Epithelial alarmins TSLP, IL-33, and IL-25 in asthma pathogenesis: mechanistic roles and therapeutic implications

Asthma is a heterogeneous chronic inflammatory airway disease affecting over 300 million people worldwide. The airway epithelium serves as the primary interface with environmental stimuli and responds by releasing epithelial-derived alarmins, thymic stromal lymphopoietin (TSLP), interleukin-33 (IL-33), and interleukin-25 (IL-25), which act as upstream regulators of both innate and adaptive immune responses. Current therapies targeting downstream inflammatory mediators demonstrate limited efficacy in severe and refractory asthma, necessitating novel approaches targeting upstream pathways.

This narrative review examines the mechanistic roles of epithelial alarmins TSLP, IL-33, and IL-25 in asthma pathogenesis, covering their regulation, receptor signalling, downstream immune activation, and contributions to airway remodelling. Current and emerging biologic therapies targeting these alarmins are summarised, with key clinical trials tabulated.

A comprehensive narrative literature review was conducted. Electronic databases searched included PubMed/MEDLINE, Scopus, Web of Science, and ClinicalTrials.gov, covering publications from January 2015 to March 2025, with seminal earlier studies included where relevant. Search terms used included: "TSLP asthma," "IL-33 asthma," "IL-25 asthma," "epithelial alarmins," "thymic stromal lymphopoietin," "tezepelumab," "itepekimab," "astegolimab," "airway inflammation," "ILC2," "airway remodelling," and "biologic therapy severe asthma." Priority was given to original research articles, randomised controlled trials, and systematic reviews; review articles were included to provide mechanistic context. Studies were selected based on relevance to alarmin biology, immunological pathways, and clinical evidence in asthma.

TSLP: Functions as a central amplifier of type-2 and mixed inflammatory responses through transcriptional regulation following epithelial stimulation. Activates dendritic cells, group 2 innate lymphoid cells (ILC2s), and granulocytes via the TSLPR/IL-7Rα complex, driving both eosinophilic and non-eosinophilic inflammation. Tezepelumab, an anti-TSLP monoclonal antibody, has achieved regulatory approval (FDA/EMA) for severe asthma with demonstrated exacerbation reduction across multiple endotypes (40-60% reduction in clinical trials). IL-33: Constitutively stored in epithelial cell nuclei, acts as an immediate danger signal upon cellular injury, triggering rapid innate immune activation through the ST2/IL-1RAcP receptor complex. Mediates acute exacerbations, airway hyperresponsiveness, and steroid-refractory inflammation. Anti-IL-33 agents including itepekimab, astegolimab, and tozorakimab are in Phase 2-3 clinical development with promising early efficacy data. IL-25: Predominantly produced by epithelial tuft cells, sustains chronic type-2 immunity, mucus hypersecretion, and corticosteroid-insensitive disease phenotypes through IL-17RA/IL-17RB receptor engagement. IL-25-targeted approaches demonstrate preclinical efficacy and are advancing toward clinical evaluation.

Epithelial alarmins represent critical upstream drivers of asthma pathogenesis, orchestrating diverse inflammatory pathways that determine disease endotypes and severity. Therapeutic targeting of TSLP, IL-33, and IL-25 offers a paradigm shift from downstream cytokine inhibition to source-directed intervention, with potential to reduce exacerbations, prevent airway remodelling, and improve disease control across different asthma phenotypes. The success of anti-TSLP therapy and ongoing clinical development of IL-33 and IL-25 inhibitors validate epithelial alarmins as essential molecular targets for precision medicine approaches in severe and refractory asthma.