Synthesis of classical pathway complement components by chondrocytes

Using immunohistochemical studies, C1q, C1s, C4 and C2 were detected in chondrocytes in normal human articular cartilage and macroscopically normal articular cartilage from the inferior surfaces of hip joints of patients with osteoarthritis. Using reverse-transcribed polymerase chain reaction (RT-PCR), mRNA for C1q, C1s, C4 and C2 was also detected in RNA extracted from articular cartilage. C1r, C3, C1-inhibitor, C4-binding protein and factor I were not detected by either technique. Articular chondrocytes cultured in vitro synthesized C1r, C1s, C4, C2, C3 and C1-inhibitor but not C1q, C4-binding protein or factor I, as assessed by enzyme-linked immunosorbent assay (ELISA) and Northern blot analysis. Thus cultured articular chondrocytes have a complement profile that is similar to that of cultured human fibroblasts rather than that of articular chondrocytes in vivo. Complement synthesis in cultured chondrocytes was modulated by the cytokines interleukin-1 beta (IL-1 beta), tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma), showing that cytokines can probably regulate complement synthesis in intact cartilage. The possible roles of local synthesis of complement components by chondrocytes in matrix turnover and the regulation chondrocyte function are discussed.