Effect of IL-12 treatment on donor T-cell expansion and activation. Lethally irradiated B6 mice received A/J BMC and spleen cells plus TCD host-type (B6) BMC with or without IL-12, 4,900 IU, administered on day 0. (A) Mean number of CD4⁺ and CD8⁺ T cells in spleens of GVHD control and IL-12–protected mice according to donor (▨;) versus host (▪) origin, as determined by separate staining with anti-CD4 versus anti-D^d and anti-CD8 versus anti-D^d. Mean values obtained from six mice per group at each time point are shown. (B) Altered expression of CD25 on donor T cells in IL-12–protected mice. Spleen cells were analyzed by two-color FCM to determine percentages of CD4⁺ and CD8⁺ T cells expressing CD25 on days 4, 5, and 7 after BMT. The mean of the products of the percentages of CD25⁺ CD4 or CD8 cells and the spleen cell yield for GVHD control (□) and IL-12–protected mice (▦) are shown (n = 3 mice per group per time point). Black bars (▪) represent splenic T cells from normal A/J mice. Similar results were obtained in two additional experiments. Because additional stains (anti-D^dv anti-CD25) showed that all CD25-expressing spleen cells were of donor origin in these animals, it can be inferred that all CD25⁺ T cells shown in this figure are of donor origin. (C) Mean number of VLA-4⁺ CD4 and CD8 cells in spleens of GVHD control (□) and IL-12–protected mice (▪) on days 4, 5, and 7 post-BMT. The average of the product of the percentage of VLA-4⁺ CD4 or CD8 cells and the spleen cell yield (n = 3 mice per group per time point) is shown.