Interactions Between Circulating Tumor Cells and the Immune System in Colorectal Cancer: Friends or Foes?

Colorectal cancer (CRC) is a leading cause of cancer death worldwide, mainly due to metastasis. Circulating tumor cells (CTC) act as the biological "seeds" of dissemination, traveling through the bloodstream to colonize distant organs. However, the blood is a hostile environment where CTC must constantly face immune pressure. This review explores the bidirectional interactions between CTC and immune cells in CRC, asking whether CTC are merely vulnerable targets of immunosurveillance or can exploit the immune system for survival and metastasis. We dissect intrinsic and extrinsic immune evasion mechanisms, including MHC-I modulation, immune checkpoint expression (PD-L1, CD47, FasL), platelet cloaking, and neutrophil extracellular traps (NET). Furthermore, we examine how CTC form heterotypic clusters with monocytes, neutrophils and lymphocytes, creating pro-metastatic niches and promoting phenotypic plasticity. The impact of CTC on systemic immunity, including reprogramming of NK cells, T lymphocytes and myeloid-derived suppressor cells (MDSC) is discussed. Importantly, we highlight the emerging role of CTC as dynamic biomarkers for immunotherapy, focusing on the predictive value of PD-L1+ CTC and the potential of CTC-derived neoantigens for personalized vaccination. Despite progress, challenges remain in standardization, detection sensitivity, and clinical validation. Understanding the equilibrium between immune elimination and evasion by CTCs is crucial to develop novel interventions that interrupt the metastatic dialogue and improve outcomes for CRC patients.