Properties of B cell stage specific and ubiquitous nuclear factors binding to immunoglobulin heavy chain gene switch regions

Genome Instability in Multiple Myeloma: Facts and Factors

Multiple myeloma (MM) is a malignant neoplasm of terminally differentiated immunoglobulin-producing B lymphocytes called plasma cells. MM is the second most common hematologic malignancy, and it poses a heavy economic and social burden because it remains incurable and confers a profound disability to patients. Despite current progress in MM treatment, the disease invariably recurs, even after the transplantation of autologous hematopoietic stem cells (ASCT). Biological processes leading to a pathological myeloma clone and the mechanisms of further evolution of the disease are far from complete understanding. Genetically, MM is a complex disease that demonstrates a high level of heterogeneity. Myeloma genomes carry numerous genetic changes, including structural genome variations and chromosomal gains and losses, and these changes occur in combinations with point mutations affecting various cellular pathways, including genome maintenance. MM genome instability in its extreme is manifested in mutation kataegis and complex genomic rearrangements: chromothripsis, templated insertions, and chromoplexy. Chemotherapeutic agents used to treat MM add another level of complexity because many of them exacerbate genome instability. Genome abnormalities are driver events and deciphering their mechanisms will help understand the causes of MM and play a pivotal role in developing new therapies.

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

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

Molecular mechanism of class switch recombination: linkage with somatic hypermutation

Class switch recombination (CSR) and somatic hypermutation (SHM) have been considered to be mediated by different molecular mechanisms because both target DNAs and DNA modification products are quite distinct. However, involvement of activation-induced cytidine deaminase (AID) in both CSR and SHM has revealed that the two genetic alteration mechanisms are surprisingly similar. Accumulating data led us to propose the following scenario: AID is likely to be an RNA editing enzyme that modifies an unknown pre-mRNA to generate mRNA encoding a nicking endonuclease specific to the stem-loop structure. Transcription of the S and V regions, which contain palindromic sequences, leads to transient denaturation, forming the stem-loop structure that is cleaved by the AID-regulated endonuclease. Cleaved single-strand tails will be processed by error-prone DNA polymerase-mediated gap-filling or exonuclease-mediated resection. Mismatched bases will be corrected or fixed by mismatch repair enzymes. CSR ends are then ligated by the NHEJ system while SHM nicks are repaired by another ligation system.

Sequences associated with the mouse Smu switch region are important for immunoglobulin heavy chain transgene expression in B cell development

Analyses of H-chain transgenes have indicated that sequences situated between the mu intronic enhancer and the Cmu exons are important for mu gene expression. We have analyzed several variant mu transgenes and find that a sequence element located within or just upstream of Smu is important for mu transgene expression in both immature and mature B cells. This Smu -associated element appears to be required for functional mu expression in small, resting pre-B cells but not in proliferating pre-B cells. Our results also indicate that this element is responsible for previously reported differential transgene expression in resting and activated/proliferating mature B cells. However, our studies of knockout mice show that deletion of the Smu -associated element from the endogenous IgH locus does not alter early B cell maturation. This indicates that other elements within the H-chain locus can replace the function of the Smu -associated element at least to the mature B cell stage. Surprisingly, we also find that Smu deletion in the IgH locus does not affect levels of the sterile germ-line mu transcripts that are involved in B cell class switching, even though S-region sequences have been indicated to be important for the production of analogous germ-line transcripts for other isotypes.

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

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

Reduction in DNA binding activity of the transcription factor Pax-5a in B lymphocytes of aged mice

Aging has been associated with intrinsic changes of the humoral immune response, which may lead to an increased occurrence of autoimmune disorders and pathogenic susceptibility. The transcription factor Pax-5 is a key regulator of B cell development. Pax-5a/B cell-specific activator protein and an alternatively spliced isoform, Pax-5d, may have opposing functions in transcriptional regulation due to the lack of a transactivation domain in Pax-5d. To study B cell-specific changes that occur during the aging process, we investigated expression patterns of Pax-5a and 5d in mature B cells of young and aged mice. RNase protection assays showed a similar transcriptional pattern for both age groups that indicates that aging has no affect on transcription initiation or alternative splicing for either isoform. In contrast, a significant reduction in the DNA binding activity of Pax-5a but not Pax-5d protein was observed in aged B cells in vitro, while Western blot analyses showed that similar levels of Pax-5a and 5d proteins were present in both age groups. The observed decrease in Pax-5a binding activity correlated with changes in expression of two Pax-5 target genes in aged B cells. Expression of the Ig J chain and the secreted form of Ig mu, which are both known to be suppressed by Pax-5a in mature B cells, were increased in B cells of aged mice. Together, our studies suggest that changes associated with the aging phenotype cause posttranslational modification(s) of Pax-5a but not Pax-5d, which may lead to an abnormal B cell phenotype in aged mice, associated with elevated levels of J chain, and secretion of IgM.

BSAP/Pax5A Expression Blocks Survival and Expansion of Early Myeloid Cells Implicating Its Involvement in Maintaining Commitment to the B-Lymphocyte Lineage

Early B lymphopoiesis is marked by plasticity between the myeloid and B lineages. An attractive model for B-lineage development is that commitment to this lineage is partly determined by the ordered expression of genes that prohibit switching to the myeloid lineage. In this regard, whereas the role of the B-cell–specific transcription factor BSAP/Pax5A in regulating B-lymphoid–restricted gene expression has been well-established, its role in maintaining B-lineage commitment is unclear. Thus, BSAP/Pax5A was constitutively expressed in the multipotent EML cell line, which can be directed toward the myeloid lineage by culture with interleukin-3 (IL-3) and retinoic acid. EML cells expressing BSAP/Pax5A successfully acquired the myeloid lineage markers CD11b and F4/80 in response to IL-3 and retinoic acid, indicating differentiation to the myeloid lineage. However, these early myeloid cells failed to expand in culture with granulocyte-macrophage colony-stimulating factor and were directed instead toward an apoptotic pathway. In parallel, primary bone marrow stem cells transduced with retrovirus constitutively expressing BSAP/Pax5A began myeloid cell differentiation, but like the transformed EML model failed to expand in response to myeloid growth factors. These studies identify a role for BSAP/Pax5A in suppressing the response to myeloid growth factors, which may be a component of the regulatory processes that limit plasticity of early B-lymphoid progenitors.

BSAP/Pax5A Expression Blocks Survival and Expansion of Early Myeloid Cells Implicating Its Involvement in Maintaining Commitment to the B-Lymphocyte Lineage

Early B lymphopoiesis is marked by plasticity between the myeloid and B lineages. An attractive model for B-lineage development is that commitment to this lineage is partly determined by the ordered expression of genes that prohibit switching to the myeloid lineage. In this regard, whereas the role of the B-cell–specific transcription factor BSAP/Pax5A in regulating B-lymphoid–restricted gene expression has been well-established, its role in maintaining B-lineage commitment is unclear. Thus, BSAP/Pax5A was constitutively expressed in the multipotent EML cell line, which can be directed toward the myeloid lineage by culture with interleukin-3 (IL-3) and retinoic acid. EML cells expressing BSAP/Pax5A successfully acquired the myeloid lineage markers CD11b and F4/80 in response to IL-3 and retinoic acid, indicating differentiation to the myeloid lineage. However, these early myeloid cells failed to expand in culture with granulocyte-macrophage colony-stimulating factor and were directed instead toward an apoptotic pathway. In parallel, primary bone marrow stem cells transduced with retrovirus constitutively expressing BSAP/Pax5A began myeloid cell differentiation, but like the transformed EML model failed to expand in response to myeloid growth factors. These studies identify a role for BSAP/Pax5A in suppressing the response to myeloid growth factors, which may be a component of the regulatory processes that limit plasticity of early B-lymphoid progenitors.

Switch Recombination in a Transfected Plasmid Occurs Preferentially in a B Cell Line That Undergoes Switch Recombination of Its Chromosomal Ig Heavy Chain Genes

Ab class switching is induced upon B cell activation in vivo by immunization or infection or in vitro by treatment with mitogens, e.g. LPS, and results in the expression of different heavy chain constant region (CH) genes without a change in the Ab variable region. This DNA recombination event allows Abs to alter their biological activity while maintaining their antigenic specificity. Little is known about the molecular mechanism of switch recombination. To attempt to develop an assay for enzymes, DNA binding proteins, and DNA sequences that mediate switch recombination, we have constructed a plasmid DNA substrate that will undergo switch recombination upon stable transfection into the surface IgM+ B cell line (I.29μ), a cell line capable of undergoing switch recombination of its endogenous genes. We demonstrate that recombination occurs between the two switch regions of the plasmid, as assayed by PCRs across the integrated plasmid switch regions, followed by Southern blot hybridization. Nucleotide sequence analysis of the PCR products confirmed the occurrence of Sμ-Sα recombination in the plasmid. Recombination of the plasmid in I.29μ cells does not require treatment with inducers of switch recombination, suggesting that recombinase activity is constitutive in I.29μ cells. Recombination does not require high levels of transcription across the switch regions of the plasmid. Fewer recombination events are detected in four different B and T cell lines that do not undergo switch recombination of their endogenous genes.

Overexpression of BSAP/Pax-5 Inhibits Switching to IgA and Enhances Switching to IgE in the I.29μ B Cell Line

B cell-specific activator protein (BSAP)/Pax-5 is a paired domain DNA-binding protein expressed in the developing nervous system, testis, and in all B lineage cells, except terminally differentiated plasma cells. BSAP regulates transcription of several genes expressed in B cells and also the activity of the 3′ IgH enhancer. As it has binding sites within or 5′ to the switch regions of nearly all Ig heavy chain C region genes and also is known to increase transcription of the germline ε RNA, BSAP has been hypothesized to be involved in regulation of Ab class switch recombination. To directly examine the effects of BSAP on isotype switching, we use a tetracycline-regulated expression system to overexpress BSAP in the surface IgM+ I.29μ B cell line, a mouse cell line that can be induced to undergo class switch recombination. We find that overexpression of BSAP inhibits switching to IgA in I.29μ cells stimulated with LPS + TGF-β1 + nicotinamide, but enhances switching to IgE in cells stimulated with LPS + IL-4 + nicotinamide. Parallel to its effects on switching, overexpression of BSAP inhibits germline α RNA expression and the transcriptional activity of the germline α promoter, while enhancing activity of the germline ε promoter. Proliferation of I.29μ cells is not affected in this system. The possible mechanisms and significance of the effect of BSAP on isotype switching are discussed.

Transcription Factor B-Cell–Specific Activator Protein (BSAP) Is Differentially Expressed in B Cells and in Subsets of B-Cell Lymphomas

The paired box containing gene PAX-5 encodes the transcription factor BSAP (B-cell–specific activator protein), which plays a key role in B-lymphocyte development. Despite its known involvement in a rare subtype of non-Hodgkin’s lymphoma (NHL), a detailed examination of BSAP expression in NHL has not been previously reported. In this study, we analyzed normal and malignant lymphoid tissues and cell lines, including 102 cases of B-cell NHL, 23 cases of T- and null-cell NHL, and 18 cases of Hodgkin’s disease. Normal lymphoid tissues showed strong nuclear BSAP expression in mantle zone B cells, less intense reactivity in follicular center B cells, and no expression in cells of the T-cell–rich zones. Monocytoid B cells showed weak expression, whereas plasma cells and extrafollicular large transformed B cells were negative. Of the 102 B-cell NHLs, 83 (81%) demonstrated BSAP expression. All of the 13 (100%) B-cell chronic lymphocytic leukemias (B-CLLs), 21 of (100%) mantle cells (MCLs), and 20 of 21 (95%) follicular lymphomas (FLs) were positive. Moderate staining intensities were found in most B-CLL and FL cases, whereas most MCLs showed strong reactions, paralleling the strong reactivity of nonmalignant mantle cells. Eight of 12 (67%) marginal zone lymphoma cases showed negative or low BSAP levels, and 17 of 24 (71%) large B-cell lymphomas displayed moderate to strong expression. None of the 23 T- and null-cell lymphomas reacted with the BSAP antisera, whereas in Hodgkin’s disease, 2 of 4 (50%) nodular lymphocytic predominance and 5 of 14 (36%) classical cases showed weak nuclear or nucleolar BSAP reactions in a fraction of the tumor cells. Western blot analysis showed a 52-kD BSAP band in B-cell lines, but not in non–B-cell or plasma cell lines. We conclude that BSAP expression is largely restricted to lymphomas of B-cell lineage and that BSAP expression varies in B-cell subsets and subtypes of B-cell NHL. The high levels of BSAP, especially those found in large-cell lymphomas and in some follicular lymphomas, may be a consequence of deregulated gene expression and suggest a possible involvement of PAX-5 in certain B-cell malignancies.

This is a US government work. There are no restrictions on its use.

Transcription Factor B-Cell–Specific Activator Protein (BSAP) Is Differentially Expressed in B Cells and in Subsets of B-Cell Lymphomas

The paired box containing gene PAX-5 encodes the transcription factor BSAP (B-cell–specific activator protein), which plays a key role in B-lymphocyte development. Despite its known involvement in a rare subtype of non-Hodgkin’s lymphoma (NHL), a detailed examination of BSAP expression in NHL has not been previously reported. In this study, we analyzed normal and malignant lymphoid tissues and cell lines, including 102 cases of B-cell NHL, 23 cases of T- and null-cell NHL, and 18 cases of Hodgkin’s disease. Normal lymphoid tissues showed strong nuclear BSAP expression in mantle zone B cells, less intense reactivity in follicular center B cells, and no expression in cells of the T-cell–rich zones. Monocytoid B cells showed weak expression, whereas plasma cells and extrafollicular large transformed B cells were negative. Of the 102 B-cell NHLs, 83 (81%) demonstrated BSAP expression. All of the 13 (100%) B-cell chronic lymphocytic leukemias (B-CLLs), 21 of (100%) mantle cells (MCLs), and 20 of 21 (95%) follicular lymphomas (FLs) were positive. Moderate staining intensities were found in most B-CLL and FL cases, whereas most MCLs showed strong reactions, paralleling the strong reactivity of nonmalignant mantle cells. Eight of 12 (67%) marginal zone lymphoma cases showed negative or low BSAP levels, and 17 of 24 (71%) large B-cell lymphomas displayed moderate to strong expression. None of the 23 T- and null-cell lymphomas reacted with the BSAP antisera, whereas in Hodgkin’s disease, 2 of 4 (50%) nodular lymphocytic predominance and 5 of 14 (36%) classical cases showed weak nuclear or nucleolar BSAP reactions in a fraction of the tumor cells. Western blot analysis showed a 52-kD BSAP band in B-cell lines, but not in non–B-cell or plasma cell lines. We conclude that BSAP expression is largely restricted to lymphomas of B-cell lineage and that BSAP expression varies in B-cell subsets and subtypes of B-cell NHL. The high levels of BSAP, especially those found in large-cell lymphomas and in some follicular lymphomas, may be a consequence of deregulated gene expression and suggest a possible involvement of PAX-5 in certain B-cell malignancies.

This is a US government work. There are no restrictions on its use.

The transcription factor NF-kappaB/p50 interacts with the blk gene during B cell activation

The B cell-specific transcription factor Pax-5 has been shown previously to interact with the promoter of the blk gene in vitro. blk encodes a tyrosine kinase associated with the B cell receptor, which is expressed during the early but not the final stages of B cell development. To investigate whether Pax-5 regulates expression of the blk gene in vivo during B cell development and/or activation, Pax-5a was overexpressed in B cell lines. Increases in blk promoter activity using a chloramphenicol acetyltransferase reporter gene system suggested a role for Pax-5a as a transcriptional activator. Subsequent site-specific mutagenesis studies showed that mutations of the Pax-5 binding site on blk significantly alter promoter activity, although results suggested that other factors could bind to this region as well. Using mobility shift assays, we detected an inducible transcription factor that interacts strongly with a sequence overlapping the Pax-5 site on the blk promoter and identified this as a homodimer of NF-kappaB/p50, a member of the NF-kappaB/Rel family of transcription factors. This factor was present at high levels in lipopolysaccharide-activated normal B cells and in plasma cell lines but either at low levels or undetectable levels in resting normal B cells or pre-B or mature B cell lines. In contrast, lipopolysaccharide induction of a pre-B cell line (703/Z) induced a complex that contained both NF-kappaB/p50 and p65. These studies suggest that different NF-kappaB complexes are able to interact with a sequence overlapping the Pax-5 site on the blk promoter and that the relative levels of "bound" factor influence levels of blk expression. Since p50 homodimers and p50/p65 heterodimers of the NF-kappaB complex should have opposing effects on blk transcription, this could provide a mechanism to differentially regulate blk expression during B cell development and activation.

Molecular comparison of cultured hybridoma cells that switch isotypes at high and low rates

We have previously reported the isolation of variants from the 36.65 and PC1.4.1 hybridoma cell lines that spontaneously switch from gamma 1 to gamma 2a and gamma 2b at high and low rates. In order to further characterize the phenotype of these variants, we have now investigated the production of germline transcripts and methylation which are two of the molecular correlates of isotype switching. While some of the correlations that exist in normal cells were present in some of the clonal variants, others were not. However, the higher switching variants of both cells lines had higher recombinational activity as measured with a shuttle vector. The distinct phenotypic characteristics of each cell line provide an opportunity to dissect the roles of individual molecular events in the process of isotype switching.

Identification of novel Pax-2 binding sites by chromatin precipitation

The Pax genes encode a family of developmental transcription factors that bind to specific DNA sequences via the paired domain and are necessary for the morphogenesis of a variety of tissues. The murine Pax-2 gene, through alternative splicing, encodes two nuclear proteins, Pax-2A and Pax-2B, which are transiently expressed during the differentiation of specific neural cell types and early kidney formation. In order to identify potential in vivo Pax-2 target sequences, chromatin from embryonic neural tube was immunoprecipitated with Pax-2 specific antibodies and cloned. Two unique immunoprecipitated clones containing three specific Pax-2 binding sites were identified by functional binding assays using Pax-2 proteins produced in both Escherichia coli and eukaryotic cells. In vitro DNA binding assays, using Pax-5 and Pax-8 DNA recognition sequences as well as the three immunopurified Pax-2 binding sites, demonstrated that both forms of the Pax-2 protein bind DNA with a similar specificity and that this binding is mediated by the paired domain. The binding sites identified in this report share significant homology among themselves and with previously defined consensus sequences for Pax-5 and Pax-2. The genomic clones can now be used as sequence tags to identify potential target loci.

Characterization of an endonuclease activity which preferentially cleaves the G-rich immunoglobulin switch repeat sequences

B lymphocytes can alter selectively their immunoglobulin (Ig) isotype expressed by deletional rearrangement of the first active immunoglobulin heavy-chain (IgH) constant region (C mu) gene with one of six other constant region genes. Recombination breakpoints occur within highly repetitive "switch" (S) regions located upstream of each IgH constant region gene except C delta. Analysis of rearranged switch DNA junctions has not detected a consensus sequence, although the predominance of two pentamer motifs (TGGGG and TGAGC) at or near these breakpoints and throughout all murine S region sequences has led to their advocacy as the S recombination signals. In this paper, we describe the characterization and partial purification of a lymphoid-specific endo-nuclease activity which cleaves preferentially murine S region DNA. Enzyme activity selectively produced single- and double-stranded breaks at TGAGC and TGGG motifs within murine S mu and S alpha DNA. Rare cryptic cleavage sites were detected also within non-switch sequences, although cleavage intensities at these sites were reduced greatly, relative to consensus S region cleavages. Analogous activity was found in murine tissue extracts, although among the tissues assayed only spleen and thymus contained detectable activity. Subsequent biochemical characterization of this activity demonstrated that the responsible enzyme (Endo-SR) represented a previously unreported tissue-specific mammalian endonuclease. Endo-SR-specific activity could be enhanced by addition of Mg2+ or Ca2+ and inhibited by addition of Zn2+. Maximal specific activity was detected at pH 5.5 and sharply declined within +/- 0.5 pH units. In view of this enzyme's sequence- and tissue-specificity, we propose that Endo-SR is a strong candidate for an endonuclease activity associated with the switch recombination process.

Genetics of IgA deficiency

IgA deficiency is the most common humoral defect in man and results in an increased susceptibility to respiratory tract and gastrointestinal infections. Both clinical and genetic data support a close relationship with common variable immunodeficiency, a disease which involves not only IgA and IgG production, but also, in half of the patients, IgM. It is likely that the two disorders represent an allelic condition with a variable expression of a common gene defect which is thought to be involved in the regulation of immunoglobulin class switching. It is possible that a single, autosomally inherited gene with a limited penetrance is responsible for the development of both these defects.

BSAP: a key regulator of B-cell development and differentiation

B-cell-specific activator protein (BSAP) is a recently identified member of the Pax-gene family of transcription factors; in the lymphoid system, BSAP is produced only in B cells. Here, Markus Neurath, Eckhard Stüber and Warren Strober describe the molecular structure of BSAP and focus on the ability of this protein to regulate the expression of B-cell-specific genes. They propose that BSAP is a key protein of B cells and that it not only influence B-cell development but also influences the balance between B-cell proliferation and immunoglobulin secretion at later stages of B-cell differentiation.

The role of BSAP (Pax-5) in B-cell development

The hierarchy of transcriptional control in B-cell development has recently been analyzed by targeted gene inactivation in the mouse. In this manner, the paired box containing gene Pax-5, encoding the B cell specific transcription factor BSAP, has been shown to play a key role in early B lymphopoiesis. Other experimental strategies have implicated BSAP in the control of cell proliferation, isotype switching and transcription of the immunoglobulin heavy-chain gene at late stages of B-cell differentiation.

The role of BSAP in immunoglobulin isotype switching and B-cell proliferation

A role for the transcription factor B cell-specific activator protein (BSAP) in switch recombination has been proposed because binding sites for this protein have been found near switch regions of several isotypes. We have attempted to assess BSAP's role by altering the expression of this protein in B cells switching in culture to IgG1. We found that a phosphorothioate oligonucleotide antisense to the BSAP translation initiation site was able, when incubated with B cells, to decrease BSAP activity in nuclear extracts, and that IgG1 expression was reduced in such cells compared to cells incubated with control oligonucleotides. However, it is not clear whether this apparent reduction in switch recombination was mediated by the known BSAP binding sites in the immunoglobulin heavy chain locus because the antisense experiments revealed an additional activity of this protein: it is a rate-limiting regulator of cell proliferation. Down-regulation of BSAP was associated with decreased proliferation, while increasing BSAP (by transfection with a BSAP expression plasmid) increased proliferation. Thus because switch recombination apparently requires cell division, the effect of BSAP down-regulation on switching might have resulted from decreased proliferation. The role of BSAP in B cell proliferation suggests that dysregulation of this protein could contribute to neoplastic transformation of B cells. Because of BSAP's many activities, experiments to elucidate the mechanisms of its effects on switching and proliferation will be challenging.

Peyer's patch CD8+ memory T cells secrete T helper type 1 and type 2 cytokines and provide help for immunoglobulin secretion

Analysis of cytokine gene expression by reverse transcription-polymerase chain reaction (RT-PCR) demonstrated high spontaneous levels of transcripts for multiple cytokines in murine Peyer's patches (PP) compared to spleen and peripheral lymph nodes. This is consistent with the presence of active germinal centers in PP and their continuous exposure to lumenal antigen including bacterial endotoxin. RT-PCR analysis of cytokine transcripts in purified PP T cell populations revealed the presence of transcripts for interleukin-4 (IL-4), IL-5 and IL-10 in addition to interferon-gamma (IFN-gamma) in CD8+ cells purified by flow cytometry. The majority of PP CD8+ T cells were also CD45RBlo (MB23G2-), suggesting that these cells were activated/memory cells. CD8+ cells in spleen and mesenteric lymph nodes (MLN) were predominantly CD45RBhi (MB23G2+) consistent with a resting/naive phenotype. PP and MLN CD8+ T cells also secreted IL-5 and IL-10 when stimulated with anti-CD3 monoclonal antibody and when co-cultured with PP B cells enhanced secretion of both IgG and IgA. These studies suggest that CD8+ T cells at mucosal sites secrete T helper type 2 cytokines and can provide functional help for B cells in these tissues.

Clonal variants of hybridoma cells that switch isotype at a high frequency

As B cells differentiate under the influence of antigen and T cells, they frequently switch from the expression of IgM antibody to the expression of other isotypes. This is accomplished by rearranging the expressed variable region gene to downstream constant region genes and deleting the intervening sequences. Some B-cell lines that represent early stages in development switch constitutively in culture at frequencies that approach those of lipopolysaccharide- or lymphokine-stimulated normal B cells. Hybridoma cells represent a later stage of development and rarely switch in culture. In contrast to early B-cell lines, hybridomas produce large amounts of immunoglobulin, and single cells can be assayed easily for the expression of new isotypes. We have used the ELISA spot assay and fluctuation analysis to determine the rate of switching of two hybridoma cell lines. By identifying subclones that switched more frequently, we have progressively enriched for cells that switch spontaneously at higher rates. These cells, like normal cells, switch by rearrangement and deletion, and the frequency of switched cells in some of the clones is comparable to that which has been observed in less differentiated B-cell lines and in normal B cells.

The B cell-specific transcription factor BSAP regulates B cell proliferation

The B cell-specific activator protein (BSAP) is a DNA-binding transcription factor expressed in pro-B, pre-B, and mature B cells, but not in plasma cells. In this study, we explored the role of BSAP in B cell function by assessing how the content of this protein varies in cells driven by proliferative stimuli and, conversely, how artificial manipulation of BSAP activity affects cell proliferation. We found that BSAP activity of nuclear extracts increased when B cells were activated by mitogen (lipopolysaccharide [LPS]), antigen receptor-mediated signaling (surface immunoglobulin D [IgD] cross-linking) or T cell-dependent stimulation (CD40 cross-linking). We could suppress BSAP activity by exposure of B cells to phosphorothioate oligonucleotides antisense to the BSAP translation initiation start site, whereas control oligonucleotides were virtually inactive. Antisense-induced BSAP suppression was associated with a striking reduction in LPS-induced proliferation of splenic B cells and in the spontaneous proliferation of B lymphoma cells (CH12.LX), but the antisense oligonucleotide had virtually no effect on proliferation of two cell lines lacking BSAP: the T lymphoma line EL-4 and the plasma cell line MOPC-315. Overexpression of BSAP in splenic B cells or de novo expression in MOPC-315 plasma cells induced by transfection of a BSAP expression plasmid stimulated cell proliferation. Taken together, these results suggest that BSAP activity is a rate-limiting regulator of B cell proliferation. We also found that treatment with the antisense BSAP oligonucleotide downregulated Ig class switching induced by interleukin 4 plus LPS. This effect may be secondary to reduced proliferation or could be mediated through BSAP binding sites in the IgH locus.

Regulation of gene expression at early stages of B-cell differentiation

The phenotype of B lymphocytes at various stages of differentiation is, in part, controlled at the transcriptional level. Recently, a number of B-cell lineage and stage-specific transcription factors have been identified as candidate determinants for the developmental regulation of gene expression in B lymphocytes.

Pax-5 encodes the transcription factor BSAP and is expressed in B lymphocytes, the developing CNS, and adult testis

BSAP has been identified previously as a transcription factor that is expressed at early, but not late, stages of B-cell differentiation. Biochemical purification and cDNA cloning has now revealed that BSAP belongs to the family of paired domain proteins. BSAP is encoded by the Pax-5 gene and has been highly conserved between human and mouse. An intact paired domain was shown to be both necessary and sufficient for DNA binding of BSAP. Binding studies with several BSAP recognition sequences demonstrated that the sequence specificity of BSAP differs from that of the distantly related paired domain protein Pax-1. During embryogenesis, the BSAP gene is transiently expressed in the mesencephalon and spinal cord with a spatial and temporal expression pattern that is distinct from that of other Pax genes in the developing central nervous system (CNS). Later, the expression of the BSAP gene shifts to the fetal liver where it correlates with the onset of B lymphopoiesis. BSAP expression persists in B lymphocytes and is also seen in the testis of the adult mouse. All of this evidence indicates that the transcription factor BSAP may not only play an important role in B-cell differentiation but also in neural development and spermatogenesis.