Low pH-induced cell fusion in flavivirus-infected Aedes albopictus cell cultures

Cell-to-cell fusion ofAedes albopictus (mosquito) cells infected with dengue and St Louis encephalitis (SLE) flaviviruses was induced by exposure to low pH. The parameters of this low pH-induced fusion were exam-ined. Syncytium formation was maximal in cultures 36 to 48 h post-infection and occurred when cultures were maintained at the acid pH for 15 min at 35 °C. The optimal pH range for fusion was 5.0 to 6.5 for dengue virus-infected cells and 5.0 to 5.5 for SLE virus-infected cells. Syncytia were not observed in vertebrate cells (Vero and BHK) under these conditions despite similar virus yields. Fusion was shown to be ATP-dependent and could be prevented by the addition of either polyclonal antiviral antibodies or monoclonal antibody to the envelope glycoprotein. The lysosomo-tropic amine ammonium chloride inhibited the replica-tion of SLE virus in both mosquito and vertebrate cells, consistent with the idea that low pH-induced fusion is necessary for virus entry into both types. The flaviviruses are small enveloped viruses which contain a single-stranded positive sense RNA genome. The virus particle contains three structural proteins: a capsid protein C (Mr approximately 14 000), an envelope glycoprotein E (Mr 50000 to 60000), and a membrane protein M (Mr approximately 8000). Most members of the family Flaviviridae are arthropod-borne and thus infect both vertebrates and invertebrates (mosquitoes or ticks) (for reviews ee Brinton, 1986; Rice et al., 1986). There have been a number of reports showing that certain flaviviruses can cause fusion of mosquito cells. The precise requirements for fusion of these cells are not known, but whether or not fusion occurs and the degree of fusion appear to be influenced by the strains of virus and cells used, the composition and pH of the medium, the age of the culture, and the surface on which cells are grown (glass or plastic) (Suitor & Paul, 1969; Ng &amp