Background/Purpose: Synovial macrophages have previously shown to play a significant role in the etiopathology of experimental collagenase-induced osteoarthritis (OA). In addition to production of pro-inflammatory proteins such as S100A8/9 and IL-1 in early OA,  synovial lining macrophages have also shown to play a crucial role in promotion of transforming growth factor-beta (TGFβ) mediated osteophyte formation. In an inflammatory milieu such as OA, accumulated low density lipoprotein (LDL) is oxidized, resulting in high intra-articular levels of oxidized LDL (oxLDL). OxLDL is taken up by macrophages via scavenger receptors, resulting in an more aggressive phenotype. In the present study we investigated whether LDL accumulation by either LDL-receptor deficiency (LDLr^-/-) or a cholesterol-rich diet leads to increased oxLDL uptake by synovial macrophages and affects synovial activation and osteophyte formation. 

Methods: LDLr^-/-mice and their wild type (WT) controls received either a high cholesterol or control diet for 120 days. Experimental OA was induced by intra-articular injection of collagenase on day 84 and 86. Paraffin sections of OA knee joints were analyzed for cartilage destruction and osteophyte formation using the Pritzker score and image analysis, respectively. ApoB and S100A8 were detected using immunohistochemistry and synovial wash-outs were tested for active TGFβ using a TGFβ reporter gene assay and gene expression. Murine bone marrow derived macrophages were stimulated with 50 μg/mL oxLDL, after which supernatant was functionally tested for active TGFβ presence.

Results:  Mice receiving a cholesterol-rich diet not only developed increased serum LDL cholesterol levels, but also showed enhanced ApoB expression in synovial lining macrophages. In line with that, LDLr^-/- mice, which already had systemically high basal levels of LDL, showed a much higher  accumulation of ApoB in the synovial lining after receiving a cholesterol-rich diet. Although increased LDL levels did not enhance thickening of the synovium, S100A8 expression within macrophages  was markedly increased, reflecting an elevated activation status. Even though no effect of LDL accumulation on cartilage destruction was found, both a cholesterol-rich diet and LDLr^-/- strongly increased cartilage and bone formation in ligaments with a fold change of 6.7 and 6.1, respectively. Moreover, an increase in osteophyte size was found at the margins of the tibial plateau (4.4 fold increase after a cholesterol-rich diet and 5.3 fold increase in LDLr deficient mice compared to WT mice). To elucidate the mechanism, we finally studied the presence of active TGFβ, which is crucial in driving osteophyte formation, in synovial wash-outs and culture supernatant of oxLDL stimulated macrophages. Using a TGFβ reporter assay, synovial wash-outs of LDLr^-/-mice and stimulation of macrophages with oxLDL showed an increased presence of functional TGFβ compared to controls (fold change of 1.4 and 2.9, respectively).

Conclusion:  Both a cholesterol-rich diet and LDLr deficiency lead to increased synovial activation and ectopic bone formation in experimental OA. Uptake of oxLDL by synovial macrophages leads to activation, rather than production of TGFβ.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/cholesterol-accumulation-by-synovial-lining-macrophages-results-in-ectopic-bone-formation-during-experimental-osteoarthritis/