Regulation of IL-33 alarmin production from lung epithelial cells via CLEC-2:Podoplanin interactions

Lung epithelial cells produce alarmins that promote allergic inflammation, however, the molecular mechanisms that regulate alarmin expression and production have not been fully elucidated. We constructed an integrated single-cell RNA-seq atlas of murine airways, lung, and vagal nerves, which we use to characterize novel lung cell subsets and infer intercellular interactions, including an interaction between podoplanin (PDPN) on alveolar epithelial cells and C-type Lectin-like Receptor-2 (CLEC-2/Clec1b) on alveolar macrophages that controls the expression of IL-33. Clec1b-/- mice spontaneously develop allergic airway inflammation with increased Il33 expression in alveolar epithelial type 2 (AT2) cells and airway-associated fibroblasts; this airway inflammation is ameliorated by co-deletion of the IL-33 receptor, Il1rl1 (ST2). Myeloid cell-specific deletion of Clec1b and acute blockade of endogenous CLEC-2:PDPN interactions in vivo also promote type 2 immune responses in airway inflammation. Organoid experiments reveal that the regulation of AT2 Il33 expression by CLEC-2:PDPN interactions is directly mediated by alveolar macrophages. Single-cell characterization of house dust mite (HDM)-induced asthma reveals a smaller population of Clec1b expressing alveolar macrophages and increased Il33 expression by AT2 cells. Thus, our study has identified the CLEC-2:PDPN pathway as an important regulator of epithelial alarmin production and induction of type 2 immunity.