Type I interferons might form the link between Toll-like receptor (TLR) 3/7 and TLR4-mediated synovial inflammation in rheumatoid arthritis (RA)

Background: Rheumatoid arthritis (RA) has been associated with an increased risk of infections, but the underlying pathways have not yet been identified. Toll-like receptors (TLR) probably play a role in synovial inflammation and may also contribute to the understanding of the role of infections in RA. Objectives: To investigate if the synovial expression of TLR3 and TLR7 in RA correlates with that of inflammatory cytokines, and to assess whether this has functional consequences for local cytokine production and to study potential links between the TLR3/7 axis and TLR4 in RA synovium. Methods: Immunohistochemistry was used to study the expression of TLR3, TLR7, interferon alpha (IFN alpha), tumour necrosis factor alpha (TNF alpha) and interleukins IL1 beta, IL12, IL17 and IL18 in RA synovium obtained by arthroscopy from 34 patients with RA. Monocytes, monocyte-derived dendritic cells (MoDCs) and RA synovial fibroblasts were stimulated via TLR3 (poly-IC) and TLR7 (loxorubin), after which IL1 beta, IL6 and TNF alpha were measured by Luminex bead array technology. Following preincubation with IFN alpha, IL1 beta and IL18, TLR3 and TLR7 mRNA expression was assessed using real-time PCR. Cytokine production after preincubation with IFN alpha and subsequent TLR stimulation was measured. Results: Synovial TLR3/7 expression was co-expressed with IFN alpha, IL1 beta and IL18, but not with TNF alpha, IL12 and IL17. Stimulation of TLR3/TLR7 on monocytes, MoDCs or synovial fibroblasts led to secretion of type I IFN but no biologically active IL1 beta or IL18 could be detected. Type I IFN alpha increased TLR3/7 mRNA expression whereas IL1 beta and IL18 did not. In spite of the fact that the mRNA level of TLR4 remained unchanged, IFN alpha enhanced the response to TLR4 agonists, a phenomenon that was clearly more marked in patients with RA. Conclusion: Type I interferons are highly co-expressed with TLR3/TLR7 in RA synovium. They enhance TLR3/TLR7-mediated cytokine production and also TLR4-mediated responses.