Forsythoside A blocks the infection of Avian Infectious Bronchitis Virus through binding S1 subunit

Avian infectious bronchitis (IB) is an acute highly contagious respiratory infectious disease caused by avian infectious bronchitis virus (IBV), causing serious economic losses to poultry industry. Forsythiaside A (FTA) is a phenylethanol glycoside compound in the fruit of Forsythia suspensa, which is widely used for inflammatory respiratory tract disease, including viral infected inflammation. In this study, the mechanism of FTA against IBV was researched. To determine the inhibitory effect of FTA on IBV infection and the interaction between FTA and IBV particles, immunofluorescence assay, plaque assay, RT-qPCR and enzyme-linked immunosorbent assay (ELISA) were performed and it was discovered that FTA inhibited IBV infection and viral attachment to host receptor under non-cytotoxic concentrations, and FTA may bind to virus particles. Then ELISA and isothermal titration calorimetry (ITC) were employed to further study the interaction between FTA and S protein. The results indicated that FTA bound to S1 subunit rather S2 subunit. Besides, the molecular docking showed that FTA form hydrogen bonds with eight amino acids in the S1 subunit. When these amino acids were mutated. the affinity disappeared between the S1 mutant and FTA. In conclusion, FTA prevents IBV infection by blocking viral attachment and contacts with the S1, but not the S2 subunit of IBV. At last, the bind sites were identified by comparing the affinity of wild S1 and mutant S1.