PDCD1 and graft versus host disease: In graft-versus-host disease, rapid mortality of PD-L1−/− mice was associated with elevated Th1 cytokine production35, and in heart transplantation, absence of PD-L1 promoted Th1-cell polarization and allograft rejection36 strongly indicating the essential role of the PD-1/PD-L1 pathway in controlling Th1 immune responses in autoimmune diseases.