Detection of COX-1 and COX-2 isoforms in synovial fluid cells from inflammatory joint diseases

OBJECTIVE: To investigate the expression of cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) in cells from synovial fluid (SF) of patients with acute or chronic arthritis. METHODS: SF was obtained from eight patients with acute crystal-induced arthritis, nine with rheumatoid arthritis and four with psoriatic arthritis. COX-1 and COX-2 gene expression was studied by reverse transcriptase-polymerase chain reaction (RT-PCR). Protein expression was detected by Western blotting and immunocytochemistry. RESULTS: There was expression of COX-1 mRNA in all and COX-2 mRNA in most of the SF samples from acute or chronic arthritis. By immunocytochemistry, both COX-1 and COX-2 immunoreactivity was restricted to a variable fraction of mononuclear cells. COX-1 staining was observed in 10-fold more cells than COX-2. By Western blotting, COX-1 protein was detected in 60% of the SF samples and COX-2 in none. There were no differences in the pattern of COX-1 and COX-2 expression between chronic and acute SF samples. CONCLUSION: In arthritis, both COX-1 and COX-2 isoforms are expressed by SF cells. COX-1 is the most abundant isoform. Since the strong COX-1 immunostaining observed in a fraction of mononuclear SF cells is not observed in peripheral blood leucocytes, it may be the result of either the activation or recruitment of a subset of mononuclear cells with a high COX-1 expression level.