Immunological characterization of breakdown peptides of the 104 kilodalton hemolysin of Actinobacillus pleuropneumoniae serotype 1.

The 104 kilodalton (kDa) hemolysin of Actinobacillus pleuropneumoniae serotype 1, strain CM-5 was precipitated from RPMI-1640 culture supernatant using ammonium sulfate to 80% saturation. In immunoblots, a rabbit polyclonal antiserum against the 104 kDa hemolysin protein, recognized not only the original 104 kDa monomeric form of the hemolysin but other proteins in the crude antigen mixture ranging in molecular mass from 43 to greater than 125 kDa. The antiserum was able to crosslink these proteins to active hemolysin in RPMI-1640 culture supernatant resulting in bands of hemolysis in blood agar used in a contact assay. Corresponding to these bands of hemolysis, denatured peptides with molecular masses of 51, 85, 104 and greater than 125 kDa were excised and injected into rabbits. In immunoblots, the resultant antibodies recognized the injected peptide and the monomeric 104 kDa protein. However, only the rabbit antisera produced against the 104 and 125 kDa proteins contained antibodies which neutralized the active 104 kDa hemolysin in culture supernatant. These results indicate that (i) the 104 kDa protein hemolysin can exist in a higher molecular weight aggregate (greater than 125 kDa) but can also break down to peptides which have molecular masses smaller than the 104 kDa parent molecule and (ii) while several epitopes are present in the hemolysin molecule, there seems to be a restricted number of antigenic determinants responsible for inducing neutralizing antibodies and these seem to reside only in the 104 kDa parent molecule. This may have consequences, in terms of vaccine development, for the control of pleuropneumonia in swine herds.