Molecular Characterization of Circulating Tumor Cells to Study Cancer Immunoevasion

Cancer cells leaving the primary tumor immunosuppressive microenvironment become vulnerable to active immune surveillance and require mechanisms of immunoevasion to survive in the circulation. Studies have identified several pathways by which circulating tumor cells (CTCs) might escape the immune system/immunotherapy attack. The PD-1/PD-L1 axis is an immune checkpoint regulator, playing a major role in maintaining self-tolerance. It is now well recognized that tumor cells co-opt the PD-1/PD-L1 axis of immune regulation to interfere with cytotoxic T lymphocyte function. Transcriptional changes in CTCs, leading to the upregulation of PD-L1, might enable them to survive in circulation. Very recent data revealed a previously unappreciated role of epithelial-mesenchymal transition (EMT) in reprogramming the immune response in the local tumor microenvironment and a mutual regulation between EMT and immunoevasion is becoming apparent. In this chapter, we will describe in detail both EpCAM-dependent and -independent approaches that allow the identification of PD-L1 expression and EMT-like features in circulating tumor cells.