Expression of programmed death ligand 1 (PD-L1) in malignant and nonmalignant pancreatic tissue

215 Background: Programmed death 1 (PD-1) protein and its ligands (PD-L1 & L2) play a key role in the evasion of the host immune system by tumor cells via inhibition of T-cells. It is of therapeutic interest that a number of tumor types selectively over-express PD-L1, since antibodies directed against PD-L1 have recently been shown to induce durable tumor regression. Methods: In this study we evaluated the expression of PD-L1 in pancreatic tissue obtained from 145 patients with localized pancreatic adenocarcinoma who underwent curative resection at our institution (stage IA – IIB). Of the 145 patients, 44 had paired pancreatic tissue samples of the tumor and adjacent non-malignant pancreatic tissue. These paired samples were used to compare expression of PD-L1 in malignant and nonmalignant tissue. Immunohistochemical staining was performed using rabbit primary antibody against PD-L1 (CD274/B7-H1) on formalin-fixed, paraffin-embedded core sections in tissue microarrays. Immunostain intensity and percentage cells stained were each scored on a scale of 0 to 3, with 3 being maximal. The product of intensity and percentage was used as a semi-quantitative measure of PD-L1 expression. Only patients with a product score greater than 0 on either the malignant or non-malignant tissue were included in this study. Results: PD-L1 expression (product score) was higher in 81% (36/44) of the malignant tissue samples than in paired non-malignant samples, equal in 7% (3/44) and lower in 11% (5/44). PD-L1 expression in malignant tissue (mean product score = 4.23) was significantly higher than the non-malignant pancreatic tissue (mean product score = 1.73) (p < 0.0001). The median product score was also statistically significantly higher for malignant versus non-malignant (4 vs. 2) tissue. Conclusions: Pancreatic adenocarcinoma tumor cells express PD-L1 at a higher level than their non-malignant counterparts. Future trials should evaluate drugs targeting the PD-1/PD-L pathway in the treatment of pancreatic cancer.