Safety, stability, and efficacy of noncapsulated mutants of Actinobacillus pleuropneumoniae for use in live vaccines.

Clonal, noniridescent mutants of Actinobacillus pleuropneumoniae serotypes 1 and 5 were isolated following chemical mutagenesis with ethyl methanesulfonate. The absence of any detectable capsule was confirmed by inhibition radioimmunoassay. There were no differences between the parent and mutant strains in lipopolysaccharide or protein electrophoretic profiles or in hemolytic activity. There was no detectable reversion to the encapsulated phenotype in vitro after passage in mice or pigs or in microporous capsules that were implanted subcutaneously in pigs for 6 weeks. The mutants were able to survive for more than 1 week in pigs following subcutaneous inoculation, which resulted in a strong immune response to whole cells and Apx toxins I and II. Intratracheal challenge of pigs with the serotype 5 mutant at a dose 1 log greater than the 50% lethal dose for the parent resulted in no clinical disease or lesions except in one pig that had slight pneumonia and pleuritis. Twenty-four hours after challenge, A. pleuropneumoniae could not be recovered from the respiratory tracts of any of the challenged pigs except for the one infected pig; this isolate remained noncapsulated. Immunization of pigs with one or both serotypes of noncapsulated mutants protected all pigs against clinical disease following intratracheal challenge with the virulent homologous or heterologous serotype. Nonimmunized control pigs and pigs immunized with a commercial bacterin died or had to be euthanized within 24 h of challenge. Thus, live noncapsulated mutants of A. pleuropneumoniae may provide safe and cost-effective protection against swine pleuropneumonia. These observations support the possibility that noncapsulated mutants of other encapsulated, toxin-producing bacteria may also prove to be efficacious live-vaccine candidates.