The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation

Umbilical cord mesenchymal stem cells suppress B-cell proliferation and differentiation

Mesenchymal stem cells (MSCs) may be obtained from umbilical cord as an abundant and noninvasive source. However, the immunomodulatory properties of umbilical cord-MSCs (UC-MSCs) were poorly studied. In this study, we aimed to investigate the effects of UC-MSCs on B-cell proliferation and differentiation. UC-MSCs were found to suppress the proliferation of B cells isolated from murine spleen. Moreover, UC-MSCs markedly suppressed B-cell differentiation as shown by the decreased number of CD138+cells and reduced levels of IgM and IgG production in coculture. As revealed by transwell experiments, soluble factors produced by UC-MSCs might be involved in mediating B-cell suppression. The Blimp-1 mRNA expression was suppressed whereas the PAX-5 mRNA expression was induced in coculture. Finally, UC-MSCs modified the phosphorylation pattern of Akt and p38 pathways, which were involved in B-cell proliferation and differentiation. These results may further support the potential therapeutic use of UC-MSCs in treating autoimmune disorders.

JAB1 is essential for B cell development and germinal center formation and inversely regulates Fas ligand and Bcl6 expression

Jun activation domain-binding protein 1 (JAB1) regulates ubiquitin-dependent protein degradation by deneddylation of cullin-based ubiquitin ligases and, therefore, plays a central role in regulating proliferation and apoptosis. Because these processes are decisive for B cell development, we investigated JAB1 functions in B cells by establishing a mouse strain with a B cell-specific JAB1 deletion. We show that JAB1 is essential for early B cell development, because the ablation of JAB1 expression blocks B cell development between the pro-B and pre-B cell stages. Furthermore, JAB1 deletion leads to aberrant expression of the apoptosis-triggering protein Fas ligand in pro-B cells. Concomitant B cell-specific overexpression of the antiapoptotic protein Bcl2 partially reverses the block in B cell development; rescued JAB1-deficient B cells reach the periphery and produce protective class-switched Abs after Borrelia burgdorferi infection. Interestingly, B cell-rescued mice exhibit no germinal centers but a striking extrafollicular plasma cell accumulation. In addition, JAB1 is essential for Bcl6 expression, a transcriptional repressor required for germinal center formation. These findings identify JAB1 as an important factor in checkpoint control during early B cell development, as well as in fate decisions in mature Ag-primed B cells.

How T cells earn the follicular rite of passage

The discovery that Bcl-6 was the transcriptional regulator of follicular helper T (Tfh) cells completed the recognition of this population as an effector subset specialized in the provision of help to B cells. Improved reagents and recent models that allow tracking of Bcl-6-expressing T cells have revealed that the decision to become a Tfh cell occurs soon after T cells are primed by dendritic cells and start dividing, before interaction with B cells. The latter are important for sustaining Bcl-6 expression. Bcl-6 coordinates a signaling program that changes expression or function of multiple guidance receptors, leading to Tfh cell localization within germinal centers. This program is not unique to CD4(+) helper T cells; FoxP3(+) regulatory T cells and NKT cells co-opt the follicular differentiation pathway to enter the follicle and become specialized follicular cells. This review will focus on recent insights into the early events that determine Tfh cell differentiation.

Early steps of follicular lymphoma pathogenesis

Follicular lymphoma (FL) pathogenesis is a complex and fascinating multi-hit process, escalating along successive derailments of the distinctive molecular and cellular mechanisms paving B-cell differentiation and activation. This progressive subversion of B-cell receptor diversification mechanisms and B-cell homeostasis likely occurs during a protracted preclinical phase of asymptomatic growth, in which premalignant clones already disseminate and establish "niches" in secondary lymphoid organs. Following FL diagnosis, a parallel indolent behavior is observed in most patients, slowly progressing over a period of many years, to eventually generate a highly refractory (and in some case transform into an aggressive subtype of) lymphoma. Novel insights in human germinal center B-cell biology recently allowed a more comprehensive understanding of the various illegitimate events sequentially involved in the premalignant progression phases. In this review, we will discuss how these new data have modified our perception of early FL pathogenesis, the new questions and challenges it opened up, and how this knowledge could impact on innovative programs of early detection, follow-up, and patient management.

Sequential class switching is required for the generation of high affinity IgE antibodies

Generation of anaphylaxis-inducing high affinity IgE requires sequential class switching.

IgE antibodies with high affinity for their antigens can be stably cross-linked at low concentrations by trace amounts of antigen, whereas IgE antibodies with low affinity bind their antigens weakly. In this study, we find that there are two distinct pathways to generate high and low affinity IgE. High affinity IgE is generated through sequential class switching (μ→γ→ε) in which an intermediary IgG phase is necessary for the affinity maturation of the IgE response, where the IgE inherits somatic hypermutations and high affinity from the IgG1 phase. In contrast, low affinity IgE is generated through direct class switching (μ→ε) and is much less mutated. Mice deficient in IgG1 production cannot produce high affinity IgE, even after repeated immunizations. We demonstrate that a small amount of high affinity IgE can cause anaphylaxis and is pathogenic. Low affinity IgE competes with high affinity IgE for binding to Fcε receptors and prevents anaphylaxis and is thus beneficial.

Pathogenesis of human B cell lymphomas

The mechanisms that drive normal B cell differentiation and activation are frequently subverted by B cell lymphomas for their unlimited growth and survival. B cells are particularly prone to malignant transformation because the machinery used for antibody diversification can cause chromosomal translocations and oncogenic mutations. The advent of functional and structural genomics has greatly accelerated our understanding of oncogenic mechanisms in lymphomagenesis. The signaling pathways that normal B cells utilize to sense antigens are frequently derailed in B cell malignancies, leading to constitutive activation of prosurvival pathways. These malignancies co-opt transcriptional regulatory systems that characterize their normal B cell counterparts and frequently alter epigenetic regulators of chromatin structure and gene expression. These mechanistic insights are ushering in an era of targeted therapies for these cancers based on the principles of pathogenesis.

What is unique about the IgE response?

IgE antibodies are involved in allergic reactions. High affinity IgE antibodies can cause anaphylaxis when cross-linked by minute amounts of antigen. The issue of how the IgE response is initiated and maintained is addressed in this review. A model has been proposed by which IgE(+) cells expressing antibodies that bind with high affinity to their antigens are generated through an IgG1 intermediate, which goes through affinity maturation in germinal centers (GC) before undergoing sequential switching to IgE. Mice deficient in IgG1 produce IgE at almost normal levels, but the IgE antibodies produced in IgG1-deficient mice lack the antigen-binding strength and the somatic mutations associated with affinity maturation. A GFP reporter strain, which expresses a modified IgE molecule, was recently developed and was utilized to challenge the sequential switching model. Several molecules that are highly expressed in GC can antagonize class switching to IgE in GC antagonize partially class switching to IgE; in addition, GC IgE(+) cells are gradually lost from GC as the immune response progresses, as shown with another recently developed, Venus-expressing IgE reporter mouse strain. In contrast, as a population, IgG1 cells thrive in the GC environment. Membrane IgE-expressing plasmablasts and plasma cells (PC) were recognized as a major component of the IgE response in secondary lymphoid organs. The swift development of IgE cells toward the PC fate, together with the affinity maturation of the IgE response via an IgG intermediate, represent the most salient features of the IgE immune responses, which make them distinct from IgG responses.

Type I interferons directly down-regulate BCL-6 in primary and transformed germinal center B cells: differential regulation in B cell lines derived from endemic or sporadic Burkitt's lymphoma

Type I interferons (IFN) exert multiple effects on both the innate and adaptive immune system in addition to their antiviral and antiproliferative activities. Little is known, however about the direct effects of type I IFNs on germinal center (GC) B cells, the central components of adaptive B cell responses. We used Burkitt's lymphoma (BL) lines, as a model system of normal human GC B cells, to examine the effect of type I IFNs on the expression of BCL-6, the major regulator of the GC reaction. We show that type I IFNs, but not IFNγ, IL-2 and TNFα rapidly down-regulate BCL-6 protein and mRNA expression, in cell lines derived from endemic, but not from sporadic BL. IFNα-induced down-regulation is specific for BCL-6, independent of Epstein-Barr virus and is not accompanied by IRF-4 up-regulation. IFNα-induced BCL-6 mRNA down-regulation does not require de novo protein synthesis and is specifically inhibited by piceatannol. The proteasome inhibitor MG132 non-specifically prevents, while inhibitors of alternate type I IFN signaling pathways do not inhibit IFNα-induced BCL-6 protein downregulation. We validate our results with showing that IFNα rapidly down-regulates BCL-6 mRNA in purified mouse normal GC B cells. Our results identify type I IFNs as the first group of cytokines that can down-regulate BCL-6 expression directly in GC B cells.

Asymmetric B cell division in the germinal center reaction

Lifelong antibody responses to vaccination require reorganization of lymphoid tissue and dynamic intercellular communication called the germinal center reaction. B lymphocytes undergo cellular polarization during antigen stimulation, acquisition, and presentation, which are critical steps for initiating humoral immunity. Here, we show that germinal center B lymphocytes asymmetrically segregate the transcriptional regulator Bcl6, the receptor for interleukin-21, and the ancestral polarity protein atypical protein kinase C to one side of the plane of division, generating unequal inheritance of fate-altering molecules by daughter cells. Germinal center B lymphocytes from mice with a defect in leukocyte adhesion fail to divide asymmetrically. These results suggest that motile cells lacking constitutive attachment can receive provisional polarity cues from the microenvironment to generate daughter cell diversity and self-renewal.

Prognostic value of immunohistochemical biomarkers at different cut-off values in patients with diffuse large B-cell lymphoma treated with CHOP chemotherapy

Many predictive models have been proposed for better stratification of diffuse large B-cell lymphoma (DLBCL). Hans' algorithm has been widely used as standard to sub-classify DLBCL into germinal center B-cell (GCB) and non-GCB origins. However, there have been disagreements in the literature regarding its prognostic significance. Here, we retrospectively analyzed Hans' algorithm and the individual immunohistochemical biomarkers at different cut-off values (5%, 30%, 50% or 75%) in 94 Korean patients with DLBCL treated with combination chemotherapy with cyclophosphamide, daunorubicin, vincristine, and prednisone. No significant differences were observed between the GCB (18 patients, 19.1%) and non-GCB (76, 80.9%) groups. Among individual biomarkers, CD10 negativity (cut point: 30%) and bcl-6 positivity (cut point: 5%) were independent good prognostic markers in progression-free survival (PFS), whereas bcl-6 (cut point: 5%) positivity was an independent good prognostic marker in overall survival irrelevant of international prognostic index. The present study showed the lack of predictability of Hans' algorithm in DLBCL patients, and that CD10, Bcl-6 may have diverse prognostic significance at different cut-off values. Our results suggest that the proposed cut-off value may not be applied universally, and that the optimal cut-off value may need to be optimized for individual laboratory.

The BTB-ZF family of transcription factors: key regulators of lineage commitment and effector function development in the immune system

Successful immunity depends upon the activity of multiple cell types. Commitment of pluripotent precursor cells to specific lineages, such as T or B cells, is obviously fundamental to this process. However, it is also becoming clear that continued differentiation and specialization of lymphoid cells is equally important for immune system integrity. Several members of the BTB-ZF family have emerged as critical factors that control development of specific lineages and also of specific effector subsets within these lineages. For example, BTB-ZF genes have been shown to control T cell versus B cell commitment and CD4 versus CD8 lineage commitment. Others, such as PLZF for NKT cells and Bcl-6 for T follicular helper cells, are necessary for the acquisition of effector functions. In this review, we summarize current findings concerning the BTB-ZF family members with a reported role in the immune system.

Analysis of p53 tumor suppressor pathway genes in chronic lymphocytic leukemia

The p53 tumor suppressor gene plays an important role in preventing tumor development. The p53 protein interacts with other p53 signal pathway members to control cell proliferation. In this study, expression of the p53, Human homolog of murine Double Minute 2 (HDM2), p14Alternating Reading Frame (ARF), Zinc Finger and BTB domain containing 7A (ZBTB7A), and B-Cell Lymphoma 6 (BCL6) genes was quantitatively investigated by real-time polymerase chain reaction (PCR) in the peripheral blood of patients with chronic lymphocytic leukemia (CLL) and healthy controls. Plasma fibronectin levels were determined by enzyme-linked immunosorbent assay. Expression of the p53, p14, and HDM2 genes were significantly higher in the patients. However, ZBTB7A and BCL6 gene expression was not detectable in both groups. A positive correlation between p14ARF and HDM2 expression and a negative correlation between p53 and p14ARF expression was observed. Expression of the p14ARF and HDM2 genes were inversely correlated in the control group. Neither HDM2 nor p14ARF gene expression was correlated with p53 expression. The p53 gene was also analyzed for the presence of mutations. A splice-site mutation was found in a single patient. Our findings indicate that expression of the p53, p14ARF, and HDM2 genes are associated with CLL. Elucidation of the mutual interactions at the protein level warrants further studies.

The role of the transcription factor Miz-1 in lymphocyte development and lymphomagenesis-Binding Myc makes the difference

The Myc interacting zinc finger protein 1 (Miz-1) is a BTB/POZ domain containing transcription factor that can function as an activator or repressor depending on its binding partners. In a complex with co-factors such as nuclophosmin or p300, Miz-1 stimulates transcription of genes that encode regulators of cell cycle progression such as p21(Cip1) or p15(Ink4b) or inhibitors of apoptosis such as Bcl-2. In contrast, Miz-1 becomes a transcriptional repressor when it binds to c-Myc or Bcl-6, which replace nucleophosmin or p300. During lymphocyte development, Miz-1 functions as a regulator of the IL-7 signaling pathway at very early steps in the bone marrow and thymus. When the IL-7 receptor (IL-7R) recognizes its cognate cytokine, a cascade of events is initiated that involves the recruitment of janus kinases (JAK) to the cytoplasmic part of the IL-7R, the phosphorylation of Stat5, its dimerization and relocation to the nucleus, enabling a transcriptional programming that governs commitment, survival and proliferation of lymphoid lineage cells. Miz-1 is critical in this signal transduction pathway, since it controls the expression of Socs1, an inhibitor of JAKs and thus of Stat5 activation and Bcl-2 expression. A lack of Miz-1 blocks IL-7 mediated signaling, which is detrimental for early B- and T-lymphoid development. These functions of Miz-1 during early lymphocyte development are c-Myc-independent. In contrast, when c-Myc is constitutively over-expressed, for instance during c-Myc induced lymphomagenesis, the interaction between Miz-1 and c-Myc becomes important and critical for the initiation and maintenance of c-Myc-dependent lymphoid malignancies.

Malignant pirates of the immune system

At great human cost, cancer is the largest genetic experiment ever conducted. This review highlights how lymphoid malignancies have genetically perverted normal immune signaling and regulatory mechanisms for their selfish oncogenic goals of unlimited proliferation, perpetual survival and evasion of the immune response.

The genetic network controlling plasma cell differentiation

Upon activation by antigen, mature B cells undergo immunoglobulin class switch recombination and differentiate into antibody-secreting plasma cells, the endpoint of the B cell developmental lineage. Careful quantitation of these processes, which are stochastic, independent and strongly linked to the division history of the cell, has revealed that populations of B cells behave in a highly predictable manner. Considerable progress has also been made in the last few years in understanding the gene regulatory network that controls the B cell to plasma cell transition. The mutually exclusive transcriptomes of B cells and plasma cells are maintained by the antagonistic influences of two groups of transcription factors, those that maintain the B cell program, including Pax5, Bach2 and Bcl6, and those that promote and facilitate plasma cell differentiation, notably Irf4, Blimp1 and Xbp1. In this review, we discuss progress in the definition of both the transcriptional and cellular events occurring during late B cell differentiation, as integrating these two approaches is crucial to defining a regulatory network that faithfully reflects the stochastic features and complexity of the humoral immune response.

BCL6-mediated repression of p53 is critical for leukemia stem cell survival in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is induced by the oncogenic BCR-ABL1 tyrosine kinase and can be effectively treated for many years with tyrosine kinase inhibitors (TKIs). However, unless CML patients receive life-long TKI treatment, leukemia will eventually recur; this is attributed to the failure of TKI treatment to eradicate leukemia-initiating cells (LICs). Recent work demonstrated that FoxO factors are critical for maintenance of CML-initiating cells; however, the mechanism of FoxO-dependent leukemia initiation remained elusive. Here, we identified the BCL6 protooncogene as a critical effector downstream of FoxO in self-renewal signaling of CML-initiating cells. BCL6 represses Arf and p53 in CML cells and is required for colony formation and initiation of leukemia. Importantly, peptide inhibition of BCL6 in human CML cells compromises colony formation and leukemia initiation in transplant recipients and selectively eradicates CD34⁺ CD38⁻ LICs in patient-derived CML samples. These findings suggest that pharmacological inhibition of BCL6 may represent a novel strategy to eradicate LICs in CML. Clinical validation of this concept could limit the duration of TKI treatment in CML patients, which is currently life-long, and substantially decrease the risk of blast crisis transformation.

Molecular pathogenesis of diffuse large B-cell lymphoma

In past years, substantial insight regarding the pathogenesis of diffuse large B-cell lymphoma has been obtained. Particularly, based on gene expression profile analysis, this disease can be classified into distinct phenotypic subgroups and specific transcriptional programs have been identified. New technologies like next-generation whole genome/exome sequencing and genome-wide single nucleotide polymorphism array analysis have revealed novel lesions involved in the pathogenesis of this disease. This review focuses on the diversity of genetic lesions identified in the different subtypes of diffuse large B-cell lymphoma.

Bcl6 protein expression shapes pre-germinal center B cell dynamics and follicular helper T cell heterogeneity

The transcription factor Bcl6 is essential for the development of germinal center (GC) B cells and follicular helper T (Tfh) cells. However, little is known about in vivo dynamics of Bcl6 protein expression during and after development of these cells. By using a Bcl6 reporter mouse strain, we found that antigen-engaged B cells upregulated Bcl6 before clustering in GCs. Two-photon microscopic analysis indicated that Bcl6 upregulation in pre-GC B cells contributed to sustaining their interactions with helper T cells and was required for their entry to GC clusters. Our data also suggested that Tfh cells gradually downmodulated Bcl6 protein over weeks after development. The Bcl6-low Tfh cells rapidly terminated proliferation and upregulated IL-7 receptor. These results clarify the role of Bcl6 in pre-GC B cell dynamics and highlight the modulation of Bcl6 expression in Tfh cells that persist in the late phase of the antibody response.

A strategy for full interrogation of prognostic gene expression patterns: exploring the biology of diffuse large B cell lymphoma

BACKGROUND

Gene expression profiling yields quantitative data on gene expression used to create prognostic models that accurately predict patient outcome in diffuse large B cell lymphoma (DLBCL). Often, data are analyzed with genes classified by whether they fall above or below the median expression level. We sought to determine whether examining multiple cut-points might be a more powerful technique to investigate the association of gene expression with outcome.

METHODOLOGY/PRINCIPAL FINDINGS

We explored gene expression profiling data using variable cut-point analysis for 36 genes with reported prognostic value in DLBCL. We plotted two-group survival logrank test statistics against corresponding cut-points of the gene expression levels and smooth estimates of the hazard ratio of death versus gene expression levels. To facilitate comparisons we also standardized the expression of each of the genes by the fraction of patients that would be identified by any cut-point. A multiple comparison adjusted permutation p-value identified 3 different patterns of significance: 1) genes with significant cut-point points below the median, whose loss is associated with poor outcome (e.g. HLA-DR); 2) genes with significant cut-points above the median, whose over-expression is associated with poor outcome (e.g. CCND2); and 3) genes with significant cut-points on either side of the median, (e.g. extracellular molecules such as FN1).

CONCLUSIONS/SIGNIFICANCE

Variable cut-point analysis with permutation p-value calculation can be used to identify significant genes that would not otherwise be identified with median cut-points and may suggest biological patterns of gene effects.

Bcl6 in pulmonary epithelium coordinately controls the expression of the CC-type chemokine genes and attenuates allergic airway inflammation

BACKGROUND

There is synteny in the CC-type chemokine gene clusters between humans (CCL2/MCP-1, CCL7MCP-3, CCL11/eotaxin, CCL8/MCP-2, CCL13/MCP-4, and CCL1/I-309) and mice (CCL2, CCL7, CCL11, CCL12/MCP-5, CCL8, and CCL1).

OBJECTIVE

As many putative Bcl6/STAT-binding sequences are observed in the clusters, we examined the roles of a transcriptional repressor Bcl6 and the regional histone modification in the expression of these chemokine genes in pulmonary epithelium.

METHODS

We generated transgenic (Tg) mice carrying the Bcl6 or the dominant-negative (DN)-Bcl6 gene under the control of the surfactant protein C (SPC) promoter that induces the exogenous gene expression in the distal lung epithelium. For in vitro studies, A549, alveolar type II-like epithelial cell line transfected with the SPC-DN-Bcl6 gene were stimulated with IL-4+TNF-α, and Bcl6 or STAT6 binding to and histone modification of the cluster in the transfectants were analysed by chromatin immunoprecipitation assays. Tg mice sensitized with ovalbumin (OVA) were challenged with OVA inhalation. The amounts of mRNAs in each sample were analysed by quantitative RT-PCR.

RESULTS

The amount of Bcl6 bound to the cluster decreased in A549 cells stimulated with IL-4 and TNF-α, whereas STAT6 binding increased in association with regional histone H3-K9/14 acetylation and H3-K4 methylation. The expression of all chemokine genes in the gene cluster was augmented in activated A549 cells transfected with the DN-Bcl6 gene. We also induced allergic airway inflammation in Tg mice. Expression of the chemokine genes and infiltrated cell numbers in the lungs of these Tg mice with allergic airway inflammation were inversely correlated with the amount of Bcl6 in the lungs.

CONCLUSION AND CLINICAL RELEVANCE

Expression of the pulmonary epithelium-derived CC-type chemokine genes in the cluster is orchestrated by the conserved machinery related to Bcl6. Thus, Bcl6 in pulmonary epithelium may be a critical regulator for pathogenesis of various pulmonary inflammatory diseases.

Follicular regulatory T cells expressing Foxp3 and Bcl-6 suppress germinal center reactions

Foxp3(+) regulatory T (T(reg)) cells suppress different types of immune responses to help maintain homeostasis in the body. How T(reg) cells regulate humoral immunity, including germinal center reactions, is unclear. Here we identify a subset of T(reg) cells expressing CXCR5 and Bcl-6 that localize to the germinal centers in mice and humans. The expression of CXCR5 on T(reg) cells depends on Bcl-6. These CXCR5(+)Bcl-6(+) T(reg) cells are absent in the thymus but can be generated de novo from CXCR5(-)Foxp3(+) natural T(reg) precursors. A lack of CXCR5(+) T(reg) cells leads to greater germinal center reactions including germinal center B cells, affinity maturation of antibodies and the differentiation of plasma cells. These results unveil a Bcl-6-CXCR5 axis in T(reg) cells that drives the development of follicular regulatory T (T(FR)) cells that function to inhibit the germinal center reactions.

Gene interaction network regulates plasma cell differentiation

Effective humoral immunity depends on B cells, plasma cells and follicular helper T cells (TFH) and secreted high-affinity antibodies. The differentiation of mature B cell into plasma cells is ultimately hardwired in a regulatory network of transcription factors. This circuitry is responding to extracellular stimuli, which leads to production of higher-affinity antibodies after germinal centre (GC) reaction. The understanding of the transcriptional regulation of GCs and the initiation of plasma cell differentiation is becoming increasingly clear. It is evident that transcriptional repressor Blimp-1 can drive the plasma cell differentiation, but the initiation of plasma cell differentiation in GCs is likely coupled to the loss of B cell characteristics maintained by transcription factors Pax5 and Bcl6.

Alternate pathways for Bcl6-mediated regulation of B cell to plasma cell differentiation

The transcription factor Bcl6 regulates germinal center formation and differentiation of B cells into high-affinity antibody-producing plasma cells. The direct double-negative regulatory circuit between Bcl6 and Blimp-1 is well established. We now reveal alternative mechanisms for Bcl6-mediated regulation of B-cell differentiation to plasma cells and show with DT40 cells that Bcl6 directly promotes the expression of Bach2, a known suppressor of Blimp-1. Moreover, Bcl6 suppresses Blimp-1 expression through direct binding to the IRF4 gene, as well as by promoting the expression of MITF, a known suppressor of IRF4. We also provide evidence that Bcl6 is needed for the expression of AID and UNG, the indispensable proteins for somatic hypermutation and class-switch recombination, and UNG appears to be a direct Bcl6 target. Our findings reveal a complex regulatory network in which Bcl6 acts as a key element dictating the transition of DT40 B cells to plasma cells.

B cell priming for extrafollicular antibody responses requires Bcl-6 expression by T cells

T follicular helper cells (Tfh cells) localize to follicles where they provide growth and selection signals to mutated germinal center (GC) B cells, thus promoting their differentiation into high affinity long-lived plasma cells and memory B cells. T-dependent B cell differentiation also occurs extrafollicularly, giving rise to unmutated plasma cells that are important for early protection against microbial infections. Bcl-6 expression in T cells has been shown to be essential for the formation of Tfh cells and GC B cells, but little is known about its requirement in physiological extrafollicular antibody responses. We use several mouse models in which extrafollicular plasma cells can be unequivocally distinguished from those of GC origin, combined with antigen-specific T and B cells, to show that the absence of T cell-expressed Bcl-6 significantly reduces T-dependent extrafollicular antibody responses. Bcl-6(+) T cells appear at the T-B border soon after T cell priming and before GC formation, and these cells express low amounts of PD-1. Their appearance precedes that of Bcl-6(+) PD-1(hi) T cells, which are found within the GC. IL-21 acts early to promote both follicular and extrafollicular antibody responses. In conclusion, Bcl-6(+) T cells are necessary at B cell priming to form extrafollicular antibody responses, and these pre-GC Tfh cells can be distinguished phenotypically from GC Tfh cells.

The LRF transcription factor regulates mature B cell development and the germinal center response in mice

B cells play a central role in immune system function. Deregulation of normal B cell maturation can lead to the development of autoimmune syndromes as well as B cell malignancies. Elucidation of the molecular features of normal B cell development is important for the development of new target therapies for autoimmune diseases and B cell malignancies. Employing B cell-specific conditional knockout mice, we have demonstrated here that the transcription factor leukemia/lymphoma-related factor (LRF) forms an obligate dimer in B cells and regulates mature B cell lineage fate and humoral immune responses via distinctive mechanisms. Moreover, LRF inactivation in transformed B cells attenuated their growth rate. These studies identify what we believe to be a new key factor for mature B cell development and provide a rationale for targeting LRF dimers for the treatment of autoimmune diseases and B cell malignancies.

WNT/β-catenin pathway is modulated in asthma patients and LPS-stimulated RAW264.7 macrophage cell line

In the present study, we investigated the possibility that the WNT/β-catenin pathway plays a role in inflammatory responses both in an human inflammatory condition and in an in vitro inflammation model. First, we analyzed gene expression patterns of the peripheral blood cells from asthma patients compared with those from normal subjects using microarray analyses. We found that intracellular signaling molecules of the WNT/β-catenin pathway were significantly changed in asthma patients compared with the levels in the controls. Next, we determined whether major components of the WNT/β-catenin pathway were involved in the lipopolysaccharide (LPS)-induced inflammatory response of the RAW264.7 macrophage cell line. Among the members of WNT/β-catenin pathway, the protein levels of low-density lipoprotein receptor-related protein (LRP) 6, dishevelled (DVL) 2, and AXIN1, which were measured using western blotting, did not significantly change in the presence of LPS. In contrast, the LPS induced a rapid phosphorylation of glycogen synthase kinase (GSK) 3β and accumulation of β-catenin protein. It was found that β-catenin plays a significant role in the LPS-induced inflammatory response through the performance of small interfering RNA (siRNA) transfection experiments. The mRNA level of IL-6 was significantly elevated in β-catenin siRNA-transfected cells compared with that in control siRNA-transfected cells after LPS treatment. Furthermore, nuclear factor-κB (NF-κB) activity was also significantly increased in β-catenin siRNA-transfected cells compared with the level seen in control siRNA-transfected cells. Taken together, these results suggest that β-catenin plays a role as a negative regulator, preventing the overproduction of inflammatory cytokines such as IL-6 in LPS-induced inflammatory responses.

BCL6 enables Ph+ acute lymphoblastic leukaemia cells to survive BCR-ABL1 kinase inhibition

Tyrosine kinase inhibitors (TKIs) are widely used to treat patients with leukaemia driven by BCR-ABL1 (ref. 1) and other oncogenic tyrosine kinases. Recent efforts have focused on developing more potent TKIs that also inhibit mutant tyrosine kinases. However, even effective TKIs typically fail to eradicate leukaemia-initiating cells (LICs), which often cause recurrence of leukaemia after initially successful treatment. Here we report the discovery of a novel mechanism of drug resistance, which is based on protective feedback signalling of leukaemia cells in response to treatment with TKI. We identify BCL6 as a central component of this drug-resistance pathway and demonstrate that targeted inhibition of BCL6 leads to eradication of drug-resistant and leukaemia-initiating subclones.

Kaposi sarcoma herpesvirus (KSHV) vFLIP oncoprotein induces B cell transdifferentiation and tumorigenesis in mice

Kaposi sarcoma herpesvirus (KSHV) is specifically associated with Kaposi sarcoma (KS) and 2 B cell lymphoproliferative diseases, namely primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). KS, PEL, and MCD are largely incurable and poorly understood diseases most common in HIV-infected individuals. Here, we have revealed the role of viral FLICE-inhibitory protein (vFLIP) in the initiation of PEL and MCD by specifically expressing vFLIP at different stages of B cell differentiation in vivo. Mice showed MCD-like abnormalities and immunological defects including lack of germinal centers (GCs), impaired Ig class switching, and affinity maturation. In addition, they showed increased numbers of cells expressing cytoplasmic IgM-λ, a thus far enigmatic feature of the KSHV-infected cells in MCD. B cell-derived tumors arose at high incidence and displayed Ig gene rearrangement with downregulated expression of B cell-associated antigens, which are features of PEL. Interestingly, these tumors exhibited characteristics of transdifferentiation and acquired expression of histiocytic/dendritic cell markers. These results define immunological functions for vFLIP in vivo and reveal what we believe to be a novel viral-mediated tumorigenic mechanism involving B cell reprogramming. Additionally, the robust recapitulation of KSHV-associated diseases in mice provides a model to test inhibitors of vFLIP as potential anticancer agents.

The genomic landscapes of inflammation

Inflammation involves the activation of a highly coordinated gene expression program that is specific for the initial stimulus and occurs in a different manner in bystander parenchymal cells and professional immune system cells recruited to the inflamed site. Recent data demonstrate that developmental transcription factors like the macrophage fate-determining Pu.1 set the stage for the activity of ubiquitous transcription factors activated by inflammatory stimuli, like NF-kB, AP-1, and interferon regulatory factors (IRFs). The intersection of lineage-determining and stimulus-activated transcription factors at enhancers explains cell type specificity in inflammatory responses.

The transcription factor B-cell lymphoma (BCL)-6 modulates pancreatic {beta}-cell inflammatory responses

Type 1 diabetes is a chronic autoimmune disease with a strong inflammatory component. We have previously shown that expression of the transcriptional repressor B-cell lymphoma (BCL)-6 is very low in pancreatic β-cells, which may favor prolonged proinflammatory responses after exposure to the cytokines IL-1β and interferon γ. Here we investigated whether cytokine-induced inflammation and apoptosis can be prevented in β-cells by BCL-6 expression using plasmid, prolactin, and adenoviral approaches. The induction of mild or abundant BCL-6 expression in β-cells by prolactin or an adenoviral BCL-6 expression construct, respectively, reduced cytokine-induced inflammatory responses in a dose-dependent manner through inhibition of nuclear factor-κB activation. BCL-6 decreased Fas and inducible nitric oxide synthase expression and nitric oxide production, but it inhibited the expression of the antiapoptotic proteins Bcl-2 and JunB while increasing the expression of the proapoptotic death protein 5. The net result of these opposite effects was an augmentation of β-cell apoptosis. In conclusion, BCL-6 expression tones down the unrestrained cytokine-induced proinflammatory response of β-cells but it also favors gene networks leading to apoptosis. This suggests that cytokine-induced proinflammatory and proapoptotic signals can be dissociated in β-cells. Further understanding of these pathways may open new possibilities to improve β-cell survival in early type 1 diabetes or after transplantation.

Follicular dendritic cell-induced microRNA-mediated upregulation of PRDM1 and downregulation of BCL-6 in non-Hodgkin's B-cell lymphomas

B-cell lymphoma 6 (BCL6) and PR domain containing 1 (PRDM1) are considered as master regulators for germinal center (GC) formation and terminal B-cell differentiation. Dysregulation of BCL6 and PRDM1 has been associated with lymphomagenesis. Here, we show for the first time that direct cell-cell contact between follicular dendritic cells (FDC) and B-lymphocytes, by influencing the expression of a set of microRNAs (miRNAs), regulates the expression of BCL6 and PRDM1. We identify that, on cell adhesion to FDC, FDC induces upregulation of PRDM1 expression through downregulation of miR-9 and let-7 families and induces downregulation of BCL-6 through upregulation of miR-30 family in B-lymphocytes and lymphoma cells. We further demonstrate that the miR-30 family directly controls BCL-6 expression and miR-9-1 and let-7a directly control PRDM-1 expression through targeting their 3'UTR, mediating the FDC effect. Our studies define a novel regulatory mechanism in which the FDC, through induction of miRNAs in B-lymphocytes, orchestrates the regulation of transcription factors, promotes germinal center B-cell survival and differentiation. Dysregulation of miRNAs may interfere with B-cell survival and maturation, thus representing a novel molecular mechanism, as well as a potential therapeutic target in B-cell lymphomas.

Bcl6 is required for the development of mouse CD4+ and CD8α+ dendritic cells

Th2-type inflammation spontaneously shown in Bcl6-knockout (KO) mice is mainly caused by bone marrow (BM)-derived nonlymphoid cells. However, the function of dendritic cells (DCs) in Bcl6-KO mice has not been reported. We show in this article that the numbers of CD4(+) conventional DCs (cDCs) and CD8α(+) cDCs, but not of plasmacytoid DCs, were markedly reduced in the spleen of Bcl6-KO mice. Generation of cDCs from DC progenitors in BM cells was perturbed in the spleen of irradiated wild-type (WT) mice transferred with Bcl6-KO BM cells, indicating an intrinsic effect of Bcl6 in cDC precursors. Although cDC precursors were developed in a Bcl6-KO BM culture with Fms-like tyrosine kinase 3 ligand, the cDC precursors were more apoptotic than WT ones. Also p53, one of the molecular targets of Bcl6, was overexpressed in the precursors. The addition of a p53 inhibitor to Bcl6-KO BM culture protected apoptosis, suggesting that Bcl6 is required by cDC precursors for survival by controlling p53 expression. Furthermore, large numbers of T1/ST2(+) Th2 cells were naturally developed in the spleen of Bcl6-KO mice. Th2 skewing was accelerated in the culture of WT CD4 T cells stimulated with Ags and LPS-activated Bcl6-KO BM-derived DCs, which produced more IL-6 and less IL-12 than did WT DCs; the addition of anti-IL-6 Abs to the culture partially abrogated the Th2 skewing. These results suggest that Bcl6 is required in cDC precursors for survival and in activated DCs for modulating the cytokine profile.

T cells and follicular dendritic cells in germinal center B-cell formation and selection

Germinal centers (GCs) are specialized microenvironments formed after infection where activated B cells can mutate their B-cell receptors to undergo affinity maturation. A stringent process of selection allows high affinity, non-self-reactive B cells to become long-lived memory B cells and plasma cells. While the precise mechanism of selection is still poorly understood, the last decade has advanced our understanding of the role of T cells and follicular dendritic cells (FDCs) in GC B-cell formation and selection. T cells and non-T-cell-derived CD40 ligands on FDCs are essential for T-dependent (TD) and T-independent GC formation, respectively. TD-GC formation requires Bcl-6-expressing T cells capable of signaling through SAP, which promotes formation of stable T:B conjugates. By contrast, differentiation of B blasts along the extrafollicular pathway is less dependent on SAP. T-follicular helper (Tfh) cell-derived CD40L, interleukin-21, and interleukin-4 play important roles in GC B-cell proliferation, survival, and affinity maturation. A role for FDC-derived integrin signals has also emerged: GC B cells capable of forming an immune synapse with FDCs have a survival advantage. This emerges as a powerful mechanism to ensure death of B cells that bind self-reactive antigen, which would not normally be presented on FDCs.

Noxa mediates p18INK4c cell-cycle control of homeostasis in B cells and plasma cell precursors

Inhibition of Cdk4/Cdk6 by p18(INK4c) (p18) is pivotal for generation of noncycling immunoglobulin (Ig)-secreting plasma cells (PCs). In the absence of p18, CD138(+) plasmacytoid cells continue to cycle and turnover rapidly, suggesting that p18 controls PC homeostasis. We now show that p18 selectively acts in a rare population of rapidly cycling CD138(hi)/B220(hi) intermediate PCs (iPCs). While retaining certain B-cell signatures, iPCs are poised to differentiate to end-stage PCs although the majority undergo apoptosis. p18 is dispensable for the development of the PC transcriptional circuitry, and Blimp-1 and Bcl-6 are expressed fully and mutually exclusively in individual iPCs. However, a minor proportion of iPCs express both, and they are preferentially protected by p18 or Bcl-xL overexpression, consistent with expansion of the iPC pool by Bcl-xL overexpression, or loss of proapoptotic Bim or Noxa. Expression of Noxa is induced during B-cell activation, peaks in iPCs, and selectively repressed by p18. It is required to promote apoptosis of cycling B cells, especially in the absence of p18. These findings define the first physiologic function for Noxa and suggest that by repressing Noxa, induction of G₁ arrest by p18 bypasses a homeostatic cell-cycle checkpoint in iPCs for PC differentiation.

Bcl-6 and NF-kappaB cistromes mediate opposing regulation of the innate immune response

In the macrophage, toll-like receptors (TLRs) are key sensors that trigger signaling cascades to activate inflammatory programs via the NF-κB gene network. However, the genomic network targeted by TLR/NF-κB activation and the molecular basis by which it is restrained to terminate activation and re-establish quiescence is poorly understood. Here, using chromatin immunoprecipitation sequencing (ChIP-seq), we define the NF-κB cistrome, which is comprised of 31,070 cis-acting binding sites responsive to lipopolysaccharide (LPS)-induced signaling. In addition, we demonstrate that the transcriptional repressor B-cell lymphoma 6 (Bcl-6) regulates nearly a third of the Tlr4-regulated transcriptome, and that 90% of the Bcl-6 cistrome is collapsed following Tlr4 activation. Bcl-6-deficient macrophages are acutely hypersensitive to LPS and, using comparative ChIP-seq analyses, we found that the Bcl-6 and NF-κB cistromes intersect, within nucleosomal distance, at nearly half of Bcl-6-binding sites in stimulated macrophages to promote opposing epigenetic modifications of the local chromatin. These results reveal a genomic strategy for controlling the innate immune response in which repressive and inductive cistromes establish a dynamic balance between macrophage quiescence and activation via epigenetically marked cis-regulatory elements.

Far from solved: a perspective on what we know about early mechanisms of left-right asymmetry

Consistent laterality is a crucial aspect of embryonic development, physiology, and behavior. While strides have been made in understanding unilaterally expressed genes and the asymmetries of organogenesis, early mechanisms are still poorly understood. One popular model centers on the structure and function of motile cilia and subsequent chiral extracellular fluid flow during gastrulation. Alternative models focus on intracellular roles of the cytoskeleton in driving asymmetries of physiological signals or asymmetric chromatid segregation, at much earlier stages. All three models trace the origin of asymmetry back to the chirality of cytoskeletal organizing centers, but significant controversy exists about how this intracellular chirality is amplified onto cell fields. Analysis of specific predictions of each model and crucial recent data on new mutants suggest that ciliary function may not be a broadly conserved, initiating event in left-right patterning. Many questions about embryonic left-right asymmetry remain open, offering fascinating avenues for further research in cell, developmental, and evolutionary biology.

Double-hit B-cell lymphomas

In many B-cell lymphomas, chromosomal translocations are biologic and diagnostic hallmarks of disease. An intriguing subset is formed by the so-called double- hit (DH) lymphomas that are defined by a chromosomal breakpoint affecting the MYC/8q24 locus in combination with another recurrent breakpoint, mainly a t(14;18)(q32;q21) involving BCL2. Recently, these lymphomas have received increased attention, which contributed to the introduction of a novel category of lymphomas in the 2008 WHO classification, "B cell lymphoma unclassifiable with features intermediate between DLBCL and BL." In this review we explore the existing literature for the most recurrent types of DH B-cell lymphomas and the involved genes with their functions, as well as their pathology and clinical aspects including therapy and prognosis. The incidence of aggressive B-cell lymphomas other than Burkitt lymphoma with a MYC breakpoint and in particular a double hit is difficult to assess, because screening by methods like FISH has not been applied on large, unselected series, and the published cytogenetic data may be biased to specific categories of lymphomas. DH lymphomas have been classified heterogeneously but mostly as DLBCL, the majority having a germinal center phenotype and expression of BCL2. Patients with DH lymphomas often present with poor prognostic parameters, including elevated LDH, bone marrow and CNS involvement, and a high IPI score. All studies on larger series of patients suggest a poor prognosis, also if treated with RCHOP or high-intensity treatment modalities. Importantly, this poor outcome cannot be accounted for by the mere presence of a MYC/8q24 breakpoint. Likely, the combination of MYC and BCL2 expression and/or a related high genomic complexity are more important. Compared to these DH lymphomas, BCL6(+)/MYC(+) DH lymphomas are far less common, and in fact most of these cases represent BCL2(+)/BCL6(+)/MYC(+) triple-hit lymphomas with involvement of BCL2 as well. CCND1(+)/MYC(+) DH lymphomas with involvement of 11q13 may also be relatively frequent, the great majority being classified as aggressive variants of mantle cell lymphoma. This suggests that activation of MYC might be an important progression pathway in mantle cell lymphoma as well. Based on clinical significance and the fact that no other solid diagnostic tools are available to identify DH lymphomas, it seems advisable to test all diffuse large B-cell and related lymphomas for MYC and other breakpoints.

The role of BCL6 in lymphomas and routes to therapy

BCL6 is a transcription factor that has essential B-cell and T-cell roles in normal antibody responses. It is involved in chromosomal translocations in diffuse large B-cell lymphoma (DBCL; including primary mediastinal B-cell lymphoma) and nodular lymphocyte predominant Hodgkin lymphoma, and is expressed in follicular lymphoma and Burkitt's lymphoma. The neoplastic T-cells of angioimmunoblastic T-cell lymphoma also express BCL6. BCL6 prevents terminal B-cell differentiation largely through repression of PRDM1. In the "cell of origin" classification of DLBCL BCL6 is associated with the germinal centre subtype, which carries a good response to modern treatments. More recently, specific BCL6 antagonists, including small molecule inhibitors, have been developed. These antagonists have demonstrated that DLBCL cells, in which BCL6 is transcriptionally active, are dependent on this gene for survival. BCL6 antagonists are active against primary DLBCL and may find future application in the treatment of lymphomas.

The long winding road toward understanding the molecular mechanisms for B-cell suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was first reported in the mid-1970s. Since this initial observation, much effort has been devoted by many laboratories toward elucidation of the cellular and molecular mechanisms responsible for the profound impairment of humoral immune responses by TCDD, which is characterized by decreased B cell to plasma cell differentiation and suppression of immunoglobulin production. These efforts have led to a significant body of research demonstrating a direct effect of TCDD on B-cell maturation and function as well as a requisite but as yet undefined role of the aryl hydrocarbon receptor (AhR) in these effects. Likewise, a number of molecular targets putatively involved in mediating B-cell dysfunction by TCDD, and other AhR ligands, have been identified. However, our current understanding has primarily relied on findings from mouse models, and the translation of this knowledge to effects on human B cells and humoral immunity in humans is less clear. Therefore, a current challenge is to determine how TCDD and the AhR affect human B cells. Efforts have been made in this direction but continued progress in developing adequate human models is needed. An in-depth discussion of these advances and limitations in elucidating the cellular and molecular mechanisms putatively involved in the suppression of B-cell function by TCDD as well as the implications on human diseases associated in epidemiological studies with exposure to TCDD and dioxin-like compounds is the primary focus of this review.

Generation of human antigen-specific monoclonal IgM antibodies using vaccinated "human immune system" mice

Background

Passive transfer of antibodies not only provides immediate short-term protection against disease, but also can be exploited as a therapeutic tool. However, the ‘humanization’ of murine monoclonal antibodies (mAbs) is a time-consuming and expensive process that has the inherent drawback of potentially altering antigenic specificity and/or affinity. The immortalization of human B cells represents an alternative for obtaining human mAbs, but relies on the availability of biological samples from vaccinated individuals or convalescent patients. In this work we describe a novel approach to generate fully human mAbs by combining a humanized mouse model with a new B cell immortalization technique.

Methodology/Principal Findings

After transplantation with CD34⁺CD38⁻ human hematopoietic progenitor cells, BALB/c Rag2^−/−IL-2Rγc^−/− mice acquire a human immune system and harbor B cells with a diverse IgM repertoire. “Human Immune System” mice were then immunized with two commercial vaccine antigens, tetanus toxoid and hepatitis B surface antigen. Sorted human CD19⁺CD27⁺ B cells were retrovirally transduced with the human B cell lymphoma (BCL)-6 and BCL-XL genes, and subsequently cultured in the presence of CD40-ligand and IL-21. This procedure allows generating stable B cell receptor-positive B cells that secrete immunoglobulins. We recovered stable B cell clones that produced IgM specific for tetanus toxoid and the hepatitis B surface antigen, respectively.

Conclusion/Significance

This work provides the proof-of-concept for the usefulness of this novel method based on the immunization of humanized mice for the rapid generation of human mAbs against a wide range of antigens.

DNA methylation prevents CTCF-mediated silencing of the oncogene BCL6 in B cell lymphomas

Aberrant DNA methylation commonly occurs in cancer cells where it has been implicated in the epigenetic silencing of tumor suppressor genes. Additional roles for DNA methylation, such as transcriptional activation, have been predicted but have yet to be clearly demonstrated. The BCL6 oncogene is implicated in the pathogenesis of germinal center-derived B cell lymphomas. We demonstrate that the intragenic CpG islands within the first intron of the human BCL6 locus were hypermethylated in lymphoma cells that expressed high amounts of BCL6 messenger RNA (mRNA). Inhibition of DNA methyltransferases decreased BCL6 mRNA abundance, suggesting a role for these methylated CpGs in positively regulating BCL6 transcription. The enhancer-blocking transcription factor CTCF bound to this intronic region in a methylation-sensitive manner. Depletion of CTCF by short hairpin RNA in neoplastic plasma cells that do not express BCL6 resulted in up-regulation of BCL6 transcription. These data indicate that BCL6 expression is maintained during lymphomagenesis in part through DNA methylation that prevents CTCF-mediated silencing.

The proto-oncogene BCL6 promotes survival of olfactory sensory neurons

For the mammalian olfactory epithelium to continually detect odorant, neuronal survival, apoptosis, and regeneration must be coordinated. Here, we showed that the proto-oncogene BCL6, which encodes a transcriptional repressor required for lymphocyte terminal differentiation, contributes to the survival of olfactory sensory neurons (OSNs). In the olfactory epithelia of the BCL6 null mutant mice, many OSNs were positive for both OMP and GAP43. The epithelium was relatively thinner, showing many apoptotic signals. These characters were phenotypically similar to those of the wild-type mice treated with nasal lectin irrigation, which acutely induces apoptosis of OSNs. Odorant receptors were expressed normally in the epithelia of the mutant mice, and their overall expression profile based on DNA microarray analyses was roughly similar to that of the apoptosis-induced olfactory epithelia of the wild-type mice. Experimental increase of BCL6 together with green fluorescent protein in OSNs using adenovirus-mediated gene transfer made the epifluorescence last longer than the control fluorescence without exogenous BCL6 after the nasal lectin irrigation, indicating that BCL6 made the infected neurons survive longer. We conclude that BCL6 plays an active role in the survival of OSNs as an anti-apoptotic factor and confers immature OSNs enough time to fully differentiate into mature ones.

Signaling pathways in T follicular helper cells

Th cell functional subsets have unique transcriptional programs that form the molecular basis for T cell differentiation and functions. T follicular helper (TFH) cells have emerged as the main providers of T cell help to B cells during the germinal center (GC) reaction, where B cells undergo selection events through competition for Ag and for access to GC T cell-mediated prosurvival and differentiation signals. Because T cell help is one limiting factor for GC B cells, the molecular mechanisms controlling TFH cell abundance and functionality are central to the GC reaction and generation of long-term humoral immunity. Two signaling pathways are absolutely critical for TFH cells: phosphoinositide-3 kinase pathway and the signaling lymphocyte activation molecule-associated protein. In this review, the molecular mechanisms constituting the signaling network in TFH cells will be explored.

BCL6 promoter interacts with far upstream sequences with greatly enhanced activating histone modifications in germinal center B cells

BCL6 encodes a transcriptional repressor that is essential for the germinal center (GC) reaction and important in lymphomagenesis. Although its promoter has been well studied, little is known concerning its possible regulation by more distal elements. To gain such information, we mapped critical histone modifications associated with active transcription within BCL6 as well as far upstream sequences at nucleosomal resolution in B-cell lines and in normal naive and GC B cells. Promoter-associated and intronic CpG islands (CGIs) in BCL6 showed a reciprocal pattern of histone modifications. Gene expression correlated with a paradoxical loss from the intronic CGI of histone H3 lysine-4 trimethylation, normally associated with transcription, suggesting that the intronic CGI may interfere with transcription. In an approximately 110-kb region extending 150-260 kb upstream of BCL6, highly active histone modifications were present only in normal GC B cells and a GC B-cell line; this region overlaps with an alternative breakpoint region for chromosomal translocations and contains a GC-specific noncoding RNA gene. By chromosome conformation capture, we determined that the BCL6 promoter interacts with this distant upstream region. It is likely that transcriptional enhancers in this region activate BCL6 and overcome strong autorepression in GC B cells.