ABSTRACT:
As infection with wild-type (wt) Sendai virus (SeV) normally activates beta interferon (IFN-β) very poorly, two unnatural SeV infections were used to study virus-induced IFN-β activation in mouse embryonic fibroblasts: (i) SeV-DI-H4, which is composed mostly of small, copyback defective interfering (DI) genomes and whose infection overproduces short 5′-triphosphorylated trailer RNAs (pppRNAs) and underproduces viral V and C proteins, and (ii) SeV-GFP(+/−), a coinfection that produces wt amounts of viral gene products but that also produces both green fluorescent protein (GFP) mRNA and its complement, which can form double-stranded RNA (dsRNA) with capped 5′ ends. We found that (i) virus-induced signaling to IFN-β depended predominantly on RIG-I (as opposed to mda-5) for both SeV infections, i.e., that RIG-I senses both pppRNAs and dsRNA without 5′-triphosphorylated ends, and (ii) it is the viral C protein (as opposed to V) that is primarily responsible for countering RIG-I-dependent signaling to IFN-β. Nondefective SeV that cannot specifically express C proteins not only cannot prevent the effects of transfected poly(I-C) or
ppp
RNAs on IFN-β activation but also synergistically enhances these effects. SeV-V
minus
infection, in contrast, behaves mostly like wt SeV and counteracts the effects of transfected poly(I-C) or
ppp
RNAs.