BTLA expressed on donor T cells as an essential target for inhibitory effects of BYK-1 in GVHD. (A) BDF1 mice were injected intravenously with 5 × 10⁷ B6 WT or BTLA-KO mouse spleen cells on day 0 and treated intraperitoneally with 200 μg BYK-1 or control Ig on days 0, 3, and 6 in the following combinations: transfer of WT donor cells and control Ig treatment (□), transfer of WT donor cells and BYK-1 treatment (■), transfer of BTLA-KO donor cells and control Ig treatment (○), and transfer of BTLA-KO donor cells and BYK-1 treatment (●). On day 9, recipient spleen cells were harvested and analyzed for CTL activity against P815 (H-2^d) and EL4 (H-2^b) cells. (B) BDF1 mice were injected with mixture of T cells (2 × 10⁷ cells) and non-T cells (4 × 10⁷ cells) purified from either B6 WT or BTLA-KO mice and treated intraperitoneally with BYK-1 or control Ig on days 0, 3, and 6 in the following combinations: transfer of WT T cells plus BTLA-KO non-T cells followed by control Ig treatment (□), transfer of WT T cells plus BTLA-KO non-T cells followed by BYK-1 treatment (■), transfer of BTLA-KO T cells plus WT non-T cells followed by control Ig treatment (○), and transfer of BTLA-KO T cells plus WT non-T cells followed by BYK-1 treatment (●). On day 9, recipient spleen cells were harvested and analyzed for CTL activity against P815 (H-2^d) and EL4 (H-2^b). CTL level against P815 in BYK-1-treated WT T cells plus BTLA-KO non-T cells (■) was significantly lower than control Ig-treated BTLA-KO T cells plus WT non-T cells (○) at an effector/target (E/T) ratio of 100, 50, 25, and 12 (P < .05). Data are representative of 3 independently repeated experiments.