Infection of human cells with murine amphotropic replication-competent retroviruses

Product Enhanced Reverse Transcriptase for assessing replication competent virus in vectors retroviral vectors pseudotyped with GALV and VSV-G envelopes

We evaluated the use of the Product Enhanced Reverse Transcriptase (PERT) assay as a means of detecting virus in retroviral vectors products pseudotyped with Gibbon Ape Leukemia Virus (GALV) and Vesicular Stomatitis Virus G (VSVG) envelopes. PERT provides greater standardization than the S+/L- assay which has been used extensively in virus detection. A challenge is that PERT will also detect residual retroviral vectors as vector particles contain reverse transcriptase. Vector products were cultured for 3 weeks on HEK293 cells to amplify any potential virus. In addition, vector supernatant and end-of-production cells were spiked with GALV to evaluate for inhibition by the test article. Results of PERT and the S+/L- assay were compared. PERT and S+/L- assays were both effective in detecting virus. Vector supernatants were negative at the end of 3 weeks of culture by PERT for both GAVL and VSVG pseudotyped vector. In contrast, end-of-production cells were positive by PERT due to persistent vector producing cells. A one-week culture of cell-free media obtained at the 3 weeks timepoint allowed distinction of virus-free test articles from those with virus. The PERT assay is suitable for detecting replication competent retrovirus in vector products pseudotyped with GALV and VSVG envelopes.

Meeting FDA Guidance recommendations for replication-competent virus and insertional oncogenesis testing

Integrating vectors are associated with alterations in cellular function related to disruption of normal gene function. This has been associated with clonal expansion of cells and, in some instances, cancer. These events have been associated with replication-defective vectors and suggest that the inadvertent exposure to a replication-competent virus arising during vector manufacture would significantly increase the risk of treatment-related adverse events. These risks have led regulatory agencies to require specific monitoring for replication-competent viruses, both prior to and after treatment of patients with gene therapy products. Monitoring the risk of cell expansion and malignancy is also required. In this review, we discuss the rational potential approaches and challenges to meeting the US FDA expectations listed in current guidance documents.

Replication competent retrovirus testing (RCR) in the National Gene Vector Biorepository: No evidence of RCR in 1,595 post-treatment peripheral blood samples obtained from 60 clinical trials

The clinical impact of any therapy requires the product be safe and effective. Gammaretroviral vectors pose several unique risks, including inadvertent exposure to replication competent retrovirus (RCR) that can arise during vector manufacture. The US FDA has required patient monitoring for RCR, and the National Gene Vector Biorepository is an NIH resource that has assisted eligible investigators in meeting this requirement. To date, we have found no evidence of RCR in 338 pre-treatment and 1,595 post-treatment blood samples from 737 patients associated with 60 clinical trials. Most samples (75%) were obtained within 1 year of treatment, and samples as far out as 9 years after treatment were analyzed. The majority of trials (93%) were cancer immunotherapy, and 90% of the trials used vector products produced with the PG13 packaging cell line. The data presented here provide further evidence that current manufacturing methods generate RCR-free products and support the overall safety profile of retroviral gene therapy.

Replication-Competent Lentivirus Analysis of Vector-Transduced T Cell Products Used in Cancer Immunotherapy Clinical Trials

Lentiviral vectors are being used in a growing number of clinical applications, including T cell immunotherapy for cancer. As this new technology moves forward, a safety concern is the inadvertent recombination and subsequent development of a replication-competent lentivirus (RCL) during the manufacture of the vector material. To assess this risk, regulators have required screening of T cell products infused into patients for RCL. Since vector particles have many of the proteins and nucleotide sequences found in RCL, a biologic assay has proven the most sensitive method for RCL detection. As regulators have required screening of up to 10⁸ cells per T cell product, this method described a procedure for assessing RCL contamination of large-volume T cell products.

The National Gene Vector Biorepository: Eleven Years of Providing Resources to the Gene Therapy Community

The National Gene Vector Biorepository (NGVB) program has been highly accessed by gene therapy investigators. The reagent repository has distributed over 1,000 reagents to 397 investigators. The Pharmacology/Toxicology Archive contains over 36,000 specimens from a variety of adeno-associated virus (AAV), adenoviral, and other pharmacology/toxicology studies. NGVB also maintains a searchable database of gene therapy pharmacology/toxicology studies to promote data sharing. NGVB has provided Food and Drug Administration (FDA)-mandated replication-competent virus testing for over 70 clinical trials. From 2008 to 2018, there have been 114 publications acknowledging the NGVB. It is unlikely that any other National Institutes of Health (NIH)-funded program has served as many gene therapy investigators as the NGVB.

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.

MicroRNA 142-3p attenuates spread of replicating retroviral vector in hematopoietic lineage-derived cells while maintaining an antiviral immune response

We are developing a retroviral replicating vector (RRV) encoding cytosine deaminase as an anticancer agent for gliomas. Despite its demonstrated natural selectivity for tumors, and other safety features, such a virus could potentially cause off-target effects by productively infecting healthy tissues. Here, we investigated whether incorporation of a hematopoietic lineage-specific microRNA target sequence in RRV further restricts replication in hematopoietic lineage-derived human cells in vitro and in murine lymphoid tissues in vivo. One or four copies of a sequence perfectly complementary to the guide strand of microRNA 142-3p were inserted into the 3' untranslated region of the RRV genome expressing the transgene encoding green fluorescent protein (GFP). Viral spread and GFP expression of these vectors in hematopoietic lineage cells in vitro and in vivo were measured by qPCR, qRT-PCR, and flow cytometry. In hematopoietic lineage-derived human cell lines and primary human stimulated peripheral blood mononuclear cells, vectors carrying the 142-3pT sequence showed a remarkable decrease in GFP expression relative to the parental vector, and viral spread was not observed over time. In a syngeneic subcutaneous mouse tumor model, RRVs with and without the 142-3pT sequences spread equally well in tumor cells; were strongly repressed in blood, bone marrow, and spleen; and generated antiviral immune responses. In an immune-deficient mouse model, RRVs with 142-3pT sequences were strongly repressed in blood, bone marrow, and spleen compared with unmodified RRV. Tissue-specific microRNA-based selective attenuation of RRV replication can maintain antiviral immunity, and if needed, provide an additional safeguard to this delivery platform for gene therapy applications.

Developing novel lentiviral vectors into clinical products

Gene therapy vectors based on murine retroviruses have now been in clinical trials for over 20 years. During that time, a variety of novel vector pseudotypes were developed in an effort to improve gene transfer. Lentiviral vectors are now in clinical trials and a similar evolution of vector technology is anticipated. These modifications present challenges for those producing large-scale clinical materials. This chapter discusses approaches to process development for novel lentiviral vectors, highlight considerations, and methods to be incorporated into the development schema.

General principles of retrovirus vector design

An understanding in the life cycle of γ-retroviruses has led to significant progress in the development of murine leukemia virus (MLV)-based vectors for gene delivery and human gene therapy. An MLV-based vector consists of the cis-acting sequences important for viral replication and gene expression. The sequence that encodes viral proteins is replaced with the gene of interest. To generate infectious retroviral vectors, viral-encoded proteins are supplied in trans for virion assembly. Here, we describe a method to rapidly generate MLV vectors from transiently transfected human 293T cells. The strategies to purify and titer the vector and to detect the presence of replication competent retrovirus (RCR) in the vector harvest are also described.

A reporter system for replication-competent gammaretroviruses: the inGluc-MLV-DERSE assay

While novel retroviral vectors for use in gene-therapy products are reducing the potential for formation of replication-competent retrovirus (RCR), it remains crucial to screen products for RCR for both research and clinical purposes. For clinical grade gammaretrovirus-based vectors, RCR screening is achieved by an extended S+L− or marker rescue assay, while standard methods for replication-competent lentivirus detection are still in development. In this report, we describe a rapid and sensitive method for replication-competent gammaretrovirus detection. We used this assay to detect three members of the gammaretrovirus family and compared the sensitivity of our assay with well-established methods for retrovirus detection, including the extended S+L− assay. Results presented here demonstrate that this assay should be useful for gene-therapy product testing.

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.

Detection of replication competent retrovirus and lentivirus

Retroviral vectors based on murine leukemia viruses (MuLV) have been used in clinical investigations for over a decade. Alternative retroviruses, most notably vectors based on HIV-1 and other lentiviruses, are now entering into clinical trials. Although vectors are designed to be replication defective, recombination events during vector production could lead to the generation of replication competent retroviruses (RCR) or replication competent lentiviruses (RCL). Careful screening of vector prior to human use must insure that patients are not inadvertently exposed to RCR or RCL. We describe methods capable of detecting low levels of virus contamination and discuss the current regulatory guidelines for screening gene therapy products intended for human use.

Retroviral vectors for gene transfer

INTRODUCTIONRetroviral vectors from the γ-retrovirus genus were the first retroviral vectors to be developed. They have been called oncoretroviral vectors or simple retroviral vectors because of their derivation from oncogenic retroviruses having a simple gag-pol-env genome structure. Later additions to the retroviral vector family include the lentiviral and foamy viral vectors derived from more complex retroviruses that contain multiple accessory genes in addition to the standard gag-pol-env genes. This article describes the advantages and disadvantages of retroviral vectors for gene therapy. It also discusses the issues that must be considered in designing retroviral vectors and in choosing retroviral packaging cell lines.

Retrovirus molecular conjugates: a versatile and efficient gene transfer vector system for primitive human hematopoietic progenitor cells

In principle, transient nongenetic modification of a noninfectious gene transfer virus enabling a one time infection and transduction of human cells could eliminate the risk of formation of replication competent virus. Formation of a molecular conjugate vector by conjugation of noninfective ecotropic murine Moloney leukemia virus to polylysine (eMMLV-PL) enabled high-efficiency transduction of human HPC using in vitro and in vivo assays. Xenotransplanted NOD-SCID mice durably expressed the transgene in human leukocytes and human progenitor cells with eMMLV-PL achieving three-fold increased transduction efficiency when directly compared to optimized amphotropic MMLV (aMMLV) transduction. Both aMMLV and eMMLV assembled conjugate vectors showed similar transduction efficiency indicating predominant polylysine-mediated uptake. Integration of retroviral sequences was determined from individual human HPC recovered from eMMLV-PL-xenotransplanted animals. This simple and versatile concept of conjugate gene transfer vectors has the potential to enhance transduction efficiency as well as to improve certain safety aspects of human gene therapy. Moreover, because it permits effective cellular internalization of particles, this concept of molecular conjugates can be used as research tool to investigate the interactions of otherwise noninfectious viruses or modified viral particles at the genomic level.

A pilot study of dose-intensified procarbazine, CCNU, vincristine for poor prognosis brain tumors utilizing fibronectin-assisted, retroviral-mediated modification of CD34+ peripheral blood cells with O6-methylguanine DNA methyltransferase

Administration of chemotherapy is often limited by myelosuppression. Expression of drug-resistance genes in hematopoietic cells has been proposed as a means to decrease the toxicity of cytotoxic agents. In this pilot study, we utilized a retroviral vector expressing methylguanine DNA methyltransferase (MGMT) to transduce hematopoietic progenitors, which were subsequently used in the setting of alkylator therapy (procarbazine, CCNU, vincristine (PCV)) for poor prognosis brain tumors. Granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood progenitor cells were collected by apheresis and enriched for CD34+ expression. Nine subjects were infused with CD34+-enriched cells treated in a transduction procedure involving a 4-day exposure to cytokines with vector exposure on days 3 and 4. No major adverse event was related to the gene therapy procedure. Importantly, the engraftment kinetics of the treated product was similar to unmanipulated peripheral blood stem cells, suggesting that the ex vivo manipulation did not significantly reduce engrafting progenitor cell function. Gene-transduced cells were detected in all subjects. Although the level and duration was limited, patients receiving cells transduced using fibronectin 'preloaded' with virus supernatant appeared to show improved in vivo marking frequency. These findings demonstrate the feasibility and safety of utilizing MGMT-transduced CD34+ peripheral blood progenitor cells in the setting of chemotherapy.

Retroviral vector production in the National Gene Vector Laboratory at Indiana University

The National Gene Vector Laboratory (NGVL) is a US National Institutes of Health initiative charged with providing clinical grade vectors for gene therapy trials. The program was started in 1995 and Indiana University has served as the production site for retroviral vectors and is also accepting applications for production of lentiviral vectors. The facility is designed to produce vectors for Phase I and Phase II clinical trials with the specific mandate to facilitate investigator-initiated research for academic institutions. To date, the facility has generated over 30 Master Cell Banks for gene therapy investigators throughout the United States. This required the facility to develop a system that can adapt to the varied needs of investigators, most of whom request different vector backbones, packaging cell lines, final product volumes, and media. In this review, we will illustrate some of the experiences of the Indiana University NGVL during the generation of retroviral vectors using murine-based packaging cell lines.

Product-Enhanced Reverse Transcriptase Assay for Replication-Competent Retrovirus and Lentivirus Detection

The product-enhanced reverse transcriptase (PERT) assay has been used to detect reverse transcriptase (RT) activity associated with retroviruses. Although the PERT assay has been proposed as a method for detection of replication-competent retrovirus (RCR) and lentivirus (RCL), it has not been rigorously compared with existing methods for RCR and RCL detection. We have assessed the PERT assay for detection of RCL and RCR that may contaminate lentiviral and retroviral vectors and compared it with published methods for RCL (p24gag ELISA/gag PCR) and RCR (S+/L-) detection. Our results suggest that the PERT assay is as sensitive as p24gag ELISA and gag PCR for detection of replication-competent HIV-1 in an RCL detection assay. Comparison of detection of replication-competent retroviruses, GALV and RD114, by extended S+/L- and PERT assays indicates that both assays can detect 1 IU of each virus. Our findings suggest that the PERT assay can be used for RCL and RCR testing of a variety of retroviral vectors regardless of the structure, sequence, and envelope of the vectors.

Human primary T lymphocytes have a low capacity to amplify MLV-based amphotropic RCR and the virions produced are largely noninfectious

The presence of replication-competent retrovirus (RCR) in retroviral-based gene therapy products poses a potential safety risk for patients. Therefore, RCR testing of clinical gene therapy products and monitoring of patients enrolled in gene therapy trials is required to assure viral safety. The requirement to test ex vivo-transduced cells originates from the presumed amplification of adventitious RCR during the transduction procedure. However, data on the capacity of different cell types to do so are lacking. In this study, we sought to analyze the amplification potential of primary human T lymphocytes after infection with amphotropic MLV-based RCR. The total number of viral particles produced after 1 or 2 weeks was measured by a quantitative 4070A env-specific RT-PCR assay. The fraction of infectious replication-competent viral particles was analyzed in the PG-4 S+L- assay. From this study, we conclude that the total number of viral particles RCR produced by T lymphocytes is 2-4 logs lower than the number produced by NIH-3T3 cells. Surprisingly, less than 1% of the viral particles produced by primary T lymphocytes appeared to be infectious, while nearly all virions produced by NIH-3T3 were. We conclude that primary human T lymphocytes are low producers of MLV-based amphotropic RCR.

Replication of enhancer-deficient amphotropic murine leukemia virus in human fibrosarcoma but not in primary human fibroblasts

Amphotropic murine leukemia virus (MLV) replicates in cells from various mammalian species including humans and is a potential contaminant in MLV vector preparations for human gene transfer studies. In general, MLV replication depends on the expression of viral genes under the control of 75 bp enhancer elements in the long terminal repeat. However, in specific human fibrosarcoma and lymphoma lines replication of amphotropic MLV is possible without these enhancers. Fibrosarcomas are malignant tumors of fibroblast origin. To test the replication potential of intact and enhancerless amphotropic MLV in untransformed cells, infection studies with these viruses were carried out in three types of primary human fibroblasts. Replication of amphotropic MLV is observed in two of three tested fibroblast strains. None of these primary human fibroblasts is permissive for enhancer-deficient MLV, suggesting that replication of this virus may be limited to transformed cells.

Detection of ecotropic replication-competent retroviruses: comparison of s(+)/l(-) and marker rescue assays

Guidelines for testing gene therapy products for ecotropic replication-competent retrovirus (Eco-RCR) have not been delineated as they have for amphotropic viruses. To evaluate biologic assays that can detect these viruses, we compared an S(+)/L(-) assay and a marker rescue assay designed specifically for Eco-RCR detection. Moloney murine leukemia virus (Mo-MuLV) obtained from the American Type Culture Collection was used as the positive control. For marker rescue, NIH 3T3 cells were transduced with a retroviral vector expressing the neomycin phosphotransferase gene (3T3/Neo). Inoculation and passage of test material in 3T3/Neo cells for 3 weeks (amplification) and subsequent testing in the S(+)/L(-) assay or the marker rescue assay increased the level of sensitivity for virus detection greater than 10-fold compared with direct inoculation of D56 S(+)/L(-) cells. When serial dilutions of Mo-MuLV stock were evaluated, six of six cultures had detectable virus by the S(+)/L(-) and marker rescue assays at dilutions of 10(-5) and 10(-6). At the 10(-7) dilution, five of six assays had detectable virus in both assays. The ability to detect virus-infected cells was also evaluated in a modification that substituted cells for supernatant. Fifteen 3T3/Neo cultures inoculated with 10(6) 293 cells containing 100 or 10 Mo-MuLV/3T3 cells were all positive by marker rescue. For dilution with 1 virus-infected cell per 10(6) 293 cells, 10 of 15 cultures were positive. At the 0.1-cell dilution only 2 of 15 cultures were positive. If we hope to detect one infected cell in a test article, the probability of detecting virus if the assay is performed in triplicate is 96.3%. In summary, after 3 weeks of amplification the S(+)/L(-) and marker rescue assays can detect virus with similar sensitivities. We prefer the marker rescue assay because of the more reliable growth features of NIH 3T3 cells compared with the D56 cell line. For laboratories analyzing clinical materials, this report may prove useful in establishing detection assays for Eco-RCR.

Amphotropic murine leukemia virus replication in human mammary epithelial cells and the formation of cytomegalovirus-promoter recombinants

Amphotropic murine leukemia virus (MLV) can replicate in human cells and is a potential contaminant in vector preparations for human gene transfer studies. We have recently shown that replication of amphotropic MLV in specific human sarcoma and lymphoma lines is possible in the absence of the viral 75-bp transcription enhancer elements. Here, we have tested the replication of an amphotropic MLV, MLV-(MOA), and an enhancer-deficient mutant of this virus in human breast carcinoma-derived cell lines. The proviral expression plasmids use a cytomegalovirus (CMV) promoter for the initial transcription of virus RNA. We found that all cells analyzed are permissive for replication of MLV-(MOA). Enhancer-deficient virus is unable to replicate. However, in two lines the replication defect can be rescued by the spontaneous insertion of a CMV promoter and enhancer into the U3 region. This recombinant virus MLV-(RCMV) replicates with kinetics similar to that of MLV-(MOA) but is restricted to specific cell lines. The potential formation of RCMV recombinants during MLV vector preparation must be considered.

Replication of enhancer-deficient amphotropic murine leukemia virus in human cells

Amphotropic murine leukemia virus (MLV) replicates in cells from various mammalian species, including humans, and is a potential contaminant in MLV vector preparations for human gene transfer studies. The generation of replication-competent virus is considered less likely with vectors that delete the viral transcription elements. This conclusion is based on data obtained in rodents, where MLV replication depends on the expression of viral genes under the control of 75-bp enhancer elements in the long terminal repeat. We demonstrate here that in some human cells replication of amphotropic MLV is possible in the absence of these enhancer elements. Replication of the enhancer-deficient virus MLV-(MOA)Delta E is observed in selected human sarcoma and B lymphoma lines and proceeds at a lower rate than that of the intact virus. No insertion of a foreign promoter or enhancer into the long terminal repeat was detected. Our data suggest the presence of a secondary enhancer element within the MLV provirus that can in selected human cells mediate virus transcription and replication in the absence of the 75-bp U3 enhancers.

Packaging cell line DNA contamination of vector supernatants: implication for laboratory and clinical research

Investigators conducting retroviral gene therapy trials are required to monitor for the presence of replication-competent retrovirus (RCR). The required testing utilizes a combination of biologic assays and molecular tests using PCR. In the course of a human clinical gene therapy trial, we detected 4070A viral envelope sequences in CD34(+) peripheral blood stem cells 2 days after transduction using a PCR-based assay, suggesting the presence of RCR. The supernatant and producer cells used for vector generation had been negative in extensive screening using the extended S(+)/L(-) assay. The presence of a replication-competent virus was subsequently excluded by a combination of biologic and PCR analyses. The source of the 4070A viral envelope sequences was determined to be packaging cell line DNA in the vector supernatant. The analysis of a variety of vector supernatants by quantitative real-time PCR revealed 4070A envelope DNA sequences from the packaging cell line in concentrations equivalent to approximately 50-500 focus-forming units per milliliter of wild-type 4070A virus. When PCR was performed after reverse transcriptase treatment of supernatant (i.e., assessing both RNA and DNA content), 4070A envelope sequence concentrations ranged from 10(2) to 3.5 x 10(3) focus-forming units per milliliter of wild-type 4070A virus. Our data indicate that PCR should not be used to analyze transduced cells for RCR within the first 2 weeks of vector exposure. Furthermore, investigators using PCR to analyze transduction efficiency shortly after vector exposure may experience false-positive findings.

Safety testing for replication-competent retrovirus associated with gibbon ape leukemia virus-pseudotyped retroviral vectors

The potential pathogenicity of replication-competent retroviruses (RCR) requires vigilant testing to exclude inadvertent contamination of clinical gene therapy vector products with RCR. Pseudotyped vectors using the gibbon ape leukemia virus (GALV) envelope have entered into clinical trials but specific recommendations regarding methods for screening of vector product and analysis of clinical samples have not been set forth. Unfortunately, current screening assays used for detecting amphotropic RCR are not suitable for GALV-pseudotyped RCR. We modified the extended S+/L- assay for RCR detection by using human 293 cells for virus amplification. Of five cell lines tested, 293 cells were selected because they combined a high transduction efficiency and an ability to generate RCR at high titer. After optimizing the amplification assay, a dilution of GALV virus could consistently be detected at a dilution of 10(-6). In coculture experiments, one GALV-infected cell could be consistently detected in 10(6) uninfected cells. A PCR-based assay was developed that was capable of detecting 100 copies of a GALV envelope containing plasmid diluted in 1 microg of DNA obtained from uninfected cells. PCR was also able to detect one GALV-infected cell in 10(6) uninfected cells. These assays will be suitable for testing of vector preparations and for monitoring of clinical samples from patients treated in clinical gene therapy protocols. The assays developed are similar in methodology and sensitivity to those currently used for certification of amphotropic retroviral vectors.

Efficient retrovirus-mediated transfer of the multidrug resistance 1 gene into autologous human long-term repopulating hematopoietic stem cells

Pre-clinical studies indicate that efficient retrovirus-mediated gene transfer into hematopoietic stem cells and progenitor cells can be achieved by co-localizing retroviral particles and target cells on specific adhesion domains of fibronectin. In this pilot study, we used this technique to transfer the human multidrug resistance 1 gene into stem and progenitor cells of patients with germ cell tumors undergoing autologous transplantation. There was efficient gene transfer into stem and progenitor cells in the presence of recombinant fibronectin fragment CH-296. The infusion of these cells was associated with no harmful effects and led to prompt hematopoietic recovery. There was in vivo vector expression, but it may have been limited by the high rate of aberrant splicing of the multidrug resistance 1 gene in the vector. Gene marking has persisted more than a year at levels higher than previously reported in humans.

Retrovirus-mediated, stable scavenger-receptor gene transfer leads to functional endocytotic receptor expression, foam cell formation, and increased susceptibility to apoptosis in rabbit aortic smooth muscle cells

The type II, class A macrophage scavenger receptor (SR-A) plays an important role in the pathogenesis of atherosclerosis and foam cell formation. However, its role in nonmacrophage cell lines remains unknown. To test the hypothesis that SR-A activity leads to proatherogenic changes in nonmacrophage cell lines, we generated Moloney murine leukemia virus- and vesicular stomatitis virus G protein-pseudotyped retroviruses containing SR-A type II cDNA, which were used for stable transfection of SR-A activity into mouse fibroblasts and rabbit aortic smooth muscle cells (SMCs). beta-Galactosidase-transfected cell lines were used as controls. Transfected cell lines expressed functional SR-A mRNA and protein. Expression of SR-A activity was stable for at least 9 months. By electron microscopy, transfected receptors were located in coated pits and in intracellular structures resembling endocytotic vesicles. Expression of SR-A on the cell surface was verified by flow cytometry and by uptake and degradation of (125)I-labeled acetylated low density lipoprotein (LDL). Increases of 5- to 25-fold and of 6- to 8-fold in the rate of acetylated LDL degradation were observed in transfected fibroblasts and SMCs, respectively, compared with beta-galactosidase-transfected control cell lines. Incubation of the transfected SMCs and fibroblasts with acetylated or oxidized LDL led to foam cell formation. Incubation with oxidized LDL also led to increased apoptosis and cell death. An altered morphology with increased cell size and granularity was observed in the most active SR-A SMC clones. It is concluded that stable overexpression of SR-A leads to foam cell formation and other proatherogenic changes in nonmacrophage cell lines. Stable SMC and fibroblast cell lines can be used as models for foam cell formation. The results also suggest that increased SR activity may play an important role in SMC-related pathology in atherosclerotic arteries.

Retrovirus-mediated transfer of human alpha-galactosidase A gene to human CD34+ hematopoietic progenitor cells

Fabry disease, caused by a deficiency of lysosomal enzyme alpha-galactosidase A (alpha-gal A), is one of the inherited disorders potentially treatable by gene transfer to hematopoietic stem cells. In this study, a high-titer amphotropic retroviral producer cell line, MFG-alpha-gal A, was established. CD34+ cells from normal umbilical cord blood were transduced by centrifugal enhancement. The alpha-gal A activity in transduced cells increased 3.6-fold above the activity in nontransduced cells. Transduction efficiency measured by PCR for the integrated alpha-gal A cDNA in CFU-GM colonies was in the range of 42-88% (average, 63%). The expression of functional enzyme in TFI erythroleukemia was sustained for as long as cells remained in culture (84 days) and for 28 days in LTC-IC cultures of CD34+ cells. The ability of the transduced CD34+ cells to secrete the enzyme and to correct enzyme-deficient Fabry fibroblasts was assessed by cocultivation of these cells. The enzyme was secreted into the medium from transduced CD34+ cells and taken up by Fabry fibroblasts through mannose 6-phosphate receptors. These findings suggest that genetically corrected hematopoietic stem/progenitor cells can be an enzymatic source for neighboring enzyme-deficient cells, and can potentially be useful for gene therapy of Fabry disease.

Human serum-resistant retroviral vector particles from galactosyl (alpha1-3) galactosyl containing nonprimate cell lines

Retroviral vector particles (RVP) which are resistant to inactivation by human serum will be needed for many in vivo gene therapy applications. Murine-based producer cell lines generate RVP which are inactivated by human serum, reportedly due to the presence of the galactosyl (alpha1-3) galactosyl carbohydrate moiety (alphaGal) on these and other nonprimate producer cells and RVP. Consequently, human cells (which lack the alphaGal moiety) have been developed as producer cell lines for generation of human serum-resistant RVP. In this study, we report that contrary to earlier reports, the presence of the alphaGal moiety on producer cells and RVP does not necessarily correlate with cell killing or RVP inactivation by human serum. We show that the alphaGal-positive ferret brain cell line, Mpf, is an excellent basal cell line for generation of RVP which have titers and serum resistance levels equal to or greater than RVP produced in human cell lines such as HT1080. Therefore, packaging cell lines need not be limited to those of human or primate origin for production of human serum-resistant RVP.

In vivo hepatocyte retrovirus-mediated gene transfer through the rat biliary tract

Delivering retroviruses targeted to hepatocytes in vivo involves the injection of retroviruses directly into the blood stream of the portal vein. The aim of this work was to delineate the conditions for delivering retroviruses in vivo by perfusing in situ the bile duct of the regenerating rat liver, and to study the hepatocyte transgene expression. At 24 hr after partial hepatectomy, during the S phase of the cell cycle, regenerating livers were perfused for 2.8+/-0.5 hr through the bile duct with 36.2+/-6.8 ml (0.3+/-01 ml/min) of fresh culture supernatant containing amphotropic recombinant retroviruses encoding the beta-galactosidase gene. The virus total titer was 1.5 x 10(8) ffu (group I) or 6.5 x 10(8) ffu (groups II and III). The hepatic artery blood flow was either maintained (groups I and II) or interrupted (group III) during bile duct perfusion. Liver biopsies taken 7 days later showed that 31.4+/-24.2% (group I), 58.7+/-23.6% (group II), and 45.1+/-21.4% (group III) of hepatocytes expressed beta-galactosidase activity, predominantly in the periportal and mediolobular zones. This study demonstrates that hepatocytes of regenerating rat livers that have entered the S phase of the cell cycle as a result of partial hepatectomy can be transduced in vivo by retroviral vectors delivered in situ by bile duct perfusion. Furthermore, the number of transduced hepatocytes closely correlated with the viral total titer and was diminished by hepatic artery blood flow occlusion during perfusion.

Diversity of HIV-1 Vpr interactions involves usage of the WXXF motif of host cell proteins

Targeting protein or RNA moieties to specific cellular compartments may enhance their desired functions and specificities. Human immunodeficiency virus type I (HIV-1) encodes proteins in addition to Gag, Pol, and Env that are packaged into virus particles. One such retroviral-incorporated protein is Vpr, which is present in all primate lentiviruses. Vpr has been implicated in different roles within the HIV-1 life cycle. In testing a new hypothesis in which viral proteins are utilized as docking sites to incorporate protein moieties into virions, we used the peptide phage display approach to search for Vpr-specific binding peptides. In the present studies, we demonstrate that most of the peptides that bind to Vpr have a common motif, WXXF. More importantly, we demonstrate that the WXXF motif of uracil DNA glycosylase is implicated in the interaction of uracil DNA glycosylase with Vpr intracellularly. Finally, a dimer of the WXXF motif was fused to the chloramphenicol acetyl transferase (CAT) gene, and it was demonstrated that the WXXF dimer-CAT fusion protein construct produces CAT activity within virions in the presence of Vpr as a docking protein. This study provides a novel potential strategy in the targeting of anti-viral agents to interfere with HIV-1 replication.

Protection of hematopoietic stem cells from chemotherapy-induced toxicity by multidrug-resistance 1 gene transfer

An increased chemotherapeutic dose intensity is believed to translate into higher survival rates among cancer patients. Pancytopenia is the dose-limiting toxic result of most anticancer agents. Overexpression of the human multidrug resistance 1 (MDR1) gene in transgenic animals resulted in complete myeloprotection against high doses of cytostatic drugs. Stem cell research, vector development, and experimental pharmacology are uniting their efforts in an attempt to achieve a similar effect in human hematopoietic stem cells. This article gives an overview of the crucial steps involved, from retroviral vector design and optimization of viral titers to vector uptake, gene integration, and expression. The authors' own results are presented with special regard in vitro and in vivo assays for the detection of hematopoietic stem cell transduction.

Adeno-associated virus type 2-mediated transfer of ecotropic retrovirus receptor cDNA allows ecotropic retroviral transduction of established and primary human cells

The cellular receptors that mediate binding and internalization of retroviruses have recently been identified. The concentration and accessibility of these receptors are critical determinants in accomplishing successful gene transfer with retrovirus-based vectors. Murine retroviruses containing ecotropic glycoproteins do not infect human cells since human cells do not express the receptor that binds the ecotropic glycoproteins. To enable human cells to become permissive for ecotropic retrovirus-mediated gene transfer, we have developed a recombinant adeno-associated virus type 2 (AAV) vector containing ecotropic retroviral receptor (ecoR) cDNA under the control of the Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter (vRSVp-ecoR). Established human cell lines, such as HeLa and KB, known to be nonpermissive for murine ecotropic retroviruses, became permissive for infection by a retroviral vector containing a bacterial gene for resistance to neomycin (RV-Neo(r)), with a transduction efficiency of up to 47%, following transduction with vRSVp-ecoR, as determined by the development of colonies that were resistant to the drug G418, a neomycin analog. No G418-resistant colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. Southern and Northern blot analyses revealed stable integration and long-term expression, respectively, of the transduced murine ecoR gene in clonal isolates of HeLa and KB cells. Similarly, ecotropic retrovirus-mediated Neo(r) transduction of primary human CD34+ hematopoietic progenitor cells from normal bone marrow was also documented, but only following infection with vRSVp-ecoR. The retroviral transduction efficiency was approximately 7% without prestimulation and approximately 14% with prestimulation of CD34+ cells with cytokines, as determined by hematopoietic clonogenic assays. No G418-resistant progenitor cell colonies were present in cultures infected with either vRSVp-ecoR or RV-Neo(r) alone. These results suggest that sequential transduction of primary human cells with two different viral vectors may overcome limitations encountered with a single vector. Thus, the combined use of AAV- and retrovirus-based vectors may have important clinical implications for ex vivo and in vivo human gene therapy.

Evaluation of recommendations for replication-competent retrovirus testing associated with use of retroviral vectors

With input from the gene therapy community, CBER is actively examining the recommendations for RCR testing during retroviral vector production, production of ex vivo-transduced cells, and in patients who receive such material. Our initial recommendations were made at a time when our experience with RCR detection assays and clinical use of retroviral vectors was limited. As the gene therapy field has matured, there is an increasing amount of data available on RCR detection assays and from monitoring of patients in clinical trials. The cumulative data give assurance that RCR detection assays in use are of sufficient sensitivity to provide a margin of safety to patients: no patients to date have evidence of RCR infection. However, CBER encourages members of the gene therapy community to continue to submit data to the FDA or to publish data that will enhance the cumulative data base on RCR testing assays, experience with different VPC, and patient monitoring. Based on the analysis of data accumulated to data, and ongoing discussions with members of the gene therapy community, CBER is proposing to discuss changes to the current RCR testing recommendations, as summarized below. RCR testing during production of retroviral vector and ex vivo-transduced cells. Development of characterized standards for RCR testing of supernatant and cells should allow comparison of assay sensitivity. One proposal under consideration is to apply statistical methods to determine how much material needs to be tested independent of the size of the production lot. Data and discussion are still needed to define a limit concentration and a value for probability of detection for RCR testing, while maintaining an appropriate margin of safety. These modifications of RCR testing strategies could lead to improvements in assay sensitivity. Additional discussion and data are also needed to evaluate the current recommendations of the testing for ex vivo-transduced cells: should both cells and supernatant be tested in all cases? RCR testing during patient follow-up. The time points required for RCR testing during patient follow-up need examination. One proposal under consideration is to sample and assay at three time points during the first year of treatment (e.g., 4-6 weeks, 3 months, and 1 year post-treatment). Further discussion is needed to define appropriate additional follow-up. Choice of assays to detect surrogate markers for RCR infection (i.e., serologic or PCR-based assays) should consider mode of vector administration and the patient population. Positive results with such assays should be pursued by direct culture assay to obtain and characterize the infectious viral isolate. These proposals will be the focal point for the discussion at the Retroviral Vector Breakout Session at the 1997 FDA/NIH Gene Therapy Conference. After the 1997 FDA/NIH Gene Therapy Conference, CBR plans to propose revised recommendations for RCR testing for public comment.

Endogenous murine leukemia virus DNA sequences in murine cell lines: implications for gene therapy safety testing by PCR

Safety testing for replication-competent retrovirus (RCR) is an important requirement in gene transfer clinical trials using retroviral vectors. A sensitive polymerase chain reaction (PCR) method is one approach to RCR detection. Only in the presence of RCR will the pol-env encoding sequences, necessary for viral replication and packaging, be amplified from proviral DNA in infected indicator cells. To avoid false-positive results in this assay it is crucial that indicator cell lines are free of endogenous retroviral sequences that could also be amplified with pol-env PCR primers. We screened candidate murine indicator cell lines and determined that while Mus dunni is free of detectable pol-env sequences, endogenous retroviral sequences do indeed exist in several cell lines and lead to false-positive results in the PCR assay for RCR. Furthermore, these endogenous retroviral sequences are expressed as RNA transcripts in NIH 3T3 and SC-1 cell lines, as determined by PCR amplification of cDNA but, nevertheless, do not give rise to replication-competent particles. We recognize the potential for murine cell lines to undergo spontaneous rearrangements of endogenous viral sequences in culture and give rise to recombinants containing newly acquired contiguous pol-env sequences. Indicator cell lines should thus be carefully selected and monitored on an ongoing basis when used in safety testing using PCR approaches for the detection of RCR.

The pCL vector system: rapid production of helper-free, high-titer, recombinant retroviruses

We describe the construction and characterization of retroviral vectors and packaging plasmids that produce helper-free retrovirus with titers of 1 X 10(6) to 5 X 10(6) within 48 h. These vectors contain the immediate early region of the human cytomegalovirus enhancer-promoter fused to the Moloney murine leukemia virus long terminal repeat at the TATA box in the 5' U3 region, yielding the pCL promoter. By selecting vectors designed to express genes from one of four promoters (dihydrofolate reductase, Rous sarcoma virus, long terminal repeat, or cytomegalovirus), the pCL system permits the investigator to control the level of gene expression in target cells over a 100-fold range, while maintaining uniformly high titers of virus from transiently transfected producer cells. The pCL packaging plasmids lack a packaging signal (delta-psi) and include an added safety modification that renders them self-inactivating through the deletion of the 3' U3 enhancer. Ecotropic, amphotropic (4070A), and amphotropic-mink cell focus-forming hybrid (10A1) envelope constructions have been prepared and tested, permitting flexible selection of vector pseudotype in accordance with experimental needs. Vector supernatants are free of helper virus and are of sufficiently high titer within 2 days of transient transfection in 293 cells to permit infection of more than 50% of randomly cycling target cells in culture. We demonstrated the efficacy of these vectors by using them to transfer three potent cell cycle control genes (the p16(INK4A), p53, and Rb1 genes) into human glioblastoma cells.

Retroviral gene transfer in autologous bone marrow transplantation for adult acute leukemia

To evaluate whether marrow contributes to relapse after autologous bone marrow transplantation (AuBMT) for acute leukemia, transplanted marrow was marked with the G1N retroviral vector (Genetic Therapy Inc.) containing the neomycin phosphotransferase gene (neo). Between April 1992 and August 1993, 4 patients were transplanted for acute myeloid leukemia (AML) in second complete remission (CR) and 1 patient for acute lymphoid leukemia in first CR. An average of 12.4% (range 5-19%) of transplanted marrow mononuclear cells were exposed to G1N vector for 4 hr. In the vector-treated portion of the marrow, 4.9% of GM-CFU and 3.6% of erythroid burst-forming units (BFU-E) were resistant to G418 in vitro. In the 5 patients, the polymerase chain reaction (PCR) detected the neo sequence on only two occasions after AuBMT. Of 4 patients surviving 1 year after transplantation, only 1 had evidence of gene marked cells by PCR. Two AML patients have relapsed, one of whom had evidence of neo sequences in the bone marrow at day 100 but not at relapse 11 months after AuBMT. The second patient relapsed 18 months after AuBMT but never had PCR evidence of neo sequences before or after relapse. Our results indicate vector-transduced autologous bone marrow from heavily pretreated adults with acute leukemia mark with low efficiency, although vector sequences have been detected in bone marrow and peripheral blood up to 1 year after transplant. Of the 2 relapsed patients, no evidence of vector-marked leukemic blasts have been detected.

Improved detection of replication-competent retrovirus

Recombinant retroviral vectors are the predominant delivery system in human gene therapy protocols. Since contaminating replication-competent retrovirus (RCR) can arise during the production of retroviral vector supernatants, sensitive assays for the screening of supernatants are necessary. In this study, we present a marker rescue assay based upon a Mus dunni cell line stably transduced with a lacZ gene. We show that detection of RCR in vector supernatants by the M. dunni lacZ marker rescue assay or PG-4 S+ L- focus-forming assay is equally sensitive. By inoculating test supernatants under centrifugation (which we term spinoculation), we increased the sensitivity of detection of RCR 10 to 100-fold with the PG-4 S+ L- and lacZ marker rescue assays. While the spinoculation protocol had no adverse effects on cells, spinoculation of high titer vector supernatants onto PG-4 cells resulted in some cytotoxicity, making identification of RCR positive cultures difficult. However, spinoculation of vector supernatants onto M. dunni lacZ cells resulted in no cytotoxicity, and also partially overcame inhibition of detection of low levels of RCR due to the presence of high titer replication-incompetent vector.

Intracellular immunization against HIV-1 infection of human T lymphocytes: utility of anti-rev single-chain variable fragments

Genetic therapy offers a potentially promising approach with which to combat human immunodeficiency virus type 1 (HIV-1) infections. Several modalities, using protein- and RNA-based systems, have recently been shown to inhibit HIV-1 replication. A single-chain variable fragment (SFv), constructed from the cDNA of a monoclonal antibody to the HIV-1 regulatory protein Rev, has been demonstrated to potently inhibit HIV-1 replication, when expressed intracellularly in an epithelial cell-line (HeLa-CD4). Murine retroviral shuttle vectors, which express the anti-Rev SFv moiety, have now been constructed. HIV-1 infection was dramatically inhibited in human T-lymphocytic cell-lines, CEM and Sup-T1, transduced with these anti-Rev SFv-expressing vectors. This resistance to high levels of HIV-1 expression was demonstrated in both mixed populations and clones of these cells. Of further potential clinical significance, HIV-1 infection was also potently inhibited in human peripheral blood mononuclear cells (PBMC), transduced with retroviral vectors expressing the anti-Rev SFv molecule. These data suggest that intracellular expression of anti-Rev SFvs, or related approaches, may be utilized as genetic therapy, or intracellular immunization, for HIV-1 infections in vivo.

Characterization of a replication-competent retrovirus resulting from recombination of packaging and vector sequences

A replication-competent retrovirus (RCR) was detected by S+/L- assays in three lots of retroviral vector G1Na that were harvested on consecutive days from a single culture of PA317/G1Na producer cells. Using a number of retrovirus-specific primer pairs, it was shown that this RCR was a novel recombinant created by exchanges between G1Na and helper sequence pPAM3 and was not an existing RCR introduced by cross-contamination. Sequencing of clones of DNA amplified in six independent PCR reactions confirmed that the 3' portion of this RCR was composed of retroviral envelope sequences unique to pPAM3 joined to a 3' long terminal repeat (LTR) unique to G1Na. Comparison of pPAM3 and G1Na sequences at the site corresponding to this junction revealed a short segment of patchy nucleotide identity (8 out of 10 bp), suggesting that these helper and vector sequences were joined by homologous recombination. Generation of RCR by exchanges between helper and vector sequences underscores the necessity of testing by efficient methods all retroviral vectors for the presence of RCR before their use. Production of 171 lots (855 liters) of various retroviral vectors that were free of RCR, including 42 lots of G1Na, however, indicates that the combination of exchanges required to generate an RCR are infrequent in this system.