Indispensable role of HVEM in allo-CTL generation and donor T-cell survival. (A) BDF1 mice were injected intravenously with 5 × 10⁷ spleen cells from WT (▪) or HVEM-KO (□) mice. Ten days later, spleen cells from the BDF1 mice were harvested and analyzed for the CTL activity against P815 and EL4, as shown in Figure 1. (B) T cells (2 × 10⁷ cells) and non–T cells (4 × 10⁷ cells) purified from spleen cells of WT or HVEM-KO mice were mixed as follows: WT T cells plus WT non–T cells (□), WT T cells plus HVEM-KO non–T cells (▪), HVEM-KO T cells plus WT non–T cells (○), HVEM-KO T cells plus HVEM-KO non–T cells (•). These combined cells were injected intravenously into BDF1 mice. Ten days later, anti–host CTL activity from recipient spleen cells were analyzed. (C-D) BDF1 mice were injected intravenously with 5 × 10⁷ spleen cells from WT or HVEM-KO mice. Ten days later, spleens from the BDF1 recipients were harvested and stained with Annexin V and anti–H-2K^d mAb, along with either anti–CD4 or anti–CD8 mAb. (C) Percentage of donor (H-2K^d-negative) CD4⁺ or CD8⁺ T cells in total spleen lymphocytes is shown. (D) Percentage of Annexin V–positive cells in donor CD4⁺ or CD8⁺ T cells is shown. (A-D) Results shown in are representative of 3 independently performed experiments. (E) BALB/c mice (n = 10 in each group) were exposed to lethal-dose irradiation (10 Gy) followed by intravenous injection of 5 × 10⁶ T cell–depleted B6 BM cells alone (•) or together with 1 × 10⁶ B6 WT (▪) or HVEM-KO (○) T cells. The survival was monitored daily. *P = .005.