IL1RL1 and Arthritis: On day 6 after induction of arthritis, serum IL-33 levels were similar in WT and ST2 KO mice (WT, mean: 310 ± 192 pg/ml, range < 1.61 to 1258 pg/ml; ST2KO, 447 ± 287 pg/ml, range < 1.61 to 2142 pg/ml) and comparable to IL-33 levels measured in naïve WT C57BL/6 mice (n = 17; 294 ± 191 pg/ml, range < 1.61 to 3062 pg/ml) housed in our conventional mouse facility.