PDCD1LG2 and graft versus host disease: PD-1 binds to PD-L1 (B7-H1; CD274), which is expressed by macrophages, DCs and non-hematopoietic cells, and PD-L2 (B7-DC; CD273), which is primarily expressed by monocytes and inflammatory macrophages in GVHD target organs (59, 60).