PDCD1 and graft versus host disease: The acute GVHD induced by STAT3–/– donor T cells in PD-L1–/– recipients (Figure 4, D and E) or by STAT3–/–PD-1–/– donor T cells in WT recipients (Figure 4, F and G) caused severe damage in the liver, small intestine, and colon (Figure 4, D and F, and Supplemental Figure 3) with extensive infiltration of donor T cells in those tissues, but not in the spleen (Figure 4, E and G).