CXCL12 and rheumatoid arthritis: The hypoxic microenvironment induces SDF-1 expression in RA FLSs, leading to the accumulation of CXCR4-expressing monocytes in the synovial membrane and their differentiation into macrophages and secretion of proinflammatory factors (IL-1β, IL-6, and TNFα) and MMP.