Small interfering RNA significantly suppresses mRNA and protein levels of IL-1Ra. Knocking down all isoforms of IL-1Ra via IL1RN siRNA treatment reduces the IL-1β induction of this gene as observed at two time points (24, 36 hours) post cell culture treatment (A, B). This also results in significant reduction of soluble IL-1Ra protein levels in primary human chondrocyte (PHC) cell supernatants after 24 hours (C). PHCs treated with a nonspecific scrambled siRNA construct is used as the control group. For these experiments, recombinant human (rh) matrilin-3 (MATN3) protein is used at 200 ng/ml. Rh IL-1β protein treatment is always 5.0 ng/ml. *P ≤ 0.05 for statistically significant differences between groups. Data are representative of three individual experiments.