Modulation by cell trypsinization of Sendai virus expression in African green monkey kidney cells: first infection and establishment of a carrier state.

Two African green monkey kidney (AGMK) cell lines, 37RC (interferon-producing) and Vero (non-interferon-producing), were infected by egg-grown Sendai virus passaged in eggs at high and low m.o.i. The appearance of haem-adsorption, and cytopathic effect (c.p.e.) as well as the presence of haemagglutinating virions in the supernates were much more pronounced with a virus seed obtained with 10(-3) diluted passages than with a seed obtained with undiluted inoculum. They were also independent of interferon production (data obtained in 37RC and Vero cells were almost superimposable). In studies carried out with the virus seed prepared at high dilution the establishment of infection was maximal when monolayers were infected as soon as 5 h after trypsinization and cell seeding, regardless of cell density. Virus in supernates obtained from cultures infected 5, 20 or 50 h after seeding exhibited a greatly reduced infectivity for monkey cells, but not apparently for chick embryos. Trypsin treatment of the virus supernates restored their infectivity for AGMK cells more efficiently for virus released from cells infected 5 h after seeding than for virus released from cells infected later after seeding. In keeping with these observations, virus in supernates from cultures infected 5, 20 or 50 h after seeding contained increasing amounts of auto-interfering virions. The decreased infectivity obtained in cell supernates was not accounted for by major differences in virus RNA synthesis. Similarly, the optimum infection established in cells seeded only for 5 h was not due to increased virus adsorption. Several lines of cells surviving first infection with egg-grown Sendai virus have been obtained and kept in cultures for 3 to 18 months. Virus release and c.p.e. apparently become reduced in the cells surviving the first infection until the newly repopulated monolayers must undergo trypsinization. Weekly protease treatments of the cells reactivate all parameters of virus infection which again will tend to disappear slowly and then reappear following each trypsinization ('intermittent' carrier state). The establishment and the 'intermittent' reactivation of these lines were not prevented by the inclusion in the medium of anti-Sendai virus serum. Temperature-sensitive ts functions do not seem to play an important role in this virus-host relationship.