TCR-based lineage tracing: no evidence for conversion of conventional into regulatory T cells in response to a natural self-antigen in pancreatic islets

Lack of IFN-γ Receptor Signaling Inhibits Graft-versus-Host Disease by Potentiating Regulatory T Cell Expansion and Conversion

IFN-γ is a pleiotropic cytokine that plays a controversial role in regulatory T cell (Treg) activity. In this study, we sought to understand how IFN-γ receptor (IFN-γR) signaling affects donor Tregs following allogeneic hematopoietic cell transplant (allo-HCT), a potentially curative therapy for leukemia. We show that IFN-γR signaling inhibits Treg expansion and conversion of conventional T cells (Tcons) to peripheral Tregs in both mice and humans. Mice receiving IFN-γR-deficient allo-HCT showed markedly reduced graft-versus-host disease (GVHD) and graft-versus-leukemia (GVL) effects, a trend associated with increased frequencies of Tregs, compared with recipients of wild-type allo-HCT. In mice receiving Treg-depleted allo-HCT, IFN-γR deficiency-induced peripheral Treg conversion was effective in preventing persistent GVHD while minimally affecting GVL effects. Thus, impairing IFN-γR signaling in Tcons may offer a promising strategy for achieving GVL effects without refractory GVHD. Similarly, in a human PBMC-induced xenogeneic GVHD model, significant inhibition of GVHD and an increase in donor Tregs were observed in mice cotransferred with human CD4 T cells that were deleted of IFN-γR1 by CRISPR/Cas9 technology, providing proof-of-concept support for using IFN-γR-deficient T cells in clinical allo-HCT.

Increased TCR signaling in regulatory T cells is disengaged from TCR affinity

Foxp3+ regulatory T cells (Tregs) are capable suppressors of aberrant self-reactivity. However, TCR affinity and specificities that support Treg function, and how these compare to autoimmune T cells remain unresolved. In this study, we used antigen agnostic and epitope-focused analyses to compare TCR repertoires of regulatory and effector T cells that spontaneously infiltrate pancreatic islets of non-obese diabetic mice. We show that effector and regulatory T cell-derived TCRs possess similar wide-ranging reactivity for self-antigen. Treg-derived TCRs varied in their capacity to confer optimal protective function, and Treg suppressive capacity was in part determined by effector TCR affinity. Interestingly, when expressing the same TCR, Tregs showed higher Nur77-GFP expression than Teffs, suggesting Treg-intrinsic ability to compete for antigen. Our findings provide a new insight into TCR-dependent and independent mechanisms that regulate Treg function and indicate a TCR-intrinsic insufficiency in tissue-specific Tregs that may contribute to the pathogenesis of type 1 diabetes.

Activation pathways that drive CD4+ T cells to break tolerance in autoimmune diseases. Immunol Rev 2022;307:161-190. [PMID: 35142369 PMCID: PMC9255211 DOI: 10.1111/imr.13071]

Autoimmune diseases are characterized by dysfunctional immune systems that misrecognize self as non-self and cause tissue destruction. Several cell types have been implicated in triggering and sustaining disease. Due to a strong association of major histocompatibility complex II (MHC-II) proteins with various autoimmune diseases, CD4+ T lymphocytes have been thoroughly investigated for their roles in dictating disease course. CD4+ T cell activation is a coordinated process that requires three distinct signals: Signal 1, which is mediated by antigen recognition on MHC-II molecules; Signal 2, which boosts signal 1 in a costimulatory manner; and Signal 3, which helps to differentiate the activated cells into functionally relevant subsets. These signals are disrupted during autoimmunity and prompt CD4+ T cells to break tolerance. Herein, we review our current understanding of how each of the three signals plays a role in three different autoimmune diseases and highlight the genetic polymorphisms that predispose individuals to autoimmunity. We also discuss the drawbacks of existing therapies and how they can be addressed to achieve lasting tolerance in patients.

A dormant T cell population with autoimmune potential exhibits low self-reactivity and infiltrates islets in type 1 diabetes

The contribution of low affinity T cells to autoimmunity in the context of polyclonal T cell responses is understudied due to the limitations in their capture by tetrameric reagents and low level of activation in response to antigenic stimulation. As a result, low affinity T cells are often disregarded as non-antigen specific cells irrelevant to the immune response. Our study aimed to assess how the level of self-antigen reactivity shapes T cell lineage and effector responses in the context of spontaneous tissue specific autoimmunity observed in NOD mice. Using multi-color flow cytometry in combination with Nur77^GFP reporter of TCR signaling we identified a dormant population of T cells that infiltrated the pancreatic islets of pre-diabetic NOD mice, which exhibited reduced level of self-tissue reactivity based on expression of CD5 and Nur77^GFP . We showed that these CD5^low T cells had a unique TCR repertoire, exhibited low activation and minimal effector function; however, induced rapid diabetes upon transfer. The CD4⁺ CD5^low T cell population displayed transcriptional signature of central memory T cells, consistent with the ability to acquire effector function post-transfer. Transcriptional profile of CD5^low T cells was similar to T cells expressing a low affinity TCR, indicating TCR affinity to be the important factor in shaping CD5^low T cell phenotype and function at the tissue site. Overall, our study suggests that autoimmune tissue can maintain a reservoir of undifferentiated central memory-like autoreactive T cells with pathogenic effector potential that might be an important source for effector T cells during long-term chronic autoimmunity. This article is protected by copyright. All rights reserved.

Function, Failure, and the Future Potential of Tregs in Type 1 Diabetes

Critical insights into the etiology of type 1 diabetes (T1D) came from genome-wide association studies that unequivocally connected genetic susceptibility to immune cell function. At the top of the susceptibility are genes involved in regulatory T-cell (Treg) function and development. The advances in epigenetic and transcriptional analyses have provided increasing evidence for Treg dysfunction in T1D. These are well supported by functional studies in mouse models and analysis of peripheral blood during T1D. For these reasons, Treg-based therapies are at the forefront of research and development and have a tangible probability to deliver a long-sought-after successful immune-targeted treatment for T1D. The current challenge in the field is whether we can directly assess Treg function at the tissue site or make informative interpretations based on peripheral data. Future studies focused on Treg function in pancreatic lymph nodes and pancreas could provide key insight into the ultimate mechanisms underlying Treg failure in T1D. In this Perspective we will provide an overview of current literature regarding Treg development and function in T1D and how this knowledge has been applied to Treg therapies.

Helios+ and Helios- Treg subpopulations are phenotypically and functionally distinct and express dissimilar TCR repertoires

The transcription factor Helios is expressed in a large subset of Foxp3⁺ Tregs. We previously proposed that Helios is a marker of thymic derived Treg (tTreg), while Helios^- Treg were induced from Foxp3^- T conventional (Tconv) cells in the periphery (pTreg). To compare the two Treg subpopulations, we generated Helios-GFP reporter mice and crossed them to Foxp3-RFP reporter mice. The Helios⁺ Treg population expressed a more activated phenotype, had a slightly higher suppressive capacity in vitro and expressed a more highly demethylated TSDR but were equivalent in their ability to suppress inflammatory bowel disease in vivo. However, Helios⁺ Treg more effectively inhibited the proliferation of activated, autoreactive splenocytes from scurfy mice. When Helios⁺ and Helios^- Treg were transferred to lymphoreplete mice, both populations maintained comparable Foxp3 expression, but Foxp3 expression was less stable in Helios^- Treg when transferred to lymphopenic mice. Gene expression profiling demonstrated a large number of differentially expressed genes and showed that Helios^- Treg expressed certain genes normally expressed in CD4⁺ Foxp3^- T cells. TCR repertoire analysis indicated very little overlap between Helios⁺ and Helios^- Treg. Thus, Helios⁺ and Helios^- Treg subpopulations are phenotypically and functionally distinct and express dissimilar TCR repertoires.

More or less-On the influence of labelling strategies to infer cell population dynamics

The adoptive transfer of labelled cell populations has been an essential tool to determine and quantify cellular dynamics. The experimental methods to label and track cells over time range from fluorescent dyes over congenic markers towards single-cell labelling techniques, such as genetic barcodes. While these methods have been widely used to quantify cell differentiation and division dynamics, the extent to which the applied labelling strategy actually affects the quantification of the dynamics has not been determined so far. This is especially important in situations where measurements can only be obtained at a single time point, as e.g. due to organ harvest. To this end, we studied the appropriateness of various labelling strategies as characterised by the number of different labels and the initial number of cells per label to quantify cellular dynamics. We simulated adoptive transfer experiments in systems of various complexity that assumed either homoeostatic cellular turnover or cell expansion dynamics involving various steps of cell differentiation and proliferation. Re-sampling cells at a single time point, we determined the ability of different labelling strategies to recover the underlying kinetics. Our results indicate that cell transition and expansion rates are differently affected by experimental shortcomings, such as loss of cells during transfer or sampling, dependent on the labelling strategy used. Furthermore, uniformly distributed labels in the transferred population generally lead to more robust and less biased results than non-equal label sizes. In addition, our analysis indicates that certain labelling approaches incorporate a systematic bias for the identification of complex cell expansion dynamics.

The Lower Limit of Regulatory CD4+ Foxp3+ TCRβ Repertoire Diversity Required To Control Autoimmunity

The TCR repertoire of regulatory T cells (Tregs) is highly diverse. The relevance of this diversity to maintain self-tolerance remains unknown. We established a model where the TCR repertoire of normal polyclonal Tregs was limited by serial transfers into IL-2Rβ^-/- mice, which lack functional Tregs. After a primary transfer, the donor Treg TCR repertoire was substantially narrowed, yet the recipients remained autoimmune-free. Importantly, upon purification and transfer of donor-derived Tregs from an individual primary recipient into neonatal IL-2Rβ^-/- mice, the secondary recipients developed autoimmunity. In this study, the Treg TCRβ repertoire was reshaped and further narrowed. In contrast, secondary IL-2Rβ recipients showed fewer symptoms of autoimmunity when they received donor Tregs that were premixed from several primary recipients to increase their TCRβ repertoire diversity. About 8-11% of the Treg TCRβ repertoire was estimated to be the minimum required to establish and maintain tolerance in primary IL-2Rβ^-/- recipients. Collectively, these data quantify where limitations imposed on the Treg TCRβ repertoire results in a population of Tregs that cannot fully suppress polyclonal autoreactive T cells. Our data favor a model where the high diversity of the Treg TCR provides a mechanism for Tregs to actively adapt and effectively suppress autoreactive T cells, which are not fixed, but are evolving as they encounter self-antigens.

Understanding Autoimmune Diabetes through the Prism of the Tri-Molecular Complex

The strongest susceptibility allele for Type 1 Diabetes (T1D) is human leukocyte antigen (HLA), which supports a central role for T cells as the drivers of autoimmunity. However, the precise mechanisms that allow thymic escape and peripheral activation of beta cell antigen-specific T cells are still largely unknown. Studies performed with the non-obese diabetic (NOD) mouse have challenged several immunological dogmas, and have made the NOD mouse a key experimental system to study the steps of immunodysregulation that lead to autoimmune diabetes. The structural similarities between the NOD I-A^g7 and HLA-DQ8 have revealed the stability of the T cell receptor (TCR)/HLA/peptide tri-molecular complex as an important parameter in the development of autoimmune T cells, as well as afforded insights into the key antigens targeted in T1D. In this review, we will provide a summary of the current understanding with regard to autoimmune T cell development, the significance of the antigens targeted in T1D, and the relationship between TCR affinity and immune regulation.

Developmental Progression and Interrelationship of Central and Effector Regulatory T Cell Subsets

Resting central Tregs (cTregs) and activated effector Tregs (eTregs) are required for self-tolerance, but the heterogeneity and relationships within and between phenotypically distinct subsets of cTregs and eTregs are poorly understood. By extensive immune profiling and deep sequencing of TCR-β V regions, two subsets of cTregs, based on expression of Ly-6C, and three subsets of eTregs, based on distinctive expression of CD62L, CD69, and CD103, were identified. Ly-6C(+) cTregs exhibited lower basal activation, expressed on average lower affinity TCRs, and less efficiently developed into eTregs when compared with Ly-6C(-) cTregs. The dominant TCR Vβs of Ly-6C(+) cTregs were shared by eTregs at a low frequency. A single TCR clonotype was also identified that was largely restricted to Ly-6C(+) cTregs, even under conditions that promoted the development of eTregs. Collectively, these findings indicate that some Ly-6C(+) cTregs may persist as a lymphoid-specific subset, with minimal potential to develop into highly activated eTregs, whereas other cTregs readily develop into eTregs. In contrast, subsets of CD62L(lo) eTregs showed higher clonal expansion and were more highly interrelated than cTreg subsets based on their TCR-β repertoires, but exhibited varied immune profiles. The CD62L(lo) CD69(-) CD103(-) eTreg subset displayed properties of a transitional intermediate between cTregs and more activated eTreg subsets. Thus, eTreg subsets appear to exhibit substantial flexibility, most likely in response to environmental cues, to adopt defined immune profiles that are expected to optimize suppression of autoreactive T cells.

Conventional and Regulatory CD4+ T Cells That Share Identical TCRs Are Derived from Common Clones

Results from studies comparing the diversity and specificity of the TCR repertoires expressed by conventional (Tconv) and regulatory (Treg) CD4⁺ T cell have varied depending on the experimental system employed. We developed a new model in which T cells express a single fixed TCRα chain, randomly rearranged endogenous TCRβ chains, and a Foxp3-GFP reporter. We purified CD4⁺Foxp3^- and CD4⁺Foxp3⁺ cells, then performed biased controlled multiplex PCR and high throughput sequencing of endogenous TCRβ chains. We identified >7,000 different TCRβ sequences in the periphery of 5 individual mice. On average, ~12% of TCR sequences were expressed by both conventional and regulatory populations within individual mice. The CD4⁺ T cells that expressed shared TCR sequences were present at higher frequencies compared to T cells expressing non-shared TCRs. Furthermore, nearly all (>90%) of the TCR sequences that were shared within mice were identical at the DNA sequence level, indicating that conventional and regulatory T cells that express shared TCRs are derived from common clones. Analysis of TCR repertoire overlap in the thymus reveals that a large proportion of Tconv and Treg sharing observed in the periphery is due to clonal expansion in the thymus. Together these data show that there are a limited number of TCR sequences shared between Tconv and Tregs. Also, Tconv and Tregs sharing identical TCRs are found at relatively high frequencies and are derived from common progenitors, of which a large portion are generated in the thymus.

Immune tolerance. Regulatory T cells generated early in life play a distinct role in maintaining self-tolerance

Aire is an important regulator of immunological tolerance, operating in a minute subset of thymic stromal cells to induce transcripts encoding peptides that guide T cell selection. Expression of Aire during a perinatal age window is necessary and sufficient to prevent the multiorgan autoimmunity characteristic of Aire-deficient mice. We report that Aire promotes the perinatal generation of a distinct compartment of Foxp3(+)CD4(+) regulatory T (Treg) cells, which stably persists in adult mice. This population has a role in maintaining self-tolerance, a transcriptome and an activation profile distinguishable from those of Tregs produced in adults. Underlying the distinct Treg populations are age-dependent, Aire-independent differences in the processing and presentation of thymic stromal-cell peptides, resulting in different T cell receptor repertoires. Our findings expand the notion of a developmentally layered immune system.

Quantification of HTLV-1 clonality and TCR diversity

Estimation of immunological and microbiological diversity is vital to our understanding of infection and the immune response. For instance, what is the diversity of the T cell repertoire? These questions are partially addressed by high-throughput sequencing techniques that enable identification of immunological and microbiological "species" in a sample. Estimators of the number of unseen species are needed to estimate population diversity from sample diversity. Here we test five widely used non-parametric estimators, and develop and validate a novel method, DivE, to estimate species richness and distribution. We used three independent datasets: (i) viral populations from subjects infected with human T-lymphotropic virus type 1; (ii) T cell antigen receptor clonotype repertoires; and (iii) microbial data from infant faecal samples. When applied to datasets with rarefaction curves that did not plateau, existing estimators systematically increased with sample size. In contrast, DivE consistently and accurately estimated diversity for all datasets. We identify conditions that limit the application of DivE. We also show that DivE can be used to accurately estimate the underlying population frequency distribution. We have developed a novel method that is significantly more accurate than commonly used biodiversity estimators in microbiological and immunological populations.

Homeostasis and function of regulatory T cells (Tregs) in vivo: lessons from TCR-transgenic Tregs

The identification of CD25 and subsequently Forkhead box protein 3 (Foxp3) as markers for regulatory T cells (Tregs) has revolutionized our ability to explore this population experimentally. In a similar vein, our understanding of antigen-specific Treg responses in vivo owes much to the fortuitous generation of T-cell receptor (TCR)-transgenic Tregs. This has permitted tracking of Tregs with a defined specificity in vivo, facilitating analysis of how encounter with cognate antigen shapes Treg homeostasis and function. Here, we review the key lessons learned from a decade of analysis of TCR-transgenic Tregs and set this in the broader context of general progress in the field. Use of TCR-transgenic Tregs has led to an appreciation that Tregs are a highly dynamic proliferative population in vivo, rather than an anergic population as they were initially portrayed. It is now clear that Treg homeostasis is positively regulated by encounter with self-antigen expressed on peripheral tissues, which is likely to be relevant to the phenomenon of peripheral repertoire reshaping that has been described for Tregs and the observation that the Treg TCR specificities vary by anatomical location. Substantial evidence has also accumulated to support the role of CD28 costimulation and interleukin-2 in Treg homeostasis. The availability of TCR-transgenic Tregs has enabled analysis of Treg populations that are sufficient or deficient in particular genes, without the comparison being confounded by repertoire alterations. This approach has yielded insights into genes required for Treg function in vivo, with particular progress being made on the role of ctla-4 in this context. As the prospect of manipulating Treg populations in the clinic becomes reality, a full appreciation of the rules governing their homeostasis will prove increasingly important.

Single cell behavior in T cell differentiation

Upon primary infection, naïve T cells that recognize their cognate antigen become activated, proliferate, and simultaneously differentiate into various subsets. A long-standing question in the field has been how this cellular diversification is achieved. Conceptually, diverse cellular output may either arise from every single cell or only from populations of naïve cells. Furthermore, such diversity may either be driven by cell-intrinsic heterogeneity or by external, niche-derived signals. In this review, we discuss how recently developed technologies have allowed the analysis of the mechanisms underlying T cell diversification at the single cell level. In addition, we outline the implications of this work on our understanding of the formation of immunological memory, and describe a number of unresolved key questions in this field.

Tolerogenic vaccination reduced effector memory CD4 T cells and induced effector memory Treg cells for type I diabetes treatment

Background

Vaccination could induce immune tolerance and protected NOD mice from the development of type I diabetes (T1D). We previously demonstrated that insulin peptide (B9-23) combined with dexamethasone (DEX) stimulated the expansion of antigen specific regulatory T (Treg) cells which in turn effectively prevented T1D in NOD mice. Here, we aimed to investigate the therapeutic effect of tolerogenic vaccination for T1D treatment.

Methodology/Principal Findings

The diabetic NOD mice (Blood glucose level ≧250 mg/dl) were treated with B9-23 and DEX twice. The tolerance was restored by blocking maturation of dendritic cells (DCs) and inducing Treg cells in treated NOD mice. Remarkably, the reduction of autoreactive effector memory CD4 T (Tm) cells and the induction of functional effector memory Treg (mTreg) cells contributed to the improvement of T1D in treated NOD mice.

Conclusions/Significance

Tolerogenic vaccination restored tolerance and ameliorated T1D by suppressing effector CD4 Tm cells and inducing effector mTreg cells. Our findings implicate the potential of tolerogenic vaccination for T1D treatment.

Thymic versus induced regulatory T cells - who regulates the regulators?

Physiological health must balance immunological responsiveness against foreign pathogens with tolerance toward self-components and commensals. Disruption of this balance causes autoimmune diseases/chronic inflammation, in case of excessive immune responses, and persistent infection/immunodeficiency if regulatory components are overactive. This homeostasis occurs at two different levels: at a resting state to prevent autoimmune disease, as autoreactive effector T-cells (Teffs) are only partially deleted in the thymus, and during inflammation to prevent excessive tissue injury, contract the immune response, and enable tissue repair. Adaptive immune cells with regulatory function (“regulatory T-cells”) are essential to control Teffs. Two sets of regulatory T cell are required to achieve the desired control: those emerging de novo from embryonic/neonatal thymus (“thymic” or tTregs), whose function is to control autoreactive Teffs to prevent autoimmune diseases, and those induced in the periphery (“peripheral” or pTregs) to acquire regulatory phenotype in response to pathogens/inflammation. The differentiation mechanisms of these cells determine their commitment to lineage and plasticity toward other phenotypes. tTregs, expressing high levels of IL-2 receptor alpha chain (CD25), and the transcription factor Foxp3, are the most important, since mutations or deletions in these genes cause fatal autoimmune diseases in both mice and men. In the periphery, instead, Foxp3⁺ pTregs can be induced from naïve precursors in response to environmental signals. Here, we discuss molecular signatures and induction processes, mechanisms and sites of action, lineage stability, and differentiating characteristics of both Foxp3⁺ and Foxp3⁻ populations of regulatory T cells, derived from the thymus or induced peripherally. We relate these predicates to programs of cell-based therapy for the treatment of autoimmune diseases and induction of tolerance to transplants.

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.

Targeting Tregs in Malignant Brain Cancer: Overcoming IDO

One of the hallmark features of glioblastoma multiforme (GBM), the most common adult primary brain tumor with a very dismal prognosis, is the accumulation of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Regulatory T cells (Tregs) segregate into two primary categories: thymus-derived natural Tregs (nTregs) that develop from the interaction between immature T cells and thymic epithelial stromal cells, and inducible Tregs (iTregs) that arise from the conversion of CD4⁺FoxP3⁻ T cells into FoxP3 expressing cells. Normally, these Treg subsets complement one another’s actions by maintaining tolerance of self-antigens, thereby suppressing autoimmunity, while also enabling effective immune responses toward non-self-antigens, thus promoting infectious protection. However, Tregs have also been shown to be associated with the promotion of pathological outcomes, including cancer. In the setting of GBM, nTregs appear to be primary players that contribute to immunotherapeutic failure, ultimately leading to tumor progression. Several attempts have been made to therapeutically target these cells with variable levels of success. The blood brain barrier-crossing chemotherapeutics, temozolomide, and cyclophosphamide (CTX), vaccination against the Treg transcriptional regulator, FoxP3, as well as mAbs against Treg-associated cell surface molecules CD25, CTLA-4, and GITR are all different therapeutic approaches under investigation. Contributing to the poor success of past approaches is the expression of indoleamine 2,3-dioxygenase 1 (IDO), a tryptophan catabolizing enzyme overexpressed in GBM, and critically involved in regulating tumor-infiltrating Treg levels. Herein, we review the current literature on Tregs in brain cancer, providing a detailed phenotype, causative mechanisms involved in their pathogenesis, and strategies that have been used to target this population, therapeutically.

Thymus-derived regulatory T cells contribute to tolerance to commensal microbiota

Peripheral mechanisms preventing autoimmunity and maintaining tolerance to commensal microbiota involve CD4(+) Foxp3(+) regulatory T (Treg) cells generated in the thymus or extrathymically by induction of naive CD4(+) Foxp3(-) T cells. Previous studies suggested that the T-cell receptor repertoires of thymic Treg cells and induced Treg cells are biased towards self and non-self antigens, respectively, but their relative contribution in controlling immunopathology, such as colitis and other untoward inflammatory responses triggered by different types of antigens, remains unresolved. The intestine, and especially the colon, is a particularly suitable organ to study this question, given the variety of self-, microbiota- and food-derived antigens to which Treg cells and other T-cell populations are exposed. Intestinal environments can enhance conversion to a regulatory lineage and favour tolerogenic presentation of antigens to naive CD4(+) T cells, suggesting that intestinal homeostasis depends on microbiota-specific induced Treg cells. Here, to identify the origin and antigen-specificity of intestinal Treg cells, we performed single-cell and high-throughput sequencing of the T-cell receptor repertoires of CD4(+) Foxp3(+) and CD4(+) Foxp3(-) T cells, and analysed their reactivity against specific commensal species. We show that thymus-derived Treg cells constitute most Treg cells in all lymphoid and intestinal organs, including the colon, where their repertoire is heavily influenced by the composition of the microbiota. Our results suggest that thymic Treg cells, and not induced Treg cells, dominantly mediate tolerance to antigens produced by intestinal commensals.

Aire-dependent thymic development of tumor-associated regulatory T cells

Despite considerable interest in the modulation of tumor-associated Foxp3(+) regulatory T cells (T(regs)) for therapeutic benefit, little is known about the developmental origins of these cells and the nature of the antigens that they recognize. We identified an endogenous population of antigen-specific T(regs) (termed MJ23 T(regs)) found recurrently enriched in the tumors of mice with oncogene-driven prostate cancer. MJ23 T(regs) were not reactive to a tumor-specific antigen but instead recognized a prostate-associated antigen that was present in tumor-free mice. MJ23 T(regs) underwent autoimmune regulator (Aire)-dependent thymic development in both male and female mice. Thus, Aire-mediated expression of peripheral tissue antigens drives the thymic development of a subset of organ-specific T(regs), which are likely coopted by tumors developing within the associated organ.

Regulatory T Cell-Based Immunotherapy

CD4+CD25+ regulatory T (Treg) cells expressing the forkhead box transcription factor Foxp3 have a vital function in the maintenance of immune homeostasis and the prevention of fatal multi-organ autoimmunity throughout life. In the last decade, Foxp3+ Treg cells have raised the hope for novel cell-based therapies to achieve tolerance in clinical settings of unwanted immune responses such as autoimmunity and graft rejection. Conceptually, the antigen-specific enhancement of Treg cell function is of particular importance because such strategies will minimize the requirements for pharmaceutical immunosuppression, sparing desired protective host immune responses to infectious and malignant insults. This chapter discusses current concepts of Treg cell-based immunotherapy with particular emphasis on antigen-specific Treg cell induction from conventional CD4+ T cells to deal with organ-specific autoimmunity.

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.

Treg cells, life history, and diversity

Regulatory T cells expressing the FoxP3 transcription factor have a profound and nonredundant role in several aspects of immunological tolerance. We will review here the specification of this lineage, its population dynamics, and the diversity of subphenotypes that correlate with their diverse roles in controlling inflammation in a variety of settings.

Down syndrome, autoimmunity and T regulatory cells

Autoimmune diseases are more represented in Down syndrome (DS) individuals compared to chromosomally normal people. Natural T regulatory cells (nT(reg) ) have been considered to be primary in the role of controlling the intensity and targets of the immune response. We have investigated the phenotypical and functional alteration of nT(reg) in a group of DS people. The phenotypical characteristic of T(reg) cells of 29 DS was analysed and compared with an age-matched healthy control group. The inhibitory potential of CD4(+) CD25(high) CD127(low) T regulatory cells was evaluated on autologous CD4(+) CD25(-) T cell proliferation in response to activation with a mytogenic pan-stimulus (anti-CD2, anti-CD3 and anti-CD28 antibodies). The CD4(+) CD25(high) cells in the DS and control groups were 2·692±0·3808%, n=29 and 1·246±0·119, n=29%, respectively (P=0.0007), with a percentage of forkhead box protein 3 (FoxP3)-expressing cells of 79·21±3·376%, n=29 and 59·75±4·496%, respectively (P=0.0015). CD4(+) CD25(+) FoxP3(+) cells were increased in peripheral blood from DS subjects (DS mean 5·231±0·6065% n=29, control mean 3·076±0·3140% n=29). The majority of CD4(+) CD25(high) were CD127(low) and expressed a high percentage of FoxP3 (natural T(reg) phenotype). While the proliferative capacity of DS T cells was not altered significantly compared to normal individuals, a reduced inhibitory potential of T(reg) compared to healthy controls was clearly observed (mean healthy control inhibition in T(eff)  : T(reg) 1:1 co-culture: 58·9%±4·157%, n=10 versus mean DS inhibition in T(eff) :T(reg) 1:1 co-culture: 39·8±4·788%, n=10, P=0.0075; mean healthy control inhibition in T(eff)  : T(reg) 1:0·5 co-culture: 45·10±5·858%, n=10 versus DS inhibition in T(eff) : T(reg) 1:0·5 co-culture: 24·10±5·517%, n=10, P=0.0177). DS people present an over-expressed peripheral nT(reg) population with a defective inhibitory activity that may partially explain the increased frequency of autoimmune disease.

Following the fate of one insulin-reactive CD4 T cell: conversion into Teffs and Tregs in the periphery controls diabetes in NOD mice

In diabetic patients and susceptible mice, insulin is a targeted autoantigen. Insulin B chain 9-23 (B:9-23) autoreactive CD4 T cells are key for initiating autoimmune diabetes in NOD mice; however, little is known regarding their origin and function. To this end, B:9-23-specific, BDC12-4.1 T-cell receptor (TCR) transgenic (Tg) mice were studied, of which, despite expressing a single TCR on the recombination activating gene-deficient background, only a fraction develops diabetes in an asynchronous manner. BDC12-4.1 CD4 T cells convert into effector (Teff) and Foxp3(+)-expressing adaptive regulatory T cells (aTregs) soon after leaving the thymus as a result of antigen recognition and homeostatic proliferation. The generation of aTreg causes the heterogeneous diabetes onset, since crossing onto the scurfy (Foxp3) mutation, BDC12-4.1 TCR Tg mice develop accelerated and fully penetrant diabetes. Similarly, adoptive transfer and bone marrow transplantation experiments showed differential diabetes kinetics based on Foxp3(+) aTreg's presence in the BDC12-4.1 donors. A single-specificity, insulin-reactive TCR escapes thymic deletion and simultaneously converts into aTreg and Teff, establishing an equilibrium that determines diabetes penetrance. These results are of particular importance for understanding disease pathogenesis. They suggest that once central tolerance is bypassed, autoreactive cells arriving in the periphery do not by default follow solely a pathogenic fate upon activation.

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.

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.

Regulatory T cells: mechanisms of differentiation and function

The immune system has evolved to mount an effective defense against pathogens and to minimize deleterious immune-mediated inflammation caused by commensal microorganisms, immune responses against self and environmental antigens, and metabolic inflammatory disorders. Regulatory T (Treg) cell-mediated suppression serves as a vital mechanism of negative regulation of immune-mediated inflammation and features prominently in autoimmune and autoinflammatory disorders, allergy, acute and chronic infections, cancer, and metabolic inflammation. The discovery that Foxp3 is the transcription factor that specifies the Treg cell lineage facilitated recent progress in understanding the biology of regulatory T cells. In this review, we discuss cellular and molecular mechanisms in the differentiation and function of these cells.

Nature and nurture in Foxp3(+) regulatory T cell development, stability, and function

Foxp3(+) regulatory T lymphocytes (Treg) are critical homeostatic regulators of immune and inflammatory responses. Their absence leads to fulminant multiorgan autoimmunity. This review explores recent studies that have altered our emerging view of the development, stability, and plasticity of these cells. Treg appear not to be a single entity, but a family of immunomodulatory cell types with shared capabilities. On a first level, Treg may alternatively form in response to developmental cues in the thymus as a distinct lineage of CD4(+) T cells or adaptively, in response to environmental cues received by mature conventional CD4(+) T lymphocytes. These 2 populations bear distinct specificity, stability, and genetic profiles and are differentially used in immune responses. Secondarily, in a manner analogous to the generation of T helper (Th)-1, Th2, and other T cell subsets, Treg may further specialize, adapting to the needs of their immunologic surroundings. Treg therefore comprise developmentally distinct, functionally overlapping cell populations that are uniquely designed to preserve immunologic homeostasis. They combine an impressive degree of both stability and adaptability.

Harnessing memory adaptive regulatory T cells to control autoimmunity in type 1 diabetes

Type 1 diabetes (T1D) results from autoimmune destruction of insulin-producing β-cells in the pancreatic islets. There is an immediate need to restore both β-cell function and immune tolerance to control disease progression and ultimately cure T1D. Currently, there is no effective treatment strategy to restore glucose regulation in patients with T1D. FoxP3-expressing CD4(+) regulatory T cells (Tregs) are potential candidates to control autoimmunity because they play a central role in maintaining self-tolerance. However, deficiencies in either naturally occurring Tregs (nTregs) themselves and/or their ability to control pathogenic effector T cells have been associated with T1D. Here, we hypothesize that nTregs can be replaced by FoxP3(+) adaptive Tregs (aTregs), which are uniquely equipped to combat autoreactivity in T1D. Unlike nTregs, aTregs are stable and provide long-lived protection. In this review, we summarize the current understanding of aTregs and their potential for use as an immunological intervention to treat T1D.

Regulatory T cells for tolerance therapy: revisiting the concept

The discovery of regulatory T cells (Tregs) as a crucial component of peripheral down-regulation of immunity to self and allogeneic antigens has raised legitimate hope for the development of Treg-based clinical protocols for tolerance to allografts. The present review addresses the question of whether therapeutic Tregs are ready to enter the clinical transplantation arena. In light of recent experimental observations, we will revisit some fundamentals of T cell and Treg biology that stress the need for further studies prior to applications and provide conceptual cues for novel therapeutic approaches.

Induction of self-antigen-specific Foxp3+ regulatory T cells in the periphery by lymphodepletion treatment with anti-mouse thymocyte globulin in mice

Lymphodepletion therapies are increasingly tested for controlling immune damage. One appealing premise for such a therapy is that it may 'reboot' the immune system and restore immune tolerance. However, the tolerogenic potential of lymphodepletion therapies remains controversial. The debate is exemplified by conflicting evidence from the studies of anti-thymocyte globulin (ATG), a prototype of immunodepleting drugs, in particular on whether it induces CD4(+) CD25(+) Foxp3(+) regulatory T (Treg) cells. To understand the impact of ATG on T cells at a clonal level in vivo, we studied the effect of anti-mouse thymocyte globulin (mATG) in a reductionist model in which the T-lymphocyte repertoire consists of a single clone of pathogenic T effector (Teff) cells specific to a physiological self-antigen. The mATG treatment led to peripheral induction of antigen-specific Treg cells from an otherwise monoclonal Teff repertoire, independent of thymic involvement. The de novo induction of Treg cells occurred consistently in local draining lymph nodes, and persistence of induced Treg cells in blood correlated with long-term protection from autoimmune destruction. This study provides in vivo evidence for clonal conversion from a pathogenic self-antigen-specific Teff cell to a Treg cell in the setting of immunodepletion therapies.

Setting the threshold for extra-thymic differentiation of Foxp3+ Tregs: TGF-β-dependent and T-cell autonomous

There is a lack of clarity regarding the conditions supporting de novo induction of Tregs. While there is widespread agreement in the literature over the need for optimal stimulation conditions and exogenous TGF-β in vitro, a number of studies indicate that sub-immunogenic conditions induce Tregs in vivo. These seemingly disparate findings have hindered the ability to pin down the conditions promoting Treg induction, including the role of co-stimulation and even the necessity for TGF-β. Two studies in this issue of the European Journal of Immunology re-examine these previous findings in detail and shed some light on the controversy. These studies demonstrate that Treg induction depends on reaching a certain threshold of signal strength: the requirement for co-stimulation is therefore not absolute but dictated by the strength of other signals received by the T cell. Furthermore, these studies demonstrate that the only source of TGF-β required for the induction of Tregs under sub-optimal condition is the T cells themselves. Overall, the picture that emerges is one where sub-immunogenicity, rather than a requirement for exogenous TGF-β, defines the conditions that support TGF-β-dependent Foxp3 induction in a T-cell autonomous fashion. The next challenge lies in utilizing this knowledge for the purpose of inducing Tregs for therapeutic gain.

Control of central and peripheral tolerance by Aire

The negative selection of self-reactive thymocytes depends on the expression of tissue-specific antigens by medullary thymic epithelial cells. The autoimmune regulator (Aire) protein plays an important role in turning on these antigens, and the absence of even one Aire-induced tissue-specific antigen in the thymus can lead to autoimmunity in the antigen-expressing target organ. Recently, Aire protein has been detected in peripheral lymphoid organs, suggesting that peripheral Aire plays a complementary role here. In these peripheral sites, Aire was found to regulate the expression of a group of tissue-specific antigens that is distinct from those expressed in the thymus. Furthermore, transgenic antigen expression in extrathymic Aire-expressing cells (eTACs) can mediate deletional tolerance, but the immunological relevance of Aire-dependent, endogenous tissue-specific antigens remains to be determined.

Cutting edge: De novo induction of functional Foxp3+ regulatory CD4 T cells in response to tissue-restricted self antigen

Naive CD4 T cells can differentiate into a number of functional subsets in response to Ag, including Foxp3(+) induced regulatory T cells (iTregs). The in vivo development and function of iTregs has been primarily demonstrated in systems involving Ag encountered systemically or delivered via the intestinal mucosa. In this study, we demonstrate that de novo Foxp3 expression in naive CD4 T cells is a critical mechanism for establishing tolerance for a tissue-restricted neo-self Ag. Naive CD4 T cells lacking a functional Foxp3 gene cannot achieve tolerance, but can be suppressed in vivo in the presence of wild type naive CD4 T cells. Exposure to nonspecific inflammation during priming undermines tolerance through impaired Foxp3 induction, suggesting that the microenvironment also has a role. These data show that de novo Foxp3 expression is an integral component of establishing and maintaining tolerance among naive peripheral CD4 T cells.

Targeting of a T cell agonist peptide to lysosomes by DNA vaccination induces tolerance in the nonobese diabetic mouse

CD4 T cells are crucial effectors in the pathology of type 1 diabetes (T1D). Successful therapeutic interventions for prevention and cure of T1D in humans are still elusive. Recent research efforts have focused on the manipulation of T cells by treatment with DNA. In this paper, we studied the effects of a DNA treatment strategy designed to target antigenic peptides to the lysosomal compartment on a monospecific T cell population termed 2.5mi(+) T cells that shares reactivity with the diabetogenic T cell clone BDC-2.5 in the NOD mouse. MHC class II tetramer analysis showed that repeated administrations were necessary to expand 2.5mi(+) T cells in vivo. This expansion was independent of Ag presentation by B cells. A single peptide epitope was sufficient to induce protection against T1D, which was not due to Ag-specific T cell anergy. Typical Th2 cytokines such as IL-10 or IL-4 were undetectable in 2.5mi(+) T cells, arguing against a mechanism of immune deviation. Instead, the expanded 2.5mi(+) T cell population produced IFN-γ similar to 2.5mi(+) T cells from naive mice. Protection against T1D by DNA treatment was completely lost in NOD.CD28(-/-) mice which are largely deficient of natural regulatory T cells (Treg). Although Ag-specific Foxp3(+) Treg did not expand in response to DNA treatment, diabetes onset was delayed in Treg-reconstituted and DNA-treated NOD.SCID mice. These observations provide evidence for a Treg-mediated protective mechanism that is independent of the expansion or de novo generation of Ag-specific Treg.

Analysis of the T-cell receptor repertoires of tumor-infiltrating conventional and regulatory T cells reveals no evidence for conversion in carcinogen-induced tumors

A significant enrichment of CD4(+)Foxp3(+) T cells (regulatory T cells, Treg) is frequently observed in murine and human carcinomas. As Tregs can limit effective antitumor immune responses, thereby promoting tumor progression, it is important that the mechanisms underpinning intratumoral accumulation of Tregs are identified. Because of evidence gathered mostly in vitro, the conversion of conventional T cells (Tconv) into Tregs has been proposed as one such mechanism. We assessed the contribution of conversion in vivo by analyzing the TCR (T-cell receptor) repertoires of Tconvs and Tregs in carcinogen-induced tumors in mice. Our results indicate that the TCR repertoires of Tregs and Tconvs within tumor-infiltrating lymphocytes (TIL) are largely distinct. Indeed, the cell population with the greatest degree of repertoire similarity with tumor-infiltrating Tregs was the Treg population from the tumor-draining lymph node. These findings demonstrate that conversion of Tconvs does not contribute significantly to the accumulation of tumor-infiltrating Tregs; rather, Tconvs and Tregs arise from different populations with unique TCR repertoires. Enrichment of Tregs within TILs most likely, therefore, reflects differences in the way that Tregs and Tconvs are influenced by the tumor microenvironment. Elucidating the nature of these influences may indicate how the balance between tumor-infiltrating Tregs and Tconvs can be manipulated for therapeutic purposes.

Host APCs augment in vivo expansion of donor natural regulatory T cells via B7H1/B7.1 in allogeneic recipients

Foxp3(+) regulatory T (Treg) cells include thymic-derived natural Treg and conventional T-derived adaptive Treg cells. Both are proposed to play important roles in downregulating inflammatory immune responses. However, the mechanisms of Treg expansion in inflammatory environments remain unclear. In this study, we report that, in an autoimmune-like graft-versus-host disease model of DBA/2 (H-2(d)) donor to BALB/c (H-2(d)) recipients, donor Treg cells in the recipients predominantly originated from expansion of natural Treg cells and few originated from adaptive Treg cells. In vivo neutralization of IFN-γ resulted in a marked reduction of donor natural Treg expansion and exacerbation of graft-versus-host disease, which was associated with downregulation of host APC expression of B7H1. Furthermore, host APC expression of B7H1 was shown to augment donor Treg survival and expansion. Finally, donor Treg interactions with host APCs via B7.1/B7H1 but not PD-1/B7H1 were demonstrated to be critical in augmenting donor Treg survival and expansion. These studies have revealed a new immune regulation loop consisting of T cell-derived IFN-γ, B7H1 expression by APCs, and B7.1 expression by Treg cells.

Thymic and peripheral differentiation of regulatory T cells

The development of regulatory T (Treg) cells is essential for the maintenance of immune tolerance and homeostasis. Here, we review recent studies that have advanced our understanding of Treg cell differentiation. In the thymus, TCR specificity to self-antigen appears to be a primary determinant for Treg cell lineage commitment, with c-Rel being an important factor that links T cell receptor (TCR) engagement and Foxp3 expression, along with cytokines and costimulatory molecules. It is also clear that postthymic events shape the peripheral Treg cell population. This includes preferential maintenance of Treg cells specific to self-antigens presented in the periphery, as well as the de novo generation of Treg cells from conventional Foxp3(-) T cells. The process of peripheral Treg cell differentiation shares some features with thymic Treg cell development, but there are notable differences. Together, thymic and peripheral Treg cell differentiation appear to generate an "imprint" of both self- and foreign antigens in the peripheral Treg cell population to provide dominant tolerance.

Model for comparative analysis of antigen receptor repertoires

In modern molecular biology one of the standard ways of analyzing a vertebrate immune system is to sequence and compare the counts of specific antigen receptor clones (either immunoglobulins or T-cell receptors) derived from various tissues under different experimental or clinical conditions. The resulting statistical challenges are difficult and do not fit readily into the standard statistical framework of contingency tables primarily due to the serious under-sampling of the receptor populations. This under-sampling is caused, on one hand, by the extreme diversity of antigen receptor repertoires maintained by the immune system and, on the other, by the high cost and labor intensity of the receptor data collection process. In most of the recent immunological literature the differences across antigen receptor populations are examined via non-parametric statistical measures of the species overlap and diversity borrowed from ecological studies. While this approach is robust in a wide range of situations, it seems to provide little insight into the underlying clonal size distribution and the overall mechanism differentiating the receptor populations. As a possible alternative, the current paper presents a parametric method that adjusts for the data under-sampling as well as provides a unifying approach to a simultaneous comparison of multiple receptor groups by means of the modern statistical tools of unsupervised learning. The parametric model is based on a flexible multivariate Poisson-lognormal distribution and is seen to be a natural generalization of the univariate Poisson-lognormal models used in the ecological studies of biodiversity patterns. The procedure for evaluating a model's fit is described along with the public domain software developed to perform the necessary diagnostics. The model-driven analysis is seen to compare favorably vis a vis traditional methods when applied to the data from T-cell receptors in transgenic mice populations.

Discrete TCR repertoires and CDR3 features distinguish effector and Foxp3+ regulatory T lymphocytes in myelin oligodendrocyte glycoprotein-induced experimental allergic encephalomyelitis

Regulatory T lymphocytes (Tregs) expressing the Foxp3 transcription factor are critical modulators of autoimmunity. Foxp3(+) Tregs may develop in the thymus as a population distinct from conventional Foxp3(-) αβ T cells (Tconvs). Alternatively, plasticity in Foxp3 expression may allow for the interconversion of mature Tregs and Tconvs. We examined >160,000 TCR sequences from Foxp3(+) or Foxp3(-) populations in the spleens or CNS of wild-type mice with experimental allergic encephalomyelitis to determine their relatedness and identify distinguishing TCR features. Our results indicate that the CNS-infiltrating Tregs and Tconvs arise predominantly from distinct sources. The repertoires of CNS Treg or Tconv TCRs showed limited overlap with heterologous populations in both the CNS and the spleen, indicating that they are largely unrelated. Indeed, Treg and Tconv TCRs in the CNS were significantly less related than those populations in the spleen. In contrast, CNS Treg and Tconv repertoires strongly intersected those of the homologous cell type in the spleen. High-frequency sequences more likely to be disease associated showed similar results, and some public TCRs demonstrated Treg- or Tconv-specific motifs. Different charge characteristics and amino acid use preferences were identified in the CDR3β of Tregs and Tconvs infiltrating the CNS, further indicating that their repertoires are qualitatively distinct. Therefore, discrete populations of Tregs and Tconvs that do not substantially interconvert respond during experimental allergic encephalomyelitis. Differences in sequence and physical characteristics distinguish Treg and Tconv TCRs and imply dissimilar Ag recognition properties.

Regulatory T-cell stability and plasticity in mucosal and systemic immune systems

Regulatory T cells (Treg) express the forkhead box p3 (Foxp3) transcription factor and suppress pathological immune responses against self and foreign antigens, including commensal microorganisms. Foxp3 has been proposed as a master key regulator for Treg, required for their differentiation, maintenance, and suppressive functions. Two types of Treg have been defined. Natural Treg (nTreg) are usually considered to be a separate sublineage arising during thymus differentiation. Induced Treg (iTreg) originate upon T cell receptor (TCR) stimulation in the presence of tumor growth factor beta. Although under homeostatic conditions most Treg in the periphery are nTreg, special immune challenges in the intestine promote more frequently the generation of iTreg. Furthermore, recent observations have challenged the notion that Treg are a stable sublineage, and they suggest that, particularly under lymphopenic and/or inflammatory conditions, Treg may lose Foxp3 and/or acquire diverse effector functions, especially in the intestine, which may contribute to uncontrolled inflammation.

An update on regulatory T cells in transplant tolerance and rejection

Several types of T cells with immunosuppressive properties have been identified, but FOXP3(+) regulatory T (T(REG)) cells have emerged as a dominant cell type; they are critically involved in the induction and maintenance of immune tolerance. Manipulation of this cell type for the induction of transplant tolerance including renal transplant tolerance has attracted considerable attention. Studies in this area have demonstrated unexpected complexities, and attempts to translate T(REG) cells towards clinical utility have met with unanticipated difficulties. In this Review, a broad overview is provided on recent progress in the study of T(REG) cells, focusing on challenges, opportunities, and emerging approaches in exploiting T(REG) cells for the induction of transplant tolerance.

Anti-CD3 therapy permits regulatory T cells to surmount T cell receptor-specified peripheral niche constraints

Treatment with anti-CD3 is a promising therapeutic approach for autoimmune diabetes, but its mechanism of action remains unclear. Foxp3⁺ regulatory T (T reg) cells may be involved, but the evidence has been conflicting. We investigated this issue in mice derived from the NOD model, which were engineered so that T reg populations were perturbed, or could be manipulated by acute ablation or transfer. The data highlighted the involvement of Foxp3⁺ cells in anti-CD3 action. Rather than a generic influence on all T reg cells, the therapeutic effect seemed to involve an ∼50–60-fold expansion of previously constrained T reg cell populations; this expansion occurred not through conversion from Foxp3⁻ conventional T (T conv) cells, but from a proliferative expansion. We found that T reg cells are normally constrained by TCR-specific niches in secondary lymphoid organs, and that intraclonal competition restrains their possibility for conversion and expansion in the spleen and lymph nodes, much as niche competition limits their selection in the thymus. The strong perturbations induced by anti-CD3 overcame these niche limitations, in a process dependent on receptors for interleukin-2 (IL-2) and IL-7.

Identification of an immediate Foxp3(-) precursor to Foxp3(+) regulatory T cells in peripheral lymphoid organs of nonmanipulated mice

CD4⁺CD25⁺ regulatory T cells (T reg cells) expressing the transcription factor Foxp3 can be induced from peripheral T cell receptor (TCR) transgenic CD4⁺CD25⁻Foxp3⁻ T cells stimulated with noninflammatory dendritic cells presenting low amounts of agonist cognate antigen. However, limited evidence exists for extra-thymic T reg cell generation from non-TCR transgenic T cells in unmanipulated mice. We compared events early during agonist-driven generation of Foxp3⁺ TCR transgenic T cells to polyclonal CD4⁺ T cell populations in unmanipulated mice. We identified an interleukin-2– and phosphatidylinositol-3-kinase–dependent precommitted Foxp3⁻ precursor to Foxp3⁺ T reg cells in peripheral lymphoid organs. Transforming growth factor β signaling played a minor role in the generation and subsequent differentiation of these T reg precursor cells.

CD28 facilitates the generation of Foxp3(-) cytokine responsive regulatory T cell precursors

The T cell costimulatory molecule CD28 plays an important role in the thymic generation of Foxp3(+) regulatory T cells (Tregs) essential for the maintenance of self-tolerance. In this study, we show that a cell-intrinsic signal from CD28 is involved in the generation of cytokine-responsive Foxp3(-) precursors using studies of mixed bone marrow chimeras as well as TCR-specific generation of Foxp3(+) cells using intrathymic transfer of TCR-transgenic thymocytes expressing a natural Treg TCR. Contrary to a previous report, the analysis of CD28 mutant knockin mice revealed that this cell-intrinsic signal is only partially dependent on the Lck-binding PYAP motif. Surprisingly, even though the absence of CD28 resulted in a 6-fold decrease in thymic Tregs, the TCR repertoires of CD28-deficient and sufficient cells were largely overlapping. Thus, these data suggest that CD28 does not operate by markedly enlarging the repertoire of TCRs available for Treg development, but rather by improving the efficiency of Treg development of thymocytes expressing natural Treg TCRs.

Different proliferative potential and migratory characteristics of human CD4+ regulatory T cells that express either CD45RA or CD45RO

Although human naturally occurring regulatory T cells (Tregs) may express either CD45RA or CD45RO, we find in agreement with previous reports that the ( approximately 80%) majority of natural Tregs in adults are CD45RO(+). The proportion of CD45RA(+) Tregs decreases, whereas CD45RO(+) Tregs increase significantly with age. Nevertheless, a small proportion of CD45RA(+) Tregs are found even in old (>80 y) adults and a proportion of these express CD31, a marker for recent thymic emigrants. We found that CD45RO(+) Tregs were highly proliferative compared with their CD45RA(+) counterparts. This was due in part to the conversion of CD45RA Tregs to CD45RO expression after activation. Another difference between these two Treg populations was their preferential migration to different tissues in vivo. Whereas CD45RA(+) Tregs were preferentially located in the bone marrow, associated with increased CXCR4 expression, CD45RO(+) Tregs were preferentially located in the skin, and this was associated with their increased expression of CLA and CCR4. Our studies therefore show that proliferation features strongly in maintenance of the adult Treg pool in humans and that the thymus may make a minor contribution to the maintenance of the peripheral pool of these cells, even in older adults. Furthermore, the different tissue compartmentalization of these cells suggests that different Treg niches exist in vivo, which may have important roles for their maturation and function.

T cell receptor CDR3 sequence but not recognition characteristics distinguish autoreactive effector and Foxp3(+) regulatory T cells

The source, specificity, and plasticity of the forkhead box transcription factor 3 (Foxp3)(+) regulatory T (Treg) and conventional T (Tconv) cell populations active at sites of autoimmune pathology are not well characterized. To evaluate this, we combined global repertoire analyses and functional assessments of isolated T cell receptors (TCR) from TCRalpha retrogenic mice with autoimmune encephalomyelitis. Treg and Tconv cell TCR repertoires were distinct, and autoantigen-specific Treg and Tconv cells were enriched in diseased tissue. Autoantigen sensitivity and fine specificity of these cells intersected, implying that differences in responsiveness were not responsible for lineage specification. Notably, autoreactive Treg and Tconv cells could be fully distinguished by an acidic versus aliphatic variation at a single TCR CDR3 residue. Our results imply that ontogenically distinct Treg and Tconv cell repertoires with convergent specificities for autoantigen respond during autoimmunity and argue against more than limited plasticity between Treg and Tconv cells during autoimmune inflammation.