Protection of chickens against infectious bronchitis virus with a multivalent DNA vaccine and boosting with an inactivated vaccine

Yan, Fang; Zhao, Yujun; Hu, Yongting; Qiu, Jianyang; Lei, Wenxin; Ji, Wenhui; Li, Xuying; Wu, Qian; Shi, Xiumin; Li, Zhong

Avian infectious bronchitis (IB) is a major disease in the poultry industry worldwide. This disease frequently occurs in vaccinated and non-vaccinated flocks, and has caused severe economic loss over the last few years [11] [12] [13] [14] 17, 21] . Protection failure is mainly due to the numerous IB virus (IBV) serotypes and frequent emergence of new variants [23, 26] . Given the economic effects of IBV on the commercial poultry industry, preventing infection with this virus has been continually pursued. Traditional IBV vaccines include inactivated or live attenuated vaccines [2] [3] [4] . However, both types of vaccines have unique disadvantages. Inactivated vaccines can induce the production of relatively high antibody titers but a very low level of cytotoxic T lymphocyte (CTL) responses [5] . Live attenuated vaccines can initiate humoral and cellular immune responses, but lead to the spread the live vaccine viruses [7, 15] . It is therefore necessary to develop novel vaccines and/or optimize current vaccination procedures. IBV belongs to the Coronaviridae family and contains a positive single-stranded RNA genome encoding four major structural proteins: a small envelope protein (E), integral membrane protein (M), phosphorylated nucleocapsid protein (N), and spike glycoprotein (S) [19] . The S protein is cleaved into two subunits (S1 and S2). The S1 protein is very important for inducing protective immunity and has been successfully used to construct IBV DNA vaccines [6, 9] . The N protein is conserved and induces CTL as well as activated B cell responses, which are critical for preventing IBV infection in poultry [8, 20] . The M glycoprotein can induce the production of detectable antibodies and delayed type hypersensitivity responses [8] . Hence, all of these proteins are primary targets for developing DNA vaccines to elicit immune responses.