Abstract P4-04-08: Determination of PD-L1 expression on tumorspheres cultured from the peripheral blood in patients with breast cancer

Abstract Background Cancer stem cells (CSCs) are the critical subpopulation of cancer cells involved in tumor formation, progression, metastasis and drug resistance. CSCs also have been considered as the main cause of cancer recurrence. The exact role of CSCs in escape of cancer from immunosurveillance is still unknown, but recent studies revealed that enhanced PDL-1 expression in cancer stem cells is linked to immune evasion and could crucially contribute to the maintenance of CSC self-renewal. Understanding the mechanisms behind this PDL-1 overexpression in cancer stem cells is critical for developing more effective anti-PD-1/PD-L1 therapy. Therefore the aim of the study was to determine the number of tumorspheres and expression of PD-L1 on tumorspheres cultured from the peripheral blood in patients with breast cancer.Methods:110 patients with breast cancer in different stages of disease were included in this study. The determination of circulating tumor cells and circulating cancer stem cells (cCSC) was performed using maintrac and tumorsphere forming assay, respectively. Additionally anti-PDL-1 antibody staining was applied to examine PDL-1 expression on breast tumorspheres.Results:We developed an innovative in vitro platform for cultivation of CSCs from peripheral blood of solid cancer patients and confirmed for the first time that tumorspheres are positive for PDL-1 in breast cancer patients. Our data also show a considerable heterogeneity in the PDL-1 expression of tumorspheres. We confirmed once again that the number of tumorspheres increased significantly with tumor progression, especially with presence of metastasis. Patients with metastatic disease had statistically more tumorspheres as compared to patients without metastasis (30 vs 10/100µl blood, p<0.05). Patients with multiple metastasis had more tumorspheres compared to patients with single metastases (60 vs 30/100µl blood, p<0.05). The number of tumorspheres was positively correlated with proliferating index Ki-67 and grade score in primary breast tumor. Moreover, patients with HER2 positive tumors had more tumorspheres compared to patients with HER2 negative primary tumors (35 vs 10/100 µl blood, p<0.005). Conclusion:The number of tumorspheres cultured from peripheral blood directly reflects aggressiveness and proliferation capacity of primary tumor. The presence of tumorspheres with expression of PDL-1 might suggest their immunregulatory potential, but further studies are needed to validate the impact of PDL-1 expression on tumorspheres in patients with breast cancer. Citation Format: Monika Pizon, Dorothea Schott, Ulrich Pachmann, Katharina Pachmann. Determination of PD-L1 expression on tumorspheres cultured from the peripheral blood in patients with breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-04-08.