Quantitative impact of thymic clonal deletion on the T cell repertoire

Design principles of adaptive immune systems

Both jawless vertebrates, such as lampreys and hagfish, and jawed vertebrates (encompassing species as diverse as sharks and humans) have an adaptive immune system that is based on somatically diversified and clonally expressed antigen receptors. Although the molecular nature of the antigen receptors and the mechanisms of their assembly are different, recent findings suggest that the general design principles underlying the two adaptive immune systems are surprisingly similar. The identification of such commonalities promises to further our understanding of the mammalian immune system and to inspire the development of new strategies for medical interventions targeting the consequences of faulty immune functions.

Regulatory T-cell differentiation versus clonal deletion of autoreactive thymocytes

The concept of clonal deletion of immune cells that carry an autoreactive antigen receptor was a central prediction of Burnet's clonal selection theory. A series of classical experiments in the late 1980s revealed that certain immature thymocytes upon encounter of 'self' are indeed removed from the T-cell repertoire before their release into the blood circulation. A second essential cornerstone of immunological tolerance, not anticipated by Burnett, has more recently surfaced through the discovery of Foxp3(+) regulatory T cells (Treg). Intriguingly, it appears that the expression of an autoreactive T-cell receptor is a shared characteristic of T cells that are subject to clonal deletion as well as of those deviated into the Treg lineage. This is all the more striking as Treg differentiation for the most part branches off from mainstream CD4T cell development during thymocyte maturation in the thymus, that is, it may neither temporally nor spatially be separated from clonal deletion. This raises the question of how an apparently identical stimulus, namely the encounter of 'self' during thymocyte development, can elicit fundamentally different outcomes such as apoptotic cell death on the one hand or differentiation into a highly specialized T-cell lineage on the other hand. Here, we will review the T-cell intrinsic and extrinsic factors that have been implicated in intrathymic Treg differentiation and discuss how these parameters may determine whether an autoreactive major histocompatibility complex class II-restricted thymocyte is deviated into the Treg lineage or subject to clonal deletion.

The role of the thymus in tolerance

The thymus serves as the central organ of immunologic self-nonself discrimination. Thymocytes undergo both positive and negative selection, resulting in T cells with a broad range of reactivity to foreign antigens but with a lack of reactivity to self-antigens. The thymus is also the source of a subset of regulatory T cells that inhibit autoreactivity of T-cell clones that may escape negative selection. As a result of these functions, the thymus has been shown to be essential for the induction of tolerance in many rodent and large animal models. Proper donor antigen presentation in the thymus after bone marrow, dendritic cell, or solid organ transplantation has been shown to induce tolerance to allografts. The molecular mechanisms of positive and negative selection and regulatory T-cell development must be understood if a tolerance-inducing therapeutic intervention is to be designed effectively. In this brief and selective review, we present some of the known information on T-cell development and on the role of the thymus in experimental models of transplant tolerance. We also cite some clinical attempts to induce tolerance to allografts using pharmacologic or biologic interventions.

B cells participate in thymic negative selection of murine auto-reactive CD4+ T cells

It is well documented that thymic epithelial cells participate in the process of negative selection in the thymus. In recent years it was reported that also dendritic cells enter the thymus and contribute to this process, thus allowing for the depletion of thymocytes that are specific to peripherally expressed self-antigens. Here we report that also B cells may take part in the elimination of auto-reactive thymocytes. Using a unique mouse model we show that B cells induce negative selection of self-reactive thymocytes in a process that leads to the deletion of these cells whereas regulatory T cells are spared. These findings have direct implication in autoimmunity, as expression of a myelin antigen by B cells in the thymus renders the mice resistant to autoimmune inflammation of the CNS.

Cutting edge: Hematopoietic-derived APCs select regulatory T cells in thymus

Recognition of self-peptide-MHC complexes by high-affinity TCRs and CD28 signaling are critical for the development of forkhead-winged helix box transcription factor 3(+) regulatory T cells (Tregs) in thymus. However, the type of APCs that are responsible for selecting Tregs has remained unclear. To dissect the role of hematopoietic-derived APCs (HCs) and thymic epithelial cells (TECs) in Treg selection, we constructed bone marrow chimeras with disrupted CD28/B7 signaling in the HC or TEC compartment and analyzed the generation of Tregs in the thymus. We found that both HCs and TECs were independently able to fully reconstitute the Treg population in the thymus of bone marrow chimeras. In addition, Treg selection requires the TCR signal and CD28 costimulation presented in cis on the same APC type in vivo. This study demonstrates a new role, to our knowledge, for HCs in the development of Tregs in thymus.

Autonomous role of medullary thymic epithelial cells in central CD4(+) T cell tolerance

Medullary thymic epithelial cells (mTECs) serve an essential function in central tolerance by expressing peripheral-tissue antigens. These antigens may be transferred to and presented by dendritic cells (DCs). Therefore, it is unclear whether mTECs, in addition to being an antigen reservoir, also serve a mandatory function as antigen-presenting cells. Here we diminished major histocompatibility complex (MHC) class II on mTECs through transgenic expression of a 'designer' microRNA specific for the MHC class II transactivator CIITA (called 'C2TA' here). This resulted in an enlarged polyclonal CD4(+) single-positive compartment and, among thymocytes specific for model antigens expressed in mTECs, enhanced selection of regulatory T cells (T(reg) cells) at the expense of deletion. Our data document an autonomous contribution of mTECs to both dominant and recessive mechanisms of CD4(+) T cell tolerance and support an avidity model of T(reg) cell development versus deletion.

Fas apoptosis inhibitory molecule regulates T cell receptor-mediated apoptosis of thymocytes by modulating Akt activation and Nur77 expression

Fas apoptosis inhibitory molecule (FAIM) has been demonstrated to confer resistance to Fas-induced apoptosis of lymphocytes and hepatocytes in vitro and in vivo. Here, we show that FAIM is up-regulated in thymocytes upon T cell receptor (TCR) engagement and that faim(-/-) thymocytes are highly susceptible to TCR-mediated apoptosis with increased activation of caspase-8 and -9. Furthermore, injection of anti-CD3 antibodies leads to augmented depletion of CD4(+)CD8(+) T cells in the thymus of faim(-/-) mice compared with wild-type control, suggesting that FAIM plays a role in thymocyte apoptosis. Cross-linking of the TCR on faim(-/-) thymocytes leads to an elevated protein level of the orphan nuclear receptor Nur77, which plays a role in thymocyte apoptosis. Interestingly, in the absence of FAIM, there are reduced ubiquitination and degradation of the Nur77 protein. Faim(-/-) thymocytes also exhibit a defective TCR-induced activation of Akt whose activity we now show is required for Nur77 ubiquitination. Further analyses utilizing FAIM-deficient primary thymocytes and FAIM-overexpressing DO-11.10 T cells indicate that FAIM acts upstream of Akt during TCR signaling and influences the localization of Akt to lipid rafts, hence affecting its activation. Taken together, our study defined a TCR-induced FAIM/Akt/Nur77 signaling axis that is critical for modulating the apoptosis of developing thymocytes.

Thymic selection and lineage commitment of CD4(+)Foxp3(+) regulatory T lymphocytes

Regulatory T lymphocytes play a central role in the control of a variety of immune-responses. Their absence in humans and in experimental animal models leads to severe autoimmune and inflammatory disorders. Consistent with their major role in prevention of autoimmune pathology, their repertoire is enriched in autospecific cells. Probably the majority of regulatory T cells develop in the thymus. How T cell-precursors choose between the conventional versus regulatory T cell lineages remains an unanswered question. More is known about selection of regulatory T cell precursors. Positive selection of these cells is favored by high affinity interactions with MHC class II/peptide ligands expressed by thymic epithelial or dendritic cells. They are also known to be relatively resistant to negative selection. These two parameters allow for the generation of the autoreactive regulatory T cell repertoire, and clearly distinguish selection-criteria of conventional versus regulatory T cell-precursors. It will now be important to elucidate the molecular mechanisms involved in the intrathymic choice of the regulatory T cell-lineage.

Constitutive ablation of dendritic cells breaks self-tolerance of CD4 T cells and results in spontaneous fatal autoimmunity

Lack of immunological tolerance against self-antigens results in autoimmune disorders. During onset of autoimmunity, dendritic cells (DCs) are thought to be critical for priming of self-reactive T cells that have escaped tolerance induction. However, because DCs can also induce T cell tolerance, it remains unclear whether DCs are required under steady-state conditions to prevent autoimmunity. To address this question, we crossed CD11c-Cre mice with mice that express diphtheria toxin A (DTA) under the control of a loxP-flanked neomycin resistance (neo^R) cassette from the ROSA26 locus. Cre-mediated removal of the neo^R cassette leads to DTA expression and constitutive loss of conventional DCs, plasmacytoid DCs, and Langerhans cells. These DC-depleted (ΔDC) mice showed increased frequencies of CD4 single-positive thymocytes and infiltration of CD4 T cells into peripheral tissues. They developed spontaneous autoimmunity characterized by reduced body weight, splenomegaly, autoantibody formation, neutrophilia, high numbers of Th1 and Th17 cells, and inflammatory bowel disease. Pathology could be induced by reconstitution of wild-type (WT) mice with bone marrow (BM) from ΔDC mice, whereas mixed BM chimeras that received BM from ΔDC and WT mice remained healthy. This demonstrates that DCs play an essential role to protect against fatal autoimmunity under steady-state conditions.

Does intra-individual major histocompatibility complex diversity keep a golden mean?

An adaptive immune response is usually initiated only if a major histocompatibility complex (MHC) molecule presents pathogen-derived peptides to T-cells. Every MHC molecule can present only peptides that match its peptide-binding groove. Thus, it seems advantageous for an individual to express many different MHC molecules to be able to resist many different pathogens. However, although MHC genes are the most polymorphic genes of vertebrates, each individual has only a very small subset of the diversity at the population level. This is an evolutionary paradox. We provide an overview of the current data on infection studies and mate-choice experiments and conclude that overall evidence suggests that intermediate intra-individual MHC diversity is optimal. Selective forces that may set an upper limit to intra-individual MHC diversity are discussed. An updated mathematical model based on recent findings on T-cell selection can predict the natural range of intra-individual MHC diversity. Thus, the aim of our review is to evaluate whether the number of MHC alleles usually present in individuals may be optimal to balance the advantages of presenting an increased range of peptides versus the disadvantages of an increased loss of T-cells.

Dendritic cells in the thymus contribute to T-regulatory cell induction

Central tolerance is established through negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T(R)s). The role of thymic dendritic cells (TDCs) in these processes has not been clearly determined. In this study, we demonstrate that in vivo, TDCs not only play a role in negative selection but in the induction of T(R)s. TDCs include two conventional dendritic cell (DC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+)) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-)) [corrected] which have different origins. We found that the CD8(hi)Sirpalpha(+) DCs represent a conventional DC subset that originates from the blood and migrates into the thymus. Moreover, we show that the CD8(lo)Sirpalpha(+) DCs demonstrate a superior capacity to induce T(R)s in vitro. Finally, using a thymic transplantation system, we demonstrate that the DCs in the periphery can migrate into the thymus, where they efficiently induce T(R) generation and negative selection.

The impact of circulating dendritic cells on the development and differentiation of thymocytes

Central tolerance is established through the negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T-regs). A role for thymic epithelial cells in mediating both negative selection and T-reg induction has been clearly shown. The role of thymic dendritic cells (DCs) in these processes has not been clearly determined but has been the focus of recent studies. Thymic DCs include two conventional DC (cDC) subtypes, CD8(lo)Sirpalpha(hi/+) (CD8(lo)Sirpalpha(+) herein) and CD8(hi)Sirpalpha(lo/-) (CD8(hi)Sirpalpha(-) herein). It has been shown that these DC subsets have distinct developmental origins, the CD8(hi)Sirpalpha(-) cDCs developing intra-thymically and the CD8(lo)Sirpalpha(+) migrating into the thymus from the periphery. Furthermore, an important role for thymic DCs in the induction of T-regs has been shown. In this review, the role of DCs in the development and education of T cells in the thymus will be reviewed, with emphasis on the role of circulatory DCs in mediating these processes.

Murine CD8+ regulatory T lymphocytes: the new era

Regulatory T lymphocytes unequivocally play a major role in the maintenance of immunologic homeostasis. The first descriptions of regulatory T lymphocytes concerned CD8(+) cells, but this field was brought into discredit when some of its central tenets turned out to be erroneous. CD4(+) regulatory T cells took over and, with the help of newly developed molecular tools, rapidly were phenotypically and functionally characterized. We now know that these cells control a large variety of immune responses. However some observations of in vitro or in vivo immune regulation could not be explained with CD4(+) regulatory T cell activity and depended on the action of a variety of CD8(+) T cell populations. In recent years, substantial progress has been made in the phenotypic and functional characterization of CD8(+) regulatory T cells. These cells play a role in the control of intestinal immunity, immunopathology, and autoimmunity, as well as in immune privilege of the eye, in oral tolerance, and in prevention of graft-versus-host disease and graft-rejection. The suppressor effector mechanisms used by these cells are in part shared with CD4(+) regulatory T cells and in part unique to this population. We here review the current literature on naturally occurring and experimentally induced murine CD8(+) regulatory T-cell populations.

Comprehensive assessment and mathematical modeling of T cell population dynamics and homeostasis

Our current view of T cell differentiation and population dynamics is assembled from pieces of data obtained from separate experimental systems and is thus patchy. We reassessed homeostasis and dynamics of T cells 1) by generating a mathematical model describing the spatiotemporal features of T cell differentiation, and 2) by fitting this model to experimental data generated by disturbing T cell differentiation through transient depletion of dividing T cells in mice. This specific depletion was obtained by administration of ganciclovir to mice expressing the conditional thymidine kinase suicide gene in T cells. With this experimental approach, we could derive quantitative parameters describing the cell fluxes, residence times, and rates of import, export, proliferation, and death across cell compartments for thymocytes and recent thymic emigrants (RTEs). Among other parameters, we show that 93% of thymocytes produced before single-positive stages are eliminated through the selection process. Then, a postselection peripheral expansion of naive T cells contributes three times more to naive T cell production than the thymus, with half of the naive T cells consisting of dividing RTEs. Altogether, this work provides a quantitative population dynamical framework of thymocyte development, RTEs, and naive T cells.

Selection of Foxp3+ regulatory T cells specific for self antigen expressed and presented by Aire+ medullary thymic epithelial cells

The parameters specifying whether autoreactive CD4(+) thymocytes are deleted (recessive tolerance) or differentiate into regulatory T cells (dominant tolerance) remain unresolved. Dendritic cells directly delete thymocytes, partly through cross-presentation of peripheral antigens 'promiscuously' expressed in medullary thymic epithelial cells (mTECs) positive for the autoimmune regulator Aire. It is unclear if and how mTECs themselves act as antigen-presenting cells during tolerance induction. Here we found that an absence of major histocompatibility class II molecules on mTECs resulted in fewer polyclonal regulatory T cells. Furthermore, targeting of a model antigen to Aire(+) mTECs led to the generation of specific regulatory T cells independently of antigen transfer to dendritic cells. Thus, 'routing' of mTEC-derived self antigens may determine whether specific thymocytes are deleted or enter the regulatory T cell lineage.

The T-cell antigen receptor: a logical response to an unknown ligand

The immune system can be roughly divided into innate and adaptive compartments. The adaptive compartment includes the B and T lymphocytes, whose antigen receptors are generated by recombination of gene segments. The consequence is that the creation of self-reactive lymphocytes is unavoidable. For the host to remain viable, the immune system has evolved a strategy for removing autoimmune lymphocytes during development. This review discusses how T lymphocytes are generated, how they recognize antigens, and how their antigen receptor directs the removal of self-reactive T cells.

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.

Central tolerance: good but imperfect

T-cell development is a highly coordinated process that depends on interactions between thymocytes, thymic epithelium, and bone marrow (BM)-derived dendritic cells (DCs). Before entering the peripheral T-cell pool, thymocytes are subject to negative selection, a process that eliminates (or deletes) T cells with high affinity toward self-antigens and therefore promotes self-tolerance. These self-antigens include those that are broadly expressed ubiquitous antigens and those whose expression is restricted to a few tissues, tissue-specific antigens (TSAs). Expression of TSAs in the thymus is mostly a property of medullary thymic epithelial cells (mTECs), and because these cells may be less capable than BM-derived DCs at mediating negative selection to ubiquitous antigens, we investigated the roles of both of these cell types in tolerance to TSAs. Here, we review our studies in which we found that mTECs were competent mediators of negative selection to a subset of TSA-reactive T cells, while thymic DCs extend the range of TSA-reactive T cells that undergo negative selection by capturing TSAs from mTECs. In addition, we recently investigated the efficiency of central tolerance to TSA during ontogeny, and we report that this process was less efficient in neonates than adult animals.

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.

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.

The effects of thymic selection on the range of T cell cross-reactivity

Based on the results of a computational model of thymic selection, we propose a mechanism that produces the observed wide range of T cell cross-reactivity. The model suggests that the cross-reactivity of a T cell that survives thymic selection is correlated with its affinity for self peptides. In order to survive thymic selection, a T cell with low affinity for all self peptides expressed in the thymus must have high affinity for major histocompatibility complex (MHC), which makes it highly cross-reactive. A T cell with high affinity for any self peptide must have low MHC affinity to survive selection, which makes it highly specific for its cognate peptide. Our model predicts that (1) positive selection reduces by only 17% the number of T cells that can detect any given foreign peptide, even though it eliminates over 95% of pre-selection cells; (2) negative selection decreases the average cross-reactivity of the pre-selection repertoire by fivefold; and (3) T cells responding to foreign peptides similar to self peptides will have a lower average cross-reactivity than cells responding to epitopes dissimilar to self.

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.

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.

EphA Receptors Inhibit Anti-CD3-Induced Apoptosis in Thymocytes

The EphA receptor tyrosine kinases interact with membrane-bound ligands of the ephrin-A subfamily. Interaction induces EphA receptor oligomerization, tyrosine phosphorylation, and, as a result, EphA receptor signaling. EphA receptors have been shown to regulate cell survival, migration, and cell-cell and cell-matrix interactions. However, their functions in lymphoid cells are only beginning to be described. We show in this study that functional EphA receptors are expressed by murine thymocytes, including CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) subpopulations. We demonstrate that activation of EphA receptors by the ephrin-A1 ligand inhibits the anti-CD3-induced apoptosis of CD4(+)CD8(+) double-positive thymocytes. Furthermore, ephrin-A1 costimulation suppresses up-regulation of both the IL-2R alpha-chain (CD25) and early activation Ag CD69 and can block IL-2 production by CD4(+) single-positive cells. In agreement, EphA receptor activation in thymocytes also inhibits TCR-induced activation of the Ras-MAPK pathway. Our findings suggest that EphA receptor activation is antithetical to TCR signaling in thymocytes, and that the level of engagement by ephrin-A proteins on thymic APCs regulates thymocyte selection.

B cells and B cell products-helping to restore cellular immunity?

T cells that provide vital protection against tumors, viruses and intracellular bacteria are thought to develop independently of B cells. However, recent discoveries suggest that development of T cells depends on B cells. One way B cells promote T cell development is by providing diverse peptides that may promote positive selection of thymocytes. Diverse peptides and B cells help in diversification of the T cell receptor repertoire and may decrease cross-reactivity in the mature T cell compartment. These new insights may provide the basis for the design of novel therapeutics.

The Role of Dendritic Cells in Selection of Classical and Nonclassical CD8+T Cells In Vivo

T cell development is determined by positive and negative selection events. An intriguing question is how signals through the TCR can induce thymocyte survival and maturation in some and programmed cell death in other thymocytes. This paradox can be explained by the hypothesis that different thymic cell types expressing self-MHC/peptide ligands mediate either positive or negative selection events. Using transgenic mice that express MHC class I (MHC-I) selectively on DC, we demonstrate a compartmentalization of thymic functions and reveal that DC induce CTL tolerance to MHC-I-positive hemopoietic targets in vivo. However, in normal and bone marrow chimeric mice, MHC-I+ DC are sufficient to positively select neither MHC-Ib (H2-M3)- nor MHC-Ia (H2-K)-restricted CD8+ T cells. Thus, thymic DC are specialized in tolerance induction, but cannot positively select the vast majority of MHC-I-restricted CD8+ T cells.

Activation-threshold tuning in an affinity model for the T-cell repertoire

Naive T cells respond to peptides from foreign proteins and remain tolerant to self peptides from endogenous proteins. It has been suggested that self tolerance comes about by a 'tuning' mechanism, i.e. by increasing the T-cell activation threshold upon interaction with self peptides. Here, we explore how such an adaptive mechanism of T-cell tolerance would influence the reactivity of the T-cell repertoire to foreign peptides. We develop a computer simulation model in which T cells are tolerized by increasing their activation-threshold dependent on the affinity with which they see self peptides presented in the thymus. Thus, different T cells acquire different activation thresholds (i.e. different cross-reactivities). In previous mathematical models, T-cell tolerance was deletional and based on a fixed cross-reactivity parameter, which was assumed to have evolved to an optimal value. Comparing these two different tolerance-induction mechanisms, we found that the tuning model performs somewhat better than an optimized deletion model in terms of the reactivity to foreign antigens. Thus, evolutionary optimization of clonal cross-reactivity is not required. A straightforward extension of the tuning model is to delete T-cell clones that obtain a too high activation threshold, and to replace these by new clones. The reactivity of the immune repertoires of such a replacement model is enchanced compared with the basic tuning model. These results demonstrate that activation-threshold tuning is a functional mechanism for self tolerance induction.

The impact of thymic antigen diversity on the size of the selected T cell repertoire

The TCR repertoire of a normal animal is shaped in the thymus by ligand-specific positive- and negative-selection events. These processes are believed to be determined at the single-cell level primarily by the affinity of the TCR-ligand interactions. The relationships among all the variables involved are still unknown due to the complexity of the interactions and the lack of quantitative analysis of those parameters. In this study, we developed a quantitative model of thymic selection that provides estimates of the fractions of positively and negatively selected thymocytes in the cortex and in the medulla, as well as upper-bound ranges for the number of selecting ligands required for the generation of a normal diverse TCR repertoire. Fitting the model to current estimates of positive- and negative-selected thymocytes leads to specific predictions. The results indicate the following: 1) the bulk of thymocyte death takes place in the cortex, and it is due to neglect; 2) the probability of a thymocyte to be negatively selected in the cortex is at least 10-fold lower than in the medulla; 3) <60 ligands are involved in cortical positive selection; and 4) negative selection in the medulla is constrained by a large diversity of selecting ligands on medullary APCs.

Back to central tolerance

The establishment and maintenance of immunological tolerance entails both central and peripheral mechanisms. The latter have been highlighted in the past several years, mostly because of great interest in the activities of regulatory T cells. However, an important role for central tolerance mechanisms has been reemphasized by recent results on human autoimmune diseases, including APECED and type 1 diabetes.

Induction of antigen-specific tolerance to bone marrow allografts with CD4+CD25+ T lymphocytes

Thymus-derived regulatory T lymphocytes of CD4(+)CD25(+) phenotype regulate a large variety of beneficial and deleterious immune responses and can inhibit lethal graft-versus-host disease in rodents. In vitro, CD4(+)CD25(+) T cells require specific major histocompatibility complex (MHC)/peptide ligands for their activation, but once activated they act in an antigen-nonspecific manner. In vivo, regulatory T cells are also activated in an antigen-specific fashion, but nothing is known about antigen specificity of their suppressor-effector function. Here we show that CD4(+)CD25(+) regulatory T lymphocytes isolated from naive mice and activated in vitro with allogeneic antigen-presenting cells (APCs) induced specific long-term tolerance to bone marrow grafts disparate for major and minor histocompatibility antigens; whereas "target" bone marrow was protected, third-party bone marrow was rejected. Importantly, in mice injected with a mix of target and third-party bone marrows, protection and rejection processes took place simultaneously. These results indicate that CD4(+)CD25(+) regulatory T cells can act in an antigen-specific manner in vivo. Our results suggest that CD4(+)CD25(+) regulatory T cells could in the future be used in clinical settings to induce specific immunosuppression.

Potential role of major histocompatibility complex class II peptides in regulatory tolerance to vascularized grafts

The inactivation of persisting T lymphocytes reactive to self- and non-self-antigens is a major arm of operational immune tolerance in mammals. Silencing of such T cells proceeds mostly by means of suppression, a process that is mediated by regulatory T-cell subsets and especially by CD4(+)CD(25high) regulatory T cells (Treg). Although Treg activation and ensuing suppressive activity appear to be major histocompatibility complex class II dependent, the fine specificity of Treg T-cell receptors has not yet been elucidated. Recent data from the author's laboratory on a class II gene therapy induction of tolerance to allogeneic kidney grafts suggest that class II peptides are involved as generic signals for Treg activation. A brief compilation of results that would support this hypothesis is discussed in the present article.

The Pro-451 to Leu polymorphism within the C-terminal tail of P2X7 receptor impairs cell death but not phospholipase D activation in murine thymocytes

The P2X family of ATP receptors (P2XR) are ligandgated channels that have been proposed to regulate cell death of immature thymocytes. However, the nature of the P2XR subtype involved has been controversial until recently. In agreement with previous studies, we found that extracellular ATP (ATPe) induces a caspase-dependent apoptosis of BALB/c thymocytes, as observed by DNA fragmentation. Additionally, ATPe induces a predominant caspase-independent thymocytes lysis characterized by plasma membrane disruption. Both responses to ATPe can be induced by a potent P2X7R agonist, benzoylbenzoyl-ATP, whereas P2X7R antagonists, oxidized ATP and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, inhibited the effect of ATPe. These results are further supported by observations where disruption of the P2X7R gene (P2X7R(-/-) mice) completely abolishes thymocytes death induced by ATPe. Interestingly, the natural P451L mutation in the C-terminal tail of P2X7R present in C57BL/6 mice, which impairs ATPe-dependent pore formation in T lymphocytes, significantly reduces thymocytes death triggered by ATPe. Furthermore, we found that P2X7R from BW5147 thymoma cells also harbors this point mutation, accounting for their insensitivity to ATPe-induced cell death. Concentrations of ATPe effective in inducing cell death also increase phosphatidylcholine-hydrolyzing phospholipase D (PC-PLD) activity in BALB/c thymocytes through the stimulation of P2X7R. However, in contrast to ATPe-induced cell death, PC-PLD activation is totally Ca(2+)-dependent. Moreover, the stimulation of PC-PLD by ATPe is not affected by the P451L mutation present in C57BL/6 thymocytes and BW5147 cells, suggesting that cell death and PC-PLD activity are regulated through distinct domains of the P2X7R. Finally, the inhibition of ATPe-induced PC-PLD stimulation does not affect thymocytes death. Altogether, these data suggest that P2X7R-induced thymocytes death is independent of the stimulation of PC-PLD activity.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

Negative selection imparts peptide specificity to the mature T cell repertoire

The T cell alphabeta receptor (TCR) recognizes foreign peptide antigens bound to proteins encoded in the MHC. The MHC portion of this complex contributes much to the footprint of the TCR on the ligand, yet T cells are usually very specific for individual foreign peptides. Here, we show that the development of peptide-specific T cells is not intrinsic to thymocytes that undergo thymic-positive selection but is an outcome of eliminating, through negative selection, thymocytes bearing TCRs with extensive peptide cross-reactivity. Hence, thymic-negative selection imposes peptide specificity on the mature T cell repertoire.

Impaired thymic negative selection causes autoimmune graft-versus-host disease

Animal models with impaired thymic negative selection do not always cause autoimmune diseases despite the development of an autoreactive T-cell repertoire. We investigated the requirements for the development of systemic autoimmune disease by using bone marrow chimeras that lacked expression of major histocompatibility complex (MHC) class II on thymic antigen-presenting cells (APCs), leading to impaired negative selection. We found that impaired negative selection mediated by absence of MHC class II, but not MHC class I, permitted the development of systemic autoimmune disease that is indistinguishable from acute graft-versus-host disease (GVHD). Thymectomy prevented disease, confirming the causal association of the thymus with its development. Adoptive transfer of CD4+ T cells caused GVHD in secondary hosts only when they were irradiated, and cotransfer of peripheral CD4+ and CD8+ T cells from naive mice prevented the disease. These results demonstrate that impaired thymic negative selection can cause lethal autoimmune disease indistinguishable from acute GVHD in the context of a proinflammatory milieu when peripheral regulatory mechanisms are absent.

Negative selection--clearing out the bad apples from the T-cell repertoire

Dead cells are a prominent feature of the thymic landscape as only 5% of developing thymocytes are exported as mature T cells. The remaining thymocytes die by one of two mechanisms; most thymocytes die because they are not positively selected and do not receive a survival signal, whereas a minority of thymocytes undergo T-cell receptor (TCR)-mediated apoptosis, a process known as negative selection. Negative selection is extremely important for establishing a functional immune system, as it provides an efficient mechanism for ridding the T-cell repertoire of self-reactive and potentially autoimmune lymphocytes. This review discusses several cellular and molecular aspects of negative selection.

Thymic function and allogeneic T-cell responses in stem-cell transplantation

The thymus is the primary site of T-cell production early in life, and has now been shown to continue to function in both healthy and immunocompromised individuals late into life. Positive and negative selection occurring in the thymus are two of the most important processes that govern the development and specificity of peripheral T cells, including their restriction to self HLA and their ability to respond in an alloreactive manner. In the chimeric state that follows successful allogeneic stem-cell transplants, the specificity of alloreactive cells may be governed by either host- or recipient-derived cellular elements, as well as maturing lymphoid cells, which are, in turn, derived from donor stem cells or host cells surviving transplant conditioning. The ability to measure recent thymic emigrants via the detection of T-cell receptor excision circles has facilitated studies of thymic function in immunodeficient individuals, including HIV-1 infected subjects and recipients of autologous or allogeneic stem-cell transplant (SCT). These studies have now demonstrated that thymic function is likely to play a beneficial role in immune reconstitution in these settings, but have yet to clearly demonstrate what clinical variables are the most important determinants of thymic persistence. It is also not yet clear how much the degree of thymic function following allogeneic SCT influences the alloreactive T-cell repertoire, although studies in animal models and early clinical studies suggest that GvHD results in thymic injury and dysfunction. Future studies will further clarify how thymic function shapes the repertoire of T cells that mediate alloreactivity, as well as protective pathogen-specific immune responses, following SCT. Finally, these studies will also demonstrate whether endogenous mediators of thymic function could be selectively applied to regulate post-SCT thymic function and alloreactivity.

Quantitative constraints on the scope of negative selection

Maturing T cells with a high affinity for self-antigens presented in the thymus are deleted in the process of negative selection. Although the expression of various "tissue-specific" antigens has been described in the thymus, it is still controversial what fraction of all self-antigens induces tolerance by this mechanism. We demonstrate that the limited duration of the negative selection phase imposes a constraint on the number of self-peptides that can be reliably selected against. The analysis supports the theory that negative selection is confined to the subset of peptides produced by dendritic cells.

T-cell compartments of prediabetic NOD mice

Given the importance of the NOD mouse as a model of type 1 diabetes, there is a surprising lack of published information on the overall composition of the thymic and peripheral T-cell compartments. In this study, we revisited some earlier reports of T-cell abnormalities in this strain and examined a number of additional parameters to provide a global view of T-cells in prediabetic NOD mice. In some cases, we concur with past conclusions, but in other important areas, we find that NOD mice closely resemble nonautoimmune strains. Specifically, and contrary to published reports, the thymocyte subset distribution, the rate and composition of thymic export, and the composition of the peripheral T-cell pool, including the proportion of CD25(+)CD4(+) T-cells, are essentially normal in prediabetic NOD mice. These factors are therefore unlikely to be involved in the loss of tolerance that leads to autoimmunity within this strain.

In vivo maintenance of T-lymphocyte unresponsiveness induced by thymic medullary epithelium requires antigen presentation by radioresistant cells

The T-cell repertoire developing in the thymus is rid of autospecific cells by the process of thymic negative selection. Recognition of major histocompatibility complex (MHC)/self-peptide complexes expressed by thymic antigen-presenting cells (APC) of bone marrow origin leads to induction of apoptotic death of autospecific thymocytes. Induction of tolerance to self-antigens not presented by thymic APC is mediated by medullary thymic epithelial cells (mTEC) which express a very wide range of proteins, e.g. inducible and tissue-specific proteins. The main type of tolerance induced by mTEC is non-deletional and the issue of how it is maintained outside the thymus is therefore of crucial interest. We have previously shown that the non-T-cell receptor (TCR) -transgenic T-cell repertoire developing in conditions in which tolerance to self-MHC/peptide ligands is exclusively induced by mTEC is tolerant to syngeneic targets in vivo but lyses such targets in vitro. Here we report that this non-deletional in vivo self-tolerance is not due to active tolerance assured by known naturally occurring regulatory or immune-modulating T lymphocytes. Importantly, we show that in vivo maintenance of this therefore probably anergic state requires continued interaction of autospecific T cells with self-MHC/peptide ligands expressed by radioresistant cells while APC are incapable of maintaining the tolerant state. Therefore, maintenance of non-deletional T-lymphocyte tolerance to the wide range of self-antigens expressed by mTEC depends on continued interaction with radioresistant cells that very probably express a much more limited repertoire of antigens. Our data may therefore have important consequences for tolerance to tissue-specific and inducible self-antigens.

Cathepsin L regulates CD4+ T cell selection independently of its effect on invariant chain: a role in the generation of positively selecting peptide ligands

CD4+ T cells are positively selected in the thymus on peptides presented in the context of major histocompatibility complex class II molecules expressed on cortical thymic epithelial cells. Molecules regulating this peptide presentation play a role in determining the outcome of positive selection. Cathepsin L mediates invariant chain processing in cortical thymic epithelial cells, and animals of the I-A(b) haplotype deficient in this enzyme exhibit impaired CD4+ T cell selection. To determine whether the selection defect is due solely to the block in invariant chain cleavage we analyzed cathepsin L-deficient mice expressing the I-A(q) haplotype which has little dependence upon invariant chain processing for peptide presentation. Our data indicate the cathepsin L defect in CD4+ T cell selection is haplotype independent, and thus imply it is independent of invariant chain degradation. This was confirmed by analysis of I-A(b) mice deficient in both cathepsin L and invariant chain. We show that the defect in positive selection in the cathepsin L-/- thymus is specific for CD4+ T cells that can be selected in a wild-type and provide evidence that the repertoire of T cells selected differs from that in wild-type mice, suggesting cortical thymic epithelial cells in cathepsin L knockout mice express an altered peptide repertoire. Thus, we propose a novel role for cathepsin L in regulating positive selection by generating the major histocompatibility complex class II bound peptide ligands presented by cortical thymic epithelial cells.