire analysis of γδT cell-deficient thymocytes revealed altered Trav segment usage relative to wild-type thymocytes, highlighting retention of the Tcra-Tcrd locus from the 129 mouse strain used to generate Tcrd-/- mice. This resulted in a moderate increase in distal Trav segment usage, including Trav1, potentially contributing to increased generation of Trav1-Traj33+ MAIT cells in the Tcrd-/- thymus. Importantly, adoptively transferred MAIT cells underwent increased homeostatic proliferation within NKT/gdT cell-deficient tissues, with MAIT cell subsets exhibiting tissue-specific homing patterns. Our data reveal a shared niche for unconventional T cells, where competition for common factors may be exploited to collectively modulate these cells in the immune response. Lastly, our findings emphasise careful assessment of studies using NKT or γδT cell-deficient mice when investigating the role of unconventional T cells in disease.

New cell sources for CAR-based immunotherapy

Chimeric antigen receptor (CAR) T cell therapy, in which a patient's own T lymphocytes are engineered to recognize and kill cancer cells, has achieved striking success in some hematological malignancies in preclinical and clinical trials, resulting in six FDA-approved CAR-T products currently available in the market. Despite impressive clinical outcomes, concerns about treatment failure associated with low efficacy or high cytotoxicity of CAR-T cells remain. While the main focus has been on improving CAR-T cells, exploring alternative cellular sources for CAR generation has garnered growing interest. In the current review, we comprehensively evaluated other cell sources rather than conventional T cells for CAR generation.

Die Kämpfe únd schláchten-the struggles and battles of innate-like effector T lymphocytes with microbes

The large majority of lymphocytes belong to the adaptive immune system, which are made up of B2 B cells and the αβ T cells; these are the effectors in an adaptive immune response. A multitudinous group of lymphoid lineage cells does not fit the conventional lymphocyte paradigm; it is the unconventional lymphocytes. Unconventional lymphocytes-here called innate/innate-like lymphocytes, include those that express rearranged antigen receptor genes and those that do not. Even though the innate/innate-like lymphocytes express rearranged, adaptive antigen-specific receptors, they behave like innate immune cells, which allows them to integrate sensory signals from the innate immune system and relay that umwelt to downstream innate and adaptive effector responses. Here, we review natural killer T cells and mucosal-associated invariant T cells-two prototypic innate-like T lymphocytes, which sense their local environment and relay that umwelt to downstream innate and adaptive effector cells to actuate an appropriate host response that confers immunity to infectious agents.

Vaccination with intravenous BCG protects macaques with pre-existing SIV infection from tuberculosis

Tuberculosis (TB) is the most common cause of death in people living with HIV. BCG delivered intradermally (ID) is the only licensed vaccine to prevent TB. However, it offers little protection from pulmonary TB in adults. Intravenous (IV) BCG, but not ID BCG, confers striking protection against Mycobacterium tuberculosis (Mtb) infection and disease in rhesus macaques. We investigated whether IV BCG could protect against TB in macaques with a pre-existing SIV infection. There was a robust influx of airway T cells following IV BCG in both SIV-infected and SIV-naïve animals, with elevated antibody titers in plasma and airways. Following Mtb challenge, all 7 SIV-naïve and 9 out of 12 SIV-infected vaccinated animals were completely protected, without any culturable bacilli in their tissues. PBMC responses post-challenge indicated early clearance of Mtb in vaccinated animals regardless of SIV infection. These data support that IV BCG is immunogenic and efficacious in SIV-infected animals.

MAIT cells and the microbiome

Mucosal associated invariant T (MAIT) cells are innate-like T lymphocytes, strikingly enriched at mucosal surfaces and characterized by a semi-invariant αβ T cell receptor (TCR) recognizing microbial derived intermediates of riboflavin synthesis presented by the MHC-Ib molecule MR1. At barrier sites MAIT cells occupy a prime position for interaction with commensal microorganisms, comprising the microbiota. The microbiota is a rich source of riboflavin derived antigens required in early life to promote intra-thymic MAIT cell development and sustain a life-long population of tissue resident cells. A symbiotic relationship is thought to be maintained in health whereby microbes promote maturation and homeostasis, and in turn MAIT cells can engage a TCR-dependent "tissue repair" program in the presence of commensal organisms conducive to sustaining barrier function and integrity of the microbial community. MAIT cell activation can be induced in a MR1-TCR dependent manner or through MR1-TCR independent mechanisms via pro-inflammatory cytokines interleukin (IL)-12/-15/-18 and type I interferon. MAIT cells provide immunity against bacterial, fungal and viral pathogens. However, MAIT cells may have deleterious effects through insufficient or exacerbated effector activity and have been implicated in autoimmune, inflammatory and allergic conditions in which microbial dysbiosis is a shared feature. In this review we summarize the current knowledge on the role of the microbiota in the development and maintenance of circulating and tissue resident MAIT cells. We also explore how microbial dysbiosis, alongside changes in intestinal permeability and imbalance between pro- and anti-inflammatory components of the immune response are together involved in the potential pathogenicity of MAIT cells. Whilst there have been significant improvements in our understanding of how the microbiota shapes MAIT cell function, human data are relatively lacking, and it remains unknown if MAIT cells can conversely influence the composition of the microbiota. We speculate whether, in a human population, differences in microbiomes might account for the heterogeneity observed in MAIT cell frequency across mucosal sites or between individuals, and response to therapies targeting T cells. Moreover, we speculate whether manipulation of the microbiota, or harnessing MAIT cell ligands within the gut or disease-specific sites could offer novel therapeutic strategies.

Mucosal-associated invariant T cells in hematological malignancies: Current knowledge, pending questions

Non-classical HLA restricted T cell subsets such as γδ T and NK-T cells are showing promises for immune-based therapy of hematological malignancies. Mucosal-Associated Invariant T cells (MAIT) belong to this family of innate-like T cell subsets and are the focus of many studies on infectious diseases, owing to their unusual recognition of bacterial/fungal metabolites. Their ability to produce type 1 cytokines (IFNγ, TNFα) as well as cytotoxic effector molecules endows them with potential anti-tumor functions. However, their contribution to tumor surveillance in solid cancers is unclear, and only few studies have specifically focused on MAIT cells in blood cancers. In this review, we wish to recapitulate our current knowledge on MAIT cells biology in hematological neoplasms, at diagnosis and/or during treatment, as well as tentative approaches to target them as therapeutic tools. We also wish to take this opportunity to briefly elaborate on what we think are important question to address in this field, as well as potential limitations to overcome in order to make MAIT cells the basis of future, novel therapies for hematological cancers.

The adaptive immune system in early life: The shift makes it count

Respiratory infectious diseases encountered early in life may result in life-threatening disease in neonates, which is primarily explained by the relatively naive neonatal immune system. Whereas vaccines are not readily available for all infectious diseases, vaccinations have greatly reduced childhood mortality. However, repeated vaccinations are required to reach protective immunity in infants and not all vaccinations are effective at young age. Moreover, protective adaptive immunity elicited by vaccination wanes more rapidly at young age compared to adulthood. The infant adaptive immune system has previously been considered immature but this paradigm has changed during the past years. Recent evidence shows that the early life adaptive immune system is equipped with a strong innate-like effector function to eliminate acute pathogenic threats. These strong innate-like effector capacities are in turn kept in check by a tolerogenic counterpart of the adaptive system that may have evolved to maintain balance and to reduce collateral damage. In this review, we provide insight into these aspects of the early life's adaptive immune system by addressing recent literature. Moreover, we speculate that this shift from innate-like and tolerogenic adaptive immune features towards formation of immune memory may underlie different efficacy of infant vaccination in these different phases of immune development. Therefore, presence of innate-like and tolerogenic features of the adaptive immune system may be used as a biomarker to improve vaccination strategies against respiratory and other infections in early life.

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

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

Mining the multifunction of mucosal-associated invariant T cells in hematological malignancies and transplantation immunity: A promising hexagon soldier in immunomodulatory

Mucosal-associated invariant T (MAIT) cells are evolutionarily conserved innate-like T cells capable of recognizing bacterial and fungal ligands derived from vitamin B biosynthesis. Under different stimulation conditions, MAIT cells can display different immune effector phenotypes, exerting immune regulation and anti-/protumor responses. Based on basic biological characteristics, including the enrichment of mucosal tissue, the secretion of mucosal repair protective factors (interleukin-17, etc.), and the activation of riboflavin metabolites by intestinal flora, MAIT cells may play an important role in the immune regulation effect of mucosal lesions or inflammation. At the same time, activated MAIT cells secrete granzyme B, perforin, interferon γ, and other toxic cytokines, which can mediate anti-tumor effects. In addition, since a variety of hematological malignancies express the targets of MAIT cell-specific effector molecules, MAIT cells are also a potentially attractive target for cell therapy or immunotherapy for hematological malignancies. In this review, we will provide an overview of MAIT research related to blood system diseases and discuss the possible immunomodulatory or anti-tumor roles that unique biological characteristics or effector phenotypes may play in hematological diseases.

The role of the gut microbiota in multiple sclerosis

During the past decade, research has revealed that the vast community of micro-organisms that inhabit the gut - known as the gut microbiota - is intricately linked to human health and disease, partly as a result of its influence on systemic immune responses. Accumulating evidence demonstrates that these effects on immune function are important in neuroinflammatory diseases, such as multiple sclerosis (MS), and that modulation of the microbiome could be therapeutically beneficial in these conditions. In this Review, we examine the influence that the gut microbiota have on immune function via modulation of serotonin production in the gut and through complex interactions with components of the immune system, such as T cells and B cells. We then present evidence from studies in mice and humans that these effects of the gut microbiota on the immune system are important in the development and course of MS. We also consider how strategies for manipulating the composition of the gut microbiota could be used to influence disease-related immune dysfunction and form the basis of a new class of therapeutics. The strategies discussed include the use of probiotics, supplementation with bacterial metabolites, transplantation of faecal matter or defined microbial communities, and dietary intervention. Carefully designed studies with large human cohorts will be required to gain a full understanding of the microbiome changes involved in MS and to develop therapeutic strategies that target these changes.

γδ T, NKT, and MAIT Cells During Evolution: Redundancy or Specialized Functions?

Innate-like T cells display characteristics of both innate lymphoid cells (ILCs) and mainstream αβ T cells, leading to overlapping functions of innate-like T cells with both subsets. In this review, we show that although innate-like T cells are probably present in all vertebrates, their main characteristics are much better known in amphibians and mammals. Innate-like T cells encompass both γδ and αβ T cells. In mammals, γδ TCRs likely coevolved with molecules of the butyrophilin family they interact with, whereas the semi-invariant TCRs of iNKT and mucosal-associated invariant T cells are evolutionarily locked with their restricting MH1b molecules, CD1d and MR1, respectively. The strong conservation of the Ag recognition systems of innate-like T cell subsets despite similar effector potentialities supports that each one fulfills nonredundant roles related to their Ag specificity.

Harnessing Unconventional T Cells and Innate Lymphoid Cells to Prevent and Treat Hematological Malignancies: Prospects for New Immunotherapy

Unconventional T cells and innate lymphoid cells (ILCs) make up a heterogeneous set of cells that characteristically show prompt responses toward specific antigens. Unconventional T cells recognize non-peptide antigens, which are bound and presented by diverse non-polymorphic antigen-presenting molecules and comprise γδ T cells, MR1-restricted mucosal-associated invariant T cells (MAITs), and natural killer T cells (NKTs). On the other hand, ILCs lack antigen-specific receptors and act as the innate counterpart to the T lymphocytes found in the adaptive immune response. The alteration of unconventional T cells and ILCs in frequency and functionality is correlated with the onset of several autoimmune diseases, allergy, inflammation, and tumor. However, depending on the physio-pathological framework, unconventional T cells may exhibit either protective or pathogenic activity in a range of neoplastic diseases. Nonetheless, experimental models and clinical studies have displayed that some unconventional T cells are potential therapeutic targets, as well as prognostic and diagnostic markers. In fact, cell-mediated immune response in tumors has become the focus in immunotherapy against neoplastic disease. This review concentrates on the present knowledge concerning the function of unconventional T cell sets in the antitumor immune response in hematological malignancies, such as acute and chronic leukemia, multiple myeloma, and lymphoproliferative disorders. Moreover, we discuss the possibility that modulating the activity of unconventional T cells could be useful in the treatment of hematological neoplasms, in the prevention of specific conditions (such as graft versus host disease), and in the formulation of an effective anticancer vaccine therapy. The exact knowledge of the role of these cells could represent the prerequisite for the creation of a new form of immunotherapy for hematological neoplasms.

MAIT and Vδ2 unconventional T cells are supported by a diverse intestinal microbiome and correlate with favorable patient outcome after allogeneic HCT

Microbial diversity is associated with improved outcomes in recipients of allogeneic hematopoietic cell transplantation (allo-HCT), but the mechanism underlying this observation is unclear. In a cohort of 174 patients who underwent allo-HCT, we demonstrate that a diverse intestinal microbiome early after allo-HCT is associated with an increased number of innate-like mucosal-associated invariant T (MAIT) cells, which are in turn associated with improved overall survival and less acute graft-versus-host disease (aGVHD). Immune profiling of conventional and unconventional immune cell subsets revealed that the prevalence of Vδ2 cells, the major circulating subpopulation of γδ T cells, closely correlated with the frequency of MAIT cells and was associated with less aGVHD. Analysis of these populations using both single-cell transcriptomics and flow cytometry suggested a shift toward activated phenotypes and a gain of cytotoxic and effector functions after transplantation. A diverse intestinal microbiome with the capacity to produce activating ligands for MAIT and Vδ2 cells appeared to be necessary for the maintenance of these populations after allo-HCT. These data suggest an immunological link between intestinal microbial diversity, microbe-derived ligands, and maintenance of unconventional T cells.

Immunoprofiling reveals cell subsets associated with the trajectory of cytomegalovirus reactivation post stem cell transplantation

Human cytomegalovirus reactivation is a major opportunistic infection after allogeneic haematopoietic stem cell transplantation and has a complex relationship with post-transplant immune reconstitution. Here, we use mass cytometry to define patterns of innate and adaptive immune cell reconstitution at key phases of human cytomegalovirus reactivation in the first 100 days post haematopoietic stem cell transplantation. Human cytomegalovirus reactivation is associated with the development of activated, memory T-cell profiles, with faster effector-memory CD4⁺ T-cell recovery in patients with low-level versus high-level human cytomegalovirus DNAemia. Mucosal-associated invariant T cell levels at the initial detection of human cytomegalovirus DNAemia are significantly lower in patients who subsequently develop high-level versus low-level human cytomegalovirus reactivation. Our data describe distinct immune signatures that emerged with human cytomegalovirus reactivation after haematopoietic stem cell transplantation, and highlight Mucosal-associated invariant T cell levels at the first detection of reactivation as a marker that may be useful to anticipate the magnitude of human cytomegalovirus DNAemia.

Human cytomegalovirus is a major cause of morbidity and mortality in transplant patients and multiple immune cells types are critical during infection and reactivation. Here the authors assess the immune cell compartments of haematopoietic stem cell recipients in the early period post transplantation and identify key features of effector memory CD4⁺ T cells and mucosal associated invariant T cells in this context.

Early life host-microbe interactions in skin

Our skin is the interface through which we mediate lifelong interactions with our surrounding environment. Initial development of the skin's epidermis, adnexal structures, and barrier function is necessary for normal cutaneous microbial colonization, immune development, and prevention of disease. Early life microbial exposures can have unique and long-lasting impacts on skin health. The identity of neonatal skin microbes and the context in which they are first encountered, i.e., through a compromised skin barrier or in conjunction with cutaneous inflammation, can have additional short- and long-term health consequences. Here, we discuss key attributes of infant skin and endogenous and exogenous factors that shape its relationship to the early life cutaneous microbiome, with a focus on their clinical implications.

Characterization of cord blood CD3+ TCRVα7.2+ CD161high T and innate lymphoid cells in the pregnancies with gestational diabetes, morbidly adherent placenta, and pregnancy hypertension diseases

PROBLEM

Although pregnant women with gestational diabetes (GD), morbidly adherent placenta (MAP), and pregnancy hypertension (pHT) diseases lead to intrauterine growth restriction (IUGR), little is known about their effect on mucosal-associated invariant T (MAIT) and innate lymphoid cells (ILC) in the umbilical cord. This study aimed to quantify and characterize MAIT cells and ILCs in the cord blood of pregnant women with GD, MAP, and pHT diseases.

METHOD OF STUDY

Cord blood mononuclear cells (CBMCs) were isolated by Ficoll-Paque gradient. CD3⁺ TCRVα7.2⁺ CD161^high cells and ILC subsets were quantified by flow cytometry. CBMCs were stimulated with PMA/Ionomycin and Golgi Plug for 4 h and stained for IFN-γ, TNF-α, and granzyme B. The stained cells were analyzed on FACS ARIA III.

RESULTS

Compared with healthy pregnancies, in the cord blood of the pHT group, elevated number of lymphocytes was observed. Moreover, the absolute number of IFN-γ producing CD4⁺ or CD4^- subsets of CD3⁺ TCRVα7.2⁺ CD161^high cells as well as those producing granzyme B were significantly elevated in the pHT group compared to healthy controls suggesting increased MAIT cell activity in the pHT cord blood. Similarly, in the MAP group, the absolute number of total CD3⁺ TCRVα7.2⁺ CD161^high cells, but not individual CD4⁺ or negative subsets, were significantly increased compared with healthy controls' cord blood. Absolute numbers of total CD3⁺ TCRVα7.2⁺ CD161^high cells and their subsets were comparable in the cord blood of the GD group compared with healthy controls. Finally, the absolute number of total ILCs and ILC3 subset were significantly elevated in only pHT cord blood compared with healthy controls. Our data also reveal that IFN-γ⁺ or granzyme B⁺ cell numbers negatively correlated with fetal birth weight.

CONCLUSIONS

CD3⁺ TCRVα7.2⁺ CD161^high cells and ILCs show unique expansion and activity in the cord blood of pregnant women with distinct diseases causing IUGR and may play roles in fetal growth restriction.

Current insights in mouse iNKT and MAIT cell development using single cell transcriptomics data

Innate T (T_inn) cells are a collection of T cells with important regulatory functions that have a crucial role in immunity towards tumors, bacteria, viruses, and in cell-mediated autoimmunity. In mice, the two main αβ T_inn cell subsets include the invariant NKT (iNKT) cells that recognize glycolipid antigens presented by non-polymorphic CD1d molecules and the mucosal associated invariant T (MAIT) cells that recognize vitamin B metabolites presented by the non-polymorphic MR1 molecules. Due to their ability to promptly secrete large quantities of cytokines either after T cell antigen receptor (TCR) activation or upon exposure to tissue- and antigen-presenting cell-derived cytokines, T_inn cells are thought to act as a bridge between the innate and adaptive immune systems and have the ability to shape the overall immune response. Their swift response reflects the early acquisition of helper effector programs during their development in the thymus, independently of pathogen exposure and prior to taking up residence in peripheral tissues. Several studies recently profiled, in an unbiased manner, the transcriptomes of mouse thymic iNKT and MAIT cells at the single cell level. Based on these data, we re-examine in this review how T_inn cells develop in the mouse thymus and undergo effector differentiation.

Assessing the safety of bioactive ingredients in infant formula that affect the immune system: recommendations from an expert panel

Bioactive ingredients for infant formula have been sought to reduce disparities in health outcomes between breastfed and formula-fed infants. Traditional food safety methodologies have limited ability to assess some bioactive ingredients. It is difficult to assess the effects of nutrition on the infant immune system because of coincident developmental adaptations to birth, establishment of the microbiome and introduction to solid foods, and perinatal environmental factors. An expert panel was convened to review information on immune system development published since the 2004 Institute of Medicine report on evaluating the safety of new infant formula ingredients and to recommend measurements that demonstrate the safety of bioactive ingredients intended for that use. Panel members participated in a 2-d virtual symposium in November 2020 and in follow-up discussions throughout early 2021. Key topics included identification of immune system endpoints from nutritional intervention studies, effects of human milk feeding and human milk substances on infant health outcomes, ontologic development of the infant immune system, and microbial influences on tolerance. The panel explored how "nonnormal" conditions such as preterm birth, allergy, and genetic disorders could help define developmental immune markers for healthy term infants. With consideration of breastfed infants as a reference, ensuring proper control groups, and attention to numerous potential confounders, the panel recommended a set of standard clinical endpoints including growth, response to vaccination, infection and other adverse effects related to inflammation, and allergy and atopic diseases. It compiled a set of candidate markers to characterize stereotypical patterns of immune system development during infancy, but absence of reference ranges, variability in methods and populations, and unreliability of individual markers to predict disease prevented the panel from including many markers as safety endpoints. The panel's findings and recommendations are applicable for industry, regulatory, and academic settings, and will inform safety assessments for immunomodulatory ingredients in foods besides infant formula.

A subset of follicular helper-like MAIT cells can provide B cell help and support antibody production in the mucosa

[Figure: see text].

Immune Suppression in Allogeneic Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative treatment for high-risk hematologic disorders. There are multiple immune-mediated complications following allo-HSCT that are prevented and/or treated by immunosuppressive agents. Principal among these immune-mediated complications is acute graft-versus-host disease (aGVHD), which occurs when the new donor immune system targets host tissue antigens. The immunobiology of aGVHD is complex and involves all aspects of the immune system. Due to the risk of aGVHD, immunosuppressive aGVHD prophylaxis is required for nearly all allogeneic HSCT recipients. Despite prophylaxis, aGVHD remains a major cause of nonrelapse mortality. Here, we discuss the clinical features of aGVHD, the immunobiology of aGVHD, the immunosuppressive therapies used to prevent and treat aGVHD, how to mitigate the side effects of these immunosuppressive therapies, and what additional immune-mediated post-allo-HSCT complications are also treated with immunosuppression.

A high-dimensional cytometry atlas of peripheral blood over the human life span

Age can profoundly affect susceptibility to a broad range of human diseases. Children are more susceptible to some infectious diseases such as diphtheria and pertussis, while in others, such as coronavirus disease 2019 and hepatitis A, they are more protected compared with adults. One explanation is that the composition of the immune system is a major contributing factor to disease susceptibility and severity. While most studies of the human immune system have focused on adults, how the immune system changes after birth remains poorly understood. Here, using high-dimensional spectral flow cytometry and computational methods for data integration, we analyzed more than 50 populations of immune cells in the peripheral blood, generating an immune cell atlas that defines the healthy human immune system from birth up to 75 years of age. We focused our efforts on children under 18 years old, revealing major changes in immune cell populations after birth and in children of schooling age. Specifically, CD4⁺ T effector memory cells, Vδ2⁺ gamma delta (γδ)T cells, memory B cells, plasmablasts, CD11c⁺ B cells and CD16⁺ CD56^bright natural killer (NK) cells peaked in children aged 5-9 years old, whereas frequencies of T helper 1, T helper 17, dendritic cells and CD16⁺ CD57⁺ CD56^dim NK cells were highest in older children (10-18 years old). The frequency of mucosal-associated invariant T cells was low in the first several years of life and highest in adults between 19 and 30 years old. Late adulthood was associated with fewer mucosal-associated invariant T cells and Vδ2⁺ γδ T cells but with increased frequencies of memory subsets of B cells, CD4⁺ and CD8⁺ T cells and CD57⁺ NK cells. This human immune cell atlas provides a critical resource to understand changes to the immune system during life and provides a reference for investigating the immune system in the context of human disease. This work may also help guide future therapies that target specific populations of immune cells to protect at-risk populations.

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

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

Early-life imprinting of unconventional T cells and tissue homeostasis

[Figure: see text].

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

The role of lymphocytes in neonatal encephalopathy

Neonatal encephalopathy is a syndrome characterised by abnormal neurological function often caused by a hypoxic insult during childbirth. Triggers such as hypoxia-ischaemia result in the release of cytokines and chemokines inducing the infiltration of neutrophils, natural killer cells, B cells, T cells and innate T cells into the brain. However, the role of these cells in the development of the brain injury is poorly understood. We review the mechanisms by which lymphocytes contribute to brain damage in NE. NK, T and innate T cells release proinflammatory cytokines contributing to the neurodegeneration in the secondary and tertiary phase of injury, whereas B cells and regulatory T cells produce IL-10 protecting the brain in NE. Targeting lymphocytes may have therapeutic potential in the treatment of NE in terms of management of inflammation and brain damage, particularly in the tertiary or persistent phases.

Human MAIT cells are devoid of alloreactive potential: prompting their use as universal cells for adoptive immune therapy

Background

Mucosal-associated invariant T (MAIT) cells are semi-invariant T cells that recognize microbial antigens presented by the highly conserved MR1 molecule. MAIT cells are predominantly localized in the liver and barrier tissues and are potent effectors of antimicrobial defense. MAIT cells are very few at birth and accumulate gradually over a period of about 6 years during the infancy. The cytotoxic potential of MAIT cells, as well as their newly described regulatory and tissue repair functions, open the possibility of exploiting their properties in adoptive therapy. A prerequisite for their use as ‘universal’ cells would be a lack of alloreactive potential, which remains to be demonstrated.

Methods

We used ex vivo, in vitro and in vivo models to determine if human MAIT cells contribute to allogeneic responses.

Results

We show that recovery of MAIT cells after allogeneic hematopoietic stem cell transplantation recapitulates their slow physiological expansion in early childhood, independent of recovery of non-MAIT T cells. In vitro, signals provided by allogeneic cells and cytokines do not induce sustained MAIT cell proliferation. In vivo, human MAIT cells do not expand nor accumulate in tissues in a model of T-cell-mediated xenogeneic graft-versus-host disease in immunodeficient mice.

Conclusions

Altogether, these results provide evidence that MAIT cells are devoid of alloreactive potential and pave the way for harnessing their translational potential in universal adoptive therapy overcoming barriers of HLA disparity.

Trial registration number

ClinicalTrials.gov number NCT02403089.

Single-Cell Analysis of the Neonatal Immune System Across the Gestational Age Continuum

Although most causes of death and morbidity in premature infants are related to immune maladaptation, the premature immune system remains poorly understood. We provide a comprehensive single-cell depiction of the neonatal immune system at birth across the spectrum of viable gestational age (GA), ranging from 25 weeks to term. A mass cytometry immunoassay interrogated all major immune cell subsets, including signaling activity and responsiveness to stimulation. An elastic net model described the relationship between GA and immunome (R=0.85, p=8.75e-14), and unsupervised clustering highlighted previously unrecognized GA-dependent immune dynamics, including decreasing basal MAP-kinase/NFκB signaling in antigen presenting cells; increasing responsiveness of cytotoxic lymphocytes to interferon-α; and decreasing frequency of regulatory and invariant T cells, including NKT-like cells and CD8⁺CD161⁺ T cells. Knowledge gained from the analysis of the neonatal immune landscape across GA provides a mechanistic framework to understand the unique susceptibility of preterm infants to both hyper-inflammatory diseases and infections.

MAIT cells numbers and frequencies in patients with acute myeloid leukemia at diagnosis: association with cytogenetic profile and gene mutations

Harnessing or monitoring immune cells is actually a major topic in pre-clinical and clinical studies in acute myeloid leukemia (AML). Mucosal-Associated Invariant T cells (MAIT) constitute one of the largest subset of innate-like, cytotoxic T cell subsets in humans. Despite some papers suggesting a role for MAIT cells in cancer, their specific involvement remains unclear, especially in myeloid malignancies. This prospective monocentric study included 216 patients with a newly diagnosed AML. Circulating MAIT cells were quantified by flow cytometry at diagnosis and during intensive chemotherapy. We observed that circulating MAIT cells show a specific decline in AML patients at diagnosis compared to healthy donors. Post-induction monitored patients presented with a drastic drop in MAIT cell numbers, with recovery after one month. We also found correlation between decrease in MAIT cells number and adverse cytogenetic profile. FLT3-ITD and IDH ½ mutations were associated with higher MAIT cell numbers. Patients with high level of activated MAIT cells are under-represented within patients with a favorable cytogenetic profile, and over-represented among patients with IDH1 mutations or bi-allelic CEBPA mutations. We show for the first time that circulating MAIT cells are affected in newly diagnosed AML patients, suggesting a link between MAIT cells and AML progression. Our work fosters new studies to deepen our knowledge about the role of MAIT cells in cancer.

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.

Extreme prematurity and sepsis strongly influence frequencies and functional characteristics of circulating γδ T and natural killer cells

Objectives

Extremely low gestational age neonates with extremely low birthweight (ELGAN/ELBW) are highly susceptible to infection. This is linked to their relatively immature immune system which is not yet fully compatible with an extra‐uterine environment. Here, we performed a longitudinal characterisation of unconventional T and natural killer (NK) cells in ELGAN/ELBW during their first months of life.

Methods

Peripheral blood mononuclear cells were collected from 97 ELGAN/ELBW at 14 and 28 days of life and at a time point corresponding to postmenstrual week 36 + 0. γδ T‐cell, NKT‐cell, mucosa‐associated invariant T‐cell and NK cell frequencies and characteristics were analysed by flow cytometry. As control, cells from 14‐day‐old full‐term (FT) infants were included.

Results

Extreme prematurity had significant bearing on γδ T‐cell and NK cell frequencies and characteristics. ELGAN/ELBW had significantly higher proportions of γδ T cells that were skewed towards effector and effector memory phenotypes, characteristics that were maintained throughout the study period. Expression of the gut homing receptor CCR9 was also more common in γδ T cells from ELGAN/ELBW. Conversely, NK cell frequencies were markedly lower and skewed towards a cytotoxic phenotype in the ELGAN/ELBW group at 14 days of age. Culture‐proven sepsis with an onset during the first 14 days after birth further manifested these differences in the γδ T‐ and NK cell populations at 14 days of age.

Conclusion

Prematurity strongly influences the levels of γδ T and NK cells, in particular in cases where sepsis debuts during the first 2 weeks of life.

The frequency and dynamics of CD4+ mucosal-associated invariant T (MAIT) cells in active pulmonary tuberculosis

MAIT cells are unconventional innate-like T lymphocytes contributing to host immune protection against Mycobacteria tuberculosis (Mtb) infection. CD4^- MAIT cells play a major role in immune protection against tuberculosis (TB), however, the role of CD4⁺ MAIT cells was elusive due to their low abundance. We firstly investigated the frequency and functions of CD4⁺ MAIT cells in pulmonary tuberculosis (PTB) patients before and after anti-TB treatment. We found that the frequency of Mtb-reactive CD4⁺ MAIT cells and IFN-γ, granzyme B (GrzB), CD69 expression on them were increased while LAG-3⁺ cells of them were decreased in PTB patients. After the treatment, the frequency of Mtb-reactive CD4⁺ MAIT cells and CD69, IFN-γ, GrzB expression on them were decreased while LAG-3 increased. The results indicated the expression profile is distinct between CD4⁺ MAIT cells and CD4^- MAIT cells in PTB patients, the increased IFN-γ and GrzB expression of CD4⁺ MAIT cells play a role in anti-TB immunity.

Title of article: Mucosal-associated invariant T cells in lung diseases

The lungs are directly connected to the external environment, which makes them more vulnerable to infection and injury. They are protected by the respiratory epithelium and immune cells to maintain a dynamic balance. Both innate and adaptive immune cells are involved in the pathogenesis of lung diseases. Mucosal-associated invariant T (MAIT) cells are a subset of unconventional T cells, which have attracted increasing attention in recent years. Although MAIT cells account for a small part of the total immune cells in the lungs, evidence suggests that these cells are activated by T cell receptors and/or cytokine receptors and mediate immune response. They play an important role in immunosurveillance and immunity against microbial infection, and recent studies have shown that subsets of MAIT cells play a role in promoting pulmonary inflammation. Emerging data indicate that MAIT cells are involved in the immune response against SARS-CoV-2 and possible immunopathogenesis in COVID-19. Here, we introduce MAIT cell biology to clarify their role in the immune response. Then we review MAIT cells in human and murine lung diseases, including asthma, chronic obstructive pulmonary disease, pneumonia, pulmonary tuberculosis and lung cancer, and discuss their possible protective and pathological effects. MAIT cells represent an attractive marker and potential therapeutic target for disease progression, thus providing new strategies for the treatment of lung diseases.