Osteoarthritis, characterized by the breakdown of articular cartilage in synovial joints, has long been viewed as the result of 'wear and tear'1. Although low-grade inflammation is detected in osteoarthritis, its role is unclear2,3,4. Here we identify a central role for the inflammatory complement system in the pathogenesis of osteoarthritis. Through proteomic and transcriptomic analyses of synovial fluids and membranes from individuals with osteoarthritis, we find that expression and activation of complement is abnormally high in human osteoarthritic joints. Using mice genetically deficient in complement component 5 (C5), C6 or the complement regulatory protein CD59a, we show that complement, specifically, the membrane attack complex (MAC)-mediated arm of complement, is crucial to the development of arthritis in three different mouse models of osteoarthritis. Pharmacological modulation of complement in wild-type mice confirmed the results obtained with genetically deficient mice. Expression of inflammatory and degradative molecules was lower in chondrocytes from destabilized joints from C5-deficient mice than C5-sufficient mice, and MAC induced production of these molecules in cultured chondrocytes. Further, MAC colocalized with matrix metalloprotease 13 (MMP13) and with activated extracellular signal-regulated kinase (ERK) around chondrocytes in human osteoarthritic cartilage. Our findings indicate that dysregulation of complement in synovial joints has a key role in the pathogenesis of osteoarthritis.
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Gene Expression Omnibus
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These studies were supported by a Rehabilitation Research and Development Merit Award from the Department of Veterans Affairs and the US National Heart, Lung, and Blood Institute Proteomics Center contract N01 HV 28183 (W.H.R.); a Northern California Chapter Arthritis Foundation Postdoctoral Fellowship Award (A.L.R.); a New York Chapter Arthritis Foundation/Merck Osteoarthritis Research Fellowship Award, the Atlantic Philanthropies, the American College of Rheumatology Research and the Education Fund, the Association of Specialty Professors and the US National Institute of Arthritis, Musculoskeletal and Skin Diseases Mentored Clinical Scientist Research Career Development Award K08 AR057859 (C.R.S.); US National Institute of Neurological Disorders and Stroke P30 Center Core grant NS069375 (M.S.); US National Institutes of Health training grant T32 AR007530 (S.Y.R.); National Institutes of Health R01 AR051749 (V.M.H.); and the Frankenthaler and Kohlberg Foundations (M.K.C.).
D.M. Lee is currently employed by Novartis Pharma. D.M. Lee and R.G. own equity in Synostics. J.M.T. and V.M.H. are consultants for Alexion Pharmaceuticals.
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