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Acute myeloid leukemia (AML) is an aggressive and heterogeneous clonal disorder of hematopoietic stem/progenitor cells (HSPCs). Leukemogenesis is characterized by rapid growth and impaired differentiation of leukemic cells, which progressively occupy and likely alter bone marrow (BM) niche where normal HSPCs reside [Kumar B., et al. Leukemia. 2018]. The homing and adhesion of AML cells to the BM niche is critical for its pathogenesis and involves many adhesion molecules, chemokine receptors, their ligands and signaling cascades. In the healthy context, the CXC chemokine receptor type 4 (CXCR4) expressed on HSPCs and its ligand, stromal derived factor-1 alpha (SDF-1α) produced by BM niche, are key mediators of HSPC-BM niche interaction inducing the homing, quiescence and survival of HSPCs, and the regulation of the HSC pool amount [Sánchez-Aguilera A., et al. Cell Mol Life Sci. 2017]. In AML context, where CXCR4/SDF-1α axis is deregulated, growth, BM niche-homing and adhesion of leukemic cells are altered also inducing AML relapse/resistance [Sánchez-Aguilera A., et al. Cell Mol Life Sci. 2017]. Despite recent improvements in therapeutic approaches, only a fraction of AML patients is cured. Therefore, it is crucial to identify new molecular mechanisms driving the AML pathogenesis. Extracellular vesicles (EVs) are lipidic bi-layer membrane particles released by normal and neoplastic cells. They shuttle a complex molecular cargo which reflects that of the origin cell [Caivano A., et al. Tumor Biol 2015; Caivano A., et al. Cell Oncol 2017] and which can be transferred to target cells altering their biological functions [De Luca L., et al. Oncotarget 2016; De Luca L., et al. Int J Mol Sci. 2017]. In particular, neoplasm derived EVs promote a tumor-supporting environment in non-malignant cells favoring cancer proliferation [Laurenzana I., et al. Stem Cells Int. 2018; De Luca L., et al. Expert Rev Mol Diagn. 2019]. We studied the effect of leukemia derived-EVs (LEVs) on healthy umbilical cord blood HSPCs to explore their potential role in dysregulating normal hematopoiesis. Specifically, the proliferation and differentiation, the quantitative and qualitative colony-forming features and the SDF-1α /CXCR4 mediated-migration of HSPCs induced by LEVs were investigated. EVs were isolated from AML cell lines by centrifugation steps and analyzed for AML surface markers by flow cytometry. Next, they were co-incubated with healthy CD34+ sorted cells for 24 hours. After treatment, cell count, apoptosis, HSPC-specific cluster of differentiation marker (CD10, CD34, CD38, CD45RA, CD90, CD123) expression [Karamitros D., et al. Nat Immunol. 2018], in vitro colony-forming unit cell assay, CXCR4 levels and SDF-1α mediated-migration assay were assessed. Our data showed: i) LEVs displaying specific AML surface markers; ii) a similar number of CD34+ cells in LEV treated-HSPCs, including HSCs, compared with controls; iii) no differences in apoptosis between the two groups; iv) an increase of multipotent progenitor (MPP, p=0.01) associated with a reduction of lymphoid-primed multipotent progenitors (LMPP, p=0.01) in treated cells; v) an increase of common myeloid progenitors (CMP, p=0.01), together with reduction of granulocyte macrophage progenitors (GMP, p=0.01) and megakaryocyte erythroid progenitors (MEP) in treated group; vi) a reduction of colony-forming ability, including erythroid, granulocytic and megakaryocytic colonies (p=0.03), in treated cells; vii) decreased levels of CXCR4 in LEV treated-HSPCs, that were accompanied by reduced HSPC migration mediated by SDF-1α. Altogether, this study suggests that AML derived-EVs can dysregulate the normal hematopoiesis promoting an increase of less differentiated precursors (MPP and CMP) which is associated with a reduction of more committed myeloid progenitors (GMP and MEP) (Figure 1). Furthermore, AML derived-EVs seem to induce less attraction of HSPCs in BM niche by interfering with SDF-1α/CXCR4 axis. Strategies to block EV production and secretion, as well as EV induced-reprogramming, could be novel exciting therapeutic approaches in AML. Disclosures Musto: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Introduction According to WHO classification, large granular lymphocyte leukemia (LGL leukemia) includes three entities: T-cell LGL leukemia, chronic NK lymphoproliferative disorders (NK-CLPD) and aggressive NK cell leukemia. Since no standard therapies for immature NK cell neoplasms have been established so far and the overall outcomes are dismal, new therapeutic options are needed. A pyrazolo[3,4-d]pyrimidine library of compounds have already been studied in a previous work by Tintori et al. It has been demonstrated that one of these compounds proved to be efficient against some member of Src-family of nonreceptor protein-tyrosine kinases, in particular against Fyn. Its abnormal activity has been shown to be related to various hematological malignancies. In this context we verified the effect of the pyrazolo[3,4-d]pyrimidine 4c compound on LGL leukemia cells and we investigated its mechanism of action. Methods The pyrazolo[3,4-d]pyrimidine 4c compound was tested in a time course MTS test (24, 48, 72 h) at different concentrations (0.1, 0.5, 1, 2, 4, 8, 10 µM) on NK leukemia cells (KHYG1 and NK92), hepatosplenic gamma-delta T cell lymphoma cells (Derl-2 and Derl-7), acute T cell leukemia cells (Jurkat), chronic myeloid leukemia cells (Jurl-MK1), acute myeloid leukemia cells (OCI), multiple myeloma cells (SKM-M1), cervical cancer cells (HeLa) and healthy stem cells (CD34+). Fyn mRNA and protein expression was evaluated in all cell lines by quantitative RT-PCR and western blotting (WB). Fyn mRNA levels were evaluated in cells from 10 healthy controls, 10 T-LGL leukemia, 5 GD-LGL leukemia and 8 NK-CLPD patients by qRT-PCR. Apoptosis and cell cycle studies were performed on KHYG1 after treatment with this compound at 4 µM for 24 h using cytometric analysis by labeling cells with annexin V and propidium iodide. Apoptosis pathway was investigated using proteome profiler human apoptosis array kit; pro-caspase 3, activated caspase 3, Akt and p70S6 kinase were validated by WB. Fyn immunoprecipitation was conducted on treated KHYG1 at 4 µM for 24 h. Results The pyrazolo[3,4-d]pyrimidine 4c compound reduced cell viability in KHYG1 cells (50% of reduction after 24 h of treatment at 4 µM), in Derl-2 cells (about 50% after 72 h at 4 µM), in Derl-7 (about 40% after 72 h at 8 µM), in Jurkat cells (50% after 72 h at 2 µM), in NK92 cells (40% after 72 h at 4 µM). No differences in cell viability were observed in Jurl-MK1, OCI, SKM-M1, HeLa and CD34+. To evaluate Fyn expression level, we performed qRT-PCR and WB showing that both transcript and protein levels were highest in NK and T cells (KHYG1>NK92>Jurkat>Derl-7>Derl-2) compared to other cells. Furthermore, we investigated the viability reduction in treated KHYG1 cells demonstrating that pyrazolo[3,4-d]pyrimidine 4c compound induced 45% of apoptosis caspase-3 mediated and cell cycle arrest in G2/M phase. In these cells we detected a reduction of Fyn phosphorylation after treatment and we also observed a decrease of total levels of Akt and p70S6 kinase, other two protein involved in apoptotic process and cell proliferation. Interestingly, qRT-PCR data showed a significant over-expression of Fyn mRNA level also in T-LGL, GD-LGL and NK-CLPD patients compared to healthy controls (p Conclusion In this study we demonstrated a higher Fyn expression in LGL leukemia patients compared to healthy controls. We also demonstrated an apoptotic effect of pyrazolo[3,4-d]pyrimidine 4c compound on NK leukemia cells and we provided in vitro preliminary evidences that Fyn is its possible cellular target. Disclosures Musto: Novartis: Honoraria; Celgene: Honoraria; Sanofi: Consultancy; Genzyme: Consultancy; Sandoz: Consultancy; Janssen: Honoraria; Mundipharma: Honoraria; Roche: Honoraria.

INTRODUCTION: Extracellular vesicles (EVs), including exosomes, shedding vesicles (microvesicles: MVs) and apoptotic bodies, are membrane vesicles of 40-3,000 nm that are released from many cell types, such as red blood cells, platelets, lymphocytes, dendritic cells, endothelial cells and tumour cells. They have a key role not only in the regulation of normal physiological processes, but also in the pathology underlying several diseases. Recently, it was discovered that EVs circulate in bodily fluids of cancer patients and could serve as potential diagnostic biomarkers. However, the presence and role of peripheral circulating EVs in hematological malignancies (HMs) remain unclear. The aim of this study was to investigate EVs in peripheral blood of HM patients, defining the EVs count, size and phenotype. METHODS: Peripheral blood samples were obtained prior to treatment from 87 subjects: 6 Chronic Lymphocytic Leukemia (CLL), 10 Non-Hodgkin's Lymphoma (NHL; 5 follicular, 5 diffuse large B-cell), 6 Waldenstrom's Macroglobulinemia (WM), 6 Hodgkin's Lymphoma (HL), 6 Multiple Myeloma (MM), 5 Acute Myeloid Lukemia (AML), 19 Myeloproliferative Neoplasms [(MPNs; 5 Essential Thrombocythemia (ET), 5 Polycythemia Vera (PV), 5 Primary Myelofibrosis (PMF), 4 Chronic Myelogenous Leukemia (CML)], 5 Myelodysplastic Syndrome (MDS) and 28 healthy controls. EVs were isolated from serum of peripheral blood by ultracentrifugation steps. For calculation of counts, TruCOUNT beads were added immediately prior to analysis by flow cytometry (FACS). For size assessment, Megamix beads with specific size (0.3,0.9, 3.0 micrometer) were used. For phenotype characterization, a panel of specific antibodies (anti CD59, CD61, CD62E, CD19, CD20, CD38, CD138, CD13, CD30) were employed in a FACS analysis. RESULTS: MVs count was significantly higher in patients with CLL, WM, HL, MM, AML and CML (median values 890, 3227, 1218, 914, 656 and 1275 MVs/microliter of serum, respectively) compared to healthy controls (median value 270 MVs/ microliter of serum, ***P In CLL and MDS, through the number of cases was small, MVs counts directly correlated with Rai stage and R-IPSS risk, respectively. As regards vesicles volume, all HMs generated a distinct population of MVs with smaller size when compared with controls. As expected, MVs were consistently Annexin V positive and expressed a common membrane protein, CD59, an ubiquitous complement regulator. Part of MVs originated from platelets (median value 400 CD61 positive/ microliter of serum) and endothelial cells (median value 60 CD62E positive/ microliter of serum). Interestingly, MV phenotype was disease specific: a relevant amount of MVs expressed surface specific proteins related to malignancy. In particular, we found MVs positive for CD19 and/or CD20 in CLL, WM and NHL, CD38 and/or CD138 in MM, CD13 in AML, MPNs and MDS, CD30 in HL. These markers were not significantly expressed on the surface of healthy subject MVs. CONCLUSIONS: In this study, for the first time, serum MVs in a panel of HMs were analysed. We found that patients with various types of HMs release elevated peripheral blood MV levels selectively displaying markers of the underlying malignancy. MVs could be therefore useful as potential novel biomarkers in HMs. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Introduction Hematopoietic stem cells (HSC) are used in transplantation for the treatment of several hematological or non-hematological diseases. HSC fate is bound to different factors and, among these, the cross-talk with the bone marrow microenvironment represents an important regulatory mechanism of aging, self-renewal, stemness and/or differentiation. Cells communicate by both direct interaction and through the secretion of soluble factor and extracellular vesicles (EVs). EVs are cell-derived vesicles enclosed by a lipid bilayer and enriched in phosphatidylserine, cytoplasmic protein, mRNA, miRNA and DNA. Recent studies suggest that the release of EVs is a highly regulated and important process for surface membrane traffic and horizontal transfer of RNAs, protein and DNA, present in different kind of cells, including tumor cells. Bone marrow mesenchimal stem cells (BM-MSC) are a component of hematopoietic microenvironment and support hematopoiesis by the constitutive ability to secrete soluble factor and EVs. On these basis, we decided to study the interaction between the umbilical cord blood (UCBs) stem cells and the BM-MSCs mediated by EVs. The overarching goal of this project was to find out if and how the EVs can influence the fate of UCBs in transplantation. Methods Primary BM-MSC were cultured in DMEM containing 10% of fetal bovine serum (FBS). EVs were isolated from supernatant of BM-MSC by ultracentrifugation at 100.000 x g for 70 min at 4 ºC. After characterization by FACS analysis, EVs were stocked at -80 ºC and partially used for RNA extraction using TRIZOL reagent. EVs smallRNA was sequenced with HiScan SQ Illumina. UCBs CD34+ cells were isolated by Miltenyi Biotec separation kit and were cultured in DMEM containing 10% of FBS with or without BM-MSC EVs. After 24h of co-culture, viability by trypan blue count, apoptosis by Propide Iodure and Annexin V test and cell differentiation by FACS analysis of different markers (CD45, CD34, CD33, CD19 and CD38) were evaluated. RNA was extracted from the same samples and a gene expression profile experiment was performed using Illumina array. All genomic data were analyzed by Ingenuity Pathways Analysis software (IPA). Results EVs isolated from BM-MSC were positive for MSC markers like CD29 (61%), CD90 (72%), CD73 (63%), CD105 (35%), CD146 (32%), CD44 (70%) and for the exosome antigen CD81 (40%); they were negative, instead, for the hematopoietic marker CD45. The sequencing of smallRNA-EVs identified 87 miRNAs in BM-MSC EVs. Analysis of gene expression profile of UCBs CD34+ cells treated with EVs revealed 103 up-regulated and 100 down-regulated genes (with padj < 0.05 and logFC=0.7), when compared to UCBs CD34+ cells not treated with EVs. Interestingly, analyzing together the sequencing of EVs and the gene expression data of UCBs CD34+ cells treated with EVs, we found a direct correlation between EVs miRNAs and down-regulated genes, identifying at least one target gene for each EVs miRNA (e.g., miR-3168/LYZ, miR-27b-3p/ZFP36, miR21-5p/ANXA1). These results indicate that the EVs RNA content modifies the genes profile of receiving cells. Gene ontology analysis, using IPA, of the miRNA targeted genes identified different down-regulated biological function, like cell death and cellular development (e.g., CDKN1B, CEBPA, ANXA1, MPL). To confirm these data, we evaluated viability and apoptosis of UCBs treated with EVs respect to the UCBs control culture, and we observed an increase of 50% of cell viability and a reduction of 42% of apoptosis (p Furthermore, using IPA, we found that some up-regulated genes (e.g., IL6, CSF2, CCL3) are under the control of miRNA targeted genes (e.g., ZFP36/miR-27b-3p). The analysis of these genes identified some biological function up-regulated in cell after EVs treatment, the most important one being chemotaxis of the cells. Conclusion This study indicates, for the first time, the existence of a cross-talk between MSC and UCBs mediated by EVs. Moreover, it identifies a gene profile modification of UCBs cells after miRNA EVs treatment, providing new insights for cord blood transplantation. Disclosures No relevant conflicts of interest to declare.

Introduction Multiple myeloma (MM) remains an incurable malignancy despite important recent advances in treatments. Neo-vascularization entails a crucial aspect of interactions between neoplastic plasma cells (PCs) and their microenvironment. Without it, MM would be unable to grow and progress, and would probably regress to a low-mass steady-state comparable to monoclonal gammopathy of undetermined significance (MGUS). To overcome drug resistance and improve clinical response to novel therapeutic approaches halting both PC growth and the increased bone marrow (BM) microvascular density are needed. In this setting, monoclonal antibodies against MM-specific cell surface antigens represent a promising therapeutic approach, which is however hampered by a lack of appropriate membrane target structures expressed across all MM cells. The Eph receptors, a large family of receptor tyrosine kinases, have been implicated in many processes involved in malignancy, including alteration of the tumour microenvironment, and in angiogenesis, in both of which EphA3 likely plays an active role. Interestingly, the over-expression of EphA3 is sufficient to confer tumorigenic potential, although probably further mechanisms can occur to abnormally activate the receptor. A first-in-class engineered IgG1 antibody targeting the EphA3 was developed and it is now under phase I clinical trials in USA and Australia for the treatment of EphA3 over-expressing hematological myeloid malignancies refractory to conventional treatment. Methods We investigated the EphA3 role in MM patients in order to define whether it may represent a potential new molecular target for a novel therapeutic approach with a specific anti EphA3 monoclonal antibody. The EphA3 expression was studied through a comparative proteomic analysis between BM endothelial cells (ECs) of patients with MM (MMECs) or with MGUS (MGECs), of control subjects (normal ECs). Moreover, the effects of anti EphA3 antibody in MM were studied in vitro and in vivo in a MM xenograft mouse model. After written informed consent, BM aspirates were collected from 26 MM and 6MGUS patients. Normal ECs were derived from 5 BM aspirates of subjects with anemia due to iron or vitamin B12 deficiency. We analyzed both mRNA and protein levels of EphA3 in normal ECs, MGECs and MMECs and in MM cell lines by absolute RT-PCR and by WB coupled to immunofluorescence and FACS analysis respectively. Immunoistochemistry was also performed on MM BM biopsies. The biological effects of EphA3 targeting were studied in vitro silencing (siRNA) the EphA3 mRNA in MMECs and using the anti EphA3 antibody testing them in series of in vitro functional assays including viability, apoptosis, adhesion, migration, wound healing and angiogenesis tests. We further examined the inhibitory capacity of anti-EphA3 Ab on tumor growth in SCID mice bearing MM tumor cell xenografts. Finally, we assessed morphology, vessel density, and apoptosis of excised xenotransplanted tumors. Results Briefly, our data showed that EphA3 mRNA and protein levels are progressively increased from ECs to MGECs, reaching the highest values in MMECs. EphA3 stained intensely and diffusely MM microvessels and PC in MM BM biopsies. The EphA3siRNA MMECs revealed a protein level reduction of approximately 80% when compared to the control. We not detected viability or apoptotic defects, whereas in vitro adhesion, migration and angiogenesis inhibition was evident when compared to the not silenced counterpart. The anti EphA3 antibody inhibited MMECs migration and reduced in vitro MM angiogenesis. In particular, tumour masses developed in xenograft mice treated with anti-EphA3 Abs were smaller in size and showed foci of ischemic-hemorrhagic necrosis, in association with a significant (P < 0.05) reduction in the number of intact tumor microvessels. The proliferative activity was not significantly different from that observed in tumors from untreated or control isotype treated mice, while the apoptotic index was significantly (P < 0.05) increased in comparison with tumors from both groups of mice. Conclusions In this study we have characterized the role of the EphA3in MM patients, providing in vitro and in vivo experimental evidences that support the possibility of using EphA3 as a new molecular target for MM. Disclosures: No relevant conflicts of interest to declare.

Abstract 3580 Introduction: Although treatment with tyrosine kinase inhibitors (TKIs) has revolutionized the management of adult patients with BCR-ABL1 -positive acute lymphoblastic leukemia (ALL) and significantly improved response rates, relapse is still an expected and early event in the majority of them. It is usually attributed to the emergence of resistant clones with mutations in BCR-ABL1 kinase domain or to BCR-ABL1 -independent pathways but many questions remain unresolved about the genetic abnormalities responsible for relapse after TKI and chemotherapy-based regimens. Patients and methods: In an attempt to better understand the genetic mechanisms responsible for this phenomenon, we have analyzed matched diagnosis-relapse samples from 30 adult BCR-ABL1 -positive ALL patients using high resolution Affymetrix single nucleotide polymorphism (SNP) arrays (GeneChip® Human Mapping 250K NspI, n=15 pairs and Genome-Wide Human SNP 6.0, n= 15 pairs). Genetic differences were analyzed in terms of copy number changes and loss of heterozygosity (LOH) events. 20 patients were enrolled in clinical trials of GIMEMA AL Working Party and treated with imatinib alone or in combination with conventional chemotherapy (40%) or dasatinib as frontline therapy (60%). The median age at diagnosis was 54 years (range 23–74) and the median blast cell count was 97% (range 60–99). The median time to relapse was 27 months (range, 9–104). 10 patients were treated according to the GMALL trials, a high-dose chemotherapy based protocol in combination with imatinib. The median age at diagnosis was 65 years (range 19–79) and the median leucocyte count was 37300/μl (range 5000 – 220000/μl). The median time to relapse was 9.8 months (range, 3 – 25). Results: First, we compared diagnosis and relapse samples for the presence of macroscopic (> 1.5 MB) copy number alterations (CNA). Novel acquired macroscopic CNAs were detected in 7/20 (35%) TKI relapse cases and included losses of 3p12-p14, 5q34, 9q34, 10q24 and 12p13-p12 and gains of 1q, 9q34-q33 and 22q and in 4/10 (40%) chemotherapy-relapse cases and included losses of 9p21 and 12q21–22 and gains of all chromosome 8 or part of it in 2 patients. Since no common patterns of acquired alterations were observed, it is likely that relapse may be due to a more generalized genetic instability rather than to specific mechanisms. Moreover, chemotherapy did not select resistant clones with higher number of alterations. 8/20 (40%) TKI resistant cases and 4/10 chemotherapy resistant patients harbored the same CNAs present in the matched diagnosis sample (losses of 9p21 in 7 cases, 7p and 22q11 in single cases and gains of chromosomes 1q, 4, 8q, 17q and 21), indicating a common clonal origin. In contrast, in 5/20 (25%) TKI resistant cases and 4/10 (40%) chemotherapy resistant patients macroscopic CNAs present at diagnosis were lost at relapse (losses of chromosomes 7, 11q, 14q, 15q, 16q and 19p and gains of 5q, 8q, 9q34 and 22q11). Thereafter, we compared diagnosis and relapse samples for microscopic CNAs (< 1.5 MB). The alteration most frequently acquired at relapse was loss of the tumor suppressor CDKN2A (53% vs 33 % of diagnosis). Other common acquired CNAs at relapse included gains of ABC transporter genes, such as ABCC1, ABCC6 (1q41) and BCL8 (15q11); losses affected EBF1 (5q33) and IGLL3 (22q11) genes involved in B-cell development, BTG1 (12q21) involved in cell cycle regulation and CHEK2 (22q12) involved in DNA repair. Interestingly, for all relapse cases analysis of IKZF1 deletions, identified in 80% of patients, demonstrated a clonal relationship between diagnostic and relapse samples, suggesting that this alteration is not acquired with relapse but it is maintained with fidelity from diagnosis working as a marker of disease. The majority (92%) of relapse samples harbored at least some of the CNAs present in the matched diagnosis sample, indicating a common clonal origin. Conclusions: Genomic copy number changes evolving from diagnosis to relapse have been identified demonstrating that a diversity of alterations contributes to relapse and with the most common alterations targeting key regulators of tumor suppression, cell cycle control, and lymphoid/B cell development. Supported by European LeukemiaNet, AIL, AIRC, Fondazione Del Monte di Bologna e Ravenna, FIRB 2006, PRIN 2009, Ateneo RFO grants, PIO program, Programma di Ricerca Regione – Università 2007 – 2009. Disclosures: Soverini: Novartis: Consultancy; ARIAD: Consultancy; Bristol-Myers Squibb: Consultancy. Baccarani:Pfizer Oncology: Consultancy; Novartis: Consultancy; BMS: Consultancy; Ariad: Consultancy; Novartis: Research Funding; Pfizer Oncology: Honoraria; Novartis: Honoraria; BMS: Honoraria; Ariad: Honoraria; Novartis: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Ariad: Membership on an entity's Board of Directors or advisory committees. Ottmann:Novartis Corporation: Consultancy, Honoraria, Research Funding, Speakers Bureau. Martinelli:Novartis: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Pfizer: Consultancy.