Induction of antigen-specific immune responses by oral vaccination with Saccharomyces cerevisiae expressing Actinobacillus pleuropneumoniae ApxIIA

An effective way of inducing both mucosal and systemic immune responses to protect against Actinobacillus pleuropneumoniae serotype 2 Korean isolate was examined in mice by oral immunization using Saccharomyces cerevisiae expressing the ApxIIA protein. The immunogenicity of the yeast-derived ApxIIA antigen was confirmed by the challenge test and ApxIIA-specific IgG antibody response assay. The group subcutaneously immunized with the protein extracted from the yeast expressing ApxIIA showed a higher survival rate after challenging with A. pleuropneumoniae serotype 2 isolate and IgG antibody level in serum than the group injected with that prepared from the yeast harboring vector only. Feeding the yeast expressing ApxIIA to mice induced both systemic and mucosal immune responses against the antigen. ApxIIA-specific IgA antibody titers and the number of IgA-secreting cells of mice vaccinated with S. cerevisiae expressing ApxIIA dose-dependently increased from the third immunization in both intestine and lung (P < 0.01). A similar tendency of ApxIIA-specific IgG antibody responses was observed in the sera. The protective efficacy of the oral immunization was then evaluated by a challenge with a minimal lethal dose (MLD, 4.5 × 10⁷ CFU/ml) of the A. pleuropneumoniae serotype 2 isolate. Fifty percent of the 30 mg administered group and 30% of the 15 mg administered group survived while none of the mice in the control groups survived after 36 h. These results suggest that feeding animals the yeast expressing the antigen can be an effective strategy to induce protective immune responses against A. pleuropneumoniae infection.