Abstract 7282: A comprehensive multiomics atlas of treatment-naïve breast cancer uncovers co-occurring tumor-immune ecosystems driving immune hot and cold phenotypes

Abstract Breast cancer is a clinically and genetically heterogeneous disease. Whilst single-cell studies have advanced our understanding of the underlying biological diversity of this disease, a major limitation in previous studies is that they fail to capture the broad spectrum of breast cancer subtypes and lack the statistical power to resolve subtype-specific differences. To address this gap, we generated a comprehensive breast cancer single-cell atlas designed to robustly characterise transcriptomic and genomic heterogeneity across all major clinical subtypes. Focusing on treatment-naïve tissues to capture disease biology prior to therapeutic intervention, we profiled nearly 200 patient samples to generate a comprehensive multiomics atlas of scRNA-seq, whole transcriptome sequencing (WTS), whole genome sequencing (WGS), and clinicopathological data, such as age, histological grade and type, ER/PR/HER2 measurements, and stromal tumour-infiltrating lymphocyte (sTIL) and PD-L1 status. To quantify inter- and intra-tumoral heterogeneity, we developed a framework of breast cancer archetyping that positions malignant epithelial cells along axes capturing the cancer intrinsic properties. Archetyping successfully resolves biologically meaningful gradients and systematically quantifies transcriptomic heterogeneity. Our results show that luminal cancers—particularly Luminal B—exhibit the largest degree of intra- and inter-tumoural diversity. We define the key gene programs that contribute to this variability and identify immune programs among luminal cancers. Using sTIL status as a measure of immunogenicity, we identified an underappreciated subset of luminal tumours exhibiting high immune infiltration—challenging the prevailing view that luminal cancers are uniformly “immune cold.” Leveraging our single-cell data, we performed integrative analysis to uncover the distinct co-occurring tumour-immune ecosystems associated with immune-hot versus immune-cold phenotypes. These observations were further validated using our matched multimodal WGS, WTS, and spatial transcriptomics data. Our single-cell multiomics breast cancer atlas spanning all major clinical subtypes provides a high-resolution framework for dissecting the multilayered heterogeneity of breast cancer. By maximizing patient representation and integrating rich clinical and genomic metadata, this resource enables robust identification of subtype-specific biology and reveals striking diversity within luminal breast cancers, including a substantial subset with immune-hot characteristics. Ongoing work will leverage this atlas and our deeply curated metadata, including survival outcomes, to refine breast cancer stratification and identify clinically actionable tumour ecosystems. Citation Format: Hani Jieun Kim, Beata Kiedik, Kate Harvey, Sehrish Kanwal, James Douglas, John Reeves, Alexander Lobanov, Daniel L. Roden, Sophie Van Der Leij, Mun N. Hui, Aziz Al’Khafaji, Elgene Lim, Sean M. Grimmond, Joakim Lundeberg, Charles M. Perou, Alexander Swarbrick. A comprehensive multiomics atlas of treatment-naïve breast cancer uncovers co-occurring tumor-immune ecosystems driving immune hot and cold phenotypes abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7282.