The alarmins S100A8 and S100A9 mediate acute pain in experimental synovitis

Synovitis-associated pain is mediated by inflammatory factors that may include S100A8/9, which is able to stimulate nociceptive neurons via Toll-like receptor 4. In this study, we investigated the role of S100A9 in pain response during acute synovitis. Acute synovitis was induced by streptococcal cell wall (SCW) injection in the knee joint of C57Bl/6 (WT) and S100A9 mice. The expression of S100A8/A9 was determined in serum and synovium by ELISA and immunohistochemistry. Inflammation was investigated by Tc accumulation, synovial cytokine release, and histology at days 1, 2, and 7. To assess pain, weight distribution, gait analysis, and mechanical allodynia were monitored. Activation markers in afferent neurons were determined by qPCR and immunohistochemistry in the dorsal root ganglia (DRG). Differences between groups were tested using a one-way or two-way analysis of variance (ANOVA). Differences in histology were tested with a non-parametric Mann-Whitney U test. p values lower than 0.05 were considered significant. Intra-articular SCW injection resulted in increased synovial expression and serum levels of S100A8/A9 at day 1. These increased levels, however, did not contribute to the development of inflammation, since this was equal in S100A9 mice. WT mice showed a significantly decreased percentage of weight bearing on the SCW hind paw on day 1, while S100A9 mice showed no reduction. Gait analysis showed increased "limping" behavior in WT, but not S100A9 mice. Mechanical allodynia was observed but not different between WT and S100A9 when measuring paw withdrawal threshold. The gene expression of neuron activation markers NAV1.7, ATF3, and GAP43 in DRG was significantly increased in arthritic WT mice at day 1 but not in S100A9 mice. S100A8/9, released from the synovium upon inflammation, is an important mediator of pain response in the knee during the acute phase of inflammation.