Mouse models in the study of chronic lymphocytic leukemia pathogenesis and therapy

The DLEU2/miR-15a/miR-16-1 cluster shapes the immune microenvironment of chronic lymphocytic leukemia

The development and progression of chronic lymphocytic leukemia (CLL) depend on genetic abnormalities and on the immunosuppressive microenvironment. We have explored the possibility that genetic drivers might be responsible for the immune cell dysregulation that shapes the protumor microenvironment. We performed a transcriptome analysis of coding and non-coding RNAs (ncRNAs) during leukemia progression in the Rag2-/-γc-/- MEC1-based xenotransplantation model. The DLEU2/miR-16 locus was found downmodulated in monocytes/macrophages of leukemic mice. To validate the role of this cluster in the tumor immune microenvironment, we generated a mouse model that simultaneously mimics the overexpression of hTCL1 and the germline deletion of the minimal deleted region (MDR) encoding the DLEU2/miR-15a/miR-16-1 cluster. This model provides an innovative and faster CLL system where monocyte differentiation and macrophage polarization are exacerbated, and T-cells are dysfunctional. MDR deletion inversely correlates with the levels of predicted target proteins including BCL2 and PD1/PD-L1 on murine CLL cells and immune cells. The inverse correlation of miR-15a/miR-16-1 with target proteins has been confirmed on patient-derived immune cells. Forced expression of miR-16-1 interferes with monocyte differentiation into tumor-associated macrophages, indicating that selected ncRNAs drive the protumor phenotype of non-malignant immune cells.

Impact of leukemia-associated macrophages on the progression and therapy response of chronic lymphocytic leukemia

The treatment landscape of chronic lymphocytic leukemia (CLL) has advanced remarkably over the past decade. The advent and approval of the BTK inhibitor ibrutinib and BCL-2 inhibitor venetoclax, as well as monoclonal anti-CD20 antibodies rituximab and obinutuzumab, have resulted in deep remissions and substantially improved survival outcomes for patients. However, CLL remains a complex disease with many patients still experiencing relapse and unsatisfactory treatment responses. CLL cells are highly dependent on their pro-leukemic tumor microenvironment (TME), which comprises different cellular and soluble factors. A large body of evidence suggests that CLL-associated macrophages shaped by leukemic cells play a pivotal role in maintaining CLL cell survival. In this review, we summarize the pro-survival interactions between CLL cells and macrophages, as well as the impact of the current first-line treatment agents, including ibrutinib, venetoclax, and CD20 antibodies on leukemia-associated macrophages.

Lessons learned from the Eµ-TCL1 mouse model of CLL

The Eµ-TCL1 mouse model has been used for over 20 years to study the pathobiology of chronic lymphocytic leukemia (CLL) and for preclinical testing of novel therapies. A CLL-like disease develops with increasing age in these mice due to a B cell specific overexpression of human TCL1. The reliability of this model to mirror human CLL is controversially discussed, as none of the known driver mutations identified in patients are found in Eµ-TCL1 mice. It has to be acknowledged that this mouse model was key to develop targeted therapies that aim at inhibiting the constitutive B cell receptor (BCR) signaling, a main driver of CLL. Inhibitors of BCR signaling became standard-of-care for a large proportion of patients with CLL as they are highly effective. The Eµ-TCL1 model further advanced our understanding of CLL biology owed to studies that crossed this mouse line with various transgenic mouse models and demonstrated the relevance of CLL-cell intrinsic and -extrinsic drivers of disease. These studies were instrumental in showing the relevance of the tumor microenvironment in the lymphoid tissues for disease progression and immune escape in CLL. It became clear that CLL cells shape and rely on stromal and immune cells, and that immune suppressive mechanisms and T cell exhaustion contribute to CLL progression. Based on this knowledge, new immunotherapy strategies were clinically tested for CLL, but so far with disappointing results. As some of these therapies were effective in the Eµ-TCL1 mouse model, the question arose concerning the translatability of preclinical studies in these mice. The aim of this review is to summarize lessons we have learnt over the last decades by studying CLL-like disease in the Eµ-TCL1 mouse model. The article focuses on pitfalls and limitations of the model, as well as the gained knowledge and potential of using this model for the development of novel treatment strategies to achieve the goal of curing patients with CLL.

The complexities of T-cell dysfunction in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a B-cell malignancy characterized by profound alterations and defects in the T-cell compartment. This observation has gained renewed interest as T-cell treatment strategies, which are successfully applied in more aggressive B-cell malignancies, have yielded disappointing results in CLL. Despite ongoing efforts to understand and address the observed T-cell defects, the exact mechanisms and nature underlying this dysfunction remain largely unknown. In this review, we examine the supporting signals from T cells to CLL cells in the lymph node niche, summarize key findings on T-cell functional defects, delve into potential underlying causes, and explore novel strategies for reversing these deficiencies. Our goal is to identify strategies aimed at resolving CLL-induced T-cell dysfunction which, in the future, will enhance the efficacy of autologous T-cell-based therapies for CLL patients.

Eμ-TCL1 adoptive transfer mouse model of chronic lymphocytic leukemia

Despite being the most common adult leukemia in the western world, Chronic Lymphocytic Leukemia (CLL) remains a life-threatening and incurable disease. Efforts to develop new treatments are highly dependent on the availability of appropriate mouse models for pre-clinical testing. The Eμ-TCL1 mouse model is the most established pre-clinical approach to study CLL pathobiology and response to treatment, backed by numerous studies highlighting its resemblance to the most aggressive form of this malignancy. In contrast to the transgenic Eμ-TCL1 model, employing the adoptive transfer of Eμ-TCL1-derived splenocytes in immunocompetent C57BL/6 mice results in a comparably rapid (e.g., leukemic development within weeks compared to months in the transgenic model) and reliable model mimicking CLL. In this chapter, we would like to provide readers with a thoroughly optimized, detailed, and comprehensive protocol to use the adoptive transfer Eμ-TCL1 model in their research.

In Vitro 3D Spheroid Culture System Displays Sustained T Cell-dependent CLL Proliferation and Survival

Chronic lymphocytic leukemia (CLL) cells are highly dependent on microenvironmental cells and signals. The lymph node (LN) is the critical site of in vivo CLL proliferation and development of resistance to both chemotherapy and targeted agents. We present a new model that incorporates key aspects of the CLL LN, which enables investigation of CLL cells in the context of a protective niche. We describe a three-dimensional (3D) in vitro culture system using ultra-low attachment plates to create spheroids of CLL cells derived from peripheral blood. Starting from CLL:T cell ratios as observed in LN samples, CLL activation was induced by either direct stimulation and/or indirectly via T cells. Compared with two-dimensional cultures, 3D cultures promoted CLL proliferation in a T cell-dependent manner, and enabled expansion for up to 7 weeks, including the formation of follicle-like structures after several weeks of culture. This model enables high-throughput drug screening, of which we describe response to Btk inhibition, venetoclax resistance, and T cell-mediated cytotoxicity as examples. In summary, we present the first LN-mimicking in vitro 3D culture for primary CLL, which enables readouts such as real-time drug screens, kinetic growth assays, and spatial localization. This is the first in vitro CLL system that allows testing of response and resistance to venetoclax and Bruton's tyrosine kinase inhibitors in the context of the tumor microenvironment, thereby opening up new possibilities for clinically useful applications.

B Cell Chronic Lymphocytic Leukemia Development in Mice with Chronic Lung Exposure to Coccidioides Fungal Arthroconidia

Links between repeated microbial infections and B cell chronic lymphocytic leukemia (B-CLL) have been proposed but not tested directly. This study examines how prolonged exposure to a human fungal pathogen impacts B-CLL development in Eµ-hTCL1-transgenic mice. Monthly lung exposure to inactivated Coccidioides arthroconidia, agents of Valley fever, altered leukemia development in a species-specific manner, with Coccidioides posadasii hastening B-CLL diagnosis/progression in a fraction of mice and Coccidioides immitis delaying aggressive B-CLL development, despite fostering more rapid monoclonal B cell lymphocytosis. Overall survival did not differ significantly between control and C. posadasii-treated cohorts but was significantly extended in C. immitis-exposed mice. In vivo doubling time analyses of pooled B-CLL showed no difference in growth rates of early and late leukemias. However, within C. immitis-treated mice, B-CLL manifests longer doubling times, as compared with B-CLL in control or C. posadasii-treated mice, and/or evidence of clonal contraction over time. Through linear regression, positive relationships were noted between circulating levels of CD5+/B220low B cells and hematopoietic cells previously linked to B-CLL growth, albeit in a cohort-specific manner. Neutrophils were positively linked to accelerated growth in mice exposed to either Coccidioides species, but not in control mice. Conversely, only C. posadasii-exposed and control cohorts displayed positive links between CD5+/B220low B cell frequency and abundance of M2 anti-inflammatory monocytes and T cells. The current study provides evidence that chronic lung exposure to fungal arthroconidia affects B-CLL development in a manner dependent on fungal genotype. Correlative studies suggest that fungal species differences in the modulation of nonleukemic hematopoietic cells are involved.

Progress in and Prospects of Genome Editing Tools for Human Disease Model Development and Therapeutic Applications

Programmable nucleases, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas, are widely accepted because of their diversity and enormous potential for targeted genomic modifications in eukaryotes and other animals. Moreover, rapid advances in genome editing tools have accelerated the ability to produce various genetically modified animal models for studying human diseases. Given the advances in gene editing tools, these animal models are gradually evolving toward mimicking human diseases through the introduction of human pathogenic mutations in their genome rather than the conventional gene knockout. In the present review, we summarize the current progress in and discuss the prospects for developing mouse models of human diseases and their therapeutic applications based on advances in the study of programmable nucleases.

PYK2 is overexpressed in chronic lymphocytic leukaemia: A potential new therapeutic target

Chronic Lymphocytic Leukaemia (CLL) is the most common adult B-cell leukaemia and despite improvement in patients' outcome, following the use of targeted therapies, it remains incurable. CLL supportive microenvironment plays a key role in both CLL progression and drug resistance through signals that can be sensed by the main components of the focal adhesion complex, such as FAK and PYK2 kinases. Dysregulations of both kinases have been observed in several metastatic cancers, but their role in haematological malignancies is still poorly defined. We characterized FAK and PYK2 expression and observed that PYK2 expression is higher in leukaemic B cells and its overexpression significantly correlates with their malignant transformation. When targeting both FAK and PYK2 with the specific inhibitor defactinib, we observed a dose-response effect on CLL cells viability and survival. In vivo treatment of a CLL mouse model showed a decrease of the leukaemic clone in all the lymphoid organs along with a significant reduction of macrophages and of the spleen weight and size. Our results first define a possible prognostic value for PYK2 in CLL, and show that both FAK and PYK2 might become putative targets for both CLL and its microenvironment (e.g. macrophages), thus paving the way to an innovative therapeutic strategy.

Integrin Signaling Shaping BTK-Inhibitor Resistance

Integrins are adhesion molecules that function as anchors in retaining tumor cells in supportive tissues and facilitating metastasis. Beta1 integrins are known to contribute to cell adhesion-mediated drug resistance in cancer. Very late antigen-4 (VLA-4), a CD49d/CD29 heterodimer, is a beta1 integrin implicated in therapy resistance in both solid tumors and haematological malignancies such as chronic lymphocytic leukemia (CLL). A complex inside-out signaling mechanism activates VLA-4, which might include several therapeutic targets for CLL. Treatment regimens for this disease have recently shifted towards novel agents targeting BCR signaling. Bruton’s tyrosine kinase (BTK) is a component of B cell receptor signaling and BTK inhibitors such as ibrutinib are highly successful; however, their limitations include indefinite drug administration, the development of therapy resistance, and toxicities. VLA-4 might be activated independently of BTK, resulting in an ongoing interaction of CD49d-expressing leukemic cells with their surrounding tissue, which may reduce the success of therapy with BTK inhibitors and increases the need for alternative therapies. In this context, we discuss the inside-out signaling cascade culminating in VLA-4 activation, consider the advantages and disadvantages of BTK inhibitors in CLL and elucidate the mechanisms behind cell adhesion-mediated drug resistance.

In Vitro and In Vivo Models of CLL–T Cell Interactions: Implications for Drug Testing

Simple Summary

Chronic lymphocytic leukemia (CLL) cells in the peripheral blood and lymphoid microenvironment display substantially different gene expression profiles and proliferative capaci-ty. It has been suggested that CLL–T-cell interactions are key pro-proliferative stimuli in immune niches. We review in vitro and in vivo model systems that mimic CLL-T-cell interactions to trigger CLL proliferation and study therapy resistance. We focus on studies describing the co-culture of leukemic cells with T cells, or supportive cell lines expressing T-cell factors, and simplified models of CLL cells’ stimulation with recombinant factors. In the second part, we summarize mouse models revealing the role of T cells in CLL biology and implications for generating patient-derived xenografts by co-transplanting leukemic cells with T cells.

Abstract

T cells are key components in environments that support chronic lymphocytic leukemia (CLL), activating CLL-cell proliferation and survival. Here, we review in vitro and in vivo model systems that mimic CLL–T-cell interactions, since these are critical for CLL-cell division and resistance to some types of therapy (such as DNA-damaging drugs or BH3-mimetic venetoclax). We discuss approaches for direct CLL-cell co-culture with autologous T cells, models utilizing supportive cell lines engineered to express T-cell factors (such as CD40L) or stimulating CLL cells with combinations of recombinant factors (CD40L, interleukins IL4 or IL21, INFγ) and additional B-cell receptor (BCR) activation with anti-IgM antibody. We also summarize strategies for CLL co-transplantation with autologous T cells into immunodeficient mice (NOD/SCID, NSG, NOG) to generate patient-derived xenografts (PDX) and the role of T cells in transgenic CLL mouse models based on TCL1 overexpression (Eµ-TCL1). We further discuss how these in vitro and in vivo models could be used to test drugs to uncover the effects of targeted therapies (such as inhibitors of BTK, PI3K, SYK, AKT, MEK, CDKs, BCL2, and proteasome) or chemotherapy (fludarabine and bendamustine) on CLL–T-cell interactions and CLL proliferation.

Longitudinal analyses of CLL in mice identify leukemia-related clonal changes including a Myc gain predicting poor outcome in patients

Chronic lymphocytic leukemia (CLL) is a B-cell malignancy mainly occurring at an advanced age with no single major genetic driver. Transgenic expression of TCL1 in B cells leads after a long latency to a CLL-like disease in aged Eµ-TCL1 mice suggesting that TCL1 overexpression is not sufficient for full leukemic transformation. In search for secondary genetic events and to elucidate the clonal evolution of CLL, we performed whole exome and B-cell receptor sequencing of longitudinal leukemia samples of Eµ-TCL1 mice. We observed a B-cell receptor stereotypy, as described in patients, confirming that CLL is an antigen-driven disease. Deep sequencing showed that leukemia in Eµ-TCL1 mice is mostly monoclonal. Rare oligoclonality was associated with inability of tumors to develop disease upon adoptive transfer in mice. In addition, we identified clonal changes and a sequential acquisition of mutations with known relevance in CLL, which highlights the genetic similarities and therefore, suitability of the Eµ-TCL1 mouse model for progressive CLL. Among them, a recurrent gain of chromosome 15, where Myc is located, was identified in almost all tumors in Eµ-TCL1 mice. Interestingly, amplification of 8q24, the chromosomal region containing MYC in humans, was associated with worse outcome of patients with CLL.

The Determinants of B Cell Receptor Signaling as Prototype Molecular Biomarkers of Leukemia

Advanced genome-wide association studies (GWAS) identified several transforming mutations in susceptible loci which are recognized as valuable prognostic markers in chronic lymphocytic leukemia (CLL) and B cell lymphoma (BCL). Alongside, robust genetic manipulations facilitated the generation of preclinical mouse models to validate mutations associated with poor prognosis and refractory B cell malignancies. Taken together, these studies identified new prognostic markers that could achieve characteristics of precision biomarkers for molecular diagnosis. On the contrary, the idea of augmented B cell antigen receptor (BCR) signaling as a transforming cue has somewhat receded despite the efficacy of Btk and Syk inhibitors. Recent studies from several research groups pointed out that acquired mutations in BCR components serve as faithful biomarkers, which become important for precision diagnostics and therapy, due to their relevant role in augmented BCR signaling and CLL pathogenesis. For example, we showed that expression of a single point mutated immunoglobulin light chain (LC) recombined through the variable gene segment IGLV3-21, named IGLV3-21^R110, marks severe CLL cases. In this perspective, we summarize the molecular mechanisms fine-tuning B cell transformation, focusing on immunoglobulin point mutations and recurrent mutations in tumor suppressors. We present a stochastic model for gain-of-autonomous BCR signaling and subsequent neoplastic transformation. Of note, additional mutational analyses on immunoglobulin heavy chain (HC) derived from non-subset #2 CLL IGLV3-21^R110 cases endorses our perspective. Altogether, we propose a model of malignant transformation in which the augmented BCR signaling creates a conducive platform for the appearance of transforming mutations.

Understanding CLL biology through mouse models of human genetics

Rapid advances in large-scale next-generation sequencing studies of human samples have progressively defined the highly heterogeneous genetic landscape of chronic lymphocytic leukemia (CLL). At the same time, the numerous challenges posed by the difficulties in rapid manipulation of primary B cells and the paucity of CLL cell lines have limited the ability to interrogate the function of the discovered putative disease "drivers," defined in human sequencing studies through statistical inference. Mouse models represent a powerful tool to study mechanisms of normal and malignant B-cell biology and for preclinical testing of novel therapeutics. Advances in genetic engineering technologies, including the introduction of conditional knockin/knockout strategies, have opened new opportunities to model genetic lesions in a B-cell-restricted context. These new studies build on the experience of generating the MDR mice, the first example of a genetically faithful CLL model, which recapitulates the most common genomic aberration of human CLL: del(13q). In this review, we describe the application of mouse models to the studies of CLL pathogenesis and disease transformation from an indolent to a high-grade malignancy (ie, Richter syndrome [RS]) and treatment, with a focus on newly developed genetically inspired mouse lines modeling recurrent CLL genetic events. We discuss how these novel mouse models, analyzed using new genomic technologies, allow the dissection of mechanisms of disease evolution and response to therapy with greater depth than previously possible and provide important insight into human CLL and RS pathogenesis and therapeutic vulnerabilities. These models thereby provide valuable platforms for functional genomic analyses and treatment studies.

The Modes of Dysregulation of the Proto-Oncogene T-Cell Leukemia/Lymphoma 1A

Simple Summary

T-cell leukemia/lymphoma 1A (TCL1A) is a proto-oncogene that is mainly expressed in embryonic and fetal tissues, as well as in some lymphatic cells. It is frequently overexpressed in a variety of T- and B-cell lymphomas and in some solid tumors. In chronic lymphocytic leukemia and in T-prolymphocytic leukemia, TCL1A has been implicated in the pathogenesis of these conditions, and high-level TCL1A expression correlates with more aggressive disease characteristics and poorer patient survival. Despite the modes of TCL1A (dys)regulation still being incompletely understood, there are recent advances in understanding its (post)transcriptional regulation. This review summarizes the current concepts of TCL1A’s multi-faceted modes of regulation. Understanding how TCL1A is deregulated and how this can lead to tumor initiation and sustenance can help in future approaches to interfere in its oncogenic actions.

Abstract

Incomplete biological concepts in lymphoid neoplasms still dictate to a large extent the limited availability of efficient targeted treatments, which entertains the mostly unsatisfactory clinical outcomes. Aberrant expression of the embryonal and lymphatic TCL1 family of oncogenes, i.e., the paradigmatic TCL1A, but also TML1 or MTCP1, is causally implicated in T- and B-lymphocyte transformation. TCL1A also carries prognostic information in these particular T-cell and B-cell tumors. More recently, the TCL1A oncogene has been observed also in epithelial tumors as part of oncofetal stemness signatures. Although the concepts on the modes of TCL1A dysregulation in lymphatic neoplasms and solid tumors are still incomplete, there are recent advances in defining the mechanisms of its (de)regulation. This review presents a comprehensive overview of TCL1A expression in tumors and the current understanding of its (dys)regulation via genomic aberrations, epigenetic modifications, or deregulation of TCL1A-targeting micro RNAs. We also summarize triggers that act through such transcriptional and translational regulation, i.e., altered signals by the tumor microenvironment. A refined mechanistic understanding of these modes of dysregulations together with improved concepts of TCL1A-associated malignant transformation can benefit future approaches to specifically interfere in TCL1A-initiated or -driven tumorigenesis.

Development and characterization of prototypes for in vitro and in vivo mouse models of ibrutinib-resistant CLL

Although ibrutinib improves the overall survival of patients with chronic lymphocytic leukemia (CLL), some patients still develop resistance, most commonly through point mutations affecting cysteine residue 481 (C481) in Bruton's tyrosine kinase (BTKC481S and BTKC481R). To enhance our understanding of the biological impact of these mutations, we established cell lines that overexpress wild-type or mutant BTK in in vitro and in vivo models that mimic ibrutinib-sensitive and -resistant CLL. MEC-1 cell lines stably overexpressing wild-type or mutant BTK were generated. All cell lines coexpressed GFP, were CD19+ and CD23+, and overexpressed BTK. Overexpression of wild-type or mutant BTK resulted in increased signaling, as evidenced by the induction of p-BTK, p-PLCγ2, and p-extracellular signal-related kinase (ERK) levels, the latter further augmented upon IgM stimulation. In all cell lines, cell cycle profiles and levels of BTK expression were similar, but the RNA sequencing and reverse-phase protein array results revealed that the molecular transcript and protein profiles were distinct. To mimic aggressive CLL, we created xenograft mouse models by transplanting the generated cell lines into Rag2-/-γc-/- mice. Spleens, livers, bone marrow, and peripheral blood were collected. All mice developed CLL-like disease with systemic involvement (engraftment efficiency, 100%). We observed splenomegaly, accumulation of leukemic cells in the spleen and liver, and macroscopically evident necrosis. CD19+ cells accumulated in the spleen, bone marrow, and peripheral blood. The overall survival duration was slightly lower in mice expressing mutant BTK. Our cell lines and murine models mimicking ibrutinib-resistant CLL will serve as powerful tools to test reversible BTK inhibitors and novel, non-BTK-targeted therapeutics.

AID overexpression leads to aggressive murine CLL and nonimmunoglobulin mutations that mirror human neoplasms

Most cancers become more dangerous by the outgrowth of malignant subclones with additional DNA mutations that favor proliferation or survival. Using chronic lymphocytic leukemia (CLL), a disease that exemplifies this process and is a model for neoplasms in general, we created transgenic mice overexpressing the enzyme activation-induced deaminase (AID), which has a normal function of inducing DNA mutations in B lymphocytes. AID not only allows normal B lymphocytes to develop more effective immunoglobulin-mediated immunity, but is also able to mutate nonimmunoglobulin genes, predisposing to cancer. In CLL, AID expression correlates with poor prognosis, suggesting a role for this enzyme in disease progression. Nevertheless, direct experimental evidence identifying the specific genes that are mutated by AID and indicating that those genes are associated with disease progression is not available. To address this point, we overexpressed Aicda in a murine model of CLL (Eμ-TCL1). Analyses of TCL1/AID mice demonstrate a role for AID in disease kinetics, CLL cell proliferation, and the development of cancer-related target mutations with canonical AID signatures in nonimmunoglobulin genes. Notably, our mouse models can accumulate mutations in the same genes that are mutated in human cancers. Moreover, some of these mutations occur at homologous positions, leading to identical or chemically similar amino acid substitutions as in human CLL and lymphoma. Together, these findings support a direct link between aberrant AID activity and CLL driver mutations that are then selected for their oncogenic effects, whereby AID promotes aggressiveness in CLL and other B-cell neoplasms.

Mouse Models in the Study of Mature B-Cell Malignancies

Over the past two decades, genomic analyses of several B-cell lymphoma entities have identified a large number of genes that are recurrently mutated, suggesting that their aberrant function promotes lymphomagenesis. For many of those genes, the specific role in normal B-cell development is unknown; moreover, whether and how their deregulated activity contributes to lymphoma initiation and/or maintenance is often difficult to determine. Genetically engineered mouse models that faithfully mimic lymphoma-associated genetic alterations represent valuable tools for elucidating the pathogenic roles of candidate oncogenes and tumor suppressors in vivo, as well as for the preclinical testing of novel therapeutic principles in an intact microenvironment. Here we summarize what has been learned about the mechanisms of oncogenic transformation from accurately modeling the most common and well-characterized genetic alterations identified in mature B-cell malignancies. This information is expected to guide the design of improved molecular diagnostics and mechanism-based therapeutic approaches for these diseases.

Exploiting B-cell Receptor Stereotypy to Design Tailored Immunotherapy in Chronic Lymphocytic Leukemia

PURPOSE

Approximately 30% of patients with chronic lymphocytic leukemia (CLL) can be grouped into subsets with stereotyped B-cell receptor immunoglobulin (BcR IG) displaying remarkable similarity in the heavy complementarity-determining region 3 (VH CDR3). Here, we investigated whether the consensus VH CDR3 sequences from CLL stereotyped subsets can be exploited for immunotherapy approaches.

EXPERIMENTAL DESIGN

Immunogenic epitopes from the consensus VH CDR3 sequence of the clinically aggressive subsets #1 and #2 and from Eμ-TCL1 mice, which spontaneously develop CLL with BcR IG stereotypy, were identified and used to generate specific HLA class I- and II-restricted T cells in vitro. T-cell reactivity was assayed in vitro as IFNγ production. Bone marrow-derived dendritic cells loaded with the peptides were used as vaccination strategy to restrain leukemia development in the Eμ-TCL1 mouse model.

RESULTS

These stereotyped epitopes were naturally processed and presented by CLL cells to the VH CDR3-specific T cells. Furthermore, we validated the efficacy of VH CDR3 peptide-based immunotherapy in the Eμ-TCL1 transplantable mouse model. Immunization of mice against defined VH CDR3 peptide epitopes, prior to the challenge with the corresponding leukemia cells, resulted in the control of CLL development in a significant fraction of mice, and increased overall survival.

CONCLUSIONS

Our data highlight the immunogenicity of stereotyped VH CDR3 sequences and support the feasibility and efficacy of their use for novel cancer vaccine in CLL. Such approach has the advantage to generate "off-the-shelf" therapeutic vaccines for relevant groups of patients belonging to stereotyped subsets.See related commentary by Seiffert, p. 659.

B cell signalling pathways-New targets for precision medicine in chronic lymphocytic leukaemia

The B cell receptor (BCR) is a master regulator of B cells, controlling cellular processes such as proliferation, migration and survival. Cell signalling downstream of the BCR is aberrantly activated in the B cell malignancy chronic lymphocytic leukaemia (CLL), supporting the pathophysiology of the disease. This insight has led to development and approval of small molecule inhibitors that target components of the BCR pathway. These advances have greatly improved the management of CLL, but the disease remains incurable. This may partly be explained by the inter-patient heterogeneity of the disease, also when it comes to treatment responses. Precision medicine is therefore required to optimize treatment and move towards a cure. Here, we discuss how the introduction of BCR signalling inhibitors has facilitated the development of functional in vitro assays to guide clinical treatment decisions on use of the same therapeutic agents in individual patients. The cellular responses to these agents can be analysed in high-throughput assays such as dynamic BH3 profiling, phospho flow experiments and drug sensitivity screens to identify predictive biomarkers. This progress exemplifies the positive synergy between basal and translational research needed to optimize patient care.

Genetic Loss of LCK Kinase Leads to Acceleration of Chronic Lymphocytic Leukemia

Most patients with chronic lymphocytic leukemia (CLL) exhibit an indolent disease course and unresponsive B cell receptors (BCRs) exemplified by an anergic phenotype of their leukemic cells. In up to 5% of patients, CLL transforms from an indolent subtype to an aggressive form of B cell lymphoma (Richter's syndrome), which is associated with worse disease outcome and severe downregulation of NFAT2. Here we show that ablation of the tyrosine kinase LCK, which has previously been characterized as a main NFAT2 target gene in CLL, leads to loss of the anergic phenotype, thereby restoring BCR signaling, which results in an acceleration of CLL. Our study identifies LCK as a main player in mediating BCR unresponsiveness and its role as a crucial regulator of anergy in CLL.

Meta-Analysis Reveals Significant Sex Differences in Chronic Lymphocytic Leukemia Progression in the Eµ-TCL1 Transgenic Mouse Model

The Eµ-TCL1 transgenic mouse model represents the most widely and extensively used animal model for chronic lymphocytic leukemia (CLL). In this report, we performed a meta-analysis of leukemia progression in over 300 individual Eµ-TCL1 transgenic mice and discovered a significantly accelerated disease progression in females compared to males. This difference is also reflected in an aggressive CLL mouse model with additional deletion of Tp53 besides the TCL1 transgene. Moreover, after serial adoptive transplantation of murine CLL cells, female recipients also succumbed to CLL earlier than male recipients. This sex-related disparity in the murine models is markedly contradictory to the human CLL condition. Thus, due to our observation we urge both careful consideration in the experimental design and accurate description of the Eµ-TCL1 transgenic cohorts in future studies.

Lung on a Chip for Drug Screening and Design

Lung-on-a-chip is a micro device that combines the techniques of bioengineering, microbiology, polymer science and microfluidics disciplines in order to mimic physicochemical features and microenvironments, multicellular constructions, cell-cell interfaces of a human lung. Specifically, most novel lung on a chip designs consist of two micro-channeled outer parts, flexible and porous Polydimethylsiloxane (PDMS) membrane to create separation of air-blood chamber and subsidiary vacuum channels which enable stretching of the PDMS membrane to mimic movement mechanisms of the lung. Therefore, studies aim to emulate both tissue and organ functionality since it shall be creating great potential for advancing the studies about drug discovery, disease etiology and organ physiology compared with 2D (two dimensional) and 3D (three dimensional) cell culture models and current organoids. In this study, history of researches on lung anatomy and physiology, techniques of recreating lung functionality such as cell cultures in 2D and 3D models, organoids were covered and finally most advanced and recent state of the art technology product lung-on-a-chips' construction steps, advantages compared with other techniques, usage in lung modeling and diseases, present and future offers were analyzed in detail.

Oxidative stress as candidate therapeutic target to overcome microenvironmental protection of CLL

Chronic lymphocytic leukemia (CLL) cells depend on microenvironmental non-malignant cells for survival. We compared the transcriptomes of primary CLL cells cocultured or not with protective bone marrow stromal cells (BMSCs) and found that oxidative phosphorylation, mitochondrial function, and hypoxic signaling undergo most significant dysregulation in non-protected CLL cells, with the changes peaking at 6-8 h, directly before induction of apoptosis. A subset of CLL patients displayed a gene expression signature resembling that of cocultured CLL cells and had significantly worse progression-free and overall survival. To identify drugs blocking BMSC-mediated support, we compared the relevant transcriptomic changes to the Connectivity Map database. Correlation was found with the transcriptomic signatures of the cardiac glycoside ouabain and of the ipecac alkaloids emetine and cephaeline. These compounds were highly active against protected primary CLL cells (relative IC₅₀'s 287, 190, and 35 nM, respectively) and acted by repressing HIF-1α and disturbing intracellular redox homeostasis. We tested emetine in a murine model of CLL and observed decreased CLL cells in peripheral blood, spleen, and bone marrow, recovery of hematological parameters and doubling of median survival (31.5 vs. 15 days, P = 0.0001). Pathways regulating redox homeostasis are thus therapeutically targetable mediators of microenvironmental support in CLL cells.

Exome sequencing of the TCL1 mouse model for CLL reveals genetic heterogeneity and dynamics during disease development

The TCL1 mouse model is widely used to study pathophysiology, clonal evolution, and drug sensitivity or resistance of chronic lymphocytic leukemia (CLL). By performing whole exome sequencing, we present the genetic landscape of primary tumors from TCL1 mice and of TCL1 tumors serially transplanted into wild-type recipients to mimic clonal evolution. We show that similar to CLL patients, mutations in mice are frequently subclonal and heterogenous among different primary TCL1 mice. We further describe that this molecular heterogeneity mirrors heterogenous disease characteristics such as organ infiltration or CLL dependent T cell skewing. Similar to human CLL, we further observed the occurrence of novel mutations and structural variations during clonal evolution and found plasticity in the expansion of B cell receptor specific subclones. Thus, our results uncover that the genetic complexity, pathway dependence and clonal dynamics in mouse CLL are in relevant agreement to human CLL, and they are important to consider in future research using the TCL1 mouse for studying CLL.

Loss of NFAT2 expression results in the acceleration of clonal evolution in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) can be defined as a clonal expansion of B cells with stereotypic BCRs. Somatic hypermutation of the BCR heavy chains (IGVH) defines a subgroup of patients with a better prognosis. In up to 10% of CLL cases, a transformation to an aggressive B cell lymphoma (Richter's syndrome) with a dismal prognosis can be observed over time. NFAT proteins are transcription factors originally identified in T cells, which also play an important role in B cells. The TCL1 transgenic mouse is a well-accepted model of CLL. Upon B cell-specific deletion of NFAT2, TCL1 transgenic mice develop a disease resembling human Richter's syndrome. Whereas TCL1 B cells exhibit tonic anergic BCR signaling characteristic of human CLL, loss of NFAT2 expression leads to readily activated BCRs indicating different BCR usage with altered downstream signaling. Here, we analyzed BCR usage in wild-type and TCL1 transgenic mice with and without NFAT2 deletion employing conventional molecular biology techniques and next-generation sequencing (NGS). We demonstrate that the loss of NFAT2 in CLL precipitates the selection of unmutated BCRs and the preferential usage of certain VDJ recombinations, which subsequently results in the accelerated development of oligoclonal disease.

Identification of Distinct Unmutated Chronic Lymphocytic Leukemia Subsets in Mice Based on Their T Cell Dependency

Chronic lymphocytic leukemia (CLL) can be divided into prognostically distinct subsets with stereotyped or non-stereotyped, mutated or unmutated B cell receptors (BCRs). Individual subsets vary in antigen specificity and origin, but the impact of antigenic pressure on the CLL BCR repertoire remains unknown. Here, we employed IgH.TEμ mice that spontaneously develop CLL, expressing mostly unmutated BCRs of which ~35% harbor V_H11-2/Vκ14-126 and recognize phosphatidylcholine. Proportions of V_H11/Vκ14-expressing CLL were increased in the absence of functional germinal centers in IgH.TEμ mice deficient for CD40L or activation-induced cytidine deaminase. Conversely, in vivo T cell-dependent immunization decreased the proportions of V_H11/Vκ14-expressing CLL. Furthermore, CLL onset was accelerated by enhanced BCR signaling in Siglec-G^−/− mice or in mice expressing constitutively active Bruton's tyrosine kinase. Transcriptional profiling revealed that V_H11 and non-V_H11 CLL differed in the upregulation of specific pathways implicated in cell signaling and metabolism. Interestingly, principal component analyses using the 148 differentially expressed genes revealed that V_H11 and non-V_H11 CLL clustered with BCR-stimulated and anti-CD40-stimulated B cells, respectively. We identified an expression signature consisting of 13 genes that were differentially expressed in a larger panel of T cell-dependent non-V_H11 CLL compared with T cell-independent V_H11/Vκ14 or mutated IgH.TEμ CLL. Parallel differences in the expression of these 13 signature genes were observed between heterogeneous and stereotypic human unmutated CLL. Our findings provide evidence for two distinct unmutated CLL subsets with a specific transcriptional signature: one is T cell-independent and B-1 cell-derived while the other arises upon antigen stimulation in the context of T-cell help.

Abnormal regulation of BCR signalling by c-Cbl in chronic lymphocytic leukaemia

Abnormalities of molecules involved in signal transduction pathways are connected to Chronic Lymphocytic Leukemia (CLL) pathogenesis and a critical role has been already ascribed to B-Cell Receptor (BCR)-Lyn axis. E3 ubiquitin ligase c-Cbl, working together with adapter protein CIN85, controls the degradation of protein kinases involved in BCR signaling. To investigate cell homeostasis in CLL, we studied c-Cbl since in normal B cells it is involved in the ubiquitin-dependent Lyn degradation and in the down-regulation of BCR signaling. We found that c-Cbl is overexpressed and not ubiquitinated after BCR engagement. We observed that c-Cbl did not associate to CIN85 in CLL with respect to normal B cells at steady state, nor following BCR engagement. c-Cbl association to Lyn was not detectable in CLL after BCR stimulation, as it happens in normal B cells. In some CLL patients, c-Cbl is constitutively phosphorylated at Y731 and in the same subjects, it associated to regulatory subunit p85 of PI3K. Moreover, c-Cbl is constitutive associated to Cortactin in those CLL patients presenting Cortactin overexpression and bad prognosis. These results support the hypothesis that c-Cbl, rather than E3 ligase activity, could have an adaptor function in turn influencing cell homeostasis in CLL.

High-throughput sequencing of murine immunoglobulin heavy chain repertoires using single side unique molecular identifiers on an Ion Torrent PGM

With the advent of high-throughput sequencing (HTS), profiling immunoglobulin (IG) repertoires has become an essential part of immunological research. Advances in sequencing technology enable the IonTorrent Personal Genome Machine (PGM) to cover the full-length of IG mRNA transcripts. Nucleotide insertions and deletions (indels) are the dominant errors of the PGM sequencing platform and can critically influence IG repertoire assessments. Here, we present a PGM-tailored IG repertoire sequencing approach combining error correction through unique molecular identifier (UID) barcoding and indel detection through ImMunoGeneTics (IMGT), the most commonly used sequence alignment database for IG sequences. Using artificially falsified sequences for benchmarking, we found that IMGT's underlying algorithms efficiently detect 98% of the introduced indels. Undetected indels are either located at the end of the sequences or produce masked frameshifts with an insertion and deletion in close proximity. The complementary determining regions 3 (CDR3s) are returned correct for up to 3 insertions or 3 deletions through conservative culling. We further show, that our PGM-tailored unique molecular identifiers result in highly accurate HTS data if combined with the presented processing strategy. In this regard, considering sequences with at least two copies from datasets with UID families of minimum 3 reads result in correct sequences with over 99% confidence. Finally, we show that the protocol can readily be used to generate homogenous datasets for bulk sequencing of murine bone marrow samples. Taken together, this approach will help to establish benchtop-scale sequencing of IG heavy chain transcripts in the field of IG repertoire research.

Antibodies conjugated with viral antigens elicit a cytotoxic T cell response against primary CLL ex vivo

Chronic lymphocytic leukemia (CLL) is the most frequent B cell malignancy in Caucasian adults. The therapeutic armamentarium against this incurable disease has recently seen a tremendous expansion with the introduction of specific pathway inhibitors and innovative immunotherapy. However, none of these approaches is curative and devoid of side effects. We have used B-cell-specific antibodies conjugated with antigens (AgAbs) of the Epstein-Barr virus (EBV) to efficiently expand memory CD4⁺ cytotoxic T lymphocytes (CTLs) that recognized viral epitopes in 12 treatment-naive patients with CLL. The AgAbs carried fragments from the EBNA3C EBV protein that is recognized by the large majority of the population. All CLL cells pulsed with EBNA3C-AgAbs elicited EBV-specific T cell responses, although the intensity varied across the patient collective. Interestingly, a large proportion of the EBV-specific CD4⁺ T cells expressed granzyme B (GrB), perforin, and CD107a, and killed CLL cells loaded with EBV antigens with high efficiency in the large majority of patients. The encouraging results from this preclinical ex vivo study suggest that AgAbs have the potential to redirect immune responses toward CLL cells in a high percentage of patients in vivo and warrant the inception of clinical trials.

Control of B-1a cell development by instructive BCR signaling

B-1a cells remain one of the most enigmatic lymphocyte subsets. In this review, we discuss recent advances in our understanding of the development of these cells and their regulation by the transcription factors Bhlhe41 and Arid3a as well as by the RNA-binding protein Lin28b. A large body of literature supports an instructive role of BCR signaling in B-1a cell development and lineage commitment, which is initiated only after signaling from an autoreactive BCR. While both fetal and adult hematopoiesis can generate B-1a cells, the contribution of adult hematopoiesis to the B-1a cell compartment is low under physiological conditions. We discuss several models that can reconcile the instructive role of BCR signaling with this fetal bias in B-1a cell development.

Epigenetic deregulation in chronic lymphocytic leukemia: Clinical and biological impact

Deregulated transcriptional control caused by aberrant DNA methylation and/or histone modifications is a hallmark of cancer cells. In chronic lymphocytic leukemia (CLL), the most common adult leukemia, the epigenetic 'landscape' has added a new layer of complexity to our understanding of this clinically and biologically heterogeneous disease. Early studies identified aberrant DNA methylation, often based on single gene promoter analysis with both biological and clinical impact. Subsequent genome-wide profiling studies revealed differential DNA methylation between CLLs and controls and in prognostics subgroups of the disease. From these studies, it became apparent that DNA methylation in regions outside of promoters, such as enhancers, is important for the regulation of coding genes as well as for the regulation of non-coding RNAs. Although DNA methylation profiles are reportedly stable over time and in relation to therapy, a higher epigenetic heterogeneity or 'burden' is seen in more aggressive CLL subgroups, albeit as non-recurrent 'passenger' events. More recently, DNA methylation profiles in CLL analyzed in relation to differentiating normal B-cell populations revealed that the majority of the CLL epigenome reflects the epigenomes present in the cell of origin and that only a small fraction of the epigenetic alterations represents truly CLL-specific changes. Furthermore, CLL patients can be grouped into at least three clinically relevant epigenetic subgroups, potentially originating from different cells at various stages of differentiation and associated with distinct outcomes. In this review, we summarize the current understanding of the DNA methylome in CLL, the role of histone modifying enzymes, highlight insights derived from animal models and attempts made to target epigenetic regulators in CLL along with the future directions of this rapidly advancing field.

Microenvironment-induced CD44v6 promotes early disease progression in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) outgrowth depends on signals from the microenvironment. We have previously found that in vitro reconstitution of this microenvironment induces specific variant isoforms of the adhesion molecule CD44, which confer human CLL with high affinity to hyaluronan (HA). Here, we determined the in vivo contribution of standard CD44 and its variants to leukemic B-cell homing and proliferation in Tcl1 transgenic mice with a B-cell-specific CD44 deficiency. In these mice, leukemia onset was delayed and leukemic infiltration of spleen, liver, and lungs, but not of bone marrow, was decreased. Competitive transplantation revealed that CLL homing to spleen and bone marrow required functional CD44. Notably, enrichment of CD44v6 variants particularly in spleen enhanced CLL engraftment and proliferation, along with increased HA binding. We recapitulated CD44v6 induction in the human disease and revealed the involvement of MAPK and NF-κB signaling upon CD40 ligand and B-cell receptor stimulation by in vitro inhibition experiments and chromatin immunoprecipitation assays. The investigation of downstream signaling after CD44v6-HA engagement uncovered the activation of extracellular signal-regulated kinase and p65. Consequently, anti-CD44v6 treatment reduced leukemic cell proliferation in vitro in human and mouse, confirming the general nature of the findings. In summary, we propose a CD44-NF-κB-CD44v6 circuit in CLL, allowing tumor cells to gain HA binding capacity and supporting their proliferation.

A combination of an anti-SLAMF6 antibody and ibrutinib efficiently abrogates expansion of chronic lymphocytic leukemia cells

The signaling lymphocyte activation molecule family [SLAMF] of cell surface receptors partakes in both the development of several immunocyte lineages and innate and adaptive immune responses in humans and mice. For instance, the homophilic molecule SLAMF6 (CD352) is in part involved in natural killer T cell development, but also modulates T follicular helper cell and germinal B cell interactions. Here we report that upon transplantation of a well-defined aggressive murine B220⁺CD5⁺ Chronic Lymphocytic Leukemia (CLL) cell clone, TCL1-192, into SCID mice one injection of a monoclonal antibody directed against SLAMF6 (αSlamf6) abrogates tumor progression in the spleen, bone marrow and blood. Similarly, progression of a murine B cell lymphoma, LMP2A/λMyc, was also eliminated by αSlamf6. But, surprisingly, αSLAMF6 neither eliminated TCL1-192 nor LMP2A/λMyc cells, which resided in the peritoneal cavity or omentum. This appeared to be dependent upon the tumor environment, which affected the frequency of sub-populations of the TCL1-192 clone or the inability of peritoneal macrophages to induce Antibody Dependent Cellular Cytotoxicity (ADCC). However, co-administering αSlamf6 with the Bruton tyrosine kinase (Btk) inhibitor, ibrutinib, synergized to efficiently eliminate the tumor cells in the spleen, bone marrow, liver and the peritoneal cavity. Because an anti-human SLAMF6 mAb efficiently killed human CLL cells in vitro and in vivo, we propose that a combination of αSlamf6 with ibrutinib should be considered as a novel therapeutic approach for CLL and other B cell tumors.

NFAT2 is a critical regulator of the anergic phenotype in chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is a clonal disorder of mature B cells. Most patients are characterised by an indolent disease course and an anergic phenotype of their leukaemia cells, which refers to a state of unresponsiveness to B cell receptor stimulation. Up to 10% of CLL patients transform from an indolent subtype to an aggressive form of B cell lymphoma over time (Richter´s syndrome) and show a significantly worse treatment outcome. Here we show that B cell-specific ablation of Nfat2 leads to the loss of the anergic phenotype culminating in a significantly compromised life expectancy and transformation to aggressive disease. We further define a gene expression signature of anergic CLL cells consisting of several NFAT2-dependent genes including Cbl-b, Grail, Egr2 and Lck. In summary, this study identifies NFAT2 as a crucial regulator of the anergic phenotype in CLL.NFAT2 is a transcription factor that has been linked with chronic lymphocytic leukaemia (CLL), but its functions in CLL manifestation are still unclear. Here the authors show, by analysing mouse CLL models and characterising biopsies from CLL patients, that NFAT2 is an important regulator for the anergic phenotype of CLL.

Anti-leukemic activity of microRNA-26a in a chronic lymphocytic leukemia mouse model

Dysregulation of microRNAs (miRNAs) plays an important role in the pathogenesis of chronic lymphocytic leukemia (CLL). The Eμ-TCL1 transgenic mouse develops a form of leukemia that is similar to the aggressive type of human B-CLL, and this valuable model has been widely used for testing novel therapeutic approaches. Here, we adopted this model to investigate the potential effects of miR-26a, miR-130an and antimiR-155 in CLL therapy. Improved delivery of miRNA molecules into CLL cells was obtained by developing a novel system based on lipid nanoparticles conjugated with an anti-CD38 monoclonal antibody. This methodology has proven to be highly effective in delivering miRNA molecules into leukemic cells. Short- and long-term experiments showed that miR-26a, miR-130a and anti-miR-155 increased apoptosis after in vitro and in vivo treatment. Of this miRNA panel, miR-26a was the most effective in reducing leukemic cell expansion. Following long-term treatment, apoptosis was readily detectable by analyzing cleavage of PARP and caspase-7. These effects could be directly attributed to miR-26a, as confirmed by significant downregulation of its proven targets, namely cyclin-dependent kinase 6 and Mcl1. The results of this study are relevant to two distinct areas. The first is related to the design of a technical strategy and to the selection of CD38 as a molecular target on CLL cells, both consenting efficient and specific intracellular transfer of miRNA. The original scientific finding inferred from the above approach is that miR-26a can elicit in vivo anti-leukemic activities mediated by increased apoptosis.

Chronic lymphocytic leukemia cells are active participants in microenvironmental cross-talk

The importance of the tumor microenvironment in chronic lymphocytic leukemia is widely accepted. Nevertheless, the understanding of the complex interplay between the various types of bystander cells and chronic lymphocytic leukemia cells is incomplete. Numerous studies have indicated that bystander cells provide chronic lymphocytic leukemia-supportive functions, but it has also become clear that chronic lymphocytic leukemia cells actively engage in the formation of a supportive tumor microenvironment through several cross-talk mechanisms. In this review, we describe how chronic lymphocytic leukemia cells participate in this interplay by inducing migration and tumor-supportive differentiation of bystander cells. Furthermore, chronic lymphocytic leukemia-mediated alterations in the interactions between bystander cells are discussed. Upon bystander cell interaction, chronic lymphocytic leukemia cells secrete cytokines and chemokines such as migratory factors [chemokine (C-C motif) ligand 22 and chemokine (CC motif) ligand 2], which result in further recruitment of T cells but also of monocyte-derived cells. Within the tumor microenvironment, chronic lymphocytic leukemia cells induce differentiation towards a tumor-supportive M2 phenotype of monocyte-derived cells and suppress phagocytosis, but also induce increased numbers of supportive regulatory T cells. Like other tumor types, the differentiation of stromal cells towards supportive cancer-associated fibroblasts is critically dependent on chronic lymphocytic leukemia-derived factors such as exosomes and platelet-derived growth factor. Lastly, both chronic lymphocytic leukemia and bystander cells induce a tolerogenic tumor microenvironment; chronic lymphocytic leukemia-secreted cytokines, such as interleukin-10, suppress cytotoxic T-cell functions, while chronic lymphocytic leukemia-associated monocyte-derived cells contribute to suppression of T-cell function by producing the immune checkpoint factor, programmed cell death-ligand 1. Deeper understanding of the active involvement and cross-talk of chronic lymphocytic leukemia cells in shaping the tumor microenvironment may offer novel clues for designing therapeutic strategies.

Two mouse models reveal an actionable PARP1 dependence in aggressive chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) remains an incurable disease. Two recurrent cytogenetic aberrations, namely del(17p), affecting TP53, and del(11q), affecting ATM, are associated with resistance against genotoxic chemotherapy (del17p) and poor outcome (del11q and del17p). Both del(17p) and del(11q) are also associated with inferior outcome to the novel targeted agents, such as the BTK inhibitor ibrutinib. Thus, even in the era of targeted therapies, CLL with alterations in the ATM/p53 pathway remains a clinical challenge. Here we generated two mouse models of Atm- and Trp53-deficient CLL. These animals display a significantly earlier disease onset and reduced overall survival, compared to controls. We employed these models in conjunction with transcriptome analyses following cyclophosphamide treatment to reveal that Atm deficiency is associated with an exquisite and genotype-specific sensitivity against PARP inhibition. Thus, we generate two aggressive CLL models and provide a preclinical rational for the use of PARP inhibitors in ATM-affected human CLL.

ATM and TP53 mutations are associated with poor prognosis in chronic lymphocytic leukaemia (CLL). Here the authors generate mouse models of Tp53- and Atm-defective CLL mimicking the high-risk form of human disease and show that Atm-deficient CLL is sensitive to PARP1 inhibition.

Invariant NKT cells contribute to chronic lymphocytic leukemia surveillance and prognosis

iNKT cells control CLL progression in both mice and patients and this inversely correlates with CD1d expression by leukemia cells. Human iNKT cells indirectly hinder CLL survival by restraining proleukemia monocyte-derived nurse-like cells.

Cell lines generated from a chronic lymphocytic leukemia mouse model exhibit constitutive Btk and Akt signaling

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature CD5⁺ B cells in blood. Spontaneous apoptosis of CLL cells in vitro has hampered in-depth investigation of CLL pathogenesis. Here we describe the generation of three monoclonal mouse cell lines, EMC2, EMC4 and EMC6, from the IgH.TEμ CLL mouse model based on sporadic expression of SV40 large T antigen. The cell lines exhibit a stable CD5⁺CD43⁺IgM⁺CD19⁺ CLL phenotype in culture and can be adoptively transferred into Rag1^−/− mice. RNA-seq analysis revealed only minor differences between the cell lines and their primary tumors and suggested that NF-κB and mTOR signaling pathways were involved in cell line outgrowth. In vitro survival and proliferation was dependent on constitutive phosphorylation of Bruton's tyrosine kinase (Btk) at Y551/Y223, and Akt(S473). Treatment of the cell lines with small molecule inhibitors specific for Btk (ibrutinib) or PI3K (idelalisib), which is upstream of Akt, resulted in reduced viability, proliferation and fibronectin-dependent cell adhesion. Treatment of cell line-engrafted Rag1^−/− mice with ibrutinib was associated with transient lymphocytosis, reduced splenomegaly and increased overall survival. Thus, by generating stable cell lines we established a novel platform for in vitro and in vivo investigation of CLL signal transduction and treatment modalities.

The PTPROt tyrosine phosphatase functions as an obligate haploinsufficient tumor suppressor in vivo in B-cell chronic lymphocytic leukemia

The tyrosine phosphatase PTPROt is a suggested tumor suppressor (TS) in B-cell chronic lymphocytic leukemia (CLL), and its expression is reduced in this disease. In order to examine how reduced PTPROt expression affects CLL in vivo we induced CLL in PTPROt-targeted mice. Unexpectedly, loss of both Ptprot alleles delayed disease detection and progression and lengthened survival relative to mice carrying two intact alleles, indicating that PTPROt fulfills a novel tumor-promoting role in CLL. Tumor cells from mice lacking PTPROt exhibited reduced B-cell receptor (BCR)-induced signaling, as well as increased apoptosis and autophagy. Inhibition of BCR/Src signaling in CLL cells induced their apoptosis, indicating that these findings are linked causally. These results suggest a cell-autonomous mechanism for the weakened CLL phenotype of PTPROt-deficient mice and uncover non-redundant roles for PTPROt in support of BCR signaling and survival of CLL cells. In contrast, loss of only one Ptprot allele induced earlier detection and progression of CLL and reduced survival, consistent with a tumor-suppressing role for PTPROt. Tumor cells from mice lacking one or both Ptprot allele exhibited increased interleukin-10 (IL-10) expression and signaling, factors known to support CLL; cells lacking one Ptprot alleles exhibited normal BCR signaling and cell death rates. We conclude that loss of one Ptprot allele promotes CLL, most likely by activating IL-10 signaling. Loss of both Ptprot alleles also reduces BCR signaling and increases cell death rates, offsetting the IL-10 effects and reducing the severity of the disease. PTPROt thus functions as an obligate haploinsufficient TS in CLL, where its expression levels determine its role as a promoter or inhibitor of the tumorigenic process in mice. Partial loss of PTPROt generates the strongest disease phenotype, suggesting that its intermediate expression levels in CLL are selected for.

Integrated Cellular and Plasma Proteomics of Contrasting B-cell Cancers Reveals Common, Unique and Systemic Signatures

Approximately 800,000 leukemia and lymphoma cases are diagnosed worldwide each year. Burkitt's lymphoma (BL) and chronic lymphocytic leukemia (CLL) are examples of contrasting B-cell cancers; BL is a highly aggressive lymphoid tumor, frequently affecting children, whereas CLL typically presents as an indolent, slow-progressing leukemia affecting the elderly. The B-cell-specific overexpression of the myc and TCL1 oncogenes in mice induce spontaneous malignancies modeling BL and CLL, respectively. Quantitative mass spectrometry proteomics and isobaric labeling were employed to examine the biology underpinning contrasting Eμ-myc and Eμ-TCL1 B-cell tumors. Additionally, the plasma proteome was evaluated using subproteome enrichment to interrogate biomarker emergence and the systemic effects of tumor burden. Over 10,000 proteins were identified (q<0.01) of which 8270 cellular and 2095 plasma proteins were quantitatively profiled. A common B-cell tumor signature of 695 overexpressed proteins highlighted ribosome biogenesis, cell-cycle promotion and chromosome segregation. Eμ-myc tumors overexpressed several methylating enzymes and underexpressed many cytoskeletal components. Eμ-TCL1 tumors specifically overexpressed ER stress response proteins and signaling components in addition to both subunits of the interleukin-5 (IL5) receptor. IL5 treatment promoted Eμ-TCL1 tumor proliferation, suggesting an amplification of IL5-induced AKT signaling by TCL1. Tumor plasma contained a substantial tumor lysis signature, most prominent in Eμ-myc plasma, whereas Eμ-TCL1 plasma contained signatures of immune-response, inflammation and microenvironment interactions, with putative biomarkers in early-stage cancer. These findings provide a detailed characterization of contrasting B-cell tumor models, identifying common and specific tumor mechanisms. Integrated plasma proteomics allowed the dissection of a systemic response and a tumor lysis signature present in early- and late-stage cancers, respectively. Overall, this study suggests common B-cell cancer signatures exist and illustrates the potential of the further evaluation of B-cell cancer subtypes by integrative proteomics.

EBI2 overexpression in mice leads to B1 B-cell expansion and chronic lymphocytic leukemia-like B-cell malignancies

Human and mouse chronic lymphocytic leukemia (CLL) develops from CD5⁺ B cells that in mice and macaques are known to define the distinct B1a B-cell lineage. B1a cells are characterized by lack of germinal center (GC) development, and the B1a cell population is increased in mice with reduced GC formation. As a major mediator of follicular B-cell migration, the G protein-coupled receptor Epstein-Barr virus-induced gene 2 (EBI2 or GPR183) directs B-cell migration in the lymphoid follicles in response to its endogenous ligands, oxysterols. Thus, upregulation of EBI2 drives the B cells toward the extrafollicular area, whereas downregulation is essential for GC formation. We therefore speculated whether increased expression of EBI2 would lead to an expanded B1 cell subset and, ultimately, progression to CLL. Here, we demonstrate that B-cell-targeted expression of human EBI2 (hEBI2) in mice reduces GC-dependent immune responses, reduces total immunoglobulin M (IgM) and IgG levels, and leads to increased proliferation and upregulation of cellular oncogenes. Furthermore, hEBI2 overexpression leads to an abnormally expanded CD5⁺ B1a B-cell subset (present as early as 4 days after birth), late-onset lymphoid cancer development, and premature death. These findings are highly similar to those observed in CLL patients and identify EBI2 as a promoter of B-cell malignancies.

LYN Kinase in the Tumor Microenvironment Is Essential for the Progression of Chronic Lymphocytic Leukemia

Survival of chronic lymphocytic leukemia (CLL) cells strictly depends on the support of an appropriate tumor microenvironment. Here, we demonstrate that LYN kinase is essential for CLL progression. Lyn deficiency results in a significantly reduced CLL burden in vivo. Loss of Lyn within leukemic cells reduces B cell receptor (BCR) signaling including BTK phosphorylation, but surprisingly does not affect leukemic cell expansion. Instead, syngeneic CLL transplantation of CLL cells into Lyn- or Btk-deficient recipients results in a strongly delayed leukemic progression and prolonged survival. Moreover, Lyn deficiency in macrophages hinders nursing functions for CLL cells, which is mediated by direct contact rather than secretion of soluble factors. Taken together, LYN and BTK seem essential for the formation of a microenvironment supporting leukemic growth.

Constitutive Kit activity triggers B-cell acute lymphoblastic leukemia-like disease in mice

Acute lymphoblastic leukemia (ALL) is the most common childhood malignancy and, in most cases, is of pro- or pre-B cell origin (B-ALL). The receptor tyrosine kinase KIT is expressed by hematopoietic stem and precursor cells. Gain-of-function mutations of KIT cause systemic mastocytosis, which is characterized by abnormal accumulations of mast cells. We previously reported a mouse model of mastocytosis based on conditional expression of a constitutively active Kit protein. Half of these animals developed leukemic disease of B-lineage origin. Herein, we report that this condition bears striking similarities to human B-ALL. The immuno-phenotype of the leukemic cells was compatible with a pro-B-cell origin, as was the finding of immunoglobulin heavy-chain gene rearrangements in all cases, whereas light-chain loci were mostly not rearranged. Leukemogenesis was independent of pre-B-cell receptor expression. Primary leukemic cells and permanent cell lines derived from these were serially transplantable and rapidly killed the recipients. In a few animals, the leukemia was of T-cell origin with abnormal CD4/8 double-positive T-cell precursors dominating in the circulation. In summary, we report a novel ALL mouse model that may prove useful for in vivo drug testing and identification of novel oncogenic mutations and principles.

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.

Factors involved in CLL pathogenesis and cell survival are disrupted by differentiation of CLL B-cells into antibody-secreting cells

Recent research has shown that chronic lymphocytic leukemia (CLL) B-cells display a strong tendency to differentiate into antibody-secreting cells (ASCs) and thus may be amenable to differentiation therapy. However, the effect of this differentiation on factors associated with CLL pathogenesis has not been reported. In the present study, purified CLL B-cells were stimulated to differentiate into ASCs by phorbol myristate acetate or CpG oligodeoxynucleotide, in combination with CD40 ligand and cytokines in a two-step, seven-day culture system. We investigated (i) changes in the immunophenotypic, molecular, functional, morphological features associated with terminal differentiation into ASCs, (ii) the expression of factors involved in CLL pathogenesis, and (iii) the expression of pro- and anti-apoptotic proteins in the differentiated cells. Our results show that differentiated CLL B-cells are able to display the transcriptional program of ASCs. Differentiation leads to depletion of the malignant program and deregulation of the apoptosis/survival balance. Analysis of apoptosis and the cell cycle showed that differentiation is associated with low cell viability and a low rate of cell cycle entry. Our findings shed new light on the potential for differentiation therapy as a part of treatment strategies for CLL.

Chronic lymphocytic leukemia: Time to go past genomics?

Recent advances in massively parallel sequencing technologies have provided a detailed picture of the mutational landscape in CLL and underscored the vast degree of interpatient and intratumor heterogeneities. These studies have led to the characterization of novel putative driver genes and recurrently affected biological pathways, and to the modeling of CLL clonal evolution. We herein review selected aspects including recent advances in the biology of CLL and present cellular and biological processes involved in the development of CLL and potentially other mature B-cell lymphoproliferative neoplasms.