Notch regulation of early thymocyte development

Isoform-specific functions of Ras in T-cell development and differentiation

Ras GTPases, well characterized for their role in oncogenesis, are the cells' molecular switches that signal to maintain immune homeostasis through cellular development, proliferation, differentiation, survival, and apoptosis. In the immune system, T cells are the central players that cause autoimmunity if dysregulated. Antigen-specific T-cell receptor (TCR) stimulation activates Ras-isoforms, which exhibit isoform-specific activator and effector requirements, functional specificities, and a selective role in T-cell development and differentiation. Recent studies show the role of Ras in T-cell-mediated autoimmune diseases; however, there is a scarcity of knowledge about the role of Ras in T-cell development and differentiation. To date, limited studies have demonstrated Ras activation in response to positive and negative selection signals and Ras isoform-specific signaling, including subcellular signaling, in immune cells. The knowledge of isoform-specific functions of Ras in T cells is essential, but still inadequate to develop the T-cell-targeted Ras isoform-specific treatment strategies for the diseases caused by altered Ras-isoform expression and activation in T cells. In this review, we discuss the role of Ras in T-cell development and differentiation, critically analyzing the isoform-specific functions.

Fate of hematopoietic stem cells determined by Notch1 signaling (Review)

Regulation of the fate of hematopoietic stem cells (HSCs), including silencing, self-renewal or differentiation into blood line cells, is crucial to maintain the homeostasis of the human blood system and prevent leukemia. Notch1, a key receptor in the Notch signaling pathway, plays an important regulatory role in these properties of HSCs, particularly in the maintenance of the stemness of HSCs. In recent decades, the ubiquitination modification of Notch1 has been gradually revealed, and also demonstrated to affect the proliferation and differentiation of HSCs. Therefore, a detailed elucidation of Notch1 and its ubiquitination modification may help to improve understanding of the maintenance of HSC properties and the pathogenesis of leukemia. In addition, it may aid in identifying potential therapeutic targets for specific leukemias and provide potential prognostic indicators for HSC transplantation (HSCT). In the present review, the association between Notch1 and HSCs and the link between the ubiquitination modification of Notch1 and HSCs were described. In addition, the association between abnormal HSCs mediated by Notch1 or ubiquitinated Notch1and T-cell acute lymphoblastic leukemia (T-ALL) was also examined, which provides a promising direction for clinical application.

MicroRNAs as Modulators of the Immune Response in T-Cell Acute Lymphoblastic Leukemia

Acute lymphoblastic leukaemia (ALL) is an aggressive haematological tumour driven by the malignant transformation and expansion of B-cell (B-ALL) or T-cell (T-ALL) progenitors. The evolution of T-ALL pathogenesis encompasses different master developmental pathways, including the main role played by Notch in cell fate choices during tissue differentiation. Recently, a growing body of evidence has highlighted epigenetic changes, particularly the altered expression of microRNAs (miRNAs), as a critical molecular mechanism to sustain T-ALL. The immune response is emerging as key factor in the complex multistep process of cancer but the role of miRNAs in anti-leukaemia response remains elusive. In this review we analyse the available literature on miRNAs as tuners of the immune response in T-ALL, focusing on their role in Natural Killer, T, T-regulatory and Myeloid-derived suppressor cells. A better understanding of this molecular crosstalk may provide the basis for the development of potential immunotherapeutic strategies in the leukemia field.

New insights into TCR β-selection

T cell receptor (TCR) β-selection (herein referred to as β-selection) is a pivotal checkpoint in mammalian T cell development when immature CD4^-CD8^- T-cells (thymocytes) express pre-TCR following successful Tcrb gene rearrangement. At this stage, αβ T cell lineage commitment and allelic exclusion to restrict one β-chain per cell take place and thymocytes undergo a proliferative burst. β-selection is known to be crucially dependent upon synchronized Notch and pre-TCR signaling; however, other necessary inputs have been identified over the past decade, expanding our knowledge and understanding of the β-selection process. In this review, we discuss recent mechanistic findings that have enabled a more detailed decoding of the molecular dynamics of the β-selection checkpoint and have helped to elucidate its role in early T cell development.

How transcription factors drive choice of the T cell fate

Recent evidence has elucidated how multipotent blood progenitors transform their identities in the thymus and undergo commitment to become T cells. Together with environmental signals, a core group of transcription factors have essential roles in this process by directly activating and repressing specific genes. Many of these transcription factors also function in later T cell development, but control different genes. Here, we review how these transcription factors work to change the activities of specific genomic loci during early intrathymic development to establish T cell lineage identity. We introduce the key regulators and highlight newly emergent insights into the rules that govern their actions. Whole-genome deep sequencing-based analysis has revealed unexpectedly rich relationships between inherited epigenetic states, transcription factor-DNA binding affinity thresholds and influences of given transcription factors on the activities of other factors in the same cells. Together, these mechanisms determine T cell identity and make the lineage choice irreversible.

PHF6 Expression Levels Impact Human Hematopoietic Stem Cell Differentiation

Transcriptional control of hematopoiesis involves complex regulatory networks and functional perturbations in one of these components often results in malignancies. Loss-of-function mutations in PHF6, encoding a presumed epigenetic regulator, have been primarily described in T cell acute lymphoblastic leukemia (T-ALL) and the first insights into its function in normal hematopoiesis only recently emerged from mouse modeling experiments. Here, we investigated the role of PHF6 in human blood cell development by performing knockdown studies in cord blood and thymus-derived hematopoietic precursors to evaluate the impact on lineage differentiation in well-established in vitro models. Our findings reveal that PHF6 levels differentially impact the differentiation of human hematopoietic progenitor cells into various blood cell lineages, with prominent effects on lymphoid and erythroid differentiation. We show that loss of PHF6 results in accelerated human T cell development through reduced expression of NOTCH1 and its downstream target genes. This functional interaction in developing thymocytes was confirmed in vivo using a phf6-deficient zebrafish model that also displayed accelerated developmental kinetics upon reduced phf6 or notch1 activation. In summary, our work reveals that appropriate control of PHF6 expression is important for normal human hematopoiesis and provides clues towards the role of PHF6 in T-ALL development.

Quantitative analysis reveals reciprocal regulations underlying recovery dynamics of thymocytes and thymic environment in mice

Thymic crosstalk, a set of reciprocal regulations between thymocytes and the thymic environment, is relevant for orchestrating appropriate thymocyte development as well as thymic recovery from various exogenous insults. In this work, interactions shaping thymic crosstalk and the resultant dynamics of thymocytes and thymic epithelial cells are inferred based on quantitative analysis and modeling of the recovery dynamics induced by irradiation. The analysis identifies regulatory interactions consistent with known molecular evidence and reveals their dynamic roles in the recovery process. Moreover, the analysis also predicts, and a subsequent experiment verifies, a previously unrecognized regulation of CD4+CD8+ double positive thymocytes which temporarily increases their proliferation rate upon the decrease in their population size. Our model establishes a pivotal step towards the dynamic understanding of thymic crosstalk as a regulatory network system.

Notch and the pre-TCR coordinate thymocyte proliferation by induction of the SCF subunits Fbxl1 and Fbxl12

Proliferation is tightly regulated during T cell development and is limited to immature CD4⁻CD8⁻ thymocytes. The major proliferative event is initiated at the ‘β-selection’ stage following successful rearrangement of Tcrβ and is triggered by and dependent on concurrent signaling by Notch and the pre-TCR; however, it is unclear how these signals cooperate to promote cell proliferation. Here we found that β-selection-associated proliferation required the combined activity of two SCF ubiquitin ligase complexes that included as substrate recognition subunits the F-box proteins Fbxl1 or Fbxl12. Both SCF complexes targeted the cyclin-dependent kinase inhibitor Cdkn1b for ubiquitinylaton and degradation. We found that Notch signals induced the transcription of Fbxl1 whereas pre-TCR signals induced the transcription of Fbxl12. Thus, concurrent Notch and pre-TCR signaling induced the expression of two genes, Fbxl1 and Fbxl12, whose products functioned identically but additively to promote degradation of Cdkn1b, cell cycle progression, and proliferation of β-selected thymocytes.

Role of Notch signaling in regulating innate immunity and inflammation in health and disease

The Notch signaling pathway is conserved from Drosophila to mammals and is critically involved in developmental processes. In the immune system, it has been established that Notch signaling regulates multiple steps of T and B cell development in both central and peripheral lymphoid organs. Relative to the well documented role of Notch signaling in lymphocyte development, less is known about its role in regulating myeloid lineage development and function, especially in the context of acute and chronic inflammation. In this review article, we will describe the evidence accumulated during the recent years to support a key regulatory role of the Notch pathway in innate immune and inflammatory responses and discuss the potential implications of such regulation for pathogenesis and therapy of inflammatory disorders.

Activated Notch counteracts Ikaros tumor suppression in mouse and human T-cell acute lymphoblastic leukemia

Activating NOTCH1 mutations occur in ~60% of human T-cell acute lymphoblastic leukemias (T-ALLs), and mutations disrupting the transcription factor IKZF1 (IKAROS) occur in ~5% of cases. To investigate the regulatory interplay between these driver genes, we have used a novel transgenic RNA interference mouse model to produce primary T-ALLs driven by reversible Ikaros knockdown. Restoring endogenous Ikaros expression in established T-ALL in vivo acutely represses Notch1 and its oncogenic target genes including Myc, and in multiple primary leukemias causes disease regression. In contrast, leukemias expressing high levels of endogenous or engineered forms of activated intracellular Notch1 (ICN1) resembling those found in human T-ALL rapidly relapse following Ikaros restoration, indicating that ICN1 functionally antagonizes Ikaros in established disease. Furthermore, we find that IKAROS mRNA expression is significantly reduced in a cohort of primary human T-ALL patient samples with activating NOTCH1/FBXW7 mutations, but is upregulated upon acute inhibition of aberrant NOTCH signaling across a panel of human T-ALL cell lines. These results demonstrate for the first time that aberrant NOTCH activity compromises IKAROS function in mouse and human T-ALL, and provide a potential explanation for the relative infrequency of IKAROS gene mutations in human T-ALL.

From inception to output, Tcf1 and Lef1 safeguard development of T cells and innate immune cells

Transcription factors have recurring roles during T cell development and activation. Tcf1 and Lef1 are known to be essential for early stages of thymocyte maturation. Recent research has revealed several novel aspects of their functionality. Tcf1 is induced at the very earliest step of specifying hematopoietic progenitors to the T cell lineage as a key target gene downstream of Notch activation. In addition to promoting maturation of T-lineage-committed thymocytes, Tcf1 functions as a tumor suppressor in developing thymocytes, and this is mediated, paradoxically, by restraining Lef1 expression. After positive selection, Tcf1 and Lef1 act together to direct CD4(+)CD8(+) double positive thymocytes to a CD4(+) T cell fate. Although not required for CD8(+) T cell differentiation, Tcf1 and Lef1 cooperate with Runx factors to achieve stable silencing of the Cd4 gene in CD8(+) T cells. Tcf1 is also found to have versatile roles in innate immune cells, which partly mirror its functions in mature T helper cells. Discrepancy in requirements of Tcf1/Lef1 and β-catenin in T cells has been a long-standing enigma. We will review other protein factors interacting with Tcf1 and Lef1 and discuss their regulatory roles independent of β-catenin.

Targeting signaling pathways in T-cell acute lymphoblastic leukemia initiating cells

Leukemia initiating cells (LICs) represent a reservoir that is believed to drive relapse and resistance to chemotherapy in blood malignant disorders. T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive neoplastic disorder of immature hematopoietic precursors committed to the T-cell lineage. T-ALL comprises about 15% of pediatric and 25% of adult ALL cases and is prone to early relapse. Although the prognosis of T-ALL has improved especially in children due to the use of new intensified treatment protocols, the outcome of relapsed T-ALL cases is still poor. Putative LICs have been identified also in T-ALL. LICs are mostly quiescent and for this reason highly resistant to chemotherapy. Therefore, they evade treatment and give rise to disease relapse. At present great interest surrounds the development of targeted therapies against signaling networks aberrantly activated in LICs and important for their survival and drug-resistance. Both the Notch1 pathway and the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) network are involved in T-ALL LIC survival and drug-resistance and could be targeted by small molecules. Thus, Notch1 and PI3K/Akt/mTOR inhibitors are currently being developed for clinical use either as single agents or in combination with conventional chemotherapy for T-ALL patient treatment. In this review, we summarize the existing knowledge of the relevance of Notch1 and PI3K/Akt/mTOR signaling in T-ALL LICs and we examine the rationale for targeting these key signal transduction networks by means of selective pharmacological inhibitors.

Transcriptional control of early T and B cell developmental choices

T and B cells share a common somatic gene rearrangement mechanism for assembling the genes that code for their antigen receptors; they also have developmental pathways with many parallels. Shared usage of basic helix-loop-helix E proteins as transcriptional drivers underlies these common features. However, the transcription factor networks in which these E proteins are embedded are different both in membership and in architecture for T and B cell gene regulatory programs. These differences permit lineage commitment decisions to be made in different hierarchical orders. Furthermore, in contrast to B cell gene networks, the T cell gene network architecture for effector differentiation is sufficiently modular so that E protein inputs can be removed. Complete T cell-like effector differentiation can proceed without T cell receptor rearrangement or selection when E proteins are neutralized, yielding natural killer and other innate lymphoid cells.

Characterization of two distinct lymphoproliferative diseases caused by ectopic expression of the Notch ligand DLL4 on T cells

Notch signaling is essential for the development of T cell progenitors through the interaction of NOTCH1 receptor on their surface with the ligand, Delta-like 4 (DLL4), which is expressed by the thymic epithelial cells. Notch signaling is quickly shut down once the cells pass β-selection, and CD4/CD8 double positive (DP) cells are unresponsive to Notch. Over the past two decades a number of papers reported that over-activation of Notch signaling causes T cell acute lymphoblastic leukemia (T-ALL), a cancer that prominently features circulating monoclonal CD4/CD8 double positive T cells in different mouse models. However, the possible outcomes of Notch over-activation at different stages of T cell development are unknown, and the fine timing of Notch signaling that results in T-ALL is poorly understood. Here we report, by using a murine model that ectopically expresses DLL4 on developing T cells, that the T-ALL onset is highly dependent on a sustained Notch activity throughout the DP stage, which induces additional mutations to further boost the signaling. In contrast, a shorter period of Notch activation that terminates at the DP stage causes a polyclonal, non-transmissible lymphoproliferative disorder that is also lethal. These observations resolved the discrepancy of previous papers on DLL4 driven hematological diseases in mice, and show the critical importance of the timing and duration of Notch activity.

Progress in understanding the relationship between Notch signaling pathway and hepatic stellate cell activation

Multiple signaling pathways are involved in the pathogenesis of hepatic fibrosis, and the Notch signaling pathway plays an important role in promoting the activation of hepatic stellate cells (HSCs). This pathway participates in the activation of HSCs mainly by cooperating with transforming growth factor β (TGF-β)/BMP, nuclear factor-kappa B (NF-κB), and Wnt signaling pathways directly or indirectly. This review aims to explore the relationship between the Notch signaling pathway and the activation of HSCs as well as the cooperative actions between TGF-β/BMP, NF-κB, and Wnt and the Notch signaling pathway in the process of the activation of HSCs.

Gauging NOTCH1 Activation in Cancer Using Immunohistochemistry

Fixed, paraffin-embedded (FPE) tissues are a potentially rich resource for studying the role of NOTCH1 in cancer and other pathologies, but tests that reliably detect activated NOTCH1 (NICD1) in FPE samples have been lacking. Here, we bridge this gap by developing an immunohistochemical (IHC) stain that detects a neoepitope created by the proteolytic cleavage event that activates NOTCH1. Following validation using xenografted cancers and normal tissues with known patterns of NOTCH1 activation, we applied this test to tumors linked to dysregulated Notch signaling by mutational studies. As expected, frequent NICD1 staining was observed in T lymphoblastic leukemia/lymphoma, a tumor in which activating NOTCH1 mutations are common. However, when IHC was used to gauge NOTCH1 activation in other human cancers, several unexpected findings emerged. Among B cell tumors, NICD1 staining was much more frequent in chronic lymphocytic leukemia than would be predicted based on the frequency of NOTCH1 mutations, while mantle cell lymphoma and diffuse large B cell lymphoma showed no evidence of NOTCH1 activation. NICD1 was also detected in 38% of peripheral T cell lymphomas. Of interest, NICD1 staining in chronic lymphocytic leukemia cells and in angioimmunoblastic lymphoma was consistently more pronounced in lymph nodes than in surrounding soft tissues, implicating factors in the nodal microenvironment in NOTCH1 activation in these diseases. Among carcinomas, diffuse strong NICD1 staining was observed in 3.8% of cases of triple negative breast cancer (3 of 78 tumors), but was absent from 151 non-small cell lung carcinomas and 147 ovarian carcinomas. Frequent staining of normal endothelium was also observed; in line with this observation, strong NICD1 staining was also seen in 77% of angiosarcomas. These findings complement insights from genomic sequencing studies and suggest that IHC staining is a valuable experimental tool that may be useful in selection of patients for clinical trials.

Specific Notch receptor-ligand interactions control human TCR-αβ/γδ development by inducing differential Notch signal strength

Jagged2 preferentially signals through Notch3 to promote γδ T cell development.

In humans, high Notch activation promotes γδ T cell development, whereas lower levels promote αβ-lineage differentiation. How these different Notch signals are generated has remained unclear. We show that differential Notch receptor–ligand interactions mediate this process. Whereas Delta-like 4 supports both TCR-αβ and -γδ development, Jagged1 induces mainly αβ-lineage differentiation. In contrast, Jagged2-mediated Notch activation primarily results in γδ T cell development and represses αβ-lineage differentiation by inhibiting TCR-β formation. Consistently, TCR-αβ T cell development is rescued through transduction of a TCR-β transgene. Jagged2 induces the strongest Notch signal through interactions with both Notch1 and Notch3, whereas Delta-like 4 primarily binds Notch1. In agreement, Notch3 is a stronger Notch activator and only supports γδ T cell development, whereas Notch1 is a weaker activator supporting both TCR-αβ and -γδ development. Fetal thymus organ cultures in JAG2-deficient thymic lobes or with Notch3-blocking antibodies confirm the importance of Jagged2/Notch3 signaling in human TCR-γδ differentiation. Our findings reveal that differential Notch receptor–ligand interactions mediate human TCR-αβ and -γδ T cell differentiation and provide a mechanistic insight into the high Notch dependency of human γδ T cell development.

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Human T-lymphoid progenitors generated in a feeder-cell-free Delta-like-4 culture system promote T-cell reconstitution in NOD/SCID/γc(-/-) mice

Slow T-cell reconstitution is a major clinical concern after transplantation of cord blood (CB)-derived hematopoietic stem cells. Adoptive transfer of in vitro-generated T-cell progenitors has emerged as a promising strategy for promoting de novo thymopoiesis and thus accelerating T-cell reconstitution. Here, we describe the development of a new culture system based on the immobilized Notch ligand Delta-like-4 (DL-4). Culture of human CD34⁺ CB cells in this new DL-4 system enabled the in vitro generation of large amounts of T-cell progenitor cells that (a) displayed the phenotypic and molecular signatures of early thymic progenitors and (b) had high T lymphopoietic potential. When transferred into NOD/SCID/γc^−/− (NSG) mice, DL-4 primed T-cell progenitors migrated to the thymus and developed into functional, mature, polyclonal αβ T cells that subsequently left the thymus and accelerated T-cell reconstitution. T-cell reconstitution was even faster and more robust when ex vivo-manipulated and nonmanipulated CB samples were simultaneously injected into NSG mice (i.e., a situation reminiscent of the double CB transplant setting). This work provides further evidence of the ability of in vitro-generated human T-cell progenitors to accelerate T-cell reconstitution and also introduces a feeder-cell-free culture technique with the potential for rapid, safe transfer to a clinical setting.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

The TCF-1 and LEF-1 transcription factors have cooperative and opposing roles in T cell development and malignancy

The TCF-1 and LEF-1 transcription factors are known to play critical roles in normal thymocyte development. Unexpectedly, we found that TCF-1-deficient (Tcf7(-/-)) mice developed aggressive T cell malignancy, resembling human T cell acute lymphoblastic leukemia (T-ALL). LEF-1 was aberrantly upregulated in premalignant Tcf7(-/-) early thymocytes and lymphoma cells. We further demonstrated that TCF-1 directly repressed LEF-1 expression in early thymocytes and that conditional inactivation of Lef1 greatly delayed or prevented T cell malignancy in Tcf7(-/-) mice. In human T-ALLs, an early thymic progenitor (ETP) subtype was associated with diminished TCF7 expression, and two of the ETP-ALL cases harbored TCF7 gene deletions. We also showed that TCF-1 and LEF-1 were dispensable for T cell lineage commitment but instead were required for early thymocytes to mature beyond the CD4(-)CD8(-) stage. TCF-1 thus has dual roles, i.e., acting cooperatively with LEF-1 to promote thymocyte maturation while restraining LEF-1 expression to prevent malignant transformation of developing thymocytes.

Therapeutic potential of Notch inhibition in T-cell acute lymphoblastic leukemia: rationale, caveats and promises

T-cell acute lymphoblastic leukemia (T-ALL) is a malignancy that presents with poor prognosis. Treatment relies on the application of aggressive therapies that produce deleterious side-effects, justifying the quest for novel, more efficient and selective molecular targeting agents. Mutations leading to abnormal Notch-1 activity are present in more than half of the T-ALL patients, underscoring the potential therapeutic relevance of targeting Notch-1 inhibition and further reinforcing the need to better comprehend the mechanisms by which Notch-1 drives T cell leukemogenesis. Clinical application of γ-secretase inhibitors to block Notch signaling in T-ALL revealed new challenges that involve improvement of the therapeutic benefit and reduction of intestinal toxicity. Here, we review the latest advances in the development and use of Notch antagonists and summarize the current knowledge on Notch function in T-ALL to understand how it may translate into novel therapeutic strategies that increment the efficiency of Notch inhibition.

Notch2 receptor signaling controls functional differentiation of dendritic cells in the spleen and intestine

Dendritic cells (DCs) in tissues and lymphoid organs comprise distinct functional subsets that differentiate in situ from circulating progenitors. Tissue-specific signals that regulate DC subset differentiation are poorly understood. We report that DC-specific deletion of the Notch2 receptor caused a reduction of DC populations in the spleen. Within the splenic CD11b(+) DC subset, Notch signaling blockade ablated a distinct population marked by high expression of the adhesion molecule Esam. The Notch-dependent Esam(hi) DC subset required lymphotoxin beta receptor signaling, proliferated in situ, and facilitated CD4(+) T cell priming. The Notch-independent Esam(lo) DCs expressed monocyte-related genes and showed superior cytokine responses. In addition, Notch2 deletion led to the loss of CD11b(+)CD103(+) DCs in the intestinal lamina propria and to a corresponding decrease of IL-17-producing CD4(+) T cells in the intestine. Thus, Notch2 is a common differentiation signal for T cell-priming CD11b(+) DC subsets in the spleen and intestine.

On becoming a T cell, a convergence of factors kick it up a Notch along the way

The thymus is seeded by bone marrow-derived progenitors, which undergo a series of differentiation and proliferation events in order to generate functional T lymphocytes. The Notch signaling pathway, together with multiple transcription factors, act in concert to commit progenitors to a T-lineage fate, extinguishing non-T cell potential, inducing thymocyte differentiation and supporting proliferation and survival along the way to becoming a mature T cell. This review focuses on recent evidence regarding the complex interplay between the Notch pathway and other key transcription factors at specific lineage-decision points during the program of T cell development.

Factors and networks that underpin early hematopoiesis

Multiple trajectories have recently been described through which hematopoietic progenitor cells travel prior to becoming lineage-committed effectors. A wide spectrum of transcription factors has recently been identified that modulate developmental progression along such trajectories. Here we describe how distinct families of transcription factors act and are linked together to orchestrate early hematopoiesis.

Differential regulation of Notch ligands in dendritic cells upon interaction with T helper cells

The Notch signalling pathway has recently been linked to T helper 1 (Th1)/T helper 2 (Th2) cell polarization via a mechanism involving differential expression of Notch ligands, Delta-like and Jagged, in antigen-presenting cells. However, whether stimuli other than pathogen-derived factors are involved in the regulation of Notch ligand expression in dendritic cells (DCs) remains unknown. Here, we address the effect of T helper cells (Th1 and Th2) on Delta-like 4 and Jagged 2 expression in bone marrow-derived DCs. We demonstrate that both Th1 and Th2 cells induce Delta-like 4 mRNA expression in DCs, in a process that is, in part, mediated by CD40 signalling. In contrast, only Th2 cells induce a significant increase in Jagged 2 mRNA levels in DCs. Additionally, we show that IL-4, a hallmark Th2 cytokine, plays a role in Jagged 2 expression, as evidenced by the fact that cholera toxin, a Th2-promoting stimulus, induces Jagged 2 mRNA expression in DCs only in the presence of IL-4. Finally, we demonstrate that DCs also express Notch 1 and that this expression is downregulated by IL-4. These data suggest that Notch ligands are differentially regulated in DCs: Delta-like 4 is regulated by T helper cells and by pathogen-derived Th1 stimuli, whereas Jagged 2 is regulated by Th2 cells and pathogen-derived Th2-promoting stimuli. Based on our results, we propose that the positive feedback loop that Th2 cells exert on T cell polarization may involve the induction of Jagged 2 expression in DCs.

Cell of origin strongly influences genetic selection in a mouse model of T-ALL

Identifying the normal cell from which a tumor originates is crucial to understanding the etiology of that cancer. However, retrospective identification of the cell of origin in cancer is challenging because of the accumulation of genetic and epigenetic changes in tumor cells. The biologic state of the cell of origin likely influences the genetic events that drive transformation. We directly tested this hypothesis by performing a Sleeping Beauty transposon mutagenesis screen in which common insertion sites were identified in tumors that were produced by mutagenesis of cells at varying time points throughout the T lineage. Mutation and gene expression data derived from these tumors were then compared with data obtained from a panel of 84 human T-cell acute lymphoblastic leukemia samples, including copy number alterations and gene expression profiles. This revealed that altering the cell of origin produces tumors that model distinct subtypes of human T-cell acute lymphoblastic leukemia, suggesting that even subtle changes in the cell of origin dramatically affect genetic selection in tumors. These findings have broad implications for the genetic analysis of human cancers as well as the production of mouse models of cancer.

The Notch signaling pathway: molecular basis of cell context dependency

Notch receptor signaling controls cell-fate specification, self-renewal, differentiation, proliferation and apoptosis throughout development and regeneration in all animal species studied to date. Its dysfunction causes several developmental defects and diseases in the adult. A key feature of Notch signaling is its remarkable cell-context dependency. In this review, we summarize the influences of the cellular context that regulate Notch activity and propose a model how the interplay between the cell-intrinsically established chromatin state and the cell-extrinsic signals that modify chromatin may select for Notch target accessibility and activation in different cellular contexts.