Conservation and antigenic cross-reactivity of the transferrin-binding proteins of Haemophilus influenzae, Actinobacillus pleuropneumoniae and Neisseria meningitidis

Haemophilus influenzae acquires iron from the iron-transporting glycoprotein transferrin via a receptor-mediated process. This involves two outer-membrane transferrin-binding proteins (Tbps) termed Tbp1 and Tbp2 which show considerable preference for the human form of transferrin. Since the Tbps are attracting considerable attention as potential vaccine components, we used transferrin affinity chromatography to examine their conservation amongst 28 H. influenzae type b strains belonging to different outer-membrane-protein subtypes as well as six non-typable strains. Whole cells of all type b and nontypable strains examined bound human transferrin; whilst most strains possessed a Tbp1 of approximately 105 kDa, the molecular mass of Tbp2 varied from 79 to 94 kDa. Antisera raised against affinity-purified native H. influenzae Tbp1/Tbp2 receptor complex cross-reacted on Western blots with the respective Tbps of all the Haemophilus strains examined. When used to probe Neisseria meningitidis Tbps, sera from each of four mice immunized with the Haemophilus Tbp1/2 complex recognized the 68 kDa Tbp2 of N. meningitidis strain B16B6 but not the 78 kDa Tbp2 of N. meningitidis strain 70942. Serum from one mouse also reacted weakly with Tbp1 of strain B16B6. Apart from a weak reaction with the Tbp2 of a serotype 5 strain, this mouse antiserum failed to recognize the Tbps of the porcine pathogen A. pleuropneumoniae. However, a monospecific polyclonal antiserum raised against the denatured Tbp2 of Neisseria meningitidis B16B6 recognized the Tbps of all Haemophilus and Actinobacillus strains examined. Since H. influenzae forms part of the natural flora of the upper respiratory tract, human sera were screened for the presence of antibodies to the Tbps. Sera from healthy adults contained antibodies which recognized both Tbp1 and Tbp2 from H. influenzae but not N. meningitidis. Convalescent sera from meningococcal meningitis patients contained antibodies which, on Western blots, recognized the Tbps2s of both pathogens. These data demonstrate the existence of shared epitopes on the Tbps of H. influenze, N. meningitidis and A. pleuropneumoniae despite their transferrin species specificity.