The resulting altered proteins typically lack normal functionality but additionally, they constitute neo-epitopes, when presented in the context of MHC molecules at the tumor cells’ surface. In recent years, our group was leading in demonstrating the high immunogenicity of MSI-induced frameshift-peptides by identifying numerous epitopes recognized by T cells. Using the classical reverse-immunology approach, T cells from healthy HLA-A0201+ donors are stimulated by synthetic FSPs. Importantly, the outgrowing T cells are mainly CD8+ cytotoxic T lymphocytes capable to effectively lyse cells harbouring the respective mutation. Based on these in vitro studies, Schwitalle et al. provided evidence for FSP-specific immune responses not only in HNPCC patients but also in still healthy HNPCC germline mutation carriers. This study additionally revealed that FSPs are recognized by the human immune system and thus represent relevant tumor antigens in vivo. Hence, FSPs are very interesting targets for specific immunological approaches both for therapeutic and even if currently hypothetical for preventive LY294002 154447-36-6 purposes. Since an effective tumor vaccine ideally contains epitopes derived from several tumorspecific antigens, the identification of additional, relevant T cell epitopes is imperative. Here, we describe the identification of HLA-A0201-restricted CTL epitopes generated by a MSI+ tumor specific frameshift mutation in a cMS of the MSH3 gene. MSH3 is one of the DNA MMR genes located on chromosome 5q14.1. It belongs to the MutS family and mutations are found in about 40% of colorectal and stomach cancers, as well as in.50% of established CRC cell cultures. Functional loss of this protein was found to play a role in the progression of MMR-deficient tumors by increasing instability. Therefore, mutated MSH3 shows several characteristics an ideal tumor antigen should have and thus constitutes another important candidate for the development of immunebased MSI+ tumor targeting strategies. Vaccination strategies aiming to specifically act on tumor target antigens have now come to the fore. Ideally, an efficient antitumor immunotherapy involves a multi-epitope strategy including T-cell epitopes from a set of highly immunogenic tumor-associated antigens and several different HLA-class I restriction elements. However, most of the tumor antigens identified so far may not constitute perfect target structures since antigen down-regulation or loss can occur during immunotherapy. Hence, there is a need for further identification of relevant T cell epitopes. Represent neo-antigens to the immune system and thus may elicit spontaneous immune responses. Indeed, MSI+ tumors often exhibit marked lymphocytic infiltration, especially at the tumor invasion front in the stromal compartment, with a predominance of activated CD8+ T cells. Consequently, it has been suggested that this may even be causative for the better prognosis observed for CRC when compared to their MSS counterpart. Whether the observed T cell responses are directly associated with the higher apoptosis rate of MSI+ tumor cells or whether this is just a coincidence is largely unsolved.