Phenotypes of St Louis encephalitis virus mutants produced in persistently infected mosquito cell cultures

Viral mutants that appeared during long-term persistent infections of mosquito cell cultures (Aedes albopictus, A. dorsalis and Culex tarsalis) with St Louis encephalitis virus were characterized. Evidence was obtained for the presence of temperature-sensitive mutants in the A. dorsalis and C. tarsalis persistently infected cultures, and small plaque mutants were predominant in all cultures except one of two cell cultures of C. tarsalis. Virus from persistently infected A. albopictus cell cultures was growth-restricted in Vero and C. tarsalis cells. One of two persistently infected A. dorsalis cell cultures also produced viral mutants that were growth-restricted in C. tarsalis cells. Further, Western blots of persistently infected A. albopictus cell extracts showed an overproduction of capsid (C) and envelope (E) structural proteins and reduced production of an Mr 27K protein (p27) which was immunologically related to the E protein. In contrast, the production of E and C proteins in persistently infected C. tarsalis cultures was consistent with the amount of infectious virus present, whereas p27 was relatively overproduced. These observations suggest that the host cell has an important influence on both the types and relative quantities of viral mutants that accumulate during long-term persistent infections.