Low Levels of Functional of 1-Proteinase Inhibitor in the Promotion of C3-Cleavage by Granulocyte-Neutral Proteases in Pleural Empyema

Abstract
It has been suggested that C3 breakdown by granulocyte-neutral proteases in pleural empyemas may be related to a decreased inhibitor potential for these enzymes. In the present study it was shown that in 17 infected pleural effusions, high proteolytic activity on 125I-labeled C3 (16.3% ± 4.4%) correlated with low functional levels of α1-proteinase inhibitor (α1-PI), as determined by trypsin-inhibitory capacity (56.2 ± 20.1 IU/ml; rs = − 0.97, P < .001), whereas in 18 sterile pleural effusions there was no such correlation (125I-labeled C3 cleavage, 2.2% ± .2%; trypsin-inhibitory capacity, 192.6 ± 26.7 IU/ml). However, α1-PI and α2-macroglobulin protein concentrations in infected and sterile effusions (as measured by immuodiffusion) were similar. Fifteen strains of three bacterial species — Streptococcus pneumoniae, Pseudomonas aeruginosa, and Proteus mirabilis — isolated from patients with pneumonia or empyema inactivated the elastase-inhibitory capacity of α1-PI in vitro. These results show that in empyemas functional levels of α1-PI were too low to inactivate granulocyte elastase and that some bacterial species may contribute to the low inhibitor potential of infected pleural fluid by direct α1-PI inactivation.