Cells persistently infected with newcastle disease virus: I. Properties of mutants isolated from persistently infected L cells.

The strain of Newcastle disease virus (NDV(pi)) present in persistently infected L cells differed markedly from the Herts strain (NDV(0)) used to initiate the infection. NDV(pi) produced small plaques (less than 1 mm) in chick embryo cell cultures, whereas the wild type (NDV(0)) produced large plaques (2 to 3 mm). The two viruses differed in a number of additional properties. Whereas 80% of adsorbed NDV(0) eluted from chicken red blood cells at 37 C, only about 20% of NDV(pi) was recovered under similar conditions. There was no significant difference in the neuraminidase content of the two viruses. The infectivity of NDV(0) was stable for 1 hr at 48 C, whereas 99.9% of the infectivity of NDV(pi) was destroyed. The two viruses also differed in lethality for chick embryos; NDV(pi) had significantly reduced lethality for 9-day-old chick embryos when compared to NDV(0). In contrast to NDV(0), which produced an abortive infection in L cells, NDV(pi) not only replicated effectively and destroyed these cells, but also induced significantly higher quantities of interferon than did NDV(0). These data furnished additional evidence for the lack of relationship of interferon production to abortive infection of L cells with NDV(0). In contrast, interferon was found to play a significant role in the maintenance of persistent infection.