Effects of inducing or inhibiting apoptosis on Sindbis virus replication in mosquito cells

Sindbis virus (SINV) is a mosquito-borne virus in the genus Alphavirus, family Togaviridae. Like most alphaviruses, SINVs exhibit lytic infection (apoptosis) in many mammalian cell types, but are generally thought to cause persistent infection with only moderate cytopathic effects in mosquito cells. However, there have been several reports of apoptotic-like cell death in mosquitoes infected with alphaviruses or flaviviruses. Given that apoptosis has been shown to be an antiviral response in other systems, we have constructed recombinant SINVs that express either pro-apoptotic or anti-apoptotic genes in order to test the effects of inducing or inhibiting apoptosis on SINV replication in mosquito cells. Recombinant SINVs expressing the pro-apoptotic genes reaper (rpr) from Drosophila or michelob_x (mx) from Aedes aegypti caused extensive apoptosis in cells from the mosquito cell line C6/36, thus changing the normal persistent infection observed with SINV to a lytic infection. Although the infected cells underwent apoptosis, high levels of virus replication were still observed during the initial infection. However, virus production subsequently decreased compared with persistently infected cells, which continued to produce high levels of virus over the next several days. Infection of C6/36 cells with SINV expressing the baculovirus caspase inhibitor P35 inhibited actinomycin D-induced caspase activity and protected infected cells from actinomycin D-induced apoptosis, but had no observable effect on virus replication. This study is the first to test directly whether inducing or inhibiting apoptosis affects arbovirus replication in mosquito cells.