Characteristics of persistently infected (PI) Vero cell lines with measles virus established in our laboratory, R-5-PI-Vero, R-5-PHI-Vero and 448-PI-Vero, were studied.The 448-PI-Vero was more stable as a carrier culture than the R-5-PI-Vero, i.e., the latter showed a cyclic fashion of the lytic infection.These PI cells were able to be stored without definite decrease of viability at -70°C for at least 94 to 125 days.In these PI cells not only the typical syncytial cytopathic effect (CPE), but also a type of swollen cells arising from the cell monolayer was observed.
Immunofluorescence studies confirmed the presence of specific measles antigens in both the cytoplasm and the nucleus of PI cells.
Electron microscopy of the 15th passaged R-5-PI-Vero revealed the presence of smooth nucleocapsids in the nucleus, rough nucleocapsids in the cytoplasm and budding virus particles. The swollen cells which were characteristic of the PI culture showed the rough nucleocapsids near the cell membrane.
The temperature sensitivity of the virus was not directly related to the maintenance of the PI cultures.Mixed culture of R-5-PHI-Vero together with fresh Vero cells developed the significant CPE, which suggested that the PI culture was composed of cells endowed with a resistance against measles CPE by means of internal or external element (s).Interferon did not apparently take part in the maintenance of PI cultures, because they were not resistant to the superinfection of vesicular stomatitis virus.
The plaque size was inversely proportional to the virus yield, when six plaque purified clones from the R-5-PI-Vero were inoculated to fresh Vero monolayer cultures.