CD3-immunotoxin mediated depletion of T cells in lymphoid tissues of rhesus macaques

Selective T-cell depletion prior to cell or organ transplantation is considered a preconditioning regimen to induce tolerance and immunosuppression. An immunotoxin consisting of a recombinant anti-CD3 antibody conjugated with diphtheria toxin was used to eliminate T-cells. It showed significant T-cell depletion activity in the peripheral blood and lymph nodes in animal models used in previous studies. To date, a comprehensive evaluation of T-cell depletion and CD3 proliferation for all lymphoid tissues has not been conducted. Here, two rhesus macaques were administered A-dmDT390-SCFBdb (CD3-IT) intravenously at 25 μg/kg twice daily for four days. Samples were collected one day prior to and four days post administration. Flow cytometry and immunofluorescence staining were used to evaluate treatment efficiency accurately. Our preliminary results suggest that CD3-IT treatment may induce higher depletion of CD3 and CD4 T-cells in the lymph nodes and spleen, but is ineffective in the colon and thymus. The data showed a better elimination tendency of CD4 T-cells in the B-cell zone relative to the germinal center in the lymph nodes. Further, CD3-IT treatment may lead to a reduction in germinal center T follicular helper CD4 cells in the lymph nodes compared to healthy controls. The number of proliferating CD3 T-cell indicated that repopulation in different lymphoid tissues may occur four days post treatment. Our results provide insights into the differential efficacy of CD3-IT treatment and T-cell proliferation post treatment in different lymphoid tissues. Overall, CD3-IT treatment shows potential efficacy in depleting T-cells in the periphery, lymph nodes, and spleen, making it a viable preconditioning regimen for cell or organ transplantation. Our pilot study provides critical descriptive statistics and can contribute to the design of larger future studies.

Pre-existing immunity does not impair the engraftment of CRISPR-Cas9-edited cells in rhesus macaques conditioned with busulfan or radiation

CRISPR-Cas9-based therapeutic genome editing approaches hold promise to cure a variety of human diseases. Recent findings demonstrate pre-existing immunity for the commonly used Cas orthologs from Streptococcus pyogenes (SpCas9) and Staphylococcus aureus (SaCas9) in humans, which threatens the success of this powerful tool in clinical use. Thus, a comprehensive investigation and potential risk assessment are required to exploit the full potential of the system. Here, we investigated existence of immunity to SpCas9 and SaCas9 in control rhesus macaques (Macaca mulatta) alongside monkeys transplanted with either lentiviral transduced or CRISPR-SpCas9 ribonucleoprotein (RNP)-edited cells. We observed significant levels of Cas9 antibodies in the peripheral blood of all transplanted and non-transplanted control animals. Transplantation of ex vivo transduced or SpCas9-mediated BCL11A enhancer-edited cells did not alter the levels of Cas9 antibodies in rhesus monkeys. Following stimulation of peripheral blood cells with SpCas9 or SaCas9, neither Cas9-specific T cells nor cytokine induction were detected. Robust and durable editing frequencies and expression of high levels of fetal hemoglobin in BCL11A enhancer-edited rhesus monkeys with no evidence of an immune response (>3 years) provide an optimistic outlook for the use of ex vivo CRISPR-SpCas9 (RNP)-edited cells.

Forced enhancer-promoter rewiring to alter gene expression in animal models

Transcriptional enhancers can be in physical proximity of their target genes via chromatin looping. The enhancer at the β-globin locus (locus control region [LCR]) contacts the fetal-type (HBG) and adult-type (HBB) β-globin genes during corresponding developmental stages. We have demonstrated previously that forcing proximity between the LCR and HBG genes in cultured adult-stage erythroid cells can activate HBG transcription. Activation of HBG expression in erythroid cells is of benefit to patients with sickle cell disease. Here, using the β-globin locus as a model, we provide proof of concept at the organismal level that forced enhancer rewiring might present a strategy to alter gene expression for therapeutic purposes. Hematopoietic stem and progenitor cells (HSPCs) from mice bearing human β-globin genes were transduced with lentiviral vectors expressing a synthetic transcription factor (ZF-Ldb1) that fosters LCR-HBG contacts. When engrafted into host animals, HSPCs gave rise to adult-type erythroid cells with elevated HBG expression. Vectors containing ZF-Ldb1 were optimized for activity in cultured human and rhesus macaque erythroid cells. Upon transplantation into rhesus macaques, erythroid cells from HSPCs expressing ZF-Ldb1 displayed elevated HBG production. These findings in two animal models suggest that forced redirection of gene-regulatory elements may be used to alter gene expression to treat disease.

Sustained fetal hemoglobin induction in vivo is achieved by BCL11A interference and coexpressed truncated erythropoietin receptor

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.

Preclinical evaluation for engraftment of CD34+ cells gene-edited at the sickle cell disease locus in xenograft mouse and non-human primate models

Sickle cell disease (SCD) is caused by a 20A > T mutation in the β-globin gene. Genome-editing technologies have the potential to correct the SCD mutation in hematopoietic stem cells (HSCs), producing adult hemoglobin while simultaneously eliminating sickle hemoglobin. Here, we developed high-efficiency viral vector-free non-footprint gene correction in SCD CD34+ cells with electroporation to deliver SCD mutation-targeting guide RNA, Cas9 endonuclease, and 100-mer single-strand donor DNA encoding intact β-globin sequence, achieving therapeutic-level gene correction at DNA (∼30%) and protein (∼80%) levels. Gene-edited SCD CD34+ cells contributed corrected cells 6 months post-xenograft mouse transplant without off-target δ-globin editing. We then developed a rhesus β-to-βs-globin gene conversion strategy to model HSC-targeted genome editing for SCD and demonstrate the engraftment of gene-edited CD34+ cells 10-12 months post-transplant in rhesus macaques. In summary, gene-corrected CD34+ HSCs are engraftable in xenograft mice and non-human primates. These findings are helpful in designing HSC-targeted gene correction trials.

NADPH oxidase correction by mRNA transfection of apheresis granulocytes in chronic granulomatous disease

Granulocytes from patients with chronic granulomatous disease (CGD) have dysfunctional phagocyte reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase that fails to generate sufficient antimicrobial reactive oxidative species. CGD patients with severe persistent fungal or bacterial infection who do not respond to antibiotic therapy may be given apheresis-derived allogeneic granulocyte transfusions from healthy volunteers to improve clearance of intractable infections. Allogeneic granulocyte donors are not HLA matched, so patients who receive the donor granulocyte products may develop anti-HLA alloimmunity. This not only precludes future use of allogeneic granulocytes in an alloimmunized CGD recipient, but increases the risk of graft failure of those recipients who go on to need an allogeneic bone marrow transplant. Here, we provide the first demonstration of efficient functional restoration of CGD patient apheresis granulocytes by messenger RNA (mRNA) electroporation using a scalable, Good Manufacturing Practice-compliant system to restore protein expression and NADPH oxidase function. Dose-escalating clinical-scale in vivo studies in a nonhuman primate model verify the feasibility, safety, and persistence in peripheral blood of infusions of mRNA-transfected autologous granulocyte-enriched apheresis cells, supporting this novel therapeutic approach as a potential nonalloimmunizing adjunct treatment of intractable infections in CGD patients.

BCL11A enhancer-edited hematopoietic stem cells persist in rhesus monkeys without toxicity

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.

Development of a forward-oriented therapeutic lentiviral vector for hemoglobin disorders

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.

Busulfan Combined with Immunosuppression Allows Efficient Engraftment of Gene-Modified Cells in a Rhesus Macaque Model

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.

Fetal hemoglobin and F-cell variance in mobilized CD34+ cell-transplanted rhesus monkeys

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.

Bone Marrow as a Hematopoietic Stem Cell Source for Gene Therapy in Sickle Cell Disease: Evidence from Rhesus and SCD Patients

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.

Total body irradiation must be delivered at high dose for efficient engraftment and tolerance in a rhesus stem cell gene therapy model

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.

Dynamics of HSPC repopulation in nonhuman primates revealed by a decade-long clonal-tracking study

In mice, clonal tracking of hematopoietic stem cells (HSCs) has revealed variations in repopulation characteristics. However, it is unclear whether similar properties apply in primates. Here, we examined this issue through tracking of thousands of hematopoietic stem and progenitor cells (HSPCs) in rhesus macaques for up to 12 years. Approximately half of the clones analyzed contributed to long-term repopulation (over 3-10 years), arising in sequential groups and likely representing self-renewing HSCs. The remainder contributed primarily for the first year. The long-lived clones could be further subdivided into functional groups contributing primarily to myeloid, lymphoid, or both myeloid and lymphoid lineages. Over time, the 4%-10% of clones with robust dual lineage contribution predominated in repopulation. HSPCs expressing a CCR5 shRNA transgene behaved similarly to controls. Our study therefore documents HSPC behavior in a clinically relevant model over a long time frame and provides a substantial system-level data set that is a reference point for future work.

Integration-specific In Vitro Evaluation of Lentivirally Transduced Rhesus CD34(+) Cells Correlates With In Vivo Vector Copy Number

Hematopoietic stem cell (HSC) gene therapy using integrating vectors has a potential leukemogenic risk due to insertional mutagenesis. To reduce this risk, a limitation of ≤2 average vector copy number (VCN) per cell is generally accepted. We developed an assay for VCN among transduced CD34(+) cells that reliably predicts in vivo VCN in 16 rhesus recipients of CD34(+) cells transduced with a green fluorescent protein (GFP) (or yellow fluorescent protein (YFP))-encoding lentiviral vector. Using GFP (or YFP)-specific probe/primers by real-time PCR, VCN among transduced CD34(+) cells had no correlation with VCN among granulocytes or lymphocytes in vivo assayed 6 months post-transplantation. This was a likely result of residual plasmids present in the vector preparation. We then designed self-inactivating long terminal repeat (SIN-LTR)-specific probe/primers, which detect only integrated provirus. Evaluation with SIN-LTR probe/primers resulted in a positive correlation of VCN among transduced CD34(+) cells with granulocytes and lymphocytes in vivo. The transduced CD34(+) cells had higher VCN (25.1 ± 5.6) as compared with granulocytes (2.8 ± 1) and lymphocytes (2.4 ± 0.7). In summary, an integrated provirus-specific real-time PCR system demonstrated nine- to tenfold higher VCN in transduced CD34(+) cells in vitro, as compared with VCN in vivo. Therefore, the restriction of ≤2 VCN before infusion might unnecessarily limit gene transfer efficacy.Molecular Therapy-Nucleic Acids (2013) 2, e122; doi:10.1038/mtna.2013.49; published online 17 September 2013.

High-efficiency transduction of rhesus hematopoietic repopulating cells by a modified HIV1-based lentiviral vector

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.

Intracoronary infusion of autologous mononuclear cells from bone marrow or granulocyte colony-stimulating factor-mobilized apheresis product may not improve remodelling, contractile function, perfusion, or infarct size in a swine model of large myocardial infarction

AIMS

In a blinded, placebo-controlled study, we investigated whether intracoronary infusion of autologous mononuclear cells from granulocyte colony-stimulating factor (G-CSF)-mobilized apheresis product or bone marrow (BM) improved sensitive outcome measures in a swine model of large myocardial infarction (MI).

METHODS AND RESULTS

Four days after left anterior descending (LAD) occlusion and reperfusion, cells from BM or apheresis product of saline- (placebo) or G-CSF-injected animals were infused into the LAD. Large infarcts were created: baseline ejection fraction (EF) by magnetic resonance imaging (MRI) of 35.3 +/- 8.5%, no difference between the placebo, G-CSF, and BM groups (P = 0.16 by ANOVA). At 6 weeks, EF fell to a similar degree in the placebo, G-CSF, and BM groups (-7.9 +/- 6.0, -8.5 +/- 8.8, and -10.9 +/- 7.6%, P = 0.78 by ANOVA). Left ventricular volumes and infarct size by MRI deteriorated similarly in all three groups. Quantitative positron emission tomography (PET) demonstrated significant decline in fluorodeoxyglucose uptake rate in the LAD territory at follow-up, with no histological, angiographic, or PET perfusion evidence of functional neovascularization. Immunofluorescence failed to demonstrate transdifferentiation of infused cells.

CONCLUSION

Intracoronary infusion of mononuclear cells from either BM or G-CSF-mobilized apheresis product may not improve or limit deterioration in systolic function, adverse ventricular remodelling, infarct size, or perfusion in a swine model of large MI.

Paradoxical drop in circulating neutrophil count following granulocyte-colony stimulating factor and stem cell factor administration in rhesus macaques

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 microg/kg or 50 microg/kg. It was also administered (10 microg/kg) in combination with stem cell factor (SCF; 200 microg/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 microg/kg G-CSF was administered in combination with SCF over 5 days, or given as a single 10 microg/kg dose. At a single 50 microg/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.

Prolonged multilineage clonal hematopoiesis in a rhesus recipient of CD34 positive cells marked with a RD114 pseudotyped oncoretroviral vector

The ability to efficiently transfer a gene into repopulating hematopoietic stem cells would create many therapeutic opportunities. We have evaluated the ability of particles bearing an alternative envelope protein, that of the feline endogenous virus (RD114), to transduce stem cells in a nonhuman primate autologous transplantation model using rhesus macaques. We have previously shown this pseudotyped vector to be superior to the amphotropic vector at transducing cells in umbilical cord blood capable of establishing hematopoiesis in immunodeficient mice. Gene transfer efficiency as reflected by the number of genetically modified cells in hematopoietic tissues varied among the five monkeys studied from low levels (<1%) in three animals to much higher levels in two (20-60%). An animal that exhibited extremely high levels for several weeks was found by vector genome insertion site analysis to have reconstitution predominantly with a single clone of cells. This variability among animals is in keeping with computer simulations of reconstitution with limiting numbers of stem cells genetically modified at about 10% efficiency. Our studies provide insights into the biology of hematopoietic reconstitution and suggest approaches for increasing stem cell targeted gene transfer efficiency.

Retroviral transduction and engraftment ability of primate hematopoietic progenitor and stem cells transduced under serum-free versus serum-containing conditions

The ability to efficiently transduce hematopoietic stem and progenitor cells under serum-free conditions would be desirable for safety and standardization of clinical gene therapy protocols. Using rhesus macaques, we studied the transduction efficiency and engraftment ability of CD34-enriched SCF/G-CSF mobilized progenitor cells (PBSC) transduced with standard amphotropic marking vectors under serum-free and serum-containing conditions. Supernatants were collected from producer cells 16 hours after serum-free medium or medium containing 10% fetal calf serum was added. Vector titers were approximately two- to threefold higher when producer cells were cultured in serum-containing medium. However, retroviral transduction of rhesus CFU-GM was improved using serum-free vector-containing medium. For analysis of engraftment with transduced cells, three macaques had CD34+ peripheral blood stem cells split into two fractions for transduction. One fraction was transduced using serum-free vector-containing medium, and the other fraction was transduced using standard serum-containing medium. The two fractions were re-infused simultaneously following total body irradiation. In all three animals, there was equivalent marking from both vectors for 7-9 months post-transplantation. These data are encouraging regarding the removal of serum-containing medium from clinical hematopoietic cell transduction protocols, given the lack of a detrimental effect on transduction and engraftment with transduced cells.

Mechanism of copper-mediated inactivation of herpes simplex virus

The inactivation of herpes simplex virus (HSV) by copper was enhanced by the following reducing agents at the indicated relative level: ascorbic acid >> hydrogen peroxide > cysteine. Treatment of HSV-infected cells with combinations of Cu(II) and ascorbate completely inhibited virus plaque formation to below 0.006% of the infectious virus input, while it maintained 30% viability for the host mammalian cells. The logarithm of the surviving fraction of HSV mediated by 1 mg of Cu(II) per liter and 100 mg of reducing agent per liter followed a linear relationship with the reaction time, in which the kinetic rate constant for each reducing agent was -0.87 min(-1) (r = 0.93) for ascorbate, -0.10 min(-1) (r = 0.97) for hydrogen peroxide, and -0.04 min(-1) (r = 0.97) for cysteine. The protective effects of metal chelators and catalase, the lack of effect of superoxide dismutase, and the partial protection conferred by free-radical scavengers suggest that the mechanism of copper-mediated HSV inactivation is similar to that previously reported for copper-mediated DNA damage. The sensitivity exhibited by HSV to Cu(II) and reducing agents, particularly ascorbate, might be useful in the development of therapeutic antiviral agents.