Abstract A028: Cycling T regulatory cells architect an immune escape hub during <i>in situ</i> to invasive breast carcinoma transition

Abstract The tumor immune microenvironment shapes breast tumor evolution. We previously described the ductal carcinoma in situ (DCIS) to invasive breast carcinoma (IBC) transition as an evolutionary bottleneck and a key step of immune escape. Here, we dissected the cellular composition and spatial topology of DCIS and IBC patient samples and rat tumors that faithfully recapitulate human breast cancer. We identified differences in T cells as the main distinguishing feature between DCIS and IBC and discovered a novel cycling regulatory T cell population (cycTreg) as the primary orchestrator of the immune suppressive environment during invasive progression, especially enriched in triple-negative histological subtype. We showed that cycTregs are locally expanding Treg precursors in response to increased antigen presentation, and further demonstrated that modulating them by OX40 agonist in combination with anti-PD-L1 enhances CD8+ T cell infiltration and suppresses tumor growth. Characterizing therapeutic responses at the single cell level revealed a Treg and cancer-associated fibroblast (CAF) crosstalk via IL33-ST2 pathway that is critical for Treg expansion. Multimodal analysis of the patient cohorts demonstrated that cycTregs predict recurrence in low-grade DCIS and poor patient outcomes in IBC, and that increased CAF-immune-DCIS proximity is prognostic of progression to IBC. In sum, our study establishes cycTregs as key drivers of immune escape during invasive progression and implies that their targeting is essential for enhancing the efficacy of immune checkpoint inhibitors in IBC. Citation Format: Triet Minh Bui, Ernesto A Rojas, Pierre Foidart, Zheqi Vaciry Li, Pengze Yan, Jun Nishida, Robert West, Shelley Hwang, Simona Cristea, Kornelia Polyak. Cycling T regulatory cells architect an immune escape hub during in situ to invasive breast carcinoma transition [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2024 Oct 18-21; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2024;12(10 Suppl):Abstract nr A028.