Abstract B034: Single-cell immune profiling reveals T cell dynamics in estrogen receptor–positive breast cancer treated with anti-PD-1 and radiation therapy

Abstract Background: The effectiveness of immunotherapy in estrogen receptor–positive (ER+) breast cancer remains limited. ER+ tumors typically exhibit low tumor-infiltrating lymphocytes and an immunosuppressive tumor microenvironment, both of which contribute to poor responsiveness to immune checkpoint blockade. Prior work in the PEARL trial (NCT03366844) showed that triple-negative breast cancer responders to anti–programmed cell death 1 (anti-PD-1) combined with radiation therapy showed an increase in intratumoral CD8+ T cells, highlighting the importance of effective T cell activation in mediating treatment response. The current study investigates whether ER+ breast tumors demonstrate similar T cell-driven mechanisms following anti-PD-1 therapy alone and in combination with radiation. Methods: Single-cell RNA sequencing (scRNA-seq) and T cell receptor sequencing (scTCR-seq) were performed on matched tumor biopsies and peripheral blood mononuclear cells collected from ER+ breast cancer patients (n= 12) at three treatment timepoints: baseline, after first cycle of anti-PD-1, and after second cycle of anti-PD-1 plus radiation. Results: We identified diverse T cell subsets in tumor and blood, including cytotoxic, exhausted, regulatory, and memory phenotypes. TCR analyses demonstrated expansion of intratumoral T cell clones following anti-PD-1 treatment, and these expanded clones persisted after anti-PD-1 plus radiation, with responders maintaining clonal expansion compared to non-responders. Notably, responders had more shared T cell clonotypes between tumor and peripheral blood after anti-PD-1 plus radiation than non-responders, suggesting an enhanced systemic immune response mediated by tumor-reactive clones in responders. Conclusion: Overall, this study identifies intratumoral T cell signatures associated with anti-PD-1 and radiation response in ER+ breast cancer. By characterizing how T cell dynamics change across treatment timepoints and between tumor and blood, these findings provide insight into mechanisms that may contribute to resistance to checkpoint blockade, informing the development of improved therapeutic strategies for ER+ breast cancer. Disclosure: Portions of this abstract were revised using generative AI under full author supervision. Citation Format: Na Jeong Kim, Vaishnavi Devarakonda, Kamal Asadipour, Isaiah Vazquez, Elvin Canseco, Julie Jang, Anthony Nguyen, Yuan Yuan, Jin Sun Bitar, Scott Karlan, Simon Knott, Stephen Shiao. Single-cell immune profiling reveals T cell dynamics in estrogen receptor–positive breast cancer treated with anti-PD-1 and radiation therapy 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 B034.