CD8A and breast carcinoma: In a breast cancer mouse model, the PD-1hi-expressing CD8+ T cell population failed to be rescued by anti-PD-1 therapy, showing increases in the Treg/CD8+ T ratio, in contrast to CD8+ T cells with PD-1lo surface expression, which were sensitive to anti-PD-1 mAb in a colon cancer mouse model (24).