Evaluation of baculovirus expression vectors with enhanced stability in continuous cascaded insect-cell bioreactors

Adeno-associated virus as a delivery vector for gene therapy of human diseases

Adeno-associated virus (AAV) has emerged as a pivotal delivery tool in clinical gene therapy owing to its minimal pathogenicity and ability to establish long-term gene expression in different tissues. Recombinant AAV (rAAV) has been engineered for enhanced specificity and developed as a tool for treating various diseases. However, as rAAV is being more widely used as a therapy, the increased demand has created challenges for the existing manufacturing methods. Seven rAAV-based gene therapy products have received regulatory approval, but there continue to be concerns about safely using high-dose viral therapies in humans, including immune responses and adverse effects such as genotoxicity, hepatotoxicity, thrombotic microangiopathy, and neurotoxicity. In this review, we explore AAV biology with an emphasis on current vector engineering strategies and manufacturing technologies. We discuss how rAAVs are being employed in ongoing clinical trials for ocular, neurological, metabolic, hematological, neuromuscular, and cardiovascular diseases as well as cancers. We outline immune responses triggered by rAAV, address associated side effects, and discuss strategies to mitigate these reactions. We hope that discussing recent advancements and current challenges in the field will be a helpful guide for researchers and clinicians navigating the ever-evolving landscape of rAAV-based gene therapy.

Evaluation of the immunogenicity of vaccine candidates developed using a baculovirus surface display system for Crimean-Congo hemorrhagic fever virus in mice

Crimean-Congo hemorrhagic fever (CCHF), which has a fatality rate of 20–30%, is widely prevalent in several regions in Asia, Europe, and Africa and has spread to a wider range of areas in recent years. At present, there is a lack of safe and effective vaccines for the prevention of CCHF. In this study, we prepared three vaccine candidates, rvAc-Gn, rvAc-Np, and rvAc-Gn-Np, that encoded the CCHF virus (CCHFV) glycoprotein Gn and the nucleocapsid protein (Np) on the surface of baculovirus using an insect baculovirus vector expression system (BVES) and evaluated their immunogenicity in BALB/c mice. The experimental results showed that both CCHFV Gn and Np were expressed by the respective recombinant baculoviruses and anchored to the viral envelope. BALB/c mice were immunized, and all three recombinant baculoviruses showed significant humoral immunity. At the cellular level, the level of immunity in the rvAc-Gn group was significantly higher than that in the rvAc-Np and rvAc-Gn-Np groups, and the rvAc-Gn-Np coexpression group exhibited the lowest level of cellular immunity. In conclusion, the strategy of coexpressing Gn and Np in the baculovirus surface display system did not result in improvements in immunogenicity, whereas the recombinant baculovirus displaying Gn alone could induce significant humoral and cellular immunity in mice, indicating that rvAc-Gn has potential as a CCHF vaccine candidate. This study thus provides new ideas for the development of a CCHF baculovirus vaccine.

Highly efficient CRISPR-mediated large DNA docking and multiplexed prime editing using a single baculovirus

CRISPR-based precise gene-editing requires simultaneous delivery of multiple components into living cells, rapidly exceeding the cargo capacity of traditional viral vector systems. This challenge represents a major roadblock to genome engineering applications. Here we exploit the unmatched heterologous DNA cargo capacity of baculovirus to resolve this bottleneck in human cells. By encoding Cas9, sgRNA and Donor DNAs on a single, rapidly assembled baculoviral vector, we achieve with up to 30% efficacy whole-exon replacement in the intronic β-actin (ACTB) locus, including site-specific docking of very large DNA payloads. We use our approach to rescue wild-type podocin expression in steroid-resistant nephrotic syndrome (SRNS) patient derived podocytes. We demonstrate single baculovirus vectored delivery of single and multiplexed prime-editing toolkits, achieving up to 100% cleavage-free DNA search-and-replace interventions without detectable indels. Taken together, we provide a versatile delivery platform for single base to multi-gene level genome interventions, addressing the currently unmet need for a powerful delivery system accommodating current and future CRISPR technologies without the burden of limited cargo capacity.

Relocation of the attTn7 Transgene Insertion Site in Bacmid DNA Enhances Baculovirus Genome Stability and Recombinant Protein Expression in Insect Cells

Baculovirus expression vectors are successfully used for the commercial production of complex (glyco)proteins in eukaryotic cells. The genome engineering of single-copy baculovirus infectious clones (bacmids) in E. coli has been valuable in the study of baculovirus biology, but bacmids are not yet widely applied as expression vectors. An important limitation of first-generation bacmids for large-scale protein production is the rapid loss of gene of interest (GOI) expression. The instability is caused by the mini-F replicon in the bacmid backbone, which is non-essential for baculovirus replication in insect cells, and carries the adjacent GOI in between attTn7 transposition sites. In this paper, we test the hypothesis that relocation of the attTn7 transgene insertion site away from the mini-F replicon prevents deletion of the GOI, thereby resulting in higher and prolonged recombinant protein expression levels. We applied lambda red genome engineering combined with SacB counterselection to generate a series of bacmids with relocated attTn7 sites and tested their performance by comparing the relative expression levels of different GOIs. We conclude that GOI expression from the odv-e56 (pif-5) locus results in higher overall expression levels and is more stable over serial passages compared to the original bacmid. Finally, we evaluated this improved next-generation bacmid during a bioreactor scale-up of Sf9 insect cells in suspension to produce enveloped chikungunya virus-like particles as a model vaccine.

On the stability of sequences inserted into viral genomes

Viruses are widely used as vectors for heterologous gene expression in cultured cells or natural hosts, and therefore a large number of viruses with exogenous sequences inserted into their genomes have been engineered. Many of these engineered viruses are viable and express heterologous proteins at high levels, but the inserted sequences often prove to be unstable over time and are rapidly lost, limiting heterologous protein expression. Although virologists are aware that inserted sequences can be unstable, processes leading to insert instability are rarely considered from an evolutionary perspective. Here, we review experimental work on the stability of inserted sequences over a broad range of viruses, and we present some theoretical considerations concerning insert stability. Different virus genome organizations strongly impact insert stability, and factors such as the position of insertion can have a strong effect. In addition, we argue that insert stability not only depends on the characteristics of a particular genome, but that it will also depend on the host environment and the demography of a virus population. The interplay between all factors affecting stability is complex, which makes it challenging to develop a general model to predict the stability of genomic insertions. We highlight key questions and future directions, finding that insert stability is a surprisingly complex problem and that there is need for mechanism-based, predictive models. Combining theoretical models with experimental tests for stability under varying conditions can lead to improved engineering of viral modified genomes, which is a valuable tool for understanding genome evolution as well as for biotechnological applications, such as gene therapy.

Identifying parameters to improve the reproducibility of transient gene expression in High Five cells

Virus-free, transient gene expression (TGE) in High Five cells was recently presented as an efficient protein production method. However, published TGE protocols have not been standardized to a general protocol. Therefore, reproducibility and implementation of the method in other labs remains difficult. The aim of this study is to analyse the parameters determining the reproducibility of the TGE in insect cells. Here, we identified that using linear 40 kDa PEI instead of 25 kDa PEI was one of the most important aspects to improve TGE. Furthermore, DNA amount, DNA:PEI ratio, growth phase of the cells before transfection, passage number, the origin of the High-Five cell isolates and the type of cultivation medium were considered. Interestingly, a correlation of the passage number to the DNA content of single cells (ploidy) and to the transfection efficacy could be shown. The optimal conditions for critical parameters were used to establish a robust TGE method. Finally, we compared the achieved product yields in High Five cells using our improved TGE method with both the baculoviral expression system and TGE in the mammalian HEK293-6E cell line. In conclusion, the presented robust TGE protocol in High Five cells is easy to establish and produces ample amounts of high-quality recombinant protein, bridging the gap in expression level of this method to the well-established mammalian TGE in HEK293 cells as well as to the baculoviral expression vector system (BEVS).

Remarkably efficient production of a highly insecticidal Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV) isolate in its homologous host

BACKGROUND

A Chrysodeixis chalcites nucleopolyhedrovirus from the Canary Islands (ChchNPV-TF1) has proved to be effective for control of Chrysodeixis chalcites on banana crops. Commercialization of this virus as a bioinsecticide requires an efficient production system.

RESULTS

The sixth instar (L₆ ) was the most suitable for virus production, producing 1.80 × 10¹¹ occlusion bodies (OB)/larva and showed a lower prevalence of cannibalism (5.4%) than fourth (L₄ ) or fifth (L₅ ) instars. Inoculation of L₆ at 24 h post molting produced six times more OB (5.72 × 10¹¹ OB/larva) than recently molted L₆ larvae (1.00 × 10¹¹ OB/larva). No significant differences were recorded in mean time to death (165-175 h) or OB production per larva (3.75 × 10¹¹ to 5.97 × 10¹¹ ) or per mg larval weight (1.30 × 10¹¹ to 2.11 × 10⁹ ), in larvae inoculated with a range of inoculum concentrations (LC₅₀ -LC₉₀ ). Groups of infected L₆ larvae reared at a density of 150 larvae/container produced a greater total number of OBs (8.07 × 10¹³ OB/container) than lower densities (25, 50 and 100 OB/container), and a similar number to containers with 200 inoculated larvae (8.43 × 10¹³ OB/container).

CONCLUSION

The processes described here allow efficient production of sufficient OBs to treat ∼ 40 ha of banana crops using the insects from a single container. © 2018 Society of Chemical Industry.

Expression and purification of classical swine fever virus E2 protein from Sf9 cells using a modified vector

OBJECTIVE

To develop a simple method for efficient expression of classical swine fever virus (CSFV) E2 protein.

RESULTS

The pFastBac HT B vector (pFastHTB-M1) was modified by adding a melittin signal peptide sequence. The E2 gene fragment without the transmembrane region was cloned into pFastHTB-M1. The modified vector has clear advantage over the original one, as evidenced by the purified recombinant E2 protein that was detected significantly by SDS-PAGE.

CONCLUSIONS

The modified vector has the potential for large-scale production and easy purification of the CSFV E2 protein or other proteins of interests.

Determinant Factors in the Production of a Co-Occluded Binary Mixture of Helicoverpa armigera Alphabaculovirus (HearNPV) Genotypes with Desirable Insecticidal Characteristics

A co-occluded binary mixture of Helicoverpa armigera nucleopolyhedrovirus genotypes HearSP1B and HearLB6 at a 1:1 ratio (HearSP1B+HearLB6) was selected for the development of a virus-based biological insecticide, which requires an efficient large-scale production system. In vivo production systems require optimization studies in each host-virus pathosystem. In the present study, the effects of larval instar, rearing density, timing of inoculation, inoculum concentration and temperature on the production of HearSP1B+HearLB6 in its homologous host were evaluated. The high prevalence of cannibalism in infected larvae (40-87%) indicated that insects require individual rearing to avoid major losses in OB production. The OB production of recently molted fifth instars (7.0 x 109 OBs/larva), combined with a high prevalence of mortality (85.7%), resulted in the highest overall OB yield (6.0 x 1011 OBs/100 inoculated larvae), compared to those of third or fourth instars. However, as inoculum concentration did not influence final OB yield, the lowest concentration, LC80 (5.5 x 106 OBs/ml), was selected. Incubation temperature did not significantly influence OB yield, although larvae maintained at 30°C died 13 and 34 hours earlier than those incubated at 26°C and 23°C, respectively. We conclude that the efficient production of HearSP1B+HearLB6 OBs involves inoculation of recently molted fifth instars with a LC80 concentration of OBs followed by individual rearing at 30°C.

Bioreactors for high cell density and continuous multi-stage cultivations: options for process intensification in cell culture-based viral vaccine production

With an increasing demand for efficacious, safe, and affordable vaccines for human and animal use, process intensification in cell culture-based viral vaccine production demands advanced process strategies to overcome the limitations of conventional batch cultivations. However, the use of fed-batch, perfusion, or continuous modes to drive processes at high cell density (HCD) and overextended operating times has so far been little explored in large-scale viral vaccine manufacturing. Also, possible reductions in cell-specific virus yields for HCD cultivations have been reported frequently. Taking into account that vaccine production is one of the most heavily regulated industries in the pharmaceutical sector with tough margins to meet, it is understandable that process intensification is being considered by both academia and industry as a next step toward more efficient viral vaccine production processes only recently. Compared to conventional batch processes, fed-batch and perfusion strategies could result in ten to a hundred times higher product yields. Both cultivation strategies can be implemented to achieve cell concentrations exceeding 10(7) cells/mL or even 10(8) cells/mL, while keeping low levels of metabolites that potentially inhibit cell growth and virus replication. The trend towards HCD processes is supported by development of GMP-compliant cultivation platforms, i.e., acoustic settlers, hollow fiber bioreactors, and hollow fiber-based perfusion systems including tangential flow filtration (TFF) or alternating tangential flow (ATF) technologies. In this review, these process modes are discussed in detail and compared with conventional batch processes based on productivity indicators such as space-time yield, cell concentration, and product titers. In addition, options for the production of viral vaccines in continuous multi-stage bioreactors such as two- and three-stage systems are addressed. While such systems have shown similar virus titers compared to batch cultivations, keeping high yields for extended production times is still a challenge. Overall, we demonstrate that process intensification of cell culture-based viral vaccine production can be realized by the consequent application of fed-batch, perfusion, and continuous systems with a significant increase in productivity. The potential for even further improvements is high, considering recent developments in establishment of new (designer) cell lines, better characterization of host cell metabolism, advances in media design, and the use of mathematical models as a tool for process optimization and control.

Tubular Bioreactor for Probing Baculovirus Infection and Protein Production

Probing the baculovirus infection process is essential in optimizing recombinant protein production. Typically, researchers monitor the infection process in stirred reactors that contain cells that have been infected at different times after virus inoculation, particularly if cells pass the primary infection and become infected by progeny virus. This chapter describes several alternative bioreactor systems for baculovirus infection. We provide an example alternative system that holds promise to avoid asynchronous distributions in infection time. Namely, we describe a two-stage reactor system consisting of an upstream continuous stirred tank reactor and a downstream tubular reactor with segmented plug flow for probing baculovirus infection and production.

Bioreactor concepts for cell culture-based viral vaccine production

Vaccine manufacturing processes are designed to meet present and upcoming challenges associated with a growing vaccine market and to include multi-use facilities offering a broad portfolio and faster reaction times in case of pandemics and emerging diseases. The final products, from whole viruses to recombinant viral proteins, are very diverse, making standard process strategies hardly universally applicable. Numerous factors such as cell substrate, virus strain or expression system, medium, cultivation system, cultivation method, and scale need consideration. Reviewing options for efficient and economical production of human vaccines, this paper discusses basic factors relevant for viral antigen production in mammalian cells, avian cells and insect cells. In addition, bioreactor concepts, including static systems, single-use systems, stirred tanks and packed-beds are addressed. On this basis, methods towards process intensification, in particular operational strategies, the use of perfusion systems for high product yields, and steps to establish continuous processes are introduced.

Use of bacterial artificial chromosomes in baculovirus research and recombinant protein expression: current trends and future perspectives

The baculovirus expression system is one of the most successful and widely used eukaryotic protein expression methods. This short review will summarise the role of bacterial artificial chromosomes (BACS) as an enabling technology for the modification of the virus genome. For many years baculovirus genomes have been maintained in E. coli as bacterial artificial chromosomes, and foreign genes have been inserted using a transposition-based system. However, with recent advances in molecular biology techniques, particularly targeting reverse engineering of the baculovirus genome by recombineering, new frontiers in protein expression are being addressed. In particular, BACs have facilitated the propagation of disabled virus genomes that allow high throughput protein expression. Furthermore, improvement in the selection of recombinant viral genomes inserted into BACS has enabled the expression of multiprotein complexes by iterative recombineering of the baculovirus genome.

Production of baculovirus defective interfering particles during serial passage is delayed by removing transposon target sites in fp25k

Accumulation of baculovirus defective interfering particle (DIP) and few polyhedra (FP) mutants is a major limitation to continuous large-scale baculovirus production in insect-cell culture. Although overcoming these mutations would result in a cheaper platform for producing baculovirus biopesticides, little is known regarding the mechanism of FP and DIP formation. This issue was addressed by comparing DIP production of wild-type (WT) Autographa californica multiple nucleopolyhedrovirus (AcMNPV) with that of a recombinant AcMNPV (denoted Ac-FPm) containing a modified fp25k gene with altered transposon insertion sites that prevented transposon-mediated production of the FP phenotype. In addition to a reduction in the incidence of the FP phenotype, DIP formation was delayed on passaging of Ac-FPm compared with WT AcMNPV. Specifically, the yield of DIP DNA in Ac-FPm was significantly lower than in WT AcMNPV up to passage 16, thereby demonstrating that modifying the transposon insertion sites increases the genomic stability of AcMNPV. A critical component of this investigation was the optimization of a systematic method based on the use of pulsed-field gel electrophoresis (PFGE) to characterize extracellular virus DNA. Specifically, PFGE was used to detect defective genomes, determine defective genome sizes and quantify the amount of defective genome within a heterogeneous genome population of passaged virus.

Enhanced expression of recombinant proteins utilizing a modified baculovirus expression vector

The baculovirus expression vector system (BEVS) has been widely used for over-expressing eukaryotic proteins due to a close resemblance in post-translational modification, processing, and transportation properties of the expressed protein, to that of the mammalian cells. In comparison to the bacterial expression system, protein yield from BEVS is relatively low, resulting in higher cost of production. To improve the existing recombinant protein expression levels, baculovirus homologous region1 (hr1) was strategically integrated into the bacmid-based transfer vectors. Luciferase reporter, human Protein Kinase B-alpha (PKB-A), and N-terminal-modified CYP-1A2 genes were independently cloned in non-hr1 and hr1 constructs for generating respective bacmids and baculoviruses. These recombinant baculoviruses were utilized for comparing the expression levels at varying multiplicity of infections (MOI) and time intervals in Spodoptera frugiperda (Sf21) or Trichoplusia ni (Tni) insect cell lines. Targeted insertion of hr1 upstream to CYP-1A2, PKB-A, and Luciferase genes, compared to the non-hr1 sets, led to 3-, 3.5-, and 4.5-fold increase in the resultant protein levels, respectively. Moreover, at equal protein concentration, the corresponding activity and inhibition characteristics of these high expression hr1 sets were comparable to that of the respective non-hr1 sets. Utilization of this modified baculovirus expression construct offers significant advantage of producing recombinant proteins in a cost-effective manner for various biotechnological and therapeutic applications.

MultiBac: multigene baculovirus-based eukaryotic protein complex production

Multiprotein complexes are an emerging focus in current biology, resulting in a demand for advanced heterologous expression systems. This unit provides protocols for the expression of eukaryotic multiprotein complexes using multigene expression vectors. Homologous and site-specific recombinases facilitate their assembly. Thus, modification of individual subunits for revised expression studies is achieved with comparative ease. The strategy outlined here employs the MultiBac baculoviral expression system for multiprotein complexes as an example. Baculoviral expression does not require particular safety precautions due to the replication incompetence of baculovirus in mammalian hosts. The MultiBac system provides for improved protein production due to deletion of specific viral genes (V-cath, chiA). Most of the steps described in this unit are tailored for high-throughput approaches. The general strategy of rapidly combining encoding DNAs by recombination into multigene expression vectors for protein complex expression can also be applied to other prokaryotic or mammalian expression systems.

Multiprotein Expression Strategy for Structural Biology of Eukaryotic Complexes

The concept of the cell as a collection of multisubunit protein machines is emerging as a cornerstone of modern biology, and molecular-level study of these machines in most cases will require recombinant production. Here, we present and validate a strategy to rapidly produce, permutate, and posttranslationally modify large, eukaryotic multiprotein complexes by using DNA recombination in a process that is fully automatable. Parallel production of 12 protein complex variants within a period of weeks resulted in specimens of sufficient quantity and homogeneity for structural biology applications.

Protein complex expression by using multigene baculoviral vectors

Elucidation of the molecular basis of protein-interaction networks, in particular in higher eukaryotes, is hampered by insufficient quantities of endogenous multiprotein complexes. Present recombinant expression methods often require considerable investment in both labor and materials before multiprotein expression, and after expression and biochemical analysis these methods do not provide flexibility for expressing an altered multiprotein complex. To meet these demands, we have recently introduced MultiBac, a modular baculovirus-based system specifically designed for eukaryotic multiprotein expression. Here we describe new transfer vectors and a combination of DNA recombination-based methods, which further facilitate the generation of multigene cassettes for protein coexpression (Fig. 1), thus providing a flexible platform for generation of protein expression vectors and their rapid regeneration for revised expression studies. Genes encoding components of a multiprotein complex are inserted into a suite of compatible transfer vectors by homologous recombination. These progenitor constructs are then rapidly joined in the desired combination by Cre-loxP-mediated in vitro plasmid fusion. Protocols for integration of the resulting multigene expression cassettes into the MultiBac baculoviral genome are provided that rely on Tn7 transposition and/or Cre-loxP reaction carried out in vivo in Escherichia coli cells tailored for this purpose. Detailed guidelines for multigene virus generation and amplification, cell culture maintenance and protein production are provided, together with data illustrating the simplicity and remarkable robustness of the present method for multiprotein expression using a composite MultiBac baculoviral vector.

Economized large-scale production of high yield of rAAV for gene therapy applications exploiting baculovirus expression system

BACKGROUND

The versatility of recombinant adeno-associated vector (rAAV) as a gene delivery system is due to the vector's ability to transduce different cell types as well as dividing and non-dividing cells. Large-scale production of rAAV remains one of the major challenges for continued development of pre-clinical and clinical studies, and for its potential commercialization. The baculovirus expression vectors (BEVS) and insect cells represent a potential method to produce rAAV economically at large scale. This technology uses three different BEVs (Bac-Rep, Bac-GFP, and Bac-VP) each at a multiplicity of infection (MOI) of 3. We reported previously the production of rAAV at 40 L scale using a stirred-tank bioreactor (STB). However, production in larger volumes is limited by the stability of the BEVs and amount of BEVs needed to achieve the target MOI of 3 per BEV. Here, the production parameters were optimized and the baculovirus stability was determined.

METHODS

The stability of the three types of baculovirus used to produce rAAV was determined for six expansion passages by protein expression analysis. To economize baculovirus, MOI and cell density at time of infection (TOI) were evaluated initially at small scale and then applied to the 10 L scale.

RESULTS

An MOI = 0.03 and TOI cell density of 1 x 10(6) cells/mL produced high titer rAAV without comprising yield. To confirm the scalability of the process, rAAV was produced in a 10 L STB using the optimized parameters obtaining a 10x increase in yield ( approximately 1 x 10(14) rAAV DNAse-resistant particles per liter).

CONCLUSION

These findings contribute to the process development for large-scale production of rAAV for gene therapy applications and its commercialization.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.

Production of adeno-associated viral vectors in insect cells using triple infection: Optimization of baculovirus concentration ratios

The production of viral vectors or virus-like particles for gene therapy or vaccinations using the baculovirus expression system is gaining in popularity. Recently, reports of a viral vector based on adeno-associated virus (AAV) produced in insect cells using the baculovirus expression vector system have been published. This system requires the triple infection of cells with baculovirus vectors containing the AAV gene for replication proteins (BacRep), the AAV gene for structural proteins (BacCap), and the AAV vector genome (BacITR). A statistical approach was used to investigate the multiplicities of infection of the three baculoviruses and the results were extended to the production of AAVs containing various transgenes. Highest AAV yields were obtained when BacRep and BacCap, the baculovirus vectors containing genes that code for proteins necessary for the formation of the AAV vector, were added in equal amounts at high multiplicities of infection. These combinations also resulted in the closest ratios of infectious to total AAV particles produced. Overexpression of the AAV structural proteins led to the production of empty AAV capsids, which is believed to overload the cellular machinery, preventing proper encapsidation of the AAV vector transgene, and decreased the viability of the insect cells. Delaying the input of BacCap, to reduce the amount of capsids produced, resulted in lower infectious AAV titers then when all three baculoviruses were put into the system at the same time. The amount of BacITR added to the system can be less than the other two without loss of AAV yield.

Baculovirus vectors for gene therapy

Since the discovery that baculoviruses can efficiently transduce mammalian cells, baculoviruses have been extensively studied as potential vectors for both in vitro and in vivo gene therapy. This chapter reviews the history of this research area, cells permissive to baculovirus transduction, factors influencing transduction and transgene expression, efforts to improve transduction, mechanisms of virus entry and intracellular trafficking, applications for in vivo and ex vivo gene therapy, as well as advantages, limitations, and safety issues concerning use of baculoviruses as gene therapy vectors. Recent progress and efforts directed toward overcoming existing bottlenecks are emphasized.

Stabilized baculovirus vector expressing a heterologous gene and GP64 from a single bicistronic transcript

The efficient scale-up of recombinant protein production in insect-cell bioreactors using baculovirus expression vectors is hampered by reductions in yield with increasing viral passage, the so-called passage effect. This phenomenon is characterized by the generation and subsequent accumulation of defective interfering baculoviruses (DIs), which interfere with the replication of genomically intact virus. A novel baculovirus expression vector is presented equipped with a bicistronic expression cassette that allows the simultaneous expression of the recombinant gene (GFP, first cistron) and an essential baculovirus gene (GP64, second cistron) from a single messenger RNA (mRNA). The translation of GP64 is mediated by an internal ribosome entry site (IRES) element from Rhopalosiphum padi virus (RhPV) while the native GP64 gene is deleted. In this way, a dominant selection pressure is placed on the entire bicistronic mRNA and hence on the maintenance of the foreign gene. The bicistronic expression vector was superior to the control baculovirus vector in that GFP expression remained at much higher levels upon continued virus passage. The versatility of this stabilized vector was demonstrated by its ability to propagate in a number of cell lines including Sf21, Sf9 and High Five cells. This novel baculovirus vector is especially valuable for large-scale recombinant protein production in insect-cell bioreactors where the number of viral passages is high.

Baculoviruses-- re-emerging biopesticides

Biological control of agricultural pests has gained importance in recent years due to increased pressure to reduce the use of agrochemicals and their residues in the environment and food. Viruses of a few families are known to infect insects but only those belonging to the highly specialized family Baculoviridae have been used as biopesticides. They are safe to people and wildlife, their specificity is very narrow. Their application as bioinsecticides was limited until recently because of their slow killing action and technical difficulties for in vitro commercial production. Two approaches for the wider application of baculoviruses as biopesticides will be implemented in future. In countries where use of genetically modified organisms is restricted, the improvements will be mainly at the level of diagnostics, in vitro production and changes in biopesticide formulations. In the second approach, the killing activity of baculoviruses may be augmented by genetic modifications of the baculovirus genome with genes of another natural pathogen. It is expected that the baculoviruses improved by genetic modifications will be gradually introduced in countries which have fewer concerns towards genetically modified organisms.

Baculovirus as a highly efficient expression vector in insect and mammalian cells

Baculovirus has been widely used for the production of recombinant proteins in insect cells. Since the finding that baculovirus can efficiently transduce mammalian cells, the applications of baculovirus have been greatly expanded. The prospects and drawbacks of baculovirus-mediated gene expression, either in insect or in mammalian cells, are reviewed. Recent progresses in expanding the applications to studies of gene regulation, viral vector preparation, in vivo and ex vivo gene therapy studies, generation of vaccine vectors, etc are discussed and the efforts directed towards overcoming the existing bottlenecks are particularly emphasized.