Abstract 6645: Temporal immune profiling reveals systemic anti tumor immunity in triple negative breast cancer(TNBC) patients treated with immunotherapy and radiation

Abstract Combining pembrolizumab (anti-PD1) with radiation therapy (RT) has shown to enhance response rates in TNBC. We hypothesize that this improved efficacy is not just localized to the tumor bed, but also involves a broader systemic immune response, although the precise mechanisms are yet to be fully understood. Therefore, in this study, we aimed to investigate T cell clonal dynamics in Tumor and peripheral blood mononuclear cells (PBMCs) in 9 patients (6 responders (R) and 3 non-responders (NR) ) from 3 different timepoints (before therapy (Baseline), after anti-PD1 alone and after anti-PD1+RT). CD45+ positive stained cells were sorted for subsequent single-cell RNA and T-cell receptor (TCR) sequencing analysis. Through single-cell analysis, we identified increased proportions of exhausted CD8+ and Treg cells from baseline to both treatment points, whereas exhaustedCD4+ T cells increased in proportions after PD1 alone in Responders. We aimed to then understand the origin of these cells, and how they might contribute to systemic immunity. Hence, by TCR overlap between Tumor and PBMC’s we detected 590 overlapped clones in CD8 and 1000 overlapped clones in CD4 T cells. Neo-TCR score was higher in these overlapped clones, suggesting tumor reactive clones can be identified in PBMC’s. Compared to exhausted CD4, exhausted CD8 T cell and Treg had higher overlapped clones between Tumor and PBMC. We also found that exhausted CD8+T cells clonal expansion after treatment was higher in pre-existing clones, compared to new clones in Responders. These pre-existing clones consisted of overlapped clones, suggesting tumor reactivity to be present in PBMC’s before treatment. Further investigation suggested that exhausted CD8+ cells also showed increased transition to TEMRA (in PBMC) and a tissue-resident memory CD8 T cell subset, under combination therapy. This pattern was not seen in exhausted CD4+Tcells or Treg cells, indicating that CD8+ T cell subsets in the blood are crucial in driving systemic anti-tumor immunity. Overall, this study has identified the systemic immune response triggered by the combination of anti-PD1 and radiation therapy highlighting its potential to inform and refine future therapeutic strategies in TNBC treatment. Citation Format: Vaishnavi Devarakonda, Jolene Viramontes, Satchel Stevens, Tahir Dar, Emily Ko, Elvin Canesco, Kaiden Anderson, Jlenia Guarnerio, Simon Knott, Stephen L. Shiao. Temporal immune profiling reveals systemic anti tumor immunity in triple negative breast cancer (TNBC) patients treated with immunotherapy and radiation abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6645.