CD274 and cancer: PD-L1 is constitutively expressed at varying levels in cells of the myeloid lineage, such as DCs, macrophages, and myeloid-derived suppressor cells (MDSCs) but also in other cell types.5,18,27–33 This includes many tumors and cancer cell lines.7,21 Moreover, PD-L1 is up-regulated in many cell types, including cancer cells, by a range of pro-inflammatory stimuli.26,34–36 This expression is regulated via the binding of transcription factors to its promoter activated by pro-inflammatory cytokines.