CD8A and leukemia: PD-1 expression increased during disease progression, as assessed by flow cytometric analysis of CD8+ cells from Eμ-TCL1 mice with either mild (20–39% leukemic cells in PB, 1–2 × 107 WBC/ml PB) or overt (>40% leukemic cells in PB, >2 × 107 WBC/ml PB) leukemia (Fig. 3C, Supplementary Fig. 3A).