Mechanisms underlying the hyperalgesic responses triggered by joint activation of TLR4

Toll-like receptors (TLRs) including TLR4 and their signal pathways contribute to the pathogenesis of arthritis. Herein, we evaluated the mechanisms underlying the hyperalgesic response caused by TLR4 activation in the tibio-tarsal joint in mice.

Joint inflammatory hyperalgesia was induced by intra-articular (ia) injection of LPS (lipopolysaccharide- TLR4 agonist) in C57BL/6, TLR4, TLR2, MyD88, TRIF, TNFR1/2 and IL-1R1 knockout (^-/-) mice. Joint hyperalgesia was evaluated using an electronic von Frey. Neutrophil recruitment was assessed by MPO activity. Joint levels of cytokines were measured by ELISA.

Firstly, it was shown that LPS injected into the joints causes a dose- and time-dependent reduction in the mechanical nociceptive threshold. The TLR4 activation in the joint triggers mechanical hyperalgesia and neutrophil migration, which was abolished in TLR4 ^-/- and MyD88^-/-, but not in TLR2^-/- and TRIF^-/- mice. Besides, joint administration of LPS increased the release of TNF-α, IL-1β, and KC/CXCL1, which were reduced in TLR4^-/- and MyD88^-/-, but not in TRIF^-/- mice. In agreement, the LPS-induced joint nociceptive effect was decreased in TNFR1/2^-/- and IL-1R1^-/- mice or in mice pre-treated with a CXCR1/2 selective antagonist (DF2156A).

These results suggest that TLR4 activation in the joint produces articular hyperalgesia via MyD88 signaling pathway. Moreover, this pathway is involved in the cascade of events of articular hyperalgesia through mechanisms dependent on cytokines and chemokines production. Thus, TLR4/MyD88 signaling pathway inhibitors might be useful for the treatment of inflammatory joint pain.