A novel family of retroviral vectors for the rapid production of complex stable cell lines

The production of stable cell lines is an important technique in cell biology, and it is often the rate-limiting step in studies involving the characterization of the function of novel genes or gene mutations. To facilitate this process, a novel family of retroviral vectors, the pE vector family, has been generated. The retroviral sequences in the pE vectors have been taken from the Moloney murine leukemia virus (MMLV) vector pMFG, which has been shown to express cDNA inserts more consistently and at higher levels than earlier generations of MMLV vectors. These vectors contain four different internal ribosome entry site-selectable markers, allowing high-efficiency selection of transductants expressing the desired cDNA. The pE vectors have an episomal design to allow long-term production of high-titer virus without the need for subcloning the producer line. Using a strategy of combinatorial infection followed by combinatorial drug selection, we demonstrate that the pE vectors can be used to generate stable, polyclonal cell lines expressing at least three novel cDNAs in less than 2 weeks. The use of these vectors will thus dramatically accelerate the production of complex stable cell lines.     

对虾白斑杆状病毒DNA聚合酶调控序列的克隆

为了揭示对虾白斑杆状病毒的致病机理,将该病毒基因组中推测的DNA聚合酶上游调控序列克隆进荧光素酶报告基因载体中,以便寻找一个能表达该病毒基因的细胞系统.     

Enhancement of enveloped virus entry by phosphatidylserine

Enveloped virus vectors are used in a wide variety of applications. We have discovered that treatment of cultured cells with phosphatidylserine (PS) liposomes can increase virus vector infection by up to 20-fold. This effect does not abrogate virus receptor requirements, is specific to PS compared to other phospholipids, and is limited to enveloped viruses. Furthermore, the enhancement of infection does not occur through increases in virus receptor levels or virus binding, indicating that virus fusion is enhanced. The liposomes are easily generated, store well, and allow enhanced infection with a variety of virus vectors and cell types.     

Production and design of more effective avian replication-incompetent retroviral vectors.

Retroviral vectors have been invaluable tools for studies of development in vertebrates. Their use has been somewhat constrained, however, by the low viral titers typically obtained with replication-incompetent vectors, particularly of the avian type. We have addressed this problem in several ways. We optimized the transient production of avian replication-incompetent viruses in a series of cell lines. One of the optimal cell lines was the mammalian line 293T, which was surprising in light of previous reports that avian viral replication was not supported by mammalian cells. We also greatly increased the efficiency of viral infection. Pseudotyping with the vesicular stomatitus virus G (VSV-G) protein led to an over 350-fold increase in the efficiency of infection in ovo relative to infection with virus particles bearing an avian retroviral envelope protein. To further increase the utility of the system, we developed new Rous sarcoma virus (RSV)-based replication-incompetent vectors, designed to express a histochemical marker gene, human placental alkaline phosphatase, as well as an additional gene. These modified retroviral vectors and the VSV-G pseudotyping technique constitute significant improvements that allow for expanded use of avian replication-incompetent viral vectors in ovo.     

Resistance to human immunodeficiency virus type 1 (HIV-1) infection in human CD4+ lymphocyte-derived cell lines conferred by using retroviral vectors expressing an HIV-1 RNA-specific ribozyme.

Toward gene therapy for the treatment of human immunodeficiency virus type 1 (HIV-1) infections in AIDS, Moloney murine leukemia virus-derived retroviral vectors were engineered to allow constitutive and tat-inducible expression of an HIV-1 5' leader sequence-specific ribozyme (Rz1). These vectors were used to infect the human CD4+ lymphocyte-derived MT4 cell line. The stable MT4 transformants expressing an HIV-1 RNA-specific ribozyme, under the control of the herpes simplex virus thymidine kinase (tk) promoter, were found to be somewhat resistant to HIV-1 infection as virus production was delayed. In cells allowing ribozyme expression under control of the simian virus 40 or cytomegalovirus promoter, the rate of HIV-1 multiplication was slightly decreased, and virus production was delayed by about 14 days. The highest level of resistance to HIV-1 infection was observed in MT4 cells transformed with a vector containing a fusion tk-TAR (trans activation-responsive) promoter to allow ribozyme expression in a constitutive and tat-inducible manner; no HIV-1 production was observed 22 days after infection of these cells. These results indicate that retroviral vectors expressing HIV-1 RNA-specific ribozymes can be used to confer resistance to HIV-1 infection.     

Pseudotyping of glycoprotein D-deficient herpes simplex virus type 1 with vesicular stomatitis virus glycoprotein G enables mutant virus attachment and entry

The use of herpes simplex virus (HSV) vectors for in vivo gene therapy will require the targeting of vector infection to specific cell types in certain in vivo applications. Because HSV glycoprotein D (gD) imparts a broad host range for viral infection through recognition of ubiquitous host cell receptors, vector targeting will require the manipulation of gD to provide new cell recognition specificities in a manner designed to preserve gD's essential role in virus entry. In this study, we have determined whether an entry-incompetent HSV mutant with deletions of all Us glycoproteins, including gD, can be complemented by a foreign attachment/entry protein with a different receptor-binding specificity, the vesicular stomatitis virus glycoprotein G (VSV-G). The results showed that transiently expressed VSV-G was incorporated into gD-deficient HSV envelopes and that the resulting pseudotyped virus formed plaques on gD-expressing VD60 cells, albeit at a 50-fold-reduced level compared to that of wild-type gD. This reduction may be related to differences in the entry pathways used by VSV and HSV or to the observed lower rate of incorporation of VSV-G into virus envelopes than that of gD. The rate of VSV-G incorporation was greatly improved by using recombinant molecules in which the transmembrane domain of HSV glycoprotein B or D was substituted for that of VSV-G, but these recombinant molecules failed to promote virus entry. These results show that foreign glycoproteins can be incorporated into the HSV envelope during replication and that gD can be dispensed with on the condition that a suitable attachment/entry function is provided.     

High-efficiency gene transfer into CD34+ cells with a human immunodeficiency virus type 1-based retroviral vector pseudotyped with vesicular stomatitis virus envelope glycoprotein G.

Currently, amphotropic retroviral vectors are widely used for gene transfer into CD34+ hematopoietic progenitor cells. The relatively low levels of transduction efficiency associated with these vectors in human cells is due to low viral titers and limitations in concentrating the virus because of the inherent fragility of retroviral envelopes. Here we show that a human immunodeficiency virus type 1 (HIV-1)-based retroviral vector containing the firefly luciferase reporter gene can be pseudotyped with a broad-host-range vesicular stomatitis virus envelope glycoprotein G (VSV-G). Higher-efficiency gene transfer into CD34+ cells was achieved with a VSV-G-pseudotyped HIV-1 vector than with a vector packaged in an amphotropic envelope. Concentration of virus without loss of viral infectivity permitted a higher multiplicity of infection, with a consequent higher efficiency of gene transfer, reaching 2.8 copies per cell. These vectors also showed remarkable stability during storage at 4 degrees C for a week. In addition, there was no significant loss of titer after freezing and thawing of the stock virus. The ability of VSV-G-pseudotyped retroviral vectors to achieve a severalfold increase in levels of transduction into CD34+ cells will allow high-efficiency gene transfer into hematopoietic progenitor cells for gene therapy purposes. Furthermore, since it has now become possible to infect CD34+ cells with pseudotyped HIV-1 with a high level of efficiency in vitro, many important questions regarding the effect of HIV-1 on lineage-specific differentiation of hematopoietic progenitors can now be addressed.     

Effects of calcium ion on adenovirus production with high densities of HEK293 cells

One of the major restrictions on the production of adenoviral vectors (AdV) is due to the decrease in virus concentration concomitant with an increase in cell concentration at infection (CCI) which is known as “cell density effect”, this could be because of the limited access to the nutrients or significant accumulation of toxic by-products. However, current strategies, such as developing robust serum-free medium and performing nutrient feeding, will partially address this issue. Therefore, the objective of this study was to further optimize serum-free culture medium by exploring the influence of calcium ion on virus production. Our studies showed that an optimal concentration of calcium ion significantly enhances AdV production, especially at a high CCI. During the virus infection process, a high concentration of calcium ion (≥ 1 mM) caused a reduction in virus infection efficiency, possibly as a result of cell aggregation. However, by optimizing the timing of the addition of calcium ion (i.e., 12 hours post-infection [hpi]), high virus infection efficiency could be maintained. The addition of 0.1 to 2 mM calcium ion at 12 hpi increased virus proliferation dose-dependently. Therefore, the optimal selection of both the concentration and the supplementation time of calcium ion during the process of AdV production could minimize the “cell density effect,” and a 2.6- and 3.2-fold increase in virus concentration could be achieved at CCI3 and CCI4, respectively.     

Production of High-Titer Human Immunodeficiency Virus Type 1 Pseudotyped with Vesicular Stomatitis Virus Glycoprotein

We describe a method for the production of high-titer stocks of human immunodeficiency virus type 1 (HIV-1) pseudotyped with vesicular stomatitis virus envelope glycoprotein (VSV G). VSV G pseudotypes provide several advantages over other retroviral envelope proteins. The VSV G envelope is mechanically stable, enabling ultracentrifugal concentration of virions to high titers, and VSV G has a broad host range, enabling infection of many mammalian and nonmammalian cell types. VSV G pseudotypes of HIV-1 are useful for the study of HIV infection and replication kinetics and for the study of the function of specific viral proteins. We describe applications for the study of HIV-1 using VSV G pseudotypes. Additionally, we describe a method for pseudotyping retroviral vectors with VSV G. The same advantages of VSV G pseudotypes of HIV-1 apply to retroviral vectors; VSV G pseudotyped retroviral vectors may be used to introduce genes of interest into a wide variety of cell lines.     

Stimulation of enveloped virus infection by beta-amyloid fibrils

Alzheimer's disease is characterized by deposition of beta-amyloid peptide (Abeta) into plaques in the brain, leading to neuronal toxicity and dementia. Human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system can also cause a dementia, and amyloid deposition in the central nervous system is significantly higher in HIV-1-infected individuals compared with uninfected controls. Here we report that Abeta fibrils stimulated, by 5-20-fold, infection of target cells expressing CD4 and an appropriate coreceptor by multiple HIV-1 isolates but did not permit infection of cells lacking these receptors. Abeta enhanced infection at the stage of virus attachment or entry into the cell. Abeta fibrils also stimulated infection by amphotrophic Moloney leukemia virus, herpes simplex virus, and viruses pseudotyped with the envelope glycoprotein of vesicular stomatitis virus. Other synthetic fibril-forming peptides similarly enhanced viral infection and may be useful in gene delivery applications utilizing retroviral vectors. These data suggest that Abeta deposition may increase the vulnerability of the central nervous system to enveloped viral infection and that amyloidogenic peptides could be useful in enhancing gene transfer by enveloped viral vectors.     

Implications of the innate immune response to adenovirus and adenoviral vectors

Adenovirus (AdV) is a common cause of respiratory illness in both children and adults. Respiratory symptoms can range from those of the common cold to severe pneumonia. Infection can also cause significant disease in the immunocompromised and among immunocompetent subjects in close quarters. Fortunately, infection with AdV in the normal host is generally mild. This is one reason why its initial use as a gene-therapy vector appeared to be so promising. Unfortunately, both innate and adaptive responses to the virus have limited the development of AdV vectors as a tool of gene therapy by increasing toxicity and limiting duration of transgene expression. This article will focus on the innate immune response to infection with wild-type AdV and exposure to AdV gene-therapy vectors. As much of the known information relates to the pulmonary inflammatory response, this organ system will be emphasized. This article will also discuss how that understanding has led to the creation of new vectors for use in gene therapy.     

Species-specific, postentry barriers to primate immunodeficiency virus infection

By using replication-defective vectors derived from human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV(mac)), and murine leukemia virus (MuLV), all of which were pseudotyped with the vesicular stomatitis virus (VSV) G glycoprotein, the efficiency of postentry, early infection events was examined in target cells of several mammalian species. Titers of HIV-1 vectors were significantly lower than those of SIV(mac) and MuLV vectors in most cell lines and primary cells from Old World monkeys. By contrast, most New World monkey cells exhibited much lower titers for the SIV(mac) vector compared with those of the HIV-1 vector. Prosimian cells were resistant to both HIV-1 and SIV(mac) vectors, although the MuLV vector was able to infect these cells. Cells from other mammalian species were roughly equivalent in susceptibility to the three vectors, with the exception of rabbit cells, which were specifically resistant to the HIV-1 vector. The level of HIV-1 vector expression was very low in transduced cells of rodent, rabbit, cow, and pig origin. Early postentry restriction of primate immunodeficiency virus infection exhibits patterns largely coincident with species borders and applies to diverse cell types within an individual host, suggesting the involvement of species-specific, widely expressed cellular factors.     

Oncolytic adenoviruses in anticancer therapy: Current status and prospects

Lytic virus infection results in production of a virus progeny and lysis of the infected cell. Tumor cells are usually more sensitive to virus infection. Studies indicate that viral oncolysis provides a promising alternative approach to cancer therapy. The ability of viruses to selectively kill cancer cells is long known, but construction of virus variants with an improved therapeutic potential was impossible until recent advances in virus and cell molecular biology and the development of modern methods for directed modification of viruses. Adenoviruses are one of the best studied models of oncolytic viruses. These DNA viruses are convenient for genetic manipulation and show minimal pathogenicity. The review summarizes the data on the directions and approaches to generation of highly efficient variants of oncolytic adenoviruses. The approaches include introduction of directed genetic modifications into the virus genome, accelerated selection of oncolytic virus variants following treatment with mutagens, the use of adenoviruses as vectors to introduce therapeutic gene products, optimization of viral delivery systems, minimization of the negative effects from the host immune system, etc. The dynamic development of studies in the field holds promise that many variants of oncolytic adenoviruses will find clinical application in the nearest future.     

Production of retrovirus and adenovirus vectors for gene therapy: a comparative study using microcarrier and stationary cell culture

In gene therapy, retrovirus and adenovirus vectors are extensively used as gene-delivery vehicles and further large-scale processing of these viral vectors will be increasingly important. This study examined stationary and microcarrier cell culture systems with respect to the production of a retrovirus vector (encoding a monounit hammerhead ribozyme gene with an intron) and an adenovirus vector (encoding a reporter lacZ gene). Cytodex 1 and Cytodex 3 solid microcarriers were found to be able to provide good cell growth and high-titer vector production in suspension cultures. Porous microcarriers such as Cytopore 2 gave slightly lower but still efficient growth but produced significantly lower titers of retrovirus and adenovirus vector from the producer cells. The specific retrovirus production was not proportionally related to the specific growth rate of the producer cells. High MOI infection was essential for high-titer production of adenovirus vector in 293 cells. Hydrodynamic shear forces on microcarrier-grown cells increased the production yield for retrovirus vector but decreased for adenovirus vector. The cellular productivity was much more efficient for adenovirus vector produced in 293 cells as compared to the retrovirus vector produced in PA317-RCM1 cells. These findings can provide further insight into the feasibility of applying microcarrier cell culture technology to produce gene-therapy virus vectors.     

Development of a Chinese hamster ovary cell line for recombinant adenovirus-mediated gene expression

Recombinant human adenovirus (rhAd) has been used extensively for functional protein expression in mammalian cells including those of human and nonhuman origin. High-level protein production by rhAd vectors is expected in their permissive host cells, such as the human embryonic kidney 293 (HEK293) cell line. This is attributed primarily to the permissiveness of HEK293 to rhAd infection and their ability to support viral DNA replication by providing the missing El proteins. However, the HEK293 cells tend to suffer from cytopathic effect (CPE) as a result of virus replication. Under these circumstances, the host cell function is compromised and the culture viability will be reduced. Consequently, newly synthesized polypeptides may not be processed properly at posttranslational levels. Therefore, the usefulness of HEK293 cells for the expression of complex targets such as secreted proteins could be limited. In the search for a more robust cell line as a production host for rhAd expression vectors, a series of screening experiments was performed to isolate clones from Chinese hamster ovary-K1 (CHO-K1) cells. First, multiple rounds of infection of CHO-K1 cells were performed utilizing an rhAd expressing GFP. After each cycle of infection, a small population of CHO cells with high GFP levels was enriched by FACS. Second, individual clones more permissive to human adenovirus infection were isolated from the highly enriched subpopulation by serial dilution. A single clone, designated CHO-K1-C5, was found to be particularly permissive to rhAd infection than the parental pool and has served as a production host in the successful expression of several secreted proteins.     

Factors Influencing Adeno-Associated Virus-Mediated Gene Transfer to Human Cystic Fibrosis Airway Epithelial Cells: Comparison with Adenovirus Vectors

Adeno-associated virus (AAV) vectors appear promising for use in gene therapy in cystic fibrosis (CF) patients, yet many features of AAV-mediated gene transfer to airway epithelial cells are not well understood. We compared the transduction efficiencies of AAV vectors and adenovirus (Ad) vectors in immortalized cell lines from CF patients and in nasal epithelial primary cultures from normal humans and CF patients. Similar dose-dependent relationships between the vector multiplicities of infection and the efficiencies of lacZ gene transfer were observed. However, levels of transduction for both Ad and recombinant AAV (rAAV) were significantly lower in the airway epithelial cell than in the control cell lines HeLa and HEK 293. Transduction efficiencies differed among cultured epithelial cell types, with poorly differentiated cells transducing more efficiently than well-differentiated cells. A time-dependent increase in gene expression was observed after infection for both vectors. For Ad, but not for AAV, this increase was dependent on prolonged incubation of cells with the vector. Furthermore, for rAAV (but not for rAd), the delay in maximal transduction could be abrogated by wild-type Ad helper infection. Thus, although helper virus is not required for maximal transduction, it increases the kinetics by which this is achieved. Expression of Ad E4 open reading frame 6 or addition of either hydroxyurea or camptothecin resulted in increased AAV transduction, as previously demonstrated for nonairway cells (albeit to lower final levels), suggesting that second-strand synthesis may not be the sole cause of inefficient transduction. Finally, the efficiency of AAV-mediated ex vivo gene transfer to lung cells was similar to that previously described for Ad vectors in that transduction was limited to regions of epithelial injury and preferentially targeted basal-like cells. These studies address the primary factors influencing rAAV infection of human airway cells and should impact successful gene delivery in CF patients.     

Oncoretrovirus and Lentivirus Vectors Pseudotyped with Lymphocytic Choriomeningitis Virus Glycoprotein: Generation, Concentration, and Broad Host Range

Lymphocytic choriomeningitis virus (LCMV) is a noncytopathic arenavirus shown to infect a broad range of different cell types. Here, we combined the beneficial characteristics of the LCMV glycoprotein (LCMV-GP) and those of retroviral vectors to generate a new, safe, and efficient gene transfer system. These LCMV-GP pseudotypes were systematically compared with vectors containing the widely used amphotropic murine leukemia virus envelope (A-MLVenv) or the vesicular stomatitis virus G protein (VSV-G). Production of LCMV-GP-pseudotyped oncoretroviral and lentiviral vectors by transient transfection resulted in vector titers similar to those with A-MLVenv or VSV-G. In contrast to A-MLVenv particles, LCMV-GP pseudotypes could be efficiently concentrated by ultracentrifugation without loss of vector titer. Unlike the cell-toxic VSV-G, a stable retroviral packaging cell line constitutively expressing LCMV-GP could be established. Vectors pseudotyped with LCMV-GP efficiently transduced many cell lines from different species and tissues relevant for gene therapy. Transduction of human glioma cells was studied in detail. These cells are a major target for cancer gene therapy and were transduced more efficiently with LCMV-GP-pseudotyped vectors than with the generally used A-MLVenv particles. The high stability, low toxicity, and broad host range make LCMV-GP-pseudotyped vectors attractive for gene transfer applications. The recombinant LCMV-GP-pseudotyped vectors will also allow functional characterization of naturally occurring and recombinant LCMV-GP variants.     

Safety aspects of insect cell culture

Conclusions The hazards posed to the environment by the accidental release of baculovirus expression vectors can be put into perspective by the results obtained from experiments in which AcNPV was released deliberately into the field (Bishop et al., 1992). Polyhedrin positive viruses will persist in soil and on leaf surfaces for periods comprising weeks and months. However, polyhedrin negative viruses (similar to those used as expression vectors) do not survive in similar situations. In consequence, accidental release of baculovirus expression vectors poses negligible hazard. The risk of such a release will largely depend on the skill of the operators. This does not take into account the hazard posed by the recombinant product which is being made by the virus-infected insect cell. Synthesis of a mammalian-specific toxin, of course, would require particularly careful manipulation of the virus-infected cell culture.The fact that insect cell lines represent an undefined risk, in terms of carriage of adventitious agents means that their containment should be maintained at a minimum of the European containment level 2. Where the tissue of origin has a high risk of infection with human pathogens or where cells may have been used in a virus culture laboratory then appropriate testing is advisable. Careful risk assessment respecting the scale of work and whole procedures (in addition to individual assessment of agents and reagents) will ensure safe working conditions for laboratory staff. If applied properly safety procedures will also succeed in encouraging clean, efficient and well documented work procedures which are synonymous with the economical use of time and resources and good science.     

Novel baculovirus expression vectors that provide sialylation of recombinant glycoproteins in lepidopteran insect cells

This report describes novel baculovirus vectors designed to express mammalian beta1,4-galactosyltransferase and alpha2,6-sialyltransferase genes at early times after infection. Sf9 cells infected with these viral vectors, unlike cells infected with a wild-type baculovirus, produced a sialylated viral glycoprotein during the late phase of infection. Thus, the two mammalian glycosyltransferases encoded by these viral vectors are necessary and sufficient for sialylation of a foreign glycoprotein in insect cells under the conditions used in this study. While some of the new baculovirus vectors described in this study produced less, one produced wild-type levels of infectious budded virus progeny.