D1.173 - Sputum Tryptase Identifies Airway Mast Cell Activation Across Asthma Phenotypes

Mast cells are implicated in asthma pathogenesis, yet their contribution remains difficult to define because they are tissue-resident and not readily detected in airway samples. We evaluated sputum tryptase as a minimally invasive biomarker of mast cell activation and examined its relationship with asthma phenotypes, airway inflammation, and clinical outcomes.

Cell-free sputum supernatant from 111 adults with moderate-to-severe asthma and 20 healthy controls was analyzed for tryptase and T2 and non-T2 cytokines by ELISA. Mast cell and eosinophil-related gene expression (CPA3, CLC, CMA1, KITLG, TPSAB1, TPSB2) were quantified in sputum cells by digital PCR. Clinical assessments included lung function, airway hyperresponsiveness, FeNO, blood eosinophils, IgE, serum tryptase, and symptom control (ACQ-5). Sputum tryptase measurements were validated by immunohistochemistry of formalin-fixed paraffin-embedded sputum plugs.

Sputum tryptase levels were significantly higher in asthmatics compared with healthy controls (median 764 pg/mL; p<0.0001) and were detectable across inflammatory phenotypes. Elevated levels were most prevalent in eosinophilic (59%) and paucigranulocytic (66%) asthma. Tryptase correlated positively with sputum IL-13 (r=0.3, p=0.002) and strongly with TPSAB1+ mast cell counts in sputum plugs (r=0.88, p=0.015), confirming its airway origin. While not directly associated with overall asthma severity, elevated sputum tryptase was linked to poorer symptom control (ACQ-5 ≥1.5), particularly in patients without sputum eosinophilia. Quartile analysis showed eosinophilic patients predominated in the highest tryptase quartile, whereas paucigranulocytic patients clustered in the third quartile. Tryptase levels were highest among patients receiving oral corticosteroids. Gene expression analyses demonstrated significant phenotypic differences, including higher CLC expression in eosinophilic asthma.

Sputum tryptase is a reliable, non-invasive biomarker of mast cell activation in asthma. Mast cell activity occurs across phenotypes, including paucigranulocytic sputa, typically associated with IL-13, and may contribute to symptoms and airflow limitation.  Measuring sputum tryptase may help to identify mast cell-driven endotypes, refine patient stratification, and enhance biomarker-guided therapeutic targeting beyond traditional eosinophilic pathways.