Sequential Linkage of Carbohydrate Antigens to Mimic Capsular Polysaccharides: Toward Semisynthetic Glycoconjugate Vaccine Candidates against Streptococcus pneumoniae Serotype 14

Abstract

Vaccines based on isolated polysaccharides successfully protect humans from bacterial pathogens such as Streptococcus pneumoniae . Because polysaccharide production and isolation can be technically challenging, glycoconjugates containing synthetic antigens are an attractive alternative. Typically, the shortest possible oligosaccharide antigen is preferable as syntheses of longer structures are more difficult and time-consuming. Combining several protective epitopes or polysaccharide repeating units as blocks by bonds other than glycosidic linkages would greatly reduce the synthetic effort if the immunological response to the polysaccharide could be retained. To explore this concept, we bridged the well-understood and immunologically potent RU of S. pneumoniae serotype 14 (ST14) with an aliphatic spacer and conjugated it to the carrier protein CRM197. Mice immunized with the spacer-bridged glycan conjugates produced high levels of specific antibodies after just one or two vaccine doses, while the tetrasaccharide repeating unit alone required three doses. The antibodies recognized specifically ST14 CPS, while no significant antibody levels were raised against the spacer or unrelated CPS. Synthetic vaccines generated antibodies with opsonic activity. Mimicking polysaccharides by coupling repeating unit antigens via an aliphatic spacer may prove useful also for the development of other glycoconjugate vaccine candidates, thereby reducing the synthetic complexity while enhancing a faster immune response.

Recyclable fluorous-tag assisted two-directional oligosaccharide synthesis enabled by interrupted Pummerer reaction mediated glycosylation

“…As a nal endeavor to demonstrate the utilization potential of the protocol, structurally more complex S. pneumonia type 14 (ST14) tetrasaccharide and octasaccharide were chosen as synthetic targets (Scheme 4). 39,40 The tetrasaccharide repeating unit is the smallest structure to induce ST14-specic antibody effects 41,42 and represents an important component for the development of synthetic or semi-synthetic pneumococcal vaccines. 43 Savage 44 and Seeberger 42,45 have developed glycoconjugate vaccine candidates, which are advantageous to clinically used vaccines on the basis of solution-phase synthesized tetrasaccharide fragments.…”

“…39,40 The tetrasaccharide repeating unit is the smallest structure to induce ST14-specic antibody effects 41,42 and represents an important component for the development of synthetic or semi-synthetic pneumococcal vaccines. 43 Savage 44 and Seeberger 42,45 have developed glycoconjugate vaccine candidates, which are advantageous to clinically used vaccines on the basis of solution-phase synthesized tetrasaccharide fragments. Besides, Salwiczak also has synthesized ST14 tetrasaccharide and octasaccharide antigens by a solid-phase method.…”

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

“…142 A polysaccharide mimic composed of a sequential synthetic tetrasaccharide unit connected through a linker has been made and shown to be immunogenic in mice. 143 A hexasaccharide conjugated to BSA induced apparently both innate (through TLR2 activation) and humoral responses in mice, with longer antibody persistence when aluminum hydroxide was used as adjuvant. However, the TLR2 expression on spleen cells from immunized mice seems not specific and caused by cytokine action rather than being the outcome of a ligand−receptor interaction.…”

Semi- and fully synthetic carbohydrate vaccines against pathogenic bacteria: recent developments

“…A glycoconjugate ST14 pneumococcal vaccine with unique carrier-pneumococcal surface adhesin A (PsaA) [ 45 ] investigated the role of the conjugation site in determining the immunogenicity of the glycoconjugate [ 46 ]. Connecting the repeating unit of ST14 to an aliphatic spacer to simplify syntheses based on glycosidic linkages between repeating units of the ST14 CP yielded a neo-glycoconjugate with CRM 197 as a promising alternative with the advantage of a more straightforward synthesis [ 47 ].…”