FRI0039 Synovial Macrophages Promote TGF-Beta Signaling but Protect against Influx of S100a8/s100a9-Producing Cells Following Intra-Articular Injections of Oxidized Low-Density Lipoproteins

BackgroundIn previous studies we found that synovial macrophages regulate joint pathology during experimental osteoarthritis (OA) and, more recently, we found that high systemic levels of low-density lipoproteins (LDL) aggravate joint pathology during experimental OA with synovitis. LDL in inflamed synovium is oxidized and taken-up by macrophages, leading to an activated macrophage phenotype.ObjectivesIn this study, we investigate whether injection of oxidized LDL directly into a murine knee joint induces joint pathology and elucidate the role of synovial macrophages in that process.MethodsSynovium was obtained from end-stage OA patients and stained for apolipoprotein B (APOB). Murine knee joints were injected five consecutive days with oxLDL, LDL, or an equal volume of vehicle (PBS). This procedure was repeated 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 flow cytometry (FCM) analysis, and RNA expression and protein production by synovium were determined using RT-PCR and luminex, respectively. Aggrecanase activity was measured using NITEGE-staining and active TGF-β was measured using a functional CAGA-luciferase assay. Data are depicted as mean ± standard deviation.ResultsSynovial macrophages and fibroblasts of end-stage OA patients showed extensive accumulation of APOB, the main protein present in LDL and oxLDL. Multiple injections of oxLDL in murine knee joints significantly increased TGF-β activity in synovial wash-outs by 28% [from 16562 ± 2326 relative light units (RLU) to 21151 ± 3823 RLU; P