A Streptococcus pneumoniae Type 2 Oligosaccharide Glycoconjugate Elicits Opsonic Antibodies and Is Protective in an Animal Model of Invasive Pneumococcal Disease

Abstract

Invasive pneumococcal diseases (IPDs) remain the leading cause of vaccine-preventable childhood death, even though highly effective pneumococcal conjugate vaccines (PCVs) are used in national immunization programs in many developing countries. Licensed PCVs currently cover only 13 of the over 90 serotypes of Streptococcus pneumoniae (Sp), so nonvaccine serotypes are a major obstacle to the effective control of IPD. Sp serotype 2 (ST2) is such a nonvaccine serotype that is the main cause of IPD in many countries, including Nepal, Bangladesh, and Guatemala. Glycoconjugate vaccines based on synthetic oligosaccharides instead of isolated polysaccharides offer an attractive alternative to the traditional process for PCV development. To prevent the IPDs caused by ST2, we identified an effective ST2 neoglycoconjugate vaccine candidate that was identified using a medicinal chemistry approach. Glycan microarrays containing a series of synthetic glycans resembling portions of the ST2 capsular polysaccharide (CPS) repeating unit were used to screen human and rabbit sera and identify epitope hits. Synthetic hexasaccharide 2, resembling one repeating unit (RU) of ST2 CPS, emerged as a hit from the glycan array screens. Vaccination with neoglycoconjugates consisting of hexasaccharide 2 coupled to carrier protein CRM197 stimulates a T-cell-dependent B-cell response that induced CPS-specific opsonic antibodies in mice, resulting in killing of encapsulated bacteria by phagocytic activity. Subcutaneous immunization with neoglycoconjugate protected mice from transnasal challenge with the highly virulent ST2 strain NCTC 7466 by reducing the bacterial load in lung tissue and blood.

Improving vaccines against Streptococcus pneumoniae using synthetic glycans

“…Minimal protective glycan epitopes are identified using synthetic homogeneous oligosaccharides that constitute one or more repeating units of bacterial CPS. Glycoconjugates containing such synthetic antigens have been previously tested individually for their antigenicity, immunogenicity, and protective effects in animal challenge models of disease, and have been shown to be effective in some cases (9,10,(17)(18)(19). We aimed to expand S. pneumoniae vaccine coverage by including serotype antigen conjugates ( Fig.…”

“…The medicinal chemistry approach has identified di-to tetrasaccharides as potential vaccine candidates (11). The immunogenicity of the CPS depends among other factors on rare sugars and labile functional groups (9,17,19). Stable oligosaccharide analogs can fix production problems such as those encountered for ST5 CPS due to the labile ketone present in the repeating unit (9).…”

“…Previously, we had demonstrated that single semisynthetic glycoconjugate vaccine candidates ST2, ST3, and ST8, in which the synthetic antigen is conjugated to carrier protein CRM197, induce opsonic antibodies, kill bacteria in vitro (OPKA), and can protect against lethal infection in an in vivo S. pneumoniae challenge model (17)(18)(19). This study was designed to address the question of whether it is possible to improve the licensed polysaccharide-based vaccines Prevnar13 and Synflorix by adding antigens of nonvaccine serotypes, and to formulate a semisynthetic pentavalent glycoconjugate vaccine consisting exclusively of synthetic antigens.…”

Microarray Strategies for Exploring Bacterial Surface Glycans and Their Interactions With Glycan-Binding Proteins

“…Clearly distinct determinants were identified in each case. Thus, in serotype 2 the GlcAα(1-6)Glcα(1-2) branch (GlcA being glucuronic acid) was found to be an important substructure of the hexasaccharide repeating unit (Emmadi et al, 2017), while in serotype 7F the two side chains that decorate the linear tetrasaccharide backbone, i.e., Galβ(1-and GlcNAcα(1-2)Rhaα(1-, played a key role (Menova et al, 2018). Gal modification with a pyruvate ketal in the linear tetrasaccharide unit of serotype 4 was observed to be an important determinant, although pyruvate-independent epitopes were also unveiled (Geissner et al, 2016), whereas in the serotype 5 pentasaccharide unit the rare aminosugar N-acetyl-L-pneumosamine (PneuNAc, Figure 2) together with branched N-acetyl-L-fucosamine (FucNAc) were essential for antibody recognition and avidity (Lisboa et al, 2017).…”

Recent Advances in the Synthesis of Glycoconjugates for Vaccine Development

“…Emmadi et al [ 76 ] reported the synthesis of the repeating unit of ST2 CPS and of series of synthetic glycans containing portions of the ST2 CPS (compounds 87a – 93a ), in order to identify the protective oligosaccharide epitope. Hexasaccharide 89a (one repeating unit) was synthesized from disaccharides 94 , 95 , 96 via a [2+2+2] glycosylation strategy ( Figure 11 ).…”