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Myelodysplastic syndromes (MDS) are myeloid neoplasms characterized by bone marrow (BM) failure and associated with aging. We have previously shown that reversal of BM microenvironment (BMME) dysfunction in MDS mitigates MDS associated marrow failure and delays progression to acute leukemia. However, the exact mechanisms driving BMME dysfunction in MDS remain unknown. We recently reported that interleukin-1 (IL1) Receptor Type1 (IL1R1) signaling is a driver in myeloid bias via disruption of BMME in aging. In addition, we have found that IL1R1 signaling is involved in disease progression of AML. Therefore, to assess the role of IL1R1 signaling in MDS associated BMME dysfunction and marrow failure, we employed an age appropriate murine transplant model for MDS utilizing NUP98-HOXD13 (NHD13) transgenic mice. Methods: BM cells (NHD13 transgenic or wild type (WT), 7 weeks) and competitor cells were transplanted into irradiated aged recipients (WT or IL1R1 KO, 60 weeks), and subsequently monitored for development of marrow failure. When marrow failure developed, mice were euthanized and peripheral blood, BM, BM extracellular fluid (BMEF), and collagenase-1 digested bone associated cells were analyzed including flow cytometry, colony forming units (CFU) assay, and cytokine analyses. Next, BM from NHD13 (8-10 weeks) and competitor cells were transplanted into lethally irradiated aged recipients (WT, 50-60 weeks). At onset of marrow failure, mice were treated with inhibitors of IL1/Toll-like receptor signaling (IL1R antagonist, MCC950, or IL1R-associated kinase 4 protein (IRAK4) inhibitor) for fourteen days, and then euthanized and analyzed as above. Finally, we evaluated cytokine profile in the BM serum from the patients with MDS and normal donors. Results: Transplant of NHD13 BM cells into aged IL1R1 wt recipients (NHD13→IL1R1 wt) was not associated with a significant difference in survival rates or levels of NHD13 engraftment compared to NHD13 into IL1R1 ko recipients (NHD13→IL1R1 ko). IL1R1 wt developed macrocytic anemia compared to IL1R1 ko recipient (Hb 11.3±0.57 v.s 13.1±0.42 g/dL, n=12 and 9, p Conclusions: Collectively, our findings demonstrate that IL1R1 signaling alters the BMME and contributes to the disease phenotype of MDS and that the effects of targeting IL1R1 pharmacologically have differing effects based on the modality of inhibition as well as the cell population. IL1R1 signaling can be a promising target to alleviate the complexity of MDS via improving inflammatory status in BMME. Disclosures No relevant conflicts of interest to declare.

Use of Cryopreserved Allogeneic PBSC Results in Delayed Engraftment And Increased Incidence of Poor Graft Function Introduction: During COVID Pandemic, national and international transplant centres agreed to use cryopreserve the donor PBSC as a safer option to deliver allogeneic transplants. Published data suggests that use of cryopreserved allogeneic PBSC is safe and comparable to use of fresh PBSC but cryopreservation of stem cells may lead to cell loss and hence efficacy. During COVID pandemic, use of cryopreserved allogeneic PBSC was adopted as policy on 01/06/2020. This look back analysis evaluates the impact of change in policy. Aims: Evaluate Engraftment time, compare with historical data, blood component support, and use of growth factors Methods and Materials: Data was collected from health records (paper and electronic) and laboratory records. Transplant features and engraftment kinetics were analysed. Results: Group A June 2020 to November 2020, 19 patients [M: 13; F: 6; median age: 50yr (range: 23-69)] who received cryopreserved allogeneic PBSC were compared to 35 patients [M:24; F:11; median age: 59yr (range: 21-71)] receiving fresh allogeneic PBSC for engraftment kinetics. There were no differences between two groups regarding underlying diagnosis (p=0.31), sex mismatch, CMV mismatch, blood group mismatch, reduced intensity conditioning [RIC](p=0.28), type of donor (p=0.98) or use of Alemtuzumab (p=0.88). Median infused Cell dose in group A was 5.3 (3.4-7.16) and in group B 4.9 (1.03-6.85), [p=0.11]. Neutrophil engraftment was significantly faster with fresh PBSC as compared to cryopreserved PBSC (16d vs. 25d, p=0.0025) predominantly with MUD (18d vs. 27d, p=0.009) and RIC (16d vs. 25d, p=0.009). Platelet engraftment to 25 was faster with fresh PBSC (13d vs. 20d, p=0.021) with delayed engraftment in MUD (20d vs. 13d, p=0.006) and RIC (23d vs. 13d, p=0.039). Day to engraftment per unit CD34 was shorter with fresh PBSC for neutrophils (median: 3.2, range: 2.0-7.7 vs. 5.3, range: 2.5-16.7; p=0.006) and platelets (median: 2.4, range: 1.7-25 vs. 3.8, range: 2.2-25; p=0.001) but only for MUD. This suggests 35-40% less efficiency with use of cryopreserved PBSC. There was no difference in the need for transfusion support [RBCs (6 units vs. 3 units, p=0.32); platelets (5 pools vs. 7 pools, p=0.33)]. G-CSF use was higher with cryopreserved PBSC in RIC (54% vs. 20%, p=0.031). Two patients experienced TRM before day 90 (3.7%). At day 90, 17/52 (32.7%) had cytopenia in one lineage and 8/52 (16%) had cytopenia in more than one lineage. Delayed engraftment was observed in 10 of 33 patients (30.3%) transplanted in 2020 and the only significant association was use of cryopreserved PBSC (0% vs. 53%, p=0.001). There was no difference in the incidence of aGVHD, hepatic VOD, microangiopathy and bacterial infections. Numbers are not sufficient to make disease specific comparisons. Conclusion: Cryopreserved PBSC result in delayed neutrophil and platelet engraftment predominantly with MUDS and RIC. Incidence of delayed engraftment and poor graft function is higher. Per unit CD34 dose, cryopreserved PBSC are 30-40% less efficient to achieve engraftment. Delayed engraftment with cryopreserved PBSC especially in MUD raises the possibility that time from harvest to cryopreservation contributes to reduced efficacy. Based on these findings it was decided to infuse higher CD34 dose (6-7x10^6/kg as compared to usual 4-5x10^6/kg) for cryopreserved MUD PBSC. Disclosures Bloor: Kite, a Gilead Company: Honoraria; Novartis: Honoraria.

Introduction: The World Health Organization declared COVID-19 a global pandemic on 03/11/20. Subsequent concerns around caring for patients with sickle cell disease who require automated red blood cell (RBC) exchange transfusions emerged, especially in the setting of physical distancing and national shortages in blood product supplies. In this vulnerable population at high risk of allo-immunization, ideal transfusion parameters (e.g., antigen optimization) will likely grow increasingly difficult to satisfy and require careful evaluation and strategic planning. Methods: Automated RBC exchange transfusions were performed at the University of Alabama at Birmingham (UAB) in patients with sickle cell disease for a variety of clinical indications with the primary objective of lowering the amount of Hemoglobin S (goal 15%) and replacing it with Hemoglobin A. We collected the number of weekly RBC exchange transfusions performed and then compared the frequencies between 01/05/20 and 03/14/20 (pre-pandemic) to those between 03/15/20 and 08/01/20 (intra-pandemic) using a one-tailed t-test. We also examined the number of RBC units ordered per week at UAB, in both the inpatient and outpatient settings, shortly before and after the declaration of a global pandemic using a one-tailed t-test. Results: The mean frequency of RBC exchange transfusions performed per week was 8.1 [standard deviation 2.3] pre-pandemic and 8.6 [2.3] intra-pandemic (Figure 1a). There was no statistically significant difference (p=0.27) in the frequency between these two periods. Shortly prior to the start of the pandemic (02/23/20-03/14/20), a mean of 77.3 [17.9] units/week were ordered for outpatient RBC exchange transfusions. Shortly after the start of the pandemic (03/15/20-04/26/20), a mean of 55.3 [22.8] units/week were ordered for outpatient RBC exchange transfusions, which was also not significantly different (p=0.09). During this time period, the mean number of RBC units per week ordered in the inpatient surgical setting significantly declined from 719.3 [43.1] to 390.0 [46.8] as elective procedures were delayed (p Conclusions/Future Directions: The frequency of automated RBC exchange transfusions performed at UAB did not decrease after the onset of the pandemic. UAB was able to continue caring for patients with sickle cell disease receiving RBC exchange transfusions as the pandemic emerged and national blood product supplies declined despite a similar overall demand. Interestingly, there was also a concomitant decrease in the demand for RBCs from inpatient surgical settings as elective procedures were delayed, possibly contributing to the blood bank's ability to maintain ideal transfusion parameters and perform antigen optimization of transfused RBCs. As the COVID-19 pandemic continues, the national shortage of blood product supplies will likely worsen and necessitate multidisciplinary efforts, including intra-institutional and inter-institutional collaborations, to continue caring for patients with sickle cell disease receiving RBC exchange transfusions. Furthermore, community education, safely structured blood drives, and other efforts to encourage donations are essential to maintain the national blood product supply. Disclosures No relevant conflicts of interest to declare.

Most cancers evolve over time as patients initially responsive to therapy acquire resistance to the same drugs at relapse. Cancer stem cells have been postulated to represent a therapy-refractory reservoir for relapse, but formal proof of this model is lacking. We prospectively characterized leukemia stem cell populations (LSCs) from a well-defined cohort of patients with acute myelogenous leukemia (AML) at diagnosis and relapse to assess the effect of the disease course on these critical populations. Leukemic samples were collected from patients with newly diagnosed AML before therapy and after relapse, and LSC frequency was assessed by limiting dilution analyses. LSC populations were identified using fluorescent-labeled cell sorting and transplantation into immunodeficient NOD/SCID/interleukin 2 receptor γ chain null mice. The surface antigen expression profiles of pretherapy and postrelapse LSCs were determined for published LSC markers. We demonstrate a 9- to 90-fold increase in LSC frequency between diagnosis and relapse. LSC activity at relapse was identified in populations of leukemic blasts that did not demonstrate this activity before treatment and relapse. In addition, we describe genetic instability and exceptional phenotypic changes that accompany the evolution of these new LSC populations. This study is the first to characterize the evolution of LSCs in vivo after chemotherapy, identifying a dramatic change in the physiology of primitive AML cells when the disease progresses. Taken together, these findings provide a new frame of reference by which to evaluate candidate AML therapies in which both disease control and the induction of more advanced forms of disease should be considered.

Introduction: Myelodysplastic syndromes (MDS) are genetically diverse and heterogeneous diseases characterized by dysplasia and cytopenias and a dismal prognosis of ˂ 50% overall survival at 5-years. In vitro and in vivo experimental data have shown that the bone marrow microenvironment (BMME) may play a role in disease progression and, importantly, in murine models, transplantation of MDS to a healthy BMME was shown to mitigate disease. Mesenchymal Stem and Progenitor Cells (MSPCs), as part of the BMME, give rise to multiple non-hematopoietic progenitor cells and provide essential support to hematopoietic stem cells. MSPCs have also been shown to be functionally altered in patients with myeloid neoplasias. Murine studies have demonstrated distinct roles for specific subsets of bone marrow mesenchymal cells in myeloid malignancies. We hypothesized that specific subsets of human bone marrow MSPCs will play differential roles in the pathogenesis of MDS. Using flow-cytometry, high-throughput sequencing and gene set enrichment analysis (GSEA), we characterized human MSPCs in the bone marrow aspirates from patients with MDS and normal healthy controls (NBM). Methods: Mononuclear cells were isolated by iliac crest bone marrow aspiration from 10-healthy donors and 14-patients at the University of Rochester Medical Center. Within the non-hematopoietic (CD45-, CD235-), non-endothelial (CD31-) bone marrow compartment, we enriched and isolated 3 distinct-subpopulations of MSPCs based on cell-surface expression of CD271/NGFR, CD106/VCAM-1 and CD146/MCAM. Populations were defined as follows: CD271+/CD146- (CD271+), CD271+/CD146+/CD106+ (DPCD106+), and CD271+/CD146+/CD106- (DPCD106-). RNA-seq analysis was performed on each subpopulation to define transcriptional signatures (TS) and gene set enrichment patterns. Statistically significant differentially expressed genes (DEGs) were defined by fold-change ≥ ±1 and p-value ˂0.05. Results: We first set out to define differences in the TS and interrogate the function of MSPC populations in NBM. Principle component analysis (PCA) demonstrated the highest variance between the DPCD106+ and the CD271+ populations. The number of DEGs were also highest between the DPCD106+ and CD271+ populations (n=3,619 genes). GSEA identified 745 and 336 gene sets with positive enrichment in the DPCD106+ and CD271+ group, respectively, and illustrated that the DPCD106+ population was significantly enriched in gene sets involved in early embryonic developmental and "stem-like" pathways whereas the CD271+ population was enriched in cell cycling, DNA and chromosomal organization. In the setting of MDS, the mean relative frequency of MDS CD271+ nearly tripled (0.4230/0.1445; p-value 0.045). Compared to NBM, MDS DPCD106+ cells had the highest variance by PCA and the highest number of DEGs (n=560). GSEA identified 19-gene sets with significant enrichment in the MDS DPCD106+ group and, intriguingly, 12 (63%) were identical to gene sets enriched in the CD271+ group. Furthermore, of the 560 DEGs in the MDS DPCD106+ MSPCs, 300 were upregulated and, of those, 160 (53%) were identical to upregulated genes in the CD271+ NBM group including the acquisition of a proliferative signature. Altogether, this data suggests a switch in the TS of theDPCD106+ population in the setting of MDS. Importantly, this TS clustered MDS DPCD106+ from NBM, regardless of MDS risk category. Conclusion: We successfully characterized 3 subtypes of MSPCs in NBM and MDS. In NBM, we demonstrate that cell surface expression of CD271, CD146 and CD106 defined the most stem-like TS within the non-hematopoietic, non-endothelial bone marrow compartment. In the setting of MDS, the increase in population frequency of the CD271+ cells and the concomitant transcriptomic aberrations observed in the MDS derived DPCD106+ population support the hypothesis that specific MSPC populations have differential roles in MDS pathogenesis. Further, we identify a TS that discriminates MDS derived MSPCs from NBM irrespective of MDS-risk category. This suggests that alterations within specific MSPC populations may represent a unifying pathway in disease pathogenesis despite heterogeneity and genetic drivers intrinsic to the MDS clone. Thus, targeting the BMME represents a potentially novel therapeutic strategy aimed at mitigating disease and restoring normal hematopoiesis in patients with MDS. Disclosures Liesveld: Onconova: Other: Data safety monitoring board; Abbvie: Membership on an entity's Board of Directors or advisory committees. Scadden:Editas Medicine: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Magenta Therapeutics: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Bone Therapeutics: Consultancy; Clear Creek Bio: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Novartis: Other: Sponsored research; Fate Therapeutics: Consultancy, Equity Ownership; Red Oak Medicines: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Fog Pharma: Consultancy; Agios Pharmaceuticals: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; LifeVaultBio: Equity Ownership, Membership on an entity's Board of Directors or advisory committees.

Background: Although diffuse large B cell lymphoma (DLBCL) can be cured using immuno-chemotherapy, 40% of patients experience relapse or refractory disease. Large-scale profiling studies have mainly focused on DLBCL at diagnosis, resolving different outcome groups based on gene expression (e.g. cell-of-origin (COO) or molecular high grade), MYC/BCL2 translocations (double-hit lymphoma) or gene mutations and copy number aberrations (Schmitz et al, NEJM 2018; Chapuy et al, NatureMedicine 2018). In comparison, longitudinal studies have been hindered by the limited availability of sequential biopsy samples. To date, the relapse-specific gene mutations identified are limited and inconsistent across studies. In our study, we have focussed attention on the changes in gene expression profile (GEP) accompanying DLBCL relapse. Methods: We retrospectively collected archival paired diagnostic/relapse formalin fixed paraffin embedded tumor biopsies from 38 de novo DLBCL patients collected from multiple UK sites treated with rituximab-based immuno-chemotherapy, where partial or complete remission was reported following treatment. COO classification was performed by the Lymph2Cx assay on NanoString to distinguish activated B-cell-like (ABC) and germinal center B-cell-like (GCB) subtypes. The Ion AmpliSeq™ Transcriptome Human Gene Expression Kit was used to measure the expression levels of > 20,000 genes on the paired samples. Results: COO remained stable from diagnosis to relapse in 17 ABC-ABC pairs, 11 GCB-GCB pairs and 4 unclassified (UNC)-UNC pairs. Frank COO switching was observed in 6 cases (1 ABC-GCB, 2 ABC-UNC, 2 GCB-UNC, 1 UNC-ABC). Pairs with stable COO were taken forward for further analysis. Gene expression analysis using the limma R package identified 163 and 136 genes as differentially expressed (DE) (p 1) between the diagnostic and relapse biopsies in ABC and GCB tumors respectively, with only a one gene overlap. Gene Set Enrichment Analysis further suggested that ABC and GCB relapses are mediated via different mechanisms, with tumor growth and proliferation signatures enriched in ABC relapses, whilst adaptive immunity-related signatures accompanied GCB relapses. Next, we aimed to utilise our relapse-specific genes to identify outcome predictors at diagnosis using publicly available GEP datasets. In order to increase our discovery power and accuracy, a larger set of DE genes from the paired differential analysis (796 genes in ABC pairs and 387 from GCB pairs) were selected (p The prognostic significance of this 30-gene discriminator was successfully validated using a linear predictor in two independent GEP datasets: 1) a population-based cohort (Lenz et al, NEJM 2008) with 93 R-CHOP-treated ABC cases identifying 47 low and 46 high-risk cases (HR=1.92, p=0.046, C-index=0.77; Fig1.C) and 2) a clinical trial dataset (REMoDL-B, Davies et al, Lancet Oncol 2019) with 255 ABC cases identifying 110 low and 145 high-risk ABC cases (HR=1.95, p=0.0051, C-index=0.70; Fig1.D). Conclusions: Here we describe a 30-gene discriminator in ABC-DLBCL, derived from genes differentially expressed between diagnosis and relapse, that allowed the definition of clinically distinct high and low risk subgroups in ABC-DLBCLs at diagnosis. The clinical translation of such a tool may be useful to guide therapy for this unfavourable subgroup of ABC-DLBCLs. Validation of this signature is currently underway in additional datasets and further study is required to understand the contribution of these genes in DLBCL pathology. Disclosures Korfi: Roche: Consultancy. Burton:Celgene: Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees. Rule:TG Therapeutics: Consultancy, Honoraria; Napp: Consultancy; Kite: Consultancy; Pharmacyclics: Consultancy, Honoraria; Gilead: Consultancy, Honoraria; Sunesis: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding; Roche: Consultancy, Honoraria, Research Funding; Astra-Zeneca: Consultancy, Honoraria; Celgene: Consultancy, Honoraria. Crosbie:Janssen: Honoraria. Scott:Celgene: Consultancy; Janssen: Consultancy, Research Funding; NanoString: Patents & Royalties: Named inventor on a patent licensed to NanoSting [Institution], Research Funding; Roche/Genentech: Research Funding. Rimsza:NanoSting: Patents & Royalties: Named inventor on a patent licensed to NanoSting [Institution]. Davies:Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Honoraria, Research Funding; Bayer: Research Funding; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Honoraria, Research Funding; Karyopharma: Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Research Funding; Acerta Pharma: Honoraria, Research Funding; ADCT Therapeutics: Honoraria, Research Funding; BioInvent: Research Funding; Kite Pharma: Membership on an entity's Board of Directors or advisory committees; MorphoSys AG: Honoraria, Membership on an entity's Board of Directors or advisory committees. Gribben:Abbvie: Consultancy, Honoraria, Research Funding; Acerta/Astra Zeneca: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding. Okosun:Gilead Sciences: Honoraria, Research Funding. Johnson:Epizyme: Honoraria, Research Funding; Novartis: Honoraria; Kite: Honoraria; Janssen: Consultancy, Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria; Boehringer Ingelheim: Honoraria; Takeda: Honoraria; Genmab: Honoraria; Celgene: Honoraria; Incyte: Honoraria. Fitzgibbon:Epizyme: Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Speakers Bureau.

Recent clinical trials have reported that in combination with hypomethylating agents, the BCL-2 inhibitor venetoclax can induce responses in over 70% of older previously untreated AML patients who are unfit for conventional chemotherapy. These findings led to the recent United States Food and Drug Administration approval of this regimen for this population, and it is now considered to be the standard care. However, a significant minority of patients do not achieve a remission and are refractory. In addition, the majority of patients who do achieve a remission ultimately relapse. It is therefore critical to identify AML patients who are likely to be resistant to venetoclax-based therapy. To initially address this question, we retrospectively reviewed 75 newly diagnosed AML patients who received venetoclax + azacitidine (VEN+AZA) at our institution and analyzed several baseline clinical features to determine the ability of each to predict disease that was refractory to treatment (defined as a lack of complete remission [CR], CR with incomplete recovery of peripheral blood counts [CRi], partial remission, or morphological leukemia free state [MLFS]). Both univariate and multivariate analyses revealed the presence of FAB-M5 to be associated with disease that was refractory to VEN+AZA (Table 1). Given that FAB-M5 represents AML with monocytic differentiation, these findings indicate a strong correlation between myeloid differentiation status and resistance to venetoclax. Using multicolor flow cytometry, we show bone marrow specimens of typical FAB-M5 patients who were refractory to VEN+AZA presented dominant monocytic disease that has an immunophenotype of CD45-bright/SSC-high/CD117-/CD11b+/CD68+ (Figure A). In contrast, bone marrow specimens of typical FAB-M0/M1/M2 patients who achieved CR with VEN+AZA presented as a single dominant disease population that is CD45-med/SSC-low/CD117+/CD11b-/CD68- (Figure B). In a subset of AML patients, we observed the co-existence of both phenotypically primitive and monocytic populations, which we term "MPM" AML (for Mixed Primitive/Monocytic). We observe that after attaining CR with VEN+AZA treatment and subsequent relapse, MPM-AML showed almost complete loss of the primitive subpopulation, and evolved to a dominant monocytic disease (Figure C). These data indicate that VEN+AZA treatment induces strong selection of the monocytic phenotype. Importantly, when we compared the immunophenotype of six pairs of diagnostic/relapse specimens from AML patients treated with conventional intensive induction chemotherapy, we observed selection of a more primitive phenotype, suggesting the drive toward a monocytic phenotype observed at relapse appears to be a unique consequence of VEN+AZA therapy. To our knowledge, selection of a monocytic phenotype at relapse has never been previously observed in AML, suggesting the relapse after VEN+AZA may represent a new clinical entity. Mechanistically, using RNA-seq we show the global transcriptome of monocytic AMLs are distinct from primitive AMLs, suggesting they represent two broad classes of AML with likely differential responses to therapy. Indeed, we demonstrate that AML with a primitive immunophenotype is dependent on BCL-2 activity as a means to drive oxidative phosphorylation, a critical requirement for survival of leukemia stem cells. Conversely, AML with a more differentiated monocytic phenotype is no longer dependent on BCL-2, but rather switches to MCL-1 as a mediator of oxidative phosphorylation. Using colony-forming and xenograft assays, we show the stem and progenitor potential of monocytic AMLs are selectively more sensitive to MCL-1 inhibition comparing to BCL-2 inhibition. Together, our study suggests a significantly higher refractory/relapse risk for monocytic AML patients treated with VEN+AZA (Figure D). Further, for those AML patients who do respond to initial VEN+AZA treatment, the therapy drives a powerful selective process resulting in emergence of more differentiated monocytic disease in some patients. Based on these findings, we propose that AML exists on a developmental spectrum that is inherently fluid, where with appropriate selective pressure the disease can acquire characteristics of a more differentiated state. Further, our data indicate that optimal AML therapy will require strategies designed to target both primitive and myeloid phenotypes. Disclosures Pollyea: Forty-Seven: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Astellas: Consultancy, Membership on an entity's Board of Directors or advisory committees; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celyad: Consultancy, Membership on an entity's Board of Directors or advisory committees; Agios: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Diachii Sankyo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Savona:TG Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Selvita: Membership on an entity's Board of Directors or advisory committees; Karyopharm Therapeutics: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Incyte Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding; Sunesis: Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees; Boehringer Ingelheim: Patents & Royalties; AbbVie: Membership on an entity's Board of Directors or advisory committees.

Rheumatoid arthritis (RA) is a debilitating autoimmune disease resulting from autoantibodies that cause damage to synovial joints. Joint damage causes increased systemic inflammatory cytokines which may lead to aberrant hematopoiesis. Indeed, RA is accompanied by many hematological complications including anemia, cytopenias, and suppressed bone marrow function. Hematopoietic stem cells (HSC) at root of the blood system can respond to inflammatory signals by activating the cell cycle and preferentially generating myeloid cells. However, chronic inflammation can also lead to HSC dysfunction. Previous studies using genetic mouse models of RA have identified myeloid overproduction in this context; however, HSC long-term reconstitution activity was maintained. To better understand hematopoietic alterations in RA, our group used the collagen induced arthritis (CIA) mouse model, which is inducible in adult mice and recapitulates many immunological features of the human disease, including elevated inflammatory cytokine levels in the bone marrow (BM) and peripheral blood (PB). Confirming prior reports, we found increased numbers of myeloid lineage cells in the PB and BM of CIA mice. We also found reduced erythroid and lymphoid lineage progenitor cell numbers in the BM, consistent with anemia and immunosenescence phenotypes in RA patients. Interestingly, these features were accompanied by a significant increase in the number of myeloid-biased multipotent progenitor-3 (MPP3) cells, suggesting increased activation of myeloid differentiation pathways. However, we found no changes to the number of activated (MPP1), short-term HSC (ST-HSC), or long-term HSC (LT-HSC) in CIA mice. Likewise, and in line with previous reports, long-term HSC potential was not reduced in CIA mice, as assessed by transplantation of either purified HSC or with unfractionated BM into irradiated recipient mice. While HSC from control and CIA donor mice displayed similar blood chimerism and lineage distribution over a 16-week period, we did observe increased proportions of donor-derived MPP3 in CIA recipient animals, further supporting an activation of myeloid HSC differentiation pathways. Overall, these results reveal underlying changes in the BM driving aberrant hematopoiesis, with the HSC pool remaining intact despite activation of a myeloid differentiation pathway. To better understand how HSC are impacted by arthritic inflammation, we assessed the molecular state of control and CIA HSC using RNA-seq. We found 292 genes significantly upregulated and 237 genes significantly downregulated in CIA HSC. Analysis of these genes using Ingenuity, GSEA, and DAVID tools revealed broad downregulation of inflammatory and proliferation signaling pathways including IL-1β, NFκB, MYC, and ERK. Genes for G1/S cell cycle transition, transcription, protein translation, and proliferation pathways were also significantly downregulated in CIA HSC. On the other hand, genes corresponding to cell cycle arrest and negative regulation of transcription were significantly upregulated. Notably, we find that IL-1β, which is produced in the BM of CIA mice, is sufficient to induce this molecular program. We find that HSC in CIA mice have a global downregulation of transcripts required for activation and proliferation, and a global upregulation of transcripts that would promote quiescence. Hence, HSC are forced back into a quiescent state by broad downregulation of cell growth and proliferation genes even during chronic inflammation caused by RA. Altogether, our data show that a mouse model of rheumatoid arthritis causes hematopoietic lineage skewing towards the myeloid lineage with simultaneous loss of lymphoid and erythroid lineage potential. Interestingly, in this chronic inflammatory setting HSC downregulate pathways involved in cytokine signaling, cell cycle activation, and translation. This mechanism can be triggered by chronic exposure to pro-inflammatory cytokines, and may serve to limit HSC proliferation and potential for damage in disease settings. These results may explain the relative rarity of outright bone marrow failure in autoimmune disease patients, while providing insight into mechanisms driving aberrant hematopoiesis in these individuals. Lastly, they provide functional evidence for cytokine blockade to normalize HSC function in the setting of RA and other chronic inflammatory diseases. Disclosures No relevant conflicts of interest to declare.

Background: Severe Aplastic Anemia (SAA) is a rare bone marrow disorder characterized by inadequate levels of peripheral, multi-lineage blood cells. Of the two available first-line treatments for SAA, allogeneic hematopoietic stem cell transplantation is limited by patient eligibility and donor availability, and immunosuppressive therapy (IST) is characterized by a significant proportion of non-responders, toxicity, and risk of transformation to diseases such as acute myelogenous leukemia. Patients who do not respond to treatments become transfusion dependent, which has a significant impact on patients' quality of life as well as healthcare costs. Eltrombopag is the only TPO-R agonist approved for the treatment of refractory SAA. In a Phase I/II clinical trial, eltrombopag, given in association with IST, showed efficacy in patients with naive-SAA, and offers a significantly improved first-line alternative to patients affected by SAA (Townsley, et al 2017). In the US healthcare environment, there is a need to compare costs and consequences to understand value. Objective: Evaluate eltrombopag as a first-line treatment versus IST alone for SAA from the American private healthcare system perspective. Methods: A responder model for newly diagnosed SAA patients was created to assess the treatment pathway and economic impact of including eltrombopag in addition to IST (antithymocyte globulin and cyclosporine A) as a first-line treatment. A simulated cohort with two treatment arms underwent 6 months of treatment with either eltrombopag in addition to IST or with IST alone and were followed for a 3-year time period. Each arm received a diagnostic test measuring response at 6 months. Patients who achieved complete or partial response in either arm received low-dose cyclosporine A as maintenance therapy for an additional 6 months of treatment. Patients who did not respond in either arm continued with eltrombopag monotherapy as a second-line therapy for an additional 6 months. First-line therapy (eltrombopag with IST, IST alone), maintenance therapy (low-dose cyclosporine A), second-line therapy (eltrombopag monotherapy), administration, routine care, mortality and adverse event costs were included in the analysis. Workplace productivity related costs were not considered. Response rates, mortality, dosing, treatment duration and adverse event rates for each arm were based on a phase I/II trial (Townsley, et al 2017). Drug costs were obtained from a large online database (REDBOOK Online). Administration costs were based on the 2017 CMS Medical Fee Schedule. Routine care rates (visits, hospitalizations, tests and transfusions) were based on published data (Peffault De Latour, et al, 2017). Routine care, mortality and adverse event costs were based on CPT codes from the American Medical Association, HCUPnet and published data (Toner, et al 2011). All cost data are reported in 2018 US dollars. See figure 1 for details. Results: In a simulated cohort with a population of one million, the annual incidence of aplastic anemia was 0.000234% and SAA accounted for 83.8% of those cases. The two treatment paths were compared for their consequences. Based on the clinical trial data, in the treatment arm with eltrombopag and IST, 94% of patients experienced treatment response relative to the IST arm where only 66% of patients experienced treatment response. Further, in the treatment arm with eltrombopag and IST, the patients experienced a reduced annual risk of mortality by 0.3% relative to the IST arm. Use of eltrombopag therapy as a first-line therapy produced a cost increase of $77,442 over 3 years. First-line drug costs accounted for an increase of $109,147, while improvements in response rates led to cost offsets for second-line drugs and produced $29,663 in savings. Adverse event, routine care and mortality costs had relatively negligible effects on either treatment arm over a 3-year time period. Sensitivity analyses confirmed the robustness of the analysis. Conclusion: When following treatment approaches specified in clinical studies, high response rates combined with reduced risk of mortality and less usage of rescue medication, and a low disease incidence are likely to lead to manageable economic consequences with eltrombopag + IST therapy from the American private healthcare system perspective. In a simulated cohort with a population of one million, this was estimated to be $77,442 over 3 years. Disclosures Said: Novartis: Employment. Cai:Novartis: Employment. Forsythe:Novartis: Consultancy. Roy:Novartis: Employment.

Introduction: Transplant recipients younger than 18 years old are believed to have a 2- to 4-fold higher risk of developing post-transplant lymphoproliferative disorder (PTLD) than adult transplant patients. Within the pediatric group, there is evidence to suggest an increased risk of PTLD in younger children. We conducted a systematic literature review (SLR) to estimate the weighted mean age (WMA) at PTLD diagnosis and describe patient demographics in the real-world setting. Methods: An SLR was performed following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines with the scope defined in terms of Population, Intervention Comparators, Outcomes, and Study design (PICOS) criteria. Studies were identified based on a systematic search using key biomedical literature databases: EMBASE, MEDLINE, and Cochrane. The literature search was conducted on July 19, 2018 and included studies published between database inception in January 1, 1959 and July 19, 2018. Relevant congress abstracts published between January 2015 and June 2018 were also identified. The PICOS-based inclusion and exclusion criteria were used to review identified citations. To ensure inclusion of all relevant evidence, no treatment limitations were imposed; however, the study designs were limited to prospective and retrospective observational studies. Case reports were included regardless of sample size. Two independent reviewers screened all citations and full-text articles; any discrepancies were resolved by a third independent reviewer. Data from included studies were extracted into a pre-defined template, and results were summarized using the PRISMA flow diagram. Using the sample size from the selected studies, the WMA for each relevant subgroup was determined and was compared to the average age at the time of diagnosis for other cancers reported in SEER Cancer Statistics (2011-2015; https://seer.cancer.gov/csr/). Results: A total of 447 studies fulfilled the search criteria: 114 adult studies, 136 pediatric studies with varied definitions (< 16, < 18 or < 21 years old), and 197 that did not specify inclusion by age. Seventy studies enrolled only PTLD patients after allogeneic hematopoietic stem cell transplantation (HCT). Among these studies, 17 included pediatric patients with a WMA of 12.5 years, 10 included adult patients with a WMA of 40.2 years, and 41 had no age restriction with a WMA of 34.4 years. For pediatric patients, the risk of post-HCT PTLD was 1.3-23.5% and the time from transplant to PTLD was 0.9-6.0 months. There was only one pediatric HCT study (N=4; mean age, 9 years), which reported median survival of 27.6 months. A total of 350 studies only enrolled PTLD patients after solid-organ transplant (SOT). Among these studies, 115 included pediatric patients with a WMA of 6.6 years, 98 included adult patients with a WMA of 46.9 years, and 136 had no age restriction with a WMA of 38.6 years. Among the pediatric SOT studies, the risk of PTLD was 0.3-25.0%, the time from transplant to PTLD was 1.4-92.8 months, and the reported survival was 0.9-37.2 months. The average age of lymphoma patients at cancer diagnosis according to the SEER Cancer Statistics database was 65 years. Conclusions: This SLR of published real-world studies demonstrates that the WMA at PTLD diagnosis (for studies with no age restrictions: 34.4 years for HCT and 38.6 years for SOT) is substantially lower than the reported average age at diagnosis for lymphomas (65 years), irrespective of study design and inclusion criteria. PTLD greatly impacts the pediatric population, with a quarter of HCT studies and a third of SOT studies focusing on this population. Currently, there is no indicated treatment for this ultra-rare disease with poor prognosis, indicating a clear unmet need for patients with PTLD that disproportionately affects younger patients. Disclosures Watson: Atara Biotherapeutics, Inc: Employment, Equity Ownership. Xu:Atara Biotherapeutics, Inc: Employment, Equity Ownership. Forsythe:Novartis: Consultancy. Barlev:Atara Biotherapeutics, Inc: Employment, Equity Ownership.

Background: Use of direct-acting oral anticoagulants (DOACs) in morbidly obese (BMI >40 kg/m2) patients for venous thromboembolism (VTE) treatment is not fully understood. Current International Society of Thrombosis and Haemostasis guidelines do not recommend DOAC use in morbidly obese patients due to limited clinical data in this patient population. Objective: Compare risk of recurrent VTE, major bleeding, healthcare resource utilization, and costs between morbidly obese VTE patients initiating rivaroxaban or warfarin treatment. Methods: This is a retrospective 1:1 propensity score matched cohort study. VTE patients initiating rivaroxaban or warfarin were identified from Truven MarketScan Commercial Claims and Encounters and Medicare Supplemental database (12/1/2011-12/31/2016). An ICD-9/ICD-10 diagnosis code for morbid obesity was required during the 12 months pre- or 3 months post-initiation. Patients were followed ≥3 months. Analyses were conducted during the entire follow-up period regardless of discontinuation as well as on-treatment prior to discontinuation of index treatment. Major bleeding was assessed using the validated claims-based algorithm developed by Cunningham et al. 2011. Conditional logistic regression and generalized linear models were used to compare recurrent VTE and major bleeding risks, all-cause healthcare resource utilization, and per patient per year (PPPY) costs. Results: Among 5,780 rivaroxaban or warfarin treated patients (2,890 in each matched cohort), mean age was 53 ±13 years; 60% were female; mean follow-up time was 10.0 months and 10.5 months, respectively. Mean time between VTE diagnosis and treatment start was 14 days. Risk of recurrent VTE was similar for both cohorts in the intent-to-treat analysis (OR: 0.99, 95% CI: 0.85-1.14, p=0.844). Major bleeding risk was numerically lower for the rivaroxaban cohort but did not reach statistical significance in the on-treatment analysis (OR: 0.75, 95% CI: 0.47-1.19, p=0.227). Rivaroxaban treated patients utilized fewer all-cause healthcare resources, specifically inpatient hospitalizations (OR: 0.86, 95% CI: 0.77-0.96, p=0.006) and outpatient visits (OR: 0.23, 95% CI: 0.10-0.56, p=0.001) compared to warfarin (Table). Rivaroxaban patients incurred an average $2,829 lower total medical costs PPPY, ($34,824 vs $37,653, p=0.020), mainly driven by hospitalization costs. Total healthcare costs (including pharmacy) showed a numerical $1,531 reduction for rivaroxaban patients, not reaching statistical significance ($43,034 vs. $44,565, p=0.237). Conclusions: Our study showed that morbidly obese VTE patients treated with rivaroxaban had similar risk of recurrent VTE and major bleeding compared to warfarin. Treatment with rivaroxaban yielded less all-cause healthcare resource utilization (i.e. inpatient hospitalizations and outpatient visits) and reduced total medical costs. Disclosures Spyropoulos: Janssen Scientific Affairs, LLC: Consultancy. Ashton:Janssen Scientific Affairs, LLC: Employment. Chen:Janssen Scientific Affairs, LLC: Employment. Wu:Janssen Scientific Affairs, LLC: Employment. Peterson:Janssen Scientific Affairs, LLC: Consultancy.

INTRODUCTION: Both solid and hematologic malignancies are associated with an increased risk of thromboembolic events, according to previous studies. One of the most common risk scoring systems was developed based on type of non-hematologic cancer, body mass index (BMI), WBC, hemoglobin, and platelet counts. However, compared to other solid tumors, acute leukemia patients commonly present with pancytopenia at the time of diagnosis and the Khorana Risk Score (KRS) has not been validated in patients with acute myeloid leukemia (AML). The goal of our study was to validate the KRS in AML patients. METHODS: Using Total Cancer Care (TCC), we retrospectively identified histologically confirmed AML patients from 2000 to 2018. Clinical and laboratory variables including age, gender, previous cancer history, use of growth factors, underlying coagulopathy, white blood cell counts, hemoglobin, and platelet counts were characterized at the time of AML diagnosis and annotated using descriptive statistics. The thrombotic event rate was estimated with the Kaplan-Meier method and compared using log-rank test. All statistical analyses were performed using SPSS v24.0 and GraphPad Prism 7. RESULTS: A total of 867 AML patients were included in this study. The median age at AML diagnosis was 75 (51-96) years and male patients were 65% (n=565). A total of 28% (n=241) patients had prior cancer history (hematologic malignancies, n=34; solid tumor, n=207). A total of 12% (n=101), 7% (n=58), and 1% (n=8) patients had EPO, G-CSF, GM-CSF treatment prior to AML diagnosis and 84% (n=728) patient were treated with any typed of growth factors after AML diagnosis. The median BMI was 27.1 (14.8-103) and 5% (n=47) patients had BMI higher than 35. A total of 26% (n=229) patients had WBC ≥11 x109/L (median 3.165 (0.08-413.74) x109/L), 67% (n=584) patients had hemoglobin 350 x109/L (median 46 (1-800) x109/L). A total of 22% (n=191) patients had Khorana score 0, 51% (n=445) patients had score 1, 24% (n=207) patients had score 2, and 3% (n=24) patients had score 3, respectively. A total of 42 thrombotic events were observed in the median follow up of 3 (0.1-307) months. Among these, a total of 3% (n=6/191), 5% (n=23/445), 6.3% (n=13/207), and 0% (n=0/24) thrombotic events occurred in patients with Khorana sore 0, 1, 2, and 3, respectively. Log-rank (Mantel-Cox) test showed no statistical difference between individual subgroup (p=0.1949). CONCLUSIONS: In AML patients, there was a higher incidence of thrombotic events in patients with higher Khorana score though the difference was not statistically significant. The proportion of patients with Khorana score ≥3 was relatively low, which could be due to pancytopenia, a common presentation in AML patients. These results suggest that the development of better thrombotic risk scoring system is warranted in patients with AML. Disclosures Komrokji: Celgene: Honoraria, Research Funding; Novartis: Honoraria, Speakers Bureau; Novartis: Honoraria, Speakers Bureau; Celgene: Honoraria, Research Funding; Novartis: Honoraria, Speakers Bureau; Novartis: Honoraria, Speakers Bureau.

Introduction: Umbilical cord blood (UCB) is a salient source of primitive hematopoietic stem progenitor cells (HSPCs) for bone marrow (BM) reconstitution in patients with hematologic and non-hematologic malignancies. However, a relatively low number of HSPCs in UCB units and poor BM homing efficiency greatly hinders the clinical application of UCB CD34+ cells for transplantation. To overcome these hurdles, we developed two independent strategies that increase CD34+ cell numbers and improve BM homing efficiency of UCB HSPCs. First, we expanded UCB HSPCs by culturing them in decellularized Wharton's jelly matrix (DWJM), a biometric scaffold mimicking the 3-dimenstional (3D) microenvironment of BM. Second, we enhanced the in vitro transmigration and in vivo BM homing efficiency of UCB CD34+ cells by blocking EPO/EPOR signaling. Both approaches enhance UCB CD34+ cell migration toward stromal cell-derived factor 1 (SDF1). In this study, we employed RNA-Seq and RT-PCR approaches to analyze UCB HSPCs treated with EPO and co-cultured with DWJM, aiming to identify molecules that regulate UCB HSPC transmigration via EPO/EPOR signaling. Methods: CD34+ cells from highly enriched UCB units (>90% purity) were treated with EPO for 24 hours and separately co-cultured with DWJM for 1 week. UCB CD34+ cells were collected and subjected to RNA-Seq and real-time PCR (RT-PCR) analyses. In vitro transmigration toward SDF-1 was assessed by transwell assay. To assess the involvement of RasGRP3 in UCB CD34+ cell mobility, cells were treated with 100 nM ingenol-3-angelate (I3A), a diacylglycerol (DAG) analog that specifically targets RAS Guanyl Releasing-Protein 3 (RasGRP3), for 16 hours followed by transwell assay. Anti-EPOR antibody-treated or EPO-treated cells were used as controls. In addition, RasGRP3 gene expression was examined in CD34+ cells from peripheral blood (PB) and BM samples collected from the same donor, and compared to RasGRP3 expression in UCB CD34+ cells. Unpaired, 2-tailed t-test was used to analyze results. Results: RasGRP3 was identified by RNA-Seq from the two independent approaches, EPO treatment and DWJM co-culture. EPO downregulated and DWJM upregulated RasGRP3 gene expression in UCB CD34+ cells. RasGRP3 expression was confirmed by qPCR. UCB CD34+ cells that migrated to the bottom chamber of the transwell assays, a population that has a higher mobility, showed an elevated RasGRP3 gene expression and a decreased EPOR cell surface expression. Activation of RasGRP3 by DAG analog I3A induced a significant increase in RasGRP3 gene expression (control: I3A treatment = 1: 202 ± 58, p=0.00012) that was associated with an enhanced transmigration capability (control: I3A = 41%+/-5: 54%+/- 3, p=0.032). Knocking-down of RasGRP3 in K562 cells, a known EPOR expressing cell line, impaired the transmigration capability of K562. CD34+ cells in peripheral blood (PB) showed a higher level of RasGRP3 gene expression compared to CD34+ cells in BM samples from the same healthy donors. RasGRP3 expression in PB CD34+ cells was significantly higher than BM and UCB CD34+ cells (qPCR signals relative to BM, BM: PB: UCB = 1: 431±65: 21±8, p=0.0012, 0.0023, and Conclusions: By employing transwell assays, flow cytometry and molecular analyses, we demonstrate for the first time that RasGRP3, a protein responsible for GDP/GTP exchange of Ras, regulates the transmigration ability of human CD34+ cells. In addition, our findings connect RasGRP3 expression to the EPOR-mediated signaling pathway in CD34+ cells. A significantly higher level of RasGRP3 expression in PB CD34+ cells than its counterparts in BM might provide an explanation for why PB HSPCs show relatively faster BM engraftment than BM HSPCs during transplantation. Ongoing follow-up studies will elucidate the molecular mechanism(s) underlying EPOR signaling, which holds clinical potential to improve the BM homing deficiency of UCB CD34+ cells via modulating EPOR and RasGRP3 expression (Figure 1). Disclosures Liesveld: Onconova: Other: DSMB; Abbvie: Honoraria. Aljitawi:Medpace: Consultancy; The University of Rochester Medical Center: Patents & Royalties: Pending patent related to decellularized Wharton's jelly matrix.

Introduction: In previous work, we have demonstrated that culturing leukemia cell lines in decellularized Wharton's jelly (WJ) matrix (DWJM), the gelatinous material in umbilical cord tissues, resulted in chemotherapy resistance. To reduce the variability in the biochemical composition between different parts of the WJ matrix and to optimize our 3-dimensional (3D) DWJM-based extracellular matrix (ECM) model for in vitro culture of primary AML cells, we fabricated DWJM-derived porous disks with uniform architecture and pore-size by homogenization followed by lyophilization of human DWJM. Herein, we examine whether DWJM disks support primary leukemia cells and result in chemotherapy resistance. Methods: AML patient samples collected by leukapheresis were cultured in DWJM disks for one week. Non-adherent cells were first aspirated and adherent cells were separately isolated and assessed for viability, apoptosis, and colony forming unit (CFU). RNA sequencing was performed at the end of culture. Response to chemotherapy following treatment with doxorubicin was also assessed. For all studies, DWJM-adherent and non-adherent cells were compared to suspension culture controls. Metabolic pathway analyses were conducted using enzyme-linked immunosorbent assay (ELISA). One-tailed t-test was used for comparison between the groups. Results: Consistent with our prior studies, adherent DWJM cells demonstrated less apoptosis (P=0.027) and greater CFU activity with larger and more dense colonies (1 vs 4/50,000 cells, P=0.0008). Co-culture of primary AML samples with DWJM reduced doxorubicin induced cell death (P=.047) preserving CFU activity (3.3 vs 0.3/300,000 cells, P=.047) compared to treatment of suspension culture cells. To understand the mechanisms by which co-culture with DWJM enhanced leukemic progenitor function and therapy resistance, we performed RNA-Seq analyses. RNA-Seq data analysis from day 7 demonstrated significant upregulation of FAM83A and MIR34A and downregulation of BPI, ZNF521, NHLH2, CD69, FKBP14, PBX1, TANC1, GRIN2b, MYO6, INHBA, SA1008, CXCL1, A1009, BLNK, MMP9, BHLHE41, and CD9 in the adherent population compared to the suspension population (adjusted P-value Conclusion: We demonstrate that DWJM disks support primary leukemia cell survival. DWJM-adherent cells demonstrate chemotherapy resistance in association with induction of glycolysis. Based on our RNA Seq data we hypothesize that leukemia cell adherence to DWJM upregulates FAM83A, which functions in the epidermal growth factor receptor (EGFR) signaling pathway. FAM83A is known to control PI3K-AKT-TOR signaling cascade. M-TOR, in turn is known to activate HIF-1α pathway, which regulates glycolysis (Figure-1). Additionally, down-regulation of mitochondria-related molecules (MT-TH and MT-TW) is consistent with a switch from oxidative phosphorylation to glycolysis in the adherent cells. Further work is ongoing to further understand the role of glycolysis in primary leukemia cell chemotherapy resistance. Disclosures Lipe: Celgene: Consultancy. Aljitawi:The University of Rochester Medical Center: Patents & Royalties: Pending patent related to decellularized Wharton's jelly matrix; Medpace: Consultancy.

Introduction: Post-transplant lymphoproliferative disorder (PTLD) is a well-recognized disease after allogeneic hematopoietic stem cell transplantation (HCT) and is one of the most common post-transplant malignancies. In most cases, PTLD is associated with Epstein-Barr Virus (EBV) infection. The management of PTLD remains a challenge, with no approved treatments for patients. Clinical practice treatment guidelines recommend rituximab as first-line therapy for PTLD post-allogeneic HCT; however, treatment options for PTLD patients who fail rituximab are not clearly defined. We conducted a systematic literature review of the published literature to better understand treatment patterns for patients with PTLD who fail rituximab post-allogeneic HCT in a real-world setting. Methods: The systematic literature review was performed following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines with the scope defined in terms of Population, Intervention Comparators, Outcomes, and Study design (PICOS) criteria. Using extensive search terms for the indication and study designs, studies were identified using key biomedical literature databases: EMBASE, MEDLINE, and Cochrane. The literature search was conducted on July 19, 2018 and included studies published between database inception in January 1, 1959 and July 19, 2018. Relevant congress abstracts published between January 2015 and June 2018 were also identified. The PICOS-based inclusion and exclusion criteria were used to review identified citations. No treatment limitations were imposed to ensure inclusion of all relevant evidence; the study designs were limited to prospective and retrospective observational studies. Case reports were included regardless of sample size. Two independent reviewers screened all citations and full-text articles; any discrepancies were resolved by a third independent reviewer. Data from included studies were extracted into a pre-defined template, and results were summarized using the PRISMA flow diagram. Results: A total of 69 studies were identified that described patients with PTLD post-allogeneic HCT. The majority (61 studies) were retrospective chart reviews, of which 54 studies were single-center studies. Forty-eight studies reported data on treatment of patients with PTLD. Among these, 5 studies included data prior to 2000, 33 studies included data from 2000-2010, and 26 studies included data from 2010-2016. The sample size for PTLD patients was between 1 and 144 patients, with only one study of > 100 patients. First-line therapy in PTLD included rituximab (41 studies), various chemotherapy regimens (15 studies), and lymphocyte infusion (10 studies). Nine studies reported treatment for patients who failed first-line rituximab (13-67% of PTLD patients); the number of patients with second-line treatments ranged from 2-10 across studies. Second-line treatments varied greatly across studies and included cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP, n=1); rituximab plus CHOP (R-CHOP, n=1-2); cyclophosphamide, etoposide/cytarabine (VP16/ARA-C, n=1); rituximab plus cyclophosphamide (n=1); rituximab plus high-dose cytarabine (n=1); chemotherapy unspecified (n=2-4), and lymphocyte infusion (n=3-5). Only 2 studies reported treatment outcomes in rituximab-refractory patients. One study (N=62, second-line n=10, median age 49 years) reported no complete or partial remission in the chemotherapy group; 60% had complete remission in the lymphocyte infusion group. The second study (N=12, second-line n=3, mean age 5 years) reported complete remission in 1 patient with lymphocyte infusion as second-line treatment. Conclusions: This systematic literature review demonstrates that data on treatment patterns for PTLD patients who failed rituximab post-allogeneic HCT are limited (9 studies with a sample size ≤ 10). Published data suggest that the percentage of patients who fail rituximab vary greatly (13-67%), there is no consistent standard of care for PTLD patients who fail rituximab, and outcomes are poor. There continues to be a significant unmet need among PTLD patients who fail rituximab, and further studies are needed to better understand rituximab response rates in the real-world setting. Disclosures Xu: Atara Biotherapeutics, Inc: Employment, Equity Ownership. Watson:Atara Biotherapeutics, Inc: Employment, Equity Ownership. Forsythe:Novartis: Consultancy. Barlev:Atara Biotherapeutics, Inc: Employment, Equity Ownership.

Abstract 243 Disruption of normal hematopoietic development is a major problem in acute myeloid leukemia (AML). Osteoblastic cells have been shown to support hematopoiesis. We hypothesized that myeloid leukemia inhibits osteoblastic cells contributing to the loss of normal hematopoiesis. To define effects of leukemia on osteoblastic cells, we employed a previously described syngeneic murine model of AML where transplant of leukemic cells does not require irradiation of recipients (Neering et al. Blood, 2007). Leukemic mice had decreased serum levels of the bone formation marker osteocalcin (OC) (92.6 ± 11.6 vs 25.9 ± 4.4 ng/ml Normal (N) vs Leukemic (L) p Disclosures: No relevant conflicts of interest to declare.

Although long considered as a disease of failed apoptosis, it is now clear that chronic lymphocytic leukemia (CLL) cells undergo extensive cell division in vivo, especially in progressive disease. Signaling via the B-cell receptor is thought to activate proliferation and survival pathways in CLL cells and also has been linked to poor outcome. Here, we have analyzed the expression of the proto-oncoprotein MYC, an essential positive regulator of the cell cycle, after stimulation of surface IgM (sIgM). MYC expression was rapidly increased after sIgM stimulation in a subset of CLL samples. The ability of sIgM stimulation to increase MYC expression was correlated with sIgM-induced intracellular calcium fluxes. MYC induction was partially dependent on the MEK/ERK signaling pathway, and MYC and phosphorylated ERK1/2 were both expressed within proliferation centers in vivo. Although stimulation of sIgD also resulted in ERK1/2 phosphorylation, responses were relatively short lived compared with sIgM and were associated with significantly reduced MYC induction, suggesting that the kinetics of ERK1/2 activation is a critical determinant of MYC induction. Our results suggest that ERK1/2-dependent induction of MYC is likely to play an important role in antigen-induced CLL cell proliferation.

The anti-CD20 mAb rituximab is central to the treatment of B-cell malignancies, but resistance remains a significant problem. We recently reported that resistance could be explained, in part, by internalization of rituximab (type I anti-CD20) from the surface of certain B-cell malignancies, thus limiting engagement of natural effectors and increasing mAb consumption. Internalization of rituximab was most evident in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), but the extent of internalization was heterogeneous within each disease. Here, we show that the inhibitory FcγRIIb on target B cells promotes this process and is largely responsible for the observed heterogeneity across a range of B-cell malignancies. Internalization correlated strongly with FcγRIIb expression on normal and malignant B cells, and resulted in reduced macrophage phagocytosis of mAb-coated targets. Furthermore, transfection of FcγRIIb into FcγRIIb negative Ramos cells increased internalization of rituximab in a dose-dependent manner. Target-cell FcγRIIb promoted rituximab internalization in a cis fashion and was independent of FcγRIIb on neighboring cells. It became phosphorylated and internalized along with CD20:anti-CD20 complexes before lysosomal degradation. In MCL patients, high FcγRIIb expression predicted less durable responses after rituximab-containing regimens. Therefore, target-cell FcγRIIb provides a potential biomarker of response to type I anti-CD20 mAb.

We developed a murine model of CNS disease to obtain a better understanding of the pathogenesis of CNS involvement in pre-B-cell acute lymphoblastic leukemia (ALL). Semiquantitative proteomic discovery–based approaches identified unique expression of asparaginyl endopeptidase (AEP), intercellular adhesion molecule 1 (ICAM1), and ras-related C3 botulinum toxin substrate 2 (RAC2), among others, in an invasive pre-B-cell line that produced CNS leukemia in NOD-SCID mice. Targeting RAC2 significantly inhibited in vitro invasion and delayed disease onset in mice. Induced expression of RAC2 in cell lines with low/absent expression of AEP and ICAM1 did not result in an invasive phenotype or murine CNS disease. Flow cytometric analysis identified an enriched population of blast cells expressing ICAM1/lymphocyte function associated antigen-1 (LFA-1)/CD70 in the CD10+/CD19+ fraction of bone marrow aspirates obtained from relapsed compared with normal controls and those with primary disease. CD10+/CD19+ fractions obtained from relapsed patients also express RAC2 and give rise to CNS disease in mice. Our data suggest that combinations of processes are involved in the pathogenesis of CNS disease in pre-B-cell ALL, support a model in which CNS disease occurs as a result of external invasion, and suggest that targeting the processes of adhesion and invasion unique to pre-B cells may prevent recurrences within the CNS.