Cooperating pre-T-cell receptor and TCF-1-dependent signals ensure thymocyte survival

Multi-modular structure of the gene regulatory network for specification and commitment of murine T cells

T cells develop from multipotent progenitors by a gradual process dependent on intrathymic Notch signaling and coupled with extensive proliferation. The stages leading them to T-cell lineage commitment are well characterized by single-cell and bulk RNA analyses of sorted populations and by direct measurements of precursor-product relationships. This process depends not only on Notch signaling but also on multiple transcription factors, some associated with stemness and multipotency, some with alternative lineages, and others associated with T-cell fate. These factors interact in opposing or semi-independent T cell gene regulatory network (GRN) subcircuits that are increasingly well defined. A newly comprehensive picture of this network has emerged. Importantly, because key factors in the GRN can bind to markedly different genomic sites at one stage than they do at other stages, the genes they significantly regulate are also stage-specific. Global transcriptome analyses of perturbations have revealed an underlying modular structure to the T-cell commitment GRN, separating decisions to lose "stem-ness" from decisions to block alternative fates. Finally, the updated network sheds light on the intimate relationship between the T-cell program, which depends on the thymus, and the innate lymphoid cell (ILC) program, which does not.

Stepwise progression of β-selection during T cell development involves histone deacetylation

During T cell development, the first step in creating a unique T cell receptor (TCR) is genetic recombination of the TCRβ chain. The quality of the new TCRβ is assessed at the β-selection checkpoint. Most cells fail this checkpoint and die, but the coordination of fate at the β-selection checkpoint is not yet understood. We shed new light on fate determination during β-selection using a selective inhibitor of histone deacetylase 6, ACY1215. ACY1215 disrupted the β-selection checkpoint. Characterising the basis for this disruption revealed a new, pivotal stage in β-selection, bookended by up-regulation of TCR co-receptors, CD28 and CD2, respectively. Within this "DN3b^Pre" stage, CD5 and Lef1 are up-regulated to reflect pre-TCR signalling, and their expression correlates with proliferation. These findings suggest a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD28, CD5 and Lef1 allows for modulating TCR signalling strength and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.

Exploring the stage-specific roles of Tcf-1 in T cell development and malignancy at single-cell resolution

Tcf-1 (encoded by Tcf7) not only plays critical roles in promoting T cell development and differentiation but also has been identified as a tumor suppressor involved in preventing T cell malignancy. However, the comprehensive mechanisms of Tcf-1 involved in T cell transformation remain poorly understood. In this study, Tcf7fl/fl mice were crossed with Vav-cre, Lck-cre, or Cd4-cre mice to delete Tcf-1 conditionally at the beginning of the HSC, DN2-DN3, or DP stage, respectively. The defective T cell development phenotypes became gradually less severe as the deletion stage became more advanced in distinct mouse models. Interestingly, consistent with Tcf7-/- mice, Tcf7fl/flVav-cre mice developed aggressive T cell lymphoma within 45 weeks, but no tumors were generated in Tcf7fl/flLck-cre or Tcf7fl/flCd4-cre mice. Single-cell RNA-seq (ScRNA-seq) indicated that ablation of Tcf-1 at distinct phases can subdivide DN1 cells into three clusters (C1, C2, and C3) and DN2-DN3 cells into three clusters (C4, C5, and C6). Moreover, Tcf-1 deficiency redirects bifurcation among divergent cell fates, and clusters C1 and C4 exhibit high potential for leukemic transformation. Mechanistically, we found that Tcf-1 directly binds and mediates chromatin accessibility for both typical T cell regulators and proto-oncogenes, including Myb, Mycn, Runx1, and Lyl1 in the DN1 phase and Lef1, Id2, Dtx1, Fyn, Bcl11b, and Zfp36l2 in the DN2-DN3 phase. The aberrant expression of these genes due to Tcf-1 deficiency in very early T cells contributes to subsequent tumorigenesis. Thus, we demonstrated that Tcf-1 plays stage-specific roles in regulating early thymocyte development and transformation, providing new insights and evidence for clinical trials on T-ALL leukemia.

The E protein-TCF1 axis controls γδ T cell development and effector fate

TCF1 plays a critical role in T lineage commitment and the development of αβ lineage T cells, but its role in γδ T cell development remains poorly understood. Here, we reveal a regulatory axis where T cell receptor (TCR) signaling controls TCF1 expression through an E-protein-bound regulatory element in the Tcf7 locus, and this axis regulates both γδ T lineage commitment and effector fate. Indeed, the level of TCF1 expression plays an important role in setting the threshold for γδ T lineage commitment and modulates the ability of TCR signaling to influence effector fate adoption by γδ T lineage progenitors. This finding provides mechanistic insight into how TCR-mediated repression of E proteins promotes the development of γδ T cells and their adoption of the interleukin (IL)-17-producing effector fate. IL-17-producing γδ T cells have been implicated in cancer progression and in the pathogenesis of psoriasis and multiple sclerosis.

The transcriptional repressor Bcl6 promotes pre-TCR-induced thymocyte differentiation and attenuates Notch1 activation

Pre-T-cell receptor (TCR) signal transduction is required for developing thymocytes to differentiate from CD4^-CD8^- double-negative (DN) cell to CD4⁺CD8⁺ double-positive (DP) cell. Notch signalling is required for T-cell fate specification and must be maintained throughout β-selection, but inappropriate Notch activation in DN4 and DP cells is oncogenic. Here, we show that pre-TCR signalling leads to increased expression of the transcriptional repressor Bcl6 and that Bcl6 is required for differentiation to DP. Conditional deletion of Bcl6 from thymocytes reduced pre-TCR-induced differentiation to DP cells, disrupted expansion and enrichment of intracellular TCRβ⁺ cells within the DN population and increased DN4 cell death. Deletion also increased Notch1 activation and Notch-mediated transcription in the DP population. Thus, Bcl6 is required in thymocyte development for efficient differentiation from DN3 to DP and to attenuate Notch1 activation in DP cells. Given the importance of inappropriate NOTCH1 signalling in T-cell acute lymphoblastic leukaemia (T-ALL), and the involvement of BCL6 in other types of leukaemia, this study is important to our understanding of T-ALL.

An integrated transcriptional switch at the β-selection checkpoint determines T cell survival, development and leukaemogenesis

In T cell development, a pivotal decision-making stage, termed β-selection, integrates a TCRβ checkpoint to coordinate survival, proliferation and differentiation to an αβ T cell. Here, we review how transcriptional regulation coordinates fate determination in early T cell development to enable β-selection. Errors in this transcription control can trigger T cell acute lymphoblastic leukaemia. We describe how the β-selection checkpoint goes awry in leukaemic transformation.

Alteration of Bcl11b upon stimulation of both the MAP kinase- and Gsk3-dependent signaling pathways in double-negative thymocytes

B-cell lymphoma/leukemia 11B (Bcl11b) is a transcription factor critical for thymocyte development. We have previously characterized the kinetic post-translational modifications (PTMs) of Bcl11b in double-positive (DP) thymocytes during stimulation of the T cell receptor-activated MAP kinase pathway. However, the PTMs of Bcl11b in thymocytes from other developmental stages in the thymus, primarily double-negative (DN) cells, have not been previously identified. We found that kinetic modifications of Bcl11b in DN cells are somewhat different than the patterns observed in DP cells. Distinct from DP thymocytes, phosphorylation and sumoylation of Bcl11b in DN cells were not oppositely regulated in response to activation of MAP kinase, even though hyper-phosphorylation of Bcl11b coincided with near complete desumoylation. Additionally, prolonged stimulation of the MAP kinase pathway in DN cells, unlike DP thymocytes, did not alter Bcl11b levels of sumoylation or ubiquitinylation, or stability. On the other hand, activation of Wnt-Gsk3-dependent signaling in DN cells resulted in composite dephosphorylation and sumoylation of Bcl11b. Moreover, stimulation of MAP kinase and (or) Wnt signaling pathways differentially affects gene expression of some Bcl11b target and maturation-associated genes. Defining the signaling pathways and regulation of sequence-specific transcription factors by PTMs at various stages of thymopoiesis may improve our understanding of leukemogenesis.

Nuclear Wiskott-Aldrich syndrome protein co-regulates T cell factor 1-mediated transcription in T cells

BACKGROUND

The Wiskott-Aldrich syndrome protein (WASp) family of actin-nucleating factors are present in the cytoplasm and in the nucleus. The role of nuclear WASp for T cell development remains incompletely defined.

METHODS

We performed WASp chromatin immunoprecipitation and deep sequencing (ChIP-seq) in thymocytes and spleen CD4⁺ T cells.

RESULTS

WASp was enriched at genic and intergenic regions and associated with the transcription start sites of protein-coding genes. Thymocytes and spleen CD4⁺ T cells showed 15 common WASp-interacting genes, including the gene encoding T cell factor (TCF)12. WASp KO thymocytes had reduced nuclear TCF12 whereas thymocytes expressing constitutively active WASp^L272P and WASp^I296T had increased nuclear TCF12, suggesting that regulated WASp activity controlled nuclear TCF12. We identify a putative DNA element enriched in WASp ChIP-seq samples identical to a TCF1-binding site and we show that WASp directly interacted with TCF1 in the nucleus.

CONCLUSIONS

These data place nuclear WASp in proximity with TCF1 and TCF12, essential factors for T cell development.

TCF-1 upregulation identifies early innate lymphoid progenitors in the bone marrow

The cellular and molecular events that drive the early development of innate lymphoid cells (ILCs) remain poorly understood. We show that the transcription factor TCF-1 is required for the efficient generation of all known adult ILC subsets and their precursors. Using novel reporter mice, we identified a new subset of early ILC progenitors (EILPs) expressing high amounts of TCF-1. EILPs lacked efficient T and B lymphocyte potential but efficiently gave rise to NK cells and all known adult helper ILC lineages, indicating that they are the earliest ILC-committed progenitors identified so far. Our results suggest that upregulation of TCF-1 expression denotes the earliest stage of ILC fate specification. The discovery of EILPs provides a basis for deciphering additional signals that specify ILC fate.

Transcription factors and target genes of pre-TCR signaling

Almost 30 years ago pioneering work by the laboratories of Harald von Boehmer and Susumo Tonegawa provided the first indications that developing thymocytes could assemble a functional TCRβ chain-containing receptor complex, the pre-TCR, before TCRα expression. The discovery and study of the pre-TCR complex revealed paradigms of signaling pathways in control of cell survival and proliferation, and culminated in the recognition of the multifunctional nature of this receptor. As a receptor integrated in a dynamic developmental process, the pre-TCR must be viewed not only in the light of the biological outcomes it promotes, but also in context with those molecular processes that drive its expression in thymocytes. This review article focuses on transcription factors and target genes activated by the pre-TCR to drive its different outcomes.

Transcriptional regulation of innate and adaptive lymphocyte lineages

The lymphocyte family has expanded significantly in recent years to include not only the adaptive lymphocytes (T cells, B cells) and NK cells, but also several additional innate lymphoid cell (ILC) types. ILCs lack clonally distributed antigen receptors characteristic of adaptive lymphocytes and instead respond exclusively to signaling via germline-encoded receptors. ILCs resemble T cells more closely than any other leukocyte lineage at the transcriptome level and express many elements of the core T cell transcriptional program, including Notch, Gata3, Tcf7, and Bcl11b. We present our current understanding of the shared and distinct transcriptional regulatory mechanisms involved in the development of adaptive T lymphocytes and closely related ILCs. We discuss the possibility that a core set of transcriptional regulators common to ILCs and T cells establish enhancers that enable implementation of closely aligned effector pathways. Studies of the transcriptional regulation of lymphopoiesis will support the development of novel therapeutic approaches to correct early lymphoid developmental defects and aberrant lymphocyte function.

Loss of IP3R-dependent Ca2+ signalling in thymocytes leads to aberrant development and acute lymphoblastic leukemia

Calcium ions (Ca(2+)) function as universal second messengers in eukaryotic cells, including immune cells. Ca(2+) is crucial for peripheral T-lymphocyte activation and effector functions, and influences thymocyte selection and motility in the developing thymus. However, the role of Ca(2+) signalling in early T-lymphocyte development is not well understood. Here we show that the inositol triphosphate receptors (IP3Rs) Ca(2+) ion channels are required for proliferation, survival and developmental progression of T-lymphocyte precursors. Our studies indicate that signalling via IP3Rs represses Sox13, an antagonist of the developmentally important transcription factor Tcf-1. In the absence of IP3R-mediated Ca(2+) signalling, repression of key Notch transcriptional targets--including Hes1--fail to occur in post β-selection thymocytes, and mice develop aggressive T-cell malignancies that resemble human T-cell acute lymphoblastic leukemia (T-ALL). These data indicate that IP3R-mediated Ca(2+) signalling reinforces Tcf-1 activity to both ensure normal development and prevent thymocyte neoplasia.

TCF-1 and LEF-1 act upstream of Th-POK to promote the CD4(+) T cell fate and interact with Runx3 to silence Cd4 in CD8(+) T cells

The transcription factors TCF-1 and LEF-1 are essential for early T cell development, but their roles beyond the CD4(+)CD8(+) double-positive (DP) stage are unknown. By specific ablation of these factors in DP thymocytes, we demonstrated that deficiency in TCF-1 and LEF-1 diminished the output of CD4(+) T cells and redirected CD4(+) T cells to a CD8(+) T cell fate. The role of TCF-1 and LEF-1 in the CD4-versus-CD8 lineage 'choice' was mediated in part by direct positive regulation of the transcription factor Th-POK. Furthermore, loss of TCF-1 and LEF-1 unexpectedly caused derepression of CD4 expression in T cells committed to the CD8(+) lineage without affecting the expression of Runx transcription factors. Instead, TCF-1 physically interacted with Runx3 to cooperatively silence Cd4. Thus, TCF-1 and LEF-1 adopted distinct genetic 'wiring' to promote the CD4(+) T cell fate and establish CD8(+) T cell identity.

T cell factor 1 is required for group 2 innate lymphoid cell generation

Group 2 innate lymphoid cells (ILC2) are innate lymphocytes that confer protective type 2 immunity during helminth infection and are also involved in allergic airway inflammation. Here we report that ILC2 development required T cell factor 1 (TCF-1, the product of the Tcf7 gene), a transcription factor also implicated in T cell lineage specification. Tcf7(-/-) mice lack ILC2, and were unable to mount ILC2-mediated innate type 2 immune responses. Forced expression of TCF-1 in bone marrow progenitors partially bypassed the requirement for Notch signaling in the generation of ILC2 in vivo. TCF-1 acted through both GATA-3-dependent and GATA-3-independent pathways to promote the generation of ILC2. These results are reminiscent of the critical roles of TCF-1 in early T cell development. Hence, transcription factors that underlie early steps of T cell development are also implicated in the development of innate lymphoid cells.

The TCF-1 and LEF-1 transcription factors have cooperative and opposing roles in T cell development and malignancy

The TCF-1 and LEF-1 transcription factors are known to play critical roles in normal thymocyte development. Unexpectedly, we found that TCF-1-deficient (Tcf7(-/-)) mice developed aggressive T cell malignancy, resembling human T cell acute lymphoblastic leukemia (T-ALL). LEF-1 was aberrantly upregulated in premalignant Tcf7(-/-) early thymocytes and lymphoma cells. We further demonstrated that TCF-1 directly repressed LEF-1 expression in early thymocytes and that conditional inactivation of Lef1 greatly delayed or prevented T cell malignancy in Tcf7(-/-) mice. In human T-ALLs, an early thymic progenitor (ETP) subtype was associated with diminished TCF7 expression, and two of the ETP-ALL cases harbored TCF7 gene deletions. We also showed that TCF-1 and LEF-1 were dispensable for T cell lineage commitment but instead were required for early thymocytes to mature beyond the CD4(-)CD8(-) stage. TCF-1 thus has dual roles, i.e., acting cooperatively with LEF-1 to promote thymocyte maturation while restraining LEF-1 expression to prevent malignant transformation of developing thymocytes.

β-catenin/TCF-1 pathway in T cell development and differentiation

T cells must undergo two critical differentiation processes before they become competent effectors that can mediate actual immune responses. Progenitor T cells undergo defined stages of differentiation in the thymus, which include positive and negative selection, to generate a repertoire of T cells that will respond to foreign but not self antigens. When these immunocompetent T cells first migrate out of thymus into peripheral lymphoid tissues, they are naïve and are unable to mediate immune responses. However, upon antigen encounter, peripheral CD4+ naïve T cells undergo another differentiation process to become armed effector T cells including Th1, Th2, Th17 or regulatory T cells, all of which are capable of regulating immune responses. A canonical Wnt/β-catenin/T cell factor (TCF) pathway has been shown to regulate T cell differentiation in both the thymus and in peripheral lymphoid tissues. Dysfunction of this pathway at any stage of T cell differentiation could lead to severe autoimmunity including experimental autoimmune encephalomyelitis or immune deficiency. Understanding the role played by β-catenin/TCF-1 in T cell differentiation will facilitate our understanding of the mechanisms that regulate T cell function and assist in identifying novel therapy targets for treating both autoimmune and immune diseases. Therefore, in this review, we will focus on the function of β-catenin/TCF-1 pathway in the regulation of thymic and peripheral T cell differentiation processes.

Transcription factor network regulating CD(+)CD8(+) thymocyte survival

More than 80% of thymocytes are CD4(+)CD8(+) double positive (DP) cells subject to positive/ negative selection. The lifespan of DP thymocytes is critical in shaping the peripheral T-cell repertoire essential for mounting immune responses against foreign, but not self, antigens. During T-cell maturation, if the first round of T-cell receptor (TCR) α chain rearrangement fails to generate a productive T-cell receptor, DP cells start another round of α chain rearrangement until positive selection or cell death intervenes. Thus, the lifespan of DP cells determines how many rounds of α chain rearrangement can be carried out, and influences the likelihood of completing positive selection. The antiapoptotic protein Bcl-x(L) is the ultimate effector regulating DP cell survival, and several transcription factors critical for T-cell development, such as TCF-1, E proteins, c-Myb, and RORγt, regulate DP survival via a Bcl-x(L)-dependent pathway. However, the relationship between these transcription factors in this process is largely unclear. Recent results are revealing an interactive network among these critical factors during regulation of DP thymocyte survival. This review will discuss how these transcription factors potentially work together to control DP thymocyte survival that is critical for successful completion of T-cell development.

HEB in the spotlight: Transcriptional regulation of T-cell specification, commitment, and developmental plasticity

The development of T cells from multipotent progenitors in the thymus occurs by cascades of interactions between signaling molecules and transcription factors, resulting in the loss of alternative lineage potential and the acquisition of the T-cell functional identity. These processes require Notch signaling and the activity of GATA3, TCF1, Bcl11b, and the E-proteins HEB and E2A. We have shown that HEB factors are required to inhibit the thymic NK cell fate and that HEBAlt allows the passage of T-cell precursors from the DN to DP stage but is insufficient for suppression of the NK cell lineage choice. HEB factors are also required to enforce the death of cells that have not rearranged their TCR genes. The synergistic interactions between Notch1, HEBAlt, HEBCan, GATA3, and TCF1 are presented in a gene network model, and the influence of thymic stromal architecture on lineage choice in the thymus is discussed.

Regulation of mature T cell responses by the Wnt signaling pathway

The canonical Wnt signaling pathway is evolutionarily conserved and plays key roles during development of many organ systems. This pathway utilizes TCF/LEF transcription factors, β-catenin coactivator, and TLE/GRG corepressors to achieve balanced regulation of its downstream gene expression. It is well established that several Wnt ligands and their effector proteins are crucial for normal T cell development. Recent studies have also revealed critical requirements for TCF-1 in generation and persistence of functional memory CD8(+) T cells, and in promoting Th2-differentiation and suppressing Th17-differentiation of activated CD4(+) T cells. Activation of β-catenin facilitated CD8(+) memory T cell formation, with enhanced protective capacity and extended survival of CD4(+) CD25(+) regulatory T cells. Upregulation of Wnt ligands was observed in Drosophila in response to Toll signaling as well as in mammalian dendritic cells and macrophages upon microbial stimulation. These new findings suggest that modulating the activity of Wnt pathway may be a powerful approach to enhance protective immunity and treat autoimmune diseases.

T-cell factor 1 is a gatekeeper for T-cell specification in response to Notch signaling

Although transcriptional programs associated with T-cell specification and commitment have been described, the functional hierarchy and the roles of key regulators in structuring/orchestrating these programs remain unclear. Activation of Notch signaling in uncommitted precursors by the thymic stroma initiates the T-cell differentiation program. One regulator first induced in these precursors is the DNA-binding protein T-cell factor 1 (Tcf-1), a T-cell-specific mediator of Wnt signaling. However, the specific contribution of Tcf-1 to early T-cell development and the signals inducing it in these cells remain unclear. Here we assign functional significance to Tcf-1 as a gatekeeper of T-cell fate and show that Tcf-1 is directly activated by Notch signals. Tcf-1 is required at the earliest phase of T-cell determination for progression beyond the early thymic progenitor stage. The global expression profile of Tcf-1-deficient progenitors indicates that basic processes of DNA metabolism are down-regulated in its absence, and the blocked T-cell progenitors become abortive and die by apoptosis. Our data thus add an important functional relationship to the roadmap of T-cell development.

A critical role for TCF-1 in T-lineage specification and differentiation

The vertebrate thymus provides an inductive environment for T-cell development. Within the mouse thymus, Notch signals are indispensable for imposing the T-cell fate on multipotential haematopoietic progenitors, but the downstream effectors that impart T-lineage specification and commitment are not well understood. Here we show that a transcription factor, T-cell factor 1 (TCF-1; also known as transcription factor 7, T-cell specific, TCF7), is a critical regulator in T-cell specification. TCF-1 is highly expressed in the earliest thymic progenitors, and its expression is upregulated by Notch signals. Most importantly, when TCF-1 is forcibly expressed in bone marrow (BM) progenitors, it drives the development of T-lineage cells in the absence of T-inductive Notch1 signals. Further characterization of these TCF-1-induced cells revealed expression of many T-lineage genes, including T-cell-specific transcription factors Gata3 and Bcl11b, and components of the T-cell receptor. Our data suggest a model where Notch signals induce TCF-1, and TCF-1 in turn imprints the T-cell fate by upregulating expression of T-cell essential genes.

Multilayered specification of the T-cell lineage fate

T-cell development from stem cells has provided a highly accessible and detailed view of the regulatory processes that can go into the choice of a cell fate in a postembryonic, stem cell-based system. But it has been a view from the outside. The problems in understanding the regulatory basis for this lineage choice begin with the fact that too many transcription factors are needed to provide crucial input: without any one of them, T-cell development fails. Furthermore, almost all the factors known to provide crucial functions during the climax of T-lineage commitment itself are also vital for earlier functions that establish the pool of multilineage precursors that would normally feed into the T-cell specification process. When the regulatory genes that encode them are mutated, the confounding effects on earlier stages make it difficult to dissect T-cell specification genetically. Yet both the positive and the negative regulatory events involved in the choice of a T-cell fate are actually a mosaic of distinct functions. New evidence has emerged recently that finally provides a way to separate the major components that fit together to drive this process. Here, we review insights into T-cell specification and commitment that emerge from a combination of molecular, cellular, and systems biology approaches. The results reveal the regulatory structure underlying this lineage decision.

T-cell lineage determination

T cells originate from hematopoietic stem cells (HSCs) in the bone marrow but complete their development in the thymus. HSCs give rise to a variety of non-renewing hematopoietic progenitors, among which a rare subset migrates to the thymus via the bloodstream. The earliest T-cell progenitors identified in the thymus are not T-lineage restricted but possess the ability to give rise to cells of many different lineages. Alternative lineage potentials are gradually lost as progenitors progress toward later developmental stages. Here, we review the early developmental events that might be involved in T-cell lineage fate determination, including the properties of possible thymus-settling progenitors, their homing into the thymus, and their T-cell lineage specification and commitment.

Developmental progression of fetal HEB(-/-) precursors to the pre-T-cell stage is restored by HEBAlt

Gene knockout studies have shown that the E-protein transcription factor HEB is required for normal thymocyte development. We have identified a unique form of HEB, called HEBAlt, which is expressed only during the early stages of T-cell development, whereas HEBCan is expressed throughout T-cell development. Here, we show that HEB(-/-) precursors are inhibited at the β-selection checkpoint of T-cell development due to impaired expression of pTα and function of CD3ε, both of which are necessary for pre-TCR signaling. Transgenic expression of HEBAlt in HEB(-/-) precursors, however, upregulated pTα and allowed development to CD4(+) CD8(+) stage in fetal thymocytes. Moreover, HEBAlt did overcome the CD3ε signaling defect in HEB(-/-) Rag-1(-/-) thymocytes. The HEBAlt transgene did not permit Rag-1(-/-) precursors to bypass β-selection, indicating that it was not acting as a dominant negative inhibitor of other E-proteins. Therefore, our results provide the first mechanistic evidence that HEBAlt plays a critical role in early T-cell development and show that it can collaborate with fetal thymic stromal elements to create a regulatory environment that supports T-cell development past the β-selection checkpoint.

Cell-type-specific activation and repression of PU.1 by a complex of discrete, functionally specialized cis-regulatory elements

The transcription factor PU.1 is critical for multiple hematopoietic lineages, but different leukocyte types require strictly distinct patterns of PU.1 regulation. PU.1 is required early for T-cell lineage development but then must be repressed by a stage-specific mechanism correlated with commitment. Other lineages require steady, low expression or upregulation. Until now, only the promoter plus a distal upstream regulatory element (URE) could be invoked to explain nearly all Sfpi1 (PU.1) activation and repression, including bifunctional effects of Runx1. However, the URE is dispensable for most Sfpi1 downregulation in early T cells, and we show that it retains enhancer activity in immature T-lineage cells even where endogenous Sfpi1 is repressed. We now present evidence for another complex of conserved noncoding elements that mediate discrete, cell-type-specific regulatory features of Sfpi1, including a myeloid cell-specific activating element and a separate, pro-T-cell-specific silencer element. These elements yield opposite, cell-type-specific responses to Runx1. T-cell-specific repression requires Runx1 acting through multiple nonconsensus sites in the silencer core. These newly characterized sites recruit Runx1 binding in early T cells in vivo and define a functionally specific scaffold for dose-dependent, Runx-mediated repression.

c-Myb promotes the survival of CD4+CD8+ double-positive thymocytes through upregulation of Bcl-xL

Mechanisms that regulate the lifespan of CD4(+)CD8(+) double-positive (DP) thymocytes help shape the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The Myb proto-oncogene encodes a transcription factor required during multiple stages of T cell development. We demonstrate that Myb mRNA expression is upregulated as thymocytes differentiate from the double-negative into the metabolically quiescent, small, preselection DP stage during T cell development. Using a conditional deletion mouse model, we demonstrate that Myb-deficient DP thymocytes undergo premature apoptosis, resulting in a limited Tcralpha repertoire biased toward 5' Jalpha segment usage. Premature apoptosis occurs specifically in the small preselection DP compartment in an alphabetaTCR-independent manner and is a consequence of decreased Bcl-xL expression. Forced Bcl-xL expression is able to rescue survival, and reintroduction of c-Myb restores both Bcl-xL expression and the small preselection DP compartment. We further demonstrate that c-Myb promotes transcription at the Bcl2l1 locus via a genetic pathway that is independent of the expression of T cell-specific factor-1 or RORgammat, two transcription factors that induce Bcl-xL expression in T cell development. Thus, Bcl-xL is a novel mediator of c-Myb activity during normal T cell development.

The serine/threonine kinase LKB1 controls thymocyte survival through regulation of AMPK activation and Bcl-XL expression

LKB1 is a serine/threonine kinase that directly activates the energy sensor AMP-activated protein kinase (AMPK) in response to bioenergetic stress, and mainly acts as a tumor suppressor that controls cell polarity and proliferation. Although LKB1 is expressed in multiple tissues including the thymus and the spleen, its roles in T-cell development and function remain unknown. Here, we show that T-cell-specific deletion of LKB1 resulted in reduced survival of double-positive (DP) thymocytes and impaired generation of both CD4 and CD8 single-positive thymocytes. Disruption of LKB1 not only prevented the activation of AMPK but also impaired the expression of anti-apoptotic protein Bcl-XL. Importantly, ectopic expression of either Bcl-XL or the constitutively active AMPK mutant significantly rescued DP thymocytes from LKB1 deficiency-induced cell death. Moreover, ectopic expression of the constitutively active AMPK mutant was found to restore the expression of Bcl-XL in LKB1-deficient DP thymocytes. These findings identify LKB1 as a critical factor for the survival of DP thymocytes through regulation of AMPK activation and Bcl-XL expression.

Beta-catenin/Tcf determines the outcome of thymic selection in response to alphabetaTCR signaling

Thymic maturation of T cells depends on the intracellular interpretation of alphabetaTCR signals by processes that are poorly understood. In this study, we report that beta-catenin/Tcf signaling was activated in double-positive thymocytes in response to alphabetaTCR engagement and impacted thymocyte selection. TCR engagement combined with activation of beta-catenin signaled thymocyte deletion, whereas Tcf-1 deficiency rescued from negative selection. Survival/apoptotis mediators including Bim, Bcl-2, and Bcl-x(L) were alternatively influenced by stabilization of beta-catenin or ablation of Tcf-1, and Bim-mediated beta-catenin induced thymocyte deletion. TCR activation in double-positive cells with stabilized beta-catenin triggered signaling associated with negative selection, including sustained overactivation of Lat and Jnk and a transient activation of Erk. These observations are consistent with beta-catenin/Tcf signaling acting as a switch that determines the outcome of thymic selection downstream the alphabetaTCR cascade.

Sonic hedgehog negatively regulates pre-TCR-induced differentiation by a Gli2-dependent mechanism

Hedgehog signaling regulates differentiation, survival, and proliferation of the earliest double-negative (DN) thymocytes, but its importance at later stages of T-cell development is controversial. Here we use loss- and gain-of-function mouse models to show that Shh, by signaling directly to the developing thymocyte, is a negative regulator of pre-TCR-induced differentiation from DN to double-positive (DP) cell. When hedgehog signaling was reduced, in the Shh(-/-) and Gli2(-/-) thymus, or by T lineage-specific transgenic expression of a transcriptional-repressor form of Gli2 (Gli2DeltaC(2)), differentiation to DP cell after pre-TCR signal transduction was increased. In contrast, when Hh signaling was constitutively activated in thymocytes, by transgenic expression of a constitutive transcriptional-activator form of Gli2 (Gli2DeltaN(2)), the production of DP cells was decreased. Gene expression profiling showed that physiologic Hh signaling in thymocytes maintains expression of the transcription factor FoxA2 on pre-TCR signal transduction.

Sustained expression of pre-TCR induced beta-catenin in post-beta-selection thymocytes blocks T cell development

Pre-TCR and IL-7R signals regulate beta-selection of thymocytes and then must be down-regulated for further development. However, the molecular events that control down-regulation remain unknown. We and others have previously shown that beta-catenin in cooperation with TCF regulates beta-selection. In this paper, we demonstrate that beta-catenin expression is stringently regulated by intrathymic signals, it is expressed at the highest levels in the pre-TCR signaled thymocytes, and is down-regulated in post-beta-selection thymocytes. Pre-TCR-induced beta-catenin regulates initial stages of pre-TCR signaling including expression of early growth response (Egr) genes but must be down-regulated to express RORgammat, which is essential for maturation to the CD4+CD8+ double positive (DP) stage. Sustained expression of beta-catenin results in the generation of IL-7R-, Egr-, and TGFbeta-expressing pre-DP thymocytes that are blocked in development. These data are consistent with a model in which post-beta-selection, pre-TCR-induced beta-catenin expression must return to background levels for efficient transition to the DP stage.

Pre-TCR-induced beta-catenin facilitates traversal through beta-selection

Pre-TCR induced signals regulate development of the alphabeta TCR lineage cells at the beta-selection checkpoint. We have previously shown that conditional deletion of beta-catenin, a central mediator of Wnt-beta-catenin-T cell factor signaling pathway, impairs traversal through the beta-selection checkpoint. We now provide a molecular basis for the impairment. We demonstrate that pre-TCR signals specifically stabilize beta-catenin in CD4-CD8- double negative thymocytes during beta-selection. Pre-TCR induced Erk activity was required to stabilize beta-catenin. Enforced expression of stabilized beta-catenin was sufficient to mediate aspects of beta-selection including sustained expression of early growth response (Egr) genes. Consistently, deletion of beta-catenin reduced induction of Egr gene expression by the pre-TCR signal and blocked efficient beta-selection. Thus, we demonstrate that pre-TCR induced beta-catenin sustains expression of Egr genes that facilitate traversal through the beta-selection checkpoint.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

The canonical Wnt signaling pathway plays an important role in lymphopoiesis and hematopoiesis

The evolutionarily conserved canonical Wnt-beta-catenin-T cell factor (TCF)/lymphocyte enhancer binding factor (LEF) signaling pathway regulates key checkpoints in the development of various tissues. Therefore, it is not surprising that a large body of gain-of-function and loss-of-function studies implicate Wnt-beta-catenin signaling in lymphopoiesis and hematopoiesis. In contrast, recent papers have reported that Mx-Cre-mediated conditional deletion of beta-catenin and/or its homolog gamma-catenin (plakoglobin) did not impair hematopoiesis or lymphopoiesis. However, these studies also report that TCF reporter activity remains active in beta-catenin- and gamma-catenin-deficient hematopoietic stem cells and all cells derived from these precursors, indicating that the canonical Wnt signaling pathway was not abrogated. Therefore, these studies in fact show that the canonical Wnt signaling pathway is important in hematopoiesis and lymphopoiesis, even though the molecular basis for the induction of the reporter activity is currently unknown. In this perspective, we provide a broad background to the field with a discussion of the available data and create a framework within which the available and future studies may be evaluated.

WNT signalling in the immune system: WNT is spreading its wings

WNT proteins are secreted morphogens that are required for basic developmental processes, such as cell-fate specification, progenitor-cell proliferation and the control of asymmetric cell division, in many different species and organs. In blood and immune cells, WNT signalling controls the proliferation of progenitor cells and might also affect the cell-fate decisions of stem cells. Recent studies indicate that WNT proteins also regulate effector T-cell development, regulatory T-cell activation and dendritic-cell maturation. WNT signalling seems to function as a universal mechanism in leukocytes to establish a pool of undifferentiated cells for further selection, effector-cell maturation and terminal differentiation. WNT signalling is therefore subject to strict molecular control, and dysregulated WNT signalling is implicated in the development of haematological malignancies.

Critical roles of the PI3K/Akt signaling pathway in T cell development

Thymocyte development requires an integration of extracellular cues to enforce lineage commitment at multiple defined checkpoints in a stage-specific manner. Critical signals from the pre-TCR, Notch, and the receptor for interleukin-7 (IL-7) dictate cellular differentiation from the CD4(-)CD8(-) (double negative) stage to the CD4+CD8+ (double positive) stage. The PI3K/Akt signaling pathway is required to translate these extracellular signaling events into multiple functional outcomes including cellular survival, proliferation, differentiation, and allelic exclusion at the beta-selection checkpoint. However, a complete understanding of the contributions made by the PI3K/Akt pathway in thymocyte development has not been straightforward. This review highlights studies that support the model that the PI3K/Akt pathway is essential for thymocyte survival. We provide new evidence that Akt-mediated survival is not solely due to the increased expression of Bcl-xL but also is a consequence of the role played by Akt to support metabolism in proliferating thymocytes.

Long-term, multilineage hematopoiesis occurs in the combined absence of β-catenin and γ-catenin

The canonical Wnt signaling pathway plays key roles in stem-cell maintenance, progenitor cell expansion, and lineage decisions. Transcriptional responses induced by Wnt depend on the association of either β-catenin or γ-catenin with lymphoid enhancer factor/T cell factor transcription factors. Here we show that hematopoiesis, including thymopoiesis, is normal in the combined absence of β- and γ-catenin. Double-deficient hematopoietic stem cells maintain long-term repopulation capacity and multilineage differentiation potential. Unexpectedly, 2 independent ex vivo reporter gene assays show that Wnt signal transmission is maintained in double-deficient hematopoietic stem cells, thymocytes, or peripheral T cells. In contrast, Wnt signaling is strongly reduced in thymocytes lacking TCF-1 or in nonhematopoietic cells devoid of β-catenin. These data provide the first evidence that hematopoietic cells can transduce canonical Wnt signals in the combined absence of β- and γ-catenin.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

Simultaneous loss of beta- and gamma-catenin does not perturb hematopoiesis or lymphopoiesis

Hematopietic stem cells (HSCs) maintain life-long hematopoiesis in the bone marrow via their ability to self-renew and to differentiate into all blood lineages. Although a central role for the canonical wnt signaling pathway has been suggested in HSC self-renewal as well as in the development of B and T cells, conditional deletion of beta-catenin (which is considered to be essential for Wnt signaling) has no effect on hematopoiesis or lymphopoiesis. Here, we address whether this discrepancy can be explained by a redundant and compensatory function of gamma-catenin, a close homolog of beta-catenin. Unexpectedly, we find that combined deficiency of beta- and gamma-catenin in hematopoietic progenitors does not impair their ability to self-renew and to reconstitute all myeloid, erythroid, and lymphoid lineages, even in competitive mixed chimeras and serial transplantations. These results exclude an essential role for canonical Wnt signaling (as mediated by beta- and/or gamma-catenin) during hematopoiesis and lymphopoiesis.

Molecular dissection of prethymic progenitor entry into the T lymphocyte developmental pathway

Notch signaling activates T lineage differentiation from hemopoietic progenitors, but relatively few regulators that initiate this program have been identified, e.g., GATA3 and T cell factor-1 (TCF-1) (gene name Tcf7). To identify additional regulators of T cell specification, a cDNA library from mouse Pro-T cells was screened for genes that are specifically up-regulated in intrathymic T cell precursors as compared with myeloid progenitors. Over 90 genes of interest were identified, and 35 of 44 tested were confirmed to be more highly expressed in T lineage precursors relative to precursors of B and/or myeloid lineage. To a remarkable extent, however, expression of these T lineage-enriched genes, including zinc finger transcription factor, helicase, and signaling adaptor genes, was also shared by stem cells (Lin(-)Sca-1(+)Kit(+)CD27(-)) and multipotent progenitors (Lin(-)Sca-1(+)Kit(+)CD27(+)), although down-regulated in other lineages. Thus, a major fraction of these early T lineage genes are a regulatory legacy from stem cells. The few genes sharply up-regulated between multipotent progenitors and Pro-T cell stages included those encoding transcription factors Bcl11b, TCF-1 (Tcf7), and HEBalt, Notch target Deltex1, Deltex3L, Fkbp5, Eva1, and Tmem131. Like GATA3 and Deltex1, Bcl11b, Fkbp5, and Eva1 were dependent on Notch/Delta signaling for induction in fetal liver precursors, but only Bcl11b and HEBalt were up-regulated between the first two stages of intrathymic T cell development (double negative 1 and double negative 2) corresponding to T lineage specification. Bcl11b was uniquely T lineage restricted and induced by Notch/Delta signaling specifically upon entry into the T lineage differentiation pathway.

Beta-catenin expression enhances IL-7 receptor signaling in thymocytes during positive selection

Differentiation of CD4+CD8+ double-positive thymocytes into CD8+ single-positive (SP) thymocytes is regulated by TCR and cytokine receptor signals. Previously, we have shown that expression of stabilized beta-catenin, the major transcriptional cofactor of T cell factor, results in increase in both CD4SP and CD8SP thymocytes with a preferential effect on CD8SP thymocytes. In this report, using mice expressing stabilized beta-catenin and mice with T cell specific deletion of beta-catenin, we show that beta-catenin expression augments IL-7Ralpha-chain expression and down-regulates suppressor of cytokine signaling-1 expression in thymocytes undergoing positive selection. Consequently, beta-catenin expression augments IL-7R signaling in thymocytes during positive selection and promotes the development of CD8SP thymocytes.

Redundant role for early growth response transcriptional regulators in thymocyte differentiation and survival

The early growth response (Egr) family of transcriptional regulators consists of four proteins that share highly conserved DNA-binding domains. In many cell types, they are coexpressed and appear to have cooperative roles in regulating gene expression during growth and differentiation. Three Egr proteins, Egr1, Egr2, and Egr3, are induced during thymocyte differentiation in response to pre-TCR signaling, suggesting they may be critical for some aspects of pre-TCR-mediated differentiation. Indeed, enforced expression of Egr proteins in developing thymocytes can recapitulate some aspects of pre-TCR signaling, but the mechanisms by which they contribute to beta-selection are still poorly understood. Egr3 stimulates proliferation of beta-selected thymocytes, and Egr3-deficient mice have hypocellular thymuses, defects in proliferation, and impaired progression from double-negative 3 to double-negative 4. Surprisingly, Egr1-deficient mice exhibit normal beta-selection, indicating that the functions of Egr1 during beta-selection are likely compensated by other Egr proteins. In this study, we show that mice lacking both Egr1 and Egr3 exhibit a more severe thymic atrophy and impairment of thymocyte differentiation than mice lacking either Egr1 or Egr3. This is due to a proliferation defect and cell-autonomous increase in apoptosis, indicating that Egr1 and Egr3 cooperate to promote thymocyte survival. Microarray analysis of deregulated gene expression in immature thymocytes lacking both Egr1 and Egr3 revealed a previously unknown role for Egr proteins in the maintenance of cellular metabolism, providing new insight into the function of these molecules during T cell development.

Regulatory factors for initial T lymphocyte lineage specification

PURPOSE OF REVIEW

Initiation of T lymphocyte development depends on balanced regulatory inputs from multiple essential transcription factors. This review highlights contributions of E2A, hematopoietic transcription factor PU.1, growth factor independence (Gfi)-1, T cell factor (TCF)-1, and Runx factors and their interactions with the Notch pathway to promote T cell development.

RECENT FINDINGS

E2A and Runx family factors have been implicated in establishing competent precursors in which Notch signaling can induce the T cell program. An early role was also indicated for PU.1. Later PU.1 activities are antagonistic to pro-T cell factors, however, including E proteins, Myb, Gfi-1, and TCF-1. Diversion to a non-T lineage can be promoted by PU.1, CCAAT/enhancer binding protein, or even GATA and TCF, but these diversion mechanisms are blocked by Notch signaling. An emergent gene network summarizes the cross-regulatory relationships among these factors.

SUMMARY

Entry into the T-cell pathway is controlled by a dynamic balance among essential regulatory factors that depend on Notch signaling not only to trigger initiation of the T-cell program but also to maintain the lineage fidelity of their collective action.

Akt1 and Akt2 are required for alphabeta thymocyte survival and differentiation

The beta-selection checkpoint in alphabetaT lymphocyte development occurs at the double negative (DN) 3 (CD4(-)CD8(-)CD25(+)c-kit(-)) stage, when further differentiation requires a signal from the newly rearranged TCR beta chain. Thymocytes with mutations in key signaling molecules in the phosphatidylinositol 3-kinase-Akt pathway manifest defects in survival, proliferation, and differentiation past the beta-selection checkpoint. However, little information is available regarding the role of Akt itself in thymocyte development. In this study, we explore the role of the two Akt isoforms most highly expressed in the thymus, Akt1 and Akt2, in early T cell development. Using several complementary approaches, we find that deletion of Akt1 results in only minor defects in thymocyte development. The Akt1(-/-)Akt2(-/-) thymocytes manifest a severe developmental block at the DN3 stage and ultimately fail to repopulate the T cell compartment of an irradiated host. Further, we show that Akt1(-/-)Akt2(-/-) DN3 cells have decreased glucose uptake and die in response to TCR stimulation in vitro. Study of thymocytes from the genetically altered mice suggests that the cause of the developmental defect is due to apoptosis, partially caused by decreased cellular growth and metabolism at the DN3 stage. Our results show that Akt protects thymocytes from cell death during the beta-selection checkpoint.

Beta-catenin regulates positive selection of thymocytes but not lineage commitment

Positive selection and lineage commitment to the cytolytic or helper lineage of T cells result in coordinated expression of MHC class I-restricted TCR and CD8 coreceptor or MHC class II-restricted TCR and CD4 molecule. Positive selection signals also regulate the survival and generation of requisite numbers of cytolytic or Th cells. beta-Catenin is the major transcriptional cofactor of T cell factor and plays a role in thymocyte development. In this study, using mice expressing stabilized beta-catenin and mice with T cell-specific deletion of beta-catenin, we show that beta-catenin regulates positive selection, but not lineage commitment of thymocytes. Furthermore, beta-catenin expression accelerates the timing of mature CD8 thymocyte generation such that CD4 and CD8 single-positive thymocytes mature with the same kinetics during development.

Functional genomics analyses of differential macaque peripheral blood mononuclear cell infections by human immunodeficiency virus-1 and simian immunodeficiency virus

The pathogenicity of the primate lentiviruses, human, and simian immunodeficiency viruses, is host-specific. Previous studies indicated that the highly pathogenic human lentivirus HIV-1 has markedly reduced pathogenicity compared to the pathogenic simian lentivirus SIV in pigtail macaques (Macaca nemestrina). We therefore hypothesized that the pigtail macaque peripheral blood mononuclear cells (mPBMCs) would respond differently to infections of HIV-1 and pathogenic SIV. To elucidate the cellular responses to the infections of HIV-1 and SIV, we infected mPBMC with these two viruses. Like infections in vivo, HIV-1 and SIV demonstrated distinct replication kinetics in mPBMCs, with HIV-1 replicating at significantly lower levels. Similarly, gene expression profiling facilitated by macaque-specific oligonucleotide microarrays also revealed distinct expression patterns of genes between the HIV-1- and SIV-infected mPBMCs; in particular, genes associated with the antigen presentation, T cell receptor, ERK/MAPK signaling, Wnt/beta-catenin signaling, and natural killer cell signaling pathways were differentially regulated between these two viruses. Most interestingly, despite the lower levels of replication, HIV-1 triggered a more robust regulation of immune response genes early after infection; the converse was true in SIV-infected mPBMCs. Our results therefore suggest that macaques may be controlling the infection of HIV-1 at an early stage through coordinated regulation of host defense pathways.

Integrated morphogen signal inputs in gammadelta versus alphabeta T-cell differentiation

Morphogens, a class of secreted proteins that regulate gene expression in a concentration-dependent manner, are responsible for directing nearly all lineage fate choices during embryogenesis. In the thymus, morphogen signal pathways consisting of WNT, Hedgehog, and the transforming growth factor-beta superfamily are active and have been implicated in various developmental processes including proliferation, survival, and differentiation of maturing thymocytes. Intriguingly, it has been inferred that some of these morphogen signal pathways differentially affect gammadelta and alphabeta T-cell development or maintenance, but their role in T-cell lineage commitment has not been directly probed. We have recently identified a modulator of morphogen signaling that significantly influences binary gammadelta versus alphabeta T-cell lineage diversification. In this review, we summarize functions of morphogens in the thymus and provide a highly speculative model of integrated morphogen signals, potentially directing the gammadelta versus alphabeta T-cell fate determination process.

Overexpression of ICAT highlights a role for catenin-mediated canonical Wnt signalling in early T cell development

Transcription factors of the T cell factor/lymphoid enhancing factor (Tcf/Lef) family are key regulators in the development of T cell precursors to the CD4+8+ stage. These factors are known targets of the canonical Wnt signalling pathway, and regulate transcription of Wnt target genes following interaction with the armadillo repeat-containing protein beta-catenin. However, as recent studies show normal thymocyte maturation in the absence of either beta-catenin or its homologue gamma-catenin, the role of Wnt signalling in Tcf/Lef activation during T cell development is controversial. To directly investigate the importance of catenin-mediated Wnt signalling in early thymocytes, we have compared the expression of beta- and gamma-catenin and analysed distinct stages of T cell precursor maturation following overexpression of inhibitor of beta-catenin and Tcf (ICAT), which inhibits Wnt signalling by preventing binding of armadillo repeat-containing proteins to Tcf/Lef. By direct retroviral gene targeting of CD4-8- and CD4+8+ precursors, we show that ICAT overexpression inhibits the CD4-8--to-CD4+8+ transition, but not the CD4+8+-to-CD4+8- or -CD4-8+ transition. Collectively, our data support a model in which canonical Wnt signalling influences T cell development in the thymus by playing an essential role in the maturation of CD4-8- but not CD4+8+ thymocytes.