Novel Pathway for Intercepting Granular Exocytosis: A13 Engages APLNR to Drive FBXO28-Mediated Ubiquitination and...

Allergic airway inflammation (AA) is primarily driven by the activation of mast cells and eosinophils, with granular exocytosis serving as a key source of pro-allergic mediators that amplify pathological responses. This unmet need highlights the importance of identifying novel, pathway-specific therapeutic targets to improve disease management. A dust mite extract (DME)-induced murine model of AA was used to assess intranasal A13 (1 mg/kg daily) efficacy; human EoL-1 eosinophils and murine P815 mast cells were stimulated with PMA/ionomycin (P&I) to induce exocytosis. In DME-induced murine AA, intranasal A13 reduced lung inflammation by 58% (p < 0.01), serum sIgE by 73% (p < 0.001), and BALF Th2 cytokines (IL-4/IL-5/IL-13) by 65%-80% (p < 0.001), while restoring BALF IFN-γ (p < 0.01). A13 inhibited granular mediator release: in P&I-challenged WT mice, it reduced BALF eosinophil peroxidase (EPX) by 81% and mast cell protease-1 by 85%, but had no effect in APLNR⁻^/⁻ mice (p > 0.05). Mechanistically, P&I induced Rab27a upregulation (P815: 3.2-fold; EoL-1: 2.8-fold), which A13 reversed in vitro; in vivo, A13 lowered lung granulocyte Rab27a by 2.5-3.1-fold (p < 0.001). A13 engaged APLNR to recruit FBXO28, promoting K48-linked Rab27a ubiquitination and proteasomal degradation. APLNR knockdown or MG132 treatment abrogated A13's effects, while A13 enhanced FBXO28-Rab27a complex formation by 4.7-fold (p < 0.001)-an interaction undetectable in APLNR^-/- cells. Intranasal A13 exhibits localised action, effectively suppressing allergic inflammation without broad systemic immunosuppression, making it a promising candidate for development as a topical biologic to treat allergic airway diseases.