Immunogenicity of candidate protective antigens Omp19, VirB8, and HlyD. A, B Anti-antigen IgG titres and antibody subclasses in serum were determined by ELISA and represented as dilution titres. C–H Cellular immune responses elicited by antigen proteins. Expression levels of TNF-α, IFN-γ, IL-4, IL-6, IL-10 and IL-17 in supernatants were subsequently quantified. Error bar, mean ± S.D.