Immunological properties of Actinobacillus pleuropneumoniae hemolysin I

The 105 kDa hemolysin I protein from Actinobacillus pleuropneumoniae serotype I type strain 4074 (HlyI) was shown by immunoblot analysis to be the predominant immunogenic protein if convalescent field sera or sera from pigs experimentally infected with A. pleuropneumoniae serotype 1 were used. SDS gel- and immunoblot-analysis using total culture, washed cells or culture supernatant showed that HlyI is essentially secreted and is not found attached to the bacteria. Proteins in the 105 kDa range that react strongly with anti-HlyI antibody, are produced by all serotypes and are presumed to be their hemolysins. Sera from pigs experimentally infected with each of the 12 serotypes strongly reacted with HlyI. In addition, some sera from pigs that were confirmed to be negative for A. pleuropneumoniae, also reacted with HlyI as well as with related proteins from Actinobacillus rossii and Actinobacillus suis. These two species produce proteins in the 105 kDa range which cross-react strongly with HlyI. They could be the source of the immunological reactions of the A. pleuropneumoniae-negative sera with HlyI. However, no cross-reactions could be found between HlyI and the Pasteurella haemolytica leukotoxin, the Escherichia coli alpha-hemolysin or related proteins from various hemolytic E. coli strains isolated from pigs. The immunological cross-reactions of HlyI with related proteins from A. rossii, A. suis and possibly from other bacterial species may create uncertainty in interpretation if HlyI is used as the antigen in serodiagnosis of A. pleuropneumoniae.