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Hematopoietic stem cells (HSCs) and distinct multipotent progenitor (MPP) populations (MPP1-4) contained within the Lin−Sca-1+c-Kit+ (LSK) compartment have previously been identified using diverse surface-marker panels. Here, we phenotypically define and functionally characterize MPP5 (LSK CD34+CD135−CD48−CD150−). Upon transplantation, MPP5 supports initial emergency myelopoiesis followed by stable contribution to the lymphoid lineage. MPP5, capable of generating MPP1-4 but not HSCs, represents a dynamic and versatile component of the MPP network. To characterize all hematopoietic stem and progenitor cells, we performed RNA-sequencing (RNA-seq) analysis to identify specific transcriptomic landscapes of HSCs and MPP1-5. This was complemented by single-cell RNA-seq analysis of LSK cells to establish the differentiation trajectories from HSCs to MPP1-5. In agreement with functional reconstitution activity, MPP5 is located immediately downstream of HSCs but upstream of the more committed MPP2-4. This study provides a comprehensive analysis of the LSK compartment, focusing on the functional and molecular characteristics of the newly defined MPP5 subset.

Hematopoietic stem cells (HSCs) and distinct multipotent progenitor (MPP) populations (MPP1-4) contained within the Lin−Sca-1+c-Kit+(LSK) compartment have previously been identified using diverse surface-marker panels. Here, we phenotypically define and functionally characterize MPP5 (LSK CD34+CD135−CD48−CD150−). Upon transplantation, MPP5 supports initial emergency myelopoiesis followed by stable contribution to the lymphoid lineage. MPP5, capable of generating MPP1-4 but not HSCs, represents a dynamic and versatile component of the MPP network. To characterize all hematopoietic stem and progenitor cells, we performed RNA-sequencing (RNA-seq) analysis to identify specific transcriptomic landscapes of HSCs and MPP1-5. This was complemented by single-cell RNA-seq analysis of LSK cells to establish the differentiation trajectories from HSCs to MPP1-5. In agreement with functional reconstitution activity, MPP5 is located immediately downstream of HSCs but upstream of the more committed MPP2-4. This study provides a comprehensive analysis of the LSK compartment, focusing on the functional and molecular characteristics of the newly defined MPP5 subset.

BACKGROUND. HCV-positive DLBCL has distinct clinical and pathologic characteristics compared to its negative counterpart: patients (pts) are usually older with more frequent splenic and extranodal involvement and elevated LDH. Differently from its clinical hallmarks, the molecular landscape of this pathological entity has been scarcely outlined.

Passamonti: Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AbbVie: Speakers Bureau; BMS: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Arcaini: Celgene: Speakers Bureau; Gilead Sciences: Research Funding; Bayer, Celgene, Gilead Sciences, Roche, Sandoz, Janssen-Cilag, VERASTEM: Consultancy; Celgene, Roche, Janssen-Cilag, Gilead: Other: Travel expenses.

Mauro: Gilead: Consultancy, Research Funding; Jannsen: Consultancy, Research Funding; Shire: Consultancy, Research Funding; Abbvie: Consultancy, Research Funding; Roche: Consultancy, Research Funding. Coscia:Abbvie: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Karyopharm Therapeutics: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding. Varettoni:ABBVIE: Other: travel expenses; Roche: Consultancy; Janssen: Consultancy; Gilead: Other: travel expenses. Rossi:Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Honoraria, Other: Scientific advisory board; Janseen: Honoraria, Other: Scientific advisory board; Roche: Honoraria, Other: Scientific advisory board; Astra Zeneca: Honoraria, Other: Scientific advisory board. Gaidano:AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sunesys: Consultancy, Honoraria; Astra-Zeneca: Consultancy, Honoraria; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Blood regeneration upon transplantation relies on the activity of long-term repopulating hematopoietic stem cells (LT-HSCs). One of the major controversies in hematopoiesis relates to the apparently different properties that HSCs have in transplantation versus unperturbed settings. In unperturbed steady state hematopoiesis, the most potent HSCs appear to be mostly dormant, and only producing platelet-lineage cells. In turn, upon transplant, even a single transplanted HSC can actively divide and regenerate hundreds of millions of blood progenitors of all lineages. It would thus appear that HSCs have different fundamental properties in each study system.

No relevant conflicts of interest to declare.

The CLL-IPI score, which combines genetic, biochemical, and clinical parameters, represents a simple worldwide model able to refine risk stratification for CLL patients. This score, developed in the era of chemo-immunotherapy, has not been gauged extensively in R/R-CLL patients treated with novel targeted agents, such as BCR and BCL2 inhibitors. Soumerai et al (Lancet Hematol 2019) assembled a novel risk model for OS in the setting of R/R-CLL receiving targeted therapies in clinical trials. This model, consisting of four accessible markers (β2M, LDH, Hb, and time from initiation of last therapy; BALL score), is able to cluster 3 groups of CLL patients with significantly different OS. This multicenter, observational retrospective study aimed to validate the proposed Soumerai (BALL) and/or CLL-IPI scores for R/R-CLL real-world patients treated with idelalisib and rituximab (IDELA-R). The primary objectives were to determine whether: i) the CLL-IPI retains its prognostic power also in R/R patients treated with IDELA-R; ii) the BALL score is of prognostic value for IDELA-treated R/R-CLL patients, and iii) the BALL score is predictive of PFS. This study, sponsored by Gilead (ISR#IN-IT-312-5339), included CLL patients collected from 12 Italian centers, who received IDELA-R (idelalisib 150 mg b.i.d. and a total of 8 rituximab infusions intravenously) outside clinical trials as salvage therapy with available data for the calculation of the CLL-IPI and BALL scores at the time of treatment start. OS was estimated for all subgroups of both scores. Additionally, risk-specific PFS was assessed. Kaplan-Meier curve, log-rank test, and Cox regression analyses were performed. The prognostic accuracy of the predictive model was assessed by Harrell's C-index. Overall, 120 CLL patients were included in this analysis. The majority of patients were Binet stage B and C (94.2%). The median age was 75 years and 83 cases (69.2%) were male. The median number of previous therapies was 3 (range 1-9) Baseline patient features are listed in Table 1. After a median follow-up of 1.6 years (1 month to 5.8 years), 33 patients had died and 39 experienced an event (death or progression). CLL-IPI scoring (115/120 evaluable cases) indicated that 6 patients (5.2%) were classified as low-risk, 24 (20.9%) as intermediate-risk, 58 (50.4%) as high-risk, and 27 (23.5%) as very high-risk. Stratification of patients according to the CLL-IPI score did not allow prediction of significant differences in OS. Thus, low-risk patients had a 2-year OS probability of 75% (HR=1), with an intermediate-risk of 68% (HR=2.9, 95%CI 0.37-23.3, P=0.3), high-risk of 83% (HR=1.58, 95%CI 0.2-12.5, P=0.66), and very high-risk of 63% (HR=5.9, 95%CI 0.78-45.2, P=0.86). Next, we tested a modified CLL-IPI by assigning a more balanced score to the original CLL-IPI variables (Soumerai et al, Leukemia Lymphoma 2019), partially overlapping previous results. Specifically, modified CLL-IPI high-risk group showed a significantly different OS as compared with intermediate- and low-risk groups. However, differently from the original report no difference was observed between low- and intermediate-risk). According to the BALL score (120/120 evaluable cases), 33 patients (27.5%) were classified as low-risk, 68 (56.7%) as intermediate-risk, and 19 (15.8%) as high-risk. Stratification of patients according to the BALL score predicted significant differences in terms of OS. Thus, low-risk patients had a 2-year OS probability of 92% (HR=1), intermediate-risk of 76% (HR=5.47, 95%CI 1.3-23.7, P=0.023), and high-risk of 54% (HR=15.1, 95%CI 3.4-67, P<0.0001) (Figure 1). Harrell's C-statistic was 0.68 (P<0.001) for predicting OS. To note, BALL score failed to significantly stratify patients in terms of PFS. As for Soumerai et al (Leukemia Lymphoma 2019), the original CLL-IPI score did not retain discriminative power in term of OS in R/R-CLL patients receiving IDELA-R. The modified CLL-IPI failed to stratify low- and intermediate-risk groups, likely due to the number of cases analysed in the current cohort and the heterogeneous IDELA-containing regimens included in the Soumerai study (Soumerai et al, Leukemia Lymphoma 2019). The CLL-IPI was designed for CLL patients treated with first-line chemo-immunotherapy. Herein, we confirm the prognostic power of the BALL score in this real-world series for OS, while losing the predictive impact of patient outcomes in terms of PFS.

No relevant conflicts of interest to declare.

The hematopoietic system follows a hierarchical organization, with multipotent long-term repopulating hematopoietic stem cells (LT-HSCs) occupying the top tier. This paradigm, developed mostly through cell transplantation assays, has recently been contested by a series of studies performed under native conditions, without transplantation. Application of systems-level single cell methods in this setting has revealed a heterogeneity of cell states within progenitors and stem cells, prompting a reevaluation of the theories of hematopoietic lineage fate decisions. We have previously described that hematopoietic stem cell fates are clonally heterogeneous under steady state and uncovered that a fraction of LT-HSCs contributes to a significant proportion of the megakaryocytic cell lineage under steady state, while rarely generating other types of progeny in unperturbed conditions. To elucidate the molecular underpinnings of this functional lineage-output heterogeneity, we developed a technique to barcode hematopoietic cells at the RNA level in order to simultaneously capture the lineage relationships and transcriptional states of HSCs. Using a droplet-based massive single cell RNAseq platform, we analyzed thousands of engrafted hematopoietic stem cells together with a sufficiently significant representation of downstream progenitor cells to measure HSC output. Inspection of the resulting “stem cell state-fate maps” revealed a variety of stem cell behaviors, including single cell quiescence, asymmetric and symmetric divisions, and clonal expansion. We also connected these behaviors with some of the previously observed heterogeneity in stem cell outcomes, including lineage bias, lineage output and clonal competition. Importantly, clustering of expression profiles revealed significant differences in the transcriptional programs related with some of these behaviors, which illuminate the molecular machineries that operate at the stem cell level to define this heterogeneity. Thus, our work has identified potential novel mediators for stem cell heterogeneity, which we are functionally analyzing in further detail to understand their molecular mechanisms.

Off Label Use: Off-label use of vemurafenib in hairy cell leukemia will be discussed as part of a clinical research protocol..

Offidani: Mundipharma, Janssen: Honoraria. Off Label Use: Bendamustine. Corvatta:Janssen: Honoraria. Ballanti:Janssen: Honoraria. Brunori:Janssen: Honoraria.

Introduction:Bendamustine, a bifunctional alkylating agent, exerts a mechanism of action different from that of other conventional alkylators despite it remains mostly unknown. In patients with newly diagnosed or relapsed-refractory MM bendamustine has proven to be active either as monotherapy or in combination with new drugs, particularly bortezomib and immunomodulatory agents.

Offidani: Mundipharma: Honoraria, Research Funding. Off Label Use: Bendamustine.

Bendamustine, a bi-functional alkylating agent comprising a purine-like benzimidazole ring an a nitrogen mustard group, exerts a peculiar mechanism of action if compared to most conventional alkylators and this may partially explain its effectiveness in alkylator-resistant cells. In patients with MM several studies demonstrated the efficacy and tolerability of bendamustine as monotherapy or in combination with new drugs, particularly bortezomib that was found to enhance the in vitro sensitivity of MM cells to bendamustine.

Bendamustine is a bifunctional alkylating agent approved for the treatment of several lymphoproliferative disorders. Studies have evidenced its efficacy in multiple myeloma (MM), but data so far available in this setting are scarce. We performed a retrospective analysis of Italian patients with relapsed/refractory MM, who had received bendamustine as salvage therapy within a national compassionate use program.Seventy-eight patients (42 males, 36 females) were collected in 19 hematological centers. Mean age was 64.2 years (range 38–84). ISS was equally distributed, with about one third of patents being represented in single stages. Twenty-three of 43 analyzed patients had cytogenetic abnormalities, the most frequent being del13q (14 patients); t(4;14) and t(11;14) were observed in 4 and 2 patients, respectively, while t(6;14), del17p or complex karyotype occurred in single patients. The median number of prior lines of therapy was 4 (range 1–10). Ninety-seven percent of patients had previously received bortezomib, 94% IMIDs, 85% melphalan, 74% cyclophosphamide, 45% anthracyclines, 26% other drugs, 33% radiotherapy. Sixty percent of patients had undergone autologous and 4% allogeneic stem cell transplantation. The last treatment before bendamustine was a bortezomib-based regimen in 31%, an IMIDs-based regimen in 42%, a combined bortezomib/IMIDs-based regimen in 9%, while 18% of patients had received other therapies. Seventy-three percent of patients were resistant to last therapy received, while 27% had relapsed. Median duration of response to last treatment received before bendamustine was 9 months (range 2–46). Median Hb value was 10.1 g/dl (range 7.6–14.9), WBC count 2.700/μl (range 550–15.200), PLT count 130.000/μl (range 6.000–410.000). Serum creatinine, calcium, beta2-microglobulin and LDH levels were increased in 12 (15%), 4 (5%), and 44 (56%) patients, respectively, while albumin levels were decreased in 27 patients (35%). The median percentage of marrow plasma cells (as evaluated in 57 cases) was 60% (range 1–100). Seventy-six percent of patients had osteolytic bone involvement and 78% extramedullary localizations, with 13% showing secondary plasma cell leukemia and 7% documented amyloidosis or proteinuria. Finally, 45% of patients presented with at least one severe comorbidity, mainly cardio-vascular, liver or pulmonary dysfunction, and diabetes.A total of 236 cycles was administered (median 3, range 1–9). In 47% of patients bendamustine was variously associated to bortezomib (23%), or IMIDs (21%), or to a combination of both (3%). In 80% of patients receiving bendamustine +/− steroids, a median dose of 90 mg/sqm for two consecutive days every 28 days was employed; the median dose was 80 mg/sqm when bendamustine was combined with bortezomib, 60 mg/sqm with IMIDs (total range: 40–140 mg/sqm). The remaining patients received single, monthly doses ranging from 60 to 150 mg/sqm. According to IMWG uniform response criteria, 21 out of 73 evaluable patients achieved a response after a median time of 3 months. In particular, there were 16 PR, 1 VGPR, 1 sCR, and 3 CR; overall response rate (ORR) was, therefore, 29%. Response rate was 10% (4/39) in bendamustine single agent +/− steroids, 38% (5/13) in bendamustine + bortezomib and 62% (10/16) in bendamustine + IMIDs subgroups, respectively. Responders had received a lower number of previous treatments than non responders (median 3 vs 4). Response rate was higher in relapsed (12/21, 57%) than in resistant patients (10/57, 17%). The time to best response ranged from 1 to 8 months. After a median follow-up of 8 months, median PFS duration was 6 months, with 13 out of 21 responding patients not yet progressed. Median OS of the entire cohort was 6.2 months (7 months in responders and 4 months in non responders, range 0–27). Grade 3–4 hematological and non-hematological toxicities occurred in 56% and 15% of patients, respectively, causing three interruptions of the treatment.Though with the clear limits due to the high heterogeneity of treatments applied and of population analyzed, our data indicate that bendamustine may be a therapeutic option in heavily pretreated MM, suggesting a possible non cross-resistance with other agents. Its earlier use with appropriate doses and combinations might further improve the results obtained in this study.Musto: Mundipharma: Honoraria. Off Label Use: Bendamustine in relapsed/refractory myeloma. Fragasso:Mundipharma: Honoraria. Baldini:Mundipharma: Honoraria. Storti:Mundipharma: Honoraria.

Abstract 2971

No relevant conflicts of interest to declare.

Abstract 2873

Off Label Use: Thalidomide, Bortezomib and Doxil.

Abstract 2828

Polymorphonuclear leukocytes (PMN) from healthy subjects can produce and store tissue factor (TF), which is expressed on PMN surface upon in vitro stimulation with P-selectin. We report here that platelets and PMN from 12 patients with myeloproliferative disorders (MPD) (6 Polycytemia Vera, 6 Essential Thrombocythemia) show up regulation of P-selectin and TF, respectively, in the absence of any in vitro challenge, in comparison to healthy subjects (31±5 vs. 2.8±0.4 percent of P-selectin-positive platelets and 6.4±1.3 vs. 0.8±0.3 percent of TF-positive PMN, both differences P<0.05). The number of circulating mixed platelet-PMN aggregates was also increased (15.1±4.4 percent of platelet-PMN aggregates in MPD vs 2.6±0.5 in healthy, P<0.05). PMN TF expression as well as mixed platelet-PMN aggregates, but not platelet P-selectin, were significantly reduced in six MPD patients after three weeks treatment with hydroxyurea (HU) (6.3±1.7 vs. 1.4±0.2 percent of TF-positive PMN and 25.1±6.3 vs. 4.7±1.1 percent of platelet-PMN aggregates, before and after HU, respectively; both differences P<0.05). In vitro studies performed on PMN separated from healthy donors confirmed HU effects (0–1,400 microM). HU prevented both P-selectin-induced TF expression and mixed cell aggregate formation. The inhibitory effect of HU was specific for P-selectin induced PMN activation, as it did not affect fMLP-induced PMN TF expression. Thus, in MPD patients, platelet P-selectin-mediated TF expression on circulating PMN may play a role in thrombus formation and represents a novel target for the antithrombotic activity of HU.

Polymorphonuclear leukocytes (PMN) from healthy subjects can produce and store tissue factor (TF), which is expressed on PMN surface upon in vitrostimulation with P-selectin. We report here that platelets and PMN from 12 patients with myeloproliferative disorders (MPD) (6 Polycytemia Vera, 6 Essential Thrombocythemia) show up regulation of P-selectin and TF, respectively, in the absence of any in vitrochallenge, in comparison to healthy subjects (31±5 vs. 2.8±0.4 percent of P-selectin-positive platelets and 6.4±1.3 vs. 0.8±0.3 percent of TF-positive PMN, both differences P<0.05). The number of circulating mixed platelet-PMN aggregates was also increased (15.1±4.4 percent of platelet-PMN aggregates in MPD vs 2.6±0.5 in healthy, P<0.05). PMN TF expression as well as mixed platelet-PMN aggregates, but not platelet P-selectin, were significantly reduced in six MPD patients after three weeks treatment with hydroxyurea (HU) (6.3±1.7 vs. 1.4±0.2 percent of TF-positive PMN and 25.1±6.3 vs. 4.7±1.1 percent of platelet-PMN aggregates, before and after HU, respectively; both differences P<0.05). In vitrostudies performed on PMN separated from healthy donors confirmed HU effects (0–1,400 microM). HU prevented both P-selectin-induced TF expression and mixed cell aggregate formation. The inhibitory effect of HU was specific for P-selectin induced PMN activation, as it did not affect fMLP-induced PMN TF expression. Thus, in MPD patients, platelet P-selectin-mediated TF expression on circulating PMN may play a role in thrombus formation and represents a novel target for the antithrombotic activity of HU.