The IL-33/ST2 Axis Promotes Post-COVID-19 Pulmonary Fibrosis Through β-catenin/PPAR-γ-mediated Epithelial-Mesenchymal Transition

Background Post-COVID-19 pulmonary fibrosis (PCPF) presents an increasingly significant public health challenge. Although fibrotic mechanisms in PCPF are extensively studied, their exact pathogenesis remains unresolved. Interleukin-33 (IL-33), a critical alarmin in viral infections, including COVID-19, has been reported to drive fibrotic processes across multiple organs, yet its role in PCPF remains undefined. This study aims to investigate the specific contribution of IL-33 to PCPF. Methods Serum cytokine profiles (IL-1β, IL-6, IL-8, IL-10, IL-33, ST2) were analyzed across healthy controls, COVID-19 patients, and PCPF cohorts. In vitro , SARS-CoV-2 spike protein was applied to A549 alveolar epithelial cells to model viral infection, observing its impact on IL-33/ST2 signaling and epithelial-mesenchymal transition (EMT). Morphological changes and migration capacity were evaluated. Functional validation was performed using exogenous recombinant human IL-33 (rhIL-33), IL-33-overexpressing, and IL-33-knockdown A549 models to determine the role of IL-33 in EMT and fibrotic phenotypes. Mechanistic studies employed PPAR-γ agonism (rosiglitazone) in conjunction with spike stimulation to assess the regulation of the pathway. Results Serum levels of IL-33 and ST2 were remarkably elevated in PCPF patients compared to control groups and COVID-19 patients. In A549 cells, treatment with Spike protein (1000 ng/mL, 48 h) upregulated both extracellular and intracellular IL-33 and significantly increased the expression of IL-33 and ST2 at both mRNA and protein levels. Spike protein treatment further induced a spindle-shaped morphology, enhanced cell migration, and promoted EMT, as evidenced by decreased E-Cadherin and increased N-Cadherin, Vimentin, and α-SMA expression. Similar pro-EMT phenotype and migratory effects were observed upon exogenous rIL-33 administration or IL-33 overexpression. Conversely, IL-33 knockdown attenuated Spike protein-induced alterations in EMT markers. Mechanistically, Spike protein upregulated β-catenin while suppressing PPAR-γ protein expression—effects reversed by IL-33 knockdown. Co-treatment with rosiglitazone partially inhibited Spike protein-induced upregulation of N-Cadherin, Vimentin, and α-SMA and restored E-Cadherin expression. Conclusions Our findings demonstrate remarkable upregulation of IL-33 in PCPF. SARS-CoV-2 Spike protein drives alveolar EMT via IL-33/ST2-dependent β-catenin activation and PPAR-γ suppression, unveiling novel therapeutic targets for PCPF intervention.