Elevated IL-6 levels in the synovial fluid of osteoarthritis patients stem from plasma cells

Objective: To analyse the levels of interleukin-6 (IL-6) in the synovial fluids and sera of patients with osteoarthritis (OA) and to identify the IL-6-secreting cells. Methods: Serum, synovial fluid, synovial tissue, and articular cartilage samples were collected from 49 OA patients with end-stage knee or hip OA who underwent joint replacement surgery. Serum and synovial fluid levels of IL-6 were measured by enzyme-linked immunosorbent assay (ELISA) and IL-6-secreting cells were identified by immunohistochemistry. Results: Eight out of 49 patients (16%) exhibited elevated IL-6 levels in the synovial fluids, averaging at 2022 +/- 526 pg/mL, while the levels in the rest of the patients averaged at 132 +/- 19 pg/mL. The sera levels of all patients were comparable in the 10 pg/mL range. Immunohistochemical analyses revealed plasma cells in the synovial lining of the high producers as the source of IL-6. Conclusions: Synovial fluid IL-6 levels may help to classify OA patients and may point to a subgroup with a particular impact from their immune system.