Abstract
No validated biomarkers beyond PD-L1 currently guide immune checkpoint inhibition (ICI) in advanced triple-negative breast cancer (TNBC), and how tumors remodel their microenvironment in response to ICI remains incompletely understood. We perform a temporal analysis of patients with advanced TNBC treated with chemo-immunotherapy in the SYNERGY trial (NCT03616886; n = 84) and the EORTC-1553-SPECTA initiative (NCT02834884; n = 38) using RNA sequencing, whole-exome sequencing, multiplex immunofluorescence, and imaging mass cytometry. A transcriptomic progression-free survival (PFS)-based risk signature stratifies outcomes and is validated across ICI-treated cohorts, while showing context-dependent behavior in non-ICI settings. Favorable outcomes are associated with immune-enriched tumors with organized lymphoid architecture, while poor outcomes are linked to fibrotic, angiogenic microenvironments with frequent PI3K alterations. Upon treatment, early progression was characterized by fibrovascular remodeling, macrophage expansion, and upregulation of alternative immunosuppressive mediators. These findings establish fibrovascular and myeloid reprogramming as a coordinated resistance program, identify actionable therapeutic targets, and support microenvironment stratification in future ICI-based trials.