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: r_s= -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.