Spindles of an entomopoxvirus facilitate its infection of the host insect by disrupting the peritrophic membrane.

The mode of action by which entomopoxvirus (EPV) spindles, proteinaceous crystalline bodies produced by EPVs, enhance EPV infection has not been clarified. We fed Anomala cuprea EPV (AcEPV) spindles to host insects; subsequent scanning electron microscopy revealed the disruption of the peritrophic membranes (PMs) of these insects. The PM is reportedly a barrier against the infection of some insects by viruses. Quantitative PCR of AcEPV DNA in the ectoperitrophic area revealed that PM disruption facilitated the passage of EPVs through the PM toward the initial infection site, the midgut epithelium. These results indicate that EPV spindles enhance infection by EPVs by disrupting the PM in the host insects. Fusolin is almost exclusively the constituent protein of the spindles and is the enhancing factor of the infectivity of nucleopolyhedroviruses (NPVs) and possibly that of EPVs. Spheroid is another type of proteinaceous crystalline structure produced by EPVs. Pseudaletia separata EPV (PsEPV) spheroids reportedly contain considerable amounts of fusolin and enhance NPV infection. We assessed the ability of AcEPV spheroids to enhance EPV infectivity and their effect on the PM and carried out immunological experiments; these experiments showed that AcEPV spheroids contain little or no fusolin and are biologically inactive, in contrasts to the situation in PsEPV.