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The Leader Proteinase of Foot-and-Mouth Disease Virus Inhibits the Induction of Beta Interferon mRNA and Blocks the Host Innate Immune Response

ABSTRACT We have previously shown that the leader proteinase (Lpro) of foot-and-mouth disease virus (FMDV) blocks cap-dependent mRNA translation and that a genetically engineered FMDV lacking the leader proteinase coding region (A12-LLV2) is attenuated in cell culture and susceptible animals. The attenuated phenotype apparently is a consequence of the inability of A12-LLV2 to block the expression of type I interferon (IFN-α/β) protein, resulting in IFN-induced inhibition of FMDV replication. Here we show that in addition to preventing IFN-α/β protein synthesis, Lpro reduces the level of immediate-early induction of IFN-β mRNA and IFN-stimulated gene products such as double-stranded RNA-dependent protein kinase R (PKR), 2′,5′-oligoadenylate synthetase, and Mx1 mRNAs in swine cells. Down-regulation of cellular PKR by RNA interference did not affect wild-type virus yield but resulted in a higher yield of A12-LLV2, indicating a direct role of PKR in controlling FMDV replication in the natural host. The observation that Lpro controls the transcription of genes involved in innate immunity reveals a novel role of this protein in antagonizing the cellular response to viral infection.