The in vivo role of endogenous tumor necrosis factor alpha (TNF-alpha) and reactive nitrogen intermediates (RNIs) in modulation of growth of Legionella pneumophila in the lung was assessed using a murine model of replicative L. pneumophila lung infection. Intratracheal inoculation of mice with L. pneumophila resulted in induction of endogenous TNF-alpha, which preceded clearance of L. pneumophila from the lung. Inhibition of endogenous TNF-alpha activity, via in vivo administration of TNF-alpha neutralizing antibody, or inhibition of endogenous RNIs, via administration of the nitric oxide (NO) synthetase inhibitor N-monomethyl-L-arginine (NMMA), resulted in enhanced growth of L. pneumophila in the lung at > or = 3 days postinfection (when compared with untreated L. pneumophila-infected mice). Because of the similar kinetics of enhanced pulmonary growth of L. pneumophila in mice treated in vivo with either anti-TNF-alpha antibody or NMMA, the immunomodulatory effect of NO on endogenous TNF-alpha activity in the lung was assessed. Administration of NMMA to L. pneumophila-infected mice resulted in a significant decrease in endogenous TNF-alpha activity in the lung during replicative L. pneumophila infections in vivo. However, administration of exogenous TNF-alpha to NMMA-treated mice failed to significantly enhance clearance of L. pneumophila from the lung. Results of these studies indicate that both endogenous NO and TNF-alpha facilitate resolution of replicative L. pneumophila lung infections and that regulation of L. pneumophila replication by TNF-alpha is mediated, at least in part, by NO.