TLR4 and rheumatoid arthritis: In RA, several TLRs, including TLR2, TLR3, TLR4 and TLR7, are highly expressed in the synovial tissues [17], and the ligands of TLRs induce the expression of cytokines (IL-1β, IL-6 and IFN-α), chemokines (IL-8) and matrix metalloproteinases on synovial fibroblasts (FLSs) [18-21], which contributes to RA synovial inflammation.