The B lymphocyte-specific coactivator BOB.1/OBF.1 is required at multiple stages of B-cell development

Characterization of Freshly Isolated Human Peripheral Blood B Cells, Monocytes, CD4+ and CD8+ T Cells, and Skin Mast Cells by Quantitative Transcriptomics

Quantitative transcriptomics offers a new way to obtain a detailed picture of freshly isolated cells. By direct isolation, the cells are unaffected by in vitro culture, and the isolation at cold temperatures maintains the cells relatively unaltered in phenotype by avoiding activation through receptor cross-linking or plastic adherence. Simultaneous analysis of several cell types provides the opportunity to obtain detailed pictures of transcriptomic differences between them. Here, we present such an analysis focusing on four human blood cell populations and compare those to isolated human skin mast cells. Pure CD19+ peripheral blood B cells, CD14+ monocytes, and CD4+ and CD8+ T cells were obtained by fluorescence-activated cell sorting, and KIT+ human connective tissue mast cells (MCs) were purified by MACS sorting from healthy skin. Detailed information concerning expression levels of the different granule proteases, protease inhibitors, Fc receptors, other receptors, transcription factors, cell signaling components, cytoskeletal proteins, and many other protein families relevant to the functions of these cells were obtained and comprehensively discussed. The MC granule proteases were found exclusively in the MC samples, and the T-cell granzymes in the T cells, of which several were present in both CD4+ and CD8+ T cells. High levels of CD4 were also observed in MCs and monocytes. We found a large variation between the different cell populations in the expression of Fc receptors, as well as for lipid mediators, proteoglycan synthesis enzymes, cytokines, cytokine receptors, and transcription factors. This detailed quantitative comparative analysis of more than 780 proteins of importance for the function of these populations can now serve as a good reference material for research into how these entities shape the role of these cells in immunity and tissue homeostasis.

The Role of the Transcriptional Coactivator BOB.1/OBF.1 in Adaptive Immunity

BOB.1/OBF.1 is a transcriptional coactivator involved in octamer-dependent transcription. Thereby, BOB.1/OBF.1 is involved in the transcriptional regulation of genes important for lymphocyte physiology. BOB.1/OBF.1-deficient mice reveal multiple B- and T-cell developmental defects. The most prominent defect of these mice is the complete absence of germinal centers (GCs) resulting in severely impaired T-cell-dependent immune responses. In humans, BOB.1/OBF.1 is associated with several autoimmune and inflammatory diseases but also linked to liquid and solid tumors. Although its role for B-cell development is relatively well understood, its exact role for the GC reaction and T-cell biology has long been unclear. Here, the contribution of BOB.1/OBF.1 for B-cell maturation is summarized, and recent findings regarding its function in GC B- as well as in various T-cell populations are discussed. Finally, a detailed perspective on how BOB.1/OBF.1 contributes to different pathologies is provided.

The transcriptional program during germinal center reaction - a close view at GC B cells, Tfh cells and Tfr cells

The germinal center (GC) reaction is a key process during an adaptive immune response to T cell specific antigens. GCs are specialized structures within secondary lymphoid organs, in which B cell proliferation, somatic hypermutation and antibody affinity maturation occur. As a result, high affinity antibody secreting plasma cells and memory B cells are generated. An effective GC response needs interaction between multiple cell types. Besides reticular cells and follicular dendritic cells, particularly B cells, T follicular helper (Tfh) cells as well as T follicular regulatory (Tfr) cells are a key player during the GC reaction. Whereas Tfh cells provide help to GC B cells in selection processes, Tfr cells, a specialized subset of regulatory T cells (Tregs), are able to suppress the GC reaction maintaining the balance between immune activation and tolerance. The formation and function of GCs is regulated by a complex network of signals and molecules at multiple levels. In this review, we highlight recent developments in GC biology by focusing on the transcriptional program regulating the GC reaction. This review focuses on the transcriptional co-activator BOB.1/OBF.1, whose important role for GC B, Tfh and Tfr cell differentiation became increasingly clear in recent years. Moreover, we outline how deregulation of the GC transcriptional program can drive lymphomagenesis.

T Cell Specific BOB.1/OBF.1 Expression Promotes Germinal Center Response and T Helper Cell Differentiation

The transcriptional co-activator BOB.1/OBF.1 is expressed in both B and T cells. The main characteristic of conventional BOB.1/OBF.1 deficient mice is the complete absence of germinal centers (GCs). This defect was mainly attributed to the defective B cell compartment. However, it is unknown whether and how BOB.1/OBF.1 expression in T cells contributes to the GC reaction. To finally clarify this question, we studied the in vivo function of BOB.1/OBF.1 in CD4⁺ T and follicular T helper (TFH) cell subpopulations by conditional mutagenesis, in the presence of immunocompetent B lymphocytes. BOB.1/OBF.1 deletion in CD4⁺ T as well as TFH cells resulted in impaired GC formation demonstrating that the impaired GC reaction described for conventional BOB.1/OBF.1-deficient mice cannot exclusively be traced back to the B cell compartment. Furthermore, we show a requirement of BOB.1/OBF.1 for T helper (TH) cell subsets, particularly for TFH cell differentiation.

Differential Requirement of Vav Proteins for Btk-dependent and –Independent Signaling During B Cell Development

Btk and Vav proteins are all components of the signalosome that builds upon B cell receptor (BCR) activation. However, the role of Vav proteins within the signalosome is quite complex and not yet fully understood. Until now, studies of these have focused predominantly on a deficiency of Vav proteins alone or in combination with other Vav protein family members. Since a physical association of Btk with Vav was shown previously, we asked whether these molecules lie in the same or independent signaling pathways. By analyzing Vav1 and Vav3 single knock-out mice and generating double-knock-out animals deficient for either Vav1 or Vav3 and Btk, we observed, in line with previous publications, no severe B cell developmental defects when either Vav1 or Vav3 alone are not expressed. However, a simultaneous deficiency of Btk together with either Vav1 or Vav3 leads to a severe reduction of splenic B cells, which exhibit an immature phenotype. B cell developmental defects of Btk/Vav1-double deficient mice in the periphery were more severe than those observed in Btk-single-deficient animals. Additionally, morphological changes in splenic microarchitecture were observed in double- but also in single-knock-out mutants. These observations were accompanied by reduced BCR-induced Ca²⁺ mobilization, proliferation, germinal center formation and immunoglobulin secretion. Although deletion of Btk alone impaired Ca²⁺ mobilization upon BCR activation, the defect was even more severe when Vav1 or Vav3 were also mutated, indicating that Btk and the Vav proteins act in separate pathways that converge on Ca2+ signaling. In vitro ASC differentiation suggests that both B and T cells contribute to the observed phenotype of a Btk/Vav-double deficiency. Our results show that Vav proteins and Btk are both components of the BCR-activated signalosome but control separate signaling pathways important for B cell development.

A broad and systematic approach to identify B cell malignancy-targeting TCRs for multi-antigen-based T cell therapy

CAR T cell therapy has shown great promise for the treatment of B cell malignancies. However, antigen-negative escape variants often cause disease relapse, necessitating the development of multi-antigen-targeting approaches. We propose that a T cell receptor (TCR)-based strategy would increase the number of potential antigenic targets, as peptides from both intracellular and extracellular proteins can be recognized. Here, we aimed to isolate a broad range of promising TCRs targeting multiple antigens for treatment of B cell malignancies. As a first step, 28 target genes for B cell malignancies were selected based on gene expression profiles. Twenty target peptides presented in human leukocyte antigen (HLA)-A∗01:01, -A∗24:02, -B∗08:01, or -B∗35:01 were identified from the immunopeptidome of B cell malignancies and used to form peptide-HLA (pHLA)-tetramers for T cell isolation. Target-peptide-specific CD8 T cells were isolated from HLA-mismatched healthy donors and subjected to a stringent stepwise selection procedure to ensure potency and eliminate cross-reactivity. In total, five T cell clones specific for FCRL5 in HLA-A∗01:01, VPREB3 in HLA-A∗24:02, and BOB1 in HLA-B∗35:01 recognized B cell malignancies. For all three specificities, TCR gene transfer into CD8 T cells resulted in cytokine production and efficient killing of multiple B cell malignancies. In conclusion, using this systematic approach we successfully identified three promising TCRs for T cell therapy against B cell malignancies.

BOB.1/OBF.1 is required during B-cell ontogeny for B-cell differentiation and germinal center function

BOB.1/OBF.1 is a lymphocyte-specific transcriptional co-activator of octamer-dependent transcription. It regulates the expression of genes important for lymphocyte physiology together with the Oct-1 and Oct-2 transcription factors. So far, BOB.1/OBF.1 has been studied in conventional knockout mice, whereby a function of BOB.1/OBF.1 in B but also in T cells was described. The main characteristic of BOB.1/OBF.1-deficient mice is the complete absence of germinal centers. However, it is entirely unsolved at which stage of B-cell development BOB.1/OBF.1 expression is essential for germinal center formation. Still, it is not known whether defects observed late in B-cell development of BOB.1/OBF.1-deficient mice are merely a consequence of defective early B-cell development. To answer the question, whether BOB.1/OBF.1 expression is required before or during the process of germinal center formation, we established a mouse system, which allows the conditional deletion of BOB.1/OBF.1 at different stages of B-cell development. Our data reveal a requirement for BOB.1/OBF.1 during both early antigen-independent and late antigen-dependent B-cell development, and further a requirement for efficient germinal center reaction during complete B-cell ontogeny. By specifically deleting BOB.1/OBF.1 in germinal center B cells, we provide evidence that the failure to form germinal centers is a germinal center B-cell intrinsic defect and not exclusively a consequence of defective early B-cell maturation.

Transcriptional regulator BOB.1: Molecular mechanisms and emerging role in chronic inflammation and autoimmunity

Lymphocytes constitute an essential and potent effector compartment of the immune system. Therefore, their development and functions must be strictly regulated to avoid inappropriate immune responses, such as autoimmune reactions. Several lines of evidence from genetics (e.g. association with multiple sclerosis and primary biliary cirrhosis), human expression studies (e.g. increased expression in target tissues and draining lymph nodes of patients with autoimmune diseases), animal models (e.g. loss of functional protein protects animals from the development of collagen-induced arthritis, experimental autoimmune encephalomyelitis, type 1 diabetes, bleomycin-induced fibrosis) strongly support a causal link between the aberrant expression of the lymphocyte-restricted transcriptional regulator BOB.1 and the development of autoimmune diseases. In this review, we summarize the current knowledge of unusual structural and functional plasticity of BOB.1, stringent regulation of its expression, and the pivotal role that BOB.1 plays in shaping B- and T-cell responses. We discuss recent developments highlighting the significant contribution of BOB.1 to the pathogenesis of autoimmune diseases and how to leverage our knowledge to target this regulator to treat autoimmune tissue inflammation.

Loss of OcaB Prevents Age-Induced Fat Accretion and Insulin Resistance by Altering B-Lymphocyte Transition and Promoting Energy Expenditure

The current demographic shift toward an aging population has led to a robust increase in the prevalence of age-associated metabolic disorders. Recent studies have demonstrated that the etiology of obesity-related insulin resistance that develops with aging differs from that induced by high-calorie diets. Whereas the role of adaptive immunity in changes in energy metabolism driven by nutritional challenges has recently gained attention, its impact on aging remains mostly unknown. Here we found that the number of follicular B2 lymphocytes and expression of the B-cell-specific transcriptional coactivator OcaB increase with age in spleen and in intra-abdominal epididymal white adipose tissue (eWAT), concomitantly with higher circulating levels of IgG and impaired glucose homeostasis. Reduction of B-cell maturation and Ig production-especially that of IgG2c-by ablation of OcaB prevented age-induced glucose intolerance and insulin resistance and promoted energy expenditure by stimulating fatty acid utilization in eWAT and brown adipose tissue. Transfer of wild-type bone marrow in OcaB^-/- mice replenished the eWAT B2-cell population and IgG levels, which diminished glucose tolerance, insulin sensitivity, and energy expenditure while increasing body weight gain in aged mice. Thus these findings demonstrate that upon aging, modifications in B-cell-driven adaptive immunity contribute to glucose intolerance and fat accretion.

TCR-based therapy for multiple myeloma and other B-cell malignancies targeting intracellular transcription factor BOB1

Immunotherapy for hematological malignancies or solid tumors by administration of monoclonal antibodies or T cells engineered to express chimeric antigen receptors or T-cell receptors (TCRs) has demonstrated clinical efficacy. However, antigen-loss tumor escape variants and the absence of currently targeted antigens on several malignancies hamper the widespread application of immunotherapy. We have isolated a TCR targeting a peptide of the intracellular B cell-specific transcription factor BOB1 presented in the context of HLA-B*07:02. TCR gene transfer installed BOB1 specificity and reactivity onto recipient T cells. TCR-transduced T cells efficiently lysed primary B-cell leukemia, mantle cell lymphoma, and multiple myeloma in vitro. We also observed recognition and lysis of healthy BOB1-expressing B cells. In addition, strong BOB1-specific proliferation could be demonstrated for TCR-modified T cells upon antigen encounter. Furthermore, clear in vivo antitumor reactivity was observed of BOB1-specific TCR-engineered T cells in a xenograft mouse model of established multiple myeloma. Absence of reactivity toward a broad panel of BOB1^- but HLA-B*07:02⁺ nonhematopoietic and hematopoietic cells indicated no off-target toxicity. Therefore, administration of BOB1-specific TCR-engineered T cells may provide novel cellular treatment options to patients with B-cell malignancies, including multiple myeloma.

Effect of antipyretic analgesics on immune responses to vaccination

While antipyretic analgesics are widely used to ameliorate vaccine adverse reactions, their use has been associated with blunted vaccine immune responses. Our objective was to review literature evaluating the effect of antipyretic analgesics on vaccine immune responses and to highlight potential underlying mechanisms. Observational studies reporting on antipyretic use around the time of immunization concluded that their use did not affect antibody responses. Only few randomized clinical trials demonstrated blunted antibody response of unknown clinical significance. This effect has only been noted following primary vaccination with novel antigens and disappears following booster immunization. The mechanism by which antipyretic analgesics reduce antibody response remains unclear and not fully explained by COX enzyme inhibition. Recent work has focused on the involvement of nuclear and subcellular signaling pathways. More detailed immunological investigations and a systems biology approach are needed to precisely define the impact and mechanism of antipyretic effects on vaccine immune responses.

The transcriptional coactivator Bob1 promotes the development of follicular T helper cells via Bcl6

Follicular T helper (Tfh) cells are key regulators of the germinal center reaction and long‐term humoral immunity. Tfh cell differentiation requires the sustained expression of the transcriptional repressor Bcl6; however, its regulation in CD4⁺ T cells is incompletely understood. Here, we report that the transcriptional coactivator Bob1, encoded by the Pou2af1 gene, promotes Bcl6 expression and Tfh cell development. We found that Bob1 together with the octamer transcription factors Oct1/Oct2 can directly bind to and transactivate the Bcl6 and Btla promoters. Mixed bone marrow chimeras revealed that Bob1 is required for the expression of normal levels of Bcl6 and BTLA, thereby controlling the pool size and composition of the Tfh compartment in a T cell‐intrinsic manner. Our data indicate that T cell‐expressed Bob1 is directly involved in Tfh cell differentiation and required for mounting normal T cell‐dependent B‐cell responses.

Oct2 and Obf1 as Facilitators of B:T Cell Collaboration during a Humoral Immune Response

The Oct2 protein, encoded by the Pou2f2 gene, was originally predicted to act as a DNA binding transcriptional activator of immunoglobulin (Ig) in B lineage cells. This prediction flowed from the earlier observation that an 8-bp sequence, the “octamer motif,” was a highly conserved component of most Ig gene promoters and enhancers, and evidence from over-expression and reporter assays confirmed Oct2-mediated, octamer-dependent gene expression. Complexity was added to the story when Oct1, an independently encoded protein, ubiquitously expressed from the Pou2f1 gene, was characterized and found to bind to the octamer motif with almost identical specificity, and later, when the co-activator Obf1 (OCA-B, Bob.1), encoded by the Pou2af1 gene, was cloned. Obf1 joins Oct2 (and Oct1) on the DNA of a subset of octamer motifs to enhance their transactivation strength. While these proteins variously carried the mantle of determinants of Ig gene expression in B cells for many years, such a role has not been borne out for them by characterization of mice lacking functional copies of the genes, either as single or as compound mutants. Instead, we and others have shown that Oct2 and Obf1 are required for B cells to mature fully in vivo, for B cells to respond to the T cell cytokines IL5 and IL4, and for B cells to produce IL6 normally during a T cell dependent immune response. We show here that Oct2 affects Syk gene expression, thus influencing B cell receptor signaling, and that Oct2 loss blocks Slamf1 expression in vivo as a result of incomplete B cell maturation. Upon IL4 signaling, Stat6 up-regulates Obf1, indirectly via Xbp1, to enable plasma cell differentiation. Thus, Oct2 and Obf1 enable B cells to respond normally to antigen receptor signals, to express surface receptors that mediate physical interaction with T cells, or to produce and respond to cytokines that are critical drivers of B cell and T cell differentiation during a humoral immune response.

A C-terminal acidic domain regulates degradation of the transcriptional coactivator Bob1

Bob1 (Obf-1 or OCA-B) is a 34-kDa transcriptional coactivator encoded by the Pou2af1 gene that is essential for normal B-cell development and immune responses in mice. During lymphocyte activation, Bob1 protein levels dramatically increase independently of mRNA levels, suggesting that the stability of Bob1 is regulated. We used a fluorescent protein-based reporter system to analyze protein stability in response to genetic and physiological perturbations and show that, while Bob1 degradation is proteasome mediated, it does not require ubiquitination of Bob1. Furthermore, degradation of Bob1 in B cells appears to be largely independent of the E3 ubiquitin ligase Siah. We propose a novel mechanism of Bob1 turnover in B cells, whereby an acidic region in the C terminus of Bob1 regulates the activity of degron signals elsewhere in the protein. Changes that make the C terminus more acidic, including tyrosine phosphorylation-mimetic mutations, stabilize the instable murine Bob1 protein, indicating that B cells may regulate Bob1 stability and activity via signaling pathways. Finally, we show that expressing a stable Bob1 mutant in B cells suppresses cell proliferation and induces changes in surface marker expression commonly seen during B-cell differentiation.

Octamer-dependent transcription in T cells is mediated by NFAT and NF-κB

The transcriptional co-activator BOB.1/OBF.1 was originally identified in B cells and is constitutively expressed throughout B cell development. BOB.1/OBF.1 associates with the transcription factors Oct1 and Oct2, thereby enhancing octamer-dependent transcription. In contrast, in T cells, BOB.1/OBF.1 expression is inducible by treatment of cells with PMA/Ionomycin or by antigen receptor engagement, indicating a marked difference in the regulation of BOB.1/OBF.1 expression in B versus T cells. The molecular mechanisms underlying the differential expression of BOB.1/OBF.1 in T and B cells remain largely unknown. Therefore, the present study focuses on mechanisms controlling the transcriptional regulation of BOB.1/OBF.1 and Oct2 in T cells. We show that both calcineurin- and NF-κB-inhibitors efficiently attenuate the expression of BOB.1/OBF.1 and Oct2 in T cells. In silico analyses of the BOB.1/OBF.1 promoter revealed the presence of previously unappreciated combined NFAT/NF-κB sites. An array of genetic and biochemical analyses illustrates the involvement of the Ca²⁺/calmodulin-dependent phosphatase calcineurin as well as NFAT and NF-κB transcription factors in the transcriptional regulation of octamer-dependent transcription in T cells. Conclusively, impaired expression of BOB.1/OBF.1 and Oct2 and therefore a hampered octamer-dependent transcription may participate in T cell-mediated immunodeficiency caused by the deletion of NFAT or NF-κB transcription factors.

Direct interactions of OCA-B and TFII-I regulate immunoglobulin heavy-chain gene transcription by facilitating enhancer-promoter communication

B cell-specific coactivator OCA-B, together with Oct-1/2, binds to octamer sites in promoters and enhancers to activate transcription of immunoglobulin (Ig) genes, although the mechanisms underlying their roles in enhancer-promoter communication are unknown. Here, we demonstrate a direct interaction of OCA-B with transcription factor TFII-I, which binds to DICE elements in Igh promoters, that affects transcription at two levels. First, OCA-B relieves HDAC3-mediated Igh promoter repression by competing with HDAC3 for binding to promoter-bound TFII-I. Second, and most importantly, Igh 3' enhancer-bound OCA-B and promoter-bound TFII-I mediate promoter-enhancer interactions, in both cis and trans, that are important for Igh transcription. These and other results reveal an important function for OCA-B in Igh 3' enhancer function in vivo and strongly favor an enhancer mechanism involving looping and facilitated factor recruitment rather than a tracking mechanism.

B lymphocyte lineage specification, commitment and epigenetic control of transcription by early B cell factor 1

Early B cell factor 1 (EBF1) is a transcription factor that is critical for both B lymphopoiesis and B cell function. EBF1 is a requisite component of the B lymphocyte transcriptional network and is essential for B lineage specification. Recent studies revealed roles for EBF1 in B cell commitment. EBF1 binds its target genes via a DNA-binding domain including a unique 'zinc knuckle', which mediates a novel mode of DNA recognition. Chromatin immunoprecipitation of EBF1 in pro-B cells defined hundreds of new, as well as previously identified, target genes. Notably, expression of the pre-B cell receptor (pre-BCR), BCR and PI3K/Akt/mTOR signaling pathways is controlled by EBF1. In this review, we highlight these current developments and explore how EBF1 functions as a tissue-specific regulator of chromatin structure at B cell-specific genes.

BMP2-induced gene profiling in dental epithelial cell line

Tooth development is regulated by epithelial-mesenchymal interactions and their reciprocal molecular signaling. Bone morphogenetic protein 2 (BMP2) is known as one of the inducers for tooth development. To analyze the molecular mechanisms of BMP2 on ameloblast differentiation (amelogenesis), we performed microarray analyses using rat dental epithelial cell line, HAT-7. After confirming that BMP2 could activate the canonical BMP-Smads signaling in HAT-7 cells, we analyzed the effects of BMP2 on 14,815 gene expressions and profiled them. Seventy-three genes were up-regulated and 28 genes were down-regulated by BMP2 treatment for 24 hours in HAT-7 cells. Functional classification revealed that 18% of up-regulated genes were ECM/adhesion molecules present in the enamel organ. Furthermore, we examined the expression of several differentiation markers in dental epithelial four cell-lineages including inner enamel epithelium (ameloblasts), stratum intermedium, stratum reticulum, and outer enamel epithelium. The results indicated that BMP2 might induce at least two different cell-lineage markers including a BMP antagonist expressed in HAT-7 cells, suggesting that BMP2 could accelerate amelogenesis via BMP signaling.

Enforced expression of the transcriptional coactivator OBF1 impairs B cell differentiation at the earliest stage of development

OBF1, also known as Bob.1 or OCA-B, is a B lymphocyte-specific transcription factor which coactivates Oct1 and Oct2 on B cell specific promoters. So far, the function of OBF1 has been mainly identified in late stage B cell populations. The central defect of OBF1 deficient mice is a severely reduced immune response to T cell-dependent antigens and a lack of germinal center formation in the spleen. Relatively little is known about a potential function of OBF1 in developing B cells. Here we have generated transgenic mice overexpressing OBF1 in B cells under the control of the immunoglobulin heavy chain promoter and enhancer. Surprisingly, these mice have greatly reduced numbers of follicular B cells in the periphery and have a compromised immune response. Furthermore, B cell differentiation is impaired at an early stage in the bone marrow: a first block is observed during B cell commitment and a second differentiation block is seen at the large preB2 cell stage. The cells that succeed to escape the block and to differentiate into mature B cells have post-translationally downregulated the expression of transgene, indicating that expression of OBF1 beyond the normal level early in B cell development is deleterious. Transcriptome analysis identified genes deregulated in these mice and Id2 and Id3, two known negative regulators of B cell differentiation, were found to be upregulated in the EPLM and preB cells of the transgenic mice. Furthermore, the Id2 and Id3 promoters contain octamer-like sites, to which OBF1 can bind. These results provide evidence that tight regulation of OBF1 expression in early B cells is essential to allow efficient B lymphocyte differentiation.

Identification of murine B cell lines that undergo somatic hypermutation focused to A:T and G:C residues

Activation-induced deaminase (AID) is the master regulator of class switch recombination (CSR) and somatic hypermutation (SHM), but the mechanisms regulating AID function are obscure. The differential pattern of switch plasmid activity in three IgM(+)/AID(+) and two IgG(+)/AID(+) B cell lines prompted an analysis of global gene expression to discover the origin of these cells. Gene profiling suggested that the IgG(+)/AID(+) B cell lines derived from germinal center B cells. Analysis of SHM potential demonstrates that the IgVkappa domains are inducibly diversified at high rate during in vitro culture. The mutation spectra focused to A:T base pairs, revealing a component of the hypermutation program that occurs preferentially during phase 2 of SHM. The A:T error spectra were analyzed and were not characteristic of polymerase eta activity. A differential pattern of three consensus motifs used for A:T base substitutions was observed in WT and Poleta-, Msh2- and Msh6-deficient B cells. Strikingly, mutations in our B cell lines recapitulated the mutable motif profile for Poleta and Msh2 deficiency, respectively, and suggest that an additional pathway for the generation of A:T mutations in SHM is conserved in mouse and human.

OBF-1 is essential for the generation of antibody-secreting cells and the development of autoimmunity in MRL-lpr mice

As reported previously, the lack of the transcriptional co-activator OBF-1 prevented development of autoimmunity in Aiolos knockout mice. To further investigate the role and mechanism of OBF-1 in autoimmunity, we crossed OBF-1 null mice with MRL-lpr mice and generated OBF-1-deficent MRL-lpr mice. OBF-1 deletion abrogated all autoantibodies in the MRL-lpr mice, including anti-dsDNA Ab and anti-Sm Ab. The failure to produce autoantibodies was not related to development of immature or mature B cells, but correlated with severely reduced antibody-secreting cells (ASCs). The loss of OBF-1 protected against hypergammaglobulinemia, immune complex deposition, glomerulonephritis, and early mortality in MRL-lpr mice. In addition, accumulation of CD4(-)CD8(-)B220(+)CD3(+) T cells that characteristically develop in Fas mutation mice were markedly reduced in MRL-lpr mice without OBF-1. These results identify OBF-1 as a critical gene in the development of autoantibodies and reveal an essential role for OBF-1 in the generation of antibody/autoantibody-secreting cells in vivo.

BOB.1/OBF.1 controls the balance of TH1 and TH2 immune responses

BOB.1/OBF.1 is a transcriptional coactivator essential at several stages of B-cell development. In T cells, BOB.1/OBF.1 expression is inducible by co-stimulation. However, a defined role of BOB.1/OBF.1 for T-cell function had not been discovered so far. Here, we show that BOB.1/OBF.1 is critical for T helper cell function. BOB.1/OBF.1(-/-) mice showed imbalanced immune responses, resulting in increased susceptibility to Leishmania major infection. Functional analyses revealed specific defects in TH1 and TH2 cells. Whereas expression levels of TH1 cytokines were reduced, the secretion of TH2 cytokines was increased. BOB.1/OBF.1 directly contributes to the IFNgamma and IL2 promoter activities. In contrast, increased TH2 cytokine production is controlled indirectly, probably via the transcription factor PU.1, the expression of which is regulated by BOB.1/OBF.1. Thus, BOB.1/OBF.1 regulates the balance of TH1 versus TH2 mediated immunity.

Vav promoter-tTA conditional transgene expression system for hematopoietic cells drives high level expression in developing B and T cells

OBJECTIVE

We previously showed that Vav promoter-tetracycline transactivator (Vav-tTA)-driven tetracycline-regulated element (TRE)-NRAS(V12) expression resulted in mastocytosis development in mice. To investigate which hematopoietic cells express TRE-driven transgenes when combined with Vav-tTA, we assayed hematopoietic cells, including bone marrow-derived mast cells (BMMC) and CD34-positive hematopoietic progenitor cells (HPC) as well as myeloid and lymphoid lineages. To determine if suppression of NRAS(V12) expression early in life would delay mastocytosis we treated developing and juvenile mice with doxycycline (Dox).

MATERIALS AND METHODS

Vav-tTA-driven luciferase expression was assayed by live mouse imaging and relative light unit measurement before or after treating Vav-tTA and TRE-luciferase (TRE-Luc) cotransgenic mice with Dox. Magnetic cell sorting and fluorescence-activating cell sorting methods were used to sort hematopoietic cells. To suppress TRE-mediated luciferase or NRAS(V12) expression in Vav-tTA cotransgenic mice, we added Dox to the drinking water.

RESULTS

B cells in the bone marrow and T cells in the thymus expressed Vav-tTA-driven luciferase at much higher levels than in myeloid cells, BMMC, and CD34-positive HPC, which showed relatively low levels. Dox treatment completely eliminated the luciferase expression from all hematopoietic cells. Repression of TRE-NRAS(V12) expression early in life was sufficient to increase the latency of mastocytosis development.

CONCLUSION

The Vav-tTA transgenic line will be very useful for conditional transgene expression in developing B and T cells. Vav-tTA-driven NRAS(V12) expression is sufficient for mastocytosis development, but not for myeloid leukemia. Lymphoid cells are resistant to NRAS(V12) transformation despite high level of expression.

Expression of the B cell-associated transcription factors PAX5, OCT-2, and BOB.1 in acute myeloid leukemia: associations with B-cell antigen expression and myelomonocytic maturation

The aberrant expression of the B-cell transcription factor PAX5 has been described in a subset of acute myeloid leukemia (AML) with t(8;21)(q22;q22) in association with B-cell antigen expression. However, the expression of other B cell-associated transcription factors, particularly OCT-2 and its B cell-specific coactivator BOB.1, has not been described in AML. In this study, expression of PAX5, OCT-2 and BOB.1 was evaluated by immunohistochemical staining of bone marrow samples from 83 cases of AML. The expression patterns were correlated with t(8;21)(q22;q22), B cell-associated antigen expression, and AML subtype. We confirmed the expression of PAX5 in AML with t(8;21)(q22;q22), but also demonstrated its expression in cases that express B-cell antigens but lack this translocation. Although OCT-2 and BOB.1 were not associated with PAX5 expression, we report expression of OCT-2 in AML with myelomonocytic/monocytic maturation and BOB.1 in normal hematopoietic elements.

The Ets factor Spi-B is a direct critical target of the coactivator OBF-1

OBF-1 (Bob.1, OCA-B) is a lymphoid-specific transcriptional coactivator that associates with the transcription factors Oct-1 or Oct-2 on the conserved octamer element present in the promoters of several ubiquitous and lymphoid-specific genes. OBF-1-deficient mice have B cell-intrinsic defects, lack germinal centers, and have severely impaired immune responses to T cell-dependent antigens. Crucial genes that are regulated by OBF-1 and that might explain the observed phenotype of OBF-1 deficiency have remained elusive to date. Here we have generated transgenic mice expressing OBF-1 specifically in T cells and examined these together with mice lacking OBF-1 to discover transcriptional targets of this coactivator. Using microarray analysis, we have identified the Ets transcription factor Spi-B as a direct target gene critically regulated by OBF-1 that can help explain the phenotype of OBF-1-deficient mice. Spi-B has been implicated in signaling pathways downstream of the B cell receptor and is essential for germinal center formation and maintenance. The present findings establish a hierarchy between these two factors and provide a molecular link between OBF-1 and B cell receptor signaling.

Nontranscriptional regulation of SYK by the coactivator OCA-B is required at multiple stages of B cell development

OCA-B was originally identified as a nuclear transcriptional coactivator that is essential for antigen-driven immune responses. The later identification of a membrane bound, myristoylated form of OCA-B suggested additional, unique functions in B cell signaling pathways. This study has shown that OCA-B also functions in the pre-B1-to-pre-B2 cell transition and, most surprisingly, that it directly interacts with SYK, a tyrosine kinase critical for pre-BCR and BCR signaling. This unprecedented type of interaction-a transcriptional coactivator with a signaling kinase-occurs in the cytoplasm and directly regulates SYK stability. This study indicates that OCA-B is required for pre-BCR and BCR signaling at multiple stages of B cell development through its nontranscriptional regulation of SYK. Combined with the deregulation of OCA-B target genes, this may help explain the multitude of defects observed in B cell development and immune responses of Oca-b-/- mice.

Regulation of peripheral B cell maturation

Although it is clear that the final phases of B cell maturation occur after newly formed B cells exit the bone marrow, the mechanisms underpinning the maturation, selection, and long-term survival of immature peripheral B cells remain poorly understood. Here, we review recent advances in our understanding of how B cell receptor (BCR)-mediated signaling events integrate with additional environmental cues to promote the selection and differentiation of immature B cells into functionally distinct subpopulations of mature B cells. We pay particular attention to the role of the Baff cytokine family and the Notch receptor-ligand family and their unique roles in promoting B cell survival and differentiation into follicular and marginal zone B cells.

Interaction of the B Cell-specific Transcriptional Coactivator OCA-B and Galectin-1 and a Possible Role in Regulating BCR-mediated B Cell Proliferation

OCA-B is a B cell-specific transcriptional coactivator for OCT factors during the activation of immunoglobulin genes. In addition, OCA-B is crucial for B cell activation and germinal center formation. However, the molecular mechanisms for OCA-B function in these processes are not clear. Our previous studies documented two OCA-B isoforms and suggested a novel mechanism for the function of the myristoylated, membrane-bound form of OCA-B/p35 as a signaling molecule. Here, we report the identification of galectin-1, and related galectins, as a novel OCA-B-interacting protein. The interaction of OCA-B and galectin-1 can be detected both in vivo and in vitro. The galectin-1 binding domain in OCA-B has been localized to the N terminus of OCA-B. In B cells lacking OCA-B expression, increased galectin-1 expression, secretion, and cell surface association are observed. Consistent with these observations, and a reported inhibitory interaction of galectin-1 with CD45, the phosphatase activity of CD45 is reduced modestly, but significantly, in OCA-B-deficient B cells. Finally, galectin-1 is shown to negatively regulate B cell proliferation and tyrosine phosphorylation upon BCR stimulation. Together, these results raise the possibility that OCA-B may regulate BCR signaling through an association with galectin-1.

Rap1 signal controls B cell receptor repertoire and generation of self-reactive B1a cells

We previously reported that the mice deficient for SPA-1, a Rap1 GTPase-activating protein, developed hematopoietic stem cell disorders. Here, we demonstrate that SPA-1(-/-) mice show an age-dependent increase in B220(high) B1a cells producing anti-dsDNA antibody and lupus-like nephritis. SPA-1(-/-) peritoneal B1 cells revealed the altered Vkappa gene repertoire, including skewed Vkappa4 usage and the significant Igkappa/Iglambda isotype inclusion indicative of extensive receptor editing. Rap1GTP induced OcaB gene activation via p38MAPK-dependent Creb phosphorylation, and consistently, SPA-1(-/-) immature BM B cells showing high Rap1GTP exhibited the augmented expression of OcaB and Vkappa4 genes. SPA-1(-/-) BM cells could transfer the autoimmunity in association with the generation of peritoneal B220(high) B1a cells in Rag-2(-/-) recipients. Finally, a portion of SPA-1(-/-) mice developed B1 cell leukemia with hemolytic autoantibody. Present results suggest that the regulated Rap1 signal in the immature B cells plays a role in modifying the B cell receptor repertoire and in maintaining the self-tolerance.

Dysregulated TCL1 requires the germinal center and genome instability for mature B-cell transformation

Most lymphomas arise by transformation of germinal center (GC) B cells. TCL1, a proto-oncogene first recognized for its role in T-cell transformation, also induces GC B-cell malignancies when dysregulated in pEmu-B29-TCL1 transgenic (TCL1-tg) mice. Clonal B-cell lymphomas develop from polyclonal populations with latencies of 4 months or more, suggesting that secondary genetic events are required for full transformation. The goals of this study were to determine the GC-related effects of TCL1 dysregulation that contribute to tumor initiation and to identify companion genetic alterations in tumors that function in disease progression. We report that compared with wild-type (WT) cells, B cells from TCL1-tg mice activated in a manner resembling a T-dependent GC reaction show enhanced resistance to FAS-mediated apoptosis with CD40 stimulation, independent of a B-cell antigen receptor (BCR) rescue signal. Mitogenic stimulation of TCL1-tg B cells also resulted in increased expression of Aicda. These GC-related enhancements in survival and Aicda expression could underlie B-cell transformation. Supporting this notion, no B-cell lymphomas developed for 20 months when TCL1-tg mice were crossed onto an Oct coactivator from B cell (OCA-B)-deficient background to yield mice incapable of forming GCs. Spectral karyotype analyses showed that GC lymphomas from TCL1-tg mice exhibit recurrent chromosome translocations and trisomy 15, with corresponding MYC overexpression. We conclude that pEmu-B29-TCL1 transgenic B cells primed for transformation must experience the GC environment and, for at least some, develop genome instability to become fully malignant.

Tetracycline-controlled transgenic targeting from the SCL locus directs conditional expression to erythrocytes, megakaryocytes, granulocytes, and c-kit-expressing lineage-negative hematopoietic cells

The stem cell leukemia gene SCL, also known as TAL-1, encodes a basic helix-loop-helix transcription factor expressed in erythroid, myeloid, megakaryocytic, and hematopoietic stem cells. To be able to make use of the unique tissue-restricted and spatio-temporal expression pattern of the SCL gene, we have generated a knock-in mouse line containing the tTA-2S tetracycline transactivator under the control of SCL regulatory elements. Analysis of this mouse using different tetracycline-dependent reporter strains demonstrated that switchable transgene expression was restricted to erythrocytes, megakaryocytes, granulocytes, and, importantly, to the c-kit-expressing and lineage-negative cell fraction of the bone marrow. In addition, conditional transgene activation also was detected in a very minor population of endothelial cells and in the kidney. However, no activation of the reporter transgene was found in the brain of adult mice. These findings suggested that the expression of tetracycline-responsive reporter genes recapitulated the known endogenous expression pattern of SCL. Our data therefore demonstrate that exogenously inducible and reversible expression of selected transgenes in myeloid, megakaryocytic, erythroid, and c-kit-expressing lineage-negative bone marrow cells can be directed through SCL regulatory elements. The SCL knock-in mouse presented here represents a powerful tool for studying normal and malignant hematopoiesis in vivo.

Btk expression is controlled by Oct and BOB.1/OBF.1

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds together with the Oct1 and Oct2 transcription factors to DNA and enhances their transactivation potential. Mice deficient for the transcriptional coactivator BOB.1/OBF.1 show several defects in differentiation, function and signaling of B cells. In search of BOB.1/OBF.1 regulated genes we identified Btk—a cytoplasmic tyrosine kinase—as a direct target of BOB.1/OBF.1. Analyses of the human as well as murine Btk promoters revealed a non-consensus octamer site close to the start site of transcription. Here we show that Oct proteins together with BOB.1/OBF.1 are able to form ternary complexes on these sites in vitro and in vivo. This in turn leads to the induction of Btk promoter activity in synergism with the transcription factor PU.1. Btk, like BOB.1/OBF.1, plays a critical role in B cell development and B cell receptor signalling. Therefore the down-regulation of Btk expression in BOB.1/OBF.1-deficient B cells could be related to the functional and developmental defects observed in BOB.1/OBF.1-deficient mice.

Transcriptional control of B cell activation

The developmental program that commits a hematopoietic stem cell to the B lymphocyte lineage employs transcriptional regulators to enable the assembly of an antigen receptor complex with a useful specificity and with signalling competence. Once a naive IgM+ B cell is generated, it must correctly integrate signals from the antigen receptor with those from cytokine receptors and co-receptors delivering T cell help. The B cell responds through the regulated expression of genes that implement specific cell expansion and differentiation, secretion of high levels of high-affinity antibody, and generation of long-term memory. The transcriptional regulators highlighted in this chapter are those for which genetic evidence of function in IgM+ B cells in vivo has been provided, often in the form of mutant mice generated by conventional or conditional gene targeting. A critical developmental step is the maturation of bone marrow emigrant "transitional" B cells into the mature, long-lived cells of the periphery, and a number of the transcription factors discussed here impact on this process, yielding B cells with poor mitogenic responses in vitro. For mature B cells, it is clear that not only the nature, but the duration and amplitude of an activating signal are major determinants of the transcription factor activities enlisted, and so the ultimate outcome. The current challenge is the identification of the target genes that are activated to implement the correct response, so that we may more precisely and safely manipulate B cell behavior to predictably and positively influence humoral immune responses.

Differential requirement for OBF-1 during antibody-secreting cell differentiation

Resting B cells can be cultured to induce antibody-secreting cell (ASC) differentiation in vitro. A quantitative analysis of cell behavior during such a culture allows the influences of different stimuli and gene products to be measured. The application of this analytical system revealed that the OBF-1 transcriptional coactivator, whose loss impairs antibody production in vivo, has two effects on ASC development. Although OBF-1 represses early T cell–dependent (TD) differentiation, it is also critical for the completion of the final stages of ASC development. Under these conditions, the loss of OBF-1 blocks the genetic program of ASC differentiation so that Blimp-1/prdm1 induction fails, and bcl-6, Pax5, and AID are not repressed as in control ASC. Retroviral complementation confirmed that OBF-1 was the critical entity. Surprisingly, when cells were cultured in lipopolysaccharide to mimic T cell–independent conditions, OBF-1–null B cells differentiated normally to ASC. In the OBF-1^−/− ASC generated under either culture regimen, antibody production was normal or only modestly reduced, revealing that Ig genes are not directly dependent on OBF-1 for their expression. The differential requirement for OBF-1 in TD ASC generation was confirmed in vivo. These studies define a new regulatory role for OBF-1 in determining the cell-autonomous capacity of B cells to undergo terminal differentiation in response to different immunological signals.

Transcriptional networks in developing and mature B cells

The development of B cells from haematopoietic stem cells proceeds along a highly ordered, yet flexible, pathway. At multiple steps along this pathway, cells are instructed by transcription factors on how to further differentiate, and several check-points have been identified. These check-points are initial commitment to lymphocytic progenitors, specification of pre-B cells, entry to the peripheral B-cell pool, maturation of B cells and differentiation into plasma cells. At each of these regulatory nodes, there are transcriptional networks that control the outcome, and much progress has recently been made in dissecting these networks. This article reviews our current understanding of this exciting field.

Cardiac transgenesis with the tetracycline transactivator changes myocardial function and gene expression

The cardiac-specific tetracycline-regulated gene expression system (tet-system) is a powerful tool using double-transgenic mice. The cardiac alpha-myosin heavy chain promoter (alphaMHC) drives lifetime expression of a tetracycline-inhibited transcription activator (tTA). Crossing alphaMHC-tTA mice with mice containing a tTA-responsive promoter linked to a target gene yields double-transgenic mice having tetracycline-repressed expression of the target gene in the heart. Using the tet-system, some studies use nontransgenic mice for the control group, whereas others use single-transgenic alphaMHC-tTA mice. However, previous studies found that high-level expression of a modified activator protein caused cardiomyopathy. Therefore, we tested whether cardiac expression of tTA was associated with altered function of alphaMHC-tTA mice compared with wild-type (WT) littermates. We monitored in vivo and in vitro function and gene expression profiles for myocardium from WT and alphaMHC-tTA mice. Compared with WT littermates, alphaMHC-tTA mice had a greater heart-to-body weight ratio (approximately 10%), ventricular dilation, and decreased ejection fraction, suggesting mild cardiomyopathy. In vitro, submaximal contractions were greater compared with WT and were associated with greater myofilament Ca2+ sensitivity. Gene expression profiling revealed that the expression of 153 genes was significantly changed by >20% when comparing alphaMHC-tTA with WT myocardium. These findings demonstrate that introduction of the alphaMHC-tTA construct causes significant effects on myocardial gene expression and major functional abnormalities in vivo and in vitro. For studies using the tet-system, these results suggest caution in the use of controls, since alphaMHC-tTA myocardium differs appreciably from WT. Furthermore, the results raise the possibility that the phenotype conferred by a target gene may be influenced by the modified genetic background of alphaMHC-tTA myocardium.

Epigenetic silencing of the immunoglobulin heavy-chain gene in classical Hodgkin lymphoma-derived cell lines contributes to the loss of immunoglobulin expression

Immunoglobulin production is impaired in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL) in spite of functional clonal rearrangements. The presence of "crippling" mutations in coding and regulatory regions, as well as down-regulation of B-cell-specific transcription factors, has been suggested as a potential reason for the lack of immunoglobulin (Ig) chain gene transcription. We have investigated the impact of epigenetic silencing in suppressing Ig heavy (H)-chain expression. Chromatin immunoprecipitation (ChIP) was used to analyze transcription factor binding to octamer motifs present in the IgH regulatory regions. Transcription factors were bound to these motifs in control cell lines, however, they were absent in the cHL-derived cell lines KMH2, L1236, and L428. Ectopic expression of octamer-binding transcription factor (Oct2) and/or B-cell Oct binding protein/Oct-binding factor (BOB.1/OBF.1) did not result in any measurable binding to these sites. Increased histone 3 Lysine 9 (H3-K9) methylation was observed in the promoter region of the IgH locus in L428 and L1236 cells. This is a typical feature of heterochromatic, transcriptionally silent regions. Treatment of cHL-derived cell lines with the DNA demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) partially reactivated IgH transcription and affected chromatin modifications. Our results suggest an important role of epigenetic silencing in the inhibition of IgH transcription in HRS cells.

Reversible lymphomagenesis in conditionally c-MYC expressing mice

It is well documented that deregulation of MYC leads to tumor development, yet many aspects of this process are only partially understood. We have established a transgenic mouse model in which c-MYC is conditionally expressed in lymphoid cells using the tetracycline-regulated system of gene regulation. Mice with continuously expressed transgenic c-MYC died of invasive T- or B-cell lymphomas within 4 months. Lymphomas developing in transgenic mice were c-MYC dependent since doxycycline treatment led to tumor regression. Using transplantation of established tumor cell lines labeled with GFP, we followed the fate of neoplastic cells in recipients upon MYC inactivation. This approach allowed us to elucidate both apoptosis and differentiation as mechanisms of tumor elimination. Comparative genomic hybridization (CGH) and FISH analyses were performed in order to analyze possible chromosomal aberrations induced by c-MYC. We observed that overexpression of c-MYC is sufficient to induce recurrent patterns of genomic instability. The main observation was a gain of genomic material that corresponded to chromosome 15 in several T-cell tumors, which could be identified as trisomy.

Myosin light chain 1 atrial isoform (MLC1A) is expressed in pre-B cells under control of the BOB.1/OBF.1 coactivator

The BOB.1/OBF.1 protein is a B-cell-specific coactivator of the Oct1 and Oct2 transcription factors. It is involved in mediating the transcriptional activity of the Oct proteins. However, animals deficient for BOB.1/OBF.1 showed virtually normal expression of genes that contain octamer motifs in their regulatory regions. To identify new genes that are regulated by BOB.1/OBF.1, we took advantage of a previously described cell system. RNAs differentially expressed in a BOB.1/OBF.1-deficient pre-B cell line and a derivative of this cell line expressing a hormone dependent BOB.1/OBF.1-estrogen receptor (BobER) fusion protein were isolated. Using the cDNA representational difference analysis method we could identify myosin light chain 1 atrial (MLC1A) isoform as a gene regulated by BOB.1/OBF.1. MLC1A was so far unknown to be expressed in tissues other than muscle. Here we demonstrate that MLC1A is indeed expressed in mouse pre-B cells. Analysis of the expressed mRNA revealed an alternative 5' promoter element and an alternative splice product, which had not yet been described for the murine gene. Cotransfection experiments with reporter constructs driven by the MLC1A promoter suggest that the regulation by BOB.1/OBF.1 is indirect. Consistent with this conclusion is the observation that transcriptional induction of the endogenous MLC1A gene by BOB.1/OBF.1 requires de novo protein synthesis.

Molecular mechanisms guiding late stages of B-cell development

In mice, large numbers of immature B cells are continuously produced in the bone marrow. To enter the pools of mature B cells, these immature B cells have to pass two checkpoints. First, B cells have to migrate from the bone marrow to the spleen. The second checkpoint involves the immature B cells differentiating to mature B cells within the spleen. As the net result of this selection and maturation, only a fraction of the newly produced B cells enters the mature B-cell pool. Recent advances in the understanding of the molecular mechanisms that operate at these two checkpoints are described and discussed.

Immunoglobulin heavy chain genes somatic hypermutations and chromosome 11q22-23 deletion in classic mantle cell lymphoma: a study of the Swiss Group for Clinical Cancer Research

Mantle cell lymphoma (MCL) shares immunophenotypic and karyotypic features with chronic lymphocytic leukaemia. The latter comprises two distinct entities with prognosis dependent upon immunoglobulin heavy chain (IgH) gene mutational status and the presence of 11q deletion. We evaluated the relevance of IgH gene mutational status, IgV gene family usage and presence of 11q deletion in a series of 42 histologically reviewed classical MCL cases to determine the prognostic impact. VH3 was the most common VH family, with VH3-21 being the most frequent individual VH gene. Approximately 30% of the cases had a IgH somatic mutation rate higher than 2%, but was only higher than 4% in <10% of cases. Half of the cases had deletion of chromosome 11q21-telomere (11q21->ter), with two minimal deleted regions, at 11q22.2 and 11q23.2. There was no association between 11q loss and IgH gene somatic mutation rate; the use of VH3-21 gene could be associated with a better prognosis.

Critical Role for the Oct-2/OCA-B Partnership in Ig-Secreting Cells

B and T lymphocytes arise from a common precursor in the bone marrow, but ultimately acquire very different functions. The difference in function is largely attributable to the expression of tissue-specific transcription factors that activate discrete sets of genes. In previous studies we and others have shown that the specialized genes expressed by Ig-secreting cells cease transcription when these cells are fused to a T lymphoma. The extinguished genes include those encoding Ig, J chain, and the transcription factors Oct-2, PU.1, and the coactivator OCA-B. Remarkably, if we sustain Oct-2 expression during cell fusion, all the other tissue-specific genes of the Ig-secreting cell simultaneously escape silencing. This suggests that Oct-2 plays a central role in maintaining the gene expression program of these cells. In the present studies we have investigated the roles of the transcription factor PU.1 and the coactivator OCA-B within the hierarchy of regulatory factors that sustain Ig-secreting cell function. Our results show that OCA-B and Oct-2 are regulatory partners in this process and that PU.1 plays a subordinate role at this cell stage.