Bacterial species-dependent inhibition of human granulocyte elastase

Lung infection with Streptococcus pneumoniae causes the recruitment of many granulocytes (PMN) to the alveoli, but little damage to lung structure ensues, as opposed to infections with other bacterial species that induce a similar PMN response and cause lung damage. Elastase, a proteolytic enzyme of PMN, has been implicated as an agent of lung injury. We studied the interaction of different bacterial species with human PMN in vitro to determine if PMN elastase activity is affected by the species of bacteria ingested. Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella pneumoniae, and S. pneumoniae were grown, opsonized, and incubated with human PMN. After 1 h of incubation, intracellular and extracellular elastase activity was measured. S. pneumoniae reduced PMN elastase activity by 48%, whereas the other 3 species tested had only minimal effects on elastase activity. Loss of elastase activity occurred with S. pneumoniae: PMN ratios as low as 2:1. Adherence or ingestion of bacteria by the PMN was necessary for the decrease in elastase activity to occur; interventions that decreased phagocytosis, such as not opsonizing the bacteria, pretreatment of the PMN with cytochalasin B, and separation of bacteria from PMN by 0.22-mu filters, increased elastase activity. ELISA and Western blot analysis of elastase levels in these experiments suggested that normal amounts of elastase were present. Thus, the loss of elastase activity we observed may be due to an elastase inhibitor present in S. pneumoniae.