The canonical (caspase-1) and noncanonical (caspase-4/5/11) inflammasomes both cleave the pore-forming protein gasdermin D (GSDMD) to induce pyroptosis. Whereas caspase-1 processes IL-1β and IL-18 for maturation in myeloid cells, no cytokine target has been firmly established for LPS-activated caspase-4/5/11. We find that activated human caspase-4, but not mouse caspase-11, directly and efficiently processes IL-18 in vitro and during bacterial infections. Such a critical proinflammatory pathway mainly operates in epithelial cells, distinct from caspase-1 processing of IL-1β and IL-18. Crystal structure of the caspase-4/pro-IL-18 complex reveals a binary substrate-recognition mechanism, including a unique exosite that binds to a specific structure formed jointly by the propeptide and post-cleavage-site sequences in pro-IL-18. In caspase-11, a structural deviation around the exosite underlies its inability to target pro-IL-18, which can be restored by rationally designed mutations. The structure of pro-IL-18 features autoinhibitory interactions between the propeptide and the post-cleavage-site region, preventing recognition by the IL-18Rα receptor. Notably, IL-1β and IL-18 both lack a secretion signal, their release absolutely requires and is mediated by the GSDMD pore upon canonical and noncanonical inflammasome activation. Such an unconventional secretion mechanism is particularly important prior to or in the absence of the final plasma membrane rupture in pyroptosis.
GSDMD belongs to an evolutionary conserved gasdermin family which additionally includes GSDMA, GSDMB, GSDMC, and GSDME in humans. The N-terminal domains of these gasdermins all bear intrinsic pore-forming activities, and the formed pores that appear to be nonselective sizes between 16 nm and 22 nm. While the pore-forming or cell-killing activity of different gasdermins may vary to a certain extent, emerging and accumulating evidence suggests they likely all can serve as conduits to release cytokines or other cytosolic factors to stimulate proinflammatory responses in immunity.