Vaccinia Virus E3 Suppresses Expression of Diverse Cytokines through Inhibition of the PKR, NF-κB, and IRF3 Pathways

The vaccinia virus double-stranded RNA binding protein E3 has been demonstrated to inhibit the expression of cytokines, including beta interferon (IFN-β) and tumor necrosis factor alpha (TNF-α). However, few details regarding the molecular mechanisms of this inhibition have been described. Using real-time PCR arrays, we found that E3 suppressed the induction of a diverse array of cytokines representing members of the IFN, interleukin (IL), TNF, and transforming growth factor cytokine families. We discovered that the factor(s) responsible for the induction of IL-6, TNF-α, and inhibin beta A (INHBA) was associated with the early and late phases of virus infection. In contrast, the factor(s) which regulates IFN-β induction was associated with the late phase of replication. We have found that expression of these cytokines can be induced by transfection of cells with RNA isolated from vaccinia virus-infected cells. Moreover, we provide evidence that E3 antagonizes both PKR-dependent and PKR-independent pathways to regulate cytokine expression. PKR-dependent activation of p38 and NF-κB was required for vaccinia virus-induced INHBA expression, whereas induction of TNF-α required only PKR-dependent NF-κB activation. In contrast, induction of IL-6 and IFN-β was largely PKR independent. IL-6 induction is regulated by NF-κB, while IFN-β induction is mediated by IFN-β promoter stimulator 1 and IFN regulatory factor 3/NF-κB. Collectively, these results indicate that E3 suppresses distinct but interlinked host signaling pathways to inhibit the expression of a diverse array of cytokines.