Epithelial microbial sensing through the non-canonical inflammasome modulates airway type 2 immunity

Asthma is often characterized by type 2 inflammation triggered by environmental aeroallergens. Epidemiological studies link respiratory microbial infections, including intracellular bacteria, to both asthma initiation and exacerbations, yet the underlying molecular mechanisms remain poorly understood. Airway epithelial cells sense both microbes and allergens, and recent studies show that common aeroallergens induce type 2 inflammation through mechanisms including protease activity and pore-forming toxins that converge on epithelial cell damage, IL-33 secretion, and mitogen-activated protein kinase (MAPK) signaling. Host defense against microbes also involves endogenous membrane pore formation through inflammasome activation and gasdermin-mediated pyroptosis, releasing interleukin (IL)-1β and IL-18 to orchestrate bacterial killing. However, whether airway epithelial microbial sensing interacts with allergen-mediated cell death mechanisms to regulate type 2 immunity through analogous membrane pore formation and IL-33 release remains unknown. Using human airway epithelial cells and mouse models, we demonstrate that activation by intracellular lipopolysaccharide (LPS) initiates caspase-4-dependent pyroptotic cell death, IL-33 secretion, and a MAPK transcriptional program. Non-canonical pyroptotic cell death is enhanced by protease allergen through calcium signaling, and mice lacking caspase-4 show reduced IL-33 secretion and diminished innate type 2 responses to airway allergen challenge. Purified LPS from different bacterial species engage the non-canonical inflammasome with differing intensity, suggesting that microbiome composition likely influences this disease mechanism. Asthmatic patients show increased airway expression of caspase-4 and its effector molecule gasdermin D compared to healthy controls. This study identifies non-canonical inflammasome-dependent IL-33 expression and secretion as a novel link between bacterial detection and type 2 inflammation, providing a potential mechanism for how airway bacterial dysbiosis might influence asthma development and offering new insights into the interplay between microbial sensing and allergic responses in airways.