A murine early thymocyte developmental sequence is marked by transient expression of the interleukin 2 receptor

Intrathymic Cell Migration: Implications in Thymocyte Development and T Lymphocyte Repertoire Formation

During the development of T cells in the thymus, differentiating thymocytes move through specific thymic compartments and interact with the cortical and medullary microenvironments of the thymic lobules. This migration is primarily controlled by adhesion molecules, such as extracellular matrix ligands and receptors, and soluble factors like chemokines that are important for thymocyte differentiation. The migration events driven by these molecules include the entry of lymphoid progenitors from the bone marrow, movement within the thymus, and the exit of mature thymocytes. Notably, the migration of developing T cells can also impact the positive and negative selection processes, which are crucial for preventing the development of self-reactive T cells. This chapter will focus on the key molecules involved in thymocyte migration and how their expression patterns may affect T cell development and the formation of T cell repertoires.

Regulation of CD4 and CD8 coreceptor expression and CD4 versus CD8 lineage decisions

During blood cell development, hematopoietic stem cells generate diverse mature populations via several rounds of binary fate decisions. At each bifurcation, precursors adopt one fate and inactivate the alternative fate either stochastically or in response to extrinsic stimuli and stably maintain the selected fates. Studying of these processes would contribute to better understanding of etiology of immunodeficiency and leukemia, which are caused by abnormal gene regulation during the development of hematopoietic cells. The CD4(+) helper versus CD8(+) cytotoxic T-cell fate decision serves as an excellent model to study binary fate decision processes. These two cell types are derived from common precursors in the thymus. Positive selection of their TCRs by self-peptide presented on either MHC class I or class II triggers their fate decisions along with mutually exclusive retention and silencing of two coreceptors, CD4 and CD8. In the past few decades, extensive effort has been made to understand the T-cell fate decision processes by studying regulation of genes encoding the coreceptors and selection processes. These studies have identified several key transcription factors and gene regulatory networks. In this chapter, I will discuss recent advances in our understanding of the binary cell fate decision processes of T cells.

Decreased Vβ8.2 T-cells in neonatal rats exposed prenatally to Staphylococcal enterotoxin B are further deleted by restimulation in an in vitro cultured thymus

Staphylococcal enterotoxin B (SEB) administration during adulthood can cause the anergy or deletion of variable portion of the ? chain (V?)?expressing T cells. However, the effect of maternal SEB administration during pregnancy on the thymocytes of neonatal rats remains to be elucidated. In the present study, pregnant rats at gestational day 16 were intravenously injected with 15 µg SEB. The present study revealed that prenatal exposure of SEB significantly increased the proportion of cluster of differentiation (CD)4?single positive (SP) T cells and decreased the proportions of CD8?SP, CD4+ V?8.2+ and CD8+ V?8.2+ T cells in the thymus of neonatal rats between day 0 and 5 after delivery. In an in vitro cultured thymus, SEB restimulation significantly increased the proportion of double positive cells and decreased the proportions of CD4?SP, CD8?SP, CD4+ V?8.2+ and CD8+ V?8.2+ T cells. Furthermore, the decreased V?8.2+ T?cells in neonatal rats exposed prenatally to SEB were further deleted by SEB restimulation in an in vitro cultured thymus. These data suggested the special response pattern of the remaining SEB?specific T cells to SEB restimulation in neonatal rats exposed prenatally to SEB.

Complete TCR-α gene locus control region activity in T cells derived in vitro from embryonic stem cells

Locus control regions (LCRs) are cis-acting gene regulatory elements with the unique, integration site-independent ability to transfer the characteristics of their locus-of-origin's gene expression pattern to a linked transgene in mice. LCR activities have been discovered in numerous T cell lineage-expressed gene loci. These elements can be adapted to the design of stem cell gene therapy vectors that direct robust therapeutic gene expression to the T cell progeny of engineered stem cells. Currently, transgenic mice provide the only experimental approach that wholly supports all the critical aspects of LCR activity. In this study, we report the manifestation of all key features of mouse TCR-α gene LCR function in T cells derived in vitro from mouse embryonic stem cells. High-level, copy number-related TCR-α LCR-linked reporter gene expression levels are cell type restricted in this system, and upregulated during the expected stage transition of T cell development. We also report that de novo introduction of TCR-α LCR-linked transgenes into existing T cell lines yields incomplete LCR activity. These data indicate that establishing full TCR-α LCR activity requires critical molecular events occurring prior to final T lineage determination. This study also validates a novel, tractable, and more rapid approach for the study of LCR activity in T cells, and its translation to therapeutic genetic engineering.

The role of AIRE in human autoimmune disease

The autoimmune regulator (AIRE) gene encodes a transcription factor involved in the presentation of tissue-restricted antigens during T-cell development in the thymus. Mutations of this gene lead to type 1 autoimmune polyglandular syndrome (APS-1), also termed autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome, which is characterized by the clinical presentation of at least two of a triad of underlying disorders: Addison disease, hypoparathyroidism and chronic mucocutaneous candidiasis. This Review describes the process of positive and negative selection of developing T cells in the thymus and the role of AIRE as a regulator of peripheral antigen presentation. Furthermore, it addresses how mutations of this gene lead to the failure to eliminate autoreactive T cells, which can lead to clinical autoimmune syndromes.

Role of CRTAM during mouse early T lymphocytes development

CRTAM was reported as a novel receptor expressed in activated NKT and CD8 T lymphocytes. However, we have recently shown that it is also expressed in several non-immune tissues. In opposition to what has been stated for lymphoid cells, CRTAM expression is constitutive in epithelia, suggesting a role in cell-cell interactions. Given the importance of cell interactions during T lymphocyte development, we evaluated CRTAM during T lymphocyte ontogeny. Here we show that CRTAM has an unexpected constitutive expression in adult thymocytes and, remarkably, it is sustained during all stages of thymocyte development. CRTAM expression is restricted to CD8 and all DN subpopulations, with a consistent pattern from E13.5 stage to adult mice. Blocking CRTAM interaction with CADM1 impairs thymus growth, uncovering a novel role in thymus development, with a consequent impact in thymocyte maturation. Thus, CRTAM interaction with CADM1 is involved in structural maintenance of the thymic lobes.

A novel TCR transgenic model reveals that negative selection involves an immediate, Bim-dependent pathway and a delayed, Bim-independent pathway

A complete understanding of negative selection has been elusive due to the rapid apoptosis and clearance of thymocytes in vivo. We report a TCR transgenic model in which expression of the TCR during differentiation occurs only after V(D)J-like recombination. TCR expression from this transgene closely mimics expression of the endogenous TCRalpha locus allowing for development that is similar to wild type thymocytes. This model allowed us to characterize the phenotypic changes that occurred after TCR-mediated signaling in self-reactive thymocytes prior to their deletion in a highly physiological setting. Self-reactive thymocytes were identified as being immature, activated and CD4(lo)CD8(lo). These cells had upregulated markers of negative selection and were apoptotic. Elimination of Bim reduced the apoptosis of self-reactive thymocytes, but it did not rescue their differentiation and the cells remained at the immature CD4(lo)CD8(lo) stage of development. These cells upregulate Nur77 and do not contribute to the peripheral T cell repertoire in vivo. Remarkably, development past the CD4(lo)CD8(lo) stage was possible once the cells were removed from the negatively selecting thymic environment. In vitro development of these cells occurred despite their maintenance of high intracellular levels of Nur77. Therefore, in vivo, negatively selected Bim-deficient thymocytes are eliminated after prolonged developmental arrest via a Bim-independent pathway that is dependent on the thymic microenvironment. These data newly reveal a layering of immediate, Bim-dependent, and delayed Bim-independent pathways that both contribute to elimination of self-reactive thymocytes in vivo.

Adoptive precursor cell therapy to enhance immune reconstitution after hematopoietic stem cell transplantation in mouse and man

Hematopoietic stem cell transplantation is a curative therapy for hematological malignancies. T cell deficiency following transplantation is a major cause of morbidity and mortality. In this review, we discuss adoptive transfer of committed precursor cells to enhance T cell reconstitution and improve overall prognosis after transplantation.

Thymic microenvironments for T-cell repertoire formation

Functionally competent immune system includes a functionally competent T-cell repertoire that is reactive to foreign antigens but is tolerant to self-antigens. The repertoire of T cells is primarily formed in the thymus through positive and negative selection of developing thymocytes. Immature thymocytes that undergo V(D)J recombination of T-cell antigen receptor (TCR) genes and that express the virgin repertoire of TCRs are generated in thymic cortex. The recent discovery of thymoproteasomes, a molecular complex specifically expressed in cortical thymic epithelial cells (cTEC), has revealed a unique role of cTEC in cuing the further development of immature thymocytes in thymic cortex, possibly by displaying unique self-peptides that induce positive selection. Cortical thymocytes that receive TCR-mediated positive selection signals are destined to survive for further differentiation and are induced to express CCR7, a chemokine receptor. Being attracted to CCR7 ligands expressed by medullary thymic epithelial cells (mTEC), CCR7-expressing positively selected thymocytes relocate to thymic medulla. The medullary microenvironment displays another set of unique self-peptides for trimming positively selected T-cell repertoire to establish self-tolerance, via promiscuous expression of tissue-specific antigens by mTEC and efficient antigen presentation by dendritic cells. Recent results demonstrate that tumor necrosis factor (TNF) superfamily ligands, including receptor activating NF-kappaB ligand (RANKL), CD40L, and lymphotoxin, are produced by positively selected thymocytes and pivotally regulate mTEC development and thymic medulla formation.

Expression profiling of immature thymocytes revealed a novel homeobox gene that regulates double-negative thymocyte development

Intrathymic development of CD4/CD8 double-negative (DN) thymocytes can be tracked by well-defined chronological subsets of thymocytes, and is an ideal target of gene expression profiling analysis to clarify the genetic basis of mature T cell production, by which differentiation of immature thymocytes is investigated in terms of gene expression profiles. In this study, we show that development of murine DN thymocytes is predominantly regulated by largely repressive rather than inductive activities of transcriptions, where lineage-promiscuous gene expression in immature thymocytes is down-regulated during their differentiation. Functional mapping of genes showing common temporal expression profiles implicates previously uncharacterized gene regulations that may be relevant to early thymocytes development. A small minority of genes is transiently expressed in the CD44(low)CD25(+) subset of DN thymocytes, from which we identified a novel homeobox gene, Duxl, whose expression is up-regulated by Runx1. Duxl promotes the transition from CD44(high)CD25(+) to CD44(low)CD25(+) in DN thymocytes, while constitutive expression of Duxl inhibits expression of TCR beta-chains and leads to impaired beta selection and greatly reduced production of CD4/CD8 double-positive thymocytes, indicating its critical roles in DN thymocyte development.

CTCF-Independent, but Not CTCF-Dependent, Elements Significantly Contribute to TCR-α Locus Control Region Activity

The mouse TCRalpha/TCRdelta/Dad1 gene locus bears a locus control region (LCR) that drives high-level, position-independent, thymic transgene expression in chromatin. It achieves this through DNA sequences that enhance transcription and protect transgene expression from integration site-dependent position effects. The former activity maps to a classical enhancer region (Ealpha). In contrast, the elements supporting the latter capacity that suppresses position effects are incompletely understood. Such elements likely play important roles in their native locus and may resemble insulator/boundary sequences. Insulators support enhancer blocking and/or chromatin barrier activity. Most vertebrate enhancer-blocking insulators are dependent on the CTCF transcription factor and its cognate DNA binding site. However, studies have also revealed CTCF-independent enhancer blocking and barrier insulator activity in the vertebrate genome. The TCRalpha LCR contains a CTCF-dependent and multiple CTCF-independent enhancer-blocking regions whose roles in LCR activity are unknown. Using randomly integrated reporter transgenes in mice, we find that the CTCF region plays a very minor role in LCR function. In contrast, we report the in vivo function of two additional downstream elements located in the region of the LCR that supports CTCF-independent enhancer-blocking activity in cell culture. Internal deletion of either of these elements significantly impairs LCR activity. These results reveal that the position-effect suppression region of the TCRalpha LCR harbors an array of CTCF-independent, positive-acting gene regulatory elements, some of which share characteristics with barrier-type insulators. These elements may help manage the separate regulatory programs of the TCRalpha and Dad1 genes.

A role for CD44 in T cell development and function during direct competition between CD44+ and CD44- cells

The role of CD44 in T cell biology remains incompletely understood. Although studies using anti-CD44 antibodies have implicated this cell adhesion molecule in a variety of important T cell processes, few T cell defects have been reported in CD44-deficient mice. We have assessed the requirement for CD44 in T cell development and mature T cell function by analyzing mice in which CD44(-/-) and WT cells were produced simultaneously. In mixed (CD44(-/-) + CD44(+/+)) bone marrow chimeras, production of CD44(-/-) T cells was shown to be reduced compared to WT cells due to inefficient intrathymic development. In addition, mature CD44(-/-) CD8(+) T cells generated a substantially lower response than WT T cells after infection of mice with lymphocytic choriomeningitis virus, with the reduction in response apparent in both lymphoid and non-lymphoid tissues. Overall, these results demonstrate a poor capacity of CD44(-/-) T lineage cells to compete with WT cells at multiple levels, implicating CD44 in normal T cell function.

Selective thymus settling regulated by cytokine and chemokine receptors

To generate T cells throughout adult life, the thymus must import hemopoietic progenitors from the bone marrow via the blood. In this study, we establish that thymus settling is selective. Using nonirradiated recipient mice, we found that hemopoietic stem cells were excluded from the thymus, whereas downstream multipotent progenitors (MPP) and common lymphoid progenitors rapidly generated T cells following i.v. transfer. This cellular specificity correlated with the expression of the chemokine receptor CCR9 by a subset of MPP and common lymphoid progenitors but not hemopoietic stem cells. Furthermore, CCR9 expression was required for efficient thymus settling. Finally, we demonstrate that a prethymic signal through the cytokine receptor fms-like tyrosine kinase receptor-3 was required for the generation of CCR9-expressing early lymphoid progenitors, which were the most efficient progenitors of T cells within the MPP population. We conclude that fms-like tyrosine kinase receptor-3 signaling is required for the generation of T lineage-competent progenitors, which selectively express molecules, including CCR9, that allow them to settle within the thymus.

Comparison of CDR3 length among thymocyte subpopulations: impacts of MHC and BV segment on the CDR3 shortening

Thymocytes are thought to be selected on the basis of antigen specificity between TCR and peptide-MHC (pMHC) ligands. The specificity depends primarily on extensive diversities of complementarity determining region 3 (CDR3), whose specificity is considered to be determined through thymocyte selection. We examined the CDR3 length profiles with 20 BV segments in thymocyte subpopulations from C57BL/6 (H-2(b)), C.B10 (Balb/c congenic, H-2(b)) and Balb/c (H-2(d)) mice. The CDR3 length was shorter in both CD4 single positive (SP) and CD8SP than in double positive (DP), but not altered among DP, double negative (DN) 4 and DN3 subpopulations. The CDR3 shortened more prominently in CD4SP than in CD8SP for C57BL/6 and C.B10, but the shortening was only slight for Balb/c. Although the shortening varied considerably among different BV segments, the greater shortening was observed in most BV segments for CD4SP and in several for CD8SP, in particular, the extent was the greatest in BV1, BV2, BV15, BV16, BV23 and BV26 for CD4SP, and in BV13-1 and BV29 for CD8SP. Moreover, the extent and the pattern of CDR3 shortening were basically the same among highly homologous BV segments (e.g. BV12-1 and 12-2; BV13-1, 13-2 and 13-3). These results taken together indicate that (1) the CDR3 shortening occurred between the DP to the SP stages but never earlier, that (2) there would be the MHC class preference for the CDR3 shortening, that (3) it was in part influenced by MHC haplotype, and finally that (4) the primary structure of particular BV segments would possibly affect the CDR3 length in selected thymocytes. It could be deduced from these results that the CDR3 shortening might play roles in ensuring geometrical disposition of CDRs unique to each BV segment and consequently allow CDRs to intimately interact with pMHC ligands.

Primitive lymphoid progenitors in bone marrow with T lineage reconstituting potential

Multiple subsets of the bone marrow contain T cell precursors, but it remains unclear which is most likely to replenish the adult thymus. Therefore, RAG-1+ early lymphoid progenitors (RAG-1+ ELP), and CD62L/L-selectin+ progenitors (LSP), as well as common lymphoid progenitors from C57BL6-Thy1.1-RAG-1/GFP mouse bone marrow were directly compared in transplantation assays. The two c-Kit(high) populations vigorously regenerated the thymus and were superior to common lymphoid progenitors in magnitude and frequency of thymic reconstitution. Regeneration was much faster than the 22 days described for transplanted stem cells, and RAG-1+ ELP produced small numbers of lymphocytes within 13 days. As previously reported, LSP were biased to a T cell fate, but this was not the case for RAG-1+ ELP. Although RAG-1+ ELP and LSP had reduced myeloid potential, they were both effective progenitors for T lymphocytes and NK cells. The LSP subset overlapped with and included most RAG-1+ ELP and many RAG-1- TdT+ ELP. LSP and RAG-1+ ELP were both present in the peripheral circulation, but RAG-1+ ELP had no exact counterpart among immature thymocytes. The most primitive of thymocytes were similar to Lin- c-Kit(high) L-selectin+ TdT+ RAG-1- progenitors present in the marrow, suggesting that this population is normally important for sustaining the adult thymus.

Alteration of T-cell receptor repertoires during thymic T-cell development

The majority of thymocytes die in the thymus, whereas small populations of T cells that are able to specifically recognize an antigen are considered to survive. Although the thymic selection is thought to have a profound effect on T-cell receptor (TCR) repertoire, little is known how TCR repertoire is formed during the thymocyte developmental process. We examined TCRalpha- and beta-chain variable regions (TCRAV and TCRBV) repertoire in thymic T-cell subpopulations from mice bearing different major histocompatibility (MHC) haplotypes. In Balb/c mice, but not C57BL/6, remarkable alterations of the TCR repertoire were observed in mature T-cell subpopulations as previously reported. In contrast, there were no significant differences of TCRBV repertoire between DN3 (CD25(+)CD44(-)) and DN4 (CD25(-)CD44(-)), and between DN4 and DP. These results suggest that (1) TCR repertoire of mature T cells was formed mainly under the influence of endogenous superantigens, while MHC haplotypes played the least role; (2) the 'beta-selection' process during immature stages had little impact on TCRBV repertoire formation; and (3) TCR repertoire in immature T-cell subpopulations was extremely similar between different strains of mice. We thus consider that pre-selection TCR repertoire in immature T cells could be determined by some genetic factors conserved among different strains.

Phosphoinositide-dependent kinase l (PDK1) haplo-insufficiency inhibits production of alpha/beta (α/β) but not gamma delta (γ/δ) T lymphocytes

In the present study, we have explored the impact of deleting a single allele of PDK1 in T cell progenitors on alpha/beta and gamma/delta T cell development. The data show that deleting a single allele of PDK1 allows differentiation of alpha/beta T cells but prevents their proliferative expansion in the thymus. Accordingly, mice with T cells that are haplo-insufficient for PDK1 have reduced numbers of thymocytes and alpha/beta peripheral T cells. T cell progenitors also give rise to gamma/delta T cells but in contrast to the loss of alpha/beta T cells in T-PDK1 null and haplo-insufficient mice, there were increased numbers of gamma/delta T cells. The production of alpha/beta T cells is dependent on the proliferative expansion of thymocytes and is determined by a balance between the frequency with which cells enter the proliferative phase of the cell cycle and rates of cell death. Herein, we show that PDK1 haplo-insufficient thymocytes have no defects in their ability to enter the cell cycle but show increased apoptosis. PDK1 thus plays a determining role in the development of alpha/beta T lymphocytes but does not limit gamma/delta T cell development.

From stem cell to T cell: one route or many?

T cells developing in the adult thymus ultimately derive from haematopoietic stem cells in the bone marrow. Here, we summarize research into the identity of the haematopoietic progenitors that leave the bone marrow, migrate through the blood and settle in the thymus to generate T cells. Accumulating data indicate that various different bone-marrow progenitors are T-cell-lineage competent and might contribute to intrathymic T-cell development. Such developmental flexibility implies a mechanism of T-cell-lineage commitment that can operate on a range of T-cell-lineage-competent progenitors, and further indicates that only those T-cell-lineage-competent progenitors able to migrate to, and settle in, the thymus should be considered physiological T-cell progenitors.

Journey through the thymus: stromal guides for T-cell development and selection

Lympho-stromal interactions in multiple microenvironments within the thymus have a crucial role in the regulation of T-cell development and selection. Recent studies have implicated that chemokines that are produced by thymic stromal cells have a pivotal role in positioning developing T cells within the thymus. In this Review, I discuss the importance of stroma-derived chemokines in guiding the traffic of developing thymocytes, with an emphasis on the processes of cortex-to-medulla migration and T-cell-repertoire selection, including central tolerance.

Trafficking from the bone marrow to the thymus: a prerequisite for thymopoiesis

T-cell development in the thymus requires periodic importation of hematopoietic progenitors from the bone marrow. Such thymus settling progenitors arise from hematopoietic stem cells (HSCs) that are retained in a specific bone marrow microenvironmental niche. Vacation of this niche is required for HSC proliferation and differentiation into downstream progenitors. In order to reach the thymus, progenitors must then be mobilized from bone marrow to blood. Finally, progenitors in blood must settle in the thymus. Here we review signals and molecular interactions that are likely to play a role in trafficking from the bone marrow to the thymus, focusing on how these interactions may regulate which progenitors physiologically contribute to thymopoiesis.

The TCRalpha locus control region specifies thymic, but not peripheral, patterns of TCRalpha gene expression

The molecular mechanisms ensuring the ordered expression of TCR genes are critical for proper T cell development. The mouse TCR alpha-chain gene locus contains a cis-acting locus control region (LCR) that has been shown to direct integration site-independent, lymphoid organ-specific expression of transgenes in vivo. However, the fine cell type specificity and developmental timing of TCRalpha LCR activity are both still unknown. To address these questions, we established a transgenic reporter model of TCRalpha LCR function that allows for analysis of LCR activity in individual cells by the use of flow cytometry. In this study we report the activation of TCRalpha LCR activity at the CD4-CD8-CD25-CD44- stage of thymocyte development that coincides with the onset of endogenous TCRalpha gene rearrangement and expression. Surprisingly, TCRalpha LCR activity appears to decrease in peripheral T cells where TCRalpha mRNA is normally up-regulated. Furthermore, LCR-linked transgene activity is evident in gammadelta T cells and B cells. These data show that the LCR has all the elements required to reliably reproduce a developmentally correct TCRalpha-like expression pattern during thymic development and unexpectedly indicate that separate gene regulatory mechanisms are acting on the TCRalpha gene in peripheral T cells to ensure its high level and fine cell type-specific expression.

Revision of T cell receptor {alpha} chain genes is required for normal T lymphocyte development

To become mature alphabeta T cells, developing thymocytes must first assemble a T cell receptor (TCR) beta chain gene encoding a TCRbeta chain that forms a pre-TCR. These cells then need to generate a TCRalpha chain gene encoding a TCRalpha chain, which, when paired with the TCRbeta chain, forms a selectable alphabeta TCR. Newly generated VJalpha rearrangements that do not encode TCRalpha chains capable of forming selectable alphabeta TCRs can be excised from the chromosome and replaced with new VJalpha rearrangements. Such replacement occurs through the process of TCRalpha chain gene revision whereby a Valpha gene segment upstream of the VJalpha rearrangement is appended to a downstream Jalpha gene segment. A multistep, gene-targeting approach was used to generate a modified TCRalpha locus (TCRalpha(sJ)) with a limited capacity to undergo revision of TCRalpha chain genes. Thymocytes from mice homozygous for the TCRalpha(sJ) allele are defective in their ability to generate an alphabeta TCR. Furthermore, those thymocytes that do generate an alphabeta TCR have a diminished capacity to be positively selected, and TCRalpha(sJ/sJ) mice have significantly reduced numbers of mature alphabeta T cells. Together, these findings demonstrate that normal T cell development relies on the ability of developing thymocytes to revise their TCRalpha chain genes.

Intrathymic administration of hematopoietic progenitor cells enhances T cell reconstitution in ZAP-70 severe combined immunodeficiency

Patients with severe combined immunodeficiency (SCID) present with opportunistic infections that are almost universally fatal in infancy. The mainstay treatment for these patients is allogeneic hematopoietic stem cell (HSC) transplantation, but sustained polyclonal T cell reconstitution is too often unsatisfactory. Although transplantation is conventionally performed by i.v. administration of HSC, we hypothesized that an intrathymic strategy would be superior. Indeed, several progenitor cell populations are incapable of homing to the thymus, the major site of T cell differentiation, and it appears that there are extensive time periods during which the thymus is refractory to progenitor cell import. To test this hypothesis, nonconditioned infant ZAP-70-deficient SCID mice were intrathymically injected with WT bone marrow progenitor cells, a procedure accomplished without surgical intervention. Upon intrathymic HSC injection, there was a more rapid T cell differentiation, with mature thymocytes detected by 4 weeks after transplantation. Intrathymic injection of HSC also resulted in significantly higher numbers of peripheral T cells, increased percentages of naïve T cells, and more diverse T cell receptor repertoires. Moreover, T cell reconstitution after intrathymic transplantation was obtained after injection of 10-fold fewer donor HSC. Thus, this intrathymic transplantation approach may improve the outcome of SCID patients by enhancing T cell reconstitution.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

A multipotent precursor in the thymus maps to the branching point of the T versus B lineage decision

Hematopoietic precursors continuously colonize the thymus where they give rise mainly to T cells, but also to B and dendritic cells. The lineage relationship between these three cell types is unclear, and it remains to be determined if precursors in the thymus are multipotent, oligopotent, or lineage restricted. Resolution of this question necessitates the determination of the clonal differentiation potential of the most immature precursors in the thymus. Using a CC chemokine receptor 9–enhanced green fluorescent protein knock-in allele like a surface marker of unknown function, we identify a multipotent precursor present in bone marrow, blood, and thymus. Single cells of this precursor give rise to T, B, and dendritic cells. A more differentiated stage of this multipotent precursor in the thymus has lost the capacity to generate B but not T, dendritic, and myeloid cells. Thus, the newly identified precursor maps to the branching point of the T versus B lineage decision in the hematopoietic lineage hierarchy.

Ikaros induces quiescence and T-cell differentiation in a leukemia cell line

Ikaros is a hematopoietic cell-specific zinc finger DNA binding protein that plays an important role in lymphocyte development. Genetic disruption of Ikaros results in T-cell transformation. Ikaros null mice develop leukemia with 100% penetrance. It has been hypothesized that Ikaros controls gene expression through its association with chromatin remodeling complexes. The development of leukemia in Ikaros null mice suggests that Ikaros has the characteristics of a tumor suppressor gene. In this report, we show that the introduction of Ikaros into an established mouse Ikaros null T leukemia cell line leads to growth arrest at the G0/G1 stage of the cell cycle. This arrest is associated with up-regulation of the cell cycle-dependent kinase inhibitor p27kip1, the induction of expression of T-cell differentiation markers, and a global and specific increase in histone H3 acetylation status. These studies provide strong evidence that Ikaros possesses the properties of a bona fide tumor suppressor gene for the T-cell lineage and offer insight into the mechanism of Ikaros's tumor suppressive activity.

Dynamic regulation of IL-7 receptor expression is required for normal thymopoiesis

Interleukin-7 receptor (IL-7R) levels are tightly controlled during ontogeny: high on double-negative (DN) cells, absent on double-positive (DP) cells, and high once again on thymocytes undergoing positive selection. To determine if loss of IL-7–mediated survival signals in DP cells is necessary for normal antigen-specific selection, we created T-lineage–specific IL-7R α chain (IL-7Rα) transgenic (Tg) mice in which IL-7R is expressed throughout ontogeny. There was no effect of the IL-7Rα Tg on negative selection. Surprisingly, however, although the thymi of IL-7Rα Tg mice were comparable at birth, there was a decrease in thymocyte number as the mice aged. This was found to be due to competition between DN and IL-7R–expressing DP cells for endogenous IL-7, which resulted in decreased levels of Bcl-2 in DN cells, increased DN apoptosis, and decreased DN cell number. Therefore, the down-regulation of IL-7R on DP cells is an “altruistic” act required for maintaining an adequate supply of local IL-7 for DN cells.

Developmental Activation of the TCR α Enhancer Requires Functional Collaboration among Proteins Bound Inside and Outside the Core Enhancer

The TCR delta enhancer (Edelta) and TCR alpha enhancer (Ealpha) play critical roles in the temporal and lineage-specific control of V(D)J recombination and transcription at the TCR alphadelta locus, working as a developmental switch controlling a transition from TCR delta to TCR alpha activity during thymocyte development. Previous experiments using a transgenic reporter substrate revealed that substitution of the 116-bp minimal Ealpha, denoted Talpha1-Talpha2, for the entire 1.4-kb Ealpha led to a premature activation of V(D)J recombination. This suggested that binding sites outside of Talpha1-Talpha2 are critical for the strict developmental regulation of TCR alpha rearrangement. We have further analyzed Ealpha to better understand the mechanisms responsible for appropriate developmental regulation in vivo. We found that a 275-bp Ealpha fragment, denoted Talpha1-Talpha4, contains all binding sites required for proper developmental regulation in vivo. This suggests that developmentally appropriate enhancer activation results from a functional interaction between factors bound to Talpha1-Talpha2 and Talpha3-Talpha4. In support of this, EMSAs reveal the formation of a large enhanceosome complex that reflects the cooperative assembly of proteins bound to both Talpha1-Talpha2 and Talpha3-Talpha4. Our data suggest that enhanceosome assembly is critical for developmentally appropriate activation of Ealpha in vivo, and that transcription factors, Sp1 and pCREB, may play unique roles in this process.

Analysis of transcription factor expression during discrete stages of postnatal thymocyte differentiation

Postnatal T lymphocyte differentiation in the thymus is a multistage process involving serial waves of lineage specification, proliferative expansion, and survival/cell death decisions. Although these are believed to originate from signals derived from various thymic stromal cells, the ultimate consequence of these signals is to induce the transcriptional changes that are definitive of each step. To help to characterize this process, high density microarrays were used to analyze transcription factor gene expression in RNA derived from progenitors at each stage of T lymphopoietic differentiation, and the results were validated by a number of appropriate methods. We find a large number of transcription factors to be expressed in developing T lymphocytes, including many with known roles in the control of differentiation, proliferation, or cell survival/death decisions in other cell types. Some of these are expressed throughout the developmental process, whereas others change substantially at specific developmental transitions. The latter are particularly interesting, because stage-specific changes make it increasingly likely that the corresponding transcription factors may be involved in stage-specific processes. Overall, the data presented here represent a large resource for gene discovery and for confirmation of results obtained through other methods.

Heterogeneity among DN1 prothymocytes reveals multiple progenitors with different capacities to generate T cell and non-T cell lineages

The nature of early T lineage progenitors in the thymus or bone marrow remains controversial. Here we assess lineage capacity and proliferative potential among five distinct components of the earliest intrathymic stage (DN1, CD25(-)44(+)). All of these express one or more hemato-lymphoid lineage markers. All can produce T lineage cells, but only two of them display kinetics of differentiation, proliferative capacity, and other traits consistent with being canonical T progenitors. The latter also appeared limited to producing cells of the T or NK lineages, while B lineage potential derived mainly from the other, less typical T progenitors. In addition to precisely defining canonical early progenitors in the thymus, this work reconciles conflicting results from numerous groups by showing that multiple progenitors with a DN1 phenotype home to the thymus and make T cells, but possess different proliferative potentials and lineage capacities.

Detailed analysis of gene expression during development of T cell lineages in the thymus

The genetic mechanisms that promote lineage commitment and eliminate autoreactive cells in the thymus are not well understood. To better understand this process, we have identified and quantitated transcripts in the two major thymocyte lineages by using serial analysis of gene expression. Approximately 25 genes displayed almost complete segregation to one or the other T cell lineage. Commitment to the CD4 lineage was marked by up-regulation of genes associated with increased survival and chaperone function followed by expression of genes that regulate nucleosome remodeling and T cell receptor signaling. Differentiation within the CD8 lineage, on the other hand, was marked by up-regulation of genes that regulate lymphocyte homing, followed by quenching of genes that inhibit apoptosis. Definition of differential gene expression during development of the two major thymocyte lineages will allow insight into mechanisms of T cell development after positive and negative selection.

A dose effect of IL-7 on thymocyte development

To study interleukin-7 (IL-7) in early thymocyte development, we generated mice transgenic (Tg) for the IL-7 gene under control of the lck proximal promoter. Founder line TgA, with the lowest level of IL-7 overexpression, showed enhanced alphabeta T-cell development. In contrast, in the highest overexpressing founder line, TgB, alphabeta T-cell development was disturbed with a block at the earliest intrathymic precursor stage. This was due to decreased progenitor proliferation as assessed by Ki-67 staining and in vivo bromodeoxyuridine (BrdU) incorporation. Bcl-2 was up-regulated in T-cell-committed progenitors in all Tg lines, and accounted for greater numbers of double positive (DP), CD4 single positive (SP), and CD8SP thymocytes in TgA mice where, in contrast to TgB mice, thymocyte progenitor proliferation was normal. Mixed marrow chimeras using TgB(+) and congenic mice as donors, and experiments using anti-IL-7 monoclonal antibody (MAb) in vivo, confirmed the role of IL-7 protein in the observed TgB phenotype. In conclusion, at low Tg overexpression, IL-7 enhanced alphabeta T-cell development by increasing thymocyte progenitor survival, while at high overexpression IL-7 reduces their proliferation, inducing a dramatic block in DP production. These results show for the first time in vivo a dose effect of IL-7 on alphabeta T-cell development and have implications for IL-7 in the clinical setting.

Lymphocyte development in neonatal and adult c-Kit-deficient (c-KitW/W) mice

Hematopoietic stem cells and lymphocyte progenitors express the receptor tyrosine kinase c-Kit. In fetal and neonatal life, c-Kit plays a redundant role in T, and no apparant role in B cell development. In neonatal mice deficient for both c-Kit and the common gamma chain (gammac), a component of the interleukin-7 (IL-7) receptor, the thymus is alymphoid, and therefore lacks T cell receptor (TCR) beta, gamma, and delta rearrangements. Thus, a critical role for c-Kit in T cell development around birth is well established. More recently, it has become possible to examine the impact of c-Kit deficiency under conditions of steady state lymphopoiesis in adult life. Such analysis has been made possible by the identification of a viable adult c-Kit-deficient (c-KitW/W) variant, termed the Vickid mouse. The Vickid mouse arose by outcrossing c-KitW-bearing mice of the WB strain, in which lack of c-Kit is lethal, to a mixed genetic background. In adult Vickid mice, mainstream alphabeta TCR+ thymocyte development, and B cell development in the bone marrow are severely c-Kit-dependent with progressive age. Analysis of other pathways of developing T cells, i.e. CD4-CD8- (double neagative [DN]) alphabeta TCR+ and DN gammadelta TCR+ thymocytes revealed that the development of both lineages is also severely affected by lack of c-Kit. However, numbers of gammadelta TCR+ T cells decline before numbers of alphabeta TCR+ T cells in the thymus. In contrast to T and B cell development, generation of NK cells is not affected in adult c-KitW/W mice.

A positive look at double-negative thymocytes

In some respects, our understanding of the cellular and molecular aspects of early T-cell differentiation is lagging behind that of B cells. Papers describing gene-knockout and reporter-transgenic mice in which thymocyte development is affected are often difficult to interpret. Progress in this field will be hampered unless a more detailed phenotypic and molecular analysis of progenitor thymocytes at the single-cell level is carried out.

Interleukin-7 inhibits pre-T-cell differentiation induced by the pre-T-cell receptor signal and the effect is mimicked by hGM-CSF in hGM-CSF receptor transgenic mice

We have previously reported that human granulocyte-macrophage colony-stimulating factor (hGM-CSF) causes a stage-specific inhibition of T-cell receptor (TCR) alphabeta cell development in the thymus of transgenic mice constitutively expressing the hGM-CSF receptor. Since it has been reported that the addition of interleukin-7 (IL-7) to fetal thymic organ culture (FTOC) has similar effects, we compared the effects of IL-7 and hGM-CSF on TCR(alphabeta) cell development in hGM-CSF receptor transgenic mice. We reconstituted fetal lobes with sorted pre-T, or post pre-T CD4(-)CD8(-) precursor cells. The addition of either IL-7 or hGM-CSF to these cultures suppressed further differentiation of pre-T cells but not post pre-T cells. At the same time, the cell number was increased, suggesting that pre-T-cell proliferation is stimulated by these cytokines. Furthermore, the differentiation of recombination-activating gene-1 (RAG-1)-deficient pre-T cells in response to anti-CD3 antibody stimulation was suppressed by either IL-7 or hGM-CSF, suggesting that these cytokines inhibit the pre-T-cell receptor (pre-TCR) signal. This inhibition is unexpected because the pre-TCR signal and the IL-7 signal have previously been considered to be co-operative. Recent analysis of the downstream events of IL-7 receptor and GM-CSF receptor revealed that they share common signal transduction molecules. Our results show that IL-7 is able to promote pre-T cell proliferation and to suppress differentiation induced by the pre-TCR signal. GM-CSF can mimic these biological activities of IL-7 when the pre-T cells express GM-CSF receptors. Our data suggest that both timing and level of activation of the IL-7 signalling pathway must be precisely regulated to facilitate the differentiation of thymocytes.

A role for CD147 in thymic development

We have previously identified a mAb that binds to a molecule expressed preferentially on the surface of cycling thymocytes. In this study the molecule recognized by this mAb has been identified in the mouse as CD147 (basigin) by expression cloning. We show that CD147 expression correlates with cycling of immature thymocytes even in the absence of TCRbeta selection and that ligation of this molecule on immature fetal thymocytes inhibits their further development into mature T cells.

CD8+ T cells induce medullary thymic epithelium and CD4+CD8+CD25+ TCRbeta- thymocytes in SCID mice

During T-cell development the transition in the thymus of CD4-CD8- double negative (DN) progenitor T cells into CD4+CD8+ double positive (DP) cells is dependent on the expression of a T-cell receptor (TCR)-beta-chain protein. In this study purified peripheral CD4+ and CD8+ T lymphocytes from the C.B-17 strain of mice were adoptively transferred into syngeneic, neonatal SCID mice, where donor cells resided at constant numbers in thymus from 2 weeks until 10 weeks post cell transfer. In the recipient thymus the CD8+ donor cells outnumbered the CD4+ cells by a factor of three to five and both subsets contained a large fraction of activated cells. During the late phase of treatment, CD8+ T cells induced high numbers of DP thymocytes in the SCID mice, a process accompanied by the maturation of medullary epithelial cells. Such thymic development in the SCID mouse was inhibited by coresiding CD4+ donor T cells. These results indicate a regulatory role by mature peripheral T cells on medullary epithelial growth and thymocyte development in the treated SCID mice.

Limited effect of chromatin remodeling on D(beta)-to-J(beta) recombination in CD4+CD8+ thymocyte: implications for a new aspect in the regulation of TCR beta gene recombination

We have generated mutant mice in which TCR beta chain enhancer (E(beta)) was replaced with the TCR alpha chain enhancer (E(alpha)). Using this mouse model, we analyzed (i) recombination status of the TCR beta chain genes after functional V(D)J rearrangements occurred in the first allele during double-negative (DN)-to-double-positive (DP) transition and (ii) involvement of E(beta) for the expression of rearranged TCR beta chain genes. Our data show that E(alpha) substituted for E(beta) function to express a similar extent of TCR beta chains exactly at the same time as did E(beta) (CD25+CD44- DN stage), although the proportion of TCR beta+ cells at this stage was low in mutant mice. At the DP stage, germline transcription and histone acetylation of D(beta)-J(beta) loci were detectable at a high degree in both mutant and wild-type mice. However, DP cells in mutant mice retained the germline D(beta)-J(beta) configuration at a higher frequency than that of wild-type mice, whereas both DP cells expressed TCR beta chains to a similar extent. These data suggest that chromatin opening has a limited impact on D(beta)-to-J(beta) recombination at the DP stage and that E(alpha) is functionally equivalent to E(beta) in promoting expression of functionally rearranged TCR beta chain genes through DN-to-DP transition.

TCRA gene rearrangement in immature thymocytes in absence of CD3, pre-TCR, and TCR signaling

During thymocyte differentiation, TCRA genes are massively rearranged only after productively rearranged TCRB genes are expressed in association with pTalpha and CD3 complex molecules within a pre-TCR. Signaling from the pre-TCR via the CD3 complex is thought to be required to promote TCRA gene accessibility and recombination. However, alphabeta(+) thymocytes do develop in pTalpha-deficient mice, showing that TCRalpha-chain genes are rearranged, either in CD4(-)CD8(-) or CD4(+)CD8(+) thymocytes, in the absence of pre-TCR expression. In this study, we analyzed the TCRA gene recombination status of early immature thymocytes in mutant mice with arrested thymocyte development, deficient for either CD3 or pTalpha and gammac expression. ADV genes belonging to different families were found rearranged to multiple AJ segments in both cases. Thus, TCRA gene rearrangement is independent of CD3 and gammac signaling. However, CD3 expression was found to play a role in transcription of rearranged TCRalpha-chain genes in CD4(-)CD8(-) thymocytes. Taken together, these results provide new insights into the molecular control of early T cell differentiation.

Mapping precursor movement through the postnatal thymus reveals specific microenvironments supporting defined stages of early lymphoid development

Cellular differentiation is a complex process involving integrated signals for lineage specification, proliferation, endowment of functional capacity, and survival or cell death. During embryogenesis, spatially discrete environments regulating these processes are established during the growth of tissue mass, a process that also results in temporal separation of developmental events. In tissues that undergo steady-state postnatal differentiation, another means for inducing spatial and temporal separation of developmental cues must be established. Here we show that in the postnatal thymus, this is achieved by inducing blood-borne precursors to enter the organ in a narrow region of the perimedullary cortex, followed by outward migration across the cortex before accumulation in the subcapsular zone. Notably, blood precursors do not transmigrate the cortex in an undifferentiated state, but rather undergo progressive developmental changes during this process, such that defined precursor stages appear in distinct cortical regions. Identification of these cortical regions, together with existing knowledge regarding the genetic potential of the corresponding lymphoid precursors, sets operational boundaries for stromal environments that are likely to induce these differentiative events. We conclude that active cell migration between morphologically similar but functionally distinct stromal regions is an integral component regulating differentiation and homeostasis in the steady-state thymus.

UEA-I-binding to thymic medullary epithelial cells selectively reduces numbers of cortical TCRalphabeta+ thymocytes in FTOCs

Thymic medullary epithelial cells (TMECs) constitute a major stromal cell type, the function of which is incompletely understood. Some TMECs express L-fucose-glycosylated proteins on their plasma membrane; these have been shown to specifically bind the lectin UEA-I. We exploited this observation to investigate the consequences of in situ blockage of TMECs in FTOCs by UEA-I. In UEA-I-treated FTOCs, we noted a decreased cellularity among TCRalphabeta+ but not TCRgammadelta+ cells. In fact, CD3- and CD3lo cortical cells were markedly depleted, while CD3hi cells were unaffected. Since the affected cell subsets are in a different compartment from that where UEA-I binding occurs, it is likely that the effect is mediated through a soluble factor. Two possible mechanisms are proposed: a reduced activation of either TMECs or of medullary thymocytes which normally bind to them, results in lowered production of soluble factors responsible for cortical thymocyte proliferation. Alternately, the binding of UEA-I to TMECs could activate the latter to produce signals inhibitory to cortical thymocytes.

Premature TCR alpha beta expression and signaling in early thymocytes impair thymocyte expansion and partially block their development

In thymocyte ontogeny, Tcr-a genes rearrange after Tcr-b genes. TCR alpha beta transgenic (Tg) mice have no such delay, consequently expressing rearranged TCR alpha beta proteins early in the ontogeny. Such mice exhibit reduced thymic cellularity and accumulate mature, nonprecursor TCR(+)CD8(-)4(-) thymocytes, believed to be caused by premature Tg TCR alpha beta expression via unknown mechanism(s). Here, we show that premature expression of TCR alpha beta on early thymocytes curtails thymocyte expansion and impairs the CD8(-)4(-) --> CD8(+)4(+) transition. This effect is accomplished by two distinct mechanisms. First, the early formation of TCR alpha beta appears to impair the formation and function of pre-TCR, consistent with recently published results. Second, the premature TCR alpha beta contact with intrathymic MHC molecules further pronounces the block in proliferation and differentiation. These results suggest that the benefit of asynchronous Tcr-a and Tcr-b rearrangement is not only to minimize waste during thymopoiesis, but also to simultaneously allow proper expression/function of the pre-TCR and to shield CD8(-)4(-) thymocytes from TCR alpha beta signals that impair thymocyte proliferation and CD8(-)4(-) --> CD8(+)4(+) transition.

TNF regulates thymocyte production by apoptosis and proliferation of the triple negative (CD3-CD4-CD8-) subset

TNF is a proinflammatory cytokine with opposing death/no-death effects in vivo and in vitro. Our studies showed that TNF regulates mouse thymocyte production, inducing both apoptosis and proliferation of the most immature CD3(-)CD4(-)CD8(-) triple negative (TN) subset within a broad range of dosages (10(1)-10(5) pg/ml) in the presence of IL-7. TNF apoptosis affected only the TN3 (CD44(-)CD25(+)) and TN4 (CD44(-)CD25(-)) subsets that expressed both TNFR-p55 and -p75. Although each TNFR alone could mediate TNF apoptosis, maximal apoptosis was seen in C57BL/6J wild type, which expressed both TNFRs. TNF also induced proliferation of TN3 cells at higher doses (10(4)-10(5) pg/ml) mediated only by TNFR-p75. Both anti-TNFR-p55 and -TNFR-p75 mAb inhibited apoptosis but only anti-p75 inhibited proliferation. TNF also regulated TN proliferation to IL-7 because TNFR knockout (KO), TNF KO, and TNF/lymphotoxin alpha and beta triple KO mice showed 2- to 3-fold increased responses not seen in C57BL/6J wild type. In vivo, TNFR KO mice showed thymic hypertrophy with a 60% increase in total thymocytes, with no effect on the CD4/CD8 subsets. We conclude that TNF maintains homeostatic control of total thymocyte production by negative selection of TN3 and TN4 prothymocytes and down-regulation of their proliferation to endogenous IL-7.

Metabolites from apoptotic thymocytes inhibit thymopoiesis in adenosine deaminase-deficient fetal thymic organ cultures

Murine fetal thymic organ culture was used to investigate the mechanism by which adenosine deaminase (ADA) deficiency causes T-cell immunodeficiency. C57BL/6 fetal thymuses treated with the specific ADA inhibitor 2'-deoxycoformycin exhibited features of the human disease, including accumulation of dATP and inhibition of S-adenosylhomocysteine hydrolase enzyme activity. Although T-cell receptor (TCR) Vbeta gene rearrangements and pre-TCR-alpha expression were normal in ADA-deficient cultures, the production of alphabeta TCR(+) thymocytes was inhibited by 95%, and differentiation was blocked beginning at the time of beta selection. In contrast, the production of gammadelta TCR(+) thymocytes was unaffected. Similar results were obtained using fetal thymuses from ADA gene-targeted mice. Differentiation and proliferation were preserved by the introduction of a bcl-2 transgene or disruption of the gene encoding apoptotic protease activating factor-1. The pan-caspase inhibitor carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone also significantly lessened the effects of ADA deficiency and prevented the accumulation of dATP. Thus, ADA substrates accumulate and disrupt thymocyte development in ADA deficiency. These substrates derive from thymocytes that undergo apoptosis as a consequence of failing to pass developmental checkpoints, such as beta selection.

Cell migration and the anatomic control of thymocyte precursor differentiation

The thymus performs several essential functions during the steady-state production of T lymphocytes in adults, including expansion of the precursor pool, differentiation into multiple lineages and screening for TCRs with restricted specificities. Other than those functions attributed to the TCR, most of the factors that control these processes remain undefined. One potential mechanism for such control may be related to the movement of precursor cells between distinct anatomical compartments in the thymus. Histological studies show that the majority of CD4- CD8- cells are found in the subcapsular region. However; vascular tissues that support the migration of precursor cells into the thymus (postcapillary venules) are located deep in the tissue, near the cortico-medullary junction. This implies that blood-borne cells entering the thymus must transit outward across the cortex in order to accumulate in the SCR. Differentiation of DN cells into the CD4+ 8+ stage correlates with a reversal in polarity and migration inward, while mature cells ultimately transit the CMJ in the opposite direction of cells first entering the organ. Here we review evidence for a model in which differentiation is induced and proliferation is controlled by this progressive translocation of immature precursors through discrete stromal compartments. In addition, we attempt to summarize what is known about the molecular mechanisms that may support polarized migration of early CD4- 8- thymocytes in the adult, as well as how and where the relevant differentiative and/or proliferative signals may be compartmentalized.