Abstract
Human respiratory syncytial virus (RSV) is a single-stranded RNA virus that causes acute, and occasionally fatal, lower respiratory illness in young infants, the elderly, and immunocompromised patients. Therapeutic interventions able to cut short viral replication and quickly return the airways to normal function are needed. An understanding of antiviral activities and their effects on host defense mechanisms is important for the design of safe and effective therapy. We targeted functionally and temporally distinct steps within the viral life cycle using small-molecule RSV inhibitors and studied their antiviral activities and their effects on innate interferon responses of airway epithelial cells in vitro. Antivirals acting upstream of RSV polymerase activity (i.e., compounds targeting the fusion protein or the nucleoprotein) reduced viral load immediately postinfection and partially attenuated interferon responses. In contrast, antivirals directed to the RSV polymerase demonstrated activity throughout the viral replication cycle and specifically modulated the RIG-I/mitochondrial antiviral signaling protein (MAVS)/TBK1/IRF3/interferon-stimulated gene (ISG) axis, causing either an upregulation or a downregulation of interferon responses, depending on the mechanism of polymerase inhibition. Notably, polymerase inhibition leading to the accumulation of abortive RNA products correlated with the amplification of interferon-stimulated genes to up to 10 times above normal infection levels. Understanding how antiviral activities and their modulation of innate immunity may affect recovery from RSV infection will help guide the development of safe and effective therapies. Importance: RSV circulates seasonally, causing acute lower respiratory disease. Therapeutic interventions with efficacy throughout the viral replication cycle, rapid viral clearance, and prevention of potentially harmful inflammatory responses are desirable. Compounds targeting the RSV polymerase inhibited virus replication late in the viral life cycle and, depending on the functional domain targeted, either attenuated or amplified RIG-I and downstream interferon pathways in infected cells. These data will help guide the development of safe and effective therapies by providing new molecular evidence that the mechanism of inhibition by an antiviral compound can directly impact innate antiviral immune responses in the airway epithelium.