Comparative T-cell proliferation and apoptosis of IL-21^−/− versus wt CD25⁻ T cells during a GVHD response. Frequency of Ki-67–positive CD4⁺ and CD8⁺ spleen and lymph node T cells (A); mean fluorescence intensity (MFI) of annexin-V–allophycocyanin on CD4⁺ and CD8⁺ spleen and lymph node T cells (B). Total cell numbers in spleen and lymph nodes; (C) CD4/CD8 ratio in spleens and lymph nodes (D). All data at day 7 after BMT of lethally irradiated recipients injected with 10⁷ T cell–depleted BM cells and 5 × 10⁶ CD25⁻ wt or IL-21^−/− CD25⁻ T cells. P = ns for all wt versus IL-21^−/− comparisons. Data from 1 experiment with 3 mice per group.