Airway inflammation in allergic asthma is a complex process, and Th2-type inflammation and excessive accumulation of eosinophils are the important features (14). At the site of airway inflammation, Th2 cells secrete large amounts of interleukin (IL)-3, -4, -5, -9 and -13 and recruit/activate eosinophils, mast cells, and basophils (15). IL-13 is crucial to the pathogenesis of asthma; overexpression of IL-13 significantly induces the occurrence of allergic asthma in a mouse model (16). Additionally, IL-13 induces not only the proliferation of goblet cells (the main effector cells for mucus production in the respiratory tract) but also subepithelial fibrosis which leads to airway remodeling (16, 17). Activated eosinophils migrate to the bronchial epithelium and release ROS and eosinophil granulocyte protein, resulting in airway hyper responsiveness (AHR) and epithelial damage that exacerbates the respiratory symptoms (18). The growth and survival signaling induced by ligand/receptor interactions in the airway smooth muscle cells are mediated through ROS (19). In addition, macrophage migration inhibitory factor (MIF), an important upstream regulator of airway inflammation, promotes eosinophil differentiation, survival, activation, and migration by binding to CD74 and CXCR4 on the surface of eosinophils (20). In the pathogenesis of allergic asthma, reduced expression of antioxidant genes is also observed, such as Nrf2. Clinical studies have shown that with the reduction of Nrf2 expression in the body, asthma becomes more serious (21, 22), and the application of Nrf2 agonists significantly relaxes the bronchi and improves the symptoms (23), suggesting that the occurrence of allergic asthma is not only related to Th2 inflammatory reaction but also has a close relationship with oxidative stress.

Clinical treatment of asthma mainly involves β2-receptor agonists, corticosteroids, and aminophylline. Although β2 agonists are currently the largest class of treatment agents for asthma, their use is controversial because of poor clinical reactions and possible life-threatening adverse events. For moderate and severe asthma, combination therapy with inhaled corticosteroids and long-acting β2-agonists is used; however, this combination cannot prevent, reverse, or treat the underlying causes of the disease. Moreover, this treatment requires continuous monitoring for side effects and resistance (24). For instance, aminophylline can often cause adverse reactions, such as palpitations, headache, and vomiting (25).

Trx1 is closely associated with asthma. Serum Trx1 levels in patients with acute asthma exacerbation are significantly high and there is a significant correlation between these levels and eosinophil cationic protein (9, 26). Exogenous Trx1 treatment can significantly improve AHR and airway inflammation in ovalbumin-sensitized mice (27). Similarly, in a mouse model of chronic asthma, the systemic use of Trx1 significantly inhibited airway remodeling, eosinophil infiltration, and AHR while reducing the expressions of eotaxin (an eosinophil chemokine), macrophage inflammatory protein-1 and IL-13 in the lungs; thus, Trx1 improves pathological changes in the airway to prevent remodeling and asthma development (28), in agreement with a recent report which indicated that Trx1 displayed pronounced protective effects on the manifestation of allergic airway inflammation (29). Trx1 also inhibits Th2 cytokine production by directly downregulating MIF production and indirectly inhibiting eosinophil chemotaxis. Notably, the realization of this process does not depend on the regulation of systemic Th1/Th2 immunity (30). The proliferation of goblet cells that secrete excessive mucus increases the morbidity and mortality of asthma patients; however, Trx1 prevents this proliferation or improves established goblet cell proliferation (31). Trx1 also regulates ARH and airway remodeling by directly reducing the production of intracellular ROS. Additionally, the clinical drug ephedrine may produce anti-asthma effects in vivo through inducing Trx1 production (32). The regulation of allergic asthma by Trx1 also involves the Trx1/Txnip system. Trx1 and Txnip are normally combined into a dimer, and the Trx1/Txnip dimer is separated in the presence of irritant factors such as inflammation. Txnip can activate such signaling pathways as ASK1, Bax, p38, and Caspase3, and induce apoptosis in the lung tissue (33). Moreover, Txnip can also activate inflammasomes to trigger inflammatory reactions (33), all of which aggravate the symptoms of allergic asthma. Trx1, on the other hand, inhibits these reactions, thereby maintaining the balance between Trx and Txnip. Overall, Trx1 may be useful for the treatment of asthma and may represent a therapeutic target for asthma control.

The inflammatory process of AR is similar to that of asthma. Many Th2 cells infiltrate the nasal mucosa and release cytokines (e.g., IL-4, IL-5 and IL-13) that promote IgE production by plasma cells. A lot of medical treatment modalities used as a treatment of AR, such as antihistamines, steroids, montelukast, and immunotherapy. However, these therapeutic modalities can fail on some occasions (34).

Generally, large-scale production and release of inflammatory cells, including eosinophils and ROS and their metabolites, plays a vital role in the pathogenesis of allergic inflammatory airway diseases (35, 36). As an endogenous antioxidant protein, Trx1 has strong antioxidative stress effects. Thus, administration of exogenous Trx1 can inhibit AHR induced by specific allergens via the inhibition of eosinophil accumulation in the airway of mouse models with asthma (27, 28). Additionally, a recent study indicated that the concentration of Trx1 in nasal tissues is significantly decreased in patients with chronic rhinosinusitis with nasal polyps and a connection between elevated ROS levels and decreased levels of Trx1 has also been observed (37). Quercetin has been suggested as a dietary supplement for improving the clinical symptoms of allergic diseases, such as AR, but its precise mechanisms of action remain unclear. Nevertheless, Trx1 levels in the nasal mucosa significantly increase after oral administration of quercetin; moreover, the frequency of nasal allergy-like symptoms, such as sneezing and nasal rubbing, are significantly reduced (38). These changes provide insights into the possible mechanism underlying the favorable effects of quercetin on AR.

Food allergies are caused by aberrant immune responses towards food antigens; these responses are skewed towards Th2 responses associated with IL-4, IL-5, and IL-13. Current treatments for IgE-mediated food allergies are largely confined to the avoidance of the suspected allergens, antihistamine treatments, and corticosteroid therapies with low efficacy and several side effects. Food allergen immunotherapy induces desensitization and promotes permanent immune tolerance to food allergens by gradually increasing exposure to the allergens (39); however, the incidence of adverse reactions is high and is a long-term treatment process (40).

Trx1 treatment has been effective against food allergies in previous studies. For example, the application of Trx1 significantly reduced allergic reactions in a wheat allergy dog model subjected to skin test. Therefore, Trx1 potentially reduces wheat sensitization by reducing the number of disulphide bonds in the major protein allergens of wheat (41). Similarly, Trx1 reduces the number of disulphide bonds in β-lactoglobulin, an allergen in bovine milk. The disulphide-reduced protein shows increased sensitivity to pepsin digestion and decreased hypersensitivity in vivo (42). Additionally, a Trx1-treated salt-soluble wheat allergen was shown to reduce IgE binding in children with asthma (43). Consistent with these results, active systemic and passive cutaneous anaphylaxis testing on guinea pigs showed that yeast extract rich in Trx1 significantly reduced egg mucin-induced anaphylaxis. It was hypothesized that the anti-allergic activity of Trx1 itself plays a role in these effects (44). Accordingly, Trx1-rich yeast extract can potentially be used to ferment foods, such as alcoholic beverages and bread. Recently, recombinant Trx1 rice has been shown to improve β-lactoglobulin digestion and decrease its allergenicity, thereby improving the feasibility and practicality of its large-scale application; a plant Trx system would be more cost-effective than those of Escherichia coli or animals (45).

Drug allergies (DAs) can be IgE- or non-IgE mediated. Some drugs, such as anesthetics, antibiotics, nonsteroidal anti-inflammatory drugs and codeine, are associated with a carrier protein through a prototype or its metabolite (46). Binding of cell-bound IgE molecules activates mast cells and releases various factors, such as histamines, leukotrienes, prostaglandins, and cytokines, which can cause extensive tissue damage. Trx1 is a stress-induced redox regulatory protein in vivo; thus, it inhibits histamine release by eliminating ROS in mast cells (47). The mechanisms of DAs may often be associated with non-IgE-mediated complement activation. Indeed, Trx1 is known to inhibit the activation of the complement cascade at different stages, e.g., suppressing C3 cleavage and C5 convertase activation (48, 49). The functions of Trx1 in mast cells and the complement system are described in section 3.5 “Suppression of Complement Activation”.

Contact dermatitis is a common inflammatory skin disorder that is usually characterized by alternating relief from and deterioration of symptoms, but it can be persistent at times (50). Contact dermatitis can be categorized as irritant contact dermatitis (ICD), a non-immunologically driven inflammatory reaction to an irritating substance, and allergic contact dermatitis, a type-IV delayed-type hypersensitivity reaction resulting from the activation of allergen-specific T cells, i.e., a second exposure to the allergen resulting in circulating memory T cells homing to the skin and eliciting an immunologic reaction that causes skin inflammation (51). Topical corticosteroid treatment is typically the first-choice treatment for contact dermatitis. However, corticosteroids are not suitable for long-term use because of multiple side effects, such as skin atrophy, telangiectasia, dermatoglyphics, and pigmentation. Oxidative stress is known to play a key role in contact dermatitis inflammation. In particular, ROS participate in dendritic cell activation (52). In addition to being an endogenous redox regulatory protein, Trx is an effective ROS scavenger (53). Because ROS regulate the function of dendritic cells that function in the sensitization phase of contact hypersensitivity, transgenic overexpression and systemic administration of exogenous Trx1 can suppress skin inflammation by inhibiting neutrophil recruitment during the elicitation phase, but not during the induction phase, in mice treated with 2,4-dinitrofluorobenzene (54). Transgenic overexpression of Trx1 and systemic administration of exogenous Trx1 can prevent the cutaneous inflammation caused by UV radiation through regulating the cellular redox status and ROS scavenging (55). We previously demonstrated that Trx1 ameliorates ICD by inhibiting epithelial production and releasing inflammatory cytokines and chemokines (56). Existing research suggests that Trx1 can be used to treat contact dermatitis; however, its exact therapeutic mechanism requires further clarification.

Trx1 can directly remove ROS produced in inflamed tissues and help maintain redox balance. Mitsui et al. showed that Trx1 transgenic mice had strong resistance to oxidative stress and a longer life span compared with wild-type (WT) animals (57). Compared with the Trx1 system, the intracellular redox system has similar antioxidant mechanisms, such as the glutathione and peroxidase systems, which defend against oxidative stress. The Trx1 and glutathione systems act as backup systems to provide electrons for each other, i.e., the two systems protect cells from oxidative damage synergistically (58, 59). In addition, Trx1 is required to provide electrons when peroxidase is used to reduce ROS in organisms (60). Thus, Trx1 plays a key role in the balance of multiple redox systems in the body, and it coordinates the normal operation and function of these systems. Moreover, Trx1 is also a downstream target molecule for the activation of many redox signals. For example, in the case of mitochondrial redox imbalance, Nrf2 signal that is important for redox is activated (61). Nrf2 signaling activates some specific targets, including Trx1 and glutathione (62).

In the allergic state, expression of Trx1 can be induced to reduce the damage caused by excessive ROS. Simultaneously, the Trx1 system restores and refolds oxidized and damaged proteins. Consequently, Trx1 likely plays an important protective role against allergic inflammation.

Human MIF, a member of the Trx1 family of proteins that displays thiol reductase activity, was first cloned from T cells in 1989 (63). It shows inhibitory properties against the migration of macrophages and plays an essential role in cellular immunity, particularly in delayed-type hypersensitivity (64). MIF is largely considered a pleiotropic inflammatory medium with a wide range of immunoregulatory and pro-inflammatory activities, including the induction of inflammatory cytokines, regulation of macrophage and lymphocyte proliferation, and functions similar to those of chemokines (64, 65). Furthermore, MIF is directly involved in eosinophil differentiation, survival, activation, and migration (20).

MIF shares the redox-active motif -Cys-Xxx-Xxx-Cys- with Trx1 (66). It has sulfhydryl reductase activity and direct redox reactions with Trx1 (67). Several preclinical studies using animal models have shown that Trx has beneficial MIF-related functions against various inflammatory diseases. For example, the serum MIF level of Trx1 transgenic mice was significantly lower than that of WT mice in a dextran sodium sulphate-induced colitis mouse model (68). In mice with systemic inflammatory reactions from smoking, MIF gene expression in the spleens of Trx1 transgenic mice was inhibited compared with the expression levels in control mice (69). Using a mouse model of asthma, Torii et al. determined that MIF production in the lungs of Trx1 transgenic mice was significantly reduced despite similar systemic Th2 responses and IgE concentrations, indicating that Trx1 can suppress airway inflammation by directly inhibiting MIF independent of systemic Th1/Th2 immune modulation (30).

In vitro studies have provided evidence on the strong anti-MIF effect of Trx1. For instance, the production of MIF in macrophages cultured with LPS and IFN-γ was significantly inhibited by Trx1 (68). MIF expression is also suppressed in Trx1-transfected cells (70), and topically applied exogenous Trx1 suppresses the expression of MIF in ICD skin tissues (56). Additionally, MIF can enter cells to induce a series of inflammatory reactions, and cell surface Trx1 is one of the target proteins for MIF internalization. Specifically, Trx1 on the cell surface binds to extracellular MIF with high affinity and blocks MIF internalization. Exogenous and intracellular Trx1 can also directly bind to MIF, thereby forming a complex that blocks MIF-induced inflammatory response (71).

In the cell microenvironment, proliferation and differentiation of Th1/Th2 cells are affected by various factors, such as cytokines, antigen properties, T cell receptor signal intensity, antigen-presenting cell types, and costimulatory molecules (72–74). In addition to these external factors, cell redox status is considered to play a role Th cell differentiation. T cells have limitations in terms of cystine uptake and require exogenous mercaptan for their activation to play a role in this process. During antigen presentation, after dendritic cells interact with T cells, the former generate and release Trx1, which reduces extracellular cystine to cysteine used by T cells; thus, the normal proliferative ability of T cells as well as an effective immune response are maintained (75, 76). Trx1 also controls the redox state of cell surface receptors, such as CD4 and CD30, and thereby affects the behavior of T cells (77, 78). When Th2 cytokine responses increase, Trx1 induces the expression of Th1-like cytokines, such as IL-1α, IL-1β, IL-1Ra, and IL-18, which in turn suppresses Th2-like cytokine expression (27). In recent studies, Trx1 has been confirmed as a specific target gene induced by the cytokine IFN-γ that directly drives the Th1 immune response (79, 80). Indeed, IFN-γ promotes Th1 differentiation and downregulates the Th2 response (81). Exogenous Trx1 can induce the expression and release of IFN-γ in Th1 cells, and the increased IFN-γ level in turn increases the Trx1 level. The intracellular Trx1 of IFN-γ-activated macrophages increases the secretion of the Th1 cytokine IL-12 by regulating the thiol redox state. Given the mutual induction and promotion of Trx1 and IFN-γ by immune cells during oxidative stress, a positive feedback mechanism could exist between Trx1 and IFN-γ as they participate in stimulating Th1 immunity (80). In addition, hTrx1 can bind to Dectin-1 and/or Dectin-2 on antigen presenting cells to secret IL-1β and IL-23, which influence Th2/Th17-polarizing milieu during the allergic sensitization in the skin (82). Recently, IL-4 has been identified as a new target of Trx1; specifically, its activity can be selectively suppressed by Trx1 (83) ( Figure 2 ); thus, the production of IgE by B cells may also be effectively blocked. However, Trx1 does not directly affect the proliferation and differentiation of Th1/Th2 cells; instead, it suppresses inflammation by regulating the production and release of Th1/Th2 cytokines because lymphocytes isolated from Trx1-transgenic (Trx1-Tg) mice are similar to those from WT mice in terms of their ability to produce Th2 cytokines, such as IL-4, IL-5 and IL-13, once they leave an in vivo environment with high Trx1 (30). The recent report showed that regulatory T cells (Tregs) play an important role in maintaining immune tolerance to allergens by inhibiting the type 2 immune cells, inducing tolerogenic dendritic cells, regulatory B cells and IgG4-producing B cells in allergic disease (84). Increased Trx1 in Tregs enhances tolerance to oxidative stress (85). Thus, Trx1 may prevent the occurrence and progression of Th2-driven allergic inflammatory conditions by adjusting the Th1/Th2 balance. In response to various cytokines released by Th2 cells, such as IL-4, IL-5, IL-5 and IL-13, many inflammatory cells are activated, which in turn elicit an inflammatory response that leads to the clinical symptoms of allergic disease. The main effector cells are eosinophils, mast cells, and neutrophils in this complex immune response. A number of studies have shown that Trx1 directly modulate these cells through various mechanisms. Next, we focus on the role and the mechanisms of Trx1 on these inflammatory cells in allergic reactions.

Excessive complement activation has been implicated in the pathogenesis of allergic inflammatory disorders, such as IgE-independent DAs, and the increased production of the anaphylactic toxins C3a and C5a contributes to the activation of mast cells or basophils, vasodilation, and smooth muscle contraction. Transgenic overexpression of Trx1 in vivo or exogenous Trx1 injection can reduce choroidal neovascularization formation in laser-injured mouse models, which is closely associated with the complement activation of the Trx1 inhibition alternative pathway (48). Complement factor H, a multidomain and multifunctional protein, functions within the negative feedback that occurs during complement alternative pathway activation. It competes with factor B for C3b binding and accelerates the degradation of C3 convertase into its component (105). Trx1 inhibits C3 cleavage into C3a and C3b in a dose-dependent manner and prevents the deposition of C3b, and it inhibits the activation of C3b and reduces the generation of C3 convertase by binding to complement factor H; thus, it enhances the inhibition of C3 cleavage by complement factor H (48).

Moreover, Trx1 inhibits the activation of C5 convertase through its active site, thereby preventing the production of C5a and the formation of the membrane attack complex (49) ( Figure 3 ). The deposition of C5b and C9 is also inhibited by Trx1 in a concentration-dependent manner in all three pathways during their early stages; however, Trx1 does not inhibit the deposition of non-allergic toxin C3b, which has a conditioning effect on bacteria and promotes phagocyte phagocytosis (106). C5a shows strong chemotactic activity in neutrophils and stimulates them to produce a large amount of oxygen free radicals, prostaglandins, and arachidonic acid. When Trx1 is intravenously injected into mice, complement-mediated neutrophil recruitment is significantly inhibited (48, 49). Therefore, blockage of complement activation by Trx1 may represent a therapeutic target for relieving IgE-independent allergic inflammation.