CCR2 and cancer: CCR2 signaling monocytes transition into mobile tumor-associated macrophages and are converted into the chemokine receptor 4 -expressing macrophages upon exposure to transforming growth factor β. CXC-chemokine ligand 12 (CXCL12), secreted by stromal fibroblasts near blood vessels, triggers migrating macrophages and cancer cells to aggregate toward blood vessels (21).