Abstract 2549: The aged tumor microenvironment limits T cell control of breast cancer

Abstract Introduction: Breast cancer is the leading cause of cancer-related death among women worldwide and the most commonly diagnosed invasive cancer. Given that the risk of developing cancer increases dramatically with age and is often paralleled with a progressive decline in immune function, breast cancer presents unique challenges for understanding how aging influences tumor development. Amid a globally aging population, the etiology and effect of age-related immune dysfunction in cancer remains unclear. Methods: To address this, we investigated the major cellular determinants of aging in an orthotopic mouse model using EO771 breast carcinoma cells on young (2-month-old) and old (27-month-old) BALB/c mice separately. 21 days post inoculation, tumors were immuno-profiled with extensive flow cytometry and multiplex immunohistochemistry. Young and aged human breast biopsies were processed to generate tumor-infiltrating lymphocytes, which were subsequently analyzed using single-cell RNA sequencing. Results: We observed that tumor size increased with mice age, while the frequencies infiltrating CD4 and CD8 T cell decreased. Specifically, CD8 T cell exhaustion is significantly induced in aged breast tumors: quantification by flow cytometry and multiplex immunohistochemistry shows a consistent upward trend in CD39+CD8 T, PD-1+CD8 T, and TOX+CD8 T cells in 27-month-old mice. Complemented with a detailed single-cell RNA sequencing analysis of over 5, 000 T cells from young and aged human breast biopsies, we verified the presence of exhausted CD8 T cell subsets in the aged tumor microenvironment, characterized by Pdcd1, Havcr2 and Entpd1 high expression. Conclusions: Our findings demonstrate that limited priming of CD8+T cells in the aged tumor microenvironment within breast cancer, primarily restricts tumor control in aging. Further studies are ongoing to expand the cohort and investigate age-related responses to immunotherapy in immunopathological context with advance single-cell spatial technologies. Citation Format: Jia Meng, Denise Goh, Kai Soon Ng, Xinru Lim, Jeffrey Lim, Lee Hong Song, Jabed Iqbal, Darren Wan-Teck Lim, Elaine Hsuen Lim, Joe Yeong. The aged tumor microenvironment limits T cell control of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2549.