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

In the present study, a prime-boost vaccination regimen that completely protected chickens against challenge with a virulent IBVSX16 strain was developed. Immune responses were evaluated in chickens immunized with the DNA vaccines either alone or with a mixture of the three vaccines. Findings from the present study agree with those of a previous investigation showing that all three IBV proteins selected for this study have their own unique and important roles in eliciting IBV immune responses [24] . The DNA vaccination with pVAX1-16M alone provoked the weakest immune response demonstrated by the lowest antibody titers (p ＜ 0.01) in comparison with pVAX1-16S1 and pVAX1-16N. In contrast, pVAX1-16S1 induced the highest antibody titers among the three constructs. Chickens immunized with pVAX1-16N had significantly higher levels of IBV-specific cellular proliferation (p ＜ 0.01) and T lymphocytes (p ＜ 0.01) than birds immunized with pVAX1-16S1 and pVAX1-16M. The challenge assay also proved that the DNA vaccine targeting the N protein is more effective. Chickens that received a combination of the three DNA vaccines also mounted stronger immune responses than the birds immunized with each DNA vaccines alone, suggesting that a combined DNA vaccination with S1, M, and N genes may provide stronger protection against IBV. CD4+ T cells may directly produce antiviral cytokines, which increasing B cell activity and increasing the proliferation, maturation, and functional activity of CD8+ CTLs. The CD8+ CTL plays a critical role in controlling IBV infection [18] . At 49 days of age, chickens immunized with plasmid constructs groups had percentages of CD4+CD3+ and CD8+CD3+ T lymphocytes subgroups higher than those of birds administered the control pVAX1 vector or inactivated vaccine (p ＜ 0.01). Increased number of T lymphocytes indicates that administration of the DNA vaccine resulted in effective cellular immunization and promoted virus clearance [2] .