Development of a subunit vaccine candidate (RBD-HA trimer) provides dual protection against multi-subtype avian influenza viruses and QX-type infectious bronchitis virus.

Avian influenza virus (AIV) and infectious bronchitis virus (IBV) are major respiratory pathogens of poultry, and their co-circulation complicates disease control. Here, we developed a chimeric subunit vaccine, RBD-HA, in which the receptor-binding domain (RBD) of the QX-type IBV spike protein replaced the immunodominant head domain of H9N2 AIV haemagglutinin. Structural analyses showed that RBD-HA formed a stable trimeric assembly, supporting the use of the HA stalk as an antigen-presenting scaffold. In chickens, RBD-HA induced humoral responses against both viruses and protected against homologous and heterologous H9N2 AIV challenge. In parallel, RBD-HA induced IBV-reactive and neutralizing antibody responses and provided protection against QX-type IBV challenge. The vaccine also elicited cross-reactive neutralizing activity and reduced viral shedding and tissue damage after H6N6 AIV challenge. These findings provide proof of concept for a bivalent subunit vaccine targeting two major avian respiratory viruses.