PD-L1^-/- vs. WT donor T cells lessen GVHD lethality, independent of donor Treg function. (A) Survival of BALB/c mice with WT B6 or PD-L1^-/- T cells. (B) Survival of B10.BR mice with WT B6 or PD-L1^-/- T cells. (C) Survival of BALB/c mice with WT B6 or PD-L1^-/- T cells, or with WT B6 or PD-L1^-/- CD25 depleted T cells (recipients of WT T cells vs. WT CD25-depleted T cells, P = 0.0003; recipients of PD-L1^-/- T cells vs. PD-L1^-/- CD25-depleted T cells, P = 0.2306; recipients of WT vs. PD-L1^-/- T cells, P < 0.0001). (D) BALB/c mice transplanted with WT B6 or PD-L1^-/- T cells. Mice were killed on d7 post-BMT and splenocytes were analyzed for LPAM-1, CCR9, and CXCR3 expression (not shown) on donor T cells. (E-F) BALB/c mice were transplanted with B6 Ly5.2 T cells plus PD-L1^-/- T cells. Mice were killed on d3 post-BMT and splenocytes were analyzed for CTLA-4 and Lag-3 expression on donor T cells.