Human embryonic lung (MRC-5), feline embryo (FEA), mink lung (Mv1Lu) and monkey kidney (BSC-1) cells infected by respiratory syncytial virus showed characteristic morphological changes when viewed by scanning electron microscopy. The surfaces of respiratory synctial virus-infected cells developed a profusion of slender filaments after 48 h incubation at 31 °C. Similar changes in surface morphology were observed in BSC-1 cells infected by murine pneumonia virus. Filament production therefore appears to be a common property of pneumo-viruses. Filaments were not observed in cells infected with either syncytial and non-syncytial herpes simplex virus, the cytocidal vesicular stomatitis and Batai (Bunyaviridae) viruses, or the focus-inducing rabbit fibroma virus.

Filament production was not observed in cells infected with mutants of respiratory syncytial (RS) virus during incubation at the restrictive temperature, or in a persistently infected culture of BSC-1 cells at 37 °C. The persistently infected cells (the RS 1/BSC-1 line) had some of the characteristics of cells transformed by oncogenic viruses, namely ability to overlap adjacent cells and agglutination by a low concentration of concanavalin A. The pseudo-transformed phenotype was temperature-dependent, however, and suppressed by raising the temperature of incubation to 39 °C. The presence of virus antigen at the cell surface was similarly temperature-dependent in these cells, diminished at high temperature (39 °C) and enhanced at low temperature (31 °C), suggesting that the changes in the host cell were the result of insertion of virus protein into the cell membrane. Evidently, persistent infection by a cytoplasmic virus can produce alterations in the host cell usually associated with transformation by nuclear viruses.


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