Recombinant Protein D from Haemophilus influenzae Induces Mouse Bactericidal Antibodies Against Typeable and Non-Typeable Haemophilus influenzae, which Partially Protect Infant Rats Against Serotype b Bacteraemia

Aim To evaluate the immunogenicity of a recombinant protein D from Haemophilus Influenzae (Hi) and the functional activities of the induced protein D antibodies in a mouse model. Methods Female Balb/c mice were immunised subcutaneously with recombinant protein D in the presence or absence of adjuvants and the serum immunoglobulin G (IgG) response to protein D was assessed by ELISA. The functional activity of the immune sera was evaluated in vitro using bactericidal assay against typeable Hi serotype b (Hib) and non-typeable Hi (NTHi) clinical isolates and in vivo using an infant rat bacteraemia model and a Hib clinical isolate. Results A dose-dependent IgG response was induced in mice immunised with the recombinant protein D and this response was further increased by the adjuvants used [CPG, AlPO4 and Al(OH)3], with the latter showing the greatest effect on the antibody response. Immune sera were very effective in bactericidal assay against several Hib and NTHi clinical isolates, with a higher serum bactericidal titre against the NTHi than against the Hib isolates. This is possibly due to the lower expression of protein D on the Hib isolates used in our study, compared to the NTHi isolates. In addition, anti-protein D antibodies were partially protective in vivo infant rat bacteraemia model against a challenge with Hib Eagan strain. Conclusion Our results suggest that recombinant protein D is a good vaccine candidate against Hi and should be given in combination with other vaccine candidates to ensure complete protection against Hib and NTHi.