ISG15 Is Upregulated in Respiratory Syncytial Virus Infection and Reduces Virus Growth through Protein ISGylation

Human Respiratory Syncytial Virus (RSV), for which neither a vaccine nor an effective therapeutic treatment is currently available, is the leading cause of severe lower respiratory tract infections in children. Interferon stimulated gene 15 (ISG15) is an ubiquitin-like protein that is highly increased during viral infections and has been reported to play an antiviral or a proviral activity, depending on the virus. Previous studies from our laboratory demonstrated a strong ISG15 up-regulation during RSV infection in vitro . In this study, an in depth analysis of the role of ISG15 in RSV infection is presented. ISG15 overexpression and siRNA silencing experiments, along with ISG15 knockout cells (ISG15 -/- ) revealed an anti-RSV effect of this molecule. Conjugation inhibition assays demonstrated that ISG15 exerts its antiviral activity via protein ISGylation. This antiviral activity requires high levels of ISG15 to be present in the cells before RSV infection. Finally, ISG15 is also up-regulated in human respiratory pseudo-stratified epithelia and in nasopharyngeal washes from infants infected with RSV, pointing to a possible antiviral role of this molecule in vivo . These results advance our understanding of the innate immune response elicited by RSV and open new possibilities to control infections by this virus. IMPORTANCE At present no vaccine or effective treatment against human respiratory syncytial virus (RSV) is available. This study shows that interferon-stimulated gene 15 (ISG15) lowers RSV growth through protein ISGylation. In addition, ISG15 accumulation highly correlates with RSV load in nasopharyngeal washes from children, indicating that ISG15 may also have an antiviral role in vivo . These results improve our understanding of the innate immune response against RSV and identify ISG15 as a potential target for virus control.