Local cytokine profiles in knee osteoarthritis: elevated synovial fluid interleukin-15 differentiates early from end-stage disease

Objective: Much of what is known about the inflammatory response in the synovial membrane (SM) of patients with osteoarthritis (OA) comes from studies of synovial tissues from end-stage disease. In this study, we sought to better characterize the inflammatory infiltrate in symptomatic patients with early signs of knee OA, and to determine how inflammatory cell populations relate to the pattern of cytokine and degradative enzyme production. Methods: Study populations comprised patients with degenerative meniscal tears and early cartilage thinning undergoing arthroscopic procedures (early OA) and patients undergoing total knee replacement for end-stage OA. Quantitative real-time polymerase chain reaction (PCR) was used to measure expression of SM cytokines and enzymes implicated in the pathogenesis of inflammatory arthritis and OA, as well as cell lineage-specific markers. We quantified synovial fluid (SF) cytokines and enzymes by enzyme-linked immunosorbent assay (ELISA) and SM cell populations by immunohistochemistry. Results: We found increased levels of SF interleukin-15 (IL-15) protein in the early knee OA patients when compared to end-stage OA. Both SF IL-15 protein and numbers of CD8 cells within SM correlated with matrix metalloproteinase-1 (MMP-1) and three levels. TNF-α, IL-6 and IL-21 were also detectable in the SF of the majority of patients, and IL-15 levels were associated with IL-6 levels. Conclusion: IL-15 is elevated in early knee OA, suggesting activation of an innate immune response in the SM. The association of IL-15 expression with CD8 transcripts and MMPs implicates this cytokine in OA pathogenesis and as a candidate therapeutic target.