Direct comparison of RD114-pseudotyped versus amphotropic-pseudotyped retroviral vectors for transduction of rhesus macaque long-term repopulating cells

Lentivector Producer Cell Lines with Stably Expressed Vesiculovirus Envelopes

Retroviral and lentiviral vectors often use the envelope G protein from the vesicular stomatitis virus Indiana strain (VSVind.G). However, lentivector producer cell lines that stably express VSVind.G have not been reported, presumably because of its cytotoxicity, preventing simple scale-up of vector production. Interestingly, we showed that VSVind.G and other vesiculovirus G from the VSV New Jersey strain (VSVnj), Cocal virus (COCV), and Piry virus (PIRYV) could be constitutively expressed and supported lentivector production for up to 10 weeks. All G-enveloped particles were robust, allowing concentration and freeze-thawing. COCV.G and PIRYV.G were resistant to complement inactivation, and, using chimeras between VSVind.G and COCV.G, the determinant for complement inactivation of VSVind.G was mapped to amino acid residues 136-370. Clonal packaging cell lines using COCV.G could be generated; however, during attempts to establish LV producer cells, vector superinfection was observed following the introduction of a lentivector genome. This could be prevented by culturing the cells with the antiviral drug nevirapine. As an alternative countermeasure, we demonstrated that functional lentivectors could be reconstituted by admixing supernatant from stable cells producing unenveloped virus with supernatant containing envelopes harvested from cells stably expressing VSVind.G, COCV.G, or PIRYV.G.

Baboon envelope pseudotyped LVs outperform VSV-G-LVs for gene transfer into early-cytokine-stimulated and resting HSCs

Hematopoietic stem cell (HSC)-based gene therapy holds promise for the cure of many diseases. The field is now moving toward the use of lentiviral vectors (LVs) as evidenced by 4 successful clinical trials. These trials used vesicular-stomatitis-virus-G protein (VSV-G)-LVs at high doses combined with strong cytokine-cocktail stimulation to obtain therapeutically relevant transduction levels; however, they might compromise the HSC character. Summarizing all these disadvantages, alternatives to VSV-G-LVs are urgently needed. We generated here high-titer LVs pseudotyped with a baboon retroviral envelope glycoprotein (BaEV-LVs), resistant to human complement. Under mild cytokine prestimulation to preserve the HSC characteristics, a single BaEV-LV application at a low dose, resulted in up to 90% of hCD34(+) cell transduction. Even more striking was that these new BaEV-LVs allowed, at low doses, efficient transduction of up to 30% of quiescent hCD34(+) cells, whereas high-dose VSV-G-LVs were insufficient. Importantly, reconstitution of NOD/Lt-SCID/γc(-/-) (NSG) mice with BaEV-LV-transduced hCD34(+) cells maintained these high transduction levels in all myeloid and lymphoid lineages, including early progenitors. This transduction pattern was confirmed or even increased in secondary NSG recipient mice. This suggests that BaEV-LVs efficiently transduce true HSCs and could improve HSC-based gene therapy, for which high-level HSC correction is needed for life-long cure.

Transduction of human primitive repopulating hematopoietic cells with lentiviral vectors pseudotyped with various envelope proteins

Lentiviral vectors are useful for transducing primitive hematopoietic cells. We examined four envelope proteins for their ability to mediate lentiviral transduction of mobilized human CD34(+) peripheral blood cells. Lentiviral particles encoding green fluorescent protein (GFP) were pseudotyped with the vesicular stomatitis virus envelope glycoprotein (VSV-G), the amphotropic (AMPHO) murine leukemia virus envelope protein, the endogenous feline leukemia viral envelope protein or the feline leukemia virus type C envelope protein. Because the relative amount of genome RNA per ml was similar for each pseudotype, we transduced CD34(+) cells with a fixed volume of each vector preparation. Following an overnight transduction, CD34(+) cells were transplanted into immunodeficient mice which were sacrificed 12 weeks later. The average percentages of engrafted human CD45(+) cells in total bone marrow were comparable to that of the control, mock-transduced group (37-45%). Lenti-particles pseudotyped with the VSV-G envelope protein transduced engrafting cells two- to tenfold better than particles pseudotyped with any of the gamma-retroviral envelope proteins. There was no correlation between receptor mRNA levels for the gamma-retroviral vectors and transduction efficiency of primitive hematopoietic cells. These results support the use of the VSV-G envelope protein for the development of lentiviral producer cell lines for manufacture of clinical-grade vector.

Cocal-pseudotyped lentiviral vectors resist inactivation by human serum and efficiently transduce primate hematopoietic repopulating cells

Lentiviral vectors are established as efficient and convenient vehicles for gene transfer. They are almost always pseudotyped with the envelope glycoprotein of vesicular stomatitis virus (VSV-G) due to the high titers that can be achieved, their stability, and broad tropism. We generated a novel cocal vesiculovirus envelope glycoprotein plasmid and compared the properties of lentiviral vectors pseudotyped with cocal, VSV-G, and a modified feline endogenous retrovirus envelope glycoprotein (RD114/TR). Cocal-pseudotyped lentiviral vectors can be produced at titers as high as with VSV-G, have a broad tropism, and are stable, allowing for efficient concentration by centrifugation. Additionally, cocal vectors are more resistant to inactivation by human serum than VSV-G-pseudotyped vectors, and efficiently transduce human CD34(+) nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse-repopulating cells (SRCs), and long-term primate hematopoietic repopulating cells. These studies establish the potential of cocal-pseudotyped lentiviral vectors for a variety of scientific and therapeutic gene transfer applications, including in vivo gene delivery and hematopoietic stem cell (HSC) gene therapy.

Reconstitution of the myeloid and lymphoid compartments after the transplantation of autologous and genetically modified CD34+ bone marrow cells, following gamma irradiation in cynomolgus macaques

Background

Prolonged, altered hematopoietic reconstitution is commonly observed in patients undergoing myeloablative conditioning and bone marrow and/or mobilized peripheral blood-derived stem cell transplantation. We studied the reconstitution of myeloid and lymphoid compartments after the transplantation of autologous CD34⁺ bone marrow cells following gamma irradiation in cynomolgus macaques.

Results

The bone marrow cells were first transduced ex vivo with a lentiviral vector encoding eGFP, with a mean efficiency of 72% ± 4%. The vector used was derived from the simian immunodeficiency lentivirus SIVmac251, VSV-g pseudotyped and encoded eGFP under the control of the phosphoglycerate kinase promoter. After myeloid differentiation, GFP was detected in colony-forming cells (37% ± 10%). A previous study showed that transduction rates did not differ significantly between colony-forming cells and immature cells capable of initiating long-term cultures, indicating that progenitor cells and highly immature hematopoietic cells were transduced with similar efficiency. Blood cells producingeGFP were detected as early as three days after transplantation, and eGFP-producing granulocyte and mononuclear cells persisted for more than one year in the periphery.

Conclusion

The transplantation of CD34⁺ bone marrow cells had beneficial effects for the ex vivo proliferation and differentiation of hematopoietic progenitors, favoring reconstitution of the T- and B-lymphocyte, thrombocyte and red blood cell compartments.

Gene therapy to inhibit xenoantibody production using lentiviral vectors in non-human primates

Xenoantibodies to the gal alpha1,3 gal (gal) epitope impede the use of pig tissues for xenotransplantation, a procedure that may help overcome the shortage of human organ donors. Stable gal chimerism and tolerance to gal(+) hearts could be achieved in alpha1,3-galactosyltransferase (alpha1,3GT)(-/-) mice using lentiviral vectors expressing porcine alpha1,3GT, the enzyme that synthesizes the gal carbohydrate. In this study, we evaluated whether chimerism sufficient to inhibit anti-gal xenoantibody responses can be achieved using lentivectors in non-human primates. Rhesus macaques were transplanted with autologous, alpha1,3GT-transduced bone marrow (BM) following sublethal irradation. Simian immunodeficiency virus (SIV)- and human immunodeficiency virus (HIV)-1-derived lentiviral constructs were compared. Chimerism was observed in several hematopoietic lineages in all monkeys. Engraftment in animals receiving SIV-based alpha1,3GT constructs was similar to that achieved using the HIV-1-derived lentivector for the first 2 months post-transplantation, but increased thereafter to reach higher levels by 5 months. Upon immunization with porcine hepatocytes, the production of anti-gal immunoglobulin M xenoantibody was substantially reduced in the gal(+) BM recipients compared to controls. This study is the first to report the application of gene therapy to achieve low-level, long-term gal chimerism sufficient to inhibit production of anti-gal antibodies after immunization with porcine cells in rhesus macaques.

High-level expression of porcine factor VIII from genetically modified bone marrow-derived stem cells

Clinical success for gene therapy of hemophilia A will be judged by achievement of sustained, therapeutic levels of coagulation factor VIII (fVIII). Previous clinical trials have suffered from transient, subtherapeutic expression of human fVIII transgenes. Porcine fVIII contains sequence elements that enable more efficient biosynthesis than human fVIII due to enhanced posttranslational transit through the secretory pathway. In this study, we evaluated ex vivo retroviral gene transfer of a high-expression porcine fVIII transgene into bone marrow-derived stromal and hematopoietic stem/progenitor cells (MSCs and HSCs, respectively) and transplantation into genetically immunocompetent hemophilia A mice. Both MSCs and HSCs demonstrated high-level expression of porcine fVIII in vivo. However, following transplantation of gene-modified MSCs, fVIII activity levels rapidly returned to baseline due to the formation of anti-porcine fVIII-neutralizing antibodies. Alternatively, transplantation of HSCs into myeloablated and nonmyeloablated hemophilia A mice resulted in high-level fVIII expression despite low-level hematopoietic reconstitution by gene-modified cells. FVIII expression was sustained beyond 10 months, indicating that immunologic tolerance to porcine fVIII was achieved. Furthermore, transplantation of bone marrow from primary recipients into naive secondary recipients resulted in sustained, high-level fVIII expression demonstrating successful genetic modification and engraftment of HSCs.

Oncolytic adenoviruses - selective retargeting to tumor cells

Virotherapy is an approach for the treatment of cancer, in which the replicating virus itself is the anticancer agent. Virotherapy exploits the lytic property of virus replication to kill tumor cells. As this approach relies on viral replication, the virus can self-amplify and spread in the tumor from an initial infection of only a few cells. The success of this approach is fundamentally based on the ability to deliver the replication-competent viral genome to target cells with a requisite level of efficiency. With virotherapy, while a number of transcriptional retargeting strategies have been utilized to restrict viral replication to tumor cells, this review will focus primarily on transductional retargeting strategies, whereby oncolytic viruses can be designed to selectively infect tumor cells. Using the adenoviral vector paradigm, there are three broad strategies useful for viral retargeting. One strategy uses heterologous retargeting ligands that are bispecific in that they bind both to the viral vector as well as to a cell surface target. A second strategy uses genetically modified viral vectors in which a cellular retargeting ligand is incorporated. A third strategy involves the construction of chimeric recombinant vectors, in which a capsid protein from one virus is exchanged for that of another. These transductional retargeting strategies have the potential for reducing deleterious side effects, and increasing the therapeutic index of virotherapeutic agents.

Gene transfer to repopulating human CD34+ cells using amphotropic-, GALV-, or RD114-pseudotyped HIV-1-based vectors from stable producer cells

A novel, stable human immunodeficiency virus type 1 vector packaging system, STAR, was tested for its ability to transduce human cord blood CD34+ progenitor cells assayed both in vitro and after transplantation into NOD/SCID mice. Vectors pseudotyped with three different gammaretrovirus envelopes were used: the amphotropic MLV envelope (MLV-A), a modified gibbon ape leukemia virus envelope (GALV+), and a modified feline endogenous virus RD114 envelope (RDpro). Gene transfer to freshly thawed CD34+ cells in the absence of cytokines was very low. Addition of cytokines increased gene transfer efficiency significantly and this was further augmented if the cells were prestimulated for 24 h. Concentration of the vectors (15-fold) by low-speed centrifugation increased gene transfer to CD34+ cells in vitro even further. More than 90% of cells were transduced with a single exposure to the RDpro vector as determined by GFP expression using flow cytometry. The two other pseudotypes transduced approximately 65-70% of the cells under the same conditions. Transplantation of CD34+ cells prestimulated for 24 h and then transduced with a single exposure to concentrated vector revealed that the RDpro vector transduced 55.1% of NOD/SCID repopulating human cells, which was significantly higher than the MLV-A (12.6%)- or GALV+ (25.1%)-pseudotyped vectors.

Improved transduction of human sheep repopulating cells by retrovirus vectors pseudotyped with feline leukemia virus type C or RD114 envelopes

Gene therapy for hematopoietic diseases has been hampered by the low frequency of transduction of human hematopoietic stem cells (HSCs) with retroviral vectors pseudotyped with amphotropic envelopes. We hypothesized that transduction could be increased by the use of retroviral vectors pseudotyped with envelopes that recognize more abundant cellular receptors. The levels of mRNA encoding the receptors of the feline retroviruses, RD114 and feline leukemia virus type C (FeLV-C), were significantly higher than the level of gibbon ape leukemia virus (GaLV) receptor mRNA in cells enriched for human HSCs (Lin- CD34+ CD38-). We cotransduced human peripheral blood CD34+ cells with equivalent numbers of FeLV-C and GALV or RD114 and GALV-pseudotyped retroviruses for injection into fetal sheep. Analysis of DNA from peripheral blood and bone marrow from recipient sheep demonstrated that FeLV-C- or RD114-pseudotyped vectors were present at significantly higher levels than GALV-pseudotyped vectors. Analysis of individual myeloid colonies demonstrated that retrovirus vectors with FeLV-C and RD114 pseudotypes were present at 1.5 to 1.6 copies per cell and were preferentially integrated near known genes We conclude that the more efficient transduction of human HSCs with either FeLV-C- or RD114-pseudotyped retroviral particles may improve gene transfer in human clinical trials.

Long-Term Clinical and Molecular Follow-up of Large Animals Receiving Retrovirally Transduced Stem and Progenitor Cells: No Progression to Clonal Hematopoiesis or Leukemia

There has been significant progress toward clinically relevant levels of retroviral gene transfer into hematopoietic stem cells (HSC), and the therapeutic potential of HSC-based gene transfer has been convincingly demonstrated in children with severe combined immunodeficiency syndrome (SCID). However, the subsequent development of leukemia in two children with X-linked SCID who were apparently cured after transplantation of retrovirally corrected CD34+ cells has raised concerns regarding the safety of gene therapy approaches utilizing integrating vectors. Nonhuman primates and dogs represent the best available models for gene transfer safety and efficacy and are particularly valuable for evaluation of long-term effects. We have followed 42 rhesus macaques, 23 baboons, and 17 dogs with significant levels of gene transfer for a median of 3.5 years (range 1-7) after infusion of CD34+ cells transduced with retroviral vectors expressing marker or drug-resistance genes. None developed abnormal hematopoiesis or leukemia. Integration site analysis confirmed stable, polyclonal retrovirally marked hematopoiesis, without progression toward mono- or oligoclonality over time. These results suggest that retroviral integrations using replication-incompetent vectors, at copy numbers achieved using standard protocols, are unlikely to result in leukemogenesis and that patient- or transgene-specific factors most likely contributed to the occurrence of leukemia in the X-SCID gene therapy trial.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.