Notch signaling controls the generation and differentiation of early T lineage progenitors

Modeling T-cell acute lymphoblastic leukemia induced by the SCL and LMO1 oncogenes

Deciphering molecular events required for full transformation of normal cells into cancer cells remains a challenge. In T-cell acute lymphoblastic leukemia (T-ALL), the genes encoding the TAL1/SCL and LMO1/2 transcription factors are recurring targets of chromosomal translocations, whereas NOTCH1 is activated in >50% of samples. Here we show that the SCL and LMO1 oncogenes collaborate to expand primitive thymocyte progenitors and inhibit later stages of differentiation. Together with pre-T-cell antigen receptor (pre-TCR) signaling, these oncogenes provide a favorable context for the acquisition of activating Notch1 mutations and the emergence of self-renewing leukemia-initiating cells in T-ALL. All tumor cells harness identical and specific Notch1 mutations and Tcrbeta clonal signature, indicative of clonal dominance and concurring with the observation that Notch1 gain of function confers a selective advantage to SCL-LMO1 transgenic thymocytes. Accordingly, a hyperactive Notch1 allele accelerates leukemia onset induced by SCL-LMO1 and bypasses the requirement for pre-TCR signaling. Finally, the time to leukemia induced by the three transgenes corresponds to the time required for clonal expansion from a single leukemic stem cell, suggesting that SCL, LMO1, and Notch1 gain of function, together with an active pre-TCR, might represent the minimum set of complementing events for the transformation of susceptible thymocytes.

Emerging strategies to boost thymic function

The thymus constitutes the primary lymphoid organ for the generation of T cells. Its function is particularly susceptible to various negative influences ranging from age-related involution to atrophy as a consequence of malnutrition, infection or harmful iatrogenic influences such as chemotherapy and radiation. The loss of regular thymus function significantly increases the risk for infections and cancer because of a restricted capacity for immune surveillance. In recent years, thymus-stimulatory, thymus-regenerative, and thymus-protective strategies have been developed to enhance and repair thymus function in the elderly and in individuals undergoing hematopoietic stem cell transplantation. These strategies include the use of sex steroid ablation, the administration of growth and differentiation factors, the inhibition of p53, and the transfer of T cell progenitors to alleviate the effects of thymus dysfunction and consequent T cell deficiency.

Flt3 ligand-receptor interaction is important for maintenance of early thymic progenitor numbers in steady-state thymopoiesis

T-cell production throughout life depends on efficient colonization and intrathymic expansion of BM-derived hematopoietic precursors. After irradiation-induced thymic damage, thymic recovery is facilitated by Flt3 ligand (FL), expressed by perivascular fibroblasts surrounding the thymic entry site of Flt3 receptor-positive progenitor cells. Whether intrathymic FL-Flt3 interactions play a role in steady-state replenishment of T cells remains unknown. Here, using competitive BM transplantation studies and fetal thymic organ cultures we demonstrated the continued numerical advantage of Flt3+ intrathymic T-cell precursors. Sub-kidney capsule thymic transplantation experiments, in which WT and FL-/- thymic lobes were grafted into FL-/- recipients, revealed that FL expression by the thymic microenvironment plays a role in steady-state thymopoiesis. The deficiency of the most immature thymic T-cell precursors correlated to upregulation of FL by thymic MTS15+ fibroblasts, suggesting that the number of Flt3+ progenitor cells may regulate the thymic expression of this cytokine. Together, these results show that FL expression by thymic stromal fibroblasts interacting with Flt3+ T-cell progenitors is important for the physiological maintenance of early T-cell development.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

Two opposing roles of RBP-J in Notch signaling

RBP-J/Su(H)/Lag1, the main transcriptional mediator of Notch signaling, binds DNA with the consensus sequence YRTGDGAD. Notch target genes can be controlled by two opposing activities of RBP-J. The interaction of the Notch intracellular domain with RBP-J induces a weak transcriptional activation and requires an additional tissue-specific transcriptional activator such as bHLH proteins or GATA to mediate strong target gene expression. For example, during Drosophila sensory organ precursor (SOP) cell development, proneural bHLH interacts with Da, a Drosophila orthologue of E2A, to form a tissue-specific activator of Su(H), the Drosophila orthologue of RBP-J. This complex and Su(H) act synergistically to promote the epidermal cell fate. In contrast, a complex of Su(H) with Hairless, a Drosophila functional homologue of MINT, has transcriptional repression activity that promotes SOP differentiation to neurons. Recent conditional loss-of-function studies demonstrated that transcriptional networks involving RBP-J, MINT, and E2A are conserved in mammalian cell differentiation, including multiple steps of lymphocyte development, and probably also in neuronal maturation in adult neurogenesis. During neurogenesis, Notch-RBP-J signaling was thought historically to be involved mainly in the maintenance of undifferentiated neural progenitors. However, the identification of a tissue-specific transcriptional activator of RBP-J-Notch has revealed new roles of RBP-J in the promotion of neuronal maturation. Finally, the Notch-independent function of RBP-J was recently discovered and will be reviewed here.

Notch signaling in the hematopoietic system

Because of its multiple effects in tissue homeostasis and cancer, Notch signaling is gaining increasing attention as a potential therapeutic target. Notch proteins belong to a family of highly conserved cell surface receptors. Ligand binding leads to proteolytic cleavage of Notch receptors by the gamma-secretase complex, followed by translocation of the active intracellular Notch domain into the nucleus and transcriptional activation. Multiple genetic and pharmacological methods are available to inhibit or activate the Notch pathway, some of which are entering human clinical trials. In this review, we discuss our current understanding of Notch signaling in the hematopoietic system. Canonical Notch signaling is essential for the generation of definitive embryonic hematopoietic stem cells, but dispensable for their maintenance during adult life. Notch controls several early steps of T cell development, as well as specific cell fate and differentiation decisions in other hematopoietic lineages. In addition, emerging evidence indicates that Notch is a potent, context-specific regulator of T cell immune responses, including in several disease models relevant to patients. This knowledge will constitute a framework to explore Notch modulation as a therapeutic strategy and to understand potential hematopoietic side effects of systemic Notch inhibition.

Multiple extrathymic precursors contribute to T-cell development with different kinetics

T-cell development in the thymus depends on continuous supply of T-cell progenitors from bone marrow (BM). Several extrathymic candidate progenitors have been described that range from multipotent cells to lymphoid cell committed progenitors and even largely T-lineage committed precursors. However, the nature of precursors seeding the thymus under physiologic conditions has remained largely elusive and it is not known whether there is only one physiologic T-cell precursor population or many. Here, we used a competitive in vivo assay based on depletion rather than enrichment of classes of BM-derived precursor populations, thereby only minimally altering physiologic precursor ratios to assess the contribution of various extrathymic precursors to T-lineage differentiation. We found that under these conditions multiple precursors, belonging to both multipotent progenitor (MPP) and common lymphoid progenitor (CLP) subsets have robust T-lineage potential. However, differentiation kinetics of different precursors varied considerably, which might ensure continuous thymic output despite gated importation of extrathymic precursors. In conclusion, our data suggest that the thymus functions to impose T-cell fate on any precursor capable of filling the limited number of progenitor niches.

CCR7 and CCR9 together recruit hematopoietic progenitors to the adult thymus

T lymphopoiesis requires settling of the thymus by bone marrow-derived precursors throughout adult life. Progenitor entry into the thymus is selective, but the molecular basis of this selectivity is incompletely understood. The chemokine receptor CCR9 has been demonstrated to be important in this process. However, progenitors lacking CCR9 can still enter the thymus, suggesting a role for additional molecules. Here we report that the chemokine receptor CCR7 is also required for efficient thymic settling. CCR7 is selectively expressed on bone marrow progenitors previously shown to have the capacity to settle the thymus, and CCR7(-/-) progenitors are defective in settling the thymus. We further demonstrate that CCR7 sustains thymic settling in the absence of CCR9. Mice deficient for both CCR7 and CCR9 have severe reductions in the number of early thymic progenitors, and in competitive assays CCR7(-/-)CCR9(-/-) double knockout progenitors are almost completely restricted from thymic settling. However, these mice possess near-normal thymic cellularity. Compensatory expansion of intrathymic populations can account for at least a part of this recovery. Together our results illustrate the critical role of chemokine receptor signaling in thymic settling and help to clarify the cellular identity of the physiologic thymic settling progenitors.

Isolation of mouse thymic dendritic cell precursors

Dendritic cells (DC) are efficient antigen-presenting cells. Their ability to present antigens via MHC class I and MHC class II molecules to T cells allows them not only to initiate an immune response to exogenous pathogens but also to induce immune tolerance to self-antigens. Thymic DC play important roles in the establishment of central immune tolerance by presenting self-antigens to developing thymocytes and subsequently deleting the self-reactive thymocytes and inducing naturally occurring regulatory T cells. DC in the thymus are comprised of plasmacytoid DC (pDC) and conventional DC (cDC) populations. The cDC can be divided into two populations based on the expression of CD8 alpha and Sirp alpha: CD8 alpha(+)Sirp alpha(l) degrees (approximately 70%) and CD8 alpha(l) degrees Sirp alpha(+) (approximately 30%). The CD8 alpha(+)Sirp alpha(l) degrees cDC are generated in the thymus by the earliest intrathymic oligo-potent progenitors that are also precursors for T-lineage cells and natural killer cells (NK cells). Whereas the CD8 alpha(l) degrees Sirp alpha(+)cDC and pDC are migratory DC and originate mainly from peripheral blood. The ability to isolate and purify the earliest intrathymic precursors allows us to generate thymic cDC in culture or in vivo upon intrathymic or intravenous injections. These experimental systems are crucial for studying the development and functions of thymic DC.

GATA-3 is required for early T lineage progenitor development

Most T lymphocytes appear to arise from very rare early T lineage progenitors (ETPs) in the thymus, but the transcriptional programs that specify ETP generation are not completely known. The transcription factor GATA-3 is required for the development of T lymphocytes at multiple late differentiation steps as well as for the development of thymic natural killer cells. However, a role for GATA-3 before the double-negative (DN) 3 stage of T cell development has to date been obscured both by the developmental heterogeneity of DN1 thymocytes and the paucity of ETPs. We provide multiple lines of in vivo evidence through the analysis of T cell development in Gata3 hypomorphic mutant embryos, in irradiated mice reconstituted with Gata3 mutant hematopoietic cells, and in mice conditionally ablated for the Gata3 gene to show that GATA-3 is required for ETP generation. We further show that Gata3 loss does not affect hematopoietic stem cells or multipotent hematopoietic progenitors. Finally, we demonstrate that Gata3 mutant lymphoid progenitors exhibit neither increased apoptosis nor diminished cell-cycle progression. Thus, GATA-3 is required for the cell-autonomous development of the earliest characterized thymic T cell progenitors.

Ly6d marks the earliest stage of B-cell specification and identifies the branchpoint between B-cell and T-cell development

Common lymphoid progenitors (CLPs) clonally produce both B- and T-cell lineages, but have little myeloid potential in vivo. However, some studies claim that the upstream lymphoid-primed multipotent progenitor (LMPP) is the thymic seeding population, and suggest that CLPs are primarily B-cell-restricted. To identify surface proteins that distinguish functional CLPs from B-cell progenitors, we used a new computational method of Mining Developmentally Regulated Genes (MiDReG). We identified Ly6d, which divides CLPs into two distinct populations: one that retains full in vivo lymphoid potential and produces more thymocytes at early timepoints than LMPP, and another that behaves essentially as a B-cell progenitor.

Regulation of Notch signaling during T- and B-cell development by O-fucose glycans

Notch signaling is required for the development of all T cells and marginal zone (MZ) B cells. Specific roles in T- and B-cell differentiation have been identified for different Notch receptors, the canonical Delta-like (Dll) and Jagged (Jag) Notch ligands, and downstream effectors of Notch signaling. Notch receptors and ligands are post-translationally modified by the addition of glycans to extracellular domain epidermal growth factor-like (EGF) repeats. The O-fucose glycans of Notch cell-autonomously modulate Notch-ligand interactions and the strength of Notch signaling. These glycans are initiated by protein O-fucosyltransferase 1 (Pofut1), and elongated by the transfer of N-acetylglucosamine (GlcNAc) to the fucose by beta1,3GlcNAc-transferases termed lunatic, manic, or radical fringe. This review discusses T- and B-cell development from progenitors deficient in O-fucose glycans. The combined data show that Lfng and Mfng regulate T-cell development by enhancing the interactions of Notch1 in T-cell progenitors with Dll4 on thymic epithelial cells. In the spleen, Lfng and Mfng cooperate to modify Notch2 in MZ B progenitors, enhancing their interaction with Dll1 on endothelial cells and regulating MZ B-cell production. Removal of O-fucose affects Notch signaling in myelopoiesis and lymphopoiesis, and the O-fucose glycan in the Notch1 ligand-binding domain is required for optimal T-cell development.

Shortening the immunodeficient period after hematopoietic stem cell transplantation

The delayed reconstitution of the T-lymphoid compartment represents a major clinical challenge after HLA-mismatched hematopoietic stem cell transplantation. The generation of new T lymphocytes deriving from transplanted hematopoietic stem cells requires several months, a period associated with an increased risk of opportunistic infections and relapses. Recently, the early steps of human lymphopoiesis and the nature of the thymus-seeding progenitors were described. Moreover several scientific groups succeeded to generate T-cell precursors from murine and human hematopoietic stem cells in vitro by transitory exposition to Notch-ligands. Here we summarize and discuss these results and their possible usage in the development of new cell therapies to shorten the immunodeficient period following hematopoietic stem cell transplantation.

Hes1 potentiates T cell lymphomagenesis by up-regulating a subset of notch target genes

BACKGROUND

Hairy/Enhancer of Split (Hes) proteins are targets of the Notch signaling pathway and make up a class of basic helix-loop-helix (bHLH) proteins that function to repress transcription. Data from Hes1 deficient mice suggested that Hes1, like Notch1, is necessary for the progression of early T cell progenitors. Constitutive activation of Notch is known to cause T cell leukemia or lymphoma but whether Hes1 has any oncogenic activity is not known.

METHODOLOGY/PRINCIPAL FINDINGS

We generated mice carrying a Hes1 transgene under control of the proximal promote of the lck gene. Hes1 expression led to a reduction in numbers of total thymocytes, concomitant with the increased percentage and number of immature CD8+ (ISP) T cells and sustained CD25 expression in CD4+CD8+ double positive (DP) thymocytes. Hes1 transgenic mice develop thymic lymphomas at about 20 weeks of age with a low penetrance. However, expression of Hes1 significantly shortens the latency of T cell lymphoma developed in Id1 transgenic mice, where the function of bHLH E proteins is inhibited. Interestingly, Hes1 increased expression of a subset of Notch target genes in pre-malignant ISP and DP thymocytes, which include Notch1, Notch3 and c-myc, thus suggesting a possible mechanism for lymphomagenesis.

CONCLUSIONS/SIGNIFICANCE

We have demonstrated for the first time that Hes1 potentiates T cell lymphomagenesis, by up-regulating a subset of Notch target genes and by causing an accumulation of ISP thymocytes particularly vulnerable to oncogenic transformation.

Pre-TCR signaling inactivates Notch1 transcription by antagonizing E2A

Precise control of the timing and magnitude of Notch signaling is essential for the normal development of many tissues, but the feedback loops that regulate Notch are poorly understood. Developing T cells provide an excellent context to address this issue. Notch1 signals initiate T-cell development and increase in intensity during maturation of early T-cell progenitors (ETP) to the DN3 stage. As DN3 cells undergo beta-selection, during which cells expressing functionally rearranged TCRbeta proliferate and differentiate into CD4(+)CD8(+) progeny, Notch1 signaling is abruptly down-regulated. In this report, we investigate the mechanisms that control Notch1 expression during thymopoiesis. We show that Notch1 and E2A directly regulate Notch1 transcription in pre-beta-selected thymocytes. Following successful beta-selection, pre-TCR signaling rapidly inhibits Notch1 transcription via signals that up-regulate Id3, an E2A inhibitor. Consistent with a regulatory role for Id3 in Notch1 down-regulation, post-beta-selected Id3-deficient thymocytes maintain Notch1 transcription, whereas enforced Id3 expression decreases Notch1 expression and abrogates Notch1-dependent T-cell survival. These data provide new insights into Notch1 regulation in T-cell progenitors and reveal a direct link between pre-TCR signaling and Notch1 expression during thymocyte development. Our findings also suggest new strategies for inhibiting Notch1 signaling in pathologic conditions.

The orphan adapter protein SLY1 as a novel anti-apoptotic protein required for thymocyte development

Background

SH3 containing Lymphocyte Protein (SLY1) is a putative adapter protein exclusively expressed in lymphocytes which is involved in antigen receptor induced activation. We previously have generated SLY1^Δ/Δ mice harbouring a partial deletion in the N-terminal region of SLY1 which revealed profound immunological defects in T and B cell functions.

Results

In this study, T cell development in SLY1^-/- and SLY1^Δ/Δ mice was analysed ex vivo and upon cultivation with the bone marrow stromal cell line OP9. SLY1-deficient thymocytes were compromised in inducing nutrient receptor expression and ribosomal protein S6 phosphorylation, indicating a defect in mTOR complex activation. Furthermore, SLY1 was identified as a novel anti-apoptotic protein required for developmental progression of T cell precursors to the CD4⁺CD8⁺ double-positive stage by protecting from premature programmed cell death initiation in developing CD4^-CD8^- double-negative thymocytes. In addition, SLY1 phosphorylation was differentially regulated upon Notch ligand-mediated stimulation and expression of the preTCR.

Conclusion

Thus, our results suggest a non-redundant role for SLY1 in integrating signals from both receptors in early T cell progenitors in the thymus.

Id1 attenuates Notch signaling and impairs T-cell commitment by elevating Deltex1 expression

Complete inhibition of E protein transcription factors by Id1 blocks the developmental transition of CD4/CD8 double-negative 1 (DN1; CD44(+) CD25(-)) thymocytes to the DN2 (CD44(+) CD25(+)) stage. To understand the underlying mechanisms, we observed that mRNA levels of Deltex1, as well as Deltex4, were dramatically elevated in Id1-expressing thymocytes, which could result in developmental arrest by attenuating Notch function. In support of this hypothesis, we found that Deltex1 ablation enabled Id1-expressing progenitors to differentiate to the DN3 (CD44(-) CD25(+)) stage, which was accompanied by enhanced Notch1 expression in T-cell progenitors. Consistently, constitutive activation of Notch1 drove the differentiation of Id1-expressing progenitors to the DN3 stage. Furthermore, we showed that Gfi1b levels decreased, whereas GATA3 levels increased in Id1 transgenic thymocytes. When overexpressed, GATA3 was able to upregulate Deltex1 transcription. Thus, T-cell commitment may be controlled by the interplay among E proteins, Gfi1b, and GATA3 transcription regulators, which influence Notch function through the expression of Deltex1.

ELDA: extreme limiting dilution analysis for comparing depleted and enriched populations in stem cell and other assays

ELDA is a software application for limiting dilution analysis (LDA), with particular attention to the needs of stem cell assays. It is the first limiting dilution analysis software to provide meaningful confidence intervals for all LDA data sets, including those with 0% or 100% responses. Other features include a test of the adequacy of the single-hit hypothesis, tests for frequency differences between multiple data sets, and the ability to take advantage of cases where the number of cells in the sample is counted exactly. A webtool at http://bioinf.wehi.edu.au/software/elda/ provides an easy user interface.

A new paradigm for hematopoietic cell lineages: revision of the classical concept of the myeloid-lymphoid dichotomy

The concept that blood cells arising from hematopoietic stem cells (HSC) can be subdivided into two major lineages, a myelo-erythroid and a lymphoid lineage, has long persisted. Indeed, it has become almost axiomatic that the first branch point from the HSC produces two progenitors, one for myelo-erythroid cells and the other for lymphoid cells. However, recent studies have provided a battery of findings that cannot be explained by this classical model. We will outline how this classical model arose before describing how we came to propose an alternative 'myeloid-based model', in which myeloid potential is retained in erythroid, T, and B cell branches even after these lineages have segregated from each other.

CSL-MAML-dependent Notch1 signaling controls T lineage-specific IL-7R{alpha} gene expression in early human thymopoiesis and leukemia

Notch1 activation is essential for T-lineage specification of lymphomyeloid progenitors seeding the thymus. Progression along the T cell lineage further requires cooperative signaling provided by the interleukin 7 receptor (IL-7R), but the molecular mechanisms responsible for the dynamic and lineage-specific regulation of IL-7R during thymopoiesis are unknown. We show that active Notch1 binds to a conserved CSL-binding site in the human IL7R gene promoter and critically regulates IL7R transcription and IL-7R α chain (IL-7Rα) expression via the CSL–MAML complex. Defective Notch1 signaling selectively impaired IL-7Rα expression in T-lineage cells, but not B-lineage cells, and resulted in a compromised expansion of early human developing thymocytes, which was rescued upon ectopic IL-7Rα expression. The pathological implications of these findings are demonstrated by the regulation of IL-7Rα expression downstream of Notch1 in T cell leukemias. Thus, Notch1 controls early T cell development, in part by regulating the stage- and lineage-specific expression of IL-7Rα.

Untangling the T branch of the hematopoiesis tree

T cells develop in the thymus. Previous work suggested an early separation of lymphoid from myeloerythroid lineages during hematopoiesis and hypothesized the thymus was settled exclusively by lymphoid-restricted hematopoietic progenitors. Recent data have instead established the existence of lymphoid-myeloid progenitors, which possess lymphoid and myeloid lineage potentials but lack erythroid potential. Myeloid and lymphoid potentials are present at the clonal level in early thymic progenitors, confirming that progenitors settling the thymus include lymphoid-myeloid progenitors. These results revise our view of the T lineage branch of hematopoiesis and focus attention on the generation, circulation, and homing of lymphoid-myeloid progenitors to the thymus.

Transcription factor expression dynamics of early T-lymphocyte specification and commitment

Mammalian T lymphocytes are a prototype for development from adult pluripotent stem cells. While T-cell specification is driven by Notch signaling, T-lineage commitment is only finalized after prolonged Notch activation. However, no T-lineage specific regulatory factor has been reported that mediates commitment. We used a gene-discovery approach to identify additional candidate T-lineage transcription factors and characterized expression of >100 regulatory genes in early T-cell precursors using realtime RT-PCR. These regulatory genes were also monitored in multilineage precursors as they entered T-cell or non-T-cell pathways in vitro; in non-T cells ex vivo; and in later T-cell developmental stages after lineage commitment. At least three major expression patterns were observed. Transcription factors in the largest group are expressed at relatively stable levels throughout T-lineage specification as a legacy from prethymic precursors, with some continuing while others are downregulated after commitment. Another group is highly expressed in the earliest stages only, and is downregulated before or during commitment. Genes in a third group undergo upregulation at one of three distinct transitions, suggesting a positive regulatory cascade. However, the transcription factors induced during commitment are not T-lineage specific. Different members of the same transcription factor family can follow opposite trajectories during specification and commitment, while factors co-expressed early can be expressed in divergent patterns in later T-cell development. Some factors reveal new regulatory distinctions between alphabeta and gammadelta T-lineage differentiation. These results show that T-cell identity has an essentially complex regulatory basis and provide a detailed framework for regulatory network modeling of T-cell specification.

Strategies for reconstituting and boosting T cell-based immunity following haematopoietic stem cell transplantation: pre-clinical and clinical approaches

Poor immune recovery is characteristic of bone marrow transplantation and leads to high levels of morbidity and mortality. The primary underlying cause is a compromised thymic function, resulting from age-induced atrophy and further compounded by the damaging effects of cytoablative conditioning regimes on thymic epithelial cells (TEC). Several strategies have been proposed to enhance T cell reconstitution. Some, such as the use of single biological agents, are currently being tested in clinical trials. However, a more rational approach to immune restoration will be to leverage the evolving repertoire of new technologies. Specifically, the combined targeting of TEC, thymocytes and peripheral T cells, together with the bone marrow niches, promises a more strategic clinical therapeutic platform.

The long road to the thymus: the generation, mobilization, and circulation of T-cell progenitors in mouse and man

The majority of T cells develop in the thymus. T-cell progenitors in the thymus do not self-renew and so progenitor cells must be continuously imported from the blood into the thymus to maintain T-cell production. Recent work has shed light on both the identity of the cells that home to the thymus and the molecular mechanisms involved. This review will discuss the cells in the bone marrow and blood that are involved in early thymopoiesis in mouse and man. Understanding the pre-thymic steps in T-cell development may translate into new therapeutics, especially in the field of hematopoietic stem cell transplantation.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

Ikaros represses the transcriptional response to Notch signaling in T-cell development

Notch activity is essential for early T-cell differentiation, but aberrant activity induces T-cell transformation. Thus, Notch target genes must be efficiently silenced in cells where Notch activity is no longer required. How these genes are repressed remains poorly understood. We report here that the Ikaros transcription factor plays a crucial role in repressing the transcriptional response to Notch signaling in T-cell development. Using the Notch target gene Hes-1 as a model, we show that Ikaros and RBP-Jkappa, the transcriptional mediator of Notch signaling, compete for binding to two elements in the Hes-1 promoter in immature thymocytes. This antagonistic interaction likely occurs at the CD4(-) CD8(-) CD3(-) double-negative 4 (DN4) stage, where Ikaros levels and binding to the Hes-1 promoter increase sharply and wild-type thymocytes lose their capacity to transcribe Hes-1 upon Notch stimulation. Nonresponsiveness to Notch signaling requires Ikaros, as Ikaros-deficient DN4 and CD4(+) CD8(+) double-positive (DP) cells remain competent to express Hes-1 after Notch activation. Further, Hes-1 promoter sequences from Ikaros-deficient DP cells show reduced trimethylated H3K27, a modification associated with silent chromatin. These results indicate that Ikaros functions as a transcriptional checkpoint to repress Notch target gene expression in T cells.

Delta-like 4 is indispensable in thymic environment specific for T cell development

The thymic microenvironment is required for T cell development in vivo. However, in vitro studies have shown that when hematopoietic progenitors acquire Notch signaling via Delta-like (Dll)1 or Dll4, they differentiate into the T cell lineage in the absence of a thymic microenvironment. It is not clear, however, whether the thymus supports T cell development specifically by providing Notch signaling. To address this issue, we generated mice with a loxP-flanked allele of Dll4 and induced gene deletion specifically in thymic epithelial cells (TECs). In the thymus of mutant mice, the expression of Dll4 was abrogated on the epithelium, and the proportion of hematopoietic cells bearing the intracellular fragment of Notch1 (ICN1) was markedly decreased. Corresponding to this, CD4 CD8 double-positive or single-positive T cells were not detected in the thymus. Further analysis showed that the double-negative cell fraction was lacking T cell progenitors. The enforced expression of ICN1 in hematopoietic progenitors restored thymic T cell differentiation, even when the TECs were deficient in Dll4. These results indicate that the thymus-specific environment for determining T cell fate indispensably requires Dll4 expression to induce Notch signaling in the thymic immigrant cells.

Mind bomb 1 in the lymphopoietic niches is essential for T and marginal zone B cell development

Notch signaling regulates lineage decisions at multiple stages of lymphocyte development, and Notch activation requires the endocytosis of Notch ligands in the signal-sending cells. Four E3 ubiquitin ligases, Mind bomb (Mib) 1, Mib2, Neuralized (Neur) 1, and Neur2, regulate the Notch ligands to activate Notch signaling, but their roles in lymphocyte development have not been defined. We show that Mib1 regulates T and marginal zone B (MZB) cell development in the lymphopoietic niches. Inactivation of the Mib1 gene, but not the other E3 ligases, Mib2, Neur1, and Neur2, abrogated T and MZB cell development. Reciprocal bone marrow (BM) transplantation experiments revealed that Mib1 in the thymic and splenic niches is essential for T and MZB cell development. Interestingly, when BM cells from transgenic Notch reporter mice were transplanted into Mib1-null mice, the Notch signaling was abolished in the double-negative thymocytes. In addition, the endocytosis of Dll1 was impaired in the Mib1-null microenvironment. Moreover, the block in T cell development and the failure of Dll1 endocytosis were also observed in coculture system by Mib1 knockdown. Our study reveals that Mib1 is the essential E3 ligase in T and MZB cell development, through the regulation of Notch ligands in the thymic and splenic microenvironments.

Competition and collaboration: GATA-3, PU.1, and Notch signaling in early T-cell fate determination

T-cell precursors remain developmentally plastic for multiple cell generations after entering the thymus, preserving access to developmental alternatives of macrophage, dendritic-cell, and even mast-cell fates. The underlying regulatory basis of this plasticity is that early T-cell differentiation depends on transcription factors which can also promote alternative developmental programs. Interfactor competition, together with environmental signals, keep these diversions under control. Here the pathways leading to several lineage alternatives for early pro-T-cells are reviewed, with close focus on the mechanisms of action of three vital factors, GATA-3, PU.1, and Notch-Delta signals, whose counterbalance appears to be essential for T-cell specification.

Delineating the cellular pathways of hematopoietic lineage commitment

The prevailing model for adult hematopoiesis postulates that the first lineage commitment step results in a strict separation of common myeloid and common lymphoid pathways. However, the recent identification of granulocyte/monocyte (GM)-lymphoid restricted lymphoid-primed multipotent progenitors (LMPPs) and primitive common myeloid progenitors (CMPs) within the "HSC" compartment provide compelling support for establishment of independent GM-megakaryocyte/erythroid (GM-MkE) and GM-lymphoid commitment pathways as decisive early lineage fate decisions. These changes in lineage potentials are corroborated by corresponding changes in multilineage transcriptional priming, as LMPPs down-regulate MkE priming but become GM-lymphoid transcriptionally primed, whereas CMPs are GM-MkE primed. These distinct biological and molecular relationships are established already in the fetal liver.

The signaling protein Wnt4 enhances thymopoiesis and expands multipotent hematopoietic progenitors through beta-catenin-independent signaling

Despite studies based on deletion or activation of intracellular components of the canonical Wingless related (Wnt) pathway, the role of Wnts in hematolymphopoiesis remains controversial. Using gain-of-function and loss-of-function models, we found that Wnt4 differentially affected diverse subsets of hematopoietic stem and progenitor cells. Bone-marrow and thymic Lin(-)Sca1(+)Kit(hi) cells (LSKs) were the key targets of Wnt4. In adult mice, Wnt4-induced expansion of Flt3(+) bone-marrow LSKs (lymphoid-primed multipotent progenitors) led to a sizeable accumulation of the most immature thymocyte subsets (upstream of beta-selection) and a major increase in thymopoiesis. Conversely, Wnt4(-/-) neonates showed low frequencies of bone-marrow LSKs and thymic hypocellularity. We provide compelling evidence that Wnt4 activates noncanonical (beta-catenin-independent) signaling and that its effects on hematopoietic cells are mainly non-cell-autonomous. Our work shows that Wnt4 overexpression has a unique ability to expand Flt3(+) LSKs in adults and demonstrates that noncanonical Wnt signaling regulates thymopoiesis.

Molecular-genetic insights in paediatric T-cell acute lymphoblastic leukaemia

Paediatric T-cell acute lymphoblastic leukaemia (T-ALL) is an aggressive malignancy of thymocytes that accounts for about 15% of ALL cases and for which treatment outcome remains inferior compared to B-lineage acute leukaemias. In T-ALL, leukemic transformation of maturating thymocytes is caused by a multistep pathogenesis involving numerous genetic abnormalities that drive normal T-cells into uncontrolled cell growth and clonal expansion. This review provides an overview of the current knowledge on onco- and tumor suppressor genes in T-ALL and suggests a classification of these genetic defects into type A and type B abnormalities. Type A abnormalities may delineate distinct molecular-cytogenetic T-ALL subgroups, whereas type B abnormalities are found in all major T-ALL subgroups and synergize with these type A mutations during T-cell pathogenesis.

E2A proteins promote development of lymphoid-primed multipotent progenitors

The first lymphoid-restricted progeny of hematopoietic stem cells (HSCs) are lymphoid-primed multipotent progenitors (LMPPs), which have little erythromyeloid potential but retain lymphoid, granulocyte, and macrophage differentiation capacity. Despite recent advances in the identification of LMPPs, the transcription factors essential for their generation remain to be identified. Here, we demonstrated that the E2A transcription factors were required for proper development of LMPPs. Within HSCs and LMPPs, E2A proteins primed expression of a subset of lymphoid-associated genes and prevented expression of genes that are not normally prevalent in these cells, including HSC-associated and nonlymphoid genes. E2A proteins also restricted proliferation of HSCs, MPPs, and LMPPs and antagonized differentiation of LMPPs toward the myeloid fate. Our results reveal that E2A proteins play a critical role in supporting lymphoid specification from HSCs and that the reduced generation of LMPPs underlies the severe lymphocyte deficiencies observed in E2A-deficient mice.

T and B lymphocyte differentiation from hematopoietic stem cell

Until the past few years, it has been thought that lymphoid and myeloid lineage segregation represents the first step of lineage restriction during hematopoiesis from hematopoietic stem cell. Recent investigation of the cell populations within multipotent progenitors in the bone marrow has led to new understanding of how hematopoietic stem cells diversify into different hematopoietic cell types. This review focuses on the recent advances in understanding the developmental events that occur during hematopoietic stem cell specification into the T and B lymphocyte lineages in adult mice.

Canonical notch signaling is dispensable for the maintenance of adult hematopoietic stem cells

Gain-of-function experiments have demonstrated the potential of Notch signals to expand primitive hematopoietic progenitors, but whether Notch physiologically regulates hematopoietic stem cell (HSC) homeostasis in vivo is unclear. To answer this question, we evaluated the effect of global deficiencies of canonical Notch signaling in rigorous HSC assays. Hematopoietic progenitors expressing dominant-negative Mastermind-like1 (DNMAML), a potent inhibitor of Notch-mediated transcriptional activation, achieved stable long-term reconstitution of irradiated hosts and showed a normal frequency of progenitor fractions enriched for long-term HSCs. Similar results were observed with cells lacking CSL/RBPJ, a DNA-binding factor that is required for canonical Notch signaling. Notch-deprived progenitors provided normal long-term reconstitution after secondary competitive transplantation. Furthermore, Notch target genes were expressed at low levels in primitive hematopoietic progenitors. Taken together, these results rule out an essential physiological role for cell-autonomous canonical Notch signals in HSC maintenance.

Notch signaling in leukemia

Recent discoveries indicate that gain-of-function mutations in the Notch1 receptor are very common in human T cell acute lymphoblastic leukemia/lymphoma. This review discusses what these mutations have taught us about normal and pathophysiologic Notch1 signaling, and how these insights may lead to new targeted therapies for patients with this aggressive form of cancer.

The stream of precursors that colonizes the thymus proceeds selectively through the early T lineage precursor stage of T cell development

T cell development in the thymus depends on continuous colonization by hematopoietic precursors. Several distinct T cell precursors have been identified, but whether one or several independent precursor cell types maintain thymopoiesis is unclear. We have used thymus transplantation and an inducible lineage-tracing system to identify the intrathymic precursor cells among previously described thymus-homing progenitors that give rise to the T cell lineage in the thymus. Extrathymic precursors were not investigated in these studies. Both approaches show that the stream of T cell lineage precursor cells, when entering the thymus, selectively passes through the early T lineage precursor (ETP) stage. Immigrating precursor cells do not exhibit characteristics of double-negative (DN) 1c, DN1d, or DN1e stages, or of populations containing the common lymphoid precursor 2 (CLP-2) or the thymic equivalent of circulating T cell progenitors (CTPs). It remains possible that an unknown hematopoietic precursor cell or previously described extrathymic precursors with a CLP, CLP-2, or CTP phenotype feed into T cell development by circumventing known intrathymic T cell lineage progenitor cells. However, it is clear that of the known intrathymic precursors, only the ETP population contributes significant numbers of T lineage precursors to T cell development.

Core binding factors are necessary for natural killer cell development and cooperate with Notch signaling during T-cell specification

CBFbeta is the non-DNA binding subunit of the core binding factors (CBFs). Mice with reduced CBFbeta levels display profound, early defects in T-cell but not B-cell development. Here we show that CBFbeta is also required at very early stages of natural killer (NK)-cell development. We also demonstrate that T-cell development aborts during specification, as the expression of Gata3 and Tcf7, which encode key regulators of T lineage specification, is substantially reduced, as are functional thymic progenitors. Constitutively active Notch or IL-7 signaling cannot restore T-cell expansion or differentiation of CBFbeta insufficient cells, nor can overexpression of Runx1 or CBFbeta overcome a lack of Notch signaling. Therefore, the ability of the prethymic cell to respond appropriately to Notch is dependent on CBFbeta, and both signals converge to activate the T-cell developmental program.