The bacterial transferrin receptor has been considered a candidate vaccine antigen for multiple important pathogens, including Neisseria meningitidis. It was previously shown that both components, transferrin binding proteins A (TbpA) and B (TbpB) are required for bacterial survival within the host and both can elicit bactericidal antibodies after immunization. Here, cross reactivity of multiple TbpB variants has been evaluated in a high-throughput ELISA-based assay that was developed to ensure all regions of TbpB are accessible during evaluation. An engineered C-lobe scaffold of TbpB has been used to host surface-exposed regions of TbpA. Antibodies developed against the hybrid antigens were able to bind TbpA on whole bacteria and inhibit TbpA- dependent transferrin-based growth. Serum bactericidal activity was elicited by antisera to these ‘hybrid’ antigens both with and without TbpB present in the targeted strain, indicating individual TbpA loops induce complement-based killing.
In N. meningitidis, asymptomatic carriage of the nasopharyngeal mucosa represents the only reservoir of disease. Conjugate capsular vaccines have been shown to reduce the burden of colonization of N. meningitidis, indicating that systemic immunization is able to effect mucosal carriage. The recent development of transgenic mice expressing human CEACAM1, a molecule necessary for bacterial adhesion, has led to the first mucosal colonization model of N. meningitidis. This model has been utilized to evaluate mucosal protection by TbpB and another prominent meningococcal vaccine antigen, factor H binding protein (FHbp). These data suggest that while TbpB is able to elicit protection from both invasive disease and mucosal colonization, FHbp is unable to prevent mucosal colonization while still eliciting systemic protection.
TbpB was evaluated as a carrier antigen for the development of conjugate capsular vaccines to target a pathogen by both the polysaccharide capsule and a required protein antigen. TbpB from N. meningitidis was conjugated to the Haemophilus influenzae type B polysaccharide and resultant sera was bactericidal against both N. meningitidis and H. influenzae with no significant decrease in TbpB cross reactivity or meningococcal protection when compared to sera against native TbpB.
These data provide a more comprehensive evaluation of the efficacy of transferrin receptor-based vaccines.