The role of cytokines in the pathogenesis of experimental Legionella pneumophila infections

Legionnaires\u27 disease is an acute lobar pneumonia caused, primarily by the facultative intracellular pathogen Legionella pneumophila. This organism when inhaled by humans descends into the lower respiratory tract and parasitizes alveolar macrophages. L. pneumophila adhered to U-937 cells, A549 cells and peritoneal macrophages from A/J mice in an opsonin-independent fashion. Following attachment, the organism penetrated the cell membrane, replicated within these cells eventually inducing lysis. To better define the adhesion of L. pneumophila to host cells, an E. coli clone (LP 116), expressing the 25 kDa major outer membrane protein (MOMP) of L. pneumophila was used in binding studies. This MOMP-expressing clone showed a 5-fold increase in opsonin-independent binding to U-937 cells as compared to the parent E. coli strain and suggests that the 25 kDa MOMP of L. pneumophila functions as an adhesin. A major thrust of this study was to better define the cytokine response of the host during L. pneumophila infection. Challenge of macrophage-like U-937 cells with live, heat-killed or formalin-fixed L. pneumophila elicited the secretion of some cytokines while down-regulating others, suggesting that L. pneumophila modulates cytokine production in vitro. To develop a better understanding of the complexity of the immune response to L. pneumophila infection, an experimental model of Legionnaires\u27 disease in A/J mice was developed. A/J mice were challenged intratracheally with a sublethal dose of L. pneumophila to investigate bacterial replication and to characterize the progression of pneumonic disease. Challenge of mice with L. pneumophila resulted in an 8-fold increase in the numbers of L. pneumophila in the lungs within 24 h followed by gradual clearing of the organism. During infection with L. pneumophila, gross and histopathological evaluation indicated pneumonic infection. In addition, mice challenged with L. pneumophila produced both proinflammatory and immunoregulating cytokines as assayed from bronchoalveolar lavage fluids, lung homogenates and sera. This biphasic approach demonstrated that L. pneumophila elicits the expression of some cytokines while modifying the levels of other cytokines produced by macrophages. Furthermore, L. pneumophila induced a specific cytokine response in A/J mice following lung infection