Abstract PS2-07-10: A Single Cell Peripheral Blood Immune Cell Atlas of Breast Cancer Disease States by Race reveals Differences in Peripheral Blood Immune Cell Composition and Functional Pathways

Abstract Background: Racial disparities in breast cancer (BC) are well documented— Non-Hispanic Black (NHB) women are more likely to develop BC at a younger age and experience worse survival outcomes than Non-Hispanic White (NHW) women. Previous studies using pan-cancer cohorts have suggested that changes in peripheral immune cells accompany tumor development. However, in breast cancer, whether the trajectory of these immune changes differs by race across key stages of disease progression, remains poorly understood. To address this gap, we systematically characterized and compared the circulating immune landscape of NHB and NHW women across the continuum of breast cancer progression, from high-risk lesions (HRL) to ductal carcinoma in situ (DCIS) to invasive breast cancer (BC). Methods: PBMCs were isolated from age-matched NHB (HRL:10, DCIS:6, BC:13) and NHW women (HRL:10, DCIS:4, BC:13). scRNA-seq was performed using the 10x Genomics 5’ v2 platform, with data processing by CellRanger (v7.1.0) and Seurat (v5.1.0). Cell types were identified via reference-guided annotation with Azimuth (v0.5.0) and canonical markers. Differentially expressed genes (DEGs) between NWB and NHW were detected using edgeR (v4.4.2, by FDR0.05) after pseudobulking by cell type in each patient, the number of DEGs was used to quantify PBMC transcriptomic differences by race and across disease stages. Pathway enrichment of DEGs was performed with Gene Set Enrichment Analysis (GSEA). Results: We analyzed 436,077 high-quality single PBMCs classified into 36 cell types. The proportions of various PBMC cell types differed between NHB and NHW women across all disease stages. For example, the ratio of classical:non-classical monocytes was lower in NHB women than NHW women at all disease stages. We observed a markedly higher number of DEGs between BC and HRL than between DCIS and HRL, suggesting a greater transcriptomic change with the transition to invasive breast cancer. These differences were mostly reflected in naïve and non-switched memory B cells, naïve and central memory CD4+ T cells, monocytes, and mature NK cells. In BC patients, for monocytes, NHB race was related to abundance of non-classical monocytes and IFN pathway in all monocytes. In CD4+ T cells, NHB showed higher PD-1 and lower TIM-3 levels, potentially suggesting an early exhaustion state towards immune suppression, which may affect immunotherapy outcome. TNF-α, apoptosis, IL2-STAT5, and TGF-β signaling pathways were consistently up-regulated in multiple cell types in NHB PBMC, and oxidative phosphorylation was constantly down-regulated. Conclusion: This study is one of the largest single cell transcriptomic profiling of circulating immune system comparing race across disease states of breast cancer. Results showed that BC gene expression (GE) diverge from HRL much more than DCIS. GE difference by race was also largest in BC, featuring increased IFN in monocytes and an early exhaustion in CD4 T cells in NHB patients. Further investigation is warranted to determine how these differences in systemic immune cell quality in NHB breast cancer patients relate to response to immunotherapy and survival. Citation Format: F. Chen, E. R. Ogayo, T. Rahman, A. Recko, A. M. Parsons, P. van Galen, S. S. McAllister, E. A. Mittendorf. A Single Cell Peripheral Blood Immune Cell Atlas of Breast Cancer Disease States by Race reveals Differences in Peripheral Blood Immune Cell Composition and Functional Pathways abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-07-10.