Project Summary Intracellular bacterial pathogens such as Legionella pneumophila, the causative agent of the severe pneumonia Legionnaires' disease, are responsible for significant disease burden in the United States every year. Successful control of Legionella and other intracellular bacterial pathogens requires a robust immune response that allows for production of inflammatory cytokines, such as Tumor Necrosis Factor (TNF), that enables macrophages to restrict intracellular bacterial replication. TNF is required for host defense against Legionella and other pathogens in mouse models of infection. TNF is also critical for the control of Legionella and other intracellular pathogens in humans, as patients taking TNF blockers to treat inflammatory disorders such as rheumatoid arthritis or ulcerative colitis are at increased risk of acquiring Legionnaires' disease and other bacterial infections. However, the precise molecular and cellular mechanisms by which TNF mediates anti-bacterial defense are still unclear. Our new preliminary data indicate that that TNF promotes caspase-8- dependent cell death during Legionella infection, and that mice and macrophages deficient for caspase-8 are defective in controlling Legionella infection. These new data provoke the hypothesis that TNF promotes caspase-8-dependent apoptosis of infected macrophages to restrict Legionella infection. Thus, Aim 1 seeks to test the hypothesis that TNF induces caspase-8-mediated death of infected macrophages to restrict intracellular bacterial replication. Aim 2 will test the hypothesis that TNF- and caspase-8-mediated death of infected alveolar macrophages promotes host control of pulmonary Legionella infection. These studies will provide fundamental insight into how TNF mediates immune control of intracellular bacterial infection, and may provide a basis for the development of improved therapeutics for the treatment of Legionella and other bacterial infections.