Apoptosis as an antiviral defense in larval Lepidoptera

Insects face a wide array of viral pathogens in their natural environments, yet very little is known about how insects are able to defend themselves against viruses. In this study, two different anti-viral defense mechanisms are described from the lepidopteran noctuid species Spodoptera frugiperda (Fall Armyworm). Larvae of S. frugiperda infected with the baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV) show a low level of resistance to infection relative to the highly susceptible species Trichoplusia ni (Cabbage Looper). Approximately 1000-fold higher doses of injected virus were required to initiate infection in S. frugiperda larvae than in T. ni and infected S. frugiperda larvae survived twice as long as T. ni larvae. S. frugiperda exhibited a slower buildup of virus in the haemolymph and a more gradual increase in the proportion of infected haemocytes.; Mutants of AcMNPV lacking the anti-apoptosic gene p35 have been shown to induce apoptosis in cell lines derived from S. frugiperda, but not in cell lines from T. ni. Such mutants were also shown to be attenuated in S. frugiperda larvae, but not in T. ni larvae, suggesting that apoptosis may be an effective defense against baculovirus infection. This hypothesis was tested by examining the pathology of AcMNPV infections using enhanced green fluorescent protein (eGFP)-tagged recombinant of AcMNPV either containing or lacking the p35 gene. Even though S. frugiperda larvae were injected with doses of p35 mutant virus approximately 1000-fold higher than the dose required for 95% lethality by wild-type AcMNPV, only about 30% of the injected insects developed any detectable eGFP expression, and only around 10% developed disseminated infections and significant levels of virus and infected haemocytes in the haemolymph. Numerous apoptotic cells were observed in the epidermis and fat body of strongly infected larvae, and the fat bodies of these insects underwent extensive damage that appeared to be due to widespread apoptosis. These results demonstrate that apoptosis can serve as an effective anti-viral defense in insects and suggest how apoptosis may combine with the low level of background resistance to AcMNPV observed in S. frugiperda to protect these caterpillars from viral infection.