Transient rescue of Sendai-6/94 cl virus from the persistently infected cell line Cl-E-8 by cocultivation

The cell line Cl-E-8 showing expression of Sendai-6/94 viral antigens after the original isolation was reexamined after approximately 160 subcultures. In the virus fraction of cell supernatants 6/94 virus particles (termed 6/94 cl) could be demonstrated; no infectivity, however, was monitored. We were also unable to activate the viral infectivity of 6/94 cl virus by trypsin treatment. The analysis of viral RNA revealed that the virus contains a high-molecular-weight (50 S) single-stranded RNA. After cocultivation of Cl-E-8 cells with several standard cell lines the production of an infectious 6/94 virus, termed 6/94 co, was detected. The infectivity titer in the supernatants was very low, about three orders of magnitude lower than in cultures infected with the egg-grown 6/94 virus (6/94 ST). Surprisingly, the production of infectious 6/94 cl virus invariably ceased several subcultures after cocultivation even in the presence of foreign cells. However, the infectivity could be repeatedly reinduced by adding fresh foreign cells.