IL15 and rheumatoid arthritis: Also of interest is that normal healthy knees from this small cohort seemed to have higher SF IL-4 levels (panel n) than patients with RA or OA, and comparable levels of IL-18 (panel l), IL-15 (panel p), IGFBP-3 (panel g), IGFBP-4 (panel k), MMP-3 (panel h) and G-CSF (panel o), suggesting that some of these proteins may be involved in normal cartilage homeostasis.