Pyroptosis

Pyroptosis (from the Greek pyro, "fire" or "fever," and ptosis, "falling," coined by Brad T. Cookson and Molly A. Brennan in 2001) is a rapid, lytic, and pro-inflammatory mode of programmed cell death mediated by pore-forming proteins of the gasdermin (GSDM) family following proteolytic cleavage by inflammatory caspases. Unlike immunologically silent apoptosis—which packages cellular contents into intact membrane-bound apoptotic bodies cleared via efferocytosis—and kinase-driven necroptosis (mediated by RIPK3 and MLKL pseudokinase pores), pyroptosis converts cytosolic pathogen detection directly into plasma membrane perforation. By assembling large β-barrel transmembrane channels (~18–21 nm inner diameter) that secrete mature interleukin-1β (IL-1β, an endogenous pyrogen that induces fever) and interleukin-18 (IL-18) prior to terminal NINJ1-mediated cell lysis, pyroptosis eliminates the replicative niche of intracellular bacteria (Salmonella, Shigella, Burkholderia) and viruses while exposing trapped microbes to neutrophil phagocytosis and extracellular traps.

Pyroptosis is initiated through four distinct upstream proteolytic cascades that converge on the cleavage of autoinhibited gasdermin zymogens in macrophages, dendritic cells, monocytes, and barrier epithelial cells:

Discovered independently in 2015 by the laboratories of Feng Shao and Vishva M. Dixit as the executioner of pyroptosis, Gasdermin D (GSDMD, a 53 kDa cytosolic protein) belongs to a six-member human gene family (GSDMA, GSDMB, GSDMC, GSDMD, GSDME, and PJVK / Pejvakin). Full-length gasdermins share a conserved two-domain architecture separated by a flexible interdomain linker:

1. Autoinhibited Basal State: In resting cells, the 22 kDa C-terminal repressor domain (GSDMD-CT, an α-helical bundle) folds back onto the 31 kDa N-terminal pore-forming domain (GSDMD-NT), masking its lipid-binding α1 helix and hydrophobic β-hairpins so that the protein remains soluble and monomeric in the cytosol.
2. Proteolytic Liberation and Acidic Phospholipid Binding: When activated caspase-1 or caspase-4/5/11 recognizes the C-terminal exosite of GSDMD and cleaves the interdomain linker after Asp275, the liberated GSDMD-NT fragment undergoes a conformational rearrangement. GSDMD-NT selectively binds acidic phospholipids restricted to the inner (cytoplasmic) leaflet of the mammalian plasma membrane—specifically phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), phosphatidylserine (PS), and cardiolipin exposed on mitochondrial and bacterial membranes. Because the outer leaflet of healthy eukaryotic plasma membranes lacks acidic phospholipids (and cholesterol further inhibits outer-leaflet insertion), extracellular GSDMD-NT released from dying cells cannot perforate bystander host cells, whereas it readily lyses intracellular bacteria containing cardiolipin.
3. 33-Mer β-Barrel Pore Assembly: Upon membrane binding, 31 to 34 GSDMD-NT monomers oligomerize into a ring-shaped pre-pore and insert two amphipathic β-hairpins per monomer (124 to 136 β-strands in total) across the lipid bilayer. Cryo-electron microscopy (cryo-EM) structures determined by Hao Wu and colleagues (2021) reveal that the mature human GSDMD pore is a 33-fold symmetric transmembrane β-barrel with an outer diameter of ~31 nm and an inner conduit diameter of ~21.5 nm (~215 Å).
4. Electrostatic Filtering of IL-1β Secretion: The inner rim of the GSDMD-NT conduit is lined with negatively charged acidic patches (Asp/Glu electrostatic filter). Because the unprocessed precursor pro-IL-1β (31 kDa) carries an acidic N-terminal pro-domain, electrostatic repulsion retards its passage through the negatively charged GSDMD pore; once caspase-1 cleaves off the acidic pro-domain, the mature IL-1β (17 kDa, diameter ~4.5 nm, positively charged surface patch) passes rapidly through the GSDMD channel into the extracellular space even before the cell lyses.

Historically, the terminal bursting of pyroptotic cells (plasma membrane rupture, PMR) was assumed to be a passive mechanical consequence of water influx and osmotic swelling through gasdermin pores. In 2021, Kayagaki and colleagues demonstrated that terminal membrane lysis is an active, genetically regulated process mediated by the 16 kDa transmembrane protein Ninjurin-1 (NINJ1):

• Sub-Lytic GSDMD Phase ("Hyperactivated" Cells): Opening of GSDMD pores dissipates transmembrane Na⁺ and K⁺ gradients, draws water into the cytosol via oncotic pressure (producing characteristic balloon-like membrane herniations or "pyroptotic bubbles"), and releases small proteins (<20–30 kDa) such as IL-1β and IL-18. If membrane repair machinery (ESCRT-III endosomal sorting complexes triggered by Ca²⁺ influx) sheds GSDMD-perforated membrane vesicles fast enough, macrophages can survive in a "hyperactivated" state that secretes IL-1β without dying.
• NINJ1 Oligomerization and Lysis: When GSDMD pore density exceeds the threshold of ESCRT-III repair, loss of membrane tension triggers NINJ1 monomers to oligomerize into amphipathic filaments that slice open megadalton-scale lesions in the plasma membrane, releasing large cytoplasmic damage-associated molecular patterns (DAMPs) including lactate dehydrogenase (LDH, a 140 kDa tetramer used as the standard clinical assay of lytic cell death), HMGB1, ATP, and mitochondrial DNA.

In acute bacterial and viral infections, pyroptosis functions as a frontline innate immune defense mechanism:

• Pore-Induced Intracellular Traps (PITs): When facultative intracellular bacteria such as Salmonella enterica serovar Typhimurium, Listeria monocytogenes, or Burkholderia thailandensis replicate inside a macrophage, NLRC4 or caspase-11 triggers pyroptosis. Rather than dispersing freely into the tissue, the viable bacteria remain physically trapped inside the collapsed, insoluble cytoskeletal and plasma-membrane corpse of the pyroptotic macrophage—a structure termed a pore-induced intracellular trap (PIT) by Edward A. Miao and colleagues (2010). The PIT exposes phosphatidylserine and complement opsonization ligands (C3b) while secreting ATP and leukotriene B₄ (LTB₄), recruiting neutrophils via scavenger and complement receptors to engulf the PIT and kill the entrapped bacteria via NADPH oxidase reactive oxygen species (ROS).
• Endotoxic Septic Shock and Cytokine Storm Syndrome: Conversely, systemic disseminated Gram-negative bacteremia delivers high concentrations of lipopolysaccharide (LPS) into the bloodstream, triggering widespread caspase-4/5/11 and GSDMD-mediated pyroptosis across vascular endothelial cells and myeloid leukocytes. Endothelial pyroptosis destroys microvascular barrier integrity, releases tissue factor (TF) microparticles that trigger disseminated intravascular coagulation (DIC), and drives hypotension and multi-organ failure in septic shock. Genetic deletion of Gsdmd or Casp11—or pharmacological blockade of GSDMD Cys191 palmitoylation/oligomerization with disulfiram or necrosulfonamide—protects mice against lethal endotoxemia and polymicrobial sepsis.

1. Shi, J., Zhao, Y., Wang, K., et al. (2015). Cleavage of GSDMD by inflammatory caspases determines pyroptotic cell death. Nature, 526(7575), 660–665.
2. Kayagaki, N., Stowe, I. B., Lee, B. L., et al. (2015). Caspase-11 cleaves gasdermin D for non-canonical inflammasome signalling. Nature, 526(7575), 666–671.
3. Xia, S., Zhang, Z., Magupalli, V. G., et al. (2021). Gasdermin D pore structure reveals preferential release of mature interleukin-1. Nature, 593(7860), 607–611.
4. Kayagaki, N., Kornfeld, O. S., Lee, B. L., et al. (2021). NINJ1 mediates plasma membrane rupture during lytic cell death. Nature, 591(7848), 131–136.