Bovine viral diarrhea virus (BVDV), which is classified in the genus Pestivirus, family Flaviviridae, can be divided into two biotypes according to its ability to induce a cytopathic effect in tissue culture cells. The mechanisms through which cytopathogenic (cp) BVDV induces cell death and non-cytopathogenic (ncp) BVDV causes persistent infection without producing cell death remain unclear. Here, it was found that the overexpression of four apoptosis-related cellular mRNAs in cells infected with cpBVDV could also be caused by synthetic dsRNA. In fact, it was found that the amount of dsRNA produced by cpBVDV considerably exceeded the amount yielded by ncpBVDV. To evaluate the possible involvement of dsRNA in the induction of apoptosis, this study examined whether RNAi-mediated depletion of two dsRNA-reactive cellular factors, dsRNA-dependent protein kinase and 2′,5′-oligoadenylate synthetase 1, resulted in the prevention of cpBVDV-induced apoptosis. Although the induction of apoptosis was reduced after the suppression of either factor alone, the simultaneous silencing of both factors resulted in an almost complete inhibition of apoptosis without affecting viral titre. These results showed that dsRNA is the main trigger of apoptosis in cpBVDV-infected cells and that the cytopathogenicity of BVDV depends on the yield potential of dsRNA. In contrast, ncpBVDV yielded minimal levels of dsRNA, thereby establishing a persistent infection without inducing apoptosis. This report supports the significance of viral dsRNA as a trigger of innate immune responses.
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