Asthma is one of the most common inflammatory lung diseases, affecting approximately 300 million people worldwide (Fortescue et al., 2020). Allergic asthma is the most common asthma phenotype, accounting for more than 50% of adult asthma cases (Akar-Ghibril et al., 2020). Inhaled corticosteroids are recommended as the first-line drug therapy in patients with asthma. However, their long-term use or high doses are associated with various local oropharyngeal or systemic adverse effects (Fuhlbrigge and Kelly, 2014; Kwda et al., 2019). Some patients with severe eosinophilic asthma are insensitive to glucocorticoid therapy (Nabe, 2020), highlighting the need for new effective compounds for asthma therapy.

Allergic asthma is a type 2 helper (Th2)-induced immune disorder. Th2 cytokines, such as interleukin (IL)-4, IL-5, and IL-13, enhance asthmatic allergic responses characterized by airway inflammation, mucus hypersecretion, and airway hyperresponsiveness (Akar-Ghibril et al., 2020). Macrophages play a key role in innate immune responses, influencing the development of Th1/Th2 adaptive immunity (Li et al., 2020b). Th2 inflammation polarizes macrophages to an M2 phenotype, which induces the expression of M2 macrophage-related genes, including arginase (Arg)-1, chitinase-like protein 3 (encodes Ym-1), and found in inflammatory zone 1 (Fizz-1). Activated M2 macrophages secrete high levels of type 2 chemokines such as C–C motif chemokine ligand (CCL)-17 and eotaxin, further exacerbating eosinophilic inflammation (Han et al., 2013; Jiang and Zhu, 2016; Kim et al., 2019). Persistent eosinophilic inflammation is a central event in asthma and contributes to the loss of airway construction and lung function. Several reports have suggested that a reduction in alternatively activated macrophage polarization attenuates asthmatic inflammation (Huang et al., 2019; Lee et al., 2021), indicating that modulation of macrophage phenotypes through drug intervention might be beneficial for the treatment of asthma.

Mollugin (MOL), a quinone compound, is a major bioactive compound isolated from Rubia cordifolia Linn (Rubiaceae) (Wang et al., 2020a). Rubia cordifolia L. root is listed in the Chinese Pharmacopoeia for treating arthritis, chronic bronchitis, uterine hemorrhage, and uteritis. Previous studies have demonstrated that Rubia cordifolia extracts exert anti-allergic activity in a murine model of peanut allergy (Lopez-Exposito et al., 2011) and suppress mast cell activation (Lee et al., 2006). However, it is unclear whether MOL is protective against allergic diseases. It has been reported that MOL has various pharmaceutical effects, such as anti-tumor (Do et al., 2013), anti-inflammatory (Li et al., 2020a; Zhu et al., 2013), and anti-platelet aggregation (Chung et al., 1994). As an inflammatory, MOL prevented dextran sulfate sodium-induced ulcerative colitis in mice by inhibiting the production of pro-inflammatory chemocytokines (Li et al., 2020a). M1 polarization responds to lipopolysaccharide stimulation. MOL in vitro inhibits the expression of inducible nitric oxide synthase, IL-1β, tumor necrosis factor-α, and IL-6 in lipopolysaccharide-induced RAW 264.7 cells (Zhu et al., 2013). We investigated whether MOL could modulate macrophage phenotypes under high Th2 cytokine stimulation.

In this study, we established a ST-induced mouse asthma model to investigate the effects of MOL on allergic airway inflammation. Its potential mechanism was further explored in IL-4-induced macrophages and allergen-stimulated splenocyte models.