Cell-to-cell spread of vaccinia virus is promoted by TGF-β-independent Smad4 signalling

The induction of Smad signalling by the extracellular ligand TGF-beta promotes tissue plasticity and cell migration in developmental and pathological contexts. Here, we show that vaccinia virus (VACV) stimulates the activity of Smad transcription factors and expression of TGF-beta/Smad-responsive genes at the transcript and protein levels. Accordingly, infected cells share characteristics to those undergoing TGF-beta/Smad-mediated epithelial-to-mesenchymal transition (EMT). Depletion of the Smad4 protein, a common mediator of TGF-beta signalling, results in an attenuation of viral cell-to-cell spread and reduced motility of infected cells. VACV induction of TGF-beta/Smad-responsive gene expression does not require the TGF-beta ligand or type I and type II TGF-beta receptors, suggesting a novel, non-canonical Smad signalling pathway. Additionally, the spread of ectromelia virus, a related orthopoxvirus that does not activate a TGF-beta/Smad response, is enhanced by the addition of exogenous TGF-beta. Together, our results indicate that VACV orchestrates a TGF-beta-like response via a unique activation mechanism to enhance cell migration and promote virus spread.