An activated form of Notch influences the choice between CD4 and CD8 T cell lineages

Notch ligation by Delta1 inhibits peripheral immune responses to transplantation antigens by a CD8+ cell-dependent mechanism

Notch signaling plays a fundamental role in determining the outcome of differentiation processes in many tissues. Notch signaling has been implicated in T versus B cell lineage commitment, thymic differentiation, and bone marrow hematopoietic precursor renewal and differentiation. Notch receptors and their ligands are also expressed on the surface of mature lymphocytes and APCs, but the effects of Notch signaling in the peripheral immune system remain poorly defined. The aim of the studies reported here was to investigate the effects of signaling through the Notch receptor using a ligand of the Delta-like family. We show that Notch ligation in the mature immune system markedly decreases responses to transplantation antigens. Constitutive expression of Delta-like 1 on alloantigen-bearing cells renders them nonimmunogenic and able to induce specific unresponsiveness to a challenge with the same alloantigen, even in the form of a cardiac allograft. These effects could be reversed by depletion of CD8+ cells at the time of transplantation. Ligation of Notch on splenic CD8+ cells results in a dramatic decrease in IFN-gamma with a concomitant enhancement of IL-10 production, suggesting that Notch signaling can alter the differentiation potential of CD8+ cells. These data implicate Notch signaling in regulation of peripheral immunity and suggest a novel approach for manipulating deleterious immune responses.

The Anti-apoptotic Effect of Notch-1 Requires p56 -dependent, Akt/PKB-mediated Signaling in T Cells

The Notch family of transmembrane receptors have been implicated in a variety of cellular decisions in different cell types. Here we investigate the mechanism underlying Notch-1-mediated anti-apoptotic function in T cells using model cell lines as the experimental system. Ectopic expression of the intracellular domain of Notch-1/activated Notch (AcN1) increases expression of anti-apoptotic proteins of the inhibitors of apoptosis (IAP) family, the Bcl-2 family, and the FLICE-like inhibitor protein (FLIP) and inhibits death triggered by multiple stimuli that activate intrinsic or extrinsic pathways of apoptosis in human and murine T cell lines. Numb inhibited the AcN1-dependent induction of anti-apoptotic proteins and anti-apoptotic function. Using pharmacological inhibitors and dominant-negative approaches, we describe a functional role for phosphatidylinositol 3-kinase (PI3K)-dependent activation of the serine-threonine kinase Akt/PKB in the regulation of AcN1-mediated anti-apoptotic function and the expression of FLIP and IAP family proteins. Using a cell line deficient for the T cell-specific, Src family protein, the tyrosine kinase p56(lck) and by reconstitution approaches we demonstrate that p56(lck) is required for the Notch-1-mediated activation of Akt/PKB function. Furthermore, the Src tyrosine kinase inhibitor, PP2, abrogated ectopically expressed AcN1-mediated anti-apoptotic function and phosphorylation of p56(lck). We present evidence that endogenous Notch-1 associates with p56(lck) and PI3K but that Akt/PKB does not co-immunoprecipitate with the Notch1.p56(lck).PI3K complex. Finally, we demonstrate that the Notch1.p56(lck).PI3K complex is present in primary T cells that have been activated in vitro and sustained in culture with the cytokine interleukin-2.

Thicker than blood: conserved mechanisms in Drosophila and vertebrate hematopoiesis

Blood development in Drosophila melanogaster shares several interesting features with hematopoiesis in vertebrates, including spatiotemporal regulation as well as the use of similar transcriptional regulators and signaling pathways. In this review, we describe what is known about hematopoietic development in Drosophila and the various cell types generated and their functions. Additionally, the molecular genetic mechanisms of hematopoietic cell fate determination and commitment within Drosophila blood cell lineages are discussed and compared to vertebrate mechanisms.

Activated Notch2 potentiates CD8 lineage maturation and promotes the selective development of B1 B cells

Although studies have shown that the Notch2 family member is critical for embryonic development, little is known concerning its role in hematopoiesis. In this study, we show that the effects of an activated form of Notch2 (N2IC) on the T-cell lineage are dosage related. High-level expression of N2IC results in the development of T-cell leukemias. In contrast, lower-level expression of N2IC does not lead to transformation but skews thymocyte development to the CD8 lineage. Underlying this skew is a dramatic enhancement in positive selection and CD8SP maturation. N2IC permits early B-cell development but blocks the maturation of conventional B2 cells at the pre-B stage, which is the limit of endogenous Notch2 protein expression in developing B cells. Most strikingly, while B2 B cell development is blocked at the pre-B-cell stage, N2IC promotes the selective development of LPS-responsive B1 B cells. This study implicates a role for Notch2 in the maturation of the CD8 lineage and suggests a novel function for Notch2 in the development of the B1 B-cell subset.

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

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

Notch signaling in development and disease

Notch receptors and ligands were first identified in flies and worms, where they were shown to regulate cell proliferation, cell differentiation, and, in particular, binary cell fate decisions in a variety of developmental contexts. The first mammalian Notch homolog was discovered to be a partner in a chromosomal translocation in a subset of human T-cell leukemias. Subsequent studies in mice and humans have shown that Notch signaling plays essential roles at multiple stages of hematopoiesis, and also regulates the development or homeostasis of cells in many tissues and organs. Thus, it is not surprising that mutations which disrupt Notch signaling cause a wide range of cancers and developmental disorders. Perhaps because it is so widely used, Notch signaling is subject to many unusual forms of regulation. In this review, we will first outline key aspects of Notch signaling and its regulation by endocytosis, glycosylation, and ubiquitination. We will then overview recent literature elucidating how Notch regulates cell-lineage decisions in a variety of developmental contexts. Finally, we will describe the roles of dysregulated Notch signaling in causing several types of cancer and other pathologies.

Overexpression of the Notch ligand, Jagged-1, induces alloantigen-specific human regulatory T cells

Graft-versus-host disease (GVHD) represents one of the major complications of allogeneic hematopoietic stem cell transplantation. Techniques to prevent GVHD have included ex vivo T-cell depletion of the graft or prolonged in vivo immunosuppression. Both reduce the frequency and severity of GVHD but also reduce T-cell-mediated graft-versus-malignancy effect, and increase the risk of infection. A major goal in transplantation is to prevent alloreactivity while preserving activity against tumors and infectious agents. We have used activation of the Notch pathway to try to generate T cells able to specifically regulate alloantigen responses. We used allogeneic Epstein-Barr virus lymphoblastoid B cells (EBV-LCLs) as stimulator cells. Such LCLs are excellent (allo) antigen-presenting cells and can be obtained in large numbers even from donors who have received extensive chemo/radiotherapy. We overexpressed a Notch ligand, Jagged-1, in these cells by adenoviral vector transduction. Stimulation of CD45RA+ naive T cells by Jagged-1 EBV-LCL reduces production of interferon-gamma, interleukin-2, and interleukin-5, but up-regulates transforming growth factor-beta 1 synthesis, consistent with induction of a regulatory T-cell phenotype. Transfer of these T cells to fresh lymphocyte cultures inhibits proliferative and cytotoxic immune responses to the priming alloantigens while sparing responses to third-party stimulator cells. Notch activation in the presence of alloantigen-presenting cells may therefore be a means of inducing specific regulatory T cells while preserving other T-cell functionality.

Notch signaling in the immune system

Notch signaling plays a preeminent role during development in not only regulating cell fate decisions, but it can also influence growth and survival of progenitor cells. In the immune system, Notch is required for the maintenance of hematopoietic stem cells and in directing T- versus B-lineage commitment. In this review, I will summarize some of the recent findings relating to the function of Notch in the immune system during lymphocyte development and in the generation and function of mature cells.

The role of Notch in tumorigenesis: oncogene or tumour suppressor?

Notch signalling participates in the development of multicellular organisms by maintaining the self-renewal potential of some tissues and inducing the differentiation of others. Involvement of Notch in cancer was first highlighted in human T-cell leukaemia, fuelling the notion that aberrant Notch signalling promotes tumorigenesis. However, there is mounting evidence that Notch signalling is not exclusively oncogenic. It can instead function as a tumour suppressor.

TCR-mediated Notch signaling regulates proliferation and IFN-gamma production in peripheral T cells

Notch genes encode membrane receptors that regulate cell fate decisions in metazoa. Notch receptors and ligands are expressed in developing lymphoid tissue and mature lymphocytes and the role of Notch signaling in early T and B cell development has been studied extensively. However, its contribution to mature T cell function is unknown. TCR-mediated T cell activation is a fundamental process of the adaptive immune system that has been studied for decades; however, the details of this process are incompletely understood. In this study, we present evidence that Notch is required for TCR-mediated activation of peripheral T cells. Inhibition of Notch activation dramatically decreases T cell proliferation in both CD4 and CD8 cells and blocks both NF-kappaB activity and IFN-gamma production in peripheral T cells. Our data reveal a new, nondevelopmental function of Notch as a previously unknown key link in peripheral T cell activation and cytokine secretion.

Surface expression of Notch1 on thymocytes: correlation with the double-negative to double-positive transition

Notch1 plays a critical role in regulating T lineage commitment during the differentiation of lymphoid precursors. The physiological relevance of Notch1 signaling during subsequent stages of T cell differentiation has been more controversial. This is due in part to conflicting data from studies examining the overexpression or targeted deletion of Notch1 and to difficulties in distinguishing between the activities of multiple Notch family members and their ligands, which are expressed in the thymus. We employed a polyclonal antiserum against the extracellular domain of Notch1 to study surface expression during thymopoiesis. We found high levels of Notch1 on the cell surface only on double negative (DN) stage 2 through the immature single-positive stage of thymocyte development, before the double-positive (DP) stage. The Notch signaling pathway, as read out by Deltex1 expression levels, is highly active in DN thymocytes. When an active Notch1 transgene, Notch1IC, is exogenously introduced into thymocytes of recombinase-activating gene 2-deficient mice, it promotes proliferation and development to the DP stage following anti-CD3 treatment without apparently affecting the intensity of pre-TCR signaling. In addition, a stromal cell line expressing the Notch ligand, Delta-like-1, promotes the in vitro expansion of wild-type DN3 thymocytes in vitro. Consistent with other recent reports, these data suggest a role for Notch1 during the DN to DP stage of thymocyte maturation and suggest a cellular mechanism by which Notch1IC oncogenes could contribute to thymoma development and maintenance.

Loss of Ly-6A.2 expression on immature developing T cells in the thymus is necessary for their normal growth and generation of the Vbeta T-cell repertoire

Stage-specific expression of a number of cell-surface and signaling proteins is critical for normal development of T cells in the thymus. Equally important may be the loss of expression/signaling of developmentally regulated proteins for proper transitioning of developing T cells into thymic subsets. Ly-6A.2 exhibits a regulated pattern of expression on T cells maturing in the thymus, and dysregulating its expression results in arrest of developing T cells within the CD3-CD4-CD8- triple negative (TN) stage where the normal expression of Ly-6A.2 is extinguished. To further characterize the mechanisms underlying this block, we examined whether cell signaling and/or cell adhesion properties of the Ly-6A.2 molecule influenced the block in T-cell development. Analysis of bone marrow chimeras generated by injecting CFSE-labeled Ly-6A.2 transgenic bone marrow cells into irradiated syngeneic non-transgenic mice revealed normal trafficking of developing T cells from the cortex into the medulla. Production of LAT but not p56lck was diminished in CD4-CD8- DN cells from Ly-6A.2 dysregulated mice when compared with control littermates. Dysregulated expression of Ly-6A.2 did not suppress endogenous TCR-Vbeta expression. Finally, dysregulated expression of Ly-6A.2 enhanced apoptosis of an immature CD4+CD8+ (DP) subset of developing cells and altered the selected TCR-Vbeta repertoire. Taken together, these observations indicate that the termination of Ly-6A.2 expression and signaling within the CD4-CD8-CD3- subset of developing T cells is an important checkpoint during normal thymic development.

Notch1 enhances B-cell receptor-induced apoptosis in mature activated B cells without affecting cell cycle progression and surface IgM expression

The transmembrane receptor Notch1 plays a crucial role in differentiation and apoptosis of hematopoietic cells. To investigate the influence of Notch1 on apoptosis and cell growth of mature murine B cells, we transduced the murine B-lymphoma line NYC 31.1 with a constitutively active, intracellular form of human Notch1 (Notch1-ICT). NYC cells represent mature activated B cells that can be induced to undergo apoptosis by crosslinking of the B-cell receptor (BCR). In contrast to investigations in immature chicken B-cell lines, transduced Notch1-ICT did not affect cell cycle progression, cell growth or surface IgM levels in NYC cells and resulted only in a slight induction of apoptosis. However, BCR-crosslinking enhanced apoptosis, but did not influence cell cycle progression in Notch1-ICT-positive NYC cells. These data imply a distinct function of Notch1 in mature murine B-cells as compared to immature chicken B cells and provide further evidence for Notch1's involvement in B-cell differentiation and development.

Notch-regulated ankyrin-repeat protein inhibits Notch1 signaling: multiple Notch1 signaling pathways involved in T cell development

We have characterized the function of Notch-regulated ankyrin-repeat protein (Nrarp) in mouse cell lines and in hematopoietic stem cells (HSCs). Nrarp overexpression is able to block Notch-induced activation of CBF-1. In AKR1010 thymoma cells, Nrarp overexpression blocks CBF-1-dependent transcriptional activation of Notch-responsive genes and inhibits phenotypic changes associated with Notch activation. Enforced expression of Nrarp in mouse HSCs results in a profound block in T lineage commitment and progression through early stages of thymocyte maturation. In contrast, Deltex-1 overexpression in HSCs can also block T lineage commitment but not progression through the early double negative stages of thymocyte maturation. The different effects of Deltex-1 and Nrarp overexpression suggest that alternate Notch signaling pathways mediate T vs B lineage commitment and thymocyte maturation.

Bcl11a is essential for normal lymphoid development

Bcl11a (also called Evi9) functions as a myeloid or B cell proto-oncogene in mice and humans, respectively. Here we show that Bcl11a is essential for postnatal development and normal lymphopoiesis. Bcl11a mutant embryos lack B cells and have alterations in several types of T cells. Phenotypic and expression studies show that Bcl11a functions upstream of the transcription factors Ebf1 and Pax5 in the B cell pathway. Transplantation studies show that these defects in Bcl11a mutant mice are intrinsic to fetal liver precursor cells. Mice transplanted with Bcl11a-deficient cells died from T cell leukemia derived from the host. Thus, Bcl11a may also function as a non-autonomous T cell tumor suppressor gene.

Perturbation of Ikaros isoform selection by MLV integration is a cooperative event in Notch(IC)-induced T cell leukemogenesis

The chromosomal translocation t(7;9)(q34;q34.3) in human T cell acute lymphoblastic leukemia (T-ALL) results in the aberrant expression of the intracellular domain of Notch (N(ic)). Consistent with the current multistep model for tumorigenesis, mice that express N(ic) in T cell progenitors develop a T-ALL-like disease with a lengthened latency. Proviral insertional mutagenesis greatly accelerated the onset of leukemia in N(ic) transgenic mice. We demonstrate that the Ikaros (Ik) locus is a common target of proviral integration in N(ic) transgenic mice, which results in the loss of Ik DNA binding activity through altered isoform expression. We propose that cooperative leukemogenesis occurs in cells that have constitutive N(ic) and altered Ik isoform expression because genes normally repressed by Ik become activated by N(ic)/CSL.

Notch activation suppresses fibroblast growth factor-dependent cellular transformation

Aberrant activations of the Notch and fibroblast growth factor receptor (FGFR) signaling pathways have been correlated with neoplastic growth in humans and other mammals. Here we report that the suppression of Notch signaling in NIH 3T3 cells by the expression of either the extracellular domain of the Notch ligand Jagged1 or dominant-negative forms of Notch1 and Notch2 results in the appearance of an exaggerated fibroblast growth factor (FGF)-dependent transformed phenotype characterized by anchorage-independent growth in soft agar. Anchorage-independent growth exhibited by Notch-repressed NIH 3T3 cells may result from prolonged FGFR stimulation caused by both an increase in the expression of prototypic and oncogenic FGF gene family members and the nonclassical export of FGF1 into the extracellular compartment. Interestingly, FGF exerts a negative effect on Notch by suppressing CSL (CBF-1/RBP-Jk/KBF2 in mammals, Su(H) in Drosophila and Xenopus, and Lag-2 in Caenorhabditis elegans)-dependent transcription, and the ectopic expression of constitutively active forms of Notch1 or Notch2 abrogates FGF1 release and the phenotypic effects of FGFR stimulation. These data suggest that communication between the Notch and FGFR pathways may represent an important reciprocal autoregulatory mechanism for the regulation of normal cell growth.

Notch and lymphopoiesis: a view from the microenvironment

The differentiation of B- and T-cells in primary lymphoid organs depends on, or is strongly influenced by, signals provided by stromal cells, extracellular matrix components as well as by direct contacts between differentiating lymphocytes and distinct environmental cells. Notch receptors and their ligands mediate intercellular contacts and are crucially important for the development of T- and B-cell lineages. Here we start by reviewing current knowledge on the expression patterns of Notch receptors and their ligands in primary lymphoid organs and the effects induced by their functional interactions. Then we shall attempt to discuss how those interactions may regulate not only lymphopoiesis per se but also morphogenesis and the functional compartmentalization of lymphopoietic organs during development.

Notch3, another Notch in T cell development

Different members of the Notch family have been described to play a critical role in T cell lineage commitment and T cell development and functions. Nevertheless, whether they act as redundant molecules, by affecting the same molecular mechanisms, or play distinct roles in T cell differentiation and/or functions is not clear. Altered Notch3 signaling impairs the developmentally regulated interplay between pre-TCR and NF-kappaB signaling and allows the disruption of early thymocyte differentiation and the development of T cell leukemia, thus identifying the crucial role of Notch3 receptor in the coordination of T cell differentiation and growth control.

Microenvironmental regulation of Notch signalling in T cell development

T cells develop in the thymus from blood-borne progenitors derived from haematopoietic tissues. Amongst the mechanisms by which stromal cells in thymic and prethymic tissues influence lymphoid progenitors, recent attention has focussed on the importance of Notch signalling in early T cell development. Here, we review evidence that developing T cells and their progenitors receive signals through Notch receptors as a result of interactions with Notch ligands expressed by stromal cells. In particular, we focus on the role of Notch ligand-expressing stromal cells in regulating key control points during pre- and intrathymic phases of T cell development.

B-cell commitment: deciding on the players

Considerable progress has been made over the past few years in determining the key molecular players that regulate the commitment and development of early B-lineage cells in mouse bone marrow (transcription factors, cytokines and their receptors). However, confusion remains as to the precise stages, surface phenotypes and lineage restrictions that are observed early in B-cell development. Increasingly powerful flow cytometry analysis of genetically altered animals (knockouts and transgenics) combined with sensitive functional assays may provide an answer, a "scorecard" for the cell stages.

The Wiskott-Aldrich syndrome protein: forging the link between actin and cell activation

The Wiskott-Aldrich syndrome protein (WASp) has emerged as a central player in the regulation of actin remodeling in T cells. The unique domain structure of WASp and other WASp family members enables these proteins to associate with a myriad of signaling effectors and to thereby regulate the coupling of T cell antigen receptor (TCR) engagement to both cytoskeletal rearrangement and transcriptional activation. This review focuses on these biochemical properties of WASp and also on the mechanisms whereby WASp interactions with its cognate ligands influence T cell activation. Because of its capacity to shift intracellular location and thereby dictate both the timing and the spatial distribution of actin polymerization following cell stimulation, WASp is well positioned to play major regulatory roles in directing a wide range of cellular processes and signaling pathways. Further dissection of the functional and biochemical properties of WASp therefore represents a promising avenue towards defining the molecular mechanisms that convey TCR stimulatory signals to the actin cytoskeleton and integrate cytoskeletal and other signaling systems so as to evoke a biological response.

Subversion of B lymphocyte signaling by infectious agents

Infectious agents and their hosts interact in a complex manner, involving not only superficially apparent mechanisms, but also the signaling machinery that governs host cells responses. Thus, signaling events, surface molecule expression, and transcriptional control may be affected in various cell types, with profound consequences for the function of individual cells and organ systems. Studies of the biochemistry of cell signaling and cell invasion by infectious agents have begun to detail the interplay between elements of infectious organisms and the host at the molecular level. Consequently, the resulting interferences with lymphocyte signaling may disturb the function of the immune system. In B cells, alterations of immune receptor signaling has implications for human diseases. By affecting the mechanisms of the host's immune defense, this may not only lead to inadequate elimination of an infectious agent, but also to autoimmunity or neoplasia.

IkappaBalpha and p65 regulate the cytoplasmic shuttling of nuclear corepressors: cross-talk between Notch and NFkappaB pathways

Notch and NFkappaB pathways are key regulators of numerous cellular events such as proliferation, differentiation, or apoptosis. In both pathways, association of effector proteins with nuclear corepressors is responsible for their negative regulation. We have previously described that expression of a p65-NFkappaB mutant that lacks the transactivation domain (p65DeltaTA) induces cytoplasmic translocation of N-CoR leading to a positive regulation of different promoters. Now, we show that cytoplasmic sequestration of p65 by IkappaBalpha is sufficient to both translocate nuclear corepressors SMRT/N-CoR to the cytoplasm and upregulate transcription of Notch-dependent genes. Moreover, p65 and IkappaBalpha are able to directly bind SMRT, and this interaction can be inhibited in a dose-dependent manner by the CREB binding protein (CBP) coactivator and after TNF-alpha treatment, suggesting that p65 acetylation is modulating this interaction. In agreement with this, TNF-alpha treatment results in downregulation of the Hes1 gene. Finally, we present evidence on how this mechanism may influence cell differentiation in the 32D myeloid progenitor system.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Is there a role for Notch signalling in human breast cancer?

Aberrant Notch signalling has been observed in several human cancers, including acute T-cell lymphoblastic leukaemia and cervical cancer, and is strongly implicated in tumourigenesis. Unregulated Notch signalling in the mouse mammary gland leads to tumour formation. These results raise the possibility that Notch signalling might play a role in human breast cancer. There are currently few reports that address this question directly and this appears to be an area worthy of further investigation.

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

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

Induction of T cell development from hematopoietic progenitor cells by delta-like-1 in vitro

The molecular interactions provided by the thymic microenvironment that predicate T cell development remain obscure. Here, we show that a bone marrow stromal cell line ectopically expressing the Notch ligand Delta-like-1 loses its ability to support B cell lymphopoiesis, but acquires the capacity to induce the differentiation of hematopoietic progenitors into CD4 CD8 double- and single-positive T cells. Both gammadelta-TCR(+) and alphabeta-TCR(+) T cells are generated, and CD8(+) TCR(hi) cells produce gamma-interferon following CD3/TCR stimulation. These results establish that expression of Delta-like-1 on stromal cells provides key signals for the induction of T cell lineage commitment, stage-specific progenitor expansion, TCR gene rearrangement, and T cell differentiation in the absence of a thymus. Thus, it is likely that Delta-like-1/Notch interactions by the thymus underpin its unique ability to promote lineage commitment and differentiation of T cells.

Notch signaling in lymphocyte development

Cytokine and antigen receptor signals play well-characterized roles in promoting the survival and maturation of T and B lymphocyte progenitors through sequential developmental stages. Emerging studies suggest equally important roles for more ancient signaling pathways that evolved prior to the adaptive immune system in jawed vertebrates. In particular, there are at least two essential functions for the highly conserved Notch signaling pathway in lymphocyte development. First, Notch signals are essential for the development of T cell progenitors in the thymus and intestinal epithelium. Second, Notch signals are required to suppress B cell development in the thymus. This review will focus on focus on recent advances in our understanding of how Notch signaling regulates this developmental switch, as well as how Notch might regulate subsequent survival and cell fate decisions in developing T cells.

T-lineage specification and commitment: a gene regulation perspective

T lymphocytes originate from pluripotent precursors and undergo lasting commitment to the T cell developmental fate during their processing in the thymus. Commitment includes both the acquisition of essential T cell characteristics and the foreclosing of other developmental options. Gain of T cell characteristics is probably mediated by separate mechanisms, at least in detail, from loss of alternative developmental potentials. Programmed shifts in survival requirements make changes irreversible. Here we review the current evidence identifying the regulatory components of this commitment pathway, and the first hints of how they work together. Roles for PU.1, GATA-3, and their target genes are highlighted.

Controlling CD4 gene expression during T cell lineage commitment

T cell lineage commitment as the double-positive (DP) thymocyte matures into the single-positive (SP) T cell requires the irreversible repression or maintenance of CD4 gene expression. Signals transmitted from the T cell antigen receptor (TCR) during thymic selection are believed to be linked to the transcriptional regulation of the CD4 gene; thus, a study of the factors that control CD4 gene expression may lead to further insight into the molecular mechanisms that drive T cell development. This review discusses the work conducted to date to identify and characterize the transcriptional control elements in the CD4 locus and the factors that mediate their function. From these studies, it is clear that the molecular mechanisms controlling CD4 gene expression are very complex and are controlled by many different signals as the thymocyte develops.

Genetic dissection of systemic lupus erythematosus pathogenesis: evidence for functional expression of Sle3/5 by non-T cells

On the non-autoimmune C57BL/6 (B6) background, the chromosome 7-derived lupus susceptibility loci Sle3 and Sle5 have been shown to mediate an elevated CD4:CD8 ratio with an increase in activated CD4(+) T cells, decreased susceptibility to apoptosis, and a break in humoral tolerance. Development of subcongenic strains has subsequently shown that the elevated CD4:CD8 ratio is due to Sle3 but that both loci contribute to the development of autoantibodies. To elucidate the functional expression patterns of these loci, adoptive transfer experiments were conducted. All possible combinations of bone marrow reconstitution, including syngenic, were conducted between the congenic B6 and B6.Sle3/5 strains. It was found that the Sle3/5 locus was functionally expressed by bone marrow-derived cells, but not by host cells, and that the elevated CD4:CD8 phenotype could be reconstituted in radiation chimeras. Using Ly5-marked congenic strains and B6 host mice, additional experiments surprisingly demonstrated that the elevated CD4:CD8 ratio was neither an intrinsic property of the T cells nor of single positive thymocytes. Allotype-marked chimeras indicated that autoantibody production by B cells was also an extrinsic property, as shown by the fact that B cells without the Sle3/5 interval contributed to autoantibody production. These experiments strongly suggest that a gene within the B6.Sle3/5 interval was expressed by a bone marrow-derived, nonlymphocyte population in the thymus and periphery and was affecting T cell selection and/or survival.

The Notch ligand Jagged-1 is able to induce maturation of monocyte-derived human dendritic cells

Notch receptors play a key role in several cellular processes including differentiation, proliferation, and apoptosis. This study investigated whether the activation of Notch signaling would affect the maturation of dendritic cells (DCs). Direct stimulation of Notch signaling in DCs with a peptide ligand induced DC maturation, similar to LPS: DCs up-regulated maturation markers, produced IL-12, lost endocytosis capacity, and became able to activate allogeneic T cells. Furthermore, coculture of DCs with cells expressing Notch ligand Jagged-1 induced up-regulation of maturation markers, IL-12 production, T cell proliferative responses, and IFN-gamma production. Our data suggest that activation of Notch by Jagged-1 plays an important role in maturation of human DCs. Additionally, they reveal a novel role for Notch signaling in cell maturation events distal to the cell fate decision fork. These data may have important medical implications, since they provide new reagents to induce DC activity, which may be beneficial as adjuvants in situations where an immune response needs to be elicited, such as tumor immunotherapy.

Stromal cells provide the matrix for migration of early lymphoid progenitors through the thymic cortex

During steady state lymphopoiesis in the postnatal thymus, migration of precursors outward from the deep cortex toward the capsule is required for normal differentiation. Such migration requires, at a minimum, expression of adhesive receptors on the migrating lymphoid cells, as well as a stable matrix of their ligands persisting throughout the region of migration. In this study, we address the nature of this adhesive matrix. Although some precursor stages bound efficiently to extracellular matrix ligands, a specific requirement for the cell surface ligand VCAM-1 was also found. In situ analysis revealed that early precursors are found in intimate contact with a matrix formed by stromal cells in the cortex, a proportion of which expresses VCAM-1. In vivo administration of an anti-VCAM-1 Ab resulted in decreased thymic size and altered distribution of early precursors within the cortex. These results indicate that precursors migrating outward through the cortex may use a cellular, rather than extracellular, matrix for adhesion, and suggest that the VCAM-1(+) subset of cortical stroma may play a crucial role in supporting the migration of early precursors in the steady state thymus.

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

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

Active form of Notch imposes T cell fate in human progenitor cells

The crucial role of Notch signaling in cell fate decisions in hematopoietic lineage and T lymphocyte development has been well established in mice. Overexpression of the intracellular domain of Notch mediates signal transduction of the protein. By retroviral transduction of this constitutively active truncated intracellular domain in human CD34+ umbilical cord blood progenitor cells, we were able to show that, in coculture with the stromal MS-5 cell line, depending on the cytokines added, the differentiation toward CD19+ B lymphocytes was blocked, the differentiation toward CD14+ monocytes was inhibited, and the differentiation toward CD56+ NK cells was favored. The number of CD7+cyCD3+ cells, a phenotype similar to T/NK progenitor cells, was also markedly increased. In fetal thymus organ culture, transduced CD34+ progenitor cells from umbilical cord blood cells or from thymus consistently generated more TCR-gammadelta T cells, whereas the other T cell subpopulations were largely unaffected. Interestingly, when injected in vivo in SCID-nonobese diabetic mice, the transduced cells generated ectopically human CD4+CD8+ TCR-alphabeta cells in the bone marrow, cells that are normally only present in the thymus, and lacked B cell differentiation potential. Our results show unequivocally that, in human, Notch signaling inhibits the monocyte and B cell fate, promotes the T cell fate, and alters the normal T cell differentiation pathway compatible with a pretumoral state.

The role of Notch signaling during hematopoietic lineage commitment

Over the last few years a vast amount of progress has been made in identifying mechanisms controlling lineage commitment and plasticity of hematopoietic precursors to different lymphoid or myeloid lineages. This has been due largely to the ability to identify and isolate rare cell populations in order to investigate their developmental potential, together with the development of inducible and/or tissue specific targeting technology. One family of proteins that has been postulated to be involved in hematopoietic stem cell maintenance as well as in multiple commitment processes during T cell development is the Notch receptors and their ligands. In this review we will summarize recent findings and controversies regarding the role of Notch signaling in the myeloid and lymphoid systems.

A reassessment of the effect of activated Notch1 on CD4 and CD8 T cell development

The Notch signaling pathway plays an important role in the early steps of T cell development and in the generation of T cell tumors, but its role in the CD4 vs CD8 lineage decision is controversial. Notch1 is not essential for CD4 or CD8 T cell development; however, there are suggestions that multiple Notch family members may act in a redundant fashion during thymic development. In theory, expressing a constitutively activated form of Notch in CD4(+)CD8(+) thymocytes could provide clues about the normal role of Notch in developing CD4 and CD8 T cells. Unfortunately, two different studies of transgenic mice expressing activated forms of Notch1 (Notch1IC) led to conflicting conclusions. In this study, we re-examine the effect of the two Notch1IC transgenes on thymocyte development. We find that both Notch1IC transgenic lines display a decrease in CD4 single positive (SP) thymocytes and a corresponding increase in CD8 SP thymocytes. The enhanced development of CD8 SP thymocytes is dependent on either class I or II MHC. Thus, data from two different Notch1IC transgenic lines indicate that Notch activity promotes CD8 and inhibits CD4 SP development. We suggest that the discrepancies in previous reports of Notch1IC transgenic mice are due to differences in the propensity of the two different transgenic lines to develop tumors.

Different role of Apaf-1 in positive selection, negative selection and death by neglect in foetal thymic organ culture

Apoptotic protease-activating factor 1 (Apaf-1) is a component of the apoptosome which is required for the activation of procaspase-9. As Apaf-1 knockout (KO) (Apaf-1-/-) mice die before birth, the role of Apaf-1 during thymic selection was investigated using 5 day foetal thymic organ culture (FTOC) of thymi obtained at gestational day 15. There was a lower ratio of CD4 single-positive (SP) to CD8 SP cells and decreased apoptosis of CD4+CD8+ (DP) thymocytes from Apaf-1-/- mice compared with wild-type. To determine if these defects resulted in increased production of neglected thymocytes, the Apaf-1-/- mice were crossed with the T-cell receptor (TCR)-alpha-chain KO mice. There was no difference in thymocyte development in the thymi of TCR-alpha-/-Apaf-1-/- and TCR-alpha-/-Apaf-1+/+ mice 5 days after FTOC. To determine if Apaf-1 is involved in apoptosis during death by negative or positive selection, FTOC of the thymus of Apaf-1-/- Db/HY TCR-alphabeta transgenic (Tg) mice was carried out. There was decreased apoptosis of the HY clonal-specific M33+ thymocytes and an increased percentage of the autoreactive CD8+M33+ thymocytes in male, but not female Apaf-1-/- Db/HY TCR Tg mice. Our data suggest that Apaf-1 is not involved in positive selection or death by neglect, but may have a partial role in negative selection during early thymic T-cell development.

Characterization of the transcriptional expression of Notch-1 signaling pathway members, Deltex and HES-1, in developing mouse thymocytes

The Notch transmembrane protein is involved in a broad range of different developmental pathways in vertebrates and invertebrates. Targeted thymocyte expression of the Notch-1 intracellular domain has been shown to affect lineage commitment decisions such as those involving T cell vs. B cell, thymocyte alpha beta vs. gamma delta TCR, as well as CD4 vs. CD8 thymocyte commitment. In this paper, we quantitatively characterize thymocyte RNA expression of two purported transcriptional markers of Notch-1 signaling activity, Deltex and HES-1. Using a semiquantitative RTPCR approach, we show that both Deltex and HES-1 transcriptional levels are developmentally regulated as thymocytes mature from the earliest CD4/CD8 double negative thymocyte stage, through the intermediate CD4/CD8 double positive stage, and finally to the mature CD4 or CD8 single positive stage. Deltex and HES-1, despite both being transcriptional markers of Notch-1 activity, express different patterns of transcriptional activity among the thymocyte subsets. Neither treatment with combined (alpha CD3)/(alpha CD28) antibodies nor the combination of the phorbol ester PMA and calcium ionophore ionomycin affects expression of Deltex in immature thymocytes; however, PMA/ionomycin treatment does downregulate expression of HES-1, an affect mostly mediated by ionomycin. Finally, a difference in HES-1 expression is seen between CD4/CD8 double positive thymocytes isolated from wild-type vs. MHC class I/II deficient mice, suggesting that Notch-1 activity is modulated during in vivo TCR/MHC-ligand selection events.

Inactivation of Notch1 impairs VDJbeta rearrangement and allows pre-TCR-independent survival of early alpha beta Lineage Thymocytes

Notch proteins influence cell fate decisions in many developmental systems. During lymphoid development, Notch1 signaling is essential to direct a bipotent T/B precursor toward the T cell fate, but the role of Notch1 at later stages of T cell development remains controversial. We have recently reported that tissue-specific inactivation of Notch1 in immature (CD44(-) CD25(+)) thymocytes does not affect subsequent T cell development. Here, we demonstrate that loss of Notch1 signaling at an earlier (CD44(+)CD25(+)) developmental stage results in severe perturbation of alpha beta but not gamma delta lineage development. Immature Notch1(-/-) thymocytes show impaired VDJ beta rearrangement and aberrant pre-TCR-independent survival. Collectively, our data demonstrate that Notch1 controls several nonredundant functions necessary for alpha beta lineage development.

The notch pathway: modulation of cell fate decisions in hematopoiesis

The hematopoietic system is maintained by a rare population of hematopoietic stem cells (HSC) that are thought to undergo self-renewal as well as continuously produce progeny that differentiate into the various hematopoietic lineages. However, the mechanisms regulating cell fate choices by HSC and their progeny have not been understood. Results of most studies support a stochastic model of cell fate determination in which growth factors support only the survival or proliferation of the progeny specified along a particular lineage. In other developmental systems, however, Notch signaling has been shown to play a central role in regulating fate decisions of numerous types of precursors, often inhibiting a particular (default) pathway while permitting self-renewal or differentiation along an alternative pathway. There is also accumulating evidence that the Notch pathway affects survival, proliferation, and cell fate choices at various stages of hematopoietic cell development, including the decisions of HSC to self-renew or differentiate and of common lymphoid precursors to undergo T- or B-cell differentiation. These data suggest that the Notch pathway plays a fundamental role in the development and maintenance of the hematopoietic system.

Inducible gene knockout of transcription factor recombination signal binding protein-J reveals its essential role in T versus B lineage decision

The transcription factor recombination signal binding protein-J (RBP-J) functions immediately downstream of the cell surface receptor Notch and mediates transcriptional activation by the intracellular domain of all four kinds of Notch receptors. To investigate the function of RBP-J, we introduced loxP sites on both sides of the RBP-J exons encoding its DNA binding domain. Mice bearing the loxP-flanked RBP-J alleles, RBP-J(f/f), were mated with Mx-Cre transgenic mice and deletional mutation of the RBP-J gene in adult mice was induced by injection of the IFN-alpha inducer poly(I)-poly(C). Here we show that inactivation of RBP-J in bone marrow resulted in a block of T cell development at the earliest stage and increase of B cell development in the thymus. Lymphoid progenitors deficient in RBP-J differentiate into B but not T cells when cultured in 2'-deoxyguanosine-treated fetal thymic lobes by hanging-drop fetal thymus organ culture. Competitive repopulation assay also revealed cell autonomous deficiency of T cell development from bone marrow of RBP-J knockout mouse. Myeloid and B lineage differentiation appears normal in the bone marrow of RBP-J-inactivated mice. These results suggest that RBP-J, probably by mediating Notch signaling, controls T versus B cell fate decision in lymphoid progenitors.

Overexpression of the Notch target genes Hes in vivo induces lymphoid and myeloid alterations

To examine the effects of Notch signaling on hematopoiesis, we transplanted mice with progenitors transduced with a constitutively active form of Notch1 (Notch1IC) or the Notch1 target genes Hes. Notch1IC-transduced cells induce T cell tumors and cannot generate B lymphocytes in vivo. Hes-transplanted mice remained healthy but cells transduced with Hes1 or Hes5 were partially impaired in their ability to differentiate into B cells. Both Hes1 and Hes5 were upregulated in the BM of Notch1IC mice and their ability to interfere with the transcriptional activity of E2A in a reporter assay was comparable to that of Notch1IC. This suggests that the inhibition of B cell development in the Notch1IC-transduced cells could be mediated by the interference of HES1/HES5 proteins with E2A. Hes1-, Hes5- and Notch1IC-transduced bone marrow cells cultured ex vivo in a colony forming assay in the presence of cytokines that promote myeloid differentiation remained very immature, indicating that the myeloid potential of these bone marrow cells was altered. Thymocytes overexpressing Hes1, Hes5 or Notch1IC matured normally into CD4 and CD8 single positive cells in vivo. Altogether our data suggest that Notch1IC induces T cell tumors independently of Hes genes but that its interference with lymphoid B and myeloid maturation is partly mediated by Hes1 and Hes5. DOI:

T-cell development and the CD4-CD8 lineage decision

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Linkage analysis of variations in CD4:CD8 T cell subsets between C57BL/6 and DBA/2

The ratio of CD4 T cells to CD8 T cells (CD4:CD8 ratio) varies over two-fold between C57BL/6 and DBA/2 mice for both T cell precursors in the thymus and mature T cells in the periphery. Correlation analysis of the CD4:CD8 ratio in thymic precursors vs peripheral T cells in F2 and backcross mice found that thymic precursor ratios are inherited independently from those in the periphery, indicating that the CD4:CD8 ratios in these populations are affected by distinct genetic mechanisms. A genome scan of progeny in the phenotypic extremes identified three quantitative trait loci (QTLs). Trmq1 (for T cell ratio modifier QTL 1) was detected in the telomeric end of c6 (peak marker D6Mit15 at 74 cM) and had a maximum LOD score of 4.6. Trmq2, in the telomeric half of c2, peaked at D2MIT483 and had a maximum LOD score of 3.41. Both of these QTLs impacted the CD4:CD8 ratios in peripheral T cells and had no impact on variation in this ratio among thymic precursors. However, heterozygosity for the H2 complex was suggestively associated (LOD score of 2.43) with increases in CD4 T cells among T cell precursors in the thymus. All of these QTLs were affected by epistatic interactions, indicating that additional modifiers in the B6 and DBA/2 genomes modulate this phenotype.

Deciphering the role of Notch signaling in lymphopoiesis

Components of the Notch signaling pathway are expressed during multiple stages of lymphoid development. Consistent with its function during invertebrate development, Notch signaling is proposed to have a central role in lymphoid cell-fate specification. Recent studies show that Notch signaling is a proximal event in T-cell commitment from a common lymphoid progenitor. The role of Notch at later stages of lymphoid development is controversial, but recent data suggest models that may help clarify observations. Current studies suggest that Notch activity is cell-context dependent and interactions between Notch and other environmental receptors are integrated during cell-fate decisions. Furthermore, the requirement for precise regulation of Notch activity is evident from human and murine neoplasms in which dysregulated Notch signaling leads to T-cell leukemia. Future studies that identify the stages of lymphoid development where Notch signaling is physiologically active and the exact targets of Notch signaling that are relevant to lymphopoiesis should significantly improve our understanding of Notch function in T- and B-cell development.