IL1RL1 and Arthritis: Our observation that endogenous IL-33 is not required for the development of joint inflammation in serum transfer-induced arthritis differs from previous conclusions based on indirect evidence obtained using ST2 KO mice and injections of recombinant IL-33 [22,23], suggesting that caution is warranted when extrapolating such data to conclude on functions of endogenous IL-33, as already highlighted by other studies [46-48].