Foxp3(+) regulatory T cells in mouse models of type 1 diabetes

Multiply restimulated human cord blood-derived Tregs maintain stabilized phenotype and suppressive function and predict their therapeutic effects on autoimmune diabetes

BACKGROUND

Regulatory T cells (Tregs) are involved in the maintenance of immune homeostasis and immune regulation. Clinical trials on the adoptive transfer of Tregs have been ongoing for > 10 years. However, many unresolved issues remain in the production of readymade Treg products and selection of patients. Hence, this study aimed to develop a method to expand off-the-shelf Tregs derived from umbilical cord blood (UCB-Tregs) in vitro without changing their phenotype and inhibitory function. In addition, the study intended to design an approach to precisely select patients who are more likely to benefit from the adoptive Treg transfer therapy.

METHODS

UCB-Tregs were isolated and cultured in a medium containing human recombinant IL-2 and rapamycin and then multiply restimulated with human T-activator CD3/CD28 dynabeads. The phenotype and suppressive capacity of Tregs were assessed on days 18 and 42. The relationship between the suppressive function of UCB-Tregs in vitro and clinical indicators was analyzed, and the ability of the in vitro suppressive capacity to predict the in vivo therapeutic effects was evaluated.

RESULTS

UCB-Tregs expanded 123-fold and 5,981-fold at 18 and 42 days, respectively. The suppressive function of UCB-Tregs on the proliferation of immune cells at 42 days was not significantly different compared with that of UCB-Tregs obtained at 18 days. The suppression rate of UCB-Tregs to PBMCs was negatively correlated with the course of diabetes. Moreover, the high-suppression group exhibited a better treatment response than the low-suppression group during the 12-month follow-up period.

CONCLUSIONS

Multiply restimulated UCB-Tregs expanded at a large scale without any alterations in their classical phenotypic features and inhibitory functions. The suppressive function of Tregs in vitro was negatively correlated with the disease duration. The present study revealed the possibility of predicting the in vivo therapeutic effects via the in vitro inhibition assay. Thus, these findings provided a method to obtain off-the-shelf Treg products and facilitated the selection of patients who are likely to respond to the treatment, thereby moving toward the goal of precision treatment.

Tipping the balance in autoimmunity: are regulatory t cells the cause, the cure, or both?

Regulatory T cells (Tregs) are a specialized subgroup of T-cell lymphocytes that is crucial for maintaining immune homeostasis and preventing excessive immune responses. Depending on their differentiation route, Tregs can be subdivided into thymically derived Tregs (tTregs) and peripherally induced Tregs (pTregs), which originate from conventional T cells after extrathymic differentiation at peripheral sites. Although the regulatory attributes of tTregs and pTregs partially overlap, their modes of action, protein expression profiles, and functional stability exhibit specific characteristics unique to each subset. Over the last few years, our knowledge of Treg differentiation, maturation, plasticity, and correlations between their phenotypes and functions has increased. Genetic and functional studies in patients with numeric and functional Treg deficiencies have contributed to our mechanistic understanding of immune dysregulation and autoimmune pathologies. This review provides an overview of our current knowledge of Treg biology, discusses monogenetic Treg pathologies and explores the role of Tregs in various other autoimmune disorders. Additionally, we discuss novel approaches that explore Tregs as targets or agents of innovative treatment options.

Novel insights regarding the role of noncoding RNAs in diabetes

Diabetes mellitus (DM) is a group of metabolic disorders defined by hyperglycemia induced by insulin resistance, inadequate insulin secretion, or excessive glucagon secretion. In 2021, the global prevalence of diabetes is anticipated to be 10.7% (537 million people). Noncoding RNAs (ncRNAs) appear to have an important role in the initiation and progression of DM, according to a growing body of research. The two major groups of ncRNAs implicated in diabetic disorders are miRNAs and long noncoding RNAs. miRNAs are single-stranded, short (17–25 nucleotides), ncRNAs that influence gene expression at the post-transcriptional level. Because DM has reached epidemic proportions worldwide, it appears that novel diagnostic and therapeutic strategies are required to identify and treat complications associated with these diseases efficiently. miRNAs are gaining attention as biomarkers for DM diagnosis and potential treatment due to their function in maintaining physiological homeostasis via gene expression regulation. In this review, we address the issue of the gradually expanding global prevalence of DM by presenting a complete and up-to-date synopsis of various regulatory miRNAs involved in these disorders. We hope this review will spark discussion about ncRNAs as prognostic biomarkers and therapeutic tools for DM. We examine and synthesize recent research that used novel, high-throughput technologies to uncover ncRNAs involved in DM, necessitating a systematic approach to examining and summarizing their roles and possible diagnostic and therapeutic uses.

Combination therapy with human amniotic epithelial cells and hyaluronic acid promotes immune balance recovery in type 1 diabetic rats through local engraftment

Stem cell engraftment is currently a promising approach for type 1 diabetes mellitus (T1DM) treatment. In our previous study, engraftment of a combination of human amniotic epithelial cells (hAECs) and hyaluronic acid (HA) showed potent anti-diabetic effect in streptozotocin (STZ)-induced T1DM mice via tail vein injection. Here, we adopted a different route of stem cell delivery, that is via pancreatic subcapsular transplantation. This combined local engraftment of hAECs and HA in STZ-induced T1DM rats showed potent anti-diabetic activity, leading to stronger hypoglycaemia, more intact islet structure and increased number of insulin-positive cells compared with those with hAECs or insulin treatments. Engraftment of hAECs alone increased the proportion of Th1 and T-reg cells and decreased the proportion of Th2 and Th17 cells to protect islet β cells in STZ-induced T1DM rats, whereas the combined engraftment of hAECs and HA showed more potent regulatory capacity, considerably decreased the level of TNF-α and IL-17 and increased the level of TGF-β1 compared with those by other treatments. The potent synergistic effect of HA contributed to the recovery of immune balance in the diabetic rat model, thereby suggesting a new strategy for effective treatment of T1DM.

Transient Depletion of Foxp3+ Regulatory T Cells Selectively Promotes Aggressive β Cell Autoimmunity in Genetically Susceptible DEREG Mice

Type 1 diabetes (T1D) represents a hallmark of the fatal multiorgan autoimmune syndrome affecting humans with abrogated Foxp3⁺ regulatory T (Treg) cell function due to Foxp3 gene mutations, but whether the loss of Foxp3⁺ Treg cell activity is indeed sufficient to promote β cell autoimmunity requires further scrutiny. As opposed to human Treg cell deficiency, β cell autoimmunity has not been observed in non-autoimmune-prone mice with constitutive Foxp3 deficiency or after diphtheria toxin receptor (DTR)-mediated ablation of Foxp3⁺ Treg cells. In the spontaneous nonobese diabetic (NOD) mouse model of T1D, constitutive Foxp3 deficiency did not result in invasive insulitis and hyperglycemia, and previous studies on Foxp3⁺ Treg cell ablation focused on Foxp3^DTR NOD mice, in which expression of a transgenic BDC2.5 T cell receptor (TCR) restricted the CD4⁺ TCR repertoire to a single diabetogenic specificity. Here we revisited the effect of acute Foxp3⁺ Treg cell ablation on β cell autoimmunity in NOD mice in the context of a polyclonal TCR repertoire. For this, we took advantage of the well-established DTR/GFP transgene of DEREG mice, which allows for specific ablation of Foxp3⁺ Treg cells without promoting catastrophic autoimmune diseases. We show that the transient loss of Foxp3⁺ Treg cells in prediabetic NOD.DEREG mice is sufficient to precipitate severe insulitis and persistent hyperglycemia within 5 days after DT administration. Importantly, DT-treated NOD.DEREG mice preserved many clinical features of spontaneous diabetes progression in the NOD model, including a prominent role of diabetogenic CD8⁺ T cells in terminal β cell destruction. Despite the severity of destructive β cell autoimmunity, anti-CD3 mAb therapy of DT-treated mice interfered with the progression to overt diabetes, indicating that the novel NOD.DEREG model can be exploited for preclinical studies on T1D under experimental conditions of synchronized, advanced β cell autoimmunity. Overall, our studies highlight the continuous requirement of Foxp3⁺ Treg cell activity for the control of genetically pre-installed autoimmune diabetes.

Progress in Translational Regulatory T Cell Therapies for Type 1 Diabetes and Islet Transplantation

Regulatory T cells (Tregs) have become highly relevant in the pathophysiology and treatment of autoimmune diseases, such as type 1 diabetes (T1D). As these cells are known to be defective in T1D, recent efforts have explored ex vivo and in vivo Treg expansion and enhancement as a means for restoring self-tolerance in this disease. Given their capacity to also modulate alloimmune responses, studies using Treg-based therapies have recently been undertaken in transplantation. Islet transplantation provides a unique opportunity to study the critical immunological crossroads between auto- and alloimmunity. This procedure has advanced greatly in recent years, and reports of complete abrogation of severe hypoglycemia and long-term insulin independence have become increasingly reported. It is clear that cellular transplantation has the potential to be a true cure in T1D, provided the remaining barriers of cell supply and abrogated need for immune suppression can be overcome. However, the role that Tregs play in islet transplantation remains to be defined. Herein, we synthesize the progress and current state of Treg-based therapies in T1D and islet transplantation. We provide an extensive, but concise, background to understand the physiology and function of these cells and discuss the clinical evidence supporting potency and potential Treg-based therapies in the context of T1D and islet transplantation. Finally, we discuss some areas of opportunity and potential research avenues to guide effective future clinical application. This review provides a basic framework of knowledge for clinicians and researchers involved in the care of patients with T1D and islet transplantation.

Metabolic adaptation orchestrates tissue context-dependent behavior in regulatory T cells

The diverse distribution and functions of regulatory T cells (Tregs) ensure tissue and immune homeostasis; however, it remains unclear which factors can guide distribution, local differentiation, and tissue context-specific behavior in Tregs. Although the emerging concept that Tregs could re-adjust their transcriptome based on their habitations is supported by recent findings, the underlying mechanisms that reprogram transcriptome in Tregs are unknown. In the past decade, metabolic machineries have been revealed as a new regulatory circuit, known as immunometabolic regulation, to orchestrate activation, differentiation, and functions in a variety of immune cells, including Tregs. Given that systemic and local alterations of nutrient availability and metabolite profile associate with perturbation of Treg abundance and functions, it highlights that immunometabolic regulation may be one of the mechanisms that orchestrate tissue context-specific regulation in Tregs. The understanding on how metabolic program instructs Tregs in peripheral tissues not only represents a critical opportunity to delineate a new avenue in Treg biology but also provides a unique window to harness Treg-targeting approaches for treating cancer and autoimmunity with minimizing side effects. This review will highlight the metabolic features on guiding Treg formation and function in a disease-oriented perspective and aim to pave the foundation for future studies.

Are Regulatory T Cells Defective in Type 1 Diabetes and Can We Fix Them?

Regulatory T cells (Tregs) are critical regulators of peripheral immune tolerance. Treg insufficiency can lead to autoimmune disorders, including type 1 diabetes (T1D). Increasing evidence in mouse models of T1D, as well as other autoimmune disorders, suggests that there are defects in Treg-mediated suppression. Indeed, whereas Treg frequency in the peripheral blood of T1D patients is unaltered, their suppressive abilities are diminished compared with Tregs in healthy controls. Although expression of the transcription factor Foxp3 is a prerequisite for Treg development and function, there are many additional factors that can alter their stability, survival, and function. Much has been learned in other model systems, such as tumors, about the mechanism and pathways that control Treg stability and function. This review poses the question of whether we can use these findings to develop new therapeutic approaches that might boost Treg stability, survival, and/or function in T1D and possibly other autoimmune disorders.

MicroRNA-26a Promotes Regulatory T cells and Suppresses Autoimmune Diabetes in Mice

Type-1 diabetes (TID) is an autoimmune disease in which the body's own immune cells attack islet β cells, the cells in the pancreas that produce and release the hormone insulin. Mir-26a has been reported to play functions in cellular differentiation, cell growth, cell apoptosis, and metastasis. However, the role of microRNA-26a (Mir-26a) in autoimmune TID has never been investigated. In our current study, we found that pre-Mir-26a (LV-26a)-treated mice had significantly longer normoglycemic time and lower frequency of autoreactive IFN-γ-producing CD4(+) cells compared with an empty lentiviral vector (LV-Con)-treated non-obese diabetic (NOD) mice. Mir-26a suppresses autoreactive T cells and expands Tregs in vivo and in vitro. Furthermore, in our adoptive transfer study, the groups receiving whole splenocytes and CD25-depleted splenocytes from LV-Con-treated diabetic NOD mice develop diabetes at 3 to 4 weeks of age. In comparison, mice injected with undepleted splenocytes obtained from LV-26a-treated reversal NOD mice develop diabetes after 6-8 weeks. And depletion of CD25(+) cells in the splenocytes of reversed mice abrogates the delay in diabetes onset. In conclusion, Mir-26a suppresses autoimmune diabetes in NOD mice in part through promoted regulatory T cells (Tregs) expression.

Exploring the induction of preproinsulin-specific Foxp3(+) CD4(+) Treg cells that inhibit CD8(+) T cell-mediated autoimmune diabetes by DNA vaccination

DNA vaccination is a promising strategy to induce effector T cells but also regulatory Foxp3⁺ CD25⁺ CD4⁺ Treg cells and inhibit autoimmune disorders such as type 1 diabetes. Little is known about the antigen requirements that facilitate priming of Treg cells but not autoreactive effector CD8⁺ T cells. We have shown that the injection of preproinsulin (ppins)-expressing pCI/ppins vector into PD-1- or PD-L1-deficient mice induced K^b/A12-21-monospecific CD8⁺ T cells and autoimmune diabetes. A pCI/ppinsΔA12-21 vector (lacking the critical K^b/A12-21 epitope) did not induce autoimmune diabetes but elicited a systemic Foxp3⁺ CD25⁺ Treg cell immunity that suppressed diabetes induction by a subsequent injection of the diabetogenic pCI/ppins. TGF-β expression was significantly enhanced in the Foxp3⁺ CD25⁺ Treg cell population of vaccinated/ppins-primed mice. Ablation of Treg cells in vaccinated/ppins-primed mice by anti-CD25 antibody treatment abolished the protective effect of the vaccine and enabled diabetes induction by pCI/ppins. Adoptive transfer of Treg cells from vaccinated/ppins-primed mice into PD-L1^−/− hosts efficiently suppressed diabetes induction by pCI/ppins. We narrowed down the Treg-stimulating domain to a 15-residue ppins76–90 peptide. Vaccine-induced Treg cells thus play a crucial role in the control of de novo primed autoreactive effector CD8⁺ T cells in this diabetes model.

Type 1 diabetes and T regulatory cells

T-regulatory cells (Tregs) play a fundamental role in the creation and maintenance of peripheral tolerance. Deficits in the numbers and/or function of Tregs may be an underlying cause of human autoimmune diseases including type 1 Diabetes Mellitus (T1D), whereas an over-abundance of Tregs can hinder immunity against cancer or pathogens. The importance of Tregs in the control of autoimmunity is well established in a variety of experimental animal models. In mice, manipulating the numbers and/or function of Tregs can decrease pathology in a wide range of contexts, including autoimmunity and it is widely assumed that similar approaches will be possible in humans. T1D, the most prevalent human autoimmune disease, has been a focus of interventions either through direct and indirect in vivo proliferations or through adoptive transfer of the in vitro generated antigen specific and non specific Treg. Some challenges still need to be addressed, including a more specific phenotype marker for Tregs; the reproducibility of satisfactory animal results in human and the reconcile of discrepancies between in vitro and in vivo studies. In this article, we will highlight the role of Tregs in autoimmune disease in general with a special focus on T1D, highlighting progress made and challenges ahead in developing Treg-based therapies.

Anti-diabetic actions of carbon monoxide-releasing molecule (CORM)-A1: Immunomodulation and regeneration of islet beta cells

We have recently shown that carbon monoxide releasing molecule (CORM)-A1 prevents type 1 diabetes induced in C57BL/6 mice with multiple low doses of streptozotocin (MLDS) by shifting the Th1/Th17/M1 balance towards a Th2/M2 response. In the present work we tested the hypothesis that CORM-A1 might influence regulatory arm of the immune response, as well as beta cell regeneration. CORM-A1 (2 mg/kg/day) was administered for 10 days to mice induced with MLDS and/or depleted of low dose cyclophosphamide (CY)-sensitive FoxP3+ T regulatory (Treg) cells. Besides monitoring hyperglycaemia, ex vivo analysis of spleen, pancreatic lymph nodes (PLN) and pancreas was performed at the end of treatment. In CORM-A1-treated MLDS-induced mice the improvement of hyperglycaemia was observed only without depletion of CY-sensitive FoxP3+ Treg cells. This was accompanied by decreased levels of interleukin (IL)-12, IL-2 and early activation marker CD25 in the spleen and PLN and increased transforming growth factor (TGF)-β, resulting in reduced lymphocyte proliferation in both organs. In parallel, decreased transcript levels of IL-2, but increased mRNA expression of TGF-β, accompanied with up-regulation of Ki-67 protein expression was observed within pancreas. Together, the data suggested that besides the immunomodulatory potential, CORM-A1 probably induces beta cell regeneration.

Rodent models for human diseases

One of the factors limiting the translation of knowledge from preclinical studies to the clinic has been the limitations of in vivo diseases models. Except in the case of highly controlled and regulated clinical trials, geneticists and scientists do not use humans for their experimental investigations because of the obvious risk to life. Instead, they use various animal, fungal, bacterial, and plant species as model organisms for their studies. Amongst these model organisms, rodent models are the most used due to the easiness for the experiments and the possibility to modify genetically these model animals. Nevertheless, due to the fact that animal models typically do not contract the same genetic diseases as people, so scientists must alter their genomes to induce human disease states and to know what kind of mutation causes the disease. In this brief review, we will discuss the interests of rodent models that have been developed to simulate human pathologies, focusing in models that employ xenografts and genetic modification. Within the framework of genetically engineered mouse (GEM) models, we will review some of the current genetic strategies for modeling diseases.

Gut microbial markers are associated with diabetes onset, regulatory imbalance, and IFN-γ level in NOD mice

Gut microbiota regulated imbalances in the host's immune profile seem to be an important factor in the etiology of type 1 diabetes (T1D), and identifying bacterial markers for T1D may therefore be useful in diagnosis and prevention of T1D. The aim of the present study was to investigate the link between the early gut microbiota and immune parameters of non-obese diabetic (NOD) mice in order to select alleged bacterial markers of T1D. Gut microbial composition in feces was analyzed with 454/FLX Titanium (Roche) pyro-sequencing and correlated with diabetes onset age and immune cell populations measured in diabetic and non-diabetic mice at 30 weeks of age. The early gut microbiota composition was found to be different between NOD mice that later in life were classified as diabetic or non-diabetic. Those differences were further associated with changes in FoxP3(+) regulatory T cells, CD11b(+) dendritic cells, and IFN-γ production. The model proposed in this work suggests that operational taxonomic units classified to S24-7, Prevotella, and an unknown Bacteriodales (all Bacteroidetes) act in favor of diabetes protection whereas members of Lachnospiraceae, Ruminococcus, and Oscillospira (all Firmicutes) promote pathogenesis.

Experimental myocardial infarction induces altered regulatory T cell hemostasis, and adoptive transfer attenuates subsequent remodeling

Background

Ischemic cardiac damage is associated with upregulation of cardiac pro-inflammatory cytokines, as well as invasion of lymphocytes into the heart. Regulatory T cells (Tregs) are known to exert a suppressive effect on several immune cell types. We sought to determine whether the Treg pool is influenced by myocardial damage and whether Tregs transfer and deletion affect cardiac remodeling.

Methods and Results

The number and functional suppressive activity of Tregs were assayed in mice subjected to experimental myocardial infarction. The numbers of splenocyte-derived Tregs in the ischemic mice were significantly higher after the injury than in the controls, and their suppressive properties were significantly compromised. Compared with PBS, adoptive Treg transfer to mice with experimental infarction reduced infarct size and improved LV remodeling and functional performance by echocardiography. Treg deletion with blocking anti-CD25 antibodies did not influence infarct size or echocardiographic features of cardiac remodeling.

Conclusion

Treg numbers are increased whereas their function is compromised in mice with that underwent experimental infarction. Transfer of exogeneous Tregs results in attenuation of myocardial remodeling whereas their ablation has no effect. Thus, Tregs may serve as interesting potential interventional targets for attenuating left ventricular remodeling.

Fluorochrome-based definition of naturally occurring Foxp3(+) regulatory T cells of intra- and extrathymic origin

Under physiological conditions, studies on the biology of naturally induced Foxp3(+) Treg cells of intra- and extrathymic origin have been hampered by the lack of unambiguous markers to discriminate the mature progeny of such developmental Treg-cell sublineages. Here, we report on experiments in double-transgenic mice, in which red fluorescent protein (RFP) is expressed in all Foxp3(+) Treg cells, whereas Foxp3-dependent GFP expression is exclusively confined to intrathymically induced Foxp3(+) Treg cells. This novel molecular genetic tool enabled us to faithfully track and characterize naturally induced Treg cells of intrathymic (RFP(+) GFP(+) ) and extrathymic (RFP(+) GFP(-) ) origin in otherwise unmanipulated mice. These experiments directly demonstrate that extrathymically induced Treg cells substantially contribute to the overall pool of mature Foxp3(+) Treg cells residing in peripheral lymphoid tissues of steady-state mice. Furthermore, we provide evidence that intra- and extrathymically induced Foxp3(+) Treg cells represent distinct phenotypic and functional sublineages.

Activated FoxM1 attenuates streptozotocin-mediated β-cell death

The forkhead box transcription factor FoxM1, a positive regulator of the cell cycle, is required for β-cell mass expansion postnatally, during pregnancy, and after partial pancreatectomy. Up-regulation of full-length FoxM1, however, is unable to stimulate increases in β-cell mass in unstressed mice or after partial pancreatectomy, probably due to the lack of posttranslational activation. We hypothesized that expression of an activated form of FoxM1 could aid in recovery after β-cell injury. We therefore derived transgenic mice that inducibly express an activated version of FoxM1 in β-cells (RIP-rtTA;TetO-hemagglutinin (HA)-Foxm1(Δ)(NRD) mice). This N-terminally truncated form of FoxM1 bypasses 2 posttranslational controls: exposure of the forkhead DNA binding domain and targeted proteasomal degradation. Transgenic mice were subjected to streptozotocin (STZ)-induced β-cell ablation to test whether activated FoxM1 can promote β-cell regeneration. Mice expressing HA-FoxM1(ΔNRD) displayed decreased ad libitum-fed blood glucose and increased β-cell mass. β-Cell proliferation was actually decreased in RIP-rtTA:TetO-HA-Foxm1(NRD) mice compared with that in RIP-rtTA mice 7 days after STZ treatment. Unexpectedly, β-cell death was decreased 2 days after STZ treatment. RNA sequencing analysis indicated that activated FoxM1 alters the expression of extracellular matrix and immune cell gene profiles, which may protect against STZ-mediated death. These studies highlight a previously underappreciated role for FoxM1 in promoting β-cell survival.

Advances in the cellular immunological pathogenesis of type 1 diabetes

Type 1 diabetes is an autoimmune disease caused by the immune-mediated destruction of insulin-producing pancreatic β cells. In recent years, the incidence of type 1 diabetes continues to increase. It is supposed that genetic, environmental and immune factors participate in the damage of pancreatic β cells. Both the immune regulation and the immune response are involved in the pathogenesis of type 1 diabetes, in which cellular immunity plays a significant role. For the infiltration of CD4(+) and CD8(+) T lymphocyte, B lymphocytes, natural killer cells, dendritic cells and other immune cells take part in the damage of pancreatic β cells, which ultimately lead to type 1 diabetes. This review outlines the cellular immunological mechanism of type 1 diabetes, with a particular emphasis to T lymphocyte and natural killer cells, and provides the effective immune therapy in T1D, which is approached at three stages. However, future studies will be directed at searching for an effective, safe and long-lasting strategy to enhance the regulation of a diabetogenic immune system with limited toxicity and without global immunosuppression.

Targeted antigen delivery to DEC-205⁺ dendritic cells for tolerogenic vaccination

Dendritic cells (DCs) and Foxp3-expressing CD4⁺ regulatory T (Treg) cells play non-redundant roles in the maintenance of peripheral tolerance to self-antigens, thereby preventing fatal autoimmunity. A common hallmark of intra- and extra-thymic Treg cell lineage commitment is the induction of Foxp3 expression as a consequence of appropriate T cell receptor engagement with MHC class II:agonist ligand. It has now become increasingly clear that agonist ligand presentation by immature DCs in the steady state induces T cell tolerance by both recessive and dominant mechanisms, rather than promoting productive T helper cell responses. In this context, the ability of steady-state DCs to promote the extrathymic conversion of initially naïve CD4⁺Foxp3⁻ T cells into Foxp3⁺ Treg cells is of particular interest as it provides novel perspectives to enhance antigen-specific Treg cell function in clinical settings of unwanted immunity, such as β-cell autoimmunity.