Abstract P3-05-03: Comparative analysis of PD-L1 expression on matched primary tumors, metastasis and peripheral blood of patients with breast cancer
Authors: Papadaki Maria A , Koutsopoulos Anastasios V , Lagoudaki Eleni , Monastirioti Alexia , Thomopoulou Konstantina , Kalbakis Kostas , Papadaki Chara , Agelaki Sofia , Mavroudis Dimitrios 
Published: March 2020

Abstract
Background PD-L1 expression on tumor and immune cells plays an important role in many cancers, including breast cancer (BC), however, discordance has been demonstrated between primary tumors and matched distant metastases. PD-L1 is expressed on circulating tumor cells (CTCs) in BC, however limited data exist on PD-L1 expression on the respective peripheral blood mononuclear cells (PBMCs). In the current study we aimed to investigate tumor and immune PD-L1 expression in primary tumors, and matched peripheral blood (PB) and metastatic sites from CTC-positive patients with BC. Methodology PB was collected from 96 BC patients (early: n=72; de novo metastatic: n=24) and PBMCs were enriched by ficoll-density gradient centrifugation. PBMC cytospins were immunofluorescently stained using antibodies for cytokeratins (Clones: AE1/AE3 & C11) to detect CTCs and PD-L1 (Clone: E1L3N). Matched primary tumor samples from CTC-positive patients were evaluated for PD-L1 expression on tumor cells and TILs by immunocytochemistry (positivity cut-off: ≥1% positive tumor cells or immune cells, respectively). In 7 patients, matched metastatic tumor tissue was also available for analysis. Results CK+ CTCs were identified in 18 out of 96 BC patients (early disease: 11/72, de novo metastatic: 7/24). Overall, PD-L1+ CTCs were detected in 22% of CTC+ patients and PDL1+ tumor cells in 28% of the respective primary tumors. However, there was no concordance for PD-L1 expression positivity between CTCs and corresponding primary tumor cells. PD-L1+ PBMCs were identified in 23% of patients, whereas PD-L1+ TILs were detected in 65% of primary tumor samples (positive concordance 17.6%). In addition, there was no concordance for PD-L1-positivity between PB samples and metastatic tumor samples, regarding either tumor cells (PD-L1+ CTCs identified in 3/7 patients and PD-L1+ tumor cells in 1/7 metastatic tumor samples), or immune cells (PD-L1+ PBMCs were detected in 1/6 and PD-L1-positive TILs in 3/6 metastatic tumors). Finally, when primary and corresponding metastatic tumor samples were compared, PD-L1+ tumor cells were detected in 3 versus 1 of 7 patients, respectively (positive concordance 14.3%), whereas, PD-L1+ TILs were observed in 6 versus 4 patients, respectively (positive concordance, 42.9%). Conclusions Discrepancies in PD-L1 positivity of tumor and immune cells are for the first time demonstrated between PB, primary and metastatic tissue samples in BC. In general, PD-L1 positivity is lower in metastatic compared to primary tumors. In addition, despite that similar rates of PD-L1 expression are observed in primary tumors and the corresponding CTCs, no positive concordance exists. PD-L1 expression on CTCs and PBMCs should be further explored to identify their potential value as prognostic and/or predictive biomarkers in BC patients treated with immunotherapy.
Citation Format: Maria A Papadaki, Anastasios V Koutsopoulos, Eleni Lagoudaki, Alexia Monastirioti, Konstantina Thomopoulou, Kostas Kalbakis, Chara Papadaki, Sofia Agelaki, Dimitrios Mavroudis. Comparative analysis of PD-L1 expression on matched primary tumors, metastasis and peripheral blood of patients with breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-05-03.