Plaque formation by a host range mutant of vaccinia virus in non-permissive cells co-infected with Yaba virus

A host-dependent conditional lethal mutant of vaccinia virus strain DIs was rescued to form plaques in a non-permissive cell line JINET co-infected with Yaba virus, a poxvirus serologically distant from the rescued virus. The efficiency of plaque formation under optimal conditions in this system was comparable to that in Vero cells which were permissive for the mutant. The plaque number of the mutant in JINET cells was influenced greatly by the multiplicity of Yaba virus, the interval between inoculations with DIs and Yaba virus and the maintenance medium for pre-infection of cells with Yaba virus. The plaque formation by DIs in JINET cells was suppressed or inhibited when the duration of pre-infection with high multiplicities of Yaba virus was prolonged. Thus, Yaba virus possessed both rescuing and inhibitory activities on DIs and the plaque number of DIs in doubly infected cells was thought to be determined by the balance between the two activities. Ultraviolet light-inactivated Yaba virus retained rescuing activity on DIs in JINET cells.