Major histocompatibility complex class II-positive cortical epithelium mediates the selection of CD4(+)25(+) immunoregulatory T cells

Regulating regulatory T cells

Regulatory T cells (Tregs) are a specialized subpopulation of T cells that act to suppress activation of other immune cells and thereby maintain immune system homeostasis, self-tolerance as well as control excessive response to foreign antigens. The mere concept of Tregs was the subject of significant controversy among immunologists for many years owing to the paucity of reliable markers for defining these cells and the ambiguity of the nature and molecular basis of suppressive phenomena. However, recent advances in the molecular characterization of this cell population have firmly established their existence and their vital role in the vertebrate immune system. Of interest, accumulating evidence from both humans and experimental animal models has implicated the involvement of Tregs in the development of graft-versus-host disease (GVHD). The demonstration that Tregs could separate GVHD from graft-versus-tumor (GVT) activity suggests that their immunosuppressive potential could be manipulated to reduce GVHD without detrimental consequence on GVT effect. Although a variety of T lymphocytes with suppressive capabilities have been reported, the two best-characterized subsets are the naturally arising, intrathymic-generated Tregs (natural Tregs) and the peripherally generated, inducible Tregs (inducible Tregs). This review summarizes our current knowledge of the generation, function and regulation of these two populations of Tregs during an immune response. Their role in the development of GVHD and their therapeutic potential for the prevention and treatment of GVHD will also be described.

Donor-derived thymic-dependent T cells cause chronic graft-versus-host disease

Chronic graft-versus-host disease (GVHD) is the most common cause of poor long-term outcomes after allogeneic bone marrow transplantation (BMT), but the pathophysiology of chronic GVHD still remains poorly understood. We tested the hypothesis that the impaired thymic negative selection of the recipients will permit the emergence of pathogenic T cells that cause chronic GVHD. Lethally irradiated C3H/HeN (H-2k) recipients were reconstituted with T-cell–depleted bone marrow cells from major histocompatibility complex [MHC] class II–deficient (H2-Ab1−/−) B6 (H-2b) mice. These mice developed diseases that showed all of the clinical and histopathological features of human chronic GVHD. Thymectomy prevented chronic GVHD, thus confirming the causal association of the thymus. CD4+ T cells isolated from chronic GVHD mice were primarily donor reactive, and adoptive transfer of CD4+ T cells generated in these mice caused chronic GVHD in C3H/HeN mice in the presence of B6-derived antigen-presenting cells. Our results demonstrate for the first time that T cells that escape from negative thymic selection could cause chronic GVHD after allogeneic BMT. These results also suggest that self-reactivity of donor T cells plays a role in this chronic GVHD, and improvement in the thymic function may have a potential to decrease chronic GVHD.

Making regulatory T cells with defined antigen specificity: role in autoimmunity and cancer

There is increasing evidence that agonist ligand presentation either intrathymically or extrathymically plays a crucial if not essential role in the generation of regulatory T cells (Tregs). Thus, it is possible to induce Tregs of any desired specificity in vivo. The same goal can be achieved in vitro by expanding antigen-specific CD4+ T cells and retrovirally transducing them. In contrast, in vitro expansion of Tregs is limited to antigens that have resulted in Treg generation in vivo. Antigen-specific Tregs can be used in cellular therapy with the goal to prevent autoimmune disease or even to interfere with established autoimmunity. The latter requires that the Tregs can suppress effector cells that have already caused harm, which is possible because of the antigen-dependent homing properties of Tregs, i.e. these cells can accumulate in antigen-draining lymph nodes and exit into inflamed tissue. Generally, the in vivo interference is dependent on cytokines such as transforming growth factor-beta and interleukin-10 that were dispensable in in vivo analysis of immunosuppression. The precise mechanisms of suppression remain enigmatic, however, but may be further elucidated by the molecular analysis of suppressed versus non-suppressed T cells.

Role of TCR specificity in CD4+ CD25+ regulatory T-cell selection

CD4+ CD25+ regulatory T cells play a crucial role in preventing autoimmune disease and can also modulate immune responses in settings such as transplantation and infection. We have developed a transgenic mouse system in which the role that T-cell receptor (TCR) specificity for self-peptides plays in the formation of CD4+ CD25+ regulatory T cells can be examined. We have shown that interactions with a single self-peptide can induce thymocytes bearing an autoreactive TCR to undergo selection to become CD4+ CD25+ regulatory T cells and that thymocytes bearing TCRs with low affinity for the selecting peptide do not appear to undergo selection into this pathway. In addition, thymocytes with identical specificity for the selecting self-peptide can undergo overt deletion versus abundant selection to become CD4+ CD25+ regulatory T cells in response to variations in expression of the selecting peptide in different lineages of transgenic mice. Finally, we have shown that CD4+ CD25+ T cells proliferate in response to their selecting self-peptide in the periphery, but these cells do not proliferate in response to lymphopenia in the absence of the selecting self-peptide. These studies are determining how the specificity of the TCR for self-peptides directs the thymic selection and peripheral expansion of CD4+ CD25+ regulatory T cells.

Role of Naturally Arising Regulatory T Cells in Hematopoietic Cell Transplantation

Naturally arising CD4(+)CD25(+) regulatory T cells (Tregs) have the potential to suppress aberrant immune responses and to regulate peripheral T-cell homeostasis. In murine models of bone marrow transplantation, Tregs promote donor bone marrow engraftment and decrease the incidence and severity of graft-versus-host-disease without abrogating the beneficial graft-versus-tumor immunologic effect. These findings, in concert with observations that Tregs in mice and humans share phenotypic and functional characteristics, have led to active investigations into the use of these cells to decrease complications associated with human hematopoietic cell transplantation. Early human studies suggest that an imbalance of Tregs and effector T cells may contribute to the development of graft-versus-host-disease. However, the mechanisms of immunoregulation, in particular the allorecognition properties of Tregs, their effects on and interaction with other immune cells, and their sites of suppressive activity, are not well understood. In this review, we discuss the current knowledge of Treg biology and the potential therapeutic strategies and barriers of Treg immunotherapy in human hematopoietic cell transplantation.

NY-ESO-1: review of an immunogenic tumor antigen

In the 9 years since its discovery, cancer-testis antigen NY-ESO-1 has made one of the fastest transitions from molecular, cellular, and immunological description to vaccine and immunotherapy candidate, already tested in various formulations in more than 30 clinical trials worldwide. Its main characteristic resides in its capacity to elicit spontaneous antibody and T-cell responses in a proportion of cancer patients. An overview of immunological findings and immunotherapeutic approaches with NY-ESO-1, as well the role of regulation in NY-ESO-1 immunogenicity, is presented here.

Antigen-Specific Regulatory T-Cell Subsets in Transplantation Tolerance

Regulatory T cells (Treg) are critical controllers of the immune response. Disturbed Treg function results in autoimmunity, whereas in transplantation Treg are crucial in graft survival and transplant tolerance. Hence therapeutic modalities that influence Treg numbers or function hold great clinical opportunity. Ahead of us are clinical trails studying in vivo Treg induction protocols and immunotherapy with ex vivo expanded Treg. Here we discuss the preferential use and/or induction of antigen-specific Treg subsets with high suppressive power and migratory capacity as a potential therapeutic tool to prevent solid organ transplantation rejection. Accordingly, ex vivoselection procedures to induce and isolate highly suppressive antigen-specific Treg (subsets) are needed. This subject, as well as the Treg-facilitating potential of immunosuppressive agents, is discussed.

Agonist ligands expressed by thymic epithelium enhance positive selection of regulatory T lymphocytes from precursors with a normally diverse TCR repertoire

CD4+CD25+ regulatory T lymphocytes play a crucial role in inhibition of autoimmune pathology. In accordance with this physiological role, it is now well established that the repertoire of these lymphocytes is strongly enriched in autospecific cells. However, despite extensive investigation, the thymic mechanisms involved in development of regulatory T cells remain incompletely defined. To address the issue of selection of regulatory T cell precursors in mice with a naturally diverse TCR repertoire, we have analyzed development of superantigen-specific regulatory T cells in hemopoietic chimeras in which endogenous super-antigens are exclusively presented by thymic epithelial cells. Our results demonstrate that recognition of agonist ligands expressed by thymic epithelium does not lead to deletion but substantially enhances development of mature regulatory T cells. Interestingly, also development of a small subpopulation of CD25-expressing T cells lacking expression of the transcription factor Foxp3, thought to be autospecific, is enhanced by expression of the agonist ligand on thymic epithelium. Based on quantitative arguments, we propose that commitment to the regulatory T cell lineage is not dictated by the specificity of precursors, but that recognition of the agonist ligand expressed by thymic epithelium substantially enhances their positive selection.

Function of tumor necrosis factor receptor family members on regulatory T-cells

Effector cells play a crucial role in the immune system of higher vertebrates in eliminating invading pathogens and transformed cells that could cause disease or death of the individual. To be effective and specific, immune responses have to distinguish between self and nonself. Mechanisms of central and peripheral tolerance have evolved to control effector cells that could respond to autoantigens. Regulatory T-cells (Treg cells) are critical modulators of effector cells in the periphery that suppress autoreactive T-cells but are also involved in modulating immune responses against invading pathogens. Identification of surface markers of Treg cells and the development of in vitro systems to study the suppressive function of Treg cells have revealed distinct phenotypic and functional subsets of Treg cells. Several tumor necrosis factor receptor (TNFR) family members have been shown to play a role in the development, homeostasis, and suppressor function of Treg cells. Recent findings suggest that TNFRs and other cell-surface molecules of Treg cells can be explored for therapeutic strategies targeting autoimmune disorders, cancer, and immune responses against pathogens.

The association of CTLA-4 and HLA class II autoimmune risk genotype with regulatory T cell marker expression in 5-year-old children

Regulatory T cells (Treg) are involved in the maintenance of peripheral tolerance by suppression of autoreactive lymphocytes that have avoided thymic depletion. The defective function of Treg cells has recently attracted attention in autoimmune diseases such as type 1 diabetes (T1D), rheumatoid arthritis and multiple sclerosis. Susceptibility to these diseases is associated with specific human leucocyte antigen (HLA) class II and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) gene polymorphisms. This study aimed to investigate the relationship between HLA class II and CTLA +49 A/G polymorphisms associated with susceptibility to T1D and the number and characteristics of Treg cells in children. Samples from 47 5-year-old children who participated in the All Babies in South-east Sweden (ABIS) follow-up study were grouped according to the presence of the T1D risk-associated HLA genotype (DQA1*0501-DQB1*0201, DQA1*0301-DQB1*0302) or neutral HLA genotypes. Lower percentages of CD4+ T cells (P = 0.03) and CD4+ CD25high cells (P = 0.06) expressing intracellular CTLA-4 were detected in samples from children with CTLA-4 +49GG compared to children with the +49AA genotype. Similarly, lower percentages of CD4+ (P = 0.002) and CD4+ CD25high (P = 0.002) cells expressing CTLA-4 were observed in children positive for HLA DQA1*0501-DQB1*0201 and DQA1*0301-DQB1*0302 (P = 0.04 for CD4+ and P = 0.02 for CD4+ CD25high) risk haplotypes when compared to children without these alleles. The percentage of CD25high cells among CD4+ cells was correlated inversely with CTLA-4 mRNA expression in PBMC (r = -0.56, P = 0.03). Decreased levels of CTLA-4 in CD4+ and CD4+ CD25high cells in individuals with CTLA-4 and HLA class II alleles associated with T1D may contribute to the initiation and/or progression of autoimmune response.

Th17: an effector CD4 T cell lineage with regulatory T cell ties

The naive CD4 T cell is a multipotential precursor with defined antigen recognition specificity but substantial plasticity for development down distinct effector or regulatory lineages, contingent upon signals from cells of the innate immune system. The range of identified effector CD4 T cell lineages has recently expanded with description of an IL-17-producing subset, called Th17, which develops via cytokine signals distinct from, and antagonized by, products of the Th1 and Th2 lineages. Remarkably, Th17 development depends on the pleiotropic cytokine TGF-beta, which is also linked to regulatory T cell development and function, providing a unique mechanism for matching CD4 T cell effector and regulatory lineage specification. Here, we review Th17 lineage development, emphasizing similarities and differences with established effector and regulatory T cell developmental programs that have important implications for immune regulation, immune pathogenesis, and host defense.

Origin and T cell receptor diversity of Foxp3+CD4+CD25+ T cells

Foxp3(+)CD4(+)CD25(+) regulatory T cells can differentiate from Foxp3(-)CD4(+) medullary thymocytes and Foxp3(-)CD4(+) naive T cells. However, the impact of these two processes on size and composition of the peripheral repertoire of regulatory T cells is unclear. Here we followed the fate of individual Foxp3(+)CD4(+)CD25(+) thymocytes and T cells in vivo in T cell receptor (TCR) transgenic mice that express a restricted but polyclonal repertoire of TCRs. By utilizing high-throughput single-cell analysis, we showed that Foxp3(+)CD4(+) peripheral T cells were derived from thymic precursors that expressed a different TCRs than Foxp3(-)CD4(+) medullary thymocytes and Foxp3(-)CD4(+) T cells. Furthermore, the diversity of TCRs on Foxp3(+)CD4(+) regulatory T cells exceeded the diversity of TCRs on Foxp3(-)CD4(+) naive T cells, even in mice that lack expression of tissue-specific antigens. Our results imply that higher TCR diversity on Foxp3(+) regulatory T cells helps these cells to match the specificities of autoreactive and naive T cells.

IPEX and the role of Foxp3 in the development and function of human Tregs

Genetic defects in the transcription factor forkhead box protein P3 (Foxp3) cause immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX). IPEX is thought to be due to a defect in naturally arising CD4+ Tregs. In this issue of the JCI, Bacchetta and colleagues demonstrate that patients with IPEX and missense mutations in Foxp3 provide insight into the role of various domains of Foxp3 in the development and function of Tregs (see the related article beginning on page 1713).

Translating the concept of suppressor/regulatory T cells to clinical applications

The in vivo expansion of suppressor/regulatory T cells (Tregs) is a desirable event in autoimmunity and transplantation. Here we summarize the general rules involved in antigen recognition by T cells and describe Tregs and their requirements, discussing different levels of immune intervention.

Induction of antigen-specific tolerance by intrathymic injection of lentiviral vectors

Immune tolerance to self-antigens is established during lymphocyte differentiation in the thymus, but a simple means to induce antigen-specific tolerance in the thymus is still elusive. We show here that intrathymic injection of a lentiviral vector expressing the hemagglutinin antigen (HA) in TCR-HA transgenic mice resulted in negative selection of HA-specific effector T cells and sustained positive selection of HA-specific regulatory T cells (Tregs). This positive selection increased the number of HA-specific Tregs 10-fold and was comparable with the one observed in TCR-HA transgenic mice crossed with transgenic mice expressing HA under the control of the insulin promoter (Ins-HA). HA expression by radioresistant thymic epithelial cells was sufficient to drive Treg generation. Intrathymic injection of the lentiviral vector also resulted in an enrichment of HA-specific Tregs in peripheral lymphoid organs, which prevented diabetes induced in Ins-HA mice by transfer of HA-specific effector T cells. In this model, HA-specific Tregs inhibited effector T-cell division in pancreatic lymph nodes. Finally, we show that intrathymic injection of a lentiviral vector expressing preproinsulin-2 could reduce the occurrence of spontaneous diabetes in nonobese diabetic mice. Intrathymic gene transfer using lentiviral vectors thus offers new means to manipulate antigen-specific tolerance.

A central role for central tolerance

Recent elucidation of the role of central tolerance in preventing organ-specific autoimmunity has changed our concepts of self/nonself discrimination. This paradigmatic shift is largely attributable to the discovery of promiscuous expression of tissue-restricted self-antigens (TRAs) by medullary thymic epithelial cells (mTECs). TRA expression in mTECs mirrors virtually all tissues of the body, irrespective of developmental or spatio-temporal expression patterns. This review summarizes current knowledge on the cellular and molecular regulation of TRA expression in mTECs, outlines relevant mechanisms of antigen presentation and modes of tolerance induction, and discusses implications for the pathogenesis of autoimmune diseases and other biological processes such as fertility, pregnancy, puberty, and tumor defense.

Modulation of graft-versus-host disease: role of regulatory T lymphocytes

Graft-versus-host (GVH) disease (GVHD) continues to be a major life-threatening complication after allogeneic bone marrow transplantation. Considerable progress has been made elucidating the pathophysiology of acute GVHD. Mature donor T cells transferred along with the marrow graft directly recognize antigenic differences on antigen-presenting cells of the host. Once activated, donor antihost-specific T cells can mediate tissue destruction. Interestingly, the failure to clonally delete autoreactive T cells in the thymus can also lead to an autoimmune syndrome mimicking the pathology of GVHD. Negative selection in the thymus may be compromised either by damage to the thymic epithelium (because of a direct attack by donor antihost alloreactive T cells) or by the use of immunosuppressive drugs that inhibit clonal deletion. An important component underlying GVHD mediated by either alloreactive or autoreactive T cells is the absence of a competent peripheral regulatory system. Studies in animal model systems clearly indicate that regulatory T cells play a vital role in down-regulating GVHD and are critically important for the establishment of active dominant tolerance to both allo- and self-major histocompatibility complex antigens. Although multiple populations of cells appear to participate in this process, CD4(+) regulatory T cells that innately express CD25(+) appear to orchestrate the regulatory control of the immune response. Evidence for regulatory T cells in clinical bone marrow transplantation, however, remains rudimentary. The recent identification that CD4(+)CD25(+) regulatory T cells preferentially express the Foxp3 nuclear transcription factor and the development of molecular reagents to isolate antigen-specific T cells have provided unique opportunities to explore immunoregulatory mechanisms after clinical marrow transplantation. Recent studies in recipients of clinical bone marrow transplantation suggest that antigen-specific CD4(+)CD25(+)Foxp3(+) T cells play a vital role in the regulatory control of GVH reactions mediated by both alloreactive and autoreactive lymphocytes. These regulatory T cells also appear to facilitate the establishment of donor antihost and donor antidonor (self) tolerance.

Foxp3+ CD25+ regulatory T cells specific for a neo-self-antigen develop at the double-positive thymic stage

Thymus-derived regulatory T cells (Tregs) expressing CD4, CD25, and the transcription factor Foxp3 play major roles in preventing autoimmunity. The Treg population is enriched in T cells expressing high-avidity self-reactive T cell receptors, and thymic epithelial cells expressing self-antigens (Ag) have been implicated in their induction and/or selection. However, the thymic selection events leading to Treg lineage commitment remain unclear. We followed the thymic development of self-Ag-specific Tregs in double-transgenic mice coexpressing a neo-self-Ag, hemagglutinin (HA) under the control of a neural tissue-specific promoter, and a transgenic class II-restricted T cell antigen receptor specific for HA111-119. Our data show that the promiscuous expression of the HA transgene in thymic epithelial cells is involved in the selective induction and/or expansion of HA-specific Foxp3(+) Treg thymic precursors as early as the double-positive stage.

Emerging possibilities in the development and function of regulatory T cells

CD25+CD4+ Regulatory T cells (Treg) represent a unique population of lymphocytes capable of powerfully suppressing immune responses. A large body of experimental data have now confirmed the essential role played by these cells in a host of clinically relevant areas such as self-tolerance, transplantation, allergy and tumor/microbial immunity. Despite this mass of knowledge, significant gaps in our understanding of fundamental Treg biology remain, particularly regarding their development and mechanisms of suppression. In this review we attempt to highlight the current controversies and directions in which this exciting field is moving.

Therapeutic potential of self-antigen-specific CD4+ CD25+ regulatory T cells selected in vitro from a polyclonal repertoire

CD4+ CD25+ regulatory T cells (Treg) play a major role in the prevention of autoimmune diseases. Converging evidence indicates that Treg specific for self-antigens expressed by target tissues have a greater therapeutic potential than polyclonal Treg. Therefore, the selective expansion of rare self-antigen-specific T(reg) naturally present in a polyclonal repertoire of Treg is of major importance. In this work, we investigated the potential of different dendritic cell (DC) subsets to expand antigen-specific Treg in mice. The in vitro selective expansion of rare islet-specific Treg from polyclonal Treg could only be achieved efficiently by stimulation with CD8+ splenic DC presenting islet antigens. These islet-specific Treg exerted potent bystander suppression on diabetogenic T cells and prevented type 1 diabetes. This approach opens new perspectives for cell therapy of autoimmune diseases.

The blind-spot of regulatory T cells

CD4+ CD25+ Foxp3+ regulatory T cells (Treg) are natural suppressors of autoimmunity but they can also dampen the effective clearance of infectious organisms. These cells have the potential to be exploited to prevent transplant rejection and to treat autoimmune disease. A paper in this issue of the European Journal of Immunology details a method to selectively expand antigen-specific Treg from a polyclonal Treg population, by using a specific dendritic cell (DC) subset. Furthermore, the authors show that such Treg can be used to prevent experimental type I diabetes; however, as Treg are positively selected by thymic epithelial cells (TEC) on the basis of self-reactivity, they would systematically suppress protective immune responses unless their repertoire is devoid of recognition towards peripheral antigen-presenting cells. This may be achieved by negative selection of developing Treg on thymic DC, thus creating a 'blind-spot' corresponding to DC-self-antigens in the mature Treg repertoire. Therefore, therapeutic use of DC subsets for the expansion of rare Treg populations should take into account this blind-spot, as peptides that are not accessible to thymic DC may be significantly more effective for the expansion of Treg.

In vivo peripheral expansion of naive CD4+CD25highFoxP3+ regulatory T cells in patients with multiple myeloma

In solid tumors, leukemias, and lymphomas, increased frequencies of functional CD4+CD25high regulatory T cells (Treg cells) have been previously demonstrated. In healthy individuals, Treg cells consist not only of memory but also of naive T cells, which can undergo peripheral expansion and are characterized by a relative enrichment for autoreactive T-cell receptors. Here, we demonstrate in patients with premalignant monoclonal gammopathy of undetermined significance and patients with multiple myeloma that functional FoxP3+ Treg cells of naive, central, and effector memory phenotype as determined by CCR7 and CD45RA expression are significantly expanded. Low frequencies of T-cell receptor excision circles in naive Treg cells in both healthy controls and multiple myeloma patients point to peripheral expansion as the prominent mechanism of increased frequencies of naive Treg cells in these cancer patients. These findings strongly suggest that the increase of functional Treg cells in cancer patients is a response to the process of malignant transformation.

Developmental regulation of Foxp3 expression during ontogeny

Thymectomy of neonatal mice can result in the development of autoimmune pathology. It has been proposed that thymic output of regulatory T (T reg) cells is delayed during ontogeny and that the development of autoimmune disease in neonatally thymectomized mice is caused by the escape of self-reactive T cells before thymectomy without accompanying T reg cells. However, the kinetics of T reg cell production within the thymus during ontogeny has not been assessed. We demonstrate that the development of Foxp3-expressing T reg cells is substantially delayed relative to nonregulatory thymocytes during ontogeny. Based on our data, we speculate that induction of Foxp3 in developing thymocytes and, thus, commitment to the T reg cell lineage is facilitated by a signal largely associated with the thymic medulla.

Antigen-induced IL-10+ regulatory T cells are independent of CD25+ regulatory cells for their growth, differentiation, and function

Recent studies have emphasized the importance of T cells with regulatory/suppressor properties in controlling autoimmune diseases. A number of different types of regulatory T cells have been described with the best characterized being the CD25(+) population. In addition, it has been shown that regulatory T cells can be induced by specific Ag administration. In this study, we investigate the relationship between peptide-induced, CD4(+) regulatory T cells and naturally occurring CD4(+)CD25(+) cells derived from the Tg4 TCR-transgenic mouse. Peptide-induced cells were FoxP3(-) and responded to Ag by secreting IL-10, whereas CD25(+) cells failed to secrete this cytokine. Both cell types were able to suppress the proliferation of naive lymphocytes in vitro although with distinct activation sensitivities. Depletion of CD25(+) cells did not affect the suppressive properties of peptide-induced regulators. Furthermore, peptide-induced regulatory/suppressor T cells could be generated in RAG(-/-), TCR-transgenic mice that do not spontaneously generate CD25(+) regulatory cells. These results demonstrate that these natural and induced regulatory cells fall into distinct subsets.

T regulatory cell suppression of colitis: the role of TGF-beta

Transforming growth factor β (TGF‐β) and interleukin 2 may be involved in IBD peripheral regulatory T cell pathophysiology, raising the possibility of therapeutic application of TGF‐β induced regulatory T cells in IBD patients

Antibody-induced transplantation tolerance that is dependent on thymus-derived regulatory T cells

Targeting of the CD45RB isoform by mAb (anti-CD45RB) effectively induces donor-specific tolerance to allografts. The immunological mechanisms underlying the tolerant state remain unclear although some studies have suggested the involvement of regulatory T cells (T-regs). Although their generative pathway remains undefined, tolerance promoting T-regs induced by systemic anti-CD45RB treatment have been assumed to originate in the peripheral immune system. We demonstrate herein that separable effects on the peripheral and central immune compartments mediate graft survival induced by anti-CD45RB administration. In the absence of the thymus, anti-CD45RB therapy is not tolerogenic though it retains peripheral immunosuppressive activity. The thymus is required for anti-CD45RB to produce indefinite graft survival and donor-specific tolerance, and this effect is accomplished through thymic production of donor-specific T-regs. These data reveal for the first time an Ab-based tolerance regimen that relies on the central tolerance pathway.

CD4+CD25+ regulatory T lymphocytes in bone marrow transplantation

Induction of immunological tolerance to alloantigens would be the treatment of choice to prevent graft-versus-host disease (GvHD) and allograft rejection in transplantation medicine. Organisms use a variety of mechanisms to avoid potentially deadly immunity to self-antigens. The most potent self-tolerance mechanism is probably dominant tolerance assured by regulatory and suppressor T lymphocytes. It appears therefore attractive to use the same mechanism to induce transplantation-tolerance. We here review and discuss recent advances in the use of one of the best-characterized regulatory T lymphocyte populations, CD4(+)CD25(+) T cells, to prevent graft-versus-host disease and bone marrow allograft rejection.

CD18 is required for optimal development and function of CD4+CD25+ T regulatory cells

CD4+CD25+ T regulatory (Treg) cells inhibit immunopathology and autoimmune disease in vivo. CD4+CD25+ Treg cells' capacity to inhibit conventional T cells in vitro is dependent upon cell-cell contact; however, the cell surface molecules mediating this cell:cell contact have not yet been identified. LFA-1 (CD11a/CD18) is an adhesion molecule that plays an established role in T cell-mediated cell contact and in T cell activation. Although expressed at high levels on murine CD4+CD25+ Treg cells, the role of LFA-1 in these cells has not been defined previously. We hypothesized that LFA-1 may play a role in murine CD4+CD25+ Treg function. To evaluate this, we analyzed LFA-1-deficient (CD18-/-) CD4+CD25+ T cells. We show that CD18-/- mice demonstrate a propensity to autoimmunity. Absence of CD18 led to diminished CD4+CD25+ T cell numbers and affected both thymic and peripheral development of these cells. LFA-1-deficient CD4+CD25+ T cells were deficient in mediating suppression in vitro and in mediating protection from colitis induced by the transfer of CD4+CD25- T cells into lymphopenic hosts. Therefore, we define a crucial role for CD18 in optimal CD4+CD25+ Treg development and function.

Developmental alterations in thymocyte sensitivity are actively regulated by MHC class II expression in the thymic medulla

Developing thymocytes are positively selected if they respond to self-MHC-peptide complexes, yet mature T cells are not activated by those same self-complexes. To avoid autoimmunity, positive selection must be followed by a period of maturation when the cellular response to TCR signals is altered. The mechanisms that mediate this postselection developmental tuning remain largely unknown. Specifically, it is unknown whether developmental tuning is a preprogrammed outcome of positive selection or if it is sensitive to ongoing interactions between the thymocyte and the thymic stroma. We probed the requirement for MHC class II-TCR interactions in postselection maturation by studying single positive (SP) CD4 thymocytes from K14/A(beta)(b) mice, in which CD4 T cells cannot interact with MHC class II in the thymic medulla. We report here that SP CD4 thymocytes must receive MHC class II signals to avoid hyperactive responses to TCR signals. This hyperactivity correlates with decreased expression of CD5; however, developmental tuning can occur independently of CD5, correlating instead with differences in the distribution of Lck. Thus, the maturation of postselection SP CD4 thymocytes is an active process mediated by ongoing interactions between the T cell and MHC class II molecules. This represents a novel mechanism by which the thymic medulla prevents autoreactivity.

Regulatory T cells

Immunologic self-tolerance is critically dependent on the induction but also on the downregulation of immune responses. Though ignored and neglected for many years, suppressor T cells, now renamed regulatory T cells (Tregs), play an important role in the negative regulation of immune responses. Several subsets of Tregs have been described. Naturally occurring CD4(+)CD25(+) Tregs are important in the prevention of autoimmune diseases. Type 1 Tregs, another subtype of Treg that is inducible, exert their suppressive activity primarily via the release of IL-10. Detailed knowledge about the phenotype and mode of action of these cells will significantly increase our understanding of the pathogenesis of autoimmune diseases and will also help to identify new therapeutic strategies.

Characterization of thymocyte phenotypic alterations induced by long-lasting beta-adrenoceptor blockade in vivo and its effects on thymocyte proliferation and apoptosis

Adult male Wistar rats were subjected to propranolol (P, 0.40 mg/100 g/day) or saline (S) administration (controls) over 14 days. The expression of major differentiation molecules on thymocytes and Thy-1 (CD90) molecules, which are shown to adjust thymocyte sensitivity to TCRalphabeta signaling, was studied. In addition, the sensitivity of thymocytes to induction of apoptosis and concanavalin A (Con A) signaling was estimated. The thymocytes from P-treated (PT) rats exhibited an increased sensitivity to induction of apoptosis, as well as to Con A stimulation. Furthermore, P treatment produced changes in the distribution of thymocyte subsets suggesting that more cells passed positive selection and further differentiated into mature CD4+ or CD8+ single positive (SP) TCRalphabeta(high) cells. These changes may, at least partly, be related to the markedly increased density of Thy-1 surface expression on TCRalphabeta(low) thymocytes from these rats. The increased frequency of cells expressing the CD4+25+ phenotype, which has been shown to be characteristic for regulatory cells in the thymus, may also indicate alterations in thymocyte selection following P treatment. Inasmuch as positive and negative selections play an important role in continuously reshaping the T-cell repertoire and maintaining tolerance, the hereby presented study suggests that pharmacological manipulations with beta-AR signaling, or chemically evoked alterations in catecholamine release, may interfere with the regulation of thymocyte selection, and consequently with the immune response.

Establishment and functioning of intrathymic microenvironments

The thymus supports the production of self-tolerant T cells from immature precursors. Studying the mechanisms regulating the establishment and maintenance of stromal microenvironments within the thymus therefore is essential to our understanding of T-cell production and ultimately immune system functioning. Despite our ability to phenotypically define stromal cell compartments of the thymus, the mechanisms regulating their development and the ways by which they influence T-cell precursors are still unclear. Here, we review recent findings and highlight unresolved issues relating to the development and functioning of thymic stromal cells.

The impact of CD4+CD25+ Treg on tumor specific CD8+ T cell cytotoxicity and cancer

There is sufficient evidence to suggest that tumor growth elicits specific immune responses, including CD8(+) and CD4(+) T cell responses that may delay tumor growth and could potentially be harnessed to eradicate cancer. Nevertheless the frequent outcome of cancer is lethality associated with uncontrolled growth and dissemination of tumor cells. The failure of the immune response may be naturally programmed and related to a specific subpopulation of CD4(+)CD25(+) regulatory T cells, whose function is to protect us against autoimmunity. Recent investigations have shed light on the in vivo behavior and functions of these cells. It is becoming evident that a major impact of these cells is on the cytolytic action of specific CD8(+) T cells that target the tumor. Inhibition of cytotoxicity is dependent on TGF-beta signaling by the effector cells. Thus, targeting immune regulation may provide a promising approach to the immune therapy of cancer. This approach however could also have unexpected deleterious consequences, as surprising new observations indicate that regulatory T cells can also delay tumor growth by independent mechanisms that relate to their cross talk with the innate immune response to cancer.

CD4+CD25+ Regulatory T Cells and Graft-Versus-Host Disease

Peripheral suppression of autoreactive T cells by specialized T-cell populations is one of several mechanisms ensuring self-tolerance within the adaptive immune system. Thymus-derived CD4+CD25+ T cells expressing the transcriptional repressor FOXP3 mediate such immunoregulatory functions and are pivotal for the prevention of autoimmunity. As peripheral tolerance induction is a prerequisite for successful treatment outcome after allogeneic hematopoietic stem cell transplantation (HSCT), the role of CD4+CD25+ T cells in transplantation models and clinical trials is now under investigation in many laboratories. Here we summarize recent results regarding protection from graft-versus-host disease (GVHD) by adoptively transferred CD4+CD25+ T cells in mice and discuss early findings from clinical studies in HSCT.

LAT-mediated signaling in CD4+CD25+ regulatory T cell development

Engagement of the T cell receptor for antigen (TCR) induces formation of signaling complexes mediated through the transmembrane adaptor protein, the linker for activation of T cells (LAT). LAT plays an important role in T cell development, activation, and homeostasis. A knock-in mutation at Tyr136, which is the phospholipase C (PLC)-γ1–binding site in LAT, leads to a severe autoimmune disease in mice. In this study, we show that CD4⁺CD25⁺ T reg cells that expressed Foxp3 transcription factor were nearly absent in both thymus and peripheral lymphoid organs of LAT^Y136F mice. This defect was not a result of the autoimmune environment as LAT^Y136F T reg cells also failed to develop in healthy LAT^−/− mice that received mixed wild-type and LAT^Y136F bone marrow cells. Moreover, adoptive transfer of normal CD4⁺CD25⁺ T reg cells protected neonatal LAT^Y136F mice from developing this disease. These T reg cells effectively controlled expansion of CD4⁺ T cells in LAT^Y136F mice likely via granzymes and/or TGF-β–mediated suppression. Furthermore, ectopic expression of Foxp3 conferred a suppressive function in LAT^Y136F T cells. Our data indicate that the LAT–PLC-γ1 interaction plays a critical role in Foxp3 expression and the development of CD4⁺CD25⁺ T reg cells

Peripheral generation and function of CD4+CD25+ regulatory T cells

The balance between immunity and tolerance is important to maintain immune homeostasis. Several mechanisms are in place to ensure that the immune response is controlled, such as T cell anergy, apoptosis and immune ignorance. A fourth mechanism of peripheral tolerance is the active suppression by regulatory or suppressor T cells. The existence of suppressor T cells was first described in the early 1970s, but these cells became discredited in the 1980s. The work of Shimon Sakaguchi and others, however, has brought these cells back into the limelight and nowadays research into regulatory/suppressor T cells is a very active field of immunology. Different types of regulatory T cells have been described, including CD4+CD25+ T cells that constitutively express CTLA-4, GITR and Foxp3, TGF-beta producing Th3 cells, IL-10 producing Tr1 cells, and CD8+CD28- T cells. This review will focus on the generation and function of CD4+CD25+ regulatory T cells. CD4+CD25+ regulatory cells were originally described as thymus-derived anergic/suppressive T cells. Recent papers, however, indicate that these cells might also be generated in the periphery. CD4+CD25+ regulatory T cells can be activated by self-antigens and non-self-antigens, and once activated can suppress T cells in an antigen nonspecific manner. Interestingly, the suppressive effects of these cells are not restricted to the adaptive immune system (T and B cells) but can also affect the activation and function of innate immune cells (monocytes, macrophages, dendritic cells). These features make the CD4+CD25+ regulatory T cell subset an interesting target for immunotherapy of chronic inflammatory or autoimmune diseases.

Selection and behavior of CD4+ CD25+ T cells in vivo: lessons from T cell receptor transgenic models

Despite great interest in CD4+ CD25+ suppressor T cells, many of the fundamental properties of these cells remain enigmatic. This is in part due to experimental limitations inherent to the study of polyclonal suppressor T cells, and the extensive use of in vitro assays. This review article intends to outline recent advances in our understanding of the biology of suppressor T cells that have emerged from the analysis of T cell receptor (TCR) transgenic models. Several laboratories have taken advantage of model systems in which suppressor T cells of defined antigen-specificity are naturally selected in order to characterize the selection and behavior of these cells in vivo. In addition to providing valuable insights into the mechanism of differentiation of suppressor T cells, these systems now offer new possibilities for understanding the mode of action of suppressor T cells. For example, adoptive transfer of small numbers of ex vivo isolated TCR transgenic suppressor T cells allows for the visualization of the fate of such cells when confronted with cognate antigen in a quasi-normal, nonlymphopenic environment. Characteristic features of the currently available TCR transgenic models of suppressor T cells will be highlighted, and particular issues pertaining to the differentiation, function, and homeostasis of this T cell subset that have emerged from these models will be discussed.

Genetic control of thymic development of CD4+CD25+FoxP3+ regulatory T lymphocytes

Among the several mechanisms known to be involved in the establishment and maintenance of immunological tolerance, the activity of CD4+CD25+ regulatory T lymphocytes has recently incited most interest because of its critical role in inhibition of autoimmunity and anti-tumor immunity. Surprisingly, very little is known about potential genetic modulation of intrathymic regulatory T lymphocyte development. We show that distinct proportions of CD4+CD25+FoxP3+ regulatory T cells are found in thymi of common laboratory mouse strains. We demonstrate that distinct levels of phenotypically identical regulatory T cells develop with similar kinetics in the mice studied. Our experimental data on congenic mouse strains indicate that differences are not caused by the distinct MHC haplotypes of the inbred mouse strains. Moreover, the responsible loci act in a thymocyte-intrinsic manner, confirming the latter conclusion. We have not found any correlation between thymic and peripheral levels of regulatory T cells, consistent with known homeostatic expansion and/or retraction of the peripheral regulatory T cell pool. Our data indicate that polymorphic genes modulate differentiation of regulatory T cells. Identification of responsible genes may reveal novel clinical targets and still elusive regulatory T cell-specific markers. Importantly, these genes may also modulate susceptibility to autoimmune disease.

CD4+CD25+ regulatory T cells limit the risk of autoimmune disease arising from T cell receptor crossreactivity

The molecular-mimicry theory proposes that immune crossreactivity between microbial and self-antigen is the initiating event in the activation of autoaggressive immune responses leading to autoimmune disease. In support of this possibility, it is now accepted that T cell recognition of antigen is highly degenerate. However, it is to be expected that the immune system would have evolved mechanisms to counter such a potential danger. We studied the influence of CD4(+)CD25(+) regulatory T cells (Treg) on the ability of suboptimal T cell receptor ligands to provoke autoimmunity. By using CD4(+) T cell-driven experimental autoimmune encephalomyelitis as a model, it was found that depletion of CD4(+)CD25(+)Foxp3(+) Treg allowed pathology to develop in response to suboptimal T cell stimulation. These data demonstrate the importance of Treg in raising the threshold of triggering of autoreactive T cell responses, thus limiting the risk of autoimmune disease due to molecular mimicry.

Molecular signature of recent thymic selection events on effector and regulatory CD4+ T lymphocytes

Natural CD4+CD25+ regulatory T lymphocytes (Treg) are key protagonists in the induction and maintenance of peripheral T cell tolerance. Their thymic origin and biased repertoire continue to raise important questions about the signals that mediate their development. We validated analysis of MHC class II capture by developing thymocytes from thymic stroma as a tool to study quantitative and qualitative aspects of the cellular interactions involved in thymic T cell development and used it to analyze Treg differentiation in wild-type mice. Our data indicate that APCs of bone marrow origin, but, surprisingly and importantly, not thymic epithelial cells, induce significant negative selection among the very autoreactive Treg precursors. This fundamental difference between thymic development of regulatory and effector T lymphocytes leads to the development of a Treg repertoire enriched in cells specific for a selected subpopulation of self-Ags, i.e., those specifically expressed by thymic epithelial cells.

Protection against autoimmunity in nonlymphopenic hosts by CD4+ CD25+ regulatory T cells is antigen-specific and requires IL-10 and TGF-beta

CD4+ CD25+ regulatory T cells (T(Reg)) play a critical role in the control of autoimmunity. However, little is known about how T(Reg) suppress self-reactive T cells in vivo, thus limiting the development of T(Reg)-based therapy for treating autoimmune diseases. This is in large part due to the dependency on a state of lymphopenia to demonstrate T(Reg)-mediated suppression in vivo and the unknown Ag specificity of T(Reg) in most experimental models. Using a nonlymphopenic model of autoimmune pneumonitis and T(Reg) with known Ag specificity, in this study we demonstrated that these T(Reg) can actively suppress activation of self-reactive T cells and protect mice from fatal autoimmune pneumonitis. The protection required T(Reg) with the same Ag specificity as the self-reactive T cells and depended on IL-10 and TGF-beta. These results suggest that suppression of autoimmunity by T(Reg) in vivo consists of multiple layers of regulation and advocate for a strategy involving Ag-specific T(Reg) for treating organ-specific autoimmunity, because they do not cause generalized immune suppression.

Central tolerance: learning self-control in the thymus

In the past few years, there has been a flurry of discoveries and advancements in our understanding of how the thymus prepares T cells to exist at peace in normal healthy tissue: that is, to be self-tolerant. In the thymus, one of the main mechanisms of T-cell central tolerance is clonal deletion, although the selection of regulatory T cells is also important and is gaining enormous interest. In this Review, we discuss the emerging consensus about which models of clonal deletion are most physiological, and we review recent data that define the molecular mechanisms of central tolerance.

Human dendritic cell subsets for vaccination

Protective immunity results from the interplay of antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity. The cells and molecules of the innate system employ non-clonal recognition pathways such as lectins and TLRs. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing Ag or peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology that shows the complexity of this cell system. Thus, presentation of antigen by immature (non-activated) DCs leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. Furthermore, DCs are composed of multiple subsets with distinct functions at the interface of the innate and adaptive immunity. Our increased understanding of DC pathophysiology will permit their rational manipulation for therapy such as vaccination to improve immunity.

Thymic regulatory T cells

Several types of T regulatory (Treg) cells have been described in both mice and humans, including natural or professional (CD4+CD25+ T cells) and adaptive (Th3 and Tr1 cells) Treg cells. The former develops in the thymus and results in an endogeneous long-lived population of self-antigen-specific T cells in the periphery poised to prevent potentially autoimmune reactions. The second subset develops as a consequence of activation of mature T cells under particular conditions of sub-optimal antigen exposure and/or costimulation. Natural Treg cells are positively selected in the cortex through their TCR interactions with self-peptides presented by thymic stromal cells. It is likely that this high-affinity recognition results in signals rendering them anergic and able to produce anti-apoptoptic molecules which protect them from negative selection. Recently, small subsets of CD4+CD25+ and of CD8+CD25+ cells sharing similar characteristics have been detected in human fetal and post-natal thymuses. Both CD4+CD25+ and CD8+CD25+ human thymocytes express Foxp3 and GITR mRNA, as well as surface CCR8 and TNFR2 and cytoplasmic CTLA-4 proteins, which are common features of mature Treg cells. Following activation they do not proliferate or produce cytokines, but express surface CTLA-4 and TGF-beta1. They suppress the proliferation of autologous CD4+CD25- thymocytes to allogeneic stimulation by a contact-dependent mechanism related to the combined action of surface CTLA-4 and TGF-beta leading to the inhibition of the IL-2R alpha chain on target T cells. Lastly, both CD4+CD25+ and CD8+CD25+ Treg thymocytes exert strong suppressive activity on Th1, but much lower on Th2 cells, since these latter may escape from suppression via their ability to respond to growth factors other than IL-2. Treg cells that develop in, and emerge from, the thymus are certainly responsible for the maintenance of self-tolerance and prevention of autoimmune disorders. The result that Th1 cells are highly susceptible to the suppressive activity of Treg thymocytes is consistent with the important role of these cells in protecting against the Th1-mediated immune response to autoantigens.

Peptide-based instruction of suppressor commitment in naïve T cells and dynamics of immunosuppression in vivo

Recent years have witnessed the revival of suppressor T cells that control immunity by interfering with the generation of effector T-cell function in vivo. The discovery that CD4 T cells with the CD25 surface marker were enriched in suppressor activity enabled further phenotypic and functional analysis of the so-called natural suppressor cells. In vitro characterization showed that these cells were anergic, i.e. did not respond to antigenic stimulation with proliferation and, instead they suppressed other cells through direct cell contact resulting in inhibition of interleukin-2 gene transcription. We have analysed the generation and function of suppressor T cells in T-cell receptor (TCR) transgenic mice. The results showed that such cells can be generated intrathymically when agonist TCR ligands are expressed on thymic epithelial cells. Thus generated cells constitute a lineage of cells committed to suppression only with the ability to survive for prolonged periods of time in the absence of the inducing ligand. Because of appropriate homing receptors such cells can accumulate and proliferate in antigen draining lymphnodes after antigenic stimulation and suppress proliferation and cytokine secretion of CD4 and CD8 T cells as well as CD8 T-cell-mediated cytotoxicity. We also attempted to generate such cells from naïve T cells in secondary lymphoid tissue under conditions where expansion of already preformed suppressor T cells could be excluded. The results showed that subimmunogenic peptide delivery by osmotic minipumps or by peptide containing DEC 205 antibodies yielded CD25+ suppressor cells that were phenotypically and functionally indistinguishable from intrathymically generated suppressor cells. The experiments with DEC205 antibodies revealed (i) dose-dependent proliferation of naïve T cells and (ii) conversion into suppressor T cells of only those T cells that underwent a limited number of cell divisions. These results are compatible with other studies that were, however, less rigorous in excluding expansion of existing cells as opposed to de novo generation of suppressor cells from naïve T cells. The fact that natural suppressor cells have an essential role in preventing autoimmunity and that they can be specifically induced by TCR agonist ligands opens new perspectives in preventing autoimmunity, transplant rejection and allergy.

Thymic commitment of regulatory T cells is a pathway of TCR-dependent selection that isolates repertoires undergoing positive or negative selection

The seminal work of Le Douarin and colleagues (Ohki et al. 1987; Ohki et al. 1988; Salaun et al. 1990; Coutinho et al. 1993) first demonstrated that peripheral tissue-specific tolerance is centrally established in the thymus, by epithelial stromal cells (TEC). Subsequent experiments have shown that TEC-tolerance is dominant and mediated by CD4 regulatory T cells (Treg) that are generated intrathymically by recognition of antigens expressed on TECs (Modigliani et al. 1995; Modigliani et al. 1996a). From these and other observations, in 1996 Modigliani and colleagues derived a general model for the establishment and maintenance of natural tolerance (MM96) (Modigliani et al. 1996b), with two central propositions: (1) T cell receptor (TCR)-dependent sorting of emergent repertoires generates TEC-specific Treg displaying the highest TCR self-affinities below deletion thresholds, thus isolating repertoires undergoing positive and negative selection; (2) Treg are intrathymically committed (and activated) for a unique differentiative pathway with regulatory effector functions. The model explained the embryonic/perinatal time window of natural tolerance acquisition, by developmental programs determining (1) TCR multireactivity, (2) the cellular composition in the thymic stroma (relative abundance of epithelial vs hemopoietic cells), and (3) the dynamics of peripheral lymphocyte pools, built by accumulation of recent thymic emigrants (RTE) that remain recruitable to regulatory functions. We discuss here the MM96 in the light of recent results demonstrating the promiscuous expression of tissue-specific antigens by medullary TECs (Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004) and indicating that Treg represent a unique differentiative pathway (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003), which is adopted by CD4 T cells with high avidity for TEC-antigens (Bensinger et al. 2001; Jordan et al. 2001; Apostolou et al. 2002). In the likelihood that autoimmune diseases (AID) result from Treg deficits, some of which might have a thymic origin, we also speculate on therapeutic strategies aiming at selectively stimulating their de novo production or peripheral function, within recent findings on Treg responses to inflammation (Caramalho et al. 2003; Lopes-Carvalho et al., submitted, Caramalho et al., submitted). In short, the MM96 argued that natural tolerance is dominant, established and maintained by the activity of Treg, which are selected upon high-affinity recognition of self-ligands on TECs, and committed intrathymically to a unique differentiative pathway geared to anti-inflammatory and antiproliferative effector functions. By postulating the intrathymic deletion of self-reactivities on hemopoietic stromal cells (THC), together with the inability of peripheral resident lymphocytes to engage in the regulatory pathway, the MM96 simultaneously explained the maintenance of responsiveness to non-self in a context of suppression mediating dominant self-tolerance. The major difficulty of the MM96 is related to the apparent tissue specificity of Treg repertoires generated intrathymically. This difficulty has now been principally solved by the work of Hanahan, Kyewski and others (Jolicoeur et al. 1994; Derbinski et al. 2001; Anderson et al. 2002; Gotter et al. 2004), demonstrating the selective expression of a variety of tissue-specific antigens by TECs, in topological patterns that are compatible with the MM96, but difficult to conciliate with recessive tolerance models (Kappler et al. 1987; Kisielow et al. 1988). While the developmentally regulated multireactivity of TCR repertoires (Gavin and Bevan 1995), as well as the peripheral recruitment of Treg among RTE (Modigliani et al. 1996a) might add to this process, it would seem that the establishment of tissue-specific tolerance essentially stems from the "promiscuous expression of tissue antigens" by TEC. The findings of AID resulting from natural mutations (reviewed in Pitkanen and Peterson 2003) or the targeted inactivation (Anderson et al. 2002; Ramsey et al. 2002) of the AIRE transcription factor that regulates promiscuous gene expression on TECs support this conclusion. The observations on the correlation of natural or forced expression of the Foxp3 transcription factor in CD4 T cells with Treg phenotype and function (Fontenot et al. 2003; Hori et al. 2003; Khattri et al. 2003) provided support for the MM96 contention that Treg represent a unique differentiative pathway that is naturally established inside the thymus. Furthermore, Caton and colleagues (Jordan et al. 2001), as well as several other groups (Bensinger et al. 2001; Apostolou et al. 2002), have provided direct evidence for our postulate that Treg are selected among differentiating CD4 T cells with high affinity for ligands expressed on TECs (Modigliani et al. 1996b). Finally, the demonstration by Caramalho et al. that Treg express innate immunity receptors (Caramalho et al. 2003) and respond to pro-inflammatory signals and products of inflammation (Caramalho et al., submitted) brought about a new understanding on the peripheral regulation of Treg function. Together with the observation that Treg also respond to ongoing activities of "naïve/effector" T cells--possibly through the IL-2 produced in these conditions--these findings explain the participation of Treg in all immune responses (Onizuka et al. 1999; Shimizu et al. 1999; Annacker et al. 2001; Curotto de Lafaille et al. 2001; Almeida et al. 2002; Shevach 2002; Bach and Francois Bach 2003; Wood and Sakaguchi 2003; Mittrucker and Kaufmann 2004; Sakaguchi 2004), beyond their fundamental role in ensuring self-tolerance (e.g., Modigliani et al. 1996a; Shevach 2000; Hori et al. 2003; Sakaguchi 2004; Thompson and Powrie 2004). Thus, anti-inflammatory and anti-proliferative Treg are amplified by signals that promote or mediate inflammation and proliferation, accounting for the quality control of responses (Coutinho et al. 2001). In turn, such natural regulation of Treg by immune responses to non-self may well explain the alarming epidemiology of allergic and AID in wealthy societies (Wills-Karp et al. 2001; Bach 2002; Yazdanbakhsh et al. 2002), where a variety of childhood infections have become rare or absent. Thus, it is plausible that Treg were evolutionarily set by a given density of infectious agents in the environment. With hindsight, it is not too surprising that natural Treg performance falls once hygiene, vaccination, and antibiotics suddenly (i.e., 100 years) plunged infectious density to below some critical physiological threshold. As the immune system is not adapted to modern clean conditions of postnatal development, clinical immunologists must now deal with frequent Treg deficiencies (allergies and AID) for which they have no curative or rational treatments. It is essential, therefore, that basic immunologists concentrate on strategies to selectively stimulate the production, survival, and activity of this set of lymphocytes that is instrumental in preventing immune pathology. We have argued that the culprit of this inability of basic research to solve major clinical problems has been the self-righteousness of recessive tolerance champions, from Ehrlich to some of our contemporaries. It is ironical, however, that none of us--including the heretic opponents of horror autotoxicus--had understood that self-tolerance, or its robustness at least, is in part determined by the frequency and intensity of the responses to non-self. In the evolution of ideas on immunological tolerance, the time might be ripe for some kinds of synthesis. First, conventional theory reduced self-tolerance to negative selection and microbial defense to positive selection, while the MM96 solution was the precise opposite: positive selection of autoreactivities for self-tolerance (Treg) and negative selection (of Treg) for ridding responses. In contrast, it would now appear that positive and negative selection of autoreactive T cells are both necessary to establish either self-tolerance or competence to eliminate microbes, two processes that actually reinforce each other in the maintenance of self-integrity. Second, V-region recognition has generally been held responsible for specific discrimination between what should be either tolerated or eliminated from the organism. In contrast again, it would now seem that both processes of self-tolerance and microbial defense (self/non-self discrimination) also operate on the basis of evolutionarily ancient, germ-line-encoded innate, nonspecific receptors (Medzhitov and Janeway 2000) capable of a coarse level of self/non-self discrimination (Coutinho 1975). It could thus be interesting to revisit notions of cooperativity between V-regions and such mitogen receptors, both in single cell functions (Coutinho et al. 1974) and in the system's evolution (Coutinho 1975, 1980) as well. After all, major transitions in evolution were cooperative (Maynard-Smith and Szathmary 1995).

The role of TCR specificity in naturally arising CD25+ CD4+ regulatory T cell biology

CD25+ CD4+ T cells (TR) are a naturally arising subset of regulatory T cells important for the preservation of self-tolerance and the prevention of autoimmunity. Although there is substantial data that TCR specificity is important for TR development and function, relatively little is known about the antigen specificity of naturally arising TR. Here, we will review the available evidence regarding naturally arising TR TCR specificity in the context of TR development, function, and homeostasis.

An Alternate Pathway for CD4 T Cell Development: Thymocyte-Expressed MHC Class II Selects a Distinct T Cell Population

Conventional understanding of CD4 T cell development is that the MHC class II molecules on cortical thymic epithelial cell are necessary for positive selection, as demonstrated in mouse models. Clinical data, however, show that hematopoietic stem cells reconstitute CD4 T cells in patients devoid of MHC class II. Additionally, CD4 T cells generated from human stem cells in immunocompromised mice were restricted to human, but not mouse, MHC class II. These studies suggest an alternative pathway for CD4 T cell development that does not normally exist in mice. MHC class II is expressed on developing human thymocytes, indicating a possible role of MHC II on thymocytes for CD4 T cell generation. Therefore, we created mice in which MHC class II is expressed only on T lineage cells. Remarkably, the CD4 compartment in such mice is efficiently reconstituted with unique specificity, demonstrating a novel thymocyte-driven pathway of CD4 T cell selection.

Peripheral CD8+CD25+ T lymphocytes from MHC class II-deficient mice exhibit regulatory activity

We characterized CD8(+) T cells constitutively expressing CD25 in mice lacking the expression of MHC class II molecules. We showed that these cells are present not only in the periphery but also in the thymus. Like CD4(+)CD25(+) T cells, CD8(+)CD25(+) T cells appear late in the periphery during ontogeny. Peripheral CD8(+)CD25(+) T cells from MHC class II-deficient mice also share phenotypic and functional features with regulatory CD4(+)CD25(+) T cells: in particular, they strongly express glucocorticoid-induced TNFR family-related gene, CTLA-4 and Foxp3, produce IL-10, and inhibit CD25(-) T cell responses to anti-CD3 stimulation through cell contacts with similar efficiency to CD4(+)CD25(+) T cells. However, unlike CD4(+)CD25(+) T cells CD8(+)CD25(+) T cells from MHC class II-deficient mice strongly proliferate and produce IFN-gamma in vitro in response to stimulation in the absence of exogenous IL-2.

CD4+CD25+-regulatory T cells from mouse to man

Immunological tolerance is one of the fundamental concepts of the immune system. During the past decade, CD4+CD25+-regulatory T cells have emerged as key players in the development of tolerance to autoantigens as well as to foreign antigens. Still many questions remain illusive regarding the basic properties of CD4+CD25+-regulatory T cells. This review aims to recapitulate some of the current understandings about the phenotype, function and clinical relevance of murine and human CD4+CD25+-regulatory T cells.