| Actinobacillus pleuropneumoniae(APP) serotype 7 is the predominant causative agent of porcine infectious pleuropneumonia in China. The hemolysin from serotype 7 strain Shen 8 has been studied. Heat stability-analysis showed that there must be two different hemolysins. One is heat-stable, the hemotysin was not inactivated by heating at 60C for 30 minutes even by autoclaving at 121 0C for 1 hour. It was also resistant against treatment with formalin and proteinase K. These results indicate that this kind of hemolysin may be polysaccharides in nature. The other is heat-labile, the hemolytic activity was lost completely by heating at 60XD for 30 minutes or at 100C for 3 minutes. SDS-PAGE-analysis following purification by ammonium sulfate precipitation and filtration chromatography showed that the other hemolysiri is a 65-Kilodalton protein. It was sensitive to proteinase K, but not to formalin.The purified 65KDa hemolysin protein and reference sera to each sercitype strain of APP were used in cross-neutralization test and agar gelimmunodiffusion test. The sera to serotypes 1 and 4 revealed high titer of hemolysin-neutralizing antibodies in the former test, which suggest that they have a close relationship with serotype 7. In agar gel-immunodiffusion testbesides homologous serotype7, other serotypes sera such as serotypel,4,5 and 9 showed precipitate lines too. These results indicate that there may be manyantigen-associated epitopes on the antibodies against the hemolysin, but epitopescontribute to neutralization may be very little, which only exist among a fewserotypes. Since there are hemolysin-neutralizing antibodies in the convalescentserum from swine that infected APP naturally, it is expected that hemolysin mayhave immunogenicity and can induce immunological protection. The 65KDahemolysin protein was used as antigen to inoculate rabbits followed by challengewith APP serotype 7. It revealed that hemolysin can only confer partial protectionagainst the challenge with the homologous strains.
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