Genetically engineered baculoviruses as agents for pest control

Hemolin increases the immune response of a caterpillar to NPV infection

Hemolin, a member of the immunoglobulin superfamily, plays a crucial role in the immune responses of insects against pathogens. However, the innate immune response of Hemolin to baculovirus infection varies among different insects, and the antiviral effects of Hemolin in Hyphantria cunea (HcHemolin) remain poorly understood. Our results showed that HcHemolin was expressed throughout all developmental stages, with higher expressions observed during pupal and adult stages of H. cunea. Additionally, HcHemolin was expressed in reproductive and digestive organs. The expression levels of the HcHemolin were induced significantly following H. cunea nucleopolyhedrovirus (HcNPV) infection. The susceptibility of H. cunea larvae to HcNPV decreased upon silencing of HcHemolin, resulting in a 40% reduction in median lifespan compared to the control group. The relative growth rate (RGR), the relative efficiency of consumption rate (RCR), the efficiency of the conversion of ingested food (ECI), and efficiency of the conversion of digested food (ECD) of silenced H. cunea larvae were significantly lower than those of the control group. Immune challenge assays showed that the median lifespan of treated H. cunea larvae was two-fold longer than the control group after HcNPV and HcHemolin protein co-injection. Therefore, we propose that HcHemolin plays a crucial role in regulating the growth, development, and food utilization of H. cunea, as well as in the antiviral immune response against HcNPV. These findings provide implications for the development of targeted nucleic acid pesticides and novel strategies for pollution-free biological control synergists for HcNPV.

Toxicity of Bioactive Molecule Andrographolide against Spodoptera litura Fab and Its Binding Potential with Detoxifying Enzyme Cytochrome P450

Spodoptera litura Fab. is a polyphagous pest causing damage to many agriculture crops leading to yield loss. Recurrent usage of synthetic pesticides to control this pest has resulted in resistance development. Plant-derived diterpenoid compound andrographolide was isolated from the leaves of Andrographis paniculata. It was analysed by gas chromatography-mass spectroscopy and quantified by HPLC. Nutritional indices and digestive enzymatic profile were evaluated. Third, fourth and fifth instar larvae were treated with different concentrations of andrographolide. At 3, 6 and 9 ppm-treated concentrations the larvae showed decreased RGR, RCR, ECI, ECD values with adverse increase in AD. The digestive enzymes were significantly inhibited when compared with control. Conspicuously, andrographolide showed pronounced mortality of S. litura by inhibition of enzyme secretion and intake of food. The binding ability of andrographolide with CYTP450 showed high affinity with low binding energy. Andrographolide has the potential to be exploited as a biocontrol agent against S. litura as an eco-friendly pesticide.

Baculovirus Utilizes Cholesterol Transporter NIEMANN-Pick C1 for Host Cell Entry

The dual roles of baculovirus for the control of natural insect populations as an insecticide, and as a tool for foreign gene expression and delivery, have called for a comprehensive understanding of the molecular mechanisms governing viral infection. Here, we demonstrate that the Bombyx mori Niemann-Pick C1 (BmNPC1) is essential for baculovirus infection in insect cells. Both pretreatment of B. mori embryonic cells (BmE) with NPC1 antagonists (imipramine or U18666A) and down-regulation of NPC1 expression resulted in a significant reduction in baculovirus BmNPV (B. mori nuclear polyhedrosis virus) infectivity. Disruption of BmNPC1 could decrease viral entry (2 hpi) rather than reduce the viral binding to the BmE cells. Furthermore, our results showed that NPC1 domain C binds directly and specifically to the viral glycoprotein GP64, which is responsible for both receptor binding and fusion. Antibody blocking assay also revealed that the domain C specific polyclonal antibody inhibited BmNPV infection, indicating that NPC1 domain C most likely plays a role during viral fusion in endosomal compartments. Our results, combined with previous studies identifying an essential role of human NPC1 (hNPC1) in filovirus infection, suggest that the glycoprotein of several enveloped viruses possess a shared strategy of exploiting host NPC1 proteins during virus intracellular entry events.

Synergistic Effect of Combining Plutella xylostella Granulovirus and Bacillus thuringiensis at Sublethal Dosages on Controlling of Diamondback Moth (Lepidoptera: Plutellidae)

Plutella xylostella granulovirus (PxGV) and Bacillus thuringiensis (Bt) are both entomo-pathogens to the diamondback moth, Plutella xylostella (L.). The purpose of the present study was to measure the effect of the combination of PxGV and Bt at sublethal dosages on the development and mortality of diamondback moth in a laboratory setting. Bt and PxGV exhibited synergistic effect on diamondback moth larval mortality and effectively controlled diamondback moth populations with low dose combination treatment. The combination of three parts per million Bt and 1.3 × 10(3) occlusion bodies per milliliter of PxGV revealed a higher larval mortality compared with the treatment of Bt or PxGV alone. Combination of Bt and PxGV at sublethal concentrations also increased larval duration, reduced oviposition and decreased adult longevity remarkably, resulting in the lowest population trend index among the treatments. The results suggested that the combination of Bt and PxGV at sublethal dosages might provide a valuable way to improve the control efficacy of diamondback moth compared with treatment of Bt or PxGV alone.

Bacillus subtilis chitinase identified by matrix-assisted laser desorption/ionization time-of flight/time of flight mass spectrometry has insecticidal activity against Spodoptera litura Fab

An extracellular chitinase was identified and purified (CS1 and CS2) from Bacillus subtilis. The 16S rRNA sequencing was submitted in GenBank (accession numbers KC336487 and KC412256). The purified crude enzymes were identified through matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF MS) analysis. The peptide sequences were matched with chitinase sequences. The peak m/z with 1297. 592 and 3094.570 mascot search resulted sequence was blasted with NCBI protein sequences and confirmed that it is a chitinase enzyme. The effects of chitinase on gut enzymes lactate dehydrogenase, acid phosphatase, alkaline phosphatase and adenosine triphosphatase of the tobacco cutworm Spodoptera litura larvae were investigated. At all concentrations tested, chitinase decreased the activities of these gut enzymes relative to the control. When chitinase treated leaves were fed to larvae in bioassays, gut tissue and gut enzymes were affected. The histological study clearly shows the chitinase treated larval gut, peritrophic membrane and epithelial cells were affected significantly. Chitinase isolated from B. subtilis has effectively reduced the gut enzyme activity and growth of S. litura. The chitin based bioformulation may serve as an effective biocide against the polyphagous pest like S. litura.

Complete genomic sequences and comparative analysis of Mamestra brassicae nucleopolyhedrovirus isolated in Korea

Mamestra brassicae nucleopolyhedrovirus-K1 (MabrNPV-K1) was isolated from naturally infected M. brassicae (Lepidoptera: Noctuidae) larvae in Korea. The full genome sequences of MabrNPV-K1 were determined, analysed and compared to those of other baculoviruses. The MabrNPV-K1 genome consisted of 152,710 bp and had an overall G + C content of 39.9%. Computer-assisted analysis predicted 158 open reading frames (ORFs) of 150 nucleotides or greater that showed minimal overlap. Two inhibitor of apoptosis (iap) and six baculovirus repeated ORFs were interspersed in the MabrNPV-K1 genome. The unique MabrNPV-K1 ORF133 was identified in the MabrNPV-K1 genome that was not previously reported in baculoviruses. The gene content and arrangement in MabrNPV-K1 had the highest similarity with those of Helicoverpa armigera MNPV (HearMNPV) and Mamestra configurata NPV-B (MacoNPV-B), and their shared homologous genes were 99% collinear. The MabrNPV-K1 genome contained four homologous repeat regions (hr1, hr2, hr3 and hr4) that accounted for 3.3% of the genome. The genomic positions of the four MabrNPV-K1 hr regions were conserved among those of HearMNPV and MacoNPV-B. The gene parity plot, percent identity of the gene homologues and a phylogenetic analysis suggested that these three viruses are closely related not only to each other but also to the same virus strains rather than different virus species.

Cell Culture Derived AgMNPV Bioinsecticide: Biological Constraints and Bioprocess Issues

We have studied parameters for optimizing the Spodoptera frugiperda (Sf9) cell culture and viral infection for the production of Anticarsia gemmatalis multiple nucleopolyhedrosis virus (AgMNPV) polyhedra inclusion bodies (PIBs) in shaker-Schott or spinner bottles and bioreactors. We have assayed the k(L)a of the systems, initial cell seeding, cell culture volume, dissolved oxygen (DO), multiplicity of infection (MOI), nutrients consumption, and metabolites production. The medium surface oxygen transfer was shown to be higher in shaker bottles than in spinner ones, which was in direct correlation to the higher cell density obtained. Best quantitative performances of PIBs production were obtained with a SF900II medium volume/shaker-bottle volume ratio of 15% and MOI of 0.5 to 1 performed at a cell concentration at infection (CCI) of 1 to 2.5x10(6) cells/ml in a medium containing enough glucose and glutamine. Upon infection, a decrease in the cell multiplication was observed to be dependent on the MOI used, and the muX at the exponential growth phase in infected and non-infected cultures were, respectively, of 0.2832 and 0.3914 (day(-1)). The glucose consumption and lactate production were higher in the infected cultures (muGlucose and muLactate of, respectively, 0.0248 and 0.0089x10(-8) g/cellxday in infected cultures and 0.0151 and 0.0046x10(-8) g/cellxday in non infected ones). The glutamine consumption did not differ in both cultures (muGlutamine of 0.0034 and 0.0037x10(-8) g/cellxday in, respectively, infected and non infected cultures). When a virus MOI of 0.1 to 1 was used for infection, a higher concentration of PIBs/ml was obtained. This was in direct correlation to a higher cell concentration present in these cultures, where a decrease in cell multiplication due to virus infection is minimized. When a MOI of 1 was used, a more effective decrease in cell multiplication was observed and a lower concentration of PIBs/ml was obtained, but with the best performance of PIBs/cell. Correlations between MOI and CCI indicate that a MOI 0.1 to 1.4 and a CCI of 10(6) to 2x10(6) cells/ml led to the best PIBs production performances. The virulence of PIBs produced in cultures infected at low or high MOI showed comparable DL(50). Culture and infection in scaling-up conditions, performed in a bioreactor, were shown to provide the cells with a better environment and be capable of potentially improving the shaker-Schott findings. For an accurate qualitative control of PIB virulence, hemolymph from AgMNPV infected Anticarsia gemmatalis was used as starting material for passages in Sf9 cells. These led to a loss of virulence among the PIBs with an increase in the DL(50). The loss of virulence was accompanied by a loss in budded virus titer, a decreased number of PIBs produced and an altered DNA restriction pattern, suggesting the generation of defective interference particles (DIPs). Transmission electron microscopy (TEM) studies revealed that after cell passages, PIBs lacking virions were progressively synthesized. The study described here point out the biological constraints and bioprocess issues for the preparation of AgMNPV PIBs for biological control.

The International Workshop on Establishment of Microbial Inocula in Soils: Cooperative Research Project on Biological Resource Management of the Organization for Economic Cooperation and Development (OECD)

Low-input agriculture is likely to be the focal point for future cropping systems. Worldwide there is increasing concern that we must decrease the use of chemicals in agriculture. A leading reason for this concern has been the effects of pesticides on food quality, consumer health, and the environment. There also is concern regarding fertilizers and energy inputs because of environmental pollution from excessive application rates and poor timing of fertilization and because of the depletion of nonrenewable energy resources. Overuse of these materials not only is an economic waste but also may require environmental cleanup. Legislation may mandate the development of alternative methods of pest control. For example, the Dutch Government has demanded a 35% decrease in the use of farm pesticides in 1995 and a 50% reduction by the year 2000. Other governments may impose similar limitations.

Host heterogeneity in susceptibility and disease dynamics: tests of a mathematical model

Most mathematical models of disease assume that transmission is linearly dependent on the densities of host and pathogen. Recent data for animal diseases, however, have cast doubt on this assumption, without assessing the usefulness of alternative models. In this article, we use a combination of laboratory dose-response experiments, field transmission experiments, and observations of naturally occurring populations to show that virus transmission in gypsy moths is a nonlinear function of virus density, apparently because of heterogeneity among individual gypsy moth larvae in their susceptibility to the virus. Dose-response experiments showed that larvae from a laboratory colony of gypsy moths are substantially less heterogeneous in their susceptibility to the virus than are larvae from feral populations, and field experiments showed that there is a more strongly nonlinear relationship between transmission and virus density for feral larvae than for lab larvae. This nonlinearity in transmission changes the dynamics of the virus in natural populations so that a model incorporating host heterogeneity in susceptibility to the virus gives a much better fit to data on virus dynamics from large-scale field plots than does a classical model that ignores host heterogeneity. Our results suggest that heterogeneity among individuals has important effects on the dynamics of disease in insects at several spatial and temporal scales and that heterogeneity in susceptibility may be of general importance in the ecology of disease.

Detection of White spot syndrome virus DNA in pond soil using a 2-step nested PCR

White spot syndrome virus (WSSV) continues to be the most pathogenic among the penaeid shrimp viruses. In this study, WSSV DNA was detected in pond soil samples using a 2-step nested PCR. Primers described previously were used for the first round of amplification and based on the sequenced amplicon, an inner primer was designed for the 2nd round of amplification. Using plasmid DNA (pET 100) containing the 211 bp target WSSV sequence, analytical sensitivity showed that the 2-step nested PCR protocol was able to detect down to 0.015 fg of the plasmid DNA, or approximately 2 copies of the target DNA sequence. Persistence of WSSV DNA in pond soil samples after various time intervals was determined. WSSV-specific PCR product (161 bp) was still present in the soil samples even after 10 months of storage. The effect of soil heat treatment on the WSSV DNA was also examined. Soils were subjected to 25, 37, 50 and 70 degrees C for 1, 3 and 5 days. The results showed that PCR amplifiable WSSV DNA was still present even after 5 days at 70 degrees C. To our knowledge, this is the first report on the detection of WSSV DNA in soil samples. Based on these findings, it is concluded that the persistence of viral DNA in soil habitats may be an important aspect of WSSV ecology and may have an implication for viral transmissibility.

Characterization of an Egyptian Spodoptera littoralis nucleopolyhedrovirus and a possible use of a highly conserved region from polyhedrin gene for nucleopolyhedrovirus detection

An Egyptian isolate of Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) was tested for its potential as biocontrol agent in comparison to Autographa californica multiple nucleopolyhedrovirus (AcMNPV). Comparative assays of SpliNPV and AcMNPV against 2nd instar larvae of Spodoptera littoralis revealed 4-fold greater susceptibility of S. littoralis to AcMNPV than to SpliNPV based on LC50 values for the two viruses. The LT50s determined for SpliNPV and AcMNPV using LC50 of the virus against 2nd instar larvae were 4.2 and 5.8 days, respectively. A DNA segment of 405 bp containing highly conserved region from polyhedrin gene of SpliNPV (Polh-cr) was successfully amplified by PCR. Subsequently, this DNA segment was cloned and sequenced. Nucleotide sequence and its deduced amino acid sequence were compared to all available sequences in GenBank. Sequence alignment results revealed that Polh-cr showed significant similarities with 91 different baculovirus isolates. The percentage of homology ranged from 78% for Plusia orichalcea NPV to 99% for SpliNPV. This highly conserved region provides a candidate that could be used in easy, fast and economic prospective systems for virus detection as well as in biological control strategies.

Open reading frame 132 of Helicoverpa armigera nucleopolyhedrovirus encodes a functional per os infectivity factor (PIF-2)

Open reading frame 132 (Ha132) of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) is a homologue of per os infectivity factor 2 (pif-2) of Spodoptera exigua multiple nucleopolyhedrovirus. Sequence analysis indicated that Ha132 encoded a protein of 383 aa with a predicted molecular mass of 44.5 kDa. Alignment of HA132 and its baculovirus homologues revealed that HA132 was highly conserved among baculoviruses, with 14 absolutely conserved cysteine residues. RT-PCR indicated that Ha132 was first transcribed at 24 h post-infection. Western blot analysis showed that a 43 kDa band was detectable in HearNPV-infected HzAM1 cells from 36 h post-infection. Western blots also indicated that HA132 was a component of the occlusion-derived virus, but not of budded virus. Deletion of Ha132 from HearNPV abolished per os infectivity, but had no effect on the infectivity of the budded virus phenotype.

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.

Characterization of Ha29, a specific gene for Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus

Open reading frame 29 (ha29) is a gene specific for Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearSNPV). Sequence analyses showed that the transcription factor Tfb2 motif, bromodomain and Half-A-TPR (HAT) repeat were present at aa 66-82, 4-76, 55-90 of the Ha29 protein respectively. The product of Ha29 was detected in HearSNPV-infected HzAM1 cells at 3 h post-infection. Western blot analysis using a polyclonal antibody produced by immunizing a rabbit with purified GST-Ha29 fusion protein indicates that Ha29 is an early gene. The size of Ha29 product in infected HzAM1 cells was about 25 kDa, which was larger than the presumed size of 20.4 kDa. Tunicamycin treatment of HearSNPV-infected HzAM1 cells suggested that the Ha29 protein is N-glycosylated. Fluorescent confocal laser scanning microscope examination, and Western blot analysis of purified budded virus (BVs), occlusion-derived virus (ODVs), cell nuclear and cytoplasmic fraction, showed that the Ha29 protein was localized in the nucleus. Our results suggested that ha29 of HearSNPV encodes a non-structurally functional protein that may be associated with virus gene transcription in Helicoverpa hosts.

Persistence and coexistence of engineered baculoviruses

Baculoviruses, and in particular, the nucleopolyhedroviruses infect a wide range of arthropod hosts and have the potential to be used as biopesticides. However, one of the major drawbacks with these pathogens as biocontrol agents is that they have a slow response time. Alterations to the speed of kill and pathogen life history characteristics can influence the competitive outcome and persistence between wildtype and modified strains. Here, we explore, theoretically, how life-history modifications of pathogens can affect the epidemiology and ecology of strain coexistence. In particular, we show how under simple mass action disease transmission, life-history difference between strains are insufficient to allow coexistence. Additional heterogeneities in transmission are shown to be necessary to facilitate coexistence of wildtype and modified pathogen strains. We also illustrate how the patterns of infectivity of wildtype and modified strains can also affect long-term coexistence, and argue that appropriate assessment of genetic modifications must be presented in terms of relevant ecological theory.

Pest control through viral disease: mathematical modeling and analysis

This paper deals with the mathematical modeling of pest management under viral infection (i.e. using viral pesticide) and analysis of its essential mathematical features. As the viral infection induces host lysis which releases more virus into the environment, on the average 'kappa' viruses per host, kappain(1,infinity), the 'virus replication parameter' is chosen as the main parameter on which the dynamics of the infection depends. We prove that there exists a threshold value kappa(0) beyond which the endemic equilibrium bifurcates from the free disease one. Still for increasing kappa values, the endemic equilibrium bifurcates towards a periodic solution. We further analyse the orbital stability of the periodic orbits arising from bifurcation by applying Poor's condition. A concluding discussion with numerical simulation of the model is then presented.

Characterization of two Autographa californica nucleopolyhedrovirus proteins, Ac145 and Ac150, which affect oral infectivity in a host-dependent manner

The genome of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) contains two homologues, orf145 and orf150, of the Heliothis armigera Entomopoxvirus (HaEPV) 11,000-kDa gene. Polyclonal antibodies raised against the Ac145 or Ac150 protein were utilized to demonstrate that they are expressed from late to very late times of infection and are within the nuclei of infected Sf-21 cells. Transmission electron microscopy coupled with immunogold labeling of Ac145 found this protein within the nucleus in areas of nucleocapsid assembly and maturation, along with some association with the enveloped bundles of virions within the developing occlusion bodies (OBs). Ac150 was found to be mainly associated with enveloped bundles of virions within OBs and also with those not yet occluded. Both Ac145 and Ac150 were found to be present in budded virus as well as OBs. Both orf145 and orf150 were deleted from the AcMNPV genome, singly or together, and these deletion mutants were assessed for oral infectivity both in Trichoplusia ni and Heliothis virescens larvae. Deletion of Ac145 led to a small but significant drop in infectivity (sixfold) compared to wild-type (wt) AcMNPV for T. ni but not for H. virescens. Deletion of Ac150 alone had no effect on infectivity of the virus for either host. However, deletion of both Ac145 and Ac150 gave a recombinant virus with a drastic (39-fold) reduction in infectivity compared to wt virus for H. virescens. Intrahemocoelic injection of budded virus from the double-deletion virus into H. virescens larvae is as infectious to this host as wt budded virus, indicating that Ac145 and Ac150 play a role in primary oral infection of AcMNPV, the extent of which is host dependent.

Poneratoxin, a neurotoxin from ant venom

Poneratoxin is a small neuropeptide found in the venom of the ant Paraponera clavata. It is stored in the venom reservoir as an inactive 25-residue peptide. Here we describe both chemically synthesized poneratoxin and poneratoxin obtained by expression in insect cells. When expressed in insect cells, poneratoxin was observed attached to cell membranes. Both synthetic and recombinant ponerotoxins were soluble below pH 4.5. The structure of synthetic poneratoxin was characterized by circular dichroism and solved by nuclear magnetic resonance. In an environment imitating a lipid bilayer, at pH within the range of insect hemolymph, synthetic poneratoxin has a V shape, with two alpha-helices connected by a beta-turn. Insect larvae were paralyzed by injection of either of the purified toxins, with the recombinant one acting faster. The recombinant toxin-producing baculovirus reduced the average survival time of the insect host by 25 h compared with unmodified virus. Mass spectrometry analysis showed that the recombinant toxin has an N-terminal 21-residue extension, possibly improving its stability and/or stabilizing the membrane-bound state. The potential use of poneratoxin for the construction of biological insecticide is discussed.

An improved baculovirus insecticide producing occlusion bodies that contain Bacillus thuringiensis insect toxin

Baculovirus occlusion bodies, large proteinaceous structures which contain virions, have recently been engineered to incorporate foreign proteins. The major constituent protein of occlusion bodies from the baculovirus Autographa californica nucleopolyhedrovirus is polyhedrin, and assembly of recombinant occlusion bodies which incorporate a foreign protein depends on an interaction between native polyhedrin and a polyhedrin-foreign protein fusion. This technology has now been applied to the generation of a recombinant baculovirus (ColorBtrus) that produces occlusion bodies incorporating the Bacillus thuringiensis (Bt) insecticidal Cry1Ac toxin protein. ColorBtrus coexpresses native polyhedrin and a fusion protein in which polyhedrin is fused to the Bt toxin, which is in turn fused to green fluorescent protein (GFP). Analysis of ColorBtrus occlusion bodies confirmed that they include both Bt toxin and GFP, yet still incorporate virions. Bioassay of ColorBtrus demonstrated that its speed of action and pathogenicity are strikingly enhanced compared to wild-type virus. ColorBtrus represents a novel, powerful biological insecticide that combines positive attributes of both Bt toxin and baculovirus based systems.

Field activity and storage stability of Anagrapha falcifera nucleopolyhedrovirus (AfMNPV) in spray-dried lignin-based formulations

A multiple-embedded nucleopolyhedrovirus isolated from Anagrapha falcifera (Kirby) (AfMNPV) has potential to be developed into a microbial bioinsecticide because the host range includes several economic pests. We tested spray-dried AfMNPV formulations after storage for insecticidal activity based on bioassays with neonate Trichoplusia ni (Hübner). Eight experimental lignin-based spray-dried formulations, a glycerin-based formulation, and an unformulated sample were made with virus stock from three commercial production lots. Samples of these formulations were stored at 30 degrees C in individually sealed sample containers for destructive sampling after 1, 3, and 6 mo whereas the remaining product was stored in glass jars under refrigeration for up to 30 mo. Spray drying did not significantly reduce the initial LC50s of AfMNPV in experimental formulations compared with unformulated virus that was not spray dried. Refrigerated storage for 6 mo did not significantly lower virus activity of formulated samples compared with the unformulated AfMNPV stored frozen, while samples stored for 30 mo had higher LC50 values determined by both droplet and leaf feeding assays. When stored at 30 degrees C, most formulations (22 of 24) maintained insecticidal activity for 3 mo, but most (21 of 24) lost significant activity after 6 mo of storage. The glycerin-based formulation also lost activity within 6 mo of storage at 30 degrees C when compared with frozen unformulated virus, but did not lose activity when stored refrigerated for up to 30 mo. These formulations were evaluated after 7 mo at 4 degrees C for residual insecticidal activity when applied to field grown cabbage. Insecticidal activity was determined against T. ni neonates for treated leaf samples collected at 3, 7, 27, and 51 h after application of 2.5 x 10(12) obs/ha. Field tests showed no differences in activity among samples of stored formulations and one freshly made formulation. Spray-dried formulations had significantly higher insecticidal activity (67.5% mortality) compared with the unformulated treatment (30% mortality) sampled 3 h after application. At 3, 7, and 27 h after application, the spray-dried formulations had higher residual activity (67%, 59%, and 42% mortality, respectively), compared with the commercial glycerin-based formulation (61%, 38%, and 23% mortality, respectively). These experiments demonstrated that AfMNPV in lignin-based spray-dried formulations had a shelf-life of up to 3 mo at 30 degrees C and up to 30 mo at 4 degrees C, and with longer residual insecticidal activity in the field compared with unformulated or a glycerin formulation.

Storage stability of Anagrapha falcifera nucleopolyhedrovirus in spray-dried formulations

A multiply embedded nucleopolyhedrovirus isolated from Anagrapha falcifera (Kirby) (AfMNPV) can lose insecticidal activity during months of dry storage in ambient room conditions. We tested the spray-dried AfMNPV formulations after storage for up to 1 year at room temperatures for insecticidal activity against neonate Trichoplusia ni (Hübner). Experimental formulations were made using combinations of corn flours, lignin, and sucrose, and were selected based on previous work which demonstrated that these formulations resisted solar degradation in field experiments. Twelve experimental formulations (organized in three groups of four formulations) compared the effect of (1) the ratio of formulation ingredients (lignin and corn flour) to virus concentration, (2) different sources of lignin, or (3) different corn flours and sugar. Based on a single-dose plant assay with these 12 formulations, none of the formulations lost significant activity due to the drying process, when compared with the unformulated wet AfMNPV. Samples of the 12 dried formulations were stored at room (22+/-3 degrees C) and refrigerated (4 degrees C) temperatures. Insecticidal activity (LC(50)) was determined with a dosage-response assay for neonates fed on treated cotton-leaf disks. After 6 (or 9) and 12 months storage, refrigerated samples maintained insecticidal activity better than corresponding samples stored at room temperatures with LC(50)s that averaged 2.0 x 10(6) polyhedral inclusion bodies per milliliter (pibs/ml) for refrigerated samples and 5.4 x 10(6) pibs/ml for samples stored at room temperatures. Compared with unformulated stock virus stored frozen, six formulations stored at room temperature and 10 formulations stored in the refrigerator did not lose significant insecticidal activity after 1 year based on overlapping 90% confidence intervals. Changing the ratio of virus to formulation ingredients did not provide a clear trend over the range of concentrations tested, and may be less important for shelf-life of virus activity compared with formulations made with different ingredients. Two of the four formulations made with different lignins were about 15 times less active after 1 year at room temperature compared with refrigerated samples, indicating that specific formulation ingredients can affect storage stability. Formulations that contained sugar generally maintained activity during storage better than formulations without sugar. Unformulated virus stock maintained insecticidal activity (ranged from 0.20 to 2.5 x 10(6) pibs/ml) better during storage than dried formulations with LC(50)s that ranged from 0.39 to 27 x 10(6) pibs/ml. Unformulated virus stock, which is essentially a suspension of virus occlusion bodies in homogenized insect cadavers, did not lose activity when stored at refrigerated or room temperature. We believe that stability of AfMNPV insecticidal activity during storage as dry formulations is related to the general composition of the formulation and that sugar may play a critical role in maintaining insecticidal activity.

Persistence of an occlusion-negative recombinant nucleopolyhedrovirus in Trichoplusia ni indicates high multiplicity of cellular infection

We use data from the serial passage of co-occluded recombinant Autographa californica nuclear polyhedrosis virus (AcMNPV) to estimate the viral multiplicity of infection of cells within infected insects. Co-occlusion, the incorporation of wild-type and mutant virus genomes in the same occlusion body, has been proposed as a strategy to deliver genetically modified viruses as insecticides in a way that contains their spread in the environment. It may also serve as a means whereby naturally occurring mutant forms of NPVs can be maintained in a stable polymorphism. Here, a recombinant strain of AcMNPV was constructed with a deletion of its polyhedrin gene, rendering it incapable of producing occlusion bodies (i.e., occlusion negative). This was co-occluded with wild-type AcMNPV and used to infect fifth-instar Trichoplusia ni larvae. The fate of both genotypes was monitored over several rounds of insect infection. Levels of the occlusion-negative virus genome declined slowly over successive rounds of infection. We applied these data to a model of NPV population genetics to derive an estimate of 4.3 +/- 0.3 viral genomes per occlusion body-producing cell.

Replication, integration, and packaging of plasmid DNA following cotransfection with baculovirus viral DNA

Infection-dependent replication assays have been used to identify numerous putative origins of baculovirus replication. However, plasmid DNA, when cotransfected into insect cells with Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) DNA, replicates independently of any viral sequence in cis (11). Cotransfection of transfer plasmids and baculovirus DNA is a common procedure used in generating recombinant viruses and in measuring the level of gene expression in transient-expression assays. We have examined the fate of a series of vector plasmids in cotransfection experiments. The data reveal that these plasmids replicate following cotransfection and the replication of plasmid DNA is not due to acquisition of viral putative origin sequences. The conformation of plasmid DNA replicating in the cotransfected cells was analyzed and found to exist as high-molecular-weight concatemers. Ten to 25% of the replicated plasmid DNA was integrated into multiple locations on the viral genome and was present in progeny virions following serial passage. Sequence analysis of plasmid-viral DNA junction sites revealed no homologous or conserved sequences in the proximity of the integration sites, suggesting that nonhomologous recombination was involved during the integration process. These data suggest that while a rolling-circle mechanism could be used for baculovirus DNA replication, recombination may also be involved in this process. Plasmid integration may generate large deletions of the viral genome, suggesting that the process of DNA replication in baculovirus may be prone to generation of defective genomes.

A field release of genetically engineered gypsy moth (Lymantria dispar L.) nuclear polyhedrosis virus (LdNPV)

The gypsy moth (Lymantria dispar L.) nuclear polyhedrosis virus was genetically engineered for nonpersistence by removal of the gene coding for polyhedrin production and stabilized using a coocclusion process. A beta-galactosidase marker gene was inserted into the genetically engineered virus (LdGEV) so that infected larvae could be tested for its presence using a colorimetric assay. In 1993, LdGEV-infected gypsy moths were released in a forested plot in Massachusetts to test for spread and persistence. A similar forested plot 2 km away served as a control. For 3 years (1993-1995), gypsy moths were established in the two plots in Massachusetts to serve as test and control populations. Each week, larvae were collected from both plots. These field-collected larvae were reared individually, checked for mortality, and then tested for the presence of beta-galactosidase. Other gypsy moth larvae were confined on LdGEV-contaminated foliage for 1 week and then treated as the field-collected larvae. The LdGEV was sought in bark, litter, and soil samples collected from each plot. To verify the presence of the LdGEV, polymerase chain reaction, slot blot DNA hybridization, and restriction enzyme analysis were also used on larval samples. Field-collected larvae infected with the engineered virus were recovered in the release plot in 1993, but not in subsequent years; no field-collected larvae from the control plot contained the engineered virus. Larvae confined on LdGEV-contaminated foliage were killed by the virus. No LdGEV was recovered from bark, litter, or soil samples from either of the plots.

Assessment of the application of baculoviruses for control of Lepidoptera

Baculoviruses, among other insect viruses, are regarded as safe and selective bioinsecticides, restricted to invertebrates. They have been used worldwide against many insect pests, mainly Lepidoptera. Their application as microbial pesticides, however, has not met their potential to control pests in crops, forests, and pastures, with the exception of the nuclear polyhedrosis virus of the soybean caterpillar (Anticarsia gemmatalis), which is used on approximately 1 million ha annually in Brazil. Problems that have limited expansion of baculovirus use include narrow host range, slow killing speed, technical and economical difficulties for in vitro commercial production, timing of application based on frequent host population monitoring, variability of field efficacy due to climatic conditions, and farmers' attitudes toward pest control, which have been based on application of fast-killing chemical insecticides. Farmer education regarding use of biological insecticides and their characteristics is considered one of the major actions necessary for increased use of baculoviruses. Strategies to counteract some of the limitations of baculoviruses, especially their slow killing activity, have been investigated and are promising. These include the use of chemical or biological substances added to virus formulations and genetic engineering of the viruses themselves to express insect toxins or hormones. Such strategies can enhance viral activity and increase speed of kill as well as reduce larval feeding activity. The use of baculoviruses against Lepidoptera is reviewed, with the utilization of the nuclear polyhedrosis virus of A. gemmatalis in Brazil serving as a case-study.

Hepatitis B virus replication in human HepG2 cells mediated by hepatitis B virus recombinant baculovirus

A novel transient mechanism for studying hepatitis B virus (HBV) gene expression and replication using recombinant HBV baculovirus to deliver the HBV genome to HepG2 cells was generated. In HBV baculovirus infected HepG2 cells, HBV transcripts, and intracellular and secreted HBV antigens are produced; replication occurs as evidenced by the presence of high levels of intracellular replicative intermediates and protected HBV DNA in the medium. Density-gradient analysis of extracellular HBV DNA indicated that the DNA was contained predominantly in enveloped HBV virions. Covalently closed circular (CCC) DNA is present indicating that, in this system, HBV core particles are capable of delivering newly synthesized HBV genomes back into the nuclei of infected cells. HBV gene expression is driven exclusively from endogenous promoters. Levels of HBV gene expression and replication can be achieved in HBV baculovirus-infected HepG2 cells which far exceed levels found in HepG2 2.2.15 cells. HBV baculovirus infection of HepG2 cells lends itself readily to experimental manipulation as follows: 1) HBV expression can be initiated any time relative to seeding of HepG2 cells; 2) levels of HBV replication can be regulated over a wide range simply by changing the baculovirus multiplicity of infection; 3) HBV replication is readily detectable by one day post infection with HBV baculovirus and persists at least through day eleven post infection; and (4) the transient nature of the infection can be extended and/or enhanced by superinfecting the cultures. We conclude that infection of HepG2 cells by HBV recombinant baculovirus represents a simple to use and highly flexible system for studying the effects of antivirals and/or cytokines on HBV production and for understanding HBV replication and pathogenesis at the molecular level.

Observations on the presence of the peritrophic membrane in larval Trichoplusia ni and its role in limiting baculovirus infection

Light microscopical examinations of dissected and stained peritrophic membranes (PMs) were conducted to determine the presence or absence of this protective structure in larvae of Trichoplusia ni, prior to and through ecdysis. Observations of fourth- and fifth-instar larvae of T. ni from two independent rearing colonies showed that PMs were present and lined the midgut prior to, during, and immediately after ecdysis in both instars. Western blot analysis of insect intestinal mucin (IIM), a major protective protein in the T. ni PM, indicated that synthesis of IIM occurred during T. ni embryonic development, or more precisely, that IIM synthesis started approximately 4 h prior to hatching. These results demonstrated that the neonate T. ni midgut is lined with a protective mucinous layer at hatching. A baculovirus enhancin from T. ni granulosis virus (TnGV) enhanced per os viral infections of budded viruses (BVs) of Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) and T. ni single nuclear polyhedrosis virus (TnSNPV) in neonate, fourth-, and fifth-instar larvae of T. ni. These results provided further evidence that the PM may serve as a partial barrier to viruses in the midgut lumen and that enhancins can facilitate the infection process.

Baculovirus-mediated expression of a scorpion depressant toxin improves the insecticidal efficacy achieved with excitatory toxins

The insecticidal efficacy towards Helicoverpa armigera lepidopteran larvae of recombinant Autographa californica M nucleopolyhedroviruses, expressing depressant and excitatory scorpion anti-insect selective toxins, was investigated. The ET50 (effective paralysis time 50%) values obtained with the recombinant viruses expressing the depressant toxin, LqhIT2, and the excitatory toxin, LqhIT1, were 59 h and 66 h, respectively, whereas the ET50 value of the wild-type virus was longer, 87 h post infection. The insecticidal effects obtained when using two distinct temporally regulated viral promoters revealed advantage for the very late p10 promoter over the p35 early promoter. The higher insecticidity of the virus expressing the depressant toxin compared to the excitatory toxin suggests that pharmacokinetic factors and/or promoter efficiency may play a role during infection of insect pest larvae by recombinant baculoviruses.

Ecological considerations for the environmental impact evaluation of recombinant baculovirus insecticides

The history of baculoviruses in insect control and the current status of recombinant baculovirus (recBV) insecticides in the laboratory and the field are briefly outlined. A conceptual model for impact evaluation is described that distinguishes between scientific impact evaluation and regulatory risk assessment. Its components are identified and reviewed in the light of existing ecological theory and experimental study under the categories of impact identification, exposure identification, and impact evaluation. Impact identification aims to identify species and populations sensitive to direct or indirect impacts by a recBV. Exposure identification examines how susceptible populations may be exposed to a recBV. Impact evaluation combines these data to predict the potential for recBV impacts in the environment.

Enhanced Bioactivity of Recombinant Baculoviruses Expressing Insect-Specific Spider Toxins in Lepidopteran Crop Pests

Two genetically enhanced isolates of the Autographa californica nuclear polyhedrosis virus (AcMNPV) expressing insect-specific neurotoxin genes from the spiders Diguetia canities and Tegenaria agrestis were evaluated for their commercial potential. Since prevention of feeding damage is of primary importance in assessing agronomic efficacy, a method for estimating the median time to cessation of feeding (FT50) was developed. Neonate droplet feeding assays with preoccluded virus samples were conducted to compare the FT50s and median survival times (ST50s) of larvae infected by the toxin-expressing recombinant viruses with those of larvae infected by wild-type AcMNPV and the appropriate polyhedrin-minus control viruses. Low dosages were used to minimize the effect of dosage on the response times, and the time to molting of noninfected larvae was used to audit variability among batches of larvae within and between tests. Appropriate statistics are discussed. To evaluate host spectrum, response times were compared in three lepidopteran insect pests, Trichoplusia ni Hubner, Spodoptera exigua (Hubner), and Heliothis virescens (Fabricius). The recombinant viruses expressing insect-specific toxin genes from T. agrestis and D. canities, designated vAcTalTX-1 and vAcDTX9.2, respectively, significantly reduced both FT50 and ST50 in all three lepidopteran pests. Reductions in feeding times compared to the wild-type virus ranged from 16 to 39% with vAcTalTX-1 and 30 to 40% with vAcDTX9.2. Reductions in survi val time were lower, ranging from 18 to 33% with vAcTalTX-1 and 9 to 24% with vAcDTX9.2. While vAcTalTX-1 tended to kill faster than vAcDTX9.2, vAcDTX9.2 stopped feeding faster than vAcTalTX-1, suggesting that it would be more effective in reducing crop damage.

A microinjection technique using Drosophila melanogaster for bioassay-guided isolation of neurotoxins in arthropod venoms

Modern analytical techniques permit isolation and structural determination of neurotoxins at the picomole level. However, bioassay-guided fractionation of the sample often relies on simple injection assays using insects, vertebrates or crustaceans of a fairly large size, thus consuming quite a large amount of the samples being investigated. In order to investigate samples of very small size, we have devised an insect microinjection method using glass micropipettes and Drosophila melanogaster adults as test insects. The validity of the method was tested with a series of six buthoid scorpion venoms (Androctonus australis, Buthotus judaicus, Buthus tamulus, Centruroides sculpturatus, Leiurus quinquestriatus hebraeus, Tityus serrulatus) and one chactoid scorpion (Scorpio maurus palmatus) as standards. The LD50S of the venoms were determined using both the microinjection method and a classical injection assay with crickets (Gryllus bimaculatus) as test insects. Results demonstrated that the new method can successfully be applied to the study of insect neurotoxic activity in arthropod venoms. The Gryllus:Drosophila ratio in amount of sample utilized is 100. However, for all Buthoid venoms tested, except L. quinquestriatus, Drosophila showed less sensitivity, thus reducing the gain by a factor of 2-10. Drosophila were several times more sensitive to the only chactoid venom tested. These results clearly demonstrate the advantage of using this microtechnique, when limited amounts of material are available for both chemical and biological work.

Identification and nucleotide sequence of an ecdysteroid UDP-glucosyltransferase gene of Spodoptera littoralis multicapsid nuclear polyhedrosis virus

The Spodoptera littoralis multicapsid nuclear polyhedrosis virus (SlMNPV) is a member of the Baculoviridae that shows a distant genetic relationship to the prototype Autographa californica MNPV (AcMNPV). Using an AcMNPV gene-specific probe, we identified and mapped an ecdysteroid UDP-glucosyltransferase (egt) gene in the genome of SlMNPV. Sequence determination of a part from the hybridizing DNA fragment revealed an open reading frame of 1548 nucleotides that exhibits 38% and 44% identity to the egt amino acid sequences of AcMNPV and Lymantria dispar MNPV (LdMNPV), respectively. Sequences flanking the SlMNPV egt gene, including the promoter region, were found to be unique to the virus. The presence of this nonstructural gene in SlMNPV and several other baculoviruses points to the importance of egt for the viral infection process.

The 1629-bp open reading frame of the Autographa californica multinucleocapsid nuclear polyhedrosis virus encodes a virion structural protein

A 1629-bp open reading frame (ORF) of Autographa californica multinucleocapsid nuclear polyhedrosis virus (AcMNPV) is shown to encode a 78-kDa virion structural protein. To determine this, polyclonal antibody was made to a fusion protein synthesized in Escherichia coli from a chimeric gene that contained 1415 bp of the 1629-bp gene. In Western blot analyses, this antibody cross-reacted with a protein of about 78 kDa in both extracellular virions (ECV) and virions isolated from polyhedra (PDV), and with a 78-kDa protein in PDV envelope preparations, but not with PDV nucleocapsids. This suggests that the protein encoded by the 1629-bp ORF is a virion envelope protein or a protein that occurs in the virion intermediate layer between the envelope and nucleocapsid.

Baculovirus systems for the expression of human gene products

Significant advances in basic and applied biology have resulted from the use of baculovirus vectors for the expression of heterologous proteins in cultured insect cells and in insect larvae. The development of improved vectors has greatly facilitated the construction of recombinant baculoviruses, both by increasing the efficiency of identifying recombinant viruses and by reducing or eliminating the tedious steps used to purify the desired recombinant virus from its non-recombinant parent virus.

Sequence, transcriptional mapping, and overexpression of p47, a baculovirus gene regulating late gene expression

A 2.8-kb region of the Autographa californica nuclear polyhedrosis virus genome was sequenced and found to contain an open reading frame (p47) which was capable of rescuing a previously characterized temperature-sensitive mutant, ts317 (S. Partington, H. Yu, A. Lu, and E. B. Carstens, Virology 157:91-102, 1990). Transcriptional mapping demonstrated that an early 4.2-kb RNA encoded the p47 open reading frame and probably overlapped the 39K delayed-early gene. The p47 open reading frame was cloned behind the polyhedrin promoter in a baculovirus transfer plasmid, which was then used to prepare a recombinant baculovirus overexpressing the p47 polypeptide. The overexpressed polypeptide was used to prepare p47-specific monoclonal antibodies. These antibodies detected a polypeptide of 47 kDa in A. californica nuclear polyhedrosis virus-infected cells, demonstrating that p47 is expressed as an authentic viral product. The p47 gene product was localized to the nucleus of infected cells, supporting the hypothesis that it is involved in regulating viral transcription at late times postinfection.

The 35-kilodalton protein gene (p35) of Autographa californica nuclear polyhedrosis virus and the neomycin resistance gene provide dominant selection of recombinant baculoviruses

Autographa californica nuclear polyhedrosis virus (AcMNPV) recombinants were constructed to test the effectiveness of the AcMNPV 35-kilodalton protein gene (35K gene) and the bacterial neomycin resistance gene (neo) as dominant selectable markers for baculoviruses. Insertion of the AcMNPV apoptosis suppressor gene (p35) into the genome of p35-deletion mutants inhibited premature host cell death and increased virus yields up to 1200-fold at low multiplicities in Spodoptera frugiperda (SF21) cell cultures. When placed under control of an early virus promoter, the bacterial neomycin resistance gene (neo) restored multiplication of AcMNPV in the same cells treated with concentrations of the antibiotic G418 that inhibited wild-type virus growth greater than 1000-fold. The selectivity of these dominant markers was compared by serial passage of recombinant virus mixtures. After four passages, the proportion of p35-containing virus increased as much as 2,000,000-fold relative to deletion mutants, whereas the proportion of neo-containing viruses increased 500-fold relative to wild-type virus under G418 selection. The strength and utility of p35 as a selectable marker was further demonstrated by the construction of AcMNPV expression vectors using polyhedrin-based transfer plasmids that contain p35. Recombinant viruses with foreign gene insertions at the polyhedrin locus accounted for 15 to 30% of the transfection progeny. The proportion of desired viruses was increased to greater than 90% by linearizing the parental virus DNA at the intended site of recombination prior to transfection. These results indicate that p35 and neo facilitate the selection of baculovirus recombinants and that p35, in particular, is an effective marker for the generation of AcMNPV expression vectors.

A cellular promoter-based expression cassette for generating recombinant baculoviruses directing rapid expression of passenger genes in infected insects

We have developed an expression cassette which allows the generation of recombinant baculoviruses that can express passenger genes under the control of a constitutive cellular promoter derived from the cytoplasmic actin gene of the silkmoth Bombyx mori. Silkmoth tissue culture cells which were infected with a recombinant B. mori nuclear polyhedrosis virus (BmNPV) containing the gene-encoding chloramphenicol acetyltransferase (CAT) under the control of this expression cassette expressed significant CAT activity beginning 5 hr postinfection (p.i.). Cells infected with a recombinant BmNPV containing the cat gene under the control of the polyhedrin gene promoter did not express CAT activity until 20 hr p.i. Silkworm larvae were also infected with the two recombinant viruses by hemocelic injections and all larval tissues examined were found to express the cat gene. While significant actin-cassette-driven CAT expression in vivo was first seen at 24 hr p.i., expression from the polyhedrin promoter was not seen until 48 hr p.i. By 60 hr p.i., tissues of larvae infected with the recombinant virus expressing cat under polyhedrin promoter control were found to exhibit sixfold higher CAT activity than those infected with recombinant virus expressing the cat gene under the control of the actin promoter. The 24-hr temporal advantage in expression of a passenger gene in infected larvae indicates that the actin-promoter-based expression cassette or other analogous cellular promoter-based cassettes could be used for generating recombinant baculovirus insecticides which could incapacitate pest insects more quickly than viruses employing the polyhedrin or other late viral promoters for expressing insect-incapacitating proteins.