BOB.1/OBF.1 deficiency affects marginal-zone B-cell compartment

OCA-B/Pou2af1 is sufficient to promote CD4+ T cell memory and prospectively identifies memory precursors

The molecular mechanisms leading to the establishment of immunological memory are inadequately understood, limiting the development of effective vaccines and durable antitumor immune therapies. Here, we show that ectopic OCA-B expression is sufficient to improve antiviral memory recall responses, while having minimal effects on primary effector responses. At peak viral response, short-lived effector T cell populations are expanded but show increased Gadd45b and Socs2 expression, while memory precursor effector cells show increased expression of Bcl2, Il7r, and Tcf7 on a per-cell basis. Using an OCA-B mCherry reporter mouse line, we observe high OCA-B expression in CD4+ central memory T cells. We show that early in viral infection, endogenously elevated OCA-B expression prospectively identifies memory precursor cells with increased survival capability and memory recall potential. Cumulatively, the results demonstrate that OCA-B is both necessary and sufficient to promote CD4 T cell memory in vivo and can be used to prospectively identify memory precursor cells.

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.

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.

Putative role of marginal zone B cells in pathophysiological processes

The maintenance of inner integrity of an organism is founded on the proper performance of two immunity branches, innate and adaptive immune responses. Recently, it became apparent that subset of splenic B cells named marginal zone B cells (MZB cells) exhibits unique developmental and functional features that bridge these two immunity branches. Strategically positioned at the site where blood and lymph are filtered, MZB cells represent a population of sentinels that rapidly proliferate and differentiate into IgM plasmablast cells when encountered with blood-borne, thymus-independent (TI) Ags. Moreover, MZB cells have intrinsic capability to induce potent CD4⁺ helper T cell response and cytokine production upon stimulation with soluble antigens. Due to their ability to overcome a time gap prior the establishment of the full adaptive response towards pathogens, MZB cells connect and direct innate and adaptive immunity. An additional interesting characteristic of MZB cells is capacity to function as regulatory cells in autoimmune processes. MZB cells may also contribute to the control of autoimmunity via the induction of tolerance by apoptotic cells. Importantly, in the clear association with inflammation and autoimmunity, MZB cells may transform into MALT lymphoma, representing a concurrence point for the infection, immunity and malignancy. This paper presents an insight into the complex biology of marginal zone B cells and their role in intertwining and directing innate and adaptive immune processes at the physiological and pathological level.

Sweet SIGNs: IgG glycosylation leads the way in IVIG-mediated resolution of inflammation

A hallmark of many chronic inflammatory and autoimmune diseases is that there is an impaired resolution of inflammation and return to the steady state. The infusion of high doses of pooled serum IgG preparations from thousands of donors [intravenous immunoglobulin (IVIG) therapy] has been shown to induce resolution of inflammation in a variety of chronic inflammatory and autoimmune diseases, suggesting that IgG molecules can instruct the immune system to stop inflammatory processes and initiate the return to the steady state. The aim of this review is to discuss how insights into the mechanism of IVIG activity may help to understand the molecular and cellular pathways underlying resolution of inflammation. We will put a special emphasis on pathways dependent on the IgG FC domain and IgG sialylation, as several recent studies have provided new insights into how this glycosylation-dependent pathway modulates innate and adaptive immune responses through different sets of C-type or I-type lectins.

Cutting edge: The transcription factor Bob1 counteracts B cell activation and regulates miR-146a in B cells

Mice lacking the lymphocyte-specific transcription factor Bob1 (also called OBF-1 or OCA-B) fail to generate germinal centers and a robust Ig response. We show that peripheral B cells in Bob1(-/-) mice bear characteristics of chronically activated or anergic-like B cells and identify the immunosuppressive microRNA-146a, together with other microRNAs, as novel transcriptional targets of Bob1. The inability to restrict B cell signaling could contribute to the immunodeficient phenotype of these mice and is consistent with an important role for Bob1 in suppressing B cell activation in vivo.

The BTB-ZF transcription factor Zbtb20 is driven by Irf4 to promote plasma cell differentiation and longevity

Zbtb20 facilitates terminal differentiation of B cells into antibody-secreting cells, and its expression is dependent on Irf4 and independent of Blimp1.

The transcriptional network regulating antibody-secreting cell (ASC) differentiation has been extensively studied, but our current understanding is limited. The mechanisms of action of known “master” regulators are still unclear, while the participation of new factors is being revealed. Here, we identify Zbtb20, a Bcl6 homologue, as a novel regulator of late B cell development. Within the B cell lineage, Zbtb20 is specifically expressed in B1 and germinal center B cells and peaks in long-lived bone marrow (BM) ASCs. Unlike Bcl6, an inhibitor of ASC differentiation, ectopic Zbtb20 expression in primary B cells facilitates terminal B cell differentiation to ASCs. In plasma cell lines, Zbtb20 induces cell survival and blocks cell cycle progression. Immunized Zbtb20-deficient mice exhibit curtailed humoral responses and accelerated loss of antigen-specific plasma cells, specifically from the BM pool. Strikingly, Zbtb20 induction does not require Blimp1 but depends directly on Irf4, acting at a newly identified Zbtb20 promoter in ASCs. These results identify Zbtb20 as an important player in late B cell differentiation and provide new insights into this complex process.

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.

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.

The follicular versus marginal zone B lymphocyte cell fate decision

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

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.

Oct2 enhances antibody-secreting cell differentiation through regulation of IL-5 receptor alpha chain expression on activated B cells

Mice lacking a functional gene for the Oct2 transcriptional activator display several developmental and functional deficiencies in the B lymphocyte lineage. These include defective B cell receptor (BCR) and Toll-like receptor 4 signaling, an absence of B-1 and marginal zone populations, and globally reduced levels of serum immunoglobulin (Ig) in naive and immunized animals. Oct2 was originally identified through its ability to bind to regulatory regions in the Ig loci, but genetic evidence has not supported an essential role for Oct2 in the expression of Ig genes. We describe a new Oct2-mediated role in B cells. Oct2 augments the ability of activated B cells to differentiate to antibody-secreting plasma cells (ASCs) under T cell-dependent conditions through direct regulation of the gene encoding the alpha chain of the interleukin (IL) 5 receptor. Ectopic expression of IL-5Ralpha in oct2-deficient B cells largely restores their ability to differentiate to functional ASCs in vitro but does not correct other phenotypic defects in the mutants, such as the maturation and specialization of peripheral B cells, which must therefore rely on distinct Oct2 target genes. IL-5 augments ASC differentiation in vitro, and we show that IL-5 directly activates the plasma cell differentiation program by enhancing blimp1 expression.

Rheumatoid arthritis fibroblast-like synoviocytes express BCMA and are stimulated by APRIL

OBJECTIVE

Fibroblast-like synoviocytes (FLS) are among the principal effector cells in the pathogenesis of rheumatoid arthritis (RA). This study was undertaken to examine the variety of stimulating effects of APRIL and its specific effect on FLS in the affected RA synovium.

METHODS

Synovium and serum samples were obtained from patients with RA, patients with osteoarthritis (OA), and healthy subjects. Soluble APRIL proteins were assayed by enzyme-linked immunosorbent assay. The relative gene expression of APRIL, BCMA, interleukin-6 (IL-6), tumor necrosis factor alpha (TNFalpha), IL-1beta, and RANKL was assessed in RA and OA FLS by polymerase chain reaction. Effects of APRIL on the production of proinflammatory cytokines and RANKL in RA FLS were investigated by flow cytometry and with the use of a BCMA-Fc fusion protein.

RESULTS

A significantly higher level of soluble APRIL was detected in RA serum compared with normal serum. Among the 3 receptors of APRIL tested, RA FLS expressed only BCMA, whereas OA FLS expressed none of the receptors. APRIL stimulated RA FLS, but not OA FLS, to produce IL-6, TNFalpha, IL-1beta, and APRIL itself. In addition, APRIL increased RA FLS expression of RANKL and also enhanced progression of the cell cycle of RA FLS. Neutralization of APRIL by the BCMA-Fc fusion protein attenuated all of these stimulating effects of APRIL on RA FLS.

CONCLUSION

RA FLS are stimulated by APRIL and express the APRIL receptor BCMA. These results provide evidence that APRIL is one of the main regulators in the pathogenesis of RA.

POU2AF1, an amplification target at 11q23, promotes growth of multiple myeloma cells by directly regulating expression of a B-cell maturation factor, TNFRSF17

Multiple myeloma (MM), a progressive hematological neoplasm, is thought to result from multiple genetic events affecting the terminal plasma cell. However, genetic aberrations related to MM are seldom reported. Using our in-house array-based comparative genomic hybridization system to locate candidate target genes with following their expression analysis, we identified POU2AF1 at 11q23.1 as a probable amplification target in MM cell lines. POU2AF1 is a B-cell-specific transcriptional co-activator, which interacts with octamer-binding transcription factors Oct-1 and Oct-2, and augments their function. Downregulation of POU2AF1 expression by specific small-interfering RNA (siRNA) inhibited MM cell growth, whereas ectopic expression of POU2AF1 promoted growth of MM cells. Among putative transcriptional targets for POU2AF1, B-cell maturation factor, TNFRSF17, enhanced its transcription by POU2AF1, and POU2AF1 directly bound to an octamer site within the 5' region of TNFRSF17. Expression level of TNFRSF17 was closely correlated with that of POU2AF1 in cell lines and primary samples of MM, and decreasing TNFRSF17 expression by means of TNFRSF17 siRNA inhibited MM cell growth. Taken together, our results suggest that POU2AF1, when activated by amplification or other mechanisms, may contribute to progression of MM by accelerating growth of MM cells through direct transactivation of one of its target genes, TNFRSF17.

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.

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.

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.

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.

Characterization of marginal zone B cell precursors

Selection of recently formed B cells into the follicular or marginal zone (MZ) compartments is proposed to occur by way of proliferative intermediates expressing high levels of CD21/35 and CD23. However, we show that CD21/35(high) CD23(+) splenocytes are not enriched for proliferative cells, and do not contribute substantially to the generation of follicular B cells. Instead, ontogenic relationships, steady-state labeling kinetics, and adoptive transfer experiments suggest that CD21/35(high) CD23(+) splenocytes serve primarily as precursors for MZ B cells, although their developmental potential seems to be broader and is influenced by environmental cues that are associated with lymphopenia. Furthermore, CD21/35(high) CD23(+) splenocytes share several key functional characteristics with MZ B cells, including their capacity to trap T-independent antigen and a heightened proliferative response to LPS. These observations challenge previous models of peripheral B cell maturation, and suggest that MZ B cells develop by way of CD21/35(high) CD23(+) intermediates.

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.

MARGINAL ZONE B CELLS

Our views regarding the origins and functions of splenic marginal zone B cells have changed considerably over the past few years. Perspectives regarding the development and function of these cells vary considerably between investigators studying human and rodent immunology. Marginal zone B cells are now recognized to constitute a distinct naive B lymphoid lineage. Considerable progress has been made regarding the mechanisms involved in marginal zone B cell development in the mouse. Many of the molecular events that participate in the retention of this lineage of B cells in the marginal zone have been identified. Here, we discuss the functions of these cells in both innate and adaptive immunity. We also attempt to reconcile differing viewpoints regarding the generation and function of marginal zone B cells in rodents and primates.

Development and selection of marginal zone B cells

It is now clear that functionally distinct subsets of mature peripheral B cells exist. Of these subsets, marginal zone (MZ) B cells in the spleen are strategically positioned at the blood-lymphoid interface and are programmed to initiate a fast and intense antibody response to blood-borne viral and bacterial agents. Their ability to respond vigorously to antigen and polyclonal activators make MZ B cells key players in the early response to pathogens in the bloodstream. The specialized functions of these innate-like lymphocytes bridge the gap between the early innate immune response and the slower adaptive antibody response, affected mainly by the more prolific follicular B cells. MZ B cells, like B1 cells, are important not only to combat infections but also in the maintenance of host homeostasis. Here we discuss some aspects of MZ B-cell selection and function in health and disease.

Positive selection and lineage commitment during peripheral B-lymphocyte development

Although it is appreciated that the antigen receptor on B cells is required for peripheral B-lymphocyte development and survival, it has been unclear whether this receptor interacts with self-antigens during development or if it signals constitutively in an antigen-independent fashion. The analysis of mutant mice in which antigen receptor signaling in B cells is either attenuated or enhanced has revealed the existence of a follicular versus marginal zone B-lymphocyte cell-fate decision. These analyses indicate that weak antigen receptor-derived signals favor marginal zone B-cell generation, and relatively strong signals favor the development of mature follicular B cells. Even stronger signals derived from the antigen receptor favor the generation of B1 B cells. This signal strength model for B-cell development supports the notion that self-antigens of varying affinity may mediate positive selection and lineage commitment. Direct evidence supporting such a view has been obtained from the analysis of antigen receptor knockin mice. Specific antigen receptors guide B cells to develop into specific lineages. Although Notch-2, nuclear factor-kappaBp50, and other genes are essential for marginal zone B-cell development, instructive signals delivered by the antigen receptor represent the primary force driving positive selection and lineage commitment in B lymphocytes.

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.

Marginal Zone, but Not Follicular B Cells, Are Potent Activators of Naive CD4 T Cells

The early involvement of marginal zone (MZ) B lymphocytes in T-independent immune responses is well established. In this study we compared the abilities of MZ and follicular (FO) B cells to collaborate with T cells. After immunization with soluble hen egg lysozyme, both MZ and FO B cells captured Ag and migrated to T cell areas in the response to hen egg lysozyme. MZ B cells were far superior to FO B cells in inducing CD4+ T cell expansion both in vitro and in vivo. MZ, but not FO, B cells, after interaction with T cells, differentiated into plasma cells, and in addition they stimulated Ag-specific CD4+ T cells to produce high levels of Th1-like cytokines upon primary stimulation in vitro. These results indicate that MZ B cells rapidly and effectively capture soluble Ag and activate CD4+ T cells to become effector T cells. The enhanced capacity of MZ B cells to prime T cells in this study appeared to be intrinsic to MZ B cells, as both MZ and FO B cell populations express an identical Ag receptor.

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.

Knockout B lymphoma cell lines as biochemical tools to explore multiple signalling pathways

Studies on B lymphocyte signalling pathways using B lymphocytes from genetically modified mice have the disadvantages of primary cell polyclonality and finite life span. B lymphoma cell lines have been generated from mice with targeted mutations in the oct-2, OBF-1, vav-1 and btk genes, as a model system that lacks these limitations and possesses additional potential for experimental manipulation. To assess their utility, activation of the B cell receptor using anti- micro, the Toll-like receptor-4 using lipopolysaccharide and the interleukin-4 receptor were assessed in these cell lines. Differential tyrosine phosphorylation of intracellular proteins was measured in the wild-type controls compared to the corresponding mutant cell lines after B cell receptor stimulation. Intracellular calcium (Ca2+i) was mobilized in the control cell lines but not in the OBF-1 and Vav1-deficient cells, while Xid B cell lines (btk mutant) showed a reduced Ca2+ mobilization. Extracellular signal-regulated kinase 1/2 phosphorylation in response to anti- micro or lipopolysaccharide stimulation was significantly reduced in Vav1-deficient cells. Interleukin-4 stimulation of wild-type cells resulted in a 2-3-fold increase in Stat-6 phosphorylation. These results indicate that the cell lines mimic the biochemical responses of the corresponding primary B cells. They therefore represent a useful model system to investigate the regulation and roles of these and other gene products in B cell signal transduction and activation.

B cell-specific transgenic expression of Bcl2 rescues early B lymphopoiesis but not B cell responses in BOB.1/OBF.1-deficient mice

Mice deficient for the transcriptional coactivator BOB.1/OBF.1 show several defects in B cell differentiation. Numbers of immature transitional B cells in the bone marrow are reduced and fewer B cells reach the periphery. Furthermore, germinal center B cells are absent and marginal zone (MZ) B lymphocytes are markedly reduced. Increased levels of B cell apoptosis in these mice prompted us to analyze expression and function of antiapoptotic proteins. Bcl2 expression is strongly reduced in BOB.1/OBF.1-deficient pre-B cells. When BOB.1/OBF.1-deficient mice were crossed with Bcl2-transgenic mice, B cell development in the bone marrow and numbers of B cells in peripheral lymphoid organs were normalized. However, neither germinal center B cells nor MZ B cells were rescued. Additionally, Bcl2 did not rescue the defects in signaling and affinity maturation found in BOB.1/OBF.1-deficient mice. Interestingly, Bcl2-transgenic mice by themselves show an MZ B cell defect. Virtually no functional MZ B cells were detected in these mice. In contrast, mice deficient for Bcl2 show a relative increase in MZ B cell numbers, indicating a previously undetected function of Bcl2 for this B cell compartment.