Eliminating chronic myeloid leukemia stem cells by IRAK1/4 inhibitors

Leukemia stem cells (LSCs) in chronic myeloid leukemia (CML) are quiescent, insensitive to BCR-ABL1 tyrosine kinase inhibitors (TKIs) and responsible for CML relapse. Therefore, eradicating quiescent CML LSCs is a major goal in CML therapy. Here, using a G0 marker (G0M), we narrow down CML LSCs as G0M- and CD27- double positive cells among the conventional CML LSCs. Whole transcriptome analysis reveals NF-κB activation via inflammatory signals in imatinib-insensitive quiescent CML LSCs. Blocking NF-κB signals by inhibitors of interleukin-1 receptor-associated kinase 1/4 (IRAK1/4 inhibitors) together with imatinib eliminates mouse and human CML LSCs. Intriguingly, IRAK1/4 inhibitors attenuate PD-L1 expression on CML LSCs, and blocking PD-L1 together with imatinib also effectively eliminates CML LSCs in the presence of T cell immunity. Thus, IRAK1/4 inhibitors can eliminate CML LSCs through inhibiting NF-κB activity and reducing PD-L1 expression. Collectively, the combination of TKIs and IRAK1/4 inhibitors is an attractive strategy to achieve a radical cure of CML.

Shedding Light on Targeting Chronic Myeloid Leukemia Stem Cells

Chronic myeloid leukemia stem cells (CML LSCs) are a rare and quiescent population that are resistant to tyrosine kinase inhibitors (TKI). When TKI therapy is discontinued in CML patients in deep, sustained and apparently stable molecular remission, these cells in approximately half of the cases restart to grow, resuming the leukemic process. The elimination of these TKI resistant leukemic stem cells is therefore an essential step in increasing the percentage of those patients who can reach a successful long-term treatment free remission (TFR). The understanding of the biology of the LSCs and the identification of the differences, phenotypic and/or metabolic, that could eventually allow them to be distinguished from the normal hematopoietic stem cells (HSCs) are therefore important steps in designing strategies to target LSCs in a rather selective way, sparing the normal counterparts.

Quinacrine-CASIN combination overcomes chemoresistance in human acute lymphoid leukemia

Chemoresistance posts a major hurdle for treatment of acute leukemia. There is increasing evidence that prolonged and intensive chemotherapy often fails to eradicate leukemic stem cells, which are protected by the bone marrow niche and can induce relapse. Thus, new therapeutic approaches to overcome chemoresistance are urgently needed. By conducting an ex vivo small molecule screen, here we have identified Quinacrine (QC) as a sensitizer for Cytarabine (AraC) in treating acute lymphoblastic leukemia (ALL). We show that QC enhances AraC-mediated killing of ALL cells, and subsequently abrogates AraC resistance both in vitro and in an ALL-xenograft model. However, while combo AraC+QC treatment prolongs the survival of primary transplanted recipients, the combination exhibits limited efficacy in secondary transplanted recipients, consistent with the survival of niche-protected leukemia stem cells. Introduction of Cdc42 Activity Specific Inhibitor, CASIN, enhances the eradication of ALL leukemia stem cells by AraC+QC and prolongs the survival of both primary and secondary transplanted recipients without affecting normal long-term human hematopoiesis. Together, our findings identify a small-molecule regimen that sensitizes AraC-mediated leukemia eradication and provide a potential therapeutic approach for better ALL treatment.

Leukemia Stem Cells as a Potential Target to Achieve Therapy-Free Remission in Chronic Myeloid Leukemia

Leukemia stem cells (LSCs, also known as leukemia-initiating cells) not only drive leukemia initiation and progression, but also contribute to drug resistance and/or disease relapse. Therefore, eradication of every last LSC is critical for a patient’s long-term cure. Chronic myeloid leukemia (CML) is a myeloproliferative disorder that arises from multipotent hematopoietic stem and progenitor cells. Tyrosine kinase inhibitors (TKIs) have dramatically improved long-term outcomes and quality of life for patients with CML in the chronic phase. Point mutations of the kinase domain of BCR-ABL1 lead to TKI resistance through a reduction in drug binding, and as a result, several new generations of TKIs have been introduced to the clinic. Some patients develop TKI resistance without known mutations, however, and the presence of LSCs is believed to be at least partially associated with resistance development and CML relapse. We previously proposed targeting quiescent LSCs as a therapeutic approach to CML, and a number of potential strategies for targeting insensitive LSCs have been presented over the last decade. The identification of specific markers distinguishing CML-LSCs from healthy HSCs, and the potential contributions of the bone marrow microenvironment to CML pathogenesis, have also been explored. Nonetheless, 25% of CML patients are still expected to switch TKIs at least once, and various TKI discontinuation studies have shown a wide range in the incidence of molecular relapse (from 30% to 60%). In this review, we revisit the current knowledge regarding the role(s) of LSCs in CML leukemogenesis and response to pharmacological treatment and explore how durable treatment-free remission may be achieved and maintained after discontinuing TKI treatment.

IL-1 Via IRAK1/4 Sustains Acute Myeloid Leukemia Stem Cells Following Treatment and Relapse

Current treatment options for relapsed acute myeloid leukemia (AML) are limited and ineffective for the majority of patients. In AML, primitive leukemia stem cells (LSCs) and pre-leukemic populations are able to maintain the disease and drive relapse. Thus, therapies targeting LSC populations may increase the overall survival of AML patients. In this study, we aim to identify the drivers favoring LSC expansion following treatment and relapse and develop potential therapies for AML. The transcriptome analyses of 12 pairs of functionally defined LSC fractions at diagnosis and relapse revealed significant changes of IL-1 signaling in AML patients. We demonstrated that the protein expression levels of interleukin-1 receptor type I (IL1R1) and its complex member interleukin-1 receptor accessory protein (IL1RAP) were both up-regulated in human leukemia stem and progenitor cells (LSPCs) at diagnosis or in relapse compared to normal hematopoietic stem and progenitor cells (HSPCs). Knockdown of IL1R1 and IL1RAP suppressed the clonogenicity and engraftment growth of primary human AML cells but showed low impacts on HSPCs in the normal bone marrow. Additionally, knockout of IL1R1 in leukemia MLL-AF9 mice significantly reduced the LSC frequency and prolonged the overall survival rate. To target IL-1/TLR signaling in LSCs, we performed iterative structure-activity relationship (SAR) guided medicinal chemistry, in silico modeling and leukemia cell line reporter assays to screen and identify a novel interleukin-1 receptor-associated kinase 1/4 (IRAK1/4) inhibitor (termed UR241-2). UR241-2 robustly inhibits IL-1/TLR signaling in AML cells including the activation of NF-κB following IL-1 stimulation. UR241-2 repressed LSPC function as assessed by colony-forming unit assays in primary human AML cells at diagnosis and in relapse while minimally impacting normal HSPC function. Taken together, our findings demonstrate the important role of IL-1/TLR signaling in supporting AML LSC expansion following treatment and relapse and suggest that targeting IL-1/TLR signaling using the novel IRAK1/4 inhibitor, UR241-2, can target LSC function to improve patient outcomes in AML. Disclosures No relevant conflicts of interest to declare.

Imetelstat Significantly Reduces Leukemia Stem Cells in Patient-Derived Xenograft Models of Pediatric AML

Introduction Acute myeloid leukemia (AML) is the deadliest malignancy in children. Despite the use of maximally intensive therapy, 20% of patients experience recurrent disease. These patients are also burdened with significant treatment-related toxicities. To improve survival in pediatric AML, novel targeted therapies that are more effective and less toxic are needed. Telomerase inhibition has been shown to be effective in reducing leukemic burden and eradicating leukemia stem cells (LSCs) in syngeneic mouse models of AML and in patient-derived xenograft (PDX) models of adult AML (Bruedigam et al., 2014). Recent transcriptome analyses demonstrate that the genomic landscape of pediatric AML is distinct from adult AML (Bolouri et al., 2018). In fact, mutations in the telomerase complex components are infrequent in pediatric AML unlike adult AML patients (Aalbers et al., 2013). However, similar to what is seen in adult patients, Aalbers et al. identified that telomere lengths in pediatric AML cells were shortened compared to normal leukocytes, and pediatric AML patients with the shortest telomere length tend to have shorter overall survival. Furthermore, the 5-year survival rate was 88% for pediatric AML patients who had lower telomerase activity, and 43% for those patients with higher telomerase activity, suggesting telomerase activity could be an important prognostic factor in pediatric AML patients (Verstovsek et al., 2003). Imetelstat is an oligonucleotide that specifically binds with high affinity to the RNA template of telomerase and is a potent, competitive inhibitor of telomerase enzymatic activity (Asai et al., 2003; Herbert et al., 2005). In this study, we evaluated if imetelstat has anti-leukemia activity in pediatric AML PDX models. Results The PDX lines tested in this study were derived using samples from pediatric AML patients who were 1-14 years old, representing different FAB subtypes. Mouse passaged pediatric AML PDX lines (n=6) were treated ex vivo with imetelstat or mismatch oligo control and the viability of LSC (CD34+CD38low population) was determined at 48 or 96 h by staining with BV785-human CD45, APC-human CD34, Pacific blue-human CD38, FITC conjugated annexin V and propidium iodide (PI). Imetelstat treatment significantly increased apoptosis/death (PI+/annexin V+) of the LSC population in a dose-dependent manner in all PDX lines evaluated (Fig. 1A, B), while it had limited activity on LSCs in normal pediatric bone marrow samples (n=4). The efficacy of imetelstat either alone or in combination with chemotherapy or azacitidine was evaluated in two distinct PDX models of pediatric AML in vivo. Mice engrafted with both NTPL-377 and DF-2 lived longer when treated with imetelstat than the untreated mice (Fig. 1C, D, n=5 each, P<0.05). Mice receiving standard chemotherapy consisting of cytarabine and daunorubicin or azacitidine showed prolonged survival compared to the untreated mice. Interestingly, sequential administration of imetelstat following chemotherapy treatment provided additional benefit over chemotherapy alone (P<0.01). Concurrent treatment of azacitidine and imetelstat further extended survival of these mice compared to azacitidine alone (P<0.05). At the end of the in vivo studies, the percentage of LSC population was evaluated in the bone marrow of mice post euthanasia. There was a significant reduction of LSC population in mice treated with imetelstat compared to those treated with the mismatch oligo (Fig. 1E, F, P<0.05). Neither chemotherapy nor azacitidine alone affected LSC population compared to untreated mice. However, imetelstat significantly reduced the LSC population when combined with chemotherapy or azacitidine compared to single agent (P<0.05). These results were confirmed by secondary transplantation in mice, which showed delayed engraftment of cells isolated from imetelstat treated mice (Fig. 1G, H). Conclusions Imetelstat treatment of pediatric AML PDX samples showed significant dose- and time-dependent effects on the viability of the LSCs to induce cell apoptosis/death. These results were corroborated in vivo in two distinct PDX models which showed reduced LSC population and increased median survival in mice with imetelstat treatment. Combining imetelstat with chemotherapy or azacitidine further enhanced activity against LSCs, suggesting imetelstat could represent an effective therapeutic strategy for pediatric AML. Figure 1 Figure 1. Disclosures Barwe: Prelude Therapeutics: Research Funding. Huang: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Gopalakrishnapillai: Geron: Research Funding.

In Vivo CRISPR-Cas9 Screens in PDX Models Reveals ADAM10 As Novel Therapeutic Target in Acute Leukemia

Acute leukemias require more accurate and effective treatments, especially upon disease relapse. In search for novel therapeutic targets for acute leukemias, we established a pipeline for CRISPR-Cas9 mediated functional genomic screens which harbor the ability to elegantly increase our knowledge about vulnerabilities and gene dependencies. For a highly patient-related setting, we performed CRISPR knockout (KO) dropout screens in patient-derived xenograft (PDX) models in vivo, combining the advantages of studying an individual patient´s tumor cell in the physiologic in vivo bone marrow microenvironment. Serially transplantable PDX models were lentivirally transduced to stably express Cas9. A customized CRISPR-Cas9 library targeting about 100 genes addressing surface molecules was designed, cloned and transduced into two PDX models of acute lymphoblastic leukemia (ALL). Enriched PDX ALL cells were transplanted into NSG mice and grown until advanced disease stage. Input versus end stage cells were subjected to next generation sequencing, followed by data analysis using MAGeCK algorithm. Data analysis revealed commonly depleted as well as sample-specific depleted genes between the two PDX models tested, and CXCR4 and ITGB1 were the top commonly depleted genes from the screens. For target validation, single sgRNA were cloned into the knockout vector; concomitant expression of recombinant fluorochromes allowed competitive growth assays in PDX models in vivo, comparing cells with KO of interest versus control KO in the same animal. In vivo competitive assay showed that both PDX models clearly depended on both CXCR4 and ITGB1, validating an essential function for both genes in the two PDX ALL models. Of note, disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) was among the list of dropout genes in the screens. ADAM10 is known for its role in the central nervous system and considered as a therapeutic target in Alzheimer disease, but poorly studied in the context of leukemia. In competitive validation assays in vivo, ADAM10 KO population showed a clear growth disadvantage compared to control KO cells in a number of PDX models of ALL, but also acute myeloid leukemia (AML); in some PDX models, ADAM10 KO cells were completely abrogated, indicating that ADAM10 plays an essential role for different types of acute leukemias and represents a yet unknown vulnerability. To better characterize the role of ADAM10 for the clinical situation, we performed further in vivo assays with PDX models. Re-expression of ADAM10 in ADAM10 KO PDX cells could partially rescue the phenotype in an in vivo competitive reconstitution assay, unequivocally proving ADAM10 essentiality in ALL cells. Interestingly, a similar rescue assay expressing a ADAM10 variant lacking the disintegrin domain resulted in the same phenotypical compensation, highlighting essentiality of the enzymatic but not adhesion domain of ADAM10 in tumor engraftment and growth in BM. Important for translating the molecular insights into clinical use, PDX cells treated with an ADAM10 chemical inhibitor ex vivo, showed reduced tumor engraftment capacity compared to the vehicle treated cells, suggesting a role for ADAM10 in tumor-niche interactions and homing to the bone marrow. Further, we performed limiting dilution transplantation assays to determine stem cell frequencies; ADAM10 loss resulted in reduced stemness and a reduced number of leukemia-initiating cells compared to control KO cells, indicating that ADAM10 is essential also in leukemia stem cells. Of clinical relevance, ADAM10 KO significantly sensitized leukemic cells towards treatment of mice with the routine chemotherapeutic drug Cyclophosphamide in vivo, suggesting a putative synergistic effect when addressing ADAM10 as a therapeutic target. In summary, our data revealed ADAM10 as attractive novel vulnerability in acute leukemias with essential function for the tumor-niche interaction, leukemia stem cells and anti-leukemia treatment. ADAM10 might be addressed as therapeutic target to treat acute leukemias in the future. Disclosures No relevant conflicts of interest to declare.

Inhibition of De Novo Pyrimidine Synthesis Depletes Acute Myleogenous Leukemia Stem Cells (LSCs) Burden and Triggers Apoptosis and Differentiation Associated with Oxidative Stress

Background: De novo nucleotide synthesis is necessary to meet the enormous demand for nucleotides, other macromolecules associated with acute myeloid leukemia (AML) progression 1, 2, 3 4. Hence, we hypothesized that targeting de novo nucleotide synthesis would lead to the depletion of the nucleotide pool, pyrimidine starvation and increase oxidative stress preferentially in leukemic cells compared to their non-malignant counterparts, impacting proliferative and differentiation pathways. Emvododstat (PTC299) is an inhibitor of dihydroorotate dehydrogenase (DHODH), a rate-limiting enzyme for de novo pyrimidine nucleotide synthesis that is currently in a clinical trial for the treatment of AML. Objectives: The goals of these studies were to understand the emvododstat-mediated effects on leukemia growth, differentiation and impact on Leukemia Stem Cells(LSCs). Comprehensive analyses of mitochondrial function, metabolic signaling in PI3K/AKT pathways, apoptotic signatures, and DNA damage responses were carried out. The rationale for clinical testing emvododstat was confirmed in an AML-PDX model. Results: Emvododstat treatment in cytarabine-resistant AML cells and primary AML blasts induced apoptosis, differentiation, and reduced proliferation, with corresponding decreased in cell number and increases in annexin V- and CD14-positive cells. Indeed, the inhibition of de novo nucleotide synthesis compromises the dynamic metabolic landscape and mitochondrial function, as indicated by alterations in the oxygen consumption rate (OCR) and mitochondrial ROS/membrane potential and corresponding differentiation, apoptosis, and/or inhibition of proliferation of LSCs. These effects can be reversed by the addition of exogenous uridine and orotate. Further immunoblotting and mass cytometry (CyTOF) analyses demonstrated changes in apoptotic and cell signaling proteins (cleaved PARP, cleaved caspase-3) and DNA damage responses (TP53, γH2AX) and PI3/AKT pathway downregulation in response to emvododstat. Importantly, emvododstat treatment reduced leukemic cell burden in a mouse model of AML PDX ( Complex karyotype ,mutation in ASXL1, IDH2, NRAS), decreased levels of leukemia stem cells frequency (1 in 522,460 Vs 1 in 3,623,599 in vehicle vs emvododstat treated mice), and improved survival. The median survival 40 days vs. 30 days, P=0.0002 in primary transplantation and 36 days vs 53.5 days, P=0.005 in secondary transpantation in a PDX mouse model of human AML. This corresponded with a reduction in the bone marrow burden of leukemia and increased expression of differentiation markers in mice treated with emvododstat (Fig. 1). These data demonstrate effect of emvododstat on mitochondrial functions . Conclusion: Inhibition of de novo pyrimidine synthesis triggers differentiation, apoptosis, and depletes LSCs in AML models. Emvododstat is a novel dihydroorotate dehydrogenase inhibitor being tested in a clinical trial for the treatment of myeloid malignancies and COVID-19. Keywords: AML, emvododstat, DHODH, apoptosis, differentiation References: 1 Thomas, D. & Majeti, R. Biology and relevance of human acute myeloid leukemia stem cells. Blood 129, 1577-1585, doi:10.1182/blood-2016-10-696054 (2017). 2 Quek, L. et al. Genetically distinct leukemic stem cells in human CD34- acute myeloid leukemia are arrested at a hemopoietic precursor-like stage. The Journal of experimental medicine 213, 1513-1535, doi:10.1084/jem.20151775 (2016). 3 Villa, E., Ali, E. S., Sahu, U. & Ben-Sahra, I. Cancer Cells Tune the Signaling Pathways to Empower de Novo Synthesis of Nucleotides. Cancers (Basel) 11, doi:10.3390/cancers11050688 (2019). 4 DeBerardinis, R. J. & Chandel, N. S. Fundamentals of cancer metabolism. Sci Adv 2, e1600200, doi:10.1126/sciadv.1600200 (2016). Figure 1 Figure 1. Disclosures Weetall: PTC therapeutics: Current Employment. Sheedy: PTC therapeutics: Current Employment. Ray: PTC therapeutics: Current Employment. Andreeff: Karyopharm: Research Funding; AstraZeneca: Research Funding; Oxford Biomedica UK: Research Funding; Aptose: Consultancy; Daiichi-Sankyo: Consultancy, Research Funding; Syndax: Consultancy; Breast Cancer Research Foundation: Research Funding; Reata, Aptose, Eutropics, SentiBio; Chimerix, Oncolyze: Current holder of individual stocks in a privately-held company; Novartis, Cancer UK; Leukemia & Lymphoma Society (LLS), German Research Council; NCI-RDCRN (Rare Disease Clin Network), CLL Foundation; Novartis: Membership on an entity's Board of Directors or advisory committees; Senti-Bio: Consultancy; Medicxi: Consultancy; ONO Pharmaceuticals: Research Funding; Amgen: Research Funding; Glycomimetics: Consultancy. Borthakur: ArgenX: Membership on an entity's Board of Directors or advisory committees; Protagonist: Consultancy; Astex: Research Funding; University of Texas MD Anderson Cancer Center: Current Employment; Ryvu: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; GSK: Consultancy.

Leukemia Stem Cells Depend on Regulated Protein Synthesis

Acute myeloid leukemia (AML) is initiated and sustained by leukemia stem cells (LSCs) which arise from progenitor cells that do not usually self-renew but become aberrantly self-renewing. It is thought that LSCs gain aberrant self-renewal potential by co-opting molecular and cellular programs from hematopoietic stem cells (HSCs) (PMID: 16862118). HSCs have been shown to require tightly regulated protein synthesis rates, where increased or decreased protein synthesis impairs self-renewal (PMID: 24670665), but it is not known if LSCs share this dependence. We have shown that human LSCs reside in the population of AML cells with the highest levels of CD99 (PMID: 28123069). In RNA-sequencing studies, we found that LSCs with high levels of CD99 are depleted for ribosomal protein transcripts. We thus reasoned that similar to HSCs, LSCs may depend on tightly regulated protein synthesis to self-renew. To test if CD99 promotes LSC function by constraining protein synthesis, we transduced c-Kit+ cells from B6-CD99 Gt(pU-21T)44lmeg (CD99 KO) or wild-type (WT) mice to express AML1-ETO9a (AE9a) and transplanted them into WT mice treated with rapamycin or vehicle. There was no difference in leukemogenesis in primary recipients, but CD99 KO-AE9a AMLs exhibited a 72% (p=0.048) increase in protein synthesis compared with WT-AE9a AMLs (Figure 1A), confirming that CD99 negatively regulates protein synthesis in AML. We next performed secondary transplants to assess LSC function, as measured by survival of secondary recipients in the absence of rapamycin treatment (Figure 1B). We furthermore performed these transplants at limiting dilution to quantify LSCs (Figure 1C). CD99 KO-AE9a vehicle treated AMLs demonstrated improved survival and a lower LSC frequency compared with WT-AE9a vehicle treated AMLs, consistent with a self-renewal defect with loss of CD99. Rapamycin treatment completely rescued this defect, leading to decreased survival and increased LSC frequency in CD99 KO-AE9a AMLs compared with vehicle. Conversely, rapamycin treatment depleted LSCs in WT-AE9a AMLs, increasing survival and decreasing LSC frequency compared with vehicle. Thus, similar to HSCs, LSCs are adversely affected by both increases or decreases in protein synthesis. MLL-AF9-induced mouse AMLs initiated in HSCs as compared with granulocyte macrophage progenitors (GMPs) exhibit increased epigenetic imprinting of HSC features resulting in disease features reminiscent of high-risk AML (PMID: 23235717). To test if HSC-derived leukemias exhibit increased dependence on regulated protein synthesis, we transduced HSCs or GMPs from CD99 KO or WT mice to express MLL-AF9 and transplanted them into WT recipients, followed by secondary transplants to assess LSC function. Loss of CD99 led to increased survival indicative of decreased LSC function in HSC-derived but not GMP-derived leukemias (Figure 1D). This suggests that HSC-derived leukemias co-opt from HSCs a more pronounced dependence on tightly regulated protein synthesis. Accordingly, WT HSC-derived leukemias exhibited decreased protein synthesis as compared with their WT GMP-derived counterparts (Figure 1E), as well as increased sensitivity to rapamycin (Figure 1F). To directly study protein synthesis in human LSCs, we transduced primary AML specimens (n=4) to express a destabilized form of GFP (dGFP) from a constitutive promoter followed by xenotransplantation (Figure 1G), allowing us to measure dGFP by flow cytometry as a surrogate for protein synthesis rates in vivo. We validated this assay by measuring protein synthesis using orthogonal O-propargyl-puromycin incorporation assays (Figure 1H). Human AML cells with low levels of dGFP demonstrated increased engraftment in secondary transplants (Figure 1I), demonstrating that human LSCs exhibit low protein synthesis rates. In conclusion, our data demonstrate that LSCs co-opt from HSCs a dependence on tightly regulated protein synthesis. This is the first description of a cellular feature co-opted from HSCs that also represents a therapeutic vulnerability. Furthermore, the types of AML that exhibit the most robust re-activation of HSC programs and increased dependence on regulated protein synthesis are also likely to represent high-risk AMLs most resistant to standard therapies. Our data suggest that such therapy resistant AMLs may be highly sensitive to strategies disrupting protein synthesis to deplete LSCs. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Immunotherapy Targeting ST2/IL-33 Signaling in Myeloid Leukemia Stem Cells

Therapies for acute myeloid leukemia (AML) has barely changed over 30 years, while treatment for other blood cancers have made remarkable leaps forward. Recent advances in genomics have allowed molecular targeted therapies (i.e. FLT3-ITD, IDH, c-KIT inhibitors) extending survival, but most patients still succumb (Burd et al. Nat Med 2020). Therefore, developing more efficient, less toxic, and immune-based therapies for AML is an urgent unmet need. Previous studies showed that stromal cell-derived IL-33 stimulates myeloproliferative neoplasms (Mager et al. J Clin Invest 2015). Stimulation-2 (ST2), IL-33 receptor, contributes to leukemia stem cells (LSCs) survival in Cbfb-MYH11 mice (Wang et al . Sci Rep 2019). We showed that ST2 blockade enhanced graft versus leukemia activity against MLL-AF9 egfp AML after hematopoietic cell transplantation (Zhang et al . Sci Transl Med 2015). We and others, also, found that ST2 is expressed on normal murine and human hematopoietic stem cells (HSCs), respectively (Capitano et al. Blood Cells Mol Dis 2020; Alt et al. Biol Blood Marrow Transplant 2019). These data suggest a leukemia-promoting role of ST2/IL-33 signaling. To determine clinical relevance of ST2 in AML, we generated Kaplan-Meier curves using TCGA (n=173) and TARGET AML (n=187) databases. Decreased survival was observed in patients with high IL1RL1 (ST2 gene) which was validated in an independent database (AMLCG 1999 trial, n=417) (Fig. 1A). Since ST2 is expressed on HSCs, we interrogated if ST2 is expressed on LSCs defined as CD34 +CD38 - in the Princess Margaret Leukemia biobank (n=192), and found ST2 is higher on LSCs vs CD34 -CD38 +/- cells (Fig. 1B). We then sought to analyze ST2 on bone marrow samples comparing complete responders vs refractory patients to note that ST2 expression was increased in refractory patients' LSCs (Fig. 1C). To scrutinize the role of ST2 in initiating leukemogenesis, we performed limiting dilution transplantation using 500, 200, and 50 Lin -Sca-1 +c-KIT +-sorted LSCs from WT vs ST2 -/- MLL-AF9 egfp transduced cells. Frequency of LSCs in ST2 -/- cells was decreased by ~15-fold as opposed to WT cells [1:2141 (1:546-1:8405) vs 1:145 (1:75-1:283), p=3.37e-05] (Fig. 2A). We also tested leukemia maintenance, secondary transplantations from the primary recipients resulted in leukemia growth delay in ST2 -/- vs WT cells which was confirmed in tertiary transplantations (Fig. 2B-E). Self-renewal ability of LSCs is correlated to reactive oxygen species (ROS) (Testa et al. Exp Hematol 2016), and we found that ROS levels in ST2 -/- leukemic cells are markedly diminished in contrast to WT leukemic cells (Fig. 2F). ST2 deficiency in leukemic cells arrests G2/S/M cell cycle progression in LSCs (Fig. 2G-J). These data indicated that ST2 is indispensable for initiating and maintaining LSCs in MLL-AF9 AML. We next developed murine and human Fc-silenced-bispecific antibodies engaging mouse or human ST2 and CD3 (BsAb) built on the IgG[L]-scFv platform with proven ability to drive T cells into human tumors for effective tumor ablation (Santich et al. Sci Transl Med 2020; Park et al. J Immunother Cancer 2021) (Fig. 3A, 3E). Both BsAbs showed >90% purity by HPLC, stability under heat stress and low endotoxin. Animals did not exhibit any in vivo toxicity at BsAb doses of 0.4, 2, 5, 10 μg ip q 3 days x 6 doses (not shown). In the immunocompetent MLL-AF9 mice, murine anti-ST2 BsAb (BC281) treatment (10 μg i.p, 4 days post-AML challenge and given every three days for a total of 6 injections) resulted in extended survival compared to isotype control mice (Fig. 3B). Leukemic cells and LSCs were accordingly decreased in treated vs control group (Fig. 3C-D). We modeled humanized leukemic mice with MOLM-14 egfp cells and weekly injection of human CD8 + T cells in NOD.Cg-Prkdc scid Il2rg tm1Wjl/SzJ (NSG) mice (Fig. 3F). Animals treated with human anti-ST2 BsAb (BC282), using a similar regimen as for the immunocompetent model, led to better survival when compared to animals treated with mutated non-functional anti-ST2 BsAb (BC283) (Fig. 3G). Frequency of MOLM-14 egfp cells was lower in the BC282 vs BC283 group (Fig. 3H). These results suggested that anti-ST2 BsAbs can inhibit AML growth to improve survival. We concluded that ST2 is a potential therapeutic target, and ST2-specific T cell engaging BsAbs represent promising immunotherapeutics for AML. Figure 1 Figure 1. Disclosures Cheung: Medical University of South Carolina: Patents & Royalties: inventor on the ST2 bispecific antibody patent application; Y-mabs Therapeutics and Abpro-Labs Inc: Patents & Royalties: inventor on multiple patents filed by MSK, including those licensed to Ymabs Therapeutics, Biotec Pharmacon, and Abpro-labs; Eureka Therapeutics: Membership on an entity's Board of Directors or advisory committees. Paczesny: Medical University of South Carolina: Patents & Royalties: inventor on the ST2 bispecific antibody patent application.