Immunopathology of experimental Mycoplasma pneumoniae disease

Experimental Mycoplasma pneumoniae pneumonia in the Syrian hamster resembles natural disease of man and is characterized by peribronchial round cells as well as exudates in the air passages. The nature of these intralumenal exudates was analyzed to assess host-parasite interaction at the respiratory epithelial surface, where infection with the mycoplasma is concentrated. Cells recovered by tracheobronchial lavage revealed changing patterns during the course of experimental disease; free macrophages decreased, polymorphonuclear leukocytes increased, and rosettes of epithelial cells surrounded by leukocytes appeared. Studies on formation of the rosettes in vitro with M. pneumoniae, sera, and cells suggested that complement-dependent immune phenomena were involved. The cellular events were maximal by 2 weeks after inoculation of normal hamsters. When previously infected animals were challenged, the course of infection was shortened, peribronchial infiltrates appeared rapidly, and development of exudates was both exaggerated and accelerated. These findings suggest that leukocytes entering the pulmonary parenchyma and lumen participate in the immune response to M. pneumoniae and contribute to host protection.