Profiling Multiple CD8⁺ T-cell Functional Dimensions Enhances Breast Cancer Immune Assessment

CD8⁺ T-cell abundance is insufficient to assess antitumor immunity and shows poor performance in predicting breast cancer prognosis and immunotherapy response, presumably owing to the complexity of CD8⁺ T-cell functionalities. Although single-cell RNA sequencing can dissect the multifaceted functions of CD8⁺ T cells for better immune assessment, its clinical application is limited. In this study, we developed bulk RNA sequencing–based FuncDimen models from integrative analysis of single-cell RNA sequencing and matched bulk RNA sequencing data to evaluate CD8⁺ T-cell functionalities across five dimensions: tumor reactivity, cytotoxicity, IFNγ secretion, proliferation, and apoptosis. The FuncDimen models quantifying different functional dimensions of CD8⁺ T cells were validated in our breast cancer cohort and external databases using immunofluorescence and imaging mass cytometry. We calculated the FuncAggre score by weighted aggregation of all five FuncDimen models to encapsulate the overall antitumor immunity. In our breast cancer cohort and external databases, the FuncAggre score demonstrated superior predictive performance for breast cancer prognosis (time-dependent AUC: 0.56–0.70) and immunotherapy response (AUC: 0.71–0.83) over other immune biomarkers, regardless of the breast cancer molecular subtype. Together, the FuncDimen models offer a refined assessment of antitumor immunity mediated by CD8⁺ T cells in the clinic, enhancing prognostic prediction and aiding personalized immunotherapy in breast cancer.