Reovirus σ3 protein limits interferon expression and cell death induction

ABSTRACT Induction of necroptosis by mammalian reovirus requires both type I interferon (IFN)-signaling and viral replication events that lead to production of progeny genomic dsRNA. The reovirus outer capsid protein µ1 negatively regulates reovirus-induced necroptosis by limiting RNA synthesis. To determine if the outer capsid protein σ3, which interacts with µ1, also functions in regulating cell death, we used siRNA-mediated knockdown. Similar to that observed by diminishment of µ1 expression, knockdown of newly synthesized σ3 enhances necroptosis. σ3 knockdown does not impact reovirus RNA synthesis. Instead, this increase in necroptosis following σ3 knockdown is accompanied by an increase in IFN production. Furthermore, ectopic expression of σ3 is sufficient to block IFN expression following infection. Surprisingly, the capacity of σ3 protein to bind dsRNA does not impact its capacity to diminish production of IFN. Consistent with this, infection with a virus harboring a mutation in the dsRNA binding domain of σ3 does not result in enhanced production of IFN or cell death. Together, these data suggest that σ3 limits the production of IFN to control innate immune signaling and cell death following infection through a mechanism that is independent of its dsRNA binding capacity. IMPORTANCE We use mammalian reovirus as a model to study how virus infection modulates innate immune signaling and cell death induction. Here we sought to determine how viral factors regulate these processes. Our work highlights a previously unknown role for the reovirus outer capsid protein σ3 in limiting the induction of a necrotic form of cell death called necroptosis. Induction of cell death by necroptosis requires production of interferon. σ3 limits the induction of necroptosis by preventing excessive production of interferon following infection.