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Keywords:

  • CD25;
  • IL-2;
  • Foxp3;
  • Immunological self-tolerance;
  • IPEX;
  • Regulatory T cells;
  • Suppressor T cells

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

It is now widely accepted that the normal immune system harbors a regulatory T-cell population specialized for immune suppression. It was found initially that some CD4+ T cells in normal animals were capable of suppressing autoimmunity. Characterization of this autoimmune-suppressive CD4+ T cell population revealed that they constitutively expressed the CD25 molecule, which made it possible to distinguish them from other T cells, delineate their developmental pathways, in particular their thymic development, and characterize their potent in vivo and in vitro immunosuppressive activity. The marker also helped to identify human regulatory T cells with similar functional and phenotypic characteristics. Recent studies have shown that CD25+CD4+ regulatory T cells specifically express the transcription factor Foxp3. Genetic anomaly of Foxp3 causes autoimmune and inflammatory disease in rodents and humans through affecting the development and function of CD25+CD4+ regulatory T cells. These findings at the cellular and molecular levels altogether provide firm evidence for Foxp3+CD25+CD4+ regulatory T cells as an indispensable cellular constituent of the normal immune system and for their crucial roles in establishing and maintaining immunologic self-tolerance and immune homeostasis. They can be exploited for clinical use to treat immunological diseases and control physiological and pathological immune responses.

Abbreviations:
ATx:

adult thymectomy

IBD:

inflammatory bowel disease

IPEX:

immune dysfunction, polyendocrinopathy, enteropathy, X-linked syndrome

NTx:

neonatal thymectomy

T1D:

type 1 diabetes mellitus

TR cells:

regulatory T cells

Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

In addition to knowing how to elicit and augment immune responses protective to the host, it is essential to understand how aberrant or excessive immune responses deleterious to the host can be suppressed, for example, against normal self-constituents in autoimmune disease, an allogeneic fetus in pregnancy, innocuous environmental substances in allergy, or commensal microbes in certain inflammatory diseases. Stable establishment of immunological tolerance to organ transplants is an urgent clinical need, while abrogation of unresponsiveness to autologous tumor cells is conducive to effective immunotherapy of cancer. Among various mechanisms for establishing and sustaining immune tolerance and homeostasis, T cell-mediated suppression of immune responses towards self and non-self antigens has recently attracted enormous interest 1. The idea of suppressor T cells, now euphemistically called regulatory T (TR) cells, is not new to immunologists since early 1970s. Yet it has been contentious whether T cells specialized for suppressive function should constitute a definite cellular entity in the immune system, and, if it is the case, to what extent they contribute to immune tolerance and homeostasis, that is, whether they are truly instrumental in controlling immune responses or whether their anomaly can be a primary cause of any immunological disease. Research endeavor for the past forty years has attempted to answer these questions.

History of suppressor T cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

In 1970, Gershon and Kondo 2 made the seminal finding that T cells not only augmented but also dampened immune responses, and that this down-regulation was mediated by T cells that were different from helper T cells. This T-cell population, called suppressor T cells, was intensively studied over the following years in various fields of immunology. The studies showed several type of suppressor T cells; some were antigen-specific others being nonspecific; some secreted antigen-specific suppressive factors and others nonspecific ones; and suppressor populations with different phenotypes and modes of suppression interacted in cascade 3. The phenotype of suppressor T cells, assessed by the expression of Lyt-1 (CD5) and Lyt-2 (CD8), was on the most part Lyt-2+, corresponding to CD8+, while some of them, in particular those suppressing delayed type hypersensitivity, were Lyt-1+2, corresponding to CD4+ cells. CD8+ suppressor T cells expressed the I-J molecule, which was supposed to be a key suppressor molecule intimately associated with their suppressive function 3. However, active research of suppressor T cells, involving many immunologists, quite abruptly collapsed in the mid 1980s when scrutiny of the mouse MHC gene by molecular biology techniques showed no existence of the I-J region, which was assumed to encode the I-J molecule and locate within the MHC gene complex 4. With this bewildering I-J episode as a turning point, immunologists’ interest in suppressor T cells rapidly waned, forming, in the late 1980s and early 90s, an atmosphere in which they even shied away from using the word “suppressor T cells” in interpreting suppressive or inhibitory immunological phenomena 5. In retrospect, there are several other reasons for this decline in the study; e.g. failure in finding reliable markers for distinguishing suppressor T cells from other T cells, ambiguity in the molecular basis of suppression, and difficulty in preparing antigen-specific suppressor T-cell clones amenable to fine cellular and molecular analyses. Clinical immunologists failed to obtain definitive evidence for anomaly of suppressor T cells as a primary cause of any immunological disease. In sharp contrast, approaches to immunologic tolerance with new molecular tools, such as TCR subfamily-specific mAb and TCR-transgenic mice, in the late 80s to the early 90s unequivocally demonstrated clonal deletion, and anergy, as key mechanisms of immunologic tolerance 68. Moreover, molecular characterization of various cytokines, including the newly found immunosuppressive IL-10, revealed their pleiotropism, cross-regulation, and redundancy in function 9. These findings altogether generated a climate in which T cell-involving suppressive phenomena were attributed to T cells secreting immunosuppressive or cross-regulatory cytokines, with little meaningful part played by suppressor T cells. In this atmosphere in the 90s, it is quite understandable that IL-10-secreting TR cells, called Tr1 cells, produced in vitro by antigenic stimulation of naive T cells in the presence of IL-10, or TGF-β-secreting TR cells, called Th3 cells, propagated from animals via oral tolerance encountered little resistance to be accepted 10, 11.

CD4+ T cells with autoimmune-suppressive activity

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

In parallel with the study of suppressor T cells briefly depicted above, there has been a different stream of endeavor to investigate T cell suppression. A notable feature of the latter is that it examined from the beginning how autoimmune disease can be produced by breaching natural self-tolerance and how it can be inhibited to develop, rather than analyzing tolerance or suppression experimentally induced towards a particular exogenous antigen. This approach led to the finding that the normal immune system naturally harbors T cells and thymocytes with autoimmune-suppressive activity 1. Nishizuka and Sakakura 12 showed in 1969 that neonatal thymectomy (NTx) of normal mice between day 2 and 4 after birth resulted in the destruction of ovaries, which they first supposed to be due to deficiency of a certain ovary-tropic hormone secreted by the thymus, hence was called “ovarian dysgenesis”. This ovarian lesion later turned out to be of autoimmune nature because subsequent investigation demonstrated that NTx produced inflammatory tissue damage in other organs. Further, it was connected with the appearance of tissue-specific autoantibodies in the circulation. Depending on the mouse strain used, NTx, which is also called 3dTx because it is most efficient if the thymus is removed 3 days after birth, leads to the development of thyroiditis, gastritis, orchitis, prostatitis, and sialadenitis 13 (Fig. 1A). In 1973, Penhale et al.14 reported that adult thymectomy (ATx) of normal rats (e.g. PVG rats) followed by four rounds of biweekly sublethal X-irradiation (2 to 2.5 Gray) produced autoimmune thyroiditis accompanied by anti-thyroglobulin autoantibody production (Fig. 1A). They and others later showed that the same protocol was able to elicit type 1 diabetes (T1D) in other strains of rats 15, 16. Importantly, inoculation of normal T cells from normal syngeneic animals inhibited disease development in both systems 17, 18. CD4+ T cells and CD4+CD8 mature thymocytes in particular inhibited NTx-induced murine autoimmune disease 17. On the other hand, once autoimmunity developed, CD4+ T cells were able to adoptively transfer disease to syngeneic T cell-deficient mice as helper T cells for autoantibody formation and effectors of cell-mediated immune destruction 19.

thumbnail image

Figure 1. (A) Induction of autoimmune disease in animals by manipulating thymus/T cells. See text for details. (B) The normal murine thymus continuously produces TR cells as well as naive T cells including potentially pathogenic self-reactive T cells. It starts to release natural TR cells around day 3 after birth. Gray and black arrows mean thymic production of TR cells and non-TR cells (including potentially pathogenic self-reactive T cells), respectively. (C) Normal adult animals harbor both TR cells and self-reactive T cells in the periphery. When thymocytes or splenic cell suspensions prepared from normal mice are depleted of TR cells (for example as CD25+CD4+ T cells) and the remaining T cells are transferred to syngeneic T cell-deficient mice (for example, athymic nude mice), the recipients spontaneously develop a variety of autoimmune diseases.

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These results altogether indicated the following scenario of autoimmune disease. The normal thymus continuously produces a population of CD4+ T cells with an autoimmune-suppressive activity; NTx of mice shortly after birth abrogates developmentally determined thymic production of autoimmune-suppressive CD4+ T cells, allowing those self-reactive CD4+ T cells that have been produced before NTx to become spontaneously activated and cause autoimmune disease because of the paucity of suppressive CD4+ T cells in the periphery (Fig. 1B). Likewise, ATx and X-irradiations abrogates thymic supply of such T cells and reduce them in the periphery presumably because they are relatively radiosensitive. The results also suggested that there might co-exist two types of CD4+ T cells in the periphery of normal untreated mice and rats, one potentially capable of mediating autoimmune diseases, the other dominantly suppressing them (Fig. 1B, C) 20.

Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

A next obvious question from above findings was how the two populations of CD4+ T cells can be distinguished in normal animals, and whether specific and direct removal of the autoimmune-suppressive population can break self-tolerance and cause autoimmune disease similar to the one produced by NTx in mice or ATx and X-irradiation in rats. Attempts were made to separate the two putative CD4+ populations in normal naive mice by the expression of cell surface molecules 2026 (Fig. 1C). Our experiments in 1985 showed that, when splenic CD4+ T cell suspensions from normal BALB/c mice were depleted of CD5highCD4+ T cells ex vivo and the remaining CD5lowCD4+ T cells were transferred to congenitally T cell-deficient BALB/c athymic nude mice, the nude mice spontaneously developed autoimmune disease in multiple organs (stomach, thyroid, ovaries, or testes) in a few months after cell transfer 20. Co-transfer of normal untreated CD4+ T cells with CD5lowCD4+ T cells inhibited autoimmunity. Likewise, transfer of CD5lowCD4+ T cells from normal C3H mice to T cell-depleted C3H mice produced autoimmune thyroiditis 21. In 1990, Powrie and Mason reconstituted PVG athymic nude rats with splenic T cell suspensions that were depleted of CD45RClowCD4+ T cells, thereby showing that the transferred CD45RChighCD4+ T cells elicited graft-versus-host disease-like systemic disease and autoimmune tissue damage in multiple organs including thyroid and Langerhans’ islets 23. McKeever et al.24 conducted a similar experiment and showed that transfer of splenic cell suspensions depleted of RT6.1+ T cells was able to produce T1D and thyroiditis in histocompatible athymic nude rats. Powrie et al. 27 and Morrisey et al28 then independently showed that transfer of BALB/c CD45RBhighCD4+ T cells to T/B cell-deficient BALB/c SCID mice induced inflammatory bowel disease (IBD).

These findings prompted us to search for a cell surface molecule which would be more specific than CD5 or CD45RB (or CD45RC) in defining such autoimmunity- and inflammation-suppressive CD4+ T cells. In 1995, we identified the CD25 molecule (the IL-2 receptor α-chain) as a candidate because CD25+ T cells, which constituted 5–10% of peripheral CD4+ T cells and less than 1% of peripheral CD8+ T cells in normal naive mice, were confined in the CD5high and CD45RBlow fraction of CD4+ T cells 25, 26. Transfer of BALB/c splenic cell suspensions depleted of CD25+CD4+ T cells indeed produced in BALB/c athymic nude mice histologically and serologically evident autoimmune diseases at higher incidences and in a wider spectrum of organs (including stomach, thyroid, ovaries, adrenal glands, and Langerhans’ islets) than the transfer of CD5low or CD45RBhigh T cells prepared from the same number of splenic cell suspensions. Co-transfer of a small number of CD25+CD4+ T cells with CD25 T cells clearly inhibited the autoimmunity. Removal of CD25+CD4+ T cells not only elicited autoimmune disease but also enhanced immune responses to non-self antigens including soluble xenogeneic proteins and allografts; reconstitution with CD25+CD4+ T cells normalized the responses 25. Transfer of CD4+CD8+ mature thymocyte suspensions depleted of CD25+ thymocytes also produced similar autoimmune diseases in syngeneic nude mice 29 (Fig. 1B, C). Furthermore, the appearance of CD25+CD4+ T cells in the spleen correlated well with the findings in NTx system. CD25+CD4+ T cells became detectable in the periphery of normal mice from around day 3 after birth, rapidly increasing to the adult level (i.e. 5–10% of CD4+ T cells) in 3 weeks, though some CD25+CD4+ T cells can already be detected in the lymph nodes of 2-day-old mice 26, 30. Further, inoculation of CD25+CD4+ T cells from normal mice within a limited period after NTx prevented autoimmune development 26. Interestingly, they were also able to suppress autoimmune disease induced by already active antigen-specific effector T cells 31.

Thus, the attempts to delineate autoimmune-suppressive CD4+ T cells by utilizing cell surface markers revealed thymus-produced CD25+CD4+ T cells that engage in the maintenance of natural self-tolerance and the control of immune responses to non-self antigens. The thymus is at any time producing functionally mature CD25+CD4+ suppressive T cells and also some potentially pathogenic self-reactive T cells. With these results that defined a specific small subset of T cells with suppressive activity, such cells were then called regulatory T cells (TR or Treg).

The functional role of IL-2 and CD25 for natural TR cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

Following the discovery of CD25 as a useful marker for operationally distinguishing endogenous TR cells from other T cells in normal naive animals, several studies revealed that the molecule was not a mere marker for natural TR cells but essential for their function. IL-2-deficient mice, which spontaneously develop severe autoimmunity/inflammation, were found to have a substantially reduced number of CD25+CD4+ T cells despite a normal number of T cells and a normal composition of CD4/CD8 subsets 32, 31. Bone marrow chimera of IL-2-deficient and IL-2-intact T cells failed to develop autoimmunity or inflammation with normal generation of CD25+CD4+ TR cells 33. CD25-deficient or CD122 (the IL-2Rβ-chain)-deficient mice were afflicted with similar autoimmunity and inflammation, which was prevented by inoculation of normal CD25+CD4+ T cells 3436. Besides, neutralization of circulating IL-2 by administration of anti-IL-2 monoclonal antibody selectively reduced CD25+CD4+ T cells in normal mice and consequently provoked autoimmune disease 37. These findings collectively indicate that IL-2 is a key growth and survival factor for natural TR cells and that CD25 as a component of the high affinity IL-2R is therefore an indispensable molecule for their maintenance.

Establishment of in vitro functional assays for natural TR cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

The discovery of CD25 as a highly TR-specific cell surface marker enabled easy isolation of natural TR cells from normal rodents and encouraged the establishment of in vitro assays for their suppressive function. In 1998, two groups showed that CD25+CD4+ T cells potently suppressed in vitro proliferation of other CD4+ and CD8+ T cells when both populations were co-cultured and stimulated with specific antigen (or polyclonal TCR stimulator such as anti-CD3 mAb and concanavalin A) in the presence of antigen-presenting cells (APC) 38, 39. Such in vitro studies also revealed the inability of TR cells to produce IL-2 upon stimulation, their in vitro hypo-proliferative response to antigenic stimulation, and their proliferation upon TCR stimulation in the presence of high dose IL-2. Further, although the mechanism of suppression is still elusive, this assay has shown that TR cells directly suppress CD4+ T cells via cell contact with no need for soluble factors 38, 39. Notably, this simple and reliable in vitro assay, together with the CD25 marker, made it possible to identify human CD25+CD4+ TR cells with similar phenotype and function as those in rodents (reviewed in 40).

The transcription factor Foxp3 as a key control molecule of TR cell development and function

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

A recent milestone in TR cell research is the discovery of the function of Foxp3. The Foxp3 gene was identified in 2001 as the disease-causative gene in Scurfy mice, which spontaneously develop severe autoimmunity/inflammation as a result of a single gene mutation on the X chromosome 41. Mutations of the human FOXP3 gene, the ortholog of murine Foxp3, were immediately found to be the cause of a similar human disease called IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome), which is characterized by autoimmune disease in multiple endocrine organs (such as T1D and thyroiditis), IBD, and severe allergy 4244. Similarities of autoimmune disease and IBD in IPEX to those produced in mice by TR cell depletion prompted several groups to investigate possible roles of Foxp3 in natural TR cells. In 2003, they reported that Foxp3 was indeed a key molecule essential for TR cell development and function. CD25+CD4+ peripheral T cells and CD25+CD4+CD8 thymocytes specifically expressed Foxp3 mRNA, and activation of CD25CD4+ T cells was unable to induce Foxp3 expression 4547. Retroviral transduction of Foxp3 to normal CD25CD4+ T cells converted them into phenotypically and functionally TR-like cells. Such transduced cells displayed in vivo and in vitro suppressive activity, in vitro hypo-proliferation and hypo-production of IL-2, and up-regulation of CD25 and other TR cell-associated molecules (such as CTLA-4 and GITR) 45, 46. In BM chimera with a mixture of BM cells from wild-type and Foxp3-deficient mice, Foxp3-deficient BM cells failed to give rise to CD25+CD4+ T cells, while Foxp3-intact BM cells generated them and suppressed disease development 46. These findings collectively indicated that the transcription factor Foxp3 could be a master controller of the development and function of natural CD25+CD4+ TR cells.

With specific expression of Foxp3 in natural TR cells, genetically engineered mice have recently been prepared which express the reporter GFP or diphtheria toxin receptor under the control of the Foxp3 promoter 4850. Use of these Foxp3-reporter mice confirmed the previous findings that were made by the use of CD25 as a specific TR cell marker; e.g. the ontogeny of TR cells, the requirement of IL-2 and CD25 for TR cell maintenance, and induction of autoimmunity by depletion of TR cells. Raising monoclonal antibody to the Foxp3 protein and its use for intracellular staining of Foxp3 has also showed that Foxp3 is abundantly expressed in natural TR cells and so far the most reliable molecular marker for them 51. This has enabled more reliable analyses than before of the dynamics of TR cells in physiological and pathological immune responses in humans and rodents.

These findings on the role of Foxp3 in TR cells pose several key questions pertinent to the function and development of TR cells. First, given that Foxp3 expression suffices to confer suppressive activity to naive T cells, how does Foxp3 control the activity? Foxp3 appears to activate or repress hundreds of genes directly or indirectly through forming a transcription complex with other key transcription factors such as NFAT and AML1/Runx1 5255. Thus, it needs to be determined whether Foxp3 controls cell-contact dependent inhibition of the activation and proliferation of T cells, killing or inactivation of APC and/or T cells, and/or suppression via cytokines such as IL-10 and TGF-β 56, 57. Secondly, how do Foxp3+ TR cells develop in the thymus and in the periphery? Besides thymic production of natural TR cells, naive T cells in the periphery can acquire Foxp3 expression and TR cell function in several experimental settings, such as in vitro antigenic stimulation in the presence of TGF-β or following in vivo chronic suboptimal antigenic stimulation 58, 59. There is also evidence that naive T cells in humans readily express Foxp3 upon TCR stimulation, though the expression is generally much lower in amount and more transient than in natural TR cells 60, 61. It is not yet clear whether such induced TR cells are functionally stable in vivo and to what extent they contribute to the peripheral pool of Foxp3+ TR cells. Finally, how are the activation, expansion, and differentiation of TR cells controlled systemically and locally? Mature DC expand Foxp3+ TR cells in a CD80/86 dependent fashion 62, 63. Activated DC secrete IL-6, which renders antigen-responding non-TR cells resistant to TR-mediated suppression in vitro64. Naive CD4+ T cells may differentiate into Foxp3+ TR cells in the presence of TGF-β, or into IL-17-secreting Th17 cells in the presence of TGF-β and IL-6 65, 66. IL-2 facilitates this differentiation of naive CD4+ T cells into Foxp3+ TR cells, but inhibits their differentiation into Th17 cells 67. Thus, costimulatory molecules expressed by APC and cytokines secreted by APC and other T cells crucially contribute to the control of various aspects of TR cell development and function in a complex manner.

Clinical perspectives

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

In contrast to IPEX, in which genetic anomaly of TR cells is primarily causative, it is obscure whether any TR cell anomaly, genetically determined or environmentally induced, should play a substantial role for the development of common immunological diseases, in particular common autoimmune diseases such as T1D, which are apparently polygenic 68. It has been well documented that polymorphisms of the CTLA-4, IL-2, and CD25 genes significantly contribute to genetic susceptibility to T1D in humans and also in NOD mice with spontaneous T1D 68, 69. Given that total genetic deficiency of these genes, in particular the IL-2 and CD25 genes, produces severe autoimmunity mainly through affecting TR cell development and function (see above), it is possible that the polymorphisms of these genes will alter TR cell development or function, and thereby render the host susceptible to autoimmune disease. Whether known polymorphisms of other autoimmune susceptibility genes, especially the CTLA-4 gene, might affect TR cells needs to be examined 70. In addition, it is a key question in autoimmune disease whether any environmental agents could affect natural TR cells and thereby elicit autoimmune disease 70.

For clinical use of TR cells, natural Foxp3+ TR cells bear unique immunological properties that make them a suitable therapeutic target. They are naturally present in the circulation and can be phenotypically distinguished from other T cells; can recognize a broad repertoire of self and non-self antigens; are phenotypically in an “antigen-primed” state already in the thymus; can be stimulated to proliferate by in vivo antigenic stimulation; and are functionally stable, retaining their suppressive activity after clonal expansion in vivo and in vitro (reviewed in 71). By exploiting this stable and robust suppressive activity and proliferative capacity, in vivo and in vitro strategies that clonally expand antigen-specific natural TR cells is useful to strengthen or re-establish self-tolerance in autoimmune disease or induce tolerance to non-self-antigens in organ transplantation, allergy and IBD, or augment feto-maternal tolerance in pregnancy. As a reciprocal approach, selective reductions in the number or function of natural TR cells while retaining or enhancing effector T cells may be a strategy for provoking and augmenting tumor immunity in cancer patients or microbial immunity in chronic infection.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. History of suppressor T cells
  5. CD4+ T cells with autoimmune-suppressive activity
  6. Naturally arising CD25+CD4+ TR cells and their crucial role in self-tolerance
  7. The functional role of IL-2 and CD25 for natural TR cells
  8. Establishment of in vitro functional assays for natural TR cells
  9. The transcription factor Foxp3 as a key control molecule of TR cell development and function
  10. Clinical perspectives
  11. Conclusions
  12. Acknowledgements
  13. Appendix

This feature has focused its discussion on how natural TR cells have been investigated from the initial characterization of CD4+ T cells with an autoimmune suppressive activity. Yet there are other important studies that have contributed to our current conceptualization of TR cells. For example, dominant transplantation tolerance can be established by administration of anti-CD4 or other mAb, the immunosuppressant cyclosporine A, or transplanting allogeneic or xenogeneic thymic epithelial cells into embryos 7274. There is recent evidence that these types of graft tolerance are maintained by suppressive CD4+ T cells, which are functionally and phenotypically similar to Foxp3+ natural TR cells 75. In addition to Foxp3-expressing TR cells, several types of Foxp3-nonexpressing TR cells have also been reported, including Tr1 and Th3 cells (reviewed in 76). Antigen-induced suppressor T cells that were intensively studied in the 70s and early 80s remain to be re-investigated from a vantage point of the present. Thus, the current active research of T cell-mediated self-tolerance and immune regulation is revealing “unity” and “diversity” of TR cells. Further investigation of TR cells, natural or adaptive, will make it a reality to use them clinically for better control of a variety of physiological and pathological immune responses.

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