Antiviral Innate Immune Response Interferes with the Formation of Replication-Associated Membrane Structures Induced by a +RNA Virus

Infection with nidoviruses like corona- and arteriviruses induces a reticulovesicular network of interconnected ER-derived double-membrane vesicles (DMVs) and other membrane structures. This network is thought to accommodate the viral replication machinery, and protect it from innate immune detection. We hypothesized that the innate immune response machinery has tools to counteract the formation of these virus-induced membrane structures in order to inhibit virus replication. Here we have investigated the effect of type I interferon on the formation of arterivirus-induced membrane structures. Our approach involved ectopic expression of arterivirus non-structural proteins nsp2 and nsp3, which formed DMVs in the absence of other viral triggers of the interferon (IFN) response such as replicating viral RNA. Thus, this set-up can be used to identify immune effectors that specifically target the (formation of) virus-induced replication structures. Using large-scale electron microscopy mosaic maps, we found that IFN-β treatment significantly reduced the formation of the membrane structures. Strikingly, we also observed abundant stretches of double-membrane sheets (a proposed intermediate of DMV formation) in IFN-β-treated samples, suggesting the disruption of DMV biogenesis. Three interferon-stimulated gene products, two of which have been reported to target the hepatitis C virus replication structures, were tested for their possible involvement, but none of them affected membrane structure formation. Our study reveals the existence of a previously unknown innate immune mechanism that antagonizes the viral hijacking of host membranes. It also provides a solid basis for further research into the poorly understood interactions between the innate immune system and virus-induced replication structures.