Toll-like receptor expression in synovial fibroblasts and normal skin fibroblasts and induction of matrix metallo-proteinase expression by various Toll-like receptor ligands
© BioMed Central Ltd 2005
Received: 11 January 2005
Published: 17 February 2005
Toll-like receptors (TLRs) are pattern recognition receptors of the innate immune system. TLR2 has been demonstrated to be expressed in synovial tissue of patients with rheumatoid arthritis (RA) as well as in cultured synovial fibroblasts of patients with RA and osteoarthritis. However, it is not known whether other members of the TLR family are expressed in synovial fibroblasts. We have therefore examined the expression of TLR1–TLR9 in these cells and performed stimulation experiments using specific TLR ligands. TLR and matrix metalloproteinase (MMP) mRNA expression was analysed by real-time PCR using 18S-cDNA as an internal control. A difference of five or more cycles between cDNA of the samples and non-RT control was considered to be the detection limit. While all fibroblasts expressed TLR1–TLR6, the expression of TLR2 and TLR3 was significantly higher in synovial fibroblasts from patients with RA as compared with normal skin fibroblasts. TLR7, TLR8 and TLR9 mRNA did not reach the detection level. To assess the function of these TLRs, cultured synovial and skin fibroblasts were stimulated with the TLR ligands bacterial lipopeptide, poly (I:C), lipopolysaccharide and flagellin. All ligands stimulated the expression of MMP1, MMP3, MMP9 and MMP13 to a certain extent. Whereas the TLR2 ligand bacterial lipopeptide preferentially induced the expression of MMP1 and MMP3, the TLR3 ligand poly (I:C) was a more efficient inducer of MMP13 and MMP3 and also induced MMP1 to a lesser extent. Normal skin fibroblasts expressed significantly lower levels of MMP3 and MMP13 mRNA as compared with synovial fibroblasts of patients with RA. Our data extend previous reports on the expression of TLRs on synovial fibroblasts to include TLR1–TLR6 and document that these TLRs are functional. Depending on the TLR ligand used, MMP expression is differentially induced. This supports the notion that activation of TLR signalling pathways might contribute to joint inflammation and destruction in RA.