Passage effect of virus infection in insect cells

Identification of Various InDel-II Variants of the White Spot Syndrome Virus Isolated from Frozen Shrimp and Bivalves Obtained in the Korean Commercial Market

White spot syndrome virus (WSSV) poses a significant threat to the global shrimp industry. We investigated the presence of WSSV in frozen shrimp (n = 86) and shellfish (n = 185) from the Korean market (2010-2018). The detection rate of first-step polymerase chain reaction (PCR) in domestic shrimp was 36.8% (7/19), whereas that in imported shrimp was 0.01% (1/67). Furthermore, the WSSV genome was amplified from domestic bivalve mollusks by first- and second-step PCR with accuracies of 3.4% (5/147) and 15.6% (23/147), respectively. The genetic relatedness of InDel-II regions among WSSVs detected in domestic shrimp groups revealed four variants (777, 5649, 11,070 and 13,046 bp insertion or deletion), and imported shrimp groups had four variants (10,778, 11,086, 11,500 and 13,210 bp) compared with the putative ancestor WSSV strain. The 5649 bp variant was the dominant type among the WSSV variants detected in domestic shrimp (54.5%, 6/11). Notably, bivalve mollusks exhibited six variants (777, 5649, 5783, 5876, 11,070 and 13,046 bp), including four variants detected in shrimp, indicating that bivalve mollusks could facilitate WSSV tracking. In a challenge test, whiteleg shrimp (Litopenaeus vannamei) exhibited varying mortality rates, indicating a link between InDel-II deletion and viral replication. These findings highlight the complexity of WSSV transmission.

Homologous recombination risk in baculovirus expression vector system

The baculovirus expression vector system (BEVS) is widely used for producing recombinant proteins. To achieve high expression level of recombinant proteins, baculoviral elements, such as enhancers, promoters, signal peptide coding sequences and 3'-UTR, have been extensively employed. There is a recombination risk derived from homologous sequences between viral genome and functional baculovirus-derived elements associated with foreign genes. Although homologous recombination have distinct biological functions, these potential adverse recombination may trigger a DNA fragment being inverted or looped out, resulting in the production of defective viruses and eventual yields declines of recombinant proteins. However, the risk of such homologous recombination has not been systematically assessed. Here, we measured the recombination rate using a promoter-less fluorescent reporter integrated with various lengths homologous of p10 coding region. Homologous fragments longer than 60 bp possess sufficient recombination probability and exerts effect on purity and integrity of virus. Shortening the length of homologous fragments and separating homologous fragments by point mutations can effectively reduce unfavorable recombination. These findings reveal a homologous recombination risk resulted from genome-homologous baculoviral elements and propose reliable strategies reducing recombination rate to facilitate viral stability and integrity in baculovirus expression vector system.

Monobac System-A Single Baculovirus for the Production of rAAV

Large-scale manufacturing of rAAV is a bottleneck for the development of genetic disease treatments. The baculovirus/Sf9 cell system underpins the first rAAV treatment approved by EMA and remains one of the most advanced platforms for rAAV manufacturing. Despite early successes, rAAV is still a complex biomaterial to produce. Efficient production of the recombinant viral vector requires that AAV replicase and capsid genes be co-located with the recombinant AAV genome. Here, we present the Monobac system, a singular, modified baculovirus genome that contains all of these functions. To assess the relative yields between the dual baculovirus and Monobac systems, we prepared each system with a transgene encoding γSGC and evaluated vectors’ potency in vivo. Our results show that rAAV production using the Monobac system not only yields higher titers of rAAV vector but also a lower amount of DNA contamination from baculovirus.

Pseudorabies virus production using a serum-free medium in fixed-bed bioreactors with low cell inoculum density

Fixed-bed bioreactors packed with macrocarriers show great potential to be used for vaccine process development and large-scale production due to distinguishing features of low shear force, high cell adhering surface area, and easy replacement of culture media in situ. As an initial step of utilizing this type of bioreactors for Pseudorabies virus production (PRV) by African green monkey kidney (Vero) cells, we developed a tube-fixed-bed bioreactor in the previous study, which represents a scale-down model for further process optimization. By using this scale-down model, here we evaluated impacts of two strategies (use of serum-free medium and low cell inoculum density) on PRV production, which have benefits of simplifying downstream process and reducing risk of contamination. We first compared Vero cell cultures with different media, bioreactors and inoculum densities, and conclude that cell growth with serum-free medium is comparable to that with serum-containing medium in tube-fixed-bed bioreactor, and low inoculum density supports cell growth only in this bioreactor. Next, we applied serum-free medium and low inoculum cell density for PRV production. By optimization of time of infection (TOI), multiplicity of infection (MOI) and the harvesting strategy, we obtained total amount of virus particles ~ 9 log₁₀ TCID₅₀ at 5 days post-infection (dpi) in the tube-fixed-bed bioreactor. This process was then scaled up by 25-fold to a Xcell 1-L fixed-bed bioreactor, which yields totally virus particles of 10.5 log₁₀ TCID_50, corresponding to ~ 3 × 10⁵ doses of vaccine. The process studied in this work holds promise to be developed as a generic platform for the production of vaccines for animal and human health.

Cryptophlebia peltastica Nucleopolyhedrovirus Is Highly Infectious to Codling Moth Larvae and Cells

Cydia pomonella granulovirus (CpGV) is a cornerstone of codling moth (Cydia pomonella) control in integrated and organic pome fruit production, though different types of resistance to CpGV products have been recorded in codling moth field populations in Europe for several years. Recently, a novel baculovirus named Cryptophlebia peltastica nucleopolyhedrovirus (CrpeNPV) was isolated from a laboratory culture of the litchi moth, Cryptophlebia peltastica, in South Africa. Along with CpGV, it is the third known baculovirus that is infectious to codling moth. In the present study, parameters of infectiveness of CrpeNPV, such as the median lethal concentration and median survival time, were determined for codling moth larvae susceptible or resistant to CpGV. In addition, the permissiveness of a codling moth cell line with respect to infection by CrpeNPV budded virus was demonstrated by infection and gene expression studies designed to investigate the complete replication cycle. Investigations of the high degree of virulence of CrpeNPV for codling moth larvae and cells are of high significant scientific and economic value and may offer new strategies for the biological control of susceptible and resistant populations of codling moth.IMPORTANCE The emergence of codling moth populations resistant to commercially applied isolates of CpGV is posing an imminent threat to organic pome fruit production. Very few CpGV isolates are left that are able to overcome the reported types of resistance, emphasizing the demand for new and highly virulent baculoviruses. Here we report the recently discovered CrpeNPV as highly infectious to all types of resistant codling moth populations with a high speed of killing, making it a promising candidate baculovirus in fighting the spread of resistant codling moth populations.

Expression- and genomic-level changes during passage of four baculoviruses derived from bacmids in permissive insect cell lines

The baculovirus-based bacmid expression vector system has been widely used for protein production in basic research and biotechnological laboratories. Since the first construction of the Autographa californica multiple nucleopolyhedrovirus bacmid (AcBacmid), three more bacmids have been created from Bombyx mori nucleopolyhedrovirus (BmBacmid), Spodoptera exigua nucleopolyhedrovirus (SeBacmid) and Helicoverpa armigera nucleopolyhedrovirus (HaBacmid). Each of these bacmid-derived viruses replicates efficiently in a range of specific and permissive cell types. Here, we investigated the relative stability of each virus derived from the bacmid during passage in permissive cell lines through assessment of their expression level and genome structure changes. Using two different reporters, the expression levels of the viruses from the AcBacmid-Sf9, AcBacmid-Tn5, BmBacmid-BmN and SeBacmid-SeE1 bacmid-cell systems were significantly reduced after five passages of the viruses, whereas the reductions were not detected in the AcBacmid-Sf21 and HaBacmid-HzAM1 systems. Pulse field gel electrophoresis (PFGE) and restriction fragment length polymorphism (RFLP) analysis of viral DNA isolated from passaged viruses from the AcBacmid-Sf21 and HaBacmid-HzAM1 systems showed no major genomic changes. In contrast, the genomes from passaged viruses in the AcBacmid-Tn5 and AcBacmid-Sf9 systems displayed reduced genome size and various mutations at individual loci, including genotypes missing one at least or more viral RNA polymerase subunits and fp25k. These genotypic changes were correlated with reduced protein expression. RFLP analysis of viral DNA from passaged viruses in the BmBacmid-BmN and SeBacmid-SeE1 systems exhibited changes in genome size, including excision of particular EcoRI fragments containing the mini-F replicon. Collectively, our data suggest that the viruses from the AcBacmid-Sf21 and HaBacmid-HzAM1 bacmid-cell systems are better for large-scale protein expression in continuous culture. Further study is needed to investigate the mechanism(s) behind the protein expression reduction in these bacmid-derived virus/cell systems.

Impact of Lateral Transfers on the Genomes of Lepidoptera

Transfer of DNA sequences between species regardless of their evolutionary distance is very common in bacteria, but evidence that horizontal gene transfer (HGT) also occurs in multicellular organisms has been accumulating in the past few years. The actual extent of this phenomenon is underestimated due to frequent sequence filtering of "alien" DNA before genome assembly. However, recent studies based on genome sequencing have revealed, and experimentally verified, the presence of foreign DNA sequences in the genetic material of several species of Lepidoptera. Large DNA viruses, such as baculoviruses and the symbiotic viruses of parasitic wasps (bracoviruses), have the potential to mediate these transfers in Lepidoptera. In particular, using ultra-deep sequencing, newly integrated transposons have been identified within baculovirus genomes. Bacterial genes have also been acquired by genomes of Lepidoptera, as in other insects and nematodes. In addition, insertions of bracovirus sequences were present in the genomes of certain moth and butterfly lineages, that were likely corresponding to rearrangements of ancient integrations. The viral genes present in these sequences, sometimes of hymenopteran origin, have been co-opted by lepidopteran species to confer some protection against pathogens.

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.

Routine Maintenance and Storage of Lepidopteran Insect Cell Lines and Baculoviruses

The various methods for maintaining (i.e., subculturing, splitting, or passaging) established lepidopteran cell lines are described. Three procedures are presented that are appropriate for different cell lines dependent upon the growth characteristics (in particular, cell attachment properties) of the cells of interest. In addition to the routine maintenance of cells in active culture, methods are also described for both short-term (low temperature) and long-term (frozen in liquid nitrogen) storage of cell lines, as well as quality control procedures for the cultures. Methods for storing baculoviruses for use in cell cultures and issues of concern when using cell cultures for their production and study are also described.

Characterizing Enterovirus 71 and Coxsackievirus A16 virus-like particles production in insect cells

Enterovirus 71 (EV71) and Coxsackievirus A16 (CVA16) are two viruses commonly responsible for hand, foot and mouth disease (HFMD) in children. The lack of prophylactic or therapeutic measures against HFMD is a major public health concern. Insect cell-based EV71 and CVA16 virus-like particles (VLPs) are promising vaccine candidates against HFMD and are currently under development. In this paper, the influence of insect cell line, incubation temperature, and serial passaging effect and stability of budded virus (BV) stocks on EV71 and CVA16 VLP production was investigated. Enhanced EV71 and CVA16 VLP production was observed in Sf9 cells compared to High Five™ cells. Lowering the incubation temperature from the standard 27°C to 21°C increased the production of both VLPs in Sf9 cells. Serial passaging of CVA16 BV stocks in cell culture had a detrimental effect on the productivity of the structural proteins and the effect was observed with only 5 passages of BV stocks. A 2.7× higher production yield was achieved with EV71 compared to CVA16. High-resolution asymmetric flow field-flow fractionation couple with multi-angle light scattering (AF4-MALS) was used for the first time to characterize EV71 and CVA16 VLPs, displaying an average root mean square radius of 15±1nm and 15.3±5.8 nm respectively. This study highlights the need for different approaches in the design of production process to develop a bivalent EV71 and CVA16 vaccine.

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.

High-titer preparation of Bombyx mori nucleopolyhedrovirus (BmNPV) displaying recombinant protein in silkworm larvae by size exclusion chromatography and its characterization

Background

Budded baculoviruses are utilized for vaccine, the production of antibody and functional analysis of transmembrane proteins. In this study, we tried to produce and purify the recombinant Bombyx mori nucleopolyhedrovirus (rBmNPV-hPRR) that displayed human (pro)renin receptor (hPRR) connected with FLAG peptide sequence on its own surface. These particles were used for further binding analysis of hPRR to human prorenin. The rBmNPV-hPRR was produced in silkworm larvae and purified from its hemolymph using size exclusion chromatography (SEC).

Results

A rapid method of BmNPV titer determination in hemolymph was performed using quantitative real-time PCR (Q-PCR). A correlation coefficient of BmNPV determination between end-point dilution and Q-PCR methods was found to be 0.99. rBmNPV-hPRR bacmid-injected silkworm larvae produced recombinant baculovirus of 1.31 × 10⁸ plaque forming unit (pfu) in hemolymph, which was 2.8 × 10⁴ times higher than transfection solution in Bm5 cells. Its purification yield by Sephacryl S-1000 SF column chromatography was 264 fold from larval hemolymph at 4 days post-injection (p.i.), but 35 or 39 fold at 4.5 or 5 days p.i., respectively. Protein patterns of rBmNPV-hPRR purified at 4 and 5 days were the same and ratio of envelope proteins (76, 45 and 35 kDa) to VP39, one of nucleocapsid proteins, increased at 5 days p.i. hPRR was detected in only purified rBmNPV-hPRR at 5 days p.i..

Conclusion

The successful purification of rBmNPV-hPRR indicates that baculovirus production using silkworm larvae and its purification from hemolymph by Sephacryl S-1000 SF column chromatography can provide an economical approach in obtaining the purified BmNPV stocks with high titer for large-scale production of hPRR. Also, it can be utilized for further binding analysis and screening of inhibitors of hPRR.

Accumulation of few-polyhedra mutants upon serial passage of Anticarsia gemmatalis multiple nucleopolyhedrovirus in cell culture

Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) has been widely used to control the velvetbean caterpillar, Anticarsia gemmatalis, in Brazil. To date, AgMNPV has been produced by larval infection and, due to in vivo production limitations and the continuing high demand for the biopesticide, attempts should be made to develop in vitro production of this virus. In order to investigate the effects caused by serial passage of AgMNPV in cell culture, we carried out a total of ten passages and analyzed the morphological and the genomic changes of the virus. After six passages, the many-polyhedra (MP) phenotype started to switch to the few-polyhedra (FP) phenotype which rapidly accumulated in the virus population. Ultrastructural analysis showed typical signs of FP mutant formation such as decrease in the number of polyhedra per cell, polyhedra aberrant morphology and low numbers of virions occluded in the protein matrix. Also enhanced BV production was observed from the fifth passage indicating that FP mutants were becoming predominant in comparison to the wild type virus. Restriction endonuclease analysis of the viral DNA revealed that lower and higher passages had similar profiles indicating that there were no large insertions or deletions or rearrangements in their genomes and indicating the generation of FP mutants instead of defective interfering viruses.

Toward exascale production of recombinant adeno-associated virus for gene transfer applications

To gain acceptance as a medical treatment, adeno-associated virus (AAV) vectors require a scalable and economical production method. Recent developments indicate that recombinant AAV (rAAV) production in insect cells is compatible with current good manufacturing practice production on an industrial scale. This platform can fully support development of rAAV therapeutics from tissue culture to small animal models, to large animal models, to toxicology studies, to Phase I clinical trials and beyond. Efforts to characterize, optimize and develop insect cell-based rAAV production have culminated in successful bioreactor-scale production of rAAV, with total yields potentially capable of approaching the exa-(10(18)) scale. These advances in large-scale AAV production will allow us to address specific catastrophic, intractable human diseases such as Duchenne muscular dystrophy, for which large amounts of recombinant vector are essential for successful outcome.

White spot syndrome virus proteins and differentially expressed host proteins identified in shrimp epithelium by shotgun proteomics and cleavable isotope-coded affinity tag

Shrimp subcuticular epithelial cells are the initial and major targets of white spot syndrome virus (WSSV) infection. Proteomic studies of WSSV-infected subcuticular epithelium of Penaeus monodon were performed through two approaches, namely, subcellular fractionation coupled with shotgun proteomics to identify viral and host proteins and a quantitative time course proteomic analysis using cleavable isotope-coded affinity tags (cICATs) to identify differentially expressed cellular proteins. Peptides were analyzed by offline coupling of two-dimensional liquid chromatography with matrix-assisted laser desorption ionization-tandem time of flight mass spectrometry. We identified 27, 20, and 4 WSSV proteins from cytosolic, nuclear, and membrane fractions, respectively. Twenty-eight unique WSSV proteins with high confidence (total ion confidence interval percentage [CI%], >95%) were observed, 11 of which are reported here for the first time, and 3 of these novel proteins were shown to be viral nonstructural proteins by Western blotting analysis. A first shrimp protein data set containing 1,999 peptides (ion score, > or =20) and 429 proteins (total ion score CI%, >95%) was constructed via shotgun proteomics. We also identified 10 down-regulated proteins and 2 up-regulated proteins from the shrimp epithelial lysate via cICAT analysis. This is the first comprehensive study of WSSV-infected epithelia by proteomics. The 11 novel viral proteins represent the latest addition to our knowledge of the WSSV proteome. Three proteomic data sets consisting of WSSV proteins, epithelial cellular proteins, and differentially expressed cellular proteins generated in the course of WSSV infection provide a new resource for further study of WSSV-shrimp interactions.

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.

Routine maintenance and storage of lepidopteran insect cell lines and baculoviruses

The various methods for maintaining (a.k.a., subculturing, splitting, or passaging) established lepidopteran cell lines are described. Three procedures are presented that are appropriate for different cell lines dependent upon the growth characteristics (in particular, cell attachment properties) of the cells of interest. In addition to the routine maintenance of cells in active culture, methods are also described for both short (low temperature) and long-term (frozen in liquid nitrogen) storage of cell lines, as well as quality control procedures for the cultures. Methods for storing baculoviruses for use in cell cultures and issues of concern when using cell cultures for their production and study are also described.

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.

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.

Fitness and virulence of an ancestral White Spot Syndrome Virus isolate from shrimp

White Spot Syndrome Virus, the type species of the virus family Nimaviridae, is a large dsDNA virus infecting shrimp and other crustaceans. Genomic analysis of three completely sequenced WSSV isolates identified two major polymorphic loci, "variable region ORF14/15" and "variable region ORF23/24". Here, we characterize a WSSV isolate originating from shrimp collected in Thailand in 1996 (TH-96-II). This isolate contains the largest WSSV genome ( approximately 312 kb) identified so far, mainly because of its sequences in both major polymorphic loci. Analysis of "variable region ORF14/15" suggests that TH-96-II may be ancestral to the WSSV isolates described to date. A comparison for virulence was made between TH-96-II and WSSV-TH, a well characterized isolate containing the smallest genome ( approximately 293 kb) identified at present. After injection of the isolates into Penaeus monodon the mortality rates showed that the median lethal time (LT50) of TH-96-II was approximately 14 days, compared to 3.5 days for WSSV-TH. When both isolates were mixed in equal amounts and serially passaged in shrimp, WSSV-TH outcompeted TH-96-II within four passages. These data suggest a higher virulence of WSSV-TH compared to TH-96-II. The molecular basis for the difference in virulence remains unclear, but a replication advantage of the 19 kb smaller WSSV-TH genome could play a role.

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

Continuous protein production with baculovirus expression vectors in insect-cell bioreactors is characterized by a dramatic drop in heterologous protein production within a few weeks. This is mainly due to the spontaneous deletion of the heterologous gene(s) from the baculovirus genome and/or to the rapid accumulation of defective interfering baculoviruses (DIs). Cell culture experiments with bacmid-derived baculoviruses showed that spontaneous deletions in the foreign bacterial artificial chromosome (BAC) sequences readily occurred. These deletions correlated with a low density of baculovirus homologous (repeat) regions (hrs), which are located dispersed throughout the baculovirus genome and are believed to act as origins of viral DNA replication (oris). To test the hypothesis that deletions are more likely to occur in regions with a low ori density, the properties of bacmid-derived baculoviruses with an additional hr in the unstable BAC sequences were compared to the standard bacmid-derived baculovirus in a continuous cascaded insect-cell bioreactor configuration. All viruses were equipped with a green fluorescent protein (GFP) gene and a gene encoding the classical swine fever virus E2 glycoprotein (CSFV-E2). The insertion of an extra hr in the BAC vector led to improved genetic stability of adjacent sequences, resulting in prolonged protein expression. The maintenance of the BAC sequences appeared to be dependent on the orientation of the inserted hr. The advantages of the utilization of hrs to improve the stability of baculovirus expression vectors for the large-scale protein production in insect-cell bioreactors are discussed.

Cell line-specific accumulation of the baculovirus non-hr origin of DNA replication in infected insect cells

Successive viral passage of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) in the S. exigua cell line Se301 leads to the rapid accumulation of the non-hr origin of DNA replication (ori) as large concatemers. Passage of SeMNPV in two other S. exigua cell lines, SeUCR1 and SeIZD2109, did not show the accumulation of such concatemers. When introduced into SeUCR1 and SeIZD2109 cells, the non-hr ori concatemers generated in Se301 cells were maintained but did not increase. This suggests that the non-hr ori confers a strong selective advantage in Se301 cells, but not or to a lesser extent in the other cell lines. The cell line-specific accumulation of non-hr ori concatemers might be due to a higher intrinsic recombination frequency in Se301 cells and may reflect tissue related differences involving some host cell factor(s). Since non-hr ori concatemers in Se301 cells were more abundant in intracellular than in extracellular viral DNA preparations, episomal replication and the requirement of a minimal DNA size for packaging into nucleocapsids is hypothesized.

Spontaneous excision of BAC vector sequences from bacmid-derived baculovirus expression vectors upon passage in insect cells

Repeated baculovirus infections in cultured insect cells lead to the generation of defective interfering viruses (DIs), which accumulate at the expense of the intact helper virus and compromise heterologous protein expression. In particular, Autographa californica multicapsid nucleopolyhedovirus (AcMNPV) DIs are enriched in an origin of viral DNA replication (ori) not associated with the homologous regions (hrs). This non-hr ori is located within the coding sequence of the non-essential p94 gene. We investigated the effect of a deletion of the AcMNPV non-hr ori on the heterologous protein expression levels following serial passage in Sf21 insect cells. Using homologous ET recombination in E. coli, deletions within the p94 gene were made in a bacterial artificial chromosome (BAC) containing the entire AcMNPV genome (bacmid). All bacmids were equipped with an expression cassette containing the green fluorescent protein gene and a gene encoding the classical swine fever virus E2 glycoprotein (CSFV-E2). For the parental (intact) bacmid only, a strong accumulation of DIs with reiterated non-hr oris was observed. This was not observed for the mutants, indicating that removal of the non-hr ori enhanced the genetic stability of the viral genome upon passaging. However, for all passaged viruses it was found that the entire BAC vector including the expression cassette was spontaneously deleted from the viral genome, leading to a rapid decrease in GFP and CSFV-E2 production. The rationale for the (intrinsic) genetic instability of the BAC vector in insect cells and the implications with respect to large-scale production of proteins with bacmid-derived baculoviruses are discussed.

Virus-like particle production at low multiplicities of infection with the baculovirus insect cell system

The baculovirus insect cell expression system (BEVS) was used for the production of self-forming Porcine parvovirus-like particles (VLPs) in serum-free medium. A low multiplicity of infection (MOI) strategy was used to overcome an extra virus amplification step, undesirable in industrial production, and to minimize the virus passage effect. It was confirmed that the time of infection (TOI) and MOI are dependent variables. Higher cell densities were obtained at low MOIs, keeping a constant TOI; however, both volumetric and specific productivities were lower. In synchronous infection, at high MOI, the specific productivity decreased when the cells were infected in the late phase of growth. Product degradation due to cell lysis strongly influenced the optimal time of harvest (TOH). Time of harvest was found to be highly dependent on the MOI, and a direct relationship with the cell yield was obtained. Analysis of the culture medium reveals that glutamine depletion occurs in the late phase of the growth. Supplementation of glutamine to uninfected cell cultures resulted in an increased cell yield. Its addition to cultures infected in the middle phase of the growth curve was also able to restore the productivity levels, but addition to cells in their stationary phase caused no observable effect on product expression. The study clearly shows that for a specific TOI it is not obvious what the correct MOI should be to obtain the best volumetric productivity.

Production of core and virus-like particles with baculovirus infected insect cells

In this paper the fundamental aspects of process development for the production of core and virus-like particles with baculovirus infected insect cells are reviewed. The issues addressed include: particle formation and monomer composition, chemical and physical conditions for optimal cell growth, baculovirus replication and product expression, multiplicity of infection strategy, and scale-up of the process. Study of the differences in the metabolic requirements of infected and non-infected cells is necessary for high cell density processes. In the bioreactor, the specific oxygen uptake rate (OURsp) plays a central role in process scale-up, leading to the specification of the bioreactor operational parameters. Shear stress can also be an important variable for bioreactor operation due to its influence on cell growth and product expression. The determination of the critical variables in process development is discussed, showing the relevance of the mathematical models that have been developed for the insect cells/baculovirus system in process implementation and control.

Pivotal role of the non-hr origin of DNA replication in the genesis of defective interfering baculoviruses

The generation of deletion mutants, including defective interfering viruses, upon serial passage of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) in insect cell culture has been studied. Sequences containing the non-homologous region origin of DNA replication (non-hr ori) became hypermolar in intracellular viral DNA within 10 passages in Se301 insect cells, concurrent with a dramatic drop in budded virus and polyhedron production. These predominant non-hr ori-containing sequences accumulated in larger concatenated forms and were generated de novo as demonstrated by their appearance and accumulation upon infection with a genetically homogeneous bacterial clone of SeMNPV (bacmid). Sequences were identified at the junctions of the non-hr ori units within the concatemers, which may be potentially involved in recombination events. Deletion of the SeMNPV non-hr ori using RecE/RecT-mediated homologous ET recombination in Escherichia coli resulted in a recombinant bacmid with strongly enhanced stability of virus and polyhedron production upon serial passage in insect cells. This suggests that the accumulation of non-hr oris upon passage is due to the replication advantage of these sequences. The non-hr ori deletion mutant SeMNPV bacmid can be exploited as a stable eukaryotic heterologous protein expression vector in insect cells.

Autographa californica baculoviruses with large genomic deletions are rapidly generated in infected insect cells

Defective interfering baculoviruses (DIs) lack considerable portions of the genome, interfere with the replication of helper virus, and cause the so-called "passage-effect" during serial passaging in insect cells and in bioreactor configurations. We investigated their origin by (nested) PCR and demonstrated that DIs lacking approximately 43% (d43) of their DNA are present in low-passage Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV)-E2 virus stocks and in polyhedra, but not in the authentic AcMNPV isolate obtained prior to passage in cell culture. To investigate whether DIs are rapidly generated de novo in Sf21 insect cells, a genetically homogeneous AcMNPV bacmid was serially passaged, resulting in the generation of d43 DIs within two passages. AT-rich sequences of up to 66 nucleotides of partly unknown origin were found at the deletion junctions in the d43 DI genomes. These data suggest that the rapid generation of DIs is an intrinsic property of baculovirus infection in insect cell culture and involves several recombination steps.

Serial passage of a Helicoverpa armigera nucleopolyhedrovirus in Helicoverpa zea cell cultures

The serial passaging of baculoviruses in cell lines numerous times can result in a variety of mutations or defective viral populations becoming predominant in the cultures. The generation of these mutants during cell culture passage, also known as "the passage effect," can seriously hinder the use of in vitro methods for large-scale production of baculoviruses for use as biopesticides. In an effort to develop a large-scale in vitro method of producing Helicoverpa armigera singly enveloped nucleopolyhedrovirus (HaSNPV), it was essential to determine whether or not the passage effect was evident when this virus is serially passaged in cell cultures. An isolate of HaSNPV was serially passaged in Helicoverpa zea cell cultures up to 10 times. The production of occlusion bodies decreased with increasing passage number and there was evidence of defective viruses becoming predominant in cultures after 5 passages. The number of virions present within cross sections of passage 3 occlusion bodies was 1.5 times higher than those from passage 10 occlusion bodies when quantified using electron microscopy. A laboratory bioassay showed that potencies of passage 3 isolates against H. armigera larvae were 8 times higher than potencies of passage 10 isolates. This study indicated that changes typical of the passage effect were evident when HaSNPV was serially passaged in H. zea cell cultures up to 10 times.

Four genotypic variants of a Spodoptera exigua Nucleopolyhedrovirus (Se-SP2) are distinguishable by a hypervariable genomic region

Four genotypes named SP2A, SP2B, SP2C and SP2D were obtained in vivo by infecting S. exigua larvae with limiting dilutions of the Spanish field isolate Spodoptera exigua Nucleopolyhedrovirus (Se-SP2) of SeMNPV. The cloning of variants SP2A, SP2B and SP2C took 1, 6, and 3 passages, respectively, before the DNA profiles showed all bands in equimolar concentrations, and they remained constant for at least six further passages indicating the stability of their genotypes. The SP2D variant isolation took over ten passages and it was genetically less stable. Physical maps of their genomes were constructed for the restriction enzymes BamHI, BglII, PstI, and XbaI. The region between 8-10 m.u. was highly variable and characteristic of each cloned genotype and, hence, can be used as RFLP markers for all four genotypic variants. This region, included in the PstI-MB fragment, was cloned and sequenced showing that all the Se-SP2 variants contained a homologous region (hr) with a variable number of 98 bp sequences tandemly repeated, which were used to distinguish genotypic variants from each other. The biological activity of the genotypic variants SP2A, SP2B, and SP2C when compared in terms of LD50 and LT50, were not significantly different. However, the SP2D genotypic variant was found to be significantly less infective (higher LD50). The emergence of new genotypes in the Se-SP2 field populations is discussed.

Naturally occurring deletion mutants are parasitic genotypes in a wild-type nucleopolyhedrovirus population of spodoptera exigua

A wild-type nucleopolyhedrovirus (NPV) isolate from Spodoptera exigua from Florida (Se-US2) is a variant of the SeMNPV type strain since it has a unique DNA profile but is closely related to other known geographical isolates of SeMNPV. It consists of several genotypic variants, of which seven were identified in a Se-US2 virus stock by a modification of the in vivo cloning method developed by Smith and Crook (Virology 166:240-244, 1988). The US2A variant was the most prevalent genotype, and it was designated the prototype Se-US2 variant, while four of the variants (US2B, US2D, US2F, and US2H) were found at low frequency. US2C and US2E were also very abundant, and their diagnostic bands were easily observed in wild-type isolate restriction endonuclease patterns. The analysis of each variant, compared to the prototype US2A, showed that US2B and US2H presented minor differences, while US2D and US2F contained slightly larger insertions or deletions. Variants US2C and US2E contained major deletions of 21.1 and 14 kb, respectively, mapping at the same genomic region (between 14.5 and 30.2 map units [m.u.] and between 12.8 and 23 m.u., respectively). This is the first report of such deletion mutants in a natural baculovirus population. Variants US2A, US2B, US2D, US2F, and US2H were isolated as pure genotypes, but we failed to clone US2C and US2E in vivo. When these two variants appeared without apparent contamination with any other variant, they lost their pathogenicity for Spodoptera exigua larvae. A further biological characterization showed evidence that these two naturally occurring deletion mutants act as parasitic genotypes in the virus population. Bioassay data also demonstrated that pure US2A is significantly more pathogenic against second-instar S. exigua larvae than the wild-type isolate. The need for precise genotypic characterization of a baculovirus prior to its development as a bioinsecticide is discussed.