Involvement of nonarticular cartilage, as demonstrated by release of a cartilage‐specific protein, in rheumatoid arthritis

Analysis of human cartilage extracts by radioimmunoassay showed that the noncollagenous 148-kd cartilage matrix protein was present in extracts of tracheal cartilage but was undetectable in normal or arthritic joint cartilage, corroborating previous results with bovine cartilage samples. Concentrations of the protein in the circulation, as studied by radioimmunoassay, were greatly elevated in patients with rheumatoid arthritis and polyarticular juvenile rheumatoid arthritis. In contrast, patients with reactive arthritis and oligoarticular juvenile rheumatoid arthritis, as well as rheumatoid arthritis patients treated with low-dose glucocorticoids, had levels similar to those in healthy controls. The serum concentrations were not related to age. A patient with polychondritis and tracheal involvement had a high serum concentration of the protein, which decreased during plasma exchange and cyclophosphamide treatment. Studies of the release of this cartilage matrix protein, which is present in nonarticular cartilage but not in articular cartilage, should aid in the understanding of the mechanisms of cartilage involvement in disease, and the protein may become a clinically useful marker.