ABSTRACT
Legionella pneumophila is a gram-negative bacterial pathogen and the causative agent of Legionnaires’ disease. Patients with Legionnaires’ disease respond to the infection with both humoral and cell-mediated immune responses. Humoral immunity appears to play a minor role in host defense. The organism is a facultative intracellular parasite that multiplies in human monocytes and alveolar macrophages. Under tissue culture conditions, and presumably in vivo, multiplication is exclusively intracellular. Activation of monocytes appears to have little influence on several features of phagosome biology. In both activated and nonactivated monocytes, L. pneumophila forms a specialized ribosome-lined phagosome, inhibits phago-some-lysosome fusion, and inhibits phagosome acidification. In the presence of complement, antibody promotes phagocytosis of L. pneumophila by human polymorphonuclear leukocytes, monocytes, and alveolar macrophages, but such phagocytes kill only a modest proportion of intracellular bacteria.
