Limonene from Citrus unshiu Peel Suppresses Neutrophil Trafficking and Alleviates Acute Lung Injury by Directly Targeting β2 Integrin

Citrus unshiu “Suavissima” peel, an abundant agricultural byproduct, is a traditional remedy for respiratory ailments, yet its mechanism of action is not well understood. This study investigated the protective effects and molecular mechanism of C. unshiu peel essential oil (CUEO) against lipopolysaccharide (LPS)-induced acute lung injury (ALI). In a murine ALI model, CUEO pretreatment significantly reduced the lung injury score from 12.31 ± 1.78 to 8.68 ± 1.26 and the wet/dry ratio from 7.58 ± 0.77 to 5.93 ± 0.66. Mechanistically, CUEO and its major constituent, limonene (71.6 ± 4.6%), specifically inhibited neutrophil adhesion and chemotaxis by disrupting the β2 integrin/intercellular adhesion molecule-1 interaction. Microscale thermophoresis confirmed that limonene directly binds to β2 integrin with a Kd of 4.28 ± 0.25 μM, while the L257A mutation reduced affinity to 89.36 ± 5.47 μM. Limonene monotherapy fully replicated CUEO’s protective effects in vivo. These findings identify limonene as a direct modulator of β2 integrin via the leucine 257 (Leu257) residue, providing a scientific basis for valorizing C. unshiu peel as a functional food ingredient for respiratory health.