Eosinophils differentiate from the eosinophil lineage-committed progenitor (EoP) cells which are a type of human common myeloid progenitor (hCMP) cell found in the bone marrow [16]. Eosinophils are stimulated to migrate to the lungs and airways by cytokines and chemokines. They play a crucial role in the immune responses to bacteria and parasites, as well as allergic responses. The effector function of eosinophils is attributed to the release of stored particles from their cytoplasm, such as eosinophil cationic proteins. Eosinophils can also form extracellular traps that capture and neutralize harmful substances [17].

Eosinophilic asthma is a unique asthma phenotype that is pathologically associated with thickening of the basement membrane zone and pharmacologically associated with the physical response to corticosteroids [18]. Eosinophilic asthma can be classed, primarily, as an allergen-dependent type 2 T helper (Th2) pulmonary immune response. This response is mediated by dendritic cells and T lymphocytes [19]. The most prevalent and distinctive form of this condition is allergic asthma, which is characterized by eosinophilic airway inflammation. Allergic asthma is associated with heightened production of type 2 cytokines, including IL-4, IL-5, and IL-13, which are secreted by group 2 innate lymphoid cells (ILC2s). Individuals with allergic asthma exhibit elevated levels of allergen-specific immunoglobulin E (IgE) antibodies [3]. Eosinophilic inflammation results in varying degrees of airflow obstruction and airway hyperresponsiveness (AHR) [20].

Th2 cytokines such as IL-4, IL-5, and IL-13 are heavily involved in eosinophilic airway inflammation. Additionally, IL-4-induced signal transducer and activator of transcription 6 (STAT6) signals are transmitted to Th2 cells during T-cell polarization [21]. IL-5 contributes significantly to the proliferation, activation, and recruitment of eosinophils to specific tissues. Its primary sources are Th2 cells and ILC2s. Along with dendritic cells, Th2 cells mediate eosinophil involvement in allergic pulmonary inflammation. ILC2s are the main cellular source of IL-5 in tissue. They interact with epithelial cells, stimulating the production of IL-25, IL-33, and thymic stromal lymphopoietin, which further enhance eosinophilia [17]. In asthma, IL-5 induces autophagy in the airways of individuals with asthma and upregulates the expression of the eosinophil cationic proteins (ECPs) [11, 15]. The upregulation of tumor necrosis factor-alpha (TNF-$\alpha$) during allergen sensitization is associated with the induction of Th2 cell responses [21]. IL-13 is involved in the regulation of epithelial cell differentiation into goblet cells, as well as the promotion of intracellular oxidative stress and secretion of the mucin MUC5AC. These processes are associated with increased autophagic activity. Knockdown of autophagy-related protein 5 (Atg5) or depletion of autophagy inhibits goblet cell secretion [22] and intracellular ROS production, which is mediated by the activation of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase dual oxidase 1 (DUOX1) [23].

Galle-Treger et al. [20] have demonstrated that activated ILC2s rely on autophagy for their effector function and homeostasis. The absence of autophagy in ILC2s inhibits the effector function and their ability to mediate AHR. A lack of autophagy in activated ILC2s can inhibit the nuclear factor kappa B (NF-$\kappa$B) pathway and induce cell apoptosis. It also hinders lipolysis, the induction of glycolytic metabolism, and the tricarboxylic acid cycle [20].

Transforming growth factor-beta 1 (TGF-$\beta$1) is a mediator of Th2-induced eosinophilic inflammation and the progression of airway remodeling [21]. Recent investigations have revealed that aryl hydrocarbon receptors (AhRs) in alveolar epithelial type II (AT2) cells are involved in the prevention of allergic airway inflammation. This is achieved through the regulation of TGF-$\beta$1 release and the control of autophagy adapters calcium binding and coiled-coil domain 2 (CALCOCO2) and S100 calcium-binding protein A9 (S100A9), which, in turn, regulate the autophagy process [24].

In a mechanistic study, Zhang et al. [12] provided evidence for an allergen-ROS-oxidized Ca${}^2$⁺/calmodulin-dependent protein kinase II (ox-CAMKII)-mitophagic axis in the development of allergic airway inflammation and asthma. The axis was observed in both allergen-treated human bronchial epithelial cells and a mouse model. Furthermore, it was discovered that CAMKII can regulate autophagy/mitophagy by directly phosphorylating Beclin 1, which is a core player in autophagosome formation and maturation [12].

Eosinophils have been shown to rapidly release mitochondrial DNA and granulin rapidly in a ROS-dependent manner when exposed to IL-5 or interferon-gamma (INF-$\gamma$). This leads to the formation of eosinophil extracellular traps (EETs), which possess bactericidal properties [25, 26]. EET generation relies on autophagy rather than cell death. Notably, inhibiting autophagy hinders the release of EETs, leading to a decrease in mucus and lung inflammation [27].

Neutrophilic asthma is characterized by elevated levels of neutrophils in the sputum. Individuals with neutrophilic asthma often have inherent immune system abnormalities, increased bacterial colonization of the airways, and elevated levels of airway endotoxins [21]. Th17 cells, a T lymphocyte subset, have been identified as significant contributors to the pathobiology of neutrophilic asthma. These cells produce IL-17A and IL-17F [28], which stimulate the recruitment of neutrophils by binding to the IL-17R on epithelial cells [29]. Consequently, the epithelial cells produce chemokines, including IL-8, which promote further neutrophil recruitment [30]. IL-8 concentrations have been found to correlate with the number of neutrophils present in the airways. This continuous recruitment and expansion of the neutrophil population contributes to the characteristic features of neutrophilic asthma, which include elevated levels of pro-inflammatory cytokines and neutrophilic inflammation of the airways. Various circulatory mediators of inflammation are also activated within the neutrophil-infiltrated airways [31, 32]. Activated neutrophils adhere to the airway epithelium and stimulate the inflammatory response [28, 33, 34]. Neutrophil autophagy and the formation of neutrophil extracellular traps (NETs) [35] exacerbate asthmatic severity. These processes can disrupt the integrity of the airway epithelium and trigger an inflammatory response in human airway epithelial cells and peripheral blood eosinophils [15]. Damage to the airway epithelium can consequently lead to ongoing airway obstruction and remodeling [36, 37].

Neutrophils migrate via the bloodstream to the site of an infection or injury by traversing damaged endothelial cells (EC) that line the blood vessels. Recent studies have revealed that acutely inflamed microvascular venules induce autophagy induction at the junctions between EC. In vivo experiments have further established that endothelial cell autophagy negatively regulates the physiological transendothelial migration of neutrophils. This suggests that the exploitation of EC autophagy has therapeutic potential as an anti-inflammatory strategy that would protect the host from excessive tissue damage caused by neutrophils [38].

Neutrophil airway inflammation in asthma has been linked to polymorphisms of the autophagy-related genes ATG5 and ATG7 [39]. A recent study by Suzuki et al. [8] demonstrated that depleted Atg5 levels in CD11c cells result in airway inflammation induced by steroid-resistant neutrophils. The authors of the study propose that enhancing pulmonary autophagy could ameliorate refractory severe asthma [8]. Bhattacharya et al. [40] investigated neutrophil biology using Atg7/Atg5-deficient mice and observed that the absence of autophagy can decrease neutrophil degranulation and inflammatory potential due to secretion defects. They identified reduced production of NADPH-induced ROS as a mechanism that contributes to autophagy-deficient neutrophils as this reduction diminishes the degranulation of neutrophils. These findings highlight the role of autophagy in regulating neutrophil functions and suggest that autophagy may serve as a potential target for modulating neutrophil-mediated inflammation [40].

An increase in the presence of NETs and NET components in the airways of asthmatic patients and, sometimes, the accumulation of excessive NETs, has been associated with the activation of the innate immune response. This implicates NETs in the pathogenesis of asthma [25, 41]. NETs are composed of mitochondrial DNA and antimicrobial proteins released by viable neutrophils [42]. NETs ensnare and degrade microorganisms [43] and can induce inflammation by disrupting airway epithelial cells (AECs), stimulating AECs to produce IL-8, and activating eosinophils to release their granules. NETs can also exacerbate asthma by causing small airway obstruction through increased formation of cellular aggregates [44]. Pham et al. [15] found that patients with severe asthma (SA) exhibit higher levels of autophagy and NET production than those with non-severe asthma (NSA). Autophagy plays a crucial role in the regulation of neutrophil survival and the formation of NETs. Inhibition of autophagy led to activation of caspases and to the appearance of morphological features of apoptosis, such as membrane blebbing [45]. Inhibition of autophagy prevents the decondensation of intracellular chromatin and this attenuates NET production. In turn, this results in apoptotic cell death. Thus, the dysregulation of autophagy and NET formation may contribute to the pathogenesis of SA [45].

The polarization of Th1 and Th17 cells is predominantly regulated by IL-12 and IL-6, both of which contribute to neutrophil-mediated inflammation. Notably, the Th1 cytokine IFN-$\gamma$ is upregulated in the induced sputum of patients with SA and is positively correlated with high sputum neutrophil levels. This suggests a potential link between Th1 polarization, neutrophilic inflammation, and the severity of asthma in these patients [46]. Transgenic mouse experiments have shown that IFN-$\gamma$ promotes the polarization of Th1 cells by upregulating the transcription factor T-bet. This, in turn, inhibits the polarization of Th2 cells. IFN-$\gamma$ also suppresses the recruitment of eosinophils into lung tissue following antigen stimulation by inhibiting the infiltration of CD4+ T cells [47]. However, elevated levels of IFN-$\gamma$ can induce neutrophilic airway inflammation, emphysema [48], and AHR [49].

IL-17, produced by the CD4+ T effector cell lineage known as Th17, plays a significant role in regulating airway remodeling in asthma. It can target various cell types and trigger the production of cytokines [50]. Moreover, IL-17 acts as a stimulator, attracting neutrophils to the site of inflammation. IL-17 also induces the production of matrix metalloproteinases (MMPs) which contribute to the degradation of inflammatory tissues [51, 52]. In individuals with SA, IL-17-induced autophagy has been shown to regulate mitochondrial dysfunction and fibrosis in bronchial fibroblasts [53]. Deficiency in the Th17-associated cytokine IL-23 has been associated with corticosteroid resistance [52]. Suzuki et al. [8] demonstrated that neutrophilic asthmatic mice with a deficiency in the autophagy-related gene Atg5 exhibit glucocorticoid resistance and dependence on IL-17A. IL-17A is a glycoprotein [54] secreted by IL-17-producing cells that is implicated in chronic inflammatory and autoimmune diseases. Mi et al. [55] revealed that particulate matter 2.5 (PM2.5) activates the TGF-$\beta$ signaling pathway by promoting the secretion of IL-17A in $\gamma$$\delta$T/Th17 cells, thereby inhibiting autophagy in bronchial epithelial cells. This activation causes epithelial-mesenchymal transition and ultimately exacerbates lung inflammation and fibrosis [55]. Targeting IL-17A creates a shift in the immune response in fibrotic lung tissue from an inhibitory state to a Th1 type, induces effective autophagy, facilitates autophagy-related cell death, and alleviates both acute and chronic pulmonary fibrosis. Moreover, IL-17A has been found to impede collagen degradation and inhibit autophagy in a TGF-$\beta$1-independent manner [55].

Kawaguchi et al. [56] have shown that IL-17, either alone or in combination with other cytokines (IL-4, IL-13, and IFN-$\gamma$), can induce IL-8 production. IL-8 is a neutrophil chemoattractant that stimulates neutrophils, increases mucus secretion, promotes the transient adhesion of eosinophils, prolongs the contact time between eosinophils and EC, and enhances the immune response in those with SA. In migration experiments, it was observed that pretreatment with LY294002(a PI3K inhibitor) and HCQ significantly decreases the migration of neutrophils toward IL-8 [11, 15]. This suggests that the suppression of autophagy could be considered an effective therapeutic approach for the management of corticosteroid-resistant neutrophilic asthma.

In comparison to other forms of asthma, paucigranulocytic asthma (PGA) is characterized by reduced levels of eosinophils and neutrophils in the airways, improved lung function, relatively low expression of inflammatory biomarkers in exhaled and induced sputum, and favorable responses to treatment [57, 58]. The remaining inflammatory cells, such as epithelial cells, macrophages, monocytes, dendritic cells, fibroblasts, mast cells, etc., are known to be involved in the initiation and propagation of this form of asthma.

Certain drugs can increase autophagy in medical conditions characterized by impaired autophagy. These drugs are potential treatment options for severe neutrophilic asthma. Rapamycin, everolimus and telrolimus activate autophagy by inhibiting mTORC1. In a study of mice with autophagy-deficient neutrophilic asthma, pretreatment with rapamycin resulted in the inhibition of Th17 cell polarization and downregulation of IL-17A expression, leading to a significant reduction in neutrophilic airway inflammation [85]. By targeting mTORC1, rapamycin has been shown to decrease the number of eosinophils and inhibit eosinophil differentiation. In vitro, this mechanism has been found to reduce cytokine levels, allergic airway inflammation, and mucus secretion, while promoting autophagy in vitro [86]. In contrast, Torin-1 inhibits mTORC1 and mTORC2 in vitro and facilitates eosinophil differentiation. Although there have been few clinical studies evaluating their efficacy in asthma treatment, rapamycin has been found that can restore steroid responsiveness involving IL-10 and IL-1$\beta$ to dexamethasone treatment on CD14++CD16+ monocytes from steroid-resistant asthma patients [87]. And low-dose therapy with mTORC1 inhibitors has been found safe for older patients. Additionally, it has been reported that such therapy improves immune function and reduces the incidence of infections in the elderly patients [88].

Cardiac glycosides, such as digoxin, digitalis, and ouabain, are widely used to treat heart disease, but also show potential for use in targeted cancer therapy. These compounds have been found to activate autophagy. In human non-small cell lung cancer (NSCLC) cells, cardiac glycosides induce autophagy by downregulating rapamycin (mTOR) signaling and activating extracellular signaling to regulate kinase 1/2 (ERK1/2) signaling [89]. Furthermore, they can restore autophagosome maturation in AECs and reverse defective autophagy fluxes [90]. However, under certain conditions, cardiac glycosides may exert inhibitory effects on autophagy and autophagic cell death by inhibiting sodium-potassium pump (Na+/K+-ATPase) activity. For instance, in situations such as starvation, autophagy-inducing peptide therapy, and hypoxic-ischemic injury of the neonatal hippocampus [91].

Statins such as atorvastatin and simvastatin, have shown promise for improved asthma control and reduced asthma exacerbations [92]. Specifically, simvastatin has been found to activate autophagy in bronchial smooth muscle cells (BSMCs) in mouse models of asthma, leading to the inhibition of airway inflammation and airway remodeling [93]. Additionally, short-term combination therapy with atorvastatin and inhaled corticosteroids has been shown to reduce inflammatory sputum mediators in smokers with mild to moderate asthma who do not respond adequately to inhaled corticosteroids alone [94]. Recent studies indicate that it is capable of controlling inflammatory responses in asthma by inhibiting macrophage autophagy and promoting the production of IL-10 [66]. Studies have found that mevalonate pathway regulates cell size homeostasis and proteostasis through autophagy involving the Rab GTPases. And statins, inhibitors of the mevalonate pathway, reduce cell proliferation and increase cell size and cellular protein density in various cell types [95].

Metformin, commonly used in the clinical treatment of diabetes and metabolic syndrome, has shown potential as a means of reducing asthma exacerbations and has been used in adult cohorts with comorbid asthma and diabetes mellitus [96]. In vivo studies using mice with sensitized asthma have demonstrated that metformin can effectively mitigate injury-induced airway inflammation and remodeling [97]. Metformin has been found to induce mitophagy by downregulating p53 and promoting Parkin activity. However, metformin also exerts various effects unrelated to AMPK, and the relationships between these effects and autophagy activation have not been confirmed. Carbamazepine is another medication that activates autophagy. This is clinically used for the treatment of seizures and bipolar disorder. It activates autophagy by inhibiting phosphoinositide 3-kinase (PI3K) signaling. However, it also inhibits various neuronal functions [98], and, to date, there have been no clinical trials on the treatment of asthma with carbamazepine.

Certain drugs have the ability to reduce autophagy in diseases, where it is excessive. These are potential candidates for eosinophilic asthma treatments. CQ, an antimalarial drug, inhibits autophagy by impeding the fusion of autophagosomes and lysosomes. It is important to note that CQ treatment does not disrupt lysosomal acidity. Conversely, the autophagy flux inhibitor HCQ elevates lysosomal pH, blocks lysosomal acidification, and inhibits autophagosome-lysosomal fusion, resulting in the accumulation of autophagic components [99]. However, CQ and HCQ have various additional effects, including disruptions to the Golgi apparatus and lysosomal network [100, 101]. Furthermore, the anticancer effects of these drugs have not been consistently observed in certain clinical trials, suggesting that their efficacy may not be solely dependent on autophagy modulation.

Baf-A1 is a macrolide antibiotic that inhibits the acidification of autolysosomes by targeting vacuolar H+-V-ATPase, effectively blocking late-stage autophagy [102, 103]. Another macrolide antibiotic, azithromycin, has been used in the long-term treatment of chronic inflammatory lung disease; however, this application carries a risk of infection with nontuberculous mycobacteria (NTM). This is attributed to the antibiotic’s ability to hinder lysosomal acidification, impede autophagosome clearance, impair autophagy, and hinder phagosome degradation [104]. However, research has shown long-term, low-dose macrolide antibiotic therapy to effectively reduce airway inflammation and steroid resistance [105]. Yet, this approach has some drawbacks, including the potential induction of drug resistance and increased susceptibility to mycobacterial infections [106].

In asthmatic patients, the inhibition of autophagy and the induction of IL-10 have been observed in alveolar macrophages treated with 3-methyladenine (3-MA) [66]. This inhibition is achieved by the suppression of autophagosome formation through the inhibition of class III PI3Ks [107]. Moreover, 3-MA has also been shown to reduce oxidative stress and lung inflammation by inhibiting the formation of EETs [14, 27, 80, 108]. But, 3-MA is also associated with cytotoxic effects and DNA damage, resulting in reduced cell viability across various cell lines [109]. Melatonin has been investigated in clinical trials as a potential therapeutic agent for various diseases. It has been shown to protect against ischemia-reperfusion (I/R) damage in rat brains by inhibiting autophagy and activating the PI3K/Akt survival pathway. However, it should be noted that melatonin can interfere with ROS-sensitive processes [110]. Furthermore, it has been observed that several current asthma medications, such as dexamethasone, montelukast, anti-IL-5, and anti-IgE antibodies, also inhibit autophagy [107]. Specifically, dexamethasone significantly decreases IL-5-induced autophagy in the peripheral blood eosinophils of patients with NSA, with less pronounced effects in patients with SA [11].

More specific inhibitors that target the autophagy pathway are currently under preclinical development. These include the lysosomal inhibitor Lys05, inhibitors of mitochondrial autophagy, ULK1 inhibitors (MRT67307, MRT68921, and SBI-0206965), and VPS34 selective inhibitors (SAR405, LY294002, VPS34-IN1, and Wortmannin) [109]. A recent study proposed that the compromised glucose metabolism and impaired cytokine secretion observed in ATG5-deficient ILC2s can be restored by treatment with exogenous pyruvate or supplementing with free fatty acid supplements. These interventions effectively target ILC2-dependent inflammation [20].

In 2020, whole human anti-IL-5 antibodies and anti-IL-5 receptor antibodies were utilized in the management of steroid-refractory asthma. These antibodies specifically target the autophagy-mediated activation pathways of eosinophils, reducing LC3 levels and the eosinophil count in bronchoalveolar lavage fluid (BALF) [80]. Treatment with anti-IL-5 monoclonal antibodies was found to improve AHR [111]. Early-stage human studies have demonstrated that IL-1$\beta$ receptor antagonists, such as anakinra, can alleviate neutrophilic airway inflammation. These antagonists can serve as a rescue treatment for acute asthma attacks [112]. Research has shown that the NLRP3 inhibitor MCC950 effectively inhibits IL-1$\beta$ and is an efficacious treatment for mouse models of severe steroid-resistant neutrophil-allergic airway disease [113]. Whole human anti-IL-4 and anti-IL-13 monoclonal antibodies have progressed to phase III clinical trials, while anti-IL-17 monoclonal antibodies are currently in phase II clinical trials. However, existing research did not find anti-IL-17R antibodies, such as brodalumab to yield therapeutic effects in individuals with asthma [114]. Conversely, anti-IL-6R antibodies, such as tocilizumab have been effectively employed as an adjunctive treatment in children suffering from severe persistent non-atopic asthma [115]. Omalizumab is a monoclonal antibody designed to treat allergic asthma patients who display elevated serum IgE levels. It has a proven ability to prevent persistent severe allergic asthma exacerbations and to reduce the need to utilize systemic corticosteroids in asthmatics, regardless there blood eosinophil count [116].

Dupilumab, a recently developed monoclonal antibody, functions by blocking the signaling pathways of both IL-4 and IL-13. Clinical evidence suggests that this antibody is highly effective in managing severe type 2 (T2) asthma and can help prevent acute exacerbations [117]. In patients aged 12 months and older with SA, treatment with tezepelumab, a humanized TSLP IgG2 antibody, has also demonstrated significant clinical benefits, including improved lung function and reduced frequency of exacerbations. However, further validation by larger phase III clinical trials is required [118]. Cahill et al. [119] have demonstrated that imatinib treatment effectively reduces AHR, mast cell count, and the release of tryptase (a marker of mast cell activation). Maun et al. [120] found that treatment with allosteric anti-tryptase antibodies can effectively decrease the incidence of anaphylaxis. Studies with murine models of asthma have found anti-nerve growth factor (anti-NGF) antibodies to effectively reduce Th2 airway inflammation through the downregulation of lung autophagy levels [121], while oral astragalus supplementation has been found to alleviate ROS-facilitated bronchial fibrosis by inhibiting the formation of autophagosomes [122]. Luteolin is known for its anti-inflammatory, anti-allergic, and immune-enhancing properties, and has recently been found to activate the PI3K/Akt/mTOR signaling pathway, inhibit autophagy, promote cell survival, improve lung function, and alleviate airway inflammation in mice with allergic asthma. Luteolin has no effect at low doses [123].

As a novel therapeutic strategy, biologics can target specific phenotypic or endotypic mechanisms of asthma pathophysiology mediated by autophagy, and precisely modulate the intermediate pathways of immune response in inflammation. This approach significantly improves the therapeutic efficacy of refractory and severe asthma patients. Therefore, it is necessary to explore and validate more new biologics in clinical practice to achieve personalized asthma treatment.