Antibody-mediated depletion of tumor necrosis factor-alpha impairs pulmonary host defenses to Legionella pneumophila

Tumor necrosis factor-alpha (TNF-alpha) has been shown to stimulate the resistance of alveolar macrophages and neutrophils to Legionella pneumophila in vitro. To determine whether endogenous TNF-alpha is necessary for host defense against legionellosis in vivo, anti-TNF-alpha IgG or control IgG was administered to rats exposed to aerosolized L. pneumophila. Treatment with anti-TNF-alpha neutralized >90% of the intrapulmonary TNF-alpha response to infection, resulting in persistent pneumonitis and failure to clear L. pneumophila from the lungs. Depletion of TNF-alpha limited the recruitment of mononuclear cells to the lungs and resulted in a progressive increase in the proportion of alveolar macrophages that were infected; neutrophil recruitment and phagocytosis were not impaired. Both systemic and intrapulmonary IFN-gamma levels were significantly higher in rats depleted of TNF-alpha. These observations indicate that TNF-alpha is required for the prompt resolution of pneumonic legionellosis and point to a direct role for TNF-alpha in the activation of phagocytes.