Synthesis of fragments of Salmonella typhi capsular polysaccharide by depolymerization of the natural polysaccharide and their zwitterionic analogues

Since capsular polysaccharides (CPS) are the key virulence factor for encapsulated bacteria, various antibacterial vaccines based on purified fragments of CPS have been developed and licensed during last decades. However, several studies showed that carbohydrates are typical T cell-independent antigens. Consequently, polysaccharide-based vaccines are poorly immunogenic in infants, young children and immune compromised patients, and induce only short-lasting antibody responses in adults. A series of recent investigations has found that a group of structurally distinct bacterial polysaccharides specifically activate T cells in vivo and in vitro through the traditional MHC-II endocytic pathway. Although these molecules show a large diversity of chemical structure, they share the common characteristic of presenting a zwitterionic charge motif distributed along the chain (zwitterionic polysaccharides, ZPS). This zwitterionic charge motif is believed to be responsible of their peculiar immunological activity, which is unique among bacterial polysaccharides. The design of new T cell dependent, ZPS-based antigens for vaccine formulation, however, requires a better understanding of how ZPS antigens stimulate the host immune system and a correlation of the ZPS structural and conformational properties with their biological activity. The CPS of Salmonella enterica serovar Typhi is an anionic polymer composed by α-(1-4)-linked N-acetyl galactosaminuronic acid repeating units mainly O-acetylated at position 3. The structure of Vi CPS makes it an ideal candidate for our investigation, since its fragments (attainable by partial depolymerisation of the native polysaccharide) can be converted into zwitterionic derivatives by N-deacetylation. However, the natural CPS is a high molecular weight polymer (about 1-4 MDa). We therefore applied a preliminary depolymerisation by prolonged ultrasonic irradiation to provide fragments of about 80 kDa molecular weight on average. The resulting mixture showed a narrower MW distribution and could be characterized by both MALDI-TOF MS and 13C-NMR. The mixture will be then subjected to autohydrolysis to obtain shorter fragments with different chain lengths. These fragments will be purified by size exclusions chromatography and also converted by N-deacetylation into zwitterionic derivatives. Both types of fragments will be used as reference compounds and biologically evaluated in order to correlate the structural features with their biological behaviour.