To evaluate the expression of Toll‐like receptors (TLRs) 3 and 7 in synovium and to study potential differences in the maturation and cytokine production mediated by TLR‐2, TLR‐3, TLR‐4, and TLR‐7/8 by dendritic cells (DCs) from rheumatoid arthritis (RA) patients and DCs from healthy controls.

Synovial expression of TLR‐3 and TLR‐7 in RA was studied using immunohistochemistry. Monocyte‐derived DCs from RA patients and healthy controls were cultured for 6 days and subsequently stimulated for 48 hours via TLR‐mediated pathways (lipoteichoic acid, Pam₃Cys, and fibroblast‐stimulating lipopeptide 1 for TLR‐2, poly[I‐C] for TLR‐3, lipopolysaccharide and extra domain A for TLR‐4, and R848 for TLR‐7/8). Phenotypic DC maturation was measured using flow cytometry. The secretion of tumor necrosis factor α (TNFα), interleukin‐6 (IL‐6), IL‐10, and IL‐12 was measured using the Bio‐Plex system. Cell lines expressing TLR‐2 and TLR‐4 were used for the detection of TLR‐2 and TLR‐4 ligands in serum and synovial fluid from RA patients.

TLR‐3 and TLR‐7 were highly expressed in RA synovium. All TLR ligands elicited phenotypic DC maturation equally between DCs from RA patients and those from healthy controls. TLR‐2– and TLR‐4–mediated stimulation of DCs from RA patients resulted in markedly higher production of inflammatory mediators (TNFα and IL‐6) compared with DCs from healthy controls. In contrast, upon stimulation of TLR‐3 and TLR‐7/8, the level of cytokine production was equal between DCs from RA patients and those from healthy controls. Remarkably, both TLR‐3 and TLR‐7/8 stimulation resulted in a skewed balance toward IL‐12. Intriguingly, the combined stimulation of TLR‐4 and TLR‐3–7/8 resulted in a marked synergy with respect to the production of inflammatory mediators. As a proof of concept, TLR‐4 ligands were increased in the serum and synovial fluid of RA patients.

TLRs are involved in the regulation of DC activation and cytokine production. We hypothesize that various TLR ligands in the joint trigger multiple TLRs simultaneously, favoring the breakthrough of tolerance in RA.