Synthesis and immunological characterization of modified hyaluronic acid hexasaccharide conjugates

The synthesis of a tetanus toxoid (TT)-conjugate of a hyaluronic acid (HA) hexasaccharide is described. The compound was intended for use in monitoring HA levels as a disease marker and as a potential vaccine against Group A Streptococcus (GAS) infections. We also report the synthesis of a chemically modified HA-hexasaccharide-TT conjugate in which the N-acetyl moiety of the N-acetyl-D-glucosamine residue is replaced with an N-propionyl unit in order to enhance immunogenicity. The oligosaccharides are synthesized in a convergent manner. The TT-conjugate syntheses rely on the reaction of the amines on the 6-aminohexyl aglycon of the hexasaccharides with diethyl squarate to give the monoethyl squarate adducts. Subsequent reactions with lysine ε-amino groups on TT then give the glycoconjugates containing an average of 8 hexasaccharide haptens per TT molecule. Immunological studies in mice show very similar antibody responses with both conjugates, suggesting that the N-acetyl groups of the glucosaminyl residues of the HA-hexasaccharide are not a critical part of the epitope recognized by the anti-HA polyclonal immune response. Furthermore, it would appear that the N-acyl moieties are not in close contact with the amino acid residues of the antibody combining sites.