Preferential recognition of self antigens despite normal thymic deletion of CD4(+)CD25(+) regulatory T cells

CD4+CD25+ regulatory T cell repertoire formation shaped by differential presentation of peptides from a self-antigen

We have used TCR transgenic mice directed to different MHC class II-restricted determinants from the influenza virus hemagglutinin (HA) to analyze how specificity for self-peptides can shape CD4+CD25+ regulatory T (Treg) cell formation. We show that substantial increases in the number of CD4+CD25+ Treg cells can occur when an autoreactive TCR directed to a major I-E(d)-restricted determinant from HA develops in mice expressing HA as a self-Ag, and that the efficiency of this process is largely unaffected by the ability to coexpress additional TCR alpha-chains. This increased formation of CD4+CD25+ Treg cells in the presence of the self-peptide argues against models that postulate selective survival rather than induced formation as mechanisms of CD4+CD25+ Treg cell formation. In contrast, T cells bearing a TCR directed to a major I-A(d)-restricted determinant from HA underwent little or no selection to become CD4+CD25+ Treg cells in mice expressing HA as a self-Ag, correlating with inefficient processing and presentation of the peptide from the neo-self-HA polypeptide. These findings show that interactions with a self-peptide can induce thymocytes to differentiate along a pathway to become CD4+CD25+ Treg cells, and that peptide editing by DM molecules may help bias the CD4+CD25+ Treg cell repertoire away from self-peptides that associate weakly with MHC class II molecules.

Enhanced engagement of CTLA-4 induces antigen-specific CD4+CD25+Foxp3+ and CD4+CD25- TGF-beta 1+ adaptive regulatory T cells

CTLA-4 is a critical negative regulator of T cell response and is instrumental in maintaining immunological tolerance. In this article, we report that enhanced selective engagement of CTLA-4 on T cells by Ag-presenting dendritic cells resulted in the induction of Ag-specific CD4(+)CD25(+)Foxp3(+) and CD4(+)CD25(-)TGF-beta1(+) adaptive Tregs. These cells were CD62L(low) and hyporesponsive to stimulation with cognate Ag but demonstrated a superior ability to suppress Ag-specific effector T cell response compared with their CD62L(high) counterparts. Importantly, treatment of mice with autoimmune thyroiditis using mouse thyroglobulin (mTg)-pulsed anti-CTLA-4 agonistic Ab-coated DCs, which results in a dominant engagement of CTLA-4 upon self-Ag presentation, not only suppressed thyroiditis but also prevented reemergence of the disease upon rechallenge with mTg. Further, the disease suppression was associated with significantly reduced mTg-specific T cell and Ab responses. Collectively, our results showed an important role for selective CTLA-4 signaling in the induction of adaptive Tregs and suggested that approaches that allow dominant CTLA-4 engagement concomitant with Ag-specific TCR ligation can be used for targeted therapy.

Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.

The role of T helper 17 (Th17) and regulatory T cells (Treg) in human organ transplantation and autoimmune disease

Uncommitted (naive) murine CD4+ T helper cells (Thp) can be induced to differentiate towards T helper 1 (Th1), Th2, Th17 and regulatory (Treg) phenotypes according to the local cytokine milieu. This can be demonstrated most readily both in vitro and in vivo in murine CD4+ T cells. The presence of interleukin (IL)-12 [signalling through signal transduction and activator of transcription (STAT)-4] skews towards Th1, IL-4 (signalling through STAT-6) towards Th2, transforming growth factor (TGF)-beta towards Treg and IL-6 and TGF-beta towards Th17. The committed cells are characterized by expression of specific transcription factors, T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for Tregs and RORgammat for Th17 cells. Recently, it has been demonstrated that the skewing of murine Thp towards Th17 and Treg is mutually exclusive. Although human Thp can also be skewed towards Th1 and Th2 phenotypes there is as yet no direct evidence for the existence of discrete Th17 cells in humans nor of mutually antagonistic development of Th17 cells and Tregs. There is considerable evidence, however, both in humans and in mice for the importance of interferon (IFN)-gamma and IL-17 in the development and progression of inflammatory and autoimmune diseases (AD). Unexpectedly, some models of autoimmunity thought traditionally to be solely Th1-dependent have been demonstrated subsequently to have a non-redundant requirement for Th17 cells, notably experimental allergic encephalomyelitis and collagen-induced arthritis. In contrast, Tregs have anti-inflammatory properties and can cause quiescence of autoimmune diseases and prolongation of transplant function. As a result, it can be proposed that skewing of responses towards Th17 or Th1 and away from Treg may be responsible for the development and/or progression of AD or acute transplant rejection in humans. Blocking critical cytokines in vivo, notably IL-6, may result in a shift from a Th17 towards a regulatory phenotype and induce quiescence of AD or prevent transplant rejection. In this paper we review Th17/IL-17 and Treg biology and expand on this hypothesis.

Influence of direct and indirect allorecognition pathways on CD4+CD25+ regulatory T-cell function in transplantation

While both direct and indirect allorecognition are involved in allograft rejection, evidence to date suggests that tolerance is primarily dependent on indirect pathway-triggered CD4+CD25+ T cell-mediated immunoregulation. However, the precise influence of these two pathways on CD4+CD25+ T-cell function has not been addressed. In the current study, we have utilized an adoptive transfer model to assess selectively how the absence of either direct or indirect allorecognition affects CD4+CD25+ T-cell function. The effects of the loss of the direct pathway were assessed by transplanting skin grafts from minor histocompatibility mismatched B10.D2 (H-2d) donors onto Balb/c (H-2d) recipients, or by placing bone marrow chimeric DBA/2 (H-2d/H-2b) allografts onto C57BL/6 (H-2b) hosts. The requirement for indirect allorecognition was tested by grafting DBA/2 skin allografts onto either C57BL/6- or MHC-II-deficient C57BL/6 recipients. We report here that although CD4+CD25+ regulatory T cells can suppress both directly and indirectly generated alloresponses, immunoregulation is favored when indirect presentation is the sole mechanism of allorecognition. Hence, in the absence of indirect presentation, net CD4+CD25+ T cell-dependent immunoregulation is weak, and high ratios of CD4+CD25+ to CD4+CD25 T cells are required to ensure graft survival.

Regulatory T Cells Are Resistant to Apoptosis via TCR but Not P2X7

Regulatory T cells (Tregs) are relatively autoreactive yet, paradoxically, have been found to display normal sensitivity to thymic deletion. The relationship between self-avidity, apoptosis, and the selection of Tregs therefore remains unclear. We show that thymic Tregs develop efficiently, even at low self-avidity, and are moderately resistant to apoptosis in comparison to conventional thymocytes. Consistent with this, although conventional self-reactive T cell populations undergo chronic peripheral deletion, self-reactive Tregs are largely spared removal. Similarly, the distribution of Tregs among peripheral CD4(+) cells exhibits a linear inverse relationship with CD45RB expression, indicating relative apoptosis resistance of Tregs in chronic responses to environmental Ags. We also show that appropriate controls for CD45RB levels are important for comparisons of Treg and conventional T cell activity. When thus controlled, and contrary to previous reports, Tregs exhibit normal sensitivity to cell death through TCR-independent stimuli, such as the purinergic receptor, P2X(7). Finally, although absence of CD45 in gene-targeted mice results in profound T cell hyporesponsiveness, there is little or no effect on thymic Treg frequency. In summary, the data support a model in which signal strength plays little part in Treg lineage specification, though moderate resistance of self-reactive Tregs to apoptosis may result in progressive biasing of peripheral Treg TCRs toward autoreactivity in comparison to those of conventional T cells.

A defect of regulatory T cells in patients with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), a monogenic recessive disease characterized by autoimmunity against multiple tissues, offers a unique possibility to study the breakdown of self-tolerance in humans. It is caused by mutations in the autoimmune regulator gene (AIRE), which encodes a transcriptional regulator. Work using Aire(-/-) mice suggests that Aire induces ectopic expression of peripheral Ags and promotes their presentation in the thymus. We have explored reasons for the difference between the comparatively mild phenotype of Aire-deficient mice and human APECED patients. We provide evidence that, unlike in the Aire(-/-) mice, in the patients a key mediator of active tolerance, the CD4(+)CD25(+) regulatory T (Treg) cell subset is impaired. This was shown by significantly decreased expression of FOXP3 mRNA and protein, decreased function, and alterations in TCR repertoire. Also, in the normal human thymus a concentric accumulation of AIRE(+) cells was seen around thymic Hassall's corpuscles, suggesting that in the patients these cells may be involved in the observed Treg cell failure. In Aire(-/-) mice the expression of FoxP3 was normal and even increased in target tissues in parallel with the lymphocyte infiltration process. Our results suggest that a Treg cell defect is involved in the pathogenesis of APECED and emphasize the importance of active tolerance mechanisms in preventing human autoimmunity.

Development of CD25– regulatory T cells following heart transplantation: Evidence for transfer of long-term survival

Donor-specific heart allograft acceptance can be induced in the MHC-mismatched LEW.1 W to LEW.1A rat by donor-specific transfusions. Whereas the induction phase of tolerance has been studied in detail, its maintenance remained poorly understood. Here, we performed a side-by-side comparison of CD25+ and CD25- splenic T cells of 100-day tolerant rats. Administration of CD25- T cells from tolerant rats to sublethally irradiated recipients transferred long-term graft survival. These CD25- T cells displayed a decreased donor-specific response in the mixed lymphocyte reaction and presented suppressive activity. These CD25- T cells accumulated IFN-gamma, IL-10 and Foxp3 transcripts. The in vitro suppressive activity of CD25- T cells required both cell contact and soluble factors (IL-10 and IFN-gamma). The CD25+ T cells from tolerant rats did not show any modification of their regulatory properties. We show that splenic CD25- T cells of tolerant rats contribute to the maintenance of tolerance following the transplantation. Our data show that regulatory T cells are not restricted to the CD4+ CD25+ T cell subset and provide new insights on the mechanisms of tolerance to allograft following donor cell priming.

Immune tolerance and control of CNS autoimmunity: from animal models to MS patients

Multiple sclerosis (MS) is a chronic inflammatory disease resulting in demyelination and axonal loss within the CNS. An autoimmune reaction directed against myelin antigens contributes to the disease process. As the CNS has long been considered an immune privileged site, how such an immune response can develop locally has remained enigmatic. Recent data, mostly based on the study of animal models for MS, have shown that the CNS is in fact more permissive to the development of immune responses than previously thought. This observation is counterbalanced by the fact that immune tolerance to myelin antigens can be induced outside the CNS. This review focuses on the mechanisms preventing CNS autoimmunity, which act in three separate tissues. In the thymus, expression of CNS autoantigens promotes partial protection, notably through elimination of autoreactive T cells. In the secondary lymphoid organs, the remaining autoreactive T cells are kept under control by the naturally occurring regulatory T cells of the CD4(+)Foxp3(+) phenotype. In the CNS, multiple mechanisms including the local activation of regulatory T cells further limit autoimmunity. A better understanding of the induction of regulatory T cells, of their mechanisms of action, and of approaches to manipulate them in vivo may offer new therapeutic opportunities for MS patients.

An MHC-linked locus modulates thymic differentiation of CD4+CD25+Foxp3+ regulatory T lymphocytes

CD4+CD25+Foxp3+ regulatory T lymphocytes are crucial for maintenance of immunological tolerance to self and innocuous non-self, are known to modulate immunity to tumors and infectious agents and can induce transplantation tolerance. Surprisingly, only a single genetic polymorphism is known to modulate regulatory T cell (Treg) development in the thymus, leading to a lethal autoimmune disorder. Here, we show that considerably different levels of Tregs are found in the thymi of distinct common laboratory mouse strains. We demonstrate that distinct levels of phenotypically and functionally identical Tregs develop with similar kinetics in the studied mice, that the responsible locus acts in a thymocyte-intrinsic manner and that levels of thymic Foxp3+ Tregs correlate to those found in the periphery. Using several congenic mouse strains, we mapped one of the at least two genetic loci capable of quantitatively modulating thymic Treg development to a Collapse

Key Words

• animals
• antigens, surface
• metabolism
• cd4-positive t-lymphocytes
• cell differentiation
• genetics
• immunology
• female
• forkhead transcription factors
• genes, mhc class ii
• physiology
• interleukin-2 receptor alpha subunit
• mice
• mice, inbred strains
• t-lymphocytes, regulatory
• cytology
• telomere
• thymus gland

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MESH Headings

• Animals
• Antigens, Surface/metabolism
• CD4-Positive T-Lymphocytes/metabolism
• Cell Differentiation/genetics
• Cell Differentiation/immunology
• Female
• Forkhead Transcription Factors/metabolism
• Genes, MHC Class II/genetics
• Genes, MHC Class II/physiology
• Interleukin-2 Receptor alpha Subunit/metabolism
• Mice
• Mice, Inbred Strains/immunology
• T-Lymphocytes, Regulatory/cytology
• T-Lymphocytes, Regulatory/immunology
• Telomere/physiology
• Thymus Gland/cytology
• Thymus Gland/immunology

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Grants Collapse

Affiliation(s)

Julie Tellier Centre de Physiopathologie de Toulouse Purpan INSERM : U563IFR30Université Paul SabatierInstitut Claude de Préval Toulouse, FR
Joost PM van Meerwijk Centre de Physiopathologie de Toulouse Purpan INSERM : U563IFR30Université Paul SabatierInstitut Claude de Préval Toulouse, FR Institut universitaire de France and Faculty of life-sciences (UFR-SVT) Université Paul SabatierToulouse,FR
Paola Romagnoli Centre de Physiopathologie de Toulouse Purpan INSERM : U563IFR30Université Paul SabatierInstitut Claude de Préval Toulouse, FR

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Peripheral T Cell Lymphopenia and Concomitant Enrichment in Naturally Arising Regulatory T Cells: The Case of the Pre-Tα Gene-Deleted Mouse

In pre-Talpha (pTalpha) gene-deleted mice, the positively selectable CD4+ CD8+ double-positive thymocyte pool is only 1% that in wild-type mice. Consequently, their peripheral T cell compartment is severely lymphopenic with a concomitant increase in proportion of CD25+ FoxP3+ regulatory T cells. Using mixed bone marrow chimeras, where thymic output was 1% normal, the pTalpha(-/-) peripheral T cell phenotype could be reproduced with normal cells. In the pTalpha(-/-) thymus and peripheral lymphoid organs, FoxP3+ CD4+ cells were enriched. Parabiosis experiments showed that many pTalpha(-/-) CD4+ single-positive thymocytes represented recirculating peripheral T cells. Therefore, the enrichment of FoxP3+ CD4+ single-positive thymocytes was not solely due to increased thymic production. Thus, the pTalpha(-/-) mouse serves as a model system with which to study the consequences of chronic decreased thymic T cell production on the physiology of the peripheral T cell compartment.

Immune privilege induced by regulatory T cells in transplantation tolerance

Immune privilege was originally believed to be associated with particular organs, such as the testes, brain, the anterior chamber of the eye, and the placenta, which need to be protected from any excessive inflammatory activity. It is now becoming clear, however, that immune privilege can be acquired locally in many different tissues in response to inflammation, but particularly due to the action of regulatory T cells (Tregs) induced by the deliberate therapeutic manipulation of the immune system toward tolerance. In this review, we consider the interplay between Tregs, dendritic cells, and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T-cell and antigen-presenting cell activity, for example, by catabolism of the amino acids tryptophan and arginine and the induction of hemoxygenase and carbon monoxide. The induction of local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.

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.

Resistance of regulatory T cells to glucocorticoid-induced [corrected] TNFR family-related protein (GITR) during Plasmodium yoelii infection

CD4+ T cells are the major effector T cells against blood-stage Plasmodium yoelii infection. On the other hand, the lethal strain of P. yoelii (PyL) has acquired an escape mechanism from host T cell immunity by activating CD4+CD25+ regulatory T cells (Treg). Although the activation of Treg during PyL infection precludes the clearance of PyL from mice, it remains unclear whether activation of Treg is attributable to a specific response against PyL infection. Thus, we examined here whether Treg proliferate in an antigen-dependent manner during PyL infection. We also investigated the effector and regulatory mechanisms of Treg. Infection with PyL increased the number of CD4+CD25+ T cells, in which expression of Foxp3 mRNA is up-regulated. The Treg that were transferred into mice infected with PyL, but not with a non-lethal strain of P. yoelii (PyNL), proliferated during the initial 5 days following infection. The Treg from PyL-infected mice showed strong suppression compared with those from naive or PyNL-infected mice, and could suppress T cell activation by recognizing PyL- but not PyNL-derived antigens. Furthermore, the suppressive function of Treg activated in PyL-infected but not in naive mice could not be inhibited by treatment with an anti-glucocorticoid-induced TNFR family-related protein (GITR) mAb. These findings indicate that PyL infection specifically activates Treg that are specific for PyL-derived antigens. The infection also induces resistance for Treg to GITR signaling, and this eventually contributes to the escape of parasites from host T cell immunity.

Naive regulatory T cells: a novel subpopulation defined by resistance toward CD95L-mediated cell death

Most CD4(+)CD25(hi)FOXP3(+) regulatory T cells (T(regs)) from adult peripheral blood express high levels of CD45RO and CD95 and are prone to CD95L-mediated apoptosis in contrast to conventional T cells (T(convs)). However, a T(reg) subpopulation remained consistently apoptosis resistant. Gene microarray and 6-color flow cytometry analysis including FOXP3 revealed an increase in naive T-cell markers on the CD95L-resistant T(regs) compared with most T(regs). In contrast to T(regs) found in adult humans, most CD4(+)CD25(+)FOXP3(+) T cells found in cord blood are naive and exhibit low CD95 expression. Furthermore, most of these newborn T(regs) are not sensitive toward CD95L similar to naive T(regs) from adult individuals. After short stimulation with anti-CD3/CD28 monoclonal antibodies (mAbs), cord blood T(regs) strongly up-regulated CD95 and were sensitized toward CD95L. This functional change was paralleled by a rapid up-regulation of memory T-cell markers on cord blood T(regs) that are frequently found on adult memory T(regs). In summary, we show a clear functional difference between naive and memory T(regs) that could result in different survival rates of those 2 cell populations in vivo. This new observation could be crucial for the planning of therapeutic application of T(regs).

Characteristics of Rat CD4+CD25+ T Cells and Their Ability to Prevent Not Only Diabetes But Also Insulitis in an Adoptive Transfer Model in BB Rats

Human and mouse CD4(+)CD25(+) T cells have been intensively studied through the last decade. However, little is known about this subset in other species. This study describes the phenotype of rat CD4(+)CD25(+) Foxp3(+) T cells and the site in which they exert regulation in a transfer-induced autoimmune diabetes model. Several proteins and mRNAs are up-regulated in unstimulated rat CD4(+)CD25(+) T cells compared with CD4(+)CD25(-) T cells, including Foxp3, Lag-3, CD80, interleukin 10 (IL-10) and CTLA-4. To investigate CD4(+)CD25(+) T cells in vivo, we transferred three million diabetogenic T cells either alone or in combination with two million CD4(+)CD25(+) T cells to 30-day-old BB rats. The pancreas and the pancreatic lymph nodes were examined as two potential regulatory sites. Time-course analysis of pancreatic histology following diabetogenic T-cell transfers revealed insulitis from about 14 days after transfer. By contrast, rats receiving both diabetogenic T cells and CD4(+)CD25(+) T cells had no insulitis at any time. Moreover, the frequency of diabetogenic T cells in the pancreatic lymph nodes 2 days after transfer was significantly reduced in rats receiving both subsets. These data indicate that the primary site of T-cell regulation is in the draining lymph nodes and not the pancreas in our model.

Conditional Ablation of MHC-II Suggests an Indirect Role for MHC-II in Regulatory CD4 T Cell Maintenance

Although the importance of MHC class II (MHC-II) in acute homeostatic proliferation of regulatory T (Treg) cells has been established, we considered here the maintenance and state of Treg cells in mice that are almost completely devoid of MHC-II in their periphery but still make their own CD4 T cells and Treg cells. The latter was accomplished by conditional deletion of a loxP-flanked MHC-II beta-chain allele using a TIE2Cre transgene, which causes a very high degree of deletion in hemopoietic/endothelial progenitor cells but without deletion among thymic epithelial cells. Such conditional MHC-II-deficient mice possess their own relatively stable levels of CD4+CD25+ cells, with a normal fraction of Foxp3+ Treg cells therein, but at a level approximately 2-fold lower than in control mice. Thus, both Foxp3low/- CD4+CD25+ cells, said to be a major source of IL-2, and IL-2-dependent Foxp3+ Treg cells are reduced in number. Furthermore, CD25 expression is marginally reduced among Foxp3+ Treg cells in conditional MHC-II-deficient mice, indicative of a lack of MHC-II-dependent TCR stimulation and/or IL-2 availability, and IL-2 administration in vivo caused greatly increased cell division among adoptively transferred Treg cells. This is not to say that IL-2 can cause Treg cell division in the complete absence of MHC-II as small numbers of MHC-II-bearing cells do remain in conditional MHC-II-deficient mice. Rather, this suggests only that IL-2 was limiting. Thus, our findings lend support to the proposal that Treg cell homeostasis depends on a delicate balance with a population of self-reactive IL-2-producing CD4+CD25+ cells which are themselves at least in part MHC-II-dependent.

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.

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.

Migration rules: functional properties of naive and effector/memory-like regulatory T cell subsets

Suppressor T cells were first described in the early 1970s, but since the hypothetical soluble suppressor factor could not be identified on a molecular level and since appropriate cellular markers were lacking, the suppressor T cell concept vanished for a long time. The discovery by Sakaguchi and co-workers, that the adoptive transfer of CD25+CD4+ -depleted T cells induced several organ-specific autoimmune diseases in immunodeficient recipients, put the suppressor T cell model back into the focus of many immunologists. CD25+CD4+ T cells were named regulatory T cells (Treg) and since then have been intensively characterized by many groups. It has now been well documented in a variety of models that CD25+CD4+ Tregs, in addition to cell-intrinsic peripheral tolerance mechanisms such as anergy induction and peripheral deletion, play indispensable roles in the maintenance of natural self-tolerance, in averting autoimmune responses as well as in controlling inflammatory reactions. However, a number of fundamental questions concerning their origin, mechanism of action, and the sites of suppression remain elusive and are currently a matter of debate. Notably, the potential heterogeneity of Tregs with respect to phenotype and function deserves attention and is a major issue discussed in this review.

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.

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.

Continuous control of autoimmune disease by antigen-dependent polyclonal CD4+CD25+ regulatory T cells in the regional lymph node

This study investigated the unresolved issue of antigen-dependency and antigen-specificity of autoimmune disease suppression by CD4⁺CD25⁺ T cells (T regs). Based on autoimmune ovarian disease (AOD) in day 3 thymectomized (d3tx) mice and polyclonal T regs expressing the Thy1.1 marker, we determined: (a) the location of recipient T cell suppression, (b) the distribution of AOD-suppressing T regs, and (c) the relative efficacy of male versus female T regs. Expansion of recipient CD4⁺ T cells, activation/memory marker expression, and IFN-γ production were inhibited persistently in the ovary-draining LNs but not elsewhere. The cellular changes were reversed upon Thy1.1⁺ T reg depletion, with emergence of potent pathogenic T cells and severe AOD. Similar changes were detected in the regional LNs during autoimmune dacryoadenitis and autoimmune prostatitis suppression. Although the infused Thy1.1⁺ T regs proliferated and were disseminated in peripheral lymphoid organs, only those retrieved from ovary-draining LNs adoptively suppressed AOD at a suboptimal cell dose. By depriving d3tx recipients of ovarian antigens, we unmasked the supremacy of ovarian antigen-exposed female over male T regs in AOD suppression. Thus, disease suppression by polyclonal T regs depends on endogenous antigen stimulation; this occurs in a location where potent antigen-specific T regs accumulate and continuously negate pathogenic T cell response.

Neonatal autoimmune disease: influence of CD4+ CD25+ regulatory T cells

Although previous studies have emphasized the tolerogenic property of murine neonatal immune system, recent studies indicate that neonatal mice are prone to autoimmune disease. This chapter will summarize the evidence for neonatal propensity to autoimmune ovarian disease (AOD) and describe the new finding that autoantibody can trigger a T cell-dependent autoimmune disease in neonatal but not adult mice. Based on depletion or addition of the CD4+ CD25+ T cells, disease resistance of older mice is explicable by the emergence of CD4+ CD25+ regulatory T-cell function after day 5, whereas disease susceptibility is associated with resistance to regulation by CD4+ CD25+ T cells.

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.

A peripheral circulating compartment of natural naive CD4 Tregs

CD4CD25 Tregs play a central role in the maintenance of peripheral self tolerance by keeping autoreactive T cells in check. Whereas the thymic origin of CD4CD25 Tregs, as a distinct lineage, has been inferred, understanding of their developmental pathways has remained elusive. In both mice and humans, peripheral CD4CD25 Treg populations have been described as composed of antigen-experienced T cells that fail to significantly proliferate following TCR stimulation but suppress proliferation and effector functions of CD25 T cells. Here we show that analysis of CD25 expression in human circulating CD4 T lymphocytes with respect to their in vivo differentiation stages identifies a distinct subset of CD25CCR7CD62LCTLA-4FOXP3 cells contained in the CD45RA/RO naive fraction. The subset, which we have named natural naive Tregs (NnTregs), is prominent in young adults and decreases with age together with the total naive CD4 population. NnTregs are anergic following stimulation in the absence of IL-2 and exert ex vivo cell-cell contact-mediated suppressor functions. In addition, they proliferate in response to stimulation with autologous APCs, which indicates a high enrichment in T cells bearing self-reactive TCRs. The definition of this subset has important implications for the analysis of human naturally occurring Tregs and for their targeting in therapeutic immune interventions.

CD4+CD25+ regulatory T cells attenuate the phosphatidylinositol 3-kinase/Akt pathway in antigen-primed immature CD8+ CTLs during functional maturation

This study was designed to determine the role of CD25(+)CD4(+) regulatory T (Tr) cells in CTL maturation and effector functions using a murine CTL line and in vitro MLC. Tr cells inhibited CTL functional maturation, but had no effect on CTL effector functions. In CD4(+) responder T cell-depleted MLC supplemented with IL-2, Tr cells suppressed mature CTL generation only when added within the first 2 days of culture. Tr cells down-regulated levels of active Akt, but not STAT5 or ZAP70 in Ag-primed immature CTLs. Down-regulation of active Akt was accompanied by a reduction in CTL cell size and IL-2Ralpha expression. In Tr cell-depleted MLC, CTLs were generated that exhibited high levels of nonspecific cytotoxicity. Our in vitro findings suggest that Tr cells regulate functional CTL maturation to generate optimal Ag-specific immune responses through the control of the PI3K/Akt pathway.

Roles of Deletion and Regulation in Creating Mixed Chimerism and Allograft Tolerance Using a Nonlymphoablative Irradiation-Free Protocol

The induction of mixed chimerism (MC) is a powerful and effective means to achieve transplantation tolerance in rodent models. Host conditioning with irradiation or cytotoxic drugs has been used in many protocols for chimeric induction across allogeneic barriers. The deletion of alloreactive T cell clones has been described as the main mechanism responsible for the induction of a stable MC. In this study, we demonstrate that a stable MC and skin allograft tolerance can be established across MHC barriers by a noncytotoxic, irradiation-free approach using costimulation blockade plus rapamycin treatment. By using an adoptive transfer model of skin allograft and using specific Vbeta TCR probes, we demonstrated that deletion of donor-reactive cytopathic T cell clones is indeed profound in tolerant hosts. Nonetheless, the challenge of tolerant mixed chimeras with 5 million mononuclear leukocytes (MNL) from naive syngeneic mice was neither able to abolish the stable MC nor to trigger skin allograft rejection, a hallmark of peripheral, not central tolerance. Furthermore, in an adoptive transfer model, MNLs harvested from tolerant hosts significantly inhibited the capacity of naive MNLs to reject same donor, but not third-party, skin allografts. Moreover, when we transplanted skin allografts from stable tolerant chimeras onto syngeneic immune-incompetent mice, graft-infiltrating T cells migrated from the graft site, expanded in the new host, and protected allografts from acute rejection by naive syngeneic MNLs. In this model, both deletional and immunoregulatory mechanisms are active during the induction and/or maintenance of allograft tolerance through creation of MC using a potentially clinically applicable regimen.

Ontogeny of CD4+CD25+ regulatory/suppressor T cells in human fetuses

Little is known about the ontogeny of naturally occurring CD4+CD25+ regulatory/suppressor T cells that play a major role in maintaining self-tolerance in mice and humans. In rodents, thymectomy on day 3 of life leads to multiple organ-specific autoimmune diseases that can be prevented by adoptive transfer of regulatory T cells, suggesting their neonatal development. We investigated regulatory T-cell ontogeny in 11 human fetuses. Together with the first mature T cells, thymic CD4+CD25+ cells were detected as early as 13 weeks of gestation. Thymic CD25+ cells appeared to be positively selected at the CD4+CD8+CD3hi differentiation stage, as assessed by CD1a and CD69 expression. The proportion of thymic CD4+CD25+ cells appeared quite stable with age, around 6% to 7%, similar to the proportion observed in infant thymi. Extrathymic CD4+CD25+ T cells could hardly be detected at 13 weeks of gestation but were present from week 14 onwards. As adult regulatory T cells, purified CD4+CD25+ fetal cells were anergic and suppressed T-cell proliferative responses; they expressed intracellular cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and Foxp3 mRNA. Altogether, our results indicate that the generation of regulatory/suppressor T cells is consubstantial to the generation of a functional and self-tolerant immune system. (Blood. 2005;105:4715-4721)

Lymph Node Occupancy Is Required for the Peripheral Development of Alloantigen-Specific Foxp3+ Regulatory T Cells

We previously demonstrated that L-selectin (CD62L)-dependent T cell homing to lymph nodes (LN) is required for tolerance induction to alloantigen. To explore the mechanisms of this observation, we analyzed the development and distribution of regulatory T cells (Treg), which play an important protective role against allograft rejection in transplantation tolerance. Alloantigen-specific tolerance was induced using either anti-CD2 plus anti-CD3 mAbs, or anti-CD40L mAbs plus donor-specific transfusion, in fully mismatched (BALB/c donor, C57BL/6 recipient) vascularized cardiac allografts. An expansion of CD4(+)CD25(+)CD62L(high) T cells was observed specifically within the LN of tolerant animals, but not in other anatomic sites or under nontolerizing conditions. These cells exhibited a substantial up-regulation of Foxp3 expression as measured by real-time PCR and by fluorescent immunohistochemistry, and possessed alloantigen-specific suppressor activity. Neither LN nor other lymphoid cells expressed the regulatory phenotype if recipients were treated with anti-CD62L mAbs, which both prevented LN homing and caused early allograft rejection. However, administration of FTY720, a sphingosine 1-phosphate receptor modulator that induces CD62L-independent T cell accumulation in the LNs, restored CD4(+)CD25(+) Treg in the LNs along with graft survival. These data suggest that alloantigen-specific Foxp3(+)CD4(+)CD25(+) Treg develop and are required within the LNs during tolerization, and provide compelling evidence that distinct lymphoid compartments play critical roles in transplantation tolerance.

TCRalphabeta repertoire diversity of human naturally occurring CD4+CD25+ regulatory T cells

We examined the alphabeta T cell receptor (TCR) repertoire of naturally occurring CD4+CD25+ regulatory T (Treg) cells isolated from healthy human blood. Three-color FACS analysis demonstrated that the usage of variable region segments of TCRbeta chains by CD4+CD25+ cells did not differ from those of CD4+CD25- cells. Complementarity-determining region 3 (CDR3) size distribution analyses demonstrated that the repertoire diversity of CDR3beta was almost identical between CD4+CD25+ and CD4+CD25- T cell subsets, and that there was no skewing of the CDR3beta repertoire of CD4+CD25+ T cells. In contrast, in vitro activated CD4+CD25+ T cells by cytomegalovirus-derived antigens showed a skewed CDR3 size distribution pattern. These findings support the hypothesis that naturally occurring CD4+CD25+ T cell subset in humans is 1argely composed of a T cell lineage positively selected in the thymus as a consequence of the interaction between self-peptides and TCRs and not derived from recent activation by a limited array of antigens.

The immunomodulatory effects of regulatory T cells: implications for immune regulation in the skin

Regulatory T cells are thought to have a critical role in the suppression of immune responses. In addition to the prevention of the development of autoimmunity, they are also thought to have a role in the prevention of allergic responses to environmental allergens, immune responses to tumours and the development of memory responses to chronic infections. They have been isolated within the skin and have been shown to express surface markers that enable skin-specific migration, suggesting that regulatory T cells have a functional role in the skin. There is accumulating evidence to suggest that regulatory T cells may be involved in numerous skin disorders and may also be modified by various therapeutic agents used to treat these disorders. We review the evidence for the presence of this T-cell subset in humans, the suppressive effects of regulatory T cells, and their role in the skin.

CD25+ regulatory cells from HLA-DQ8 transgenic mice are capable of modulating collagen-induced arthritis

In the last decade, CD4+CD25+ T regulatory cells have been implicated in the protection against autoimmune diseases. The human DQ8 major histocompatibility complex (MHC) class II molecule is associated with rheumatoid arthritis (RA) and various other autoimmune diseases in humans. The human leukocyte antigen (HLA)-DQ8 transgenic mouse, containing the human DQ8 MHC class II molecule, is predisposed toward collagen-induced arthritis. However, the biologic pathways responsible for DQ8-associated autoimmunity have yet to be defined, including possible defects in the CD4+CD25+ T regulatory cell compartment. To explore this concept, we examined the suppressive capacity of CD4+CD25+ T regulatory cells from DQ8 transgenic mice in vitro and, using CD25-specific depleting antibodies, investigated their influence on collagen-induced arthritis in vivo. CD4+CD25+ T regulatory cells isolated from DQ8 transgenic mice were found to be sufficient suppressors of splenocyte proliferation and interferon (INF)-gamma production. Furthermore, depletion of these cells before immunization led to significant increases in arthritis severity, collagen-specific antibodies, and INF-gamma production. These results indicate that HLA-DQ8 mice contain naturally occurring CD25+ regulatory cells that modulate collagen-induced arthritis and imply that DQ8 expression does not hinder the development of CD25+ T regulatory cells.

Donor CD4+CD25+ T cells promote engraftment and tolerance following MHC-mismatched hematopoietic cell transplantation

Allogeneic bone marrow transplantation (BMT) is a potentially curative treatment for both inherited and acquired diseases of the hematopoietic compartment; however, its wider use is limited by the frequent and severe outcome of graft-versus-host disease (GVHD). Unfortunately, efforts to reduce GVHD by removing donor T cells have resulted in poor engraftment and elevated disease recurrence. Alternative cell populations capable of supporting allogeneic hematopoietic stem/progenitor cell engraftment without inducing GVHD could increase numbers of potential recipients while broadening the pool of acceptable donors. Although unfractionated CD4(+) T cells have not been shown to be an efficient facilitating population, CD4(+)CD25(+) regulatory cells (T-reg's) were examined for their capacity to support allogeneic hematopoietic engraftment. In a murine fully major histocompatibility complex (MHC)-mismatched BMT model, cotransplantation of donor B6 T-reg's into sublethally conditioned BALB/c recipients supported significantly greater lineage-committed and multipotential donor progenitors in recipient spleens 1 week after transplantation and significantly increased long-term multilineage donor chimerism. Donor engraftment occurred without GVHD-related weight loss or lethality and was associated with tolerance to donor and host antigens by in vitro and in vivo analyses. Donor CD4(+)CD25(+) T cells may therefore represent a potential alternative to unfractionated T cells for promotion of allogeneic engraftment in clinical hematopoietic cell transplantation.

Recognition of the peripheral self by naturally arising CD25+ CD4+ T cell receptors

Naturally arising CD25+ CD4+ regulatory T cells (TR) play an important role in the prevention of autoimmunity. TCR specificity is thought to play a critical role in TR development and function, but the repertoire and specificity of TR TCRs remain largely unknown. We find by sequencing of TRAV14 (Valpha2) TCRalpha chains associated with a transgenic TCRbeta chain that the TRand CD25- CD4+ TCR repertoires are similarly diverse, yet only partially overlapping. Retroviral expression of TCRalpha genes in TCR transgenic RAG-deficient T cells revealed that a high frequency of TCRs derived from CD25+ but not CD25- CD4+ T cells confers the ability to rapidly expand upon transfer into a lymphopenic host. Thus, these data show that a large proportion of naturally arising TR have substantially more efficient interactions with MHC class II bound peptides from the peripheral self than CD25- T cells.

Induction of foxP3+ regulatory T cells in the periphery of T cell receptor transgenic mice tolerized to transplants

Transplantation tolerance can be induced in mice by grafting under the cover of nondepleting CD4 plus CD8 or CD154 mAbs. This tolerance is donor Ag specific and depends on a population of CD4(+) regulatory T cells that, as yet, remain poorly defined in terms of their specificity, origin, and phenotype. Blocking of the Ag-specific response in vitro with an anti-CD4 mAb allowed T cells from monospecific female TCR-transgenic mice against the male Ag Dby, presented by H-2E(k), to express high levels of foxP3 mRNA. foxP3 induction was dependent on TGF-beta. The nondepleting anti-CD4 mAb was also able to induce tolerance in vivo in such monospecific TCR-transgenic mice, and this too was dependent on TGF-beta. As in conventional mice, acquired tolerance was dominant, such that naive monospecific T cells were not able to override tolerance. Splenic T cells from tolerant mice proliferated normally in response to Ag, and secreted IFN-gamma and some IL-4, similar to control mice undergoing primary or secondary graft rejection. High levels of foxP3 mRNA, and glucocorticoid-induced TNFR superfamily member 18 (GITR)(+) CD25(+) T cells were found within the tolerated skin grafts of long-term tolerant recipients. These data suggest that regulatory T cells maintaining transplantation tolerance after CD4 Ab blockade can be induced de novo through a TGF-beta-dependent mechanism, and come to accumulate in tolerated grafts.

Role for suppressor T cells in the pathogenesis of autoimmune diseases (including rheumatoid arthritis). Facts and hypotheses

Although uncontrolled clones of autoreactive T cells play a central role in the pathogenesis of autoimmunity, another mechanism potentially involved in many autoimmune diseases is deficiency of suppressor T cells, most notably those belonging to the antiidiopeptide TH3/Tr1 TCD4+CD25+(high) subset. Failure of suppressor mechanisms may be in part primary, due to defective positive selection of suppressor T cells in the thymus, and in part acquired, secondary to chronic infections promoted by deficiencies in innate immunity. Renewed interest in suppressor TCD4+ cells has generated plausible explanations for many events including paradoxical induction of autoimmune disorders by immunosuppressive agents or thymectomy. Insights into the physiology of these regulatory T-cell clones might suggest new treatment options, although many currently used drugs (including anti-TNF alpha agents) enhance the activity of several suppressor T-cell clones. Investigation of these suppressor clones in rheumatoid arthritis is still in its infancy and faces obstacles such as the need for identifying key clones in each individual patient and the presence of T-cell repertoire contraction. This last phenomenon exists at disease onset and may stem from early thymus dysfunction, which may also lead to a reduction in suppressor TCD4+ cell counts. Thus, although restoring deficient suppressor clones may provide a full recovery in animals, the high prevalence of T-cell repertoire contraction in humans with rheumatoid arthritis may severely limit the beneficial effects of this therapeutic approach.

Ig-Reactive CD4+CD25+ T Cells from Tolerized (New Zealand Black × New Zealand White)F1 Mice Suppress In Vitro Production of Antibodies to DNA

We have recently shown that tolerogenic administration of an artificial peptide (pConsensus) that is based on sequences within the V(H) regions of several murine anti-dsDNA Ig delays appearance of autoantibodies in female (New Zealand Black (NZB) x New Zealand White (NZW))F(1) (NZB/W F(1)) mice and significantly prolongs their survival. The aim of this study was to characterize the T cell population(s) involved in pConsensus-induced down-regulation of autoimmune responses in tolerized NZB/W F(1) mice. Using MHC class II dimers loaded with tolerogenic peptide, we found that pCons favored expansion of peptide-reactive CD4(+)CD25(+) regulatory T cells (T(R)) that inhibited in vitro production of anti-dsDNA Ab-forming cells. Suppression by T(R) was abrogated by the presence in culture of Ab to glucocorticoid-induced TNFR family member 18 or to TGFbeta latency-associated protein. These findings suggest possible relevance of Ag specificity in the mechanism of T(R)-mediated immune tolerance to Ig-derived peptides in NZB/W F(1) mice.

Naturally arising CD4+ regulatory t cells for immunologic self-tolerance and negative control of immune responses

Naturally occurring CD4+ regulatory T cells, the majority of which express CD25, are engaged in dominant control of self-reactive T cells, contributing to the maintenance of immunologic self-tolerance. Their depletion or functional alteration leads to the development of autoimmune disease in otherwise normal animals. The majority, if not all, of such CD25+CD4+ regulatory T cells are produced by the normal thymus as a functionally distinct and mature subpopulation of T cells. Their repertoire of antigen specificities is as broad as that of naive T cells, and they are capable of recognizing both self and nonself antigens, thus enabling them to control various immune responses. In addition to antigen recognition, signals through various accessory molecules and via cytokines control their activation, expansion, and survival, and tune their suppressive activity. Furthermore, the generation of CD25+CD4+ regulatory T cells in the immune system is at least in part developmentally and genetically controlled. Genetic defects that primarily affect their development or function can indeed be a primary cause of autoimmune and other inflammatory disorders in humans. Based on recent advances in our understanding of the cellular and molecular basis of this T cell-mediated immune regulation, this review discusses how naturally arising CD25+CD4+ regulatory T cells contribute to the maintenance of immunologic self-tolerance and negative control of various immune responses, and how they can be exploited to prevent and treat autoimmune disease, allergy, cancer, and chronic infection, or establish donor-specific transplantation tolerance.

[Thymus function and autoimmunity]

PURPOSE

Thymus is the site of T-cell development and is essential for the induction of self-tolerance, by deletion of autoreactive T lymphocytes (negative selection) and by generation of regulatory T cells. Defect of the selection mechanism of both types of lymphocytes lead to autoimmune diseases.

CURRENT KNOWLEDGE AND KEY POINTS

Elimination of potentially self-reactive T cells in the thymus requires the intrathymic expression of ubiquitous and "tissue-specific" antigens. Some thymic antigen expressions are dependent on AIRE expression. Mutations in the AIRE gene that are associated with the absence of autoantigen expression in the thymus, defects in the peptide presentation or in apoptosis can allow autoreactive T cells to escape negative selection, and are associated with autoimmune diseases. Recent data are now available concerning the thymic selection of autoreactive regulatory T cells. The Foxp3 gene was recently shown to be predominantly expressed in regulatory T cells and could be a more specific marker of regulatory T cells than phenotypic markers.

FUTURE PROSPECTS AND PROJECTS

Animal models show that regulatory T cells injection or intrathymic inoculation of antigen lead to immunological tolerance in autoimmunity and transplantation. These novel strategies could be used in human.

Could rheumatoid arthritis result from an abnormal T cell response towards lubricin/superficial zone protein?

The lack of some suppressor T cells (including TCD4+CD25+(high) positively selected first in thymic medulla) specific to a restricted set of autoantigens may be the common link between all patterns of rheumatoid arthritis. In other words, instead of a 'peak' of TCD4+ effector T cells common to all patients with rheumatoid arthritis (which has so far never been demonstrated), a 'hole' in TCD4+CD25+(high) responses towards a limited set of autoantigens responsible for the normal maintenance of tolerance within the joints may be shared by many patients with rheumatoid arthritis. The hallmark of this disorder is the involvement of tissues subjected to friction stress bathed in a lubricating fluid (rheumatoid nodules and bursae, tendinous sheaths, pleura, pericardium, sclera, and joints covered by hyaline cartilage). Consequently, autoantigens shared by all forementioned places may be better candidates than autoantigens restricted to the hyaline cartilage (like collagen II). Tenosynovitis, bursitis and rheumatoid nodules can herald rheumatoid arthritis, and rheumatoid pericarditis is very frequent at the histological level. Lubricin and superficial zone protein (SZP), which are closely related products of the megacaryocyte stimulating factor (MSF) gene, are among the best candidate autoantigens for such a positive selection of suppressor T cells. Lubricin is responsible for most of the lubricating properties of synovial fluid, and SZP (expressed by the superficial articular chondrocytes from diarthrodial cartilages and lining cells of synovial villi) also shares lubricating and cytoprotective properties. Moreover, the expression of lubricin is very probable in pericardium and pleura, and can be induced by friction stress. Although this mucinous glycoprotein may already share close similarities at the antigenic levels with mucins previously demonstrated in Hassall's corpuscles of the thymus, evidence for the ectopic expression of lubricin/SZP within normal human thymus may further support this hypothesis. The prenatal positive selection within the thymus of a functional pool of TCD4+CD25+(high) clones specific for most peripheral tissues is critical (at least in mice) for the quality of tolerance for the rest of the organism's lifespan. Therefore, a poor expression of lubricin/SZP early in life within the human thymus may also favour a lack of suppressor T cells specific to tissues bathed with synovial fluid, i.e. the onset of rheumatoid arthritis later on in life. As studies of human thymus long before the onset of rheumatoid arthritis are hampered for obvious reasons, studies of murine thymus could be a first step. In as much as the human counterpart of lubricin is expressed in the thymic medulla of mice, the generation of knocked-out mice for its expression within the thymus could be one of the best models to test the above hypothesis. The stimulation of TCD4+CD25+(high) clones specific for immunodominant epitopes from the joints/synovial fluid (belonging perhaps to lubricin or SZP) could help restore a normal balance between effector T cells and suppressor T cells in rheumatoid arthritis patients.

Human CD4+CD25+ Regulatory T Cells Share Equally Complex and Comparable Repertoires with CD4+CD25− Counterparts

CD4(+)CD25(+) T cells are critical mediators of peripheral immune tolerance. However, many developmental and functional characteristics of these cells are unknown, and knowledge of human regulatory T cells is particularly limited. To better understand how human CD4(+)CD25(+) T cells develop and function, we examined the diversity of CD4(+)CD25(+) and CD4(+)CD25(-) T cell repertoires in both thymus and peripheral blood. Levels of T receptor excision circles (TREC) were comparable in purified CD4(+)CD25(+) and CD4(+)CD25(-) thymic populations, but were significantly higher than those in samples derived from peripheral blood, consistent with murine studies demonstrating thymic development of CD4(+)CD25(+) regulatory T cells. Surprisingly, CD4(+)CD25(-) T cells isolated from peripheral blood had greater TREC quantities than their CD4(+)CD25(+) counterparts, supporting the possibility of extrathymic expansion as well. CD4(+)CD25(+) and CD4(+)CD25(-) T cells from a given individual showed overlapping profiles with respect to diversity by Vbeta staining and spectratyping. Interestingly, CD4(+)CD25(+) T cells have lower quantities of CD3 than CD4(+)CD25(-) T cells. Collectively, these data suggest that human CD4(+)CD25(+) T cells recognize a similar array of Ags as CD4(+)CD25(-) T cells. However, reduced levels of TCR on regulatory T cells suggest different requirements for activation and may contribute to how the immune system regulates whether a particular response is suppressed or augmented.

CD4+ CD25+ Treg: divide and rule?

Research on CD4+ CD25+ regulatory T cells (Treg) has gathered momentum over the last five years but many aspects of their fundamental biology remain elusive. Treg have been considered to be 'naturally anergic' based on their failure to proliferate in response to T-cell receptor ligation in vitro. Several recent studies challenge this view and demonstrate a robust proliferative capacity for CD25+ cells. The significance of this finding for Treg homeostasis and function is considered below.

Alloantigen-induced CD25+CD4+ regulatory T cells can develop in vivo from CD25-CD4+ precursors in a thymus-independent process

The capacity of naturally occurring autoreactive CD25+CD4+ regulatory T cells (Treg) to control immune responses both in vivo and in vitro is now well established. It has been demonstrated that these cells undergo positive selection within the thymus and appear to enter the periphery as committed CD25+CD4+ Treg. We have shown previously that CD25+CD4+ Treg with the capacity to prevent skin allograft rejection can be generated by pretreatment with donor alloantigen under the cover of anti-CD4 therapy. Here we demonstrate that this process does not require an intact thymus. Furthermore, generation of these Treg is not dependent on the expansion of CD25+CD4+ thymic emigrants, because depletion of CD25+ cells before pretreatment does not prevent Treg development, and Treg can be generated from CD25-CD4+ precursors. Taken together, these results clearly demonstrate that CD25+CD4+ Treg can be generated in the periphery from CD25-CD4+ precursors in a pathway distinct to that by which naturally occurring autoreactive CD25+CD4+ Treg develop. These observations may have important implications for the design of protocols, both experimental and clinical, for the induction of tolerance to autoantigens or alloantigens in adults with limited thymic function.

Revival of the regulatory T cell: new targets for drug development

Compelling new evidence supports the idea that regulatory T cells play a major role in our immune system. Several subsets of these regulators have been identified recently. Differences in the phenotypical and functional characteristics of these subsets have immunological implications. From our growing knowledge of the field of immunology, we could potentially generate a new class of therapeutic agents that target immune-related diseases.

Specificity of regulatory CD4+CD25+ T cells for self-T cell receptor determinants

Although the phenotypic and regulatory properties of the CD4(+)CD25(+) T cell lineage (Treg cells) have been well described, the specificities remain largely unknown. We demonstrate here that the CD4(+)CD25(+) Treg population includes the recognition of a broad spectrum of human TCR CDR2 determinants found in the germline V gene repertoire as well as that of a clonotypic nongermline-encoded CDR3beta sequence present in a recombinant soluble T cell receptor (TCR) protein. Regulatory activity was demonstrated in T cell lines responsive to TCR but not in T cell lines responsive to control antigens. Inhibitory activity of TCR-reactive T cells required cell-cell contact and involved CTLA-4, GITR, IL-10, and IL-17. Thus, the T-T regulatory network includes Treg cells with specificity directed toward self-TCR determinants.

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Control of autoimmunity by naturally arising regulatory CD4+ T cells

Naturally acquired immunological self-tolerance is not entirely accounted for by clonal deletion, anergy, and ignorance. It is now well established that the T cell-repertoire of healthy individuals harbors self-reactive lymphocytes with a potential to cause autoimmune disease and these lymphocytes are under dominant control by a unique subpopulation of CD4+ T cells now called regulatory T cells. Efforts to delineate these Treg cells naturally present in normal individuals have revealed that they are enriched in the CD25+ CD4+ population. The identification of the CD25 molecule as a useful marker for naturally arising CD4+ regulatory T cells has made it possible to investigate many key aspects of their immunobiology, including their antigen specificities and the cellular/molecular pathways involved in their development and their mechanisms of action. Furthermore, reduction or dysfunction of the CD25+ CD4+ regulatory T cell population can be responsible for certain autoimmune diseases in humans.

Whither T-suppressors: if they didn’t exist would we have to invent them?

Arriving at an understanding of the role of suppressor T-cells (regulatory T-cells, CD4(+)CD25+) depends on whether their functional repertoire is somatically selected to be anti-Self or anti-Nonself. Immunologists are ambivalent; often publications espousing opposite views share an author. Here the arguments are detailed that the suppressor repertoire is not somatically selected to be anti-Self, but rather it is anti-Nonself. Therefore, suppression cannot regulate the Self-Nonself discrimination; its function is to regulate the magnitude and class of the anti-Nonself effector response.