Abstract P4-01-15: CTC with EMT phenotype are associated with PD-L1 expression in tumor associated stroma in primary breast cancer patients

Background: Circulating tumor cells (CTCs) play role in tumor dissemination and are an independent survival predictor in breast cancer (BC) patients. PD-L1 is prognostic factor in BC and PD-L1 inhibitors prolonged survival in metastatic triple-negative breast cancer. The aim of this study was to assess correlation between CTCs and tumor PD-L1 in BC. Methods: This study included 299 primary BC patients treated by surgery from March 2012 to February 2015. Peripheral blood mononuclear cells (PBMC) were depleted of hematopoietic cells using RossetteSepTM negative selection kit. RNA extracted from CD45-depleted PBMC was interrogated for expression of EMT (TWIST1, SNAIL1, SLUG, ZEB1) and epithelial (CK19) gene transcripts by qRT-PCR. Patient samples with higher epithelial and/or mesenchymal gene transcripts than those of healthy donors (n=60) were considered as CTC positive. Expression of PD-L1 in surgical specimens was evaluated by immunohistochemistry and quantified by multiplicative score (proportion of cells with staining x intensity of staining). Results: CTCs were detected in 75 (25.1%) of patients. CTCs exhibiting only epithelial markers were present in 28 (9.4%) of patients, whereas CTCs with only EMT markers were observed in 53 (17.7%) of patients. In 6 patients, CTCs exhibit both epithelial and EMT markers. Patients with EMT_CTCs had significantly higher mean ± SEM score of PD-L1 expression in tumor associated stroma than those of patients without EMT_CTCs (2.66 ± 0.62 vs. 0.88 ± 0.29, p=0.048). There was no association between CTCs and PD-L1 expression in cancer cells including CTCs subtypes. Tumors without PD-L1 expression had significantly better disease-free survival compared to patients with PD-L1 expression (HR = 0.50, 95%CI 0.29 - 0.86, P = 0.001), however, this prognostic value was restricted to CTC positive subpopulation (HR = 0.24, 95%CI 0.10 - 0.56, P = 0.002), while, in CTC negative patients PD-L1 lack prognostic value (HR = 0.77, 95%CI 0.39 - 1.54, P = 0.45) Conclusion: Our data suggest link between PD-L1 expression in tumor associated stroma and CTCs with EMT phenotype and thus further support connection between immunosuppression and EMT. PD-L1 expression is prognostic only in patients with detectable CTC in peripheral blood suggesting different biology of tumors with detectable CTCs. Citation Format: Michal Mego, Katarina Kalavska, Marian Karaba, Gabriel Minarik, Juraj Benca, Tatiana Sedlackova, Denisa Manasova, Daniel Pindak, Jozef Mardiak, Zuzana Cierna. CTC with EMT phenotype are associated with PD-L1 expression in tumor associated stroma in primary breast cancer patients [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 P4-01-15.