Structural Basis for TCR Recognition of a Rac1 Neoantigen Arising from Anchor Residue Mutation

T cell receptor (TCR)-based targeted immunotherapy can mediate cancer regression primary targeting neoantigens derived from mutations in self-proteins. Most of neoantigens arise from the solvent-exposed residue mutation that generate neoepitope for TCR exquisite recognition. Here, we report a melanoma neoantigen (Rac1 ^P29S ) that derived from the primary anchor residue mutation, conferring immunogenicity for TCR recognition. We also determine the structure of the mutant Rac1 ^P29S –HLA-A2 ligand, as well as the structure of the tumor-specific TCR 5934 in complex with Rac1 ^P29S –HLA-A2. These structures reveal how the Rac1 P29S mutation enables a self- peptide visible to T cells. TCR 5934 adopts a recognition strategy distinct from the conventional recognition mode, which highly focus on the C-terminal non-mutated portion of mutant Rac1 ^P29S . The structure of the TCR 5934–Rac1 ^P29S –HLA-A2 complex provides a crucial framework for TCR design, facilitating the enhancement of its cytotoxic efficacy for targeted therapy without compromising specificity.