Us3 and Us9 proteins contribute to the stromal invasion of bovine herpesvirus 1 in the respiratory mucosa.

Bovine herpesvirus 1 (BHV-1) infection may lead to conjunctivitis, upper respiratory tract problems, pneumonia, genital disorders and abortion. BHV-1 is able to spread quickly in a plaque-wise manner and invade by breaching the basement membrane (BM) barrier in the respiratory mucosa. BHV-1 Us3, a serine/threonine kinase, induces a dramatic cytoskeletal reorganization and BHV-1 Us9, a tail-anchored membrane protein, is required for axonal transport of viruses in neurons. In this study, we investigated the role of Us3 and Us9 during BHV-1 infection in the respiratory mucosa. First, we constructed and characterized BHV-1 Us3 null, Us9 null and revertant viruses. Then, we analysed the viral replication and plaque size (latitude) in Madin-Darby bovine kidney (MDBK) cells and the respiratory mucosa as well as viral penetration depth underneath the BM of the respiratory mucosa when inoculated with these recombinant viruses. Knockout of Us3 resulted in a 1 log ₁₀ reduction in viral titre and plaque size (latitude) in MDBK cells and the trachea mucosa. There were no defects in the cell-to-cell spread observed for BHV-1 Us9 null virus. Both BHV-1 Us3 null and Us9 null viruses showed a significant reduction of plaque penetration underneath the BM; however, penetration was not completely inhibited. In conclusion, the current findings demonstrated that Us3 and Us9 play an important role in the invasion of BHV-1 through the BM of the respiratory mucosa, which shows the way forward for research-based attenuation of viruses in order to make safer and better-performing vaccines.