Synthetic carbohydrate vaccines: synthesis and immunogenicity of Tn antigen conjugates

A tumor-associated carbohydrate antigen, Tn antigen (GalNAc alpha 1-->O-Ser), was synthesized with a spacer arm, and assembled to dimeric and trimeric structures using N-tert-butyloxycarbonyl-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-alpha- D-galactopyranosyl)-L-serine as a key building block. The synthetic antigens were conjugated with OSA and their immunogenicity examined in mice. Mice immunized with dimeric or trimeric Tn antigen showed a stronger antibody (IgM) response to a Tn-glycoprotein (asialo-ovine submaxillary mucin) than mice immunized with monomeric Tn antigen. The dimeric and trimeric Tn antigens also induced measurable IgG responses. The dimeric Tn antigen was further coupled to a Starburst dendrimer (5th generation) and to tripalmitoyl-S-glycerylcysteinyl-serine, a synthetic lipopeptide of the active moiety of a major lipoprotein of Escherichia coli. Unexpectedly, the Starburst dendrimer conjugate did not stimulate any immune response specific to Tn antigen. On the other hand, immunization of mice with the lipopeptide conjugate produced not only a high IgM response but also significant IgG anti-Tn response without any carrier molecules or additional adjuvants. The production of IgG antibody is quite significant since carbohydrate antigens are in general known to produce only IgM antibody response. Being a totally synthetic, low-molecular weight, and carrier-free immunogen, the lipopeptide conjugate could be a prototype of synthetic carbohydrate vaccines.