A self-assembled Fiber2 nanoparticle vaccine confers superior protection against fowl adenovirus serotype 4 infection.

Since 2015, hepatitis-pericardial effusion syndrome caused by fowl adenovirus serotype 4 (FAdV-4) has resulted in serious economic losses to the poultry industry in China, highlighting the urgent need for safe and effective vaccines. In this study, the Fiber2 protein of the FAdV-4 was solubly expressed in Escherichia coli and covalently conjugated to Mi3 which self-assembling nanoparticles (forming Fiber2-Mi3) via the SpyTag-SpyCatcher system. The optimal ratio for self-assembly of Fiber2 SpyTag with SpyCatcher-Mi3 was 3:1. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analyses indicated that the average particle sizes of Fiber2-Mi3 and Mi3 nanoparticles were approximately 32.0 nm and 20.8 nm, respectively. Compared to the Fiber2 monomer and a commercial inactivated vaccine, the Fiber2-Mi3 nanoparticle vaccine elicited earlier antibody production, detectable at 7 days post-vaccination (dpv). Furthermore, the neutralizing antibody titers in the Fiber2-Mi3 group were significantly higher than those in the other groups at 14, 21, and 28 dpv. Although challenge experiments demonstrated that chickens immunized with Fiber2-Mi3, Fiber2 monomer, or commercial vaccine were protected against FAdV-4 infection, the Fiber2-Mi3 nanoparticle group exhibited less weight loss, lower viral load, and reduced viral shedding in different tissues. In addition, based on hepatitis-pericardial effusion syndrome scoring and biochemical analysis, the Fiber2-Mi3 nanoparticle significantly inhibited liver damage caused by FAdV-4 infection and more effectively blocked its pathogenicity in target organs. Overall, the Fiber2-Mi3 nanoparticle vaccine conferred superior immune protection compared to the Fiber2 monomer and commercial vaccine. These findings provide a foundation for developing effective nanoparticle-based vaccines against FAdV-4.