PDCD1LG2 and neoplasm: PD-L1 is a key regulator for peripheral immune tolerance, is induced upon cytokine stimulation (e.g., IFN-y) and expressed by different hematopoietic cells, but also other cells such as endothelium and tumor cells [12], while PD-L2 expression is mainly restricted to antigen-presenting cells (APC), e.g., dendritic cells and macrophages [14].