In vivo occupancy of the kappa light chain enhancers in primary pro- and pre-B cells: a model for kappa locus activation

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.

PU.1 Regulates Ig Light Chain Transcription and Rearrangement in Pre-B Cells during B Cell Development

B cell development and Ig rearrangement are governed by cell type- and developmental stage-specific transcription factors. PU.1 and Spi-B are E26-transformation-specific transcription factors that are critical for B cell differentiation. To determine whether PU.1 and Spi-B are required for B cell development in the bone marrow, Spi1 (encoding PU.1) was conditionally deleted in B cells by Cre recombinase under control of the Mb1 gene in Spib (encoding Spi-B)-deficient mice. Combined deletion of Spi1 and Spib resulted in a lack of mature B cells in the spleen and a block in B cell development in the bone marrow at the small pre-B cell stage. To determine target genes of PU.1 that could explain this block, we applied a gain-of-function approach using a PU.1/Spi-B-deficient pro-B cell line in which PU.1 can be induced by doxycycline. PU.1-induced genes were identified by integration of chromatin immunoprecipitation-sequencing and RNA-sequencing data. We found that PU.1 interacted with multiple sites in the Igκ locus, including Vκ promoters and regions located downstream of Vκ second exons. Induction of PU.1 induced Igκ transcription and rearrangement. Upregulation of Igκ transcription was impaired in small pre-B cells from PU.1/Spi-B-deficient bone marrow. These studies reveal an important role for PU.1 in the regulation of Igκ transcription and rearrangement and a requirement for PU.1 and Spi-B in B cell development.

A novel pax5-binding regulatory element in the igκ locus

The Igκ locus undergoes a variety of different molecular processes during B cell development, including V(D)J rearrangement and somatic hypermutations (SHM), which are influenced by cis regulatory regions (RRs) within the locus. The Igκ locus includes three characterized RRs termed the intronic (iEκ), 3′Eκ, and Ed enhancers. We had previously noted that a region of DNA upstream of the iEκ and matrix attachment region (MAR) was necessary for demethylation of the locus in cell culture. In this study, we further characterized this region, which we have termed Dm, for demethylation element. Pre-rearranged Igκ transgenes containing a deletion of the entire Dm region, or of a Pax5-binding site within the region, fail to undergo efficient CpG demethylation in mature B cells in vivo. Furthermore, we generated mice with a deletion of the full Dm region at the endogenous Igκ locus. The most prominent phenotype of these mice is reduced SHM in germinal center B cells in Peyer’s patches. In conclusion, we propose the Dm element as a novel Pax5-binding cis regulatory element, which works in concert with the known enhancers, and plays a role in Igκ demethylation and SHM.

Noncoordinate expression of J-chain and Blimp-1 define nurse shark plasma cell populations during ontogeny

B-lymphocyte-induced maturation protein 1 (Blimp-1) is the master regulator of plasma cell development, controlling genes such as those encoding J-chain and secretory Ig heavy chain. However, some mammalian plasma cells do not express J-chain, and mammalian B1 cells secrete "natural" IgM antibodies without upregulating Blimp-1. While these results have been controversial in mammalian systems, here we describe subsets of normally occurring Blimp-1(-) antibody-secreting cells in nurse sharks, found in lymphoid tissues at all ontogenic stages. Sharks naturally produce large amounts of both pentameric (classically "19S") and monomeric (classically "7S") IgM, the latter an indicator of adaptive immunity. Consistent with the mammalian paradigm, shark Blimp-1 is expressed in splenic 7S IgM-secreting cells, though rarely detected in the J-chain(+) cells producing 19S IgM. Although IgM transcript levels are lower in J-chain(+) cells, these cells nevertheless secrete 19S IgM in the absence of Blimp-1, as demonstrated by ELISPOT and metabolic labeling. Additionally, cells in the shark BM equivalent (epigonal) are Blimp-1(-). Our data suggest that, in sharks, 19S-secreting cells and other secreting memory B cells in the epigonal are maintained for long periods without Blimp-1, but like in mammals, Blimp-1 is required for terminating the B-cell program following an adaptive immune response in the spleen.

miR290-5p/292-5p activate the immunoglobulin kappa locus in B cell development

Regulated expression of miRNAs influences development in a wide variety of contexts. We report here that miR290-5p (100049710) and miR292-5p (100049711) are induced at the pre-B stage of murine B cell development and that they influence assembly of the Igκ light chain gene (243469) by contributing to the activation of germline Igκ transcription (κGT). We found that upon forced over-expression of miR290-5p/292-5p in Abelson Murine Leukemia Virus (AMuLV) transformed pro-B cells, two known activators of κGT, E2A (21423) and NF-κB (19697), show increased chromosomal binding to the kappa intronic enhancer. Conversely, knockdown of miR290-5p/292-5p in AMuLV pro-B cells blunts drug-induced activation of κGT. Furthermore, miR290-5p/292-5p knockdown also diminishes κGT activation, but not Rag1/2 (19373, 19374) expression, in an IL-7 dependent primary pro-B cell culture system. In addition, we identified a deficiency in κGT induction in miR290 cluster knockout mice. We hypothesize that increased expression of miR290-5p and miR292-5p contributes to the induction of κGT at the pre-B stage of B cell development through increased binding of NF-κB and E2A to kappa locus regulatory sequences.

Exploiting synthetic lethality for the therapy of ABC diffuse large B cell lymphoma

Knowledge of oncogenic mutations can inspire therapeutic strategies that are synthetically lethal, affecting cancer cells while sparing normal cells. Lenalidomide is an active agent in the activated B cell-like (ABC) subtype of diffuse large B cell lymphoma (DLBCL), but its mechanism of action is unknown. Lenalidomide kills ABC DLBCL cells by augmenting interferon β (IFNβ) production, owing to the oncogenic MYD88 mutations in these lymphomas. In a cereblon-dependent fashion, lenalidomide downregulates IRF4 and SPIB, transcription factors that together prevent IFNβ production by repressing IRF7 and amplify prosurvival NF-κB signaling by transactivating CARD11. Blockade of B cell receptor signaling using the BTK inhibitor ibrutinib also downregulates IRF4 and consequently synergizes with lenalidomide in killing ABC DLBCLs, suggesting attractive therapeutic strategies.

A bistable switch underlying B-cell differentiation and its disruption by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin

The differentiation of B cells into antibody-secreting plasma cells upon antigen stimulation, a crucial step in the humoral immune response, is disrupted by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Several key regulatory proteins in the B-cell transcriptional network have been identified, with two coupled mutually repressive feedback loops among the three transcription factors B-cell lymphoma 6 (Bcl-6), B lymphocyte-induced maturation protein 1(Blimp-1), and paired box 5 (Pax5) forming the core of the network. However, the precise mechanisms underlying B-cell differentiation and its disruption by TCDD are not fully understood. Here we show with a computational systems biology model that coupling of the two feedback loops at the Blimp-1 node, through parallel inhibition of Blimp-1 gene activation by Bcl-6 and repression of Blimp-1 gene deactivation by Pax5, can generate a bistable switch capable of directing B cells to differentiate into plasma cells. We also use bifurcation analysis to propose that TCDD may suppress the B-cell to plasma cell differentiation process by raising the threshold dose of antigens such as lipopolysaccharide required to trigger the bistable switch. Our model further predicts that high doses of TCDD may render the switch reversible, thus causing plasma cells to lose immune function and dedifferentiate to a B cell-like state. The immunotoxic implications of these predictions are twofold. First, TCDD and related compounds would disrupt the initiation of the humoral immune response by reducing the proportion of B cells that respond to antigen and differentiate into antibody-secreting plasma cells. Second, TCDD may also disrupt the maintenance of the immune response by depleting the pool of available plasma cells through dedifferentiation.

Blimp-1/Prdm1 alternative promoter usage during mouse development and plasma cell differentiation

The zinc-finger PR domain transcriptional repressor Blimp-1/Prdm1 plays essential roles in primordial germ cell specification, placental, heart, and forelimb development, plasma cell differentiation, and T-cell homeostasis. The present experiments demonstrate that the mouse Prdm1 gene has three alternative promoter regions. All three alternative first exons splice directly to exon 3, containing the translational start codon. To examine possible cell-type-specific functional activities in vivo, we generated targeted deletions that selectively eliminate two of these transcriptional start sites. Remarkably, mice lacking the previously described first exon develop normally and are fertile. However, this region contains NF-kappaB binding sites, and as shown here, NF-kappaB signaling is required for Prdm1 induction. Thus, mutant B cells fail to express Prdm1 in response to lipopolysaccharide stimulation and lack the ability to become antibody-secreting cells. An alternative distal promoter located approximately 70 kb upstream, giving rise to transcripts strongly expressed in the yolk sac, is dispensable. Thus, the deletion of exon 1B has no noticeable effect on expression levels in the embryo or adult tissues. Collectively, these experiments provide insight into the organization of the Prdm1 gene and demonstrate that NF-kappaB is a key mediator of Prdm1 expression.

Involvement of Blimp-1 and AP-1 dysregulation in the 2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated suppression of the IgM response by B cells

B cell differentiation and humoral immune responses are markedly suppressed by the persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The suppression of humoral immune responses by TCDD occurs by direct actions on the B cell and involves activation of the aryl hydrocarbon receptor. Transcriptional regulation of paired box gene 5 (Pax5), an important regulator of B cell differentiation, is altered by TCDD in concordance with the suppression of B cell differentiation and humoral immunoglobulin M response. We hypothesized that TCDD treatment leads to dysregulation of Pax5 transcription by interfering with the basic B cell differentiation mechanisms and aimed to determine the effects of TCDD on upstream regulators of Pax5. A critical regulator of B cell differentiation, B lymphocyte-induced maturation protein-1 (Blimp-1) acts as a transcriptional repressor of Pax5. In lipopolysaccharide (LPS)-activated murine B cell lymphoma, CH12.LX, Blimp-1 messenger RNA, and DNA-binding activity within the Pax5 promoter were suppressed by TCDD. Furthermore, LPS activation of CH12.LX cells upregulated DNA-binding activity of activator protein 1 (AP-1) at three responsive element-like motifs within the Blimp-1 promoter. TCDD treatment of LPS-activated CH12.LX cells suppressed AP-1 binding to these motifs between 24 and 72 h, in concordance with the suppression of Blimp-1 by TCDD. A more comprehensive analysis at 72 h demonstrated that the suppression of AP-1 binding within the Blimp-1 promoter by TCDD was concentration dependent. In summary, our findings link the TCDD-mediated suppression of Blimp-1 through AP-1 to the dysregulation of Pax5, which ultimately leads to the suppression of B cell differentiation and humoral immune responses.

Molecular pathways and mechanisms regulating the recombination of immunoglobulin genes during B-lymphocyte development

The hallmark of B-cell development is the ordered recombination of immunoglobulin (Ig) genes. Recently, considerable progress has been achieved in assembling gene regulatory networks comprised of signaling components and transcription factors that regulate B-cell development. In this chapter we synthesize experimental evidence to explain how such signaling pathways and transcription factors can orchestrate the ordered recombination of immunoglobulin (Ig) genes. Recombination of antigen-receptor loci is regulated both by the developmentally controlled expression of the Rag1 and Rag2 genes and the accessibility of particular loci and their gene segments to recombination. A new framework has emerged that invokes nuclear compartmentalization, large-scale chromatin dynamics and localized changes in chromatin structure in regulating the accessibility of Ig loci at specific stages of B-cell development. We review this emergent framework and discuss new experimental approaches that will be needed to explore the underlying molecular mechanisms.

Mesenchymal stromal cell-derived CCL2 suppresses plasma cell immunoglobulin production via STAT3 inactivation and PAX5 induction

We demonstrate that the secretome of mesenchymal stromal cells (MSCs) suppresses plasma cell (PC) immunoglobulin (Ig) production, induces plasmablast proliferation, and leads to interleukin-10-mediated blockade in vitro. We found that these effects are the result of MSC-derived CC chemokine ligands CCL2 and CCL7. More specifically, MSCs further processed these CC chemokines by the activity of matrix metalloproteinases (MMPs), leading to the generation of proteolytically processed antagonistic CCL2 variant. Neutralizing CCL2 or inhibiting MMP enzymatic activity abolished the PC-suppressive effect of MSCs. We also observed that MMP-processed CCL2 suppresses signal transducer and activator of transcription 3 (STAT3) activation in PC. As a result, the transcription factor PAX5 is induced, thus explaining the inhibition of Ig synthesis. The absence of inhibitory effects by MSC on the humoral response of CCR2(-/-) mice to xenoantigen suggests that MMP-cleaved CCL2/CCR2 interaction as well as downstream phosphatase activity is necessary for antagonistic effect. We tested syngeneic MSCs in hemophilic B6 mice with predeveloped antihuman factor VIII (hFVIII) antibodies and demonstrated a robust decrease in hFVIII-specific IgG levels. Thus, MSCs may play a role in modulating Ig production by PCs via MMP processing of CCL2 and may represent an appealing cell therapy approach for pathologic humoral responses.

Regulation of immunoglobulin light-chain recombination by the transcription factor IRF-4 and the attenuation of interleukin-7 signaling

Productive rearrangement of the immunoglobulin heavy-chain locus triggers a major developmental checkpoint that promotes limited clonal expansion of pre-B cells, thereby culminating in cell-cycle arrest and rearrangement of light-chain loci. By using Irf4-/-Irf8-/- pre-B cells, we demonstrated that two pathways converge to synergistically drive light-chain rearrangement, but not simply as a consequence of cell-cycle exit. One pathway was directly dependent on transcription factor IRF-4, whose expression was elevated by pre-B cell receptor signaling. IRF-4 targeted the immunoglobulin 3'Ekappa and Elambda enhancers and positioned a kappa allele away from pericentromeric heterochromatin. The other pathway was triggered by attenuation of IL-7 signaling and activated the iEkappa enhancer via binding of the transcription factor E2A. IRF-4 also regulated expression of chemokine receptor Cxcr4 and promoted migration of pre-B cells in response to the chemokine ligand CXCL12. We propose that IRF-4 coordinates the two pathways regulating light-chain recombination by positioning pre-B cells away from IL-7-expressing stromal cells.

Chromatin opening is tightly linked to enhancer activation at the kappa light chain locus

Enhancers play an important role in chromatin opening but the temporal relationship between enhancer activation and the generation of an accessible chromatin structure is poorly defined. Recombination enhancers are essential for chromatin opening and subsequent V(D)J recombination at immunoglobulin loci. In mice, the kappa light chain locus displays an open chromatin structure before the lambda locus yet the same proteins, PU.1/PIP, trigger full enhancer activation of both loci. Using primary B cells isolated from distinct developmental stages and an improved method to quantitatively determine hypersensitive site formation, we find the kappa and lambda recombination enhancers become fully hypersensitive soon after transition to large and small pre-B-II cells, respectively. This correlates strictly with the stages at which these loci are activated. Since these cells are short-lived, these data imply that there is a close temporal relationship between full enhancer hypersensitive site formation and locus chromatin opening.

Phospholipase Cgamma2 contributes to light-chain gene activation and receptor editing

Phospholipase Cgamma2 (PLCgamma2) is critical for pre-B-cell receptor (pre-BCR) and BCR signaling. Current studies discovered that PLCgamma2-deficient mice had reduced immunoglobulin lambda (Iglambda) light-chain usage throughout B-cell maturation stages, including transitional type 1 (T1), transitional type 2 (T2), and mature follicular B cells. The reduction of Iglambda rearrangement by PLCgamma2 deficiency was not due to specifically increased apoptosis or decreased proliferation of mutant Iglambda+ B cells, as lack of PLCgamma2 exerted a similar effect on apoptosis and proliferation of both Iglambda+ and Igkappa+ B cells. Moreover, PLCgamma2-deficient IgHEL transgenic B cells exhibited an impairment of antigen-induced receptor editing among both the endogenous lambda and kappa loci in vitro and in vivo. Importantly, PLCgamma2 deficiency impaired BCR-induced expression of IRF-4 and IRF-8, the two transcription factors critical for lambda and kappa light-chain rearrangements. Taken together, these data demonstrate that the PLCgamma2 signaling pathway plays a role in activation of light-chain loci and contributes to receptor editing.

Immunoglobulin kappa enhancers are differentially regulated at the level of chromatin structure

The kappa intronic and the kappa 3' enhancers synergize to regulate recombination and transcription of the Ig kappa locus. Although these enhancers have overlapping functions, the kappa i enhancer appears to predominate during receptor editing, while the kappa 3' enhancer may be more important for initiating Ig kappa germline transcription to target locus recombination and, later in development, somatic hypermutation. Changes in chromatin structure appear to regulate both enhancers, and previous reports suggest that both enhancers are packaged into an accessible chromatin structure only in B lineage cells. Why these enhancers cannot activate the demethylated, accessible, protein-associated Ig kappa allele in pro-B cells is not known. Furthermore, how the enhancers function to reactivate the locus for receptor editing or to quantitatively promote hypermutation in B cells is vague. Quantitative analysis of Ig enhancer chromatin structure in murine pro-, pre-and splenic B cells demonstrated that the kappa i enhancer maintains a highly accessible chromatin structure under a variety of conditions. This stable chromatin structure mirrored the highly accessible structure characterizing the Ig mu intronic enhancer, despite the fact that Ig mu is activated prior to Ig kappa during B cell development. Surprisingly, parallel analysis of the kappa 3' enhancer demonstrated its accessible chromatin structure is markedly unstable, as characterized by sensitivity to changes in environmental conditions. These data unexpectedly suggest that kappa locus regulation is compartmentalized along the gene in B lineage cells. Furthermore, these findings raise the possibility that environmentally dependent regulation of kappa 3' enhancer structure underlies changes in kappa activation during B cell development.

IFN regulatory factor 4 and 8 promote Ig light chain kappa locus activation in pre-B cell development

Previous studies have shown that B cell development is blocked at the pre-B cell stage in IFN regulatory factor (IRF)4 (pip) and IRF8 (IFN consensus sequence binding protein) double mutant mice (IRF4,8(-/-)). In this study, the molecular mechanism by which IRF4,8 regulate pre-B cell development was further investigated. We show that IRF4,8 function in a B cell intrinsic manner to control pre-B cell development. IRF4,8(-/-) mice expressing a Bcl-2 transgene fail to rescue pre-B cell development, suggesting that the defect in B cell development in IRF4,8(-/-) mice is not due to a lack of survival signal. IRF4,8(-/-) pre-B cells display a high proliferation index that may indirectly inhibit the L chain rearrangement. However, forced cell cycle exit induced by IL-7 withdrawal fails to rescue the development of IRF4,8(-/-) pre-B cells, suggesting that cell cycle exit by itself is not sufficient to rescue the development of IRF4,8(-/-) pre-B cells and that IRF4,8 may directly regulate the activation of L chain loci. Using retroviral mediated gene transduction, we show that IRF4 and IRF8 function redundantly to promote pre-B cell maturation and the generation of IgM(+) B cells. Molecular analysis indicates that IRF4, when expressed in IRF4,8(-/-) pre-B cells, induces kappa germline transcription, enhances V(D)J rearrangement activity at the kappa locus, and promotes L chain rearrangement and transcription. Chromatin immunoprecipitation assay further reveals that IRF4 expression leads to histone modifications and enhanced chromatin accessibility at the kappa locus. Thus, IRF4,8 control pre-B cell development, at least in part, by promoting the activation of the kappa locus.

Impaired intracellular calcium mobilization and NFATc1 availability in tolerant anti-insulin B cells

B lymphocytes that recognize soluble self-Ags are routinely found in normal individuals in a functionally inactive or anergic state. Current models indicate that this tolerant state is maintained by interactions with self-Ags that uncouple the BCR from downstream signaling pathways and increase levels of free calcium. Contrary to this expectation, B cells that harbor anti-insulin Ig transgenes (125Tg) are maintained in a tolerant state even though free calcium levels remain normal and tyrosine kinase substrate phosphorylation is preserved following BCR stimulation. Under basal conditions, intracellular levels of inositol 1,4,5-trisphosphate are increased and NFATc1 levels are reduced in 125Tg B cells. The 125Tg B cells are markedly impaired in their ability to mobilize calcium upon stimulation with ionomycin, and BCR-induced calcium mobilization from internal stores is decreased. In contrast, poisoning intracellular calcium pumps with thapsigargin increases calcium mobilization in 125Tg B cells. Changes in calcium signaling are accompanied by a failure of 125Tg B cells to translocate NFATc1 into the nucleus following stimulation with either anti-IgM or ionomycin. Thus, disassociation of BCR from multiple signaling pathways is not essential for maintaining tolerance in anti-insulin 125Tg B cells. Rather, BCRs that are occupied by autologous insulin deliver signals that induce changes in intracellular calcium mobilization and maintain tolerance by preventing activation of key transcription factors such as NFAT.

Changes in immunoglobulin–nucleoprotein complex structure mapped by chromatin immunoprecipitation

Transcription factor-mediated immunoglobulin (Ig) enhancer activation has been analyzed extensively outside the physiological constraints of chromatin. Towards understanding the role sequence-specific DNA binding proteins identified by these methods play in activating Ig genes during B cell development, we have investigated in vivo interaction between the Ig enhancer activator PU.1 and two target elements, the Igmu and kappa3' enhancers, by chromatin immunoprecipitation (ChIP). By using two antibodies recognizing different PU.1 epitopes in murine B cells, these analyses demonstrate that ChIP results may depend on the availability of the epitope(s) targeted by the immunoprecipitating antibody. Specifically, PU.1 epitope availability at the mu and kappa3' enhancers does not accurately quantitate total PU.1 association. This result suggests the nucleoprotein complexes formed at these various active enhancers is cell type-specific. Interestingly, RAG1-/- but not RAG2-/- pro-B cells lack PU.1/kappa3' association, probably due to limited accessibility of the kappa locus in the former. The more robust association of PU.1 with the kappa3' versus mu enhancer in all but RAG1-/- B lineage cells is not explained by differences in PCR primer efficiency, but likely reflects the different structures formed by the complexes at mu versus kappa3' enhancers. Finally, PU.1 is not associated with an inaccessible mu or kappa3' enhancer chromatin structure in macrophages, again emphasizing the importance cellular protein context plays in PU.1/Ig enhancer association. The demonstration that changes in epitope availability, hence nucleoprotein structure, can be monitored by ChIP suggests using this technique to monitor biologically important changes in nucleoprotein complex structure/composition in situ.

E2A and IRF-4/Pip promote chromatin modification and transcription of the immunoglobulin kappa locus in pre-B cells

The immunoglobulin kappa light chain (Igkappa) locus is regulated in a lineage- and stage-specific manner during B-cell development. The highly restricted timing of V to J gene recombination at the pre-B-cell stage is under the control of two enhancers, the intronic enhancer (kappaEi) and the 3' enhancer (kappaE3'), flanking the constant exon. E2A transcription factors have been indicated to be directly involved in the regulation of Igkappa locus activation. In this study, we utilize E2A-deficient pre-B cells to directly investigate the mechanism of E2A-mediated Igkappa activation. We demonstrate that Igkappa germ line transcription is severely impaired and recombination is blocked in the absence of E2A. Reconstitution of E2A-/- pre-B cells with inducible human E2A (E47R) is sufficient to promote chromatin modification of Igkappa and rescue Igkappa germ line transcription and Jkappa gene recombinase accessibility. Furthermore, we show that increased E2A recruitment to kappaEi and kappaE3' correlates with activation of Igkappa in pre-B cells and that recruitment of E2A to kappaE3' is in part dependent on the transcription factor IRF-4. Inhibition of IRF-4 expression in pre-B cells leads to a significant reduction of Igkappa germ line transcription and enhancer acetylation. In the absence of E2A, increased IRF-4 expression is not sufficient to promote Igkappa enhancer chromatin modification or transcription, suggesting that the sequential involvement of IRF-4 and E2A is necessary for the activation of the Igkappa locus. Finally, we provide genetic evidence in the mouse that E2A gene dosage can influence the development of pre-B cells during the phase of Igkappa gene activation.

Pre-BCR signals and the control of Ig gene rearrangements

Progenitor B lymphocytes that successfully assemble a heavy chain gene encoding an immunoglobulin capable of pairing with surrogate light chain proteins trigger their own further differentiation by signaling via the pre-BCR complex. The pre-BCR signals several rounds of proliferation and, in this expanded population, directs a complex, B cell-specific set of epigenetic changes resulting in allelic exclusion of the heavy chain locus and activation of the light chain loci for V(D)J recombination.

Accessibility control of V(D)J recombination

Mammals contend with a universe of evolving pathogens by generating an enormous diversity of antigen receptors during lymphocyte development. Precursor B and T cells assemble functional immunoglobulin (Ig) and T cell receptor (TCR) genes via recombination of numerous variable (V), diversity (D), and joining (J) gene segments. Although this combinatorial process generates significant diversity, genetic reorganization is inherently dangerous. Thus, V(D)J recombination must be tightly regulated to ensure proper lymphocyte development and avoid chromosomal translocations that cause lymphoid tumors. Each genomic rearrangement is mediated by a common V(D)J recombinase that recognizes sequences flanking all antigen receptor gene segments. The specificity of V(D)J recombination is due, in large part, to changes in the accessibility of chromatin at target gene segments, which either permits or restricts access to recombinase. The chromatin configuration of antigen receptor loci is governed by the concerted action of enhancers and promoters, which function as accessibility control elements (ACEs). In general, ACEs act as conduits for transcription factors, which in turn recruit enzymes that covalently modify or remodel nucleosomes. These ACE-mediated alterations are critical for activation of gene segment transcription and for opening chromatin associated with recombinase target sequences. In this chapter, we describe advances in understanding the mechanisms that control V(D)J recombination at the level of chromatin accessibility. The discussion will focus on cis-acting regulation by ACEs, the nuclear factors that control ACE function, and the epigenetic modifications that establish recombinase accessibility.

Protein acetylation regulates both PU.1 transactivation and Ig kappa 3' enhancer activity

Igkappa gene expression and chromatin structure change during B cell development. At the pre-B cell stage, the locus is relatively hypoacetylated on histone H3, whereas it is hyperacetylated at the plasma cell stage. We find in this study that the histone deacetylase inhibitor, trichostatin A (TSA) stimulated 3' enhancer activity through the PU.1 binding site. TSA also stimulated PU.1 transactivation potential. PU.1 activity was increased by the coactivator acetyltransferase protein, p300, and p300 physically interacted with PU.1 residues 7-30. PU.1 served as a substrate for p300 and was acetylated on lysine residues 170, 171, 206, and 208. Mutation of PU.1 lysines 170 and 171 did not affect PU.1 DNA binding, but did lower the ability of PU.1 to activate transcription in association with p300. Lysine 170 was acetylated in pre-B cells and plasmacytoma cells, but TSA treatment did not stimulate PU.1 acetylation at this residue arguing that a second mechanism can stimulate 3' enhancer activity. Using chromatin immunoprecipitation assays we found that TSA caused preferential acetylation of histone H3 at the 3' enhancer. The relevance of these studies for PU.1 function in transcription and hemopoietic development is discussed.

A role for nuclear factor kappa B/rel transcription factors in the regulation of the recombinase activator genes

In developing B cells, expression of surface immunoglobulin is an important signal to terminate recombinase activator gene (RAG) expression and V(D)J recombination. However, autoreactive antigen receptors instead promote continued gene rearrangement and receptor editing. The regulation by B cell receptor (BCR) signaling of RAG expression and editing is poorly understood. We report that in editing-competent cells BCR ligand-induced RAG mRNA expression is regulated at the level of RAG transcription, rather than mRNA stability. In immature B cells carrying innocuous receptors, RAG expression appears to be under rapidly reversible negative regulation. Studies involving transduction of a superrepressive (sr) I kappa B alpha protein indicate that NF-kappaB/Rel proteins promote RAG transcription. Interestingly, NF kappa B1-deficient cells overexpress RAG and undergo an exaggerated receptor editing response. Our data implicate NF kappa B transcription factors in the BCR-mediated regulation of RAG locus transcription. Rapidly activated NF kappa B pathways may facilitate prompt antigen receptor-regulated changes in RAG expression important for editing and haplotype exclusion.

Long-range interactions between three transcriptional enhancers, active Vkappa gene promoters, and a 3' boundary sequence spanning 46 kilobases

The mouse immunoglobulin kappa (Igkappa) gene contains an intronic enhancer and two enhancers downstream of its transcription unit. Using chromosome conformation capture technology, we demonstrate that rearranged and actively transcribed Igkappa alleles in MPC-11 plasmacytoma cells exhibit mutual interactions over 22 kb between these three enhancers and Vkappa gene promoters. In addition, the 5' region of the active transcription unit exhibits a continuum of interactions with downstream chromatin segments. We also observe interactions between Ei and E3' with 3' boundary sequences 24 kb downstream of Ed, adjacent to a neighboring housekeeping gene. Very similar interactions between the enhancers are also exhibited by normal B cells isolated from mouse splenic tissue but not by germ line transcriptionally inactive alleles of T cells or P815 mastocytoma cells, which exhibit a seemingly linear chromatin organization. These results fit a looping mechanism for enhancer function like in the beta-globin locus and suggest a dynamic modulation of the spatial organization of the active Igkappa locus. Chromatin immunoprecipitation experiments reveal that the interacting Igkappa gene cis-acting sequences are associated with AP-4, E47, and p65NF-kappaB, potential protein candidates that may be responsible for initiating and/or maintaining the formation of these higher-order complexes. However, S107 plasmacytoma cells that lack NF-kappaB still exhibit mutual interactions between the Igkappa gene enhancers.

Regulation of plasma-cell development

Plasma cells are the terminally differentiated, non-dividing effector cells of the B-cell lineage. They are cellular factories devoted to the task of synthesizing and secreting thousands of molecules of clonospecific antibody each second. To respond to microbial pathogens with the necessary specificity and rapidity, B cells are exquisitely regulated with respect to both development in the bone marrow and activation in the periphery. This review focuses on the terminal differentiation of B cells into plasma cells, including the different subsets of B cells that become plasma cells, the mechanism of regulation of this transition, the transcription factors that control each developmental stage and the characteristics of long-lived plasma cells.

The Ig kappa 3' enhancer is activated by gradients of chromatin accessibility and protein association

The Igkappa locus is recombined following initiation of a signaling cascade during the early pre-B stage of B cell development. The Ig kappa3' enhancer plays an important role in normal B cell development by regulating kappa locus activation. Quantitative analyses of kappa3' enhancer chromatin structure by restriction endonuclease accessibility and protein association by chromatin immunoprecipitation in a developmental series of primary murine B cells and murine B cell lines demonstrate that the enhancer is activated progressively through multiple steps as cells mature. Moderate kappa3' chromatin accessibility and low levels of protein association in pro-B cells are increased substantially as the cells progress from pro- to pre-B, then eventually mature B cell stages. Chromatin immunoprecipitation assays suggest transcriptional regulators of the kappa3' enhancer, specifically PU.1 and IFN regulatory factor-4, exploit enhanced accessibility by increasing association as cells mature. Characterization of histone acetylation patterns at the kappa3' enhancer and experimental inhibition of histone deacetylation suggest changes therein may determine changes in enzyme and transcription factor accessibility. This analysis demonstrates kappa activation is a multistep process initiated in early B cell precursors before Igmu recombination and finalized only after the pre-B cell stage.

NF-kappaB and the immunoglobulin kappa gene enhancer

Enhancers regulate lineage choice and the developmental timing of antigen receptor gene rearrangements. The transcription factor NF-kappaB has been implicated as a key component of the recombination and transcription activation potential of the immunoglobulin kappa chain gene intronic enhancer. Here, I discuss the implications of the new observation that an NF-kappaB binding site-mutated enhancer in the correct biological context does not appear to affect kappa gene expression.

Regulation of activation and recombination of the murine Igkappa locus

The murine immunoglobulin (Ig) kappa locus has been intensively studied in an attempt to understand its developmentally regulated activation for both transcription and V(D)J recombination. A variety of signaling proteins, cis-acting DNA elements, and trans-acting DNA-binding proteins have been discovered and shown to be involved in the regulated changes in chromatin structure, which are associated with recombinase accessibility. In addition, key roles have been suggested for DNA methylation and replication in kappa-locus expression and rearrangement. This review summarizes data in this area and considers what studies of the murine kappa locus have revealed about the lineage specificity, order, and allelic exclusion of lymphoid V(D)J recombination.

A mechanistic insight into a proteasome-independent constitutive inhibitor kappaBalpha (IkappaBalpha) degradation and nuclear factor kappaB (NF-kappaB) activation pathway in WEHI-231 B-cells

Inducible activation of the transcription factor NF-kappaB (nuclear factor kappaB) is classically mediated by proteasomal degradation of its associated inhibitors, IkappaBalpha (inhibitory kappaBalpha) and IkappaBbeta. However, certain B-lymphocytes maintain constitutively nuclear NF-kappaB activity (a p50-c-Rel heterodimer) which is resistant to inhibition by proteasome inhibitors. This activity in the WEHI-231 B-cell line is associated with continual and preferential degradation of IkappaBalpha, which is also unaffected by proteasome inhibitors. Pharmacological studies indicated that there was a correlation between inhibition of IkappaBalpha degradation and constitutive p50-c-Rel activity. Domain analysis of IkappaBalpha by deletion mutagenesis demonstrated that an N-terminal 36-amino-acid sequence of IkappaBalpha represented an instability determinant for constitutive degradation. Moreover, domain grafting studies indicated that this sequence was sufficient to cause IkappaBbeta, but not chloramphenicol acetyltransferase, to be rapidly degraded in WEHI-231 B-cells. However, this sequence was insufficient to target IkappaBbeta to the non-proteasome degradation pathway, suggesting that there was an additional cis-element(s) in IkappaBalpha that was required for complete targeting. Nevertheless, the NF-kappaB pool associated with IkappaBbeta now became constitutively active by virtue of IkappaBbeta instability in these cells. These findings further support the notion that IkappaB instability governs the maintenance of constitutive p50-c-Rel activity in certain B-cells via a unique degradation pathway.

XBP1, downstream of Blimp-1, expands the secretory apparatus and other organelles, and increases protein synthesis in plasma cell differentiation

The differentiation of B cells into immunoglobulin-secreting plasma cells is controlled by two transcription factors, Blimp-1 and XBP1. By gene expression profiling, we defined a set of genes whose induction during mouse plasmacytic differentiation is dependent on Blimp-1 and/or XBP1. Blimp-1-deficient B cells failed to upregulate most plasma cell-specific genes, including xbp1. Differentiating xbp1-deficient B cells induced Blimp-1 normally but failed to upregulate genes encoding many secretory pathway components. Conversely, ectopic expression of XBP1 induced a wide spectrum of secretory pathway genes and physically expanded the endoplasmic reticulum. In addition, XBP1 increased cell size, lysosome content, mitochondrial mass and function, ribosome numbers, and total protein synthesis. Thus, XBP1 coordinates diverse changes in cellular structure and function resulting in the characteristic phenotype of professional secretory cells.

Variegated transcriptional activation of the immunoglobulin kappa locus in pre-b cells contributes to the allelic exclusion of light-chain expression

Regulated gene rearrangement is thought to underlie allelic exclusion, the observation that an individual B cell expresses only a single immunoglobulin molecule. Previous data has implicated transcriptional activation of rearranging loci in the regulation of their accessibility to the V(D)J recombinase. Using homologous recombination in ES cells, we have generated "knockin" mice which express a GFP cDNA from an unrearranged immunoglobulin kappa light-chain allele. Surprisingly, we find that only a small fraction of kappa alleles are highly transcribed in a population of pre-B cells, that such transcription is monoallelic, and that these highly transcribed alleles account for the vast majority of kappa light-chain gene rearrangement. These data lead us to suggest that probabilistic enhancer activation and allelic competition are part of the mechanism of kappa locus allelic exclusion and may be a general mechanism contributing to cellular differentiation during development.

Genomic Ly49A transgenes: basis of variegated Ly49A gene expression and identification of a critical regulatory element

Several gene families are known in which member genes are expressed in variegated patterns in differentiated cell types. Mechanisms responsible for imposition of a variegated pattern of gene expression are unknown. Members of the closely linked Ly49 inhibitory receptor gene family are expressed in a variegated fashion by NK cells. Variegated expression of these genes results in subsets of NK cells that differ in specificity for MHC class I molecules. To address the mechanisms underlying variegation, a 30-kb genomic fragment containing a single Ly49 gene was used to generate a panel of murine transgenic lines. The results demonstrated that, in almost all of the lines, the isolated Ly49A gene was expressed in a variegated pattern, remarkably similar in nearly all respects to the expression pattern of the endogenous Ly49A gene. Furthermore, the developmental timing of gene expression and regulation by host MHC molecules closely mirrored that of the endogenous Ly49A gene. Therefore, Ly49 variegation does not require competition in cis between different Ly49 genes, and the sequences imposing variegation are located proximally to Ly49 genes. Efforts to define regulatory elements of the Ly49A gene led to the identification of a DNase I hypersensitive site 4.5 kb upstream of the Ly49A gene transcription initiation site, which was shown to be essential for transgene expression. Highly related sequence elements were found upstream of other Ly49 genes, suggesting that a similar regulatory element controls each Ly49 gene.

Pax-5 Is Essential for κ Sterile Transcription during Igκ Chain Gene Rearrangement

Pax-5 is the key regulator in B cell development. Pax-5-deficient mice show defects in B cell commitment and recombination of IgH chain gene rearrangement from DJ to VDJ. Previously, we found that Pax-5 bound to KI and KII sites, which play a crucial role in kappa-chain gene rearrangement. However, the function of Pax-5 in Ig kappa chain gene rearrangement has not been investigated. To address this issue, we newly established pre-BI cell lines expressing the pre-B cell receptor from Pax-5-deficient mice and used them in an in vitro culture system, in which kappa-chain gene rearrangement is induced by removing IL-7. By examining the Pax-5-deficient pre-BI (knockout (KO)) cells, we show in this study that, despite recombination-activating gene 1 and 2 expression, these KO cells did not rearrange the kappa-chain gene following the absence of kappa sterile transcription. Consistent with these data, fluorescent in situ hybridization analyses revealed that the J(kappa) locus in KO cells was located at the nuclear periphery as a repressive compartment. Transfection of KO cells with Pax-5 constructs indicated that the transactivation domain of Pax-5 was required for kappa sterile transcription and kappa-chain gene rearrangement. Moreover, the hormone-inducible system in KO cells demonstrated that Pax-5 directly functioned in kappa sterile transcription. These results indicate that Pax-5 is necessary for kappa sterile transcription during Ig kappa chain gene rearrangement.

NF-kB inhibitor blocks B cell development at two checkpoints

Members of the NF-kB transcription factor family are differentially expressed in the B cell lineage. Disruption of individual or two NF-kB subunits exhibits distinct defects in B lymphocyte development, activation, and survival. However, the role each NF-kB plays during B cell development has been obscured by molecular compensation. To address this issue, a trans-dominant form of IkBα was transduced into bone marrow cells to act as a pan-inhibitor of NF-kB using a retroviral system. While the development of T-lymphocytes and myeloid cell lineages was not grossly affected by the transduced IkBα gene, a significant reduction in the number and percentage of B lineage cells was apparent in IkBα transduced chimeric mice. IkBα expression decreased the percentage of pre-B and immature B cell subsets in the bone marrow and further impaired the development of follicular mature B cells and marginal zone B cells in the periphery. Introduction of the Bcl-X transgene completely restored the pre-B and immature B cell pool in the bone marrow. However, despite a significant improvement of overall viability of the B cell lineage, Bcl-X expression was insufficient to overcome the maturation block resulting from NF-kB inhibition. Together, our study suggests that NF-kB activity is required for two distinct checkpoints during B cell development: one is for pre-B/immature B cell viability, the other is to provide both survival and maturation signals to ensure the proper development of follicular mature B cells.

PU.1 as a chromatin accessibility factor for immunoglobulin genes

The hematopoietic-specific transcription factor PU.1 is a chromatin accessibility factor, based on analysis of the immunoglobulin heavy chain intronic (mu) enhancer. Whether PU.1 functions as an accessibility factor for additional PU.1-regulated genes is unknown. Outside the constraints of chromatin, PU.1 binds and activates transcription through both mu and kappa3' immunoglobulin enhancers, among others. The DNA-binding ETS domain of PU.1 is sufficient for activating both enhancers in an extrachromosomal context. New data show that the ETS domain of PU.1 is sufficient for increasing accessibility of a closed mu enhancer chromatin structure proximal to the PU.1-binding site. In contrast, PU.1 does not alter widespread chromatin accessibility. Furthermore, PU.1 does not induce accessibility proximal or distal to its binding site on the kappa3' enhancer. Taken together the data demonstrate that PU.1 induces chromatin accessibility proximal to its binding site at a locus activated early in development, the mu locus. PU.1 does not function as an accessibility factor for the kappa3' enhancer, which regulates a locus important for later stages of B cell development. We conclude that PU.1 is a context-dependent chromatin accessibility factor that, alone, cannot establish widespread accessibility required for critical developmental processes such as antigen receptor recombination.

Corecruitment of the Grg4 repressor by PU.1 is critical for Pax5-mediated repression of B-cell-specific genes

PU.1 and Pax5 are important regulators of immunoglobulin heavy-chain (IgH) gene expression in B lineage cells. We have previously shown that PU.1 can potentiate the transcription of an IgH HS1,2 enhancer-linked reporter gene, and that Pax5 represses the same enhancer in transient transfection assays. Here we report that PU.1, like Pax5, can recruit and physically interact with a member of the Groucho family of co-repressors, Grg4. As a consequence, PU.1 in conjunction with Pax5 represses enhancer function in a position-dependent manner when Grg4 is recruited. Interestingly, Grg4 levels decrease following B-cell activation, suggesting temporal regulation of Grg4. Moreover, the joining-chain promoter, with an activity pattern and architecture resembling HS1,2 can also be repressed by the combinatorial action of Pax5/PU.1/Grg4. These data indicate that Pax5 depends on PU.1, acting in cis, for stable recruitment of Grg co-repressors to B-cell-specific genes.

Blimp-1 is required for the formation of immunoglobulin secreting plasma cells and pre-plasma memory B cells

Blimp-1 is a transcriptional repressor able to drive the terminal differentiation of B cells into Ig-secreting plasma cells. We have created mice with a B cell-specific deletion of prdm1, the gene encoding Blimp-1. B cell development and the number of B cells responding to antigen appear to be normal in these mice. However, in response to either TD or TI antigen, serum Ig, short-lived plasma cells, post-GC plasma cells, and plasma cells in a memory response are virtually absent, demonstrating that Blimp-1 is required for plasmacytic differentiation and Ig secretion. In the absence of Blimp-1, CD79b(+)B220(-) pre-plasma memory B cell development is also defective, providing evidence that this subset is an intermediate in plasma cell development. B cells lacking Blimp-1 cannot secrete Ig or induce muS mRNA when stimulated ex vivo. Furthermore, although prdm1-/- B cells fail to induce XBP-1, XBP-1 cannot rescue plasmacytic differentiation without Blimp-1.

CD40 stimulation induces Pax5/BSAP and EBF activation through a APE/Ref-1-dependent redox mechanism

CD40 is a member of the growing tumor necrosis factor receptor family that has been shown to play important roles in T cell-mediated B lymphocyte activation. Ligation of B cell CD40 by CD154, mainly expressed on activated T cells, stimulates B cell proliferation, differentiation, isotype switching, up-regulation of surface molecules contributing to antigen presentation, development of the germinal center, and the humoral memory response. In this study we demonstrate that the redox factor APE/Ref-1 acts as a key signaling intermediate in response to CD40-mediated B cell activation. The transcription factors Pax5a or BSAP (B cell lineage-specific activator protein) and EBF (early B cell factor) are constitutively expressed in spleen B cells and CD40 cross-linking induces increases in Pax5a and EBF binding activity compared with nonstimulated B cells. We show that upon CD40 antibody-mediated cross-linking, APE/Ref-1 translocates from the cytoplasm to the nucleus of activated B cells, where it modulates the DNA binding activity of both Pax5a and EBF. Moreover, we show that the repression of APE/Ref-1 protein production is able to block CD40-mediated Pax5a activation. We also provide evidence that APE/Ref-1 can modulate the cooperative activation of the blk promoter operated by Pax5a and EBF and that APE/Ref-1 might directly regulate EBF functional activity. Finally, we show that the interaction between Pax5a and EBF enhances EBF binding activity to its consensus sequence, suggesting that Pax5a can physically interact with EBF and modulate its DNA binding activity.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

IRF-4,8 orchestrate the pre-B-to-B transition in lymphocyte development

B-lymphocyte development involves sequential DNA rearrangements of immunoglobulin (Ig) heavy (mu) and light (kappa, lambda) chain loci and is dependent on transient expression of mu containing pre-antigen receptor complexes (pre-BCR). To date, genetic analysis has not identified transcription factors that coordinate the pre-B-to-B transition. We demonstrate that the related interferon regulatory factors IRF-4 (Pip) and IRF-8 (ICSBP) are required for Ig light but not heavy-chain gene rearrangement. In the absence of these transcription factors, B-cell development is arrested at the cycling pre-B-cell stage and the mutant cells fail to down-regulate the pre-BCR. On the basis of molecular analysis, we propose that IRF-4,8 function as a genetic switch to down-regulate surrogate light-chain gene expression and induce conventional light-chain gene transcription and rearrangement.

A small molecule Abl kinase inhibitor induces differentiation of Abelson virus-transformed pre-B cell lines

Abelson murine leukemia virus-transformed cell lines have provided a critical model system for studying the regulation of B cell development. However, transformation by v-Abl blocks B cell development, resulting in the arrest of these transformants in an early pre-B cell-like state. We report here that treatment of Abelson virus-transformed pre-B cell lines with the small molecule Abl kinase inhibitor (STI571) results in their differentiation to a late pre-B cell-like state characterized by induction of immunoglobulin (Ig) light chain gene rearrangement. DNA microarray analyses enabled us to identify two genes inhibited by v-Abl that encode the Igk 3' enhancer-binding transcription factors Spi-B and IRF-4. We show that enforced expression of these two factors is sufficient to induce germline Igk transcription in Abelson-transformed pro-B cell lines. This suggests a key role for these factors, and perhaps for c-Abl itself, in the regulated activation of Ig light chain gene rearrangement.

Chromatin structural analyses of the mouse Igkappa gene locus reveal new hypersensitive sites specifying a transcriptional silencer and enhancer

To identify new regulatory elements within the mouse Igkappa locus, we have mapped DNase I hypersensitive sites (HSs) in the chromatin of B cell lines arrested at different stages of differentiation. We have focused on two regions encompassing 50 kilobases suspected to contain new regulatory elements based on our previous high level expression results with yeast artificial chromosome-based mouse Igkappa transgenes. This approach has revealed a cluster of HSs within the 18-kilobase intervening sequence, which we cloned and sequenced in its entirety, between the Vkappa gene closest to the Jkappa region. These HSs exhibit pro/pre-B cell-specific transcriptional silencing of a Vkappa gene promoter in transient transfection assays. We also identified a plasmacytoma cell-specific HS in the far downstream region of the locus, which in analogous transient transfection assays proved to be a powerful transcriptional enhancer. Deletional analyses reveal that for each element multiple DNA segments cooperate to achieve either silencing or enhancement. The enhancer sequence is conserved in the human Igkappa gene locus, including NF-kappaB and E-box sites that are important for the activity. In summary, our results pinpoint the locations of presumptive regulatory elements for future knockout studies to define their functional roles in the native locus.

Transcription factor complex formation and chromatin fine structure alterations at the murine c-fms (CSF-1 receptor) locus during maturation of myeloid precursor cells

Expression of the gene for the macrophage colony stimulating factor receptor (CSF-1R), c-fms, has been viewed as a hallmark of the commitment of multipotent precursor cells to macrophages. Lineage-restricted expression of the gene is controlled by conserved elements in the proximal promoter and within the first intron. To investigate the developmental regulation of c-fms at the level of chromatin structure, we developed an in vitro system to examine the maturation of multipotent myeloid precursor cells into mature macrophages. The dynamics of chromatin fine structure alterations and transcription factor occupancy at the c-fms promoter and intronic enhancer was examined by in vivo DMS and UV-footprinting. We show that the c-fms gene is already transcribed at low levels in early myeloid precursors on which no CSF-1R surface expression can be detected. At this stage of myelopoiesis, the formation of transcription factor complexes on the promoter was complete. By contrast, occupancy of the enhancer was acutely regulated during macrophage differentiation. Our data show that cell-intrinsic differentiation decisions at the c-fms locus precede the appearance of c-fms on the cell surface. They also suggest that complex lineage-specific enhancers such as the c-fms intronic enhancer regulate local chromatin structure through the coordinated assembly and disassembly of distinct transcription factor complexes.

Allelic exclusion of immunoglobulin gene rearrangement and expression: why and how?

Since the discovery of the allelic exclusion of immunoglobulin (Ig) gene expression by Pervis in the 1960s [J. Exp. Med. 122 (1965) 853], much attention has been focused on its mechanism. Much less attention has been paid, however, to the question of why B cells demonstrate such unusual genetic regulation of antigen receptor gene expression. A large body of literature implicates the Ig gene products as feedback regulators of their own genetic rearrangement [Adv. Immunol.78 (2001)169; Science 236 (1987)816]. While a role for Ig gene products in the regulation of V(D)J recombination is beyond debate, it is extremely unlikely that such a feedback mechanism would be fast enough to avoid occasional near-simultaneous rearrangement of allelic loci leading to dual receptor gene expression. This review will suggest an hypothesis to answer the 'why bother' aspect of allelic exclusion and then go on to propose a mechanism, distinct from feedback regulation, which may contribute to the allelic exclusion of Ig gene expression.

Dissociation of Pax-5 from KI and KII sites during kappa-chain gene rearrangement correlates with its association with the underphosphorylated form of retinoblastoma

The KI and KII sites play a crucial role in kappa-chain gene rearrangement, which was investigated in mice deficient for these sites. Previously, we found that Pax-5 can bind to the KI and KII sites; however, the function of Pax-5 in kappa-chain gene rearrangement has not been investigated. Here, we have used an in vitro culture system in which differentiation from pre-B cells to immature B cells is induced by removing IL-7. We showed that, after the induction of differentiation, Pax-5 dissociated from the KI and KII revealed by EMSA analyses, and this dissociation occurred specifically at the KI and KII sites, but not at the Pax-5 binding site, in the CD19 promoter because of a lower binding affinity of Pax-5 for the KI and KII sites. During differentiation induced by removing IL-7, the underphosphorylated form of retinoblastoma preferentially associated with Pax-5, which caused dissociation of Pax-5 from KI and KII sites. These results suggest that the dissociation of Pax-5 from the KI and KII sites is important in the induction of kappa-chain gene rearrangement.