The B cell-specific coactivator OBF-1 (OCA-B, Bob-1) is inducible in T cells and its expression is dispensable for IL-2 gene induction

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 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.

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.

The regulatory interplay between Oct-1 isoforms contributes to hematopoiesis and the isoforms imbalance correlates with a malignant transformation of B cells

Oct-1(POU2F1) is a DNA-binding transcription regulator and its level being highly increased in many human cancers. Oct-1 is present in the human cells as a family of functionally different isoforms which are transcribed from alternative promoters. Here, we have demonstrated that expression patterns of Oct-1 isoforms change during differentiation of hematopoetic progenitor cells (CD34+) (HPCs) to the B (CD19+) and T (CD3+) cells. While Oct-1L is expressed at a high level in the CD34+ HPCs, its expression level drops dramatically during the T-cell differentiation, although remains nearly the same in B-cells. We have described the novel human Oct-1R isoform which is conserved in mammals and is B cell-specific. Oct-1R was found in B cells, but not in HPCs. Oct-1R is transcribed from the same promoter as Oct-1L, another lymphocyte-specific isoform. Overexpression of Oct-1R and Oct-1L in the Namalwa cells leads to the repression of many genes involved in B-lymphocyte differentiation and signal transduction. Thus these isoforms may regulate the particular stages of development of normal B cells and maintain their proper differentiation status. However the extremely high level of Oct-1L isoform observed in the B-lymphoblast tumor cell lines indicated that the excess of Oct-L seem likely to considerably decrease the differentiation ability of these cells. Oct-1 may serve as a therapeutic target for many tumors, but it should be noted that in a tumor the content of a certain isoform Oct-1, rather than the total Oct-1 protein, can be increased.

Germline variation in the 3'-untranslated region of the POU2AF1 gene is associated with susceptibility to lymphoma

Genetic variations in certain genes may alter the susceptibility to lymphoma. We searched electronic databases and selected candidate single-nucleotide polymorphisms (SNPs) located within 3'-untranslated regions (3'-UTRs) that might affect miRNA-binding ability in the 50 most dysregulated genes in lymphoma for further study. We found that rs1042752-located in the 3'-UTR of POU2AF1, which plays a vital role in lymphomagenesis-was significantly associated with lymphoma risk in a case-control study with 793 patients and 939 controls. Compared with individuals with the rs1042752TT genotype, those with the rs1042752CC genotype had a higher risk of lymphoma (OR = 2.14, 95% CI: 1.55-2.95, P < 0.001), even in stratified analysis for non-Hodgkin lymphoma (OR = 4.58, 95% CI: 2.38-8.81, P < 0.001), B-cell lymphoma (OR = 4.89, 95% CI: 2.46-9.73, P < 0.001), T-cell lymphoma (OR = 4.20, 95% CI: 1.76-10.05, P = 0.001), and Hodgkin lymphoma (OR = 3.62, 95% CI: 1.25-10.46, P = 0.018). Similar results were also observed in a recessive genetic model. Database findings suggested that rs1042752 might affect the interaction of POU2AF1 mRNA with hsa-miR-633. Functional assays confirmed that rs1042752C altered the binding site of hsa-miR-633 and increased POU2AF1 expression in Ramos, HuT 102, and Jurkat E6-1 cell lines. These findings demonstrate for the first time that functional polymorphism in the 3'-UTR of POU2AF1 is associated with susceptibility, and that SNP interaction with hsa-miR-633 affects gene expression and increases the risk of lymphoma.

Different N-terminal isoforms of Oct-1 control expression of distinct sets of genes and their high levels in Namalwa Burkitt's lymphoma cells affect a wide range of cellular processes

Oct-1 transcription factor has various functions in gene regulation. Its expression level is increased in several types of cancer and is associated with poor survival prognosis. Here we identified distinct Oct-1 protein isoforms in human cells and compared gene expression patterns and functions for Oct-1A, Oct-1L, and Oct-1X isoforms that differ by their N-terminal sequences. The longest isoform, Oct-1A, is abundantly expressed and is the main Oct-1 isoform in most of human tissues. The Oct-1L and the weakly expressed Oct-1X regulate the majority of Oct-1A targets as well as additional sets of genes. Oct-1X controls genes involved in DNA replication, DNA repair, RNA processing, and cellular response to stress. The high level of Oct-1 isoforms upregulates genes related to cell cycle progression and activates proliferation both in Namalwa Burkitt's lymphoma cells and primary human fibroblasts. It downregulates expression of genes related to antigen processing and presentation, cytokine-cytokine receptor interaction, oxidative metabolism, and cell adhesion, thus facilitating pro-oncogenic processes.

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

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

Oct1 and OCA-B are selectively required for CD4 memory T cell function

Shakya et al. identify the transcription factor Oct1 and its cofactor OCA-B as central mediators for generating memory T cell responses in mice.

Epigenetic changes are crucial for the generation of immunological memory. Failure to generate or maintain these changes will result in poor memory responses. Similarly, augmenting or stabilizing the correct epigenetic states offers a potential method of enhancing memory. Yet the transcription factors that regulate these processes are poorly defined. We find that the transcription factor Oct1 and its cofactor OCA-B are selectively required for the in vivo generation of CD4⁺ memory T cells. More importantly, the memory cells that are formed do not respond properly to antigen reencounter. In vitro, both proteins are required to maintain a poised state at the Il2 target locus in resting but previously stimulated CD4⁺ T cells. OCA-B is also required for the robust reexpression of multiple other genes including Ifng. ChIPseq identifies ∼50 differentially expressed direct Oct1 and OCA-B targets. We identify an underlying mechanism involving OCA-B recruitment of the histone lysine demethylase Jmjd1a to targets such as Il2, Ifng, and Zbtb32. The findings pinpoint Oct1 and OCA-B as central mediators of CD4⁺ T cell memory.

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.

B and T cells collaborate in antiviral responses via IL-6, IL-21, and transcriptional activator and coactivator, Oct2 and OBF-1

Transcriptional activator Oct2 and cofactor OBF-1 regulate B cell IL-6 to induce T cell production of IL-21, to support Tfh cell development in antiviral immunity.

A strong humoral response to infection requires the collaboration of several hematopoietic cell types that communicate via antigen presentation, surface coreceptors and their ligands, and secreted factors. The proinflammatory cytokine IL-6 has been shown to promote the differentiation of activated CD4⁺ T cells into T follicular helper cells (T_FH cells) during an immune response. T_FH cells collaborate with B cells in the formation of germinal centers (GCs) during T cell–dependent antibody responses, in part through secretion of critical cytokines such as IL-21. In this study, we demonstrate that loss of either IL-6 or IL-21 has marginal effects on the generation of T_FH cells and on the formation of GCs during the response to acute viral infection. However, mice lacking both IL-6 and IL-21 were unable to generate a robust T_FH cell–dependent immune response. We found that IL-6 production in follicular B cells in the draining lymph node was an important early event during the antiviral response and that B cell–derived IL-6 was necessary and sufficient to induce IL-21 from CD4⁺ T cells in vitro and to support T_FH cell development in vivo. Finally, the transcriptional activator Oct2 and its cofactor OBF-1 were identified as regulators of Il6 expression in B cells.

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.

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

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

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

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

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.

Oct transcription factors mediate t(14;18) lymphoma cell survival by directly regulating bcl-2 expression

Oct-1 and Oct-2 are members of the POU homeodomain family of transcriptional regulators and are critical for normal embryonic development. Gene-targeting studies showed that Oct-1 and Oct-2 are largely dispensable for B-cell development and immunoglobulin production, although both Oct-2 and Bob-1 are required for a proper immune response and germinal center formation. In these studies, we investigated the role of Oct factors in B-cell lymphomas. Recent investigations have shown increased expression of Oct-2 and Bob-1 in lymphomas, and we observed greatly increased levels of Oct-2 in lymphoma cells with the t(14;18) translocation. Decreased expression of Oct-1, Oct-2, or Bob-1 by RNA interference resulted in apoptosis and down-regulation of bcl-2 expression. Furthermore, Oct-2 induced bcl-2 promoter activity and mediated this effect through three regions in the bcl-2 P2 promoter. Although these regions did not contain canonical octamer motifs, we observed the direct interaction of Oct-2 with all three sites both in vitro by EMSA and in vivo by chromatin immunoprecipitation assay. Moreover, by mutation analysis we found that the ability of Oct-2 to activate bcl-2 required C/EBP, Cdx, and TATA-binding sites. Oct-2, therefore, acts as a cell survival factor in t(14;18) lymphoma cells by directly activating the antiapoptotic gene bcl-2.

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.

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

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

Novel mechanisms of class II major histocompatibility complex gene regulation

Class II MHC molecules present processed peptides from exogenous antigens to CD4+ helper T lymphocytes. In so doing, they are central to immunity, driving both the humoral and cell mediated arms of the immune response. Class II MHC molecules, and the genes encoding them, are expressed primarily in cells of the immune system (B cells, thymic epithelial cells, activated T cells and professional antigen presenting cells). The expression is also under developmental control. Research over the past 20 years have provided a clear understanding of the cis-elements and transcription factors that regulate the expression of Class II MHC genes. Perhaps the most critical advance has been the discovery of CIITA, a non- DNA binding activator of transcription that is a master control gene for class II gene expression. Current research is focused on understanding the situations where class II MHC gene expression occurs in a CIITA-independent pathway, and the molecular basis for this expression. Finally, significant emphasis is being placed on targeting class II MHC transcription factors to either inhibit or stimulate the immune response to transplanted tissue or in cell based vaccines. This communication outlines recent advances in this field and discusses likely areas for future research.

Expression of B-lymphocyte-associated transcription factors in human T-cell neoplasms

In this study we have investigated the expression of three B-cell-associated transcription factors in normal lymphoid tissue and in T-cell neoplasms (three cell lines, and more than 50 biopsy samples). Nuclear OCT-1 immunoreactivity was seen in normal B cells, in many extrafollicular T cells, and in a heterogeneous pattern (ranging in intensity from weak to moderate) in most T-cell neoplasms. OCT-2 immunostaining was primarily restricted in normal lymphoid tissue to B cells, and was absent from most T-cell neoplasms. In contrast, immunostaining for BOB-1/OCA-B--essentially restricted to B cells in normal lymphoid tissue, with the exception of activated T-lymphocytes--was seen in all of the T-cell lines tested and the majority of the tumor cells in all categories of T-cell lymphoma. Thus labeling for each of these three B-cell-associated transcription factors can be seen to varying degrees in T-cell neoplasms. However, the high frequency of BOB-1 expression in T-cell neoplasms, in contrast to its absence from resting peripheral T cells, suggests that its expression might be a prerequisite for neoplastic transformation, and prompts a search for the transcriptional target(s) of this factor in T cells.

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

Marginal-zone (MZ) B cells represent a first line of defense against particulate blood-borne antigens. Together with the B1 cells, they are responsible for the early response against type II T-independent antigens. The molecular pathways controlling the development of MZ B cells are only poorly understood. We found that these cells are virtually absent in mice deficient in the BOB.1/OBF.1 coactivator. Loss of these B cells was demonstrated by the lack of cells showing the appropriate cell surface phenotype but also by histological analyses and tri-nitro-phenol-Ficoll capturing. The lack of these cells is a B-cell-intrinsic defect, as shown by bone marrow complementation experiments. We also show that the expression of BOB.1/OBF.1 in peripheral B cells is required for the development of MZ B lymphocytes. Our analysis of BOB.1/OBF.1-deficient splenic B cells reveals alterations in cell motility, tumor necrosis factor receptor expression, and B-cell receptor (BCR) signaling. These changes could contribute to the loss of MZ B lymphocytes by altering the maturation of the cells. Interestingly, development of and BCR signaling in B1 B cells are completely normal in BOB.1/OBF.1 mutant mice.

Regulation of BOB.1/OBF.1 stability by SIAH

The BOB.1/OBF.1 coactivator is critically involved in mediating octamer-dependent transcriptional activity in B lymphocytes. Mice lacking this coactivator show various defects in B-cell development, most notably they completely lack germinal centers. Consistent with this phenotype, BOB.1/OBF.1 levels are massively upregulated in germinal center B cells as compared with resting B cells. We have addressed the mechanism of upregulation and found that only a minor part of this regulation can be attributed to increased levels of BOB.1/OBF.1-specific mRNA. Apparently, BOB.1/OBF.1 is also regulated at the protein level. In support of this suggestion we have been able to identify two related BOB.1/OBF.1 interacting proteins, SIAH1 and SIAH2, in a yeast two-hybrid screen. SIAH1 and SIAH2 are known regulators of protein stability. Cotransfection experiments revealed that coexpression of SIAH results in a destabilization of BOB.1/OBF.1 protein without affecting mRNA levels. Further more, proteasome inhibitors block the degradation of BOB.1/OBF.1 protein. Finally, B-cell receptor cross-linking also resulted in the degradation of BOB.1/OBF.1 and consequently reduced transcriptional activation of BOB.1/OBF.1-dependent reporters.

Reconstitution of T cell-specific transcription directed by composite NFAT/Oct elements

The complex nature of most promoters and enhancers makes it difficult to identify key determinants of tissue-specific gene expression. Furthermore, most tissue-specific genes are regulated by transcription factors that have expression profiles more widespread than the genes they control. NFAT is an example of a widely expressed transcription factor that contributes to several distinct patterns of cytokine gene expression within the immune system and where its role in directing specificity remains undefined. To investigate distinct combinatorial mechanisms employed by NFAT to regulate tissue-specific transcription, we examined a composite NFAT/AP-1 element from the widely active GM-CSF enhancer and a composite NFAT/Oct element from the T cell-specific IL-3 enhancer. The NFAT/AP-1 element was active in the numerous cell types that express NFAT, but NFAT/Oct enhancer activity was T cell specific even though Oct-1 is ubiquitous. Conversion of the single Oct site in the IL-3 enhancer to an AP-1 enabled activation outside of the T cell lineage. By reconstituting the activities of both the IL-3 enhancer and its NFAT/Oct element in a variety of cell types, we demonstrated that their T cell-specific activation required the lymphoid cofactors NIP45 and OCA-B in addition to NFAT and Oct family proteins. Furthermore, the Oct family protein Brn-2, which cannot recruit OCA-B, repressed NFAT/Oct enhancer activity. Significantly, the two patterns of combinatorial regulation identified in this study mirror the cell-type specificities of the cytokine genes that they govern. We have thus established that simple composite transcription factor binding sites can indeed establish highly specific patterns of gene expression.

Coactivator OBF-1 makes selective contacts with both the POU-specific domain and the POU homeodomain and acts as a molecular clamp on DNA

The lymphoid-specific transcriptional coactivator OBF-1 (also known as OCA-B or Bob-1) is recruited to octamer site-containing promoters by interacting with Oct-1 or Oct-2 and thereby enhances the transactivation potential of these two Oct factors. For this interaction the POU domain is sufficient. By contrast, OBF-1 does not interact with the POU domains of other POU proteins, such as Oct-4, Oct-6, or Pit-1, even though these factors bind efficiently to the octamer motif. Here we examined the structural requirements for selective interaction between the POU domain and OBF-1. Previous data have shown that formation of a ternary complex among OBF-1, the POU domain, and the DNA is critically dependent on residues within the octamer site. By methylation interference analysis we identified bases that react differently in the presence of OBF-1 compared to the POU domain alone, and using phosphothioate backbone-modified probes in electrophoretic mobility shift assays, we identified several positions influencing ternary complex formation. We then used Oct-1/Pit-1 POU domain chimeras to analyze the selectivity of the interaction between OBF-1 and the POU domain. This analysis indicated that both the POU specific domain (POUS) and the POU homeodomain (POUH) contribute to complex formation. Amino acids that are different in the Pit-1 and Oct-1 POU domains and are considered to be solvent accessible based on the Oct-1 POU domain/DNA cocrystal structure were replaced with alanine residues and analyzed for their influence on complex formation. Thereby, we identified residues L6 and E7 in the POUS and residues K155 and I159 in the POUH to be critical in vitro and in vivo for selective interaction with OBF-1. Furthermore, in an in vivo assay we could show that OBF-1 is able to functionally recruit two artificially separated halves of the POU domain to the promoter DNA, thereby leading to transactivation. These data allow us to propose a model of the interaction between OBF-1 and the POU domain, whereby OBF-1 acts as a molecular clamp holding together the two moieties of the POU domain and the DNA.