XIAP deficiency sensitizes mice to LBW242-induced T-cell death. C57BL/6 WT and XIAP^−/− mice were infected intravenously with 200 PFU LCMV and treated daily with 5 mg/kg LBW242 or solvent intraperitoneally. At day 8 post-infection, (A) total leukocyte counts and (B) percent and total counts of CD8 T cells were determined in the spleen. (C) Differentiation of effector T-cell subpopulations was analyzed by staining with anti-KLRG-1. (D) At day 8 post-infection, GP33-specific T cells were determined in the spleen. (E) Proliferation of XIAP^−/− CD8 T cells treated with LBW242. A total of 3 × 10⁵ CFSE-labeled WT splenocytes were stimulated with plate-bound anti-CD3 antibody. Splenocytes were exposed to LBW242 (red line) or solvent (green line). After 72 hours, cultures were harvested completely and stained with anti-CD8 antibodies. Cells were resuspended in equal volumes of FACS buffer and counted in the flow cytometer for a defined time interval. The area under each curve indicates the total number of cells collected, and the dilution steps of CFSE fluorescence represent the number of cell divisions. Red numbers represent a statistical analysis of the proportion of total T cells exposed to IAP antagonists relative to solvent control from 4 experiments. Horizontal lines represent mean values. *P < .05; **P < .005; ***P < .001. NS, not significant (Student unpaired t test). Data are representative of 2 independent experiments for in vivo and 4 for CFSE in vitro experiments.