A nuclear envelop-associated baculovirus protein promotes intranuclear lipid accumulation during infection

Although it has been well-accepted that baculoviruses produce a virus envelop within the nucleus, the redistribution of membrane lipids in infected cells has not been demonstrated. Here, we characterize a baculovirus protein (Bm5/Ac13: renamed BION; baculovirus protein associated with both the inner- and outer nuclear membranes) that localizes to both the inner- and outer nuclear membranes and show that the nuclear membrane (NE) protein promotes formation of a virus-induced intranuclear structure, the peristromal region (PR). Consistent with its role in virus envelopment, the PR was found to contain viral membrane proteins and lipids, suggesting PR formation proceeds through intranuclear lipid accumulation. About 50% of the cells infected with a bion-deficient virus exhibited no polyhedra production due to lack of the PR. Association of BION with the NE rather than the PR may contribute to the formation of the PR and polyhedra via NE-to-PR lipid transport.

The complete genome sequence of a second alphabaculovirus from the true armyworm, Mythimna unipuncta: implications for baculovirus phylogeny and host specificity

The Mythimna unipuncta nucleopolyhedrovirus isolate KY310 (MyunNPV-KY310) is an alphabaculovirus isolated from a true armyworm (Mythimna unipuncta) population in Kentucky, USA. Occlusion bodies of this virus were examined by electron microscopy and the genome sequence was determined by 454 pyrosequencing. MyunNPV-KY310 occlusion bodies consisted of irregular polyhedra measuring 0.8-1.8 µm in diameter and containing multiple virions, with one to six nucleocapsids per virion. The genome sequence was determined to be 156,647 bp with a nucleotide distribution of 43.9% G+C. 152 ORFs and six homologous repeat (hr) regions were annotated for the sequence, including the 38 core genes of family Baculoviridae and an additional group of 26 conserved alphabaculovirus genes. BLAST queries and phylogenetic inference confirmed that MyunNPV-KY310 is most closely related to the alphabaculovirus Leucania separata nucleopolyhedrovirus isolate AH1, which infects Mythimna separata. In contrast, MyunNPV-KY310 did not exhibit a close relationship with Mythimna unipuncta nucleopolyhedrovirus isolate #7, an alphabaculovirus from the same host species. MyunNPV-KY310 lacks the gp64 envelope glycoprotein, which is a characteristic of group II alphabaculoviruses. However, this virus and five other alphabaculoviruses lacking gp64 are placed outside the group I and group II clades in core gene phylogenies, further demonstrating that viruses of genus Alphabaculovirus do not occur in two monophyletic clades. Potential instances of MyunNPV-KY310 ORFs arising by horizontal transfer were detected. Although there are now genome sequences of four different baculoviruses from M. unipuncta, comparison of their genome sequences provides little insight into the genetic basis for their host specificity.

Complete study demonstrating the absence of rhabdovirus in a distinct Sf9 cell line

A putative novel rhabdovirus (SfRV) was previously identified in a Spodoptera frugiperda cell line (Sf9 cells [ATCC CRL-1711 lot 58078522]) by next generation sequencing and extensive bioinformatic analysis. We performed an extensive analysis of our Sf9 cell bank (ATCC CRL-1711 lot 5814 [Sf9L5814]) to determine whether this virus was already present in cells obtained from ATCC in 1987. Inverse PCR of DNA isolated from Sf9 L5814 cellular DNA revealed integration of SfRV sequences in the cellular genome. RT-PCR of total RNA showed a deletion of 320 nucleotides in the SfRV RNA that includes the transcriptional motifs for genes X and L. Concentrated cell culture supernatant was analyzed by sucrose density gradient centrifugation and revealed a single band at a density of 1.14 g/ml. This fraction was further analysed by electron microscopy and showed amorphous and particulate debris that did not resemble a rhabdovirus in morphology or size. SDS-PAGE analysis confirmed that the protein composition did not contain the typical five rhabdovirus structural proteins and LC-MS/MS analysis revealed primarily of exosomal marker proteins, the SfRV N protein, and truncated forms of SfRV N, P, and G proteins. The SfRV L gene fragment RNA sequence was recovered from the supernatant after ultracentrifugation of the 1.14 g/ml fraction treated with diethyl ether suggesting that the SfRV L gene fragment sequence is not associated with a diethyl ether resistant nucleocapsid. Interestingly, the 1.14 g/ml fraction was able to transfer baculovirus DNA into Sf9L5814 cells, consistent with the presence of functional exosomes. Our results demonstrate the absence of viral particles in ATCC CRL-1711 lot 5814 Sf9 cells in contrast to a previous study that suggested the presence of infectious rhabdoviral particles in Sf9 cells from a different lot. This study highlights how cell lines with different lineages may present different virosomes and therefore no general conclusions can be drawn across Sf9 cells from different laboratories.

Electron tomography and simulation of baculovirus actin comet tails support a tethered filament model of pathogen propulsion

Several pathogens induce propulsive actin comet tails in cells they invade to disseminate their infection. They achieve this by recruiting factors for actin nucleation, the Arp2/3 complex, and polymerization regulators from the host cytoplasm. Owing to limited information on the structural organization of actin comets and in particular the spatial arrangement of filaments engaged in propulsion, the underlying mechanism of pathogen movement is currently speculative and controversial. Using electron tomography we have resolved the three-dimensional architecture of actin comet tails propelling baculovirus, the smallest pathogen yet known to hijack the actin motile machinery. Comet tail geometry was also mimicked in mixtures of virus capsids with purified actin and a minimal inventory of actin regulators. We demonstrate that propulsion is based on the assembly of a fishbone-like array of actin filaments organized in subsets linked by branch junctions, with an average of four filaments pushing the virus at any one time. Using an energy-minimizing function we have simulated the structure of actin comet tails as well as the tracks adopted by baculovirus in infected cells in vivo. The results from the simulations rule out gel squeezing models of propulsion and support those in which actin filaments are continuously tethered during branch nucleation and polymerization. Since Listeria monocytogenes, Shigella flexneri, and Vaccinia virus among other pathogens use the same common toolbox of components as baculovirus to move, we suggest they share the same principles of actin organization and mode of propulsion.

High-contrast observation of unstained proteins and viruses by scanning electron microscopy

Scanning electron microscopy (SEM) is an important tool for the nanometre-scale analysis of the various samples. Imaging of biological specimens can be difficult for two reasons: (1) Samples must often be left unstained to observe detail of the biological structures; however, lack of staining significantly decreases image contrast. (2) Samples are prone to serious radiation damage from electron beam. Herein we report a novel method for sample preparation involving placement on a new metal-coated insulator film. This method enables obtaining high-contrast images from unstained proteins and viruses by scanning electron microscopy with minimal electron radiation damage. These images are similar to those obtained by transmission electron microscopy. In addition, the method can be easily used to observe specimens of proteins, viruses and other organic samples by using SEM.

Nuclear transport of baculovirus: revealing the nuclear pore complex passage

Baculoviruses are one of the largest viruses that replicate in the nucleus of their host cells. During an infection the capsid, containing the DNA viral genome, is released into the cytoplasm and delivers the genome into the nucleus by a mechanism that is largely unknown. Here, we used capsids of the baculovirus Autographa californica multiple nucleopolyhedrovirus in combination with electron microscopy and discovered this capsid crosses the NPC and enters into the nucleus intact, where it releases its genome. To better illustrate the existence of this capsid through the NPC in its native conformation, we reconstructed the nuclear import event using electron tomography. In addition, using different experimental conditions, we were able to visualize the intact capsid interacting with NPC cytoplasmic filaments, as an initial docking site, and midway through the NPC. Our data suggests the NPC central channel undergoes large-scale rearrangements to allow translocation of the intact 250-nm long baculovirus capsid. We discuss our results in the light of the hypothetical models of NPC function.

Isolation and characterization of a baculovirus associated with the insect parasitoid wasp, Cotesia marginiventris, or its host, Trichoplusia ni

A multiple nucleopolyhedrovirus (MNPV) was isolated from Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) larvae that had been stung by the parasitoid Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae). The wild type virus was plaque purified by infecting a Heliothis subflexa (BCIRL- HsAM1) cell line and isolating several clones. The mean estimated genomic size of this virus based on PstI, BstEII, StyI, HindIII restriction profiles was estimated to be 106 +/- 2.5 kbp (mean+/-SE). A clone designated as TnMNPV/CmBCL9 was used in bioassays against several lepidopteran pests and in comparative studies with the baculoviruses AcMNPV, AgMNPV, AfMNPV, PxMNPV and HzSNPV of Autographa califomica, Anticarsia gemmatalis, Anagrapha falcifera, Plutella xylostella, and Helicoverpa zea, respectively. Infectivity studies showed that TnMNPV/CmBCL9 was highly infectious for Heliothis subflexa and T. ni, with an LC(50) value 0.07 occlusion bodies/mm(2) in both species and also infectious for H. zea and Heliothis virescens with LC(50) values of 0.22 and 0.27 occlusion bodies/mm(2), respectively. Restriction endonuclease analysis of the isolate and selected baculoviruses revealed profiles that were very similar to AfMNPV but different from the restriction endonuclease profiles of the other baculoviruses. Hybridization studies suggest that the TnMNPV/CmBCL9 was closely related to AfMNPV and AcMNPV-HPP. Further support for this comes from a phylogenetic analysis employing a split-graphs network, comparing the polh, egt, and p10 genes from TnMNPV/CmBCL9 with those from other baculoviruses and suggests that this virus is closely related to the AcMNPV variants, AfMNPV and RoMNPV of Rachiplusia ou.

Avian influenza virus hemagglutinin display on baculovirus envelope: cytoplasmic domain affects virus properties and vaccine potential

Hemagglutinin (HA) is the major immunogen on the envelope of avian influenza virus (AIV). Therefore we constructed two recombinant baculoviruses: Bac-HA, expressing histidine-tagged HA with the cytoplasmic domain (CTD) derived from HA, and Bac-HA64, expressing histidine-tagged HA with the CTD derived from baculovirus envelope protein gp64. After infection, HA with either CTD was expressed and anchored on the plasma membrane of Sf-9 cells, as revealed by confocal microscopy. Immunogold electron microscopy demonstrated that both Bac-HA and Bac-HA64 displayed HA on the viral surface. However, analyses of purified viruses revealed that significantly more HA was incorporated into Bac-HA64 than into Bac-HA. In comparison with Bac-HA, Bac-HA64 significantly improved the gene delivery and transgene expression in mammalian cells, as determined by quantitative real-time polymerase chain reaction and flow cytometry. Bac-HA64 elicited significantly higher hemagglutination inhibition titers in mouse models than Bac-HA and the negative controls. These data collectively confirmed that the gp64 CTD, in comparison with HA CTD, resulted in more efficient HA incorporation into baculovirus, more efficient transgene delivery and expression, and elevated immunogenicity. This is the first report demonstrating the potential of HA-pseudotyped baculovirus as an avian influenza vaccine and that the choice of CTD tremendously affects baculovirus properties and vaccine efficacy.

The baculoviruses occlusion-derived virus: virion structure and function

Baculoviruses play an important ecological role regulating the size of insect populations. For many years, baculoviruses have been applied as targeted biocontrol agents against forestry and agriculture pests. Baculovirus insecticides are effective against insect pests such as velvetbean caterpillar (Anticarsia gemmatalis ), cotton bollworm (Helicoverpa zea ), and gypsy moth (Lymantria dispar ). Baculoviruses are transmitted to insects by the oral route mediated by the occlusion-derived virus (ODV). The ODV is also specialized to exploit the insect midgut that is one of the most extreme biological environments where the viruses are subject to caustic pH and digestive proteases. The molecular biology of the ODV reveals new frontiers in protein chemistry. Finally, ODVs establishes infection in insect gut tissues that are virtually nonsupportive to virus replication and which are continuously sloughed away. ODVs carry with them a battery of proteins that enable them to rapidly exploit and harness these unstable cells for virus replication.

Magnetic resonance imaging of viral particle biodistribution in vivo

We describe here a technique for the visualization of viral vector delivery by magnetic resonance imaging (MRI) in vivo. By conjugating avidin-coated baculoviral vectors (Baavi) with biotinylated ultra-small superparamagnetic iron oxide particles (USPIO), we are able to produce vector-related MRI contrast in the choroid plexus cells of rat brain in vivo over a period of 14 days. Ten microlitres of 2.5 x 10(10) PFU/ml nuclear-targeted LacZ-encoding Baavi with bUSPIO coating was injected into rat brain ventricles and visualized by MRI at 4.7 T. As baculoviruses exhibit restricted cell-type specificity in the rat brain, altered MRI contrast was detected in the choroid plexus of the injected ventricles. No specific signal loss was detected when wild-type baculoviruses or intact biotinylated USPIO particles were injected into the lateral ventricles. Cryosectioned brains were stained for nuclear-targeted beta-galactosidase gene expression, which was found to colocalize with MRI contrast. This study provides the first proof of principle for robust and non-invasive viral vector MRI by using avidin-displaying viruses in vivo. Considering the widespread use of MRI in current medical imaging, the approach is likely to provide numerous future applications in imaging of therapeutic gene transfer.

Disassembly of structurally modified viral nanoparticles: characterization by fluorescence correlation spectroscopy

Analysis of the breakdown products of engineered viral particles can give useful information on the particle structure. We used various methods to breakdown both a recombinant enveloped virus and virus-like particles (VLPs) from two non-enveloped viruses and analysed the resulting subunits by fluorescence correlation spectroscopy (FCS). Analysis of the enveloped baculovirus, Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV), displaying the green fluorescent protein (GFP) fused to its envelope protein gp64 was performed in the presence and absence of 5 mM SDS and 25 mM DTT. Without treatment, the viral particle showed a diffusion time of 3.3 ms. In the presence of SDS, fluorescent subunits with diffusion times of 0.2 ms were observed. Additional treatment with DTT caused a drop in the diffusion time to 0.1 ms. Changes in the amplitude of the autocorrelation function suggested a 3-fold increase in fluorescent particle number when viral particles were treated with SDS, and a further 1.5-fold increase with additional treatment with DTT. Thus, the data showed that an average of 4.5 molecules of gp64-GFP was incorporated in the membrane of the modified baculovirus. Further, this suggests that each fluorescent gp64 trimer carries on average 1.5 fluorescent units. Similar experiments were carried out with two non-enveloped fluorescent virus-like particles (fVLPs) that displayed enhanced green fluorescent protein (EGFP). These, fVLPs of canine and human B19 parvoviruses were treated with 6 M urea and 5 mM SDS, respectively. Correspondingly, the original hydrodynamic radii of 17 and 14 nm were reduced to 9 and 5 nm after treatment. Here, the change in the amplitude of the autocorrelation curve suggested a 10-fold increase in particle number when viral particles of CPV were treated with 6 M urea at 50 degrees C for 10 min. For EGFP-B19, there was a decrease in the amplitude, accompanied by a 9-fold increase in the number of fluorescent units with SDS treatment. The results showed that approximately 10 and 9 fluorescent units were associated with the corresponding CPV and B19 VLPs. In summary, we were able to estimate the number of fluorescent subunits in a baculovirus containing a GFP-fusion with its gp64 envelope protein and in two different parvo-VLPs containing EGFP-fused with their VP2 capsid proteins.

Response of crayfish, Procambarus clarkii, haemocytes infected by white spot syndrome virus

White spot syndrome virus (WSSV) is a serious pathogen of aquatic crustaceans. Little is known about its transmission in vivo and the immune reaction of its hosts. In this study, the circulating haemocytes of crayfish, Procambarus clarkii, infected by WSSV, and primary haemocyte cultures inoculated with WSSV, were collected and observed by transmission electron microscopy and light microscopy following in situ hybridization. In ultra-thin sections of infected haemocytes, the enveloped virions were seen to be phagocytosed in the cytoplasm and no viral particles were observed in the nuclei. In situ hybridization with WSSV-specific probes also demonstrated that there were no specific positive signals present in the haemocytes. Conversely, strong specific positive signals showed that WSSV replicated in the nuclei of gill cells. As a control, the lymphoid organ of shrimp, Penaeus monodon, infected by WSSV was examined by in situ hybridization which showed that WSSV did not replicate within the tubules of the lymphoid organ. In contrast to previous studies, it is concluded that neither shrimp nor crayfish haemocytes support WSSV replication.

Improved display of synthetic IgG-binding domains on the baculovirus surface

Improved display of foreign protein moieties in combination with beneficial alteration of the viral surface properties should be of value for targeted and enhanced gene delivery. Here, we describe a vector based on Autographa californica multiple nucleopolyhedrovirus (AcMNPV) displaying synthetic IgG-binding domains (ZZ) of protein A fused to the transmembrane anchor of vesicular stomatitis virus (VSV) G protein. This display vector was equipped with a GFP/EGFP expression cassette enabling fluorescent detection in both insect and mammalian cells. The virus construct displayed the biologically active fusion protein efficiently and showed increased binding capacity to IgG. As the display is carried out using a membrane anchor of foreign origin, gp64 is left intact for virus entry, which may increase gene expression in the transduced mammalian cells. In addition, the viral vector can be targeted to any desired cell type via binding of ZZ domains when an appropriate IgG antibody is available.

Pathology and ultrastructure of an intranuclear bacilliform virus (IBV) infecting brown shrimp Crangon crangon (Decapoda: Crangonidae)

The brown shrimp Crangon crangon supports an important fishery in Europe (over 25000 t, valued at 80 million euros in 2000). Through the course of histopathological screening of crustaceans from the Clyde estuary, western Scotland, for the biological effect of contaminants, we have discovered a highly prevalent (up to 100%) non-occluded intranuclear bacilliform virus (IBV) infection in the hepatopancreatic tubule epithelia and midgut epithelia of wild C. crangon. This is the first report of an IBV in this family. We have termed this virus Crangon crangon bacilliform virus (CcBV). Histological and ultrastructural observations suggest that this virus is similar to other IBVs previously described from crabs and penaeid shrimps. The nuclei of virus-infected epithelial cells contained an eosinophilic, hypertrophied viroplasm that marginalised the chromatin of the host nucleus. Infected cells were often separated from their neighbouring cells and their nuclei appeared apoptotic. In heavily infected shrimp, apoptotic cells were expelled into the lumen of the hepatopancreatic tubule or the midgut. Following this stage, some hepatopancreatic tubules became degenerate, with remnants of the basement membrane and myoepithelial lining remaining. Transmission electron microscopy of hypertrophic nuclei revealed the presence of rod-shaped and cylindrical, envelope-bound virions. These virions did not form arrays and were not embedded within occlusion bodies, but did appear to be partially occluded in an amorphous matrix that corresponded to a granular viroplasm. The ultrastructure, morphology and size of the nucleocapsid and the complete virion aligns the virus most closely to the IBVs previously reported from other decapod crustaceans. Due to the pathological manifestation of IBV infection in C. crangon, it appears likely that it can act as a population modulator, particularly at sites where infection prevalence is high, such as that observed in the Clyde estuary.

[Expression of hepatitis A virus procapsids in the insect cells infected by recombinant baculovirus]

The recombinant baculocvirus containing genome P1-2A-P3 of hepatitis A virus (HAV) was constructed and used for infecting the Sf9 insect cells. It was demonstrated that the deletion of 2BC from HAV polyprotein and the insertion of a new 3C protease cleavage site between P1-2A and P3 did not interfere with the processing of polyprotein or with forming the 70S-procapsids. The identity of the protein contents as well as of morphological and antigen characteristics, obtained in Sf9-cells, to HAV empty capsids, which take shape in the infected mammal cells, proves that it is possible to use them in making the vaccine and diagnostic preparations.

Assembly of single-shelled cores and double-shelled virus-like particles after baculovirus expression of major structural proteins P3, P7 and P8 of Rice dwarf virus

Expression of the core capsid protein P3 of Rice dwarf virus in a baculovirus system resulted in the formation of single-shelled core-like particles in insect cells in the absence of any other capsid proteins. Double-shelled virus-like particles were also observed upon mixing or co-expression of P3 and the major outer capsid protein P8, suggesting that P3 and P8 have the ability to form double-shelled particles both in vivo and in vitro. Core protein P7 expressed in a similar manner was incorporated into the virus-like particles.

High efficiency gene transfer into cultured primary rat and human hepatic stellate cells using baculovirus vectors

BACKGROUND/AIMS

Gene transfer into hepatic stellate cells (HSC) is inefficient when using plasmid-based transfection methods; viral-based systems are therefore being developed. A baculovirus system has recently been shown to be useful for expressing genes in mammalian cells. The aim of this study was to determine if baculovirus vectors can infect and express target genes in rat and human HSC and to assess potential cytotoxic and modulatory effects of infection.

METHODS

A recombinant baculovirus vector (AcCALacZ) carrying the LacZ gene was used to infect HSC. beta-Galactosidase assays and electron microscopy were used to determine efficiency of infection and gene expression. Counting of trypan blue negative cells was used to assess cytotoxic/cytostatic effects of infection. Measurement of protein content of cells and alpha-smooth muscle actin expression were performed to assess the effects of baculovirus on cell function/phenotype.

RESULTS

Baculovirus infection of activated HSC was highly efficient (> 90%) and provided long-term LacZ gene expression (15 days) in the absence of cytotoxic, cytostatic or modulatory effects. Infection of freshly isolated cells was also observed but at lower levels (20%).

CONCLUSIONS

Baculovirus vectors can therefore be used to deliver target genes to cultured rat and human HSC with high efficiency and longevity in the absence of detrimental effects on cell function.

[A viral infection in the neck glandula of larvae of Dione junio (Lepidoptera: Nymphalidae)]

Lepidopteran larvae may be attacked by different viruses, many of which belong to the Baculoviridae family. Whilst studying the ultrastructure of the neck gland in Dione junio larvae we found that in later instars the larvae showed symptoms of attack by two types of virus. The glands were prepared for optical and electron microscopy using sodium cacodylate buffer and standard procedures (0.1M, pH 7.2). The neck gland is composed of two oval internal sacks which communicate with the exterior via an extracellular channel. Each sack contains, in its external region, cells with large, irregular nuclei and a dense cytoplasm containing numerous small mitochondria. In infected larvae, the tissues are damaged and the nuclear polyhedrosis virus can be observed in several of the nuclei. In the cytoplasm another "rickettsia type" virus, may be observed. The pathogenic viruses present in D. junio larvae could be studied as potential biological controls of this pest.

Individual rotavirus-like particles containing 120 molecules of fluorescent protein are visible in living cells

Rotaviruses are large, complex icosahedral particles consisting of three concentric capsid layers. When the innermost capsid protein VP2 is expressed in the baculovirus-insect cell system it assembles as core-like particles. The amino terminus region of VP2 is dispensable for assembly of virus-like particles (VLP). Coexpression of VP2 and VP6 produces double layered VLP. We hypothesized that the amino end of VP2 could be extended without altering the auto assembly properties of VP2. Using the green fluorescent protein (GFP) or the DsRed protein as model inserts we have shown that the chimeric protein GFP (or DsRed)-VP2 auto assembles perfectly well and forms fluorescent VLP (GFP-VLP2/6 or DsRed-VLP2/6) when coexpressed with VP6. The presence of GFP inside the core does not prevent the assembly of the outer capsid layer proteins VP7 and VP4 to give VLP2/6/7/4. Cryo-electron microscopy of purified GFP-VLP2/6 showed that GFP molecules are located at the 5-fold vertices of the core. It is possible to visualize a single fluorescent VLP in living cells by confocal fluorescent microscopy. In vitro VLP2/6 did not enter into permissive cells or in dendritic cells. In contrast, fluorescent VLP2/6/7/4 entered the cells and then the fluorescence signal disappear rapidly. Presented data indicate that fluorescent VLP are interesting tools to follow in real time the entry process of rotavirus and that chimeric VLP could be envisaged as "nanoboxes" carrying macromolecules to living cells.

Morphological and molecular evidence that Culex nigripalpus baculovirus is an unusual member of the family Baculoviridae

We present evidence that a newly discovered mosquito virus from Culex nigripalpus is an unusual member of the family BACULOVIRIDAE: Development of this virus was restricted to nuclei of midgut epithelial cells in the gastric caeca and posterior stomach. The globular occlusion bodies were not enveloped, measured around 400 nm in diameter, occurred exclusively in nuclei of infected cells and typically contained four, sometimes up to eight, virions. The developmental sequence involved two virion phenotypes: an occluded form (ODV) that initiated infection in the midgut epithelial cells, and a budded form that spread the infection in the midgut. Each ODV contained one rod-shaped enveloped nucleocapsid (40x200 nm). The double-stranded DNA genome was approximately 105-110 kbp with an estimated GC content of 52%. We have sequenced approximately one-third of the genome and detected 96 putative ORFs of 50 amino acids or more including several genes considered to be unique to baculoviruses. Phylogenetic analysis of the amino acid sequences of DNApol and p74 placed this virus in a separate clade from the genera NUCLEOPOLYHEDROVIRUS: and GRANULOVIRUS: We provisionally assign this virus in the genus NUCLEOPOLYHEDROVIRUS:, henceforth abbreviated as CuniNPV (for Culex nigripalpus nucleopolyhedrovirus), and suggest that, awaiting additional data to clarify its taxonomic status, it may be a member of a new genus within the family BACULOVIRIDAE:

Epizootiology and transmission of a newly discovered baculovirus from the mosquitoes Culex nigripalpus and C. quinquefasciatus

Reports of mosquito baculoviruses are extremely uncommon and epizootics in field populations are rarely observed. We describe a baculovirus that was responsible for repeated and extended epizootics in field populations of Culex nigripalpus and C. quinquefasciatus over a 2 year period. These mosquito species are important vectors of St Louis and Eastern equine encephalitis in the United States. Our initial attempts to transmit this baculovirus to mosquitoes in the laboratory were unsuccessful. A salt mixture similar to that found in water supporting infection in the field was used in laboratory bioassays and indicated that certain salts were crucial to transmission of the virus. Further investigations revealed conclusively that transmission is mediated by divalent cations: magnesium is essential, whereas calcium inhibits virus transmission. These findings represent a major advancement in our understanding of the transmission of baculoviruses in mosquitoes and will allow characterization of the virus in the laboratory. In addition, they can explain, in great part, conditions that support epizootics in natural populations of mosquitoes that vector life-threatening diseases of man and animals.

Characterization of a granulovirus from the cassava hornworm (Erinnyis ello: Sphingidae)

A Colombian isolate of Erinnyis ello granulovirus (EeGV) was characterized by electron microscopy, restriction endonuclease digestion, and SDS-PAGE. Electron microscopy showed the occlusion bodies to have a morphology typical of granuloviruses. The restriction patterns of DNA from EeGV and the granuloviruses of Trichoplusia ni (TnGV) and Pieris rapae (PrGV) show little or no similarity, indicating little relatedness among these viruses. EeGV was estimated to possess a relatively small genome of 90.5 +/- 0.5 kbp. SDS-PAGE analysis compared the occulsion body and enveloped nucleocapsid proteins of EeGV and TnGV, and the polypeptide patterns also showed little similarity between these viruses. These analyses, as well as comparison of our results to those reported for other granuloviruses, indicate that EeGV represents a new granulovirus isolate.

[Multiplication of the shrimp baculovirus HHNBV with primary cell cultures from lymphoid organ of Penaeus chinensis]

The hypodermal and hematopoietic necrosis baculovirus HHNBV has been confirmed to be the causative agent for the explosive epidermic disease of farmed shrimp Penaeus chinensis since 1993. The virus was isolated and multiplied successfully in the primary cell cultures from the lymphoid organ of the shrimp. A cell monolayer was formed in three days in the medium MPS and could be maintained for 1-3 months as the medium replaced every 4-5 days. The cytopathic effect occurred in 5 days after inoculation of the tissue extract from the diseased shrimp. The transmission electron microscopy showed many rod-shaped virions of HHNBV in the nuclei of infected cells.

Monodon baculovirus from Australia: ultrastructural observations

The cytopathology, virogenesis and replication of monodon baculovirus (MBV) in Penaeus monodon from Australia are described. Electron-dense unenveloped nucleocapsids, not previously described for MBV, are shown in the cytoplasm and attached to the nuclear envelope of infected hepatopancreatocytes. These nucleocapsids comprise a missing link in the published literature on the replication cycle of MBV by providing evidence for the means by which the viral genome travels from the plasma membrane of the hepatopancreatocyte to the nucleus. Features similar to those of MBV from other areas, but not previously reported for MBV from Australia include empty capsids attached to the nuclear pore, central filaments in developing capsids, capsids partly filled with nucleic acid, and filaments in subapical envelope expansions. A model for virogenesis and replication is illustrated which takes into account the new observations as well as previously described ultrastructural characteristics of the developing viral particle.

Transient and stable gene expression in mammalian cells transduced with a recombinant baculovirus vector

Recombinant baculoviruses can serve as gene-transfer vehicles for transient expression of recombinant proteins in a wide range of mammalian cell types. Furthermore, by inclusion of a dominant selectable marker in the viral vector, cell lines can be derived that stably express recombinant genes. A virus was constructed containing two expression cassettes controlled by constitutive mammalian promoters: the cytomegalovirus immediate early promoter/enhancer directing expression of green fluorescent protein and the simian virus 40 (SV40) early promoter controlling neomycin phosphotransferase II. Using this virus, efficient gene delivery and expression was observed and measured in numerous cell types of human, primate, and rodent origin. In addition to commonly used transformed cell lines such as HeLa, CHO, Cos-7, and 293, this list includes primary human keratinocytes and bone marrow fibroblasts. In all cases, addition of butyrate or trichostatin A (a selective histone deacetylase inhibitor) to transduced cells markedly enhanced the levels of reporter protein expression observed. When transduced cells are put under selection with the antibiotic G418, cell lines can be obtained at high frequency that stably maintain the expression cassettes of the vector DNA and exhibit stable, high-level expression of the reporter gene. Stably transduced derivatives have been selected from a substantial number of different cell types, suggesting that stable lines can be derived from any cell type that exhibits transient expression.

The morphology of the polyhedra of a host range-expanded recombinant baculovirus and its parents

The host range-expanded recombinant baculovirus, RecB-8 was isolated from BmN-4 cells coinfected with Autographa californica and Bombyx mori nuclear polyhedrosis viruses. Its genome was compared with those of its parents by restriction endonuclease digestion and their polyhedra compared in an electron microscope. Interestingly, the polyhedra of RecB-8 were tetrahedral although the polyhedrin gene was the same as that of the BmNPV parent which has icosahedral polyhedra. Thus the morphology of the RecB-8 polyhedra resulted from host cell factors and/or another viral genome in the host cells.

A comparative study of three different isolates of white spot virus

Three separate isolates of white spot virus (WSV) purified from 3 different penaeid shrimp species from different countries were compared morphologically, biochemically, and genomically using the following techniques; negative stain electron microscopy, sodium dodecyl-sulfate polyacrylamide gel electrophoresis/western blot, and restriction fragment length polymorphism (RFLP), respectively. Under the electron microscope, the 3 isolates were indistinguishable. Their nucleoprotein cores exhibited the unique striated structure characteristic of the baculovirus-like agents associated with white spot syndrome. The dimensions of the nucleoprotein cores were also identical for all 3 isolates. SDS-PAGE gels of purified virus preparations showed all 3 to be identical in the position of at least 3 of the most prominent protein bands of WSV, with approximate molecular weights of 19, 23.5, and 27.5 kDa. Western blot analyses also revealed these 3 same protein bands in identical positions for all 3 isolates. RFLP analyses of the viral genomes using Hind III and EcoR 1 enzymes revealed that although the 3 isolates were identical when cut with EcoR I, the isolate from Penaeus japonicus from China was distinguishable from the other 2 genomes (P. monodon from Indonesia and P. setiferus from the U.S.) when cut with Hind III.

Characterization of a non-occluded baculovirus-like agent pathogenic to penaeid shrimp

A non-occluded baculovirus-like agent recently isolated by this laboratory from moribund Penaeus japonicus shrimps obtained from China and named Chinese baculovirus (CBV) was purified and some of its properties characterized. Under the electron microscope, negatively stained virus particles were rod-shaped, enveloped, and measured 322 to 378 nm in length and 130 to 159 nm in diameter. The nucleoprotein core exhibited a unique striated structure and measured 316 to 350 nm in length and 65 to 66 nm in diameter. The striations appear to be the result of the stacking of ring-like structures. These rings consisted of 2 rows of 12 to 14 globular subunits. Each globular subunit measured approximately 10 nm in diameter. SDS-PAGE gels of purified virus preparations showed, among several, 4 prominent protein bands with approximate molecular weights of 19, 23.5, 27.5 and 75 kDa. The structural viral proteins were identified by western blot analysis using polyclonal hyperimmune serum made against purified CBV. The 19, 27.5, and 75 kDa structural proteins were determined to be non-glycosylated components associated with the viral envelope. The 23.5 kDa protein, also non-glycosylated, was identified with the capsid structure. Viral genomic DNA digested with Hind III restriction endonuclease revealed at least 29 different fragments with a conservatively estimated total size of at least 183 kb.

Characterization of a granulosis virus from the castor semilooper, Achaea janata L

The granulosis virus isolated from Achaea janata is composed of capsules (463 +/- 25 x 280 +/- 22 nm) which contain single virus particles. The virus particles (274 +/- 9 x 95 +/- 4 nm) contain nucleocapsids (297 +/- 7 x 52 +/- 2 nm) within an envelope. The empty capsids measured 265 +/- 9 x 63 +/- 3 nm. SDS-PAGE electrophoresis was carried out and the granulin polypeptide (28.9 +/- 0.5 kDa) was found to contain two other associated polypeptides of molecular weight 58.2 +/- 2.3 and 55.2 +/- 1.3 kDa, respectively. Protein gel electrophoresis of granulin gave degradation products from 27.2 to 15.4 kDa, when protease associated with capsules was not inhibited. The virus particles were found to contain 16 polypeptides with molecular weights ranging from 106.4 +/- 2.1 to 15. 5 +/- 0.8 kDa and 12 of these polypeptides were contained in the nucleocapsids. The major polypeptide of the capsids had molecular weight of 34.9 +/- 0.3 kDa. Restriction profiles of viral DNA with seven enzymes were obtained and the average molecular weight was found to be approximately 92 +/- 7 kb. Southern hybridization of the restriction profiles of the viral DNA with SalI fragment from pUCTnGV (containing granulin gene) was performed and hybridization was found on single discrete fragments with molecular weights ranging from 12.0 to 2.7 kb. The GC content of AjGV DNA was found to be 65 +/- 1% and the genome size as determined from reassociation kinetics was 92 kb.

Polyhedron-like inclusion body formation by a mutant nucleopolyhedrovirus expressing the granulin gene from a granulovirus

The polyhedrin gene in Bombyx mori nucleopolyhedrovirus (BmNPV) was replaced with the granulin gene of Trichoplusia ni granulovirus (TnGV). The substitution was verified by Southern hybridization, and expression of granulin by the mutant virus, BmGran, was demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by amino acid sequencing of the predominant protein of BmGran inclusion bodies (IBs). Light and electron microscopy examination of BmGran-infected B. mori and BmN cells revealed large, cuboidal, polyhedron-like IBs in the nucleus and cytoplasm, but granules were not seen. IBs contained small, parallel, electron-dense streaks, which defined the geometric pattern of crystallization. Geometric patterns of nuclear IBs were frequently disrupted by occlusion of polyhedron envelope fragments, resulting in IB instability and fracturing. Virions were not embedded in most of the polyhedron-like IBs, but accumulated with polyhedron envelope fragments. Some virions were coated with matrix protein and were partially wrapped by polyhedron envelope. These results suggested that (1) the amino acid sequence of granulin insufficient for determining IB morphology in TnGV-infected cells, and TnGV may have genes, not present in BmNPV, that control granule formation, and (2) interactions among the virion, the IB envelope, and the matrix protein may be important in virion occlusion and IB morphology and stability.

Electron microscopic observation on a non-occluded baculo-like virus in shrimps

A severe disease of farm-raised shrimp, Penaeus chinensis has spread throughout Chinese coasts since 1993. Recently, a baculo-like virus has been diagnosed as the causative agent for this epidemic disease. The electron microscopic observation of the virus is described. Thin sections of hepatopancreatic and hypodermic tissue of diseased P. chinensis showed many rod-shaped, enveloped, baculo-like virions in hypertrophied nuclei of infected cells. The virion was filled with a highly electron dense core. No occlusion bodies have been found. Negative stained intact virions, purified from infected tissues by centrifugation on sucrose discontinuous gradients, demonstrated that the viral envelops had been broken, but the cylindrical nucleocapside could be observed clearly. The nucleocapsid of average 62 nm x 314 nm was composed of a helix system of capsomers, giving rise to an open stacked ring structure and repeating approximately every third ring. The number of repeating unit was 13 to 15. We propose that the virus described here could be designated as Non-Occluded Shrimp Virus (NOSV) or Penaeus chinensis Baculo-Like Virus (PcBLV).

Efficient transduction of mammalian cells by a recombinant baculovirus having the vesicular stomatitis virus G glycoprotein

Baculovirus vectors recently have been shown to be capable of efficient transduction of human hepatoma cells and primary hepatocytes in culture. This paper describes the generation of a novel recombinant baculovirus (VGZ3) in which the vesicular stomatitis virus glycoprotein G (VSV G) is present in the viral envelope. The gene encoding VSV G was inserted into the baculovirus genome under the control of the polyhedrin promoter such that it was expressed at very high levels in infected insect cells but not in mammalian cells. Expression of the lacZ reporter gene was driven by a promoter that is functional in mammalian cells (the Rous sarcoma virus long terminal repeat). We show by Western analysis that VSV G protein was present in purified baculovirus preparations. A VSV G monoclonal antibody blocked transduction of mammalian cells by VGZ3. This virus was morphologically distinct from baculovirus lacking VSV G, with virions adopting an oval rather than rod-shaped morphology. VGZ3 transduced human hepatoma cells in vitro at an efficiency roughly 10-fold greater than baculovirus lacking VSV G (the virus Z4). VGZ3 was also capable of transducing cell lines that could not be transduced efficiently by Z4. We provide evidence that VSV G protein may enhance transduction by increasing the efficiency of escape of baculovirus from intracellular vesicles rather than by increasing cell binding or uptake of the virus. The possible use of this and related baculoviruses in gene therapy is discussed.

Multiple interactions control the intracellular localization of the herpes simplex virus type 1 capsid proteins

Herpes simplex virus type 1 (HSV-1) capsid assembly takes place in the nucleus of infected cells. However, when each of the outer capsid shell proteins, VP5, VP23 and VP26, is expressed in the absence of any other HSV-1 proteins, it does not localize to the nucleus but is distributed throughout the cell. We have previously shown that the HSV-1 capsid scaffolding protein, preVP22a, can relocate VP5 into the nucleus but does not influence the distribution of VP23. We now demonstrate that the outer capsid shell protein, VP19C, is able to relocate both VP5 and VP23 separately into the nucleus. However, nuclear localization of VP26 is only observed when VP5 is present together with either VP19C or preVP22a. Thus, pair-wise interactions involving all of the abundant capsid proteins have now been identified. Electron microscope examination of insect cells coinfected with recombinant baculoviruses expressing VP19C and VP5 reveals the presence of 70 nm diameter 'capsid-like' structures, suggesting that these two proteins can form the basic capsid shell.

Protein retention in the endoplasmic reticulum of insect cells is not compromised by baculovirus infection

High level expression of the major auxin-binding protein (ABP1) from maize (Zea mays L.) has been used to demonstrate that the machinery for retaining proteins in the endoplasmic reticulum (ER) of insect cells functions efficiently throughout the baculovirus infection cycle. Immunolocalization showed wild-type ABP1 (ABP1-KDEL) to be targeted to the lumen of the ER, in accordance with its signal peptide and carboxyterminal KDEL ER-retention signal. The protein accumulated in dilations of the ER, and none was detected at the cell surface. Immunoblotting of concentrated culture medium confirmed that ABP1-KDEL was not secreted at a detectable level. In contrast, when the carboxyterminus was mutated to KEQL, secretion of the baculovirus-expressed protein was readily detected. Immunolocalization and immunoblotting demonstrated that a high proportion of the ABP1-KEQL protein was secreted at the cell surface and into the culture medium. The data demonstrate that the ER of insect cells has a great capacity to retain proteins and that this property is largely unaffected by the cellular disruption caused by baculovirus replication.

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.

Fullerene-like organization of HIV gag-protein shell in virus-like particles produced by recombinant baculovirus

Virus-like particles produced by a recombinant baculovirus containing the HIV gag gene were examined by negative staining after delipidization. This technique demonstrated that the gag-protein shell consisted of radially arranged short rods which formed a network of ring-like structures. Similar structures were observed at the plasma membrane of infected cells which had been opened by wet-cleaving. Occasionally five or six subunits were observed forming a ring. These findings suggest that the gag-encoded precursor (pr55) is a rod-like molecule about 34 A in diameter and 85 A in length. A protein cylinder of such dimensions would have a molecular weight of 56K. The center-to-center distance of two neighboring rings formed by the rods was 66 +/- 8 A (N = 200) by direct measurements and 65 A as obtained from averaged images. This morphology and these dimensions indicate that the virus-like particles contain the gag precursor in the form of a near-spherical "fullerene-like" icosahedral shell. Our data indicate that the triangulation number of the rings equals 63. However, since one rod of pr55 is shared by two rings, the number of copies of the precursor will be 1890 as opposed to 2522 if the molecules were closely packed. The particle diameter of 102 nm deduced from the proposed model was close to the diameter obtained from thin sections of low-temperature-embedded specimens (103-108 nm).

Genetically engineered multi-component virus-like particles as veterinary vaccines

Multiprotein structures can be constructed to mimic virus particles. These engineered particles lack genetic material and are not infectious but they can elicit protective immune responses in animals against challenges with infectious viruses. As a prototype, insect cells were co-infected with two recombinant baculoviruses. One recombinant baculovirus contained an insert of L2 and M5 genes, which encode for outer capsid proteins VP2 and VP5, respectively, from bluetongue virus (BTV) downstream of duplicated copies of the baculovirus (Autographa californica nuclear polyhedrosis virus, AcNPV) polyhedrin promoter. Another recombinant baculovirus expressed two major core proteins (VP3 and VP7) from BTV virions. The co-infected cells synthesized non-infectious, double-shelled, virus-like particles (VLP). The VLP resembled the authentic BTV in size, appearance, and biochemical constitution but they lacked the double-stranded-RNA genome and three minor polypeptides normally contained in the icosahedral inner capsid. The VLP consisted of an outer shell of VP2 and VP5 from US BTV-10 attached to an icosahedral framework formed by the two major core proteins VP3 and BTV-17 and VP7 from US BTV-10. Sheep immunized with VLP expressing BTV capsid proteins produced antibodies that neutralized homologous serotypes of BTV. The assembly of VLP from different proteins simultaneously expressed indicates the potential of this novel approach for producing safe and effective vaccines against several viral agents.

The 1,629-nucleotide open reading frame located downstream of the Autographa californica nuclear polyhedrosis virus polyhedrin gene encodes a nucleocapsid-associated phosphoprotein

A 78-kDa protein was produced in bacteria from a clone of the 1,629-nucleotide open reading frame located immediately downstream from the polyhedrin gene of Autographa californica nuclear polyhedrosis virus. The identity of this protein was confirmed by its reactivity with peptide antiserum and amino terminal peptide sequencing after purification from transformed bacteria. The polypeptide was used to produce polyclonal antisera in rabbits. Immunoblot analysis of insect cells infected with the baculovirus indicated that two related proteins with molecular masses of 78 and 83 kDa were synthesized late in infection. Biochemical fractionation studies indicated that both of these proteins were present in purified nucleocapsids from budded and occluded virus preparations. Immunoprecipitation of 32P-labeled proteins and treatment of purified nucleocapsids with alkaline phosphatase demonstrated that the 83-kDa protein was a phosphorylated derivative of the 78-kDa protein. Furthermore, immunoelectron microscopy revealed that the proteins were localized to regions of nucleocapsid assembly within the infected cell and appeared to be associated with the end structures of mature nucleocapsids.

Purification and amplification of DNA from Penaeus monodon-type baculovirus (MBV)

The purpose of this paper was to purify and amplify the DNA fragment of Penaeus monodon-type baculovirus (MBV). Using 30-50% caesium chloride gradients, MBV virions and occlusion bodies with density parameters of 1.28-1.29 and 1.32-1.33 g/ml, respectively, were purified. Two oligonucleotide primers have been successfully designed and utilized for the amplification of a DNA fragment of MBV. After 35 amplification cycles of the MBV DNA fragment, a large amount of amplified product with an approximate molecular weight of 600 bp was obtained. This is the first successfully published work on the amplification of MBV using the polymerase chain reaction (PCR). Using the same primers, DNA extracted from MBV noninfected P. monodon, P. japonicus, and P. orientalis had a negative PCR response. However, a positive PCR response was obtained from DNA extracted from MBV-infected postlarval P. monodon. DIG-dot blot hybridization technique using PCR product obtained from the present study as a probe further confirmed that the product is originated from a portion of MBV polyhedrin gene. It is also suggested that PCR product may be beneficial for an accurate and early diagnosis of MBV infection in larval shrimp.

Identification and immunocharacterization of a 25K structural protein in Autographa californica nuclear polyhedrosis virus (AcMNPV)

A 25K protein was a major component of the Triton X-100 solubilized fraction prepared for the isolation of nuclear matrices from Spodoptera frugiperda cells infected with Autographa californica nuclear polyhedrosis virus (AcMNPV). Polyclonal antisera to p25 were produced in rabbits and used in the characterization of p25. Western blots of proteins probed with the p25 antiserum demonstrated the presence of the p25 in both the polyhedral-derived and extracellular viral phenotypes. Using an indirect fluorescent antibody assay, p25 was detected within the infected host cell as early as 6 h p.i. and throughout the time course examined. The intensity in fluorescence increased from the periphery of the infected cells to within the nucleus at late times post-infection. On electron micrographs, the p25 was observed as dense gold particles in localized pockets and around virions in the nucleus at 16 and 36 h p.i., respectively. By 48 h p.i., p25 was concentrated over the surface of nucleocapsids in the intranuclear region and within polyhedra. p25 was not associated with any infected cell structures and did not appear to be involved in polyhedron formation. These studies demonstrated that p25 was a structural protein which was expressed very early during infection and displayed characteristics similar to In major capsid proteins of the baculoviruses.

Replication of Autographa californica baculovirus (AcMNPV) in a coleopteran cell line

A coleopteran cell line (AGE) derived from the cotton boll weevil Anthonomus grandis supported replication of Autographa californica multiple nuclear polyhedrosis virus (AcMNPV). The titer of extracellular virus (ECV) and the number of occlusion bodies (OB) produced in AGE cells were approximately equal to those produced by a Trichoplusia ni cell line (TN-CL1), and the OB produced by both cell lines were equally infectious for T. ni larvae. The identity of the AGE cell line was established by chromosome and isoenzyme analyses.

Analysis of polyhedra morphology mutants of Autographa californica nuclear polyhedrosis virus: molecular and ultrastructural features

Two new mutants of Autographa californica multiple nuclear polyhedrosis virus affected in the morphogenesis of their polyhedra, designated M276 and M934, were investigated. Marker transfer experiments demonstrated that the observed phenotype was due exclusively to alterations in the polyhedrin gene. M276 contained a 229 base insertion near the carboxyl terminus coding region which resulted in synthesis of a truncated protein; M934 had a point mutation substituting phenylalanine for leucine at amino acid 183. Both mutations occurred in highly conserved regions of the protein and prevented the occlusion of virus particles, but did not affect targeting for the intranuclear ring zone. M276 was distinct in that it had prominent cytosolic condensations of polyhedrin, although these were probably due to a decreased protein solubility. M934 polyhedrin condensations associated prematurely with calyx material such that it became incorporated into the condensation rather than at the surface. Results confirm that occlusion size and shape are features inherent to the polyhedrin protein, and suggest that polyhedrin conformation may help regulate the occlusion process.