VSIR and myeloid leukemia: In addition to overexpressing PD-1H in C1498 cells, we also performed PD-1H knockdown in murine myeloid leukemia cell line WEHI3, which constitutively expresses PD-1H, using shRNA (WEHI3–PD-1Hlo versus WEHI–PD-1Hhi) (Supplemental Figure 3D), and tested the effect of PD-1H knockdown on leukemia growth in vivo.