PD-1 is short for "programmed cell death 1". It's a cell surface protein found on the surface of immune cells, such as cytotoxic T-cells.PD-L1, the ligand for PD-1, is another cell surface protein often overexpressed by tumor cells.When PD-1 on T-cells comes into contact with PD-L1 on tumor cells, it causes deactivation or death of the T-cells.PD-L1 expression levels can be determined in tumor tissue by simple immunohistochemical staining.
If I may, I'll also add my general understanding to Stephen's brilliant scientific explanation. PD-1 is an immune checkpoint, one of the mechanisms built into cells of the immune system which prevent them from attacking body's regular cells (so called autoimmunity). Tumor cells exploit this mechanism to defend themselves from the immune cells. PD-1 inhibitors (e.g., nivolumab/Opdivo) disable this mechanism, making the tumor cells more vulnerable against the immune system.
Click here for a NY Times Article with an excellent graphic that shows exactly how it works. A picture is worth a 1,000 words and you'll grasp it immediately.http://www.nytimes.com/2016/07/31/health/harnessing-the-immune-system-to-fight-cancer.html
And last month, they announced that Opvido failed clinical trials for lung cancer. Very upsetting:http://www.nytimes.com/2016/08/06/business/lung-cancer-drug-opdivo-fails-clinical-trial-to-expand-use.html?_r=0
Thanks Logan! BTW- I loved your letter to Joe Biden. Right on target.