Mopeia and Lassa virus produce different RIG-I- agonists in the absence of a functional viral exoribonuclease domain
All viruses of the genus Mammarenavirus possess an exoribonuclease (ExoN) domain in their nucleoproteins (NP). Through this domain, NP efficiently prevents the activation of the interferon (IFN) response presumably by degrading dsRNA produced during the viral replication. While the importance of this ExoN activity regarding Mammarenavirus virulence is well established, little is known about the dsRNA molecules that are targeted by NP for degradation and the contribution of cellular sensors activated by these molecules are yet to be described. Here, we addressed these questions using recombinant viruses with abrogated ExoN domains that are no longer able to control the IFN response. We infected RIG-I, MDA5 and MAVS deficient cells and demonstrated that ExoN mutants activate the interferon response through RIG-I, but not MDA5. We then purified RIG-I-associated RNA from infected cells and confirmed their immunostimulatory activity in transfected cells. We sequenced the RIG-I associated RNA and identified different enriched sequences in the MOPV or LASV RNA. RIG-I activating sequences corresponded to the extremities to the 5' ends and intergenic regions of the MOPV genome and to a GPC sequence in the LASV genome. We produced corresponding synthetic RNA molecules and confirmed their RIG-I dependent activation of the IFN response. These results underline the central role of the ExoN domain in MOPV and LASV NP for immune escape, and identifies new virus-derived RNA molecules with high immunostimulatory properties.