Acute lung injury (ALI) is a common respiratory disease caused by multiple etiologies, and the main clinical manifestations are progressive dyspnea and acute onset of bilateral pulmonary infiltrates with hypoxemia [[1], [2], [3]]. The essential pathological feature of ALI is the excessive accumulation and activation of neutrophils in the lung tissue [4]. These neutrophils then release harmful substances such as reactive oxygen species (ROS) and myeloperoxidase (MPO), exacerbating the inflammation [5,6]. Even with treatment, ALI usually leads to severe respiratory failure, which causes an in-hospital mortality rates up to 30 %-40 % [7,8]. Therefore, effective therapeutic approaches are urgently needed to control the pulmonary inflammatory cascade to reduce the mortality and long-term morbidity.

Two primary causes of ALI are infectious and non-infectious factors which can induce direct or indirect damage to the lungs. ALI is also induced by indirect systemic diseases, including sepsis, bacterial pneumonia, and severe trauma [[9], [10], [11], [12]]. Currently, a series of drug studies against ALI have been conducted based on the pathological mechanism. In the early stage of ALI, the dominant pathogen can excessively activate the body's innate immune system, thus producing excessive inflammatory response. At this time anti-inflammatory drugs can be used to control the inflammatory response. Clinically used anti-inflammatory drugs for ALI include specific and non-specific anti-inflammatory drugs [13]. Oxidative stress plays an important role in the pathogenesis of ALI. Excessive production of ROS during ALI will aggravate the inflammatory process. The large amount of oxygen free radicals can lead to an imbalance between oxidation and anti-oxidation in the body of ALI patients [14].

The signal transducer and activator of transcription 3 (STAT3) is a member of the STAT protein family which plays an important role in cell homeostasis and signal transduction through cytokines, hormones, and growth factors. They participate in regulating biological processes such as cell proliferation, survival, differentiation, and immune regulation [15,16]. STAT proteins, especially STAT3, have become important targets in the treatment of cancer, infectious diseases and autoimmune diseases. Moreover, STAT3 signaling pathway is closely related to inflammatory responses and oxidative stress. Under normal circumstances, the activation of intracellular STAT3 is fast and tightly controlled. Phosphorylated STAT3 (p-STAT3) returns to the cytoplasm through nuclear pores after being dephosphorylated to complete the fast activation-inactivation cycle. The abnormal activation of STAT3 is associated with a variety of inflammatory diseases, which can be stimulated by inflammatory cytokines, such as interleukin-6 (IL-6) and transforming growth factor-β (TGF-β) [17]. Under inflammatory conditions, STAT3 can be phosphorylated by receptor-related kinases to form homodimers or heterodimers. The activation and high expression of STAT3 are also closely related to poor prognosis of inflammatory diseases [18,19].

In the lung, alveolar epithelial cells (AECs) which content of alveolar epithelial type I cells (AECIs) and alveolar epithelial type II cells (AECIIs) are crucial components of the respiratory system [20,21]. The inflammatory environment during ALI can cause apoptosis or necrosis of AECs, thereby reducing the production of pulmonary surfactant and destroying the stability of the alveolar barrier [22,23]. Disruption of the epithelial barrier increases alveolar cavity protein leakage and aggregation of inflammation, causing pulmonary oedema and consequently refractory hypoxemia [24]. Several studies have shown that the proliferation and reconstruction of AECIs are essential for maintaining the integrity of the epithelium, especially during the process of repair after lung injury [[25], [26], [27]]. AECIIs have important metabolic and biosynthetic functions. They can re-enter into the cell cycle and act as progenitors to replace old or damaged AECIs [[28], [29], [30]]. The concern is that AECIIs will lose their specific markers, upregulate multiple keratins and become transitional state cells which eventually differentiate into AECIs in the process of AECIIs transdifferentiating into AECIs [31]. Furthermore, the transdifferentiation of transitional state cells which are marked by keratin 8 (Krt8) into AECIs which are marked by aquaporin 5 (AQP5) has been shown to be the critical step for lung repair [32]. Fa Wang et al. showed that most AECs assumed the transitional state in mouse models of pulmonary fibrosis, which was maintained by inflammation and fibrosis [33]. However, the role of transitional state cells in ALI has not been elucidated and reported.

Mesenchymal stem cells (MSCs) are multilineage cells with remarkable capacity for self-renewal and the ability to differentiate into multiple cell types [34]. They have many biological functions such as promotion of tissue repair, anti-inflammation, immunosuppression and nerve protection [[35], [36], [37], [38]] and have been widely investigated for clinical applications in recent years [[39], [40], [41], [42], [43], [44]]. Moreover, MSCs-based therapy is also prospective in the treatment of lung disease including ALI [[45], [46], [47], [48]]. A number of clinical trials have been carried out to explore the use of MSCs in the treatment of ALI. However, the application of MSCs needs special requirements of storage at a temperature of −80 °C [49] and has risk of obstruction of small‐diameter pulmonary arteries as cells pass through after transplanted into body [50].

The therapeutic effect of MSCs transplantation therapy is mainly attributed to the paracrine mode [51]. Compared with MSCs, MSC exosomes have the advantages of lower tumorigenicity, lack of transmission of secondary infections and easy manipulation [[52], [53], [54], [55], [56]]. Exosomes, a subgroup of extracellular vesicles with a diameter of 30–150 nm, can be efficiently isolated from MSCs and have been reported to participate in intercellular communication, immune modulation, and tissue regeneration. In addition to their therapeutic potential in lung injury, exosomes play critical roles in various other diseases, particularly cancer, where they are involved in tumor progression, metastasis, drug resistance, and modulation of the tumor microenvironment [[57], [58], [59], [60], [61]]. Our previous study has revealed that MSC exosomes possess anti-oxidative activity and alleviate the pathological process of dry AMD induced by oxidative stress [62]. Moreover, substantial researches have shown that MSC exosomes ameliorate lung permeability, improve the metabolic function of alveolar macrophages, decrease inflammatory cell infiltration, downregulate proinflammatory mediators and upregulate anti-inflammatory mediators to reduce lung injury in ALI models [[63], [64], [65], [66]]. Due to the role of MSC exosomes in alleviating ALI and in the promotion of tissue repair, we speculated that the effect of MSC exosomes in ALI may be related to transitional state cells.

Key signaling pathways including NF-kB, Hedgehog, PI3K/Akt, MAPK and STAT3 [[67], [68], [69], [70], [71]] have been shown to be regulated by MSC exosomes in ALI models. Tao et al. found that BMSCs-derived exosomes can downregulate STAT3, thereby inhibiting macrophage pyroptosis and alleviating sepsis-associated ALI [71]. For the treatment of ALI, it is of great significance to clarify how MSC exosomes regulate STAT3 to promote the transdifferentiation of transitional state cells into AECIs via STAT3/Krt8/AQP5 axis in ALI.

The inhalation of nebulized drug has become the most important route in treating lung diseases [[72], [73], [74]]. Oral inhalation results in lung drug targeting and thereby reduces systemic side effects, making it the preferred means of drug delivery for the treatment of lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) or cystic fibrosis (CF) [75,76]. Elia Bari et al. proposed that the possibility to administer MSC secretome by inhalation would bring to profound clinical consequences [77]. However, the studies of producing inhalable MSC exosomes have not been carried out.

This study aims to explore the therapeutic potential of MSC exosomes as a nanotherapeutic strategy for ALI. By focusing on the regulation of transitional state cells and the promotion of lung epithelial regeneration, this study is expected to provide new theoretical insights and contribute to the development of effective treatments for ALI.