Cytoplasmic polyhedrosis virus classification by electropherotype; validation by serological analyses and agarose gel electrophoresis

Ovine and murine T cell epitopes from the non-structural protein 1 (NS1) of bluetongue virus serotype 8 (BTV-8) are shared among viral serotypes

Bluetongue virus (BTV) is a non-enveloped dsRNA virus that causes a haemorrhagic disease mainly in sheep. It is an economically important Orbivirus of the Reoviridae family. In order to estimate the importance of T cell responses during BTV infection, it is essential to identify the epitopes targeted by the immune system. In the present work, we selected potential T cell epitopes (3 MHC-class II-binding and 8 MHC-class I binding peptides) for the C57BL/6 mouse strain from the BTV-8 non-structural protein NS1, using H2^b-binding predictive algorithms. Peptide binding assays confirmed all MHC-class I predicted peptides bound MHC-class I molecules. The immunogenicity of these 11 predicted peptides was then determined using splenocytes from BTV-8-inoculated C57BL/6 mice. Four MHC-class I binding peptides elicited specific IFN-γ production and generated cytotoxic T lymphocytes (CTL) in BTV-8 infected mice. CTL specific for 2 of these peptides were also able to recognise target cells infected with different BTV serotypes. Similarly, using a combination of IFN-γ ELISPOT, intracellular cytokine staining and proliferation assays, two MHC-class II peptides were identified as CD4⁺ T cell epitopes in BTV-8 infected mice. Importantly, two peptides were also consistently immunogenic in sheep infected with BTV-8 using IFN-γ ELISPOT assays. Both of these peptides stimulated CD4⁺ T cells that cross-reacted with other BTV serotypes. The characterisation of these T cell epitopes can help develop vaccines protecting against a broad spectrum of BTV serotypes and differentiate infected from vaccinated animals.

Characterization of the complete genome segments from BmCPV-SZ, a novelBombyx moricypovirus 1 isolate

A novel Bombyx mori cypovirus 1 isolated from infected silkworm larvae and tentatively assigned as Bombyx mori cypovirus 1 isolate Suzhou (BmCPV-SZ). The complete nucleotide sequences of genomic segments S1–S10 from BmCPV-SZ were determined. All segments possessed a single open reading frame; however, bioinformatic evidence suggested a short overlapping coding sequence in S1. Each BmCPV-SZ segment possessed the conserved terminal sequences AGUAA and GUUAGCC at the 5′ and 3′ ends, respectively. The conserved A/G at the –3 position in relation to the AUG codon could be found in the BmCPV-SZ genome, and it was postulated that this conserved A/G may be the most important nucleotide for efficient translation initiation in cypoviruses (CPVs). Examination of the putative amino acid sequences encoded by BmCPV-SZ revealed some characteristic motifs. Homology searches showed that viral structural proteins VP1, VP3, and VP4 had localized homologies with proteins of Rice ragged stunt virus , a member of the genus Oryzavirus within the family Reoviridae. A phylogenetic tree based on RNA-dependent RNA polymerase sequences demonstrated that CPV is more closely related to Rice ragged stunt virus and Aedes pseudoscutellaris reovirus than to other members of Reoviridae, suggesting that they may have originated from common ancestors.

The evolution of two homologues of the core protein VP6 of epizootic haemorrhagic disease virus (EHDV), which correspond to the geographical origin of the virus

Epizootic haemorrhagic disease virus is a 10-segmented, double-stranded RNA virus. When these ten segments of dsRNA are run on 1% agarose, eastern (Australia, Japan) and western (North America, Africa, Middle-East) strains of the virus can be separated phenotypically based on the migration of genome segments 7-9. In western strains, segments 7-9 are roughly the same size and co-migrate as a single RNA band. In eastern strains, segment 9 is smaller, so while segments 7 and 8 co-migrate, the segment 9 RNA runs faster than its western homologue. Translation experiments demonstrated that these two segment 9 homologues are both functional and produce proteins (VP6) of different sizes-something that has not been reported in any other orbivirus species to date. Sequence analysis suggests that eastern and western versions of segment 9 (VP6) have likely evolved as a response to adaptive selection in different geographical regions via gene duplication and subsequent mutation. These significant findings are considered unusual given the conserved nature of VP6 and its presumed role as the viral helicase. It is not currently known what the biological relevance of each homologue is to the virus.

Genetic and phylogenetic analysis of the core proteins VP1, VP3, VP4, VP6 and VP7 of epizootic haemorrhagic disease virus (EHDV)

The core proteins of epizootic haemorrhagic disease virus (EHDV) have important roles to perform in maintaining the structure and function of the virus. A complete genetic and phylogenetic analysis was therefore performed on these proteins (and the genes that code for them) to allow comparison of the selective pressures acting on each. Accession numbers, gene and protein sizes, ORF positions, G+C contents, terminal hexanucleotides, start and stop codons and phylogenetic relationships are all presented. The inner core proteins (VP1, VP3, VP4 and VP6) were characterised by high levels of sequence conservation, and the ability to topotype isolates very strongly into eastern or western groups. This is particularly evident in genome segment 9 (VP6) which exists as two different sized homologues. VP7 did not topotype, but rather exhibited a more random, radial phylogeny suggestive of genetic drift. With the exception of VP6, all of the core proteins also showed high numbers of synonymous mutations in the third base position, suggesting they have been evolving for a long period of time. Interestingly, VP6 did not show this, and possible reasons for this are discussed.

Characterisation and partial sequence analysis of two novel cypoviruses isolated from the winter moth Operophtera brumata (Lepidoptera: Geometridae)

The complete nucleotide sequences of genomic segments S5 to S10 from Operophtera brumata cypovirus 18 (OpbuCPV18), and the complete nucleotide sequences of genomic segments S2, S5, S9 and S10 from Operophtera brumata cypovirus 19 (OpbuCPV19) have been determined. Each genome segment contained a single open reading frame (ORF). Conserved motifs 5' (AGUAAA....GUUAGCU) 3' were found at the ends of each segment of OpbuCPV18, whilst conserved motifs 5' (AACAAA....UUUGC) 3' were found at each segment terminus of OpbuCPV19. The putative proteins were compared with those of other members of the Reoviridae family. Phylogenetic analysis using the polyhedrin gene (S10) indicated that OpbuCPV18 was most closely related to Dendrolimus punctatus cypovirus 1, whilst OpbuCPV19 was most closely related to Trichoplusia ni cypovirus 15. In addition, analysis of S2, which encoded a putative RNA-dependant RNA polymerase gene, confirmed OpbuCPV19 belonged to the genus Cypovirus. Following the expression of the ORF from OpbuCPV19 S10, using a baculovirus expression vector, occlusion bodies were observed in insect cell culture. This demonstrated that segment 10 coded for the polyhedrin gene, capable of forming a polyhedral crystalline matrix.

Biological and molecular studies of a cypovirus from the black fly Simulium ubiquitum (Diptera: Simuliidae)

A cypovirus from the black fly Simulium ubiquitum (SuCPV) was isolated and examined using biological and molecular techniques. SuCPV produces small (typically 0.25mum), polyhedral shaped inclusion bodies (polyhedra), in which the virus particles become multiply embedded. SuCPV is the third cypovirus isolated from Diptera, but the first from Simuliidae that has been characterized using molecular analyses. SuCPV has a genome composed of 10 segments of dsRNA, with an electrophoretic migration pattern that is different from those of recent UsCPV-17 and CrCPV-17 isolates from the mosquitoes Uranotaenia sapphirina and Culex restuans, respectively. The SuCPV electropherotype appears to show significant differences from those of the previously characterized lepidopteran cypoviruses. Sequence analysis of SuCPV segment 10 shows that it is unrelated to either of the two CPV isolates from Diptera or to the CPV species for which Seg-10 has been previously characterized from Lepidoptera. A comparison of the terminal regions of SuCPV genome segments to those of CPV-1, 2, 4, 5 14, 15, 16, 17, 18, and 19 also revealed only low levels of conservation. We therefore, propose that SuCPV is classified within a new Cypovirus species, which we have tentatively identified as Cypovirus-20. We have therefore referred to this virus isolate as S. ubiquitum CPV-20 (SuCPV-20).

Phylogenetic analysis of Heliothis armigera cytoplasmic polyhedrosis virus type 14 and a series of dwarf segments found in the genome

Full-length nucleotide sequences for the genome segments (S1-S6) of Heliothis armigera cytoplasmic polyhedrosis virus type 14 (HaCPV-14) have been characterized. Each segment consists of a single open reading frame with conserved motifs AGAA and AGCU at the 5' and 3' ends, respectively. Comparison of the proteins of HaCPV-14 with those of other members of the family Reoviridae suggests that S1 encodes an RNA-dependent RNA polymerase (RdRp), whilst S2 encodes a major capsid protein of the virus. Phylogenetic analysis of RdRps from 16 viruses in the family Reoviridae reveals that the genera Cypovirus and Oryzavirus may have originated from a common insect virus ancestor. A series of viable dwarf segments originating from S5 of HaCPV-14 has been identified. Analysis of the predicted secondary structures for these dwarf segments suggests that the signals essential for replication and packaging are located within the terminal sequences of these segments.

Identification and genome characterization of Heliothis armigera cypovirus types 5 and 14 and Heliothis assulta cypovirus type 14

Genomic characterization of Heliothis armigera cypovirus (HaCPV) isolated from China showed that insects were co-infected with several cypoviruses (CPVs). One of the CPVs (HaCPV-5) could be separated from the others by changing the rearing conditions of the Heliothis armigera larvae. This finding was further confirmed by nucleotide sequencing analysis. Genomic sequences of segments S10-S7 from HaCPV-14, S10 and S7 from HaCPV-5, and S10 from Heliothis assulta CPV-14 were compared. Results from database searches showed that the nucleotide sequences and deduced amino acid sequences of the newly identified CPVs had high levels of identity with those of reported CPVs of the same type, but not with CPVs of different types. Putative amino acid sequences of HaCPV-5 S7 were similar to that of the protein from Rice ragged stunt virus (genus Oryzavirus, family Reoviridae), suggesting that CPVs and oryzaviruses are related more closely than other genera of the family Reoviridae. Conserved motifs were also identified at the ends of each RNA segment of the same virus type: type 14, 5'-AGAAUUU...CAGCU-3'; and type 5, 5'-AGUU...UUGC-3'. Our results are consistent with classification of CPV types based on the electrophoretic patterns of CPV double-stranded RNA.

Detection and characterisation of three novel species of reovirus (Reoviridae), isolated from geographically separate populations of the winter moth Operophtera brumata (Lepidoptera: Geometridae) on Orkney

Geographically separate populations of winter moth (Operophtera brumata L.) were sampled in heather habitats on the Orkney Isles in order to investigate the prevalence of virus pathogens. Reoviruses were isolated in 11 of the 13 winter moth populations sampled, with 3 novel species being detected. Two species of Cypoviridae (CPV) were isolated, Operophtera brumata CPV18 and O. brumata CPV19, with one host population suffering 46% infection prevalence of OpbuCPV19. A third virus, O. brumata Reovirus (OpbuRV), was isolated from both winter moth and a hymenopteran parasitoid wasp, Phobocampe tempestiva, which is abundant in these populations. This was identified as a non-occluded reovirus, which was clearly able to infect and persist in both the lepidopteran and the hymenopteran host. The genomes of the three viruses were characterised using gel electrophoresis and the virus structure was investigated using transmission electron microscopy. The relationship of these viruses with a baculovirus that also infects winter moth, OpbuNPV, was investigated, as well as the association of OpbuRV with P. tempestiva. The detection of such viruses is discussed with reference to studies of similar viruses in other lepidopteran and hymenopteran host systems.

Molecular and biological characterization of a Cypovirus from the mosquito Culex restuans

A cypovirus from the mosquito Culex restuans (named CrCPV) was isolated and its biology, morphology, and molecular characteristics were investigated. CrCPV is characterized by small (0.1-1.0 microm), irregularly shaped inclusion bodies that are multiply embedded. Laboratory studies demonstrated that divalent cations influenced transmission of CrCPV to Culex quinquefasciatus larvae; magnesium enhanced CrCPV transmission by approximately 30% while calcium inhibited transmission. CrCPV is the second cypovirus from a mosquito that has been confirmed by using molecular analysis. CrCPV has a genome composed of 10 dsRNA segments with an electropherotype similar to the recently discovered UsCPV-17 from the mosquito Uranotaenia sapphirina, but distinct from the lepidopteran cypoviruses BmCPV-1 (Bombyx mori) and TnCPV-15 (Trichoplusia ni). Nucleotide and deduced amino acid sequence analysis of CrCPV segment 10 (polyhedrin) suggests that CrCPV is closely related (83% nucleotide sequence identity and 87% amino acid sequence identity) to the newly characterized UsCPV-17 but is unrelated to the 16 remaining CPV species from lepidopteran hosts. A comparison of the terminal segment regions of CrCPV and UsCPV-17, an additional method for differentiating various Cypovirus species, revealed a high level of conservation. Therefore, we propose that CrCPV is a member of the Cypovirus-17 group and designate this species as CrCPV-17.

Characterization of the RNA-binding domain in the Dendrolimus punctatus cytoplasmic polyhedrosis virus nonstructural protein p44

Dendrolimus punctatus cytoplasmic polyhedrosis virus (DpCPV-1) belongs to the Cypovirus genus in the Reoviridae family. The ORF of genome segment 8 (S8) of DpCPV-1 was cloned into vector pMAL-c2X and used to express a 44kDa protein (p44) in E. coli, which was detected by Western blotting. The gel mobility shift assays showed that p44 had ssRNA-binding activity. Competitive assay indicated that this protein only bind to ssRNA and could not interact with DNA and dsRNA. The binding of p44 to ssRNA is sequence non-specific. To identify the domain(s) important for RNA binding of the protein, a number of deletions were made. These truncated proteins were expressed in E. coli and purified. The affinity of each truncated protein towards ssRNA was then assayed by electrophoretic mobility shift assays and northwestern blot. The results indicated that glutamic acid-rich domain in the central region of p44 from residues 104 to 201 was the ssRNA-binding domain.

Morphological and molecular characterization of a Cypovirus (Reoviridae) from the mosquito Uranotaenia sapphirina (Diptera: Culicidae)

A novel cypovirus has been isolated from the mosquito Uranotaenia sapphirina (UsCPV) and shown to cause a chronic infection confined to the cytoplasm of epithelial cells of the gastric ceca and posterior stomach. The production of large numbers of virions and inclusion bodies and their arrangement into paracrystalline arrays gives the gut of infected insects a distinctive blue iridescence. The virions, which were examined by electron microscopy, are icosahedral (55 to 65 nm in diameter) with a central core that is surrounded by a single capsid layer. They are usually packaged individually within cubic inclusion bodies (polyhedra, approximately 100 nm across), although two to eight virus particles were sometimes occluded together. The virus was experimentally transmitted per os to several mosquito species. The transmission rate was enhanced by the presence of magnesium ions but was inhibited by calcium ions. Most of the infected larvae survived to adulthood, and the adults retained the infection. Electrophoretic analysis of the UsCPV genome segments (using 1% agarose gels) generated a migration pattern (electropherotype) that is different from those of the 16 Cypovirus species already recognized. UsCPV genome segment 10 (Seg-10) showed no significant nucleotide sequence similarity to the corresponding segment of the other cypoviruses that have previously been analyzed, and it has different "conserved" termini. A BLAST search of the UsCPV deduced amino acid sequence also showed little similarity to Antheraea mylitta CPV-4 (67 of 290 [23%]) or Choristoneura fumiferana CPV-16 (33 of 111 [29%]). We conclude that UsCPV should be recognized as a member of a new Cypovirus species (Cypovirus 17, strain UsCPV-17).

Overt viral diseases induced from apparent latency following parasitization by the ichneumonid wasp, Hyposoter exiguae

Latent and/or asymptomatic viral infections are commonplace in insects, but factors inducing overt disease are poorly understood. Here we show that in Trichoplusia ni larvae parasitized by the ichneumonid wasp, Hyposoter exiguae, overt, lethal disease may on occasion be observed; however, disease has been consistently absent in control (non-parasitized) larvae. Thus far, we have detected two such diseases, one of which is caused by a granulosis virus affecting primarily fat body tissue. The other is associated with the presence of two viruses replicating together in larval midgut epithelial cells; of these, one has been identified as a non-occluded form of TnCPV. Since H. exiguae carries a polydnavirus, which is delivered to host larvae during oviposition, it is tempting to speculate that viral latency may in some cases be broken through immunosuppressive activity resulting from insect parasitism.

Molecular characterization of Bombyx mori cytoplasmic polyhedrosis virus genome segment 4

The complete nucleotide sequence of the genome segment 4 (S4) of Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) was determined. The 3,259-nucleotide sequence contains a single long open reading frame which spans nucleotides 14 to 3187 and which is predicted to encode a protein with a molecular mass of about 130 kDa. Western blot analysis showed that S4 encodes BmCPV protein VP3, which is one of the outer components of the BmCPV virion. Sequence analysis of the deduced amino acid sequence of BmCPV VP3 revealed possible sequence homology with proteins from rice ragged stunt virus (RRSV) S2, Nilaparvata lugens reovirus S4, and Fiji disease fijivirus S4. This may suggest that plant reoviruses originated from insect viruses and that RRSV emerged more recently than other plant reoviruses. A chimeric protein consisting of BmCPV VP3 and green fluorescent protein (GFP) was constructed and expressed with BmCPV polyhedrin using a baculovirus expression vector. The VP3-GFP chimera was incorporated into BmCPV polyhedra and released under alkaline conditions. The results indicate that specific interactions occur between BmCPV polyhedrin and VP3 which might facilitate BmCPV virion occlusion into the polyhedra.

Lipofectin increases the specific activity of cypovirus particles for cultured insect cells

Cytoplasmic polyhedrosis viruses (CPV) are classified as 14 distinct species (electropherotypes) within the genus Cypovirus, family Reoviridae. Cypovirus research has been limited by a lack of appropriate cell culture systems (for each of these virus species) in which the majority of cells can become productively infected. Lipofection increased the infection rate of Lymantria dispar 652 cells, by virus particles (derived from polyhedra) of Orgyia pseudosugata type 5 cypovirus (Op-5 CPV), from 3 to 44%. Lipofection also significantly increased the percentage of Trichoplusia ni 368 cells infected with the same virus (from < 1 to approximately 7%). The spread of cypovirus infection between cells was either very slow or insignificant, and infected cells appeared to remain viable for long periods. Virus infection was detected by the observation of polyhedra formation in individual cells and it was therefore possible to develop a simple quantitative assay system to measure virus titre (TCID50). Cryo-electron microscopy showed that cypovirus particles formed a complex with the lipid, involving their envelopment within the liposome membrane. It was concluded that the increased infectivity of the virus by lipofection was due to a more efficient cell entry mechanism, probably involving fusion between liposome and cell membranes.

Sequence analysis and expression of the polyhedrin gene of Choristoneura fumiferana cytoplasmic polyhedrosis virus (CfCPV)

The segmented double-stranded RNA genome of Choristoneura fumiferana cytoplasmic polyhedrosis virus (CfCPV) was extracted, polyadenylated, reverse-transcribed into cDNA and cloned. The cDNA clones that hybridized to the smallest genomic segment (segment 10) were identified, and its nucleotide sequence was determined. Genome segment 10 of CfCPV was found to be 1171 nucleotides in length with a single open reading frame in one strand capable of coding a predicted protein of 258 residues (Mr of 29,795), consistent with an apparent Mr of 30.5 kDa determined by SDS-PAGE of purified polyhedrin. Comparison of the nucleotide and amino acid sequences of the polyhedrin gene of CfCPV with those of other CPVs and with several nuclear polyhedrosis viruses revealed no particular homology. Analysis of the hydrophilic profiles and predicted secondary structures of Bombyx mori (BmCPV), Euxoa scandens (EsCPV) and CfCPV indicated the presence of seven similar regions located at the amino terminus of the polyhedrin polypeptide of the three viruses. The expression of the cloned CfCPV polyhedrin gene in Escherichia coli demonstrated that this polyhedrin has the property of self-assembly, since the production of crystal-like occlusion with a well-defined crystalline lattice structure was observed.

The Reoviridae family

The members of the Reoviridae family are extremely varied in host ranges and have such diverse natural histories that it is compelling to conclude that their structural asset and replication strategy are uniquely successful in evolutionary terms. It follows that their study addresses fundamental aspects of virology, besides the ones which are customary with important pathogens affecting humans, animals and plants. We deal here with the present taxonomy of the family Reoviridae and of its genera, two of which (Orbivirus and Rotavirus) are treated separately in this issue. Along with a cursory presentation of the remaining genera, we give a concise update of recent findings on the genus Orthoreovirus.