However, the PD-1/PD-L1 axis plays an important role in maintenance of immune tolerance after allogeneic HSCT as revealed by preclinical studies (11–14) and supported by the high incidence of severe, and in some cases fatal, Graft-vs.-Host-Disease (GvHD) in patients receiving PD-1 blockade for post-transplant relapse (8–10). Here, PDCD1 is linked to graft versus host disease.