JIA-2019-11

2602 ZHANG Jun-qin et al. Journal of Integrative Agriculture 2019, 18(11): 2598–2604 observed around the myocardial cells (Fig. 3). Infiltrated red blood cells and basophilic intranuclear inclusion body was observed in hepatic cells (Fig. 3). 4. Discussion As for the prevention and control of HHS, formalin- inactivated liver homogenate, killed vaccines produced in tissue culture and live vaccines attenuated in a fibroblast cell line (QT35) have been used (Kumar et al . 1997; Schonewille et al . 2010; Mughal et al . 2011; Kim et al . 2014). However, incomplete inactivation and return of virulence of these inactivated or live vaccines still compromise biosecurity threats. So developing a subunit vaccine to prevent the disease is necessary. To develop a subunit vaccine against HHS, capsid proteins like fiber-1, fiber-2, hexon loop-1, and penton base, have been expressed in E . coli or baculovirus or recombinant human adenovirus expression system. Previously studies have similar findings that penton base and fiber-2 protein expressed in E . coli provided full protection again virulent FAdV-4 challenge. One study showed that a dosage of 25 μg penton base protein expressed in E . coli provided 100% protection compared to 60% that of inactivated vaccine (Shah et al . 2012). Another study has also shown that a dosage of 10 μg fiber-2 protein expressed in E . coli induced 100% survival rate compared to 90% that of inactivated vaccine (Chen et al . 2018). Our results are in consistency with the above two studies though we did not use inactivated vaccine as control. However, another study showed that different dosages of E . coli expressed fiber-2 induce complete protection and only at a high dosage of 200 μg penton base could induce 90% protection with the control group with 45% survival rate, which is different to our results (Wang et al . 2017). Since the amino acid sequences of fiber-2 and penton base in this study are exactly the same to ours, we speculate that baculovirus expression system may be more suitable for penton base protein than E . coli for some unknown reason. It is interesting that in another expression system the results are also different from other researches. In this study, fiber-1, Fiber-2, and penton base protein of FAdV-4 were expressed in HEK293Acells or HK-2 cells by recombinant human adenovirus (Wang et al . 2018). These three proteins assembled penton-dodecahedron (Pt-Dd) and fiber-1 provide complete protection, while fiber-2 and penton base only induce 80 and 67% protection, respectively. The mechanisms why fiber-1 provided better protection than did fiber-2 and penton base are still unknown and need further research. Base on previous studies, penton base protein is a promising vaccine candidate against FAdV-4. And the protective efficiency and immunogenicity of baculovirus- expressed penton base protein have not been assessed before. The baculovirus expression system is a promising expression platform, which has been extensively used to produce various candidate subunit vaccines for human and animals; several of these vaccines are commercially available (Lin et al . 2014). In the present study, we evaluated the protective efficiency and immunogenicity of baculovirus-expressed penton base protein. Comparing with the study conducted by Schachner et al . (2014), fowl adenovirus antibodies were detected in groups vaccinated with the recombinant proteins before the challenge by using a commercial test kit as in our study (Fig. 2). Two weeks after the first immunization, chickens in groups immunized with penton and fiber-2 were all seropositive. One week after the second immunization, the antibody titers in group A and group B (immunized with penton and fiber-2, respectively) reached the highest level before challenge. And all chickens immunized with penton+fiber-2 in group C were seropositive. Heart Penton Fiber-2 Penton base+Fiber-2 Challenge control Negative control Liver 100 μm 100 μm 100 μm 100 μm 100 μm 100 μm 100 μm 100 μm 100 μm 100 μm Fig. 3 Histological lesions of heart and liver in chickens after fowl adenovirus serotype 4 (FAdV-4) challenge. No apparent lesion of heart and liver in groups immunized with penton, fiber-2 and penton base+fiber-2. Lesions were shown in the challenge control group including lymphocytes infiltration around the myocardial cells, red blood cells infiltration and basophilic intranuclear inclusion body in the hepatic cells. The negative control group showed no lesions.