An "off-the-shelf" CD2 universal CAR-T therapy for T-cell malignancies.

T-cell malignancies are associated with frequent relapse and high morbidity, which is partly due to the lack of effective or targeted treatment options. To broaden the use of CAR-T cells in pan T-cell malignancies, we developed an allogeneic "universal" CD2-targeting CAR-T cell (UCART2), in which the CD2 antigen is deleted to prevent fratricide, and the T-cell receptor is removed to prevent GvHD. UCART2 demonstrated efficacy against T-ALL and CTCL and prolonged the survival of tumor-engrafted NSG mice in vivo. To evaluate the impact of CD2 on CAR-T function, we generated CD19 CAR-T cells (UCART19) with or without CD2 deletion, single-cell secretome analysis revealed that CD2 deletion in UCART19 reduced frequencies of the effector cytokines (Granzyme-B and IFN-γ). We also observed that UCART19ΔCD2 had reduced anti-tumor efficacy compared to UCART19 in a CD19+NALM6 xenograft model. Of note is that the reduced efficacy resulting from CD2 deletion was reversed when combined with rhIL-7-hyFc, a long-acting recombinant human interleukin-7. Treatment with rhIL-7-hyFc prolonged UCART2 persistence and increased survival in both the tumor re-challenge model and primary patient T-ALL model in vivo. Together, these data suggest that allogeneic fratricide-resistant UCART2, in combination with rhIL-7-hyFc, could be a suitable approach for treating T-cell malignancies.

Anti-myeloma efficacy of CAR-iNKT is enhanced with a long-acting IL-7, rhIL-7-hyFc.

Multiple myeloma (MM), a malignancy of mature plasma cells, remains incurable. B-cell maturation antigen (BCMA) is the lead protein target for chimeric antigen receptor (CAR) therapy because of its high expression in most MM, with limited expression in other cell types, resulting in favorable on-target, off tumor toxicity. The response rate to autologous BCMA CAR-T therapy is high; however, it is not curative and is associated with risks of cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome. Outcomes in patients treated with BCMA CAR-T cells (CAR-Ts) may improve with allogeneic CAR T-cell therapy, which offer higher cell fitness and reduced time to treatment. However, to prevent the risk of graft-versus-host disease (GVHD), allogenic BCMA CAR-Ts require genetic deletion of the T-cell receptor (TCR), which has potential for unexpected functional or phenotype changes. Invariant natural killer T cells (iNKTs) have an invariant TCR that does not cause GVHD and, as a result, can be used in an allogeneic setting without the need for TCR gene editing. We demonstrate significant anti-myeloma activity of BCMA CAR-iNKTs in a xenograft mouse model of myeloma. We found that a long-acting interleukin-7 (IL-7), rhIL-7-hyFc, significantly prolonged survival and reduced tumor burden in BCMA CAR-iNKT-treated mice in both primary and re-challenge settings. Furthermore, in CRS in vitro assays, CAR-iNKTs induced less IL-6 than CAR-Ts, suggesting a reduced likelihood of CAR-iNKT therapy to induce CRS in patients. These data suggest that BCMA CAR-iNKTs are potentially a safer, effective alternative to BCMA CAR-Ts and that BCMA CAR-iNKT efficacy is further potentiated with rhIL-7-hyFc.

Single-Cell Discovery and Multiomic Characterization of Therapeutic Targets in Multiple Myeloma.

Multiple myeloma (MM) is a highly refractory hematologic cancer. Targeted immunotherapy has shown promise in MM but remains hindered by the challenge of identifying specific yet broadly representative tumor markers. We analyzed 53 bone marrow (BM) aspirates from 41 MM patients using an unbiased, high-throughput pipeline for therapeutic target discovery via single-cell transcriptomic profiling, yielding 38 MM marker genes encoding cell-surface proteins and 15 encoding intracellular proteins. Of these, 20 candidate genes were highlighted that are not yet under clinical study, 11 of which were previously uncharacterized as therapeutic targets. The findings were cross-validated using bulk RNA sequencing, flow cytometry, and proteomic mass spectrometry of MM cell lines and patient BM, demonstrating high overall concordance across data types. Independent discovery using bulk RNA sequencing reiterated top candidates, further affirming the ability of single-cell transcriptomics to accurately capture marker expression despite limitations in sample size or sequencing depth. Target dynamics and heterogeneity were further examined using both transcriptomic and immuno-imaging methods. In summary, this study presents a robust and broadly applicable strategy for identifying tumor markers to better inform the development of targeted cancer therapy. SIGNIFICANCE: Single-cell transcriptomic profiling and multiomic cross-validation to uncover therapeutic targets identifies 38 myeloma marker genes, including 11 transcribing surface proteins with previously uncharacterized potential for targeted antitumor therapy.

A long-acting interleukin-7, rhIL-7-hyFc, enhances CAR T cell expansion, persistence, and anti-tumor activity.

Chimeric antigen receptor (CAR) T cell therapy is routinely used to treat patients with refractory hematologic malignancies. However, a significant proportion of patients experience suboptimal CAR T cell cytotoxicity and persistence that can permit tumor cell escape and disease relapse. Here we show that a prototype pro-lymphoid growth factor is able to enhance CAR T cell efficacy. We demonstrate that a long-acting form of recombinant human interleukin-7 (IL-7) fused with hybrid Fc (rhIL-7-hyFc) promotes proliferation, persistence and cytotoxicity of human CAR T cells in xenogeneic mouse models, and murine CAR T cells in syngeneic mouse models, resulting in long-term tumor-free survival. Thus, rhIL-7-hyFc represents a tunable clinic-ready adjuvant for improving suboptimal CAR T cell activity.

CS1 CAR-T targeting the distal domain of CS1 (SLAMF7) shows efficacy in high tumor burden myeloma model despite fratricide of CD8+CS1 expressing CAR-T cells.

Despite improvement in treatment options for myeloma patients, including targeted immunotherapies, multiple myeloma remains a mostly incurable malignancy. High CS1 (SLAMF7) expression on myeloma cells and limited expression on normal cells makes it a promising target for CAR-T therapy. The CS1 protein has two extracellular domains - the distal Variable (V) domain and the proximal Constant 2 (C2) domain. We generated and tested CS1-CAR-T targeting the V domain of CS1 (Luc90-CS1-CAR-T) and demonstrated anti-myeloma killing in vitro and in vivo using two mouse models. Since fratricide of CD8 + cells occurred during production, we generated fratricide resistant CS1 deficient Luc90- CS1- CAR-T (ΔCS1-Luc90- CS1- CAR-T). This led to protection of CD8 + cells in the CAR-T cultures, but had no impact on efficacy. Our data demonstrate targeting the distal V domain of CS1 could be an effective CAR-T treatment for myeloma patients and deletion of CS1 in clinical production did not provide an added benefit using in vivo immunodeficient NSG preclinical models.

Ablation of VLA4 in multiple myeloma cells redirects tumor spread and prolongs survival.

Multiple myeloma (MM) is a cancer of bone marrow (BM) plasma cells, which is increasingly treatable but still incurable. In 90% of MM patients, severe osteolysis results from pathological interactions between MM cells and the bone microenvironment. Delineating specific molecules and pathways for their role in cancer supportive interactions in the BM is vital for developing new therapies. Very Late Antigen 4 (VLA4, integrin α4ß1) is a key player in cell-cell adhesion and signaling between MM and BM cells. We evaluated a VLA4 selective near infrared fluorescent probe, LLP2A-Cy5, for in vitro and in vivo optical imaging of VLA4. Furthermore, two VLA4-null murine 5TGM1 MM cell (KO) clones were generated by CRISPR/Cas9 knockout of the Itga4 (α4) subunit, which induced significant alterations in the transcriptome. In contrast to the VLA4+ 5TGM1 parental cells, C57Bl/KaLwRij immunocompetent syngeneic mice inoculated with the VLA4-null clones showed prolonged survival, reduced medullary disease, and increased extramedullary disease burden. The KO tumor foci showed significantly reduced uptake of LLP2A-Cy5, confirming in vivo specificity of this imaging agent. This work provides new insights into the pathogenic role of VLA4 in MM, and evaluates an optical tool to measure its expression in preclinical models.

Liposomal phytohemagglutinin: In vivo T-cell activator as a novel pan-cancer immunotherapy.

Immunotherapy is an attractive approach for treating cancer. T-cell engagers (TCEs) are a type of immunotherapy that are highly efficacious; however, they are challenged by weak T-cell activation and short persistence. Therefore, alternative solutions to induce greater activation and persistence of T cells during TCE immunotherapy is needed. Methods to activate T cells include the use of lectins, such as phytohemagglutinin (PHA). PHA has not been used to activate T cells in vivo, for immunotherapy, due to its biological instability and toxicity. An approach to overcome the limitations of PHA while also preserving its function is needed. In this study, we report a liposomal PHA which increased PHA stability, reduced toxicity and performed as an immunotherapeutic that is able to activate T cells for the use in future cancer immunotherapies to circumvent current obstacles in immunosuppression and T-cell exhaustion.

Co-evolution of tumor and immune cells during progression of multiple myeloma.

Multiple myeloma (MM) is characterized by the uncontrolled proliferation of plasma cells. Despite recent treatment advances, it is still incurable as disease progression is not fully understood. To investigate MM and its immune environment, we apply single cell RNA and linked-read whole genome sequencing to profile 29 longitudinal samples at different disease stages from 14 patients. Here, we collect 17,267 plasma cells and 57,719 immune cells, discovering patient-specific plasma cell profiles and immune cell expression changes. Patients with the same genetic alterations tend to have both plasma cells and immune cells clustered together. By integrating bulk genomics and single cell mapping, we track plasma cell subpopulations across disease stages and find three patterns: stability (from precancer to diagnosis), and gain or loss (from diagnosis to relapse). In multiple patients, we detect "B cell-featured" plasma cell subpopulations that cluster closely with B cells, implicating their cell of origin. We validate AP-1 complex differential expression (JUN and FOS) in plasma cell subpopulations using CyTOF-based protein assays, and integrated analysis of single-cell RNA and CyTOF data reveals AP-1 downstream targets (IL6 and IL1B) potentially leading to inflammation regulation. Our work represents a longitudinal investigation for tumor and microenvironment during MM progression and paves the way for expanding treatment options.

Strategies to Implement a Competency Assessment Verification Program.

Professional role competence is an essential element of nursing practice and an integral component of providing safe perioperative patient care. In the health care setting, verifying professional role competence and managing the associated documentation can be complex. Educators can use a variety of modalities (eg, flipped classrooms, gaming, podcasts) to present information in a manner that supports adult learning principles. When developing a competency assessment verification program, perioperative leaders should use a structured model to provide consistency; they also should partner with staff members and other key stakeholders (eg, surgeons, risk management personnel) to identify and prioritize ongoing competencies. The leaders and educators should identify competency verification methods, and leaders should designate qualified observers if needed. Documentation of competency activities should be stored in an easily accessible location. Implementing a standardized competency assessment verification program is a best practice that should result in improved patient outcomes.

Nanoparticle T-cell engagers as a modular platform for cancer immunotherapy.

T-cell-based immunotherapy, such as CAR-T cells and bispecific T-cell engagers (BiTEs), has shown promising clinical outcomes in many cancers; however, these therapies have significant limitations, such as poor pharmacokinetics and the ability to target only one antigen on the cancer cells. In multiclonal diseases, these therapies confer the development of antigen-less clones, causing tumor escape and relapse. In this study, we developed nanoparticle-based bispecific T-cell engagers (nanoBiTEs), which are liposomes decorated with anti-CD3 monoclonal antibodies (mAbs) targeting T cells, and mAbs targeting the cancer antigen. We also developed a nanoparticle that targets multiple cancer antigens by conjugating multiple mAbs against multiple cancer antigens for T-cell engagement (nanoMuTEs). NanoBiTEs and nanoMuTEs have a long half-life of about 60 h, which enables once-a-week administration instead of continuous infusion, while maintaining efficacy in vitro and in vivo. NanoMuTEs targeting multiple cancer antigens showed greater efficacy in myeloma cells in vitro and in vivo, compared to nanoBiTEs targeting only one cancer antigen. Unlike nanoBiTEs, treatment with nanoMuTEs did not cause downregulation (or loss) of a single antigen, and prevented the development of antigen-less tumor escape. Our nanoparticle-based immuno-engaging technology provides a solution for the major limitations of current immunotherapy technologies.

VLA4-Targeted Nanoparticles Hijack Cell Adhesion-Mediated Drug Resistance to Target Refractory Myeloma Cells and Prolong Survival.

PURPOSE: In multiple myeloma, drug-resistant cells underlie relapse or progression following chemotherapy. Cell adhesion-mediated drug resistance (CAM-DR) is an established mechanism used by myeloma cells (MMC) to survive chemotherapy and its markers are upregulated in residual disease. The integrin very late antigen 4 (VLA4; α4ß1) is a key mediator of CAM-DR and its expression affects drug sensitivity of MMCs. Rather than trying to inhibit its function, here, we hypothesized that upregulation of VLA4 by resistant MMCs could be exploited for targeted delivery of drugs, which would improve safety and efficacy of treatments. EXPERIMENTAL DESIGN: We synthetized 20 nm VLA4-targeted micellar nanoparticles (V-NP) carrying DiI for tracing or a novel camptothecin prodrug (V-CP). Human or murine MMCs, alone or with stroma, and immunocompetent mice with orthotopic multiple myeloma were used to track delivery of NPs and response to treatments. RESULTS: V-NPs selectively delivered their payload to MMCs in vitro and in vivo, and chemotherapy increased their uptake by surviving MMCs. V-CP, alone or in combination with melphalan, was well tolerated and prolonged survival in myeloma-bearing mice. V-CP also reduced the dose requirement for melphalan, reducing tumor burden in association with suboptimal dosing without increasing overall toxicity. CONCLUSIONS: V-CP may be a safe and effective strategy to prevent or treat relapsing or refractory myeloma. V-NP targeting of resistant cells may suggest a new approach to environment-induced resistance in cancer.

Development of [89Zr]DFO-elotuzumab for immunoPET imaging of CS1 in multiple myeloma.

PURPOSE: Multiple myeloma (MM) is a bone marrow malignancy that remains mostly incurable. Elotuzumab is an FDA-approved therapeutic monoclonal antibody targeted to the cell surface glycoprotein CS1, which is overexpressed in MM cells. Identifying patients who will respond to CS1-targeted treatments such as elotuzumab requires the development of a companion diagnostic to assess the presence of CS1. Here, we evaluated [89Zr]DFO-elotuzumab as a novel PET tracer for imaging CS1 expression in preclinical MM models. METHODS: Conjugation of desferrioxamine-p-benzyl-isothiocyanate (DFO-Bz-NCS) to elotuzumab enabled zirconium-89 radiolabeling. MM.1S-CG cells were intravenously injected in NOD SCID gamma (NSG) mice. Small animal PET imaging with [89Zr]DFO-elotuzumab (1.11 MBq/mouse, 7 days post-injection), [89Zr]DFO-IgG (1.11 MBq/mouse, 7 days post-injection), and [18F]FDG (7-8 MBq, 1 h post-injection) was performed. Additionally, biodistribution of [89Zr]DFO-elotuzumab post-imaging at 7 days was also done. In vivo specificity of [89Zr]DFO-elotuzumab was further evaluated with a blocking study and ex vivo autoradiography. RESULTS: [89Zr]DFO-elotuzumab was produced with high specific activity (56 ± 0.75 MBq/nmol), radiochemical purity (99% ± 0.5), and yield (93.3% ± 1.5). Dissociation constant of 40.4 nM and receptor density of 126 fmol/mg was determined in MM.1S-CG cells. Compared to [89Zr]DFO-IgG, [89Zr]DFO-elotuzumab localized with a significantly higher standard uptake value in tumor-bearing bone tissue (8.59 versus 4.77). Blocking with unlabeled elotuzumab significantly reduced (P < 0.05) uptake of [89Zr]DFO-elotuzumab in the bones. Importantly, while [18F]FDG demonstrated similar uptake in the bone and muscle, [89Zr]DFO-elotuzumab showed > 3-fold enhanced uptake in bones. CONCLUSION: These data demonstrate the feasibility of [89Zr]DFO-elotuzumab as a companion diagnostic for CS1-targeted therapies.

EZH2 Overexpression in Multiple Myeloma: Prognostic Value, Correlation With Clinical Characteristics, and Possible Mechanisms.

BACKGROUND: EZH2 is a histone methyltransferase that suppresses genes involved in cell cycle control. Overexpression of EZH2 has been associated with a poor prognosis in various malignancies. Pawlyn et al recently reported poor outcomes in patients with multiple myeloma and overexpression of EZH2. In order to validate these findings, we analyzed EZH2 expression and outcomes among patients from the CoMMpass study. PATIENTS AND METHODS: We extracted clinical, expression, and genomic data from Interim Analysis 13 of the MMRF CoMMpass study, which harbors data from over 1000 patients with multiple myeloma. Correlations were drawn between EZH2 expression and common genetic mutations. We analyzed the association of EZH2 overexpression with progression-free (PFS) and overall survival (OS). RESULTS: The estimated median PFS for patients with EZH2 overexpression was 20.2 months (95% confidence interval [CI], 16.3-25.5 months) compared with 37.2 months (95% CI, 31.5-40.7 months) for patients without (P < .001). The estimated median OS for patients with EZH2 overexpression was 52.3 months (95% CI, 38.5 months to unable to quantitate), whereas the median OS had not been reached for those without (P < .001). EZH2 overexpression was more common in those with 17p and 1q deletions, TP53 missense mutations, and certain KRAS mutations. Coinciding BRAF and EZH2 amplification occurred frequently. CONCLUSION: EZH2 overexpression is associated with worse outcomes among patients with multiple myeloma from the CoMMpass study. Its known association with p53 and other drivers of malignancy support further lab-based and clinical study in multiple myeloma.

A multiple myeloma-specific capture sequencing platform discovers novel translocations and frequent, risk-associated point mutations in IGLL5.

Multiple myeloma (MM) is a disease of copy number variants (CNVs), chromosomal translocations, and single-nucleotide variants (SNVs). To enable integrative studies across these diverse mutation types, we developed a capture-based sequencing platform to detect their occurrence in 465 genes altered in MM and used it to sequence 95 primary tumor-normal pairs to a mean depth of 104×. We detected cases of hyperdiploidy (23%), deletions of 1p (8%), 6q (21%), 8p (17%), 14q (16%), 16q (22%), and 17p (4%), and amplification of 1q (19%). We also detected IGH and MYC translocations near expected frequencies and non-silent SNVs in NRAS (24%), KRAS (21%), FAM46C (17%), TP53 (9%), DIS3 (9%), and BRAF (3%). We discovered frequent mutations in IGLL5 (18%) that were mutually exclusive of RAS mutations and associated with increased risk of disease progression (p = 0.03), suggesting that IGLL5 may be a stratifying biomarker. We identified novel IGLL5/IGH translocations in two samples. We subjected 15 of the pairs to ultra-deep sequencing (1259×) and found that although depth correlated with number of mutations detected (p = 0.001), depth past ~300× added little. The platform provides cost-effective genomic analysis for research and may be useful in individualizing treatment decisions in clinical settings.

An "off-the-shelf" fratricide-resistant CAR-T for the treatment of T cell hematologic malignancies.

T cell malignancies represent a group of hematologic cancers with high rates of relapse and mortality in patients for whom no effective targeted therapies exist. The shared expression of target antigens between chimeric antigen receptor (CAR) T cells and malignant T cells has limited the development of CAR-T because of unintended CAR-T fratricide and an inability to harvest sufficient autologous T cells. Here, we describe a fratricide-resistant "off-the-shelf" CAR-T (or UCART7) that targets CD7+ T cell malignancies and, through CRISPR/Cas9 gene editing, lacks both CD7 and T cell receptor alpha chain (TRAC) expression. UCART7 demonstrates efficacy against human T cell acute lymphoblastic leukemia (T-ALL) cell lines and primary T-ALL in vitro and in vivo without the induction of xenogeneic GvHD. Fratricide-resistant, allo-tolerant "off-the-shelf" CAR-T represents a strategy for treatment of relapsed and refractory T-ALL and non-Hodgkin's T cell lymphoma without a requirement for autologous T cells.

Inhibition of 6-phosphofructo-2-kinase (PFKFB3) suppresses glucose metabolism and the growth of HER2+ breast cancer.

PURPOSE: Human epidermal growth factor receptor-2 (HER2) has been implicated in the progression of multiple tumor types, including breast cancer, and many downstream effectors of HER2 signaling are primary regulators of cellular metabolism, including Ras and Akt. A key downstream metabolic target of Ras and Akt is the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 isozyme (PFKFB3), whose product, fructose-2,6-bisphosphate (F26BP), is a potent allosteric activator of a rate-limiting enzyme in glycolysis, 6-phosphofructo-1-kinase (PFK-1). We postulate that PFKFB3 may be regulated by HER2 and contribute to HER2-driven tumorigenicity. METHODS: Immunohistochemistry and Kaplan-Meier analysis of HER2+ patient samples investigated the relevance of PFKFB3 in HER2+ breast cancer. In vitro genetic and pharmacological inhibition of PFKFB3 was utilized to determine effects on HER2+ breast cancer cells, while HER2 antagonist treatment assessed the mechanistic regulation on PFKFB3 expression and glucose metabolism. Administration of a PFKFB3 inhibitor in a HER2-driven transgenic breast cancer model evaluated this potential therapeutic approach in vivo. RESULTS: PFKFB3 is elevated in human HER2+ breast cancer and high PFKFB3 transcript correlated with poorer progression-free (PFS) and distant metastatic-free (DFMS) survival. Constitutive HER2 expression led to elevated PFKFB3 expression and increased glucose metabolism, while inhibition of PFKFB3 suppressed glucose uptake, F26BP, glycolysis, and selectively decreased the growth of HER2-expressing breast cancer cells. In addition, treatment with lapatinib, an FDA-approved HER2 inhibitor, decreased PFKFB3 expression and glucose metabolism in HER2+ cells. In vivo administration of a PFKFB3 antagonist significantly suppressed the growth of HER2-driven breast tumors and decreased 18F-2-deoxy-glucose uptake. CONCLUSIONS: Taken together, these data support the potential clinical utility of PFKFB3 inhibitors as chemotherapeutic agents against HER2+ breast cancer.

Collaborative Control of Cell Cycle Progression by the RNA Exonuclease Dis3 and Ras Is Conserved Across Species.

Dis3 encodes a conserved RNase that degrades or processes all RNA species via an N-terminal PilT N terminus (PIN) domain and C-terminal RNB domain that harbor, respectively, endonuclease activity and 3'-5' exonuclease activity. In Schizosaccharomyces pombe, dis3 mutations cause chromosome missegregation and failure in mitosis, suggesting dis3 promotes cell division. In humans, apparently hypomorphic dis3 mutations are found recurrently in multiple myeloma, suggesting dis3 opposes cell division. Except for the observation that RNAi-mediated depletion of dis3 function drives larval arrest and reduces tissue growth in Drosophila, the role of dis3 has not been rigorously explored in higher eukaryotic systems. Using the Drosophila system and newly generated dis3 null alleles, we find that absence of dis3 activity inhibits cell division. We uncover a conserved CDK1 phosphorylation site that when phosphorylated inhibits Dis3's exonuclease, but not endonuclease, activity. Leveraging this information, we show that Dis3's exonuclease function is required for mitotic cell division: in its absence, cells are delayed in mitosis and exhibit aneuploidy and overcondensed chromosomes. In contrast, we find that modest reduction of dis3 function enhances cell proliferation in the presence of elevated Ras activity, apparently by accelerating cells through G2/M even though each insult by itself delays G2/M. Additionally, we find that dis3 and ras genetically interact in worms and that dis3 can enhance cell proliferation under growth stimulatory conditions in murine B cells. Thus, reduction, but not absence, of dis3 activity can enhance cell proliferation in higher organisms.

Deletion of Rb1 induces both hyperproliferation and cell death in murine germinal center B cells.

The retinoblastoma gene (RB1) has been implicated as a tumor suppressor in multiple myeloma (MM), yet its role remains unclear because in the majority of cases with 13q14 deletions, un-mutated RB1 remains expressed from the retained allele. To explore the role of Rb1 in MM, we examined the functional consequences of single- and double-copy Rb1 loss in germinal center B cells, the cells of origin of MM. We generated mice without Rb1 function in germinal center B cells by crossing Rb1(Flox/Flox) with C-γ-1-Cre (Cγ1) mice expressing the Cre recombinase in class-switched B cells in a p107(-/-) background to prevent p107 from compensating for Rb1 loss (Cγ1-Rb1(F/F)-p107(-/-)). All mice developed normally, but B cells with two copies of Rb1 deleted (Cγ1-Rb1(F/F)-p107(-/-)) exhibited increased proliferation and cell death compared with Cγ1-Rb1(+/+)-p107(-/-) controls ex vivo. In vivo, Cγ1-Rb1(F/F)-p107(-/-) mice had a lower percentage of splenic B220+ cells and reduced numbers of bone marrow antigen-specific secreting cells compared with control mice. Our data indicate that Rb1 loss induces both cell proliferation and death in germinal center B cells. Because no B-cell malignancies developed after 1 year of observation, our data also suggest that Rb1 loss is not sufficient to transform post-germinal center B cells and that additional, specific mutations are likely required to cooperate with Rb1 loss to induce malignant transformation.

Whole Genome Sequence of Multiple Myeloma-Prone C57BL/KaLwRij Mouse Strain Suggests the Origin of Disease Involves Multiple Cell Types.

Monoclonal gammopathy of undetermined significance (MGUS) is the requisite precursor to multiple myeloma (MM), a malignancy of antibody-producing plasma B-cells. The genetic basis of MGUS and its progression to MM remains poorly understood. C57BL/KaLwRij (KaLwRij) is a spontaneously-derived inbred mouse strain with a high frequency of benign idiopathic paraproteinemia (BIP), a phenotype with similarities to MGUS including progression to MM. Using mouse haplotype analysis, human MM SNP array data, and whole exome and whole genome sequencing of KaLwRij mice, we identified novel KaLwRij gene variants, including deletion of Samsn1 and deleterious point mutations in Tnfrsf22 and Tnfrsf23. These variants significantly affected multiple cell types implicated in MM pathogenesis including B-cells, macrophages, and bone marrow stromal cells. These data demonstrate that multiple cell types contribute to MM development prior to the acquisition of somatic driver mutations in KaLwRij mice, and suggest that MM may an inherently non-cell autonomous malignancy.