Receptor editing and marginal zone B cell development are regulated by the helix-loop-helix protein, E2A

Kidins220 regulates the development of B cells bearing the λ light chain

The ratio between κ and λ light chain (LC)-expressing B cells varies considerably between species. We recently identified Kinase D-interacting substrate of 220 kDa (Kidins220) as an interaction partner of the BCR. In vivo ablation of Kidins220 in B cells resulted in a marked reduction of λLC-expressing B cells. Kidins220 knockout B cells fail to open and recombine the genes of the Igl locus, even in genetic scenarios where the Igk genes cannot be rearranged or where the κLC confers autoreactivity. Igk gene recombination and expression in Kidins220-deficient B cells is normal. Kidins220 regulates the development of λLC B cells by enhancing the survival of developing B cells and thereby extending the time-window in which the Igl locus opens and the genes are rearranged and transcribed. Further, our data suggest that Kidins220 guarantees optimal pre-BCR and BCR signaling to induce Igl locus opening and gene recombination during B cell development and receptor editing.

TCF3 haploinsufficiency defined by immune, clinical, gene-dosage, and murine studies

BACKGROUND

TCF3 is a transcription factor contributing to early lymphocyte differentiation. Germline monoallelic dominant negative and biallelic loss-of-function (LOF) null TCF3 mutations cause a fully penetrant severe immunodeficiency. We identified 8 individuals from 7 unrelated families with monoallelic LOF TCF3 variants presenting with immunodeficiency with incomplete clinical penetrance.

OBJECTIVE

We sought to define TCF3 haploinsufficiency (HI) biology and its association with immunodeficiency.

METHODS

Patient clinical data and blood samples were analyzed. Flow cytometry, Western blot analysis, plasmablast differentiation, immunoglobulin secretion, and transcriptional activity studies were conducted on individuals carrying TCF3 variants. Mice with a heterozygous Tcf3 deletion were analyzed for lymphocyte development and phenotyping.

RESULTS

Individuals carrying monoallelic LOF TCF3 variants showed B-cell defects (eg, reduced total, class-switched memory, and/or plasmablasts) and reduced serum immunoglobulin levels; most but not all presented with recurrent but nonsevere infections. These TCF3 LOF variants were either not transcribed or translated, resulting in reduced wild-type TCF3 protein expression, strongly suggesting HI pathophysiology for the disease. Targeted RNA sequencing analysis of T-cell blasts from TCF3-null, dominant negative, or HI individuals clustered away from healthy donors, implying that 2 WT copies of TCF3 are needed to sustain a tightly regulated TCF3 gene-dosage effect. Murine TCF3 HI resulted in a reduction of circulating B cells but overall normal humoral immune responses.

CONCLUSION

Monoallelic LOF TCF3 mutations cause a gene-dosage-dependent reduction in wild-type protein expression, B-cell defects, and a dysregulated transcriptome, resulting in immunodeficiency. Tcf3+/- mice partially recapitulate the human phenotype, underscoring the differences between TCF3 in humans and mice.

Asymmetrical forward and reverse developmental trajectories determine molecular programs of B cell antigen receptor editing

During B lymphopoiesis, B cell progenitors progress through alternating and mutually exclusive stages of clonal expansion and immunoglobulin (Ig) gene rearrangements. Great diversity is generated through the stochastic recombination of Ig gene segments encoding heavy and light chain variable domains. However, this commonly generates autoreactivity. Receptor editing is the predominant tolerance mechanism for self-reactive B cells in the bone marrow (BM). B cell receptor editing rescues autoreactive B cells from negative selection through renewed light chain recombination first at Igκ then Igλ loci. Receptor editing depends on BM microenvironment cues and key transcription factors such as NF-κB, FOXO, and E2A. The specific BM factor required for receptor editing is unknown. Furthermore, how transcription factors coordinate these developmental programs to promote usage of the λ chain remains poorly defined. Therefore, we used two mouse models that recapitulate pathways by which Igλ light chain-positive B cells develop. The first has deleted J kappa (Jκ) genes and hence models Igλ expression resulting from failed Igκ recombination (Igκdel). The second models autoreactivity by ubiquitous expression of a single-chain chimeric anti-Igκ antibody (κ-mac). Here, we demonstrated that autoreactive B cells transit asymmetric forward and reverse developmental trajectories. This imparted a unique epigenetic landscape on small pre-B cells, which opened chromatin to transcription factors essential for Igλ recombination. The consequences of this asymmetric developmental path were both amplified and complemented by CXCR4 signaling. These findings reveal how intrinsic molecular programs integrate with extrinsic signals to drive receptor editing.

Tnpo3 enables EBF1 function in conditions of antagonistic Notch signaling

Transcription factor EBF1 (early B cell factor 1) acts as a key regulator of B cell specification. The transcriptional network in which EBF1 operates has been extensively studied; however, the regulation of EBF1 function remains poorly defined. By mass spectrometric analysis of proteins associated with endogenous EBF1 in pro-B cells, we identified the nuclear import receptor Transportin-3 (Tnpo3) and found that it interacts with the immunoglobulin-like fold domain of EBF1. We delineated glutamic acid 271 of EBF1 as a critical residue for the association with Tnpo3. EBF1E271A showed normal nuclear localization; however, it had an impaired B cell programming ability in conditions of Notch signaling, as determined by retroviral transduction of Ebf1 -/- progenitors. By RNA-seq analysis of EBF1E271A-expressing progenitors, we found an up-regulation of T lineage determinants and down-regulation of early B genes, although similar chromatin binding of EBF1E271A and EBF1wt was detected in pro-B cells expressing activated Notch1. B lineage-specific inactivation of Tnpo3 in mice resulted in a block of early B cell differentiation, accompanied by a down-regulation of B lineage genes and up-regulation of T and NK lineage genes. Taken together, our observations suggest that Tnpo3 ensures B cell programming by EBF1 in nonpermissive conditions.

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.

Bursty gene expression and mRNA decay pathways orchestrate B cell activation

It is well established that the helix-loop-helix proteins, E2A and E2-2, promote B cell activation. Here, we examined how during the course of B cell activation E2A and E2-2 gene expression is regulated. We found that E2A and E2-2 mRNA abundance concomitantly increased in activated B cells. The increase in E2A and E2-2 mRNA abundance correlated with increased cell growth. Elevated E2A and E2-2 mRNA abundance was instructed by increased transcriptional bursting frequencies and elevated E2A and E2-2 mRNA half-lives. The increase in E2A and E2-2 bursting frequencies often occurred at shared interchromosomal transcriptional hubs. We suggest that in naïve B cells low E2A and E2-2 bursting frequencies and high E2A and E2-2 mRNA decay rates instruct noisy gene expression that allows a clonal and swift response to invading pathogens whereas in activated B cells increased transcriptional bursting and low mRNA decay rates dictate an activated B lineage gene program.

Transcriptional Control of Mature B Cell Fates

The mature naïve B cell repertoire consists of three well-defined populations: B1, B2 (follicular B, FOB), and marginal zone B (MZB) cells. FOB cells are the dominant mature B cell population in the secondary lymphoid organs and blood of both humans and mice. The driving forces behind mature B lineage selection have been linked to B cell receptor (BCR) signaling strength and environmental cues, but how these fate-determination factors are transcriptionally regulated remains poorly understood. We summarize emerging data on the role of transcription factors (TFs) - particularly the ETS and IRF families - in regulating MZB and FOB lineage selection. Indeed, genomic analyses have identified four major groups of target genes that are crucial for FOB differentiation, revealing previously unrecognized pathways that ultimately determine biological responses specific to this lineage.

Enhancers as regulators of antigen receptor loci three-dimensional chromatin structure

Enhancers are defined as regulatory elements that control transcription in a cell-type and developmental stage-specific manner. They achieve this by physically interacting with their cognate gene promoters. Significantly, these interactions can occur through long genomic distances since enhancers may not be near their cognate promoters. The optimal coordination of enhancer-regulated transcription is essential for the function and identity of the cell. Although great efforts to fully understand the principles of this type of regulation are ongoing, other potential functions of the long-range chromatin interactions (LRCIs) involving enhancers are largely unexplored. We recently uncovered a new role for enhancer elements in determining the three-dimensional (3D) structure of the immunoglobulin kappa (Igκ) light chain receptor locus suggesting a structural function for these DNA elements. This enhancer-mediated locus configuration shapes the resulting Igκ repertoire. We also propose a role for enhancers as critical components of sub-topologically associating domain (subTAD) formation and nuclear spatial localization.

LSD1 Cooperates with Noncanonical NF-κB Signaling to Regulate Marginal Zone B Cell Development

Marginal zone B cells (MZB) are a mature B cell subset that rapidly respond to blood-borne pathogens. Although the transcriptional changes that occur throughout MZB development are known, the corresponding epigenetic changes and epigenetic modifying proteins that facilitate these changes are poorly understood. The histone demethylase LSD1 is an epigenetic modifier that promotes plasmablast formation, but its role in B cell development has not been explored. In this study, a role for LSD1 in the development of B cell subsets was examined. B cell-conditional deletion of LSD1 in mice resulted in a decrease in MZB whereas follicular B cells and bone marrow B cell populations were minimally affected. LSD1 repressed genes in MZB that were normally upregulated in the myeloid and follicular B cell lineages. Correspondingly, LSD1 regulated chromatin accessibility at the motifs of transcription factors known to regulate splenic B cell development, including NF-κB motifs. The importance of NF-κB signaling was examined through an ex vivo MZB development assay, which showed that both LSD1-deficient and NF-κB-inhibited transitional B cells failed to undergo full MZB development. Gene expression and chromatin accessibility analyses of in vivo- and ex vivo-generated LSD1-deficient MZB indicated that LSD1 regulated the downstream target genes of noncanonical NF-κB signaling. Additionally LSD1 was found to interact with the noncanonical NF-κB transcription factor p52. Together, these data reveal that the epigenetic modulation of the noncanonical NF-κB signaling pathway by LSD1 is an essential process during the development of MZB.

The Concerted Action of E2-2 and HEB Is Critical for Early Lymphoid Specification

The apparition of adaptive immunity in Gnathostomata correlates with the expansion of the E-protein family to encompass E2-2, HEB, and E2A. Within the family, E2-2 and HEB are more closely evolutionarily related but their concerted action in hematopoiesis remains to be explored. Here we show that the combined disruption of E2-2 and HEB results in failure to express the early lymphoid program in Common lymphoid precursors (CLPs) and a near complete block in B-cell development. In the thymus, Early T-cell progenitors (ETPs) were reduced and T-cell development perturbed, resulting in reduced CD4 T- and increased γδ T-cell numbers. In contrast, hematopoietic stem cells (HSCs), erythro-myeloid progenitors, and innate immune cells were unaffected showing that E2-2 and HEB are dispensable for the ancestral hematopoietic lineages. Taken together, this E-protein dependence suggests that the appearance of the full Gnathostomata E-protein repertoire was critical to reinforce the gene regulatory circuits that drove the emergence and expansion of the lineages constituting humoral immunity.

Helix-loop-helix proteins and the advent of cellular diversity: 30 years of discovery

In this review from Murre, the evolution of HLH genes, the structures of HLH domains, and the elaborate activities of HLH proteins in multicellular life are discussed.

Helix–loop–helix (HLH) proteins are dimeric transcription factors that control lineage- and developmental-specific gene programs. Genes encoding for HLH proteins arose in unicellular organisms >600 million years ago and then duplicated and diversified from ancestral genes across the metazoan and plant kingdoms to establish multicellularity. Hundreds of HLH proteins have been identified with diverse functions in a wide variety of cell types. HLH proteins orchestrate lineage specification, commitment, self-renewal, proliferation, differentiation, and homing. HLH proteins also regulate circadian clocks, protect against hypoxic stress, promote antigen receptor locus assembly, and program transdifferentiation. HLH proteins deposit or erase epigenetic marks, activate noncoding transcription, and sequester chromatin remodelers across the chromatin landscape to dictate enhancer–promoter communication and somatic recombination. Here the evolution of HLH genes, the structures of HLH domains, and the elaborate activities of HLH proteins in multicellular life are discussed.

Mechanisms of central tolerance for B cells

Immune tolerance hinders the potentially destructive responses of lymphocytes to host tissues. Tolerance is regulated at the stage of immature B cell development (central tolerance) by clonal deletion, involving apoptosis, and by receptor editing, which reprogrammes the specificity of B cells through secondary recombination of antibody genes. Recent mechanistic studies have begun to elucidate how these divergent mechanisms are controlled. Single-cell antibody cloning has revealed defects of B cell central tolerance in human autoimmune diseases and in several human immunodeficiency diseases caused by single gene mutations, which indicates the relevance of B cell tolerance to disease and suggests possible genetic pathways that regulate tolerance.

Id3 Orchestrates Germinal Center B Cell Development

Previous studies have demonstrated that E proteins induce activation-induced deaminase (AID) expression in activated B cells. Here, we examined the role of Id3 in germinal center (GC) cells. We found that Id3 expression is high in follicular B lineage cells but declines in GC cells. Immunized mice with Id3 expression depleted displayed a block in germinal center B cell maturation, showed reduced numbers of marginal zone B cells and class-switched cells, and were associated with decreased antibody titers and lower numbers of plasma cells. In vitro, Id3-depleted B cells displayed a defect in class switch recombination. Whereas AID levels were not altered in Id3-depleted activated B cells, the expression of a subset of genes encoding signaling components of antigen receptor-, cytokine receptor-, and chemokine receptor-mediated signaling was significantly impaired. We propose that during the GC reaction, Id3 levels decline to activate the expression of genes encoding signaling components that mediate B cell receptor- and or cytokine receptor-mediated signaling to promote the differentiation of GC B cells.

Analysis of IgM antibody production and repertoire in a mouse model of Sjögren's syndrome

This study tested the hypothesis that B cells from salivary tissue are distinct in terms of proliferative capacity, immunoglobulin M secretion, repertoire, and autoantibody enrichment in Sjögren's syndrome. We sorted purified B cells from the spleen, cervical lymph nodes, and submandibular glands of a primary Sjögren's syndrome mouse model (Id3(-/-)). Enzyme-linked immunospot and proliferation assays were performed with stimulated B cells. We single-cell sorted B cells from the spleen, cervical lymph nodes, and submandibular gland tissue from Sjögren's syndrome mice and sequenced immunoglobulin M heavy-chain variable regions. Finally, autoantigen arrays were performed using immunoglobulin M derived from sera, cervical lymph nodes, spleens, and submandibular gland tissue of Id3(-/-) animals. Results suggest B cells from salivary tissue of Sjögren's syndrome mice are similar to those from secondary immune sites in terms of proliferative and secretory capacity. However, differences in repertoire usage, heavy chain complementarity-determining region 3 length, mutational frequency, and N region addition were observed among B cells derived from submandibular gland, cervical lymph node, and spleen tissue. Moreover, autoantigen array data show immunoglobulin M from salivary B cells have enriched specificity for Ro (Sjögren's syndrome A) and La (Sjögren's syndrome B). All together, these data suggest salivary B cells have unique repertoire characteristics that likely influence autoantigen binding and contribute to Sjögren's syndrome disease in a tissue-specific manner.

CNOT3 contributes to early B cell development by controlling Igh rearrangement and p53 mRNA stability

Inoue et al. report that CNOT3, a subunit of the CCR4–NOT deadenylase complex regulating mRNA decay and translational repression, controls Igh gene rearrangement and destabilizes the mRNA of the tumor suppressor p53. Loss of CNOT3 results in a block of pro- to pre–B cell transition.

The CCR4–NOT deadenylase complex plays crucial roles in mRNA decay and translational repression induced by poly(A) tail shortening. Although the in vitro activities of each component of this complex have been well characterized, its in vivo role in immune cells remains unclear. Here we show that mice lacking the CNOT3 subunit of this complex, specifically in B cells, have a developmental block at the pro- to pre–B cell transition. CNOT3 regulated generation of germline transcripts in the V_H region of the immunoglobulin heavy chain (Igh) locus, compaction of the locus, and subsequent Igh gene rearrangement and destabilized tumor suppressor p53 mRNA. The developmental defect in the absence of CNOT3 could be partially rescued by ablation of p53 or introduction of a pre-rearranged Igh transgene. Thus, our data suggest that the CCR4–NOT complex regulates B cell differentiation by controlling Igh rearrangement and destabilizing p53 mRNA.

Chronic lymphocytic leukaemia: could immunological tolerance mechanisms be the origin of lymphoid neoplasms?

Immunological tolerance theory in chronic lymphocytic leukaemia (CLL): we suggest that B cells that express B-cell receptors (BCR) that recognize their own BCR epitopes are viewed by immune system as 'dangerous cells'. BCR autonomous signalling may induce constant receptor editing and mistakes in allelic exclusion. The fact that whole BCR recognizes a self-antigen or foreing antigen may be irrelevant in early B cell development. In early B cells, autonomous signalling induced by recognition of the BCR's own epitopes simulates an antigen-antibody engagement. In the bone marrow this interaction is viewed as recognition of self-molecules and induces receptor editing. In mature B cells autonomous signalling by the BCR may promote 'reversible anergy' and also may correct self-reactivity induced by the somatic hypermutation mechanisms in mutated CLL B cells. However, in unmutated CLL B cells, BCR autonomous signalling in addition to self-antigen recognition augments B cell activation, proliferation and genomic instability. We suggest that CLL originates from a coordinated normal immunologic tolerance mechanism to destroy self-reactive B cells. Additional genetic damage induced by tolerance mechanisms may immortalize self-reactive B cells and transform them into a leukemia.

Id2 and Id3 maintain the regulatory T cell pool to suppress inflammatory disease

Regulatory T (Treg) cells suppress the development of inflammatory disease, but our knowledge of transcriptional regulators that control this function remains incomplete. Here we show that expression of Id2 and Id3 in Treg cells was required to suppress development of fatal inflammatory disease. We found that T cell antigen receptor (TCR)-driven signaling initially decreased the abundance of Id3, which led to the activation of a follicular regulatory T (TFR) cell-specific transcription signature. However, sustained lower abundance of Id2 and Id3 interfered with proper development of TFR cells. Depletion of Id2 and Id3 expression in Treg cells resulted in compromised maintenance and localization of the Treg cell population. Thus, Id2 and Id3 enforce TFR cell checkpoints and control the maintenance and homing of Treg cells.

The Bright Side of Hematopoiesis: Regulatory Roles of ARID3a/Bright in Human and Mouse Hematopoiesis

ARID3a/Bright is a DNA-binding protein that was originally discovered for its ability to increase immunoglobulin transcription in antigen-activated B cells. It interacts with DNA as a dimer through its ARID, or A/T-rich interacting domain. In association with other proteins, ARID3a increased transcription of the immunoglobulin heavy chain and led to improved chromatin accessibility of the heavy chain enhancer. Constitutive expression of ARID3a in B lineage cells resulted in autoantibody production, suggesting its regulation is important. Abnormal ARID3a expression has also been associated with increased proliferative capacity and malignancy. Roles for ARID3a in addition to interactions with the immunoglobulin locus were suggested by transgenic and knockout mouse models. Over-expression of ARID3a resulted in skewing of mature B cell subsets and altered gene expression patterns of follicular B cells, whereas loss of function resulted in loss of B1 lineage B cells and defects in hematopoiesis. More recent studies showed that loss of ARID3a in adult somatic cells promoted developmental plasticity, alterations in gene expression patterns, and lineage fate decisions. Together, these data suggest new regulatory roles for ARID3a. The genes influenced by ARID3a are likely to play pivotal roles in lineage decisions, highlighting the importance of this understudied transcription factor.

Gpr97 is essential for the follicular versus marginal zone B-lymphocyte fate decision

Gpr97 is an orphan adhesion GPCR and is highly conserved among species. Up to now, its physiological function remains largely unknown. Here, we show that Gpr97 deficiency results in an extensive reduction in B220⁺ lymphocytes in mice. More intensive analyses reveal an expanded marginal zone but a decreased follicular B-cell population in Gpr97^−/−spleen, which displays disorganized architecture characterized by diffuse, irregular B-cell areas and the absence of discrete perifollicular marginal and mantle zones. In vivo functional studies reveal that the mutant mice could generate antibody responses to T cell-dependent and independent antigens, albeit enhanced response to the former and weakened response to the latter. By screening for the molecular events involved in the observed phenotypes, we found that lambda 5 expression is downregulated and its upstream inhibitor Aiolos is increased in the spleen of mutant mice, accompanied by significantly enhanced phosphorylation and nuclear translocation of cAMP response element-binding protein. Interestingly, increased constitutive Nf-κb p50/p65 expression and activity were observed in Gpr97^−/− spleen, implicating a crucial role of Gpr97 in regulating Nf-κb activity. These findings uncover a novel biological function of Gpr97 in regulating B-cell development, implying Gpr97 as a potential therapeutic target for treatment of immunological disorders.

Notch-regulated periphery B cell differentiation involves suppression of E protein function

Notch signaling pathway plays important roles in promoting the generation of marginal zone (MZ) B cells at the expense of follicular (FO) B cells during periphery B cell maturation, but the underlying molecular mechanisms are not well understood. We hypothesize that Notch favors the generation of MZ B cells by downregulating E protein activity. In this study, we demonstrated that expression of Id2 and ankyrin-repeat SOCS box-containing protein 2 was elevated in MZ B cells and by Notch signaling. Id2 inhibits the DNA binding activity of E proteins, whereas ankyrin-repeat SOCS box-containing protein 2 facilitates E protein ubiquitination. Next, we examined the phenotypes of splenic B cells in mice expressing constitutively active Notch1 and/or two gain-of-function mutants of E proteins that counteract Id2-mediated inhibition or Notch-induced degradation. We found that upregulation of E proteins promoted the formation of FO B cells, whereas it suppressed the maturation of MZ B cells. In contrast, excessive amounts of Notch1 stimulated the differentiation of MZ B cells and inhibited the production of FO B cells. More interestingly, the effects of Notch1 were reversed by gain of E protein function. Furthermore, high levels of Bcl-6 expression in FO B cells was shown to be diminished by Notch signaling and restored by E proteins. In addition, E proteins facilitated and Notch hindered the differentiation of transitional B cells. Taken together, it appears that Notch regulates peripheral B cell differentiation, at least in part, through opposing E protein function.

The endocytic adaptor Eps15 controls marginal zone B cell numbers

Eps15 is an endocytic adaptor protein involved in clathrin and non-clathrin mediated endocytosis. In Caenorhabditis elegans and Drosophila melanogaster lack of Eps15 leads to defects in synaptic vesicle recycling and synapse formation. We generated Eps15-KO mice to investigate its function in mammals. Eps15-KO mice are born at the expected Mendelian ratio and are fertile. Using a large-scale phenotype screen covering more than 300 parameters correlated to human disease, we found that Eps15-KO mice did not show any sign of disease or neural deficits. Instead, altered blood parameters pointed to an immunological defect. By competitive bone marrow transplantation we demonstrated that Eps15-KO hematopoietic precursor cells were more efficient than the WT counterparts in repopulating B220⁺ bone marrow cells, CD19⁻ thymocytes and splenic marginal zone (MZ) B cells. Eps15-KO mice showed a 2-fold increase in MZ B cell numbers when compared with controls. Using reverse bone marrow transplantation, we found that Eps15 regulates MZ B cell numbers in a cell autonomous manner. FACS analysis showed that although MZ B cells were increased in Eps15-KO mice, transitional and pre-MZ B cell numbers were unaffected. The increase in MZ B cell numbers in Eps15 KO mice was not dependent on altered BCR signaling or Notch activity. In conclusion, in mammals, the endocytic adaptor protein Eps15 is a regulator of B-cell lymphopoiesis.

Notch-RBP-J-independent marginal zone B cell development in IgH transgenic mice with VH derived from a natural polyreactive antibody

Both the B cell antigen receptor (BCR) signaling and Notch signaling pathway play important roles in marginal zone (MZ) B cell development; however, if and how these two signaling pathways engage in crosstalk with each other remain unclear. In the present study, IgH transgenic mice (TgV(H)3B4) were crossed with mice with Notch downstream transcription factor RBP-J floxed alleles (RBP-J(f/f)) and Mx-Cre transgene. Subsequently, MZ B cell development was analyzed in 3B4/Cre/RBP-J(f/f) mice that expressed the transgenic 3B4 IgH and exhibited a deficiency in Notch signaling in B cells upon poly (I:C) injection. We observed that MZ B cell numbers were severely reduced, but still detectable in 3B4/Cre/RBP-J(f/f) mice, in contrast to increased numbers of MZ B cells in TgV(H)3B4 mice and almost no MZ B cells in Cre/RBP-J(f/f) mice. The majority of the MZ B cells in the 3B4/Cre/RBP-J(f/f) mice had the same antigen specificity with that of 3B4 antibody, indicating that a particular BCR specificity might direct MZ B cell development in the absence of Notch signaling. The number of MZ B precursor (MZP) cells was reduced sharply in 3B4/Cre/RBP-J(f/f) mice, and the number of transitional stage 1 and transitional stage 2 cells did not change that much, indicating that the interaction between BCR and Notch signaling likely occurred during the T2-MZP stage. Based on the transgenic mouse model, our data indicate that MZ B cells with certain BCR specificity can develop in a Notch-RBP-J independent manner.

Transcription factor Ebf1 regulates differentiation stage-specific signaling, proliferation, and survival of B cells

The transcription factor Ebf1 is an important determinant of early B lymphopoiesis. To gain insight into the functions of Ebf1 at distinct stages of differentiation, we conditionally inactivated Ebf1. We found that Ebf1 is required for the proliferation, survival, and signaling of pro-B cells and peripheral B-cell subsets, including B1 cells and marginal zone B cells. The proliferation defect of Ebf1-deficient pro-B cells and the impaired expression of multiple cell cycle regulators are overcome by transformation with v-Abl. The survival defect of transformed Ebf1(fl/fl) pro-B cells can be rescued by the forced expression of the Ebf1 targets c-Myb or Bcl-x(L). In mature B cells, Ebf1 deficiency interferes with signaling via the B-cell-activating factor receptor (BAFF-R)- and B-cell receptor (BCR)-dependent Akt pathways. Moreover, Ebf1 is required for germinal center formation and class switch recombination. Genome-wide analyses of Ebf1-mediated gene expression and chromatin binding indicate that Ebf1 regulates both common and distinct sets of genes in early and late stage B cells. By regulating important components of transcription factor and signaling networks, Ebf1 appears to be involved in the coordination of cell proliferation, survival, and differentiation at multiple stages of B lymphopoiesis.

Biochemical and phosphoproteomic analysis of the helix-loop-helix protein E47

Numerous in vitro as well as genetic studies have demonstrated that the activities of the E2A proteins are regulated at multiple levels, including modulation of DNA binding by the Id proteins, association with the transcriptional modulators p300 and ETO, and posttranslational modifications. Here, we use affinity purification of tagged E47 combined with mass spectrometry in order to show that E47 interacts with the entire ensemble of Id proteins, namely, Id1, Id2, Id3, and Id4. Furthermore, we find that the lysine-specific histone demethylase 1 (LSD1), the protein arginine N-methyltransferase 5 (PRMT5), the corepressor CoREST, and the chaperones of the 14-3-3 family associate with affinity-purified E47. We also identify a spectrum of amino acid residues in E47 that are phosphorylated, including an AKT substrate site. We did, however, find that mutation of the identified AKT substrate site by itself did not perturb B cell development. In sum, these studies show that the entire ensemble of Id proteins has the ability to interact with E47, identify factors that associate with E47, and reveal a spectrum of phosphorylated residues in E47, including an AKT substrate site.

Epigenetic repression of the Igk locus by STAT5-mediated recruitment of the histone methyltransferase Ezh2

During B lymphopoiesis, recombination of the locus encoding the immunoglobulin κ-chain complex (Igk) requires expression of the precursor to the B cell antigen receptor (pre-BCR) and escape from signaling via the interleukin 7 receptor (IL-7R). By activating the transcription factor STAT5, IL-7R signaling maintains proliferation and represses Igk germline transcription by unknown mechanisms. We demonstrate that a STAT5 tetramer bound the Igk intronic enhancer (E(κi)), which led to recruitment of the histone methyltransferase Ezh2. Ezh2 marked trimethylation of histone H3 at Lys27 (H3K27me3) throughout the κ-chain joining region (J(κ)) to the κ-chain constant region (C(κ)). In the absence of Ezh2, IL-7 failed to repress Igk germline transcription. H3K27me3 modifications were lost after termination of IL-7R-STAT5 signaling, and the transcription factor E2A bound E(κi), which resulted in acquisition of H3K4me1 and acetylated histone H4 (H4Ac). Genome-wide analyses showed a STAT5 tetrameric binding motif associated with transcriptional repression. Our data demonstrate how IL-7R signaling represses Igk germline transcription and provide a general model for STAT5-mediated epigenetic transcriptional repression.

The opposing roles of the transcription factor E2A and its antagonist Id3 that orchestrate and enforce the naive fate of T cells

It is established that E2A and its antagonist, Id3, modulate developmental progression at the pre-TCR receptor (pre-TCR) and TCR checkpoints. Here we demonstrate that Id3 expression is elevated beyond the pre-TCR checkpoint, remains high in naive T cells and shows a bimodal pattern in the effector/memory population. We show how E2A promotes T-lineage specification and how pre-TCR mediated signaling affects E2A genome-wide occupancy. Thymi in Id3-deficient mice exhibited aberrant development of effector/memory cells, increased CXCR5 and Bcl6 expression, T-B cell conjugates and remarkably B cell follicles. Collectively, these data show how E2A acts globally to orchestrate T-lineage development and that Id3 antagonizes E2A activity beyond the pre-TCR checkpoint to enforce the naïve T cell fate.

Deletion of genes encoding PU.1 and Spi-B in B cells impairs differentiation and induces pre-B cell acute lymphoblastic leukemia

The E26 transformation-specific (Ets) transcription factor PU.1 is required to generate lymphoid progenitor cells from hematopoietic stem cells, but it is not required to generate B cells from committed B-cell lineage progenitors. We hypothesized that PU.1 function in B-cell differentiation is complemented by the related Ets transcription factor Spi-B. To test this hypothesis, mice were generated lacking both PU.1 and Spi-B in the B-cell lineage. Unlike mice lacking PU.1 or Spi-B, mice deficient in both PU.1 and Spi-B in the B-cell lineage had reduced frequencies of B cells as well as impaired B-cell differentiation. Strikingly, all PU.1 and Spi-B-deficient mice developed pre-B cell acute lymphoblastic leukemia before 30 weeks of age. Pre-B cells accumulated in the thymus resulting in massive thymic enlargement and dyspnea. These findings demonstrate that PU.1 and Spi-B are essential transcriptional regulators of B-cell differentiation as well as novel tumor suppressors in the B-cell lineage.

The B lineage transcription factor E2A regulates apoptosis in chronic lymphocytic leukemia (CLL) cells

Chronic lymphocytic leukemia (CLL) is a common malignancy characterized by the accumulation of B lymphocytes with an antigen-experienced activated CD19(+)CD5(+) clonal phenotype. Clinically, ∼50% of cases will behave more aggressively. Here, we investigate the role of the major B-cell transcription factor E2A, a known regulator of B-cell survival and proliferation, to CLL persistence. We show that E2A is elevated at the mRNA and protein levels relative to normal B-cell subsets. E2A silencing in primary CLL cells leads to a significant increase in spontaneous apoptosis in both CD38(+) (aggressive) and CD38(-) (indolent) cases. Moreover, E2A knockdown synergizes with the immunomodulatory drug lenalidomide to reduce CLL viability. E2A is known to restrain the proliferation of primary B and T lymphocytes at multiple stages of maturation and we report that targeted E2A disruption increases the frequency of Ki-67(+) CLL cells in the absence of effects on de novo proliferation. At the molecular level, E2A siRNA-treated CLL cells display reduced expression of key genes associated with survival and cell cycling including p27, p21 and mcl-1, of which the former two are known E2A target genes. Thus, E2A, a key transcription factor associated with the B-cell activation profile, regulates apoptosis in CLL and may contribute to disease pathology.

E2A is a transcriptional regulator of CD38 expression in chronic lymphocytic leukemia

CD38, a nucleotide-metabolizing ectoenzyme and a receptor, is a negative prognostic marker for chronic lymphocytic leukemia (CLL) patients. CD38 has a genetic polymorphism, with a C → G variation in a putative E-box located in a regulatory region. E2A, the predominant E-box factor in B lymphocytes, was found to be highly expressed by CD38(+) CLL patients. The highest CD38 levels scored by E2A(+)/G carrier patients suggested that E2A is (i) directly associated with CD38 expression, and that (ii) the binding of the transcription factor is influenced by the CD38 genotype. Chromatin immunoprecipitation indicated that E2A directly interacts with the CD38 regulatory region. Furthermore, E2A binding was stronger in the presence of the G allele. Experiments of E2A silencing led to a significant reduction of surface levels of CD38, confirming the working hypothesis. A direct functional interplay between E2A and CD38 was shown by exposing CLL cells to interleukin-2 and TLR-9 ligands, both inducers of CD38 expression. Under these conditions, CD38 upregulation was primarily conditioned by the presence of E2A and then by the G allele. The results of this study link E2A and CD38 expression within a common pathway, in which E-protein activity is required for the efficient induction of CD38 transcription.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

IFN regulatory factor 8 restricts the size of the marginal zone and follicular B cell pools

Transcriptional control of marginal zone (MZ) and follicular (FO) B cell development remains incompletely understood. The transcription factor, IFN regulatory factor (IRF)8, is known to play important roles in the differentiation of early B cells. In this article, we demonstrate that IRF8 is also required for normal development of MZ and FO B cells. Mice with a conventional knockout of Irf8 (IRF8(-/-)) or a point mutation in the IRF association domain of IRF8 had increased numbers of MZ B cells. To determine the B cell-intrinsic effects of IRF8 deficiency, we generated mice with a conditional allele of Irf8 crossed with CD19-Cre mice (designated IRF8-conditional knockout [CKO]). These mice had enlarged MZ and increased numbers of MZ and FO B cells compared with controls. The FO B cells of CKO mice exhibited reduced expression of CD23 and moderately increased expression of CD21. Gene-expression profiling showed that increased B cell production in IRF8-CKO mice was associated with changes in expression of genes involved in regulation of transcription, signaling, and inflammation. Functional studies showed that IRF8-CKO mice generated normal Ab responses to T-independent and T-dependent Ags. Thus, IRF8 controls the expansion and maturation of MZ and FO B cells but has little effect on B cell function.

E2A-positive gastric MALT lymphoma has weaker plasmacytoid infiltrates and stronger expression of the memory B-cell-associated miR-223: possible correlation with stage and treatment response

Extranodal marginal-zone lymphoma of mucosa-associated lymphoid tissue of the stomach (gastric MALT lymphoma) is derived from memory B cells of the marginal zone. Normal memory B cells do not express markers of germinal-center B cells, such as E2A (immunoglobulin enhancer-binding factor E12/E47), B-cell chronic lymphocytic leukemia/lymphoma 6 (BCL6), or activation-induced cytidine deaminase (AID). E2A is a transcription factor that induces somatic hypermutations and blocks plasma cell differentiation. In 50 stage-I(E)/II(E1) gastric MALT lymphomas, we confirmed that all cases were BCL6(-)/AID(-), but a subset (50%, 25/50) was E2A(+). As E2A(-) and E2A(+) gastric MALT lymphomas had similar numbers of somatic hypermutations without intraclonal variations, which implied an origin from memory B cells, the expression of E2A was best regarded as a marker of aberrant follicular differentiation. Although the status of somatic hypermutation was not affected by E2A, E2A(+) gastric MALT lymphoma showed less plasmacytoid infiltrates and higher expressions of miRNA-223, a microRNA associated with memory B cells. Clinically, E2A(+) gastric MALT lymphomas were more likely to spread to perigastric lymph nodes and were less responsive to Helicobacter eradication therapy than were E2A(-) gastric MALT lymphomas. Taken together, aberrant E2A expression is a diagnostic feature of a subtype of gastric MALT lymphoma with weaker plasmacytoid infiltrates and stronger miR-223 expression. A prospective study would be necessary to verify the association between E2A expression and a poor response to Helicobacter eradication therapy.

Disrupted Mer receptor tyrosine kinase expression leads to enhanced MZ B-cell responses

Control of lymphocyte homeostasis is essential to ensure efficient immune responses and to prevent autoimmunity. Splenic marginal zone B cells are important producers of autoantibodies, and are subject to stringent tolerance mechanisms to prevent autoimmunity. In this paper, we explore the role of the Mer tyrosine kinase (Mertk) in regulating autoreactive B cells. This receptor tyrosine kinase serves to bind apoptotic cells, to mediate their phagocytosis, and to regulate subsequent cytokine production. Mice lacking Mertk suffer from impaired apoptotic cell clearance and develop a lupus-like autoimmune syndrome. Here we show that such Mertk-KO mice have expanded numbers of splenic marginal zone B cells. Mertk-KO mice bearing a DNA-specific immunoglobulin heavy-chain transgene (3H9) produced anti-DNA antibodies that appeared to be secreted largely by marginal zone B cells. Finally, Mertk-KO mice developed greater antibody responses after NP-Ficoll immunization than their B6 counterparts. Taken together, our data show that Mertk has a major effect on the development of the marginal zone B-cell compartment. Mertk is also important in establishing DNA-specific B-cell tolerance in 3H9 anti-DNA transgenic mice.

The MAPK/ERK and PI3K pathways additively coordinate the transcription of recombination-activating genes in B lineage cells

Rag-1 and Rag-2 are essential for the construction of the BCR repertoire. Regulation of Rag gene expression is tightly linked with BCR expression and signaling during B cell development. Earlier studies have shown a major role of the PI(3)K/Akt pathway in regulating the transcription of Rag genes. In this study, by using the 38c13 murine B cell lymphoma we show that transcription of Rag genes is also regulated by the MEK/ERK pathways, and that both pathways additively coordinate in this regulation. The additive effect is observed for both ligand-dependent (upon BCR ligation) and ligand independent (tonic) signals. However, whereas the PI(3)K/Akt regulation of Rag transcription is mediated by Foxo1, we show in this study that the MEK/ERK pathway coordinates with the regulation of Rag by controlling the phosphorylation and turnover of E47 and its consequential binding to the Rag enhancer regions. Our results suggest that the PI(3)K and MEK/ERK pathways additively coordinate in the regulation of Rag transcription in an independent manner.

A global network of transcription factors, involving E2A, EBF1 and Foxo1, that orchestrates B cell fate

It is now established that the transcription factors E2A, EBF1 and Foxo1 play critical roles in B cell development. Here we show that E2A and EBF1 bound regulatory elements present in the Foxo1 locus. E2A and EBF1 as well as E2A and Foxo1, in turn, were wired together by a vast spectrum of cis-regulatory codes. These associations were dynamic during developmental progression. Occupancy by the E2A isoform, E47, directly elevated the abundance as well as the pattern of histone H3K4 monomethylation across putative enhancer regions. Finally, the pro-B cell epigenome was divided into clusters of loci that show E2A, EBF and Foxo1 occupancy. From this analysis a global network consisting of transcriptional regulators, signaling and survival factors, was constructed that we propose orchestrates the B cell fate.

Distinct regulatory mechanism of immunoglobulin gene transcription in epithelial cancer cells

The restriction of immunoglobulin (Ig) expression to B lymphocytes is well established. However, several reports have confirmed that the Ig gene can be expressed in many non-B cancer cells and/or some normal cells. Our aim is to determine whether the Ig gene promoter can be activated in non-B cancer cells and to identify the regulatory mechanism for Ig gene expression. Our results show that the Ig promoter of VH4-59 was activated in several non-B cancer cell lines. Moreover, two novel positive regulatory elements, an enhancer-like element at -800 to -610 bp and a copromoter-like element at -610 to -300 bp, were identified in two epithelial cancer cell lines, HeLa S3 and HT-29. The octamer element (5'-ATGCAAAT-3') located in the Ig promoter, a crucial element for B-cell-derived Ig gene transcription, was also very important for non-B-cell-derived Ig gene transcription. More importantly, we confirmed that octamer-related protein-1 (Oct-1), but not Oct-2, was a crucial transcriptional factor for Ig gene transcription due to its ability to bind to the octamer element of the Ig promoter in epithelial cancer cells. These results suggested the presence of a distinct regulatory mechanism for Ig gene expression in non-B cancer cells.

Rap signaling in normal lymphocyte development and leukemia genesis

Although Rap GTPases of the Ras family remained enigmatic for years, extensive studies in this decade have revealed diverse functions of Rap signaling in the control of cell proliferation, differentiation, survival, adhesion, and movement. With the use of gene-engineered mice, we have uncovered essential roles of endogenous Rap signaling in normal lymphocyte development of both T- and B-lineage cells. Deregulation of Rap signaling, on the other hand, results in the development of characteristic leukemia in manners highly dependent on the contexts of cell lineages. These results highlight crucial roles of Rap signaling in the physiology and pathology of lymphocyte development.

Two opposing roles of RBP-J in Notch signaling

RBP-J/Su(H)/Lag1, the main transcriptional mediator of Notch signaling, binds DNA with the consensus sequence YRTGDGAD. Notch target genes can be controlled by two opposing activities of RBP-J. The interaction of the Notch intracellular domain with RBP-J induces a weak transcriptional activation and requires an additional tissue-specific transcriptional activator such as bHLH proteins or GATA to mediate strong target gene expression. For example, during Drosophila sensory organ precursor (SOP) cell development, proneural bHLH interacts with Da, a Drosophila orthologue of E2A, to form a tissue-specific activator of Su(H), the Drosophila orthologue of RBP-J. This complex and Su(H) act synergistically to promote the epidermal cell fate. In contrast, a complex of Su(H) with Hairless, a Drosophila functional homologue of MINT, has transcriptional repression activity that promotes SOP differentiation to neurons. Recent conditional loss-of-function studies demonstrated that transcriptional networks involving RBP-J, MINT, and E2A are conserved in mammalian cell differentiation, including multiple steps of lymphocyte development, and probably also in neuronal maturation in adult neurogenesis. During neurogenesis, Notch-RBP-J signaling was thought historically to be involved mainly in the maintenance of undifferentiated neural progenitors. However, the identification of a tissue-specific transcriptional activator of RBP-J-Notch has revealed new roles of RBP-J in the promotion of neuronal maturation. Finally, the Notch-independent function of RBP-J was recently discovered and will be reviewed here.

The follicular versus marginal zone B lymphocyte cell fate decision

Bone marrow-derived B cells make an important cell fate choice to develop into either follicular B cells or marginal zone B cells in the spleen, which depends on signalling through the B cell receptor, Notch2, the receptor for B cell-activating factor and the canonical nuclear factor-kappaB pathway, as well as signals involved in the migration and anatomical retention of marginal zone B cells. Recent information discussed in this Review reconciles some of the controversies regarding the role of the B cell receptor in this cell fate decision and a clearer picture has also emerged regarding the anatomical location of ligands for Notch2 in the spleen. This cell fate decision could provide mechanistic insights that are relevant to other commitment events in lymphocytes.

A tamoxifen inducible knock-in allele for investigation of E2A function

Background

E-proteins are transcription factors important for the development of a variety of cell types, including neural, muscle and lymphocytes of the immune system. E2A, the best characterized E-protein family member in mammals, has been shown to have stage specific roles in cell differentiation, lineage commitment, proliferation, and survival. However, due to the complexity of E2A function, it is often difficult to separate these roles using conventional genetic approaches. Here, we have developed a new genetic model for reversible control of E2A protein activity at physiological levels. This system was created by inserting a tamoxifen-responsive region of the estrogen receptor (ER) at the carboxyl end of the tcfe2a gene to generate E2AER fusion proteins. We have characterized and analyzed the efficiency and kinetics of this inducible E2A^ER system in the context of B cell development.

Results

B cell development has been shown previously to be blocked at an early stage in E2A deficient animals. Our E2A^ER/ER mice demonstrated this predicted block in B cell development, and E2AER DNA binding activity was not detected in the absence of ligand. In vitro studies verified rapid induction of E2AER DNA binding activity upon tamoxifen treatment. While tamoxifen treatment of E2A^ER/ER mice showed inefficient rescue of B cell development in live animals, direct exposure of bone marrow cells to tamoxifen in an ex vivo culture was sufficient to rescue and support early B cell development from the pre-proB cell stage.

Conclusion

The E2A^ER system provides inducible and reversible regulation of E2A function at the protein level. Many previous studies have utilized over-expression systems to induce E2A function, which are complicated by the toxicity often resulting from high levels of E2A. The E2A^ER model instead restores E2A activity at an endogenous level and in addition, allows for tight regulation of the timing of induction. These features make our E2A^ER ex vivo culture system attractive to study both immediate and gradual downstream E2A-mediated events.

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.

Rap signaling is crucial for the competence of IL-7 response and the development of B-lineage cells

Rap family GTPases consist of multiple members with substantial functional redundancy. With the use of transgenic mice conditionally expressing a bona fide dominant-negative Rap1 mutant, Rap1A17, capable of inhibiting the activation of all Rap family members in B-lineage cells (mb.1-Rap1A17 Tg), we demonstrate that these mice show a defective generation of pre-B cells in bone marrow, resulting in a significant diminution of peripheral mainstream B cells. The effect is attributed to the impaired survival and expansion of B-lineage progenitors in response to IL-7, despite normal IL-7Ralpha expression. The pre-B cells from mb.1-Rap1A17 Tg mice showed a significantly reduced expression of c-myc and E2A, and the competence of IL-7 response was restored by the transduction of c-myc, but not by constitutively active (CA) Stat5a, CA PI3K-p100, or bcl-2. The residual follicular B cells with complete Cre-mediated recombination proliferated normally in response to B-cell receptor stimulation and showed efficient germinal center reaction in vivo. These results show that endogenous Rap signaling plays a crucial role selectively in B-lineage cell development by sustaining the competence for IL-7 response, whereas it is dispensable for mature B-cell function.

NK cells in the CD19- B220+ bone marrow fraction are increased in senescence and reduce E2A and surrogate light chain proteins in B cell precursors

E2A encoded proteins, key transcriptional regulators in B lineage specification and commitment, have been shown to decrease in B cell precursors in old age. E2A regulates genes encoding the surrogate light chain proteins lambda5 and VpreB. In old age, B cell precursors express less surrogate light chain and this results in compromised pre-B cell receptor function and diminished expansion of new pre-B cells in senescence. Herein, we show that aged bone marrow has increased Hardy Fraction A (CD19(-) B220(+)) cells, including NK cells, that can inhibit both E47 (E2A) protein and surrogate light chain protein expression in B cell precursors. In vitro, NK-associated inhibition of E47 protein is contact-independent and partially reversed by neutralization of TNFalpha. In vivo, depletion of NK cells in aged mice by treatment with anti-asialo GM1 antibody led to restoration of surrogate light chain protein levels to that typical of young B cell precursors. These studies suggest that NK cells, within the CD19(-) B220(+) bone marrow cell fraction, may contribute to a bone marrow microenvironment that has the potential to negatively regulate E47 (E2A) as well as surrogate light chain levels in B cell precursors in old age.

E47 controls the developmental integrity and cell cycle quiescence of multipotential hematopoietic progenitors

Little is known about the transcriptional regulators that control the proliferation of multipotent bone marrow progenitors. Understanding the mechanisms that restrict proliferation is of significant interest since the loss of cell cycle integrity can be associated with hematopoietic exhaustion, bone marrow failure, or even oncogenic transformation. Herein, we show that multipotent LSKs (lineage(-)Sca(high)c-kit(+)) from E47-deficient mice exhibit a striking hyperproliferation associated with a loss of cell cycle quiescence and increased susceptibility to in vivo challenge with a mitotoxic drug. Total LSKs contain long-term self-renewing hematopoietic stem cells and downstream multipotential progenitors (MPPs) that possess very limited or no self-renewal ability. Within total LSKs, we found specific developmental and functional deficits in the MPP subset. E47 knockout mice have grossly normal numbers of self-renewing hematopoietic stem cells but a 50-70% reduction in nonrenewing MPPs and downstream lineage-restricted populations. The residual MPPs in E47 knockout mice fail to fully up-regulate flk2 or initiate V(D)J recombination, hallmarks of normal lymphoid lineage progression. Consistent with the loss of normal cell cycle restraints, we show that E47-deficient LSKs have a 50% decrease in p21, a cell cycle inhibitor and known regulator of LSK proliferation. Moreover, enforced expression studies identify p21 as an E47 target gene in primary bone marrow LSKs. Thus, E47 appears to regulate the developmental and functional integrity of early hematopoietic subsets in part through effects on p21-mediated cell cycle quiescence.

Regulation of V(D)J recombination by E-protein transcription factors

Extensive study of the E-proteins E2A and HEB duringlymphocyte development has revealed various functions for these bHLH transcription factors in regulating V(D)J recombination in both B- and T-cells. The study of E-proteins in mammals began with the identification of E2A by its ability to bind immunoglobulin heavy and light chain enhancers. Subsequent analysis has identified numerous roles for E2A and HEB at the immunoglobulin and T-cell receptor loci. E-protein targets also include the rag genes and other factors critical for recombination and for regulation of the developmental windows when cells undergo recombination. E-proteins appear to be master regulators that coordinate antigen receptor gene rearrangement and expression. This chapter focuses on how E-proteins regulate V(D)J recombination by activating transcription, initiating rearrangement and driving differentiation during B- and T-cell development.

Transcription factor E2-2 is an essential and specific regulator of plasmacytoid dendritic cell development

Plasmacytoid dendritic cells (PDCs) represent a unique immune cell type specialized in type I interferon (IFN) secretion in response to viral nucleic acids. The molecular control of PDC lineage specification has been poorly understood. We report that basic helix-loop-helix transcription factor (E protein) E2-2/Tcf4 is preferentially expressed in murine and human PDCs. Constitutive or inducible deletion of murine E2-2 blocked the development of PDCs but not of other lineages and abolished IFN response to unmethylated DNA. Moreover, E2-2 haploinsufficiency in mice and in human Pitt-Hopkins syndrome patients was associated with aberrant expression profile and impaired IFN response of the PDC. E2-2 directly activated multiple PDC-enriched genes, including transcription factors involved in PDC development (SpiB, Irf8) and function (Irf7). These results identify E2-2 as a specific transcriptional regulator of the PDC lineage in mice and humans and reveal a key function of E proteins in the innate immune system.

Instructive role of the transcription factor E2A in early B lymphopoiesis and germinal center B cell development

The transcription factor E2A controls the initiation of B lymphopoiesis, which is arrested at the pre-pro-B cell stage in E2A-deficient mice. Here, we demonstrate by conditional mutagenesis that E2A is essential for the development of pro-B, pre-B, and immature B cells in the bone marrow. E2A is, however, dispensable for the generation of mature B cells and plasma cells in peripheral lymphoid organs. In contrast, germinal center B cell development is impaired in the absence of E2A despite normal AID expression and class-switch recombination. Molecular analysis revealed that E2A is required not only for initiating but also for maintaining the expression of Ebf1, Pax5, and the B cell gene program in pro-B cells. Notably, precocious Pax5 transcription from the Ikzf1 locus promotes pro-B cell development in E2A-deficient mice, demonstrating that ectopic Pax5 expression is sufficient to activate the B lymphoid transcription program in vivo in the absence of E2A.

A role for interferon regulatory factor 4 in receptor editing

Receptor editing is the primary means through which B cells revise antigen receptors and maintain central tolerance. Previous studies have demonstrated that interferon regulatory factor 4 (IRF-4) and IRF-8 promote immunoglobulin light-chain rearrangement and transcription at the pre-B stage. Here, the roles of IRF-4 and -8 in receptor editing were analyzed. Our results show that secondary rearrangement was impaired in IRF-4 but not IRF-8 mutant mice, suggesting that receptor editing is defective in the absence of IRF-4. The role of IRF-4 in receptor editing was further examined in B-cell-receptor (BCR) transgenic mice. Our results show that secondary rearrangement triggered by membrane-bound antigen was defective in the IRF-4-deficient mice. Our results further reveal that the defect in secondary rearrangement is more severe at the immunoglobulin lambda locus than at the kappa locus, indicating that IRF-4 is more critical for the lambda rearrangement. We provide evidence demonstrating that the expression of IRF-4 in immature B cells is rapidly induced by self-antigen and that the reconstitution of IRF-4 expression in the IRF-4 mutant immature B cells promotes secondary rearrangement. Thus, our studies identify IRF-4 as a nuclear effector of a BCR signaling pathway that promotes secondary rearrangement at the immature B-cell stage.