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Abstract Abstract 1802 Poster Board I-828 Background Heterogeneity in multiple myeloma (MM) leads to disparate outcomes with a wide variation in overall survival (OS). Based upon the evidence predominantly gathered from newly diagnosed patients treated with standard chemotherapy and/or transplantation, the current Mayo (mSMART) prognostic model uses a combination of metaphase cytogenetics, fluorescence in-situ hybridization (FISH) and plasma cell labeling index (PCLI) for risk-stratification at diagnosis of MM. The objective of our study was to evaluate the significance of this stratification model in the era of novel therapies, and to assess the independent prognostic value of each of the components in the model. Methods Data were abstracted from a cohort of 1556 consecutive patients with MM who presented to our institution between January 1999 and March 2009 within ninety days of diagnosis of MM A total of three hundred and eighty six patients had been evaluated with conventional cytogenetics near diagnosis in addition to interphase FISH. Out of this cohort, three hundred and sixty six patients had availability of all the three tests, including the slide-based PCLI. High-risk MM was defined as presence of any one or more of the following: cytogenetics: hypodiploidy, monosomy of chromosome 13 or deletion 13q ; FISH: deletion of p53 (locus 17p13) or immunoglobulin heavy chain (IgH) translocations, t(4;14) (p16.3;q32) or t(14;16)(q32;q23); PCLI ≥3%. Survival estimates were created using the Kaplan-Meier method and compared by log-rank tests. The impact of each of the three tests was assessed by a multivariate (MV) analysis using Cox proportional hazards model. Results The median OS of the cohort of 366 patients was 65 months from presentation. One hundred and twelve (31%) patients were deemed high risk by any one or more of the three tests: Metaphase cytogenetics, FISH or PCLI (current mSMART definition of high-risk MM). The median OS of high risk patients was 52 months versus 71 months for standard-risk (P=0.007). We then studied the prognostic value of the three tests separately. Patients classified as high risk by FISH had a median OS of 35 months versus 65 months for the standard risk, HR= 2.3, P<0.001. Similarly, cytogenetics alone was also able to classify patients into high and standard risk with a comparable HR of 2.1 (P=0.008, median survival 37 vs. 64 months). In contrast, PCLI did not have a significant prognostic value (HR =1.41, P=0.32). This was confirmed on multivariate analysis in a model that included all 3 tests. FISH (HR=2.36, P=0.001) and cytogenetics (HR=1.99; P=0.017) emerged as independent determinants of prognosis in the multivariable analysis, while PCLI (HR=1.03, P=0.93) did not provide additional prognostic value. We then examined whether the prognostic value of FISH contributed additionally in patients classified into high and standard risk by conventional cytogenetics, and vice versa. Those classified as high risk by cytogenetics (n=44, 11 %), were further categorized into 2 groups by additional prognostic information obtained by FISH (median survival 18 vs. 52 months; P=0.005). Similarly, those classified as standard risk by cytogenetics (n=342, 89 %), were further categorized into 2 groups (median survival 52 vs. 71 months; P=0.017) by additional FISH-based stratification. Similarly, high-risk patients defined by FISH (n=65, 17 %) were further classified into 2 groups with significant difference in median OS (18 vs. 52 months, P=0.004), based on the presence or absence of high-risk features on cytogenetics. In contrast, no additional prognostic information was gained by cytogenetics in patients who were stratified as standard risk by FISH alone (n=321, 83%). Conclusion Our study validates high risk features defined by FISH and conventional cytogenetics in the mSMART risk stratification model among patients treated after the introduction of novel therapies. Either test identifies a high risk cohort with a hazard ratio of approximately 2 (median survival ∼37 versus ∼65 months). In patients known to have high risk disease by one of the two tests, knowledge of the other appears to add additional prognostic value. Disclosures Fonseca: Amgen, BMS, Celgene, Otsuka, Genzyme and Medtronic: Consultancy. Gertz:celgene, millennium: Honoraria. Dispenzieri:celgene: Research Funding. Kumar:celgene, genzyme, millennium, novartis, bayer: Research Funding; genzyme: Membership on an entity's Board of Directors or advisory committees.

Abstract Abstract 194 Background: Immunomodulatory drugs (IMiDs) are key components of treatment for hematologic malignancies, especially multiple myeloma (MM). Recently, we demonstrated that CRBN expression is the major mediator of IMiD action operating at least in part via interferon regulatory factor 4 (IRF4), however the clinical significance of this finding remains unclear. Methods: To better understand clinical impact, we retrospectively reviewed 148 MM patients whose tumor cells were tested for CRBN expression by Gene Expression Profiling (GEP) prior to treatment with an IMiD containing therapy. Results: GEP studies show no significant differences in CRBN expression between MGUS, smoldering MM (SMM), untreated symptomatic MM and normal plasma cells. However, differences were observed between MM cell lines of varying drug sensitivity to IMiDs and between other B-cell Neoplasia (eg low grade lymphoma has higher levels). Within the genetic subtypes of MM, levels were significantly higher for hyperdiploid patients (median 1.26) as opposed to significantly lower levels for TC class D2 (median 0.76) and average for the 4p16 (median 0.97). We next screened the University of Arkansas Medical School (UAMS) GEP database of MM patients treated with combination therapies and did not find significant correlation between CRBN expression and survival in this multi-agent regimen. We then focused on patients that received only single agent IMiD therapy with low dose dexamethasone. Review of the twelve lowest expressing patients demonstrated a lack of response to IMiDs in any patient. Finally we analyzed 53 relapsed or relapsed/refractory patients treated homogeneously in a study with pomalidomide 2–4mg daily and weekly dexamethasone 40mg. CRBN levels were assessed prior to therapy initiation. Response rates varied significantly based on gene expression level. The percentage of patients that demonstrated at least a partial response was 0% for CRBN <0.81, 19% for CRBN expression of 0.81–0.90, and 33% for CBN expression >0.9. Significant differences in PFS (3.0 months vs. 8.9 months, p = 0.0006) and in OS (9.1 months vs. 27.2 months, p = 0.01) were observed when the lowest quartile of CRBN expression was compared to the top three quartiles. Values used for the cut offs are as follows: (25%: 0.889687; 50%: 1.026542, 75%: 1.211133). While this study demonstrates a positive correlation between CRBN expression and response to pomalidomide and dexamethasone, an important caveat with respect to PFS and OS is that CRBN mRNA level is primarily a reflection of the number of copies of CRBN. Since focal deletions are uncommon, this mostly reflects chromosome 3p copy number. Since trisomy 3 is common in hyperdiploid, good prognosis disease, CRBN not only is required for IMiD function but higher levels also serve as a surrogate for low risk disease. Optimal gene expression cut offs for survival were determined using the Contal and O'Quigley method. The optimal cut point for PFS was 1.18443 and for OS was 1.17816. Conclusion: In summary, this study demonstrates, for the first time, a clinical correlation between CRBN expression and response to IMiD and dexamethasone therapy. CRBN expression is also predictive of survival outcomes and when quantified prior to therapy could potentially be utilized as a predictive biomarker for response to an IMiD based treatment. Disclosures: Stewart: Celgene: Consultancy, Honoraria.

Abstract Abstract 832 We hypothesized that new therapeutic targets for multiple myeloma (MM) could be discovered through the integrative computational analysis of genomic data. Accordingly, we generated gene expression profiling and copy number data on 250 clinically-annotated MM patient samples. Utilizing an outlier statistical approach, we identified HOXA9 as the top candidate gene for further investigation. HOXA9 expression was particularly high in patients lacking canonical MM chromosomal translocations, and allele-specific expression analysis suggested that this overexpression was mono-allelic. Indeed, focal copy number amplifications at the HOXA locus were observed in some patients. Outlier HOXA9 expression was further validated in both a collection of 52 MM cell lines and 414 primary patient samples previously described. To further verify the aberrant expression of HOXA9 in MM, we performed quantitative RT-PCR, which confirmed expression in all MM patients and cell lines tested, with high-level expression in a subset. To further investigate the mechanism of aberrant HOXA9 expression, we interrogated the pattern of histone modification at the HOXA locus because HOXA gene expression is particularly regulated by such chromatin marks. Accordingly, immunoprecipitation studies showed an aberrantly low level of histone 3 lysine 27 trimethylation marks (H3K27me3) at the HOXA9 locus. H3K27me3 modification is normally associated with silencing of HOXA9 in normal B-cell development. As such, it appears likely that the aberrant expression of HOXA9 in MM is due at least in part to defects in histone modification at this locus. To determine the functional consequences of HOXA9 expression in MM, we performed RNAi-mediated knock-down experiments in MM cell lines. Seven independent HOXA9 shRNAs that diminished HOXA9 expression resulted in growth inhibition of 12/14 MM cell lines tested. Taken together, these experiments indicate that HOXA9 is essential for survival of MM cells, and that the mechanism of HOXA9 expression relates to aberrant histone modification at the HOXA9 locus. The data thus suggest that HOXA9 is an attractive new therapeutic target for MM. Disclosures: No relevant conflicts of interest to declare.

Abstract Abstract 132 Multiple myeloma (MM), an incurable neoplasia of terminally differentiated plasma cells, are critically dependent on their interactions with bone marrow stromal cells (BMSC) for essential survival signals, growth and immunosuppressive factors. Very little is known about the specific BM cell type or the molecular elements in these interactions, an understanding of which could provide novel targets that could be interdicted to enhance conventional chemotherapy. A potential MM surface protein that could be involved in these interactions is CD28, based on its known pro-survival role in T cells. Clinical studies have shown that expression of CD28 in multiple myeloma highly correlates (p=0.006) with myeloma tumoral expansion. Moreover, CD28+ MM cells invariably express the CD28 ligand CD86. A survival role for MM-CD28 might involve interactions with BM cells that express B7 (CD80/CD86) such as dendritic cells (DCs, that are known to be closely associated with MM cells in the BM) or with CD86+ MM cells themselves. We had previously shown (ASH2008, #I-769) that blocking CD28-CD86 interactions between myeloma cells with high affinity B7 ligand CTLA4Ig (Abatacept®) sensitized myeloma cells to chemotherapy. Now we show that myeloma cells co-cultured with myeloid DCs in vitro derive both direct and indirect survival signals from DCs, and this can be partially blocked by commercially available reagents. Our data show that flow cytometric analysis of mononuclear cells (MNC) from BM aspirates of myeloma patients with increased CD138+ plasma cell populations (9-58%), show an increased CD11b+ (myeloid) population (20-37%) as well, which is in contrast to healthy transplant donor controls (12-15% CD11b+, 4–6% CD138+). Moreover, a larger fraction (11-47%) of the myeloma CD138+ plasma cells expressed CD28 compared to healthy control (3.3-7.7%). Also, when we analyzed gene expression datasets (NCBI #GSE5900 and GSE4204) from plasma cells (PC) of normal donors, monoclonal gammopathy of undetermined significance (MGUS), smoldering myeloma (SM) and newly diagnosed multiple myeloma (MM), we found a progressive increase in patients showing CD28 expression with increasing severity of disease (normal<MGUS<SM<MM) (Fig 1A). When we sorted the highest scoring MM group (n=538) into 8 genetic subgroups as defined earlier, CD28 expression was found to peak in the MF subgroup (typically associated with poor survival in myeloma patients) (Zhan et al. 2006, Blood 108, pp. 2020) relative to total population (p<0.0001) (Fig 1B). Antibody mediated activation of MM-CD28 over 48 hrs increased viability of myeloma cell line MM.1S cultured under serum starvation (3.7%) or with drugs ATO (1.9%), melphalan (18%) or dexamethasone (3.3%) to 66%, 21%, 33% and 11% respectively. Viability of MM.1S cells or primary CD138+ plasma cells (isolated from myeloma BM aspirates) cultured under serum starvation was enhanced >3 fold (p<0.001) when co-cultured with monocyte derived DCs, and in MM.1S this was partially reversed when either MM-CD28 or DC-B7 was blocked (Fig 2). Similar protection of MM.1S was also observed against a gradient of dexamethasone or melphalan. CD28 activation was accompanied by rapid tyrosine phosphorylation of CD28, association of p85 (PI3K), activation of Vav-1 and increase in CD28 associated tyrosine kinase activity, as shown by immunoprecipitation, western and kinase activity assays. We had previously shown that MM-CD28 interaction drive DC production of pro-survival factor IL-6 and immunosuppressive factor IDO via DC-B7 “backsignaling” (ASH2008 #I-769). Now we show that MM induced DC production of IL-6 (8 ng/ml) was partially inhibited in presence of CD28 blocking αCD28(Fab) fragments (3 ng/ml) or with protein kinase C (PKC) inhibitor Bisindolylmaleimide-I (2.1ng/ml). Activity of the immunosuppressive enzyme IDO in these co-cultures was completely inhibited in the presence of a novel IDO inhibitor from Incyte corporation, and this helped partially reverse IDO mediated suppression of T-cell proliferation in proliferation assays using co-culture supernatants. In conclusion, our data characterizes CD28-B7 pathway and DCs in the BM as vital for myeloma survival and also as possible targets to include in future strategies in the treatment of myeloma. FIGURE 1 FIGURE 1. FIGURE 2 FIGURE 2. Disclosures: Boise: University of Chicago: Patents & Royalties.

Abstract Abstract 1808 Poster Board I-834 We have developed the first mouse model of multiple myeloma (MM), Vk*MYC, in an immune competent mouse, with indolent plasma cell (PC) growth confined to the bone marrow, and in which the major biological and clinical features of the human disease are recapitulated, including the post germinal center origin of the tumor cells, the presence of a M-spike in the serum, anemia, diffuse bone disease and drug sensitivity. To further characterize karyotypically and phenotypically the Vk*MYC tumors, we performed gene expression profile (GEP) and aCGH analysis on CD138+ purified MM cells from Vk*MYC mice. For GEP, we analyzed 33 individual tumors on the Affymetrix 430_2 array for which publicly available datasets can be used to compare levels of gene expression. For aCGH, we used the Agilent 244K array to analyze 19 individual tumors. By GEP we first confirmed the PC origin of the Vk*MYC tumors, that clearly express the typical PC markers, including CD138, IRF4, BLIMP, XBP1, IgG or IgA, but not B cell markers like IgM, IgD, PAX5, or BCL6. Next, we analyzed the expression of the three cyclin D, that are almost universally dysregulated in human myeloma patients and cell lines, but are not expressed in normal human or murine PCs. We found that the Vk*MYC tumors express high levels of cyclin D2, and occasionally cyclin D1, but never cyclin D3 that, instead, is commonly expressed in B cells. Interestingly, in one sample lacking any D type cyclin expression we detected loss of Rb expression (and homozygous deletion of the gene, see below), as in the “TC none” group of human MM patients, highlighting once again the importance of this pathway in both human and mouse myeloma. aGCH analysis indicated that the genomic complexity of Vk*MYC tumors is low, and complex rearrangements were rarely observed. In contrast with the recurrent trisomy of odd number chromosomes present in approximately 50% of human MM patients, we found no evidences of hyperdiploidy in Vk*MYC mice, except perhaps in one tumor with trisomy 15 and 17. Instead, we detected monosomy 5 in 10/19, monosomy 14 in 4/19 and trisomy 16 in 4/19 tumors. Interestingly a large portion of the murine chr.14 is syntenic to the human chr.13, frequently lost in MM patients. After monosomy 13, the most frequent chromosomal change in human MM is loss of a sex chromosome. In the mouse, the most common numerical abnormality identified was loss of a sex chromosome: 10/11 male had loss of Y and 5/8 female had loss of X. From the analysis of aCGH data publicly available on the MMRC portal, we have generated a list of the most common bi-allelic deletions present in human myeloma patients and cell lines. Interestingly, several of these lesions are also present in Vk*MYC tumors. The Rb1 gene, frequently affected by monosomy 13 in human MM, is bi-allelically lost in 3/19 Vk*MYC tumors. In one case, we also detected a focal monoallelic deletion encompassing Rb1, in addition to the four tumors with monosomy 14. Other focal bi-allelic deletion identified are: Cdkn2a (p16 and p19ARF), FHIT and PTEN. The latter was found in a Vk*MYC tumor selected in-vivo to be bortezomib resistant. On chr.X we identified recurrent (5/19 tumors) bi-allelic deletions of UTX, the histone demethylase gene recently reported to be deleted in various cancers, including MM tumors and human myeloma cell lines. While we are in the process of performing functional studies in human cell lines to identify a role for UTX in myeloma pathogenesis, we took advantage of the relative genetic uniformity of the Vk*MYC mice to identify by GEP a signature associated with UTX lost. We separated our cohort of Vk*MYC tumors for which GEP data are available in two groups based on expression of UTX and generated by volcano plot a list of genes that distinguished the two groups. Strikingly, loss of UTX was associated with loss of expression of a subset of lymphoid genes and acquisition of a “macrophage signature”, suggesting a role for UTX in lineage commitment. These data provide a genetic basis for tumor plasticity similar to epithelial mesenchymal transition observed in some solid tumors, in which the tumor gradually acquires supporting characteristics of its microenvironment. We conclude that the Vk*MYC provide a remarkably faithful model of MM not only phenotypically but also genetically, that provides an excellent framework for functional characterization of the molecular pathogenesis of MM Disclosures Bergsagel: Genentech: Consultancy; Amgen: Consultancy; Celgene: Consultancy; Merck: Research Funding.

High levels of circulating miR-16 in the serum of multiple myeloma (MM) patients are independently associated with longer survival. Although the tumor suppressor function of intracellular miR-16 in MM plasma cells (PCs) has been elucidated, its extracellular role in maintaining a nonsupportive cancer microenvironment has not been fully explored. Here, we show that miR-16 is abundantly released by MM cells through extracellular vesicles (EVs) and that differences in its intracellular expression as associated with chromosome 13 deletion (Del13) are correlated to extracellular miR-16 levels. We also demonstrate that EVs isolated from MM patients and from the conditioned media of MM-PCs carrying Del13 more strongly differentiate circulating monocytes to M2-tumor supportive macrophages (TAMs), compared with MM-PCs without this chromosomal aberration. Mechanistically, our data show that miR-16 directly targets the IKKα/β complex of the NF-κB canonical pathway, which is critical not only in supporting MM cell growth, but also in polarizing macrophages toward an M2 phenotype. By using a miR–15a-16-1–KO mouse model, we found that loss of the miR-16 cluster supports polarization to M2 macrophages. Finally, we demonstrate the therapeutic benefit of miR-16 overexpression in potentiating the anti-MM activity by a proteasome inhibitor in the presence of MM-resident bone marrow TAM.

Abstract Multiple myeloma (MM) is a predominantly incurable form of plasma cell cancer. Although immunomodulatory imide drugs (IMiDs) such as thalidomide, lenalidomide, and pomalidomide constitute the backbone of most MM therapies, little is understood about how these drugs function to alter the tumor/microenvironment in vivo. Recently, it was demonstrated that IMiDs bind to the substrate receptor protein cereblon (CRBN) leading to the ubiquitination and degradation of multiple neoantigen protein targets, often DNA-binding zinc finger regulatory proteins such as ZFP91, IKZF1 and IKZF3. Interestingly, CRBN differs in amino acid sequence between mice and humans rendering mice non-responsive to IMiDs’ effects. This difference has made it difficult to study the complex pleotropic actions of IMiDs in vivo. Consequently, our lab has generated an immunocompetent transgenic mouse that expresses full-length human CRBN under the control of its endogenous regulatory elements (hCRBN+). We crossed this mouse with our well-established Vk*MYC mouse to generate a novel humanized Vk*MYChCRBN+ strain that develops human-like multiple myeloma disease, is sensitive to IMiD therapy, and maintains a fully competent immune system. The purpose of generating our Vk*MYChCRBN+mouse is to determine the mechanism of action of IMiDs on the tumor only, the host only, and to understand the cross-talk between tumor and microenvironment in a clinically relevant in vivo model of MM. We find that hCRBN+ mice are sensitive to IMiD treatment in vivo by displaying degradation of known target proteins such as Ikzf1/Ikzf3 in splenocytes. We also show that non-tumor immune cells such as T and NK cells respond to IMiD treatment by increased proliferation and expression of effector molecules such as IL-2 and IFN-γ. Furthermore, by engrafting IMiD-sensitive Vk*MYChCRBN+ MM cells into an IMiD-insensitive WT host we establish that IMiDs have tumor-intrinsic effects in vivo that are independent of the host’s immune system. These tumor-intrinsic IMiD effects synergize not only with dexamethasone, which has been shown clinically, but also with enhancer-targeting BET or p300/CBP inhibitors. Conversely, when engrafting IMiD-insensitive Vk*MYC MM cells into an IMiD-sensitive hCRBN+ host we were unable to demonstrate significant IMiD-induced host immune activation or tumor suppression. We conclude, based on our preliminary data, that IMiD activity is predominantly tumor-intrinsic. Moving forward, we are currently utilizing single-cell RNA sequencing to map the cellular and transcriptional changes across time in both tumor cells and the immune microenvironment during IMiD treatment. Ultimately, our goal is to gain mechanistic understanding in order to optimize clinical practice. Citation Format: Seth J. Welsh, Meaghen E. Sharik, Victoria M. Garbitt, Link R. Taylor, Daniel L. Riggs, Caleb Stein, Grady Day, Courtney J. Hillukka, Zach J. Hammond, P Leif Bergsagel, Marta Chesi. A clinically relevant mouse model to understand how IMiDs modulate the host-tumor immunolandscape in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4628.

Abstract MYC dysregulation, the most common genetic aberration in multiple myeloma, is frequently due to the translocation of super-enhancers to the MYC locus. Several drugs target proteins that are enriched at many of these enhancers including BRD4 (BET inhibitors, BETi) and Ikaros (IMiDs), although their mechanism of action remains poorly understood. Here we present a characterization of the responses to these drugs in a collection of over sixty myeloma cell lines having a diversity of MYC rearrangements. We found that the anti-proliferative effects of these drugs significantly correlated with changes in MYC protein levels, consistent with both drugs targeting MYC expression. Despite this common target, there was no statistically significant correlation between the individual BETi and IMiD responses, suggesting that they act through different mechanisms. Of those lines having extremes of sensitivity or resistance, there were two major groups (BETiS/IMiDS and BETiS/IMiDR), a smaller group of four lines resistant to both drugs individually (BETiR/IMiDR) and only one line was BETiR/IMiDS. In the BETiR/IMiDR group, resistance to BETi was mediated by a BRD4-independent mechanism as BRD4 was efficiently released from the MYC-associated enhancers. In all three of BETiR/IMiDR cell lines that we examined, treatment with BETi and IMiD together abolished proliferation and down-regulated MYC, consistent with parallel BRD4- and Ikaros-dependent pathways driving MYC expression. These resistant lines all expressed high levels of the transcription factor ETV4 and knocking out its gene sensitized a line to each drug individually. Thus ETV4 appears to be necessary for the parallel pathways driving MYC expression. There were nine lines in the BETiS/IMiDR group. Sensitivity to BETi in these lines could be explained by either low ETV4 expression or by BETi repressing Ikaros levels (which was only observed in BETiS lines, suggesting that BRD4 drives IKZF1 expression in these lines). Thus, in ETV4-containing lines, BETi sensitivity is due to the simultaneously targeting of the BRD4- and Ikaros-dependent pathways. IMiD resistance likely was due to several reasons. In one cell line, OCIMY5, IMiD had little effect on Ikaros levels, likely due to the previously reported low levels of Cereblon. Seven of the eight remaining lines expressed high levels of either ETV4, or the other potential super-enhancer binding factors IRF4 or RUNX1. In the eight BETiS/IMiDS cell lines, IMiD strongly reduced both Ikaros and Aiolos protein levels, which likely caused IMiD sensitivity. As with the BETiS lines described above, the lines in this group either lacked ETV4 or BETi repressed Ikaros levels. In conclusion, by examining drug response in a collection of genetically annotated myeloma cell lines we have been able to identify factors that contribute the broad range of responses to BETi and IMiDs in myeloma cells. Citation Format: Daniel L. Riggs, Camille Herzog, Victoria M. Garbitt, Niamh Keane, Courtney J. Hillukka, Zachary J. Hammond, Julia E. Wiedmeier, Seth J. Welsh, Shulan Tian, Huihuang Yan, Ranjan Maity, Nizar Bahlis, Paola Neri, W Michael Kuehl, Marta Chesi, P Leif Bergsagel. IMiDs and BET inhibitors target distinct pathways of MYC dysregulation by super-enhancers in multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3015.

Immunotherapy holds promise for patients with multiple myeloma (MM), but little is known about how MM-induced immunosuppression influences response to therapy. Here, we investigated the impact of disease progression on immunotherapy efficacy in the Vk*MYC mouse model. Treatment with agonistic anti-CD137 (4-1BB) mAbs efficiently protected mice when administered early but failed to contain MM growth when delayed more than 3 weeks after Vk*MYC tumor cell challenge. The quality of the CD8 + T cell response to CD137 stimulation was not altered by the presence of MM, but CD8 + T cell numbers were profoundly reduced at the time of treatment. Our data suggest that an insufficient ratio of CD8 + T cells to MM cells (CD8/MM ratio) accounts for the loss of anti-CD137 mAb efficacy. We established serum M-protein levels prior to therapy as a predictive factor of response. Moreover, we developed an in silico model to capture the dynamic interactions between CD8 + T cells and MM cells. Finally, we explored two methods to improve the CD8/MM ratio: anti-CD137 mAb immunotherapy combined with Treg depletion or administered after chemotherapy treatment with cyclophosphamide or melphalan efficiently reduced MM burden and prolonged survival. Together, our data indicate that consolidation treatment with anti-CD137 mAbs might prevent MM relapse.

Marrow adipocytes reprogrammed by myeloma cells induce bone lesions.