T-cell Progenitors Research Articles

Tumor necrosis factor promotes T-cell at the expense of B-cell lymphoid development from cultured human CD34 + cord blood cells

Human CD34+ cord blood (CB) cells are hematopoietic progenitors useful for stem cell transplantation, even after ex vivo expansion. We investigated the effect of tumor necrosis factor (TNF) on lymphoid development from cultured CD34+ CB cells. Human CD34+ CB cells were cultured in cytokine mixes with or without TNF. Preculture during 60 hours was followed by in vitro differentiation assays, including fetal thymus organ culture and coculture on murine stromal MS-5 cells. In a next step, experiments were extended to CD34+CD38- and CD34+CD38+ CB cells and prolonged preculture. Preculture in the presence of TNF improved differentiation into T cells and diminished the ability to generate B cells, while NK potential and myeloid development were unaffected. Sorted CD34+CD38- CB cells were more potent T-cell precursors after preculture in TNF, compared to CD34+CD38+ CB cells. In precultured CD34+CD38- CB cells, TNF increased GATA3 but decreased EBF1 expression, in line with the skewed lymphoid differentiation induced by TNF. However, when preculture in the presence of TNF was extended to 1 week, T-cell precursors were lost. After short-term culture of CD34+ CB cells in the presence of TNF, T-cell generation is stimulated at the expense of B-cell generation. T-cell progenitors are enriched in the CD34+CD38- fraction. These results have implications on the culture conditions to be used for CB CD34+ cells prior to transplantation.

Growth factor independent 1B (Gfi1b) is an E2A target gene that modulates Gata3 in T-cell lymphomas

AbstractThe E2A transcription factors are required for normal T lymphopoiesis and to prevent T-lymphocyte progenitor transformation. Ectopic expression of E2A proteins in E2A-deficient lymphomas results in growth arrest and apoptosis, indicating that these cells remain responsive to the targets of E2A. Here we identify the transcriptional repressor growth factor independent 1B (Gfi1b) as a target of E2A that promotes growth arrest and apoptosis in lymphomas. Gfi1b expression in primary T-lymphocyte progenitors is dependent on E2A and excess Gfi1b prevents the outgrowth of T lymphocyte progenitors in vitro. Gfi1b represses expression of Gata3, a transcription factor whose appropriate regulation is required for survival of lymphomas and T-lymphocyte progenitors. We also show that ectopic expression of Gata3 in lymphomas promotes expression of Gfi1b, indicating that these proteins may function in an autoregulatory loop that maintains appropriate levels of Gata3. Therefore, we propose that E2A proteins prevent lymphoma cell expansion, at least in part through regulation of Gfi1b and modulation of Gata3 expression.

Germline transcription from T-cell receptor Vβ gene is uncoupled from allelic exclusion

Allelic exclusion operates in B and T lymphocytes to ensure clonal expression of antigen receptors after V(D)J recombination. Germline transcription, which proceeds V(D)J recombination, has been postulated to provide an instructive signal for allelic exclusion. Here, we use a genetic marker to track germline transcription from a Vbeta gene within the TCRbeta locus. We find that developing thymocytes exhibit uniformed, bi-allelic activation of the Vbeta gene before V-DJ recombination, a process subject to allelic exclusion. We further show that V-DJ rearrangement promotes activation rather than silencing of germline transcription from the remaining Vbeta genes on either the functionally or non-functionally rearranged chromosome. Results presented here suggest that germline transcription, although necessary for V(D)J recombination, is not sufficient to instruct allelic exclusion.

Hematopoietic stem cells did not have a synchronized defect with age-related thymic involution during murine aging (81.5)

Abstract Previous reports suggested that aging early T-lymphocyte progenitors (ETPs) derived from bone marrow (BM) hematopoietic stem cells (HSCs) have an intrinsic defect and insufficiency in aged thymus, which triggers age-related thymic involution, but our studies suggest otherwise. We grafted fetal thymus into a kidney capsule of the aged mice, in which aging HSCs could fully generate all thymocyte subpopulations and restore normal thymopoiesis under intact physiological conditions in aged animals. We found that aging ETPs were not deficient initially because both BM HSC and early-stage ETPs—DN1a absolute cell numbers were not significantly reduced although late-stage ETPs—DN1b cells significantly decreased. Moreover, proliferation of the aging ETPs remained unchanged although their apoptotic rate increased. Conversely, provision of sufficient young HSCs/ETPs intravenously or intrathymically failed to restore thymopoiesis in aging host mice. Meanwhile, we demonstrated that young progenitors could seed young and aged thymi without significant difference via intravenous injection. The results exclude insufficient and defective HSCs/ETPs as causes for age-related thymic involution. Our findings indicate that an intrinsic defect in aging T-lymphocyte progenitors is unlikely and HSCs are not synchronal defective with age-related thymic involution during murine aging. (This work was supported by US NIH grant R21AI064939 to DMS).

Differential expression of the human MIXL1 gene product in non-Hodgkin and Hodgkin lymphomas

The Mix1 homeobox-like (MIXL1) gene encodes a paired class homeobox transcription factor that is involved in embryogenesis. Previous studies have shown that the MIXL1 gene product is expressed in B- and T-cell progenitors of normal bone marrow and, in some cell lines derived from hematopoietic neoplasms. The status of MIXL1 expression and subcellular localization in human lymphomas is unknown. Using a highly specific antibody, we assessed for MIXL1 expression in lymphoma cell lines of B- and T-cell lineage by reverse transcriptase-polymerase chain reaction, Western blot analysis, and immunohistochemistry. We also assessed for MIXL1 expression using immunohistochemical methods in 193 lymphoid tumors, including 140 B-cell non-Hodgkin lymphomas (NHL), 36 T-cell NHL, and 17 Hodgkin lymphomas (HL). MIXL1 was detected predominantly in the nuclear fraction of all cell lines tested and was predominantly nuclear in primary tumor specimens. Based on the distribution of the staining results (histogram), a 50% cutoff was selected for high versus low MIXL1 expression. High MIXL1 expression was detected more frequently in Burkitt lymphoma and diffuse large B-cell lymphoma compared with other types of B-cell NHL (P < .0001, chi(2) test). Most cases of T-cell NHL and all cases of HL also highly expressed MIXL1. Most plasma cell myelomas were negative for MIXL1, but rare cases had low MIXL1 expression. MIXL1 expression significantly correlated with proliferation index (Ki-67) in B-cell NHL (P < .0001). The frequent and high expression of MIXL1 in aggressive B-cell NHL, T-cell NHL, and HL suggests that MIXL1 may be involved in lymphomagenesis.

Transcriptome analysis of age-, gender- and diet-associated changes in murine thymus

The loss of thymic function with age may be due to diminished numbers of T-cell progenitors and the loss of critical mediators within the thymic microenvironment. To assess the molecular changes associated with this loss, we examined transcriptomes of progressively aging mouse thymi, of different sexes and on caloric-restricted (CR) vs. ad libitum (AL) diets. Genes involved in various biological and molecular processes including transcriptional regulators, stress response, inflammation and immune function significantly changed during thymic aging. These differences depended on variables such as sex and diet. Interestingly, many changes associated with thymic aging are either muted or almost completely reversed in mice on caloric-restricted diets. These studies provide valuable insight into the molecular mechanisms associated with thymic aging and emphasize the need to account for biological variables such as sex and diet when elucidating the genomic correlates that influence the molecular pathways responsible for thymic involution.

Is there a difference in childhood T-cell acute lymphoblastic leukaemia and T-cell lymphoblastic lymphoma?

To distinguish the similarities or differences between T-cell acute lymphoblastic leukaemia (T-ALL) and T-cell lymphoblastic lymphoma (T-LBL), we retrospectively analyzed the clinical, immunophenotypic, cytogenetic, and molecular characteristics in 37 children diagnosed between December 1990 and December 2003. Comparative Expressed Sequence Hybridisation (CESH) was used to determine gene expressing profile in both diseases. Twenty two patients suffered from T-ALL and 15 patients were diagnosed as T-LBL. Immunophenotyping demonstrated a more immature phenotype in T-ALL and a more mature phenotype in T-LBL. Cytogenetic and molecular genetic aberrations were found in 82% of T-ALL compared with 73% of T-LBL. By CESH gene expression profiling, the investigated cases were segregated into two groups that largely corresponded with T-ALL and T-LBL. The clinical presentation and cytogenetic characteristics are largely similar for T-ALL and T-LBL supporting the concept that both represent a spectrum of one single disease. The differences that were found between both neoplasms, in particular in their phenotype and in their expression profile may suggest that most T-ALL derive from a T-cell progenitor of the bone marrow, while thymocytes represent the normal counterpart of T-LBL.

Differential Activation of Notch1 and Notch2 by Delta1 and Jagged1 Determines Hematopoietic Cell Fate.

Spatially restricted in vivo expression of the Notch ligands Delta1 and Jagged1 suggests their differential roles in inducing hematopoietic cell fates, and studies have shown that each induces alternative fates during in vitro culture. We hypothesize that the ligands induce alternative fates via differential activation of Notch1 and/or Notch2. To address this, we assessed fate outcomes of Notch1- and Notch2-deficient murine bone marrow derived lin−Sca-1+c-kit+ Hoescht side population progenitors (LSKSP) after 14-days incubation with either purified Delta1 or Jagged1 in serum containing medium. Ligands consisted of purified Delta1 and Jagged1 extracellular domains fused to HumanIgG1 (Delta1ext-IgG or Jagged1ext-IgG). Equal amounts of ligand, as determined by ELISA, were immobilized to plastic surfaces of culture wells along with fibronectin. Notch deficient cells were generated by infecting either Notch1fl/fl or Notch2fl/fl LSKSP with lentivirus encoding cre recombinase. We show here that both ligands inhibit myeloid differentiation, since after 14 days, LSKSP incubated with either ligand generated multi-log increased numbers of immature progeny with significantly reduced percentages of GR1+ and/or F480+ cells compared to LSKSP incubated with control-IgG. However, only Delta1ext-IgG promotes T-cell progenitor differentiation, since a higher percentage of progeny cultured with Delta1ext-IgG expressed CD25 (37.0+/−0.6%) compared to Jagged1ext-IgG (1.7+/−1.0%; p=0.01). In contrast, Jagged1ext-IgG is less effective at inhibiting myeloid differentiation, since a higher percentage of progeny cultured with Jagged1ext-IgG (2.1+/−0.9%) expressed GR1 and F4/80 compared to Delta1ext-IgG (56.5+/−6.3; p=0.02). Furthermore, Delta1ext-IgG is more effective than Jagged1ext-IgG at inducing Notch activation as measured by increased expression of Notch target Hes1, since we found 3.3-fold more expression of Hes1 mRNA following incubation of LSKSP cells with Delta1ext-IgG compared to Jagged1ext-IgG. We further show that Notch2 is required to prevent myeloid differentiation, since Notch2 deficient LSKSP incubated with either Delta1ext-IgG or Jagged1ext-IgG generated cultures containing fewer numbers of cells and a higher percentage of GR1+ and/or F480+ myeloid progeny (83% with Delta1ext-IgGor 86% with Jagged1ext-IgG) similarly to those generated with control-IgG. Likewise, we found a reduced percentage of immature Sca-1+c-kit+ cells (19.3+/−4.4 or 13.0+/−5.2) than wild-type cells incubating with Delta1ext-IgG or Jagged1ext-IgG (92.2+/−3.0 or 71.0+/−15.0: p=0.004 or p=0.05). We found that Notch1 is required to induce T-cell differentiation, since Notch1 deficient LSKSP incubated with Delta1ext-IgG had a reduced percentage of CD25+ cells compared to wild-type cells (4.4+/−1.9% to 30.3+/− 3.3) even though myeloid differentiation was inhibited. In summary, we show that both Delta1ext-IgG and Jagged1ext-IgG induce signaling via Notch2 to prevent myeloid differentiation, whereas only Delta1ext-IgG induces signaling via Notch1 to promote generation of T-cell progenitors. Our results indicate unique Notch ligands differentially activate Notch1 or Notch2, resulting in alternative cell fate choices and lay a framework for investigating the mechanisms underlying differential activation, including determining the role of Notch modifiers such as Fringe.

Early events in the thymus affect the balance of effector and regulatory T cells

In cellular immunology the critical balance between effector and regulatory mechanisms is highlighted by serious immunopathologies attributable to mutations in Foxp3, a transcription factor required for a major subset of regulatory T (Tr) cells. Thus, many studies have focused on the developmental origin of Tr cells, with the prevailing view that they emerge in the thymus from late-stage T-cell progenitors whose T-cell receptors (TCRs) engage high affinity (agonist) ligands. This study questions the completeness of that interpretation. Here we show that without any obvious effect on TCR-mediated selection, the normal differentiation of mouse gammabeta T cells into potent cytolytic and interferon-gamma-secreting effector cells is switched towards an aggregate regulatory phenotype by limiting the capacity of CD4+CD8+ T-cell progenitors to influence in trans early gammabeta cell progenitors. Unexpectedly, we found that the propensity of early TCR-alphabeta+ progenitors to differentiate into Foxp3+ Tr cells is also regulated in trans by CD4+CD8+ T-cell progenitor cells, before agonist selection.

T Cells Develop Normally in the Absence of both Deltex1 and Deltex2

Deltex1, Deltex2, and Deltex4 form a family of related proteins that are the mammalian homologues of Drosophila Deltex, a known regulator of Notch signals. Deltex1 is highly induced by Notch signaling in thymocytes, and overexpression of Deltex1 in T-cell progenitors can block Notch signals, suggesting that Deltex1 may play an important role in regulating Notch signals during T-cell development. A recent report found that T cells develop normally in mice carrying a targeted deletion in the Deltex1 gene (S. Storck, F. Delbos, N. Stadler, C. Thirion-Delalande, F. Bernex, C. Verthuy, P. Ferrier, J. C. Weill, and C. A. Reynaud, Mol. Cell. Biol. 25: 1437-1445, 2005), suggesting that other Deltex homologues may compensate in Deltex1-deficient T cells. We generated mice that lack expression of both Deltex1 and Deltex2 by gene targeting and further reduced expression of Deltex4 in Deltex1/Deltex2 double-deficient T-cell progenitors using RNA interference. Using a sensitive in vitro assay, we found that Notch signaling is more potent in cells expressing lower levels of Deltex proteins. Nevertheless, we were unable to detect any significant defects in thymocyte maturation in Deltex1/Deltex2 double-knockout mice. Together these data suggest that Deltex can act as a negative regulator of Notch signals in T cells but that endogenous levels of Deltex1 and Deltex2 are not important for regulating Notch signals during thymocyte development.

Identification of Notch target genes in uncommitted T-cell progenitors: no direct induction of a T-cell specific gene program

Deregulated Notch signaling occurs in the majority of human T-ALL. During normal lymphoid development, activation of the Notch signaling pathway poses a T-cell fate on hematopoietic progenitors. However, the transcriptional targets of the Notch pathway are largely unknown. We sought to identify Notch target genes by inducing Notch signaling in human hematopoietic progenitors using two different methods: an intracellular signal through transfection of activated Notch and a Notch-receptor dependent signal by interaction with its ligand Delta1. Gene expression profiles were generated and evaluated with respect to expression profiles of immature thymic subpopulations. We confirmed HES1, NOTCH1 and NRARP as Notch target genes, but other reported Notch targets, including the genes for Deltex1, pre-T-cell receptor alpha and E2A, were not found to be differentially expressed. Remarkably, no induction of T-cell receptor gene rearrangements or transcription of known T-cell specific genes was found after activation of the Notch pathway. A number of novel Notch target genes, including the transcription factor TCFL5 and the HOXA cluster, were identified and functionally tested. Apparently, Notch signaling is essential to open the T-cell pathway, but does not initiate the T-cell program itself.

Increasing Flt3L availability alters composition of a novel bone marrow lymphoid progenitor compartment

AbstractWe have recently described a CD19– B220+CD117low bone marrow subpopulation with B, T, and myeloid developmental potential, which we have called “early progenitors with lymphoid and myeloid potential” or EPLM. These cells also expressed Fms-like tyrosine kinase 3, Flt3, or CD135. Treatment of mice with the corresponding ligand, Flt3L, showed a 50-fold increase in EPLM. In addition to the expected increase in dendritic cell numbers, Flt3L treatment had a reversible inhibitory effect on B lymphopoiesis. Limiting dilution analysis of sorted EPLM from Flt3L-treated mice showed that B-lymphocyte progenitor activity was reduced 20-fold, but that myeloid and T-cell progenitor activity was largely preserved. EPLM from treated mice transiently reconstituted the thymus and bone marrow of recipient mice, generating cohorts of functional T and B cells in peripheral lymphoid organs. Thus, Flt3L treatment results in a dramatic increase in a novel bone marrow cell with lymphoid and myeloid progenitor activity.

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.

Disabled Receptor Signaling and New Primary Immunodeficiency Disorders

During the past decade, we have witnessed major advances in the identification of mutations that cause primary immunodeficiency disorders — exciting results of the successful blending of fundamental and clinical research. Among the disturbances that disable the immune system is severe combined immunodeficiency (SCID), which occurs at a rate of 1 per 75,000 births. Babies born with SCID have greatly increased susceptibility to many environmental pathogens, and without treatment, these babies die of infection within 6 to 12 months. The curative treatment is allogeneic hematopoietic stem-cell transplantation, which provides the T-cell progenitors that are missing in patients with SCID.The . . .

Human thymus contains multipotent progenitors with T/B lymphoid, myeloid, and erythroid lineage potential

AbstractIt is a longstanding question which bone marrow–derived cell seeds the thymus and to what level this cell is committed to the T-cell lineage. We sought to elucidate this issue by examining gene expression, lineage potential, and self-renewal capacity of the 2 most immature subsets in the human thymus, namely CD34+CD1a– and CD34+CD1a+ thymocytes. DNA microarrays revealed the presence of several myeloid and erythroid transcripts in CD34+CD1a– thymocytes but not in CD34+CD1a+ thymocytes. Lineage potential of both subpopulations was assessed using in vitro colony assays, bone marrow stroma cultures, and in vivo transplantation into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. The CD34+CD1a– subset contained progenitors with lymphoid (both T and B), myeloid, and erythroid lineage potential. Remarkably, development of CD34+CD1a– thymocytes toward the T-cell lineage, as shown by T-cell receptor δ gene rearrangements, could be reversed into a myeloid-cell fate. In contrast, the CD34+CD1a+ cells yielded only T-cell progenitors, demonstrating their irreversible commitment to the T-cell lineage. Both CD34+CD1a– and CD34+CD1a+ thymocytes failed to repopulate NOD/SCID mice. We conclude that the human thymus is seeded by multipotent progenitors with a much broader lineage potential than previously assumed. These cells resemble hematopoietic stem cells but, by analogy with murine thymocytes, apparently lack sufficient self-renewal capacity.

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

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

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.

Regulation of T‐cell progenitor survival and cell‐cycle entry by the pre‐T‐cell receptor

Pre-T-cell receptor (pre-TCR) functions and the study of early thymocyte development continue to fascinate immunologists more than 10 years after the first description and cloning of the receptor. Although multiple reports have addressed several aspects of pre-TCR signaling and function, its ability to regulate diverse functions, including proliferation, survival, and allelic exclusion of the TCR-beta locus, remains an open question. What fascinates us is its central role in the fine balance between physiological differentiation and thymocyte transformation that leads to T-cell leukemia and lymphomas. In this review, we integrate pre-TCR signaling pathways and study their effects on the regulation of T-cell progenitor cell-cycle entry and cell survival. We also connect aberrant pre-TCR signaling to deregulated proliferation and apoptotic balances and thymocyte transformation.

Transcriptional Regulation of the LMO2 T-Cell Leukaemia Oncogene.

Transcriptional control has long been identified as a key mechanism regulating the formation and subsequent behaviour of haematopoietic stem cells. We have used a comparative genomics approach to identify transcriptional regulatory elements of the LMO2 gene, a transcriptional cofactor originally identified through its involvement in T-cell leukaemia and subsequently shown to be critical for the formation of haematopoietic stem cells and endothelial development. An initial stringent search for homology between evolutionary distant species demonstrated that, apart from the coding exons, high level of identity between mammalian, amphibian and fish sequences was restricted to the proximal promoter region of LMO2. Real-time RT-PCR expression analysis identified this promoter as the predominant source of transcription in haematopoietic tissue. Transient and stable transfections indicated that the proximal promoter was active in haematopoietic progenitor and endothelial cell lines and this activity was shown to depend on three conserved Ets sites which were bound in vivo by Elf1, Fli1 and Ets1. Transgenic analysis demonstrated that the LMO2 proximal promoter was sufficient for expression in endothelial cells in vivo. However, no haematopoietic expression was observed indicating that additional enhancers are required to mediate transcription from the proximal promoter in haematopoietic cells.To identify additional elements involved in haematopoietic expression of LMO2, we have performed a less restrictive search for conserved sequences by comparing the human, dog, rat and mouse LMO2 loci to the marsupial opossum LMO2 locus. The addition of the opossum locus, and removal of the more distant fish and amphibian sequences from the alignment, resulted in the discovery of eleven conserved regions. These sequences represent candidate haematopoietic regulatory regions as they contain conserved transcription factor binding sites (E boxes, Ets and Gata sites) previously shown to regulate several other haematopoietic genes. We will present results from a systematic analysis of these regions for enhancer activity in both haematopoietic cell lines and transgenic mice, which suggest that several of these elements indeed act as enhancers. Taken together, our experiments will provide a framework for the transcriptional hierarchies within which LMO2 exerts its function in normal haematopoietic cells. Moreover, the current studies will serve as a platform to examine potential molecular mechanisms that can cause ectopic expression of LMO2 in T-cell progenitors with the ultimate consequence of developing T-ALL.

Transcriptional Regulatory Networks Downstream of NOTCH1 in T-Cell Acute Lymphoblastic Leukemia.

The NOTCH1 signaling pathway plays a critical role in the regulation of hematopoietic stem cell homeostasis and is required for lymphocytes to adopt a T-cell fate. Importantly, aberrant activation of NOTCH signaling due to activating mutations in the NOTCH1 gene is involved in the pathogenesis of over 50% human T-cell acute lymphoblastic leukemias (T-ALL) and the therapeutic efficacy of inhibition of NOTCH signaling via gamma-secretase inhibitors (GSI) is currently being tested in clinical trials. However, little is known about the transcriptional programs activated downstream of NOTCH1 activation that contribute to the transformation of T-cell progenitors. Here we used chromatin immunoprecipitation and promoter microarrays (ChIP-on-chip) to identify direct transcriptional targets of NOTCH1; and microarray gene expression analysis to decipher the oncogenic transcriptional network activated by the NOTCH1 oncoprotein in T-ALL cells. Using the Hu19K arrays that contain over 13,000 human promoter regions, we have identified 134 candidate direct targets (p<0.0001) of NOTCH1 in the HPB-ALL T-ALL cell line by ChIP-on-chip. NOTCH1 targets identified by ChIP -on-chip included known direct targets of NOTCH1 such as preTCRA, as well as genes involved in T-cell signaling (CD3D, ASE-1/CD3E associated protein), proliferation (CDK5, CDC25A, RBL1, BUB3 and ING3) and survival (Survivin).Inhibition of NOTCH1 signaling with compound E, a strong GSI, induced a gene expression signature characterized by the downregulation of known NOTCH1 direct target genes such as DELTEX1, HES1 and HEY1. Analysis of different cell lines representative of different stages of maturation arrest and different oncogenic groups of T-ALL identified a core signature of transcriptional responses to NOTCH signaling inhibition, which included the downregulation of the MYC oncogene as well as numerous genes involved in nucleotide metabolism and protein synthesis. In addition, cell cycle inhibitors p27/KIP1 and p18/INK4D were upregulated. Importantly, 13 of the top direct target genes of NOTCH1 identified by ChIP-on-chip were consistently downregulated (p<0.005) upon NOTCH signaling inhibition in multiple cell lines. However, more restricted responses to NOTCH1 signaling inhibition, which included important developmental regulators of T-cell development such as the pre-TCRA, were present in specific groups of samples. These results demonstrate that NOTCH1 activation induces a complex transcriptional response in T-ALL cells, which is in part dependent on the stage of T-cell development and/or the interaction with other T-ALL transcription factor oncogenes. Common effector pathways downstream of NOTCH signaling may represent novel therapeutic targets for the treatment of T-ALL, while cell type specific responses may influence the cellular effects and the clinical efficacy of NOTCH signaling inhibitors currently under evaluation in clinical trials. Overall, our results indicate that NOTCH1 acts as a master transcriptional regulator at the top of a complex regulatory network that contributes to leukemogenesis by regulating multiple critical pathways involved in the regulation of T-cell differentiation, proliferation and survival.