PD-1 and PD-L1 expression in cardiac transplantation.

Programmed death-ligand 1 (PD-L1), a transmembrane protein and member of the CD28 T cell family is associated with lymphocyte activation.1 PD-L1 expression is upregulated on activated antigen presenting cells such as monocytes, myeloid and dendritic cells.2 When bound to its cognate receptor programmed cell death (PD-1), inhibition of immune responses including downregulation of T cell proliferation occurs.3 Mechanistically, such inhibition would be hypothetically favorable in the setting of a transplanted organ undergoing allograft rejection. However, there is a paucity of data addressing the role of PD-L1 and PD-1 expression in the human transplanted heart.