MON-287 Expression of Programmed Death-Ligand 1 (PD-L1) in Human Pituitary Neuroendocrine Tumor

Introduction Some Pituitary NeuroEndocrine Tumors (PitNET) present an aggressive evolution and are resistant to standard management. Immunotherapy have shown durable efficacy in a variety of malignancies. The aim of this study was to explore the programmed death-ligand 1 (PD-L1) expression in varied subtypes of pituitary adenomas with assessment of their clinical behavior at diagnosis and follow-up. Methods We conducted a retrospective monocentric study, including all patients operated a PitNET between 2012 and 2018. PDL-1 immunostaining were performed using an European Conformity-In-Vitro-Diagnostic labeled anti-PDL1 antibody (Clone 22C3). PD-L1 immunostaining was evaluated as the percentage of tumor cell showing positive membrane staining, into four grades: grade 0 = <1%, grade 1 = 1 to 5%, grade 2 = 6 to 49% and grade 3 = ≥ 50%. PD-L1 expression was compared with tumor features (secretion, proliferation, invasion) and outcome. Results The study included one hundred and thirty-nine PitNET, including 84 (60%) nonfunctioning adenomas. Twenty-five PitNET were PD-L1 positive (18%), including 3 grade 3, 8 grade 2 and 14 grade 1. PD-L1 expression was not different between functioning and non-functioning adenomas (p=0.26). Among sixteen tumors with proliferative markers (Ki-67 ≥ 3% and p53 positive), only one was PD-L1 positive. Conclusion In our series, pituitary tumors rarely exhibit PD-L1 expression and this immune marker did not seem to be associated with any biological characteristic or behavior of the pituitary tumors. Thus, PD-L1 staining is necessary before considering PD-L1 blockage in PitNET, in case of therapeutic impasse.