Ultra-short cyclic peptide Cy<sub>RL-QN15</sub> acts as a TLR4 antagonist to expedite oral ulcer healing

Oral ulcers (OUs) are among the most common lesions of the oral mucosa, typically associated with pain and burning sensations, and remain clinically challenging due to the scarcity of effective treatment options. Cy _RL-QN15, a novel ultra-short cyclic heptapeptide recently shown to promote skin repair, diabetic wound healing, and follicle neogenesis, was evaluated for its therapeutic potential in mucosal repair. Using a rat OU model and a primary oral epithelial cell inflammation model, Cy _RL-QN15 significantly accelerated wound closure through coordinated modulation of immune-epithelial crosstalk, including suppression of inflammatory cytokine release from macrophages and neutrophils, reduction of pro-inflammatory factor secretion by oral epithelial cells, and enhancement of their proliferation and migration. Mechanistic studies employing alanine scanning mutagenesis and microscale thermophoresis revealed that Cy _RL-QN15 directly interacted with Toll-like receptor 4 (TLR4) via a methionine-dependent binding interface (K _d=2.64 µmol/L), thereby inhibiting downstream MyD88/NF-κB signaling. As the first ultra-short cyclic heptapeptide identified to antagonize TLR4, Cy _RL-QN15 represents a mechanistically distinct immunomodulatory scaffold that restores mucosal homeostasis and offers a promising therapeutic candidate for TLR4-based OU intervention.