MOLECULAR CHARACTERIZATION OF A COMMON 48-KILODALTON OUTER-MEMBRANE PROTEIN OF ACTINOBACILLUS-PLEUROPNEUMONIAE

Previous studies have shown that a vaccine prepared from outer membran es of Actinobacillus pleuropneumoniae serotype 5 can elicit protective immunity in swine against challenge with either serotype 5 or serotyp e 1. These results suggest the presence of common subcapsular surface antigens, such as outer membrane proteins, that contribute to cross-pr otective immunity. We have identified a 48-kDa outer membrane protein that is common to all 12 capsular serotypes of A. pleuropneumoniae but is not present in tile outer membranes of related species of gram-neg ative swine pathogens. This protein is immunogenic in swine infected w ith either A. pleuropneumoniae serotype 5 or 1A, as well as in swine v accinated with A. pleuropneumoniae serotype 5 outer membranes. This 48 -kDa protein is readily detected in outer membranes produced by sucros e density gradient centrifugation, but it is sarcosyl soluble and ther efore not found in outer membranes prepared by detergent treatment. Th e gene encoding the 48-kDa outer membrane protein has been cloned from A. pleuropneumoniae serotype 5 and has been designated aopA, for Acti nobacillus outer membrane protein A. The gene is 1,347 bp in length an d encodes a protein, designated AopA, of 449 amino acids with a predic ted molecular weight of 48,603. Southern blot analysis under high-stri ngency conditions showed that strains of all 12 serotypes of A. pleuro pneumoniae contain DNA homologous to this gene, as do strains of two c losely related species, Actinobacillus suis and Pasteurella multocida. Whether antibodies against the AopA antigen contribute to cross-prote ctive immunity against A. pleuropneumoniae infection remains to be det ermined.