Pathogenesis of Autographa californica multiple nucleopolyhedrovirus in fifth-instar Anticarsia gemmatalis larvae

Fecal Transmission of Spodoptera frugiperda Multiple Nucleopolyhedrovirus (SfMNPV; Baculoviridae)

The production of viable nucleopolyhedrovirus in the feces of infected lepidopteran larvae represents a poorly understood route for virus transmission prior to host death. In the present study, we examined the presence of viable virus in the feces of fourth-instar Spodoptera frugiperda larvae infected with the Nicaraguan isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV-NIC). Feces production increased in samples taken at 2 to 6 days post-inoculation but was significantly lower in infected insects compared to controls. Second instars experienced 3.9 to 68.3% of polyhedrosis disease following consumption of feces collected at 2-5 days post-inoculation, which subsequently fell to 29.1% in the 6-day sample. Calibration of the insect bioassay using OB-spiked samples of feces indicated that the concentration of OBs varied between 5.4 × 102 and 4.4 × 105 OBs/100 mg of feces in infected fourth instars. Quantitative PCR analysis of fecal samples indicated the presence of 0 to 7629 copies/mg feces following amplification targeted at the polyhedrin gene. However, no correlation was detected between qPCR estimates of virus concentration and time of sample collection or the quantity of feces collected. The qPCR estimates were positively correlated with the prevalence of lethal infection observed in the insect bioassay, but the correlation was weak and several samples did not amplify. Calibration of the qPCR assay using OB-spiked samples of feces provided estimates that were 5- to 10-fold lower than the insect bioassay, indicating inhibition of the amplification reaction or loss of material during processing. In a greenhouse experiment, 2.5-48.3% of second-instar larvae acquired lethal infection following a 24 h period of feeding on maize plants on which fourth instar larvae had deposited their feces at 3 days and 4 days post-infection, respectively. These findings highlight the potential of OB-contaminated feces as a source of biologically significant quantities of inoculum for virus transmission prior to the death of infected insects and represent an additional contribution to the biological control of lepidopteran pests by these pathogens.

Midgut-based resistance to oral infection by a nucleopolyhedrovirus in the laboratory-selected strain of the smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae)

A strain of Adoxophyes honmai resistant to Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) was established from a field-collected colony by repeated selection. Fifth-instar larvae of this resistant strain (R-strain) had over 66 666-fold greater resistance in terms of 50 % lethal concentration values to oral infection of AdhoNPV than non-selected strain larvae (susceptible for AdhoNPV; S2-strain). In this study, the mechanism of resistance to AdhoNPV was determined in R-strain larvae. An assessment of viral genome replication in AdhoNPV-infected S2- and R-strain larvae by quantitative PCR showed no viral genome replication occurring in R-strain larvae. Transcription of AdhoNPV ie-1, vp39 and polyhedrin genes was also not detected in R-strain midgut cells. Besides, a fluorescent brightener had no effect on AdhoNPV infection in either S2- or R-strain. However, binding and fusion of occlusion-derived virus with R-strain were significantly lower than those of S2-strain. These findings suggest that R-strain Adoxophyeshonmai larvae possess a midgut-based resistance to oral infection by AdhoNPV in which midgut epithelial cells are infected less efficiently.

The silencing suppressor (NSs) protein of the plant virus Tomato spotted wilt virus enhances heterologous protein expression and baculovirus pathogenicity in cells and lepidopteran insects

In this work, we showed that cell death induced by a recombinant (vAcNSs) Autographa californica multiple nucleopolyhedrovirus (AcMNPV) expressing the silencing suppressor (NSs) protein of Tomato spotted wilt virus (TSWV) was enhanced on permissive and semipermissive cell lines. The expression of a heterologous gene (firefly luciferase) during co-infection of insect cells with vAcNSs and a second recombinant baculovirus (vAgppolhfluc) was shown to increase when compared to single vAgppolhfluc infections. Furthermore, the vAcNSs mean time-to-death values were significantly lower than those for wild-type AcMNPV on larvae of Spodoptera frugiperda and Anticarsia gemmatalis. These results showed that the TSWV-NSs protein could efficiently increase heterologous protein expression in insect cells as well as baculovirus pathogenicity and virulence, probably by suppressing the gene-silencing machinery in insects.

Inactivation of the budded virus of Autographa californica M nucleopolyhedrovirus by gloverin

Antimicrobial peptides are generated in insects exposed to pathogens for combating infection. Gloverin is a small cationic antibacterial protein whose expression is induced in the hemocytes and fat body cells of Trichoplusia ni larvae exposed to bacteria. The purpose of this study was to determine the role of gloverin during baculovirus infection. We found that gloverin expression is induced in T. ni systemically infected with the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV). Two gloverin genes were cloned using RNA isolated from the hemocytes of T. ni larvae that were systemically infected with AcMNPV budded virus (BV) and C-terminal 6x-His and V5 epitope tags were incorporated to facilitate gloverin isolation, detection and functional studies. The supernatants of Sf9 cells stably transfected with the two gloverin expression plasmids and affinity purified gloverin proteins reduced the quantity of infectious AcMNPV BV as measured in vitro by plaque assay with untransfected Sf9 cells. Nanomolar concentrations of affinity column purified gloverin protein caused calcein to be rapidly released from unilamellar vesicles comprised of phosphatidylglycerol, but not from vesicles made up of phosphatidylcholine, suggesting that gloverin interaction with membranes is rapid and affected by membrane charge. Both the BV inactivation and calcein release activities of gloverin increased with higher concentrations of gloverin. These results demonstrate that gloverin is an antiviral protein that interacts with vesicle membranes to cause the contents to be released.