Instruction of Treg commitment in peripheral T cells is suited to reverse autoimmunity

Cancer immunotherapy: A comprehensive appraisal of its modes of application

Conventional cancer treatments such as chemotherapy and radiation therapy have reached their therapeutic potential, leaving a gap for developing more effective cancer therapeutics. Cancer cells evade the immune system using various mechanisms of immune tolerance, underlying the potential impact of immunotherapy in the treatment of cancer. Immunotherapy includes several approaches such as activating the immune system in a cytokine-dependent manner, manipulating the feedback mechanisms involved in the immune response, enhancing the immune response via lymphocyte expansion and using cancer vaccines to elicit long-lasting, robust responses. These techniques can be used as monotherapies or combination therapies. The present review describes the immune-based mechanisms involved in tumor cell proliferation and maintenance and the rationale underlying various treatment methods. In addition, the present review provides insight into the potential of immunotherapy used alone or in combination with various types of therapeutics.

The IL-2 cytokine family in cancer immunotherapy

The use of cytokines from the IL-2 family (also called the common γ chain cytokine family) such as interleukin (IL)-2, IL-7, IL-15, and IL-21 to activate the immune system of cancer patients is one of the most important areas of current cancer immunotherapy research. The infusion of IL-2 at low or high doses for multiple cycles in patients with metastatic melanoma and renal cell carcinoma was the first successful immunotherapy for cancer proving that the immune system could completely eradicate tumor cells under certain conditions. The initial clinical success observed in some IL-2-treated patients encouraged further efforts focused on developing and improving the application of other IL-2 family cytokines (IL-4, IL-7, IL-9, IL-15, and IL-21) that have unique biological effects playing important roles in the development, proliferation, and function of specific subsets of lymphocytes at different stages of differentiation with some overlapping effects with IL-2. IL-7, IL-15, and IL-21, as well as mutant forms or variants of IL-2, are now also being actively pursued in the clinic with some measured early successes. In this review, we summarize the current knowledge on the biology of the IL-2 cytokine family focusing on IL-2, IL-15 and IL-21. We discuss the similarities and differences between the signaling pathways mediated by these cytokines and their immunomodulatory effects on different subsets of immune cells. Current clinical application of IL-2, IL-15 and IL-21 either as single agents or in combination with other biological agents and the limitation and potential drawbacks of these cytokines for cancer immunotherapy are also described. Lastly, we discuss the future direction of research on these cytokines, such as the development of new cytokine mutants and variants for improving cytokine-based immunotherapy through differential binding to specific receptor subunits.

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.

ATP inhibits the generation and function of regulatory T cells through the activation of purinergic P2X receptors

Extracellular nucleotides are pleiotropic regulators of mammalian cell function. Adenosine triphosphate (ATP) released from CD4(+) helper T cells upon stimulation of the T cell receptor (TCR) contributes in an autocrine manner to the activation of mitogen-activated protein kinase (MAPK) signaling through purinergic P2X receptors. Increased expression of p2rx7, which encodes the purinergic receptor P2X7, is part of the transcriptional signature of immunosuppressive CD4(+)CD25(+) regulatory T cells (T(regs)). Here, we show that the activation of P2X7 by ATP inhibits the suppressive potential and stability of T(regs). The inflammatory cytokine interleukin-6 (IL-6) increased ATP synthesis and P2X7-mediated signaling in T(regs), which induced their conversion to IL-17-secreting T helper 17 (T(H)17) effector cells in vivo. Moreover, pharmacological antagonism of P2X receptors promoted the cell-autonomous conversion of naïve CD4(+) T cells into T(regs) after TCR stimulation. Thus, ATP acts as an autocrine factor that integrates stimuli from the microenvironment and cellular energetics to tune the developmental and immunosuppressive program of the T cell in adaptive immune responses.

Analysis of the transcriptional program of developing induced regulatory T cells

CD25+ regulatory T cells develop in the thymus (nTregs), but may also be generated in the periphery upon stimulation of naive CD4 T cells under appropriate conditions (iTregs). To gain insight into the mechanisms governing iTreg development, we performed longitudinal transcriptional profiling of CD25+ T cells during their differentiation from uncommitted naive CD4 T cells. Microarray analysis of mRNA from CD25+ iTregs early after stimulation revealed expression of genes involved in cell cycle progression and T cell activation, which largely overlapped with genes expressed in CD25+ effector T cells (Teffs) used as a control. Whereas expression of these genes remained elevated in Teffs, it declined gradually in developing iTregs, resulting in a more quiescent phenotype in mature iTregs. A similar pattern of kinetics was observed for biological processes and for intracellular pathways over-represented within the expressed genes. A maximum dichotomy of transcriptional activity between iTregs and Teffs was reached at late stages of their maturation. Of interest, members of the FoxO and FoxM1 transcription factor family pathways exhibited a reciprocal expression pattern in iTregs and Teffs, suggesting a role of these transcription factors in determining T cell fate.

Neuroantigen-specific CD8+ regulatory T-cell function is deficient during acute exacerbation of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system (CNS). MS is thought to be T-cell-mediated, with prior research predominantly focusing on CD4+ T-cells. There is a high prevalence of CNS-specific CD8+ T-cell responses in MS patients and healthy subjects. However, the role of neuroantigen-specific CD8+ T-cells in MS is poorly understood, with the prevalent notion that these may represent pathogenic T-cells. We show here that healthy subjects and MS patients demonstrate similar magnitudes of CD8+ and CD4+ T-cell responses to various antigenic stimuli. Interestingly, CD8+ T-cells specific for CNS autoantigens, but not those specific for control foreign antigens, exhibit immune regulatory ability, suppressing proliferation of CD4+CD25- T-cells when stimulated by their cognate antigen. While CD8+ T-cell-mediated immune suppression is similar between healthy subjects and clinically quiescent treatment-naïve MS patients, it is significantly deficient during acute exacerbation of MS. Of note, the recovery of neuroantigen-specific CD8+ T-cell suppression correlates with disease recovery post-relapse. These studies reveal a novel immune suppressor function for neuroantigen-specific CD8+ T-cells that is clinically relevant in the maintenance of peripheral tolerance and the intrinsic regulation of MS immune pathology.

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.

Mechanistic insights into immunomodulation by hepatic stellate cells in mice: a critical role of interferon-gamma signaling

The liver is considered to be an immune-privileged organ that favors the induction of tolerance. The underlying mechanisms are not completely understood. Interestingly, liver transplants are spontaneously accepted in several animal models, but hepatocyte transplants are acutely rejected, suggesting that liver nonparenchymal cells may effectively protect the parenchymal cells from immune attack. We have shown the profound T cell inhibitory activity of hepatic stellate cells (HSCs). Thus, cotransplantation with HSCs effectively protects islet allografts from rejection in mice. In this study, using T cell receptor transgenic and gene knockout approaches, we provided definitive evidence that HSCs protected cotransplanted islet allografts by exerting comprehensive inhibitory effects on T cells, including apoptotic death in graft-infiltrating antigen-specific effector T cells and marked expansion of CD4(+) Forkhead box protein (Foxp)3(+) T regulatory (Treg) cells. All these effects required an intact interferon-gamma (IFN-gamma) signaling in HSCs, demonstrated by using HSCs isolated from IFN-gamma receptor 1 knockout mice. B7-H1 expression on HSCs, a product molecule of IFN-gamma signaling, was responsible for induction of T cells apoptosis, but had no effect on expansion of Treg cells, suggesting that undetermined effector molecules produced by IFN-gamma signaling is involved in this process.

CONCLUSION

Upon inflammatory stimulation, specific organ stromal cells (such as HSCs in the liver) demonstrate potent immune regulatory activity. Understanding of the mechanisms involved may lead to development of novel strategies for clinical applications in transplantation and autoimmune diseases.

Prevention of spontaneous immune-mediated diabetes development in the LEW.1AR1-iddm rat by selective CD8+ T cell transfer is associated with a cytokine shift in the pancreas-draining lymph nodes

AIMS/HYPOTHESIS

The LEW.1AR1-iddm rat is an animal model of spontaneous type 1 diabetes mellitus. This study analysed how adoptive transfer of selective T cell subpopulations affects the incidence of diabetes.

METHODS

CD4(+) or CD8(+) T cells were isolated from diabetic LEW.1AR1-iddm rats or diabetes-resistant LEW.1AR1 rats. Cells were selectively transferred into athymic LEW.1AR1-Whn ( rnu ) or prediabetic LEW.1AR1-iddm rats. The animals were monitored for blood glucose, islet infiltration and immune cell composition of pancreas-draining lymph nodes.

RESULTS

After adoptive transfer of CD4(+) T cells from diabetic LEW.1AR1-iddm rats into athymic LEW.1AR1-Whn ( rnu ) rats, 50% of the recipients developed diabetes. Transfer of CD8(+) T cells failed to induce diabetes. Only 10% of the athymic recipients became diabetic after co-transfer of CD4(+) and CD8(+) T cells. Adoptive transfer of CD8(+) T cells from LEW.1AR1 or diabetic LEW.1AR1-iddm rats into prediabetic LEW.1AR1-iddm rats significantly reduced the incidence of diabetes. In protected normoglycaemic animals regulatory CD8(+)/CD25(+) and CD4(+)/CD25(+) T cell subpopulations that were also FOXP3-positive accumulated in the pancreas-draining lymph nodes. In this lymphatic organ, gene expression of anti-inflammatory cytokines was significantly higher than in diabetic rats.

CONCLUSIONS/INTERPRETATION

Our results show that adoptive transfer of CD4(+) but not CD8(+) T cells from diabetic LEW.1AR1-iddm rats induced diabetes development. Importantly, CD8(+) T cells from diabetic LEW.1AR1-iddm rats and diabetes-resistant LEW.1AR1 rats provided protection against beta cell destruction. The accumulation of regulatory T cells in the pancreas-draining lymph nodes from protected rats indicates that transferred CD8(+) T cells may have beneficial effects in the control of beta cell autoimmunity.

Peripheral induction of tolerance by retinal antigen expression

The contribution of peripheral expression of tissue-specific CNS Ags to the generation of tolerance is uncertain. To study this question, we examined mice transgenic (Tg) for expression of beta-galactosidase (beta gal) on the retinal photoreceptor cell arrestin promoter, in conjunction with TCR Tg mice producing CD4(+) T cells specific for beta gal (beta galTCR). Several strategies were used to test the hypothesis that betagal expressed in the retina supported thymus-independent tolerance and regulatory T cell development. Retinal expression generated an immunoregulatory response that depressed development of immune responses to beta gal following systemic immunization with beta gal. This regulation was transferable to naive mice by CD3(+)4(+)25(+) T cells from naive retinal beta gal(+) donors. Experiments that removed the beta gal(+) retina by enucleation showed that subsequent development of a regulatory response was lost. Adoptive transfer of CD25(-) beta galTCR T cells into retinal beta gal Tg mice on the Rag(-/-) background led to regulatory activity that limited lymphopenia-induced proliferation of beta galTCR T cells in mice with retinal expression of beta gal and inhibited the ear-swelling assay for delayed type hypersensitivity. These results show that retinal expression of very small amounts of a tissue-specific Ag can generate tolerance that includes regulatory T cells.

How the immune system achieves self-nonself discrimination during adaptive immunity

We propose an "Avidity Model of Self-Nonself Discrimination" in which self-nonself discrimination is achieved by both central thymic selection and peripheral immune regulation. The conceptual framework that links these two events is the understanding that both in the thymus and in the periphery the survival or the fate of T cells is determined by the avidity of the interactions between T cell receptors (TCRs) on T cells, specific to any antigens and MHC/antigen peptides presented by antigen-presenting cells (APCs). We envision that the immune system achieves self-nonself discrimination, during adaptive immunity, not by recognizing the structural differences between self versus foreign antigens, but rather by perceiving the avidity of T cell activation. Intrathymic deletion of high avidity T cell clones responding to the majority of self-antigens generates a truncated peripheral self-reactive repertoire composed of mainly intermediate and low but devoid of high avidity T cells compared with the foreign-reactive repertoire. The existence of intermediate avidity self-reactive T cells in the periphery represents a potential danger of pathogenic autoimmunity inherited in each individual because potentially pathogenic self-reactive T cells are included in the pool of intermediate avidity T cells and can often be functionally activated to elicit autoimmune diseases. The distinct composition of peripheral T cell repertoires to self versus to foreign antigens provides a unique opportunity for the immune system to discriminate self from nonself, in the periphery, by selectively downregulating intermediate avidity T cells to both self and foreign antigens. Selective downregulation of the intermediate avidity T cell populations containing the potentially pathogenic self-reactive T cells enables the immune system to specifically control autoimmune diseases without damaging the effective anti-infection immunity, which is, largely, mediated by high avidity T cells specific to the infectious pathogens. In this regard, it has been recently shown that Qa-1-restricted CD8(+) T cells selectively downregulate intermediate avidity T cells, to both self and foreign antigens, and as a consequence, specifically dampen autoimmunity yet optimize the immune response to foreign antigens. Selective downregulation of intermediate avidity T cells is accomplished via specific recognition, by the Qa-1-restricted CD8(+) T cells, of particular Qa-1/self-peptide complexes, such as Qa-1/Hsp60sp, which function as a common surrogate target structure and preferentially expressed on the activated intermediate avidity T cells. This regulatory pathway thus represents one example of the peripheral mechanisms that the immune system evolved to complete self-nonself discrimination that is achieved, imperfectly, by thymic negative selection, in order to maintain self-tolerance. The conceptual framework of the "Avidity Model" differs from, but contains intellectual wisdom of certain conceptual elements of, the "Tunable Activation Thresholds Hypothesis," the "Danger Model," and the "Ergotypic Regulation Phenomenon." It provides a unified and simple paradigm to explain various seemingly unrelated biomedical problems inherent in immunological disorders that cannot be uniformly interpreted by any currently existing paradigms. The potential impact of the conceptual framework of the "Avidity Model" on our understanding of the development and control of commonly seen autoimmune diseases is also discussed.

Qa-1/HLA-E-restricted regulatory CD8+ T cells and self-nonself discrimination: an essay on peripheral T-cell regulation

By discriminating self from nonself and controlling the magnitude and class of immune responses, the immune system mounts effective immunity against virtually any foreign antigens but avoids harmful immune responses to self. These are two equally important and related but distinct processes, which function in concert to ensure an optimal function of the immune system. Immunologically relevant clinical problems often occur because of failure of either process, especially the former. Currently, there is no unified conceptual framework to characterize the precise relationship between thymic negative selection and peripheral immune regulation, which is the basis for understanding self-non-self discrimination versus control of magnitude and class of immune responses. In this article, we explore a novel hypothesis of how the immune system discriminates self from nonself in the periphery during adaptive immunity. This hypothesis permits rational analysis of various seemingly unrelated biomedical problems inherent in immunologic disorders that cannot be uniformly interpreted by any currently existing paradigms. The proposed hypothesis is based on a unified conceptual framework of the "avidity model of peripheral T-cell regulation" that we originally proposed and tested, in both basic and clinical immunology, to understand how the immune system achieves self-nonself discrimination in the periphery.

The IL-2/CD25 pathway determines susceptibility to T1D in humans and NOD mice

Although the interleukin-2 (IL-2)/IL-2R signaling pathway has been the focus of numerous studies, certain aspects of its molecular regulation are not well characterized, especially in non-T cells, and a more complete understanding of the pathway is necessary to discern the functional basis of the genetic association between the IL-2-IL-21 and IL-2RA/CD25 gene regions and T1D in humans. Genetic variation in these regions may promote T1D susceptibility by influencing transcription and/or splicing and, hence, IL-2 and IL-2RA/CD25 expression at the protein level in different immune cell subsets; thus, there is a need to establish links between the genetic variation and immune cell phenotypes and functions in humans, which can be further investigated and validated in mouse models. The detection and characterization of genetically determined immunophenotypes should aid in elucidating disease mechanisms and may enable future monitoring of disease initiation and progression in prediabetic subjects and of responses to therapeutic intervention.

Polyclonal adaptive regulatory CD4 cells that can reverse type I diabetes become oligoclonal long-term protective memory cells

Type 1 diabetes is a CD4 cell-dependent disease that results from destruction of insulin-producing beta cells in pancreatic islets. An ideal therapy would reverse diabetes shortly after onset when islet function in not yet fully ablated, and also prevent re-emergence of disease through the generation of memory cells that control the autoimmune response. In this study, we show that adaptive/induced polyclonal regulatory (TR) cells, which contain islet-reactive cells, fulfill these criteria in the NOD mouse model. CD4 cells induced to express FoxP3, IL-10, and TGF-beta1 in response to TCR signaling and TGF-beta1 can reverse diabetes with clinical restoration of prediabetic serum levels of IL-10. Unlike naturally occurring TR cells, these adaptive TR cells persist indefinitely (>1 year) as FoxP3(+), CD25(-) memory cells that self-renew. Establishment of memory is accompanied by narrowing of the T cell repertoire to usage of a single TCR beta-chain, Vbeta11, implying selection by Ag. With islet-specific adaptive TR cells, we show that memory is functionally stable and transferable. Therefore, adaptive TR cells, which can be readily generated from normal CD4 populations and become focused by Ag with induction of memory, may provide a treatment and a vaccine for the long-term cure of diabetes making them attractive as immunotherapeutic agents.

The biology of interleukin-2

Much data support an essential role for interleukin (IL)-2 in immune tolerance. This idea is much different from the early paradigm in which IL-2 is central for protective immune responses. This change in thinking occurred when a T regulatory cell defect was shown to be responsible for the lethal autoimmunity associated with IL-2/IL-2R deficiency. This realization allowed investigators to explore immune responses in IL-2-nonresponsive mice rendered autoimmune-free. Such studies established that IL-2 sometimes contributes to optimal primary immune responses, but it is not mandatory. Emerging findings, however, suggest an essential role for IL-2 in immune memory. Here, the current understanding of the dual role of IL-2 in maintaining tolerance and contributing to immunity in vivo is reviewed with some emphasis on T regulatory cell production and homeostasis. Also discussed are implications of this new appreciation concerning the immunobiology of IL-2 with respect to targeting IL-2 or its receptor in immunotherapy.

Antagonistic nature of T helper 1/2 developmental programs in opposing peripheral induction of Foxp3+ regulatory T cells

Recent studies have highlighted the importance of peripheral induction of Foxp3-expressing regulatory T cells (Tregs) in the dominant control of immunological tolerance. However, Foxp3(+) Treg differentiation from naïve CD4(+) T cells occurs only under selective conditions, whereas the classical T helper (Th) 1 and 2 effector development often dominate T cell immune responses to antigen stimulation in the periphery. The reason for such disparity remains poorly understood. Here we report that Th1/Th2-polarizing cytokines can potently inhibit Foxp3(+) Treg differentiation from naïve CD4(+) precursors induced by TGF-beta. Furthermore, antigen receptor-primed CD4(+) T cells are resistant to Treg induction because of autocrine production of IFNgamma and/or IL-4, whereas neutralizing IFNgamma and IL-4 not only can potentiate TGF-beta-mediated Foxp3 induction in vitro but can also enhance antigen-specific Foxp3(+) Treg differentiation in vivo. Mechanistically, inhibition of Foxp3(+) Treg development by Th1/Th2-polarizing cytokines involves the activation of Th1/Th2 lineage transcription factors T-bet and GATA-3 through the canonical Stat1-, Stat4-, and Stat6-dependent pathways. Using IFNgamma and IL-4 knockouts and retrovirus-mediated transduction of T-bet and GATA-3, we further demonstrate that enforced expression of the Th1/Th2 lineage-specific transcription factors is sufficient to block Foxp3 induction and Treg differentiation independent of the polarizing/effector cytokines. Thus, our study has unraveled a previously unrecognized mechanism of negative cross-regulation of Foxp3(+) Treg fate choice by Th1/Th2 lineage activities. In addition, these findings also provide an attainable explanation for the general paucity of antigen-triggered de novo generation of Foxp3(+) Tregs in the periphery.

G protein-coupled receptor 83 is dispensable for the development and function of regulatory T cells

Global analyses of gene expression in regulatory T (Treg) cells, whose development is critically dependent upon the transcription factor Foxp3, have provided many clues as to the molecular mechanisms these cells employ to control immune responses and establish immune tolerance. Through these studies, G protein-coupled receptor 83 (GPR83) was found to be expressed at high levels in Treg-cell populations. However, its function remained unclear. Recently, it has been suggested that GPR83 is involved in the induction of Foxp3 expression in the peripheral nonregulatory Foxp3- CD4 T cells. To examine a role for GPR83 in Treg-cell biology, we generated and characterized GPR83-deficient mice. We have shown that GPR83 abolition does not result in measurable pathology or changes in the numbers or function of Foxp3+ Treg cells. Furthermore, while in vitro analysis suggested a potential involvement of GPR83 in transforming growth factor beta-dependent Foxp3 induction, there was no difference in the ability of nonregulatory GPR83-deficient and nondeficient Foxp3- T cells to acquire Foxp3 expression in vivo. Collectively, our results demonstrate that GPR83 is dispensable for Treg-cell development and function.

Regulatory T cells in health and disease

The healthy host does not normally develop tissue destructive autoimmunity in part because of the presence of natural regulatory T cells. These cells are best identified by their expression of a unique transcription factor forkhead box transcription factor (Foxp3) that controls their regulatory function. Several other types of regulatory T cells also occur most of which are induced in response to antigen stimulation. Some of these express the Foxp3 transcription factor but many do not. The role of natural T-regulatory cells as well as induceable regulatory cells in autoimmunity, cancer, allergy and infectious disease is described. The current status of therapeutic approaches that modulate regulatory T-cell responses on the outcome of experimental animal and human disease is also discussed.

Increased expression of the FoxP3 functional marker of regulatory T cells following B cell depletion with rituximab in patients with lupus nephritis

B cell depletion may affect T cell activation and costimulation status in rituximab-treated patients with SLE. We examined whether rituximab administration in patients with active lupus nephritis is related to changes in mRNA expression of genes that define regulatory T cells (Tregs) in peripheral blood lymphocytes, measured by real-time PCR. At the early phase of B cell depletion mRNA levels of CD25, CTLA-4, GITR and the bona fide Treg functional marker FOXP3 increased significantly in all 7 patients examined. In contrast, mRNA levels of the costimulatory/activation T cell molecule CD40L were profoundly reduced, while mRNA levels of TGF-beta, a cytokine contributing to Treg induction, increased significantly in all. During follow-up, increased FOXP3 mRNA persisted in those patients in clinical remission, while in those patients with active disease subsequent decreases were noted. Further studies should examine whether modulation of Tregs by therapeutic B cell depletion contributes and/or predicts lupus disease remission.

Vaccination against the forkhead family transcription factor Foxp3 enhances tumor immunity

Depletion of CD4+CD25+ regulatory T cells (Treg) by treatment with alphaCD25 antibody synergizes with vaccination protocols to engender protective immunity in mice. The effectiveness of targeting CD25 to eliminate Treg is limited by the fact that CD25, the low-affinity interleukin-2 receptor, is up-regulated on conventional T cells. At present, foxp3 is the only product known to be exclusively expressed in Treg of mice. However, foxp3 is not expressed on the cell surface and hence cannot be targeted with antibodies. In this study, we tested the hypothesis that vaccination of mice against foxp3, a self-antigen expressed also in the thymus, is capable of stimulating foxp3-specific CTL that will cause the depletion of Treg and enhanced antitumor immunity. Vaccination of mice with foxp3 mRNA-transfected dendritic cells elicited a robust foxp3-specific CTL response and potentiated vaccine-induced protective immunity comparably with that of alphaCD25 antibody administration. In contrast to alphaCD25 antibody treatment, repeated foxp3 vaccination did not interfere with vaccine-induced protective immunity. Importantly, foxp3 vaccination led to the preferential depletion of foxp3-expressing Treg in the tumor but not in the periphery, whereas alphaCD25 antibody treatment led to depletion of Treg in both the tumor and the periphery. Targeting foxp3 by vaccination offers a specific and simpler protocol for the prolonged control of Treg that may be associated with reduced risk of autoimmunity, introducing an approach whereby specific depletion of cells is not limited to targeting products expressed on the cell surface.