Shimon Sakaguchi

Regulatory T Cells and Immune Tolerance

Regulatory T cells (Tregs) play an indispensable role in maintaining immunological unresponsiveness to self-antigens and in suppressing excessive immune responses deleterious to the host. Tregs are produced in the thymus as a functionally mature subpopulation of T cells and can also be induced from naive T cells in the periphery. Recent research reveals the cellular and molecular basis of Treg development and function and implicates dysregulation of Tregs in immunological disease.

Naturally arising Foxp3-expressing CD25 + CD4 + regulatory T cells in immunological tolerance to self and non-self

Naturally arising CD25+CD4+ regulatory T cells actively maintain immunological self-tolerance. Deficiency in or dysfunction of these cells can be a cause of autoimmune disease. A reduction in their number or function can also elicit tumor immunity, whereas their antigen-specific population expansion can establish transplantation tolerance. They are therefore a good target for designing ways to induce or abrogate immunological tolerance to self and non-self antigens.

Regulatory T Cells: Key Controllers of Immunologic Self-Tolerance

The phenomena of T cell–mediated suppression in immunologic tolerance have been controversial and remain an exciting area of active research. An approach from the analysis of autoimmune-preventive activity of normal T cells is now revealing a unique regulatory T cell population dominantly engaged in the maintenance of immunologic self-tolerance. Production of such regulatory T cells can be another key function of the thymus in mediating self-tolerance. Recent research has also revealed that a part of the suppressive phenomena can be attributed to immunosuppressive cytokines secreted by particular types of effector T cells. Therefore, more than one population of regulatory T cells seem to be engaged in the maintenance of self-tolerance and these populations function in different ways—some are locally induced as a result of immune responses, while others are naturally produced. Further characterization of the function and development of these regulatory T cells will contribute to our understanding of immunologic self-tolerance as an acquired process and of the cause and mechanism of autoimmune disease. Manipulation of regulatory T cells will lead to new strategies for the treatment or prevention of autoimmune disease, transplant rejection, and cancer.*E-mail: shimon@frontier.kyoto-u.ac.jp

CTLA-4 Control over Foxp3+ Regulatory T Cell Function

Naturally occurring Foxp3+CD4+ regulatory T cells (Tregs) are essential for maintaining immunological self-tolerance and immune homeostasis. Here, we show that a specific deficiency of cytotoxic T lymphocyte antigen 4 (CTLA-4) in Tregs results in spontaneous development of systemic lymphoproliferation, fatal T cell–mediated autoimmune disease, and hyperproduction of immunoglobulin E in mice, and it also produces potent tumor immunity. Treg-specific CTLA-4 deficiency impairs in vivo and in vitro suppressive function of Tregs—in particular, Treg-mediated down-regulation of CD80 and CD86 expression on dendritic cells. Thus, natural Tregs may critically require CTLA-4 to suppress immune responses by affecting the potency of antigen-presenting cells to activate other T cells.

Stimulation of CD25 + CD4 + regulatory T cells through GITR breaks immunological self-tolerance

CD25+CD4+ regulatory T cells in normal animals are engaged in the maintenance of immunological self-tolerance. We show here that glucocorticoid-induced tumor necrosis factor receptor family–related gene (GITR, also known as TNFRSF18)—a member of the tumor necrosis factor–nerve growth factor (TNF-NGF) receptor gene superfamily—is predominantly expressed on CD25+CD4+ T cells and on CD25+CD4+CD8− thymocytes in normal naïve mice. We found that stimulation of GITR abrogated CD25+CD4+ T cell–mediated suppression. In addition, removal of GITR-expressing T cells or administration of a monoclonal antibody to GITR produced organ-specific autoimmune disease in otherwise normal mice. Thus, GITR plays a key role in dominant immunological self-tolerance maintained by CD25+CD4+ regulatory T cells and could be a suitable molecular target for preventing or treating autoimmune disease.

Immunologic tolerance maintained by CD25+ CD4+ regulatory T cells: their common role in controlling autoimmunity, tumor immunity, and transplantation tolerance

Summary: There is accumulating evidence that T‐cell‐mediated dominant control of self‐reactive T‐cells contributes to the maintenance of immunologic self‐tolerance and its alteration can cause autoimmune disease. Efforts to delineate such a regulatory T‐cell population have revealed that CD25+ cells in the CD4+ population in normal naive animals bear the ability to prevent autoimmune disease in vivo and, upon antigenic stimulation, suppress the activation/proliferation of other T cells in vitro. The CD25+ CD4+ regulatory T cells, which are naturally anergic and suppressive, appear to be produced by the normal thymus as a functionally distinct subpopulation of T cells. They play critical roles not only in preventing autoimmunity but also in controlling tumor immunity and transplantation tolerance.

FOXP3 + regulatory T cells in the human immune system

Forkhead box P3 (FOXP3)+ regulatory T (TReg) cells are potent mediators of dominant self tolerance in the periphery. But confusion as to the identity, stability and suppressive function of human TReg cells has, to date, impeded the general therapeutic use of these cells. Recent studies have suggested that human TReg cells are functionally and phenotypically diverse. Here we discuss recent findings regarding human TReg cells, including the ontogeny and development of TReg cell subsets that have naive or memory phenotypes, the unique mechanisms of suppression mediated by TReg cell subsets and factors that regulate TReg cell lineage commitment. We discuss future studies that are needed for the successful therapeutic use of human TReg cells.

Foxp3+ CD25+ CD4+ natural regulatory T cells in dominant self-tolerance and autoimmune disease.

Summary:  Naturally arising CD25+CD4+ regulatory T (Treg) cells, most of which are produced by the normal thymus as a functionally mature T‐cell subpopulation, play key roles in the maintenance of immunologic self‐tolerance and negative control of a variety of physiological and pathological immune responses. Natural Tregs specifically express Foxp3, a transcription factor that plays a critical role in their development and function. Complete depletion of Foxp3‐expressing natural Tregs, whether they are CD25+ or CD25–, activates even weak or rare self‐reactive T‐cell clones, inducing severe and widespread autoimmune/inflammatory diseases. Natural Tregs are highly dependent on exogenously provided interleukin (IL)‐2 for their survival in the periphery. In addition to Foxp3 and IL‐2/IL‐2 receptor, deficiency or functional alteration of other molecules, expressed by T cells or non‐T cells, may affect the development/function of Tregs or self‐reactive T cells, or both, and consequently tip the peripheral balance between the two populations toward autoimmunity. Elucidation of the molecular and cellular basis of this Treg‐mediated active maintenance of self‐tolerance will facilitate both our understanding of the pathogenetic mechanism of autoimmune disease and the development of novel methods of autoimmune disease prevention and treatment via enhancing and re‐establishing Treg‐mediated dominant control over self‐reactive T cells.

Regulatory T cells in transplantation tolerance.

The identification and characterization of regulatory T (TReg) cells that can control immune responsiveness to alloantigens have opened up exciting opportunities for new therapies in transplantation. After exposure to alloantigens in vivo, alloantigen-specific immunoregulatory activity is enriched in a population of CD4+ T cells that express high levels of CD25. In vivo, common mechanisms seem to underpin the activity of CD4+CD25+ TReg cells in both naive and manipulated hosts. However, the origin, allorecognition properties and molecular basis for the suppressive activity of CD4+CD25+ TReg cells, as well as their relationship to other populations of regulatory cells that exist after transplantation, remain a matter of debate.

Functional Delineation and Differentiation Dynamics of Human CD4+ T Cells Expressing the FoxP3 Transcription Factor

FoxP3 is a key transcription factor for the development and function of natural CD4(+) regulatory T cells (Treg cells). Here we show that human FoxP3(+)CD4(+) T cells were composed of three phenotypically and functionally distinct subpopulations: CD45RA(+)FoxP3(lo) resting Treg cells (rTreg cells) and CD45RA(-)FoxP3(hi) activated Treg cells (aTreg cells), both of which were suppressive in vitro, and cytokine-secreting CD45RA(-)FoxP3(lo) nonsuppressive T cells. The proportion of the three subpopulations differed between cord blood, aged individuals, and patients with immunological diseases. Terminally differentiated aTreg cells rapidly died whereas rTreg cells proliferated and converted into aTreg cells in vitro and in vivo. This was shown by the transfer of rTreg cells into NOD-scid-common gamma-chain-deficient mice and by TCR sequence-based T cell clonotype tracing in peripheral blood in a normal individual. Taken together, the dissection of FoxP3(+) cells into subsets enables one to analyze Treg cell differentiation dynamics and interactions in normal and disease states, and to control immune responses through manipulating particular FoxP3(+) subpopulations.