Fibroblast-Derived PLA2G12A Amplifies Mast Cell Activation Through Extracellular Phospholipid Remodeling

Extracellular vesicles (EVs) mediate intercellular communication, yet whether extracellular phospholipid metabolism on EV membranes regulates immune activation remains incompletely understood. Through comparative phenotypic analyses of multiple secreted phospholipase A₂ (sPLA₂)-deficient mouse strains, we identified PLA2G12A as a stromal regulator of extracellular lipid metabolism that promotes IgE/antigen-dependent mast cell activation. Pla2g12a deficiency attenuated passive cutaneous and systemic anaphylaxis and selectively impaired IgE-dependent, but not IgE-independent, mast cell responses. Single-cell RNA-sequencing of mouse and human skin revealed that PLA2G12A is expressed in broad subsets of stromal fibroblasts, including papillary fibroblasts adjacent to mast cells, implicating a specialized stromal niche in mast cell regulation. Consistently, mast cell-intrinsic PLA2G12A was dispensable, whereas fibroblast-targeted deletion of Pla2g12a phenocopied global Pla2g12a deficiency. Lipidomic analyses during IgE-mediated anaphylaxis revealed a significant reduction in lysophospholipids, accompanied by minimal changes in oxylipins, in Pla2g12a-deficient skin. Mechanistically, fibroblast-derived PLA2G12A hydrolyzed phospholipids in EV-enriched fractions to generate multiple lysophospholipids, and treatment with either the PLA2G12A-treated EV-enriched fraction or exogenous lysophospholipids restored impaired IgE-dependent anaphylactic responses in Pla2g12a-deficient mice. Collectively, these findings establish sPLA₂-driven extracellular phospholipid remodeling as a mechanism by which stromal niches regulate allergic inflammation.