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Ex vivo hematopoietic stem and progenitor cell (HSPC) expansion platforms are under active development, designed to increase HSPC numbers and thus engraftment ability of allogeneic cord blood grafts or autologous HSPCs for gene therapies. Murine and in vitro models have not correlated well with clinical outcomes of HSPC expansion, emphasizing the need for relevant pre-clinical models. Our rhesus macaque HSPC competitive autologous transplantation model utilizing genetically barcoded HSPC allows direct analysis of the relative short and long-term engraftment ability of lentivirally transduced HSPCs, along with additional critical characteristics such as HSPC clonal diversity and lineage bias. We investigated the impact of ex vivo expansion of macaque HSPCs on the engineered endothelial cell line (E-HUVECs) platform regarding safety, engraftment of transduced and E-HUVEC-expanded HSPC over time compared to non-expanded HSPC for up to 51 months post-transplantation, and both clonal diversity and lineage distribution of output from each engrafted cell source. Short and long-term engraftment were comparable for E-HUVEC expanded and the non-expanded HSPCs in both animals, despite extensive proliferation of CD34+ cells during 8 days of ex vivo culture for the E-HUVEC HSPCs, and optimization of harvesting and infusion of HSPCs co-cultured on E-HUVEC in the second animal. Long-term hematopoietic output from both E-HUVEC expanded and unexpanded HSPCs was highly polyclonal and multilineage. Overall, the comparable HSPC kinetics of macaques to humans, the ability to study post-transplant clonal patterns, and simultaneous multi-arm comparisons of grafts without the complication of interpreting allogeneic effects makes our model ideal to test ex vivo HSPC expansion platforms, particularly for gene therapy applications., Graphical Abstract, The rhesus macaque barcoded autologous competitive repopulation preclinical transplantation model was utilized to study the engraftment and safety of hematopoietic stem and progenitor cells expanded in co-culture with E4ORF-1 gene transduced human umbilical vein endothelial cell as compared to non-expanded cells, documenting polyclonal long-term multilineage hematopoiesis.

Intrabone (IB) injection of umbilical cord blood has been proposed as a potential mechanism to improve transplant engraftment and prevent graft failure. However, conventional IB techniques produce low retention of transplanted cells in the marrow. To overcome this barrier, we developed an optimized IB (OIB) injection method using low-volume, computer-controlled slow infusion that promotes cellular retention in the marrow. Here, we compare engraftment of CD34+ cells transplanted in a myeloablative rhesus macaque (RM) model using the OIB method compared with IV delivery. RM CD34+ cells obtained by apheresis were split equally for transduction with lentiviral vectors encoding either green fluorescent protein or yellow fluorescent protein reporters. Following conditioning, one marked autologous population of CD34+ cells was injected directly IB using the OIB method and the other was injected via slow IV push into the same animal (n = 3). Daily flow cytometry of blood quantified the proportion of engrafting cells deriving from each source. Marrow retention was examined using positron emission tomography/computed tomography imaging of 89Zirconium (89Zr)-oxine–labeled CD34+ cells. CD34+ cells injected via the OIB method were retained in the marrow and engrafted in all 3 animals. However, OIB-transplanted progenitor cells did not engraft any faster than those delivered IV and contributed significantly less to hematopoiesis than IV-delivered cells at all time points. Rigorous testing of our OIB delivery system in a competitive RM myeloablative transplant model showed no engraftment advantage over conventional IV infusion. Given the increased complexity and potential risks of IB vs IV approaches, our data do not support IB transplantation as a strategy to improve hematopoietic engraftment.

Gene editing of the erythroid-specific BCL11A enhancer in hematopoietic stem and progenitor cells (HSPCs) from patients with sickle cell disease (SCD) induces fetal hemoglobin (HbF) without detectable toxicity, as assessed by mouse xenotransplant. Here, we evaluated autologous engraftment and HbF induction potential of erythroid-specific BCL11A enhancer–edited HSPCs in 4 nonhuman primates. We used a single guide RNA (sgRNA) with identical human and rhesus target sequences to disrupt a GATA1 binding site at the BCL11A +58 erythroid enhancer. Cas9 protein and sgRNA ribonucleoprotein complex (RNP) was electroporated into rhesus HSPCs, followed by autologous infusion after myeloablation. We found that gene edits persisted in peripheral blood (PB) and bone marrow (BM) for up to 101 weeks similarly for BCL11A enhancer– or control locus–targeted (AAVS1-targeted) cells. Biallelic BCL11A enhancer editing resulted in robust γ-globin induction, with the highest levels observed during stress erythropoiesis. Indels were evenly distributed across PB and BM lineages. Off-target edits were not observed. Nonhomologous end-joining repair alleles were enriched in engrafting HSCs. In summary, we found that edited HSCs can persist for at least 101 weeks after transplant and biallelic-edited HSCs provide substantial HbF levels in PB red blood cells, together supporting further clinical translation of this approach.

The classical model of hematopoietic hierarchies is being reconsidered on the basis of data from in vitro assays and single cell expression profiling. Recent experiments suggested that the erythroid lineage might differentiate directly from multipotent hematopoietic stem cells / progenitors or from a highly biased subpopulation of stem cells, rather than transiting through common myeloid progenitors or megakaryocyte-erythrocyte progenitors. We genetically barcoded autologous rhesus macaque stem and progenitor cells, allowing quantitative tracking of the in vivo clonal output of thousands of individual cells over time following transplantation. CD34+ cells were lentiviral-transduced with a high diversity barcode library, with the barcode in an expressed region of the provirus, allowing barcode retrieval from DNA or RNA, with each barcode representing an individual stem or progenitor cell clone. Barcode profiles from bone marrow CD45-CD71+ maturing nucleated red blood cells were compared with other lineages purified from the same bone marrow sample. There was very high correlation of barcode contributions between marrow nucleated red blood cells and other lineages, with the highest correlation between nucleated red blood cells and myeloid lineages, whether at earlier or later time points post transplantation, without obvious clonal contributions from highly erythroid-biased or restricted clones. A similar profile occurred even under stressors such as aging or erythropoietin stimulation. RNA barcode analysis on circulating mature red blood cells followed over long time periods demonstrated stable erythroid clonal contributions. Overall, in this nonhuman primate model with great relevance to human hematopoiesis, we documented continuous production of erythroid cells from multipotent, non-biased hematopoietic stem cell clones at steady-state or under stress.

Lentiviral vectors (LVs) are used for delivery of genes into hematopoietic stem and progenitor cells (HSPCs) in clinical trials worldwide. LVs, in contrast to retroviral vectors, are not associated with insertion site-associated malignant clonal expansions and, thus, are considered safer. Here, however, we present a case of markedly abnormal dysplastic clonal hematopoiesis affecting the erythroid, myeloid, and megakaryocytic lineages in a rhesus macaque transplanted with HSPCs that were transduced with a LV containing a strong retroviral murine stem cell virus (MSCV) constitutive promoter-enhancer in the LTR. Nine insertions were mapped in the abnormal clone, resulting in overexpression and aberrant splicing of several genes of interest, including the cytokine stem cell factor and the transcription factor PLAG1. This case represents the first clear link between lentiviral insertion-induced clonal expansion and a clinically abnormal transformed phenotype following transduction of normal primate or human HSPCs, which is concerning, and suggests that strong constitutive promoters should not be included in LVs.

Hematopoietic stem cell gene therapy for hemoglobin disorders, including sickle cell disease, requires high-efficiency lentiviral gene transfer and robust therapeutic globin expression in erythroid cells. Erythropoietin is a key cytokine for erythroid proliferation and differentiation (erythropoiesis), and truncated human erythropoietin receptors (thEpoR) have been reported in familial polycythemia. We reasoned that coexpression of thEpoR could enhance the phenotypic effect of a therapeutic vector in erythroid cells in xenograft mouse and autologous nonhuman primate transplantation models. We generated thEpoR by deleting 40 amino acids from the carboxyl terminus, allowing for erythropoietin-dependent enhanced erythropoiesis of gene-modified cells. We then designed lentiviral vectors encoding both thEpoR and B cell lymphoma/leukemia 11A (BCL11A)–targeting microRNA-adapted short hairpin RNA (shmiR BCL11A) driven by an erythroid-specific promoter. thEpoR expression enhanced erythropoiesis among gene-modified cells in vitro. We then transplanted lentiviral vector gene-modified CD34+ cells with erythroid-specific expression of both thEpoR and shmiR BCL11A and compared to cells modified with shmiR BCL11A only. We found that thEpoR enhanced shmiR BCL11A–based fetal hemoglobin (HbF) induction in both xenograft mice and rhesus macaques, whereas HbF induction with shmiR BCL11A only was robust, yet transient. thEpoR/shmiR BCL11A coexpression allowed for sustained HbF induction at 20 to 25% in rhesus macaques for 4 to 8 months. In summary, we developed erythroid-specific thEpoR/shmiR BCL11A–expressing vectors, enhancing HbF induction in xenograft mice and rhesus macaques. The sustained HbF induction achieved by addition of thEpoR and shmiR BCL11A may represent a viable gene therapy strategy for hemoglobin disorders.

Hematopoietic stem cell gene therapy for hemoglobin disorders, including sickle cell disease, requires high-efficiency lentiviral gene transfer and robust therapeutic globin expression in erythroid cells. Erythropoietin is a key cytokine for erythroid proliferation and differentiation (erythropoiesis), and truncated human erythropoietin receptors (thEpoR) have been reported in familial polycythemia. We reasoned that coexpression of thEpoR could enhance the phenotypic effect of a therapeutic vector in erythroid cells in xenograft mouse and autologous nonhuman primate transplantation models. We generated thEpoR by deleting 40 amino acids from the carboxyl terminus, allowing for erythropoietin-dependent enhanced erythropoiesis of gene-modified cells. We then designed lentiviral vectors encoding both thEpoR and B cell lymphoma/leukemia 11A (

Age-associated changes in hematopoietic stem and progenitor cells (HSPCs) have been carefully documented in mouse models but poorly characterized in primates and humans. To investigate clinically relevant aspects of hematopoietic aging, we compared the clonal output of thousands of genetically barcoded HSPCs in aged vs young macaques after autologous transplantation. Aged macaques showed delayed emergence of output from multipotent (MP) clones, with persistence of lineage-biased clones for many months after engraftment. In contrast to murine aging models reporting persistence of myeloid-biased HSPCs, aged macaques demonstrated persistent output from both B-cell and myeloid-biased clones. Clonal expansions of MP, myeloid-biased, and B-biased clones occurred in aged macaques, providing a potential model for human clonal hematopoiesis of indeterminate prognosis. These results suggest that long-term MP HSPC output is impaired in aged macaques, resulting in differences in the kinetics and lineage reconstitution patterns between young and aged primates in an autologous transplantation setting.

Hematopoietic stem cell (HSC) gene therapy is now curative for multiple genetic diseases; however, it is limited by morbidity and mortality from cytotoxic chemotherapy-based conditioning. To overcome these limitations, we developed an antibody drug conjugate (ADC) targeting CD117 (c-Kit) to specifically deplete both HSCs and progenitor cells. In our preliminary study, 0.2 mg/kg CD117-ADC conditioning resulted in >99% bone marrow depletion, detectable engraftment of gene-modified cells (vector copy number per cell (VCN) ~0.01), and minimal toxicities in a rhesus HSC gene therapy model (ASH 2019). In this study, we investigated escalating doses of CD117-ADC to determine the optimum conditioning dose to enable engraftment of gene-modified CD34+ HSCs in rhesus macaques. We evaluated autologous CD34+ cell transplantation with lentiviral gene marking following conditioning using a single injection of CD117-ADC at the 0.3 mg/kg dose for ZL13 and ZJ62, and the 0.4 mg/kg dose for H635 and H96G. The extent of gene marking was compared with myeloablative busulfan conditioning (5.5 mg/kg x 4 days) for 12U018 and 12U020. Mobilized rhesus CD34+ cells (ADC 3.8±1.9x10e7 vs. Busulfan 2.9±0.2x10e7, n.s.) were transduced with a lentiviral vector encoding BCL11A-targeting microRNA-adapted short hairpin RNA (shmiR-BCL11A) co-encoding a truncated human erythropoietin receptor (thEpoR) for stable fetal hemoglobin (HbF) induction (Sci Transl Med. 2021). These cells (in vitro VCN 10.1±3.8 vs. 10.2±7.3, n.s.) were transplanted into autologous animals 6 or 10 days after ADC conditioning (0.3 or 0.4 mg/kg, respectively) or 1 day after busulfan conditioning. Blood counts, gene-marking levels, and HbF induction were evaluated for 0.3-1.2 years post-transplant in ADC conditioning and for 1.5 years in busulfan conditioning. After a reduction of blood counts post-transplantation with ADC or busulfan conditioning, all lineages recovered. Granulocyte (>500/μl, day 6-9 vs. day 8-9), reticulocyte (>50,000/μl, day 10-14 vs. day 11), and platelet (>30,000/μl, day 2-8 vs. no reduction) recoveries were similar for ADC and busulfan conditioning, respectively. Only ADC conditioning resulted in a reduction of platelet counts as well as a novel transient rebound in all major lineages. Two months post-transplant, efficient gene marking (VCN in granulocytes 0.28±0.16 vs. 0.44±0.17, n.s.) was observed in 3 of 4 animals in ADC-conditioning (ZJ62 with 0.3 mg/kg ADC, and H635 and H96G with 0.4 mg/kg ADC). This marking level was similar to busulfan conditioning (Left panel in Figure). Robust and durable HbF induction was also detected by both HbF-positive percentages (F-cell 8.5±1.8% vs. 13.7±5.8%, n.s.) and HPLC-quantitated HbF amounts (8.0±2.9% vs. 11.1±5.2%, n.s.) in these 3 animals, similar to busulfan conditioning (Right panel in Figure). In ZL13 (1 of 2 animals in 0.3 mg/kg ADC), lower gene marking (VCN in granulocytes 0.02) was obtained, along with low HbF induction (F-cell 1.0% and HbF amounts 0.9%), suggesting that 0.3 mg/kg ADC is marginal and 0.4 mg/kg ADC is sufficient for robust engraftment of gene-modified cells. Importantly, CD117-ADC conditioning resulted in minimal toxicities unlike busulfan conditioning. In summary, we demonstrated that a single dose of CD117-ADC allows for efficient engraftment of gene-modified CD34+ HSCs in a rhesus gene therapy model, achieving a similar level as myeloablative busulfan conditioning. Robust HbF induction was also confirmed at the protein levels in this rhesus gene therapy model with ADC conditioning. This targeted approach for safer conditioning could improve the risk benefit profile in HSC gene therapy. Figure 1 Figure 1. Disclosures Latimer: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Bhattarai: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Yoder: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Palchaudhuri: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company, Patents & Royalties. Li: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Bertelsen: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Olson: Magenta Therapeutics: Current Employment, Current holder of stock options in a privately-held company.

Steady state bone marrow (BM) is the preferred hematopoietic stem cell (HSC) source for gene therapy in sickle cell disease (SCD) due to the recognized risk of vaso-occlusive crisis during granulocyte colony–stimulating factor mobilization. We previously established clinically relevant HSC gene transfer in the rhesus model following transplantation of mobilized peripheral blood (PB) CD34+ cells transduced with lentiviral vectors. In this study, we examined steady state bone marrow (BM) in the rhesus competitive repopulation model and demonstrate similar gene marking in vitro and in vivo, as compared with mobilized PB CD34+ cells. We then evaluated PB and steady state BM in subjects with SCD and observed a higher frequency of CD34+ cells when compared with controls, likely due to enhanced hematopoiesis. However, CD34+ cell counts were reduced in both the PB and BM in patients treated with hydroxyurea, and hydroxyurea treatment strongly inhibited iPS cell generation from SCD subjects. Our data support that steady state BM is a useful HSC source for SCD gene therapy with similar transduction. The lower CD34+ percentages observed with hydroxyurea treatment warrants withholding hydroxyurea temporarily prior to harvesting HSCs. Our results are important for the design of gene targeting strategies for SCD.

Elevated fetal hemoglobin (HbF) is associated with reduced severity of sickle cell disease. Therefore, γ-globin protein levels and F-cell (HbF-positive red blood cell) percentages are used for estimation of clinical benefit. Here, we monitored transplantation-related changes in HbF and F-cell percentages for rhesus macaques (Macaca mulatta) following total body irradiation or busulfan conditioning prior to CD34(+) cell transplantation. HbF protein expression peaked in the first 4–9 weeks posttransplant (0.99%–2.53%), and F-cells increased in the first 6–17 weeks posttransplant (8.7%–45.3%). HbF and F-cell ratios gradually decreased and stabilized to levels similar to those of control animals (1.96 ± 1.97% for F cells and 0.49 ± 0.19% γ-globin expression) 4–7 months post-transplant. These findings confirm and expand on previous reports of transient induction in HbF and F-cell percentages in rhesus macaques following CD34(+) cell transplantation, an observation that must be taken into consideration when evaluating therapeutic strategies that aim to specifically elevate HbF expression, which are currently in clinical development. Published by Elsevier Inc. on behalf of ISEH – Society for Hematology and Stem Cells.

Lentiviral vectors (LV) have been used for the delivery of genes into hematopoietic stem and progenitor cells (HSPC) in clinical trials worldwide. LV, in contrast to retroviral vectors, have not been associated with insertion site-associated malignant clonal expansions, and thus have been considered safer. Here, however, we present a case of markedly abnormal dysplastic clonal hematopoiesis impacting the erythroid, myeloid and megakaryocytic lineages in a rhesus macaque transplanted with HSPCs that were transduced with a LV containing a strong retroviral murine stem cell virus (MSCV) constitutive promoter-enhancer in the LTR. 9 insertions were mapped in the abnormal clone, resulting in overexpression and aberrant splicing of several genes of interest, including the cytokine stem cell factor and the transcription factor PLAG1. This case represents the first clear link between a lentiviral insertion-induced clonal expansion and a clinically abnormal transformed phenotype following transduction of normal primate or human HSPC, and are thus concerning, and suggest that strong constitutive promoters should not be included within LV vectors.

Autologous hematopoietic stem cell transplantation (Auto-HSCT) with gene-modification represents a potential cure for multiple genetic diseases, but its broad curative potential, is limited because of morbidity/mortality from cytotoxic chemotherapy-based conditioning. To overcome these limitations, we developed antibody drug conjugates (ADC) targeting CD117 (C-KIT) to specifically deplete the hematopoietic stem and progenitor cells (HSPC). To validate CD117 ADC-mediated depletion prior to HSCT, we developed an optimized non-human primate (NHP) tool anti-CD117 ADC and evaluated it in an auto-gene modified HSCT in a rhesus model. The CD117-ADC is potent on primary human and NHP CD34+ cells in vitro (Figure 1A). Humanized NSG mice treated with a single dose had full depletion of human HSPCs in the bone marrow, while maintaining peripheral immune cells. In rhesus a single administration was fully myeloablative (>99% HSPC depletion) and comparable to HSPC depletion observed following busulfan conditioning (6 mg/kg/day x4). There was no effect of the ADC on the peripheral and bone marrow lymphocytes and the ADC was well tolerated compared to busulfan where the animals had diarrhea, loss of appetite and weight loss. To facilitate use in HSCT, the CD117-ADC was engineered to have a fast clearance and the half-life was We next explored whether the tool CD117-ADC could enable auto gene modified HSCT in the rhesus model. Two rhesus NHP were mobilized with GCSF and plerixafor. The selected CD34+ cells were transduced with β-globin encoded lentivirus and cryopreserved. The tool CD117-ADC was dosed on day -6 and the CD34+ cells were thawed and infused on day 0. Bone marrow aspirates analyzed on the day of infusion (day 0) demonstrated >99% depletion of the HSPCs and maintenance of the bone marrow lymphocytes (Figure 1B). The primates engrafted neutrophils (day 8 and 10) and platelets (day 10 and 11), and the peripheral lymphocytes were maintained throughout the transplant (Figure 1C-D). The gene marking in the granulocytes is comparable to busulfan conditioned animals previously reported (Tisdale, Molecular Therapy 2019). Longer follow up and data from additional animals will be presented. In summary, we have developed a tool CD117 ADC that shows potent activity on NHP CD34+ cells, is fully myeloablative, has a favorable safety profile, spares the immune system and is cleared rapidly as designed. In a rhesus model of auto-gene modified HSCT, a single dose of the ADC enables engraftment of gene modified HSC. These proof of concept studies validate the use of CD117-ADC for targeted HSPC depletion prior to transplant and support its use as a new conditioning agent for auto-gene modified HSCT. This targeted approach for safer conditioning could improve the risk benefit profile for patients undergoing HSCT and enable more patients to benefit from these potentially curative therapies.

Busulfan conditioning is utilized for hematopoietic stem cell (HSC) depletion in the context of HSC gene-therapy conditioning but may result in insufficient immunosuppression. In this study, we evaluated whether additional immunosuppression is required for efficient engraftment of gene-modified cells using a rhesus HSC lentiviral gene-therapy model. We transduced half of rhesus CD34(+) cells with an enhanced green fluorescent protein (GFP)-encoding vector (immunogenic) and the other half with a γ-globin-encoding vector (no predicted immunogenicity). After autologous transplantation of both transduced cell populations following myeloablative busulfan conditioning (5.5 mg/kg/day for 4 days), we observed immunological rejection of GFP-transduced cells up to 3 months post-transplant and stable engraftment of γ-globin-transduced cells in two animals, demonstrating that ablative busulfan conditioning is sufficient for engraftment of gene-modified cells producing non-immunogenic proteins but insufficient to permit engraftment of immunogenic proteins. We then added immunosuppression with abatacept and sirolimus to busulfan conditioning and observed engraftment of both GFP- and γ-globin-transduced cells in two animals, demonstrating that additional immunosuppression allows for engraftment of gene-modified cells expressing immunogenic proteins. In conclusion, myeloablative busulfan conditioning should permit engraftment of gene-modified cells producing non-immunogenic proteins, while additional immunosuppression is required to prevent immunological rejection of a neoantigen.

Hematopoietic stem cell (HSC) gene therapy is being evaluated for hemoglobin disorders including sickle cell disease (SCD). Therapeutic globin vectors have demanding requirements including high-efficiency transduction at the HSC level and high-level, erythroid-specific expression with long-term persistence. The requirement of intron 2 for high-level β-globin expression dictates a reverse-oriented globin-expression cassette to prevent its loss from RNA splicing. Current reverse-oriented globin vectors can drive phenotypic correction, but they are limited by low vector titers and low transduction efficiencies. Here we report a clinically relevant forward-oriented β-globin-expressing vector, which has sixfold higher vector titers and four to tenfold higher transduction efficiency for long-term hematopoietic repopulating cells in humanized mice and rhesus macaques. Insertion of Rev response element (RRE) allows intron 2 to be retained, and β-globin production is observed in transplanted macaques and human SCD CD34+ cells. These findings bring us closer to a widely applicable gene therapy for hemoglobin disorders., Vectors used in gene therapy for hemoglobin disorders carry globin in a reverse-orientation to prevent the loss of key regulatory elements by RNA splicing, but this limits their efficiency. Here, the authors develop a vector carrying β-globin in a forward orientation and show that it has improved titers and transduction efficiency in humanized mice and nonhuman primates.

Ionizing irradiation is used routinely to induce myeloablation and immunosuppression. However, it has not been possible to evaluate the extent of ablation without invasive biopsy. For lymphoid recovery, peripheral blood (PB) lymphocytes (PBLs) have been used for analysis, but they represent

Autologous hematopoietic stem cell transplantation (Auto-HSCT) with gene-modification techniques represents a potential cure for multiple genetic blood diseases. Despite its broad curative potential, auto-gene modified HSCT is currently limited due to morbidity/mortality from cytotoxic chemotherapy-based conditioning, including risks of secondary malignancies, organ toxicity, and infertility. To overcome these limitations, we have developed antibody drug conjugates (ADC) targeting CD117 (C-KIT) to specifically deplete the hematopoietic stem and progenitor cells (HSPC) prior to auto-gene modified HSCT. We have previously shown that the anti-CD117 ADC is highly effective at killing human CD117+ cells in vitro and in vivo (Pearse et al., Blood 2018 132:3314). To validate CD117 as an appropriate antigen for targeted ADC-mediated depletion prior to HSCT, we developed an optimized non-human primate (NHP) tool anti-CD117 ADC and evaluated it in an auto-gene modified HSCT in the rhesus macaque model. The tool CD117-ADC is potent on primary human and NHP CD34+ cells in vitro with EC50 of 0.2 and 0.09 pM respectively (Figure 1A). Humanized NSG mice treated with the tool CD117-ADC had full depletion of human HSPCs in the bone marrow 21 days after a single administration of the ADC, while maintaining the peripheral immune cells. We next tested the efficacy and safety of the tool CD117-ADC in NHPs. A single administration of the tool CD117-ADC was fully myeloablative (>99% HSPC depletion) and comparable to HSPC depletion observed following busulfan conditioning (6 mg/kg, once daily for 4 consecutive days). There was no effect on the peripheral and bone marrow lymphocytes and the ADC was well tolerated. To facilitate the use in HSCT, the tool CD117-ADC was engineered to have a fast clearance and in this study the half-life was Based on these encouraging results, we explored whether the tool CD117-ADC could enable engraftment of autologous gene modified hematopoietic stem cells in the rhesus macaque model. A single rhesus macaque was mobilized with granulocyte-colony stimulating factor (G-CSF, 20 mcg/kg/day x 5) and plerixafor (1 mg/kg on day 5 of G-CSF) prior to apheresis. The isolated CD34+ cells were transduced with a lentivirus encoding the β-globin gene and cryopreserved. The tool CD117-ADC was dosed on day -6 and the cryopreserved gene modified cells were thawed and infused (3.3 x 106 CD34+ cells/kg) on day 0. A bone marrow aspirate analyzed on the day of infusion (day 0) demonstrated >99% depletion of the HSPCs and preserved of the bone marrow lymphocytes (Figure 1B). The primate engrafted neutrophils and platelets on day 8 and 10 respectively, and the peripheral lymphocytes were maintained throughout the transplant (Figure 1C). The gene marking in the granulocytes was detectable at day 9, and additional follow up and data from additional animals will be presented. In summary, we have developed a tool CD117 ADC that shows potent activity on NHP CD34+ cells. This optimized CD117-ADC is fully myeloablative with a single dose in NHPs, has a favorable safety profile, spares the immune system and is cleared rapidly as designed. In a rhesus model of autologous gene modified HSCT, a single dose of the ADC enables engraftment of auto-gene modified HSC. These proof of concept studies validate the use of CD117-ADC for targeted HSPC depletion prior to transplant and support its use as a new conditioning agent for autologous gene modified HSCT. This targeted approach for safer conditioning could improve the risk benefit profile for patients undergoing stem cell transplant and enable more patients to benefit from these potentially curative therapies. Disclosures Pearse: Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. McDonough:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Proctor:Magenta Therapeutics: Employment, Equity Ownership. Panwar:Magenta Therapeutics: Employment, Equity Ownership. Sarma:Magenta Therapeutics: Employment, Equity Ownership. Kien:Magenta Therapeutics: Employment, Equity Ownership. Latimer:Magenta Therapeutics: Employment, Equity Ownership. Dushime:Magenta Therapeutics: Employment, Equity Ownership. Hyzy:Magenta Therapeutics: Employment, Equity Ownership. Brooks:Magenta Therapeutics: Employment, Equity Ownership. Palchaudhuri:Magenta Therapeutics: Employment, Equity Ownership. Li:Magenta Therapeutics: Employment, Equity Ownership. Sawant:Magenta Therapeutics: Employment, Equity Ownership. McDonagh:Magenta Therapeutics: Employment. Boitano:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties. Cooke:Magenta Therapeutics: Employment, Equity Ownership, Patents & Royalties.

Hematopoietic stem cell gene therapy for hemoglobin disorders, such as sickle cell disease, requires high-level gene marking and robust therapeutic globin expression in erythroid cells (>20% of γ- or β-globin production) for widespread successful clinical application. We previously demonstrated that lentiviral transduction of a truncated human erythropoietin receptor (thEpoR) gene allows for erythropoietin-dependent selective proliferation of gene-modified human erythroid cells during in vitro differentiation (ASH 2017). In this study, we sought to evaluate whether thEpoR can enhance the phenotypic effect of a therapeutic vector in erythroid cells in xenograft mouse and autologous non-human primate transplantation models. To investigate this hypothesis, we designed lentiviral vectors encoding both thEpoR and BCL11A-targeting micro RNA-adapted short hairpin RNA (shmiBCL11A), driven off an erythroid specific ankyrin 1 (ANK1) promoter. Both selective proliferation and high-level fetal hemoglobin (HbF) induction were observed in in vitro erythroid differentiation cultures using transduced human CD34+ cells. Healthy donor CD34+ cells were transduced with shmiBCL11A vector, thEpoR-shmiBCL11A vector, and GFP vector (control). Transduced cells were transplanted into immunodeficient NBSGW mice. Five months post-transplant, xenograft bone marrow cells were evaluated for human cell engraftment (human CD45+) and vector copy number (VCN) in both human CD34+ progenitor cells and glycophorin A+ (GPA+) erythroid cells. HbF production was also measured in GPA+ erythroid cells by reverse phase HPLC. We observed efficient transduction in transduced CD34+ cells in vitro (VCN 2.1-5.1) and similar human cell engraftment among all groups (84-89%). The VCN with thEpoR-shmiBCL11A transduction was 3-fold higher in human erythroid cells when compared to CD34+ cells (p We then performed autologous rhesus CD34+ cell transplantation using either shmiBCL11A vector (142562 and RA0706, n=2, compared to a GPA promoter-derived shmiBCL11A vector) or thEpoR-shmiBCL11A vector (ZL50 and ZM24, n=2, compared to a Venus-encoding vector). Transduced CD34+ cells were transplanted into autologous rhesus macaques following 2x5Gy total body irradiation. Efficient transduction was observed in CD34+ cells in vitro among all 4 macaques (VCN 3.8-8.7) using a high-density culture protocol (Uchida N, Mol Ther Methods Clin Dev. 2019). In shmiBCL11A transduction animals, engraftment of gene-modified cells (VCN 0.2-1.0) and robust HbF induction (14-16%) were observed 1 month post-transplant. However, VCN and HbF levels were reduced down to VCN ~0.1 and HbF ~0.4% in both animals 6 months post-transplant. In contrast, a thEpoR-shmiBCL11A transduction animal (ZL50) resulted in engraftment of gene-modified cells (VCN 0.8-1.0) and robust HbF induction (~18%) 1 month post-transplant, with both gene marking and HbF levels remaining high at VCN 0.6-0.7 and HbF ~15% 4 months post-transplant. These data suggest that shmiBCL11A transduction results in transient HbF induction in gene-modified erythroid cells, while thEpoR-based selective advantage allows for sustained HbF induction with shmiBCL11A. In summary, we developed erythroid-specific thEpoR-shmiBCL11A expressing vectors, enhancing HbF induction in gene-modified erythroid cells in xenograft mice and rhesus macaques. While further in vivo studies are desirable, the use of thEpoR appears to provide a selective advantage for gene-modified erythroid cells in gene therapy strategies for hemoglobin disorders. Disclosures No relevant conflicts of interest to declare.

Elevated fetal hemoglobin (HbF, α2γ2) levels are clinically beneficial for patients with β-hemoglobinopathies. Editing of the erythroid-specific BCL11A enhancer induces HbF, inhibiting sickling and restoring globin chain balance in erythroid cells derived from hematopoietic stem and progenitor cells (HSPCs) from SCD and β-thalassemia patients respectively, without detectable genotoxicity or adverse effects on hematopoietic stem cell (HSC) function (Wu Y, Nat Med, 2019). Here, we sought to evaluate engraftment and HbF induction potential of erythroid-specific BCL11A enhancer edited CD34+ HSPCs in a non-human primate transplantation model in which hemoglobin switching is conserved. We targeted the erythroid-specific +58 DNAse I hypersensitive site of BCL11A, which has identical human and rhesus sequences at the spacer and protospacer adjacent motif (PAM) of the potent #1617 sgRNA. Ribonucleoprotein complex (RNP) composed of 3x-NLS SpCas9 protein and either BCL11A enhancer targeting (#1617) or AAVS1 targeting sgRNA was electroporated into rhesus CD34+ HSPCs (n=3). Following erythroid differentiation, substantial γ-globin expression (54-77%, p We tested BCL11A enhancer editing with autologous HSC transplant in two cohorts, with two macaques per cohort. For cohort 1, we performed competitive engraftment of BCL11A enhancer and AAVS1 edited HSPCs to test long-term reconstitution. For cohort 2, we evaluated BCL11A enhancer editing alone to evaluate HbF induction and hematopoietic reconstitution. For each cohort, purified CD34+ HSPCs were electroporated with RNP one day after G-CSF and plerixafor mobilization and cultured for two days prior to cryopreservation. HSPCs were thawed and infused following 2×5 Gy total body irradiation. For cohort 1 (n=2, ZL25 and ZL22, 1.34-1.39×106 CD34+ HSPCs/kg), we observed reduced indel frequencies (8-41%) at early post-infusion time points compared to cell products (18-49%), suggesting indels in unfractionated HSPCs may overestimate those in engrafting cells and/or hematopoietic ablation was incomplete. From weeks 6 to 83, stable indel frequencies were detected in both BCL11A (~3-18%) and AAVS1 (~10-45%), suggesting no selective advantage for BCL11A enhancer edited, AAVS1 edited, or non-edited HSCs. For cohort 2 (BCL11A enhancer editing alone (n=2, ZM17 and ZM26, 1.78-6.06×106 CD34+ cells/kg), cell products showed improved editing with ~95% indels and ~65-78% γ-globin protein after in vitro erythroid culture. Animals engrafted with typical kinetics and displayed stable indel ratios up to 28 weeks post-transplantation. A significant correlation was detected between γ-globin level and indel frequency comparing all 4 transplanted animals and unedited controls (R2=0.76, p In summary, we evaluated the clinical potential of autologous BCL11A erythroid enhancer editing in rhesus macaques. BCL11A enhancer edited HSCs can persist for at least 83 weeks post-transplant and provide therapeutic levels of HbF in peripheral RBCs without anemia or other apparent hematologic toxicity. Furthermore, these results emphasize input CD34+ HSPC dose and conditioning intensity as critical variables that influence gene editing following autologous HSCT. Overall, these findings support BCL11A erythroid enhancer genome editing as a promising strategy for therapeutic HbF induction. Disclosures Weiss: GlaxoSmithKline: Consultancy; Cellarity INC: Consultancy; Esperian: Consultancy; Beam Therapeutics: Consultancy; Rubius INC: Consultancy.

Sickle cell disease (SCD) is caused by a 20A>T mutation in the β-globin gene. State-of-the-art genome editing technologies have the potential to correct the SCD mutation in hematopoietic stem cells (HSCs), producing adult hemoglobin (Hb) while simultaneously eliminating sickle Hb. We have demonstrated efficient gene correction in SCD CD34+ cells with SCD mutation-specific guide RNA, Cas9 mRNA/protein, and single strand donor DNA, resulting in ~30% gene correction and ~50% indels at the DNA level, and ~60% normal β-globin production at the protein level in in vitro erythroid differentiation (ASH 2018). Gene correction by homology directed repair is thought to be enhanced by cell proliferation; however, cell proliferation might reduce stemness of HSCs. To investigate this hypothesis, we sought to evaluate engraftment of gene-edited CD34+ HSCs in a non-human primate model. To model SCD gene correction, a β-to-βs globin conversion was designed in rhesus macaques. Mobilized rhesus CD34+ cells (n=2) were electroporated using the GMP-compliant, FDA Master File-supported, and scalable MaxCyte GT System to deliver rhesus β-globin-targeting guide RNA (the same target site as the SCD mutation-specific guide RNA), SpCas9 protein, and single strand donor DNA including a SCD mutation (20A>T). We also added an adjuvant to improve gene conversion efficiencies. Following erythroid differentiation, gene correction efficiency was evaluated at DNA levels by deep sequencing and at protein levels by reverse-phase HPLC. We observed high-efficiency genome editing without the adjuvant (20-30% gene conversion and 61-64% indels), and further enhanced genome editing with the adjuvant (51-59% gene conversion and 36-39% indels). After erythroid differentiation, we observed production of βs-globin protein (~100%) but not normal β-globin in gene-edited cells. We then evaluated engraftment of gene-edited rhesus CD34+ cells with β-to-βs globin conversion (n=2, 13U005 and 12U011). Mobilized rhesus CD34+ cells (3.4-3.8e7) were pre-stimulated for 2 days, and edited cells were cryopreserved after electroporation with editing tools. Small aliquots of edited cells (before and after cryopreservation) were differentiated into erythroid cells in vitro, resulting in 17-26% of gene conversion and 57-71% of indels at the DNA level and 50-100% of β-globin production at the protein level, with no difference observed between aliquots taken before and after cryopreservation. Following 9.5 Gy total body irradiation, the frozen edited CD34+ cells (1.6-2.2e7) were injected into autologous macaques. We observed robust recovery of blood counts in 13U005, while peripheral blood recovery was delayed in 12U011, who was supported by serial whole blood transfusion. We observed 7-11% of gene conversion and 44-54% of indels in both granulates and lymphocytes in 13U005 1 month post-transplant. Around 15% sickle Hb production in red blood cells was detected by Hb electrophoresis in 13U005 three months post-transplant and ~7% in 12U011 two months post-transplant. Interestingly, ~10% of fetal Hb production was observed in 12U001, likely due to stress hematopoiesis. In summary, we developed a rhesus β-to-βs globin conversion model with HSC-targeted genome editing strategies. The gene-edited rhesus CD34+ cells are engraftable for at least 3 months post-transplant. Although further follow-up is necessary for transplanted animals, these findings are helpful in designing HSC-targeted gene correction trials. Figure Disclosures Li: MaxCyte, Inc: Employment. Allen:MaxCyte, Inc: Employment. Peshwa:MaxCyte, Inc: Employment.

Reduced intensity conditioning (RIC) is desirable for hematopoietic stem cell (HSC) targeted gene therapy; however, RIC may be insufficient for efficient engraftment and inducing immunological tolerance to transgenes. We previously established long-term gene marking in our rhesus macaque autologous HSC transplantation model following 10 Gy total body irradiation (TBI). In this study, we evaluated RIC transplantation with 4 Gy TBI in two rhesus macaques that received equal parts of CD34(+) cells transduced with green fluorescent protein (GFP)-expressing lentiviral vector and empty vector not expressing transgenes. In both animals, equivalently low gene marking between GFP and empty vectors was observed 6 months post-transplantation, even with efficient transduction of CD34(+) cells in vitro. Autologous lymphocyte infusion with GFP marking resulted in an increase of gene marking in lymphocytes in a control animal with GFP tolerance, but not in the two RIC-transplanted animals. In vitro assays revealed strong cellular and humoral immune responses to GFP protein in the two RIC-transplanted animals, but this was not observed in controls. In summary, 4 Gy TBI is insufficient to permit engraftment of genetically modified HSCs and induce immunological tolerance to transgenes. Our findings should help in the design of conditioning regimens in gene therapy trials.

Reduced intensity conditioning (RIC) is desirable for hematopoietic stem cell (HSC) gene therapy applications. However, low gene marking was previously observed in gene therapy trials, suggesting that RIC might be insufficient for (i) opening niches for efficient engraftment and/or (ii) inducing immunological tolerance for transgene-encoded proteins. Therefore, we evaluated both engraftment and tolerance for gene-modified cells using our rhesus HSC gene therapy model following RIC. We investigated a dose de-escalation of total body irradiation (TBI) from our standard dose of 10Gy (10, 8, 6, and 4Gy), in which rhesus CD34 + cells were transduced with a VSVG-pseudotyped chimeric HIV-1 vector encoding enhanced green fluorescent protein (GFP) (or enhanced yellow fluorescent protein (YFP)). At ∼6 months after transplantation, higher-dose TBI resulted in higher gene marking with logarithmic regression in peripheral blood cells. We then evaluated immunological tolerance for gene-modified cells, and found that lower-dose TBI allowed vigorous anti-GFP antibody production with logarithmic regression, while no significant anti-VSVG antibody formation was observed among all TBI groups. These data suggest that higher-dose TBI improves both engraftment and immunological tolerance for gene-modified cells. Additional immunosuppression might be required in RIC to induce tolerance for transgene products. Our findings should be valuable for developing conditioning regimens for HSC gene therapy applications.

Human immunodeficiency virus type 1 (HIV1) vectors poorly transduce rhesus hematopoietic cells due to species-specific restriction factors, including the tripartite motif-containing 5 isoformα (TRIM5α) which targets the HIV1 capsid. We previously developed a chimeric HIV1 (χHIV) vector system wherein the vector genome is packaged with the simian immunodeficiency virus (SIV) capsid for efficient transduction of both rhesus and human CD34(+) cells. To evaluate whether χHIV vectors could efficiently transduce rhesus hematopoietic repopulating cells, we performed a competitive repopulation assay in rhesus macaques, in which half of the CD34(+) cells were transduced with standard SIV vectors and the other half with χHIV vectors. As compared with SIV vectors, χHIV vectors achieved higher vector integration, and the transgene expression rates were two- to threefold higher in granulocytes and red blood cells and equivalent in lymphocytes and platelets for 2 years. A recipient of χHIV vector-only transduced cells reached up to 40% of transgene expression rates in granulocytes and lymphocytes and 20% in red blood cells. Similar to HIV1 and SIV vectors, χHIV vector frequently integrated into gene regions, especially into introns. In summary, our χHIV vector demonstrated efficient transduction for rhesus long-term repopulating cells, comparable with SIV vectors. This χHIV vector should allow preclinical testing of HIV1-based therapeutic vectors in large animal models.

Introduction: Umbilical cord blood (UCB) grafts are the only option for a significant minority of patients who require hematopoietic stem cell transplantation (HSCT) but lack a suitable related or unrelated donor. While UCB can serve as a suitable 'off the shelf' graft for many patients, units with the best HLA match often contain low and sometimes insufficient numbers of CD34+ cells for use in transplantation, particularly for adult patients. Furthermore, UCB grafts contain lower CD34+ cells number compared to BM or PBSC grafts, which leads to longer engraftment times and a higher risk of graft failure. BM and PBSC grafts are usually injected intravenously, homing to the bone marrow after several hours in the circulation. During this time, CD34+ cells are lost in the lungs, liver and spleen with typically < 20% making it to the bone marrow.1 Investigators have sought to overcome the limitation of low cell dose in UCB grafts and loss of CD34+ cells in the circulation by injecting CD34+ cells directly into the bone marrow space. However, we have recently shown that conventional intrabone delivery methods used in investigational trials to transplant UCB, result in low-level retention of hematopoietic progenitor cells in the intrabone space. Recently, we developed an optimized intrabone (OIB) transplant method using computer controlled low pressure and low volume injection (controlled infusion rate Methods: Rhesus macaques received GCSF and plerixafor mobilization prior to apheresis. Products were CD34+ selected using Miltenyi beads. CD34+ cells were split equally for transduction with lentiviral vectors encoding the reporters GFP and YFP. After myeloablative conditioning with 10Gy total body irradiation, half the graft was injected directly intra-bone using the OIB method, with the other half of the autograft simultaneously being injected intravenously via slow iv push. Peripheral blood samples were measured daily by flow cytometry to assess the proportion of engrafting cells deriving from each source. To address whether OIB transplantation could allow engraftment of low CD34+ cell numbers, as found in UCB units, the cell doses transplanted in one rhesus macaque recipient were reduced to only 0.5 x 106CD34+ cells/kg. Results: CD34+ cells injected intrabone utilizing the OIB method engrafted in all 3 animals. However, flow cytometric analysis gating on GFP vs YFP positive neutrophils showed CD34+ cells injected utilizing OIB delivery did not engraft quicker than IV transplanted cells. Sequential monitoring of neutrophils over 30 days showed the contribution to hematopoiesis of OIB delivered cells was significantly lower than the cells injected IV (Table 1.) Conclusions: We developed a novel intrabone delivery system that optimizes the retention of CD34+ cells into the bone marrow space. Although autologous CD34+ cells injected using this OIB transplant method were capable of engrafting in rhesus macaques that had undergone myeloablative conditioning, they did not engraft faster and contributed less to hematopoiesis than CD34+ cells simultaneously transplanted using conventional IV infusion. These data raise questions over whether intrabone delivery, even when using techniques to optimize intrabone retention, has utility in improving CD34+ cell engraftment. References: 1. Van der Loo JC et al. Marrow and spleen seeding efficiencies of all murine hematopoietic stem cell subsets are de- creased by preincubation with hematopoietic growth factors. Blood. 1995; 85:2598-2606 2. Pantin et. al. Optimization of intrabone delivery of hematopoietic progenitor cells in a swine model using cell radiolabeling with 89zirconium. American J Transplant. 2015 Mar;15(3):606-17. Disclosures Davidson: Macrogenics: Employment. Pantin:NIH: Patents & Royalties: a patent application for an intrabone delivery device. Dunbar:National Institute of Health: Research Funding.

Human immunodeficiency virus type 1 (HIV-1) vectors transduce rhesus blood cells poorly due to a species-specific block by TRIM5α and APOBEC3G, which target HIV-1 capsid and viral infectivity factor (Vif), respectively. We sought to develop a lentiviral vector capable of transducing both human and rhesus blood cells by combining components of both HIV-1 and simian immunodeficiency virus (SIV), including SIV capsid (sCA) and SIV Vif. A chimeric HIV-1 vector including sCA (χHIV) was superior to the conventional SIV in transducing a human blood cell line and superior to the conventional HIV-1 vector in transducing a rhesus blood cell line. Among human CD34 + hematopoietic stem cells (HSCs), the χHIV and HIV-1 vectors showed similar transduction efficiencies; in rhesus CD34 + HSCs, the χHIV vector yielded superior transduction rates. In in vivo competitive repopulation experiments with two rhesus macaques, the χHIV vector demonstrated superior marking levels over the conventional HIV-1 vector in all blood lineages (first rhesus, 15 to 30% versus 1 to 5%; second rhesus, 7 to 15% versus 0.5 to 2%, respectively) 3 to 7 months postinfusion. In summary, we have developed an HIV-1-based lentiviral vector system that should allow comprehensive preclinical testing of HIV-1-based therapeutic vectors in the rhesus macaque model with eventual clinical application.

Inherited disorders of globin synthesis remain desirable targets for hematopoietic stem cell (HSC)-based therapies. Gene transfer using retroviral vectors offers an alternative to allogeneic HSC transplantation by the permanent integration of potentially therapeutic genes into primary autologous HSCs. Although proof of principle has been demonstrated in humans, this approach has been met by formidable obstacles, and large-animal models have become increasingly important for the preclinical development of gene addition strategies. Here we report lentiviral gene transfer of the human beta-globin gene under the control of the globin promoter and large fragments of the globin locus control region (LCR) in the nonhuman primate. Using an HIV-1, vesicular stomatitis virus glycoprotein G (VSV-G)-pseudotyped vector, modified to overcome a species-specific restriction to HIV-1, gene transfer to colony-forming units (CFU) derived from mobilized peripheral blood (PB) rhesus CD34+ cells was 84.4 +/- 2.33%. Erythroid cells derived from transduced rhesus CD34+ cells expressed human beta-globin at high levels as assessed by flow cytometry with a human beta-globin-specific antibody. Two rhesus macaques (RQ3586 and RQ3583) were transplanted with mobilized PB CD34+ cells transduced with our modified HIV vector at a multiplicity of infection of 80. High gene transfer rates to CFUs were achieved in vitro (RQ3586, 87.5%; RQ3583, 83.3%), with efficient human beta-globin expression among erythroid progeny generated in vitro. Early posttransplantation, gene transfer rates of 5% or higher were detectable and confirmed by genomic Southern blotting, with equivalent-level human beta-globin expression detected by flow cytometry. Long-term gene marking levels among mononuclear cells and granulocytes assessed by quantitative polymerase chain reaction gradually decreased to about 0.001% at 2 years, likely due to additional HIV-1 restrictive elements in the rhesus macaque. No evidence of clonal hematopoiesis has occurred in our animals in up to 2 years. Current efforts are aimed at developing a lentiviral vector capable of efficiently transducing both human and rhesus HSCs to allow preclinical modeling of globin gene transfer.

Genome-wide integration site analyses showed that Moloney murine leukemia virus (MoMLV)- and lentivirus-derived vectors integrate preferentially into the coding regions of genes, posing a risk of insertional mutagenesis. Avian sarcoma and leukosis viruses (ASLVs) were previously reported to have a weak preference for gene-coding regions in a cell line study as compared with human immunodeficiency virus and MoMLV; however, thus far these vectors have not been studied for their potential efficacy in transduction of hematopoietic progenitor and stem cells. In this study we investigated for the first time the ability of ASLV-derived RCAS (replication-competent ALV LTR [avian leukosis virus long terminal repeat] with a splice acceptor) vectors to transduce rhesus macaque hematopoietic progenitors and long-term repopulating cells, in an autologous transplantation model. RCAS vectors can efficiently and stably transduce rhesus macaque CD34+ hematopoietic progenitor cells with an efficiency of transduction of up to 34% ex vivo. In two animals transplanted with RCAS vector-transduced autologous CD34+ cells, highly polyclonal hematopoietic reconstitution with sustained gene-marking levels in myeloid and lymphoid lineages was observed up to 18 months post-transplantation. These findings are encouraging and suggest that this vector system should be explored and further optimized for gene therapy applications targeting hematopoietic stem and progenitor cells.

Objective Granulocyte colony-stimulating factor (G-CSF) is frequently used therapeutically to treat chronic or transient neutropenia and to mobilize hematopoietic stem cells. Shortly following G-CSF administration, we observed a dramatic transient drop in circulating neutrophil number. This article characterizes this effect in a rhesus macaque animal model. Methods Hematologic changes were monitored following subcutaneous (SQ) administration of G-CSF. G-CSF was administered as a single SQ dose at 10 μg/kg or 50 μg/kg. It was also administered (10 μg/kg) in combination with stem cell factor (SCF; 200 μg/kg) over 5 days. Flow cytometry was performed on serial blood samples to detect changes in cell surface adhesion protein expression. Results Neutrophil count dramatically declined 30 minutes after G-CSF administration. This decline was observed whether 10 μg/kg G-CSF was administered in combination with SCF over 5 days, or given as a single 10 μg/kg dose. At a single 50 μg/kg dose, the decline accelerated to 15 minutes. Neutrophil count returned to baseline after 120 minutes and rapidly increased thereafter. An increase in CD11a and CD49d expression coincided with the drop in neutrophil count. Conclusion A transient paradoxical decline in neutrophil count was observed following administration of G-CSF either alone or in combination with SCF. This decline accelerated with the administration of a higher dose of G-CSF and was associated with an increase in CD11a and CD49d expression. It remains to be determined whether this decline in circulating neutrophils is associated with an increase in endothelial margination and/or entrance into extravascular compartments.

Immunodeficiency after peripheral blood progenitor cell (PBPC) transplantation may be influenced by graft composition, underlying disease, and/or pre-treatment. These factors are difficult to study independently in humans. Ex vivo culture and genetic manipulation of PBPC grafts may also affect immune reconstitution, with relevance to gene therapy applications. We directly compared the effects of three clinically relevant autologous graft compositions on immune reconstitution after myeloblative total body irradiation in rhesus macaques, the first time these studies have been performed in a large animal model with direct clinical relevance. Animals received CD34+ cell dose-matched grafts of either peripheral blood mononuclear cells, purified CD34+ PBPCs, or purified CD34+ PBPCs expanded in vitro and retrovirally transduced. We evaluated the reconstitution of T, B, natural killer, dendritic cells, and monocytes in blood and lymph nodes for up to 1 year post-transplantation. Animals receiving selected-transduced CD34+ cells had the fastest recovery of T-cell numbers, along with the highest T-cell-receptor gene rearrangement excision circles levels, the fewest proliferating Ki-67+ T-cells in the blood, and the best-preserved thymic architecture. Selected-transduced CD34+ cells may therefore repopulate the thymus more efficiently and promote a higher output of naïve T-cells. These results have implications for the design of gene therapy trials, as well as for the use of expanded PBPCs for improved T-cell immune reconstitution after transplantation.

Hematopoietic cells can be highly enriched for repopulating ability based upon the efflux of the fluorescent Hoechst 33342 dye by sorting for SP (side population) cells, a phenotype attributed to expression of ABCG2, a member of the ABC transporter superfamily. Intriguingly, murine studies suggest that forced ABCG2 expression prevents hematopoietic differentiation. We cloned the full-length rhesus ABCG2 and introduced it into a retroviral vector. ABCG2-transduced human peripheral blood progenitor cells (PBPCs) acquired the SP phenotype but showed significantly reduced growth compared with control. Two rhesus macaques received autologous PBPCs split for transduction with the ABCG2 or control vectors. Marking levels were similar between fractions with no discrepancy between bone marrow and peripheral blood marking. Analysis for the SP phenotype among bone marrow and mature blood populations confirmed ABCG2 expression at levels predicted by vector copy number long term, demonstrating no block to differentiation in the large animal. In vitro studies showed selective protection against mitoxantrone among ABCG2-transduced rhesus PBPCs. Our results confirm the existence of rhesus ABCG2, establish its importance in conferring the SP phenotype, suggest no detrimental effect of its overexpression upon differentiation in vivo, and imply a potential role for its overexpression as an in vivo selection strategy for gene therapy applications.

The high risk of insertional oncogenesis reported in clinical trials using integrating retroviral vectors to genetically modify hematopoietic stem and progenitor cells (HSPCs) requires the development of safety strategies to minimize risks associated with novel cell and gene therapies. The ability to ablate genetically modified cells in vivo is desirable, should an abnormal clone emerge. Inclusion of “suicide genes” in vectors to facilitate targeted ablation of vector-containing abnormal clones in vivo is one potential safety approach. We tested whether the inclusion of the “inducible Caspase-9” (iCasp9) suicide gene in a gamma-retroviral vector facilitated efficient elimination of vector-containing HSPCs and their hematopoietic progeny in vivo long-term, in an autologous non-human primate transplantation model. Following stable engraftment of iCasp9 expressing hematopoietic cells in rhesus macaques, administration of AP1903, a chemical inducer of dimerization able to activate iCasp9, specifically eliminated vector-containing cells in all hematopoietic lineages long-term, suggesting activity at the HSPC level. Between 75% and 94% of vector-containing cells were eliminated by well-tolerated AP1903 dosing, but lack of complete ablation was linked to lower iCasp9 expression in residual cells. Further investigation of resistance mechanisms demonstrated upregulation of Bcl-2 in hematopoietic cell lines transduced with the vector and resistant to AP1903 ablation. These results demonstrate both the potential and the limitations of safety approaches using iCasp9 to HSPC-targeted gene therapy settings, in a model with great relevance to clinical development. Stem Cells 2015;33:91–100

Reduced intensity conditioning (RIC) regimens are desirable for hematopoietic stem cell (HSC)-targeted gene therapy. However, in previous gene therapy trials, low gene marking was reported in peripheral blood cells, raising the question that RIC might be insufficient for (1) opening niches for efficient engraftment and (2) inducing immunological tolerance to transgenes. Therefore, we sought to evaluate both engraftment and tolerance for genetically modified cells using our rhesus HSC gene therapy model following RIC.We performed a dose de-escalation of total body irradiation (TBI) (10, 8, 6, and 4Gy) as RIC transplantation (total 19 animals), in which mobilized rhesus CD34+ cells were transduced with a VSV-G-pseudotyped chimeric HIV-1 vector encoding GFP (or YFP) and these cells were transplanted into autologous animals following TBI. We evaluated GFP-positive rates (%GFP), average vector copy number per cell (VCN), and antibody titers against GFP and VSV-G in peripheral blood 6 months after transplantation.When we evaluated in vitro %GFP in transduced rhesus CD34+ cells, efficient transduction (22-71%) was observed among all TBI groups (without correlation). After transplantation of the transduced CD34+ cells, GFP-positive cells were detected in peripheral blood cells in all animals. Increasing doses of TBI resulted in higher %GFP with logarithmic regression in both granulocytes (R2=0.64, p

Ameliorating hemoglobin disorders such as sickle cell disease (SCD) using hematopoietic stem cell (HSC) gene therapy is under development. Unlike in other diseases, therapeutic globin vectors have demanding requirements including high-level β-globin expression, tissue specificity among erythroid cells, long-term persistence, and high-level modification at the HSC level. These demanding requirements necessitate the inclusion of complex genetic elements including the locus control region (LCR), β-globin promoter, β-globin gene, and the 3' untranslated region (3'UTR), all now feasible using lentiviral vectors. The additional requirement of intron 2 for high-level β-globin expression dictates a reverse-oriented globin-expression cassette to prevent loss by RNA splicing during viral preparation. This reverse-orientation is in contrast to all other therapeutic vectors under clinical development. Current reverse-oriented globin vectors can drive phenotypic correction in mouse models for both b-thalassemia and SCD, while they are limited by lower viral titers and lower transduction efficiency in primary human HSCs, limiting their prospects, especially in SCD. We hypothesized that the reverse-orientation impedes both viral preparation and vector transduction, as despite deletion of cryptic polyadenylation (polyA) signals to optimize a conventional reverse-oriented globin vector, titers were still 10-fold lower than a standard GFP-vector. We thus designed a forward-oriented globin-expressing vector, which was further optimized by minimizing the size of the LCR, inclusion of a large segment of the β-globin promoter and an enhancer region of the 3'UTR lacking the polyA signal. Viral titers of the forward-oriented vectors (1.0±0.2x10e9 IU/mL) were 6-fold higher than the optimized vector in the reverse orientation (1.6±0.2x10e8 IU/mL, p To evaluate transduction efficiency for long-term HSCs, we transduced rhesus CD34+ cells with our optimized reverse-oriented vector and our forward-oriented vector including GFP or YFP genes (instead of β-globin gene) in a competitive repopulation assay following 10 Gy total body irradiation. In two animals, GFP and YFP signals from both vectors were detected exclusively in red blood cells, documenting tissue specificity. Gene marking levels were 10-fold higher out to 4 years with the forward-oriented vector, compared to the reverse-oriented vector, and were comparable to standard GFP or YFP-marking vectors in 2 other animals. We then replaced the GFP gene with the β-globin gene containing intron 2 in the forward-oriented vector construct. To positively select intron-2-containing β-globin vectors, essential viral components (packaging signal, rev response element (RRE), or central polypurine tract (cPPT)) were deleted in the backbone of the forward-oriented vector, and the deleted viral components were inserted into intron 2 of the β-globin gene. We observed that half of the forward-oriented vectors lost intron 2 during vector preparation when no elements were included into intron 2. Insertion of the RRE resulted in positive selection of intron-2-containing β-globin vectors. We confirmed β-globin expression from the forward-oriented vector by hemoglobin A production in human erythroid cells derived from transduced peripheral blood mononuclear cells from SCD patients. Finally, human β-globin expression was detected in rhesus erythroid cells following transplantation of transduced CD34+ cells in 2 animals. In summary, we have developed a clinically relevant forward-oriented globin-expressing vector, which has 6 fold higher viral titers and 4-10 fold higher transduction efficiency for hematopoietic repopulating cells, as compared to the optimized reverse-oriented vector. RRE insertion allowed positive selection of intron-2-containing β-globin vectors, and human β-globin production was observed in transplanted rhesus macaques with the forward-oriented β-globin vector transduction. These findings bring us closer to a curative gene therapy for hemoglobin disorders. Disclosures No relevant conflicts of interest to declare.

The development of ex-vivo HSPC expansion techniques is particulary relevant for improving cord blood transplantation and gene therapy. Despite successful long-term, multilineage reconstitution of expanded human cord blood HSPC in immunodeficient mice, early phase clinical trials have failed to demonstrate improved outcomes. Thus, it is critical to develop a robust pre-clinical model to study ex-vivo expansion strategies, particularly function of long term HSPC and engrafment of all lineages, difficult in xenograft models. We have previously used retroviral insertion site retrieval and autologous competitive transplantation in rhesus macaques to track hematopoietic engraftment and ontogeny at a single HSPC level, in addition to comparing ex-vivo expanded and unexpanded HSPC (Gomes et al, Mol Ther). More recently we have developed retrieval of 31bp diverse barcodes as a more robust and quantitative in vivo HSPC tracking approach (Wu et al, Cell Stem Cell). Modified human endothelial cells (HUVEC) with the ability to be maintained in serum free media for prolonged periods have shown to support and expand human HSPC in murine Butler et al, Blood). We have developed a barcoded rhesus autologous transplant model to evaluate expansion of HPSC on HUVEC versus unexpanded or cytokine expanded rhesus HSPC. We first tested the feasibility of rhesus CD34+ cell expansion on HUVEC versus cytokines/fibronectin for 8 days. We also evaluated for the diversity of expanded HSPC by retrieving transduced viral barcode DNA tags from individual cells by low cycle PCR followed by Illumina sequencing. On average, rhesus CD34+ cells expanded 76 fold (± 53) on HUVEC versus 13 fold (±7) in cytokines (n=4; p=0.03). By morphologic assessment, the HUVEC expanded cell fraction contained higher numbers of progenitors and differentiating cells, with up to 97% CD34+CD45+ cells in the HUVEC expanded fraction and 65% in the cytokine expanded fraction. The HUVEC expanded rhesus CD34+ cells were capable of multi-lineage colony formation after 8 days of expansion. Within the HUVEC expanded cell fractions, fold expansion was similar whether CD34+ cells were frozen after (C1) or before (C2) transduction-expansion. However, viability and percentages of CD34+CD45+ cells were higher in C2 compared to C1 (55% versus 26%). GFP transduction efficiency was also higher in C2 compared to C1 (43% vs. 28%). We demonstrated a slightly lower percentage but higher absolute number of CD34+CD38-CD45RA-CD90+CD49f+Rho-low putative long-term HSC after expansion on HUVEC compared to in cytokines over 26 days in culture (0.018% vs. 0.016% pre and post expansion, respectively). Retrieved barcode analysis and quantitation revealed that in the HUVEC expanded fraction, the top 20 clones constituted up 15% of the total valid reads by day 8, in contrast to only 3.9% of total clones in the cytokine expanded fraction. Cells have been transplanted into autologous macaques, in a competitive model comparing non-expanded, cytokine-expanded and HUVEC-expanded conditions, and in vivo expansion and barcode analysis will be presented. Our data thus far show that peripheral blood CD34+ cells from rhesus macaques expand more robustly in the presence of HUVEC and cytokines as compared to expansion in cytokines alone, and barcode analysis suggests that the HUVEC fraction selectively expanded a subset of highly proliferative cells, which may represent true HSCs within the CD34+ cell fraction.

Despite the genotoxic complications encountered in clinical gene therapy trials for primary immunodeficiency diseases targeting hematopoietic cells with integrating vectors; this strategy holds promise for the cure of several monogenic blood, metabolic and neurodegenerative diseases. In this study, we asked whether the inclusion of a suicide gene in a standard retrovirus vector would allow elimination of vector-containing stem and progenitor cells and their progeny in vivo following transplantation, using our rhesus macaque transplantation model. Following stable engraftment with autologous CD34(+) cells transduced with a retrovirus vector encoding a highly sensitive modified Herpes simplex virus thymidine kinase SR39, the administration of the antiviral prodrug ganciclovir (GCV) was effective in completely eliminating vector-containing cells in all hematopoietic lineages in vivo. The sustained absence of vector-containing cells over time, without additional GCV administration, suggests that the ablation of TkSR39 GCV-sensitive cells occurred in the most primitive hematopoietic long-term repopulating stem or progenitor cell compartment. These results are a proof-of-concept that the inclusion of a suicide gene in integrating vectors, in addition to a therapeutic gene, can provide a mechanism for later elimination of vector-containing cells, thereby increasing the safety of gene transfer.

Objective Granulocyte colony-stimulating factor (G-CSF) in combination with plerixafor produces significant mobilization of CD34 + cells in rhesus macaques. We sought to evaluate whether these CD34 + cells can stably reconstitute blood cells with lentiviral gene marking. Materials and Methods We performed hematopoietic stem cell transplantation using G-CSF and plerixafor-mobilized rhesus CD34 + cells transduced with a lentiviral vector, and these data were compared with those of G-CSF and stem cell factor mobilization. Results G-CSF and plerixafor mobilization resulted in CD34 + cell yields that were twofold higher than yields with G-CSF and stem cell factor. CD123 (interleukin-3 receptor) expression was greater in G-CSF and plerixafor-mobilized CD34 + cells when compared to G-CSF alone. Animals transplanted with G-CSF and plerixafor-mobilized cells showed engraftment of all lineages, similar to animals who received G-CSF and stem cell factor−mobilized grafts. Lymphocyte engraftment was accelerated in animals receiving the G-CSF and plerixafor-mobilized CD34 + cells. One animal in the G-CSF and plerixafor group developed cold agglutinin-associated skin rash during the first 3 months of rapid lymphocyte recovery. One year after transplantation, all animals had 2% to 10% transgene expression in all blood cell lineages. Conclusions G-CSF and plerixafor-mobilized CD34 + cells accelerate lymphocyte engraftment and contain hematopoietic stem cell capable of reconstituting multilineage blood cells. These findings indicate important differences to consider in plerixafor-based hematopoietic stem cell mobilization protocols in rhesus macaques.

Activation of proto-oncogenes by retroviral insertion is an important issue delaying clinical development of gene therapy. We have reported the nonrandom persistence of hematopoietic clones with vector insertions within the MDS1/EVI1 locus following transplantation of rhesus macaques. We now ask whether prolonged culture of transduced CD34(+) cells before transplantation selects for clones with insertions in the MDS1/EVI11 or other proto-oncogene loci. CD34(+) cells were transduced with standard retroviral vectors for 4 days and then continued in culture for an additional 6 days before transplantation. A 15% of insertions identified in granulocytes 6 months post-transplant were in MDS1/EVI11, significantly increased compared to the frequency in animals transplanted with cells immediately following transduction. MDS1/EVI1 clones became more dominant over time post-transplantation in one animal that was followed long term, accompanied by an increased overall copy number of vector-containing granulocytes, with one MDS1/EVI1 clone eventually accounting for 100% of transduced granulocytes and marrow colony-forming unit (CFU). This vector insertion increased the expression of Evi1 mRNA. There was no overrepresentation of MDS1/EVI1 insertions contributing to lymphoid lineages. Strategies involving prolonged ex vivo expansion of transduced cells may increase the risk of genotoxicity.

Major limitations to gene therapy using HSCs are low gene transfer efficiency and the inability of most therapeutic genes to confer a selective advantage on the gene-corrected cells. One approach to enrich for gene-modified cells in vivo is to include in the retroviral vector a drug resistance gene, such as the P140K mutant of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT*). We transplanted 5 rhesus macaques with CD34+ cells transduced with lentiviral vectors encoding MGMT* and a fluorescent marker, with or without homeobox B4 (HOXB4), a potent stem cell self-renewal gene. Transgene expression and common integration sites in lymphoid and myeloid lineages several months after transplantation confirmed transduction of long-term repopulating HSCs. However, all animals showed only a transient increase in gene-marked lymphoid and myeloid cells after O6-benzylguanine (BG) and temozolomide (TMZ) administration. In 1 animal, cells transduced with MGMT* lentiviral vectors were protected and expanded after multiple courses of BG/TMZ, providing a substantial increase in the maximum tolerated dose of TMZ. Additional cycles of chemotherapy using 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) resulted in similar increases in gene marking levels, but caused high levels of nonhematopoietic toxicity. Inclusion of HOXB4 in the MGMT* vectors resulted in no substantial increase in gene marking or HSC amplification after chemotherapy treatment. Our data therefore suggest that lentivirally mediated gene transfer in transplanted HSCs can provide in vivo chemoprotection of progenitor cells, although selection of long-term repopulating HSCs was not seen.

RNAi is a powerful method for suppressing gene expression that has tremendous potential for therapeutic applications. However, because endogenous RNAi plays a role in normal cellular functions, delivery and expression of siRNAs must be balanced with safety. Here we report successful stable expression in primates of siRNAs directed to chemokine (c-c motif) receptor 5 (CCR5) introduced through CD34+ hematopoietic stem/progenitor cell transplant. After hematopoietic reconstitution, to date 14 months after transplant, we observe stably marked lymphocytes expressing siRNAs and consistent down-regulation of chemokine (c-c motif) receptor 5 expression. The marked cells are less susceptible to simian immunodeficiency virus infection ex vivo. These studies provide a successful demonstration that siRNAs can be used together with hematopoietic stem cell transplant to stably modulate gene expression in primates and potentially treat blood diseases such as HIV-1.

Hematopoietic stem and progenitor cells (HSPC) primarily reside in bone marrow (BM) niches, but are also found in peripheral blood (PB) in small numbers. Large numbers of HSPC can be pharmacologically mobilized from the BM into the PB and home back to BM niches following transplantation. We used the rhesus macaque to study the process of hematopoiesis in both space and time following autologous transplantation, as a model with great relevance to humans. We labeled individual CD34+ HSPC and their progeny with genetic "barcodes" via lentiviral transduction with a very high diversity barcode library, allowing tracking of the output from individual HSPC clones in vivo following autologous transplantation, in a quantitative and sensitive manner (Wu et al., 2014). In this study we investigated the pattern of HSPC clonal output in various anatomic niches, sampling left (L) vs right (R) iliac crest BM, PB and lymph node (LN) over time post ablative autologous transplantation, for all major hematopoietic lineages and CD34+ HSPC. We tracked thousands of HSPC clones in 6 rhesus macaques from 3.5 months (m) to up to 18.5m post transplantation. L and R iliac crest BM and PB were serially collected and CD34+ HSPC along with CD3+CD4+ T, CD3+CD8+ T, CD20+ B, CD14+ monocytes (Mo), CD33+ granulocytes (Gr), and two NK cell subsets (CD3-CD20-CD14-CD16+/ or CD56+) were purified. In all animals, there was marked geographic segregation of CD34+ HSPC for at least 6m post-transplant, with individual clones localized only to the L or R side but not both (Pearson correlation at 3.5-6m: r=0.019±0.08 for CD34+ L vs R, n=6), despite rapid expansion of CD34+ HSPC during early engraftment. This result suggests that during this phase of recovery, HSPC spread contiguously in the BM, and there is little geographic "mixing" via egress into and re-entry from the PB. With time between 7m and 18.5m post-transplant, the distribution of CD34+ HSPC became more homogeneous, with clones detected on both L and R (r=0.52±0.22 at 12-18.5m, n=3). The geographic restriction of clones at earlier time points suggests that release and homing may be dependent on local contiguous niche occupancy status, with migration only occurring following hematopoietic recovery. We next examined the clonal distribution of the more mature lineage-committed cells in the L vs R BM and in PB. Gr, Mo and B cells were produced locally in BM, with the clonal pattern of each lineage matching the CD34+ cells collected from the same side BM time point through 6m (r>0.80 for each lineage vs same side CD34+ cells), and distinct from same lineages (r Our model for the first time documents the dynamics of HSPC geographic distribution and migration in primates following transplantation, findings with direct clinical relevance, and provides new insights into hematopoietic lineage development, including a potential novel T cell development pathway in the BM. Our findings may also help explain the extremely patchy distribution of hematopoiesis in humans following transplantation or in the setting of marrow failure or aging, and suggest that analysis of individual BM samples may not fully reflect ongoing global hematopoiesis. Disclosures No relevant conflicts of interest to declare.

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4, has been shown to induce rapid mobilization of CD34(+) hematopoietic cells in mice, dogs, and humans, offering an alternative to G-CSF mobilization of peripheral-blood hematopoietic stem cells. In this study, AMD3100-mobilized CD34(+) cells were phenotypically analyzed, marked with Neo(R)-containing retroviral vectors, and subsequently transplanted into myeloablated rhesus macaques. We show engraftment of transduced AMD3100-mobilized CD34(+) cells with Neo(R) gene marked myeloid and lymphoid cells up to 32 months after transplantation, demonstrating the ability of AMD3100 to mobilize true long-term repopulating hematopoietic stem cells. More AMD3100-mobilized CD34(+) cells are in the G(1) phase of the cell cycle and more cells express CXCR4 and VLA-4 compared with G-CSF-mobilized CD34(+) cells. In vivo gene marking levels obtained with AMD3100-mobilized CD34(+) cells were better than those obtained using CD34(+) cells mobilized with G-CSF alone. Overall, these results indicate that AMD3100 mobilizes a population of hematopoietic stem cells with intrinsic characteristics different from those of hematopoietic stem cells mobilized with G-CSF, suggesting fundamental differences in the mechanism of AMD3100-mediated and G-CSF-mediated hematopoietic stem cell mobilization. Thus, AMD3100-mobilized CD34(+) cells represent an alternative source of hematopoietic stem cells for clinical stem cell transplantation and genetic manipulation with integrating retroviral vectors.

Objective Gene transfer to hematopoietic stem cells has recently been demonstrated to benefit a small number of patients in whom a selective advantage is conferred upon genetically modified cells; however, in disorders where no such selective advantage is conferred, conditioning appears necessary to allow adequate engraftment. To decrease the toxicity profile, we sought to develop nonmyeloablative conditioning regimens and in this work, explored the use of intravenous busulfan in a large animal model. Methods Busulfan pharmacokinetics and toxicity were monitored in young rhesus macaques at two dosing levels (4 and 6 mg/kg). These doses were then employed to condition two animals at each dose level prior to autologous transplantation of genetically modified cells using our standard methods. Results Busulfan pharmacokinetic (PK) data showed the area under the curve (AUC), drug half-life, and drug clearance were consistent within each dose group and similar to those reported in children. Single doses of busulfan were well tolerated and produced dose-dependent myelosuppression, most notably in the neutrophil and platelet counts. Although marking levels reached over 1% early in one animal, the long-term marking was low but detectable at 0.01 to 0.001%. Conclusions We conclude that low-dose intravenous bolus infusion of busulfan is well tolerated, has dose-dependent effects on peripheral blood counts, and allows long-term engraftment of genetically modified cells, but at levels too low for most clinical disorders.

Hematopoietic cells can be highly enriched for repopulating ability based upon the efflux of the fluorescent Hoechst 33342 dye by sorting for SP (side population) cells, a phenotype attributed to expression of ABCG2, a member of the ABC transporter superfamily. Intriguingly, murine studies suggest that forced ABCG2 expression prevents hematopoietic differentiation. We cloned the full-length rhesus ABCG2 and introduced it into a retroviral vector. ABCG2-transduced human peripheral blood progenitor cells (PBPCs) acquired the SP phenotype but showed significantly reduced growth compared with control. Two rhesus macaques received autologous PBPCs split for transduction with the ABCG2 or control vectors. Marking levels were similar between fractions with no discrepancy between bone marrow and peripheral blood marking. Analysis for the SP phenotype among bone marrow and mature blood populations confirmed ABCG2 expression at levels predicted by vector copy number long term, demonstrating no block to differentiation in the large animal. In vitro studies showed selective protection against mitoxantrone among ABCG2-transduced rhesus PBPCs. Our results confirm the existence of rhesus ABCG2, establish its importance in conferring the SP phenotype, suggest no detrimental effect of its overexpression upon differentiation in vivo, and imply a potential role for its overexpression as an in vivo selection strategy for gene therapy applications.