S100A8/A9 increases the mobilization of pro-inflammatory Ly6C monocytes to the synovium during experimental osteoarthritis

Background: Monocytes are dominant cells present within the inflamed synovium during osteoarthritis (OA). In mice, two functionally distinct monocyte subsets are described: pro-inflammatory Ly6C and patrolling Ly6C monocytes. Alarmins S100A8/A9 locally released by the synovium during inflammatory OA for prolonged periods may be dominant proteins involved in stimulating recruitment of Ly6C monocytes from the circulation to the joint. Our objective was to investigate the role of S100A8/A9 in the mobilization of Ly6C and Ly6C monocytic populations to the inflamed joint in collagenase-induced OA (CiOA). Method: S100A8 was injected intra-articularly to investigate monocyte influx. CiOA was induced by injection of collagenase into knee joints of wild-type C57BL/6 (WT), and S100a9 mice. Mice were sacrificed together with agematched saline-injected control mice (n = 6/group), and expression of monocyte markers, pro-inflammatory cytokines, and chemokines was determined in the synovium using ELISA and RT-qPCR. Cells were isolated from the bone marrow (BM), spleen, blood, and synovium and monocytes were identified using FACS. Results: S100A8/A9 was highly expressed during CiOA. Intra-articular injection of S100A8 leads to elevated expression of monocyte markers and the monocyte-attracting chemokines CCL2 and CX3CL1 in the synovium. At day 7 (d7) after CiOA induction in WT mice, numbers of Ly6C, but not Ly6C monocytes, were strongly increased (7.6-fold) in the synovium compared to saline-injected controls. This coincided with strong upregulation of CCL2, which preferentially attracts Ly6C monocytes. In contrast, S100a9 mice showed a significant increase in Ly6C monocytes (twofold) within the synovium at CiOA d7, whereas the number of Ly6C monocytes remained unaffected. In agreement with this finding, the Ly6C mobilization marker CX3CL1 was significantly higher within the synovium of S100a9 mice. Next, we studied the effect of S100A8/A9 on release of Ly6C monocytes from the BM into the circulation. A 14% decrease in myeloid cells was found in WT BM at CiOA d7. No decrease in myeloid cells in S100a9 BM was found, suggesting that S100A8/A9 promotes the release of myeloid populations from the BM. Conclusion: Induction of OA locally leads to strongly elevated S100A8/A9 expression and an elevated influx of Ly6C monocytes from the BM to the synovium.