S led at first to the consensus that full-length IL-33 is the active form (Figure 3). Yet, it was since proposed that IL-33 is cleaved to a mature and more proinflammatory form.196 In fact, in vitro studies demonstrated the cleavage of IL-33 by neutrophil elastase and cathepsin G. The resulting C-terminal cleavage products, containing the entire IL-1-like domain, had stronger proinflammatory activ