Background Synovitis-associated pain is an important aspect of arthritis pathology. Inflammatory mediators released by the synovium have been implicated in the regulation of pain, including S100A8 and S100A9, which may act via stimulation of TLR4 on the nerve endings in the synovium. In this study we investigated the role of S100A9 in the pain response after induction of an acute synovitis using streptococcal cell walls (SCW) as a trigger, comparing S100A9^-/- mice and their WT controls.

Materials and methods Acute synovitis was induced by a single i.a. SCW injection in the knee joint of C57Bl6 (WT) and S100A9^-/- mice. Serum S100A8/A9 levels were investigated by ELISA and S100A8 and S100A9 expression in synovium by immunohistochemistry. Joint swelling and cell influx was assessed by ^99mTc accumulation and histology, respectively. Pain response was investigated by Incapacitance Tester (weight bearing), Catwalk (gait analysis) and von Frey’s filaments (mechanical allodynia). Gene expression of neuron activation markers in dorsal root ganglia (DRG) were determined by q-PCR.

Results A single i.a. SCW injection resulted in increased synovial expression of S100A8 and S100A9 as well as significant increased serum S100A8/A9 levels (2.6-fold, p<0.001) 1 day p.i.. These increased levels however, did not contribute to the development of inflammation since joint swelling and cell influx were similar in both mice strains. WT mice showed a significant decrease in percentage of weight bearing on the SCW hindpaw (28%, p<0.001) 1 day p.i., while S100A9^-/- mice showed no reduction. In line with that, gait analysis showed that the stand-phase of the unaffected paws was significantly increased in WT mice 1 day p.i., but not in S100A9^-/- mice. No difference in mechanical allodynia was observed, both mouse strains showed a similar reduction of paw withdrawal threshold. Gene expression of neuron activation markers NAV1.7, ATF3 and GAP43 in DRG were significantly increased in SCW injected WT mice 1 day p.i (p=0.022, 0.004 and 0.030, respectively) but not in S100A9^-/- mice.

Conclusions These findings show that S100A9, which is released from the synovium upon inflammation, is an important mediator of pain response in the knee during the acute phase of inflammation.