Type C retrovirus inactivation by human complement is determined by both the viral genome and the producer cell

Sodium-dependent neutral amino acid transporter type 1 is an auxiliary receptor for baboon endogenous retrovirus

The baboon endogenous retrovirus (BaEV) belongs to a large, widely dispersed interference group that includes the RD114 feline endogenous virus and primate type D retroviruses. Recently, we and another laboratory independently cloned a human receptor for these viruses and identified it as the human sodium-dependent neutral amino acid transporter type 2 (hASCT2). Interestingly, mouse and rat cells are efficiently infected by BaEV but only become susceptible to RD114 and type D retroviruses if the cells are pretreated with tunicamycin, an inhibitor of protein N-linked glycosylation. To investigate this host range difference, we cloned and analyzed NIH Swiss mouse ASCT2 (mASCT2). Surprisingly, mASCT2 did not mediate BaEV infection, which implied that mouse cells might have an alternative receptor for this virus. In addition, elimination of the two N-linked oligosaccharides from mASCT2 by mutagenesis, as substantiated by protein N-glycosidase F digestions and Western immunoblotting, did not enable it to function as a receptor for RD114 or type D retroviruses. Based on these results, we found that the related ASCT1 transporters of humans and mice are efficient receptors for BaEV but are relatively inactive for RD114 and type D retroviruses. Furthermore, elimination of the two N-linked oligosaccharides from extracellular loop 2 of mASCT1 by mutagenesis enabled it to function as an efficient receptor for RD114 and type D retroviruses. Thus, we infer that the tunicamycin-dependent infection of mouse cells by RD114 and type D retroviruses is caused by deglycosylation of mASCT1, which unmasks previously buried sites for viral interactions. In contrast, BaEV efficiently employs the glycosylated forms of mASCT1 that occur normally in untreated mouse cells.

VSV-G pseudotyped lentiviral vector particles produced in human cells are inactivated by human serum

Lentiviral vectors transduce dividing and postmitotic cells and thus are being developed toward therapies for many diseases affecting diverse tissues. One essential requirement for efficacy will be that vector particles are resistant to inactivation by human serum complement. Most animal studies with lentiviral vectors have utilized VSV-G pseudotyped envelopes. Here we demonstrate that VSV-G pseudotyped HIV and FIV vectors produced in human cells are inactivated by human serum complement, suggesting that alternative envelopes may be required for therapeutic efficacy for many clinical applications of lentiviral vectors.

Study of full-length porcine endogenous retrovirus genomes with envelope gene polymorphism in a specific-pathogen-free Large White swine herd

Specific-pathogen-free (SPF) swine appear to be the most appropriate candidate for pig to human xenotransplantation. Still, the risk of endogenous retrovirus transmission represents a major obstacle, since two human-tropic porcine endogenous retroviruses (PERVs) had been characterized in vitro (P. Le Tissier, J. P. Stoye, Y. Takeuchi, C. Patience, and R. A. Weiss, Nature 389:681-682, 1997). Here we addressed the question of PERV distribution in a French Large White SPF pig herd in vivo. First, PCR screening for previously described PERV envelope genes envA, envB, and envC (D. E. Akiyoshi, M. Denaro, H. Zhu, J. L. Greenstein, P. Banerjee, and J. A. Fishman, J. Virol. 72:4503-4507, 1998; Le Tissier et al., op. cit.). demonstrated ubiquity of envA and envB sequences, whereas envC genes were absent in some animals. On this basis, selective out-breeding of pigs of remote origin might be a means to reduce proviral load in organ donors. Second, we investigated PERV genome carriage in envC negative swine. Eleven distinct full-length PERV transcripts were isolated. The sequence of the complete envelope open reading frame was determined. The deduced amino acid sequences revealed the existence of four clones with functional and five clones with defective PERV PK-15 A- and B-like envelope sequences. The occurrence of easily detectable levels of PERV variants in different pig tissues in vivo heightens the need to assess PERV transmission in xenotransplantation animal models.

Using a tropism-modified adenoviral vector to circumvent inhibitory factors in ascites fluid

Peritoneal compartmentalization of advanced stage ovarian cancer provides a rational scenario for gene therapy strategies. Several groups are exploring intraperitoneal administration of adenoviral (Ad) vectors for this purpose. We examined in vitro gene transfer in the presence of ascites fluid from ovarian cancer patients and observed significant inhibition of Ad-mediated gene transfer. The inhibitory activity was not identified as either complement or cellular factors, but depletion of IgG from ascites removed the inhibitory activity, implicating neutralizing anti-Ad antibodies. A wide range of preexisting anti-Ad antibody titers in patient ascites fluid was measured by ELISA. Western blot analysis demonstrated that the antibodies were directed primarily against the Ad fiber protein. To circumvent inhibition by neutralizing antibodies, a genetically modified adenoviral vector was tested. The Ad5Luc.RGD vector has an Arg-Gly-Asp (RGD) peptide sequence inserted into the fiber knob domain and enters cells through a nonnative pathway. Compared with the conventional Ad5 vector, Ad5Luc.RGD directed efficient gene transfer to cell lines and primary ovarian cancer cells in the presence of ascites fluid containing high-titer neutralizing anti-Ad antibodies. These results suggest that such modified Ad vectors will be needed to achieve efficient gene transfer in the clinical setting.

Retroviral vectors

Traditionally, the retrovirus is regarded as an enemy to be overcome. However, for the past two decades retroviruses have been harnessed as vehicles for transferring genes into eukaryotic cells, a process known as transduction. During this time, the technology has moved from being a scientific laboratory tool to a potential clinical molecular medicine to be used in gene therapy. This review explains the strategy for harnessing the retrovirus life cycle, the scientific research and clinical applications of this methodology, and its limitations, as well as possible future developments.

Chimeric retroviral helper virus and picornavirus IRES sequence to eliminate DNA methylation for improved retroviral packaging cells

Most retroviral packaging cell lines were established by a helper virus plasmid cotransfected with a separate plasmid encoding a selection marker. Since this selection marker coexisted in trans with the helper virus sequence, helper virus gene expression could be inactivated by host DNA methylation despite selection for the cotransfected selection marker. We have reported that DNA methylation could occur in the long terminal repeat (LTR) region of helper virus in vector producer cells (VPC) in up to 2% of the population per day (W. B. Young, G. L. Lindberg, and C. J. Link, Jr., J. Virol. 74:3177-3187, 2000). To overcome host cell DNA methylation that suppresses viral gene expression, we constructed a chimeric retroviral helper virus, pAM3-IRES-Zeo, that contains Moloney murine leukemia virus as a helper virus and a picornavirus internal ribosome entry site (IRES) sequence followed by a Zeocin selection marker at the 3' end of the env sequence. This pAM3-IRES-Zeo permitted selection for intact and functional helper virus in transfected cells without subcloning. By selection with Zeocin, a mixed population of pAM3-IRES-Zeo-transfected NIH3T3 cells (AMIZ cells) was maintained with little or no DNA methylation of the helper virus 5' LTR. The high level of pAM3-IRES-Zeo gene expression resulted in no detectable vector superinfection and in high vector titers (2 x 10(6) to 1.5 x 10(7) CFU/ml) after introduction of a retroviral vector. When Zeocin selection was withdrawn from AMIZ cells, methylation of the 5' LTR increased from 17 to 36% of the population during 67 days of continuous culture and the cells became susceptible to superinfection. During this period, gene expression of pAM3-IRES-Zeo decreased and vector titer production was reduced to 2 x 10(4) CFU/ml. These data demonstrate an important role of DNA methylation in the genetic instability of VPC. The chimeric helper virus allows the establishment of a mixed population of packaging cells capable of high-level and sustained vector production without cloning procedures.

Neonatal gene therapy. transfer and expression of exogenous genes in neonatal sheep following direct injection of retroviral vectors into the bone marrow space

We investigated whether gene transfer into hematopoietic cells could be achieved by direct injection of retroviral vector supernatant into the bone marrow space of newborn sheep. Six sheep (5 weeks old) were injected bilaterally with either 1 mL of G1nBgSvNa8.1 vector supernatant (titer: 1 x 10(7)) in each hip (n = 5) or with 3 mL of the same vector preparation/hip (n = 1). In addition, one 3-month-old sheep was injected unilaterally with 1 mL of the same vector preparation. Blood and marrow of these animals were analyzed for the transgene before injection and at intervals thereafter. At 1 week postinjection, an average of 11.6% of the lymphocytes and 25.5% of the granulocytes/monocytes in the marrow, and an average of 0.9% of the lymphocytes and 1.8% of the granulocytes/monocytes in the blood contained and expressed the LacZ gene. The presence/expression of the transgene has persisted for at least 13 months within the blood and bone marrow of these animals. These findings demonstrate that the direct injection of small volumes of high-titer retroviral supernatant into the bone marrow of newborn sheep results in transduction of hematopoietic cells that persists for at least 13 months postinjection.

Progress with retroviral gene vectors

Retroviral vectors have become a standard tool for gene transfer technology. Compared with other gene transfer systems, retroviral vectors have several advantages, including their ability to transduce a variety of cell types, to integrate efficiently into the genomic DNA of the recipient cells and to express the transduced gene at high levels. The relatively well understood biology of retroviruses has made possible the development of packaging cell lines which provide in trans all the viral proteins required for viral particle formation. The design of different types of packaging cells has evolved to reduce the possibility of helper virus production. The host range of retroviruses has been expanded by pseudotyping the vectors with heterologous viral glycoproteins and receptor-specific ligands. The development of lentivirus vectors has allowed efficient gene transfer to quiescent cells. This review describes different strategies adopted for developing vectors to be used in gene therapy applications.

An envelope glycoprotein of the human endogenous retrovirus HERV-W is expressed in the human placenta and fuses cells expressing the type D mammalian retrovirus receptor

A new human endogenous retrovirus (HERV) family, termed HERV-W, was recently described (J.-L. Blond, F. Besème, L. Duret, O. Bouton, F. Bedin, H. Perron, B. Mandrand, and F. Mallet, J. Virol. 73:1175-1185, 1999). HERV-W mRNAs were found to be specifically expressed in placenta cells, and an env cDNA containing a complete open reading frame was recovered. In cell-cell fusion assays, we demonstrate here that the product of the HERV-W env gene is a highly fusogenic membrane glycoprotein. Transfection of an HERV-W Env expression vector in a panel of cell lines derived from different species resulted in formation of syncytia in primate and pig cells upon interaction with the type D mammalian retrovirus receptor. Moreover, envelope glycoproteins encoded by HERV-W were specifically detected in placenta cells, suggesting that they may play a physiological role during pregnancy and placenta formation.

Latest developments in gene transfer technology: achievements, perspectives, and controversies over therapeutic applications

Over the last decade, more than 300 phase I and phase II gene-based clinical trials have been conducted worldwide for the treatment of cancer and monogenic disorders. Lately, these trials have been extended to the treatment of AIDS and, to a lesser extent, cardiovascular diseases. There are 27 currently active gene therapy protocols for the treatment of HIV-1 infection in the USA. Preclinical studies are currently in progress to evaluate the possibility of increasing the number of gene therapy clinical trials for cardiopathies, and of beginning new gene therapy programs for neurologic illnesses, autoimmuno diseases, allergies, regeneration of tissues, and to implement procedures of allogeneic tissues or cell transplantation. In addition, gene transfer technology has allowed for the development of innovative vaccine design, known as genetic immunization. This technique has already been applied in the AIDS vaccine programs in the USA. These programs aim to confer protective immunity against HIV-1 transmission to individuals who are at risk of infection. Research programs have also been considered to develop therapeutic vaccines for patients with AIDS and generate either preventive or therapeutic vaccines against malaria, tuberculosis, hepatitis A, B and C viruses, influenza virus, La Crosse virus, and Ebola virus. The potential therapeutic applications of gene transfer technology are enormous. However, the effectiveness of gene therapy programs is still questioned. Furthermore, there is growing concern over the matter of safety of gene delivery and controversy has arisen over the proposal to begin in utero gene therapy clinical trials for the treatment of inherited genetic disorders. From this standpoint, despite the latest significant achievements reported in vector design, it is not possible to predict to what extent gene therapeutic interventions will be effective in patients, and in what time frame.

Incorporation of simian virus 5 fusion protein into murine leukemia virus particles and its effect on the co-incorporation of retroviral envelope glycoproteins

We describe the generation of murine leukemia virus (MLV) virus particles carrying the paramyxovirus fusion protein F from simian virus 5 (SV5-F). This glycoprotein was expressed in cells providing Moloney MLV (MoMLV) Gag and Pol proteins and a lacZ retroviral vector. SV5-F was correctly expressed, processed, and efficiently incorporated into retroviral particles. SV5-F-bearing retroviruses were not infectious although a weak binding to primate and rodent cells could be detected and SV5-F could mediate cell to cell fusion. We then co-expressed the SV5-F glycoprotein in retroviral particles with chimeric and wild-type MoMLV envelope glycoproteins. Our results show that F strongly inhibited infection via the retroviral envelopes although the mechanism of inhibition was different depending on the retroviral envelope used.

In utero transfer and expression of exogenous genes in sheep

OBJECTIVE

We have previously reported that directly injecting low-titer retroviral vector supernatant into pre-immune sheep fetuses resulted in the transfer and long-term expression of the bacterial NeoR gene within the hematopoietic system of these animals for over 5 years. In the present studies, we investigated whether using a higher titer vector would enable more efficient transduction and expression of the transgenes within the hematopoetic cells in sheep injected in utero.

MATERIALS AND METHODS

Sixteen pre-immune sheep fetuses were injected intraperitoneally with the G1nBgSvNa8.1 helper-free retroviral vector supernatant encoding the bacterial NeoR and LacZ genes (titer: 1x10(7) cfu/mL).

RESULTS

Over the 2-year time course of these studies, the presence and expression of the NeoR and LacZ genes were demonstrated in 12 of the 14 animals evaluated by several immunological and biochemical methods. Seven of the 12 sheep examined by flow cytometric analysis contained > or =6% transduced peripheral blood lymphocytes. Vector distribution was widespread without any detectable pathology. Importantly, PCR analyses and breeding experiments demonstrated that the germ line was not altered.

CONCLUSIONS

These studies confirmed that direct injection of an engineered retrovirus is a feasible means of safely delivering foreign genes into a developing fetus and thus achieving long-term expression of the transgenes within the recipient's hematopoietic cells. Furthermore, expression of the NeoR gene from these studies was higher than that reported in our previous study in which a lower titer vector was used.

Myocardial injury-induced fibroblast proliferation facilitates retroviral-mediated gene transfer to the rat heart in vivo

BACKGROUND

Efficient and stable transfer of therapeutic DNA into injured myocardium would be an initial step towards a genetic treatment aimed at myocardial repair after myocardial infarction. Proliferating cardiac fibroblasts in the healing myocardium could be a compelling target for retroviral infection. We evaluated the feasibility of direct in vivo gene transfer into injured myocardium using a high-titer, stable retroviral vector.

METHODS

Using the TE-FLY-A-based MFG retroviral vector harboring nlsLacZ reporter, the gene transfer efficiency was assessed first in vitro in rat cardiac fibroblasts, followed by in vivo evaluation in healing rat myocardium after local freeze-thaw injury. A total of 2.5 x 10(7) infectious units of retrovirus were injected into the injured region of a beating rat heart. The transduced cells were identified by X-gal staining and immunohistochemistry.

RESULTS

Highly efficient transduction of cardiac fibroblasts was observed in vitro with 98% of the cells transduced with single infection. The cell proliferation index in the cardiac granulation tissue appeared maximal 3 days after cryoinjury. Retroviral injection into the injured beating heart induced gene expression localized to the wound repair region. One week after retrovirus injection, 14% of the cells in the reparative tissue were beta-gal-positive, while 4% were beta-gal-positive after 4 weeks. The transduced cells were mostly myofibroblasts.

CONCLUSIONS

Local gene transfer to the healing rat heart is feasible by retrovirus in vivo. This observation may serve as a useful guide for the development of gene therapy aimed at myocardial repair after myocardial infarction.

In situ use of suicide genes for therapy of brain tumours

Suicide gene therapy represents a new therapeutic approach to the treatment of patients with otherwise incurable malignant brain tumours. This strategy involves the introduction of a gene that renders the tumour cell susceptible to an otherwise nontoxic prodrug. The most often used genetic prodrug activation system is the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) paradigm. An important aspect of this system is the 'bystander effect', the extension of cytotoxic effects to untransduced cells. For gene delivery, retroviral, adenoviral vectors and HSV-1 mutants have been used. Clinical studies have revealed that the HSV-tk/GCV approach is safe, but also that responses are observed only in very small brain tumours, indicating insufficient vector distribution and very low transduction efficiency with replication-deficient vector systems. To improve treatment efficacy, the use of replication-competent oncolytic vectors in combination with new or improved prodrug-suicide gene systems as a part of a multimodal approach is warranted. In the context of replication-competent vectors, suicide genes might also be used as fail-safe genes in the case of runaway infection.

In vivo cell type-specific gene delivery with retroviral vectors that display single chain antibodies

Cell type-specific gene delivery will be essential for in vivo gene therapy. Our laboratory has previously developed retroviral vector particles, derived from spleen necrosis virus, SNV, which display the antigen-binding site of an antibody on the viral surface. Such particles infected only human cells in vitro, which expressed a receptor recognized by the antibody. To test cell type-specific gene delivery in vivo, a mouse model system has been developed. Antibiotic resistant human target and non-target cells were injected into the peritoneum of SCID mice. Subsequently, a vector solution containing 106 infectious particles, which display scAs against the human her2neu cell surface protein, was injected. Cells were recovered from the peritoneum, subjected to antibiotic selection, and tested for the expression of a lacZ gene transduced by the retroviral vector. We found that human target cells, which express her2neu, were infected in vivo. However, neither human cells that do not express her2neu, nor normal mouse cells were infected by such viral particles. These data give proof of principle that retroviral vector-mediated, cell type-specific gene delivery can be obtained in vivo.

Viral gene delivery

Experimental studies of viral gene delivery generally support the principle that virus-mediated gene transfer is indeed possible. However, the field of gene therapy has not yet been realised as a practicable clinical intervention. The delay in translation of laboratory work to clinical utility largely reflects the inability of gene delivery vectors to convey adequate genetic material to a desired location, with adequate durability and low enough toxicity to be effective. Current studies of viral gene therapy vehicles have focused on re-engineering viruses being tested as vectors at present, treating the host to facilitate viral gene transfer and the development of new vectors. Initial enthusiasm for oncoretroviral and adenoviral vectors has cooled, while adeno-associated virus and lentiviral vectors are attracting more interest. Experimental studies with modified SV40-based vectors have also been very promising. The future of gene therapy will probably entail using an array of gene delivery vehicles, each with its own strengths and weaknesses. The vector systems will probably be as diverse as the applications to which they will be put.

Protection of MLV vector particles from human complement

Murine cell-derived MLV vector particles usually are highly sensitive to human complement-mediated lysis. Expression of the human complement inhibitor CD59 on murine packaging cells resulted in partial protection of these cells from lysis caused by human complement proteins. Furthermore, CD59 was incorporated into MLV vector particles released by these packaging cells, leading to an improved resistance of the virions against human complement-mediated inactivation.

Selective transduction of protease-rich tumors by matrix-metalloproteinase-targeted retroviral vectors

We recently showed that retroviral vectors can be targeted through protease substrate interactions. Infectivity is blocked by a polypeptide fused to the viral envelope glycoprotein (SU) and is restored when a protease cleaves the connecting linker, releasing the inhibitory polypeptide from the viral surface. Protease specificity is achieved by engineering the sequence of the linker. Here, using two different matrix-metalloproteinase (MMP)-activatable vectors, we demonstrated highly efficient and selective transduction of MMP-rich target cells in a heterogeneous cell population. In vivo, the MMP-targeted vectors showed strong selectivity for MMP-rich tumor xenografts. Protease-activatable vectors offer new possibilities for in vivo targeting of gene delivery.

Truncation of the human immunodeficiency virus-type-2 envelope glycoprotein allows efficient pseudotyping of murine leukemia virus retroviral vector particles

The incorporation of human immunodeficiency virus-type-2 (HIV-2) envelope glycoprotein into murine leukemia virus (MuLV) particles was studied in a transient transfection packaging cell system. We observed that wild-type HIV-2 envelope protein or a frameshift mutant with 187 unrelated carboxyl-terminal residues did not allow the formation of infectious retroviral particles. In view of recent findings that an HIV-1 envelope protein variant with a shortened cytoplasmic domain was incorporated into MuLV particles, we constructed carboxyl-terminal truncations of the HIV-2 envelope protein. An envelope variant with 18 cytoplasmic amino acids formed only very few viral pseudotypes. The further removal of an additional 11 amino acids allowed the efficient pseudotyping of MuLV particles. As with the HIV-1 envelope protein, an HIV-2 envelope variant with 7 cytoplasmic amino acids was incorporated into functional MuLV particles. The pseudotyped vectors obtained are able to infect human CD4/CXCR4-expressing cells. Cell lines expressing human CD4 and other coreceptors could not be infected. This retroviral vector will prove useful for the study of HIV infection events mediated by the HIV-2 envelope glycoproteins, as well as for the targeting of CD4+ cells in the context of gene therapy of AIDS.

Gene transfer technology in therapy: current applications and future goals

Gene therapy has attracted much interest since the first submissions of phase I clinical trials in the early 1990s, for the treatment of inherited genetic diseases. Preliminary results were very encouraging and prompted many investigators to submit protocols for phase I and phase II clinical trials for the treatment of inherited genetic diseases and cancer. The possible application of gene transfer technology to treat AIDS, cardiopathies, and neurologic diseases is under evaluation. Some viral vectors have already been used to deliver HIV-1 subunits to immunize volunteers who are participating in the AIDS vaccine programs in the USA. However, gene delivery systems still need to be optimized in order to achieve effective therapeutic interventions. The purpose of this review is to summarize the latest achievements in improving gene delivery systems, their current application in preclinical studies and in therapy, and the most pressing issues that must be addressed in the area of vector design.

The resistance of retroviral vectors produced from human cells to serum inactivation in vivo and in vitro is primate species dependent

The ability to deliver genes as therapeutics requires an understanding of the vector pharmacokinetics similar to that required for conventional drugs. A first question is the half-life of the vector in the bloodstream. Retroviral vectors produced in certain human cell lines differ from vectors produced in nonhuman cell lines in being substantially resistant to inactivation in vitro by human serum complement (F. L. Cosset, Y. Takeuchi, J. L. Battini, R. A. Weiss, and M. K. Collins, J. Virol. 69:7430-7436, 1995). Thus, use of human packaging cell lines (PCL) may produce vectors with longer half-lives, resulting in more-efficacious in vivo gene therapy. However, survival of human PCL-produced vectors in vivo following systemic administration has not been explored. In this investigation, the half-lives of retroviral vectors packaged by either canine D17 or human HT1080 PCL were measured in the bloodstreams of macaques and chimpanzees. Human PCL-produced vectors exhibited significantly higher concentrations of circulating biologically active vector at the earliest time points measured (>1, 000-fold in chimpanzees), as well as substantially extended half-lives, compared to canine PCL-produced vectors. In addition, the circulation half-life of human PCL-produced vector was longer in chimpanzees than in macaques. This was consistent with in vitro findings which demonstrated that primate serum inactivation of vector produced from human PCL increased with increasing phylogenetic distance from humans. These results establish that in vivo retroviral vector half-life correlates with in vitro resistance to complement. Furthermore, these findings should influence the choice of animal models used to evaluate retroviral-vector-based therapies.

Retrovirus targeting by tropism restriction to melanoma cells

Targeted vectors will be necessary for many gene therapy applications. To target retroviruses to melanomas, we fused a single-chain variable fragment antibody (scFv) directed against the surface glycoprotein high-molecular-weight melanoma-associated antigen (HMW-MAA) to the amphotropic murine leukemia virus envelope. A proline-rich hinge and matrix metalloprotease (MMP) cleavage site linked the two proteins. The modified viruses bound only to HMW-MAA-expressing cells, as inclusion of the proline-rich hinge prevented viral binding to the amphotropic viral receptor. Following attachment to HMW-MAA, MMP cleavage of the envelope at the melanoma cell surface removed the scFv and proline-rich hinge, allowing infection. Complexing of targeted retroviruses with 2, 3-dioleoyloxy-N-[2(spermine-carboxamido)ethyl]N, N-dimethyl-1-propanaminium trifluoroacetate-dioleoyl phosphatidylethanolamine liposomes greatly increased their efficiency without affecting their target cell specificity. In a cell mixture, 40% of HMW-MAA-positive cells but less than 0.01% of HMW-MAA-negative cells were infected. This approach can therefore produce efficient, targeted retroviruses suitable for in vivo gene delivery and should allow specific gene delivery to many human cell types by inclusion of different scFv and protease combinations.

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.

Complement-protected amphotropic retroviruses from murine packaging cells

The application of retroviruses generated from murine cells for in vivo gene therapy is restricted primarily because of the rapid inactivation of these viruses by the human complement system. To circumvent this disadvantageous property of murine retroviruses we have generated infectious amphotropic retroviruses that exhibit strong protection against human complement attack. The membrane of these viruses contains a fusion protein, DAFF2A, that is composed of the catalytic domain of the human complement regulatory protein (CRP) decay-accelerating factor (DAF) and the envelope protein of the amphotropic murine leukemia virus (MuLV) 4070A (EnvA). The fusion of two other CRPs, MCP and CD59, to the same amphotropic Env moiety did not lead to equivalent results. The fusion protein DAFF2A was stably expressed in mouse NIH 3T3-based helper cells and independently identified with either alpha-DAF MAb or alpha-Env PAb on the cell membrane. Western blot analysis confirmed the expected molecular weight of the fusion protein. Viral titers obtained from NIH 3T3 helper cell pools were 5 x 10(5) CFU for wild-type amphotropic EnvA virus and 1 x 10(5) CFU for DAFF2A virus, respectively. By blocking the catalytic domain of DAF by pretreatment with alpha-DAF MAb DAFF2A, recombinant virions could be converted to wild-type with respect to sensitivity against human serum. Since the method for producing virions that are protected against human serum should be applicable to any cell type it offers a novel tool for human in vivo gene therapy.

Minimum requirements for efficient transduction of dividing and nondividing cells by feline immunodeficiency virus vectors

The development of gene delivery vectors based on feline immunodeficiency virus (FIV) is an attractive alternative to vectors based on primate sources for the delivery of genes into humans. To investigate the requirements for efficient transduction of dividing and nondividing cells by vector particles based on FIV, a series of packaging and vector constructs was generated for which viral gene expression was minimized and from which unnecessary cis-acting sequences were deleted. Pseudotyped vector particles produced in 293T cells were used to transduce various target cells, including contact-inhibited human skin fibroblasts and growth-arrested HT1080 cells. FIV vectors in which the U3 promoter was replaced with the cytomegalovirus promoter gave rise to over 50-fold-higher titers than FIV vectors containing the complete FIV 5' long terminal repeat (LTR). Comparison of the transduction efficiencies of vectors containing different portions of the FIV Gag coding region indicates that at least a functional part of the FIV packaging signal (Psi) is located within an area which includes the 5' LTR and the first 350 bp of gag. Transduction efficiencies of vectors prepared without FIV vif and orf2 accessory gene expression did not differ substantially from those of vectors prepared with accessory gene expression in either dividing or nondividing cells. The requirement for FIV rev-RRE was, however, demonstrated by the inefficient production of vector particles in the absence of rev expression. Together, these results demonstrate the efficient transduction of nondividing cells in vitro by a multiply attenuated FIV vector and contribute to an understanding of the minimum requirements for efficient vector production and infectivity. In addition, we describe the ability of an FIV vector to deliver genes in vivo into hamster muscle tissue.

A sodium-dependent neutral-amino-acid transporter mediates infections of feline and baboon endogenous retroviruses and simian type D retroviruses

The type D simian retroviruses cause immunosuppression in macaques and have been reported as a presumptive opportunistic infection in a patient with AIDS. Previous evidence based on viral interference has strongly suggested that the type D simian viruses share a common but unknown cell surface receptor with three type C viruses: feline endogenous virus (RD114), baboon endogenous virus, and avian reticuloendotheliosis virus. Furthermore, the receptor gene for these viruses has been mapped to human chromosome 19q13.1-13.2. We now report the isolation and characterization of a cell surface receptor for this group of retroviruses by using a human T-lymphocyte cDNA library in a retroviral vector. Swiss mouse fibroblasts (NIH 3T3), which are naturally resistant to RD114, were transduced with the retroviral library and then challenged with an RD114-pseudotyped virus containing a dominant selectable gene for puromycin resistance. Puromycin selection yielded 12 cellular clones that were highly susceptible to a beta-galactosidase-encoding lacZ(RD114) pseudotype virus. Using PCR primers specific for vector sequences, we amplified a common 2.9-kb product from 10 positive clones. Expression of the 2.9-kb cDNA in Chinese hamster ovary cells conferred susceptibility to RD114, baboon endogenous virus, and the type D simian retroviruses. The 2.9-kb cDNA predicted a protein of 541 amino acids that had 98% identity with the previously cloned human Na+-dependent neutral-amino-acid transporter Bo. Accordingly, expression of the RD114 receptor in NIH 3T3 cells resulted in enhanced cellular uptake of L-[3H]alanine and L-[3H]glutamine. RNA blot (Northern) analysis suggested that the RD114 receptor is widely expressed in human tissues and cell lines, including hematopoietic cells. The human Bo transporter gene has been previously mapped to 19q13.3, which is closely linked to the gene locus of the RD114 receptor.

In pursuit of new developments for gene therapy of human diseases

Gene therapy aims at transferring a therapeutic gene into human somatic cells in order to treat a disease. Originally addressed to hereditary genetic disorders, gene therapy has found therapeutic applications in cancer, infectious diseases and degenerative disorders, particularly those of the nervous system. Although gene transfer into humans has been demonstrated in several clinical trials, with more than 300 currently underway worldwide, there is still no single outcome that undoubtedly showed a consistent benefit for the patient. Nevertheless, the expectations for gene therapy are still high, and the prospects of future clinical success are increasing together with the growing of the field. The development of better delivery systems specifically tailored to individual diseases, with sustained expression of the therapeutic gene in the appropriate cells, will in the end make possible true therapeutic applications of human gene transfer.

Modification of retroviral tropism by display of IGF-I

We have displayed insulin-like growth factor I (IGF-I) as an N-terminal extension of 4070A (amphotropic) retroviral envelope protein. Western blot demonstrated that chimaeric envelope proteins were incorporated into retroviral particles. Interaction between the displayed IGF-I and cell-surface receptors impaired gene delivery. The magnitude of this inhibitory effect was smallest on NIH 3T3 cells, greater on NIH 3T3 cells over-expressing insulin receptor, and greatest on NIH 3T3 cells over-expressing human type-I IGF receptor. Hence, both the number of ligand receptors and their affinity for the displayed ligand influenced the level of gene delivery. The inhibitory effect was abrogated by cleaving the displayed domain from the underlying envelope protein with factor Xa protease, and by the addition of free ligand to the infection. Addition of IGF-I or insulin caused a dose-dependent increase in titre. Possible mechanisms for receptor-mediated inhibition of gene delivery by IGF-displaying vectors are discussed, together with the implications of these results for practical applications of retroviral display and for understanding the mechanism of virus entry.

Complement and anti-alpha-galactosyl natural antibody-mediated inactivation of murine retrovirus occurs in adult serum but not in umbilical cord serum

Many retroviral vectors for hematopoietic cell and other clinical gene therapy are derived from murine packaging cell lines. The exposure of these retroviruses and packaging cell lines to adult human serum (AS) inactivates them by complement and anti-alpha-galactosyl natural antibody-mediated mechanisms. We show that virus stability and infection efficiency of CRIP/BAG, a murine packaging cell line derived amphotropic retrovirus vector is reduced > 95% following a 30-min incubation in AS. This inactivation is prevented by replacing AS with umbilical cord serum (CS), wherein full retroviral transduction efficiency is maintained after 30 min of incubation. The loss of retrovirus transduction efficiency in AS was smaller upon blockage of anti-alpha-galactosyl antibodies with galactose alpha 1-3-galactose. Serum levels of CH 100, as well as C1q complement which inactivates retroviruses by an antibody-independent mechanism were similar in AS and CS. The high stability of CRIP/BAG retrovirus vector in CS is likely due to its lower levels of maternally derived anti-alpha-galactosyl antibodies. These results have implications for in vivo gene transfer in adults and also in newborns since neonates do not produce natural antibodies during the initial months of life.

Generation of retroviral vector for clinical studies using transient transfection

Transient transfection of 293T cells was utilized to produce high-titer murine recombinant retroviral vectors for clinical studies. This system was initially optimized by gene transfer using different retroviral envelope proteins into activated human CD4+ T lymphocytes in vitro. Higher titer and infectivity were obtained than with stable murine producer lines; titers of 0.3-1 x 10(7) infectious units per milliliter for vectors encoding the green fluorescent protein (GFP) were achieved. Virions pseudotyped with envelope proteins from gibbon ape leukemia virus or amphotropic murine leukemia virus resulted in gene transfer of > or = 50% in CD4+ human T lymphocytes with this marker. Gene transfer of Rev M10 with this vector conferred resistance to HIV infection compared with negative controls in the absence of drug selection. Thus, the efficiency of transduction achieved under these conditions obviated the need to include selection to detect biologic effects in T cells. Finally, a protocol for the production of large-scale supernatants using transient transfection was optimized up to titers of 1.9 x 10(7) IU/ml. These packaging cells can be used to generate high-titer virus in sufficient quantities for clinical studies and will facilitate the rapid, cost-effective generation of improved retroviral, lentiviral, or other viral vectors for human gene therapy.

Host range and interference studies of three classes of pig endogenous retrovirus

Recent interest in the use of porcine organs, tissues, and cells for xenotransplantation to humans has highlighted the need to characterize the properties of pig endogenous retroviruses (PERVs). Analysis of a variety of pig cells allowed us to isolate and identify three classes of infectious type C endogenous retrovirus (PERV-A, PERV-B, and PERV-C) which have distinct env genes but have highly homologous sequences in the rest of the genome. To study the properties of these env genes, expression plasmids for the three env genes were constructed and used to generate retrovirus vectors bearing corresponding Env proteins. Host range analyses by the vector transduction assay showed that PERV-A and PERV-B Envs have wider host ranges, including several human cell lines, compared with PERV-C Env, which infected only two pig cell lines and one human cell line. All PERVs could infect pig cells, indicating that the PERVs have a potential to replicate in pig transplants in immunosuppressed patients. Receptors for PERV-A and PERV-B were present on cells of some other species, including mink, rat, mouse, and dog, suggesting that such species may provide useful model systems to study infection and pathogenicity of PERV. In contrast, no vector transduction was observed on nonhuman primate cell lines, casting doubt on the utility of nonhuman primates as models for PERV zoonosis. Interference studies showed that the three PERV strains use receptors distinct from each other and from a number of other type C mammalian retroviruses.

A proline-rich motif downstream of the receptor binding domain modulates conformation and fusogenicity of murine retroviral envelopes

The entry of retroviruses into cells depends on receptor recognition by the viral envelope surface subunit SU followed by membrane fusion, which is thought to be mediated by a fusion peptide located at the amino terminus of the envelope transmembrane subunit TM. Several fusion determinants have been previously identified in murine leukemia virus (MLV) envelopes, but their functional interrelationships as well as the processes involved in fusion activation upon retroviral receptor recognition remain unelucidated. Despite both structural and functional similarities of their envelope glycoproteins, ecotropic and amphotropic MLVs display two different postbinding properties: (i) while amphotropic MLVs fuse the cells at neutral pH, penetration of ecotropic MLVs is relatively acid pH dependent and (ii) ecotropic envelopes are more efficient than amphotropic envelopes in inducing cell-to-cell fusion and syncytium formation. By exploiting the latter characteristic in the analysis of chimeras of ecotropic and amphotropic MLV envelopes, we show here that substitution of the ecotropic MLV proline-rich region (PRR), located in the SU between the amino-terminal receptor binding domain and the TM-interacting SU carboxy-terminal domains, is sufficient to revert the amphotropic low-fusogenic phenotype into a high-fusogenic one. Furthermore, we have identified potential beta-turns in the PRR that control the stability of SU-TM associations as well as the thresholds required to trigger either cell-to-cell or virus-to-cell fusion. These data, demonstrating that the PRR functions as a signal which induces envelope conformational changes leading to fusion, have enabled us to derive envelopes which can infect cells harboring low levels of available amphotropic receptors.

Inhibitory effect of biphalin and AZT on murine Friend leukemia virus infection in vitro

Biphalin is a bivalent opioid analogue containing two tyrosine residues. We have examined the effect of biphalin's anti-retroviral potency in vitro using a murine model. Biphalin, in non-cytotoxic concentrations, suppressed in a dose-dependent fashion the replication of Friend leukemia virus (FLV) in Mus dunni cells as determined using a focus forming assay. FLV replication was substantially reduced by biphalin at 10(-4) M concentration. When biphalin was combined with 3'-azido-3'-deoxythymidine (AZT) the two acted synergistically in inhibiting FLV replication compared to either used alone. Using a reverse transcriptase (RT) assay, FLV RT levels also were noted to be reduced in the presence of biphalin. These observations indicate that biphalin possesses anti-retroviral activity in vitro, suggesting that this opioid peptide should be examined further in vivo to determine if it is a candidate for combined therapy with AZT and possibly other drugs for retrovirus infections including the human immunodeficiency virus (HIV).

CXCR4 is required by a nonprimate lentivirus: heterologous expression of feline immunodeficiency virus in human, rodent, and feline cells

A heterologous feline immunodeficiency virus (FIV) expression system permitted high-level expression of FIV proteins and efficient production of infectious FIV in human cells. These results identify the FIV U3 element as the sole restriction to the productive phase of replication in nonfeline cells. Heterologous FIV expression in a variety of human cell lines resulted in profuse syncytial lysis that was FIV env specific, CD4 independent, and restricted to cells that express CXCR4, the coreceptor for T-cell-line-adapted strains of human immunodeficiency virus. Stable expression of human CXCR4 in CXCR4-negative human and rodent cell lines resulted in extensive FIV Env-mediated, CXCR4-dependent cell fusion and infection. In feline cells, stable overexpression of human CXCR4 resulted in increased FIV infectivity and marked syncytium formation during FIV replication or after infection with FIV Env-expressing vectors. The use of CXCR4 is a fundamental feature of lentivirus biology independent of CD4 and a shared cellular link to infection and cytopathicity for distantly related lentiviruses that cause AIDS. Their conserved use implicates chemokine receptors as primordial lentivirus receptors.

Cationic liposomes enhance the rate of transduction by a recombinant retroviral vector in vitro and in vivo

Cationic liposomes enhanced the rate of transduction of target cells with retroviral vectors. The greatest effect was seen with the formulation DC-Chol/DOPE, which gave a 20-fold increase in initial transduction rate. This allowed an efficiency of transduction after brief exposure of target cells to virus plus liposome that could be achieved only after extensive exposure to virus alone. Enhancement with DC-Chol/DOPE was optimal when stable virion-liposome complexes were preformed. The transduction rate for complexed virus, as for virus used alone or with the polycation Polybrene, showed first-order dependence on virus concentration. Cationic liposomes, but not Polybrene, were able to mediate envelope-independent transduction, but optimal efficiency required envelope-receptor interaction. When virus complexed with DC-Chol/DOPE was used to transduce human mesothelioma xenografts, transduction was enhanced four- to fivefold compared to that for virus alone. Since the efficacy of gene therapy is dependent on the number of cells modified, which is in turn dependent upon the balance between transduction and biological clearance of the vector, the ability of cationic liposomes to form stable complexes with retroviral vectors and enhance their rate of infection is likely to be important for in vivo application.

Retroviral vector targeting human cells via c-Kit-stem cell factor interaction

Targeted gene transfer into hematopoietic stem cells by retroviral vectors would greatly facilitate the development of in vivo strategies for stem cell gene therapy. We engineered a recombinant retroviral vector that can target human cells expressing a c-Kit receptor via a ligand-receptor interaction. The ecotropic (Moloney murine leukemia virus) envelope protein was modified by insertion of a sequence encoding the N-terminal 161 amino acids of murine stem cell factor (mSCF), the ligand for murine c-Kit. The chimeric envelope protein was correctly processed and incorporated into viral particles as efficiently as the wild-type envelope protein. Virions pseudotyped with the chimeric envelope proteins bound to 293 cells expressing murine c-Kit (293KIT) preferentially; however, they could not transduce any c-Kit-positive cells under conventional conditions. They could transduce 293KIT cells in the presence of chloroquine, and HEL cells expressing human c-Kit on a fibronectin fragment (CH296)-coated dish. The fact that recombinant mSCF in the medium at the time of transduction greatly reduced the efficiency of both gene deliveries implies that the vector utilized the mSCF-c-Kit interaction for the initial step of transduction in either case. The vector may prove useful for targeting cells expressing c-Kit on their surface.

Retroviral vector targeting to melanoma cells by single-chain antibody incorporation in envelope

Two strategies for targeting recombinant retroviruses to melanoma cells were compared. One was to extend the tropism of an ecotropic envelope to human melanoma cells, the other was to enhance the tropism of an amphotropic envelope for melanoma cells. Chimeric retroviral envelopes, incorporating a single-chain antibody (ScFv) directed against high-molecular-weight melanoma-associated antigen (HMWMAA) at the amino terminus are correctly processed and incorporated into virions. ScFv-ecotropic envelope chimeras allow specific, but low-titer, targeting of HMWMAA-positive cells, when co-expressed with ecotropic envelopes. ScFv-amphotropic envelope chimeras bind specifically to HMWMAA-positive cells and allow preferential infection at high titer.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

Efficient transduction of nondividing human cells by feline immunodeficiency virus lentiviral vectors

The molecular bases for species barriers to lentiviral replication are not well understood, but are of interest for explaining lentiviral pathogenesis, devising therapeutic strategies, and adapting lentiviruses to gene therapy. HIV-1-based lentiviral vectors efficiently transduce nondividing cells, but present complex safety concerns. Nonprimate (ungulate or feline) lentiviruses might provide safer alternatives, but these viruses display highly restricted tropisms, and their potential for adaptation as replication-defective vectors capable of transducing human cells is unknown. Feline immunodeficiency virus (FIV) does not infect humans or other non-Felidae despite prevalent natural exposure. Although long terminal repeat (LTR)-directed FIV expression was found to be negligible in human cells, promoter substitution enabled an env-deleted, three-plasmid, human cell-FIV lentiviral vector system to express high levels of FIV proteins and FIV vectors in human cells, thus bypassing the hazards of feline vector producer cells. Pseudotyped FIV vectors efficiently transduced dividing, growth-arrested, and postmitotic human targets. The experiments delineate mechanisms involved in species-restricted replication of this lentivirus and show that human cells support both productive- and infective-phase mechanisms of the FIV life cycle needed for efficient lentiviral vector transduction. Nonprimate lentiviral vectors may offer safety advantages, and FIV vectors provide unique experimental opportunities.

Ex vivo hepatic gene therapy of a mouse model of Hereditary Tyrosinemia Type I

Previously, this lab has reported the use of hepatocyte transplantation and in vivo gene therapy for the correction of a mouse model of Hereditary Tyrosinemia Type I (HT1). Here, we demonstrate repopulation of fumarylacetoacetate hydrolase (FAH)-deficient livers with cultured hepatocytes. Correction of the disease phenotype was achieved by retrovirally transducing cultured FAH- hepatocytes ex vivo, followed by transplantation and selective repopulation. Treated mice were phenotypically normal and had corrected plasma amino acid levels and liver function tests. Our results demonstrate that efficient hepatic repopulation using ex vivo genetically manipulated hepatocytes is feasible.

Construction of retroviral vectors with improved safety, gene expression, and versatility

Murine leukemia virus (MLV)-based retroviral vectors are the most frequently used gene delivery vehicles. However, the current vectors are still not fully optimized for gene expression and viral titer, and many genetic and biochemical features of MLV-based vectors are poorly understood. We have previously reported that the retroviral vector MFG, where the gene of interest is expressed as a spliced mRNA, is superior in the level of gene expression with respect to other vectors compared in the study. As one approach to developing improved retroviral vectors, we have systematically performed mutational analysis of the MFG retroviral vector. We demonstrated that the entire gag coding sequence, together with the immediate upstream region, could be deleted without significantly affecting viral packaging or gene expression. To our knowledge, this region is included in all currently available retroviral vectors. In addition, almost the entire U3 region could be replaced with the heterologous human cytomegalovirus immediately-early promoter without deleterious effects. We could also insert internal ribosome entry sites (IRES) and multicloning sites into MFG without adverse effects. Based on these observations, we have constructed a series of new, improved retroviral constructs. These vectors produced viral titers comparable to MFG, expressed high levels of gene expression, and stably transferred genes to the target cells. Our vectors are more convenient to use because of the presence of multicloning sites and IRESs, and they are also more versatile because they can be readily converted to various applications. Our results have general implications regarding the design and development of improved retroviral vectors for gene therapy.

A new system for stringent, high-titer vesicular stomatitis virus G protein-pseudotyped retrovirus vector induction by introduction of Cre recombinase into stable prepackaging cell lines

We report here on stable prepackaging cell lines which can be converted into packaging cell lines for high-titer vesicular stomatitis virus G protein (VSV-G)-pseudotyped retrovirus vectors by the introduction of Cre recombinase-expressing adenovirus. The generated prepackaging cell lines constitutively express the gag-pol genes and contain an inducible transcriptional unit for the VSV-G gene. From this unit, the introduced Cre recombinase excised both a neomycin resistance (Neo(r)) gene and a poly(A) signal flanked by a tandem pair of loxP sequences and induced transcription of the VSV-G gene from the same promoter as had been used for Neo(r) expression. By inserting an mRNA-destabilizing signal into the 3' untranslated region of the Neo(r) gene to reduce the amount of Neo(r) transcript, we were able efficiently to select the clones capable of inducing VSV-G at high levels. Without the introduction of Cre recombinase, these cell lines produce neither VSV-G nor any detectable infectious virus at all, even after the transduction of a murine leukemia virus-based retrovirus vector encoding beta-galactosidase. They reproducibly produced high-titer virus stocks of VSV-G-pseudotyped retrovirus (1.0 x 10(6) infectious units/ml) from 3 days after the introduction of Cre recombinase. We also present evidence that VSV-G-producing cells are still fully susceptible to transduction by VSV-G pseudotypes. However, in this vector-producing system, which regulates VSV-G pseudotype production in an all-or-none manner, the integration of vector DNA into packaging cell lines would be minimized. We further show that heparin significantly inhibits retransduction of VSV-G pseudotypes in the culture fluids of packaging cell lines, leading to a two- to fourfold increase in the yield of the pseudotypes after induction. This vector-producing system was very stable and should be advantageous in human gene therapy.