CPA3 and asthma: CPA3 is a metalloexopeptidase specifically expressed in a particular subtype of mast cells in combination with tryptase.35 Expression of CPA3 has been associated with TH2‐high asthma in sputum and epithelial brushings of steroid naïve asthma.36, 37 Berthon et al15 reported reduction of CPA3 expression following treatment with oral corticosteroid, suggesting responsiveness to treatment.