EVALUATION OF PROTECTIVE EFFICACY OF AN ACTINOBACILLUS-PLEUROPNEUMONIAE SEROTYPE-1 LIPOPOLYSACCHARIDE-PROTEIN CONJUGATE IN MICE

Actinobacillus pleuropneumoniae is the causative agent of porcine pleu ropneumonia. The major adhesin of A. pleuropneumoniae has previously b een identified as a lipopolysaccharide (LPS), and more recently, we de monstrated that high molecular mass LPS were involved in A. pleuropneu moniae adherence to porcine respiratory tract cells. We postulated tha t immunization with a LPS-based vaccine may confer a protective immuni ty. The high molecular mass O-polysaccharides obtained after acid hydr olysis and chromatographic separation were conjugated to bovine serum albumin (BSA) as a protein carrier. Groups of mice were injected twice with the following antigen preparations: whole-cell preparation, oute r membrane preparation, O-polysaccharide-BSA conjugate, hydrolyzed LPS and phenol/water extracted LPS. A combination of different adjuvants was also used during these immunization procedures to induce a stronge r immunological response to the polysaccharide antigen. Two weeks afte r the second injection, the mice were challenged intranasally with eit her homologous A. pleuropneumoniae serotype 1 strain or a serotype 5 s train. The highest survival rate, up to 80%, compared to the control g roups (P < 0.05), was recorded when the mice were injected twice with 15 mu g of carbohydrates of O-polysaccharide-BSA conjugate mixed with the saponin-derived adjuvant Quil A. Survival rates of between 60 and 70%, twice those observed in the control groups immunized with PBS, we re recorded in mice injected with the O-polysaccharide-BSA conjugate m ixed with other adjuvant preparations such as alhydrogel, peanut oil a nd Freund's incomplete adjuvant. However, the protection induced by th e conjugate antigen preparation was serotype specific, because mice ch allenged with a serotype 5 strain were killed. Taken together, these r esults confirm the important role of A. pleuropneumoniae LPS in pathog enesis. (C) 1996 Elsevier Science Ltd.