Regulation of thymocyte development from immature progenitors

IL-15 Prevents the Development of T-ALL from Aberrant Thymocytes with Impaired DNA Repair Functions and Increased NOTCH1 Activation

We previously reported that NOD.Scid mice lacking interleukin-15 (IL-15), or IL-15 receptor alpha-chain, develop T-acute lymphoblastic leukemia (T-ALL). To understand the mechanisms by which IL-15 signaling controls T-ALL development, we studied the thymocyte developmental events in IL-15-deficient Scid mice from NOD and C57BL/6 genetic backgrounds. Both kinds of mice develop T-ALL characterized by circulating TCR-negative cells expressing CD4, CD8 or both. Analyses of thymocytes in NOD.Scid.Il15^-/- mice prior to T-ALL development revealed discernible changes within the CD4^-CD8^- double-negative (DN) thymocyte developmental stages and increased frequencies of CD4⁺CD8⁺ double-positive cells with a high proportion of TCR-negative CD4⁺ and CD8⁺ cells. The DN cells also showed elevated expressions of CXCR4 and CD117, molecules implicated in the expansion of DN thymocytes. T-ALL cell lines and primary leukemic cells from IL-15-deficient NOD.Scid and C57BL/6.Scid mice displayed increased NOTCH1 activation that was inhibited by NOTCH1 inhibitors and blockers of the PI3K/AKT pathway. Primary leukemic cells from NOD.Scid.Il15^-/- mice survived and expanded when cultured with MS5 thymic stromal cells expressing Delta-like ligand 4 and supplemented with IL-7 and FLT3 ligand. These findings suggest that IL-15 signaling in the thymus controls T-ALL development from aberrant thymocytes with an impaired DNA repair capacity and increased NOTCH1 activation.

N-Glycan Branching Is Required for Development of Mature B Cells

Galectins have been implicated in inhibiting BCR signaling in mature B cells but promoting pre-BCR signaling during early development. Galectins bind to branched N-glycans attached to cell surface glycoproteins to control the distribution, clustering, endocytosis, and signaling of surface glycoproteins. During T cell development, N-glycan branching is required for positive selection of thymocytes, inhibiting both death by neglect and negative selection via enhanced surface retention of the CD4/CD8 coreceptors and limiting TCR clustering/signaling, respectively. The role of N-glycan branching in B cell development is unknown. In this study, we report that N-glycan branching is absolutely required for development of mature B cells in mice. Elimination of branched N-glycans in developing B cells via targeted deletion of N-acetylglucosaminyl transferase I (Mgat1) markedly reduced cellularity in the bone marrow and/or spleen and inhibited maturation of pre-, immature, and transitional stage 2 B cells. Branching deficiency markedly reduced surface expression of the pre-BCR/BCR coreceptor CD19 and promoted spontaneous death of pre-B cells and immature B cells in vitro. Death was rescued by low-dose pre-BCR/BCR stimulation but exacerbated by high-dose pre-BCR/BCR stimulation as well as antiapoptotic Bcl_xL overexpression in pre-B cells. Branching deficiency also enhanced Nur77 induction, a marker of negative selection. Together, these data suggest that, as in T cells, N-glycan branching promotes positive selection of B cells by augmenting pre-BCR/BCR signaling via CD19 surface retention, whereas limiting negative selection from excessive BCR engagement.

Plasma membrane sphingomyelin modulates thymocyte development by inhibiting TCR-induced apoptosis

Sphingomyelin (SM) in combination with cholesterol forms specialized membrane lipid microdomains in which specific receptors and signaling molecules are localized or recruited to mediate intracellular signaling. SM-microdomain levels in mouse thymus were low in the early CD4+CD8+ double-positive (DP) stage prior to thymic selection and increased >10-fold during late selection. T-cell receptor (TCR) signal strength is a key factor determining whether DP thymocytes undergo positive or negative selection. We examined the role of SM-microdomains in thymocyte development and related TCR signaling, using SM synthase 1 (SMS1)-deficient (SMS1-/-) mice which display low SM expression in all thymocyte populations. SMS1 deficiency caused reduced cell numbers after late DP stages in TCR transgenic models. TCR-dependent apoptosis induced by anti-CD3 treatment was enhanced in SMS1-/- DP thymocytes both in vivo and in vitro. SMS1-/- DP thymocytes, relative to controls, showed increased phosphorylation of TCR-proximal kinase ZAP-70 and increased expression of Bim and Nur77 proteins involved in negative selection following TCR stimulation. Addition of SM to cultured normal DP thymocytes led to greatly increased surface expression of SM-microdomains, with associated reduction of TCR signaling and TCR-induced apoptosis. Our findings indicate that SM-microdomains are increased in late DP stages, function as negative regulators of TCR signaling and modulate the efficiency of TCR-proximal signaling to promote thymic selection events leading to subsequent developmental stages.

Alteration of Bcl11b upon stimulation of both the MAP kinase- and Gsk3-dependent signaling pathways in double-negative thymocytes

B-cell lymphoma/leukemia 11B (Bcl11b) is a transcription factor critical for thymocyte development. We have previously characterized the kinetic post-translational modifications (PTMs) of Bcl11b in double-positive (DP) thymocytes during stimulation of the T cell receptor-activated MAP kinase pathway. However, the PTMs of Bcl11b in thymocytes from other developmental stages in the thymus, primarily double-negative (DN) cells, have not been previously identified. We found that kinetic modifications of Bcl11b in DN cells are somewhat different than the patterns observed in DP cells. Distinct from DP thymocytes, phosphorylation and sumoylation of Bcl11b in DN cells were not oppositely regulated in response to activation of MAP kinase, even though hyper-phosphorylation of Bcl11b coincided with near complete desumoylation. Additionally, prolonged stimulation of the MAP kinase pathway in DN cells, unlike DP thymocytes, did not alter Bcl11b levels of sumoylation or ubiquitinylation, or stability. On the other hand, activation of Wnt-Gsk3-dependent signaling in DN cells resulted in composite dephosphorylation and sumoylation of Bcl11b. Moreover, stimulation of MAP kinase and (or) Wnt signaling pathways differentially affects gene expression of some Bcl11b target and maturation-associated genes. Defining the signaling pathways and regulation of sequence-specific transcription factors by PTMs at various stages of thymopoiesis may improve our understanding of leukemogenesis.

Role of WW Domain-containing Oxidoreductase WWOX in Driving T Cell Acute Lymphoblastic Leukemia Maturation

Whether tumor suppressor WWOX (WW domain-containing oxidoreductase) stimulates immune cell maturation is largely unknown. Here, we determined that Tyr-33-phosphorylated WWOX physically binds non-phosphorylated ERK and IκBα in immature acute lymphoblastic leukemia MOLT-4 T cells and in the naïve mouse spleen. The IκBα·ERK·WWOX complex was shown to localize, in part, in the mitochondria. WWOX prevents IκBα from proteasomal degradation. Upon stimulating MOLT-4 with ionophore A23187/phorbol myristate acetate, endogenous IκBα and ERK undergo rapid phosphorylation in <5 min, and subsequently WWOX is Tyr-33 and Tyr-287 de-phosphorylated and Ser-14 phosphorylated. Three hours later, IκBα starts to degrade, and ERK returns to basal or non-phosphorylation, and this lasts for the next 12 h. Finally, expression of CD3 and CD8 occurs in MOLT-4 along with reappearance of the IκBα·ERK·WWOX complex near 24 h. Inhibition of ERK phosphorylation by U0126 or IκBα degradation by MG132 prevents MOLT-4 maturation. By time-lapse FRET microscopy, IκBα·ERK·WWOX complex exhibits an increased binding strength by 1-2-fold after exposure to ionophore A23187/phorbol myristate acetate for 15-24 h. Meanwhile, a portion of ERK and WWOX relocates to the nucleus, suggesting their role in the induction of CD3 and CD8 expression in MOLT-4.

Effects of catecholamines on thymocyte apoptosis and proliferation depend on thymocyte microenvironment

The present study, through quantification of tyrosine hydroxylase (TH) expression and catecholamine (CA) content in the presence and in the absence of α-methyl-p-tyrosine (AMPT), a TH inhibitor, in adult thymic organ (ATOC) and thymocyte culture, demonstrated that thymic cells produce CAs. In addition, in ATOC an increase in β2-adrenoceptor (AR) mRNA expression and β2-AR thymocyte surface density was registered. Furthermore, AMPT (10(-4)M), as propranolol (10(-4)M), augmented thymocyte apoptosis and diminished thymocyte proliferation in ATOC. Propranolol exerted these effects acting on CD3(high) thymocytes. However, in thymocyte cultures, propranolol (10(-6)M) acting on the same thymocyte subset exerted the opposing effect on thymocyte apoptosis and ConA-stimulated proliferation. This suggested that, depending on thymocyte microenvironment, differential effects can be induced through the same type of AR. Additionally, arterenol (10(-8) to 10(-6)M), similar to propranolol, diminished apoptosis, but increased ConA-stimulated thymocyte proliferation in thymocyte culture. However, differently from propranolol, arterenol affected manly CD3- thymocyte subset, which harbors majority of α1-AR+thymocytes. Additionally, arterenol showed a dose-dependent decrease in efficiency of thymocyte apoptosis and proliferation modulation with the rise in its concentration. Considering greater affinity of arterenol for α1-ARs than for β2-ARs, the previous findings could be attributable to increased engagement of β2-ARs with the rise of arterenol concentration. Consistently, in the presence of propranolol (10(-6)M), a β-AR blocker, the arterenol (10(-8)M) effects on thymocytes were augmented. In conclusion, thymic endogenous CAs, acting through distinct AR types and, possible, the same AR type (but in different cell microenvironment) may exert the opposing effects on thymocyte apoptosis/proliferation.

Genetic basis of altered central tolerance and autoimmune diseases: a lesson from AIRE mutations

The thymus is a specialized organ that provides an inductive environment for the development of T cells from multipotent hematopoietic progenitors. Self-nonself discrimination plays a key role in inducing a productive immunity and in preventing autoimmune reactions. Tolerance represents a state of immunologic nonresponsiveness in the presence of a particular antigen. The immune system becomes tolerant to self-antigens through the two main processes, central and peripheral tolerance. Central tolerance takes place within the thymus and represents the mechanism by which T cells binding with high avidity self-antigens, which are potentially autoreactive, are eliminated through so-called negative selection. This process is mostly mediated by medullary thymic epithelia cells (mTECs) and medullary dendritic cells (DCs). A remarkable event in the process is the expression of tissue-specific antigens (TSA) by mTECs driven by the transcription factor autoimmune regulator (AIRE). Mutations in this gene result in autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), a rare autosomal recessive disease (OMIM 240300). Thus far, this syndrome is the paradigm of a genetically determined failure of central tolerance and autoimmunty. Patients with APECED have a variable pattern of autoimmune reactions, involving different endocrine and nonendocrine organs. However, although APECED is a monogenic disorder, it is characterized by a wide variability of the clinical expression, thus implying a further role for disease-modifying genes and environmental factors in the pathogenesis. Studies on this polyreactive autoimmune syndrome contributed enormously to unraveling several issues of the molecular basis of autoimmunity. This review focuses on the developmental, functional, and molecular events governing central tolerance and on the clinical implication of its failure.

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.

Direct comparison of Dll1- and Dll4-mediated Notch activation levels shows differential lymphomyeloid lineage commitment outcomes

In the thymus, Notch signaling is essential for T lymphopoiesis, with Delta-like (Dll)4 uniquely involved in this process. However, using cocultures, either Dll4 or Dll1 were shown to support T lymphopoiesis. To address which Dll is more effective at inducing hematopoietic progenitor cells to give rise to T lineage cells in vitro, we generated OP9 cells expressing a series of incrementally discrete and equivalent levels of Dll1 or Dll4. In keeping with previous findings, OP9 cells expressing high levels of either Dll1 or Dll4 gave rise to T lineage cells with similar efficacy, and prevented the differentiation of B and myeloid-lineage cells. However, at limiting levels, Dll4 maintained its ability to inhibit B lineage choice and induce T lineage commitment and differentiation at lower levels than Dll1. This manifest property of Dll4 is evident despite lower levels of steady-state surface expression than Dll1 on OP9 cells. The heightened effectiveness of Dll4 over Dll1 also corresponded to the induction of Notch target genes, and inhibition of B and myeloid-specific transcription factors. Furthermore, we show that OP9 cells expressing levels of Dll4 equivalent to those present in thymic epithelial cells, as expected, gave rise to T lineage cells, but were also permissive for the differentiation of myeloid cells; whereas, still inhibiting B lymphopoiesis. Our findings show that Dll4 expressed at physiological levels on OP9 cells is functionally distinct from similarly expressed levels of Dll1, illustrating the unique properties of Dll4 in supporting the combined T lineage and specific myeloid-lineage outcomes that underpin its function within the thymus.

Peptide-specific, TCR-alpha-driven, coreceptor-independent negative selection in TCR alpha-chain transgenic mice

As thymocytes differentiate, Ag sensitivity declines, with immature CD4-CD8- double-negative (DN) cells being most susceptible to TCR signaling events. We show that expression of alphabetaTCR from the DN3 stage lowers the threshold for activation, allowing recognition of MHC peptides independently of the TCR beta-chain and without either T cell coreceptor. The MHC class I-restricted C6 TCR recognizes the Y-chromosome-derived Ag HYK(k)Smcy. Positive selection in C6 alphabetaTCR females is skewed to the CD8 compartment, whereas transgenic male mice exhibit early clonal deletion of thymocytes. We investigated the effect of the HYK(k)Smcy complex on developing thymocytes expressing the C6 TCR alpha-chain on a TCR-alpha(-/-) background. On the original selecting haplotype, the skew to the CD8 lineage is preserved. This is MHC dependent, as the normal bias to the CD4 subset is seen on an H2b background. In male H2k C6 alpha-only mice, the presence of the HYK(k)Smcy complex leads to a substantial deletion of thymocytes from the DN subset. This phenotype is replicated in H2k C6 alpha-only female mice expressing an Smcy transgene. Deletion is not dependent on the beta variable segment of the C6 TCR or on a restricted TCR-beta repertoire. In contrast, binding of HYK(k)Smcy and Ag-specific activation of mature CD8+ T cells is strictly dependent on the original C6 beta-chain. These data demonstrate that, in comparison with mature T cells, alphabetaTCR+ immature thymocytes can recognize and transduce signals in response to specific MHC-peptide complexes with relaxed binding requirements.

Essential role of the RNA-binding protein HuR in progenitor cell survival in mice

The RNA-binding protein HuR (also known as ELAV1) binds to the 3'-untranslated region of mRNAs and regulates transcript stability and translation. However, the in vivo functions of HuR are not well understood. Here, we report that murine HuR is essential for life; postnatal global deletion of Elavl1 induced atrophy of hematopoietic organs, extensive loss of intestinal villi, obstructive enterocolitis, and lethality within 10 days. Upon Elavl1 deletion, progenitor cells in the BM, thymus, and intestine underwent apoptosis, whereas quiescent stem cells and differentiated cells were unaffected. The survival defect of hematopoietic progenitor cells was cell intrinsic, as transplant of Elavl1-/- BM led to compromised hematopoietic reconstitution but did not cause lethality. Expression of p53 and its downstream effectors critical for cell death were induced in progenitor cells as HuR levels declined. In mouse embryonic fibroblasts, HuR bound to and stabilized the mRNA for Mdm2, a critical negative regulator of p53. Furthermore, cell survival was restored by expression of Mdm2 in Elavl1-/- cells, suggesting that HuR keeps p53 levels in check in progenitor cells and thereby promotes cell survival. This regulation of cell stress response by HuR in progenitor cells, which we believe to be novel, could potentially be exploited in cytotoxic anticancer therapies as well as stem cell transplant therapy.

ZAP-70 restoration in mice by in vivo thymic electroporation

Viral and non-viral vectors have been developed for gene therapy, but their use is associated with unresolved problems of efficacy and safety. Efficient and safe methods of DNA delivery need to be found for medical application. Here we report a new monopolar system of non-viral electro-gene transfer into the thymus in vivo that consists of the local application of electrical pulses after the introduction of the DNA. We assessed the proof of concept of this approach by correcting ZAP-70 deficient severe combined immunodeficiency (SCID) in mice. The thymic electro-gene transfer of the pCMV-ZAP-70-IRES-EGFP vector in these mice resulted in rapid T cell differentiation in the thymus with mature lymphocytes detected by three weeks in secondary lymphoid organs. Moreover, this system resulted in the generation of long-term functional T lymphocytes. Peripheral reconstituted T cells displayed a diversified T cell receptor (TCR) repertoire, and were responsive to alloantigens in vivo. This process applied to the thymus could represent a simplified and effective alternative for gene therapy of T cell immunodeficiencies.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

Genetic disorders of programmed cell death in the immune system

Human genetics offers new possibilities for understanding physiological regulatory mechanisms and disorders of the immune system. Genetic abnormalities of lymphocyte cell death programs have provided insights into mechanisms of receptor biology and principles of immune homeostasis and tolerance. Thus far, there are two major diseases of programmed cell death associated with inherited human mutations: the autoimmune lymphoproliferative syndrome and the caspase-eight deficiency state. We describe the details of their molecular pathogenesis and discuss how these diseases illustrate important concepts in immune regulation and genetics.

The Scurfy mutation of FoxP3 in the thymus stroma leads to defective thymopoiesis

The Scurfy mutation of the FoxP3 gene (FoxP3^sf) in the mouse and analogous mutations in human result in lethal autoimmunity. The mutation of FoxP3 in the hematopoietic cells impairs the development of regulatory T cells. In addition, development of the Scurfy disease also may require mutation of the gene in nonhematopoietic cells. The T cell–extrinsic function of FoxP3 has not been characterized. Here we show that the FoxP3^sf mutation leads to defective thymopoiesis, which is caused by inactivation of FoxP3 in the thymic stromal cells. FoxP3 mutation also results in overexpression of ErbB2 in the thymic stroma, which may be involved in defective thymopoiesis. Our data reveal a novel T cell–extrinsic function of FoxP3. In combination, the T cell–intrinsic and –extrinsic defects provide plausible explanation for the severity of the autoimmune diseases in the scurfy mice and in patients who have immunodysregulation, polyendocrinopathy, enteropathy, and X-linked syndrome.

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.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.

Probing p53 biological functions through the use of genetically engineered mouse models

The p53 tumor suppressor gene is rendered dysfunctional in the majority of human cancers. To model the effects of p53 dysfunction in an experimentally manipulable organismal context, genetically engineered inbred mice have been the models of choice. Transgenic and knock-out technologies have been utilized to generate an array of different p53 germ line alterations. As expected, many (though not all) of the mutant p53 mouse models are susceptible to enhanced spontaneous and carcinogen-induced tumors of a variety of types. A number of different variables affect the incidence and spectrum of tumors in p53 mutant mice. These include strain background, the nature of the p53 mutation, the presence of wild-type p53 (in addition to mutant p53), exposure to physical and chemical mutagens, or introduction of other cancer-associated genes into the mutant p53 background. In addition to their role in furthering our understanding of the mechanisms of cancer initiation and progression, these models have led to unexpected insights into p53 function in embryogenesis and aging. With the development of ever more sophisticated methods for manipulating the mouse genome, new p53 models are on the horizon, which should deliver advances that will provide not only important mechanistic insights but also discoveries of great clinical relevance.

Up-Regulation of IL-7, Stromal-Derived Factor-1α, Thymus-Expressed Chemokine, and Secondary Lymphoid Tissue Chemokine Gene Expression in the Stromal Cells in Response to Thymocyte Depletion: Implication for Thymus Reconstitution

Three in vivo adult mouse models were established to study which signals are required to restore the postnatal thymus. Single administration of dexamethasone, estradiol, or exposure to sublethal dose of gamma irradiation served as prototype thymus-ablating therapies. In all models, transient thymic atrophy was manifested due to the loss of the predominant portion of CD4- CD8- double negative and CD4+ CD8+ double positive thymocytes and was followed by a complete regeneration of the thymuses. Acute atrophy/regeneration was observed in the dexamethasone and irradiation models; in the estradiol-treated animals, slow kinetics of atrophy and regeneration was observed. Importantly, in both acute and chronic models, high levels of IL-7 mRNA were detected in the thymuses isolated from mice during maximum atrophy. In addition, chemokine gene array analysis of involuted thymuses revealed high levels of mRNA expression of stromal-derived factor-1alpha (SDF-1alpha), thymus-expressed chemokine (TECK), and secondary lymphoid tissue chemokine (SLC) but not of other chemokines. The levels of IL-7, SDF-1alpha, TECK, and SLC mRNA inversely correlated with the kinetics of regeneration. RT-PCR analysis of stromal cells purified from involuted thymuses confirmed increased IL-7, SDF-1alpha, and SLC gene expression in MHC class II+ CD45- epithelial cells and increased IL-7 and TECK gene expression in class II+ CD45+ CD11c+ dendritic cells. Thus, our data showed for the first time that expression of IL-7, SDF-1alpha, TECK, and SLC mRNA is induced in the thymic stroma during T cell depletion and may play an important role in the reconstitution of the adult thymus.

Assembly of silent chromatin during thymocyte development

Epigenetic events that contribute to the assembly and maintenance of silent chromatin structures have been defined through genetic, molecular, and cytological studies in a variety of eukaryotic model organisms. However, the precise cascade of events responsible for converting a developmentally regulated gene from an active euchromatic state to a heritably silent heterochromatic state remains to be elucidated. To establish a molecular framework for studying this cascade, we examined the temporal order of events associated with silencing of the murine terminal transferase (Dntt) gene during thymocyte maturation. This article describes our findings in the context of current knowledge of gene silencing mechanisms.

Maintenance of T cell specification and differentiation requires recurrent notch receptor-ligand interactions

Notch signaling has been shown to play a pivotal role in inducing T lineage commitment. However, T cell progenitors are known to retain other lineage potential long after the first point at which Notch signaling is required. Thus, additional requirements for Notch signals and the timing of these events relative to intrathymic differentiation remain unknown. Here, we address this issue by culturing subsets of CD4 CD8 double negative (DN) thymocytes on control stromal cells or stromal cells expressing Delta-like 1 (Dll1). All DN subsets were found to require Notch signals to differentiate into CD4+ CD8+ T cells. Using clonal analyses, we show that CD44+ CD25+ (DN2) cells, which appeared committed to the T cell lineage when cultured on Dll1-expressing stromal cells, nonetheless gave rise to natural killer cells with a progenitor frequency similar to that of CD44+ CD25- (DN1) thymocytes when Notch signaling was absent. These data, together with the observation that Dll1 is expressed on stromal cells throughout the thymic cortex, indicates that Notch receptor-ligand interactions are necessary for induction and maintenance of T cell lineage specification at both the DN1 and DN2 stages of T cell development, suggesting that the Notch-induced repression of the B cell fate is temporally separate from Notch-induced commitment to the T lineage.

CD4+CD8+ human small intestinal T cells are decreased in coeliac patients, with CD8 expression downregulated on intra-epithelial T cells in the active disease

BACKGROUND/OBJECTIVE

The intestinal lesion of coeliac disease is thought to be initiated and exacerbated by dysregulation of local T-lymphocyte sub-populations. This study examines changes in intestinal T cells from coeliac patients, with a particular focus on CD4CD8 T cells, immunoregulatory cells normally found in relatively high proportions in the small intestine.

METHODS

Cells were obtained from duodenal biopsies from active and treated coeliac patients using chelating and reducing agents (epithelial layer) followed by collagenase treatment (lamina propria). Cell yield and viability were assessed and flow cytometric analysis was used to examine CD4CD8 T cells and to quantify CD8 expression.

RESULTS

Surprisingly, total T-cell yields in the epithelial layer did not increase in active coeliac disease although enterocyte counts decreased significantly, giving an appearance of infiltration. In active coeliac patients, CD4CD8 T cell percentages were significantly decreased in both the epithelial layer and lamina propria. Levels of CD8 expression by CD4CD8 T cells in the epithelial layer were decreased significantly in patients with active coeliac disease. CD4CD8 T cell proportions did not return to normal in treated coeliac patients whose villous architecture had responded to gluten withdrawal.

CONCLUSIONS

No increase of intra-epithelial lymphocytes in the coeliac lesion may require us to reconsider the definition of coeliac disease as an inflammatory condition. Low CD4CD8 populations in treated as well as untreated coeliac patients indicate that these T cells are inherently absent in individuals genetically predisposed to coeliac disease.

Regulated Expression of the Receptor-Like Tyrosine Phosphatase CD148 on Hemopoietic Cells

CD148 is a receptor-like protein tyrosine phosphatase expressed on a wide variety of cell types. Through the use flow cytometry and immunofluorescence microscopy on tissue sections, we examined the expression of CD148 on multiple murine hemopoietic cell lineages. We found that CD148 is moderately expressed during all stages of B cell development in the bone marrow, as well as peripheral mature B cells. In contrast, CD148 expression on thymocytes and mature T cells is substantially lower. However, stimulation of peripheral T cells through the TCR leads to an increase of CD148 expression. This up-regulation on T cells can be partially inhibited by reagents that block the activity of src family kinases, calcineurin, MEK, or PI3K. Interestingly, CD148 levels are elevated on freshly isolated T cells from MRL lpr/lpr and CTLA-4-deficient mice, two murine models of autoimmunity. Together, these expression data along with previous biochemical data suggest that CD148 may play an important regulatory role to control an immune response.

Thymocytes between the beta-selection and positive selection checkpoints are nonresponsive to IL-7 as assessed by STAT-5 phosphorylation

Interleukin-7 is widely accepted as a major homeostatic factor involved in T cell development. To assess the IL-7 responsiveness of thymocytes involved in selection processes, we used a new sensitive flow cytometry-based assay to detect intracellular phosphorylation of STAT-5 induced by IL-7 in defined mouse thymocyte subsets. Using this method, we found the earliest thymocyte subset (CD4(-)CD8(-)CD25(-)CD44(+)) to contain both IL-7-responsive and nonresponsive cells. Transition through the next stages of development (CD4(-)CD8(-)CD25(+)CD44(+ and -)) was associated with responsiveness of all thymocytes within these populations. Passage of thymocytes through beta-selection resulted in a significant reduction in IL-7 sensitivity. In the next phases of development (TCR(-) and TCR(low)CD69(-)), thymocytes were completely insensitive to the effects of IL-7. STAT-5 phosphorylation in response to IL-7 was again observed, however, in thymocytes involved in the positive selection process (TCR(low)CD69(+) and TCR(intermediate)). As expected, CD4 and CD8 single-positive thymocytes were responsive to IL-7. These findings delineate an IL-7-insensitive population between the beta-selection and positive selection checkpoints encompassing thymocytes predicted to die by neglect due to failure of positive selection. This pattern of sensitivity suggests a two-signal mechanism by which survival of thymocytes at these checkpoints is governed.

A new model of Hantaan virus persistence in mice: the balance between HTNV infection and CD8(+) T-cell responses

We established a viral persistence model that involves the adoptive transfer of spleen cells from immunocompetent mice (H-2(d)) into Hantaan virus (HTNV)-infected severe combined immunodeficient (SCID, H-2(d)) mice. The infection is maintained despite the presence of neutralizing antibodies, without apparent signs of disease, and there is a correlation between HTNV persistence and the lack of HTNV-specific CD8(+) T cells. In addition, disseminated HTNV infection before the initiation of immune responses appears to be important for virus persistence. The suppression of HTNV-specific CD8(+) T cells in the present model appears to occur at the periphery. The present study also demonstrates that CD8(+) T cells contribute to the clearance of HTNV. Thus, it seems that HTNV-specific CD8(+) T cells play a key role in HTNV persistence in mice. This model of viral persistence is useful for studies of immune responses and immunocytotherapy against viral infection.

Loss of CARM1 results in hypomethylation of thymocyte cyclic AMP-regulated phosphoprotein and deregulated early T cell development

The coactivator-associated arginine methyltransferase, CARM1, is a positive regulator of transcription. Using high density protein arrays, we have previously identified in vitro substrates for CARM1. One of these substrates, TARPP (thymocyte cyclic AMP-regulated phosphoprotein), is expressed specifically in immature thymocytes. Here, we have demonstrated that TARPP is arginine-methylated at a single residue, Arg(650), both in vitro and in vivo. In addition, recombinant TARPP is not methylated by extracts from Carm1(-/-) cells, indicating that there is no redundancy in this pathway. We show that thymi from Carm1(-/-) embryos (E18.5) have a 5-10-fold reduction in cellularity compared with wild type littermates. Flow cytometric analysis of thymocytes revealed a decrease in the relative proportion of double negative thymocytes in Carm1(-/-) embryos because of a partial developmental arrest in the earliest thymocyte progenitor subset. These results demonstrate that CARM1 plays a significant role in promoting the differentiation of early thymocyte progenitors, possibly through its direct action on TARPP.

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.

E2A proteins enforce a proliferation checkpoint in developing thymocytes

E2A proteins regulate multiple stages of thymocyte development and suppress T-cell lymphoma. The activity of E2A proteins throughout thymocyte development is modulated by signals emanating from the pre-TCR and TCR. Here we demonstrate that E2A is required for the complete arrest in both differentiation and proliferation observed in thymocytes with defects in proteins that mediate pre-TCR signaling, including LAT, Lck and Fyn. We show that E2A proteins are required to prevent the accumulation of TCRbeta negative cells beyond the pre-TCR checkpoint. E2A-deficient thymocytes also exhibit abnormal cell-cycle progression prior to pre-TCR expression. Furthermore, we demonstrate that E47 can act in concert with Bcl-2 to induce cell-cycle arrest in vitro. These observations indicate that E2A proteins function during early thymocyte development to block cell-cycle progression prior to the expression of TCRbeta. In addition, these data provide further insight into how deficiencies in E2A lead to T lymphoma.

Suppressor of cytokine signaling 1 attenuates IL-15 receptor signaling in CD8+ thymocytes

SOCS1-/- mice die prematurely of increased interferon-gamma (IFNgamma) signaling with severe thymic atrophy and accelerated maturation of T cells. However, it was unclear whether the thymic defects were caused by SOCS1 deficiency or by increased IFNgamma signaling. Using SOCS1-/- IFNgamma-/- mice, we show in this study that SOCS1 deficiency skews thymocyte development toward CD8 lineage independently of IFNgamma. Fetal thymic organ cultures and intrathymic transfer of CD4-CD8- precursors into Rag1-/- mice show that the lineage skewing in SOCS1-/- mice is a T-cell autonomous defect. Interestingly, SOCS1 is not required for attenuating interleukin-7 (IL-7) signaling at the CD4-CD8- stage but is essential for regulating IL-15 and IL-2 signaling in CD8+ thymocytes. IL-15 selectively stimulates SOCS1-/- CD8+ thymocytes, inducing sustained signal transducer and activator of transcription 5 (STAT5) phosphorylation and massive proliferation. IL-15 also strongly up-regulates Bcl-xL and CD44 in CD8+ thymocytes lacking SOCS1. The SOCS1 gene is induced in CD4+ thymocytes by gammac cytokines, whereas CD8+ thymocytes constitutively express SOCS1 mRNA even in the absence of cytokine stimulation. Because many different cell types express IL-15, our results strongly suggest that SOCS1 functions as an indispensable attenuator of IL-15 receptor signaling in developing CD8+ thymocytes.

Transferrin receptor 1 is differentially required in lymphocyte development

Transferrin receptor (TfR) facilitates cellular iron uptake by mediating endocytosis of its ligand, iron-loaded transferrin. Although TfR is widely believed to be important for iron acquisition by all mammalian cells, direct experimental evidence is lacking. We have previously shown that mouse embryos homozygous for a disrupted transferrin receptor allele (TfR-/-) die of anemia before embryonic day 12.5, although most other embryonic tissues appear to be developing normally. Here, we have investigated the importance of TfR postnatally, by using TfR-/- embryonic stem cells to produce chimeric animals. We find that TfR-/- embryonic stem cells give rise to most tissues and organs, but do not contribute to hematopoietic tissues on a wild-type C57BL/6J background, indicating that both adult erythropoiesis and lymphopoiesis require TfR. On an immunodeficient RAG2-/- background, TfR-/- B-cell development proceeds at least to the IgM+ stage, although significantly fewer IgM+ cells are present in peripheral lymphoid organs. Conversely, T cells lacking TfR are arrested very early in their development, at the CD4-8-3- stage. These results indicate that TfR is necessary for the normal maturation of thymocytes, but that B-cell development is less severely affected by the absence of TfR.

Early defect prethymic in bone marrow T cell progenitors in athymic nu/nu mice

nu/nu mice fail to develop a thymus and mature T cells due to a defect in the whn gene encoding a transcription factor necessary for terminal epithelial cell differentiation. We investigated whether early T cell progenitor development in the nu/nu bone marrow is also defective. We demonstrated a maturation arrest of nu/nu marrow T cell progenitors associated with a lack of pTalpha gene expression and a failure to give rise to mature T cells in adoptive euthymic hosts. Wild-type hemopoietic stem cells rapidly matured into functional T cell progenitors in the marrow of euthymic or thymectomized but not nu/nu hosts. We show that defects in bone marrow prethymic T cell development can also contribute to T cell deficiency in nu/nu mice.

Flow cytometric analysis of cytokine receptor signal transduction

Cytokines are critical regulators of the development and maturation of hematopoietic cells. Signal transduction via cytokine receptors proceeds through activation of the JAK-STAT pathway to stimulate cell proliferation, differentiation and effector functions. Phosphorylation of intracellular STAT molecules by the receptor-associated JAK kinases is one of the very early events following cytokine stimulation. Western blot detection of tyrosine phosphorylated STAT molecules is widely used as a hallmark of cytokine receptor activation. However, this is not feasible when cells of interest are limiting, or represent a small fraction within a mixed population of different cell types. To circumvent this technical obstacle, we have developed techniques to detect phosphorylated STAT molecules in fixed cells by flow cytometry. The fixation and permeabilization protocols preserve the antigenicity of cell surface markers allowing us to distinguish distinct cell populations responding to cytokine stimulation. In this report, we demonstrate the use of this technique to rapidly and reliably identify, and quantify thymocyte subsets activated by interleukin-7. We envisage that this technique will find wide application in studying cytokine receptor signal transduction, particularly in identifying cytokine-dependent developmental checkpoints during hematopoiesis.

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

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

Regulation of cytokine receptor signaling by SOCS1

The suppressor of cytokine signaling (SOCS) family of proteins is a novel class of negative feedback regulators of cytokine receptor signaling. SOCS1 is rapidly induced following stimulation by several type I and type II cytokines, and it attenuates their signaling by its ability to bind and inhibit all four of the Janus family of intracellular tyrosine kinases (JAKs). Studies from our own and other laboratories have documented another important function of SOCS1 in facilitating ubiquitination of protein substrates and their subsequent proteasomal degradation. SOCS1 also functions as a potential tumor suppressor by inhibiting several hematopoietic oncogenes. In addition to these negative regulatory functions, we have recently shown a positive regulatory role for SOCS1 in increasing the stability of major histocompatibility complex (MHC) class II proteins by preventing their degradation. These findings illustrate multiple roles for SOCS1 in cytokine receptor signaling, and provide groundwork for detailed analysis of the role of SOCS1 in pre-T cell receptor (TCR) and TCR signaling, and regulation of T helper (Th)1 and Th2 differentiation.

TCRbeta enhancer activation occurs in some but not all cells with T cell lineage developmental potential

Previous studies have shown that murine bone marrow contains a fraction of CD3(-)/B220(-)/Thy1(lo) cells that have pre T cell activity following adoptive transfer and produce sterile transcripts of the T cell receptor beta chain gene. The relationship between progenitors and TCRbeta transcription has not been examined. Transgenic mice were generated that express green fluorescent protein under the control of the TCRbeta enhancer (Ebeta). Phenotypic analysis of the founders revealed faithful expression of GFP in populations that express TCRbeta transcripts. Examination of the bone marrow showed two populations, CD3(-)/B220(-)/Thy1(-) and CD3(-)/B220(-)/Thy1(lo), which were GFP(+). Both populations were analyzed for their developmental potential following intrathymic transfer into recipient mice. Surprisingly, the GFP(+)/CD3(-)/B220(-)/Thy1(lo) cells failed to reconstitute; however, the GFP(+)/CD3(-)/B220(-)/Thy1(-) cells exhibited thymic repopulation. These data demonstrate that Ebeta is active pre-thymically; however, pre-thymic transcription of the TCRbeta chain gene is neither required for T cell development, nor is it limited to pre T cells.

Down-regulation of the myeloid homeobox protein Hex is essential for normal T-cell development

The haematopoietic homeobox gene Hex (also called Prh) is expressed in myeloid cells and B cells but not T cells. To investigate whether Hex levels might play a role in myeloid versus T-cell development, two types of transgenic mouse lines were constructed, each with ectopic expression of Hex in T cells (CD11a/Hex and Lck/Hex). Both these types of transgenic mouse had the same defects in T-cell maturation, indicating that proper T-cell development may be dependent not just on the up-regulation of lymphoid-specific transcriptional regulators but also on the co-ordinated down-regulation of myeloid-specific transcriptional regulators such as Hex. In addition, Hex over-expression significantly increased myeloid progenitor cycling, which may explain its role in retrovirally induced murine leukaemia.

Soluble factor-mediated differentiation of CD4+CD8+ thymocytes to single positives in vitro

Although the thymic microenvironment provides the necessary elements for T-cell differentiation, the precise role of individual components remains to be determined. In this paper, attempts were made to address the possibility that CD4 or CD8 single-positive (SP) thymocytes could be developed from immature CD4+CD8+ (double-positive; DP) thymocytes in a suspension culture in the presence of soluble factors. We observed that IL-4 and IFN-gamma weakly induced DP cells to differentiate to CD4 cells, but not to CD8. In contrast, IL-2 weakly induced differentiation to CD8. Interestingly, Con A sup strongly induced differentiation to CD8 SP from the purified DP thymocytes prepared from C57BL/6 or LCMV TCRtg mice. In particular, it was found that thymocyte culture with Con A sup generated CD69+DP cells, and the CD69+DP differentiated to CD8 SP under the suspension culture with soluble factors. Thus, Con A sup or combinations of IL-2, IL-4 and IL-7 strongly induced differentiation of CD69+DP to CD8 SP, whereas individual cytokines did not. These results suggest that soluble factors like cytokines play an important role in the generation of SP thymocytes in the absence of thymic stromal cells, at least from a distinctive subpopulation like CD69+DP thymocytes, and perhaps from those of broader range when in conjunction with TCR/MHC interaction.

Regulation of surface expression of the human pre-T cell receptor complex

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

Regulation of thymocyte differentiation: pre-TCR signals and beta-selection

The specificity of the adaptive immune response is, in part, dependent on the clonal expression of the mature T cell receptor (TCR) on T lymphocytes. One mechanism regulating the clonality of the TCR occurs at the level of TCR-beta gene rearrangements during lymphocyte development. Expression of a nascent TCR-beta chain together with pre-Talpha (pTalpha) and CD3 molecules to form the pre-TCR complex, represents a critical checkpoint in T cell differentiation known as beta-selection. Indeed, failure to generate a functionally rearranged TCR-beta chain at this stage of development results in apoptosis. Signals derived from the pre-TCR complex trigger a maturation program within developing thymocytes that includes: rescue from apoptosis; inhibition of further DNA recombination at the TCR-beta gene locus (allowing for the clonality of antigen receptor expression; allelic exclusion); and induction of proliferation and differentiation. The signaling mechanisms that control this developmental program remain largely undefined. Here, we discuss recent evidence investigating the molecular mechanisms that regulate thymocyte differentiation downstream of pre-TCR formation.

The ETS protein MEF plays a critical role in perforin gene expression and the development of natural killer and NK-T cells

We utilized gene targeting by homologous recombination to define the role that MEF, a transcriptional activating member of the ETS family of transcription factors, plays in lymphopoiesis. MEF-/- mice have a profound reduction in the number of NK-T and NK cells. Purified MEF-/- NK cells cannot lyse tumor cell targets and secrete only minimal amounts of IFNgamma. Perforin protein expression is severely impaired in MEF-deficient NK cells, likely accounting for the lack of tumor cell cytotoxicity. Promoter studies and chromatin immunoprecipitation analyses demonstrate that MEF and not ETS-1 directly regulates transcription of the perforin gene in NK cells. Our results uncover a specific role of MEF in the development and function of NK cells and in innate immunity.

Deficiency in Bak and Bax perturbs thymic selection and lymphoid homeostasis

Bak and Bax are required and redundant regulators of an intrinsic mitochondrial cell death pathway. To analyze this pathway in T cell development and homeostasis, we reconstituted mice with Bak(-/-)Bax<(-/-) hematopoietic cells. We found that the development and selection of Bak(-/-)Bax(-/-) thymocytes was disrupted, with altered representation of thymic subsets and resistance to both death-by-neglect and antigen receptor-induced apoptosis. Elimination of Bak(-/-)Bax(-/-) T cells that responded to endogenous superantigen was also reduced. Despite more efficient early reconstitution and apoptotic resistance of Bak(-/-)Bax(-/-) thymocytes, thymic cellularity declined over time. Reduced thymic cellularity resulted from a progressive cessation of thymopoiesis. However, animals developed splenomegaly as a result of accumulated memory T cells that were not deleted after antigen-driven expansion. These data indicate that Bak and Bax are required for thymic selection and peripheral lymphoid homeostasis and suggest that thymopoiesis can be negatively regulated by the accumulation of cells that would normally be eliminated by pro-apoptotic Bcl-2-related genes.

Lineage plasticity and commitment in T-cell development

The earliest stages of intrathymic T-cell development include not only the acquisition of T-cell characteristics but also programmed loss of potentials for B, natural killer, and dendritic cell development. Evidence from genetics and cell-transfer studies suggests an order and some components of the mechanisms involved in loss of these options, but some of the interpretations conflict. The conflicts can be resolved by a view that postulates overlapping windows of developmental opportunity and individual mechanisms regulating progression along each pathway. This view is consistent with molecular evidence for the expression patterns of positive regulators of non-T developmental pathways, SCL, PU.1 and Id2, in early thymocytes. To some extent, overexpression of such regulators redirects thymocyte development in vitro. Specific commitment functions may normally terminate this developmental plasticity. Both PU.1 overexpression and stimulation of ectopically expressed growth factor receptors can perturb T- and myeloid/dendritic-cell divergence, but only in permissive stages. A cell-line system that approximates DN3-stage thymocytes reveals that PU.1 can alter specification even in a homogeneous population. However, the response of the population to PU.1 is sharply discontinuous. These studies show a critical role for regulatory context in restricting plasticity, which is probably maintained by interacting transcription factor networks.

E protein function in lymphocyte development

Lymphocytes arise from hematopoietic stem cells through the coordinated action of transcription factors. The E proteins (E12, E47, HEB and E2-2) have emerged as key regulators of both B and T lymphocyte differentiation. This review summarizes the current data and examines the various functions of E proteins and their antagonists, Id2 and Id3, throughout lymphoid maturation. Beyond an established role in B and T lineage commitment, E proteins continue to be essential at subsequent stages of development. E protein activity regulates the expression of surrogate and antigen receptor genes, promotes Ig and TCR rearrangements, and coordinates cell survival and proliferation with developmental progression in response to TCR signaling. Finally, this review also discusses the role of E47 as a tumor suppressor.

Heterogeneity of the DN4 (CD44-CD25-) subset of CD4-CD8- double negative thymocytes; dependence on CD3 signaling

Recent studies have shown that apoptotic cell death associated with selection for thymocytes that express clonotypic TCRbeta or TCRgammadelta proteins takes place in the DN4 (CD44-CD25-) subset of CD4-CD8- double negative (DN) thymocytes. A detailed analysis of the DN4 subset is therefore of interest. Using intracellular (IC) staining for clonotypic TCR and CD3varepsilon proteins we find that DN4 cells consist of five subpopulations: TCRbetaIC(high)/CD3varepsilonIC(high)/TCRgammadeltaIC-, TCRbetaI-C-/CD3varepsilonIC(high)/TCRgammadeltaIC(+), TCRbetaIC(high)/CD3varepsilonIC(high)/TCRgammadeltaIC(+), TCRbetaIC(low)/CD3varepsilonIC(low)/TCRgammadeltaIC(-), and TCRbetaIC(-)/CD3varepsilonIC(-)/TCRgammadeltaIC(-). Expression levels of IC TCRbeta/CD3varepsilon, and of Thy1.2, CD2, and CD69 at the cell surface suggest that the TCRbetaIC(low)/CD3varepsilonIC(low)/TCRgammadeltaIC(-) subset harbors the direct precursors of DP cells, and is critical for life/death decisions in early thymic selection. TCRbeta/CD3varepsilon downregulation is less pronounced in DN4 and DP cells of mice deficient for CD3zeta or for p56(lck), suggesting that the dynamics of TCR protein regulation in the DN4 subset is dependent on CD3 signaling.

Transgenic expression of numb inhibits notch signaling in immature thymocytes but does not alter T cell fate specification

The conserved adaptor protein Numb is an intrinsic cell fate determinant that functions by antagonizing Notch-mediated signal transduction. The Notch family of membrane receptors controls cell survival and cell fate determination in a variety of organ systems and species. Recent studies have identified a role for mammalian Notch-1 signals at multiple stages of T lymphocyte development. We have examined the role of mammalian Numb (mNumb) as a Notch regulator and cell fate determinant during T cell development. Transgenic overexpression of mNumb under the control of the Lck proximal promoter reduced expression of several Notch-1 target genes, indicating that mNumb antagonizes Notch-1 signaling in vivo. However, thymocyte development, cell cycle, and survival were unperturbed by mNumb overexpression, even though transgenic Numb was expressed at an early stage in thymocyte development (CD4(-)CD8(-)CD3(-) cells that were CD44(+)CD25(+) or CD44(-)CD25(+); double-negative 2/3). Moreover, bone marrow from mNumb transgenic mice showed no defects in thymopoiesis in competitive repopulation experiments. Our results suggest that mNumb functions as a Notch-1 antagonist in immature thymocytes, but that suppression of Notch-1 signaling at this stage does not alter gammadelta/alphabeta or CD4/CD8 T cell fate specification.