iNKT and MAIT Cell Alterations in Diabetes

The proliferation of human mucosal-associated invariant T cells requires a MYC-SLC7A5-glycolysis metabolic axis

Mucosal-associated invariant T (MAIT) cells are an abundant population of innate T cells that recognize bacterial ligands and play a key role in host protection against bacterial and viral pathogens. Upon activation, MAIT cells undergo proliferative expansion and increase their production of effector molecules such as cytokines. In this study, we found that both mRNA and protein abundance of the key metabolism regulator and transcription factor MYC was increased in stimulated MAIT cells. Using quantitative mass spectrometry, we identified the activation of two MYC-controlled metabolic pathways, amino acid transport and glycolysis, both of which were necessary for MAIT cell proliferation. Last, we showed that MAIT cells isolated from people with obesity showed decreased MYC mRNA abundance upon activation, which was associated with defective MAIT cell proliferation and functional responses. Collectively, our data uncover the importance of MYC-regulated metabolism for MAIT cell proliferation and provide additional insight into the molecular basis for the functional defects of MAIT cells in obesity.

New insights into MAIT cells in autoimmune diseases

Mucosal-associated invariant T (MAIT) cells are resident T cells that express semi-invariant TCR chains and are restricted by monomorphic major histocompatibility complex (MHC) class I-related molecules (MR1). MAIT cells can be activated by microbial-specific metabolites (MR1-dependent mode) or cytokines (MR1-independent mode). Activated MAIT cells produce chemokines, cytotoxic molecules (granzyme B and perforin), and proinflammatory cytokines (IFN-γ, TNF-α, and IL-17), to clear pathogens and target infected cells involved in the pro-inflammatory, migratory, and cytolytic properties of MAIT cells. MAIT cells produce pro-inflammatory cytokines in the target organs of autoimmune diseases and contribute to the development and progression of autoimmune diseases. This article reviews the biological characteristics, activation mechanism, dynamic migration, and dual functions of MAIT cells, and focuses on the mechanism and potential application of MAIT cells in the early diagnosis, disease activity monitoring, and therapeutic targets of autoimmune diseases, to lay a foundation for future research.

Dysfunction of circulating CD3+CD56+ NKT-like cells in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is associated with increased incidence and mortality of many cancers and infectious diseases. CD3⁺CD56⁺ NKT-like cells play pivotal roles in tumor surveillance and infection control. However, little is known about potential alterations in circulating NKT-like cells in T2DM patients. In this study, we found that the frequency and absolute counts of circulating NKT-like cells were significantly lower in patients with T2DM compared to healthy volunteers. Moreover, in T2DM patients, NKT-like cells were impaired in their production of IFN-γ and TNF-α as well as degranulation capacity. The expression of activating receptor NKG2D was markedly decreased on NKT-like cells in T2DM patients, while the expression of inhibitory receptors Tim-3 and LAG-3 was upregulated. In detail, Tim-3⁺NKT-like cells expressed higher LAG-3 and less IFN-γ and TNF-α compared to Tim-3^-NKT-like cells. Importantly, we further found that the expression of Tim-3 in NKT-like cells from T2DM patients correlated positively with glycated hemoglobin (HbA1c) and fasting blood glucose (FBG) levels, as well as with diabetes duration. In conclusion, these results indicate that NKT-like cells from T2DM patients display an exhausted phenotype and reduced functionality. Moreover, Tim-3 expression on NKT-like cells likely serves a novel biomarker for duration of T2DM.

Involvement of impaired CD8+ mucosal-associated invariant T cells and myeloid-derived suppressor cells in polycystic ovary syndrome

BACKGROUND

Immune dysfunction is one of the mechanisms to promote polycystic ovary syndrome (PCOS). Various immune cells have been reported to be involved in the development of PCOS. Meanwhile, the disturbance of metabolism is closely related to PCOS. The aim of this study is to explore the association of mucosal-associated invariant T (MAIT) cells and myeloid-derived suppressor cells (MDSCs) with the metabolic dysfunction in PCOS.

METHODS

68 PCOS patients and 40 controls were recruited in this study and we collected the peripheral blood of participants' during their follicular phase. The frequencies of MAIT cells and MDSCs were determined by flow cytometry after being stained with different monoclonal antibodies. And the concentrations of cytokines were determined by ELISA.

RESULTS

Compared to controls with normal metabolism, the frequency of MDSCs, CD8⁺MAIT cells and CD38⁺CD8⁺MAIT cells were significantly decreased in PCOS patients with normal metabolism, however, proportion of CD4⁺MAIT cells exhibited a noticeable increase. Similar results of CD8⁺MAIT, CD38⁺CD8⁺MAIT cells and reduced expression of IL-17 were observed in PCOS patients with metabolic dysfunction as compared to controls with metabolic disorders. PCOS patients with excessive testosterone levels displayed significantly decreased levels of CD8⁺MAIT, CD38⁺CD8⁺MAIT cells, MDSCs and Mo-MDSCs as compared to PCOS patients with normal testosterone concentrations. PCOS patients with abnormal weight showed a lower level and activation of CD8⁺MAIT cells. On the contrary, they displayed an enrichment of CD4⁺MAIT cells. PCOS patients with glucose metabolic disorder displayed a remarkable dysregulation of MDSCs and Mo-MDSCs. MDSCs were positively correlated with MAIT cells. Negative correlations between the frequency of CD8⁺MAIT cells, CD38⁺CD8⁺MAIT cells and body mass index were revealed. CD4⁺MAIT cells positively correlated with BMI. Mo-MDSCs were found to be negatively related to the levels of 2hour plasma glucose and HOMA-IR index.

CONCLUSION

The impairment of CD8+MAIT cells and MDSCs is involved in the metabolic dysfunction of PCOS.

Impact of Bariatric Surgery on Adipose Tissue Biology

Bariatric surgery (BS) procedures are actually the most effective intervention to help subjects with severe obesity achieve significant and sustained weight loss. White adipose tissue (WAT) is increasingly recognized as the largest endocrine organ. Unhealthy WAT expansion through adipocyte hypertrophy has pleiotropic effects on adipocyte function and promotes obesity-associated metabolic complications. WAT dysfunction in obesity encompasses an altered adipokine secretome, unresolved inflammation, dysregulated autophagy, inappropriate extracellular matrix remodeling and insufficient angiogenic potential. In the last 10 years, accumulating evidence suggests that BS can improve the WAT function beyond reducing the fat depot sizes. The causal relationships between improved WAT function and the health benefits of BS merits further investigation. This review summarizes the current knowledge on the short-, medium- and long-term outcomes of BS on the WAT composition and function.

Mucosal-Associated Invariant T Cell Response to Acute Exercise and Exercise Training in Older Obese Women

(1) Background: Obesity is a major global public health concern as it is associated with many of the leading causes of preventable deaths. Exercise reduces obesity-induced inflammation; however, it is unknown how exercise training may impact mucosal associated invariant T (MAIT) cells in overweight/obese (OW) post-menopausal women. Therefore, the purpose of this study was to investigate (i) circulating MAIT-cells at rest in OW vs. Lean women, (ii) the response of MAIT-cells to a single bout of combined aerobic and resistance exercise, and (iii) the effects of 12 weeks of exercise training (EX) or educational program (ED) on the MAIT-cell response in OW. (2) Methods: OW completed an acute exercise session or sitting control, underwent 12 weeks of exercise training or received educational materials, and then repeated the exercise session/sitting control. Lean post-menopausal women provided a baseline comparison. (3) Results: OW had lower circulating MAIT-cells at rest than Lean prior to exercise training; however, after training EX displayed improved MAIT-cell frequency. Additionally, prior to training EX did not exhibit MAIT-cell mobilization/egress, however, both improved after training. (4) Conclusions: Reduced MAIT-cell frequency and ability to mobilize/egress were potentially partially rescued in EX after 12 weeks of exercise training; however, further research is needed to elucidate age or obesity-induced attenuations in MAIT-cells.

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

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

Altered distribution and enhanced osteoclastogenesis of mucosal-associated invariant T cells in gouty arthritis

OBJECTIVE

This study was designed to investigate the role of mucosal-associated invariant T (MAIT) cells in gouty arthritis (GA) and their effects on osteoclastogenesis.

METHODS

Patients with GA (n = 61), subjects with hyperuricaemia (n = 11) and healthy controls (n = 30) were enrolled in this study. MAIT cells, cytokines, CD69, programmed death-1 (PD-1) and lymphocyte-activation gene 3 (LAG-3) levels were measured by flow cytometry. In vitro osteoclastogenesis experiments were performed using peripheral blood mononuclear cells in the presence of M-CSF and RANK ligand.

RESULTS

Circulating MAIT cell levels were significantly reduced in GA patients. However, their capacities for IFN-γ, IL-17 and TNF-α production were preserved. Expression levels of CD69, PD-1 and LAG-3 in MAIT cells were found to be elevated in GA patients. In particular, CD69 expression in circulating MAIT cells was increased by stimulation with MSU crystals, suggesting that deposition of MSU crystals might contribute to MAIT cell activation. Interestingly, MAIT cells were found to be accumulated in synovial fluid and infiltrated into gouty tophus tissues within joints. Furthermore, activated MAIT cells secreted pro-resorptive cytokines (i.e. IL-6, IL-17 and TNF-α) and facilitated osteoclastogenesis.

CONCLUSION

This study demonstrates that circulating MAIT cells are activated and numerically deficient in GA patients. In addition, MAIT cells have the potential to migrate to inflamed tissues and induce osteoclastogenesis. These findings provide an important role of MAIT cells in the pathogenesis of inflammation and bone destruction in GA patients.

Sex discrepancy in the reduction of mucosal-associated invariant T cells caused by obesity

Introduction

Gut microbiota has been reported to contribute to obesity and the pathology of obesity‐related diseases but the underlying mechanisms are largely unknown. Mucosal‐associated invariant T (MAIT) cells are a unique subpopulation of T cells characterized by the expression of a semi‐invariant T cell receptor (TCR) α chain (Vα19 in mice; Vα7.2 in humans). The expansion and maturation of MAIT cells require the gut microbiota and antigen‐presenting molecule MR1, suggesting that MAIT cells may play a unique role in bridging gut microbiota, obesity, and obesity‐associated inflammation.

Methods

The levels of human MAIT cells from obese patients, as well as mouse MAIT cells from obese mouse models, were determined by flow cytometry. By comparing to controls, we analyzed the change of MAIT cells in obese subjects.

Results

We found obese patients had fewer circulating MAIT cells than healthy‐weight donors and the difference was more distinct in male patients. Consistently, male mice (but not female mice) have shown reduced MAIT cells in the liver and adipose tissue after a 10‐week Western diet compared to mice on a control diet. We also explored the possibility of utilizing high‐throughput technology (i.e., quantitative polymerase chain reaction [qPCR]), other than flow cytometry, to determine the expression levels of the invariant TCR of human MAIT cells. But a minimal correlation (R² = 0.23, p = .11) was observed between qPCR and flow cytometry data.

Conclusion

Our study suggests that there is a sex discrepancy in the impact of obesity on MAIT cells: MAIT cells in male (but not female) humans and male mice are reduced by obesity.

Concentration of NK cells after β-glucan and vitamin D supplementation in patients with diabetic retinopathy

In our study, we focused on possible effects of supplementation with glucan and vitamin D on total numbers of NK cells in patients with diabetic retinopathy. We evaluated possible relations among nutritional state (BMI), leptin levels, and total numbers of NK cells in patients supplemented with (1) glucan and vitamin D, (2) vitamin D and placebo, and (3) vitamin D alone. Our results show that 3 months of supplementation with both glucan and vitamin D resulted in significant improvements of NK cell numbers. In addition, we found statistically significant correlation between NK cell numbers and leptin levels. Based on these results, we propose that the molecule responsible for these changes is glucan, as vitamin D alone or together with placebo caused no effects.

Distinct Dysfunctional States of Circulating Innate-Like T Cells in Metabolic Disease

The immune system plays a significant role in controlling systemic metabolism. Innate-like T (ILT) cells in particular, such as mucosal-associated invariant T (MAIT) cells, invariant natural killer T (iNKT) cells and γδ T cell receptor expressing cells, have been reported to promote metabolic homeostasis. However, these different ILT cell subsets have, to date, been generally studied in isolation. Here we conducted a pilot study assessing the phenotype and function of circulating MAIT, iNKT, and Vδ2⁺ T cells in a small cohort of 10 people with obesity and type 2 diabetes (T2D), 10 people with obesity but no diabetes, and 12 healthy individuals. We conducted phenotypic analysis by flow cytometry ex vivo, and then functional analysis after in vitro stimulation using either PMA/ionomycin or synthetic agonists, or precursors thereof, for each of the cell-types; use of the latter may provide important knowledge for the development of novel therapeutics aimed at activating human ILT cells. The results of our pilot study, conducted on circulating cells, show clear dysfunction of all three ILT cell subsets in obese and obese T2D patients, as compared to healthy controls. Importantly, while both iNKT and Vδ2⁺ T cell dysfunctions were characterized by diminished IL-2 and interferon-γ production, the distinct dysfunctional state of MAIT cells was instead defined by skewed subset composition, heightened sensitivity to T cell receptor engagement and unchanged production of all measured cytokines.

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.

The Immune Landscape of Visceral Adipose Tissue During Obesity and Aging

Obesity and aging represent major health burdens to the global adult population. Both conditions promote the development of associated metabolic diseases such as insulin resistance. The visceral adipose tissue (VAT) is a site that becomes dysfunctional during obesity and aging, and plays a significant role during their pathophysiology. The changes in obese and aging VAT are now recognized to be partly driven by a chronic local inflammatory state, characterized by immune cells that typically adopt an inflammatory phenotype during metabolic disease. Here, we summarize the current knowledge on the immune cell landscape of the VAT during lean, obese, and aged conditions, highlighting their similarities and differences. We also briefly discuss possible linked mechanisms that fuel obesity- and age-associated VAT dysfunction.

CD4+/CD8+ mucosa-associated invariant T cells foster the development of endometriosis: a pilot study

BACKGROUND

Immune dysregulation is one of the mechanisms to promote endometriosis (EMS). Various T cell subpopulations have been reported to play different roles in the development of EMS. The mucosa-associated invariant T cell (MAIT) is an important T cell subset in the pathogenesis of various autoimmune diseases. Evidence has indicated that there are three functionally distinct MAIT subsets: CD4+, CD8+ and CD4/CD8-/- (double negative, DN) MAIT cells. Till now, the associations between endometriosis and MAIT have not been studied. Our research investigates different MAIT subpopulations in peripheral blood (PB) and peritoneal fluid (PF) from EMS patients.

METHODS

Thirty-two EMS patients and eighteen controls were included. PB and PF were collected. Tests of cytokines in plasma and PF were performed by ELISA kit. Characterisations of MAIT were done by flow cytometry. MAIT cells have been defined as CD3 + CD161 + Vα7.2+ cells. Based on CD4 and CD8 expression, they were divided into CD8⁺MAIT, CD4⁺MAIT and DN MAIT.

RESULTS

Enrichments of MAIT cells, especially CD4 and CD8 MAIT subsets were found. Moreover, CD8 MAIT cells had a high activation in the EMS group. EMS patients produced higher level of IL-8/12/17 as compared to these from controls. On the contrary, control patients exhibited an impressive upregulation of DN MAIT cells, however, these DN MAIT cells from controls showed a higher expression of PD-1. Lastly, we performed the relevance analysis, and discovered that the accumulation of PB MAIT cells positively correlated with an elevated level of serum CA125 production in EMS group.

CONCLUSION

These results suggest that different MAIT subsets play distinct roles in the progression of endometriosis.

Adipose Tissue-Resident Immune Cells in Obesity and Type 2 Diabetes

Inflammation is an important contributor to the pathogenesis of obesity-related type 2 diabetes (T2D). Adipose tissue-resident immune cells have been observed, and the potential contribution of these cells to metabolic dysfunction has been appreciated in recent years. This review focused on adipose tissue-resident immune cells that are dysregulated in the context of obesity and T2D. We comprehensively overviewed emerging knowledge regarding the phenotypic and functional properties of these cells and local factors that control their development. We discussed their function in controlling the immune response cascade and disease progression. We also characterized the metabolic profiles of these cells to explain the functional consequences in obese adipose tissues. Finally, we discussed the potential therapeutic targeting of adipose tissue-resident immune cells with the aim of addressing novel therapeutic approaches for the treatment of this disease.

The Differential Roles of T Cells in Non-alcoholic Fatty Liver Disease and Obesity

Non-alcoholic fatty liver disease (NAFLD) constitutes a spectrum of disease states characterized by hepatic steatosis and is closely associated to obesity and the metabolic syndrome. In non-alcoholic steatohepatitis (NASH), additionally, inflammatory changes and hepatocellular damage are present, representing a more severe condition, for which the treatment is an unmet medical need. Pathophysiologically, the immune system is one of the main drivers of NAFLD progression and other obesity-related comorbidities, and both the innate and adaptive immune system are involved. T cells form the cellular component of the adaptive immune system and consist of multiple differentially active subsets, i.e., T helper (Th) cells, regulatory T (Treg) cells, and cytotoxic T (Tc) cells, as well as several innate T-cell subsets. This review focuses on the role of these T-cell subsets in the pathogenesis of NAFLD, as well as the association with obesity and type 2 diabetes mellitus, reviewing the available evidence from both animal and human studies. Briefly, Th1, Th2, Th17, and Th22 cells seem to have an attenuating effect on adiposity. Th2, Th22, and Treg cells seem to decrease insulin resistance, whereas Th1, Th17, and Tc cells have an aggravating effect. Concerning NAFLD, both Th22 and Treg cells appear to have an overall tempering effect, whereas Th17 and Tc cells seem to induce more liver damage and fibrosis progression. The evidence regarding the role of the innate T-cell subsets is more controversial and warrants further exploration.

Role of Adaptive and Innate Immunity in Type 2 Diabetes Mellitus

After the recognition of the essential role of the immune system in the progression of type 2 diabetes mellitus, more studies are focused on the effects produced by the abnormal differentiation of components of the immune system. In patients suffering from obesity or T2DM, there were alterations in proliferation of T cells and macrophages, and impairment in function of NK cells and B cells, which represented abnormal innate and adaptive immunity. The abnormality of either innate immunity, adaptive immunity, or both was involved and interacted with each other during the progression of T2DM. Although previous studies have revealed the functional involvement of T cells in T2DM, and the regulation of metabolism by the innate or adaptive immune system during the pathogenesis of T2DM, there has been a lack of literature reviewing the relevant role of adaptive and innate immunity in the progression of T2DM. Here, we will review their relevant roles, aiming to provide new thought for the development of immunotherapy in T2DM.

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

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

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.

Human MAIT cells exit peripheral tissues and recirculate via lymph in steady state conditions

Mucosal-associated invariant T cells (MAIT cells) recognize bacterial metabolites as antigen and are found in blood and tissues, where they are poised to contribute to barrier immunity. Recent data demonstrate that MAIT cells located in mucosal barrier tissues are functionally distinct from their blood counterparts, but the relationship and circulation of MAIT cells between blood and different tissue compartments remains poorly understood. Previous studies raised the possibility that MAIT cells do not leave tissue and may either be retained or undergo apoptosis. To directly address if human MAIT cells exit tissues, we collected human donor-matched thoracic duct lymph and blood and analyzed MAIT cell phenotype, transcriptome, and T cell receptor (TCR) diversity by flow cytometry and RNA sequencing. We found that MAIT cells were present in the lymph, despite being largely CCR7- in the blood, thus indicating that MAIT cells in the lymph migrated from tissues and were capable of exiting tissues to recirculate. Importantly, MAIT cells in the lymph and blood had highly overlapping clonotype usage but distinct transcriptome signatures, indicative of differential activation states.

Cytotoxic and regulatory roles of mucosal-associated invariant T cells in type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease that results from the destruction of pancreatic β-cells by the immune system that involves innate and adaptive immune cells. Mucosal-associated invariant T cells (MAIT cells) are innate-like T-cells that recognize derivatives of precursors of bacterial riboflavin presented by the major histocompatibility complex (MHC) class I-related molecule MR1. Since T1D is associated with modification of the gut microbiota, we investigated MAIT cells in this pathology. In patients with T1D and mice of the non-obese diabetic (NOD) strain, we detected alterations in MAIT cells, including increased production of granzyme B, which occurred before the onset of diabetes. Analysis of NOD mice that were deficient in MR1, and therefore lacked MAIT cells, revealed a loss of gut integrity and increased anti-islet responses associated with exacerbated diabetes. Together our data highlight the role of MAIT cells in the maintenance of gut integrity and the control of anti-islet autoimmune responses. Monitoring of MAIT cells might represent a new biomarker of T1D, while manipulation of these cells might open new therapeutic strategies.

MHC class I-related molecule, MR1, and mucosal-associated invariant T cells

The MHC-related 1, MR1, molecule presents a new class of microbial antigens (derivatives of the riboflavin [Vitamin B2] biosynthesis pathway) to mucosal-associated invariant T (MAIT) cells. This raises many questions regarding antigens loading and intracellular trafficking of the MR1/ligand complexes. The MR1/MAIT field is also important because MAIT cells are very abundant in humans and their frequency is modified in many infectious and non-infectious diseases. Both MR1 and the invariant TCRα chain expressed by MAIT cells are strikingly conserved among species, indicating important functions. Riboflavin is synthesized by plants and most bacteria and yeasts but not animals, and its precursor derivatives activating MAIT cells are short-lived unless bound to MR1. The recognition of MR1 loaded with these compounds is therefore an exquisite manner to detect invasive bacteria. Herein, we provide an historical perspective of the field before describing the main characteristics of MR1, its ligands, and the few available data regarding its cellular biology. We then summarize the current knowledge of MAIT cell differentiation and discuss the definition of MAIT cells in comparison to related subsets. Finally, we describe the phenotype and effector activities of MAIT cells.

Sarcopenic obesity or obese sarcopenia: A cross talk between age-associated adipose tissue and skeletal muscle inflammation as a main mechanism of the pathogenesis

Sarcopenia, an age-associated decline in skeletal muscle mass coupled with functional deterioration, may be exacerbated by obesity leading to higher disability, frailty, morbidity and mortality rates. In the combination of sarcopenia and obesity, the state called sarcopenic obesity (SOB), some key age- and obesity-mediated factors and pathways may aggravate sarcopenia. This review will analyze the mechanisms underlying the pathogenesis of SOB. In obese adipose tissue (AT), adipocytes undergo hypertrophy, hyperplasia and activation resulted in accumulation of pro-inflammatory macrophages and other immune cells as well as dysregulated production of various adipokines that together with senescent cells and the immune cell-released cytokines and chemokines create a local pro-inflammatory status. In addition, obese AT is characterized by excessive production and disturbed capacity to store lipids, which accumulate ectopically in skeletal muscle. These intramuscular lipids and their derivatives induce mitochondrial dysfunction characterized by impaired β-oxidation capacity and increased reactive oxygen species formation providing lipotoxic environment and insulin resistance as well as enhanced secretion of some pro-inflammatory myokines capable of inducing muscle dysfunction by auto/paracrine manner. In turn, by endocrine manner, these myokines may exacerbate AT inflammation and also support chronic low grade systemic inflammation (inflammaging), overall establishing a detrimental vicious circle maintaining AT and skeletal muscle inflammation, thus triggering and supporting SOB development. Under these circumstances, we believe that AT inflammation dominates over skeletal muscle inflammation. Thus, in essence, it redirects the vector of processes from "sarcopenia→obesity" to "obesity→sarcopenia". We therefore propose that this condition be defined as "obese sarcopenia", to reflect the direction of the pathological pathway.

Does bariatric surgery improve adipose tissue function?

Bariatric surgery is currently the most effective treatment for obesity. Not only do these types of surgeries produce significant weight loss but also they improve insulin sensitivity and whole body metabolic function. The aim of this review is to explore how altered physiology of adipose tissue may contribute to the potent metabolic effects of some of these procedures. This includes specific effects on various fat depots, the function of individual adipocytes and the interaction between adipose tissue and other key metabolic tissues. Besides a dramatic loss of fat mass, bariatric surgery shifts the distribution of fat from visceral to the subcutaneous compartment favoring metabolic improvement. The sensitivity towards lipolysis controlled by insulin and catecholamines is improved, adipokine secretion is altered and local adipose inflammation as well as systemic inflammatory markers decreases. Some of these changes have been shown to be weight loss independent, and novel hypothesis for these effects includes include changes in bile acid metabolism, gut microbiota and central regulation of metabolism. In conclusion bariatric surgery is capable of improving aspects of adipose tissue function and do so in some cases in ways that are not entirely explained by the potent effect of surgery. © 2016 World Obesity.

HIV-Infected Children Have Lower Frequencies of CD8+ Mucosal-Associated Invariant T (MAIT) Cells that Correlate with Innate, Th17 and Th22 Cell Subsets

Mucosal-associated invariant T cells (MAIT) are innate T cells restricted by major histocompatibility related molecule 1 (MR1) presenting riboflavin metabolite ligands derived from microbes. Specificity to riboflavin metabolites confers MAIT cells a broad array of host-protective activity against gram-negative and -positive bacteria, mycobacteria, and fungal pathogens. MAIT cells are present at low levels in the peripheral blood of neonates and gradually expand to relatively abundant levels during childhood. Despite no anti-viral activity, MAIT cells are depleted early and irreversibly in HIV infected adults. Such loss or impaired expansion of MAIT cells in HIV-positive children may render them more susceptible to common childhood illnesses and opportunistic infections. In this study we evaluated the frequency of MAIT cells in perinatally HIV-infected children, their response to antiretroviral treatment and their associations with HIV clinical status and related innate and adaptive immune cell subsets with potent antibacterial effector functions. We found HIV+ children between ages 3 to 18 years have significantly decreased CD8+ MAIT cell frequencies compared to uninfected healthy children. Remarkably, CD8 MAIT levels gradually increased with antiretroviral therapy, with greater recovery when treatment is initiated at a young age. Moreover, diminished CD8+ MAIT cell frequencies are associated with low CD4:CD8 ratios and elevated sCD14, suggesting a link with HIV disease progression. Last, CD8+ MAIT cell levels tightly correlate with other antibacterial and mucosa-protective immune subsets, namely, neutrophils, innate-like T cells, and Th17 and Th22 cells. Together these findings suggest that low frequencies of MAIT cells in HIV positive children are part of a concerted disruption to the innate and adaptive immune compartments specialized in sensing and responding to pathogenic or commensal bacteria.

Interleukin-13 Pathway Alterations Impair Invariant Natural Killer T-Cell-Mediated Regulation of Effector T Cells in Type 1 Diabetes

Many studies have shown that human natural killer T (NKT) cells can promote immunity to pathogens, but their regulatory function is still being investigated. Invariant NKT (iNKT) cells have been shown to be effective in preventing type 1 diabetes in the NOD mouse model. Activation of plasmacytoid dendritic cells, modulation of B-cell responses, and immune deviation were proposed to be responsible for the suppressive effect of iNKT cells. We studied the regulatory capacity of human iNKT cells from control subjects and patients with type 1 diabetes (T1D) at disease clinical onset. We demonstrate that control iNKT cells suppress the proliferation of effector T cells (Teffs) through a cell contact-independent mechanism. Of note, suppression depended on the secretion of interleukin-13 (IL-13) by iNKT cells because an antibody blocking this cytokine resulted from the abrogation of Teff suppression; however, T1D-derived iNKT cells showed impaired regulation that could be attributed to the decrease in IL-13 secretion. Thus, alteration of the IL-13 pathway at disease onset may lead to the progression of the autoimmune response in T1D. Advances in the study of iNKT cells and the selection of agonists potentiating IL-13 secretion should permit new therapeutic strategies to prevent the development of T1D.

Distinct activation thresholds of human conventional and innate-like memory T cells

Conventional memory CD8⁺ T cells and mucosal-associated invariant T cells (MAIT cells) are found in blood, liver, and mucosal tissues and have similar effector potential following activation, specifically expression of IFN-γ and granzyme B. To better understand each subset's unique contributions to immunity and pathology, we interrogated inflammation- and TCR-driven activation requirements using human memory CD8⁺ T and MAIT cells isolated from blood and mucosal tissue biopsies in ex vivo functional assays and single cell gene expression experiments. We found that MAIT cells had a robust IFN-γ and granzyme B response to inflammatory signals but limited responsiveness when stimulated directly via their TCR. Importantly, this is not due to an overall hyporesponsiveness to TCR signals. When delivered together, TCR and inflammatory signals synergize to elicit potent effector function in MAIT cells. This unique control of effector function allows MAIT cells to respond to the same TCR signal in a dichotomous and situation-specific manner. We propose that this could serve to prevent responses to antigen in noninflamed healthy mucosal tissue, while maintaining responsiveness and great sensitivity to inflammation-eliciting infections. We discuss the implications of these findings in context of inflammation-inducing damage to tissues such as BM transplant conditioning or HIV infection.

The Immunology of CD1- and MR1-Restricted T Cells

CD1- and MHC-related molecule-1 (MR1)-restricted T lymphocytes recognize nonpeptidic antigens, such as lipids and small metabolites, and account for a major fraction of circulating and tissue-resident T cells. They represent a readily activated, long-lasting population of effector cells and contribute to the early phases of immune response, orchestrating the function of other cells. This review addresses the main aspects of their immunological functions, including antigen and T cell receptor repertoires, mechanisms of nonpeptidic antigen presentation, and the current evidence for their participation in human and experimental diseases.