Immunogenicity and protective efficacy of recombinant fiber-2 protein in protecting SPF chickens against fowl adenovirus 4

Since a novel hyper-virulent fowl adenovirus serotype 4 (FAdV-4) infection occurred in 2015, the novel FAdV-4 has been widely spreading across China, causing significant economic losses to the poultry industry. As the urgency of the issue calls for effective and efficient solutions, the present study investigated the possibility of the fiber-2 protein of the FAdV-4 to serve as a vaccine candidate against the novel FAdV-4. In the research, fiber-2 proteins were expressed in Escherichia coli, and then purified. To evaluate the immunogenicity of the recombinant fiber-2 protein, we investigated both the humoral and cellular immune responses in chickens immunized with fiber-2. The humoral immunity was assessed by detecting IgY antibodies and virus-neutralizing antibodies in chicken serum at 7, 14, 21 days post-immunization (dpi). We examined cellular immune responses by detecting CD3+CD4+ and CD3+CD8+ changes in chickens' peripheral blood through using flow cytometry at 7, 14, 21 dpi. The cytokine production in the serum of the immunized chickens was detected by ELISA at 7, 14, 21 dpi to further explore the impact of the recombinant protein on the regulation of cytokines. The protective efficacy was determined by the survival rate of the immunized chickens challenged with the novel FAdV-4. The results show that the level of IgY antibodies of the chickens immunized with fiber-2 protein was significantly higher than that of the chickens immunized with an inactivated vaccine against FAdV-4. Moreover, 7 days after immunization, the CD4+ T-cell proliferative response of the chickens immunized with fiber-2 was significantly higher than that of the chickens immunized with the inactivated vaccine. Challenge experiment showed that the fiber-2 protein could provide full protection and the inactivated vaccine could provide 90 percent protection against the FAdV-4. These results suggest that the recombinant fiber-2 protein can be an ideal candidate for subunit vaccines against the disease.