Identification of Spodoptera exigua nucleopolyhedrovirus genes involved in pathogenicity and virulence

Generation of Variability in Chrysodeixis includens Nucleopolyhedrovirus (ChinNPV): The Role of a Single Variant

The mechanisms generating variability in viruses are diverse. Variability allows baculoviruses to evolve with their host and with changes in their environment. We examined the role of one genetic variant of Chrysodeixis includens nucleopolyhedrovirus (ChinNPV) and its contribution to the variability of the virus under laboratory conditions. A mixture of natural isolates (ChinNPV-Mex1) contained two genetic variants that dominated over other variants in individual larvae that consumed high (ChinNPV-K) and low (ChinNPV-E) concentrations of inoculum. Studies on the ChinNPV-K variant indicated that it was capable of generating novel variation in a concentration-dependent manner. In cell culture, cells inoculated with high concentrations of ChinNPV-K produced OBs with the ChinNPV-K REN profile, whereas a high diversity of ChinNPV variants was recovered following plaque purification of low concentrations of ChinNPV-K virion inoculum. Interestingly, the ChinNPV-K variant could not be recovered from plaques derived from low concentration inocula originating from budded virions or occlusion-derived virions of ChinNPV-K. Genome sequencing revealed marked differences between ChinNPV-K and ChinNPV-E, with high variation in the ChinNPV-K genome, mostly due to single nucleotide polymorphisms. We conclude that ChinNPV-K is an unstable genetic variant that is responsible for generating much of the detected variability in the natural ChinNPV isolates used in this study.

Relocation of the attTn7 Transgene Insertion Site in Bacmid DNA Enhances Baculovirus Genome Stability and Recombinant Protein Expression in Insect Cells

Baculovirus expression vectors are successfully used for the commercial production of complex (glyco)proteins in eukaryotic cells. The genome engineering of single-copy baculovirus infectious clones (bacmids) in E. coli has been valuable in the study of baculovirus biology, but bacmids are not yet widely applied as expression vectors. An important limitation of first-generation bacmids for large-scale protein production is the rapid loss of gene of interest (GOI) expression. The instability is caused by the mini-F replicon in the bacmid backbone, which is non-essential for baculovirus replication in insect cells, and carries the adjacent GOI in between attTn7 transposition sites. In this paper, we test the hypothesis that relocation of the attTn7 transgene insertion site away from the mini-F replicon prevents deletion of the GOI, thereby resulting in higher and prolonged recombinant protein expression levels. We applied lambda red genome engineering combined with SacB counterselection to generate a series of bacmids with relocated attTn7 sites and tested their performance by comparing the relative expression levels of different GOIs. We conclude that GOI expression from the odv-e56 (pif-5) locus results in higher overall expression levels and is more stable over serial passages compared to the original bacmid. Finally, we evaluated this improved next-generation bacmid during a bioreactor scale-up of Sf9 insect cells in suspension to produce enveloped chikungunya virus-like particles as a model vaccine.

Inhibition of expression of BmNPV cg30 by bmo-miRNA-390 is a host response to baculovirus invasion

Bombyx mori larvae exhibit in vivo defensive reactions immediately after invasion by a virus. One of these defense systems is to express appropriate microRNAs (miRNAs) to respond to the infection. A novel Bombyx mori-encoded miRNA, bmo-miR-390, was identified previously by high-throughput sequencing. Based on bioinformatic predictions, the Bombyx mori nuclear polyhedrosis virus cg30 gene (BmNPV-cg30) is one of the target genes of bmo-miR-390. In this study, expression vectors with an enhanced green fluorescence protein (EGFP) or a luciferase (luc) reporter gene together with bm-miR-390 or the cg30 3' UTR were constructed and used to co-transfect BmN cells. Using a dual luciferase reporter (DLR) assay, we found that bmo-miR-390 significantly downregulates the expression of BmNPV-cg30 (P < 0.05) in vitro. Moreover, artificially synthesized bmo-miR-390 mimics enhanced the regulatory effect of bmo-miR-390, while an inhibitor eliminated the inhibitory effect. These results show for the first time that bmo-miR-390 can effectively downregulate the expression of BmNPV-cg30 in BmNPV-infected BmN cells.

Genome Analysis and Genetic Stability of the Cryptophlebia leucotreta Granulovirus (CrleGV-SA) after 15 Years of Commercial Use as a Biopesticide

Thaumatotibia leucotreta Meyrick (Lepidoptera: Tortricidae) is an indigenous pest in southern Africa which attacks citrus fruits and other crops. To control T. leucotreta in South Africa, an integrated pest management (IPM) programme incorporating the baculovirus Cryptophlebialeucotreta granulovirus (CrleGV-SA) as a biopesticide has been implemented. This study investigated the genetic stability of a commercially produced CrleGV-SA product that has been applied in the field since 2000. Seven representative full-genome sequences of the CrleGV-SA isolate spanning a 15-year period were generated and compared with one another. Several open reading frames (ORFs) were identified to have acquired single nucleotide polymorphisms (SNPs) during the 15-year period, with three patterns observed and referred to as "stable", "reversion", and "unstable switching". Three insertion events were also identified, two of which occurred within ORFs. Pairwise multiple alignments of these sequences showed an identity ranging from 99.98% to 99.99%. Concentration-response bioassays comparing samples of CrleGV-SA from 2000 and 2015 showed an increase in virulence toward neonate T. leucotreta larvae. The CrleGV-SA genome sequence generated from the 2015 sample was compared to the Cape Verde reference genome, CrleGV-CV3. Several fusion events were identified between ORFs within these genomes. These sequences shared 96.7% pairwise identity, confirming that CrleGV-SA is a genetically distinct isolate. The results of this study indicate that the genome of CrleGV-SA has remained stable over many years, with implications for its continued use as a biopesticide in the field. Furthermore, the study describes the first complete baculovirus genome to be sequenced with the MinION (Oxford Nanopore, Oxford, UK) platform and the first complete genome sequence of the South African CrleGV isolate.

In vitro production of Spodoptera exigua multiple nucleopolyhedrovirus with enhanced insecticidal activity using a genotypically defined virus inoculum

Defective virus accumulations during baculovirus passages in insect cell culture are impediments to large scale baculovirus production. A genotypically defined virus inoculum comprises of stable genotypes was proposed for production of a Thailand isolated SeMNPV in Se-UCR1 insect cells. Targeted genotypes were from wild-type SeMNPV containing naturally mixed genotypes. Plaque assays, PCR screening and XbaI restriction analysis were employed for genotype purification, genotype selection and genome analysis, respectively. A selective marker was pif2 encoded per os infection factor which predominantly deleted, along with the adjacent pif1, in defective viruses. A purified, genetically stable pif2+ (and pif1+) genotype, namely SeThpif2+, was the first tryout. SeThpif2+ occlusion bodies (OBs) possessed insecticidal activity but at lower level than the wild-type. When the SeThpif2+ was co-infected with another purified, genetically stable pif1- (and pif2-) genotype, SeThpif2-, at ratio of 3:1, respectively, mixed genotypes OBs had 2.8 times greater insecticidal activity than the SeThpif2+ alone. Dilution of deleterious PIF1 of SeThpif2+ by the pif1 deletion genotypes, SeThpif2-, was the key for this enhanced activity. A promising approach was described for SeMNPV production in vitro using the virus inoculum whose genotypes compositions were designed to mimic virus interactions in the wild-type, to generate per oral infective baculovirus.

Genetic and Biological Characterization of Four Nucleopolyhedrovirus Isolates Collected in Mexico for the Control of Spodoptera exigua (Lepidoptera: Noctuidae)

This study describes four multiple nucleocapsid nucleopolyhedrovirus isolates recovered from infected larvae of beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on crops in two different geographical regions of Mexico. Molecular and biological characterization was compared with characterized S. exigua multiple nucleopolyhedrovirus (SeMNPV) isolates from the United States (SeUS1 and SeUS2) and Spain (SeSP2). Restriction endonuclease analysis of viral DNA confirmed that all Mexican isolates were SeMNPV isolates, but molecular differences between the Mexican and the reference isolates were detected using PCR combined with restriction fragment length polymorphism (RFLP). Amplification of the variable region V01 combined with RFLP distinguished the two Mexican isolates, SeSLP6 and SeSIN6. BglII digestions showed that the majority of the isolates contained submolar bands, indicating the presence of genetic heterogeneity. Amplification of the variable regions V04 and V05 distinguished between American and the Spanish isolates. Biological characterization was performed against two laboratory colonies of S. exigua, one from Mexico, and another from Switzerland. Insects from the Mexican colony were less susceptible to infection than insects from Se-Swiss colony. In the Se-Mex colony, SeSP2 was the most pathogenic isolate followed by SeSIN6, although their virulence was similar to most of the isolates tested. In Se-Swiss colony, similar LD50 values were observed for the five isolates, although the virulence was higher for the SeSLP6 isolate, which also had the highest OB (occlusion body) yield. We conclude that the Mexican isolates SeSIN6 and SeSLP6 possess insecticidal traits of value for the development of biopesticides for the control of populations of S. exigua.

Covert Infection of Insects by Baculoviruses

Baculoviruses (Baculoviridae) are occluded DNA viruses that are lethal pathogens of the larval stages of some lepidopterans, mosquitoes, and sawflies (phytophagous Hymenoptera). These viruses have been developed as biological insecticides for control of insect pests and as expression vectors in biotechnological applications. Natural and laboratory populations frequently harbor covert infections by baculoviruses, often at a prevalence exceeding 50%. Covert infection can comprise either non-productive latency or sublethal infection involving low level production of virus progeny. Latency in cell culture systems involves the expression of a small subset of viral genes. In contrast, covert infection in lepidopterans is associated with differential infection of cell types, modulation of virus gene expression and avoidance of immune system clearance. The molecular basis for covert infection may reside in the regulation of host-virus interactions through the action of microRNAs (miRNA). Initial findings suggest that insect nudiviruses and vertebrate herpesviruses may provide useful analogous models for exploring the mechanisms of covert infection by baculoviruses. These pathogens adopt mixed-mode transmission strategies that depend on the relative fitness gains that accrue through vertical and horizontal transmission. This facilitates virus persistence when opportunities for horizontal transmission are limited and ensures virus dispersal in migratory host species. However, when host survival is threatened by environmental or physiological stressors, latent or persistent infections can be activated to produce lethal disease, followed by horizontal transmission. Covert infection has also been implicated in population level effects on host-pathogen dynamics due to the reduced reproductive capacity of infected females. We conclude that covert infections provide many opportunities to examine the complexity of insect-virus pathosystems at the organismal level and to explore the evolutionary and ecological relationships of these pathogens with major crop and forest pests.

Nucleopolyhedrovirus infection and/or parasitism by Microplitis pallidipes Szepligeti affect hemocyte apoptosis of Spodoptera exigua (Hübner) larvae

We determined the effects of parasitism by the endoparasitoid Microplitis pallidipes Szepligeti and/or nucleopolyhedrovirus (NPV) infection on hemocyte apoptosis of Spodoptera exigua (Hübner) larvae. Compared to healthy (control) larvae, larvae that were parasitized, virus-infected, or both all showed a significant increase in hemocyte apoptosis during 48-h observation period. The peaks of hemocyte apoptosis in parasitized, virus-infected and parasitized+infected larvae were at 12, 24 and 48 h after treatment, and were 86.7±1.9, 87.4±3.6 and 76.5±1.6%, respectively. Meanwhile, compared to parasitized larvae, hemocyte apoptosis in jointly parasitized and infected larvae increased by 12.9%, 18.7% and 2.8% at 8, 36 and 48 h respectively, and decreased by 39.0% and 9.1% at 12 and 24h. Compared to virus-infected larvae, hemocyte apoptosis in jointly parasitized and infected larvae increased by 13.4%, 2.4% and 15.3% at 8, 36 and 48 h, respectively, and decreased by 4.0% and 29.9% at 12 and 24h. Our study found that joint and separate parasitism and SeNPV infection induced hemocyte apoptosis of S. exigua larvae. It also revealed that NPV infection promoted host hemocyte apoptosis induced by parasitism at early egg and larval stages of M. pallidipes in host larvae, but inhibited the same effect at late egg stage of M. pallidipes in host larvae, and that parasitism promoted host hemocyte apoptosis induced by NPV infection at early egg and larval stages of M. pallidipes in host larvae, but inhibited the same effect at late egg stage of M. pallidipes in host larvae.