Humoral antibody response and protective immunity in swine following immunization with the 104-kilodalton hemolysin of Actinobacillus pleuropneumoniae.

Five cesarean-derived, colostrum-deprived pigs were given three adjuvant-supplemented subcutaneous and one intravenous injection of the purified 104-kDa hemolysin from serotype 1 Actinobacillus pleuropneumoniae CM-5. Six control animals received phosphate-buffered saline only. Five of six control pigs died within 24 h after challenge. The sixth control pig was moribund and euthanized after 48 h. All six pigs had pleuropneumonia, and A. pleuropneumoniae was isolated from all six lungs. None of the vaccinated pigs died as a result of challenge. After being euthanized, two pigs in this group had no lung lesions but three had chronic pleuropneumonia involving 10, 20, and 40% of the lung tissue. A. pleuropneumoniae was isolated from lung lesions of these three animals but not from the two pigs without lesions. The prechallenge hemolysin-neutralizing antibody titers in the vaccinated pigs were 1:10,900, 1:10,600, 1:4,800, 1:3,900, and 1:3,000, in order of increasing lung involvement. None of the control pigs had neutralizing antibodies. Enzyme-linked immunosorbent assay (ELISA) antibodies to capsule, lipopolysaccharide, and hemolysin were not detected in serum samples collected from the control pigs. In the vaccinated group, prechallenge sera did not contain ELISA antibodies to capsule or lipopolysaccharide. ELISA antibodies to the hemolysin were detected only in the prechallenge and postchallenge serum samples. These results indicate that pigs immunized with the 104-kDa hemolysin of serotype 1 A. pleuropneumoniae are protected against challenge with virulent bacteria. The association between neutralizing antibodies and protection indicates indirectly that the hemolysin is an important virulence factor.