Abstract A072: The T cell precursor frequency determines the immunogenicity of cancer neoantigens

Abstract Neoantigens have become a prevalent target for cancer immunotherapy, however, their definition and reliable identification remain challenging. Moreover, immunodominance may mask the true immunogenicity of a neoantigen, which can only be accurately analyzed in vivo. In this study, we analyzed the immunogenicity of naturally occurring human cancer neoantigens, which had been shown to elicit specific CD8 T cell responses in respective patients. We employed a mouse model with a diverse human HLA-A*02:01-restricted T cell receptor (TCR) repertoire and implanted a spontaneous, syngeneic, low-immunogenic tumor into it. We genetically engineered the tumor cells to simultaneously express 16 neoantigens, to assess their immunogenicity in vivo. This resulted in either tumor cell rejection or selection of antigen-negative variants. The CD8 T cell response was dominated by a single neoantigen, with occasional low-level responses to one additional neoantigen. Analysis of the TCRs specific for the dominant neoantigen revealed several uniform features. Most TCRs utilized a restricted set of Vβ/Vα gene segments, contained almost no nucleotide additions, and displayed short CDR3s. T cells against the dominant neoantigen were present at a high precursor frequency in the naïve repertoire. Shuffling Vβ/Vα chains between individual T cell clones retained specificity as long as the Vβ CDR3 contained a certain germline D-segment motif. In contrast, such “germline” (public-like) TCRs were absent in responses to recessive neoantigens. The dominant neoantigen suppressed T cell responses against recessive neoantigens, which were characterized by a low T precursor frequency. Additionally, we analyzed the TCRs isolated from both primary and recall (memory) responses against the most dominant neoantigen. While germline (public-like) TCRs significantly dominated the primary response, most of these clones failed to transition into the memory pool. This demonstrates that TCR clones with high precursor frequency are recruited into the neoantigen response earliest. In conclusion, the immunogenicity and immunodominance of human cancer neoantigens can in part be determined by the T cell precursor frequency. Citation Format: Tzu-Jiun Kuo, Elmehdi Belbaraka, Ioannis Gavvovidis, Julianna Volko, Amin Ben Hamza, Niklas Fellmer, Mira Heichenko, Gerald Willimsky, Thomas Kammertoens, Leo Hansmann, Thomas Blankenstein. The T cell precursor frequency determines the immunogenicity of cancer neoantigens [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr A072.