Assessment of PhtD C-Terminal Immunogenicity by Opsonophagocytosis Assay (OPA) with OMVs as Adjuvants

Streptococcus pneumoniae causes invasive and non-invasive diseases in children and adults. Currently, there are two types of pneumococcal vaccines: 23-valent pneumococcal polysaccharide vaccine and 13-valent pneumococcal conjugate vaccine which have caused many failures. Therefore, a new generation of pneumococcal vaccines is being pursued.

An improved version of our previous study was performed using recombinant C-terminal of pneumococcal polyhistidine triad protein D (PhtD-C) as a vaccine antigen. The antigen was combined with meningococcal outer membrane vesicle (OMV) and alum as adjuvants to immunize BALB/c mice intraperitoneally. The generated total IgG, specific IgG, IgG1 and IgG2a antibodies and the killing ability of pneumococci by an opsonophagocytosis assay were then assayed.

Immunization by 30 µg PhtD-C and 50 µg OMV as an adjuvant, induced higher amounts of functional antibodies compared to our previous study while killing 50-55% of pneumococci cells.

At optimized concentrations, PhtD-C and meningococcal OMV could be considered as a potent immunogens and the induced specific IgGs were effective and functional for killing pneumococci.