ABSTRACT: 1) Interaction of mast cells (MCs) with fibroblasts is essential for MC maturation within tissue microenvironments, although the underlying mechanism is incompletely understood. Through a phenotypic screening of >30 mouse lines deficient in lipid-related genes, we found that deletion of the lysophosphatidic acid (LPA) receptor LPA1, like that of the phospholipase PLA2G3, the prostaglandin D2 (PGD2) synthase L-PGDS, or the PGD2 receptor DP1, impairs MC maturation and thereby anaphylaxis. Mechanistically, MC-secreted PLA2G3 acts on extracellular vesicles (EVs) to supply lysophospholipids, which are converted by fibroblast-derived autotaxin (ATX) to LPA. Fibroblast LPA1 then integrates multiple pathways required for MC maturation by facilitating integrin-mediated MC-fibroblast adhesion, IL-33-ST2 signaling, L-PGDS-driven PGD2 generation, and feedforward ATX-LPA1 amplification. Defective MC maturation resulting from PLA2G3 deficiency is restored by supplementation with LPA1 agonists or PLA2G3-modified EVs. Thus, the lipid-orchestrated paracrine circuit involving PLA2G3-driven lysophospholipid, eicosanoid, integrin, and cytokine signaling fine-tunes MC-fibroblast communication, ensuring MC maturation. 2) Systemic scleroderma (SSc) is a devastating fibrotic disease with poor prognosis, yet its underlying mechanisms remain poorly understood. Given that lipid dysregulation is often associated with skin disorders, we screened cutaneous expression of various lipid hydrolases in SSc patients and found that ABHD12B, a putative hydrolase expressed in terminally differentiated epidermal keratinocytes, was most robustly downregulated in the affected skin. ABHD12B deficiency disturbed skin barrier and exaggerated skin fibrosis in mouse scleroderma models. Single-cell RNA-sequencing and untargeted lipidomic analyses of the skin revealed that ABHD12B deficiency facilitated epidermal- and endothelial-to-mesenchymal transitions and selectively reduced the levels of 1-O-acylceramides with very-long-chain fatty acids. Topical application of 1-O-acylceramide to ABHD12B-deficient mice reversed skin barrier abnormality and fibrosis. Thus, our results reveal a functional connection of ABHD12B with 1-O-acylceramide biosynthesis, highlight an unexplored anti-fibrotic role of this unique lipid, and point to this lipid-driven pathway as a potential drug target for SSc.
