Host Factors for Human Respiratory Synctial Virus Cell-to-Cell Spread

Human respiratory syncytial virus (RSV) is the most common viral cause of serious lower respiratory disease in infants and young children worldwide, with an estimated 3.4 million RSV-associated hospitalizations annually among children younger than 5 years of age. There is currently no licensed RSV vaccine or suitable antiviral drug available. Dr. Mehedi and his team studied host-pathogen interactions to identify possible host or viral targets for therapeutic interventions. In a high-throughput human genome-wide siRNA screening on human lung epithelial A549 cells infected with RSV-GFP, approximately 21,500 genes were screened to identify genes with a role in the RSV replicative cycle. The researchers identified actin-related protein 2 (ARP2) as one of the host factors involved in the RSV life cycle. ARP2 is part of the ARP2/3 complex, which regulates actin polymerization and affects cellular shape, structure, and motility. ARP2 knockdown in A549 cells did not reduce RSV entry or gene expression at early time points after infection; however, virus budding, the release of infectious virions, and syncytia formation were reduced at late time points. This suggested that ARP2 plays a role in viral spread. Staining of A549 cells for filamentous actin, microtubulin, and RSV fusion (F) protein revealed that infection induced the extensive formation of finger-like actin protrusions that were mostly microtubulin-deficient, identifying them as filopodia. Using stimulated emission depletion (STED) microscopy, we found that the filopodia appeared to shuttle RSV particles to nearby uninfected cells. ARP2-knockdown reduced RSV-induced filopodia formation and infected-cell motility. Thus, RSV uses two previously unrecognized ARP2-dependent features to facilitate viral spread, namely filopodia formation and cell motility. This provides a new target for the development of RSV antivirals.