WH01-3A: a DIVA-compliant, ApxIA, ApxIIA, and ApxIIIA expressing Actinobacillus pleuropneumoniae live attenuated vaccine strain that protects mice and pigs against homologous and heterologous serovars.

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a major economic burden to the worldwide swine industry. Many available vaccines protect against one or a few of the 19 serovars known. Live attenuated vaccines can potentially protect against many different serovars. In this study, the highly virulent parental A. pleuropneumoniae serovar 1 strain WH01 was used to construct WH01-3A, expressing nontoxic (nonacylated) versions of the three major toxin antigens ApxIA, ApxIIA, and ApxIIIA. In addition, the vivo-induced toxin ApxIVA was deleted, enabling use in a differentiation of vaccinated from infected animals (DIVA) strategy. Compared with WH01, the virulence of WH01-3A in mice was significantly reduced, and, when used as a live attenuated vaccine, WH01-3A protected 87.5%, 75%, and 87.5% animals against A. pleuropneumoniae challenge with serovars 1, 5, and 15, respectively. Pigs immunized with up to 5 × 108 CFU WH01-3A had normal body temperatures and behaviors, and normal lung and tonsil pathology. WH01-3A immunization of pigs at 1 × 108 CFU resulted in 66.7% and 100% protection after challenge with A. pleuropneumoniae serovars 1 and 15, respectively. WH01-3A immunization induced antibodies against ApxI, ApxII, ApxIII, and whole-cell proteins of serovars 1 and 15. Compared to the unvaccinated group, the average lung lesion scores were reduced by 42.6% and 67.8% after challenge with serovars 1 and 15, respectively. We conclude that WH01-3A is a safe and effective live attenuated vaccine, providing effective immunological protection against A. pleuropneumoniae serovars 1 and 15.