Amarnath et al.36 and Fujiwara et al.37 used mouse models of acute and chronic graft-versus-host disease (GVHD) and found that regulatory T cells (Tregs) promote immunosuppression through PD-L1 upregulation leading to GVHD and that the PD-1/PD-L1 pathway inhibits Th17/Th1-mediated chronic GVHD. Here, PDCD1 is linked to graft versus host disease.