The immune phenotype of circulating tumor cells: from escape mechanisms to immunotherapy navigation.

Circulating tumor cells (CTCs) must survive immune attack in the bloodstream before they can contribute to distant metastasis. Their immune phenotype, including checkpoint ligands, antigen-presentation molecules, and immunomodulatory features, can be assessed through serial blood sampling and may provide information complementary to established immunotherapy biomarkers. This review organizes CTC immune evasion across intravasation, circulatory transit, and colonization, and examines CTC-associated immune interactions while distinguishing direct CTC observations from evidence derived from tumor tissue, cell culture, or animal models. We review technologies for immune phenotyping and the clinical evidence linking CTC immune features to immunotherapy outcomes across cancer types. Current studies support further investigation of longitudinal CTC monitoring, but heterogeneous assays, small or retrospective cohorts, and limited prospective validation prevent routine treatment selection based on these readouts. We also discuss experimental approaches to disrupting CTC immune protection during vascular transit and outline a four-stage pathway from discovery and analytical validation to clinical utility and implementation. CTC immune phenotyping remains a research strategy whose clinical value must be established beyond tissue biomarkers, ctDNA, and standard clinical assessment.