BCL6: master regulator of the germinal center reaction and key oncogene in B cell lymphomagenesis

Cyclin D1-positive diffuse large B-cell lymphoma with IGH-CCND1 translocation and BCL6 rearrangement: a report of two cases

OBJECTIVES

To demonstrate and confirm the existence of cyclin D1-positive diffuse large B-cell lymphoma (DLBCL) with IGH-CCND1 rearrangement and discuss the rationale of differentiating this entity from blastoid and pleomorphic variants of mantle cell lymphoma (MCL).

METHODS

Two cyclin D1-positive lymphomas with morphologic features of DLBCL and IGH-CCND1 translocations were characterized with respect to clinical features, as well as morphologic, immunophenotypic, cytogenetic, and molecular findings.

RESULTS

The large tumor cells were CD20+, CD5-, CD10-, BCL6+, MUM1+, and cyclin D1+ in both cases. SOX11 was negative. Epstein-Barr virus-encoded RNA in situ hybridization demonstrated diffuse positivity in case 1. BCL6 and IGH-CCND1 rearrangements were identified by fluorescence in situ hybridization in both cases. Specifically, the diagnosis of a relapsed DLBCL with acquisition of IGH-CCND1 was rendered for case 1, molecularly confirmed by the detection of identical monoclonal IGH rearrangements between the initial diagnostic DLBCL and relapse lymphoma.

CONCLUSIONS

Our study demonstrates convincingly that IGH-CCND1 rearrangement leading to cyclin D1 overexpression can occur in DLBCL and pose a potential diagnostic pitfall, requiring thorough knowledge of the clinicopathologic findings to allow accurate discrimination from a blastoid or pleomorphic MCL. The coexistence of IGH-CCND1 and IGH-BCL6 rearrangements suggest that BCL6 and cyclin D1 may cooperate in the pathogenesis of DLBCL.

EBV Persistence--Introducing the Virus

Persistent infection by EBV is explained by the germinal center model (GCM) which provides a satisfying and currently the only explanation for EBVs disparate biology. Since the GCM touches on every aspect of the virus, this chapter will serve as an introduction to the subsequent chapters. EBV is B lymphotropic, and its biology closely follows that of normal mature B lymphocytes. The virus persists quiescently in resting memory B cells for the lifetime of the host in a non-pathogenic state that is also invisible to the immune response. To access this compartment, the virus infects naïve B cells in the lymphoepithelium of the tonsils and activates these cells using the growth transcription program. These cells migrate to the GC where they switch to a more limited transcription program, the default program, which helps rescue them into the memory compartment where the virus persists. For egress, the infected memory cells return to the lymphoepithelium where they occasionally differentiate into plasma cells activating viral replication. The released virus can either infect more naïve B cells or be amplified in the epithelium for shedding. This cycle of infection and the quiescent state in memory B cells allow for lifetime persistence at a very low level that is remarkably stable over time. Mathematically, this is a stable fixed point where the mechanisms regulating persistence drive the state back to equilibrium when perturbed. This is the GCM of EBV persistence. Other possible sites and mechanisms of persistence will also be discussed.

Convergent transcription at intragenic super-enhancers targets AID-initiated genomic instability

Activation-induced cytidine deaminase (AID) initiates both somatic hypermutation (SHM) for antibody affinity maturation and DNA breakage for antibody class switch recombination (CSR) via transcription-dependent cytidine deamination of single-stranded DNA targets. Though largely specific for immunoglobulin genes, AID also acts on a limited set of off-targets, generating oncogenic translocations and mutations that contribute to B cell lymphoma. How AID is recruited to off-targets has been a long-standing mystery. Based on deep GRO-seq studies of mouse and human B lineage cells activated for CSR or SHM, we report that most robust AID off-target translocations occur within highly focal regions of target genes in which sense and antisense transcription converge. Moreover, we found that such AID-targeting "convergent" transcription arises from antisense transcription that emanates from super-enhancers within sense transcribed gene bodies. Our findings provide an explanation for AID off-targeting to a small subset of mostly lineage-specific genes in activated B cells.

Histone deacetylase inhibitors upregulate B cell microRNAs that silence AID and Blimp-1 expression for epigenetic modulation of antibody and autoantibody responses

Class-switch DNA recombination (CSR) and somatic hypermutation (SHM), which require activation-induced cytidine deaminase (AID), and plasma cell differentiation, which requires B lymphocyte-induced maturation protein-1 (Blimp-1), are critical for the generation of class-switched and hypermutated (mature) Ab and autoantibody responses. We show that histone deacetylase inhibitors valproic acid and butyrate dampened AICDA/Aicda (AID) and PRDM1/Prdm1 (Blimp-1) mRNAs by upregulating miR-155, miR-181b, and miR-361 to silence AICDA/Aicda, and miR-23b, miR-30a, and miR-125b to silence PRDM1/Prdm1, in human and mouse B cells. This led to downregulation of AID, Blimp-1, and X-box binding protein 1, thereby inhibiting CSR, SHM, and plasma cell differentiation without altering B cell viability or proliferation. The selectivity of histone deacetylase inhibitor-mediated silencing of AICDA/Aicda and PRDM1/Prdm1 was emphasized by unchanged expression of HoxC4 and Irf4 (important inducers/modulators of AICDA/Aicda), Rev1 and Ung (central elements for CSR/SHM), and Bcl6, Bach2, or Pax5 (repressors of PRDM1/Prdm1 expression), as well as unchanged expression of miR-19a/b, miR-20a, and miR-25, which are not known to regulate AICDA/Aicda or PRDM1/Prdm1. Through these B cell-intrinsic epigenetic mechanisms, valproic acid blunted class-switched and hypermutated T-dependent and T-independent Ab responses in C57BL/6 mice. In addition, it decreased class-switched and hypermutated autoantibodies, ameliorated disease, and extended survival in lupus MRL/Fas(lpr/lpr) mice. Our findings outline epigenetic mechanisms that modulate expression of an enzyme (AID) and transcription factors (Blimp-1 and X-box binding protein 1) that are critical to the B cell differentiation processes that underpin Ab and autoantibody responses. They also provide therapeutic proof-of-principle in autoantibody-mediated autoimmunity.

Follicular helper T lymphocytes in health and disease

A correct antibody response requires the participation of both B and T lymphocytes and antigen presenting cells. In this review we address the role of follicular helper T lymphocytes (T_FH) in this reaction. We shall focus on the regulation of their development and function in health and disease. T_FH can be characterized on the basis of their phenotype and the pattern of secretion of cytokines. This fact is useful to study their participation in the generation of antibody deficiency in primary immunodeficiency diseases such as common variable immunodeficiency, X-linked hyper IgM syndrome or X-linked lymphoproliferative disease. Increased numbers of T_FH have been demonstrated in several autoimmune diseases and are thought to play a role in the development of autoantibodies. In chronic viral infections caused by the human immunodeficiency virus, hepatitis B or C virus, increased circulating T_FH have been observed, but their role in the protective immune response to these agents is under discussion. Likewise, an important role of T_FH in the control of some experimental protozoan infections has been proposed, and it will be important to assess their relevance in order to design effective vaccination strategies.

Inter- and intratumoral heterogeneity of BCL2 correlates with IgH expression and prognosis in follicular lymphoma

Most follicular lymphomas (FLs) are genetically defined by the t(14;18)(q32;q21) translocation that juxtaposes the BCL2 gene to the immunoglobulin heavy chain (IgH) 3' regulatory regions (IgH-3'RRs). Despite this recurrent translocation, FL cases are heterogeneous in terms of intratumoral clonal diversity for acquired mutations and variations in the tumor microenvironment. Here we describe an additional mechanism that contributes to inter- and intratumoral heterogeneity in FLs. By applying a novel single-molecule RNA fluorescence-based in situ hybridization (FISH) technique to detect mRNA molecules of BCL2 and IgH in single cells, we found marked heterogeneity in the number of BCL2 mRNA transcripts within individual lymphoma cells. Moreover, BCL2 mRNA molecules correlated with IgH mRNA molecules in individual cells both in t(14;18) lymphoma cell lines and in patient samples. Consistently, a strong correlation between BCL2 and IgH protein levels was found in a series of 205 primary FL cases by flow cytometry and immunohistochemistry. Inter- and intratumoral heterogeneity of BCL2 expression determined resistance to drugs commonly used in FL treatment and affected overall survival of FL patients. These data demonstrate that BCL2 and IgH expressions are heterogeneous and coregulated in t(14;18)-translocated cells, and determine the response to therapy in FL patients.

Pellino 1 promotes lymphomagenesis by deregulating BCL6 polyubiquitination

The signal-responsive E3 ubiquitin ligase pellino 1 (PELI1) regulates TLR and T cell receptor (TCR) signaling and contributes to the maintenance of autoimmunity; however, little is known about the consequence of mutations that result in upregulation of PELI1. Here, we developed transgenic mice that constitutively express human PELI1 and determined that these mice have a shorter lifespan due to tumor formation. Constitutive expression of PELI1 resulted in ligand-independent hyperactivation of B cells and facilitated the development of a wide range of lymphoid tumors, with prominent B cell infiltration observed across multiple organs. PELI1 directly interacted with the oncoprotein B cell chronic lymphocytic leukemia (BCL6) and induced lysine 63-mediated BCL6 polyubiquitination. In samples from patients with diffuse large B cell lymphomas (DLBCLs), PELI1 expression levels positively correlated with BCL6 expression, and PELI1 overexpression was closely associated with poor prognosis in DLBCLs. Together, these results suggest that increased PELI1 expression and subsequent induction of BCL6 promotes lymphomagenesis and that this pathway may be a potential target for therapeutic strategies to treat B cell lymphomas.

Bcl-6 directly represses the gene program of the glycolysis pathway

Despite the increasing knowledge of the molecular events that induce the glycolysis pathway in effector T cells, very little is known about the transcriptional mechanisms that dampen the glycolysis program in quiescent cell populations such as memory T cells. Here we found that the transcription factor Bcl-6 directly repressed genes encoding molecules involved in the glycolysis pathway, including Slc2a1, Slc2a3, Pkm and Hk2, in type 1 helper T cells (TH1 cells) exposed to low concentrations of interleukin 2 (IL-2). Thus, Bcl-6 had a role opposing the IL-2-sensitive glycolytic transcriptional program that the transcription factors c-Myc and HIF-1α promote in effector T cells. Additionally, the TH1 lineage-specifying factor T-bet functionally antagonized the Bcl-6-dependent repression of genes encoding molecules in the glycolysis pathway, which links the molecular balance of these two factors to regulation of the metabolic gene program.

Reciprocal occupancy of BCL6 and STAT5 on Growth Hormone target genes: contrasting transcriptional outcomes and promoter-specific roles of p300 and HDAC3

Expression of the Growth Hormone (GH)-stimulated gene Socs2 (Suppressor of Cytokine Signaling 2) is mediated by the transcription activator STAT5 (Signal Transducer and Activator of Transcription 5) and the transcription repressor BCL6 (B-Cell Lymphoma 6). ChIP-Sequencing identified Cish (Cytokine-Inducible SH2-containing protein) and Bcl6 as having similar patterns of reciprocal occupancy by BCL6 and STAT5 in response to GH, though GH stimulates Cish and inhibits Bcl6 expression. The co-activator p300 occupied Socs2, Cish and Bcl6 promoters, and enhanced STAT5-mediated activation of Socs2 and Cish. In contrast, on Bcl6, p300 functioned as a repressor and inhibited in conjunction with STAT5 or BCL6. The co-repressor HDAC3 (Histone deacetylase 3) inhibited the Socs2, Cish and Bcl6 promoters in the presence of STAT5. Thus transcriptional outcomes on GH-regulated genes occupied by BCL6 and STAT5 are determined in a promoter-specific fashion by co-regulatory proteins which mediate the distinction between activating and repressive transcription factors.

Deficiency of the transcriptional repressor B cell lymphoma 6 (Bcl6) is accompanied by dysregulated lipid metabolism

The transcriptional repressor B-cell Lymphoma 6 (Bcl6) was recently identified in a profile of genes regulated in adipocytes, suggesting a relationship between Bcl6 and metabolic regulation. As a representative target gene repressed by Bcl6, Suppressor of Cytokine Signaling (Socs) 2 expression was elevated in Bcl6 deficient (KO) mice, including metabolic tissues liver, adipose tissue and muscle, as well as in spleen and thymus. Bcl6 occupied the Socs2 promoter in wild-type, but not Bcl6 KO mice, suggesting direct regulation of Socs2 by Bcl6 in vivo. Mice deficient in Bcl6 were found to exhibit multiple features of dysregulated lipid metabolism. Adipose tissue mass was dramatically reduced or absent in Bcl6 KO mice. Further, hepatic and serum triglycerides were low. Bcl6 deficiency was accompanied by decreased hepatic expression of Stearoyl-CoA desaturase 1 (Scd1) and Fatty acid synthase (Fasn) genes which encode lipogenic enzymes. Expression of the gene for the transcription factor Carbohydrate-Responsive Element Binding Protein (Chrebp), which regulates expression of lipogenic genes, was also reduced in liver of Bcl6 KO mice. Bcl6 deficiency disrupted fasting-induced increases in hepatic triglyceride deposition, but not decreases in lipogenic gene expression. Taken together, these findings suggest that in addition to its well-recognized roles in immune regulation, Bcl6 plays a role in regulatory events of lipid metabolism, and that in the absence of Bcl6, lipid metabolism in liver and adipose tissue is dysregulated.

STAT6-mediated BCL6 repression in primary mediastinal B-cell lymphoma (PMBL)

Primary mediastinal B-cell lymphoma (PMBL) is characterized by aberrant activation of JAK/STAT-signaling resulting in constitutive presence of phosphorylated STAT6 (pSTAT6). In primary PMBL samples pSTAT6 is only expressed in a sub-population of lymphoma cells in a pattern that is reminiscent of that of the BCL6 oncogene. Double-fluorescence staining was carried out to determine the association between these two proteins in ten primary PMBL cases and three available PMBL cell line models. Surprisingly, only a minute fraction of double-positive nuclei was observed, while each sample contained considerable fractions of single-positive pSTAT6 and BCL6 nuclei. The intratumoral coexistence of BCL6+/pSTAT6− and BCL6−/pSTAT6+ subpopulations suggests a negative interaction between these factors. In silico screening of the STAT6 /BCL6 promoters for DNA consensus binding sites identified five STAT-binding-sites in the BCL6 promoter. We confirmed STAT6 binding to the BCL6 promoter in vitro and in vivo by band shift / super shift assays and chromatin immunoprecipitations. Using BCL6 luciferase reporter assays, depletion of STAT6 by siRNA, and ectopic overexpression of a constitutive active STAT6 mutant, we proved that pSTAT6 is sufficient to transcriptionally repress BCL6. Recently developed small molecule inhibitors 79-6 and TG101348 that increases BCL6 target gene expression and decreases pSTAT6 levels, respectively, demonstrate that a combined targeting results in additive efficacy regarding their negative effect on cell viability.

The delineated pSTAT6-mediated molecular repression mechanism links JAK/STAT to BCL6-signaling in PMBL and may carry therapeutic potential.

BTB-ZF transcription factors, a growing family of regulators of early and late B-cell development

The differentiation of early B-cell precursors in the bone marrow into the variety of mature and effector B-cell subsets of the periphery is a complex process that requires tight regulation at the transcriptional level. Different members of the broad complex, tramtrack, bric-à-brac and zinc finger (BTB-ZF) family of transcription factors have recently been shown to have key roles in many phases of B-cell development, including early B-cell development in the bone marrow, peripheral B-cell maturation and specialization into effector cells during an immune response. This review highlights the critical functions mediated by BTB-ZF transcription factors within the B-cell lineage and emphasizes how the deregulation of these transcription factors can lead to B-cell malignancies.

Moving beyond rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone for diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is the most common B-cell non-Hodgkin lymphoma (NHL). While the de facto treatment standard R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone) is curative in most cases, it is ineffective for a significant proportion of patients, particularly those with intermediate and high-risk disease. Efforts to improve upon the results of R-CHOP have principally explored dose intensification of chemotherapy and resulted in considerable additive toxicity without clear benefit. DLBCL is not a uniform disease, however, and can be dissected into distinct molecular subtypes by gene expression profiling. These subtypes are characterized by distinct oncogenic mechanisms of activation and addictions to aberrant intracellular signaling pathways. Novel therapeutic agents that target these pathway addictions are emerging, and may have specific activity within molecular subtypes of DLBCL. To move beyond R-CHOP for all patients with DLBCL, targeted therapies added to the most effective chemotherapy platforms must be studied within the context of molecularly defined subsets.

Long-term B cell depletion in murine lupus eliminates autoantibody-secreting cells and is associated with alterations in the kidney plasma cell niche

Autoantibodies to dsDNA, produced by autoreactive plasma cells (PCs), are a hallmark of systemic lupus erythematosus and play a key role in disease pathogenesis. Recent data suggest that autoreactive PCs accumulate not only in lymphoid tissues, but also in the inflamed kidney in lupus nephritis. We hypothesized that the variable efficacy of anti-CD20 (rituximab)-mediated B cell depletion in systemic lupus erythematosus may be related to the absence of an effect on autoreactive PCs in the kidney. In this article, we report that an enrichment of autoreactive dsDNA Ab-secreting cells (ASCs) in the kidney of lupus-prone mice (up to 40% of the ASCs) coincided with a progressive increase in splenic germinal centers and PCs, and an increase in renal expression for PC survival factors (BAFF, a proliferation-inducing ligand, and IL-6) and PC attracting chemokines (CXCL12). Short-term treatment with anti-CD20 (4 wk) neither decreased anti-dsDNA nor IgG ASCs in different anatomical locations. However, long-term treatment (12 wk) significantly reduced both IgG- and dsDNA-specific ASCs. In addition, long-term treatment substantially decreased splenic germinal center and PC generation, and unexpectedly reduced the expression for PC survival factors in the kidney. These results suggest that prolonged B cell depletion may alter the PC survival niche in the kidney, regulating the accumulation and maintenance of autoreactive PCs.

PTEN C-terminal deletion causes genomic instability and tumor development

Tumor suppressor PTEN controls genomic stability and inhibits tumorigenesis. The N-terminal phosphatase domain of PTEN antagonizes the PI3K/AKT pathway, but its C-terminal function is less defined. Here, we describe a knockin mouse model of a nonsense mutation that results in the deletion of the entire Pten C-terminal region, referred to as Pten(ΔC). Mice heterozygous for Pten(ΔC) develop multiple spontaneous tumors, including cancers and B cell lymphoma. Heterozygous deletion of the Pten C-terminal domain also causes genomic instability and common fragile site rearrangement. We found that Pten C-terminal disruption induces p53 and its downstream targets. Simultaneous depletion of p53 promotes metastasis without influencing the initiation of tumors, suggesting that p53 mainly suppresses tumor progression. Our data highlight the essential role of the PTEN C terminus in the maintenance of genomic stability and suppression of tumorigenesis.

Systems biology of primary CNS lymphoma: from genetic aberrations to modeling in mice

Primary lymphoma of the central nervous system (CNS, PCNSL) is a specific diffuse large B cell lymphoma entity arising in and confined to the CNS. Despite extensive research since many decades, the pathogenetic mechanisms underlying the remarkable tropism of this peculiar malignant hematopoietic tumor remain still to be elucidated. In the present review, we summarize the present knowledge on the genotypic and phenotypic characteristics of the tumor cells of PCNSL, give an overview over deregulated molecular pathways in PCNSL and present recent progress in the field of preclinical modeling of PCNSL in mice. With regard to the phenotype, PCNSL cells resemble late germinal center exit IgM+IgD+ B cells with blocked terminal B cell differentiation. They show continued BCL6 activity in line with ongoing activity of the germinal center program. This together with the pathways deregulated by genetic alterations may foster B cell activation and brisk proliferation, which correlated with the simultaneous MYC and BCL2 overexpression characteristic for PCNSL. On the genetic level, PCNSL are characterized by ongoing aberrant somatic hypermutation that, besides the IG locus, targets the PAX5, TTF, MYC, and PIM1 genes. Moreover, PCNSL cells show impaired IG class switch due to sμ region deletions, and PRDM1 mutations. Several important pathways, i.e., the B cell receptor (BCR), the toll-like receptor, and the nuclear factor-κB pathway, are activated frequently due to genetic changes affecting genes like CD79B, SHIP, CBL, BLNK, CARD11, MALT1, BCL2, and MYD88. These changes likely foster tumor cell survival. Nevertheless, many of these features are also present in subsets of systemic DLBLC and might not be the only reasons for the peculiar tropism of PCNSL. Here, preclinical animal models that closely mimic the clinical course and neuropathology of human PCNSL may provide further insight and we discuss recent advances in this field. Such models enable us to understand the pathogenetic interaction between the malignant B cells, resident cell populations of the CNS, and the associated inflammatory infiltrate. Indeed, the immunophenotype of the CNS as well as tumor cell characteristics and intracerebral interactions may create a micromilieu particularly conducive to PCNSL that may foster aggressiveness of tumor cells and accelerate the fatal course of disease. Suitable animal models may also serve as a well-defined preclinical system and may provide a useful tool for developing new specific therapeutic strategies.

Diffuse large B-cell lymphoma-treatment approaches in the molecular era

Diffuse large B-cell lymphoma (DLBCL) is an aggressive B-cell non-Hodgkin lymphoma that affects patients of all ages with a wide range of clinical presentations. Although DLBCL is curable even in advanced stages, up to one-third of patients will not achieve cure with initial therapy. In the modern era of rituximab-based therapy as the first-line treatment, the prognoses of patients who require salvage therapy are poor and most will eventually succumb to their disease. Insight into the complex molecular circuitry of DLBCL reveals a diverse range of somatic mutations and aberrant intracellular signalling pathways that characterize distinct molecular subsets of the disease. The next major breakthrough in DLBCL therapy during this 'molecular era' of disease definition will be the identification of combinations of novel agents that target the oncogenic drivers of these subsets. Well-conducted clinical trials, with translational molecular investigations, will be essential to achieve the goal of precision medicine and expand the number of patients with DLBCL who achieve a cure.

Novel CTCF binding at a site in exon1A of BCL6 is associated with active histone marks and a transcriptionally active locus

BCL6 is a zinc-finger transcriptional repressor, which is highly expressed in germinal centre B-cells and is essential for germinal centre formation and T-dependent antibody responses. Constitutive BCL6 expression is sufficient to produce lymphomas in mice. Deregulated expression of BCL6 due to chromosomal rearrangements, mutations of a negative autoregulatory site in the BCL6 promoter region and aberrant post-translational modifications have been detected in a number of human lymphomas. Tight lineage and temporal regulation of BCL6 is, therefore, required for normal immunity, and abnormal regulation occurs in lymphomas. CCCTC-binding factor (CTCF) is a multi-functional chromatin regulator, which has recently been shown to bind in a methylation-sensitive manner to sites within the BCL6 first intron. We demonstrate a novel CTCF-binding site in BCL6 exon1A within a potential CpG island, which is unmethylated both in cell lines and in primary lymphoma samples. CTCF binding, which was found in BCL6-expressing cell lines, correlated with the presence of histone variant H2A.Z and active histone marks, suggesting that CTCF induces chromatin modification at a transcriptionally active BCL6 locus. CTCF binding to exon1A was required to maintain BCL6 expression in germinal centre cells by avoiding BCL6-negative autoregulation. Silencing of CTCF in BCL6-expressing cells reduced BCL6 mRNA and protein expression, which is sufficient to induce B-cell terminal differentiation toward plasma cells. Moreover, lack of CTCF binding to exon1A shifts the BCL6 local chromatin from an active to a repressive state. This work demonstrates that, in contexts in which BCL6 is expressed, CTCF binding to BCL6 exon1A associates with epigenetic modifications indicative of transcriptionally open chromatin.

Molecular diagnosis of hematopoietic neoplasms

Cytogenetic abnormalities are considered to be common events in hematologic malignancies. These abnormalities generally consist of structural chromosomal abnormalities or gene mutations, which often are integral to the pathogenesis and subsequent evolution of an individual malignancy. Improvements made in identifying and interpreting these molecular alterations have resulted in advances in the diagnosis, prognosis, monitoring, and therapy for cancer. As a consequence of the increasingly important role of molecular testing in hematologic malignancy management, this article presents an update on the importance and use of molecular tests, detailing the advantages and disadvantages of each test when applicable.

Tertiary lymphoid neogenesis is a component of pulmonary lymphoid hyperplasia in patients with common variable immunodeficiency

BACKGROUND

Despite reducing pneumonia and other infections, antibody replacement does not appear to treat pulmonary lymphoid hyperplasia (PLH) in patients with common variable immunodeficiency (CVID). The pathogenesis and optimal treatments remain to be clarified.

OBJECTIVE

We aimed to better understand the pathology of CVID-associated lung disease. Tertiary lymphoneogenesis, although a component of interstitial lung disease associated with autoimmune diseases, has not previously been explored in patients with CVID.

METHODS

We examined the clinical characteristics and pathologic findings of 6 patients with CVID with nodular/infiltrative lung disease who had biopsy specimens demonstrating PLH.

RESULTS

In these subjects regions of PLH contained distinct B- and T-cell zones, with B-cell predominance in 1 patient and T-cell predominance in the others. Colocalization of Ki67, Bcl6, and CD23 within this ectopic lymphoid architecture demonstrated tertiary lymphoneogenesis with active centers of cellular proliferation. One patient received rituximab with improved pulmonary radiologic findings.

CONCLUSION

Ectopic lymphoid tissue forming germinal centers suggest tertiary lymphoneogenesis in CVID-associated lung disease. B cell-targeted therapy might disrupt CVID-associated lymphoid hyperplasia.

Molecular pathways: targeting MALT1 paracaspase activity in lymphoma

MALT1 mediates the activation of NF-κB in response to antigen receptor signaling. MALT1, in association with BCL10 and CARD11, functions as a scaffolding protein to activate the inhibitor of IκB kinase (IKK) complex. In addition, MALT1 is a paracaspase that targets key proteins in a feedback loop mediating termination of the NF-κB response, thus promoting activation of NF-κB signaling. Activated B-cell subtype of diffuse large B-cell lymphomas (ABC-DLBCL), which tend to be more resistant to chemotherapy, are often biologically dependent on MALT1 activity. Newly developed MALT1 small-molecule inhibitors suppress the growth of ABC-DLBCLs in vitro and in vivo. This review highlights the recent advances in the normal and disease-related functions of MALT1. Furthermore, recent progress targeting MALT1 proteolytic activity raises the possibility of deploying MALT1 inhibitors for the treatment of B-cell lymphomas and perhaps autoimmune diseases that involve increased B- or T-cell receptor signaling.

Evidence of dynamically dysregulated gene expression pathways in hyperresponsive B cells from African American lupus patients

Recent application of gene expression profiling to the immune system has shown a great potential for characterization of complex regulatory processes. It is becoming increasingly important to characterize functional systems through multigene interactions to provide valuable insights into differences between healthy controls and autoimmune patients. Here we apply an original systematic approach to the analysis of changes in regulatory gene interconnections between in Epstein-Barr virus transformed hyperresponsive B cells from SLE patients and normal control B cells. Both traditional analysis of differential gene expression and analysis of the dynamics of gene expression variations were performed in combination to establish model networks of functional gene expression. This Pathway Dysregulation Analysis identified known transcription factors and transcriptional regulators activated uniquely in stimulated B cells from SLE patients.

LITAF, a BCL6 target gene, regulates autophagy in mature B-cell lymphomas

We have previously reported that LITAF is silenced by promoter hypermethylation in germinal centre-derived B-cell lymphomas, but beyond these data the regulation and function of lipopolysaccharide-induced tumour necrosis factor (TNF) factor (LITAF) in B cells are unknown. Gene expression and immunohistochemical studies revealed that LITAF and BCL6 show opposite expression in tonsil B-cell subpopulations and B-cell lymphomas, suggesting that BCL6 may regulate LITAF expression. Accordingly, BCL6 silencing increased LITAF expression, and chromatin immunoprecipitation and luciferase reporter assays demonstrated a direct transcriptional repression of LITAF by BCL6. Gain- and loss-of-function experiments in different B-cell lymphoma cell lines revealed that, in contrast to its function in monocytes, LITAF does not induce lipopolysaccharide-mediated TNF secretion in B cells. However, gene expression microarrays defined a LITAF-related transcriptional signature containing genes regulating autophagy, including MAP1LC3B (LC3B). In addition, immunofluorescence analysis co-localized LITAF with autophagosomes, further suggesting a possible role in autophagy modulation. Accordingly, ectopic LITAF expression in B-cell lymphoma cells enhanced autophagy responses to starvation, which were impaired upon LITAF silencing. Our results indicate that the BCL6-mediated transcriptional repression of LITAF may inhibit autophagy in B cells during the germinal centre reaction, and suggest that the constitutive repression of autophagy responses in BCL6-driven lymphomas may contribute to lymphomagenesis.

Landscape of transcriptional deregulations in the preeclamptic placenta

Preeclampsia is a pregnancy disease affecting 5 to 8% of pregnant women and a leading cause of both maternal and fetal mortality and morbidity. Because of a default in the process of implantation, the placenta of preeclamptic women undergoes insufficient vascularization. This results in placental ischemia, inflammation and subsequent release of placental debris and vasoactive factors in the maternal circulation causing a systemic endothelial activation. Several microarray studies have analyzed the transcriptome of the preeclamptic placentas to identify genes which could be involved in placental dysfunction. In this study, we compared the data from publicly available microarray analyses to obtain a consensus list of modified genes. This allowed to identify consistently modified genes in the preeclamptic placenta. Of these, 67 were up-regulated and 31 down-regulated. Assuming that changes in the transcription level of co-expressed genes may result from the coordinated action of a limited number of transcription factors, we looked for over-represented putative transcription factor binding sites in the promoters of these genes. Indeed, we found that the promoters of up-regulated genes are enriched in putative binding sites for NFkB, CREB, ANRT, REEB1, SP1, and AP-2. In the promoters of down-regulated genes, the most prevalent putative binding sites are those of MZF-1, NFYA, E2F1 and MEF2A. These transcriptions factors are known to regulate specific biological pathways such as cell responses to inflammation, hypoxia, DNA damage and proliferation. We discuss here the molecular mechanisms of action of these transcription factors and how they can be related to the placental dysfunction in the context of preeclampsia.

HDAC3 is essential for DNA replication in hematopoietic progenitor cells

Histone deacetylase 3 (HDAC3) contributes to the regulation of gene expression, chromatin structure, and genomic stability. Because HDAC3 associates with oncoproteins that drive leukemia and lymphoma, we engineered a conditional deletion allele in mice to explore the physiological roles of Hdac3 in hematopoiesis. We used the Vav-Cre transgenic allele to trigger recombination, which yielded a dramatic loss of lymphoid cells, hypocellular bone marrow, and mild anemia. Phenotypic and functional analysis suggested that Hdac3 was required for the formation of the earliest lymphoid progenitor cells in the marrow, but that the marrow contained 3-5 times more multipotent progenitor cells. Hdac3(-/-) stem cells were severely compromised in competitive bone marrow transplantation. In vitro, Hdac3(-/-) stem and progenitor cells failed to proliferate, and most cells remained undifferentiated. Moreover, one-third of the Hdac3(-/-) stem and progenitor cells were in S phase 2 hours after BrdU labeling in vivo, suggesting that these cells were impaired in transit through the S phase. DNA fiber-labeling experiments indicated that Hdac3 was required for efficient DNA replication in hematopoietic stem and progenitor cells. Thus, Hdac3 is required for the passage of hematopoietic stem/progenitor cells through the S phase, for stem cell functions, and for lymphopoiesis.

Notch-regulated periphery B cell differentiation involves suppression of E protein function

Notch signaling pathway plays important roles in promoting the generation of marginal zone (MZ) B cells at the expense of follicular (FO) B cells during periphery B cell maturation, but the underlying molecular mechanisms are not well understood. We hypothesize that Notch favors the generation of MZ B cells by downregulating E protein activity. In this study, we demonstrated that expression of Id2 and ankyrin-repeat SOCS box-containing protein 2 was elevated in MZ B cells and by Notch signaling. Id2 inhibits the DNA binding activity of E proteins, whereas ankyrin-repeat SOCS box-containing protein 2 facilitates E protein ubiquitination. Next, we examined the phenotypes of splenic B cells in mice expressing constitutively active Notch1 and/or two gain-of-function mutants of E proteins that counteract Id2-mediated inhibition or Notch-induced degradation. We found that upregulation of E proteins promoted the formation of FO B cells, whereas it suppressed the maturation of MZ B cells. In contrast, excessive amounts of Notch1 stimulated the differentiation of MZ B cells and inhibited the production of FO B cells. More interestingly, the effects of Notch1 were reversed by gain of E protein function. Furthermore, high levels of Bcl-6 expression in FO B cells was shown to be diminished by Notch signaling and restored by E proteins. In addition, E proteins facilitated and Notch hindered the differentiation of transitional B cells. Taken together, it appears that Notch regulates peripheral B cell differentiation, at least in part, through opposing E protein function.

Genetic differences between Asian and Caucasian chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a common hematological malignancy in Western countries. However, this disease is very rare in Asian countries. It is not clear whether the mechanisms of development of CLL in Caucasians and Asians are the same. We compared genetic abnormalities in Asian and Caucasian CLL using 250k GeneChip arrays. Both Asian and Caucasian CLL had four common genetic abnormalities: deletion of 13q14.3, trisomy 12, abnormalities of ATM (11q) and abnormalities of 17p. Interestingly, trisomy 12 and deletion of 13q14.3 were mutually exclusive in both groups. We also found that deletions of miR 34b/34c (11q), caspase 1/4/5 (11q), Rb1 (13q) and DLC1 (8p) are common in both ethnic groups. Asian CLL more frequently had gain of 3q and 18q. These suggest that classic genomic changes in the Asian and Caucasina CLL are same. Further, we found amplification of IRF4 and deletion of the SP140/SP100 genes; these genes have been reported as CLL-associated genes by previous genome-wide-association study. We have found classic genomic abnormalities in Asian CLL as well as novel genomic alteration in CLL.

Control of the STAT6-BCL6 antagonism by SWAP-70 determines IgE production

Asthma and allergies are major health concerns in which Ig isotype E plays a pivotal role. Ag-bound IgE drives mast cells and basophils into exocytosis, thereby promoting allergic and potentially anaphylactic reactions. The importance of tightly regulated IgE production is underscored by severe immunological conditions in humans with elevated IgE levels. Cytokines direct IgH class-switching to a particular isotype by initiation of germline transcription (GLT) from isotype-specific intronic (I) promoters. The switch to IgE depends on IL-4, which stimulates GLT of the Iε promoter, but is specifically and strongly impaired in Swap-70(-/-) mice. Although early events in IL-4 signal transduction (i.e., activation of the JAK/STAT6 pathway) do not require SWAP-70, SWAP-70 deficiency results in impaired Iε GLT. The affinity of STAT6 to chromatin is reduced in absence of SWAP-70. Chromatin immunoprecipitation revealed that SWAP-70 binds to Iε and is required for association of STAT6 with Iε. BCL6, known to antagonize STAT6 particularly at Iε, is increased on Iε in absence of SWAP-70. Other promoters bound by BCL6 and STAT6 were found unaffected. We conclude that SWAP-70 controls IgE production through regulation of the antagonistic STAT6 and BCL6 occupancy of Iε. The identification of this mechanism opens new avenues to inhibit allergic reactions triggered by IgE.

Downregulation of FOXP1 is required during germinal center B-cell function

B-cell maturation and germinal center (GC) formation are dependent on the interplay between BCL6 and other transcriptional regulators. FOXP1 is a transcription factor that regulates early B-cell development, but whether it plays a role in mature B cells is unknown. Analysis of human tonsillar B-cell subpopulations revealed that FOXP1 shows the opposite expression pattern to BCL6, suggesting that FOXP1 regulates the transition from resting follicular B cell to activated GC B cell. Chromatin immunoprecipitation-on-chip and gene expression assays on B cells indicated that FOXP1 acts as a transcriptional activator and repressor of genes involved in the GC reaction, half of which are also BCL6 targets. To study FOXP1 function in vivo, we developed transgenic mice expressing human FOXP1 in lymphoid cells. These mice exhibited irregular formation of splenic GCs, showing a modest increase in naïve and marginal-zone B cells and a significant decrease in GC B cells. Furthermore, aberrant expression of FOXP1 impaired transcription of noncoding γ1 germline transcripts and inhibited efficient class switching to the immunoglobulin G1 isotype. These studies show that FOXP1 is physiologically downregulated in GC B cells and that aberrant expression of FOXP1 impairs mechanisms triggered by B-cell activation, potentially contributing to B-cell lymphomagenesis.

A lesson for cancer research: placental microarray gene analysis in preeclampsia

Tumor progression and pregnancy share many common features, such as immune tolerance and invasion. The invasion of trophoblasts in the placenta into the uterine wall is essential for fetal development, and is thus precisely regulated. Its deregulation has been implicated in preeclampsia, a leading cause for maternal and perinatal mortality and morbidity. Pathogenesis of preeclampsia remains to be defined. Microarray-based gene profiling has been widely used for identifying genes responsible for preeclampsia. In this review, we have summarized the recent data from the microarray studies with preeclamptic placentas. Despite the complex of gene signatures, suggestive of the heterogeneity of preeclampsia, these studies identified a number of differentially expressed genes associated with preeclampsia. Interestingly, most of them have been reported to be tightly involved in tumor progression. We have discussed these interesting genes and analyzed their potential molecular functions in preeclampsia, compared with their roles in malignancy development. Further investigations are warranted to explore the involvement in molecular network of each identified gene, which may provide not only novel strategies for prevention and therapy for preeclampsia but also a better understanding of cancer cells. The trophoblastic cells, with their capacity for proliferation and differentiation, apoptosis and survival, migration, angiogenesis and immune modulation by exploiting similar molecular pathways, make them a compelling model for cancer research.

Helper T-cell identity and evolution of differential transcriptomes and epigenomes

CD4(+) T cells are critical for the elimination of an immense array of microbial pathogens. Among the ways they accomplish this task is to generate progeny with specialized, characteristic patterns of gene expression. From this perspective, helper cells can be viewed as pluripotent precursors that adopt distinct cell fates. Although there are aspects of helper cell differentiation that can be modeled as a classic cell fate commitment, CD4(+) T cells also maintain considerable flexibility in their transcriptional program. This makes sense in terms of host defense, but raises the question of how these remarkable cells balance both these requirements, a high degree of specific gene expression and the capacity for plasticity. In this review, we discuss recent advances in our understanding of CD4(+) T-cell specification, focusing on how genomic perspectives have influenced our views of these processes. The relative contributions of sensors of the cytokine milieu, especially the signal transducer and activator of transcription family transcription factors, 'master regulators', and other transcription factors are considered as they relate to the helper cell transcriptome and epigenome.

PRDM Proteins: Molecular Mechanisms in Signal Transduction and Transcriptional Regulation

PRDM (PRDI-BF1 and RIZ homology domain containing) protein family members are characterized by the presence of a PR domain and a variable number of Zn-finger repeats. Experimental evidence has shown that the PRDM proteins play an important role in gene expression regulation, modifying the chromatin structure either directly, through the intrinsic methyltransferase activity, or indirectly through the recruitment of chromatin remodeling complexes. PRDM proteins have a dual action: they mediate the effect induced by different cell signals like steroid hormones and control the expression of growth factors. PRDM proteins therefore have a pivotal role in the transduction of signals that control cell proliferation and differentiation and consequently neoplastic transformation. In this review, we describe pathways in which PRDM proteins are involved and the molecular mechanism of their transcriptional regulation.

Cys2His2 zinc finger protein family: classification, functions, and major members

Cys2His2 (C2H2)-type zinc fingers are widespread DNA binding motifs in eukaryotic transcription factors. Zinc fingers are short protein motifs composed of two or three β-layers and one α-helix. Two cysteine and two histidine residues located in certain positions bind zinc to stabilize the structure. Four other amino acid residues localized in specific positions in the N-terminal region of the α-helix participate in DNA binding by interacting with hydrogen donors and acceptors exposed in the DNA major groove. The number of zinc fingers in a single protein can vary over a wide range, thus enabling variability of target DNA sequences. Besides DNA binding, zinc fingers can also provide protein-protein and RNA-protein interactions. For the most part, proteins containing the C2H2-type zinc fingers are trans regulators of gene expression that play an important role in cellular processes such as development, differentiation, and suppression of malignant cell transformation (oncosuppression).

Fate determination of mature autoreactive B cells

A large antibody repertoire is generated in developing B cells in the bone marrow. Before these B cells achieve immunocompetence, those expressing autospecificities must be purged. To that end, B cells within the bone marrow and just following egress from the bone marrow are subject to tolerance induction. Once B cells achieve immunocompetence, the antibody repertoire can be further diversified by somatic hypermutation of immunoglobulin genes in B cells that have been activated by antigen and cognate T cell help and have undergone a germinal center (GC) response. This process also leads to the generation of autoreactive B cells which must be again purged to protect the host. Thus, B cells within the GC and just following egress from the GC are also subject to tolerance induction. Available data suggest that B cell intrinsic processes triggered by signaling through the B cell receptor activate tolerance mechanisms at both time points. Recent data suggest that GC and post-GC B cells are also subject to B cell extrinsic tolerance mechanisms mediated through soluble and membrane-bound factors derived from various T cell subsets.

Global gene expression changes of in vitro stimulated human transformed germinal centre B cells as surrogate for oncogenic pathway activation in individual aggressive B cell lymphomas

Background

Aggressive Non-Hodgkin lymphomas (NHL) are a group of lymphomas derived from germinal centre B cells which display a heterogeneous pattern of oncogenic pathway activation. We postulate that specific immune response associated signalling, affecting gene transcription networks, may be associated with the activation of different oncogenic pathways in aggressive Non-Hodgkin lymphomas (NHL).

Methodology

The B cell receptor (BCR), CD40, B-cell activating factor (BAFF)-receptors and Interleukin (IL) 21 receptor and Toll like receptor 4 (TLR4) were stimulated in human transformed germinal centre B cells by treatment with anti IgM F(ab)₂-fragments, CD40L, BAFF, IL21 and LPS respectively. The changes in gene expression following the activation of Jak/STAT, NF-кB, MAPK, Ca²⁺ and PI3K signalling triggered by these stimuli was assessed using microarray analysis. The expression of top 100 genes which had a change in gene expression following stimulation was investigated in gene expression profiles of patients with Aggressive non-Hodgkin Lymphoma (NHL).

Results

αIgM stimulation led to the largest number of changes in gene expression, affecting overall 6596 genes. While CD40L stimulation changed the expression of 1194 genes and IL21 stimulation affected 902 genes, only 283 and 129 genes were modulated by lipopolysaccharide or BAFF receptor stimulation, respectively. Interestingly, genes associated with a Burkitt-like phenotype, such as MYC, BCL6 or LEF1, were affected by αIgM. Unique and shared gene expression was delineated. NHL-patients were sorted according to their similarity in the expression of TOP100 affected genes to stimulated transformed germinal centre B cells The αIgM gene module discriminated individual DLBCL in a similar manner to CD40L or IL21 gene modules. DLBCLs with low module activation often carry chromosomal MYC aberrations. DLBCLs with high module activation show strong expression of genes involved in cell-cell communication, immune responses or negative feedback loops. Using chemical inhibitors for selected kinases we show that mitogen activated protein kinase- and phosphoinositide 3 kinase-signalling are dominantly involved in regulating genes included in the αIgM gene module.

Conclusion

We provide an in vitro model system to investigate pathway activation in lymphomas. We defined the extent to which different immune response associated pathways are responsible for differences in gene expression which distinguish individual DLBCL cases. Our results support the view that tonic or constitutively active MAPK/ERK pathways are an important part of oncogenic signalling in NHL. The experimental model can now be applied to study the therapeutic potential of deregulated oncogenic pathways and to develop individual treatment strategies for lymphoma patients.

Gastrointestinal B-cell lymphomas: From understanding B-cell physiology to classification and molecular pathology

The gut is the most common extranodal site where lymphomas arise. Although all histological lymphoma types may develop in the gut, small and large B-cell lymphomas predominate. The sometimes unexpected finding of a lymphoid lesion in an endoscopic biopsy of the gut may challenge both the clinician (who is not always familiar with lymphoma pathogenesis) and the pathologist (who will often be hampered in his/her diagnostic skill by the limited amount of available tissue). Moreover, the past 2 decades have spawned an avalanche of new data that encompasses both the function of the reactive B-cell as well as the pathogenic pathways that lead to its neoplastic counterpart, the B-cell lymphoma. Therefore, this review aims to offer clinicians an overview of B-cell lymphomas in the gut, and their pertinent molecular features that have led to new insights regarding lymphomagenesis. It addresses the question as how to incorporate all presently available information on normal and neoplastic B-cell differentiation, and how this knowledge can be applied in daily clinical practice (e.g., diagnostic tools, prognostic biomarkers or therapeutic targets) to optimalise the managment of this heterogeneous group of neoplasms.

DNA released by leukemic cells contributes to the disruption of the bone marrow microenvironment

Reciprocal interactions between a tumor and its microenvironment control expansion of tumor cells. Here we show a specific type of interaction in which blasts of experimental leukemia destroy the bone marrow (BM) structures and kill stromal cells. The in vitro experiments showed that the cytotoxic agent released by leukemic cells is the fragmented DNA derived from their genome and occurring in nucleosome-like complexes. This DNA entered nuclei of BM or other cells and induced H2A.X phosphorylation at serine 139, similar to double-strand break-inducing agents. There was a correlation between large amounts of acquired DNA and death of recipient cells. Moreover, the DNA integrated into chromosomal DNA of recipient cells. Primary human acute myeloid leukemia cells also released fragmented DNA that penetrated the nuclei of other cells both in vitro and in vivo. We suggest that DNA fragments released from leukemic and also perhaps other types of tumor cells can activate DNA repair mechanisms or death in recipient cells of a tumor microenvironment, depending on the amount of the acquired DNA. This can impair DNA stability and viability of tumor stromal cells, undermine homeostatic capacity of tumor microenvironment and facilitate tumor progression.

U-2932: two clones in one cell line, a tool for the study of clonal evolution

Genetic heterogeneity is common in tumors, explicable by the development of subclones with distinct genetic and epigenetic alterations. We describe an in vitro model for cancer heterogeneity, comprising the diffuse large B-cell lymphoma cell line U-2932 which expresses two sets of cell surface markers representing twin populations flow-sorted by CD20 vs CD38 expression. U-2932 populations were traced to subclones of the original tumor with clone-specific immunoglobulin IgVH4-39 hypermutation patterns. BCL6 was overexpressed in one subpopulation (R1), MYC in the other (R2), both clones overexpressed BCL2. According to the combined results of immunoglobulin hypermutation and cytogenetic analysis, R1 and R2 derive from a mother clone with genomic BCL2 amplification, which acquired secondary rearrangements leading to the overexpression of BCL6 (R1) or MYC (R2). Some 200 genes were differentially expressed in R1/R2 microarrays including transcriptional targets of the aberrantly expressed oncogenes. Other genes were regulated by epigenetic means as shown by DNA methylation analysis. Ectopic expression of BCL6 in R2 variously modulated new candidate target genes, confirming dual silencing and activating functions. In summary, stable retention of genetically distinct subclones in U-2932 models tumor heterogeneity in vitro permitting functional analysis of oncogenes against a syngenic background.

miR-155 targets histone deacetylase 4 (HDAC4) and impairs transcriptional activity of B-cell lymphoma 6 (BCL6) in the Eμ-miR-155 transgenic mouse model

Multiple studies have established that microRNAs (miRNAs) are involved in the initiation and progression of cancer. Notably, miR-155 is one of the most overexpressed miRNAs in several solid and hematological malignancies. Ectopic miR-155 expression in mice B cells (Eμ-miR-155 transgenic mice) has been shown to induce pre-B-cell proliferation followed by high-grade lymphoma/leukemia. Loss of miR-155 in mice resulted in impaired immunity due to defective T-cell-mediated immune response. Here we provide a mechanistic insight into miR-155-induced leukemogenesis in the Eμ-miR-155 mouse model through genome-wide transcriptome analysis of naïve B cells and target studies. We found that a key transcriptional repressor and proto-oncogene, Bcl6 is significantly down-regulated in Eμ-miR-155 mice. The reduction of Bcl6 subsequently leads to de-repression of some of the known Bcl6 targets like inhibitor of differentiation (Id2), interleukin-6 (IL6), cMyc, Cyclin D1, and Mip1α/ccl3, all of which promote cell survival and proliferation. We show that Bcl6 is indirectly regulated by miR-155 through Mxd1/Mad1 up-regulation. Interestingly, we found that miR-155 directly targets HDAC4, a corepressor partner of BCL6. Furthermore, ectopic expression of HDAC4 in human-activated B-cell-type diffuse large B-cell lymphoma (DLBCL) cells results in reduced miR-155-induced proliferation, clonogenic potential, and increased apoptosis. Meta-analysis of the diffuse large B-cell lymphoma patient microarray data showed that miR-155 expression is inversely correlated with Bcl6 and Hdac4. Hence this study provides a better understanding of how miR-155 causes disruption of the BCL6 transcriptional machinery that leads to up-regulation of the survival and proliferation genes in miR-155-induced leukemias.

The lupus-prone NZM2410/NZW strain-derived Sle1b sublocus alters the germinal center checkpoint in female mice in a B cell-intrinsic manner

C57BL/6 (B6) mice carrying the Sle1b sublocus (named B6.Sle1b), which harbors the lupus-associated NZM2410/NZW SLAM family genes, produce antinuclear Abs (ANAs). However, the role and mechanism(s) involved in the alteration of the germinal center (GC) tolerance checkpoint in the development of ANAs in these mice is not defined. In this study, we show significantly higher spontaneously formed GCs (Spt-GCs) in B6.Sle1b female mice compared with B6 controls. We also found a significant increase in CD4(+)CXCR5(hi)PD-1(hi) spontaneously activated follicular Th cells in B6.Sle1b female mice. Compared with B6 controls, B6.Sle1b female mice had increased numbers of proliferating B cells predominantly located in Spt-GCs. The elevated Spt-GCs in B6.Sle1b female mice were strongly associated with increased ANA-specific Ab-forming cells and ANA titers. The increased numbers of Spt-GCs and spontaneously activated follicular Th cells in B6.Sle1b mice were not the result of a generalized defect in B cells expressing Sle1b. Consistent with the elevated spontaneous response in B6.Sle1b mice, the attenuated GC response characteristic of DNA and p-azophenylarsonate reactive B cells from Ig V(H) knock-in mice (termed HKIR) were relieved in adoptively transferred recipients in the presence of Sle1b. Finally, by generating mixed bone marrow chimeras, we showed that the effect of Sle1b on Spt-GC, follicular Th cell, and autoantibody responses in B6.Sle1b mice was B cell autonomous. These data indicate that the NZM2410/NZW-derived Sle1b sublocus in conjunction with the female sex primarily affects B cells, leading to the alteration of the GC tolerance checkpoint and the generation of ANA-specific Ab-forming cells.

BTB-ZF factors recruit the E3 ligase cullin 3 to regulate lymphoid effector programs

The differentiation of several T and B cell effector programs in the immune system is directed by signature transcription factors that induce rapid epigenetic remodeling. We report that PLZF, the BTB-ZF transcription factor directing the innate-like effector program of NKT thymocytes ^1,2 was prominently associated with cullin 3 (Cul3), an E3 ubiquitin ligase previously shown to use BTB domain-containing proteins as adaptors for substrate binding ^3–7. PLZF transported Cul3 to the nucleus where the two proteins were associated within a chromatin modifying complex. Furthermore, PLZF expression resulted in selective changes of ubiquitination of multiple components of this complex. Cul3 was also found associated with another BTB-ZF transcription factor, Bcl6, which directs the B cell germinal center and the T follicular helper programs. Conditional deletion in mice demonstrated an essential role of Cul3 for the development of PLZF- and Bcl6-dependent lineages. We conclude that distinct lineage-specific BTB-ZF transcription factors recruit Cul3 to alter the ubiquitination pattern of their associated chromatin modifying complex. We propose that this novel function is essential to direct the differentiation of several T and B lymphocyte effector programs, and may also be involved in the oncogenic role of PLZF and Bcl6 in leukemias and lymphomas ^8,9.

Master regulators or lineage-specifying? Changing views on CD4+ T cell transcription factors

There is an emerging body of research demonstrating that the co-expression of key lineage-specifying transcription factors, commonly referred to as 'master regulators', affects the functional capabilities and flexibility of CD4(+) T cell subsets. Here, we discuss how the natural co-expression of these lineage-specifying transcription factors has challenged the concept that the expression of a single 'master regulator' strictly establishes an absolute CD4(+) T cell phenotype. Instead, it is becoming clear that the interplay between the lineage-specifying (or lineage-defining) transcription factors, including T-bet, GATA3, RORγt, BCL-6 and FOXP3, contributes to the fate and flexibility of CD4(+) T cell subtypes. This in turn has led to the realization that CD4(+) T cell phenotypes are more diverse than previously recognized.

Distinct cellular pathways select germline-encoded and somatically mutated antibodies into immunological memory

B cell memory is generated along two fundamentally distinct cellular differentiation pathways.

One component of memory in the antibody system is long-lived memory B cells selected for the expression of somatically mutated, high-affinity antibodies in the T cell–dependent germinal center (GC) reaction. A puzzling observation has been that the memory B cell compartment also contains cells expressing unmutated, low-affinity antibodies. Using conditional Bcl6 ablation, we demonstrate that these cells are generated through proliferative expansion early after immunization in a T cell–dependent but GC-independent manner. They soon become resting and long-lived and display a novel distinct gene expression signature which distinguishes memory B cells from other classes of B cells. GC-independent memory B cells are later joined by somatically mutated GC descendants at roughly equal proportions and these two types of memory cells efficiently generate adoptive secondary antibody responses. Deletion of T follicular helper (Tfh) cells significantly reduces the generation of mutated, but not unmutated, memory cells early on in the response. Thus, B cell memory is generated along two fundamentally distinct cellular differentiation pathways. One pathway is dedicated to the generation of high-affinity somatic antibody mutants, whereas the other preserves germ line antibody specificities and may prepare the organism for rapid responses to antigenic variants of the invading pathogen.