Closely related reovirus lab strains induce opposite expression of RIG-I/IFN-dependent versus -independent host genes, via mechanisms of slow replication versus polymorphisms in dsRNA binding σ3 respectively

The Dearing isolate of Mammalian orthoreovirus (T3D) is a prominent model of virus-host relationships and a candidate oncolytic virotherapy. Closely related laboratory strains of T3D, originating from the same ancestral T3D isolate, were recently found to exhibit significantly different oncolytic properties. Specifically, the T3D(PL) strain had faster replication kinetics in a panel of cancer cells and improved tumor regression in an in vivo melanoma model, relative to T3D(TD). In this study, we discover that T3D(PL) and T3D(TD) also differentially activate host signalling pathways and downstream gene transcription. At equivalent infectious dose, T3D(TD) induces higher IRF3 phosphorylation and expression of type I IFNs and IFN-stimulated genes (ISGs) than T3D(PL). Using mono-reassortants with intermediate replication kinetics and pharmacological inhibitors of reovirus replication, IFN responses were found to inversely correlate with kinetics of virus replication. In other words, slow-replicating T3D strains induce more IFN signalling than fast-replicating T3D strains. Paradoxically, during co-infections by T3D(PL) and T3D(TD), there was still high IRF3 phosphorylation indicating a phenodominant effect by the slow-replicating T3D(TD). Using silencing and knock-out of RIG-I to impede IFN, we found that IFN induction does not affect the first round of reovirus replication but does prevent cell-cell spread in a paracrine fashion. Accordingly, during co-infections, T3D(PL) continues to replicate robustly despite activation of IFN by T3D(TD). Using gene expression analysis, we discovered that reovirus can also induce a subset of genes in a RIG-I and IFN-independent manner; these genes were induced more by T3D(PL) than T3D(TD). Polymorphisms in reovirus σ3 viral protein were found to control activation of RIG-I/ IFN-independent genes. Altogether, the study reveals that single amino acid polymorphisms in reovirus genomes can have large impact on host gene expression, by both changing replication kinetics and by modifying viral protein activity, such that two closely related T3D strains can induce opposite cytokine landscapes.