Thymic Rudiment Research Articles

Limits to in vivo fate changes of epithelia in thymus and parathyroid by ectopic expression of transcription factors Gcm2 and Foxn1

The development of the parathyroid and the thymus from the third pharyngeal pouch depends on the activities of the Gcm2 and Foxn1 transcription factors, respectively, whose expression domains sharply demarcate two regions in the developing third pharyngeal pouch. Here, we have generated novel mouse models to examine whether ectopic co-expression of Gcm2 in the thymic epithelium and of Foxn1 in the parathyroid perturbs the establishment of organ fates in vivo. Expression of Gcm2 in the thymic rudiment does not activate a parathyroid-specific expression programme, even in the absence of Foxn1 activity. Co-expression of Foxn1 in the parathyroid fails to impose thymopoietic capacity. We conclude that the actions of Foxn1 and Gcm2 transcription factors are cell context-dependent and that they each require permissive transcription factor landscapes in order to successfully interfere with organ-specific cell fate.

Isolation of Embryonic Tissues and Formation of Quail-Chicken Chimeric Organs Using The Thymus Example.

The capacity to isolate embryonic tissues was an essential step for establishing the quail-chicken chimera system, which in turn has provided undisputed contributions to unveiling key processes in developmental biology. Herein is described an optimized method to isolate embryonic tissues from quail and chickens by microsurgery and enzymatic digestion while preserving its biological properties. After isolation, tissues from both species are associated in an in vitro organotypic assay for 48 h. Quail and chicken tissues can be discriminated by distinct nuclear features and molecular markers allowing the study of the cellular cross-talk between heterospecific association of tissues. This approach is, therefore, a useful tool for studying complex tissue interactions in developmental processes with highly dynamic spatial modifications, such as those occurring during pharyngeal morphogenesis and the formation of the foregut endoderm-derived organs. This experimental approach was first developed to study the epithelial-mesenchymal interactions during early-stages of thymus formation. In this, the endoderm-derived prospective thymic rudiment and mesoderm-derived mesenchyme, were isolated from quail and chicken embryos, respectively. The capacity of the associated tissues to generate organs can be further tested by grafting them onto the chorioallantoic membrane (CAM) of a chicken embryo. The CAM provides nutrients and allows gas exchanges to the explanted tissues. After 10 days of in ovo development, the chimeric organs can be analyzed in the harvested explants by conventional morphological methods. This procedure also allows studying tissue-specific contributions during organ formation, from its initial development (in vitro development) to the final stages of organogenesis (in ovo development). Finally, the improved isolation method also provides three-dimensionally (3D) preserved embryonic tissues, that can also be used for high-resolution topographical analysis of tissue-specific gene-expression patterns.

Three-Dimensional Ameliorated Biologics Elicit Thymic Renewal in Tumor-Bearing Hosts

Cancer-initiating/sustaining stem cell subsets (CSCs) have the potential to regenerate cancer cell populations and are resistant to routine therapeutic strategies, thus attracting much attention in anticancer research. In this study, an innovative framework of endogenous microenvironment-renewal for addressing such a dilemma has been just developed. CSCs in three-dimensional multipotent spheroid-engineered biologics were prepared with 150 Gy radiation and inoculated into 15-mo-old BALB/c and C57BL/6 mice bearing diverse advanced tumors covering Mammary 4T1, liver Hepa, lung LL/2, and colon C26 tumors and distant metastases. Subsequently, the systematic microenvironment of tumor-bearing hosts was rapidly remodeled to resettle thymic cortex and medulla rudiment as an endogenous foxn1-thymosin reprogramming TCR-repertoire for resetting MHC-unrestricted multifunction renewal. Postrenewal Vγ4γδT-subsets would bind and lead migrating CSCs into apoptosis. Moreover, TCR repertoire multifunction renewal could reverse tumor metastases from tumoricidal resistance into eventual regression as a blockade of cancer-sustaining Bmi-1/Nanog-Oct4-Sox2 renewal loop with sequent multivalent depletion of both migrating/in situ CSCs and non-stem terminal cancer cell subsets. This study represents a promising start to set up a generalizable strategy of three-dimensional biologics evoking an endogenous integral microenvironment into pluripotent renewal versus advanced cancer.

Initial seeding of the embryonic thymus by immune-restricted lympho-myeloid progenitors.

The final stages of restriction to the T cell lineage occur in the thymus after the entry of thymus-seeding progenitors (TSPs). The identity and lineage potential of TSPs remains unclear. Because the first embryonic TSPs enter a non-vascularized thymic rudiment, we were able to directly image and establish the functional and molecular properties of embryonic thymopoiesis-initiating progenitors (T-IPs) before their entry into the thymus and activation of Notch signaling. T-IPs did not include multipotent stem cells or molecular evidence of T cell-restricted progenitors. Instead, single-cell molecular and functional analysis demonstrated that most fetal T-IPs expressed genes of and had the potential to develop into lymphoid as well as myeloid components of the immune system. Moreover, studies of embryos deficient in the transcriptional regulator RBPJ demonstrated that canonical Notch signaling was not involved in pre-thymic restriction to the T cell lineage or the migration of T-IPs.

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.

Expression analyses revealed thymic stromal co-tansporter/Slc46A2 is in stem cell populations and is a putative tumor suppressor (HEM8P.237)

Abstract By combining flow cytometry and database searches, we extended the expression profiling of TSCOT/Slc46A2/Ly110, that was shown to be expressed in the bipotent precursor and cortical thymic epithelial cells. Genome scale analysis verified that TSCOT is tissue- and cell type- specific in thymus and is also expressed in other epithelial tissues. Coexpression profiling with genes, Foxn1 and Hoxa3, revealed the role of TSCOT during the tissue formation including early thymic organogenesis. TSCOT expression was detected in all thymic epithelial (TEC), but not in the CD31+ endothelial lineage of fetal thymus. In addition, ABC transporter-dependent side population and Sca-1+ of fetal TEC population 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 the searches for expression variation, 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 was diminished in the lung cancers suggesting TSCOT may be involved in the suppression of lung tumor development. From these results, a model for TEC differentiation from the stem cells was proposed in the context of multiple epithelial organ formation.

Long-term persistence of functional thymic epithelial progenitor cells in vivo under conditions of low FOXN1 expression.

Normal thymus function reflects interactions between developing T-cells and several thymic stroma cell types. Within the stroma, key functions reside in the distinct cortical and medullary thymic epithelial cell (TEC) types. It has been demonstrated that, during organogenesis, all TECs can be derived from a common thymic epithelial progenitor cell (TEPC). The properties of this common progenitor are thus of interest. Differentiation of both cTEC and mTEC depends on the epithelial-specific transcription factor FOXN1, although formation of the common TEPC from which the TEC lineage originates does not require FOXN1. Here, we have used a revertible severely hypomorphic allele of Foxn1, Foxn1R, to test the stability of the common TEPC in vivo. By reactivating Foxn1 expression postnatally in Foxn1R /− mice we demonstrate that functional TEPCs can persist in the thymic rudiment until at least 6 months of age, and retain the potential to give rise to both cortical and medullary thymic epithelial cells (cTECs and mTECs). These data demonstrate that the TEPC-state is remarkably stable in vivo under conditions of low Foxn1 expression, suggesting that manipulation of FOXN1 activity may prove a valuable method for long term maintenance of TEPC in vitro.

Umbilical cord-derived mesenchymal stem cells regulate thymic epithelial cell development and function in Foxn1−/− mice

Thymic microenvironments are essential for the maturation of thymocytes, which can be anatomically compartmentalized into cortical and medullar regions. The absence of the gene encoding the transcription factor forkhead box n1 (Foxn1) causes epithelial differentiation to stall in the precursor stage, resulting in the formation of an abnormal thymus. In this study, we used human umbilical cord-derived mesenchymal stem cells (UC-MSCs) to treat Foxn1(-/-) mice, and then analyzed the maturation and distribution of thymic epithelial cells in the Foxn1(-/-) thymic rudiment and the thymopoiesis of this newly developed rudiment. Our data showed a well-organized cortex-medulla architecture and an obvious improvement in the maturation of thymic epithelial cells along with the appearance of UEA-1(+)MHCII(hi) thymic epithelial cells in the rudiment. We further demonstrated improved thymopoiesis and the enhanced export of mature T cells with increased numbers of regulatory T cells into the peripheral blood. Furthermore, we observed that MSCs can engraft into thymic tissue and express many cytokines or proteins, particularly keratinocyte growth factor (KGF) and CD248, which are essential for thymic development. Collectively, our data identified a new mechanism for MSCs, which may provide a proper microenvironment for the reconstitution and functional maturation of the thymus in Foxn1(-/-) mice. Additionally, we elicited additional insights into the therapeutic efficacy of MSCs in several autoimmune diseases.

Zebrafish model for allogeneic hematopoietic cell transplantation not requiring preconditioning

Recent work on vertebrate hematopoiesis has uncovered the presence of deeply rooted similarities between fish and mammals at molecular and cellular levels. Although small animal models such as zebrafish are ideally suited for genetic and chemical screens, the study of cellular aspects of hematopoietic development in lower vertebrates is severely hampered by the complex nature of their histocompatibility-determining genes. Hence, even when hosts are sublethally irradiated before hematopoietic cell transplantation, stable and long-term reconstitution by allogeneic stem cells often fails. Here, we describe the unexpected observation that transplantation and maintenance of allogeneic hematopoietic stem cells in zebrafish homozygous for the c-myb(t25127) allele, carrying a missense mutation (Ile181Asn) in the DNA binding domain can be achieved without prior conditioning. Using this model, we examined several critical parameters of zebrafish hematopoiesis in a near-physiological setting. Limiting dilution analysis suggests that the kidney marrow of adult zebrafish harbors about 10 transplantable hematopoietic stem cells; this tissue also contains thymus-settling precursors that colonize the thymic rudiment within days after transplantation and initiate robust T-cell development. We also demonstrate that c-myb mutants can be stably reconstituted with hematopoietic cells carrying specific genetic defects in lymphocyte development, exemplifying one of the many potential uses of this model in experimental hematology.

Misguided Migration of C Cell Precursors to Extra-Thyroidal Locations Related to Defective Pharyngeal Pouch Development in Shh Deficient Mice

A possible role of sonic hedgehog (Shh) in recruitment of C cell precursors to the Ultimobranchial Body (UB) and embryonic thyroid was investigated in Shh-/- mice. Nkx2.1 and Foxa2 co-expression distinguished UB originating in the fourth pharyngeal pouch from other derivatives of pharyngeal endoderm. In mutants UB formed a single structure that failed to bud and instead of fusing with the midline thyroid primordium adhered to the thymic rudiments. Mature C cells appeared in the UB remnant and ectopically in the thymic parenchyma, foregut endoderm and trachea. Thyroid did not contain C cells except minute numbers close to the tracheal interface. Tracing progeny in Shh-CRE/Rosa26R mice showed the vast majority of both UB and thyroid progenitors derived from Shh negative endoderm, but Shh expressing cells appeared in both thyroid primordia before fusion of the two. The findings indicate that Shh determines the endoderm territory for C cell differentiation and guides the migration of C cell precursors into the thyroid, presumably by regulating the separation of glandular domains in the pharyngeal pouch endoderm. A cell-autonomous role of Shh in thyroid morphogenesis is suggested.

Foxn1 maintains thymic epithelial cells to support T-cell development via mcm2 in zebrafish

The thymus is mainly comprised of thymic epithelial cells (TECs), which form the unique thymic epithelial microenvironment essential for intrathymic T-cell development. Foxn1, a member of the forkhead transcription factor family, is required for establishing a functional thymic rudiment. However, the molecular mechanisms underlying the function of Foxn1 are still largely unclear. Here, we show that Foxn1 functions in thymus development through Mcm2 in the zebrafish. We demonstrate that, in foxn1 knockdown embryos, the thymic rudiment is reduced and T-cell development is impaired. Genome-wide expression profiling shows that a number of genes, including some known thymopoiesis genes, are dysregulated during the initiation of the thymus primordium and immigration of T-cell progenitors to the thymus. Functional and epistatic studies show that mcm2 and cdca7 are downstream of Foxn1, and mcm2 is a direct target gene of Foxn1 in TECs. Finally, we find that the thymus defects in foxn1 and mcm2 morphants might be attributed to reduced cell proliferation rather than apoptosis. Our results reveal that the foxn1-mcm2 axis plays a central role in the genetic regulatory network controlling thymus development in zebrafish.

Human FOXN1-Deficiency Is Associated with αβ Double-Negative and FoxP3+ T-Cell Expansions That Are Distinctly Modulated upon Thymic Transplantation

Forkhead box N1 (FOXN1) is a transcription factor crucial for thymic epithelium development and prevention of its involution. Investigation of a patient with a rare homozygous FOXN1 mutation (R255X), leading to alopecia universalis and thymus aplasia, unexpectedly revealed non-maternal circulating T-cells, and, strikingly, large numbers of aberrant double-negative αβ T-cells (CD4negCD8neg, DN) and regulatory-like T-cells. These data raise the possibility that a thymic rudiment persisted, allowing T-cell development, albeit with disturbances in positive/negative selection, as suggested by DN and FoxP3+ cell expansions. Although regulatory-like T-cell numbers normalized following HLA-mismatched thymic transplantation, the αβDN subset persisted 5 years post-transplantation. Involution of thymus allograft likely occurred 3 years post-transplantation based on sj/βTREC ratio, which estimates intrathymic precursor T-cell divisions and, consequently, thymic explant output. Nevertheless, functional immune-competence was sustained, providing new insights for the design of immunological reconstitution strategies based on thymic transplantation, with potential applications in other clinical settings.

Modulation of Bmp4 signalling in the epithelial–mesenchymal interactions that take place in early thymus and parathyroid development in avian embryos

Epithelial–mesenchymal interactions are crucial for the development of the endoderm of the pharyngeal pouches into the epithelia of thymus and parathyroid glands. Here we investigated the dynamics of epithelial–mesenchymal interactions that take place at the earliest stages of thymic and parathyroid organogenesis using the quail-chick model together with a co-culture system capable of reproducing these early events in vitro. The presumptive territories of thymus and parathyroid epithelia were identified in three-dimensionally preserved pharyngeal endoderm of embryonic day 4.5 chick embryos on the basis of the expression of Foxn1 and Gcm2, respectively: the thymic rudiment is located in the dorsal domain of the third and fourth pouches, while the parathyroid rudiment occupies a more medial/anterior pouch domain. Using in vitro quail-chick tissue associations combined with in ovo transplantations, we show that the somatopleural but not the limb bud mesenchyme, can mimic the role of neural crest-derived pharyngeal mesenchyme to sustain development of these glands up to terminal differentiation. Furthermore, mesenchymal-derived Bmp4 appears to be essential to promote early stages of endoderm development during a short window of time, irrespective of the mesenchymal source. In vivo studies using the quail-chick system and implantation of growth factor soaked-beads further showed that expression of Bmp4 by the mesenchyme is necessary during a 24h-period of time. After this period however, Bmp4 is no longer required and another signalling factor produced by the mesenchyme, Fgf10, influences later differentiation of the pouch endoderm. These results show that morphological development and cell differentiation of thymus and parathyroid epithelia require a succession of signals emanating from the associated mesenchyme, among which Bmp4 plays a pivotal role for triggering thymic epithelium specification.

Three chemokine receptors cooperatively regulate homing of hematopoietic progenitors to the embryonic mouse thymus

The thymus lacks self-renewing hematopoietic cells, and thymopoiesis fails rapidly when the migration of progenitor cells to the thymus ceases. Hence, the process of thymus homing is an essential step for T-cell development and cellular immunity. Despite decades of research, the molecular details of thymus homing have not been elucidated fully. Here, we show that chemotaxis is the key mechanism regulating thymus homing in the mouse embryo. We determined the number of early thymic progenitors in the thymic rudiments of mice deficient for one, two, or three of the chemokine receptor genes, chemokine (C-C motif) receptor 9 (Ccr9), chemokine (C-C motif) receptor 7 (Ccr7), and chemokine (C-X-C motif) receptor 4 (Cxcr4). In the absence of all three chemokine receptors, thymus homing was reduced about 100-fold both before and after vascularization of the thymic rudiment. In the absence of only two of these three chemokine receptor genes, thymus homing was much less affected (only two- to 10-fold), indicating that the chemotactic regulation of thymus homing is remarkably robust. Our results reveal the redundant roles of Ccr9, Ccr7, and Cxcr4 for thymic homing and provide a framework to examine the regulation of progenitor homing in the postnatal thymus.

EphB–ephrin-B2 interactions are required for thymus migration during organogenesis

Thymus organogenesis requires coordinated interactions of multiple cell types, including neural crest (NC) cells, to orchestrate the formation, separation, and subsequent migration of the developing thymus from the third pharyngeal pouch to the thoracic cavity. The molecular mechanisms driving these processes are unclear; however, NC-derived mesenchyme has been shown to play an important role. Here, we show that, in the absence of ephrin-B2 expression on thymic NC-derived mesenchyme, the thymus remains in the cervical area instead of migrating into the thoracic cavity. Analysis of individual NC-derived thymic mesenchymal cells shows that, in the absence of ephrin-B2, their motility is impaired as a result of defective EphB receptor signaling. This implies a NC-derived cell-specific role of EphB-ephrin-B2 interactions in the collective migration of the thymic rudiment during organogenesis.

FoxN1 in K14 promoter-driven epithelium is required for generation and maintenance of 3D-thymus medulla and preventing nude phenotype in the skin (36.33)

Abstract An organized 3D thymic epithelial cell (TEC) meshwork is crucial for T-lymphocyte development. Expansion of thymic epithelial cystic cavities represents disruption of this 3D-structure. Abnormal culture, lacking lympho-stromal interactions, thymic aging, and gene mutation induce 2D-thymus cysts. Since germline FoxN1 gene mutation develops a 2D-cystic thymic rudiment, while abnormal culture caused 2D-thymus cyst-lining was reported to be non-FoxN1-dependent, it is unclear whether loss of FoxN1 promotes generation of thymic cysts, whether FoxN1 regulates maintenance of the 3D-thymus structure. Using loxP-floxed FoxN1 mouse model, we demonstrated that deletion of FoxN1 in K-14 promoter-driven epithelium induced loss of 3D-thymus medullary structure by producing a large number of alveolus-like 2D-epithelial cysts. The cyst-lining epithelium showed immature TEC phenotype by expressing Cld3,4 and K8, but low levels of MHC-II and K5. These 2D-thymus cysts expanded with ages, associated with reduced UEA-1+ and Aire+ TECs, and FoxN1 mutant Cld3,4+ TECs cannot develop mature medullary TECs. Meanwhile, K14-specific FoxN1 deletion sufficiently induced hair follicle defect and caused FoxN1-null nude phenotype in the skin, but did not cause athymia. These findings provide new insight into gene regulation on 3D-thymus epithelial structure and separate roles of FoxN1 in the thymus and skin. Supported by the NIH grants (R01AI081995, R21AG031880, and R21AI079747) to D-M S.

WNT Proteins: Environmental Factors Regulating HSC Fate in the Niche

The Wnt signaling pathway has been implicated in regulation of hematopoiesis through a plethora of studies from many different laboratories. However, different inducible gain- and loss-of-function approaches retrieved controversial and sometimes contradictory results. Different levels of activation of the pathway, dosages of Wnt signaling required, and the interference by other signals in the context of Wnt activation collectively explain these controversies. Gain-of-function or in vitro exposure to WNT proteins and more specifically WNT3a was shown to enhance hematopoietic stem cell (HSC) activity, but its exact role was still not completely understood. In a recent study we analyzed the hematopoietic system of mice deficient for this specific Wnt gene. Wnt3a deficiency results in early embryonic lethality around embryonic day 12.5 (E12.5), precluding analysis in adult mice, but allowing hematopoiesis to be studied in fetal liver (FL) and in the just colonized thymic rudiment. Notably, we showed that long-term HSCs and multipotent progenitors are reduced in FL and have severely reduced long-term reconstitution capacity as observed in serial transplantation assays. Of interest, deficiency in Wnt3a leads to complete abolition of canonical Wnt signaling in FL hematopoietic stem and progenitor cells. This HSC deficiency is not explained by altered cell cycle or survival and is irreversible, since it cannot be restored by transplantation into Wnt3a-competent mice. In addition, Wnt3a deficiency differentially affects myeloid and B-lymphoid lineages, with myeloid cells being affected at the progenitor level, while B lymphopoiesis is apparently unaffected. Immature thymocytes, however, were reduced in cell numbers due to lack of Wnt3a production by the thymic microenvironment. Our results show that while in the thymus Wnt3a provides cytokine-like, proliferative stimuli to developing thymocyte Wnt3a regulates cell fate decisions of FL HSC in a nonredundant way.

Increased thymus- and decreased parathyroid-fated organ domains in Splotch mutant embryos

Embryos that are homozygous for Splotch, a null allele of Pax3, have a severe neural crest cell (NCC) deficiency that generates a complex phenotype including spina bifida, exencephaly and cardiac outflow tract abnormalities. Contrary to the widely held perception that thymus aplasia or hypoplasia is a characteristic feature of Pax3Sp/Sp embryos, we find that thymic rudiments are larger and parathyroid rudiments are smaller in E11.5–12.5 Pax3Sp/Sp compared to Pax3+/+ embryos. The thymus originates from bilateral third pharyngeal pouch primordia containing endodermal progenitors of both thymus and parathyroid glands. Analyses of Foxn1 and Gcm2 expression revealed a dorsal shift in the border between parathyroid- and thymus-fated domains at E11.5, with no change in the overall cellularity or volume of each shared primordium. The border shift increases the allocation of third pouch progenitors to the thymus domain and correspondingly decreases allocation to the parathyroid domain. Initial patterning in the E10.5 pouch was normal suggesting that the observed change in the location of the organ domain interface arises during border refinement between E10.5 and E11.5. Given the well-characterized NCC defects in Splotch mutants, these findings implicate NCCs in regulating patterning of third pouch endoderm into thymus- versus parathyroid-specified domains, and suggest that organ size is determined in part by the number of progenitor cells specified to a given fate.

Expression of early activation antigen (CD69) during human thymic development

The novel early activation antigen, EA1, has been shown to be induced by mitogens, antigens and the tumour promoter, phorbol myristate acetate (PMA), on human lymphocytes. This antigen has been designated to be CD69. EA1 has also been shown to be expressed on thymocytes without exogenous activation stimuli. In order to characterize further the expression of EA1 on thymocytes, the ontogeny of its expression was studied. EA1 appeared between 7 and 9.5 weeks of gestation, after colonization of the thymic rudiment with CD7+ T cell precursors, but before the onset of compartmentalization of the thymus into cortical and medullary zones. After cortico-medullary differentiation, the majority of medullary thymocytes expressed EA1 while only a fraction of the cortical thymocytes expressed this antigen. In the fetal and post-natal cortex, EA1 expression appeared to cluster in the subcapsular cortex. EA1+ cells were also scattered throughout the inner cortex. By two-colour fluorocytometric analysis of post-natal thymocytes, it was shown that EA1 was expressed on 30 to 65% of thymocytes. EA1 was expressed on CD4+ CD8+ as well as on the more immature CD4- CD8- thymocytes. In contrast to circulating T cells, thymocytes were much less responsive to PMA stimulation for the expression of EA1. Molecular characterization showed that EA1 on thymocytes had the same structure as that of activated peripheral T cells. In addition, thymic EA1 was constitutively phosphorylated. Thus, EA1 expression is acquired early during thymic development after colonization of the thymic rudiment by CD7+ T cell precursors. However, the specific role that EA1 may play in the activation and function of developing thymocytes remains to be determined.

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.