Cellular response of insect cells to virus infection

Abstract

RNA interference (RNAi) is the dsRNA-triggered gene regulatory mechanism that is evolutionally conserved in most eukaryotic cells. It has been widely used as a powerful tool for functional genomics in various organisms. In flies, mosquitoes or other insect cells, gene functional analysis by RNAi is usually performed through introduced dsRNAs that are synthesized by in vitro transcription. RNAi serves as an important innate immunity against viruses in plants and invertebrates. It has recently been shown that Aedes albopictus mosquito C6/36 cells, commonly used for arbovirus propagation, possess an impaired RNAi pathway. In this study, we developed in vitro Dicer assay using extracts prepared from mosquito cells. Our results confirmed the inability of C6/36 cells to process dsRNAs into siRNAs, which is consistent with the loss-of-function of Dcr-2 due to a frameshift mutation. However, such a defect could not be complemented by introduction of Drosophila Dicer-2. To evaluate the RNAi-based antiviral mechanism in C6/36 cells, we analyzed the replication of a mutant Nodamura virus (NoV) genomic RNA1 of which viral RNAi suppressor B2 is not expressed (NoVR1ΔB2) and cannot accumulate to a detectable level in RNAi-competent cells. In C6/36 cells, the defective RNAi gives rise to complete restoration of NoVR1ΔB2 replication, suggesting that RNAi is the primary antiviral immunity in mosquito cells. At present, dsRNA, as the trigger of the antiviral RNAi pathway in invertebrate and plants, is the major efficiency limitation factor in RNAi. In this study, a plasmid-based system was developed to express dsRNA intracellular from a DNA cassette containing two convergent T7 promoters in Drosophila S2 cells. Efficient knockdown of a transiently expressed reporter gene or an endogenous gene can be achieved by dsRNA expressed from the system. A random cDNA library was constructed in the dsRNA expression cassette and initial screening led to identification of two host factors that are involved in antiviral response in Drosophila S2 cells. The plasmid-based dsRNA expression system provides an alternative tool for functional genomics in Drosophila and other insect cells.