Murine Bone Marrow Transplantation Research Articles

Investigating the effects of radiation, T cell depletion, and bone marrow transplantation on murine gut microbiota.

Microbiome research has gained much attention in recent years as the importance of gut microbiota in regulating host health becomes increasingly evident. However, the impact of radiation on the microbiota in the murine bone marrow transplantation model is still poorly understood. In this paper, we present key findings from our study on how radiation, followed by bone marrow transplantation with or without T cell depletion, impacts the microbiota in the ileum and caecum. Our findings show that radiation has different effects on the microbiota of the two intestinal regions, with the caecum showing increased interindividual variation, suggesting an impaired ability of the host to regulate microbial symbionts, consistent with the Anna Karenina principle. Additionally, we observed changes in the ileum composition, including an increase in bacterial taxa that are important modulators of host health, such as Akkermansia and Faecalibaculum. In contrast, radiation in the caecum was associated with an increased abundance of several common commensal taxa in the gut, including Lachnospiraceae and Bacteroides. Finally, we found that high doses of radiation had more substantial effects on the caecal microbiota of the T-cell-depleted group than that of the non-T-cell-depleted group. Overall, our results contribute to a better understanding of the complex relationship between radiation and the gut microbiota in the context of bone marrow transplantation and highlight the importance of considering different intestinal regions when studying microbiome responses to environmental stressors.

Aromatase deficiency in transplanted bone marrow cells improves vertebral trabecular bone quantity, connectivity, and mineralization and decreases cortical porosity in murine bone marrow transplant recipients.

Estradiol is an important regulator of bone accumulation and maintenance. Circulating estrogens are primarily produced by the gonads. Aromatase, the enzyme responsible for the conversion of androgens to estrogen, is expressed by bone marrow cells (BMCs) of both hematopoietic and nonhematopoietic origin. While the significance of gonad-derived estradiol to bone health has been investigated, there is limited understanding regarding the relative contribution of BMC derived estrogens to bone metabolism. To elucidate the role of BMC derived estrogens in male bone, irradiated wild-type C57BL/6J mice received bone marrow cells transplanted from either WT (WT(WT)) or aromatase-deficient (WT(ArKO)) mice. MicroCT was acquired on lumbar vertebra to assess bone quantity and quality. WT(ArKO) animals had greater trabecular bone volume (BV/TV p = 0.002), with a higher trabecular number (p = 0.008), connectivity density (p = 0.017), and bone mineral content (p = 0.004). In cortical bone, WT(ArKO) animals exhibited smaller cortical pores and lower cortical porosity (p = 0.02). Static histomorphometry revealed fewer osteoclasts per bone surface (Oc.S/BS%), osteoclasts on the erosion surface (ES(Oc+)/BS, p = 0.04) and low number of osteoclasts per bone perimeter (N.Oc/B.Pm, p = 0.01) in WT(ArKO). Osteoblast-associated parameters in WT(ArKO) were lower but not statistically different from WT(WT). Dynamic histomorphometry suggested similar bone formation indices' patterns with lower mean values in mineral apposition rate, label separation, and BFR/BS in WT(ArKO) animals. Ex vivo bone cell differentiation assays demonstrated relative decreased osteoblast differentiation and ability to form mineralized nodules. This study demonstrates a role of local 17β-estradiol production by BMCs for regulating the quantity and quality of bone in male mice. Underlying in vivo cellular and molecular mechanisms require further study.

Signaling Pathway Mutations Cooperate with the PICALM/MLLT10 Fusion in a Knock-in AML Mouse Model

The PICALM/MLLT10 (or CALM/AF10; C/A) is a rare but recurring fusion gene detected in patients with T-cell acute lymphoblastic leukemia, malignant lymphoma and acute myeloid leukemia (AML). Our lab has previously established a C/A knock-in mouse model, where the fusion gene preceded by a loxP-flanked transcriptional stop cassette was inserted into the Rosa26 locus (the R26LSLCA strain). To achieve tissue-specific expression of C/A, R26LSLCA mice were crossed with three different Cre-inducer lines, namely the Vav-Cre, MB1-Cre and CD19-Cre. In the Vav-Cre strain, Cre recombinase is expressed in the entire hematopoietic system, while in the MB1-Cre and CD19-Cre lines, Cre is expressed in the B-cell compartment. Using this model, Dutta et al. showed that C/A leads to leukemia only when expressed in all hematopoietic cells, including hematopoietic stem cells and does not lead to induction of leukemia when expressed in B-cells alone ( Dutta et al., Leukemia 2016). To further characterise leukemias driven by the C/A fusion, we established new R26LSLCA and Vav-Cre crosses. Similar to what was found previously, the Vav-Cre/R26LSLCA mice developed leukemia with a long median latency of 1 year (366.5 days; N=24) and complete penetrance. On post-mortem analysis, the Vav-Cre/R26LSLCA mice were found to have splenomegaly and leucocytosis. Expression of the fusion gene was confirmed in these mice by RT-PCR on the cDNA from bone marrow (BM) cells. Immunophenotyping analysis of the BM cells and splenocytes of these mice revealed a predominantly myeloid phenotype. To assess the effect of expression of C/A on the transcriptome, we performed RNA-seq analysis using BM cells from 17 leukemic mice and 12 healthy controls. In accordance with reports on C/A patient samples, the Hoxa genes Hoxa7 and Hoxa9 as well as the Hox-cofactor Meis1 were significantly upregulated in our C/A expressing mice. In addition, genes involved in B-cell development, such as Cd19, Pax5 and Ikzf3, and Gata1, which is essential for the development of erythrocytes, were downregulated. To prove that the Vav-Cre/R26LSLCA mice were indeed leukemic and had the potential to propagate the disease, we serially transplanted BM cells from 4 Vav-Cre/R26LSLCA mice into congenic recipients. The secondary (N=20) and tertiary (N=19) transplanted mice developed leukemia with a significantly shorter median latency of 21 and 17 days, respectively. Our group and others have shown that a single genetic lesion is in most cases not sufficient to cause leukemia. Thus, to determine if additional somatic mutations were acquired by the C/A knock-in mice, we performed whole exome sequencing on the DNA from BM cells of the 4 knock-in mice, some of their daughter secondary and tertiary leukemias as well as the corresponding germline DNA samples (N=19). We identified between 2-32 mutations per leukemia exome, including mutations in genes and/or members of gene families that have been implicated in human leukemias. Mutations were detected in the transcription factors Sfpi1, Zeb1, and Sox9; Aff2, a member of the Af4/Fmr2 gene family; and Sf3a2, a component of the RNA-splicing machinery, among others. In addition, in two C/A sister secondary leukemias and the corresponding tertiary leukemias, we detected a missense mutation at amino acid 507 (G507A) of the Ptpn11 gene, which is homologous to G507 of human PTPN11, a known hotspot mutation site in AML and juvenile myelomonocytic leukemia. Interestingly, Dutta et al. also reported a G507 mutation (G507V) in Ptpn11 in a C/A knock-in mouse. PTPN11 regulates the RAS/MAPK signaling pathway. This finding is reminiscent of what we have previously reported in a C/A driven murine bone marrow transplantation model, where we detected somatic mutations in several AML-associated signal transduction genes, including Flt3, Cbl and Kras ( Desai et al., Leukemia 2020). Taken together, our results strongly suggest that mutations in genes involved in signaling cascades, particularly those in the RAS/MAPK pathway, cooperate with the C/A fusion and lead to complete malignant transformation. We aim to functionally validate these findings by establishing mouse models that harbor the C/A fusion as well as mutations in Ptpn11 and other signal transduction genes. These murine models will be immensely valuable for gaining a deeper insight into PICALM/MLLT10-mediated leukemogenesis, studying cooperating mutations and for testing new targeted therapies.

Glycolysis-Driven IFN-γ Expression in T-Cell Via Histone Lactylation Promotes Acute Graft-Versus-Host Disease

Background Cell metabolism is a highly regulated process essential for sustaining life and driving evolution. Cellular metabolites and metabolic flux intricately govern post-translational modifications of proteins. In the context of acute graft-versus-host disease (aGVHD), T cells undergo enhanced glycolysis, resulting in increased lactate production. Consequently, lactate may act as a substrate for lysine lactylation (Kla), influencing gene expression by modulating histone biology. This study aims to assess T cell glucose metabolism during aGVHD, investigate the role of histone lactylation, and explore its impact on T cell functionality. Methods We conducted a comparative analysis of histone lactylation between aGVHD patients and individuals undergoing allo hematopoietic stem cell transplantation (control). Subsequently, we established a murine bone marrow transplant model and isolated T cells for analysis. Glucose metabolism status was assessed using extracellular acidification rate (ECAR) and oxygen consumption rate (OCR) assays, along with the 2-NBDG glucose uptake assay. To comprehensively investigate histone lactylation in T cells, we employed the cleavage under targets and tagmentation (CUT&Tag) technique to explore its regulatory effects on gene expression. To investigate the impact of lactatylation on aGVHD pathology, we constructed an aGVHD model by applying T cells with a specific deletion of LDHA. Results By analyzing the serum lactate at the onset of aGVHD, we found that patients with grade III-IV aGVHD had significantly higher lactate levels compared to those with grade I-II aGVHD ( Figure 1A). In light of this finding, we further examined the histone lactylation levels in T cells from aGVHD patients. The results demonstrated an enhanced histone lactylation in these T cells compared to controls ( Figure 1B). In the aGVHD mice, the ECAR of T cells exhibited a significant increase ( Figure 1C). Intriguingly, the OCR were also significantly higher in the aGVHD mice compared to the bone marrow T cell-depleted (BM-TCD) mice ( Figure 1D). Additionally, the 2-NBDG revealed a significantly higher fluorescence intensity in aGVHD mice ( Figure 1E). Through analysis of T cell RNA-seq data, we discovered upregulation of most glycolytic enzymes in aGVHD mice similar to human data ( Figure 1F). Notably, LDHA, glucose transporter 1 ( Slc2a1), and monocarboxylate transporter 4 ( Slc16a3) were highly upregulated ( Figure 1G). The upregulation of LDHA in T cells from aGVHD mice prompted us to investigate the presence of histone lactylation. The results revealed that histone in T cells from aGVHD mice exhibited a higher level of lactylation ( Figure 1H). Because lactylation was particularly prominent at the H4K12 site, we conducted CUT&Tag experiments. We found promoters of key regulatory genes of the Th1 subset, such as Tbx21 and Ifng, along with several genes associated with IFN-γ production, including Stim2, Crtam, Il18rap, Fasl, and Phf11a, Cxcr3 were all lactylated ( Figure 1I). Specifically, the Call Peak analysis confirmed the elevated H4K12la levels on the promoters Tbx21 and Ifng ( Figure 1J). The expression of IFN-γ and CXCR3 in CD4 + and CD8 + T cells of aGVHD mice was significantly higher than in BM-TCD mice ( Figure 1K). To confirm the role of histone lactylation in IFN-γ generation, we intervened with T cells using the p300 lactylation writer inhibitor (A485). The results demonstrated a significant reduction in histone lactylation upon A485 treatment, which was effectively restored by the addition of neutral lactate sodium (Nala) ( Figure 1L). Notably, A485 treatment led to a decrease in IFN-γ + T cells, while Nala administration resulted in a substantial recovery of IFN-γ expression ( Figure 1M). Additionally, the infusion of Cd4creLdhafl/fl T cells into recipient mice improved the survival of aGVHD mice ( Figure 1N). Conclusion This study represents the first global investigation of T cell histone lactylation in aGVHD. Importantly, we have uncovered a novel link between histone lactylation and the regulation of IFN-γ-related gene expression in the context of aGVHD. Considering the heightened glycolysis and the pivotal role of IFN-γ in aGVHD pathology, it is plausible to postulate that T cell-produced lactate induces histone lactylation, thereby promoting the transcription of IFN-γ-related genes and contributing to the development of aGVHD.

Intestinal Epithelial-Derived IL-34 Regulates Macrophage Polarization in an Apolipoprotein E Dependent Manner and Mitigates the Severity of Graft Versus Host Disease

Acute graft versus host disease (GVHD) of the gastrointestinal (GI) tract is a major complication of allogeneic bone marrow transplantation. While donor T cells are the proximate trigger of GVHD, innate cells also contribute to disease pathogenesis, although the role of these cell populations is not as well defined. Macrophages, for example, are known contributors in the pathophysiology of chronic GVHD, but their role in acute GVHD is not well delineated. Fundamentally, macrophages are dependent upon exposure to m-CSF and IL-34 which serve as critical trophic factors necessary for their development through binding to the CSF-1R. IL-34 is a more recently discovered cytokine produced by macrophages, but whose role in GVHD pathogenesis is unknown. To examine this question, we employed a murine bone marrow transplantation model (B10.BR [H-2k]®B6[H-2b]) and observed that that there was no difference in GVHD-induced lethality when recipients were reconstituted with marrow grafts from wild type (WT) or IL-34 -/- donors. In contrast, recipient IL-34 -/- animals had significantly worse survival compared to WT mice indicating that host IL-34 expression protected animals from lethal GVHD. We observed no difference in the absolute number of donor macrophages in the colons of WT versus IL-34 -/- mice; however, IL-34 -/- recipient mice had a significant increase in the number of inflammatory Ly6C + macrophages, and a corresponding decrease in Ly6C - cells. Single cell RNA sequence (scRNAseq) analysis of donor macrophages from the colons of WT and IL-34 -/- recipients revealed that macrophages from WT animals had increased expression of anti-inflammatory genes (i.e., CD83, ApoE, Selenop, Lyz1 and Mrc1), whereas macrophages from IL-34 -/- recipients had increased expression of pro-inflammatory genes (i.e., Ifitm1, S100A8, S100A9 and GOs2), indicating that absence of host IL-34 skewed donor macrophages towards an inflammatory phenotype. Since the CSF-1R is not expressed on T cells, we sought to determine the effect of IL-34/CSF-1R signaling on proinflammatory T cells in GVHD target organs. scRNAseq analysis of colonic CD4+ T cells revealed that Csf2 (GM-CSF) was the most differentially expressed gene in IL-34 -/- CD4+ T cells. Functionally, this was confirmed by the observation of a significantly increased number of CD4 + GM-CSF+ T cells in the colons of these mice. Notably, there was no difference in proinflammatory CD8 + T cells, nor any effect on T cell phenotype in the liver or lung, demonstrating that the mitigating effect of host IL-34 expression was confined to the GI tract. To determine the cellular source of IL-34 production, we employed IL-34 LacZ reporter mice and showed that β-Gal co-localized with the epithelial marker, E-cadherin, indicating IL-34 is produced primarily by intestinal epithelial cells. Additional studies using chimeric mice in which IL-34 was restricted to either the hematopoietic or non-hematopoietic compartmentrevealed that absence of IL-34 in the non-hematopoietic compartment phenocopied findings observed with global IL-34 -/- animals, confirming that intestinal epithelial cells were the primary source of this cytokine. The administration of exogenous IL-34 prolonged survival, abrogated pathological damage in the colon and resulted in a corresponding decrease in pro-inflammatory macrophages and T cells, demonstrating this pathway could be pharmacologically targeted. Re-analysis of the scRNAseq data on donor macrophages from the colons of WT and IL-34 -/- mice revealed that apolipoprotein E ( Apoe) was the most differentially expressed gene in macrophages from WT GVHD animals. To confirm a functional role for ApoE, mice were transplanted with T cell depleted (TCD) BM from either WT or ApoE -/- mice and treated with IL-34. These studies revealed that the protective effect conferred by IL-34 was abrogated in animals that received TCD ApoE -/- BM grafts, indicating that ApoE expression in donor macrophages was necessary for agonist driven IL-34-mediated regulation of GVHD. Finally, we confirmed these studies have translational relevance by demonstrating IL-34 and Apoe have increased expression in intestinal epithelial cells and macrophages, respectively, in the colons of humans with GVHD. Thus, these results identify IL-34 as a novel intestinal epithelial cell-derived cytokine that regulates macrophage polarization and mitigates GVHD in an ApoE-dependent manner.

Integrated Genomic Analysis Identifies a Downstream Regulatory Element Oncogenically Activating Hematopoietically Expressed Homeobox ( HHEX) in ETP-ALL

Hematopoietically-expressed Homeobox ( HHEX) is required for the maintenance of hematopoietic stem cells (HSPCs) and common lymphoid progenitor cells. HHEX is also the second most frequent integration site in retroviral insertional mutagenesis screens of leukemias and lymphomas arising in AKXD recombinant inbred mouse strains, implying that it is an important oncogene. Enforced expression of HHEX induces T-cell acute lymphoblastic leukemias (T-ALL) in murine bone marrow transplantation models. In human studies, HHEXmRNA expression is upregulated in human T-ALL studies, especially in Early T-cell Precursor (ETP-) ALL subtypes where it is concordantly expressed with LMO2. The HHEX locus is not rearranged in ETP-ALL where it is upregulated so the mechanism of HHEX activation is not clear. We analyzed T-ALL induced by retroviral mutagenesis and found an intergenic site of frequent integration, 65kb 3‘ of the Hhex coding exons. Integrations clustered within a 3 kb genomic area that is also an open chromatin region (OCR) by ATAC-seq analysis and that induced Hhex upregulation. The syntenic human region was highly enriched for H3K27 acetylation and for occupancy by multiple transcription factors, notably GATA1 and TAL1, per ENCODE. Our prior data had shown that HHEX was a downstream target of the LMO2 complex comprised of GATA1/2, TAL1 or LYL1, and nucleated by the scaffolding protein, LIM domain binding protein 1 (LDB1). We had previously shown by ChIP-PCR that the LMO2 complex was bound to the intron 1 enhancer. Thus, we performed ChIP-exonuclease analysis of LDB1 in the human ETP-ALL model cell line, LOUCY. We confirmed its occupancy at the intron 1 enhancer but we also observed LDB1 occupancy 68kb 3‘ of the HHEX coding exons, in the exact region syntenic to the murine common insertion sites. ChIP-exo analysis allowed us to pinpoint a core element with composite GATA/E box sites that was highly conserved across multiple mammalian species. LDB1 occupancy at intron 1 and at +68kb of the HHEX locus was reminiscent of LDB1's occupancy at the beta globin locus where LDB1's occupancy and homodimerization mediates chromatin looping between composite E box/GATA sites in the locus control region (LCR) and the beta globin proximal promoters. To test whether a similar interaction was occurring at the HHEX locus, we analyzed virtual 4C (chromatin conformation capture) data. We confirmed looping between the +68kb element and the proximal promoter of HHEX in human ETP-ALL primary samples and in human HSPCs. The experiments showed that looping occurs in certain developmental contexts in HSPCs and is recapitulated in ETP-ALL and mediated by the LMO2/LDB1 protein complex. HHEX is a downstream oncogene of the LMO2/LDB1 complex and is activated through this distal +68kb regulatory element. Our studies raise the possibility that dissection of this regulatory element and interference with chromatin looping are potential therapeutic mechanisms that could disrupt oncogene expression.

Impaired thymic iNKT cell differentiation at early precursor stage in murine haploidentical bone marrow transplantation with GvHD.

Early recovery of donor-derived invariant natural killer T (iNKT) cells are associated with reduced risk of graft-versus-host disease (GvHD) and overall survival. Patients with severe GvHD, however, had much slower iNKT cell reconstitution relative to conventional T cells. To characterize the delay of iNKT cell reconstitution and explore its possible causes, we used a haploidentical bone marrow transplantation (haplo-BMT) mouse model with GvHD. We found the delayed recovery of thymic and peripheral iNKT cell numbers with markedly decreased thymic NKT1 subset in GvHD mice. The defective generation of thymic iNKT precursors with egress capability contributed to the reduced peripheral iNKT cells in GvHD mice. We further identified intermediate NK1.1- NKT1 precursor subpopulations under steady-state conditions and found that the differentiation of these subpopulations was impaired in the thymi of GvHD mice. Detailed characterization of iNKT precursors and thymic microenvironment showed a close association of elevated TCR/co-stimulatory signaling provided by double positive thymocytes and macrophages with defective down-regulation of proliferation, metabolism, and NKT2 signature in iNKT precursor cells. Correspondingly, NKT2 but not NKT1 differentiation was favored in GvHD mice. These data underline the important roles of TCR and co-stimulatory signaling in the differentiation of thymic iNKT subsets under transplantation conditions.

Peritransplantation Glucocorticoid Haploidentical Stem Cell Transplantation Is a Promising Strategy for AML Patients With High Leukemic Burden: Comparison With Transplantations Using Other Donor Types

Using a murine haploidentical bone marrow transplantation (BMT) model, we recently showed that peritransplantation administration of glucocorticoid (GC) redistributed donor T cells from the gastrointestinal tract to bone marrow, which resulted in a significant reduction of graft-versus-host disease (GVHD) while promoting graft-versus-leukemia effects. Furthermore, in a retrospective clinical study of patients with acute myelogenous leukemia (AML) undergoing transplantation in non-remission, we also showed that haploidentical stem cell transplantation (haplo-SCT) using peritransplantation GC administration led to a significantly lower relapse rate and better overall survival rate compared with haplo-SCT using post-transplantation cyclophosphamide. In the present study, using the same dataset of patients undergoing GC haplo-SCT, we retrospectively compared with patients with AML undergoing transplantation in non-remission using 3 other donor types: matched sibling donor (MSD), matched unrelated donor (MUD), and umbilical cord blood (UCB). For GC haplo-SCT, 44 patients underwent peripheral blood stem cell transplantation in a single center (Hyogo College of Medicine), with the conditioning treatment consisting of fludarabine, melphalan, anti-thymocyte globulin (2.5 mg/kg), and TBI 3 Gy. Methylprednisolone was given from the start of conditioning treatment, and the GVHD prophylaxis consisted of tacrolimus and methylprednisolone (1 mg/kg). The transplantation outcomes were compared with data of 1889 patients undergoing MSD-SCT (n = 449), MUD-BMT (n = 493), or UCB transplantation (UCBT) (n = 947) in non-CR, which were extracted from the Transplant Registry Unified Management Program data, the largest data registry in Japan. For donor engraftment, significantly faster neutrophil and platelet engraftment was achieved with GC haplo-SCT compared with allo-SCT using the 3 other donor types. Neutrophil engraftment was achieved at a median of 10 days for GC haplo-SCT, and 20 days for MSD-, MUD-, and UCB-transplants. Platelet engraftment was achieved at a median of 19.5 days for GC haplo-SCT, 42 days for MSD-SCT and MUD-BMT, and 43 days for UCBT, respectively. The incidence of grade II-IV acute GVHD was lower after allo-SCTs using MSD (hazard ratio [HR] = 0.465, P = .003), MUD (HR = 0.524, P = .010), and UCB (HR = 0.647, P = .067) compared with GC haplo-SCT. There was no significant difference in the incidence of chronic GVHD between GC haplo-SCT and allo-SCT using the other 3 donor types. Regarding relapse, GC haplo-SCT was associated with a significantly lower risk compared with MSD-SCT (P < .001) or MUD-BMT (P = .004). GC haplo-SCT tended to have a lower risk compared with UCBT (P = .063). Especially, all the 43 evaluable GC haplo-SCT recipients achieved CR after transplantation, whereas 23.9%, 22.8%, and 27.0% of patients who underwent MSD-SCT, MUD-BMT, and UCBT could not achieve CR after transplantation, respectively. Regarding non-relapse mortality, GC haplo-SCT was associated with a significantly higher risk compared with MUD-BMT (P = .014), and tended to have a higher risk compared with MSD-SCT (P = .061). There was no significant difference between GC haplo-SCT and UCBT (P = .600). Allo-SCTs using MSD (HR = 2.548, P < .001), MUD (HR = 2.134, P = .005), and UCB (HR = 2.376, P = .001) lead to significantly higher overall mortality compared with GC haplo-SCT; the adjusted overall survival at 3 years was 19.8% for MSD, 26.1% for MUD, 28.0% for UCB, and 65.1% for GC haplo. Thus GC haplo-SCT is a promising treatment option for patients with AML with a high leukemic burden.

Isocitrate Dehydrogenase 2 Mutation Drives Bone Marrow Macrophage Dysfunction without a Complete Block in Hematopoietic Differentiation

Myelodysplastic syndromes (MDS) represent a broad category of myeloid malignancies characterized by myeloid skewed hematopoiesis, blood cytopenias and increased risk of transformation to leukemia. While not common, isocitrate dehydrogenase 2 (IDH2) mutations are represented in clonal hematopoiesis and in MDS, are found in patients with higher International Prognosis Scoring System (IPSS) disease scores and confer a higher risk of transformation to leukemia. IDH2 mutations lead to metabolic dysfunction and DNA hypermethylation due to the accumulation of D-2-hydroxyglutarate (2HG), an oncometabolite which disrupts hematopoiesis. However, the mechanisms that induces clonal progression and transformation remains incompletely understood. Furthermore, the intrinsic and extrinsic impact of IDH mutation on mature myeloid cell functions and the bone marrow microenvironment (BMME) remains to be elucidated. Using RAW264.7 cells, a monocyte/macrophage-like murine cell line, we retrovirally infected cells with human wt IDH2 or the R172K IDH2 mutant, a common IDH2 mutation. After confirming a 30-fold increase of 2HG (p<.005) and observing evidence of hypermethylation in the IDH2 mutant RAW cells, we tested several key functions of bone marrow macrophages: ability to engulf apoptotic cells (efferocytosis), erythropoietic support, and cytokine production. After labeled apoptotic neutrophils were fed as efferocytic targets to RAW cells at a 10:1 ratio for 6 hours, we observed a 10% decrease of efferocytosis in IDH2 mutants (p < 0.05). Next, we cocultured RAW cells with primary murine late-stage erythroid progenitors and quantified their capacity to form colonies (CFU-E) in the presence of erythropoietin. Compared to wt, IDH2 mutant RAW cells inhibited CFU-E formation by 25% (p < 0.0075). Cytokine array on cell culture supernatant from mutant RAW cells had elevated proinflammatory chemokines/cytokines (CCL5, CXCL10, TNF-a) and decreased cytokines related to inflammation resolution and efferocytosis (GDF-15, Gas6, Endostatin, CCL2). To test these findings further in vivo we generated a murine bone marrow transplant model with retrovirally infected hematopoietic stem cells (HSC: lineage-, c-Kit+, Sca-1+) containing the human wt or R172K IDH2 mutation with an mCherry reporter and transplanted into CD45.1 congenic mice (10 mutant, 10 wt, 1:1 male:female). Mutant IDH2 HSCs demonstrated better engraftment (85-90%) compared to wt (70-80%) and also had higher levels of mCherry positivity within CD45.2+ cells (80-90% mutant vs 40-60% in wt). Longitudinal blood draws showed macrocytic anemia, thrombocytopenia, and elevated reticulocytes in IDH2 mutants by 16 weeks post-transplant. In contrast to current literature, IDH2 mutant cells did not exhibit a complete block in myeloid cell differentiation, as the mutant clone gave rise to mature monocytes, macrophages, and neutrophils. We also harvested bone marrow at 16 weeks post transplantation. Compared to wt IDH2, the mutant IDH2 bone marrow cells exhibited myeloid skewing hematopoiesis and a 3-fold increase in bone marrow monocytes (CD45+, Ly6G-, Ly6C++, F4/80+, p < 0.00001). In the IDH2 mutant bone marrow cells, myeloid progenitors were also expanded: common myeloid progenitors (CMPs: 4-fold increase p < 0.001, CD45+, lineage-, c-Kit+, Sca-1-, CD34+, CD16/32-), and granulocytic monocytic precursors (GMPs: 3-fold increase p < 0.0001, CD45+, c-Kit+, Sca-1-, CD34+, CD16/32+). We also observed a previously unreported loss of long-term hematopoietic stem cells (LT-HSC: 13.5-fold reduction, p < 0.00001, CD45+, lineage-, c-Kit+, Sca-1+, Flt3-, CD48-, CD150+). When cultured ex vivo, there was loss of efferocytic capacity in IDH2 mutant bone marrow derived macrophages fed normal apoptotic neutrophils at a 10:1 ratio for 3 or 24 hours. In summary, we identify defects in efferocytic function and cytokine production in monocytes and macrophages derived from IDH2 mutant hematopoietic stem and progenitor cells. We have previously shown that defects in the efferocytic function of bone marrow macrophages induce increases in inflammatory signals and contribute to skewing of hematopoiesis. Since these macrophage dysfunctions have not previously been attributed to IDH2 single mutation, they represent a potential novel mechanism that could alter the bone marrow microenvironment and impact progression of MDS and transformation to leukemia.

Melatonin Improves Ischemia-Induced Circulation Recovery Impairment in Mice with Streptozotocin-Induced Diabetes by Improving the Endothelial Progenitor Cells Functioning.

Patients with diabetes mellitus tend to develop ischemia-related complications and have compromised endothelial progenitor cell (EPC) function. Melatonin protects against ischemic injury, possibly via EPC modulation. We investigated whether melatonin pretreatment could restore EPC function impairment and improve circulation recovery in a diabetic critical limb ischemia mouse model. Under 25 mM high-glucose medium in vitro, EPC proliferation, nitric oxide production, tube formation, and endothelial nitric oxide synthase (eNOS) phosphorylation were significantly suppressed. Hyperglycemia promoted EPC senescence and apoptosis as well as increased reactive oxygen species (ROS) production. Melatonin treatment reversed the harmful effects of hyperglycemia on EPC through adenosine monophosphate–activated protein kinase-related mechanisms to increase eNOS phosphorylation and heme oxygenase-1 expression. In an in-vivo study, after a 4-week surgical induction of hindlimb ischemia, mice with streptozotocin (STZ)-induced diabetes showed significant reductions in new vessel formation, tissue reperfusion, and EPC mobilization in ischemic hindlimbs compared to non-diabetic mice. Mice with STZ-induced diabetes that received melatonin treatment (10 mg/kg/day, intraperitoneal) had significantly improved blood perfusion ratios of ischemic to non-ischemic limb, EPC mobilization, and densities of capillaries. In addition, a murine bone marrow transplantation model to support these findings demonstrated that melatonin stimulated bone marrow-originated EPCs to differentiate into vascular endothelial cells in femoral ligation-induced ischemic muscles. In summary, this study suggests that melatonin treatment augments EPC function along with neovascularization in response to ischemia in diabetic mice. We illustrated the protective effects of melatonin on EPC H2O2 production, senescence, and migration through melatonin receptors and modulating eNOS, AMPK, and HO-1 activities at the cellular level. Thus, melatonin might be used to treat the impairment of EPC mobilization and circulation recuperation in response to ischemic injury caused by chronic hyperglycemia. Additional studies are needed to elucidate the applicability of the results in humans.

P438: ARYL HYDROCARBON RECEPTOR SIGNALLING IN NORMAL HAEMATOPOIETIC (HSC) AND LEUKEMIA STEM CELLS (LSC): DIFFERENTIAL EFFECT OF AHR ANTAGONIST ON LSC COMPARED TO HSC

Background: Acute myeloid leukaemia (AML) is a devastating disease with poor prognosis. It is driven by leukemia stem cells (LSCs), which are able to survive treatment and drive relapse. This highlights the importance of characterizing LSCs. LSC-targeted treatment strategies are limited because there are no known surface markers that are specific for LSCs in every leukaemia. Targeting a signalling pathway that is altered in all LSCs could be a novel, promising treatment strategy. The aryl hydrocarbon receptor (AHR) pathway was shown to be critical for the self-renewal properties of normal haematopoietic stem cells (HSCs). Aims: We aimed to study the consequences of manipulating the AHR pathway in vivo in a murine leukaemia model. Methods: We used the following methods: murine bone marrow transplantation leukemia model (MBMTLM) - CALM-AF10 with limiting dilution assay (LDA); competitive repopulation assay (CRA) with LDA (CD45.2 donors into CD45.1 recipient mice), transcriptome analysis (sequencing with the Illumina platform and analisys with the DESeq2). Treatment of the cells was performed at various experiments with the AHR agonist, ITE, and an AHR antagonist, CH-223191 (a vehicle control is DMSO). Results: We used a murine retroviral transduction bone marrow transplantation leukemia model (MBMTLM)) with the CALM-AF10 minimal fusion gene (CAMF) as the oncogenic driver, to study the effect of AHR agonist (ITE) and an AHR antagonist (CH-223191) treatment on LSCs. Interestingly, both AHR agonist and antagonist treatment affected LSC frequency and survival of mice transplanted with treated cells. The LSC frequency as measured by limiting dilution transplantation assays (LDA) was 3.7 times lower after agonist treatment (1:163, blue survival curve in fig. 1, A) compared to a vehicle control (1:44, DMSO, black survival curve)). Very interestingly, after antagonist treatment the LSC frequency was so low that could not be measured as none of the mice transplanted with antagonist-treated cells in the LDAs developed leukaemia (n=9, CH-223191 (red curve in fig. 1, A)). Of note, our CAMF MBMTLM has a very high LSC frequency of about 1:4 in cells that were not cultured after thawing. We performed RNA-Seq on 90 samples to determine changes in gene expression patterns after treatment of healthy bone marrow cells and CAMF leukemia cells with the AHR agonist and antagonist. Preliminary differential expression analysis with DESeq2 revealed differential expression of genes involved in apoptosis and proliferation in leukaemia cells treated with CH-223191. As leukemia did not develop in recipients of cells treated with the antagonist CH-223191, genes differentially regulated in these cells might be critical and specific to LSC function and will be candidate targets for LSC-specific treatments. To confirm that AHR antagonist treatment affects LSCs differently than normal HCS, we performed competitive repopulation assay (CRA). Our preliminary data (n = 12) show that treatment of HSCs with CH-223191 results in a steady increase in the engraftment over a 47-week period following transplantation (coral bars, fig. 1, B), as opposed to the grafts from the ITE-treated arm (blue bars), vehicle or untreated HSCs (black and green bars, respectively). Image:Summary/Conclusion: In summary, our data show that the LSCs react differently than normal HSCs to changes in AHR signalling. These differences suggest it might be possible to specifically inhibit LSCs while sparing normal HSCs, thus paving the way to develop strategies to treat leukemia based on manipulating AHR signalling.

Expression of glucocorticoid and androgen receptors in bone marrow–derived hematopoietic and nonhematopoietic murine endometrial cells

To determine whether bone marrow (BM)-derived cells engrafting the murine endometrium express the glucocorticoid receptor (GR) and androgen receptor (AR). Recent data demonstrate that BM is a long-term source of multiple hematopoietic and nonhematopoietic endometrial cell types. Important roles for glucocorticoids and androgens in regulating endometrial functions, including decidualization and early embryo attachment/invasion, have very recently emerged. Whether endometrial cells of BM origin express glucocorticoid or ARs has not been previously studied. Animal study. Basic science laboratory. Wild-type C57BL/6J male mice expressing enhanced green fluorescent protein (GFP) and syngeneic wild-type C57BL/6J female mice aged 6-9 weeks. Murine bone marrow transplant. Bone marrow cells were harvested from adult wild-type C57BL/6 mice and subjected to flow cytometry to identify the percentage of hematopoietic and nonhematopoietic cells expressing GR or AR. Uterine tissue sections from lethally irradiated syngeneic adult female C57BL/6 mice that had been recipients of BM transplants from adult male transgenic donor mice ubiquitously expressing GFP were studied. Immunohistochemistry was performed in the uterine tissue sections of the recipient mice at 5, 9, and 12 months after transplant using specific anti-GR, anti-AR, anti-GFP, anti-CD45 (pan leukocyte marker), and anti-F4/80 (murine macrophage marker) primary antibodies. Confocal laser microscopy was used to localize and quantitate BM-derived (GFP+) cell types in the endometrial stromal and epithelial compartments and determine whether BM-derived cell types in the murine endometrium express GR or AR. Hematopoietic cells comprised 93.6%-96.6% of all cells in the BM, of which 98.1% ± 0.2% expressed GR and 92.2% ± 4.4% expressed AR. Nonhematopoietic cells comprised 0.4%-1.3% of BM, of which 52.8% ± 5.9% expressed GR and 48.9% ± 3.4% expressed AR. After BM transplant, the proportion of cells originating from BM in the endometrial stromal compartment increased over time, reaching 13.5% ± 2.3% at 12 months after transplant. In the epithelial compartments, <1% of the cells were of BM origin at 12 months after transplant. Most (60%-72%) GR+ and/or AR+ BM-derived cells in the stroma were hematopoietic (CD45+) cells, of which 37%-51% were macrophages. Nonetheless, 28%-33% of GR+ cells, and 28%-40% of AR+ BM-derived cells, were nonhematopoietic (CD45-) stromal cells of BM origin. A substantial number of BM-derived cells express GR and AR, suggesting a role for these cells in both glucocorticoid-regulated and androgen-regulated endometrial functions, such as proliferation and/or decidualization.

Donor plasmacytoid dendritic cells limit graft-versus-host disease through vasoactive intestinal polypeptide expression

AbstractVasoactive intestinal polypeptide (VIP), an anti-inflammatory neuropeptide with pleiotropic cardiovascular effects, induces differentiation of hematopoietic stem cells into regulatory dendritic cells that limit graft-versus-host disease (GVHD) in allogeneic hematopoietic stem cell transplant (HSCT) recipients. We have previously shown that donor plasmacytoid dendritic cells (pDCs) in bone marrow (BM) donor grafts limit the pathogenesis of GVHD. In this current study we show that murine and human pDCs express VIP, and that VIP-expressing pDCs limit T-cell activation and expansion using both in vivo and in vitro model systems. Using T cells or pDCs from transgenic luciferase+ donors in murine bone marrow transplantation (BMT), we show similar homing patterns of donor pDCs and T cells to the major sites for alloactivation of donor T cells: spleen and gut. Cotransplanting VIP-knockout (KO) pDCs with hematopoietic stem cells and T cells in major histocompatibility complex mismatched allogeneic BMT led to lower survival, higher GVHD scores, and more colon crypt cell apoptosis than transplanting wild-type pDCs. BMT recipients of VIP-KO pDCs had more T helper 1 polarized T cells, and higher plasma levels of granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-α than recipients of wild-type pDCs. T cells from VIP-KO pDC recipients had increasing levels of bhlhe40 transcripts during the first 2 weeks posttransplant, and higher levels of CyclophilinA/Ppia transcripts at day 15 compared with T cells from recipients of wild-type pDCs. Collectively, these data indicate paracrine VIP synthesis by donor pDCs limits pathogenic T-cell inflammation, supporting a novel mechanism by which donor immune cells regulate T-cell activation and GVHD in allogeneic BMT.

Chimerism through the activation of invariant natural killer T cells prolongs graft survival after transplantation of induced pluripotent stem cell-derived allogeneic cardiomyocytes.

The loss of functional cells through immunological rejection after transplantation reduces the efficacy of regenerative therapies for cardiac failure that use allogeneic induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Recently, mixed-chimera mice with donor-specific immunotolerance have been established using the RGI-2001 (liposomal formulation of α-galactosyl ceramide) ligand, which activates invariant natural killer T (iNKT) cells. The present study aimed to investigate whether mixed chimerism, established using RGI-2001, prolongs graft survival in allogeneic iPSC-CM transplantation. Mixed-chimera mice were established via combinatorial treatment with RGI-2001 and anti-CD154 antibodies in an irradiated murine bone marrow transplant model. Luciferase-expressing allogeneic iPSC-CMs were transplanted into mixed-chimera and untreated mice, followed by in vivo imaging. RGI-2001 enhanced iNKT cell activation in mice, and mixed chimerism was successfully established. In vivo imaging revealed that while the allografts were completely obliterated within 2 weeks when transplanted to untreated mice, their survivals were not affected in the mixed-chimera mice. Furthermore, numerous CD3+ cells infiltrated allografts in untreated mice, but fewer CD3+ cells were present in mixed-chimera mice. We conclude that mixed-chimera mice established using RGI-2001 showed prolonged graft survival after allogeneic iPSC-CM transplantation. This donor-specific immunotolerance might increase the efficacy of regenerative therapies for heart failure with allogeneic iPSC-CMs.

Modeling acute graft-versus-host disease (aGVHD) in murine bone marrow transplantation (BMT) models with MHC disparity.

For more than 50years, hematopoietic stem cell transplantation (HSCT) has been the major curative therapy for hematological malignancies and genetic disorders, but its success is limited by the development of graft-versus-host disease (GVHD). GVHD represents a post-transplantation disorder representing the immune-mediated attack of transplant-derived T cells against recipient tissue finally leading to increased morbidity and mortality of the recipient. GVHD develops if donor and recipient are disparate in major or minor histocompatibility antigens (MHC, miHA). Most of the initial knowledge about the biology of GVHD is derived from murine bone marrow transplantation (BMT) models. Of course, GVHD mouse models do not reflect one to one the human situation, but they contribute significantly to our understanding how conditioning and danger signals activate the immune system, enlighten the role of individual molecules, e.g., cytokines, chemokines, death-inducing ligands, define the function of lymphocytes subpopulations for GVHD development and have significant impact on establishing new treatment and prevention strategies used in clinical HSCT. This chapter describes in detail the procedure of allogeneic BMT and the development of GVHD in two commonly used allogeneic murine BMT models (B6→B6.bm1, B6→B6D2F1) with different MHC disparities, which can be used as a basis for advanced studies of GVHD pathology or the development of new treatment strategies.

Fecal metabolomic profiles in experimental murine bone marrow transplantation

Graft-versus-host disease (GVHD) is a life-threating complication of allogeneic hematopoietic cell transplantation (allo-HCT). Prior studies have shown that gastrointestinal (GI) GVHD is associated with a reduction in intestinal microbiota diversity and a change in microbial metabolites. We conducted fecal metabolome analyses using a murine bone marrow transplantation. From this analysis, 290 metabolites were identified; of these, 18 metabolites were significantly or specifically higher and 12 were significantly or specifically lower in the allogeneic group than in the syngeneic one. Particularly, several metabolites in the choline metabolism and tryptophan metabolism were altered in the allogeneic group. Hierarchical clustering analysis demonstrated that the changed metabolites in the allogeneic group had similar profiles. Conclusively, we suggest that alloimmune responses are related to microbial metabolites in recipients receiving allo-HCT. The relationship between metabolites involved in GI GVHD and the intestinal microbiota and its physiological significance warrant further investigations.

Magnetism-Induced Cell-Targeting Transplantation Improves Early Hematopoietic Reconstitution in a Murine Bone Marrow Transplantation Model

BackgroundEarly hematopoietic reconstitution is essential for improving survival and reducing complications after hematopoietic stem/progenitor cell (HSC/HPC) transplantation (HSCT/HPCT). Increasing HSC/HPC homing to the bone marrow is a potential approach for promoting hematopoietic reconstitution.MethodsWe proposed the transplantation of HSCs/HPCs with a magnetism-induced cell-targeting transplantation (MagIC-TT) strategy. HSCs/HPCs were magnetized with CD45 microbeads. The biological characteristics (morphology, proliferation, viability, and ferroptosis) and target migration ability of these cells were studied in vitro. The hematopoietic reconstitution experiments were constructed in vivo in autologous and allogeneic bone marrow transplantation models with grouping showed as Table 1. The therapeutic effects were assessed by survival, donor chimerism, routine blood examination and histological analysis. We also performed transcriptomic sequencing for further mechanistic studies.ResultsThe biological characteristics was found no significant difference between the MagIC-TT and non-MagIC-TT groups, while migration ability was greatly improved with MagIC-TT (Data not showed). The survival rate was higher in the MagIC-TT group and significantly different in the allogeneic model (P<0.05). Hematopoietic reconstitution of donor chimerism, WBCs, HGB, and PLTs was faster in the MagIC-TT groups (within 22 days) (Figure 1). Confocal observation showed that more donor cells (eGFP +) were retained in the injected femur of the MagIC-TT group than in the femur of the non-MagIC-TT group 7 days after transplantation (P<0.05) (Figure 2). The severity of aGVHD (Assessed by survival, body weight, back arching, fur losting, diarrhea etc.) was reduced in the MagIC-TT groups in the allogeneic model (P<0.05) (Data not showed). Transcriptome sequencing revealed differentially expressed genes (DEGs) involved in localization/locomotion and pathways, cytokine-cytokine receptor interactions and chemokine signaling pathways between the two groups (Figure 3).ConclusionThe MagIC-TT strategy improves HSC/HPC homing, resulting in faster hematopoietic reconstitution in a murine bone marrow transplantation model.Key wordsMagnetism; Hematopoietic Stem Cell Transplantation (HSCT); Cell Homing; Hematopoietic Reconstitution; Ferroptosis [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Abrogation of a Histone Chaperone Pathway Mitigates Inflammation-Driven AML Progression

Background: Genetic heterogeneity makes clinical interventions challenging for acute myeloid leukemia (AML) patients. Identifying and targeting microenvironment-driven pathways that are active across AML genetic subtypes should allow the development of more broadly effective therapies. Previously, we have shown that AML microenvironment is rich in proinflammatory cytokine interleukin-1β (IL-1β) and significantly promotes the growth of AML progenitors while suppressing healthy progenitors. To elucidate this paradoxical effect, we performed transcriptome (RNA-seq) analysis from IL-1β-stimulated CD34+ AML and normal progenitors and found that ASF1B (anti-silencing function-1B) is one of the most differentially expressed genes. ASF1B is a histone chaperone, which recruits H3-H4 histones onto the replication fork during S-phase. This process is regulated by tousled-like kinase 1 and 2 (TLKs). TLKs and ASF1B are overexpressed in multiple solid tumors and associated with poor prognosis. However, their functional roles in hematopoiesis and inflammation-driven leukemia are unexplored. Here, we reveal a novel molecular mechanism that IL-1β promotes leukemia progression by activating the TLK-ASF1B pathway.Methods and Results: We first confirmed that IL-1β stimulation upregulates ASF1B expression at both mRNA and protein levels in FLT3-ITD, MLL-ENL, and NPM1 positive primary AML samples. ASF1B upregulation is abolished upon treatment with a p38MAPK inhibitor, further suggesting that ASF1B is downstream of IL-1β/p38 signaling. Next, we stably knocked down ASF1B in an AML cell line MOLM-14 using doxycycline-inducible shRNA. ASF1B-depleted AML cells exhibited reduced cell viability, inhibited growth, blocked cell cycle progression, and impaired colony formation ability. We xenografted shASF1B expressing AML cells into NSG mice and induced knockdown in vivo. Flow cytometry analysis of bone marrow cells 3 weeks post-engraftment showed 80 percent reduction in leukemia burden following ASF1B silencing compared to controls.To determine whether upregulation of ASF1B contributes to IL-1β-driven leukemic growth, we overexpressed ASF1B with MLL-ENL oncogene in a murine bone marrow transplantation model. We found that daily IL-1β exposure accelerated leukemia progression compared to vehicle-treated group (median survival = 64 vs. 85 days, p<0.05), and this effect was phenocopied by overexpression of ASF1B (median survival = 62 vs. 85 days, p<0.05). Conversely, heterozygous and complete Asf1b deletion in the MLL-ENL AML model delayed the leukemia progression compared to wildtype mice. Furthermore, Asf1b deletion attenuated IL-1β-mediated AML progression compared to wildtype controls (median survival = 63 vs. 47 days, p <0.01). Immunophenotyping of Asf1b-deficient mice using flow cytometry suggested that ASF1B is dispensable for normal hematopoiesis. Together, these data suggested that targeting of the ASF1B pathway may spare healthy cells. Additionally, we found that TLK2 which regulates ASF1B activation, is also upregulated in AML progenitors compared to healthy cells at the baseline levels and upon IL-1β stimulation. We next knocked down TLKs in human AML cells and observed a similar pattern with growth arrest, higher replication stress and DNA damage response. Finally, we generated Vav-Cre+ Tlk2 mice allowing Tlk2 deletion only in hematopoietic cells and found that Tlk2 deletion prolongs survival of leukemic mice in a dose dependent manner with and without IL-1β stimulation.Conclusions: We demonstrate that increased TLK-ASF1B expression promotes survival of AML cells. We also provide the first in vitro and in vivo evidence that the TLK-ASF1B pathway plays a critical role in potentiating IL-1β-dependent AML growth. Therefore, we establish TLK-ASF1B pathway as a novel route for therapeutic strategy to suppress inflammation-driven growth in various AML genetic subtypes. DisclosuresNo relevant conflicts of interest to declare.

Clonal Evolution in a Murine CALM-AF10 Leukemia: Evidence of Functional Heterogeneity of Leukemia Stem Cells

Myeloid leukemia is caused by acquired genetic changes in haematopoietic stem cells. The combination of stepwise acquisition of genetic changes together with selection of the fittest clones results in great genetic and clonal heterogeneity. We used a CALM-AF10-driven retroviral transduction murine bone marrow transplantation leukemia model (MBMTLM) to study clonal hierarchy and clonal evolution starting with a primary leukaemia (Fig 1: Leu7) which developed after 131 days and had B220 marker expression on 4% of its cells. Limiting dilution assays (LDAs) showed that the leukemia stem cell (LSC) frequency of Leu7 was 1:2339 (95% confidence interval: 1:794-1:6885). Whole exome sequencing (WES) and analysis of the variant allele fraction of somatic mutations revealed that Leu7 was composed of a main clone (Fig 1: grey) with two subclones (blue and red). Half a million leukemic cells from Leu7 were transplanted into 4 sublethally irradiated recipients, which all developed secondary leukemias after a latency of 19 days (Leu7Sec1 to 4). All secondary leukemias showed similar B220 expression levels to Leu7, and all showed an expansion of the blue subclone. When again half a million cells each of one of the secondary leukemias (Leu7Sec2) were transplanted into 4 recipients, the expansion of the blue subclone continued, the red subclone vanished and, surprisingly, the proportion of B220 expressing cells increased to between 16 to 26%. LDAs showed that the LSC frequency of Leu7Sec2 had not changed. However, several of the leukemias from the LDAs had greatly varying latencies (27 to 193 days) and B220 marker expression (2 to 85%). Four of these tertiary LDA leukemias (Leu7Sec2Ter5 to 8), which each arose from a single LSC, were analysed more closely using WES. Leu7Sec2Ter5 showed a similar latency (27 days) and B220 expression levels like Leu7SecTer1 to 4 and also had the expansion of the blue subclone. Leu7Sec2Ter6 had a long latency of 69 days and a very low B220 expression. Leu7Sec2Ter6 was driven by a new, third subclone (pink), and both the blue and the red subclone disappeared. Very interestingly, Leu7Sec2Ter7 and Leu7Sec2Ter8 had a very long latency of 193 days, and showed an expansion of a subclone (green) of the red subclone. The B220 expression was high (37%) to very high (85%) in these two leukemias.Taken together, these observations paint an interesting picture with the blue subclone outcompeting the red subclone, as leukemias arising from the red subclone only appear after a long latency and in leukemias initiated by a single LSC, when there is no blue subclone LSC present. As the four leukemias (Leu7Sec2Ter5 to 8), which each were derived from a single LSC, showed striking differences in latency and surface marker expression, it can be concluded that this variation in phenotype is an intrinsic property of an individual LSCs most likely a consequence of the distinct combination of somatic mutations present in the individual LSCs. These observations also suggest that distinct LSCs with different properties might be present in a single human leukemia. [Display omitted] DisclosuresBrowett: Janssen: Membership on an entity's Board of Directors or advisory committees; MSD: Membership on an entity's Board of Directors or advisory committees; AbbVie: Honoraria.

Synergism between IGF2BP1 and ETV6-RUNX1 in the Pathogenesis of ETV6-RUNX1 Positive B-Acute Lymphoblastic Leukaemia

Acute lymphoblastic leukaemia (ALL) is the most common pediatric haematological malignancy with a prevalence of around 85% cases of B cell origin. ETV6-RUNX1 is the most common genetic aberration in childhood B-ALL with a prevalence of around 25% in Western countries and a lower prevalence in Asian countries. ETV6-RUNX1 presence portends a good prognosis in B-ALL. ETV6-RUNX1 translocation has been observed in 1-5% of healthy newborns, indicating that it is not sufficient for leukemogenesis, and the need to accumulate secondary mutations for leukemogenesis.We have previously demonstrated the specific overexpression of Insulin like Growth Factor 2 Binding Protein 1 (IGF2BP1), an oncofetal RNA binding protein, in ETV6-RUNX1 positive B-ALL. Overexpression of the ETV6-RUNX1 fusion protein in 7OZ/3, a murine B-ALL cell line increased endogenous Igf2bp1 levels in a dose dependent manner.Conversely, knockout of IGF2BP1 in Reh-Cas9-GFP expressing B-ALL cell line, a modified version of Reh (cell line positive for ETV6-RUNX1 translocation), using 2 different guide RNAs led to a decrease in the ETV6-RUNX1 transcript expression in the IGF2BP1 KO Reh-Cas9 cells. This was accompanied by a decrease in cell proliferation and increased chemosensitivity to prednisolone.A similar phenotype was observed in Reh-Cas9 cells after knockout of ETV6-RUNX1 fusion transcript hinting towards a synergism between the two proteins in tumour cell survival and chemosensitivity. Interestingly, there appears to be a positive correlation between prednisolone resistance and ETV6-RUNX1 positivity in our patient cohort too, thus supporting the in vitro finding. Previous studies have shown the binding of IGF2BP1 to the ETV6-RUNX1 fusion transcript which was also confirmed by us.To study the in vivo synergistic effect between ETV6-RUNX1 fusion protein and IGF2BP1, we used a murine bone marrow transplant model. The individual genes were overexpressed alone (IGF2BP1/ETV6-RUNX1) or together (IGF2BP1+ETV6-RUNX1) in separate groups of lethally irradiated mice (n=8 per group). In the group where both the genes were overexpressed together, there was clonal expansion and hypercellularity in the bone marrow. This was accompanied by an increase in the number of lineage negative cells (Lin-), progenitors including HSCs, LMPPs and c-kit+ cells which also showed high Ki67 positivity. The marrow hypercellularity led to extramedullary haematopoiesis which was seen as significant splenomegaly.In addition, analysis of the endogenous levels of Igf2bp1 in the bone marrow of the primary murine samples belonging to the ETV6-RUNX1 fusion protein overexpression group resulted in an increase in its levels, thus confirming our in vitro finding suggesting its induction by ETV6-RUNX1 fusion protein.RNA-Seq of IGF2BP1 KO cells identified enrichment of targets of the TNF alpha/NFκB signalling and K-Ras pathways hinting towards their dependency on IGF2BP1. The direct targets of IGF2BP1 were identified by RIP-Seq (RNA-Immunoprecipitation-Seq) analysis in Reh cell line. Numerous genes from the oncogenic signalling pathways including the TNF alpha/ NFκB signalling pathway were identified as IGF2BP1 targets. We validated some overexpressed genes from the TNF alpha/NFκB signalling and PI3K pathway in primary patient samples. We found significant overexpression of some of the genes (IL6ST, NFAT5, CDK6, MDM2, CCND1, NGFR) belonging to these pathways in the ETV6-RUNX1 positive group (n=39) compared to the negative cohort (n=111) as well as MACS sorted CD19+ B-cells (n=5). The decrease in the ETV6-RUNX1 transcript levels after downregulation of IGF2BP1 and overlap between the oncogenic pathways clearly shows an interdependency between the two genes.Overall, our results suggest a potential feedback mechanism between ETV6-RUNX1 fusion protein and IGF2BP1 in the pathogenesis of leukemia development and IGF2BP1 can be utilised as an ideal therapeutic target for this particular subtype. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.