BACKGROUND & AIMS: DNA mismatch repair deficiency drives microsatellite instability (MSI). Cells with MSI accumulate numerous frameshift mutations. Frameshift mutations affecting cancer-related genes may promote tumorigenesis and, therefore, are shared among independently arising MSI tumors. Consequently, such recurrent frameshift mutations can give rise to shared immunogenic frameshift peptides (FSPs) that represent ideal candidates for a vaccine against MSI cancer. Pathogenic germline variants of mismatch repair genes cause Lynch syndrome (LS), a hereditary cancer syndrome affecting approximately 20-25 million individuals worldwide. Individuals with LS are at high risk of developing MSI cancer. Previously, we demonstrated safety and immunogenicity of an FSP-based vaccine in a phase I/IIa clinical trial in patients with a history of MSI colorectal cancer. However, the cancer-preventive effect of FSP vaccination in the scenario of LS has not yet been demonstrated. METHODS: A genome-wide database of 488,235 mouse coding mononucleotide repeats was established, from which a set of candidates was selected based on repeat length, gene expression, and mutation frequency. In silico prediction, in vivo immunogenicity testing, and epitope mapping was used to identify candidates for FSP vaccination. RESULTS: We identified 4 shared FSP neoantigens (Nacad [FSP-1], Maz [FSP-1], Senp6 [FSP-1], Xirp1 [FSP-1]) that induced CD4/CD8 T cell responses in naïve C57BL/6 mice. Using VCMsh2 mice, which have a conditional knockout of Msh2 in the intestinal tract and develop intestinal cancer, we showed vaccination with a combination of only 4 FSPs significantly increased FSP-specific adaptive immunity, reduced intestinal tumor burden, and prolonged overall survival. Combination of FSP vaccination with daily naproxen treatment potentiated immune response, delayed tumor growth, and prolonged survival even more effectively than FSP vaccination alone. CONCLUSIONS: Our preclinical findings support a clinical strategy of recurrent FSP neoantigen vaccination for LS cancer immunoprevention.