Study on the Immunogenicity of N-terminal Polypeptide of RTX Toxin I of Actinobacillus pleuropneumoniae

ApxIA is one of the most important virulence factors of Actinobacillus pleuropneumoniae (APP). To study the immunogenicity of ApxIA, its complete coding sequence (3 146 bp) and 5′- terminal 1 140 bp fragment were separately cloned into the prokaryotic expression vector pET-28a and expressed in the E. coli BL21 (DE3). The expression products purified from the inclusion bodies (designated as rApxIA and rApxIAN) showed a similar immunological reactivity as the native ApxIA purified from the culture media (designated as nApxI) in Western-blot analysis. Balb/c mice were intraperitoneally immunized with rApxIA, rApxIAN and nApxIA respectively and boosted once two weeks later. The serum antibody levels of the rApxIAN immunized mice were significantly lower than those immunized with rApxIA or nApxIA in an ApxIA-specific ELISA, whereas serum neutralization test demonstrated that mice immunized with rApxIAN, rApxIA and nApxI could generate similar levels of antibodies neutralizing the hemolytic activity of the native ApxIA. rApxIAN was able to elicit 80 % and 100 % protection when challenged at 14 days post the second immunization with a LD50 of APP serotype 1 and 2.