Immunity analysis against Fowl Adenovirus serotype 4 (FAdV-4) based on Fiber-2 trimer Protein with the different virulence.

Since June 2015, Fowl adenovirus outbreaks have occurred in China, causing significant economic losses to poultry industry. The FAdV-4 Fiber-2 proteins could induce effective protection, but the precise mechanism of immune protection remains unknown. Here, we have compared the biological characteristics of Fiber-2 protein of the very virulent WZ strain of FAdV-4 (vvFAdV-4) with that of non-virulent ON1 strain. The sequence analysis revealed natural deletions and sequence differences between the classical non-pathogenic strain ON1 and the vvFAdV-4 isolate. These two Fiber-2 proteins successfully expressed in E. coli resemble in structure and function to the native-like trimeric protein. The trimeric structure and bioreactivity of the recombinant Fiber-2 proteins to FAdV-4 specific antibodies were characterized. The immune protection induced by Fiber-2 proteins of FAdV-4 WZ and ON1 strains were compared in SPF chickens. All birds in the WZ-Fiber-2 immunized group generated systemic specific antibodies compared with both ON1-Fiber-2 protein and PBS immunized groups. According to the results of attack mortalities, viral shedding and tissue gross lesion, the WZ Fiber-2 protein induced complete protection at a dose of 2 μg per chicken, whereas the ON1-Fiber-2 protein induced 0 protection at 3 dpc. In view of the characteristics of Fiber-2 proteins of different strains, this study can help us to further understand the mechanism of protective immunity and provide a basis for the prevention and control of FAdV-4 in chickens.