Abstract PO2-15-04: Fas/FasL expression on circulating tumor and immune cells in the peripheral blood of patients with metastatic breast cancer (BC): correlation with clinical outcome
Authors: Papadaki Maria , Vassilakopoulou Maria , Papadaki Eleni , Aggouraki Despoina , Zioudas Chrysostomos , Mala Anastasia , Agelaki Sofia , Mavroudis Dimitrios 
Published: May 2024

Background: The activation of Fas (CD95/APO-1) death receptor by Fas ligand (FasL/CD95L) can trigger a signal transduction pathway leading to apoptosis, which is a common pathway used by immune cells for tumor elimination. Cancer cells also exploit FasL expression to induce immune cell apoptosis, known as "Fas counterattack", and thus to increase their invasion and migration capacity. We herein assessed the expression of Fas and FasL on circulating tumor cells (CTCs) and immune cells in the peripheral blood (PB) of patients with metastatic BC. Methods: PB was obtained from 98 patients with metastatic BC at baseline before first-line treatment. Peripheral blood mononuclear cells (PBMCs) were isolated and immunofluorescently stained with antibodies against cytokeratins (CK; clones: AE1/AE3 & C11) and CD45 for CTC detection, along with antibodies for Fas, FasL and dapi. Fas/FasL expression was individually assessed on CTCs and PBMCs using the Ariol microscopy system (2x10^6 PBMCs were analyzed per patient). Results: CTCs were detected in 26/98 (26.5%) patients (total CTC count: n=70). Fas+ CTCs and FasL+ CTCs were identified in 88.5% and 92.3% of CTC-positive patients, respectively, representing the 57.1% and 82.9% of total CTCs. Regarding co-expression at single cell level, Fas+/FasL+ CTCs were detected in 84.6% of patients, whereas Fas+/FasL- CTCs, Fas-/FasL+ CTCs and Fas-/FasL- CTCs were identified in 7.7%, 19.2% and 11.5% of patients, respectively. Regarding PBMCs, the Fas+/FasL+ phenotype was identified in 70.3%, of patients, whereas Fas-/FasL+ PBMCs and Fas-/FasL- PBMCs were detected in 24.2% and 5.5% of patients, respectively; interestingly the Fas+/FasL- expression pattern was not observed in any patient. A reduced progression-free survival (PFS) was demonstrated among CTC-positive as compared to CTC-negative patients (median PFS: 9.5 versus 13.4 months; p=0.004), and specifically those harboring Fas+/FasL+ CTCs (median PFS: 9.5 vs 13.4 months; p=0.009). Increased overall survival (OS) was demonstrated among patients harboring Fas+/FasL+ PBMCs, as compared to those with Fas-/FasL+ PBMCs and Fas-/FasL- PBMCs (median OS: 35.7 vs 25.9 vs 14.4 months; p=0.008). Conclusion: Herein we provide for the first time evidence on Fas/FasL expression on CTCs and PBMCs with significant prognostic relevance for patients with metastatic BC. The findings highlight the Fas/FasL signaling pathway as a putative mechanism of immune evasion and metastatic progression of BC. Their role as prognostic biomarkers in BC patients merits further investigation.

Citation Format: Maria Papadaki, Maria Vassilakopoulou, Eleni Papadaki, Despoina Aggouraki, Chrysostomos Zioudas, Anastasia Mala, Sofia Agelaki, Dimitrios Mavroudis. Fas/FasL expression on circulating tumor and immune cells in the peripheral blood of patients with metastatic breast cancer (BC): correlation with clinical outcome [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-15-04.