Downregulation of a protective Actinobacillus pleuropneumoniae antigen during the course of infection

The persistence of Actinobacillus pleuropneumoniae in convalescent pigs sig nificantly contributes to the distribution of disease. The downregulation o f protective antigens in vivo as one possible mechanism responsible for thi s phenomenon was investigated using the small iron-regulated transferrin bi nding protein (TbpB-protein) as exemplary protective antigen. From a total of 21 pigs experimentally infected with A. pleuropneumoniae serotype 7 in t hree trials, bronchoalveolar lavage fluid (BALF) was obtained on day 1 or 2 , day 7, day 14 and day 21. Employing double immunofluorescence of BALF wit h a monoclonal anti-TbpB antibody and an A. pleuropneumoniae-specific anti- polysaccharide antiserum a statistically significant decrease of the percen tage of A. pleuropneumoniae bacteria strongly expressing TbpB protein was o bserved during the course of infection. These results were supported by in vitro incubation of A. pleuropneumoniae in medium supplemented with BALF. I n addition, it was found that TbpB-expression in BALF from day 7 after infe ction could not be inhibited by the substitution of iron. These results suggest (i) the downregulation of protective antigens is one possible mechanism allowing bacterial persistence, (ii) in vitro induction in the presence of BALF mimics the in vivo situation, and (iii) TbpB expres sion is additionally regulated by an iron-independent mechanism. (C) 1999 A cademic Press.