IL1RL1 and Arthritis: In mouse models of experimental arthritis involving active immunization, such as collagen- and antigen-induced arthritis, the use of ST2 knockout (KO) mice, ST2 blockade or injection of sST2 led to decreased immune responses and severity of arthritis, while injection of recombinant IL-33 increased arthritis severity, suggesting a pathogenic role for IL-33, signaling through ST2, in these experimental models [18-21].