Streptococcus pneumoniae can cause life-threatening
infections mostly in infants, children, and elderly people. Capsular
polysaccharide conjugate vaccines provide serotype-dependent protection against S. pneumoniae
infections but fail to protect against new emerging serotypes. To overcome these limitations,
pneumolysin (Ply), a serotype-independent and conserved
protein was selected. As such
subunit vaccines lack immunogenicity, we engineered Ply to be attached to self-assembled
polyester beads in order to boost immunogenicity. To display Ply at the surface of these
polyester beads, it was translationally fused to the N-terminus of the
polyhydroxybutyrate (
PHB) synthase (PhaC), which mediates PHB bead assembly inside recombinant Escherichia coli. We also chemically conjugated the capsular
polysaccharide (CPS) 19F to isolated PHB beads to further assess their
antigen carrier properties. CPS conjugated to soluble
tetanus toxoid served as control. Balb/c mice immunized with Ply-PhaC beads and 19F-PhaC beads induced specific and higher
IgG levels than the respective soluble counterparts. The induced
IgG antibodies recognized Ply in whole cell lysates of six different serotypes of S. pneumoniae. Additionally, restimulated splenocytes from animals immunized with Ply-PhaC beads produced a balanced INF-γ/IL-17A profile unlike animals immunized with soluble Ply. The 19F-PhaC beads induced production of
antibodies showing high opsonophagocytic titers against the homologous strain, serotype 19F, while CPS 19F only mixed with PhaC beads did not elicit any detectable immune response. This study provided insight into the design of PHB beads as a carrier of proteinaceous
antigens and CPS in order to induce immune responses for the prevention of
pneumococcal infections.