Menthol citrate ester modulates epithelial alarmin–type 2 immune responses in OVA-induced allergic airway inflammation

Menthol citrate ester (MC) is a chemically defined menthol-derived compound, but its inhalation safety and timing-dependent effects on allergic airway inflammation remain unclear. Here, the inhalation safety and biological effects of aerosolized MC were evaluated through in vitro and in vivo approaches. Acute inhalation exposure resulted in no mortality, exposure-related clinical signs, hematological or biochemical alterations, or evident histopathological injury in mice, while MC maintained cytocompatibility in RAW264.7 cells at concentrations up to 500 μg/mL. In an ovalbumin (OVA)-induced allergic airway inflammation model, post-challenge MC treatment reduced serum total and OVA-specific immunoglobulin E (IgE), suppressed IL-13, IL-5, IL-33, TSLP, CCL11, IL-6, and IL-1β levels in bronchoalveolar lavage fluid (BALF), and decreased eosinophil and neutrophil accumulation. Histological analyses showed reduced airway inflammation, mucus hypersecretion, and collagen deposition. In contrast, pre-challenge administration failed to provide protection and further increased several inflammatory indices. Bulk RNA sequencing showed that high-dose therapeutic MC partially reversed OVA-induced transcriptional programs associated with epithelial alarmin signaling, type 2 inflammation, eosinophil recruitment, mucus production, airway remodeling, inflammatory amplification, and sensory-response regulation. Quantitative polymerase chain reaction (qPCR) confirmed downregulation of Il33, Il1rl1, Mmp9, and Htr3a, while Western blotting showed reduced IL-33, ST2/IL1RL1, MMP9, and HTR3A protein expression. These findings support the preliminary inhalation safety of MC and demonstrate a timing-dependent therapeutic effect in established allergic airway inflammation.