Differential responses of old human cartilage explants to synovial- and mononuclear-cell factors

To investigate mechanisms of cartilage destruction that may apply to rheumatoid arthritis, young and old human, and young porcine, articular cartilage was cultured for 8 days and the effects on proteoglycan (PG) metabolism of normal synovium supernatant (NSS), rheumatoid synovial fluid (RFL), and blood mononuclear cell supernatant (MCF) were studied. The effects were chondrocyte-mediated. An inverse correlation was found between baseline net PG synthesis and the effect of NSS on PG synthesis. Responses of young (porcine and human) cartilage were similar. In young cartilage the three agents induced PG depletion by suppression of net PG synthesis. In old cartilage NSS and RFL induced PG depletion, whereas MCF did not. In cartilage of low baseline net PG synthesis, NSS and MCF stimulated both PG release and PG synthesis; NSS stimulated predominantly PG release, and MCF predominantly PG synthesis. In conclusion, young and old human cartilage differ in the quality of their in vitro response to potentially catabolic factors. This may be due to the difference in baseline net PG synthesis. Synovial extracts differ from mononuclear-cell supernatants in their effects on old cartilage. It is suggested that this is caused by the presence, in different relative amounts, of factors that influence either PG synthesis or PG release.