PD-1 expression on CD8 T cells regulates their differentiation within lung allografts and is critical for tolerance induction.

Immunological requirements for rejection and tolerance induction differ between various organs. While memory CD8(+) T cells are considered a barrier to immunosuppression-mediated acceptance of most tissues and organs, tolerance induction after lung transplantation is critically dependent on central memory CD8(+) T lymphocytes. Here we demonstrate that costimulation blockade-mediated tolerance after lung transplantation is dependent on programmed cell death 1 (PD-1) expression on CD8(+) T cells. In the absence of PD-1 expression, CD8(+) T cells form prolonged interactions with graft-infiltrating CD11c(+) cells; their differentiation is skewed towards an effector memory phenotype and grafts are rejected acutely. These findings extend the notion that requirements for tolerance induction after lung transplantation differ from other organs. Thus, immunosuppressive strategies for lung transplant recipients need to be tailored based on the unique immunological properties of this organ.