Synthetic Oligomers Mimicking Capsular Polysaccharide Diheteroglycan are Potential Vaccine Candidates against Encapsulated <i>Enterococcal</i> Infections

Abstract

Infections caused by <i>Enterococcus</i> spp. are a major concern in the clinical setting. In <i>Enterococcus faecalis</i>, the capsular polysaccharide diheteroglycan (DHG), composed of ß-d-galactofuranose-(1 → 3)-ß-d-glucopyranose repeats, has been described as an important virulence factor and as a potential vaccine candidate against encapsulated strains. Synthetic structures emulating immunogenic polysaccharides present many advantages over native polysaccharides for vaccine development. In this work, we described the synthesis of a library of DHG oligomers, differing in length and order of the monosaccharide constituents. Using suitably protected thioglycoside building blocks, oligosaccharides up to 8-mer in length built up from either Gal<i>f</i>-Glc<i>p</i> or Glc<i>p</i>-Gal<i>f</i> dimers were generated, and we evaluated their immunoreactivity with antibodies raised against DHG. After the screening, we selected two octasaccharides, having either a galactofuranose or glucopyranose terminus, which were conjugated to a carrier protein for the production of polyclonal antibodies. The resulting antibodies were specific toward the synthetic structures and mediated <i>in vitro</i> opsonophagocytic killing of different encapsulated <i>E. feacalis</i> strains. The evaluated oligosaccharides are the first synthetic structures described to elicit antibodies that target encapsulated <i>E. faecalis</i> strains and are, therefore, promising candidates for the development of a well-defined enterococcal glycoconjugate vaccine.