As the most abundant stromal cells in the TME, cancer-associated fibroblasts (CAFs) promote the recruitment of monocytes and their transformation into M2 phenotype via secreting multiple regulatory molecules, such as IL-6, M-CSF, monocyte chemoattractant protein-1 (MCP-1, or CCL2), and stromal cell–derived factor 1 (SDF-1, or CXCL12) (107). Here, CCL2 is linked to cancer.