PDCD1 and graft versus host disease: It was suggested that PD-1 signaling augments production of mitochondrial reactive oxygen species (Mito-ROS) because pathogenic alloreactive T cells in GVHD target tissues were PD-1hiROShi (19, 20), although PD-1 signaling also reduces Mito-ROS production by activated T cells in vitro (21).