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SummaryThe bone marrow 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.

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.

Progression and persistence of malignancies are influenced by the local tumor microenvironment, and future eradication of currently incurable tumors will, in part, hinge on our understanding of malignant cell biology in the context of their nourishing surroundings. Here, we generated paired single-cell transcriptomic datasets of tumor cells and the bone marrow immune and stromal microenvironment in multiple myeloma. These analyses identified myeloma-specific inflammatory mesenchymal stromal cells, which spatially colocalized with tumor cells and immune cells and transcribed genes involved in tumor survival and immune modulation. Inflammatory stromal cell signatures were driven by stimulation with proinflammatory cytokines, and analyses of immune cell subsets suggested interferon-responsive effector T cell and CD8+ stem cell memory T cell populations as potential sources of stromal cell–activating cytokines. Tracking stromal inflammation in individuals over time revealed that successful antitumor induction therapy is unable to revert bone marrow inflammation, predicting a role for mesenchymal stromal cells in disease persistence. Multiple myeloma disease progression and therapy response are influenced by the bone marrow niche in which the tumor cells reside. To characterize this supportive niche, Cupedo and colleagues use single-cell transcriptomic analysis of bone marrow stromal cell populations from individuals with multiple myeloma. They identify a myeloma-specific inflammatory mesenchymal stromal cell (iMSC) population that spatially colocalizes with tumor cells. Anti-myeloma induction therapy does not influence iMSC presence, suggesting a role for bone marrow inflammation in myeloma persistence or relapse.

Multiple myeloma (MM) is a plasma-cell neoplasm that is treated with high-dose chemotherapy, autologous stem cell transplant (ASCT) and long-term immunomodulatory drug (IMiD) maintenance. The presence of somatic mutations in the peripheral blood is termed clonal hematopoiesis of indeterminate potential (CHIP) and is associated with adverse outcomes. Targeted sequencing of the stem cell product from 629 MM patients treated by ASCT at the Dana-Farber Cancer Institute (2003–2011) detects CHIP in 136/629 patients (21.6%). The most commonly mutated genes are DNMT3A, TET2, TP53, ASXL1 and PPM1D. Twenty-one from fifty-six patients (3.3%) receiving first-line IMiD maintenance develop a therapy-related myeloid neoplasm (TMN). However, regardless of CHIP status, the use of IMiD maintenance associates with improved PFS and OS. In those not receiving IMiD maintenance, CHIP is associated with decreased overall survival (OS) (HR:1.34, p = 0.02) and progression free survival (PFS) (HR:1.45, p, Multiple myeloma (MM) is treated with induction chemotherapy, autologous stem cell transplant (ASCT) and long-term immunomodulatory drug (IMiD) maintenance. Here, the authors show that the presence of clonal haematopoiesis of indeterminate potential (CHIP) at time of ASCT is associated with adverse outcomes in MM patients.

Somatic mutations in cancer genes have been ubiquitously detected in clonal expansions across healthy human tissue, including in clonal hematopoiesis. However, mutated and wildtype cells are morphologically and phenotypically similar, limiting the ability to link genotypes with cellular phenotypes. To overcome this limitation, we leveraged multi-modality single-cell sequencing, capturing the mutation with transcriptomes and methylomes in stem and progenitors from individuals with DNMT3A R882 mutated clonal hematopoiesis. DNMT3A mutations resulted in myeloid over lymphoid bias, and in expansion of immature myeloid progenitors primed toward megakaryocytic-erythroid fate. We observed dysregulated expression of lineage and leukemia stem cell markers. DNMT3A R882 led to preferential hypomethylation of polycomb repressive complex 2 targets and a specific sequence motif. Notably, the hypomethylation motif is enriched in binding motifs of key hematopoietic transcription factors, serving as a potential mechanistic link between DNMT3A R882 mutations and aberrant transcriptional phenotypes. Thus, single-cell multi-omics pave the road to defining the downstream consequences of mutations that drive human clonal mosaicism.

Patients with smoldering multiple myeloma (SMM) are observed until progression, but early treatment may improve outcomes. We conducted a phase II trial of elotuzumab, lenalidomide, and dexamethasone (EloLenDex) in patients with high-risk SMM and performed single-cell RNA and T cell receptor (TCR) sequencing on 149 bone marrow (BM) and peripheral blood (PB) samples from patients and healthy donors (HDs). We find that early treatment with EloLenDex is safe and effective and provide a comprehensive characterization of alterations in immune cell composition and TCR repertoire diversity in patients. We show that the similarity of a patient's immune cell composition to that of HDs may have prognostic relevance at diagnosis and after treatment and that the abundance of granzyme K (GZMK)

Introduction Clonal hematopoiesis (CH) is associated with adverse outcomes in patients with non-Hodgkin lymphoma (NHL) and multiple myeloma undergoing autologous stem cell transplantation. Still, its implications for patients with indolent NHL have not been well studied. Here, we report the prevalence of CH in patients with Waldenström Macroglobulinemia (WM) and its association with clinical outcomes. Methods We retrospectively reviewed clinical data of 602 patients with IgM monoclonal gammopathy of undetermined significance (MGUS), smoldering WM (SWM), and WM who had clinical next-generation sequencing (NGS) performed on bone marrow aspirates or peripheral blood obtained between October 2014 and February 2020 at the Dana-Farber Cancer Institute. An Illumina Truseq amplicon-based NGS assay of 95 genes recurrently mutated in myeloid and lymphoid neoplasms was utilized. Each specimen yielded ~2 million reads and ~1500X average coverage, with 90% of amplicons having >200X coverage. Pathogenic driver variants were identified based on mutation type, position, and frequency in published reports and public databases. To unambiguously differentiate CH mutations from those in the WM clone, CH was defined by the presence of somatic mutations in DNMT3A, TET2, or ASXL1 (DTA). Results The cohort included 147 patients with MGUS or asymptomatic WM and 453 patients with symptomatic WM, with a median age of 66 years (range = 40-89) and 68 years (range = 33-93), respectively, at the time of first NGS assay. The prevalence of CH-DTA was 14% in symptomatic WM patients and did not differ significantly in MGUS (13%) or SWM (14%). Among precursor patients, there was an increased risk of progression to symptomatic WM for those with CH-DTA [7/20 patients with vs. 11/116 without CH-DTA progressed over a median of 54 months (P = 0.002)]. In symptomatic WM patients, CH-DTA was positively associated with older age (P < 0.001) at the time of NGS, with a median age of 72 vs. 67 years for patients with versus those without CH-DTA. CH-DTA was not associated with inferior overall survival (OS) with a relatively short median follow-up from diagnosis and NGS assay. OS was 6.7 years (95% CI = 6.1-7.6) for patients with CH-DTA and 2.5 years for patients without DH-DTA (95% CI = 2.2-2.8). The most common cause of death was disease progression, with no significant difference between those with or without CH-DTA. Patients with CH-DTA had an increased risk of cardiovascular disease (30% vs. 18%, P = 0.036). Conclusions We demonstrate that CH is common in WM patients and is associated with an increased risk of progression from precursor states but not with inferior survival. Further work is needed to determine how the presence of CH might promote progression to WM and whether it can be incorporated into future risk stratification models. Importantly, our data do not support changes in clinical management or alterations in therapy for patients with WM and coexistent CH and reinforce the need to interpret NGS results within their specific clinical context. Disclosures Steensma: Novartis: Current Employment. Castillo: Abbvie: Consultancy, Research Funding; BeiGene: Consultancy, Research Funding; Pharmacyclics: Consultancy, Research Funding; Janssen: Consultancy; Roche: Consultancy; TG Therapeutics: Research Funding. Treon: X4: Research Funding; Dana Farber Cancer Institute: Current Employment; Janssen: Consultancy, Research Funding; Pharmacyclics: Consultancy, Research Funding; BeiGene: Consultancy, Research Funding; Self: Patents & Royalties: Holder of multiple patents related to testing and treatment of MYD88 and CXCR4 mutated B-cell malignancies; BMS: Consultancy, Research Funding; AbbVie: Consultancy, Research Funding. Ghobrial: AbbVie, Adaptive, Aptitude Health, BMS, Cellectar, Curio Science, Genetch, Janssen, Janssen Central American and Caribbean, Karyopharm, Medscape, Oncopeptides, Sanofi, Takeda, The Binding Site, GNS, GSK: Consultancy. Sperling: Adaptive: Consultancy.

Background Multiple Myeloma (MM) disease progression and therapy response are influenced by cues of the microenvironment including tumor control by a cytotoxic immune response. Natural killer (NK) cells are notable mediators of the cytotoxic immune response to MM, and important effector cells in recent immune-mediated therapies. NK cells are drivers of antibody-dependent-cellular cytotoxicity in therapies based on anti-CD38 monoclonal antibodies such as Daratumumab. Classically, NK cells are divided based upon CD56 expression into a cytokine-producing CD56bright subset, releasing cytokines such as IFN-γ, TNF-a and GM-CSF, and a cytotoxic CD56dim subset. However, accumulating evidence suggests much larger heterogeneity in the NK cell compartment and modulation of these NK cell subsets could impact response to NK cell-driven immunotherapies. Here, we used single-cell RNA sequencing to investigate the heterogeneity and MM-driven alterations of the NK cell compartment in the bone marrow of newly diagnosed MM patients undergoing first-line Daratumumab-containing therapy. Methods We performed single-cell RNA sequencing of the CD38+ and CD38- fractions of viably frozen bone marrow aspirates from 19 newly diagnosed MM patients and 5 non-cancer control patients. NK cells were identified in silico by transcription of KLRF1, KLRD1, GNLY and NKG7 resulting in a single-cell transcriptomic dataset of 30,373 NK cells from MM patients and 8,865 NK cells from control patients. Results After integration of the datasets, bone marrow NK cells formed eight distinct transcriptomic clusters. Conventional CD56bright and CD56dim NK-cells were identified by increased transcription of GZMK or GZMB, respectively. The GZMK+CD56bright NK cells contained both a cluster of naive and a cluster of activated NK cells. The GZMB+CD56dim NK cells consisted of 5 subclusters. To identify MM-induced alterations in NK cell subsets we compared GZMK+CD56bright vs GZMB+CD56dim cluster composition and distribution between control and MM patients. Control bone marrow was dominated by GZMB-transcribing cytotoxic CD56dim NK cells, represented by a low cytokine-producing GZMK+CD56bright vs cytotoxic GZMB+CD56dim ratio. In contrast, MM bone marrow was characterized by heterogeneity in the ratio of cytokine-producing GZMK+CD56bright vs cytotoxic GZMB+CD56dim NK cells. A subset of patients presented with a complete reversal of this ratio. This altered composition was due to a loss of cytotoxic GZMB+CD56dim NK cells, and more specifically a loss of NK cells with a transcriptome suggesting recent activation. Conclusion Here we present a transcriptomic overview of NK cells in MM bone marrow at the single-cell level. A subset of MM patients has a loss of activated cytotoxic GZMB+CD56dim NK cells, suggestive of reduced cytotoxic anti-tumor responses. Current analyses are focused on clinical implications of NK cell alterations in response to Daratumumab-based therapies.

Background Multiple myeloma (MM) disease progression is influenced by signals from the bone marrow (BM) microenvironment. Recently, we showed that the MM BM is characterized by inflammatory mesenchymal stromal cells (iMSCs) that transcribe MM survival factors and are predicted to recruit proliferating myeloma cells via CCL2-CCR2 interactions (de Jong et al. Nat Immunol. 2021). iMSCs also transcribed high levels of chemokines that can bind to CXCR1 and 2. Myeloid cells are known to express CXCR1/2, and have been implicated in both pro- and anti-tumor responses in various malignancies. Therefore, we hypothesized that iMSCs attract and influence myeloid populations in the MM BM. Methods Using flow cytometry, we verified expression of CXCR1/2 on myeloid cell populations in the BM of 5 newly diagnosed MM (NDMM) patients. Results CD15+ neutrophils were the most dominant population expressing these receptors, as 22.4% (± 9.8%) of cells expressed CXCR2 alone, and 72.6% (± 8.0%) expressed both CXCR1 and CXCR2. CD14+ monocytes only expressed CXCR2 (86.9% ± 15.8%). Importantly, less than 1% of myeloma cells expressed these receptors (n = 17 NDMM). As these findings suggested neutrophils as a potential target of iMSC-mediated chemotaxis, we set out to identify MM-associated alterations in this population by performing single cell RNA sequencing of the full neutrophilic lineage (n = 5 NDMM and 2 controls). Interestingly, CXCR1 and CXCR2 transcription was increased in mature neutrophils of MM patients compared to controls. Additionally, mature neutrophils of MM patients had an activated transcriptome as defined by increased transcription of C3AR1, SLPI, and IL6R, the plasma cell supportive factor TNFSF13B (encoding BAFF), and the inflammatory cytokines IL1B and IL18. Transcription of IL1B and IL18 can be regulated by pattern-recognition receptors (PRRs) binding damage-associated molecular patterns (DAMPs) resulting from e.g. matrix breakdown. Transcription of PRRs as TLR1, 2 and 4 was increased in mature neutrophils of MM patients compared to controls. Secretion of IL-1 β and IL-18 relies on the cleavage of pro-forms of these cytokines by the inflammasome, a multiprotein complex that is assembled in response to alarmins. Transcription of inflammasome components PYCARD, NLRP3 and CASP1 was increased in mature neutrophils of patients with MM. Additionally, protein levels of both IL-18 and IL-1 β are increased in BM plasma from MM patients, implicating activated neutrophils as a potential source of these cytokines. Conclusion In MM, mature neutrophils are activated and might interact with iMSCs via CXCR1/2. Moreover, neutrophils are inflammasome-primed and are likely to be a source of increased IL-1 β levels in the MM BM. As IL-1 β can activate normal MSCs to become iMSC, neutrophils and iMSCs may form a feed-forward loop in which activated neutrophils contribute to a pro-MM environment by maintaining iMSC and by directly providing BAFF to tumor cells.

Background Circulating tumor cells (CTCs) are present in the blood of all patients with (precursor)stages of Multiple Myeloma (MM). The levels of CTCs vary widely between patients, yet the biology behind this variability is unknown. Importantly, MM patients with a higher percentage of CTCs have an inferior prognosis independent of high-risk cytogenetics, suggesting that CTC numbers are a relevant reflection of tumor cell biology. Here, we set out to identify differences in CTCs and paired bone marrow plasma cells (BM PCs). Additionally, we compared BM PCs from patients with high and low levels of CTCs with the goal of identifying mechanistic drivers of high-risk disease. Methods We isolated PCs from viably frozen mononuclear cells of peripheral blood and bone marrow aspirates of newly diagnosed MM patients. Common high-risk mutations were present in both groups. We performed single cell RNA sequencing of paired CTCs and BM PCs from five patients with a high percentage of CTCs (0.5%-8%). In addition, we generated single cell transcriptomes of BM PCs of eight patients with a high percentage of CTCs (2-22%) and 13 patients with low percentage of CTCs (0.004%-0.08%) Results Single cell transcriptomes were generated from 44,779 CTCs and 35,697 bone marrow PCs. When paired CTCs and BM PCs were integrated, we identified 9 common clusters, no cell specific cluster for either source was detected. Moreover, only 25 genes were significantly differentially expressed between CTCs and BM PCs. The absence of unique clusters in either CTCs or BM PCs, and the transcriptional overlap between these two sources indicate that CTC levels are not driven by the emergence of a transcriptionally different migratory clone, but are likely a reflection of altered BM PC biology. To identify such alterations, we compared bone marrow PCs from patients with high and low percentages of CTCs. Single cell transcriptomes were generated from 74,830 bone marrow PCs. Integration of all patients lead to the identification of 8 distinct PC clusters, one of which was characterized by active proliferation as defined by transcription of STMN1 and MKI67. Interestingly, this proliferative cluster was larger 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 Conclusions Through single cell transcriptomic analyses we reveal that CTCs and MM cells from BM 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 entry into the cell cycle as one of the mechanisms driving CTC levels and MM disease pathobiology.

Background Multiple myeloma (MM) is an incurable plasma cell malignancy and identifying mechanisms driving disease progression and relapse is crucial for development of novel therapies. Although cytotoxic immunity is an important factor in controlling tumor progression, the cells and signals that constitute an effective anti-myeloma immune response within the bone marrow (BM) are incompletely defined. In this study we set out to identify immune-related mechanisms of disease progression in a murine model of MM. Methods C57Bl/6 and KaLwRij mice received 106 5TGM1-GFP murine myeloma cells intravenously. Tumor development was monitored through weekly measurements of M-protein levels. Results Three weeks after tumor injection all KaLwRij mice (18/18) developed myeloma, as defined by >5% tumor cells in BM (“unrestrained tumor”) and serum M-protein >2mg/ml, and in all mice tumor cells had infiltrated the spleen. Interestingly, while 39% (7/18) of C57Bl/6 mice had no tumor, 44% (8/18) had low but detectable levels of MM cells (0.1-5% of BM cells, “restrained tumor”) while 17% (3/18) presented with an unrestrained myeloma with BM tumor load similar to that seen in KaLwRij mice. In all C57Bl/6 mice MM was confined to the bone marrow with no splenic involvement at day 21. When tumor development in 5TGM1-injected C57BL/6 mice was allowed to proceed for a prolonged period of time, the incidence of mice developing a tumor did not change (59%, 7/12 mice). However, the number of mice with unrestrained tumor increased to 42% (5/12 mice), and 2 out of these 5 mice also developed splenic MM involvement. In addition, 17% (2/12) of mice were characterized by restrained tumor, as they contained low but detectable tumor levels up until 42 days post tumor cell transfer. The time-dependent increase in incidence of mice with high tumor burden suggests an initial immune-mediated restraint of MM that eventually fails, leading first to unrestrained BM disease and then to systemic spread of the tumor. The C57Bl/6 mice with restrained bone marrow myeloma had an expansion of activated mature (CD69+ CD11b+) NK cells as well as activated (Lsel-CD44+CD69+) CD8+ cytotoxic T cells. In contrast, high bone marrow tumor burden and spread of MM cells to the spleen was associated with a sharp decline in the absolute numbers of both these cytotoxic immune cell subsets. Conclusion Here we provide evidence that loss of innate and adaptive cytotoxic immune populations is associated with progression and dissemination of experimental MM in mice. Transfer of 5TGM1 myeloma cells into C57Bl/6 mice leads to initial tumor control via expansion of innate and adaptive cytotoxic immune cell populations. However, with time, immune-mediated restraint of MM fails, exemplified by loss of NK cell and CD8+ T cell expansion, culminating in tumor outgrowth in the BM and eventual systemic dissemination. This in vivo model will allow for generation of novel insights into the mechanisms of immune escape by MM cells.

Monoclonal Gammopathy of Undetermined Significance (MGUS) is considered to be a benign precursor condition that may progress to a lymphoproliferative disease or multiple myeloma. Most patients do not progress to an overt condition, but nevertheless, MGUS is associated with a shortened life expectancy and, in a minority of cases, a number of co-morbid conditions that include an increased fracture risk, renal impairment, peripheral neuropathy, secondary immunodeficiency, and cardiovascular disease. This review aims to consolidate current evidence for the significance of these co-morbidities before considering how best to approach these symptoms and signs, which are often encountered in primary care or within a number of specialties in secondary care.

Purpose of review The aim of the review is to describe recent advances in our understanding of how multiple myeloma interacts with its cellular and molecular neighbours in the bone marrow microenvironment, and how this may provide targets for prognostication and prevention. Recent findings The bone marrow microenvironment in myeloma is beginning to yield targets that are amenable to therapy. A number of trials demonstrate some clinical efficacy in heavily pretreated disease. The challenge remains for how and when these therapeutic interventions are of particular benefit early in disease progression. Summary Multiple myeloma is rarely curable and its interactions with the bone marrow microenvironment are evident. However, separating cause from effect remains a challenge. We propose that targeting specific niches within the bone marrow will yield therapies that have the potential for significant benefit in myeloma and may facilitate earlier intervention to disrupt an environment that is permissive for myeloma progression.

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 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.

Introduction Multiple Myeloma (MM) disease progression and therapy response are the net result of tumor cell-intrinsic features and tumor cell-extrinsic cues from the bone marrow (BM) microenvironment. Natural killer (NK) cells are mediators of the cytotoxic immune response against MM and are important effector cells in antibody-based immune therapies, especially anti-CD38 monoclonal antibodies such as Daratumumab. Classically, NK cells are divided into a cytotoxic CD56 dim subset, important for antibody-dependent cellular cytotoxicity, and a cytokine-producing CD56 bright subset releasing inflammatory mediators such as IFNγ, TNFα and GM-CSF. However, accumulating evidence suggests greater heterogeneity in the NK cell compartment and modulation of these NK cell subsets could impact disease progression and response to NK cell-driven immunotherapies. Here, we combined the 5TGM1 murine model of MM with single-cell RNA sequencing of bone marrow (BM) NK cells of newly diagnosed MM patients to map NK cell heterogeneity and to investigate their role in MM progression. Results To gain insight in NK cell heterogeneity in MM disease we performed single-cell RNA sequencing on immune cells of viably frozen BM aspirates from 19 newly diagnosed MM patients and 5 non-cancer control patients. NK cells were identified in silico by transcription of KLRF1, KLRD1, GNLY and NKG7 resulting in a single-cell transcriptomic dataset of 30,373 NK cells from MM patients and 8,865 NK cells from control patients. Conventional CD56 bright and CD56 dim NK-cells were identified by increased transcription of GZMK or GZMB, respectively. The GZMK +CD56 bright NK cells contained clusters of naïve and activated NK cells. The GZMB+CD56 dim NK cells consisted of 5 subclusters. To identify MM-induced alterations in NK cell subsets, we compared GZMK +CD56 bright vs GZMB+CD56 dim cluster composition and distribution between controls and MM patients. Control BM was dominated by GZMB-transcribing cytotoxic CD56 dim NK cells, resulting in a low ratio of cytokine-producing GZMK +CD56 bright vs cytotoxic GZMB+CD56 dim NK cells. In contrast, MM bone marrow was characterized by heterogeneity of this ratio with a subset of patients presenting with complete reversal of this ratio . In this subset of patients, the altered composition was due to a loss of cytotoxic GZMB +CD56 dim NK cells, and more specifically a loss of NK cells with a transcriptome suggesting recent activation. To better examine the significance of cytotoxic NK cells in MM disease course we utilized the well-established 5TGM1 mouse model. C57Bl/6 and KaLwRij mice both received 10 6 5TGM1-GFP cells intravenously. Three weeks after tumor injection all KaLwRij mice (18/18) developed MM, defined by >5% tumor cells in BM ("unrestrained tumor") and serum M-protein >2mg/ml. Interestingly, while 39% (7/18) of C57Bl/6 mice had no tumor, 44% (8/18) had low but detectable levels of MM cells (0.1-5% of BM cells, "restrained tumor") and 17% (3/18) presented with an unrestrained MM with BM tumor load similar to that seen in KaLwRij mice. With time the percentage of mice with unrestrained tumor increased (5/12, 42%) at the expense of restrained tumor (2/12, 16%). We hypothesized that C57Bl/6 mice with low tumor load could represent a model of immune-mediated tumor control. Detailed analysis of the NK cell compartment revealed an expansion of activated mature (CD69 + CD11b +CD27 +) NK cells in C57Bl/6 mice with restrained BM MM (p=0.0031). In contrast, high BM tumor burden in both genotypes was associated with a sharp decline in absolute numbers of activated NK cells. Conclusion: Through a combination of single-cell transcriptomic analyses of the BM immune microenvironment in MM patients and experimental mouse models we found a loss of activated NK cells in a subset of patients and mice. Our data suggests that loss of these activated NK cells is associated with MM progression in vivo. A subset of MM patients presented with a loss of activated cytotoxic GZMB +CD56 dim NK cells in the BM, suggestive of reduced cytotoxic anti-tumor responses. Meanwhile, in vivo, high disease burden only occurred in mice with an absence of activated NK cells. Current analyses are focused on differences in human disease progression and efficacy of Daratumumab-based therapies in patients with various NK cell phenotypes. Disclosures Broijl: Janssen, Amgen, Sanofi, Celgene/BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees. Sonneveld: Janssen: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding.

Background: Multiple myeloma (MM) disease progression is influenced by signals from the bone marrow (BM) microenvironment. Recently, we showed that the MM BM is characterized by inflammatory mesenchymal stromal cells (iMSCs) that transcribe MM survival factors and are predicted to recruit proliferating myeloma cells via CCL2-CCR2 interactions (de Jong et al. Nat Immunol. 2021). iMSCs also transcribed high levels of chemokines that can bind to CXCR1 and 2. Myeloid cells are known to express CXCR1/2, and have been implicated in both pro- and anti-tumor responses in various malignancies. Therefore, we hypothesized that iMSCs attract and influence myeloid populations in the MM BM. Results: Using flow cytometry, we verified expression of CXCR1/2 on myeloid cell populations in the BM of 5 newly diagnosed MM (NDMM) patients. CD15 + neutrophils were the most dominant population expressing these receptors, as 22.4% (± 9.8%) of cells expressed CXCR2 alone, and 72.6% (± 8.0%) expressed both CXCR1 and CXCR2. CD14 + monocytes only expressed CXCR2 (86.9% ± 15.8%). Importantly, less than 1% of myeloma cells expressed these receptors (n = 17 NDMM). As these findings suggested neutrophils and monocytes as potential targets of iMSC-mediated chemotaxis, we set out to identify MM-associated alterations in this population by performing single cell RNA sequencing of the full neutrophilic and monocytic lineages (n = 5 NDMM and 2 controls). In line with our flow cytometric data, CXCR1 transcripts were absent in monocytes, while CXCR2 was transcribed by classical monocytes of both myeloma patients and controls. Interestingly, CXCR1 and CXCR2 transcription was increased in mature neutrophils of MM patients compared to controls. Additionally, both mature classical monocytes as well as mature neutrophils of MM patients had an activated transcriptome as defined by increased transcription of C3AR1, SLPI, and IL6R, the plasma cell supportive factor TNFSF13B (encoding BAFF), and the inflammatory cytokines IL1B and IL18. Transcription of IL1B and IL18 can be regulated by pattern-recognition receptors (PRRs) binding damage-associated molecular patterns (DAMPs) resulting from e.g. matrix breakdown. Transcription of PRRs as TLR1, 2 and 4 was increased in mature neutrophils and classical monocytes of MM patients compared to controls. Secretion of IL-1β and IL-18 relies on the cleavage of pro-forms of these cytokines by the inflammasome, a multiprotein complex that is assembled in response to alarmins. Transcription of inflammasome components PYCARD, NLRP3 and CASP1 was increased in mature neutrophils and classical monocytes of patients with MM. Additionally, protein levels of both IL-18 and IL-1β are increased in BM plasma from MM patients, implicating activated neutrophils and monocytes as a potential sources of these cytokines. Conclusion: In MM, mature neutrophils and classical monocytes are activated and might interact with iMSCs via CXCR1 and/or 2. Moreover, these myeloid cells are inflammasome-primed and are likely to be sources of the increased IL-1β levels in the MM BM. Therefore, myeloid cells and iMSCs may form a feed-forward loop in which myeloid cells contribute to a pro-MM environment by maintaining iMSC and by directly providing BAFF to tumor cells. Disclosures Broyl: Celgene/BMS: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Janssen: Consultancy, Honoraria. Sonneveld: Janssen: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Honoraria, Research Funding; SkylineDx: Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; Celgene/BMS: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding.

Background Patients with Smoldering Multiple Myeloma (SMM) are typically observed until progression to overt Multiple Myeloma (MM), but early treatment of high-risk patients may improve outcomes. Clinical and genomic biomarkers can help to identify SMM patients at high risk of progression, but whether parallel profiling of the tumor immune microenvironment can further improve our models remains to be determined. Methods We performed single-cell RNA sequencing on CD138- immune cells from 40 samples of 14 patients with high-risk SMM enrolled in a Phase II trial of Elotuzumab, Lenalidomide, and Dexamethasone (NCT02279394), to develop biomarkers for optimal patient selection and monitoring of response to treatment. This study was approved by the Dana-Farber Cancer Institute's Institutional Review Board (#14-338). Specifically, we used the Chromium Single Cell 3' Gene Expression system by 10X Genomics to profile 33 magnetically sorted (Miltenyi Biotec) CD138- bone marrow (BM) samples collected at baseline (n=11), cycle 9 day 1 (C9D1, n=13), and end of treatment (EOT, n=9), and 7 peripheral blood mononuclear cell (PBMC) samples collected at baseline (n=4) and C9D1 (n=3). We integrated this data with our cohort of non-trial patients with Monoclonal Gammopathy of Undetermined Significance (n=5), low-risk SMM (n=3), high-risk SMM (n=8), newly diagnosed MM (n=9) and healthy donors (n=10), reaching a total of 75 samples. Results We found that higher abundance of mature B-cells, Th17, and Granzyme K (GZMK)+ T-cells, as well as gene expression signatures marked by type 17 genes and GZMK, are associated with significantly longer progression-free survival (PFS) in SMM patients under treatment. Although immunoparesis is a significant predictor of PFS in our cohort and could thus be confounding our B-cell signal, we showed that immunoparesis is not associated with lower number of B-cells in the BM and is instead associated with the upregulation of transcription factor Kruppel-like Factor 2 (KLF2, q=2e-05), which we hypothesize may oppose differentiation into antibody-secreting plasma cells. We found that these differences in abundance can be summarized by how normal-like the patients' immune composition is at baseline, whereby patients whose immune composition is not normal-like have significantly longer PFS (p=0.031). This model suggests that at least some of the compositional changes observed in disease may reflect the immune system's capacity to react successfully to the immune challenge posed by the tumor, which we termed 'immune reactivity'. Baseline immune reactivity may help to identify patients who will benefit the most from early treatment. Furthermore, we found that the expansion of tissue-resident NK cells and exhausted GZMK+ CD8+ T-cells at C9D1 of treatment, as well as higher levels of a gene expression signature marked by amphiregulin (AREG), are associated with significantly shorter PFS (p=0.039). These biomarkers may improve monitoring of response to immunotherapy, which may not be fully explained by residual tumor burden alone. Lastly, patients whose immune profile was normal-like at EOT (Post-therapy Immune Normalization, PIN), potentially signifying the resolution of the immune challenge, had significantly longer biochemical PFS (p=0.04). Assessment of PIN at EOT may improve stratification of patients with minimal residual disease. Importantly, we found that biomarker status could be assessed in both the patients' peripheral blood and their BM, suggesting that minimally invasive immune profiling for prognostication and monitoring may be feasible. Conclusions Our study has nominated novel immune biomarkers for optimal patient selection and assessment of response to immunotherapy and uncovered a previously unappreciated mechanism of B-cell immunosuppression in MM, which could lead to the development of novel therapeutics. Our findings may usher in a next generation of clinical assays that assess both tumor biology and immune state, as well as common clinical biomarkers, in the marrow or blood, to accurately predict who may benefit from early treatment, monitor response to immunotherapy, and improve patient outcomes. Disclosures Dutta: Menarini Silicon Biosystems: Consultancy. Haradhvala: Constellation Pharmaceuticals a MorphoSys Company: Consultancy. Zavidij: Constellation Pharmaceuticals: Current Employment. Bustoros: Janssen, Bristol Myers Squibb: Honoraria, Speakers Bureau; Takeda: Consultancy, Honoraria. Rosenblatt: Attivare: Consultancy; Imaging Endpoints: Consultancy; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Parexel: Consultancy; Wolters Kluwer Health Inc: Consultancy, Patents & Royalties. Zonder: Caelum Biosciences: Consultancy; Intellia: Consultancy; Alnylam: Consultancy; Regeneron: Consultancy; Janssen: Consultancy; BMS: Consultancy, Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy. Bhutani: Janssen, MedImmune, Takeda, Celgene, BMS, Cerecor, Celularity: Research Funding; Sanofi: Consultancy; Amgen, BMS, Takeda: Speakers Bureau. Usmani: Array BioPharma: Consultancy, Research Funding; Abbvie: Consultancy; Celgene/BMS: Consultancy, Research Funding, Speakers Bureau; GSK: Consultancy, Research Funding; EdoPharma: Consultancy; Janssen: Consultancy, Research Funding, Speakers Bureau; Sanofi: Consultancy, Research Funding, Speakers Bureau; Merck: Consultancy, Research Funding; Pharmacyclics: Consultancy, Research Funding; Seattle Genetics: Consultancy, Research Funding; SkylineDX: Consultancy, Research Funding; Takeda: Consultancy, Research Funding, Speakers Bureau; Janssen Oncology: Consultancy, Research Funding; Bristol-Myers Squibb: Research Funding; Amgen: Consultancy, Research Funding, Speakers Bureau. Yee: Bristol Myers Squibb: Consultancy; Oncopeptides: Consultancy; Amgen: Consultancy; Janssen: Consultancy; GSK: Consultancy; Adaptive: Consultancy; Sanofi: Consultancy; Takeda: Consultancy; Karyopharm: Consultancy. Jakubowiak: Janssen: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Gracell: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; GSK: Membership on an entity's Board of Directors or advisory committees; Sanofi: Membership on an entity's Board of Directors or advisory committees; Karyopharm: Membership on an entity's Board of Directors or advisory committees. Manier: Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene - Bristol Myers Squibb: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Adaptive Biotechnologies: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; GSK: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Oncopeptides: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Regeneron: Consultancy, Research Funding; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Nadeem: GSK: Consultancy; Adaptive: Consultancy; Bristol Myer Squibb: Consultancy; Takeda: Consultancy; Karyopharm: Consultancy. Richardson: GlaxoSmithKline: Consultancy; Takeda: Consultancy, Research Funding; AbbVie: Consultancy; AstraZeneca: Consultancy; Celgene/BMS: Consultancy, Research Funding; Karyopharm: Consultancy, Research Funding; Janssen: Consultancy; Sanofi: Consultancy; Oncopeptides: Consultancy, Research Funding; Regeneron: Consultancy; Secura Bio: Consultancy; Protocol Intelligence: Consultancy; Jazz Pharmaceuticals: Consultancy, Research Funding. Badros: J&J: Research Funding; BMS: Research Funding; Janssen: Research Funding; GlaxoSmithKline: Research Funding. Mateos: Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bluebird bio: Honoraria; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; GSK: Honoraria; Oncopeptides: Honoraria; Regeneron: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sea-Gen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene - Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Oncopeptides: Honoraria, Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria. Getz: IBM, Pharmacyclics: Research Funding; Scorpion Therapeutics: Consultancy, Current holder of individual stocks in a privately-held company, Membership on an entity's Board of Directors or advisory committees. Ghobrial: AbbVie, Adaptive, Aptitude Health, BMS, Cellectar, Curio Science, Genetch, Janssen, Janssen Central American and Caribbean, Karyopharm, Medscape, Oncopeptides, Sanofi, Takeda, The Binding Site, GNS, GSK: Consultancy.

Background: Recent studies have identified clinical and genomic factors contributing to worse clinical outcomes in patients with multiple myeloma (MM). Clonal hematopoiesis (CH) reflects the presence of somatic driver mutations in the blood or marrow of otherwise asymptomatic individuals. Using a variant allele frequency (VAF) cutoff of 2%, we recently reported CH in 21.6% of MM patients at the time of autologous stem cell transplant (ASCT) and found it was associated with shorter overall survival (OS) and progression-free survival (PFS) in those who did not receive maintenance therapy with an immunomodulatory drug (IMiD). However, this finding was based on a single tertiary center and only included MM patients who received ASCT. Methods: We studied a larger cohort of 986 newly diagnosed MM cases. Whole-exome sequencing (WES) data of peripheral blood and bone marrow samples of 986 MM patients (523 transplanted and 463 non-transplanted) from the Multiple Myeloma Research Foundation (MMRF) Clinical Outcomes in MM to Personal Assessment of Genetic Profile (CoMMpass, NCT0145429) study were analyzed. Both peripheral blood and tumor samples were analyzed to filter out myeloma mutations that could be contaminating the peripheral blood. Given the lower depth of coverage compared to prior targeted sequencing studies, small clones with a VAF below 2% were not detected. Altogether, the WES samples had a total depth of coverage of 117.68X. All data were analyzed using R version 3.5.0 (R Core Team). Results: Among the total cohort, 113 CH mutations were detected in 101/986 (10.24%) patients. CH was detected in 42/523 (8.03%) transplanted patients, compared to 59/463 (12.74%) non-transplanted patients. The most commonly mutated genes were DNMT3A, TET2, ASXL1, PPM1D, and TP53. The median age of the cohort was 63 years (range: 27 - 93), 60% were male, and median follow-up was 3.9 years (95% CI: 3.7 - 4.0). The presence of CH was associated with age (69 vs. 62 years, P < 0.001). As expected, the median age of transplanted patients was lower (60 vs. 67 years) than in the non-transplanted group, which likely explains the higher prevalence of CH detected in the non-transplanted group. CH was associated with recurrent bacterial infections (P = 0.01) and increased cardiovascular disease (P = 0.006), but not with cerebrovascular disease (P = 0.74) or coagulopathies (P = 0.65). There was a trend towards worse PFS in non-ASCT patients with CH who were not treated with IMiDs (1.8 years) compared to non-CH IMiD-treated patients (2.7 years) (P < 0.001). A CH effect on PFS was not detected in ASCT patients. OS was not different in those with or without CH in both ASCT and non-ASCT groups. 8 (0.8%) patients developed a second hematologic malignancy. CH at the time of MM diagnosis was not associated with an increased risk of developing a second hematologic malignancy (P = 0.58). To determine whether CH clones emerged or evolved during treatment, we examined serial samples from 52 patients (36 ASCT patients and 16 non-transplanted patients) with sequential samples. The median time between the first and second time point was 3.1 years (range: 1.0 - 5.4 years). At the first time point, only 3/52 (5.8%) patients had CH, but that number increased to 13/52 (25.0%) at the second time point. Five out of the 13 (38%) were non-transplanted patients. All but 1 patient were exposed to IMiDs. The most common emerging mutated gene was DNMT3A, found in 7 patient samples at the second time point, compared to 2 patients at the first time point. Conclusion: Using WES in a large cohort of newly diagnosed MM patients, we detected CH in 10.2% (VAF ≥ 2%) of patients. CH and non-IMiD treatment confers a shorter PFS in non-transplanted MM patients. However, throughout IMiD-based treatment, MM patients tend to acquire and/or expand previously undetected CH clones, particularly DNMT3A. The clinical significance of this clonal expansion during therapy is yet to be elucidated, and for now, this observation does not yet change clinical management. Figure 1 Figure 1. Disclosures Steensma: Novartis: Current Employment. Ebert: Deerfield: Research Funding; GRAIL: Consultancy; Exo Therapeutics: Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; Skyhawk Therapeutics: Membership on an entity's Board of Directors or advisory committees. Soiffer: NMPD - Be the Match, USA: Membership on an entity's Board of Directors or advisory committees; Gilead, USA: Other: Career Development Award Committee; Rheos Therapeutics, USA: Consultancy; Kiadis, Netherlands: Membership on an entity's Board of Directors or advisory committees; Juno Therapeutics, USA: Other: Data Safety Monitoring Board; Precision Biosciences, USA: Consultancy; Jazz Pharmaceuticals, USA: Consultancy; Jasper: Consultancy; Takeda: Consultancy. Sperling: Adaptive: Consultancy. Getz: Scorpion Therapeutics: Consultancy, Current holder of individual stocks in a privately-held company, Membership on an entity's Board of Directors or advisory committees; IBM, Pharmacyclics: Research Funding. Ghobrial: AbbVie, Adaptive, Aptitude Health, BMS, Cellectar, Curio Science, Genetch, Janssen, Janssen Central American and Caribbean, Karyopharm, Medscape, Oncopeptides, Sanofi, Takeda, The Binding Site, GNS, GSK: Consultancy.

The extracellular matrix (ECM) is a major component of the tumor microenvironment, contributing to the regulation of cell survival, proliferation, differentiation and metastasis. In multiple myeloma (MM), interactions between MM cells and the bone marrow (BM) microenvironment, including the BM ECM, are critical to the pathogenesis of the disease and the development of drug resistance. Nevertheless, composition of the ECM in MM and its role in supporting MM pathogenesis has not been reported. We have applied a novel proteomic-based strategy and defined the BM ECM composition in patients with monoclonal gammopathy of undetermined significance (MGUS), newly diagnosed and relapsed MM compared with healthy donor-derived BM ECM. In this study, we show that the tumor ECM is remodeled at the mRNA and protein levels in MGUS and MM to allow development of a permissive microenvironment. We further demonstrate that two ECM-affiliated proteins, ANXA2 and LGALS1, are more abundant in MM and high expression is associated with a decreased overall survival. This study points to the importance of ECM remodeling in MM and provides a novel proteomic pipeline for interrogating the role of the ECM in cancers with BM tropism.

Leukemia driver mutations have been identified in clonal hematopoiesis (CH; Jaiswal et al, NEJM, 2014). This provides a window of opportunity to interrogate the downstream impact of driver mutations in the earliest stages of neoplasia, before the accumulation of additional drivers that lead to frank malignancy. However, CH mutated cells are morphologically and phenotypically similar to normal cells. Thus, previous characterization in primary human samples have largely focused on genetic identification of these clonal outgrowths. To overcome this limitation and define the downstream effects of CH mutations, we leveraged multi-omic single-cell sequencing to profile the mutation status and whole transcriptome for the same cells at high-throughput (GoT; Nam et al, Nature, 2019). To identify CH samples, we screened 136 stem cell grafts collected for autologous transplant from multiple myeloma patients in remission (Mouhieddine et al, Nat Comm, 2020). We identified four individuals with DNMT3AR882 mutations with VAF > 0.05, allowing the profiling of 27,324 CD34+ cells (Fig. 1a-b). DNMT3A R882 mutations in these human CH samples did not result in a significant differentiation block in the hematopoietic stem cells, as assessed by pseudotime analysis (Fig. 1c). However, specific differentiation skews were apparent in multi-lineage progenitors, corresponding to the highest expression of DNMT3A in mutated immature myeloid progenitors (IMP, i.e. common myeloid progenitors/granulocyte-monocyte progenitors, Fig. 1d). We observed the expansion of mutated IMPs (Fig. 1e) that showed priming toward the megakaryocytic-erythroid (ME) lineage (Fig. 1f). DNMT3AR882 lympho-myeloid primed progenitors showed a myeloid bias and disfavored lymphoid development (Fig. 1g). In these CD34+ progenitors, we identified dysregulated expressions of megakaryocytic lineage markers, such as CD9 and PLEK, consistent with the ME-bias (Fig. 1h). We also observed downregulation of lymphoid differentiation gene ZBTB1, corresponding to the myeloid over lymphoid skewing (Fig. 1h). Finally, we identified dysregulated expression of genes involved in TNF-alpha signaling (e.g. TNFRSF4, TNFSF13B), suggesting a pro-inflammatory state. To identify activated pathways, we performed a gene set enrichment analysis that revealed activation of MYC and IL-6 signaling (FDR To determine how aberrant DNA methylation may serve as a link between DNMT3A mutations and the observed transcriptional dysregulation, we performed single-cell multi-omics that simultaneously profiles the cells' methylome and transcriptome, linked with mutation status (Gaiti et al, Nature, 2019; Fig. 1i). By comparing the methylation rates in mutated vs. wildtype CD34+ cells within the same sample, we found that DNMT3AR882 result in selective hypomethylation that preferentially impacts CpG islands (Fig. 1j) and PRC2 targets (Fig. 1k), which may underlie the dysregulated expression of PRC2 targets in DNMT3AR882 progenitors. Notably, DNMT3AR882-induced hypomethylation was enriched in a specific sequence context in which the CpG is followed by a T nucleotide (Fig. 1l). This DNMT3AR882 motif was enriched in the DNA binding motifs of central hematopoietic TFs, including MYC/MAX (Fig. 1m), suggesting that DNMT3AR882-induced hypomethylation may serve as a mechanism to preferentially increase the activity of these TFs. Consistent with this hypothesis, joint single-cell methylome and transcriptome data revealed that the expression of MYC targets increased with hypomethylation of its binding motifs (Fig. 1n), providing a mechanism of enhanced MYC activity due to DNMT3AR882 mutations. Altogether, we report the direct examination of the consequences of DNMT3AR882 mutations in primary CD34+ cells in CH. We discovered that DNMT3A is most highly expressed in mutated multi-lineage progenitors, promoting their expansion and biasing their downstream differentiation. This is accompanied by a disruption to differentiation through PRC2 and MYC target reactivation via selective hypomethylation. Disclosures Abdel-Wahab: Envisagenics Inc.: Current equity holder in private company; H3 Biomedicine Inc.: Consultancy, Research Funding; Janssen: Consultancy; Merck: Consultancy. Ghobrial:Takeda: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Bristol-Myers Squibb: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Karyopharm Therapeutics: Consultancy, Honoraria; Cellectar: Honoraria; Adaptive Biotechnologies: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria; Novartis: Consultancy; Noxxon Pharma: Consultancy; Genentech: Consultancy; GlaxoSmithKline: Consultancy; GNS Healthcare: Consultancy; AbbVie: Consultancy. Landau:Bristol Myers Squibb: Research Funding; Illumina: Research Funding.

Background This study investigated the prevalence of pulmonary hypertension (PH) in cystic fibrosis (CF) patients awaiting lung transplantation (LTx) and its influence on survival. We also explored the feasibility of using echocardiography as a first assessment for diagnosing PH. Methods The study included 93 CF patients (46 women [50%]) evaluated for LTx between 2001 and 2010. Median age was 29 years. PH was defined as a mean pulmonary artery pressure (mPAP) measured by right heart catheterization (mPAP cath ) of ≥ 25mm Hg with a wedge pressure of ≤ 15mm Hg. Echocardiographic results were divided into 3 categories based on current guidelines as "unlikely," "possible," or "likely" to have PH. Results In 23 patients (25%) the mPAP cath was between 25 and 35mm Hg, and 1 (1%) had severe PH (mPAP cath of ≥ 35mm Hg). PH did not influence survival after enlistment ( p = 0.7) and after LTx ( p = 0.8). For 62 patients (67%), the sPAP echo could be measured, and PH was unlikely in 24 (39%). In another 19 patients (20%), PH was unlikely based on the absence of tricuspid regurgitation. The negative-predictive value (NPV) of measuring PH by echocardiography was 88% in whom PH was estimated to be unlikely ( n = 43); whereas in 24 patients with a measurable low sPAP echo , the NPV was 96%. Conclusions PH exists in 26% of end-stage CF patients and has no effect on survival on the waiting list for LTx or after LTx. Echocardiography might be used as the first tool to rule out PH, showing a NPV of 88%.

Introduction: Multiple Myeloma (MM) is an incurable malignancy characterized by the proliferation of clonal plasma cells in the bone marrow (BM). MM almost always progresses from the precursor states of monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM), which indicates the presence of a gradual clonal evolution underlying progression from the original stages of tumor development to the time of clinical presentation. Clonal heterogeneity adds another layer of complexity to that, by introducing interclonal competition in the context of disease progression or therapeutic bottlenecks. Here we developed a mouse model to investigate the impact of multiple clonal mutations on tumor development, as well as the competitive expansion of individual clones. Methods: Primary mouse MM Vk*Myc cells stably expressing Cas9 were infected with validated sgRNAs to knockout (KO) genes of interest (P53, Cyld, Rb1, Dis3, Prdm1, Traf3 and Fam46c) that are significantly mutated in human MM. KO cells were mixed at a 1:1 ratio with control cells infected with control sgRNA and injected intravenously into 8-week-old RAG2 KO mice. Vk*Myc cells were then isolated from bone marrow and spleen through CD138 positive selection, followed by genomic DNA extraction and NGS sequencing to understand the dynamic changes in abundance of mutants from injection to early and late timepoints. Results: In vitro, most knockout Vk*Myc cells had a similar proliferation rate to control cells with the exception of P53 and Rb1 knockout cells, which grew faster as expected; both P53 and RB1 are known cell cycle regulators. However, when co-injected into RAG2 KO mice (Vk*Myc cells constructed with Cas9 do not engraft in C57BL/6 mice), although P53 and Rb1 knockout cells remained the strongest competitors, occupying the majority of the tumor, most KO cells exhibited significantly enhanced proliferation over control cells. These results indicate that certain mutations only become advantageous in the context of the tumor microenvironment, while mutations that directly affect the tumor cell's proliferation rate give rise to more flexible, potent clones. To better understand these differences, we took advantage of the CRISPR-induced heterogeneous pool of genomic edits per gene, and looked at clonal abundancy rates within each knockout population separately. Interestingly, we found mutants with certain insertions/deletions grew faster than others and were overrepresented at the late stage of disease, even when they were generated from the same double-stranded break. Although it is well established that mutations in different regions of the same gene might have different effects, these results indicate that different mutations in the exact same spot can give rise to clones of variable potency and beg the question of whether mutation sequence is as important as mutation hotspot. Conclusion: We established a mouse model to study clonal competition in vivo, utilizing the CRISPR-Cas9 genome editing toolset. Through our model, we were able to witness a range of competitive potential among genes that are significantly mutated in multiple myeloma, with P53- and RB1-mutants as the strongest competitors. Furthermore, we observed that competitive potential can be conditional, with certain mutants conferring fitness advantage only in the context of tumor microenvironment. Adding another layer of complexity to differential fitness, we found that different mutations in the same spot of the same gene give rise to clones of varied potency, implicating mutation sequence as a novel fitness variable. In this study, we thus demonstrate that mutational candidates can be prioritized based on competitive potential, a process of the utmost importance given multiple myeloma's marked genetic heterogeneity. Disclosures Ghobrial: Celgene: Consultancy; Takeda: Consultancy; Janssen: Consultancy; BMS: Consultancy.

Introduction: Multiple Myeloma (MM) is a genetically complex and evolutionary process with well defined precursor states, which offer a unique opportunity to study the sequential evolution of the disease. A small number of detectable pre-malignant clones are present in early stage and continue to acquire more genomic abnormalities leading to overt disease. The interaction between cancer cells and their environment is reciprocal, multiple components in the tissue environment can influence cancer clonal evolution and cancer cells in turn can also remodel the microenvironment and further disseminate to spatially separated areas of BM. To accurately predict the course of disease with the presence of BM environment, we require methods to estimate clone-specific growth rates and define clones that have the propensity of dissemination. Methods: We developed a novel 'bone chip' MM metastatic xenograft model using fluorescent protein tagged 'rainbow' system which enables both molecular profiling and functional tracking of clonal dissemination of tumor cells by performing tumor-bearing bone chip implantation subcutaneously to SCID-beige mice (SCID-murine model). Rainbow MM cells with equal proportion of all 15 colors were injected into donor femurs and implanted into recipient mice. After paralysis, the mice were sacrificed and tumor cells were analyzed using flow cytometry and confocal microscopy. Tumor clones in the implanted bone chip (primary sites) and distant host BM (metastatic sites) were purified by sorting and underwent RNA sequencing. By intersecting differentially expressed genes, we identified a set of genes, the expression of which were altered during disease dissemination and designated this set of genes as 'metastatic signature'. In addition, we also performed genome-wide CRISPR/Cas9-mediated loss-of-function screen in a subcutaneous xenograft mouse model to investigate the essential drivers of tumor growth and metastasis in MM. The cell library infected with human sgRNA library was injected subcutaneously into SCID-Beige mice on both flanks. When metastasis was established, the fractions of each sgRNA of the primary and metastatic tumors were calculated to identify genes that facilitate tumor metastasis. Results: We found that the 15 rainbow subpopulations were present with equal distribution in the primary sites but not at the metastatic sites. Confocal imaging showed the difference in cluster structures between primary and metastatic tumors. Most of the clusters in the metastatic sites consisted of cells of single colors. RNA sequencing analysis of two human MM cell lines derived from SCID-murine model demonstrated a distinct gene expression profile of the metastatic tumors. Gene Set Enrichment Analysis of the metastatic signature in publicly available MM patient datasets (GSE6477 and GSE2658) demonstrated that this signature is significantly correlated with overall survival and with clinical progression from MGUS/smoldering MM to overt myeloma and relapsed disease. Through genome-wide CRISPR screening in vivo, we found that the gene targets of the most enriched sgRNAs in the BM samples were preferentially involved in important cellular processes, such as cell cycle regulation and several oncogenic signaling pathways. Additionally, many sgRNAs that remained the implanted sites until late stage were depleted during dissemination, indicating their targeted genes were important for progression. These depleted sgRNAs mainly targeted genes involved in mTORC1 and DNA repair pathways, many of which are regulated by MYC and cell cycle related targets of E2F transcription factors. By using a network-based inference of protein activity method, we chose 4 genes (HMGA1, KLF6, TRIM28 and PA2G4) and validated in SCID-murine model using CRISPR mediated loss-of-function screen which prioritized HMGA1 as the key regulator in MM dissemination. Conclusions: Here, we demonstrate that in vivo clonal evolution can be characterized using an in vivo model of MM. The data defines specific subclones that have a higher metastatic potential and are likely driver clones for tumor metastasis in MM. We then established a platform for future invivo CRISPR screens to investigate essential genes of response to targeted therapies and/or immunotherapies. Furthermore, a metastatic gene signature was identified and among these, HMGA1 was validated as potential regulator of MM metastasis. Disclosures Roccaro: AMGEN: Other: Advisory Board; GILEAD: Research Funding. Ghobrial:Takeda: Consultancy; Celgene: Consultancy; BMS: Consultancy; Janssen: Consultancy.

Introduction: Recent data show that Multiple Myeloma (MM) always progresses from a precursor state (monoclonal gammopathy of undetermined significance [MGUS]/smoldering multiple myeloma [SMM]) to overt MM indicating that there is continuous dissemination/clonal evolution of tumor cells from the original stages of tumor development to the time of clinical presentation. A major challenge in understanding the progression and metastasis of MM is to distinguish alterations driving the tumor growth and evolution from passenger mutations. Genetic screens are powerful tools for assaying phenotypes and identifying causal genes in various hallmarks of cancer progression. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system has emerged as a powerful technology to efficiently and simultaneously perform genome editing of multiple genes. Here we report a genome-wide CRISPR/Cas9-mediated loss of function screen in a xenograft mouse model to investigate the essential drivers of tumor growth and metastasis in MM. Methods: Lentiviral particles from 2 subpools of a human sgRNA library (Avana), each containing 1 sgRNA per gene were introduced into MM1.S (Cas9+/GFP+/Luc+) cell line with the pre-determined amount of virus to achieve 30-50% infection efficiency, corresponding to a multiplicity of infection (MOI) of ~0.5-1. Cells were selected with puromycin for 5-7 days following infection to remove uninfected cells. Selected cells were injected subcutaneously into SCID-Beige mice on both flanks. Genomic DNA from pre-transplantation cells, early primary tumors (~3 weeks post tumor cell injection), late stage primary tumors and metastatic bone marrow samples were extracted. gDNA was amplified following adaptor ligation and barcoding of the samples and PCR products were subsequently sequenced on a HiSeq2000 (Illumina). Results: To investigate the sgRNA library dynamics in different sample types (pre-transplantation cells, early primary tumor, late primary tumor, and bone marrow metastasis), we compared the overall distributions of sgRNAs from all sequenced samples. The early tumor sample replicates of both subpools on average retained 77.3% and 94.7% of the sgRNAs found in the pre-transplanted cell populations, while the late primary tumors retained 59.4% and 65.6% of the sgRNAs respectively, compared to early tumors. Interestingly, only a small fraction of sgRNAs (1.1% and 3.4% of sgRNAs in the pre-transplantation cells, 10.7% and 7.2% of sgRNAs in the late primary tumors for the 2 subpools respectively) were detected in the metastatic bone marrow samples. Using gene set enrichment analysis (GSEA), we found that the gene targets of the most enriched sgRNAs in the bone marrow samples were preferentially involved in important cellular processes, such as cell cycle regulation, protein translation, and several signaling pathways. Additionally we compared sgRNAs present in early primary tumor versus pre-transplantation cells and late primary tumor and found that many sgRNAs were depleted during tumor progression, indicating that their target genes were important for progression. These depleted sgRNAs in both stages mainly targeted genes involved in mTORC1 and DNA repair pathways, many of which are regulated by MYC and cell cycle related targets of E2F transcription factors. Conclusion: We established a platform for future in vivo Cas9 screens using the genome-wide CRISPR screening libraries to explore potential new targets in regulating tumor dissemination, colonization and metastasis in MM. In addition, this in vivo screening could potentially be used to investigate essential genes of response to targeted therapies or/and immunotherapies. Thus, CRISPR/Cas9-based in vivo screening is a powerful tool for functional genomics discoveries. Disclosures Roccaro: Takeda Pharmaceutical Company Limited: Honoraria. Ghobrial:BMS: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Novartis: Honoraria; Takeda: Honoraria; Noxxon: Honoraria; Amgen: Honoraria.

In fertile women, the laparoscopic Roux-en-Y gastric bypass (LRYGB) is being increasingly performed. Pregnancy and LRYGB both give an increased risk of intussusception, which can lead to bowel necrosis, sepsis and preterm labour. We describe two pregnant women with a history of LRYGB who presented to the emergency department with non-specific abdominal pain. Both were diagnosed with intussusception. These cases illustrate that intussusception should be considered in pregnant women with a history of LRYGB who present with non-specific abdominal pain. Only MRI, CT scan or diagnostic laparoscopy is sufficient for diagnosis. Early diagnosis may prevent serious complications.