CCL2 and rheumatoid arthritis: Despite this important finding, clinical trials aimed at blocking CCL2/CCR2 interaction in Rheumatoid Arthritis (RA) failed to reach phase III (5), most likely due to the complexity of the disease in humans (e.g.: synovial infiltrate with predominant follicle like structure, monocyte/macrophages, or fibroblasts) and the different chemokines produced.