A basic tenet of most religious faiths is the belief that those who have died will again be brought to life or resurrected. The past 15 years have seen a resurrection of T suppressor cells, although their soul is now in the body of a T regulatory cell. The studies presented by S. Sakaguchi et al. (1) in the Pillars of Immunology article in this issue are the major reasons for this rebirth. To understand the resurgence of interest in T cell-mediated suppression/regulation, one must first review the factors responsible for its untimely demise. It was proposed more than 40 years ago that a distinct subset of T cells is responsible for immune suppression. Most immunologists credit the discovery of suppressor T cells to the late Dr. Richard Gershon and his colleagues at Yale University (2, 3), but Nishizuka and Sakakura also identified suppressor T cells in a landmark article published in Science in 1969 (4). The studies from the Gershon laboratory were designed to address the phenomenon of “high-zone” tolerance. They noted that when spleen cells from tolerized animals were transferred into secondary recipients together with normal thymocytes and bone marrow cells, T cells from the tolerant animals suppressed the Ab response of the recipient animals to SRBC. Gershon hypothesized at that time that these T suppressors were a distinct population with a fully differentiated gene program that allowed them to perform this very specialized function. In contrast, Nishizuka and Sakakura were studying the pathogenesis of autoimmune oophoritis that developed after neonatal thymectomy (Tx). They noted that mice that were thymectomized on the third day of life (d3Tx) experienced development of organ-specific autoimmune diseases. Autoimmunity was not seen if the mice were thymectomized on day 1 or 7 of life, and disease could be completely prevented if the d3Tx mouse received a thymus transplant between days 10 and 15 of life. The conclusions drawn from these studies led to the hypothesis that autoreactive T cells were exported from the thymus during the first 3 d of life, but that somewhat later in ontogeny a population of suppressor T cells emigrated from the thymus and controlled the autoreactive T cells. d3Tx prevented the suppressor cells from reaching the periphery and resulted in autoimmune disease.

Taken together, these pioneering observations should have been an auspicious beginning for the role of the suppressor T cells in immune responses. However, the approach used by Gershon and multiple other laboratories to further analyze T cell-mediated suppression shifted from studies of the function of intact T cells to studies of their soluble products. T cell suppression was regarded as being mediated by numerous soluble Ag-specific and nonspecific factors that comprised a functionally unique network (5). Many of these soluble factors expressed a product of the MHC, termed I-J, whereas others were claimed to express Ig VH gene products. However, the inability to identify a marker specific for suppressor T cells and the failure to purify these cells raised doubts about the existence of a distinct functional lineage of suppressor T cells. The suppressor T cell field was dealt two deathblows in the mid-1980s. First, the region of the MHC complex to which I-J mapped did not contain a gene that could encode a unique I-J polypeptide (6). Second, when the genes encoding the TCR were isolated, they were completely unrelated to the genes encoding Ig H chains (7). Several articles subsequently appeared questioning the existence of suppressor T cells (8). “Suppression” became a dirty word that was not to be used by cellular immunologists (9).

The studies of Nishizuka and Sakakura (4) were ignored by most immunologists, and subsequent studies from this group concentrated on the characterization of the immunopathology of the different autoimmune diseases generated after d3Tx and ignored characterization of the suppressor cell populations. The only other studies that supported the relation of suppressor T cells and the development of autoimmunity were a series of insightful experiments by Penhale and coworkers in the 1970s (10, 11). They demonstrated that spontaneous thyroiditis developed in 60% of rats after the partial depletion of T cells by adult Tx followed by irradiation. Autoimmune disease could be prevented if the Tx-irradiated mice were reconstituted with lymphoid cells from normal donors on the last day of irradiation. These studies demonstrated that suppression was not unique to the neonate, and that suppressor cells also exist in the normal adult animals that are capable of preventing the activation of autoreactive effector T cells. Further characterization of the suppressor population was not performed.

It was not until the early 1980s that Sakaguchi and coworkers (12) began to characterize the suppressor cells that prevented disease development post-d3Tx. A single injection of spleen cells from an adult mouse given within 2 wk of the Tx protected, but cells from newborn, 7-d-old, or 14-d-old mice failed to protect against autoimmunity. Spleen cells treated with anti–Lyt-1 and complement could not protect, but anti–Lyt-1+ cells that remained after treatment with anti–Lyt-2,3 and complement completely prevented disease. A major theoretical advance in 1985 (13) was to extend the model from Tx studies in the neonate to the normal adult mouse. When spleen cells were treated with anti–Lyt-1 and complement and transferred to nu/nu mice, recipients experienced development of organ-specific autoimmune diseases. Treatment of spleen cells with anti–Lyt-2 and complement had no effect, but cotransfer of Lyt-1+ cells suppressed the ability of Lyt-1 cells to induce disease. Although we now know that Lyt-1 (CD5) is expressed on all CD4+ T cells, the operational definition of Lyt-1+ in this study was based on susceptibility to complement-dependent lysis. This article conceptually extended the studies in the neonate and the studies using adult Tx-irradiation to demonstrate for the first time that suppressor cells played a dominant role in maintaining self-tolerance in the normal animal and formed a solid theoretical basis for the Pillars of Immunology article.

It is somewhat surprising that 10 years passed before the Sakaguchi laboratory continued their characterization of the T suppressor population with the publication of the Pillars of Immunology article. The model described in the 1985 article (13) was ideally suited for continued study because many mAbs to murine cell surface Ags were raised during that time. Parenthetically, the anti-CD25 mAb used in the Pillars of Immunology article to define the CD4+CD25+ suppressor population was described in 1983 (14) and was widely available. A number of publications from Don Mason's laboratory in the early 1990s influenced Sakaguchi to better define the suppressor cells in his model. Mason's group demonstrated that rat CD4+ T cells could be divided into two subpopulations based on the differential expression of CD45RB. CD45RBhigh cells induced graft-versus-hose disease and appeared to be Th1 cells capable of producing IFN-γ, whereas the CD45RBlow cells provided the majority of help for Ab formation and produced primarily IL-4. The CD45RBlow cells inhibited the ability of the CD45RBhigh cells to induce a wasting disease in nu/nu recipients (15) and also protected against the development of diabetes when given to adult Tx-irradiated animals (16). The initial interpretation of these experiments was that the CD45RBlow population functioned to deviate the immune response to a protective Th2 type. However, when this model was extended to a mouse colitis model by Powrie et al. (17), the CD45RBhigh cells that induced the disease were IFN-γ producers, but the CD45RBlow cells did not produce IL-4, suggesting that a more complex process was involved in the interaction between the two T cell subsets.

Sakaguchi et al. in the Pillars of Immunology article (1) attempted to further define a surface marker whose expression correlated with high levels of CD5 expression and with low levels of CD45RB expression. Peripheral T cells expressing CD25 (IL-2R α-chain) were CD5highCD45RBlow and comprised 10–15% of CD4 cells in normal mice. CD25 expression appeared to be more specific than Lyt-1high or CD45RBlow, because depletion of CD25+ cells eliminated only a fraction of the Lyt-1high cells or the CD45RBlow cells. CD4+CD25 T cells readily induced disease in nu/nu recipients, whereas cotransfer of CD4+CD25+ T cells prevented disease. Nu/nu mice reconstituted with CD4+CD25 T cells also rejected skin grafts more rapidly and developed higher Ab titers to a foreign Ag when immunized with BSA than mice reconstituted with total CD4+ T cells. Thus, CD4+CD25+ T cells downregulated not only autoimmune responses, but also cellular and humoral responses to non–self-Ags. The most important conceptual advance in this article is the demonstration that removal of a minor T cell subpopulation is sufficient to allow activation of autoreactive effector T cells in an environment where the target self-Ags are normally expressed at physiologic concentrations. A follow-up article (18) correlated the recent studies with earlier experiments (2) and demonstrated that CD4+CD25+ T cells did not appear in peripheral sites until after day 3 of life, and that CD4+CD25+ T cells could prevent d3Tx-induced autoimmunity if given 1 wk after the Tx. Most importantly, the Pillars of Immunology article raises the issue of the potential mechanisms of action of the CD25+ T cells and their potential target cells, including whether they act on APC or T cells, whether they absorb IL-2 and function as cytokine sinks, or whether they produce suppressor cytokines. Many of these questions still remain unanswered (19).

Although the Pillars of Immunology article has been cited 2182 times at the time of this writing, not many immunologists jumped on the CD4+CD25+ suppressor T cell “bandwagon” at the time of its publication. I have previously (20) summarized the influence this article had on the direction of my own research. However, it is clear that the immunologic climate in 1995 was so antisuppressor T cell that it took 6 y for the first publications identifying a similar population in humans, even though reagents were readily available to perform these studies in 1995 (reviewed in Ref. 21). In contrast, the initial description of Th17 cells in 2006 was rapidly followed by numerous publications (22). Climates do change and the discovery (23) of Foxp3 as the molecular marker for the CD4+CD25+ T cell suppressor population solidified their status as a bona fide lineage. It is always difficult to predict the future direction of research, but there is little doubt in my mind that Foxp3+ T regulatory cells (as they are now called) are now ready for widespread clinical application, including cellular therapy of graft-versus-host disease (24) and graft rejection, depletion to enhance tumor immunity (25), and pharmacologic enhancement of their suppressor function for treatment of autoimmune diseases (26).

Abbreviations used in this article:

d3Tx

thymectomy on the third day of life

Tx

thymectomy.

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The author has no financial conflicts of interest.