Original Article PD-L1 Expression in Non Small Cell Lung Cancer and Prognosis Şahin et al. PD-L1 Expression in Non Small Cell Lung Cancer and Prognosis Songül Şahin 1 , Şebnem Batur 2 , Övgü Aydın 2 , Tülin Öztürk 2 , Akif Turna 3 , Büge Öz 2 1 Pathology Laboratory, Çankırı State Hospital, Çankırı, Turkey 2 Department of Pathology, İstanbul University, Cerrahpaşa School of Medicine, İstanbul, Turkey 3 Department of Thoracic Surgery, İstanbul University, Cerrahpaşa School of Medicine, İstanbul, Turkey Address for Correspondence: Songül Şahin, Pathology Laboratory, Çankırı State Hospital, Çankırı, Turkey Phone: +90 544 536 50 09 e-mail: songulsahin34@gmail.com Received: 26 March 2018 Accepted: 26 December 2018 DOI: 10.4274/balkanmedj.2018.0392 Cite this article as: Şahin S, Batur Ş, Aydın Ö, Öztürk T, Turna A, Öz B. PD-L1 Expression in Non Small Cell Lung Cancer and Prognosis. Balkan Med J This study was specified in the 25 th National Pathology Congress on October 17, 2015. Background: Prognostic significance of programmed death-ligand-1 (PD-L1) status in non-small cell lung carcinoma (NSCLC) is controversial. Aims: In this study, we aimed to show the PD-L1 expression status in our patient population with NSCLC and its effect on prognosis and relationship with clinicopathologic data. Study Design: The study was retrospective cross-sectional study and included 208 cases (107 had squamous cell carcinoma (SCC), 72 had adenocarcinoma (AC), and 29 had other types of NSCLC) who underwent surgery between 2001 and 2012. Methods: PD-L1 (SP142 clone) was applied on the sections acquired from the microarray paraffin block with immunohistochemistry (IHC). Results: Four different threshold values were used in our study and all clinical and pathologic parameters were compared with the PD-L1 results obtained from these threshold values. PD-L1 expression was observed in patients with NSCLS independent of the histological type or subtype. Conclusion: In conclusion, PD-L1 expression is observed in NSCLC in parallel to the literature and it can be used as a negative prognostic indicator independent from the treatment method selected. Keywords: PD-L1, lung, NSCLC, prognosis Lung cancer is the most commonly observed cancer with the highest death rate in men and women amongst all cancers (1). Treatment selection for patients with lung cancer is based on the cell type, molecular characteristics of the tumor, tumor stage, and the patient’s performance status. Survival rates remain low although important developments have been made in recent years in multimodal treatments with the emergence of targeted therapies (2). New studies are being conducted on lung cancer related to tumor immunotherapy, which has been the subject of studies on many tumors in recent years (3). Impressive and long-term responses have started to be achieved with monoclonal antibodies, which target the checkpoints of the immune system (check-point inhibitors) (4). Effective protective immunity against cancer is dependent on the compatibility of the activity of cytotoxic T lymphocytes. T cell activity is related to the balance of negative and positive signals. CD28 and ICOS (Inducible T cell co-stimulator) are positive co-stimulators and they provide T cell activation and proliferation by binding to the ligand from the B7 family. On the other hand, there are negative regulatory molecules on the cell surface that inhibit T cell activation or prompt apoptosis.PD-L1 and PD-L2 are members of the B7 super family. These decrease the T cell activation by binding to the PD-1 receptors. Normally, it is an important step for the immune response to prevent tissue damage caused by the immune system induced by inflammation. However, PD-L1 and PD-L2 in the cancer cells suppress the T cell attack and provide its escape from the immune system. Thus the tumor cells effectively form an appropriate tumor microenvironment and continue proliferation. (5). Uncorrected Proof