Synthetic carbohydrate vaccines: Synthesis and immunogenicity of Tn antigen conjugates

Abstract

A tumor-associated carbohydrate antigen, Tn antigen (GalNAcα1→ 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-α- d-galactop 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.