Cutting edge: lack of peripheral B cells and severe agammaglobulinemia in mice simultaneously lacking Bruton's tyrosine kinase and the B cell-specific transcriptional coactivator OBF-1

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.

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.

MTA2/NuRD Regulates B Cell Development and Cooperates with OCA-B in Controlling the Pre-B to Immature B Cell Transition

The NuRD complex contains both chromatin remodeling and histone deacetylase activities. Mice lacking the MTA2 subunit of NuRD show developmental defects in pro-B, pre-B, immature B, and marginal zone B cells, and abnormal germinal center B cell differentiation during immune responses. Mta2 inactivation also causes a derepression of Igll1 and VpreB1 genes in pre-B cells. Furthermore, MTA2/NuRD interacts directly with AIOLOS/IKAROS and shows a striking overlap with AIOLOS/IKAROS target genes in human pre-B cells, suggesting a functional interdependence between MTA2/NuRD and AIOLOS. Mechanistically, MTA2 deficiency in mice leads to increased H3K27 acetylation at both Igll1 and VpreB1 promoters. Gene profiling analyses also identify distinct MTA2-dependent transcription programs in pro-B and pre-B cells. In addition, we find a strong synergy between MTA2 and OCA-B in repressing Igll1 and VpreB1 at the pre-B cell stage, and in regulating both the pre-B to immature B transition and splenic B cell development.

Lu et al. examine B cell developmental defects in MTA2-deficient mice. MTA2 interacts with AIOLOS/IKAROS, represses Igll1 expression, co-binds to most AIOLOS/IKAROS target genes in pre-B cells, and cooperates with OCA-B in the pre-B to immature B transition. These data suggest that AIOLOS/IKAROS functions through MTA2/NuRD during B cell development.

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.

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.

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.

Blood-brain barrier damage and brain penetration of antiepileptic drugs: role of serum proteins and brain edema

PURPOSE

Increased blood-brain barrier (BBB) permeability is radiologically detectable in regions affected by drug-resistant epileptogenic lesions. Brain penetration of antiepileptic drugs (AEDs) may be affected by BBB damage. We studied the effects of BBB damage on brain distribution of hydrophilic [deoxy-glucose (DOG) and sucrose] and lipophilic (phenytoin and diazepam) molecules. We tested the hypothesis that lipophilic and hydrophilic drug distribution is differentially affected by BBB damage.

METHODS

In vivo BBB disruption (BBBD) was performed in rats by intracarotid injection of hyperosmotic mannitol. Drugs (H3-sucrose, 3H-deoxy-glucose, 14C-phenytoin, and C14-diazepam) or unlabeled phenytoin was measured and correlated to brain water content and protein extravasation. In vitro hippocampal slices were exposed to different osmolarities; drug penetration and water content were assessed by analytic and densitometric methods, respectively.

RESULTS

BBBD resulted in extravasation of serum protein and radiolabeled drugs, but was associated with no significant change in brain water. Large shifts in water content in brain slices in vitro caused a small effect on drug penetration. In both cases, total drug permeability increase was greater for lipophilic than hydrophilic compounds. BBBD reduced the amount of free phenytoin in the brain.

DISCUSSION

After BBBD, drug binding to protein is the main controller of total brain drug accumulation. Osmotic BBBD increased serum protein extravasation and reduced free phenytoin brain levels. These results underlie the importance of brain environment and BBB integrity in determining drug distribution to the brain. If confirmed in drug-resistant models, these mechanisms could contribute to drug brain distribution in refractory epilepsies.

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.

Silencing and nuclear repositioning of the lambda5 gene locus at the pre-B cell stage requires Aiolos and OBF-1

The chromatin regulator Aiolos and the transcriptional coactivator OBF-1 have been implicated in regulating aspects of B cell maturation and activation. Mice lacking either of these factors have a largely normal early B cell development. However, when both factors are eliminated simultaneously a block is uncovered at the transition between pre-B and immature B cells, indicating that these proteins exert a critical function in developing B lymphocytes. In mice deficient for Aiolos and OBF-1, the numbers of immature B cells are reduced, small pre-BII cells are increased and a significant impairment in immunoglobulin light chain DNA rearrangement is observed. We identified genes whose expression is deregulated in the pre-B cell compartment of these mice. In particular, we found that components of the pre-BCR, such as the surrogate light chain genes λ5 and VpreB, fail to be efficiently silenced in double-mutant mice. Strikingly, developmentally regulated nuclear repositioning of the λ5 gene is impaired in pre-B cells lacking OBF-1 and Aiolos. These studies uncover a novel role for OBF-1 and Aiolos in controlling the transcription and nuclear organization of genes involved in pre-BCR function.

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.

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.

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 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.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.

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.

Expression of the aldehyde dehydrogenase 2-like gene is controlled by BOB.1/OBF.1 in B lymphocytes

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds to the Oct1 and Oct2 transcription factors and increases their transactivation potential. Targeted gene disruption experiments revealed that BOB.1/OBF.1 is critical at different stages of B cell development. A large number of genes expressed in B cells contain octamer motifs in their regulatory regions. However, only few genes have been described so far whose expression is dependent on BOB.1/OBF.1. To understand the molecular basis of BOB.1/OBF.1 function in B cell development, we searched for BOB.1/OBF.1 target genes by expression profiling. We have identified genes both induced and repressed by BOB.1/OBF.1. Using different genetic systems, we demonstrate regulation of a selection of these genes. Identified targets included genes encoding Ahd2-like, AKR1C13, Rbp1, Sdh, Idh2, protocadherin gamma, alpha-catenin, Ptprs, Id3, and Creg. Classification of BOB.1/OBF.1 target genes by function suggests that they affect various aspects of B cell physiology such as cellular metabolism, cell adhesion, and differentiation. To better understand the mechanism of BOB.1/OBF.1 action, we cloned the promoter of the gene encoding Ahd2-like, the gene showing the strongest regulation by BOB.1/OBF.1. This promoter indeed contains a perfect octamer motif. Furthermore, the motif was recognized by the Oct transcription factors as well as BOB.1/OBF.1 in vitro and in vivo, as shown by electromobility shift and chromatin immunoprecipitation assays. Transient transfections confirm that this promoter is activated by BOB.1/OBF.1. Our observations suggest that by regulating genes in different functional pathways, BOB.1/OBF.1 has a widespread effect on B cell development and function.

OCA-B regulation of B-cell development and function

The transcriptional co-activator OCA-B [for Oct co-activator from B cells, also known as OBF-1 (OCT-binding factor-1) and Bob1] is not required for B-cell genesis but does regulate subsequent B-cell development and function. OCA-B deficient mice show strain-specific, partial blocks at multiple stages of B-cell maturation and a complete disruption of germinal center formation in all strains, causing humoral immune deficiency and susceptibility to infection. OCA-B probably exerts its effects through the regulation of octamer-motif controlled gene expression. The OCA-B gene encodes two proteins of distinct molecular weight, designated p34 and p35. The p34 isoform localizes in the nucleus, whereas the p35 isoform is myristoylated and is bound to the cytoplasmic membrane. p35 can traffic to the nucleus and probably activates octamer-dependent transcription, although this OCA-B isoform might regulate B cells through membrane-related signal transduction.

OcaB is required for normal transcription and V(D)J recombination of a subset of immunoglobulin kappa genes

OcaB, a transcriptional coactivator also known as Bob-1 or OBF-1, was isolated on the basis of its ability to enhance transcription of immunoglobulin (Ig) genes in vitro. Paradoxically, OcaB(-/-) mice showed no apparent deficiency in Ig gene transcription, only cellular immune defects including absence of germinal centers (GC) and decreased numbers of immature B cells; the genes targeted by OcaB were not determined. Here we report that OcaB is essential for V(D)J recombination of a subset of Igkappa genes. We show that OcaB modulates recombination by directly enhancing Igkappa gene transcription in vivo.

Expression profiling in transformed human B cells: influence of Btk mutations and comparison to B cell lymphomas using filter and oligonucleotide arrays

We have used both Clontech Atlas Human Hematology/Immunology cDNA microarrays, containing 588 genes, and Affymetrix oligonucleotide U95Av2 human array complementary to more than 12,500 genes to get a global view of genes expressed in Epstein-Barr virus (EBV)-transformed B cells and genes regulated by Bruton's tyrosine kinase (Btk). We compared EBV-transformed wild-type (WT) B cells from a healthy individual, WT1 and an X-linked agammaglobulinemia (XLA) patient cell line, XLA1, using the Clontech filters arrays. Eleven genes were > or =1.9-fold induced in absence of functional Btk. Furthermore, we analyzed a second patient cell line, XLA2, and compared this to two WT cell lines using oligonucleotide arrays. A total of 391 genes were found to be differentially expressed, including kinases and transcriptions factors. Furthermore, one expressed sequence tag and eight complementary DNA clones with unknown function were down-regulated in XLA2, indicating their biological role. Higher-fold inductions, Fyn (39.5), Hck (15.5) and Cyp1B1 (5.8), were observed using oligonucleotide array and were confirmed using real-time PCR for Fyn (20.8), Hck (6.7) and Cyp1B1 (10). Two genes, B cell translocation gene1 (BTG1) and B cell-specific OCT binding factor-1 (OBF-1) were induced > or =1.9-fold in both XLA1 and XLA2 analyzed by Atlas filter arrays andAffymetrix chips, respectively. Data from both filter and oligonucleotide arrays were compared to the gene clusters of a previously published lymphoma expression profile by linking to the UniGene transcript database. Our findings demonstrate for the first time the use of microarray to study the influence of Btk mutations and the use of functional annotation and validation of expression data by comparison of microarray analyses.

Selection events operating at various stages in B cell development

B cells have to progress through various checkpoints during their process of development. The three transcription factors E2A, EBF (early B cell factor) and Pax5 play essential roles in B cell commitment checkpoints. The various forms of the BCR and their downstream signaling molecules, which are expressed at different stages of B cell development, act as critical checkpoint guards allowing (positive selection) or preventing (negative selection) developmental progression. The recent advances on the molecular mechanisms operating at these various checkpoints are here summarized and discussed.

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

The transcriptional coactivator BOB.1/OBF.1 confers B-cell specificity on the transcription factors Oct1 and Oct2 at octamer site-containing promoters. A hallmark of the BOB.1/OBF.1 mutation in the mouse is the absence of germinal center development in secondary lymphoid organs, demonstrating the requirement for BOB.1/OBF.1 in antigen-dependent stages of B-cell differentiation. Here we analyzed earlier stages of B lymphopoiesis in BOB.1/OBF.1-deficient mice. Examination of B-cell development in the bone marrow revealed that the numbers of transitional immature (B220(+) IgM(hi)) B cells were reduced and that B-cell apoptosis was increased. When in competition with wild-type cells, BOB.1/OBF.1(-/-) bone marrow cells exhibited defects in repopulating the bone marrow B-cell compartment and were unable to establish a presence in the periphery of host mice. The defective bone marrow populations in BOB.1/OBF.1(-/-) mice were rescued by conditional expression of a BOB.1/OBF.1 transgene controlled by the tetracycline gene expression system. However, the restored populations did not restore the numbers of IgD(hi) B cells in the periphery, where the BOB.1/OBF.1 transgene was not expressed. These results show that BOB.1/OBF.1(-/-) B cells exhibit multistage defects in B-cell development, including impaired production of transitional B cells and defective maturation of recirculating B cells.

A new class of errant DNA polymerases provides candidates for somatic hypermutation

The mechanism of somatic hypermutation of the immunoglobulin genes remains a mystery after nearly 30 years of intensive research in the field. While many clues to the process have been discovered in terms of the genetic elements required in the immunoglobulin genes, the key enzymatic players that mediate the introduction of mutations into the variable region are unknown. The recent wave of newly discovered eukaryotic DNA polymerases have given a fresh supply of potential candidates and a renewed vigour in the search for the elusive mutator factor governing affinity maturation. In this paper, we discuss the relevant genetic and biochemical evidence known to date regarding both somatic hypermutation and the new DNA polymerases and address how the two fields can be brought together to identify the strongest candidates for further study. In particular we discuss evidence for the in vitro biochemical misincorporation properties of human Rad30B/Pol iota and how it compares to the in vivo somatic hypermutation spectra.

B cell development and immunoglobulin gene transcription in the absence of Oct-2 and OBF-1

Oct-2 and OBF-1 (also called OCA-B or Bob-1) are B cell-specific transcription factors that bind to the conserved octamer site of immunoglobulin promoters, yet their role in immunoglobulin transcription has remained unclear. We generated mice in which the lymphoid compartment was reconstituted with cells that lack both Oct-2 and OBF-1. Even in the absence of these two transcription factors, B cells develop normally to the membrane immunoglobulin M-positive (IgM+) stage and immunoglobulin gene transcription is essentially unaffected. These observations imply that the ubiquitous factor Oct-1 plays a previously unrecognized role in the control of immunoglobulin gene transcription and suggest the existence of another, as yet unidentified, cofactor. In addition, both factors are essential for germinal center formation, although OBF-1 is more important than Oct-2 for IgG production after immunization.

Genetic analyses of NFKB1 and OCA-B function: defects in B cells, serum IgM level, and antibody responses in Nfkb1-/-Oca-b-/- mice

Defined patterns of gene expression during cell differentiation are likely to be ensured by multiple factors playing redundant roles. By generating mice deficient in both NFKB1 and OCA-B, we show here that the two transcription factors are required for B-1 cell differentiation and serum IgM production. In addition, relative to Nfkb1(-/-) or Oca-b(-/-) mice, the Nfkb1(-/-)Oca-b(-/-) mice show a decrease in conventional B cell frequencies in the spleen and augmented reductions in T-independent and T-dependent Ab responses. These results suggest that NFKB1 and OCA-B play compensatory roles in multiple aspects of B cell differentiation.

A sensitized genetic system for the analysis of murine B lymphocyte signal transduction pathways dependent on Bruton's tyrosine kinase

Modifier screens have been powerful genetic tools to define signaling pathways in lower organisms. The identification of modifier loci in mice has begun to allow a similar dissection of mammalian signaling pathways. Transgenic mice (Btk(lo)) expressing 25% of endogenous levels of Bruton's tyrosine kinase (Btk) have B cell functional responses between those of wild-type and Btk(-/-) mice. We asked whether reduced dosage or complete deficiency of genes previously implicated as Btk regulators would modify the Btk(lo) phenotype. We used two independent assays of Btk-dependent B cell function. Proliferative response to B cell antigen receptor cross-linking in vitro was chosen as an example of a relatively simple, well-defined signaling system. In vivo response to type II T-independent antigens (TI-II) measures complex interactions among multiple cell types over time and may identify additional Btk pathways. All modifiers identified differentially affected these two assays, indicating that Btk mediates these processes via distinct mechanisms. Loss of Lyn, PTEN (phosphatase and tensin homolog), or SH2-containing inositol phosphatase suppressed the Btk(lo) phenotype in vitro but not in vivo, whereas CD19 and the p85alpha form of phosphoinositide 3-kinase behaved as Btk(lo) enhancers in vivo but not in vitro. Effects of Lyn, PTEN, or p85alpha haploinsufficiency were observed. Haploinsufficiency or complete deficiency of protein kinase C beta, Fyn, CD22, Galphaq, or Galpha11 had no detectable effect on the function of Btk(lo) B cells. A transgenic system creating a reduction in dosage of Btk can therefore be used to identify modifier loci that affect B cell responses and quantitatively rank their contribution to Btk-mediated processes.