Extracellular matrix cancer‐associated fibroblasts promote stromal fibrosis and immune exclusion in triple‐negative breast cancer

Abstract The impact of high heterogeneity of cancer‐associated fibroblasts (CAFs) on triple‐negative breast cancer (TNBC) immunotherapy response has not been fully elucidated, restricting progress in precision immuno‐oncology. We integrated single‐cell transcriptomic data from 18 TNBC patients and analyzed fibroblast subpopulations. Extracellular matrix CAFs (ecmCAFs) were identified as a fibroblast subpopulation with distinct ECM‐associated characteristics. The ecmCAFs were significantly enriched in TNBC patients with residual disease after neoadjuvant immunotherapy and contributed to a fibrotic tumor microenvironment and T‐cell exclusion. Secreted phosphoprotein 1 ( SPP1 ) positive macrophages (SPP1 + Mφs) were closely localized to ecmCAFs and produced more transforming growth factor beta ( TGFB1 ), interleukin 1 beta ( IL1B ), and SPP1 under hypoxic conditions. SPP1 + Mφs were found to facilitate the differentiation of normal breast fibroblasts to ecmCAFs, thus promoting ECM remodeling and stromal fibrosis. Our work revealed the critical role of ecmCAFs in generating a desmoplastic architecture and driving immunosuppression in TNBC. © 2025 The Pathological Society of Great Britain and Ireland.