Surface-Displayed Expression of a Neutralizing Epitope of ApxIIA Exotoxin in<i>Saccharomyces cerevisiae</i>and Oral Administration of It for Protective Immune Responses against Challenge by<i>Actinobacillus pleuropneumoniae</i>

A neutralizing epitope fragment of ApxIIA toxin (ApxIIA#5) of the Korean Actinobacillus pleuropneumoniae serotype 2 strain was expressed and immobilized on the cell surface of Saccharomyces cerevisiae for efficient vaccine development. Expression of ApxIIA#5 was confirmed by Western blot analysis using cell-wall proteins, and the surface display of ApxIIA#5 was further visualized under confocal microscopy. Quantitative ELISA revealed that the recombinant ApxIIA#5 directed to the cell surface consisted of approximately 16% cell-wall proteins, estimated to be 35 mg of ApxIIA#5 protein per liter of cultured cells. An immunoassay revealed that antigen-specific antibodies against ApxIIA#5 were present in the sera of mice fed recombinant ApxIIA#5-expressing yeast, but not in mice fed the wild-type nor the vector-only transformant. Moreover, the mice fed the recombinant epitope-expressing yeast were protected from injection of a lethal dose of A. pleuropneumoniae.