A Novel Chimeric Fiber-C4/D11 Subunit Vaccine Induces Cross-Neutralizing Antibodies and Provides Better Protection Against Fowl Adenovirus (FAdV) Type 4 and Type 11 Than the Fiber-D11/C4 Subunit Vaccine

The widespread prevalence of different serotypes of fowl adenoviruses (FAdVs) has led to diverse vaccine demands, especially for subunit vaccines targeting FAdV-4 and FAdV-11, which cause hydropericardium-hepatitis syndrome (HHS) and inclusion body hepatitis (IBH), respectively. Although the Fiber protein is known to elicit robust immune protection, further exploration is needed to enhance the production of cross-neutralizing antibodies. This study utilized structural prediction and homology modeling techniques, employing domain-swapping strategy to integrate neutralizing epitope-containing amino acid sequences (274-451aa and 364-543aa) into the shaft domain of the Fiber protein. Two novel chimeric proteins were recombinantly expressed and developed into subunit vaccines: Fiber-C4/D11 and Fiber-D11/C4. Immunogenicity assessments revealed that the Fiber-C4/D11 vaccine group rapidly induced an antibody response against FAdV-11 within 7 days post-vaccination. By 28 days post-vaccination (dpv), the Fiber-C4/D11 vaccine group exhibited significantly higher levels of cross-neutralizing antibodies compared to the Fiber-D11/C4 group (<i>p</i> < 0.05). Challenge experiments demonstrated that both vaccines effectively alleviated clinical symptoms and prevented mortality in SPF chickens. Compared to Fiber-D11/C4, Fiber-C4/D11 significantly reduced body weight loss, liver lesions, viral titers in tissues, and viral shedding. Notably, no cross-neutralizing antibodies were detected following FAdV-4 or FAdV-11 infection, indicating a lack of natural cross-protection between the two serotypes. The chimeric vaccine addressed this gap, offering a promising multivalent approach to mitigate FAdV infections and advancing fowl adenoviral subunit vaccine strategies.