Lymphoid atrophy in IFN-γRKO hosts is due to FasL-mediated apoptosis and inhibition of cell proliferation. (A) Proportion of apoptotic cells in the spleen on day 15 after transplantation. Apoptotic cells were defined as annexin V–positive propidium iodine-negative cells (Ann V⁺/PI⁻). Mean and SD of at least 3 mice per group from one representative experiment out of 2. (B) Day 8 FasL transcripts in CD8 T cells from wt and IFN-γRKO hosts. Levels are expressed relative to FasL transcripts found in unmanipulated C3H.SW CD8 T cells (arbitrary level of 1). Three mice per group from 3 independent experiments. Absolute cell numbers (C) and relative increase (D) in spleen cellularity on day 15 after transplantation after injection of anti-FasL monoclonal antibody MFL4 (FasL neutralization⁺) or a control antibody (FasL neutralization⁻; 7-9 mice per group). (E) Percentage of donor cells in S/G₂-M phases of the cell cycle on day 15 after transplantation. Data represent the mean and SD of at least 3 mice per group from 1 representative experiment out of 2. *P < .05, **P < .01, and ***P < .001.