A TGF βRII frameshift-mutation-derived CTL epitope recognised by HLA-A2-restricted CD8+ T cells

Microsatellite instability (MSI) is recognised as genome-wide alterations in repetitive DNA sequences caused by defects in the DNA mismatch repair machinery. Such mutation patterns have been found in almost all analysed malignancies from patients with hereditary non-polyposis colorectal cancer, and in approximately 15% of sporadic colorectal cancers. In cancers with the MSI phenotype, micro satellite-like sequences in coding regions of various cancer-related genes, including transforming growth factor beta receptor type II (TGF beta RII), are targets for mutations. The TGF beta RII gene harbours a 10-bp polyadenine tract, and mutations within this region are found in 90% of colorectal cancers with MSI. The frameshift mutations result in new amino acid sequences in the C-terminal part of the proteins, prematurely terminating where a novel stop codon appears. In this study we have defined a new cytotoxic T lymphocyte (CTL) epitope (RLSSCVPVA), carrying a good HLA-A*0201 binding motif. and resulting from the most common frameshift mutation in TGF beta RII. A CTL line and several CTL clones were generated from an HLA-A2(+) normal donor by repeated stimulation of T cells with dendritic cells pulsed with the peptide. One of the CTL clones was able to kill an HLA-A2(+) colon cancer cell line harbouring mutant TGF beta RII. This epitope may be a valuable component in cancer vaccines directed at MSI-positive carcinomas.