An early T cell lineage commitment checkpoint dependent on the transcription factor Bcl11b

[CTIP2, a multifunctional protein: cellular physiopathology and therapeutic implications]

The transcription factor CTIP2 (BCL11B) is a multifunctional protein involved in numerous cell physiological processes. To date, many molecular mechanisms underlying this process have been discovered, which highlighted the importance of the epigenetic regulation of genes and the regulation of the elongation factor P-TEFb. Furthermore studies of the deregulation of CTIP2 showed the association of CTIP2 to numerous pathologies including cancer and cardiac hypertrophy. A better comprehension of the physiopathology of these diseases might lead to the design of therapeutical strategies intending to prevent CTIP2 deregulation. Moreover, CTIP2 and its associated proteins constitute potential targets in strategies aiming to reduce and/or purge HIV-1 cell reservoirs.

CD1d-unrestricted NKT cells are endowed with a hybrid function far superior than that of iNKT cells

Invariant natural killer T (iNKT) cells to date represent the best example of cells known to have a hybrid function, representing both innate and adaptive immunity. Shared phenotypic similarities with NK cells together with a rapid response to a cytokine stimulus and a productive TCR engagement are the features that underline the hybrid nature of iNKT cells. Using these criteria, we provide molecular and functional evidence demonstrating that CD1d-independent (CD1d(ind)) NKT cells, a population of CD1d-unrestricted NKT cells, are endowed with a hybrid function far superior to that of iNKT cells: (i) an extensive shared program with NK cells, (ii) a closer Euclidian distance with NK cells, and (iii) the ability to respond to innate stimuli (Poly:IC) with cytotoxic potential in the same manner as NK cells identify a hybrid feature in CD1d(ind)NKT cells that truly fulfills the dual function of an NK and a T cell. Our finding that CD1d(ind)NKT cells are programmed to act like NK cells in response to innate signals while being capable of adaptive responses is unprecedented, and thus might reemphasize CD1d-unrestricted NKT cells as a subset of lymphocytes that could affect biological processes of antimicrobial and tumor immunity in a unique way.

Developmental gene networks: a triathlon on the course to T cell identity

Cells acquire their ultimate identities by activating combinations of transcription factors that initiate and sustain expression of the appropriate cell type-specific genes. T cell development depends on the progression of progenitor cells through three major phases, each of which is associated with distinct transcription factor ensembles that control the recruitment of these cells to the thymus, their proliferation, lineage commitment and responsiveness to T cell receptor signals, all before the allocation of cells to particular effector programmes. All three phases are essential for proper T cell development, as are the mechanisms that determine the boundaries between each phase. Cells that fail to shut off one set of regulators before the next gene network phase is activated are predisposed to leukaemic transformation.

Gene expression profile analysis of SUDHL6 cells with siRNA-mediated BCL11A downregulation

Our previous study has shown that downregulation of B-cell chronic lymphocytic leukemia (CLL)/lymphoma11A (BCL11A) gene by small interfering RNA (siRNA) resulted in the growth inhibition and apoptosis of B cell lymphoma cell line SUDHL6. To gain further insight into the molecular mechanisms of this process and identify the differentially expressed genes in SUDHL6 cells after BCL11A downregulation, the global gene expression profile was identified and analyzed using the Affymetrix HG-U133 Plus 2.0 array. Twenty-one differentially expressed genes were validated and analyzed from the BCL11A siRNA-treated SUDHL6 cells. There was a significant dysregulation in the global gene expression of the BCL11A-suppressed SUDHL6 cells. There were 1903 genes differentially expressed with >2-fold changes between the BCL11A siRNA- and negative control-transfected cells. Of these, there were 916 upregulated genes and 987 downregulated genes. The differential genes are involved in various molecular functions and signaling pathways. QRT-PCR validation of the selected differentially expressed genes demonstrated there was a good correlation with the microarray analysis. There was a significant deregulation of expression in the apoptosis-related genes such as BCL-2, BCL2L11 and involved in TGFβ, MAPK, WNT signaling pathways after BCL11A was downregulated in SUDHL6 cells. Our results show that the suppression of BCL11A by RNA interference altered gene expression profile of SUDHL6 cells. The apoptosis-related genes BCL-2, BCL2L11 and the gene alterations in TGFβ, MAPK, WNT signaling pathways might be important in BCL11A siRNA-induced apoptosis of SUDHL6 cells, suggesting BCL11A is involved in gene networks associated with apoptosis.

Low expression of T-cell transcription factor BCL11b predicts inferior survival in adult standard risk T-cell acute lymphoblastic leukemia patients

Background

Risk stratification, detection of minimal residual disease (MRD), and implementation of novel therapeutic agents have improved outcome in acute lymphoblastic leukemia (ALL), but survival of adult patients with T-cell acute lymphoblastic leukemia (T-ALL) remains unsatisfactory. Thus, novel molecular insights and therapeutic approaches are urgently needed.

Methods

We studied the impact of B-cell CLL/lymphoma 11b (BCL11b), a key regulator in normal T-cell development, in T-ALL patients enrolled into the German Multicenter Acute Lymphoblastic Leukemia Study Group trials (GMALL; n = 169). The mutational status (exon 4) of BCL11b was analyzed by Sanger sequencing and mRNA expression levels were determined by quantitative real-time PCR. In addition gene expression profiles generated on the Human Genome U133 Plus 2.0 Array (affymetrix) were used to investigate BCL11b low and high expressing T-ALL patients.

Results

We demonstrate that BCL11b is aberrantly expressed in T-ALL and gene expression profiles reveal an association of low BCL11b expression with up-regulation of immature markers. T-ALL patients characterized by low BCL11b expression exhibit an adverse prognosis [5-year overall survival (OS): low 35% (n = 40) vs. high 53% (n = 129), P = 0.02]. Within the standard risk group of thymic T-ALL (n = 102), low BCL11b expression identified patients with an unexpected poor outcome compared to those with high expression (5-year OS: 20%, n = 18 versus 62%, n = 84, P < 0.01). In addition, sequencing of exon 4 revealed a high mutation rate (14%) of BCL11b.

Conclusions

In summary, our data of a large adult T-ALL patient cohort show that low BCL11b expression was associated with poor prognosis; particularly in the standard risk group of thymic T-ALL. These findings can be utilized for improved risk prediction in a significant proportion of adult T-ALL patients, which carry a high risk of standard therapy failure despite a favorable immunophenotype.

The CD4/CD8 lineages: central decisions and peripheral modifications for T lymphocytes

CD4(+) helper and CD8(+) cytotoxic T cells, two major subsets of αβTCR expressing lymphocytes, are differentiated from common precursor CD4(+)CD8(+) double-positive (DP) thymocytes. Bifurcation of the CD4(+)/CD8(+) lineages in the thymus is a multilayered process and is thought to culminate in a loss of developmental plasticity between these functional subsets. Advances in the last decade have deepened our understanding of the transcription control mechanisms governing CD4 versus CD8 lineage commitment. Reciprocal expression and antagonistic interplay between two transcription factors, ThPOK and Runx3, is crucial for driving thymocyte decisions between these two cell fates. Here, we first focus on the regulation of ThPOK expression and its role in directing helper T cell development. We then discuss a novel aspect of the ThPOK/Runx3 axis in modifying CD4(+) T cell function upon exposure to gut microenvironment.

Combination of BCL11A siRNA with vincristine increases the apoptosis of SUDHL6 cells

Background

B cell chronic lymphocytic leukemia/lymphoma 11 A (BCL11A) is associated with human B cell malignancy initiation. Our previous study has shown that downregulation of BCL11A mRNA by small interfering RNA (siRNA) is capable of inducing apoptosis in the SUDHL6 cell line. To further explore the effects of BCL11A siRNA on the enhanced cytotoxicity of a chemotherapeutic drug, we investigated the effects of BCL11A siRNA combined with vincristine (VCR) on SUDHL6 cell proliferation and apoptosis.

Methods

Chemically synthesized BCL11A siRNA was transfected into SUDHL6 cells using the HiPerFect Transfection Reagent in combination with VCR. Cell proliferation was measured by the CCK8 assay. The morphology of apoptotic cells was observed with Hoechst 33258 staining. The rate of cell apoptosis was determined by annexin V-fluorescein isothiocyanate/propidium iodide double staining using fluorescence-activated cell sorting (FACS) analysis.

Results

After BCL11A siRNA plus VCR treatment, cell proliferation was significantly decreased in comparison with VCR or BCL11A siRNA treatment alone and negative control siRNA plus VCR treatment (P <0.05). The apoptotic rate of BCL11A siRNA plus VCR treated cells was significantly increased compared with BCL11A siRNA and VCR treatment alone and negative control siRNA plus VCR treatment (P <0.05).

Conclusions

The combination of BCL11A siRNA and VCR increases apoptosis in SUDHL6 cells. Our study implies that BCL11A siRNA in combination with VCR may be a useful approach for improving effective treatment for B cell lymphoma.

An overview of the intrathymic intricacies of T cell development

The generation of a functional and diverse repertoire of T cells occurs in the thymus from precursors arriving from the bone marrow. In this article, we introduce the various stages of mouse thymocyte development and highlight recent work using various in vivo, and, where appropriate, in vitro models of T cell development that led to discoveries in the regulation afforded by transcription factors and receptor-ligand signaling pathways in specifying, maintaining, and promoting the T cell lineage and the production of T cells. This review also discusses the role of the thymic microenvironment in providing a niche for the successful development of T cells. In particular, we focus on advances in Notch signaling and developments in Notch ligand interactions in this process.

Epigenetic control of immunity

Immunity relies on the heterogeneity of immune cells and their ability to respond to pathogen challenges. In the adaptive immune system, lymphocytes display a highly diverse antigen receptor repertoire that matches the vast diversity of pathogens. In the innate immune system, the cell's heterogeneity and phenotypic plasticity enable flexible responses to changes in tissue homeostasis caused by infection or damage. The immune responses are calibrated by the graded activity of immune cells that can vary from yeast-like proliferation to lifetime dormancy. This article describes key epigenetic processes that contribute to the function of immune cells during health and disease.

Lmo2 induces hematopoietic stem cell-like features in T-cell progenitor cells prior to leukemia

LIM domain only 2 (Lmo2) is frequently deregulated in sporadic and gene therapy-induced acute T-cell lymphoblastic leukemia (T-ALL) where its overexpression is an important initiating mutational event. In transgenic and retroviral mouse models, Lmo2 expression can be enforced in multiple hematopoietic lineages but leukemia only arises from T cells. These data suggest that Lmo2 confers clonal growth advantage in T-cell progenitors. We analyzed proliferation, differentiation, and cell death in CD2-Lmo2 transgenic thymic progenitor cells to understand the cellular effects of enforced Lmo2 expression. Most impressively, Lmo2 transgenic T-cell progenitor cells were blocked in differentiation, quiescent, and immortalized in vitro on OP9-DL1 stromal cells. These cellular effects were concordant with a transcriptional signature in Lmo2 transgenic T-cell progenitor cells that is also present in hematopoietic stem cells (HSCs) and early T-cell precursor ALL. These results are significant in light of the crucial role of Lmo2 in the maintenance of the HSC. The cellular effects and transcriptional effects have implications for LMO2-dependent leukemogenesis and the treatment of LMO2-induced T-ALL.

Leucine-rich repeat containing 8A (LRRC8A) is essential for T lymphocyte development and function

Lrrc8a is a ubiquitously expressed gene that encodes a leucine-rich repeat (LRR)-containing protein detected at higher levels on the surface of thymocytes than on other immune cells. We generated Lrrc8a(-/-) mice to investigate the role of LRRC8A in lymphocyte development and function. Lrrc8a(-/-) mice had increased prenatal and postnatal mortality, growth retardation, and multiple tissue abnormalities. Lrrc8a(-/-) mice displayed a modest block in B cell development but intact intrinsic B cell function. In contrast, both Lrrc8a(-/-) mice and Lrrc8a(-/-)→Rag2(-/-) bone marrow chimeras exhibited a severe cell-intrinsic block in early thymic development, with decreased proliferation and increased apoptosis of thymocytes, and impaired peripheral T cell function. Thymic epithelial cells expressed an LRRC8A ligand that was critical for double-negative to double-positive thymocyte differentiation and survival in vitro. LRRC8A constitutively associated with the GRB2-GAB2 complex and lymphocyte-specific protein tyrosine kinase (LCK) in thymocytes. LRRC8A ligation activated AKT via the LCK-ZAP-70-GAB2-PI3K pathway, and AKT phosphorylation was markedly reduced in the thymus of Lrrc8a(-/-) mice. These findings reveal an essential role for LRRC8A in T cell development, survival, and function.

Interleukin-7 receptor mutants initiate early T cell precursor leukemia in murine thymocyte progenitors with multipotent potential

Early T cell precursor acute lymphoblastic leukemia (ETP-ALL) exhibits lymphoid, myeloid, and stem cell features and is associated with a poor prognosis. Whole genome sequencing of human ETP-ALL cases has identified recurrent mutations in signaling, histone modification, and hematopoietic development genes but it remains to be determined which of these abnormalities are sufficient to initiate leukemia. We show that activating mutations in the interleukin-7 receptor identified in human pediatric ETP-ALL cases are sufficient to generate ETP-ALL in mice transplanted with primitive transduced thymocytes from p19(Arf-/-) mice. The cellular mechanism by which these mutant receptors induce ETP-ALL is the block of thymocyte differentiation at the double negative 2 stage at which myeloid lineage and T lymphocyte developmental potential coexist. Analyses of samples from pediatric ETP-ALL cases and our murine ETP-ALL model show uniformly high levels of LMO2 expression, very low to undetectable levels of BCL11B expression, and a relative lack of activating NOTCH1 mutations. We report that pharmacological blockade of Jak-Stat signaling with ruxolitinib has significant antileukemic activity in this ETP-ALL model. This new murine model recapitulates several important cellular and molecular features of ETP-ALL and should be useful to further define novel therapeutic approaches for this aggressive leukemia.

The many lives of CTIP2: from AIDS to cancer and cardiac hypertrophy

CTIP2 is a key transcriptional regulator involved in numerous physiological functions. Initial works have shown the importance of CTIP2 in the establishment and persistence of HIV latency in microglial cells, the main latent/quiescent viral reservoir in the brain. Recent studies have highlighted the importance of CTIP2 in several other pathologies, such as cardiac hypertrophy and various types of human malignancies. Targeting CTIP2 may therefore constitute a new approach in the treatment of these pathologies.

Illegitimate V(D)J recombination-mediated deletions in Notch1 and Bcl11b are not sufficient for extensive clonal expansion and show minimal age or sex bias in frequency or junctional processing

Illegitimate V(D)J recombination at oncogenes and tumor suppressor genes is implicated in formation of several T cell malignancies. Notch1 and Bcl11b, genes involved in developing T cell specification, selection, proliferation, and survival, were previously shown to contain hotspots for deletional illegitimate V(D)J recombination associated with radiation-induced thymic lymphoma. Interestingly, these deletions were also observed in wild-type animals. In this study, we conducted frequency, clonality, and junctional processing analyses of Notch1 and Bcl11b deletions during mouse development and compared results to published analyses of authentic V(D)J rearrangements at the T cell receptor beta (TCRβ) locus and illegitimate V(D)J deletions observed at the human, nonimmune HPRT1 locus not involved in T cell malignancies. We detect deletions in Notch1 and Bcl11b in thymic and splenic T cell populations, consistent with cells bearing deletions in the circulating lymphocyte pool. Deletions in thymus can occur in utero, increase in frequency between fetal and postnatal stages, are detected at all ages examined between fetal and 7 months, exhibit only limited clonality (contrasting with previous results in radiation-sensitive mouse strains), and consistent with previous reports are more frequent in Bcl11b, partially explained by relatively high Recombination Signal Information Content (RIC) scores. Deletion junctions in Bcl11b exhibit greater germline nucleotide loss, while in Notch1 palindromic (P) nucleotides are more abundant, although average P nucleotide length is similar for both genes and consistent with results at the TCRβ locus. Non-templated (N) nucleotide insertions appear to increase between fetal and postnatal stages for Notch1, consistent with normal terminal deoxynucleotidyl transferase (TdT) activity; however, neonatal Bcl11b junctions contain elevated levels of N insertions. Finally, contrasting with results at the HPRT1 locus, we find no obvious age or gender bias in junctional processing, and inverted repeats at recessed coding ends (Pr nucleotides) correspond mostly to single-base additions consistent with normal TdT activity.

IFN-γ-producing and IL-17-producing γδ T cells differentiate at distinct developmental stages in murine fetal thymus

γδ T cells develop at the double-negative (DN) 2 and DN3 stages and acquire functions to produce IL-17 and IFN-γ in fetal thymus. However, the relationship between differentiation stages and their functions was unclear. In this study, we found that, although IFN-γ-producing and IL-17-producing γδ T cells developed from DN2 cells, only IFN-γ-producing γδ T cells developed from DN3 cells, indicating the direct generation of IL-17-producing γδ T cells from the DN2 stage, not through the DN3 stage. Single-cell analysis revealed that DN2 cells contained heterogeneous γδ T cell precursors with or without an ability to develop IL-17 producers. Inactivation of B cell leukemia/lymphoma 11b, a zinc finger transcription factor responsible for transition from early to late stages of DN2 cells, completely abrogated the development of IL-17-producing γδ T cells, although a unique subset of IFN-γ-producing γδ T cells expressing a high level of promyelocytic leukemia zinc finger was able to develop. Thus, our results reveal that γδ T cells are functionally differentiated to IFN-γ and IL-17 producers at different developmental stages in fetal thymus.

Transcriptional control of early T and B cell developmental choices

T and B cells share a common somatic gene rearrangement mechanism for assembling the genes that code for their antigen receptors; they also have developmental pathways with many parallels. Shared usage of basic helix-loop-helix E proteins as transcriptional drivers underlies these common features. However, the transcription factor networks in which these E proteins are embedded are different both in membership and in architecture for T and B cell gene regulatory programs. These differences permit lineage commitment decisions to be made in different hierarchical orders. Furthermore, in contrast to B cell gene networks, the T cell gene network architecture for effector differentiation is sufficiently modular so that E protein inputs can be removed. Complete T cell-like effector differentiation can proceed without T cell receptor rearrangement or selection when E proteins are neutralized, yielding natural killer and other innate lymphoid cells.

Genetic and epigenetic control of early lymphocyte development

T, B, and NK lymphocytes are generated from pluripotent hematopoietic stem cells through a successive series of lineage restriction processes. Many regulatory components, such as transcription factors, cytokines/cytokine receptors, and signal transduction molecules orchestrate cell fate specification and determination. In particular, transcription factors play a key role in regulating lineage-associated gene programs. Recent findings suggest the involvement of epigenetic factors in the maintenance of cell fate. Here, we review the early developmental events during lymphocyte lineage determination, focusing on the transcriptional networks and epigenetic regulation. Finally, we also discuss the developmental relationship between acquired and innate lymphoid cells.

Tcrδ translocations that delete the Bcl11b haploinsufficient tumor suppressor gene promote atm-deficient T cell acute lymphoblastic leukemia

ATM is the master regulator of the cellular response to DNA double strand breaks (DSBs). Deficiency of ATM predisposes humans and mice to αβ T lymphoid cancers with clonal translocations between the T cell receptor (TCR) α/δ locus and a 450 kb region of synteny on human chromosome 14 and mouse chromosome 12. While these translocations target and activate the TCL1 oncogene at 14q32 to cause T cell pro-lymphocytic leukemia (T-PLL), the TCRα/δ;14q32 translocations in ATM-deficient T cell acute lymphoblastic leukemia (T-ALL) have not been characterized and their role in cancer pathogenesis remains unknown. The corresponding lesion in Atm-deficient mouse T-ALLs is a chromosome t(12;14) translocation with Tcrδ genes fused to sequences on chromosome 12; although these translocations do not activate Tcl1, they delete the Bcl11b haploinsufficient tumor suppressor gene. To assess whether Tcrδ translocations that inactivate one copy of Bcl11b promote transformation of Atm-deficient cells, we analyzed Atm(-/-) mice with mono-allelic Bcl11b deletion initiating in thymocytes concomitant with Tcrδ recombination. Inactivation of one Bcl11b copy had no effect on the predisposition of Atm(-/-) mice to clonal T-ALLs. Yet, none of these T-ALLs had a clonal chromosome t(12;14) translocation that deleted Bcl11b indicating that Tcrδ translocations that inactivate a copy of Bcl11b promote transformation of Atm-deficient thymocytes. Our data demonstrate that antigen receptor locus translocations can cause cancer by deleting a tumor suppressor gene. We discuss the implications of these findings for the etiology and therapy of T-ALLs associated with ATM deficiency and TCRα/δ translocations targeting the 14q32 cytogenetic region.

Diverting T helper cell trafficking through increased plasticity attenuates autoimmune encephalomyelitis

Naive T helper cells differentiate into functionally distinct effector subsets that drive specialized immune responses. Recent studies indicate that some of the effector subsets have plasticity. Here, we used an EAE model and found that Th17 cells deficient in the transcription factor BCL11B upregulated the Th2-associated proteins GATA3 and IL-4 without decreasing RAR-related orphan receptor γ (RORγt), IL-17, and GM-CSF levels. Surprisingly, abnormal IL-4 production affected Th17 cell trafficking, diverting migration from the draining lymph nodes/CNS route to the mesenteric lymph nodes/gut route, which ameliorated EAE without overt colitis. T helper cell rerouting in EAE was dependent on IL-4, which enhanced retinoic acid (RA) production by dendritic cells, which further induced expression of gut-homing receptors CCR9 and α4β7 on Bcl11b-deficient CD4+ T cells. Furthermore, IL-4 treatment or Th2 immunization of wild-type mice with EAE caused no alteration in Th17 cytokines or RORγt, but diverted T helper cell trafficking to the gut, which improved EAE outcome without overt colitis. Our data demonstrate that Th17 cells are permissive to Th2 gene expression without affecting Th17 gene expression. This Th17 plasticity has an impact on trafficking, which is a critical component of the immune response and may represent a possible avenue for treating multiple sclerosis.

Novel somatic mutations in large granular lymphocytic leukemia affecting the STAT-pathway and T-cell activation

T-cell large granular lymphocytic (T-LGL) leukemia is a clonal disease characterized by the expansion of mature CD3+CD8+ cytotoxic T cells. It is often associated with autoimmune disorders and immune-mediated cytopenias. Our recent findings suggest that up to 40% of T-LGL patients harbor mutations in the STAT3 gene, whereas STAT5 mutations are present in 2% of patients. In order to identify putative disease-causing genetic alterations in the remaining T-LGL patients, we performed exome sequencing from three STAT mutation-negative patients and validated the findings in 113 large granular lymphocytic (LGL) leukemia patients. On average, 11 CD8+ LGL leukemia cell-specific high-confidence nonsynonymous somatic mutations were discovered in each patient. Interestingly, all patients had at least one mutation that affects either directly the STAT3-pathway (such as PTPRT) or T-cell activation (BCL11B, SLIT2 and NRP1). In all three patients, the STAT3 pathway was activated when studied by RNA expression or pSTAT3 analysis. Screening of the remaining 113 LGL leukemia patients did not reveal additional patients with same mutations. These novel mutations are potentially biologically relevant and represent rare genetic triggers for T-LGL leukemia, and are associated with similar disease phenotype as observed in patients with mutations in the STAT3 gene.

T cell development requires constraint of the myeloid regulator C/EBP-α by the Notch target and transcriptional repressor Hes1

Notch signaling induces gene expression of the T cell lineage and discourages alternative fate outcomes. Hematopoietic deficiency in the Notch target Hes1 results in severe T cell lineage defects; however, the underlying mechanism is unknown. We found here that Hes1 constrained myeloid gene-expression programs in T cell progenitor cells, as deletion of the myeloid regulator C/EBP-α restored the development of T cells from Hes1-deficient progenitor cells. Repression of Cebpa by Hes1 required its DNA-binding and Groucho-recruitment domains. Hes1-deficient multipotent progenitor cells showed a developmental bias toward myeloid cells and dendritic cells after Notch signaling, whereas Hes1-deficient lymphoid progenitor cells required additional cytokine signaling for diversion into the myeloid lineage. Our findings establish the importance of constraining developmental programs of the myeloid lineage early in T cell development.

Transcriptional establishment of cell-type identity: dynamics and causal mechanisms of T-cell lineage commitment

Precursor cell entry into the T-cell developmental pathway can be divided into two phases by the closure of T-lineage commitment. As cells decide against the last alternative options to the T-cell fate, they turn on the transcription factor Bcl11b and silence expression of a group of multipotent progenitor regulatory factors that include hematopoietic transcription factor PU.1. Functional perturbation tests show that Bcl11b is needed for commitment while PU.1 actively participates in keeping open access to alternative fates, until it is silenced; however, PU.1 and Bcl11b both contribute positively to T-cell development. Our recent work reviewed here sheds light on the transcriptional regulatory network that determines the timing and irreversibility of Bcl11b activation, the ways that Notch signaling from the thymic microenvironment restricts the action of PU.1 to prevent it from diverting cells to non-T fates, and the target genes that PU.1 still regulates under the influence of Notch signaling to contribute to T-cell generation. We argue that T-cell development depends on the sequential operation of two interlaced, but mutually antagonistic, gene regulatory networks, one initially supporting expansion before commitment and the other imposing a "terminal" differentiation process on committed cells.

Single-cell analysis of thymocyte differentiation: identification of transcription factor interactions and a major stochastic component in αβ-lineage commitment

T cell commitment and αβ/γδ lineage specification in the thymus involves interactions between many different genes. Characterization of these interactions thus requires a multiparameter analysis of individual thymocytes. We developed two efficient single-cell methods: (i) the quantitative evaluation of the co-expression levels of nine different genes, with a plating efficiency of 99–100% and a detection limit of 2 mRNA molecules/cell; and (ii) single-cell differentiation cultures, in the presence of OP9 cells transfected with the thymus Notch1 ligand DeltaL4. We show that during T cell commitment, Gata3 has a fundamental, dose-dependent role in maintaining Notch1 expression, with thymocytes becoming T-cell-committed when they co-express Notch1, Gata3 and Bc11b. Of the transcription factor expression patterns studied here, only that of Bcl11b was suggestive of a role in Pu1 down-regulation. Individual thymocytes became αβ/γδ lineage-committed at very different stages (from the TN2a stage onwards). However, 20% of TN3 cells are not αβ/γδ-lineage committed and TN4 cells comprise two main subpopulations with different degrees of maturity. The existence of a correlation between differentiation potential and expression of the pre-TCR showed that 83% of αβ-committed cells do not express the pre-TCR and revealed a major stochastic component in αβ-lineage specification.

CTIP2 is a negative regulator of P-TEFb

The positive transcription elongation factor b (P-TEFb) is involved in physiological and pathological events including inflammation, cancer, AIDS, and cardiac hypertrophy. The balance between its active and inactive form is tightly controlled to ensure cellular integrity. We report that the transcriptional repressor CTIP2 is a major modulator of P-TEFb activity. CTIP2 copurifies and interacts with an inactive P-TEFb complex containing the 7SK snRNA and HEXIM1. CTIP2 associates directly with HEXIM1 and, via the loop 2 of the 7SK snRNA, with P-TEFb. In this nucleoprotein complex, CTIP2 significantly represses the Cdk9 kinase activity of P-TEFb. Accordingly, we show that CTIP2 inhibits large sets of P-TEFb- and 7SK snRNA-sensitive genes. In hearts of hypertrophic cardiomyopathic mice, CTIP2 controls P-TEFb-sensitive pathways involved in the establishment of this pathology. Overexpression of the β-myosin heavy chain protein contributes to the pathological cardiac wall thickening. The inactive P-TEFb complex associates with CTIP2 at the MYH7 gene promoter to repress its activity. Taken together, our results strongly suggest that CTIP2 controls P-TEFb function in physiological and pathological conditions.

A far downstream enhancer for murine Bcl11b controls its T-cell specific expression

Bcl11b is a T-cell specific gene in hematopoiesis that begins expression during T-lineage commitment and is required for this process. Aberrant expression of BCL11B or proto-oncogene translocation to the vicinity of BCL11B can be a contributing factor in human T-ALL. To identify the mechanism that controls its distinctive T-lineage expression, we corrected the identified Bcl11b transcription start site and mapped a cell-type-specific differentially methylated region bracketing the Bcl11b promoter. We identified a 1.9-kb region 850 kb downstream of Bcl11b, "Major Peak," distinguished by its dynamic histone marking pattern in development that mirrors the pattern at the Bcl11b promoter. Looping interactions between promoter-proximal elements including the differentially methylated region and downstream elements in the Major Peak are required to recapitulate the T-cell specific expression of Bcl11b in stable reporter assays. Functional dissection of the Major Peak sequence showed distinct subregions, in which TCF-1 sites and a conserved element were required for T-lineage-specific activation and silencing in non-T cells. A bacterial artificial chromosome encompassing the full Bcl11b gene still required the addition of the Major Peak to exhibit T-cell specific expression. Thus, promoter-proximal and Major Peak sequences are cis-regulatory elements that interact over 850 kb to control expression of Bcl11b in hematopoietic cells.

The transcriptional landscape of αβ T cell differentiation

αβT cell differentiation from thymic precursors is a complex process, explored here with the breadth of ImmGen expression datasets, analyzing how differentiation of thymic precursors gives rise to transcriptomes. After surprisingly gradual changes though early T commitment, transit through the CD4⁺CD8⁺ stage involves a shutdown or rare breadth, and correlating tightly with MYC. MHC-driven selection promotes a large-scale transcriptional reactivation. We identify distinct signatures that mark cells destined for positive selection versus apoptotic deletion. Differential expression of surprisingly few genes accompany CD4 or CD8 commitment, a similarity that carries through to peripheral T cells and their activation, revealed by mass cytometry phosphoproteomics. The novel transcripts identified as candidate mediators of key transitions help define the “known unknown” of thymocyte differentiation.

Architecture of a lymphomyeloid developmental switch controlled by PU.1, Notch and Gata3

Hematopoiesis is a classic system with which to study developmental potentials and to investigate gene regulatory networks that control choices among alternate lineages. T-cell progenitors seeding the thymus retain several lineage potentials. The transcription factor PU.1 is involved in the decision to become a T cell or a myeloid cell, and the developmental outcome of expressing PU.1 is dependent on exposure to Notch signaling. PU.1-expressing T-cell progenitors without Notch signaling often adopt a myeloid program, whereas those exposed to Notch signals remain in a T-lineage pathway. Here, we show that Notch signaling does not alter PU.1 transcriptional activity by degradation/alteration of PU.1 protein. Instead, Notch signaling protects against the downregulation of T-cell factors so that a T-cell transcriptional network is maintained. Using an early T-cell line, we describe two branches of this network. The first involves inhibition of E-proteins by PU.1 and the resulting inhibition of Notch signaling target genes. Effects of E-protein inhibition can be reversed by exposure to Notch signaling. The second network is dependent on the ability of PU.1 to inhibit important T-cell transcription factor genes such as Myb, Tcf7 and Gata3 in the absence of Notch signaling. We show that maintenance of Gata3 protein levels by Myb and Notch signaling is linked to the ability to retain T-cell identity in response to PU.1.

BCL11B suppression does not influence CD34(+) cell differentiation and proliferation

The B-cell chronic lymphocytic leukemia (CLL)/lymphoma 11B (BCL11B) gene plays a critical role in T-cell differentiation and proliferation. However, little is understood about the role of BCL11B in human hematopoietic stem/progenitor cells. Small interfering RNA (siRNA)-mediated suppression of the BCL11B was shown to induce apoptosis in human T-cell acute lymphoblastic leukemia cells. To further characterize the role of BCL11B in hematopoietic stem/progenitor cells and assess the safety of siRNA-mediated targeted therapy, the in vitro differentiation and proliferation of CD34(+) cells after BCL11B-siRNA935 treatment were studied. CD34(+) cells were sorted from three cases of umbilical cord blood by the magnetic activated cell sorting technique, and the purity was identified by flow cytometry. BCL11B-siRNA935 was delivered into CD34(+) cells by nucleofection and the BCL11B expression level was analyzed by quantitative real-time polymerase chain reaction. Erythroid burst-forming units (BFU-E), granulocyte/macrophage colony-forming units (CFU-GM), and megakaryocyte colony-forming units (CFU-Meg) were assessed using BCL11B-siRNA935-treated CD34(+) cells by the methylcellulose semi-solid culture method. The BCL11B expression level in CD34(+) cells was significantly lower than that in Molt-4 cells and peripheral blood mononuclear cells from healthy individuals. An approximate one-fold reduction in the BCL11B mRNA level was observed 24 hours post-transfection with BCL11B-siRNA935. However, there was no significant difference on the colony formation ability of BFU-E, CFU-GM, and CFU-Meg for CD34(+) cells between the BCL11B-siRNA935-treated and mock-transfected groups (P > 0.05). BCL11B suppression by RNA interference had no significant influence on the differentiation and proliferation of CD34(+) cells. In conclusion, the BCL11B-siRNA935 used in this study may be safe, and BCL11B may be considered a new candidate for targeted gene therapy in T-cell malignancies.

Increased expression of miR-221 is associated with shorter overall survival in T-cell acute lymphoid leukemia

Background

CD56 expression has been associated with a poor prognosis in lymphoid neoplasms, including T-cell acute lymphoblastic leukemia (T-ALL). MicroRNAs (miRNAs) play an important role in lymphoid differentiation, and aberrant miRNA expression has been associated with treatment outcome in lymphoid malignancies. Here, we evaluated miRNA expression profiles in normal thymocytes, mature T-cells, and T-ALL samples with and without CD56 expression and correlated microRNA expression with treatment outcome.

Methods

The gene expression profile of 164 miRNAs were compared for T-ALL/CD56⁺ (n=12) and T-ALL/CD56^- (n=36) patients by Real-Time Quantitative PCR. Based on this analysis, we decided to evaluate miR-221 and miR-374 expression in individual leukemic and normal samples.

Results

miR-221 and miR-374 were expressed at significantly higher levels in T-ALL/CD56⁺ than in T-ALL/CD56^- cells and in leukemic blasts compared with normal thymocytes and peripheral blood (PB) T-cells. Age at diagnosis (15 or less vs grater than 15 years; HR: 2.19, 95% CI: 0.98-4.85; P=0.05), miR-221 expression level (median value as cut off in leukemic samples; HR: 3.17, 95% CI: 1.45-6.92; P=0.004), and the expression of CD56 (CD56^-vs CD56⁺; HR: 2.99, 95% CI: 1.37-6.51; P=0.006) were predictive factors for shorter overall survival; whereas, only CD56 expression (HR: 2.73, 95% CI: 1.03-7.18; P=0.041) was associated with a shorter disease-free survival rate.

Conclusions

miR-221 is highly expressed in T-ALL and its expression level may be associated with a poorer prognosis.

BCL11B is up-regulated by EWS/FLI and contributes to the transformed phenotype in Ewing sarcoma

The EWS/FLI translocation product is the causative oncogene in Ewing sarcoma and acts as an aberrant transcription factor. EWS/FLI dysregulates gene expression during tumorigenesis by abnormally activating or repressing genes. The expression levels of thousands of genes are affected in Ewing sarcoma, however, it is unknown which of these genes contribute to the transformed phenotype. Here we characterize BCL11B as an up-regulated EWS/FLI target that is necessary for the maintenance of transformation in patient derived Ewing sarcoma cells lines. BCL11B, a zinc finger transcription factor, acts as a transcriptional repressor in Ewing's sarcoma and contributes to the EWS/FLI repressed gene signature. BCL11B repressive activity is mediated by the NuRD co-repressor complex. We further demonstrate that re-expression of SPRY1, a repressed target of BCL11B, limits the transformation capacity of Ewing sarcoma cells. These data define a new pathway downstream of EWS/FLI required for oncogenic maintenance in Ewing sarcoma.

Human T-lymphoid progenitors generated in a feeder-cell-free Delta-like-4 culture system promote T-cell reconstitution in NOD/SCID/γc(-/-) mice

Slow T-cell reconstitution is a major clinical concern after transplantation of cord blood (CB)-derived hematopoietic stem cells. Adoptive transfer of in vitro-generated T-cell progenitors has emerged as a promising strategy for promoting de novo thymopoiesis and thus accelerating T-cell reconstitution. Here, we describe the development of a new culture system based on the immobilized Notch ligand Delta-like-4 (DL-4). Culture of human CD34⁺ CB cells in this new DL-4 system enabled the in vitro generation of large amounts of T-cell progenitor cells that (a) displayed the phenotypic and molecular signatures of early thymic progenitors and (b) had high T lymphopoietic potential. When transferred into NOD/SCID/γc^−/− (NSG) mice, DL-4 primed T-cell progenitors migrated to the thymus and developed into functional, mature, polyclonal αβ T cells that subsequently left the thymus and accelerated T-cell reconstitution. T-cell reconstitution was even faster and more robust when ex vivo-manipulated and nonmanipulated CB samples were simultaneously injected into NSG mice (i.e., a situation reminiscent of the double CB transplant setting). This work provides further evidence of the ability of in vitro-generated human T-cell progenitors to accelerate T-cell reconstitution and also introduces a feeder-cell-free culture technique with the potential for rapid, safe transfer to a clinical setting.

Loss of T cell progenitor checkpoint control underlies leukemia initiation in Rag1-deficient nonobese diabetic mice

NOD mice exhibit major defects in the earliest stages of T cell development in the thymus. Genome-wide genetic and transcriptome analyses were used to investigate the origins and consequences of an early T cell developmental checkpoint breakthrough in Rag1-deficient NOD mice. Quantitative trait locus analysis mapped the presence of checkpoint breakthrough cells to several known NOD diabetes susceptibility regions, particularly insulin-dependent diabetes susceptibility genes (Idd)9/11 on chromosome 4, suggesting common genetic origins for T cell defects affecting this trait and autoimmunity. Genome-wide RNA deep-sequencing of NOD and B6 Rag1-deficient thymocytes revealed the effects of genetic background prior to breakthrough, as well as the cellular consequences of the breakthrough. Transcriptome comparison between the two strains showed enrichment in differentially expressed signal transduction genes, prominently tyrosine kinase and actin-binding genes, in accord with their divergent sensitivities to activating signals. Emerging NOD breakthrough cells aberrantly expressed both stem cell-associated proto-oncogenes, such as Lmo2, Hhex, Lyl1, and Kit, which are normally repressed at the commitment checkpoint, and post-β-selection checkpoint genes, including Cd2 and Cd5. Coexpression of genes characteristic of multipotent progenitors and more mature T cells persists in the expanding population of thymocytes and in the thymic leukemias that emerge with age in these mice. These results show that Rag1-deficient NOD thymocytes have T cell defects that can collapse regulatory boundaries at two early T cell checkpoints, which may predispose them to both leukemia and autoimmunity.

The DN2 Myeloid-T (DN2mt) Progenitor is a Target Cell for Leukemic Transformation by the TLX1 Oncogene

INTRODUCTION

Inappropriate activation of the TLX1 (T-cell leukemia homeobox 1) gene by chromosomal translocation is a recurrent event in human T-cell Acute Lymphoblastic Leukemia (T-ALL). Ectopic expression of TLX1 in murine bone marrow progenitor cells using a conventional retroviral vector efficiently yields immortalized cell lines and induces T-ALL-like tumors in mice after long latency.

METHODS

To eliminate a potential contribution of retroviral insertional mutagenesis to TLX1 immortalizing and transforming function, we incorporated the TLX1 gene into an insulated self-inactivating retroviral vector.

RESULTS

Retrovirally transduced TLX1-expressing murine bone marrow progenitor cells had a growth/survival advantage and readily gave rise to immortalized cell lines. Extensive characterization of 15 newly established cell lines failed to reveal a common retroviral integration site. This comprehensive analysis greatly extends our previous study involving a limited number of cell lines, providing additional support for the view that constitutive TLX1 expression is sufficient to initiate the series of events culminating in hematopoietic progenitor cell immortalization. When TLX1-immortalized cells were co-cultured on OP9-DL1 monolayers under conditions permissive for T-cell differentiation, a latent T-lineage potential was revealed. However, the cells were unable to transit the DN2 myeloid-T (DN2mt)-DN2 T-lineage determined (DN2t) commitment step. The differentiation block coincided with failure to upregulate the zinc finger transcription factor gene Bcl11b, the human ortholog of which was shown to be a direct transcriptional target of TLX1 downregulated in the TLX1+ T-ALL cell line ALL-SIL. Other studies have described the ability of TLX1 to promote bypass of mitotic checkpoint arrest, leading to aneuploidy. We likewise found that diploid TLX1-expressing DN2mt cells treated with the mitotic inhibitor paclitaxel bypassed the mitotic checkpoint and displayed chromosomal instability. This was associated with elevated expression of TLX1 transcriptional targets involved in DNA replication and mitosis, including Ccna2 (cyclin A2), Ccnb1 (cyclin B1), Ccnb2 (cyclin B2) and Top2a (topoisomerase IIα). Notably, enforced expression of BCL11B in ALL-SIL T-ALL cells conferred resistance to the topoisomerase IIα poison etoposide.

CONCLUSION

Taken together with previous findings, the data reinforce a mechanism of TLX1 oncogenic activity linked to chromosomal instability resulting from dysregulated expression of target genes involved in mitotic processes. We speculate that repression of BCL11B expression may provide part of the explanation for the observation that aneuploid DNA content in TLX1+ leukemic T cells does not necessarily portend an unfavorable prognosis. This TLX1 hematopoietic progenitor cell immortalization/T-cell differentiation assay should help further our understanding of the mechanisms of TLX1-mediated evolution to malignancy and has the potential to be a useful predictor of disease response to novel therapeutic agents in TLX1+ T-ALL.

Dendritic cell development: a choose-your-own-adventure story

Dendritic cells (DCs) are essential components of the immune system and contribute to immune responses by activating or tolerizing T cells. DCs comprise a heterogeneous mixture of subsets that are located throughout the body and possess distinct and specialized functions. Although numerous defined precursors from the bone marrow and spleen have been identified, emerging data in the field suggests many alternative routes of DC differentiation from precursors with multilineage potential. Here, we discuss how the combinatorial expression of transcription factors can promote one DC lineage over another as well as the integration of cytokine signaling in this process.

Regulation of murine natural killer cell commitment

Natural killer (NK) cells can derive from the same precursors as B and T cells, however, to achieve lineage specificity, several transcription factors need to be activated or annulled. While a few important transcription factors have been identified for NK genesis the mechanisms of how this is achieved is far from resolved. Adding to the complexity of this, NK cells are found and potentially develop in diverse locations in vivo and it remains to be addressed if a common NK cell precursor seeds diverse niches and how transcription factors may differentially regulate NK cell commitment in distinct microenvironments. Here we will summarize some recent findings in NK cell commitment and discuss how a NK cell transcriptional network might be organized, while addressing some misconceptions and anomalies along the way.

Stem cell biology is population biology: differentiation of hematopoietic multipotent progenitors to common lymphoid and myeloid progenitors

The hematopoietic stem cell (HSC) system is a demand control system, with the demand coming from the organism, since the products of the common myeloid and lymphoid progenitor (CMP, CLP respectively) cells are essential for activity and defense against disease. We show how ideas from population biology (combining population dynamics and evolutionary considerations) can illuminate the feedback control of the HSC system by the fully differentiated products, which has recently been verified experimentally. We develop models for the penultimate differentiation of HSC Multipotent Progenitors (MPPs) into CLP and CMP and introduce two concepts from population biology into stem cell biology. The first concept is the Multipotent Progenitor Commitment Response (MPCR) which is the probability that a multipotent progenitor cell follows a CLP route rather than a CMP route. The second concept is the link between the MPCR and a measure of Darwinian fitness associated with organismal performance and the levels of differentiated lymphoid and myeloid cells. We show that many MPCRs are consistent with homeostasis, but that they will lead to different dynamics of cells and signals following a wound or injury and thus have different consequences for Darwinian fitness. We show how coupling considerations of life history to dynamics of the HSC system and its products allows one to compute the selective pressures on cellular processes. We discuss ways that this framework can be used and extended.

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.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

Nuclear location and the control of developmental progression

It is now well established that the mammalian genome is highly organized. Chromosomes are structured as territories that only sporadically intermingle. Chromosome territories themselves are segregated into distinct environments, that is, the transcriptionally inert/repressive (heterochromatic) and permissive (euchromatic) compartments. The transcriptionally permissive compartment is organized into domains (∼0.5-3 Mb) that consist of bundles of loops, are gene-rich and closely associated by activating epigenetic marks. During ontogeny and developmental progression chromatin states are highly dynamic. Recent studies have shown that loci and domains readily switch compartments. Switching nuclear neighborhoods is closely associated with changes in transcriptional activity and extensive chromatin reorganization. Here we discuss the implications of a dynamic genome and how it relates to the control of developmental progression.

Bcl11a is essential for lymphoid development and negatively regulates p53

Bcl11a regulates development of lymphoid cells in adult mice in part by inhibiting expression of p53.

Transcription factors play important roles in lymphopoiesis. We have previously demonstrated that Bcl11a is essential for normal lymphocyte development in the mouse embryo. We report here that, in the adult mouse, Bcl11a is expressed in most hematopoietic cells and is highly enriched in B cells, early T cell progenitors, common lymphoid progenitors (CLPs), and hematopoietic stem cells (HSCs). In the adult mouse, Bcl11a deletion causes apoptosis in early B cells and CLPs and completely abolishes the lymphoid development potential of HSCs to B, T, and NK cells. Myeloid development, in contrast, is not obviously affected by the loss of Bcl11a. Bcl11a regulates expression of Bcl2, Bcl2-xL, and Mdm2, which inhibits p53 activities. Overexpression of Bcl2 and Mdm2, or p53 deficiency, rescues both lethality and proliferative defects in Bcl11a-deficient early B cells and enables the mutant CLPs to differentiate to lymphocytes. Bcl11a is therefore essential for lymphopoiesis and negatively regulates p53 activities. Deletion of Bcl11a may represent a new approach for generating a mouse model that completely lacks an adaptive immune system.

Global changes in the nuclear positioning of genes and intra- and interdomain genomic interactions that orchestrate B cell fate

The genome is folded into domains located in either transcriptionally inert or permissive compartments. Here we used genome-wide strategies to characterize domains during B cell development. Structured Interaction Matrix Analysis revealed that CTCF occupancy was primarily associated with intra-domain interactions, whereas p300, E2A and PU.1 bound sites were associated with intra- and inter-domain interactions that are developmentally regulated. We identified a spectrum of genes that switched nuclear location during early B cell development. In progenitors the transcriptionally inactive Ebf1 locus was sequestered at the nuclear lamina, thereby preserving multipotency. Upon development into the pro-B cell stage Ebf1 and other genes switched compartments to establish de novo intra- and inter-domain interactions that are associated with a B lineage specific transcription signature.

Notch signalling inhibits CD4 expression during initiation and differentiation of human T cell lineage

The Delta/Notch signal transduction pathway is central to T cell differentiation from haemopoietic stem cells (HSCs). Although T cell development is well characterized using expression of cell surface markers, the detailed mechanisms driving differentiation have not been established. This issue becomes central with observations that adult HSCs exhibit poor differentiation towards the T cell lineage relative to neonatal or embryonic precursors. This study investigates the contribution of Notch signalling and stromal support cells to differentiation of adult and Cord Blood (CB) human HSCs, using the Notch signalling OP9Delta co-culture system. Co-cultured cells were assayed at weekly intervals during development for phenotype markers using flow cytometry. Cells were also assayed for mRNA expression at critical developmental stages. Expression of the central thymocyte marker CD4 was initiated independently of Notch signalling, while cells grown with Notch signalling had reduced expression of CD4 mRNA and protein. Interruption of Notch signalling in partially differentiated cells increased CD4 mRNA and protein expression, and promoted differentiation to CD4⁺ CD8⁺ T cells. We identified a set of genes related to T cell development that were initiated by Notch signalling, and also a set of genes subsequently altered by Notch signal interruption. These results demonstrate that while Notch signalling is essential for establishment of the T cell lineage, at later stages of differentiation, its removal late in differentiation promotes more efficient DP cell generation. Notch signalling adds to signals provided by stromal cells to allow HSCs to differentiate to T cells via initiation of transcription factors such as HES1, GATA3 and TCF7. We also identify gene expression profile differences that may account for low generation of T cells from adult HSCs.

Using mouse models to study function of transcriptional factors in T cell development

Laboratory mice have widely been used as tools for basic biological research and models for studying human diseases. With the advances of genetic engineering and conditional knockout (CKO) mice, we now understand hematopoiesis is a dynamic stepwise process starting from hematopoietic stem cells (HSCs) which are responsible for replenishing all blood cells. Transcriptional factors play important role in hematopoiesis. In this review we compile several studies on using genetic modified mice and humanized mice to study function of transcriptional factors in lymphopoiesis, including T lymphocyte and Natural killer (NK) cell development. Finally, we focused on the key transcriptional factor Bcl11b and its function in regulating T cell specification and commitment.

Ctip2 is a dynamic regulator of epidermal proliferation and differentiation by integrating EGFR and Notch signaling

Epidermal morphogenesis results from a delicate balance between keratinocyte proliferation and differentiation, and this balance is perturbed upon deletion of transcription factor Ctip2. Here we demonstrate that Ctip2, in a cell autonomous manner, controls keratinocyte proliferation and cytoskeletal organization, and regulates the onset and maintenance of differentiation in keratinocytes in culture. Ctip2 integrates keratinocyte proliferation and the switch to differentiation by directly and positively regulating EGFR transcription in proliferating cells and Notch1 transcription in differentiating cells. In proliferative cells, the EGFR promoter is occupied by Ctip2, whereas Ctip2 is only recruited to the Notch1 promoter under differentiating conditions. Activation of EGFR signaling downregulates Ctip2 at the transcript level, whereas high calcium signaling triggers SUMOylation, ubiquitination and proteasomal degradation of Ctip2 at the protein level. Together, our findings demonstrate a novel mechanism(s) of Ctip2-mediated, coordinated control of epidermal proliferation and terminal differentiation, and identify a pathway of negative feedback regulation of Ctip2 during epidermal development.

Downregulation of BCL11A by siRNA induces apoptosis in B lymphoma cell lines

The B-cell chronic lymphocytic leukemia (CLL)/lymphoma 11A gene (BCL11A) encodes a krüppel-like zinc finger protein, which is important in thymopoiesis and has been associated with hematopoietic malignancies. In this study, we investigated whether the downregulation of BCL11A mRNA by small interference RNA (siRNA) was capable of inducing apoptosis, and tested the effect of BCL11A siRNA combined with BCL2 siRNA in B lymphoma cell lines (SUDHL6, EB1). BCL11A siRNA was transfected into SUDHL6, EB1 cells with HiPerfect transfection reagents. After transient transfection with BCL11A siRNA, the expression levels of BCL11A mRNA and protein were assayed by quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot analysis. The cell proliferation was determined by a cell counting kit-8 (CCK8) assay. Apoptosis was determined by morphological observation and flow cytometric analysis. The results showed that the expression levels of BCL11A mRNA and protein from SUDHL6, EB1 cells transfected with BCL11A siRNA decreased, compared with either the scrambled negative control siRNA group or untransfected cells group (P<0.05). Viability of cells transfected with BCL11A siRNA was less compared to cells transfected with control siRNA and untransfected SUDHL6, EB1 cells, respectively (P<0.05). BCL11A siRNA induced apoptosis in both SUDHL6 and EB1 cells. BCL11A siRNA combined with BCL2 siRNA significantly inhibited cell growth. Apoptotic rates of SUDHL6, EB1 cells treated with BCL11A siRNA combined with BCL2 siRNA significantly increased (P<0.05), compared with either the scrambled control (Sc) siRNA and BCL2 siRNA combination or BCL2 or BCL11A siRNA-treated cells alone. Findings of this study suggest the downregulation of BCL11A mRNA by siRNA was able to induce apoptosis. Moreover, BCL11A siRNA combined with BCL2 siRNA increased apoptosis in SUDHL6, EB1 cells. Thus, suppression of BCL11A expression may be a useful approach in the treatment of B lymphoma.

The role of BCL11B in hematological malignancy

The B-cell leukemia/lymphoma 11B (BCL11B) gene is a member of the BCL family which plays a crucial role in the development, proliferation, differentiation and subsequent survival of T cells. BCL11B gene alterations are related to malignant T cell transformation that occurs in hematological malignancies. Remarkably, the BCL11B gene is responsible for the regulation of the apoptotic process and cell proliferation. This review summarizes current data and knowledge concerning the alteration of BCL11B in hematological malignancies and its role as a potential target for therapies directed against T cell malignancies.

High levels of IL-7 cause dysregulation of thymocyte development

IL-7 signaling is required for thymocyte development and its loss has a severe deleterious effect on thymus function. Thymocyte-stromal cell interactions and other mechanisms tightly regulate IL-7 expression. We show that disruption of that regulation by over-expression of IL-7 inhibits T-cell development and promotes extensive B-cell lymphopoiesis in the thymus. Our data reveal that high levels of IL-7 negate Notch-1 function in thymocytes found in IL-7 transgenic mice and in co-culture with OP9-DL1 cells. While high levels of IL-7R are present on thymocytes, increased suppressor of cytokine signaling-1 expression blunts IL-7 downstream signaling, resulting in hypo-phosphorylation of proteins in the PI3K-Akt pathway. Consequently, GSK3β remains active and inhibits Notch-1 signaling as observed by decreased Hes-1 and Deltex expression in thymic progenitors. This is the first demonstration that high levels of IL-7 antagonize Notch-1 signaling and suggest that IL-7 may affect T- versus B-lineage choice in the thymus.

Coordinated regulation of transcription factor Bcl11b activity in thymocytes by the mitogen-activated protein kinase (MAPK) pathways and protein sumoylation

The transcriptional regulatory protein Bcl11b is essential for T-cell development. We have discovered a dynamic, MAPK-regulated pathway involving sequential, linked, and reversible post-translational modifications of Bcl11b in thymocytes. MAPK-mediated phosphorylation of Bcl11b was coupled to its rapid desumoylation, which was followed by a subsequent cycle of dephosphorylation and resumoylation. Additionally and notably, we report the first instance of direct identification by mass spectrometry of a site of small ubiquitin-like modifier (SUMO) adduction, Lys-679 of Bcl11b, in a protein isolated from a native, mammalian cell. Sumoylation of Bcl11b resulted in recruitment of the transcriptional co-activator p300 to a Bcl11b-repressed promoter with subsequent induction of transcription. Prolonged treatment of native thymocytes with phorbol 12,13-dibutyrate together with the calcium ionophore A23187 also promoted ubiquitination and proteasomal degradation of Bcl11b, providing a mechanism for signal termination. A Bcl11b phospho-deSUMO switch was identified, the basis of which was phosphorylation-dependent recruitment of the SUMO hydrolase SENP1 to phospho-Bcl11b, coupled to hydrolysis of SUMO-Bcl11b. These results define a regulatory pathway in thymocytes that includes the MAPK pathways and upstream signaling components, Bcl11b and the associated nucleosome remodeling and deacetylation (NuRD) complex, SENP proteins, the Bcl11b protein phosphatase 6, the sumoylation machinery, the histone acetyltransferase p300, and downstream transcriptional machinery. This pathway appears to facilitate derepression of repressed Bcl11b target genes as immature thymocytes initiate differentiation programs, biochemically linking MAPK signaling with the latter stages of T-cell development.