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