PDCD1 and breast cancer: While the expression of PD-1 represents a hallmark of CD8+ T cell exhaustion, recent reports reveal that varying degrees of PD-1 expression enable the identification of different exhausted CD8+ T cell subsets.22 For instance, high PD-1 expression correlates with a terminal exhaustion stage, while intermediate PD-1 expression corresponds to a pre-exhaustion stage.7 To identify the heterogeneity within the exhausted CD8+ T cell population in tumors and different tissues of BC patients, CD8+ T cells were classified as PD-1High or PD-1Int and compared to PD-1Neg (Figure 2a).