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Introduction The introduction of new treatment regimens has significantly increased the progression free survival (PFS) of newly diagnosed multiple myeloma (MM) patients. However, even with these novel treatments, for some the disease remains refractory, highlighting the need to identify the pathobiology of high-risk MM. In MM patients, high levels of circulating tumor cells (CTCs) is associated with an inferior prognosis independent of high-risk cytogenetics (Chakraborty et al., 2016), suggesting that CTC numbers are a relevant reflection of tumor cell biology. We hypothesized that high levels of CTCs in MM patients are either the result of a transcriptionally distinct tumor clone with enhanced migration capacities, or driven by transcriptional differences present in the bone marrow (BM) tumor cells. To test these hypotheses, we 1) compared MM cells from paired blood and BM samples, and 2) compared BM tumor cells of patients with high and low CTC levels, using single cell RNA-sequencing. Results We isolated plasma cell (PCs) from viably frozen mononuclear cells of paired peripheral blood (PB) and BM aspirates from five newly diagnosed MM patients (0.5%-8% CTCs) to determine the presence of a distinct CTC subclone. We generated single cell transcriptomes from 44,779 CTCs and 35,697 BM PCs. In the total 9 clusters common to BM PCs and CTCs were identified upon single cell data integration, but no cluster specific for either source was detected. Only 25 genes were significantly differential expressed between CTCs and BM PCs. The absence of transcriptional clusters unique to either CTCs or BM PCs, and the transcriptional similarity between these two anatomical sites makes it highly unlikely that CTC levels are driven by the presence of a transcriptionally-primed migratory clone. We next set out to identify possible transcriptional differences in BM PCs from eight patients with high (2-22%) versus thirteen patients with low (0.004%-0.08%) percentages of CTCs. Recurrent high-risk mutations were present in both groups. Single cell transcriptomes were generated from 74,830 BM PCs. Single cell data integration across all patients led to the identification of 8 distinct PC clusters, one of which was characterized by enhanced proliferation as defined by STMN1 and MKI67 transcription. Interestingly, this proliferative cluster was increased in patients with a high percentage of CTCs. Furthermore, cell cycle analyses based on canonical G2M and S phase markers revealed that actively cycling PCs were more frequent in the BM of patients with a high percentage of CTCs (64% versus 30%, p<0.001), irrespective of the transcriptional cluster of origin. We hypothesized that plasma cell-extrinsic cues from the bone marrow micro-environment might be driving tumor proliferation. In order to substantiate this, we isolated BM immune cells from the same 21 patients and generated a library of 301,045 single immune cell transcriptomes. This library contained all major immune cell subsets, including CD4 + and CD8 + T cells, NK cells, B cells and monocytes. Comparative analyses of these cell populations in patients with either high or low levels of CTC are ongoing. Conclusion Through single cell transcriptomic analyses, we demonstrate that CTCs and BM PCs are transcriptionally similar. Importantly, we identify increased BM PC proliferation as a significant difference between patients with high and low levels of CTCs, implicating an increased tumor proliferation as one of the potential mechanisms driving CTC levels and MM disease pathobiology. The relation of the BM immune micro-environment to this altered proliferative state is currently under investigation. Disclosures van der Velden: Janssen: Other: Service Level Agreement; BD Biosciences: Other: Service Level Agreement; Navigate: Other: Service Level Agreement; Agilent: Research Funding; EuroFlow: Other: Service Level Agreement, Patents & Royalties: for network, not personally. Sonneveld: SkylineDx: Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding. Broyl: Sanofi: Honoraria; Janssen Pharmaceuticals: Honoraria; Celgene: Honoraria; Bristol-Meyer Squibb: Honoraria; Amgen: Honoraria.

Multiple myeloma is a disease of malignant plasma cells residing in the bone marrow, where interactions with local immune cells are thought to contribute to disease pathobiology. However, since a multiple myeloma diagnosis is virtually always preceded by an asymptomatic precursor phase, identifying early alterations in the bone marrow micro-environment following occupation by multiple myeloma cells remains challenging. Here we used the 5TGM1 transfer model of murine myeloma in combination with myeloma-permissive KaLwRij mice and myeloma-resistant C57Bl/6 mice and hypothesized that differential sensitivity to myeloma in these HLA-identical mouse strains has an immunological basis and might allow for dissection of early immune responses to myeloma cells. Using flow cytometry and single-cell RNA sequencing we show that C57Bl/6 mice can restrain tumor growth for prolonged periods, associated with activation of cytotoxic immune responses that were absent from KaLwRij mice. Transcriptional analysis of immune cells and stromal cells identified a central role for IFN-signaling in tumor containment, and antibody-mediated neutralization of IFNγ increased both incidence and outgrowth of multiple myeloma in C57Bl/6 mice. Together these findings highlight the ability of a fully functional immune system to control multiple myeloma progression in an IFNγ-dependent manner and suggest that transfer of 5TGM1 cells into parental C57Bl/6 mice can serve as a faithful model to track anti-myeloma immune responses in immune competent and genetically modifiable mice.

SummaryThe bone marrow is permanently harbors high numbers of neutrophils, and a tumor-supportive bias of these cells could significantly impact bone marrow-confined malignancies. In multiple myeloma, the bone marrow is characterized by inflammatory stromal cells with the potential to influence neutrophils. We investigated myeloma-associated alterations in marrow neutrophils and the impact of stromal inflammation on neutrophil function. Mature neutrophils in myeloma marrow are activated and tumor-supportive, transcribing increased levels of IL-1β, and myeloma cell survival factor BAFF. Interactions with inflammatory stromal cells can induce neutrophil activation, including BAFF secretion, in a STAT3-dependent manner and once activated, neutrophils gain the ability to reciprocally induce stromal activation. After first-line myeloid-depleting treatment, patient bone marrow retains residual stromal inflammation and newly-formed neutrophils are reactivated. Combined, we identify a neutrophil-stromal cell feed-forward loop driving tumor-supportive inflammation that persists after treatment and warrants novel strategies to target both stromal and immune microenvironments in multiple myeloma.\

Introduction Peripheral neuropathy (PNP) remains one of the most common adverse events during multiple myeloma (MM) treatment. The immunomodulatory agent thalidomide and proteasome inhibitor bortezomib are particularly prone to induce PNP (Dimopoulos MA et al., Leukemia., 2010). Both agents are part of standard treatment regimens for newly diagnosed transplant-eligible MM patients. PNP varies from mild symptoms to severe disability, depending on timely dose reduction or discontinuation of treatment. Currently, incidence or severity of PNP cannot be predicted. Therefore, it is of utmost importance to monitor incidence of PNP in different treatment combinations, and in order to identify risk factors for developing PNP. Aims To investigate the incidence of PNP in patients treated in the Cassiopeia trial, to evaluate the role of CD38 antibody (daratumumab) treatment in development of PNP, and to identify risk factors for the development of PNP. Methods We retrospectively analysed incidence of PNP grade 2 to 4, scored according to common terminology criteria for adverse events version 4 (CTCAE) in the Cassiopeia study, a phase III trial conducted by IFM/HOVON, investigating the efficacy of adding daratumumab to bortezomib, thalidomide and dexamethasone (VTD). 1074 newly diagnosed MM patients were randomised. Patients received 4 induction cycles and 2 post transplantation consolidation cycles of 28 days each. Cycles included subcutaneous bortezomib (1.3 mg/m2 days 1,4,8,11), oral thalidomide (100 mg daily), dexamethasone (20-40 mg) and daratumumab intravenously (16 mg/kg and weekly during induction cycles 1 and 2 and once every two weeks during induction cycles 3,4 and consolidation). This trial was registered as ClinicalTrials.gov NCT02541383 and was supported by the French IFM and Dutch HOVON groups (Moreau et al., Lancet, 2019). Multivariate analysis was performed including sex, age, arm, body mass index (BMI), cytogenetics, ISS stage, country, diabetes mellitus (DM), creatinine clearance, liver function, ECOG, baseline PNP and disease characteristics. Results Baseline characteristics in dara-VTD and VTD arms were similar. Overall, 380/1074 (35%) patients developed grade ≥2 PNP and 102/1074 (9%) patients developed grade ≥3 PNP. Multivariate analysis indicated that the cumulative incidence of PNP grade ≥2 was significantly lower in the dara-VTD arm (33%) when compared to the VTD arm (38%) (hazard ratio (HR)=0.73, 95% confidence interval (CI) 0.59-0.91, P=0.004). Furthermore, risk factors associated with a higher cumulative incidence of PNP grade ≥2 included older age (HR=1.03; P=0.020), grade 1 PNP at baseline (HR= 2.75; P= 0.002) and higher BMI (HR=1.46, P=0.003 for BMI 25-30 to HR=2.02, P=0.004 for BMI > 35). Progression free survival (PFS) from the end of induction was similar (86% vs 80% at 2 years, HR = 0.74, 95% CI 0.41-1.33, P=0.32) for patients developing grade ≥2 PNP during induction (179 pts, 17%). An unexpected finding was the difference in cumulative incidence between countries participating in this trial: in the Netherlands 68/141 (49%) of patients developed grade ≥2 PNP, while in France this was 280/846 (33%) and in Belgium 31/87 (36%) (p<0.001). The protocol of the Cassiopeia trial included instructions of discontinuation and dose modification, when PNP grade ≥2 was observed. However, in a subset of patients reaching PNP grade ≥ 2 the (temporary) discontinuation or adjustment of dose as described in the treatment protocol had not been applied (respectively in 148/352 (42%) with PNP ≥ 2 and in 39/97 (40%) with PNP ≥3). Conclusions Despite bortezomib being administered subcutaneously and clear instructions on discontinuation and dose modification, we observed a clinically relevant incidence of grade ≥2 PNP (35%) and grade ≥3 PNP (9%) in patients treated in the Cassiopeia trial. Patients in the dara-VTD arm showed less grade ≥2 PNP, suggesting a possible positive effect of daratumumab. Risk factors for the development of grade ≥2 PNP included older age, PNP at baseline and BMI > 25. Differences in incidence between countries were observed, however no clear explanation was found. Furthermore, standard measures for grading PNP, such as CTCAE criteria, are subject to interpretation bias of both the patient and the treating physician. Continuous screening and correct grading of PNP and strict compliance with guidelines is warranted. Disclosures Moreau: Amgen: Consultancy, Honoraria; Celgene/Bristol-Myers Squibb: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Takeda: Honoraria; Sanofi: Consultancy, Honoraria; Novartis: Honoraria. Vermeulen:Janssen: Current Employment, Current equity holder in publicly-traded company. Broyl:Janssen, Celgene, Takeda, Amgen: Honoraria. Sonneveld:Sanofi: Consultancy; Takeda: Consultancy, Honoraria, Research Funding; Bristol-Myers Squibb: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; Skyline Dx: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding.