BACKGROUND Cystic fibrosis (CF) is characterised by an excess of free proteinases that destroy lung tissue. Despite this, previous studies have shown that patients with CF with a mild deficiency variant of the proteinase inhibitor α₁-antitrypsin have less, rather than more, severe pulmonary disease. Alpha₁-antichymotrypsin is another important serine proteinase inhibitor that protects the lung against proteolytic attack, and point mutations in the α₁-antichymotrypsin gene that result in plasma deficiency are associated with chronic obstructive pulmonary disease.

RESULTS One hundred and fifty seven patients with CF were screened and 10 were identified with a plasma deficiency of α₁-antichymotrypsin (plasma concentration <0.2 g/l). In a multivariate analysis these individuals had significantly less severe lung disease than those who had normal or raised levels of α₁-antichymotrypsin: forced expiratory volume in one second (FEV₁) 69.9% predicted versus 53.2% predicted (p=0.04) and chest radiographic score of 7.2 versus 9.7 (p=0.03) for those with and without α₁-antichymotrypsin deficiency, respectively. The –15 signal peptide genotype did not affect plasma levels, but the –15 Ala/Ala signal peptide genotype was over-represented in individuals with CF compared with healthy blood donor controls.

CONCLUSION These data indicate that deficiency of α₁-antichymotrypsin is associated with less severe pulmonary disease in patients with CF, and support our previous observations that mild genetic deficiency of a proteinase inhibitor is associated with an improved outcome.