A novel Notch ligand, Dll4, induces T-cell leukemia/lymphoma when overexpressed in mice by retroviral-mediated gene transfer

Normalization of the tumor microenvironment by harnessing vascular and immune modulation to achieve enhanced cancer therapy

Solid tumors are complex entities that actively shape their microenvironment to create a supportive environment for their own growth. Angiogenesis and immune suppression are two key characteristics of this tumor microenvironment. Despite attempts to deplete tumor blood vessels using antiangiogenic drugs, extensive vessel pruning has shown limited efficacy. Instead, a targeted approach involving the judicious use of drugs at specific time points can normalize the function and structure of tumor vessels, leading to improved outcomes when combined with other anticancer therapies. Additionally, normalizing the immune microenvironment by suppressing immunosuppressive cells and activating immunostimulatory cells has shown promise in suppressing tumor growth and improving overall survival. Based on these findings, many studies have been conducted to normalize each component of the tumor microenvironment, leading to the development of a variety of strategies. In this review, we provide an overview of the concepts of vascular and immune normalization and discuss some of the strategies employed to achieve these goals.

A double-negative thymocyte-specific enhancer augments Notch1 signaling to direct early T cell progenitor expansion, lineage restriction and β-selection

T cell differentiation requires Notch1 signaling. In the present study, we show that an enhancer upstream of Notch1 active in double-negative (DN) mouse thymocytes is responsible for raising Notch1 signaling intrathymically. This enhancer is required to expand multipotent progenitors intrathymically while delaying early differentiation until lineage restrictions have been established. Early thymic progenitors lacking the enhancer show accelerated differentiation through the DN stages and increased frequency of B, innate lymphoid (IL) and natural killer (NK) cell differentiation. Transcription regulators for T cell lineage restriction and commitment are expressed normally, but IL and NK cell gene expression persists after T cell lineage commitment and T cell receptor β VDJ recombination, Cd3 expression and β-selection have been impaired. This Notch1 enhancer is inactive in double-positive (DP) thymocytes. Its aberrant reactivation at this stage in Ikaros mutants is required for leukemogenesis. Thus, the DN-specific Notch1 enhancer harnesses the regulatory architecture of DN and DP thymocytes to achieve carefully orchestrated changes in Notch1 signaling required for early lineage restrictions and normal T cell differentiation.

Low-density-lipoprotein-receptor-related protein 1 mediates Notch pathway activation

The Notch signaling pathway controls cell growth, differentiation, and fate decisions, and its dysregulation has been linked to various human genetic disorders and cancers. To comprehensively understand the global organization of the Notch pathway and identify potential drug targets for Notch-related diseases, we established a protein interaction landscape for the human Notch pathway. By combining and analyzing genetic and phenotypic data with bioinformatics analysis, we greatly expanded this pathway and identified many key regulators, including low-density-lipoprotein-receptor-related protein 1 (LRP1). We demonstrated that LRP1 mediates the ubiquitination chain linkage switching of Delta ligands, which further affects ligand recycling, membrane localization, and stability. LRP1 inhibition led to Notch signaling inhibition and decreased tumorigenesis in leukemia models. Our study provides a glimpse into the Notch pathway interaction network and uncovers LRP1 as one critical regulator of the Notch pathway, as well as a possible therapeutic target for Notch-related cancers.

DLL1 orchestrates CD8+ T cells to induce long-term vascular normalization and tumor regression

The immunosuppressive and hypoxic tumor microenvironment (TME) remains a major obstacle to impede cancer immunotherapy. Here, we showed that elevated levels of Delta-like 1 (DLL1) in the breast and lung TME induced long-term tumor vascular normalization to alleviate tumor hypoxia and promoted the accumulation of interferon γ (IFN-γ)-expressing CD8⁺ T cells and the polarization of M1-like macrophages. Moreover, increased DLL1 levels in the TME sensitized anti-cytotoxic T lymphocyte-associated protein 4 (anti-CTLA4) treatment in its resistant tumors, resulting in tumor regression and prolonged survival. Mechanically, in vivo depletion of CD8⁺ T cells or host IFN-γ deficiency reversed tumor growth inhibition and abrogated DLL1-induced tumor vascular normalization without affecting DLL1-mediated macrophage polarization. Together, these results demonstrate that elevated DLL1 levels in the TME promote durable tumor vascular normalization in a CD8⁺ T cell- and IFN-γ-dependent manner and potentiate anti-CTLA4 therapy. Our findings unveil DLL1 as a potential target to persistently normalize the TME to facilitate cancer immunotherapy.

Spleen plays a major role in DLL4-driven acute T-cell lymphoblastic leukemia

The Notch pathway is highly active in almost all patients with T-cell acute lymphoblastic leukemia (T-ALL), but the implication of Notch ligands in T-ALL remains underexplored. Methods: We used a genetic mouse model of Notch ligand delta like 4 (DLL4)-driven T-ALL and performed thymectomies and splenectomies in those animals. We also used several patient-derived T-ALL (PDTALL) models, including one with DLL4 expression on the membrane and we treated PDTALL cells in vitro and in vivo with demcizumab, a blocking antibody against human DLL4 currently being tested in clinical trials in patients with solid cancer. Results: We show that surgical removal of the spleen abrogated T-ALL development in our preclinical DLL4-driven T-ALL mouse model. Mechanistically, we found that the spleen, and not the thymus, promoted the accumulation of circulating CD4⁺CD8⁺ T cells before T-ALL onset, suggesting that DLL4-driven T-ALL derives from these cells. Then, we identified a small subset of T-ALL patients showing higher levels of DLL4 expression. Moreover, in mice xenografted with a DLL4-positive PDTALL model, treatment with demcizumab had the same therapeutic effect as global Notch pathway inhibition using the potent γ-secretase inhibitor dibenzazepine. This result demonstrates that, in this PDTALL model, Notch pathway activity depends on DLL4 signaling, thus validating our preclinical mouse model. Conclusion: DLL4 expression in human leukemic cells can be a source of Notch activity in T-ALL, and the spleen plays a major role in a genetic mouse model of DLL4-driven T-ALL.

Notch1 Drives the Formation and Proliferation of Intrahepatic Cholangiocarcinoma

The molecular mechanisms underlying the development of intrahepatic cholangiocarcinoma (ICC) are not clear yet. In this study, we investigated the involvement of Notch1 in the development of ICC. The cDNA microarray analysis showed that Notch1 expression was higher in ICC tissues than in normal biliary epithelial cells. Stable transfection of Notch1 receptor intracellular domain (NICD1) by hydrodynamic tail vein injection induced ICC formation in mice. Western blotting confirmed that Notch1 signaling was activated in human ICC cell lines and mouse ICC tissues. Silencing Notch1 with specific short interfering RNA (siRNA) inhibited the proliferation of ICC cells. Flow cytometry and Western blotting indicated that apoptosis was induced in Notch1-silenced ICC cells compared with controls. Additionally, Notch1 silencing was associated with the inhibition of hairy and enhancer of split-1 (Hes1) and activation of the phosphatase and tensin homolog (PTEN)/p53 pathway. Taken together, these data suggest that Notch1 drives ICC formation and proliferation; downregulation of Notch1 induces apoptosis in ICC cells; Notch1 signaling may serve as a novel therapeutic target for the treatment of ICC.

Notch1 signaling in NOTCH1-mutated mantle cell lymphoma depends on Delta-Like ligand 4 and is a potential target for specific antibody therapy

Background

NOTCH1 gene mutations in mantle cell lymphoma (MCL) have been described in about 5–10% of cases and are associated with significantly shorter survival rates. The present study aimed to investigate the biological impact of this mutation in MCL and its potential as a therapeutic target.

Methods

Activation of Notch1 signaling upon ligand-stimulation and inhibitory effects of the monoclonal anti-Notch1 antibody OMP-52M51 in NOTCH1-mutated and -unmutated MCL cells were assessed by Western Blot and gene expression profiling. Effects of OMP-52M51 treatment on tumor cell migration and tumor angiogenesis were evaluated with chemotaxis and HUVEC tube formation assays. The expression of Delta-like ligand 4 (DLL4) in MCL lymph nodes was analyzed by immunofluorescence staining and confocal microscopy. A MCL mouse model was used to assess the activity of OMP-52M51 in vivo.

Results

Notch1 expression can be effectively stimulated in NOTCH1-mutated Mino cells by DLL4, whereas in the NOTCH1-unmutated cell line JeKo-1, less effect was observed upon any ligand-stimulation. DLL4 was expressed by histiocytes in both, NOTCH1-mutated and –unmutated MCL lymph nodes. Treatment of NOTCH1-mutated MCL cells with the monoclonal anti-Notch1 antibody OMP-52M51 effectively prevented DLL4-dependent activation of Notch1 and suppressed the induction of numerous direct Notch target genes involved in lymphoid biology, lymphomagenesis and disease progression. Importantly, in lymph nodes from primary MCL cases with NOTCH1/2 mutations, we detected an upregulation of the same gene sets as observed in DLL4-stimulated Mino cells. Furthermore, DLL4 stimulation of NOTCH1-mutated Mino cells enhanced tumor cell migration and angiogenesis, which could be abolished by treatment with OMP-52M51. Importantly, the effects observed were specific for NOTCH1-mutated cells as they did not occur in the NOTCH1-wt cell line JeKo-1. Finally, we confirmed the potential activity of OMP-52M51 to inhibit DLL4-induced Notch1-Signaling in vivo in a xenograft mouse model of MCL.

Conclusion

DLL4 effectively stimulates Notch1 signaling in NOTCH1-mutated MCL and is expressed by the microenvironment in MCL lymph nodes. Our results indicate that specific inhibition of the Notch1-ligand-receptor interaction might provide a therapeutic alternative for a subset of MCL patients.

From the outside, from within: Biological and therapeutic relevance of signal transduction in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological cancer that arises from clonal expansion of transformed T-cell precursors. In this review we summarize the current knowledge on the external stimuli and cell-intrinsic lesions that drive aberrant activation of pivotal, pro-tumoral intracellular signaling pathways in T-cell precursors, driving transformation, leukemia expansion, spread or resistance to therapy. In addition to their pathophysiological relevance, receptors and kinases involved in signal transduction are often attractive candidates for targeted drug development. As such, we discuss also the potential of T-ALL signaling players as targets for therapeutic intervention.

Role of the RB-Interacting Proteins in Stem Cell Biology

Human retinoblastoma gene RB1 is the first tumor suppressor gene (TSG) isolated by positional cloning in 1986. RB is extensively studied for its ability to regulate cell cycle by binding to E2F1 and inhibiting the transcriptional activity of the latter. In human embryonic stem cells (ESCs), only a minute trace of RB is found in complex with E2F1. Increased activity of RB triggers differentiation, cell cycle arrest, and cell death. On the other hand, inactivation of the entire RB family (RB1, RBL1, and RBL2) in human ESC induces G2/M arrest and cell death. These observations indicate that both loss and overactivity of RB could be lethal for the stemness of cells. A question arises why inactive RB is required for the survival and stemness of cells? To shed some light on this question, we analyzed the RB-binding proteins. In this review we have focused on 27 RB-binding partners that may have potential roles in different aspects of stem cell biology.

DLL4 regulates NOTCH signaling and growth of T acute lymphoblastic leukemia cells in NOD/SCID mice

Activation of the NOTCH pathway occurs commonly in T acute lymphoblastic leukemia (T-ALL) mainly due to mutations in NOTCH1 or alterations in FBW7 and is involved in the regulation of cell proliferation and survival. Since mutations hit different domains of the receptor, they are predicted to heterogeneously perturb ligand-induced NOTCH1 activity. Moreover, T-ALL cells also co-express NOTCH3 receptors which could be triggered by different ligands. In this study, we aimed to investigate the role of DLL4 in the regulation of NOTCH signaling in T-ALL cells in the context of different types of NOTCH1 mutation or wild-type NOTCH receptor, as well as the effects of DLL4 neutralization on T-ALL engraftment in mice. We found that NOTCH signaling can be stimulated in T-ALL cells in vitro by either human or murine DLL4 with heterogeneous effects, according to NOTCH1/FBW7 mutation status, and that these effects can be blocked by antibodies neutralizing DLL4, NOTCH1 or NOTCH2/3. In vivo, DLL4 is expressed in the spleen and the bone marrow (BM) of NOD/SCID mice bearing T-ALL xenografts as well as the BM of T-ALL patients. Importantly, DLL4 blockade impaired growth of T-ALL cells in NOD/SCID mice and increased leukemia cell apoptosis. These results show that DLL4 is an important component of the tumor microenvironment which contributes to the early steps of T-ALL cell growth.

Pleiotropic roles of Notch signaling in normal, malignant, and developmental hematopoiesis in the human

The Notch signaling pathway is evolutionarily conserved across species and plays an important role in regulating cell differentiation, proliferation, and survival. It has been implicated in several different hematopoietic processes including early hematopoietic development as well as adult hematological malignancies in humans. This review focuses on recent developments in understanding the role of Notch signaling in the human hematopoietic system with an emphasis on hematopoietic initiation from human pluripotent stem cells and regulation within the bone marrow. Based on recent insights, we summarize potential strategies for treatment of human hematological malignancies toward the concept of targeting Notch signaling for fate regulation.

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.

Context- and cell-dependent effects of Delta-like 4 targeting in the bone marrow microenvironment

Delta-like 4 (Dll4) is a ligand of the Notch pathway family which has been widely studied in the context of tumor angiogenesis, its blockade shown to result in non-productive angiogenesis and halted tumor growth. As Dll4 inhibitors enter the clinic, there is an emerging need to understand their side effects, namely the systemic consequences of Dll4:Notch blockade in tissues other than tumors. The present study focused on the effects of systemic anti-Dll4 targeting in the bone marrow (BM) microenvironment. Here we show that Dll4 blockade with monoclonal antibodies perturbs the BM vascular niche of sub-lethally irradiated mice, resulting in increased CD31⁺, VE-Cadherin⁺ and c-kit⁺ vessel density, and also increased megakaryocytes, whereas CD105⁺, VEGFR3⁺, SMA⁺ and lectin⁺ vessel density remained unaltered. We investigated also the expression of angiocrine genes upon Dll4 treatment in vivo, and demonstrate that IGFbp2, IGFbp3, Angpt2, Dll4, DHH and VEGF-A are upregulated, while FGF1 and CSF2 are reduced. In vitro treatment of endothelial cells with anti-Dll4 reduced Akt phosphorylation while maintaining similar levels of Erk 1/2 phosphorylation. Besides its effects in the BM vascular niche, anti-Dll4 treatment perturbed hematopoiesis, as evidenced by increased myeloid (CD11b⁺), decreased B (B220⁺) and T (CD3⁺) lymphoid BM content of treated mice, with a corresponding increase in myeloid circulating cells. Moreover, anti-Dll4 treatment also increased the number of CFU-M and -G colonies in methylcellulose assays, independently of Notch1. Finally, anti-Dll4 treatment of donor BM improved the hematopoietic recovery of lethally irradiated recipients in a transplant setting. Together, our data reveals the hematopoietic (BM) effects of systemic anti-Dll4 treatment result from qualitative vascular changes and also direct hematopoietic cell modulation, which may be favorable in a transplant setting.

LRF-mediated Dll4 repression in erythroblasts is necessary for hematopoietic stem cell maintenance

Hematopoietic stem cells (HSCs) are the most primitive cells in the hematopoietic system and are under tight regulation for self-renewal and differentiation. Notch signals are essential for the emergence of definitive hematopoiesis in mouse embryos and are critical regulators of lymphoid lineage fate determination. However, it remains unclear how Notch regulates the balance between HSC self-renewal and differentiation in the adult bone marrow (BM). Here we report a novel mechanism that prevents HSCs from undergoing premature lymphoid differentiation in BM. Using a series of in vivo mouse models and functional HSC assays, we show that leukemia/lymphoma related factor (LRF) is necessary for HSC maintenance by functioning as an erythroid-specific repressor of Delta-like 4 (Dll4) expression. Lrf deletion in erythroblasts promoted up-regulation of Dll4 in erythroblasts, sensitizing HSCs to T-cell instructive signals in the BM. Our study reveals novel cross-talk between HSCs and erythroblasts, and sheds a new light on the regulatory mechanisms regulating the balance between HSC self-renewal and differentiation.

Role of the ubiquitin ligase Fbw7 in cancer progression

Fbw7 is a member of F-box family proteins, which constitute one subunit of Skp1, Cul1, and F-box protein (SCF) ubiquitin ligase complex. SCF(Fbw7) targets a set of well-known oncoproteins, including c-Myc, cyclin E, Notch, c-Jun, and Mcl-1, for ubiquitylation and degradation. Fbw7 provides specificity of the ubiquitylation of these substrate proteins via recognition of a consensus phosphorylated degron. Through regulation of several important proteins, Fbw7 controls diverse cellular processes, including cell-cycle progression, cell proliferation, differentiation, DNA damage response, maintenance of genomic stability, and neural cell stemness. As reduced Fbw7 expression level and loss-of-function mutations are found in a wide range of human cancers, Fbw7 is generally considered as a tumor suppressor. However, the exact mechanisms underlying Fbw7-induced tumor suppression is unclear. This review focuses on regulation network, biological functions, and genetic alteration of Fbw7 in connection with its role in cancer development.

Anti-DLL4 has broad spectrum activity in pancreatic cancer dependent on targeting DLL4-Notch signaling in both tumor and vasculature cells

PURPOSE

We previously showed that targeting Delta-like ligand 4 (DLL4) in colon and breast tumors inhibited tumor growth and reduced tumor initiating cell frequency. In this report, we have extended these studies to pancreatic cancer and probed the mechanism of action in tumor and stromal cells involved in antitumor efficacy.

EXPERIMENTAL DESIGN

Patient-derived pancreatic xenograft tumor models were used to evaluate the antitumor effect of anti-DLL4. To investigate the mechanism of action, we compared the activity of targeting DLL4 in tumor cells with an anti-human DLL4 antibody (anti-hDLL4) and in the host stroma/vasculature with an anti-mouse DLL4 antibody (anti-mDLL4). The effect of these antibodies on cancer stem cell frequency was examined by in vivo limiting dilution assays.

RESULTS

The combination of anti-hDLL4 and anti-mDLL4 was efficacious in a broad spectrum of pancreatic tumor xenografts and showed additive antitumor activity together with gemcitabine. Treatment with either anti-hDLL4 or anti-mDLL4 delayed pancreatic tumor recurrence following termination of gemcitabine treatment, and the two together produced an additive effect. Anti-hDLL4 had a pronounced effect in reducing the tumorigenicity of pancreatic cancer cells based on serial transplantation and tumorsphere assays. In contrast, disruption of tumor angiogenesis with anti-mDLL4 alone or with anti-VEGF had minimal effects on tumorigenicity. Gene expression analyses indicated that anti-DLL4 treatment regulated genes that participate in Notch signaling, pancreatic differentiation, and epithelial-to-mesenchymal transition.

CONCLUSIONS

Our findings suggest a novel therapeutic approach for pancreatic cancer treatment through antagonism of DLL4/Notch signaling.

POK/ZBTB proteins: an emerging family of proteins that regulate lymphoid development and function

The germinal center (GC) is a unique histological structure found in peripheral lymphoid organs. GCs provide an important source of humoral immunity by generating high affinity antibodies against a pathogen. The GC response is tightly regulated during clonal expansion, immunoglobulin modification, and affinity maturation, whereas its deregulation has a detrimental effect on immune function, leading to development of diseases, such as lymphoma and autoimmunity. LRF (lymphoma/leukemia-related factor), encoded by the ZBTB7A gene, is a transcriptional repressor belonging to the POK (POZ and Krüppel)/ZBTB (zing finger and BTB) protein family. LRF was originally identified as a PLZF (promyelocytic leukemia zinc finger) homolog that physically interacts with BCL6 (B-cell lymphoma 6), whose expression is required for GC formation and associated with non-Hodgkin's lymphoma. Recently, our group demonstrated that LRF plays critical roles in regulating lymphoid lineage commitment, mature B-cell development, and the GC response via distinct mechanisms. Herein, we review POK/ZBTB protein function in lymphoid development, with particular emphasis on the role of LRF in GC B cells.

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.

Notch1 regulates chemotaxis and proliferation by controlling the CC-chemokine receptors 5 and 9 in T cell acute lymphoblastic leukaemia

Tumour cells often express deregulated profiles of chemokine receptors that regulate cancer cell migration and proliferation. Notch1 pathway activation is seen in T cell acute lymphoblastic leukaemia (T-ALL) due to the high frequency of Notch1 mutations affecting approximately 60% of patients, causing ligand-independent signalling and/or prolonging Notch1 half-life. We have investigated the possible regulative role of Notch1 on the expression and function of chemokine receptors CCR5, CCR9 and CXCR4 that play a role in determining blast malignant properties and localization of extramedullary infiltrations in leukaemia. We inhibited the pathway through γ-Secretase inhibitor and Notch1 RNA interference and analysed the effect on the expression and function of chemokine receptors. Our results indicate that γ-Secretase inhibitor negatively regulates the transcription level of the CC chemokine receptors 5 and 9 in T-ALL cell lines and patients' primary leukaemia cells, leaving CXCR4 expression unaltered. The Notch pathway also controls CCR5- and CCR9-mediated biological effects, ie chemotaxis and proliferation. Furthermore, engaging CCR9 through CCL25 administration rescues proliferation inhibition associated with abrogation of Notch activity. Finally, through RNA interference we demonstrated that the oncogenic isoform in T-ALL, Notch1, plays a role in controlling CCR5 and CCR9 expression and functions. These findings suggest that Notch1, acting in concert with chemokine receptors pathways, may provide leukaemia cells with proliferative advantage and specific chemotactic abilities, therefore influencing tumour cell progression and localization.

Resuscitating cancer immunosurveillance: selective stimulation of DLL1-Notch signaling in T cells rescues T-cell function and inhibits tumor growth

Deficiencies in immune function that accumulate during cancer immunoediting lead to a progressive escape from host immunosurveillance. Therapies that correct or overcome these defects could have a powerful impact on cancer management, but current knowledge of the types and mechanisms of immune escape is still incomplete. Here, we report a novel mechanism of escape from T-cell immunity that is caused by reduction in levels of the Delta family Notch ligands DLL1 and DLL4 in hematopoietic microenvironments. An important mediator of this effect was an elevation in the levels of circulating VEGF. Selective activation of the DLL1-Notch signaling pathway in bone marrow precursors enhanced T-cell activation and inhibited tumor growth. Conversely, tumor growth led to inhibition of Delta family ligand signaling through Notch in the hematopoietic environment, resulting in suppressed T-cell function. Overall, our findings uncover a novel mechanism of tumoral immune escape and suggest that a soluble multivalent form of DLL1 may offer a generalized therapeutic intervention to stimulate T-cell immunity and suppress tumor growth.

E proteins and the regulation of early lymphocyte development

Lymphopoiesis generates mature B, T, and NK lymphocytes from hematopoietic stem cells via a series of increasingly restricted developmental intermediates. The transcriptional networks that regulate these fate choices are composed of both common and lineage-specific components, which combine to create a cellular context that informs the developmental response to external signals. E proteins are an important factor during lymphopoiesis, and E2A in particular is required for normal T- and B-cell development. Although the other E proteins, HEB and E2-2, are expressed during lymphopoiesis and can compensate for some of E2A's activity, E2A proteins have non-redundant functions during early T-cell development and at multiple checkpoints throughout B lymphopoiesis. More recently, a role for E2A has been demonstrated in the generation of lymphoid-primed multipotent progenitors and shown to favor their specification toward lymphoid over myeloid lineages. This review summarizes both our current understanding of the wide-ranging functions of E proteins during the development of adaptive lymphocytes and the novel functions of E2A in orchestrating a lymphoid-biased cellular context in early multipotent progenitors.

A cell autonomous role for the Notch ligand Delta-like 3 in αβ T-cell development

Notch signalling is critical to help direct T-cell lineage commitment in early T-cell progenitors and in the development of αβ T-cells. Epithelial and stromal cell populations in the thymus express the Notch DSL (Delta, Serrate and Lag2)ligands Delta-like 1 (Dll1), Delta-like 4 (Dll4), Jagged 1 and Jagged 2, and induce Notch signalling in thymocytes that express the Notch receptor. At present there is nothing known about the role of the Delta-like 3 (Dll3) ligand in the immune system. Here we describe a novel cell autonomous role for Dll3 in αβ T-cell development. We show that Dll3 cannot activate Notch when expressed in trans but like other Notch ligands it can inhibit Notch signalling when expressed in cis with the receptor. The loss of Dll3 leads to an increase in Hes5 expression in double positive thymocytes and their increased production of mature CD4(+) and CD8(+) T cells. Studies using competitive irradiation chimeras proved that Dll3 acts in a cell autonomous manner to regulate positive selection but not negative selection of autoreactive T cells. Our results indicate that Dll3 has a unique function during T-cell development that is distinct from the role played by the other DSL ligands of Notch and is in keeping with other recent studies indicating that Dll1 and Dll3 ligands have non-overlapping roles during embryonic development.

NF-κB in T-cell Acute Lymphoblastic Leukemia: Oncogenic Functions in Leukemic and in Microenvironmental Cells

Two main NF-κB signaling pathways, canonical and noncanonical, performing distinct functions in organisms have been characterized. Identification of mutations in genes encoding components of these NF-κB signaling pathways in lymphoid malignancies confirmed their key role in leukemogenesis. T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of thymocytes that despite significant therapeutic advances can still be fatal. Although mutations in NF-κB genes have not been reported in T-ALL, NF-κB constitutive activation in human T-ALL and in acute T-cell leukemia mouse models has been observed. Although these studies revealed activation of members of both canonical and noncanonical NF-κB pathways in acute T-cell leukemia, only inhibition of canonical NF-κB signaling was shown to impair leukemic T cell growth. Besides playing an important pro-oncogenic role in leukemic T cells, NF-κB signaling also appears to modulate T-cell leukemogenesis through its action in microenvironmental stromal cells. This article reviews recent data on the role of these transcription factors in T-ALL and pinpoints further research crucial to determine the value of NF-κB inhibition as a means to treat T-ALL.

Prognostic significance of Notch 3 gene expression in ovarian serous carcinoma

The Notch signaling pathway is an important cell signaling system, which regulates cell differentiation, proliferation, and apoptosis, and is aberrantly activated in a wide range of cancer, including ovarian cancers. However, it remains unclear as to whether Notch signaling plays a role in the progression and prognosis of ovarian cancer. We examined the mRNA and protein expression of Notch 3, Jagged 1, and Jagged 2 in 98 ovarian epithelial tumors via real-time PCR and in 175 tumors with immunohistochemical analysis, and then correlated their expression levels with clinicopathological parameters and patient survival. In this study, we detected high levels of Notch3 mRNA and protein expression especially in serous ovarian carcinomas compared to their benign counterparts, accompanied by a positive correlation with the expressions of Jagged 1 and Jagged 2. High levels of Notch 3 mRNA expression (>2-fold than that of benign tumor) were noted in 63% of the serous carcinomas (mean level: 17-fold, P = 0.032). Additionally, Notch 3 protein overexpression was significantly associated with advanced stage (P = 0.0008), lymph node (P = 0.001), and distant metastasis (P = 0.003). Notably, high Notch 3 mRNA and protein expressions were correlated with chemoresistance (P = 0.033) and poor overall survival (P = 0.027, P = 0.042) in these patients. Our results indicate that the Notch 3 signaling pathway is involved in the tumor progression of ovarian serous carcinoma, and higher Notch 3 expression may be an independent poor prognostic factor in this subset of tumors.

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.

Spatial mapping of thymic stromal microenvironments reveals unique features influencing T lymphoid differentiation

Interaction of hematopoietic progenitors with the thymic microenvironment induces them to proliferate, adopt the T lineage fate, and asymmetrically diverge into multiple functional lineages. Progenitors at various developmental stages are stratified within the thymus, implying that the corresponding microenvironments provide distinct sets of signals to progenitors migrating between them. These differences remain largely undefined. Here we used physical and computational approaches to generate a comprehensive spatial map of stromal gene expression in the thymus. Although most stromal regions were characterized by a unique gene expression signature, the central cortex lacked distinctive features. Instead, a key function of this region appears to be the sequestration of unique microenvironments found at the cortical extremities, thus modulating the relative proximity of progenitors moving between them. Our findings compel reexamination of how cell migration, lineage specification, and proliferation are controlled by thymic architecture and provide an in-depth resource for global characterization of this control.

Notch signaling in solid tumors

In recent years a substantial body of evidence derived from not only preclinical but also clinical studies has accumulated in support of Notch signaling playing important oncogenic roles in several types of cancer. The finding that activating Notch mutations are frequently found in patients suffering from acute lymphoblastic leukemia is one of the best examples for a critical role of Notch signaling in cancer, a fact that motivated many researchers and clinicians to study the role of Notch also in solid tumors. Hence Notch signaling has gained increasing attention as a potential therapeutic target. In this book chapter we would like to discuss our current knowledge of Notch signaling within different types of solid cancers as well as advantages and disadvantages of potential new therapies that try to target the oncogenic properties of Notch signaling.

DLL4 blockade inhibits tumor growth and reduces tumor-initiating cell frequency

Previous studies have shown that blocking DLL4 signaling reduced tumor growth by disrupting productive angiogenesis. We developed selective anti-human and anti-mouse DLL4 antibodies to dissect the mechanisms involved by analyzing the contributions of selectively targeting DLL4 in the tumor or in the host vasculature and stroma in xenograft models derived from primary human tumors. We found that each antibody inhibited tumor growth and that the combination of the two antibodies was more effective than either alone. Treatment with anti-human DLL4 inhibited the expression of Notch target genes and reduced proliferation of tumor cells. Furthermore, we found that specifically inhibiting human DLL4 in the tumor, either alone or in combination with the chemotherapeutic agent irinotecan, reduced cancer stem cell frequency, as shown by flow cytometric and in vivo tumorigenicity studies.

Cross-talk between tumor and endothelial cells involving the Notch3-Dll4 interaction marks escape from tumor dormancy

The Notch ligand Dll4 has a recognized role during both physiologic and tumor angiogenesis, as it contributes to regulate Notch activity in endothelial cells (EC). The effects of Dll4 on Notch signaling in tumor cells expressing Notch receptors remain, however, largely unknown. Here, we report that escape of human T-cell acute lymphoblastic leukemia (T-ALL) cells or colorectal cancer cells from dormancy is associated with Dll4 expression in the tumor microenvironment and increased Notch3 signaling in tumor cells. Dll4 was expressed at early time points during the angiogenic process, and its expression preceded perfusion of the newly established vessels. Treatment of EC with angiogenic factors induced Dll4 expression and increased Notch3 activation in cocultured T-ALL cells. Neutralization of Dll4 greatly reduced EC-mediated activation of Notch 3 signaling in T-ALL cells and blocked tumorigenesis. Moreover, silencing Notch3 by RNA interference had marked antiproliferative and proapoptotic effects on T-ALL cells in vitro and reduced tumorigenicity in vivo. Our results elucidate a novel mechanism by which a direct interplay between endothelial and tumor cells promotes survival and triggers tumor growth.

Delta-like 4 is the essential, nonredundant ligand for Notch1 during thymic T cell lineage commitment

Thymic T cell lineage commitment is dependent on Notch1 (N1) receptor–mediated signaling. Although the physiological ligands that interact with N1 expressed on thymic precursors are currently unknown, in vitro culture systems point to Delta-like 1 (DL1) and DL4 as prime candidates. Using DL1- and DL4-lacZ reporter knock-in mice and novel monoclonal antibodies to DL1 and DL4, we show that DL4 is expressed on thymic epithelial cells (TECs), whereas DL1 is not detected. The function of DL4 was further explored in vivo by generating mice in which DL4 could be specifically inactivated in TECs or in hematopoietic progenitors. Although loss of DL4 in hematopoietic progenitors did not perturb thymus development, inactivation of DL4 in TECs led to a complete block in T cell development coupled with the ectopic appearance of immature B cells in the thymus. These immature B cells were phenotypically indistinguishable from those developing in the thymus of conditional N1 mutant mice. Collectively, our results demonstrate that DL4 is the essential and nonredundant N1 ligand responsible for T cell lineage commitment. Moreover, they strongly suggest that N1-expressing thymic progenitors interact with DL4-expressing TECs to suppress B lineage potential and to induce the first steps of intrathymic T cell development.

Notch-1 mutations are secondary events in some patients with T-cell acute lymphoblastic leukemia

PURPOSE

Activating Notch-1 mutations are frequent in T-cell acute lymphoblastic leukemia (T-ALL), occurring in >50% of patients. In murine models of T-ALL, Notch-1 activation can both directly initiate leukemia and cooperate secondarily to other primary events. Whether acquisition of Notch-1 mutations is an early initiating event or a secondary event in the pathogenesis of human T-ALL is unclear.

EXPERIMENTAL DESIGN

We used denaturing high-performance liquid chromatography, sequencing, and fragment analysis to analyze Notch-1 mutational status and mutant level in 62 patients at presentation as well as 16 matched presentation-relapse samples.

RESULTS

We detected Notch-1 mutations in 47 patients (76%). Seven of these were low-level mutations (quantified at < or =10%), despite high blast counts, suggesting that they were acquired as a secondary event in a subclone. Of 16 matched presentation-relapse samples studied, 7 were wild-type at both presentation and relapse. Five of nine mutant-positive patients at presentation relapsed with the same mutation(s) at the same high level. Four patients had evidence of a change in mutant at relapse. One lost a PEST mutation and became wild-type. Two others lost mutations at relapse but acquired different mutations, despite unchanged T-cell receptor rearrangements, suggesting that the latter event predated the acquisition of the Notch-1 mutation. One relapsed with a secondary T-cell leukemia and different Notch mutation.

CONCLUSIONS

These results suggest that Notch-1 mutations can sometimes be acquired as secondary events in leukemogenesis and must be used cautiously as solitary minimal residual disease markers.

Aberrant signaling in T-cell acute lymphoblastic leukemia: biological and therapeutic implications

T-cell acute lymphoblastic leukemia (T-ALL) is a biologically heterogeneous disease with respect to phenotype, gene expression profile and activation of particular intracellular signaling pathways. Despite very significant improvements, current therapeutic regimens still fail to cure a portion of the patients and frequently implicate the use of aggressive protocols with long-term side effects. In this review, we focused on how deregulation of critical signaling pathways, in particular Notch, PI3K/Akt, MAPK, Jak/STAT and TGF-beta, may contribute to T-ALL. Identifying the alterations that affect intracellular pathways that regulate cell cycle and apoptosis is essential to understanding the biology of this malignancy, to define more effective markers for the correct stratification of patients into appropriate therapeutic regimens and to identify novel targets for the development of specific, less detrimental therapies for T-ALL.

Genomewide expression analysis in zebrafish mind bomb alleles with pancreas defects of different severity identifies putative Notch responsive genes

Background

Notch signaling is an evolutionarily conserved developmental pathway. Zebrafish mind bomb (mib) mutants carry mutations on mib gene, which encodes a RING E3 ligase required for Notch activation via Delta/Jagged ubiquitylation and internalization.

Methodology/Principal Findings

We examined the mib mutants for defects in pancreas development using in situ hybridization and GFP expression analysis of pancreas-specific GFP lines, carried out the global gene expression profile analysis of three different mib mutant alleles and validated the microarray data using real-time PCR and fluorescent double in situ hybridization. Our study showed that the mib mutants have diminished exocrine pancreas and this defect was most severe in mib^ta52b followed by mib^m132 and then mib^tfi91, which is consistent with the compromised Notch activity found in corresponding mib mutant alleles. Global expression profile analysis of mib mutants showed that there is a significant difference in gene expression profile of wt and three mib mutant alleles. There are 91 differentially expressed genes that are common to all three mib alleles. Through detailed analysis of microarray data, we have identified several previously characterized genes and some putative Notch-responsive genes involved in pancreas development. Moreover, results from real-time PCR and fluorescent double in situ hybridization were largely consistent with microarray data.

Conclusions/Significance

This study provides, for the first time, a global gene expression profile in mib mutants generating useful genomic resources and providing an opportunity to identify the function of novel genes involved in Notch signaling and Notch-regulated developmental processes.

Notch signaling induces cytoplasmic CD3 epsilon expression in human differentiating NK cells

It has been proposed that heterogeneity in natural killer (NK)-cell phenotype and function can be achieved through distinct thymic and bone marrow pathways of NK-cell development. Here, we show a link between Notch signaling and the generation of intracellular CD3epsilon (cyCD3)-expressing NK cells, a cell population that can be detected in vivo. Differentiation of human CD34(+) cord blood progenitors in IL-15-supplemented fetal thymus organ culture or OP9-Delta-like 1 (DL1) coculture resulted in a high percentage of cyCD3(+) NK cells that was blocked by the gamma-secretase inhibitor DAPT. The requirement for Notch signaling to generate cyCD3(+) NK cells was further illustrated by transduction of CD34(+) cord blood (CB) cells with either the active intracellular part of Notch or the dominant-negative mutant of mastermind-like protein 1 that resulted in the generation of NK cells with respectively high or low frequencies of cyCD3. Human thymic CD34(+) progenitor cells displayed the potential to generate cyCD3(+) NK cells, even in the absence of Notch/DL1 signaling. Peripheral blood NK cells were unable to induce cyCD3 expression after DL1 exposure, indicating that Notch-dependent cyCD3 expression can only be achieved during the early phase of NK-cell differentiation.

Detection of NOTCH1 mutations in adult T-cell leukemia/lymphoma and peripheral T-cell lymphoma

We analyzed NOTCH1 gene mutation in 53 adults with mature T-cell leukemia/lymphoma: 21 patients with adult T-cell leukemia (ATL), 25 with T-cell non-Hodgkin's lymphoma (T-NHL), and 7 with T-cell prolymphocytic leukemia. We detected a nonsense mutation, C7249T (resulting in Q2417X, where X is a termination codon) in the PEST domain of NOTCH1 in an ATL patient and detected a 3-bp deletion (positions 7234-7236) that resulted in deletion of a proline codon at codon 2412 in the PEST domain of NOTCH1 in a patient with a T-NHL, peripheral T-cell lymphoma-unspecified (PTCL-u). We also analyzed the expression of NOTCH1 target genes (HES1, CCND1, and MYC), all of which were expressed in the sample of the PTCL-u patient with the NOTCH1 mutation, but found only MYC to be expressed in the sample from the ATL patient. These findings suggest that nonsense mutation in the PEST domain in the ATL case was associated with NOTCH1 signaling through a pathway different from that for T-cell acute lymphoblastic leukemia (T-ALL). Although NOTCH1 mutation occurs infrequently in mature T-cell leukemia/lymphoma, NOTCH1 may be involved in leukemogenesis associated with various forms of T-cell leukemia/lymphoma rather than only with T-ALL.

Zoned out: functional mapping of stromal signaling microenvironments in the thymus

All hematopoietic cells, including T lymphocytes, originate from stem cells that reside in the bone marrow. Most hematopoietic lineages also mature in the bone marrow, but in this respect, T lymphocytes differ. Under normal circumstances, most T lymphocytes are produced in the thymus from marrow-derived progenitors that circulate in the blood. Cells that home to the thymus from the marrow possess the potential to generate multiple T and non-T lineages. However, there is little evidence to suggest that, once inside the thymus, they give rise to anything other than T cells. Thus, signals unique to the thymic microenvironment compel multipotent progenitors to commit to the T lineage, at the expense of other potential lineages. Summarizing what is known about the signals the thymus delivers to uncommitted progenitors, or to immature T-committed progenitors, to produce functional T cells is the focus of this review.

Notch signaling in development and cancer

Notch is an evolutionarily conserved local cell signaling mechanism that participates in a variety of cellular processes: cell fate specification, differentiation, proliferation, apoptosis, adhesion, epithelial-mesenchymal transition, migration, and angiogenesis. These processes can be subverted in Notch-mediated pathological situations. In the first part of this review, we will discuss the role of Notch in vertebrate central nervous system development, somitogenesis, cardiovascular and endocrine development, with attention to the mechanisms by which Notch regulates cell fate specification and patterning in these tissues. In the second part, we will review the molecular aspects of Notch-mediated neoplasias, where Notch can act as an oncogene or as a tumor suppressor. From all these studies, it becomes evident that the outcome of Notch signaling is strictly context-dependent and differences in the strength, timing, cell type, and context of the signal may affect the final outcome. It is essential to understand how Notch integrates inputs from other signaling pathways and how specificity is achieved, because this knowledge may be relevant for future therapeutic applications.

IL-2-independent generation of FOXP3+CD4+CD8+CD25+ cytotoxic regulatory T cell lines from human umbilical cord blood

OBJECTIVE

Since the existence of mouse naturally occurring CD4(+)CD25(+) T regulatory (Treg) cells was demonstrated, a variety of human Treg subsets have been identified as distinct T cell populations. Here we show the establishment of novel Treg cell lines possessing unique characteristics.

METHODS

Novel Treg cell lines, designated HOZOT, were generated by coculturing human umbilical cord blood cells with mouse stromal cell lines in the absence of exogenous IL-2 or other cytokines. HOZOT were characterized and compared with CD4(+)CD25(+) Treg cells in terms of the CD phenotype, FOXP3 expression, suppressor activity against allogeneic MLR, anergy property, and IL-10 production.

RESULTS

HOZOT were generated and expanded as normal lymphoblastoid cells with cytotoxic activity against the cocultured stromal cells. HOZOT consisted of three subpopulations as defined by phenotype: CD4(+)CD8(+), CD4(+)CD8(dim), and CD4(-)CD8(+). All three subpopulations showed both suppressor and cytotoxic activities. While HOZOT's expression of FOXP3, CD25, GITR, and cytoplasmic CTLA-4 implied a similarity to naturally occurring CD4(+)CD25(+) Treg cells, these two Treg cells differed in IL-2 responsiveness and IL-10 production.

CONCLUSIONS

Our studies introduce a new method of generating Treg cells in an IL-2-independent manner and highlight a unique Treg cell type with cytotoxic activity and a phenotype of FOXP3(+)CD4(+)CD8(+)CD25(+).

Hierarchy of Notch-Delta interactions promoting T cell lineage commitment and maturation

Notch1 (N1) receptor signaling is essential and sufficient for T cell development, and recently developed in vitro culture systems point to members of the Delta family as being the physiological N1 ligands. We explored the ability of Delta1 (DL1) and DL4 to induce T cell lineage commitment and/or maturation in vitro and in vivo from bone marrow (BM) precursors conditionally gene targeted for N1 and/or N2. In vitro DL1 can trigger T cell lineage commitment via either N1 or N2. N1- or N2-mediated T cell lineage commitment can also occur in the spleen after short-term BM transplantation. However, N2-DL1-mediated signaling does not allow further T cell maturation beyond the CD25(+) stage due to a lack of T cell receptor beta expression. In contrast to DL1, DL4 induces and supports T cell commitment and maturation in vitro and in vivo exclusively via specific interaction with N1. Moreover, comparative binding studies show preferential interaction of DL4 with N1, whereas binding of DL1 to N1 is weak. Interestingly, preferential N1-DL4 binding reflects reduced dependence of this interaction on Lunatic fringe, a glycosyl transferase that generally enhances the avidity of Notch receptors for Delta ligands. Collectively, our results establish a hierarchy of Notch-Delta interactions in which N1-DL4 exhibits the greatest capacity to induce and support T cell development.

Notch signaling in hematopoiesis and lymphopoiesis: lessons from Drosophila

The evolutionarily conserved Notch signaling pathway regulates a broad spectrum of cell fate decisions and differentiation processes during fetal and postnatal life. It is involved in embryonic organogenesis as well as in the maintenance of homeostasis of self-renewing systems. In this article, we review the role of Notch signaling in the hematopoietic system with particular emphasis on lymphocyte development and highlight the similarities in Notch function between Drosophila and mammalian differentiation processes. Recent studies indicating that aberrant NOTCH signaling is frequently linked to the induction of T leukemia in humans will also be discussed.

T-cell development, doing it in a dish

The thymus provides a unique environment for the development of T lymphocytes from bone marrow-derived progenitor cells. Several environmental factors have been identified that influence the development of T cells in the thymus. In particular, the Notch pathway has emerged as critical for the induction of T-lineage commitment and differentiation. Until recently, however, the precise nature of the thymus-derived signals that drive T-cell development were unclear, and the only reliable in vitro culture system that supported T-cell differentiation required the use of thymus organ cultures. Here, we discuss recent advances in the identification of critical Notch receptor ligands that have facilitated the development of a simple in vitro model for the differentiation of T cells 'in a dish', providing an alternate approach for studying T lymphopoiesis.

Regulation of intrathymic T-cell development by Lunatic Fringe- Notch1 interactions

Intrathymic Notch1 signaling critically regulates T-lineage specification and commitment as well as T-cell progenitor survival and differentiation. Notch1 activation is continuously required during progression of early CD4/CD8-double-negative thymocytes to the CD4/CD8-double-positive stage. This developmental transition occurs as thymocytes migrate from the corticomedullary junction (CMJ) to the outer subcapsular zone (SCZ) of the thymus. Members of two families of structurally distinct Notch ligands, Delta-like 1 and Jagged-1, are expressed by cortical thymic epithelial cells, but it is not known which ligands are functionally required within the CMJ and SCZ microenvironmental niches. Our laboratory has investigated this question by genetically manipulating thymocyte expression of Lunatic Fringe (L-Fng), a glycosyltransferase that enhances sensitivity of Notch receptors to Delta-like ligands. This approach has revealed that low-threshold intrathymic Notch1 signals instruct multipotent thymus-seeding progenitors to suppress their B-cell potential and choose the T-cell fate. This strategy has also revealed that Delta-like Notch ligands are functionally limiting in both the CMJ and SCZ microenvironmental niches. Finally, we discuss our recent demonstration that L-Fng-mediated competition for Delta-like ligands is an important mechanism for regulating thymus size.

Expression of JAGGED1 in T-lymphocytes results in thymic involution by inducing apoptosis of thymic stromal epithelial cells

Proper development of the thymus and differentiation of T-lymphocytes requires cell-cell interactions between the developing T-lymphocytes and the thymic epithelia. The Delta/Serrate/Lag-2 (DSL)/Notch signal-transduction pathway is known to govern cell fate decisions required for proper development through direct cell-cell interactions. The functional consequences of specific DSL/Notch interactions during the development of a complex organ, such as the thymus, have not been thoroughly elucidated, however. In order to examine the role of DSL proteins during thymus development and T-lymphocyte differentiation, we targeted expression of JAGGED1 in T-lymphocyte progenitors via the control of the proximal lck promoter. Here, we report that expression of JAGGED1 in T cells causes premature involution of the thymus by directing thymic epithelial cells to undergo an apoptotic program. Adoptive transfer of JAGGED1 transgenic bone marrow into non-transgenic mice revealed that JAGGED1 expression on T cells does not alter T-cell differentiation, but is directly responsible for involution of the thymus. We propose that the phenotype of the lck-JAGGED1 transgenic mice is a direct result of specific DSL/Notch interactions and improper cell-to-cell signaling.

Deregulated NOTCH signaling in acute T-cell lymphoblastic leukemia/lymphoma: new insights, questions, and opportunities

Recent work has shown that the majority of human acute T-cell lymphoblastic leukemias and lymphomas (T-ALL) have gain-of-function mutations in NOTCH1, a type I transmembrane receptor that normally signals through a gamma-secretase-dependent mechanism that relies on ligand-induced regulated intramembranous proteolysis. Cleavage by gamma-secretase releases the intracellular domain of NOTCH1 (ICN1), permitting it to translocate to the nucleus and form a short-lived transcriptional activation complex that is essential for normal T-cell development. Two types of mutations are prevalent in human T-ALL: extracellular domain mutations that increase ICN1 production and C-terminal mutations that sustain ICN1 action. Inhibitors of ICN1 production and activity abrogate the growth of established T-ALL cell lines, and a clinical trial of a NOTCH pathway inhibitor in patients with refractory T-ALL has opened recently. These insights raise a number of new questions relevant to T-ALL pathogenesis and offer exciting opportunities for rational targeted therapy.

Notch-dependent T-lineage commitment occurs at extrathymic sites following bone marrow transplantation

Early T-lineage progenitors (ETPs) arise after colonization of the thymus by multipotent bone marrow progenitors. ETPs likely serve as physiologic progenitors of T-cell development in adult mice, although alternative T-cell differentiation pathways may exist. While we were investigating mechanisms of T-cell reconstitution after bone marrow transplantation (BMT), we found that efficient donor-derived thymopoiesis occurred before the pool of ETPs had been replenished. Simultaneously, T lineage-restricted progenitors were generated at extrathymic sites, both in the spleen and in peripheral lymph nodes, but not in the bone marrow or liver. The generation of these T lineage-committed cells occurred through a Notch-dependent differentiation process. Multipotent bone marrow progenitors efficiently gave rise to extrathymic T lineage-committed cells, whereas common lymphoid progenitors did not. Our data show plasticity of T-lineage commitment sites in the post-BMT environment and indicate that Notch-driven extrathymic Tlineage commitment from multipotent progenitors may contribute to early T-lineage reconstitution after BMT.

Recent insights into the role of Notch signaling in tumorigenesis

Members of the Notch family of transmembrane receptors play an important role in cell fate determination. Over the past decade, a role for Notch in the pathogenesis of hematologic and solid malignancies has become apparent. Numerous cellular functions and microenvironmental cues associated with tumorigenesis are modulated by Notch signaling, including proliferation, apoptosis, adhesion, epithelial-to-mesenchymal transition, and angiogenesis. It is becoming increasingly evident that Notch signaling can be both oncogenic and tumor suppressive. This review highlights recent findings regarding the molecular and functional aspects of Notch-mediated neoplastic transformation. In addition, cellular mechanisms that potentially explain the complex role of Notch in tumorigenesis are discussed.

Notch signaling is a potent inducer of growth arrest and apoptosis in a wide range of B-cell malignancies

Although Notch receptor expression on malignant B cells is widespread, the effect of Notch signaling in these cells is poorly understood. To investigate Notch signaling in B-cell malignancy, we assayed the effect of Notch activation in multiple murine and human B-cell tumors, representing both immature and mature subtypes. Expression of constitutively active, truncated forms of the 4 mammalian Notch receptors (ICN1-4) inhibited growth and induced apoptosis in both murine and human B-cell lines but not T-cell lines. Similar results were obtained in human precursor B-cell acute lymphoblastic leukemia lines when Notch activation was achieved by coculture with fibroblasts expressing the Notch ligands Jagged1 or Jagged2. All 4 truncated Notch receptors, as well as the Jagged ligands, induced Hes1 transcription. Retroviral expression of Hairy/Enhancer of Split-1 (Hes1) recapitulated the Notch effects, suggesting that Hes1 is an important mediator of Notch-induced growth arrest and apoptosis in B cells. Among the B-cell malignancies that were susceptible to Notch-mediated growth inhibition/apoptosis were mature B-cell and therapy-resistant B-cell malignancies, including Hodgkin, myeloma, and mixed-lineage leukemia (MLL)-translocated cell lines. These results suggest that therapies capable of activating Notch/Hes1 signaling may have therapeutic potential in a wide range of human B-cell malignancies.

Regulation of lymphoid development, differentiation, and function by the Notch pathway

The Notch pathway is gaining increasing recognition as a key regulator of developmental choices, differentiation, and function throughout the hematolymphoid system. Notch controls the generation of hematopoietic stem cells during embryonic development and may affect their subsequent homeostasis. Commitment to the T cell lineage and subsequent stages of early thymopoiesis is critically regulated by Notch. Recent data indicate that Notch can also direct the differentiation and activity of peripheral T and B cells. Thus, the full spectrum of Notch effects is just beginning to be understood. In this review, we discuss this explosion of knowledge as well as current controversies and challenges in the field.