CXCL12 and rheumatoid arthritis: In addition, the mechanisms of RA-FLS inducing the activation and proliferation of T cells by interacting between CD47 expressed on T cell surface and its ligand (thrombospondin-1) expressed by FLS or interacting between CXCR4 expressed on T cell surface and its ligand (stromal cell-derived factor-1, SDF-1) expressed by FLS [12].