Background and objectives A significant role for inflammation during osteoarthritis (OA) is increasingly recognised, which involves the recruitment of immune cells, including monocytes, towards the inflamed synovium. In mice two functionally distinct monocyte populations are described; Ly6C-high monocytes, which express CCR2 and are considered pro-inflammatory and Ly6C-low monocytes, which express CX3CR1 and are suggested to be involved in repair processes. These monocytes arise from the bone marrow (BM) where monocyte chemoattractant protein-1 (MCP-1) is a key molecule that drives the monocyte efflux. As second tissue from which monocytes may originate is the spleen. The aim of this study is to investigate systemic effects of locally induced OA on BM and splenic monocyte subpopulations and the recruitment of these monocytes to the OA joint synovium in collagenase induced osteoarthritis (CiOA).
Materials and methods CiOA was locally 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, spleen, blood and knee synovial tissue were isolated and analysed by FACS. Ly6C-high monocytes were identified as (B220/CD90/CD49b/NK1.1/Ly6G)lowCD11bhigh(F4/80/MHCII/CD11c)lowLy6Chigh and Ly6C-low monocytes as (B220/CD90/CD49b/NK1.1/Ly6G)lowCD11bhigh(F4/80/MHCII/CD11c)lowLy6Clow. 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 (1012 ± 925 Ly6C-high and 621 ± 488 Ly6C-low monocytes), which is likely an artefact of residual blood. At day 7 after CiOA induction, the number of Ly6C-high and -low monocytes in the OA synovium was 420% and 300% increased, respectively, compared to naive synovium. In blood, monocyte subpopulations were not changed, but in BM, the number of Ly6C-high monocytes was 160% increased, while Ly6C-low monocytes were decreased by 170%. Furthermore, expression of MCP-1 and CCR2 was increased by 3.2 and 2.8 times, while CX3CR1 expression remained unchanged. Even at day 42 increased levels of both monocyte subpopulations were observed in the OA synovium (Ly6C-high; 280% and Ly6C-low; 220%). In the BM, no change in both monocyte subpopulations was observed anymore as well as no change in expression of MCP-1, CCR2 and CX3CR. In spleen no changes in Ly6C-high or -low monocytes were observed throughout the course of CiOA, indicating no role for splenic monocytes in the recruitment of monocytes towards the OA synovium.
Conclusions These data indicate that compared to naïve synovium, increased numbers of both Ly6C-high and -low monocytes are present in the OA synovium throughout the course of CiOA, but that a systemic effect on the BM monocyte subpopulations and their efflux is only observed in the early phase of OA. In the BM a clear skew towards a pro-inflammatory monocyte subset is visible, indicating that locally induced OA may also be systemically regulated.