Human iNKT and MAIT cells exhibit a PLZF-dependent proapoptotic propensity that is counterbalanced by XIAP. Blood. 2013;121:614-623. [PMID: 23223428 DOI: 10.1182/blood-2012-09-456095]

Genetic Regulation of Cell Death: Insights from Autoinflammatory Diseases

Metazoans have evolved innate antimicrobial defenses that promote cellular survival and proliferation. Countering the inevitable molecular mechanisms by which microbes sabotage these pathways, multicellular organisms rely on an alternative, perhaps more ancient, strategy that is the immune equivalent of suicide bombing: Infection triggers cell death programs that summon localized or even systemic inflammation. The study of human genetics has now unveiled a level of complexity that refutes the naive view that cell death is merely a blunt instrument or an evolutionary afterthought. To the contrary, findings from patients with rare diseases teach us that cell death-induced inflammation is a sophisticated, tightly choreographed process. We herein review the emerging body of evidence describing a group of illnesses-inborn errors of cell death, which define many of the molecular building blocks and regulatory elements controlling cell death-induced inflammation in humans-and provide a possible road map to countering this process across the spectrum of rare and common illnesses.

Patients With Immunoglobulin A Nephropathy Show Abnormal Frequencies of B Cell Subsets, Unconventional T Cells, and High Levels of Galactose-Deficient IgA1-Coated Gut Bacteria

Introduction

Mucosal inflammation is involved in the pathophysiology of immunoglobulin-A nephropathy (IgAN); however, peripheral immune phenotype analyses of patients with IgAN often do not include unconventional T cells, the major subset in mucosal immunity.

Methods

We measured serum total IgA, galactose-deficient IgA1 (gd-IgA1), secretory IgA (SIgA), B cell-activating factor (BAFF), and A proliferation-inducing ligand (APRIL) in 66 patients with IgAN and 30 healthy controls (HCs). We also quantified the total IgA and gd-IgA1 in stool supernatant along with the same coated on bacteria. In 35 patients and 14 controls, we performed extensive phenotyping using cytometry by Time-of-Flight (CyTOF) of circulating immune cells, including unconventional T cells (mucosal associated invariant T [MAIT] cells, γδ T, and natural killer [NK] T cells). The results were validated using RNAseq data from a larger cohort of 179 patients with IgAN, 140 patients with minimal change disease, and 91 HCs.

Results

Patients with IgAN had higher circulating levels of total IgA, gd-IgA1, and APRIL, and higher IgA and gd-IgA1-coated gut bacteria than controls, whereas serum levels of SIgA and BAFF did not differ between groups. Patients with IgAN showed more class-switched memory (CSM) and double negative (DN) B cells than controls. MAIT cells and γδ T cells were significantly lower, and CD4-CD8- NK T cells were significantly higher in patients with IgAN than in HCs. We validated the significant decrease in MAIT cells in an independent cohort of patients with IgAN.

Conclusion

The data indicate that patients with IgAN have increased circulating CSM and DN B cells associated with abnormal T cell immunity, involving defects in unconventional T cell frequency. This may suggest putative alterations at mucosal sites because of cell migration leading to altered IgA production.

Thymic development of human natural killer T cells: recent advances and implications for immunotherapy

Invariant natural killer T (iNKT) cells are a subset of lipid-reactive, unconventional T cells that have anti-tumor properties that make them a promising target for cancer immunotherapy. Recent studies have deciphered the developmental pathway of human MAIT and Vγ9Vδ2 γδ-T cells as well as murine iNKT cells, yet our understanding of human NKT cell development is limited. Here, we provide an update in our understanding of how NKT cells develop in the human body and how knowledge regarding their development could enhance human treatments by targeting these cells.

Somatic RAP1B gain-of-function variant underlies isolated thrombocytopenia and immunodeficiency

The ubiquitously expressed small GTPase Ras-related protein 1B (RAP1B) acts as a molecular switch that regulates cell signaling, cytoskeletal remodeling, and cell trafficking and activates integrins in platelets and lymphocytes. The residue G12 in the P-loop is required for the RAP1B-GTPase conformational switch. Heterozygous germline RAP1B variants have been described in patients with syndromic thrombocytopenia. However, the causality and pathophysiological impact remained unexplored. We report a boy with neonatal thrombocytopenia, combined immunodeficiency, neutropenia, and monocytopenia caused by a heterozygous de novo single nucleotide substitution, c.35G>A (p.G12E) in RAP1B. We demonstrate that G12E and the previously described G12V and G60R were gain-of-function variants that increased RAP1B activation, talin recruitment, and integrin activation, thereby modifying late responses such as platelet activation, T cell proliferation, and migration. We show that in our patient, G12E was a somatic variant whose allele frequency decreased over time in the peripheral immune compartment, but remained stable in bone marrow cells, suggesting a differential effect in distinct cell populations. Allogeneic hematopoietic stem cell transplantation fully restored the patient's hemato-immunological phenotype. Our findings define monoallelic RAP1B gain-of-function variants as a cause for constitutive immunodeficiency and thrombocytopenia. The phenotypic spectrum ranged from isolated hematological manifestations in our patient with somatic mosaicism to complex syndromic features in patients with reported germline RAP1B variants.

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.

Understanding the molecular mechanism of pathogenic variants of BIR2 domain in XIAP-deficient inflammatory bowel disease

X-linked inhibitor of apoptosis protein (XIAP) deficiency causes refractory inflammatory bowel disease. The XIAP protein plays a pivotal role in the pro-inflammatory response through the nucleotide-binding oligomerization domain-containing signaling pathway that is important in mucosal homeostasis. We analyzed the molecular mechanism of non-synonymous pathogenic variants (PVs) of XIAP BIR2 domain. We generated N-terminally green fluorescent protein-tagged XIAP constructs of representative non-synonymous PVs. Co-immunoprecipitation and fluorescence cross-correlation spectroscopy showed that wild-type XIAP and RIP2 preferentially interacted in live cells, whereas all non-synonymous PV XIAPs failed to interact properly with RIP2. Structural analysis showed that various structural changes by mutations, such as hydrophobic core collapse, Zn-finger loss, and spatial rearrangement, destabilized the two loop structures (174-182 and 205-215) that critically interact with RIP2. Subsequently, it caused a failure of RIP2 ubiquitination and loss of protein deficiency by the auto-ubiquitination of all XIAP mutants. These findings could enhance our understanding of the role of XIAP mutations in XIAP-deficient inflammatory bowel disease and may benefit future therapeutic strategies.

Understanding the Spectrum of Immune Dysregulation Manifestations in Autoimmune Lymphoproliferative Syndrome and Autoimmune Lymphoproliferative Syndrome-like Disorders

As a disorder of immune dysregulation, autoimmune lymphoproliferative syndrome (ALPS) stems from pathogenic variants in the first apoptosis signal-mediated apoptosis (Fas) and Fas-ligand pathway that result in elevations of CD3+ TCRαβ+ CD4- CD8- T cells along with chronic lymphoproliferation, a heightened risk for malignancy, and importantly for the rheumatologist, increased risk of autoimmunity. While immune cytopenias are the most encountered autoimmune phenomena, there is increasing appreciation for ocular, musculoskeletal, pulmonary and renal inflammatory manifestations similar to more common rheumatology diseases. Additionally, ALPS-like conditions that share similar clinical features and opportunities for targeted therapy are increasingly recognized via genetic testing, highlighting the need for rheumatologists to be facile in the recognition and diagnosis of this spectrum of disorders. This review will focus on clinical and laboratory features of both ALPS and ALPS-like disorders with the intent to provide a framework for rheumatologists to understand the pathophysiologic drivers and discriminate between diagnoses.

Enhanced ZBTB16 Levels by Progestin-Only Contraceptives Induces Decidualization and Inflammation

Progestin-only long-acting reversible-contraceptive (pLARC)-exposed endometria displays decidualized human endometrial stromal cells (HESCs) and hyperdilated thin-walled fragile microvessels. The combination of fragile microvessels and enhanced tissue factor levels in decidualized HESCs generates excess thrombin, which contributes to abnormal uterine bleeding (AUB) by inducing inflammation, aberrant angiogenesis, and proteolysis. The- zinc finger and BTB domain containing 16 (ZBTB16) has been reported as an essential regulator of decidualization. Microarray studies have demonstrated that ZBTB16 levels are induced by medroxyprogesterone acetate (MPA) and etonogestrel (ETO) in cultured HESCs. We hypothesized that pLARC-induced ZBTB16 expression contributes to HESC decidualization, whereas prolonged enhancement of ZBTB16 levels triggers an inflammatory milieu by inducing pro-inflammatory gene expression and tissue-factor-mediated thrombin generation in decidualized HESCs. Thus, ZBTB16 immunostaining was performed in paired endometria from pre- and post-depo-MPA (DMPA)-administrated women and oophorectomized guinea pigs exposed to the vehicle, estradiol (E2), MPA, or E2 + MPA. The effect of progestins including MPA, ETO, and levonorgestrel (LNG) and estradiol + MPA + cyclic-AMP (E2 + MPA + cAMP) on ZBTB16 levels were measured in HESC cultures by qPCR and immunoblotting. The regulation of ZBTB16 levels by MPA was evaluated in glucocorticoid-receptor-silenced HESC cultures. ZBTB16 was overexpressed in cultured HESCs for 72 h followed by a ± 1 IU/mL thrombin treatment for 6 h. DMPA administration in women and MPA treatment in guinea pigs enhanced ZBTB16 immunostaining in endometrial stromal and glandular epithelial cells. The in vitro findings indicated that: (1) ZBTB16 levels were significantly elevated by all progestin treatments; (2) MPA exerted the greatest effect on ZBTB16 levels; (3) MPA-induced ZBTB16 expression was inhibited in glucocorticoid-receptor-silenced HESCs. Moreover, ZBTB16 overexpression in HESCs significantly enhanced prolactin (PRL), insulin-like growth factor binding protein 1 (IGFBP1), and tissue factor (F3) levels. Thrombin-induced interleukin 8 (IL-8) and prostaglandin-endoperoxide synthase 2 (PTGS2) mRNA levels in control-vector-transfected HESCs were further increased by ZBTB16 overexpression. In conclusion, these results supported that ZBTB16 is enhanced during decidualization, and long-term induction of ZBTB16 expression by pLARCs contributes to thrombin generation through enhancing tissue factor expression and inflammation by enhancing IL-8 and PTGS2 levels in decidualized HESCs.

RIPK3 controls MAIT cell accumulation during development but not during infection

Cell death mechanisms in T lymphocytes vary according to their developmental stage, cell subset and activation status. The cell death control mechanisms of mucosal-associated invariant T (MAIT) cells, a specialized T cell population, are largely unknown. Here we report that MAIT cells express key necroptotic machinery; receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL) protein, in abundance. Despite this, we discovered that the loss of RIPK3, but not necroptotic effector MLKL or apoptotic caspase-8, specifically increased MAIT cell abundance at steady-state in the thymus, spleen, liver and lungs, in a cell-intrinsic manner. In contrast, over the course of infection with Francisella tularensis, RIPK3 deficiency did not impact the magnitude of the expansion nor contraction of MAIT cell pools. These findings suggest that, distinct from conventional T cells, the accumulation of MAIT cells is restrained by RIPK3 signalling, likely prior to thymic egress, in a manner independent of canonical apoptotic and necroptotic cell death pathways.

Activation-induced pyroptosis contributes to the loss of MAIT cells in chronic HIV-1 infected patients

BACKGROUND

Mucosal-associated invariant T (MAIT) cells are systemically depleted in human immunodeficiency virus type 1 (HIV-1) infected patients and are not replenished even after successful combined antiretroviral therapy (cART). This study aimed to identify the mechanism underlying MAIT cell depletion.

METHODS

In the present study, we applied flow cytometry, single-cell RNA sequencing and immunohistochemical staining to evaluate the characteristics of pyroptotic MAIT cells in a total of 127 HIV-1 infected individuals, including 69 treatment-naive patients, 28 complete responders, 15 immunological non-responders, and 15 elite controllers, at the Fifth Medical Center of Chinese PLA General Hospital, Beijing, China.

RESULTS

Single-cell transcriptomic profiles revealed that circulating MAIT cells from HIV-1 infected subjects were highly activated, with upregulation of pyroptosis-related genes. Further analysis revealed that increased frequencies of pyroptotic MAIT cells correlated with markers of systemic T-cell activation, microbial translocation, and intestinal damage in cART-naive patients and poor CD4+ T-cell recovery in long-term cART patients. Immunohistochemical staining revealed that MAIT cells in the gut mucosa of HIV-1 infected patients exhibited a strong active gasdermin-D (GSDMD, marker of pyroptosis) signal near the cavity side, suggesting that these MAIT cells underwent active pyroptosis in the colorectal mucosa. Increased levels of the proinflammatory cytokines interleukin-12 (IL-12) and IL-18 were observed in HIV-1 infected patients. In addition, activated MAIT cells exhibited an increased pyroptotic phenotype after being triggered by HIV-1 virions, T-cell receptor signals, IL-12 plus IL-18, and combinations of these factors, in vitro.

CONCLUSIONS

Activation-induced MAIT cell pyroptosis contributes to the loss of MAIT cells in HIV-1 infected patients, which could potentiate disease progression and poor immune reconstitution.

Mucosal-associated invariant T cells are activated in an interleukin-18-dependent manner in Epstein-Barr virus-associated T/natural killer cell lymphoproliferative diseases

Mucosal-associated invariant T (MAIT) cells are a type of innate immune cells that protect against some infections. However, the involvement of MAIT cells in Epstein-Barr virus-associated T/natural killer cell lymphoproliferative diseases (EBV-T/NK-LPD) is unclear. In this study, we found that MAIT cells were highly activated in the blood of patients with EBV-T/NK-LPD. MAIT cell activation levels correlated with disease severity and plasma IL-18 levels. Stimulation of healthy peripheral blood mononuclear cells with EBV resulted in activation of MAIT cells, and this activation level was enhanced by exogenous IL-18. MAIT cells stimulated by IL-18 might thus be involved in the immunopathogenesis of EBV-T/NK-LPD.

Human MAIT cells respond to and suppress HIV-1

Human MAIT cells sit at the interface between innate and adaptive immunity, are polyfunctional and are capable of killing pathogen infected cells via recognition of the Class IB molecule MR1. MAIT cells have recently been shown to possess an antiviral protective role in vivo and we therefore sought to explore this in relation to HIV-1 infection. There was marked activation of MAIT cells in vivo in HIV-1-infected individuals, which decreased following ART. Stimulation of THP1 monocytes with R5 tropic HIVBAL potently activated MAIT cells in vitro. This activation was dependent on IL-12 and IL-18 but was independent of the TCR. Upon activation, MAIT cells were able to upregulate granzyme B, IFNγ and HIV-1 restriction factors CCL3, 4, and 5. Restriction factors produced by MAIT cells inhibited HIV-1 infection of primary PBMCs and immortalized target cells in vitro. These data reveal MAIT cells to be an additional T cell population responding to HIV-1, with a potentially important role in controlling viral replication at mucosal sites.

Preferential and persistent impact of acute HIV-1 infection on CD4+ iNKT cells in colonic mucosa

Evidence suggests that HIV-1 disease progression is determined in the early stages of infection. Here, preinfection invariant natural killer T (iNKT) cell levels were predictive of the peak viral load during acute HIV-1 infection (AHI). Furthermore, iNKT cells were preferentially lost in AHI. This was particularly striking in the colonic mucosa, where iNKT cells were depleted more profoundly than conventional CD4⁺ T cells. The initiation of antiretroviral therapy during AHI-prevented iNKT cell dysregulation in peripheral blood but not in the colonic mucosa. Overall, our results support a model in which iNKT cells are early and preferential targets for HIV-1 infection during AHI.

Acute HIV-1 infection (AHI) results in the widespread depletion of CD4⁺ T cells in peripheral blood and gut mucosal tissue. However, the impact on the predominantly CD4⁺ immunoregulatory invariant natural killer T (iNKT) cells during AHI remains unknown. Here, iNKT cells from peripheral blood and colonic mucosa were investigated during treated and untreated AHI. iNKT cells in blood were activated and rapidly depleted in untreated AHI. At the time of peak HIV-1 viral load, these cells showed the elevated expression of cell death–associated transcripts compared to preinfection. Residual peripheral iNKT cells suffered a diminished responsiveness to in vitro stimulation early into chronic infection. Additionally, HIV-1 DNA, as well as spliced and unspliced viral RNA, were detected in iNKT cells isolated from blood, indicating the active infection of these cells in vivo. The loss of iNKT cells occurred from Fiebig stage III in the colonic mucosa, and these cells were not restored to normal levels after initiation of ART during AHI. CD4⁺ iNKT cells were depleted faster and more profoundly than conventional CD4⁺ T cells, and the preferential infection of CD4⁺ iNKT cells over conventional CD4⁺ T cells was confirmed by in vitro infection experiments. In vitro data also provided evidence of latent infection in iNKT cells. Strikingly, preinfection levels of peripheral blood CD4⁺ iNKT cells correlated directly with the peak HIV-1 load. These findings support a model in which iNKT cells are early targets for HIV-1 infection, driving their rapid loss from circulation and colonic mucosa.

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.

Infection-induced inflammation from specific inborn errors of immunity to COVID-19

Inborn errors of immunity (IEIs) are a group of genetically defined disorders leading to defective immunity. Some IEIs have been linked to mutations of immune receptors or signaling molecules, resulting in defective signaling of respective cascades essential for combating specific pathogens. However, it remains incompletely understood why in selected IEIs, such as X-linked lymphoproliferative syndrome type 2 (XLP-2), hypo-immune response to specific pathogens results in persistent inflammation. Moreover, mechanisms underlying the generation of anticytokine autoantibodies are mostly unknown. Recently, IEIs have been associated with coronavirus disease 2019 (COVID-19), with a small proportion of patients that contract severe COVID-19 displaying loss-of-function mutations in genes associated with type I interferons (IFNs). Moreover, approximately 10% of patients with severe COVID-19 possess anti-type I IFN-neutralizing autoantibodies. Apart from IEIs that impair immune responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), SARS-CoV-2 encodes several proteins that suppress early type I IFN production. One primary consequence of the lack of type I IFNs during early SARS-CoV-2 infection is the increased inflammation associated with COVID-19. In XLP-2, resolution of inflammation rescued experimental subjects from infection-induced mortality. Recent studies also indicate that targeting inflammation could alleviate COVID-19. In this review, we discuss infection-induced inflammation in IEIs, using XLP-2 and COVID-19 as examples. We suggest that resolving inflammation may represent an effective therapeutic approach to these diseases.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

ZBTB Transcription Factors: Key Regulators of the Development, Differentiation and Effector Function of T Cells

The development and differentiation of T cells represents a long and highly coordinated, yet flexible at some points, pathway, along which the sequential and dynamic expressions of different transcriptional factors play prominent roles at multiple steps. The large ZBTB family comprises a diverse group of transcriptional factors, and many of them have emerged as critical factors that regulate the lineage commitment, differentiation and effector function of hematopoietic-derived cells as well as a variety of other developmental events. Within the T-cell lineage, several ZBTB proteins, including ZBTB1, ZBTB17, ZBTB7B (THPOK) and BCL6 (ZBTB27), mainly regulate the development and/or differentiation of conventional CD4/CD8 αβ⁺ T cells, whereas ZBTB16 (PLZF) is essential for the development and function of innate-like unconventional γδ⁺ T & invariant NKT cells. Given the critical role of T cells in host defenses against infections/tumors and in the pathogenesis of many inflammatory disorders, we herein summarize the roles of fourteen ZBTB family members in the development, differentiation and effector function of both conventional and unconventional T cells as well as the underlying molecular mechanisms.

Incorporating mucosal-associated invariant T cells into the pathogenesis of chronic liver disease

Mucosal-associated invariant T (MAIT) cells have been described in liver and non-liver diseases, and they have been ascribed antimicrobial, immune regulatory, protective, and pathogenic roles. The goals of this review are to describe their biological properties, indicate their involvement in chronic liver disease, and encourage investigations that clarify their actions and therapeutic implications. English abstracts were identified in PubMed by multiple search terms, and bibliographies were developed. MAIT cells are activated by restricted non-peptides of limited diversity and by multiple inflammatory cytokines. Diverse pro-inflammatory, anti-inflammatory, and immune regulatory cytokines are released; infected cells are eliminated; and memory cells emerge. Circulating MAIT cells are hyper-activated, immune exhausted, dysfunctional, and depleted in chronic liver disease. This phenotype lacks disease-specificity, and it does not predict the biological effects. MAIT cells have presumed protective actions in chronic viral hepatitis, alcoholic hepatitis, non-alcoholic fatty liver disease, primary sclerosing cholangitis, and decompensated cirrhosis. They have pathogenic and pro-fibrotic actions in autoimmune hepatitis and mixed actions in primary biliary cholangitis. Local factors in the hepatic microenvironment (cytokines, bile acids, gut-derived bacterial antigens, and metabolic by-products) may modulate their response in individual diseases. Investigational manipulations of function are warranted to establish an association with disease severity and outcome. In conclusion, MAIT cells constitute a disease-nonspecific, immune response to chronic liver inflammation and infection. Their pathological role has been deduced from their deficiencies during active liver disease, and future investigations must clarify this role, link it to outcome, and explore therapeutic interventions.

Mucosal-associated invariant T cell responses differ by sex in COVID-19

BACKGROUND

Sexual dimorphisms in immune responses contribute to coronavirus disease 2019 (COVID-19) outcomes, but the mechanisms governing this disparity remain incompletely understood.

METHODS

We carried out sex-balanced sampling of peripheral blood mononuclear cells from hospitalized and non-hospitalized individuals with confirmed COVID-19, uninfected close contacts, and healthy control individuals for 36-color flow cytometry and single-cell RNA sequencing.

FINDINGS

Our results revealed a pronounced reduction of circulating mucosal-associated invariant T (MAIT) cells in infected females. Integration of published COVID-19 airway tissue datasets suggests that this reduction represented a major wave of MAIT cell extravasation during early infection in females. Moreover, MAIT cells from females possessed an immunologically active gene signature, whereas cells from males were pro-apoptotic.

CONCLUSIONS

Our findings uncover a female-specific protective MAIT cell profile, potentially shedding light on reduced COVID-19 susceptibility in females.

FUNDING

This work was supported by NIH/NIAID (U01AI066569 and UM1AI104681), the Defense Advanced Projects Agency (DARPA; N66001-09-C-2082 and HR0011-17-2-0069), the Veterans Affairs Health System, and Virology Quality Assurance (VQA; 75N93019C00015). The content is solely the responsibility of the authors and does not necessarily represent the official view of the National Institutes of Health. COVID-19 samples were processed under Biosafety level 2 (BSL-2) with aerosol management enhancement or BSL-3 in the Duke Regional Biocontainment Laboratory, which received partial support for construction from NIH/NIAID (UC6AI058607).

A fetal wave of human type 3 effector γδ cells with restricted TCR diversity persists into adulthood

Accumulating evidence suggests that the mouse embryonic thymus produces distinct waves of innate effector γδ T cells. However, it is unclear whether this process occurs similarly in humans and whether it comprises a dedicated subset of innate-like type 3 effector γδ T cells. Here, we present a protocol for high-throughput sequencing of TRG and TRD pairs that comprise the clonal γδTCR. In combination with single-cell RNA sequencing, multiparameter flow cytometry, and TCR sequencing, we reveal a high heterogeneity of γδ T cells sorted from neonatal and adult blood that correlated with TCR usage. Immature γδ T cell clusters displayed mixed and diverse TCRs, but effector cell types segregated according to the expression of either highly expanded individual Vδ1⁺ TCRs or moderately expanded semi-invariant Vγ9Vδ2⁺ TCRs. The Vγ9Vδ2⁺ T cells shared expression of genes that mark innate-like T cells, including ZBTB16 (encoding PLZF), KLRB1, and KLRC1, but consisted of distinct clusters with unrelated Vγ9Vδ2⁺ TCR clones characterized either by TBX21, FCGR3A, and cytotoxicity-associated gene expression (type 1) or by CCR6, RORC, IL23R, and DPP4 expression (type 3). Effector γδ T cells with type 1 and type 3 innate T cell signatures were detected in a public dataset of early embryonic thymus organogenesis. Together, this study suggests that functionally distinct waves of human innate-like effector γδ T cells with semi-invariant Vγ9Vδ2⁺ TCR develop in the early fetal thymus and persist into adulthood.

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.

Evolution of Our Understanding of XIAP Deficiency

X-linked inhibitor of apoptosis (XIAP) deficiency is a rare inborn error of immunity first described in 2006. XIAP deficiency is characterised by immune dysregulation and a broad spectrum of clinical manifestations, including haemophagocytic lymphohistiocytosis (HLH), inflammatory bowel disease (IBD), hypogammaglobulinemia, susceptibility to infections, splenomegaly, cytopaenias, and other less common autoinflammatory phenomena. Since the first description of the disease, many XIAP deficient patients have been identified and our understanding of the disease has grown. Over 90 disease causing mutations have been described and more inflammatory disease manifestations, such as hepatitis, arthritis, and uveitis, are now well-recognised. Recently, following the introduction of reduced intensity conditioning (RIC), outcomes of allogeneic haematopoietic stem cell transplantation (HSCT), the only curative treatment option for XIAP deficiency, have improved. The pathophysiology of XIAP deficiency is not fully understood, however it is known that XIAP plays a role in both the innate and adaptive immune response and in immune regulation, most notably through modulation of tumour necrosis factor (TNF)-receptor signalling and regulation of NLRP3 inflammasome activity. In this review we will provide an up to date overview of both the clinical aspects and pathophysiology of XIAP deficiency.

Mucosal associated invariant T cells are differentially impaired in tolerant and immunosuppressed liver transplant recipients

Mucosal associated invariant T (MAIT-) cells represent a semi-invariant T cell population responsive to microbial vitamin B metabolite and innate cytokine stimulation, executing border tissue protection and particularly contributing to human liver immunity. The impact of immunosuppressants on MAIT cell biology alone and in context with solid organ transplantation has not been thoroughly examined. Here, we demonstrate that in vitro cytokine activation of peripheral MAIT cells from healthy individuals was impaired by glucocorticoids, whereas antigen-specific stimulation was additionally sensitive to calcineurin inhibitors. In liver transplant (LTx) recipients, significant depletion of peripheral MAIT cells was observed that was largely independent of the type and dosage of immunosuppression, equally applied to tolerant patients, and was reproducible in kidney transplant recipients. However, MAIT cells from tolerant LTx patients exhibited a markedly diminished ex vivo activation signature, associated with individual regain of functional competence toward antigenic and cytokine stimulation. Still, MAIT cells from tolerant and treated liver recipients exhibited high levels of PD1, accompanied by functional impairment particularly toward bacterial stimulation that also affected polyfunctionality. Our data suggest interlinked effects of primary liver pathology and immunosuppressive treatment on overall MAIT cell fitness after transplantation and propose their monitoring in context with tolerance induction protocols.

Efficient 5-OP-RU-Induced Enrichment of Mucosa-Associated Invariant T Cells in the Murine Lung Does Not Enhance Control of Aerosol Mycobacterium tuberculosis Infection

Mucosa-associated invariant T (MAIT) cells are an innate-like T cell subset in mammals that recognize microbial vitamin B metabolites presented by the evolutionarily conserved major histocompatibility complex class I (MHC I)-related molecule, MR1. Emerging data suggest that MAIT cells may be an attractive target for vaccine-induced protection against bacterial infections because of their rapid cytotoxic responses at mucosal services to a widely conserved bacterial ligand. In this study, we tested whether a MAIT cell priming strategy could protect against aerosol Mycobacterium tuberculosis infection in mice. Intranasal costimulation with the lipopeptide Toll-like receptor (TLR)2/6 agonist, Pam2Cys (P2C), and the synthetic MR1 ligand, 5-OP-RU, resulted in robust expansion of MAIT cells in the lung. Although MAIT cell priming significantly enhanced MAIT cell activation and expansion early after M. tuberculosis challenge, these MAIT cells did not restrict M. tuberculosis bacterial load. MAIT cells were depleted by the onset of the adaptive immune response, with decreased detection of granzyme B⁺ and gamma interferon (IFN-γ)⁺ MAIT cells relative to that in uninfected P2C/5-OP-RU-treated mice. Decreasing the infectious inoculum, varying the time between priming and aerosol infection, and testing MAIT cell priming in nitric oxide synthase 2 (NOS2)-deficient mice all failed to reveal an effect of P2C/5-OP-RU-induced MAIT cells on M. tuberculosis control. We conclude that intranasal MAIT cell priming in mice induces early MAIT cell activation and expansion after M. tuberculosis exposure, without attenuating M. tuberculosis growth, suggesting that MAIT cell enrichment in the lung is not sufficient to control M. tuberculosis infection.

Immune surveillance of the liver by T cells

The liver is the target of several infectious, inflammatory, and neoplastic diseases, which affect hundreds of millions of people worldwide and cause an estimated death toll of more than 2 million people each year. Dysregulation of T cell responses has been implicated in the pathogenesis of these diseases; hence, it is critically important to understand the function and fate of T cells in the liver. Here, we provide an overview of the current knowledge on liver immune surveillance by conventional and invariant T cells and explore the complex cross-talk between immune cell subsets that determines the balance between hepatic immunity and tolerance.

Mucosal-Associated Invariant T Cells Develop an Innate-Like Transcriptomic Program in Anti-mycobacterial Responses

Conventional T cells exhibit a delayed response to the initial priming of peptide antigens presented by major histocompatibility complex (MHC) proteins. Unlike conventional T cells, mucosal-associated invariant T (MAIT) cells quickly respond to non-peptidic metabolite antigens presented by MHC-related protein 1 (MR1). To elucidate the MR1-dependent activation program of MAIT cells in response to mycobacterial infections, we determined the surface markers, transcriptomic profiles, and effector responses of activated human MAIT cells. Results revealed that mycobacterial-incubated antigen-presenting cells stimulated abundant human CD8⁺ MAIT cells to upregulate the co-expression of CD69 and CD26, as a combinatorial activation marker. Further transcriptomic analyses demonstrated that CD69⁺CD26⁺⁺ CD8⁺MAIT cells highly expressed numerous genes for mediating anti-mycobacterial immune responses, including pro-inflammatory cytokines, cytolytic molecules, NK cell receptors, and transcription factors, in contrast to inactivated counterparts CD69^+/−CD26^+/− CD8⁺MAIT cells. Gene co-expression, enrichment, and pathway analyses yielded high statistical significance to strongly support that activated CD8⁺ MAIT cells shared gene expression and numerous pathways with NK and CD8⁺ T cells in activation, cytokine production, cytokine signaling, and effector functions. Flow cytometry detected that activated CD8⁺MAIT cells produced TNFα, IFNγ, and granulysin to inhibit mycobacterial growth and fight mycobacterial infection. Together, results strongly support that the combinatorial activation marker CD69⁺CD26⁺⁺ labels the activated CD8⁺MAIT cells that develop an innate-like activation program in anti-mycobacterial immune responses. We speculate that the rapid production of anti-mycobacterial effector molecules facilitates MAIT cells to fight early mycobacterial infection in humans.

Sestrins induce natural killer function in senescent-like CD8+ T cells

Aging is associated with remodeling of the immune system to enable the maintenance of life-long immunity. In the CD8+ T cell compartment, aging results in the expansion of highly differentiated cells that exhibit characteristics of cellular senescence. Here we found that CD27-CD28-CD8+ T cells lost the signaling activity of the T cell antigen receptor (TCR) and expressed a protein complex containing the agonistic natural killer (NK) receptor NKG2D and the NK adaptor molecule DAP12, which promoted cytotoxicity against cells that expressed NKG2D ligands. Immunoprecipitation and imaging cytometry indicated that the NKG2D-DAP12 complex was associated with sestrin 2. The genetic inhibition of sestrin 2 resulted in decreased expression of NKG2D and DAP12 and restored TCR signaling in senescent-like CD27-CD28-CD8+ T cells. Therefore, during aging, sestrins induce the reprogramming of non-proliferative senescent-like CD27-CD28-CD8+ T cells to acquire a broad-spectrum, innate-like killing activity.

MAIT cells are functionally impaired in a Mauritian cynomolgus macaque model of SIV and Mtb co-infection

Mucosal-associated invariant T (MAIT) cells can recognize and respond to some bacterially infected cells. Several in vitro and in vivo models of Mycobacterium tuberculosis (Mtb) infection suggest that MAIT cells can contribute to control of Mtb, but these studies are often cross-sectional and use peripheral blood cells. Whether MAIT cells are recruited to Mtb-affected granulomas and lymph nodes (LNs) during early Mtb infection and what purpose they might serve there is less well understood. Furthermore, whether HIV/SIV infection impairs MAIT cell frequency or function at the sites of Mtb replication has not been determined. Using Mauritian cynomolgus macaques (MCM), we phenotyped MAIT cells in the peripheral blood and bronchoalveolar lavage (BAL) before and during infection with SIVmac239. To test the hypothesis that SIV co-infection impairs MAIT cell frequency and function within granulomas, SIV+ and -naïve MCM were infected with a low dose of Mtb Erdman, and necropsied at 6 weeks post Mtb-challenge. MAIT cell frequency and function were examined within the peripheral blood, BAL, and Mtb-affected lymph nodes (LN) and granulomas. MAIT cells did not express markers indicative of T cell activation in response to Mtb in vivo within granulomas in animals infected with Mtb alone. SIV and Mtb co-infection led to increased expression of the activation/exhaustion markers PD-1 and TIGIT, and decreased ability to secrete TNFα when compared to SIV-naïve MCM. Our study provides evidence that SIV infection does not prohibit the recruitment of MAIT cells to sites of Mtb infection, but does functionally impair those MAIT cells. Their impaired function could have impacts, either direct or indirect, on the long-term containment of TB disease.

MAIT Cells in Health and Disease

Mucosal-associated invariant T (MAIT) cells have been attracting increasing attention over the last few years as a potent unconventional T cell subset. Three factors largely account for this emerging interest. Firstly, these cells are abundant in humans, both in circulation and especially in some tissues such as the liver. Secondly is the discovery of a ligand that has uncovered their microbial targets, and also allowed for the development of tools to accurately track the cells in both humans and mice. Finally, it appears that the cells not only have a diverse range of functions but also are sensitive to a range of inflammatory triggers that can enhance or even bypass T cell receptor–mediated signals—substantially broadening their likely impact in health and disease. In this review we discuss how MAIT cells display antimicrobial, homeostatic, and amplifier roles in vivo, and how this may lead to protection and potentially pathology.

Mucosal-associated invariant T cells and disease

Mucosal-associated invariant T (MAIT) cells are unique innate-like T cells that bridge innate and adaptive immunity. They are activated by conserved bacterial ligands derived from vitamin B biosynthesis and have important roles in defence against bacterial and viral infections. However, they can also have various deleterious and protective functions in autoimmune, inflammatory and metabolic diseases. MAIT cell involvement in a large spectrum of pathological conditions makes them attractive targets for potential therapeutic approaches.

Reconstitution of Circulating Mucosal-Associated Invariant T Cells after Allogeneic Hematopoietic Cell Transplantation: Its Association with the Riboflavin Synthetic Pathway of Gut Microbiota in Cord Blood Transplant Recipients

Mucosal-associated invariant T (MAIT) cells are a type of innate lymphocyte and recognize riboflavin (vitamin B2) synthesis products presented by MHC-related protein 1. We investigated long-term reconstitution of MAIT cells and its association with chronic graft-versus-host disease (cGVHD) in a cross-sectional cohort of 173 adult patients after allogeneic hematopoietic cell transplantation. According to donor source, the number of MAIT cells significantly correlated with time after cord blood transplantation (CBT) but not with time after bone marrow transplantation or peripheral blood stem cell transplantation. The number of MAIT cells was significantly lower in patients with cGVHD compared with patients without cGVHD. We also examined the association between MAIT cell reconstitution and gut microbiota as evaluated by 16S ribosomal sequencing of stool samples 1 mo post-CBT in 27 adult patients undergoing CBT. The diversity of gut microbiota was positively correlated with better MAIT cell reconstitution after CBT. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States analysis indicated that amounts of ribB and ribA genes were significantly higher in the microbiomes of patients with subsequent MAIT cell reconstitution after CBT. In conclusion, long-term MAIT cell reconstitution is dependent on the type of donor source. Our data also unveiled an important role for the interaction of circulating MAIT cells with gut microbiota in humans.

Functional MAIT Cells Are Associated With Reduced Simian-Human Immunodeficiency Virus Infection

Mucosa-associated invariant T (MAIT) cells are recently characterized as a novel subset of innate-like T cells that recognize microbial metabolites as presented by the MHC-1b-related protein MR1. The significance of MAIT cells in anti-bacterial defense is well-understood but not clear in viral infections such as SIV/HIV infection. Here we studied the phenotype, distribution, and function of MAIT cells and their association with plasma viral levels during chronic SHIV infection in rhesus macaques (RM). Two groups of healthy and chronic SHIV-infected macaques were characterized for MAIT cells in blood and mucosal tissues. Similar to human, we found a significant fraction of macaque T cells co-expressing MAIT cell markers CD161 and TCRVα-7.2 that correlated directly with macaque MR1 tetramer. These cells displayed memory phenotype and expressed high levels of IL-18R, CCR6, CD28, and CD95. During chronic infection, the frequency of MAIT cells are enriched in the blood but unaltered in the rectum; both blood and rectal MAIT cells displayed higher proliferative and cytotoxic phenotype post-SHIV infection. The frequency of MAIT cells in blood and rectum correlated inversely with plasma viral RNA levels and correlated directly with total CD4 T cells. MAIT cells respond to microbial products during chronic SHIV infection and correlated positively with serum immunoreactivity to flagellin levels. Tissue distribution analysis of MAIT cells during chronic infection showed significant enrichment in the non-lymphoid tissues (lung, rectum, and liver) compared to lymphoid tissues (spleen and LN), with higher levels of tissue-resident markers CD69 and CD103. Exogenous in vitro cytokine treatments during chronic SHIV infection revealed that IL-7 is important for the proliferation of MAIT cells, but IL-12 and IL-18 are important for their cytolytic function. Overall our results demonstrated that MAIT cells are enriched in blood but unaltered in the rectum during chronic SHIV infection, which displayed proliferative and functional phenotype that inversely correlated with SHIV plasma viral RNA levels. Treatment such as combined cytokine treatments could be beneficial for enhancing functional MAIT cells during chronic HIV infection in vivo.

Mucosal-Associated Invariant T cell in liver diseases

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

Alcohol Abstinence Does Not Fully Reverse Abnormalities of Mucosal-Associated Invariant T Cells in the Blood of Patients With Alcoholic Hepatitis

OBJECTIVES

Alcoholic hepatitis (AH) develops in approximately 30% of chronic heavy drinkers. The immune system of patients with AH is hyperactivated, yet ineffective against infectious diseases. Mucosal-associated invariant T (MAIT) cells are innate-like lymphocytes that are highly enriched in liver, mucosa, and peripheral blood and contribute to antimicrobial immunity. We aimed to determine whether MAIT cells were dysregulated in heavy drinkers with and without AH and the effects of alcohol abstinence on MAIT cell recovery.

METHODS

MR1 tetramers loaded with a potent MAIT cell ligand 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil were used in multiparameter flow cytometry to analyze peripheral blood MAIT cells in 59 healthy controls (HC), 56 patients with AH, and 45 heavy drinkers without overt liver disease (HDC) at baseline and 6- and 12-month follow-ups. Multiplex immunoassays were used to quantify plasma levels of cytokines related to MAIT cell activation. Kinetic Turbidimetric Limulus Amebocyte Lysate Assay and ELISA were performed to measure circulating levels of 2 surrogate markers for bacterial translocation (lipopolysaccharide and CD14), respectively.

RESULTS

At baseline, patients with AH had a significantly lower frequency of MAIT cells than HDC and HC. HDC also had less MAIT cells than HC (median 0.16% in AH, 0.56% in HDC, and 1.25% in HC). Further, the residual MAIT cells in patients with AH expressed higher levels of activation markers (CD69, CD38, and human leukocyte antigen [HLA]-DR), the effector molecule granzyme B, and the immune exhaustion molecule PD-1. Plasma levels of lipopolysaccharide and CD14 and several cytokines related to MAIT cell activation were elevated in patients with AH (interferon [IFN]-α, interleukin [IL]-7, IL-15, IL-17, IL-18, IL-23, IFN-γ, and tumor necrosis factor α). Decreased MAIT cell frequency and upregulated CD38, CD69, and HLA-DR correlated negatively and positively, respectively, with aspartate aminotransferase level. MAIT cell frequency negatively correlated with IL-18. HLA-DR and CD38 levels correlated with several cytokines. At follow-ups, abstinent patients with AH had increased MAIT cell frequency and decreased MAIT cell activation. However, MAIT cell frequency was not fully normalized in patients with AH (median 0.31%).

DISCUSSION

We showed that HDC had a reduction of blood MAIT cells despite showing little evidence of immune activation, whereas patients with AH had a severe depletion of blood MAIT cells and the residual cells were highly activated. Alcohol abstinence partially reversed those abnormalities.

The CD4-CD8- MAIT cell subpopulation is a functionally distinct subset developmentally related to the main CD8+ MAIT cell pool

Mucosa-associated invariant T (MAIT) cells are unconventional innate-like T cells recognizing microbial riboflavin metabolites presented by the monomorphic MR1 molecule. Here, we show that the CD8⁺CD4⁻ and CD8⁻CD4⁻ subpopulations of human MAIT cells represent transcriptionally and phenotypically discrete subsets with distinct functional profiles. Furthermore, T cell receptor repertoire analysis, as well as MAIT cell data based on human fetal tissues, umbilical cord blood, and culture systems indicate that the CD8⁻CD4⁻ subset may derive from the main CD8⁺CD4⁻ MAIT cell pool. Thus, MAIT cells, a major antimicrobial effector T cell population in humans, segregate into two functionally distinct but developmentally related subsets separated by the expression of CD8. This functional difference may have significant implications in infectious and inflammatory diseases.

Mucosa-associated invariant T (MAIT) cells are unconventional innate-like T cells that recognize microbial riboflavin metabolites presented by the MHC class I-like protein MR1. Human MAIT cells predominantly express the CD8α coreceptor (CD8⁺), with a smaller subset lacking both CD4 and CD8 (double-negative, DN). However, it is unclear if these two MAIT cell subpopulations distinguished by CD8α represent functionally distinct subsets. Here, we show that the two MAIT cell subsets express divergent transcriptional programs and distinct patterns of classic T cell transcription factors. Furthermore, CD8⁺ MAIT cells have higher levels of receptors for IL-12 and IL-18, as well as of the activating receptors CD2, CD9, and NKG2D, and display superior functionality following stimulation with riboflavin-autotrophic as well as riboflavin-auxotrophic bacterial strains. DN MAIT cells display higher RORγt/T-bet ratio, and express less IFN-γ and more IL-17. Furthermore, the DN subset displays enrichment of an apoptosis gene signature and higher propensity for activation-induced apoptosis. During development in human fetal tissues, DN MAIT cells are more mature and accumulate over gestational time with reciprocal contraction of the CD8⁺ subset. Analysis of the T cell receptor repertoire reveals higher diversity in CD8⁺ MAIT cells than in DN MAIT cells. Finally, chronic T cell receptor stimulation of CD8⁺ MAIT cells in an in vitro culture system supports the accumulation and maintenance of the DN subpopulation. These findings define human CD8⁺ and DN MAIT cells as functionally distinct subsets and indicate a derivative developmental relationship.

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.

Insights Into Mucosal-Associated Invariant T Cell Biology From Studies of Invariant Natural Killer T Cells

Mucosal-associated invariant T (MAIT) cells and invariant natural killer T (iNKT) cells are innate-like T cells that function at the interface between innate and adaptive immunity. They express semi-invariant T cell receptors (TCRs) and recognize unconventional non-peptide ligands bound to the MHC Class I-like molecules MR1 and CD1d, respectively. MAIT cells and iNKT cells exhibit an effector-memory phenotype and are enriched within the liver and at mucosal sites. In humans, MAIT cell frequencies dwarf those of iNKT cells, while in laboratory mouse strains the opposite is true. Upon activation via TCR- or cytokine-dependent pathways, MAIT cells and iNKT cells rapidly produce cytokines and show direct cytotoxic activity. Consequently, they are essential for effective immunity, and alterations in their frequency and function are associated with numerous infectious, inflammatory, and malignant diseases. Due to their abundance in mice and the earlier development of reagents, iNKT cells have been more extensively studied than MAIT cells. This has led to the routine use of iNKT cells as a reference population for the study of MAIT cells, and such an approach has proven very fruitful. However, MAIT cells and iNKT cells show important phenotypic, functional, and developmental differences that are often overlooked. With the recent availability of new tools, most importantly MR1 tetramers, it is now possible to directly study MAIT cells to understand their biology. Therefore, it is timely to compare the phenotype, development, and function of MAIT cells and iNKT cells. In this review, we highlight key areas where MAIT cells show similarity or difference to iNKT cells. In addition, we discuss important avenues for future research within the MAIT cell field, especially where comparison to iNKT cells has proven less informative.

Mucosal-Associated Invariant T Cells in Autoimmune Diseases

Mucosal-associated invariant T (MAIT) cells are innate T cells restricted by MHC-related molecule 1 (MR1). MAIT cells express semi-invariant T-cell receptors TRAV1-2-TRAJ33/12/20 in humans and TRAV1-TRAJ33 in mice. MAIT cells recognize vitamin B2 biosynthesis derivatives presented by MR1. Similar to other innate lymphocytes, MAIT cells are also activated by cytokines in the absence of exogenous antigens. MAIT cells have the capacity to produce cytokines, such as IFNγ, TNFα, and IL-17, and cytotoxic proteins, including perforin and granzyme B. MAIT cells were originally named after their preferential location in the mucosal tissue of the gut, but they are also abundant in other peripheral organs, including the liver and lungs. In humans, the frequency of MAIT cells is high in peripheral blood, and these cells constitute approximately 5% of circulating CD3⁺ cells. Their abundance in tissues and rapid activation following stimulation have led to great interest in their function in various types of immune diseases. In this review, first, we will briefly introduce key information of MAIT cell biology required for better understating their roles in immune responses, and then describe how MAIT cells are associated with autoimmune and other immune diseases in humans. Moreover, we will discuss their functions based on information from animal models of autoimmune and immunological diseases.

Inherited Immunodeficiencies With High Predisposition to Epstein-Barr Virus-Driven Lymphoproliferative Diseases

Epstein–Barr Virus (EBV) is a gamma-herpes virus that infects 90% of humans without any symptoms in most cases, but has an oncogenic potential, especially in immunocompromised individuals. In the past 30 years, several primary immunodeficiencies (PIDs) associated with a high risk to develop EBV-associated lymphoproliferative disorders (LPDs), essentially consisting of virus-associated hemophagocytic syndrome, non-malignant and malignant B-cell LPDs including non-Hodgkin and Hodgkin’s types of B lymphomas have been characterized. Among them are SH2D1A (SAP), XIAP, ITK, MAGT1, CD27, CD70, CTPS1, RASGRP1, and CORO1A deficiencies. Penetrance of EBV infection ranges from 50 to 100% in those PIDs. Description of large cohorts and case reports has refined the specific phenotypes associated with these PIDs helping to the diagnosis. Specific pathways required for protective immunity to EBV have emerged from studies of these PIDs. SLAM-associated protein-dependent SLAM receptors and MAGT1-dependent NKG2D pathways are important for T and NK-cell cytotoxicity toward EBV-infected B-cells, while CD27–CD70 interactions are critical to drive the expansion of EBV-specific T-cells. CTPS1 and RASGRP1 deficiencies further strengthen that T-lymphocyte expansion is a key step in the immune response to EBV. These pathways appear to be also important for the anti-tumoral immune surveillance of abnormal B cells. Monogenic PIDs should be thus considered in case of any EBV-associated LPDs.

Proteomic definition of human mucosal-associated invariant T cells determines their unique molecular effector phenotype

Mucosal-associated invariant T cells (MAIT) constitute the most abundant anti-bacterial CD8⁺ T-cell population in humans. MR1/TCR-activated MAIT cells were reported to organize cytotoxic and innate-like responses but knowledge about their molecular effector phenotype is still fragmentary. Here, we have examined the functional inventory of human MAIT cells (CD3⁺ Vα7.2⁺ CD161⁺ ) in comparison with those from conventional non-MAIT CD8⁺ T cells (cCD8⁺ ) and NK cells. Quantitative mass spectrometry characterized 5500 proteins of primary MAIT cells and identified 160 and 135 proteins that discriminate them from cCD8⁺ T cells and NK cells donor-independently. Most notably, MAIT cells showed a unique exocytosis machinery in parallel to a proinflammatory granzyme profile with high levels of the granzymes A, K, and M. Furthermore, 24 proteins were identified with highest abundances in MAIT cells, including CD26, CD98, and L-amino-oxidase (LAAO). Among those, expression of granzyme K and CD98 were validated as MAIT-specific with respect to non-MAIT CD8⁺ effector subsets and LAAO was found to be recruited together with granzymes, perforin, and CD107a at the immunological synapse of activated MAIT cells. In conclusion, this study complements knowledge on the molecular effector phenotype of MAIT cells and suggest novel immune regulatory functions as part of their cytotoxic responses.

Late Development of FcεRγneg Adaptive Natural Killer Cells Upon Human Cytomegalovirus Reactivation in Umbilical Cord Blood Transplantation Recipients

In human natural killer (NK) cells, human cytomegalovirus (HCMV) has been shown to be a driving force capable of inducing the expansion of a highly differentiated NKG2C⁺CD57⁺ subset, persisting over time in both HCMV⁺ healthy subjects and umbilical cord blood transplantation (UCBT) recipients experiencing HCMV viral reactivation. In HCMV⁺ healthy subjects, such expanded NK-cells are characterized by epigenetic modifications that modulate their phenotypic and functional characteristics. In particular, an enhanced ADCC activity is detectable in NK cells lacking the signaling protein FcεRγ. Timing and mechanisms involved in the acquisition of HCMV-induced, adaptive-like features by NK cells are currently unknown. In this study, we investigated the de novo acquisition of several adaptive features in NK cells developing after UCBT by monitoring NK-cell differentiation for at least 2 years after transplant. In UCBT recipients experiencing HCMV reactivation, a rapid phenotypic reconfiguration occurred resulting in the expected expansion of CD56^dim NKG2C⁺CD57⁺ NK cells. However, while certain HCMV-driven adaptive hallmarks, including high KIR, LILRB1, CD2 and low/negative NKG2A, Siglec-7, and CD161 expression, were acquired early after UCBT (namely by month 6), downregulation of the signaling protein FcεRγ was detected at a later time interval (i.e., by month 12). This feature characterized only a minor fraction of the HCMV-imprinted NKG2C⁺CD57⁺ CD56^dim NK cell subset, while it was detectable in higher proportions of CD57⁺ NK cells lacking NKG2C. Interestingly, in patients developing a hyporesponsive CD56^-CD16^bright NK-cell subset, FcεRγ downregulation occurred in these cells earlier than in CD56^dim NK cells. Our data suggest that the acquisition of a fully "adaptive" profile requires signals that may lack in UCBT recipients and/or longer time is needed to obtain a stable epigenetic reprogramming. On the other hand, we found that both HCMV-induced FcεRγ^neg and FcεRγ⁺ NK cells from these patients, display similar CD107a degranulation and IFN-γ production capabilities in response to different stimuli, thus indicating that the acquisition of specialized effector functions can be achieved before the "adaptation" to HCMV is completed. Our study provides new insights in the process leading to the generation of different adaptive NK-cell subsets and may contribute to develop new approaches for their employment as novel immunotherapeutic tools.

Mucosal-associated invariant T (MAIT) cells are depleted and prone to apoptosis in cardiometabolic disorders

The disruption of systemic immune homeostasis is a key mediator in the progression of cardiometabolic diseases (CMDs). We aimed to extend knowledge regarding the clinical relevance of CMD-associated variation of circulating mucosal-associated invariant T (MAIT) cell abundance and to explore underlying cellular mechanisms. We analyzed cross-sectional data from 439 participants of the Metagenomics in Cardiometabolic Diseases (MetaCardis) study, stratified into 6 groups: healthy control subjects and patients with metabolic syndrome (MS), obesity, type 2 diabetes mellitus (T2DM), and coronary artery disease (CAD) without, or with congestive heart failure (CAD-CHF). Blood MAIT cell frequency was significantly decreased in all CMD groups, including early (MS) and later (CAD and CAD-CHF) stages of disease progression. Reduced MAIT cell abundance was associated with increased glycosylated hemoglobin, inflammation markers, and deterioration of cardiac function. Glucose dose dependently promoted MAIT cell apoptosis in vitro, independently of anti-CD3 and cytokine-mediated activation. This outcome suggests the prominence of metabolic over an antigenic or cytokine-rich environment to promote MAIT cell reduction in patients with CMD. In summary, all stages of CMDs are characterized by reduced circulating MAIT cells. Chronically elevated blood glucose levels could contribute to this decline. These data extend the pathologic relevance of MAIT cell loss and suggest that MAIT cell abundance may serve as an indicator of cardiometabolic health.-Touch, S., Assmann, K. E., Aron-Wisnewsky, J., Marquet, F., Rouault, C., Fradet, M., Mosbah, H., MetaCardis Consortium, Isnard, R., Helft, G., Lehuen, A., Poitou, C., Clément, K., André, S. Mucosal-associated invariant T (MAIT) cells are depleted and prone to apoptosis in cardiometabolic disorders.

Influence of post-transplant mucosal-associated invariant T cell recovery on the development of acute graft-versus-host disease in allogeneic bone marrow transplantation

Mucosal-associated invariant T (MAIT) and invariant natural killer T (iNKT) cells are T cell subpopulations that possess innate-like properties. We examined the impact of post-hematopoietic stem cell transplantation (HSCT) MAIT and iNKT cell recovery on the clinical outcomes of 69 patients who underwent allogeneic HSCT at Kyoto University Hospital. Multivariate analyses identified the absolute number of MAIT cells (< 0.48/μL on day 60 post-HSCT) as the sole independent risk factor for grade I-IV and grade II-IV acute graft-versus-host disease (aGVHD) among patients who underwent bone marrow transplantation; no correlation was observed between post-HSCT iNKT cell recovery and the development of aGVHD. Six of the 15 patients in the MAIT^high (≥ 0.48/μL) group developed aGVHD, five within the first 30 days post HSCT. In contrast, 13 of the 15 patients in the MAIT^low (< 0.48/μL) group developed aGVHD, seven after day 30 post HSCT. The overall survival of the MAIT^low group was slightly shorter than that of the MAIT^high group. Thus, the post-HSCT recovery of MAIT cells is closely related to the development of delayed onset aGVHD and the outcome of post-HSCT, suggesting its utility for identifying a subset of patients that requires more prolonged and/or intense GVHD prophylaxis.

MAIT cells and viruses

Mucosal associated invariant T cells (MAIT cells) bear a T cell receptor (TCR) that specifically targets microbially derived metabolites. Functionally, they respond to bacteria and yeasts, which possess the riboflavin pathway, essential for production of such metabolites and which are presented on MR1. Viruses cannot generate these ligands, so a priori, they should not be recognized by MAIT cells and indeed this is true when considering recognition through the TCR. However, MAIT cells are distinctive in another respect, since they respond quite sensitively to non‐TCR signals, especially in the form of inflammatory cytokines. Thus, a number of groups have shown that virus infection can be “sensed” by MAIT cells and a functional response invoked. Since MAIT cells are abundant in humans, especially in tissues such as the liver, the question has arisen as to whether this TCR‐independent MAIT cell triggering by viruses plays any role in vivo. In this review, we will discuss the evidence for this phenomenon and some common features which emerge across different recent studies in this area.

Synthesis, stabilization, and characterization of the MR1 ligand precursor 5-amino-6-D-ribitylaminouracil (5-A-RU)

Mucosal-associated invariant T (MAIT) cells are an abundant class of innate T cells restricted by the MHC I-related molecule MR1. MAIT cells can recognize bacterially-derived metabolic intermediates from the riboflavin pathway presented by MR1 and are postulated to play a role in innate antibacterial immunity through production of cytokines and direct bacterial killing. MR1 tetramers, typically stabilized by the adduct of 5-amino-6-D-ribitylaminouracil (5-A-RU) and methylglyoxal (MeG), are important tools for the study of MAIT cells. A long-standing problem with 5-A-RU is that it is unstable upon storage. Herein we report an efficient synthetic approach to the HCl salt of this ligand, which has improved stability during storage. We also show that synthetic 5-A-RU•HCl produced by this method may be used in protocols for the stimulation of human MAIT cells and production of both human and mouse MR1 tetramers for MAIT cell identification.

Mucosal-Associated Invariant T Cells in Chronic Inflammatory Liver Disease

The broadening field of microbiome research has led to a substantial reappraisal of the gut-liver axis and its role in chronic liver disease. The liver is a central immunologic organ that is continuously exposed to food and microbial-derived antigens from the gastrointestinal tract. Mucosal-associated invariant T (MAIT) cells are enriched in the human liver and can be activated by inflammatory cytokines and microbial antigens. In chronic inflammatory liver disease, MAIT cells are depleted suggesting an impaired MAIT cell-dependent protection against bacterial infections.

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

There are very few human MAIT cells in cord blood. Ben Youssef et al. show that they slowly expand during childhood and point to a critical role of the TCRαβ repertoire in determining their unique ability to recognize MR1-restricted microbial antigens.

Mucosal-associated invariant T (MAIT) cells are semi-invariant Vα7.2⁺ CD161^highCD4⁻ T cells that recognize microbial riboflavin precursor derivatives such as 5-OP-RU presented by MR1. Human MAIT cells are abundant in adult blood, but there are very few in cord blood. We longitudinally studied Vα7.2⁺ CD161^high T cell and related subset levels in infancy and after cord blood transplantation. We show that Vα7.2⁺ and Vα7.2⁻ CD161^high T cells are generated early during gestation and likely share a common prenatal developmental program. Among cord blood Vα7.2⁺ CD161^high T cells, the minority recognizing MR1:5-OP-RU display a TRAV/TRBV repertoire very similar to adult MAIT cells. Within a few weeks of life, only the MR1:5-OP-RU reactive Vα7.2⁺ CD161^high T cells acquire a memory phenotype. Only these cells expand to form the adult MAIT pool, diluting out other Vα7.2⁺ CD161^high and Vα7.2⁻ CD161^high populations, in a process requiring at least 6 years to reach adult levels. Thus, the high clonal size of adult MAIT cells is antigen-driven and likely due to the fine specificity of the TCRαβ chains recognizing MR1-restricted microbial antigens.