Synthetic glycopeptides from the mucin family as potential tools in cancer immunotherapy

Compared to glycoproteins of healthy cells, glycoproteins of tumor cells are often aberrantly glycosylated. Thus, glycopeptide fragments of surface glycoproteins of tumor cells are of interest as tumor-associated antigens for the distinction between normal and tumor cells. Cancer immunotherapy directed at selectively targeting these tumor-associated glycoprotein structure alterations--deficient glycosylation and, thus, exposure of peptide epitopes which are masked in normal cells--is considered a promising approach for the treatment of cancer. For this purpose, glycoproteins from the mucin family are of particular interest. Mucins belong to a class of heavily O-glycosylated, high-molecular weight glycoproteins present on the surface of many epithelial cells. The mucin core protein consists of numerous tandem repeats rich in serine, threonine and proline. In their tumor-associated forms, epithelial mucins carry cryptic saccharide structures such as T(N)-, T-, sialyl-T(N)- and sialyl-T antigens and more complex oligosaccharides (e.g. Lewis(y)). In contrast to glycoproteins isolated from natural sources, synthetic glycopeptides can be obtained in high purity and with exactly defined structure. In this review, methodologies for the synthesis of mucin-type glycopeptides containing complex tumor-associated antigen structures are described. Due to the low immunogenicity often exhibited by synthetic tumor-associated glycopeptide antigens, their conjugation to carrier proteins or suitable T-cell epitopes is essential for the development of anti-tumor vaccines. The results of immunological evaluations of synthetic (glyco)peptides and oligosaccharides are described. Some of these synthetic vaccines show promising activities inducing proliferation of T-cells and cytotoxic T-cell responses.