Positive role for rApxIVN in the immune protection of pigs against infection by Actinobacillus pleuropneumoniae.

The role of in vivo-induced ApxIV toxin of Actinobacillus pleuropneumoniae in protective immunity was evaluated in pigs by administering it alone or added to a multicomponent recombinant subunit vaccine composed of recombinant ApxI, ApxII, ApxIII toxin, and 42-kDa outer membrane protein (OMP). The pigs were immunized with vaccine I (rApxIVN), vaccine II (rApxI+rApxII+rApxIII+rApxIVN+rOMP), vaccine III (rApxI+rApxII+rApxIII+rOMP), or placebo (phosphate-buffered saline+adjuvant). A. pleuropneumoniae serovar 1 field isolate JMS 06 and serovar 2 field strain FX 01 were used as the challenge strains. Pigs that were immunized with vaccine I or vaccine II all developed high antibody titers against rApxIVN. The antibody titers against rApxI, rApxII, rApxIII, and rOMP in pigs immunized with vaccine II were higher than those in pigs vaccinated with vaccine III. Following the challenge, the pigs immunized with rApxIVN alone showed similar results to the pigs in the control group, such as severe respiratory symptoms and severe lung lesions. Pigs that had been immunized with vaccine II or vaccine III were protected against challenge with A. pleuropneumoniae serovar 1 and serovar 2. The pigs immunized with vaccine II had slighter lung lesions and fewer bacterial recovery than those of pigs immunized with vaccine III. These results indicate that rApxIVN contributes to the production of high level of antibodies directed against the vaccination antigens, and thus confers strong protection against challenges with different serovars of A. pleuropneumoniae.