A Semi-synthetic Oligosaccharide Conjugate Vaccine Candidate Confers Protection against Streptococcus pneumoniae Serotype 3 Infection

Abstract

The identification of immunogenic glycotopes that render glycoconjugate vaccines protective is key to improving vaccine efficacy. Synthetic oligosaccharides are an attractive alternative to the heterogeneous preparations of purified polysaccharides that most marketed glycoconjugate vaccines are based on. To investigate the potency of semi- synthetic glycoconjugates, we chose the least efficient serotype in the current pneumococcal conjugate vaccine Prevnar 13™, Streptococcus pneumoniae serotype 3 (ST3). Glycan arrays containing synthetic ST3 repeating unit oligosaccharides were used to screen a human reference serum for antibodies and to define the recognition site of two ST3-specific protective monoclonal antibodies. The glycan array screens identified a tetrasaccharide that was selected for in-depth immunological evaluation. The tetrasaccharide-CRM197 carrier protein conjugate elicited protective immunity as evidenced by opsonophagocytosis assays and protection against pneumonia caused by ST3 in mice. Formulation of the defined protective lead candidate glycotope has to be further evaluated to elicit optimal long-term immunity.

A semisynthetic Streptococcus pneumoniae serotype 8 glycoconjugate vaccine

“…1A). Octasaccharides derived from ST8 CPS hydrolysis have been found to induce protective immunity (16), but based on the findings that glycans as small as di- to tetrasaccharides induced protective immunity against other S. pneumoniae serotypes (8, 9, 17), we reasoned that a collection of all four tetrasaccharide frameshifts would cover most glycotopes in ST8 CPS that are relevant for antibody reverse engineering. We used automated glycan assembly (AGA) to synthesize conjugation-ready variants of all four frameshifts A( 1 ), B( 2 ), C( 3 ) and D( 4 ) from polystyrene resin 5 and orthogonally protected building blocks 6–11 (Fig.…”

“…3B) and monitored the immune response by glycan microarray. We included disaccharide 13 and tetrasaccharide 14 , a synthetic dimer of cellobiuronic acid that is an ST3 vaccine candidate (6, 17), in the microarray experiment to evaluate the glycan binding characteristics of immune sera (Fig. 3C).…”

“…We next determined the nature of protective as well as non-protective glycotopes found in ST8 frameshift C using a collection of synthetic, ST8 and ST3 CPS-related oligosaccharides (Fig. 6, A and B) (17). Antisera that were raised against frameshift C( 12 ) and bound to ST3 tetrasaccharide 14 interacted with cellobiuronic acid 18 , whereas mAb 1H8 did not recognize disaccharide 18 by glycan microarray and surface plasmon resonance (Fig.…”

Synthesis and Immunological Studies of Oligosaccharides that Consist of the Repeating Unit of Streptococcus pneumoniae Serotype 3 Capsular Polysaccharide

“…ST3 CPS is a linear polysaccharide that is composed of a repeating disaccharide made of β‐linked d ‐glucuronic acid (GlcA) and d ‐glucose (Glc), specifically, →3)‐β‐ d ‐GlcA‐(1→4)‐β‐ d ‐Glc‐(1→. Several short oligosaccharide fragments of ST3 CPS and their protein conjugates have been synthesized and immunologically evaluated . It was disclosed that these semisynthetic conjugate vaccines could elicit robust ST3 CPS‐specific IgG antibodies, whose titers increased with the ST3 oligosaccharide chain length .…”

“…It was disclosed that these semisynthetic conjugate vaccines could elicit robust ST3 CPS‐specific IgG antibodies, whose titers increased with the ST3 oligosaccharide chain length . Furthermore, glycan array screening revealed that an ST3 tetrasaccharide could be well‐recognized by ST3‐specific protective human monoclonal antibodies, and opsonophagocytosis assays showed that a CRM 197 protein conjugate of the tetrasaccharide could induce immunoprotection . In general, it was also revealed that ST3 CPS oligosaccharide–protein conjugates could form effective vaccines to protect mice from intraperitoneal challenge with lethal doses of ST3 .…”