An MSI Tumor Specific Frameshift Mutation in a Coding Microsatellite of MSH3 Encodes for HLA-A0201-Restricted CD8+ Cytotoxic T Cell Epitopes

BACKGROUND Microsatellite instability (MSI) resulting from inactivation of the DNA mismatch repair system (MMR) characterizes a highly immunological subtype of colorectal carcinomas. Those tumors express multiple frameshift-mutated proteins which present a unique pool of tumor-specific antigens. The DNA MMR protein MSH3 is frequently mutated in MSI(+) colorectal tumors, thus making it an attractive candidate for T cell-based immunotherapies. METHODOLOGY/PRINCIPAL FINDINGS FSP-specific CD8(+) T cells were generated from a healthy donor using reverse immunology. Those T cells specifically recognized T2 cells sensitized with the respective peptides. Specific recognition and killing of MSI(+) colorectal carcinoma cells harbouring the mutated reading frame was observed. The results obtained with T cell bulk cultures could be reproduced with T cell clones obtained from the same cultures. Blocking experiments (using antibodies and cold target inhibition) confirmed peptide as well as HLA-A0201-specificity. CONCLUSIONS We identified two novel HLA-A0201-restricted cytotoxic T cell epitopes derived from a (-1) frameshift mutation of a coding A(8) tract within the MSH3 gene. These were (386)-FLLALWECSL (FSP18) and (387)-LLALWECSL (FSP19) as well as (403)-IVSRTLLLV (FSP23) and (402)-LIVSRTLLLV (FSP31), respectively. These results suggest that MSH3(-1) represents another promising MSI(+)-induced target antigen. By identifying two distinct epitopes within MSH3(-1), the sustained immunogenicity of the frameshift mutated sequence was confirmed. Our data therefore encourage further exploitation of MSH3 as a piece for peptide-based vaccines either for therapeutic or--even more important--preventive purposes.