Rescue of a Vaccinia Virus Mutant Lacking IFN Resistance Genes K1L and C7L by the Parapoxvirus Orf Virus

The parapoxvirus Orf virus ORF116 gene encodes an antagonist of the interferon response

Orf virus (ORFV) is the type species of the Parapoxvirus genus of the Poxviridae family. Genetic and functional studies have revealed ORFV has multiple immunomodulatory genes that manipulate innate immune responses, during the early stage of infection. ORF116 is a novel gene of ORFV with hitherto unknown function. Characterization of an ORF116 deletion mutant showed that it replicated in primary lamb testis cells with reduced levels compared to the wild-type and produced a smaller plaque phenotype. ORF116 was shown to be expressed prior to DNA replication. The potential function of ORF116 was investigated by gene-expression microarray analysis in HeLa cells infected with wild-type ORFV or the ORF116 deletion mutant. The analysis of differential cellular gene expression revealed a number of interferon-stimulated genes (ISGs) differentially expressed at either 4 or 6 h post infection. IFI44 showed the greatest differential expression (4.17-fold) between wild-type and knockout virus. Other ISGs that were upregulated in the knockout included RIG-I, IFIT2, MDA5, OAS1, OASL, DDX60, ISG20 and IFIT1 and in addition the inflammatory cytokine IL-8. These findings were validated by infecting HeLa cells with an ORF116 revertant recombinant virus and analysis of transcript expression by quantitative real time-PCR (qRT-PCR). These observations suggested a role for the ORFV gene ORF116 in modulating the IFN response and inflammatory cytokines. This study represents the first functional analysis of ORF116.