B-cell receptor translocation to lipid rafts and associated signaling differ between prognostically important subgroups of chronic lymphocytic leukemia

Identification of a New Cholesterol-Binding Site within the IFN-γ Receptor that is Required for Signal Transduction

The cytokine interferon-gamma (IFN-γ) is a master regulator of innate and adaptive immunity involved in a broad array of human diseases that range from atherosclerosis to cancer. IFN-γ exerts it signaling action by binding to a specific cell surface receptor, the IFN-γ receptor (IFN-γR), whose activation critically depends on its partition into lipid nanodomains. However, little is known about the impact of specific lipids on IFN-γR signal transduction activity. Here, a new conserved cholesterol (chol) binding motif localized within its single transmembrane domain is identified. Through direct binding, chol drives the partition of IFN-γR2 chains into plasma membrane lipid nanodomains, orchestrating IFN-γR oligomerization and transmembrane signaling. Bioinformatics studies show that the signature sequence stands for a conserved chol-binding motif presented in many mammalian membrane proteins. The discovery of chol as the molecular switch governing IFN-γR transmembrane signaling represents a significant advance for understanding the mechanism of lipid selectivity by membrane proteins, but also for figuring out the role of lipids in modulating cell surface receptor function. Finally, this study suggests that inhibition of the chol-IFNγR2 interaction may represent a potential therapeutic strategy for various IFN-γ-dependent diseases.

The Traf2DNxBCL2-tg Mouse Model of Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma Recapitulates the Biased IGHV Gene Usage, Stereotypy, and Antigen-Specific HCDR3 Selection of Its Human Counterpart

Chronic lymphocytic leukemia (CLL)/Small lymphocytic lymphoma (SLL) is a heterogeneous disease consisting of at least two separate subtypes, based on the mutation status of the immunoglobulin heavy chain variable gene (IGHV) sequence. Exposure to antigens seems to play a role in malignant transformation and in the selection and expansion of more aggressive CLL clones. Furthermore, a biased usage of particular IGHV gene subgroups and the existence of stereotyped B-cell receptors (BCRs) are distinctive characteristics of human CLL. We have previously described that Traf2DN/BCL2 double-transgenic (tg, ^+/+) mice develop CLL/SLL with high incidence with aging. In this model, TNF-Receptor Associated Factor (TRAF)-2 deficiency cooperates with B cell lymphoma (BCL)-2 in promoting CLL/SLL in mice by specifically enforcing marginal zone (MZ) B cell differentiation and rendering B cells independent of BAFF for survival. In this report, we have performed the sequencing of the IGHV-D-J rearrangements of B cell clones from the Traf2DN/BCL2-tg^+/+ mice with CLL/SLL. The results indicate that these mice develop oligoclonal and monoclonal B cell expansions. Allotransplantation of the oligoclonal populations into immunodeficient mice resulted in the preferential expansion of one of the parental clones. The analysis of the IGHV sequences indicated that 15% were mutated (M) and 85% unmutated (UM). Furthermore, while the Traf2DN/BCL2-tg^-/- (wild-type), ^-/+ (BCL2 single-tg) and ^+/- (Traf2DNDN single-tg) littermates showed the expression of various IGHV gene subgroups, the CLL/SLL expanded clones from the Traf2DN/BCL2-tg^+/+ (double-transgenic) mice showed a more restricted IGHV gene subgroup usage and an overrepresentation of particular IGHV genes. In addition, the HCDR3-encoded protein sequence indicates the existence of stereotyped immunoglobulin (Ig) in the BCRs and strong similarities with BCR recognizing autoantigens and pathogen-associated antigens. Altogether, these results highlight the remarkable similarities between the CLL/SLL developed by the Traf2DN/BCL2-tg^+/+ mice and its human counterpart.

Genome-wide association study identifies risk loci for progressive chronic lymphocytic leukemia

Prognostication in patients with chronic lymphocytic leukemia (CLL) is challenging due to heterogeneity in clinical course. We hypothesize that constitutional genetic variation affects disease progression and could aid prognostication. Pooling data from seven studies incorporating 842 cases identifies two genomic locations associated with time from diagnosis to treatment, including 10q26.13 (rs736456, hazard ratio (HR) = 1.78, 95% confidence interval (CI) = 1.47-2.15; P = 2.71 × 10-9) and 6p (rs3778076, HR = 1.99, 95% CI = 1.55-2.55; P = 5.08 × 10-8), which are particularly powerful prognostic markers in patients with early stage CLL otherwise characterized by low-risk features. Expression quantitative trait loci analysis identifies putative functional genes implicated in modulating B-cell receptor or innate immune responses, key pathways in CLL pathogenesis. In this work we identify rs736456 and rs3778076 as prognostic in CLL, demonstrating that disease progression is determined by constitutional genetic variation as well as known somatic drivers.

Proliferative Signals in Chronic Lymphocytic Leukemia; What Are We Missing?

Chronic lymphocytic leukemia (CLL) cells cycle between lymphoid tissue sites where they actively proliferate, and the peripheral blood (PB) where they become quiescent. Strong evidence exists for a crucial role of B cell receptor (BCR) triggering, either by (self-)antigen or by receptor auto-engagement in the lymph node (LN) to drive CLL proliferation and provide adhesion. The clinical success of Bruton's tyrosine kinase (BTK) inhibitors is widely accepted to be based on blockade of the BCR signal. Additional signals in the LN that support CLL survival derive from surrounding cells, such as CD40L-presenting T helper cells, myeloid and stromal cells. It is not quite clear if and to what extent these non-BCR signals contribute to proliferation in situ. In vitro BCR triggering, in contrast, leads to low-level activation and does not result in cell division. Various combinations of non-BCR signals delivered via co-stimulatory receptors, Toll-like receptors (TLRs), and/or soluble cytokines are applied, leading to comparatively modest and short-lived CLL proliferation in vitro. Thus, an unresolved gap exists between the condition in the patient as we now understand it and applicable knowledge that can be harnessed in the laboratory for future therapeutic applications. Even in this era of targeted drugs, CLL remains largely incurable with frequent relapses and emergence of resistance. Therefore, we require better insight into all aspects of CLL growth and potential rewiring of signaling pathways. We aim here to provide an overview of in vivo versus in vitro signals involved in CLL proliferation, point out areas of missing knowledge and suggest future directions for research.

Lck is a relevant target in chronic lymphocytic leukaemia cells whose expression variance is unrelated to disease outcome

Pathogenesis of chronic lymphocytic leukaemia (CLL) is contingent upon antigen receptor (BCR) expressed by malignant cells of this disease. Studies on somatic hypermutation of the antigen binding region, receptor expression levels and signal capacity have all linked BCR on CLL cells to disease prognosis. Our previous work showed that the src-family kinase Lck is a targetable mediator of BCR signalling in CLL cells, and that variance in Lck expression associated with ability of BCR to induce signal upon engagement. This latter finding makes Lck similar to ZAP70, another T-cell kinase whose aberrant expression in CLL cells also associates with BCR signalling capacity, but also different because ZAP70 is not easily pharmacologically targetable. Here we describe a robust method of measuring Lck expression in CLL cells using flow cytometry. However, unlike ZAP70 whose expression in CLL cells predicts prognosis, we find Lck expression and disease outcome in CLL are unrelated despite observations that its inhibition produces effects that biologically resemble the egress phenotype taken on by CLL cells treated with idelalisib. Taken together, our findings provide insight into the pathobiology of CLL to suggest a more complex relationship between expression of molecules within the BCR signalling pathway and disease outcome.

Lipoprotein lipase in chronic lymphocytic leukemia: function and prognostic implications

Chronic lymphocytic leukemia (CLL) is a clinically heterogeneous disease characterized by the accumulation of a clonal population of B cells in peripheral blood, bone marrow, and lymphoid organs. More than 10 years ago, lipoprotein lipase (LPL) mRNA was identified as being strongly expressed in patients experiencing a more aggressive phenotype, while CLL patients with an indolent disease course lack expression of this marker. Since then, several reports confirmed the capability of LPL to predict CLL disease evolution at the moment of diagnosis. In contrast, data on the functional implications of LPL in CLL are scarce. LPL exerts a central role in overall lipid metabolism and transport, but plays additional, non-catalytic roles as well. Which of those is more important in the pathogenesis of CLL remains largely unclear. Here, we review the current knowledge on the prognostic and biological relevance of LPL in CLL.

Functional Differences between IgM and IgD Signaling in Chronic Lymphocytic Leukemia

BCR signaling is a central pathogenetic pathway in chronic lymphocytic leukemia (CLL). Most CLL cells express BCRs of IgM and IgD isotypes, but the contribution of these isotypes to functional responses remains incompletely defined. We therefore investigated differences between IgM and IgD signaling in freshly isolated peripheral blood CLL cells and in CLL cells cultured with nurselike cells, a model that mimics the lymph node microenvironment. IgM signaling induced prolonged activation of ERK kinases and promoted CLL cell survival, CCL3 and CCL4 chemokine secretion, and downregulation of BCL6, the transcriptional repressor of CCL3 In contrast, IgD signaling induced activation of the cytoskeletal protein HS1, along with F-actin polymerization, which resulted in rapid receptor internalization and failure to support downstream responses, including CLL cell survival and chemokine secretion. IgM and IgD receptor downmodulation, HS1 and ERK activation, chemokine secretion, and BCL6 downregulation were also observed when CLL cells were cocultured with nurselike cells. The Bruton's tyrosine kinase inhibitor ibrutinib effectively inhibited both IgM and IgD isotype signaling. In conclusion, through a variety of functional readouts, we demonstrate very distinct outcomes of IgM and IgD isotype activation in CLL cells, providing novel insight into the regulation of BCR signaling in CLL.

Simvastatin and downstream inhibitors circumvent constitutive and stromal cell-induced resistance to doxorubicin in IGHV unmutated CLL cells

The immunoglobulin heavy-chain variable region (IGHV) mutational status is a strong determinant of remission duration in chronic lymphocytic leukemia (CLL). The aim of this work was to compare the multidrug resistance (MDR) signature of IGHV mutated and unmutated CLL cells, identifying biochemical and molecular targets potentially amenable to therapeutic intervention.

We found that the mevalonate pathway-dependent Ras/ERK1–2 and RhoA/RhoA kinase signaling cascades, and the downstream HIF-1α/P-glycoprotein axis were more active in IGHV unmutated than in mutated cells, leading to a constitutive protection from doxorubicin-induced cytotoxicity. The constitutive MDR phenotype of IGHV unmutated cells was partially dependent on B cell receptor signaling, as shown by the inhibitory effect exerted by ibrutinib. Stromal cells further protected IGHV unmutated cells from doxorubicin by upregulating Ras/ERK1–2, RhoA/RhoA kinase, Akt, HIF-1α and P-glycoprotein activities. Mevalonate pathway inhibition with simvastatin abrogated these signaling pathways and reversed the resistance of IGHV unmutated cells to doxorubicin, also counteracting the protective effect exerted by stromal cells. Similar results were obtained via the targeted inhibition of the downstream molecules ERK1–2, RhoA kinase and HIF-1α.

Therefore, targeting the mevalonate pathway and its downstream signaling cascades is a promising strategy to circumvent the MDR signature of IGHV unmutated CLL cells.

The MYC/miR-17-92 axis in lymphoproliferative disorders: A common pathway with therapeutic potential

MicroRNAs (miRNAs) represent a class of small non-coding single-stranded RNA molecules acting as master regulators of gene expression post transcriptionally by inhibiting the translation or inducing the degradation of target messenger RNAs (mRNAs). In particular, the miR-17-92 cluster is widely expressed in many different cell types and is essential for many developmental and pathogenic processes. As a strong oncogene, miR-17-92 can regulate multiple cellular processes that favor malignant transformation, promoting cell survival, rapid cell proliferation, and increased angiogenesis. The miR-17-92 cluster has been reported to be involved in hematopoietic malignancies including diffuse large B-cell lymphoma, mantle cell lymphoma, Burkitt's lymphoma, and chronic lymphocytic leukemia. Given the multiple and potent effects on cellular proliferation and apoptosis exerted by the miR-17-92 cluster, miRNAs belonging to the cluster surely represent attractive targets for cancer therapy also in the context of lymphoproliferative disorders. In the present review, we focus on the role of the miR-17-92 cluster in lymphoproliferative disorders, including diagnostic/prognostic implications, and on the potential applications of anti-miRNAs based therapies targeting miRNAs belonging to the cluster.

Targeting B-cell receptor signaling in leukemia and lymphoma: how and why?

B-lymphocytes are dependent on B-cell receptor (BCR) signaling for the constant maintenance of their physiological function, and in many B-cell malignancies this signaling pathway is prone to aberrant activation. This understanding has led to an ever-increasing interest in the signaling networks activated following ligation of the BCR in both normal and malignant cells, and has been critical in establishing an array of small molecule inhibitors targeting BCR-induced signaling. By dissecting how different malignancies signal through BCR, researchers are contributing to the design of more customized therapeutics which have greater efficacy and lower toxicity than previous therapies. This allows clinicians access to an array of approaches to best treat patients whose malignancies have BCR signaling as a driver of pathogenesis.

Microenvironment interactions and B-cell receptor signaling in Chronic Lymphocytic Leukemia: Implications for disease pathogenesis and treatment

Chronic Lymphocytic Leukemia (CLL) is a malignancy of mature B lymphocytes which are highly dependent on interactions with the tissue microenvironment for their survival and proliferation. Critical components of the microenvironment are monocyte-derived nurselike cells (NLCs), mesenchymal stromal cells, T cells and NK cells, which communicate with CLL cells through a complex network of adhesion molecules, chemokine receptors, tumor necrosis factor (TNF) family members, and soluble factors. (Auto-) antigens and/or autonomous mechanisms activate the B-cell receptor (BCR) and its downstream signaling cascade in secondary lymphatic tissues, playing a central pathogenetic role in CLL. Novel small molecule inhibitors, including the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib and the phosphoinositide-3-kinase delta (PI3Kδ) inhibitor idelalisib, target BCR signaling and have become the most successful new therapeutics in this disease. We here review the cellular and molecular characteristics of CLL cells, and discuss the cellular components and key pathways involved in the cross-talk with their microenvironment. We also highlight the relevant novel treatment strategies, focusing on immunomodulatory agents and BCR signaling inhibitors and how these treatments disrupt CLL-microenvironment interactions. This article is part of a Special Issue entitled: Tumor Microenvironment Regulation of Cancer Cell Survival, Metastasis, Inflammation, and Immune Surveillance edited by Peter Ruvolo and Gregg L. Semenza.

Excessive antigen reactivity may underlie the clinical aggressiveness of chronic lymphocytic leukemia stereotyped subset #8

Subset #8 is a distinctive subset of patients with chronic lymphocytic leukemia (CLL) defined by the expression of stereotyped IGHV4-39/IGKV1(D)-39 B-cell receptors. Subset #8 patients experience aggressive disease and exhibit the highest risk for Richter transformation among all CLL. In order to obtain biological insight into this behavior, we profiled the antigen reactivity and signaling capacity of subset #8 vs other clinically aggressive stereotyped subsets, namely subsets #1 and #2. Twenty-seven monoclonal antibodies (mAbs) from subsets #1, #2, and #8 CLL clones were prepared as recombinant human immunoglobulin G1 and used as primary antibodies in enzyme-linked immunosorbent assays against representatives of the major classes of established antigenic targets for CLL. Subset #8 CLL mAbs exhibited broad polyreactivity as they bound to all antigens tested, in clear contrast with the mAbs from the other subsets. Antigen challenge of primary CLL cells indicated that the promiscuous antigen-binding activity of subset #8 mAbs could lead to significant cell activation, again in contrast to the less responsive CLL cells from subsets #1 and #2. These features constitute a distinctive profile for CLL subset #8, supporting the existence of distinct mechanisms of aggressiveness in different immunogenetic subsets of CLL.

Increased chronic lymphocytic leukemia proliferation upon IgM stimulation is sustained by the upregulation of miR-132 and miR-212

To assess the involvement of microRNAs (miRNAs) in B-cell receptor (BCR) stimulation, we first evaluated miRNA profiling following IgM cross-linking in chronic lymphocytic leukemia (CLL) cells and in normal B lymphocytes. Second, we combined miRNA and gene expression data to identify putative miRNA functional networks. miRNA profiling showed distinctive patterns of regulation after stimulation in leukemic versus normal B lymphocytes and identified a differential responsiveness to BCR engagement in CLL subgroups according to the immunoglobulin heavy chain variable region mutational status and clinical outcome. The most significantly modulated miRNAs in stimulated CLL are miR-132 and miR-212. Notably, these miRNAs appeared regulated in progressive but not in stable CLL. Accordingly, gene profiling showed a significant transcriptional response to stimulation exclusively in progressive CLL. Based on these findings, we combined miRNA and gene expression data to investigate miR-132 and miR-212 candidate interactions in this CLL subgroup. Correlation analysis pointed to a link between these miRNAs and RB/E2F and TP53 cascades with proproliferative effects, as corroborated by functional analyses. Finally, basal levels of miR-132 and miR-212 were measured in an independent cohort of 20 unstimulated CLL cases and both showed lower expression in progressive compared to stable patients, suggesting an association between the expression of these molecules and disease prognosis. Overall, our results support a model involving miR-132 and miR-212 upregulation in sustaining disease progression in CLL. These miRNAs may therefore provide new valuable strategies for therapeutic intervention.

Rewiring of sIgM-Mediated Intracellular Signaling through the CD180 Toll-like Receptor

Chronic lymphocytic leukemia (CLL) development and progression are thought to be driven by unknown antigens/autoantigens through the B cell receptor (BCR) and environmental signals for survival and expansion including toll-like receptor (TLR) ligands. CD180/RP105, a membrane-associated orphan receptor of the TLR family, induces normal B cell activation and proliferation and is expressed by approximately 60% of CLL samples. Half of these respond to ligation with anti-CD180 antibody by increased activation/phosphorylation of protein kinases associated with BCR signaling. Hence CLL cells expressing both CD180 and the BCR could receive signals via both receptors. Here we investigated cross-talk between BCR and CD180-mediated signaling on CLL cell survival and apoptosis. Our data indicate that ligation of CD180 on responsive CLL cells leads to activation of either prosurvival Bruton tyrosine kinase (BTK)/phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT-mediated, or proapoptotic p38 mitogen-activated protein kinase (p38MAPK)-mediated signaling pathways, while selective immunoglobulin M (sIgM) ligation predominantly engages the BTK/PI3K/AKT pathway. Furthermore, pretreatment of CLL cells with anti-CD180 redirects IgM-mediated signaling from the prosurvival BTK/PI3K/AKT toward the proapoptotic p38MAPK pathway. Thus preengaging CD180 could prevent further prosurvival signaling mediated via the BCR and, instead, induce CLL cell apoptosis, opening the door to therapeutic profiling and new strategies for the treatment of a substantial cohort of CLL patients.

Egg intake during carbohydrate restriction alters peripheral blood mononuclear cell inflammation and cholesterol homeostasis in metabolic syndrome

Egg yolk contains bioactive components that improve plasma inflammatory markers and HDL profiles in metabolic syndrome (MetS) under carbohydrate restriction. We further sought to determine whether egg yolk intake affects peripheral blood mononuclear cell (PBMC) inflammation and cholesterol homeostasis in MetS, as HDL and its associated lipid transporter ATP-binding cassette transporter A1 (ABCA1) reduce the inflammatory potential of leukocytes through modulation of cellular cholesterol content and distribution. Thirty-seven men and women classified with MetS consumed a moderate carbohydrate-restricted diet (25%–30% of energy) for 12 weeks, in addition to consuming either three whole eggs per day (EGG) or the equivalent amount of yolk-free egg substitute (SUB). Interestingly, lipopolysaccharide-induced PBMC IL-1β and TNFα secretion increased from baseline to week 12 in the SUB group only, despite increases in PBMC toll-like receptor 4 (TLR4) mRNA expression in the EGG group. Compared to baseline, ABCA1 and 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase mRNA expression increased by week 12 in the EGG group only, whereas changes in PBMC total cholesterol positively correlated with changes in lipid raft content. Together, these findings suggest that intake of whole eggs during carbohydrate restriction alters PBMC inflammation and cholesterol homeostasis in MetS.

Does B cell receptor signaling in chronic lymphocytic leukaemia cells differ from that in other B cell types?

Chronic lymphocytic leukaemia (CLL) is an incurable malignancy of mature B cells. CLL is important clinically in Western countries because of its commonality and because of the significant morbidity and mortality associated with the progressive form of this incurable disease. The B cell receptor (BCR) expressed on the malignant cells in CLL contributes to disease pathogenesis by providing signals for survival and proliferation, and the signal transduction pathway initiated by engagement of this receptor is now the target of several therapeutic strategies. The purpose of this review is to outline current understanding of the BCR signal cascade in normal B cells and then question whether this understanding applies to CLL cells. In particular, this review studies the phenomenon of anergy in CLL cells, and whether certain adaptations allow the cells to overcome anergy and allow full BCR signaling to take place. Finally, this review analyzes how BCR signals can be therapeutically targeted for the treatment of CLL.

The outcome of B-cell receptor signaling in chronic lymphocytic leukemia: proliferation or anergy

Biologists and clinicians agree that the B-cell receptor influences the behavior of chronic lymphocytic leukemia, and promising new drugs are aimed at receptor-associated kinases. Engagement of surface immunoglobulin by antigen is a key driver of malignant cells with outcome influenced by the nature of the cell, the level of stimulation and the microenvironment. Analysis of surface immunoglobulin-mediated signaling in the two major disease subsets defined by IGHV mutational status reveals bifurcation of responses toward proliferation or anergy. Mutated chronic lymphocytic leukemia, generally of relatively good prognosis, is mainly, but not exclusively, driven towards anergy in vivo. In contrast, unmutated chronic lymphocytic leukemia shows less evidence for anergy in vivo retaining more responsiveness to surface immunoglobulin M-mediated signaling, possibly explaining increased tumor progression. Expression and function of surface immunoglobulin M in unmutated chronic lymphocytic leukemia appear rather homogeneous, but mutated chronic lymphocytic leukemia exhibits a highly heterogeneous profile that may relate to further variable clinical behavior within this subset. Anergy should increase susceptibility to apoptosis but, in leukemic cells, this may be countered by overexpression of the B-cell lymphoma-2 survival protein. Maintained anergy spreads to chemokines and adhesion molecules, restraining homing and migration. However, anergy is not necessarily completely benign, being able to reverse and regenerate surface immunoglobulin M-mediated responses. A two-pronged attack on proliferative and anti-apoptotic pathways may succeed. Increased understanding of how chronic lymphocytic leukemia cells are driven to anergy or proliferation should reveal predictive biomarkers of progression and of likely response to kinase inhibitors, which could assist therapeutic decisions.

Heterogeneous functional effects of concomitant B cell receptor and TLR stimulation in chronic lymphocytic leukemia with mutated versus unmutated Ig genes

We recently reported that chronic lymphocytic leukemia (CLL) subgroups with distinct clonotypic BCRs present discrete patterns of TLR expression, function, and/or tolerance. In this study, to explore whether specific types of BCR/TLR collaboration exist in CLL, we studied the effect of single versus concomitant BCR and/or TLR stimulation on CLL cells from mutated (M-CLL) and unmutated CLL (U-CLL) cases. We stimulated negatively isolated CLL cells by using anti-IgM, imiquimod, and CpG oligodeoxynucleotide for BCR, TLR7, and TLR9, respectively, alone or in combination for different time points. After in vitro culture in the absence of stimulation, differences in p-ERK were identified at any time point, with higher p-ERK levels in U-CLL versus M-CLL. Pronounced p-ERK induction was seen by single stimulation in U-CLL, whereas BCR/TLR synergism was required in M-CLL, in which the effect was overall limited in scale. An opposite pattern was observed regarding induction of apoptosis, as studied by Western blotting for the cleaved fragment of poly(ADP-ribose) polymerase, and the active isoform of caspase-8, with M-CLL responding even to single stimulation, contrasting with U-CLL that showed minimal response. Our findings suggest that concomitant engagement of BCR and TLR leads to differential responses in CLL depending on the mutational status of the BCR. Differential intensity and duration of responses in M-CLL versus U-CLL indicates that the differences in signal transduction between the two subgroups may be primarily quantitative rather than qualitative.

Targeted therapy in chronic lymphocytic leukemia: past, present, and future

BACKGROUND

Chronic lymphocytic leukemia (CLL) is the most prevalent leukemia in the western world. Recent advances in understanding the biology of B-cell malignancies have resulted in the development of novel agents targeting key prosurvival pathways in the neoplastic B cell.

OBJECTIVE

The goal of this article was to summarize current literature on the emerging therapeutic approaches in CLL and B-cell malignancies.

METHODS

A literature review was performed, identifying pathways and key clinical trials involving novel therapies in CLL, with special emphasis on B-cell receptor (BCR)-targeting agents.

RESULTS

Understanding the biology of the BCR-signaling pathway has led to identification of novel molecular targets. Most notably, inhibitors of Bruton tyrosine kinase and phosphatidylinositide 3-kinase-δ have entered clinical trials and demonstrated high response rates in CLL, including high-risk disease. Cyclin-dependent kinase inhibitors may evolve into an alternative therapeutic approach in CLL. New drugs that target molecules within and outside of the BCR-signaling pathway have shown promise in preclinical studies.

CONCLUSIONS

Both preclinical and early clinical trial results involving novel targeted therapies suggest that the standard treatment paradigm in CLL and B-cell malignancies will soon change. Particular attention should be paid to the BCR-targeting agents, whose favorable adverse effect profile may improve the lives of elderly patients with CLL.

B-cell antigen receptor signaling in chronic lymphocytic leukemia: therapeutic targets and translational opportunities

B-cell chronic lymphocytic leukemia (CLL) is characterized by clonally expanded and molecularly heterogeneous populations of B lymphocytes with impaired apoptotic mechanisms. This occurs as a result of multiple genetic and epigenetic abnormalities, including chromosomal aberrations and enhancer region hypomethylation, often impinging on intracellular signaling pathways that are essential to normal B-cell activation, proliferation, and survival. The B-cell antigen receptor (BCR) signaling is one such pathway usurped by malignant B cells, as exemplified by the early phase clinical success achieved by small-molecule agents targeting key players involved in the pathway. Such new targeted agents, including those that inhibit the function of Spleen tyrosine kinase (SYK), Bruton's tyrosine kinase (BTK), phosphatidylinositol 3-kinases (PI3K), and B-cell lymphoma 2 (BCL-2), along with the current standard therapy comprising chemo-immunotherapies with or without B-cell depleting biologic agent rituximab (anti-CD20 monoclonal antibody), should expand the armamentarium for CLL therapy. We review the therapeutic agents currently in clinical development which target different effectors of the malignant BCR signaling, and discuss their overlapping and discriminating translational opportunities in the context of CLL treatment.

LCK is an important mediator of B-cell receptor signaling in chronic lymphocytic leukemia cells

B-cell receptor (BCR) signals promote survival of chronic lymphocytic leukemia (CLL) cells, and it is believed that overexpressed and constitutively active Lyn mediates this signaling. Here, we show that CLL cells express lymphocyte-specific protein tyrosine kinase (LCK) and that inhibition of this Src family tyrosine kinase with the specific inhibitor [4-amino-5-(4-phenoxyphenyl)-7H-pyrrolo[3,2-d]pyrimidin-7-yl-cyclopentane (Lck-i)], or reduction of its expression with siRNA, blocks the induction of CD79a, Syk, inhibitor of IκB kinase (IKK), Akt, and extracellular signal-regulated kinase (ERK) phosphorylation by BCR cross-linking in these cells. Furthermore, we show that CLL cells with high levels of LCK expression have higher levels of BCR-mediated IKK, Akt, and ERK phosphorylation as well as cell survival than CLL cells with low levels of LCK expression. We also show that treatment of CLL cells with Lck-i inhibits BCR cross-linking-induced cell survival. Taken together, these data show a major role for LCK in proximal and distal BCR-mediated signaling in CLL cells and suggest that LCK expression is important in the pathogenesis of this disease. On a clinical level, these studies advocate the use of specific LCK inhibitors in the treatment of progressive CLL.

IgD cross-linking induces gene expression profiling changes and enhances apoptosis in chronic lymphocytic leukemia cells

Gene profile and functional changes upon IgD cross-linking were evaluated in chronic lymphocytic leukemia (CLL). Microarrays highlighted responsiveness to IgD in all cases, independently of clinico-biological characteristics. Stimulated samples exhibited the down-regulation of transcripts of B-cell receptor signaling and cell-adhesion at 24h and the up-modulation of differentiation and apoptosis genes at 48 h. A significant increase in apoptosis upon ligation was also documented. Furthermore, comparison between IgD and IgM stimulation displayed a differential transcriptional/functional response. In conclusion, CLL respond to IgD displaying expression changes and cell-death enhancement, indicating the apoptosis induction via-IgD as an alternative approach for CLL management.

Signaling pathways activated by the B-cell receptor in chronic lymphocytic leukemia

Over the past decade, several features of the B-cell receptor (BCR) complex have emerged as major markers for prognostic classification of B-cell chronic lymphocytic leukemia (B-CLL). In particular, the absence of somatic mutations within the immunoglobulin variable heavy chain genes (IGHV), the presence of ZAP-70 and a higher ability of the BCR to translate signals within the cell have been associated with an aggressive clinical course. Indeed, the stratification of patients with B-CLL based on BCR features suggests that heterogeneity of B-CLL clinical courses may reflect BCR signaling differences that have arisen during the evolution of leukemia. Therefore, characterizing BCR signaling profiles may help to identify signaling markers useful for patient stratification, disease monitoring and therapeutic targeting in B-CLL.

The miR-17∼92 family regulates the response to Toll-like receptor 9 triggering of CLL cells with unmutated IGHV genes

Chronic lymphocytic leukemia (CLL) cells from clinically aggressive cases have a greater capacity to respond to external microenvironmental stimuli, including those transduced through Toll-like-receptor-9 (TLR9). Concomitant microRNA and gene expression profiling in purified CLL cells (n=17) expressing either unmutated (UM) or mutated (M) IGHV genes selected microRNAs from the miR-17∼92 family as significantly upregulated and in part responsible for modifications in the gene expression profile of UM CLL cells stimulated with the TLR9 agonist CpG. Notably, the stable and sustained upregulation of miR-17∼92 microRNAs by CpG was preceded by a transient induction of the proto-oncogene MYC. The enforced expression of miR-17, a major member from this family, reduced the expression of the tumor suppressor genes E2F5, TP53INP1, TRIM8 and ZBTB4, and protected cells from serum-free-induced apoptosis (P ≤ 0.05). Consistently, transfection with miR-17∼92 family antagomiRs reduced Bromo-deoxy-uridine incorporation in CpG-stimulated UM CLL cells. Finally, miR-17 expression levels, evaluated in 83 CLL samples, were significantly higher in UM (P=0.03) and ZAP-70(high) (P=0.02) cases. Altogether, these data reveal a role for microRNAs of the miR-17∼92 family in regulating pro-survival and growth-promoting responses of CLL cells to TLR9 triggering. Overall, targeting of this pathway may represent a novel therapeutic option for management of aggressive CLL.

Mechanisms and clinical significance of BIM phosphorylation in chronic lymphocytic leukemia

B-cell receptor and microenvironment-derived signals promote accumulation of chronic lymphocytic leukemia (CLL) cells through increased proliferation and/or decreased apoptosis. In this study, we investigated the regulation of BIM, a proapoptotic BCL2-related protein, which is tightly regulated by phosphorylation. Surface IgM stimulation increased phosphorylation of 2 BIM isoforms, BIM(EL) and BIM(L), in a subset of CLL samples. In contrast, in normal B cells, anti-IgM triggered selective phosphorylation of BIM(EL) only. In CLL, anti-IgM-induced BIM phosphorylation correlated with unmutated IGHV gene status and with progressive disease. Strikingly, it was also associated with progressive disease within the mutated IGHV gene subset. BIM phosphorylation was dependent on MEK1/2 kinase activity, and we identified BIM(EL) serine 69, previously linked to pro-survival responses, as the major site of phosphorylation in CLL and in Ramos cells. BIM(EL)/BIM(L) phosphorylation was associated with release of the pro-survival protein MCL1. Coculture of CLL cells with HK cells, a model of the CLL microenvironment, promoted CLL cell survival and was associated with MEK1/2 activation and BIM(EL) phosphorylation. Hence, BIM phosphorylation appears to play a key role in apoptosis regulation in CLL cells, potentially coordinating antigen and microenvironment-derived survival signals. Antigen-mediated effects on BIM may be an important determinant of clinical behavior.

Surface IgM stimulation induces MEK1/2-dependent MYC expression in chronic lymphocytic leukemia cells

Although long considered as a disease of failed apoptosis, it is now clear that chronic lymphocytic leukemia (CLL) cells undergo extensive cell division in vivo, especially in progressive disease. Signaling via the B-cell receptor is thought to activate proliferation and survival pathways in CLL cells and also has been linked to poor outcome. Here, we have analyzed the expression of the proto-oncoprotein MYC, an essential positive regulator of the cell cycle, after stimulation of surface IgM (sIgM). MYC expression was rapidly increased after sIgM stimulation in a subset of CLL samples. The ability of sIgM stimulation to increase MYC expression was correlated with sIgM-induced intracellular calcium fluxes. MYC induction was partially dependent on the MEK/ERK signaling pathway, and MYC and phosphorylated ERK1/2 were both expressed within proliferation centers in vivo. Although stimulation of sIgD also resulted in ERK1/2 phosphorylation, responses were relatively short lived compared with sIgM and were associated with significantly reduced MYC induction, suggesting that the kinetics of ERK1/2 activation is a critical determinant of MYC induction. Our results suggest that ERK1/2-dependent induction of MYC is likely to play an important role in antigen-induced CLL cell proliferation.

Phase II study of dasatinib in relapsed or refractory chronic lymphocytic leukemia

PURPOSE

Chronic lymphocytic leukemia (CLL) cells treated with dasatinib in vitro undergo apoptosis via inhibition of Lyn kinase. Thus, in this study we tested the activity of dasatinib in patients with relapsed CLL.

EXPERIMENTAL DESIGN

Patients were eligible for this phase II trial if they had documented CLL/SLL and had failed at least 1 prior therapy with a fludarabine-containing regimen and if they required therapy according to NCI-WG criteria. The starting dose of dasatinib was 140 mg daily.

RESULTS

Fifteen patients were enrolled, with a median age of 59 and a median of 3 prior regimens. All patients had received fludarabine, and 5 were fludarabine-refractory. Eleven of the 15 (73%) had high risk del(11q) or del(17p) cytogenetics. The primary toxicity was myelosuppression, with grade 3 or 4 neutropenia and thrombocytopenia in 10 and 6 patients, respectively. Partial responses by NCI-WG criteria were achieved in 3 of the 15 patients (20%; 90% CI: 6-44). Among the remaining 12 patients, 5 had nodal responses by physical exam, and 1 patient had a nodal and lymphocyte response but with severe myelosuppression. Pharmacodynamic studies indicated apoptosis in peripheral blood CLL cells within 3 to 6 hours after dasatinib administration, associated with downregulation of Syk (spleen tyrosine kinase) mRNA.

CONCLUSIONS

Dasatinib as a single agent has activity in relapsed and refractory CLL.

The potential for dasatinib in treating chronic lymphocytic leukemia, acute myeloid leukemia, and myeloproliferative neoplasms

Dasatinib is a kinase inhibitor that inhibits BCR-ABL, Src family kinases, c-Kit, and platelet-derived growth factor receptor kinase. It is licensed for the first- and second-line treatment of chronic myeloid leukemia and second-line treatment of Philadelphia chromosome-positive acute lymphoblastic leukemia on the basis of BCR-ABL inhibition, but the activity of dasatinib against additional molecular targets may enable treatment of other hematologic disorders. Potential targets for dasatinib in chronic lymphocytic leukemia (CLL) include Lyn (a Src family kinase), ABL, and the associated CD40 pathway. Although dasatinib monotherapy has modest clinical activity in CLL, ongoing studies are evaluating combination treatment. In acute myeloid leukemia (AML), FLT3, Lyn, c-Kit, and BCR-ABL are expressed in a subpopulation of patients. To date, clinical responses to dasatinib in patients with unselected AML have been mixed and larger studies are needed, particularly correlating clinical response to molecular markers. Imatinib has been used successfully to treat patients with chronic eosinophilic disorders with the FIP1L1-PDGFRA fusion kinase; limited clinical data indicate that dasatinib could be active in imatinib-resistant disease. Ongoing clinical studies should further define the value of dasatinib in these disorders.

Dipeptidyl peptidase 2 apoptosis assay determines the B-cell activation stage and predicts prognosis in chronic lymphocytic leukemia

OBJECTIVE

Dipeptidyl peptidase 2 (DPP2/DPP7) is a regulator of quiescence as inhibition of DPP2 results in apoptosis of resting, but not activated lymphocytes. The purpose of the present study was to investigate the prognostic value of DPP2 inhibition and the role of DPP2 in cell cycle in chronic lymphocytic leukemia (CLL).

MATERIALS AND METHODS

We screened 152 peripheral blood samples from patients with CLL in an apoptosis assay with AX8819, a DPP2-specific inhibitor. The apoptotic response was correlated with B-cell receptor signaling and cell cycle and molecular prognostic factors.

RESULTS

We categorized CLL into two prognostic subgroups. Inhibition of DPP2 induced apoptosis in 60% of CLL, while 40% were resistant to apoptosis. Resistance to apoptosis correlated with unmutated IgV(H) and increased ZAP-70 expression and was associated with unfavorable clinical outcomes. Sensitive CLL B cells expressed high p27, low c-Myc protein levels and decreased Syk phosphorylation, indicative of a resting phenotype. DPP2 inhibition in those cells resulted in apoptosis accompanied by enhanced phosphorylation of Syk, degradation of p27 and p130, and upregulation of c-Myc, indicative of activation and inappropriate cell cycle entry. Resistant CLL demonstrated baseline low p27 and high c-Myc protein levels and increased pSyk, indicative of an activated phenotype. Inhibition of heat shock protein 90 in this subset of CLL partially reversed apoptosis resistance.

CONCLUSIONS

The DPP2 apoptosis assay provides a reliable prognostic factor in CLL. CLL B cells sensitive to DPP2 inhibition are in true G(0), while resistant CLL B-cells are partially activated. DPP2 inhibition alone or with concomitant inhibition of heat shock protein 90 warrants investigation as a therapeutic modality in CLL.

ZAP-70: an essential kinase in T-cell signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that plays a critical role in the events involved in initiating T-cell responses by the antigen receptor. Here we review the structure of ZAP-70, its regulation, its role in development and in disease. We also describe a model experimental system in which ZAP-70 function can be interrupted by a small chemical inhibitor.

The proliferative response to CpG-ODN stimulation predicts PFS, TTT and OS in patients with chronic lymphocytic leukemia

We recently reported that leukemic cells from IgVH-unmutated/progressive CLL more frequently proliferate in response to CpG-ODN stimulation than their corresponding counterparts. Here we evaluated the prognostic impact of this proliferative response in 91 CLL patients. The proliferative response was highly predictive of PFS, TTT and OS in the whole series and refined prognosis in patients with M-CLL. BCR stimulation modulated the response to CpG-ODN, suggesting that the proliferative capacity of the leukemic cells is related to antigen-encounter history. These data support the hypothesis that the capacity of the leukemic cells to respond to external stimuli influences disease progression in CLL.

Urokinase-receptor-mediated phenotypic changes in vascular smooth muscle cells require the involvement of membrane rafts

The cholesterol-enriched membrane microdomains lipid rafts play a key role in cell activation by recruiting and excluding specific signalling components of cell-surface receptors upon receptor engagement. Our previous studies have demonstrated that the GPI (glycosylphosphatidylinositol)-linked uPAR [uPA (urokinase-type plasminogen activator) receptor], which can be found in lipid rafts and in non-raft fractions, can mediate the differentiation of VSMCs (vascular smooth muscle cells) towards a pathophysiological de-differentiated phenotype. However, the mechanism by which uPAR and its ligand uPA regulate VSMC phenotypic changes is not known. In the present study, we provide evidence that the molecular machinery of uPAR-mediated VSMC differentiation employs lipid rafts. We show that the disruption of rafts in VSMCs by membrane cholesterol depletion using MCD (methyl-beta-cyclodextrin) or filipin leads to the up-regulation of uPAR and cell de-differentiation. uPAR silencing by means of interfering RNA resulted in an increased expression of contractile proteins. Consequently, disruption of lipid rafts impaired the expression of these proteins and transcriptional activity of related genes. We provide evidence that this effect was mediated by uPAR. Similar effects were observed in VSMCs isolated from Cav1Z(-/-) (caveolin-1-deficient) mice. Despite the level of uPAR being significantly higher after the disruption of the rafts, uPA/uPAR-dependent cell migration was impaired. However, caveolin-1 deficiency impaired only uPAR-dependent cell proliferation, whereas cell migration was strongly up-regulated in these cells. Our results provide evidence that rafts are required in the regulation of uPAR-mediated VSMC phenotypic modulations. These findings suggest further that, in the context of uPA/uPAR-dependent processes, caveolae-associated and non-associated rafts represent different signalling membrane domains.

High TCL1 levels are a marker of B-cell receptor pathway responsiveness and adverse outcome in chronic lymphocytic leukemia

Although activation of the B-cell receptor (BCR) signaling pathway is implicated in the pathogenesis of chronic lymphocytic leukemia (CLL), its clinical impact and the molecular correlates of such response are not clearly defined. T-cell leukemia 1 (TCL1), the AKT modulator and proto-oncogene, is differentially expressed in CLL and linked to its pathogenesis based on CD5(+) B-cell expansions arising in TCL1-transgenic mice. We studied here the association of TCL1 levels and its intracellular dynamics with the in vitro responses to BCR stimulation in 70 CLL cases. The growth kinetics after BCR engagement correlated strongly with the degree and timing of induced AKT phospho-activation. This signaling intensity was best predicted by TCL1 levels and the kinetics of TCL1-AKT corecruitment to BCR membrane activation complexes, which further included the kinases LYN, SYK, ZAP70, and PKC. High TCL1 levels were also strongly associated with aggressive disease features, such as advanced clinical stage, higher white blood cell counts, and shorter lymphocyte doubling time. Higher TCL1 levels independently predicted an inferior clinical outcome (ie, shorter progression-free survival, P < .001), regardless of therapy regimen, especially for ZAP70(+) tumors. We propose TCL1 as a marker of the BCR-responsive CLL subset identifying poor prognostic cases where targeting BCR-associated kinases may be therapeutically useful.

Down-regulation of CXCR4 and CD62L in chronic lymphocytic leukemia cells is triggered by B-cell receptor ligation and associated with progressive disease

Progressive cases of B-cell chronic lymphocytic leukemia (CLL) are frequently associated with lymphadenopathy, highlighting a critical role for signals emanating from the tumor environment in the accumulation of malignant B cells. We investigated on CLL cells from 30 untreated patients the consequence of B-cell receptor (BCR) triggering on the membrane expression of CXCR4 and CD62L, two surface molecules involved in trafficking and exit of B-lymphocytes from lymph nodes. BCR stimulation promoted a strictly simultaneous down-regulation of CXCR4 and CD62L membrane expression to a variable extent. The variable BCR-dependent decrease of the two proteins was strikingly representative of the heterogeneous capacity of the CLL cells to respond to BCR engagement in a given patient. Functionally, cells down-regulating CXCR4 and CD62L in response to BCR engagement displayed a reduction in both migration toward CXCL12 and adhesion to lymphatic endothelial cells. Remarkably, the ability of CLL cells to respond to BCR ligation was correlated with unfavorable prognostic markers and short progression-free survival. In conclusion, BCR signaling promotes decrease of CXCR4 and CD62L membrane expression in progressive cases only. These results are consistent with the hypothesis that BCR-mediated signaling pathways favor accumulation of a proliferative pool within the lymph nodes of progressive CLL cases.

Spleen tyrosine kinase is overexpressed and represents a potential therapeutic target in chronic lymphocytic leukemia

B-cell receptor signaling contributes to apoptosis resistance in chronic lymphocytic leukemia (CLL), limiting the efficacy of current therapeutic approaches. In this study, we investigated the expression of spleen tyrosine kinase (SYK), a key component of the B-cell receptor signaling pathway, in CLL and its role in apoptosis. Gene expression profiling identified enhanced expression of SYK and downstream pathways in CLL compared with healthy B cells. Immunoblotting showed increased expression and phosphorylation of SYK, PLCgamma(2), signal transducers and activators of transcription 3, and extracellular signal regulated kinase 1/2 in CLL compared with healthy B cells, suggesting enhanced activation of these mediators in CLL. SYK inhibitors reduced phosphorylation of SYK downstream targets and induced apoptosis in primary CLL cells. With respect to prognostic factors, SYK inhibitors exerted stronger cytotoxic effects in unmutated and ZAP70(+) cases. Cytotoxic effects of SYK inhibitors also associated with SYK protein expression, potentially predicting response to therapy. Combination of fludarabine with SYK Inhibitor II or R406 increased cytotoxicity compared with fludarabine therapy alone. We observed no stroma-contact-mediated drug resistance for SYK inhibitors as described for fludarabine treatment. CD40 ligation further enhanced efficacy of SYK inhibition. Our data provide mechanistic insight into the recently observed therapeutic effects of the SYK inhibitor R406 in CLL. Combination of SYK inhibitors with fludarabine might be a novel treatment option particularly for CLL patients with poor prognosis and should be further evaluated in clinical trials.

Molecular and cellular mechanisms of CLL: novel therapeutic approaches

The mainstay of therapy of chronic lymphocytic leukemia (CLL) is cytotoxic chemotherapy; however, CLL is still an incurable disease with resistance to therapy developing in the majority of patients. In recent years, our understanding of the biological basis of CLL pathogenesis has substantially improved and novel treatment strategies are emerging. Tailoring and individualizing therapy according to the molecular and cellular biology of the disease is on the horizon, and advances with targeted agents such as monoclonal antibodies combined with traditional chemotherapy have lead to improved remission rates. The proposed key role of the B-cell receptor (BCR) in CLL pathogenesis has led to a number of possible opportunities for therapeutic exploitation. We are beginning to understand that the microenvironment is of utmost importance in CLL because certain T-cell subsets and stromal cells support the outgrowth and development of the malignant clone. Furthermore, an increase in our understanding of the deregulated cell-death machinery in CLL is a prerequisite to developing new targeted strategies that might be more effective in engaging with the cell-death machinery. This Review summarizes the progress made in understanding these features of CLL biology and describes novel treatment strategies that have also been exploited in current clinical trials.

The structure, regulation, and function of ZAP-70

The tyrosine ZAP-70 (zeta-associated protein of 70 kDa) kinase plays a critical role in activating many downstream signal transduction pathways in T cells following T-cell receptor (TCR) engagement. The importance of ZAP-70 is evidenced by the severe combined immunodeficiency that occurs in ZAP-70-deficient mice and humans. In this review, we describe recent analyses of the ZAP-70 crystal structure, revealing a complex regulatory mechanism of ZAP-70 activity, the differential requirements for ZAP-70 and spleen tyrosine kinase (SyK) in early T-cell development, as well as the role of ZAP-70 in chronic lymphocytic leukemia and autoimmunity. Thus, the critical importance of ZAP-70 in TCR signaling and its predominantly T-cell-restricted expression pattern make ZAP-70 an attractive drug target for the inhibition of pathological T-cell responses in disease.

Constitutive activation of distinct BCR-signaling pathways in a subset of CLL patients: a molecular signature of anergy

Stimulation through the B-cell antigen receptor (BCR) is believed to be involved in the natural history of chronic lymphocytic leukemia (CLL). Some cases respond to the in vitro cross-linking of surface immunoglobulin (sIg) with effective activation. In contrast, the remaining cases do not respond to such stimulation, thereby resembling B cells anergized after antigen encounter in vivo. However the biochemical differences between the 2 groups are ill defined, and in humans the term B-cell anergy lacks a molecular definition. We examined the expression and activation of key molecules involved in signaling pathways originating from the BCR, and we report that a proportion of CLL patients (a) expresses constitutively phosphorylated extracellular signal-regulated kinase (ERK)1/2 in the absence of AKT activation; (b) displays constitutive phosphorylation of MEK1/2 and increased nuclear factor of activated T cells (NF-AT) transactivation; and (c) is characterized by cellular unresponsiveness to sIg ligation. This molecular profile recapitulates the signaling pattern of anergic murine B cells. Our data indicate that constitutive activation of mitogen activated protein (MAP) kinase signaling pathway along with NF-AT transactivation in the absence of AKT activation may also represent the molecular signature of anergic human B lymphocytes. CLL cases with this signature may be taken as a human model of anergic B cells aberrantly expanded.

The Aiolos transcription factor is up-regulated in chronic lymphocytic leukemia

The Aiolos transcription factor, member of the Ikaros family of zinc finger proteins, plays an important role in the control of mature B lymphocyte differentiation and proliferation, and its function appears to be modulated through alternative splicing. To assess Aiolos isoform role in humans' pathologies, we studied Aiolos variant distribution and expression in mature B lymphoproliferative disorders (chronic lymphocytic leukemia [CLL] and other B-cell lymphomas). We demonstrated that more than 80% of expressed Aiolos in normal as well as in malignant B cells is of the hAio1 type, and we showed for the first time a homogeneous overexpression of the total amounts of Aiolos transcripts in the B cells of CLL patients, independently of ZAP-70 and IgVH mutational status prognosis factors. This up-regulation of Aiolos, confirmed at protein level, seems independent of Aiolos promoter H3K9 acetylation and H3K4 trimethylation.

ZAP-70 enhances IgM signaling independent of its kinase activity in chronic lymphocytic leukemia

We transduced chronic lymphocytic leukemia (CLL) cells lacking ZAP-70 with vectors encoding ZAP-70 or various mutant forms of ZAP-70 and monitored the response of transduced CLL cells to treatment with F(ab)(2) anti-IgM (anti-mu). CLL cells made to express ZAP-70, a kinase-defective ZAP-70 (ZAP-70-KA(369)), or a ZAP-70 unable to bind c-Cbl (ZAP-YF(292)) experienced greater intracellular calcium flux and had greater increases in the levels of phosphorylated p72(Syk), B-cell linker protein (BLNK), and phospholipase C-gamma, and greater activation of the Ig accessory molecule CD79b in response to treatment with anti-mu than did mock-transfected CLL cells lacking ZAP-70. Transfection of CLL cells with vectors encoding truncated forms of ZAP-70 revealed that the SH2 domain, but not the SH1 domain, was necessary to enhance intracellular calcium flux in response to treatment with anti-mu. We conclude that ZAP-70 most likely acts as an adapter protein that facilitates B-cell receptor (BCR) signaling in CLL cells independent of its tyrosine kinase activity or its ability to interact with c-Cbl.

Signaling pathways activated by antigen-receptor engagement in chronic lymphocytic leukemia B-cells

Several features of the B-cell receptor (BCR) have emerged as major prognostic factors in chronic lymphocytic leukemia (CLL). In particular, the absence of somatic mutations in the immunoglobulin variable region genes and expression of the protein tyrosine kinase ZAP-70 are strongly associated with an aggressive clinical course, and both features have been correlated with a greater capacity of the BCR to transmit antigen-induced signals. Additionally, differences in BCR structure and reactivity indicate that CLL B-cells from the two prognostic subsets may recognize different types of antigens. Antigens that are not rapidly internalized induce sustained BCR signaling that can promote the survival of the leukemic B-cells. The BCR signal is initially transmitted by the Syk kinase and modulated by ZAP-70, which shows inefficient or absent tyrosine kinase activation in B-cells. However, both ZAP-70 expression and sustained BCR engagement have been associated with prolonged activation of the Akt and ERK kinases, which is required for the induction of several antiapoptotic proteins, including Mcl-1, Bcl-xL and XIAP. Therefore, targeting components along the BCR signaling pathway may be a promising strategy for the treatment of CLL.

Expression levels of CD38 in T cells predict course of disease in male patients with B-chronic lymphocytic leukemia

CD38 expression of tumor cells has been identified as an important prognostic factor in B-cell chronic lymphocytic leukemia (B-CLL). Although CD38 is involved in effector functions of T cells, the prognostic value of CD38+ T cells has not yet been addressed in B-CLL. In the present study, CD38-expression levels in B-CLL cells and T cells from 204 patients were analyzed by flow cytometry and correlated with clinical and molecular risk parameters. CD38 expression significantly differed in the neoplastic clone from patients with low versus advanced stage, irrespective of the sex of patients. In contrast, CD38 expression was generally higher in T cells from female compared with male patients but only increased in male patients in a stage-dependent manner. In male patients, combined analysis of CD38 in T cells and B-CLL cells identified 4 subgroups with significantly different treatment-free survival. Multivariate analysis including Rai stage and molecular risk parameters of the neoplastic clone identified CD38-expression levels in T cells as an independent prognostic factor in male patients. Combined analysis of CD38 in B-CLL and T cells is superior in predicting outcome of male B-CLL patients than either parameter alone. Further studies are needed to elucidate the underlying mechanisms of the sex-specific role of CD38+ T cells in B-CLL.

B-cell receptor signaling in chronic lymphocytic leukemia cells is regulated by overexpressed active protein kinase CβII

Signals through the B-cell antigen receptor (BCR) are important for the survival of chronic lymphocytic leukemia (CLL) cells. Therefore, factors that influence these signals have important pathophysiological roles in this disease. One key mediator of BCR signaling is protein kinase C β (PKCβ), which regulates the activation of I-κB kinases and the deactivation of Bruton tyrosine kinase within the signaling pathways initiated by BCR engagement. The present study demonstrates that overexpression of the PKCβII isoform is a feature of CLL cells and that activity of this enzyme strongly correlates with CLL cell response to BCR engagement. Thus, intracellular Ca2+ release and increases in cell survival after BCR cross-linking were significantly greater in CLL patients with low levels than in CLL patients with high levels of active PKCβII. Furthermore, BCR-induced Ca2+ fluxes could be restored in CLL patients with high levels of active PKCβII by pretreating the cells with the PKCβ-specific inhibitor LY379196. Conversely, BCR-mediated intracellular Ca2+ release could be inhibited in CLL cells with low levels of active PKCβII by pretreatment with the PKC agonist bryostatin. Taken together, these results demonstrate that overexpressed active PKCβII plays a role in the regulation and outcome of BCR signals that can be important for the progression of CLL.

c-Abl Expression in Chronic Lymphocytic Leukemia Cells: Clinical and Therapeutic Implications

c-Abl is important for normal B-cell development, but little is known about the function of this nonreceptor tyrosine kinase in chronic lymphocytic leukemia (CLL). Therefore, the aim of the present study was to examine the clinical, therapeutic, and pathogenetic importance of c-Abl in this disease. We show that the malignant cells of CLL predominantly express the type 1b splice variant of c-Abl and that the expression of c-Abl protein is higher in CLL cells than in normal peripheral blood B cells. Moreover, we show that the levels of c-Abl protein expression correlate positively with tumor burden and disease stage, and negatively with IgVH mutation. We also show that STI-571, an inhibitor of c-Abl kinase activity, induces apoptosis of CLL cells with high c-Abl expression levels through a mechanism involving inhibition of nuclear factor κB. We conclude that overexpression of c-Abl is likely to play a pathogenetic role in CLL and that STI-571 may be of potential use in the treatment of this disease. (Cancer Res 2006; 66(15): 7801-9)