Lung cancer, like most other cancers, primarily arise due to the imbalance of cell proliferation and apoptosis caused by gene mutations, mainly due to the increased expression of C-myc, N-myc, and L-myc belonging to the myc gene family (6–8). There are two main subtypes of lung cancer, with small cell lung cancer (SCLC) accounting for 15% and non-small cell lung cancer (NSCLC) accounting for 85% of all the diagnosed cases (9). It has been reported that deletions or mutations of genes such as p53 and RB1 were mainly observed in SCLC (10, 11). As for NSCLC, mutations in epidermal growth factor receptor (EGFR), kirsten rat sarcoma viral oncogene (KRAS), mesenchymal-epithelial transition factor (MET) and other genes have been mainly observed (12).

There are several main treatment options for NSCLC. Lobectomy, which removes a single lobe, is considered to be the optimal procedure for early-stage NSCLC. And the survival is generally higher with lobectomy than with limited resection (13, 14). Administration of chemotherapy before surgery, which is also known as neoadjuvant chemotherapy, has the potential of inhibiting micrometastases, downstaging of the tumor as well as improving tolerability compared with the adjuvant approach. According to existing results, neoadjuvant chemotherapy is feasible and immunotherapies may be an optimal treatment strategy for early‐stage NSCLC in the neoadjuvant setting (15). On the contrary, adjuvant chemotherapy that administrates chemotherapy after surgery has been reported to help prolong the survival time of patients receiving surgical resection. Targeted therapy, which includes EGFR inhibitors or antibodies as well as vascular endothelial growth factor receptor (VEGFR) inhibitors, is another option for NSCLC patients (16). Immunotherapy has dramatically changed the landscape of treatment of NSCLC. To limit damage to healthy cells, the process of the immune system recognizing and destroying neoplastic cells is highly regulated by an equilibrium of activating and inhibitory pathways (17). One of these pathways is the programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) axis. The PD-1 is a transmembrane receptor that is expressed in a variety of tissues. Its ligand, PD-L1, is expressed in T cells, and this interaction leads to the inactivation of T cells. Malignancies can overexpress PD-L1 as a mechanism of defense against the host’s immune system (16). Immune checkpoint inhibitors (ICIs) such as PD-1 or PD-L1 blockades have been approved for the treatment of NSCLC, FDA-approved immunotherapeutic drugs and their targets are listed in ( Table 1 ) (3).

As for SCLC, no major advances in its treatment have occurred over the past 30 years (18). The first‐line treatment in SCLC recommended by the United States and Europe is 4‐6 cycles of etoposide plus cisplatin or carboplatin and the FDA has approved the PD‐L1 inhibitor atezolizumab in combination with carboplatin and etoposide as a first‐line therapy for SCLC (19). Concurrent chemoradiotherapy is the standard of care in limited‐stage disease (LD) SCLC, but the optimal radiotherapy schedule and dose remains unclear (19). Surgery is an option for LD SCLC patients that have gone through mediastinal sample collection and diagnostic computed tomography analysis (20). Prophylactic cranial irradiation could reduce the incidence of brain metastasis in both limited‐ and extensive‐stage patients, but the patients would have lower quality of life due to the toxicity of the therapy (21–23). Palliative care on extensive-stage disease (ED) SCLC is expected to prolong the survival time, improve quality of life, as well as minimize the risk of symptoms associated with disease.

Surgery is a standard option for early-stage NSCLC patients. The pathologic complete response rate in a survey of NSCLC patients undergoing surgery was reported to be 60% (24). As for radiotherapy, a clinical survey of patients with stage I NSCLC who received stereotactic radiotherapy (SRT), the median overall survival was 2.8 years, and the 1- and 2-year survival rates were observed to be 81% and 64%, respectively (25). In a survey of NSCLC patients receiving chemotherapy, the objective response rate was 32% (26), which was much lower compared with other treatments. As chemotherapy alone is not as effective as other treatments, it is usually combined with other therapies for lung cancer. Unlike chemotherapy, targeted therapy significantly helps improve patient survival. For instance, the median progression-free survival (PFS) of patients with stage IIIB/IV lung adenocarcinoma treated with afatinib was 13.6 months (27). Additionally, a meta-analysis summarized 13 phase 3 trials in which an EGFR TKI was compared with platinum-based chemotherapy, PFS was significantly prolonged, whereas no effect on overall survival was observed (28). Immunotherapy combined with chemotherapy showed impressive potential in lung cancer treatment. A study which analyzed the efficacy of Pembrolizumab (Pembro) and Atezolizumab (Atezo) combined with carboplatin and nab-paclitaxel (CnP) as first-line treatment of advanced squamous NSCLC indicated that the overall survival and progression-free survival of treatment Pembro + CP/CnP reached 15.9 months and 6.4 months, and Atezo + CnP reached 14.0 months and 6.3 months (29). These findings show that immunotherapy is quite effective, so it makes sense to seek new immunotherapy for lung cancer treatment.

IL-33 protein consists of three distinct functional domains; nuclear domain, central domain and IL-1-like cytokine domain (30). Such a structure makes IL-33 a dual-type protein, which can act both as a pro-inflammatory cytokine and a nuclear factor capable of regulating transcription. Under basal conditions, the chromatin-binding motifs can localize IL-33 protein into the nucleus by interacting with histone H2A-H2B dimers and bind IL-33 to chromatin (31). The central domain of IL-33 contains protease cleavage sites that are sensitive to neutrophil-derived and mast-cell-derived proteases (32, 33). The IL-1-like cytokine domain of IL-33 binds to ST2 on target cells and mediates cytokine activity (34, 35).

In human lung, IL-33 is constitutively and abundantly expressed in endothelial cells, epithelial cells, fibroblasts, myofibroblasts, mesenchymal as well as smooth muscle cells (1). Epithelial progenitor cells were reported to express IL-33 during chronic obstructive pulmonary disease (36). And in lung inflammation such as asthma, IL-33 expression increased in epithelial cells and airway smooth muscle cells (37, 38). Serum level of IL-33 in lung cancer patients was reported similar, higher or lower compared with that in healthy volunteers in different studies (39–41), reflecting that the role of IL-33 in lung cancer is complex.

As a nuclear factor, IL-33 can effectively bind to the chromatin to inhibit the various inflammatory responses. As a cytokine, given the right combination of signals and exposure to the cellular damage, stored IL-33 is released to interact with its receptor ST2, thus triggering danger-associated responses and act as a cellular “alarmin” (42). Constitutive expression of preformed IL-33 in the endothelium and epithelial barriers allows it to respond rapidly to damage caused by viruses or parasites, allergens or intrinsic injury (1). The full-length IL-33_1-270 in the nucleus can thereafter inhibit the inflammatory response after interacting with histones, but when the cell is damaged or necrotic, it is sheared to generate a series of active fragments that can be released outside the cell to promote inflammatory responses, activate the immune system, and thereby provide damage signal (32, 33, 43). Although the mechanism remains unclear, it has been reported that ATP, uric acid, oxidative stress as well as protease-activated receptor protein activation lead to IL-33 secretion (44–46), which involves an increase in intracellular calcium and/or P2 purinergic receptor (47).

IL-33/ST2 signaling causes phosphorylation of GATA3, which recruits more GATA3 and RNA polymerase II to the Foxp3 promoter. GATA3 binds to the ST2 promoter, enhancing ST2 on the surface Tregs, thus promoting Treg function through enhancing TGF-β1-mediated differentiation (48). A study reported that peroxisome proliferator-activated receptor gamma (PPARγ) is involved in the interaction between IL-33 and group 2 innate lymphoid cells (ILC2) (49). By activating ornithine decarboxylase, which catalyses the synthesis of polyamines, IL-33 induces M2-like macrophage polarization (50). IL-33 is also able to induce the gene expression of Th2-associated cytokines such as IL-5 and IL-13 (34).

Suppression of tumorigenicity-2 (ST2) is divided into four different subtypes: sST2, ST2L, ST2V and ST2LV. Among them, sST2 possesses no transmembrane sequence and can be secreted outside the cell. It is a soluble ST2 and is only expressed in mast cells and fibroblasts. ST2L has a transmembrane sequence and cannot be secreted outside the cell. It is a transmembrane ST2 and is primarily expressed in fibroblasts, mast cells, eosinophils, DCs, NK cells, NKT cells, helper T cell 2 (Th2) lymphocytes, and activated macrophages (51). ST2V is characterized by the absence of an immunoglobulin-like motif and alternative splicing of the C-terminal portion of ST2 inserting a new exon (52). ST2LV is a new secreted soluble and N-glycosylated variant of the ST2 gene product (53). ST2 primarily exists as a splice variant in two distinct forms: sST2 acts as a decoy receptor to sequester free IL-33 and blocks IL-33 signal, whereas ST2L can activate the MyD88/NF-κB signaling pathway, consequently enhancing the function of Th2 lymphocytes, Tregs, mast cells and ILC2s (54).

The mechanism of action of IL-33/ST2 pathway can be summarized as follows (55): (A) The action of IL-33 signaling. IL-33 binds to ST2L and leads to the recruitment of IL1-RAcP (56). ST2L interact with IL1-RAcP to facilitate the recruitment of MyD88, IRAK1, IRAK4, and TRAF6. These molecules activate NF-κB, JNK, ERK, and p38, leading to the expression of different gene-encoding cytokines, chemokines, and growth factors. (B). The attenuation of IL-33 signaling. Single-immunoglobulin interleukin-1 receptor-related molecule (SIGIRR) is able to disrupt the ST2L/IL1RAcP heterodimer. Extracellular IL-33 can be sequestered by sST2 acting as a molecular decoy.

On one hand, binding of IL-33 to ST2L can promote the tumor cell growth, proliferation and metastasis by inducing Th2-type cytokines and promoting the accumulation of immunosuppressive cells in tumors (57). It is reported that Rab37 small G protein regulates exocytosis of sST2, resulting in the shift of macrophages phenotype from M2-like to M1-like, and thereby inhibiting tumor growth (58). On the other hand, it leads to an inflammatory gene transcription and ultimately to the production of inflammatory cytokines/chemokines and mounting of an adequate immune response (59). IL-33 signaling enhances CD8⁺ T cell activity response to inhibit the tumor growth (60). It is also reported that IL-33 in combination with IL-12 promoted IFN-γ production by both invariant natural killer T (iNKT) cell and NK cell, thus enhancing innate cellular immune responses (61). However, the specific role of IL-33/ST2 axis that might predominate in tumor remains to be further studied. Ongoing phase II clinical studies (NCT03267316 and NCT05116891) on IL-1RAcP antibody CAN04, which showed the potency of selective cytokine signaling inhibition of IL-1 family cytokines in NSCLC (62), may hopefully find a new method to treat lung cancer.

Lung cancer is a common intrapulmonary complication found in patients with pulmonary fibrosis, so the mechanisms that can promote pulmonary fibrosis are likely to simultaneously contribute to the development of lung cancer. For instance, it is reported that the interaction between IL-33 and ST2 on airway epithelial cells stimulated by house dust mite (HDM) under asthma conditions can promote the expression of cluster of differentiation 146 (CD146) and further amplify the inflammatory response, epithelial-mesenchymal transition (EMT) and airway remodeling (63). In pulmonary fibrosis, IL-33 was able to polarize M2 macrophages to produce IL-13 and TGF-β₁, and could induce ILC2 to produce IL-13 (64). TGF-β and IL-13 can induce myofibroblast differentiation and stimulate the production of extracellular matrix components, which contribute to pulmonary fibrosis (65, 66). Moreover, it is found that Akt1 (Akt is also known as protein kinase B) as well as Akt2 are involved in the IL-33-induced secretion of IL-13 by macrophage, while Akt2 also took part in IL-33-induced secretion of TGF-β (67, 68). Yi, X., et al. (69) reported that ubiquitin-specific protease 38 (USP 38) was able to cause the autophagic degradation of ST2, thereby inhibiting IL-33 signaling and attenuating bleomycin-induced pulmonary fibrosis. In summary, the current studies have established that IL-33 can play a pivotal role in promoting the process of pulmonary fibrosis. Thus, the application of IL-33 inhibitors or enhancement of mast cells and fibroblasts to produce the decoy receptor sST2 might significantly slow the progression of pulmonary fibrosis development, and inhibiting this process can help to reduce the possibility of pulmonary fibrosis patients developing lung cancer.

Akimoto, M., et al. (70) found that low-metastatic cells derived from Lewis lung carcinoma expressed functional ST2L while high-metastatic cells did not. Their study also found that recombinant IL-33 (rIL-33) increased the cell death in low-metastatic Lewis lung cancer cells under nutrient depletion and hypoxic/anoxic conditions. Interestingly, IL-1β-induced IL-33 was also involved in promoting the death of low-metastatic cells. ST2L-negative highly metastatic lung cancer cells were found to be insensitive to IL-33. IL-33 thus provides a selective pressure for ST2L-negative cells that might expand into regions adjacent to the tumor blood vessels following hypoxia-inducible factor 1 (HIF-1)-dependent translocation (71), thereby increasing the proportion of the malignant cells ( Figure 1 ). Moreover, another study by Babadi AS et al. (72) showed that IL-33-ST2 signaling resulted in programmed death of low-metastatic Lewis lung cancer cells, whereas high-metastatic cells were not substantially affected, which eventually facilitated tumor growth. Therefore, Akimot et al. concluded that anti-IL-33 antibody or sST2 may be useful for inhibiting malignant progression and alleviating lung cancer.

It is reported that up-regulating the expression level of IL-33 increased the malignant growth of lung cancer tumor cells in vitro. They also found that the tumor volume of lung cancer tumor cells transfected with IL-33 expression vector increased significantly in immunodeficient mice (73). Their research findings further indicated that the interaction between IL-33 and ST2 could up-regulate the level of glucose transporter type 1 (GLUT1) in the cell membrane, thereby enhancing the glucose uptake and glycolysis of lung cancer cells, and providing optimal energy support for the malignant growth of lung cancer cells. They concluded that enhancing IL-33/ST2 signaling was able to markedly promote the malignant growth of lung cancer cells, while down-regulating or blocking IL-33/ST2 signaling could inhibit this process.

Jiang, Y., et al. (74) found that IL-33 increased the secretion of the antimicrobial peptide LL-37 in macrophages. LL-37 cooperated with IL-33 to increase the phosphorylation of p38 MAPK as well as NF-κB p65 pathways, and augmented IL-6 and IL-1β secretion, thus resulting in the proliferation of lung cancer cells in vitro. Zhou, X. et al. (75) reported that IL-33 inhibited the expression of cell death-inducing p53 target protein 1 (CDIP1) by regulating the miR-128-3p/CDIP1 signaling pathway, thereby effectively reducing the apoptosis of NSCLC cells. Yang, Z. et al. (57) found that IL-33 treatment promoted the lung cancer cell migration as well as invasion and increased the expression of matrix metalloproteinase-2 (MMP2) and matrix metalloproteinase-9 (MMP9) in a ST2-dependent manner. The AKT signaling pathway was also found to be involved in this process. A study by Wang, K. et al. (76) further demonstrated that NSCLC-derived IL-33, which caused immune escape of the tumor cells, supported tumor growth. IL-33 exerted a positive effect on FoxP3 induction, thus increasing the frequency of Tregs in the tumor-infiltrating lymphocytes, which was partially dependent on M2 TAMs (57). Another study on Kras-mutant mice showed that treatment with anti-ST2 antibody could reduce Tregs in tumor, which led to restoration of NK cell activity and enhanced Th1 activity, with increased CD8⁺ T cell response (77). It is also reported that relying on IL-5-induced lung eosinophilia, IL-33-dependent ILC2 activation could suppress the metabolic fitness of NK cells ( Figure 2 ), which could be reversed by high glucose condition (78). In conclusion, IL-33 can potentially enhance the malignant growth ability of lung cancer tumor cells under specific conditions.

Li, (79) used human peripheral blood to confirm that IL-33 can promote the secretion of IFN-γ and GzmB in PBMCs in vitro, and further confirmed that IL-33 can enhance the cytotoxic effect of PBMCs on tumor cells and increase the activity of PMBCs to eliminate the lung cancer cells. They also found that IL-33 was able to downregulate PD-1 expression on CD3-positive and CD8-positive cell subsets in PMBCs, thereby improving the function of CD8⁺ T cells in immune responses and reducing T cell exhaustion, which was consistent with the findings of Liang, Y. et al. (80).

It is reported that IL-33 promoted the proliferation and activation of CD8⁺ T cells and NK cells through modulating the NF-κB signaling pathway, and promoted their infiltration in the tumor lung metastasis, thereby producing a significant effect of inhibiting tumor growth and lung metastasis (81). Qi, L. et al. (82) found that IL-33 was able to activate and recruit NK cells in a TNF-α-dependent manner to inhibit the development of lung metastases, which was consistent with the findings of previous studies. It is found that IL-33 induced cylindromatosis (CYLD)-activated DCs, thus inducing T cell proliferation and IL-17 secretion in lung adenocarcinoma and thereby enhanced antitumor effects (83). In addition, it is reported that IL-33 could promote the differentiation and maturation of DCs, which augmented the antitumor effect of CD8⁺ T cells and NK cells and inhibited the proliferation of lung cancer cells through affecting MyD88 pathway (84).

Moreover, mechanistic studies by Liang, Y. et al. (80) indicated that IL-33 treatment increased the expression of GLUT1 and increased the transcription of several glycolytic enzymes in CD8⁺ T cells. Their findings also suggested that IL-33 might promote CD8 effector T cell responses by activating mTORC1 and driving aerobic glycolysis. This mechanism might effectively help to inhibit both the tumor growth and metastasis.

In conclusion, IL-33 can inhibit lung tumor metastasis and tumor cell proliferation by increasing the proliferation and activation of immune cells and promoting their infiltration into various tissues.

Akimoto et al. (70) found that the expression level of IL-33 in the tumor tissue of lung cancer patients gradually decreased with an increase of the clinical grade of the cancer. It is reported that the expression of IL-33 in the tumor tissue samples of NSCLC patients significantly decreased compared with that in adjacent tissue (85). A study selected 125 NSCLC patients and found that patients with relatively higher expression of IL-33 in the paracancerous tissue were more likely to develop the poorly differentiated lung cancer and lymph node metastasis (86). Hu, L. et al. (87) analyzed 262 NSCLC patients, and found that the serum IL-33 level of the patients could be significantly higher than that of the healthy control. They also concluded that baseline serum IL-33 was an independent prognostic factor in NSCLC patients. However,Yang, M. et al. (88) found in a study of surgically resected lung cancer tissue and adjacent normal tissue specimens from 127 NSCLC patients that relatively higher IL-33 expression in the tumor was associated with longer survival in patients with NSCLC adenocarcinoma but not in squamous cell carcinoma. They also observed that in adenocarcinoma and squamous cell carcinoma, the expression level of IL-33 in tumor tissues was significantly lower as compared with its adjacent normal tissue, which was consistent with the findings of Feng, Y. et al. In conclusion, the concentration of IL-33 in the tumor was reported to be lower than that in the paracancerous tissue, and the high level of serum IL-33 in NSCLC patients was associated with tumor malignancy, but the high level of IL-33 in lung adenocarcinoma tumor tissue could predict longer survival time.

In a recent study, which used mouse model of squamous cell carcinoma, Taniguchi, S. et al. (89) found IL-33-TGF-β niche signaling loops between tumor-initiating cells (TICs) and FcϵRIα⁺ macrophages, thus indicating that IL-33 can aid to create a microenvironment that can promote the tumor growth, and whether this mechanism exists in lung cancer remains to be further investigated. In summary, if the IL-33-TGF-β niche signaling loop is also present in lung cancer, it will be necessary to investigate that why IL-33 levels were relatively lower in lung cancer tumor tissues. In addition, whether a specific mechanism allows the tumor cells to proliferate to a certain level before it stops releasing pro-proliferative substances by itself, thereby reducing the death of tumor cells caused by insufficient nutrient supply, and finally showing the phenomenon of stable growth of tumors in advanced lung cancer needs to be further analyzed.

Both EGFR and HER2 have been reported to be involved in autonomous cell proliferation and clonal expansion, accelerated intraepithelial proliferation, growth factor (GF)-induced basement membrane rupture as well as invasive growth, apoptosis evasion, and vascular growth in tumor progression (90). EGFR is frequently mutated and overexpressed in various cancer cells, especially non-small cell lung cancer. Mutated EGF tyrosine kinase receptor is constitutively expressed, thereby resulting in uncontrolled cell growth and proliferation. It has been established that by inhibiting this growth factor receptor, gefitinib can block the intracellular Ras signaling cascade, thereby suppressing the growth of the malignant cells (91). The U.S. FDA has already approved gefitinib for the treatment of NSCLC (3).

The effects of EGFR inhibition include impaired growth and migration of keratinocytes, as well as inflammatory chemokine expression by these cells (92). These effects lead to inflammatory cell recruitment and subsequent skin damage (93). A number of studies have also shown that transgenic mouse models overexpressing IL-31 develop severe pruritus with alopecia and skin lesions such as hyperkeratosis, acanthosis, inflammatory cell infiltration, as well as mastocytosis (94). The receptors for IL-31, IL-31RA, and the receptor for IL-33, ST2, are commonly expressed on dermal fibroblasts, and it is speculated that IL-31 and IL-33 can synergistically stimulate basophils that interact with fibroblasts to release atopic dermatitis (AD)-related chemokines (95). Combining the above research results, Sebastiano Gangemi et al. (96) based on the study of patients treated with gefitinib, proposed that EGFR-TK inhibitors can cause extensive keratinocyte damage, thereby resulting in the release of IL-33, which in turn can interact with the receptors on mast cells, thus stimulating the secretion of several cytokines, including IL-31, that can cause skin manifestations. We assume that downregulating the expression of IL-33 may alleviate the substantial skin damage caused by gefitinib treatment.

There are two distinct mechanisms of action of doxorubicin in cancer cells: DNA intercalation and destruction of topoisomerase-II-mediated DNA repair and generation of free radicals and their damage to cell membranes, DNA and proteins (97). Doxorubicin can be oxidized to semiquinone, an unstable metabolite, which in turn is converted to doxorubicin during the release of reactive oxygen species (ROS). ROS can cause extensive lipid peroxidation and membrane damage, DNA damage, oxidative stress, and trigger apoptotic pathways of cell death (98). It has been approved by the U.S. FDA for the treatment of both NSCLC and SCLC (3, 99).

Deng, Y. et al. (100) reported that ILC2 can expand and produce IL-4 immediately before activation of macrophages, dendritic cells and IL-4⁺ T after doxorubicin-induced myocardial necrosis. They speculated that in response to myocardial injury, cardiac ILC2s act as first-line responders and produce IL-4 to promote the responses during inflammation and cardiac tissue repair. While IL-33 can promote the production of IL-4 by Th2 cells (42), they proposed that IL-33-triggered ILC2 responses have the potential to alleviate cardiac injury from doxorubicin.

Among the kinases activated by ROS, ERK, p38 and JNK have been shown to play key roles in the process of oxidative apoptosis (101, 102). Yao, Y. et al. (103) found that doxorubicin could increase the phosphorylation of ERK, p38 and JNK, while IL-33 inhibited JNK activation. Moreover, other studies have also found that co-treatment with IL-33 did not affect ERK phosphorylation, but reduced the angiotensin II-induced p38 and JNK phosphorylation (104). In addition, targeted inhibition of JNK can attenuate doxorubicin-induced cardiomyocyte injury (105). Taken together, they proposed that IL-33 can protect the cardiomyocytes from doxorubicin-induced cardiomyocyte apoptosis by inhibiting the ASK1/JNK signaling pathway. Sabapathy, V. et al. (106) reported in a model of doxorubicin-induced nephrotoxic kidney injury that IL-233 (a molecule with the activity of IL-2 and IL-33) can potentiate Treg and ILC2 to inhibit renal injury while promoting repair. In conclusion, IL-33 combined with doxorubicin might reduce the cardiac damage and kidney injury observed during the lung cancer treatment.