Pathogenic bacteria and their hosts have had a two way conversation for millions of years. The host has developed sophisticated mechanisms to protect itself against pathogens and in turn pathogenic bacteria have developed mechanisms to alter and evade the host immune response. Francisella tularensis is a facultative intracellular organism and the causative agent of tularemia. Due to the organism's low infective dose, high morbidity and mortality, and the fact that it was weaponized by both the U.S.A. and the USSR it has been designated a category A bioterrorism agent. Consequently, there is an urgent need to understand the biology of the organism and the underlying molecular and cellular mechanisms by which F. tularensis alters the immune system of its hosts. Recent studies clearly demonstrate that F. tularensis is able to modulate the host immune response and create an environment in favor of its survival in the host. However, we have only sketchy knowledge about the mechanisms by which F. tularensis alters the host immune system. F. tularensis clearly alters the immune response by inhibiting cytokine and T cell responses. F. tularensis infection results in prostaglandin E2 (PGE2) production by infected antigen presenting cells (APC). PGE2 produced by APC inhibits the proliferation and activation of T cells. Furthermore, the prolonged induction of PGE2 during respiratory tularemia dampens the generation of a robust adaptive T cell response and as a result, significantly delays the clearance of the organism. In preliminary findings a Francisella transposon mutant library was screened and 20 candidate genes associated with induction of PGE2 were identified. The first goal of this proposal will identify the effector molecule responsible for the induction of PGE2. This will be done by utilizing nonpolar markerless clean deletions of candidate genes, as well as eukaryotic expression of candidate genes. The second goal will be to determine the eukaryotic sensor responsible for identifying the F. tularensis product. This will be accomplished with the use of a yeast two hybrid screen. The identification and characterization of this molecular interaction could be conceivably exploited for antimicrobial chemotherapy targets.