Cutting edge: size and diversity of CD4+CD25high Foxp3+ regulatory T cell repertoire in humans: evidence for similarities and partial overlapping with CD4+CD25- T cells

Obesity-induced thymic involution and cancer risk

Declining thymic functions associated either with old age (i.e., age-related thymic involution), or with acute involution as a result of stress, infectious disease, or cytoreductive therapies (e.g., chemotherapy/radiotherapy), have been associated with cancer development. A key mechanism underlying such increased cancer risk is the thymus-dependent debilitation of adaptive immunity, which is responsible for orchestrating immunoediting mechanisms and tumor immune surveillance. In the past few years, a blooming set of evidence has intriguingly linked obesity with cancer development and progression. The majority of such studies has focused on obesity-driven chronic inflammation, steroid/sex hormone and adipokine production, and hyperinsulinemia, as principal factors affecting the tumor microenvironment and driving the development of primary malignancy. However, experimental observations about the negative impact of obesity on T cell development and maturation have existed for more than half a century. Here, we critically discuss the molecular and cellular mechanisms of obesity-driven thymic involution as a previously underrepresented intermediary pathology leading to cancer development and progression. This knowledge could be especially relevant in the context of childhood obesity, because impaired thymic function in young individuals leads to immune system abnormalities, and predisposes to various pediatric cancers. A thorough understanding behind the molecular and cellular circuitries governing obesity-induced thymic involution could therefore help towards the rationalized development of targeted thymic regeneration strategies for obese individuals at high risk of cancer development.

Human papilloma virus-16-specific CD8+ T-cell expansions characterize different clinical forms of lichen planus and not lichen sclerosus et atrophicus

Lichen planus (LP) is a cutaneomucosal chronic inflammatory disease characterized by a CD8+ cytotoxic T-lymphocytes (CTL) infiltrate. In erosive oral LP, we found HPV16-specific activated CTL in lesions, supporting a pathogenic contribution of HPV16. Here, we investigated whether a similar scenario occurs in other clinical forms of LP and in lichen sclerosus et atrophicus (LSA), another chronic disease also affecting the mucosa and/or the skin. Blood CTL from LP and LSA patients expressed significant higher levels of granzyme B, perforin and CD107a proteins than healthy donors. Expansions of TCRVß3+ CTL, with presence of TCR clonotypes identical to those previously detected in erosive oral LP, were found both in blood and mucosal/skin lesions of LP, and not of LSA patients. These expansions were enriched with HPV16-specific CD8+ T-cells as shown by their recognition of the E711-20 immunodominant epitope. In LSA patients, the peripheral repertoire of CTL was oligoclonal for TCRVß6+ CTL. Finally, although patients with LP and LSA have developed antibodies against HPV16 capsid L1, antibodies against HPV16 E6 were only observed in patients with LP. Overall, our data collectively suggest an involvement of HPV16-specific CTL in different clinical forms of LP, not only in erosive oral LP, while a different scenario operates in LSA.

Research advances on targeted-Treg therapies on immune-mediated kidney diseases

The primary function of regulatory T cells (Tregs) is blocking the pathogenic immunological response mediated by autoreactive cells, establishing and maintaining immune homeostasis in tissues. Kidney diseases are often caused by Immune imbalance, including alloimmune graft damage after renal transplantation, direct immune-mediated kidney diseases like membranous nephropathy (MN) and anti-glomerular basement membrane (anti-GBM) glomerulonephritis, as well as indirect immune-mediated ones like Anti-neutrophil cytoplasmic antibody-associated vasculitis (AAVs), IgA nephropathy (IgAN) and lupus nephritis (LN). Treg cells are deficient numerically and/or functionally in those kidney diseases. Targeted-Treg therapies, including adoptive Tregs transfer therapy and low-dose IL-2 therapy, have begun to thrive in treating autoimmune diseases in recent years. However, the clinical use of targeted Treg-therapies is rarely mentioned in those kidney diseases above except for kidney transplantation. This article mainly discusses the newest progressions of targeted-Treg therapies in those specific examples of immune-mediated kidney diseases. Meanwhile, we also reviewed the main factors that affect Treg development and differentiation, hoping to inspire new strategies to develop target Tregs-therapies. Lastly, we emphasize the significant impediments and prospects to the clinical translation of target-Treg therapy. We advocate for more preclinical and clinical studies on target Tregs-therapies to decipher Tregs in those diseases.

Antigen-Specific Regulatory T Cell Therapy in Autoimmune Diseases and Transplantation

Regulatory T (Treg) cells are a heterogenous population of immunosuppressive T cells whose therapeutic potential for the treatment of autoimmune diseases and graft rejection is currently being explored. While clinical trial results thus far support the safety and efficacy of adoptive therapies using polyclonal Treg cells, some studies suggest that antigen-specific Treg cells are more potent in regulating and improving immune tolerance in a disease-specific manner. Hence, several approaches to generate and/or expand antigen-specific Treg cells in vitro or in vivo are currently under investigation. However, antigen-specific Treg cell therapies face additional challenges that require further consideration, including the identification of disease-relevant antigens as well as the in vivo stability and migratory behavior of Treg cells following transfer. In this review, we discuss these approaches and the potential limitations and describe prospective strategies to enhance the efficacy of antigen-specific Treg cell treatments in autoimmunity and transplantation.

Tregs self-organize into a computing ecosystem and implement a sophisticated optimization algorithm for mediating immune response

Regulatory T cells (Tregs) play a crucial role in mediating immune response. Yet an algorithmic understanding of the role of Tregs in adaptive immunity remains lacking. Here, we present a biophysically realistic model of Treg-mediated self-tolerance in which Tregs bind to self-antigens and locally inhibit the proliferation of nearby activated T cells. By exploiting a duality between ecological dynamics and constrained optimization, we show that Tregs tile the potential antigen space while simultaneously minimizing the overlap between Treg activation profiles. We find that for sufficiently high Treg diversity, Treg-mediated self-tolerance is robust to fluctuations in self-antigen concentrations but lowering the Treg diversity results in a sharp transition-related to the Gardner transition in perceptrons-to a regime where changes in self-antigen concentrations can result in an autoimmune response. We propose an experimental test of this transition in immune-deficient mice and discuss potential implications for autoimmune diseases.

Regulatory T Cells in the Tumor Microenvironment

Regulatory T cells (T_regs) are an immunosuppressive subpopulation of CD4⁺ T cells that are endowed with potent suppressive activity and function to limit immune activation and maintain homeostasis. These cells are identified by the hallmark transcription factor FOXP3 and the high-affinity interleukin-2 (IL-2) receptor chain CD25. T_regs can be recruited to and persist within the tumor microenvironment (TME), acting as a potent barrier to effective antitumor immunity. This chapter will discuss [i] the history and hallmarks of T_regs; [ii] the recruitment, development, and persistence of T_regs within the TME; [iii] T_reg function within TME; asnd [iv] the therapeutic targeting of T_regs in the clinic. This chapter will conclude with a discussion of likely trends and future directions.

Treg cell-based therapies: challenges and perspectives

Cellular therapies using regulatory T (Treg) cells are currently undergoing clinical trials for the treatment of autoimmune diseases, transplant rejection and graft-versus-host disease. In this Review, we discuss the biology of Treg cells and describe new efforts in Treg cell engineering to enhance specificity, stability, functional activity and delivery. Finally, we envision that the success of Treg cell therapy in autoimmunity and transplantation will encourage the clinical use of adoptive Treg cell therapy for non-immune diseases, such as neurological disorders and tissue repair.

The establishment of immune infiltration based novel recurrence predicting nomogram in prostate cancer

Prostate cancer (PCa), a severe health burden for males, accounts for the second frequent cancer and fifth tumor specific death cancer around the world. Several studies on tumor-infiltrating immune cells (TIICs) have shown inconsistent and controversial results to PCa. We downloaded a gene expression matrix and clinical information from TCGA, and CIBERSORT was used to identify the proportion of TIICs. Wilcoxon's Sign Rank Test evaluated different gene expression levels in PCa and normal tissues. Kaplan-Meier curves were used to evaluate the associations of TIICs and recurrence-free survival (RFS). Finally, based on the preset P-value of .05, the distribution of TIICs in 73 PCa tissues and 11 normal tissues was illustrated. Activated CD4⁺ T cells and M0 macrophages account for a high proportion in PCa tissues, while neutrophils and monocytes were found at a high density in normal tissues. Further results showed that the density of plasma cells, Treg cells and resting mast cells were associated with advanced PCa. Additionally, M2 macrophages affected the RFS of PCa patients, and AR was also involved. In the current study, we first evaluated the immune infiltration among PCa and revealed that M2 macrophages could predict the prognosis of PCa patients. Meanwhile, based on the LASSO regression analysis, we established a novel nomogram to assess the recurrence risk of PCa based on immune cell proportions and clinical features.

Human LAP+GARP+FOXP3+ regulatory T cells attenuate xenogeneic graft versus host disease

Adoptive transfer of regulatory T cells (FOXP3⁺ Tregs) has been developed as a potential curative immune therapy to prevent and treat autoimmune and graft-versus-host diseases (GVHD). A major limitation that has hindered the use of Treg immunotherapy in humans is the difficulty of consistently isolating and obtaining highly purified Tregs after ex vivo expansion. Methods: We isolated bona fide Tregs from expansion cultures based on their selective surface expression of latency-associated peptide (LAP). The TCR Vβ diversity and intracellular cytokine production of Tregs were determined by flow cytometer. The TSDR methylation was determined by epigenetic human FOXP3 qPCR Assay. Their in vitro and in vivo potency was confirmed with suppression assay and humanized xenogeneic GVHD (xGVHD) murine model, respectively. Results: LAP⁺ repurification results in >90% LAP⁺FOXP3⁺ Tregs, leaving behind FOXP3^- and FOXP3⁺ nonTregs within the LAP^- population. After 4-week expansion, the LAP⁺ Tregs were >1 billion cells, highly suppressive and anergic in vitro, >90% demethylated in the TSDR and able to maintain TCR Vβ diversity. In the xGVHD model, exogenous CD25^-PBMC administered alone results in a median survival of 32 days. The co-transfer of LAP⁺ Tregs increased median survival to 47 days, while the LAP parent (CD25⁺) and LAP^- nonTregs had median survival of 39 and 31 days, respectively. Conclusions: These preclinical data together provide evidence that LAP⁺ Tregs are highly purified with fully suppressive function for cell therapy. This population results in a more effective and safer product for immunotherapy to treat GVHD and provides the necessary preclinical data for transition into a clinical trial with LAP⁺ Tregs to prevent or treat GVHD and other autoimmune diseases.

Several Follicular Regulatory T Cell Subsets With Distinct Phenotype and Function Emerge During Germinal Center Reactions

An efficient B cell immunity requires a dynamic equilibrium between positive and negative signals. In germinal centers (GCs), T follicular helper cells are supposed to be the positive regulator while T follicular regulatory (Tfr) cells were assigned to be the negative regulators. Indeed, Tfr cells are considered as a homogenous cell population dedicated to dampen the GC extent. Moreover, Tfr cells prevent autoimmunity since their dysregulation leads to production of self-reactive antibodies (Ab). However, a growing corpus of evidence has revealed additional and unexpected functions for Tfr cells in the regulation of B cell responses. This review provides an overview of the Tfr cell contribution and presents Tfr cell proprieties in the context of vaccination.

Clonal Bifurcation of Foxp3 Expression Visualized in Thymocytes and T Cells

Regulatory T cells (Tregs) are crucial for suppressing autoimmunity and inflammation mediated by conventional T cells. To be useful, some Tregs should have overlapping specificity with relevant self-reactive or pathogen-specific clones. Whether matching recognition between Tregs and non-Tregs might arise through stochastic or deterministic mechanisms has not been addressed. We tested the hypothesis that some Tregs that arise in the thymus or that are induced during Ag-driven expansion of conventional CD4⁺ T cells might be clonally related to non-Tregs by virtue of asymmetric Foxp3 induction during cell division. We isolated mouse CD4⁺ thymocytes dividing in vivo, wherein sibling cells exhibited discordant expression of Foxp3 and CD25. Under in vitro conditions that stimulate induced Tregs from conventional mouse CD4⁺ T cells, we found a requirement for cell cycle progression to achieve Foxp3 induction. Moreover, a substantial fraction of sibling cell pairs arising from induced Treg stimulation also contained discordant expression of Foxp3. Division-linked yet asymmetric induction of Treg fate offers potential mechanisms to anticipate peripheral self-reactivity during thymic selection as well as produce precise, de novo counterregulation during CD4⁺ T cell–mediated immune responses.

Clonal Analysis of Regulatory T Cell Defect in Patients with Autoimmune Polyendocrine Syndrome Type 1 Suggests Intrathymic Impairment

Mutations in the autoimmune regulator gene disrupt thymic T cell development and negative selection, leading to the recessively inherited polyendocrine autoimmune disease autoimmune polyendocrine syndrome type 1 (APS-1). The patients also have a functional defect in the FOXP3+ regulatory T cell population, but its origin is unclear. Here, we have used T cell receptor sequencing to analyse the clonal relationship of major CD4+ T cell subsets in three patients and three healthy controls. The naive regulatory T cells showed little overlap with helper T cell subsets, supporting divergence in the thymus. The activated/memory regulatory T cell subset displayed more sharing with helper T cells, but was mainly recruited from the naive regulatory T cell population. These clonal patterns were very similar in both patients and controls. However, naive regulatory T cells isolated from the patients had a significantly longer T cell receptor complementarity-determining region 3 than any other population, suggesting failure of thymic selection. These data indicate that the peripheral differentiation of regulatory T cells in APS-1 patients is not different from that in healthy controls. Rather, the patients' naive regulatory T cells may have an intrinsic defect imprinted already in the thymus.

Next-Generation Sequencing Reveals Restriction and Clonotypic Expansion of Treg Cells in Juvenile Idiopathic Arthritis

OBJECTIVE

Treg cell-mediated suppression of Teff cells is impaired in juvenile idiopathic arthritis (JIA); however, the basis for this dysfunction is incompletely understood. Animal models of autoimmunity and immunodeficiency demonstrate that a diverse Treg cell repertoire is essential to maintain Treg cell function. The present study was undertaken to investigate the Treg and Teff cell repertoires in JIA.

METHODS

Treg cells (CD4+CD25+CD127(low) ) and Teff cells (CD4+CD25-) were isolated from peripheral blood and synovial fluid obtained from JIA patients, healthy controls, and children with Lyme arthritis. Treg cell function was measured in suppressive assays. The T cell receptor β chain (TRB) was amplified by multiplex polymerase chain reaction and next-generation sequencing was performed, with amplicons sequenced using an Illumina HiSeq platform. Data were analyzed using ImmunoSEQ, International ImMunoGeneTics system, and the Immunoglobulin Analysis Tools.

RESULTS

Compared to findings in controls, the JIA peripheral blood Treg cell repertoire was restricted, and clonotypic expansions were found in both blood and synovial fluid Treg cells. Skewed usage and pairing of TRB variable and joining genes, including overuse of gene segments that have been associated with other autoimmune conditions, was observed. JIA patients shared a substantial portion of synovial fluid Treg cell clonotypes that were private to JIA and not identified in Lyme arthritis.

CONCLUSION

We identified restriction and clonotypic expansions in the JIA Treg cell repertoire with sharing of Treg cell clonotypes across patients. These findings suggest that abnormalities in the Treg cell repertoire, possibly engendered by shared antigenic triggers, may contribute to disease pathogenesis in JIA.

Deep sequencing of the TCR-β repertoire of human forkhead box protein 3 (FoxP3)+ and FoxP3- T cells suggests that they are completely distinct and non-overlapping

Maintenance of peripheral tolerance requires a balance between autoreactive conventional T cells (T_conv ) and thymically derived forkhead box protein 3 (FoxP3)⁺ regulatory T cells (tT_regs ). Considerable controversy exists regarding the similarities/differences in T cell receptor (TCR) repertoires expressed by T_conv and tT_regs . We generated highly purified populations of human adult and cord blood T_conv and tT_regs based on the differential expression of CD25 and CD127. The purity of the sorted populations was validated by intracellular staining for FoxP3 and Helios. We also purified an overlap group of CD4 T cells from adult donors to ensure that considerable numbers of shared clonotypes could be detected when present. We used deep sequencing of entire TCR-β CDR3 sequences to analyse the TCR repertoire of T_conv and tT_regs . Our studies suggest that both neonatal and adult human T_conv and tT_reg cells are, in fact, entirely distinct CD4 T cell lineages.

T cell epitope redundancy: cross-conservation of the TCR face between pathogens and self and its implications for vaccines and autoimmunity

T cells are extensively trained on 'self' in the thymus and then move to the periphery, where they seek out and destroy infections and regulate immune response to self-antigens. T cell receptors (TCRs) on T cells' surface recognize T cell epitopes, short linear strings of amino acids presented by antigen-presenting cells. Some of these epitopes activate T effectors, while others activate regulatory T cells. It was recently discovered that T cell epitopes that are highly conserved on their TCR face with human genome sequences are often associated with T cells that regulate immune response. These TCR-cross-conserved or 'redundant epitopes' are more common in proteins found in pathogens that have co-evolved with humans than in other non-commensal pathogens. Epitope redundancy might be the link between pathogens and autoimmune disease. This article reviews recently published data and addresses epitope redundancy, the "elephant in the room" for vaccine developers and T cell immunologists.

Autologous stem cell transplantation aids autoimmune patients by functional renewal and TCR diversification of regulatory T cells

Autologous HSCT induces functional renewal of regulatory T cells as well as a strong Treg TCR diversification in autoimmune patients. Adding regulatory T cells to the graft does not lead to additional clinical improvement but results in delayed donor T-cell reconstitution.

Human CD45RA(-) FoxP3(hi) Memory-Type Regulatory T Cells Show Distinct TCR Repertoires With Conventional T Cells and Play an Important Role in Controlling Early Immune Activation

Adoptive immunotherapy with regulatory T cells (Treg) is a new option to promote immune tolerance following solid organ transplantation (SOT). However, Treg from elderly patients awaiting transplantation are dominated by the CD45RA(-) CD62L(+) central memory type Treg subset (TregCM), and the yield of well-characterized and stable naïve Treg (TregN) is low. It is, therefore, important to determine whether these TregCM are derived from the thymus and express high stability, suppressive capacity and a broad antigen repertoire like TregN. In this study, we showed that TregCM use a different T cell receptor (TCR) repertoire from conventional T cells (Tconv), using next-generation sequencing of all 24 Vβ families, with an average depth of 534 677 sequences. This showed almost no contamination with induced Treg. Furthermore, TregCM showed enhanced suppressive activity on Tconv at early checkpoints of immune activation controlling activation markers expression and cytokine secretion, but comparable inhibition of proliferation. Following in vitro expansion under mTOR inhibition, TregCM expanded equally as well as TregN without losing their function. Despite relatively limited TCR repertoire, TregCM also showed specific alloresponse, although slightly reduced compared to TregN. These results support the therapeutic usefulness of manufacturing Treg products from CD45RA(-) CD62L(+) Treg-enriched starting material to be applied for adoptive Treg therapy.

Regulatory T-Cell Development in the Human Thymus

The thymus generates a lineage-committed subset of regulatory T-cells (Tregs), best identified by the expression of the transcription factor FOXP3. The development of thymus-derived Tregs is known to require high-avidity interaction with MHC-self peptides leading to the generation of self-reactive Tregs fundamental for the maintenance of self-tolerance. Notwithstanding their crucial role in the control of immune responses, human thymic Treg differentiation remains poorly understood. In this mini-review, we will focus on the developmental stages at which Treg lineage commitment occurs, and their spatial localization in the human thymus, reviewing the molecular requirements, including T-cell receptor and cytokine signaling, as well as the cellular interactions involved. An overview of the impact of described thymic defects on the Treg compartment will be provided, illustrating the importance of these in vivo models to investigate human Treg development.

TCR sequences and tissue distribution discriminate the subsets of naïve and activated/memory Treg cells in mice

Analyses of the regulatory T (Treg) cell TCR repertoire should help elucidate the nature and diversity of their cognate antigens and thus how Treg cells protect us from autoimmune diseases. We earlier identified CD44(hi) CD62L(low) activated/memory (am) Treg cells as a Treg-cell subset with a high turnover and possible self-specificity. We now report that amTreg cells are predominantly distributed in lymph nodes (LNs) draining deep tissues. Multivariate analyses of CDR3 spectratyping first revealed that amTreg TCR repertoire is different from that of naïve Treg cells (nTreg cells) and effector T (Teff) cells. Furthermore, in deep- versus superficial LNs, TCR-β deep sequencing further revealed diversified nTreg-cell and amTreg-cell repertoires, although twofold less diverse than that of Teff cells, and with repertoire richness significantly lower in deep-LN versus superficial-LN Treg cells. Importantly, expanded clonotypes were mostly detected in deep-LN amTreg cells, some accounting for 20% of the repertoire. Strikingly, these clonotypes were absent from nTreg cells, but found at low frequency in Teff cells. Our results, obtained in nonmanipulated mice, indicate different antigenic targets for naïve and amTreg cells and that amTreg cells are self-specific. The data we present are consistent with an instructive component in Treg-cell differentiation.

The bone marrow of myeloma patients is steadily inhabited by a normal-sized pool of functional regulatory T cells irrespectiveof the disease status

Conflicting data have been reported about the frequency and function of regulatory T cells in multiple myeloma. Most studies have investigated peripheral blood rather than bone marrow Tregs and side-by-side comparisons with bone marrow from healthy donors have still not been made. In this study, we show that regulatory T-cells total count, subset distribution, and expression of chemokine receptors are similar in the bone marrow of myeloma patients and healthy donors. Regulatory T cells are not recruited by myeloma cells in the bone marrow and their counts are unaffected by the tumor burden and the disease status. The diversity of T-cell receptor repertoire is highly preserved ensuring broad reactivity and effective suppressor function. Our results indicate that regulatory T cells may not be the main players of immunological tolerance to myeloma cells under base-line conditions, but their fully preserved immune competence may promote their inadvertent activation and blunt T-cell driven anti-myeloma immune interventions even after myeloma cells have successfully been cleared by chemotherapy.

Induced regulatory T cells in inhibitory microenvironments created by cancer

INTRODUCTION

Regulatory T cells (Tregs) accumulating in the peripheral circulation and tumor sites of patients contribute to tumor escape from the host immune system. Tregs encompass subsets of immune cells with distinct phenotypic and functional properties. Whereas natural (n) or thymic-derived (t) Tregs regulate responses to self-antigens, inducible (i) or peripheral (p) Tregs generated and expanded in regulatory microenvironments control immune responses to a broad variety of antigens.

AREAS COVERED

Tregs accumulating in the tumor microenvironment (TME) are contextually regulated. They acquire phenotypic and functional attributes imposed by the inhibitory molecular pathways operating in situ. Several molecular pathways active in human cancer are reviewed. The pathways may differ from one tumor to another, and environmentally induced Tregs may be functionally distinct. Potential therapeutic strategies for selective silencing of iTregs are considered in the light of the newly acquired understanding of their phenotypic and functional diversity.

EXPERT OPINION

Human Tregs accumulating in cancer comprise 'bad' subsets, which inhibit antitumor immunity, and 'good' anti-inflammatory subsets, which maintain tolerance to self and benefit the host. Future therapeutic strategies targeting Tregs will need to discriminate between these Treg subsets and will need to consider reprogramming strategies instead of Treg elimination. Re-establishment of effective antitumor immune responses in cancer patients without disturbing a normal homeostatic T-cell balance will greatly benefit from insights into inhibitory pathways engaged by human tumors.

Memory regulatory T cells reside in human skin

Regulatory T cells (Tregs), which are characterized by expression of the transcription factor Foxp3, are a dynamic and heterogeneous population of cells that control immune responses and prevent autoimmunity. We recently identified a subset of Tregs in murine skin with properties typical of memory cells and defined this population as memory Tregs (mTregs). Due to the importance of these cells in regulating tissue inflammation in mice, we analyzed this cell population in humans and found that almost all Tregs in normal skin had an activated memory phenotype. Compared with mTregs in peripheral blood, cutaneous mTregs had unique cell surface marker expression and cytokine production. In normal human skin, mTregs preferentially localized to hair follicles and were more abundant in skin with high hair density. Sequence comparison of TCRs from conventional memory T helper cells and mTregs isolated from skin revealed little homology between the two cell populations, suggesting that they recognize different antigens. Under steady-state conditions, mTregs were nonmigratory and relatively unresponsive; however, in inflamed skin from psoriasis patients, mTregs expanded, were highly proliferative, and produced low levels of IL-17. Taken together, these results identify a subset of Tregs that stably resides in human skin and suggest that these cells are qualitatively defective in inflammatory skin disease.

Clonotype-specific avidity influences the dynamics and hierarchy of virus-specific regulatory and effector CD4(+) T-cell responses

A key component of immunity against viruses, CD4(+) T cells expand and differentiate into functional subsets upon primary infection, where effector (Teff) cells facilitate infection control and regulatory (Treg) cells mitigate immunopathology. After secondary infection, Teff cells mount a robust response from the memory pool. Here, we show that Treg-cell responses are diminished upon secondary infection, and Treg-cell response dynamics are associated more with T-cell receptors (TCRs) repertoire and avidity than with epitope specificity. In the murine model, the IA(b) M209 epitope of respiratory syncytial virus is recognized by both CD4(+) Treg and Teff cells, while the IA(b) M226 epitope is recognized almost exclusively by CD4(+) Teff cells expressing high avidity TCR Vβ8.1/8.2 and dominating the CD4(+) T-cell response during primary and secondary infections. IA(b) M209 -Teff cells express relatively low avidity TCRs during early primary infection, but high avidity TCR Vβ7-expressing IA(b) M209 -Teff cells emerge during the late phase, and become dominant after secondary infection. The emerging high avidity IA(b) M209 -Teff cells outcompete IA(b) M209 -Treg cells that share the same epitope, but have low avidity and are restricted to TCR Vβ2 and Vβ6 subpopulations. These data indicate that MHC-peptide-TCR interactions can produce different kinetic and functional profiles in CD4(+) T-cell populations even when responding to the same epitope.

Peripherally induced tregs - role in immune homeostasis and autoimmunity

Thymically derived Foxp3(+) regulatory T cells (tTregs) constitute a unique T cell lineage that is essential for maintaining immune tolerance to self and immune homeostasis. However, Foxp3 can also be turned on in conventional T cells as a consequence of antigen exposure in the periphery, under both non-inflammatory and inflammatory conditions. These so-called peripheral Tregs (pTregs) participate in the control of immunity at sites of inflammation, especially at the mucosal surfaces. Although numerous studies have assessed in vitro generated Tregs (termed induced or iTregs), these cells most often do not recapitulate the functional or phenotypic characteristics of in vivo generated pTregs. Thus, there are still many unanswered questions regarding the T cell receptor (TCR) repertoire and function of pTregs as well as conditions under which they are generated in vivo, and the degree to which these characteristics identify specialized features of pTregs versus features that are shared with tTregs. In this review, we summarize the current state of our understanding of pTregs and their relationship to the tTreg subset. We describe the recent discovery of unique cell surface markers and transcription factors (including Neuropilin-1 and Helios) that can be used to distinguish tTreg and pTreg subsets in vivo. Additionally, we discuss how the improved ability to distinguish these subsets provided new insights into the biology of tTregs versus pTregs and suggested differences in their function and TCR repertoire, consistent with a unique role of pTregs in certain inflammatory settings. Finally, these recent advances will be used to speculate on the role of individual Treg subsets in both tolerance and autoimmunity.

Thymus transplantation restores the repertoires of forkhead box protein 3 (FoxP3)+ and FoxP3- T cells in complete DiGeorge anomaly

The development of T cells with a regulatory phenotype after thymus transplantation has not been examined previously in complete DiGeorge anomaly (cDGA). Seven athymic infants with cDGA and non-maternal pretransplantation T cell clones were assessed. Pretransplantation forkhead box protein 3 (Foxp3)(+) T cells were detected in five of the subjects. Two subjects were studied in greater depth. T cell receptor variable β chain (TCR-Vβ) expression was assessed by flow cytometry. In both subjects, pretransplantation FoxP3(+) and total CD4(+) T cells showed restricted TCR-Vβ expression. The development of naive T cells and diverse CD4(+) TCR-Vβ repertoires following thymic transplantation indicated successful thymopoiesis from the thymic tissue grafts. Infants with atypical cDGA develop rashes and autoimmune phenomena before transplantation, requiring treatment with immunosuppression, which was discontinued successfully subsequent to the observed thymopoiesis. Post-transplantation, diverse TCR-Vβ family expression was also observed in FoxP3(+) CD4(+) T cells. Interestingly, the percentages of each of the TCR-Vβ families expressed on FoxP3(+) and total CD4(+) T cells differed significantly between these T lymphocyte subpopulations before transplantation. By 16 months post-transplantation, however, the percentages of expression of each TCR-Vβ family became significantly similar between FoxP3(+) and total CD4(+) T cells. Sequencing of TCRBV DNA confirmed the presence of clonally amplified pretransplantation FoxP3(+) and FoxP3(-) T cells. After thymus transplantation, increased polyclonality was observed for both FoxP3(+) and FoxP3(-) cells, and pretransplantation FoxP3(+) and FoxP3(-) clonotypes essentially disappeared. Thus, post-transplantation thymic function was associated with the development of a diverse repertoire of FoxP3(+) T cells in cDGA, corresponding with immunological and clinical recovery.

High-throughput sequencing of B- and T-lymphocyte antigen receptors in hematology

Application of high-throughput DNA sequencing to the analysis of B- and T-lymphocyte antigen receptors has great potential for improving the monitoring of lymphoid malignancies, assessing immune reconstitution after hematopoietic cell transplantation, and characterizing the composition of lymphocyte repertoires. Current technology can define the number and frequency of immunoglobulin heavy, T-cell receptor (TCR)α, TCRβ, or TCRγ chains expressed in a population of lymphocytes; techniques for determining the number of antigen receptor heterodimers, such as TCRαβ pairs, expressed in the population are under development.

A cancer vaccine induces expansion of NY-ESO-1-specific regulatory T cells in patients with advanced melanoma

Cancer vaccines are designed to expand tumor antigen-specific T cells with effector function. However, they may also inadvertently expand regulatory T cells (Treg), which could seriously hamper clinical efficacy. To address this possibility, we developed a novel assay to detect antigen-specific Treg based on down-regulation of surface CD3 following TCR engagement, and used this approach to screen for Treg specific to the NY-ESO-1 tumor antigen in melanoma patients treated with the NY-ESO-1/ISCOMATRIX^TM cancer vaccine. All patients tested had Treg (CD25^bright FoxP3⁺ CD127^neg) specific for at least one NY-ESO-1 epitope in the blood. Strikingly, comparison with pre-treatment samples revealed that many of these responses were induced or boosted by vaccination. The most frequently detected response was toward the HLA-DP4-restricted NY-ESO-1_157–170 epitope, which is also recognized by effector T cells. Notably, functional Treg specific for an HLA-DR-restricted epitope within the NY-ESO-1_115–132 peptide were also identified at high frequency in tumor tissue, suggesting that NY-ESO-1-specific Treg may suppress local anti-tumor immune responses. Together, our data provide compelling evidence for the ability of a cancer vaccine to expand tumor antigen-specific Treg in the setting of advanced cancer, a finding which should be given serious consideration in the design of future cancer vaccine clinical trials.

Immunological tolerance during fetal development: from mouse to man

The development of the adaptive immune system has been studied in the mouse primarily because it is easier to access fetal tissues and because there exists a rich array of probes for analysis of various components of the immune system. While much has been learned from this exercise, it is also clear that different species show substantial temporal variation in the development of the immune system during early life. In mice, for instance, mature α/β T cells first appear in the periphery during the final stages of fetal gestation and only increase in number after birth (Friedberg and Weissman, 1974); in humans, on the other hand, the first mature α/β T cells are seen in peripheral tissues at 10-12 gestational weeks (g.w.) and are circulating in significant numbers by the end of the second trimester (Ceppellini et al., 1971; Haynes et al., 1988; Hayward and Ezer, 1974; Kay et al., 1970). Although the functional implications of these differences remain unclear, it is likely that there are significant biological consequences associated with the relatively early development of the peripheral adaptive immune system in humans, for example, with respect to the development of peripheral tolerance as well as to the response to antigens that might cross the placenta from the mother (e.g., cells bearing noninherited maternal alloantigens, infectious agents, food antigens, and the like). Here, we will review studies of immune system ontogeny in the mouse and in humans, and then focus on the possible functional roles of fetal T cell populations during development and later in life in humans.

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.

Becoming self-aware: the thymic education of regulatory T cells

The generation of Foxp3(+) regulatory T (Treg) cells in the thymus is essential for immune homeostasis. In the past several years, substantial progress has been made in understanding the mechanisms by which a minor portion of developing thymocytes are selected to become Treg cells. Although previously controversial, recent data support the importance of TCR specificity as a primary determinant for selecting self-reactive thymocytes to become Treg cells in a multi-step process involving cytokines, co-stimulatory molecules, and a variety of antigen-presenting cells. Importantly, the antigenic niche for Treg cell development appears to be typically quite small, implying the recognition of tissue-specific, rather than ubiquitous, self-antigens. Finally, it appears that an NF-κB transcription factor, c-Rel, may be the link between TCR recognition and the induction of Foxp3 expression, which is required for the function and stability of the natural Treg cell population.

Regulatory T cell defect in APECED patients is associated with loss of naive FOXP3(+) precursors and impaired activated population

The pathogenetic mechanisms of organ-specific autoimmune diseases remain obscured by the complexity of the genetic and environmental factors participating in the breakdown of tolerance. A unique opportunity to study the pathogenesis of human autoimmunity is provided by autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), a rare inherited autoimmune disease caused by mutations in Autoimmune Regulator (AIRE) gene. Loss of AIRE function disrupts the deletion of autoreactive T cells and impairs the suppressive function of regulatory T (Treg) cells. Here we show by multiparameter flow cytometry that in healthy controls the peripheral naive Treg cell subset forms a slowly dividing, persistent reservoir of recent thymic emigrants (RTEs). In APECED patients the RTE Treg cells show accelerated turnover and shift to the activated pool and the RTE reservoir is depleted. Moreover, the activated Treg cell population in the patients expresses significantly less Forkhead box protein P3 (FOXP3) than in the healthy controls, consistent with the impairment of peripheral activation. Our results indicate that in addition to their thymic effects, loss-of-function mutations in AIRE disrupt the peripheral homeostasis and activation of Treg cells. This may synergize with failed negative selection to cause APECED.

Early lymphocyte recovery after intensive timed sequential chemotherapy for acute myelogenous leukemia: peripheral oligoclonal expansion of regulatory T cells

Few published studies characterize early lymphocyte recovery after intensive chemotherapy for acute myelogenous leukemia (AML). To test the hypothesis that lymphocyte recovery mirrors ontogeny, we characterized early lymphocyte recovery in 20 consecutive patients undergoing induction timed sequential chemotherapy for newly diagnosed AML. Recovering T lymphocytes were predominantly CD4(+) and included a greatly expanded population of CD3(+)CD4(+)CD25(+)Foxp3(+) T cells. Recovering CD3(+)CD4(+)CD25(+)Foxp3(+) T cells were phenotypically activated regulatory T cells and showed suppressive activity on cytokine production in a mixed lymphocyte reaction. Despite an initial burst of thymopoiesis, most recovering regulatory T cells were peripherally derived. Furthermore, regulatory T cells showed marked oligoclonal skewing, suggesting that their peripheral expansion was antigen-driven. Overall, lymphocyte recovery after chemotherapy differs from ontogeny, specifically identifying a peripherally expanded oligoclonal population of activated regulatory T lymphocytes. These differences suggest a stereotyped immunologic recovery shared by patients with newly diagnosed AML after induction timed sequential chemotherapy. Further insight into this oligoclonal regulatory T-cell population will be fundamental toward developing effective immunomodulatory techniques to improve survival for patients with AML.

Regulatory T cell suppressive potency dictates the balance between bacterial proliferation and clearance during persistent Salmonella infection

The pathogenesis of persistent infection is dictated by the balance between opposing immune activation and suppression signals. Herein, virulent Salmonella was used to explore the role and potential importance of Foxp3-expressing regulatory T cells in dictating the natural progression of persistent bacterial infection. Two distinct phases of persistent Salmonella infection are identified. In the first 3-4 weeks after infection, progressively increasing bacterial burden was associated with delayed effector T cell activation. Reciprocally, at later time points after infection, reductions in bacterial burden were associated with robust effector T cell activation. Using Foxp3(GFP) reporter mice for ex vivo isolation of regulatory T cells, we demonstrate that the dichotomy in infection tempo between early and late time points is directly paralleled by drastic changes in Foxp3(+) Treg suppressive potency. In complementary experiments using Foxp3(DTR) mice, the significance of these shifts in Treg suppressive potency on infection outcome was verified by enumerating the relative impacts of regulatory T cell ablation on bacterial burden and effector T cell activation at early and late time points during persistent Salmonella infection. Moreover, Treg expression of CTLA-4 directly paralleled changes in suppressive potency, and the relative effects of Treg ablation could be largely recapitulated by CTLA-4 in vivo blockade. Together, these results demonstrate that dynamic regulation of Treg suppressive potency dictates the course of persistent bacterial infection.

Lifelong dynamics of human CD4+CD25+ regulatory T cells: insights from in vivo data and mathematical modeling

Despite their limited proliferation capacity, regulatory T cells (T(regs)) constitute a population maintained over the entire lifetime of a human organism. The means by which T(regs) sustain a stable pool in vivo are controversial. Using a mathematical model, we address this issue by evaluating several biological scenarios of the origins and the proliferation capacity of two subsets of T(regs): precursor CD4(+)CD25(+)CD45RO(-) and mature CD4(+)CD25(+)CD45RO(+) cells. The lifelong dynamics of T(regs) are described by a set of ordinary differential equations, driven by a stochastic process representing the major immune reactions involving these cells. The model dynamics are validated using data from human donors of different ages. Analysis of the data led to the identification of two properties of the dynamics: (1) the equilibrium in the CD4(+)CD25(+)FoxP3(+)T(regs) population is maintained over both precursor and mature T(regs) pools together, and (2) the ratio between precursor and mature T(regs) is inverted in the early years of adulthood. Then, using the model, we identified three biologically relevant scenarios that have the above properties: (1) the unique source of mature T(regs) is the antigen-driven differentiation of precursors that acquire the mature profile in the periphery and the proliferation of T(regs) is essential for the development and the maintenance of the pool; there exist other sources of mature T(regs), such as (2) a homeostatic density-dependent regulation or (3) thymus- or effector-derived T(regs), and in both cases, antigen-induced proliferation is not necessary for the development of a stable pool of T(regs). This is the first time that a mathematical model built to describe the in vivo dynamics of regulatory T cells is validated using human data. The application of this model provides an invaluable tool in estimating the amount of regulatory T cells as a function of time in the blood of patients that received a solid organ transplant or are suffering from an autoimmune disease.

Human tumor antigen-specific helper and regulatory T cells share common epitope specificity but exhibit distinct T cell repertoire

CD4(+) regulatory T cells (Tregs) accumulate at tumor sites and play a critical role in the suppression of immune responses against tumor cells. In this study, we show that two immunodominant epitopes derived from the tumor Ags (TAs) NY-ESO-1 and TRAG-3 stimulate both CD4+ Th cells and Tregs. TA-specific Tregs inhibit the proliferation of allogenic T cells, act in a cell-to-cell contact dependent fashion and require activation to suppress IL-2 secretion by T cells. TRAG-3 and NY-ESO-1-specific Tregs exhibit either a Th1-, a Th2-, or a Th0-type cytokine profile and dot not produce IL-10 or TGF-beta. The Foxp3 levels vary from one Treg clone to another and are significantly lower than those of CD4+CD25high Tregs. In contrast to NY-ESO-1-specific Th cells, the NY-ESO-1-specific and TRAG-3-specific Treg clonotypes share a common TCR CDR3 Vbeta usage with Foxp3+CD4+CD25high and CD4+CD25- T cells and were not detectable in PBLs of other melanoma patients and of healthy donors, suggesting that their recruitment occurs through the peripheral conversion of CD4+CD25- T cells upon chronic Ag exposure. Collectively, our findings demonstrate that the same epitopes spontaneously stimulate both Th cells and Tregs in patients with advanced melanoma. They also suggest that TA-specific Treg expansion may be better impaired by therapies aimed at depleting CD4+CD25high Tregs and preventing the peripheral conversion of CD4+CD25- T cells.

Disturbed regulatory T cell homeostasis in multiple sclerosis

The pathological features of multiple sclerosis (MS), a chronic inflammatory disorder of the central nervous system, support an autoimmune etiology. Strong evidence has been provided for a potential functional defect of CD4(+)CD25(+)FOXP3(+) regulatory T cells (Tregs) in patients with relapsing-remitting MS. More recently, alterations in homeostatic parameters related to the development and function of naive and memory-like Tregs were discovered in MS patients. In this review, we evaluate the evidence for disturbed Treg homeostasis in MS and discuss the role of potential compensatory mechanisms in the chronic disease phase. Better insights into the processes underlying the compromised immune regulation in MS patients will be important to understand the potential of Treg-based therapies.

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.

Development of thymically derived natural regulatory T cells

Natural regulatory T cells (nTregs) are defined by their inherent ability to establish and maintain peripheral self-tolerance. In recent years, the development of nTregs has come under close examination with the advent of Forkhead Box P3 protein (FOXP3)-green fluorescent protein reporter mice that pinpointed the initiation of FOXP3 expression within the thymus. The mechanism and pathway of nTreg development has only recently been studied in detail and to a large degree remains unclear. In this review, we will discuss our current understanding of nTreg lineage choice and development from a cellular and intracellular standpoint.

Mammalian target of rapamycin inhibition and alloantigen-specific regulatory T cells synergize to promote long-term graft survival in immunocompetent recipients

Minimization of immunosuppression and donor-specific tolerance to MHC-mismatched organ grafts are important clinical goals. The therapeutic potential of regulatory T cells (Tregs) has been demonstrated, but conditions for optimizing their in vivo function posttransplant in nonlymphocyte-depleted hosts remain undefined. In this study, we address mechanisms through which inhibition of the mammalian target of rapamycin (Rapa) synergizes with alloantigen-specific Treg (AAsTreg) to permit long-term, donor-specific heart graft survival in immunocompetent hosts. Crucially, immature allogeneic dendritic cells allowed AAsTreg selection in vitro, with minimal expansion of unwanted (Th17) cells. The rendered Treg potently inhibited T cell proliferation in an Ag-specific manner. However, these AAsTreg remained unable to control T cells stimulated by allogeneic mature dendritic cells, a phenomenon dependent on the release of proinflammatory cytokines. In vivo, Rapa administration reduced danger-associated IL-6 production, T cell proliferation, and graft infiltration. Based on these observations, AAsTreg were administered posttransplant (day 7) in combination with a short course of Rapa and rendered >80% long-term (>150 d) graft survival, a result superior to that achieved with polyclonal Treg. Moreover, graft protection was alloantigen-specific. Significantly, long-term graft survival was associated with alloreactive T cell anergy. These findings delineate combination of transient mammalian target of Rapa inhibition with appropriate AAsTreg selection as an effective approach to promote long-term organ graft survival.

CD4(+) CD25(+) Foxp3(+) T regulatory cells with limited TCR diversity in control of autoimmunity

The importance of high TCR diversity of T regulatory (Treg) cells for self-tolerance is poorly understood. To address this issue, TCR diversity was measured for Treg cells after transfer into IL-2Rbeta(-/-) mice, which develop lethal autoimmunity because of failed production of Treg cells. In this study, we show that high TCR diversity of pretransferred Treg cells led to selection of therapeutic Treg cells with lower TCR diversity that prevented autoimmunity. Pretransferred Treg cells with lower diversity led to selection of Treg cells through substantial peripheral reshaping with even more restricted TCR diversity that also suppressed autoimmune symptoms. Thus, in a setting of severe breakdown of immune tolerance because of failed production of Treg cells, control of autoimmunity is achieved by only a fraction of the Treg TCR repertoire, but the risk for disease increased. These data support a model in which high Treg TCR diversity is a mechanism to ensure establishing and maintaining self-tolerance.

T cell receptor beta chain (TCR-Vbeta) repertoire of circulating CD4(+) CD25(-), CD4(+) CD25(low) and CD4(+) CD25(high) T cells in patients with long-term renal allograft survival

The mechanisms underlying maintenance of renal allografts in humans under minimal or conventional immunosuppression are poorly understood. There is evidence that CD4(+) CD25(+) regulatory T cells and clonal deletion, among other mechanisms of tolerance, could play a key role in clinical allograft survival. Twenty-four TCR-Vbeta families were assessed in CD4(+) CD25(-), CD4(+) CD25(low) and CD4(+) CD25(high) T cells from patients with long-term renal allograft survival (LTS), patients exhibiting chronic rejection (ChrRx), patients on dialysis (Dial) and healthy controls (HC) by flow cytometry. LTS patients presented a higher variability in their TCR-Vbeta repertoire, such decreased percentage of Vbeta2(+), Vbeta8a(+) and Vbeta13(+) in CD4(+) CD25(low) and (high) compared with CD4(+) CD25(-) subset and increased Vbeta4 and Vbeta7 families in CD4(+) CD25(high) T cells exclusively. Additionally, LTS patients, particularly those that were not receiving calcineurin inhibitors (CNI), had increased percentages of CD4(+) CD25(high) T cells when compared with Dial (P < 0.05) and ChrRx (P < 0.05) patients. Our results suggest that a differential expression of particular TCR-Vbeta families and high levels of circulating CD4(+) CD25(high) T cells in long-term surviving renal transplant patients could contribute to an active and specific state of immunologic suppression. However, the increase in this T cell subset with regulatory phenotype can be affected by CNI.

Modified peptides in anti-cancer vaccines: are we eventually improving anti-tumour immunity?

The discovery of tumour antigens recognized by T cells and the features of immune responses directed against them has paved the way to a multitude of clinical studies aimed at boosting anti-tumour T cell immunity as a therapeutic tool for cancer patients. One of the different strategies explored to ameliorate the immunogenicity of tumour antigens in vaccine protocols is represented by the use of optimized peptides or altered peptide ligands, whose amino acid sequence has been modified for improving HLA binding or TCR interaction with respect to native epitopes. However, despite the promising results achieved with preclinical studies, the clinical efficacy of this approach has not yet met the expectations. Although multiple reasons could explain the relative failure of altered peptide ligands as more effective cancer vaccines, the possibility that T cells primed by modified tumour peptides might may be unable to effectively cross-recognize tumour cells has not been sufficiently addressed. Indeed, the introduction of conservative amino acid substitutions may still produce diverse and unpredictable changes in the HLA/peptide interface, with consequent modifications of the TCR repertoire that can interact with the complex. This could lead to the expansion of a broad array of T cells whose TCRs may not necessarily react with equivalent affinity with the original antigenic epitope. Considering the results presently achieved with this vaccine approach, and the emerging availability of alternative strategies for boosting anti-tumour immunity, the use of modified tumour peptides could be reconsidered.

Therapeutic potential of FOXP3(+) regulatory T cells and their interactions with dendritic cells

FOXP3(+) regulatory T cells, a unique subset of T cells, are critical for orchestrating an immune response and preventing self-reactivity. With the increasing prevalence and unsatisfactory treatment of autoimmunity, allergic diseases, cancer and chronic infections, much attention has been focused on understanding their mechanisms of action in order to manipulate their function. One goal is to develop drugs or biologics that can enhance or abrogate their functions. Another approach is to utilize Tregs in adoptive cell-based therapy to treat autoimmune diseases or transplant-related complications. This review will focus on their therapeutic potential and mechanisms of action, particularly their interaction with dendritic cells.

Selective priming and expansion of antigen-specific Foxp3- CD4+ T cells during Listeria monocytogenes infection

The Foxp3-expressing subset of regulatory CD4(+) T cells have defined Ag specificity and play essential roles in maintaining peripheral tolerance by suppressing the activation of self-reactive T cells. Similarly, during chronic infection, pathogen-specific Foxp3-expressing CD4(+) T cells expand and actively suppress pathogen-specific effector T cells. Herein, we used MHC class II tetramers and Foxp3(gfp) knockin mice to track the kinetics and magnitude whereby pathogen-specific Foxp3(+)CD4(+) and Foxp3(-)CD4(+) cells are primed and expand after acute infection with recombinant Listeria monocytogenes (Lm) expressing the non-"self"-Ag 2W1S(52-68). We demonstrate that Lm infection selectively primes proliferation, expansion, and subsequent contraction of Lm-specific Foxp3(-) effector CD4(+) cells, while the numbers of Lm-specific Foxp3(+)CD4(+) regulatory cells remain essentially unchanged. In sharp contrast, purified 2W1S(52-68) peptide primes coordinated expansion of both Foxp3(+) regulatory and Foxp3(-) effector T cells with the same Ag specificity. Taken together, these results indicate selective priming and expansion of Foxp3(-) CD4 T cells is a distinguishing feature for acute bacterial infection.

CD4+ T-cell development in a mouse expressing a transgenic TCR derived from a Treg

CD4(+)Foxp3(+) Treg maintain peripheral tolerance and influence immune responses to foreign antigens. The thymus is an important source of Treg, but controversy exists as to whether T cells are selected into the Treg lineage based on signals received through TCR specific for self-peptides. To examine the specificity of TCR expressed by Treg and its effect on CD4(+) T-cell development, we generated Treg-TCR transgenic mice. Deletion of >90% of CD4(+) T cells in RAG-sufficient mice, and nearly 100% deletion in RAG(-/-) mice expressing this TCR indicate that the TCR is specific for an unknown, naturally expressed peptide in the thymus. Deletion occurs late in development, suggesting this peptide is presented by APC in the thymic medulla. These studies are the first to describe the effects of expressing a Treg-TCR on CD4(+) T-cell development. The implications of our data for models of Treg selection are discussed.

Prevention of allograft rejection by amplification of Foxp3(+)CD4(+)CD25(+) regulatory T cells

CD4(+)CD25(+) T cells were identified originally as potent suppressors of autoimmunity and were later termed "natural regulatory T cells" or nTreg cells. Subsequently, a transcription factor called forkhead box protein 3 (Foxp3) was identified to be a critical regulator for Treg differentiation and function. Foxp3(+)CD4(+)CD25(+) Treg cells have been increasingly documented to suppress allograft rejection and to mediate allograft tolerance in transplantation. In this article, the authors review current approaches for amplification of allo-specific Foxp3(+)CD4(+)CD25(+) Treg cells for prevention of allograft rejection and induction of allo-specific transplant tolerance.