IL-7Ralpha and E47: independent pathways required for development of multipotent lymphoid progenitors

Smarcd1 subunit of SWI/SNF chromatin-remodeling complexes collaborates with E2a to promote murine lymphoid specification

Lymphocyte development from murine hematopoietic stem cells (HSCs) entails a loss of self-renewal capacity and a progressive restriction of developmental potential. Previous research from our laboratory suggests that specialized assemblies of ATP-dependent SWI/SNF chromatin-remodeling complexes play lineage-specific roles during murine hematopoiesis. Here, we demonstrate that the Smarcd1 subunit is essential for specification of lymphoid cell fate from multipotent progenitors. Acute deletion of Smarcd1 in murine adult hematopoiesis leads to lymphopenia, characterized by a near-complete absence of early lymphoid progenitors and mature B and T cells, while the myeloid and erythroid lineages remain unaffected. Mechanistically, we demonstrate that Smarcd1 is essential for the coordinated activation of a lymphoid gene signature in murine multipotent progenitors. This is achieved by interacting with the E2a transcription factor at proximal promoters and by regulating the activity of distal enhancers. Globally, these findings identify Smarcd1 as an essential chromatin remodeler that governs lymphoid cell fate.

Loss of thymocyte competition underlies the tumor suppressive functions of the E2a transcription factor in T-ALL

T lymphocyte acute lymphoblastic leukemia (T-ALL) is frequently associated with increased expression of the E protein transcription factor inhibitors TAL1 and LYL1. In mouse models, ectopic expression of TAL1 or LYL1 in T cell progenitors, or inactivation of E2A, is sufficient to predispose mice to develop T-ALL. How E2A suppresses thymocyte transformation is currently unknown. Here, we show that early deletion of E2a, prior to the DN3 stage, was required for robust leukemogenesis and was associated with alterations in thymus cellularity, T cell differentiation, and gene expression in immature CD4+CD8+ thymocytes. Introduction of wild-type thymocytes into mice with early deletion of E2a prevented leukemogenesis, or delayed disease onset, and impacted the expression of multiple genes associated with transformation and genome instability. Our data indicate that E2A suppresses leukemogenesis by promoting T cell development and enforcing inter-thymocyte competition, a mechanism that is emerging as a safeguard against thymocyte transformation. These studies have implications for understanding how multiple essential regulators of T cell development suppress T-ALL and support the hypothesis that thymocyte competition suppresses leukemogenesis.

Loss of thymocyte competition underlies the tumor suppressive functions of the E2a transcription factor in T lymphocyte acute lymphoblastic leukemia

T lymphocyte acute lymphoblastic leukemia (T-ALL) is frequently associated with increased expression of the E protein transcription factor inhibitors TAL1 and LYL1. In mouse models, ectopic expression of Tal1 or Lyl1 in T cell progenitors or inactivation of E2a, is sufficient to predispose mice to develop T-ALL. How E2a suppresses thymocyte transformation is currently unknown. Here, we show that early deletion of E2a , prior to the DN3 stage, was required for robust leukemogenesis and was associated with alterations in thymus cellularity, T cell differentiation, and gene expression in immature CD4+CD8+ thymocytes. Introduction of wild-type thymocytes into mice with early deletion of E2a prevented leukemogenesis, or delayed disease onset, and impacted the expression of multiple genes associated with transformation and genome instability. Our data indicate that E2a suppresses leukemogenesis by promoting T cell development and enforcing inter-thymocyte competition, a mechanism that is emerging as a safeguard against thymocyte transformation. These studies have implications for understanding how multiple essential regulators of T cell development suppress T-ALL and support the hypothesis that thymus cellularity is a determinant of leukemogenesis.

Helix-Loop-Helix Proteins in Adaptive Immune Development

The E/ID protein axis is instrumental for defining the developmental progression and functions of hematopoietic cells. The E proteins are dimeric transcription factors that activate gene expression programs and coordinate changes in chromatin organization. Id proteins are antagonists of E protein activity. Relative levels of E/Id proteins are modulated throughout hematopoietic development to enable the progression of hematopoietic stem cells into multiple adaptive and innate immune lineages including natural killer cells, B cells and T cells. In early progenitors, the E proteins promote commitment to the T and B cell lineages by orchestrating lineage specific programs of gene expression and regulating VDJ recombination of antigen receptor loci. In mature B cells, the E/Id protein axis functions to promote class switch recombination and somatic hypermutation. E protein activity further regulates differentiation into distinct CD4+ and CD8+ T cells subsets and instructs mature T cell immune responses. In this review, we discuss how the E/Id proteins define the adaptive immune system lineages, focusing on their role in directing developmental gene programs.

Tumor suppressors BTG1 and IKZF1 cooperate during mouse leukemia development and increase relapse risk in B-cell precursor acute lymphoblastic leukemia patients

Deletions and mutations affecting lymphoid transcription factor IKZF1 (IKAROS) are associated with an increased relapse risk and poor outcome in B-cell precursor acute lymphoblastic leukemia. However, additional genetic events may either enhance or negate the effects of IKZF1 deletions on prognosis. In a large discovery cohort of 533 childhood B-cell precursor acute lymphoblastic leukemia patients, we observed that single-copy losses of BTG1 were significantly enriched in IKZF1-deleted B-cell precursor acute lymphoblastic leukemia (P=0.007). While BTG1 deletions alone had no impact on prognosis, the combined presence of BTG1 and IKZF1 deletions was associated with a significantly lower 5-year event-free survival (P=0.0003) and a higher 5-year cumulative incidence of relapse (P=0.005), when compared with IKZF1-deleted cases without BTG1 aberrations. In contrast, other copy number losses commonly observed in B-cell precursor acute lymphoblastic leukemia, such as CDKN2A/B, PAX5, EBF1 or RB1, did not affect the outcome of IKZF1-deleted acute lymphoblastic leukemia patients. To establish whether the combined loss of IKZF1 and BTG1 function cooperate in leukemogenesis, Btg1-deficient mice were crossed onto an Ikzf1 heterozygous background. We observed that loss of Btg1 increased the tumor incidence of Ikzf1^+/− mice in a dose-dependent manner. Moreover, murine B cells deficient for Btg1 and Ikzf1^+/− displayed increased resistance to glucocorticoids, but not to other chemotherapeutic drugs. Together, our results identify BTG1 as a tumor suppressor in leukemia that, when deleted, strongly enhances the risk of relapse in IKZF1-deleted B-cell precursor acute lymphoblastic leukemia, and augments the glucocorticoid resistance phenotype mediated by the loss of IKZF1 function.

Flt3 signaling regulates the proliferation, survival, and maintenance of multipotent hematopoietic progenitors that generate B cell precursors

Flt3 signaling plays a crucial role in regulating the survival and differentiation of lymphoid progenitors into B cell precursors (BCPs) in bone marrow. To define further the role of Flt3 signaling in lymphoid progenitor survival, mice deficient in Flt3 ligand that also expressed a Bcl2 transgene (Eμ-bcl2tg flt3l(-/-)) were generated. Intracellular flow cytometry established transgene expression in primitive hematopoietic progenitors, including lineage-negative Sca-1(+) c-kit(+) (LSK(+)) CD27(-) cells enriched for functional hematopoietic stem cells. Compared with flt3l(-/-) mice, Eμ-bcl2tg flt3l(-/-) mice had significantly increased multipotential progenitors (MPPs), IL-7R(+) common lymphoid progenitors, and B cell precursors. To determine whether forced expression of Bcl2 was sufficient to restore lymphoid priming in the absence of Flt3 signaling Eμ-bcl2tg flt3l(-/-)rag1-gfp(+) mice were generated. Analysis of Eμ-bcl2tg flt3l(-/-)rag1-gfp(+) mice revealed that the Bcl2 transgene had no effect on lymphoid priming before CD19 expression. Thus, forced expression of a survival gene can bypass the requirement for threshold levels of Flt3 signaling requisite for lymphoid priming. Temporal Flt3 ligand (FL) replacement therapy in flt3l(-/-) mice revealed specific requirements for Flt3 signaling in the expansion and maintenance of Flt3(+hi) MPP and Flt3(+) all lymphoid progenitors, but not Flt3(+) B lymphoid progenitors (BLPs), the immediate precursors of BCPs. BCPs were restored after temporal in vivo FL treatment, albeit with delayed kinetics. Together, these results show that Flt3 regulates the proliferation, survival, and maintenance of developmental stage-specific hematopoietic progenitors that give rise to BCPs.

E2A transcription factors limit expression of Gata3 to facilitate T lymphocyte lineage commitment

The E2A transcription factors promote the development of thymus-seeding cells, but it remains unknown whether these proteins play a role in T lymphocyte lineage specification or commitment. Here, we showed that E2A proteins were required to promote T-lymphocyte commitment from DN2 thymocytes and to extinguish their potential for alternative fates. E2A proteins functioned in DN2 cells to limit expression of Gata3, which encodes an essential T-lymphocyte transcription factor whose ectopic expression can arrest T-cell differentiation. Genetic, or small interfering RNA-mediated, reduction of Gata3 rescued T-cell differentiation in the absence of E2A and restricted the development of alternative lineages by limiting the expanded self-renewal potential in E2A−/− DN2 cells. Our data support a novel paradigm in lymphocyte lineage commitment in which the E2A proteins are necessary to limit the expression of an essential lineage specification and commitment factor to restrain self-renewal and to prevent an arrest in differentiation.

Increased expression of bHLH transcription factor E2A (TCF3) in prostate cancer promotes proliferation and confers resistance to doxorubicin induced apoptosis

E2A (TCF3) is a multifunctional basic helix loop helix (bHLH), transcription factor. E2A regulates transcription of target genes by homo- or heterodimerization with cell specific bHLH proteins. In general, E2A promotes cell differentiation, acts as a negative regulator of cell proliferation in normal cells and cancer cell lines and is required for normal B-cell development. Given the diverse biological pathways regulated/influenced by E2A little is known about its expression in cancer. In this study we investigated the expression of E2A in prostate cancer. Unexpectedly, E2A immuno-histochemistry demonstrated increased E2A expression in prostate cancer as compared to normal prostate. Silencing of E2A in prostate cancer cells DU145 and PC3 led to a significant reduction in proliferation due to G1 arrest that was in part mediated by increased CDKN1A(p21) and decreased Id1, Id3 and c-myc. E2A silencing in prostate cancer cell lines also resulted in increased apoptosis due to increased mitochondrial permeability and caspase 3/7 activation. Moreover, silencing of E2A increased sensitivity to doxorubicin induced apoptosis. Based on our results, we propose that E2A could be an upstream regulator of Id1 and c-Myc which are highly expressed in prostate cancer. These results for the first time demonstrate that E2A could in fact acts as a tumor promoter at least in prostate cancer.

Aging impairs murine B cell differentiation and function in primary and secondary lymphoid tissues

Age-related changes in humoral immunity are responsible for the reduced vaccine responses observed in elderly individuals. Although aging has been shown to affect T cells, dendritic cells and macrophages and these effects significantly impact humoral responses, intrinsic alterations in B cells also occur. We here provide an overview of age-related changes in mouse B cells. In particular, we summarize data from the literature showing age-related changes in B cell differentiation in the bone marrow, in B cell marker expression and cell survival in the periphery and in the ability to make specific antibodies in both splenic and mucosal tissues. Moreover, we summarize the results from our studies showing that the ability to undergo class switch recombination, the enzyme activation-induced cytidine deaminase and the transcription factor E47 are all decreased in stimulated B cells from old mice. The defects presented in this review for aged B cells should allow the discovery of strategies for improvement of humoral immune responses in both humans and mice in the near future.

Age effects on B cells and humoral immunity in humans

Both humoral and cellular immune responses are impaired in aged individuals, leading to decreased vaccine responses. Although T cell defects occur, defects in B cells play a significant role in age-related humoral immune changes. The ability to undergo class switch recombination (CSR), the enzyme for CSR, AID (activation-induced cytidine deaminase) and the transcription factor E47 are all decreased in aged stimulated B cells. We here present an overview of age-related changes in human B cell markers and functions, and also discuss some controversies in the field of B cell aging.

Mtg16/Eto2 contributes to murine T-cell development

Mtg16/Eto2 is a transcriptional corepressor that is disrupted by t(16;21) in acute myeloid leukemia. Using mice lacking Mtg16, we found that Mtg16 is a critical regulator of T-cell development. Deletion of Mtg16 led to reduced thymocyte development in vivo, and after competitive bone marrow transplantation, there was a nearly complete failure of Mtg16(-/-) cells to contribute to thymocyte development. This defect was recapitulated in vitro as Mtg16(-/-) Lineage(-)/Sca1(+)/c-Kit(+) (LSK) cells of the bone marrow or DN1 cells of the thymus failed to produce CD4(+)/CD8(+) cells in response to a Notch signal. Complementation of these defects by reexpressing Mtg16 showed that 3 highly conserved domains were somewhat dispensable for T-cell development but required the capacity of Mtg16 to suppress E2A-dependent transcriptional activation and to bind to the Notch intracellular domain. Thus, Mtg16 integrates the activities of signaling pathways and nuclear factors in the establishment of T-cell fate specification.

IL-7 and lymphopenia

Interleukin-7 (IL-7) is a growth and anti-apoptotic factor for T-lymphocytes, with potential for clinical use in the treatment of immunodeficiencies due to loss of T-cells. Lymphopenia induced by disease (HIV infection, hemodialysis or Idiopathic CD4+ lymphopenia) or by treatment (high dose chemotherapy or depleting antibodies) for cancer or auto-immune diseases results in increased circulating levels of IL-7 which decline with T-cell recovery, however, the mechanism of such response remains to be elucidated. Furthermore, IL-7 is a major player in the regulation of peripheral T-cell homeostasis and as such is an important candidate cytokine for therapy aimed at improving T-cell reconstitution following lymphopenia. Anti- IL-7 is on the other hand proposed to treat conditions where IL-7 may play a more direct role in pathogenesis such as autoimmune disease like Rheumatoid Arthritis, Multiple Sclerosis or Inflammatory Bowel disease.

Distinct roles for E12 and E47 in B cell specification and the sequential rearrangement of immunoglobulin light chain loci

The E2A gene products, E12 and E47, are critical regulators of B cell development. However, it remains elusive whether E12 and E47 have overlapping and/or distinct functions during B lymphopoiesis. We have generated mice deficient for either E12 or E47 and examined their roles in B cell maturation. We show that E47 is essential for developmental progression at the prepro–B cell stage, whereas E12 is dispensable for early B cell development, commitment, and maintenance. In contrast, both E12 and E47 play critical roles in pre–B and immature B cells to promote immunoglobulin λ (Igλ) germline transcription as well as Igλ VJ gene rearrangement. Furthermore, we show that E12 as well as E47 is required to promote receptor editing upon exposure to self-antigen. We demonstrate that increasing levels of E12 and E47 act to induce Igλ germline transcription, promote trimethylated lysine 4 on histone 3 (H3) as well as H3 acetylation across the Jλ region, and activate Igλ VJ gene rearrangement. We propose that in the pre–B and immature B cell compartments, gradients of E12 and E47 activities are established to mechanistically regulate the sequential rearrangement of the Ig light chain genes.

Effects of fenbendazole on the murine humoral immune system

Pinworms are highly contagious parasites that have been effectively treated in laboratory rodents with fenbendazole (FBZ). Whether FBZ has any detrimental side effects that may compromise experimental results is unknown. Here we asked whether the immune systems from young and aged mice are altered under FBZ treatment. We compared control and FBZ-treated groups of young (age, 2 to 4 mo) and old (age, 22 to 24 mo) BALB/cN mice. The treated mice received a total of 4 wk (alternating-week treatment regimen) of FBZ-medicated feed. Spleen and bone marrow were collected for immunologic assays, and heart, stomach, intestines, kidneys, and liver were evaluated by histopathology. Our results indicate that FBZ treatment has significant effects on the immune systems of mice; these effects are greater in aged mice. FBZ treatment adversely affected mRNA and protein expression of E2A (a transcription factor crucial for B lymphocytes) in activated precursor B lymphocytes obtained from the bone marrow of young and old mice. These effects were reversed by 6 wk on regular feed after the end of treatment. Activated B lymphocytes from the spleens of young and old mice showed decreased function (cell proliferation, E2A mRNA and protein expression) through the last time point of FBZ treatment but recovered by 2 to 4 wk after treatment. Our findings suggest that FBZ treatment may alter sensitive immune and molecular measures as presented here, and postponing the experimental use of mice until at least 6 wk after treatment should be considered.

The effects of c-Abl mutation on developing B cell differentiation and survival

c-Abl is a widely expressed Src family protein tyrosine kinase that is activated by chromosomal translocation in certain human leukemias. While shown in various experimental systems to regulate cell division and stress responses, its biological functions remain poorly understood. Although expressed at similar levels throughout B cell development, we found that the fraction of phosphorylated, active c-Abl peaks at the pro-B stage. We went on to perform a detailed analysis of B cell development in c-Abl-deficient mice. We confirmed a striking but variable decrease in pro- and pre-B cell numbers, a decrease in pre-B cell growth and an increase in pre-B cell apoptosis. This phenotype was not rescued by transgenic expression of a functional IgHC transgene and only partially rescued by the anti-apoptosis gene Bcl-x. Unlike their wild-type counterparts, c-Abl-deficient pre-B cells show a defect in Ca(2+) flux upon cross-linking of CD19, a co-receptor known to be involved in pre-B cell receptor signaling and failed to express CD25 on the cell surface. Despite these pre-B cell-signaling defects, selection for in-frame heavy-chain rearrangements was intact in the mutant mice. Remarkably, we were able to rescue the proliferative defect by culturing cells in vitro with large amounts of rIL-7. We conclude that c-Abl is required for normal B cell differentiation and survival.

E proteins are required to activate germline transcription of the TCR Vbeta8.2 gene

Each TCR Vbeta gene is regulated by an individual Vbeta promoter, which becomes active prior to V(D) J recombination and drives germline transcription. It has been shown that Vbeta gene locus activation and recombination are dependent on the Vbeta promoter. However, transcription factors that regulate Vbeta germline transcription remain largely undefined. A major challenge in studying Vbeta gene germline transcription is the quantitative assessment of relatively low-level transcripts in T-cell progenitors. Here we used the established Vbeta8.2(CD2) knock-in mouse model to assess functions of E-protein transcription factors in Vbeta8.2 germline transcription. We show that E proteins are required for the activation but not the maintenance of the Vbeta8.2 germline transcription during thymocyte development. The activation of Vbeta8.2 germline transcription depends more on the E proteins encoded by the E2A gene than by the HEB gene. We further show that IL-7 receptor (IL-7R)-mediated signals are essential for Vbeta8.2 germline transcription. We provide evidence that IL-7R expression is only partially controlled by E2A, suggesting a role for E2A in driving Vbeta8.2 germline transcription independent of IL-7R activation.

E2A proteins promote development of lymphoid-primed multipotent progenitors

The first lymphoid-restricted progeny of hematopoietic stem cells (HSCs) are lymphoid-primed multipotent progenitors (LMPPs), which have little erythromyeloid potential but retain lymphoid, granulocyte, and macrophage differentiation capacity. Despite recent advances in the identification of LMPPs, the transcription factors essential for their generation remain to be identified. Here, we demonstrated that the E2A transcription factors were required for proper development of LMPPs. Within HSCs and LMPPs, E2A proteins primed expression of a subset of lymphoid-associated genes and prevented expression of genes that are not normally prevalent in these cells, including HSC-associated and nonlymphoid genes. E2A proteins also restricted proliferation of HSCs, MPPs, and LMPPs and antagonized differentiation of LMPPs toward the myeloid fate. Our results reveal that E2A proteins play a critical role in supporting lymphoid specification from HSCs and that the reduced generation of LMPPs underlies the severe lymphocyte deficiencies observed in E2A-deficient mice.

Extrinsic and intrinsic regulation of early natural killer cell development

Natural killer (NK) cells are lymphocytes that play a critical role in both adaptive and innate immune responses. These cells develop from multipotent progenitors in the embryonic thymus and neonatal or adult bone marrow and recent evidence suggests that a subset of these cells may develop in the thymus. Thymus- and bone marrow-derived NK cells have unique phenotypes and functional abilities supporting the hypothesis that the microenvironment dictates the outcome of NK cell development. A detailed understanding of the mechanisms controlling this developmental program will be required to determine how alterations in NK cell development lead to disease and to determine how to harness this developmental program for therapeutic purposes. In this review, we discuss some of the known extrinsic stromal-cell derived factors and cell intrinsic transcription factors that function in guiding NK cell development.

Aging down-regulates the transcription factor E2A, activation-induced cytidine deaminase, and Ig class switch in human B cells

Elderly humans have compromised humoral and cellular immune responses, which lead to reduced protection to infectious agents and to vaccines. Currently, available vaccines suboptimally protect the elderly population. The capacity to class switch the Ig H chain is critical to the effectiveness of humoral immune responses in mice and humans. We have previously shown in mice that the E2A-encoded transcription factor E47, which regulates many B cell functions, is down-regulated in old splenic B cells. This leads to a reduction in the activation-induced cytidine deaminase (AID), which is known to induce class switch recombination and Ig somatic hypermutation. The old activated murine B cells also have less AID and less switched Abs. We have extended our study here to investigate whether aging also affects Ab production and E47 and AID expression in B cells isolated from the peripheral blood of human subjects (18-86 years). Our results obtained with activated CD19(+) B cells show that the expression of E47, AID, and Iggamma1 circle transcripts progressively decrease with age. We also show an age-related decline in the percentage of switch memory B cells (IgG(+)/IgA(+)), an increase in that of naive B cells (IgG(-)/IgA(-)/CD27(-)) for most individuals, and no decrease in that of IgM memory cells in peripheral blood, consistent with our data on the decrease seen in class switch recombination in vitro. Our results provide a possible molecular mechanism for a B cell intrinsic defect in the humoral immune response with aging and suggest avenues for improvement of vaccine response in elderly humans.

Core binding factors are necessary for natural killer cell development and cooperate with Notch signaling during T-cell specification

CBFbeta is the non-DNA binding subunit of the core binding factors (CBFs). Mice with reduced CBFbeta levels display profound, early defects in T-cell but not B-cell development. Here we show that CBFbeta is also required at very early stages of natural killer (NK)-cell development. We also demonstrate that T-cell development aborts during specification, as the expression of Gata3 and Tcf7, which encode key regulators of T lineage specification, is substantially reduced, as are functional thymic progenitors. Constitutively active Notch or IL-7 signaling cannot restore T-cell expansion or differentiation of CBFbeta insufficient cells, nor can overexpression of Runx1 or CBFbeta overcome a lack of Notch signaling. Therefore, the ability of the prethymic cell to respond appropriately to Notch is dependent on CBFbeta, and both signals converge to activate the T-cell developmental program.

The ontogeny and fate of NK cells marked by permanent DNA rearrangements

A subset of NK cells bears incomplete V(D)J rearrangements, but neither the consequence to cell activities nor the precise developmental stages in which recombination occurs is known. These are important issues, as recombination errors cause cancers of the B and T lineages. Using transgenic recombination reporter mice to examine NK cell dynamics in vivo, we show that recombination(+) NK cells have distinct developmental patterns in the BM, including reduced homeostatic proliferation and diminished Stat5 phosphorylation. In the periphery, both recombination(+) and recombination(-) NK cells mediate robust functional responses including IFN-gamma production, cytolysis, and tumor homing, suggesting that NK cells with distinct developmental histories can be found together in the periphery. We also show that V(D)J rearrangement marks both human cytolytic (CD56(dim)) and immunoregulatory (CD56(bright)) populations, demonstrating the distribution of permanent DNA rearrangements across major NK cell subsets in man. Finally, direct quantification of rag transcripts throughout NK cell differentiation in both mouse and man establishes the specific developmental stages that are susceptible to V(D)J rearrangement. Together, these data demonstrate that multipotent progenitors rather than lineage-specified NK progenitors are targets of V(D)J recombination and that NK cells bearing the relics of earlier V(D)J rearrangements have different developmental dynamics but robust biological capabilities in vivo.

Basic helix-loop-helix protein E47-mediated p21Waf1/Cip1 gene expression regulates apoptosis of intestinal epithelial cells

Inhibition of ornithine decarboxylase by DFMO (alpha-difluromethylornithine) and subsequent polyamine depletion increases p21Cip1 protein, induces cell cycle arrest and confers resistance to apoptosis on intestinal epithelial cells. However, the mechanism by which polyamines regulate p21Cip1 expression and apoptosis is unknown. On the basis of the involvement of p21Cip1 as an anti-apoptotic protein, we tested the role of p21Cip1 in providing protection from apoptosis. Simultaneously, we investigated the role of E47, a basic helix-loop-helix protein, in the regulation of p21Cip1 gene transcription. Gene-specific siRNA (small interfering RNA) decreased E47 protein levels, increased p21Cip1 promoter activity and protein levels and protected cells from TNFalpha (tumour necrosis factor alpha)-induced apoptosis. Knockdown of p21Cip1 protein by siRNA resulted in cells becoming more susceptible to apoptosis. In contrast, incubation with EGF (epidermal growth factor) stimulated p21Cip1 mRNA and protein levels and rescued cells from apoptosis. During apoptosis, the level of E47 mRNA increased, causing a concomitant decrease in p21Cip1 mRNA and protein levels. Polyamine depletion decreased E47 mRNA levels and cell survival. Caspase 3-mediated cleavage of p130Cas has been implicated in p21Cip1 transcription. The progression of apoptosis led to a caspase 3-dependent cleavage of p130Cas and generated a 31 kDa fragment, which translocated to the nucleus, associated with nuclear E47 and inhibited p21Cip1 transcription. Polyamine depletion inhibited all these effects. Transient expression of the 31 kDa fragment prevented the expression of p21Cip1 protein and increased apoptosis. These results implicate p21Cip1 as an anti-apoptotic protein and suggest a role for polyamines in the regulation of p21Cip1 via the transcription repressor E47. Caspase-mediated cleavage of p130Cas generates a 31 kDa fragment, inhibits p21Cip1 transcription and acts as an amplifier of apoptotic signalling.

Tristetraprolin, a negative regulator of mRNA stability, is increased in old B cells and is involved in the degradation of E47 mRNA

We have previously shown that the E2A-encoded transcription factor E47, which regulates class switch in splenic B cells, is down-regulated in old B cells, due to increased E47 mRNA decay. At least part of the decreased stability of E47 mRNA seen in aged B cells is mediated by proteins. We have herein looked at the specific proteins responsible for the degradation of the E47 mRNA and found that tristetraprolin (TTP), a physiological regulator of mRNA expression and stability, is involved in the degradation of the E47 mRNA. Although many studies have characterized TTP expression and function in macrophages, monocytes, mast cells, and T cells, little is known about the expression and function of TTP in primary B cells. We show herein that TTP mRNA and protein expression are induced by LPS in B cells from young and old mice, the levels of TTP in old B cells always being higher than those in young B cells. Although TTP mRNA is degraded at a significantly higher rate in old B cells, TTP mRNA expression is higher in old than in young, likely due to its increased transcription. Like in macrophages, TTP protein expression and function in B cells are dependent upon p38 MAPK. We found that there is less phospho-TTP (inactive form), as well as phospho-p38, in old than in young splenic-activated B cells. This is the first report showing that TTP is involved in the degradation of the E47 mRNA and is up-regulated in old B cells.

E2A and HEB are required to block thymocyte proliferation prior to pre-TCR expression

Thymocytes undergoing TCRbeta gene rearrangements are maintained in a low or nonproliferating state during early T cell development. This block in cell cycle progression is not released until the expression of a functional pre-TCR, which is composed of a successfully rearranged TCRbeta-chain and the Pre-Talpha-chain. The regulatory molecules responsible for the coordination of these differentiation and proliferation events are currently unknown. E2A and HEB are structurally and functionally related basic helix-loop-helix transcription factors involved in T cell development. To reveal the function of E2A and HEB through the stage of pre-TCR expression and alleviate functional compensation between E2A and HEB, we use a double-conditional knockout model. The simultaneous deletion of E2A and HEB in developing thymocytes leads to a severe developmental block before pre-TCR expression and a dramatic reduction of Pre-Talpha expression. These developmentally arrested thymocytes exhibit increased proliferation in vivo and dramatic expansion ex vivo in response to IL-7 signaling. These results suggest that E2A and HEB are not only critical for T cell differentiation but also necessary to retain developing thymocytes in cell cycle arrest before pre-TCR expression.

E47 is required for V(D)J recombinase activity in common lymphoid progenitors

Common lymphoid progenitors (CLPs) are the first bone marrow precursors in which V(D)J recombinase activity is up-regulated. Here, we show that loss of the transcription factor E47 produces a reduced CLP population that lacks V(D)J recombinase activity and D-J_H rearrangements in vivo. Apart from a profound arrest before the pro–B cell stage, other downstream lymphoid progeny of CLPs are still intact in these mice albeit at reduced numbers. In contrast to the inhibition of recombinase activity in early B lineage precursors in E47-deficient animals, loss of either E47 or its cis-acting target Erag (enhancer of rag transcription) has little effect on recombinase activity in thymic T lineage precursors. Taken together, this work defines a role for E47 in regulating lineage progression at the CLP stage in vivo and describes the first transcription factor required for lineage-specific recombinase activity.

RNA Stability of the E2A-Encoded Transcription Factor E47 Is Lower in Splenic Activated B Cells from Aged Mice

We have demonstrated previously that DNA binding and protein expression of the E2A-encoded transcription factor E47 are lower in nuclear extracts of activated splenic B cells from old mice. In the present study, we address how E47 protein expression is regulated in aging. Results herein show that E2A mRNA levels were decreased in stimulated splenic B cells from old as compared with young mice. RNA stability assays showed that the rate of E2A mRNA decay was accelerated in stimulated splenic B cells from old mice, but E47 protein degradation rates were comparable in young vs aged B cells, indicating that the regulation of E47 expression in activated splenic B cells occurs primarily by mRNA stability. The rates of decay of other mRNAs showed that the increased mRNA degradation in aged splenic activated B cells is not a general phenomenon but restricted to a subset of mRNAs. We next investigated the signal transduction pathways controlling E2A mRNA expression and stability and found that p38 MAPK regulates E2A mRNA expression through increased mRNA stability and is down-regulated in aged activated B cells. Results show that inhibition of p38 MAPK significantly reduces E2A mRNA stability in both young and old B cells, further stressing the role of p38 MAPK in E2A RNA stabilization. These studies demonstrate that the transcription factor E2A, critical for many aspects of B cell function, is regulated by a novel mechanism in aging.

Helix-loop-helix proteins and lymphocyte development

Helix-loop-helix (HLH) proteins are transcriptional regulators that control a wide variety of developmental pathways in both invertebrate and vertebrate organisms. Results obtained in the past decade have shown that HLH proteins also contribute to the development of lymphoid lineages. A subset of HLH proteins, the 'E proteins', seems to be particularly important for proper lymphoid development. Members of the E protein family include E12, E47, E2-2 and HEB. The E proteins contribute to B lineage- and T lineage-specific gene expression programs, regulate lymphocyte survival and cellular proliferation, activate the rearrangement of antigen receptor genes and control progression through critical developmental checkpoints. This review discusses HLH proteins in lymphocyte development and homeostasis.

Humoral immune response and B-cell functions including immunoglobulin class switch are downregulated in aged mice and humans

Vaccinations are powerful tools for combating infections. Because of the age-related impairment in immune functions, the currently available vaccines are protecting only a small proportion of the elderly population. We, here, provide an overview of age-related changes in innate and adaptive immunity with particular emphasis to changes in antibody production with aging. We also summarize our results showing that the E2A-encoded transcription factor E47, which regulates many B cell functions including class switch recombination (CSR) and somatic hypermutation (SHM), is downregulated in splenic B cells from old mice. This leads to a reduction in the activation-induced cytidine deaminase (AID), which directly induces CSR and SHM, and, in turn, to reduced amounts of switched antibodies produced by splenic activated B cells. Our preliminary results in humans indicate similar reductions: we show herein that the expression of E2A and AID progressively decline with age. Our results provide a possible molecular basis for a decrease in the humoral immune response in aging mice and humans.

Helix-loop-helix proteins in lymphocyte lineage determination

The cells of the lymphoid system develop from multipotent hematopoietic stem cells through a series of intermediate progenitors with progressively restricted developmental options. Commitment to a given lymphoid lineage appears to be controlled by numerous transcriptional regulatory proteins that activate lineage-specific gene expression programs and extinguish expression of lineage-inappropriate genes. In this review I discuss the function of transcription factors belonging to the helix-loop-helix protein family in the control of lymphoid cell fate decisions. A model of lymphocyte lineage determination based on the antagonistic activity of transcriptional activating and repressing helix-loop-helix proteins is presented.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

Cytokine functions in the formative stages of a lymphocyte's life

Five core cytokines that control lymphocyte differentiation and maintenance have been identified and studied in depth. IL-7 sits at the apex of this cytokine hierarchy in terms of functional significance during lymphocyte development. The IL-7-dominant phase of lymphopoiesis is preceded by the actions of c-Kit ligand (also called stem cell factor; SCF) and fetal liver kinase 2 ligand (Flk-2L); the function of both of these cytokines is essential for the maintenance and development of the progenitor compartment of multiple lineages. IL-7 activity is complemented by two cytokines whose receptors share components of the IL-7 receptor: thymic stromal lymphopoietin (TSLP) and IL-15. The influences of these core cytokines on precursor lymphocyte subsets overlap during development and are often synergistic. Recent studies are beginning to uncover the molecular mechanisms of these interrelated core cytokine functions.

Early B cell factor promotes B lymphopoiesis with reduced interleukin 7 responsiveness in the absence of E2A

The basic helix-loop-helix transcription factors encoded by the E2A gene function at the apex of a transcriptional hierarchy involving E2A, early B cell factor (EBF), and Pax5, which is essential for B lymphopoiesis. In committed B lineage progenitors, E2A proteins have also been shown to regulate many lineage-associated genes. Herein, we demonstrate that the block in B lymphopoiesis imposed by the absence of E2A can be overcome by expression of EBF, but not Pax5, indicating that EBF is the essential target of E2A required for development of B lineage progenitors. Our data demonstrate that EBF, in synergy with low levels of alternative E2A-related proteins (E proteins), is sufficient to promote expression of most B lineage genes. Remarkably, however, we find that E2A proteins are required for interleukin 7-dependent proliferation due, in part, to a role for E2A in optimal expression of N-myc. Therefore, high levels of E protein activity are essential for the activation of EBF and N-myc, whereas lower levels of E protein activity, in synergy with other B lineage transcription factors, are sufficient for expression of most B lineage genes.

Transcriptional control of early B cell development

The generation of B-lymphocytes from hematopoietic stem cells is controlled by multiple transcription factors regulating distinct developmental aspects. Ikaros and PU.1 act in parallel pathways to control the development of lymphoid progenitors in part by regulating the expression of essential signaling receptors (Flt3, c-Kit, and IL-7R alpha). The generation of the earliest B cell progenitors depends on E2A and EBF, which coordinately activate the B cell gene expression program and immunoglobulin heavy-chain gene rearrangements at the onset of B-lymphopoiesis. Pax5 restricts the developmental options of lymphoid progenitors to the B cell lineage by repressing the transcription of lineage-inappropriate genes and simultaneously activating the expression of B-lymphoid signaling molecules. LEF1 and Sox4 contribute to the survival and proliferation of pro-B cells in response to extracellular signals. Finally, IRF4 and IRF8 together control the termination of pre-B cell receptor signaling and thus promote differentiation to small pre-B cells undergoing light-chain gene rearrangements.

Leukaemia - a developmental perspective

Leukaemia is characterized by the accumulation of malignant haematopoietic precursors. Recent studies have revealed that acquired alterations in genes that regulate normal haematopoiesis are frequently detected in leukaemia. The progression to leukaemia depends on additional mutations that promote the survival of developmentally arrested cells. This review describes three examples of this general paradigm of leukaemogenesis: RUNX1 abnormalities in acute leukaemias, GATA1 mutations in the leukaemias of Down syndrome, and SCL and LMO2 ectopic expression in T cell acute lymphoblastic leukaemia.

Age-related differences in the E2A-encoded transcription factor E47 in bone marrow-derived B cell precursors and in splenic B cells

We have investigated the effects of aging on the E2A-encoded transcription factor E47, a key regulator of B cell functions, in B cell precursors and in splenic B cells. Here, we show that old mice can be classified as severely depleted, moderately depleted or not depleted mice, according to the percentage of pre-B cells in their bone marrow. IL-7-expanded populations of pro-B/early pre-B cells from bone marrow of both severely depleted and moderately depleted old mice exhibit a reduced E47 DNA-binding and expression compared to young mice, and this defect in severely depleted old mice is more dramatic than that in moderately depleted old mice. However, mRNA levels were comparable, suggesting that E47 in the bone marrow is not transcriptionally regulated. In the spleen, activated B cells from both severely depleted and moderately depleted old mice show a lower E47 DNA-binding and expression than young mice. However, in contrast to precursor B cells, E47 DNA-binding and expression are similarly and only moderately reduced in both severely depleted and in moderately depleted mice. The mRNA levels were found to be decreased in stimulated splenic B cells from old as compared to young mice, suggesting that E47 mRNA in the spleen may be both transcriptionally and/or post-transcriptionally regulated.

B lineage-specific regulation of V(D)J recombinase activity is established in common lymphoid progenitors

Expression of V(D)J recombinase activity in developing lymphocytes is absolutely required for initiation of V(D)J recombination at antigen receptor loci. However, little is known about when during hematopoietic development the V(D)J recombinase is first active, nor is it known what elements activate the recombinase in multipotent hematopoietic progenitors. Using mice that express a fluorescent transgenic V(D)J recombination reporter, we show that the V(D)J recombinase is active as early as common lymphoid progenitors (CLPs) but not in the upstream progenitors that retain myeloid lineage potential. Evidence of this recombinase activity is detectable in all four progeny lineages (B, T, and NK, and DC), and rag2 levels are the highest in progenitor subsets immediately downstream of the CLP. By single cell PCR, we demonstrate that V(D)J rearrangements are detectable at IgH loci in ∼5% of splenic natural killer cells. Finally, we show that recombinase activity in CLPs is largely controlled by the Erag enhancer. As activity of the Erag enhancer is restricted to the B cell lineage, this provides the first molecular evidence for establishment of a lineage-specific transcription program in multipotent progenitors.

Reduced Ig Class Switch in Aged Mice Correlates with Decreased E47 and Activation-Induced Cytidine Deaminase

The capacity to class switch the IgH chain is critical to the effectiveness of humoral immune responses. We show that in vitro-stimulated splenic B cells from senescent mice are deficient in production of multiple class switch isotypes (IgG1, G2a, G3, and E), class switch recombination (CSR), and induction of the E2A-encoded transcription factor E47. E47 has previously been shown to be required for CSR, at least in part via expression of the activation-induced cytidine deaminase. Our studies show that impaired induction of E47, and subsequently activation-induced cytidine deaminase, contribute to poor CSR and production of secondary isotypes in senescence.

The 31-kDa Caspase-generated Cleavage Product of p130 Functions as a Transcriptional Repressor of E2A in Apoptotic Cells

In response to integrin receptor binding to the extracellular matrix, the multidomain docking protein p130(cas) (Crk-associated substrate) activates various signaling cascades modulating such cellular processes as proliferation, migration, and apoptosis. During apoptosis, caspase-mediated cleavage of p130(cas) generated a C-terminal 31-kDa fragment (31-kDa) and promoted morphological changes characteristic of apoptosis, including loss of focal adhesions, cell rounding, and nuclear condensation and fragmentation. By contrast, a p130(cas) D748E mutant, which was unable to produce 31-kDa, attenuated the disassembly of focal adhesions at the bottom of the cell. 31-kDa contains a helix-loop-helix (HLH) domain that shows greater sequence homology with Id proteins than with basic HLH proteins, which enabled heterodimerization with E2A. Once coupled to E2A, 31-kDa was translocated to the cell nucleus, where it inhibited E2A-mediated p21(Waf1/Cip1) transcription. Moreover, overexpression of 31-kDa led to cell death that could be inhibited by treatment with the caspase inhibitor ZVAD-fluoromethyl ketone or by ectopic expression of E2A or p21(Waf1/Cip1). These data suggest that during etoposide-induced apoptosis, 31-kDa promotes caspase-mediated cell death by inhibiting E2A-mediated activation of p21(Waf1/Cip1) transcription.

Regulatory coding of lymphoid lineage choice by hematopoietic transcription factors

During lymphopoiesis, precursor cells negotiate a complex regulatory space, defined by the levels of several competing and cross-regulating transcription factors, before arriving at stable states of commitment to the B-, T- and NK-specific developmental programs. Recent perturbation experiments provide evidence that this space has three major axes, corresponding to the PU.1 versus GATA-1 balance, the intensity of Notch signaling through the CSL pathway, and the ratio of E-box transcription factors to their Id protein antagonists.

Effects of aging on proliferation and E47 transcription factor activity induced by different stimuli in murine splenic B cells

In the present paper, we have investigated the effects of aging on the expression and function of the E2A-encoded transcription factor E47 in splenic B lymphocytes, unactivated or activated with different stimuli (LPS, anti-CD40, anti-IgM, alone or in combination with IL-4). Results indicate that unstimulated splenic B cells show very low E47 protein levels as well as E47 DNA-binding activity and that, upon B cell activation, E47 expression and DNA-binding activity are strongly induced in young and, to a significantly lesser extent, in old mice. The level of E47 protein expression in stimulated splenic B cells was found significantly higher in young than in old mice, suggesting that DNA-binding activity correlates with protein expression. These results altogether suggest that the reduced expression of the transcriptional regulator E47 could help explain the reduced B cell functions in aging mice.

Regulation of early lymphocyte development by E2A family proteins

Lymphocytes develop from hematopoietic stem cells through a series of highly regulated differentiation events in the bone marrow and thymus. A number of transcription factors are known to collaborate in controlling the timing and specificity of gene expression required for these developmental processes to occur. The basic helix-loop-helix (bHLH) proteins encoded by the E2A gene have been shown to play particularly important roles in the initiation and progression of lymphocyte differentiation. Gene targeting experiments in mice have demonstrated a requirement for E2A proteins at the onset of B lymphocyte development. More recent studies have broadened our view on the function of E2A proteins at multiple stages of lymphopoiesis and in the regulation of lymphoid-specific gene expression. Here we review the mammalian E2A proteins and the accumulated evidence demonstrating central roles for E2A throughout early B and T lymphocyte development. We also speculate on the direction of future research on the mechanisms underlying the lineage and stage-specific functions of E2A in lymphopoiesis.

T-lineage specification and commitment: a gene regulation perspective

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