MMP-9, which is produced mainly by macrophages and neutrophils, is involved in airway and lung remodelling, with elevated levels reported in blood, sputum, and bronchoalveolar lavage fluid of patients with asthma.44, 45, 46 Cathepsin S which is a lysosomal cysteine protease plays an important role in the regulation of oxidant-induced airway hyperresponsiveness and neutrophil recruitment in mice47 and increased levels have been shown in allergic lung inflammation with a role in lung remodelling being proposed.48 Here, CTSS is linked to airway hyperresponsiveness.