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Abstract

Immune checkpoint blockade has become a standard of care for triple-negative breast cancer, yet only around half of patients benefit. Compensatory immunoregulatory pathways may limit the efficacy of PD-1/PD-L1 blockade. ICOS, an inducible co-stimulatory receptor enriched on activated regulatory T cells (Tregs), represents a potential route of such immune regulation, but its role in breast cancer immunotherapy remains unclear. Here, we identify ICOS as selectively upregulated on tumor-infiltrating Tregs and show that ICOS Tregs localize within a tumor niche enriched for exhausted CD8 T cells. We confirmed this cellular and spatial association across 1,359 tumor regions from 279 triple-negative breast cancer patients profiled by imaging mass cytometry, and single-cell transcriptomic data from 129 breast cancer donors. Genetic ablation or pharmacological blockade of ICOS disrupted this intratumoral Treg niche, reducing Treg activation and tissue-residency programs while expanding cytotoxic CD8 T cells with a tissue-resident memory phenotype. In syngeneic orthotopic breast cancer models, ICOS blockade restricted primary tumor growth, reduced spontaneous metastasis and prolonged survival without disrupting systemic immune homeostasis. Therapeutic activity was observed with both neoadjuvant and adjuvant treatment, while combined ICOS and PD-1 blockade further enhanced tumor control. These findings support ICOS blockade as a therapeutic strategy to improve checkpoint immunotherapy.

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