Tumor Associated Tn-MUC1 Glycoform is Internalized through the Macrophage Galactose-type C-type Lectin and delivered to the HLA Class I and II Compartments in Dendritic Cells.

The type of interaction between tumor-associated antigens and specialized antigen-presenting cells such as dendritic cells (DCs) is critical for the type of immunity that will be generated. MUC1,a highly O-glycosylated mucin,is overexpressed and aberrantly glycosylated in several tumor histotypes. This results in the expression of tumor-associated glycoforms and in MUC1 carrying the tumor-specific glycan Tn (GalNAcA1-O-Ser/Thr). Glycopeptides corresponding to three tandem repeats of MUC1,enzymatically glycosylated with 9 or 15 mol of GalNAc,were shown to specifically bind and to be internalized by immature monocyte-derived DCs (iDCs). Binding required calcium and the GalNAc residue and was competed out by GalNAc polymer and Tn-MUC1 or Tn-MUC2 glycopeptides. The macrophage galactose-type C-type lectin (MGL) receptor expressed on iDCs was shown to be responsible for the binding. Confocal analysis and ELISA done on subcellular fractions of iDCs showed that the Tn-MUC1 glycopeptides colocalized with HLA class I and II compartments after internalization. Importantly,although Tn-MUC1 recombinant protein was bound and internalized by MGL,the glycoprotein entered the HLA class II compartment,but not the HLA class I pathway. These data indicate that MGL expressed on iDCs is an optimal receptor for the internalization of short GalNAcs carrying immunogens to be delivered into HLA class I and II compartments. Such glycopeptides therefore represent a new way of targeting the HLA class I and II pathways of DCs. These results have possible implications in designing cancer vaccines.