Specific and effective T-cell recognition of cells transfected with a truncated human mucin cDNA

Epithelial cell mucin has been characterized as a tumor-specific antigen in patients with pancreatic and breast cancer. Mucins are high molecular weight glycoproteins consisting of a heavily glycosylated tandemly repeating 20-amino acid sequence. Aberrant glycosylation of mucins on carcinomatous epithelial cells leads to the exposure of novel core epitopes that are recognized by cytotoxic T lymphocytes (CTLs). We previously reported the establishment of mucin-specific CTL clones that recognize mucin expressed on the surface of EBV-immortalized B cells transfected with the mucin cDNA (MUC1). This recognition was characterized as major histocompatibility complex (MHC)-unrestricted, because of the multivalent nature of mucin. The transfectants had to be incubated with an inhibitor of O-linked glycosylation, phenyl-N-acetyl-alpha-galactosaminide (phenyl-GalNAc) in order to unmask the tandem repeat core epitope recognized by CTLs. In the present study, we examined whether mucin molecules with fewer tandem repeats are capable of MHC-unrestricted recognition by mucin-specific CTL clones. A mucin cDNA expression vector expressing a "truncated" mucin molecule that contains only two tandem repeats was constructed. We found that mucin-specific CTL clones recognize the "truncated" mucin on allogeneic target cells, showing that recognition in this case was MHC-unrestricted as well. In addition, CTL clones lysed "truncated" mucin transfectants significantly better than full-length mucin transfectants treated with phenyl-GalNAc, and controls. The "truncated" construct may represent an effective means of immunizing patients with breast and pancreatic cancer, enabling them to mount a strong and efficient immune response against mucin-bearing tumor cells.