A Synthetic Virus-Like Particle Streptococcal Vaccine Candidate Using B-Cell Epitopes from the Proline-Rich Region of Pneumococcal Surface Protein A

Abstract

Alternatives to the well-established capsular polysaccharide-based vaccines against Streptococcus pneumoniae that circumvent limitations arising from limited serotype coverage and the emergence of resistance due to capsule switching (serotype replacement) are being widely pursued. Much attention is now focused on the development of recombinant subunit vaccines based on highly conserved pneumococcal surface proteins and virulence factors. A further step might involve focusing the host humoral immune response onto protective protein epitopes using as immunogens structurally optimized epitope mimetics. One approach to deliver such epitope mimetics to the immune system is through the use of synthetic virus-like particles (SVLPs). SVLPs are made from synthetic coiled-coil lipopeptides that are designed to spontaneously self-assemble into 20–30 nm diameter nanoparticles in aqueous buffer. Multivalent display of epitope mimetics on the surface of SVLPs generates highly immunogenic nanoparticles that elicit strong epitope-specific humoral immune responses without the need for external adjuvants. Here, we set out to demonstrate that this approach can yield vaccine candidates able to elicit a protective immune response, using epitopes derived from the proline-rich region of pneumococcal surface protein A (PspA). These streptococcal SVLP-based vaccine candidates are shown to elicit strong humoral immune responses in mice. Following active immunization and challenge with lethal doses of streptococcus, SVLP-based immunogens are able to elicit significant protection in mice. Furthermore, a mimetic-specific monoclonal antibody is shown to mediate partial protection upon passive immunization. The results show that SVLPs combined with synthetic epitope mimetics may have potential for the development of an effective vaccine against Streptococcus pneumoniae.

A systematic mutational analysis identifies a 5‐residue proline tag that enhances the in vivo immunogenicity of a non‐immunogenic model protein

“…Thus, we hypothesize that the molecular mechanisms of the C5P-induced immunogenicity enhancement might be different from the previously reported high immunogenicity of proline-rich epitopes because the epitopes were located within the protein [37,38] which was in contrast to our observation that the immune response increased only with the 5-pro attached at the C terminus of the protein. We also hypothesize that the 5-arg might have enhanced the entry of BPTI into the cells during the endocytosis/ phagocytosis process [39][40][41], which might have increased the immunogenicity [42] of BPTI.…”

Evaluation of Protective Efficacy of Selected Immunodominant B-Cell Epitopes within Virulent Surface Proteins of Streptococcus pneumoniae

“…This finding may reflect that the immune response against the simultaneous administration of these epitopes could have complementary or synergistic effects by targeting different stages of pneumococcal invasion (34). This hypothesis is in line with recent animal studies demonstrating that when VPPs or VPP-derived protein fragments are combined as a mixture or as a macromolecular complex, the overall protection against pneumococcal infection is increased significantly (23)(24)(25).…”

“…Our study revealed the in vitro functional potential of human antibodies targeting selected epitopes derived from CbpD, PhtD, PhtE, and ZmpB VPPs by OPKA and the in vivo potential of these epitopes to elongate significantly the survival of mice challenged with a lethal dose of virulent clinical pneumococcal serotype 3. Our findings contribute significantly to the development of serotype-independent pneumococcal vaccines, since to date only a few VPP peptide/protein fragments that could exhibit such promising immunological characteristics have been used as vaccine candidates (20)(21)(22)(23)(24)(25).…”

“…Aiming to overcome these barriers, efforts have been directed toward the identification of major immunogenic epitopes within VPPs and construction of immunogenic peptides that could be used for the development of peptide-based pneumococcal vaccines. Some studies identified peptide epitopes or larger antigenic fragments located within pneumococcal surface protein A (PspA), pneumococcal surface protein C (PspC), and phosphoenolpyruvate protein phosphotransferase (PtsA), with promising in vitro and in vivo protective characteristics (20)(21)(22)(23)(24)(25). Similarly, we had previously identified and characterized 4 immunoreactive B-cell epitopes located within the pneumococcal surface proteins (VPPs) choline-binding protein D (CbpD), pneumococcal histidine triad proteins (PhtD and PhtE), and zinc metalloproteinase B (ZmpB), e.g., CbpD-pep4 (amino acids [aa] 291 to 310), PhtD-pep19 (aa 200 to 219), PhtE-pep40 (aa 79 to 98), and ZmpB-pep125 (aa 431 to 450), which had shown sequence homology among different pneumococcal serotypes and consistent surface binding with various pneumococcal strains and were recognized by sera obtained from pediatric patients convalescing from invasive pneumococcal disease (26).…”

Vaccine technologies: From whole organisms to rationally designed protein assemblies

“…Based on basic biophysical properties 60-90 monomers can self-assemble into a 20-25 nm particle that contains a lipid core and a surface decorated by antigens. SVLP function as repetitive antigen displays and are capable of inducing humoral responses in vaccinated animals [185,186].…”