CCL2 and neoplasm: MDSCs are recruited into the surrounding environment of the tumor by chemokines (for example, CCL2, CCL5, CXCL1, CXCL5, CXCL6, CXCL8, and CXCL12), followed with the activation of MDSCs by the granulocyte-macrophage colony stimulating factor (GM-CSF), granulocyte-colony stimulating factor (G-CSF), and VEGF [111–113].