One for all and all for one: thymic epithelial stem cells and regeneration

Chemical Strategies to Enhance the Therapeutic Efficacy of Toll-like Receptor Agonist Based Cancer Immunotherapy

Recent developments in the fields of biomedical chemistry and immune bioengineering have enabled innovative therapeutic approaches that can enhance the efficacy, accuracy, and safety of cancer immunotherapy. Among the numerous strategies utilized in cancer immunotherapy, Toll-like receptor (TLR) agonist-based approaches have been studied for a long time since they trigger the innate immune system and generate antigen-specific T cell responses to fight against tumors. In addition to these immunostimulatory functions, TLR agonists also contribute to the reprogramming of immune suppressive tumor microenvironments. Although TLR agonists are now being intensively studied in clinical trials due to their substantial immunomodulatory properties, they still show a low therapeutic index. Nonspecific and random stimulation of various immune cells produces excess levels of proinflammatory cytokines, resulting in cytokine storms and chronic diseases. Therefore, the development of chemical strategies to enhance the therapeutic efficacy as well as the safety of TLR agonist-based immunotherapy is essential and in high demand.In this Account, we summarize and discuss recent developments in biomedical chemistry and bioengineering techniques for the immunomodulation of TLR agonists that have addressed the limitations in current cancer immunotherapy. Immunomodulation of TLR agonists can be classified into two different approaches: (1) molecular modulation via chemical structure modification and (2) macroscopic modulation via an engineered drug delivery system. In molecular modulation, based on prodrug and antedrug principles, activity is modulated (active or inactive) through immolative chemical linkers that can respond to extrinsic or intrinsic biological stimulation and the plasmatic environment, respectively. To increase the effectiveness of TLR agonists as immunostimulatory agents, researchers have conjugated TLR agonists with other immunotherapeutic moieties (antigen, antibody, other TLR agonist, etc.). For macroscopic modulation, bioengineering of delivery carriers differing in size or with albumin hitchhiking moieties has been utilized to increase the efficiency of the targeting of these carriers to secondary lymphoid organs (lymph nodes (LNs) and spleen). The conjugation of specific targeting ligands and incorporation of stimulus-triggering moieties can promote the delivery of TLR agonists into specific cells or intracellular compartments. Implantable porous scaffolds for specific immune cell recruitment and in situ depot-forming gel systems for controlled release of immunomodulatory drugs can increase the therapeutic efficacy of TLR agonists while reducing systemic toxicity. Taken together, these findings show that well-designed and precisely controlled chemical strategies for the immunomodulation of TLR agonists at both the molecular and macroscopic levels are expected to play key roles in improving the therapeutic efficacy of cancer immunotherapy while minimizing immune-related toxicity.

Molecular regulatory networks of thymic epithelial cell differentiation

Functional mature T cells are generated in the thymus. Thymic epithelial cells (TECs) provide the essential microenvironment for T cell development and maturation. According to their function and localization, TECs are roughly divided into cortical TECs (cTECs) and medullary TECs (mTECs), which are responsible for positive and negative selection, respectively. This review summarizes the current understanding of TEC biology, the identification of fetal and adult bipotent TEC progenitors, and the signaling pathways that control the development and maturation of TECs. The understanding of the ontogeny, differentiation, maturation and function of cTECs lags behind that of mTECs. Better understanding TEC biology will provide clues about TEC development and the applications of thymus engineering.

Effect of cryopreservation on viability and growth efficiency of stromal-epithelial cells derived from neonatal human thymus

The thymus is the major site of T lymphocyte generation and so is critical for a functional adaptive immune system. Since, thymectomy is a component of neonatal surgery for congenital heart diseases, it provides great potential for collection and storage of thymic tissue for autologous transplantation. However, specific investigation into the optimum parameters for thymic tissue cryopreservation have not been conducted. In this research, we evaluated the effect of different cryoprotective media compositions, which included penetrating (Me₂SO, glycerol) and non-penetrating (dextran-40, sucrose, hydroxyethyl starch) components, on the viability and functionality of frozen-thawed human thymic samples to select an optimal cryoprotective medium suitable for long-term storage of thymic tissue and a stromal-epithelial enriched population. Our primary focus was on receiving, low-temperature storage, culturing and evaluation of thymic tissue samples from newborns and infants with congenital heart diseases, who had undergone thymectomy as a part of standard surgical procedure. Thus, this work builds the platform for autologous clinical intervention into the thymus-deficient patients with congenital heart diseases. From our data, we conclude that although there were no significant differences in efficiency of tested cryoprotective media compositions, the combination of Me₂SO and dextran-40 compounds was the most suitable for long-term storage both thymic cell suspensions and thymic fragments based on the viability of CD326⁺ epithelial cells and stromal-epithelial cell monolayer formation.

Medullary thymic epithelial stem cells: role in thymic epithelial cell maintenance and thymic involution

The thymus consists of two distinct anatomical regions, the cortex and the medulla; medullary thymic epithelial cells (mTECs) play a crucial role in establishing central T-cell tolerance for self-antigens. Although the understanding of mTEC development in thymic organogenesis as well as the regulation of their differentiation and maturation has improved, the mechanisms of postnatal maintenance remain poorly understood. This issue has a central importance in immune homeostasis and physiological thymic involution as well as autoimmune disorders in various clinicopathological settings. Recently, several reports have demonstrated the existence of TEC stem or progenitor cells in the postnatal thymus, which are either bipotent or unipotent. We identified stem cells specified for mTEC-lineage that are generated in the thymic ontogeny and may sustain mTEC regeneration and lifelong central T-cell self-tolerance. This finding suggested that the thymic medulla is maintained autonomously by its own stem cells. Although several issues, including the relationship with other putative TEC stem/progenitors, remain unclear, further examination of mTEC stem cells (mTECSCs) and their regulatory mechanisms may contribute to the understanding of postnatal immune homeostasis. Possible relationships between decline of mTECSC activity and early thymic involution as well as various autoimmune disorders are discussed.

Adult thymic epithelial cell (TEC) progenitors and TEC stem cells: Models and mechanisms for TEC development and maintenance

The thymus is the primary lymphoid organ for generating self-restricted and self-tolerant functional T cells. Its two distinct anatomical regions, the cortex and the medulla, are involved in positive and negative selection, respectively. Thymic epithelial cells (TECs) constitute the framework of this tissue and function as major stromal components. Extensive studies for more than a decade have revealed how TECs are generated during organogenesis; progenitors specific for medullary TECs (mTECs) and cortical TECs (cTECs) as well as bipotent progenitors for both lineages have been identified, and the signaling pathways required for the development and maturation of mTECs have been elucidated. However, little is known about the initial commitment of mTECs and cTECs during ontogeny, and how regeneration of both lineages is sustained in the postnatal/adult thymus. Recently, stem cell activities in TECs have been demonstrated, and TEC progenitors have been identified in the postnatal thymus. In this review, recent advances in studying the development and maintenance of TECs are summarized, and the possible mechanisms of thymic regeneration and involution are discussed.

Expression Analyses Revealed Thymic Stromal Co-Transporter/Slc46A2 Is in Stem Cell Populations and Is a Putative Tumor Suppressor

By combining conventional single cell analysis with flow cytometry and public database searches with bioinformatics tools, we extended the expression profiling of thymic stromal cotransporter (TSCOT), Slc46A2/Ly110, that was shown to be expressed in bipotent precursor and cortical thymic epithelial cells. Genome scale analysis verified TSCOT expression in thymic tissue- and cell type- specific fashion and is also expressed in some other epithelial tissues including skin and lung. Coexpression profiling with genes, Foxn1 and Hoxa3, revealed the role of TSCOT during the organogenesis. TSCOT expression was detected in all thymic epithelial cells (TECs), but not in the CD31(+) endothelial cell lineage in fetal thymus. In addition, ABC transporter-dependent side population and Sca-1(+) fetal TEC populations both contain TSCOT-expressing cells, indicating TEC stem cells express TSCOT. TSCOT expression was identified as early as in differentiating embryonic stem cells. TSCOT expression is not under the control of Foxn1 since TSCOT is present in the thymic rudiment of nude mice. By searching variations in the expression levels, TSCOT is positively associated with Grhl3 and Irf6. Cytokines such as IL1b, IL22 and IL24 are the potential regulators of the TSCOT expression. Surprisingly, we found TSCOT expression in the lung is diminished in lung cancers, suggesting TSCOT may be involved in the suppression of lung tumor development. Based on these results, a model for TEC differentiation from the stem cells was proposed in context of multiple epithelial organ formation.

Acute ablation of DP thymocytes induces up-regulation of IL-22 and Foxn1 in TECs

Thymic epithelial cells (TECs) provide the basic architecture for the development of thymocytes. TEC is regenerative after impairment in thymus of young mice. However, how this regeneration program is governed remains unclear. Transcription factor Foxn1 is a central mediator of the differentiation and function of TEC. We examined the relation between thymic injury and Foxn1 in TEC. Total body irradiation (TBI) treatments induced up-regulation of Foxn1 in TEC, which was abolished when thymic function recovered. Specific depletion of double positive (DP) thymocytes triggered the up-regulation of Foxn1. On the other hand, extracellular IL-22 is a potential regulator of homeostasis of TEC. We demonstrated that TBI treatments also induced the up-regulation of intrathymic IL-22. Expression pattern of Foxn1 shares similar characteristics with IL-22. Furthermore, Foxn1 related genes that regulate the function of TEC were also up-regulated. Thus, our data reveal that TBI treatment triggers regeneration program of TEC.

From murine to human nude/SCID: the thymus, T-cell development and the missing link

Primary immunodeficiencies (PIDs) are disorders of the immune system, which lead to increased susceptibility to infections. T-cell defects, which may affect T-cell development/function, are approximately 11% of reported PIDs. The pathogenic mechanisms are related to molecular alterations not only of genes selectively expressed in hematopoietic cells but also of the stromal component of the thymus that represents the primary lymphoid organ for T-cell differentiation. With this regard, the prototype of athymic disorders due to abnormal stroma is the Nude/SCID syndrome, first described in mice in 1966. In man, the DiGeorge Syndrome (DGS) has long been considered the human prototype of a severe T-cell differentiation defect. More recently, the human equivalent of the murine Nude/SCID has been described, contributing to unravel important issues of the T-cell ontogeny in humans. Both mice and human diseases are due to alterations of the FOXN1, a developmentally regulated transcription factor selectively expressed in skin and thymic epithelia.

Thymic microenvironment reconstitution after postnatal human thymus transplantation

A functional thymus develops after cultured thymus tissue is transplanted into subjects with complete DiGeorge anomaly. To gain insight into how the process occurs, 7 post-transplantation thymus biopsy tissues were evaluated. In 5 of 7 biopsies, the thymus appeared to be predominantly cortex with thymocytes expressing cortical markers. Unexpectedly, the epithelium expressed both cortical [cortical dendritic reticulum antigen 2 (CDR2)] and medullary [cytokeratin (CK) 14] markers. Early medullary development was suggested by epithelial cell adhesion molecule (EpCAM) reactivity in small areas of biopsies. Two other biopsies had distinct mature cortex and medulla with normal restriction of CK14 to the medulla and subcapsular cortex, and of CDR2 to cortex. These data are consistent with a model in which thymic epithelium contains CK14+ "progenitor epithelial cells". After transplantation these cells proliferate as CK14+CDR2+ thymic epithelial cells that are associated with cortical thymocytes. Later these cells differentiate into distinct cortical and medullary epithelia.

Expression of podoplanin in thymoma: its correlation with tumor invasion, nodal metastasis, and poor clinical outcome

Recent studies have shown that podoplanin overexpression is associated with lymph node metastasis and poor clinical outcome in several malignant tumors. To investigate the role of podoplanin in thymoma, we examined 111 thymomas by immunohistochemistry using monoclonal antibody D2-40, which recognizes podoplanin. The tumors consisted of 8 type A, 40 type AB, 15 type B1, 23 type B2, 15 type B3, and 10 combined thymomas according to the World Health Organization histological classification system and of 41 stage I, 28 stage II, 16 stage III, 20 stage IVa, and 6 stage IVb thymomas according to the Masaoka staging system. We have found podoplanin expression in 0 (0%) type A, 4 (10%) type AB, 4 (27%) type B1, 16 (70%) type B2, 10 (67%) type B3, and 7 (70%) combined thymomas and in 5 (12%) cases of stage I, 7 (25%) of stage II, 11 (69%) of stage III, 12 (60%) of stage IVa, and all (100%) of stage IVb thymomas. Podoplanin was significantly expressed in B2/B3/combined thymomas and advanced stage thymomas (P < .0001). On survival analysis, podoplanin expression was significantly associated with an increased risk of death for the whole group of thymomas (P = .0002), although it was not identified as an independent prognostic factor in multivariate analysis. The significant survival curve differences of podoplanin expression were also seen for stage III/IVa/IVb thymomas (P = .0409) and B2/B3/combined thymomas (P = .0478). In conclusion, D2-40 immunostaining seems to be valuable for predicting the aggressive and metastatic potential of thymomas and the prognosis of the patients.

Microenvironmental reprogramming of thymic epithelial cells to skin multipotent stem cells

The thymus develops from the third pharyngeal pouch of the anterior gut and provides the necessary environment for thymopoiesis (the process by which thymocytes differentiate into mature T lymphocytes) and the establishment and maintenance of self-tolerance. It contains thymic epithelial cells (TECs) that form a complex three-dimensional network organized in cortical and medullary compartments, the organization of which is notably different from simple or stratified epithelia. TECs have an essential role in the generation of self-tolerant thymocytes through expression of the autoimmune regulator Aire, but the mechanisms involved in the specification and maintenance of TECs remain unclear. Despite the different embryological origins of thymus and skin (endodermal and ectodermal, respectively), some cells of the thymic medulla express stratified-epithelium markers, interpreted as promiscuous gene expression. Here we show that the thymus of the rat contains a population of clonogenic TECs that can be extensively cultured while conserving the capacity to integrate in a thymic epithelial network and to express major histocompatibility complex class II (MHC II) molecules and Aire. These cells can irreversibly adopt the fate of hair follicle multipotent stem cells when exposed to an inductive skin microenvironment; this change in fate is correlated with robust changes in gene expression. Hence, microenvironmental cues are sufficient here to re-direct epithelial cell fate, allowing crossing of primitive germ layer boundaries and an increase in potency.

Gata3-deficient mice develop parathyroid abnormalities due to dysregulation of the parathyroid-specific transcription factor Gcm2

Heterozygous mutations of GATA3, which encodes a dual zinc-finger transcription factor, cause hypoparathyroidism with sensorineural deafness and renal dysplasia. Here, we have investigated the role of GATA3 in parathyroid function by challenging Gata3+/- mice with a diet low in calcium and vitamin D so as to expose any defects in parathyroid function. This led to a higher mortality among Gata3+/- mice compared with Gata3+/+ mice. Compared with their wild-type littermates, Gata3+/- mice had lower plasma concentrations of calcium and parathyroid hormone (PTH) and smaller parathyroid glands with a reduced Ki-67 proliferation rate. At E11.5, Gata3+/- embryos had smaller parathyroid-thymus primordia with fewer cells expressing the parathyroid-specific gene glial cells missing 2 (Gcm2), the homolog of human GCMB. In contrast, E11.5 Gata3-/- embryos had no Gcm2 expression and by E12.5 had gross defects in the third and fourth pharyngeal pouches, including absent parathyroid-thymus primordia. Electrophoretic mobility shift, luciferase reporter, and chromatin immunoprecipitation assays showed that GATA3 binds specifically to a functional double-GATA motif within the GCMB promoter. Thus, GATA3 is critical for the differentiation and survival of parathyroid progenitor cells and, with GCM2/B, forms part of a transcriptional cascade in parathyroid development and function.

Spatial (Tbata) expression in mature medullary thymic epithelial cells

The Spatial gene is expressed in highly polarized cell types such as testis germ cells, brain neurons and thymic epithelial cells (TEC). Its expression was documented in testis and brain but poorly characterized in thymus. Here, we characterize for the first time Spatial-expressing TEC throughout ontogeny and adult mouse thymus. Spatial is expressed in thymic-fated domain by embryonic day E10.5 and persists in subcapsular, cortical, medullary epithelial cells and in MTS24(+) progenitor TEC. Using mouse strains in which thymocyte development is blocked at various stages, we show that Spatial expression is independent of thymocyte-derived signals during thymus organogenesis. Analyses on purified thymic cell subsets show that Spatial short isoforms are expressed in cortical TEC (cTEC) and mature medullary TEC (mTEC). Spatial long isoforms were detected in the same TEC population. Spatial presents a nuclear distribution specific to mature mTEC expressing UEA1 and Aire. Aire- and RANKL-deficient mice revealed that Spatial expression is drastically reduced in the thymus of these mutants. These findings reveal a critical function of Aire in regulating Spatial expression, which is compatible with promiscuous Spatial gene expression.

Presence of donor-derived thymic epithelial cells in [B6-->MRL/lpr] mice after allogeneic intra-bone marrow-bone marrow transplantation (IBM-BMT)

We have previously shown that allogeneic intra-bone marrow-bone marrow transplantation (IBM-BMT) can be used to treat autoimmune diseases in MRL/lpr (H-2(K)) mice with replacing not only hematolymphoid cells but also stromal cells by normal C57BL/6 (B6: H-2(b)) mouse cells. In the present study, we examined for existence of donor-derived thymic epithelial cells (TECs) in the host thymus using green fluorescent protein (GFP)-B6 (H-2(b)) mice. In [GFP-B6-->MRL/lpr] chimeric mice, splenocytes and thymocytes were completely replaced by donor-type cells, and levels of serum autoantibodies and proteinuria were significantly - reduced to those levels of normal donors. Interestingly, GFP-expressing TECs - not only medullary TECs, which express mouse thymus stromal (MTS)-10, but also cortical TECs, which express cytokeratin 18 - were found. Also, the number of autoimmune regulator (AIRE) expressing TECs, which regulates tissue-specific antigens to delete autoreactive cells, was reduced in the chimeric mice to that of the donor, whereas the number of forkhead box N1 (FOXN1) expressing TECs, which are crucial in the terminal differentiation of TECs, remained unchanged. These findings suggest that BMCs contain the precursors of functional TECs, and that they can differentiate into TECs, thereby correcting thymic function.

Phenotypical and morphological changes in the thymic microenvironment from ageing mice

The thymus is crucial for T-cell output and the age-associated involution of this organ, is thought to have a major impact in the decline in immunity that is seen in later life. The mechanism that underlines thymic involution is not known, however, we have evidence to suggest that this is may be due to changes in the thymic microenvironment. To further test this hypothesis, we quantified the in situ changes to markers that identify cortical and medullary thymic epithelial cells. This analysis revealed an age-dependent decline in cortical and medullary markers together with an increase in Notch and Delta expression, in older mice, as judged by immunohistochemistry. This was accompanied by alterations of the archetypal staining patterns and three dimensional analysis revealed changes in the morphology of the thymic microenvironment. These studies suggest that there are age-associated alterations in the thymic microenvironment, which may therefore play a role in thymic involution.

Identification of Plet-1 as a specific marker of early thymic epithelial progenitor cells

The thymus is essential for a functional immune system, because the thymic stroma uniquely supports T lymphocyte development. We have previously identified the epithelial progenitor population from which the thymus arises and demonstrated its ability to generate an organized functional thymus upon transplantation. These thymic epithelial progenitor cells (TEPC) are defined by surface determinants recognized by the mAbs MTS20 and MTS24, which were also recently shown to identify keratinocyte progenitor cells in the skin. However, the biochemical nature of the MTS20 and MTS24 determinants has remained unknown. Here we show, via expression profiling of fetal mouse TEPC and their differentiated progeny and subsequent analyses, that both MTS20 and MTS24 specifically bind an orphan protein of unknown function, Placenta-expressed transcript (Plet)-1. In the postgastrulation embryo, Plet-1 expression is highly restricted to the developing pharyngeal endoderm and mesonephros until day 11.5 of embryogenesis, consistent with the MTS20 and MTS24 staining pattern; both MTS20 and MTS24 specifically bind cell lines transfected with Plet-1; and antibodies to Plet-1 recapitulate MTS20/24 staining. In adult tissues, we demonstrate expression in a number of sites, including mammary and prostate epithelia and in the pancreas, where Plet-1 is specifically expressed by the major duct epithelium, providing a specific cell surface marker for this putative reservoir of pancreatic progenitor/stem cells. Plet-1 will thus provide an invaluable tool for genetic analysis of the lineage relationships and molecular mechanisms operating in the development, homeostasis, and injury in several organ/tissue systems.

Thymic generation and regeneration: a new paradigm for establishing clinical tolerance of stem cell-based therapies

Tolerance to tissue-engineering products is a major obstacle hindering the clinical application of this rapidly advancing technology. Manipulation of central tolerance, by establishing thymus chimerism of both donor and host-derived haemopoietic cells (haemopoietic stem cell transplant--HSCT), should purge any T cells reactive to potential donor organ or tissue transplant. A functional thymus, however, is required to induce chimerism and repopulate the peripheral T cell pool, but age-related thymic atrophy and damage caused by ablative conditioning regimes significantly reduce thymic function and increase incident of infection-dependent morbidity and mortality. Thus rejuvenation of the thymus alongside HSCT may potentiate the use of this strategy in the clinic. In addition, the use of thymic epithelial progenitor cell technology may allow growth of ex vivo thymic tissue for use in clinical situations of immunodeficiency as well as in establishing tolerance to tissue/organ products derived from the same source.

Thymic epithelial progenitor cells and thymus regeneration: an update

The thymus provides the essential microenvironment for T-cell development and maturation. Thymic epithelial cells (TECs), which are composed of thymic cortical epithelial cells (cTECs) and thymic medullary epithelial cells (mTECs), have been well documented to be critical for these tightly regulated processes. It has long been controversial whether the common progenitor cells of TECs could give rise to both cTECs and mTECs. Great progress has been made to characterize the common TEC progenitor cells in recent years. We herein discuss the sole origin paradigm with regard to TEC differentiation as well as these progenitor cells in thymus regeneration.

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.

Formation of a functional thymus initiated by a postnatal epithelial progenitor cell

The thymus is essential for the generation of self-tolerant effector and regulatory T cells. Intrathymic T-cell development requires an intact stromal microenvironment, of which thymic epithelial cells (TECs) constitute a major part. For instance, cell-autonomous genetic defects of forkhead box N1 (Foxn1) and autoimmune regulator (Aire) in thymic epithelial cells cause primary immunodeficiency and autoimmunity, respectively. During development, the thymic epithelial rudiment gives rise to two major compartments, the cortex and medulla. Cortical TECs positively select T cells, whereas medullary TECs are involved in negative selection of potentially autoreactive T cells. It has long been unclear whether these two morphologically and functionally distinct types of epithelial cells arise from a common bi-potent progenitor cell and whether such progenitors are still present in the postnatal period. Here, using in vivo cell lineage analysis in mice, we demonstrate the presence of a common progenitor of cortical and medullary TECs after birth. To probe the function of postnatal progenitors, a conditional mutant allele of Foxn1 was reverted to wild-type function in single epithelial cells in vivo. This led to the formation of small thymic lobules containing both cortical and medullary areas that supported normal thymopoiesis. Thus, single epithelial progenitor cells can give rise to a complete and functional thymic microenvironment, suggesting that cell-based therapies could be developed for thymus disorders.

The quantitative assessment of MHC II on thymic epithelium: implications in cortical thymocyte development

The dynamics of MHC II expression in various thymic stromal compartments was investigated. By including MHC II in flow cytometry in addition to the cortical CDR1, medullary UEA-1 and pan-epithelial G8.8 markers, thymic stromal compartments were subdivided into at least six different populations. The total level of surface and cytoplasmic MHC II from fresh cortical thymic epithelial cells (cTECs) of normal mouse was as high as MHC II levels in medullary thymic epithelial cells (mTECs). MHC II levels as well as the percentages and cycling status of thymic epithelial cell populations expressing MHC II were not static during post-natal development, suggesting quantitative flexibility in presenting signals to the developing thymocytes. Although there was no evidence found for regulation of surface MHC II levels by TCR or by IFN-gamma, the absence of class II transactivator reduced both the level of MHC II expression and the number of MHC II+ cells. Surprisingly, MHC II molecules were found to form distinct focal aggregates on the surface of cTEC but not mTEC using high-resolution analysis by confocal microscopy. Moreover, these aggregates were formed independent of TCR or TCR-bearing cells in the thymus. These aggregates could potentially generate a functional unit containing a much higher local MHC II concentration to yield a higher avidity interaction. We discuss possible mechanisms for positive selection by weak interactions in the presence of such preformed MHC II aggregate units in cTEC.

CCR7-Dependent Cortex-to-Medulla Migration of Positively Selected Thymocytes Is Essential for Establishing Central Tolerance

Immature CD4+CD8+ thymocytes, which are generated in the thymic cortex, are induced upon positive selection to differentiate into mature T lymphocytes and relocate to the thymic medulla. It was recently shown that a chemokine signal via CCR7 is essential for the cortex-to-medulla migration of positively selected thymocytes in the thymus. However, the role of the cortex-to-medulla migration in T cell development and selection has remained unclear. The present study shows that the developmental kinetics and the thymic export of mature thymocytes were undisturbed in adult mice lacking CCR7 or its ligands (CCR7L). The inhibition of sphingosine-1-phosphate-mediated lymphocyte egress from the thymus led to the accumulation of mature thymocytes in the cortex of CCR7- or CCR7L-deficient mice, unlike the accumulation in the medulla of normal mice, thereby suggesting that mature thymocytes may be exported directly from the cortex in the absence of CCR7 signals. However, the thymocytes that were generated in the absence of CCR7 or CCR7L were potent in causing autoimmune dacryoadenitis and sialadenitis in mice and were thus incapable of establishing central tolerance to organ-specific antigens. These results indicate that CCR7-mediated cortex-to-medulla migration of thymocytes is essential for establishing central tolerance rather than for supporting the maturation or export of thymocytes.

BMP signaling is required for normal thymus development

The microenvironment of the thymus fosters the generation of a diverse and self-tolerant T cell repertoire from a pool of essentially random specificities. Epithelial as well as mesenchymal cells contribute to the thymic stroma, but little is known about the factors that allow for communication between the two cells types that shape the thymic microenvironment. In this study, we investigated the role of bone morphogenetic protein (BMP) signaling in thymus development. Transgenic expression of the BMP antagonist Noggin in thymic epithelial cells under the control of a Foxn1 promoter in the mouse leads to dysplastic thymic lobes of drastically reduced size that are ectopically located in the neck at the level of the hyoid bone. Interestingly, the small number of thymocytes in these thymic lobes develops with normal kinetics and shows a wild-type phenotype. Organ initiation of the embryonic thymic anlage in these Noggin transgenic mice occurs as in wild-type mice, but the tight temporal and spatial regulation of BMP4 expression is abrogated in subsequent differentiation stages. We show that transgenic Noggin blocks BMP signaling in epithelial as well as mesenchymal cells of the thymic anlage. Our data demonstrate that BMP signaling is crucial for thymus development and that it is the thymic stroma rather than developing thymocytes that depends on BMP signals.

Oncogenetic tree models based on cytogenetic data: new insights into the development of epithelial tumors of the thymus

Epithelial tumors of the thymus are rare neoplasms typically arising in the anterior mediastinum. There is an ongoing discussion whether thymomas of different histological subtypes form a biological continuum or represent distinct biological entities. To further investigate this question, we performed a statistical analysis of CGH data of 65 previously published cases. Losses of 3p, 6p, 6q, 13q, 16q, and 17p, as well as gains on 1q, were found in at least 10% of the cases. Comparing the data from B2, B3, and C thymomas, we noted an increasing complexity of karyotypes that may be well explained by a sequential order of these types. The frequencies of losses on 16q and 17p show a significant trend with respect to the sequence from B2 to B3 and C thymomas, indicating that these aberrations may be important events in the transition between these tumor types. To identify pathways of genetic development and progression of thymomas, we used oncogenetic tree models representing the dependencies between recurrent chromosomal aberrations. This analysis suggests that gains on the long arm of chromosome 1 occur early in tumor development and are correlated with losses on 6p and 6q. There is a weak correlation with losses on 16q and 17p, which appear to be late events. An independent pathway leads to losses on 3p and 13q, which are closely correlated. Our results indicate that the development of thymomas seems to be in some part a multistep mechanism. Oncogenetic tree models are a helpful means to determine developmental pathways of tumors arising from the same progenitor cell, as shown here for thymomas.

Mutations affecting thymus organogenesis in Medaka, Oryzias latipes

The thymus is an organ for T lymphocyte maturation and is indispensable for the establishment of a highly developed immune system in vertebrates. In order to genetically dissect thymus organogenesis, we carried out a large-scale mutagenesis screening for Medaka mutations affecting recombination activating gene 1 (rag1) expression in the developing thymus. We identified 24 mutations, defining at least 13 genes, which led to a marked reduction of rag1 expression in the thymus. As thymus development depends on pharyngeal arches, we classified those mutations into three classes according to the defects in the pharyngeal arches. Class 1 mutants had no or slight morphological abnormalities in the pharyngeal arches, implying that the mutations may include defects in such thymus-specific events as lymphocyte development and thymic epithelial cell maturation. Class 2 mutants had abnormally shaped pharyngeal arches. Class 3 mutants showed severely attenuated pharyngeal arch development. In Class 2 and Class 3 mutants, the defects in thymus development may be due to abnormal pharyngeal arch development. Those mutations are expected to be useful for identifying the molecular mechanisms underlying thymus organogenesis.

Reduction in the developmental potential of intrathymic T cell progenitors with age

Current models of thymic involution propose that intrinsic developmental defects in intrathymic T cell precursors do not contribute to age-related declines in thymopoiesis. This premise was reassessed in a murine model in light of the recent definition of the early T lineage progenitor (ETP), which appears to be the earliest intrathymic precursor defined to date. The results demonstrate that the frequency of ETP declines with age and their potential to reconstitute the thymus is diminished. These findings are consistent with the fact that ETP from aged mice proliferate less and have a higher rate of apoptosis than their counterparts from young animals. Taken together, these data suggest that age-associated changes in T cell precursors should be considered when attempts to rejuvenate the involuted thymus are made.

Total Deletion ofin VivoTelomere Elongation Capacity: An Ambitious but Possibly Ultimate Cure for All Age-Related Human Cancers

Despite enormous effort, progress in reducing mortality from cancer remains modest. Can a true cancer "cure" ever be developed, given the vast versatility that tumors derive from their genomic instability? Here we consider the efficacy, feasibility, and safety of a therapy that, unlike any available or in development, could never be escaped by spontaneous changes of gene expression: the total elimination from the body of all genetic potential for telomere elongation, combined with stem cell therapies administered about once a decade to maintain proliferative tissues despite this handicap. We term this therapy WILT, for whole-body interdiction of lengthening of telomeres. We first argue that a whole-body gene-deletion approach, however bizarre it initially seems, is truly the only way to overcome the hypermutation that makes tumors so insidious. We then identify the key obstacles to developing such a therapy and conclude that, while some will probably be insurmountable for at least a decade, none is a clear-cut showstopper. Hence, given the absence of alternatives with comparable anticancer promise, we advocate working toward such a therapy.

Age-related changes in lymphocyte development and function

The effects of aging on the immune system are widespread and extend from hematopoietic stem cells and lymphoid progenitors in the bone marrow and thymus to mature lymphocytes in secondary lymphoid organs. These changes combine to result in a diminution of immune responsiveness in the elderly. This review aims to provide an overview of age-related changes in lymphocyte development and function and discusses current controversies in the field of aging research.

Hepatoid Thymic Carcinoma

We describe the clinicopathologic findings in a so far unrecognized thymic tumor. The tumor occurred in a 70-year-old woman with respiratory distress but neither myasthenia gravis nor other symptoms. Metastases or another primary tumor were absent. The well-circumscribed neoplasm was located in the thymic region, measured 18 x 12 x 8 cm, and showed a homogeneous, tan-colored, soft cut surface. By histology, the tumor lacked a true capsule and a lobular growth pattern, was almost devoid of stroma, and infiltrated among remnant thymus lobules. The polygonal tumor cells formed solid sheets, trabeculae, or occurred as single cells that resembled hepatocytes. Proliferative activity was low. Portal structures, sinuses, and bile were absent as were areas of conventional thymoma, adenocarcinoma, or germ cell tumor. The tumor expressed cytokeratins 7 and 19, alpha1-antitrypsin, alpha1-antichymotrypsin, and hep-Par-1. Alpha-fetoprotein (AFP), human beta-chorionic gonadotropin (beta-HCG), placental alkaline phosphatase, CD5, CD30, CD31, CD34, CD45, CD68, CD99, S-100, HMB45, desmin, actin, or neuroendocrine markers were not expressed, and intratumorous CD1a+ or TdT+ immature T cells were absent. AFP was repeatedly undetectable in the blood. Mediastinal tumor recurrence was detected 6 months after surgery. Following radiochemotherapy, the patient has remained free of disease for 26 months. We conclude that this tumor is a thymic carcinoma (WHO type C thymoma). A diagnosis of hepatoid yolk sack tumor appears unlikely considering absence of a bona fide germ cell component, lack of AFP expression, and the patient's female gender. Because of its morphologic and immunohistochemical features, we propose the term "hepatoid thymic carcinoma" for this new type of thymic carcinoma.

Thymus cell-cell interactions

The recent advances in molecular biology and genetics, as well as the progress of in vitro techniques, have provided a more coherent image of the thymic function on the molecular level. But they have shifted the attention away from studies on the cellular level, which are necessary to clarify the biological roles of different cell types of the thymic microenvironment. The structure and function of the normal thymus depend on mutual interactions between thymocytes and nonlymphocyte cells. In this review a detailed description of morphological and phenotypic features of both maturing thymocytes and nonlymphocyte cells is given. The recent genetic and biochemical data are presented in conjunction with cytological results to enlighten the thymus cell-cell interactions during thymopoiesis and organization of thymic microstructure. Special emphasis is put on the experimental approaches, which may be used to study the interactions between thymocytes and nonlymphocyte cells in vivo.

Vertebrate thymus and the neurotrophin system

An immunomodulary role has been proposed for growth factors included in the family of neurotrophins. This is supported by the presence of both neurotrophins and neurotrophin receptors in the immune organs and some immunocompetent cells, the in vitro and in vivo effects of the neurotrophins on the immune cells, and the structural changes of lymphoid organs in mice deficient in neurotrophins and their receptors. The current data strongly indicate that neurotrophins regulate the biology of thymic stromal cells and T cells, including survival, and are involved in the thymic organogenesis. This review compiles the available data about the occurrence and distribution of neurotrophins and their signaling receptors (Trk proteins and p75(NTR)) in the vertebrate thymus and the possible contribution of these molecules to the thymic microenvironment and, therefore, to the T cells differentiation.

A domain of Foxn1 required for crosstalk-dependent thymic epithelial cell differentiation

Thymic epithelial cells (TECs) are required for T cell maturation within the thymus. In the nude (Foxn1(nu/nu)) mouse, TECs fail to differentiate. We have generated a hypomorphic allele called Foxn1(Delta), from which an N-terminal domain was deleted. The phenotype was thymus specific, identifying a tissue-specific activity for this domain. Foxn1(Delta/Delta) mice showed abnormal thymic architecture, lacking cortical and medullary domains. In contrast to thymi in mice with the null allele, the Foxn1(Delta/Delta) thymus promoted T cell development, but with specific defects at both the double-negative and double-positive stages. Thus, initiation and progression of TEC differentiation are genetically separable functions of Foxn1, and the N-terminal domain is required for crosstalk-dependent TEC differentiation.

Genetic dissection of thymus development in mouse and zebrafish

Lymphoid organs represent a specialized microenvironment for interaction of stromal and lymphoid cells. In primary lymphoid organs, these interactions are required to establish a self-tolerant repertoire of lymphocytes. While detailed information is available about the genes that control lymphocyte differentiation, little is known about the genes that direct the establishment and differentiation of principal components of such microenvironments. Here, we discuss genetic studies addressing the role of thymic epithelial cells (TECs) during thymopoiesis. We have identified an evolutionarily conserved key regulator of TEC differentiation, Foxn1, that is required for the immigration of prothymocytes into the thymic primordium. Because Foxn1 specifies the prospective endodermal domain that gives rise to thymic epithelial cells, it can be used to identify the evolutionary origins of this specialized cell type. In the course of these studies, we have found that early steps of thymus development in zebrafish are very similar to those in mice. Subsequently, we have used chemical mutagenesis to derive zebrafish lines with aberrant thymus development. Strengths and weaknesses of mouse and zebrafish models are largely complementary such that genetic analysis of mouse and zebrafish mutants may lead to a better understanding of thymus development.

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.

Development of thymic microenvironments in vitro is oxygen-dependent and requires permanent presence of T-cell progenitors

Development of a mature T-cell repertoire in the thymus depends on lympho-stromal interaction between thymocytes and stromal cells. To facilitate intercellular contact, the epithelium in the thymus has differentiated into a unique three-dimensionally (3D)-oriented network. Here we analyze factors influencing induction and maintenance of the 3D configuration of the epithelial network in fetal thymic lobes in vitro. We show that the 3D configuration of the thymic stroma depends on (a) the oxygen pressure in vitro and (b) permanent physical contact between stromal cells and developing thymocytes. This latter feature is demonstrated by incubation of fetal thymic lobes with deoxyguanosine (d-Guo), inducing a 2D-organized thymic stroma, with thymic cysts appearing. Reconstitution of d-Guo-treated lobes with a limited number of flow-sorted T-cell progenitors restores the 3D configuration of the thymic epithelium, but only at high oxygen pressure. This study underlines the plasticity of thymic epithelium and shows that the unique organization of the thymic epithelium is dependent on both oxygen and crosstalk signals derived from developing thymocytes.