The transferrin receptor of Actinobacillus pleuropneumoniae: quantitation of expression and structural characterization using a peptide-specific monoclonal antibody

When Actinobacillus pleuropneumoniae (A. pp) is grown under iron-restricted conditions in vitro, transferrin binding proteins (Tbps) are induced. The functional transferrin receptor of A. pp is composed of two outer membrane proteins (Tbp1 and Tbp2) and shows an exquisite specificity for porcine transferrin. This complex was studied using a monoclonal antibody (Mab 1.48) raised against a synthetic peptide corresponding to a hydrophilic domain of Tbp2 common to several A. pp serotypes. The antibody reacted specifically with a 60-70kDa Tbp2-antigen found in all serotypes of A. pp obtained from iron-restricted culture. It was found that Tbp2 was not expressed in iron replete medium by any serotype except serotypes 5a, 5b and 6 where a weak expression was seen. There was a weak expression of related antigens in Actinobacillus indolicus and Actinobacillus suis under iron-depleted conditions while no similar antigens were detected with the Mab in iron-starved Actinobacillus lignieresii, Actinobacillus porcinus, Actinobacillus minor, Haemophilus influenzae, and Haemophilus parasuis.Using an enzyme-linked immunosorbent assay (ELISA) based on the Mab 1.48, Tbp2 could be detected in both recombinant E. coli expressing Tbp2 and in wild type A. pp grown under iron restricted conditions. The subcellular location of Tbp2 in A. pp was studied by immunoelectron microscopy using the Mab 1.48. Interestingly, all antibody binding was found inside the A. pp cells, while Tbp2 expressed in recombinant E. coli was found both in the cytosol and on the outer membrane. These results indicate that the Mab 1.48-reactive epitope of Tbp2 is surface exposed when it is expressed without Tbp1 in E. coli while the inaccessibility of this epitope of Tbp2 in A. pp could be due to shading by the association between Tbp2 and Tbp1.