Vaccination and protection of pigs against pleuropneumonia with a vaccine strain of Actinobacillus pleuropneumoniae produced by site-specific mutagenesis of the ApxII operon

The production of toxin (Aps)-neutralizing antibodies during infection play s a major role in the induction of protective immunity to Actinobacillus pl europneumoniae reinfection. In the present study, the gene encoding the Apx II-activating protein, apxIIC, was insertionally inactivated on the chromos ome of a serovar 7 strain, HS93. Expression of the structural toxin, ApxIIA , and of the two genes required for its secretion, apxIB and apxID, still o ccurs in this strain. The resulting mutant strain, HS93C(-) Amp(r), was fou nd to secrete the unactivated toxin. Pigs vaccinated with live HS93C(-) Amp (r) via the intranasal route were protected against a cross-serovar challen ge with a virulent serovar I strain of A. pleuropneumoniae. This is the fir st reported vaccine strain of A. pleuropneumoniae which can be delivered li ve to pigs and offers cross-serovar protection against porcine pleuropneumo nia.