Bunyamwera virus replication in cultured Aedes albopictus (mosquito) cells: establishment of a persistent viral infection

Abstract

Bunyamwera virus replication was examined in Aedes albopictus (mosquito) cell cultures in which a persistent infection is established and in cytopathically infected BHK cells. During primary infection of A. albopictus cells, Bunyamwera virus reached relatively high titers ( approximately 10(7) PFU/ml), and autointerference was not observed. Three virus-specific RNAs (L, M, and S) and two virion proteins (N and G1) were detected in infected cells. Maximum rates of viral RNA synthesis and viral protein synthesis were extremely low, corresponding to <2% of the synthetic capacities of uninfected control cells. Viral protein synthesis was maximal at 12 h postinfection and was shut down to barely detectable levels at 24 h postinfection. Virus-specific RNA and nucleocapsid syntheses showed similar patterns of change, but later in infection. The proportions of cells able to release a single PFU at 3, 6, and 54 days postinfection were 100, 50, and 1.5%, respectively. Titers fell to 10(3) to 10(5) PFU/ml in carrier cultures. Persistently infected cultures were resistant to superinfection with homologous virus but not with heterologous virus. No changes in host cell protein synthesis or other cytopathic effects were observed at any stage of infection. Small-plaque variants of Bunyamwera virus appeared at approximately 7 days postinfection and increased gradually until they were 75 to 95% of the total infectious virus at 66 days postinfection. Temperature-sensitive mutants appeared between 23 and 49 days postinfection. No antiviral activity similar to that reported in A. albopictus cell cultures persistently infected with Sindbis virus (R. Riedel and D. T. Brown, J. Virol. 29: 51-60, 1979) was detected in culture fluids by 3 months after infection. Bunyamwera virus replicated more rapidly in BHK cells than in mosquito cells but reached lower titers. Autointerference occurred at multiplicities of infection of approximately 10. Virus-specific RNA and protein syntheses were at least 20% of the levels in uninfected control cells. Host cell protein synthesis was completely shut down, and nucleocapsid protein accumulated until it was 4% of the total cell protein. We discuss these results in relation to possible mechanisms involved in determining the outcome of arbovirus infection of vertebrate and mosquito cells.