Severity of serum transfer-induced arthritis is reduced in ST2, but not in IL-33 KO mice. (A) Incidence of arthritis is shown for WT (n = 14, dashed line), IL-33 KO (n = 12, gray line), and ST2 KO (n = 10, black line) mice. Incidence was significantly retarded in ST2 KO, as compared to WT or IL-33 KO mice (P < 0.01, longitudinal model for binomial data). (B-C) Arthritis severity scores (B) and number of affected paws (C) are shown as the mean ± SEM for WT (dashed line), IL-33 KO (gray line), and ST2 KO (black line) mice. The evolution of severity scores (B; P = 0.001, mixed model for repeated measures) and final disease severity were significantly decreased in ST2 KO, as compared to WT (*P < 0.05) and IL-33 KO (&P < 0.05) mice. Evolution of the number of affected paws tended to be lower in ST2 KO, as compared to IL-33 KO and WT mice (P = 0.05, longitudinal model for ordinal data). (D) Clinical scores for inflammation of the right hind paw, which was processed for histological evaluation, and histological scores for inflammation, polymorphonulcear cell (PMN) infiltration, cartilage and bone erosion are shown as the mean ± SEM for WT (open columns), IL-33 KO (gray columns), and ST2 KO (black columns) mice. Cartilage and bone erosion were significantly reduced in ST2 KO, as compared to IL-33 KO mice; &P < 0.05. KO, knockout; WT, wild-type.