Breast cancer is the most common malignant tumor in women around the world. Most breast cancer-related deaths are from CTC (circulating tumor cells) metastasis. CTC is associated with the breast cancer patients' prognosis. But recently, circulating clusters were found and its metastasis and tumor formation ability is 23-50 times as CTC. However, its mechanism has not been clarified. These days, researchers have successfully completed CTC cluster separation, CTC cell culture, and PD-L1 was found to be related with histological grading of tumor. Meanwhile, the high expression of PD-L1 in CTC surface has also been reported. Since PD-L1 can mediate Treg to play the role of immunosuppression, we propose that CTC with positive PD-L1 is easier to connect PD-L1, immune cells (Treg regulatory T cells, MDSC bone marrow inhibitory cells) and CK cytokines etc. On one hand, Treg cells can protect CTC from being attacked by the immune system through the immunosuppression, on the other hand, they can weaken CTL killing ability and trigger more MDSC. Finally, CTC formed the metastatic lesion.
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