Efficacy of a cell extract from Actinobacillus (Haemophilus) pleuropneumoniae serotype 1 against disease in swine.

We partially characterized a cell extract (CE) from Actinobacillus pleuropneumoniae serotype 1 and used the CE to test the efficacy of secreted proteins against disease. Secreted products from 4-h culture supernatants were precipitated with 20% polyethylene glycol. Analysis of the CE indicated the presence of protein, endotoxin, and carbohydrate. Hemolytic activity to bovine erythrocytes and cytotoxic activity to porcine mononuclear leukocytes was also demonstrated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the CE from a 4-h culture showed a major band at 110 kilodaltons (kDa), while a CE of a 26-h culture indicated the presence of a number of additional proteins, including the 110-kDa protein. The 110-kDa protein was also identified as a glycoprotein by periodic acid-Schiff and silver staining. A single band precipitated against convalescent-phase pig antiserum when the polyethylene glycol precipitate was used in an Ouchterlony plate. Vaccination with CE conferred greater protection against challenge with the homologous serotype than either a commercial bacterin or an outer membrane protein vaccine. Hemolysin-neutralizing titers were higher both pre- and postchallenge in the group vaccinated with the CE compared with in all other groups. We believe that this demonstrates the importance of secreted factors in protection against disease and suggests that the 110-kDa protein is an important immunogen.