An immunosorbent assay based on the recombinant ApxIa, ApxIIa, and ApxIIIa toxins of Actinobacillus pleuropneumoniae and its application to field sera

Actinobacillus pleuropneumoniae is the etiologic agent of porcine pleuropneumonia, a highly contagious pulmonary disease in pigs with major economic losses for pig producers worldwide. Whereas A. pleuropneumoniae isolates are divided into 15 serotypes, the isolates secrete 4 types of exotoxins (ApxI, ApxII, ApxIII, and ApxIV), which are known as major virulence factors. In the current study, the ApxIA, ApxIIA, and ApxIIIA genes were amplified and their recombinant proteins expressed in Escherichia coli M15 cells. The antigenicity of each recombinant protein was demonstrated by Western blot and enzyme-linked immunosorbent assay (ELISA) using sera from pigs vaccinated with a subunit vaccine. When ELISAs using the recombinant antigens were optimized and then applied to sera from 320 randomized pigs in Korea, an observed increase in seroprevalence was found among sows in comparison with weaned piglets and growing pigs, indicating an age-dependent seroprevalence. The results obtained in the study suggest that the developed ELISAs may be useful for A. pleuropneumoniae vaccination strategy as a screening tool for pig herds as well as for detection of specific antibodies to Apx exotoxins.