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 (Apx)-neutralizing antibodies during infection plays a major role in the induction of protective immunity to Actinobacillus pleuropneumoniae reinfection. In the present study, the gene encoding the ApxII-activating protein, apxIIC, was insertionally inactivated on the chromosome 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 occurs in this strain. The resulting mutant strain, HS93C(−) Amp(r), was found to secrete the unactivated toxin. Pigs vaccinated with live HS93C(−) Amp(r) via the intranasal route were protected against a cross-serovar challenge with a virulent serovar 1 strain of A. pleuropneumoniae. This is the first reported vaccine strain of A. pleuropneumoniae which can be delivered live to pigs and offers cross-serovar protection against porcine pleuropneumonia.