Increasing the Antigenicity of Synthetic Tumor-Associated Carbohydrate Antigens by Targeting Toll-Like Receptors

摘要

Epithelial cancer cells often over express mucins that are aberrantly glycosylated. Although it has been realized that these compounds offer exciting opportunities for the development of immunotherapy for cancer, their use is hampered by the low antigenicity of classical immunogens composed of a glycopeptide derived from a mucin conjugated to a foreign carrier protein. We have designed, chemically synthesized, and immunologically evaluated a number of fully synthetic vaccine candidates to establish a strategy to overcome the poor immunogenicity of tumor-associated carbohydrates and glycopeptides. The compounds were also designed to study in detail the importance of TLR engagement for these antigenic responses. We have found that covalent attachment of a TLR2 agonist, a promiscuous peptide T-helper epitope, and a tumor-associated glycopeptide, gives a compound (>1) that elicit in mice exceptionally high titers of IgG antibodies which recognize MCF7 cancer cells expressing the tumor-associated carbohydrate. Immunizations with glycolipopeptide (>2), which contains lipidated amino acids instead of a TLR2 ligand, gave significantly lower titers of IgG antibodies demonstrating that TLR engagement is critical for optimum antigenic responses. Although mixtures of compound >2 with Pam3CysSK4 (>3) or monophosphoryl lipid A (>4) elicited similar titers of IgG antibodies compared to >1, the resulting antisera had an impaired ability to recognize cancer cells. It was also found that it is essential to covalently link the helper T-epitope to B-epitope probably because internalization of the helper T-epitope by B-cells requires assistance of the B-epitope. The results presented here show that synthetic vaccine development is amenable to structure activity relationship studies for successful optimization of carbohydrate-based cancer vaccines.