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Summary: The immune system plays a critical role in protecting the host against infection but is subject to numerous levels of control that are necessary to prevent pathological, tissue-damaging responses. Inappropriate inflammatory immune responses to self-antigens, innocuous commensal microorganisms, or environmental antigens can lead to chronic, debilitating, and degenerative diseases. Regulatory T cells have an essential, non-redundant, and dominant function in preventing pathological immune responses, as shown by the development of systemic fatal autoimmunity in humans and animals with a genetic deficiency in regulatory T cells. In addition to controlling immune responses, there is a growing understanding that regulatory T cells also contribute directly to tissue homeostasis by promoting tissue regeneration and repair. For these reasons, the prospect of enhancing regulatory T-cell numbers and/or function in patients represents an appealing therapeutic opportunity with potential applications in many diseases, including some where the pathological role of the immune system has only recently been recognized. Approaches to enhance regulatory T cells are now starting to be explored in clinical studies in humans. This review series brings together papers highlighting the Treg-enhancing approaches that are most advanced clinically and examples of therapeutic opportunities based on our growing understanding of regulatory T-cell functions. [ABSTRACT FROM AUTHOR]

Summary: Regulatory T cells (Tregs) are a subtype of CD4+ T cells that can mediate immune tolerance by a multitude of immunomodulatory mechanisms. Treg-based adoptive immunotherapy is currently being tested in multiple phases I and II clinical trials in transplantation and autoimmune diseases. We have learned from the work done on conventional T cells that distinct mechanistic states can define their dysfunctions, such as exhaustion, senescence, and anergy. All three can negatively impact the therapeutic effectiveness of T-cell-based therapies. However, whether Tregs are susceptible to such dysfunctional states is not well studied, and results are sometimes found to be controversial. In addition, Treg instability and loss of FOXP3 expression is another Treg-specific dysfunction that can decreasein their suppressive potential. A better understanding of Treg biology and pathological states will be needed to compare and interpret the results of the different clinical and preclinical trials. We will review herein Tregs' mechanisms of action, describe different T-cell dysfunction subtypes and how and if they apply to Tregs (exhaustion, senescence, anergy, and instability), and finally how this knowledge should be taken into consideration when designing and interpreting Treg adoptive immunotherapy trials. Recent advances in Treg-based immunotherapy, as an immune tolerance approach for transplantation and autoimmune diseases, exhibited the need for a better understanding of Treg biology. Classical mechanisms of dysfunction in T cells such as exhaustion, senescence, and anergy are poorly understood in Tregs. Previous works in auto- and alloimmune diseases, as well as in cell-engineering, have shown susceptibility of Tregs to such complications, furthering the importance of this knowledge for future design and interpretation of Treg adoptive immunotherapy trials. [ABSTRACT FROM AUTHOR]

Summary: The Forkhead Box P3 (FOXP3) protein is an essential transcription factor for the development and function of regulatory T cells (Tregs), involved in the maintenance of immunological tolerance. Although extensive research over the last decade has investigated the critical role of FOXP3+ cells in preserving immune homeostasis, our understanding of their specific functions remains limited. Therefore, unveiling the molecular mechanisms underpinning the up- and downstream transcriptional regulation of and by FOXP3 is crucial for developing Treg-targeted therapeutics. Dysfunctions in FOXP3+ Tregs have also been found to be inherent drivers of autoimmune disorders and have been shown to exhibit multifaceted functions in the context of cancer. Recent research suggests that these cells may also be involved in tissue-specific repair and regeneration. Herein, we summarize current understanding of the thymic-transcriptional regulatory landscape of FOXP3+ Tregs, their epigenetic modulators, and associated signaling pathways. Finally, we highlight the contributions of FOXP3 on the functional development of Tregs and reflect on the clinical implications in the context of pathological and physiological immune responses. A subset of CD4+ T cells, recognized as FOXP3+ regulatory T (Treg) cells, are involved in preserving immune tolerance. Defects in these cells drive the pathogenesis of autoimmune diseases. In this review, we discuss the functions and dysfunctions in the developmental pathways of FOXP3+ Tregs, their emerging roles, and explore potential therapeutic targets. [ABSTRACT FROM AUTHOR]

Summary: The actions of the immune system are finely tuned, involving complex communication and coordination between diverse immune and non-immune cells across the tissues of the body. A healthy immune system requires a precise balance between immunity and tolerance. Regulatory T cells (Tregs) have long been appreciated as one of the master regulators of this balance; their importance is underscored by the autoimmunity that develops in mice and humans when Tregs are missing or dysfunctional. In addition to the immunoregulatory roles of Tregs in suppressing autoimmunity and inflammation via control of adaptive and innate immune responses, several non-immune modulatory functions of Tregs have been identified in recent years. In this review, we have highlighted the growing literature on the action of Tregs in metabolism, stem cell maintenance, tissue repair, and angiogenesis. Alongside Tregs' immune suppressive role, these non-suppressive activities comprise a key function of Tregs in regulating health and disease. As Tregs receive increasing attention as therapeutic targets, understanding their non-canonical functions may become an important feature of Treg-directed interventions. Diverse functions of Tregs beyond immune suppression. As detailed in the canonical suppression box, Tregs use several mechanisms to dampen proliferation and function of effector T cells and other immune cells. Tregs also have myriad effects on tissue repair, angiogenesis, basal metabolism, and maintenance of the stem cell niche. [ABSTRACT FROM AUTHOR]

Summary: Immune homeostasis is heavily dependent on the action of regulatory T cells (Tregs) which act to suppress the activation of many immune cell types including autoreactive conventional T cells. A body of evidence has shown that Tregs are intrinsically defective in many common autoimmune diseases, and gene polymorphisms which increase the susceptibility of autoimmune disease development have implicated the interleukin-2 (IL-2) signaling pathway as a key dysregulated mechanism. IL-2 is essential for Treg function and survival, and Tregs are highly sensitive to low levels of this cytokine in their environment. This review will revisit the rationale behind using low-dose IL-2 as a therapy to treat autoimmune diseases and evaluate the outcomes of trials to date. Furthermore, novel engineered IL-2 therapies with increased Treg specificity have shown promise in pre-clinical studies and human clinical trials for some agents have begun. Future studies will determine whether low-dose IL-2 or engineered IL-2 therapies can change the course of autoimmune and inflammatory diseases in patients. [ABSTRACT FROM AUTHOR]

Although B cells are essential for humoral immunity and show noteworthy immunomodulatory activity through antibody-independent functions, the role of B cells in regulating Treg cell responses remains controversial. Tregs (CD4+CD25+Foxp3+) are considered to play an immunoprotective role in viral myocarditis (VMC) by controlling autoimmune effector T cells. Here, we proved that B-cell knockout can not only lead to significant reductions in Tregs in the spleen, blood, and heart of VMC mice but also decrease the activation and immune function of splenic Tregs, which was reversed by adoptive transfer of B cells; the transcription levels of TGF-β and Foxp3 in the myocardium were also significantly reduced. B-cell depletion by anti-CD20 impaired the anti-inflammatory function of splenic Tregs and the homeostasis of myocardial Tregs population. Moreover, B cells can convert CD4+CD25− T cells into Foxp3+ and Foxp3−, two functionally suppressive Treg subgroups. Although the reduction in myocardial inflammation in BKO mice indicates that B cells may play a proinflammatory role, the beneficial side of B cells cannot be ignored, that is, to control autoimmunity by maintaining Treg numbers. The results observed in the animal model of VMC highlight the potential harm of rituximab in the nonselective depletion of B cells in clinical applications. B cell depletion by anti-CD20 and gene knockout impaired the anti-inflammatory function of splenic Tregs and the homeostasis of myocardial Tregs population in the animal model of VMC. B cells could promote the transformation of Teffs into Foxp3+ and Foxp3-Tregs in vitro. [ABSTRACT FROM AUTHOR]

The mechanism of antigen‐specific regulatory T cell (Treg) induction is not yet fully understood. Curcumin has an immune regulatory function. This study aims to induce antigen‐specific Tregs by employing extracellular vesicles (EVs) that carry two types of T cell activators. Two types of T cell activators, ovalbumin (OVA)/major histocompatibility complex‐II (MHC‐II) and tetramethylcurcumin (FLLL31) (a curcumin analog) were carried by dendritic cell‐derived extracellular vesicles, designated OFexo. A murine model of allergic rhinitis (AR) was developed with OVA as the specific antigen. AR mice were treated with a nasal instillation containing OFexo. We observed that OFexo recognized antigen‐specific T cell receptors (TCR) on CD4+ T cells and enhanced Il10 gene transcription in CD4+ T cells. Administration of the OFexo‐containing nasal instillation induced antigen‐specific type 1 Tregs (Tr1 cells) in the mouse airway tissues. OFexo‐induced Tr1 cells showed immune suppressive functions on CD4+ T cell proliferation. Administration of OFexo efficiently alleviated experimental AR in mice. In conclusion, OFexo can induce antigen‐specific Tr1 cells that can efficiently alleviate experimental AR. The results suggest that OFexo has the translational potential to be employed for the treatment of AR or other allergic disorders. [ABSTRACT FROM AUTHOR]

Summary: Programmed death‐1 (PD‐1) and interactions with PD‐ligand 1 (PD‐L1) play critical roles in the tumour evasion of immune responses through different mechanisms, including inhibition of effector T cell proliferation, reducing cytotoxic activity, induction of apoptosis in tumour‐infiltrating T cells and regulatory T cell (Treg) expansion. Effective blockade of immune checkpoints can therefore potentially eliminate these detrimental effects. The aim of this study was to investigate the effect of anti‐PD‐1 antibody, pembrolizumab, on various Treg subpopulations. Peripheral blood mononuclear cells (PBMC) from healthy donors (HD) and primary breast cancer patients (PBC) were treated in vitro with pembrolizumab, which effectively reduced PD‐1 expression in both cohorts. We found that PD‐1 was expressed mainly on CD4+CD25+ T cells and pembrolizumab had a greater effect on PD‐1 expression in CD4+CD25− T cells, compared to CD4+CD25+ cells. In addition, pembrolizumab did not affect the expression levels of Treg‐related markers, including cytotoxic T lymphocyte antigen‐4 (CTLA‐4), CD15s, latency‐associated peptide (LAP) and Ki‐67. Moreover, we report that CD15s is expressed mainly on forkhead box P3 (FoxP3)−Helios+ Treg in HD, but it is expressed on FoxP3+Helios− Treg subset in addition to FoxP3−Helios+ Treg in PBC. Pembrolizumab did not affect the levels of FoxP3+/−Helios+/− Treg subsets in both cohorts. Taken together, our study suggests that pembrolizumab does not affect Treg or change their phenotype or function but rather blocks signalling via the PD‐1/PD‐L1 axis in activated T cells. [ABSTRACT FROM AUTHOR]

Forkhead box P3 (FoxP3)+ regulatory T cells (Tregs) are functionally deficient in systemic lupus erythematosus (SLE), characterized by reduced surface CD25 [the interleukin (IL)-2 receptor alpha chain]. Low-dose IL-2 therapy is a promising current approach to correct this defect. To elucidate the origins of the SLE Treg phenotype, we studied its role through developmentally defined regulatory T cell (Treg) subsets in 45 SLE patients, 103 SLE-unaffected first-degree relatives and 61 unrelated healthy control subjects, and genetic association with the CD25-encoding IL2RA locus. We identified two separate, uncorrelated effects contributing to Treg CD25. (1) SLE patients and unaffected relatives remarkably shared CD25 reduction versus controls, particularly in the developmentally earliest CD4+FoxP3+CD45RO-CD31+ recent thymic emigrant Tregs. This first component effect influenced the proportions of circulating CD4+FoxP3highCD45RO+ activated Tregs. (2) In contrast, patients and unaffected relatives differed sharply in their activated Treg CD25 state: while relatives as control subjects up-regulated CD25 strongly in these cells during differentiation from naive Tregs, SLE patients specifically failed to do so. This CD25 up-regulation depended upon IL2RA genetic variation and was related functionally to the proliferation of activated Tregs, but not to their circulating numbers. Both effects were found related to T cell IL-2 production. Our results point to (1) a heritable, intrathymic mechanism responsible for reduced CD25 on early Tregs and decreased activation capacity in an extended risk population, which can be compensated by (2) functionally independent CD25 up-regulation upon peripheral Treg activation that is selectively deficient in patients. We expect that Treg-directed therapies can be monitored more effectively when taking this distinction into account. [ABSTRACT FROM AUTHOR]

Eosinophilic esophagitis (EoE) is an antigen-driven T cell-mediated chronic inflammatory disease where food and environmental antigens are thought to have a role. Human eosinophils express the immunoregulatory protein galectin-10 and have T cell suppressive capacity similar to regulatory T cells (Tregs). We hypothesized that one function of eosinophils in EoE might be to regulate the T cell-driven inflammation in the oesophagus. This was tested by evaluating the suppressive capacity of eosinophils isolated from the blood of adult EoE patients in a mixed lymphocyte reaction. In addition, eosinophilic expression of forkhead box protein 3 (FOXP3), the canonical transcription factor of Tregs, was determined by conventional and imaging flow cytometry, quantitative polymerase chain reaction (qPCR), confocal microscopy and immunoblotting. It was found that blood eosinophils from EoE patients had T cell suppressive capacity, and that a fraction of the eosinophils expressed FOXP3. A comparison of EoE eosinophils with healthy control eosinophils indicated that the patients' eosinophils had inferior suppressive capacity. Furthermore, a higher percentage of the EoE eosinophils expressed FOXP3 protein compared with the healthy eosinophils, and they also had higher FOXP3 protein and mRNA levels. FOXP3 was found in the cytosol and nucleus of the eosinophils from both the patients and healthy individuals, contrasting with the strict nuclear localization of FOXP3 in Tregs. To conclude, these findings suggest that the immunoregulatory function of eosinophils may be impaired in EoE. [ABSTRACT FROM AUTHOR]

Regulatory T cells (Tregs) constitute a fascinating subpopulation of CD4+ T cells due to their ability to limit the immune response against self and non-self antigens. Murine models and antibodies directed against surface and intracellular molecules have allowed elucidation of the mechanisms that govern their development and function. However, these markers used to their classification lack of specificity, as they can be expressed by activated T cells. Similarly, there are slight differences between animal models, in steady state and pathological conditions, anatomical localization and strategy of analysis by flow cytometry. Here, we revised the most common markers utilized for Treg typification by flow cytometry such as CD25, forkhead box protein 3 (FoxP3) and CD127, along with our data obtained in different body compartments of humans, mice and rats. Furthermore, we revised and determined the expression of other molecules important for the phenotypical characterization of Treg cells. We draw attention to the drawbacks of those markers used in chronic states of inflammation. However, until a specific marker for the identification of Tregs is discovered, the best combination of markers will depend upon the tissue or the degree of inflammation from which Tregs derive. [ABSTRACT FROM AUTHOR]

The predisposition of preterm neonates to invasive infection is, as yet, incompletely understood. Regulatory T cells (Tregs) are potential candidates for the ontogenetic control of immune activation and tissue damage in preterm infants. It was the aim of our study to characterize lymphocyte subsets and in particular CD4+CD25+forkhead box protein 3 (FoxP3)+ Tregs in peripheral blood of well-phenotyped preterm infants ( n = 117; 23 + 0 - 36 + 6 weeks of gestational age) in the first 3 days of life in comparison to term infants and adults. We demonstrated a negative correlation of Treg frequencies and gestational age. Tregs were increased in blood samples of preterm infants compared to term infants and adults. Notably, we found an increased Treg frequency in preterm infants with clinical early-onset sepsis while cause of preterm delivery, e.g. chorioamnionitis, did not affect Treg frequencies. Our data suggest that Tregs apparently play an important role in maintaining maternal-fetal tolerance, which turns into an increased sepsis risk after preterm delivery. Functional analyses are needed in order to elucidate whether Tregs have potential as future target for diagnostics and therapeutics. [ABSTRACT FROM AUTHOR]

Neurocysticercosis is caused by the establishment of Taenia solium cysticerci in the central nervous system. It is considered that, during co-evolution, the parasite developed strategies to modulate the host's immune response. The action mechanisms of regulatory T cells in controlling the immune response in neurocysticercosis are studied in this work. Higher blood levels of regulatory T cells with CD4+CD45RO+forkhead box protein 3 (FoxP3)high and CD4+CD25highFoxP3+CD95high phenotype and of non-regulatory CD4+CD45RO+FoxP3med T cells were found in neurocysticercosis patients with respect to controls. Interestingly, regulatory T cells express higher levels of cytotoxic T lymphocyte antigen 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), programmed death 1 (PD-1) and glucocorticoid-induced tumour necrosis factor receptor (GITR), suggesting a cell-to-cell contact mechanism with dendritic cells. Furthermore, higher IL-10 and regulatory T cell type 1 (Tr1) levels were found in neurocysticercosis patients' peripheral blood, suggesting that the action mechanism of regulatory T cells involves the release of immunomodulatory cytokines. No evidence was found of the regulatory T cell role in inhibiting the proliferative response. Suppressive regulatory T cells from neurocysticercosis patients correlated negatively with late activated lymphocytes (CD4+CD38+). Our results suggest that, during neurocysticercosis, regulatory T cells could control the immune response, probably by a cell-to-cell contact with dendritic cells and interleukin (IL)-10 release by Tr1, to create an immunomodulatory environment that may favour the development of T. solium cysticerci and their permanence in the central nervous system. [ABSTRACT FROM AUTHOR]

Sarcoidosis is a granulomatous inflammatory disorder of unknown aetiology. The increased frequency of activated lung CD4+ T cells with a T helper type 1 (Th1) cytokine profile in sarcoidosis patients is accompanied by a reduced proportion and/or impaired function of regulatory T cells (Tregs). Here we evaluated the expression of the inducible co-stimulator (ICOS) on lung and blood CD4+ T cell subsets in sarcoidosis patients with different prognosis, by flow cytometry. Samples from the deep airways were obtained by bronchoalveolar lavage (BAL). We show that Tregs from the inflamed lung of sarcoidosis patients were characterized by a unique ICOShigh phenotype. High-level ICOS expression was restricted to Tregs from the inflamed lung and was absent in blood Tregs of sarcoidosis patients as well as in lung and blood Tregs of healthy volunteers. In addition, lung Tregs exhibited increased ICOS expression compared to sarcoid-specific lung effector T cells. Strikingly, ICOS expression on Tregs was in particularly high in the lungs of Löfgren's syndrome (LS) patients who present with acute disease which often resolves spontaneously. Moreover, blood monocytes from LS patients revealed increased ICOS-L levels compared to healthy donors. Sarcoidosis was associated with a shift towards a non-classical monocyte phenotype and the ICOS-Lhigh phenotype was restricted to this particular monocyte subset. We propose a potential implication of the ICOS/ICOS-L immune-regulatory axis in disease activity and resolution and suggest to evaluate further the suitability of ICOS as biomarker for the prognosis of sarcoidosis. [ABSTRACT FROM AUTHOR]

Systemic autoimmune diseases, such as systemic lupus erythematosus (SLE), are often characterized by a failure of self-tolerance and result in an uncontrolled activation of B cells and effector T cells. Interleukin (IL)-2 critically maintains homeostasis of regulatory T cells (Treg) and effector T cells in the periphery. Previously, we identified the cAMP-responsive element modulator α (CREMα) as a major factor responsible for decreased IL-2 production in T cells from SLE patients. Additionally, using a transgenic mouse that specifically over-expresses CREMα in T cells (CD2CREMαtg), we provided in-vivo evidence that CREMα indeed suppresses IL-2 production. To analyse the effects of CREMα in an autoimmune prone mouse model we introduced a Fas mutation in the CD2CREMαtg mice (FVB/Fas-/-CD2CREMαtg). Overexpression of CREMα strongly accelerated the lymphadenopathy and splenomegaly in the FVB/Fas-/- mice. This was accompanied by a massive expansion of double-negative (DN) T cells, enhanced numbers of interferon (IFN)-γ-producing T cells and reduced percentages of Tregs. Treatment of FVB/Fas-/-CD2CREMαtg mice with IL-2 restored the percentage of Tregs and reversed increased IFN-γ production, but did not affect the number of DNTs. Our data indicate that CREMα contributes to the failure of tolerance in SLE by favouring effector T cells and decreasing regulatory T cells, partially mediated by repression of IL-2 in vivo. [ABSTRACT FROM AUTHOR]

Regulatory T cells ( Tregs) are known to play an immunosuppressive role in the response of contact hypersensitivity ( CHS), but neither the dynamics of Tregs during the CHS response nor the exaggerated inflammatory response after depletion of Tregs has been characterized in detail. In this study we show that the number of Tregs in the challenged tissue peak at the same time as the ear-swelling reaches its maximum on day 1 after challenge, whereas the number of Tregs in the draining lymph nodes peaks at day 2. As expected, depletion of Tregs by injection of a monoclonal antibody to CD25 prior to sensitization led to a prolonged and sustained inflammatory response which was dependent upon CD8 T cells, and co-stimulatory blockade with cytotoxic T lymphocyte antigen-4-immunoglobulin ( CTLA-4- Ig) suppressed the exaggerated inflammation. In contrast, blockade of the interleukin ( IL)-10-receptor ( IL-10 R) did not further increase the exaggerated inflammatory response in the Treg-depleted mice. In the absence of Tregs, the response changed from a mainly acute reaction with heavy infiltration of neutrophils to a sustained response with more chronic characteristics (fewer neutrophils and dominated by macrophages). Furthermore, depletion of Tregs enhanced the release of cytokines and chemokines locally in the inflamed ear and augmented serum levels of the systemic inflammatory mediators serum amyloid ( SAP) and haptoglobin early in the response. [ABSTRACT FROM AUTHOR]

The role of T cells in idiopathic inflammatory myopathies ( IIM) is not yet clear. Some alterations in certain subsets have been reported in inflamed muscle cells. However, a broad quantitative assessment of peripheral T cell subsets has not been evaluated. The aim of this study was to address the quantitative profile of potential pathogenic T cell subsets, namely follicular helper T cells ( Tfh), T helper type 17 ( Th17), CD28null and regulatory T cells ( Tregs) in peripheral blood from IIM patients. Thirty IIM patients and 30 age- and gender-matched healthy donors were included. Peripheral blood mononuclear cells were isolated. T cell subsets were evaluated by flow cytometry, as follows: Tfh ( CD4+ CXCR5+) and its subsets Tfh1 ( CXCR3+ CCR6−), Tfh2 ( CXCR3− CCR6−), Tfh17 ( CXCR3− CCR6+), Th17 ( CD4+ IL17 A+), CD28null ( CD4+ CD28− CD244+) and Tregs ( CD4+ CD25high forkhead box protein 3 (FoxP3+); CD8+ CD25high FoxP3+). Percentage, absolute numbers and mean fluorescence intensity were analysed. We found increased numbers of total Tfh cells (28 ± 8·16 versus 6·64 ± 1·29, P = 0·031) in IIM patients when compared to healthy controls. Moreover, this increment was dependent upon Tfh2 and Tfh17 ( Tfh2:9·49 ± 2·19 versus 1·66 ± 0·46, P = 0·005; Tfh17 9·48 ± 2·83 versus 1·18 ± 0·21, P = 0·014). Also, IIM patients showed higher numbers of Th17 cells (30·25 ± 6·49 versus 13·46 ± 2·95, P = 0·031) as well as decreased number of Tregs (5·98 ± 1·61 versus 30·82 ± 8·38, P = 0·009). We also found an expansion of CD28null cells (162·88 ± 32·29 versus 64 ± 17·35, P = 0·015). Our data suggest that IIM patients are characterized by an expansion of peripheral proinflammatory T cells, such as Tfh and Th17, as well as pro-apoptotic CD28 null cells and a deficiency of suppressor populations of Tregs ( CD4+ and CD8+). [ABSTRACT FROM AUTHOR]