PDCD1 and neoplasm: Furthermore, PD-L1 is expressed in different cell types, including the immune cells (APCs), non-lymphoid organs (the lung, heart, and placenta), and non-hematopoietic cells (epithelial cells, endothelial cells, and tumor cells), as opposed to the PD-1 that is primarily expressed in the immune cells (T- and B-lymphocytes) (Zhang et al., 2019).