The Tumor Microenvironment Affects Circulating Tumor Cells Metastasis and the Efficacy of Immune Checkpoint Blockade in Non-small Cell Lung Cancer

Lung cancer is one of the most lethal malignancies, with non-small cell lung cancer (NSCLC) being the most common histologic subtype. Metastasis leads to poor prognosis for patients with cancer. Tumor cells leave the tumor lesions, invade the surrounding stroma, and enter the bloodstream as circulating tumor cells (CTCs). The development of CTCs is the beginning of metastasis. The internal environment in which tumor cells grow and survive is called the tumor microenvironment (TME). It includes tumor cells, fibroblasts, immune cells, and the extracellular matrix. The TME is complex and dynamic. Moreover, the TME plays an important role in tumor development and metastasis and significantly impacts therapeutic outcomes. Immune checkpoint blockade (ICB) aims to inhibit the interaction of ligands with their corresponding receptors. ICB has the function of restoring the anti-tumor effect of immune cells. This review examines how TME interacts with CTCs, allowing CTCs to evade immunity and facilitating CTC metastasis. TME not only affects the progression of tumor metastasis but also interacts with tumor cells, which may affect the efficacy of immunotherapy.