Expression of the Apx toxins of Actinobacillus pleuropneumoniae in Saccharomyces cerevisiae and its induction of immune response in mice

Actinobacillus pleuropneumoniae is an important pig pathogen, which is responsible for swine pleuropneumonia, a highly contagious respiratory infection. To develop subunit vaccines for A. pleuropneumoniae infection, the Apx toxin genes, apxI and apxII, which are thought to be important for protective immunity, were expressed in Saccharomyces cerevisiae, and the induction of immune responses in mice was examined. The apxI and apxII genes were placed under the control of a yeast hybrid ADH2-GPD promoter (AG), consisting of alcohol dehydrogenase II (ADH2) and the GPD promoter. Western blot analysis confirmed that both toxins were successfully expressed in the yeast. The ApxIA and ApxIIA-specific IgG antibody response assays showed dose dependent increases in the antigen-specific IgG antibody titers. The challenge test revealed that ninety percent of the mice immunized with ApxIIA or a mixture of ApxIA and ApxIIA, and sixty percent of mice immunized with ApxIA survived, while none of those in the control groups survived longer than 36 h. These results suggest that vaccination of the yeast expressing the ApxI and ApxII antigens is effective for the induction of protective immune responses against A. pleuropneumoniae infections in mice.