Role of S100A9 in Arthritis Figure 3. Histological assessment of joint infiltration and destruction in anti-S100A9-, anti-TNFa-, and isotype control-treated animals. (A) H&E-stained sections of wrist at 406 magnification and safranin/fast green-stained sections of the distal phalange joints at 1006 magnification. The enlargement was taken at 1000 6magnification. B: bone, C: cartilage, J: joint. (B) Bone destruction, collagen degradation, and cell infiltration as assessed by two blinded observers (on a scale of 0–3, 0–2, 0–2, respectively). *p,0.05, one-way ANOVA test (n = 10 paws/group, 5 forepaws and 5 hind paws), Tukey’s multiple comparison test. doi:10.1371/journal.pone.0045478.g003 Discussion The key role played by DAMPs in acute and chronic inflammation is well known. The DAMPs S100A9 and S100A8/ A9 actively participate in acute inflammatory processes [14,19,20,25,26]. They are highly upregulated in the serum and inflamed tissues of patients with chronic inflammatory diseases like RA [10,27,28]. Indeed, it has been previously demonstrated that monocytes and neutrophils infiltrating the synovium express S100A9 and calprotectin [29], and both proteins are found at high concentrations in the cartilage-pannus junction, which is the main site of cartilage destruction and bone erosion [23,27]. Moreover, Shunahori et al have shown that S100A8/A9 secreted from tissues macrophage may amplify proinflammatory cytokine responses through the activation of NF-kB and p38 MAPK pathways [30]. These clinical observations and previous studies suggest that S100A9 and calprotectin participate in innate immunity-related mechanisms observed in RA partly through their local production at inflamed joints. Decreased arthritis score was recently reported in S100A9-null mice compared to wild-type animals in antigen-induced arthritis [31], while no significant impact of S100A9 gene deletion has been reported in the K/BxN arthritis model [32]. These discrepancies might be explained by the effect of S100A9 gene deletion on S100A8. Indeed, S100A8 exhibit anti-inflammatory functions once oxidized [33], and this activity is likely not present in S100A9-deficient mice as these mice do not express S100A8 protein. In addition, both S100A8 and S100A9 have intracellular and extracellular activities which are equally affected by gene deletion. We therefore elected to specifically deplete S100A9 using an antibody to address the extracellular role of S100A9 in arthritis. In the present study, we have identified S100A9 as a key player in the initiation and progression of arthritis. Direct inhibition of S100A9 decreased swelling and protected bone/cartilage integrity in a murine LPS-CIA model. S100A9 appears to contribute to PLOS ONE | www.plosone.org 6 September 2012 | Volume 7 | Issue 9 | e45478