Construction and characterization of a recombinant invertebrate iridovirus

Characterization of an envelope protein 118L in invertebrate iridescent virus 6 (IIV6)

Invertebrate iridescent virus 6 (IIV6) is a nucleocytoplasmic insect virus and a member of the family Iridoviridae. The IIV6 genome consists of 212,482 bp of linear dsDNA with 215 non-overlapping and putative protein-encoding ORFs. The IIV6 118L ORF is conserved in all sequenced members of the Iridoviridae and encodes a 515 amino acid protein with three predicted transmembrane domains and several N-glycosylation/N-myristoylation sites. In this study, we characterized the 118L ORF by both deleting it from the viral genome and silencing its expression with dsRNA in infected insect cells. The homologous recombination method was used to replace 118L ORF with the green fluorescent protein (gfp) gene. Virus mutants in which the 118L gene sequence had been replaced with gfp were identified by fluorescence microscopy but could not be propagated separately from the wild-type virus in insect cells. Unsuccessful attempts to isolate the mutant virus with the 118L gene deletion suggested that the protein is essential for virus replication. To support this result, we used dsRNA to target the 118L gene and showed that treatment resulted in a 99% reduction in virus titer. Subsequently, we demonstrated that 118L-specific antibodies produced against the 118L protein expressed in the baculovirus vector system were able to neutralize the virus infection. All these results indicate that 118L is a viral envelope protein that is required for the initiation of virus replication.

Identification of the potential matrix protein of invertebrate iridescent virus 6 (IIV6)

Invertebrate iridescent virus 6 (IIV6) is a nucleocytoplasmic virus with a ∼212 kb linear dsDNA genome that encodes 215 putative open reading frames (ORFs). Proteomic analysis has revealed that the IIV6 virion consists of 54 virally encoded proteins. Interactions among the structural proteins were investigated using the yeast two-hybrid system, revealing that the protein of 415R ORF interacts reciprocally with the potential envelope protein 118L and the major capsid protein 274L. This result suggests that 415R might be a matrix protein that plays a role as a bridge between the capsid and the envelope proteins. To elucidate the function of 415R protein, we determined the localization of 415R in IIV6 structure and analyzed the properties of 415R-silenced IIV6. Specific antibodies produced against 415R protein were used to determine the location of the 415R protein in the virion structure. Both western blot hybridization and immunogold electron microscopy analyses showed that the 415R protein was found in virions treated with Triton X-100, which degrades the viral envelope. The 415R gene was silenced by the RNA interference (RNAi) technique. We used gene-specific dsRNA's to target 415R and showed that this treatment resulted in a significant drop in virus titer. Silencing 415R with dsRNA also reduced the transcription levels of other viral genes. These results provide important data on the role and location of IIV6 415R protein in the virion structure. Additionally, these results may also shed light on the identification of the homologs of 415R among the vertebrate iridoviruses.

IIV-6 Inhibits NF-κB Responses in Drosophila

The host immune response and virus-encoded immune evasion proteins pose constant, mutual selective pressure on each other. Virally encoded immune evasion proteins also indicate which host pathways must be inhibited to allow for viral replication. Here, we show that IIV-6 is capable of inhibiting the two Drosophila NF-κB signaling pathways, Imd and Toll. Antimicrobial peptide (AMP) gene induction downstream of either pathway is suppressed when cells infected with IIV-6 are also stimulated with Toll or Imd ligands. We find that cleavage of both Imd and Relish, as well as Relish nuclear translocation, three key points in Imd signal transduction, occur in IIV-6 infected cells, indicating that the mechanism of viral inhibition is farther downstream, at the level of Relish promoter binding or transcriptional activation. Additionally, flies co-infected with both IIV-6 and the Gram-negative bacterium, Erwinia carotovora carotovora, succumb to infection more rapidly than flies singly infected with either the virus or the bacterium. These findings demonstrate how pre-existing infections can have a dramatic and negative effect on secondary infections, and establish a Drosophila model to study confection susceptibility.

Extraction of Hemocytes from Drosophila melanogaster Larvae for Microbial Infection and Analysis

During the pathogenic infection of Drosophila melanogaster, hemocytes play an important role in the immune response throughout the infection. Thus, the goal of this protocol is to develop a method to visualize the pathogen invasion in a specific immune compartment of flies, namely hemocytes. Using the method presented here, up to 3 × 10⁶ live hemocytes can be obtained from 200 Drosophila 3^rd instar larvae in 30 min for ex vivo infection. Alternatively, hemocytes can be infected in vivo through injection of 3^rd instar larvae followed by hemocyte extraction up to 24 h post-infection. These infected primary cells were fixed, stained, and imaged using confocal microscopy. Then, 3D representations were generated from the images to definitively show pathogen invasion. Additionally, high-quality RNA for qRT-PCR can be obtained for the detection of pathogen mRNA following infection, and sufficient protein can be extracted from these cells for Western blot analysis. Taken together, we present a method for definite reconciliation of pathogen invasion and confirmation of infection using bacterial and viral pathogen types and an efficient method for hemocyte extraction to obtain enough live hemocytes from Drosophila larvae for ex vivo and in vivo infection experiments.

Invertebrate Iridoviruses: A Glance over the Last Decade

Members of the family Iridoviridae (iridovirids) are large dsDNA viruses that infect both invertebrate and vertebrate ectotherms and whose symptoms range in severity from minor reductions in host fitness to systemic disease and large-scale mortality. Several characteristics have been useful for classifying iridoviruses; however, novel strains are continuously being discovered and, in many cases, reliable classification has been challenging. Further impeding classification, invertebrate iridoviruses (IIVs) can occasionally infect vertebrates; thus, host range is often not a useful criterion for classification. In this review, we discuss the current classification of iridovirids, focusing on genomic and structural features that distinguish vertebrate and invertebrate iridovirids and viral factors linked to host interactions in IIV6 (Invertebrate iridescent virus 6). In addition, we show for the first time how complete genome sequences of viral isolates can be leveraged to improve classification of new iridovirid isolates and resolve ambiguous relations. Improved classification of the iridoviruses may facilitate the identification of genus-specific virulence factors linked with diverse host phenotypes and host interactions.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Chilo iridescent virus (CIV) ORF 012L encodes a protein with both exonuclease and endonuclease functions

Chilo iridescent virus (CIV) is the type member of the genus Iridovirus within the family Iridoviridae. The virions of CIV contain a single linear dsDNA molecule that is circularly permuted and terminally redundant. The genome of CIV contains an open reading frame (ORF 012L) encoding a protein homologous to exonuclease II of Schizosaccharomyces pombe. In this study, we focused on the characterization of CIV ORF 012L. The target ORF was cloned into the pET28a vector, expressed in E. coli strain BL21 (DE3) pLysS with an N-terminal His tag and purified to homogeneity by using Ni-NTA affinity chromatography. Biochemical characterization of the purified CIV 012L confirmed that this viral protein is a functional 5'-3' exonuclease that digests 3'-biotin-labelled oligonucleotides and linear double-stranded DNA (dsDNA) molecules from their 5' termini in a highly processive manner. CIV 012L also has a potent endonuclease activity on dsDNA in vitro. In addition, CIV 012L converted supercoiled plasmid DNA (replicative form I, RFI) into the open circular form (RFII) and then open circular form into linear form (RFIII). Endonuclease activity of CIV 012L was optimal in the presence of 10 mM Mg(2+) or 30 mM Mn(2+) ions and at 150 mM NaCl or KCl salt concentrations. The highest endonuclease activity was obtained at pH 8, and it reached a maximum at 55 °C. The CIV 012L protein showed deficiencies for both double- and single-stranded RNAs.

Rana grylio virus TK and DUT gene locus could be simultaneously used for foreign gene expression

Ranaviruses (family Iridoviridae, genus Ranavirus) have been recognized as emerging infectious pathogens and caused a great loss to the global biodiversity. Thymidine kinase (TK) and deoxyuridine triphosphatase (dUTPase, DUT, encoded by ORF 67R) are ubiquitous, existing in iridoviruses and other organisms. Previous studies showed that TK and DUT could be individually knocked out without impeding viral replication. In this study, we tried to insert two fluorescence genes into the above loci. We started with Δ67R-RGV, a recently generated recombinant Rana grylio virus (RGV) with the whole DUT replaced by enhanced green fluorescence protein (EGFP) gene. Then, a red fluorescence protein (RFP) gene initiated by RGV immediate-early (IE) ICP18 gene promoter was inserted into TK locus through homologous recombination. A novel recombinant virus, ΔDUT, TK-RGV, was generated by nine successive rounds of plaque isolation using RFP selection. All of the plaques produced by this recombinant virus could emit both green and red fluorescence. Furthermore, one-step and multiple-step growth curves of ΔDUT, TK-RGV were similar to those of wt-RGV and Δ67R-RGV. In conclusion, a novel dual-fluorescence labeled recombinant iridovirus in which DUT and TK gene locus were simultaneously used for foreign gene expression was constructed.