Hematopoietic Repopulation Research Articles

FANCD2 and HES1 Synergistically Regulate the Immunometabolism of Hematopoietic Stem Cells through Transcriptional Suppression of Pparg- mediated Fatty Acid Oxidation and Oxidative Phosphorylation

Emerging evidence has revealed that resting quiescent hematopoietic stem cells (HSCs) possess a distinct metabolic profile with a preference for anaerobic glycolysis rather than mitochondrial oxidative phosphorylation (OXPHOS). Recent few scattered studies on the relationship between metabolism and HSCs raise a completely different perspective on the immunological/metabolic interface between HSC defect and human diseases. How immunometabolism influences HSC function, and what signaling cascades help drive cell-intrinsic immunometabolic modes in HSCs, has begun to emerge as an area of intense interest. Here we have investigated the immnometabolic regulation of HSCs using a model of Fanconi anemia (FA), a cancer-prone disease known to impact the immune system and promote inflammation. We and others have demonstrated that inflammation in FA hematopoietic stem and progenitor cells (HSPCs) plays a crucial role in FA pathophysiology. Recently, we have shown that FA HSCs are more dependent on OXPHOS relative to glycolysis for energy metabolism. However, the mechanism underpinning the link between inflammation and the altered metabolic program in FA HSCs has not been defined. More recently, we exploited the repopulating defect of FA HSCs to conduct an unbiased in vivo shRNA screen in transplant recipients, and found enrichment of shRNAs targeting genes involved in the Pparg pathway. Pparg is a central transcriptional factor regulating adipocyte differentiation and energy metabolism but has not been linked to HSC homeostasis. Pparg inhibition by shRNA or chemical compounds significantly improved the repopulating ability of Fancd2-KO HSCs. Conversely, activation of Pparg in wild-type HSCs impaired hematopoietic repopulation. Suppression of inflammation-induced expression of Pparg required both Fancd2 and the Notch target Hes1. Deletion of Hes1 de-repressed Pparg expression in wild-type LSK cells, which are enriched for HSPCs, but had no further effect on Fancd2-KO LSK cells. Mechanistic studies showed that Fancd2 and Hes1 acted synergistically to repress the Pparg promoter. Furthermore, transcriptomic study revealed that inflammation orchestrated an overlapping transcriptional program in HSPCs deficient for Fancd2 and Hes1, featuring upregulated genes in fatty acid oxidation (FAO) and OXPHOS. Consistently, we observed a marked increase in FAO and OXPHOS in mouse HSPCs deficient for Fancd2 or Hes1 and in the BM samples of FA patients. Using a newly developed Fancd2-KI mouse model, we identified a novel and inflammation-responsive interaction between Fancd2 and Hes1. This inflammation-responsive FANCD2-HES1 interaction was abolished in FA-D2 patient-derived lymphoblast cells but rescued in those constituted with a functional FANCD2 (with a 3×FLAG tag) gene. Finally, a Fancd2 mutation lacking the Hes1-interacting domain abolished the activity of the repression of Pparg expression, FAO and OXPHOS. Our results indicate that the FA pathway may constitute a key component of a novel immunometabolism axis, connecting inflammation, cellular metabolism and HSC function. DisclosuresNo relevant conflicts of interest to declare.

Engraftment and in vivo proliferation advantage of gene-corrected mobilized CD34+ cells from Fanconi anemia patients

AbstractPrevious Fanconi anemia (FA) gene therapy studies have failed to demonstrate engraftment of gene-corrected hematopoietic stem and progenitor cells (HSPCs) from FA patients, either after autologous transplantation or infusion into immunodeficient mice. In this study, we demonstrate that a validated short transduction protocol of G-CSF plus plerixafor-mobilized CD34+ cells from FA-A patients with a therapeutic FANCA-lentiviral vector corrects the phenotype of in vitro cultured hematopoietic progenitor cells. Transplantation of transduced FA CD34+ cells into immunodeficient mice resulted in reproducible engraftment of myeloid, lymphoid, and CD34+ cells. Importantly, a marked increase in the proportion of phenotypically corrected, patient-derived hematopoietic cells was observed after transplantation with respect to the infused CD34+ graft, indicating the proliferative advantage of corrected FA-A hematopoietic repopulating cells. Our data demonstrate for the first time that optimized protocols of hematopoietic stem cell collection from FA patients, followed by the short and clinically validated transduction of these cells with a therapeutic lentiviral vector, results in the generation of phenotypically corrected HSPCs capable of repopulating and developing proliferation advantage in immunodeficient mice. Our results suggest that clinical approaches for FA gene therapy similar to those used in this study will facilitate hematopoietic repopulation in FA patients with gene corrected HSPCs, opening new prospects for gene therapy of FA patients.

Neonatal umbilical cord blood transplantation halts skeletal disease progression in the murine model of MPS-I

Umbilical cord blood (UCB) is a promising source of stem cells to use in early haematopoietic stem cell transplantation (HSCT) approaches for several genetic diseases that can be diagnosed at birth. Mucopolysaccharidosis type I (MPS-I) is a progressive multi-system disorder caused by deficiency of lysosomal enzyme α-L-iduronidase, and patients treated with allogeneic HSCT at the onset have improved outcome, suggesting to administer such therapy as early as possible. Given that the best characterized MPS-I murine model is an immunocompetent mouse, we here developed a transplantation system based on murine UCB. With the final aim of testing the therapeutic efficacy of UCB in MPS-I mice transplanted at birth, we first defined the features of murine UCB cells and demonstrated that they are capable of multi-lineage haematopoietic repopulation of myeloablated adult mice similarly to bone marrow cells. We then assessed the effectiveness of murine UCB cells transplantation in busulfan-conditioned newborn MPS-I mice. Twenty weeks after treatment, iduronidase activity was increased in visceral organs of MPS-I animals, glycosaminoglycans storage was reduced, and skeletal phenotype was ameliorated. This study explores a potential therapy for MPS-I at a very early stage in life and represents a novel model to test UCB-based transplantation approaches for various diseases.

InVivo RNAi Screen Unveils PPARγ as a Regulator of Hematopoietic Stem Cell Homeostasis.

SummaryHematopoietic stem cell (HSC) defects can cause repopulating impairment leading to hematologic diseases. To target HSC deficiency in a disease setting, we exploited the repopulating defect of Fanconi anemia (FA) HSCs to conduct an in vivo short hairpin RNA (shRNA) screen. We exposed Fancd2−/− HSCs to a lentiviral shRNA library targeting 947 genes. We found enrichment of shRNAs targeting genes involved in the PPARγ pathway that has not been linked to HSC homeostasis. PPARγ inhibition by shRNA or chemical compounds significantly improves the repopulating ability of Fancd2−/− HSCs. Conversely, activation of PPARγ in wild-type HSCs impaired hematopoietic repopulation. In mouse HSCs and patient-derived lymphoblasts, PPARγ activation is manifested in upregulating the p53 target p21. PPARγ and co-activators are upregulated in total bone marrow and stem/progenitor cells from FA patients. Collectively, this work illustrates the utility of RNAi technology coupled with HSC transplantation for the discovery of novel genes and pathways involved in stress hematopoiesis.

HDAC8 Regulates Long-Term Hematopoietic Stem Cell Quiescence and Maintenance

Long-term (LT) hematopoietic stem cells (HSC) are responsible for life-long production of mature blood cells of all lineages through tightly concerted cell fate decisions including quiescence, self-renewal, differentiation and apoptosis. Histone deacetylase 8 (HDAC8) is a member of class I HDAC enzymes that remove acetyl moieties from lysine residues on histones and a variety of non-histone proteins. Specifically, HDAC8 has been shown to modulate the acetylation cycle of cohesin complex protein SMC3. Loss-of-function mutations in HDAC8, located on the X chromosome q13, have been found in patients with Cornelia de Lange Syndrome (CdLS) and those with CdLS-like features. These HDAC8 mutations are associated with severely skewed X-inactivation (100% wild type allele) in the peripheral blood of female patients, possibly due to selection against the mutant alleles. However, the expression and function of HDAC8 in normal HSCs and hematopoiesis remain unknown. In this study, we show that Hdac8 is highly expressed in the phenotypic LT-HSC (Lin-cKit+Sca1+CD150+CD48-) population in adult mice. To determine the function of HDAC8 in adult hematopoiesis, we generated conditional Hdac8 deficient mice using the Mx1-Cre and a floxed Hdac8 allele (Mx1-Cre/Hdac8f/f(y)) andconfirmed that Hdac8 is successfully deleted by polyinosinic-polycytidylic acid [poly (I:C)] treatment. Phenotypic analysis of Hdac8 deficient mice showed increased LT-HSC population compared to similarly treated control mice. However, largely normal steady state hematopoietic profile was found in Hdac8 deficient mice at 6 weeks and 1 year after induction. To further track Hdac8-deleted cells, we generated Cre/Hdac8f/f(y) mice with a dual fluorescence Rosa26mT/mG (mT/mG) Cre reporter allele, which expresses dTomato prior to Cre induction and becomes GFP+ after Cre-mediated recombination. We assessed hematopoietic repopulation by transplanting bone marrow cells from Cre/Hdac8f/f(y)/mTmG+ mice (2 x 105) together with wild type support cells (2 x 105) into lethally irradiated CD45.1+ congenic recipients. Hdac8 deletion was induced by treating the recipients with 7 does (14 m□g/kg/dose) of poly (I:C). We found that Hdac8 deletion did not affect CD45.2+ or GFP+ donor-derived overall engraftment or lineage repopulation up to 16 weeks. There was also no change in the frequency or number of GFP+ donor-derived phenotypic LT-HSCs in the bone marrow. Serial transplantation was performed to further assess long-term repopulating activity of HSCs. Hdac8 deficient cells were significantly (p=0.019; n=3) compromised in multi-lineage repopulation in secondary transplant recipients. Except a modest reduction in Pre-GM, there was no change in the overall composition of Hdac8 deficient CD45.2+-derived populations. Upon tertiary transplantation, no donor engraftment was observed for Hdac8 deficient cells (0 out of 4) compared to 50% positive engraftment in control group (4 out of 8). These results indicate that HDAC8 is crucial for maintaining long-term serial-repopulating activity over time. Cell cycle analysis revealed that Hdac8 deficient LT-HSCs display reduced quiescence and increased cycling, consistent with the increased number of phenotypic LT-HSC seen in Hdac8 deleted mice. Therefore, we further tested the sensitivity of Hdac8 deficient mice to serial ablation with 5-fluorouracil (5FU), an S phase-specific cytotoxic chemotherapeutic agent. Impaired hematopoietic recovery and increased lethality (p<0.001; n=23) was seen in Hdac8 deficient mice treated with 5-FU (100 mg/kg) every 7 days, indicating that Hdac8 deletion renders hypersensitivity to serial ablation. There were significnatly less phenotypic LT-HSCs in Hdac8 deficient mice 6 days after 5-FU treatment (p<0.01; n=4). In parallel, we observed increased DNA strand beaks as indicated by γ-H2AX staining and comet assays (p<0.001; n>100 cells). Analysis of p53 activation, cell cycle regulators and DNA dmage response are ongoing. Collectively, our study indicates that HDAC8 plays a pivotal role in LT-HSC quiescence and maintenance. DisclosuresNo relevant conflicts of interest to declare.

Exploring the Function of Sestrin/Gator As Novel Regulators of Hematopoiesis

The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that responds to multiple environmental cues such as reactive oxygen species (ROS) and thereby regulates many fundamental biological processes including cell growth and autophagy. mTOR is found in two distinct multiprotein complexes, mTORC1 and mTORC2, of which mTORC1 has been established to play an important role in the regulation of hematopoiesis. For example, mTORC1 inhibition, combined with activation of canonical Wnt-signaling, was shown to increase long term repopulating (LT)-HSC self-renewal, whereas its activation depletes LT-HSCs.The activity of mTORC1 is tightly controlled by multiple layers of upstream regulators, including the recently discovered GTPase activating protein (GAP) activity towards Rags (GATOR) complex, which is an AMP-Kinase/Tuberous sclerosis complex (TSC)-independent mTORC1 inhibitor induced by amino acid deprivation. GATOR consists of two subcomplexes, GATOR1 and GATOR2, whereby GATOR2 inhibits GATOR1. Inactivation of GATOR2 prevents mTORC1 activation by amino acids, whereas inactivation of GATOR1 constitutively activates mTORC1.Sestrins (Sesn1, Sesn2 and Sesn3) are a family stress-inducible, redox-sensitive proteins that are involved in cellular- or organism-level adaptation to diverse metabolic challenges. They have been identified as direct interactors of GATOR2 and shown to inhibit mTORC1 by preventing GATOR2 from inhibiting GATOR1 in presence of amino acids. In quantitative affinity purification-mass spectrometry (AP-MS) and coimmunoprecipitation experiments with HeLa- and mouse embryonic stem cells harboring in situ GFP-tagged Sesn2, WDR59 and NPRL3 alleles, we could confirm the Sesn2/GATOR interaction under nearly physiological conditions.To analyze the function of Sestrin/GATOR during hematopoiesis in more detail, we isolated Lin- Sca+ hematopoietic cells from the bone marrow of Sesn2-/- mice and performed serial replating experiments and competitive hematopoietic repopulation experiments in lethally irradiated mice and observed that Sesn2-/- progenitor cells proliferate significantly faster than their wild type counterparts albeit only during the initial engraftment phase. At later stages, the wild type cells took over, exceeding the Sesn2-/- cells by 3-4 fold in peripheral blood, bone marrow and spleen three months after transplantation. This suggests that the increased proliferative potential of Sesn2-/- progenitor cells leads to a depletion of the LT-HSC pool, strikingly resembling the phenotype of activated mTORC1. DisclosuresNo relevant conflicts of interest to declare.

Mitigation of radiation-induced hematopoietic injury by the polyphenolic acetate 7, 8-diacetoxy-4-methylthiocoumarin in mice.

Protection of the hematopoietic system from radiation damage, and/or mitigation of hematopoietic injury are the two major strategies for developing medical countermeasure agents (MCM) to combat radiation-induced lethality. In the present study, we investigated the potential of 7, 8-diacetoxy-4-methylthiocoumarin (DAMTC) to ameliorate radiation-induced hematopoietic damage and the associated mortality following total body irradiation (TBI) in C57BL/6 mice. Administration of DAMTC 24 hours post TBI alleviated TBI-induced myelo-suppression and pancytopenia, by augmenting lymphocytes and WBCs in the peripheral blood of mice, while bone marrow (BM) cellularity was restored through enhanced proliferation of the stem cells. It stimulated multi-lineage expansion and differentiation of myeloid progenitors in the BM and induced proliferation of splenic progenitors thereby, facilitating hematopoietic re-population. DAMTC reduced the radiation-induced apoptotic and mitotic death in the hematopoietic compartment. Recruitment of pro-inflammatory M1 macrophages in spleen contributed to the immune-protection linked to the mitigation of hematopoietic injury. Recovery of the hematopoietic compartment correlated well with mitigation of mortality at a lethal dose of 9 Gy, leading to 80% animal survival. Present study establishes the potential of DAMTC to mitigate radiation-induced injury to the hematopoietic system by stimulating the re-population of stem cells from multiple lineages.

The non-canonical Wnt receptor Ryk regulates hematopoietic stem cell repopulation in part by controlling proliferation and apoptosis.

The development of blood and immune cells requires strict control by various signaling pathways in order to regulate self-renewal, differentiation and apoptosis in stem and progenitor cells. Recent evidence indicates critical roles for the canonical and non-canonical Wnt pathways in hematopoiesis. The non-canonical Wnt pathway is important for establishment of cell polarity and cell migration and regulates apoptosis in the thymus. We here investigate the role of the non-canonical Wnt receptor Ryk in hematopoiesis and lymphoid development. We show that there are dynamic changes in Ryk expression during development and in different hematopoietic tissues. Functionally, Ryk regulates NK cell development in a temporal fashion. Moreover, Ryk-deficient mice show diminished, but not absent self-renewal of hematopoietic stem cells (HSC), via effects on mildly increased proliferation and apoptosis. Thus, Ryk deficiency in HSCs from fetal liver reduces their quiescence, leading to proliferation-induced apoptosis and decreased self-renewal.

Correction: Functional screen identifies regulators of murine hematopoietic stem cell repopulation.

Vol. 213, No. 3, March 7, 2016. Pages [433–449][1]. The authors regret that in their original paper, the gene Zfp521 appeared as Zfp251 . The online HTML and PDF versions of this article have been corrected. Figs. 1, 3, and 5 have also been edited to correct this mistake. The error only remains

Genetic hierarchy and temporal variegation in the clonal history of acute myeloid leukaemia.

In acute myeloid leukaemia (AML) initiating pre-leukaemic lesions can be identified through three major hallmarks: their early occurrence in the clone, their persistence at relapse and their ability to initiate multilineage haematopoietic repopulation and leukaemia in vivo. Here we analyse the clonal composition of a series of AML through these characteristics. We find that not only DNMT3A mutations, but also TET2, ASXL1 mutations, core-binding factor and MLL translocations, as well as del(20q) mostly fulfil these criteria. When not eradicated by AML treatments, pre-leukaemic cells with these lesions can re-initiate the leukaemic process at various stages until relapse, with a time-dependent increase in clonal variegation. Based on the nature, order and association of lesions, we delineate recurrent genetic hierarchies of AML. Our data indicate that first lesions, variegation and treatment selection pressure govern the expansion and adaptive behaviour of the malignant clone, shaping AML in a time-dependent manner.

Activated KRAS Cooperates with MLL-AF4 to Promote Extramedullary Engraftment and Migration of Cord Blood CD34+ HSPC But Is Insufficient to Initiate Leukemia.

The MLL-AF4 (MA4) fusion gene is the genetic hallmark of an aggressive infant pro-B-acute lymphoblastic leukemia (B-ALL). Our understanding of MA4-mediated transformation is very limited. Whole-genome sequencing studies revealed a silent mutational landscape, which contradicts the aggressive clinical outcome of this hematologic malignancy. Only RAS mutations were recurrently detected in patients and found to be associated with poorer outcome. The absence of MA4-driven B-ALL models further questions whether MA4 acts as a single oncogenic driver or requires cooperating mutations to manifest a malignant phenotype. We explored whether KRAS activation cooperates with MA4 to initiate leukemia in cord blood-derived CD34(+) hematopoietic stem/progenitor cells (HSPC). Clonogenic and differentiation/proliferation assays demonstrated that KRAS activation does not cooperate with MA4 to immortalize CD34(+) HSPCs. Intrabone marrow transplantation into immunodeficient mice further showed that MA4 and KRAS(G12V) alone or in combination enhanced hematopoietic repopulation without impairing myeloid-lymphoid differentiation, and that mutated KRAS did not cooperate with MA4 to initiate leukemia. However, KRAS activation enhanced extramedullary hematopoiesis of MA4-expressing cell lines and CD34(+) HSPCs that was associated with leukocytosis and central nervous system infiltration, both hallmarks of infant t(4;11)(+) B-ALL. Transcriptional profiling of MA4-expressing patients supported a cell migration gene signature underlying the mutant KRAS-mediated phenotype. Collectively, our findings demonstrate that KRAS affects the homeostasis of MA4-expressing HSPCs, suggesting that KRAS activation in MA4(+) B-ALL is important for tumor maintenance rather than initiation. Cancer Res; 76(8); 2478-89. ©2016 AACR.

Caught in a Wnt storm: Complexities of Wnt signaling in hematopoiesis.

The Wnt signaling pathway is an evolutionary conserved pathway that is involved in the development of almost every organ system in the body and provides self-renewal signals for most, if not all, adult stem cell systems. In recent years, this pathway has been studied by various research groups working on hematopoietic stem cells, resulting in contradicting conclusions. Here, we discuss and interpret the results of these studies and propose that Wnt dosage, the source of hematopoietic stem cells, and interactions with other pathways explain these disparate results.

Functional screen identifies regulators of murine hematopoietic stem cell repopulation.

Understanding the molecular regulation of hematopoietic stem and progenitor cell (HSPC) engraftment is paramount to improving transplant outcomes. To discover novel regulators of HSPC repopulation, we transplanted >1,300 mice with shRNA-transduced HSPCs within 24 h of isolation and transduction to focus on detecting genes regulating repopulation. We identified 17 regulators of HSPC repopulation: Arhgef5, Armcx1, Cadps2, Crispld1, Emcn, Foxa3, Fstl1, Glis2, Gprasp2, Gpr56, Myct1, Nbea, P2ry14, Smarca2, Sox4, Stat4, and Zfp251. Knockdown of each of these genes yielded a loss of function, except in the cases of Armcx1 and Gprasp2, whose loss enhanced hematopoietic stem cell (HSC) repopulation. The discovery of multiple genes regulating vesicular trafficking, cell surface receptor turnover, and secretion of extracellular matrix components suggests active cross talk between HSCs and the niche and that HSCs may actively condition the niche to promote engraftment. We validated that Foxa3 is required for HSC repopulating activity, as Foxa3(-/-) HSC fails to repopulate ablated hosts efficiently, implicating for the first time Foxa genes as regulators of HSPCs. We further show that Foxa3 likely regulates the HSC response to hematologic stress. Each gene discovered here offers a window into the novel processes that regulate stable HSPC engraftment into an ablated host.

Recent advances in molecular pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are characterized by activation of the JAK-STAT pathway due to driver mutations including JAK2V617F and MPLW515K/L, as well as to mutations in CALR. Driver mutations phosphorylate multiple STAT proteins that lead to proliferations, differentiations and cytokine secretions of various hematopoietic cells. However, hematopoietic cells carrying JAK2V617F, which causes excessive cellular proliferation and differentiation, do not necessarily have a clonal growth advantage in terms of hematopoietic repopulation. Alterations of epigenetic modifiers involving histone modifications and DNA methylations, which often co-exist with driver mutations and eventually upregulate several oncogenes, may play crucial roles in long-term clinical courses characterized by progression to myelofibrosis or acute leukemia in MPNs. In addition to JAK2 inhibition, molecules altered by abnormal epigenetic modifications may be worth exploring as potential new therapeutic targets in MPNs.

Endothelium-targeted human Delta-like 1 enhances the regeneration and homing of human cord blood stem and progenitor cells.

BackgroundUmbilical cord blood (UCB) is becoming an alternative cell source for hematopoietic stem cell transplantation (HSCT). However, umbilical cord blood transplantation (UCBT) has been severely limited by low and finite numbers of hematopoietic stem cells and their delayed engraftment. New strategies are needed to improve ex vivo expansion efficiency and in vivo haematopoietic recovery.MethodsWe produced an endothelium-targeted soluble Notch ligand, the Delta-Serrate-Lag-2 (DSL) domain of human Delta-like 1 fused with a RGD motif (hD1R), and tested the effects of this protein on human umbilical cord blood hematopoietic stem and progenitor cell (UCB-HSPC) ex vivo and in vivo.ResultshD1R-mediated ex vivo expansion system was able to significantly increase the absolute number of UCB-HSPCs. The hD1R-expanded cells had the enhanced homing and maintained long-term hematopoietic stem cell repopulation capacity in the bone marrow of immunodeficient nonobese diabetic-severe combined immunodeficient (NOD/SCID) mice. Moreover, systemic administration of hD1R promoted the in vivo regeneration of donor cells in recipient mice and accelerated hematopoietic recovery, particularly in settings wherein the HSPCs dose was limiting.ConclusionsOur results indicated that hD1R might be applied in improving hematopoietic recovery and HSC engraftment in human UCBT.Electronic supplementary materialThe online version of this article (doi:10.1186/s12967-015-0761-0) contains supplementary material, which is available to authorized users.

Mice Deficient in Rap1A GTPase Display Reduced Adhesion of Hematopoietic and Mesenchymal Stromal Cells but Intact Hematopoietic Repopulation Potential

Background: The Guanine Triphosphatase (GTPase) Rap1A has been implicated in cell proliferation and migration in different tissues. Its role in the production, adhesion and migration of bone marrow-derived hematopoietic cells is however incompletely understood. Methods: We analysed mice lacking one or both Rap1A alleles and measured their adhesion function and their hematopoietic repopulation capacity using competitive bone marrow transplantation in the CD45.1/CD45.2 transplant model. We further employed flow cytometry and shear stress-dependent flow chamber adhesion assays for phenotypic characterization and analysis of adhesion functions. Results: Rap1A-/- mice showed decreased numbers of lymphocytes and hematopoietic colony forming progenitors in the blood, while the numbers of granulocytes, monocytes or platelets were not significantly altered compared to controls. Chemokine CXCL12-induced adhesion of lineage marker depleted (Lin-) bone marrow cells to Vascular Cell Adhesion Molecule (VCAM)-1 and to Intercellular Adhesion Molecule (ICAM)-1 was significantly reduced in Rap1A-/- mice. However, competitive repopulation by Rap1A deficient bone marrow cells was not inferior to wild type bone marrow 9 months after transplantation. No outgrowth of Mesenchymal Stromal Cells (MSCs) was observed from bone marrow of Rap1A-/- mice, whereas Rap1A+/- MSCs displayed reduced expression of adhesion molecules involved in progenitor homing and impaired adhesion to primary endothelial cells and VCAM-1 under shear stress in vitro, as compared to their wild-type counterparts. Conclusions: Rap1A regulates adhesion of immature hematopoietic cells and mesenchymal stromal cells, but appears to be redundant for hematopoietic engraftment.

Growth Factor Receptor-Bound Protein 10 (Grb10) Regulates Hematopoietic Stem Cell (HSC) Self-Renewal Via Control of mTOR Signaling

Elucidation of the mechanisms governing HSC regeneration has been impeded by difficulty in isolating HSCs early following genotoxic injury, such as total body irradiation (TBI). Using multiparametric flow cytometric cell sorting of BM ckit+sca-1+lin- cells coupled with gene expression analysis, we identified growth factor receptor-bound protein 10 (Grb10), a co-receptor which regulates Insulin Receptor/IGF-1 signaling, to be significantly overexpressed by BM KSL cells at the earliest detectable point of regeneration (day +10) following TBI (3.3-fold, p<0.0001).Grb10 is a member of the imprinted gene family which is predominately expressed in the stem cells of a variety of tissues, including embryonic stem cells, bone marrow, skin and muscle. Viral shRNA-mediated knockdown of Grb10 in BM KSL cells caused a significant decrease in KSL cells and colony forming cells (CFCs) detected in 7-day culture (p=0.03 and p=0.002, respectively). Furthermore, mice competitively transplanted with Grb10-deficient HSCs displayed 10-fold lower donor, multilineage hematopoietic cell engraftment than mice transplanted with Grb10-expressing HSCs (p=0.007 for %CD45.1+ donor cells). Secondary competitive repopulation assays confirmed a greater than 10-fold deficit in long-term repopulating capacity in Grb10-deficient KSL cells compared to Grb10-expressing KSL cells (p=0.006 for %CD45.1+ donor cells).In order to determine if Grb10 was necessary for HSC maintenance and normal hematopoiesis in vivo, we generated maternally-derived Grb10-deficient mice. Heterozygous 8 week old Grb10m/+ (1 mutant allele, 1 wild type allele) had 10-fold decreased Grb10 expression in BM lin-cells. BM CFCs and SLAM+ KSL cells were significantly decreased in Grb10m/+ mice compared to Grb10+/+ mice (p=0.006 and p=0.04, respectively). Competitive repopulation assays demonstrated significantly decreased donor hematopoietic cell repopulation in recipient mice transplanted with Grb10m/+ BM cells versus mice transplanted with Grb10+/+ BM cells (p=0.003 for %CD45.1+ donor cells). Mice transplanted with BM cells from homozygous Grb10-/- mice showed a similar decrease in donor-derived hematopoietic repopulation compared to mice transplanted with BM cells from Grb10+/+ mice (p=0.02 at 20 weeks post-transplantation). These results confirmed that Grb10 regulates HSC self-renewal capacity in vivo.To determine whether Grb10 regulates HSC regeneration after myelotoxic injury, we irradiated Grb10m/+ mice with 550cGy TBI, and monitored hematopoietic recovery over time in comparison to Grb10+/+ controls. Interestingly, Grb10m/+ mice displayed accelerated hematopoietic regeneration early following TBI. At day+10 after 550cGy, Grb10m/+ mice contained significantly increased numbers of BM SLAM+ KSL cells (p=0.04) and CFCs (p=0.009), compared to Grb10+/+ littermates. Similarly, mice transplanted with BM cells from irradiated, Grb10m/+ mice displayed 5-fold increased donor hematopoietic repopulation at 20 weeks post-transplantation compared to mice transplanted with BM cells from irradiated, Grb10+/+ mice (p=0.006). These data suggest that Grb10 deficiency accelerates hematopoietic recovery in the early period following myelosuppressive radiation injury.Mechanistically, Grb10-deficiency caused an increase in the percentage of BM KSL cells in G1 and G2/S/M phase of cell cycle compared to Grb10+/+ KSL cells (p=0.003). We also observed significantly increased levels of mTOR activation in Grb10m/+ BM KSL cells compared to Grb10+/+ BM KSL cells (p=0.001 for pS6, p=0.001 for pS6k and p=0.02 for p4EBP1). Furthermore, mTOR inhibition via siRNA-mTOR targeting rescued the defect in BM hematopoietic progenitor content (colony forming cells) in Grb10-deficient BM cells (p<0.0001). Taken together, our results suggest that Grb10 is necessary for HSC maintenance in steady state, while, paradoxically, Grb10 inhibition accelerates HSC regeneration early following injury. Furthermore, our data suggest that Grb10 mediates these effects via regulation of mTOR signaling. Selective modulation of Grb10 signaling has the potential to augment HSC self-renewal in steady state and to accelerate HSC regeneration following myelotoxic injury. DisclosuresHimburg:Duke University: Patents & Royalties: Patent Application for use of Pleiotrophin as a hematopoietic stem cell growth factor. Chute:C2 Regenerate: Equity Ownership; Duke University: Patents & Royalties: Application to use PTN as growth factor as hematopoietic stem cell growth factor.

A Point-of-Care Platform for Hematopoietic Stem Cell Gene Therapy

Hematopoietic stem and progenitor cell (HSPC) gene therapy holds great promise but requires highly technical and dedicated facilities. Current state-of-the-art requires ex vivo HSPC gene transfer in dedicated Good Manufacturing Practices (GMP) facility infrastructure, limiting treatment to highly developed countries. A patient point-of-care strategy would therefore make HSPC gene therapy available to patients worldwide. We developed a short, semi-automated, mostly-closed platform for ex vivo isolation and lentivirus (LV) gene modification of CD34+ HSPCs from either bone marrow or mobilized peripheral blood sources using the CliniMACS Prodigy® (Figure 1). Experiments were performed with a biosafety cabinet and personal protective equipment to simulate anticipated conditions in clinical facilities of underdeveloped countries. A total of 7 custom programs were developed for bone marrow or mobilized peripheral blood CD34+ cell isolation and LV transduction. Addition of a pyrimidoindole derivative, UM 729, permitted efficient transduction of CD34+ HSPCs in <18 hours. Complete semi-automated production took <34 hours from collection to infusion of the gene modified cell product, requiring very little operator hands-on time. Autologous, LV gene-modified CD34+ HSPCs from two nonhuman primates produced using this platform engrafted and supported multilineage hematopoietic repopulation after myeloablative total body irradiation at 1020 cGy. Total cell doses achieved were 27 x 106 and 5.4 x 106 CD34+ cells/kg body weight, respectively. We observed stable, persistent and polyclonal gene marking in peripheral blood granulocytes up to 40% and lymphocytes up to 15% within 4 months after infusion. Neither animal displayed evidence for increased toxicity, including potential contamination from the cell products. We then validated processing of human bone marrow and mobilized apheresis products from healthy adult donors. We demonstrate efficient isolation of human CD34+ HSPCs and up to 60% transduction efficiency with a clinical-grade LV currently being tested in a phase I clinical trial for treatment of HIV-associated lymphoma. Products tested met current FDA approved specifications for infusion. Xenotransplantation of these products into immunodeficient mice resulted in polyclonal engraftment over 12 weeks. These data demonstrate preclinical safety and feasibility of a patient point-of-care strategy for ex vivo LV gene transfer into HSPCs. This platform represents a major advance in global portability of LV mediated HSPC gene therapy. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Identification of CD34’ and CD34− leukemia-initiating cells in MLL-rearranged human acute lymphoblastic leukemia

AbstractTranslocation of the mixed-lineage leukemia (MLL) gene with AF4, AF9, or ENL results in acute leukemia with both lymphoid and myeloid involvement. We characterized leukemia-initiating cells (LICs) in primary infant MLL-rearranged leukemia using a xenotransplantation model. In MLL-AF4 patients, CD34’CD38’CD19’ and CD34−CD19’ cells initiated leukemia, and in MLL-AF9 patients, CD34−CD19’ cells were LICs. In MLL-ENL patients, either CD34’ or CD34− cells were LICs, depending on the pattern of CD34 expression. In contrast, in patients with these MLL translocations, CD34’CD38−CD19−CD33− cells were enriched for normal hematopoietic stem cells (HSCs) with in vivo long-term multilineage hematopoietic repopulation capacity. Although LICs developed leukemic cells with clonal immunoglobulin heavy-chain (IGH) rearrangement in vivo, CD34’CD38−CD19−CD33− cells repopulated recipient bone marrow and spleen with B cells, showing broad polyclonal IGH rearrangement and recipient thymus with CD4’ single positive (SP), CD8’ SP, and CD4’CD8’ double-positive (DP) T cells. Global gene expression profiling revealed that CD9, CD32, and CD24 were over-represented in MLL-AF4, MLL-AF9, and MLL-ENL LICs compared with normal HSCs. In patient samples, these molecules were expressed in CD34’CD38’ and CD34− LICs but not in CD34’CD38−CD19−CD33− HSCs. Identification of LICs and LIC-specific molecules in primary human MLL-rearranged acute lymphoblastic leukemia may lead to improved therapeutic strategies for MLL-rearranged leukemia.

Fancd2 Is Required for Nuclear Retention of Foxo3a in Hematopoietic Stem Cell Maintenance

Functional maintenance of hematopoietic stem cells (HSCs) is constantly challenged by stresses like DNA damage and oxidative stress. Here we show that the Fanconi anemia protein Fancd2 and stress transcriptional factor Foxo3a cooperate to prevent HSC exhaustion in mice. Deletion of both Fancd2 and Foxo3a led to an initial expansion followed by a progressive decline of bone marrow stem and progenitor cells. Limiting dilution transplantation and competitive repopulating experiments demonstrated a dramatic reduction of competitive repopulating units and progressive decline in hematopoietic repopulating ability of double-knockout (dKO) HSCs. Analysis of the transcriptome of dKO HSCs revealed perturbation of multiple pathways implicated in HSC exhaustion. Fancd2 deficiency strongly promoted cytoplasmic localization of Foxo3a in HSCs, and re-expression of Fancd2 completely restored nuclear Foxo3a localization. By co-expressing a constitutively active CA-FOXO3a and WT or a nonubiquitinated Fancd2 in dKO bone marrow stem/progenitor cells, we demonstrated that Fancd2 was required for nuclear retention of CA-FOXO3a and for maintaining hematopoietic repopulation of the HSCs. Collectively, these results implicate a functional interaction between the Fanconi anemia DNA repair and FOXO3a pathways in HSC maintenance.