Immunogenicity of a recombinant Saccharomyces cerevisiae - vectored multi-epitope vaccine targeting the infectious bronchitis virus S1 protein in broiler chickens

Abstract

Background: The infectious bronchitis virus (IBV) remains a major constraint on poultry production because rapid viral evolution can reduce cross-protection between circulating variants and vaccine strains. A mucosal vaccine platform capable of coordinating antigen-specific humoral, cellular, and local responses may complement conventional vaccination programs. Aim: This study aimed to evaluate the immunogenicity and growth-associated effects of a recombinant Saccharomyces cerevisiae-vectored multi-epitope vaccine derived from the IBV S1 protein after oral or intranasal administration to broiler chickens. Methods: Ninety-one-day-old broiler chicks were randomly assigned to six groups (15 birds/group): recombinant vaccine by the oral route (G1), recombinant vaccine by the intranasal route (G2), commercial IBV vaccine (G3), empty-vector yeast by the oral route (G4), empty-vector yeast by the intranasal route (G5), and phosphate-buffered saline (G6). Primary and booster administrations were given on days 10 and 20. Body weight, weekly gain, feed conversion ratio (FCR), IBV S-specific IgM, serum interferon-gamma (IFN-γ), and fecal IgA were measured. Data were summarized as mean ± standard error and group differences were interpreted using the least significant difference thresholds reported in the experimental dataset at P < 0.05. Results: Body weight differences remained below the reported LSD through day 14 but exceeded it from day 21 onward. G1 reached the highest day-35 weight (2650 ± 44.5 g), followed by G4 (2500 ± 44.09 g). After boosting, IBV-specific IgM was highest in G3 (6.174 ± 0.05 ng/mL) and G2 (5.639 ± 0.24 ng/mL), both exceeding the reported LSD by more than the vector and PBS controls. IFN-γ peaked after priming in G2 (45.489 ± 3.76 pg/mL), whereas fecal IgA increased most strongly in G1 and G2 after priming and remained higher than control values after boosting. No challenge experiment was performed; therefore, the findings demonstrate immunogenicity rather than protection. Conclusion: The recombinant yeast-vectored construct elicited route-dependent systemic, cellular, and mucosal immune responses, with intranasal administration favoring IFN-γ and IBV-specific IgM and oral administration sustaining fecal IgA. Before protective efficacy can be established, controlled IBV challenge, antigen-expression confirmation, duration-of-immunity studies, and field validation are required.