ECM-Induced IL-23 Drives Immune Suppression in Breast Cancer via Regulating PD-1 on Tregs.

BACKGROUND: High-grade breast cancer (HGBC) is an aggressive disease with poor prognosis, underscoring the need for new treatment strategies. The tumor microenvironment (TME), particularly the extracellular matrix (ECM), plays a pivotal role in tumor progression, therapy resistance, and immune regulation. An ECM-related gene signature (defined ECM3), found in approximately 35% of HGBC cases, is associated with aggressive tumors, epithelial-to-mesenchymal transition (EMT), poor clinical outcome and increased infiltration of immunosuppressive myeloid-derived suppressor cells (MDSCs). METHODS: In this study, we investigated the impact of the ECM on T cell regulation in HGBC patients, focusing on the relationship between ECM3 + tumors and T cell phenotypes. We employed mouse models to dissect the molecular mechanisms linking ECM components to T cell regulation, with particular attention to the role of the matricellular protein SPARC, a key component of the ECM3 signature. RESULTS: We revealed a significant correlation between highly suppressive programmed cell death-1 (PD-1) negative regulatory T cells (Tregs) and ECM3 + tumors. In mouse models, SPARC was found to down-regulate PD-1 on Tregs by promoting IL-23 release, which in turn induced SATB1 expression, a repressor of the pdcd1 gene. The selective expression of the IL-23 receptor on Tregs accounted for the targeted effect of IL-23 on these cells. Notably, blocking IL-23 with monoclonal antibodies restored PD-1 expression on Tregs and activated T effector cells. CONCLUSION: These findings extend the immune-regulatory role of the ECM to include regulatory T cells and identify potential new therapeutic targets for high-grade breast cancers. Moreover, they highlight ECM3 as a potential biomarker of resistance to PD-1/PD-L1 immune checkpoint blockade (ICB), suggesting that ECM3⁺ patients may benefit from alternative checkpoint inhibitor therapies beyond PD-1/PD-L1.