A new phenolic acid isolated from Salvia miltiorrhiza ameliorates OVA-induced allergic asthma by regulation of Th17/Treg cells and inflammation through the TLR4 pathway

Objective: Salvia miltiorrhiza was a key part of the traditional Chinese medicine, with the effect of activating blood circulation and removing blood stasis, and was also commonly used in the treatment of asthma. In this study, a new phenolic acid (S1) was isolated from S. miltiorrhiza and its anti-asthmatic activity, as well as the mechanisms, were investigated for the first time. Methods: An allergic asthma (AA) model was developed based on ovalbumin (OVA). Multi-factor joint analysis, flow cytometry, and co-culture system were used to investigate the mechanism by which S1 affects AA, thus facilitating clinical treatment of allergic asthma. S1 (10 or 20 mg/kg) was administered daily in mice with OVA-induced allergic asthma (OVA-AA) during day 21-25. The airway responsiveness, lung damage, inflammation, and the levels of IgE, PGD2, interleukins (e.g., IL-4, 5, 10, 13, 17A), TNF-α, GM-CSF, CXCL1, CCL11, and mMCP-1 were investigated; activation and degranulation of mast cells were explored; Th17/Treg immune cells and the levels of biomarkers of the TLR4 pathway were analyzed. The antagonistic activity of TAK-242 (1 µM), a specific TLR4 blocker against S1 (10 μM) in co-cultured 16HBE cells and bone marrow-derived cells (BMDCs) or splenic lymphocytes (SLs) induced with LPS (1 μg/mL), was explored to clarify the mediation role of the TLR4 pathway. Results: As indicated, S1 led to reduced airway responsiveness, lung damage and inflammation, downregulations of IgE, PGD2, interleukins, TNF-α, GM-CSF, CXCL1, CCL11 and mMCP-1, impeded activation and degranulation of mast cells, and upregulation of IL-10, as well as transformation of Th17/Treg immune cells following OVA challenge. Moreover, S1 inhibited the TLR4 pathway in OVA-AA mice, and the antagonism of TLR4 led to increased positive effects of S1. Conclusions: Analysis based on an OVA-AA mouse model revealed that S1 could relieve the clinical symptoms of AA, recover lung function, and inhibit airway response. S1 could relieve AA by regulating Th17/Treg immune cells and inflammation or at least in part can be attributed to the TLR4 pathway. This study provides molecular justification for S1 in treatment of AA.