A specific proteinase 3 activity footprint in α1-antitrypsin deficiency

α1-Antitrypsin (α1-AT) deficiency is a risk factor for emphysema due to tissue damage by serine proteases. Neutrophil elastase (NE) has long been considered the enzyme responsible. However, proteinase 3 (PR3) also produces the pathological features of chronic obstructive pulmonary disease (COPD), is present in the same granules in the neutrophil and is inhibited after NE. We developed a specific footprint assay for PR3 activity and assessed its relationship to an NE footprint in α1-AT deficiency. An ELISA was developed for the specific PR3 fibrinogen cleavage site Aα-Val541. Levels were measured in plasma from 239 PiZZ patients, 94 PiSZ patients, 53 nondeficient healthy smokers and 78 individuals with usual COPD. Subjects underwent extensive demographic characterisation including full lung function and lung computed tomography scanning. Aα-Val541 was greater than the NE footprint in all cohorts, consistent with differential activity. Values were highest in the PiZZ α1-AT-deficient patients and correlated with the NE marker Aα-Val360, but were ∼17 times higher than for the NE footprint, consistent with a greater potential contribution to lung damage. Aα-Val541 was related cross-sectionally to the severity of lung disease (forced expiratory volume in 1 s % pred: rs=−0.284; p<0.001) and was sensitive to augmentation therapy, falling from 287.2 to 48.6 nM ( p<0.001). An in vivo plasma footprint of PR3 activity is present in greater quantities than an NE footprint in patients with α1-AT deficiency, is sensitive to augmentation therapy and represents a likely biomarker for dose-ranging studies.