Primitive Hematopoietic Progenitor Research Articles

Abstract 2778: Influence of the differentiation strategy on the phenotypic and genotypic features of iPSC derived NK cells

Abstract The iPSC-derived NK (iNK) cells offer new perspectives to produce large-scale of homogeneous immunotherapeutic cellular products and open the way towards manufacturing off-the-shelf cancer immunotherapies. Numerous strategies and protocols of differentiation of iPSCs toward NK lineage have been published in the last decade. Nevertheless, the embryological development of the NK cells and the impact of the differentiation strategy on the activity of the NK cells are still not clearly understood. Furthermore, very few studies have compared different NK differentiation strategies from iPSC. this study, we compared two previously reported protocols for NK cell generation from the same iPSC line, one feeder-based and the other feeder-free. In the first protocol, 3D hematopoietic differentiation was followed by lymphoid differentiation, using an OP9 DLL4 expressing feeder cell line. The second protocol employed a clinical-grade approach, utilizing the “spin embryoid body” method to generate primitive hematopoietic progenitors. Subsequently, these progenitors were differentiated towards the NK lineage in a feeder-free environment. Hematopoietic progenitors produced by these protocols have distinct phenotypes, suggesting variations in their maturity levels. Furthermore, the protocols have different kinetics of differentiation revealed by CD56 and CD7 phenotypes. Although, both protocols were able to efficiently generate functional and mature NK cells, characterized by the expression of activating and maturity ligands but with a distinct transcriptomic profile. iNK cells generated using feeders exhibit a more pronounced inflammatory profile with the expression of hallmark NFKB and STAT3 signaling pathways. In contrast, feeder-free iNK cells have a more proliferative profile as evidenced by increased expression of cell cycle factors such as E2F or Myc. Feeder-based iNK cells exhibit increased expression of transcription factors involved in NK cell differentiation, such as GATA2, STAT1, and IRF1. These transcriptional variances are reinforced by superior functionality observed in feeder-based NK cells compared to feeder-free NK cells, evidenced by increased degranulation (p-value < 0.0001) and cytotoxicity (p-value = 0.0004) at a 1:2 (effector-to-target) ratio against the K562 cell line. Our findings indicate that, despite the advantages of the feeder-free strategy, the feeder-based protocol remains more efficient and produces iPSC-derived NK cells with an enhanced cytotoxic profile in vitro. Our comprehensive transcriptional analysis offers significant insights into the biological and functional properties of iNK cells. This information has the potential to oncover novel mechanisms for NK differentiation strategies from iPSCs. Citation Format: Matthias Huyghe, Christophe Desterke, Jusuf Imeri, Nathan Belliard, Ali G. Turhan, Annelise Bennaceur Griscelli, Frank Griscelli. Influence of the differentiation strategy on the phenotypic and genotypic features of iPSC derived NK cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2778.

A germline point mutation in the MYC-FBW7 phosphodegron initiates hematopoietic malignancies.

Oncogenic activation of MYC in cancers predominantly involves increased transcription rather than coding region mutations. However, MYC-dependent lymphomas frequently acquire point mutations in the MYC phosphodegron, including at threonine 58 (T58), where phosphorylation permits binding via the FBW7 ubiquitin ligase triggering MYC degradation. To understand how T58 phosphorylation functions in normal cell physiology, we introduced an alanine mutation at T58 (T58A) into the endogenous c-Myc locus in the mouse germline. While MYC-T58A mice develop normally, lymphomas and myeloid leukemias emerge in ∼60% of adult homozygous T58A mice. We found that primitive hematopoietic progenitor cells from MYC-T58A mice exhibit aberrant self-renewal normally associated with hematopoietic stem cells (HSCs) and up-regulate a subset of MYC target genes important in maintaining stem/progenitor cell balance. In lymphocytes, genomic occupancy by MYC-T58A was increased at all promoters compared with WT MYC, while genes differentially expressed in a T58A-dependent manner were significantly more proximal to MYC-bound enhancers. MYC-T58A lymphocyte progenitors exhibited metabolic alterations and decreased activation of inflammatory and apoptotic pathways. Our data demonstrate that a single point mutation stabilizing MYC is sufficient to skew target gene expression, producing a profound gain of function in multipotential hematopoietic progenitors associated with self-renewal and initiation of lymphomas and leukemias.

792-P: GLP-1 RA Therapy Increases Circulating Vascular Regenerative Cell Content and Decreases Inflammatory Burden in People with T2D

CV outcome trials have established that long-term use of GLP-1 receptor agonists (GLP-1RAs) yields important CV benefits in people with T2D. Similar to SGLT2i, GLP-1RAs are recommended for people with T2D who have established CVD and heart failure regardless of their A1C. In a randomized trial of people with T2D and coronary artery disease, use of the SGLT2i empagliflozin for 6 months increased the frequency of vascular regenerative (VR) cells in the circulation. To determine if GLP-1RA therapy has a similar impact on the same VR cell populations, we retrospectively analyzed data from an ongoing research program with adults >40 years of age with T2D. Peripheral blood mononuclear cells were isolated and profiled with an established flow cytometry assay that was founded on the activity of the anti-oxidative enzyme aldehyde dehydrogenase (ALDH) that is highly expressed in progenitor (ALDHhi) versus mature (ALDHlow) cells. At baseline, 17 participants were on a GLP-1RA (9 of whom were also on an SGLT2i), 33 were on an SGLT2i but not a GLP-1RA, and 28 were on neither drug class. Mean age, weight, BMI, A1C, and duration of T2D was balanced across the groups. The frequency of ALDHhiSSClow cells (primitive hematopoietic and endothelial progenitor cells that coordinate angiogenesis) was higher in participants who were on a GLP-1RA (0.064%) or an SGLT2i (0.048%) relative to those not on either drug class (0.037%; P<0.05 for both). The frequencies of VR ALDHhiSSCmid monocytes in the three groups were comparable. The frequency of pro-inflammatory ALDHhiSSChi granulocyte precursor cells in participants on a GLP-1RA (2.9%) was lower than that in individuals on an SGLT2i (4.3%; P=0.05) or not on either drug class (5.2%; P<0.01). GLP-1RA therapy in people with T2D was associated with greater VR cell content and lower inflammatory burden. Augmented vessel repair mediated by higher VR cell content may account in part for the CV benefits observed with GLP-1RA in the setting of T2D. Disclosure A.Krishnaraj: None. B.Park: None. E.Bakbak: None. Y.Pan: None. A.Quan: None. H.Teoh: None. S.Verma: Advisory Panel; Amgen Canada, AstraZeneca, Bayer Inc., Boehringer Ingelheim and Eli Lilly Alliance, HLS Therapeutics Inc., Janssen Pharmaceuticals, Inc., Novartis Canada, Novartis, Novo Nordisk, Novo Nordisk Canada Inc., Consultant; AstraZeneca, Other Relationship; Amarin Corporation, AstraZeneca, Bayer Inc., Boehringer Ingelheim and Eli Lilly Alliance, Boehringer Ingelheim International GmbH, Canadian Medical and Surgical Knowledge Translation Research Group, EOCI Pharmacomm, HLS Therapeutics Inc., Janssen Pharmaceuticals, Inc., Novartis Canada, Novo Nordisk, Novo Nordisk Canada Inc., Pfizer Inc., PhaseBio Pharmaceuticals, Inc., S & L Solutions Event Management Inc, Sanofi, Sun Pharmaceutical Industries Ltd., Toronto Knowledge Translation Working Group, Research Support; Amarin Corporation, Amgen Canada, AstraZeneca, Bayer Inc., Boehringer Ingelheim International GmbH, HLS Therapeutics Inc., Novartis, Novo Nordisk, Pfizer Inc., PhaseBio Pharmaceuticals, Inc. D.A.Hess: None.

Impact of the Aryl Hydrocarbon Receptor on Aurora A Kinase and the G2/M Phase Pathway in Hematopoietic Stem and Progenitor Cells

Recent evidence suggests that the environment-sensing transcription factor aryl hydrocarbon receptor (AHR) is an important regulator of hematopoiesis. Yet, the mechanisms and extent of AHR-mediated regulation within the most primitive hematopoietic cells, hematopoietic stem and progenitor cells (HSPCs), are poorly understood. Through a combination of transcriptomic and flow cytometric approaches, this study provides new insight into how the AHR influences hematopoietic stem and progenitor cells. Comparative analysis of intraphenotypic transcriptomes of hematopoietic stem cells (HSCs) and multipotent progenitor (MPP) cells from AHR knockout (AHR KO) and wild type mice revealed significant differences in gene expression patterns. Notable among these were differences in expression of cell cycle regulators, specifically an enrichment of G2/M checkpoint genes when Ahr was absent. This included the regulator Aurora A kinase (Aurka, AurA). Analysis of AurA protein levels in HSPC subsets using flow cytometry, in combination with inducible AHR KO or in vivo AHR antagonism, showed that attenuation of AHR increased levels of AurA in HSCs and lineage-biased MPP cells. Overall, these data highlight a potential novel mechanism by which AHR controls HSC homeostasis and HSPC differentiation. These findings advance the understanding of how AHR influences and regulates primitive hematopoiesis.

IPSC-Derived MSCs Are a Distinct Entity of MSCs with Higher Therapeutic Potential than Their Donor-Matched Parental MSCs

Mesenchymal stromal cells derived from induced pluripotent stem cells (iMSCs) have been proposed as alternative sources of primary MSCs with various advantages for cell therapeutic trials. However, precise evaluation of the differences between iMSCs and primary MSCs is lacking due to individual variations in the donor cells, which obscure direct comparisons between the two. In this study, we generated donor-matched iMSCs from individual bone marrow-derived MSCs and directly compared their cell-autonomous and paracrine therapeutic effects. We found that the transition from primary MSCs to iMSCs is accompanied by a functional shift towards higher proliferative activity, with variations in differentiation potential in a donor cell-dependent manner. The transition from MSCs to iMSCs was associated with common changes in transcriptomic and proteomic profiles beyond the variations of their individual donors, revealing expression patterns unique for the iMSCs. These iMSC-specific patterns were characterized by a shift in cell fate towards a pericyte-like state and enhanced secretion of paracrine cytokine/growth factors. Accordingly, iMSCs exhibited higher support for the self-renewing expansion of primitive hematopoietic progenitors and more potent immune suppression of allogenic immune responses than MSCs. Our study suggests that iMSCs represent a separate entity of MSCs with unique therapeutic potential distinct from their parental MSCs, but points to the need for iMSC characterization in the individual basis.

A Novel Role for PLK1 in Leukemia Stem Cell Function in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy with poor prognosis. While most patients achieve initial remission following standard chemotherapy, the majority will relapse and ultimately die. Poor outcomes in AML are tied to the properties of leukemia stem cells (LSC), which drive therapy resistance and relapse. For successful therapeutic targeting, it is critical to understand the pathways that operate in LSC function and survival and how they are impacted by drug activity. We carried out an innovative stemness-focused high-throughput drug screen employing a hierarchical AML cell model and a Nanostring-based LSC signature assay to probe for genes and pathways that impact LSC function. All 4 small molecule inhibitors of Polo-like Kinase 1 (PLK1) included in the compound library were among the top hits. PLK1 inhibitor treatment selectively targeted the CD34⁺CD38⁻ fraction of OCI-AML-8227 cells, which is enriched for quiescent LSCs. PLK1 is an important cell cycle regulator; although there are reports for cell cycle-independent roles, they have not been systematically characterized. Importantly, the role of PLK1 has not been previously studied in the context of cancer stem cells. To investigate the role of PLK1 in LSCs, we performed single-cell (sc) multi-omics incorporating both scRNAseq and scATACseq on the CD34⁺CD38⁻ compartment of OCI-AML-8227 cells. Enrichment analysis of previously-identified chromatin accessibility signatures of primitive long-term hematopoietic stem cells (LT-HSC) and progenitors was used to identify cell clusters with distinct stemness and differentiation properties. Interestingly, PLK1 mRNA expression in OCI-AML-8227 was restricted to cells enriched for the LT-HSC epigenetic signature (Fig 1A). In healthy human bone marrow, PLK1 expression was restricted to committed progenitors and not detected in HSCs (Fig 1B). These data strongly suggest a previously unexplored role of PLK1 in regulating stemness that may be specific LSCs and not normal HSCs. To determine the effects of PLK1 against LSCs in vivo, we carried out PLK1 inhibitor treatment using the specific PLK1 inhibitor volasertib in a cohort of 9 primary AML samples in PDX models. Volasertib treatment significantly reduced leukemic burden in 7 of 9 samples, all of which had a high LSC17 score (associated with poor prognosis and standard chemotherapy response). To quantify effects on LSCs, we serially transplanted 2 samples at limiting doses into untreated secondary recipients and demonstrated a 5- to 25-fold reduction in LSC frequency following volasertib treatment, indicating that functional LSCs were targeted by PLK1 inhibition. Together, these data strongly suggest that PLK1 impacts LSC function and stemness, and that PLK1 inhibitors are acting through an uncharacterized cell-cycle independent mechanism to target quiescent LSCs. To discover novel PLK1 signaling pathways, we carried out proximity-dependent biotinylation (PDB) to characterize the PLK1 interactome in serum-starved cells. Preliminary analysis has revealed a number of previously uncharacterized PLK1 interactors, including components of autophagy and apoptosis machinery, that are currently being validated in the hematopoietic cell context. Our study highlights PLK1 as a specific vulnerability of LSCs, and furthers the understanding of malignant PLK1 signaling in non-cell cycle related processes. Further insights into the underlying mechanisms will enable the development of effective therapies to target these pathways and improve patient outcomes in AML. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Acetyl-CoA Carboxylase 1 Is Essential for Hematopoietic Stem Cell Homeostasis

Though fatty acid oxidation has been shown to be essential for hematopoietic stem cell maintenance, the importance of fatty acid uptake from the bone marrow microenvironment vs. de novo lipogenesis has not been elucidated. The process of de novo lipogenesis begins with the generation of malonyl-CoA from acetyl-CoA and is catalyzed by the enzyme acetyl-CoA carboxylase 1 (ACC1), encoded by Acaca. In leukemia, both ACC1 activation and degradation of ACC1 have been shown to enhance disease progression, suggesting an important role in leukemogenesis. The function of ACC1 in normal hematopoiesis remains unknown. To characterize the role of ACC1 in normal hematopoiesis, we bred mice that harbored an Acaca allele in which exons 22 through 26 were flanked by loxp sites. We crossed these animals to a mouse line harboring the Mx1-Cre transgene, generating Acaca fl/fl; Mx1-Cre + (ACC1 KO) and Acaca fl/fl; Mx1-Cre -(ACC1 WT) mice. Animals were then treated with intraperitoneal poly(I:C) injection to drive Mx1-Cre expression and Acaca excision. By 4-6 weeks after poly(I:C) injection, ACC1 KO animals developed expansion of hematopoietic stem cells and progenitors, including Lineage - Sca-1 +cKit hi (LSK) and the LSK CD48 -CD150 + long-term hematopoietic stem cell (LT-HSC) populations. In addition, these animals developed splenomegaly with extramedullary LSK expansion and enhanced myelopoiesis. ACC1 KO LSKs showed increased cell cycle activity and LT-HSCs demonstrated a reduced quiescent fraction compared to ACC1 WT controls. These data suggest that ACC1 deficiency results in increased cell cycle activity among primitive hematopoietic progenitors and biases cell fate to the myeloid lineage. To further test the impact of ACC1 deletion on LT-HSC function, we established competitive chimerism maintenance assays in which equal numbers of CD45.2 +Acaca fl/fl; Mx1-Cre +or CD45.2 +Acaca fl/fl; Mx1-Cre - bone marrow cells were mixed with CD45.1 + competitor bone marrow and transplanted into lethally irradiated recipient mice. Transplants were then given 8 weeks to establish stable engraftment. After documenting comparable engraftment, 5-poly(I:C) injections were administered every other day. Over the course of 16 weeks of tracking, the ACC1 KO graft showed a significant reduction in trilineage hematopoietic output as indicated by a reduction in B cell, T cell, and myeloid populations in the peripheral blood. After 16 weeks, animals were sacrificed and the LT-HSC compartment was analyzed, showing a significant reduction in ACC1 KO LT-HSCs. Together, these data suggest that the loss of ACC1 significantly impairs the ability of LT-HSCs to maintain long term hematopoiesis in competitive transplants. Collectively, these data are the first report of a critical role for ACC1 in hematopoiesis and LT-HSC function. Mechanistically, in the absence of ACC1, FACS sorted LSK demonstrated increased ATP content compared to WT controls, suggesting an altered metabolic state in these progenitor cells. Further studies are ongoing to determine if these findings are due to an essential function for ACC1 within the context of de novo lipogenesis, or if ACC1 participates in an as of yet uncharacterized tumor suppressive role. DisclosuresNo relevant conflicts of interest to declare.

BAALC Is a Key Mediator of Leukemia Development in Congenital Neutropenia

We recently reported high frequency of cooperating acquired CSF3R and RUNX1 mutations in severe congenital neutropenia (CN) patients that developed AML or MDS. CN is a pre-leukemic bone marrow failure syndrome (BMFS) and longitudinal retrospective analysis of CN bone marrow samples allows identification of the pathological intracellular events that lead to leukemogenic transformation.In the present study, we report the identification of a key mechanism of leukemogenesis in CN. We made use of the induced pluripotent stem cell (iPSC) model and embryoid body (EB)-based in vitro hematopoietic differentiation of iPSCs generated from a CN patient harboring ELANE mutation before (CN-iPSCs) and after (CN/AML-iPSCs) overt AML. CN/AML-iPSCs clones revealed CSF3R and RUNX1 mutations and trisomy 21. EB-based hematopoietic differentiation revealed that CN-iPSCs produced similar amounts of CD34+ cells, but approximately 2-fold reduction of mature granulocytes and CFU-G colonies in CFU-assay, as compared to healthy donor-derived iPSCs. CN/AML-iPSCs produced CD34+ cells but did not differentiate into mature granulocytes at all.Analysis of the gene expression profiles of five CN/AML patients including our index patient, revealed that genes upregulated in early hematopoietic stem/progenitor cells (HSPCs), such as Brain And Acute Leukemia, Cytoplasmic (BAALC), Hematopoietic Prostaglandin D Synthase (HPGDS), Natriuretic Peptide Receptor 3 (NPR3) and CD109 were markedly increased in CN/AML blasts harboring both RUNX1 and CSF3R mutations, as compared to hematopoietic cells prior to leukemia. This genetic signature suggests transformation of hematopoietic cells into more primitive hematopoietic progenitors after acquisition of CSF3R and RUNX1 mutations. We were able to confirm strong increase of BAALC, HPGDS, NPR3 and CD109 mRNA levels in CD34+ cells derived from CN/AML-iPSCs, as compared to CN-iPSCs, suggesting that our iPSC-model is reliable to study leukemogenesis.In order to evaluate which of these candidate genes (BAALC, HPGDS, NPR3 and CD109)is responsible for the leukemogenic transformation in CN, we generated CN/AML-iPSCs clones deficient for one selected genes. We established CRISPR-Cas9-mediated gene knockout using transfection of CN/AML-iPSCs with pSpCas9(BB)-2A-GFP vector expressing Cas9 and sgRNA, that were designed to specifically target first exon of each candidate gene. CN/AML-iPSCs transfected with specific sgRNAs were sorted and single cell clones were analyzed further.Knockout of the selected genes did not affect survival and pluripotency of CN/AML-iPSCs and we were able to study EB-based hematopoietic differentiation of gene-edited CN/AML-iPSCs clones. Interestingly, knockout of HPGDS, NPR3 or CD109 did not correct defective granulocytic differentiation of CN/AML-iPSCs, whereas knockout of BAALC in CN/AML-iPSCs resulted in the tremendous increase of the granulocytic differentiation, as assessed by FACS, CFU assay, morphological analysis of cytospin preparations and nitroblue tetrazolium (NBT) assay. Granulocytic differentiation of BAALC -deficient CN/AML-iPSCs was comparable to CN-iPSCs, but was still reduced in comparison to healthy donor-derived iPSCs.These data clearly showed that knockout of BAALC in CN/AML-iPSCs overcomes differentiation block at the early hematopoietic progenitor cell stage, demonstrating a fundamental role of BAALC in leukemia development in CN. BAALC, HPGDS and CD109 are known to be markedly up-regulated in RUNX1 -mutated de novo AML and qRT-PCR of CD34+ cells generated from BAALC -knockout CN/AML-iPSCs revealed a decrease of mRNA expression levels of BAALC, HPGDS and CD109 . At the same time, knockout of RUNX1 in CN/AML-iPSCs using CRISPR/Cas9-editing revealed even higher decrease of mRNA levels of BAALC, NPR3 and CD109 . These data suggest that HPGDS and CD109 up-regulation is downstream of RUNX1 -mediated BAALC induction in AML.Taken together, we established a reliable in vitro model to study leukemogenesis in the pre-leukemic BMFS, using CN as an example. With the help of this model, we identified BAALC up-regulation as a key “leukemogenic” factor downstream of RUNX1 and CSF3R mutations. Based on these observations, we conclude that inhibition of BAALC in hematopoietic cells of CN patients may prevent leukemogenic transformation of hematopoiesis or eliminate AML blasts in CN-AML. DisclosuresNo relevant conflicts of interest to declare.

Customized Induced Pluripotent Stem Cell-Derived Red Cell Reagents

Red blood cell transfusion is an essential therapy for patients with sickle cell disease (SCD). A major problem is the complexity of antibodies found in patients with SCD, in large part due to RH gene variation. Rh antibodies are common despite provision of serologic Rh-matched transfusions. The antibodies may be mistaken for autoantibodies when in fact they are alloantibodies to high prevalence Rh antigens that are absent or have altered Rh epitopes on the patient's RBCs. Finding compatible red cell units is often complicated by a lack of reagent red cells to properly identify the fine antibody specificity. This can delay patient care, increase costs, and make transfusion therapy impossible for some patients. We hypothesized that human induced pluripotent stem cells (iPSCs) could be reprogrammed from rare donors or genetically engineered to produce standard and reliable red cell reagents to aid identification for patients with complex antibodies. However, most human iPSC-derived red blood cells (iRBCs) are developmentally primitive red cells that differ from donor-derived red cells with high levels of embryonic globin expression, are larger in size and fail to enucleate in culture. We aimed to improve iPSC hematopoietic differentiation protocols to generate iRBCs compatible with standard blood bank assays.We designed a panel of customized iPSCs that included 1) Rh null cells, 2) cells lacking the high prevalence Rh antigen termed hrS, 3) cells expressing a partial C antigen and lacking the high prevalence Rh antigen termed hrB and, 4) cells expressing the low prevalence Rh antigens V, VS and lacking hrB. For the Rh null line, we identified wild type iPSC lines that were group O, RhD- by homozygous RHD deletion. We used CRISPR/Cas9 genetic engineering to disrupt the RHCE alleles either by a large deletion from exon 1 to exon 2 and/or via insertion-deletion mutations resulting in a frameshift and early stop codon. For lines lacking high prevalence Rh antigens, we reprogrammed group O donor cells whose genotypes were heterozygous for RHD*DAU0/RHCE*ceMO and RHD*DOL/RHCE*ceBI, homozygous for RHD*DIIIa-CE(4-7)-D/RHCE*ceS, and homozygous for RHD/RHCE*ce733G, respectively. At least 2 independent clones from each customized line were characterized for pluripotency by standard flow cytometry and gene expression markers, and a normal karyotype confirmed.We performed hematopoietic differentiation of customized iPSCs by a 3-step embryoid body protocol in defined, serum-free media with combinations of hematopoietic cytokines. We obtained CD41+235+ primitive hematopoietic progenitors on day 8 and cultured these cells in liquid culture with erythroid specific cytokines to generate iRBCs. We assessed erythroid cell maturation by morphology on cytospin preparations and by flow cytometry. We found an almost pure population of CD71+235+ iRBCs with erythroblast morphology by day 6 of culture, and progressive maturation to Band 3 high, alpha integrin low iRBCs that were smaller and had condensed nuclei that resemble erythroid cultures of primary CD34+ cells. Comparison of total Rh protein expression by flow cytometry on untargeted group O, D+ iRBCs compared to donor-derived RBCs demonstrated comparable cell surface Rh expression. As proof of principal, we assessed RBC agglutination by gel card assay of untargeted group O, D+, E-, e+ and the genetically engineered Rh null iRBCs using standard anti-D, -E, and -e reagents for Rh typing (Ortho Bioclone). As expected, the untargeted O, D+ iRBCs agglutinated with anti-D and anti-e while the engineered Rh-null iRBCs showed no agglutination with all 3 antibodies. Only 0.5 e6 iRBCs were required per gel assay.We have designed a panel of customized iPSCs reprogrammed from rare donors or genetically engineered to express rare blood group antigen phenotypes or combinations that are difficult or impossible to find as donor red cells. Any number of combinations not found in natural populations can be produced and generated in quantities sufficient for reagents. iRBC produced from these customized iPSCs can be used with standard blood bank assays and potentially provide the means to streamline and standardize antibody identification in alloimmunized patients with complex antibody specificities. In the future, when technology for scale-up is available, Rh null iRBCs could be used as “universal” donor cells for future therapeutic applications. DisclosuresNo relevant conflicts of interest to declare.

Effects of Rigosertib (RIGO) Alone or in Combination with Azacitidine or Vorinostat on Epigenetic Reprogramming of CD34+ Cells in the Myelodysplastic Syndrome

Rigosertib (RIG) which functions as a RAS-mimetic, binds to the RAS-binding domain (RBD) impacting many pathways that involve RBD-mediated interactions, including the RAS-Raf and AKT/PI3-kinase pathways (Divakar et al., Cell 2016). Recently, we demonstrated that RIG also functions as an epigenetic modifier in MDS. RIG alone or in combination with Azacitidine (AZA) exerted epigenetic effects on the global histone post-translational modifications (PTMs), chromatin remodelers, HDACs, and DNMTs and caused enhanced apoptosis in MDS (MDS-L) and AML (BW-90) cell lines as well as in bone marrow cells (BM) derived from pts with MDS (Chaurasia et al ASCO 2016). In clinical trials, the combination of RIG and AZA demonstrated an overall response rate of 76% in all MDS patients; 62% response in pts who had failed prior hypomethyating agent (HMA) (Navada et al ASH 2016) and 85% in HMA naive pts. The growing evidence suggests that combined therapy may be more effective than either agent alone. To further study these effects, we explored activity of RIG in combination with Vorinostat (VOR), a histone deacetylase inhibitor, on the cell lines. Combined treatment in sequential order VOR/RIG and RIG/VOR potentiates far greater effects on the global PTMs, HDACs, DNMTs, chromatin remodelers, cell cycle checkpoint proteins and PI3/AKT pathway than RIG/AZA or AZA/RIG.RIG in combination with AZA or VOR (AZA/RIG, RIG/AZA, VOR/RIG or RIG/VOR) demonstrated differential effects on the association of RNA polymerase II (Pol II) with active histone marks (H3K4me3 and H3K4me2) in MDS-L and BW-90 cells. An overall decrease in association of Pol II/H3K4me2 was observed with the combinations (AZA/RIG, VOR/RIG or vice versa) compared to control in MDS-L and BW-90, 10-33% (ANOVA, p=0.0006), 9-20% (ANOVA, p=0.0004), respectively. Significant differences were observed in association of H3K4me3/Pol II in BW-90 cells (7-30%; ANOVA, p<0.0001). Similarly, in BM from a pt with MDS after 1 cycle of RIG and AZA treatment demonstrated a decrease in association of Pol II with H3K4me2 (67%) and H3K4me3 (28%).Treatment of MDS-L cells with RIG alone or in combination with AZA failed to induce expansion of CD34+ cells, but the RIG/VOR combination induced 1.9-fold expansion of CD34+ cells (ANOVA p=0.002). Individually AZA or VOR treatment led to an enhanced expansion of CD34+ cells, 3.8 and 4.0 fold, respectively (ANOVA, p=0.004). RIG alone or with AZA yielded maximum aldehyde dehydrogenase (ALDH) activity, a marker of primitive hematopoietic stem and progenitor cells (HSPCs) (ANOVA, p=0.006). However, a marked decrease in ALDH activity was observed in AZA or VOR or RIG/VOR, that was inversely proportion to the expansion of CD34+ cells.In a single MDS pt treated with RIG/AZA, expansion of primitive HPSCs expressing low levels of CD34 was observed. A highly expressed subpopulation of HPSCs, which co-existed prior to treatment of the pt and disappeared upon treatment, suggests either an induced cell death or commitment of HSPCs towards improved lineage differentiation in conjunction with clinical response.In order to evaluate effects of RIG on expansion of CD34+ cells, we examined the pluripotent genes (SOX2, OCT4 NANOG and ZIC3) in the BM from MDS pts, prior and after RIG/AZA treatment. RNA expression levels of SOX2, OCT4, NANOG and ZIC3 were elevated post treatment (≥2 fold). In MDS-L cells the pluripotency genes were highly upregulated in the presence of RIG or RIG/AZA or RIG/VOR (1.7-34 folds). These findings indicate that expression of pluripotency genes is a consequence of epigenetic reprogramming that favors expansion of more primitive HSPCs in a pt, while increase in ALDH activity was inversely proportional to the enhanced expansion of CD34+ cells in MDS-L suggesting a more primitive state of HSPC.The epigenetic events modulated by RIG in combination with either AZA or VOR led to global histone PTMs, differential Pol II association with active histone marks, epigenetic reprogramming of pluripotency genes and expansion of primitive HSPC with downregulation of the PI3K/AKT pathway and cell cycle checkpoint proteins. Collectively, these studies indicate that epigenetic effects of RIG on chromatin alterations lead to HSPC reprogramming that may lead to improved hematopoietic function and response in the clinical setting. These preclinical models suggest potential novel clinical strategies to improve outcomes for patients with higher-risk MDS. DisclosuresReddy:Onconova Therapeutics: Equity Ownership.

HOTAIR expression and prognostic impact in acute myeloid leukemia patients

BackgroundAcute myeloid leukemia (AML) is a disorder characterized by a rapid onset of symptoms attributable to bone marrow failure due to clonal proliferation of primitive hematopoietic stem cells or progenitor cells. Epigenetic abnormalities play an important role in the development and progression of acute leukemia. Long non-coding ribonucleic acid (lncRNA) plays an important role in epigenetic regulation. Homeobox (Hox) transcript antisense intergenic RNA (HOTAIR) is a lncRNA which has been determined to be a negative prognostic indicator in various solid-tumor patients. However, its role in hematopoietic tumors as AML is to be assessed. This study aimed at measuring lncRNA HOTAIR expression level on bone marrow (BM) mononuclear cells in newly diagnosed AML patients and correlating its expression with their outcome and different prognostic variables. This provides new prospective for a novel marker involved in development and progression of AML which can be used as a diagnostic marker and a target of therapy. The current study included 65 subjects divided into 35 newly diagnosed AML adult patients (before initiation of chemotherapy) and 30 non-leukemic adult patients who are candidates for BM aspiration for causes other than hematological malignancies as immune thrombocytopenic purpura and hypersplenism as controls. HOTAIR expression was measured on BM mononuclear cells by quantitative reverse transcription polymerase chain reaction (qRT-PCR).ResultsHOTAIR expression was found to be significantly upregulated in AML patients (probability (p) value = 0.000) and it can be used as a diagnostic biomarker of AML as confirmed by a significant difference between cases and controls using receiver operating characteristic curve (ROC) analysis. However, it was not significantly correlated with event free survival (EFS) or prognostic variables.ConclusionThis study showed that the expression of HOTAIR is upregulated in de novo AML patients and can be used as a diagnostic marker. However, highly expressed HOTAIR is not associated with poor prognosis.

TET2 and DNMT3A Mutations Exert Divergent Effects on DNA Repair and Sensitivity of Leukemia Cells to PARP Inhibitors

Somatic variants in TET2 and DNMT3A are founding mutations in hematological malignancies that both affect the epigenetic regulation of DNA methylation. Although the proteins antagonistically regulate the epigenetic mark of 5-methylcytosine (5-mC), where DNMT3A catalyzes addition of 5-mC while TET2 oxidizes 5-mC as a first step in DNA demethylation, mutations in both genes appear in a similar spectrum of human hematopoietic malignancies. Mutations in both genes often co-occur with activating mutations in oncogenic tyrosine kinases (OTKs) such as FLT3ITD, BCR-ABL1, JAK2V617F, MPLW515L or mutations affecting related signaling pathways such as NRASG12D and CALRdel52. Moreover, while the mutations exert divergent effect on primitive hematopoietic progenitor cells, they lead to similar disease phenotypes, suggesting the roles of these mutations in hematopoietic malignancies may relate to mechanisms outside of DNA methylation. OTK-positive malignant cells accumulate high numbers of spontaneous and drug-induced DNA double-strand breaks (DSBs) in comparison to normal cells, but they manage to survive because of their enhanced/altered ability to repair these breaks. DSBs, the most lethal DNA lesions, are repaired by two major mechanisms, BRCA1/2-dependent homologous recombination (HR) and DNA-PK -mediated non-homologous end-joining (D-NHEJ). Both HR and D-NHEJ repair DSBs in proliferating cells, while D-NHEJ plays a major role in quiescent cells. PARP1 -dependent alternative NHEJ (Alt-NHEJ) serves as back-up in both proliferating and quiescent cells. The existence of these redundant pathways creates the opportunity to employ a phenomenon called "synthetic lethality", which was originally applied to eliminate cancer cells with mutations in BRCA1 and BRCA2 by PARP inhibitor (PARPi). Our previous report [M. Nieborowska-Skorska et al., Gene expression and mutation-guided synthetic lethality eradicates proliferating and quiescent leukemia cells. J Clin Invest 127, 2392-2406 (2017)] suggested that certain leukemias are sensitive to PARPi-triggered synthetic lethality. Here we show that TET2 and DNMT3A mutations exert divergent roles in regulating DNA repair activities in leukemia cells expressing OTKs. Malignant TET2-deficient cells display downregulation of BRCA1 and LIG4 resulting in reduced activity of HR and D-NHEJ, respectively, and rely on Alt-NHEJ to protect them from the toxic effects of replication stress and drug-induced DSBs. Conversely, DNMT3A-deficient cells favor HR/D-NHEJ owing to downregulation of PARP1 and reduction of Alt-NHEJ. Consequently, malignant TET2-deficient cells are sensitive to PARPi treatment in vitro and in vivo, whereas DNMT3A-deficient cells were resistant. Disruption of TET2 dioxygenase activity and/or TET2 - Wilms tumor 1 (WT1) binding ability were responsible for DNA repair defects and sensitivity to PARPi associated with TET2 deficiency. Moreover, mutation or deletion of WT1 mimicked the effect of TET2 mutation on DSB repair activity and sensitivity to PARPi. Our findings reveal that TET2 and WT1 mutations may serve as biomarkers of synthetic lethality triggered by PARPi, which should be explored therapeutically. Disclosures Valent: Allcyte GmbH: Research Funding; Pfizer: Honoraria; Cellgene: Honoraria, Research Funding. Tallman:Abbvie: Research Funding; Cellerant: Research Funding; Orsenix: Research Funding; ADC Therapeutics: Research Funding; BioSight: Membership on an entity's Board of Directors or advisory committees, Research Funding; Glycomimetics: Research Funding; Rafael: Research Funding; Amgen: Research Funding; Bioline rx: Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Membership on an entity's Board of Directors or advisory committees; KAHR: Membership on an entity's Board of Directors or advisory committees; Rigel: Membership on an entity's Board of Directors or advisory committees; Delta Fly Pharma: Membership on an entity's Board of Directors or advisory committees; Oncolyze: Membership on an entity's Board of Directors or advisory committees; Jazz Pharma: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; UpToDate: Patents &amp; Royalties. Martinelli:Roche: Consultancy; Pfizer: Consultancy, Research Funding, Speakers Bureau; Incyte: Consultancy; Jazz: Consultancy; Janssen: Consultancy; Daichii Sankyo: Consultancy, Research Funding; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy; AbbVie: Consultancy, Research Funding. Vassiliou:Kymab Ltd - Monoclonal antibody company. Currently not working in myeloid cancers or clonal haematopoiesis.: Consultancy.

Mutational screening of RTK-BRAF genes in de novo adult acute myeloid leukemia

BackgroundAcute myeloid leukemia (AML) is derived from primitive hematopoietic stem cells or progenitor cells due to genetic variations such as chromosomal abnormalities and molecular alterations. Mutations in the RTK-BRAF genes are involved in leukaemogenesis. Few reports simultaneously concerning the mutation alteration of the RTK-BRAF genes have been publicized in AML subjects. The current study aimed to screen the mutations of RTK-BRAF genes in Iranian de novo adult AML subjects. MethodsMutations of RTK-BRAF genes were investigated in bone marrow specimens from 58 patients with de novo adult AML. High-Resolution Melt (HRM) curve analysis was performed for the identification of FLT3-ITD and D835 mutations. Mutations in the NPM1 (exon12), BRAF (exons11 and 15), PTPN11(exons 3 and 13) and c-FMS (exon22) were detected using Sanger sequencing method. ResultsTotally, FLT3 and NPM1 mutations were found in 34.48% and 25.86% of AML patients, respectively. Moreover, mutations in the FLT3 and NPM1 genes were significantly higher in normal karyotype AML subjects (52.38% and 38%, respectively) than patients with abnormal karyotype (p-value = 0.001 and p-value = 0.002, respectively). No mutations were detected in other RTK-BRAF genes. ConclusionAccording to the findings of this research, in addition to the chromosomal abnormalities in the patients with acute myeloid leukemia, FLT3 and NPM1 mutations were the most recurrent genetic alterations. Identification of these alterations may help to choose appropriate treatment and improve the patients' outcome.

Modulating TNFα activity allows transgenic IL15-Expressing CLL-1 CAR T cells to safely eliminate acute myeloid leukemia

BackgroundC-type lectin-like molecule 1 (CLL-1) is highly expressed in acute myeloid leukemia (AML) but is absent in primitive hematopoietic progenitors, making it an attractive target for a chimeric antigen receptor...

2018 – EOMESODERMIN GOVERNS THE HEMOGENIC COMPETENCE OF MURINE YOLK-SAC MESODERMAL PROGENITORS

The gene regulatory networks that coordinate hematopoietic commitment in the developing murine yolk-sac (YS) remain ill-defined. Here we report that the T-box transcription factor Eomesodermin (Eomes) is transiently expressed in mesodermal progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and definitive hematopoietic progenitors but dispensable for the development of endothelial cells. Bulk RNA-seq and single-cell-RNA-seq experiments demonstrate that in the absence of Eomes function Flk-1+ hematovascular mesoderm is specified but upon further differentiation it is diverted towards an endothelial rather than hematopoietic fate. Utilizing ESC reporter lines, we show that Eomes is expressed prior to both SCL and Runx1 during mesoderm patterning. Interestingly, Eomes activity is dispensable for the expression of SCL but is essential for the normal development of Runx1+ hemogenic endothelium (HE). ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify the hematopoietic fate. Finally, ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signalling at mesodermal stages of development. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, to promote hemogenic competence of the YS mesodermal lineage. These results suggest that hemogenic competence is endowed earlier during murine embryogenesis than previously appreciated and have widespread implications for the generation of HE from pluripotent stem cell sources.

MYB bi-allelic targeting abrogates primitive clonogenic progenitors while the emergence of primitive blood cells is not affected.

MYB is a key regulator of definitive hematopoiesis and it is dispensable for the development of primitive hematopoietic cells in vertebrates. In order to delineate definitive versus primitive hematopoiesis during differentiation of human embryonic stem cells, we have introduced reporters into the MYB locus and inactivated the gene by bi-allelic targeting. In order to recapitulate the early developmental events more adequately, mutant and wild-type human embryonic stem cell lines were differentiated in defined culture conditions without the addition of hematopoietic cytokines. The differentiation of the reporter cell lines demonstrated that MYB is specifically expressed throughout emerging hematopoietic cell populations. Here we show that the disruption of the MYB gene leads to severe defects in the development and proliferation of primitive hematopoietic progenitors while the emergence of primitive blood cells is not affected. We also provide evidence that MYB is essential for neutrophil and T-cell development and the upregulation of innate immunity genes during hematopoietic differentiation. Our results suggest that the endothelial origin of primitive blood cells is direct and does not include the intermediate step of primitive hematopoietic progenitors.

Ryk modulates the niche activity of mesenchymal stromal cells by fine-tuning canonical Wnt signaling

The importance of modulating the intensity of Wnt signaling has been highlighted in various biological models, but their mechanisms remain unclear. In this study, we found that Ryk—an atypical Wnt receptor with a pseudokinase domain—has a Wnt-modulating effect in bone marrow stromal cells to control hematopoiesis-supporting activities. We first found that Ryk is predominantly expressed in the mesenchymal stromal cells (MSCs) of the bone marrow (BM) compared with hematopoietic cells. Downregulation of Ryk in MSCs decreased their clonogenic activity and ability to support self-renewing expansion of primitive hematopoietic progenitors (HPCs) in response to canonical Wnt ligands. In contrast, under high concentrations of Wnt, Ryk exerted suppressive effects on the transactivation of target genes and HPC-supporting effects in MSCs, thus fine-tuning the signaling intensity of Wnt in BM stromal cells. This ability of Ryk to modulate the HPC-supporting niche activity of MSCs was abrogated by induction of deletion mutants of Ryk lacking the intracellular domain or extracellular domain, indicating that the pseudokinase-containing intracellular domain mediates the Wnt-modulating effects in response to extracellular Wnt ligands. These findings indicate that the ability of the BM microenvironment to respond to extracellular signals and support hematopoiesis may be fine-tuned by Ryk via modulation of Wnt signaling intensity to coordinate hematopoietic activity.

The role of phosphorylation of MLF2 at serine 24 in BCR-ABL leukemogenesis.

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder defined by the presence of the fusion gene BCR-ABL1 in primitive hematopoietic progenitors. The myeloid leukemia factors (MLFs) were identified in the fly and human, and are involved in acute leukemia and enhancing the myeloid factor; however, the function of MLF2 in CML is poorly understood. In this study, we demonstrated that MLF2 may play an oncogenic role in CML. The expression level of MLF2 was related to the proliferation, colony-formation ability, and sensitivity to imatinib in K562 cells. Moreover, phosphorylation at serine 24, detected through Phos-tag sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was required to maintain the activity of MLF2 in CML. The effects of MLF2 overexpression on the colony-formation ability in vitro and mouse survival in vivo could be alleviated by point mutation of MLF2 at serine 24. These findings uncover the oncogenic role of MLF2 through phosphorylation at serine 24 and provide a novel therapeutic target in CML.

Inflammatory bone marrow microenvironment.

Self-renewing hematopoietic stem cells and their progeny, lineage-specific downstream progenitors, maintain steady-state hematopoiesis in the bone marrow (BM). Accumulating evidence over the last few years indicates that not only primitive hematopoietic stem and progenitor cells (HSPCs), but also cells defining the microenvironment of the BM (BM niche), sense hematopoietic stress signals. They respond by directing and orchestrating hematopoiesis via not only cell-intrinsic but also cell-extrinsic mechanisms. Inflammation has many beneficial roles by activating the immune system in tissue repair and as a defense mechanism. However, chronic inflammation can have detrimental effects by stressing HSPCs, leading to cell (DNA) damage resulting in BM failure or even to leukemia. Emerging data have demonstrated that the BM microenvironment plays a significant role in the pathogenesis of hematopoietic malignancies, in particular, through disrupted inflammatory signaling, specifically in niche (microenvironmental) cells. Clonal selection in the context of microenvironmental alterations can occur in the context of toxic insults (eg, chemotherapy), not only aging but also inflammation. In this review, we summarize mechanisms that lead to an inflammatory BM microenvironment and discuss how this affects normal hematopoiesis. We pay particular attention to the process of aging, which is known to involve low-grade inflammation and is also associated with age-related clonal hematopoiesis and potentially malignant transformation.

Potently Cytotoxic Natural Killer Cell Potential Initially Emerges from Erythro-Myeloid Progenitors during Mammalian Development

Natural killer (NK) cells are innate immune cells that target and kill virally infected and malignant cells, making them an attractive target for adoptive immunotherapies. An alternative to donor-derived NK cells is the use of human pluripotent stem cell (hPSC)-derived NK cells, as a renewable "off the shelf" product. Previous studies have identified hPSC-derived NK cells as potently cytotoxic, compared to donor-derived NK cells. As the differentiation of hPSCs mimics early embryonic development, this raises the possibility that hPSC-derived NK cells are ontogenically distinct from adult NK cells. NK cells are present during embryonic hematopoiesis, but their ontogenic origins are poorly understood. NK cells are thought to arise from a common lymphoid progenitor (CLP), lying downstream of hematopoietic stem cells (HSCs), but evidence exists that NK cells may arise from HSC-independent progenitors as NK cells are found in the early murine fetal liver, and NK cell progenitors are found in the early human yolk sac (YS). In this study, we investigated the emergence of NK cells during murine and human embryonic hematopoietic development. During murine embryogenesis, overlapping HSC-independent waves of hematopoietic progenitors occur in the YS that give rise to hematopoietic cells prior to HSC emergence at E10.5. The "primitive" wave occurs at E7.5, followed by an "erythro-myeloid progenitor" (EMP) wave at E8.5. To study NK cell potential during murine YS hematopoiesis, we cultured total YS and sorted hematopoietic progenitors under NK cell promoting conditions. Strikingly, we found that the YS contains NK cell potential. Further, sorted E8.5 kit+CD41+CD16/32+ EMP progenitors, but not primitive hematopoietic progenitors, contain robust NK cell potential. EMP-derived NK (EMP-NK) cells were larger and more granular than adult CLP-derived NK cells. Additionally, NK cells from the E15.5 fetal liver were larger and more granular than NK cells from the adult spleen. Both EMP-NK cells and E15.5 fetal liver NK cells had a more robust degranulation response than their HSC-derived counterparts. Together, these data support the concept that EMP in the YS serve as an initial source of physiologically relevant, functional embryonic NK cells that are phenotypically and functionally distinct from adult NK cells. As hPSC-derived NK cells were described as potently cytotoxic, and we observed that murine HSC-independent NK cells robustly degranulate, we next asked whether NK cell development from hPSCs recapitulates that found in the murine embryo. We have demonstrated previously, using a stage-specific WNT signal manipulation approach that specifies ontogenically distinct hematopoietic progenitors, that hPSC-derived NK cell progenitors can be obtained from two distinct progenitors in vitro. In this study, we sought to better understand the development and function of these two NK cell populations. Stage-specific WNT inhibition (WNTi) during hPSC mesodermal patterning yielded extra-embryonic-like HOXA-/low CD34+ populations that possessed erythroid, myeloid and NK cell potential, but lacked T cell potential. The CD56+ NK cells in these cultures co-emerged with CD15+ granulocytes, indicating that these NK cells may arise from a committed myeloid progenitor. In contrast, HOXA+ CD34+ cells, obtained in a WNT-dependent (WNTd) manner, harbored erythro-myelo-lymphoid multi-lineage potential, including NK cell potential. Phenotypically, WNTi-NK cells were larger, more granular and more mature, compared to WNTd-NK and cord blood (CB)-derived NK cells, reminiscent of murine EMP-NK cells. Further, following multiple stimulation assays, WNTi-NK and WNTd-NK cells had different effector biases. WNTi-NK cells are biased for potent cytotoxic degranulation and exhibited superior cell killing in an ADCC assay. In contrast, WNTd-NK and CB-NK had an attenuated degranulation response, but robustly produced inflammatory cytokines. Finally, RNA-seq analysis demonstrated that WNTd-NK cells were most similar to CB-NK cells. Collectively, these studies identify for the first time that the murine EMP harbor NK cell potential, and these NK cells are functionally unique. These observations raise new questions regarding which ontogenic origin of NK cells should be used in future hPSC-derived adoptive immunotherapy strategies. Disclosures Fehniger: Cyto-Sen Therapeutics: Consultancy; Horizon Pharma PLC: Other: Consultancy (Spouse). Palis:Rubius Therapeutics: Consultancy.