CD274 and neoplasm: Another key checkpoint pathway mediating tumor-induced immune suppression is the programmed death-1 (PD-1); PD-1 is a cell surface inhibitory receptor expressed on T cells, B cells, monocytes, and natural killer T cells, following activation.22 It has two ligands: PD-L1 (B7-H1) and PD-L2 (B7-DC), both expressed on antigen-presenting cells (APCs).23 The ligand PD-L1, thought to be the main mediator of PD-1-dependent immuno-suppression, is also expressed on some non-hematopoietic cells.