MHC recognition in thymic development: distinct, parallel pathways for survival and lineage commitment

NOTCH and phosphatidylinositide 3-kinase/phosphatase and tensin homolog deleted on chromosome ten/AKT/mammalian target of rapamycin (mTOR) signaling in T-cell development and T-cell acute lymphoblastic leukemia

Activating mutations in NOTCH1 consitute the most prominent genetic abnormality in T-cell acute lymphoblastic leukemia (T-ALL). However, most T-ALL cell lines with NOTCH1 mutations are resistant to treatment with γ-secretase inhibitors (GSIs). The spotlight is now shifting to the phosphatidylinositide 3-kinase (PI3K)/phosphatase and tensin homolog deleted on chromosome ten (PTEN)/AKT/mammalian target of rapamycin (mTOR) pathway as another key potential target. These two signaling routes are deregulated in many types of cancer. In this review we discuss these two pathways with respect to their signaling mechanisms, functions during T-cell development, and their mutual roles in the development of T-ALL.

Notch targets and their regulation

The proteolytic cleavages elicited by activation of the Notch receptor release an intracellular fragment, Notch intracellular domain, which enters the nucleus to activate the transcription of targets. Changes in transcription are therefore a major output of this pathway. However, the Notch outputs clearly differ from cell type to cell type. In this review we discuss current understanding of Notch targets, the mechanisms involved in their transcriptional regulation, and what might underlie the activation of different sets of targets in different cell types.

Notch signaling in CD4 and CD8 T cell development

Because Notch often acts in concert with other signaling pathways, it is able to regulate a diverse set of biological processes in a cell-context dependent manner. In lymphocytes, Notch is essential for specifying the T cell fate and for promoting early stages of T cell differentiation. At later stages of development, Notch signaling is proposed to direct CD4 versus CD8 T lineage commitment. This hypothesis has been challenged by recent studies of conditional Presenilin-deficient mice showing that Notch promotes the selection and maturation of CD4 and CD8 T cells by potentiating TCR signal transduction in immature thymocytes. While similar conclusions have not been reported with conditional mutation of other downstream mediators of Notch activation, it appears that functional inhibition may not have been achieved at a comparable stage of development and/or analogous issues have not been addressed. The differences also question whether in thymocytes Notch signals only through the canonical pathway. Further study of conditional mutants, signaling intermediates, and transcriptional regulators are needed to elucidate how Notch facilitates TCR signaling in generating mature T cells.

NOTCH1 pathway activation is an early hallmark of SCL T leukemogenesis

The acquired activation of stem cell leukemia (SCL) during T lymphopoiesis is a common event in T-cell acute lymphoblastic leukemia (T-ALL). Here, we generated tamoxifen (TAM)-inducible transgenic mice (lck-ER(T2)-SCL) to study the consequences of acquired SCL activation during T-cell development. Aberrant activation of SCL in thymocytes resulted in the accumulation of immature CD4(+)CD8(+) (double-positive, DP) cells by preventing normal surface expression of the T-cell receptor alphabeta (TCRalphabeta) complex. SCL-induced immature DP cells were further characterized by up-regulated NOTCH1 and generated noncycling polyclonal CD8(+)TCRbeta(low) cells. The prevalence of these cells was SCL dependent because TAM withdrawal resulted in their disappearance. Furthermore, we observed that SCL activation led to a dramatic up-regulation of NOTCH1 target genes (Hes-1, Deltex1, and CD25) in thymocytes. Strikingly, NOTCH1 target gene up-regulation was already observed after short-term SCL induction, implying that enhanced NOTCH signaling is mediated by SCL and is not dependent on secondary genetic events. These data represent the basis for a novel pathway of SCL-induced leukemogenesis and provide a functional link between SCL and NOTCH1 during this process.

TCR and Notch signaling in CD4 and CD8 T-cell development

The generation of CD4 and CD8 alphabeta T-cell lineages from CD4+ CD8+ double-positive (DP) thymocyte precursors is a complex process initiated by engagement of major histocompatibility complex (MHC) by T-cell receptor (TCR) and coreceptor. Quantitative differences in TCR signaling induced by this interaction impose an instructional bias on CD4/CD8 lineage commitment that must be reinforced by MHC recognition and TCR signaling over subsequent selection steps in order for the thymocyte to progress and mature in the adopted lineage. Our studies show that the transmembrane receptor Notch plays a role in this process by modifying TCR signal transduction in DP thymocytes. In this review, we consider the functional relationship of TCR and Notch signaling pathways in the selection and specification of CD4 and CD8 T-cell lineages.

Notch1 signaling influences v2 interneuron and motor neuron development in the spinal cord

The Notch signaling pathway plays a variety of roles in cell fate decisions during development. Previous studies have shown that reduced Notch signaling results in premature differentiation of neural progenitor cells, while increased Notch activities promote apoptotic death of neural progenitor cells in the developing brain. Whether Notch signaling is involved in the specification of neuronal subtypes is unclear. Here we examine the role of Notch1 in the development of neuronal subtypes in the spinal cord using conditional knockout (cKO) mice lacking Notch1 specifically in neural progenitor cells. Notch1 inactivation results in accelerated neuronal differentiation in the ventral spinal cord and gradual disappearance of the ventral central canal. These changes are accompanied by reduced expression of Hes1 and Hes5 and increased expression of Mash1 and Neurogenin 1 and 2. Using markers (Nkx2.2, Nkx6.1, Olig2, Pax6 and Dbx1) for one or multiple progenitor cell types, we found reductions of all subtypes of progenitor cells in the ventral spinal cord of Notch1 cKO mice. Similarly, using markers (Islet1/2, Lim3, Sim1, Chox10, En1 and Evx1/2) specific for motor neurons and distinct classes of interneurons, we found increases in the number of V0-2 interneurons in the ventral spinal cord of Notch1 cKO mice. Specifically, the number of Lim3+/Chox10+ V2 interneurons is markedly increased while the number of Lim3+/Islet+motor neurons is decreased in the Notch1 cKO spinal cord, suggesting that V2 interneurons are generated at the expense of motor neurons in the absence of Notch1. These results provide support for a role of Notch1 in neuronal subtype specification in the ventral spinal cord.

Gammadelta T cell development--having the strength to get there

Gammadelta T cells play critical roles in immune regulation, tumour surveillance and specific primary immune responses. Mature gammadelta cells derive from thymic precursors that also generate alphabeta T cells. Recent reports have highlighted the impact of the strength of signal received via the T cell receptor on T cell lineage commitment, and the importance of cross-talk between committed alphabeta thymocytes and bipotential progenitors for normal gammadelta T cell differentiation. Studies on T cell receptor-mediated selection of gammadelta cells have supported the view that these unconventional T cells are positively rather than negatively selected on cognate self antigen.

Distinct transcriptional programs in thymocytes responding to T cell receptor, Notch, and positive selection signals

T cell antigen receptor (TCR) signaling is necessary but not sufficient to promote the positive selection of CD4+CD8+ thymocytes into CD4+ or CD8+ mature T cells. Notch signaling has also been implicated as a potential regulator of both CD4/CD8 T cell development and TCR signaling. However, the relationship between positive selection, TCR signaling, and Notch remains unclear. Here we use DNA microarray analysis to compare gene expression changes in CD4+CD8+ double-positive thymocytes undergoing positive selection, TCR stimulation, and Notch activation. We find that the genes induced during positive selection can be resolved into two distinct sets. One set, which we term "TCR-induced," is also induced by in vitro TCR stimulation and contains a large proportion of transcription factors. A second set, which we term "positive-selection-induced," is not induced by in vitro TCR simulation and contains a large proportion of genes involved in signal transduction pathways. Genes induced by Notch activity overlap substantially with genes induced during positive selection. We also find that Notch activity potentiates the effects of TCR stimulation on gene expression. These results help to identify TCR- and positive-selection-specific transcriptional events and help to clarify the relationship between positive selection and Notch.

p53 regulates thymic Notch1 activation

Notch is crucial for multiple stages of T cell development, including the CD4+CD8+ double positive (DP)/CD8+ single positive (SP) transition, but regulation of Notchactivation is not well understood. p53 regulates Presenilin1 (PS1) expression, and PS1 cleaves Notch, releasing its intracellular domain (NIC), leading to the expression of downstream targets, e.g. the HES1 gene. We hypothesize that p53 regulates Notch activity during T cell development. We found that Notch1 expression and activation were negatively regulated by p53in several thymoma lines. Additionally, NIC was elevated in Trp53(-/-) thymocytes as compared to Trp53(+/+) thymocytes. To determine if elevated Notch1 activation in Trp53(-/-) thymocytes had an effect on T cell development, CD4 and CD8 expression were analyzed. The CD4+ SP/CD8+ SP T cell ratio was decreased in Trp53(-/-) splenocytes and thymocytes. This alteration in T cell development correlated with the increased Notch1 activation observed in the absence of p53. These data indicate that p53 negatively regulates Notch1 activation during T cell development. Skewing of T cell development toward CD8+SP T cells in Trp53(-/-) mice is reminiscent of the phenotype of NIC-overexpressing mice. Thus, we suggest that p53 plays a role in T cell development, in part by regulating Notch1 activation.

The control of apoptosis in lymphocyte selection

The stochastic nature of rearrangement and diversification of the gene segments encoding immunoglobulins (Igs) and T cell receptors (TCRs) inevitably gives rise to immature B and T lymphocytes that lack antigen receptors or express useless or dangerous (self-antigen-specific) ones. Signaling through antigen receptors promotes survival, proliferative expansion and further differentiation of useful cells and deletion of the useless and dangerous ones. During immune responses, pathogen-specific B and T lymphocytes, as well as cells of the innate immune system, undergo extensive proliferation and develop effector functions, such as antibody secretion, cytotoxicity or cytokine production. To prevent tissue damage by these effector molecules, activated lymphocytes are removed when an infection has been overcome. Together with other mechanisms, including developmental arrest and induction of unresponsiveness (anergy), programmed cell death (apoptosis) of autoreactive lymphocytes safeguards immunological tolerance to self and assists in the development of an effective immune system. We have been investigating the molecular mechanisms that control programmed cell death. This review describes some of our experiments using transgenic and knockout mice, which overexpress or lack apoptosis regulators, that led to discoveries on how life and death decisions are made during development and functioning of the immune 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.

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.

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.

Interactions between MHC molecules and co-receptors of the TCR

Genetic experiments indicate similarity between binding sites on MHC class I (MHCI) for CD8 and on MHCII for CD4, but the crystal structures of CD8/MHCI and CD4/MHCII complexes suggest critical differences between the interfaces in the two complexes. Biophysical analyses using ectodomains of co-receptors and MHC molecules demonstrate extremely fast kinetics and low-affinity interactions. Experiments with soluble multimeric MHC ligands suggest that CD4 and CD8 may differ in the mechanisms by which they promote the formation of ternary TCR/MHC/co-receptor complexes. Co-receptor-influenced duration of TCR signaling controls thymocyte selection. In naïve T cells, CD4/MHCII interactions may promote T-cell survival. Temporal and spatial analysis of TCR and CD4 co-clustering in the immunological synapse suggests that CD4 recruitment is regulated by the half-life of the initial TCR/MHCII complex. Diverse experimental systems have yielded conflicting data that have helped to formulate revised mechanistic models of co-receptor function.

Notch signaling in lymphocyte development

Signaling through Notch has been implicated in many cell-fate decisions during lymphocyte development. Recent studies have provided new clues--and raised new controversies--regarding the exact role that Notch signaling plays in the commitment of cells to the T-cell lineage. Progress has also been made in deducing the transcriptional program induced by Notch and the mechanism of oncogenic transformation by Notch in lymphocytes.

Notch signaling in leukemia

Mammalian Notch homologs were first identified from the involvement of Notch1 in a recurrent chromosomal translocation in a subset of human T-cell leukemias. The effect of the translocation was twofold: Notch expression was placed under the control of a T-cell-specific element, and Notch was truncated, resulting in a constitutively active protein. Subsequent work has shown that Notch1 is required for T cell commitment and is exclusively oncotropic for T cells. During the past year, several murine models have been used to dissect the function of Notch signaling in lymphoid development and leukemia. These models show that Notch1 drives the earliest stages of T cell commitment and that Notch signaling must be downregulated by the double positive stage for proper T cell development to occur. Constitutive Notch signaling mediated by Notch1, Notch2, or Notch3 predisposes to T-cell leukemia. Future studies are expected to elucidate the mechanisms by which Notch leads to transformation. Identification of the transcriptional targets of Notch signaling is likely to yield important insights.

Distinct mechanisms contribute to generate and change the CD4:CD8 cell ratio during thymus development: a role for the Notch ligand, Jagged1

In adult life, the high CD4:CD8 cell ratio observed in peripheral lymphoid organs originates in the thymus. Our results show that the low peripheral CD4:CD8 cell ratio seen during fetal life also has an intrathymic origin. This distinct production of CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes is regulated by the developmental age of the thymic stroma. The differential expression of Notch receptors and their ligands, especially Jagged1, throughout thymus development plays a key role in the generation of the different CD4:CD8 cell ratios. We also show that the intrathymic CD4:CD8 cell ratio sharply changes from fetal to adult values around birth. Differences in the proliferation and emigration rates of the mature thymocyte subsets contribute to this change.

Signal strength in thymic selection and lineage commitment

During development, alphabeta T cells undergo positive or negative selection and CD4(+)/CD8(+) lineage commitment-events that have a major impact on the functionality of the T cell repertoire. The precise mechanisms of these differentiative steps remain elusive. Research this year has focused on quantitative models of signaling. For positive selection, the timing and extent of ERK activation may be important. For lineage commitment, the extent of Lck recruitment and activation may be the decisive factor. Next, the search is on for the genes that commit the cell to the fate determined by these quantitative differences in signals.