OP0152 Synovial Macrophages Promote TGF-β Signaling After Intra-Articular Injections of Oxidized LDL in Naïve Murine Knee Joints, Preventing Production of Pro-Inflammatory Factors S100A8/9, Chemokines and Aggrecanase-Induced Neo-Epitopes
Authors: W. de Munter, A.B. Blom, P.M. van der Kraan, J. Roth, T. Vogl, W.B. van den Berg, P.L. van Lent
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background</h3> In previous studies we found that synovial macrophages regulate joint pathology during experimental inflammatory osteoarthritis (OA) and that high systemic levels of Low density lipoproteins (LDL) aggravate OA joint pathology.[1-4] LDL in inflamed synovium is prone to be oxidized (oxLDL) and taken-up by macrophages, leading to an activated phenotype.[5] <h3>Objectives</h3> In this study, we investigate whether direct injection of oxLDL into a murine knee joint induces pathology and elucidate the role of synovial macrophages in that process. <h3>Methods</h3> Knee joints of C57BL/6 mice were injected five consecutive days with 1.2 mg/mL oxLDL, LDL, or an equal volume of vehicle (PBS). This same procedure was performed in mice depleted of synovial macrophages by intra-articular injection of clodronate liposomes seven days prior to the consecutive injections. Joint pathology was investigated by immunohistochemistry and RNA expression and protein production by synovium were determined using RT-PCR and luminex, respectively. TGF-β activity was measured using a functional CAGA-luciferase assay. Data are depicted as mean ± standard deviation. <h3>Results</h3> LDL and oxLDL injection in naïve knee joints did neither increase synovial thickening, nor production of pro-inflammatory factors (IL-1β, IL-6 and S100A8/9) compared to vehicle injection. TGF-β activity in synovial wash-outs was, however, significantly increased by 33% (from 84.7 ng/mL/g synovium ±14.4 to 113.0 ng/mL/g synovium ±33.3; p&lt;0.05). Immunohistochemistry of total knee joints showed that oxLDL injection decreased formation of aggrecanase-induced neo-epitopes (NITEGE) compared with vehicle injections, especially in areas along the bone margins that are prone to develop osteophytes (from arbitrary score 1.19±0.57 to 0.33±0.30; p&lt;0.05). Repeated injections of oxLDL in macrophage-depleted knee joints led to a 3.1 fold increase of synovial thickening, compared with injection of vehicle (p&lt;0.01), while LDL injections did not alter synovial thickening. Protein levels of S100A8/A9, markers for inflammation, were significantly increased in synovial wash-outs of oxLDL injected joints, compared with LDL (fold increase 5.6; p&lt;0.05) or vehicle (fold increase 8.3; p&lt;0.01) injection. RNA levels of chemokines MCP-1 and MIP-1α were also significantly upregulated after oxLDL injections (6.7 fold and 4.6 fold, respectively; p&lt;0.01). No raise in TGF-β activity was measured in macrophage-depleted joints. NITEGE expression was markedly increased (fold increase 1.92) in the synovium-cartilage contact areas after oxLDL injection (p&lt;0.05). <h3>Conclusions</h3> Synovial macrophages promote anabolic effects after oxLDL injections, supporting earlier studies which show increased ectopic bone formation during LDL-rich conditions in experimental osteoarthritis.[4] In absence of synovial macrophages, however, oxLDL induces cell influx, production of pro-inflammatory mediators and aggrecanase activity. <h3>References</h3> Bondeson J, Arthritis and rheumatism 2010;62:647-657 Blom AB, Arthritis and rheumatism 2007;56:147-157 van Lent PL, Arthritis and rheumatism 2004;50:103-111 de Munter W, Arthritis research &amp; therapy 2013;15:R178 Maiolino G, Mediators of inflammation 2013;2013:714653 <h3>Disclosure of Interest</h3> None declared