EFFECTOR MECHANISMS RESPONSIBLE FOR GAMMA-INTERFERON-MEDIATED HOST-RESISTANCE TO LEGIONELLA-PNEUMOPHILA LUNG INFECTION - THE ROLE OF ENDOGENOUS NITRIC-OXIDE DIFFERS IN SUSCEPTIBLE AND RESISTANT MURINE HOSTS

To facilitate identification of the effector mechanism(s) responsible for gamma interferon (IFN-gamma)-mediated host resistance to Legionell a pneumophila, a murine model of legionellosis in BALB/c mice with a t argeted disruption in the IFN-gamma gene (gamma knockout [GKO] mice) w as developed, Immunocompetent BALB/c mice and GKO mice were inoculated intratracheally with virulent L. pneumophila (10(6) bacteria per mous e), and bacterial clearance and the pulmonary inflammatory response we re assessed. L. pneumophila did not replicate in, and was rapidly clea red from, the lungs of immunocompetent BALB/c mice, demonstrating that immunocompetent BALB/c mice are resistant to replicative L. pneumophi la pulmonary infections, In contrast, similarly infected GKO mice deve loped persistent, replicative intrapulmonary L. pneumophila infections with extrapulmonary dissemination of the bacteria to the spleen, Hist opathologic and flow cytometric analysis of L. pneumophila-infected lu ng tissue demonstrated that while immunocompetent BALB/c mice develop multifocal pneumonitis which resolves, similarly infected GKO mice dev elop diffuse pneumonitis with persistent neutrophil recruitment into t he lung, Intratracheal administration of exogenous IFN-gamma to L. pne umophila-infected GKO mice facilitated intrapulmonary clearance of the bacteria, confirming the pivotal role of IFN-gamma in innate host def enses to L. pneumophila lung infection In this murine host, The potent ial role of endogenous reactive nitrogen intermediates, including nitr ic oxide (NO), in IFN-gamma-mediated resistance to L. pneumophila pulm onary infections in immunocompetent BALB/c mice was subsequently asses sed, Macrophage inducible nitric oxide synthetase (an enzyme responsib le for the production of NO) was induced in alveolar cells from L. pne umophila-infected immunocompetent BALB/c mice (with maximal expression at 48 h postinfection) but was not induced in similarly infected GKO mice, However, administration of the NO synthetase inhibitor N-monomet hyl-L-arginine did not significantly inhibit clearance of L. pneumophi la from the lung of immunocompetent BALB/c mice (compared with that in similarly infected mice not administered N-monomethyl-L-arginine), In contrast, we have previously demonstrated that IFN-gamma-induced host resistance to replicative L. pneumophila lung infections in a suscept ible murine host (A/J mice) is mediated, in part, by endogenous NO. Ta ken together, these studies identify a differing role of endogenous NO in IFN-gamma-mediated resistance to L. pneumophila pulmonary infectio n in susceptible and resistant murine hosts.