CD274 and neoplasm: PD-L1 is mainly expressed on human tumor-associated antigen-presenting cells including tumor environmental dendritic cells (DCs) [43], monocyte-derived myeloid DCs [44], macrophages [43], neutrophils [45], fibroblasts [46], mast cells [47], and other non-tumor cells such as vascular endotheliocytes, keratinocytes, pancreatic islet cells, astrocytes, and corneal epithelial cells [48].