Replication-defective chimeric helper proviruses and factors affecting generation of competent virus: expression of Moloney murine leukemia virus structural genes via the metallothionein promoter

Screening Clinical Cell Products for Replication Competent Retrovirus: The National Gene Vector Biorepository Experience

Replication-competent retrovirus (RCR) is a safety concern for individuals treated with retroviral gene therapy. RCR detection assays are used to detect RCR in manufactured vector, transduced cell products infused into research subjects, and in the research subjects after treatment. In this study, we reviewed 286 control (n = 4) and transduced cell products (n = 282) screened for RCR in the National Gene Vector Biorepository. The transduced cell samples were submitted from 14 clinical trials. All vector products were previously shown to be negative for RCR prior to use in cell transduction. After transduction, all 282 transduced cell products were negative for RCR. In addition, 241 of the clinical trial participants were also screened for RCR by analyzing peripheral blood at least 1 month after infusion, all of which were also negative for evidence of RCR infection. The majority of vector products used in the clinical trials were generated in the PG13 packaging cell line. The findings suggest that screening of the retroviral vector product generated in PG13 cell line may be sufficient and that further screening of transduced cells does not provide added value.

Absence of Replication-Competent Lentivirus in the Clinic: Analysis of Infused T Cell Products

Exposure to replication-competent lentivirus (RCL) is a theoretical safety concern for individuals treated with lentiviral gene therapy. For certain ex vivo gene therapy applications, including cancer immunotherapy trials, RCL detection assays are used to screen the vector product as well as the vector-transduced cells. In this study, we reviewed T cell products screened for RCL using methodology developed in the National Gene Vector Biorepository. All trials utilized third-generation lentiviral vectors produced by transient transfection. Samples from 26 clinical trials totaling 460 transduced cell products from 375 subjects were evaluated. All cell products were negative for RCL. A total of 296 of the clinical trial participants were screened for RCL at least 1 month after infusion of the cell product. No research subject has shown evidence of RCL infection. These findings provide further evidence attesting to the safety of third-generation lentiviral vectors and that testing T cell products for RCL does not provide added value to screening the lentiviral vector product.

Development of a cytokine-modified allogeneic whole cell pancreatic cancer vaccine

Management of patients with pancreatic cancer is a multidisciplinary approach that presents enormous challenges to the clinician. Overall 5-year survival for all patients remains <3%. Symptoms of early pancreas cancer are nonspecific. As such, only a fraction of patients are candidates for surgery. While surgical resection provides the only curative option, most patients will develop tumor recurrence and die of their disease. To date, the clinical benefits of chemotherapy and radiation therapy have been important but have led to modest improvements. Tumor vaccines have the potential to specifically target the needle of pancreas cancer cells amidst the haystack of normal tissue. The discovery of pancreas tumor-specific antigens and the subsequent ability to harness this technology has become an area of intense interest for tumor immunologists and clinicians alike. Without knowledge of specific antigen targets, the whole tumor cell represents the best source of immunizing antigens. This chapter will focus on the development of whole tumor cell vaccine strategies for pancreas cancer.

Replication-competent lentivirus analysis of clinical grade vector products

Lentiviral vectors are now in clinical trials for a variety of inherited and acquired disorders. A challenge for moving any viral vector into the clinic is the ability to screen the vector product for the presence of replication-competent virus. Assay development for replication-competent lentivirus (RCL) is particularly challenging because recombination of vector packaging plasmids and cellular DNA leading to RCL has not been reported with the current viral vector systems. Therefore, the genomic structure of a RCL remains theoretical. In this report, we describe a highly sensitive RCL assay suitable for screening vector product and have screened large-scale vector supernatant, cells used in vector production, and cells transduced with clinical grade vector. We discuss the limitations and challenges of the current assay, and suggest modifications that may improve the suitability of this assay for screening US Food and Drug Administration (US FDA)-licensed products.

A murine leukemia virus with Cre-LoxP excisible coding sequences allowing superinfection, transgene delivery, and generation of host genomic deletions

BACKGROUND

To generate a replication-competent retrovirus that could be conditionally inactivated, we flanked the viral genes of the Akv murine leukemia virus with LoxP sites. This provirus can delete its envelope gene by LoxP/Cre mediated recombination and thereby allow superinfection of Cre recombinase expressing cells.

RESULTS

In our studies, the virus repeatedly infected the cell and delivered multiple copies of the viral genome to the host genome; the superinfected cells expressed a viral transgene on average twenty times more than non-superinfected cells. The insertion of multiple LoxP sites into the cellular genome also led to genomic deletions, as demonstrated by comparative genome hybridization.

CONCLUSION

We envision that this technology may be particularly valuable for delivering transgenes and/or causing deletions.

Stable alphavirus packaging cell lines for Sindbis virus and Semliki Forest virus-derived vectors

Alphavirus vectors are being developed for possible human vaccine and gene therapy applications. We have sought to advance this field by devising DNA-based vectors and approaches for the production of recombinant vector particles. In this work, we generated a panel of alphavirus vector packaging cell lines (PCLs). These cell lines were stably transformed with expression cassettes that constitutively produced RNA transcripts encoding the Sindbis virus structural proteins under the regulation of their native subgenomic RNA promoter. As such, translation of the structural proteins was highly inducible and was detected only after synthesis of an authentic subgenomic mRNA by the vector-encoded replicase proteins. Efficient production of biologically active vector particles occurred after introduction of Sindbis virus vectors into the PCLs. In one configuration, the capsid and envelope glycoproteins were separated into distinct cassettes, resulting in vector packaging levels of 10(7) infectious units/ml, but reducing the generation of contaminating replication-competent virus below the limit of detection. Vector particle seed stocks could be amplified after low multiplicity of infection of PCLs, again without generating replication-competent virus, suggesting utility for production of large-scale vector preparations. Furthermore, both Sindbis virus-based and Semliki Forest virus-based vectors could be packaged with similar efficiency, indicating the possibility of developing a single PCL for use with multiple alphavirus-derived vectors.

Retroviral vectors. From laboratory tools to molecular medicine

The majority of clinical trials for gene therapy currently employ retroviral-mediated gene delivery. This is because the life cycle of the retrovirus is well understood and can be effectively manipulated to generate vectors that can be efficiently and safely packaged. Here, we review the molecular technology behind the generation of recombinant retroviral vectors. We also highlight the problems associated with the use of these viruses as gene therapy vehicles and discuss future developments that will be necessary to maintain retroviral vectors at the forefront of gene transfer technology.

Advanced mammalian gene transfer: high titre retroviral vectors with multiple drug selection markers and a complementary helper-free packaging cell line

We report the development of an advanced system for transfer and expression of exogenous genes in mammalian cells based on Moloney murine leukemia virus (Mo MuLV). Extensive deletion/mutagenesis analysis to identify cis-acting signals involved in virus transmission has led to the design of a family of novel, highly efficient retroviral vectors and a partner helper-free packaging cell line. The pBabe retroviral vector constructs transmit inserted genes at high titres and express them from the Mo MuLV Long Terminal Repeat (LTR). Each of these vectors has been constructed with one of four different dominantly acting selectable markers, allowing the growth of infected mammalian cells in the presence of G418, hygromycin B, bleomycin/phleomycin or puromycin, respectively. The high titre ecotropic helper free packaging cell line, omega E, was designed in conjunction with the pBabe vectors to reduce the risk of generation of wild type Mo MuLV via homologous recombination events. The omega E cell line was generated with separate gagpol and ecotropic env expression constructs with minimal sequence overlap and decreased sequence homology achieved by 'codon wobbling'. Homologous env coding sequences were deleted from the pBabe vectors without diminishing recombinant vector titre. Together, the pBabe vectors and omega E cell line should prove useful in experiments where highest frequencies of gene transfer, or concomitant expression of several different genes within a single cell are required with minimal risk of helper virus contamination.

A new avian leukosis virus-based packaging cell line that uses two separate transcomplementing helper genomes

An avian leukosis virus-based packaging cell line was constructed from the genome of the Rous-associated virus type 1. The gag, pol, and env genes were separated on two different plasmids; the packaging signal and the 3' long terminal repeat were removed. On a plasmid expressing the gag and pol genes, the env gene was replaced by the hygromycin resistance gene. The phleomycin resistance gene was inserted in the place of the gag-pol genes on a plasmid expressing the env gene. The plasmid containing the gag, pol, and Hygror genes was transfected into QT6 cells. Clones that produced high levels of p27gag were transfected with the plasmid containing the Phleor and env genes. Clones that produced high levels of env protein (as measured by an interference assay) were tested for their ability to package NeoR-expressing replication-defective vectors (TXN3'). One of the clones (Isolde) was able to transfer the Neo+ phenotype to recipient cells at a titer of 10(5) resistance focus-forming units per ml. Titers of supernatants of cells infected with Rous-associated virus type 1 prior to transfection by Neor vectors were similar. Tests for recombination events that might result in intact helper virus showed no evidence for the generation of replication-competent virus. The use of selectable genes inserted next to the viral genes to generate high-producer packaging cell lines is discussed.

A variant Schmidt-Ruppin strain of Rous sarcoma virus with increased affinity for mammalian cells

SR-RSV-D(H), a variant virus with extremely high tropism for mammalian cells, was isolated by passage of the Schmidt-Ruppin strain of Rous sarcoma virus of subgroup D (SR-RSV-D) through hamster cells. This variant virus has acquired an altered envelope glycoprotein, encoded by the env gene, that has high affinity for receptors on the surface of mammalian cells. The variant virus transforms rat cells at about 100 times the efficiency of the parental virus, SR-RSV-D(S), as assayed by focus formation. Addition of amphotericin B (Fungizone) to the medium at a concentration of 0.2 micrograms/ml completely inhibited rat cell transformation by SR-RSV-D(H), possibly by blocking virus penetration into the cells, whereas the drug showed no inhibitory effect on transformation of chick embryo fibroblast (CEF) cells by the variant virus or on transformation of rat cells by the parental virus. The efficiency of transformation of rat cells by the variant virus was much less than its efficiency of transformation of CEF cells. Analysis of infection of rat cells suggested that the virus can infect rat cells as efficiently as CEF cells but that rat cells were not transformed by the virus as fully as CEF cells because of inefficiency of some post-penetrational step involved in viral gene expression. The finding that E1AY cells, rat cells expressing adenovirus E1A gene, were transformed by SR-RSV-D(H) as efficiently as CEF cells supports this conclusion and suggests that expression of the E1A gene in rat cells may overcome the defect in the transforming step(s) in rat cells.

New retrovirus helper cells with almost no nucleotide sequence homology to retrovirus vectors

We prepared retrovirus packaging cell lines containing gag-pol genes from spleen necrosis virus (expressed from a cytomegalovirus promoter and the simian virus 40 (SV40) polyadenylation sequences) and, on a separate vector, either the env gene from spleen necrosis virus (expressed from the Rous sarcoma virus promoter and the SV40 polyadenylation sequences) or the env gene from amphotropic murine leukemia virus (expressed from a cytomegalovirus promoter and the SV40 polyadenylation sequences). The nucleotide sequences in these packaging cell lines have almost no homology to the retrovirus vectors we used. Retrovirus vectors were produced from these new helper cell lines without any genetic interactions between the vectors and sequences in the helper cells and without transfer of the packaging sequences.

Replication-defective vectors of reticuloendotheliosis virus transduce exogenous genes into somatic stem cells of the unincubated chicken embryo

Replication-defective vectors derived from reticuloendotheliosis virus were used to transduce exogenous genes into early somatic stem cells of the chicken embryo. One of these vectors transduced and expressed the chicken growth hormone coding sequence. The helper cell line, C3, was used to generate stocks of vector containing about 10(4) transducing units per ml. Injection of 5- to 20-microliters volumes of vector directly beneath the blastoderm of unincubated chicken embryos led to infection of somatic stem cells. Infected embryos and adults contained unrearranged integrated proviral DNAs. Embryos expressed the transduced chicken growth hormone gene and contained high levels of serum growth hormone. Blood, brain, muscle, testis, and semen contained from individuals injected as embryos contained vector DNA. Replication-defective vectors of the reticuloendotheliosis virus transduced exogenous genes into chicken embryonic stem cells in vivo.

Safe and efficient generation of recombinant retroviruses with amphotropic and ecotropic host ranges

We have constructed a set of packaging cell lines useful for the generation of helper-free recombinant retroviruses with amphotropic and ecotropic host ranges. To eliminate the problems of transfer of packaging functions and helper virus formation encountered with the previously available packaging systems, two mutant Moloney murine leukemia virus-derived proviral genomes carrying complementary mutations in the gag-pol or env regions were sequentially introduced into NIH 3T3 cells by cotransformation. Both genomes contained a deletion of the psi sequence necessary for the efficient encapsidation of retroviral genomes into virus particles and additional alterations at the 3' end of the provirus. We show that the resulting packaging cell lines psi CRIP and psi CRE can be used to isolate clones that stably produce high titers (10(6) colony-forming units/ml) of recombinant retroviruses with amphotropic and ecotropic host ranges, respectively. More importantly, we demonstrate that viral producers derived from the packaging cell lines do not transfer the packaging functions, or yield helper virus, even under conditions where existing packaging cell lines can be shown to yield transfer of packaging functions and/or helper virus. These properties of the psi CRIP and psi CRE packaging lines make them particularly valuable reagents for in vivo gene transfer studies aimed at cell lineage analysis and the development of human gene replacement therapies.

A safe packaging line for gene transfer: separating viral genes on two different plasmids

A retrovirus packaging cell line was constructed by using portions of the Moloney murine leukemia virus in which the gag, pol, and env genes of the helper virus were separated onto two different plasmids and in which the psi packaging signal and 3' long terminal repeat were removed. The plasmid containing the gag and pol genes and the plasmid containing the env gene were cotransfected into NIH 3T3 cells. Clones that produced high levels of reverse transcriptase and env protein were tested for their ability to package the replication-defective retrovirus vectors delta neo and N2. One of the gag-pol and env clones (GP+E-86) was able to transfer G418 resistance to recipient cells at a titer of as high as 1.7 X 10(5) when it was used to package delta neo and as high as 4 X 10(6) when it was used to package N2. Supernatants of clones transfected with the intact parent gag-pol-env plasmid 3P0 had comparable titers (as high as 6.5 X 10(4) with delta neo; as high as 1.7 X 10(5) with N2). Tests for recombination events that might result in intact retrovirus showed no evidence for the generation of replication-competent virus. These results suggest that gag, pol, and env, when present on different plasmids, may provide an efficient and safe packaging line for use in retroviral gene transfer.