Acute Myeloid Leukemia Engraftment Research Articles

Targeting IL-1/IRAK1/4 signaling in Acute Myeloid Leukemia Stem Cells Following Treatment and Relapse.

Therapies for acute myeloid leukemia (AML) face formidable challenges due to relapse, often driven by leukemia stem cells (LSCs). Strategies targeting LSCs hold promise for enhancing outcomes, yet paired comparisons of functionally defined LSCs at diagnosis and relapse remain underexplored. We present transcriptome analyses of functionally defined LSC populations at diagnosis and relapse, revealing significant alterations in IL-1 signaling. Interleukin-1 receptor type I (IL1R1) and interleukin-1 receptor accessory protein (IL1RAP) were notably upregulated in leukemia stem and progenitor cells at both diagnosis and relapse. Knockdown of IL1R1 and IL1RAP reduced the clonogenicity and/or engraftment of primary human AML cells. In leukemic MLL-AF9 mice, Il1r1 knockout reduced LSC frequency and extended survival. To target IL-1 signaling at both diagnosis and relapse, we developed UR241-2, a novel interleukin-1 receptor-associated kinase 1 and 4 (IRAK1/4) inhibitor. UR241-2 robustly suppressed IL-1/IRAK1/4 signaling, including NF-κB activation and phosphorylation of p65 and p38, following IL-1 stimulation. UR241-2 selectively inhibited LSC clonogenicity in primary human AML cells at both diagnosis and relapse, while sparing normal hematopoietic stem and progenitor cells. It also reduced AML engraftment in leukemic mice. Our findings highlight the therapeutic potential of UR241-2 in targeting IL-1/IRAK1/4 signaling to eradicate LSCs and improve AML outcomes.

Genetic Determinants of Dependency on Regulated Protein Synthesis in Leukemia Stem Cells

Acute myeloid leukemia (AML) is initiated and sustained by rare self-renewing leukemia stem cells (LSCs). LSCs do not resemble HSCs, but rather more mature hematopoietic progenitor cells that usually have no self-renewal potential. However, during leukemogenesis, LSCs become aberrantly self-renewing, but the mechanisms that allow for this are poorly understood. We recently used models of AML driven by AML-ETO9a and MLL-AF9 to show that LSCs may co-opt from HSCs a dependence on regulation of protein synthesis mediated by CD99 (ASH 2021, #2222), which we identified as a therapeutic target highly expressed in LSCs (PMID: 28123069). However, it remains unclear if LSCs from specific subtypes of AML are particularly dependent on regulated protein synthesis. In an analysis of transcriptome data from the ECOG 1900 cohort, we found that certain mutations ( KIT, PTEN, AML-ETO, IDH1/2 and FLT3-ITD) were associated with the highest levels of CD99, raising the possibility that such AML subtypes are more dependent on CD99. In the TCGA AML cohort, CD99 expression was significantly higher in AML patients with FLT3-ITD mutations but not NRAS mutations ( Fig. 1A). To test if LSCs from AMLs driven by mutations in FLT3 or NRAS differ in their dependence on regulation of protein synthesis by CD99, we crossed Cd99 -/-(CD99 KO) mice with two mouse models of AML driven by these alleles to generate Cd99 -/-; Mx1-Cre; Tet2 fl/fl; Flt3 ITD/+and Cd99 -/-; Mx1-Cre; Tet2 fl/fl; Nras LSL-G12D/+ mice (CD99 KO TET2/FLT3 and CD99 KO TET2/NRAS). TET2 was chosen as a cooperating allele given its neutral association with CD99 in the above human datasets, and it additionally co-occurs frequently with mutations in FLT3 or NRAS. To generate AML in both models, Tet2 was deleted with polyinosine-polycytidylic acid injections, followed by transplant of 4 million bone marrow (BM) cells into irradiated wild-type (WT) mice. At 16 weeks, recipients of CD99 WT TET2/FLT3 BM developed an expansion of aberrant donor-derived Mac-1 + c-kit + cells in the peripheral blood (PB), and BM aspirates revealed an accumulation of LSK CD48 +CD150 - cells that harbor LSC activity in this model ( Fig. 1B). In contrast, recipients of CD99 KO TET2/FLT3 BM exhibited significant depletion of these aberrant populations (P=0.0055 and P=0.019 respectively, n=10 for each group). Recipients of CD99 WT TET2/FLT3 BM also exhibited increased leukocytosis and thrombocytopenia, which were attenuated in recipients of CD99 KO TET2/FLT3 BM (P=0.046 and P=0.042, respectively). In contrast, there was no difference between recipients of CD99 WT TET2/NRAS and CD99 KO TET2/NRAS BM in emergence of the aberrant Mac-1 + c-kit + cells and thrombocytopenia (P=0.3237 and P=0.9752, respectively, n=10 for each group). These data suggest that loss of CD99 may impair leukemogenesis in TET2/FLT3 but not TET2/NRAS AMLs. To determine if CD99 is required for LSC function, we performed secondary transplants of one million BM cells from primary recipients into lethally irradiated WT recipients. Mice transplanted with CD99 KO TET2/FLT3 BM, compared with those transplanted with CD99 WT TET2/FLT3 BM, demonstrated fewer donor-derived cells in the PB. While CD99 WT TET2/FLT3-transplanted mice succumbed to AML at a median of 11 weeks, all CD99 KO TET2/FLT3-transplanted mice survived beyond 16 weeks. PB smears from CD99 KO TET2/FLT3 mice demonstrated leukocytes with condensed chromatin and lobulated nuclei, indicative of restored hematopoietic maturation. Finally, in vivo O-propargyl-puromycin assays showed that loss of CD99 resulted in increased protein synthesis in LSCs (LSK CD48 +CD150 -) (fold-change=1.69±0.18, n=3 for each group). In contrast, mice secondarily transplanted with CD99 KO TET2/NRAS BM exhibited similar engraftment, survival, and morphologic evidence of AML as those transplanted with CD99 WT TET2/NRAS BM. In conclusion, our data demonstrate that LSC function in AML driven by mutated FLT3, but not mutated NRAS, is dependent on maintenance of regulated protein synthesis by CD99. We speculate that this differential dependency may reflect a different cell-of-origin, as recent work has shown that AMLs associated with FLT3-ITD exhibit particularly primitive cellular hierarchies (PMID: 35618837). Finally, our results suggest that certain high-risk AML sub-types associated with elevated CD99 expression may be highly sensitive to strategies disrupting protein synthesis to deplete LSCs.

A Necroptosis Inhibitor Functions As a Ferroptosis Inducer in Drug-Resistant Myeloid Malignancies

Introduction: Acute myeloid leukemia (AML) is associated with poor prognosis, especially in elderly patients unfit for intensive chemotherapy. AML chemotherapy agents, including venetoclax, a BH3-mimetic, are designed to induce cell death through apoptosis. Unfortunately, AML cells eventually develop a resistance to BCL-2-dependent apoptosis which causes clinical disease recurrence. Alternative cell death pathways like ferroptosis and necroptosis are lytic forms of cell death mediated by plasma membrane lipid oxidation or MLKL phosphorylation (p-MLKL), respectively, both of which disrupt plasma membrane integrity. Because apoptosis and ferroptosis involve different molecular pathways, ferroptosis-inducing therapies provide an opportunity to overcome therapy resistance in AML patients. Here, we show how necroptosis inhibitor targeting basal necroptosis signal selectively induces ferroptosis in venetoclax-resistant AML cells causing cell death. Methods: We established venetoclax-resistant myeloid cell lines by exposing cell lines to increasing doses of venetoclax. We evaluated resistance to venetoclax-induced apoptosis by measuring IC50 after Annexin-V/PI staining. We screened venetoclax-resistant myeloid cell lines and primary AML cells (n=77) for basal activation of necroptosis by pMLKL or phosphorylated RIP3 (pRIP3) western blotting. Xenotransplantation was performed to evaluate the bone marrow engraftment potentials of basal activated necroptosis primary AMLs. Necrosome inhibitor (Zharp-99) was tested to estimate IC50 for cell death induction in myeloid cell lines after Annexin-V/PI staining. Ferroptosis inhibitors (ferrostatin-1 and lipoxstatin-1) and apoptosis inhibitor (zVAD-FMK) were used to determine which death pathway was induced by necroptosis inhibitors in AML. Results: We established venetoclax-resistant myeloid cell lines (THP1-VR and HL60-VR), which showed a significant shift in IC50 for venetoclax-induced apoptosis compared to parent cell lines. Venetoclax-resistant cell lines acquired an upregulation of known anti-apoptotic proteins; BCL-2, BCL-XL, and MCL-1. Venetoclax-resistant cell lines upregulated pRIP3 signal, a known necroptosis driver, by western blot. Necrosome screening of primary AML samples revealed 73 AMLs (94%) showed basal necroptosis signal confirmed by pMLKL with heterogeneous intensities (Figure. A). Xenotransplantation confirmed engraftment of basal necroptosis-activated primary AMLs is achievable and with retention of necroptosis signal at engraftment. The basal necroptosis signal in venetoclax-resistant cell lines and primary AML samples drove us to evaluate necroptosis inhibitor Zharp-99 (RIP3K inhibitor) IC50 for venetoclax-resistant cell lines. Compared to parent cell lines, venetoclax-resistant cell lines became susceptible to cell death by necrosome inhibition. Surprisingly, the cell death induced by Zharp-99 was not from apoptosis but through ferroptosis, confirmed by the venetoclax-resistant myeloid cells being rescued from cell death by ferroptosis inhibitor (ferrostatin-1 and lipoxstatin-1) (Figure B). Conclusion: Basal activation of necrosome is observed in venetoclax-resistant myeloid cell lines and primary AML patient samples. We found necroptosis inhibitor Zharp-99 promoted cell death in necrosome-activated venetoclax-resistant cell lines. Our data suggest necroptosis inhibition targeting RIP3K activity has a novel function of inducing ferroptosis in myeloid malignancies resistant to apoptosis. RIP3K inhibitor, as a novel function of ferroptosis inducer in apoptosis-resistant myeloid cell lines, opens a new therapy opportunity to treat apoptosis resistance in AML patients. We plan to investigate in vivo effects of necrosome inhibitors using our xenotransplant model.

Calibrated Release IL15 Bivalent CD33 and/or FLT3 and NOT Emcn Logic Gated Gene Circuit CAR-NK Cell Therapy (SENTI-202) in Venetoclax Resistant Patient Derived Xenograft Acute Myeloid Leukemia Models

Venetoclax (VEN) as combination therapy has improved response rates and overall survival of patients with acute myeloid leukemia (AML). However, once AML relapse prognosis is dire with a 5.3 month median survival (Brandwein, 2020). While chimeric antigen receptor (CAR) cell therapies have revolutionized the treatment landscape for lymphoid hematological malignancies, development of such therapies for AML has been challenging, in part due to the heterogeneity of the disease. Currently, there is a paucity of individual AML targets that are consistently expressed across AML subpopulations. Furthermore, the expression of these AML targets is not restricted to tumor cell populations, often resulting in off-tumor toxicity against healthy cell populations. SENTI-202 employs OR and NOT Logic Gating to overcome these challenges. SENTI-202 represents an innovative preclinical CAR-NK cell therapy, engineered to exploit a powerful CD33 OR FLT3 NOT EMCN Logic Gate gene circuit, in conjunction with calibrated release IL-15 (crIL15) expression. The CD33 OR FLT3 (OR GATE) activating CAR (aCAR) concurrently targets two AML antigens, thus permitting a broader therapeutic window against AML LSCs (CD33+/-, FLT3+), blasts (CD33+, FLT3+/-), and more specifically, VEN-resistant (VEN-r) AML with a CD33+ monocytic phenotype (Pei, 2020). The NOT EMCN (NOT GATE) inhibitory CAR (iCAR) is a pivotal safeguard, selectively shielding healthy EMCN+ Hematopoietic Stem Cells and Progenitor Cells (HSCs/PCs) from potential off-tumor toxicity. Previously, SENTI-202 exhibited remarkable efficacy in targeting and eliminating CD33 and/or FLT3 expressing AML cell lines, while concurrently safeguarding healthy HSCs/PCs (Garrison et al., ASH, 2022). To further validate the efficacy and broaden the utility of SENTI-202, we conducted assessments on clinically relevant preclinical AML models, along with VEN-r AML patient-derived xenograft (PDX) models. We first assessed the in vitro cytotoxicity and cytokine profiles of CAR-NK cells possessing the SENTI-202 OR Gate and crIL15 against primary R/R AML patient samples (n=11), where we observed significantly higher cytotoxicity compared to unengineered NK-cells (68% vs 19.7%) (p=0.0005, E:T ratio 1:1 for 24h) and increased TNF-a and IFN-g secretion. To further explore CAR-NK cell anti-tumor activity, we screened 10 different PDX samples and discovered that while the median frequency of CD33 and/or FLT3 expression was 99.9% (97.48-100), individual CD33 and FLT3 expression level varied, underscoring AML proteomic heterogeneity and the clinical need for OR Gate approaches. Our initial PDX studies used clinically relevant VEN/HMA-r AML PDX cells (designated as PDX1) obtained from a patient with FLT3-ITD, GATA2, and NRAS mutations, who initially responded to VEN/decitabine treatment but later relapsed. To facilitate in vivo tracking of AML and disease progression, the VEN-r PDX cells were engineered with Akaluc, an exceptionally brighter luciferase compared to conventional luciferase. Following confirmation of AML engraftment in bone marrow through bioluminescence imaging (BLI), the CAR-NK cells, along with unengineered NK-cells, were infused at a dose of 30 million cells per infusion, once a week for three consecutive weeks. The persistence of the CAR-NK cells was detectable for up to 2 weeks after the last infusion. Notably, the infusion of CAR-NK cells resulted in the clearance of bone marrow (BM) disease as evident from BLI one week after the second infusion of CAR-NK cells while unengineered NK-cells failed to clear bone marrow disease. Assessment of CAR-NK cell efficacy in two different AML PDX models is ongoing, along with additional in vitro and in vivo studies that further demonstrate the ability of the SENTI-202 NOT EMCN Logic Gate to significantly protect healthy HSCs from off-tumor toxicity. In conclusion, CAR-NK cells expressing the SENTI-202 OR Gate and crIL15 demonstrate robust anti-tumor activity against primary AML cells within in vitro cytotoxicity assays and in vivo AML PDX models, including VEN-r AML. These promising preclinical results highlight the potential of SENTI-202 as a targeted and selective therapy for AML, offering a new avenue for overcoming the challenges posed by the heterogeneity of the AML proteomic landscape.

Developing a Humanized Murine Model for Co-Transplantation of Acute Myeloid Leukemia

Humanized mouse models and primary disease xenografts have produced significant translational insight into the biology of the bone marrow (BM) niche and aided drug discovery in acute myeloid leukemia (AML). To enable the study of cell-cell interactions between hematopoietic stem and progenitor cells (HSPCs) and AML as an emerging source of drug resistance, we describe two complementary approaches to humanized AML xenotransplantation models. One strategy utilizes the sequential transplantation of humanized hCD34+ cells followed by human AML cells. Recipient NBSGW mice (NOD.Cg-Kit W-41J Tyr + Prkdcscid Il2rgtm1Wjl/ThomJ), known for high rates of medullary engraftment, were conditioned with 15mg/kg busulfan followed by injection with either granulocyte colony-stimulating (G-CSF)-mobilized or BM-derived human CD34+ HSPCs. Flow cytometric analysis of the BM following injection of hCD34+ cells revealed efficient BM engraftment by both G-CSF- and BM- derived hCD34+ cells, demonstrated by BM human chimerism (hCD45%) of 63.7±10.0% by 8-weeks post-injection. Further BM hCD45 subtype analyses revealed B-cell (hCD45+ hCD19+) dominant repopulation (78.3±4.48%), followed by myeloid cell (hCD45+ hCD33+) repopulation (18.12±4.19%). T-cell repopulation was predictably minimal (0.008±0.01%). At 9- and 15- weeks post hCD34+ cells injection, GFP-expressing MOLM-14 AML cells (MOLM-14-GFP; 1E5, 1E6, or 2E6 cells) were injected. Flow cytometric analyses of MOLM-14-GFP BM chimerism up to 21 days post-injection showed complete absence of MOLM-14-GFP cells in the BM with no significant changes in overall hCD45 chimerism and hCD45 subtype composition compared to a PBS-injected control. These observations are consistent with a potential role for residual healthy hematopoietic cells in suppressing AML engraftment in the BM, previously shown in congenic studies. To test this hypothesis, we developed a second humanized AML model designed to investigate immunotherapeutic approaches for AML, where AML blasts are engrafted first, then followed by a human immune cell challenge. FA-AML1 or Kasumi-1 AML cells were injected into NSG mice (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ) at 8 weeks of age. Injected mice were monitored for AML engraftment by HLA-DR expression in peripheral blood (PB). Engraftment occurred 6-12 weeks post-injection. After AML engraftment, we reconstituted a human immune system in the mice with a one-time injection of peripheral blood mononuclear cells (PBMCs), confirmed by flow cytometry for CD3+ T-cells (14.7±9.2%), CD33+ myeloid cells, and CD19+ B-cells. AML burden in the PB decreased 5-10% upon PBMC engraftment. In this model, a PB burden of <10% AML engraftment confers durable leukemic remission, whereas at >60% AML engraftment, leukemic cells persist in the periphery for over 100 days. Flow cytometric analysis demonstrated immune cell numbers peak in week 1 post-injection (30%), then decrease by week 4 to less than 1% in both the BM and PB, but persist at this low level for over 100 days. Altogether, these models confirm the ability of human hematopoietic cells to actively suppress leukemic engraftment in the BM microenvironment. More broadly, we demonstrate the development of two complementary humanized AML co-transplantation models that can attain dynamic ranges of human immune cell and AML cell engraftment to facilitate preclinical therapeutic development studies.

METTL3 mediates chemoresistance by enhancing AML homing and engraftment via ITGA4

Chemoresistant leukemia relapse is one of the most common causes of death for acute myeloid leukemia (AML) patients and the homing/engraftment in bone marrow (BM) are crucial steps for AML cells to acquire chemoresistance by interacting with stromal cell components. No crosstalk between m6A modification and homing/engraftment has been reported. Here, we performed comprehensive high-throughput analyses, including RNA sequencing of CR (complete remission) and relapsed AML patients, and reverse-phase protein arrays of chemoresistant cells to identify METTL3 as a key player regulating AML chemoresistance. Then, METTL3-mediated m6A modification was proved to induce the chemoresistance in vitro and in vivo. Furthermore, AML homing/engraftment was discovered being enhanced by upregulated-METTL3 in chemoresistant cells. And the homing/engraftment and drug-resistance associated phenotypes of chemoresistant cells could be reversed by a METTL3 inhibitor. Mechanistically, METTL3 extended the half-life of ITGA4 mRNA by m6A methylation, and then, increased expression of ITGA4 protein to enhance homing/engraftment of AML cells. The results provide insights into the function of m6A modification on the interaction between AML cells and BM niches and clarify the relationship between METTL3 and AML homing/engraftment, suggesting a therapeutic strategy for the treatment of refractory/relapsed AML with METTL3 inhibitors.

Homoharringtonine is synergistically lethal with BCL-2 inhibitor APG-2575 in acute myeloid leukemia

BackgroundDespite advances in targeted agent development, effective treatment of acute myeloid leukemia (AML) remains a major clinical challenge. The B-cell lymphoma-2 (BCL-2) inhibitor exhibited promising clinical activity in AML, acute lymphoblastic leukemia (ALL) and diffuse large B-cell lymphoma (DLBCL) treatment. APG-2575 is a novel BCL-2 selective inhibitor, which has demonstrated anti-tumor activity in hematologic malignancies. Homoharringtonine (HHT), an alkaloid, exhibited anti-AML activity.MethodsThe synergistic effects of APG-2575 and HHT were studied in AML cell lines and primary samples. MTS was used to measure the cell viability. Annexin V/propidium iodide staining was used to measure the apoptosis rate by flow cytometry. AML cell xenografted mouse models were established to evaluate the anti-leukemic effect of BCL-2 inhibitor, HHT and their combination in vivo. Western blot was used to determine the expression of related proteins.ResultsAPG-2575 showed comparable anti-leukemic effect to the FDA-approved BCL-2 inhibitor ABT-199 in vitro and in vivo. Combined treatment of HHT with APG-2575 synergistically inhibited AML cell growth and engraftment. Mechanistically, HHT promoted degradation of myeloid cell leukemia-1 (MCL-1), which was reported to induce BCL-2 inhibitor resistant, through the PI3K/AKT/GSK3β signaling pathway.ConclusionOur results provide an effective AML treatment strategy through combination of APG-2575 and HHT, which is worthy of further clinical research.

Release of IFNγ by Acute Myeloid Leukemia Cells Remodels Bone Marrow Immune Microenvironment by Inducing Regulatory T Cells.

The stromal and immune bone marrow (BM) landscape is emerging as a crucial determinant for acute myeloid leukemia (AML). Regulatory T cells (Treg) are enriched in the AML microenvironment, but the underlying mechanisms are poorly elucidated. Here, we addressed the effect of IFNγ released by AML cells in BM Treg induction and its impact on AML prognosis. BM aspirates from patients with AML were subdivided according to IFNG expression. Gene expression profiles in INFγhigh and IFNγlow samples were compared by microarray and NanoString analysis and used to compute a prognostic index. The IFNγ release effect on the BM microenvironment was investigated in mesenchymal stromal cell (MSC)/AML cell cocultures. In mice, AML cells silenced for ifng expression were injected intrabone. IFNγhigh AML samples showed an upregulation of inflammatory genes, usually correlated with a good prognosis in cancer. In contrast, in patients with AML, high IFNG expression was associated with poor overall survival. Notably, IFNγ release by AML cells positively correlated with a higher BM suppressive Treg frequency. In coculture experiments, IFNγhigh AML cells modified MSC transcriptome by upregulating IFNγ-dependent genes related to Treg induction, including indoleamine 2,3-dioxygenase 1 (IDO1). IDO1 inhibitor abrogated the effect of IFNγ release by AML cells on MSC-derived Treg induction. In vivo, the genetic ablation of IFNγ production by AML cells reduced MSC IDO1 expression and Treg infiltration, hindering AML engraftment. IFNγ release by AML cells induces an immune-regulatory program in MSCs and remodels BM immunologic landscape toward Treg induction, contributing to an immunotolerant microenvironment. See related commentary by Ferrell and Kordasti, p. 2986.

A Targetable Bone Marrow-Niche Axis for the Treatment of Acute Myeloid Leukemia

Converging lines of evidence show that the endosteal niche within the bone marrow (BM) microenvironment plays a crucial role in the pathogenesis and chemoresistance of myeloid malignancies. Dysplastic cells can exploit niche-dependent, cell non-autonomous pathways to favor their growth. Molecular delineation and targeting of those pathways may help overcome resistance to targeted therapies. In particular, acute myeloid leukemia (AML) remains recalcitrant to conventional chemo- and targeted-therapies leading to relapse and low overall survival rates (5-year survival rate <30%). These shortcomings highlight the urgency of proposing novel and adapted therapies for the disease.Leveraging genetic mouse models, patient-derived xenografts (PDX), and patient samples, herein we show the mechanism and therapeutic targeting of a cell non-autonomous progression pathway in AML. Specifically, we show that conditional ablation of serotonin receptor 1b (HTR1B) in osteoblasts -during or after AML engraftment- hampers or even prevents AML progression, increasing overall survival. Mass spectrometry-based untargeted metabolomics identified the tryptophan catabolite kynurenine (Kyn) as an oncometabolite secreted by AML cells. Remarkably, the AML proliferative pathway that exploits peripheral serotonin signaling in osteoblasts, is not driven by serotonin but by Kyn, which acts as a previously unrecognized HTR1B ligand.To explore in vivo the significance of Kyn for leukemia progression, we inhibited its synthesis by suppressing indoleamine 2,3-dioxygenase-1 (IDO1) activity in mouse and human AML cells. CRISPR-Cas9-mediated Ido1 targeting suppressed AML growth in a dose-dependent manner. Next, we sought to identify the downstream molecular targets of Kyn in human osteoblasts that render the BM niche permissive to AML engraftment and support proliferation of leukemia cells. RNAseq analysis of co-cultures between primary human osteoblasts and leukemia cells shows that AML cells induce a pro-inflammatory remodeling of the osteoblastic niche that depends on HTR1B engagement by Kyn. Among the plethora of secreted pro-inflammatory molecules induced in osteoblasts, AML-secreted Kyn (but not the canonical HTR1B ligand serotonin) induces secretion of the acute-phase protein serum amyloid-A (SAA), which in turn acts in a positive feedback-loop on leukemia cells by increasing IDO1 expression, the rate-limiting enzyme for Kyn synthesis. Acting in an autocrine manner -in a mode distinct from its established immunomodulatory properties in cancer- Kyn activates the aryl hydrocarbon receptor (AHR), thereby supporting further AML proliferation.Highlighting a clinical relevance of these findings, the Kyn-HTR1B-SAA-IDO1 AML-promoting axis identified in leukemia mouse models, is recapitulated in PDX as well as myelodysplastic syndromes (MDS) and AML patients. Preferential and progressive production of Kyn over serotonin by leukemic cells, as well as increased levels of SAA1 in the BM plasma, occur as the disease pathogenesis proceeds from MDS to AML in patients. Moreover, genetic or pharmacological (Epacadostat) IDO1 inhibition hinders AML progression in PDX models. Of note, inhibition of IDO1 synergizes with chemotherapy to reduce AML burden. Overall, these data suggest that the leukemia-osteoblast crosstalk conferred by the Kyn-HTR1B-SAA-IDO1 axis has potential as a therapeutic target. This niche-dependent, cell non-autonomous AML axis, may impact the management of myeloid malignancies, opening also new opportunities for cancer treatment in conjunction with chemo/immunotherapies. DisclosuresBorot: Vor Biopharma: Consultancy, Patents & Royalties: coinventor on issued and pending patent applications licensed to Vor Biopharm. Ali: Vor Biopharma: Consultancy, Patents & Royalties: coinventor on issued and pending patent applications licensed to Vor Biopharm. Roth: UNC: Patents & Royalties: UNC has licensed a patent with B.L.R. listed as an inventor on biased opioid compounds. . Mukherjee: Vor Biopharma: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties: coinventor on issued and pending patent applications licensed to Vor Biopharma. S.M. has equity ownership and is on the Scientific Advisory Board of Vor Biopharma., Research Funding. Rabadan: AimedBio: Membership on an entity's Board of Directors or advisory committees; Genotwin: Other: Raul Rabadan is founder of Genotwin. Carroll: Incyte Pharmaceuticals: Research Funding; Janssen Pharmaceutical: Consultancy. Raza: Celgene Inc: Research Funding, Speakers Bureau; Novartis: Speakers Bureau; Genoptix: Speakers Bureau; Kura Oncology: Research Funding; Janssen R&D: Research Funding; Syros Pharmaceuticals: Research Funding; Onconova Therapeutics: Research Funding, Speakers Bureau.

Sirtuin 3 Inhibition Targets AML Stem Cells through Perturbation of Fatty Acid Oxidation

Acute myeloid leukemia (AML) in adults has a 5-year survival of approximately 30% and a high rate of disease recurrence in part due to our inability to eliminate the disease-initiating leukemic stem cells (LSCs) (Shlush et al. Nature, 2017). Previous studies have shown that LSCs uniquely rely on oxidative phosphorylation (OXPHOS) for survival (Lagadinou et al. Cell Stem Cell, 2013). Thus, novel therapies that are designed to target LSC metabolism have the potential to improve patient outcomes.Work from our group and others has demonstrated that a critical metabolite for OXPHOS regulation in LSCs is the coenzyme NAD + (Jones et al. Cell Stem Cell, 2020; Mitchell et al. Blood Advances 2019). One family of NAD + dependent proteins important in cancer biology, and AML specifically (Yan et al. Blood Cancer Discovery, 2021), are sirtuins. To determine if any sirtuins are important in LSC function we knocked down each sirtuin family member (sirtuin 1-7) with siRNA in four primary AML specimens and measured viability and colony forming ability. Knockdown of sirtuin 3 (SIRT3) decreased viability and colony forming potential of all AML specimens tested. SIRT3 is a mitochondrial de-acetylase with a multi-faceted role in metabolic regulation and oncogenesis (Finley, et al. Trends in Molecular Medicine, 2016). SIRT3 interacts with pathways upstream of OXPHOS including the tricarboxylic acid (TCA) and fatty acid oxidation (FAO). Importantly, a SIRT3 inhibitor (YC8-02) has been developed and has been shown to be effective pre-clinically for the treatment of B-cell lymphoma (Li et al. Cancer Cell, 2019).To further understand the significance of SIRT3 in LSCs, we assessed viability and colony forming potential upon YC8-02 treatment. LSCs were enriched from primary specimens based upon relative reactive oxygen species (ROS) level as previously described (Lagadinou et al. Cell Stem Cell, 2013). LSCs and blasts enriched from ten primary AML, and four AML cell lines (MOLM13, TEX, OCI-AML2, OCI-AML3) were cultured for 48 hours with or without YC8-02 before assessing viability and colony forming ability. YC8-02 treatment resulted in a significant decrease in colony forming potential of AML cells compared to control (data not shown). Similarly, LSCs, blasts, and cell lines showed a significant decrease in viability upon YC8-02 treatment (Fig 1A and data not shown). Cord blood and mobilized peripheral blood samples conversely did not show a change in colony forming potential following SIRT3 knockdown or YC8-02 treatment, respectively (data not shown). To assess YC8-02's effect on LSC function, three AML samples were treated with 10µM of drug for 24 hours and transplanted into NSG-S mice. YC8-02 treatment resulted in a significant decrease in AML engraftment, indicating a decrease in LSC function (Fig 1B).To determine the mechanism by which SIRT3 inhibition causes cell death, LSCs enriched from three primary specimens were treated with YC8-02; metabolite and lipid levels were determined by mass spectrometry. This analysis revealed a significant accumulation of fatty acids post YC8-02 treatment. To further characterize these changes, MOLM13 cells were treated with 13C 16-palmitic acid following 4 hours of incubation with 10µM YC8-02. Cells were collected 4 and 16 hours after introduction of palmitic acid and metabolic tracing was assessed by mass spectrometry. We found an accumulation of long and very long chain fatty acids and a decrease in TCA cycle intermediates (Fig 1C). FAO normally supplies TCA with intermediate acetyl-CoA; thus, these data indicate a decrease in FAO upon YC8-02 treatment. Accordingly, we measured changes in OXPHOS in response to treatment with YC8-02, in primary LSCs (Fig 1D) and AML cell lines (data not shown) and found a significant decrease in basal oxygen consumption. Further, ATP levels were significantly decreased upon YC8-02 treatment in LSCs (Fig 1E).In conclusion, we show that SIRT3 plays a pivotal role in FAO and LSC function. When SIRT3 is inhibited, FAO activity decreases resulting in the accumulation of long and very long chain fatty acids. This change in FAO activity reduces the availability of products for the TCA cycle, limiting necessary intermediates for OXPHOS, decreasing ATP production, and ultimately causing cell death. Therefore, our data suggests that SIRT3 is a potential therapeutic target for LSCs and should be considered in future pre-clinical and clinical investigations. [Display omitted] DisclosuresMelnick: Constellation: Consultancy; Epizyme: Consultancy; Daiichi Sankyo: Research Funding; Sanofi: Research Funding; Janssen Pharmaceuticals: Research Funding; KDAC Pharma: Membership on an entity's Board of Directors or advisory committees. Minden: Astellas: Consultancy. D'Alessandro: Omix Thecnologies: Other: Co-founder; Rubius Therapeutics: Consultancy; Forma Therapeutics: Membership on an entity's Board of Directors or advisory committees.

Acute Myeloid Leukemia Cell-Intrinsic PD-1 Functions Promoting Leukemia Development By Recruiting SHP2

Acute myeloid leukemia (AML) is a devastating hematopoietic malignancy. With current therapies, only approximately 30% of patients achieve long-term survival. Therefore, novel, more active and less toxic treatments are urgently needed. Programmed death-1 (PD-1) is a cell surface receptor that functions as a T cell checkpoint and plays a central role in regulating T cell exhaustion. Binding of PD-1 to its ligand, programmed death-ligand 1 (PD-L1), activates downstream signaling pathways and inhibits T cell activation. Abnormally high PD-L1 expression on tumor cells and antigen-presenting cells in the tumor microenvironment mediates tumor immune escape, and PD-1/PD-L1 immune checkpoint blockade has showed promising results in cancer patients. Recently, PD-1 expressed on melanoma cells was also shown to play a pivotal role in tumor growth. To date, in AML, the function of PD-1 has been mainly studied in the host T cells, while little is known regarding the role of PD-1 in AML cells. Herein, we examined the level and role of PD-1 in AML cells using AML murine model and patient samples.We used MLL PTD/WT/Flt3 ITD/ITD knock-in mouse in B6 background, a well characterized AML model, to study the expression and function of PD-1 in AML. Flow cytometric analysis of LSK (Lin -Sca-l1 +c-kit +) cells from the bone marrow (BM) of wild-type (WT, n=5) and AML (n=10) mice showed that 20.9%-61.9% of AML LSKs versus (vs) <5.0% of normal LSKs are PD-1 positive (P< 0.0001). Western blot and Q-RT-PCR analysis confirmed higher levels of PD-1 in AML LSKs than in normal LSKs. The PD-1 levels on LSKs increased over time and associated with disease progression. Similar results were obtained in AML patients, showing PD-1 + in 0.2%-14.6% of AML CD34 + cells vs < 1.0% of normal CD34 + cells (P< 0.05). Data analysis based on TCGA showed that higher PD-1 levels are associated with shorter survival in AML patients (P=0.0125).To assess the functional role of PD-1 in AML, we sorted PD-1 + and PD-1 - fractions fromAML LSKs and observed a lower frequency of quiescent cells (G0, 16.60% vs 44.87%, P< 0.05) and a higher cell growth rate in the PD-1 + vs PD-1 - cells. Further in vivo study showed that PD-1 + AML LSKs (CD45.2) generated higher white blood cell (WBC) counts (P< 0.0001), higher AML engraftment (P< 0.0001) and a shorter survival (median survival 57.5 vs >75 days, P< 0.001) in recipient mice (CD45.1) compared with PD-1 - AML LSKs. Similar results were observed in human samples. Compared to PD-1 - CD34 + cells, PD-1 + CD34 + cells are less quiescent and more proliferative (P< 0.01). PD-1 + AML blasts had higher engraftment rate (13.18% vs 2.68%, p=0.0002) and shorter survival (median survival: 27 vs 45 days, P= 0.0008) in NSGS mice than PD-1 - AML blasts.To evaluate if these in vivo differences observed in PD-1 + vs PD-1 - AML LSKs were mediated by interactions between PD-1 + AML and T cells, PD-1 + and PD-1 -LSKs from AML mouse were transplanted into T-, B- cell-deficient NSG mice. Recipient mice transplanted with PD-1 + AML LSKs had higher WBC counts (P< 0.01), higher AML engraftment (P< 0.0001) and a shorter survival (median survival: 76 vs >130 days, P< 0.0001) than recipients with PD-1 - AML LSKs, suggesting that these differences were T cell-independent.Next, we examined whether blocking PD-1 could affect leukemic cell growth. We sorted LSK cells from AML mice and performed colony forming cell (CFC) assay in the presence of anti-PD1 antibody or isotype antibody. Blocking PD-1 with anti-PD-1 antibody significantly suppressed CFC and cell growth in vitro but did not induce apoptosis compared to isotype control antibody. To explore the molecular mechanism by which PD-1 contributes to AML growth, we then sorted PD-1 + and PD-1 - LSKs from AML mice for molecular studies. Western blot assays revealed higher levels of SHP-2 and phosphorylated (p) -ERK in PD-1 + vs PD-1 - AML LSKs. We validated these results in primary human AML cells by immunofluorescence staining. Confocal microscopy of PD-1 + and PD-1 - human AML CD34 +cells demonstrated that PD-1 localized at the cell membrane and in the cytoplasm and p-ERK was markedly enhanced in the PD-1 + CD34 + cells.In conclusion, we showed here that a subpopulation of murine and human AML blasts expresses high levels of PD-1 which mediated disease initiation and growth through activation of the MAPK/ERK signaling pathways. PD-1 blocking antibody reversed these activities and might contribute to the clinical efficacy of anti-PD-1 therapy in AML. DisclosuresMarcucci: Novartis: Other: Speaker and advisory scientific board meetings; Agios: Other: Speaker and advisory scientific board meetings; Abbvie: Other: Speaker and advisory scientific board meetings.

RUNX1 Enhances Leukemia Cell Engraftment in the Vascular Niche through up-Regulating E-Selectin

Although the function of runt-related transcription factor 1 (RUNX1) has been well-characterized in leukemogenesis and regarded as an ideal target in anti-leukemia strategies, neither the effect of RUNX1 inhibition therapy on the bone marrow niche cells nor its impact on the engraftment of acute myeloid leukemia (AML) cells has largely been unknown. Here we show evidence that RUNX1 inhibition attenuates the expression of E-selectin in the vascular endothelial cells of bone marrow niche, thereby disrupting the homing of AML cells to the bone marrow niche, ultimately extending the overall survival period in AML mice models. In order to investigate the effect of RUNX1 inhibition on endothelial vascular niche cells, firstly we identified and focused on E-selectin from significant affected factors, such as E-selectin, P-selectin, VCAM 1 and TIE 2 through the screening the expressions of representative surface molecules in human umbilical vein endothelial cells (HUVEC) treated with our selective RUNX inhibitor : Chb-M'. E-selectin has recently been recognized as a vital component of vascular niche and fundamentally involved in the housing of hematopoietic stem cells (HSCs). We also genetically attenuated the expression of RUNX1 in HUVEC with doxycycline-inducible short hairpin RNA (shRNA). As we have expected, the expression of E-selectin was tightly and most profoundly down-regulated both at the transcript and surface expression levels upon RUNX1 -silencing. Of note, neither Chb-M'-treatment nor shRNA-mediated RUNX1 knockdown resulted in significant change in the expressions of niche-associated surface molecules in AML-derived MV4-11 cells. In addition, neither Chb-M'-treatment nor RUNX1 knockdown resulted in significant change in the expression of osteoblastic niche components in osteosarcoma-derived HOS cells. These results suggest that RUNX1 potentially and primordially be involved in the regulation of E-selectin in the endothelial vascular niche cells. Besides, the activity of E-selectin proximal promoter was increased upon additional RUNX1 expression, while the modified E-selectin promoter which lacks RUNX consensus binding site (5'-TGTGGT-3') dramatically decreased its luciferase reporter activity, confirming that RUNX1 directly transactivates E-selectin expression. We next examined the effect of RUNX1 attenuation on E-selectin expression in murine endothelial vascular niche cells through several assays. Treatment of C56BL/6 mice for 2 weeks with either E-selectin inhibitor: andrographolide, A205804 or RUNX-inhibitor: Chb-M' showed effective decreasing of E-selectin expression on endothelial vascular niche cells. We next retrovirally-transduced MLL-ENL to c-kit+ mice bone marrow cells and obtained mice leukemic cells. Using these mice leukemic cells, we investigated the effect of Runx1 inhibition therapy on the homing ability of leukemia cells to their bone marrow niche. We studied and confirmed the effect of RUNX1 inhibition on the homing ability of leukemic cells by both in vitro static assays and in vivo mouse models. Mise treated with andrographolide, A 205804 or Chb-M' for 2 weeks, mouse leukemic cells of MLL-ENL+ mice were transplanted. All these treatments significantly decreased the number of homed leukemic cells in the bone marrow compared to the vehicle-treated mice, underscoring the importance of E-selectin in the engraftment of AML cells in the vascular niche. Consistent with these observations, conditioning bone marrow vascular niche with Chb-M' before transplantation of leukemic cells significantly prolonged the overall survival period of AML mice with Runx inhibition therapy by Chb-M' or vehicle control treatment. Our work collectively indicates that RUNX -silencing drugs such as Chb-M' not only directly control the growth of AML cells themselves, but also indirectly enhance its anti-leukemic potential through attenuating E-selectin expressions in the bone marrow vascular niche. These drugs potentially bring the leukemic stem-like cells out from deep in the bone marrow niche to the peripheral blood stream and expose these primordial leukemia cells to chemotherapeutic drugs at much higher concentrations. DisclosuresNo relevant conflicts of interest to declare.

Hypomethylating Agent Leads to Enhanced Immunogenicity of a Dendritic Cell/Acute Myeloid Leukemia Fusion Vaccine and Prolonged Survival in an Immunocompetent Mouse Model

Immunotherapy for acute myeloid leukemia (AML) holds great promise as a modality to overcome tolerance and elicit memory responses needed for durable remissions. Our group pioneered a novel personalized vaccine that targets a broad array of leukemic antigens (Ags) by fusing whole patient-derived AML cells to autologous dendritic cells (DCs). In a clinical trial of AML patients in chemotherapy-induced remission, vaccination induced an expansion of tumor-specific T cells and resulted in prolonged remissions in a subset of patients. Enhancing response to vaccination relies on increasing Ag presentation and overcoming factors in the microenvironment that contribute to tolerance.While the mechanism of action of hypomethylating agents (HMAs) is multifactorial, HMAs are increasingly recognized as exhibiting immunomodulatory functions thought to contribute to therapeutic efficacy. We have previously shown that the HMA SGI-110 augments AML Ag processing and increases AML susceptibility to T cell-mediated killing. In the present study, we aimed to elucidate the effect of HMAs on enhancing Ag presentation and modulating the AML microenvironment as well as to investigate the impact of HMA on the immunogenicity of a DC/AML vaccine.We first investigated the ability of SGI-110 to reverse critical aspects of the AML immunosuppressive milieu and enhance vaccine efficacy. C57BL/6J mice were retro-orbitally inoculated with 1x105 syngeneic TIB-49 AML cells and treated with low-dose SGI-110 (1mg/kg) to assess immunologic effects without preventing engraftment. AML engraftment in the spleen and bone marrow (BM) was similarly observed in treated and control animals. However, SGI-110-treated animals showed decreased PD-1 expression on CD4 T cell splenocytes (mean 3.1% vs. 6.3%; n=4; p=0.01). SGI-110 therapy was also associated with decreased myeloid-derived suppressor cell (MDSC) burden in the BM (n=5; p=0.01) and spleen (n=4; p=0.03) compared to control.Consistent with these findings, treated mice demonstrated increased AML-specific immunity via increased T cells isolated from the BM or spleen that expressed intracellular IFNϒ following ex-vivo exposure to TIB-49 lysate (mean 3% vs. 0.5%; n=5; p=0.02).We then examined the potential synergy of SGI-110 + vaccine in this model. Mice treated with SGI-110 + vaccine showed a significant increase in BM and spleen CD4 (n=5; p=0.05) and CD8 (n=5; p=0.02) T cell IFNϒ in response to tumor lysate as compared to vaccine or SGI-110 alone.To further elucidate the mechanism behind the immune activating effects of HMAs, we investigated the effect of HMA on the endogenous retroviral (ERV) pathway. Activation of this pathway results in increased IFN ß expression leading to increased MHC HLA Class I expression and subsequently, immune stimulation in a solid tumor model (Chiappinelli et al. Cell . 2015). MOLM-14 cells were exposed to low-dose HMA daily X 3 days and analyzed for ERV-3 expression using western blotting (WB). The results showed significant upregulation of ERV-3 expression after treatment. Due to activation of the ERV pathway, increased IFN ß expression was observed with WB analysis. Indeed, analysis of an HLA-A2 positive human AML cell line, THP1, showed significant HLA-A2 upregulation after HMA exposure. Most importantly, HLA-A2 upregulation led to a significant rise in tumor Ag presentation, detected by measuring the binding capacity of a unique T-cell receptor-like antibody using flow cytometric analysis in THP-1 cells.Given these effects, we sought to examine whether treatment with vaccine + SGI-110 would enhance survival in this model. Mice were treated with: vehicle, SGI-110 X 5 days, vaccine, or both agents. Monotherapy with either agent led to a survival advantage compared to untreated mice (p<0.05). Notably, the combination arm further prolonged survival as compared to untreated mice (p<0.05).In conclusion, SGI-110 modulates critical factors contributing to the immunosuppressive milieu in AML by decreasing T cell PD-1 expression and promoting MDSC expansion, resulting in enhanced anti-AML immunity. Moreover, SGI-110 augments the immune response to an autologous DC/AML vaccine in a murine model resulting in a survival benefit. In addition, HMA enhances Ag presentation, in part due to activation of the ERV pathway. The combination of a novel personalized DC/AML fusion vaccine and HMA holds great potential and a clinical trial is being initiated. DisclosuresAvigan:Genus Oncology: Research Funding.

Identification of Myeloperoxidase As a Leukemia-Associated Antigen and Therapeutic Target for Acute Myeloid Leukemia

The prognosis for patients with acute myeloid leukemia (AML) remains poor despite treatments. The need for new effective treatments in AML has led to the development of therapies that harness the immune system to attack the malignant cells. We have previously shown that neutrophil elastase (NE), proteinase 3 (P3) and cathepsin G (CG), serine proteases localized within azurophilic granules of myeloid cells, including myeloid leukemia, are immunotherapeutic targets for myeloid malignancies.To determine whether other azurophilic granule components can be targeted in AML, we studied myeloperoxidase (MPO). MPO is maximally expressed at the promyelocytic stage of myeloid development and is later stored in the azurophilic granules of mature neutrophils. MPO function is implicated in the degradation and elimination of pathogenic bacteria during infection. MPO is also involved in the pathogenesis of certain autoimmune diseases such as granulomatosis with polyangiitis where it is targeted by humoral and cellular immunity. Importantly, MPO has been shown to be overexpressed in many types of leukemia, including CML, AML, and MDS, which suggests that it could be targeted if overexpression could break immune tolerance.In this study, we identified two MPO-derived, HLA-A2-restricted peptides, MY4 and MY8, as novel leukemia-associate antigens (LAA). To identify these peptides, we first scanned the MPO sequence and screened potential peptides in silico against the binding motif of the HLA-A2 molecule. Ten peptides were predicted to have high binding affinity to HLA-A2 (Dissociation half-life over 100 min), and binding was confirmed by peptide-pulsed T2 binding assays with synthetic MPO peptides and flow cytometry. Next, we elicited cytotoxic T lymphocytes (CTL) from healthy HLA-A2+ donors and tested the resulting CTL for specificity and effector function in 4-hour cytotoxicity fluorescence assays against peptide-pulsed T2 cells and primary bone marrow cells from patients with refractory/relapsed AML (Fig 1 A+B). MY4- and MY8-specific cytotoxic T cells (CTL) are able to kill leukemia blasts while sparing healthy PBMCs from HLA-A2+ donors (Percentages of killing for MY4 CTL: 20, 20, 17, 8 vs. 2, 2, 1, 6 at 1:4,1:8,1:16:1:32 Effector: Target ratios respectively p<0.0001, Percentages of killing for MY8 CTL at the same E:T ratios: 15, 16,11,8 vs. 9,7,5,2 p=0.0023).To examine potential in vivo activity against patient AML, we studied whether MPO-specific CTL could prevent or inhibit engraftment of primary AML in patient-derived mouse xenograft (PDX) models. Co-injection of MY4 or MY8-specific CTL with AML into sublethally irradiated NSG mice drastically reduced bone marrow tumor burden in vivo (Fig. 1C: 24%±6 for MY4 treated mice, n=4 vs. 65%±7 in the control group, n=2, p=0.01 and 19%±9 for MY8 treated mice, n=4 vs. 65%±7 in the control group n=2, p=0.03). Thus, these findings supported the hypothesis that MPO can be a LAA. To confirm whether MPO-specific immunity is linked to anti-leukemia immunity in patients, we studied post-allogeneic stem cell transplant (SCT) patients, where the graft-versus-leukemia (GVL) effect is critical for inducing and maintaining remission after SCT. We enumerated MPO-specific lymphocytes in the peripheral blood of patients post-SCT with MY4- and MY8/HLA-A2 tetramers. We found that MY4- and MY8-specific T cells are significantly increased in AML patients after SCT. These antigen specific T cells' frequency increased in post-transplant AML patients (n=14) as compared to healthy donors (n= 10) (Fig. 2: 0.043% ±0.006 vs. 0.01% ±0.002 of all peripheral -blood CD8 cells for MY4, p=0.0008 and 0.014% ±0.001 vs. 0.006% ±0.001 for MY8, p=0.01).These result suggest that both MY4 and MY8 are immunogenic and MY4- and that MY8-specific CTL may play a role in targeting leukemia in patients after SCT. Thus, the discovery of MY4 and MY8 as novel LAAs paves the way for using these antigens in immunotherapy against myeloid leukemia. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Next Generation Sequencing of Immunized Mouse Splenocytes to Develop an Anti-TIM3 Chimeric Antigen Receptor for Acute Myeloid Leukemia

The field of anti-cancer T cell therapy has had major advances in recent years with the development of the chimeric antigen receptor (CAR). Our group has developed and demonstrated that CARs against CD19 have complete remission rates of greater than 90% against B cell acute lymphoblastic leukemia (B-ALL). However, the progress against B-ALL has not been matched in other types of cancers. Acute Myeloid Leukemia (AML) is the most common acute leukemia in adults, presenting greater than 20,000 new cases per annum and representing 80% of acute leukemia. Additionally, prognosis is poor with as much as 70% of patients, 65 years or older, dying within 1 year of diagnosis. A recent study examining AML has shown that a TIM3 monoclonal antibody (mAb) blocks AML engraftment and eliminates leukemic stem cells (LSC). Additionally, TIM3 mAbs have anti-tumor effects such as slowing tumor progression. Gene-expression analysis demonstrates an 8-fold increase of TIM3 in LSC compared to hematopoietic stem cells. Our own evaluation of TIM3 expression on AML blasts and LSC demonstrated that TIM3 is commonly co-expressed on CD33+ blasts. These results led us to hypothesize anti-TIM3 CAR T cells can target and kill AML, which we will evaluate by the development of novel anti-TIM3 CARs. Generation of new CARs rely on a time consuming and/or costly process of phage display screening or hybridoma production, hybridoma screening, and immunoglobulin (Ig) sequencing. Here, we describe a novel CAR design strategy by next generation sequencing (NGS) followed by CAR synthesis and functionalvalidation against TIM3+ targets. NGS allows for direct identification of rearranged immunoglobulin genes, mutation and repertoire analysis, and comparison of multiple immunized mice. Furthermore, by eliminating the need for hybridoma production and screening we have developed a rapid, economical system for the development of novel CARs.After immunization of four separate mice with CHO-TIM3, NGS demonstrated that nearly 70% of Ig heavy (IgH) and 61% of Ig light (IgL) sequences were accounted for by 2 and 3 V-J rearrangements, respectively. This suggested that these rearrangements were enriched for TIM3-reactivity and we designed pairs of these IgH and IgL rearrangements. By this process we generated 8 anti-TIM3 CARs (4 VHS1-based CARs and 4 VH8-based CARs). After their gene-synthesis and cloning into a genetic construct we produced retrovirus and transduced Jurkat, as well as primary T cells. Robust gene-transfer was observed with most constructs (14-64%). Furthemore, we detected antigen-specific cytokine production by Jurkat stimulated with TIM3+ targets (Fig. 1). We also examined in vitro killing by primary anti-TIM3 human T cells using a Real-Time cell Analysis cytotoxicity assay. We determined that anti-TIM3 CAR VH8-461 had the highest transduction efficiency, IFNγ production, and in vitro killing of target cells. We are now evaluating the in vivo efficacy of anti-TIM3 CAR T cells in immune deficient animal models implanted with TIM3+ targets. This work demonstrates NGS can be used to identify new sequences that can be developed rapidly and inexpensively into CARs. Using this process we created 8 anti-TIM3 CARs and by in vitro screening selected 1 of them (VH8-461) for in vivo validation. We are using this same NGS CAR generation system to develop additional anti-AML CARs which can be used in tandem with anti-TIM3 CARs to treat AML. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Blocking CD70/CD27 Signaling in Combination with Hypomethylating Agents Eradicates Human CD34+ AML Stem and Progenitor Cells in Vitro and In Vivo

Leukemia stem cells (LSCs) are the origin of acute myeloid leukemia (AML) and are resistant to standard therapeutic regimens resulting in frequent relapse of the disease and poor prognosis especially for AML patients not fit for intensive chemotherapy. Consequently, LSCs represent a major obstacle for AML therapy. We recently identified the interaction of the TNF ligand CD70 and its receptor CD27 on LSCs as a promising therapeutic strategy to target LSCs.In this study, we demonstrate for the first time that treatment with hypomethylating agents (HMA, azacitidine or decitabine) significantly up-regulates CD70 mRNA and protein expression on primary CD34+ stem/progenitor cells from newly diagnosed AML patients independent of the cytogenetic/molecular risk category in vitro (mRNA: 4.3±1.6 fold increase, FACS: 4.0±0.9 fold increase vs. vehicle treatment, n=15). Similarly, CD34+ stem/progenitor cells from AML patients after 5 days decitabine administration (20mg/kg by continuous intravenous infusion over 1 hour repeated daily) exhibited a significant increase in CD70 expression as analyzed by flow-cytometry (6.8±1.1 fold increase vs. diagnostic sample, n=6). In contrast, CD70 expression of CD34+ stem/progenitor cells from healthy donor bone marrow (n=3) was not affected by the treatment.Consequently, co-treatment of CD34+ stem/progenitor cells from AML patients of each risk group (n=3) with the HMA decitabine and a blocking αCD70 monoclonal antibody strongly reduced colony-forming and re-plating capacity in vitro compared to single agent treatment. To confirm and extend our findings towards disease-initiating CD34+ AML LSCs in vivo, we performed patient-derived AML xenografts (PDX) from one patient with intermediate and one patient with adverse cytogenetic risk profile. Combining decitabine treatment with CD70 blockade significantly reduced AML engraftment in blood, spleen and BM compared to controls. Moreover, co-treatment significantly reduced frequency and absolute numbers of CD34+CD38- and CD34+CD38-CD45RA- AML LSCs in the BM as analyzed phenotypically by FACS and eliminated human LSCs as analyzed in limiting dilution PDX experiments compared to single treatments. Importantly, CD34+ hematopoietic stem/progenitor cells from healthy human bone marrow (n=3) were only marginally affected by the co-treatment.In summary, combining HMAs with blocking CD70/CD27 signaling could represent a novel strategy to eradicate human LSCs. DisclosuresVan Rompaey:argenX: Employment. Leupin:argenX: Employment. De Haard:argenX: Employment.

IGFBP7 activates retinoid acid-induced responses in acute myeloid leukemia stem and progenitor cells.

Treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) in combination with low doses of arsenic trioxide or chemotherapy leads to exceptionally high cure rates (>90%). ATRA forces APL cells into differentiation and cell death. Unfortunately, ATRA-based therapy has not been effective among any other acute myeloid leukemia (AML) subtype, and long-term survival rates remain unacceptably low; only 30% of AML patients survive 5 years after diagnosis. Here, we identified insulin-like growth factor binding protein 7 (IGFBP7) as part of ATRA-induced responses in APL cells. Most importantly, we observed that addition of recombinant human IGFBP7 (rhIGFBP7) increased ATRA-driven responses in a subset of non-APL AML samples: those with high RARA expression. In nonpromyelocytic AML, rhIGFBP7 treatment induced a transcriptional program that sensitized AML cells for ATRA-induced differentiation, cell death, and inhibition of leukemic stem/progenitor cell survival. Furthermore, the engraftment of primary AML in mice was significantly reduced following treatment with the combination of rhIGFBP7 and ATRA. Mechanistically, we showed that the synergism of ATRA and rhIGFBP7 is due, at least in part, to reduction of the transcription factor GFI1. Together, these results suggest a potential clinical utility of IGFBP7 and ATRA combination treatment to eliminate primary AML (leukemic stem/progenitor) cells and reduce relapse in AML patients.

Venetoclax Enhances Anti-Leukemia Activity of CD123-Specific BiTE-Secreting T-Cells in AML

Background: CD123 is frequently expressed in hematologic malignancies including AML. CD123 has been a potential immunotherapeutic target in AML due to its association with leukemic stem cells that play an essential role in disease progression and relapse. Our previous study using T-cells secreting CD123/CD3-bispecific T-cell engagers (BiTEs) (CD123-ENG T-cells) has shown activity in preclinical studies, recognizing and killing acute myeloid leukemia (AML) blasts in vitro and in vivo. CD123-ENG T-cells secrete bispecific molecules that recognize CD3 (T-cells) and CD123 (AML blasts), and are able to direct transduced T-cells and recruit bystander T-cells to kill CD123-positive blasts. Venetoclax is a BCL-2 inhibitor that can restore functional apoptosis signaling in AML cells, and has been FDA approved for the treatment of AML patients in combination with hypomethylating agents. To improve the efficacy of CD123-ENG T-cells we explored efficacy in AML by combining targeted immunotherapy (CD123-ENG T cells) with targeted inhibition of anti-apoptotic BCL-2 (venetoclax) in vitro and in vivo models of AML. Methods : CD123-ENG T-cells were generated by retroviral transduction and in vitro expansion. Non-transduced (NT) T-cells served as control. In vitro, GFP+ MOLM-13 AML cells were pretreated with venetoclax (0, 10µM, and 20µM) for 24 hours prior to co-culture with CD123-ENG or NT T-cells at an effector/target ratio of 1:10. After 16 hours, MOLM-13 AML cells were analyzed by flow cytometry and quantitated using counting beads; cytotoxicity was calculated relative to untreated MOLM-13 control. The anti-AML activity of the combination was further evaluated in a MOLM-13-luciferase xenograft AML mouse model. Leukemia progression was assessed by bioluminescence imaging. The frequency of MOLM13 AML and human T cells in periphera blod (PB) was determined by flow cytometry. Results: In vitro, we demonstrated that pretreatment of Molm13 AML cells with venetoclax enhanced the cytolytic activity of CD123-ENG T-cells compared to NT- or no T-cell controls. Interestingly, venetoclax sensitized Molm13 to CD123-ENG T-cell killing in a dose-dependent manner (Fig.1; 50%/31% killing by CD123-ENG T-cells versus 27%/14% of killing by NT T cells post pretreatment with 10µM or 20µM ventoclax, p&amp;lt;0.001). In the Molm13 luciferase xenograft model, NSGS mice were randomized into 5 groups after AML engraftment was confirmed: 1) vehicle control, 2) Venetoclax (Ven) only, 3) CD123-ENG T-cells only, 4) Ven+CD123-ENG T-cells, 5) Ven+CD123-ENG T-cells/2-day-off Ven post T-cell infusion (Ven[2-day-off]+CD123-ENG). Venetoclax treatment (100 µg/kg daily via oral gavage) was started on day 4 post Molm13 injection, and on day 7, mice received one i.v. dose of CD123-ENG T-cells (5x106 cells/mouse). Venetoclax or CD123-ENG T-cell monotherapy reduced leukemia burden compared to the control group, and combinational treatments further inhibited leukemia progression as judged by BLI and circulating AML cells (%GFP+mCD45-/total live cells) by flow cytometry on day 15 post MOLM-13 injection: vehicle control: 19.6%; Ven+: 3.4%; CD123-ENG T-cells:1.2 %; Ven+CD123-ENG T-cells: 0.3%; Ven[2-day-off]+CD123-ENG T-cells (p&amp;lt;0.01 Ven+ or CD123-ENG T-cells versus control; p&amp;lt;0.001 Ven+CD123-ENG or Ven[2-day-off]+CD123-ENG T cells versus CD123-ENG T cells, n=5). The enhanced anti-AML activity of combining venetoclax and CD123-ENG T-cells translated into a significant survival benefit in comparison to single treatment alone (Fig. 2). However, while Ven+CD123-ENG and Ven[2-day-off]+CD123-ENG T-cell treated mice had a survival advantage, they had reduced circulating numbers of human CD3+ T cells on day 8 post T-cells infusion compared to mice that received CD123-ENG T-cells, indicative of potential adverse effect of venetoclax on T-cell survival in vivo. Conclusion: Our data support a concept of combining pro-apoptotic targeted and immune therapy using venetoclax and CD123-ENG T-cells in AML. While it has been reported that venetoclax does not impair T-cell functionality, more in-depth analysis of the effect of Bcl-2 inhibition on T-cell function and survival appears warranted, as it could diminish survival not only of AML blasts but also of immune cells. Disclosures Bonifant: Patents filed in the field of engineered cellular therapies: Patents &amp; Royalties: Patents filed in the field of engineered cellular therapies. Gottschalk:Patents and patent applications in the fields of T-cell &amp; Gene therapy for cancer: Patents &amp; Royalties; Inmatics and Tidal: Membership on an entity's Board of Directors or advisory committees; Merck and ViraCyte: Consultancy; TESSA Therapeutics: Other: research collaboration. Velasquez:Rally! Foundation: Membership on an entity's Board of Directors or advisory committees; St. Jude: Patents &amp; Royalties. Andreeff:Amgen: Research Funding; Daiichi-Sankyo; Jazz Pharmaceuticals; Celgene; Amgen; AstraZeneca; 6 Dimensions Capital: Consultancy; Daiichi-Sankyo; Breast Cancer Research Foundation; CPRIT; NIH/NCI; Amgen; AstraZeneca: Research Funding; Centre for Drug Research &amp; Development; Cancer UK; NCI-CTEP; German Research Council; Leukemia Lymphoma Foundation (LLS); NCI-RDCRN (Rare Disease Clin Network); CLL Founcdation; BioLineRx; SentiBio; Aptose Biosciences, Inc: Membership on an entity's Board of Directors or advisory committees.

A Stemness-Based Screen Identifies PLK1 Inhibitors for Targeting Leukemia Stem Cells in AML

The main barrier to curing acute myeloid leukemia (AML) is disease relapse, which occurs due to therapy resistance and persistence of leukemic stem cells (LSCs) after conventional induction chemotherapy. Thus drug discovery efforts must focus on identifying agents that effectively target LSCs and not just bulk blasts. To this end, we employed a multi-parametric stemness screen of 1200 bioactive small molecules to identify drugs targeting LSCs, based on reduction of the stem cell compartment of a functionally-characterized hierarchical AML model (OCI-AML-8227) assessed by flow cytometry. In this cell line, self-renewing LSCs are restricted to the CD34+CD38- fraction. The screen identified a number of compound classes with the potential to antagonize LSC properties, including those already in clinical use for AML as well as classes of compounds whose effects in AML have not been previously reported (Figure 1A). Top hits were further validated based on treatment-induced alteration of the expression profile of 104 LSC genes (LSC104) differentially expressed between LSC+ and LSC- fractions of primary AML, which captures stemness properties. The LSC17 score, which is strongly associated with survival and response to standard therapy in AML, was derived from the LSC104 gene set. Notably, all Polo-like kinase 1 (PLK1) inhibitors in the library were identified as top hits in the screen. In vitro treatment of OCI-AML-8227 cells with PLK1 inhibitors over 3 days selectively inhibited the CD34+CD38- fraction enriched in LSCs (Figure 1B), and decreased correlation of gene expression to the LSC104 signature in bulk cells (Figure 1C). Together, these data support a role for PLK1 in regulating leukemic stemness, and we prioritized this class of compounds for validation studies. PLK1 is an important regulator of cell cycle and its best studied role is in the regulation of mitotic entry. However, PLK1 is expressed in and likely plays an important role in all phases of the cell cycle. For instance, PLK1 has been described to regulate cilia disassembly at G0/1. The PLK1 inhibitor volasertib was previously tested in a Phase III trial against AML in combination with low-dose cytarabine (LDC) for elderly patients not eligible for induction chemotherapy. In this trial, although efficacy was observed, significant toxicity in the volasertib+LDC treatment arm resulted in poor survival outcomes for this group of patients. We evaluated the toxicity of volasertib treatment in vitro against two hierarchical AML cell lines (OCI-AML-8227 and OCI-AML-21) as well as normal cord blood (CB). Similar to CB, self-renewing stem cells for these two AML cell lines are restricted to the CD34+CD38- fraction. Treatment with volasertib at 20nM over three days resulted in significantly more cytotoxicity to the AML cell lines compared to CB (Figure 1D), especially in the CD34+CD38- compartment, suggesting that a therapeutic window exists. To evaluate the effects of PDK1 inhibitors against LSCs in vivo, we treated mice bearing AML patient xenografts with single-agent volasertib at a low dose (10mg/kg twice weekly for 4 weeks by oral gavage) starting 4 weeks post-transplant. The gene expression profile for 2 of 4 samples tested showed decreased correlation to the LSC104 signature after volasertib treatment, supporting an effect on stemness (Figure 1E). Volasertib treatment significantly reduced AML engraftment in 4 of 7 samples (Figure 1F). To evaluate the effect of volasertib on LSCs in primary treated mice, we performed secondary transplantation at limiting doses. Volasertib treatment significantly reduced LSC frequency in 2 of 4 samples tested (Figure 1G). Notably, sample AML5 showed a 31.8-fold reduction in LSC frequency compared to controls (p = 0.039) despite no significant reduction in bulk engraftment in primary treated mice, suggesting that volasertib may selectively target LSCs in this sample. In conclusion, our data indicate that the PLK1 inhibitor volasertib, identified as a top hit in a stemness-based drug screen, can target LSCs and decrease stemness properties in some primary AML samples. These findings support further studies of the potential of PLK1 inhibitors for the treatment of AML. Figure Disclosures Wang: Trilium Therapeutics: Patents &amp; Royalties.

Preclinical Assessment of Non-Virally Engineered CD33.CAR Cytokine-Induced Killer (CIK) Cells in Chemoresistant AML Patient-Derived Xenografts

Background Chimeric Antigen Receptor (CAR)-T cell therapy has been successfully clinically deployed in the context of B-cell malignancies, paving the way for further development also in Acute Myeloid Leukemia (AML), a still unmet clinical need in the field of oncohematology. Among the potential AML targetable antigens, CD33 is so far one of the main validated molecule. Objectives The aim of the present study was to optimize a non-viral gene transfer method to engineer Cytokine-Induced Killer (CIK) cells with a CD33.CAR by using a novel version of the Sleeping Beauty (SB) transposon system, named "SB100X-pT4". Further, a preclinical assessment of SB-modified CD33.CAR-CIK cells was performed in chemoresistant AML Patient-Derived Xenografts (PDX), in order to address the unmet need of targeting drug-resistant AML cells. Methods Donor derived-CIK cells were stably transduced with a CD33.CAR by exploiting the novel hyperactive SB100X transposase and the pT4 transposon (SB100X-pT4). The novel SB system has been in vitro compared to the previous established SB11-pT. In vitro anti-AML activity of CD33.CAR-CIK cells was assessed by flow cytometry-based cytotoxicity (AnnV-7AAD), proliferation (CFSE) and cytokine production (intracellular IFNg and IL2 detection) assays. In vivo efficacy was evaluated in NSG mice transplanted with MA9-NRas AML cell line or PDX samples. A xenograft chemotherapy model mimicking induction therapy ("5+3" Ara-C and doxorubicin) was exploited to examine the potential benefit of CD33.CAR-CIK cells on chemoresistant/residual AML cells. Results By significantly reducing the amount of DNA transposase, the novel SB100X-pT4 combination resulted in higher CAR levels than the SB11-pT. SB100X-pT4-modified CD33.CAR CIK cells showed efficient expansion after 3 weeks (median fold increase of 38.89, n=4). Both transpositions conferred to CD33.CAR-CIK cells a specific killing (up to 70%) against CD33+ AML target cell lines and primary AML cells. The anti-AML proliferative response of SB-modified CD33.CAR-CIK cells was also considerable (up to 70% of CFSE diluted CAR-CIK cells), as well as the cytokine production (up to 35% for IFN-γ and up to 25% for IL-2). To evaluate the effect of SB100X-pT4-modified CD33.CAR-CIK cells particularly on Leukemia Initiating Cells (LICs), CD33.CAR-CIK cells were administered as an "early treatment" in mice transplanted with the MA9-NRas cell line, which retains a high frequency of LICs. At sacrifice, CD33.CAR-CIK cell-treated mice showed a significant bone marrow (BM) engraftment reduction (median 27.80 for the untreated group and 22.60 for the unmanipulated CIK cells vs 6.45 for CD33.CAR-CIK cell, n=4 NSG mice per group, p= 0.02). PDX of two different AML samples at the onset were established to be used as models mimicking different disease conditions. In an "early treatment" model using secondary transplanted PDX, a setting which presumably reflects the typical LIC properties, a clear engraftment reduction in the treated cohort was observed, nearly undetectable in 2/5 mice, as compared to the untreated mice (up to 70% in BM). A significant leukemia reduction was also measured in the peripheral blood and spleen of treated mice, showing CD33.CAR-CIK cell potential of reducing AML dissemination in the periphery. When ex vivo re-exposed to CD33.CAR-CIK cells residual AML cells were still sensitive to the treatment, indicating that no resistance mechanisms occurred. CD33.CAR-CIK cells were also effective in a second model by which the treatment started when AML engraftment was clearly manifested in the BM (&gt; 1%). Finally, when starting CD33.CAR-CIK cell treatment after disease recurrence post induction therapy, a significant disease reduction was observed in the CD33.CAR-CIK-treated group, reaching undetectable levels in half of the mice, as compared to chemotherapy-only treated mice (up to 60% of engraftment in BM)(Figure 1). Conclusions The employment of a non-viral SB-based CD33.CAR-gene transfer approach, which is overall associated to less cumbersome protocols and reduces the cost of goods, offers a unique alternative to current viral-based strategies to be explored in the setting of resistant forms of AML. Disclosures No relevant conflicts of interest to declare.