Human respiratory syncytial virus (RSV) is highly contagious and the leading cause of severe respiratory tract illness in infants, elderly, and immunocompromised individuals. Toll-like receptor 7 (TLR7), a pattern recognition receptor recognising the ssRNA of RSV, activates proinflammatory pathways and triggers secretion of interferons (IFNs). On the one hand, the inflammatory responses help clear out virus. On the other hand, they lead to severe lung damage. Banlangen is a traditional Chinese herbal medicine commonly prescribed for respiratory virus infection treatment, but the mechanisms of action and active components remain largely unknown. In the present study, we investigated the effects of the main active components of total alkaloids from banlangen (epigoitrin, indole-3-carboxaldehyde, indole-3-acetonitrile and 4-methoxyindole-3-acetonitrile) on the RSV-induced inflammatory responses in mouse macrophage cells (RAW264.7). Our results demonstrated that RSV-induced IFN-α excessive secretion was moderately inhibited by indole-3-carboxaldehyde through downregulation of mRNA expression in a dose-dependent manner, in comparison, the inhibitory effects of ribavirin were too strong. Furthermore, we revealed that indole-3-carboxaldehyde suppressed transcription of IFN-α by inhibiting RSV-induced TLR7 expression in RAW264.7 cells. Additionally, indole-3-carboxaldehyde inhibited RSV-induced NF-κB signalling activation in a TLR7-MyD88-dependent manner. Together, our findings suggest that indole-3-carboxaldehyde inhibited RSV-induced inflammatory injury by moderate regulation of TLR7 signaling pathway and did not significantly affect the viral clearance competence of the innate immune system.

Graphic abstract