Inhalation of as few as 10-50 F. tularensis organisms can lead to glandular tularemia as well as to acute and severe bronchiolitis, pneumonitis or pleuropneumonia. This extreme infectivity of F. tularensis, and the significant morbidity and mortality that can result in the absence of prompt antibiotic treatment, are reasons why F. tularensis has traditionally been considered to have potential as an agent of biological warfare. Little is known about the pathogenic mechanisms of F. tularensis, including the basis for its extraordinary infectivity. This infectivity may be at least partially explained by efficient uptake, survival and growth within host macrophages. The ability of F. tularensis to propagate intracellularly is enhanced by failure of lysosomes to fuse with the phagosome and the lack of stimulation of a respiratory burst. However, little has been described pertaining to the initial uptake of F. tularensis by macrophages early in infection. In general, macrophages can recognize intact bacteria via several different receptors; whether or not a respiratory burst is stimulated depends on which receptor(s) mediates uptake. We hypothesize that specific components on the surface of F. tularensis facilitate uptake by macrophages and that the identification of these interactions will lead to an understanding of why macrophages fail to undergo a respiratory burst. We further postulate that the identification of these surface components will suggest targets for modulating the immune response. In order to test this hypothesis we propose the following specific aims: 1) Generate and screen mutants of F. tularensis that are deficient in being taken up by macrophages, 2) Generate and screen mutants of F. tularensis that become capable of stimulating the production of a respiratory burst or alter cytokine secretion in unstimulated macrophages, 3) Examine the mutants identified in Aims 1 and 2 in an animal model of infection noting changes in virulence, pathology, and immunity. The completion of these Aims will directly support at least two other subprojects, including the testing of vaccine candidates.