Mechanisms of viral manipulation of integrins and the TGF-β signalling pathway

Abstract

Viruses manipulate host signalling to optimise replication, spread and immune evasion. The TGF-β pathway is of particular interest given its roles in epithelial integrity, fibrosis, wound repair and suppression of antiviral responses, and its dysregulation is increasingly recognised as a hallmark of severe viral disease. However, how distinct viruses engage this pathway and what consequences they have remain poorly defined. This work investigates how vaccinia virus (VACV), the live smallpox vaccine, and SARS-CoV-2, two impactful pathogens from unrelated families, exploit TGF-β signalling to promote replication and persistence.

Using CRISPR-Cas9 KOs, we found that VACV activates the R-SMAD transcription factors (SMAD2 and SMAD3) independently of the canonical TβR1 kinase. R-SMADs played contrasting, non-redundant roles: loss of SMAD3 impaired replication and plaque expansion, whereas loss of SMAD2 increased cell motility and cell–cell spread. A kinase screen identified CK2 as a candidate non-canonical activator, providing a mechanism for TβR1-independent signalling.

For SARS-CoV-2, the Spike glycoprotein potently induced TGF-β signalling through integrin motifs. The ancestral RGD sequence within the receptor-binding domain activated SMAD3, induced SERPINE1/PAI-1 and suppressed type I interferon responses. These effects occurred with the recombinant S protein, pseudotyped lentivirus, and authentic infection, and were abolished by an RGD mutation or the antagonist ATN-161. Despite RGD loss in Omicron beyond BA.2, the S protein retained activity through an emergent LDV site, with motif mutation and α4 antibody neutralisation implicating α4 integrins. We propose that integrin-binding motifs and TGF-β signalling are intrinsically linked and essential for viral fitness.

Together, these findings reveal convergent strategies by which poxviruses and coronaviruses hijack TGF-β signalling, defining two mechanistically distinct routes and highlighting new therapeutic targets.