Low levels of functional of alpha 1-proteinase inhibitor in the promotion of C3-cleavage by granulocyte-neutral proteases in pleural empyema.

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 alpha 1-proteinase inhibitor (alpha 1-PI), as determined by trypsin-inhibitory capacity (56.2 +/- 20.1 IU/ml; rs = -0.97, P less than .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, alpha 1-PI and alpha 2-macroglobulin protein concentrations in infected and sterile effusions (as measured by immunodiffusion) 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 alpha 1-PI in vitro. These results show that in empyemas functional levels of alpha 1-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 alpha 1-PI inactivation.