The B cell antigen receptor and overexpression of MYC can cooperate in the genesis of B cell lymphomas

Ibrutinib Plus RCHOP versus RCHOP Only in Young Patients with Activated B-Cell-like Diffuse Large B-Cell Lymphoma (ABC-DLBCL): A Cost-Effectiveness Analysis

The standard treatment for Diffuse Large B-Cell Lymphoma (DLBCL) is rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (RCHOP). However, many patients require subsequent treatment after relapsed disease. The ABC subtype of DLBCL (ABC-DLBCL) has a worse prognosis, and the PHOENIX trial explored adding ibrutinib to RCHOP for this patient population. The trial showed favorable outcomes for younger patients, and our study aimed to inform clinical decision-making via a cost-effectiveness model to compare RCHOP with and without ibrutinib (I-RCHOP). A Markov decision analysis model was designed to compare the treatments for patients younger than 60 years with ABC-DLBCL. The model considered treatment pathways, adverse events, relapses, and death, incorporating data on salvage treatments and novel therapies. The results indicated that I-RCHOP was more cost-effective, with greater quality-adjusted life years (QALY, 15.48 years vs. 14.25 years) and an incremental cost-effectiveness ratio (ICER) of CAD 34,111.45/QALY compared to RCHOP only. Sensitivity analyses confirmed the model's robustness. Considering the high market price for ibrutinib, I-RCHOP may be more costly. However, it is suggested as the preferred cost-effective strategy for younger patients due to its benefits in adverse events, overall survival, and quality of life. The decision analytic model provided relevant and robust results to inform clinical decision-making.

Human CD19-specific switchable CAR T-cells are efficacious as constitutively active CAR T-cells but cause less morbidity in a mouse model of human CD19+ malignancy

Current Food and Drug Administration (FDA)-approved CD19-specific chimeric antigen receptor (CAR) T-cell therapies for B-cell malignancies are constitutively active and while efficacious, can cause morbidity and mortality. Their toxicities might be reduced if CAR T-cell activity was regulatable rather than constitutive. To test this, we compared the efficacies and morbidities of constitutively active (conventional) and regulatable (switchable) CAR (sCAR) T-cells specific for human CD19 (huCD19) in an immune-competent huCD19+ transgenic mouse model.Conventional CAR (CAR19) and sCAR T-cells were generated by retrovirally transducing C57BL/6 (B6) congenic T-cells with constructs encoding antibody-derived single chain Fv (sFv) fragments specific for huCD19 or a peptide neoepitope (PNE), respectively. Transduced T-cells were adoptively transferred into huCD19 transgenic hemizygous (huCD19Tg/0 ) B6 mice; healthy B-cells in these mice expressed huCD19Tg Prior to transfer, recipients were treated with a lymphodepleting dose of cyclophosphamide to enhance T-cell engraftment. In tumor therapy experiments, CAR19 or sCAR T-cells were adoptively transferred into huCD19Tg/0 mice bearing a syngeneic B-cell lymphoma engineered to express huCD19. To regulate sCAR T cell function, a switch protein was generated that contained the sCAR-specific PNE genetically fused to an anti-huCD19 Fab fragment. Recipients of sCAR T-cells were injected with the switch to link sCAR effector with huCD19+ target cells. Mice were monitored for survival, tumor burden (where appropriate), morbidity (as measured by weight loss and clinical scores), and peripheral blood lymphocyte frequency.CAR19 and sCAR T-cells functioned comparably regarding in vivo expansion and B-cell depletion. However, sCAR T-cells were better tolerated as evidenced by the recipients' enhanced survival, reduced weight loss, and improved clinical scores. Discontinuing switch administration allowed healthy B-cell frequencies to return to pretreatment levels.In our mouse model, sCAR T-cells killed huCD19+ healthy and malignant B-cells and were better tolerated than CAR19 cells. Our data suggest sCAR might be clinically superior to the current FDA-approved therapies for B-cell lymphomas due to the reduced acute and chronic morbidities and mortality, lower incidence and severity of side effects, and B-cell reconstitution on cessation of switch administration.

LncRNA FIRRE stimulates PTBP1-induced Smurf2 decay, stabilizes B-cell receptor, and promotes the development of diffuse large B-cell lymphoma

Sustained expression of B-cell receptor (BCR) critically contributes to the development of diffuse large B-cell lymphoma (DLBCL). However, little is known on the mechanism regulating BCR expression. In the present study, we explored the biological significance of functional intergenic repeating RNA element (FIRRE) in DLBCL and its regulation on BCR. Functional impacts of FIRRE on cell viability, transformation, and apoptosis were examined by MTT, colony formation, and flow cytometry, respectively. The interaction between FIRRE and polypyrimidine tract binding protein 1 (PTBP1) was identified by RNA pull-down and verified using RNA immunoprecipitation assays. The effects of FIRRE and PTBP1 on Smurf2 mRNA were examined by RNA immunoprecipitation, RNA pull-down, and mRNA stability assays. Smurf2-mediated BCR ubiquitination was investigated using co-immunoprecipitation, ubiquitination, and protein stability assays. In vivo, xenograft models were used to assess the impacts of targeting FIRRE on DLBCL growth. FIRRE was specifically up-regulated in and essentially maintained multiple malignant behaviors of BCR-dependent DLBCL cells. Through the interaction with PTBP1, FIRRE promoted the mRNA decay of Smurf2, a ubiquitin ligase for the degradation BCR protein. Targeting FIRRE was sufficient to regulating Smurf2 and BCR expressions and inhibit DLBCL malignancy both in vivo and in vitro. FIRRE-PTBP1 interaction, by simulating Smurf2 mRNA decay and stabilizing BCR, promotes the development of DLBCL. Consequently, targeting this signaling mechanism may provide therapeutic benefits for DLBCL. This article is protected by copyright. All rights reserved.

MYD88L265P and CD79B double mutations type (MCD type) of diffuse large B-cell lymphoma: mechanism, clinical characteristics, and targeted therapy

MYD88/CD79B-mutated (MCD) genotype is a genetic subgroup of diffuse large B-cell lymphoma (DLBCL) with the co-occurrence of MYD88^L265P and CD79B mutations. MCD genotype is characterized by poor prognosis and extranodal involvement especially in immune-privileged sites. MCD model is dominated by activated B-cell (ABC)-like subtype of DLBCLs. It is generally accepted that the pathogenesis of MCD DLBCL mainly includes chronic active B-cell receptor (BCR) signaling and oncogenic MYD88 mutations, which drives pathological nuclear factor kappa B (NF-κB) activation in MCD lymphoid malignancies. CD79B and MYD88^L265P mutations are frequently and contemporaneously founded in B-cell malignancies. The collaboration of the two mutations may explain the unique biology of MCD. Meanwhile, standard immunochemotherapy combine with different targeted therapies worth further study to improve the prognosis of MCD, according to genetic, phenotypic, and clinical features of MCD type. In this review, we systematically described mechanism, clinical characteristics, and targeted therapy of MCD DLBCL.

SHMT2 inhibition disrupts the TCF3 transcriptional survival program in Burkitt lymphoma

Burkitt lymphoma (BL) is an aggressive lymphoma type that is currently treated by intensive chemoimmunotherapy. Despite the favorable clinical outcome for most patients with BL, chemotherapy-related toxicity and disease relapse remain major clinical challenges, emphasizing the need for innovative therapies. Using genome-scale CRISPR-Cas9 screens, we identified B-cell receptor (BCR) signaling, specific transcriptional regulators, and one-carbon metabolism as vulnerabilities in BL. We focused on serine hydroxymethyltransferase 2 (SHMT2), a key enzyme in one-carbon metabolism. Inhibition of SHMT2 by either knockdown or pharmacological compounds induced anti-BL effects in vitro and in vivo. Mechanistically, SHMT2 inhibition led to a significant reduction of intracellular glycine and formate levels, which inhibited the mTOR pathway and thereby triggered autophagic degradation of the oncogenic transcription factor TCF3. Consequently, this led to a collapse of tonic BCR signaling, which is controlled by TCF3 and is essential for BL cell survival. In terms of clinical translation, we also identified drugs such as methotrexate that synergized with SHMT inhibitors. Overall, our study has uncovered the dependency landscape in BL, identified and validated SHMT2 as a drug target, and revealed a mechanistic link between SHMT2 and the transcriptional master regulator TCF3, opening up new perspectives for innovative therapies.

Forward and Reverse Genetics of B Cell Malignancies: From Insertional Mutagenesis to CRISPR-Cas

Cancer genome sequencing has identified dozens of mutations with a putative role in lymphomagenesis and leukemogenesis. Validation of driver mutations responsible for B cell neoplasms is complicated by the volume of mutations worthy of investigation and by the complex ways that multiple mutations arising from different stages of B cell development can cooperate. Forward and reverse genetic strategies in mice can provide complementary validation of human driver genes and in some cases comparative genomics of these models with human tumors has directed the identification of new drivers in human malignancies. We review a collection of forward genetic screens performed using insertional mutagenesis, chemical mutagenesis and exome sequencing and discuss how the high coverage of subclonal mutations in insertional mutagenesis screens can identify cooperating mutations at rates not possible using human tumor genomes. We also compare a set of independently conducted screens from Pax5 mutant mice that converge upon a common set of mutations observed in human acute lymphoblastic leukemia (ALL). We also discuss reverse genetic models and screens that use CRISPR-Cas, ORFs and shRNAs to provide high throughput in vivo proof of oncogenic function, with an emphasis on models using adoptive transfer of ex vivo cultured cells. Finally, we summarize mouse models that offer temporal regulation of candidate genes in an in vivo setting to demonstrate the potential of their encoded proteins as therapeutic targets.

Reframing Burkitt lymphoma: virology not epidemiology defines clinical variants

In 1964, Epstein-Barr virus (EBV) was identified in a biopsy from a patient with Burkitt lymphoma (BL) launching a new field of study into this ubiquitous human virus. Almost 60 years later, insights into the role of EBV in lymphomagenesis are still emerging. While all BL carry the hallmark c-myc translocation, the epidemiologic classification of BL (e.g., endemic, sporadic or immunodeficiency-associated) has traditionally been used to define BL clinical variants. However, recent studies using molecular methods to characterize the transcriptional and genetic landscape of BL have identified several unique features are observed that distinguish EBV+ BL including a high level of activation induced deaminase mutation load, evidence of antigen selection in the B cell receptor, and a decreased mutation frequency of TCF3/ID3, all found predominantly in EBV+ compared to EBV- BL. In this review, the focus will be on summarizing recent studies that have done in depth characterization of genetic and transcriptional profiles of BL, describing the differences and similarities of EBV+ and EBV- BL, and what they reveal about the etiology of BL. The new studies put forth a compelling argument that the association with EBV should be the defining etiologic feature of clinical variants of BL. This reframing of BL has important implications for therapeutic interventions for BL that distinguish the EBV+ from the EBV- lymphomas.

Repurposing a novel anti-cancer RXR agonist to attenuate murine acute GVHD and maintain graft-versus-leukemia responses

The nuclear receptor (NR) subclass, retinoid X receptors (RXRs), exert immunomodulatory functions that control inflammation and metabolism via homodimers and heterodimers, with several other NRs, including retinoic acid receptors. IRX4204 is a novel, highly specific RXR agonist in clinical trials that potently and selectively activates RXR homodimers, but not heterodimers. In this study, in vivo IRX4204 compared favorably with FK506 in abrogating acute graft-versus-host disease (GVHD), which was associated with inhibiting allogeneic donor T-cell proliferation, reducing T-helper 1 differentiation, and promoting regulatory T-cell (Treg) generation. Recipient IRX4204 treatment reduced intestinal injury and decreased IFN-γ and TNF-α serum levels. Transcriptional analysis of donor T cells isolated from intestines of GVHD mice treated with IRX4204 revealed significant decreases in transcripts regulating proinflammatory pathways. In vitro, inducible Treg differentiation from naive CD4+ T cells was enhanced by IRX4204. In vivo, IRX4204 increased the conversion of donor Foxp3- T cells into peripheral Foxp3+ Tregs in GVHD mice. Using Foxp3 lineage-tracer mice in which both the origin and current FoxP3 expression of Tregs can be tracked, we demonstrated that IRX4204 supports Treg stability. Despite favoring Tregs and reducing Th1 differentiation, IRX4204-treated recipients maintained graft-versus-leukemia responses against both leukemia and lymphoma cells. Notably, IRX4204 reduced in vitro human T-cell proliferation and enhanced Treg generation in mixed lymphocyte reaction cultures. Collectively, these beneficial effects indicate that targeting RXRs with IRX4204 could be a novel approach to preventing acute GVHD in the clinic.

The Eµ-hnRNP K Murine Model of Lymphoma: Novel Insights into the Role of hnRNP K in B-Cell Malignancies

B-cell lymphomas are one of the most biologically and molecularly heterogeneous group of malignancies. The inherent complexity of this cancer subtype necessitates the development of appropriate animal model systems to characterize the disease with the ultimate objective of identifying effective therapies. In this article, we discuss a new driver of B-cell lymphomas - hnRNP K (heterogenous nuclear ribonucleoprotein K)-an RNA-binding protein. We introduce the Eµ-Hnrnpk mouse model, a murine model characterized by hnRNP K overexpression in B cells, which develops B-cell lymphomas with high penetrance. Molecular analysis of the disease developed in this model reveals an upregulation of the c-Myc oncogene via post-transcriptional and translational mechanisms underscoring the impact of non-genomic MYC activation in B-cell lymphomas. Finally, the transplantability of the disease developed in Eµ-Hnrnpk mice makes it a valuable pre-clinical platform for the assessment of novel therapeutics.

Pathogenic B-cell receptor signaling in lymphoid malignancies: New insights to improve treatment

Signals emanating from the B-cell receptor (BCR) promote proliferation and survival in diverse forms of B-cell lymphoma. Precision medicine strategies targeting the BCR pathway have been generally effective in treating lymphoma, but often fail to produce durable responses in diffuse large B-cell lymphoma (DLBCL), a common and aggressive cancer. New insights into DLBCL biology garnered from genomic analyses and functional proteogenomic studies have identified novel modes of BCR signaling in this disease. Herein, we describe the distinct roles of antigen-dependent and antigen-independent BCR signaling in different subtypes of DLBCL. We highlight mechanisms by which the BCR cooperates with TLR9 and mutant isoforms of MYD88 to drive sustained NF-κB activity in the activated B-cell-like (ABC) subtype of DLBCL. Finally, we discuss progress in detecting and targeting oncogenic BCR signaling to improve the survival of patients with lymphoma.

JNK/AP1 Pathway Regulates MYC Expression and BCR Signaling through Ig Enhancers in Burkitt Lymphoma Cells

In Burkitt lymphoma (BL), a chromosomal translocation by which the MYC gene is fused to an immunoglobulin (Ig) gene locus is frequently found. The translocated MYC gene is overexpressed, which is the major driver of BL tumorigenesis. Studies have shown that Ig enhancers are essential for MYC overexpression, but the involved mechanisms are not fully understood. In addition, the survival of BL cells relies on B-cell receptor (BCR) signaling, which is determined by the levels of Ig molecules expressed on the cell surface. However, whether MYC has any impact on Ig expression and its functional relevance in BL has not been investigated. Herein, we show that MYC upregulates Ig kappa (Igκ) expression in BL cells through two Igκ enhancers, the intronic enhancer (Ei) and the 3ʹ enhancer (E3ʹ). Mechanistically, by activating the JNK pathway, MYC induces the phosphorylation of c-Fos/c-Jun and their recruitment to AP1 binding sites in the Igκ enhancers, leading to the activation of the enhancers and subsequent Igκ upregulation. The AP1-mediated activation of the Igκ enhancers is also required for the expression of the translocated MYC gene, indicating positive feedback for the MYC overexpression in BL cells. Importantly, interrupting the JNK pathway inhibits both Igκ and MYC gene expression and suppresses BL cell proliferation. Our study not only reveals a novel mechanism underlying MYC overexpression in BL but also suggests that targeting the JNK pathway may provide a unique strategy to suppress BL tumorigenesis.

Human CD19-Targeted Mouse T Cells Induce B Cell Aplasia and Toxicity in Human CD19 Transgenic Mice

The clinical success of chimeric antigen receptor (CAR) T cell therapy for CD19+ B cell malignancies can be limited by acute toxicities and immunoglobulin replacement needs due to B cell aplasia from persistent CAR T cells. Life-threatening complications include cytokine release syndrome and neurologic adverse events, the exact etiologies of which are unclear. To elucidate the underlying toxicity mechanisms and test potentially safer CAR T cells, we developed a mouse model in which human CD19 (hCD19)-specific mouse CAR T cells were adoptively transferred into mice whose normal B cells express a hCD19 transgene at hemizygous levels. Compared to homozygous hCD19 transgenic mice that have ∼75% fewer circulating B cells, hemizygous mice had hCD19 frequencies and antigen density more closely simulating human B cells. Hemizygous mice given a lethal dose of hCD19 transgene-expressing lymphoma cells and treated with CAR T cells had undetectable tumor levels. Recipients experienced B cell aplasia and antigen- and dose-dependent acute toxicities mirroring patient complications. Interleukin-6 (IL-6), interferon γ (IFN-γ), and inflammatory pathway transcripts were enriched in affected tissues. As in patients, antibody-mediated neutralization of IL-6 (and IFN-γ) blunted toxicity. Apparent behavioral abnormalities associated with decreased microglial cells point to CAR-T-cell-induced neurotoxicity. This model will prove useful in testing strategies designed to improve hCD19-specific CAR T cell safety.

Genes and pathways associated with the occurrence of malignancy in benign lymphoepithelial lesions

There is increasing evidence concerning the occurrence of malignant lymphoma among people suffering from Mikulicz disease, also termed benign lymphoepithelial lesion (BLEL) and immunoglobulin G4-associated disease. However, the underlying molecular mechanism of the malignant transformation remains unclear. The present study aimed to investigate the gene expression profile between BLEL and malignant lymphoepithelial lesion (MLEL) conditions using tissue microarray analysis, to identify genes and pathways which may be associated with the risk of malignant transformation. Comparing gene expression profiles between BLEL tissues (n=13) and MLEL (n=14), a total of 1,002 differentially expressed genes (DEGs) were identified including 364 downregulated and 638 upregulated DEGs in BLEL. The downregulated DEGs in BLEL were frequently associated with immune-based functions, immune cell differentiation, proliferation and survival, and metabolic functions, whereas the upregulated DEGs were primarily associated with organ, gland and tissue developmental processes. The B cell receptor signaling pathway, the transcription factor p65 signaling pathway, low affinity immunoglobulin γ Fc region receptor II-mediated phagocytosis, the high affinity immunoglobulin ε receptor subunit γ signaling pathway and Epstein-Barr virus infection, and pathways in cancer, were the pathways associated with the downregulated DEGs. The upregulated DEGs were associated with three pathways, including glutathione metabolism, salivary secretion and mineral absorption pathways. These results suggested that the identified signaling pathways and their associated genes may be crucial for understanding the molecular mechanisms underlying malignant transformation from BLEL, and they may be considered to be markers for predicting malignancy among the BLEL group.

EBV latent membrane protein 2A orchestrates p27kip1 degradation via Cks1 to accelerate MYC-driven lymphoma in mice

Epstein-Barr virus (EBV) establishes lifelong infection in B lymphocytes of most human hosts and is associated with several B lymphomas. During latent infection, EBV encodes latent membrane protein 2A (LMP2A) to promote the survival of B cells by mimicking host B-cell receptor signaling. By studying the roles of LMP2A during lymphoma development in vivo, we found that LMP2A mediates rapid MYC-driven lymphoma onset by allowing B cells to bypass MYC-induced apoptosis mediated by the p53 pathway in our transgenic mouse model. However, the mechanisms used by LMP2A to facilitate transformation remain elusive. In this study, we demonstrate a key role of LMP2A in promoting hyperproliferation of B cells by enhancing MYC expression and MYC-dependent degradation of the p27^kip1 tumor suppressor. Loss of the adaptor protein cyclin-dependent kinase regulatory subunit 1 (Cks1), a cofactor of the SCF^Skp2 ubiquitin ligase complex and a downstream target of MYC, increases p27^kip1 expression during a premalignant stage. In mice that express LMP2A, Cks1 deficiency reduces spleen weights, restores B-cell follicle formation, impedes cell cycle progression of pretumor B cells, and eventually prolongs MYC-driven tumor onset. This study demonstrates that LMP2A uses the role of MYC in the cell cycle, particularly in the p27^kip1 degradation process, to accelerate lymphomagenesis in vivo. Thus, our results reveal a novel mechanism of EBV in diverting the functions of MYC in malignant transformation and provide a rationale for targeting EBV's roles in cell cycle modulation.

Functional imaging in combination with mutation status aids prediction of response to inhibiting B-cell receptor signaling in lymphoma

Aberrant B-cell receptor (BCR) signaling is known to contribute to malignant transformation. Two small molecule inhibitors targeting BCR pathway signaling include ibrutinib, a Bruton’s tyrosine kinase (BTK) inhibitor, and idelalisib, a specific Phosphatidylinositol-4,5-bisphosphate 3-kinase delta (PI3Kδ) inhibitor, both of which have been approved for use in haematological malignancies. Despite the identification of various diffuse large B-cell lymphoma (DLBCL) subtypes, mutation status alone is not sufficient to predict patient response and therapeutic resistance can arise. Herein we apply early molecular imaging across alternative activated B-cell (ABC) and germinal center B-cell (GCB) DLBCL subtypes to investigate the effects of BCR pathway inhibition. Treatment with both inhibitors adversely affected cell growth and viability. These effects were partially predictable based upon mutation status. Accordingly, very early 2-deoxy-2-[18F]fluoro-D-glucose positron emission tomography (¹⁸F-FDG-PET) and 3’-deoxy-3’[18F]-fluorothymidine positron emission tomography (¹⁸F-FLT-PET) reported tumour regression and reductions in tumour metabolism and proliferation upon treatment. Furthermore, matrix-assisted laser desorption ionization imaging mass spectrometry (MALDI IMS) identified alterations in the proteome of a model of ABC DLBCL upon treatment with ibrutinib or idelalisib. In conclusion we demonstrate that very early molecular imaging adds predictive value in addition to mutational status of DLBCL that may be useful in directing patient therapy.

Identification of 11(13)-dehydroivaxillin as a potent therapeutic agent against non-Hodgkin's lymphoma

Despite great advancements in the treatment of non-Hodgkin lymphoma (NHL), sensitivity of different subtypes to therapy varies. Targeting the aberrant activation NF-κB signaling pathways in lymphoid malignancies is a promising strategy. Here, we report that 11(13)-dehydroivaxillin (DHI), a natural compound isolated from the Carpesium genus, induces growth inhibition and apoptosis of NHL cells. Multiple signaling cascades are influenced by DHI in NHL cells. PI3K/AKT and ERK are activated or inhibited in a cell type dependent manner, whereas NF-κB signaling pathway was inhibited in all the NHL cells tested. Applying the cellular thermal shift assay, we further demonstrated that DHI directly interacts with IKKα/IKKβ in NHL cells. Interestingly, DHI treatment also reduced the IKKα/IKKβ protein level in NHL cells. Consistent with this finding, knockdown of IKKα/IKKβ inhibits cell proliferation and enhances DHI-induced proliferation inhibition. Overexpression of p65, p52 or RelB partially reverses DHI-induced cell growth inhibition. Furthermore, DHI treatment significantly inhibits the growth of NHL cell xenografts. In conclusion, we demonstrate that DHI exerts anti-NHL effect in vitro and in vivo, through a cumulative effect on NF-κB and other pathways. DHI may serve as a promising lead compound for the therapy of NHL.

Tonic B-cell receptor signaling in diffuse large B-cell lymphoma

We used clustered regularly interspaced short palindromic repeats/Cas9-mediated genomic modification to investigate B-cell receptor (BCR) signaling in cell lines of diffuse large B-cell lymphoma (DLBCL). Three manipulations that altered BCR genes without affecting surface BCR levels showed that BCR signaling differs between the germinal center B-cell (GCB) subtype, which is insensitive to Bruton tyrosine kinase inhibition by ibrutinib, and the activated B-cell (ABC) subtype. Replacing antigen-binding BCR regions had no effect on BCR signaling in GCB-DLBCL lines, reflecting this subtype's exclusive use of tonic BCR signaling. Conversely, Y188F mutation in the immunoreceptor tyrosine-based activation motif of CD79A inhibited tonic BCR signaling in GCB-DLBCL lines but did not affect their calcium flux after BCR cross-linking or the proliferation of otherwise-unmodified ABC-DLBCL lines. CD79A-GFP fusion showed BCR clustering or diffuse distribution, respectively, in lines of ABC and GCB subtypes. Tonic BCR signaling acts principally to activate AKT, and forced activation of AKT rescued GCB-DLBCL lines from knockout (KO) of the BCR or 2 mediators of tonic BCR signaling, SYK and CD19. The magnitude and importance of tonic BCR signaling to proliferation and size of GCB-DLBCL lines, shown by the effect of BCR KO, was highly variable; in contrast, pan-AKT KO was uniformly toxic. This discrepancy was explained by finding that BCR KO-induced changes in AKT activity (measured by gene expression, CXCR4 level, and a fluorescent reporter) correlated with changes in proliferation and with baseline BCR surface density. PTEN protein expression and BCR surface density may influence clinical response to therapeutic inhibition of tonic BCR signaling in DLBCL.

The B-cell receptor controls fitness of MYC-driven lymphoma cells via GSK3β inhibition

Similar to resting mature B cells, where the B-cell antigen receptor (BCR) controls cellular survival, surface BCR expression is conserved in most mature B-cell lymphomas. The identification of activating BCR mutations and the growth disadvantage upon BCR knockdown of cells of certain lymphoma entities has led to the view that BCR signalling is required for tumour cell survival. Consequently, the BCR signalling machinery has become an established target in the therapy of B-cell malignancies. Here we study the effects of BCR ablation on MYC-driven mouse B-cell lymphomas and compare them with observations in human Burkitt lymphoma. Whereas BCR ablation does not, per se, significantly affect lymphoma growth, BCR-negative (BCR^-) tumour cells rapidly disappear in the presence of their BCR-expressing (BCR⁺) counterparts in vitro and in vivo. This requires neither cellular contact nor factors released by BCR⁺ tumour cells. Instead, BCR loss induces the rewiring of central carbon metabolism, increasing the sensitivity of receptor-less lymphoma cells to nutrient restriction. The BCR attenuates glycogen synthase kinase 3 beta (GSK3β) activity to support MYC-controlled gene expression. BCR^- tumour cells exhibit increased GSK3β activity and are rescued from their competitive growth disadvantage by GSK3β inhibition. BCR^- lymphoma variants that restore competitive fitness normalize GSK3β activity after constitutive activation of the MAPK pathway, commonly through Ras mutations. Similarly, in Burkitt lymphoma, activating RAS mutations may propagate immunoglobulin-crippled tumour cells, which usually represent a minority of the tumour bulk. Thus, while BCR expression enhances lymphoma cell fitness, BCR-targeted therapies may profit from combinations with drugs targeting BCR^- tumour cells.

Myc enhances B-cell receptor signaling in precancerous B cells and confers resistance to Btk inhibition

Dysregulation of the oncogenic transcription factor MYC induces B cell transformation and is a driver for B cell non-Hodgkin lymphoma (B-NHL). MYC overexpression in B-NHL is associated with more aggressive phenotypes and poor prognosis. Although genomic studies suggest a link between MYC overexpression and B cell receptor (BCR) signaling molecules in B-NHL, signaling pathways essential to Myc-mediated B-cell transformation have not been fully elucidated. We utilized intracellular phospho-flow cytometry to investigate the relationship between Myc and BCR signaling in pre-malignant B cells. Utilizing the Eμ-myc mouse model, where Myc is overexpressed specifically in B cells, both basal and stimulated BCR signaling were increased in precancerous B lymphocytes from Eμ-myc mice compared to wild-type littermates. B cells overexpressing Myc displayed constitutively higher levels of activated CD79α, Btk, Plcγ2, and Erk1/2. Notably, Myc overexpressing B cells maintained elevated BCR signaling despite treatment with ibrutinib, a Bruton’s tyrosine kinase inhibitor. Furthermore, PI3K/Akt pathway signaling was also increased in Eμ-myc B cells, and this increase was partially suppressed with ibrutinib. Additionally, experiments with Btk-null B cells revealed off-target effects of ibrutinib on BCR signaling. Our data show that in pre-malignant B cells, Myc overexpression is sufficient to activate BCR and PI3K/Akt signaling pathways and further enhances signaling following BCR ligation. Therefore, our results indicate precancerous B cells have already acquired enhanced survival and growth capabilities prior to transformation, and that elevated MYC levels confer resistance to pharmacologic inhibitors of BCR signaling, which has significant implications for B-NHL treatment.

Elucidation of tonic and activated B-cell receptor signaling in Burkitt's lymphoma provides insights into regulation of cell survival

Burkitt's lymphoma (BL) is a highly proliferative B-cell neoplasm and is treated with intensive chemotherapy that, because of its toxicity, is often not suitable for the elderly or for patients with endemic BL in developing countries. BL cell survival relies on signals transduced by B-cell antigen receptors (BCRs). However, tonic as well as activated BCR signaling networks and their relevance for targeted therapies in BL remain elusive. We have systematically characterized and compared tonic and activated BCR signaling in BL by quantitative phosphoproteomics to identify novel BCR effectors and potential drug targets. We identified and quantified ∼16,000 phospho-sites in BL cells. Among these sites, 909 were related to tonic BCR signaling, whereas 984 phospho-sites were regulated upon BCR engagement. The majority of the identified BCR signaling effectors have not been described in the context of B cells or lymphomas yet. Most of these newly identified BCR effectors are predicted to be involved in the regulation of kinases, transcription, and cytoskeleton dynamics. Although tonic and activated BCR signaling shared a considerable number of effector proteins, we identified distinct phosphorylation events in tonic BCR signaling. We investigated the functional relevance of some newly identified BCR effectors and show that ACTN4 and ARFGEF2, which have been described as regulators of membrane-trafficking and cytoskeleton-related processes, respectively, are crucial for BL cell survival. Thus, this study provides a comprehensive dataset for tonic and activated BCR signaling and identifies effector proteins that may be relevant for BL cell survival and thus may help to develop new BL treatments.

Pathogenic role of B-cell receptor signaling and canonical NF-κB activation in mantle cell lymphoma

To interrogate signaling pathways activated in mantle cell lymphoma (MCL) in vivo, we contrasted gene expression profiles of 55 tumor samples isolated from blood and lymph nodes from 43 previously untreated patients with active disease. In addition to lymph nodes, MCL often involves blood, bone marrow, and spleen and is incurable for most patients. Recently, the Bruton tyrosine kinase (BTK) inhibitor ibrutinib demonstrated important clinical activity in MCL. However, the role of specific signaling pathways in the lymphomagenesis of MCL and the biologic basis for ibrutinib sensitivity of these tumors are unknown. Here, we demonstrate activation of B-cell receptor (BCR) and canonical NF-κB signaling specifically in MCL cells in the lymph node. Quantification of BCR signaling strength, reflected in the expression of BCR regulated genes, identified a subset of patients with inferior survival after cytotoxic therapy. Tumor proliferation was highest in the lymph node and correlated with the degree of BCR activation. A subset of leukemic tumors showed active BCR and NF-κB signaling apparently independent of microenvironmental support. In one of these samples, we identified a novel somatic mutation in RELA (E39Q). This sample was resistant to ibrutinib-mediated inhibition of NF-κB and apoptosis. In addition, we identified germ line variants in genes encoding regulators of the BCR and NF-κB pathway previously implicated in lymphomagenesis. In conclusion, BCR signaling, activated in the lymph node microenvironment in vivo, appears to promote tumor proliferation and survival and may explain the sensitivity of this lymphoma to BTK inhibitors.

MYD88 L265P Mutations, But No Other Variants, Identify a Subpopulation of DLBCL Patients of Activated B-cell Origin, Extranodal Involvement, and Poor Outcome

PURPOSE

Mutations in MYD88 are found in different lymphoproliferative disorders associated with particular biologic characteristics and clinical impact. The aim of this study was to analyze the incidence of MYD88 mutations and its clinical impact in diffuse large B-cell lymphoma (DLBCL).

EXPERIMENTAL DESIGN

The incidence, clinicobiological features, and outcome of 213 patients (115 M/98 F; median age, 65 years) with DLBCL treated with immunochemotherapy in a single institution according to MYD88 mutational status as assessed by an allele-specific PCR assay were analyzed. The cell of origin (COO) was determined in 129 cases by gene expression.

RESULTS

MYD88 mutations were found in 47 cases (22%), including L265P in 39 and S219C and M232F in 4 cases, respectively. Patients with MYD88 L265P were older, presenting frequent extranodal involvement, and mostly corresponded to activated B-cell like (ABC) subtype, whereas no preference in COO was observed in patients with other MYD88 mutations. Five-year overall survival (OS) for MYD88 wild-type, MYD88 L265P, and other variants was 62%, 52%, and 75%, respectively (P = 0.05). International Prognostic Index (IPI) (HR, 2.71; P < 0.001) and MYD88 L265P (HR, 1.786; P = 0.023) were independent variables predicting OS in the multivariate analysis. However, MYD88 L265P lost its independent value when COO was included in the model.

CONCLUSIONS

Our findings indicate that MYD88 L265P mutations, but no other variants, identify a subgroup of DLBCL mainly of ABC origin, with extranodal involvement and poor outcome. Clin Cancer Res; 22(11); 2755-64. ©2016 AACR.

Clonality Analysis of Immunoglobulin Gene Rearrangement by Next-Generation Sequencing in Endemic Burkitt Lymphoma Suggests Antigen Drive Activation of BCR as Opposed to Sporadic Burkitt Lymphoma

Objectives: Recent studies using next-generation sequencing (NGS) analysis disclosed the importance of the intrinsic activation of the B-cell receptor (BCR) pathway in the pathogenesis of sporadic Burkitt lymphoma (sBL) due to mutations of TCF3/ID3 genes. Since no definitive data are available on the genetic landscape of endemic Burkitt (eBL), we first assessed the mutation frequency of TCF3/ID3 in eBL compared with sBL and subsequently the somatic hypermutation status of the BCR to answer whether an extrinsic activation of BCR signaling could also be demonstrated in Burkitt lymphoma.

Methods: We assessed the mutations of TCF3/ID3 by RNAseq and the BCR status by NGS analysis of the immunoglobulin genes (IGs).

Results: We detected mutations of TCF3/ID3 in about 30% of the eBL cases. This rate is significantly lower than that detected in sBL (64%). The NGS analysis of IGs revealed intraclonal diversity, suggesting an active targeted somatic hypermutation process in eBL compared with sBL.

Conclusions: These findings support the view that the antigenic pressure plays a key role in the pathogenetic pathways of eBL, which may be partially distinct from those driving sBL development.

Avian pathogenic Escherichia coli (APEC) infection alters bone marrow transcriptome in chickens

Background

Avian pathogenic Escherichia coli (APEC) is a major cause of disease impacting animal health. The bone marrow is the reservoir of immature immune cells; however, it has not been examined to date for gene expression related to developmental changes (cell differentiation, maturation, programming) after APEC infection. Here, we study gene expression in the bone marrow between infected and non-infected animals, and between infected animals with mild (resistant) versus severe (susceptible) pathology, at two times post-infection.

Results

We sequenced 24 bone marrow RNA libraries generated from the six different treatment groups with four replicates each, and obtained an average of 22 million single-end, 100-bp reads per library. Genes were detected as differentially expressed (DE) between APEC treatments (mild pathology, severe pathology, and mock-challenged) at a given time point, or DE between 1 and 5 days post-infection (dpi) within the same treatment group. Results demonstrate that many immune cells, genes and related pathways are key contributors to the different responses to APEC infection between susceptible and resistant birds and between susceptible and non-challenged birds, at both times post-infection. In susceptible birds, lymphocyte differentiation, proliferation, and maturation were greatly impaired, while the innate and adaptive immune responses, including dendritic cells, monocytes and killer cell activity, TLR- and NOD-like receptor signaling, as well as T helper cells and many cytokine activities, were markedly enhanced. The resistant birds’ immune system, however, was similar to that of non-challenged birds.

Conclusion

The DE genes in the immune cells and identified signaling models are representative of activation and resolution of infection in susceptible birds at both post-infection days. These novel results characterizing transcriptomic response to APEC infection reveal that there is combinatorial activity of multiple genes controlling myeloid cells, and B and T cell lymphopoiesis, as well as immune responses occurring in the bone marrow in these early stages of response to infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1850-4) contains supplementary material, which is available to authorized users.

Clustering of Expression Data in Chronic Lymphocytic Leukemia Reveals New Molecular Subdivisions

Although the identification of inherent structure in chronic lymphocytic leukemia (CLL) gene expression data using class discovery approaches has not been extensively explored, the natural clustering of patient samples can reveal molecular subdivisions that have biological and clinical implications. To explore this, we preprocessed raw gene expression data from two published studies, combined the data to increase the statistical power, and performed unsupervised clustering analysis. The clustering analysis was replicated in 4 independent cohorts. To assess the biological significance of the resultant clusters, we evaluated their prognostic value and identified cluster-specific markers. The clustering analysis revealed two robust and stable subgroups of CLL patients in the pooled dataset. The subgroups were confirmed by different methodological approaches (non-negative matrix factorization NMF clustering and hierarchical clustering) and validated in different cohorts. The subdivisions were related with differential clinical outcomes and markers associated with the microenvironment and the MAPK and BCR signaling pathways. It was also found that the cluster markers were independent of the immunoglobulin heavy chain variable (IGVH) genes mutational status. These findings suggest that the microenvironment can influence the clinical behavior of CLL, contributing to prognostic differences. The workflow followed here provides a new perspective on differences in prognosis and highlights new markers that should be explored in this context.

KSHV Latency Locus Cooperates with Myc to Drive Lymphoma in Mice

Kaposi sarcoma-associated herpesvirus (KSHV) has been linked to Kaposi sarcoma and B-cell malignancies. Mechanisms of KSHV-induced oncogenesis remain elusive, however, in part due to lack of reliable in vivo models. Recently, we showed that transgenic mice expressing the KSHV latent genes, including all viral microRNAs, developed splenic B cell hyperplasia with 100% penetrance, but only a fraction converted to B cell lymphomas, suggesting that cooperative oncogenic events were missing. Myc was chosen as a possible candidate, because Myc is deregulated in many B cell lymphomas. We crossed KSHV latency locus transgenic (latency) mice to Cα Myc transgenic (Myc) mice. By itself these Myc transgenic mice develop lymphomas only rarely. In the double transgenic mice (Myc/latency) we observed plasmacytosis, severe extramedullary hematopoiesis in spleen and liver, and increased proliferation of splenocytes. Myc/latency mice developed frank lymphoma at a higher rate than single transgenic latency or Myc mice. These data indicate that the KSHV latency locus cooperates with the deregulated Myc pathways to further lymphoma progression.

Kaposi’s sarcoma-associated herpesvirus (KSHV) is associated with Kaposi sarcoma as well as the B-cell malignancies primary effusion lymphoma (PEL) and multicentric Castleman’s disease (MCD). Only a few KSHV genes, including all micro RNAs, are expressed in latent infection of B cells. We already showed that KSHV latency locus transgenic mice consistently develop B cell hyperplasia. To find out possible host contributions to lymphomagenesis we evaluated the Myc oncogene. Compound KSHV latency locus and Myc mice developed plasmacytosis exemplified by increased frequency of plasma cells in the spleen, a high accelerated lymphoma development, and severe extramedullary hematopoiesis. These data show that the KSHV latency locus can cooperate with Myc activation in viral lymphomagenesis.

Multilevel BCR signals toward CLL

In this issue of Blood, Iacovelli et al provide the first in vivo experimental evidence on the proleukemogenic relevance of autonomous (exo-antigen–independent) B-cell receptor (BCR) stimulation in conjunction with ligand (autoantigen)- mediated BCR signaling in chronic lymphocytic leukemia (CLL).

Expansion of human and murine hematopoietic stem and progenitor cells ex vivo without genetic modification using MYC and Bcl-2 fusion proteins

The long-term repopulating hematopoietic stem cell (HSC) population can self-renew in vivo, support hematopoiesis for the lifetime of the individual, and is of critical importance in the context of bone marrow stem cell transplantation. The mechanisms that regulate the expansion of HSCs in vivo and in vitro remain unclear to date. Since the current set of surface markers only allow for the identification of a population of cells that is highly enriched for HSC activity, we will refer to the population of cells we expand as Hematopoietic Stem and Progenitor cells (HSPCs). We describe here a novel approach to expand a cytokine-dependent Hematopoietic Stem and Progenitor Cell (HSPC) population ex vivo by culturing primary adult human or murine HSPCs with fusion proteins including the protein transduction domain of the HIV-1 transactivation protein (Tat) and either MYC or Bcl-2. HSPCs obtained from either mouse bone marrow, human cord blood, human G-CSF mobilized peripheral blood, or human bone marrow were expanded an average of 87 fold, 16.6 fold, 13.6 fold, or 10 fold, respectively. The expanded cell populations were able to give rise to different types of colonies in methylcellulose assays in vitro, as well as mature hematopoietic populations in vivo upon transplantation into irradiated mice. Importantly, for both the human and murine case, the ex vivo expanded cells also gave rise to a self-renewing cell population in vivo, following initial transplantation, that was able to support hematopoiesis upon serial transplantation. Our results show that a self-renewing cell population, capable of reconstituting the hematopoietic compartment, expanded ex vivo in the presence of Tat-MYC and Tat-Bcl-2 suggesting that this may be an attractive approach to expand human HSPCs ex vivo for clinical use.

Targeting the B-cell receptor signaling pathway in B lymphoid malignancies

PURPOSE OF REVIEW

Normal B cells that have failed to productively rearrange immunoglobulin V region genes encoding a functional B-cell receptor (BCR) are destined to die. Likewise, the majority of B-cell malignancies remain dependent on functional BCR signaling, whereas in some subtypes BCR expression is missing and, apparently, counterselected. Here, we summarize the recent experimental evidence for the importance of BCR signaling and clinical concepts to target the BCR pathway in B-cell leukemia and lymphoma.

RECENT FINDINGS

Although the dependency on pre-BCR signaling in pre-B acute lymphoblastic leukemia (ALL) seems to be limited to few ALL subtypes (e.g. TCF3-PBX1), most mature B-cell lymphomas rely on BCR signaling provided by different stimuli, for example tonic B-cell signaling, chronic (auto)-antigen exposure, and self-binding properties of the BCR. The finding that in chronic lymphocytic leukemia, BCRs bind to an epitope on the BCR itself unravels a novel concept for chronic lymphocytic leukemia pathogenesis.

SUMMARY

Targeting of the B-cell receptor tyrosine kinases spleen tyrosine kinase, Bruton's tyrosine kinase, and phosphatidylinositol 3-kinase achieve promising clinical responses in various mature B-cell malignancies and might also be useful in defined subsets of ALL. However, further understanding of the BCR signal integration in the different disease groups is required to accurately predict which groups of patients will benefit from BCR pathway inhibition.

Not like a wrecking ball: EBV fine-tunes MYC lymphomagenesis

In this issue of Blood, Fish et al uncover how Epstein-Barr virus (EBV) enhances MYC-driven B-cell lymphoma by crossing EBV Em-EBV latent membrane protein 2A (LMP2A) transgenic mice with immunoglobulin-l (Igl)-MYC transgenic mice.

Oncogenic mechanisms in Burkitt lymphoma

Burkitt lymphoma is a germinal center B-cell-derived cancer that was instrumental in the identification of MYC as an important human oncogene more than three decades ago. Recently, new genomics technologies have uncovered several additional oncogenic mechanisms that cooperate with MYC to create this highly aggressive cancer. The transcription factor TCF-3 is central to Burkitt lymphoma pathogenesis. TCF-3 is rendered constitutively active in Burkitt lymphoma by two related mechanisms: (1) somatic mutations that inactivate its negative regulator ID3, and (2) somatic mutations in TCF-3 that block the ability of ID3 to bind and interfere with its activity as a transcription factor. TCF-3 is also a master regulator of normal germinal center B-cell differentiation. Within the germinal center, TCF-3 up-regulates genes that are characteristically expressed in the rapidly dividing centroblasts, the putative cell of origin for Burkitt lymphoma, while repressing genes expressed in the less proliferative centrocytes. TCF-3 promotes antigen-independent (tonic) B-cell-receptor signaling in Burkitt lymphoma by transactivating immunoglobulin heavy- and light-chain genes while repressing PTPN6, which encodes the phosphatase SHP-1, a negative regulator of B-cell-receptor signaling. Tonic B-cell-receptor signaling sustains Burkitt lymphoma survival by engaging the PI3 kinase pathway. In addition, TCF-3 promotes cell-cycle progression by transactivating CCND3, encoding a D-type cyclin that regulates the G1-S phase transition. Additionally, CCND3 accumulates oncogenic mutations that stabilize cyclin D3 protein expression and drive proliferation. These new insights into Burkitt lymphoma pathogenesis suggest new therapeutic strategies, which are sorely needed in developing regions of the world where this cancer is endemic.

Regulation of MYC expression and differential JQ1 sensitivity in cancer cells

High level MYC expression is associated with almost all human cancers. JQ1, a chemical compound that inhibits MYC expression is therapeutically effective in preclinical animal models in midline carcinoma, and Burkitt's lymphoma (BL). Here we show that JQ1 does not inhibit MYC expression to a similar extent in all tumor cells. The BL cells showed a ∼90% decrease in MYC transcription upon treatment with JQ1, however, no corresponding reduction was seen in several non-BL cells. Molecularly, these differences appear due to requirements of Brd4, the most active version of the Positive Transcription Elongation Factor B (P-TEFb) within the Super Elongation Complex (SEC), and transcription factors such as Gdown1, and MED26 and also other unknown cell specific factors. Our study demonstrates that the regulation of high levels of MYC expression in different cancer cells is driven by unique regulatory mechanisms and that such exclusive regulatory signatures in each cancer cells could be employed for targeted therapeutics.

Transcriptome characterization by RNA sequencing identifies a major molecular and clinical subdivision in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) has heterogeneous clinical and biological behavior. Whole-genome and -exome sequencing has contributed to the characterization of the mutational spectrum of the disease, but the underlying transcriptional profile is still poorly understood. We have performed deep RNA sequencing in different subpopulations of normal B-lymphocytes and CLL cells from a cohort of 98 patients, and characterized the CLL transcriptional landscape with unprecedented resolution. We detected thousands of transcriptional elements differentially expressed between the CLL and normal B cells, including protein-coding genes, noncoding RNAs, and pseudogenes. Transposable elements are globally derepressed in CLL cells. In addition, two thousand genes—most of which are not differentially expressed—exhibit CLL-specific splicing patterns. Genes involved in metabolic pathways showed higher expression in CLL, while genes related to spliceosome, proteasome, and ribosome were among the most down-regulated in CLL. Clustering of the CLL samples according to RNA-seq derived gene expression levels unveiled two robust molecular subgroups, C1 and C2. C1/C2 subgroups and the mutational status of the immunoglobulin heavy variable (IGHV) region were the only independent variables in predicting time to treatment in a multivariate analysis with main clinico-biological features. This subdivision was validated in an independent cohort of patients monitored through DNA microarrays. Further analysis shows that B-cell receptor (BCR) activation in the microenvironment of the lymph node may be at the origin of the C1/C2 differences.

Epstein-Barr virus latent membrane protein 2A enhances MYC-driven cell cycle progression in a mouse model of B lymphoma

Elevated expression of MYC is a shared property of many human cancers. Epstein-Barr virus (EBV) has been associated with lymphoid malignancies, yet collaborative roles between MYC and EBV in lymphomagenesis are unclear. EBV latent membrane protein 2A (LMP2A) functions as a B-cell receptor (BCR) mimic known to provide survival signals to infected B cells. Co-expression of human MYC and LMP2A in mice (LMP2A/λ-MYC) accelerates B lymphoma onset compared with mice expressing human MYC alone (λ-MYC mice). Here we show a novel role of LMP2A in potentiating MYC to promote G1-S transition and hyperproliferation by downregulating cyclin-dependent kinase inhibitor p27(kip1) in a proteasome-dependent manner. Expressing a gain-of-function S10A mutant of p27(kip1) has minor effect on tumor latency. However, pretumor B cells from λ-MYC mice expressing homozygous S10A mutant show a significant decrease in the percentage of S-phase cells. Interestingly, LMP2A is able to counteract the antiproliferative effect of the S10A mutant to promote S-phase entry. Finally, we show that LMP2A expression correlates with higher levels of MYC expression and suppression of p27(kip1) before lymphoma onset. Our study demonstrates a novel function of EBV LMP2A in maximizing MYC expression, resulting in hyperproliferation and cellular transformation into cancer cells in vivo.

The Myc-miR-17-92 axis amplifies B-cell receptor signaling via inhibition of ITIM proteins: a novel lymphomagenic feed-forward loop

The c-Myc oncoprotein regulates >15% of the human transcriptome and a limited number of microRNAs (miRNAs). Here, we establish that in a human B-lymphoid cell line, Myc-repressed, but not Myc-stimulated, genes are significantly enriched for predicted binding sites of Myc-regulated miRNAs, primarily those comprising the Myc-activated miR-17~92 cluster. Notably, gene set enrichment analysis demonstrates that miR-17∼92 is a major regulator of B-cell receptor (BCR) pathway components. Many of them are immunoreceptor tyrosine inhibitory motif (ITIM)-containing proteins, and ITIM proteins CD22 and FCGR2B were found to be direct targets of miR-17∼92. Consistent with the propensity of ITIM proteins to recruit phosphatases, either MYC or miR-17~92 expression was necessary to sustain phosphorylation of spleen tyrosine kinase (SYK) and the B-cell linker protein (BLNK) upon ligation of the BCR. Further downstream, stimulation of the BCR response by miR-17-92 resulted in the enhanced calcium flux and elevated levels of Myc itself. Notably, inhibition of the miR-17~92 cluster in diffuse large B-cell lymphoma (DLBCL) cell lines diminished the BCR response as measured by SYK and BLNK phosphorylation. Conversely, human DLBCLs of the BCR subtype express higher Myc and mir17hg transcript levels than other subtypes. Hence, the Myc-miR-17-92-BCR axis, frequently affected by genomic rearrangements, constitutes a novel lymphomagenic feed-forward loop.

B cell receptor signaling in chronic lymphocytic leukemia

B cell receptor (BCR) signaling plays an important pathogenic role in chronic lymphocytic leukemia (CLL) and B cell lymphomas, based on structural restrictions of the BCR, and BCR-dependent survival and growth of the malignant B cells. In CLL and lymphoma subtypes, ligand-independent ('tonic') and ligand-dependent BCR signaling have been characterized, which can involve mutations of BCR pathway components or be triggered by (auto)antigens present in the tissue microenvironment. In CLL, based on high response rates and durable remissions in early-stage clinical trials, there is rapid clinical development of inhibitors targeting BCR-associated kinases [Bruton's tyrosine kinase (BTK), phosphoinositide 3-kinase (PI3K)δ], which will change treatment paradigms in CLL and other B cell malignancies. Here, we discuss the evolution of this field, from BCR-related prognostic markers, to mechanisms of BCR activation, and targeting of BCR-associated kinases, the emerging Achilles' heel in CLL pathogenesis.

Germinal centre protein HGAL promotes lymphoid hyperplasia and amyloidosis via BCR-mediated Syk activation

The human germinal centre associated lymphoma (HGAL) gene is specifically expressed in germinal centre B-lymphocytes and germinal centre-derived B-cell lymphomas, but its function is largely unknown. Here we demonstrate that HGAL directly binds Syk in B-cells, increases its kinase activity upon B-cell receptor stimulation and leads to enhanced activation of Syk downstream effectors. To further investigate these findings in vivo, HGAL transgenic mice were generated. Starting from 12 months of age these mice developed polyclonal B-cell lymphoid hyperplasia, hypergammaglobulinemia and systemic reactive AA amyloidosis, leading to shortened survival. The lymphoid hyperplasia in the HGAL transgenic mice are likely attributable to enhanced B-cell receptor signalling as shown by increased Syk phosphorylation, ex vivo B-cell proliferation and increased RhoA activation. Overall, our study shows for the first time that the germinal centre protein HGAL regulates B-cell receptor signalling in B-lymphocytes which, without appropriate control, may lead to B-cell lymphoproliferation.

Gain-of-function of Stat5 leads to excessive granulopoiesis and lethal extravasation of granulocytes to the lung

The Signal Transducer and Activator of Transcription 5 (Stat5) plays a significant role in normal hematopoiesis and a variety of hematopoietic malignancies. Deficiency in Stat5 causes impaired cytokine-mediated proliferation and survival of progenitors and their differentiated descendants along major hematopoietic lineages such as erythroid, lymphoid, and myeloid cells. Overexpression and persistent activation of Stat5 are sufficient for neoplastic transformation and development of multi-lineage leukemia in a transplant model. Little is known, however, whether a continuous activation of this signal transducer is essential for the maintenance of hematopoietic malignancies. To address this issue, we developed transgenic mice that express a hyperactive mutant of Stat5 in hematopoietic progenitors and derived lineages in a ligand-controlled manner. In contrast to the transplant model, expression of mutant Stat5 did not adversely affect normal hematopoiesis in the presence of endogenous wildtype Stat5 alleles. However, the gain-of-function of this signal transducer in mice that carry Stat5a/b hypomorphic alleles resulted in abnormally high numbers of circulating granulocytes that caused severe airway obstruction. Downregulation of hyperactive Stat5 in diseased animals restored normal granulopoiesis, which also resulted in a swift clearance of granulocytes from the lung. Moreover, we demonstrate that Stat5 promotes the initiation and maintenance of severe granulophilia in a cell autonomous manner. The results of this study show that the gain-of-function of Stat5 causes excessive granulopoiesis and prolonged survival of granulocytes in circulation. Collectively, our findings underline the critical importance of Stat5 in maintaining a normal balance between myeloid and lymphoid cells during hematopoiesis, and we provide direct evidence for a function of Stat5 in granulophilia–associated pulmonary dysfunction.

Synergy between PI3K signaling and MYC in Burkitt lymphomagenesis

In Burkitt lymphoma (BL), a germinal center B-cell-derived tumor, the pro-apoptotic properties of c-MYC must be counterbalanced. Predicting that survival signals would be delivered by phosphoinositide-3-kinase (PI3K), a major survival determinant in mature B cells, we indeed found that combining constitutive c-MYC expression and PI3K activity in germinal center B cells of the mouse led to BL-like tumors, which fully phenocopy human BL with regard to histology, surface and other markers, and gene expression profile. The tumors also accumulate tertiary mutational events, some of which are recurrent in the human disease. These results and our finding of recurrent PI3K pathway activation in human BL indicate that deregulated c-MYC and PI3K activity cooperate in BL pathogenesis.

iNKT cells suppress the CD8+ T cell response to a murine Burkitt's-like B cell lymphoma

The T cell response to B cell lymphomas differs from the majority of solid tumors in that the malignant cells themselves are derived from B lymphocytes, key players in immune response. B cell lymphomas are therefore well situated to manipulate their surrounding microenvironment to enhance tumor growth and minimize anti-tumor T cell responses. We analyzed the effect of T cells on the growth of a transplantable B cell lymphoma and found that iNKT cells suppressed the anti-tumor CD8⁺ T cell response. Lymphoma cells transplanted into syngeneic wild type (WT) mice or Jalpha18^−/− mice that specifically lack iNKT cells grew initially at the same rate, but only the mice lacking iNKT cells were able to reject the lymphoma. This effect was due to the enhanced activity of tumor-specific CD8⁺ T cells in the absence of iNKT cells, and could be partially reversed by reconstitution of iNKT cells in Jalpha 18^−/− mice. Treatment of tumor-bearing WT mice with alpha -galactosyl ceramide, an activating ligand for iNKT cells, reduced the number of tumor-specific CD8⁺ T cells. In contrast, lymphoma growth in CD1d1^−/− mice that lack both iNKT and type II NKT cells was similar to that in WT mice, suggesting that type II NKT cells are required for full activation of the anti-tumor immune response. This study reveals a tumor-promoting role for iNKT cells and suggests their capacity to inhibit the CD8⁺ T cell response to B cell lymphoma by opposing the effects of type II NKT cells.

Homeostatic chemokines guide lymphoma cells to tumor growth-promoting niches within secondary lymphoid organs

The interaction between lymphoid tumor cells and their tissue microenvironment is thought to promote dissemination and progression of lymphoma. Those type of interactions consists of at least three cornerstones, among them mesenchymal- or bone marrow-derived stromal cells, cells of the innate or adaptive immune response, and the lymphoma cells themselves. The molecular pathways of crosstalk between the lymphoma cells and their nursing stroma are not well understood and their dissection is challenging because of (1) the complexity of stroma cell subpopulations, (2) kinetic and developmental transitions/switches of stroma composition, and (3) inherent technical difficulties to isolate and analyze defined stroma cell subsets. However, recent studies of bone marrow stroma interaction with leukemia or lymphoma cells have revealed therapeutic targets involved in regulating tumor cell mobilization. Release of tumor cells from their supportive niches could be effectuated by inhibition of homing and retention signals. The present review focuses on the effects of homing receptors and cytokines attributed to lymphoid tissue formation in tumor-stroma interactions within secondary lymphoid tissues. We discuss possible cellular and molecular mechanisms of lymphoma-stroma crosstalk and highlight novel therapeutic strategies based on the disruption of tumor-stroma interaction in secondary lymphoid organs.

CD19 is a major B cell receptor-independent activator of MYC-driven B-lymphomagenesis

PAX5, a B cell-specific transcription factor, is overexpressed through chromosomal translocations in a subset of B cell lymphomas. Previously, we had shown that activation of immunoreceptor tyrosine-based activation motif (ITAM) proteins and B cell receptor (BCR) signaling by PAX5 contributes to B-lymphomagenesis. However, the effect of PAX5 on other oncogenic transcription factor-controlled pathways is unknown. Using a MYC-induced murine lymphoma model as well as MYC-transformed human B cell lines, we found that PAX5 controls c-MYC protein stability and steady-state levels. This promoter-independent, posttranslational mechanism of c-MYC regulation was independent of ITAM/BCR activity. Instead it was controlled by another PAX5 target, CD19, through the PI3K-AKT-GSK3β axis. Consequently, MYC levels in B cells from CD19-deficient mice were sharply reduced. Conversely, reexpression of CD19 in murine lymphomas with spontaneous silencing of PAX5 boosted MYC levels, expression of its key target genes, cell proliferation in vitro, and overall tumor growth in vivo. In human B-lymphomas, CD19 mRNA levels were found to correlate with those of MYC-activated genes. They also negatively correlated with the overall survival of patients with lymphoma in the same way that MYC levels do. Thus, CD19 is a major BCR-independent regulator of MYC-driven neoplastic growth in B cell neoplasms.

A defect of the INK4-Cdk4 checkpoint and Myc collaborate in blastoid mantle cell lymphoma-like lymphoma formation in mice

Mantle cell lymphoma (MCL) is a B-cell malignancy characterized by a monoclonal proliferation of lymphocytes with the co-expression of CD5 and CD43, but not of CD23. Typical MCL is associated with overexpression of cyclin D1, and blastoid MCL variants are associated with Myc (alias c-myc) translocations. In this study, we developed a murine model of MCL-like lymphoma by crossing Cdk4(R24C) mice with Myc-3'RR transgenic mice. The Cdk4(R24C) mouse is a knockin strain that expresses a Cdk4 protein that is resistant to inhibition by p16(INK4a) as well as other INK4 family members. Ablation of INK4 control on Cdk4 does not affect lymphomagenesis, B-cell maturation, and functions in Cdk4(R24C) mice. Additionally, B cells were normal in numbers, cell cycle activity, mitogen responsiveness, and Ig synthesis in response to activation. By contrast, breeding Cdk4(R24C) mice with Myc-3'RR transgenic mice prone to develop aggressive Burkitt lymphoma-like lymphoma (CD19(+)IgM(+)IgD(+) cells) leads to the development of clonal blastoid MCL-like lymphoma (CD19(+)IgM(+)CD5(+)CD43(+)CD23(-) cells) in Myc/Cdk4(R24C) mice. Western blot analysis revealed high amounts of Cdk4/cyclin D1 complexes as the main hallmark of these lymphomas. These results indicate that although silent in nonmalignant B cells, a defect in the INK4-Cdk4 checkpoint can participate in lymphomagenesis in conjunction with additional alterations of cell cycle control, a situation that might be reminiscent of the development of human blastoid MCL.

Pathogenesis of human B cell lymphomas

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

Anti-tumor activity of a B-cell receptor-targeted peptide in a novel disseminated lymphoma xenograft model

Receptor-targeted therapies have become standard in the treatment of various lymphomas. In view of its unparalleled specificity for the malignant B-cell clone, the B-cell receptor (BCR) on B cell lymphoma cells is a potential therapeutic target. We have used two BCR epitope mimicking peptides binding to the Burkitt's lymphoma cell lines CA46 and SUP-B8. We proved their functionality by demonstrating calcium flux and BCR-mediated endocytosis upon peptide receptor binding. Toxicity experiments in vitro via cross-linking of the BCR with tetramerized epitope mimics lead to apoptosis in both cell lines but was far more effective in SUP-B8 cells. We established a SUP-B8-based disseminated Burkitt's lymphoma model in NOD/SCID mice. Treatment of tumor-bearing mice with tetramerized epitope mimics had significant anti-tumor effects in vivo. We conclude that peptide-mediated, BCR-targeted therapy is a promising approach which may be explored and further developed for application in highly aggressive lymphoma.

Myc roles in hematopoiesis and leukemia

Hematopoiesis is a process capable of generating millions of cells every second, as distributed in many cell types. The process is regulated by a number of transcription factors that regulate the differentiation along the distinct lineages and dictate the genetic program that defines each mature phenotype. Myc was first discovered as the oncogene of avian leukemogenic retroviruses; it was later found translocated in human lymphoma. From then on, evidence accumulated showing that c-Myc is one of the transcription factors playing a major role in hematopoiesis. The study of genetically modified mice with overexpression or deletion of Myc has shown that c-Myc is required for the correct balance between self-renewal and differentiation of hematopoietic stem cells (HSCs). Enforced Myc expression in mice leads to reduced HSC pools owing to loss of self-renewal activity at the expense of increased proliferation of progenitor cells and differentiation. c-Myc deficiency consistently results in the accumulation of HSCs. Other models with conditional Myc deletion have demonstrated that different lineages of hematopoietic cells differ in their requirement for c-Myc to regulate their proliferation and differentiation. When transgenic mice overexpress c-Myc or N-Myc in mature cells from the lymphoid or myeloid lineages, the result is lymphoma or leukemia. In agreement, enforced expression of c-Myc blocks the differentiation in several leukemia-derived cell lines capable of differentiating in culture. Not surprising, MYC deregulation is recurrently found in many types of human lymphoma and leukemia. Whereas MYC is deregulated by translocation in Burkitt lymphoma and, less frequently, other types of lymphoma, MYC is frequently overexpressed in acute lymphoblastic and myeloid leukemia, through mechanisms unrelated to chromosomal translocation, and is often associated with disease progression.