In vivo production, stabilization, and infectivity of baculovirus preoccluded virions

Baculovirus IAP1 induces caspase-dependent apoptosis in insect cells

Baculoviruses encode inhibitors of apoptosis (IAPs), which are classified into five groups, IAP1-5, based on their sequence homology. Most of the baculovirus IAPs with anti-apoptotic functions belong to the IAP3 group, with certain exceptions. The functional roles of IAPs from other groups during virus infection have not been well established. We have previously shown that Hyphantria cunea multiple nucleopolyhedrovirus (HycuMNPV) encodes three iap genes, hycu-iap1, hycu-iap2 and hycu-iap3, and that only Hycu-IAP3 has anti-apoptotic activity against actinomycin D-induced apoptosis of Spodoptera frugiperda Sf9 cells. In the present study, we demonstrate that transient expression of Hycu-IAP1 is capable of inducing apoptosis and/or stimulating caspase-3-like protease activity in various lepidopteran and dipteran cell lines. Transient-expression assay analysis also demonstrates that not only Hycu-IAP1 but also IAP1s from Autographa californica MNPV, Bombyx mori NPV and Orgyia pseudotsugata MNPV (OpMNPV) are capable of inducing apoptosis, and that apoptosis induced by Hycu-IAP1 is precluded by the functional anti-apoptotic baculovirus protein Hycu-IAP3. In HycuMNPV-infected Spilosoma imparilis (SpIm) cells and OpMNPV-infected Ld652Y cells, caspase-3-like protease activity is markedly stimulated during the late stages of infection, and the caspase-3-like protease activity stimulated in HycuMNPV-infected SpIm cells is repressed by RNA interference-mediated silencing of hycu-iap1. In addition, initiator caspase Bm-Dronc, the B. mori homologue of Dronc, is cleaved upon transfection of BM-N cells with a plasmid expressing Hycu-IAP1. These results provide the first evidence that baculovirus IAP1s act to induce caspase-dependent apoptosis, possibly by replacing the cellular IAP1 that prevents Dronc activation.

Juvenile hormone analog technology: effects on larval cannibalism and the production of Spodoptera exigua (Lepidoptera: Noctuidae) nucleopolyhedrovirus

The production of a multiple nucleopolyhedrovirus (SeMNPV) of the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), has been markedly increased by using juvenile hormone analog (JHA) technology to generate a supernumerary sixth instar in the species. In the current study we compared the incidence of cannibalism in S. exigua fifth and sixth instars reared at low (two larvae per dish) and a high density (10 larvae per dish). The incidence of cannibalism was significantly higher in fifth instars compared with sixth instars and increased with rearing density on both instars. Infected larvae were more prone to become victims of cannibalism than healthy individuals in mixed groups comprising 50% healthy + 50% infected larvae in both instars reared at high density. Instar had a marked effect on occlusion body (OB) production because JHA-treated insects produced between 4.8- and 5.6-fold increase in OB production per dish compared with fifth instars at high and low densities, respectively. The insecticidal characteristics of OBs produced in JHA-treated insects, as indicated by LD50 values, were similar to those produced in untreated fourth or fifth instars. Because JHA technology did not increase the prevalence of cannibalism and had no adverse effect on the insecticidal properties of SeMNPV OBs, we conclude that the use of JHAs to generate a supernumerary instar is likely to be compatible with mass production systems that involve gregarious rearing of infected insects.

Trypsin cleavage of the baculovirus occlusion-derived virus attachment protein P74 is prerequisite in per os infection

Baculovirus occlusion-derived virions (ODVs) contain a number of infectivity factors essential for the initiation of infection in larval midgut cells. Deletion of any of these factors neutralizes infectivity by the per os route. We have observed that P74 of the group I alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is N-terminally cleaved when a soluble form of the protein was incubated with insect midgut tissues under alkaline conditions and that cleavage was prevented by soybean trypsin inhibitor (SBTI). Presently, biological assays were carried out that suggest SBTI inhibits and trypsin enhances baculovirus per os infectivity. We developed a method to rescue per os infectivity of a P74 null virus involving co-transfection of viral DNA with a plasmid that transiently expresses p74. We used this plasmid rescue method to functionally characterize P74. A series of site-directed mutants were generated at the N terminus to evaluate if trypsin cleavage sites were necessary for function. Mutagenesis of R195, R196 and R199 compromised per os infectivity and rendered P74 resistant to midgut trypsin.

Effect of light energy on alkali-released virions from Anagrapha falcifera nucleopolyhedrovirus

We compared the insecticidal activities of occluded and nonoccluded AfMNPV baculovirus obtained by dissolving the occlusion bodies (OB) with sodium carbonate. Droplet feeding and cotton leaf feeding bioassay techniques were used to determine the dose response against neonate Trichoplusia ni (Hübner) and loss of insecticidal activity when the virus was exposed to simulated sunlight from a xenon light source. Using droplet bioassays to determine a dose response, nonoccluded virus (NOV) was 20 times more active (LC(50) = 4.8 x 10(3) OB/ml, dissolved) than occluded virus (LC(50) = 9.6 x 10(4) OB/ml) when the samples remained wet. However, NOV lost activity when air dried before being tested by droplet (LC(50) > 1.0 x 10(6) OB/ml) or leaf feeding (LC(50) > 3.0 x 10(6) OB/ml) bioassays. Adding sucrose to NOV prevented the loss of insecticidal activity when samples were dried. The activity of NOV with 2% sucrose was similar to that of occluded virus samples, with or without sucrose, in both droplet feeding and leaf feeding assays. These results indicate that the OB protected the insecticidal activity of virions from the detrimental effects of drying. The OB also provided some protection from the detrimental effects of simulated sunlight (xenon) exposure. NOV samples exposed to xenon light had significantly greater loss of insecticidal activity than did similar samples of occluded virus. Without advancement in technologies, such as formulations, possible benefits of increased insecticidal activity from the use of nonoccluded virus is probably not sufficient to offset the rapid loss of activity due to drying or light exposure.