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

Pax5--a critical inhibitor of plasma cell fate

Paired box protein 5 (Pax5) is essential for early B cell commitment as well as for B cell development, and continuous expression of Pax5 is required throughout the B cell lineage to maintain the functional identity of B cells. During B cell activation, Pax5 is downregulated before terminal differentiation into antibody-secreting plasma cells, and enforced expression of Pax5 prevents plasmacytic development. Recently, loss of Pax5 was shown to result in the substantial transition to a plasma cell state, demonstrating a functionally significant role for Pax5 in the regulation of terminal B cell differentiation. Here we elucidate the current understanding about the function of Pax5 as a key inhibitor of plasma cell differentiation.

Somatic stem cells and the origin of cancer

Most human cancers derive from a single cell targeted by genetic and epigenetic alterations that initiate malignant transformation. Progressively, these early cancer cells give rise to different generations of daughter cells that accumulate additional mutations, acting in concert to drive the full neoplastic phenotype. As we have currently deciphered many of the gene pathways disrupted in cancer, our knowledge about the nature of the normal cells susceptible to transformation upon mutation has remained more elusive. Adult stem cells are those that show long-term replicative potential, together with the capacities of self-renewal and multi-lineage differentiation. These stem cell properties are tightly regulated in normal development, yet their alteration may be a critical issue for tumorigenesis. This concept has arisen from the striking degree of similarity noted between somatic stem cells and cancer cells, including the fundamental abilities to self-renew and differentiate. Given these shared attributes, it has been proposed that cancers are caused by transforming mutations occurring in tissue-specific stem cells. This hypothesis has been functionally supported by the observation that among all cancer cells within a particular tumor, only a minute cell fraction has the exclusive potential to regenerate the entire tumor cell population; these cells with stem-like properties have been termed cancer stem cells. Cancer stem cells can originate from mutation in normal somatic stem cells that deregulate their physiological programs. Alternatively, mutations may target more committed progenitor cells or even mature cells, which become reprogrammed to acquire stem-like functions. In any case, mutated genes should promote expansion of stem/progenitor cells, thus increasing their predisposition to cancer development by expanding self-renewal and pluripotency over their normal tendency towards relative quiescency and proper differentiation.

Prognostic Impact of Germinal Center–Associated Proteins and Chromosomal Breakpoints in Poor-Risk Diffuse Large B-Cell Lymphoma

Purpose

Outcome of diffuse large B-cell lymphoma (DLBCL) with a germinal center B-cell (GCB) expression profile is superior to that of non-GCB DLBCL. This conclusion is mainly derived from patients with mixed international prognostic index (IPI) risk profiles treated with CHOP-like therapy (cyclophosphamide, doxorubicin, vincristine, and prednisone). We wondered whether the prognostic impact of the expression profile would hold out in a homogeneous cohort of poor-risk DLBCL patients treated with high-dose sequential therapy (HDT) and autologous stem-cell transplantation (ASCT) as first-line therapy.

Patients and Methods

DLBCL from 66 newly diagnosed poor-risk patients, treated in two sequential prospective Dutch Hemato-Oncology Association (HOVON) trials, were studied retrospectively for expression of CD10, bcl6, MUM1/IRF4, bcl2, Ki67, and CD21+follicular dendritic cells (FDC) by immunohistochemistry, and for the breakpoints of BCL2, BCL6, and MYC by fluorescent in situ hybridization (FISH). Lymphomas with any follicular component were excluded.

Results

A GCB immunophenotype profile was found in 58% and non-GCB immunophenotype profile in 42% of the tumors. Clinical characteristics of both groups were similar. Complete response (CR) rate was higher in patients with CD10+tumors (58% v 30%; P = .03). A GCB immunophenotype profile, its constituting markers CD10 more than 30% and MUM1 less than 70%, and bcl2 less than 10% were each associated with a better overall survival (OS). FDC networks, equally present in GCB and non-GCB tumors, had superior CR (73% v 31%; P = .01), but disease-free survival rates were lower and there was no difference in OS rates. None of the breakpoints had a prognostic impact on outcome.

Conclusion

Also in patients with poor-risk DLBCL treated with HDT and ASCT, the GCB immunophenotype and bcl2 expression retained a major impact on survival.

Chromosomal translocations fusing the BCL6 gene to different partner loci are recurrent in primary central nervous system lymphoma and may be associated with aberrant somatic hypermutation or defective class switch recombination

Primary central nervous system lymphomas (PCNSLs) are diffuse large B cell lymphomas confined to the brain. Only minimal data exist on chromosomal aberrations underlying PCNSLs. We studied 41 PCNSLs by fluorescence in situ hybridization for breakpoints affecting the BCL6 locus in chromosomal band 3q27. Of 37 cases evaluable, 14 (38%) carried a breakpoint in the BCL6 locus. Two of these showed juxtaposition of BCL6 to the IGH locus. In 4 cases, the BCL6 breakpoints were cloned using long-distance inverse polymerase chain reaction. All breakpoints were located within the BCL6 major translocation cluster. The translocation partners were the IGH gene in 14q32.33, the IGL gene in 22q11.22, and the histone 1 H4I gene in 6p22.1. In the fourth case, a deletion in 3q leads to loss of an 837-kb fragment extending from the first intron of BCL6 to the third intron of the lipoma-preferred partner (LPP) gene. This deletion may bring the BCL6 gene under the control of regulatory elements of the LPP gene or the miRNA-28 gene located in intron 4 of LPP. DNA sequence analysis of the junctional sequences provided evidence that aberrant class switch recombination or somatic hypermutation may be involved in the generation of BCL6 translocations.

The BCL6 proto-oncogene: a leading role during germinal center development and lymphomagenesis

The BCL6 proto-oncogene encodes a nuclear transcriptional repressor, with pivotal roles in germinal center (GC) formation and regulation of lymphocyte function, differentiation, and survival. BCL6 suppresses p53 in GCB-cells and its constitutive expression can protect B-cell lines from apoptosis induced by DNA damage. BCL6-mediated expression may allow GCB-cells to sustain the low levels of physiological DNA breaks related to somatic mutation (SM) and immunoglobulin class switch recombination which physiologically occur in GCB-cells. Three types of genetic events occur in the BCL6 locus and involve invariably the 5' non-coding region and include translocations, deletions and SM actively targeted to the 5' untranslated region. These acquired mutations occur independently of translocations but may be involved in the deregulation of the gene and/or translocation mechanisms. The favorable prognostic value of high levels of BCL6 gene expression in NHL seems well-established. By contrast, the relevance of SM or translocation of the gene remains unclear. However, it is likely that non-Hodgkin's lymphomas (NHL) harboring the most frequent translocation involving BCL6, i.e. t(3;14), are characterized by a common cell of origin and similar oncogenic mechanisms. Several experiments and mouse models mimicking BCL6 translocation occurring in human lymphoma have demonstrated the oncogenic role of BCL6 and constitute a rational to consider BCL6 as a new therapeutic target in NHL. BCL6 blockade can be achieved by different strategies which include siRNA, interference by specific peptides or regulation of BCL6 acetylation by pharmacological agents such as SAHA or niacinamide and would be applicable to most type of B-cell NHL.

Novel treatment strategies for aggressive non-Hodgkin's lymphoma

The World Health Organization has included different types of lymphoma under the aggressive category. In the US, diffuse large B-cell lymphoma is the most common aggressive lymphoma and accounts for > 30% of the 55,000 new cases diagnosed annually. Recent advances in the knowledge of the molecular biology have provided an increased understanding of the heterogeneity of non-Hodgkin's lymphoma. New treatments, especially those with the use of monoclonal antibodies, are improving both the survival and the response rate.

The rationale for the IL‐2‐independent generation of the self‐renewing central memory CD8 + T cells

Clones of CD8+ T cells that have been selected in the primary response must have a mechanism by which they can continuously or intermittently generate new effector cells. Several years ago, this mechanism was proposed to involve a self-renewing, stem cell-like subset that could avoid the differentiating effects of interleukin-2 (IL-2). The model considered the stem cell subset to be contained within the central memory population of CD8+ T cells (T(CM)). This proposal was inconsistent with subsequent findings suggesting that all antigen-activated CD8+ T cells differentiated to effector cells (T(EFF)) during the primary response and that T(CM) developed during the memory phase by de-differentiating from effector memory cells (T(EM)). However, findings have since been reported that support the stem cell model. First, studies indicate that T(EM) do not serve as the precursors of T(CM). Second, transcriptional repressors of IL-2 signaling do enhance the memory response. Third, memory cells lacking effector functions and with a capacity to replicate in a secondary response develop in the absence of signaling through the IL-2/IL-15 receptor. Taken together, these findings suggest that antigen-activated CD8+ T cells with a stem cell-like capability for maintaining proliferative potential develop by an unknown IL-2-independent process. The challenge is now to identify this unknown pathway of clonal expansion.

High BCL6 expression predicts better prognosis, independent of BCL6 translocation status, translocation partner, or BCL6-deregulating mutations, in gastric lymphoma

To investigate the role of BCL6 in the pathogenesis of gastric lymphoma, we analyzed the BCL6 promoter region for BCL6 translocations, somatic hypermutations, and deregulating mutations in 43 gastric lymphomas, including 4 extranodal marginal-zone B-cell lymphomas of mucosa-associated lymphoid tissues (MALT lymphomas), 33 diffuse large B-cell lymphomas (DLBCLs), and 6 composite DLBCLs with residual MALT lymphoma (DLCLMLs). BCL6 promoter substitutions by immunoglobulin (Ig) and non-Ig translocation partners, resulting in its deregulation, were frequently involved in DLBCL (36.4%) and DLCLML (50%). Two novel BCL6 translocation partner genes, 28S rRNA and DMRT1, and a new BCL6 translocation breakpoint in intron 2 were also identified. Deregulating mutations were found only in DLBCL (24.2%), which correlated significantly with high BCL6 protein expression. Significantly, high BCL6 expression correlated strongly with longer overall survival (OS), independent of mechanism in gastric DLBCL and DLCLML. Gastric DLBCLs were further subclassified into germinal center B-cell-like (GCB) and non-GCB subgroups immunohistochemically. High BCL6 expression was detected in all GCB cases, irrespective of BCL6 genetic alterations. In the non-GCB subgroup, BCL6-deregulating mutations correlated significantly with high BCL6 expression level. No significant correlation was found between the BCL6 expression level and OS in the non-GCB subgroup, which had significantly poorer prognosis than the GCB subgroup.

Transcription factor IRF4 controls plasma cell differentiation and class-switch recombination

B cells producing high-affinity antibodies are destined to differentiate into memory B cells and plasma cells, but the mechanisms leading to those differentiation pathways are mostly unknown. Here we report that the transcription factor IRF4 is required for the generation of plasma cells. Transgenic mice with conditional deletion of Irf4 in germinal center B cells lacked post-germinal center plasma cells and were unable to differentiate memory B cells into plasma cells. Plasma cell differentiation required IRF4 as well as the transcriptional repressor Blimp-1, which both acted 'upstream' of the transcription factor XBP-1. In addition, IRF4-deficient B cells had impaired expression of activation-induced deaminase and lacked class-switch recombination, suggesting an independent function for IRF4 in this process. These results identify IRF4 as a crucial transcriptional 'switch' in the generation of functionally competent plasma cells.

JunD/AP-1 and STAT3 are the major enhancer molecules for high Bcl6 expression in germinal center B cells

The Bcl6 proto-oncogene, which encodes a transcriptional repressor, is ubiquitously expressed and predominantly in germinal center (GC) B cells. Although the promoter region of the human Bcl6 gene has been reported, enhancer molecules for its high expression in GC B cells were largely unknown. Here we show that transcriptional start sites of the murine Bcl6 gene were different from the reported human one. DNA sequence around the new promoter region is highly conserved between mice and humans and has no canonical TATA or CCAAT box. Two AP-1-binding elements in the promoter region were the major enhancer elements in GC-derived B lymphoma cells, and JunD/AP-1 was detected in GC B cells. In addition, we identified the silencer region with three Bcl6-binding elements around the start site. Bcl6 bound to the silencer elements and its over-expression repressed the promoter activity through the elements. Activated STAT factors (STATs), especially activated STAT3, also bound to the silencer elements in GC B cells and competed with Bcl6 for the binding, suggesting that JunD/AP-1 and activated STATs drive high Bcl6 expression in GC B cells. Since stimulation of splenic B cells with IL-4 or IL-21 induced high Bcl6 expression with induction of junD and activation of STATs, these cytokines may be inducers for its high expression in GC B cells. However, IL-21 but not IL-4 stimulation activated STAT3 in splenic B cells. Thus, IL-21 may be a major inducer for high Bcl6 expression in GC B cells.

Peripheral T-cell lymphoma with involvement of the expanded mantle zone

Peripheral T-cell lymphoma (PTCL) with a nodular architecture is rare. Recently, two variants have been described with infiltration of the B-cell follicle, one variant that localizes to the marginal zone with a so-called perifollicular growth pattern, and a variant that localizes to the germinal center. These lymphomas have a CD4+ phenotype and may express Bcl-6. We have studied five similar cases of PTCL with involvement of the B-cell follicle. However, our cases differ from the cases previously described by their predominant and frequently patchy involvement of the expanded mantle zone of the B-cell follicle at onset. Later biopsies in three of the cases show diffuse infiltration of the lymph node, without features of angioimmunoblastic TCL (AILT). All cases expressed Bcl-6 in addition to CD4. Cytogenetics was available in four of the cases but revealed no recurrent chromosomal aberrations or changes associated with other types of PTCL. No mutations of the BCL-6 gene were observed. Together, the cases seem to have an intermediately aggressive clinical behavior. Whether our cases are part of a spectrum of PTCLs that encompasses previously described variants with predominant marginal zone or germinal center infiltration or they represent a separate T-cell lymphoma type remains to be demonstrated by a study of more of such cases.

A peptide aptamer to antagonize BCL-6 function

BCL-6 is a transcription factor essential for germinal centre B-cell development. The BCL-6 gene is involved in diffuse large-cell lymphoma and overexpressed in other types of non-Hodgkin's lymphoma and in high-grade breast cancer. BCL-6 is a transcriptional repressor whose N-terminal POZ domain mediates protein-protein interactions to exert its effects. Reasoning that disruption of POZ domain-mediated interactions may be an effective route to antagonizing the effects of BCL-6 in lymphoma, we screened a library for peptide aptamers that specifically bind to BCL-6 POZ and not the POZ domains of related proteins and describe here the first of these reagents, Apt48. Apt48 binds BCL-6 POZ in a manner distinct from the transcriptional corepressor SMRT, yet was found to prevent BCL-6-mediated repression of a luciferase reporter gene. Apt48 also reproduced several previously validated effects of BCL-6 inhibition. Notably, expression of the differentiation markers CD69, Blimp-1 and cyclin D2 was increased in B-cell lines when Apt48 was expressed. We also show that expression of Apt48 restores cytokine-mediated growth arrest to BCL-6 overexpressing cells. Thus, we have identified a peptide aptamer that affects a function of BCL-6 that is required to prevent differentiation of proliferating B cells.

Dominant negative retinoic acid receptor initiates tumor formation in mice

Background

Retinoic acid suppresses cell growth and promotes cell differentiation, and pharmacological retinoic acid receptor (RAR) activation is anti-tumorigenic. This begs the question of whether chronic physiological RAR activation by endogenous retinoids is likewise anti-tumorigenic.

Results

To address this question, we generated transgenic mice in which expression of a ligand binding defective dominant negative RARα (RARαG303E) was under the control of the mouse mammary tumor virus (MMTV) promoter. The transgene was expressed in the lymphoid compartment and in the mammary epithelium. Observation of aging mice revealed that transgenic mice, unlike their wild type littermates, developed B cell lymphomas at high penetrance, with a median latency of 40 weeks. MMTV-RARαG303E lymphomas were high grade Pax-5+, surface H+L Ig negative, CD69+ and BCL6- and cytologically and phenotypically resembled human adult high grade (Burkitt's or lymphoblastic) lymphomas. We postulated that mammary tumors might arise after a long latency period as seen in other transgenic models of breast cancer. We tested this idea by transplanting transgenic epithelium into the cleared fat pads of wild type hosts, thus bypassing lymphomagenesis. At 17 months post-transplantation, a metastatic mammary adenocarcinoma developed in one of four transplanted glands whereas no tumors developed in sixteen of sixteen endogenous glands with wild type epithelium.

Conclusion

These findings suggest that physiological RAR activity may normally suppress B lymphocyte and mammary epithelial cell growth and that global RAR inactivation is sufficient to initiate a stochastic process of tumor development requiring multiple transforming events. Our work makes available to the research community a new animal resource that should prove useful as an experimental model of aggressive sporadic lymphoma in immunologically uncompromised hosts. We anticipate that it may also prove useful as a model of breast cancer.

IL-21 induces differentiation of human naive and memory B cells into antibody-secreting plasma cells

IL-21 is a type I cytokine that influences the function of T cells, NK cells, and B cells. In this study, we report that IL-21 plays a major role in stimulating the differentiation of human B cells. When human B cells were stimulated through the BCR, IL-21 induced minimal proliferation, IgD down-modulation, and small numbers of plasma cells. In contrast, after CD40 engagement, IL-21 induced extensive proliferation, class switch recombination (CSR), and plasma cell differentiation. Upon cross-linking both BCR and CD40, IL-21 induced the largest numbers of plasma cells. IL-21 drove both postswitch memory cells as well as poorly responsive naive cord blood B cells to differentiate into plasma cells. The effect of IL-21 was more potent than the combination of IL-2 and IL-10, especially when responsiveness of cord blood B cells was examined. IL-21 costimulation potently induced the expression of both B lymphocyte-induced maturation protein-1 (BLIMP-1) and activation-induced cytidine deaminase as well as the production of large amounts of IgG from B cells. Despite the induction of activation-induced cytidine deaminase and CSR, IL-21 did not induce somatic hypermutation. Finally, IL-2 enhanced the effects of IL-21, whereas IL-4 inhibited IL-21-induced plasma cell differentiation. Taken together, our data show that IL-21 plays a central role in CSR and plasma cell differentiation during T cell-dependent B cell responses.

Downstream of Tyrosine Kinases-1 and Src Homology 2-Containing Inositol 5′-Phosphatase Are Required for Regulation of CD4+CD25+ T Cell Development

The adaptor protein, downstream of tyrosine kinases-1 (Dok-1), and the phosphatase SHIP are both tyrosine phosphorylated in response to T cell stimulation. However, a function for these molecules in T cell development has not been defined. To clarify the role of Dok-1 and SHIP in T cell development in vivo, we compared the T cell phenotype of wild-type, Dok-1 knockout (KO), SHIP KO, and Dok-1/SHIP double-knockout (DKO) mice. Dok-1/SHIP DKO mice were runted and had a shorter life span compared with either Dok-1 KO or SHIP KO mice. Thymocyte numbers from Dok-1/SHIP DKO mice were reduced by 90%. Surface expression of both CD25 and CD69 was elevated on freshly isolated splenic CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO, suggesting these cells were constitutively activated. However, these T cells did not proliferate or produce IL-2 after stimulation. Interestingly, the CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO mice produced higher levels of TGF-beta, expressed Foxp3, and inhibited IL-2 production by CD3-stimulated CD4(+)CD25(-) T cells in vitro. These findings suggest Dok-1 and SHIP function in pathways that influence regulatory T cell development.

M17, a gene specific for germinal center (GC) B cells and a prognostic marker for GC B-cell lymphomas, is dispensable for the GC reaction in mice

In T-cell-dependent antibody responses, antigen-specific B cells undergo a phase of secondary antibody diversification in germinal centers (GCs). Somatic hypermutation (SHM) introduces mutations into the rearranged immunoglobulin (Ig) variable (V) region genes, and class-switch recombination (CSR) alters the Ig heavy (H) chain constant region. Aberrant SHM or CSR is thought to contribute to the development of GC-derived B-cell malignancies. Diffuse large B-cell lymphomas (DLBCLs) are a heterogeneous group of such GC-derived tumors. Based on their gene expression profile, DLBCLs can be divided into activated B-cell-like and GC-like subgroups. The human gene HGAL is predominantly expressed in GCs. It is also part of the gene expression signature of GC-like DLBCL, and its high expression in DLBCL has been associated with a better clinical prognosis. We have generated mice deficient of the HGAL homologue M17 in order to investigate its functional significance. The mutant animals form normal GCs, undergo efficient CSR and SHM, and mount T-cell-dependent antibody responses similar to wild-type controls. Thus, M17 is dispensable for the GC reaction, and its potential function in the pathogenesis of DLBCL remains elusive.

Prognostic Biomarkers in Diffuse Large B-Cell Lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin's lymphoma. Although it represents a curable disease, less than half of the patients are cured with conventional chemotherapy. The highly variable outcome reflects a heterogeneous group of tumors, with different genetic abnormalities and response to therapy. The International Prognostic Index (IPI) is useful in predicting the outcome of DLBCL patients. However, patients with identical IPI still exhibit marked variability in survival, suggesting the presence of significant residual heterogeneity within each IPI category. The discovery of specific genetic alterations and the assessment of protein expression led to the identification of multiple novel single molecular markers capable of predicting the outcome of DLBCL patients independently of clinical variables. The recent application of DNA microarrays and tissue array technologies allowed a better understanding of the biology of lymphoma and the development of novel diagnostic tools capable of improving the current models for outcome prediction. However, much confusion exists in the literature regarding the importance of different prognostic biomarkers and their applicability in routine practice. This review summarizes the recent advances in our understanding of prognostic biomarkers in DLBCL and discusses whether this is the right time for biomarkers-guided risk-adjusted therapy.

Molecular prognostic factors in diffuse large B-cell lymphoma

The treatment of patients with diffuse large B-cell lymphoma (DLBCL) has been guided traditionally by clinical parameters such as the Ann Arbor Staging Classification for Hodgkin's disease. Although the International Prognostic Index (IPI) represents the most widely accepted prognostic model, there is still a marked variability in outcome within identical IPI subgroups, reflecting the heterogeneity of this malignancy. Use of DNA microarray, real-time reverse transcription polymerase chain reaction, and tissue array immunohistochemistry methodologies makes the development of new classifications possible based on molecular profiling. The molecular classification of DLBCL may lead to the grouping of specific disease entities sharing similar biologic features, clinical behavior, and outcome. Once tested and validated, this new generation of prognostic models should become an integral part of the daily practice, providing valuable additional information to the currently existing clinically based predictive models. To accomplish these goals and to be in a position in which existing or new prognostic models can be easily tested and validated, there is a strong need to collect frozen and paraffin-embedded material that can be used for RNA extraction and construction of tissue arrays, respectively. Such materials should be gathered as an integral part of any planned study.

FDC-Specific Functions of p55TNFR and IKK2 in the Development of FDC Networks and of Antibody Responses

The FDC-specific molecular signals required in the formation of FDC networks, B cell follicles, and germinal centers (GCs) have remained poorly understood. We used FDC-specific gene targeting to investigate the function of p55TNFR and IKK2 in lymphoid organ structure and function. Here we show that FDC-specific expression of p55TNFR is necessary and sufficient to promote FDC network and B cell follicle formation, restore the expression of CXCL13 and VCAM-1/ICAM-1 in FDCs, and lead to productive GCs. Notably, FDC-specific disruption of IKK2 does not affect formation of FDC networks. Yet, after antigen engagement or immune complex (IC) deposition, FDCs lacking IKK2 fail to upregulate VCAM-1 and ICAM-1, and GCs remain sterile. These findings demonstrate that IKK2-independent function of p55TNFR on FDCs is sufficient to support the development of FDC networks and GCs, while FDC-specific IKK2 is indispensable for the generation of efficient humoral immune responses.

Pathogenetic and Clinical Implications of Non-Immunoglobulin; BCL6 Translocations in B-Cell Non-Hodgkin's Lymphoma

Chromosomal translocations affecting band 3q27, where BCL6 gene is located, are among the most common genetic abnormalities in non-Hodgkin's lymphoma of B-cell type (B-NHL). The BCL6 gene encodes a BTB/POZ zinc finger transcription factor, which exerts repressive activity by recruiting corepressor molecules. The 3q27/BCL6 translocation is unique in that it can involve not only immunoglobulin (Ig) genes but also non-Ig chromosomal loci as a partner. To date, around 20 non-Ig partner genes have been identified. As a result of non-Ig ; BCL6 translocations, many types of regulatory sequences of each partner gene substitute for the 5' untranslated region of BCL6, and the rearranged BCL6 comes under the control of the replaced promoter. The introduction of non-Ig ; BCL6 constructs into transformed cells led to high-level Bcl-6 protein expression in the nucleus, while BCL6 mRNA levels in clinical materials of diffuse large B-cell lymphoma (DLBCL) with non-Ig ; BCL6 translocations were unexpectedly low. A comparative study suggested that non-Ig ; BCL6 translocation and a low level of BCL6 mRNA expression are concordant indicators of a poor clinical outcome in cases of DLBCL. The coexistence of a non-Ig ; BCL6 translocation with t(14 ; 18)(q32 ; q21) in a single clone did not significantly affect the clinical features of follicular lymphoma. The pathogenetic and clinical implications of non-Ig ; BCL6 translocations in B-NHL subtypes may not be identical to those of Ig ; BCL6.

Identification of genes involved in the initiation of human Th1 or Th2 cell commitment

The differentiation of naïve T helper (Th) cells is induced by TCR activation and IL-12/STAT4 or IL-4/STAT6 signaling pathways, forming Th1 and Th2 cells, respectively. In this study, oligonucleotide arrays were used to identify genes regulated during the initiation of human Th1 and Th2 cell differentiation at 2 and 6 h in presence or absence of immunosuppressive TGF-beta. As a result the immediate targets of IL-12, IL-4 and TGF-beta were identified. The effects of IL-12 at this early stage were minimal and consistent with the known kinetics of IL-12Rbeta2 expression. IL-4, however, was observed to rapidly regulate 63 genes, 26 of which were differentially expressed at both the 2- and 6-h time points. Of these IL-4 regulated genes, one-third have previously been observed to display expression changes in the later phases of the polarization process. Similarly to the key regulators, TBX21 and GATA3, the transcription factors SATB1, TCF7 and BCL6 were differentially regulated at the protein level during early Th1 and Th2 cell polarization. Moreover, the developing Th1 and Th2 cells were demonstrated to be responsive to the immunosuppressive TGF-beta and IL-10. In this study, a panel of novel factors that may be important regulators of the differentiation process was identified.

The transcription factor Pokemon: a new key player in cancer pathogenesis

Learning how critical cell regulatory pathways are controlled may lead to new opportunities for cancer treatment. We recently identified the transcription factor Pokemon as a central regulator of the important tumor suppressor ARF. Pokemon is overexpressed in multiple human cancers and cells lacking Pokemon are refractory to oncogenic transformation. These findings suggest that Pokemon may offer an effective new target for cancer therapeutics.

Defined blocks in terminal plasma cell differentiation of common variable immunodeficiency patients

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by defective Ab production and recurrent bacterial infections. The largely unknown causes are likely to comprise a diverse set of genetic or acquired defects. In this study, we investigated terminal B cell differentiation in lymph nodes from CVID patients. Up to the germinal center B cell stage, B cell differentiation was normal but terminal plasma cell development was found to be impaired. Using differential Blimp-1 and Syndecan-1 expression in controls, we defined three different plasma cell subsets that correspond to progressive developmental stages locating to different sites in the lymph node. In the CVID patients, we could only detect one or two of these subsets indicating a defective differentiation. Thus, terminal plasma cell differentiation was found to be impaired despite normal expression of Blimp-1. B cells reaching only the first stage of plasma cell differentiation were further unable to undergo isotype switching and to up-regulate activation markers on B cells stimulated in vitro.

Peripheral B cells latently infected with Epstein-Barr virus display molecular hallmarks of classical antigen-selected memory B cells

Epstein-Barr virus (EBV) establishes a lifelong persistent infection within peripheral blood B cells with the surface phenotype of memory cells. To date there is no proof that these cells have the genotype of true germinal-center-derived memory B cells. It is critical to understand the relative contribution of viral mimicry versus antigen signaling to the production of these cells because EBV encodes proteins that can affect the surface phenotype of infected cells and provide both T cell help and B cell receptor signals in the absence of cognate antigen. To address these questions we have developed a technique to identify single EBV-infected cells in the peripheral blood and examine their expressed Ig genes. The genes were all isotype-switched and somatically mutated. Furthermore, the mutations do not cause stop codons and display the pattern expected for antigen-selected memory cells based on their frequency, type, and location within the Ig gene. We conclude that latently infected peripheral blood B cells display the molecular hallmarks of classical antigen-selected memory B cells. Therefore, EBV does not disrupt the normal processing of latently infected cells into memory, and deviations from normal B cell biology are not tolerated in the infected cells. This article provides definitive evidence that EBV in the peripheral blood persists in true memory B cells.

Follicular B helper T cells in antibody responses and autoimmunity

T-cell help for B cells is essential for high-affinity antibody responses and B-cell memory. Recently, the identity of a discrete follicular population of T cells that has a crucial role in this process has become clearer. Similar to primed CD4(+) T cells in the tonsils and memory CD4(+) T cells in the peripheral blood, this follicular population of T cells expresses CXC-chemokine receptor 5 (CXCR5). Owing to their distinct homing preferences and helper function, these T cells differ from T helper 1 and T helper 2 cells and have been denoted follicular B helper T cells. Here, we outline the central role of this subset in normal and pathological immune responses.

Analysis of BCL6-interacting Proteins by Tandem Mass Spectrometry

B-cell lymphoma 6 (BCL6) is a 95-kDa nuclear phosphoprotein and member of the Pox virus zinc finger/bric-a-brac, tramtrack, broad complex (POZ/BTB) family of transcription factors. BCL6 is a transcriptional repressor required for germinal center formation, and the gene encoding it is frequently altered in diffuse large B-cell and follicular lymphomas. The dysregulation of BCL6 has therefore been implicated in lymphomagenesis. A limited number of proteins is known to interact with BCL6 and modulate its activity or participate in its role in transcriptional regulation. Identification of additional BCL6-binding proteins could reveal potential signaling targets and previously undescribed functional roles for BCL6. We used a functional proteomic approach to determine the identity of proteins that interact with BCL6. Proteins were isolated by co-immunoprecipitation with an anti-BCL6 antibody and identified using MS/MS. We identified 61 proteins in the BCL6 immunocomplex from the following Gene Ontology categories: transcription regulator activity (n = 18), binding activity (n = 11), signal transducer activity (n = 10), catalytic activity (n = 8), structural molecule activity (n = 3), enzyme regulator activity (n = 3), transporter activity (n = 2), motor activity (n = 2), chaperone activity (n = 1), and unknown function (n = 3). Importantly we identified BCL6 and several previously reported BCL6-interacting proteins in the BCL6 immunocomplex. The remaining proteins have not been shown previously to be associated with BCL6. MS/MS results were validated on four proteins using immunoprecipitation and Western blotting. Two of these protein interactions were further confirmed by reciprocal immunoprecipitation. This study demonstrates the utility of antibody immunoprecipitation and subsequent peptide identification by MS/MS for the elucidation of BCL6-binding proteins. Many of the novel proteins identified in this study suggest additional functional roles for BCL6 beyond transcriptional repression.

A Putative Exonic Splicing Polymorphism in the BCL6 Gene and the Risk of Non-Hodgkin Lymphoma

Recent studies have shown that the B-cell lymphoma 6 gene (BCL6) is an oncogene that contributes to lymphomagenesis. Exon 6 of BCL6 contains a common single nucleotide polymorphism (SNP) (-195 C>T; dbSNP ID: rs1056932) that alters a potential binding site for an exonic splicing enhancer. We used unconditional logistic regression models to examine the association between this SNP and the risk of non-Hodgkin lymphoma (NHL) in a population-based case-control study of women residing in Connecticut (461 case patients and 535 control subjects). The risk of NHL among women with the CC genotype was more than double that of women with the TT genotype (odds ratio [OR] = 2.2, 95% confidence interval [CI] = 1.5 to 3.3). Higher risks were observed for two NHL subtypes, namely B-cell chronic lymphatic leukemia/prolymphocytic leukemia/small lymphocytic lymphoma (OR = 3.5, 95% CI = 1.6 to 7.8) and T-cell lymphoma (OR = 5.2, 95% CI = 2.0 to 13.3). Our results support the hypothesis that a genetic variant that could alter mRNA transcripts of BCL6 may contribute to the etiology of NHL and suggest that this variant warrants further investigation.

The proto-oncogene BCL-6 is expressed in olfactory sensory neurons

Mammalian olfactory sensory neurons harbor a large repertoire of odorant receptors. Yet, each cell chooses a single odorant receptor gene to express, which may then serve as the molecular identification of that cell. This process of differentiation or receptor gene regulation of olfactory sensory neurons has largely been enigmatic. Here, we showed that the important proto-oncogene in B lymphocyte terminal differentiation, B-Cell Leukemia/Lymphoma 6 (BCL-6), which codes for a sequence-specific transcription repressor, is expressed in olfactory sensory neurons in the mouse. We detected BCL-6 mRNA in most, if not all, mature olfactory sensory neurons by in situ hybridization histochemistry. Western blot analysis also revealed the presence of BCL-6 protein in the olfactory epithelium, but immunohistochemical analysis using anti-BCL-6 antibodies showed that only a subset of olfactory sensory neurons highly expressed BCL-6 protein. BCL-6 mRNA was detected as early as embryonic day 14 (E14) in most olfactory sensory neurons as in adults, and at E15, BCL-6 protein was detected in most cells that were likely to be differentiating into mature olfactory sensory neurons. Artificial induction of apoptosis of olfactory sensory neurons resulted in the decrease of BCL-6 mRNA, but during the subsequent phase of regeneration and differentiation, it markedly increased. Taken together, our results suggest the possibility that BCL-6 plays an important role in terminal differentiation not only in B lymphocytes but also in olfactory sensory neurons.

Allergic dysregulation and hyperimmunoglobulinemia E in Foxp3 mutant mice

BACKGROUND

Regulatory T cells have been proposed to play an important role in regulating allergic inflammation. The transcription factor Foxp3 is a master switch gene that controls the development and function of natural and adaptive CD4(+)CD25(+) regulatory T (T(R)) cells. In human subjects loss-of-function Foxp3 mutations trigger lymphoproliferation, autoimmunity, and intense allergic inflammation in a disease termed immune dysregulation polyendocrinopathy enteropathy-X-linked syndrome.

OBJECTIVE

We sought to examine the evolution and attributes of allergic inflammation in mice with a targeted loss-of-function mutation in the murine Foxp3 gene that recapitulates a known disease-causing human Foxp3 mutation.

METHODS

Foxp3 mutant mice were generated by means of knock-in mutagenesis and were analyzed for histologic, immunologic, and hematologic abnormalities. The role of signal transducer and activator of transcription 6 (Stat6) in disease pathogenesis was analyzed by using Stat6 and Foxp3 double-mutant mice.

RESULTS

Foxp3 mutant mice developed an intense multiorgan inflammatory response associated with allergic airway inflammation, a striking hyperimmunoglobulinemia E, eosinophilia, and dysregulated T(H)1 and T(H)2 cytokine production in the absence of overt T(H)2 skewing. Concurrent Stat6 deficiency reversed the hyperimmunoglobulinemia E and eosinophilia and delayed mortality, which is consistent with a pathogenic role for allergic inflammation in Foxp3 deficiency.

CONCLUSION

Allergic dysregulation is a common and fundamental consequence of loss of CD4(+)CD25(+) T(R) cells caused by Foxp3 deficiency in different species. Abnormalities affecting T(R) cells might contribute to a variety of allergic diseases.

The NF-κB signalling pathway: a therapeutic target in lymphoid malignancies?

Nuclear factor-kappaB/reticuloendotheliosis (NF-kappaB/Rel) designates a family of transcription factors that influence the activation of a multitude of genes critically involved in immune and inflammatory responses. Recently, genetic and biochemical evidence has accumulated, suggesting that constitutive activation of NF-kappaB/Rel proteins plays an important role in the development/progression of B and T cell lymphoid malignancies. In particular, genetic and molecular alterations of NF-kappaB family members and their transcriptional target genes have been implicated in the development of diffuse large B cell lymphoma and mucosa-associated lymphoid tissue lymphoma. Although NF-kappaB/Rel proteins represent an integrating point of several pathways, potentially contributing to several diseases, their unique activation depends on cell type and stimulus. Considering the NF-kappaB specificity in lymphoid cells, molecules that finely modulate the activity of these NF-kappaB components and dampen the inappropriate proliferation of lymphocytes may represent a novel pharmacological intervention to several lymphoid malignancies.

Targeted somatic mutation of the BCL6 proto-oncogene and its impact on lymphomagenesis

Cloning translocation breakpoints which cluster suspiciously to specific chromosomal loci has proved fruitful, leading to the identification of genes implicated in the onset of hematological malignancy. One of the most notable is BCL6, located on chromosome 3q27. The BCL6 is now known to encode a nuclear transcriptional repressor, with pivotal roles in germinal center (GC) formation and regulation of lymphocyte function, differentiation and survival. Unusually, the BCL6 gene locus is also actively targeted by the somatic mutation (SM) mechanism, at a rate indicative of specific, regulated events in both normal and malignant B-cells. These mutations occur in approximately 30% of normal centrocytes and centroblasts, but not in naive or pre-GC B-cells. They are also observed in approximately 70% of diffuse large B-cells lymphomas, approximately 30% of follicular lymphomas (FL) and at various frequencies in many lymphoma subtypes. Mutations are generated in the 5' proximity of the BCL6 promoter, including the first intron and are mainly single nucleotide substitutions, but with insertions and deletions also observed. Mutations in BCL6 occur independently of translocations, although mutational levels can be dramatically influenced by aberrantly translocated chromosomal elements, which map in the vicinity of the gene. Indeed, SMs are directly implicated in the generation of chromosomal translocations, as suggested by the overlap of the breakpoint cluster region and the mutational cluster domain. The prognostic value of the overall level of BCL6 mutations in specific lymphoma populations is, in the main, not as yet fully resolved. The accumulation of mutations in BCL6 during high grade transformation of FL, a mutational clustering and specific recurrent mutations suggest that some mutations may be selected for by their effect on the survival of the tumoral clone. In fact, it is now clear that SM can target and disrupt regulatory motifs in BCL6 to result in upregulated gene expression. Exogenous factors can also perturbate SM in BCL6. Viral infection elevates BCL6 mutational activity, suggesting a potential link with onset of virus-associated lymphoma. These findings to date reveal several mechanisms which can influence specific mutations targeting BCL6, and which may contribute to lymphomagenesis by dysregulating control of BCL6 expression.

Molecular Pathogenesis of Diffuse Large B-Cell Lymphoma

Diffuse large B-cell lymphoma (DLBCL) is a heterogeneous clinicopathologic entity accounting for 30% of non-Hodgkin's lymphomas. The pathogenesis of DLBCL is complex and heterogeneous. Recent studies using analysis of global gene expression with DNA microarrays and the classical molecular approaches demonstrate presence of several DLBCL subtypes characterized by different cells of origin, cytogenetic and molecular aberrations, and distinct pathogenesis. This review summarizes the progress in understanding of DLBCL biology and presents a state-of-the-art overview of DLBCL molecular pathogenesis.

Targeting Histones and Proteasomes: New Strategies for the Treatment of Lymphoma

Our ever-increasing understanding of cancer cell biology has begun to provide a variety of new, and potentially drugable targets for the treatment of many forms of cancer. Nowhere else is this more apparent than in the treatment of the lymphomas. A rapidly emerging experience in gene expression profiling has begun to suggest that we can define different subtypes of lymphoma on the basis of unique molecular signatures. These signatures can define important signaling pathways that may help account for the biology of different subsets of lymphoma, and are teaching us that the lymphomas are truly a heterogeneous set of diseases. What remains equally as interesting is the idea that empiric observations of novel targeted drugs in select subtypes of lymphoma can teach us much about the biology of different lymphomas. A priori assumptions about the anticipated activity of novel targeted agents in select subtypes of lymphoma have been turned upside down. Two pathways that have emerged recently as potentially important targets for new agents in lymphoma include the ubiquitin proteasome pathway and the biochemical reactions that control histone acetylation. New classes of drugs that affect these targets, such as bortezomib, depsipeptide, suberoylanilide hydroxamic acid, and a host of other compounds, though affecting a unique target in the cell, are associated with a remarkable panoply of different downstream biologic effects. In this article, we will review some of the prevailing theories about how these novel targeted drugs affect lymphoma biology, and how these compounds are changing the face of lymphoma therapy.

BCL6 interacts with the transcription factor Miz-1 to suppress the cyclin-dependent kinase inhibitor p21 and cell cycle arrest in germinal center B cells

The BCL6 proto-oncogene encodes a transcriptional repressor that is required for germinal center formation and has been linked to lymphomagenesis. BCL6 functions by directly binding to specific DNA sequences and suppressing the transcription of target genes. Here we report an alternative mechanism by which BCL6 controls the transcription of genes lacking a BCL6 binding site and show that this mechanism was required for the prevention of tumor suppressor p53-independent cell cycle arrest in germinal center B cells. BCL6 interacted with the transcriptional activator Miz-1 and, via Miz-1, bound to the promoter and suppressed transcription of the cell cycle arrest gene CDKN1A. Through this mechanism, BCL6 may facilitate the proliferative expansion of germinal centers during the normal immune response and, when deregulated, the pathological expansion of B cell lymphomas.

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.

Molecular cloning of the BCL-6 gene, a transcriptional repressor for B-cell differentiation, in torafugu (Takifugu rubripes)

B-cell lymphoma-6 (BCL-6) is a transcriptional repressor that prevents the terminal differentiation of mature B-cells to plasma cells, and is essential for germinal center formation in the primary lymphoid organs of mammals. In this study, we identified the BCL-6 gene in torafugu (Takifugu rubripes) using the torafugu genome database, and analyzed the expression of BCL-6 mRNA in various tissues of torafugu, using RT-PCR. The BCL-6 gene consisted of eight exons and seven introns spanning a genome of ca. 3.3 kb. BCL-6 mRNA contained a 2112 bp open reading frame encoding 703 amino acids, with a predicted protein size of 78.8 kDa. The predicted torafugu BCL-6 primary structure contains two conserved specific motifs, the BTB/POZ domain at the N-terminus and the sixC2H2-type zinc finger motifs at the C-terminal region. The homology of torafugu BCL-6 to those of zebrafish (Danio rerio), Xenopus laevis, mouse (Mus musculus) and human (Homo sapiens) is 76, 59, 60 and 60%, respectively. RT-PCR analysis revealed that BCL-6 mRNA is highly expressed in pronephros, thymus, intestine, ovary, brain, nasal cavity and muscle. These results imply that torafugu BCL-6 is involved in regulation of B-cell differentiation in torafugu.

Epigenetic control of B cell differentiation

Gene expression, differentiation and the specialized function of various cell types are controlled epigenetically by post-translational histone modifications. These modifications establish a "histone code" that is recognized by various regulatory proteins, thereby creating a stable pattern of gene expression. The focus of this review is to discuss how the chromatin modifications regulate immunoglobulin gene rearrangement and B cell differentiation.

The t(14;18) in Diffuse Large B-Cell Lymphoma

The clinical and biologic relevance of the t(14;18) and features of germinal center (GC) differentiation in diffuse large B-cell lymphoma (DLBCL) remain controversial. The authors examined the association of t(14;18) with GC-associated markers and clinical features in 44 de novo DLBCLs (22 nodal and 22 primary extranodal). CD10, bcl-2, and bcl-6 were expressed in 50%, 62%, and 54% of cases respectively. There were no significant differences in expression of these markers between nodal and extranodal cases. Coexpression of CD10 and bcl-6 was seen in 12 of 41 cases, and was more frequent in nodal than extranodal DLBCL (9 of 21 vs. 3 of 20; P = 0.05). A CD10+/bcl-6+ phenotype was not significantly associated with bcl-2 expression, stage, complete remission rate, or survival. The t(14;18) was found in 7 of 44 (16%) cases (6 nodal, 1 extranodal; P = 0.09). It was associated with a CD10+/bcl-6+ phenotype (5 of 7 vs. 7 of 27; P = 0.015) and a trend toward more frequent bcl-6 expression (6 of 7 vs. 15 of 34; P = 0.09), but no association with bcl-2 expression, CD10, clinical stage, complete remission, or survival. Among nodal or high-stage (III-IV) DLBCL, cases with the t(14;18) showed a trend toward decreased survival (P = 0.12).

BCL6 suppresses RhoA activity to alter macrophage morphology and motility

BCL6 is a potent transcriptional repressor that plays important roles in germinal center formation, T helper cell differentiation and lymphomagenesis and regulates expression of several chemokine genes in macrophages. In a further investigation of its role in macrophages, we show that BCL6 inactivation in primary bone marrow-derived macrophages leads to decreased polarization, motility and cell spreading accompanied by an increase in peripheral focal complexes, anchored F-actin bundles and cortical F-actin density. These changes were associated with excess RhoA activation. C3 transferase inhibition of RhoA activity reverted the adhesion structure phenotype, which was not affected by Rho kinase inhibitors, suggesting that other downstream effectors of Rho maintain this Bcl6–/– phenotype. Excess RhoA activation in BCL6-deficient macrophages is associated with a decrease in the p120RasGAP (RASA1)-mediated translocation of p190RhoGAP (GRLF1) to active RhoA at the plasma membrane and a reduction in cell surface expression of the CSF1R that has been reported to recruit RasGAP to the plasma membrane. Reconstitution of BCL6 expression in Bcl6–/– macrophages results in complete reversion of the morphological phenotype and a significant increase in cell surface CSF1R expression whereas overexpression of the CSF1R corrects the polarization and adhesion structure defects. These results demonstrate that BCL6 suppresses RhoA activity, largely through upregulation of surface CSF1R expression, to modulate cytoskeletal and adhesion structures and increase the motility of macrophages.

Deregulated BCL6 expression recapitulates the pathogenesis of human diffuse large B cell lymphomas in mice

Diffuse large B cell lymphomas (DLBCL) derive from germinal center (GC) B cells and display chromosomal alterations deregulating the expression of BCL6, a transcriptional repressor required for GC formation. To investigate the role of BCL6 in DLBCL pathogenesis, we have engineered mice that express BCL6 constitutively in B cells by mimicking a chromosomal translocation found in human DLBCL. These mice display increased GC formation and perturbed post-GC differentiation characterized by a decreased number of post-isotype switch plasma cells. Subsequently, these mice develop a lymphoproliferative syndrome that culminates with the development of lymphomas displaying features typical of human DLBCL. These results define the oncogenic role of BCL6 in the pathogenesis of DLBCL and provide a faithful mouse model of this common disease.

Advances in the biology and therapy of diffuse large B-cell lymphoma: moving toward a molecularly targeted approach

Diffuse large B-cell lymphoma (DLBCL) displays striking heterogeneity at the clinical, genetic, and molecular levels. Clinical prognostic models can define a population at high risk for relapse following empiric chemotherapy, although such models do not account for underlying biologic differences among tumors. Commonly observed genetic abnormalities that likely contribute to pathogenesis include translocations of BCL6, BCL2, cMYC, and FAS(CD95) mutations, and aberrant somatic hypermutation. Despite recent advances in empiric chemotherapy, including interval reduction of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) and the incorporation of anti-CD20 monoclonal antibodies, a significant proportion of patients still die of their disease. Gene expression profiling has shed light on the molecular heterogeneity within DLBCL by highlighting similarities between subsets of tumors and normal B cells, identifying features associated with unfavorable responses to empiric combination chemotherapy, and defining robust subtypes with comprehensive transcriptional signatures. Such strategies have suggested distinct routes to lymphomagenesis and have identified promising rational therapeutic targets. Additional novel therapies under investigation include those targeting BCL6 and BCL2, as well as development of novel monoclonal antibody-based therapies. Our increasing molecular understanding of the heterogeneous subsets within DLBCL will likely improve the current empiric therapy of DLBCL by identifying rational therapeutic targets in specific disease subtypes.

BCL6b mediates the enhanced magnitude of the secondary response of memory CD8+ T lymphocytes

A characteristic of the secondary response of CD8(+) T cells that distinguishes it from the primary response is the generation of greater numbers of effector cells. Because effector CD8(+) T cells are derived from a pool of less differentiated, replicating cells in secondary lymphoid organs, and because IL-2 mediates effector differentiation, the enhanced secondary response may reflect the enlargement of this generative pool by the transient repression of IL-2-mediated differentiation. We have examined for this function the transcriptional repressor BCL6b, a homologue of BCL6 that represses IL-2-induced B cell differentiation. BCL6b is expressed in a small subset of antigen-experienced CD8(+) T cells. Ectopic expression of BCL6b in CD8(+) T cells diminishes their growth in response to IL-2 in vitro. Female mice in which the BCL6b gene has been interrupted have normal primary responses of CD8(+) T cells to infection with vaccinia expressing the H-Y epitope, Uty, but Uty-specific, BCL6b(-/-), memory CD8(+) T cells have diminished recall proliferative responses to this epitope in vitro. BCL6b(-/-) mice also have normal primary CD8(+) T cell responses to influenza infection, but nucleoprotein peptide-specific, BCL6b(-/-), memory CD8(+) T cells have a cell autonomous defect in the number of effector cells generated in response to reinfection. Therefore, BCL6b is required for the enhanced magnitude of the secondary response of memory CD8(+) T cells.

Repression of an Interleukin-4-responsive Promoter Requires Cooperative BCL-6 Function

BCL-6 functions as a potent transcriptional repressor that binds with specificity to DNA elements bearing marked similarity to STAT recognition sequences. Previous studies have demonstrated that BCL-6 and Stat6 can both bind and regulate the Iepsilon promoter that controls immunoglobulin heavy chain class switching to IgE. Examination of BCL-6-/- and BCL-6-/-Stat6-/- mice has demonstrated that BCL-6 is a repressor of IgE and that Stat6 is still required for the interleukin-4 (IL-4) induction of class switching to IgE in B cells lacking BCL-6. To define the mechanisms by which BCL-6 represses IL-4 function, we analyzed the role of BCL-6 in repressing the Iepsilon promoter. There are three BCL-6-binding sites within this IL-4-responsive promoter. Analysis of Iepsilon promoters that have mutated BCL-6-binding sites demonstrates that at least two of these sites are required for maximal BCL-6 repression of this locus. Footprinting analysis demonstrates that BCL-6 binds cooperatively to the two upstream binding sites in the Iepsilon promoter. This cooperative binding requires the POZ domain of BCL-6. Furthermore, activated Stat6 molecules can displace BCL-6 from one of these binding sites. These data demonstrate that cooperative interaction between BCL-6 molecules is required for repression of the Iepsilon promoter.

Bcl6 regulates Th2 type cytokine productions by mast cells activated by FcepsilonRI/IgE cross-linking

Bcl6-deficient (Bcl6-/-) mice displayed Th2 type inflammation, which caused by abnormality of non-lymphoid cells. However, initiators for the Th2 type inflammation were not clear. In order to elucidate the initiators, we investigated property and function of mast cells derived from Bcl6-/- mice. Mast cells were developed from bone marrow cells cultured with IL-3 (BMMCs). Although the development of BMMCs from Bcl6-/- mice was similar to that from wild-type mice, proliferation of Bcl6-/- BMMCs stimulated with IL-3 was slightly lower than that of wild-type BMMCs. When these BMMCs were stimulated by FcepsilonRI/IgE cross-linking, Bcl6-/- BMMCs produced Th2 cytokines more than wild-type BMMCs did. Thus, Bcl6-/- mast cells are one of the initiators for Th2 type inflammation in Bcl6-/- mice, and Bcl6 may be a molecular target for Th2 type allergic diseases.

STAT5 regulates the self-renewal capacity and differentiation of human memory B cells and controls Bcl-6 expression

It is unknown how B cells that mature during a germinal center reaction 'decide' between plasma or memory cell fate. Here we describe a previously unknown subpopulation of B cells in the human germinal center that is characterized by tyrosine phosphorylated transcriptional activator STAT5. These cells had an activated centrocyte phenotype and had abundant expression of BCL6 but low expression of PRDM1, both encoding transcriptional repression proteins. Using RNA interference and ectopic expression of constitutively activated forms of STAT5, we demonstrate here a function for STAT5 in the self-renewal of B cells in vitro. STAT5b isoform seemed to directly upregulate Bcl-6, and ectopic expression of Bcl-6 in B cells resulted in self-renewal and inhibition of plasma cell differentiation. These data indicate that activation of STAT5 is involved in regulation of memory B cell differentiation.

Role of the proto-oncogene Pokemon in cellular transformation and ARF repression

Aberrant transcriptional repression through chromatin remodelling and histone deacetylation has been postulated to represent a driving force underlying tumorigenesis because histone deacetylase inhibitors have been found to be effective in cancer treatment. However, the molecular mechanisms by which transcriptional derepression would be linked to tumour suppression are poorly understood. Here we identify the transcriptional repressor Pokemon (encoded by the Zbtb7 gene) as a critical factor in oncogenesis. Mouse embryonic fibroblasts lacking Zbtb7 are completely refractory to oncogene-mediated cellular transformation. Conversely, Pokemon overexpression leads to overt oncogenic transformation both in vitro and in vivo in transgenic mice. Pokemon can specifically repress the transcription of the tumour suppressor gene ARF through direct binding. We find that Pokemon is aberrantly overexpressed in human cancers and that its expression levels predict biological behaviour and clinical outcome. Pokemon's critical role in cellular transformation makes it an attractive target for therapeutic intervention.

The ARF tumor suppressor inhibits BCL6-mediated transcriptional repression

The ARF tumor suppressor gene antagonizes generation of various tumors. ARF-mediated tumor suppression occurs in a p53-independent manner as well as in a p53-dependent manner. We here demonstrate that BCL6 is a target of the ARF tumor suppressor. Either mouse p19(ARF) or human p14(ARF) binds to BCL6 and downregulates BCL6-induced transcriptional repression. ARF-mediated downregulation of the BCL6 activity may account in part for ARF-mediated tumor suppression.

Induction of BCL-6 gene expression by interferon-gamma and identification of an IRE in exon I

BCL-6 is a POZ domain zinc-finger transcription factor that appears to play important roles in the development of the immune system and its regulation. Mutations within BCL-6 gene can therefore contribute to the genesis of a variety of lymphomas, and can also manifest as a classic Th2-type hyperimmune response. In addition to its roles in B- and T-cell development, and in germinal centre formation, the factor is also critical for the development of peripheral memory T cells. In this study, we report that BCL-6 expression is induced by IFN-gamma in Jurkat cells and in nontransformed T cells polarized toward the Th1 phenotype. The IFN-gamma-responsive region has been mapped within the first exon between nucleotide +180 and +200. In vivo footprinting of the first exon reveals that a stretch of DNA between nucleotide +180 to +195 (which we term the X-box) is constitutively occupied in vivo in the presence or absence of IFN-gamma. A guanine at +195 residing at the boundary of the X-box and a downstream IFN-gamma-activated sequence (GAS 1; between nucleotides +192 to +200) is occupied in IFN-gamma-treated cells, indicating the interaction of an IFN-gamma-inducible/modified factor to this region. Consistent with this, electrophoretic mobility shift assays detect STAT-1alpha interactions with the downstream GAS1 motif. The cumulative data suggest that the X-box-binding protein facilitate the binding of STAT-1alpha to the GAS 1 site. The discovery that the BCL-6 transcription factor is inducible by IFN-gamma may help explain some of the postulated biological roles of BCL-6 in T cell development and differentiation, and help explain the Th2-biased phenotype of BCL-6-deficient mice.