Abstract: Bone destruction is a hallmark of myeloma, and has a severe impact on patients’ quality of life and survival. Unfortunately, current treatment only offers moderate palliative effects, and this disease remains incurable. The bone changes in myeloma patients results from increased osteoclast-mediated bone resorption and decreased osteoblast-mediated bone formation. In particular, new bone formation that usually occurs at sites of previously resorbed bones is deeply suppressed; as a result, areas of bone destruction rarely heal. Previous studies have shown that myeloma cells inhibit osteoblast differentiation from mesenchymal stem cells (MSCs), and the Wnt/b-catenin signaling pathway is suppressed via myeloma-produced Wnt antagonists such as dickkopf-1. However, the role of dickkopf-1 in myeloma-induced inhibition of bone formation remains controversial since myeloma cells alone do not produce sufficient dickkopf-1 to suppress osteoblast differentiation. In addition, the administration of an antibody against dickkopf-1 in myeloma patients failed to restore new bone formation, indicating there must be an additional mechanism for inhibition of osteoblast differentiation seen in myeloma. While MSCs can differentiate into mature osteoblasts, they are also capable of differentiating into adipocytes, which is a major cell type in marrow stroma. We observed that myeloma cells (cell lines and primary cells isolated from myeloma patients’ bone marrow) injected into human or mouse bone not only reduced osteoblast number, but also increased adipocyte number and activity in bone marrow. Similar observations were seen in the clinical setting where collections of adipocytes were found in the bone marrow of newly diagnosed, untreated myeloma patients. Patients with greater bone destruction had higher adipocyte numbers than those in patients with less bone destruction, indicating a relationship among myeloma cells, adipogenesis, and osteoblastogenesis. We hypothesized that inhibition of osteoblast differentiation is a consequence of myeloma-dependent alterations in the control of the MSCs’ fate into osteoblasts or into adipocytes. In our studies, we co-cultured MSCs with myeloma cells in a mixed medium (that contained both adipocyte and osteoblast media), and we observed co-culture with myeloma cells induced more adipocyte than osteoblast formation. Moreover, co-culture with myeloma cells enhanced adipocyte differentiation in vitro. Interestingly, separation of the cells by transwell inserts significantly reduced such effect. By analysis of the adhesion molecules in myeloma cells, we identified integrin α4β1 as a novel contributor in regulation of adipogenesis and osteoblastogenesis. Thus, our studies indicate that in the presence of myeloma cells, MSCs may be more prone to differentiate into adipocytes than into osteoblasts via α4β1. Our studies also suggest the development of new strategies to improve the care of myeloma patients with bone destruction by targeting α4β1 and its signaling pathways. Disclosures No relevant conflicts of interest to declare.

Abstract: Factor VIII (FVIII) replacement products enable comprehensive care in hemophilia A. Treatment goals in severe hemophilia A are expanding beyond low annualized bleed rates to include long-term outcomes associated with high sustained FVIII levels. Endogenous von Willebrand factor (VWF) stabilizes and protects FVIII from degradation and clearance, but it also subjects FVIII to a half-life ceiling of ∼15 to 19 hours. Increasing recombinant FVIII (rFVIII) half-life further is ultimately dependent upon uncoupling rFVIII from endogenous VWF. We have developed a new class of FVIII replacement, rFVIIIFc-VWF-XTEN (BIVV001), that is physically decoupled from endogenous VWF and has enhanced pharmacokinetic properties compared with all previous FVIII products. BIVV001 was bioengineered as a unique fusion protein consisting of a VWF-DʹD3 domain fused to rFVIII via immunoglobulin-G1 Fc domains and 2 XTEN polypeptides (Amunix Pharmaceuticals, Inc, Mountain View, CA). Plasma FVIII half-life after BIVV001 administration in mice and monkeys was 25 to 31 hours and 33 to 34 hours, respectively, representing a three- to fourfold increase in FVIII half-life. Our results showed that multifaceted protein engineering, far beyond a few amino acid substitutions, could significantly improve rFVIII pharmacokinetic properties while maintaining hemostatic function. BIVV001 is the first rFVIII with the potential to significantly change the treatment paradigm for severe hemophilia A by providing optimal protection against all bleed types, with less frequent doses. The protein engineering methods described herein can also be applied to other complex proteins.

Abstract: Introduction: Multiple Myeloma (MM) is a clonal plasma cell malignancy, accounting for 10% of all hematological malignancies. Genetic analyses of large populations revealed that blood-specific somatic mutations in hematopoietic stem cells (HSCs) are commonly acquired during aging, a new entity labeled: clonal hematopoiesis of indeterminate potential (CHIP). We sought to determine the role of CHIP on survival of MM patients, specifically those receiving immunomodulator (IMiD) maintenance (Lenalidomide or Thalidomide) post autologous stem cell transplant (ASCT). Methods: We collected the cryopreserved, growth factor mobilized peripheral blood of 629 MM patients who underwent ASCT between 2003 and 2011 at the Dana-Farber Cancer Institute (DFCI). Then, we performed targeted next-generation sequencing using a 224-gene panel at a mean depth of coverage of 978X and ultra-low pass whole-genome sequencing at 0.1X to account for tumor contamination. We downloaded (dbGAP # phs000748.v6.p4) the whole-exome sequencing (WES) data of a cohort of 1144 newly diagnosed, untreated MM patients from the Multiple Myeloma Research Foundation (MMRF) Clinical Outcomes in MM to Personal Assessment of Genetic Profile (CoMMpass, NCT0145429) study (MMRC) and the WES data of a cohort of 205 newly diagnosed, untreated MM patients from the Broad Institute dataset. We analyzed their peripheral blood (average coverage of 108X) and tumor (average coverage of 107X) data separately, looking for the same CHIP genes included in our target bait panel. Results: The DFCI cohort had a median age of 58 years [range, 24-83] at time of ASCT and median follow up post ASCT of 8 years [range, 0.1-14.5] . 204 patients (32%) in the DFCI cohort had CHIP at time of ASCT. The most commonly detected mutated genes were DNMT3A, TET2, TP53, ASXL1 and PPM1D. 24 patients (3.8%) developed a second hematological malignancy at a median of 4 years [range, 1-10] post ASCT, half of whom had CHIP. Around 48% of the DFCI cohort received IMiDs as part of induction therapy. Different induction regimens had no effect on CHIP prevalence at time of ASCT. Around 56% of the DFCI cohort received IMiD maintenance, 22% of which received maintenance for at least 3 years [range, 0.06-12.8] . Among those who did not receive IMiD maintenance, patients with CHIP had worse progression free survival (PFS) (p-value 〈 0.001) and overall survival (OS) (p-value = 0.005). In patients receiving IMiD maintenance, having CHIP had no effect on PFS or OS. On the other hand, the MMRF cohort had a median age of 63 years [range, 27-93] and median follow up of 3.03 years [range, 0-5.9] from time of diagnosis. Around 52% of that cohort underwent ASCT and around 76% of those received IMiD maintenance with a median follow up of 2.7 years [range, 0-5.5] from time of ASCT. Furthermore, 200 patients of the MMRF cohort have follow-up samples of both tumor and peripheral blood that had targeted sequencing done by a 562-gene panel that included our genes of interest. Similarly, when studying the genomic results of 139 out of 1144 MMRF patients, as well as the 205 patients from the Broad Institute dataset, we detected CHIP in 25.6% of them and the top 5 most commonly mutated genes were similar to those of our cohort. Conclusion: CHIP is a common entity among MM patients, reaching a prevalence of up to 32%, that predicts a worse PFS and OS in those who do not receive IMiD maintenance therapy post ASCT. As expected, IMiD maintenance improves outcome in MM patients, with and without CHIP. In patients with CHIP, the use of IMiDs abrogated the deleterious effect imposed by CHIP to a point that outcome is identical to that of patients without CHIP. Figure Figure. Disclosures Bustoros: Dava Oncology: Honoraria. Munshi:OncoPep: Other: Board of director. Anderson:Gilead: Membership on an entity's Board of Directors or advisory committees; OncoPep: Equity Ownership, Other: Scientific founder; Celgene: Consultancy; C4 Therapeutics: Equity Ownership, Other: Scientific founder; Bristol Myers Squibb: Consultancy; Millennium Takeda: Consultancy. Richardson:Oncopeptides: Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; Karyopharm: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding. Soiffer:Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. Ghobrial:Celgene: Consultancy; Janssen: Consultancy; BMS: Consultancy; Takeda: Consultancy.

Abstract: The leukemogenic AML1-ETO fusion protein is produced by the t(8;21) translocation, which is one of the most common chromosomal abnormalities in acute myeloid leukemia (AML). In leukemic cells, AML1-ETO resides in and functions through a stable protein complex, AETFC, that contains multiple transcription factors and cofactors. Among these AETFC components, E2A (also known as TCF3) and HEB (also known as TCF12), two members of the ubiquitously expressed E proteins, directly interact with AML1-ETO, confer new DNA (E-box) binding capacity to AETFC, and are functionally essential for leukemogenesis. However, we find that the third E protein, E2-2 (also known as TCF4), is specifically silenced in AML1-ETO-expressing leukemic cells, suggesting E2-2 as a negative factor of leukemogenesis. Indeed, ectopic expression of E2-2 selectively inhibits the growth of AML1-ETO-expressing leukemic cells, and this inhibition requires the basic helix-loop-helix (bHLH) DNA-binding domain of E2-2. Gene expression profiling and ChIP-seq analysis reveal that, despite some overlap, the three E proteins differentially regulate many target genes. In particular, consistent with the fact that E2-2 is a critical transcription factor in dendritic cell (DC) development, our studies show that E2-2 both redistributes AETFC to, and activates, some genes associated with DC differentiation, and that restoration of E2-2 triggers a partial differentiation of the AML1-ETO-expressing leukemic cells into the DC lineage. Meanwhile, E2-2, but not E2A or HEB, represses MYC target genes, which may also contribute to leukemic cell differentiation and apoptosis. In AML patients, the expression of E2-2 is relatively lower in the t(8;21) subtype, and an E2-2 target gene, THPO, is identified as a potential predictor of relapse. In a mouse model of human t(8;21) leukemia, E2-2 suppression accelerates the development of leukemia. Taken together, these results reveal that, in contrast to HEB and E2A, which facilitate AML1-ETO-mediated leukemogenesis, E2-2 compromises the function of AETFC and negatively regulates leukemogenesis. The three E proteins thus define a molecular heterogeneity of AETFC, which merits further study in different t(8;21) AML patients, as well as in its potential regulation of cellular heterogeneity of AML. These studies should improve our understanding of the precise mechanism of leukemogenesis and assist development of diagnostic and therapeutic strategies. Disclosures No relevant conflicts of interest to declare.

Abstract: Relapse after allogeneic hematopoietic stem cell transplantation (HSCT) remains an unmet medical need. The ETCTN 9204 study evaluated in 71 study subjects if immune checkpoint blockade with anti-CTLA-4 (Ipilimumab (Ipi), n = 43) or anti-PD-1 (Nivolumab (Nivo), n = 28) antibodies could stimulate graft-versus-leukemia (GvL) responses in this setting. We focused mainly on patients (pts) with relapsed AML/MDS, which constituted the majority of pts (n = 38; 54%). Ipi induced both long-term complete remissions (CR; n = 3) and transient CRs (TR; n = 3), while Nivo did not generate any CRs, but 4 patients demonstrated partial remissions (PR). To define the molecular features associated with response to Ipi, we performed bulk transcriptomic analyses of formalin-fixed paraffin-embedded biopsies of sites of AML/MDS involvement (n = 35) collected before and after Ipi treatment from 13 pts. Our analysis of matched pre- and post-Ipi samples of patients with CRs identified 50 differentially expressed genes. By gene ontology analysis, these were enriched for signatures of 'lymphocyte activation' and 'antigen-receptor signaling'. Principal component analysis using these genes separated post-Ipi CR samples as a distinct cluster, transcriptionally apart from pre-Ipi CR samples and from non-responder (NR, n = 15) pre and post-Ipi samples. Of note, post-Ipi CR samples were similar to both pre- and post-Ipi TR samples as well as samples from GvHD/toxicity biopsies (n = 9). Consistent with these findings, we detected increased CD8+ T cell abundance post-Ipi in CR but not NR samples with CIBERSORTx (pre vs post, median score 0.043 vs. 0.56, p & lt; 0.01). Likewise, reconstruction of T cell receptor (TCR) CDR3 sequences using the tool TRUST showed an increase in TCR clonotypes per million reads post-Ipi in CR samples (pre vs post, median 0.33 vs. 1.65, p & lt; 0.05). In sum, response to CTLA-4 blockade is characterized by transcriptional evidence of T cell infiltration and activation within the tumor microenvironment, with similar gene programs observed in the setting of GvHD. To determine if the changes within tissue sites following Ipi are also observed in peripheral blood (PB), we analyzed the immunophenotype and TCR repertoire of T cells from serially collected PB samples from pts with AML/MDS (n = 14) or non-myeloid malignancy (n = 6). In responders and non-responders, exposure to Ipi was associated with decreased naïve, increased effector memory, and increased expression of HLA-DR on central memory T cells (p & lt; 0.05), consistent with T cell differentiation and activation. From 9 of 14 AML/MDS pts (CR = 4, NR = 5), bulk TCR sequencing before and after 1 cycle of Ipi yielded 572,017 PB TCR sequences. Only 776 clonotypes demonstrated significant change in frequency, and these TCRs were detected in greater proportion among responders (613/776 versus NR 163/776, p & lt; 0.01). Thus, Ipi alters the differentiation states of circulating T cells irrespective of response and leads to higher TCR repertoire turnover in CR patients. Of the AML patients achieving PR following Nivo, we also observed transcriptional evidence of CD8+ T cell infiltration in one patient. This signature, however, was not detected in a second such patient. In a separate instance, we had the opportunity to dissect the molecular features of a partial response in a patient with JAK2V617F+ secondary AML following Nivo through single-cell RNA and ATAC-sequencing (scRNA-seq and scATAC-seq, 10x Genomics) of PB mononuclear cells collected serially across 6 timepoints. In total, we analyzed the transcriptomes and chromatin accessibility data from of 27,777 and 28,713 individual cells, respectively. At time of response, non-exhausted CD4+ T cells expanded while both exhausted CD8+ T cells and a malignant subpopulation with increased expression of PD-L1 and features of megakaryocytic differentiation preferentially contracted. The subsequent expansion of a PD-L1- malignant population at progression suggests that decreased leukemic PD-L1 expression was associated with relapse. Altogether, these data highlight the molecular and cellular features of successful reinvigoration of GvL using CTLA-4 blockade, from increased local T cell infiltration and activation in the leukemic microenvironment to peripheral T cell turnover. In addition, the selection of therapy-resistant subclones after PD-1 blockade underscores the need for further high-resolution studies of GvL responses. Disclosures Zeiser: Novartis: Honoraria; Incyte: Honoraria; Malinckrodt: Honoraria. Ritz:Rheos Medicines: Consultancy; LifeVault Bio: Consultancy; Infinity Pharmaceuticals: Consultancy; Falcon Therapeutics: Consultancy; Avrobio: Consultancy; Kite Pharma: Research Funding; Talaris Therapeutics: Consultancy; Equillium: Research Funding; Amgen: Research Funding; TScan Therapeutics: Consultancy. Neuberg:Madrigak Pharmaceuticals: Current equity holder in publicly-traded company; Celgene: Research Funding; Pharmacyclics: Research Funding. Soiffer:alexion: Consultancy; Gilead: Consultancy; Be the Match/ National Marrow Donor Program: Membership on an entity's Board of Directors or advisory committees; Rheos Therapeutics: Consultancy; Juno: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy; Mana Therapeutics: Consultancy; Precision Bioscience: Consultancy; VOR Biopharma: Consultancy; Kiadis: Membership on an entity's Board of Directors or advisory committees; Cugene: Consultancy; Celgene: Membership on an entity's Board of Directors or advisory committees. Liu:GV20 Oncotherapy: Consultancy, Membership on an entity's Board of Directors or advisory committees; 3DMed Care: Consultancy; Sanofi: Research Funding; Takeda: Research Funding. Davids:AbbVie: Consultancy; Gilead Sciences: Consultancy; Sunesis: Consultancy; BeiGene: Consultancy; AstraZeneca: Consultancy, Research Funding; Syros Pharmaceuticals: Consultancy; Research to Practice: Honoraria; Pharmacyclics: Consultancy, Research Funding; TG Therapeutics: Consultancy, Research Funding; Verastem: Consultancy, Research Funding; Adaptive Biotechnologies: Consultancy; Janssen: Consultancy; Surface Oncology: Research Funding; Novartis: Consultancy, Research Funding; MEI Pharma: Consultancy, Research Funding; Genentech: Consultancy, Research Funding; Eli Lilly: Consultancy; Zentalis: Consultancy; Celgene: Consultancy; Bristol Myers Squibb: Research Funding; Merck: Consultancy; Ascentage Pharma: Consultancy, Research Funding. Wu:Pharmacyclics: Research Funding; BionTech: Current equity holder in publicly-traded company. OffLabel Disclosure: Ipilimumab and Nivolumab were tested in the setting of relapsed hematological malignancies after allogeneic stem cell transplantation.