Programmed death 1 (PD-1) is an inhibitory molecule expressed on the surface of T cells. The PD-1 ligand (PD-L1 (B7-H1)) is expressed on nonhematopoietic cells as well as antigenpresenting cells and placental cells located in an inflammatory microenvironment, as PD-L1 is in general upregulated by interferons. The PD-1/PD-L1 pathway is thought to assure peripheral T-cell tolerance and is involved in controlling the proliferation and cytokine production of T cells. 1 PD-L1 has been described to be expressed on cancer cells in many different hematological malignancies, where it contributes to protection of the malignant cells from immune destruction. 2, 3 As an example, in aggressive B-cell lymphomas both malignant