Tumoral PD-1^hiCD8⁺ T cells are partially exhausted and predict favorable outcome in triple-negative breast cancer

Tumor-infiltrating PD-1^hi dysfunctional CD8⁺ T cells have been identified in several tumors but largely unexplored in breast cancer (BC). Here we aimed to extensively explore PD-1^hiCD8⁺ T cells in BC, focusing on the triple-negative BC (TNBC) subtype. Flow cytometry was used to study the phenotypes and functions of CD8⁺ T-cell subsets in peripheral blood and surgical specimens from treatment-naive BC patients. RNA-seq expression data generated to dissect the molecular features of tumoral PD-1^neg, PD-1^lo and PD-1^hi CD8⁺ T cells. Further, the associations between tumoral PD-1^hi CD8⁺ T cells and the clinicopathological features of 503 BC patients were explored. Finally, multiplexed immunohistochemistry (mIHC) was performed to evaluate in situ PD-1^hiCD8⁺ T cells on the tissue microarrays (TMAs, n=328) for prognostic assessment and stratification of TNBC patients. PD-1^hiCD8⁺ T cells found readily detectable in tumor tissues but rarely in peripheral blood. These cells shared the phenotypic and molecular features with exhausted and tissue-resident memory T cells (T_RM) with a skewed TCR repertoire involvement. Interestingly, PD-1^hiCD8⁺ T cells are in the state of exhaustion characterized by higher T-BET and reduced EOMES expression. PD-1^hiCD8⁺ T cells found preferentially enriched within solid tumors, but predominant stromal infiltration of PD-1^hiCD8⁺ T subset was associated with improved survival in TNBC patients. Taken together, tumoral PD-1^hiCD8⁺ T-cell subpopulation in BC is partially exhausted, and their abundance signifies ‘hot’ immune status with favorable outcomes. Reinvigorating this population may provide further therapeutic opportunities in TNBC patients.