The roles of immune cells and non-immune cells in pre-metastatic niche of breast cancer

Distant metastasis is a pivotal and important event in patients with breast cancer, and inhibition of metastasis has always been the focus of clinical research. Recent advances have established that the metastasis of breast cancer is exacerbated not only by cancer cells and the tumor microenvironment but also by the pre-metastatic niche (PMN). Primary tumor secretory factors, immune cells including bone marrow-derived cells mobilized by tumors and non-immune cells within the local matrix microenvironment of the host are three key factors for PMN formation. This article reviews the roles of bone marrow-derived cells, lymphocytes, fibroblasts, endothelial cells, epithelial cells and cancer stem cells in the establishment of PMN before metastasis to further understand the metastasis mechanism of breast cancer and to explore clues for the inhibition of distant metastasis. Different cells play distinct but important roles in the establishment of the PMN and the induction of breast cancer metastasis. The interaction between different cells and tumor cells determines whether CTCs can be attached, survive and proliferate to promote distant metastasis.