Abstract
ObjectiveTo investigate complement(C) factors(F) and their activation fragments expression in OA joint tissues.DesignImmunohistochemistry and quantitative imaging were performed to analyze C3, C4, and CF (factor) B expression on osteochondral biopsies (43 patients) collected during arthroplasty. Isolated chondrocytes and synoviocytes, cartilage and synovial tissues obtained from surgical specimens of OA patients (15 patients) were cultured with or without IL-1β. Real time PCR for CFB, C3, and C4 was performed. Culture supernatants were analyzed for C3a, C5a, CFBa, and terminal complement complex (TCC) production.ResultsIn osteochondral biopsies, C factor expression was located in bone marrow, in a few subchondral bone cells and chondrocytes. C3 was the most expressed while factor C4 was the least expressed factor. Gene expression showed that all C factors analyzed were expressed both in chondrocytes and synoviocytes. In chondrocyte cultures and cartilage explants, CFB expression was significantly higher than C3 and C4. Furthermore, CFB, but not C3 and C4 expression was significantly induced by IL-1β. As to C activation factors, C3a was the most produced and CFBa was induced by IL-1β in synovial tissue. TCC production was undetectable in isolated chondrocytes and synoviocytes cell culture supernatants, whereas it was significantly augmented in cartilage explants.ConclusionC factors were locally produced and activated in OA joint with the contribution of all tissues (cartilage, bone, and synovium). Our results support the involvement of innate immunity in OA and suggest an association between some C alternative pathway component and joint inflammation.
Highlights
Local and systemic low-grade inflammation is recognized as one of the major triggering factors in Osteoarthritis (OA) pathogenesis in combination with age, biomechanical stress, and metabolic derangement [1,2,3]
We found that CFB was the most expressed component followed by C3, that the activation fragments C3a, C5a, and CFBa were produced in vitro mainly by isolated chondrocytes and that Terminal Complement Complex (TCC) was mainly produced by cartilage tissue and to a lesser extent by synovial tissue
Complement factors staining was mainly located in the bone marrow, with positive areas widespread among bone trabeculae and adipose tissue, whereas cartilage positivity was found only in few samples (Figure 1)
Summary
Local and systemic low-grade inflammation is recognized as one of the major triggering factors in Osteoarthritis (OA) pathogenesis in combination with age, biomechanical stress, and metabolic derangement [1,2,3]. Complement activation proceeds through three distinct pathways: the classical, the lectin (mannose-activated) and the alternative pathway which converge in the cleavage of C3 component by C3 convertase This cleavage results in the formation of C5 convertase which in turn activates C5 with the formation of C5b, the first component of C5b-9 complex, known as Terminal Complement Complex (TCC) or Membrane Attack Complex (MAC), responsible for cell lysis [9]. A large number of non-lytic functions (such as the release of proinflammatory cytokines and chemokines or the expression of Platelet-derived or b-Fibroblast growth factors) can result by the generation of C5b-9 During this process, the production of potent inflammatory fragments (C3a and C5a) amplify the inflammatory response by binding to the cognate receptors on several target cells [10]
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