ABSTRACT
Legionella Pneumophila, identified as the causative agent of Legionnaires’ diseases in 1976, is an aerobic, motile, Gram-negative bacterium and a facultative intracellular parasite. L. pneumophila infection of human mononuclear phagocytes is an excellent in vitro model for studies of host cell-pathogen interaction. The model offers many important advantages for laboratory investigation. L. pneumophila enters phagocytes, including alveolar macrophages, monocytes, and polymorphonuclear leukocytes, by a novel process termed “coiling phagocytosis”, in which phagocyte micropseudopods coil around the organism as it is internalized. Phagocytosis of L. pneumophila is mediated by complement receptors on the monocyte. Patients with Legionnaires’ disease develop a strong humoral immune response to L. pneumophila. Cell-mediated immunity, in contrast to humoral immunity, appears to play a major role in host defense against L. pneumophila as it does against other intracellular parasites. Interferon gamma, a potent macrophage-activating factor found in mitogen- and antigen-stimulated lymphocyte supernates, is one of the most important mediators of the effector arm of cell-mediated immunity.
