Effect of oral vitamin A supplementation on host immune response to infectious bronchitis virus infection in specific pathogen-free chicken

Vitamin A is a fat-soluble vitamin that is a crucial mediator of the immune system. In this study, we evaluated the effect of oral vitamin A supplementa-tion on host immune responses to infectious bronchitis virus (IBV) infection in chickens. Forty 1-day-old spe-cific pathogen-free (SPF) chickens were fed a basal diet and randomly divided into 2 groups (n = 20 birds per group). Chickens in the experimental group were treated orally with vitamin A (dissolved in 0.1 mL soybean oil, at a dose of 8,000 IU per kg diet) daily. Birds in the con-trol group were orally administered 0.1 mL soybean oil without vitamin A until 21 d of age. On d 21 after birth, all chickens were infected with 0.1 mL of 106.5 50% median embryo infectious dose of a pathogenic IBV strain (CK/CH/LDL/091022) by intraocular and intra-nasal routes. The results demonstrated that oral vitamin A supplementation did not affect the clinical course of disease and growth performance of SPF chickens. How-ever, vitamin A supplementation increased the IBV-spe-cific IgG serum levels and decreased the viral load in some tissues of IBV-infected chickens. In addition, the results demonstrated that vitamin A supplementation decreased the expression levels of most immune-related molecules in some tissues of IBV-infected chickens. Vita-min A supplementation decreased the mRNA expression levels of some avian b-defensins (AvBD2, 3, 6, 7, 11, and 13) and increased the expression levels of AvBD9 and AvBD12 in some tissues of IBV-infected chickens. Simi-larly, vitamin A supplementation decreased the mRNA expression levels of some cytokines (interferon -g, inter-leukin-1b [IL-1b], and IL-6) and increased the mRNA expression levels of IL-2 in some tissues of IBV-infected chickens. Furthermore, vitamin A supplementation decreased the mRNA expression levels of myeloid differ-entiation primary response protein 88, nuclear factor-kB p65, toll-like receptor 3, toll-like receptor 7, and CD4. In summary, the present study suggests that vitamin A supplementation enhances the immune function of SPF chickens against IBV infection by inhibiting viral repli-cation, increasing the IBV-specific antibody titer, and suppressing the excessive inflammatory responses to IBV infection.