Human Hematopoietic Engraftment Research Articles

Abstract 2918: In vivo delivery of CD3-directed CD19-CAR lentivectors leads to the generation of CAR T and NK-like (CAR-TaNK) cells capable of complete ablation of B cells in the blood, bone marrow, and tissue of NSG-SGM3 CD34+ humanized mice

Abstract Introduction: Direct in vivo delivery of lentiviral vectors (LV) to generate CD19 CAR+ cells without the need for ex vivo preparation represents a promising approach to transform autologous CAR therapy into an off-the-shelf treatment. In these studies, direct administration of a new LV encoding a CD19 CAR into humanized NOD SCID gamma (NSG) mice expressing human IL-3, GM-CSF, and SCF (NSG-SGM3) resulted in a dose-dependent elimination of B cells in the peripheral blood, peritoneal fluid, bone marrow, and tissue of treated mice. Methods: CD3-directed LV encoding a CD19 CAR with a novel synthetic driver element was manufactured utilizing a 25L clinical scale suspension-based process. NSG-SGM3 mice transplanted with human CD34+ cells from cord blood were injected with LV doses (1E6 IU, 1E7 IU, or 5E7 IU) intraperitoneally (IP) or 1E7 IU intravenously (IV). Quantification of CD19 CAR+ cells and CD20+ B cells in peripheral blood, peritoneal fluid, and bone marrow was assessed by flow cytometry. Additionally, immunohistochemical analysis was performed to evaluate the tissue-resident human B cells and for any other histopathological observations following test article administration. Results: All CD34+ humanized NSG-SGM3 mice were confirmed to exhibit efficient human hematopoietic engraftment by flow cytometry prior to test article administration (peripheral blood hCD45: 68.9% ± 4.93%; hCD19+ B cells: 53.7% ± 5.11%). Direct LV administration at the 1E7 IU and 5E7 IU doses demonstrated a dose-dependent reduction in circulating human B cells compared to the control and 1E6 IU dose (p < 0.05). The synthetic driver elements co-expressed with the CAR led to the formation of unique CD3+ CD8+ CD56+ T and NK-like (TaNK) CD19 CAR+ cells in circulation. The 1E7 IU IP dose demonstrated ablation of B cells (total cells/uL) in peripheral blood (control: 9.67 ± 3.72 vs. LV treated: 0.157 ± 0.117), intraperitoneal fluid (control: 0.322 ± 0.244 vs. LV treated: 0.038 ± 0.029), bone marrow (control: 9.16± 1.83 vs. LV treated: 0.734 ± 0.864), and splenic tissue. Both IP and IV routes of administration showed significant B cell depletion at the 1E7 IU dose. However, complete B cell elimination in splenic tissue was only observed at the 5E7 IU dose. Non-treated CD34+ humanized NSG-SGM3 mice exhibited hepatic portal inflammation and moderate graft-versus-host disease (GVHD) in the colon. Interestingly, mice treated with LV encoding CD19 CAR exhibited decreased inflammatory pathology, suggesting potential B cell involvement in the inflammatory response in this model. Conclusion: In this study, direct in vivo delivery of LV encoding CD19 CAR resulted in the generation of functionally active CD19 CAR TaNK cells capable of eliminating target B cells in peripheral blood, peritoneal fluid, bone marrow, and tissue. Citation Format: Frederic Vigant, Ani Kundu, Ramya Yarlagadda, Cody Gowan, Michael Betts, Jonathan Kato, Renata Soares, Alan Ponce, Lintao Liu, Junyi Zhang, Ewa Jaruga-Killeen, Michelle Andraza, Suraj Kachgal, Gregory Schreiber, Wei Zhang, Gregory Wade, Gregory I. Frost, Sid P. Kerkar. In vivo delivery of CD3-directed CD19-CAR lentivectors leads to the generation of CAR T and NK-like (CAR-TaNK) cells capable of complete ablation of B cells in the blood, bone marrow, and tissue of NSG-SGM3 CD34+ humanized mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2918.

Characterization and Targeting TET2-Mutant HSPCs and AML

Understanding the contribution of abnormal epigenetic programs to acute myeloid leukemia (AML) is necessary for the integrated design of epigenetic-targeted therapies. The protein encoded by TET2 gene is an enzyme that catalyzes the conversion of 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC) as part of the process of DNA demethylation and is frequently mutated in pre-leukemic stem cells. From mice studies, TET2 knockdown/knockout models exhibit a myeloid differentiation bias and an expansion of hematopoietic stem cells (HSCs) with increased myeloproliferation. Notably, no studies to date have specifically examined the cell context effect of TET2 mutations on human hematopoiesis.To understand the mechanism of TET2 mutation-driven leukomogenesis, we have developed an in vitro/ in vivo platform using human CD34+ stem and progenitor cells that mimics TET2-mutated pre-leukemia. Using double chemically modified guide RNAs (gRNAs) coupled with Cas9 recombinant protein, we can disrupt the TET2 gene in cord blood-derived CD34+ cells (average indel frequencies=94.3%, n=6). Beyond disruption, we can modify the TET2 locus through insertion of a GFP cassette using homology driven repair to generate traceable cells. Thus, we have developed a tractable and cell-traceable model that faithfully recapitulates TET2-mutated pre-leukemia and clonal hematopoiesis of indeterminate potential (CHIP).First, we examined the effects of TET2-disruption on human erythroid differentiation in vitro by culturing bulk CD34+GFP+ cells under conditions that promote erythroid differentiation. TET2-disrupted cells produced reduced numbers of glycophorin A/ CD71 double positive erythroid cells compared to AAVS1-targeted control cells. This underscores the importance of TET2 in promoting normal erythroid differentiation, and demonstrates that our methods can give biological effects in a relevant primary human cell system.Next, we investigated the effects of TET2-disruption on hematopoietic colony formation in methylcellulose. Initial colony formation did not differ between TET2-mutant and control cells in the first plating; however, upon the second and third replating, increased numbers of colonies were observed with TET2-disruption. Analysis of indels in the colonies showed that with serial replating, there was enrichment of a 65 base pair deletion that disrupts the entire TET2 locus, suggesting that TET2 knockout cells outcompete normal unmutated hematopoietic stem/progenitor cells (HSPCs) during a month in vitro . We now know that ascorbic acid is an additional co-factor for TET enzymes to hydroxylate 5-mC to 5-hmC. Thus, we also cultured the cells overnight in high dose ascorbic acid prior to plating the cells. In both normoxic and hypoxic conditions, fewer colonies were detected in the TET2 knockout group, suggesting that high dose ascorbic acid cannot restore the TET2 enzymatic function in these cells.Lastly, we conducted in vivo engraftment studies through transplantation of CD34+ cells transfected with TET2-targeting Cas9 RNP (using 2 gRNAs without GFP), and demonstrated successful engraftment of human cells. Four months after transplant, we detected human hematopoietic engraftment with T cells, B cells, and myeloid cells, indicating that Cas9 RNP transfection did not impair engraftment ability. These cells showed high frequencies of indels within the TET2 gene. Surprisingly, we detected a significant proportion of human T cells (approximately 5%, even after transplantation of T cell-depleted CD34+ cells) that were not present with transplantation of control cells. These pilot data demonstrate that Cas9-RNP transfection did not impair engraftment ability of human HSPCs and that TET2-disrupted HSCs can give rise to hematopoietic progeny in vivo with possibly increased production of T cells. Secondary transplants are ongoing to assess for self-renewal.Here, we report the development of a CRISPR/Cas9 approach to target the TET2 gene in primary human HSPCs. We show that TET2 disruption leads to a block in erythroid differentiation, increased colony formation and replating in vitro, and increased engraftment in vivo . This model will enable further studies of effects on the epigenome through DNA methylation and hydroxymethylation studies, as well as provide an in vivo model of pre-leukemia/CHIP associated with TET2 mutations. DisclosuresNo relevant conflicts of interest to declare.

Advances in Human Immune System Mouse Models for Studying Human Hematopoiesis and Cancer Immunotherapy.

Immunotherapy has established itself as a promising tool for cancer treatment. There are many challenges that remain including lack of targets and some patients across various cancers who have not shown robust clinical response. One of the major problems that have hindered the progress in the field is the dearth of appropriate mouse models that can reliably recapitulate the complexity of human immune-microenvironment as well as the malignancy itself. Immunodeficient mice reconstituted with human immune cells offer a unique opportunity to comprehensively evaluate immunotherapeutic strategies. These immunosuppressed and genetically modified mice, with some overexpressing human growth factors, have improved human hematopoietic engraftment as well as created more functional immune cell development in primary and secondary lymphoid tissues in these mice. In addition, several new approaches to modify or to add human niche elements to further humanize these immunodeficient mice have allowed a more precise characterization of human hematopoiesis. These important refinements have opened the possibility to evaluate not only human immune responses to different tumor cells but also to investigate how malignant cells interact with their niche and most importantly to test immunotherapies in a more preclinically relevant setting, which can ultimately lead to better success of these drugs in clinical trials.

ZNF410 Represses Fetal Globin By Devoted Control of CHD4/NuRD

Several major effectors of adult-stage fetal globin silencing have been identified, including the transcription factors (TFs) BCL11A and ZBTB7A/LRF and the NuRD chromatin complex, although each has potential on-target liabilities for rational β-hemoglobinopathy therapeutic inhibition. Here through CRISPR screening of 1591 transcription factors, we discover ZNF410 to be a novel fetal hemoglobin (HbF) repressing TF. ZNF410 does not bind directly to the γ-globin genes but rather its chromatin occupancy is solely concentrated at CHD4, encoding the NuRD nucleosome remodeler, itself required for HbF repression. CHD4 has two ZNF410-bound regulatory elements, one at its promoter and the other at an enhancer 6 kb upstream, with 27 combined ZNF410 binding motifs constituting unparalleled genomic clusters. Peaks at these clusters display up to 57- and 77-fold enrichment respectively for ZNF410 binding compared to controls. Footprint analysis shows that the ZNF410 motifs are protected from nuclease cleavage. No other bona fide peaks could be detected across the genome in HUDEP-2 cells or primary CD34+ HSPC-derived erythroid precursors. We demonstrate that the ZNF410-bound elements at CHD4 completely account for ZNF410's effects on γ-globin repression. Knockout of ZNF410 reduces CHD4 by 60%, enough to substantially de-repress HbF while avoiding the cellular toxicity of complete CHD4 loss. Mice with constitutive deficiency of the homolog Zfp410 are born at expected Mendelian ratios (6 homozygous Zfp410Gt/Gt gene-trap allele mice out of 20 live births) with unremarkable hematology. ZNF410 is dispensable for human hematopoietic engraftment potential and erythroid maturation unlike known HbF repressors. Engrafting human erythrocytes induce HbF from ~2.5% in controls to ~17% in ZNF410 edited recipients, by hemoglobin HPLC. These studies identify a new rational target for HbF induction for the β-hemoglobin disorders with a wide therapeutic index. More broadly, ZNF410 represents a special class of gene regulator, a conserved transcription factor with singular devotion to regulation of a chromatin subcomplex. FIGURE LEGEND: ZNF410 chromatin occupancy is restricted to CHD4. (A) Dense mutagenesis of ZNF410 shows its five C2H2 zinc-finger domains (red rectangles) are especially critical for HbF repression. (B) Genome-wide ZNF410 binding was identified by CUT&RUN in HUDEP-2 cells. Enrichment for ZNF410 binding was concentrated at just two sites in the genome - at the promoter and at an enhancer ~6 kb upstream of CHD4. (C) Genome-wide ZNF410 motif occurrences (JASPAR motif shown in center) were mapped using a 3 kb sliding window. Clusters of ZNF410 motifs were found only at three locations in the genome, including the CHD4 promoter and CHD4 -6 kb enhancer. (D) CHD4 locus showing ZNF410 binding (red peaks) at the CHD4 promoter and CHD4 -6 kb enhancer regions with anti-HA antibody recognizing epitope-tagged ZNF410 and anti-ZNF410 antibody recognizing endogenous ZNF410 in both HUDEP-2 cells and CD34+ HSPC derived erythroid precursors. CUT&RUN footprints demonstrate ZNF410 motif protection, ATAC-seq chromatin accessibility and phyloP DNA sequence conservation. Disclosures Bauer: Pfizer: Consultancy; Syros Pharmaceuticals: Consultancy; Fulcrum Therapeutics: Consultancy; Sanofi: Research Funding.

Significant impact of mesenchymal stromal cell co-transplantation in a challenging hematopoietic engraftment scenario comparing two administration routes

Background & Aim Previous studies have shown that the co-infusion of mesenchymal stromal cells (MSCs) with hematopoietic stem cells (HSCs) enhances HSC engraftment in animal models and even in human transplants with a high risk of graft failure. These studies suggest that MSCs interact with HSCs and transport them towards the bone marrow. Although in conventional hematopoietic transplants HSCs are infused by the intravenous route (IV), some studies propose the intrabone route (IB) as an effective alternative procedure to improve HSCs homing. However, few studies have been published comparing the effectiveness of both alternatives in co-transplantation of HSCs with MSCs and the results are inconclusive. For that reason, the aim of this study was to go into detail about the comparison of both administration routes in order to enhance the hematopoietic engraftment. Methods, Results & Conclusion We investigated the possible beneficial effect of 500,000 human adipose tissue-derived MSCs improving the engraftment of a very limited number of human cord blood HSCs (4,000 CD34+ HSCs) in a xenogeneic transplant model in NSG mice, comparing both routes of transplant administration: IV and IB. Analyzing the percentage of human CD45+ cells in peripheral blood, bone marrow and spleen, the results revealed that co-transplantation with MSCs improved hematopoietic engraftment in NSG recipients conditioned with a submyeloablative irradiation dose, regardless of the route used for transplantation. In all instances, engrafted cells were also able to differentiate towards the lymphoid and myeloid linages. Furthermore, we showed that, in this model, the most efficient route of transplant administration is the IV route, obtaining higher levels of human hematopoietic engraftment in all the organs analyzed compared to those obtained by the IB route. Finally, we demonstrated that HSCs transplanted by IB route were not retained in the transplantation site and could migrate to the contralateral leg and also to the spleen. This observation could reveal that IB transplantation directly into the bone marrow niche is not preventing the recirculation and the systemic loss of the HSCs that occur when cells are IV transplanted. Taken together, our results demonstrate that IV co-transplantation of MSCs significantly increase the engraftment of very low numbers of HSCs, suggesting that this approach could be relevant in the clinics, particularly in gene therapy clinical trials in which a limited amount of HSCs is available. Previous studies have shown that the co-infusion of mesenchymal stromal cells (MSCs) with hematopoietic stem cells (HSCs) enhances HSC engraftment in animal models and even in human transplants with a high risk of graft failure. These studies suggest that MSCs interact with HSCs and transport them towards the bone marrow. Although in conventional hematopoietic transplants HSCs are infused by the intravenous route (IV), some studies propose the intrabone route (IB) as an effective alternative procedure to improve HSCs homing. However, few studies have been published comparing the effectiveness of both alternatives in co-transplantation of HSCs with MSCs and the results are inconclusive. For that reason, the aim of this study was to go into detail about the comparison of both administration routes in order to enhance the hematopoietic engraftment. We investigated the possible beneficial effect of 500,000 human adipose tissue-derived MSCs improving the engraftment of a very limited number of human cord blood HSCs (4,000 CD34+ HSCs) in a xenogeneic transplant model in NSG mice, comparing both routes of transplant administration: IV and IB. Analyzing the percentage of human CD45+ cells in peripheral blood, bone marrow and spleen, the results revealed that co-transplantation with MSCs improved hematopoietic engraftment in NSG recipients conditioned with a submyeloablative irradiation dose, regardless of the route used for transplantation. In all instances, engrafted cells were also able to differentiate towards the lymphoid and myeloid linages. Furthermore, we showed that, in this model, the most efficient route of transplant administration is the IV route, obtaining higher levels of human hematopoietic engraftment in all the organs analyzed compared to those obtained by the IB route. Finally, we demonstrated that HSCs transplanted by IB route were not retained in the transplantation site and could migrate to the contralateral leg and also to the spleen. This observation could reveal that IB transplantation directly into the bone marrow niche is not preventing the recirculation and the systemic loss of the HSCs that occur when cells are IV transplanted.

Natural estrogens enhance the engraftment of human hematopoietic stem and progenitor cells in immunodeficient mice.

Hematopoietic stem and progenitor cells (HSPC) are crucial in the maintenance of lifelong production of all blood cells. These stem cells are highly regulated to maintain homeostasis through a delicate balance between quiescence, self-renewal and differentiation. However, this balance is altered during the recovery after HSPC transplantation. Transplantation efficacy can be limited by inadequate hematopoietic stem cell number, poor homing, low level of engraftment, or limited self-renewal. As recent evidence indicates that estrogens are involved in regulating hematopoiesis, we sought to examine whether natural estrogens (estrone or E1, estradiol or E2, estriol or E3 and estetrol or E4) modulate human HSPC. Our results show that human HSPC subsets express estrogen receptors, and that signaling is activated by E2 and E4 on these cells. Additionally, these natural estrogens cause different effects on human progenitors in vitro. We found that both E2 and E4 expand human HSPC. However, E4 was the best tolerated estrogen and promoted cell cycling of human hematopoietic progenitors. Furthermore, we found that E2 and, more significantly, E4 doubled human hematopoietic engraftment in immunodeficient mice without altering other HSPC properties. Finally, the impact of E4 on promoting human hematopoietic engraftment in immunodeficient mice might be mediated through the regulation of mesenchymal stromal cells in the bone marrow niche. Collectively, our data demonstrate that E4 is well tolerated and enhances human reconstitution in immunodeficient mice directly, by modulating human hematopoietic progenitor properties, and indirectly, by interacting with the bone marrow niche. This might have particular relevance for improving hematopoietic recovery after myeloablative conditioning, especially when limited numbers of HSPC are available.

Estrogens Enhance Human Hematopoietic Stem Cell Engraftment in a Xenogenic Transplantation Model

Hematopoietic Stem Cells (HSCs) is a rare cell population that sits atop a hierarchy of progenitors that become progressively restricted to several or a single blood lineage. HSCs are capable of self-renewal and multipotent differentiation to all blood cell lineages. HSCs are crucial in the maintenance of lifelong production of all blood cells. HSCs are highly regulated to maintain homeostasis through a delicate balance between quiescence, self-renewal and differentiation. However, this balance is altered during the hematopoietic recovery after Hematopoietic Stem Cell Transplantation (HSCT). HSCT is routinely used to reconstitute hematopoiesis after myeloablation, being the most commonly-used cell therapy. HSCT efficacy and multilineage reconstitution can be limited by inadequate HSC number, poor homing, engraftment, or limited self-renewal.Recent evidence indicates that estrogens are involved in regulating the hematopoietic system homeostasis. Estrogens are the primary female sex hormones and are responsible for controlling many cellular processes including growth, differentiation and function of the reproductive system. However, estrogens have also been proposed to regulate HSCs. b-Estradiol (E2) was shown to promote the cell cycle of HSCs and multipotent progenitors (MPPs) and increase erythroid differentiation in females (1). On the other hand, tamoxifen reduces the number of MPPs and short-term HSCs but activates proliferation of long-term HSCs (2). The potential clinical application of estrogens in HSCT mainly derives from the possibility that these drugs may enhance the engraftment of transplanted HSCs, thus reducing side effects associated to myeloablative conditioning.Here, we show that a short-term treatment of immunodeficient mice transplanted with hCD34+ cells with estrogens such as E2 and estetrol (E4) improves human hematopoietic engraftment. Fifty-thousand cord blood CD34+ cells (CB-CD34+) were transplanted into sublethally irradiated immunodeficient NSG mice. Three days after transplantation, mice were treated for four days with daily subcutaneous doses of E2, E4 or vehicle. Human hematopoietic engraftment was evaluated in the BM of transplanted mice at four months later. E2 and E4 estrogens increased the proportion of hCD45+ cells 1.8-fold and 2.4-fold as compared to values determined in control mice, without modifying the proportion of myeloid and lymphoid lineages. Significantly, animals treated with either estrogen had significantly higher levels of human hematopoietic progenitors (hCD45+CD34+). To study the engraftment of long-term engraftment HSCs in transplanted mice, human CD45+ cells from primary recipients were sorted and transplanted in secondary NSG recipients. Three months after transplants, the proportion of human hematopoietic cells in secondary recipients was also higher when primary recipients were treated with E2 or E4 than in vehicle-treated animals. Improved engraftments associated to the administration of E2 or E4 estrogens were confirmed when very low doses of CB-CD34+ cells were transplanted (5x103 hCD34+/mouse) in recipients of either sex.Collectively, our data support a new application of estrogens to improve the hematopoietic recovery after HSCT. This application may have particular relevance to enhance the hematopoietic recovery after myeloablative conditioning and when limiting numbers of HSCs are available. DisclosuresBueren:Rocket Pharmaceuticals Inc: Consultancy, Equity Ownership, Patents & Royalties, Research Funding. Segovia:Rocket Pharmaceuticals Inc: Consultancy, Equity Ownership, Patents & Royalties, Research Funding.

MGTA-456, a First-in-Class Cell Therapy Produced from a Single Cord Blood Unit, Enables a Reduced Intensity Conditioning Regimen and Enhances Speed and Level of Human Microglia Engraftment in the Brains of NSG Mice

Background. Allogeneic bone marrow transplant (BMT) is a promising, curative approach for patients with inherited metabolic disorders (IMDs), a class of pediatric diseases characterized by a single enzyme deficiency. The goal of transplant is to provide cells that produce functional enzymes otherwise deficient in these patients, and thereby prevent or ameliorate neurological complications associated with selected IMDs. Donor-derived microglial cells are protective, limiting neurological disease progression. For IMD patients who do not have an HLA matched, non-carrier related donor, cord blood (CB) is the preferred HSPC source given its rapid availability and superior clinical outcomes compared to other graft sources. CB, however, is associated with delayed hematopoietic recovery and relatively poor engraftment due to the limited numbers of hematopoietic stem cells (HSCs) in a CB unit, delaying enzyme/protein reconstitution and cross-correction of non-hematopoietic cells.An aryl hydrocarbon receptor antagonist (AHRa)-based culture has been shown to expand CB CD34+ and CD34+CD90+ cells 330-fold and 100-fold, respectively, leading to rapid hematopoietic recovery after infusion of the clinical product, MGTA-456 (Wagner et al., Cell Stem Cell 2016 and Orchard et al., ASH 2018). As microglia are thought to be derived from HSCs, we hypothesized that MGTA-456 might lead to faster and greater microglial engraftment and potentially enable reduced intensity conditioning. Here, we assessed human hematopoietic and brain engraftment in NSG mice after transplant with MGTA-456 and showed that microglia engrafted faster with MGTA-456, less conditioning was needed, and that, mechanistically, these cells are derived from the CD34+CD90+ cell fraction.Methods. CB CD34+ cells were expanded in growth factor-supplemented media with or without an AHRa for 10 days. NSG mice were transplanted with unmanipulated CB CD34+ cells or the expanded product after 200 cGy total body irradiation or busulfan (BU) dosed at 20 or 40 mg/kg ip. Microglial engraftment was measured by flow cytometry of homogenized brains, quantitating the number of CD45+CD11b+Iba1+ cells, and by immunohistochemistry of brain sections.Results. Relative to naïve, unmanipulated CB CD34+ cells, transplant of MGTA-456 into sublethally irradiated mice led to an 8-fold increase in hematopoietic engraftment and a 10-fold increase in microglial engraftment in the brain (p<0.0001, n=15 mice), with histology consistent with engrafting microglia. As high dose BU enables enhanced microglia engraftment relative to irradiation by crossing the blood brain barrier and clearing host microglia (Capotondo et al., PNAS 2013), we evaluated the effectiveness of MGTA-456 after BU conditioning at 20 or 40 mg/kg. Transplant of MGTA-456 led to a 37-fold increase in engraftment relative to mice transplanted with unmanipulated CB CD34+ cells (p<0.001, n=8). Notably, transplant of MGTA-456 into mice conditioned with low-dose BU (20 mg/kg) led to a 15-fold increase in engraftment relative to high-dose BU (40 mg/kg)-conditioned animals transplanted with unmanipulated CB CD34+ cells (p<0.001, n=8). To evaluate speed of microglial engraftment, we evaluated brains weekly to 16 weeks after transplant. A 28-fold increase in microglial engraftment was demonstrated as early as 2 weeks post-transplant with MGTA-456 (p<0.0001, n=8). Number of engrafting hematopoietic cells in the periphery correlated with number of engrafting microglia in the brain (p<0.0001). Lastly, subpopulations of MGTA-456 were evaluated to determine the source of microglial engraftment. Only CD34+CD90+ cells, but not CD34+CD90- or CD34- cells, led to brain engraftment, consistent with the subpopulation of cells that result in hematopoietic engraftment following transplant of unexpanded cells (Radtke et al., Sci Trans Med 2017 and Goncalves et al., Blood 2017 130:659).Conclusions. These studies demonstrate that microglial engraftment is faster and greater in recipients of MGTA-456 even after lower dose BU conditioning, that microglial engraftment correlates with peripheral blood recovery, and that microglia cells are derived from CD34+CD90+ cells. These results suggest that lower dose BU may improve safety without jeopardizing efficacy in IMD recipients of MGTA-456. A Phase 2 clinical trial is ongoing to evaluate transplant of MGTA-456 in patients with select IMDs. [Display omitted] DisclosuresGoncalves:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Li:Magenta Therapeutics: Employment, Equity Ownership. Brooks:Magenta Therapeutics: Employment, Equity Ownership. Hyzy:Magenta Therapeutics: Employment, Equity Ownership. Boitano:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Cooke:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties.

Bioengineering of Humanized Bone Marrow Microenvironments in Mouse and Their Visualization by Live Imaging.

Human hematopoietic stem cells (HSCs) reside in the bone marrow (BM) niche, an intricate, multifactorial network of components producing cytokines, growth factors, and extracellular matrix. The ability of HSCs to remain quiescent, self-renew or differentiate, and acquire mutations and become malignant depends upon the complex interactions they establish with different stromal components. To observe the crosstalk between human HSCs and the human BM niche in physiological and pathological conditions, we designed a protocol to ectopically model and image a humanized BM niche in immunodeficient mice. We show that the use of different cellular components allows for the formation of humanized structures and the opportunity to sustain long-term human hematopoietic engraftment. Using two-photon microscopy, we can live-image these structures in situ at the single-cell resolution, providing a powerful new tool for the functional characterization of the human BM microenvironment and its role in regulating normal and malignant hematopoiesis.

Bioengineering of Humanized Bone Marrow Microenvironments in Mouse and Their Visualization by Live Imaging

Human hematopoietic stem cells (HSCs) reside in the bone marrow (BM) niche, an intricate, multifactorial network of components producing cytokines, growth factors, and extracellular matrix. The ability of HSCs to remain quiescent, self-renew or differentiate, and acquire mutations and become malignant depends upon the complex interactions they establish with different stromal components. To observe the crosstalk between human HSCs and the human BM niche in physiological and pathological conditions, we designed a protocol to ectopically model and image a humanized BM niche in immunodeficient mice. We show that the use of different cellular components allows for the formation of humanized structures and the opportunity to sustain long-term human hematopoietic engraftment. Using two-photon microscopy, we can live-image these structures in situ at the single-cell resolution, providing a powerful new tool for the functional characterization of the human BM microenvironment and its role in regulating normal and malignant hematopoiesis.

A Small Molecule Inhibitor of Protein Tyrosine Phosphatase-Sigma (PTPσ) Promotes Hematopoietic Stem Cell (HSC) Regeneration

Receptor tyrosine kinases (RTKs), such as c-kit, Flt-3 and Tie2, regulate hematopoietic stem cell (HSC) proliferation, differentiation and maintenance. Substantially less is known regarding the function of receptor protein tyrosine phosphatases (PTPs) in regulating HSC fate. We recently discovered that receptor protein tyrosine phosphatase sigma (PTPσ) is highly expressed by murine and human HSCs. Interestingly, constitutive deletion of PTPσ caused a marked increase in HSC repopulating capacity as measured in primary and secondary competitive repopulation assays (J Clin Invest 2015;125:177-182). Further, negative selection of human cord blood (CB) HSCs for PTPσ surface expression (PTPσ - negative CB HSCs) conferred more than 10-fold increased human CB hematopoietic engraftment through 20 weeks in transplanted NSG mice. Additionally, PTPσ-deficient mice displayed significantly augmented recovery of phenotypic bone marrow (BM) HSCs and colony forming cells at day +10 following 550 cGy total body irradiation. Based on these observations, we hypothesized that PTPσ may function as a negative regulator of HSC self-renewal and regeneration. We sought to develop pharmacologic strategies to inhibit PTPσ function as a means to augment HSC functional capacity. Based on structure-activity-relationship analysis of PTPσ, we screened candidate small molecules for ability to modulate PTPσ function on BM HSCs. We identified a small molecule, 5483071 (Chembridge), as a candidate PTPσ inhibitor. In silico simulation indicated that 5483071 rigidly docked into the binding site of PTPσ intracellular domain through hydrogen bonding and electrostatic interactions. In a model of HSC injury and regeneration, we irradiated C57Bl6 mice with 700 cGy TBI and treated mice systemically with either 10 μcg of 5483071 or water subcutaneously every other day from day +1 to day +14. Irradiated mice treated with 5483071 displayed significantly increased 60 day survival compared to controls (P=0.0007). Irradiated mice treated with 5483071 showed accelerated recovery of BM SLAM+kit+sca-1+lin- HSCs (P=0.02), BM KSL cells (P=0.01), and colony forming cells (CFCs) (P=0.0003). In vitro culture of BM KSL cells with 5483071 significantly increased the levels of activated Rac1 (P=0.0004), which recapitulated the effects of PTPσ deletion on Rac1 activation in HSCs. Importantly, treatment of BM KSL cells from PTPσ -/- mice with 5483071 caused no change in Rac1 activation, suggesting that 5483071 acted specifically on PTPσ and was not mediating effects via inhibition of other phosphatases. Systemic administration of 5483071 to irradiated mice caused an increase in BM KSL cell cycling at 72 hours compared to irradiated control mice (p=0.02), while also decreasing BM KSL cell apoptosis at 24 hours after TBI (p=0.02). Subsequent to these findings, we have generated several new small molecule inhibitors of PTPσ with novel composition of matter and have taken a lead compound into pre-clinical studies for investigational new drug (IND) development. PTPσ represents a novel receptor tyrosine phosphatase that regulates HSC self-renewal and regeneration. Targeted inhibition of PTPσ has high therapeutic potential to promote hematopoietic regeneration in patients receiving myelosuppressive chemotherapy and/or radiotherapy or undergoing myeloablative hematopoietic cell transplantation. DisclosuresNo relevant conflicts of interest to declare.

Peripheral blood CD34+ cells efficiently engraft human cytokine knock-in mice

AbstractHuman CD34+ hematopoietic stem and progenitor cells (HSPCs) can reconstitute a human hemato-lymphoid system when transplanted into immunocompromised mice. Although fetal liver-derived and cord blood–derived CD34+ cells lead to high engraftment levels, engraftment of mobilized, adult donor-derived CD34+ cells has remained poor. We generated so-called MSTRG and MISTRG humanized mice on a Rag2−/−Il2rg−/− background carrying a transgene for human signal regulatory protein α (SIRPα) and human homologs of the cytokine macrophage colony-stimulating factor, thrombopoietin, with or without interleukin-3 and granulocyte-macrophage colony-stimulating factor under murine promoters. Here we transplanted mobilized peripheral blood (PB) CD34+ cells in sublethally irradiated newborn and adult recipients. Human hematopoietic engraftment levels were significantly higher in bone marrow (BM), spleen, and PB in newborn transplanted MSTRG/MISTRG as compared with nonobese diabetic/severe combined immunodeficient Il2rg−/− or human SIRPα-transgenic Rag2−/−Il2rg−/− recipients. Furthermore, newborn transplanted MSTRG/MISTRG mice supported higher engraftment levels of human phenotypically defined HSPCs in BM, T cells in the thymus, and myeloid cells in nonhematopoietic organs such as liver, lung, colon, and skin, approximating the levels in the human system. Similar results were obtained in adult recipient mice. Thus, human cytokine knock-in mice might open new avenues for personalized studies of human pathophysiology of the hematopoietic and immune system in vivo.

Possible role of minor h antigens in the persistence of donor chimerism after stem cell transplantation; relevance for sustained leukemia remission.

Persistent complete donor chimerism is an important clinical indicator for remissions of hematological malignancies after HLA-matched allogeneic stem cell transplantation (SCT). However, the mechanisms mediating the persistence of complete donor chimerism are poorly understood. The frequent coincidence of complete donor chimerism with graft-versus-leukemia effects and graft-versus-host disease suggests that immune responses against minor histocompatibility antigens (mHags) are playing an important role in suppressing the host hematopoiesis after allogeneic SCT. Here, we investigated a possible relationship between donor immune responses against the hematopoiesis-restricted mHag HA-1 and the long-term kinetics of host hematopoietic chimerism in a cohort of 10 patients after allogeneic HLA-matched, HA-1 mismatched SCT. Functional HA-1 specific CTLs (HA-1 CTLs) were detectable in 6/10 patients lysing host-type hematopoietic cells in vitro. Presence of HA-1 CTLs in the peripheral blood coincided with low host hematopoiesis levels quantified by highly sensitive mHag specific PCR. Additionally, co-incubation of host type CD34+ cells with HA-1 CTLs isolated after allogeneic SCT prevented progenitor and cobblestone area forming cell growth in vitro and human hematopoietic engraftment in immunodeficient mice. Conversely, absence or loss of HA-1 CTLs mostly coincided with high host hematopoiesis levels and/or relapse. In summary, in this first study, presence of HA-1 CTLs paralleled low host hematopoiesis levels. This coincidence might be supported by the capacity of HA-1 CTLs isolated after allogeneic SCT to specifically eliminate host type hematopoietic stem/progenitor cells. Additional studies involving multiple mismatched mHags in more patients are required to confirm this novel characteristic of mHag CTLs as factor for the persistence of complete donor chimerism and leukemia remission after allogeneic SCT.

Intra-Osseous Co-Transplantation of CD34-Selected Umbilical Cord Blood and Mesenchymal Stromal Cells

Introduction. Intra-Osseous (IO) transplantation has been proposed as a strategy to enhance human UCB (umbilical cord blood; hUCB) engraftment in NOD SCID-gamma (Nude Obesity Diabetic Severe Combined Immunodeficient, NSG) mice. Human Mesenchymal Stromal Cells (hMSC) have been shown to support the growth and differentiation of hematopoietic stem cells. We hypothesize that IO co-transplantation of hMSC and CD34+ selected hUCB may be effective in improving early engraftment in a NOD/SCID-gamma (NSG) mouse model. This concept would provide the basis for postulating that IO co-transplantation of hUCB and hMSC may improve engraftment in patients undergoing umbilical cord transplant.Methods. Umbilical cord grafts were obtained from our cord blood procurement program at University Hospitals Case Medical Center under an IRB approved protocol. CD34+ cells were labeled using the Miltenyi CD34+ Microbead Kit (Miltenyi Biotec, Bergiiisch Gladbach, Germany) and magnetically separated using the Miltenyi MACS system. CD34+ cells recovered from the column were re-suspended to the appropriate concentration per cohort and infused fresh.Human MSC (CD105+ CD73+ CD45- CD14-) were obtained from bone marrow donors using Percoll gradient isolation and culture-expanded to a homogeneous population under approved protocols in the Cellular Therapy Laboratory. MSCs were preserved in DMSO and when needed were thawed and re-suspended at the appropriate concentration per cohort.We used a NSG mouse (Jackson Laboratories) model of transplant to compare the rates of engraftment amongst 6 mouse cohorts by examining bone marrow and peripheral blood cellular composition at 6 weeks following transplant. Following non-lethal irradiation (300rads), 6 groups of NSG mice (5 mice per cohort) were studied: 1) intravenous (IV) 5x105 hUCB CD34+ cells, 2) IV 5x105 hUBC CD34+ cells and 1x106 hMSC, 3) IO 5x105 hUCB CD34+ cells, 4) IO 5x105 hUCB CD34+ cells and IO 1x106 hMSC, 5) IO 5x105 hUCB CD34+ cells and IV 1x106 hMSC, and 6) IV 5x105 hUCB CD34+ and IO 1x106 hMSC. hMSC dose was arbitrarily set at 2:1 with hUCB CD34+ cell dose. IV injections: cells were administered via tail vein injection suspended in a total volume of 200ul. IO injections: cells were administered via bilateral tibia IO injections suspended in a total volume of 40ul. At 6 weeks, the mice were sacrificed and their peripheral blood and bone marrow were analyzed for human hematopoietic cells using antibodies against human CD45, CD3, CD13, CD14 and CD19 (Becton Dickinson Biosciences). Human engraftment was expressed as percentage of CD45+ cells compared to total bone marrow cells.Results. All cohorts showed evidence of human hematopoietic engraftment. Analysis of human CD45, CD3 and CD19 revealed the highest level of engraftment in the IO hUCB /IO hMSC cohort, which was significantly improved over the IV hUCB cohort (Table 1, Percent human CD45, CD3 and CD19 in the right tibial bone marrow at 6 weeks, p values as compared to IV hUCB cohort). The right tibial bone marrow analysis of human CD13 and CD14 revealed no significant difference between cohorts.We observed that IO co-transplantation of hMSC and hUCB led to superior engraftment of CD45, CD3 and CD19 lineage cells in the bone marrow at 6 weeks as compared with the IV hUCB cohort controls (Figure 1, human CD45, CD3 and CD19 analysis of right tibial bone marrow at 6 weeks; * = p <0.05, ** = p<0.001). The frequency of human hematopoietic cells within the peripheral blood was not statistically different among various cohorts at 6 weeks.Conclusions. This study suggests that intra-osseous co-transplantation of hMSC and hUCB may improve early engraftment in NSG mice. The hypothesis that this strategy may improve engraftment of single unit CB transplantation in humans in intriguing and will be investigated in a phase I clinical trial. [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Antioxidant N-acetyl-l-cysteine increases engraftment of human hematopoietic stem cells in immune-deficient mice

AbstractImmunocompromised mice, such as the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, have been widely used to examine the self-renewal and differentiation potential of human hematopoietic stem cells (HSCs) in vivo. However, the efficiency of human HSC engraftment remains very low. Here, we report that NOD/SCID mice had higher levels of reactive oxygen species (ROS) in their bone marrow (BM) than other commonly used mouse strains (C57BL/6 and BALB/C). Treatment with the antioxidant N-acetyl-l-cysteine (NAC) decreased ROS levels in the BM of NOD/SCID mice. Furthermore, the NAC-treated mice displayed a significant increase in human HSC engraftment and multilineage hematopoietic differentiation in the mice. In comparison with the control mice, NAC-treated recipients displayed a 10.8-fold increase in hematopoietic engraftment in the injected tibiae. A beneficial effect of NAC for human hematopoietic engraftment was also observed in an additional immunodeficient mouse strain, namely NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NOD/SCID/γc−/− or NSG). Thus, this study uncovers a previously unappreciated negative effect of ROS on human stem cell engraftment in immunodeficient mice.

Mouse Versus Human Extrinsic Cues Dictate Transformation Potential In BCR-ABL/BMI1-Induced Leukemia In Humanized Xenograft Models

The NSG [NOD/Lt-scid/IL2Rγnull] xenotransplantation mouse model is currently the model of choice to evaluate human hematopoietic engraftment and to study development of human leukemia. Indeed, we have previously shown that co-expression of BCR-ABL together with the polycomb repression complex 1 (PRC1) member BMI1 in human cord blood (CB) derived CD34+ cells was sufficient to induce a serially transplantable lymphoid leukemia (Rizo et al., Blood 2010). This leukemia was characterized by high levels of CD34+/CD19+/CD20-/IgM-/CD33-/CD15- lymphoid blasts in the bone marrow and a high degree of infiltration of blasts in spleen and liver. Clonal analysis revealed that similar clones gave rise to leukemia in primary and secondary recipients. Although in vivo no myeloid leukemias were observed, in vitro both lymphoid as well as myeloid immortalized long-term cultures could readily be established, in line with phenotypes observed in chronic myeloid leukemia patients whereby a chronic myeloid phase can egress into a myeloid or lymphoid blast crisis. It is very plausible that differences between murine and human hematopoietic stem cell niches underlie these observed differences. Human engraftment in NSG mice is typically lymphoid biased, and since many growth factors and cytokines are species-specific it is clear that the murine niche is not ideal to evaluate human hematopoietic engraftment and leukemic transformation potential.In our current study we have evaluated the in vivo leukemic transformation potential of human CB derived CD34+ cells expressing BCR-ABL and/or BMI1 in NSG mice in which scaffolds coated with culture-expanded human mesenchymal stromal cells (MSCs) were implanted subcutaneously 8 weeks prior to injection of transduced cells, to allow the development of a humanized niche containing mineralized bone-matrix, osteoblasts, stromal cells, as well as appropriate vascularization (Groen et al., Blood 2012). BCR-ABL/BMI1 transduced human CB derived CD34+ cells or primary blast crisis CML patient cells were injected either intravenously or directly into the humanized scaffolds, and leukemia development was evaluated.Our data indicate that in a humanized niche, in contrast to a murine niche, BCR-ABL was sufficient to induce leukemia as a single hit without overexpression of exogenous BMI1. Furthermore, both ALL as well as erythro/myeloid leukemias could be induced. The ALL could be transplanted to secondary recipients and besides the lymphoid marker CD19, the cells also expressed CD33 and CD15, but not CD11b or GPA. These data are in sharp contrast to results obtained in xenograft mouse models without human niches, where BCR-ABL expression alone in human cells was not sufficient to induce leukemia, and secondary hits such as BMI1 were essential.Efficient engraftment of a blast-crisis CML patient sample was also observed in the human niche model, whereby the immature blast-like phenotype was maintained in the human scaffold niche, while more differentiated cells were observed in the mouse bone marrow niche. In vitro, long-term self-renewing cultures could readily be established with cells retrieved from the human scaffold niche of these leukemic mice, while no long-term cultures could be initiated with cells retrieved from the murine bone marrow niche, from the same mouse. These data indicate that a human niche is required to maintain appropriate in vivo self-renewal of human BC CML cells. Interestingly, the endogenous BMI1 levels were significantly higher in cells retrieved from the human scaffold niche as compared to the mouse BM niche.In conclusion, our data indicate that BCR-ABL transformed cells needs secondary event such as over expression of oncogene like BMI1 for its full transformation potential, most likely to overcome or repress oncogene-induced senescence. The mouse environment is not able to provide these secondary events in human cells, whereas the human niche is able to provide signals that together with BCR-ABL are sufficient to fully transform human cells in xenograft models. Disclosures:No relevant conflicts of interest to declare.

Self-Renewing Human Bone Marrow Mesenspheres Promote Hematopoietic Stem Cell Expansion

Strategies for expanding hematopoietic stem cells (HSCs) include coculture with cells that recapitulate their natural microenvironment, such as bone marrow stromal stem/progenitor cells (BMSCs). Plastic-adherent BMSCs may be insufficient to preserve primitive HSCs. Here, we describe a method of isolating and culturing human BMSCs as nonadherent mesenchymal spheres. Human mesenspheres were derived from CD45- CD31- CD71- CD146+ CD105+ nestin+ cells but could also be simply grown from fetal and adult BM CD45--enriched cells. Human mesenspheres robustly differentiated into mesenchymal lineages. In culture conditions where they displayed a relatively undifferentiated phenotype, with decreased adherence to plastic and increased self-renewal, they promoted enhanced expansion of cord blood CD34+ cells through secreted soluble factors. Expanded HSCs were serially transplantable in immunodeficient mice and significantly increased long-term human hematopoietic engraftment. These results pave the way for culture techniques that preserve the self-renewal of human BMSCs and their ability to support functional HSCs.

Organotypic Epigenetic Signature Predicts Bone and Marrow Niche Forming Capacity of Stromal Progenitors in a Novel Mouse Model in Vivo.

AbstractAbstract 2987Mesenchymal stem/progenitor cells (MSPCs) from numerous tissues are currently tested in clinical trials despite a limited understanding of their in vivo behavior. In this study we used MSPCs from adult and fetal tissues to select the appropriate source for clinical application. We asked whether MSPCs derived from human bone marrow (BM), white adipose tissue (WAT) and umbilical cord (UC), compared to skin fibroblasts, bear an equivalent bone and marrow niche formation potential with of in vivo. Furthermore we evaluated attraction and engraftment of murine as well as human hematopoietic stem/progenitor cells (HSPCs) into newly formed MSPC-derived niches. To elucidate potential mechanisms responsible for a tissue-specific MSPC potential after transplantation gene expression profiling and DNA methylation analysis on a novel high resolution 450K-CpG methylation array were employed.MSPCs were transplanted subcutaneously to test for their spontaneous bone and marrow niche formation potential in immune-deficient NSG mice (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ). BM-derived MSPC transplantation reproducibly led to the development of mature bone (17/17 donors) through an endochondral ossification process leading to subsequent marrow niche formation. Additionally, these newly formed hematopoietic microenvironments attracted complete mouse hematopoiesis including immature lineage negative, Sca-1 positive, c-kit positive (LSK) HSPCs. Non-BM derived MSPCs completely lacked bone and marrow niche-forming potential and did not attract hematopoietic cells (0/9 donors). Induction of human hematopoietic chimerism through transplantation of umbilical cord blood (UCB)-derived human CD34+ HSPCs in advance of subcutaneous ectopic bone and marrow development resulted in immigration of re-transplantable human hematopoiesis into extra-medullary ossicles. Comparative 450K-CpG methylation array profiling of MSPCs revealed a tissue-specific epigenetic signature virtually corresponding to the in vivo differentiation. MSPCs from BM but neither fibroblasts nor MSPCs from WAT or UC showed epigenetically imprinted human bone and marrow niche (HuNiche) formation capacity favoring BM-MSPCs for skeletal regeneration. This novel HuNiche model should be ideally suited for studying normal and malignant hematopoiesis regulation in an ectopic human marrow with subsequent human hematopoietic engraftment that mimics clinical BM transplantation reality. Disclosures:No relevant conflicts of interest to declare.

Membrane-bound human SCF/KL promotes in vivo human hematopoietic engraftment and myeloid differentiation

AbstractIn recent years, advances in the humanized mouse system have led to significantly increased levels of human hematopoietic stem cell (HSC) engraftment. The remaining limitations in human HSC engraftment and function include lymphoid-skewed differentiation and inefficient myeloid development in the recipients. Limited human HSC function may partially be attributed to the inability of the host mouse microenvironment to provide sufficient support to human hematopoiesis. To address this problem, we created membrane-bound human stem cell factor (SCF)/KIT ligand (KL)–expressing NOD/SCID/IL2rgKO (hSCF Tg NSG) mice. hSCF Tg NSG recipients of human HSCs showed higher levels of both human CD45+ cell engraftment and human CD45+CD33+ myeloid development compared with NSG recipients. Expression of hSCF/hKL accelerated the differentiation of the human granulocyte lineage cells in the recipient bone marrow. Human mast cells were identified in bone marrow, spleen, and gastrointestinal tissues of the hSCF Tg NSG recipients. This novel in vivo humanized mouse model demonstrates the essential role of membrane-bound hSCF in human myeloid development. Moreover, the hSCF Tg NSG humanized recipients may facilitate investigation of in vivo differentiation, migration, function, and pathology of human mast cells.

Cotransplantation of Ex Vivo Expanded and Unexpanded Cord Blood Units in Immunodeficient Mice Using Insulin Growth Factor Binding Protein-2–Augmented Mesenchymal Cell Cocultures

Ex vivo expansion of cord blood (CB) hematopoietic stem cells and cotransplantation of 2 CB units (CBUs) could enhance the applicability of CB transplantation in adult patients. We report an immunodeficient mouse model for cotransplantation of ex vivo expanded and unexpanded human CB, showing enhanced CB engraftment and provide proof of concept for this transplantation strategy as a means of overcoming the limiting cell numbers in each CBU. CBUs were expanded in serum-free medium supplemented with stem cell factor, Flt-3 ligand, thrombopoietin, and insulin growth factor binding protein-2 together with mesenchymal stromal cell coculture. Unexpanded and expanded CB cells were cotransplanted by tail vein injection into 45 sublethally irradiated nonobese diabetic SCID-IL2γ(-/-) (NSG) mice. Submandibular bleeding was performed monthly, and mice were sacrificed 4 months after transplantation to analyze for human hematopoietic engraftment. Expansion of non-CD34(+) selected CB cells yielded 40-fold expansion of CD34(+) cells and 3.1-fold expansion of hematopoietic stem cells based on limiting dilution analysis of NSG engraftment. Mice receiving expanded grafts exhibited 4.30% human cell repopulation, compared with 0.92% in mice receiving only unexpanded grafts at equivalent starting cell doses, even though the unexpanded graft predominated in long-term hematopoiesis (P = .07). Ex vivo expanded grafts with lower initiating cell doses also showed equivalent engraftment to unexpanded grafts with higher cell dose (8.0% versus 7.9%; P = .93). In conclusion, ex vivo expansion resulted in enhanced CB engraftment despite eventual rejection by the unexpanded CBU.