CD274 and cancer: Thus, PD-L1 is expressed in various cell types, including normal, hematopoietic and non-hematopoietic cells as well as cancer cells, whereas PD-L2 expression is mainly restricted to hematopoietic cells such as dendritic cells (DCs), macrophages, B cells, mast cells and activated T cells, but also tumor and stromal cells [20,30].