The presence of OX-40⁺ T cells in tissues from rats with aGVHD. Splenocytes, liver, and lymph node cells were obtained on day 15 posttransplantation and stained with antibodies to CD4 (ordinate) and OX-40 (absissa). The top row represents tissue obtained from control syngeneic transplanted rats. The bottom row represents tissue obtained from allogeneic transplanted rats with aGVHD. Tissue from control rats had 3%, 4%, and 3% double-positive cells in spleen, lymph node, and liver, respectively. Tissue from rats with GVHD had 22%, 33%, and 42% double-positive T cells from spleen, lymph node, and liver, respectively.