Session Type: ACR Poster Session B
Session Time: 9:00AM-11:00AM
Background/Purpose: In previous studies we found that synovial macrophages regulate joint pathology during experimental osteoarthritis (OA). Recently, we found that high systemic levels of LDL aggravate joint pathology during experimental OA with synovitis. LDL in inflamed synovium is oxidized and taken-up by macrophages via scavenger receptor A and CD36, leading to an activated macrophage phenotype. In this study, we investigate whether direct injection of oxLDL into a normal murine knee joint induces joint pathology and elucidate the role of synovial macrophages in that process.
Methods: Knee joints of C57BL/6 mice were injected five consecutive days with 6 μl PBS, or PBS containing 1.2 mg/mL oxLDL or LDL. This same procedure was done in mice which were depleted of synovial macrophages by intra-articular injection of clodronate liposomes seven days prior to the (ox)LDL or vehicle injections. Joint pathology was investigated by immunohistochemistry and RNA expression and protein production by synovium were determined using RT-PCR and luminex, respectively. Active TGF-β was measured using a functional CAGA-luciferase assay. Data are depicted as mean ± standard deviation.
Results: LDL and oxLDL injection in naïve knee joints did not increase synovial thickening, or production of pro-inflammatory factors (IL-1β, IL-6 and S100A8/9) compared to LDL injection. Levels of active TGF-β in synovial wash-outs was, however, significantly increased by 33% (from 84.7 ± 14.4 ng/mL/g synovium to 113.0 ± 33.3 ng/mL/g synovium; p<0.05). Immunohistochemistry of knee joints showed that repeated injections of PBS caused low expression of aggrecanase- (NITEGE) and matrix metalloproteinase- (VDIPEN) induced neo-epitopes. Repeated injections of oxLDL could reduce expression of NITEGE and VDIPEN in areas that are prone to develop osteophytes, in contrast to LDL-injections (arbitrary VDIPEN score 0.33 ± 0.30 for oxLDL-injections and 0.96 ± 0.36 for LDL-injections; p<0.05).
In contrast, repeated injections of oxLDL in macrophage-depleted knee joints led to a 5.3 fold increase of synovial thickening (due to cell influx), compared to injection of LDL (p<0.001). Protein levels of S100A8/A9, markers for inflammation, were significantly increased in synovial wash-outs of oxLDL injected joints, compared to LDL-injection (fold increase 5.6; p<0.05). Protein and mRNA levels of chemokines CXCL1, CCL2 and CCL3 were also significantly upregulated after oxLDL-injections compared to LDL-injections. Further immunohistochemical investigation of infiltrating cells revealed that these were NIMP.R14-negative, mononuclear monocyte-like cells.
No raise in active TGF-β was measured in macrophage-depleted joints. Remarkably, NITEGE expression was increased in these joints at the synovial-cartilage contact areas after oxLDL injection (fold increase compared to LDL-injection 2.7; p<0.05).
Conclusion: Synovial macrophages promote anabolic effects after oxLDL injections in knee joints. In absence of synovial macrophages, however, oxLDL induces cell influx, production of pro-inflammatory mediators and aggrecanase activity.
To cite this abstract in AMA style:de Munter W, van den Bosch MH, Blom A, Walgreen B, Helsen M, Joosten L, Roth J, Vogl T, van de Loo F, Koenders M, van den Berg W, van der Kraan P, van Lent P. Synovial Macrophages Promote TGF-β Signaling but Protect Against Influx of S100A8/S100A9-Producing Cells after Intra-Articular Injections of Oxidized Low-Density Lipoproteins [abstract]. Arthritis Rheumatol. 2015; 67 (suppl 10). https://acrabstracts.org/abstract/synovial-macrophages-promote-tgf-signaling-but-protect-against-influx-of-s100a8s100a9-producing-cells-after-intra-articular-injections-of-oxidized-low-density-lipoproteins/. Accessed January 22, 2022.
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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovial-macrophages-promote-tgf-signaling-but-protect-against-influx-of-s100a8s100a9-producing-cells-after-intra-articular-injections-of-oxidized-low-density-lipoproteins/