Inverted-U-shaped association between synovial IL-33 and MIG, IP-10, IL-6, IL-8, TNF-α and IL-1β in patients with rheumatoid arthritis. Second-order polynomial regression curves were fitted with SPSS 24.0. MIG, monokine-induced by γ-IFN; IP-10, IFN-γ-induced protein 10.