Equilibrium and kinetic analysis of Autographa californica nuclear polyhedrosis virus attachment to different insect cell lines

Entomopathogenic Viruses in the Neotropics: Current Status and Recently Discovered Species

The market for biological control of insect pests in the world and in Brazil has grown in recent years due to the unwanted ecological and human health impacts of chemical insecticides. Therefore, research on biological control agents for pest management has also increased. For instance, insect viruses have been used to protect crops and forests around the world for decades. Among insect viruses, the baculoviruses are the most studied and used viral biocontrol agent. More than 700 species of insects have been found to be naturally infected by baculoviruses, with 90% isolated from lepidopteran insects. In this review, some basic aspects of baculovirus infection in vivo and in vitro infection, gene content, viral replication will be discussed. Furthermore, we provide examples of the use of insect viruses for biological pest control and recently characterized baculoviruses in Brazil.

Identification of insect genes involved in baculovirus AcMNPV entry into insect cells

The baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a model enveloped DNA virus that infects and replicates in lepidopteran insect cells, and can efficiently enter a wide variety of non-host cells. Budded virions of AcMNPV enter cells by endocytosis and traffic to the nucleus where the virus initiates gene expression and genome replication. While trafficking of nucleocapsids by actin propulsion has been studied in detail, other important components of trafficking during entry remain poorly understood. We used a recombinant AcMNPV virus expressing an EGFP reporter in combination with an RNAi screen in Drosophila DL1 cells, to identify host proteins involved in AcMNPV entry. The RNAi screen targeted 86 genes involved in vesicular trafficking, including genes coding for VPS and ESCRT proteins, Rab GTPases, Exocyst proteins, and Clathrin adaptor proteins. We identified 24 genes required for efficient virus entry and reporter expression, and 4 genes that appear to restrict virus entry.

Baculovirus Entry and Egress from Insect Cells

Baculoviruses are large DNA viruses of insects that are highly pathogenic in many hosts. In the infection cycle, baculoviruses produce two types of virions. These virion phenotypes are physically and functionally distinct, and each serves a critical role in the biology of the virus. One phenotype, the occlusion-derived virus (ODV), is occluded within a crystallized protein that facilitates oral infection of the host. A large complex of at least nine ODV envelope proteins called per os infectivity factors are critically important for ODV infection of insect midgut epithelial cells. Viral egress from midgut cells is by budding to produce a second virus phenotype, the budded virus (BV). BV binds, enters, and replicates in most other tissues of the host insect. Cell recognition and entry by BV are mediated by a single major envelope glycoprotein: GP64 in some baculoviruses and F in others. Entry and egress by the two virion phenotypes occur by dramatically different mechanisms and reflect a life cycle in which ODV is specifically adapted for oral infection while BV mediates dissemination of the infection within the animal.

Baculovirus-mediated gene delivery and RNAi applications

Baculoviruses are widely encountered in nature and a great deal of data is available about their safety and biology. Recently, these versatile, insect-specific viruses have demonstrated their usefulness in various biotechnological applications including protein production and gene transfer. Multiple in vitro and in vivo studies exist and support their use as gene delivery vehicles in vertebrate cells. Recently, baculoviruses have also demonstrated high potential in RNAi applications in which several advantages of the virus make it a promising tool for RNA gene transfer with high safety and wide tropism.

Unraveling the entry mechanism of baculoviruses and its evolutionary implications

The entry of baculovirus budded virus into host cells is mediated by two distinct types of envelope fusion proteins (EFPs), GP64 and F protein. Phylogenetic analysis suggested that F proteins were ancestral baculovirus EFPs, whereas GP64 was acquired by progenitor group I alphabaculovirus more recently and may have stimulated the formation of the group I lineage. This study was designed to experimentally recapitulate a possible major step in the evolution of baculoviruses. We demonstrated that the infectivity of an F-null group II alphabaculovirus (Helicoverpa armigera nucleopolyhedrovirus [HearNPV]) can be functionally rescued by coinsertion of GP64 along with the nonfusogenic F(def) (furin site mutated HaF) from HearNPV. Interestingly, HearNPV enters cells by endocytosis and, less efficiently, by direct membrane fusion at low pH. However, this recombinant HearNPV coexpressing F(def) and GP64 mimicked group I virus not only in its EFP composition but also in its abilities to enter host cells via low-pH-triggered direct fusion pathway. Neutralization assays indicated that the nonfusogenic F proteins contribute mainly to binding to susceptible cells, while GP64 contributes to fusion. Coinsertion of GP64 with an F-like protein (Ac23) from group I virus led to efficient rescue of an F-null group II virus. In summary, these recombinant viruses and their entry modes are considered to resemble an evolutionary event of the acquisition of GP64 by an ancestral group I virus and subsequent adaptive inactivation of the original F protein. The study described here provides the first experimental evidence to support the hypothesis of the evolution of baculovirus EFPs.

Biochemical analysis of Hyphantria cunea NPV attachment to Spodoptera frugiperda 21 cells

Binding characteristics of Hyphantria cunea nuclear polyhedrosis virus (HcNPV) to Spodoptera frugiperda 21 (Sf21) cells was determined. The cells displayed an affinity of 0.9 x 10(10) M(-1) with about 8900 binding sites per cell. The biochemical nature of HcNPV-binding sites on the cell surface was also partially elucidated. There were 45 to 49% reductions in HcNPV binding following the pretreatment of cells with three proteases, suggesting the involvement of a cellular protein component in virus binding. Tunicamycin, which inhibits N-linked glycosylation and the expression of some membrane proteins on the cell surface, reduced virus binding suggesting a role for glycoprotein(s) in binding. Treatment of cells with wheat germ agglutinin or neuraminidase did not measurably reduce virus binding, indicating that oligosaccharides containing N-acetylglucosamine or sialic acid are not directly involved in HcNPV attachment. The negative effect of methylamine on HcNPV binding seems to be due to the fact that HcNPV entry via an endocytic pathway is blocked by the increased pH of the endosome. Data on energy inhibitors (sodium azide and dinitrophenol) indicates that HcNPV attachment to Sf21 cells may be closely linked to viral entry via receptor-mediated endocytosis. These findings suggest that the binding site moiety has a glycoprotein component, but that direct involvement of oligosacccharides containing N-acetylglucosamine or sialic acid residues in binding is unlikely, and that HcNPV attachment to Sf21 cells might be via receptor-mediated endocytosis.

A model of the binding, entry, uncoating, and RNA synthesis of Semliki Forest virus in baby hamster kidney (BHK-21) cells

A quantitative understanding of viral trafficking would be useful in treating viral-mediated diseases, designing protocols for viral gene therapy, and optimizing heterologous protein production. In this article, a model for the trafficking of Semliki Forest virus and its RNA synthesis in baby hamster kidney (BHK-21) cells is presented. This model includes the various steps leading to infection such as attachment, endocytosis, and viral fusion in the endosome. The model estimates a mean fusion time of 4 to 6 min for the wild-type virus, and 38 min for Fus-1, an SFV mutant which requires a lower pH for fusion. These mean fusion times are consistent with the time-scale of endosomal acidification, suggesting viruses fuse almost instantaneously with the endosomal membrane as soon as the pH of the endosome drops below the pH threshold of the virus. Infection is most likely controlled at the level of viral uncoating, as shown by the close agreement between the efficiency of uncoating and the experimentally determined fraction of viruses that is infectious. The viral RNA synthesized per cell is best described by assuming that it depends on the number of uncoated viruses prior to the onset of replication according to a saturation-type expression. A Poisson distribution is used to determine the distribution of uncoated viruses among the cells. Because attachment is the rate-limiting step in the uncoating of the virus, increasing the attachment rate can lead to enhanced RNA synthesis and, hence, new virion production. Such an increase in the attachment rate may be obtained by lowering the medium pH or the addition of a polycation. (c) 1995 John Wiley & Sons, Inc.

Production of the Anticarsia gemmatalis multiple nucleopolyhedrovirus in serum-free suspension cultures of the saUFL-AG-286 cell line in stirred reactor and airlift reactor

The velvetbean caterpillar, Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae), is one of the main plagues for soybean crops. Velvetbean caterpillar larvae are susceptible to be infected by occlusion bodies of the baculovirus Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV), a biological insecticide. The insect cell line saUFL-AG-286 produces very high yields of occlusion bodies of AgMNPV in suspension cultures done in the low-cost serum-free medium UNL-10 in shake-flasks. However, its ability to adapt to conditions of industrial production in bioreactors was unknown. The aim of this study was to characterize the growth of saUFL-AG-286 cell cultures in UNL-10 medium, as well as its capability to replicate AgMNPV in two different bio-reactors at laboratory scale. The cell line was able to adapt to conditions that can be used at industrial scale, both in an airlift reactor and a stirred reactor, although the former was better than the last to support the cell growth. The infection with AgMNPV in the airlift reactor produced a high yield of occlusion bodies, with very low production of budded virus, the progeny used as inoculums. On the other hand, infection in the stirred reactor yielded high titers of budded virus. These results suggest that a feasible strategy for scaling-up the production of AgMNPV might involve the use of airlift reactors for the scaling-up of cell suspension cultures and the final production of occlusion bodies, while the scaling-up of the viral inoculums being carried out under conditions as those existing in stirred reactors.

Partial functional rescue of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus infectivity by replacement of F protein with GP64 from Autographa californica multicapsid nucleopolyhedrovirus

Two distinct envelope fusion proteins (EFPs) (GP64 and F) have been identified in members of the Baculoviridae family of viruses. F proteins are found in group II nucleopolyhedroviruses (NPVs) of alphabaculoviruses and in beta- and deltabaculoviruses, while GP64 occurs only in group I NPVs of alphabaculoviruses. It was proposed that an ancestral baculovirus acquired the gp64 gene that conferred a selective advantage and allowed it to evolve into group I NPVs. The F protein is a functional analogue of GP64, as evidenced from the rescue of gp64-null Autographa californica multicapsid nucleopolyhedrovirus (MNPV) (AcMNPV) by F proteins from group II NPVs or from betabaculoviruses. However, GP64 failed to rescue an F-null Spodoptera exigua MNPV (SeMNPV) (group II NPV). Here, we report the successful generation of an infectious gp64-rescued group II NPV of Helicoverpa armigera (vHaBacΔF-gp64). Viral growth curve assays and quantitative real-time PCR (Q-PCR), however, showed substantially decreased infectivity of vHaBacΔF-gp64 compared to the HaF rescue control virus vHaBacΔF-HaF. Electron microscopy further showed that most vHaBacΔF-gp64 budded viruses (BV) in the cell culture supernatant lacked envelope components and contained morphologically aberrant nucleocapsids, suggesting the improper BV envelopment or budding of vHaBacΔF-gp64. Bioassays using pseudotyped viruses with a reintroduced polyhedrin gene showed that GP64-pseudotyped Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) significantly delayed the mortality of infected H. armigera larvae.

A mathematical model of the trafficking of acid-dependent enveloped viruses: application to the binding, uptake, and nuclear accumulation of baculovirus

A quantitative understanding of virus trafficking would be useful in treating viral-mediated diseases, developing protocols for viral gene therapy, designing infection regimens for viral expression systems, and optimizing vaccine and recombinant protein production. Here, we present a mathematical model of the attachment, internalization, endosomal fusion, lysosomal routing, and nuclear accumulation of baculovirus in SF21 insect cells. The model accounts for multivalent bond formation of the virus with cell surface receptors. The model mimics accurately the experimental trafficking dynamics of the virus at both low and high virion to cell ratios, and estimates a receptor number of 11,000 per cell. A significant amount of virus was degraded intracellularly. Independent of the virion to cell ratio, half of the internalized virus was degraded with the rest accumulating in the nucleus. The formalism used in the model may be generally useful for other acid-dependent enveloped viruses. A subset of the model has been used previously to describe the trafficking of Semliki Forest virus, an acid-dependent enveloped RNA virus.Two pathways have previously been implicated for the in vitro entry of the budded form of the baculovirus: adsorptive endocytosis and plasma membrane fusion. Experimental evidence is presented which strongly suggests that the physical number of viruses entering by plasma membrane fusion is not significant relative to receptor-mediated endocytosis.

Inducing single-cell suspension of BTI-TN5B1-4 insect cells: II. The effect of sulfated polyanions on baculovirus infection

Sulfated polyanions can be used to rapidly induce and maintain single-cell suspensions of BTI-TN5B1-4 insect cells, a cell line which clumps in suspension. Elimination of cell clumping results in a significant increase in volumetric yield of the baculovirus expression vector system. Sulfated polyanions, however, inhibited baculovirus infection of BTI-TN5B1-4. Data from binding studies and fusion assays suggest that the inhibition of infection was not due to the observed reduction in viral attachment rate but to inhibition of viral membrane fusion in the endosome.The three most effective polyanions for inducing single cells are dextran sulfate, pentosan sulfate, and polyvinyl sulfate. At concentrations required for single-cell formation, dextran sulfate and pentosan sulfate did not affect viral infection at multiplicities of infection greater than one plaque forming unit per cell. In contrast, polyvinyl sulfate blocked viral infection even at a high multiplicity of infection of 20 plaque-forming units per cell. To bypass this inhibition, polyvinyl sulfate can be removed by resuspending the cells in fresh medium before virus addition, and then added back to the cell suspension after a substantial amount of virus has been internalized. Alternatively, polyvinyl sulfate can be neutralized with a polycation before virus addition, and an equivalent amount of polyvinyl sulfate added back after most of the virus has been internalized. We present a simple mathematical model of the attachment and entry of baculovirus in BTI-TN5B1-4, which can be used to design appropriate infection regimens.

Baculovirus interactions in vitro and in vivo

Baculoviruses are promising viral insecticides and are safe for the environment. Interaction of baculoviruses in vitro and in vivo is a basic molecular and ecological question that has practical applications in agriculture. Cellular secretion is also a fundamental property in cell-cell communication. Here, we review recent investigations on how baculoviruses interact with insect cells and insect hosts. We focus particularly on a new interaction mechanism in which a secretion from cells infected with one virus enhances infection by a second virus. We also discuss a hypothesis that the secreted signals may serve as ligands that bind to the receptors on the surface of the cells that harbor the suppressed genomes of Thysanoplusia orichalcea MNPV (ThorMNPV) in Sf21 and Spodoptera exigua MNPV (SeMNPV) in High 5 to initiate signal transduction leading to the activation of genome replication of ThorMNPV in Sf21 and SeMNPV in High 5. We also discuss how the enhanced replication of SeMNPV replication by Autographa californica MNPV (AcMNPV) in nonpermissive insect cells depends on the types of cells. Interaction of baculoviruses in insects focused on mutualism and antagonism, even though the mechanism is not clear on mutualism. The antagonism of a Nucleopolyhedrovirus (NPV) with a Granulovirus (GV) has been extensively studied by a metalloprotein in the capsule of GV that disrupts the peritrophic membrane, a physical barrier to NPV entry to the midgut of larvae, to facilitate NPV infection.

Development of a hybrid baculoviral vector for sustained transgene expression

Baculovirus is a promising gene delivery vector but its widespread application is impeded as it only mediates transient transgene expression in mammalian cells. To prolong the expression, we developed a dual baculovirus system whereby one baculovirus expressed FLP recombinase while the other harbored an Frt-flanking cassette encompassing the transgene and oriP/EBNA1 derived from Epstein-Barr virus. After cotransduction of cells, the expressed FLP cleaved the Frt-flanking cassette off the baculovirus genome and catalyzed circular episome formation, then oriP/EBNA1 within the cassette enabled the self-replication of episomes. The excision/recombination efficiency was remarkably enhanced by sodium butyrate, reaching 75% in human embryonic kidney-293 (HEK293) cells, 85% in baby-hamster kidney (BHK) cells, 77% in primary chondrocytes, and 48% in mesenchymal stem cells (MSCs). The hybrid baculovirus substantially prolonged the transgene expression to approximately 48 days without selection and >63 days with selection, thanks to the maintenance of replicons and transgene transcription. In contrast to the replicating episomes, the baculovirus genome was rapidly degraded. Furthermore, an osteoinductive growth factor gene was efficiently delivered into MSCs using this system, which not only prolonged the growth factor expression but also potentiated the osteogenesis of MSCs. These data collectively implicate the potential of this hybrid baculovirus system in gene therapy applications necessitating sustained transgene expression.

Identification of a GP64 subdomain involved in receptor binding by budded virions of the baculovirus Autographica californica multicapsid nucleopolyhedrovirus

Enveloped virus entry into host cells is typically initiated by an interaction between a viral envelope glycoprotein and a host cell receptor. For budded virions of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus, the envelope glycoprotein GP64 is involved in host cell receptor binding, and GP64 is sufficient to mediate low-pH-triggered membrane fusion. To better define the role of GP64 in receptor binding, we generated and characterized a panel of antisera against subdomains of GP64. Eight subdomain-specific antisera were generated, and their reactivities with GP64 proteins and neutralization of virus infectivity and binding were examined. Antibodies directed against the N-terminal region of GP64 (amino acids 21 to 159) showed strong neutralization of infectivity and effectively inhibited binding of (35)S-labeled budded virions to Sf9 cells. In addition, we generated virions displaying truncated GP64 constructs. A construct displaying the N-terminal 274 amino acids (residues 21 to 294) of the ectodomain was sufficient to mediate virion binding. Additional studies of antisera directed against small subdomains revealed that an antiserum against a 40-amino-acid region (residues 121 to 160) neutralized virus infectivity. Site-directed mutagenesis was subsequently used for functional analysis of that region. Recombinant viruses expressing GP64 proteins with single amino acid substitutions within amino acids 120 to 124 and 142 to 148 replicated to high titers, suggesting that those amino acids were not critical for receptor binding or other important GP64 functions. In contrast, GP64 proteins with single amino acid substitutions of residues 153 and 156 were unable to substitute for wild-type GP64 and did not rescue a gp64 knockout virus. Further analysis showed that these substitutions substantially reduced binding of recombinant virus to Sf9 cells. Thus, the amino acid region from positions 21 to 159 was identified as a putative receptor binding domain, and amino acids 153 and 156 appear to be important for receptor binding.

Baculovirus envelope fusion proteins F and GP64 exploit distinct receptors to gain entry into cultured insect cells

Group II nucleopolyhedroviruses (NPVs), e.g. Helicoverpa armigera (Hear) NPV and Spodoptera exigua (Se) MNPV (multiple NPV), lack a GP64-like protein that is present in group I NPVs, e.g. Autographa californica (Ac)MNPV, but have an unrelated envelope fusion protein named F. Three AcMNPV viruses were constructed by introducing AcMNPV gp64, HearNPV f or SeMNPV f genes, respectively, into a gp64-negative AcMNPV bacmid. Sf21 cells were incubated with different amounts of inactivated budded virus to occupy receptors and were subsequently infected with a fixed amount of infectious virus to compete for attachment. The results suggest that GP64 and F act on their own and use different receptors, while the two different F proteins exploit the same receptor. Additionally, gp64-null AcMNPV pseudotyped with baculovirus F was, in contrast to GP64, unable to transduce mammalian cells, indicating that mammalian cells do not possess baculovirus F protein receptors despite the structural similarity of baculovirus F to vertebrate viral fusion proteins.

Baculovirus vector for gene delivery and vaccine development

The baculovirus Autographa californica multiple nucleopolyhedrovirus has been widely used not only to acheive a high level of foreign gene expression in insect cells, but also for efficient gene transduction into mammalian cells. Recombinant and pseudotyped baculoviruses possessing chimeric or foreign ligands have been constructed to improve the efficiency of gene transduction and to confer specificity for gene delivery into mammalian cells, respectively. Baculoviral DNA CpG motifs induce proinflammatory cytokines through a Toll-like receptor (TLR9)/MyD88-dependent signaling pathway. Other baculovirus components produce type I interferons via a TLR-independent pathway. Baculovirus exhibits a strong adjuvant property and recombinant baculoviruses encoding microbial antigens elicit antibodies to the antigens and provide protective immunity in mice. This review deals with recent progress in the application of baculovirus vectors to gene delivery and vaccine development, and discusses the future prospects of baculovirus vectors.

Modeling rotavirus-like particles production in a baculovirus expression vector system: Infection kinetics, baculovirus DNA replication, mRNA synthesis and protein production

Rotavirus is the most common cause of severe diarrhoea in children worldwide, responsible for more than half a million deaths in children per year. Rotavirus-like particles (Rota VLPs) are excellent vaccine candidates against rotavirus infection, since they are non-infectious, highly immunogenic, amenable to large-scale production and safer to produce than those based on attenuated viruses. This work focuses on the analysis and modeling of the major events taking place inside Spodoptera frugiperda (Sf-9) cells infected by recombinant baculovirus that may be critical for the expression of rotavirus viral proteins (VPs). For model validation, experiments were performed adopting either a co-infection strategy, using three monocistronic recombinant baculovirus each one coding for viral proteins VP(2), VP(6) and VP(7), or single-infection strategies using a multigene baculovirus coding for the three proteins of interest. A characteristic viral DNA (vDNA) replication rate of 0.19+/-0.01 h(-1) was obtained irrespective of the monocistronic or multigene vector employed, and synthesis of progeny virus was found to be negligible in comparison to intracellular vDNA concentrations. The timeframe for vDNA, mRNA and VP synthesis tends to decrease with increasing multiplicity of infection (MOI) due to the metabolic burden effect. The protein synthesis rates could be ranked according to the gene size in the multigene experiments but not in the co-infection experiments. The model exhibits acceptable prediction power of the dynamics of intracellular vDNA replication, mRNA synthesis and VP production for the three proteins involved. This model is intended to be the basis for future Rota VLPs process optimisation and also a means to evaluating different baculovirus constructs for Rota VLPs production.

Abortive replication of Bombyx mori nucleopolyhedrovirus in Sf9 and High Five cells: defective nuclear transport of the virions

Despite close genetic relationship, Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multicapsid NPV (AcMNPV) display a distinct host range property. Here, BmNPV replication was examined in Sf9 and High Five cells that were nonproductive for BmNPV infection but supported high titers of AcMNPV replication. Recombinant BmNPV, vBm/gfp/lac, containing bm-ie1 promoter-driven egfp showed that few Sf9 and High Five cells infected with vBm/gfp/lac expressed EGFP, while large proportion of EGFP-expressing cells was observed when transfected with vBm/gfp/lac DNA. Immunocytochemical analysis showed that BmNPV was not imported into the nucleus of these two cell lines, while recombinant BmNPV, vBmDelta64/ac-gp64 possessing AcMNPV gp64 was imported into the nucleus, yielding progeny virions in High Five cells, but not Sf9 cells. These results indicate that the defective nuclear import of infected virions due to insufficient BmNPV GP64 function is involved in the restricted BmNPV replication in Sf9 and High Five cells.

Lentiviral vectors pseudotyped with baculovirus gp64 efficiently transduce mouse cells in vivo and show tropism restriction against hematopoietic cell types in vitro

The envelope glycoprotein from vesicular stomatitis virus (VSV-G) has been used extensively to pseudotype lentiviral vectors, but has several drawbacks including cytotoxicity, potential for priming of immune responses against transgene products through efficient transduction of antigen-presenting cells (APCs) and sensitivity to inactivation by human complement. As an alternative to VSV-G, we extensively characterized lentiviral vectors pseudotyped with the gp64 envelope glycoprotein from baculovirus both in vitro and in vivo. We demonstrated for the first time that gp64-pseudotyped vectors could be delivered efficiently in vivo in mice via portal vein injection. Following delivery, the efficiency of mouse cell transduction and the transgene expression is comparable to VSV-G-pseudotyped vectors. In addition, we found that gp64-pseudotyped lentiviral vectors could efficiently transduce a variety of cell lines in vitro, although gp64 showed a more restricted tropism than VSV-G, with especially poor ability to transduce hematopoietic cell types including dendritic cells (DCs). Although we found that gp64-pseudotyped vectors are also sensitive to inactivation by human complement, gp64 nevertheless has advantages over VSV-G, because of its lack of cytotoxicity and narrower tropism. Consequently, gp64 is an attractive alternative to VSV-G because it can efficiently transduce cells in vivo and may reduce immune responses against the transgene product or viral vector by avoiding transduction of APCs such as DCs.

Scale-up of virus-like particles production: effects of sparging, agitation and bioreactor scale on cell growth, infection kinetics and productivity

The baculovirus-insect cells expression system was used for the production of self-forming Porcine parvovirus (PPV) like particles (virus-like particles, VLPs) in serum-free medium. At 2l bioreactor scale an efficient production was achieved by infecting the culture at a concentration of 1.5 x 10(6)cells/ml using a low multiplicity of infection of 0.05 pfu per cell. In a continuous bioreactor, it was shown that the uninfected insect cells were not sensitive to local shear stress values up to 2.25 N/m2 at high Reynolds numbers (1.5 x 10(4)) in sparging conditions. Uninfected insect cells can be grown at scaled-up bioreactor at high agitation and sparging rates as long as vortex formation is avoided and bubble entrapment is minimized. An efficient process scale-up to 25 l bioreactor was made using constant shear stress criteria for scale-up. The kinetics of baculovirus infection at low multiplicity of infection, either at different cell concentration or at different scales, are very reproducible, despite the different turbulence conditions present in the bioreactor milieu. The results suggest that the infection kinetics is controlled by the rate of baculovirus-cell receptor attachment and is independent of the bioreactor hydrodynamic conditions. Furthermore, the achieved specific and volumetric productivities were higher at the 25 l scale when compared to the smaller scale bioreactor. Different rates of cell lysis after infection were observed and seem to fully explain both the shift in optimal harvest time and the increase in cell specific productivity. The results emphasize the importance of integrated strategies and engineering concepts in process development at bioreactor stage with the baculovirus insect cell system.

Strategies for manipulating the relative concentration of recombinant rotavirus structural proteins during simultaneous production by insect cells

Adequate production strategies of virus-like particles are among the challenges that must be addressed before such complex multimeric structures find practical applications as vaccines. Attainment of the correct stoichiometric relation between proteins that constitute virus-like particles should result in an increased productivity by maximizing the concentration of assembled proteins and preventing the accumulation of waste monomers. In this work, strategies for manipulating the relative concentration between two of the structural proteins that constitute rotavirus-like particles (VP2 and VP6) were explored using the insect cell baculovirus expression vector system. It was shown that multiplicity of infection is a useful tool for manipulating protein production rates and maximum concentrations in cultures expressing one or two recombinant proteins. Thus, multiplicity of infection can be employed for improving production of rotavirus-like particles. VP2 and VP6 production rates obtained during individual infections remained unchanged when both were simultaneously produced, indicating that such rates can be utilized for estimating protein concentrations during coexpression. Manipulation of the time of infection between the two recombinant baculoviruses, proposed here for the first time, also proved to be effective for controlling the relative protein concentrations. The use of such sequential infections constituted an effective production alternative that does not require high amounts of virus stocks and is easy to implement. In addition to VP2 and VP6, kinetic parameters for the individual production of the other two proteins (VP4 and VP7) that constitute rotavirus-like particles were also obtained.

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.

Parameters that determine virus adsorption kinetics: toward the design of better infection strategies for the insect cell - baculovirus expression system

High productivities of bioprocesses involving viruses can be attained through infection strategies based on adequate understanding of parameters ruling cell-virus interactions. Two factors that affect virus binding and infection efficiency were studied: the utilization of an adsorption step, where infection volume at constant cell/virus ratio was varied; and the concentration of fetal bovine serum (FBS). The insect cell-baculovirus expression system and recombinant protein VP4 of rotavirus were used as models. Virus binding kinetics were adequately described by a sigmoidal response curve. The adsorption step, with or without FBS, increased virus attachment rate, whereas it increased bound virus at equilibrium only in FBS-free infections. A first-order dependance of virus attachment on cell concentration was found above 5 x 10(6) cell/mL in infections with 10% FBS. Addition of 10% FBS decreased maximum bound baculovirus and binding rate by as much as 3 times and VP4 concentration up to 4 times. In contrast, heat inactivation of FBS increased bound virus from 20% to over 90%, an increase of 1.4 times compared to FBS-free infections. A direct linear relation was found between attached virus and maximum VP4 concentration for the different FBS concentrations tested, indicating that baculovirus-cell attachment was the limiting step for recombinant protein production. Interestingly, virus progeny accumulation was not affected by differences in virus binding. In conclusion, infection strategies aimed at increasing productivity should be performed at high cell concentrations and without FBS, or with heat-inactivated FBS.

Characterization of cell-surface determinants important for baculovirus infection

Baculovirus gp64 envelope glycoprotein is a major component of the envelope of the budded virus and is involved in virus entry into the host cells by endocytosis. To investigate the cell-surface molecules important for infection of baculovirus into mammalian cells, we constructed a recombinant baculovirus, Ac64-CAluc, which has gp64 and luciferase genes under the polyhedrin and the CAG promoter, respectively. For controls, we constructed recombinant viruses possessing vesicular stomatitis virus (VSV) G protein, mouse hepatitis virus (MHV) S protein, or green fluorescent protein (GFP) gene under the polyhedrin promoter and the luciferase gene under the CAG promoter (AcVSVG-CAluc, AcMHVS-CAluc, and AcGFP-CAluc). Treatment of HepG2 cells with phospholipase C markedly reduced the reporter gene expression by Ac64-CAluc or AcVSVG-CAluc in a dose-dependent manner, whereas AcMHVS-CAluc was shown to be resistant to the treatment. Inhibition with purified lipids and susceptibility to the mutant CHO hamster cell lines deficient in phospholipids synthesis suggest that the interaction of gp64 and phospholipids on the cell surface might play an important role in baculovirus infection into mammalian cells.

Culture in the rotating-wall vessel affects recombinant protein production capability of two insect cell lines in different manners

The production of recombinant secreted alkaline phosphatase protein in virally infected insect cells was studied in shaker flask and high aspect rotating-wall vessel (HARV) culture. Two commonly used cell lines, Spodoptera frugiperda Sf-9 (Sf-9) and a nonaggregating isolate of the Trichoplusia ni BTI-Tn-5B1-4 (Tn-5B1-4) cell line, Trichoplusia ni Tn-5B1-4-NA (Tn-5B1-4-NA), were used and monitored for 120-h postinfection. Different responses to culture in the HARV were seen in the two cell lines. While the Sf-9 cell line was able to produce slightly greater amounts of recombinant protein in the HARV than in shaker flask controls, the Tn-5B1-4-NA cell line produced significantly lesser amounts in the HARV than in the shaker flasks. Both cell lines exhibited longer life spans and longer periods of protein production in HARV culture than in shaker flask culture, presumably due to lower levels of shear encountered in the HARV. The important difference was in the protein production rate responses of the two cell lines. While the protein production rates of Sf-9 cells were comparable in both HARV and shaker flask cultures, the protein production rates of Tn-5B1-4-NA cells were much lower in HARV culture than in shaker flask cultures. The conclusion is drawn that cell line-specific adaptation to the HARV strongly influences recombinant protein production.

Host cell receptor binding by baculovirus GP64 and kinetics of virion entry

GP64 is the major envelope glycoprotein from budded virions of the baculoviruses Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV). To examine the potential role of GP64 as a viral attachment protein in host cell receptor binding, we generated, overexpressed, and characterized a soluble form of the OpMNPV GP64 protein, GP64solOp. Assays for trimerization, sensitivity to proteinase K, and reduction by dithiothreitol suggested that GP64solOp was indistinguishable from the ectodomain of the wild-type OpMNPV GP64 protein. Virion binding to host cells was analyzed by incubating virions with cells at 4 degrees C in the presence or absence of competitors, using a single-cell infectivity assay to measure virion binding. Purified soluble GP64 (GP64solOp) competed with a recombinant AcMNPV marker virus for binding to host cells, similar to control competition with psoralen-inactivated wild-type AcMNPV and OpMNPV virions. A nonspecific competitor protein did not similarly inhibit virion binding. Thus specific competition by GP64solOp for virion binding suggests that the GP64 protein is a host cell receptor-binding protein. We also examined the kinetics of virion internalization into endosomes and virion release from endosomes by acid-triggered membrane fusion. Using a protease sensitivity assay to measure internalization of bound virions, we found that virions entered Spodoptera frugiperda Sf9 cells between 10 and 20 min after binding, with a half-time of approximately 12.5 min. We used the lysosomotropic reagent ammonium chloride to examine the kinetics of membrane fusion and nucleocapsid release from endosomes after membrane fusion. Ammonium chloride inhibition assays indicated that AcMNPV nucleocapsids were released from endosomes between 15 and 30 min after binding, with a half-time of approximately 25 min.

Production and characterization of a soluble, active form of Tva, the subgroup A avian sarcoma and leukosis virus receptor

The receptor for the subgroup A avian sarcoma and leukosis viruses [ASLV(A)] is the cellular glycoprotein Tva. A soluble form of Tva, sTva, was produced and purified with a baculovirus expression system. Using this system, 7 to 10 mg of purified sTva per liter of cultured Sf9 cells was obtained. Characterization of the carbohydrate modification of sTva revealed that the three N glycosylation sites in sTva were differentially utilized; however, the O glycosylation common to Tva produced in mammalian and avian cells was not observed. Purified sTva demonstrates significant biological activity, specifically blocking infection of avian cells by ASLV(A) with a 90% inhibitory concentration of approximately 25 pM. A quantitative enzyme-linked immunosorbent assay, developed to assess the binding of sTva to ASLV envelope glycoprotein, demonstrates that sTva has a high affinity for EnvA, with an apparent dissociation constant of approximately 0.3 nM. Once they are bound, a very stable complex is formed between EnvA and sTva, with an estimated complex half-life of 6 h. The soluble receptor protein described here represents a valuable tool for analysis of the receptor-envelope glycoprotein interaction and for structural analysis of Tva.

Baculovirus vector requires electrostatic interactions including heparan sulfate for efficient gene transfer in mammalian cells

BACKGROUND

Recently, several reports have described the ability of recombinant baculoviruses to transduce a variety of mammalian cells. Yet, mechanisms involved in baculovirus entry in those cells remain largely unexplored, particularly at the primary binding step of the virions to the cell membrane.

METHODS

This report focused on the primary virus-cell interactions that lead to in vitro transduction of human 293 cells using a polyhedrin-deleted baculovirus harboring a CMV-driven beta-galactosidase gene (BacLacZ).

RESULTS

Infection rate monitored for 8 h and transduction rate with a multiplicity of infection of up to 800 were, both, non-saturable. Temperatures from 37 degrees C to 4 degrees C dramatically impaired BacLacZ but not adenovirus cell attachment. Competitive infections performed with an excess of a non LacZ-expressing baculovirus hardly competed at a 1/1 ratio. Consistent with an adsorptive binding process onto the cell surface, interactions through electrostatic charges between both viral and cell membranes appeared to be critical for BacLacZ transduction. The addition of polybrene to the cells prior to or during the infection prevented both virus binding and LacZ gene transfer, suggesting the involvement of negatively charged epitopes exposed at the cell surface. The simultaneous presence of the highly charged heparin abrogated BacLacZ binding to the cell surface and subsequent gene transfer. Lastly, direct in vitro binding of BacLacZ to heparin but not BSA columns could be demonstrated after elution of infectious BacLacZ virus in high salt molarity.

CONCLUSION

Electrostatic charges play a critical role during the first step in mammalian cell transduction mediated by a recombinant baculovirus.

Fast and high-affinity binding of B-lymphotropic papovavirus to human B-lymphoma cell lines

Binding of B-lymphotropic papovavirus (LPV) to host cells differing in susceptibility to viral infection was determined by a newly established, direct, nonradioactive virus binding assay, which allows quantitative description of the binding characteristics by receptor saturation and Scatchard analysis. LPV binding to the highly susceptible human B-lymphoma cell line BJA-B K88 is specific, saturable, and noncooperative. Binding occurs very fast, with an association rate constant (k1) of 6.7 x 10(7) M-1s-1, and is of high affinity, with a dissociation constant (Kd) of 2.9 x 10(-12) M; and the virus-receptor complex is stable, with a half life of 70 min. The binding affinities of receptors on four other highly, moderately, or weakly susceptible human B-lymphoma cell lines were similar, with up to twofold variation around a mean Kd value of 3 x 10(-12) M, suggesting the presence of the same LPV receptor on all of these cell lines. This view is further supported by the finding that in all cases a terminal sialic acid is necessary for LPV binding. Tunicamycin has been shown to drastically induce LPV susceptibility and LPV binding in weakly and moderately susceptible B-lymphoma cell lines (O.T. Keppler, M. Herrmann, M. Oppenländer, W. Meschede, and M. Pawlita, J. Virol. 68:6933-6939, 1994). The hypothesis that the constitutively expressed and tunicamycin-induced LPV receptors are identical is strengthened by our finding that both receptor types displayed the same high affinity. LPV susceptibility of different B-lymphoma cell lines was correlated with receptor number but not with receptor affinity. The numbers of receptors per cell on highly and moderately susceptible cell lines ranged from 2,000 to 400 and were directly proportional to LPV susceptibility. This indicates that the number of high-affinity receptors per cell is a key regulating factor for the LPV host range.

Binding kinetics of ecotropic (Moloney) murine leukemia retrovirus with NIH 3T3 cells

A quantitative analysis of the binding kinetics of intact Moloney murine leukemia retrovirus (MoMuLV) particles with NIH 3T3 cells was performed with an immunofluorescence flow cytometry assay. The virus-cell binding equilibrium dissociation constant (KD), expressed in terms of virus particle concentration, was measured to be 8.5 (+/- 6.4) x 10(-12) M at 4 degrees C and was three- to sixfold lower at temperatures above 15 degrees C. The KD of virus binding is about 1,000-fold lower than the KD of purified MoMuLV envelope. The association rate constant was determined to be 2.5 (+/- 0.9) x 10(9) M-1 min-1 at 4 degrees C and was 5- to 10-fold higher at temperatures above 15 degrees C. The apparent dissociation rate constant at 4 degrees C was 1.1 (+/- 0.4) x 10(-3) min-1 and was doubled for every 10 degrees C increase in temperature over the range tested (15 to 37 degrees C).

Pharmacokinetics of adenoviral vector-mediated gene delivery to vascular smooth muscle cells: modulation by poloxamer 407 and implications for cardiovascular gene therapy

Regional in vivo delivery of therapeutic genes to the cardiovascular system at sites of localized vascular disease is feasible by catheter-mediated delivery of recombinant adenoviral vectors. Vascular smooth muscle cell (SMC) proliferation, which follows angioplasty and contributes to restenosis, is one process that may be amenable to such a gene therapy strategy. The clinical utility of localized delivery strategies such as this critically depends upon successful gene transfer to sufficient numbers of vascular cells, locally, within a clinically acceptable time period. Relatively limited information is available concerning the kinetics of gene transfer by first-generation, replication-deficient, recombinant adenovirus (Av1) vectors. In this context, we evaluated the pharmacokinetics of adenoviral vector-mediated gene delivery to vascular SMC using an Av1 reporter vector (Av1LacZ4) expressing a nuclear-targeted beta-galactosidase (beta-Gal) reporter. Bovine aortic SMC were exposed to Av1LacZ4 for various times at a range of concentrations and multiplicities of infection (MOI). After exposure, cells were washed and evaluated for transduction at 48 hr by X-Gal staining. Transduction occurred with a rate constant typically determined in the range of 10(-10) to 10(-11) events.ml/cell.virion.min. The rate of transduction was directly dependent on virion concentration, but not substantially on the virion-to-cell ratio. Relatively low fractions of the total input vector were found to be consumed, even after prolonged adsorption times. We hypothesized that the cellular transduction rate (and thus overall efficiency) would be improved by agents that could maintain a prolonged, high pericellular vector concentration. To evaluate this, cells were exposed to the vector in the presence of 15 grams/dl poloxamer 407, a viscous biocompatibile polyol, for various times followed by washout and evaluation as described above. Both cells and vector remained viable under these conditions, and poloxamer was found to increase the apparent transduction rate 10-fold or more (1-5 x 10(-9) transduction events.ml/cell.virion.min), with remarkable increases in numbers of cells transduced even after brief exposure periods. These observations demonstrate that the pharmacokinetics of adenoviral-mediated gene delivery to vascular SMC can be modulated by agents such as poloxamer 407, which may improve gene delivery by maintaining high pericellular concentrations of vector. Such modulation may permit achievement of desired levels of gene transfer while requiring lower total viral dosage and exposure time, and in turn may have important implications for in vivo gene delivery to vascular tissues.

Extension of Autographa californica nuclear polyhedrosis virus host range by interspecific replacement of a short DNA sequence in the p143 helicase gene

Recombinant baculoviruses obtained by coinfection of insect cells with Autographa californica and Bombyx mori nuclear polyhedrosis viruses (AcNPV and BmNPV, respectively) possess a wider in vitro host range than either parent virus. To localize the DNA sequences responsible for this species specificity, we used a two-step method of production and selection of recombinant viruses with altered specificity. Sf9 cells, which are permissive for AcNPV, were first cotransfected with genomic AcNPV DNA and a complete or incomplete set of BmNPV restriction fragments. AcNPV-BmNPV recombinants from the Sf9 supernatant were then selected on the basis of ability to replicate in B. mori Bm5 cells, which are not permissive for AcNPV. Cotransfection of AcNPV DNA with the 7.6-kbp BmNPV Sma I-C fragment was sufficient to produce recombinants able to infect both Sf9 and Bm5 cells. A series of cotransfections with subclones of this fragment defined a 79-nt sequence within the p143 helicase gene capable of extending AcNPV host range in vitro. In this 79-nt region, BmNPV and AcNPV differ at six positions, corresponding to four amino acid substitutions. The involvement of the 79-nt region in species specificity control was confirmed by cotransfecting AcNPV DNA and gel-purified polymerase chain reaction products derived from the BmNPV p143 gene. Replacement in the AcNPV genome of three AcNPV-specific amino acids by the three corresponding BmNPV-specific amino acids at positions 556, 564, and 577 of the p143 protein extends AcNPV host range to B. mori larvae.