Utilization of an Eilat Virus-Based Chimera for Serological Detection of Chikungunya Infection

In December of 2013, chikungunya virus (CHIKV), an alphavirus in the family Togaviridae, was introduced to the island of Saint Martin in the Caribbean, resulting in the first autochthonous cases reported in the Americas. As of January 2015, local and imported CHIKV has been reported in 50 American countries with over 1.1 million suspected cases. CHIKV causes a severe arthralgic disease for which there are no approved vaccines or therapeutics. Furthermore, the lack of a commercially available, sensitive, and affordable diagnostic assay limits surveillance and control efforts. To address this issue, we utilized an insect-specific alphavirus, Eilat virus (EILV), to develop a diagnostic antigen that does not require biosafety containment facilities to produce. We demonstrated that EILV/CHIKV replicates to high titers in insect cells and can be applied directly in enzyme-linked immunosorbent assays without inactivation, resulting in highly sensitive detection of recent and past CHIKV infection, and outperforming traditional antigen preparations.

We have developed an innovative approach to production of alphavirus antigens for use in diagnostic assays that results in reduced production complexity as well as improved sensitivity in application. By generating recombinant viruses that contain the structural protein genes of pathogenic alphaviruses and the nonstructural protein genes of an insect-specific alphavirus, Eilat virus, we have been able to produce insect-restricted viruses that are antigenically identical to their pathogenic counterparts. The insect-specific nature of these chimeric viruses yields an advantageous safety profile and allows for safe handling of the antigen at the bench top. Traditional antigens, produced from wild-type virus, require extensive processing, from growth at biosafety level 3 to concentration and inactivation, followed by lyophilization, which often compromises antigen reactivity and is financially costly. Furthermore, current inactivation methods are imperfect processes that have historically resulted in residual live virus and subsequent breach of containment when used in a diagnostic setting. Other approaches, such as recombinant antigens generated from viral particle subunits, are missing conformational epitopes and their application results in reduced sensitivity. Here we describe the development of a diagnostic assay using this technology for the detection of chikungunya infection in humans.

A new insect cell line from pupal ovary of Spodoptera exigua established by stimulation with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)

A continuous cell line derived from the pupal ovary of Spodoptera exigua was established by treating primary cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Three days after treating cells with 3.0 μg/ml of MNNG, the cells formed a monolayer and were initially subcultured 60 d after the MNNG was removed, followed by subculturing for 30 passages. The established cell line, designated IOZCAS-Spex 12, consisted of a mixture of three types of cells, including spherical, spindle-shaped, and oval cells. The population doubling time of the cell line during its logarithmic growth phase was found to be 71 h. DNA amplification fingerprinting polymerase chain reaction analysis confirmed that the new cell line originated from S. exigua. Susceptibility of IOZCAS-Spex 12 cells to infection by certain nucleopolyhedroviruses was investigated. The results showed that the cell line was highly susceptible to infection by S. exigua nucleopolyhedrovirus and Autographa californica multiple nucleopolyhedrovirus, slightly susceptible to infection by Spodoptera litura nucleopolyhedrovirus, and not susceptible to infection by Helicoverpa armigera nucleopolyhedroviruses or Hyphantria cunea nucleopolyhedroviruses. The results of this study suggest that MNNG treatment may overcome existing limitations to obtaining continually proliferating cells and may open up the possibilities for immortalizing isolated insect cells.

Disease dynamics and persistence of Musca domestica salivary gland hypertrophy virus infections in laboratory house fly (Musca domestica) populations

Past surveys of feral house fly populations have shown that Musca domestica salivary gland hypertrophy virus (MdSGHV) has a worldwide distribution, with an average prevalence varying between 0.5% and 10%. How this adult-specific virus persists in nature is unknown. In the present study, experiments were conducted to examine short-term transmission efficiency and long-term persistence of symptomatic MdSGHV infections in confined house fly populations. Average rates of disease transmission from virus-infected to healthy flies in small populations of 50 or 100 flies ranged from 3% to 24% and did not vary between three tested geographical strains that originated from different continents. Introduction of an initial proportion of 40% infected flies into fly populations did not result in epizootics. Instead, long-term observations demonstrated that MdSGHV infection levels declined over time, resulting in a 10% infection rate after passing through 10 filial generations. In all experiments, induced disease rates were significantly higher in male flies than in female flies and might be explained by male-specific behaviors that increased contact with viremic flies and/or virus-contaminated surfaces.

Host phylogeny determines viral persistence and replication in novel hosts

Pathogens switching to new hosts can result in the emergence of new infectious diseases, and determining which species are likely to be sources of such host shifts is essential to understanding disease threats to both humans and wildlife. However, the factors that determine whether a pathogen can infect a novel host are poorly understood. We have examined the ability of three host-specific RNA-viruses (Drosophila sigma viruses from the family Rhabdoviridae) to persist and replicate in 51 different species of Drosophilidae. Using a novel analytical approach we found that the host phylogeny could explain most of the variation in viral replication and persistence between different host species. This effect is partly driven by viruses reaching a higher titre in those novel hosts most closely related to the original host. However, there is also a strong effect of host phylogeny that is independent of the distance from the original host, with viral titres being similar in groups of related hosts. Most of this effect could be explained by variation in general susceptibility to all three sigma viruses, as there is a strong phylogenetic correlation in the titres of the three viruses. These results suggest that the source of new emerging diseases may often be predictable from the host phylogeny, but that the effect may be more complex than simply causing most host shifts to occur between closely related hosts.

Emerging infectious diseases such as SARS, HIV and swine-origin influenza have all been recently acquired by humans from other species. Understanding the reasons why parasites jump between different host species is essential to allow us to predict future threats and understand the causes of disease emergence. Here we ask how host-relatedness might determine when host-shifts can occur in the most important group of emerging diseases—RNA viruses. We show that the relationship between host species is the primary factor in determining a virus's ability to persist and replicate in a novel host following exposure. This can be broken down into two components. Firstly, species closely related to the virus's natural host are more susceptible than distantly related species. Secondly, independent of the distance effect, groups of closely related host species have similar levels of susceptibility. This has important implications for our understanding of disease-emergence, and until now the only large-scale studies of viruses have been correlative rather than experimental. We also found groups of related species that are susceptible to these viruses but are distantly related to the natural hosts, which may explain why viruses sometimes jump between distantly related species.

Spread of infectious chronic bee paralysis virus by honeybee (Apis mellifera L.) feces

Knowledge of the spreading mechanism of honeybee pathogens within the hive is crucial to our understanding of bee disease dynamics. The aim of this study was to assess the presence of infectious chronic bee paralysis virus (CBPV) in bee excreta and evaluate its possible role as an indirect route of infection. Samples of paralyzed bees were (i) produced by experimental inoculation with purified virus and (ii) collected from hives exhibiting chronic paralysis. CBPV in bee heads or feces (crude or absorbed onto paper) was detected by reverse transcription-PCR. CBPV infectivity was assessed by intrathoracic inoculation of bees with virus extracted from feces and by placement of naive bees in cages previously occupied by contaminated individuals. CBPV RNA was systematically detected in the feces of naturally and experimentally infected bees and on the paper sheets that had been used to cover the floors of units containing bees artificially infected with CBPV or the floor of one naturally infected colony. Both intrathoracic inoculation of bees with virus extracted from feces and placement of bees in contaminated cages provoked overt disease in naive bees, thereby proving that the excreted virus was infectious and that this indirect route of infection could lead to overt chronic paralysis. This is the first experimental confirmation that infectious CBPV particles excreted in the feces of infected bees can infect naive bees and provoke overt disease by mere confinement of naive bees in a soiled environment.

In vitro host range of the Hz-1 nonoccluded virus in insect cell lines

A total of 13 insect cell lines spanning 4 orders (Lepidoptera, Coleoptera, Diptera, and Homoptera) were tested for their ability to replicate the nonoccluded virus Hz-1. Only the Lepidopteran cell lines supported replication of the virus with TN-CL1 and BCIRL-HZ-AM1 producing the highest titers of 2.4 x 10(8) tissue culture infective dose (TCID)50/ml and 2.0 x 10(8) TCID50/ml, respectively. A codling moth cell line (CP-169) was the only Lepidopteran cell line that did not replicate the virus and transfection of this cell line with Hz-1 DNA failed to replicate the virus. Also, transfection with DNA from a recombinant baculovirus carrying the red fluorescent protein gene (AcMNPVhsp70 Red) was not expressed in CP-169 cells. The replication cycle of Hz-1 in BCIRL-HZ-AM1 cells showed that this virus replicated rapidly starting at 16 h postinoculation (p.i.) and reaching a peak titer of 1.0 x 10(8) TCID50/ml 56 h postinoculation. Hz-1 when compared with several other baculoviruses has the widest in vitro host spectrum.

A glassy-winged sharpshooter cell line supports replication of Rhopalosiphum padi virus (Dicistroviridae)

Rhopalosiphum padi virus (RhPV) (family Dicistroviridae; genus Cripavirus) is an icosahedral aphid virus with a 10kb positive-sense RNA genome. To study the molecular biology of RhPV, identification of a cell line that supports replication of the virus is essential. We screened nine cell lines derived from species within the Lepidoptera, Diptera and Hemiptera for susceptibility to RhPV following RNA transfection. We observed cytopathic effects (CPE) only in cell lines derived from hemipterans, specifically GWSS-Z10 cells derived from the glassy winged sharp shooter, Homalodisca coagulata and DMII-AM cells derived from the corn leaf hopper, Dalbulus maidis. Translation and appropriate processing of viral gene products, RNA replication and packaging of virus particles in the cytoplasm of GWSS-Z10 cells were examined by Western blot analysis, Northern blot hybridization and electron microscopy. Infectivity of the GWSS-Z10 cell derived-virus particles to the bird cherry-oat aphid, R. padi, was confirmed by RT-PCR and Western blot. The GWSS-Z10 cell line provides a valuable tool to investigate replication, structure and assembly of RhPV.

Small compounds targeted to subunit interfaces arrest maturation in a nonenveloped, icosahedral animal virus

Nudaurelia omega capensis virus (N omega V) capsids were previously characterized in two morphological forms, a T=4, 485-A-diameter round particle with large pores and a tightly sealed 395-A icosahedrally shaped particle with the same quasi-symmetric surface lattice. The large particle converts to the smaller particle when the pH is lowered from 7.6 to 5, and this activates an autocatalytic cleavage of the viral subunit at residue 570. Here we report that both 1-anilino-8 naphthalene sulfonate (ANS) and the covalent attachment of the thiol-reactive fluorophore, maleimide-ANS (MIANS), inhibit the structural transition and proteolysis at the lower pH. When ANS is exhaustively washed from the particles, the maturation proceeds normally; however, MIANS-modified particles are still inhibited after the same washing treatment, indicating that covalent attachment targets MIANS to a critical location for inhibition. Characterization of the low-pH MIANS product by electron cryo-microscopy (cryo-EM) and image reconstruction demonstrated a morphology intermediate between the two forms previously characterized. A pseudoatomic model of the intermediate configuration was generated by rigid body refinement of the X-ray structure of the subunits (previously determined in the assembled capsid) into the cryo-EM density, allowing a quantitative description of the inhibited intermediate and a hypothesis for the mechanism of the inhibition.

Genetic and phenotypic characterization of the newly described insect flavivirus, Kamiti River virus

We have described in the accompanying paper by Sang, et al., ([57], Arch Virol 2003, in press) the isolation and identification of a new flavivirus, Kamiti River virus (KRV), from Ae. macintoshi mosquitoes that were collected as larvae and pupae from flooded dambos in Central Province, Kenya. Among known flaviviruses, KRV was shown to be most similar to, but genetically and phenotypically distinct from, Cell fusing agent virus (CFAV). KRV was provisionally identified as an insect-only flavivirus that fails to replicate in vertebrate cells or in mice. We report here the further characterization of KRV. Growth in cell culture was compared to that of CFAV; although growth kinetics were similar, KRV did not cause the cell fusion that is characteristic of CFAV infection. The KRV genome was found to be 11,375 nucleotides in length, containing a single open reading frame encoding 10 viral proteins. Likely polyprotein cleavage sites were identified, which were most similar to those of CFAV and were comparable to those of other flaviviruses. Sequence identity with other flaviviruses was low; maximum identity was with CFAV. Possible terminal secondary structures for the 5' and 3' non-coding regions (NCR) were similar to those predicted for other flaviviruses. Whereas CFAV was isolated from insect cells in the laboratory, the isolation of KRV demonstrates the presence of an insect-only flavivirus in nature and raises questions regarding potential interactions between this virus and other mosquito-borne viruses in competent vector populations. Additionally, this virus will be an important tool in future studies to determine markers associated with flavivirus host specificity.

Sensitivity of Invertebrate iridescent virus 6 to organic solvents, detergents, enzymes and temperature treatment

The sensitivity of Invertebrate iridescent virus 6 (IIV-6) to a selection of organic solvents, detergents, enzymes and heat treatment was assayed in Spodoptera frugiperda (Sf9) cells and by injection of inoculum into larvae of Galleria mellonella. In several cases, the degree of sensitivity of the virus depended on the method of assay; cell culture assays indicated greater losses of activity than insect bioassay. IIV-6 was sensitive to chloroform but sensitivity to ether was only detected by cell culture assay. Sensitivity (defined as a reduction of at least 1 log activity) was detected following treatment by 1 and 0.1% SDS, 1% Triton-X100, 70% ethanol, 70% methanol, 1% sodium deoxycholate, pH 11.1 and 3.0. No sensitivity was detected to 1% Tween 80, 1 M MgCl2, 100 mM EDTA, lipase, phospholipase A2, proteinase K, or trypsin at the concentrations tested. Viral activity was reduced by approximately 4 logs following heating to 70 degrees C for 60 min or 80 degrees C for 30 min. The above observations highlight the need for studies on the role of the virus lipid component in the process of particle entry into cells, and may explain why vertebrate and invertebrate iridoviruses have been reported to differ in their sensitivity to organic solvents and enzymes.

A refined method for the detection of baculovirus occlusion bodies in forest terrestrial and aquatic habitats

A sensitive and efficient method was developed for the detection of genetically modified and wild-type baculovirus occlusion bodies (OB) in forest terrestrial and aquatic habitats. The protocol facilitates the analysis of a large number of samples collected and frozen to maintain viral integrity. Lyophilization was used to standardize the size of both field-collected soil samples and test substrates inoculated with OBs for the determination of minimum detection threshold. To simulate natural conditions, terrestrial test substrates were inoculated at a standardized moisture content determined using a soil pressure plate apparatus. OBs, extracted from lyophilized test substrates by washing, sieving and centrifugation, were subjected to alkaline lysis and viral DNA isolated using a purchased DNA purification kit. PCR amplified DNA was visualized using agarose gel electrophoresis. Minimum detection thresholds in terrestrial substrates were 10(3), 10(2), 10(2) and 10(1) OBs from 0.5 g of lyophilized L, F-H and mineral soil horizons, and 1.0 ml of leachate, respectively. Detection thresholds in aquatic substrates were 10(0) and 10(3) OBs from 1.0 ml of pond water and 1.0 g of bottom sediment, respectively.

Specific encapsidation of nodavirus RNAs is mediated through the C terminus of capsid precursor protein alpha

Flock house virus (FHV) is a small icosahedral insect virus with a bipartite, messenger-sense RNA genome. Its T=3 icosahedral capsid is initially assembled from 180 subunits of a single type of coat protein, capsid precursor protein alpha (407 amino acids). Following assembly, the precursor particles undergo a maturation step in which the alpha subunits autocatalytically cleave between Asn363 and Ala364. This cleavage generates mature coat proteins beta (363 residues) and gamma (44 residues) and is required for acquisition of virion infectivity. The X-ray structure of mature FHV shows that gamma peptides located at the fivefold axes of the virion form a pentameric helical bundle, and it has been suggested that this bundle plays a role in release of viral RNA during FHV uncoating. To provide experimental support for this hypothesis, we generated mutant coat proteins that carried deletions in the gamma region of precursor protein alpha. Surprisingly, we found that these mutations interfered with specific recognition and packaging of viral RNA during assembly. The resulting particles contained large amounts of cellular RNAs and varying amounts of the viral RNAs. Single-site amino acid substitution mutants showed that three phenylalanines located at positions 402, 405, and 407 of coat precursor protein alpha were critically important for specific recognition of the FHV genome. Thus, in addition to its hypothesized role in uncoating and RNA delivery, the C-terminal region of coat protein alpha plays a significant role in recognition of FHV RNA during assembly. A possible link between these two functions is discussed.

The production of cricket paralysis virus in suspension cultures of insect cell lines

The yields of cricket paralysis virus from two insect cell lines propagated in suspension culture were examined. Although Trichoplusia ni (TN368) cells produced more virus per cell than Drosophila line 2 (DL2) cells, the smaller DL2 cells reached a higher cell density. Thus the difference in production between TN368 and DL2 cultures was minimal. In mixed infections of Flock House virus and cricket paralysis virus (CrPV), CrPV was the predominant virus produced despite being present at a much lower multiplicity of infection.

Nonpathogenic Nilaparvata lugens reovirus is transmitted to the brown planthopper through rice plant

Nilaparvata lugens reovirus (NLRV) was found in a colony of the brown planthopper, N. lugens, an important plant virus vector. Unlike with usual phytopathogenic reoviruses, there are no visible symptoms on a rice plant which has been attacked by NLRV-infected planthoppers. Manner of transmission, host range, and multiplication of NLRV in rice plant were studied by enzyme-linked immunosorbent assay or polymerase chain reaction using reverse transcriptase. NLRV was transovumly transmitted to about 15% of nymphs. NLRV could further spread to planthoppers via rice plants through sucking by viruliferous insects. This horizontal transmission was apparently of primary importance for continuous NLRV infection to a colony of N. lugens. By corearing with viruliferous N. lugens on rice seedlings, Laodelphax striatellus acquired NLRV but Sogatella furcifera, Nephotettix cincticeps, and N. malayanus did not during the period studied. NLRV inoculated onto rice plant by viruliferous N. lugens failed to multiply, which accounts for the lack of symptoms on the plant even after an attack by viruliferous planthoppers. These biological properties and our previous data, which revealed a molecular similarity between NLRV and fijiviruses, suggest that NLRV is an ancestral virus of fijiviruses.

Plaque assay and replication of Tipula iridescent virus in Spodoptera frugiperda ovarian cells

A plaque assay was developed for the study of Tipula iridescent virus (TIV) replication using a cell line derived from the fall army worm Spodoptera frugiperda (Sf9). Infection and plaque formation were monitored with time by phase contrast microscopy, video and fluorescent light microscopy. Structure of virions, viroplasmic centres and organelles of infected cells were examined by transmission electron microscopy (TEM). After 4 h postinfection, plaques were visibly detected within the cell monolayer by the presence of localized cell damage and production of numerous vesicular-like cytoplasmic structures. Quantitation of virions present per A260 unit of TIV preparation was determined by TEM. The number of visible plaques corresponded to virus concentration and 1 A260 produced approximately 10(5) plaques. DNA hybridization analysis revealed no gross differences in genomic DNA from TIV propagated in either Sf9 cells or wax moth Galleria mellonella larvae. These findings indicate that Sf9 is permissive for replication of TIV and superior by some parameters to other cell lines currently in use for the study of host cell/TIV interactions.

Reconstitution of Flock House provirions: a model system for studying structure and assembly

Assembly of Flock House virus in infected Drosophila cells proceeds through an intermediate, the provirion, which lacks infectivity until the coat precursor protein, alpha, undergoes a spontaneous "maturation" cleavage (A. Schneemann, W. Zhong, T. M. Gallagher, and R. R. Rueckert, J. Virol 6:6728, 1992). We describe here methods for purifying provirions in a state which permitted dissociation and reassembly. Dissociation, to monomeric alpha protein and free RNA, was accomplished by freezing at pH 9.0 in the presence of 0.5 M salt and 0.1 M urea. When dialyzed at low ionic strength and pH 6.5, the dissociation products reassembled spontaneously to form homogeneous provirions with a normal complement of RNA as judged by cosedimentation with authentic virions and by ability to undergo maturation cleavage with acquisition of substantial, though subnormal, infectivity. Reconstitution experiments, i.e., remixing components after separating RNA from capsid protein, generated abnormal particles, suggesting the presence in the unfractionated dissociation products of an unidentified "nucleating" component.

Vaccinia virion surface polypeptide Ag35 expressed from a baculovirus vector is targeted to analogous poxvirus and insect virus components

Polypeptide Ag35, a major early component of the vaccinia surface, is integrated into the formative viral lipoprotein tegument. To ascertain whether positioning of Ag35 is due to its general affinity for newly assembled viral membranes we created a recombinant A12 vector to express the vaccinia protein. The baculovirus system was chosen because intranuclear virions of this agent are likewise enclosed inside newly formed envelopes. Comparable infections of two insect cell lines established that more abundant synthesis occurred in High Five (H5) than in SF9 cells. We, therefore, used H5 cells for most experiments reported here. Combined analyses by PAGE, Western blotting, and immunocytology, using light and electron microscopy, revealed a dissemination of Ag35 throughout the cell. Higher concentrations were evident at the cell surface, nuclear perimeter, and within intranuclear virogenic stroma. The association with the virogenic stroma was of specific interest with respect to vaccinia development because it showed a similarity in the targeting of Ag35 toward intranuclear DNA-protein foci of baculovirus which are analogous to the vaccinia-specified cytoplasmic "factories." A further remarkable analogy concerns association of Ag35 with intranuclear baculovirus envelopes, revealing a propensity of Ag35 for nascent viral lipoprotein membranes.

Flock house virus: a simple model for studying persistent infection in cultured Drosophila cells

Flock house virus (FHV), isolated from twenty Drosophila melanogaster cell lines, persistently infected with the virus, were examined during successive serial passages by plaque assay and sequence analysis. No phenotypic or genotypic changes in the virus were observed during the establishment of persistent infection, suggesting that it was a cellular modification that led to the first step in establishing the persistent state. Once this state was initiated, the virus was relieved of the need for a functional coat protein to propagate itself and mutations began to accumulate selectively in RNA2, the gene for the coat protein. These changes were manifested by a gradual drift to a smaller plaque population. The replicase activity, coded by RNA1, remained unaltered.

Study of the ref(2)P locus of Drosophila melanogaster. II. Genetic studies of the 37DF region

The ref(2)P gene of Drosophila melanogaster is implicated in sigma rhabdovirus multiplication. Two common alleles of ref(2)P are known, ref(2)P0 which permits sigma virus multiplication and ref(2)Pp which is restrictive for most sigma virus strains. This gene maps to the cytogenetic region 37E3-F3. Using Df(2L)E55 (= Df(2L)37D2-E1;37F5-38A1), we have screened for lethal, semi-lethal and visible mutations following diepoxybutane (DEB) or ethyl methanesulfonate (EMS) mutagenesis. Our data confirm than DEB is more efficient than EMS at inducing deletions. The mutations obtained in this region define 14 complementation groups. One of them, l(2)37Dh, appears to be a general enhancer of Minute and Minute-like mutations. None of the mutations were allelic to the ref(2)P locus. Loss-of-function alleles of ref(2)P (called null) were selected following DEB mutagenesis. Homozygous or hemizygous ref(2)Pnull flies are male sterile. These flies, like homozygous or hemizygous ref(2)P0 flies, are fully permissive for sigma virus replication. We suggest that the ref(2)P products interact with viral products, but that this interaction is not necessary for an efficient viral cycle.

Use of recombinant baculoviruses in synthesis of morphologically distinct viruslike particles of flock house virus, a nodavirus

Flock house virus (FHV) is a small icosahedral insect virus of the family Nodaviridae. Its genome consists of two messenger-sense RNA molecules, both of which are encapsidated in the same particle. RNA1 (3.1 kb) encodes proteins required for viral RNA replication; RNA2 (1.4 kb) encodes protein alpha (43 kDa), the precursor of the coat protein. When Spodoptera frugiperda cells were infected with a recombinant baculovirus containing a cDNA copy of RNA2, coat protein alpha assembled into viruslike precursor particles (provirions) that matured normally by autocatalytic cleavage of protein alpha into polypeptide chains beta (38 kDa) and gamma (5 kDa). The particles were morphologically indistinguishable from authentic FHV and contained RNA derived from the coat protein message. These results showed that RNA1 was required neither for virion assembly nor for maturation of provirions. Expression of mutants in which Asn-363 at the beta-gamma cleavage site of protein alpha was replaced by either aspartate, threonine, or alanine resulted in assembly of particles that were cleavage defective. For two of the mutants, unusual structural features were observed after preparation for electron microscopy. Particles containing Asp at position 363 were labile and showed a strong tendency to break into half-shells. Particles in which Asn-363 was replaced by Ala displayed a distinct hole in an otherwise complete shell. The third mutant, containing Thr at position 363, was indistinguishable in morphology from authentic FHV.

Continuous production of baculovirus in a cascade of insect-cell reactors

Insect cells (Spodoptera frugiperda) were cultured in a continuous stirred-tank reactor. The effluent was led to a cascade of another two reactors, each containing half the volume of the cell-growth reactor, where the cells were infected with Autographa californica nuclear polyhedrosis virus. For about 10 days production of 10(7) polyhedra (virus particles embedded in a protein capsule) per cm3 was achieved. This short production time compared to previous experiments involving an analogous system with a single infection vessel of equal volume to the cell-growth vessel is ascribed to the accelerated occurrence of the so-called passage effect (a decrease of infectious virus with time). From the results of a computer model it was concluded that this passage effect was accelerated by the change in residence time distribution as compared to the earlier experiments.

Characterization of the DA26 gene in a hypervariable region of the Autographa californica nuclear polyhedrosis virus genome

A region of the baculovirus Autographa californica nuclear polyhedrosis virus genome that is frequently found to be altered after serial passage of the virus in cell culture was characterized. Sequence analysis of this region of the genome in wild-type and mutant viruses revealed that some of the mutations affected a 675 bp open reading frame, designated DA26. The DA26 gene was disrupted both by deletion and by insertion of sequences that resembled transposable elements. Northern blot analysis of DA26 showed that it was expressed very early after infection. DA26-specific transcripts could be detected after the 1 h viral adsorption period upon infection of cultured Trichoplusia ni cells. These transcripts were mapped by nuclease protection assays. A recombinant virus was constructed in which DA26 was disrupted by insertion of the Escherichia coli lacZ gene. This virus was viable in both T. ni and Spodoptera frugiperda cells and analysis of the kinetics of protein synthesis revealed no differences between wild-type and recombinant viruses. The disruption of DA26 also did not interfere with the ability of the virus to infect T. ni or S. frugiperda larvae.

Mass production and purification of Euproctis chrysorrhoea (L.) nuclear polyhedrosis virus

The rearing of larval Euproctis chrysorrhoea (L.) on an artificial wheatgerm diet is described. The larvae grew well on the diet, but took over ten weeks to complete their development, reflecting their prolonged life-span in the field. Using these larvae it was possible to mass produce a nuclear polyhedrosis virus (NPV) isolated from this species, with a view towards its use as a biological insecticide. A mean of 5.86 x 10(9) semi-pure NPV polyhedral inclusion bodies/g virus-killed larva was produced. The purification method described (simple centrifugation) removed over 99% of contaminant urticaceous hairs. Further purification through a sucrose gradient removed more hairs, but also resulted in a loss of NPV of about 35%.

Signals important for high-level expression of foreign genes in Autographa californica nuclear polyhedrosis virus expression vectors

The transcriptional and translational signals required for efficient expression of the chloramphenicol acetyltransferase, beta-galactosidase, and tissue plasminogen activator genes, under the control of the polyhedrin promoter in Spodoptera frugiperda cells infected with Autographa californica nuclear polyhedrosis virus, were investigated by SDS-PAGE and RNA dot blot analysis. The recombinant baculoviruses all contained alterations in the leader sequence or 5' proximal coding region of the polyhedrin gene. Highest levels of foreign proteins and polyhedrin-linked mRNAs were observed when portions of the coding sequence of the polyhedrin gene were fused in phase with the foreign gene. Recombinant viruses in which the foreign gene was inserted upstream from the polyhedrin ATG start codon expressed nonfused products but at lower levels than contructs which produced fusion proteins. A corresponding decrease in the levels of mRNAs produced by such constructs was also observed. Some constructs in which the foreign gene was inserted out of phase downstream from the polyhedrin start codon expressed nonfused protein products at low levels but produced polyhedrin-linked mRNA at levels comparable to vectors which produced protein fusions. These data suggest that reinitiation of translation can take place at AUG start codons a short distance downstream from the primary polyhedrin start codon. These results indicate that sequences immediately upstream from the polyhedrin start codon are important for regulation of transcription and that additional sequences near the AUG start codon can have a dramatic influence on the levels of translation observed.

Assembly-dependent maturation cleavage in provirions of a small icosahedral insect ribovirus

Extracts from nodavirus-infected Drosophila cells contained detergent-labile 140S "young" particles much richer than mature virions in their content of protein alpha, a precursor of coat proteins beta and gamma. Incorporation studies in infected cells showed that most newly synthesized alpha protein was assembled into young particles within a few minutes. Incubation of the particles, either in cytoplasmic extracts or after purification, resulted in spontaneous first-order cleavage of alpha protein to form beta-plus-gamma chains. Alpha protein that was not associated with particles failed to cleave. Cleavage was accompanied by a marked increase in detergent stability of the particles and was unaffected by a broad spectrum of protease inhibitors or by coating with precipitating antibody. We conclude (i) that alpha chains are cleaved only after assembly into provirions, (ii) that cleavage occurs internally and is likely therefore autocatalytic, and (iii) that cleavage stabilizes the mature virus particles.

Resistance of the 64K protein of budded Autographa californica nuclear polyhedrosis virus to functional inactivation by proteolysis

The 64K surface protein of budded Autographa californica nuclear polyhedrosis virus (AcMNPV BV) is known to play a role in the functional entry of AcMNPV BV into Spodoptera frugiperda IPLB-SF-21 cells by adsorptive endocytosis. AcV1, a neutralizing monoclonal antibody, reacts with the 64K protein and in doing so prevents efficient entry. In this communication we report that treatment of AcMNPV BV with either trypsin or proteinase K cleaves the 64K protein into one major fragment of 34.6K and two minor fragments of 36K to 37.2K that are retained with the virus. All of the fragments are glycosylated. Protease treatment does not reduce viral infectivity, but it does result in the destruction of the AcV1-reactive epitope; thus AcV1 is not able to neutralize protease-treated AcMNPV BV. Polyclonal antiserum to BV is able to recognize both cleaved and uncleaved 64K and neutralize both protease-treated and untreated virus. Protease treatment does not diminish the sensitivity of AcMNPV BV to chloroquine, but it does cause the virus to become more susceptible to inactivation by 2-mercaptoethanol (2-ME) even though exposure to 2-ME does not result in dissociation of the fragments from the virus.

Studies on polydnavirus transmission

Polydnaviruses are thought to replicate only in the ovaries of certain hymenopteran species. Nevertheless, in the present study, polydnaviral DNA was found to exist in males of the braconid parasitoid species Cotesia melanoscela and in both male and female non-ovarian tissue of an ichneumonid, Hyposoter fugitivus; preliminary results suggest that viral DNA may be present in an unintegrated form, but whether or not it is encapsidated is unknown. Using interstrain genetic crosses, we demonstrated that C. melanoscela males can apparently transmit at least some viral DNA to female progeny. We suggest that polydnavirus DNAs may be present in most if not all tissues of certain parasitoid species, and are probably maintained within parasitoid populations by vertical transmission through the germ line. In parallel experiments, manually injected eggs of the ichneumonid parasitoid (H. fugitivus) survived and hatched in Malacosoma americanum larvae in the apparent absence of exogenous polydnavirus; female parasitoids reared in this manner nevertheless carried virus in their ovaries. Experiments utilizing different strains of C. melanoscela also suggest that per os transmission of polydnaviruses (to parasitoid larvae) does not occur, despite the fact that inoculum viral DNA can be shown to persist for several days in the tissues of parasitized host larvae.

Changes in the nucleoprotein complexes of a baculovirus DNA during infection

The nature of the DNA-protein complexes assumed by Autographa californica nuclear polyhedrosis virus (AcNPV) DNA during infection of Spodoptera frugiperda cells was investigated by micrococcal nuclease digestion of infected nuclei. Both parental viral DNA and progeny viral DNA assumed a chromatin-like structure early in infection. By late times (24 hr) p.i., the viral DNA acquired a unique nucleoprotein structure. In addition to fragments of mononucleosome size (185 bp), two subnucleosomal bands of 120 and 90 bp were observed. The subnucleosomal bands contained exclusively viral DNA. No alteration in the nature of the host chromatin structure following AcNPV infection was observed. An examination of the basic chromatin-associated proteins revealed two major viral-induced proteins having molecular weights of 15K and 39K. The induction of the basic 15K protein between 10 and 24 hr p.i. coincided with the appearance of the altered nucleoprotein structure observed by 24 hr p.i. and the cessation of histone synthesis.

Studies on Autographa californica nuclear polyhedrosis virus replication in Spodoptera littoralis cells including virus-induced protein synthesis

The replication of the Autographa californica nuclear polyhedrosis virus in Spodoptera littoralis cells has been investigated. Various cytopathic changes were detected by light and electron microscopy and progeny enveloped virus particles, some occluded within polyhedra, were later seen in the nucleus of infected cells. Infectious virus was released into the growth medium and increased exponentially from ca. 10 to 24 hours post infection and then slowly increased over the next 4 days. In comparison, virus was released ca. 3.5 hours earlier from Spodoptera frugiperda cells. Total rates of DNA and protein synthesis were drastically reduced during the late stages of virus replication when cell death was occurring. By ca. 18 hours post infection, a clear switch from host to virus induced protein synthesis had occurred and a total of 39 virus-induced polypeptides of M.W. 12 to 120 X 10(3) were detected. These included polyhedrin of M.W. 33 X 10(3), which was particularly prominent during the late stages of virus replication, and a major virus structural protein of M.W. 42 X 10(3).

Baculovirus (MNPV) genomic variants: characterization of Spodoptera exempta MNPV DNAs and comparison with other Autographa californica MNPV DNAs

A nuclear polyhedrosis virus (NPV) strain from Spodoptera exempta (SeMNPV-25 baculovirus) is a restriction endonuclease DNA map variant similar to Autographa californica NPV (AcMNPV baculovirus). Fourteen restriction endonuclease variants were identified and isolated from a SeMNPV baculovirus stock with 12 of the variants found at low frequency (less than 3%). The DNA from each variant was compared to the prototype SeMNPV-25 for insertions, deletions and new restriction sites. Regions of variation were defined on the prototype SeMNPV-25 genome, and the nature of the variation within these regions was determined. These data are discussed and compared with the existing data on other variants of AcMNPV. A comparison of the physical maps revealed that all the SeMNPV variants were different from those reported for AcMNPV. Although the SeMNPV variants were distinctive, they were clearly genomic AcMNPV variants. The regions of the baculovirus genomic variation were identified, and three separate mechanisms are suggested for their generation. Five regions (hr1 to hr5) were associated with intragenic homologous viral sequences, five regions (vI to vIII) may be associated with the insertion of DNA sequences of cellular origin, and two regions (pI and pII) were associated with mutations resulting in the addition or loss of a PstI site. Physical maps were generated for SeMNPV variant regions vI, hr2, vII and vIII.

Isolation of large and small plaque variants of the Autographa californica nuclear polyhedrosis virus. Brief report

Variants of the Autographa californica nuclear polyhedrosis virus which produce large (1.4 mm) or small (0.5 mm) plaques on Spodoptera littoralis cells have been isolated. Yields of extracellular virus and polyhedra by the large plaque variant were six-fold higher and 140-fold lower, respectively, than those obtained with the small plaque variant. However the two variants could not be distinguished when Spodoptera frugiperda cells were used.

In vitro host range of five baculoviruses in lepidopteran cell lines

The in vitro host range of five nuclear polyhedrosis viruses (NPV) was assessed in five lepidopteran cell lines from three genera. Multiple-enveloped baculoviruses of Autographa californica (ACMNPV), Trichoplusia ni (TNMNPV), and Galleria mellonella (GMMNPV) replicated in cells of T. ni, Spodoptera frugiperda, and Heliothis virescens to a titer of approximately 10(7) TCID50/ml. The multiple-enveloped baculovirus of S. frugiperda (SFMNPV) replicated only in S. frugiperda cells. The single-enveloped baculovirus of H. zea (HZSNPV) replicated in cells of H. zea and H. virescens but not in cells of H. armigera, T. ni, or S. frugiperda. Low levels of replication of ACMNPV, TNMNPV, and SFMNPV in cultures of H.zea, H. virescens, and T. ni, respectively, could not be detected by using a sensitive tritiated thymidine technique. However, two characteristically labelled peaks at densities of 1.145 and 1.245 g/ml were obtained in H. virescens cells inoculated with ACMNPV.60 min postinoculation ACMNPV particles were observed both entering and inside S. frugiperda cells but were not observed in H. zea or H. armigera cells. None of the five baculoviruses replicated in H. armigera cells.

Infectivity assays for the Autographa californica nuclear polyhedrosis virus using Spodoptera littoralis cells

Tissue culture ID50 and plaque assays for the Autographa californica nuclear polyhedrosis virus, using the Spodoptera littoralis cell line HPB-SL, were developed. Direct comparison using these assay methods showed that these cells were as susceptible to infection as the more commonly used Spodoptera frugiperda cell line IPLB-SF-21. Both infectious tissue culture supernatants or virus isolated directly from polyhedra could be titrated. It was important to use low cell seeding densities in the assays so that clear centres of infection formed. Dose-response curves indicated that one infectious particle was capable of initiating an infection. Virus could be cloned using either method even though, for the plaque assay, plates had been stained. The tissue culture ID50 assay was performed using 96-well plates and required an incubation period of about 10 days. The plaque assay used a simple nutrient agarose overlay and an incubation period of 5-6 days. Easily countable plaques of 0.3-1.2 mm diameter were detected after staining with iodonitrote-trazolium chloride. The plaques comprised areas of inhibited cell division and round or dead cells. Most plaques contained only some cells with polyhedra and yields averaged about 1/cell. Occasionally plaques or infected wells were found in which no polyhedra could be seen. These infectivity assays are therefore not dependent on polyhedra formation.

Plaque assay for black beetle virus

A rapidly growing strain of virus was used to develop a reliable plaque assay for Black beetle virus on monolayers of cultured Drosophila cells. Cell density of the monolayer was critical for successful plaque formation. The dose-response curve for plaque formation was linear, supporting earlier proposals that both RNA segments of the split genome reside in the same particle. The method greatly facilitates isolation of reassortant and variant strains of virus.

Electron microscopic observation of a newly isolated flavivirus-like virus from field-caught mosquitoes

Of many unidentified virus strains which were isolated from field-caught mosquitoes by using C6/36 cells (a virus-sensitive clone of Aedes albopictus cells), three strains which formed small size plaques (SP virus) in C6/36 cells were investigated by electron microscopy. Although the SP virus strains did not react with antisera against known arboviruses in serological tests, they closely resembled flaviviruses in morphology. However, when they were compared to Japanese encephalitis (JE) virus, several differences in morphogenesis were observed. Proliferating membranous structures and electron-dense amorphous areas involving precursors of the virus were observed only in cells infected with the SP virus strains. Enlarged areas of endoplasmic reticulum containing mature virions were often observed adjacent to these structures. Since the SP virus strains were isolated from wild mosquitoes and multiplied only in mosquito cells, it seems appropriate to classify them as insect viruses which resemble togaviruses morphologically.

Replication and serial passage of a singly enveloped baculovirus of Orgyia leucostigma in homologous cell lines

A singly enveloped nuclear polyhedrosis virus (SNPV) of the white-marked tussock moth, Orgyia leucostigma, was successfully grown in four continuous cell lines developed from minced neonate larvae of this insect. Level of infection in two cell lines, IPRI-OL-12 and IPRI-OL-13, was 65-90%, but in the other two, IPRI-OL-4 and IPRI-OL-9, it was about 3%. Polyhedral inclusion bodies (PIBs) appeared in the nuclei of cells within 24 h postinoculation. Cytopathological changes and morphogenesis of the virus, as revealed by light and electron microscopy, were in general typical of an SNPV. However, some of the PIBs contained very few virions, and some were fractured. Rate zonal centrifugation of alkali-released occluded virions further confirmed the singly enveloped characteristics of the virus. The SNPV was serially passaged 60 times each in OL-12 and OL-13 cells. Percentage infected cells and PIB production stayed generally high throughout serial passaging in OL-13 cells, but declined sharply after the 41st passage in OL-12 cells. PIBs from the 4th passage of the virus in OL-12 cells were tested and found to be pathogenic to O. leucostigma larvae.

The process of virus assembly in insect virus mixed infections

Interference occurred upon infection of the cabbage moth caterpillars (Mamestra brassicae L.) with a mixture of nuclear polyhedrosis virus (NPV) and cytoplasmic polyhedrosis virus (CPV), resulting in the impairment of virus assembly, and formation of abnormal nucleocapsids. At the same time protein supercapsids were produced normally, but contained no infectious virions. When insects were infected with related viruses, the virions developed as usual, but the protein supercapsids revealed abnormal forms.

Persistent baculovirus infections: Spodoptera frugiperda NPV and Autographa californica NPV in Spodoptera frugiperda cells

Establishment of a persistent infection of Spodoptera frugiperda nuclear polyhedrosis virus (NPV) in Spodoptera frugiperda (S.f.) cells occurred in three phases: the first phase was characterised by high levels of cell infection and death, the second phase by decreasing cell infection levels leading to the final phase where less than one per cent of the cells were infected during any subculture. The virus persisted at this level of infection provided the cells were maintained by regular subculturing and incubated at the optimum growth temperature of 27 degrees C. Because of the low proportion of cells infected, cultures of virus-free cells could be selected ('cured') by dilution of the persistent infection without the use of viral antiserum. Unlike the parent S.f. cells, cultures of cured cells were partially resistant to infection with S. frugiperda NPV or infection with an unrelated baculovirus Autographa californica NPV. A. californica NPV, which is cytolytic for the parent S.f. cell line, established a persistent infection in the cured cells. The establishment pattern was similar to that previously found for S. frugiperda NPV and only one to five per cent of the cells were infected at equilibrium. Cured cells from the A. californica NPV persistent infection were highly resistant to infection with both S. frugiperda NPV and A. californica NPV. All attempts to find a viral interference phenomenon to explain the resistance of the cured cells were unsuccessful. All cell types adsorbed virus equally well. Slower growth of S.f. cells cured from the persistent A. californica NPV infection is the only difference so far observed between any of the S.f. cell types.

Morphogenesis of nuclear polyhedrosis virus from Autographa californica in a cell line from Mamestra brassicae (cabbage moth). Further aspects on baculovirus assembly

Cultures of cell line IZD-MB 0503 from Mamestra brassicae were inoculated with nonoccluded nuclear polyhedrosis virus of Autographa californica (AcNOV). At 1 h postinoculation (p.i.) nucleocapsids were found in the cytoplasm near nuclear pores and within the nucleoplasm. Formation of virogenic stroma was observed at 7 h p.i. The first short empty capsids were seen at 10 h postinoculation (p.i.), followed by partially and completely filled nucleocapsids (11-12 h p.i.) with most capsids filled at 12 h and later. This suggests that nucleocapsid components, such as capsid and DNA-core, assemble in successive stages rather than simultaneously. Viral progeny, being released from the cells by budding, were observed at 13 h p.i.

Coevolution of hosts and parasites

The coevolution of parasites and their hosts has both general biological interest and practical implications in agricultural, veterinary and medical fields. Surprisingly, most medical, parasitological and ecological texts dismiss the subject with unsupported statements to the effect that ‘successful’ parasite species evolve to be harmless to their hosts. Recently, however, several people have explored theoretical aspects of the population genetics of host-parasite associations; these authors conclude that such associations may be responsible for much of the genetic diversity found within natural populations, from blood group polymorphisms (Haldane, 1949) to protein polymorphisms in general (Clarke, 1975, 1976) and to histocompatibility systems (Duncan, Wakeland & Klein, 1980). It has also been argued that pathogens may constitute the selective force responsible for the evolution and maintenance of sexual reproduction in animal and plant species (Jaenike, 1978; Hamilton, 1980, 1981, 1982; Bremermann, 1980).

Factors affecting the frequency of infection by the sigma virus in experimental populations of Drosophila melanogaster

The experiments reported in this paper deal with the maintenance of the non contagious, hereditary virus sigma in populations of its host, Drosophila melanogaster. Evidence was previously provided of the existence of two viral Types I and II, depending on their sensitivity to the ref(2)Pp allele (the ref(2)P locus interferes with the multiplication of the virus in the fly). The viral Type I which is the most sensitive to the ref(2)Pp allele, is eliminated in the presence of this allele, even when most of the flies were originally infected in the population. On the contrary, the presence of the ref(2)Pp allele does not prevent a viral Type II, introduced in a population, from infecting most of the flies. The possibility that a change has occurred recently in French natural populations of Drosophila melanogaster is discussed.

Transmission efficiency of the sigma virus in natural populations of its host, Drosophila melanogaster

A study of the viral samples collected in French natural populations of Drosophila melanogaster since 1969, indicates that natural populations include, as expected, both stabilized and non stabilized infected individuals. In agreement with previous observations made on other characters of the virus, the viral samples collected appear to be homogeneous for the efficiency of the hereditary transmission. However, this efficiency is greater than the average value observed with virus perpetuated in infected laboratory fly strains. One sample collected in Gabon and three in the U.S.A. appear to differ from the French samples for one at least of the traits studied in these experiments.

Growth of nuclear polyhedrosis virus in larvae of the cabbage moth, Mamestra brassicae L

Growth of nuclear polyhedrosis virus (NPV) in 5 larval instars of cabbage moth, Mamestra brassicae, has been quantified using 2 methods. Numbers of polyhedra were estimated by light microscope counts while concentrations of virus protein antigen were estimated using ELISA. Virus growth was rapid initially but slowed during its later stages, although ELISA protein concentrations decreased once a peak had been reached. There was a linear correlation between polyhedral counts and virus protein during the initial growth phase. Maximum polyhedral production ranged from 2 x 107 (first instar) to 3.4 x 109 (fifth instar) and could be correlated directly to increasing larval weight. Using ELisa, virus antigen was detectable at least 24 hours before polyhedra were observed under the light microscope. Productivity ratios ranged from 83,500 in the first instar to 1352 in the fifth instar.

Biological and biochemical investigations on five European isolates of Mamestra brassica nuclear polyhedrosis virus

Five multiply enveloped European isolates of Mamestra brassicae nuclear polyhedrosis virus (Oxford, German, French, Dutch and Danish) were found to be very closely related serologically using the enzyme linked immunosorbent assay (ELISA) double antibody sandwich method and immunodiffusion. By SDS-polyacrylamide gel electrophoresis of viral proteins and restriction endonuclease analysis of DNA using seven enzymes there appeared to be two variants as the Oxford and German isolates were distinct from the other three. The German isolate was shown to be more susceptible to Nonidet P40 detergent treatment affecting some nucleocapsid structural polypeptides which also reduced antigenicity in gel immunodiffusion plates. In bioassays of polyhedra, the Dutch isolate showed a higher LD50 than the other viruses although this was not statistically significant.

Applied and molecular aspects of insect granulosis viruses

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