RNA Interference Restricts Rift Valley Fever Virus in Multiple Insect Systems

Abstract

The emerging bunyavirus Rift Valley fever virus (RVFV) is transmitted to humans and livestock by a large number of mosquito species. RNA interference (RNAi) has been characterized as an important innate immune defense mechanism used by mosquitoes to limit replication of positive-sense RNA flaviviruses and togaviruses; however, little is known about its role against negative-strand RNA viruses such as RVFV. We show that virus-specific small RNAs are produced in infected mosquito cells, in <i>Drosophila melanogaster</i> cells, and, most importantly, also in RVFV vector mosquitoes. By addressing the production of small RNAs in adult <i>Aedes</i> sp. and <i>Culex quinquefasciatus</i> mosquitoes, we showed the presence of virus-derived Piwi-interacting RNAs (piRNAs) not only in <i>Aedes</i> sp. but also in <i>C. quinquefasciatus</i> mosquitoes, indicating that antiviral RNA interference in <i>C. quinquefasciatus</i> mosquitoes is similar to the described activities of RNAi in <i>Aedes</i> sp. mosquitoes. We also show that these have antiviral activity, since silencing of RNAi pathway effectors enhances viral replication. Moreover, our data suggest that RVFV does not encode a suppressor of RNAi. These findings point toward a significant role of RNAi in the control of RVFV in mosquitoes. <b>IMPORTANCE</b> Rift Valley fever virus (RVFV; <i>Phlebovirus</i>, <i>Bunyaviridae</i>) is an emerging zoonotic mosquito-borne pathogen of high relevance for human and animal health. Successful strategies of intervention in RVFV transmission by its mosquito vectors and the prevention of human and veterinary disease rely on a better understanding of the mechanisms that govern RVFV-vector interactions. Despite its medical importance, little is known about the factors that govern RVFV replication, dissemination, and transmission in the invertebrate host. Here we studied the role of the antiviral RNA interference immune pathways in the defense against RVFV in natural vector mosquitoes and mosquito cells and draw comparisons to the model insect <i>Drosophila melanogaster</i>. We found that RVFV infection induces both the exogenous small interfering RNA (siRNA) and piRNA pathways, which contribute to the control of viral replication in insects. Furthermore, we demonstrate the production of virus-derived piRNAs in <i>Culex quinquefasciatus</i> mosquitoes. Understanding these pathways and the targets within them offers the potential of the development of novel RVFV control measures in vector-based strategies.