IL-1β and IL-6 stimulate IL-1Ra production. (a) Chondrocytes (passage 3) were left unstimulated (open squares), or were stimulated with 1 ng/ml IL-1β (filled diamonds), with 10 ng/ml IL-6 and 100 ng/ml sIL-6R (filled circles) or with the combination of 1 ng/ml IL-1β, 10 ng/ml IL-6 and 100 ng/ml sIL-6R (filled triangles). IL-1Ra concentrations in culture supernatants were measured by ELISA. *P < 0.01 versus control at the same time point, †P < 0.01 versus IL-1β-treated cells at the same time point. (b) Chondrocytes (passage 1) were stimulated with the indicated doses of IL-1β alone (white bars) or with IL-1β combined with 10 ng/ml IL-6 and 100 ng/ml sIL-6R (grey bars) for 72 hours. *P < 0.01 versus control, †P < 0.01 versus the same dose of IL-1β alone.