89Zr-ivuxolimab successfully detected T-cell activation in the spleen, mesenteric lymph node, and gastrointestinal tract of mice with graft-versus-host disease induced by transgenic murine T cells expressing human OX40, compared with control groups (total body irradiation, P < 0.0001; bone marrow, P < 0.001). The gene discussed is TNFRSF4; the disease is graft versus host disease.