Structural basis for TCR recognition of a Rac1 neoantigen arising from anchor residue mutation.

T cell receptor (TCR)-based immunotherapy can drive cancer regression by targeting neoantigens derived from mutations in self-proteins. Most neoantigens result from mutations in solvent-exposed residues creating neoepitopes that allow highly specific TCR recognition. Here, we describe a melanoma neoantigen (Rac1P29S) caused by a mutation at a primary anchor residue. Unlike typical cases, the immunogenicity of Rac1P29S stems from this anchor mutation, which permits MHC presentation of the mutant peptide but not the wild-type counterpart. We determined the structures of both the mutant Rac1P29S-HLA-A2 complex and its complex with the tumor-specific TCR 5934. These structures show how the P29S mutation makes a Rac1 self- peptide visible to T cells. Notably, TCR 5934 primarily engages the C-terminal, non-mutated P8 threonine residue of Rac1P29S -far from the N-terminal mutated P2 serine. This contrasts with most neoantigen-specific TCRs, which typically focus on the mutated residue to distinguish mutant from wild-type peptides. Together, these findings provide a structural framework to guide the development of TCR-based cancer immunotherapies.