Role of S100A9 in Arthritis Figure 4. Decreased neutrophil and monocyte antigen expression in the paws of anti-S100A9-treated mice. (A) Western blot analysis of 7/4 antigen (Ag), a neutrophil marker (top). Quantification of 7/4 Ag on an immunoblot by densitometry analysis (bottom). (B) Western blot analysis of Gr-1 Ag, a marker of granulocytes and monocytes (top). Quantification of Gr-1 Ag on an immunoblot by densitometric analysis (bottom). doi:10.1371/journal.pone.0045478.g004 inflammation and joint destruction not only by promoting neutrophil transendothelial migration but also by inducing the secretion of pro-inflammatory cytokines (as described in the studies of Sunahori et al [30]) and matrix-modifying enzymes by innate immune cells. In RA, inflammatory mediators derived from resident and innate immune cells play crucial roles in amplifying and perpetuating inflammation, independent of the trigger. Downreg- ulating this circuit may prevent acute phase inflammation and limit the progression of the disease. Here, we have shown that S100A9 and calprotectin are detected early in serum after the induction of arthritis. Interestingly, while anti-S100A9 eliminated the presence of these molecules in the serum, anti-TNFa did not prevent their release, suggesting that secretion of TNFa occurs after the export of these proteins. Neutrophils are rapidly mobilised to the inflammatory site, and since they are rich in Figure 5. S100A9 treatment decreases the secretion of pro-inflammatory cytokines (A) 5 days after LPS injection, sera were collected and tested for IL-6 by ELISA. Values are the mean 6 SEM of 10 mice. *p,0.05, t test (n = 10 serum samples/group) (B) Thirty micoliters from a pool of 5 paw homogenates were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and tested for the presence of IL-6 and TNF-a. F: forepaw, H: hind paw. doi:10.1371/journal.pone.0045478.g005 PLOS ONE | www.plosone.org 7 September 2012 | Volume 7 | Issue 9 | e45478