CXCL12 and rheumatoid arthritis: Because SDF-1-CXCR4 system is also active in B cells to activate and to migrate [47], in osteoclasts to produce MMP-9 [48], in chondrocytes to produce MMP-3 [49], in endothelium to induce angiogenesis [50], and in Th1 cells to migrate into synovial tissues [35], this system may play very important roles in a multiple ways in the development of RA.