Recombinant FAdV-4 fiber-2 protein protects chickens against hepatitis-hydropericardium syndrome (HHS).

Virulent fowl adenovirus (FAdV) serotype 4 strains are the etiological agents of hepatitis-hydropericardium syndrome (HHS), a highly infectious disease in chickens with severe economic impact. In the present study, three different FAdV-4 derived capsid proteins, fiber-1, fiber-2, and hexon loop-1, were expressed in a baculovirus system and tested for their capacity to induce protection in chickens. Purified recombinant proteins were administered to day-old specific pathogen-free (SPF) chickens allocated in three separate groups and challenged with virulent FAdV-4 at 21 days of life. Two additional groups served as controls, a challenge control group with mock-vaccinated but infected birds and a negative control group with PBS injection substituting both vaccination and challenge. The fiber-2 vaccinated group displayed high resistance against the adverse effects of the challenge with only one dead bird out of 28, as compared to the challenge control group where the infection caused 78% mortality. A moderate protective effect resulting in 38% mortality was observed for fiber-1, whereas the hexon loop-1 vaccinated group was not effectively protected as manifested by 73% mortality. While a fiber-2 specific ELISA showed a gradual antibody increase after immunization of birds with the homologous protein, a commercial ELISA did not detect vaccination-induced antibodies in any of the groups but displayed a difference in challenge virus-directed response in protected and non-protected birds. Although immunoblotting confirmed the presence of specific antibodies in all vaccinated groups, the anti-protein sera did not exhibit neutralizing activity. Fecal excretion of challenge virus DNA was detected with a real-time PCR in the majority of tested birds until termination of the study independent of protection, indicating the prevention of clinical symptoms, but not infection, by vaccination. In conclusion, recombinant fiber-2 was identified as a protective immunogen and is proposed as an attractive candidate for a subunit vaccine to prevent hepatitis-hydropericardium syndrome in chickens.