Upregulation of Fas on donor CD4 cells in IL-12–protected mice. Lethally irradiated B6 mice received A/J BMC and spleen cells plus TCD B6 BMC without or with IL-12 on day 0. At 36 hours and on days 3, 5, and 7 post-BMT, spleen cells were analyzed by three-color FCM to examine the expression of Fas on 5F1-negative, ie, donor CD4⁺ cells. Solid lines represent the Fas density on donor CD4 cells recovered from spleens of GVHD mice and dotted lines represent Fas expression on splenic donor CD4 cells recovered from IL-12–protected mice. Similar results were obtained from six mice per group at each time point in three independent experiments.