IL-2 controls the stability of Foxp3 expression in TGF-beta-induced Foxp3+ T cells in vivo

Effects of dietary glutamine on the homeostasis of CD4+ T cells in mice with dextran sulfate sodium-induced acute colitis

This study investigated the effects of dietary glutamine (Gln) on T-helper (Th) and T regulatory (Treg) cell homeostasis and colonic inflammatory mediator expression in mice with dextran sulfate sodium (DSS)-induced colitis. Mice were randomly assigned to 4 groups with 2 normal control (C and G) and 2 DSS-treated groups (DC and DG). The C and DC groups were fed a common semipurified diet, while the G and DG groups received an identical diet except that part of the casein was replaced by Gln, which provided 25% of the total amino acid nitrogen. Mice were fed the diets for 10 days. On day 6, mice in the normal control groups were given distilled water, while those in the DSS groups were given distilled water containing 1.5% DSS for 5 d. At the end of the experiment, the mice were sacrificed for further examination. Results showed that DC group had higher plasma haptoglobin, colonic weight, immunoglobulin G, inflammatory cytokine and nuclear factor (NF)-κB protein levels. Gln administration lowered inflammatory mediators and NF-κB/IκBα ratio in colitis. Compared with the DC group, the percentages of interleukin-17F and interferon-γ in blood and transcription factors, T-bet and RAR-related orphan receptor-γt, gene expressions in mesenteric lymph nodes were lower, whereas blood Foxp3 was higher in the DG group. Also, DG group had lower colon injury score. These results suggest that Gln administration suppressed Th1/Th17 and Th-associated cytokine expressions and upregulated the expression of Tregs, which may modulate the balance of Th/Treg and reduce inflammatory reactions in DSS-induced colitis.

Transcriptional control of regulatory T cells

Regulatory T cells (Tregs) constitute unique T cell lineage that plays a key role for immunological tolerance. Tregs are characterized by the expression of the forkhead box transcription factor Foxp3, which acts as a lineage-specifying factor by determining the unique suppression profile of these immune cells. Here, we summarize the recent progress in understanding how Foxp3 expression itself is epigenetically and transcriptionally controlled, how the Treg-specific signature is achieved and how unique properties of Treg subsets are defined by other transcription factors. Finally, we will discuss recent studies focusing on the molecular targeting of Tregs to utilize the specific properties of this unique cell type in therapeutic settings.

Regulation of regulatory T cells: epigenetics and plasticity

Regulatory T (Treg) cells, as central mediators of immune suppression, play crucial roles in many aspects of immune system's physiology and pathophysiology. The transcription factor Foxp3 has been characterized as a master gene of Tregs. Yet Treg cells possess a distinct pattern of gene expression, including upregulation of immune-suppressive genes and silencing of inflammatory cytokine genes. Recent studies have revealed the molecular mechanisms that establish and maintain such gene regulation in Treg cells. This review discusses recent progress in our understanding of molecular features of Treg cells, with particular attention to Treg-cell lineage commitment and stability.

Antigen-specific induced T regulatory cells impair dendritic cell function via an IL-10/MARCH1-dependent mechanism

Foxp3(+) T regulatory cells (Tregs) are critically important for the maintenance of immunological tolerance, immune homeostasis, and prevention of autoimmunity. Dendritic cells (DCs) are one of the major targets of Treg-mediated suppression. Some studies have suggested that Treg-mediated suppression of DC function is mediated by the interaction of CTLA-4 on Tregs with CD80/CD86 on the DCs resulting in downregulation of CD80/CD86 expression and a decrease in costimulation. We have re-examined the effects of Tregs on mouse DC function in a model in which Ag-specific, induced Tregs (iTregs) are cocultured with DCs in the absence of T effector cells. iTreg-treated DCs are markedly defective in their capacity to activate naive T cells. iTregs from CTLA-4-deficient mice failed to induce downregulation of CD80/CD86, but DCs treated with CTLA-4-deficient iTregs still exhibited impaired capacity to activate naive T cells. The iTreg-induced defect in DC function could be completely reversed by anti-IL-10, and IL-10-deficient iTregs failed to downregulate DC function. iTreg-treated DCs expressed high levels of MARCH1, an E3 ubiquitin ligase, recently found to degrade CD86 and MHC class II on the DCs and expressed lower levels of CD83, a molecule involved in neutralizing the function of MARCH1. Both the enhanced expression of MARCH1 and the decreased expression of CD83 were mediated by IL-10 produced by the iTregs. Taken together, these studies demonstrate that a major suppressive mechanism of DC function by iTregs is secondary to the effects of IL-10 on MARCH1 and CD83 expression.

Use of gene-modified regulatory T-cells to control autoimmune and alloimmune pathology: is now the right time?

Adoptive immunotherapy using genetically targeted T-cells has recently begun to achieve impressive clinical impact in selected tumor types. Furthermore, long-term follow-up studies indicate thus far that integrating viral vectors do not elicit clinically evident genotoxicity in T-cells, unlike hematopoietic stem cells. The optimism engendered by this clinical experience provides a platform for consideration of the extended use of this technology in other disease types. One area of particular interest entails the harnessing of regulatory T-cells (Tregs) in order to down-regulate unwanted immune responses. Increasing evidence supports the efficacy of this approach in pre-clinical models of autoimmune disease and allograft rejection. Nonetheless, questions remain about optimal host cell, transgene cargo, phenotypic stability of engineered cells in vivo and potential for toxicity. Here, we review the evidence that genetically engineered Tregs can effectively dampen pathogenic immune responses and critically evaluate the prospects for clinical development of this approach.

IL-6 controls susceptibility to helminth infection by impeding Th2 responsiveness and altering the Treg phenotype in vivo

IL-6 plays a pivotal role in favoring T-cell commitment toward a Th17 cell rather than Treg-cell phenotype, as established through in vitro model systems. We predicted that in the absence of IL-6, mice infected with the gastrointestinal helminth Heligmosomoides polygyrus would show reduced Th17-cell responses, but also enhanced Treg-cell activity and consequently greater susceptibility. Surprisingly, worm expulsion was markedly potentiated in IL-6-deficient mice, with significantly stronger adaptive Th2 responses in both IL-6^−/− mice and BALB/c recipients of neutralizing anti-IL-6 monoclonal Ab. Although IL-6-deficient mice showed lower steady-state Th17-cell levels, IL-6-independent Th17-cell responses occurred during in vivo infection. We excluded the Th17 response as a factor in protection, as Ab neutralization did not modify immunity to H. polygyrus infection in BALB/c mice. Resistance did correlate with significant changes to the associated Treg-cell phenotype however, as IL-6-deficient mice displayed reduced expression of Foxp3, Helios, and GATA-3, and enhanced production of cytokines within the Treg-cell population. Administration of an anti-IL-2:IL-2 complex boosted Treg-cell proportions in vivo, reduced adaptive Th2 responses to WT levels, and fully restored susceptibility to H. polygyrus in IL-6-deficient mice. Thus, in vivo, IL-6 limits the Th2 response, modifies the Treg-cell phenotype, and promotes host susceptibility following helminth infection.

Induced regulatory T cells promote tolerance when stabilized by rapamycin and IL-2 in vivo

Natural regulatory T cells (nTregs) play an important role in tolerance; however, the small numbers of cells obtainable potentially limit the feasibility of clinical adoptive transfer. Therefore, we studied the feasibility and efficacy of using murine-induced regulatory T cells (iTregs) for the induction of tolerance after bone marrow transplantation. iTregs could be induced in large numbers from conventional donor CD4 and CD8 T cells within 1 wk and were highly suppressive. During graft-versus-host disease (GVHD), CD4 and CD8 iTregs suppressed the proliferation of effector T cells and the production of proinflammatory cytokines. However, unlike nTregs, both iTreg populations lost Foxp3 expression within 3 wk in vivo, reverted to effector T cells, and exacerbated GVHD. The loss of Foxp3 in iTregs followed homeostatic and/or alloantigen-driven proliferation and was unrelated to GVHD. However, the concurrent administration of rapamycin, with or without IL-2/anti-IL-2 Ab complexes, to the transplant recipients significantly improved Foxp3 stability in CD4 iTregs (and, to a lesser extent, CD8 iTregs), such that they remained detectable 12 wk after transfer. Strikingly, CD4, but not CD8, iTregs could then suppress Teff proliferation and proinflammatory cytokine production and prevent GVHD in an equivalent fashion to nTregs. However, at high numbers and when used as GVHD prophylaxis, Tregs potently suppress graft-versus-leukemia effects and so may be most appropriate as a therapeutic modality to treat GVHD. These data demonstrate that CD4 iTregs can be produced rapidly in large, clinically relevant numbers and, when transferred in the presence of systemic rapamycin and IL-2, induce tolerance in transplant recipients.

Therapeutic evaluation of ex vivo-generated versus natural regulatory T-cells in a mouse model of chronic gut inflammation

The objectives of this study were to (a) evaluate and compare the ability of ex vivo-generated induced regulatory T cells (iTregs) and freshly isolated natural Tregs (nTregs) to reverse/attenuate preexisting intestinal inflammation in a mouse model of chronic colitis and (b) quantify the Treg-targeted gene expression profiles of these two Treg populations. We found that ex vivo-generated iTregs were significantly more potent than nTregs at attenuating preexisting colitis. This superior therapeutic activity was associated with increased accumulation of iTregs within the mesenteric lymph nodes and large and significant reductions in interleukin (IL)-6 and IL-17A expression in the colons of iTreg- versus nTreg-treated mice. The enhanced immunosuppressive activity of iTregs was not because of increased expression or stability of Foxp3 as iTregs and nTregs obtained from the mesenteric lymph nodes, and colons of reconstituted mice expressed similar levels of this important transcription factor. In addition, we observed a total of 27 genes that were either upregulated or downregulated in iTregs when compared with nTregs. Although iTregs were found to be superior at reversing established disease, their message levels of IL-10 and IL-35 and surface expression of the gut-homing molecules CCR9 and α4β7 were significantly reduced when compared with nTregs. Taken together, our data demonstrate that ex vivo-generated iTregs are significantly more potent than nTregs at attenuating preexisting gut inflammation despite reduced expression of classical regulatory cytokines and gut-homing molecules. Our data suggest that the immunosuppressive activity of iTregs may be because of their ability to directly or indirectly decrease expression of IL-6 and IL-17A within the inflamed bowel.

Generation of highly effective and stable murine alloreactive Treg cells by combined anti-CD4 mAb, TGF-β, and RA treatment

The transfer of alloreactive regulatory T (aTreg) cells into transplant recipients represents an attractive treatment option to improve long-term graft acceptance. We recently described a protocol for the generation of aTreg cells in mice using a nondepleting anti-CD4 antibody (aCD4). Here, we investigated whether adding TGF-β and retinoic acid (RA) or rapamycin (Rapa) can further improve aTreg-cell generation and function. Murine CD4(+) T cells were cultured with allogeneic B cells in the presence of aCD4 alone, aCD4+TGF-β+RA or aCD4+Rapa. Addition of TGF-β+RA or Rapa resulted in an increase of CD25(+)Foxp3(+)-expressing T cells. Expression of CD40L and production of IFN-γ and IL-17 was abolished in aCD4+TGF-β+RA aTreg cells. Additionally, aCD4+TGF-β+RA aTreg cells showed the highest level of Helios and Neuropilin-1 co-expression. Although CD25(+)Foxp3(+) cells from all culture conditions displayed complete demethylation of the Treg-specific demethylated region, aCD4+TGF-β+RA Treg cells showed the most stable Foxp3 expression upon restimulation. Consequently, aCD4+TGF-β+RA aTreg cells suppressed effector T-cell differentiation more effectively in comparison to aTreg cells harvested from all other cultures, and furthermore inhibited acute graft versus host disease and especially skin transplant rejection. Thus, addition of TGF-β+RA seems to be superior over Rapa in stabilising the phenotype and functional capacity of aTreg cells.

Peripheral and thymic foxp3(+) regulatory T cells in search of origin, distinction, and function

Over the past decade, much has been learnt and much more to discover about Foxp3(+) regulatory T cells (Tregs). Initially, it was thought that Tregs were a unique entity that originates in the thymus. It is now recognized that there is a fraternal twin sibling that is generated in the periphery. The difficulty is in the distinction between these two subsets. The ability to detect, monitor, and analyze these two subsets in health and disease will provide invaluable insights into their functions and purposes. The plasticity and mechanisms of action can be unique and not overlapping within these subsets. Therefore, the therapeutic targeting of a particular subset of Tregs might be more efficacious. In the past couple of years, a vast amount of data have provided a better understanding of the cellular and molecular components essential for their development and stability. Many studies are implicating their preferential involvement in certain diseases and immunologic tolerance. However, it remains controversial as to whether any phenotypic markers have been identified that can differentiate thymic versus peripheral Tregs. This review will address the validity and controversy regarding Helios, Lap/Garp and Neuropilin-1 as markers of thymic Tregs. It also will discuss updated information on distinguishing features of these two subsets and their critical roles in maternal-fetal tolerance and transplantation.

Generation and immunosuppressive functions of p53-induced human adaptive regulatory T cells

Inducible regulatory T cells (iTregs, also called Tr1 cells) are generated in the periphery (circulation or tissue) of cancer patients upon the encounter of naïve CD4⁺ T cells with tumor-associated antigens. As p53 is often inactivated by genetic or epigenetic events during oncogenesis, p53-induced Tr1 cells might play a key role in establishing immunosuppressive networks in cancer patients. Tr1 cells were generated by co-culturing circulating CD4⁺CD25⁻ T cells with autologous immature dendritic cells pulsed with a wild-type (WT) p53-derived peptide or an unrelated peptide derived from mucin 1 (MUC1). The Tr1 phenotype and the specificity for p53 of these cells were confirmed by multicolor flow cytometry. Moreover, the Tr1 cell-mediated suppression of T-cell proliferation was evaluated by CFSE-based flow cytometry, while their ability to alter the T-cell cytokine profile by ELISA and Luminex assays. The capacity of p53-induced Tr1 cells to suppress the generation and function of cytotoxic T lymphcoytes (CTLs) was assessed by flow cytometry and ELISPOT. Of note, low doses of the p53-derived peptide (p53_low) induced greater numbers of Tr1 cells than the same peptide employed at high doses (p53_high). Moreover, Tr1/p53_low cells not secreted higher levels of interleukin-10 and transforming growth factor β1, but also mediated more robust suppressive effects on CTL proliferation than Tr1/p53_high cells. Tr1/p53_low cells, Tr1/p53_high cells, as well as Tr1 cells generated with low doses of an unrelated MUC1-derived peptide were equally effective in suppressing the expansion and antitumor activity of p53-reactive CTLs. p53_low induced the expansion of highly suppressive p53-reactive Tr1 cells. However, the capacity of these Tr1 cells to suppress the generation and function of p53-reactive CTLs was independent of their antigen-specificity.

Regulatory T cells and transplantation tolerance

The success of clinical organ transplantation relies on life-long use of immunosuppressive drugs that target immune responses associated with graft rejection. Preclinical studies in mice have convincingly demonstrated that robust, long-term transplantation tolerance can be achieved after a short-term treatment with T-cell coreceptor and costimulation blockade even for a fully mismatched graft. Such therapeutically induced tolerance requires the induction of Foxp3+ Tregs, which are essential for both the development and maintenance of the tolerant state. Recent advances in understanding the molecular and epigenetic mechanisms underlying the induction and stabilization of Foxp3 expression, thus guiding Foxp3+ Treg differentiation, have revealed novel therapeutic targets in animal models that can be translated to harness Foxp3+ Tregs from within the patient. Such in vivo induced Foxp3+ Tregs can also induce the tolerant state. Pharmacological compounds are available to exploit these targets and their further development holds great promise for clinical translation.

Evaluation of in vivo T cell kinetics: use of heavy isotope labelling in type 1 diabetes

CD4(+) memory cell development is dependent upon T cell receptor (TCR) signal strength, antigen dose and the cytokine milieu, all of which are altered in type 1 diabetes (T1D). We hypothesized that CD4(+) T cell turnover would be greater in type 1 diabetes subjects compared to controls. In vitro studies of T cell function are unable to evaluate dynamic aspects of immune cell homoeostasis. Therefore, we used deuterium oxide ((2) H(2)O) to assess in vivo turnover of CD4(+) T cell subsets in T1D (n = 10) and control subjects (n = 10). Serial samples of naive, memory and regulatory (T(reg)) CD4(+) T cell subsets were collected and enrichment of deoxyribose was determined by gas chromatography-mass spectrometry (GC-MS). Quantification of T cell turnover was performed using mathematical models to estimate fractional enrichment (f, n = 20), turnover rate (k, n = 20), proliferation (p, n = 10) and disappearance (d*, n = 10). Although turnover of T(regs) was greater than memory and naive cells in both controls and T1D subjects, no differences were seen between T1D and controls in T(reg) or naive kinetics. However, turnover of CD4(+) memory T cells was faster in those with T1D compared to control subjects. Measurement and modelling of incorporated deuterium is useful for evaluating the in vivo kinetics of immune cells in T1D and could be incorporated into studies of the natural history of disease or clinical trials designed to alter the disease course. The enhanced CD4(+) memory T cell turnover in T1D may be important in understanding the pathophysiology and potential treatments of autoimmune diabetes.

Novel roles for complement receptors in T cell regulation and beyond

Complement receptors are expressed on cells of the innate and the adaptive immune system. They play important roles in pathogen and danger sensing as they translate the information gathered by complement fluid phase sensors into cellular responses. Further, they control complement activation on viable and apoptotic host cells, clearance of immune complexes and mediate opsonophagocytosis. More recently, evidence has accumulated that complement receptors form a complex network with other innate receptors systems such as the Toll-like receptors, the Notch signaling system, IgG Fc receptors and C-type lectin receptors contributing to the benefit and burden of innate and adaptive immune responses in autoimmune and allergic diseases as well as in cancer and transplantation. Here, we will discuss recent developments and emerging concepts of complement receptor activation and regulation with a particular focus on the differentiation, maintenance and contraction of effector and regulatory T cells.

Role of cytokines in thymus- versus peripherally derived-regulatory T cell differentiation and function

CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) are essential players in the control of immune responses. Recently, accordingly to their origin, two main subsets of Tregs have been described: thymus-derived Tregs (tTregs) and peripherally derived Tregs (pTregs). Numerous signaling pathways including the IL-2/STAT5 or the TGF-β/Smad3 pathways play a crucial role in segregating the two lineages. Here, we review some of the information existing on the distinct requirements of IL-2, TGF-β, and TNF-α three major cytokines involved in tTreg and pTreg generation, homeostasis and function. Today it is clear that signaling via the IL-2Rβ chain (CD122) common to IL-2 and IL-15 is required for proper differentiation of tTregs and for tTreg and pTreg survival in the periphery. This notion has led to the development of promising therapeutic strategies based on low-dose IL-2 administration to boost the patients’ own Treg compartment and dampen autoimmunity and inflammation. Also, solid evidence points to TGF-β as the master regulator of pTreg differentiation and homeostasis. However, therapeutic administration of TGF-β is difficult to implement due to toxicity and safety issues. Knowledge on the role of TNF-α on the biology of Tregs is fragmentary and inconsistent between mice and humans. Moreover, emerging results from the clinical use of TNF-α inhibitors indicate that part of their anti-inflammatory effect may be dependent on their action on Tregs. Given the profusion of clinical trials testing cytokine administration or blocking to modulate inflammatory diseases, a better knowledge of the effects of cytokines on tTregs and pTregs biology is necessary to improve the efficiency of these immunotherapies.

Generation and function of induced regulatory T cells

CD4⁺ CD25⁺ Foxp3⁺ regulatory T (Treg) cells are essential to the balance between pro- and anti-inflammatory responses. There are two major subsets of Treg cells, “natural” Treg (nTreg) cells that develop in the thymus, and “induced” Treg (iTreg) cells that arise in the periphery from CD4⁺ Foxp3⁻ conventional T cells and can be generated in vitro. Previous work has established that both subsets are required for immunological tolerance. Additionally, in vitro-derived iTreg cells can reestablish tolerance in situations where Treg cells are decreased or defective. This review will focus on iTreg cells, drawing comparisons to nTreg cells when possible. We discuss the molecular mechanisms of iTreg cell induction, both in vivo and in vitro, review the Foxp3-dependent and -independent transcriptional landscape of iTreg cells, and examine the proposed suppressive mechanisms utilized by each Treg cell subset. We also compare the T cell receptor repertoire of the Treg cell subsets, discuss inflammatory conditions where iTreg cells are generated or have been used for treatment, and address the issue of iTreg cell stability.

Molecular determinants of regulatory T cell development: the essential roles of epigenetic changes

Regulatory T (Treg) cells constitute a distinct T cell subset, which plays a key role in immune tolerance and homeostasis. The transcription factor Foxp3 controls a substantial part of Treg cell development and function. Yet its expression alone is insufficient for conferring developmental and functional characteristics of Treg cells. There is accumulating evidence that concurrent induction of Treg-specific epigenetic changes and Foxp3 expression is crucial for lineage specification and functional stability of Treg cells. This review discusses recent progress in our understanding of molecular features of Treg cells, in particular, the molecular basis of how a population of developing T cells is driven to the Treg cell lineage and how its function is stably maintained.

Regulatory T-cell therapy for transplantation: how many cells do we need?

PURPOSE OF REVIEW

As regulatory T-cell (Treg) therapy begins to enter the clinic and more clinical trials of Treg therapy are being actively planned for solid organ transplantations, a thorough quantitative assessment of therapeutic dosing is essential for the design of an effective Treg-therapy trial in the solid organ transplant setting.

RECENT FINDINGS

Considering the requirement for a high percentage of Tregs to control transplant rejection in mouse models of transplantation and the total cellularity of the human T-cell compartment, we estimate that it would take billions of Tregs, preferably alloantigen-reactive Tregs, to effectively control transplant rejection in humans. Donor dendritic cells and B cells can be used to selectively expand donor alloantigen-reactive Tregs. Recent improvements in manufacturing alloantigen-reactive Tregs demonstrate that billions of alloantigen-reactive T cells can be manufactured in short-term cultures.

SUMMARY

It is feasible to grow human alloantigen-reactive Tregs up to billions, an optimal number to achieve therapeutic efficacy. Better understanding of Treg lineage commitment and further technological investments are needed to ease the implementation and ensure consistency in Treg manufacturing.

GARP-TGF-β complexes negatively regulate regulatory T cell development and maintenance of peripheral CD4+ T cells in vivo

The role of surface-bound TGF-β on regulatory T cells (Tregs) and the mechanisms that mediate its functions are not well defined. We recently identified a cell-surface molecule called Glycoprotein A Repetitions Predominant (GARP), which is expressed specifically on activated Tregs and was found to bind latent TGF-β and mediate a portion of Treg suppressive activity in vitro. In this article, we address the role of GARP in regulating Treg and conventional T cell development and immune suppression in vivo using a transgenic mouse expressing GARP on all T cells. We found that, despite forced expression of GARP on all T cells, stimulation through the TCR was required for efficient localization of GARP to the cell surface. In addition, IL-2 signals enhanced GARP cell surface expression specifically on Tregs. GARP-transgenic CD4(+) T cells and Tregs, especially those expressing higher levels of GARP, were significantly reduced in the periphery. Mature Tregs, but not conventional CD4(+) T cells, were also reduced in the thymus. CD4(+) T cell reduction was more pronounced within the effector/memory subset, especially as the mouse aged. In addition, GARP-overexpressing CD4(+) T cells stimulated through the TCR displayed reduced proliferative capacity, which was restored by inhibiting TGF-β signaling. Furthermore, inhibiting TGF-β signals greatly enhanced surface expression of GARP on Tregs and blocked the induction of Foxp3 in activated CD4(+) T cells overexpressing GARP. These findings suggest a role for GARP in natural and induced Treg development through activation of bound latent TGF-β and signaling, which negatively regulates GARP expression on Tregs.

Regulatory T cells vs Th17: differentiation of Th17 versus Treg, are the mutually exclusive?

Naive CD4(+) cells differentiate into T helper (Th1, Th2, Th9, Th17) and regulatory T (Treg) cells to execute their immunologic function. Whereas TGF-β suppresses Th1 and Th2 cell differentiation, this cytokine promotes Th9, Th17 and Foxp3(+) regulatory T cells depending upon the presence of other cytokines. IL-6 promotes Th17, but suppresses regulatory T cell differentiation. Moreover, natural but not TGF-β-induced regulatory T cells convert into Th17 cells in the inflammatory milieu. Here an update of T cell differentiation and conversion, as well as underlying mechanisms are given.

Active demethylation of the Foxp3 locus leads to the generation of stable regulatory T cells within the thymus

Stable expression of Foxp3 in regulatory T cells (Tregs) depends on DNA demethylation at the Treg-specific demethylated region (TSDR), a conserved, CpG-rich region within the Foxp3 locus. The TSDR is selectively demethylated in ex vivo Tregs purified from secondary lymphoid organs, but it is unclear at which stage of Treg development demethylation takes place. In this study, we show that commitment to a stable lineage occurred during early stages of murine thymic Treg development by engraving of lineage-specific epigenetic marks in parallel with establishment of a Treg-specific gene expression profile. TSDR demethylation was achieved through an active mechanism and involved enzymes of the ten-eleven-translocation family and hydroxylation of methylated cytosines, a modification that is implicated as an initiating step of mitosis-independent DNA demethylation pathways and has not yet been observed at specific loci during immune cell differentiation. Together, our results demonstrate that initiating TSDR demethylation during early stages of thymic Treg development commences stabilization of Foxp3 expression and guarantees full functionality and long-term lineage stability of Tregs.

Postthymic expansion in human CD4 naive T cells defined by expression of functional high-affinity IL-2 receptors

As the thymus involutes with age, the maintenance of peripheral naive T cells in humans becomes strongly dependent on peripheral cell division. However, mechanisms that orchestrate homeostatic division remain unclear. In this study we present evidence that the frequency of naive CD4 T cells that express CD25 (IL-2 receptor α-chain) increases with age on subsets of both CD31(+) and CD31(-) naive CD4 T cells. Analyses of TCR excision circles from sorted subsets indicate that CD25(+) naive CD4 T cells have undergone more rounds of homeostatic proliferation than their CD25(-) counterparts in both the CD31(+) and CD31(-) subsets, indicating that CD25 is a marker of naive CD4 T cells that have preferentially responded to survival signals from self-Ags or cytokines. CD25 expression on CD25(-) naive CD4 T cells can be induced by IL-7 in vitro in the absence of TCR activation. Although CD25(+) naive T cells respond to lower concentrations of IL-2 as compared with their CD25(-) counterparts, IL-2 responsiveness is further increased in CD31(-) naive T cells by their expression of the signaling IL-2 receptor β-chain CD122, forming with common γ-chain functional high-affinity IL-2 receptors. CD25 plays a role during activation: CD25(+) naive T cells stimulated in an APC-dependent manner were shown to produce increased levels of IL-2 as compared with their CD25(-) counterparts. This study establishes CD25(+) naive CD4 T cells, which are further delineated by CD31 expression, as a major functionally distinct immune cell subset in humans that warrants further characterization in health and disease.

Advances in distinguishing natural from induced Foxp3(+) regulatory T cells

For more than a decade now, the regulatory T (Treg) cell has widely been considered as a critical subpopulation of T cells which can suppress effector T cell responses as well as suppressing the activity of other immune cells, such as mast cell, dendritic cells, and B cells. Treg cells have been broadly characterized as comprising of two main populations: thymus-derived natural Treg (nTreg) cells, and peripherally generated induced Treg (iTreg) cells. Both subsets have similar phenotypic characteristics and comparable suppressive function against T cell-mediated immune response and diseases. However, both Foxp3 positive Treg subsets exhibit some specific differences such as different mRNA transcripts and protein expression, epigenetic modification, and stability. These subtle differences reinforce the notion that they represent unique and distinct subsets. Accurately distinguishing iTregs from nTregs will help to clarify the biological features and contributions of each Treg subsets in peripheral tolerance, autoimmunity and tumor immunity. One difficult problem is that it has not been possible to distinguish iTregs from nTregs using surface markers until two recent articles were published to address this possibility. This review will focus on very recent advances in using molecular markers to differentiate these Treg subsets.

Absence of signaling into CD4⁺ cells via C3aR and C5aR enables autoinductive TGF-β1 signaling and induction of Foxp3⁺ regulatory T cells

C3a and C5a receptor (C3aR and C5aR) signaling by dendritic cells and CD4⁺ cells provides costimulatory and survival signals to T effector cells. Here, we demonstrate that when C3aR and C5aR signals are not transduced into CD4⁺ cells, PI-3Kγ-AKT-mTOR signaling ceases, PKA activation increases, auto-inductive transforming growth factor- β1 (TGF-β1) signaling initiates, and CD4⁺ cells become Foxp3⁺ T regulatory cells (iT_regs). Endogenous TGF-β1 suppresses C3aR and C5aR signaling by preventing C3a and C5a production and upregulating C5L2, an alternate C5a receptor. Absent C3aR and C5aR signaling decreases costimulatory molecule and interleukin-6 production and augments interleukin-10 production. The resulting iT_regs exert robust suppression, possess enhanced stability, and suppress ongoing autoimmune disease. Human iT_regs with potent suppressor activity can be induced exploiting this insight.

TNF-α impairs differentiation and function of TGF-β-induced Treg cells in autoimmune diseases through Akt and Smad3 signaling pathway

Deficiency in the TGF-β-induced regulatory T (iTreg) cell differentiation is associated with compromised immune homeostasis and plays a key role in many autoimmune diseases. Therapeutic intervention to enhance in situ iTreg differentiation has become a promising treatment modality for autoimmune diseases. Here we describe that the development of autoimmune inflammation in experimental autoimmune encephalomyelitis (EAE) is associated with selective impairment of iTreg differentiation largely due to the increased production of TNF-α. The neutralization of TNF-α markedly increases iTreg differentiation, leading to the amelioration of EAE, whereas the depletion of iTreg cells abolishes the therapeutic effect of an anti-TNF-α antibody. The inhibition of iTreg differentiation by TNF-α is mediated through a signaling cascade involving the induction of TNF receptor II (TNFR2) expression and the activation of Akt. The activated Akt in turn interacts with Smad3, resulting in the inhibition of TGF-β-induced Smad3 phosphorylation and consequently the reduction of p-Smad3 results in the decreased binding to the specific binding site of the foxp3 promoter, and finally foxp3 transcription itself. Interestingly, this regulatory pathway is iTreg cell specific as TNF-α does not activate Akt in naturally occurring regulatory T cells, therefore conferring a selective effect of TNF-α and its antagonism on iTreg cells. The study sheds new light on the critical role and underlying mechanism of TNF-α in the regulation of iTreg differentiation and provides a novel rationale for TNF-α antagonistic therapy for autoimmune diseases.

mTORC2-PKBα/Akt1 Serine 473 phosphorylation axis is essential for regulation of FOXP3 Stability by chemokine CCL3 in psoriasis

The connection between infections and acute guttate psoriasis (AGP) outbreaks/chronic plaque psoriasis (CPP) exacerbation has been known for years. Impaired function of FOXP3+Tregs in psoriasis has been identified. However, the mechanisms behind these two observations have not been fully interpreted. In the present study, we provide evidence to support chemokine CCL3 as one of the vital links between infections and FOXP3 stability in the psoriatic microenvironment. We found that serum CCL3, strongly induced by microorganism infections including streptococcus, was closely correlated with FOXP3 levels in CD4+CD25+T cells of patients with psoriasis. CCL3 manipulated FOXP3 stability in a concentration-dependent bidirectional manner. High-concentration CCL3 decreased FOXP3 stability by promoting FOXP3's degradation through K48-linkage ubiquitination. This degradation was mainly dependent on upregulation of Serine 473 phosphorylation of the PKBα/Akt1 isoform, and almost independent of mTORC1 (mammalian target of rapamycin complex 1) activity. On the other hand, low-concentration CCL3 could enhance FOXP3 stability by the maintenance of the PKC pathway and the restriction of the PKB/Akt pathway. We further demonstrated that enhancing FOXP3 stability by low-concentration CCL3 attributed, at least partly, to the prevention of cytoplasmic Sin1, a vital component of mTORC2, nuclear translocation. Our results suggest vital roles for CCL3-mTORC2-isoform PKB/Akt1 S473 phosphorylation axis in FOXP3+Tregs and the development of psoriasis.

Antigen-specific transforming growth factor β-induced Treg cells, but not natural Treg cells, ameliorate autoimmune arthritis in mice by shifting the Th17/Treg cell balance from Th17 predominance to Treg cell predominance

OBJECTIVE

Transferred CD4+CD25+FoxP3+ Treg cells can prevent autoimmune disease, but generally fail to ameliorate established disease. This study was undertaken to compare the effects of antigen-specific Treg cells induced with interleukin-2 (IL-2) and transforming growth factor β (TGFβ) ex vivo (induced Treg [iTreg] cells) to the effects of equivalent expanded thymus-derived natural Treg (nTreg) cells on established collagen-induced arthritis (CIA).

METHODS

CIA was induced in DBA/1 mice by immunization with type II collagen (CII), and before or shortly after immunization, mice were treated with iTreg or nTreg cells that were generated or expanded in vitro. Clinical scores were determined. Inflammatory responses were determined by measuring the levels of anti-CII antibody in the serum and examining the histologic features of the mouse joints. The Th1/Th17-mediated autoreactive response was evaluated by determining the cytokine profile of the draining lymph node (LN) cells of the mice by flow cytometry.

RESULTS

Following transfer, nTreg cells exhibited decreased FoxP3 and Bcl-2 expression and decreased suppressive activity, and many converted to Th17 cells. In contrast, transferred iTreg cells were more numerous, retained FoxP3 expression and their suppressive activity in the presence of IL-6, and were resistant to Th17 conversion. Notably, 10 days after the transfer of donor iTreg cells, predominance was shifted from Th17 cells to Treg cells in the draining LNs of recipient mice.

CONCLUSION

These findings provide evidence that transferred TGFβ-induced iTreg cells are more stable and functional than nTreg cells in mice with established autoimmunity. Moreover, iTreg cells can have tolerogenic effects even in the presence of ongoing inflammation. The therapeutic potential of human iTreg cells in subjects with chronic, immune-mediated inflammatory diseases should be investigated.

IL-10 produced by induced regulatory T cells (iTregs) controls colitis and pathogenic ex-iTregs during immunotherapy

"Natural" regulatory T cells (nTregs) that express the transcription factor Foxp3 and produce IL-10 are required for systemic immunological tolerance. "Induced" regulatory T cells (iTregs) are nonredundant and essential for tolerance at mucosal surfaces, yet their mechanisms of suppression and stability are unknown. We investigated the role of iTreg-produced IL-10 and iTreg fate in a treatment model of inflammatory bowel disease. Colitis was induced in Rag1(-/-) mice by the adoptive transfer of naive CD4(+) T cells carrying a nonfunctional Foxp3 allele. At the onset of weight loss, mice were treated with both iTregs and nTregs where one marked subset was selectively IL-10 deficient. Body weight assessment, histological scoring, cytokine analysis, and flow cytometry were used to monitor disease activity. Transcriptional profiling and TCR repertoire analysis were used to track cell fate. When nTregs were present but IL-10 deficient, iTreg-produced IL-10 was necessary and sufficient for the treatment of disease, and vice versa. Invariably, ∼85% of the transferred iTregs lost Foxp3 expression (ex-iTregs) but retained a portion of the iTreg transcriptome, which failed to limit their pathogenic potential upon retransfer. TCR repertoire analysis revealed no clonal relationships between iTregs and ex-iTregs, either within mice or between mice treated with the same cells. These data identify a dynamic IL-10-dependent functional reciprocity between regulatory T cell subsets that maintains mucosal tolerance. The niche supporting stable iTregs is limited and readily saturated, which promotes a large population of ex-iTregs with pathogenic potential during immunotherapy.

Context and location dependence of adaptive Foxp3(+) regulatory T cell formation during immunopathological conditions

Circulating Foxp3(+) regulatory T cells (Treg) may arise in the thymus (natural Treg, nTreg) or through the adaptive upregulation of Foxp3 after T cell activation (induced Treg, iTreg). In this brief review, we explore evidence for the formation and function of iTreg during pathologic conditions. Determining the ontogeny and function of Treg populations has relied on the use of manipulated systems in which either iTreg or nTreg are absent, or lineage tracing of T cell clones through repertoire analyses. iTreg appear particularly important at mucosal interfaces. iTreg can also ameliorate tissue-specific autoimmunity and are a prominent source of tumor-infiltrating Treg in some models. However, under many conditions, including in CNS autoimmunity, diabetes, and some tumor systems, iTreg formation appears limited. The immunological contribution of iTreg is thus highly context dependent. Deciphering immune parameters responsible for iTreg formation and their role in modulating pathologic immune responses will be important.

Characterization of Foxp3 gene from grass carp (Ctenopharyngodon idellus): a rapamycin-inducible transcription factor in teleost immune system

In this study, we cloned grass carp foxp3 (gcfoxp3) gene including 5' flanking region and determined its expression profiles in vivo under immunosuppressive conditions. Sequence analysis revealed that the promoter of gcfoxp3 contains AP-1, AML-1/Runx1, NF-κb and GATA-3 binding sites, which positively or negatively regulate mammalian foxp3 expression. In addition, the intron II of gcfoxp3 contains some putative binding sites including AP-1, NFAT, Smad3, RAR, CREB/ATF and FOXO1, which are corresponding to their locations in the proximal intronic enhancers of human foxp3. In an in vivo model of grass carp, an immunosuppressive agent rapamycin was showed to stimulate gcfoxp3 mRNA expression in thymus, gill, head kidney and spleen after bacterial challenge. Moreover, rapamycin increased gcFoxp3 protein levels with an additive manner in the infected fish. These findings support the involvement of fish Foxp3 in immune response and highlight a possible signaling pathway that regulates teleost Foxp3 expression.

STAT3 transcription factor promotes instability of nTreg cells and limits generation of iTreg cells during acute murine graft-versus-host disease

Acute graft-versus-host disease (GvHD) is a major cause of mortality in allogeneic bone marrow transplantation (BMT), for which administration of FoxP3(+) regulatory T (Treg) cells has been proposed as a therapy. However, the phenotypic stability of Treg cells is controversial, and STAT3-dependent cytokines can inhibit FoxP3 expression. We assessed whether the elimination of STAT3 in T cells could limit the severity of GvHD. We found STAT3 limited FoxP3(+) Treg cell numbers following allogeneic BMT by two pathways: instability of natural Treg (nTreg) cells and inhibition of induced Treg (iTreg) cell polarization from naive CD4(+) T cells. Deletion of STAT3 within only the nTreg cell population was not sufficient to protect against lethal GvHD. In contrast, transfer of STAT3-deficient naive CD4(+) T cells increased FoxP3(+) Treg cells post-BMT and prevented lethality, suggesting that the consequence of STAT3 signaling may be greater for iTreg rather than nTreg cells during GvHD.

Functional stability of Foxp3+ regulatory T cells

Significant evidence demonstrates that CD4(+) regulatory T cells (T(reg)) expressing the Forkhead box P3 (Foxp3) transcription factor are a distinct lineage of CD4(+) T cells that are essential for maintaining self-tolerance and modulating immunity to various nonself-antigens under changing inflammatory settings. Stable Foxp3 expression ensures T(reg) function in a variety of inflammatory contexts. However, the model of T(reg) cells as a stable, long-lived lineage is controversial. Whereas some studies have observed long-lived T(reg) function, recent studies suggest that T(reg) cells adapt to microenvironmental changes and consequently manifest functional plasticity by reprogramming into inflammatory T cells. Here, we review the evidence addressing the functional stability or plasticity of Foxp3(+) T(reg) cells and the implications for immune homeostasis and disease.

Histone deacetylases 6 and 9 and sirtuin-1 control Foxp3+ regulatory T cell function through shared and isoform-specific mechanisms

Therapeutic inhibition of the histone deacetylases HDAC6, HDAC9, or sirtuin-1 (Sirt1) augments the suppressive functions of regulatory T cells (T(regs)) that contain the transcription factor Foxp3 (Forkhead box P3) and is useful in organ transplant patients or patients with autoimmune diseases. However, it is unclear whether distinct mechanisms are involved for each HDAC or whether combined inhibition of HDACs would be more effective. We compared the suppressive functions of T(regs) from wild-type C57BL/6 mice with those from mice with either complete or cell-specific deletion of various HDACs, as well as with those of T(regs) treated with isoform-selective HDAC inhibitors. The improvement of T(reg) suppressive function mediated by inhibition of HDAC6, but not Sirt1, required an intact heat shock response. Although HDAC6, HDAC9, and Sirt1 all deacetylated Foxp3, each protein had different effects on transcription factors that control expression of the gene encoding Foxp3. For example, loss of HDAC9, but not other HDACs, was associated with stabilization of the acetylated form of signal transducer and activator of transcription 5 (STAT5) and promoted its transcriptional activity. Thus, targeting different HDACs increased T(reg) function through multiple and additive mechanisms, which suggests the therapeutic potential for using combinations of HDAC inhibitors in the management of autoimmunity and organ transplantation.

An in vivo IL-7 requirement for peripheral Foxp3+ regulatory T cell homeostasis

All T cells are dependent on IL-7 for their development and for homeostasis. Foxp3(+) regulatory T cells (Tregs) are unique among T cells in that they are dependent on IL-2. Whether such IL-2 dependency is distinct from or in addition to an IL-7 requirement has been a confounding issue, particularly because of the absence of an adequate experimental system to address this question. In this study, we present a novel in vivo mouse model where IL-2 expression is intact but IL-7 expression was geographically limited to the thymus. Consequently, IL-7 is not available in peripheral tissues. Such mice were generated by introducing a thymocyte-specific IL-7 transgene onto an IL-7 null background. In these mice, T cell development in the thymus, including Foxp3(+) Treg numbers, was completely restored, which correlates with the thymus-specific expression of transgenic IL-7. In peripheral cells, however, IL-7 expression was terminated, which resulted in a general paucity of T cells and a dramatic reduction of Foxp3(+) Treg numbers. Loss of Tregs was further accompanied by a significant reduction in Foxp3(+) expression levels. These data suggest that peripheral IL-7 is not only necessary for Treg survival but also for upregulating Foxp3 expression. Collectively, we assessed the effect of a selective peripheral IL-7 deficiency in the presence of a fully functional thymus, and we document a critical requirement for in vivo IL-7 in T cell maintenance and specifically in Foxp3(+) cell homeostasis.

Protein kinase C-θ inhibits inducible regulatory T cell differentiation via an AKT-Foxo1/3a-dependent pathway

Protein kinase C (PKC)-θ has been shown to be a critical TCR signaling molecule that promotes the activation and differentiation of naive T cells into inflammatory effector T cells. In this study, we demonstrate that PKC-θ-mediated signals inhibit inducible regulatory T cell (iTreg) differentiation via an AKT-Foxo1/3A pathway. TGF-β-induced iTreg differentiation was enhanced in PKC-θ(-/-) T cells or wild-type cells treated with a specific PKC-θ inhibitor, but was inhibited by the PKC-θ activator PMA, or by CD28 crosslinking, which enhances PKC-θ activation. PKC-θ(-/-) T cells had reduced activity of the AKT kinase, and the expression of a constitutively active form of AKT in PKC-θ(-/-) T cells restored the ability to inhibit iTreg differentiation. Furthermore, knockdown or overexpression of the AKT downstream targets Foxo1 and Foxo3a was found to inhibit or promote iTreg differentiation in PKC-θ(-/-) T cells accordingly, indicating that the AKT-Foxo1/3A pathway is responsible for the inhibition of iTreg differentiation of iTregs downstream of PKC-θ. We conclude that PKC-θ is able to control T cell-mediated immune responses by shifting the balance between the differentiation of effector T cells and inhibitory Tregs.

Cell-autonomous role of TGFβ and IL-2 receptors in CD4+ and CD8+ inducible regulatory T-cell generation during GVHD

FoxP3(+) regulatory T cells (Tregs) suppress GVHD while preserving graft-versus-tumor effects, making them an attractive target for GVHD therapy. The donor-derived Treg pool can potentially be derived from the expansion of preexisting natural Tregs (nTregs) or from de novo generation of inducible Tregs (iTregs) from donor Tconvs in the transplantation recipient. Using an MHC-mismatched model of acute GVHD, in the present study we found that the Treg pool was comprised equally of donor-derived nTregs and iTregs. Experiments using various combinations of T cells from wild-type and FoxP3-deficient mice suggested that both preexisting donor nTregs and the generation of iTregs in the recipient mice contribute to protection against GVHD. Surprisingly, CD8(+)FoxP3(+) T cells represented approximately 70% of the iTreg pool. These CD8(+)FoxP3(+) T cells shared phenotypic markers with their CD4(+) counterparts and displayed suppressive activity, suggesting that they were bona fide iTregs. Both CD4(+) and CD8(+) Tregs appeared to be protective against GVHD-induced lethality and required IL-2 and TGFβ receptor expression for their generation. These data illustrate the complex makeup of the donor-derived FoxP3(+) Treg pool in allogeneic recipients and their potential role in protection against GVHD.

Surfactant protein A modulates induction of regulatory T cells via TGF-β

TCR signaling plays a critical role in regulatory T cell (Treg) development. However, the mechanism for tissue-specific induction of Tregs in the periphery remains unclear. We observed that surfactant protein A (SP-A)-deficient mice have impaired expression of Foxp3 and fewer CD25(+)Foxp3(+) Tregs after ex vivo stimulation and after stimulation with LPS in vivo. The addition of exogenous SP-A completely reversed this phenotype. Although SP-A is known to inhibit T cell proliferation under certain activation conditions, both IL-2 levels as well as active TGF-β levels increase on extended culture with exogenous SP-A, providing a key mechanism for the maintenance and induction of Tregs. In addition, kinetic suppression assays demonstrate that SP-A enhances the frequency of functional Foxp3(+) Tregs in responder T cell populations in a TGF-β-dependent manner. In mice treated with LPS in vivo, Tregs increased ∼160% in wild-type mice compared with only a 50% increase in LPS-treated SP-A(-/-) mice 8 d after exposure. Taken together, these findings support the hypothesis that SP-A affects T cell immune function by the induction of Tregs during activation.

Biological functions of regulatory T cells

The subpopulation of CD4(+) T lymphocytes that co-express the transcription factor Foxp3 plays a unique role as regulatory T lymphocytes (Tregs) that modulate many aspects of the immune response. Multiple mechanisms have been proposed for the suppressor function of CD4(+)Foxp3(+) T cells based on in vitro studies, but much less is known about how Tregs suppress immune responses in vivo. Both polyclonal Tregs and antigen-specific Tregs are capable of exerting potent suppressive effects in vivo, and it is likely that they mediate their biologic functions using different mechanisms. Antigen-specific Tregs primarily target dendritic cells and inhibit dendritic cell functions including the expression of costimulatory molecules and the presentation of antigen early during the generation of the immune response. The end result is a complete inhibition of both the expansion and the differentiation of T effector cells. Polyclonal Tregs also act on dendritic cells, but at a later phase, and do not inhibit expansion of T effector cells, but appear to modulate differentiation and cell trafficking. The cell surface molecules involved in the interaction of Tregs with dendritic cells, as well as the biochemical pathways modified by this interaction remain to be fully elucidated. A complete understand of the biological functions of Tregs in vivo should facilitate the development of pharmacologic and biologic agents that can be used to modulate Treg function in a therapeutic setting.

Stability of regulatory T-cell lineage

Regulatory T (Treg) cells expressing the transcription factor Foxp3 constitute a unique T-cell lineage committed to suppressive functions and play a central role in maintaining self-tolerance and immune homeostasis. While their differentiation state is remarkably stable in the face of various perturbations from the extracellular environment, recent studies have also revealed their adaptability to the changing environment; in response to extrinsic cues, Treg cells differentiate further into distinct substates to regulate different classes of immune responses effectively. In contrast, some other recent studies have challenged this notion of a committed Treg cell lineage and suggested that Treg cells might lose their identity and be reprogrammed to various effector helper T cells under certain circumstances, although this issue of environment-induced Treg cell reprogramming remains highly controversial. This review will focus on recent advances in our understanding of how the stability and adaptability of Treg cell lineage is regulated and how it might be perturbed in a changing environment.

The role of interleukin-2 during homeostasis and activation of the immune system

Interleukin-2 (IL-2) signals influence various lymphocyte subsets during differentiation, immune responses and homeostasis. As discussed in this Review, stimulation with IL-2 is crucial for the maintenance of regulatory T (T(Reg)) cells and for the differentiation of CD4(+) T cells into defined effector T cell subsets following antigen-mediated activation. For CD8(+) T cells, IL-2 signals optimize both effector T cell generation and differentiation into memory cells. IL-2 is presented in soluble form or bound to dendritic cells and the extracellular matrix. Use of IL-2 - either alone or in complex with particular neutralizing IL-2-specific antibodies - can amplify CD8(+) T cell responses or induce the expansion of the T(Reg) cell population, thus favouring either immune stimulation or suppression.

Oligodeoxynucleotides stabilize Helios-expressing Foxp3+ human T regulatory cells during in vitro expansion

Foxp3(+) regulatory T cells (Tregs) maintain self-tolerance and adoptive therapy, and using Foxp3(+) Tregs has been proposed as treatment for autoimmune diseases. The clinical use of Tregs will require large numbers of cells and methods for in vitro expansion of Tregs are being developed. Foxp3(+) Tregs can be divided into 2 subpopulations based on expression of the transcription factor, Helios. Foxp3(+)Helios(+) Tregs (70%) are thymic-derived, whereas Foxp3(+)Helios(-) Tregs (30%) are induced in the periphery. Foxp3(+)Helios(+) Tregs differ from Foxp3(+)Helios(-) Tregs in terms of epigenetic changes at the Foxp3 locus, their capacity to produce effector cytokines, and their stability of Foxp3 expression on days to weeks of expansion in vitro. Addition of a 25 mer DNA oligonucleotide of random composition for a short period during the expansion of Foxp3(+) Tregs in vitro results in prolonged stabilization of the Foxp3(+)Helios(+) subpopulation and yields an optimal population for use in cellular biotherapy.

Induced regulatory T cells: mechanisms of conversion and suppressive potential

Thymus-derived, naturally occurring CD4(+) Forkhead Box P3(+) regulatory T cells (nTreg) have suppressive activity that is important for the establishment and maintenance of immune homeostasis in the healthy state. Abundant reports have demonstrated that they can suppress pathogenic processes in autoimmune diseases and inhibit transplant rejection and graft-versus-host disease. Far less is known about induced regulatory T cells (iTreg) that are generated from naive T cells in the periphery or in vitro by directing naive T cells to acquire suppressive function under the influence of transforming growth factor-β and other factors. In this review, we describe mechanisms by which naive T cells are thought to be converted into iTreg. We also discuss the suppressive potential of iTreg, particularly in comparison with their naturally occurring counterparts, focusing on those reports in which direct comparisons have been made. Based on current knowledge, we consider the rationale for using iTreg versus nTreg in clinical trials.

Clinical perspectives for regulatory T cells in transplantation tolerance

Three main types of CD4+ regulatory T cells can be distinguished based upon whether they express Foxp3 and differentiate naturally in the thymus (natural Tregs) or are induced in the periphery (inducible Tregs); or whether they are FoxP3 negative but secrete IL-10 in response to antigen (Tregulatory type 1, Tr1 cells). Adoptive transfer of each cell type has proven highly effective in mouse models at preventing graft vs. host disease (GVHD) and autoimmunity. Although clinical application was initially hampered by low Treg frequency and unfavorable ex vivo expansion properties, several phase I trials are now being conducted to assess their effect on GVHD following hematopoietic stem cell transplantation (HSCT) and in type I diabetes. Human Treg trials for HSCT recipients have preceded other indications because GVHD onset is precisely known, the time period needed for prevention relatively short, initial efficacy is likely to provide life-long protection, and complications of GVHD can be lethal. This review will summarize the clinical trials conducted to date that have employed Tregs to prevent GVHD following HSCT and discuss recent advances in Treg cellular therapy.

Induced Foxp3(+) regulatory T cells: a potential new weapon to treat autoimmune and inflammatory diseases?

Foxp3(+) T regulatory cells (Tregs) consisting of natural and induced Treg subsets play a crucial role in the maintenance of immune homeostasis against self-antigen. The actions designed to correct defects in numbers or functions of Tregs may be therapeutic in the treatment of autoimmune diseases. While recent studies demonstrated that natural Tregs are instable and dysfunctional in the inflammatory condition, induced Tregs (iTregs) may have a different feature. Here we review the progress of iTregs, particularly focus on their stability and function in the established autoimmune diseases. The advantage of iTregs as therapeutics used under inflammatory conditions is highlighted. Proper generation and manipulation of iTregs used for cellular therapy may provide a promise for the treatment of many autoimmune and inflammatory diseases.