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

The persistence of Actinobacillus pleuropneumoniae in convalescent pigs significantly contributes to the distribution of disease. The downregulation of protective antigens in vivo as one possible mechanism responsible for this phenomenon was investigated using the small iron-regulated transferrin binding protein (TbpB-protein) as exemplary protective antigen. From a total of 21 pigs experimentally infected with A. pleuropneumoniae serotype 7 in three trials, bronchoalveolar lavage fluid (BALF) was obtained on day 1 or 2, day 7, day 14 and day 21. Employing double immunofluorescence of BALF with a monoclonal anti-TbpB antibody and an A. pleuropneumoniae-specific anti-polysaccharide antiserum a statistically significant decrease of the percentage of A. pleuropneumoniae bacteria strongly expressing TbpB protein was observed during the course of infection. These results were supported by in vitro incubation of A. pleuropneumoniae in medium supplemented with BALF. In addition, it was found that TbpB-expression in BALF from day 7 after infection 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 expression is additionally regulated by an iron-independent mechanism. (C) 1999 Academic Press.