L. pneumophila is an obligate or facultative intracellularpathogen.
The capability of the bacteria to multiply in macrophages and their ability to prevent phagolysosome fusion are the main determinants of virulence of the bacteria. L. pneumophilaorganisms possess many virulence factors, such as protease, hemolysin, and cytotoxin. Protease is an important virulence factor, which plays an important role in causing infection.
The bacteria infect human monocytes and alveolar macro-phages. The bacteria multiply in macrophages and mono-cytes by binding complement to an outer membrane porin protein and subsequent deposition of C3b, a component of the complement on the surface of the bacteria. This facilitates them to combine with CR3 complement receptors on phago-cytes and enter the cell through endocytosis. Once inside the cell, the bacteria are not killed because they prevent fusion of phagolysosome. Subsequently, the bacteria multiply in large numbers in the intracellular vacuoles and produce many enzymes, such as phosphatase, lipase, nuclease, and proteo-lytic enzymes. Subsequently they lyse and kill the infected host cell.
◗ Host immunity
Cell-mediated immunity plays an important role in the host immunity by preventing the intracellular growth of Legionella spp. Sensitized T cells stimulate the increased antimicrobial activity of parasitized macrophages. Humoral immunity plays a minor role.