Lentiviral Vectors For Gene Therapy Research Articles

In-Vivo Gene Therapy with Foamy Virus Vectors.

Foamy viruses (FVs) are nonpathogenic retroviruses that infect various animals including bovines, felines, nonhuman primates (NHPs), and can be transmitted to humans through zoonotic infection. Due to their non-pathogenic nature, broad tissue tropism and relatively safe integration profile, FVs have been engineered as novel vectors (foamy virus vector, FVV) for stable gene transfer into different cells and tissues. FVVs have emerged as an alternative platform to contemporary viral vectors (e.g., adeno associated and lentiviral vectors) for experimental and therapeutic gene therapy of a variety of monogenetic diseases. Some of the important features of FVVs include the ability to efficiently transduce hematopoietic stem and progenitor cells (HSPCs) from humans, NHPs, canines and rodents. We have successfully used FVV for proof of concept studies to demonstrate safety and efficacy following in-vivo delivery in large animal models. In this review, we will comprehensively discuss FVV based in-vivo gene therapy approaches established in the X-linked severe combined immunodeficiency (SCID-X1) canine model.

Lentiviral Gene Therapy with Autologous Hematopoietic Stem and Progenitor Cells (HSPCs) for the Treatment of Severe Combined Immune Deficiency Due to Adenosine Deaminase Deficiency (ADA-SCID): Results in an Expanded Cohort

Lentiviral Gene Therapy with Autologous Hematopoietic Stem and Progenitor Cells (HSPCs) for the Treatment of Severe Combined Immune Deficiency Due to Adenosine Deaminase Deficiency (ADA-SCID): Results in an Expanded Cohort

Assessing the Impact of Cyclosporin A on Lentiviral Transduction and Preservation of Human Hematopoietic Stem Cells in Clinically Relevant Ex Vivo Gene Therapy Settings.

Improving hematopoietic stem and progenitor cell (HSPC) permissiveness to lentiviral vector (LV) transduction without compromising their biological properties remains critical for broad-range implementation of gene therapy as a treatment option for several inherited diseases. This study demonstrates that the use of one-hit ex vivo LV transduction protocols based on either cyclosporin A (CsA) or rapamycin enable as efficient gene transfer as the current two-hit clinical standard into bone marrow–derived CD34+ cells while better preserving their engraftment capacity in vivo. CsA was additive with another enhancer of transduction, prostaglandin E2, suggesting that tailored enhancer combinations may be applied to overcome multiple blocks to transduction simultaneously in HSPC. Interestingly, besides enhancing LV transduction, CsA also significantly reduced HSPC proliferation, preserving the quiescent G0 fraction and the more primitive multipotent progenitors, thereby yielding the highest engraftment levels in vivo. Importantly, no alterations in the vector integration profiles could be detected between CsA and control transduced HSPC. Overall, the present findings contribute to the development of more efficient and sustainable LV gene therapy protocols, underscoring the benefits of scaling down required vector doses, as well as shortening the HSPC ex vivo culture time.

Recent Advances in Gene Therapy and Modeling of Chronic Granulomatous Disease

The Chronic granulomatous disease (CGD) is a primary immunodeficiency that characterized by mutations in phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, resulting in deficient antimicrobial activity of phagocytic cells and recurrent childhood infections. Hematopoietic stem cell transplantation (HSCT) is a curative option for patients with human leukocyte antigen (HLA) matched donor, when conventional cares and therapies fail. However, in many cases when the patients have not an HLA-matched donor, they need to a method to recapitulate the function of the affected gene within the patient's own cells. Gene therapy is a promising approach for CGD. While, the success of retroviral or lentiviral vectors in gene therapy for CGD has been hampered by random integration and insertional activation of proto-oncogenes. These serious adverse events led to improvement and generations of viral vectors with increased safety characteristics. Gene therapy continues to progress and the advent of new technologies, such as engineered endonucleases that have shown a great promise for the treatment of genetic disease. This review focuses on the application of gene therapy for the CGD, the limitations encountered in current clinical trials, advantages and disadvantages of endonucleases in gene correction and modeling with CRISPR/Cas9 approach.

The Engraftment of Lentiviral Vector-Transduced Human CD34+ Cells into Humanized Mice.

Humanized mouse models have been developed to study human hematopoiesis and therapeutic application of hematopoietic stem cell transplantation. To evaluate the safety and efficacy of lentiviral vectors for gene therapy, human CD34+ cells have been transduced with lentiviral vectors and transplanted into the humanized mice. Recipient mice are monitored over time and sacrificed for bone marrow analyses with regard to human cell engraftment, lineage distribution, and vector transduction. This chapter details the procedure for lentiviral transduction and transplantation of human hematopoietic stem/progenitor cells into humanized mice to study inherited human hematological disorders.

Preclinical Development of a Hematopoietic Stem and Progenitor Cell Bioengineered Factor VIII Lentiviral Vector Gene Therapy for Hemophilia A.

Genetically modified, autologous hematopoietic stem and progenitor cells (HSPCs) represent a new class of genetic medicine. Following this therapeutic paradigm, we are developing a product candidate, designated CD68-ET3-LV CD34+, for the treatment of the severe bleeding disorder, hemophilia A. The product consists of autologous CD34+ cells transduced with a human immunodeficiency virus 1-based, monocyte lineage-restricted, self-inactivating lentiviral vector (LV), termed CD68-ET3-LV, encoding a bioengineered coagulation factor VIII (fVIII) transgene, termed ET3, designed for enhanced expression. This vector was shown capable of high-titer manufacture under clinical scale and Good Manufacturing Practice. Biochemical and immunogenicity testing of recombinant ET3, as well as safety and efficacy testing of CD68-ET3-LV HSPCs, were utilized to demonstrate overall safety and efficacy in murine models. In the first model, administration of CD68-ET3-LV-transduced stem-cell antigen-1+ cells to hemophilia A mice resulted in sustained plasma fVIII production and hemostatic correction without signs of toxicity. Patient-derived, autologous mobilized peripheral blood (mPB) CD34+ cells are the clinical target cells for ex vivo transduction using CD68-ET3-LV, and the resulting genetically modified cells represent the investigational drug candidate. In the second model, CD68-ET3-LV gene transfer into mPB CD34+ cells isolated from normal human donors was utilized to obtain in vitro and in vivo pharmacology, pharmacokinetic, and toxicology assessment. CD68-ET3-LV demonstrated reproducible and efficient gene transfer into mPB CD34+ cells, with vector copy numbers in the range of 1 copy per diploid genome equivalent without affecting clonogenic potential. Differentiation of human CD34+ cells into monocytes was associated with increased fVIII production, supporting the designed function of the CD68 promoter. To assess in vivo pharmacodynamics, CD68-ET3-LV CD34+ cell product was administered to immunodeficient mice. Treated mice displayed sustained plasma fVIII levels and no signs of product related toxicity. Collectively, the findings of the current study support the preclinical safety and efficacy of CD68-ET3-LV CD34+.

Long-Term Follow-Up of a Phase I/II Study of ProSavin, a Lentiviral Vector Gene Therapy for Parkinson's Disease

Parkinson's disease is typically treated with oral dopamine replacement therapies. However, long-term use is complicated by motor fluctuations from intermittent stimulation of dopamine receptors and off-target effects. ProSavin, a lentiviral vector based gene therapy that delivers local and continuous dopamine, was previously shown to be well tolerated in a Phase I/II first-in-human study, with significant improvements in motor behavior from baseline at 1 year. Here, patients with Parkinson's disease from the open-label trial were followed up in the long term to assess the safety and efficacy of ProSavin after bilateral injection into the putamen. Fifteen patients who were previously treated with ProSavin have been followed for up to 5 years, with some having been seen for 8 years. Eight patients received deep brain stimulation at different time points, and their subsequent assessments continued to assess safety. Ninety-six drug-related adverse events were reported (87 mild, 6 moderate, 3 severe) of which more than half occurred in the first year. The most common drug-related events were dyskinesias (33 events, 11 patients) and on–off phenomena (22 events, 11 patients). A significant improvement in the defined “off” Unified Parkinson's Disease Rating Scale part III motor scores, compared to baseline, was seen at 2 years (mean score 29 · 2 vs. 38 · 4, n = 14, p < 0.05) and at 4 years in 8/15 patients. ProSavin continued to be safe and well tolerated in patients with Parkinson's disease. Moderate improvements in motor behavior over baseline continued to be reported in the majority of patients who could still be evaluated up to 5 years of follow-up.

Generation of a Novel Gene Therapy Vector Encoding Beta-Cell–Specific Insulin for Diabetes Treatment

Insulin injection is the main treatment modality especially for patients with type 1 diabetes. Despite the development of several insulin analogs, recent meta-analysis indicated that rapid and long acting insulin analogs provide little benefit compared to conventional insulins regarding to glycemic control. Therefore, gene therapy methods are investigated to develop a novel treatment approach to reconstitute a natural endogenous insulin expression profile in diabetic patients. For this purpose, Multisite Gateway Technology was employed to generate a transfer vector carrying lentiviral backbone with minimal insulin promotor hooked up to proinsulin gene sequence. Following confirmation of the transfer plasmid by restriction enzyme digestion and DNA sequence analysis, high-titer LentiINS vectors at a concentration of 10e9 TU/ml were successfully produced by CaPO4 cotransfection of 293T cells with packaging and transfer plasmids. While transduction of NIT1 mouse pancreatic beta cell lines produced 3 fold increase in insulin gene expression compared to controls, no insulin expression was detected in 293T kidney cell line. These results indicate that we successfully constructed an HIV-based lentiviral gene therapy vector being capable of beta cell-specific insulin gene expression and secretion (TUBITAK 215S820). Disclosure E.O. Sahin: None. F. Erendor: None. M. Balci: None. S. Sanlioglu: None.

Lentiviral gene therapy vector with UCOE stably restores function in iPSC-derived neutrophils of a CDG patient.

A recent gamma-retroviral clinical Chronic Granulomatous Disease (CGD) gene therapy (GT) trial achieved proof-of-concept but was accompanied by activation of oncogenes and transgene silencing. The ubiquitous chromatin opening element (UCOE) comprises the sequences of two divergently oriented house-keeping gene promoters and is known to have anti-silencing properties. In a screen we identified two novel UCOE constructs that prevent adjacent promoter methylation in P19 cells. Experiments were continued with the shorter UCOE constructs in induced pluripotent stem cells (iPSC) derived from a p47phox-deficient CGD patient. The iPSC line was transduced with the lentiviral GT vectors expressing P47 under the constitutively active SFFV promoter with UCOE element (UCOE_SF) and without UCOE element (SF) adjacent to the SFFV promoter. The iPSC were expanded before propagation towards neutrophils. 20 days after transduction the UCOE_SF vector was protected from methylation in iPSC as previously shown in P19 cells, whereas the SF vector was heavily methylated in iPSC. The UCOE_SF vector maintained stable transgene expression in iPSC, macrophages and neutrophils, whereas the SF vector was strongly silenced. The UCOE_SF vector stably restored ROS production in neutrophils, whereas for the SF vector the count of ROS producing cells was marginal. To conclude, we have shown that the prevention of transgene silencing by UCOE is functionally and mechanistically preserved upon terminal neutrophil differentiation.

Preclinical Development of a Lentiviral Vector for Gene Therapy of X-Linked Severe Combined Immunodeficiency

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the interleukin-2 receptor γ chain gene (IL2RG), and it is characterized by profound defects in T, B, and natural killer (NK) cell functions. Transplantation of hematopoietic stem/progenitor cells (HSPCs) genetically corrected with early murine leukemia retrovirus (MLV)-derived gammaretroviral vectors showed restoration of T cell immunity in patients, but it resulted in vector-induced insertional oncogenesis. We developed a self-inactivating (SIN) lentiviral vector carrying a codon-optimized human IL2RG cDNA driven by the EF1α short promoter (EFS-IL2RG), and we tested its efficacy and safety in vivo by transplanting transduced Il2rg-deficient Lin− HSPCs in an Il2rg−/−/Rag2−/− mouse model. The study showed restoration of T, B, and NK cell counts in bone marrow and peripheral blood and normalization of thymus and spleen cellularity and architecture. High-definition insertion site analysis defined the EFS-IL2RG genomic integration profile, and it showed no sign of vector-induced clonal selection or skewing in primarily and secondarily transplanted animals. The study enables a phase I/II clinical trial aimed at restoring both T and B cell immunity in SCID-X1 children upon non-myeloablative conditioning.

A New Era for Hemoglobinopathies: More Than One Curative Option.

Hemoglobinopathies, including severe β-thalassemia and sickle cell disease, represent the most common monogenic disorders worldwide. Allogeneic hematopoietic stem cell transplantation (allo-HCT) is the only approved curative option for these syndromes, albeit limited to patients having a suitable donor. Gene therapy, by making use of the patient's own hematopoietic stem cells to introduce a normal copy of the β-globin gene by viral vectors, bridged the gap between the need for cure of patients with hemoglobinopathies and the lack of a donor, without incurring the immunological risks of allo-HSCT. However, gene therapy for hemoglobinopathies proved a difficult and elusive goal for decades and only recently, lenti-viral vector gene therapy was successfully transferred to the clinic. Importantly, during the last years, additional curative options for patients with thalassemia and sickle cell disease are being developed, based on the ability to manipulate the genome by employing programmable nucleases and next-generation genome-modifying tools, thus providing the exciting prospects of targeted in-situ gene correction. In this review, we will summarize current developments in the new era of treatment for hemoglobinopathies, elaborate on lessons gained from gene therapy trials and discuss the exciting prospects and challenges of genome editing.

Scale-up of lentiviral vectors for gene therapy: advances and challenges

Scale-up of lentiviral vectors for gene therapy: advances and challenges

Evaluation of tolerance to lentiviral LV-RPE65 gene therapy vector after subretinal delivery in non-human primates

Several approaches have been developed for gene therapy in RPE65-related Leber congenital amaurosis. To date, strategies that have reached the clinical stages rely on adeno-associated viral vectors and two of them documented limited long-term effect. We have developed a lentiviral-based strategy of RPE65 gene transfer that efficiently restored protein expression and cone function in RPE65-deficient mice. In this study, we evaluated the ocular and systemic tolerances of this lentiviral-based therapy (LV-RPE65) on healthy nonhuman primates (NHPs), without adjuvant systemic anti-inflammatory prophylaxis. For the first time, we describe the early kinetics of retinal detachment at 2, 4, and 7days after subretinal injection using multimodal imaging in 5 NHPs. We revealed prolonged reattachment times in LV-RPE65-injected eyes compared to vehicle-injected eyes. Low- (n=2) and high-dose (n=2) LV-RPE65-injected eyes presented a reduction of the outer nuclear and photoreceptor outer segment layer thickness in the macula, that was more pronounced than in vehicle-injected eyes (n=4). All LV-RPE65-injected eyes showed an initial perivascular reaction that resolved spontaneously within 14days. Despite foveal structural changes, full-field electroretinography indicated that the overall retinal function was preserved over time and immunohistochemistry identified no difference in glial, microglial, or leucocyte ocular activation between low-dose, high-dose, and vehicle-injected eyes. Moreover, LV-RPE65-injected animals did not show signs of vector shedding or extraocular targeting, confirming the safe ocular restriction of the vector. Our results evidence a limited ocular tolerance to LV-RPE65 after subretinal injection without adjuvant anti-inflammatory prophylaxis, with complications linked to this route of administration necessitating to block this transient inflammatory event.

Eliminating HIV-1 Packaging Sequences from Lentiviral Vector Proviruses Enhances Safety and Expedites Gene Transfer for Gene Therapy

Lentiviral vector genomic RNA requires sequences that partially overlap wild-type HIV-1 gag and env genes for packaging into vector particles. These HIV-1 packaging sequences constitute 19.6% of the wild-type HIV-1 genome and contain functional cis elements that potentially compromise clinical safety. Here, we describe the development of a novel lentiviral vector (LTR1) with a unique genomic structure designed to prevent transfer of HIV-1 packaging sequences to patient cells, thus reducing the total HIV-1 content to just 4.8% of the wild-type genome. This has been achieved by reconfiguring the vector to mediate reverse-transcription with a single strand transfer, instead of the usual two, and in which HIV-1 packaging sequences are not copied. We show that LTR1 vectors offer improved safety in their resistance to remobilization in HIV-1 particles and reduced frequency of splicing into human genes. Following intravenous luciferase vector administration to neonatal mice, LTR1 sustained a higher level of liver transgene expression than an equivalent dose of a standard lentivirus. LTR1 vectors produce reverse-transcription products earlier and start to express transgenes significantly quicker than standard lentiviruses after transduction. Finally, we show that LTR1 is an effective lentiviral gene therapy vector as demonstrated by correction of a mouse hemophilia B model.

210 - Analysis of the Functional Impact of RUNX1 Mutations in MDS Stem Cells

Improving hematopoietic stem and progenitor cell (HSPC) permissiveness to HIV-derived lentiviral vectors (LVs) remains a challenge for the field of gene therapy as high vector doses and prolonged ex vivo culture are still required to achieve clinically relevant transduction levels. We report here that Cyclosporin A (CsA) and Rapamycin (Rapa) significantly improve LV gene transfer in human and murine HSPC. Both compounds increased LV but not gammaretroviral transduction and acted independently of calcineurin and autophagy. Improved gene transfer was achieved across all CD34+ subpopulations, including in long-term SCID repopulating cells. Effects of CsA were specific of HSPC and opposite to its known impact on HIV replication. Mutating the Cyclophilin A binding pocket of the viral capsid (CA) further improved transduction in combination with CsA. Tracking of the LV genome fate revealed that CsA relieves a CA-dependent early block and increases integration, while Rapa acts early in LV infection independently of the viral CA. In agreement, only Rapa was able to improve transduction by an integrase-defective LV harboring wild-type CA. Overall, our findings pave the way for more efficient and sustainable LV gene therapy in human HSPCs and shed light on the multiple innate barriers specifically hampering LV transduction in these cells.

Development of a pCCLChim Lentiviral Vector for Gene Therapy of Patients with Chronic Granulomatous Disease (CGD) due to p47-phox Deficiency

Development of a pCCLChim Lentiviral Vector for Gene Therapy of Patients with Chronic Granulomatous Disease (CGD) due to p47-phox Deficiency

State of the art: gene therapy of haemophilia

Clinical gene therapy has been practiced for more than a quarter century and the first products are finally gaining regulatory/marketing approval. As of 2016, there have been 11 haemophilia gene therapy clinical trials of which six are currently open. Each of the ongoing phase 1/2 trials is testing a variation of a liver-directed adeno-associated viral (AAV) vector encoding either factor VIII (FVIII) or factor IX (FIX) . As summarized herein, the clinical results to date have been mixed with some perceived success and a clear recognition of the immune response to AAV as an obstacle to therapeutic success. We also attempt to highlight promising late-stage preclinical activities for AAV-FVIII where, due to inherent challenges with manufacture, delivery and transgene product biosynthesis, more technological development has been necessary to achieve results comparable to what has been observed previously for AAV-FIX. Finally, we describe the development of a stem cell-based lentiviral vector gene therapy product that has the potential to provide lifelong production of FVIII and provide a functional 'cure' for haemophilia A. Integral to this program has been the incorporation of a blood cell-specific gene expression element driving the production of a bioengineered FVIII designed for optimal efficiency. As clearly outlined herein, haemophilia remains at the forefront of the rapidly advancing clinical gene therapy field where there exists a shared expectation that transformational advances are on the horizon.

280. Lentiviral-Mediated Gene Therapy Restores B Cell Homeostasis and Tolerance in Wiskott-Aldrich Syndrome Patients

Wiskott-Aldrich Syndrome (WAS) is a severe X-linked primary immunodeficiency characterized by micro-thrombocytopenia, eczema and increased risk of infections, autoimmunity and tumors. Allogeneic hematopoietic stem cell (HSC) transplantation is a recognized curative treatment for WAS, but when a matched donor is not available, administration of WAS gene-corrected autologous HSCs represents a valid alternative therapeutic approach. Since alterations of WAS protein (WASp)-deficient B lymphocytes contribute to immunodeficiency and autoimmunity in WAS, we followed the B cell reconstitution in 4 WAS patients treated by lentiviral vector-gene therapy (GT) after a reduced-intensity conditioning regimen combined with anti-CD20 administration. We analyzed the B cell subset distribution in the bone marrow and peripheral blood by flow cytometry and the autoantibody profile by a high-throughput autoantigen microarray platform before and after GT. Lentiviral vector-transduced progenitor cells were able to repopulate the B cell compartment with a normal distribution of transitional, naive and memory B cells. The reduction in the proportion of autoimmune-associated CD21low B cells and in the plasma levels of B cell-activating factor was associated with the decreased autoantibody production in WAS patients after GT. Then, we evaluated the functionality of B cell tolerance checkpoints by testing the reactivity of recombinant antibodies isolated from single B cells. Before GT, we found a decreased frequency of autoreactive new emigrant/transitional B cells in WAS patients, suggesting a hyperfunctional central B cell checkpoint in the absence of WASp. In contrast, high frequency of polyreactive and Hep2 reactive clones were found in mature naive B cells of WAS patients, indicating a defective peripheral B cell checkpoint. Both central and peripheral B cell tolerance checkpoints were restored after GT, further supporting the qualitative efficacy of this treatment. In conclusion, WASp plays an important role in the regulation of B cell homeostasis and in the establishment of B cell tolerance in humans and lentiviral-mediated GT is able to ameliorate the functionality of B cell compartment contributing to the clinical and immunological improvement in WAS patients.

286. Genome Editing of Inducible Cell Lines for Scalable Production of Improved Lentiviral Vectors for Human Gene Therapy

Lentiviral vectors (LVs) represent efficient and versatile vehicles for gene therapy. Current manufacturing of clinical-grade LVs mostly relies on transient transfection of plasmids expressing the multiple vector components. This method is labor and cost intensive and becomes challenging when facing the need of scale-up and standardization. The development of stable LV producer cell lines will greatly facilitate overcoming these hurdles. We have generated an inducible LV packaging cell line, carrying the genes encoding for third-generation vector components stably integrated in the genome under the control of tetracycline-regulated promoters. These LV packaging cells are stable in culture even after single-cell cloning and can be scaled up to large volumes. In order to minimize the immunogenicity of LVs for in vivo administration, we set out to remove the highly polymorphic class-I major histocompatibility complexes (MHC-I) expressed on LV packaging cells and incorporated in the LV envelope. We performed genetic disruption of the β-2 microglobulin (B2M) gene, a required component for the assembly and trafficking of all MHC-I to the plasma membrane in LV producer cells, exploiting the RNA-guided Cas9 nuclease. The resulting B2M-negative cells were devoid of surface-exposed MHC-I and produced MHC-free LVs. These LVs retain their infectivity on all tested cells in vitro and efficiently transduced the mouse liver upon intravenous administration. Strikingly, the MHC-free LVs showed significantly reduced immunogenicity in a T-cell activation assay performed on human primary T cells co-cultured with syngeneic monocytes exposed to LV, from several (n=7) healthy donors. To reproducibly generate LV-producer cell lines from these cells, we insert the LV genome of interest in the AAVS1 locus, chosen for robust expression, exploiting engineered nucleases and homology-directed repair. By this strategy, we have obtained several independent producer cell lines for LVs that express marker or therapeutic genes and are devoid of plasmid DNA contamination. LVs produced by these cells reproducibly show titer and infectivity within the lower bound range of standard optimized transient transfection, and effectively transduce relevant target cells, such as hematopoietic stem/progenitor cells and T cells ex vivo and the mouse liver in vivo. Overall, we provide evidence that rationally designed targeted genome engineering can be used to improve the yield, quality, safety and sustainability of LV production for clinical use.

530. Lentiviral Vector Particles Pseudotyped with Wild-Type Baboon Endogenous Retrovirus (BAEV) Glycoprotein Outperform VSV-G Particles in Transducing Human CD34+ Cells Isolated from Cytokine-Mobilized Peripheral Blood or Bone Marrow

Lentiviral vector gene therapy for hematopoietic disorders caused by single gene mutation or deficiency has demonstrated success in clinical trials. However, efficient gene delivery is still challenging, requiring high multiplicity of infection (MOI) to achieve average vector copy numbers of at least 1 per cell. Conditions utilizing high MOI result in improved transduction rates and adequate copy number but these benefits can coincide with potential insertional mutagenesis. Attempting to achieve optimal gene transfer at reduced MOI, we directly compared self-inactivating (SIN) lentiviral vector particles encoding eGFP under control of a murine stem cell virus (MSCV) promoter/enhancer pseudotyped with envelope glycoproteins for BaEV or vesicular stomatitis virus (VSV). BaEV proteins were either full-length (BaWT) or lacked the terminal R-peptide (BaR-less), a version previously reported to improve infectious titer (TU, transducing units per mL) using transient production procedures. Using a standardized four-plasmid transient production protocol, cells transfected with R-less expression plasmids produced syncitia; an issue that resolved using BaWT. Titers were significantly higher for virus packaged with BaWT compared to R-less (BaR-less: 3.6 × 105 ± 6.6 × 104 N=12; BaWT 2.4 × 106 ± 4.5 × 105; n=10; p<0.001) determined by flow cytometry analysis of GFP expression in transduced HEK-293T cells. However, these values were 10-to 20-fold less than those observed for VSV-G (3.5 × 107 ± 4.5 × 106). Human CD34+ hematopoietic stem/progenitor cells isolated from bone marrow (BM) or cytokine-mobilized peripheral blood (mPB) of healthy donors were prestimulated and transduced overnight on retronectin-coated plates with BaWT particles at an MOI of 0.5, 1, 2, or 4. The following day, cells were either plated in methylcellulose to assess colony-formation (CFU), maintained in liquid culture (Bulk), or transplanted into NOD-scid IL2Rgnull (NSG) mice pre-conditioned with busulfan (n=10 mice/1×106 cells each). Gene transfer efficiency was gauged by expression of GFP following flow cytometry analysis of cells maintained in culture or visual inspection of colonies growing in methylcellulose using an inverted fluorescence microscope after 6 or 12 days, respectively. Transduction efficiency increased with viral MOI reaching peak levels using an MOI of 2 (Bulk, 93% GFP+; CFU, 98% GFP+). Six NSG mice survived to 14 weeks post-transplant and demonstrated engraftment of CD45+ cells ranging from 19 to 54% (39 ± 13%; mean ± SD) and GFP marking ranging from 32 to 59% (41 ± 10%) determined by flow cytometry. Finally, we compared GFP-encoding particles pseudotyped with VSV-G or BaWT normalizing for levels of p24 quantified by ELISA (BaWT, 1155.0 pg/ml, 3.3 × 106 TU; VSV-G, 1164.9 pg/ml, 5.1 × 107 TU) to account for differences in calculated titer. Prestimulated mPB CD34+ cells were transduced overnight and placed into methylcellulose or transplanted into 15 NSG mice each. Methylcellulose colonies were analyzed for GFP after 12 days of growth (BaWT, 95.6%; VSV-G, 76.6%). Transplanted mice will be analyzed at 16-18 weeks post-transplant to measure engraftment and GFP expression. We believe the stability of producer cells expressing BaWT, the high efficiency of transduction at low MOI, and the ability to transduce adequate numbers of cells with unconcentrated virus supernatant warrant continued optimization of transient production methods and development of a stable producer cell line utilizing BaWT envelope as an alternative to VSV-g.