Erest because B7-H1/PD-L1, unlike B71/-2, is selectively upregulated by many human cancers.71 The PD-1 pathway normally plays a protective role in modulating immunemediated tissue destruction but can be exploited by cancer to protect itself from tumor-specific T cells.56 Although CTLA-4 regulates de novo immune responses, the PD-1 pathway exerts its major influence on ongoing (effector) immune res