Francisella tularensis, the causative agent for tularemia, can infect humans by a number of routes, including vector-borne transmission. However, it is inhalation of the bacterium, and the resulting pneumonic tularemia, that represents the most dangerous form of disease. This is due to the short incubation time (3-5 days), non-specific symptoms, and a high mortality rate (greater than 80%) in untreated individuals. Furthermore, F. tularensis has been weaponized by both the United States and the former Soviet Union making it a viable candidate for use as a biological weapon. Despite over 80 years of research on F. tularensis around the world, very little is understood about the dynamic interaction of this bacterium with the host, especially following aerosol infection. In the last several years my laboratory has provided abundant evidence that one of the primary mechanisms by which F. tularensis successfully infects and replicates in the host is via active suppression of the host immune response in the lungs. We have a developed a reproducible murine model in which mice intranasally infected with 10 CFU to study the dynamic changes and progress of infection. This model has revealed several important points concerning pneumonic tularemia. One of the most important observations is that, unlike more attenuated strains, virulent F. tularensis actively suppresses the host immune response, including pulmonary dendritic cells, during the first few days of infection. Although we have not identified the primary mechanism of suppression there are several host molecules that appear to be involved, including Transforming Growth Factor-beta (TGF-beta) and Interferon-beta (IFN-beta). Furthermore, we have recently shown that CD14 is a critical player in the elicitation of inflammation following exposure of cells to F. tularensis. Cells that lack CD14 are still susceptible to infection, but fail to produce pro-inflammatory cytokines. Furthermore, these cells become refractory to further stimulation by other microbial components. The specific role and the mechanism in which F. tularensis and its components interacts with CD14 is currently under investigation in the laboratory. In addition to CD14, we have made surprising and important observations involving the host plasminogen system (PAS) and its manipulation by virulent F. tularensis. Using both in vitro and in vivo systems we have shown that F. tularensis bind plasminogen and then converts it to the active serine protease plasmin in the presence of the host enzyme urokinase plasminogen activator (uPA). These plasmin coated bacteria readily degrade immunoglobulin in vitro. Importantly, we also observed that mice lacking uPA, and thus would not allow formation of plasmin coated bacteria, readily control F. tularensis infection, have higher numbers of B cells in specific target organs and develop F. tularensis specific IgG. Together these data provide both important insight into the role of the host PAS during Tularemia infections as well as important new understanding of how uPA might control B cell development and proliferation. In addition to understanding the way in which F. tularensis manipulates the host innate immune response we are investigating host components required for development of a protective adaptive response. To date, the only vaccine available (although not licensed in the United States) is an attenuated, Type B strain of F. tularensis known as Live Vaccine Strain or LVS. However, there are a number of problems associated in the use of this vaccine including an unpredictable phase shift in its LPS which renders the bacterium completely ineffective against pneumonic tularemia. Furthermore, the specific mechanism by which this vaccine protects against tularemia is not known. We have initiated experiments to identify host cellular and molecular components required for protection against F. tularensis. Since LVS offers incomplete protection we have developed an additional model of Francisella immune mice. By treating mice that have been infected with virulent F. tularensis for several days with antibiotic we have generated mice that develop an effective immune response against virulent F. tularensis without the concern of additional, potentially non-specific, responses generated in LVS vaccinated animals.
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