Correlation between plasma, synovial fluid and articular cartilage Interleukin-18 with radiographic severity in 33 patients with osteoarthritis of the knee

Osteoarthritis (OA) is a complex disease characterized by cartilage degeneration, secondary synovial membrane inflammation and subchondral bone changes. In recent years, many studies have confirmed that interleukin-18 (IL-18) is involved in the inflammatory process of inflammatory joint diseases. In the present study, we investigated IL-18 levels in plasma, synovial fluid and articular cartilage of patients with primary knee OA (n = 33) to analyze their relationship with radiographic severity. Compared to healthy controls (n = 15), OA patients had higher plasma and synovial fluid IL-18 concentrations (45.8 ± 22.1 vs. 23.7 ± 13.6 pg/ml, P < 0.001 and 75.2 ± 40.1 vs. 28.3 ± 11.6 pg/ml, P < 0.001) as measured by enzyme-linked immunosorbent assay. Also, the percentage of immunofluorescent IL-18 positive cells in articular cartilage was significantly increased in OA compared to controls (46.5 ± 10.3 vs. 2.9 ± 1.7, P < 0.001). Moreover, plasma, synovial fluid and articular cartilage IL-18 significantly positively correlated with radiographic severity, respectively (r = 0.663, P < 0.001, r = 0.56, P = 0.001 and r = 0.884, P < 0.001). Subsequent analysis revealed that plasma, synovial fluid and articular cartilage IL-18 levels positively correlated with each other (r = 0.632, P < 0.001, r = 0.489, P = 0.004 and r = 0.620, P < 0.001). These data suggested that plasma, synovial fluid and articular cartilage IL-18 levels were significantly increased in OA patients, and these elevated levels were positively correlated with radiographic severity. Accordingly, our study supports the role of IL-18 in the pathophysiology of OA.