Enhanced fusion of a nucleopolyhedrovirus with cultured cells by a virus enhancing factor from an entomopoxvirus.

Fusion of Pseudaletia unipuncta nucleopolyhedrovirus with an armyworm cell line (SIE-MSH-805-F) was studied by means of three fluorescence assays that are based on the relief of fluorescence self-quenching of octadecylrhodamine B chloride (R18). A gradual increase in fluorescence intensity indicative of virus-cell fusion was observed by spectrofluorometry when R18-labeled polyhedron-derived virus was incubated with cultured cells. The fusion was enhanced by the virus enhancing factor (EF) from Pseudaletia separata entomopoxvirus. Lysosomotropic agents had little effect on the virus-cell fusion. The percentage of positively fluorescent cells, as determined by flow cytometry, gradually increased after the addition of labeled virus and was higher in the presence of the EF than in its absence. Confocal microscopy of cultured cells that had been combined with labeled virus showed that the fluorescence appeared first on their surface. The plasma membrane of cultured cells had specific affinity to the EF, as revealed by indirect immunofluorescence microscopy.