Background Low grade synovitis characterised by monocyte influx has been associated with joint pathology in inflammatory osteoarthritis (OA). Alarmins S100A8/S100A9 are highly increased in the synovium after induction of experimental inflammatory OA and remain elevated for prolonged periods. Release of S100A8/A9 locally within the OA-joint and leakage into the blood may contribute to recruitment of monocytes. In mice, two functionally distinct monocyte populations are described: pro-inflammatory Ly6C^high and patrolling Ly6C^low monocytes. Our objective was to investigate the role of S100A8/A9 in the recruitment of the different monocyte populations during collagenase-induced OA (CiOA).

Methods CiOA was induced by injection of collagenase in knee joints of wild type C57BL/6 (WT), and S100A9KO mice. After 7 days, mice were sacrificed together with age-matched saline-injected control mice (n=6/group), and expression of chemokines, and adhesion molecules were measured in the synovium, and bone marrow (BM). Cells were isolated from BM and synovium and analysed by FACS. Monocytes were identified as (B220/CD90/CD49b/NK1.1/Ly6G)^lowCD11b^high(F4/80/MHCII/CD11c)^low, which were further divided into subsets based on their Ly6C expression.

Results At CiOA d7 in WT mice, numbers of Ly6C^high, but not Ly6C^low monocytes, were strongly increased (7.6-fold) in the synovium when compared to saline-injected controls. In contrast, S100A9KO mice showed a significant increase in Ly6C^low monocytes (2-fold) at CiOA d7, whereas the number of Ly6C^high monocytes remained unaffected. Although a strong upregulation of several chemokines (MCP1, CX3CL1, KC, and MIP1α) was observed in the synovium at CiOA d7, there were no differences in expression between WT and S100A9KO mice. Next we studied the effect of S100A8/A9 on the BM during CiOA. We found a 14% decrease of myeloid cells in WT BM at CiOA d7, whereas the number of myeloid cells in S100A9KO BM was not changed, suggesting that S100A8/A9 released by the inflamed synovium promotes the release of myeloid populations from the BM. In line with that, expression of adhesion molecules (LFA-1, VCAM, VE-cadherin, PECAM1, and l-selectin) was lower at CiOA d7 in S100A9KO BM when compared to WT BM.

Conclusions Local induction of OA leads to S100A8/A9 production and mobilises influx of particularly Ly6C^high monocytes, which contribute to OA pathology.