GATA3 controls the expression of CD5 and the T cell receptor during CD4 T cell lineage development

Realization of the T Lineage Program Involves GATA-3 Induction of Bcl11b and Repression of Cdkn2b Expression

The zinc-finger transcription factor GATA-3 plays a crucial role during early T cell development and also dictates later T cell differentiation outcomes. However, its role and collaboration with the Notch signaling pathway in the induction of T lineage specification and commitment have not been fully elucidated. We show that GATA-3 deficiency in mouse hematopoietic progenitors results in an early block in T cell development despite the presence of Notch signals, with a failure to upregulate Bcl11b expression, leading to a diversion along a myeloid, but not a B cell, lineage fate. GATA-3 deficiency in the presence of Notch signaling results in the apoptosis of early T lineage cells, as seen with inhibition of CDK4/6 (cyclin-dependent kinases 4 and 6) function, and dysregulated cyclin-dependent kinase inhibitor 2b (Cdkn2b) expression. We also show that GATA-3 induces Bcl11b, and together with Bcl11b represses Cdkn2b expression; however, loss of Cdkn2b failed to rescue the developmental block of GATA-3-deficient T cell progenitor. Our findings provide a signaling and transcriptional network by which the T lineage program in response to Notch signals is realized.

Protein phosphatase 2A has an essential role in promoting thymocyte survival during selection

The development of thymocytes to mature T cells in the thymus is tightly controlled by cellular selection, in which only a small fraction of thymocytes equipped with proper quality of TCRs progress to maturation. It is pivotal to protect the survival of the few T cells, which pass the selection. However, the signaling events, which safeguard the cell survival in thymus, are not totally understood. In this study, protein Ser/Thr phosphorylation in thymocytes undergoing positive selection is profiled by mass spectrometry. The results revealed large numbers of dephosphorylation changes upon T cell receptor (TCR) activation during positive selection. Subsequent substrate analysis pinpointed protein phosphatase 2A (PP2A) as the enzyme responsible for the dephosphorylation changes in developing thymocytes. PP2A catalytic subunit α (Ppp2ca) deletion in the T cell lineage in Ppp2ca ^flox/flox-Lck-Cre mice (PP2A cKO) displayed dysregulated dephosphorylation of apoptosis-related proteins in double-positive (DP) cells and caused substantially decreased numbers of DP CD4⁺ CD8⁺ cells. Increased levels of apoptosis in PP2A cKO DP cells were found to underlie aberrant thymocyte development. Finally, the defective thymocyte development in PP2A cKO mice could be rescued by either Bcl2 transgene expression or by p53 knockout. In summary, our work reveals an essential role of PP2A in promoting thymocyte development through the regulation of cell survival.

TCR signal strength controls thymic differentiation of iNKT cell subsets

During development in the thymus, invariant natural killer T (iNKT) cells commit to one of three major functionally different subsets, iNKT1, iNKT2, and iNKT17. Here, we show that T cell antigen receptor (TCR) signal strength governs the development of iNKT cell subsets, with strong signaling promoting iNKT2 and iNKT17 development. Altering TCR diversity or signaling diminishes iNKT2 and iNKT17 cell subset development in a cell-intrinsic manner. Decreased TCR signaling affects the persistence of Egr2 expression and the upregulation of PLZF. By genome-wide comparison of chromatin accessibility, we identify a subset of iNKT2-specific regulatory elements containing NFAT and Egr binding motifs that is less accessible in iNKT2 cells that develop from reduced TCR signaling. These data suggest that variable TCR signaling modulates regulatory element activity at NFAT and Egr binding sites exerting a determinative influence on the dynamics of gene enhancer accessibility and the developmental fate of iNKT cells.

Invariant natural killer T (iNKT) cells can be subsetted by their cytokine profiles, but how they develop in the thymus is unclear. Here the authors show, by analysing mice carrying mutant Zap70 genes, that T cell receptor signaling strength induces epigenetic changes of genes to modulate iNKT lineages.

Invariant Natural Killer T Cell Subsets-More Than Just Developmental Intermediates

Invariant natural killer T (iNKT) cells are a CD1d-restricted T cell population that can respond to lipid antigenic stimulation within minutes by secreting a wide variety of cytokines. This broad functional scope has placed iNKT cells at the frontlines of many kinds of immune responses. Although the diverse functional capacities of iNKT cells have long been acknowledged, only recently have distinct iNKT cell subsets, each with a marked functional predisposition, been appreciated. Furthermore, the subsets can frequently occupy distinct niches in different tissues and sometimes establish long-term tissue residency where they can impact homeostasis and respond quickly when they sense perturbations. In this review, we discuss the developmental origins of the iNKT cell subsets, their localization patterns, and detail what is known about how different subsets specifically influence their surroundings in conditions of steady and diseased states.

GATA-3 function in innate and adaptive immunity

The zinc-finger transcription factor GATA-3 has received much attention as a master regulator of T helper 2 (Th2) cell differentiation, during which it controls interleukin-4 (IL-4), IL-5, and IL-13 expression. More recently, GATA-3 was shown to contribute to type 2 immunity through regulation of group 2 innate lymphoid cell (ILC2) development and function. Furthermore, during thymopoiesis, GATA-3 represses B cell potential in early T cell precursors, activates TCR signaling in pre-T cells, and promotes the CD4(+) T cell lineage after positive selection. GATA-3 also functions outside the thymus in hematopoietic stem cells, regulatory T cells, CD8(+) T cells, thymic natural killer cells, and ILC precursors. Here we discuss the varied functions of GATA-3 in innate and adaptive immune cells, with emphasis on its activity in T cells and ILCs, and examine the mechanistic basis for the dose-dependent, developmental-stage- and cell-lineage-specific activity of this transcription factor.

Thpok-independent repression of Runx3 by Gata3 during CD4+ T-cell differentiation in the thymus

CD4(+) helper T cells are essential for immune responses and differentiate in the thymus from CD4(+) CD8(+) "double-positive" (DP) thymocytes. The transcription factor Runx3 inhibits CD4(+) T-cell differentiation by repressing Cd4 gene expression; accordingly, Runx3 is not expressed in DP thymocytes or developing CD4(+) T cells. The transcription factor Thpok is upregulated in CD4-differentiating thymocytes and required to repress Runx3. However, how Runx3 is controlled at early stages of CD4(+) T-cell differentiation, before the onset of Thpok expression, remains unknown. Here we show that Gata3, a transcription factor preferentially and transiently upregulated by CD4(+) T-cell precursors, represses Runx3 and binds the Runx3 locus in vivo. Accordingly, we show that high-level Gata3 expression and expression of Runx3 are mutually exclusive. Furthermore, whereas Runx3 represses Cd4, we show that Gata3 promotes Cd4 expression in Thpok-deficient thymocytes. Thus, in addition to its previously documented role in promoting CD4-lineage gene-expression, Gata3 represses CD8-lineage gene expression. These findings identify Gata3 as a critical pivot of CD4-CD8 lineage differentiation.

GATA-3 promotes T-cell specification by repressing B-cell potential in pro-T cells in mice

Transcription factors orchestrate T-lineage differentiation in the thymus. One critical checkpoint involves Notch1 signaling that instructs T-cell commitment at the expense of the B-lineage program. While GATA-3 is required for T-cell specification, its mechanism of action is poorly understood. We show that GATA-3 works in concert with Notch1 to commit thymic progenitors to the T-cell lineage via 2 distinct pathways. First, GATA-3 orchestrates a transcriptional “repertoire” that is required for thymocyte maturation up to and beyond the pro-T-cell stage. Second, GATA-3 critically suppresses a latent B-cell potential in pro–T cells. As such, GATA-3 is essential to sealing in Notch-induced T-cell fate in early thymocyte precursors by promoting T-cell identity through the repression of alternative developmental options.

CD5-dependent CK2 activation pathway regulates threshold for T cell anergy

CD5 activates casein kinase 2 (CK2), a serine/threonine kinase that constitutively associates with the CK2-binding domain at the end of its cytoplasmic tail. To determine the physiological significance of CD5-dependent CK2 activation in T cells, we generated a knock-in mouse that expresses a CD5 protein containing a microdeletion with selective inability to interact with CK2 (CD5ΔCK2BD). The levels of CD5 on developing and mature T cell populations from CD5ΔCK2BD mice and CD5 wild-type (WT) mice were similar. The thymus of CD5ΔCK2BD mice contained fewer double-positive thymocytes than did that of both CD5WT and CD5 knockout (KO) mice, although the numbers of all other immature and mature T cell populations were unaltered. CD5ΔCK2BD T cells hypoproliferated and exhibited enhanced activation-induced cell death when stimulated with anti-CD3 or cognate peptide in comparison with CD5WT T cells. We also found that functional CD5-dependent CK2 signaling was necessary for efficient differentiation of naive CD4+ T cells into Th2 and Th17 cells, but not Th1 cells. We previously showed that experimental autoimmune encephalomyelitis (EAE) in CD5KO mice was less severe and delayed in onset than in CD5WT mice. Remarkably, CD5ΔCK2BD mice recapitulated both EAE severity and disease onset of CD5KO mice. Increasing the immunization dose of myelin oligodendrocyte glycoprotein 35-55 peptide, a model that mimics high-dose tolerance, led to decreased severity of EAE in CD5WT mice but not in CD5KO or CD5ΔCK2BD mice. This property was recapitulated in in vitro restimulation assays. These results demonstrate that CD5-CK2 signaling sets the threshold for T cell responsiveness and is necessary for efficient generation of Th2 and Th17 cells.

Role of Hedgehog signalling at the transition from double-positive to single-positive thymocyte

In the thymus, developing T cells receive signals that determine lineage choice, specificity, MHC restriction and tolerance to self-antigen. One way in which thymocytes receive instruction is by secretion of Sonic hedgehog (Shh) from thymic epithelial cells. We have previously shown that Hedgehog (Hh) signalling in the thymus decreases the CD4:CD8 single-positive (SP) thymocyte ratio. Here, we present data indicating that double-positive (DP) thymocytes are Hh-responsive and that thymocyte-intrinsic Hh signalling plays a role in modulating the production of CD4(+) (SP4), CD8(+) (SP8) and unconventional T-cell subsets. Repression of physiological Hh signalling in thymocytes altered the proportions of DP and SP4 cells. Thymocyte-intrinsic Hh-dependent transcription also attenuated both the production of mature SP4 and SP8 cells, and the establishment of peripheral T-cell compartments in TCR-transgenic mice. Additionally, stimulation or withdrawal of Hh signals in the WT foetal thymus impaired or enhanced upregulation of the CD4 lineage-specific transcription factor Gata3 respectively. These data together suggest that Hh signalling may play a role in influencing the later stages of thymocyte development.

TOX is required for development of the CD4 T cell lineage gene program

The factors that regulate thymic development of the CD4(+) T cell gene program remain poorly defined. The transcriptional regulator ThPOK is a dominant factor in CD4(+) T cell development, which functions primarily to repress the CD8 lineage fate. Previously, we showed that nuclear protein TOX is also required for murine CD4(+) T cell development. In this study, we sought to investigate whether the requirement for TOX was solely due to a role in ThPOK induction. In apparent support of this proposition, ThPOK upregulation and CD8 lineage repression were compromised in the absence of TOX, and enforced ThPOK expression could restore some CD4 development. However, these "rescued" CD4 cells were defective in many aspects of the CD4(+) T cell gene program, including expression of Id2, Foxo1, and endogenous Thpok, among others. Thus, TOX is necessary to establish the CD4(+) T cell lineage gene program, independent of its influence on ThPOK expression.

The immunomodulatory properties of the CD5 lymphocyte receptor in health and disease

CD5 is a scavenger-like receptor expressed in association with the antigen-specific receptors on T and B-1a lymphocytes. Recent studies reveal a broader biology for CD5 that includes its role as regulator of cell death and as a receptor for pathogen-associated molecular patterns, in addition to its previously described function as an inhibitory receptor. These findings shed new light into the mechanistic role of CD5 in leukemias and effector cells to exogenous (infectious) or endogenous (autoimmune, tumoral) antigens. The newly identified properties make this receptor a potential candidate to be targeted for therapeutic intervention as well as immune modulation. This review describes the current knowledge on the function of CD5 as an immunomodulatory receptor both in health and in disease.

The enigma of CD4-lineage specification

CD4(+) T cells are essential for defenses against pathogens and affect the functions of most cells involved in the immune response. Although CD4(+) T cells generally recognize peptide antigens bound to MHC-II molecules, important subsets are restricted by other MHC or MHC-like molecules, including CD1d-restricted "invariant" iNK T cells. This review discusses recently identified nodes in the transcriptional circuits that are involved in controlling CD4(+) T-cell differentiation, notably the commitment factor Thpok and its interplay with Runx transcriptional regulators, and focuses on how transcription factors acting upstream of Thpok, including Gata3, Tox and E-box proteins, promote the emergence of CD4-lineage-specific gene expression patterns.

PU.1 regulates TCR expression by modulating GATA-3 activity

The Ets transcription factor PU.1 is a master regulator for the development of multiple lineages during hematopoiesis. The expression pattern of PU.1 is dynamically regulated during early T lineage development in the thymus. We previously revealed that PU.1 delineates heterogeneity of effector Th2 populations. In this study, we further define the function of PU.1 on the Th2 phenotype using mice that specifically lack PU.1 in T cells using an lck-Cre transgene with a conditional Sfpi1 allele (Sfpi1(lck-/-)). Although deletion of PU.1 by the lck-Cre transgene does not affect T cell development, Sfpi1(lck-/-) T cells have a lower activation threshold than wild-type T cells. When TCR engagement is limiting, Sfpi1(lck-/-) T cells cultured in Th2 polarizing conditions secrete higher levels of Th2 cytokines and have greater cytokine homogeneity than wild-type cells. We show that PU.1 modulates the levels of TCR expression in CD4(+) T cells by regulating the DNA-binding activity of GATA-3 and limiting GATA-3 regulation of TCR gene expression. GATA-3-dependent regulation of TCR expression is also observed in Th1 and Th2 cells. In CD4(+) T cells, PU.1 expression segregates into subpopulations of cells that have lower levels of surface TCR, suggesting that PU.1 contributes to the heterogeneity of TCR expression. Thus, we have identified a mechanism whereby increased GATA-3 function in the absence of the antagonizing activity of PU.1 leads to increased TCR expression, a reduced activation threshold, and increased homogeneity in Th2 populations.

Gene expression profiling in mice with enforced Gata3 expression reveals putative targets of Gata3 in double positive thymocytes

The zinc-finger transcription factors Gata3 and ThPOK have both been implicated in positive selection of double positive (DP) thymocytes towards the CD4 lineage. As in the absence of Gata3, expression of ThPOK is lacking, Gata3 may directly regulate ThPOK expression. As ThPOK failed to promote CD4(+) lineage differentiation of Gata3-deficient cells, ThPOK cannot be the only Gata3 target gene essential for the induction of the CD4(+) lineage program. Therefore, it is conceivable that Gata3 is essential for selected DP T cells to reach the developmental stage at which ThPOK expression is induced. Here, we show that Gata3 overexpression does not affect ThPOK expression levels in DP or CD4(+) thymocytes, providing evidence that Gata3 does not directly regulate ThPOK. To identify additional target genes that clarify Gata3 function at the DP thymocyte stage, we performed gene expression profiling assays in wild-type mice and transgenice mice with enforced expression of Gata3, in the presence or absence of the MHC class II-restricted DO11.10 TCR. We found that Gata3 expression in DP cells undergoing positive selection was associated with downregulation of the V(D)J-recombination machinery genes Rag1, Rag2 and TdT. Moreover, Gata3 overexpression was associated with downregulation of many signaling molecules and the induction of modulators of TCR signaling, including Ctla-4 and thrombospondin 2. Together with our previous finding that Gata3 reduces expression of CD5, a negative regulator of TCR signaling, and upregulates TCR expression, these findings indicate that Gata3 in DP cells mainly functions to (i) terminate TCRalpha gene rearrangement, and (ii) regulate TCR signal intensity or duration in cells undergoing positive selection towards the CD4 lineage.

CD4-CD8 lineage differentiation: Thpok-ing into the nucleus

The mature alphabeta T cell population is divided into two main lineages that are defined by the mutually exclusive expression of CD4 and CD8 surface molecules (coreceptors) and that differ in their MHC restriction and function. CD4 T cells are typically MHC-II restricted and helper (or regulatory), whereas CD8 T cells are typically cytotoxic. Several transcription factors are known to control the emergence of CD4 and CD8 lineages, including the zinc finger proteins Thpok and Gata3, which are required for CD4 lineage differentiation, and the Runx factors Runx1 and Runx3, which contribute to CD8 lineage differentiation. This review summarizes recent advances on the function of these transcription factors in lineage differentiation. We also discuss how the "circuitry" connecting these factors could operate to match the expression of the lineage-committing factors Thpok and Runx3, and therefore lineage differentiation, to MHC specificity.

Ectopically expressed PIR-B on T cells constitutively binds to MHC class I and attenuates T helper type 1 responses

Activated mature T cells induce various inhibitory receptors implicated in maintaining peripheral tolerance in response to the trans-acting ligands. Interestingly, paired Ig-like receptor (PIR)-B, an inhibitory MHC class I receptor on B cells and myeloid cells, could be involved in regulating early T cell development because epitope for PIR is detected on pre-thymic T/NK progenitors but not on thymocytes or mature T cells. We hypothesized that PIR-B is not only a regulator for T cell development but is also detrimental if expressed on mature T cells. Here we demonstrated, using PIR-B-deficient fetuses, that PIR-B is indeed expressed on the T cell progenitors but failed to identify its distinctive roles in the development. Forced expression of PIR-B in thymocytes and mature T cells also resulted in no abnormalities in development. However, upon antigenic or allogeneic stimulation, peripheral T cells with the ectopic PIR-B showed reduced T(h) type 1 responses due to the suppression of proximal TCR signaling by constitutive binding of PIR-B to MHC class I on the same cell surface. Our findings suggest that T cell expression of PIR-B with the cis-interacting MHC class I is strictly prohibited in periphery so as to secure prompt immune responses.

Cyclooxygenase-2 in mucosal DC mediates induction of regulatory T cells in the intestine through suppression of IL-4

Oral intake of protein leads to tolerance through the induction of regulatory T cells (Tr cells) in mesenteric lymph nodes (MLNs). Here we show that the inhibition of cyclooxygenase-2 (COX-2) in vivo suppressed oral tolerance and was associated with enhanced differentiation of interleukin (IL)-4-producing T cells and reduced Foxp3(+) Tr-cell differentiation in MLN. As a result, the functional suppressive capacity of these differentiated mucosal T cells was lost. IL-4 was causally related to loss of tolerance as treatment of mice with anti-IL-4 antibodies during COX-2 inhibition restored tolerance. Dendritic cells (DCs) in the MLN differentially expressed COX-2 and reductionist experiments revealed that selective inhibition of the enzyme in these cells inhibited Foxp3(+) Tr-cell differentiation in vitro. Importantly, the inhibition of COX-2 in MLN-DC caused increased GATA-3 expression and enhanced IL-4 release by T cells, which was directly related to impaired Tr-cell differentiation. These data provide crucial insights into the mechanisms driving de novo Tr-cell induction and tolerance in the intestine.

GATA-3 protects against severe joint inflammation and bone erosion and reduces differentiation of Th17 cells during experimental arthritis

OBJECTIVE

Rheumatoid arthritis is associated with the infiltration of T helper cells into the joints. It is unclear whether interferon-gamma (IFNgamma)-producing Th1 cells or the novel T helper subset, interleukin-17 (IL-17)-producing Th17 cells, are the pathogenic mediators of joint inflammation in chronic nonautoimmune arthritis. Therefore, this study was aimed at examining whether the Th2-specific transcription factor GATA-3 can regulate arthritis, in an experimental murine model, by modulating Th1 and/or Th17 cell polarization.

METHODS

Arthritis was induced with methylated bovine serum albumin (mBSA) in both wild-type and CD2 T cell-specific GATA-3 (CD2-GATA-3)-transgenic mice. At days 1 and 7 after the induction of arthritis, knee joints were scored macroscopically for arthritis severity and for histologic changes. Single-cell suspensions were generated from the spleens, lymph nodes, and inflamed knee joints. Cytokine expression by CD4+ T cells was determined using flow cytometry, and IL-17 expression in the inflamed knee joints was determined by enzyme-linked immunosorbent assay. Analyses of gene expression were performed for Th17-associated factors.

RESULTS

Wild-type mice developed severe joint inflammation, including massive inflammatory cell infiltration and bone erosion that increased significantly over time, reaching maximal arthritis scores at day 7. In contrast, only mild joint inflammation was observed in CD2-GATA-3-transgenic mice. This mild effect was further accompanied by systemic and local reductions in the numbers of IL-17+IFNgamma- and IL-17+IFNgamma+, but not IL-17-IFNgamma+, CD4+ T cells, and by induction of Th2 cytokine expression. Moreover, GATA-3 overexpression resulted in reduced gene expression of the Th17-associated transcription factor retinoic acid-related orphan receptor gammat.

CONCLUSION

These results indicate that enforced GATA-3 expression protects against severe joint inflammation and bone erosion in mice, accompanied by reduced differentiation of Th17 cells, but not Th1 cells, during mBSA-induced arthritis.

Enforced expression of GATA3 allows differentiation of IL-17-producing cells, but constrains Th17-mediated pathology

The zinc-finger transcription factor GATA3 serves as a master regulator of T-helper-2 (Th2) differentiation by inducing expression of the Th2 cytokines IL-4, IL-5 and IL-13 and by suppressing Th1 development. Here, we investigated how GATA3 affects Th17 differentiation, using transgenic mice with enforced GATA3 expression. We activated naïve primary T cells in vitro in the presence of transforming growth factor-beta and IL-6, and found that enforced GATA3 expression induced co-expression of Th2 cytokines in IL-17-producing T cells. Although the presence of IL-4 hampered Th17 differentiation, transforming growth factor-beta/IL-6 cultures from GATA3 transgenic mice contained substantial numbers of IL-17(+) cells, partially because GATA3 supported Th17 differentiation by limiting IL-2 and IFN-gamma production. GATA3 additionally constrained Th17 differentiation in vitro through IL-4-independent mechanisms, involving downregulating transcription of STAT3, STAT4, NFATc2 and the nuclear factor RORgammat, which is crucial for Th17 differentiation. Remarkably, upon myelin oligodendrocyte glycoprotein immunization in vivo, GATA3 transgenic mice contained similar numbers of IL-17-producing T cells in their lymph nodes as wild-type mice, but were not susceptible to autoimmune encephalomyelitis, possibly due to concomitant production of IL-4 and IL-10 induction. We therefore conclude that although GATA3 allows Th17 differentiation, it acts as an inhibitor of Th17-mediated pathology, through IL-4-dependent and IL-4-independent pathways.

Distinct functions for the transcription factors GATA-3 and ThPOK during intrathymic differentiation of CD4(+) T cells

The transcription factors GATA-3 and ThPOK are required for intrathymic differentiation of CD4(+) T cells, but their precise functions in this process remain unclear. Here we show that, contrary to previous findings, Gata3 disruption blocked differentiation into the CD4(+) T cell lineage before commitment to the CD4(+) lineage and in some contexts permitted the 'redirection' of major histocompatibility complex class II-restricted thymocytes into the CD8(+) lineage. GATA-3 promoted ThPOK expression and bound to a region of the locus encoding ThPOK established as being critical for ThPOK expression. Finally, ThPOK promoted differentiation into the CD4(+) lineage in a way dependent on GATA-3 but inhibited differentiation into the CD8(+) lineage independently of GATA-3. We propose that GATA-3 acts as a specification factor for the CD4(+) lineage 'upstream' of the ThPOK-controlled CD4(+) commitment checkpoint.

Cooperation of Gata3, c-Myc and Notch in malignant transformation of double positive thymocytes

Gata transcription factors are critical regulators of proliferation and differentiation implicated in various human cancers, but specific genes activated by Gata proteins remain to be identified. We previously reported that enforced expression of Gata3 during T cell development in CD2-Gata3 transgenic mice induced CD4(+)CD8(+) double-positive (DP) T cell lymphoma. Here, we show that the presence of the DO11.10 T-cell receptor transgene, which directs DP cells towards the CD4 lineage, resulted in enhanced lymphoma development and a dramatic increase in thymocyte cell size in CD2-Gata3 transgenic mice. CD2-Gata3 DP cells expressed high levels of the proto-oncogene c-Myc but the Notch1 signaling pathway, which is known to induce c-Myc, was not activated. Gene expression profiling showed that in CD2-Gata3 lymphoma cells transcription of c-Myc and its target genes was further increased. A substantial fraction of CD2-Gata3 lymphomas had trisomy of chromosome 15, leading to an increased c-Myc gene dose. Interestingly, most lymphomas showed high expression of the Notch targets Deltex1 and Hes1, often due to activating Notch1 PEST domain mutations. Therefore, we conclude that enforced Gata3 expression converts DP thymocytes into a pre-malignant state, characterized by high c-Myc expression, whereby subsequent induction of Notch1 signaling cooperates to establish malignant transformation. The finding that Gata3 regulates c-Myc expression levels, in a direct or indirect fashion, may explain the parallel phenotypes of mice with overexpression or deficiency of either of the two transcription factors.