Similarities and Differences in Antagonism of Neuron Alpha/Beta Interferon Responses by Venezuelan Equine Encephalitis and Sindbis Alphaviruses

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Abstract
Venezuelan equine encephalitis virus (VEEV) is highly virulent in adult laboratory mice, while Sindbis virus (SINV) is avirulent regardless of dose or inoculation route, dependent upon functioning alpha/beta interferon (IFN-α/β) responses. We have examined each virus9 resistance to and/or antagonism of IFN-α/β responses in neurons, a cell type targeted by both viruses in mice, by infecting IFN-α/β-treated or untreated primary cultures with viruses or virus-derived replicons that lacked the structural proteins. Priming with IFN-α/β prior to infection revealed that VEEV replication and progeny virion production were resistant to an established antiviral state while those of SINV were more sensitive. Postinfection IFN-α/β treatment revealed that phosphorylation of STAT1 and STAT2 was partially blocked by infection with either virus, dependent upon expression of nonstructural proteins (nsP), but not structural proteins (sP). However, inhibition of STAT phosphorylation by VEEV replicons was not correlated with inhibition of IFN-stimulated gene (ISG) mRNA induction, yet ISG induction was inhibited when sP were present. Host translation was inhibited by VEEV nsP even when cells were pretreated with IFN-α/β. SINV blocked ISG induction and translation, associated with nsP-mediated shutoff of macromolecular synthesis, but both activities were sensitive to IFN-α/β pretreatment. We conclude that both VEEV and SINV limit ISG induction in infected neurons through shutoff of host transcription and translation but that inhibition by VEEV is more resistant to IFN-α/β priming. Likewise, both viruses inhibit IFN receptor-initiated signaling, although the effect upon host responses is not clear. Finally, VEEV appears to be more resistant to effectors of the preestablished antiviral state.