A homologous polyclonal antibody was produced in a rabbit to the 104-kilodalton (kDa) protein hemolysin of Actinobacillus pleuropneumoniae serotype 1 strain CM-5. In immunoblots, this antibody recognized a similar 104-kDa protein produced in culture supernatants by A. pleuropneumoniae serotypes 1 to 12 and taxon "Minor group" in addition to Pasteurella haemolytica, Actinobacillus suis, and alpha-hemolysin-producing Escherichia coli (but only weakly in the latter two organisms). These results were reproduced by using a mouse monoclonal antibody to the CM-5 104-kDa protein hemolysin, except that the monoclonal antibody bound more strongly to the alpha-hemolysin produced by E. coli, only weakly to the 104-kDa protein produced by "Minor group," and not at all to any extracellular antigens produced by A. suis. Pigs experimentally infected with A. pleuropneumoniae serotypes 1 to 10 and A. suis produced an antibody that recognized the 104-kDa hemolysin produced by CM-5. A pig challenged with a "Minor group" strain did not have such antibodies. Rabbit antiserum produced against the leukotoxin of P. haemolytica and alpha-hemolysin-producing E. coli also recognized the CM-5 hemolysin, but the latter only weakly. The hemolytic activity produced by CM-5 in culture supernatant was neutralized strongly by the pig serum to serotypes 1, 2, 5, 6, 9, and 10 and A. suis, only partially by serotype 8 antiserum and the rabbit antiserum to P. haemolytica leukotoxin, and not all by the antiserum to serotypes 3, 4, and 7 and "Minor group" and the E. coli alpha-hemolysin. These results indicate that a similar but not identical 104-kDa protein is produced in vitro and in vivo by all serotypes of A. pleuropneumoniae and may be related to cytolysins produced by other gram-negative bacteria
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