Pulmonary damage caused by a protease from Legionella pneumophila

Intranasal (or intratracheal) administration of a tissue-destructive protease from Legionella pneumophila to guinea-pigs produced areas of haemorrhagic pneumonia in the lungs after 1/2 h. By 24 h there was confluent consolidation in all lobes. Histological and ultrastructural studies showed alveolar haemorrhage, vesiculation and necrosis of type I alveolar epithelium and endothelium, followed by progressive exudation of oedema fluid, fibrin, PMN and macrophages. Damage to type II cell lamellated bodies and discharge of lamellar material were significant features of the lesion. Collagenase activity was indicated by morphological degradation of collagen fibres in severely affected interalveolar septa. The pathological changes of Legionnaires' disease pneumonia can thus be reproduced experimentally by the administration of a L. pneumophila tissue-destructive protease, suggesting that production of this protease in vivo during L. pneumophila infection may play an important role in causing the pneumonia.