Immunity to Pneumonic Tularemia

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Summary: 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 are exposed to a low dose, whole body, aerosol (30-50 CFU) in which 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), Prostaglandin E2 (PGE2) and Vascular Endothelial Growth Factor (VEGF). The specific role and the mechanism by which F. tularensis modulates expression of these molecules is currently under investigation in the laboratory. One elusive goal in combatting pneumonic tularemia is the development of effective non-antibiotic based therapeutics that can provide protection shortly before or after infection. Previous reports have suggested that appropriate immunogens may be able to overcome the immunosuppression invoked by Francisella and enable the host to effectively eradicate the bacterium. For example, LPS purified from the attenuated F. tularensis Live Vaccine Strain (LVS) injected 3 days prior to a lethal LVS challenge protects all infected animals. A similar phenomenon has been observed following injections of CpG nucleic acid motifs or cationic-lipid DNA complexes (CLDC). While it has been previously shown that LVS LPS does not protect against infections with fully virulent F. tularensis, it is not known if CpG or CLDC can engender protective immunity against similar strains. In collaboration with Juvaris Biotherapeutics, we are currently examining the protective efficacy of CLDC against aerosol challenge with virulent F. tularensis with the goal of developing an easily administrated therapeutic that could be quickly distributed following a natural outbreak of terror event. 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. In collaboration with Dr. John Belisle (Colorado State University) we are testing acellular vaccines derived from LVS. Using crude sub-cellular fractions we have been able to generate protection against low dose aerosols nearly equivalent as that observed in animals vaccinated with LVS. Furthermore, we are currently identifying correlates of immunity for survival of pneumonic tularemia using these vaccines combined in adjuvants designed to skew the immune response in a polarized fashion. This approach will allow us to identify specific requirements in the host without the complications of using genetically modified mice.