Analysis of the immunological responses to transferrin and lactoferrin receptor proteins from Moraxella catarrhalis

Moraxella catarrhalis expresses surface receptor proteins that specifically bind host transferrin (Tf) and lactoferrin (Lf) in the first step of the i ron acquisition pathway. Acute- and convalescent-phase antisera from a seri es of patients with M. catarrhalis pulmonary infections were tested against Tf and Lf receptor proteins purified from the corresponding isolates. Afte r the purified proteins had been separated by sodium dodecyl sulfate-polyac rylamide gel electrophoresis and Western blotting, we observed strong react ivity against Tf-binding protein B (TbpB; also called OMP1) and Lf-binding protein B (LbpB) but little or no reactivity against Tf-binding protein A ( TbpA) or Lf-binding protein A (LbpA), using the convalescent-phase antisera , Considerable antigenic heterogeneity was observed when TbpBs and LbpBs is olated from different strains were tested with the convalescent-phase antis era, Comparison to the reactivity against electroblotted total cellular pro teins revealed that the immune response against LbpB and TbpB constitutes a significant portion of the total detectable immune response to M. catarrha lis proteins. Preparations of affinity-isolated TbpA and LbpA reacted with convalescent-phase antisera in a solid-phase binding assay, but blocking wi th soluble TbpB, soluble LbpB, or extracts from an LbpA(-) mutant demonstra ted that this reactivity was attributed to contaminants in the TbpA and Lbp A preparations. These studies demonstrate the immunogenicity of M. catarrha lis TbpB and LbpB in humans and support their potential as vaccine candidat es.