Anti-pneumolysin activity of commercially available α₁-antitrypsin is due to cholesterol impurities

Pneumolysin (PLY), the principal cytolytic toxin of Streptococcus pneumoniae, may be important in the pathogenesis of acute lung injury during pneumococcal pneumonia. However, the local host defenses that limit PLY injury to lung tissues have not been characterized. We investigated the ability of a commercial preparation of α₁-antitrypsin (α₁-AT), a major plasma anti-proteinase, to inhibit PLY. At normal plasma concentrations, the α₁-AT preparation prevented PLY injury to bovine pulmonary artery endothelial cells, rat alveolar epithelial cells, and human erythrocytes. The α₁-AT preparation selectively inhibited thiol-activated bacterial toxins; it was inactive against snake venom hemolysins, mastoparan, and oxygen-stable bacterial toxins. Biochemical characterization of the α₁-AT preparation and comparison with other available α₁-AT preparations revealed that this inhibitory activity was due to contamination with nanomolar concentrations of cholesterol. Characterization of non-immune human plasma anti-pneumolysin activity showed that β-lipoprotein fractions contain the major inhibitory activity. We caution other investigators that the inhibition of bacterial virulence by these α₁-AT preparations may indicate toxin-mediated, rather than protease-mediated, mechanisms.