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Potent virucidal activity in larval Heliothis virescens plasma against Helicoverpa zea single capsid nucleopolyhedrovirus

USDA, Agricultural Research Service, Biological Control of Insects Research Laboratory, 1503 S. Providence Road, Columbia, MO 65203, USA

Correspondence
Holly J. R. Popham
pophamh{at}missouri.edu

Lepidopteran larvae resist baculovirus infection by selective apoptosis of infected midgut epithelial cells and by sloughing off infected cells from the midgut. Once the infection breaches the midgut epithelial barrier and propagates from infective foci to the haemocoel, however, there are few mechanisms known to account for the resistance and clearance of infection observed in some virus–host combinations. The hypothesis that factors present in the plasma of infected pest larvae act to limit the spread of virus from initial infective foci within the haemocoel was tested. An in vitro bioassay was developed in which Helicoverpa zea single capsid nucleopolyhedrovirus (HzSNPV) was incubated with plasma collected from uninfected Heliothis virescens larvae. Infectious HzSNPV particles were then titrated on HzAM1 cells. Diluted plasma from larval Heliothis virescens exhibited a virucidal effect against HzSNPV in vitro, reducing the TCID₅₀ ml^–1 by more than 64-fold (from 4·3±3·6x10⁵ to 6·7±0·6x10³). The antiviral activity was heat-labile but was unaffected by freezing. In addition, protease inhibitors and specific chemical inhibitors of phenol oxidase or prophenol oxidase activation added to diluted plasma eliminated the virucidal activity. Thus, in the plasma of larval lepidopterans, the enzyme phenol oxidase may act as a constitutive, humoral innate antiviral immune response.

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