Sputum tryptase as a marker of airway mast cell activity in patients with asthma

Rationale Severe asthma affects approximately 10% of patients with asthma and is characterized by persistent airway inflammation. Mast cells contribute to asthma pathogenesis, but their role is not defined due to limited access to airway tissue. We evaluated tryptase in expectorated sputum as a non-invasive marker of mast cell activity and assessed its association with asthma phenotypes and clinical outcomes. Methods In a cross-sectional study, 111 adults with moderate-to-severe asthma and 20 healthy controls underwent sputum induction and cytometry based phenotyping. Sputum tryptase was quantified by enzyme-linked immunosorbent assay (ELISA) and validated using immunohistochemistry of formalin-fixed, paraffin-embedded (FFPE) sputum plugs. Associations with cytokines and clinical indices were analysed. Results Asthmatics had significantly higher sputum tryptase levels than healthy controls (median: 764 pg·mL −1 , 0 pg·mL −1 , p <0.0001). Elevated tryptase was observed across phenotypes with greatest prevalence in eosinophilic (59%) and paucigranulocytic (66%) asthmatics. Tryptase showed a weak positive association with sputum IL-13 (r=0.3, p =0.002) but not clinical indices. Tryptase strongly correlated with Tryptase Alpha-1, Beta-1 positive (TPSAB1+) mast cell counts in sputum plugs (r=0.88, p =0.015), supporting its detection in sputum. Conclusion Sputum tryptase is a measurable, minimally invasive marker of mast cell activity that is elevated across asthma phenotypes. A subset of patients (∼17%) without sputum eosinophilia exhibited elevated sputum tryptase, suggesting a role for mast cell associated pathways in this group. These findings support further investigation of mast cells in asthma and the potential role of sputum tryptase in identifying biologically distinct patient subgroups.