P151 Nociceptive pain in acute experimental synovitis is partly mediated by the alarmin S100A9
Authors: AB Blom, MH van den Bosch, EJ Geven, EN Blaney Davidson, PM van der Kraan, PL van Lent
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Career situation of first and presenting author</h3> Post-doctoral fellow. <h3>Introduction</h3> Inflammatory mediators like S100A8 and -A9 released by the synovium have been implicated in the regulation of pain. They may regulate pain either via direct stimulation of nerve endings in the synovium or via stimulation at the site of the dorsal root ganglia (DRG), enabling an increased phagocyte infiltration and causing sensitization. <h3>Objectives</h3> To elucidate the role of S100A9 in the pain response after induction of an acute synovitis using streptococcal cell walls (SCW) as a trigger. <h3>Methods</h3> Acute synovitis was induced by a single i.a. injection of SCW in the knee joint of C57Bl6 (WT) mice and S100A9<sup>-/-</sup> mice, control mice received a saline injection. Serum S100A8/A9 levels were investigated by ELISA. Joint swelling and cell influx was assessed by <sup>99m</sup>Tc accumulation and histology. Pain response were investigated using an Incapacitance Tester (weight bearing), Catwalk (gait analysis) and von Frey's filaments (mechanical allodynia). Gene expression of inflammatory mediators and neuron activation markers in DRG were determined by q-PCR. Monocyte influx and protein expression was monitored by immunohistochemistry (IHC). <h3>Results</h3> A single i.a. injection of SCW resulted in increased synovial and serum levels of S100A8 and S100A9 at day 1, which returned to basal levels at day 7. Joint swelling and cell influx were similar in WT and S100A9<sup>-/-</sup> mice at day 1 day excluding a role for S100A8/9 in synovitis. WT mice showed a marked and significant decrease in percentage of weight bearing on the SCW injected hindpaw (28%) compared to saline injection (47%, p&lt;0.001) at day 1, whereas S100A9<sup>-/-</sup> mice did not. In addition, gait analysis showed increased 'limping' in the WT mice, whereas the S100A9<sup>-/-</sup> mice did not. Both mouse strains showed a similar reduction of paw withdrawal threshold, excluding a role for S100A8/9 in allodynia. DRG showed no increased phagocyte infiltration after SCW injection and no change in gene expression of MCP-1, KC, IL-1β or TNF was measured. In addition, F4/80 staining was unchanged in both WT and S100A9<sup>-/-</sup> mice. However, neuron activation markers NAV1.7, ATF3 and GAP43 were significantly increased at 1 day after SCW injection in WT mice, compared to saline injected mice (p=0.022, 0.004 and 0.030, respectively) and not in S100A9<sup>-/-</sup> mice, which is in line in with the reduced pain response observed earlier in S100A9<sup>-/-</sup> mice. The difference in NAV1.7 expression in the DRG was further confirmed at protein level with IHC. <h3>Conclusions</h3> These findings show that S100A9 is an important mediator of inflammatory nociceptive pain response, and not of peripheral sensitization. During the acute phase of inflammation S100A8/A9 is likely involved via direct activation of nerve endings in the synovium and not via monocyte infiltration in the DRG. <h3>Disclosure of Interest</h3> None declared.