Abstract 1937: How circulating tumor cells disguise

Background: The most threatening aspect of malignant tumors is their ability to develop metastases in distant organs eventually leading to fatal outcome. These metastases must be derived from cells seeded into the circulation from the primary tumor. It is, however, well known that the majority of metastases do not develop during growth of the primary tumor but only months or even years after the removal of the primary tumor. Circulating tumor cells have been claimed to rapidly disappear from the blood after surgical removal of the primary tumor leading to the assumption that they are eradicated with a half- life of few hours. Therefore, already the presence of circulating tumor cells has been (over) interpreted as a sign of metastatic disease. Indeed, initially, we, like most other approaches for circulating tumor cell detection, did not find any tumor cells right after surgery. However, using a standardized approach (1) we were able to detect increased numbers of tumor cells in blood days after surgery not only in lung (2) but also in breast cancer (3). Rapid disappearance of cells shed from the tumor is not necessarily due to eradication of these cells by the immune system but dormant cells obviously can survive due to recirculation into foreign tissues or encryption. The question is: How can cancer cells conceal in the circulation? Methods: Using the maintrac® approach including resting the samples for 24 hours at room temperature on the shelf lead to detection of cells previously concealed from detection with anti-EpCAM antibody. In addition the presence of platelets was monitored using red fluorescing anti CD36 or anti-CD41/CD61 antibody. Results: The maintrac® approach allowed detection of live tumor cells seeded at any time into the circulation. When, accidentally, we stained cells and platelets right after blood draw we found aggregations of red fluorescing platelets which upon inspection in transmitted light were shown to be clustered on cells. In time lapse photography on individual cells for 24 hrs it was observed that platelets were increasingly released from the cells and anti-EpCAM staining became more and more apparent. This indicates that platelets initially clustered around epithelial tumor cells prevent anti-EpCAM from binding to the cells and only if platelets are released the cells can be identified as positive cells. Most possibly hypoxic conditions induce release of the platelets. Conclusion: Rapid disappearance of cells shed from the tumor may be due to occultation of the cells by platelets. Since platelets express PD-L1 this may contribute to suppress anti-cancer immune cell activity. Citation Format: Dorothea Schott, Monika Pizon, Andreas Riess, Katharina Pachmann. How circulating tumor cells disguise [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1937.