PDCD1LG2 and neoplasm: Conventionally, PD-L1 or PD-L2 expressed on the surface of antigen presenting cells or tumor cells engages in trans interactions with PD-1 expressed on the surface of T cells to induce inhibitory signaling.48–50 Tumor cells can also secrete PD-L1-containing extracellular vesicles, mainly in the form of exosomes, to activate PD-1 pathway.