Abstract P3-01-13: Co-expression of molecules associated with innate and adaptive immune response on single CTCs of patients with metastatic breast cancer (mBC)

Abstract

Abstract Introduction The expression of CD47 on tumor cells can act as a "don't eat me signal" against phagocytosis by macrophages and dendritic cells. Moreover, PD-L1-expressing tumor cells inhibit the anti-tumor activity of cytotoxic T cells. Circulating tumor cells (CTCs) expressing these molecules could overcome elimination by the immune system. In the current study, we evaluated for the first time the co-expression of CD47 and PD-L1 on single CTCs from patients with metastatic breast cancer (mBC). Methods Triple immunofluorescence staining was performed on peripheral blood mononuclear cells (PBMC) cytospin preparations from 18 CTC-positive patients with mBC, using antibodies against cytokeratins (for CTC detection), CD47 and PD-L1. Blood samples were obtained before the initiation of first-line chemotherapy. A total of 1*106 PBMCs were analyzed per patientusing the Ariol microscopy system. The expression levels of CD47 and PD-L1 were characterized as high or low/-, after quantification by the Ariol system, using the MDA.MB.231 breast cancer cell line as positive control. Results A total of 23 CTCs (median: 1, range: 1-4) were identified. CD47-expressing CTCs were detected in 94.4% of patients and represented 91.3% of total CTCs. However, high CD47 expression was confirmed in 38.9% and 43.5% of patients and CTCs, respectively. PD-L1 expression was evident in 27.8% of patients and in 21.7% of CTCs, whereas CTCs expressing high levels of PD-L1 were identified in 16.% of patients and represented 13% of total CTCs. Co-expression of CD47 and PD-L1 (CD47+/PD-L1+) was observed in 21.7% of CTCs, whereas 69.6% of CTCs expressed CD47 only (CD47+/PD-L1-). No CTCs expressing only PD-L1 (CD47-/PD-L1+) were detected and 2 of 23 cells were negative for both markers (CD47-/PD-L1- ). Regarding the differential expression levels of CD47 and PD-L1, the phenotype CD47low/-/PD-L1low/- was the most abundant both at the patient (61.1%) and the CTC level (52.2%). CD47high/PD-L1low/- CTCswere observed in 33.3% and 34.8% of patients and CTCs, respectively whereas only 1 CD47low/-/PD-L1high CTC was detected in one patient. Interestingly, CD47high/PD-L1high CTCs were identified in only 8.7% of CTCs. Conclusions CD47 expression is identified in the great majority of CTCs in mBC and could represent a potent signal to facilitate the escape from innate immune response. PD-L1 expression on CTCs is less commonly observed and could serve for the attenuation of an adaptive anti-tumor immune response. Interestingly, CD47 and PD-L1 are co-expressed in a subset of CTCs, whereas simultaneous high expression on single CTCs is even less common. The expression of these molecules is currently further investigated in a larger cohort of patients with mBC and in patients with early disease, in order to evaluate their differential distribution that potentially reflects the equilibrium and/or escape from the immune surveillance. Citation Format: Mavroudis D, Papadaki MA, Tsoulfas PG, Troullinou K, Apostolopoulou CA, Papadaki C, Agelaki S. Co-expression of molecules associated with innate and adaptive immune response on single CTCs of patients with metastatic breast cancer (mBC) [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-01-13.