Kinetics of OX40 expression on rat PBL. The percentage of CD4⁺ T cells expressing OX40 was determined on PBL by FACScan analysis using specific monoclonal antibodies on days 3, 5, 8, 12, 15, 20, and 25 posttransplantation. PBL were obtained from three rats per group and the data are presented as the mean ± standard deviation. Quadrants were set using nontransplanted Buffalo PBL as a negative control for OX-40 expression. GVHD was first detected on day 10 (event A) as erythroderma around the eyes. As the disease progressed, blistering of the feet and ear swelling (day 15, event B), hair loss (day 20, event C), and diarrhea (day 25, event D) were also observed.