CCL2 and systemic sclerosis: Multiple factors and signaling pathways are involved in the development or persistence of skin involvement in SSc, such as TGF-β, IL-4, IL-6, platelet-derived growth factor (PDGF), IL-1, IL-13, IL-17, IL-5, monocyte chemoattractant protein (MCP)-1, VEGF and connective tissue growth factor (CTGF) [6].