Usutu virus NS4A suppresses the host interferon response by disrupting MAVS signaling

Usutu virus (USUV) is an emerging flavivirus that can infect birds and mammals. In humans, in severe cases, it may cause neuroinvasive disease. The innate immune system, and in particular the interferon response, functions as the important first line of defense against invading pathogens such as USUV. Many, if not all, viruses have developed mechanisms to suppress and/or evade the interferon response in order to facilitate their replication. The ability of USUV to antagonize the interferon response has so far remained largely unexplored. Using dual-luciferase reporter assays we observed that multiple of the USUV nonstructural (NS) proteins were involved in suppressing IFN-β production and signaling. In particular NS4A was very effective at suppressing IFN-β production. We found that NS4A interacted with the mitochondrial antiviral signaling protein (MAVS) and thereby blocked its interaction with melanoma differentiation-associated protein 5 (MDA5), resulting in reduced IFN-β production. The TM1 domain of NS4A was found to be essential for binding to MAVS. By screening a panel of flavivirus NS4A proteins we found that the interaction of NS4A with MAVS is conserved among flaviviruses. The increased understanding of the role of NS4A in flavivirus immune evasion could aid the development of vaccines and therapeutic strategies.