Background A significant role for inflammation during osteoarthritis (OA) is increasingly recognized, which involves the recruitment of immune cells, including monocytes, to the inflamed synovium. Monocytes have been shown to regulate joint destruction in OA, by the release of pro-inflammatory molecules, like S100-DAMPs. In mice two functionally distinct monocyte populations are described; Ly6C-high monocytes, which express high levels of CCR2 and are considered pro-inflammatory and Ly6C-low monocytes, which express low levels of CCR2 but high levels of CX3CR1 and are suggested to be involved in repair processes. Both monocytes arise from the bone marrow (BM) where monocyte chemoattractant protein-1 (MCP-1) is a key molecule that drives the monocyte efflux via binding with CCR2.

Objectives The objective of this study is to investigate systemic effects of locally induced OA on BM monocyte subpopulations and the recruitment of these monocytes to the OA joint synovium in collagenase induced osteoarthritis (CiOA).

Methods CiOA was induced in C57BL/6 mice by injection of collagenase in the right knee joint. Seven and 42 days after induction, mice (n=6) were sacrificed, together with age-matched naive C57BL/6 mice. Cells from BM, blood and knee synovial tissue were isolated and analyzed by FACS. Ly6C-high monocytes were identified as (B220/CD90/CD49b/NK1.1/Ly6G)^lowCD11b^high(F4/80/MHCII/CD11c)^lowLy6C^high cells and Ly6C-low monocytes as (B220/CD90/CD49b/NK1.1/Ly6G)^lowCD11b^high(F4/80/MHCII/CD11c)^lowLy6C^low cells. BM expression of MCP-1, CCR2 and CX3CR1 mRNA was determined at day 7 and 42 by q-PCR.

Results In naive synovium few monocytes were present (645±146 Ly6C-high and 231±32 Ly6C-low monocytes per synovium). At day 7 after CiOA induction, the number of Ly6C-high and -low monocytes in the OA synovium was increased by 398% and 299%, respectively, compared to naive synovium. In blood, monocyte subpopulations were not changed, but in BM, a clear shift in the subpopulations was observed; the number of Ly6C-high monocytes was increased by 164%, while Ly6C-low monocytes were decreased 1.7 fold. Furthermore, expression of MCP-1 and CCR2 was increased 3.2 and 2.8 fold respectively, while CX3CR1 expression remained unchanged. When measured at day 42, levels of both monocyte subpopulations were still significantly increased in the OA synovium (Ly6C-high; 510% and Ly6C-low; 222%), while in the BM, no change in monocyte subpopulations and expression of MCP-1, CCR2 and CX3CR1 was observed anymore.

Conclusions These data show that, compared to naive synovium, both Ly6C-high and -low monocytes are increased in the OA synovium throughout the course of CiOA, but that particularly the pro-inflammatory Ly6C-high monocytes are increased. This indicates an important role for Ly6C-high monocytes in late stage OA pathology, possibly by increased release of S100-DAMPs. Indeed serum S100A8/A9 levels are elevated for a longer period than other pro-inflammatory molecules during CiOA. A systemic effect of CiOA on the BM monocyte subpopulations is only observed in the early phase of OA, here a clear skew towards the pro-inflammatory monocyte subset is visible, indicating that locally induced OA may have systemic effects on BM monocytosis.

Disclosure of Interest None declared