IL-33 deficiency does not reduce severity of arthritis induced by IgG purified from K/BxN serum. (A) Incidence of arthritis induced by total IgG purified from K/BxN serum is shown for WT C57BL/6 mice (n = 10, dashed line), IL-33 KO mice (n = 9, gray line), and ST2 KO mice (n = 10, black line). The incidence of arthritis was significantly retarded in ST2 KO, as compared to WT or IL-33 KO mice (P < 0.01, longitudinal model for binomial data). Arthritis severity was evaluated by clinical assessment of arthritis severity scores (B) and number of affected paws (C) in WT C57BL/6 mice (n = 10, dashed line), IL-33 KO mice (n = 9, gray line), and ST2 KO mice (n = 10, black line). Results shown represent the mean ± SEM for each group of mice. The evolution of severity scores (B; P < 0.01, mixed model for repeated measures) and of the number of affected paws (C, P < 0.001, longitudinal model for ordinal data) was significantly decreased in ST2 KO, as compared to IL-33 KO and WT mice. In contrast, incidence and severity of arthritis were similar in IL-33 KO and WT mice. IgG, immunoglobulin G; KO, knockout; WT, wild-type.