ALBERT

Description: Signal regulatory protein-α (SIRPA) is an immunoglobulin superfamily transmembrane protein with intracellular docking sites for two Src homology domain containing tyrosine phosphatases, and most abundantly expressed in neurons and myeloid cells. SIRPA is a critical immune inhibitory receptor on macrophages. CD47 is a ligand for SIRPA, and CD47 interaction with SIRPA serves as a ‘self-recognition’ that prevents phagocytosis of the cells expressing CD47. Recently, we (Nature immunology 2007) identified polymorphism in murine Sirpa as a critical modulator of engraftment in xenogeneic model of human-to-mouse hematopoietic stem cell transplantation, and non-obese diabetic (NOD) mouse Sirpa polymorphism has far greater reactivity with human CD47 than that of the respective alleles of other strains resulting in effective engraftment of human hematopoietic cells. In addition, as well as the mouse, human SIRPA is highly polymorphic in the IgV domain. Then, we examined whether polymorphism in SIRPA induced the difference for suppressive effect of human macrophage on hematopoiesis. First, we examined sequence alignment of SIRPA IgV domain (exon 3 of SIRPA) of healthy control 18 people. We identified two major variants; 6 people with variant1 (V1) and 12 people with variant2 (V2). The SIRPA amino acid sequences of V1 and V2 are different in 13 residues, and its residues in orthologous position between species that is polymorphic between NOD and other strains as well as between V2 and V1. Next, we examined that whether there is a difference in the effect on hematopoiesis between V1 macrophage and V2 macrophage by long term culture-initiating cells (LTC-IC) assay on MS-5 stromal cells. For macrophage preparation, peripheral mononuclear cells from healthy donors were purified by positive selection using MACS CD14 Micro Beads (Miltenyi Biotec), and they were incubated with M-CSF for 3 days. For LTC-IC assay, 5×102 differentiated macrophages were seeded onto established MS-5 stroma in 96-well tissue culture plates. The next day, human hematopoietic CD34+ cells were seeded at doses of 102 to 5×103 cells per well and cultured for 4 weeks. At the end of the culture, cells were detached and plated into methylcellulose progenitor assays. Macrophage had a suppressive effect on the number of LTC-IC in all cultures. There was tendency that V1 macrophage had a greater suppression compared to V2 macrophage, however the differences between V1 and V2 were marginal (p=0.03). These data suggests that human macrophages have suppressive effect on hematopoiesis, and human SIRPA polymorphism modulates macrophage-mediated suppression of hematopoiesis in allogenic model, likewise in xenogeneic model of human-to-mouse hematopoietic stem cell transplantation. Moreover, SIRPA polymorphism might be related to graft failure in the allogenic hematopoietic stem cell transplantation.

Description: Acute myeloid leukemia (AML) is an aggressive hematologic malignancy and is the most common type of acute leukemia in adults. Although a majority of patients achieve remission following cytotoxic chemotherapy, most will relapse and ultimately die. Therapy resistance and relapse are driven by leukemia stem cells (LSC). Evidence of genetic and functional heterogeneity in the LSC compartment underscores the importance of developing therapeutic strategies that will target all subclones effectively. We previously showed that LSCs in AML depend on CD47-SIRPα interaction to evade immune surveillance (Theocharides et al, JEM 2012). CD47 acts as a "do not eat me" signal that binds to the inhibitory receptor SIRPα on macrophages and masks cancer cells from macrophage-mediated phagocytosis. TTI-621 (Trillium Therapeutics Inc., Ontario, Canada) is a human SIRPαFc protein formed by fusing the IgV doman of human SIRPα to a human IgG1-Fc moiety; it is designed to bind CD47 on leukemia cells and disrupt its interaction with SIRPα on host macrophages. Our previous studies in AML cell lines and a small number of primary AML samples demonstrated increased phagocytosis in vitro and decreased engraftment in xenotransplant models following SIRPαFc treatment (Theocharides et al, JEM 2012, Petrova et al, Clin Cancer Res 2017). Here, we tested the efficacy of TTI-621 against a broad panel of primary AML samples in xenotransplantation models to determine efficacy and response rates in this heterogeneous disease. Bulk cells obtained from the peripheral blood of 30 AML patients representing a broad range of cytogenetic and molecular subtypes were transplanted intrafemorally into sublethally-irradiated NSG mice. After a 2-week engraftment period, mice were treated with either SIRPαFc or control IgG by intraperitoneal injection 3×/week for 4 weeks, following which leukemic engraftment was determined by flow cytometry. In all but 1 sample, a significant reduction in AML engraftment was seen in SIRPαFc-treated mice compared to controls. For 23 samples defined as good responders, SIRPαFc treatment resulted in 91% (range 53-100%, p

Description: Despite high survival rates for children with acute lymphoblastic leukemia (ALL), only 40% of adult patients will achieve long-term disease-free survival, and relapses in both pediatric and adult ALL are often fatal. Most current therapies are directed at molecular markers or dominant pathways present in the bulk of neoplastic cells, yet recent studies have identified many genetically distinct subclones co-existing within a single neoplasm. The functional properties and clinical relevance of these neoplastic subclones remain undefined. Genome wide copy number analysis of matched diagnostic and relapse ALL samples identified that in 50% of patients, the clones present at relapse are not the dominant clones at diagnosis, but have evolved from an ancestral pre-leukemic clone (Mullighan et al., 2008). In order to investigate the functional consequences of clonal evolution in disease progression and therapy resistance, we performed limiting dilution analysis of 3 diagnostic and 14 paired diagnostic/relapse samples from adult and pediatric B-ALL patients of varying cytogenetics, by transplantation into immune-deficient mice (xenografts). In one patient, the leukemia-initiating cell (LIC) frequency was 7.65 fold higher in the relapse sample than at diagnosis, while another patient showed the reverse with a 5.81 fold higher LIC frequency in the diagnostic sample. Two patients showed no significant differences in LIC frequency from diagnosis to relapse. LIC frequency varied from 1 in 14.2 to 1 in 4802 CD19+ blast cells. Interestingly, in 50% of the paired patient samples, transplantation of cells from the relapse sample gave rise to greater leukemic dissemination to the spleen and/or central nervous system of recipient mice in comparison to the diagnostic sample, despite similar levels of engraftment in the bone marrow. This data suggests that although the LIC frequency in B-ALL remains high and relatively static between diagnosis and relapse, relapse cells acquire increased invasive properties. To investigate the clonal composition of 3 diagnostic B-ALL samples, we undertook copy number variation (CNV) analysis of xenografts generated at both limiting and high transplanted cell doses. In all 3 samples, we detected subclones in the xenografts that were distinct from the predominant clone in the primary patient sample. We performed network analysis on these subclones and identified differentially enriched pathways, including differential expression of anti-apoptotic and apoptosis regulation pathways, providing evidence of putative functional differences. These results support the existence of functionally diverse subclones within diagnostic samples as well as functional diversity between the subclones present at diagnosis and relapse. Ongoing in depth genomic analysis of the diagnosis/relapse paired samples will add to our understanding of the functional role of the subclones identified at diagnosis in the establishment of disease relapse. In summary, these experiments will provide further insight into the functional heterogeneity present in B-ALL and the drivers of lymphoid leukemogenesis that lead to therapy failure and disease relapse. Disclosures: Danska: Trillium Therapeutics/Stem Cell Therapeutics: Research Funding.

Description: Survival rates for pediatric B-Cell Acute Lymphoblastic Leukemia (B-ALL) have improved dramatically over the past 40 years approaching a current long-term survival rate of 85%. However childhood B-ALL patients continue to confront co-morbidities and their long-term consequences. For example, osteopenia and osteoporosis associated fractures are a common complication of pediatric leukemia at diagnosis, during treatment and in long-term B-ALL survivors. The STeroid-associated Osteoporosis in the Pediatric Population (STOPP) study reported that at ALL diagnosis, 16% of children and adolescents present with bone pain, vertebral compression and low vertebral Bone Mineral Density (BMD) scores, with the greatest incidence of vertebral fractures (VF) seen in the first year following diagnosis (J Clin Endocrinol Metab. 2015, 100:3408-17). Glucocorticoid treatment further elevated fracture risk in this population. These data underscore the need to identify molecular mechanism by which leukemic cells contribute to bone loss, and provide targeted therapies to limit these effects. Our laboratory previously showed that Rag2-/- p53-/- Prkdcscid/scid triple mutant (TM) and p53-/- Prkdcscid/scid double mutant (DM) mice develop spontaneous B-ALL, but only TM animals exhibit dissemination of leukemic blasts to the leptomeninges of the CNS, a poor prognosis feature observed in pediatric and adult ALL patients. We observed that TM leukemic mice also displayed fragile vertebral bones. Using comparative transcriptome analysis, we found that RANKL (Receptor Activator of the Nuclear factor-kB Ligand), a Tumor Necrosis Factor (TNF) superfamily member ligand and a key regulator of B cell and osteoclast differentiation, was expressed at greater levels in TM compared to the DM leukemia cells. RANKL binds to its receptor RANK, which is expressed in osteoclast precursor cells. RANK-RANKL interaction induces signaling in the osteoclast precursors and drives their differentiation into mature bone resorbing osteoclasts (Proc. Natl. Acad. Sci. 1999, 96:3540-3545). Upon adoptive leukemia cell transfer into immune deficient mice, RANKL+ TM but not DM cells caused decreased vertebral trabecular bone density in the recipients. Treatment with the recombinant RANKL antagonist protein Osteoprotegerin (OPG-Fc) inhibited the growth and dissemination of RANKL+TM leukemic cells and attenuated bone destruction in the recipient mice. These data suggested that TM mouse leukemia cells cause bone loss in the absence of glucocorticoid or other chemotherapy agents. We then examined the potential role of RANKL in osteoporosis associated with human B-ALL. RANKL mRNA was expressed by a majority of primary human adult and pediatric B-ALL. To determine whether primary patient B-ALL can cause bone loss, we transplanted RANKL+ human B-ALL samples of multiple cytogenetic high-risk subgroups (Complex, hypo-diploid and Mixed Lineage Leukemia (MLL) rearranged) into NOD.SCID.gC-/-(NSG) recipient mice. Micro-CT imaging and bone density measures in the xenotransplant recipients revealed extensive vertebral trabecular bone destruction. Immuno-histological analysis of the human B-ALL recipient mice demonstrated extensive osteoporotic damage of the long bones and marked RANKL protein expression in the long bones of mice harboring extensive human B-ALL cell burden compared to NSG control mice. To determine whether RANKL-RANK interaction was required for the B-ALL mediated bone destruction, cohorts of NSG mice engrafted with human B-ALL were treated with recombinant OPG-Fc compared to a matched Fc control protein. OPG-Fc treatment did not attenuate leukemia cell expansion and bone marrow burden, but despite bulky disease, the treatment conferred robust protection from bone destruction suggesting that RANKL was a critical mediator of this clinical complication. Our data demonstrate a central role of the RANK-RANKL axis in B-ALL-mediated bone disease and identify an actionable therapeutic target to reduce acute and long-term morbidity. Denosumab, an anti-RANKL antibody has been approved for the treatment of bone metastasis by solid tumors and for post-menopausal osteoporosis. Our pre-clinical studies suggest that Denosumab and other agents that inhibit the RANK-RANKL pathway may be efficacious in patients with B-ALL associated bone degeneration. Disclosures No relevant conflicts of interest to declare.

Description: Lunatic Fringe (Lfng) enhances Notch1 activation by Delta-like 4 (DL4) to promote Notch1-dependent T-lineage commitment of thymus-seeding progenitors. Subsequently, Notch1 and T-cell receptor-β (TCRβ)–containing pre-TCR complexes signal CD4/CD8 double-negative 3 (DN3) committed T-cell progenitors to survive, proliferate, and differentiate into CD4/CD8 double-positive (DP) αβ T-cell precursors. Few DP thymocytes develop without Notch1 or pre-TCR signals, whereas ectopic Notch1 activation causes T-cell leukemia. However, mechanisms of a Notch-pre-TCR collaboration during this “β-selection” process are poorly understood. We genetically manipulated Lfng to attenuate or enhance Notch1 activation in DN3 thymocytes without inducing leukemogenesis. We show that Lfng temporally sustains DL-induced Notch1 signaling to prolong proliferative self-renewal of pre-DP thymocytes. Pre-TCR signaling greatly augmented Notch trophic functions to promote robust proliferation of pre-DP progenitors. In contrast, in the absence of DL/Notch signaling, pre-TCR-expressing progenitors rapidly atrophied and differentiated into DP thymocytes. Thus, Lfng prolongs Notch1 signaling to promote self-renewal more than differentiation during the early stages of β-selection. Our data provide novel insights into the Notch-pre-TCR collaboration, and suggest that decreasing Lfng expression during the DN3-DP transition minimizes the potent leukemogenic potential of Notch1 signaling.

Description: Without prophylactic therapy, B-cell Acute Lymphoblastic Leukemia (B-ALL) spreads to the leptomeninges of the central nervous system (CNS) in up to 70% of patients. CNS involvement is more common in certain high risk B-ALL subgroups, including patients with KMT2A (MLL)-translocations, and disease relapse in the CNS carries a poor prognosis. The genetic determinants and biology of B-ALL dissemination to the CNS are poorly defined and therefore therapies targeting the drivers of CNS disease are lacking. Whereas B-ALL exhibits significant subclonal diversity that contributes to functional heterogeneity and disease relapse, recent reports suggest similar clonal composition of bone marrow (BM) and CNS disease, with the potential for CNS dissemination being a universal property of B-ALL cells (Williams et al. 2016, Bartram et al. 2018). Furthermore, functional studies of leptomeningeal disease have focused on the invasion of B-ALL cells into the CNS but limited studies have addressed the selection of genetic clones with the ability to grow within the subarachnoid space. To better define the evolutionary history and biology of leptomeningeal B-ALL we performed targeted DNA, SNP copy number, RNA sequencing, and functional analysis on cells isolated from matched BM and CNS tissue of patient derived xenografts (PDX) generated from a cohort of paired diagnosis and relapse samples from 14 pediatric and adult B-ALL patients of varying cytogenetics. The majority of primary patient samples yielded CNS disease 20 weeks after intrafemoral injection into NSG mice. CNS disease burden was higher in PDXs derived from relapsed B-ALL samples. Human B-ALL cells isolated from the CNS of PDXs retained competence to repopulate disease in the BM, spleen, and CNS upon serial transplantation. Targeted DNA sequencing results analyzed using a Bayesian clustering method revealed different genetic clonal composition between matched BM and CNS cells in approximately half of the xenografts. PDXs from relapse samples were more likely to show genetic discordance between the BM and CNS. Copy number analysis also confirmed frequent genetic discordance between cells isolated from the BM and CNS from individual PDXs. Interestingly, in one patient all PDXs generated from the relapse sample displayed chromosome 6p and 17p hemi-deletions that were unique to the CNS. In total, PDXs from four patients showed recurrent enrichment of specific lesions in CNS-engrafting cells, suggesting that transit to and/or survival within the subarachnoid space can be the product of selection for genetic clones with increased CNS tropism. RNA-seq of matched BM and CNS cells derived from 45 of the primary PDXs demonstrated that CNS-isolated cells were transcriptionally distinct from their matched BM. These differences were most pronounced in samples from patients with MLL-AF4 translocations, whose CNS isolated cells grouped together in multi-dimensional scaling. Using GSEA, the most highly CNS-enriched gene sets in MLL samples were related to mRNA translation initiation and polypeptide elongation. Translation-related gene sets are similarly enriched in the blasts of MLL B-ALL patients with CNS disease in the COG 9906 study. CNS-isolated cells from PDXs of MLL patients exhibited altered rates of protein synthesis compared to matched BM-isolated cells. Treatment of PDXs with the clinically-approved translational inhibitor omacetaxine mepesuccinate (OMA) effectively decreased rates of translation in CNS-engrafting cells. Moreover, OMA reduced leukemia burden nearly 4-fold in PDXs bearing established CNS infiltration generated from two MLL patients. Our data represent an advance in the understanding of B-ALL CNS disease. We present a rich resource of genomic and transcriptomic data from xenografts spanning multiple B-ALL subgroups across diagnosis and relapse and have identified selection for genetically and biologically distinct clones in the CNS, contrary to the current model. Furthermore, we demonstrate that in MLL patients, dysregulation of protein synthesis occurs at CNS dissemination and targeting this process is a novel therapeutic paradigm that may benefit patients with CNS disease. Disclosures Mullighan: Cancer Prevention and Research Institute of Texas: Consultancy; Pfizer: Honoraria, Research Funding, Speakers Bureau; Loxo Oncology: Research Funding; Amgen: Honoraria, Speakers Bureau; Abbvie: Research Funding.

Description: Abstract 3092 Poster Board III-29 Introduction Early B cell acute lymphoblastic leukemia (B-ALL) is the most common type of childhood malignancy, characterized by abnormal accumulation and proliferation of progenitor-B or precursor-B (pre-B) cells. Current challenges associated with B-ALL treatment include fatal relapses, treatment-related toxicities and long-term morbidities underscoring a need to develop new targeted therapies aimed at eradicating leukemia cells and their stem cells. To achieve this, a better understanding of molecular mechanisms involved in leukemia initiation and progression is required. Our laboratory developed p53-/- PrkdcSCID/SCID double mutant (DM) strain as a mouse model of early B-ALL. We showed that DM leukemias progress through discrete developmental stages of leukemogenesis despite the absence of a pre-B cell receptor (pre-BCR), a crucial checkpoint in B cell development. Spleen tyrosine kinase (SYK), a key proximal component of pre-BCR signaling, was activated in the DM leukemias despite the absence of pre-BCR and was required for their survival. Approximately 70% of pediatric pre-B-ALLs also do not express pre-BCR, which lead us to investigate SYK signaling in human pre-B-ALL and to test potential therapeutic application of SYK inhibition in these leukemias. Patients and Methods We examined 22 viably frozen primary pediatric pre-B-ALL bone marrow samples to test their responses to SYK inhibition in vitro and in vivo and have investigated the molecular basis for aberrant SYK-mediated signaling in B-ALL. Results Western blot analyses revealed that SYK and BLNK, a dominant target of SYK, were expressed in pre-B-ALL patient samples. The majority of human pre-B ALL samples tested (14/22) displayed significantly attenuated proliferation in the presence of SYK inhibitors suggesting that SYK is necessary for their survival and/or proliferation. Treatment with SYK inhibitor R406 prevented phosphorylation of downstream SYK targets including BLNK and PLC-γ2. We are continuing to study the effects of SYK inhibition using phospho-flow cytometry and genome wide expression arrays. Preliminary data will also be presented on therapeutic efficacy of an orally bioavailable form of R406-mediated SYK inhibition in vivo by xenotransplantation of human leukemias into immuno-deficient mice. Conclusions Understanding the molecular mechanisms of pre-BCR-independent SYK activation involved in proliferation and survival of leukemic blasts may provide a rational basis for development of effective treatment for ALL. Specifically, targeted therapeutic inhibition of SYK signaling may be effective B-ALL treatment that may improve outcomes of current treatment regiments with minimal additional treatment-related toxicity. Disclosures Pine: Rigel Pharmaceuticals: Employment, Equity Ownership. Hitoshi:Rigel Pharmaceuticals: Employment, Equity Ownership.

Description: Abstract 476 Introduction: A large body of work has shown that acute myeloid leukemia (AML) clones are hierarchically organized and maintained by leukemia initiating cells (AML-LSC). However, little is known about molecular regulators that govern AML-LSC fate. Using the non-obese diabetic (NOD)–severe combined immunodeficiency (SCID) xenotransplantation model, our group recently found that CD47-SIRPα protein interaction is essential for repopulation of normal hematopoietic stem cells (HSC) in mice (Takenaka et al, Nat Immunol 2007). The NOD background conferred the best support for human engraftment, whereas mice with other polymorphisms of Sirpa could not be engrafted (i.e. NOD.NOR-Idd13.SCID). CD47 on human AML contributes to pathogenesis by inhibiting phagocytosis of leukemia cells through CD47-SIRPα interaction (Jaiswal et al, Cell 2009). CD47 is increased on human AML LSCs compared to normal HSCs, and high levels of CD47 are associated with poor patient outcome (Majeti et al, Cell 2009). This indicates that CD47 may function as an important molecular regulator of AML-LSC fate through communication with SIRPα protein expressed on cells of the innate immune system. Results: Consistent with published results, we observed increased CD47 expression in primary AML cells compared to normal cord blood cells. We next investigated the functional relevance of CD47 for AML-LSCs by xenografting primary human AMLs into NOD.SCID and NOD.NOR-Idd13.SCID (Idd) mice. Following intravenous (i.v.) transplantation, none of three primary human AML samples could engraft Idd mice while robust engraftment in NOD.SCID mice was observed, consistent with our previous data using normal human HSCs. When the same samples were transplanted intrafemorally (i.f.), 2 of 6 Idd mice showed engraftment in the injected femur but no engraftment in other bones or the spleen; evidence of the latter is linked to stem cell function. To assess the role of innate immunity, we pre-treated mice with antibody against murine CD122 which depletes host natural killer (NK) cells and macrophages. Engraftment in the injected femur was observed in NOD.SCID mice (43/43) for all 10 AML samples tested, and interestingly in 31 of 42 Idd mice (8/10 AML samples tested), with similar engraftment levels (Idd 37.4±6% vs NOD.SCID 53.8±5%, p=0.33). As expected, NOD.SCID mice supported migration to non-injected bones (38/43 mice, 10/10 AML samples tested). In contrast to results obtained in the absence of anti-CD122 pre-treatment, engraftment in non-injected bones was now detectable in 8 of 42 Idd mice (2/10 AML samples tested), pointing to improved migration of AML-LSCs following depletion of NK cells and macrophages. However, the engraftment level in non-injected bones was significantly lower in Idd compared to NOD.SCID mice (2.4±4% vs 63.2±6%, p=0.001). Moreover, AML-LSCs were unable to repopulate the spleens of Idd mice. Homing assays revealed decreased homing to BM and spleen 16 hours following i.v. injection in Idd compared to NOD.SCID mice. Conclusion: Our results support the hypothesis that CD47-SIRPα interaction is critical for engraftment of AML-LSCs. Attenuation of CD47-SIRPα interaction (as in Idd mice) leads to decreased engraftment ability of AML-LSCs. Enhancement of engraftment in the injected femur and in some cases migration to other bones in Idd mice following anti-CD122 treatment is likely mediated through interference with cells of the innate immune system. Interruption of CD47-SIRPα signaling through targeting of either CD47 or SIRPα provides a potential therapeutic approach for eradication of AML-LSCs. Disclosures: Dick: Roche: Research Funding; CSL Ltd: Research Funding.