Dissociation of thymic positive and negative selection in transgenic mice expressing major histocompatibility complex class I molecules exclusively on thymic cortical epithelial cells

The thymoproteasome in shaping the CD8+ T-cell repertoire

The thymoproteasome is a type of proteasome expressed specifically in thymic cortical epithelial cells. Thymoproteasome affects antigen processing of major histocompatibility complex (MHC)-I-associated peptides and optimizes positive selection of CD8+ T cells. However, it remains unanswered whether and how thymoproteasome-dependent MHC-I-associated self-peptides contribute to positive selection of cortical thymocytes. This short piece discusses the potential mechanisms of thymoproteasome contribution to positive selection of MHC-I-restricted CD8+ T cells.

Pathophysiology and human cancer risk assessment of pharmaceutical-induced thymoma in carcinogenicity studies

Thymoma, a tumor of thymic lymphocytes or thymic epithelial cells (TECs), is a common spontaneous tumor in Wistar Han rats, especially in females with up to 18% incidence in controls. In addition to sex, there are rat strain differences in background incidence of thymomas such as Sprague Dawley versus Wistar Han rats. Human thymomas are very rare and without clear differences in incidence between males and females. Immunomodulatory and anti-inflammatory pharmaceutical drug classes, including Janus kinase inhibitors, increase the incidence of benign thymoma in two-year rat carcinogenicity studies. Potential non-genotoxic mechanisms that might contribute to the pathogenesis of thymoma development in one sex (female) Wistar Han rats include: (1) hormonal differences, (2) high proliferation rate of TECs, (3) delayed physiologic thymic involution, and/or (4) significant level of immunosuppression at high doses of a pharmaceutical drug. Factors to consider in the human cancer risk assessment of pharmaceutical-induced thymoma are: the genotoxicity of the test article, sex and strain of rats, exposure safety margins, and pathophysiologic differences and similarities of thymoma between rats and humans. Totality of weight of evidence approach and available data suggest thymomas observed in carcinogenicity studies of pharmaceutical drugs are not relevant for human risk at clinically relevant therapeutic doses.

Regulation of thymic T regulatory cell differentiation by TECs in health and disease

The thymus produces self-limiting and self-tolerant T cells through the interaction between thymocytes and thymus epithelial cells (TECs), thereby generating central immune tolerance. The TECs are composed of cortical and medullary thymic epithelial cells, which regulate the positive and negative selection of T cells, respectively. During the process of negative selection, thymocytes with self-reactive ability are deleted or differentiated into regulatory T cells (Tregs). Tregs are a subset of suppressor T cells that are important for maintaining immune homeostasis. The differentiation and development of Tregs depend on the development of TECs and other underlying molecular mechanisms. Tregs regulated by thymic epithelial cells are closely related to human health and are significant in autoimmune diseases, thymoma and pregnancy. In this review, we summarize the current molecular and transcriptional regulatory mechanisms by which TECs affect the development and function of thymic Tregs. We also review the pathophysiological models of thymic epithelial cells regulating thymic Tregs in human diseases and specific physiological conditions.

IL-2 and IL-15 dependent thymic development of Foxp3-expressing regulatory T lymphocytes

Immunosuppressive regulatory T lymphocytes (Treg) expressing the transcription factor Foxp3 play a vital role in the maintenance of tolerance of the immune-system to self and innocuous non-self. Most Treg that are critical for the maintenance of tolerance to self, develop as an independent T-cell lineage from common T cell precursors in the thymus. In this organ, their differentiation requires signals from the T cell receptor for antigen, from co-stimulatory molecules, as well as from cytokine-receptors. Here we focus on the cytokines implicated in thymic development of Treg, with a particular emphasis on the roles of interleukin-2 (IL-2) and IL-15. The more recently appreciated involvement of TGF-β in thymic Treg development is also briefly discussed. Finally, we discuss how cytokine-dependence of Treg development allows for temporal, quantitative, and potentially qualitative modulation of this process.

Generation of Peptides That Promote Positive Selection in the Thymus

To establish an immunocompetent TCR repertoire that is useful yet harmless to the body, a de novo thymocyte repertoire generated through the rearrangement of genes that encode TCR is shaped in the thymus through positive and negative selection. The affinity between TCRs and self-peptides associated with MHC molecules determines the fate of developing thymocytes. Low-affinity TCR engagement with self-peptide-MHC complexes mediates positive selection, a process that primarily occurs in the thymic cortex. Massive efforts exerted by many laboratories have led to the characterization of peptides that can induce positive selection. Moreover, it is now evident that protein degradation machineries unique to cortical thymic epithelial cells play a crucial role in the production of MHC-associated self-peptides for inducing positive selection. This review summarizes current knowledge on positive selection-inducing self-peptides and Ag processing machineries in cortical thymic epithelial cells. Recent studies on the role of positive selection in the functional tuning of T cells are also discussed.

Antigen processing and presentation in the thymus: implications for T cell repertoire selection

The processing and presentation of major histocompatibility complex (MHC)-associated antigens depend on the intracellular digestion of self- and nonself-proteins, the loading of digested peptides onto MHC molecules, and the traffic of peptide-MHC complexes to plasma membrane surface for display to interacting T cells. Recent studies have revealed unique machineries for antigen processing and presentation in thymic antigen-presenting cells that display self-antigens to developing thymocytes for the formation of functionally competent yet self-tolerant T cell repertoire. Here, we briefly summarize those machineries, focusing on the biology of cortical and medullary thymic epithelial cells.

TCR affinity for thymoproteasome-dependent positively selecting peptides conditions antigen responsiveness in CD8(+) T cells

In the thymus, low-affinity T cell antigen receptor (TCR) engagement facilitates positive selection of a useful T cell repertoire. Here we report that TCR responsiveness of mature CD8(+) T cells is fine tuned by their affinity for positively selecting peptides in the thymus and that optimal TCR responsiveness requires positive selection on major histocompatibility complex class I-associated peptides produced by the thymoproteasome, which is specifically expressed in the thymic cortical epithelium. Thymoproteasome-independent positive selection of monoclonal CD8(+) T cells results in aberrant TCR responsiveness, homeostatic maintenance and immune responses to infection. These results demonstrate a novel aspect of positive selection, in which TCR affinity for positively selecting peptides produced by thymic epithelium determines the subsequent antigen responsiveness of mature CD8(+) T cells in the periphery.

The Contribution of Chemokines and Migration to the Induction of Central Tolerance in the Thymus

As T cells develop, they migrate throughout the thymus where they undergo essential bi-directional signaling with stromal cells in distinct thymic microenvironments. Immature thymocyte progenitors are located in the thymic cortex. Following T cell receptor expression and positive selection, thymocytes undergo a dramatic transition: they become rapidly motile and relocate to the thymic medulla. Antigen-presenting cells (APCs) within the cortex and medulla display peptides derived from a wide array of self-proteins, which promote thymocyte self-tolerance. If a thymocyte is auto-reactive against such antigens, it undergoes either negative selection, via apoptosis, or differentiation into the regulatory T cell lineage. This induction of central tolerance is critical for prevention of autoimmunity. Chemokines and adhesion molecules play an essential role in tolerance induction, as they promote migration of developing thymocytes through the different thymic microenvironments and enhance interactions with APCs displaying self-antigens. Herein, we review the contribution of chemokines and other regulators of thymocyte localization and motility to T cell development, with a focus on their contribution to the induction of central tolerance.

Long-term prevention of chronic allograft rejection by regulatory T-cell immunotherapy involves host Foxp3-expressing T cells

Administration of donor-specific regulatory T cells prevents chronic rejection of BM and skin allografts in the mouse. Injected regulatory T cells induce the emergence of host regulatory T cells with similar specificity thus ensuring persistence of tolerance.

Probing gene function in thymic epithelial cells

Thymic epithelial cells (TECs) provide a highly specialized microenvironment for the generation of a functional and self-tolerant T cell repertoire. Much of our current view of TEC biology is derived from gain- or loss-of-function approaches, which have significantly contributed to our understanding of gene function in TEC development and T cell repertoire selection. Here, we will review transgenic and viral strategies that have been used to manipulate gene expression in TECs, highlight some of the shortcomings of particular currently available tools and provide a brief outline of our own attempts to more rapidly and/or more specifically assess gene function in TECs.

Fas (CD95/APO-1) limits the expansion of T lymphocytes in an environment of limited T-cell antigen receptor/MHC contacts

Fas-deficient mice (Fas(lpr/lpr)) and humans have profoundly dysregulated T lymphocyte homeostasis, which manifests as an accumulation of CD4(+) and CD8(+) T cells as well as an unusual population of CD4(-)CD8(-)TCRαβ(+) T cells. To date, no unifying model has explained both the increased T-cell numbers and the origin of the CD4(-)CD8(-)TCRαβ(+) T cells. As Fas(lpr/lpr) mice raised in a germ-free environment still manifest lymphadenopathy, we considered that this process is primarily driven by recurrent low-avidity TCR signaling in response to self-peptide/MHC as occurs during homeostatic proliferation. In these studies, we developed two independent systems to decrease the number of self-peptide/MHC contacts. First, expression of MHC class I was reduced in OT-I TCR transgenic mice. Although OT-I Fas(lpr/lpr) mice did not develop lymphadenopathy characteristic of Fas(lpr/lpr) mice, in the absence of MHC class I, OT-I Fas(lpr/lpr) T cells accumulated as both CD8(+) and CD4(-)CD8(-) T cells. In the second system, re-expression of β(2)m limited to thymic cortical epithelial cells of Fas(lpr/lpr) β(2)m-deficient mice yielded a model in which polyclonal CD8(+) thymocytes entered a peripheral environment devoid of MHC class I. These mice accumulated significantly greater numbers of CD4(-)CD8(-)TCRαβ(+) T cells than conventional Fas(lpr/lpr) mice. Thus, Fas shapes the peripheral T-cell repertoire by regulating the survival of a subset of T cells proliferating in response to limited self-peptide/MHC contacts.

The regulatory role of dendritic cells in the induction and maintenance of T-cell tolerance

The induction and maintenance of T-cell tolerance to tissue antigens is essential to prevent autoimmunity. Combinations of central and peripheral mechanisms act in parallel to inactivated, eliminated or control autoreactive T cells. Both centrally and peripherally, a key requirement for self-tolerance is the presentation of self-antigens in a correct context. There is now evidence to suggest that dendritic cells (DCs) play a fundamental role in the development of central and peripheral tolerance. In this review, we summarize recent progress toward the definition of the multiple roles of DCs in these processes. We will also discuss the association between defects in the DC compartment and the development of autoimmune responses, with particular reference to DC deregulation in the context of type I diabetes.

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.

Spontaneous tumour development in human papillomavirus type 8 E6 transgenic mice and rapid induction by UV-light exposure and wounding

Cutaneous human papillomavirus type 8 (HPV8) is carcinogenic in patients with epidermodysplasia verruciformis. Transgenic mice with the complete early region (CER) of HPV8 spontaneously developed papillomas, dysplasia and squamous cell carcinomas of the skin. To characterize the role of individual early genes in carcinogenesis, the E6 and E6/E7 genes were expressed separately in transgenic mice. Nearly all HPV8-E6-positive mice spontaneously developed multifocal tumours, characterized by papillomatosis, hyperkeratosis and varying degrees of epidermal dysplasia. In 6 % of the cases, the tumours became malignant, comparable with HPV8-CER mice. Thus, in the murine epidermis, E6 is the major oncogene necessary and sufficient to induce spontaneous tumour development up to the level of squamous cell carcinoma. To evaluate the synergistic effects of UV light and wound healing, the skin of HPV8 mice was irradiated with UVA/UVB light or wounded with punch biopsies. These treatments induced papillomatosis in HPV8-CER and -E6 mice within 3 weeks. Irradiation with UVA alone did not induce papillomatosis and UVB alone had a weaker effect than UVA/UVB, indicating a synergistic role of UVA in UVB-induced papillomatosis. An HPV8 infection persisting over decades in interaction with sun burns and wound healing processes may be a relevant cause of skin cancer in humans.

TSCOT+ thymic epithelial cell-mediated sensitive CD4 tolerance by direct presentation

Although much effort has been directed at dissecting the mechanisms of central tolerance, the role of thymic stromal cells remains elusive. In order to further characterize this event, we developed a mouse model restricting LacZ to thymic stromal cotransporter (TSCOT)-expressing thymic stromal cells (TDLacZ). The thymus of this mouse contains approximately 4,300 TSCOT+ cells, each expressing several thousand molecules of the LacZ antigen. TSCOT+ cells express the cortical marker CDR1, CD40, CD80, CD54, and major histocompatibility complex class II (MHCII). When examining endogenous responses directed against LacZ, we observed significant tolerance. This was evidenced in a diverse T cell repertoire as measured by both a CD4 T cell proliferation assay and an antigen-specific antibody isotype analysis. This tolerance process was at least partially independent of Autoimmune Regulatory Element gene expression. When TDLacZ mice were crossed to a novel CD4 T cell receptor (TCR) transgenic reactive against LacZ (BgII), there was a complete deletion of double-positive thymocytes. Fetal thymic reaggregate culture of CD45- and UEA-depleted thymic stromal cells from TDLacZ and sorted TCR-bearing thymocytes excluded the possibility of cross presentation by thymic dendritic cells and medullary epithelial cells for the deletion. Overall, these results demonstrate that the introduction of a neoantigen into TSCOT-expressing cells can efficiently establish complete tolerance and suggest a possible application for the deletion of antigen-specific T cells by antigen introduction into TSCOT+ cells.

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

Clonal deletion of thymocytes can occur in the cortex with no involvement of the medulla

The thymic medulla is generally held to be a specialized environment for negative selection. However, many self-reactive thymocytes first encounter ubiquitous self-antigens in the cortex. Cortical epithelial cells are vital for positive selection, but whether such cells can also promote negative selection is controversial. We used the HY^cd4 model, where T cell receptor for antigen (TCR) expression is appropriately timed and a ubiquitous self-antigen drives clonal deletion in male mice. We demonstrated unambiguously that this deletion event occurs in the thymic cortex. However, the kinetics in vivo indicated that apoptosis was activated asynchronously relative to TCR activation. We found that radioresistant antigen-presenting cells and, specifically, cortical epithelial cells do not efficiently induce apoptosis, although they do cause TCR activation. Rather, thymocytes undergoing clonal deletion were preferentially associated with rare CD11c⁺ cortical dendritic cells, and elimination of such cells impaired deletion.

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.

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.

Thymocyte-Thymocyte Interaction for Efficient Positive Selection and Maturation of CD4 T Cells

Despite numerous reports on MHC class II expression by T cells from a wide spectrum of mammalian species including humans, the biological relevance of this phenomenon has never been tested with appropriately designed animal models. To address this issue, we developed mouse models in which immature thymocytes are the only positively selecting antigen-presenting cells in the thymus. In these mice, CD4+ T cells were generated with the appropriate maturation phenotype and showed a diverse repertoire of TCR Vbetas. The CD4+ T cells were functionally competent, mediating effective allogeneic responses that involved polyclonal TCR Vbetas. These results suggest that the thymocyte-thymocyte (T-T) interaction operates as an independent pathway for CD4+ T cell selection in the thymi of species with MHC II-positive thymocytes. This T-T interaction appears to be the basis for the generation of donor MHC-restricted CD4+ T cells in xenogeneic hosts.

MHC class I-positive dendritic cells (DC) control CD8 T cell homeostasis in vivo: T cell lymphopenia as a prerequisite for DC-mediated homeostatic proliferation of naive CD8 T cells

The sizes of peripheral T cell pools are regulated by competition for environmental signals within a given ecological T cell niche. Cytokines and MHC molecules have been identified as resources for which naive T cells compete to proliferate homeostatically in lymphopenic hosts to fill up their respective compartments. However, it still remains unclear to what extent CD4 and CD8 T cells intercompete for these resources and which role dendritic cells (DC) play in this scenario. Using transgenic mice in which only DC express MHC class I, we demonstrate that this type of APC is sufficient to trigger complete homeostatic proliferation of CD8 T cells in vivo. However, normal numbers of endogenous naive CD4 T cells, but not CD25(+)CD4(+) T regulatory cells, efficiently suppress this expansion in vivo. These findings identify DC as a major resource and a possible target for homeostatic competition between naive CD4 and CD8 T cells.

CD4+ T cell-mediated antigen-specific immunotherapy in a mouse model of cervical cancer

A major agenda for tumor immunology is the generation of specific immune responses leading to the destruction of incipient and frank neoplasia. In this report, we show that a novel HPV16 E7 fusion protein can produce objective therapeutic responses against incipient cervical cancer in genetically engineered mice that express in the cervix the HPV16 early region genes implicated as causative agents in human cervical cancer. Although nonresponsive toward the HPV16 E7 oncoprotein in the CD8+ T-cell compartment by virtue of MHC haplotype, the mice were capable of mounting an induced CD4+ T-cell response against E7, and in addition developed spontaneous anti-E7 antibodies. HPV16/CD4-/- mice showed increased tumor burden indicative of CD4-mediated immune surveillance. Seeking to enhance the CD4 response, we immunized mice bearing incipient cervical cancer with a recombinant protein fusing E7 with a mycobacterial heat shock protein. The incidences of cervical carcinoma and of high-grade dysplasia (CIN 3) were consequently reduced by comparison to control mice. Thus, an HPV16 E7 immunogen holds promise for noninvasive treatment and prevention of human cervical cancer.

Thymic cortical epithelium induces self tolerance

Because of its role in positive selection, the ability of cortical epithelium to induce tolerance is controversial. On the one hand, experiments with transplanted thymuses showed that the recipients were functionally tolerant of all the antigens expressed by the cells of those thymuses, including cells of the cortical epithelium. On the other, the keratin 14 (K14) transgenic mouse strain, which expresses MHC class II on cortical epithelium under the control of the K14 promoter, does not seem to be tolerant of the transgenic MHC molecule. Here we tested whether the lack of tolerance in the K14 mouse might be more apparent than real. We found that K14 mice are indeed completely tolerant of K14 cortical thymic epithelium, whereas they remain reactive to tissues that express the same MHC allele under normal genetic control. These results establish the ability of cortical epithelium to induce central tolerance, and impinge on several of the models concerning positive selection of newly developing T cells.

Dendritic Cell Biology

Dendritic cells (DCs) are a special type of leukocytes able to alert the immune system to the presence of infections. They play a central role in the initiation of both innate and adaptive immune responses. This particular DC feature is regulated by the activation of specific receptors at the cell surface called Toll-like receptors (TLRs) that bind a number of microbial products collectively referred to as microbial-associated molecular patterns (MAMP). TLRs initiate a cascade of events, which together define the process of DC maturation. This phenomenon allows DCs to progressively acquire varying specific functions. DC maturation depends on the nature of the perturbation and permits unique and efficient immune responses for each pathogen. In this review the discussion is focused on DCs in the context of interactions with pathogens and DC-specific functions are highlighted.

Central tolerance to tissue-specific antigens mediated by direct and indirect antigen presentation

Intrathymic expression of tissue-specific antigens (TSAs) by medullary thymic epithelial cells (Mtecs) leads to deletion of autoreactive T cells. However, because Mtecs are known to be poor antigen-presenting cells (APCs) for tolerance to ubiquitous antigens, and very few Mtecs express a given TSA, it was unclear if central tolerance to TSA was induced directly by Mtec antigen presentation or indirectly by thymic bone marrow (BM)-derived cells via cross-presentation. We show that professional BM-derived APCs acquire TSAs from Mtecs and delete autoreactive CD8 and CD4 T cells. Although direct antigen presentation by Mtecs did not delete the CD4 T cell population tested in this study, Mtec presentation efficiently deleted both monoclonal and polyclonal populations of CD8 T cells. For developing CD8 T cells, deletion by BM-derived APC and by Mtec presentation occurred abruptly at the transitional, CD4^high CD8^low TCR^intermediate stage, presumably as the cells transit from the cortex to the medulla. These studies reveal a cooperative relationship between Mtecs and BM-derived cells in thymic elimination of autoreactive T cells. Although Mtecs synthesize TSAs and delete a subset of autoreactive T cells, BM-derived cells extend the range of clonal deletion by cross-presenting antigen captured from Mtecs.

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.

Development of CD4+Macrophages from Intrathymic T Cell Progenitors Is Induced by Thymic Epithelial Cells

It was recently demonstrated that there are CD4(+) macrophages, which exhibit strong phagocytic activity, in the thymus. They are suggested to play an important role for the elimination of apoptotic thymocytes. However, the origin and nature of CD4(+) macrophages in the thymus remain unexplored. In this study, we describe that the most immature intrathymic progenitors (CD25(-)/CD44(+)/FcR(+)) give rise to CD4(+) macrophages by oncostatin M-responsive thymic epithelial cells (ORTEC) in an IL-7-dependent manner. Neither conditioned medium of ORTEC nor a mixture of cytokines induced CD4(+) macrophages, and oncostatin M receptor was not expressed in thymocytes, suggesting that the development of CD4(+) macrophages from the immature thymocytes requires a direct interaction with ORTEC. These results collectively suggest that the development of CD4(+) macrophages from the intrathymic T cell progenitors is induced by thymic epithelial cells.

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.

The thymic stromal cell line MTSC4 induced thymocyte apoptosis in a non-MHC-restricted manner

Mouse thymic stromal cell line 4 (MTSC4) is one of the stromal cell lines established in our laboratory. While losing the characteristics of epithelial cells, they express some surface markers shared with thymic dendritic cells (TDCs). To further study the biological functions of these cells, we compared the capability of MTSC4 with TDCs in the induction of thymocyte apoptosis, using thymic reaggregation culture system. Apoptosis of thymocytes induced by MTSC4 and TDCs was measured by Annexin V and PI staining and analyzed by flow cytometry. We found that MTSC4 selectively augmented the apoptosis of CD4+8+ (DP) thymocytes. This effect was Fas/FasL independent and could not be blocked by antibodies to MHC class I and class II molecules. In addition, MTSC4 enhanced the apoptosis of DP thymocytes from different strains of mice, which implies that MTSC4-induced thymocyte apoptosis is not mediated by the TCR recognition of self peptide/MHC molecules. In contrast to MTSC4, thymocyte apoptosis induced by TDCs was MHC-restricted. Thus, MHC-independent fashion of stromal-DP thymocyte interaction may be one of the ways to induce thymocyte apoptosis in thymus. Our study has also shown that the interaction of MTSC4 stromal cells and thymocytes is required for the induction of thymocyte apoptosis.

Central tolerance to self-antigen expressed by cortical epithelial cells

The exposure of developing thymocytes to high-affinity self-Ag results in T cell tolerance. A predominant mechanism for this is clonal deletion; though receptor editing, anergy induction, and positive selection of regulatory T cells have also been described. It is unclear what signals are involved in determining different tolerance mechanisms. In particular, OT-I mice displayed receptor editing when the high-affinity self-Ag was expressed in cortical epithelial cells (cEC) using the human keratin 14 promoter. To test the hypothesis that receptor editing is a consequence of a unique instruction given by cEC presenting self-Ag, we created mice expressing the 2C and HY ligands under control of the keratin 14 promoter. Alternatively, we studied the fate of developing T cells in OT-I mice where Ag was presented by all thymic APC. Surprisingly, we found that the tolerance mechanism was not influenced by the APC subset involved in presentation. Clonal deletion was observed in 2C and HY models even when Ag was presented only by cEC; and receptor editing was observed in OT-I mice even when Ag was presented by all thymic APC. These results suggest that different TCRs show intrinsic differences in thymic tolerance mechanism.

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.

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.

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.

Increased thymic output during acute measles virus infection

Measles virus infects thymic epithelia, induces a transient lymphopenia, and impairs cell-mediated immunity, but thymic function during measles has not been well characterized. Thirty Zambian children hospitalized with measles were studied at entry, hospital discharge, and at 1-month follow-up and compared to 17 healthy children. During hospitalization, percentages of naïve (CD62L+, CD45RA+) CD4+ and CD8+ T lymphocytes decreased (P = 0.01 for both), and activated (HLA-DR+, CD25+, or CD69+) CD4+ and CD8+ T lymphocytes increased (P = 0.02 and 0.03, respectively). T-cell receptor rearrangement excision circles (TRECs) in measles patients were increased in CD8+ T cells at entry compared to levels at hospital discharge (P = 0.02) and follow-up (P = 0.04). In CD4+ T cells, the increase in TRECS occurred later but was more sustained. At discharge, TRECs in CD4+ T cells (P = 0.05) and circulating levels of interleukin-7 (P = 0.007) were increased compared to control values and remained elevated for 1 month, similar to observations in two measles virus-infected rhesus monkeys. These findings suggest that a decrease in thymic output is not the cause of the lymphopenia and depressed cellular immunity associated with measles.

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.

Thymic and extrathymic T cell development pathways follow different rules

Separation between primary and secondary lymphoid organs is a universal feature in jawed vertebrates. Strikingly, oncostatin M (OM)-transgenic mice present massive extrathymic T cell development, localized exclusively in the lymph nodes (LN). According to the prevailing paradigm, the thymus is the main source of T lymphocytes in gnathostomes mainly because thymic epithelial cells have a unique ability to support early steps in T cell development. It is therefore remarkable that productive T cell development occurs in the OM(+) LN, despite the absence of epithelial cells. The present study shows that in the OM(+) LN: 1) MHC class I expression strictly on hemopoietic cells is sufficient to support the development of a diversified repertoire of CD8 T cells; 2) the efficiency of positive selection of specific TCR-transgenic T cells is not the same as in the thymus; 3) negative selection is very effective, despite the lack of an organized thymic-like medulla. Furthermore, our data suggest that extrathymic T lymphocytes developing in the OM(+) LN undergo extensive postselection expansion because they live in the microenvironment in which they were positively selected. This work illustrates how the division of labor between primary and secondary lymphoid organs influences the repertoire and homeostasis of T lymphocytes.

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

T cell tolerance to self Ags is in part established in the thymus by induction of apoptosis or anergy of potentially autoreactive thymocytes. Some autospecific T cells nevertheless migrate to peripheral lymphoid organs but are kept under control by the recently identified CD4(+)CD25(+) regulatory T cell subset. Because these cells inhibit autoimmunity more efficiently than useful non-self Ag-specific immune responses, they are probably autospecific, posing important questions as to how they develop in the thymus. In this study we show that significantly more peripheral CD4(+)CD25(+) regulatory T cells recognize self than non-self Ags. However, we also show for a large panel of endogenous superantigens as well as for self peptide/MHC complexes that autospecific CD4(+)CD25(+) thymocyte precursors are normally deleted during ontogeny. Combined, our data firmly establish that the repertoire of regulatory T cells is specifically enriched in autospecific cells despite the fact that their precursors are normally susceptible to thymic deletion.

A low affinity TCR ligand restores positive selection of CD8+ T cells in vivo

The T cell repertoire is shaped by the processes of positive and negative selection. During development, the TCR binds self peptide-MHC complexes in the thymus, and the kinetics of this interaction are thought to determine the thymocyte's fate. For development of CD8(+) T cells, the data supporting such a model have been obtained using fetal thymic organ culture. To confirm the fidelity of this model in vivo, we studied development of OT-I TCR-transgenic mice that expressed different individual K(b) binding peptides in thymic epithelial cells under the control of the human keratin 14 promoter. We used a system that allowed TAP-independent expression of the peptide-MHC complex, such that the ability of given peptides to restore positive selection in TAP(o) mice could be assessed. We found that transgenic expression of a TCR antagonist peptide (E1) in vivo efficiently restored positive selection of OT-I T cells in TAP(o) mice. An unrelated transgenic peptide (SIY) did not restore selection of OT-I T cells, nor did the E1-transgenic peptide restore selection of an unrelated receptor (2C), showing that positive selection is peptide specific in vivo, as observed in organ cultures. Neither E1 nor SIY transgenes increased the polyclonal CD8 T cell repertoire size in non-TCR-transgenic animals, arguing that single class I binding peptides do not detectably affect the size of the CD8 T cell repertoire when expressed at low levels. We also observed that OT-I T cells selected in TAP(o)-E1 mice were functional in their response to Ag; however, there was a lag in this response, suggesting that the affinity of the TCR interaction with MHC-self peptide can result in fine-tuning of the T cell response.

Stat3 in thymic epithelial cells is essential for postnatal maintenance of thymic architecture and thymocyte survival

This study describes abnormalities of the thymus in mice in which the Stat3 gene has been specifically disrupted behind the keratin 5 promoter. In these mice, virtually all of the thymic epithelial cells (TEC) were deficient for Stat3 activation. Adult mutant mice developed severe thymic hypoplasia, which included alterations in the cortical TEC architecture that coincided with the loss of thymocytes. Even during the asymptomatic period of preadolescence, these mice exhibited a higher susceptibility of the thymus to suboptimal doses of dexamethasone or gamma-irradiation, while their thymocytes per se were no more sensitive than controls. These results indicate that Stat3 in TEC plays an essential role in maintaining thymic architecture and thymocyte survival.