Inflammation is increasingly recognised to be involved in osteoarthritis (OA) pathology. In response to pro-inflammatory cytokines, monocytes can be recruited from the bone marrow (BM) where monocyte chemoattractant protein-1 (MCP-1) is a key molecule that drives monocyte efflux via binding with the C-C chemokine receptor type 2 (CCR2). In mice, two functionally distinct monocyte populations are described: pro-inflammatory Ly6C-high monocytes (CCR2^high) and patrolling Ly6C-low monocytes (CCR2^low). The objectives of our study are to investigate the systemic effects of locally induced OA on BM monocyte populations and their recruitment to the OA joint in collagenase induced OA (CiOA), and the role of the alarmins S100A8/A9 in that.

CiOA was induced in C57BL/6 mice by unilateral-articular collagenase-injection and were sacrificed at day 7, 21 and 42, together with age-matched naive mice (n = 6/group), and synovial mRNA expression of several pro-inflammatory cytokines were measured. During CiOA, the absolute amount of cells in the BM per femur was measured and the mRNA expression of BM MCP-1 and CCR2 was determined. Cells from BM, blood and synovial tissue were isolated and analysed by FACS. Monocyte subsets were identified as (B220/CD90/CD49b/NK1.1/Ly6G)^lowCD11b^high(F4/80/MHCII/CD11c)^low and further distinguished by their Ly6C expression. In addition, we investigated synovitis in S100A9^-/-mice during CiOA.

Synovial expression of IL-1β, IL-6, TNF-α, S100A8 and S100A9 was increased at day 7, but only expression of S100A8 and S100A9 remained high until day 21. Local induction of CiOA resulted in systemic effects within the BM showing decreased cell numbers at day 7 and 21 (15% and 14% respectively). Concurrently, the expression of MCP1 at day7 in BM was increased 3.3-fold, suggesting increased BM-efflux. Relative number of BM-Ly6C-high monocytes was increased at day 7 (164%), being reflected by increased CCR2 expression (2.8-fold). This suggests a specific regulation of BM Ly6C-high monocytosis and recruitment during CiOA. During the course of CiOA increased number of Ly6C-high monocytes were observed in the synovium: 398% at day 7 and 510% at day 42, compared to naïve mice. These effects may be caused by the sustained S100A8/A9 levels, we therefore investigated synovitis in S100A9^-/- mice during CiOA. The number of cell layers and cell influx in the synovium of S100A9^-/- mice was decreased compared to C57BL/6 mice

Local induction of OA induces systemic release of BM-derived cells and increased Ly6C-high monocyte populations systemically and locally in the synovium, a process that may be regulated by the sustained release of S100A8/A9 from the synovium.

Disclosure of Interest None declared.