Host range expansion of Autographa californica nuclear polyhedrosis virus (NPV) following recombination of a 0.6-kilobase-pair DNA fragment originating from Bombyx mori NPV

Expression profiling of baculovirus genes in permissive and nonpermissive cell lines

The baculoviruses Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multicapsid NPV (AcMNPV) are highly homologous at the genomic level, but they have essentially nonoverlapping host ranges. In order to characterize baculovirus replication in permissive and nonpermissive cell lines, the expression profiles of baculovirus-specific genes (at 2, 6, 12, 24, 36, 48 or 72 h post-infection) were examined in BmN (BmNPV-permissive) or Sf-9 (AcMNPV-permissive) cells that were inoculated with BmNPV or AcMNPV. Surprisingly, nearly all of the 154 genes of AcMNPV appeared to be expressed in both Sf-9 and BmN cells although the peak expression levels of these genes were delayed by roughly 12 h in the nonpermissive BmN cells. In addition, the expression levels of the very late AcMNPV polyhedrin and p10 genes were dramatically reduced in BmN cells, which presumably led to the inability of AcMNPV to form polyhedral inclusion bodies in BmN cells. Nearly all of the 136 genes of BmNPV appeared to be expressed in BmN cells, however, BmNPV gene expression was dramatically reduced in Sf-9 cells inoculated with BmNPV. Experiments in which BmNPV DNAs were transfected to Sf-9 cells suggested that this dramatic reduction in gene expression was not the result of poor attachment, penetration or uncoating of the BmNPV virion into Sf-9 cells. In conclusion, we established a system to monitor global gene expression patterns during baculovirus infection in permissive and nonpermissive cell lines. This system was used to identify global trends in the transcription of baculovirus genes during productive and nonproductive infection.

Host range factor 1 from Lymantria dispar Nucleopolyhedrovirus (NPV) is an essential viral factor required for productive infection of NPVs in IPLB-Ld652Y cells derived from L. dispar

Host range factor 1 (HRF-1) of Lymantria dispar multinucleocapsid nucleopolyhedrovirus promotes Autographa californica MNPV replication in nonpermissive Ld652Y cells derived from L. dispar. Here we demonstrate that restricted Hyphantria cunea NPV replication in Ld652Y cells was not due to apoptosis but was likely due to global protein synthesis arrest that could be restored by HRF-1. Our data also showed that HRF-1 promoted the production of progeny virions for two other baculoviruses, Bombyx mori NPV and Spodoptera exigua MNPV, whose replication in Ld652Y cells is limited to replication of viral DNA without successful production of infectious progeny virions. Thus, HRF-1 is an essential viral factor required for productive infection of NPVs in Ld652Y cells.

Virus entry or the primary infection cycle are not the principal determinants of host specificity of Spodoptera spp. nucleopolyhedroviruses

The multicapsid nucleopolyhedroviruses (NPVs) of Spodoptera exigua (SeMNPV), Spodoptera frugiperda (SfMNPV), and Spodoptera littoralis (SpliNPV) are genetically similar (78 % similarity) but differ in their degree of host specificity. Infection by each of the three NPVs in these three Spodoptera host species was determined by oral inoculation of larvae with occlusion bodies (OBs) or intrahaemocoelic injection with occlusion derived virions (ODVs). RT-PCR analysis of total RNA from inoculated insects, targeted at immediate early (ie-0), early (egt, DNA polymerase), late (chitinase) and very late genes (polyhedrin), indicated that each of the NPVs initiated an infection in all three host species tested. SpliMNPV produced a fatal NPV disease in both heterologous hosts, S. frugiperda and S. exigua, by oral inoculation or injection. SfMNPV was lethal to heterologous hosts, S. exigua and S. littoralis, but infected larvae did not melt and disintegrate, and progeny OBs were not observed. SeMNPV was able to replicate in heterologous hosts and all genes required for replication were present in the genome, as the virus primary infection cycle was observed. However, gene expression was significantly lower in heterologous hosts. SeMNPV pathogenesis in S. frugiperda and S. littoralis was blocked at the haemocoel transmission stage and very nearly cleared. SeMNPV mixtures with SpliMNPV or SfMNPV did not extend the host range of SeMNPV; in all cases, only the homologous virus was observed to proliferate. It is concluded that entry and the primary virus infection cycle are not the only, or the major determinants, for SeMNPV infection of heterologous Spodoptera species.

Analysis of host specificity of two closely related baculoviruses in permissive and nonpermissive cell lines

The baculoviruses Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multinucleocapsid nucleopolyhedrovirus (AcMNPV) share about 90% identity at the genomic level but they have non-overlapping host range and show a high degree of host specificity. We have demonstrated here that AcMNPV undergoes DNA replication and early gene expression in Bombyx-derived BmN cells but fails to show very late gene expression or produce budded virion (BV) particles. Coinfection with BmNPV supported BV production from AcMNPV in BmN cells at low levels but not very late gene expression or polyhedral inclusion body formation. BV production and very late gene expression from BmNPV, on the contrary, were adversely affected in coinfections. In Spodoptera frugiperda-derived Sf21 cell lines, BmNPV DNA replication, BV production, and very late gene expression took place only when coinfected with AcMNPV. BmNPV exerted a less profound effect on AcMNPV multiplication and very late gene expression in permissive host cell lines. AcMNPV shuts down cellular and viral protein synthesis completely when infected alone or coinfected with BmNPV in BmN cells, whereas BmNPV infection did not affect cellular and viral protein synthesis in Sf21 cells. Overall, AcMNPV showed a more dominant effect by complementing the multiplication of BmNPV in nonpermissive host cells while inhibiting it in BmN cells.

Large-scale production of porcine mature interleukin-18 (IL-18) in silkworms using a hybrid baculovirus expression system

In this report, a hybrid baculovirus expression system, which means a hybrid virus of the Autographa californica nuclear polyhedrosis virus and the Bombyx mori nuclear polyhedrosis virus, was used for the large-scale production of porcine mature interleukin-18 (IL-18) in silkworms. Two recombinant hybrid baculoviruses containing cDNA of the porcine precursor IL-18 and the porcine caspase-1 were constructed and were used to infect silkworm larvae. After the co-infection of the two viruses, porcine mature IL-18 was efficiently produced in the haemolymph. The concentration of IL-18 in the haemolymph was 80-100 microg/ml, as determined by porcine IL-18 specific ELISA. This yield was twenty-times more than that of the insect cell expression system described previously. The porcine mature IL-18 produced by the silkworms strongly induced interferon-gamma (IFN-gamma) production from porcine PBMC. An insect factory system for the large-scale production of useful cytokines for livestock animals will be available in the near future.

Reduced expression of the immediate-early protein IE0 enables efficient replication of Autographa californica multiple nucleopolyhedrovirus in poorly permissive Spodoptera littoralis cells

Infection of Spodoptera littoralis SL2 cells with the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) results in apoptosis and low yields of viral progeny, in contrast to infection with S. littoralis nucleopolyhedrovirus (SlNPV). By cotransfecting SL2 cells with AcMNPV genomic DNA and a cosmid library representing the complete SlNPV genome, we were able to rescue AcMNPV replication and to isolate recombinant virus vAcSL2, which replicated efficiently in SL2 cells. Moreover, vAcSL2 showed enhanced infectivity for S. littoralis larvae compared to AcMNPV. The genome of vAcSL2 carried a 519-bp insert fragment that increased the distance between the TATA element and the transcriptional initiation site (CAGT) of immediate-early gene ie0. This finding correlated with low steady-state levels of IE0 and higher steady-state levels of IE1 (the product of the ie1 gene, a major AcMNPV transactivator, and a multifunctional protein) than of IE0. Mutagenesis of the ie0 promoter locus by insertion of the chloramphenical acetyltransferase (cat) gene yielded a new recombinant AcMNPV with replication properties identical to those of vAcSL2. Thus, the analysis indicated that increasing the steady-state levels of IE1 relative to IE0 should enable AcMNPV replication in SL2 cells. This suggestion was confirmed by constructing a recombinant AcMNPV bearing an extra copy of the ie1 gene under the control of the Drosophila hsp70 promoter. These results suggest that IE0 plays a role in the regulation of AcMNPV infection and show, for the first time, that significant improvement in the ability of AcMNPV to replicate in a poorly permissive cell line and organism can be achieved by increasing the expression of the main multiple functional protein, IE1.

Pseudotyping Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV): F proteins from group II NPVs are functionally analogous to AcMNPV GP64

GP64, the major envelope glycoprotein of budded virions of the baculovirus Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), is involved in viral attachment, mediates membrane fusion during virus entry, and is required for efficient virion budding. Thus, GP64 is essential for viral propagation in cell culture and in animals. Recent genome sequences from a number of baculoviruses show that only a subset of closely related baculoviruses have gp64 genes, while other baculoviruses have a recently discovered unrelated envelope protein named F. F proteins from Lymantria dispar MNPV (LdMNPV) and Spodoptera exigua MNPV (SeMNPV) mediate membrane fusion and are therefore thought to serve roles similar to that of GP64. To determine whether F proteins are functionally analogous to GP64 proteins, we deleted the gp64 gene from an AcMNPV bacmid and inserted F protein genes from three different baculoviruses. In addition, we also inserted envelope protein genes from vesicular stomatitis virus (VSV) and Thogoto virus. Transfection of the gp64-null bacmid DNA into Sf9 cells does not generate infectious particles, but this defect was rescued by introducing either the F protein gene from LdMNPV or SeMNPV or the G protein gene from VSV. These results demonstrate that baculovirus F proteins are functionally analogous to GP64. Because baculovirus F proteins appear to be more widespread within the family and are much more divergent than GP64 proteins, gp64 may represent the acquisition of an envelope protein gene by an ancestral baculovirus. The AcMNPV pseudotyping system provides an efficient and powerful method for examining the functions and compatibilities of analogous or orthologous viral envelope proteins, and it could have important biotechnological applications.

Diversity, distribution, and mobility of bro gene sequences in Bombyx mori nucleopolyhedrovirus

Genetic differences between strains of a baculovirus are often limited to some restriction sites, short DNA deletions or absence of some nonessential genes. The recently coined bro gene family, represents a new major source of intraspecific variability. A comparison between two bro gene sets of Bombyx mori nucleopolyhedroviruses (NPV) shows that bro genes are distributed in three regions for the -T3 and -SC7 virus strains. In BmNPV T3, five bro genes are distributed in three genome locations, whereas the BmNPV SC7 strain possess a single bro copy in each region. In addition, each of the BmNPV SC7 bro genes belongs to one of the three subfamilies present in BmNPV T3. Analysis of bro copy sequences and of adjacent sequences suggests an active redistribution of sequences due to intraspecific recombination. The maintenance of one allele of each subfamily suggests that they play different roles in the viral cycle, and that they are essential.

DNA-independent ATPase activity of the Trichoplusia ni granulovirus DNA helicase

DNA helicases of baculoviruses are essential for virus replication and have been implicated as molecular determinants of host range. Although these proteins contain seven motifs (I, Ia, II-VI) characteristic of DNA helicases, the two most important characteristics of helicases - duplex-DNA unwinding and ATPase activity - have not been demonstrated. In the present study, a recombinant putative DNA helicase (rP137) of Trichoplusia ni granulovirus (TnGV) was purified from insect cells infected with a recombinant Autographa californica multicapsid nucleopolyhedrovirus that overproduced rP137. The rP137 protein exhibited an intrinsic DNA-independent ATPase activity that required Mg(2+) as a co-factor, an activity that was reduced in the presence of TnGV and phage lambda DNAs. These results provide further evidence that baculovirus helicase genes encode proteins with biochemical properties similar to those of classical DNA helicases.

The Trichoplusia ni granulovirus helicase is unable to support replication of Autographa californica multicapsid nucleopolyhedrovirus in cells and larvae of T. ni

Baculovirus DNA helicases are essential for replication and are determinants of host range. Helicases of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Trichoplusia ni granulovirus (TnGV) differ markedly, although both viruses replicate efficiently in the cabbage looper, T. ni. To determine whether the TnGV helicase (P137) could support replication of AcMNPV in T. ni cells or larvae, the native AcMNPV helicase gene (p143) was disrupted and substituted with p137. P137 did not support replication when synthesized by the P143-deficient AcMNPV. Moreover, P137 did not inhibit AcMNPV replication when co-synthesized in the presence of the AcMNPV P143. These results suggest that although TnGV and AcMNPV replicate efficiently in T. ni, specific protein-protein or protein-DNA interactions between baculoviral helicases and viral-specific factors which form the replicase complex are required for virus replication. A novel and rapid method for disrupting AcMNPV genes in E. coli using the commercial Bac-to-Bac AcMNPV baculovirus expression vector is described.

High-level expression of a foreign gene by a recombinant baculovirus with an expanded host range

The usefulness of host range expanded viruses as an expressionvector system was investigated by following the expression ofthe E. coli lacZ gene. The host range expanded recombinantviruses were obtained from Sf-21 or BmN-4 cells coinfected withAutographa californica and Bombyx mori nuclearpolyhedrosis viruses. Among the host range expanded viruses,RecB-8 and RecS-B6 have similar enzyme digestion profiles butdifferent infection characteristics in cells. Therefore, tostudy the foreign gene expression efficiency of these twoviruses, we constructed recombinant viruses RecB8-LacZ andRecSB6-LacZ containing the lacZ gene instead of the polyhedringene. Also, the host range expanded recombinant AcNPV, Bac-BH,containing lacZ gene in the polyhedrin gene locus was constructedby substitution of the 0.6 kb region within the helicase gene ofBacPAK6 with that of BmNPV. beta-Galactosidase expressionefficiency by these viruses were determined and compared in Sf-21and BmN-4 cells. The result showed that Bac-BH has highexpression efficiency only in Sf-21 cells, whereas RecB8-LacZhas high expression efficiency both in Sf-21 and BmN-4 cells.Also, in BmN-4 cells, beta-galactosidase expressionefficiency of RecB8-LacZ was higher than that of recombinantBmNPV (BmK1-LacZ containing lacZ gene in polyhedrin gene locus).In addition, the expression efficiency was not correlated withvirus titer.

Global protein synthesis shutdown in Autographa californica nucleopolyhedrovirus-infected Ld652Y cells is rescued by tRNA from uninfected cells

Global protein synthesis arrest occurs in Autographa californica nucleopolyhedrovirus (AcNPV)-infected Ld652Y cells at late times postinfection (p.i.). A Lymantria dispar nucleopolyhedrovirus gene, hrf-1, precludes this protein synthesis arrest. We used in vitro translation assays to characterize the translation defect. Cell-free lysates prepared from uninfected Ld652Y cells, AcNPV-infected cells harvested at early times p.i., and cells infected with vAchrf-1, a recombinant AcNPV bearing hrf-1, all supported translation. Lysates prepared from AcNPV-infected Ld652Y cells at late times p.i. did not support translation, but activity was restored by adding small RNA species from mock-, vAchrf-1- (24 or 48 h p.i.), and AcNPV- (6 h p.i. ) infected cells. Small RNA species (24 and 48 h p.i.) from AcNPV-infected cells did not rescue translation. Assays of RNA species further fractionated by ion exchange chromatography demonstrated that tRNA rescued translation. Although specific defective tRNA species were not revealed by comparative two-dimensional gel analysis, analysis of (32)P-labeled tRNAs showed a reduction in de novo synthesis of small RNA isolated from AcNPV-infected cells compared with mock- and vAchrf-1-infected cells. This study suggests a mechanism of translation arrest involving defective or depleted tRNA species in AcNPV-infected Ld652Y cells.

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.

Colocalization of baculovirus IE-1 and two DNA-binding proteins, DBP and LEF-3, to viral replication factories

We have recently identified a DNA-binding protein (DBP) from the baculovirus Bombyx mori nucleopolyhedrovirus (BmNPV) which can destabilize double-stranded DNA (V. S. Mikhailov, A. L. Mikhailova, M. Iwanaga, S. Gomi, and S. Maeda, J. Virol. 72:3107-3116, 1998). DBP was found to be an early gene product that was not present in budded or occlusion-derived virions. In order to characterize the localization of DBP during viral replication, BmNPV-infected BmN cells were examined by immunostaining and confocal microscopy with DBP antibodies. DBP first appeared as diffuse nuclear staining at 4 to 6 h postinfection (p.i.) and then localized to several specific foci within the nucleus at 6 to 8 h p.i. After the onset of viral DNA replication at around 8 h p.i., these foci began to enlarge and eventually occupied more than half of the nucleus by 14 h p.i. After the termination of viral DNA replication at about 20 h p.i., the DBP-stained regions appeared to break down into approximately 100 small foci within the nucleus. At 8 h p.i., the distribution of DBP as well as that of IE-1 or LEF-3 (two proteins involved in baculovirus DNA replication) overlapped well with that of DNA replication sites labeled with bromodeoxyuridine incorporation. Double-staining experiments with IE-1 and DBP or IE-1 and LEF-3 further confirmed that, between 8 and 14 h p.i., the distribution of IE-1 and LEF-3 overlapped with that of DBP. However, IE-1 localized to the specific foci prior to DBP or LEF-3 at 4 h p.i. In the presence of aphidicolin, an inhibitor of DNA synthesis, immature foci containing IE-1, LEF-3, and DBP were observed by 8 h p.i. However, the subsequent enlargement of these foci was completely suppressed, suggesting that the enlargement depended upon viral DNA replication. At 4 h p.i., the number of IE-1 foci correlated with the multiplicity of infection (MOI) between 0.4 and 10. At higher MOIs (e.g., 50), the number of foci plateaued at around 15. These results suggested that there are about 15 preexisting sites per nucleus which are associated with the initiation of viral DNA replication and assembly of viral DNA replication factories.

A novel expression system based on host-range expansion of baculovirus

A host range expanded recombinant Autographa californica multiple-nucleocapsid nucleopolyhedrosis virus AcMNPV/r2 was obtained by cotransfection of the bacmid DNA from Escherichia coli DH10Bac along with a plasmid pBmH-M containing HindIII M fragment of Bombyx mori nuclear polyhedrosis virus (BmNPV) genomic DNA. A recombinant transposon vector carrying a mutant green fluorescent protein gene (GFP) and a polyhedrin gene was constructed. Transposition was carried out in both E. coli DH10Bac and E. coli DH10BmH, which contains AcMNPV/r2 and a helper plasmid. Recombinant DNAs were transfected into Sf-9 cells to generate recombinant virus AcMNPV/r3 and AcMNPV/r4 respectively. Viral stock of AcMNPV/r4 was then infected into Bombyx mori cells (BmN) and Bombyx mori larvae (silkworm). Analysis shows that GFP was highly expressed in Bombyx mori larvae. This expression system, is practicable therefore for mass production of foreign gene products.

Yield and activity of Autographa californica multicapsid nucleopolyhedrovirus and Phthorimaea operculella granulosis virus in cloned and uncloned cell lines of P. operculella

Three selected uncloned Pop 2, Pop 3, Pop 4 and two cloned cell lines Pop cl1A and Pop cl2B were derived from the original cell line established from Phthorimaea operculella (ORS-Pop-93). Three new non-selected cell lines ORS-Pop-94A, ORS-Pop-94B and ORS-Pop-95 were also established from embryos of the same insect. Differences in morphology, growth rate and polypeptide profile were determined between these cell lines. All the cell lines were susceptible to the Autographa californica nucleopolyhedrovirus (AcMNPV). The cloned cell lines produced higher levels of AcMNPV (TCID-50 and PIB) than the parental cells and at the same rate as the Sf9 reference cell line. Substantial amounts of viral DNA were synthesized in the clone Pop cl 2B after infection with the granulosis virus of the potato tuber moth P. operculella (PTMGV) and a complete multiplication was obtained in the ORS-Pop-95 cell line. The comparison between Pop cell lines which support limited or complete replication of certain baculoviruses can offer insights into some of the molecular barriers which restrict the host range of these viruses. These cell lines with variable susceptibility to baculoviruses could also be used for in vitro recombinations, increasing their virus host range to be used for the control of this pest.

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.

Expression of the Autographa californica nuclear polyhedrosis virus apoptotic suppressor gene p35 in nonpermissive Spodoptera littoralis cells

Apoptosis was postulated as the main barrier to replication of the Autographa californica nuclear polyhedrosis virus (AcMNPV) in a Spodoptera littoralis SL2 cell line (N. Chejanovsky and E. Gershburg, Virology 209:519-525, 1995). Thus, we hypothesized that the viral apoptotic suppressor gene p35 is either poorly expressed or nonfunctional in AcMNPV-infected SL2 cells. These questions were addressed by first determining the steady-state levels of the p35 product, P35, in AcMNPV-infected SL2 cells. Indeed, very low levels of P35 were found in infected SL2 cells in comparison with those in SF9 cells. Overexpression of p35, in transient-transfection and recombinant-virus infection experiments, inhibited actinomycin D- and AcMNPV-induced apoptosis, as determined by reduced cell blebbing and release of oligonucleosomes and increased cell viability of SL2. However, SL2 budded-virus (BV) titers of a recombinant AcMNPV which highly expressed p35 did not improve significantly. Also, injection of S. littoralis larvae with recombinant and wild-type AcMNPV BVs showed similar 50% lethal doses. These data suggest that apoptosis is not the only impediment to AcMNPV replication in these nonpermissive S. littoralis cells, and probably in S. littoralis larvae, so p35 may not be the only host range determinant in this system.

Responses of insect cells to baculovirus infection: protein synthesis shutdown and apoptosis

Protein synthesis is globally shut down at late times postinfection in the baculovirus Autographa californica M nuclear polyhedrosis virus (AcMNPV)-infected gypsy moth cell line Ld652Y. A single gene, hrf-1, from another baculovirus, Lymantria dispar M nucleopolyhedrovirus, is able to preclude protein synthesis shutdown and ensure production of AcMNPV progeny in Ld652Y cells (S. M. Thiem, X. Du, M. E. Quentin, and M. M. Berner, J. Virol. 70:2221-2229, 1996; X. Du and S. M. Thiem, Virology 227:420-430, 1997). AcMNPV contains a potent antiapoptotic gene, p35, and protein synthesis arrest was reported in apoptotic insect cells induced by infection with AcMNPV lacking p35. In exploring the function of host range factor 1 (HRF-1) and the possible connection between protein synthesis shutdown and apoptosis, a series of recombinant AcMNPVs with different complements of p35 and hrf-1 were employed in apoptosis and protein synthesis assays. We found that the apoptotic suppressor AcMNPV P35 was translated prior to protein synthesis shutdown and functioned to prevent apoptosis. HRF-1 prevented protein synthesis shutdown even when the cells were undergoing apoptosis, but HRF-1 could not functionally substitute for P35. The DNA synthesis inhibitor aphidicolin could block both apoptosis and protein synthesis shutdown in Ld652Y cells infected with p35 mutant AcMNPVs but not the protein synthesis shutdown in wild-type AcMNPV-infected Ld652Y cells. These data suggest that protein synthesis shutdown and apoptosis are separate responses of Ld652Y cells to AcMNPV infection and that P35 is involved in inducing a protein synthesis shutdown response in the absence of late viral gene expression in Ld652Y cells. A model was developed for these responses of Ld652Y cells to AcMNPV infection.

Prospects for altering host range for baculovirus bioinsecticides

Advances in the understanding of baculovirus replication and the identification of genes that affect host range set the stage for constructing recombinant baculoviruses for specific past insects. The modification of baculovirus host specificity has recently been achieved by inserting or deleting genes that affect virus replication or cellular defenses.

Sequencing of the putative DNA helicase-encoding gene of the Bombyx mori nuclear polyhedrosis virus and fine-mapping of a region involved in host range expansion

The complete sequence of a 3666-nucleotide (nt) open reading frame (ORF) and its flanking regions (58.1-62.1 map units (m.u.) from Bombyx mori nuclear polyhedrosis virus (BmNPV)) was determined. This ORF, BmNPV dnahel, encoded a predicted protein of 143623 Da which possessed seven consensus motifs found in proteins which unwind duplex DNAs, indicating that it is a DNA helicase. A 572-bp SacI-HindIII fragment, BmScH, that was previously shown to expand the host range of Autographa californica NPV (AcNPV) following homologous recombination [Maeda et al. (1993) J. Virol. 67, 6234-6238], was localized within BmNPV dnahel. By cotransfection experiments, two adjacent nt (A and T) that appeared to be the minimal essential sequence necessary to expand the host range of AcNPV, were mapped within BmScH. These adjacent nt encoded a single amino acid difference between BmNPV (Asp) and AcNPV (Ser).

Deletion analysis of four of eighteen late gene expression factor gene homologues of the baculovirus, BmNPV

Nucleotide sequence analysis of the genome of the baculovirus Bombyx mori nuclear polyhedrosis virus (BmNPV) identified 18 homologues of the Autographa californica NPV (AcNPV) lefs (late expression factor genes). These BmNPV lefs showed high (73-98%) amino acid sequence identities to AcNPV lefs and were localized to similar positions in the genome. One lef, p35, was previously characterized in AcNPV and BmNPV deletion experiments. Functional deletion of each of the BmNPV lef homologues was attempted here by insertion of a beta-galactosidase gene cassette into the coding region of each lef. Four of 18 BmNPV lef (39K, ie-2, lef-7, and p35) deletion mutants were successfully isolated, indicating that the other 14 BmNPV lefs were likely essential for viral replication in cell culture. Further analysis showed that deletion of lef-7, p35, and ie-2 resulted in lower levels of viral DNA replication, indicating that the BmNPV lef-7, p35, and ie-2 products have stimulating effects on DNA replication. Deletion of 39K resulted in a significantly lower level of late gene transcription and extremely low (over 10(2)-fold less at 48-80 hr p.i.) production of progeny budded virus in BmN cells. In contrast, the deletion did not affect viral DNA replication, indicating that BmNPV 39K is involved in late gene transcription. Reduced late gene expression presumably affected production and/or release of progeny budded virus particles. This was corroborated by transmission electron microscopy, which showed that virus replication was abnormal in BmN cells infected with a BmNPV mutant lacking 39K and virion production was low. Even though 39K deletion resulted in a loss of oral infectivity, the 39K deletion mutant replicated in silkworm larvae when injected into the body cavity, as did the ie-2, lef7, and p35 deletion mutants. In addition, a BmNPV homologue of the baculovirus very late expression factor gene (vif-1) found in AcNPV was essential, implying an essential function of the BmNPV vif-1 homologue at a step before the onset of very late gene expression.

Autographa californica nuclear polyhedrosis virus p143 gene product is a DNA-binding protein

We have identified the protein product of the Autographa californica nuclear polyhedrosis virus (AcMNPV) p143 gene by constructing a recombinant baculovirus overexpressing the gene product P143. The overexpressed protein exhibited a relative mobility of approximately 140 kDa and was stable for at least 12 hr after synthesis. Immunoblotting using a monoclonal antibody developed against the overexpressed protein identified a similar polypeptide in AcMNPV-infected cells which was detectable by 4 hr postinfection. P143 was present within infected cell nuclei at relatively constant amounts until at least 72 hr after infection, suggesting that P143 may perform other functions at late times after infection. P143, purified from infected cell nuclei by chromatography over hydroxylapatite and DNA cellulose, bound in a sequence-independent fashion to double-stranded but not to single-stranded DNA to form a ladder of retarded protein-DNA complexes. Together, these data are consistent with the essential role of P143 for viral DNA replication and suggest that P143 may function by direct binding to DNA.

Infection of a Spodoptera frugiperda cell line with Bombyx mori nucleopolyhedrovirus

The interactions of Bombyx mori nucleopolyhedrovirus (BmNPV) with Spodoptera frugiperda cells (Sf9) were investigated. S. frugiperda cells are usually considered nonpermissive for BmNPV. However, in the present study, BmNPV DNA replication was observed and an increasing infectious titre, reaching 10(4) TCID50/ml on B. mori permissive cells by 6 days post-transfection, developed in the supernatant of infected Sf9 cells. Infection of Sf9 cells by BmNPV did not induce a discernible shutoff of cellular protein synthesis and no overt cytopathic effects were observed. These data indicate that the low permissivity of Sf9 cells for BmNPV replication is associated with an inapparent infection.

Nucleopolyhedrovirus interactions with their insect hosts

It is clear from this brief review that our understanding of the molecular cross-talk between insects and their baculovirus pathogens is still very limited. Studies in cell culture have taught us a great deal about the basic baculovirus molecular machinery and how it is regulated, and in many cases this information has been predictive of what occurs in infected insects. Frequently, however, studies in cell culture do not adequately predict the infection process in insect hosts, as demonstrated by viral mutants (some of which were discussed in this review) that behave identically to wild-type virus in cell culture but differ markedly in larvae. More baculovirus studies, therefore, need to be conducted in vivo if we are to improve our understanding of the complex interactions between baculoviruses and their hosts. Conducting baculovirus studies in insects (or at least in primary cell culture) also offers the opportunity to address questions that reach beyond the baculovirus community in significance. For example, almost all of our knowledge of viral fusion mechanisms comes from infection of cells in culture where the pH is neutral or acidic and the temperature is constant at 27 degrees or 37 degrees C. An answer to the question of how the ODV envelope fuses with the microvillar membrane of columnar epithelial cells in the highly alkaline midgut environment at low temperatures will not only be important for an improved understanding of baculovirus infection in the natural world, but will also constitute a new chapter on viral entry mechanisms. Similarly, the answer to the question of how baculovirus nucleocapsids move basally within microvilli promises to involve factors and/or a mechanism not yet described by cell biologists, and so will constitute a valuable contribution to both baculovirology and cell biology. There are many more such examples of biological mechanisms that can be uniquely explored within the context of baculoviruses and their insect hosts, some of which have been highlighted in this review. As more and more young investigators realize the importance of combining a knowledge of virology, molecular technology, and insect biology, however, many of the outstanding mysteries will be solved.

Abortive infection of the baculovirus Autographa californica nuclear polyhedrosis virus in Sf-9 cells after mutation of the putative DNA helicase gene

Homologous recombination between the Autographa californica nuclear polyhedrosis virus (AcNPV) genome and a 0.6-kbp-long DNA fragment derived from the putative DNA helicase gene of Bombyx mori nuclear polyhedrosis virus generates eh2-AcNPV, an expanded-host-range AcNPV mutant (S. Maeda, S.G. Kamita, and A. Kondo, J. Virol. 67:6234-6238, 1993). After inoculation at a high multiplicity of infection (MOI), eh2-AcNPV replicates efficiently in both the Sf-9 (AcNPV-permissive) and BmN (non-AcNPV-permissive) cell lines. In this study, we found that after the inoculation of Sf-9 cells at a low MOI (i.e., 1 and 0.1 PFU per cell), the release of eh2-AcNPV virions was dramatically reduced (approximately 900- and 10,000-fold, respectively, at 72 h postinoculation) compared with that of wild-type AcNPV. In addition, the titer of eh2-AcNPV determined by plaque assay on Sf-9 cells was approximately 200-fold lower than that determined by plaque assay on BmN cells. Analyses of gene expression and viral DNA replication after low-MOI eh2-AcNPV inoculation of Sf-9 cells indicated that viral early genes were expressed normally. However, DNA replication and late-gene expression were significantly reduced. These findings suggested that abortive infection occurred at the stage of viral DNA replication in nearly all low-MOI eh2-AcNPV-infected Sf-9 cells. In the larvae of Spodoptera frugiperda, the organism from which Sf-9 cells are derived, the infectivity of eh2-AcNPV was lower than that of AcNPV; however, abortive infection was not found.

Species-specific effects of the hcf-1 gene on baculovirus virulence

The host cell-specific factor 1 gene (hcf-1) of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) is required for the efficient replication and/or stability of reporter plasmids carrying an AcMNPV-derived origin of DNA replication in a cell-specific manner; hcf-1 is required for reporter plasmid replication or stability in TN-368 cells, a cell line derived from the cabbage looper Trichoplusia ni, but not in IPLB-SF-21 (SF-21) cells, a cell line derived from the fall armyworm Spodoptera frugiperda (A. Lu and L. K. Miller, J. Virol. 69:6265-6272, 1995). To further define the function of hcf-1, recombinant viruses with null mutations in hcf-1 were constructed in SF-21 cells and the phenotype of the mutants was determined in selected cell lines as well as in insect larvae. In S.frugiperda larvae and SF-21 cells, the phenotype of hcf-1 mutants was indistinguishable from that of wild-type AcMNPV. In T. ni larvae as well as T. ni-derived cell lines, hcf-1 mutants exhibited a mutant phenotype. In TN-368 cells, the replication of hcf-1 mutants was extremely impaired; the phenotype included a defect in viral DNA replication, late gene transcription, and virus production as well as a complete cessation of host and viral protein synthesis. In another cell line derived from T. ni, the BTI-TN5B1-4 cell line, the hcf-1 mutants exhibited a less severe phenotype. In T. ni larvae, the infectivity of the budded form of hcf-1 mutants was decreased significantly (50-fold), although no difference in the oral infectivity of the occluded form was observed. T. ni larvae infected with hcf-1 mutants by either oral or hemocoelic routes, however, died 20 to 30% more slowly than those infected with wild-type AcMNPV. These data indicate that there is a host-specific requirement for hcf-1 and that it exerts cell line-specific effects and possibly tissue-specific effects on the rate at which the virus replicates, thereby affecting the virulence of the virus in a species-specific manner.

Identification of baculovirus gene that promotes Autographa californica nuclear polyhedrosis virus replication in a nonpermissive insect cell line

A gene that promotes Autographa californica M nuclear polyhedrosis virus (AcMNPV) replication in IPLB-Ld652Y cells, a cell line that is nonpermissive for AcMNPV, was identified in Lymantria dispar M nuclear polyhedrosis virus (LdMNPV). Cotransfection of AcMNPV DNA and a plasmid carrying the LdMNPV gene into IPLB-Ld652Y cells results in AcMNPV replication. The gene maps between 43.3 and 43.8 map units on the 162-kbp genome of LdMNPV. It comprises a 218-codon open reading frame and encodes a polypeptide with a predicted molecular mass of 25.7 kDa. The predicted polypeptide is glutamic acid and valine rich and negatively charged, with a pI of 4.61. No protein sequence motifs were identified, and no matches with known nucleotide or peptide sequences were found in the AcMNPV genome or database searches that suggest how this gene might function. A recombinant AcMNPV bearing the LdMNPV gene overcomes a block in protein synthesis observed in AcMNPV-infected IPLB-Ld652Y cells. Using Southern blotting techniques, we were unable to identify a homolog in Orgyia pseudotsugata M nuclear polyhedrosis virus, a baculovirus that is routinely propagated in IPLB-Ld652Y cells. This suggests that the LdMNPV host range is unique among the baculoviruses studied to date. We named this gene hrf-1 (for host range factor 1).

Baculovirus--insect cell interactions

Baculovirus interactions with host cells range from the physical interactions that occur during viral binding and entry, to the complex and subtle mechanisms that regulate host gene expression and modify and regulate cellular and organismal physiology and defenses. Fundamental studies of baculovirus biochemistry and molecular biology have yielded many interesting and important discoveries on the mechanisms of these virus-host interactions. Information from such studies has also resulted in exciting new strategies for environmentally sound insect pest control, and in the development and improvement of a valuable eukaryotic expression vector system. In addition a number of important and valuable model biological systems have emerged from studies of baculoviruses. These include robust systems for studies of eukaryotic transcription, viral DNA replication, membrane fusion, and apoptosis. Because functions have been identified for only a small number of baculovirus genes, we can expect many exciting new discoveries in the future and an unfolding of the complex and intricate relationship between baculoviruses and insect cells.

Differential requirements for baculovirus late expression factor genes in two cell lines

A plasmid library of 18 late expression factor (LEF) genes (LEF library) from the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) supports transient expression from a late viral promoter in the SF-21 cell line, derived from Spodoptera frugiperda. We found, however, that this LEF library was unable to support expression from the same promoter in the TN-368 cell line, derived from Trichoplusia ni, which is also permissive for AcMNPV replication. To identify the additional factor(s) required for expression in TN-368 cells, we cotransfected the LEF library with clones representing portions of the AcMNPV genome not represented in the LEF library. A single additional gene was identified; this gene corresponded to ORF70 of the complete AcMNPV sequence and potentially encodes a 34-kDa cysteine-rich polypeptide. Because of its differential effect on late gene expression in the two cell lines, we renamed ORF70 hcf-1 (for host cell-specific factor 1). hcf-1 was involved in expression from reporter plasmids under late and very late but not early promoter control, indicating that it was also a LEF gene. Plasmid DNA replication assays indicated that HCF-1 was involved in virus origin-specific DNA replication in TN-368 cells. Three LEF genes, ie-2, lef-7, and p35, required for optimal virus origin-specific plasmid DNA replication or stability in SF-21 cells had little or no influence in TN-368 cells. Thus, as determined by transient-expression assays, cell line-specific and potentially host-specific factors are required for origin-specific DNA replication or stability.

Inhibition of Bombyx mori nuclear polyhedrosis virus (NPV) replication by the putative DNA helicase gene of Autographa californica NPV

Coinfection of Bombyx mori nuclear polyhedrosis virus (BmNPV) with Autographa californica NPV (AcNPV) in the BmNPV-permissive BmN cell line resulted in the complete inhibition of BmNPV replication. Coinfected BmN cells exhibited an atypical cytopathic effect (CPE) and synthesis of viral and host proteins was dramatically attenuated by 5 h postinfection (p.i.) and nearly completely blocked by 24 h p.i. Viral transcription, however, appeared to occur normally during both early (5-h-p.i.) and late (24-h-p.i.) stages of infection. Superinfection of BmN cells with AcNPV at 5 and 12 h post-BmNPV infection resulted in limited inhibition of BmNPV replication. BmN cells singly infected with AcNPV also showed similar CPE, premature inhibition of viral and host protein synthesis, and apparently normal viral transcription. BmNPV replication occurred normally following coinfection of BmNPV and eh2-AcNPV, an AcNPV mutant identical to AcNPV except for a 572-bp region in its putative DNA helicase gene originating from BmNPV (S. Maeda, S. G. Kamita, and A. Kondo, J. Virol. 67:6234-6238, 1993). Furthermore, atypical CPE and premature attenuation of host and viral protein synthesis were not observed. These results indicated that the inhibition of BmNPV replication was caused either directly or indirectly at the translational level by the putative AcNPV DNA helicase gene.