CD8+ T cells in breast cancer tumors and draining lymph nodes: PD-1 levels, effector functions and prognostic relevance.

CD8⁺ T cells shape the antitumor immune response. Here, we evaluated CD8⁺ T cells expressing different levels of PD-1, their functional status, and distribution in different tissues of luminal breast cancer (BC) patients. We characterized the exhaustion stages of CD8⁺ T cells in tumors, juxtatumoral tissues (JTs), and tumor-draining lymph nodes (TDLNs). Terminal exhausted CD8⁺ T cells (PD-1^High CD8⁺) were predominant in tumors and nearly absent in other tissues. However, in all tissues evaluated, most CD8⁺ T cells exhibited a pre-exhausted phenotype (PD-1^Int CD8⁺) or did not express PD-1. PD-1^High and PD-1^Int CD8⁺ T cells from tumors and JTs presented central and effector memory phenotypes, while in TDLNs were primarily central memory. TCR-β sequencing revealed higher clonality among CD8⁺ T cells from tumor than TDLNs, with tumor-enriched clones also detected in TDLNs. Analysis of scRNA-seq datasets from tumors and JTs of colorectal and non-small cell lung cancer patients, identified a CD8⁺ terminal exhaustion and a CD8⁺ pre-exhausted signatures. High expression of exhaustion-associated genes in BC tumors correlated with improved overall survival. Overall, PD-1 expression effectively distinguishes exhaustion stages in CD8⁺ T cells. PD-1^Int cells found in tumors, JTs, and TDLNs represent a promising therapeutic target for cancer immunotherapy.